Large-scale fabrication of silk fibroin fibers with aligned porous microstructure for thermal insulation textiles

Polar bears living in extremely cold environments such as the Artic circle are gifted with a natural capability of keeping them warm. The effective thermal insulation is provided by their thick fat fur covered by hollow hairs consisting of a unique microstructure of hollow core and aligned shells with large pore volume. Mimicking such characteristics in synthetic fibers could make a huge impact in the development of smart textiles for thermal insulation. Researchers at Zhejiang University, China lead by Prof. H. Bai have used a “freeze-spinning” method to convert silk fibroin to continuous and large-scale fabrication of fibers with aligned porous microstructure, mimicking the structural and functional features of the hair of a polar bear.

The “freeze-spinning” method involves a combination of “directional freezing” and “solution spinning” (Fig. 1). A well-dispersed viscous aqueous silk fibroin solution (50 mg/ml) with a small amount of chitosan (w_{silk fibroin} : w_chitosan = 9:1), is extruded using a syringe at a constant speed to form a stable liquid wire. When the wire slowly passes through a cold copper ring (green colour ring in Fig. 1), ice crystals grew directionally with a lamellar pattern within the wire that enables expelling and assembling of the solutes to template the ice morphology. When the extrusion speed becomes equal to the freezing speed, a stable solid–liquid interface is formed above the cold copper ring. The collected frozen fiber is freeze dried to preserve its porous microstructure. Subsequently, these fibers are woven into a textile.

Fig. 1 Schematic illustration of the “freeze-spinning” technique, combining “directional freezing” with “solution spinning” to realize continuous and large-scale fabrication of biomimetic fibers with aligned porous structure (Image credit: Cui et al., Adv. Mater. 2018, 1706807; DOI: 10.1002/adma.201706807)

The microstructure of the fibers can be controlled by a careful choice of parameters such as solution concentration/viscosity, extrusion speed and freezing temperature. Scanning electron micrographic images acquired at the axial cross-section of fibers prepared at −40, −60, −80, and −100 °C indicate an aligned porous structure while those prepared at −196 °C possess a random porous structure (Fig. 2). The degree of variation in porosity of these fibers suggests that it would be possible to prepare fibers with different pore size by simply varying the freezing temperature. The aligned porous microstructure imparts a better strength and modulus for fibers prepared at −40, −60, −80, and −100 °C than the one with random pores obtained at −196 °C.

Fig. 2 Radial cross-sectional SEM images showing different porous structures of biomimetic fibers prepared at different freezing temperatures (Image credit: Cui et al., Adv. Mater. 2018, 1706807; DOI: 10.1002/adma.201706807)

Assessment of ability of the fibers with different pore size for thermal insulation, evaluated by the change in surface temperature of the fibers using infrared images and a measure of the temperature difference (|ΔT |) between the fiber surface and the stage indicates that for a given stage temperature, the smaller the pore size of fiber, the better is its insulating property. The insulation ability of the woven textiles with different layers (1, 3 and 5 layers) indicate that the one with more layers offer better thermal insulation property (Fig. 3(a)). In spite of the free fibers, a better insulation property is also observed for textiles woven using fibers with a smaller pore size, as evidenced by the infrared images and a higher |ΔT | (Fig. 3(b)). A comparison of the infrared images of rabbits covered with a single layer (~ 0.4 mm thick) of polyester and the woven textile clearly demonstrate the better thermal insulation ability of the latter. The small difference between the surface temperature and the background makes the rabbit covered with the woven textile almost invisible to the infrared camera (Fig 4(a)). The ability of the woven textiles to demonstrate this effect over a wide range of temperature from −10 to 40 °C (Fig. 4(b)) suggest that they can very well be explored as a thermal sheath material for military applications.

Fig. 3 (a) Infrared images of textiles woven from different porous fibers. Temperature of the textile surface is measured based on the infrared images when changing the stage temperature from −20 to 80 °C; (b) Temperature difference (|ΔT|) between the textile surface and the stage against the stage temperature for different textiles (Image credit: Cui et al., Adv. Mater. 2018, 1706807; DOI: 10.1002/adma.201706807)

Fig. 4 (a) Photographic and infrared images of a rabbit before and after wearing the commercial polyester textile and the textile woven with biomimetic porous fibers; (b) Rabbit wearing the biomimetic thermal stealth textile becomes invisible by the infrared camera, regardless of the background temperature (Image credit: Cui et al., Adv. Mater. 2018, 1706807; DOI: 10.1002/adma.201706807)

When carbon nanotubes (CNTs) are dispersed along with the silk fibroin solution, it would be possible to impart a conductive network for the fiber without damaging its aligned porous structure (Fig. 5(a)). The incorporation of CNTS helps to induce electrical conductivity (~1.1 S/m) and upon impressing an applied voltage of 5 V using a portable power source, the surface temperature of the CNT-doped textile can be increased from ~24 to 36.1 °C within 45 s (Fig. 5(b)). The temperature of the CNT-doped textile can be easily manipulated by an appropriate choice of applied voltage (Fig. 5(c)). By combining two layers of textiles, one with CNTs (for electrical heating) and another one without CNTs (for thermal insulation) it would be possible to develop a hybrid textile.

Fig. 5 (a) Photographic and SEM images of the CNT-doped textile; (b) Infrared images of a CNT-doped textile during the heating process at an applied voltage of 5 V; and (c) Extent of  increase in temperature versus time after applying a voltage of 1, 3, and 5 V to a 5 × 2 cm CNT-doped textile (Image credit: Cui et al., Adv. Mater. 2018, 1706807; DOI: 10.1002/adma.201706807)

The excellent thermal insulation property of the silk fibroin fibers and the woven textiles using them, the feasibility to impart electrical heating by incorporating CNT along with the fibers, good breathability and comfort in wearing the woven textiles seems to be promising towards the development thermal sheath materials for military applications and materials for personal thermal management.

T.S.N. Sankara Narayanan

For more details, the reader may kindly refer Y. Cui et al., A Thermally Insulating Textile Inspired by Polar Bear Hair, Adv. Mater. 2018, 1706807, DOI: 10.1002/adma.201706807

 

Advances in Nanotechnology Eliminates the Use of Surgical Blades for Controlled Remodeling of Oral Connective Tissues

Malocclusion is a misalignment between the teeth in the upper and lower dental arches when they approach each other as the jaws close. In India, more than one million people are being treated for malocclusion every year. Though misalignment of teeth could occur during teeth development, it could be aggravated by childhood habits such as thumb sucking. The most common cause of misalignment of teeth is considered to be due to relatively smaller size of the jaw when compared to the size of the teeth. The recommended treatments for misaligned teeth could involve a minor surgical procedure and the use of braces to prevent relapse of the repaired teeth. In the gingiva, the teeth and underlying alveolar bone are connected by collagen type-I supracrestal fibers. For patients with severe malocclusion, sectioning of the collagen fibers using a scalpel is necessary to maneuver the teeth to a proper position (Fig. 1(a)). The invasive nature of the surgical procedure and severe pain encountered by patients warrants development of alternate procedures. In recent years, the advancements in nanotechnology has reached new heights in revolutionizing medical care. Researchers at Technion – Israel Institute of Technology lead by Prof. Avi Schroeder along with other researchers at Rambam Medical Center, Moriah Animal Companion Center and Tel Aviv Sourasky Medical Center, Israel have demonstrated a nanotechnology based approach that enabled controlled delivery of proteolytic enzymes, which eliminates the use of surgical blades to correct malocclusion (Fig. 1(b)). They have tested the ability of nanoparticles loaded with a proteolytic enzyme to replace surgical procedures by directly targeting collagen type-I fibers in the oral cavity (Fig. 1(c)).

Fig. 1 (a) Pictorial representation of teeth confined to their natural orientation by soft and hard tissue. Specifically, collagen type-I fibers anchor the teeth to the underlying bone; (b) Nanoparticles (blue spheres) loaded with collagenase, a proteolytic enzyme with specificity towards collagen, are inserted into the sulcus; (c) The nanoparticles maintain the enzyme’s therapeutic release profile and confine the biodistribution to the treatment site.

Collagenase is a proteolytic enzyme and once activated by calcium, it is capable of cleaving the collagen backbone by detaching the peptide link between glycine and leucine. Since the collagenase needs to be activated only after it is placed at the surgical site, it is loaded inside 100 nm liposomes (nanoscale vesicles with an inner aqueous core surrounded by a lipid bilayer membrane). The liposome lipids were not susceptible to degradation by collagenase. Since the liposomal lipid bilayer composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine is impermeable to Ca²⁺ ions, an early activation of the enzyme is prevented. Once placed in the sulcus, diffusion of collagenase out of the liposomes occurs. Calcium, which is naturally present in the oral cavity, activates the collagenase, which in turn helps to relax the collagen fibers. The degradative activity of the collagenase is responsible for weakening of the collagen fiber, which is evidenced by the extent of increase in weakening of the fibers with an increase in concentration of collagenase. It is important that during treatment, the collagen fibers must be relaxed but should not tear and a collagenase concentration of 0.05-0.1 mg/mL is considered to be optimal.

The change in morphological features of the collagen fiber before treatment, during treatment and during collagen regeneration indicate that the tightly packed fiber structure of collagen (Fig. 2(a)) tends to relax after treatment with collagenase (Fig. 2(b)) but resumes their initial morphology except in some regions wherein the fibers lack a perfect alignment (Fig. 2(c)). It is also important to ensure that as the collagen fibers tend to relax, the bonding between collagen fibers and fibroblasts should be retained. Following the collagenase treatment, the morphology of the fibroblast is changed from an elongated structure to a round structure. Fortunately, the collagen fibers neither detached from the fibroblasts nor impacted their viability. Hence, the adherent fibroblasts could perform the natural reparative processes.

Fig. 2 HR-SEM images of collagen fiber (a) before; (b) during; and (c) after treatment with collagenase indicating the regeneration of collagen.

Comparison of the efficiency of treatments performed on rats by traditional surgical protocol using a scalpel and by the controlled delivery of proteolytic enzymatic surgery over a period of 15 day treatment indicates a similar enhancement in tooth alignment trajectory motion. Groups treated with empty liposomes (without collagenase enzyme) and groups treated with free enzymes (without loading them in liposomes) fails to display any significant improvement in tooth alignment while those treated using liposomal nanoparticulate loaded with the collagenase enzyme exhibits a three-fold improvement in  tooth alignment (Fig. 3). The degree of inflammation appears to be similar; a mild inflammation is observed among all the groups tested. Bone recovery is found to be faster for the liposomal nanoparticulate system when compared to other groups treated with ordinary braces. The occurrence of tooth relapse is relatively less for groups treated using liposomal nanoparticulate loaded with the collagenase enzyme when compared to the control groups. The ability of the liposomal system to protect the collagenase enzyme from deactivation, to prolong its release profile and to confine the spatial distribution of the enzyme to the treatment site is considered responsible for the observed improvement.

Fig. 3 Comparison of the efficiency of treatment for different groups in terms of cumulative tooth movement as a function of time

This nanotechnology based approach enables a controlled delivery of proteolytic enzymes and eliminates the use of surgical blades to correct malocclusion. The success of this treatment approach lies in the appropriate choice of the proteolytic enzyme that can be biologically tailored towards the target organ and ability of the liposomal system to enable a controlled delivery of proteolytic enzymes with a required therapeutic dose confined to the treatment site.

T.S.N. Sankara Narayanan

For more details, the reader may kindly refer Assaf Zinger et al., Proteolytic Nanoparticles Replace a Surgical Blade by Controllably Remodeling the Oral Connective Tissue, ACS Nano, 2018, DOI: 10.1021/acsnano.7b07983

 

A Multi-Analyte Blood Test for Cancer Detection and Localization

Cancer, usually referred to uncontrolled growth of abnormal cells, leading to the formation of a tumour. It is strongly believed that the best chance to arrest cancer is its early detection. Nevertheless, it is not possible to detect many tumors until it is grown sufficiently or it spreads across other parts of the body. Numerous efforts are constantly being made by several researchers to develop effective methods for cancer detection. During the uncontrolled growth of cancer cells some of them would die and shed their mutated DNA into the bloodstream. Liquid biopsy test could detect the DNA carrying mutations, which are associated with cancer. However, development of liquid biopsy test that is capable of screening healthy people remains a big challenge. In addition, the inability of the liquid biopsy test to detect the location of the cancer is a major limitation. A team of researchers at Johns Hopkins University School of Medicine, Baltimore led by Nickolas Papadopoulos and Bert Vogelstein have developed a multi-analyte blood test, referred as “CancerSEEK” for early detection of cancer. CancerSEEK is a ‘liquid biopsy test” that examines mutations in cell-free DNA and proteins circulating in the bloodstream (Fig. 1). This research work is funded by National Institute of Health (NIH), USA and the findings of this study is published recently in Science (J. D. Cohen et al., Science 10.1126/science.aar3247 (2018)).

Fig. 1 Schematic of the liquid biopsy test – Tumour cells shed protein and DNA into the blood stream that can be used as biomarkers for early cancer detection

About 1,005 patients diagnosed with ovary, liver, stomach, pancreas, esophagus, colorectum, lung, or breast cancers (Stage I to III) were used to check the ability of CancerSEEK in which none of them have received chemotherapy prior to blood sample collection and none had evident distant metastasis at the time of study. CancerSEEK evaluates the levels of 8 proteins and the presence of mutations in 2,001 genomic positions in 16 different genes, which helps to identify at least eight common types of cancers. Since the test uses a combination of protein biomarkers along with genetic biomarkers, a better sensitivity is achieved without compromising specificity. CancerSEEK is capable of not only identifying the presence of tumours but also localize the organ at which the cancer cells are grown.

CancerSEEK was found to be 98% accurate for tumours in ovary and liver. The median sensitivity of CancerSEEK was estimated to be 73% and 78% for stage II and stage III cancers, respectively. Unfortunately, the success rate of  CancerSEEK for stage I cancer was limited to 43% (Fig. 2(a)). In spite of its low detection ability for stage I cancer, its ability to narrow down the localization of the cancer in 83% of the patients (Fig. 2(b)) makes CancerSEEK as a most reliable method for cancer detection.

Fig. 2 Performance of CancerSEEK: (a) Sensitivity of CancerSEEK by stage; Bars represent the median sensitivity of the eight cancer types and error bars represent standard errors of the median; and (b) Sensitivity of CancerSEEK by tumor type. Error bars represent 95% confidence intervals.

CancerSEEK is expected to be available in the next few years at an estimated cost of less than US$500. Since cancer-related proteins used by Cancer-SEEK could also appear in people with inflammatory diseases such as arthritis, the applicability of this test for such patients is questioned. As an early detection is the key to surgically remove cancer cells before they metastasise, the detection level of 43% for stage I cancers needs to be improved by a large margin.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer to: J. D. Cohen et al., Science 10.1126/science.aar3247 (2018)

Direct delivery of drugs to the brain using ultrathin needle

Parkinson’s disease (PD) is a long-term neurodegenerative disorder of the central nervous system that predominately affects the motor system. Patients affected by PD are deprived of dopamine-producing neurons in the substantia nigra, which leads to a decrease in dopamine levels in their brain. Drugs such as I-dopa often interact with neurotransmitters or the cell receptors. Since the application of these drugs cannot be localized only to the affected site, they could cause severe side effects on all parts of the brain. Will it be possible to deliver the drug within a cubic millimeter of the brain so that we can treat the PD while limiting the side effects of the drug? Researchers at MIT have developed  a miniaturized system that is capable of delivering a very small amount of drug to any specific region of the brain confined to a small space of 1 mm³, without interfering with the normal function of the rest of the brain.

The device was fabricated by microfabrication technique and it consisted of several tubes (diameter: ~30 µm; length: ~ 10 cm) and they are contained within a stainless steel needle (diameter: ~150 µm). The tubes can be connected to small pumps to deliver hundreds of nanoliters of drugs. The device is very stable and robust, and it can be implanted under the skin. Since the device consists of several tubes contained within a needle, which is as thin as a human hair, it would be possible to deliver one or more drugs deep within the brain, with very precise control of the amount of drug and where it should be administrated. In a rat model, they delivered “muscimol” through one of the channels of the device to substantia nigra (one of the regions of the brain) and identified symptoms similar to those seen in PD. However, by delivering a dose of saline thorough another channel, which washes away “muscimol”, the Parkinsonian behaviour is altered. The researchers believe that the device can be customized with different channels to deliver drugs targeting tumours or in treating Parkinson’s or Alzheimer’s disease

T.S.N. Sankara Narayanan

 C. Dagdeviren el al., Science Translational Medicine  24 Jan 2018: Vol. 10, Issue 425, eaan2742

Non-Endoscopic Balloon-Based Device for Sampling Cancer Detection

Esophageal Adenocarcinoma (EAC) – the cancer that occurs in the lower portion of the esophagus (the food pipe that runs between throat and stomach) is the most common form of cancer in the United States. In spite of a steady increase in the incidence of EAC over the past 3 decades, the prognosis remains poor. Barrett’s esophagus (BE) is the only known precursor for EAC and it is currently diagnosed using an endoscope. Sanford Markowitz at Case Western Reserve University, Ohio, and his colleagues have demonstrated the feasibility of a non-endoscopic molecular cytology screening method for BE and EAC (Moinova et al., Science Translational Medicine, Vol. 10, Issue 424, eaao5848)

The non-endoscopic swallowable balloon-based esophageal sampling device consists of a pill-sized capsule (16 × 9 mm) attached to a thin 2.16 mm silicone catheter (Fig. 1, A and B), which can be easily swallowed. After delivery into the stomach, the balloon is inflated by injecting 5 cm³ of air through the catheter (Fig. 1C). The inflated balloon can be gently moved through the distal esophagus to collect samples from the luminal epithelial surface. Subsequently, the balloon is deflated and inverted back into the capsule (Fig. 1D). After complete retrieval of the capsule through the mouth, DNA is extracted from the balloon surface for molecular analysis. One of the prime advantages of this balloon-based sampling device is its ability to deploy rapidly by inflation unlike the conventional sponge-based devices, which requires sufficient waiting time for the coating to dissolve. The ability of the balloon to retract back into its capsule after sampling protects the sample from dilution or contamination from the proximal esophagus or oral cavity. The swallowable balloon-based device enables a simple and rapid method to collect DNA samples from the distal esophagus of unsedated outpatients. A combination of this balloon-based sampling device with bisulfite sequencing for detecting DNA methylation, provides a highly sensitive and specific yet minimally invasive screening protocol that could be clinically used for the detection and screening of BE.

Fig. 1 Non-endoscopic balloon-based device: (A) Device capsule and catheter (a vitamin pill and a dime are included for size comparison); (B) Capsule containing inverted balloon for swallowing; (C) Capsule with inflated balloon for esophageal sampling; and (D) Capsule containing inverted balloon for device and biospecimen retrieval.

T.S.N. Sankara Narayanan

 

Cellulose Particles – An Emerging Alternative for Microplastics?

Microplastics have caused a massive problem in the ocean and it is a major environmental concern. Cosmetic products often contain microplastics made of polyethylene (PE) and polypropylene (PP), particularly in dental and skincare products, due to their abrasive nature and ability to assist in cleaning. Since they possess a better chemical stability, they are used as stabilizers and fillers. Nevertheless, their non-biodegradable nature becomes a serious limitation. The microplastic particles due to their small size are easily absorbed by sea-living organisms and thereby enters our food chain. As the use of microplastic particles or microbeads has to be phased out by the end of 2020 (deadline set by Cosmetics Europe), many companies are seeking for suitable alternatives.

Researchers at the Fraunhofer Institute for Microstructure of Materials and Systems IMWS have developed and tested biodegradable cellulose particles as an alternative for microplastic particles that could meet the requirements of  abrasive particles in dental care products as well as provide better cleaning performance in skincare products (Fig. 1(a)). When tested, the biodegradable cellulose particles incorporated toothpaste exhibited a low abrasion and efficient cleaning. The biodegradable cellulose particles present in the toothpaste are found to be effective in mechanical removal of bacterial plaque, tooth discoloration and food residues, with no damage to the tooth enamel (Figs. 1(b-d)). The cellulose particles are highly comparable to the microplastic particles, in terms of effectiveness of plaque removal and cleaning.

Fig. 1 (a) SEM image of the cellulose particles prepared from beech wood; (b-d) Photographic images of tooth enamel samples: (b) in their initial state; (c) after discoloration; and (d) after cleaning using toothpaste containing cellulose particles (Source: Fraunhofer IMWS)

Currently, the researchers are aiming at a cost-effective and large scale production of biodegradable cellulose particles from beech wood, oats, wheat and maize. The easy biodegradability and low production cost are the major advantages in using cellulose particles in place of microplastic particles. Designing the cellulose particles with a suitable size, shape, hardness and surface profile to meet the requirements of the product is the main challenge ahead. The researchers are optimistic that these cellulose particles could also find application in cosmetic products such as mascara, powder and lipstick.

T.S.N. Sankara Narayanan

Flexible and Shape-Reconfigurable Hydrogel Interlocking Adhesives – The Game Changer to Achieve Excellent Adhesion in Wet Environments

Achieving good adhesion between surfaces under wet conditions is often difficult. Nevertheless, it is an important requirement for a variety of applications. The performance of commercially available wet adhesives deteriorates with time due to hydration-induced softening and dissolution. Researchers at Department of Mechanical Engineering, Ulsan National Institute of Science and Technology (UNIST) and Department of Mechanical Engineering, Incheon National University, Republic of Korea have demonstrated a flexible, wet-responsive, and shape reconfigurable hydrogel adhesive with microhook arrays that is capable of exhibiting a strong and reversible interlocking adhesion in wet or underwater conditions. The interlocking adhesion mechanism is based on the hydration-triggered swelling behavior of the hydrogel polymers (Hyun-Ha Park et al., ACS Macro Lett., 2017, 6 (12), pp 1325–1330).

Polyethylene glycol dimethacrylate (PEGDMA) (molecular weight: 550) was used to fabricate the microscale hook arrays (Fig. 1(a)). The choice of PEGDMA hydrogel is based on its ability to strike a balance between the degree of swelling and the mechanical strength, absorb large quantities of water or physiological solutions, amenability for UV curing. The PEGDMA microscale hook arrays were designed with protruding heads that enables an effective interlocking between two mating arrays (Fig. 1(b)). When two identical hydrogel microhook arrays are brought into contact with each other, only a relatively weak adhesion between them could be realized under dry conditions. However, a significant volume expansion and shape transformation of the hydrogel microhooks occurs upon exposure to water (Fig. 1(c)). This is due to the anisotropic swelling of the hydrogel that provides a higher structural bending strength, contact surface, and friction with the neighboring microstructures. The adhesion strength is further increased with an increase in swelling time due to swelling-induced shape changes in the microhook arrays. During swelling, the size of the microhooks is increased in the direction normal to the surface rather than in the lateral direction due to the mechanical constraint provided by the substrate. The PEGDMA adhesives with smaller pitches offered a higher shear and normal strengths since they could exert a larger overlapping areas and provide tighter contact with the neighboring structures. This water-responsive shape change of the hydrogel adhesive is highly reversible upon removal of water by drying (de-swelling). During repeated cycles of swelling and de-swelling, the PEGDMA hydrogel adhesive exhibit excellent performance without any notable degradation in the extent of adhesion.

Fig. 1 (a) Fabrication protocol of the microhook arrays by photolithography using two layers of the photoresist (i.e., LOR30B and AZ 4330); (b) illustration of the reversible swelling and de-swelling process of the PEGDMA microhooks; and (c) illustration of the reversible interlocking of the PEGDMA microhook arrays via the hydration-induced shape reconfiguration of the array for high adhesion under wet conditions

T.S.N. Sankara Narayanan

Screen-Printed Paper Microbial Fuel Cell Biosensor Comes in Handy for Detecting the Presence of Toxic Compounds in Water

Microbial fuel cell (MFC) technology directly converts chemical energy contained in organic matter into electricity through the metabolic processes of microorganisms and it is shown to be promising to assess the quality of water. The development of a biofilm consisting of an electroactive bacteria on the surface of the anode is capable of transferring electrons generated from the oxidation of organic compounds to the electrode. Thus, the current generated by the MFC can be correlated to the metabolic activity of the anodic bacteria and any possible disturbances to its metabolic pathways, caused by environmental changes, such as organic load, or due to the presence of toxic compound(s), is likely to be reflected in the measured current. In spite of their ability, implementation of MFCs as sensors is limited by device designs and high cost involved in the fabrication process. Researchers at University of Bath, United Kingdom have developed a simple and cost-effective single-component paper-based MFC (pMFC) sensor device by screen printing carbon-based electrodes onto a single sheet of paper (Jon Chouler et al., Biosensors and Bioelectronics, 102 (2018) 49-56).

The paper-based MFCs (pMFC) sensor was fabricated by screen-printing. The conductive ink contains a solution mixture with 20 mg α-cellulose dissolved in 1-ethyl-3-methylimidazolium and dimethyl sulfoxide with 92:8% w/w ratio in which 40 mg carbon nanofibers and 40 mg graphite powder were thoroughly dispersed. Three layers of the conductive ink were screen-printed (43–80 μm mesh) onto the paper to form the electrodes (Fig. 1). Since the paper substrate itself acts as the separator between the two electrodes, the cellulose fibers within the paper were cross-linked with glyoxal (0-24% w/v at 20 °C for 3 h), which helped to increase robustness and operational lifetime of the sensor device. The electrochemical performance of the paper-MFC sensor for detecting formaldehyde, a potential toxic compound, was evaluated. Two pMFCs were folded back-to-back to fabricate fpMFC which enriched the performance of the sensor device. Portability, facile use, and biodegradability are the unique advantages of this paper-based MFC sensor.

Fig. 1 (a) Schematic of the pMFC and electrical connection; (b) Photograph of the actual pMFC, showing size; (c) Principle of operation of the pMFC; and (d) Assembly of the fpMFC by folding two pMFCs back-to-back (1), with parallel electrical connection (2).

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Jon Chouler et al., Biosensors and Bioelectronics, 102 (2018) 49-56

Tau – an abnormal protein has been identified as the reason for spreading of Alzheimer’s disease throughout the brain

Alzheimer’s disease (AD) is a chronic neurodegenerative disorder and worldwide about 47 million people are affected by this disease. The exact reasons for the occurrence of AD is not yet clearly understood. Genetic issues and deficiency of magnesium have been considered as possible reasons. In a recent study published in Brain, Thomas Cope et al., provide the first evidence that “tau” – an abnormal protein spreads between connected neurons in humans. They have used two advanced brain imaging techniques viz., positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) to identify the presence of tau and how it influences the brain connectivity. PET scan helps to identify the presence of tau and its distribution in different parts of the brain. The ability of fMRI to measure the blood flow in brain in real time helps to identify the connectivity between different regions of the brain. Besides these imaging techniques, they have also used a mathematical technique referred as graph analysis, which splits the brain in to 598 regions of equal size for a better analysis of brain connectivity. A combination of these methods provides meaningful information on the presence of tau, its distribution, the connectivity between different regions of the brain, all of which can very well be correlated to the Alzheimer’s disease.

Fig. 1 (a) Spreading of tau protein; and (b) Artist’s impression of tau spreading between connected neurons (Source: Thomas E. Cope)

Based on the inferences made in their study, Thomas Cope et al., have suggested that tau causes the neurons as well as their connections to die that prevents communication between different regions of the brain. Initially, tau affects the memory centers – entorhinal cortex and hippocampal formation while with an increase in time, difficulty in thinking and behavioral activity crops in, leading to a loss of independence. With an increase in the amount of tau, the neurons become less connected and the connections between them becomes random. The study by Thomas Cope et al., provides a clear evidence for the transneuronal spread of tau in humans. Hence, strategies to avoid the spreading of tau will be the next step towards the progress of preventing AD.

T.S.N. Sankara Narayanan

For further details, the reader may kindly refer: Thomas E Cope et al., Tau burden and the functional connectome in Alzheimer’s disease and progressive supranuclear palsy, Brain, awx347, https://doi.org/10.1093/brain/awx347

Fabrication of Copper Nanowires Through Multi-step Anodizing, Electrochemical Barrier Layer Thinning and Electrodeposition

Nanoporous anodic aluminum oxide (AAO) is one of the most commonly employed templates for nanofabrication since the pore diameter, interpore distance, thickness of the oxide, barrier layer and walls can be precisely controlled by a proper choice of anodizing conditions. For the fabrication of metallic nanowires (NW), it is necessary to decrease the barrier layer thickness. In addition, to achieve sufficient electrical contacts at the bottom of the pores, a thin layer of Au has to be deposited either by sputtering or by electrodeposition (ED) using a cyanide bath. Researchers at Military University of Technology, Poland and Delft University of Technology, The Netherlands have proposed a methodology that combines multi-step anodizing, electrochemical barrier layer thinning (BLT) and ED for the fabrication of Cu NW.

Commercial purity aluminum alloy (AA 1050 alloy) was degreased and electropolished (EtOH:HClO₄ 4:1, 0 °C, 20 V, 120 s, Pt grid cathode). A multi-step anodizing protocol was employed to obtain nanoporous anodic aluminium oxide (AAO) templates with a desired nanoporous structure. Mild anodization (MA) in 0.5 M H₂SO₄ with 20 vol.% ethylene glycol (EG) at 0 °C, 20 V and 60 min (Fig. 1, reaction I) was carried out as the first step. The voltage was increased up to 45 V with 0.5 V steps for each 5 s and hard anodizing was performed at 45 V for 1 h (Fig. 1, reaction II). To thin down the bottom of the barrier layer, mild anodizing was carried out in 0.3 M oxalic acid, at 30 °C, 45 V and 30 min (Fig. 1, reaction III). Electrochemical barrier layer thinning (BLT) of multi-step anodized Al alloy was performed in 0.3 M oxalic acid (Fig. 1, reaction IV). A step-wise decrease in voltage and the duration of each voltage step was varied and the suitable conditions for BLT were optimized. For effective opening of the pores at the bottom, the Al alloy at the base was chemically etched using 0.1 M CuCl₂ in HCl. The applicability of the membranes formed using a combination of MA, HA and BLT was ascertained through electrodeposition (ED) of Cu using 0.3 M CuSO₄ and 0.1 M H₃BO₃ at -0.3 V vs. Ag/AgCl for 30 min (Fig. 1, reaction V). The Cu nanowires (NW) were liberated from the AAO template by chemical etching in 5% H₃PO₄ at 30 °C for 45 min (Fig. 1, reaction VI).

Fig. 1 Schematic representation of the various stages involved in the fabrication of Cu nanowires

MA in 0.5 M H₂SO₄ with 20 vol.% EG at 0 °C, 20 V and 60 min enables the formation of a protective oxide layer. The presence of this oxide layer as well as a steady step wise increase in voltage (0.5 V steps for each 5 s) up to 45 V prevents destruction of the Al alloy anode by the high current density avalanche generated during HA at 45 V for 1 h. The MA/HA combination though improved ordering of nanoporous structure in the resultant AAO, the thickness of the barrier layer at the bottom is a critical issue. MA in 0.3 M oxalic acid,    at 30 °C, 45 V and 30 min decreased the thickness of the barrier layer by a reasonable extent, which is suitable for subsequent BLT process. Since the conditions of anodization are mild, the interpore distance and ordering of the pores are maintained. A step-wise decrease in voltage enables BLT of the anodized Al alloy in 0.3 M oxalic acid. Thinning of the barrier layer is effective at selective voltage step and time (Un+1 = 0.75.Un; Δt = 60 s). Under such conditions of BLT, the hexagonal honeycomb-like morphology is maintained at the bottom of the pores (Fig. 2(a)). The applicability of the AAO formed using a combination of MA, HA, MA and BLT is confirmed by ED of Cu NW with a high aspect ratio (Fig. 2(b)). The successful ED of Cu NW confirms efficient BLT and sufficient electrical contact at the electrolyte–aluminum interface that could have facilitated the reduction of Cu²⁺ ions at bottom of the pores.

Fig. 2 FE-SEM micrographs of (a) bottom side of the AAO (after removal of Al alloy using 0.1 M CuCl₂ in HCl) indicating the effectiveness of BLT performed at U_n+1 = 0.75 Un; Δt = 60 s; and (b) ED Cu NW

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: W.J. Stepniowski et al., Journal of Electroanalytical Chemistry, 809 (2018) 59–66.

Atmospheric Pressure Mass Spectrometric Imaging Method for Sub-cellular Imaging of Live Biological Tissue Slices with High Spatial Resolution

Atmospheric pressure matrix-assisted laser desorption ionization (AP-MALDI) method has been used for imaging of biological samples. The sampling protocol of AP-MALDI method involves heating of the samples to improve desorption and ionization of volatile organic compounds, which damages the biological samples and the limits the spatial resolution of the acquired images. Researchers at Department of New Biology, Companion Diagnostics and Medical Technology Research Group, DGIST, Daegu, Republic of Korea, Graduate School of Analytical Science and Technology (GRAST), Chungnam National University, Daejeon, Republic of Korea and KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul, Republic of Korea have reported a high spatial resolution atmospheric pressure mass spectrometric imaging method (referred as AP-nanoPALDI MS) for subcellular imaging of living biological tissue slices with a sampling depth down to several tens of μm.

The AP-nanoPALDI MS system used for high-resolution imaging of biological samples consists of a mass analyzer, a sampling stage, a femtosecond (fs) laser oscillator, an AP plasma equipment, and airflow-assisted ion transport equipment. The schematic of the AP-nanoPALDI MS system and imaging device are shown in Fig. 1.

Fig. 1 Schematic of the AP-nanoPALDI mass spectrometric analysis using a combination of fs laser oscillator, atmospheric pressure plasma jet, airflow-assisted ion transfer equipment and imaging systems

The focused fs laser enables desorption of the neutral molecules while the non-thermal AP helium plasma jet device helps to maintain a helium plasma medium above the sample, thus promoting desorption and ionization of the sample. The airflow-assisted ion transfer tube facilitates effective transport of molecules and ions to the mass analyzer. Since biological samples do not effectively absorb the NIR light, rod-shaped gold nanorods modified by polyethylene glycol (mPEG-AuNRs) are embedded inside the tissues, which served as hot spots, rapidly converts the absorbed photon energy into thermal energy, promotes absorption of the NIR light, enhanced the extent of desorption of neutral molecules, increased the intensity of the mass spectra and favours acquisition of high-quality MS images. Analysis of MS images of a mouse hippocampal tissue slice indicates that most of the strong ion signals are under m/z = 500 and the detected ions can be assigned to particular lipids and metabolites such as adenine, cholesterol and monoacylglycerol ions.

Fig. 2 (a) Optical images and (b, c) mass spectrometric images of a mouse hippocampal tissue slice

In spite of the continuous irradiation with a large number of laser shots, the use of mPEG-AuNRs and non-thermal AP plasma jet as post-ionization source prevents thermal damage of the biological specimens. In the absence of AuNRs, mass spectra could not be recorded. Since the images acquired from live tissue slices provide plenty of spatial information for metabolites, the AP-nanoPALDI MS method can also be used for tissue-based drug screening, which is evidenced by a decrease in cholesterol level in hippocampus tissues treated with methyl β-cyclodextrin. The AP nanoPALDI MS imaging technique can be applied for label-free bioimaging applications and tissue-based drug screening.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Jae Young Kim et al., Atmospheric pressure mass spectrometric imaging of live hippocampal tissue slices with subcellular spatial resolution, Nature Communications, 8 (2017) 2113, DOI: 10.1038/s41467-017-02216-6

Detecting cancer from urine sample – Will the development of a nanowire based device open the gates for early diagnoses and timely medical checkups for cancer?

MicroRNAs (miRNAs) encapsulated by extracellular vesicles (EVs) are found in body fluids of patients with malignant diseases as well as with those having a better health. The difference in the EV-encapsulated miRNAs between these two groups of people could be used as a signature to identify various diseases. The miRNAs present in urine could serve as biomarkers for detecting cancer. Unfortunately, the concentration of EVs in urine is extremely low (<0.01 volume %) and hence the most commonly used method of extraction – ultracentrifugation is not capable of extracting nearly 90% of the miRNA species, due to their low abundance. Recently, a nanowire-based device anchored to a microfluidic substrate is fabricated for the efficient collection of EVs and in situ extraction of various miRNAs of different sequences, which is believed to open the gates for urine-based early diagnoses and medical checkups for cancer (Yasui et al., Sci. Adv. 2017;3: e1701133).

Si (100) served as the base substrate (Fig. 1(a)), which was coated with a positive photoresist followed by channel patterning using photo lithography (Fig. 1(b)). A 140 nm thick Cr layer was sputter deposited (Fig. 1(c)), followed by removal of the photoresist layer and thermal oxidation of the Cr layer at 400 °C for 2 h (Fig. 1(d)). The thermally oxidized Cr layer served as the seed layer for subsequent growth of ZnO nanowires using a solution mixture of 15 mM hexamethylenetetramine (HMTA) and 15 mM zinc nitrate hexahydrate at 95 °C for 3 h (Fig. 1(e)). PDMS was poured over the ZnO nanowire grown substrate and cured (Fig. 1(f)). Subsequently, the PDMS was removed from the Si substrate (Fig. 1(g)) and the ZnO nanowires in the PDMS were transferred to another PDMS substrate. The transferred nanowires were uniformly and deeply buried into PDMS while their slightly emerged heads served as growth points for the second nanowire growth (Fig. 1(h)), which was carried out by immersing the PDMS in a solution mixture of 15 mM HMTA and 15 mM zinc nitrate hexahydrate at 95 °C for 3 h (Fig. 1(i)). To enhance contact events between the ZnO nanowires and the EVs as well as to avoid any pressure drop, the ZnO nanowire embedded PDMS substrate was anchored to a herringbone-structured PDMS substrate (Fig. 1(j)).

Fig. 1 (a-j) Various stages involved in the fabrication of nanowire-based device; and (k) schematic of the collection and extraction of EV–encapsulated miRNAs.

The nanowire-based device is capable of detecting around 1000 types of species of miRNAs when compared to the conventional ultracentrifugation method. Using this device, it is possible to extract EV–encapsulated miRNAs within 40 min (collection, 20 min; extraction, 20 min) by introducing just 1 ml of urine sample followed by 1 ml of lysis buffer into the device (Fig. 1(k)). In contrast, the ultracentrifugation method requires 20 ml of urine sample and more than 5 h for collection and extraction. The device enables a four-fold increase in the miRNA expression level with a larger variety of extracted species of miRNAs. The ZnO nanowire-based device is found to be superior to the commonly used ultracentrifugation method in terms of treatment time and RNA extraction efficiency. This attribute is due to its large surface area of ZnO nanowires and their positively charged surface (isoelectric point of 9.50 at pH 6 to 8), which electrostatically attracts the negatively charged EVs in urine sample at pH 6-8. In addition, the mechanical stability of ZnO nanowires, which are firmly anchored to the PDMS substrate helps to retain their strength during buffer flow and enhances the extraction efficiency. The positively charged surface of ZnO nanowires offers benefit in collecting negatively charged objects in urine samples, including exosomes, microvesicles, and EV-free miRNAs.

The ZnO nanowire-based device is believed to help in the early diagnoses and timely medical checkups based on urine miRNA analysis. The method is capable of identifying urinary miRNAs that could potentially serve as biomarkers for detecting bladder, prostate lung, pancreas, and liver cancer.

T.S.N. Sankara Narayanan

Will exercise helps to slow down the progression of Parkinson’s disease?

Parkinson’s disease (PD) is a long-term neurodegenerative disorder of the central nervous system that predominately affects the motor system. Patients affected by PD are deprived of dopamine-producing neurons in the substantia nigra, a specific area of the brain, which leads to a decrease in dopamine levels in their brain. Since low dopamine levels in the brain is linked with the central nervous system, they experience “tremor or shaking, muscle stiffness and slowness of movements”, the common symptoms of PD (Neurology, 2017). With the progress of this disease, particularly with ageing, the patients experience difficulty in walking, talking and even in completing simple tasks (JAMA Neurology, 2017).

Corcos and colleagues at Northwestern Medicine and University of Denver have found out that for patients with early-stage PD, performing high-intensity exercise three times a week, decreased the worsening of the motor symptoms (JAMA Neurology, 2017). According to them “exercise is the medicine” and the optimal exercise regimes for patients affected by PD should be designed based on a cardiologist-supervised graded exercise test.

Zhou, Barkow and Freed at the University of Colorado Anschutz Medical Campus using a mice model have shown that exercise could stop accumulation of the neuronal protein alpha-synuclein in brain cells (PLOS ONE, 2017). They believed that clumps of alpha-synuclein play a central role in the death of brain cells associated with PD. Keqiang Ye at Emory University, USA and his colleagues have discovered that alpha-synuclein, a sticky protein, is a pivot for the damage of brain cells in patient’s affected by PD and blocks signals that are important for brain growth. They have demonstrated that alpha-synuclein binds and interferes with TrkB, the receptor for BDNF (brain derived neurotrophic factor) thus creating a “tug of war” between the alpha-synuclein and BDNF for their dominance over the TrkB (PNAS, 2017). According to Zhou and Freed, lack of DJ-1 gene has declined the ability of the mice to run while exercise increased brain and muscle expression by turning on the protective gene DJ-1 and prevents the accumulation of the neuronal protein alpha-synuclein in the brain cells (PLOS ONE, 2017).

Based on their experiments Zhou, Barkow and Freed showed that exercise is likely to prevent brain cells from dying (PLOS ONE, 2017). Since dopamine is the critical component determining the activity of brain cells, they are working towards the conversion of human embryonic stem cells to dopamine neurons as this will open up new avenues to produce sufficient amount of dopamine cells that are necessary for transplant. Researchers at the Department of Medical Biochemistry and Biophysics, Karolinska Institute, lead by Prof. Ernest Arenas, have shown that it possible to manipulate the gene expression of non-neuronal cells (glial cells) in the brain to produce new dopamine neurons (Nature Biotechnology, 2017).

T.S.N. Sankara Narayanan

Template-assisted deposition of metal nanowires

Fabrication of metal nanowires have received considerable attention and among them template-assisted (ion tracked etched polymers or porous aluminium oxide templates) electrodeposition of metal nanowires assumed significance. Usually, a thin film (< 200 nm) is sputtered over the template and it is subsequently reinforced with a thick (up to 10 μm) metallic layer by plating. However, the difficulty in making electrical connection with the thin and fragile sputtered film as well as in removing the electrodeposited layer to facilitate the release of metal nanowires are the major limitations. Researchers at Centre for Manufacturing and Materials Engineering and Faculty of Health and Life Sciences, Coventry University, UK and Energy Technology Research Group, University of Southampton, UK for the first time have described a new procedure for fabrication of metal nanowires (Cu nanowires) by template-assisted electrodeposition using porous polycarbonate templates.

Polycarbonate templates (pore sizes: 60 nm, 100 nm and 200 nm; thickness: 25 μm) were washed using 1 v./v. % of Neutracon at 40 ºC for 5 min, rinsed, air-dried. They were sputter coated with silver for 3 min on one side of the template (Ar bombardment gas, 15 mA current) followed by electroless plating of Cu using an electroless copper bath at 46 ºC for 10 min to form the electrode layer, rinsed and air dried. Subsequently a layer of Cu was deposited by electrodeposition at -75 mV vs. saturated calomel electrode (SCE) for 120 min to grow the Cu nanowires. A titanium/mixed metal oxide mesh served as a counter electrode. After plating, the coated template was removed from the plating cell, rinsed and air dried. The Cu nanowires were freed from the template by etching away the bottom electrode layer using a 3 v./v.% solution of hydrogen peroxide/sulphuric acid and then by dissolving the polycarbonate template in dichloromethane. The various stages involved in the fabrication of Cu nanowire is schematically illustrated in Fig. 1.

Fig. 1 Schematic illustration of template-assisted deposition of Cu nanowires

The sputtered Ag acts as a seed layer and served as an effective catalyst for electroless deposition of Cu. After 3 min sputtering, a uniform but porous layer of Ag (thickness: ≈ 15 nm) is deposited (Fig. 2(a)). Electroless plating of Cu over the sputtered Ag film for 10 min completely covered and sealed the pores and provides an excellent coverage (Figs. 2(b) and 2(c)). Analysis performed at the reverse side of the electrode layer after dissolving the template indicates that the electroless deposited Cu starts to fill the bottom of the pores and forms the base of the nanowire. The sputtering process directs the Ag atoms into the pores wherein they adhere to the side of the walls and trigger deposition of Cu. Filling-up the bottom and side walls of the porous structure provides an ideal base for subsequent electroplating step to build uniform Cu nanowires (Fig. 2(d)). Plating of Cu into the pores offers an additional advantage of mechanically keying the electrode layer to the smooth surface of the template. A magnified acquired by SEM at the bottom of the Cu nanowires (Fig. 2(e)) clearly indicate the electroless Cu and sputter-coated Ag layers and the interconnection between the electroless Cu and electroplated Cu layer is good.

Fig. 2 SEM images (a) after sputter coating with Ag for 3 min (thickness: ≤ 15 nm); (b, c) after electroless plating with Cu for 10 min (thickness: 300–500 nm); (d) Cu nanowires formed after 120 min of electrodeposition of Cu at -75 V vs. SCE over the sputtered Ag seed layer/electroless Cu; and (e) bottom portion of the Cu nanowire showing a good interconnection between the electroless Cu and electroplated Cu layer

A simple protocol is suggested for the fabrication of template-assisted electrodeposition of metal nanowires. Sputter deposited Ag thin film (≤15 nm) on one side of the polycarbonate template acts as a seed layer and catalyze electroless deposition of a uniform and highly conductive Cu layer (300–500 nm) for subsequent electrolytic deposition of a thick Cu layer. Removal of the electrode layer at the bottom of the template by chemical etching as well as dissolution of the template in dichloromethane yields free standing Cu nanowires.

T.S.N. Sankara Narayanan

For more details, the reader may kindly refer: J.E. Graves et al., A new procedure for the template synthesis of metal nanowires, Electrochemistry Communications (2017) (article in press), doi:10.1016/j.elecom.2017.11.022

Probing the biomechanics of blood clotting

We all might have experienced a small incision in our fingers during cutting/slicing vegetables. Though it is not very painful, loss of some blood is inevitable. Fortunately, the bleeding stops quickly. The platelets in the blood is doing this trick; by sticking together, they form clots and stops further loss of blood from the wound site. Researchers at  Department of Chemistry and Emory’s School of Medicine, Emory University, USA, lead by Prof. Khalid Salaita have successfully identified the key molecular forces responsible to activate blood clotting for the first time. Their findings are published in the Proceedings of the National Academy of Sciences and in Nature Methods.

Fibrinogen (third most abundant protein in blood) acts like a glue and stick the platelets together during clotting. In spite of the presence of ~70,000 copies of receptors on the surface of each platelet to latch on with the fibrinogen, the platelets will not perform this action under normal conditions (any abnormal clotting would lead to strokes). The platelets will be in flow until we experience an injury wherein the receptors of platelets rapid bind with the fibrinogen, resulting in agglomeration of platelets followed by clotting.

Anchoring fibrinogen ligands on the surface of a lipid membrane has enabled them to slip and slide laterally, but resisted their movement perpendicular to the surface. When platelets are introduced to this surface, they failed to activate and stick together. On the contrary, when the fibrinogen ligands are anchored on a glass slide, they are unable to move laterally and under such conditions, the platelets are rapidly activated. To activate clotting, the blood platelets require a targeted force of the order of 5-20 piconewtons. When the platelets starts to stick with each other to form a clot they contract toward a line, or central axis, in each cell. However, they are not pulled together toward a shared central axis, rather they are pulled in such a way to form clusters with a random orientation.

T.S.N. Sankara Narayanan

Activating surface lattice oxygen in single-atom Pt/CeO2 catalysis brings industrial applications a step closer

Achieving better fuel efficiency with low greenhouse gas emissions has been the main focus in the development of advanced combustion engines. In order to achieve this goal, the catalyst materials must be active at temperatures < 150 °C. Single-atom heterogeneous catalysts have been shown to offer excellent low-temperature reactivity. Nevertheless, they lack durability at high-temperatures. Researchers at Pacific Northwest National Laboratory, USA, University of New Mexico, USA, Beijing University of Chemical Technology, China and Washington State University, USA have demonstrated that activation of atomically dispersed Pt²⁺ on CeO₂ by steam treatment (hydrothermal aging) at 750 °C decreased the temperature required to achieve 100% conversion (T₁₀₀) of CO from 320 °C to 148 °C with no evidence of deactivation in the catalytic ability besides providing a better thermal stability.

Ce(NO₃)₃·6H₂O was used as the precursor to prepare the CeO₂ polyhedra by thermal treatment at 350 °C for 2 h at in air atmosphere. Platinum was loaded on CeO₂ by incipient wetness impregnation method followed by drying at 80 °C for 12 h. Thermal and hydrothermal aging treatments were performed to prepare the catalysts (Fig.1(a)). The thermally aged (800 °C for 12 h in flowing air) and hydrothermally aged (10% H₂O in Argon at 750 °C for 9 h) CeO₂ catalysts are designated as Pt/CeO₂ and Pt/CeO₂_S, respectively. In the Pt/CeO₂ catalyst (Fig. 1(b)), Pt is atomically dispersed while in the Pt/CeO₂_S catalyst, no sintering of Pt has occurred, and Pt remained atomically dispersed (Fig. 1(c)), even after the steam treatment at 750 °C. The presence of Pt NPs is not evident either in Pt/CeO₂ or Pt/CeO₂_S even in high-resolution STEM images, suggesting atomic dispersion of Pt, which is further substantiated by XRD, EXAFS and XPS.

Fig. 1 (a) Protocol for preparing Pt/CeO₂ and Pt/CeO₂_S catalysts; (b, c)  aberration corrected–STEM images of (b) thermally aged Pt/CeO₂; and (c) hydrothermally aged Pt/CeO₂_S (Circles: Single atoms of Pt)

The mechanism of CO oxidation is evaluated in terms of adsorption of CO over the Pt sites of both Pt/CeO₂ and Pt/CeO₂_S catalysts at 180 °C using diffuse reflectance infrared Fourier-transform spectroscopy. The IR bands at 2096 and 2098 cm^–1 are assigned to the linearly adsorbed CO on isolated ionic Pt²⁺ over the catalysts (Figs. 2(a) and 2(b)). The intensity of the IR band at 2098 cm^–1 did not change appreciably after the flow of CO is stopped over the Pt/CeO₂ catalyst (Fig. 2(a)), suggesting a strong adsorption of CO on the ionic Pt site. In contrast, the substantial decrease in the intensity of the IR band at 2096 cm^–1 suggests that the CO adsorbed on single-ion Pt²⁺ is readily oxidized to CO₂ (Fig. 2(b)). Hence, it is clear that the CO adsorbed on Pt²⁺ in Pt/CeO₂_S catalyst is more reactive than those adsorbed on Pt²⁺ in the Pt/CeO₂ catalyst. Since the Pt on both catalysts exhibits the same atomic dispersion and valence (Pt²⁺), the difference in low-temperature reactivity between these catalysts could be attributed to neighboring lattice oxygen, which is part of the active site and it is reflected in the reactivity of the ionic Pt sites. When compared to the Pt/CeO₂ catalyst, steam treatment roughly doubled the amount of active lattice oxygen in Pt/CeO­₂_S catalyst (Fig. 2(c)). H₂ temperature-programmed reduction (H₂-TPR) analysis (Fig. 2(d)) indicates the presence of two major reduction peaks for the Pt/CeO₂ catalyst: (i) reduction of the surface lattice oxygen in the vicinity of Pt (Pt–O–Ce bond), centered at 184 °C; and (ii) reduction of surface lattice oxygen on CeO₂ distant from Pt, centered at 348 °C. For Pt/CeO₂_S catalyst, in addition to these peaks, an extra peak at 162 °C is also observed, which is due to a new type of active surface lattice oxygen generated during steam treatment.

Fig. 2 Identification of Pt single atoms (Pt²⁺) and active surface lattice oxygen. CO adsorption DRIFTS for (a) Pt/CeO₂ and (b) Pt/CeO₂_S. (c) Time-resolved CO oxidation with surface active lattice oxygen of Pt/CeO₂ catalysts at 300 °C; and (d) H₂-TPR profiles of Pt/CeO₂ catalysts.

Density functional theory calculations and reaction kinetic analyses indicate that the oxygen vacancies from the CeO₂ bulk is redistributed to the CeO₂(111) surface (Fig. 3(a)) as a result of exposure to water at 380 °C. Under steam-treatment conditions, H₂O molecules fill out the oxygen vacancy (VO) over the atomically dispersed Pt/CeO₂ surface, generating two neighboring active O_lattice[H] in the vicinity of Pt (Fig. 3(a)), which are thermodynamically stable up to 767 °C. The proposed reaction mechanism for CO oxidation on isolated Pt on CeO₂(111) surface and the calculated energy profile are shown in Fig. 3(b).  Coordination of only one catalytically active O_lattice[H] site with a Pt atom (Pt²⁺) occurs during the initial stage (Fig. 3(b), intermediate I). The surface O_lattice[H] reacts with CO adsorbed on Pt and creates an VO (Fig. 3(b), intermediate III), which is subsequently filled by adsorption of an oxygen molecule. Deprotonation of the carboxyl intermediate assisted by the newly adsorbed oxygen molecule [(Fig. 3(b), transition state 2 (TS2)] enables generation of CO₂. Reaction between the OO[H] species (Fig. 3(b), intermediate V) and another adsorbed CO results in the formation of an additional CO₂ molecule (Fig. 3(b), TS3). The atomically dispersed Pt/CeO₂_S catalyst surface is recovered after desorption of CO₂, and the catalytic cycle over the steam-treated catalyst surface (2O_lattice[H]) is closed. The improved low-temperature activity of Pt/CeO₂_S catalyst is due to the activation of surface lattice oxygen that is bonded to H, resulting in the formation of hydroxyls on the CeO₂ support in the vicinity of atomically dispersed Pt.

Fig. 3 (a) Steam-treatment on the atomically dispersed Pt/CeO₂ catalyst – generation of active sites by steam treatment is responsible for low-temperature CO oxidation activity (highlighted by dashed green circles); (b) Proposed reaction mechanism for CO oxidation on isolated Pt on a CeO₂(111) surface

The importance of activation of the catalyst support by steam treatment to achieve high reactivity and durability has been demonstrated. The enhanced reactivity of the Pt/CeO₂_S catalyst is not due to the formation of Pt NPs, rather it has been attributed to the activation of surface oxygen on the ceria support. The excellent low-temperature reactivity and better high-temperature durability bring single-atom catalysis of Pt/CeO₂_S closer to reality for many industrial applications.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Lei Nie et al., Activation of surface lattice oxygen in single-atom Pt/CeO₂ for low-temperature CO oxidation, Science 358, 1419–1423 (2017)

 

3D Printing of Nanotwinned Copper

Nanotwinned (NT)-metals exhibit superior mechanical and electrical properties when compared to their coarse-grained and nano-grained counterparts. NT-metals, either as a film or in bulk, are usually obtained by pulsed electrodeposit­­­­ion (PED), plastic deformation and sputter deposition. However, 3D printing of NT-metals has not been explored. Researchers at Department of Mechanical Engineering and Alan G. MacDiarmid NanoTech Institute, The University of Texas at Dallas, USA have reported a localized pulse electrodeposition (L-PED) process for 3D printing of NT-Cu, which is fully dense, almost free of impurities and low microstructural defects, with no obvious interface between deposited layers, with good mechanical and electrical properties and without the requirement of any post-treatment.

The process is based on L-PED that occurs at the tip of an electrolyte-filled nozzle steered in 3D-XYZ directions, by precisely positioning the stage relative to the substrate for omnidirection (complex 3D, Fig. 1(a)) or layer-by-layer (lateral, Fig. 1(b))) metal printing. The 3D printed Cu structures and patterns are shown in Figs. 1(c) – 1(f). The FIB image of a micropillar printed by vertical deposition at 0.5 V and at an average current density of ≈0.018 A/cm² is shown in Fig. 2(a). The SEM image of a four-layer hollow square shape printed by layer-by-layer process at 0.7 V and at an average current density of 0.045 A/cm² is shown in Fig. 2(b). Both structures exhibit the presence of high density aligned twin boundaries (TBs) within their grains in which most of the TBs are aligned perpendicular to the electric field direction. The TEM image (Fig. 2(c)) confirmed the formation of densely packed nanotwins, mostly aligned in one direction in almost all grains. No noticeable interlayer is observed in the 3D printed Cu by L-PED. With the exception of a few grains, stacking faults, in general, are not frequently observed in 3D-printed Cu.

Fig. 1 (a) Schematic of L-PED process; (b) Schematic side-view of the meniscus between the nozzle tip and the growth front for layer-by-layer deposition of nt-metals; (c-f) SEM images of several 3D-printed Cu structures. (c) A 24-layer structure printed by layer-by-layer DC-ED process, printing time ≈200 min; (d) UTD letter printed, printing time ≈16 min; (e) A micropillar with diameter of ≈10 μm 3D printed by PED, printing time ≈60 min; (f) A helical structure fabricated by pulsed voltage, printing time ≈12 min

Fig. 2 (a) FIB ion channeling contrast image of cross-section of a 3D printed micropillar, printing time ≈60 min; (b) SEM image of a layer-by-layer structure (four-layer) cross-sectioned by FIB, printing time ≈35 min; and (c) FIB ion channel image of the cross-section that shows high-density parallel TBs.

The formation and presence of inter-layers during a step-wise 3D printing of six-layer structure of Cu using a nozzle diameter of 10 μm at 0.5 V pulsed voltage with a duty cycle of 1/100 is studied (Fig. 3(a)). Each layer is printed starting from a point with a shift to the right from the starting point of the former layer. FIB ion channeling contrast images acquired at the cross section (Figs. 3(b) and 3(c)) indicate that the grains in each new layer has continued growing from the grains in the previous layer without the formation of any new grains at each layer. One columnar grain initiated from the first layer grew to the fifth layer (boundary of the grain is shown by a dashed line) with no interlayer could be observed at any step and between any two layers. In L-PED process, the preceding layer functions as the seed layer for the deposition of the next layer, which helps development of an interface-free structure, which is further confirmed by the EDS map of Cu (Fig. 3(d).

Fig. 3 (a) Plan-view FIB image of a six-layer printed Cu structure, printing time ≈35 min; (b, c) Zoomed-in views of the layers that show each layer is deposited on the previous layer, without any noticeable interlayer; (d) EDS map of Cu

The high quality of 3D printed NT-Cu with minimal porosity and structural defects as well as the absence of any interface between the printed layers offer good mechanical property (elastic modulus: 128.2 ± 10.9 GPa; hardness: 2.0 ± 0.19 GPa) and electrical resistance (3.9×10⁻⁷ Ω.m). By modifying the geometry of the nozzle, the geometry of the meniscus between the nozzle tip and the substrate can be varied, which would facilitate fabrication of geometries with more sharp corners and helical pitches. Layer-by-layer deposition seems to be necessary to fabricate entangled structures. The reproducibility of the L-PED process largely depends on the stability of the meniscus, which is influenced by humidity and temperature. In addition, moving speed of the nozzle also influences the reproducibility; high speed could result in breakage of the meniscus while too slow displacement might lead to clogging of the nozzle tip by the deposited metal. The L-PED process can be used for direct 3D printing of layer-by-layer and complex 3D microscale NT-Cu structures for various applications including electronics, micro/nano electromechanical systems (MEMS, and NEMS), metamaterials, plasmonic, and sensors.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Adv. Mater. 2017, 1705107, DOI: 10.1002/adma.201705107

Sequential Surface-Modification of Graphene Oxide

The formation and growth of ice crystals is considered to be a critical issue in aerospace and automotive industries as well as in cryopreservation of cells. Pure water undergoes homogenous nucleation of ice at ~ −40 °C. However, the presence of dusts, minerals, birch and conifer pollen and some species of fungus could serve as a nucleator and promotes nucleation of ice much above this temperature. Carbon nanotubes, graphene nano-flakes and carbon soots (from burning of fuels) are promising candidates to promote nucleation of ice crystals.

Base-washing has been shown to be effective in removing oxidative debris from graphene oxide (GO) and enables effective functionalization of the surface of GO with thiols, Au nanoparticles and polymers. Base-washed graphene oxide (bwGO) is a distinct graphene-like material with better qualities than the normal GO. Researchers at Department of Chemistry, Warwick Medical School and Department of Physics, University of Warwick, UK have suggested that surface modification of bwGO would offer a versatile template to evaluate the potential of 2D carbon nanomaterials as ice-nucleating agents as well as to serve as a versatile scaffold to probe the role of surface chemistry.

GO was synthesized by Hummer’s method. About 140 mg of GO was re-dispersed in 250 ml of deionized H₂O by mild sonication followed by addition of 0.140 g of NaOH and heating of the solution to 70 °C for 1 h. The resultant dark brown solution was centrifuged (@12,500 rpm for 30 min). The dark brown solid was washed with water and re-centrifuged. The solid was re-protonated using 0.014 M HCl at 70 °C for 1 h, filtered, thoroughly washed with deionized H₂O and dried under vacuum to yield bwGO (a black solid), which was dispersed in a H₂O/CH₃CN mixture via sonication. Poly(N-isopropylacrylamide), (pNIPAM) with degree of polymerization of 55 and 140 were prepared by polymerization of N-isopropylacrylamide (Fig. 1). pNIPAM hexanethiol, dodecanethiol and octadecanethiol were grafted on the surface of bwGO under Schlenk conditions in N₂ atmosphere (Fig. 2).

Fig. 1 Scheme depicting polymerization of N-isopropylacrylamide

Fig. 2 Scheme depicting polymerization of N-isopropylacrylamide and grafting of polymers and thiols on the surface of base-washed graphene oxide

The ice nucleation activity of unmodified and surface modified GO was quantified by determining the average nucleation temperature to freeze a droplet (1 μL) of water. The droplets were cooled under an atmosphere of dry nitrogen, and the freezing point of each droplet was recorded by visual observation using a microscope. When tested for the nucleation activity, ultra-pure Milli-Q water nucleated at -26 °C, suggesting a heterogeneous nucleation (Fig. 3); Both bwGO and bwGO-Cyst increased the nucleation temperature by over 5 °C, to -20 and -18 °C (Fig. 4).

Fig. 3 Ice nucleation assay: No water droplet is frozen at -20 °C; At -23 °C, two water droplets (marked by red circles) are frozen while all water droplets are frozen at -30 °C.

Fig. 4 Comparison of ice nucleation activity of Milli-Q water, GO and cysteine-functionalized GO

A remarkable nucleation promotion activity is observed for bwGO surface modified with alkane thiols; octadecanethiol modified bwGO increased the nucleation temperature by > 15 °C to –12 °C (Fig. 5(a)). All the alkyl modified GOs are more active than bwGO and the cysteine modified bwGO, which suggests that the increased hydrophobicity plays a dominant role in determining the ice nucleation. The similar activity of pNIPAM-bwGO with that of bwGO (Fig. 5(b)) suggests that modification of the surface of bwGO with polymer molecules exert a very little influence on the ice nucleation temperature.

Fig. 5 Comparison of ice nucleation activity of (a) Milli Q water, GO and GO functionalized with hexanethiol, octadecanethiol and dodecanethiol; and (b) Milli Q water, GO, pNIPAM55 and pNIPAM140.

The surface modified bwGO may find application in cryopreservation and cloud seeding.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Caroline I. Biggs et al., Impact of sequential surface-modification of graphene oxide on ice nucleation, Phys. Chem. Chem. Phys., 2017,19, 21929-21932

Titanium Fiber Plates with Suitable Elastic Modulus and Porous Structure Facilitate Bone Tissue Repair

Titanium (Ti) is one of the most commonly used biomedical materials in orthopedics and dentistry. The excellent biocompatibility of Ti with high bone affinity makes it a suitable material for biomedical applications. Nevertheless, the higher Young’s modulus of Ti (≈110 GPa) than that of the cortical bone (10-30 GPa) causes stress shielding, leading to bone embrittlement. Since stress shielding is unavoidable with the use of Ti plates, it is generally recommended to remove them from the bone after completion of bone repair. However, removal of the Ti plate involves many risks; bone formation around the plate poses difficulty in removal of the plate besides pain and infections due to surgery. Researchers at Department of Orthopaedic Surgery, Shinshu University School of Medicine, Mechanical Systems Engineering, Shinshu University, Faculty of Engineering, Institute for Biomedical Sciences, Interdisciplinary Cluster for Cutting Edge Research, Shinshu University and Department of Applied Physical Therapy, Shinshu University School of Health Sciences, Japan have suggested the use of Ti fiber plates in place of the conventional Ti plates. Since the Young’s modulus of Ti fiber plates are similar to that of the cortical bone, stress shielding effect can be minimized. In addition, the porous structure of Ti fiber plates could act as scaffolds and promotes cell attachment.

The Ti fiber plates were prepared using Ti fibers (ASTM Grade 1 with 99.52% purity) with a mean diameter of 20 μm and a mean length of 500 μm (Fig. 1(a)). The Ti fibers were molded into a plate by simultaneously applying a compression stress of 1000 MPa and a shearing load of 400 kN at room temperature (Fig. 1(b)) followed by sintering at 300 K. The resultant Ti fiber plates had a thickness of 0.2 mm, Young’s modulus of ≈30 GPa and uniform porous structure with 30–40% porosity and 60–80 μm pore diameter (Fig. 1(c). The utility of the plates for repair of bone fracture and bone tissue regeneration was evaluated under in vitro and in vivo conditions.

Fig. 1 (a) SEM image of Ti fibers; (b) schematic diagram of the process used for preparing Ti fiber plates; and (c) SEM image of the Ti fiber plate

In vitro test results indicate that the extent of osteoblast adhesion and cell proliferation on the Ti fiber plates are quite similar to that of the conventional Ti plates. However, the difference in expression levels of cell-adhesion-related genes between cells on the Ti fiber plates and the cells on conventional Ti plates, suggests the existence of a difference in the mode of cell adhesion at the gene level between these two plates. The unique 3D structure of the Ti fiber plate is considered responsible for the increased level of osteoblast adhesion than those observed for the conventional titanium plates with a simple planar structure. In vivo study in rabbits with comminuted fracture at the center of the ulnar stem indicates that placing the titanium fiber plate in close contact with the fractured bone helps to immobilize and repair of small bone fragments.

Fig. 2 (a) Fixing of titanium fiber plate to the ulna using miniature screws for the repair of comminuted fracture (arrow mark) in rabbits; (b) Scout radiograms and (c) μCT images taken at Week 4 post-operation indicate complete bone union in the titanium fiber plate group but not with the control group

Unlike the conventional Ti plate, the Ti fiber plates could be easily prepared by compressing Ti fibers at room temperature without changing the fiber shape, which makes the process cost-effective and commercially viable. By suitably altering the length and thickness of the Ti fibers as well as the extent of compression and shear stress, Ti fiber plates with varying thickness, surface properties, porosity, Young’s modulus and strength can be prepared. Due to its malleability, the Ti fiber plates can be manually reshaped into a curved 3D structure and customized to the required size and shape of the fixation site for bone regeneration. The porous structure of Ti fibers with 30–40% porosity and 60–80 μm pore diameter is considered to be suitable for bone regeneration. Since the Young’s modulus of Ti fiber plate is similar to that of cortical bone, the deleterious stress shielding effect can be minimized and hence, it can remain implanted even after the fracture is healed. The use of pure Ti fibers ensures a better biosafety.

The Ti fiber plate is easy to deform manually and hence it can be shaped optimally during surgery to prevent loss of bone fragments from comminuted fractures. Since the titanium fiber plate is thin and easily deformable, it is suitable for fractures at sites where the space around the plate is limited such as ulnar and phalangeal fractures. The titanium fiber plates also allow holes to be drilled for insertion of small screws at given sites during surgery. Hence, the Ti fiber plates can be used for a wide variety of fracture treatments including bone regeneration. Titanium fiber plates are not so tough to withstand high mechanical loads. In addition, rubbing of Ti fibers against one another could produce wear particles. These limitations still remain to be solved.

T.S.N. Sankara Narayanan

For more information, the reader may kind refer: Takashi Takizaw et al., Titanium Fiber Plates for Bone Tissue Repair, Adv. Mater. 2017, 1703608, DOI: 10.1002/adma.201703608

Plating and stripping calcium in an organic electrolyte – Implications on the development of calcium-ion batteries

Multivalent-ion batteries are gaining attention as energy storage devices. The major limitation in multivalent metal anodes is the reduction of aprotic-based electrolytes, resulting in the formation of passivating layers that inhibit plating and stripping of the metal. In Ca-ion battery systems using propylene carbonate, butyrolactone and acetonitrile based electrolytes, Ca(OH)₂, CaCO₃ and calcium alkoxides are formed during electrochemical reduction while the decomposition products passivate the electrode surface and inhibit further plating of Ca. Plating and stripping of Ca has been shown to be feasible in electrolyte solutions containing Ca(BF₄)₂ in ethylene carbonate and propylene carbonate mixtures only at elevated temperatures of the order of 75 to 100 °C, with small capacities of the order of 0.165 mAh/cm², accompanied by the formation of CaF₂. Since Ca(BH₄)₂ is readily soluble in THF at room temperature, researchers at Departments of Materials and Chemistry, University of Oxford, UK have investigated plating and stripping of Ca in this electrolyte to obtain capacities of 1 mAh/cm² at a rate of 1 mA/cm², with low polarization (~100 mV) and in excess of 50 cycles.

Electrochemistry of plating/stripping of Ca in 1.5 M Ca(BH₄)₂ in THF, evaluated by cyclic voltammetry (CV) using a three-electrode assembly indicates plating of Ca during reduction and its stripping on subsequent oxidation with a Coulombic efficiency of 94.8% (Fig. 1). The X-ray diffraction pattern and FT-IR spectrum confirms that the dominant product after deposition is Ca metal, along with a small amount of CaH₂. The total amount of Ca deposited (sum of Ca as metal as well as CaH₂) corresponds to 98% of the charge passed (1.828 µmol for 0.1 mAh of charge). For a charge/mass ratio of 2.04 e^–/Ca, majority of charge passed is utilized for the deposition of Ca metal while only a small amount of the deposited Ca metal (5.0 mole%) reacts with the electrolyte to form CaH₂.

The morphology acquired at the cross section during plating and stripping of Ca at 1^st, 5^th and 10^th cycles at 1 mAh/cm² is shown in Fig. 2. Deposition of a thick film of Ca is evident after the first plating (Fig. 2(a)), which upon stripping leaves some CaH₂ as residue on the surface of the electrode (Fig. 2(b)). The morphology acquired after 5^th and 10^th cycles of plating and stripping of Ca indicates the presence of CaH₂ at the end of each stripping process (Figs. 2(c)-2(f)). Time-of-flight secondary ion mass spectroscopy and gas chromatography mass spectrometry results confirm the formation of CaH₂ following the reaction of freshly deposited Ca with THF, which is distributed throughout the film. Allowing the Ca deposit in contact with the electrolyte to rest at its open circuit potential enables the growth of CaH₂ until a protective film of CaH₂ with sufficient thickness to suppress further reaction between Ca and the electrolyte.

Fig. 1 (a) CV of plating/stripping of Ca in 1.5 M Ca(BH₄)₂/THF using Au, Ca and Pt as the working, reference and counter electrodes, respectively at a scan rate 25 mV/s. (Inset: charge passed on plating/stripping)

Fig. 2 Cross-sectional morphology at the end of (a, c) 1^st, (b, d) 5^th and (c, e) 10^th cycle of (a, c, e) plating and (b, d, f) stripping of Ca in 1.5 M Ca(BH₄)₂/THF on Au electrode.

The cycling efficiency of a metal anode for rechargeable batteries needs to be as high as 99.98% per cycle. To achieve this level of efficiency in Ca-ion battery system, the SEI layer should be electronically insulating while the Ca²⁺ ions are conducting. The formation of CaH₂ acts as a passivating layer at open circuit and mitigates further reaction of Ca with the electrolyte. However, the SEI layer fails to acquire the required properties, thus limiting the cycling efficiency to 96%, which is not sufficient for practical applications. The proposed Ca(BH₄)₂/THF electrolyte for the plating and stripping of Ca though not solves all the problems of the Ca anode for rechargeable batteries, it opens up an avenue to make further progress towards achieving Ca-ion batteries with better cycling efficiency.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Da Wang et al., Plating and stripping calcium in an organic electrolyte, Nature Materials, doi:10.1038/nmat5036

Freeze-drying of TiO2 nanorod arrays improves charge transport properties and performance of solar cell

TiO₂ nanorod arrays (NRAs) prepared by hydrothermal treatment, anodic oxidation and sol-gel synthesis have received considerable attention in solar cells, photoelectrochemical water splitting and gas sensors. Conventional air drying of the TiO₂ NRAs could cause aggregation of neighboring nanostructures and distortion of their morphological features, which deleteriously influence their charge transport properties and surface area. In addition, chemically adsorbed halides and alkyl chains might change the surface properties and  influence the interfacial charge transfer process and hence the overall performance of the device. It is important to preserve the vertical alignment of the nanostructures as well as to avoid chemically adsorbed impurities to achieve fast electron transportation, conformal heterojunctions with guest materials and enhanced light scattering. Researchers at School of Advanced Materials and Nanotechnology and Key Lab of Wide Band-Gap Semiconductor Materials and Devices, Xidian University, People’s Republic of China have employed a freeze drying method to dry the TiO₂ nanorod arrays obtained by a two-step hydrothermal process. The freeze-drying post-treatment renders a clean TiO₂ surface and preserves the vertically-aligned nanostructures.

The TiO₂ NRAs were synthesized on FTO (SnO₂:F) substrates by a two-step hydrothermal method. In the first step, a mixture of 1.5 g TiCl₄, 15 ml DI water and 15 ml HCl (36.5 wt%) was subjected to hydrothermal treatment at 150 °C for 6 h. The resultant powder obtained from the first step was treated with a similar solution without TiCl₄ at 150 °C for 3 h in the second step. The air-dried TiO₂ NRAs were directly collected by air-gun blowing, while the freeze-dried samples were obtained after freeze-drying them for 5 h.

The morphological features of air- and freeze-fried TiO₂ NRAs are shown in Fig. 1(a) and Fig. 1(b), respectively. In spite of the method of drying, there is not much difference in the surface morphologies of the TiO₂ NRAs. Nevertheless, the cross-sectional morphology of the air-dried TiO₂ NRAs indicates collapse of the nanorods and destruction of morphology, probably induced by surface tension effect. In contrast, the vertically-aligned nanorod arrays are well preserved for the freeze-dried TiO₂ NRAs since this methodology enables removal of water by sublimation and desorption under vacuum, which avoids solid/liquid interfaces and eliminates the surface tension effect. During freeze-drying, localized energy generated through intermolecular heat transfer enables breaking of the Ti-Cl bonds, desorption and collision of chlorine atoms, resulting in the formation of molecular chlorine. Thus, the freeze-drying post-treatment leads to a “clean” TiO₂ surface with a well-defined morphology of NRAs. The length of the air- and freeze-dried TiO₂ NRAs are ~7.1 μm and ~7.8 μm, respectively. In spite of a slight difference in their colour shade, no apparent difference is observed in their crystallinity.

Fig. 1 Surface and cross-sectional (top insets) SEM images of TiO₂ NRAs obtained by: (a) air-drying; and (b) freeze drying methods (bottom insets: optical images)

The air- and freeze-dried TiO₂ NRAs show a similar UV-visible absorption spectra before loading the dyes. However, after loading the dyes, the freeze-dried TiO₂ NRAs exhibits a decrease in absorption when compared to that of the air-dried ones (Fig. 2(a)). The amount of dye loaded in air-dried TiO₂ NRAs is 69.6 nmol/cm² while for the freeze-dried TiO₂ NRAs it is decreased to 44.3 nmol/cm², following a decrease in its surface area by ~ 36%. However, the perseverance of the ordered nanostructures enables the freeze-dried TiO₂ NRAs to exhibit an enhanced visible-NIR light-scattering performance when compared to that of the air-dried ones (upper inset of Fig. 2(a)). The band gaps of freeze- and air-dried TiO₂ NRAs are 2.91 eV and 2.95 eV, respectively (bottom inset of Fig.2(a)). The flatband potential (E_fb) of the freeze-dried TiO₂ NRA shows a negative shift (~ 0.05 V) when compared to that of the air-dried one. The donor density (N_d) of the freeze-dried TiO₂ NRA is slightly increased when compared to that of the air-dried TiO₂ NRA` from 0.53×10¹⁷/cm³ to 0.6×10¹⁷/cm³. The schematic energy level diagram (Fig.2(b)) indicates that the negative shift in CB and E_F as well as disappearance of the deep acceptor level of freeze-dried TiO₂ NRAs are due to the removal of the adsorbed species that could facilitate charge separation and transport.

Fig. 2 (a) UV-Vis-NIR absorption of the TiO₂ NRAs formed on FTO substrates, (top inset: diffused reflectance spectra; bottom inset: Kubelka-Munk function vs. energy); and (b)  schematic energy level diagrams

The sharp decay in photocurrent with in a second observed for the freeze-dried TiO₂ NRAs when compared to the prolonged duration of decay of photocurrent over 100 s (Fig. 3(a)) observed for the air-dried one suggests the occurrence of an efficient charge extraction across the freeze-dried TiO₂/electrolyte interface. The model dye sensitized solar cells device (top inset of Fig. 3(b)) fabricated using air- and freeze-dried TiO₂ NRAs and the corresponding J-V curves are shown in Fig. 3(b). The PV parameters of the device fabricated using air-dried TiO₂ NRAs are as follows: open-circuit voltage (V_oc) = 0.70 V, short-circuit current density (J_sc) = 6.68 mA/cm², fill factor (FF) = 66%, and power conversion efficiency (PCE) = 3.08%. For the device fabricated using freeze-dried TiO₂ NRAs, the PV parameters are improved as follows: V_oc= 0.68 V, J_sc=8.11 mA/cm², FF= 65%, and PCE=3.60%. The improvement in the PCE of freeze-dried TiO₂ NRAs by ~20% is mainly due to an increase in its J_s.

Fig. 3 (a) Photocurrent decay curves (inset: parallel capacitance vs. applied potential plots); (b) J-V curves (top inset: model of DSSCs using TiO₂ NRAs)

Freeze-drying post-treatment of TiO₂ NRAs preserves the vertically-aligned nanoarchitecture and provides a clean surface, which helps to improve the electronic properties and the performance of the solar cell.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: P. Zhong et al., Freeze-drying as a novel approach to improve charge transport in titanium dioxide nanorod arrays, ChemElectroChem 10.1002/celc.201700572

Detection of Mesopores Using Liquid Photonic Crystals

Determination of pore characteristics of porous materials are very vital for their practical application in drug delivery, oil-water separation, etc. Isothermal adsorption and desorption of N₂, a method commonly employed for the measurement of mesopore volume, pore diameter and surface area, is time-consuming besides involving the use of expensive instrument and consumption of liquid N₂. Researchers at School of Chemistry and Molecular Engineering, Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, China, have developed a novel method to measure pore characteristics of mesoporous materials by mixing them with supersaturated SiO₂ colloidal solution at different temperatures, followed by quick measurement of “absorption induced reflection wavelength changes” of the precipitated liquid photonic crystals (PC). The positive relationship between pore volume (V) and unit mass reflection wavelength change (Δλ/m), and the negative relationship between pore diameter (D) and average absorption temperature (T) are used to determine the pore volume and pore diameter.

Supersaturated solution of SiO₂ particles in various organic solvents spontaneously precipitate to form liquid PCs in which the particle size controls their structural colors and reflection peaks. A supersaturated ethylene glycol-ethanol (EG-EtOH) solution of SiO₂ particles can be used as a “mesopore indicator” and when it is mixed with mesoporous materials, part of the solvent will be irreversibly absorbed into the mesopores, which increases the volume fraction of SiO₂ particles in solution, shrinks the lattice constant of precipitated PCs and monotonically decreases the reflection wavelength.

The “absorption induced reflection blueshift” is schematically illustrated in Fig. 1(a). A supersaturated SiO₂/EG-EtOH solution is placed for several minutes to precipitate red PCs. When pieces of porous silica are carefully spread into the solution (Fig. 1(b)), the red PCs around the porous powders turns green immediately (Fig. 1(c)). The microscopic reflection exhibits a blue shift of 56 nm (from 553 to 609), which indicates shrinkage of lattice constant of PCs that occurs along with solvent transfer to porous substance (Fig. 1(d)).

Fig. 1 (a) Schematic illustration of the measurement of reflection wavelength change of PCs; (b) dried porous silica powders added to the supersaturated SiO₂/EG-EtOH solution; (c) precipitated red photonic crystals turned green around the porous powder; and (d) reflection spectra of colloidal PCs

The pore volume (V) has a positive relationship with the reflection wavelength changes induced by unit mass of porous materials (Δλ/m), which is schematically represented in Fig. 2(a). For mesoporous silica standards, an increase in mesopore mass (m), increases the wavelength change (Δλ) following the “absorption induced reflection blueshift”, which is also reflected by an increase in slope of the “Δλ-m” curve (Figs. 2(b) and 2(c)). A measure of the reflection wavelength change actually is a precise indicator of the amount of absorbed solvent as well as the pore volume (V).

Fig. 2 (a) Schematic representation of the positive relationship between pore volume (V) and the reflection wavelength changes induced by unit mass of porous materials (Δλ/m); (b) increase in wavelength change (Δλ) and an increase in slope of the “Δλ-m” curve with an increase in mesopore mass (m)

The pore diameter (D) has a negative relationship with the average absorption temperature (T) of mesopores, which is schematically represented in Fig. 3(a). When the SiO₂/EG-EtOH solution is mixed with a silica standard with a pore diameter of 1.92 nm, the reflection change of liquid PC (Δλ) is close to zero at low temperature. An increase in temperature from 60 ℃ to 120 ℃ increases the thermal motion and the kinetic energy of the solvent molecules and promotes absorption of solvent into the mesopores, resulting in a gradual increase in Δλ. At temperatures >120 ℃, Δλ reaches a maximum value following a saturation in the absorption of solvent molecules. Simulation of Δλ vs. T curve using Boltzmann function indicates a negative correlation between the average absorption temperature (T) and the pore diameter (D). Accordingly, T is decreased from 93.5℃, to -12.5℃ for mesopore silica standards with D ranging from 1.92 nm to 34.8 nm (Fig. 3(b)-3(g)). It is evident that larger mesopores are easily filled at low temperatures while the smaller pores are filled only at high temperature. The negative correlation between T and D can be used to determine the pore size.

Fig. 3 Schematic representation of negative relationship of pore diameter (D) with average absorption temperature (T) of mesopores SiO₂; (b-g) temperature evolution of reflection change caused by the addition of mesoporous silica standards with different pore volumes

The methodology is validated by testing two well-known mesoporous silica, viz.,  MCM41 and SBA15. The pore volumes and pore diameters estimated by N₂ adsorption-desorption isotherms and BJH pore distributions are 0.898 cm³/g and 2.43 nm for MCM41, and 0.96 cm³/g and 8.04 nm for SBA15. The average absorption temperature measured using the reflection change of the PCs for MCM41 and SBA15 are 88.1°C and 8.5°C, which indicates their pore diameters are 2.33 nm and 10.86 nm. The mass evolution of reflection change shows that the Δλ/m for MCM41 and SBA15 are 2.49 nm/mg and 2.56 nm/mg, which indicate that their pore volumes are about 0.955 cm³/g and 1.00 cm³/g. The pore characteristics of MCM41 and SBA15 measured by N₂ adsorption-desorption and based on the “absorption induced reflection wavelength changes” of PCs are close, suggesting the suitability of the later method as an alternative for the N₂ adsorption-desorption method.

The method based on the measurement of “absorption induced reflection wavelength changes” of the precipitated liquid PC provides a convenient, efficient way to characterize mesoporous materials. This methodology could also be explored for non-silica based porous materials with suitable modification for wettability.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Biting Zhu et al., Liquid Photonic Crystals for Mesopore Detection, Angew. Chem. Int. Ed. 10.1002/anie.201710456

Developing Strong Texture in Lithium Deposits Helps Designing Future Anode Materials for Li-Metal Batteries

Lithium–sulfur and lithium–air batteries have been used as high-energy storage systems for electric vehicles. Poor cycling efficiency and safety are the major limitations of Li-metal batteries and they arise due to the uncontrollable Li deposition process. Researchers at Department of Materials Science and Engineering and Stanford Nano Shared Facilities, Stanford University, USA and Stanford Institute for Materials and Energy Sciences, Stanford Linear Accelerator Center National Accelerator Laboratory, USA have performed some fundamental studies on electrodeposition of Li and characterized the resultant films using X-ray diffraction (XRD), morphological characteristics and Pole-figure analysis to establish a correlation between the crystallographic texture with the morphology of Li deposits. The fundamental understanding of electrocrystallization of Li helps to rationalize the use of suitable additives or inhibitors in the Li battery electrolytes and provides an insight on the design of future lithium anode materials for high-energy-density batteries.

The morphological features and texture of electrodeposited Li on lithium anodes of Li–S and Li–O₂ full-cell batteries using carbonate and ether based electrolytes with and without additives (inhibitors) viz., lithium polysulfides and LiNO₃ were investigated. Whisker-shaped elongated Li deposits are obtained using ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 vol/vol), 1 M LiPF₆ (Figs. 1(a) and 1(b)). In spite of a decrease in size with an increase in current density from 0.1 to 5 mA/cm², the characteristic shape of the Li deposits does not change, suggesting that current density has a less-pronounced role on the morphology of Li deposits. Li deposits obtained using 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME, 1:1 vol/vol), 1 M LiTFSI, 1% LiNO₃ possess a characteristic rounded shape (Figs. 1(c) and 1(d)) and a similar effect of current density on the size and morphology of Li deposits is also observed in this electrolyte as that of the carbonate electrolyte. Li deposits obtained using 5 M poly-sulfide as catholyte in Li–S full battery are round-shaped and addition of polysulfide in DOL/DME with 1% LiNO₃ leads to the formation of Li deposits that are uniform and round-shaped, even at higher current density and on the edge of the current collector (Figs. 1(e) ad 1(f)). Polysulfide and LiNO₃ exert synergetic effects to prevent the growth of whisker-shaped deposits. Li deposits obtained using ether-based electrolytes in a Li–O₂ full battery also exhibits round particle morphologies without any dendrite formation (Figs. 1(g) and 1(h)). The morphological features reveal that beyond current density and electrolyte solvent identity, the specific additives (e.g., LiNO₃) or the cross-over molecules (O₂, polysulfide) from the cathode side play a major role in determining the morphology of Li deposits.

Fig. 1 Morphology of Li deposits obtained using various electrolyte systems at 0.1 mA/cm², 1 mAh/cm²: (a and b) EC/DEC 1 M LiPF6; (c and d) DOL/DME 1 M LiTFSI, 1% LiNO₃; (e and f) Sulfur catholyte 5 M S8 dissolved in DOL/DME 1 M LiTFSI, 1% LiNO₃; (g and h) TEGDME 1 M LiTFSI with Li₂O₂ as cathode (Scale bars: a, c, and g, 5 μm; e, 20 μm; d and f, 2 μm; b and h, 1 μm.)

XRD pattern indicates that Li (110) and Li (200) are the two major peaks of Li. Hence, pole figures were collected at 2θ angles of 36.19° and 51.97° corresponding to the locations of (110) and (200) Bragg peaks, respectively. The pole figures for plain Li metal foil show [100] out-of-plane preferred orientation (Fig. 2(a)). The (110) and (200) pole figures for Li electrodeposits obtained using various electrolytes are shown in Figs. 2(b)-2(d). The (110) pole figure of Li deposits obtained using EC/DEC electrolyte shows a disk-shaped, radially uniform diffraction intensity distribution, which indicates that the film’s texture is not clearly pronounced (Fig. 2(b)). It is evident that random orientations of the whiskers result in a rather broad distribution of crystallographic grain orientations. The (110) pole figures of Li deposits exhibit a sharp intensity concentration around ψ = 0°, indicating that the round shaped Li deposits are mostly textured with (110) planes parallel to the electrode substrate (Figs. 2(c) and 2(d)). The more pronounced [110] texture is due to the strong adsorption of the LiNO₃ and polysulfide additives during the crystal growth. It is difficult to establish a correlation between the morphology and texture of lithium deposits with the current density as well as with their SEI layer. Nevertheless, adsorption of inhibitor molecules (additives) in the electrolyte seems to be the dominant factor that leads to texturing of electrochemically deposited Li.

Fig. 2 Pole-figure [left side: (110) and right side: (200)] analysis of Li films: (a) Li metal foil; (b) Li deposit in EC/DEC 1 M LiPF₆; (c) Li deposit in DOL/DME 1 M LiTFSI, 1% LiNO₃; and (d) Li deposit in sulfur catholyte 5 M S8 dissolved in DOL/DME 1 M LiTFSI, 1% LiNO₃.

A growth diagram of Li is proposed consolidating the effects of commonly used additives and current density on lithium morphology (Fig. 3). Li deposits grown over the limiting current densities (J_limiting) is ramified and dendritic, a common inference in cells with large electrode spacings (Fig. 3(a)). At current densities far below the limit (0.1% J_limiting), in the case of no/weak inhibition (EC/DEC and DOL/DME electrolyte without any additives), the deposits usually show a whisker-like shape (Figs. 3(b) and 3(c)). In presence of inhibitors (HF and CsPF₆ as additives), when the extent of inhibition is increased, a large number of elongated crystals grow perpendicular to the substrate, resulting in the formation of coherent Li deposits (Fig. 3(d)). In presence of strong inhibitors, such as LiNO₃ or polysulfides, field oriented texture-type deposits are emerged (Figs. 3(e) and 3(f)). Such strong texturing generates compact Li deposits with a reduced surface area, lesser SEI formation, lower electrolyte consumption, and less dead lithium, which consequently improves cycling efficiency.

Fig. 3 Growth of lithium electrodeposits as a function of current density J and additives in electrolyte with inhibition intensity increasing in the horizontal direction: (a) optical image of dendritic and ramified Li deposit in EC/DEC 1 M LiPF6, below 0.1% of diffusion-limited current density, J_limiting. (b-e) SEM images of lithium deposit in (b) DOL/DME 1 M LiTFSI; (c) EC/DEC 1 M LiPF₆; (d) EC/DEC 1 M LiPF6, 100 ppm H₂O; (e) DOL/DME 1 M LiTFSI, 1% LiNO₃; and (f) sulfur catholyte, 5 M S8 dissolved in DOL/DME 1 M LiTFSI, 1% LiNO₃. (Scale bar: A, 200 μm; B, E, and F, 2 μm; C and D, 1 μm.)

It has been established that the morphology of the electrodeposited Li film is intrinsically determined by its crystallographic texture. Strongly textured Li represents compact, well-aligned deposits, while weak/non-textured Li points out mossy and whisker-like structure. Additives in electrolytes and the cross-over molecules from the cathode (O₂, polysulfide) play a critical role in determining the crystallographic texture because they hinder the cathodic process and selectively adsorb on different crystal planes. A growth diagram is proposed to correlate the texture and morphology of Li deposits. The electrolytes with additives of lower exchange current density is likely to generate Li deposits with a stronger texture and a uniform morphology. The fundamental understanding obtained from the correlation of texture with morphology helps designing new types of additives to produce Li deposition with controllable texture, which will form the basis for development of future lithium metal batteries.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: F. Shi et al., Strong texturing of lithium metal in batteries, PNAS, 114 (46) (2017) 12138-12143, November 14, 2017, doi: 10.1073/pnas.1708224114

Achieving Enhanced Antibacterial Activity by Suitably Aligning Graphene Oxide Nanosheets

Graphene-based nanomaterials (GBNs) due to their exceptional mechanical, electronic, and thermal properties assumed significance in a variety of applications. The cytotoxic properties of GBNs are also important for their biomedical applications. GBNs have been shown to be cytotoxic toward a variety of cell types. However, the impact of alignment of nanosheets on the antibacterial activity has not been established. Researchers at Department of Chemical and Environmental Engineering, Yale University, USA, have investigated orientation-dependent interaction of graphene oxide (GO) nanosheets aligned in different orientations using a magnetic filed with Escherichia coli (E. coli). The GO nanosheets with vertical orientation exhibit an enhanced antibacterial activity when compared to those with random and horizontal orientations and the mechanism responsible is also suggested.

The schematic illustration of alignment of GO nanosheets with different orientations using magnetic field and alignment quality of GO nanosheets suspended in the monomer solution at different field strengths evaluated by 2D small-angle X-ray scattering (SAXS) are shown in Fig. 1.

Fig. 1 (a) Schematic illustration of alignment of GO nanosheets with different orientations using magnetic field; and (b) alignment quality of GO nanosheets

The various stages involved in the fabrication of GO composite films is shown in Fig. 2. Suspensions of GO nanosheets (with a thickness of ∼0.8 nm) in 2-hydroxyethyl methacrylate (HEMA), doped with cross-linker and photo initiator, were sealed between two glass substrates with a 300-μm spacer and aligned in a magnetic field of 6 T. Samples were subsequently cross-linked under UV irradiation to form polymer films, which preserved the orientation of the aligned GO nanosheets. The composite films were then detached from the glass substrates and irradiated using UV/O₃ to etch away the outer polymer and expose GO nanosheets on the surface. The resultant films are tough, mechanically coherent and resistant to water swelling, which are critical in preserving the GO orientation in aqueous environments.

Fig. 2 Various stages involved in the fabrication of GO composite film

The GO composite films were contacted with E. coli in suspension for 3 h. The bacteria attached on the surface were stained using SYTO 9 dye and propidium iodide and evaluated for live and dead cells. The vertical-GO film showed a lower cell viability (56.0 ± 8.7%) when compared to those with random (75.3 ± 3.5%) and planar (81.8 ± 5.1%) orientation. Morphological features indicate that E. coli on No-GO film showed an intact cell morphology, indicating no cytotoxicity of the pure polymer. E. coli on planar- and random-GO films largely retained their morphological integrity whereas cells on vertical-GO films became flattened and wrinkled, suggesting loss of viability and possible damage to the cell membrane (Fig. 3).Fig. 3 SEM micrographs of E. coli cells on etched GO composite films. The scale bar is 1 μm.

The mechanism for the enhanced antibacterial activity of vertically aligned GO nanosheets is explained based on (i) physical disruption; and (ii) chemical oxidation using lipid vesicles and oxidation of glutathione, respectively. GO nanosheets with a vertical orientation induced physical disruption of the lipid bilayer structure, resulting in loss of membrane integrity of the GO/lipid vesicle system. GO nanosheets with a vertical orientation also increased the extent of oxidation of glutathione (27.6%) with limited generation of reactive oxygen species, suggesting that the oxidation occurs through a direct electron transfer mechanism. Thus, both mechanisms contribute to the enhanced antibacterial activity of the vertical-GO film. Nevertheless, both of them require direct, edge-mediated contact with cells. The exposed edges of GO nanosheets with a vertical orientation could induce enhanced physical penetration and promote greater levels of electron transfer. Hence, the enhanced antibacterial activity of the film with vertically aligned GO nanosheets can be attributed to the increased density of edges with a preferential orientation for membrane disruption. The orientation-dependent cytotoxicity of GO nanosheets has direct implications on the design of engineering surfaces using graphene based nanomaterials.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: X. Lu et al., Enhanced antibacterial activity through the controlled alignment of graphene oxide nanosheets, PNAS 2017 114: E9793-E9801

 

Stretchable, Compressible and Conductive Metal-Coated PDMS Sponges

Development of flexible, highly conductive electrodes or interconnects with excellent mechanical stability has been a challenging issue in device fabrication for wearable electronics, flexible displays, etc. Researchers at College of Chemistry and Environmental Engineering, Shenzhen University, P. R. China have developed 3D stretchable, compressible and electrically conductive conductors by surface modification of poly(dimethylsiloxane) (PDMS) sponges with poly[2-(methacryloyloxy)ethyl-trimethylammoniumchloride] (PMETAC) polymers followed by electroless deposition of metals.

The PDMS sponges were fabricated using sugar templating method. The sugar cubes were immersed in a mixture of Sylgard 184 and curing agent, degassed in a vacuum desiccator for 2 h followed by baking at 65 °C for 3 h, removal of sugar template by immersion in water at 60 °C for 24 h and drying at 100 °C for 2 h. The PDMS sponges were activated by air plasma treatment for 5 min, functionalized with vinyltrimethoxysilane (VTMS) via silanization followed by in situ free radical polymerization with METAC monomer and potassium persulfate as initiator, leading to the formation of PMETAC-modified PDMS sponges with 3D-interconnected porous structures. Electroless deposition of metals enables the formation of metal-coated PDMS sponges. The schematic of the fabrication process and images of electroless Cu-, Ag/Cu-, and Au/Cu-coated PDMS sponges are shown in Fig. 1.

Fig. 1 (a) Schematic illustration of the fabrication of metal-coated PDMS sponges; (b, c) Optical images of the PDMS sponge and Cu-, Ag/Cu-, and Au/Cu-coated PDMS sponge

The PDMS sponges consist of highly interconnected 3-D porous structures and the sponge-like structure is retained even after coating them with Cu, Ag/Cu and Au/Cu by electroless deposition. The coated metal particles exhibit a close-packed arrangement on the surface of PDMS sponges. The elastomeric property of PDMS sponges enable them to be stretched and compressed while metal coating makes them conductive, thus making them suitable for the fabrication of electrically conductive, stretchable and compressible electrodes. The metal-coated PDMS sponges exhibit remarkable mechanical stability and electrical conductivity, which is evidenced by the overlap of I–V characteristics of Ag/Cu-PDMS sponges while stretching or compressing them from 0% to 50% as well as by the continuous glowing of LED lamps at different extents of stretching, bending, and twisting (Fig. 2). In addition, the metal-coated PDMS sponges remain conducting even after cutting them to two pieces, which indicates that the metal coating is uniform not only on the outer side but also on the inner side of PDMS sponges.

Fig. 2 Flexible LED circuits made of Ag/Cu-PDMS sponge interconnects: (a, b) I–V characteristics of the LED circuit at different (a) tensile; and (b) compressive strains; (c-e) Optical images of the LED circuits with two LEDs at different extent of (c) stretching; (d) bending; and (e) twisting.

The ability of metal-coated PDMS sponges to offer no change in resistance at 40% tensile strain and only ≈20% increments at 50% strain after 5000 cycles of stretching suggests that they can be used for stretchable, compressible, and bendable interconnects or soft electronic contacts.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: S.Q. Liang et al., 3D Stretchable, Compressible, and Highly Conductive Metal-Coated Polydimethylsiloxane Sponges, Adv. Mater. Technol. 2016, 1600117, DOI: 10.1002/admt.201600117

 

Synthesis of Reduced Graphene Oxide by Microwave Exfoliation Using Graphite as a Catalyst

Achieving large scale synthesis of high quality graphene is a critical step to exploit the practical application of graphene in a variety of fields. Researchers at State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, China have reported a environmental friendly and ultrafast catalytic microwave method in which a small amount of graphite flake served as the catalyst to promote microwave exfoliation and reduction of graphene oxide (GO).

Microwave irradiation of GO powder leads to the formation of microwave exfoliated graphene oxide (MEGO), which usually occurs in ~15 min. This reaction is triggered in presence of a small amount of graphite powder (< 1 mg) and the exfoliation process is completed within 5 s with the formation of a large volume of  catalytic microwave exfoliated graphite oxide (CMEGO) (Fig. 1). The graphite flakes with highly extended π-system efficiently absorb the microwave as a susceptor and convert the energy to activate the nearby gas molecules while the microwave plasma generates a local ultrahigh energy environment.

Fig. 1 Schematic of the synthesis of CMEGO

The exfoliation and reduction of GO in the ultrahigh energy environment created by catalytic microwave plasma leads to a more complete removal of oxygen functional groups and yields CMEGO with a lower lattice defects, higher specific surface area (886 m²/g), large C/O ratio (19.4), good electrical conductivity (53180 S/m) as well as excellent solvent dispersability and processability. The morphology of CMEGO indicates that it is thoroughly exfoliated as evidenced by the formation of smooth, thinner and transparent graphene sheets with a weak lattice distortion (Figs. 2(a) and 2(b)). HR-TEM images show that the CMEGO has a more regular lattice and fewer graphene layers (Figs. 2(c) and 2(d)).

Fig. 2 (a, b) SEM; and (c, d) HR-TEM images of CMEGO

Use of CMEGO as an anode material in lithium-ion batteries has enabled very high reversible capacities of 2260 mAh/g and 469 mAh/g at a charge/discharge rate of 0.1 A/g and 30 A/g, respectively, and an outstanding capacity retention of 91.4% after 1000 cycles at 5.0 A/g. Similarly, use of CMEGO as an anode material in sodium-ion batteries offered very high reversible capacities of 424 mAh/g and 218 mAh/g at 0.1 A/g and 30 A/g, respectively, and a stable capacity retention of 85.7% after 1000 cycles at 5.0 A/g.

The catalytic microwave irradiation strategy employed for the large-scale synthesis of high quality graphene is promising for applications in energy storage and conversion.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Runze Liu, A Catalytic Microwave Process for Superfast Preparation of High-Quality Reduced Graphene Oxide, Angew. Chem. Int. Ed. 10.1002/anie.201708714

Polymer Microfiber Bundles for Oil/Water Separation

Cleanup of large-scale oil spills or organic pollutants from water is indeed a challenging problem. Techniques hitherto proposed for this purpose are not efficient and involves high operational cost. Researchers at College of Materials Science and Engineering and National Engineering Research Center for Advanced Polymer Processing Technology, Zhengzhou University, PR China and Integrated Composites Laboratory (ICL), University of Tennessee, USA have developed a cost-effective and environmentally friendly method to fabricate high-density polyethylene (HDPE) microfiber bundles (PMBs) with interconnected microchannels with hydrophobicity and oleophilicity for oil/water separation.

High-density polyethylene (HDPE) and polyethylene oxide (PEO) granules (50/50 wt.%) were melt mixed in a micro twin-screw extruder (Fig. 1(a)). The HDPE/PEO extrudates were hot-drawn with a given draw ratio, air-cooled and wounded up by a winder (Fig. 1(b)). The polyethylene microfiber bundles (PMBs) containing interconnected microchannels were obtained by aqueous leaching of PEO in water (Fig. 1(c)). The resultant PMBs are very soft and can be knitted in the form of a mat (Fig. 1(d)).

Fig. 1 Various step involved in the fabrication of polyethylene microfiber bundles (PMBs): (a) Schematic of the micro twin-screw extruder; (b) HDPE/PEO fibers; (c) PMBs after leaching of PEO in water; and (d) PMBs knitted in the form of a mat

The morphology of PMBs acquired at the cross section indicates the formation of interconnected microfiber networks in bundles, which are aligned along the extrusion direction with many pores and gaps between the microfibers (Fig. 2(a)). The formation of random lamellae, particularly at the edges, imparts multi-scale roughness on surface of PMBs These attributes change the contact angle. Cyclohexane droplets spread and penetrated into the 3D interconnected structure of PMBs whereas water droplets show a nearly sphere with a high contact angle (Figs. 2(b) and 2(c)).

Fig. 2 (a) Morphology acquired at the cross section of PMBs; (b, c) contact angle images of water (dyed with potassium permanganate) and cyclohexane (dyed with Sudan III) droplets on the (b) surface and (c) cross-section of  PMBs

The interconnected structure and highly hydrophobic surface enable the PMBs with an excellent absorption capacity for oils/organic pollutants. When the PMBs is placed on a cyclohexane–water mixture, the cyclohexane is quickly and selectively absorbed from water surface in several seconds (Fig. 3(a)). The underwater chloroform is also absorbed quickly by PMBs (Fig. 3(b)). The oil absorption mechanism of PBMs is attributed to the capillary force through which the oil is absorbed into the interconnected microchannels of PMBs and replaces the air within microchannels. The absorbent capacity of PMBs for various kinds of oils and organic solvents (Fig. 3(c)) indicate a maximum absorption capacity of up to 7 times their weight. Alternate immersion of PMBs in soybean oil for absorption and centrifugation for removal of oil indicates (Fig. 3(d)) no apparent deterioration in absorption capacity for 100 cycles and nearly 90% of the absorbed oil is centrifuged during each cycle. In spite of a large deformation in the shape after centrifugation, the PMBs are capable of recovering its original shape after oil absorption to saturation (Fig. 3(e)) without damaging the oil absorption performance. Due to the excellent recyclability and recovery, PMBs can be considered as a potential absorbent for dealing with oil spills or oil/water separation. The ability of PMBs for continuous oil–water separation is also demonstrated (Fig. 4).

Fig. 3 Snapshots of removal process of (a) cyclohexane floating on water; (b) chloroform sinking underwater; (c) Mass absorption capacities of the PMBs for various organic solvents and oils; (d) Oil-absorption capacity of PMBs with different absorption/centrifugation cycles; and (e) The shape change of PMBs during absorption and centrifugation process

Fig. 4 Continuous removal of cyclohexane (dyed with Sudan III) from water (dyed with methylene blue) using PMBs.

PMBs with 3D interconnected structures, high absorption capacity and excellent reusability have been developed by combining melt extrusion molding and leaching techniques. The ability of PMBs to selectively remove oils and organic solvents from water, to retain a high absorption capacity even after 100 absorption/centrifugation cycles and to offer continuous oil–water separation, suggest that they can be a promising candidate material for cleanup of oil spills and chemicals leaks.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Yayun Wang, , Applied Materials Today, 9 (2017) 77-81.

Flexible, High-Wetting and Fire-Resistant Separators based on Hydroxyapatite Nanowires for Lithium Ion Batteries

Lithium ion batteries (LIBs) have find widespread use as power sources for electric vehicles, grids, and other large-scale energy storage systems. Polyolefin membranes, the most commonly used separators in LIBs, though capable of offering good electrochemical stability, suitable mechanical strength and pore size, suffer from large shrinkages at high temperatures due to the low glass transformation temperature and melting point of the polymer moiety. The incompatibility between the polar organic solvents and nonpolar polyolefin membranes results in poor electrolyte wettability and thus low ionic conductivity. Researchers at Shanghai Institute of Ceramics, Chinese Academy of Sciences, China and School of Materials Science and Engineering, Huazhong University of Science and Technology, China have designed and fabricated a highly flexible and porous separator by self-assembling  hydroxyapatite nanowires (HAP NW) with cellulose fibers (CFs). The HAP/CF separator with a hierarchically cross-linked structure exhibits a good combination of high flexibility, robust mechanical strength, highly porous structure, superior electrolyte wettability, excellent thermal stability and fire resistance. Batteries fabricated using the HAP/CF separator exhibit enhanced cyclability and rate capability when bench marked against the commonly used polypropylene separator in LIBs.

HAP NWs were prepared by solvo-thermal method using calcium oleate as the precursor (Fig. 1(a)). The HAP NWs and CFs (Fig. 1(b)) were subjected to a self-assembly (hybridization) process (Fig. 1 (c)) in aqueous solution wherein the branched CFs were wrapped with network-structured HAP NWs through hydrogen bonding and van der Waals force. The self-assembled hierarchically cross-linked hybrid fibers were filtrated under vacuum suction (Fig. 1(d)) and dried. The HAP-CF separator was used to assemble the LiFePO₄/separator/Li half cells (Fig. 1(e)).

Fig. 1 Various stages involved in the fabrication of HAP/CF separator

The morphological features of HAP NW, CF and HAP/CF separator (Fig. 2) indicate that the CFs are uniformly embedded in the porous HAP NW networks, leading to the formation of HAP/CF separator with open, continuous, and interconnected nanopores. The porosity of the HAP/CF separator is ~81% with an average pore size of 120.9 nm and a narrow pore size distribution ranging from about 110 to 130 nm, suggesting its suitability as separators for LIBs. The HAP/CF separator can be rolled, twisted, folded, scrunched and unscrunched with no visible damages (Fig. 3), which indicate its good strength and high flexibility. The synergistic combination of the van der Waals force and hydrogen bonding enables the HAP/CF separator to achieve a higher tensile strength of 13.21 MPa. The HAP/CF separator exhibits good mechanical strength even at 200 °C, demonstrating its excellent thermal stability.

Fig. 2 Morphological features of (a) HAP NW networks; (b) CFs; and (c, d) HAP/CF separator

Fig. 3 Flexibility of the HAP/CF separator under different bending conditions: (a) rolled; (b) twisted; (c) folded; and (d) scrunched.

The rapid penetration of the electrolyte droplet within 5 s indicates the high wettability, which enables the HAP/CF separator with an electrolyte uptake of 253%. The ability of HAP/CF separator to maintain 77% of the initial weight even at a high temperature of 900 °C, suggests its excellent thermal stability. When ignited, the HAP/CF separator wetted with electrolyte gets self-extinguished due to the non-flammable nature and strong affinity of HAP NW for the electrolytes. In contrast, the commonly used PP separator wetted with electrolyte gets ignited and continuously combusted (Fig. 4).

Fig. 4 Fire-resistant characteristics of (a, b) PP separator; and (c, d) HAP/CF separator; (a, c) before burning; and (b, d) after burning.

The cycling performance and rate capability of the LiFePO₄/separator/Li half cells assembled using the HAP/CF separator and commercial PP separator (Fig. 5) indicate that the initial discharge capacity of the cell with HAP/CF separator (138 mAh/g) is higher than those obtained using the PP separator (130.1 mAh/g) at 0.5 C. Cells with the HAP/CF separator exhibit a higher discharge capacity of 135.4 mAh/g when compared to those obtained with the PP separator (129.5 mAh/g) after 145 cycles at 1 C. Cells with the HAP/CF separator exhibit a capacity retention of 85.7%, which is much higher than those exhibited by the cells with PP separators (65.7%).

Fig. 5 (a) Cycling performance; and (b) rate capability of theLiFePO₄/separator/ Li half cells using the HAP/CF and PP separators

The electrochemical performance of the batteries prepared using HAP/CF and commercial PP separators, for the initial 5 cycles at room temperature and subsequent 20 cycles at 150 °C (Fig. 6 (a)) indicates that the battery constructed with the HAP/CF separator exhibits a good cycling performance with a higher discharge capacity of 157.8 mAh/g and an average Coulombic efficiency of >98%. In contrast, the battery constructed with PP separator fails to offer a good performance at 150 °C (Fig. 6(a)). The large shrinkage of the PP separator at 150 °C causes internal short circuit, which is evidenced by the sudden drop in open-circuit voltage (OCV) while the battery with the HAP/CF separator could maintain its initial voltage throughout the whole testing process (Fig. 6(b)). The battery equipped with HAP/CF separator can safely light up two 3.0 V LED lamps at a temperature as high as 150 °C, suggesting the extraordinary thermal stability of the HAP/CF separator and the great potential for its application in high-temperature-related batteries (Figs. 6 (c) and 6(d)).

Fig. 6 (a) Cycling performance of the batteries constructed using HAP/CF and PP separators at 2 C for the initial 5 cycles at room temperature and subsequent 20 cycles at 150 °C; (b) OCV curves of the LiFePO4/separator/Li batteries with the HAP/CF separator and the PP separator at 150 °C; (c, d) battery prepared using the HAP/CF separator working at 150 °C.

The superior electrolyte wettability, mechanical robustness, high thermal stability, and fire resistance of HAP/CF separator appears to be promising for LIBs with enhanced performance and safety.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Heng Li et al., Adv. Mater. 2017, 1703548, DOI: 10.1002/adma.201703548

Flexible sensors for real-time monitoring of temperature, pressure and flow

Development of strain/stress sensors for human-motion detection and implantable devices for human health monitoring assumed significance in recent years. Multi-functionality, poor interface stability and flexibility of such sensors still remain as a major challenge. Researchers at State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, and Engineering Research Center of Advanced Glasses Manufacturing Technology, College of Materials Science and Engineering, Donghua University, China have reported fabrication of a mechanically strong and flexible vessel-like sensor that is capable of continuously monitoring applied temperature, strain, and the frequency, pressure, and temperature of the in-vessel flow.

Braided cotton yarn served as the base and it was coated with single-walled carbon nanotubes (SWCNTs) by dip-coating (cotton@SWNTs) to function as a current collector/electrode. ZnO nanoseeds were nucleated on the surface of cotton@SWNTs to promote the growth of ZnO nanorods, which could serve as temperature sensing arrays. A polyvinylidene fluoride (PVDF) film was formed over the cotton@SWCNTs/ZnO through dip-coating. A PVDF fibrous membrane was further prepared on the PVDF surface through electrospinning. The PVDF fibrous membrane was immersed in ethyl acetate for 12 h to obtain a compact film. The PVDF membrane could serve as piezoelectric nanofibers. Finally, silver paste was uniformly coated on the surface of the PVDF membrane. The various stages involved in the fabrication are shown in Fig. 1.

Fig. 1 various stages involved in the fabrication of flexible sensor

The morphological features of cotton@SWNTs indicate a uniform distribution of SWCNTs on the surface of cotton yarn (Fig. 2(a)). Similarly, the ZnO nanorods arrays@PVDF are also uniformly developed on the surface of cotton@SWCNTs (Figs. 2(b) and 2(c)).

Fig. 2 Morphological features of (a) SWCNTs coated cotton yarn; (b, c) cotton@SWCNTs/ZnO@PVDF; (c) magnified image of ZnO nanorods

Structural representation of the tubular sensor used for monitoring temperature, motions and liquid flow is depicted in Fig 3(a). The thermosensitivity of ZnO grown between the fibers of SWCNTs helps monitoring the temperature variation, which is detected through resistance measurement of the tubular inner electrode (Fig. 3(a)). The I-V curves measured using the functionalized inner electrode of the sensor at different temperatures indicate the change in resistance with variation in temperature (Fig. 3(b)).

Fig. 3 (a) Schematic representation of the sensor used for monitoring temperature, motions and liquid flow; (b) Linear I-V curves of the sensor measured under different fluid temperature.

The sensor is also capable of monitoring human body motions by sensing both bending and pressing. When a force is applied on the surface of the PVDF nanofibers, they get polarized. The equivalent bound charges accumulated on the surface of PVDF nanofibers are measured as electrical signals with the help of bonding wires between the two electrodes – cotton@SWCNTs/ZnO layer and the silver paste layer. The output voltage is increased from 0.038V to 0.1V with an increase in bending angle from 1.4 rad to 2.1 rad (Figs. 4(a) and 4(c)). Similarly, an increase in output voltage is observed with an increase in frequency during compression (Figs. 4(b) and 4(d)). These inferences suggest that human motions such as bending and compression can be directly monitored without any external power supply.

Fig. 4 (a, b) Demonstration of the bending and pressing motion; (c) output voltage generated by the sensor under different bending angles; and (d) output voltage generated by the sensor under compression at different frequencies.

The ability of the sensor to monitor blood temperature and blood pressure is also evaluated. Fig. 5(a) shows the experimental set-up used to monitor liquid flow. The temperature of the test fluid (deionized water to mimic blood flow) that flows into the sensor is accurately controlled using microfluidic channels heated by sample heater. The variation in the temperature of the fluid under flow from 35 to 40 °C is clearly reflected by the change in resistance measured using the sensor (Fig. 5(b)). The sensor could also detect the variation in the input pressure and frequency of the pulsed flow. The increase in the output voltage with an increase in applied pressure indicates the ability of the sensor to measure the variation in pressure under flow conditions (Fig. 5(c)). Similarly, the variation in the output current is sufficient enough to qualify the sensor for its ability to measure the pulse frequency of fluid (Fig. 5(d)).

Fig. 5 (a) Experimental set-up used to monitor liquid flow using the sensor; (b) I-V curves of the sensor measured under a narrow fluid temperature range; (c) output voltage generated by the sensor under different fluid input pressure; and (d) output current generated by the sensor under different pulse frequencies.

The cotton@SWCNTs/ZnO@PVDF sensor is flexible and durable. The output signal of the sensor remains steady during 1920 circles of the bending circle test. The sensor facilities detection and continuous monitoring of the applied temperature, strain, and the frequency, pressure, and temperature of pulsed fluids. It can be used for implantable physical sensing applications and monitoring of human health.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Wei Zhang et al., A strong and flexible electronic vessel for real-time monitoring temperature, motions and flow, Nanoscale, 2017, DOI: 10.1039/C7NR05575G

Playing Billiards using Protons Generated by Electrolytic Dissociation of Hydrogen for Intercalation and Ion Substitution

TaS₂ is a well-known 2D layered material and intercalation extends its window of opportunity for a wide variety of applications. Achieving homogeneous intercalation while maintaining high crystallinity using liquid phase method is difficult. Chemical vapor transport (CVT), which involves a direct reaction between the guest vapor and host solid requires high temperature to vaporize the guest materials. Na₃V₂(PO₄)₃ (NVP), due to its high redox potential, excellent thermal stability,  and strong (PO₄)³⁻ polyanion networks, is considered as a promising cathode material for Na ion battery. Substitution of K⁺ ions in place of Na⁺ ions is likely to improve the rate capability and cycling stability. Nevertheless, the large difference in size between K⁺ and Na⁺ ions makes such substitution difficult. Researchers at Hokkaido University, Japan, Xi’an University of Technology, China, Kyushu University, Japan, Tokyo University of Science, Japan and Kyushu Institute of Technology, Japan have demonstrated a new synthesis method, referred as proton-driven ion introduction (PDII), which is based on a solid-state electrochemical reaction for intercalation into TaS₂ and ion substitution of Na₃V₂(PO₄)₃.

Intercalation of alkali metal ions (K⁺, Na⁺, and Li⁺) into TaS₂

A single crystal of plate-shaped TaS_­2 was placed on a carbon cathode. Disk-shaped phosphate glass containing alkali metal ions (K⁺, Na⁺, and Li⁺), referred as the ion-source material was placed above the TaS₂ single crystal. The electrodes were arranged inside a chamber with H₂ atmosphere. The schematic of the experimental set-up used for proton-driven ion introduction (PDII) is shown in Fig. 1. Upon application of a high between the needle-shaped anode and the carbon cathode, electrolytic dissociation of H₂ leads to the formation of protons and electrons. The protons are accelerated into the ion-source material. Penetration of protons pushes the alkali metal ions from the top to the bottom side of the ion source material. In order to maintain charge neutrality, the ion-source material releases the alkali metal ions through its bottom side, which reach the surface of TaS₂. Simultaneously, electrons produced by the corona discharge move from the needle-shaped anode to the TaS₂ single crystal placed over the carbon cathode through the electrode. The electric current flows around the circuit enables electrochemically driven intercalation of TaS_­2, according to the following reaction: TaS₂ + xA⁺ + xe⁻ = A_xTaS₂ (A = alkali metal ion).

Fig. 1 Schematic of the experimental set-up used for proton-driven ion introduction (PDII) – intercalation of alkali metal ions into TaS₂

Intercalation of transition metal ions (Cu⁺ and Ag⁺) into TaS₂

Besides alkali metal ions, transition metal ions such as Cu⁺ and Ag⁺ can also be intercalated  into  TaS₂ by  PDII,  using  CuI and  AgI as  the  respective source materials. Phosphate glasses containing Na, CuI, and TaS₂ were stacked on the carbon  cathode in that  order  from the top. The  schematic of the  experimental set-up used for the intercalation of Cu and Ag into TaS₂ by PDII is shown in Fig. 2(a). The phosphate glass is essential to prevent the formation of poisonous  HI gas. Optical image of the top side of CuI (Fig. 2(b)) indicate a change in colouration of CuI following the replacement of Cu⁺ with Na⁺ ions. The protons drive the release of Na⁺ ions from the phosphate glass, which is then substituted for the Cu⁺ ions on the top surface of CuI. At the bottom side, alkali metal ions and Cu ions are homogeneously intercalated into TaS₂. The three black single crystals represent Cu_xTaS₂ while the red products formed around Cu_xTaS₂ are indeed Cu metal (Fig 2(b)).

The supply rate of Cu⁺ ions from CuI and the diffusion coefficient of Cu⁺ ions in TaS₂ determines the nature of intercalation. When the rate of supply of Cu⁺ ions is  much smaller than the diffusion coefficient of Cu⁺ ions, Cu⁺ ions spread effectively throughout the TaS₂. On the contrary, when the rate of supply of Cu⁺ ions is  much larger than the diffusion coefficient, the surplus Cu⁺ ions are precipitated as Cu metal and the migration of Cu⁺ ions in to TaS₂ is prevented. Hence, to achieve a homogeneous intercalation, the supply rate of Cu⁺ ions should be smaller than its diffusion coefficient. Accordingly, a combination of low treatment temperature and high voltage fails to produce homogeneous intercalation of Cu intoTaS₂ even after 50 h. However, a high treatment temperature and a low voltage allow the formation of a homogeneous single crystal of Cu_2/3TaS₂, within a short treatment time of ~ 12 h (Fig. 2(c)).

Fig. 2 (a) Schematic of the Cu intercalation; (b) Optical images of the top and bottom surfaces of CuI after PDII. (c) Conditions under which homogeneous an partial intercalation of Cu into TaS₂ occurs

Ion substitution (K+ in place of Na+) in Na₃V₂(PO₄)₃

Powdered sample of Na₃V₂(PO₄)₃ (NVP) taken in a Al₂O₃ cylinder was placed on the carbon cathode. Potassium-containing phosphate glass (K⁺ source material) was placed over the Al₂O₃ cylinder (Fig. 3). Upon application of the voltage, the protons replace the K⁺ ions in the phosphate glass and push them into NVP. Unlike the intercalation of ions into TaS₂, during ion substitution, the K⁺ ions will not receive electrons between interfaces since NVP is electrically insulating.  Conversely, the Na⁺ ions discharged from the bottom side of NVP receive electrons and precipitates. Cross-sectional optical images (Fig. 3) indicate a change in colouration of the NVP powder to dark green in the upper region following substitution of K⁺ ions in place of Na⁺ ions. Based on the chemical composition (K_1.69Na_1.20V_2.00P_3.88O_13.83), the upper region is referred as K-NVP which is distinctly different from the NVP in lower region (Na_3.18V_2.00P_3.02O_10.86). Post-annealing at 600 °C improved the crystallization of K-NVP. When compared to conventional solid-state reaction, it would be possible to increase the amount of K substitution by 15 times using PDII.

Fig. 3 Schematic of the ion substitution process – K⁺ ion substitution in the Na⁺ site in Na₃V₂(PO₄)₃ and cross-sectional optical images of Na_3−xK_xV₂(PO₄)₃

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Masaya Fujioka et al., Proton-Driven Intercalation and Ion Substitution Utilizing Solid-State Electrochemical Reaction, J. Am. Chem. Soc., Article in press, DOI: 10.1021/jacs.7b09328

Electrochemical 3D Printing of Copper

Additive manufacturing (AM), also referred attractively as 3D printing, is a well-established process of creating complex 3D geometries through a layer-by-layer building approach. 3D printing has received considerable attention in automotive, aerospace, and biomedical sectors. Direct metal laser sintering (DMLS) is the most commonly used method for 3D printing of metals which involves selective laser sintering of layers of metal powders. However, the high capital cost, defects in the manufactured components and inability to work with multiple materials, limit 3D printing of metals by DMLS and warrant the development of novel non-laser based 3D printing techniques.

Electrochemical additive manufacturing (ECAM) (electrochemical 3D printing) involves deposition of thin and highly adherent layers of metal with a layer-by-layer approach onto a conducting surface through reduction of metal ions from the electrolyte. It combines the basic principles of electrodeposition and 3D printing. Two different approaches, viz., localized electrochemical deposition (LCD) and meniscus confined electrode (MCE), have been used for electrochemical 3D printing of metals. These methods, however, require the use of expensive piezo-based movement stages and nanopippettes/ultrafine electrodes. The slow rate deposition rate of metals (Cu: from ≈100 nm/s to ≈0.18 μm/s), inability to develop components of complex geometries, porosity and roughness are some of the major limitations. Moreover, majority of the structures fabricated using these methods are simple wire-based architectures. Methods for 3D electrochemical printing of metals with improved deposition rates and capable of printing complex structures are warranted. In this perspective, researchers at Dyson School of Design Engineering and Department of Earth Science and Engineering, Imperial College London, UK, have reported a novel strategy for ECAM using a MCE approach.

The electrochemical 3D printer assembly comprised of a plastic syringe and nozzle (diameter 400 μm) with a porous sponge filled with 1 M CuSO₄ was mounted on a carriage and its movement in the x-, y- and z- directions was precisely controlled using computer controlled stepper motors. Two copper rods suspended in the CuSO₄ electrolyte served as the counter and reference electrodes while a copper plate served as the working electrode. The print head was moved to contact the copper plate and retract back by a small amount. The sufficient back pressure provided by the porous sponge to the hydraulic head enabled the formation of a stable meniscus (localized electrolyte). When a positive potential was applied between the working and counter electrodes, deposition of Cu occurs on the working electrode through reduction of Cu²⁺ ions with a simultaneous replenishment of Cu²⁺ ions from the counter electrode. To create 3D printed structures, the print head was moved in x-, y- and z-directions with varying traverse speed as directed by the 3D model. The various components of the electrochemical 3D printer assembly is schematically represented in Figs. 1 (a), 1(b) and 1(c). Optical image of the 3D printed Cu with the shape of the letters “I”, “C”, and “L” using 1 M CuSO₄ at 4 V at a print head speed of 0.4 mm/s is shown in Fig. 1(d).

Fig. 1 Schematic illustration of the electrochemical 3D printer assembly: (a) Print head set-up; (b) electrode arrangement; (c) print nozzle and sponge in the tip, highlighting how they act during the deposition of Cu; and (d) optical images of the printed Cu structures featuring the letters “I”, “C”, and “L” printed using 1 M CuSO₄ at 4 V at a print head speed of 0.4 mm/s.

Electrochemical 3D printing of Cu dots and lines (lateral print head velocity: 0.4 mm/s) were made using 1 M CuSO₄ as the electrolyte at varying voltages from 1 to 6 V for 1 h. The Cu dots printed at 1 V exhibited a dense structure with a high degree of concentricity. In spite of a faster rate of growth along with a dense structure, those printed at 2 V exhibited a convex shape due to preferential deposition at the center. Formation of Cu dendrites becomes apparent at 3V due to the limitations imposed by the mass transport and the a continuous increase in porosity is observed with a further increase in deposition potential (Fig. 2(a)). Hence, achieving dimensionally accurate structures would be difficult using electrochemical printing of Cu dots at potentials beyond 2 V. Irrespective of the deposition potentials in the rage of 1-6 V, electrochemically printed Cu lines fails to exhibit any Cu dendrites since the relative position of the print head aids mass transport of Cu²⁺ ions or might have assisted mechanical removal of the dendrites (Fig. 2(b)). The thickness of the Cu lines is increased from 3 μm at 1 V to 15 μm at 4 V, beyond which it decreased due to the decrease in deposition efficiency. For a given lateral speed of 0.4 mm/s for 3600 s, Cu lines can be printed for 144 passes over a distance of 10 mm.

Fig. 2 SEM images of (a) single Cu dot; and (b) Cu lines (lateral print head speed: 0.4 mm/s) electrochemically printed using 1 M CuSO₄ at 3-6 V for 1 h

The electrochemical 3D printed Cu dots and Cu lines exhibit a higher hardness than cold worked cast Cu. The Vickers hardness of Cu dots varies from 184 to 196 MPa, Cu lines ranging from 211 to 228 MPa when compared to the hardness of cold worked cast Cu, which varies from 50 to 176 MPa; the smaller the grain size, the higher is the hardness. The electrical conductivity of Cu lines is ranging between 1.31×10⁶ and 6.86×10⁶ S/m, which agrees well with that of nanocrystalline Cu (5.41×10⁶ S/m) but relatively lower than coarse grained Cu.

Electrochemical 3D printing of Cu builds a foundation for future work to achieve high speed deposition as well as printing of multi-metal functional structures. The z-height resolution opens up new avenues for the fabrication of functional electronics such as sensors.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Xiaolong Chen et al., A Low Cost Desktop Electrochemical Metal 3D Printer, Adv. Mater. Technol. 2017, 1700148, DOI: 10.1002/admt.201700148

Sn/SnOx-loaded Hollow Carbon Spheres on Graphene as an Anode for Lithium-ion Batteries

Lithium-ion batteries (LIBs), due to their ability to offer a high capacity and long cyclability have established themselves as a potential high-energy storage device. Numerous developments are constantly being made to overcome the current limitations of LIBs such as volume expansion, diffusion of Li⁺ ions, etc. Researchers at Graduate School of Convergence Science and Technology and Advanced Institutes of Convergence Technology, Seoul National University, Republic of Korea have synthesized Sn/SnO_x-loaded uniform-sized hollow carbon spheres on graphene nanosheets (Sn-UHCS/G) and demonstrated its utility as a lithium-ion battery anode to overcome the limitations in LIBs.

The graphene nanosheets (G) were prepared by thermal reduction of exfoliated graphene oxide in vacuum at 300 °C for 3 h. Uniform-sized carbon coated iron oxides on graphene sheets (C@Iron-oxides/G) were prepared by heating a mixture of Fe(acac)₃, oleic acid and graphene nanosheets at 600 °C for 5 h in Ar atmosphere. The C@Iron-oxides/G spheres were etched using 3M HCl for 24 h, rinsed with DI water followed by ethanol and heated at 800 °C for 5 h to obtain uniform-sized hollow carbon spheres/graphene composite (UHCS/G) with a higher conductivity. The UHCS/G was mixed with Sn powder in a weight ratio of 5:5 and heated at 250 °C for 5 h under Ar atmosphere, which enabled loading of Sn in the UHCS/G by melt diffusion to yield Sn-UHCS/G composites. The various steps involved in the synthesis of Sn-UHCS/G is represented in Fig. 1.

Fig. 1 Various steps involved in the synthesis of Sn-UHCS/G

The SEM and TEM images of Sn-UHCS/G indicate uniform-sized hollow carbon spheres (diameter: ~ 10 nm) anchored on graphene nanosheets and the absence of any agglomerated Sn (Fig. 2). The uniform-sized carbon spheres provide a closed structure for Sn that helps to mitigate its direct contact with the electrolyte.

Fig. 2 (a) SEM; and (b, c) TEM images of Sn-UHCS/G

The first and second voltage profile curves of Sn-UHCS/G in the voltage range of 0.01 V to 3.00 V at 0.1 C are shown in Fig. 3(a). Although  the specific capacity of Sn-UHCS/G is relatively lower during the 1^st cycle due to the formation of SEI on the surface of Sn particles, its capacity is increased during the 2^nd cycle. The coulombic efficiency of Sn-UHCS/G is increased from 61.8 % for the initial cycle to 91.2 % during the subsequent cycles. Sn-UHCS/G exhibits a relatively stable capacity retention in the current density range of  0.1 to 3.0 A/g when compared to Sn/G, and pristine Sn (Fig. 3(b)). The discharge capacities of Sn-UHCS/G at 2.0 and 3.0 A/g are ~342 mA h/g and ~275 mA h/g, respectively. Cycling tests performed at 1.0 A/g indicate a relatively stable performance of Sn-UHCS/G for 1000 cycles when compared to Sn/G, and pristine Sn (Fig. 3(c)). The ability of UHCS/G to provide good electronic conductivity through improved coverage of the Sn particles on conductive carbon helps to achieve an improved performance.

Fig. 3 (a) 1^st and 2^nd discharge/charge curves of Sn-UHCS/G electrode; and (b, c) cycling performance of Sn-UHCS/G, Sn/G, and pristine Sn electrodes.

Sn-UHCS/G exhibited a good rate performance (290 mA/g at 3.0 A/g) and excellent cycle stability (284.1 mA h/g after 1000 cycles at 1.0 A/g). The better electrochemical performance of Sn-UHCS/G is due to the ability of (i) the nanosized Sn/SnO_x powders to mitigate the volume expansion during continuous cycles; and (ii) UHCS/G with a high surface area, good electrical conductivity and uniform distribution of Sn/SnO_x, which improves diffusion of Li⁺ ions as well as electrons and promotes diffusion/penetration of electrolyte in the electrode.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Jeongyeon Lee et al., Sn/SnO_x-loaded uniform-sized hollow carbon spheres on graphene nanosheets as an anode for lithium-ion batteries, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.11.127

 

Aqueous Rechargeable Chloride ion Battery

The ever increasing demand for energy warrants the development of long-lasting energy storage devices capable of delivering high energy densities. Researchers at Singapore University of Technology and Design, Singapore have demonstrated an aqueous rechargeable battery using chloride ions in an aqueous NaCl solution with BiOCl anode and silver cathode, for the first time.

The anode was prepared by coating graphite papers using a slurry consisting of a mixture of BiOCl, polyvinylidene difluoride (PVDF) and carbon black in a ratio of 8:4:4 with N-methylpyrrolidone (NMP) as the solvent (BiOCl electrode). Graphite papers coated with Ag paste was used as the cathode (Ag electrode). The cathode and anode were dried in a vacuum oven at 100 °C for two days before assembling them in the battery. 1M NaCl (pH adjusted to 8.0) was used as the electrolyte and it was purged with Argon gas for 15 min before battery assembling. GB100R glass fiber membranes were used as separators.

The mechanism of the aqueous rechargeable chloride ion battery during charging and discharging involves reversible transport of chloride ions through the electrolyte and its reaction with the electrodes via redox electrochemistry (Fig. 1). During  charging (Fig. 1(a)), the chloride ions are deintercalated from the BiOCl electrode, transferred through the electrolyte and intercalated into the Ag electrode to form AgCl while the chloride ions will be inserted back into the anode with the recovery of BiOCl during discharging (Fig. 1(b)).

Fig. 1 Schematic illustration of the aqueous rechargeable chloride ion battery during: (a) charging; and (b) discharging process.

The charge and discharge curves of BiOCl-Silver system using 1 M NaCl as  the electrolyte is shown in Fig. 2(a). In spite of the low coulombic efficiency during the initial cycles (37.2%), the efficiency is increased to ~99% during the subsequent cycling (Fig. 2(b)), suggesting the high reversibility of the BiOCl-Ag battery. The discharge capacity is decreased from 92.1 mAh/g to 24.9 mAh/g with an increase in current density from 400 mA/g to 1200 mA/g (Fig. 2(c)). However, its ability to restore back a stable capacity when the current density is reverted back from 1200 mA/g to 400 mA/g (Fig. 2(c)) indicates that the performance of the aqueous chloride ion battery is stable and reversible. Volume contractions or expansions during the electrode phase transformation still remains as a problem that need to be solved.

Fig. 2 (a, b) Charge-discharge curves; (c) cycling performance; and (d) rate capabilities, of the BiOCl/Ag system in 1 M aqueous NaCl electrolyte.

The aqueous rechargeable chloride ion battery exhibits a stable discharge capacity of 92.1 mAh/g at 400 mA/g with a coulombic efficiency of ~100% and maintains its stability for 45 cycles without decay in performance.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Fuming Chen et al., Energy Storage Materials 7 (2017) 189–194.

Ultrathin Epidermal Piezoelectric Sensors for Real-Time Pulse Monitoring

Real-time monitoring of heart rate, blood pressure, and respiration rate assume significance since these data can provide the status of one’s personal health and early warning about deterioration of health so that necessary therapeutic treatments can be given to the patient. In spite of the numerous developments of sensors for real-time monitoring, meeting their requirements such as flexibility, sensitivity, response time and mechanical stability and biocompatibility remains to be really challenging. Researchers at Korea Advanced Institute of Science and Technology (KAIST), Republic of Korea, Severance Cardiovascular Hospital, Yonsei University College of Medicine, Republic of Korea, KAIST Institute for NanoCentury (KINC), Republic of Korea, The Pennsylvania State University, USA and ROBOPRINT Co., Ltd., Republic of Korea have demonstrated a self-powered flexible piezoelectric pulse sensor based on PZT thin film for real-time healthcare monitoring.

The fabrication process of self-powered flexible pressure sensor on an ultrathin polyethylene terephthalate (PET) substrate is described schematically in Fig. 1. A high-quality PZT thin film was deposited on sapphire using a 0.6 M sol-gel PZT solution by spin coating at 2000 rpm for 30 s, annealed at 700 °C for 2 h (Fig. 1(a)), followed by attachment of a thermal release tape (Fig. 1(b)). Subsequently, the PZT thin film was exfoliated by an inorganic-based laser lift-off (ILLO) technique, which involves irradiation at the backside of the sapphire substrate using a XeCl Excimer laser (Fig. 1(c)). The exfoliated PZT thin film was then transferred to an ultrathin PET substrate using an UV-cured adhesive polymer and the transfer medium was detached by thermal treatment (Fig. 1(d)). Photolithography and wet etching process were employed to develop gold interdigitated electrodes (IDEs) (Fig. 1(e)) followed by deposition of a photocurable epoxy passivation layer on the PZT thin film. The ultrathin piezoelectric sensor is highly flexible, which enables a conformal contact of the sensor to the human skin topography and improves the sensing ability of tiny pressure arising near the surface region of epidermis. The epoxy passivation of the piezoelectric sensor offers negligible cytotoxicity.

Fig. 1 Schematic illustration of the various stages involved in the fabrication of self-powered flexible pressure sensor on an ultrathin PET substrate

The flexible ultrathin piezoelectric sensor can be conformally attached on human wrist (Fig. 2(a)) and carotid artery (Fig. 2(b)) using a biocompatible liquid bandage spray. Besides, it can be integrated with the medical mask (Fig. 2(c)).

Fig. 2 (a-c) Photographs of piezoelectric pulse sensor conformally attached on (a) human wrist; and (b) carotid artery position (top) and the middle of the throat (bottom) using a biocompatible liquid bandage; (c) sensor integrated with the medical mask

The variation in the radial artery pulse signals before (red line) and after (blue line) physical exercise (running for 10 min) (Fig. 3(a)) indicate the effectiveness of the sensor to respond to blood vessel movements. Similarly, the flexible ultrathin piezoelectric sensor can be used to monitor carotid artery pulse and muscle movements (Fig. 3(b)) and respiratory activities (Fig. 3(c)).

Fig. 3 (a) Radial artery pulse signals showing different heart rates and generated output voltages before and after physical exercise; (b) output voltage in response to carotid arterial pressure (top) and saliva swallowing actions (bottom); and (c) output voltage response of the pressure sensor to normal (right bottom, blue) and deep oral breathing (right top, red) during periodic oral breathing.

The applicability of the self-powered flexible pulse sensor for real-time pulse monitoring is validated by integrating signal amplification, frequency filtering,  signal processing circuit to identify arterial pulse signals, decision making module and a microcontroller unit for wireless transmission of the signal to a smart phone, in the assembly (Fig. 4). The outputs from the system confirm that the self-powered pulse sensor can be effectively utilized for continuous real-time monitoring.

Fig. 4 (a) Photograph of the LED and speaker unit operated synchronously corresponding to the radial artery pulse (inset: output voltage from the first (bottom, red) and second (top, blue) amplifier stage; (b) Photograph of wireless transmission of the pulse to a smart phone, showing capability for a real-time arterial pulse monitoring system.

The sensitivity (0.018 kPa⁻¹), fast response time (60 ms), good mechanical stability (5000 pushing cycles), response to lower frequency vibrations (0.2 to 5.0 Hz) and higher frequency sound waves (240 Hz), ability to conformally attach on human epidermis of the flexible piezoelectric sensor helps effective  monitoring of radial/carotid artery pulse, respiratory activities, and trachea movements. Development of such sensors is believed to make significant impact in medical diagnosis towards achieving a better human health care.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Dae Yong Park et al., Self-Powered Real-Time Arterial Pulse Monitoring Using Ultrathin Epidermal Piezoelectric Sensors, Adv. Mater. 2017, 1702308, DOI: 10.1002/adma.201702308

Converting Wood to Graphene by Laser Scribing

Graphene has proved its supremacy in a wide variety of applications due to its high electrical conductivity, and excellent chemical and mechanical properties. The conventional method of synthesis of graphene by chemical vapor deposition (CVD) has certain limitations such as need for high temperature, type of substrates, requirement for post-treatments such as etching and critical point drying, or aerogel formation. Laser-induced graphene (LIG) can be formed on commercial polyimide (PI) films, which is considered as an alternate method for CVD to prepare graphene. Researchers at Rice University, USA and Beihang University, China have demonstrated that LIG can be formed on the surface of wood with a high electrical conductivity. Moreover, the graphene layer can be further modified by electrodeposition of polyaniline (PANI) to make it as a supercapacitor or plate it with Co-P and Ni-Fe hydroxides to catalyze hydrogen evolution and oxygen evolution reactions (OERs).

The 3D porous LIG graphene was formed on the surface of pine wood by irradiating it with a 10.6 μm CO₂ laser under Ar or H₂ atmosphere (Fig. 1(a)). The LIG graphene is formed only on the area of the pine wood, which is scribed by the laser whereas other areas remain unchanged (inset of Fig. 1(a)). This attribute helps patterning of LIG with different shapes (Fig. 1(b)). The inert atmosphere helps in the development of graphene with a stable structure while ablation of wood in air has lead to decomposition of the lignocellulose structure. The LIGs were prepared from pine wood using 10% (1.6 W), 30% (4.1 W), 50% (6.3 W), 70% (7.8 W), and 90% (8.6 W). The LIGs prepared using pine, oak and birch woods are designated as P-LIG-x, O-LIG-x and B-LIG-x, respectively, where x signifies the power percentage of the laser.

Fig. 1 (a) Schematic illustration of the formation of LIG using CO₂ laser under Ar or H₂ atmosphere; (b) Photograph of LIG patterned with letter ‘R’ on wood.

The morphology, chemical composition, structure, porosity, crystallite size, electrical properties and I_D/I_G ratio of P-LIG show a strong dependence on the laser power. Morphological features of the pine wood, laser scribed at varying laser powers indicate the evolution of a porous structure due to the liberation of gas during laser irradiation. The pore size of the P-LIGs is decreased with an increase in laser power from 10% to 70% (Figs. 2). The thermal stability and electrical conductivity of P-LIGs are increased with an increase in laser power.

Fig. 2 Morphological features of the laser-scribed pine wood at varying powers: (a) 30%; (b) 50%; and (f) 70%.

The chemical composition of the P-LIG is changed during laser irradiation; an increase in laser power leads to a decrease in C-O content and an increase an in C-C and carboxyl contents (Fig. 3(a)). The structural changes of P-LIG with laser power are confirmed by Raman spectra and TEM. At 10% laser power, no apparent Raman signal is detected while at 30% formation of amorphous carbon is evident by the broad D peak at ≈1350 cm⁻¹ and the weak 2D peak at ≈2700 cm⁻¹. As the laser power is increased from 50% to 90%, sharpening of the D and G peaks along with an enhancement in  the intensity of 2D peak indicate the formation of the graphene structure (Fig. 3(b)).

Fig. 3 (a) Change in chemical composition derived from XPS; and (b) Raman spectra of P-LIG as a function of laser power

The predominance of amorphous carbon without a clear graphene lattice at 30% laser power and graphene carbon over 50% laser power is confirmed by TEM (Fig. 4). The crystalline size of P-LIG reach a maximum at 70% power. The large I_2D/I_G ratio indicate that P-LIG-70 has a stacking of fewest-layered graphene. An increase in laser power, in general, has resulted in the formation of graphene with desired characteristics. Nevertheless, at 90% laser power, overheating of the pine wood has lead to an inferior LIG structure.

Fig. 4 TEM images of (a) P-LIG-30; (b) P-LIG-50; and (c) P-LIG-70

Similar to pine wood, 3D porous LIG is also formed using oak and birch woods. A comparison of the characteristics of LIGs formed using pine, oak and birch woods at a laser power of 70% indicates a lower I_D/I_G ratio of 0.48 for O-LIG-70, followed by at 0.73 and 0.85 for B-LIG-70 and P-LIG-70, respectively. Since aliphatic carbon moieties are more reactive than aromatic carbon, the hemicellulose and cellulose of oak and birch woods are easily decomposed during laser irradiation, thus resulting in O-LIG-70 and B-LIG-70 with more defects. In contrast, a higher aromatic lignin content favours the  generation of P-LIG-70 with a lower degree of defects. In spite of the lignin component, the formation of LIG also depends on the inherent composite structure of wood consisting of cellulose, hemicelluloses, and lignin.

The P-LIG is converted to a supercapacitor by electrodepositing polyaniline (P-LIG-PANI). The cyclic voltammetry curves of P-LIG-PANI in the potential window of −0.2 to 0.8 V indicate two characteristic pairs of redox peaks that correspond to leucoemeraldine/emeraldine and emeraldine/ pernigraniline transition of PANI, thus confirming the pseudocapacitive characteristics of PANI upon P-LIG (Fig. 5(a)). The galvanostatic charge-discharge curves of P-LIG-PANI indicate a specific areal capacitance of ≈780 and ≈320 mF/cm² at 1 and 10 mA/cm², respectively (Fig. 5(b)).

Fig. 5 (a) CV of P-LIG-PANI and P-LIG in 1 M H₂SO₄ at a scan rate of 20 mV/s; and (b) Galvanostatic charge–discharge curves of P-LIG-PANI at varying current densities.

Electrodeposition of Co-P on P-LIG-70 (P-LIG-Co-P) shows that if can be tuned to catalyze the hydrogen and oxygen evolution reaction (HER and OER). The polarization curves of P-LIG-Co-P in 1 M KOH delivers a HER current density of ≈62 mA/cm² at 200 mV overpotential and an OER current density of ≈20 mA/cm² at 400 mV overpotential over an area of ≈0.5 cm² (Fig. 6(a)). The Tafel slopes extrapolated from the polarization curves are ≈35 and 280 mV/decade for HER and OER, respectively (Fig. 6(b)). The OER performance can be improved by electrodepositing NiFe hydroxides on P-LIG (P-LIG-NiFe). Photographic image of P-LIG-Co-P as cathode and P-LIG-NiFe as anode, powered by two 1.5 V batteries in series are shown in Fig. 6(c)).

Fig. 6 (a) HER and OER windows of P-LIG-Co-P and P-LIG-NiFe in 1 M KOH; (b) HER and OER Tafel slopes of P-LIG-Co-P and P-LIG-NiFe; and (c) Photograph of P-LIG-Co-P and P-LIG-NiFe are powered by two 1.5 V batteries in series.

By a suitable choice of materials that can be deposited on LIG, it can be tailored to suit diverse applications such as supercapacitors and water splitting systems. The methodology opens up new avenues to engineer woods surfaces for diverse electronic applications.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Ruquan Ye et al., Laser-Induced Graphene Formation on Wood, Adv. Mater. 2017, 1702211, DOI: 10.1002/adma.201702211

Hierarchical Three-layered TiO2@carbon@MoS2 Tubular Nanostructures as Anode Materials for Lithium Ion Batteries

Lithium-ion batteries (LIBs) have received considerable attention as the power source for portable electronic devices. The anode materials used in LIBs suffer from limitations such as poor intrinsic electronic conductivity, sluggish Li⁺ ion transport kinetics and the inevitable volume change that occurs during the lithium insertion/de-insertion process. To overcome these limitations, researchers at School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore have demonstrated a multi-step synthesis route to prepare hierarchical tubular nanostructures by sequentially coating nitrogen-doped carbon (NC) layer and ultrathin MoS₂ nanosheets on TiO₂ nanotubes (designated as TiO₂@NC@MoS₂).

The multistep process involved in the synthesis of TiO₂@NC@MoS₂ tubular nanostructures is schematically represented in Fig. 1. MnO₂ nanowires (average diameter ≈40 nm) with a high-aspect ratio synthesized by hydrothermal method served as the starting template. A TiO₂ layer was deposited on the MnO₂ nanowires to develop core shell MnO₂@TiO₂ nanowires (step I). A layer of polydopamine (PDA) (thickness: 10 nm) was deposited over the MnO₂@TiO₂ nanowires to produce coaxial MnO₂@TiO₂@PDA nanowires (step II). Subsequently, the MnO₂@TiO₂@PDA nanowires were carbonized at 500 °C for 3 h under N₂ atmosphere followed by acid etching to remove the MnO₂ template (step III). In the meantime, the outer PDA layer is converted into NC shell for the core–shell TiO₂@NC nanotubes (step III). Finally, a layer of ultrathin MoS₂ nanosheets was grown on the surface of TiO₂@NC nanotubes by a hydrothermal reaction, which upon subsequent annealing (H₂/Ar atmosphere at 700 °C for 2 h) yields three-layered hierarchical TiO₂@NC@MoS₂ tubular nanostructures (step IV).

Fig. 1 Schematic of the multi-step synthesis process of TiO₂@NC@ MoS₂ tubular nanostructures: (I) TiO₂ coating; (II) PDA coating; (III) carbonizing and acid etching; and (IV) deposition of MoS₂ nanosheets and annealing.

The morphological features of TiO₂@NC@MoS₂ tubular nanotubes indicate that the hierarchical MoS₂ shell is composed of randomly assembled ultrathin nanosheets (Figs. 2(a) and 2(b)) while the TEM image (Fig. 2(c)) reveals its hollow structure.

Fig. 2 (a, b) FE-SEM; and (c) TEM images of TiO₂@NC@MoS₂ nanotubes

Galvanostatic charge/discharge voltage profiles indicate that the TiO₂@NC@MoS₂ electrode delivers high initial discharge and charge capacities of 1410 and 838 mAh/g, respectively, with a Coulombic efficiency (CE) of 59.4%. Pre-lithiation of TiO₂@NC@MoS₂ electrode is a viable option to bring the initial CE to ~100%. In spite of the low CE, the capacity quickly stabilizes after the 1^st cycle. The coincidence of the discharge–charge curves points out that the electrochemical reactions are highly stable and reversible after the first cycle (Fig. 3(a)). The average specific discharge capacity is decreased from  ≈925 to 612 mAh/g with an increase in current density from 0.1 to 2.0 A/g. However, the capacity of the electrode reverts back to 955 mAh/g when the current density is decreased from 2.0 to 0.1 A/g, thus confirming its good reversibility (Fig. 3(b)). The cycling performance of the TiO₂@NC@MoS₂ electrode indicate that it can retain a high reversible capacity of 590 mAh/g after 200 cycles.

The improved performance of the TiO₂@NC@MoS₂ nanotube electrode is due to synergetic effect of the three functional layers. In the sandwich-like structural arrangement, the inner layer of TiO₂ nanotubes serves as a skeleton of the hybrids, buffers the large volume variation of the electrode for stable cycling performance and shortens the diffusion distance of Li⁺ ions to achieve high rate capacities. The highly conductive N-doped C layer in the middle facilitates electron transfer within the hybrid, protects the overall 1D hollow structure, and prevents the MoS₂ nanosheets from restacking. The outer layer of ultrathin MoS₂ nanosheets with high surface area provides sufficient electrode/electrolyte contact area and reduces the diffusion length for the transfer of electrons and Li⁺ ions to realize a high specific capacity.

Fig. 3 Electrochemical performance of TiO₂@NC@MoS₂ tubular nanotubes for lithium storage: (a) Discharge/charge voltage profiles for the first 5 cycles at 0.2 A/g; and (b) Rate performance at various current densities.

The TiO₂@NC@MoS₂ tubular nanostructures exhibit enhanced lithium storage in terms of high capacity, long cycle life, and good rate performance and hence it can be considered as an effective electrode material for LIBs.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Sibo Wang et al., Rational Design of Three-Layered TiO₂@Carbon@MoS₂ Hierarchical Nanotubes for Enhanced Lithium Storage, Adv. Mater. 2017, 1702724, DOI: 10.1002/adma.201702724

 

 

Printable Conducting Inks for Bioresorbable Electronics through Electrochemically Induced Sintering of Zinc Microparticles

Among the biodegradable metals, Zn is attractive for the development of printable conducting inks due to its low activation energy for atomic self-diffusion. When exposed to ambient conditions, Zn spontaneously forms a native oxide layer (thickness of ZnO: tens of nanometer), which is insulting in nature, possess a high melting point (~1975 °C) and low diffusivity. The presence of the native oxide layer on Zn poses difficulty in sintering and limits the use of Zn microparticles for the development of printable conducting inks.

Researchers at University of Illinois at Urbana-Champaign, USA, Kwangwoon University, Republic of Korea and Northwestern University, USA have described a process that enables a dilute acid-induced  dissolution of the native oxide layers on Zn followed by an electrochemical self-exchange reaction between Zn and Zn²⁺ ions that promotes rapid sintering of Zn metal particles under ambient conditions, without any heating or mechanical loading.

An aqueous solution of acetic acid (H₂O:CH₃COOH = 10:1 by volume, pH 2.3) is used to dissolve the native passive oxide layer on Zn. This dissolution promotes self-exchange between Zn and Zn²⁺ ions at the Zn/H₂O interfaces between the particles and enables cold welding of the Zn particles, resulting in the formation of a conductive network. The acetate anion (CH₃COO⁻(ac), pK_a = 4.8) serves as the buffer until the ink is dried and at this stage, the welded compact solid is covered with a new passivation layer (Zn(ac)₂). (Fig. 1)

Fig. 1 Electrochemical sintering of Zn microparticles in CH₃COOH/H₂O

The change in morphological features of Zn particles before and after immersion in H₂O:CH₃COOH (10:1 by volume, pH 2.3) for < 1 min at room temperature and ambient conditions is shown in Fig. 2. Before immersion the Zn particle remain intact (Figs. 2 (a) and 2(d)). In contrast, after immersion, formation of necks at points of near contact between the particles, that corresponds to regions of high local concentration of Zn is evident (Figs. 2(b) and 2(e)). When the interparticle distances becomes sufficiently short, the sintered particles are transformed into a solid compact covered with a thick passivation layer of Zn(ac)₂ (Figs. 2(c) and 2(f)).

Fig. 2 Morphological features of Zn particles before and after exposure to CH₃COOH/H₂O (10:1 by volume, pH 2.3) for < 1 min at room temperature

The Zn ink is mixed with polyvinylpyrrolidone (PVP) as a binder in isopropyl alcohol (IPA) (Zn:PVP:IPA = 30:1:10 by weight) to facilitate printing using a stencil mask while attachment of Au contacts enables measurement of resistance (Fig. 3(a)). Patterns generated with a 1 mm wide, 6 cm long, and ≈50 μm thick lines exhibit a decrease in resistance from >10 MOhm to <10 Ohm in < 1 min following treatment using <100 μL of H₂O:CH₃COOH (10:1 by volume, pH 2.3). Use of dilute HCl and HNO₃ has lead a decrease in resistance during the initial period following the removal of native oxide layer. Nevertheless, the resistance is increased again after drying due to the reformation of the oxide layer. Use of IPA in place of H₂O is not found to be effective in decreasing the resistance, due to a low rate of self-exchange of Zn²⁺/Zn and/or a low solubility of Zn(ac)₂ in IPA (Fig. 3(b)).

Fig. 3 (a) Schematic of the stencil mask and printing using zinc ink using PVP and Au contact arrangement; (b) Change in resistance after treatment with CH₃COOH, HCl, and HNO₃ (pH 2.3).

A near-field communication (NFC) device is screen printed using the Zn ink (800 μm line width). A flexible sheet of biodegradable (poly lactic-co-glycolic acid (PLGA)) prepared by drop casting of PLGA (20 w/v% in ethyl acetate) on a glass substrate served as the base (Fig. 4(a) (i)). It was screen printed with the Zn ink formulation (Zn:PVP:IPA = 3:0.1:1 by weight). The white printed lines correspond to the Zn ink in its high resistance state (Fig. 4(a) (ii)), which becomes conductive after treatment with a solution of water:CH₃COOH:PVP = 10:0.5:2 w/v% (Fig. 4(a) (iii)). Interconnecting the two terminals of the antenna, mounting an NFC chip and a light-emitting diode (LED), and drop-casting PLGA (20 w/v% in ethyl acetate, ≈100 μm) on top as an encapsulation layer complete the device (Fig. 4(a) (iv)). The validity of the circuit is verified by the glowing LED using a wireless power transfer through the RF antenna (Fig. 4(b)). The device is highly flexible and degradable (Fig. 4(c)). Upon immersion in water, the device remains functional for several hours due to the slow degradation of the 100 μm thick PLGA coating (Fig. 4(d)). Degradation of Zn is accompanied with the evolution of hydrogen (Fig. 4(e)) after several days.

Fig. 4 (a) Schematic illustration of the fabrication process of NFC device with Zn ink; (b) flexibility of the fabricated NFC device; (c) LED operated by wireless power transfer through the RF antenna; (d, e) stability of the device: (d) slow degradation of PLGA coating during the initial periods of immersion in water; and (e) degradation of Zn accompanied with the evolution of H₂.

The electrically conductivity, ability to print, and degradability of Zn ink  could find applications in environmentally sustainable electronic devices and resorbable biomedical implants.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: Yoon Kyeung Le et al., Room Temperature Electrochemical Sintering of Zn Microparticles and Its Use in Printable Conducting Inks for Bioresorbable Electronics, Adv. Mater. 2017, 1702665, DOI: 10.1002/adma.201702665

Freestanding, Hydrophobic, Flexible, Lightweight 2D Transition-Metal Carbide Foams for Electromagnetic-Interference Shielding

The deleterious effect of electromagnetic radiation on human health and sensitive electronic devices is matter of concern. The vast growth in use of portable and wearable smart electronics warrant development of thin, lightweight and flexible electromagnetic-interference (EMI) shielding materials.

Researchers at State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, China and School of Materials Science and Engineering, Henan Polytechnic University, China have demonstrated  the fabrication of freestanding, hydrophobic, lightweight, and flexible 2D transition-metal carbide (MXene) foams by assembling MXene sheets into films followed by a hydrazine-induced foaming process.

A stack of Ti₃AlC₂ sheets were used as the precursor. The Al in Ti₃AlC₂ sheets was selectively etched using LiF/HCl. Delamination of the sheets induced during etching has resulted in the formation of loosely stacked structure of Ti₃C₂T_x (MXene) with weakened interlayer interactions. The MXene film was prepared by vacuum-assisted filtration of an aqueous suspension of MXene using a polypropylene membrane. MXene films with desired thickness were obtained by suitably adjusting the concentration and volume of the MXene suspension. The freestanding MXene film exhibits excellent mechanical flexibility and withstand repeated folding and stretching. The MXene film sandwiched between two ceramic wafers was treated with hydrazine at 90 °C. Infiltration of hydrazine molecules into the interior of the MXene film through the numerous tiny channels created during vacuum filtration process has enabled the formation of a lightweight MXene foam with a cellular structure. The various stages involved in the fabrication of MXene foam is schematically illustrated in Fig. 1 along with the photographs of MXene suspension, film and foam.

Fig. 1 Schematic illustration of the various stages involved in the fabrication of MXene foam along with photographs of MXene suspension, film and foam

The morphological features acquired at the cross-section indicate that the MXene film possesses a compact structure with its layers arranged parallel to each other (Figs. 2(a) and 2(b)). This structural arrangement enables the MXene film a good flexibility and excellent mechanical properties. During hydrazine treatment, introduction of numerous small pores between the parallel layers which is accompanied by volume expansion has enabled the formation of MXene foam with a cellular structure (Figs. 2(c) and 2(d)). The reaction of hydrazine with the oxygen-containing groups of MXene accompanied by the rapid release large amounts of gaseous species overcome the van der Waals forces that hold the sheets together, resulting in a lightweight and flexible MXene foam with a cellular structure containing numerous pores.

Fig. 2 Cross-sectional SEM of: (a, b) MXene film; and (c, d) MXene foam

The MXene film and foam exhibit distinct wetting behaviors due to their difference in chemical composition. The MXene film is hydrophilic (water contact angle: 59.5°), an attribute which is originated from the MXene sheets containing oxygen and fluorine terminal groups. In contrast, the MXene foam is hydrophobic  (water contact angle: 94.0°), resulting from the reaction of hydrazine with the oxygen-containing groups in the MXene film. The hydrophobic nature and porous structure of the MXene foam will be useful for selective absorption of organic solvents and oils.

The MXene film possesses a very high electrical conductivity of 400000 S/m   and offers an excellent EMI-shielding performance at different thicknesses; ≈29 dB (1 μm), ≈47 dB (3 μm), and ≈53 dB (6 μm). During the preparation of MXene foams, the sample thickness is increased from 1 to 6 μm, 3 to 18 μm, and 6 to 60 μm and the introduction of insulating pores has lead to a decrease in their electrical conductivity to 58820, 62500, and 58000 S/m, respectively. It is difficult to retain the high electrical conductivity while increasing the thickness of MXene films by foaming. Nevertheless, the increment in thickness of the MXene foam outweighs the decrease in conductivity and improves its EMI-shielding performance. A 6 μm thick MXene foam offers an EMI-shielding effect of 70 dB as opposed to 53 dB for MXene film of similar thickness (Fig. 3).

    Fig. 3 EMI-shielding efficiency: (a) MXene films; and (b) MXene foams

The lightweight, flexible, hydrophobic MXene foam with high strength, reasonable electrical conductivity and excellent EMI-shielding performance will be suitable for applications in defense, aerospace, and wearable electronics.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Ji Liu et al., Hydrophobic, Flexible, and Lightweight MXene Foams for High-Performance Electromagnetic-Interference Shielding, Adv. Mater. 2017, 1702367, DOI: 10.1002/adma.201702367

Achieving Improved Stability of Li Anode through Dendrite Free Li Deposition using Ag Nanoparticles

Lithium metal batteries due to their high energy densities assume significance in portable electronics and electric vehicles. Nevertheless, dendrite formation during deposition, instability of the Li metal interface, and huge volume change are the major limitations that need to be solved for their effective utilization. The uncontrolled dendrite-growth of Li is considered responsible for the low reversibility, short cycle life, internal short circuiting, and safety hazards. Researchers at Department of Materials Science and Engineering, University of Maryland at College Park, USA have demonstrated a rapid Joule heating method to anchor Ag nanoparticles (Ag NPs) on carbon nanofibers (CNFs) to guide seeded nucleation and growth of Li to obtain smooth Li metal anode without dendrite growth.

CNFs prepared by electrospinning served as the host material. They were soaked in silver acetate and rapidly heated by Joule heating setup (Fig. 1(a)). When heated above the melting point of Ag (962 K) for only 0.1 s, the molten Ag gets self-assembled as Ag NPs. The high temperature promotes a strong bonding between Ag NPs and CNFs. The defects in the CNFs constrain the migration of Ag NPs. Rapid quenching of the CNFs seeded with Ag NPs below the melting point of Ag prevents agglomeration of Ag NPs. Fortunately, the CNFs could withstand the  thermal shock and preserved its graphitic structure.

Fig. 1 (a) Schematic of the Joule heating method for coating Ag NPs on CNFs (inset: morphology of CNFs prepared by electrospinning); (b) Digital image of the Joule heating set up. The sample was connected to Cu electrodes and heated by a current pulse in Ar-filled glove box.

The morphologies of Ag NPs on CNFs obtained by Joule heating for 0.05, 0.1, 0.5, and 4 s (Fig. 2) indicate that they are homogenous with an average size of 29–57 nm. The size of Ag NPs show a strong dependence on the thermal shock time; the shorter the time, the lesser the particle size (Fig. 2).

Fig. 2 SEM images of Ag NPs deposited on CNFs by Joule heating method  for (a) 0.05 s; (b) 0.5 s; and (c) 4 s.

The nucleation and growth of Li seeded by Ag NPs on CNFs is schematically represented in Fig. 3(a). Due to the zero nucleation overpotential, selective nucleation of Li occurs on AgNP/CNFs (Fig. 3(c)). Plating of Li is proceeded by alloying of Li with Ag NPs. The strong anchoring of Ag NPs on CNFs guides the formation of a smooth Li coating. During the growth stage, Li from AgNP/CNFs gradually fills the voids between the CNFs, resulting in the formation of an even Li metal anode without dendrite growth (Fig. 3(d)). The ability of the Ag NPs strongly bound on to CNFs to retain itself on the surface of the anode even after stripping of Li (Fig. 3(e)), could repeatedly guide the seeded nucleation of Li. Figs. 3 (f) and 3(g) show the inability of bare CNFs to promote uniform deposition of Li metal, due to the poor wettability of CNFs with Li, thus justifying the beneficial role of Ag NPs.

Fig. 3 (a) Schematic of Li nucleation and growth seeded by Ag NPs on CNFs; (b-g) SEM images: (b) pristine AgNP/CNFs without Li deposition; (c) initial Li nucleation on AgNP/CNFs; (d) Li deposited on CNFs guided by Ag NPs at 1 mA h/cm² of; (e) AgNP/CNFs after the first plating/stripping cycle: (f) bare CNFs without Ag nanoseeds; and (g) Li deposited on bare CNFs

The cycling performance of Li metal anodes using AgNP/CNFs as host (size of Ag NPs ≈40 nm) indicate an exceptional cycling stability at 0.5 mA/cm² for 500 h without short-circuiting with a high Coulombic efficiency of ≈98%. In contrast, self-nucleation of Li resulting in the formation of pillars and dendrites of Li metal dramatically decrease cycling stability of bare CNFs to 100 h. The discharge/charge profiles of Li anode seeded by AgNP/CNFs show a low overpotential (≈25 mV) with a negligible nucleation overpotential at 0.5 mA/cm², which is likely to promote a controlled growth of Li. In contrast, plating or stripping of Li on bare CNFs is accompanied with an initial nucleation overpotential.

The CNFs host modified by Ag NPs effectively regulates the deposition of Li, thus enabling the formation of a smooth Li anode without dendrites. The Li metal anodes developed using AgNP/CNFs exhibits a low voltage overpotential, an exceptional cycling stability and avoids problems due to short-circuiting.

T.S.N. Sankara Narayanan.

For more information, the reader may kindly refer: Chunpeng Yang et al., Ultrafine Silver Nanoparticles for Seeded Lithium Deposition toward Stable Lithium Metal Anode, Adv. Mater. 2017, 1702714, DOI: 10.1002/adma.201702714

Metal-Organic Framework based Filters for the Removal of Particulate Matter

Particulate matters (PMs) are the major source of air pollution, particularly in developing countries. Long-term exposure to PM could cause respiratory problems. PM emitted from power plants and refineries can be toxic. Metal-organic framework-based membranes are found to be effective to control air pollution. The presence of various ions and water vapor makes PM highly polar. The unbalanced metal ions on the surface of metal-organic framework and the defects present in it could impart positive charges. Hence, the electrostatic interactions between metal-organic framework and PM could be exploited for the removal PM using metal-organic framework based membranes/filters. In this perspective, researchers at Beijing Key Laboratory of Photoelectronic/ Electrophotonic Conversion Materials, Beijing Institute of Technology, China have developed a roll-to-roll hot pressing method for the preparation of MO Filters (MOF) for the removal of particulate matter (PM).

The roll-to-roll hot pressing method enables mass production of MO filters (Fig 1). Three different zeolite imidazolate framework, viz., ZIF-8, ZIF-67, and Ni-ZIF-8 were used to develop the filters on substrates such as plastic mesh, glass cloth, metal mesh, nonwoven fabric, and melamine foam. ZIF-8 is also coated on the plastic mesh by a layer-by-layer fashion. ZIF-8@plastic mesh was prepared by covering the plastic mesh (thickness: 300 μm; width: 10 cm) with ZIF-8 precursors (Zn(OAc)₂·2H₂O, 2-methylimidazole, and polyethylene glycol-200) and rolled between two rollers ~80 °C at 15 rpm. With repeated cycles of operation, ZIF-8@Plastic mesh with one coating layer (ZIF-8@Plastic mesh-1^st) to seven coating layers (ZIF-8@Plastic mesh-7^th) were prepared. The SEM images and photographs of representative MO filters are shown in Fig. 2.

Fig. 1 Schematic representation of the roll-to-roll production of various MOF-based filters (MO Filters) for the removal of PM.

Fig. 2 SEM images (a, c, e, g, i) and photographs (b, d, f, h, j) of different MO filters: (a, b) ZIF-8@Plastic mesh-1^st; (c, d) ZIF-8@Melamine foam-3^rd; (e, f) ZIF-8@Nonwoven fabric-3^rd; (g, h) ZIF-8@Glass cloth-3^rd; and (i, j) ZIF-8@Metal mesh-3^rd.

The MO filters are highly robust and offer excellent PM removal efficiency. ZIF-8@Plastic mesh-7^th filter maintained its crystallinity and morphology after several cycles. Similarly, ZIF-8@Melamine foam-3^rd tolerated 1000 cycles of bending and twisting with negligible weight loss.

For ZIF-8@Melamine foam-3^rd, the removal efficiency for PM_2.5 and PM₁₀ is 99.5% ± 1.7%, and 99.3% ± 1.2%, respectively (PM_2.5 and PM₁₀ refer to PM with an aerodynamic diameter < 2.5 and 10 μm). When tested for its long-term efficiency using a simulated pipe system with a large amount of PM (PM_2.5 > 800 μg/m³ and PM₁₀ > 1000 μg/m³), ZIF-8@Melamine foam-3^rd retained >95.4% efficiency after 12 h. Most importantly, the tested ZIF-8@Melamine foam-3^rd can be easily cleaned using tap water and ethanol, and dried at 60 °C for 3 h. ZIF-8@Melamine foam-3^rd is promising for pipe filtration systems for fine PM removal.

ZIF-8@Glass cloth and ZIF-8@Metal mesh are found to be suitable for removal of PM at high temperatures. When tested at 200 °C, both of them exhibit a good efficiency for PM removal (ZIF-8@Glass cloth-3^rd, PM_2.5: 96.8% ± 1.3%, PM₁₀: 95.8% ± 1.4% and ZIF-8@Metal mesh-3^rd, PM_2.5: 91.6% ± 1.3%, PM₁₀: 90.7% ± 1.1%). ZIF-8@Glass cloth and ZIF-8@Metal mesh are suitable for baghouse dust collectors, pipe filters, and inlet barrier filters and exhaust pipes filters for vehicle or aircraft engine systems.

ZIF-8@Plastic mesh-7^th offered a reasonably good efficiency (PM_2.5: 56.3% ± 1.6%, PM₁₀: 58.4% ± 2.1%) for PM removal in simulated living environment. Long-term testing indicates that ZIF-8@Plastic mesh-7^th could retain >90% of PM removal after a month. The used ZIF-8@Plastic mesh-7^th can be easily recycled by brush cleaning using water and ethanol. The excellent long-term stability and reusability make ZIF-8@Plastic mesh-7^th as a promising filter for the removal of PM in residential environments.

The easy scalability of the roll-to-roll hot pressing method for mass production, the efficiency, robustness, stability, long-term performance and reusability of MO filters  are promising and they are suitable for the removal of PM from residential as well as industrial environments.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Yifa Chen et al., Roll-to-Roll Production of Metal-Organic Framework Coatings for Particulate Matter Removal, Adv. Mater. 2017, 1606221, DOI: 10.1002/adma.201606221

Restraining Dendrite Growth in Li Anodes through Vertically Aligned Microchannels Developed on Copper Collectors

Lithium-ion batteries (LIBs) have received considerable attention in portable electronics. Uneven plating/stripping and uncontrolled dendrite growth of Li that could induce internal short circuits and explosion of the battery are the major limitations. Researchers at CAS Key Laboratory of Molecular Nanostructure and Nanotechnology,  Chinese Academy of Sciences (CAS), School of Chemistry and Chemical Engineering University of Chinese Academy of Sciences (CAS) and Beijing Institute of Nanoenergy and Nanosystems, China have developed a novel porous Cu current collector with vertically aligned microchannels (VAMCs), which regulates the current density distributions to prevents dendrite growth of Li metal anodes.

Vertically aligned Cu micro-channels with varying pore radius, pore depth and pore spacing were fabricated using a laser micro-processing system (Fig. 1(a)). The sample of porous Cu with a pore radius of 5 μm, a pore depth of 50 μm, and a pore spacing of 12 μm is designated as porous Cu-5-50-12. The porous Cu current collector with VAMCs due to its large specific surface area and low local current density successfully prevents the dendrite growth of Li. The large surface area of VAMCs enables uniform deposition of Li not only on their surface but also in the microchannels.

For VAMCs with a fixed pore radius of 5 μm, for a pore spacing of 10 μm the current efficiency is lower at locations away from the channels (Fig. 1(b)). When the pore spacing is decreased to 6 and 2 μm, the current efficiency at these locations is increased but the extent of increase is much lower than those experienced in the mouth of the channels (Figs. 1(c) and 1(d)). Since the current density within the microchannels is much larger than that on the upper surface of the porous Cu, preferential nucleation of Li occurs inside the mouth of channels (Fig. 1(e)).

Fig. 1 (a) Schematic of the porous Cu current collectors; (b–d) current density distribution on the surface of porous Cu collectors obtained from COSMOL simulation: (b) Cu-5-50-20; (c) Cu-5-50-16; (d) Cu-5-50-12; and (e) schematic diagram depicting preferential deposition of Li inside the mouth of channels.

For VAMCs with a fixed pore radius of 5 μm, an increase in pore depth increases the current density around the entire mouth of the channel. The current density distribution gradient is inevitable, which helps to accommodate most of the Li inside the channels. Hence, systems having a highest current efficiency in the pores and lowest current efficiency at the locations away from the pores is expected to effectively suppress the Li dendrite formation.

The morphology of Li deposits formed using 1 M lithium bis(trifluoromethane-sulfonyl)imide in 1:1 1,3-dioxolane/1,2-dimethoxyethane as the electrolyte containing 1 wt% LiNO₃ on porous Cu with different pore radii as well as those formed on planar Cu at 3 mA h/cm² is compared (Figs. 2(a)–2(e)). Li deposits on porous Cu-5-50-12 indicate enrichment of Li in the VAMCs (Fig. 2(a)) while an increase in pore radius decreased the ability of VAMCs to restrict Li deposition within the microchannels (Figs 2(b)-2(d)). Formation of Li deposits with a spherical shape could not be observed on planar Cu (Fig. 2e). The voltage profiles of Li deposition on porous and planar Cu current collectors (Fig. 2(f)) indicate a lower overpotential of ≈144 mV for porous 5-50-12 whereas for planar Cu, the overpotential is 280 mV under similar conditions. The lower overpotential values obtained for porous Cu points out a decrease in local current density due to the larger specific surface area of the VAMCs.

Fig. 2 Morphology of Li deposits formed on porous and planar Cu current collectors: (a–d) SEM images of Li deposits formed on the porous Cu with varying pore radii: (a) 5 μm; (b) 7.5 μm; (c) 10 μm; (d) 15 μm; (e) SEM image of Li deposits form on the planar Cu; and (f) Voltage profiles of Li deposition on Cu current collectors.

Galvanostatic cycling measurements performed using symmetrical cells indicate that porous Cu-7.5-50-17 possesses a low voltage hysteresis of ≈20 mV and improved cycling stability even after 300 h. In contrast, under similar conditions, planar Cu exhibits a gradual rise in voltage hysteresis after 50 h.  The cycling stability and CE of porous Cu collectors are also ascertained by cells assembled using commercial lithium foil as the counter electrodes. For porous Cu-5-50-12, the CE of the cell remains stable for 200 cycles with an average CE of 98.5%. In contrast, for planar copper, the CE of the cell reaches 98.2% after 13 cycles and exhibits a rapid decline to 68.1% after 81 cycles, with an average CE of 94%.

Full cell galvanostatic cycling of Li/LFP cells performed using planar Cu as well as porous Cu-5-50-12 anodes indicate that cells with porous Cu anode exhibits good cycling behavior, delivering a capacity of 134 mA h/g (Fig. 3(a) with a capacity retention of ≈90% after 100 cycles (Fig. 3(b). The cell with planar Cu anode exhibits a larger polarization effect of 177 mV (Fig. 3(a)) and a poor capacity retention of 80% after 100 cycles (Fig. 3(b)).

Fig. 3 (a, b) Electrochemical performance of Li/LiFePO₄ cells with (a) porous Cu-5-50-12 anode; and (b) planar Cu anode; and (c) cycling performance of the Li/LFP cells

The ability of porous Cu current collector with VAMCs to control the dendrite growth of Li is mainly due to the larger specific surface area, lower local current density, restriction in lithium volume change, lower charge transfer resistance, and high Li⁺ transport properties in the cell. The porous structure served as a cage for Li, thus accommodating large amounts of Li in the pores.

T.SN. Sankara Narayanan

For more information, the reader may kindly refer: Shu-Hua Wang et al., Stable Li Metal Anodes via Regulating Lithium Plating/Stripping in Vertically Aligned Microchannels, Adv. Mater. 2017, 1703729, DOI: 10.1002/adma.201703729

Ni(OH)2 Nanosheet Ink for Wearable Energy Storage Devices

Flexible electronics have received considerable attention, particularly in energy storage devices. Among the various techniques available for their fabrication, solution based methods such as ink-jet printing, screen printing, and roll-to-roll printing assume significance in terms of their low-cost, high processing speed, and scalability. Nevertheless, formulation of inks containing suitable active materials with better dispersion and stability remains a challenge. Researchers at Nanjing Tech University, China, Nanyang Technological University, Singapore, Lanzhou University, China, Nanjing University of Posts and Telecommunications, China and Northwestern Polytechnical University, China have developed a facile method to prepare a highly concentrated ink comprised of 2D ultrathin Ni(OH)₂ nanosheets, which can be easily coated on commercial printing paper as well as carbon fiber yarns (CFYs). Using CFY@Ni(OH)₂ as a weavable electrode, they have fabricated wearable energy storage devices.

The Ni(OH)₂ nanosheets were prepared by co-precipitation method. Since the precipitated Ni(OH)₂ nanosheets tend to aggregate, they were exfoliated by ultrasonication. The Ni(OH)₂ nanosheets dispersed in water, ethanol, and DMF exhibit excellent stability. The Ni(OH)₂ ink (20 mg/mL in water) (Fig. 1(a)) was directly coated on printing paper using a brush. The desired thickness of the Ni(OH)₂ coating can be tuned by a careful choice of the number of cycles. The Ni(OH)₂ coated paper is highly flexible and it can be easily wound around a glass rod (Fig. 1(b)). Selective removal of Ni(OH)₂ from the coated paper (Fig. 1(c)) using 3M HCl enables patterning of suitable designs (Fig. 1(d)).

Fig. 1 (a) Ni(OH)₂ ink (20 mg/mL in water); (b, c) flexible and plain printing papers coated with Ni(OH)₂ nanosheets; and (d) selective removal of Ni(OH)₂ from the coated printing paper using 3M HCl for patterning of suitable designs

The Ni(OH)₂ nanosheet ink was also coated on carbon fiber yarn (CFY). Ethanol was used as the solvent to improve wettability of CFY with the ink. Nafion (Nf) was used as an ionic binder to promote the interaction between Ni(OH)₂ nanosheets and CFY. The CFY@Nf-Ni(OH)₂ electrode was prepared by impregnation-dyeing method (Fig. 2(a)) in which the CFY was repetitively impregnated in Nf-Ni(OH)₂ ink (20 mg/mL in ethanol) for 1 min for each cycle and dried at 80 °C. The CFY@Nf-Ni(OH)₂ maintains excellent flexibility (Fig. 2(b)). Morphological features reveal that the Ni(OH)₂ is uniformly coated on CFY (300 nm thick for 6 impregnations) (Figs. 2(c) and 2(d)).

Fig. 2 (a) Schematic illustration of the preparation of CFY@Nf-Ni(OH)₂ by impregnation-dyeing method; (b) Photograph of highly bended CFY@Nf-Ni(OH)₂; (c) SEM image of a single yarn CF@Nf-Ni(OH)₂; (d) Magnified image of (c) 300 nm thick Ni(OH)₂ coated layer obtained after 6 impregnations.

Galvanostatic charge–discharge curves of CFY@Nf-Ni(OH)₂ indicate that a maximum  specific volumetric capacitance (CV) is obtained after 6 times of impregnation whereas gravimetric capacitance (Cg) is decreased from 1010.0 to 640.0 F/g with an increase in impregnation times from 2 to 10. Repetitive impregnations though enable an increase in the mass loading of Ni(OH)₂ nanosheets, it leads to a reduction in the conductivity of CFY@Nf-Ni(OH)₂. The CFY@Nf-Ni(OH)₂ electrode exhibits good cycling stability, which is evidenced by its 89% capacitance retention after 5000 charge–discharge cycles.

A hybrid supercapacitor was fabricated using CFY@Nf-Ni(OH)₂ as positive electrode, CFY@CPs (carbon particles) as negative electrode and poly(vinyl alcohol-KOH gel as the solid-state electrolyte, enclosed in a thermal plastic tube to seal the device (Fig. 3). The hybrid supercapacitor exhibits a wide potential window of 1.5 V and delivers a high energy density of 11.3 mWh/cm³ at a power density of 0.3 W/cm³.

Fig. 3 Schematic of the fabrication of yarn-based hybrid supercapacitor

The CFY-based hybrid supercapacitor can be woven in the form of a glove (Fig. 4), which at different bending angles is capable of retaining 96% capacitance after 5000 bending–unbending cycles. This behaviour seems to be promising for their potential use as energy storage devices in e-textiles.

Fig. 4 (a) Photographs of yarn-based hybrid supercapacitor woven in the form of a glove at different bending states; (b) Capacitance retention of the hybrid supercapacitor at different bending angles.

The Ni(OH)₂ nanosheet inks seem to be promising for large-scale production of high-performance energy storage devices for flexible electronics.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer to Peipei Shi et al., Highly Concentrated, Ultrathin Nickel Hydroxide Nanosheet Ink for Wearable Energy Storage Devices, Adv. Mater. 2017, 1703455, DOI: 10.1002/adma.201703455

Healing of Conducting PEDOT:PSS Films

Polyethylenedioxythiophene doped with polystyrene sulfonate (PEDOT:PSS) is a well-known conducting polymer that possesses high air stability, high electrical conductivity, and biocompatibility. Nevertheless, the healability of PEDOT:PSS has not been explored much. Researchers at Department of Chemical Engineering, Polytechnique Montréal, Canada have demonstrated that damages in PEDOT:PSS films could be electrically healed by simply wetting the damaged areas with a few drops of water or by wetting the films with water, which enables a self-healing nature for the PEDOT:PSS films, without the need for any external stimulation.

The PEDOT:PSS films were prepared by drop-casting PEDOT:PSS suspension onto glass, uniform spreading and sequential baking at 80 °C for 1 h, 110 °C for 1 h, and 140 °C for 4 h to eliminate bubble formation. In electrically-assisted healing experiments, the  PEDOT:PSS film is biased at 0.2 V. Damage of the film with a razor blade leads to an interruption in the current flow. Wetting the damaged area of the film with a drop of DI water has lead to complete recovery of the current to its initial value within 150 ms (Fig.1(a) and inset of Fig. 1(a)).

The current-time characteristics of wet PEDOT:PSS films (soaked in DI water for 5 s) under an electrical bias of 0.2 V indicate no significant change in current for repeated cuts at different regions of the film (Fig. 1(b)). The thickness of the PEDOT:PSS films should be at least 1 μm to trigger electrically-assisted healing of a 40 μm damage within 150 ms. For 1 to 10 μm thick films, no significant dependence of healing time could be observed with film thickness. Repeated damage and repair at different regions of the same film has produced a similar effect in terms of current recovery and response time, thus substantiating the high reproducibility and reliability of the process.

Fig. 1 Current-time transients of PEDOT:PSS films biased at 0.2 V: (a) effect of damage and healing with a drop of DI water (inset: surge in current response due to rapid healing of the damage); (b) effect of wet film to damage

The PEDOT:PSS films prepared using glycerol (conductivity enhancer) and Capstone FS-30 (plasticizer) exhibit water-induced healing behavior without any electrical bias. The film was cut using a razor blade to create a gap of about 40 μm and the damaged area was healed after addition of water (Fig. 2). The damaging and repairing process of the PEDOT:PSS film was demonstrated by connecting it in a simple circuit with light-emitting diode (LED) bulb (Fig. 2). The healing effect is also ascertained after exposure of damaged PEDOT:PSS film to water vapor in a humidity chamber. At RH between 50% and 70%, no significant recovery in current is observed even after 30 min. The current is recovered in ~5 min at 80% RH whereas complete recovery of current to the initial value is observed only at ≥ 90% RH. The healing of damages in the PEDOT:PSS film in water vapor is much slower than in liquid.

Fig. 2 SEM images of the damaged area of PEDOT:PSS film (a) before; and (b) after healing with a 10 μL drop of DI water; (c) schematic representation of water-induced mechanical and electrical healing; and (d) demonstration of damage and healing effect on PEDOT:PSS film connected in a circuit with  a LED bulb at 3 V: (i) intact film; (ii) damaged film; and (iii) after dropping DI water on the damage that enables repair of the circuit within 150 ms.

The exact mechanism of water-assisted healing of the damages in PEDOT:PSS films is not clear. It is presumed that the healing effect is due to the swelling of PSS⁻ chains upon water exposure, which increases the viscoelasticity and softness of the film. The swelling of PSS⁻ chains simultaneously enables the PEDOT⁺ chains to shift, thus allowing formation of PEDOT⁺-PEDOT⁺ conducting paths across the damage, leading to healing of the damage with a total restoration of electrical conductivity of the film. The slower current recovery upon exposure to water vapor is due to the lower water absorption rate. Water-induced reversible hydrogen bond breaking and restoring could have also contributed to the separation and propagation of PSS⁻ and PEDOT⁺ grains to the damaged area. The inability of other solvents such as a fluorinated solvent, glycerol and Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) in place of water, to swell PSS⁻ or break the hydrogen bonding between PSS⁻ chains excludes the possibility of healing and current recovery either by mechanical movements of the film or by transport of conducting debris to the damaged area.

It is also possible to obtain free-standing PEDOT:PSS films by using a water-assisted wedging method, which exhibits excellent conformability on various surfaces. Moreover, the detached wet free-standing films can be easily shaped on objects even with irregular shapes.

Fig. 3 (a) Illustration of water-assisted wedging method to obtain free-standing PEDOT:PSS films; (b, c) the film is not deteriorated during its detachment from the glass substrate; (d, e) excellent conformability of PEDOT:PSS free-standing films (10 μm thickness) on finger and fingertip.

The ultrafast electrically-assisted healing of wet PEDOT:PSS films will be useful in application such as electronic skin, self-healable large-scale electronics, and epidermal electronics. The free-standing PEDOT:PSS films can be effectively used as healable electrodes or electronic welding patches.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Shiming Zhang and Fabio Cicoira, Water-Enabled Healing of Conducting Polymer Films, Adv. Mater. 2017, 1703098, DOI: 10.1002/adma.201703098

Fabrication of Wide-Angle Selective Solar Absorber for High-Efficiency Solar–Thermal Energy Conversion

The increasing trend for global energy demand and environmental concerns have enforced us to seek for renewable energy, particularly from the most abundant sunlight. Hence, development of selective solar absorbers (SSAs) with a high absorptance in the solar wavelengths (0.3 to ≈2.5 μm) and low emittance in the infrared thermal radiation wavelengths (≈2.5–40 μm) are being explored. Researchers at Department of Applied Physics and Applied Mathematics, Columbia University, Department of Chemistry, Columbia University Department of Materials Science and Engineering, Stanford University, USA, have demonstrated a simple, “dip and dry” technique based on galvanic displacement reaction to fabricate solar absorbing plasmonic nanoparticle coated foils (PNFs) for SSAs at room temperature.

The fabrication process involves immersion of Zn foil in aqueous CuSO₄ solution for 30–60 s, in which the Cu²⁺ ions are reduced to metallic Cu nanoparticles by Zn on its surface (Figs. 1(a) and 1(b)). The galvanically deposited Cu nanoparticles on Zn appears as a black layer (Fig. 1(c)), with a strong solar absorptance (Fig. 1(d)) and an excellent optical selectivity (­α = 0.96 and ­ε = 0.08) (Fig. 1(e)).

Fig. 1 (a) Schematic of the deposition process – formation of Cu nanoparticles on Zn by galvanic displacement reaction; (b) SEM image of the Cu nanoparticle layer on the PNF; c) Photograph of PNF; (d) Schematic depicting the high solar absorptance and low thermal emittance of PNF (Thickness of the arrows indicates their intensity); and (e) Spectral reflectance of PNF (­α = 0.96, ε= 0.08) and the ideal SSA at 100 °C.

The effect of immersion time, concentration of CuSO₄ and temperature on the galvanic deposition of Cu nanoparticles on Zn is reflected in the spectral reflectance at normal incidence in the wavelength range of 400 nm to 14 μm (Figs. 2(a)-2(c)). It is evident that longer immersion time, higher concentrations of CuSO₄, and higher temperatures employed for deposition of Cu nanoparticles have lead to a lower reflectance across the wavelength. An increase in thickness as well as the diameter of the Cu nanoparticle layer lead to a lower reflectance. An increase in surface roughness of the Cu nanoparticle layer also causes a lower reflectance. The morphology of the Cu nanoparticles could also be altered by varying the type of anions as well as with the addition of surface active agents in the solution.

Fig. 2 Variation in spectral reflectance across the wavelength as a function of (a) immersion time; (b) concentration of CuSO₄; and (c) temperature.

The extent of change in solar absorptance, emittance and efficiency as a function of immersion time, concentration of CuSO₄ and temperature are shown in Figs. 3(a)-3(c). Only a small variation in the α (≈0.83) and ε (0.03 to 0.06) are observed with an increase in immersion time from 15 s to 45 s (Fig. 3(a)). Both α (0.43 to 0.94) and ε­ (0.02 to 0.24) are increased with an increase in concentration of CuSO₄ from 2.5 mM to 50 mM (Fig. 3(b)). Similarly, a reasonable increase in α (0.86 to 0.93) and ε­ ­(0.02 to 0.17) are observed with an increase in temperature up to 0 °C to 75 °C (Fig. 3(c)). For efficient harvesting of solar energy, an SSA must have possess a high ­α(θ) and a low ε­ ­at all incidence angles. The SSAs developed in this work exhibit an excellent wide-angle solar absorptance, with ­α(θ) ranging from 0.96 at 15°, to a peak of 0.97 at ≈35°, to 0.79 at 80°.

The adhesion between the Cu nanoparticle layer and the Zn substrate is very strong. Reflectance measurements fails to indicate any significant change in the solar absorptance, emittance and efficiency before and after the adhesion testing. Accelerated thermal aging at 200 °C in air up to 96 h indicates only a small decrease in α/ε­ from 0.94/0.13 to 0.90/0.09, suggesting its better thermal stability. The observed variation in ­and ­with experimental parameters employed for the deposition of Cu nanoparticles indicate that it would be possible to maximize the efficiency of PNFs’ by suitably tuning the experimental parameters. Thus the “dip and dry” technique proposed in this study offers many avenues to the optical selectivity of the SSAs.

T.S.N. Sankara Narayanan

Fig. 3 Extent of change in solar absorptance, emittance and efficiency, as a function of (a) immersion time; (b) concentration of CuSO₄; and (c) temperature.

For more information, the reader may kindly refer: Jyotirmoy Mandal et al., Scalable, “Dip-and-Dry” Fabrication of a Wide-Angle Plasmonic Selective Absorber for High-Efficiency Solar–Thermal Energy Conversion, Adv. Mater. 2017, 1702156, DOI: 10.1002/adma.201702156

 

Multi-shelled Al2O3 coated CaO microspheres for CO2 capture

Emissions of carbon dioxide (CO₂) is considered as the main reason for global warming and ocean acidification and hence many technologies for CO₂ capture are currently emerging. Limestone-derived CaO possesses a high CO₂ uptake capacity (≈0.78 g CO₂/g of CaO) and exhibits fast kinetics of the CO₂ capture and release. Nevertheless, the poor cyclic stability stems from high sintering temperatures, 600–700 °C for CO₂ capture and ≥ 900 °C for sorbent regeneration causes irreversible detrimental changes in their textural properties. Incorporation of stabilizers such as Al₂O₃ though helps to improve the cyclic stability of CaO, the quantity of such stabilizers should be minimized to retain a high CO₂ uptake capacity. Mass transport limitation is yet another issue and for optimal performance, the ideal grain/particle size of CaO should be <100 nm.

Researchers at Department of Mechanical and Process Engineering and Department of Chemistry and Applied Biosciences, ETH Zürich, Switzerland have developed Al₂O₃ coated CaO microspheres for CO₂ capture. In their design approach, porous hollow spherical microstructures composed of nanostructured CaO served as the CO₂ sorbent. The voids in CaO microspheres enhance the surface-to-volume ratio, decrease the mass transport length for CO₂ and act as a buffer to accommodate large volume changes originated from the difference in molar volumes of CaCO₃ (36.9 cm³/mol) and CaO (16.7 cm³/mol). To increase the sintering resistance, the CaO microspheres are coated with a thin layer of Al₂O₃ (< 3 nm) by atomic layer deposition (ALD). The structural design is schematically represented in Fig. 1.

Fig. 1 Structural design of Al₂O₃ coated CaO microspheres for CO₂ capture

6.10 g of glucose and 4 g of Ca(NO₃)₂.4H₂O were dissolved in 15 ml of deionized (DI) water. Then, varying concentrations of urea (0 M, 2 M, 3 M and 6 M) dissolved in 3 ml of DI water was added to it. This reaction mixture in a glass vial was transferred to a 45 ml PTFE-lined stainless steel autoclave and subjected to hydrothermally treatment  at 170 °C for 24 h. The resultant black powder was filtered, thoroughly washed with DI water and ethanol, dried overnight at 80 °C and calcined at 800 °C for 1 h. The CaO sorbents prepared using 0 M, 2 M, 3 M and 6 M urea were denoted as Ca-0M, Ca-2M, Ca-3M and Ca-6M, respectively.

Atomic layer deposition (ALD) was employed for the coat conformal deposition of the Al₂O₃ over CaO. The CaO sorbent sample was alternatively exposed to pulse injections of trimethylaluminum (TMA) and DI water at 300 °C in which the pulse and purge times were set as 1 s and 10 s, respectively. Nitrogen served as purge as well as carrier gas. The deposition process was carried out for 10, 20 and 30 cycles to vary the thickness of the Al₂O₃ coating as 0.9, 1.8, and 2.7 nm, respectively. The Al₂O₃ coated CaO sorbents were denoted as Ca-xM-Al(10), Ca-xM-Al(20), and Ca-xM-Al(30), respectively where xM refers to the molarity of the urea and the number in the parenthesis represent the number of cycles employed for Al₂O₃ coating.

During hydrothermal treatment at 170 °C for 24 h, hydrolysis of urea increase the pH of the reaction mixture, resulting in the precipitation of CaCO₃. Glucose enables development of an interconnected network while Ca(NO₃)₂ increases the diameter of carbonaceous microspheres. The hydrolysis of urea enables a homogenous distribution of Ca within the carbonaceous spheres; the higher the concentration of urea, the greater is the level of incorporation of Ca, which helps to inhibit the oxidative decomposition of the inner core and increase the decomposition temperature. The mechanism involves simultaneous occurrence of condensation, polymerization, and carbonization of glucose with the binding of Ca²⁺ ions to the template surface and precipitation of CaCO₃ nanoparticles, resulting in a homogeneous distribution of Ca compounds within the carbonaceous matrix. Calcination at 800 °C for 1 h leads to the formation of a hollow, multi-shell structure (Fig. 2).

Fig. 2 Hydrothermal treatment of an aqueous solution of glucose, urea, and the Ca precursor after calcination results in multi-shelled hollow microspheres.

The CO₂ uptake performance of the sorbents assessed by TGA reveals that after 10 cycles the synthesized CaO sorbents outperforms limestone-derived CaO by several folds. Nevertheless, all of them experience deactivation (18.7%, 19.9%, and 31.5% decrease in capacity for Ca-2M, Ca-3M, and Ca-6M, respectively, over 10 cycles) due to the formation of smaller CaO microspheres with a reduced central void volume in the absence of a structural stabilizer. The cyclic stability of Al₂O₃ coated CO­₂ sorbents is significantly improved;  the capacity retention of the sorbents is increased to 87.1%, 92.1%, and 92.4% for 0.9, 1.8, and 2.7 nm thick Al₂O₃ coated CO­₂ sorbents, over 10 cycles of operation. After 30 cycles of operation, 0.9 nm thick Al₂O₃ coated CO­₂ sorbent  exhibits a 80.5% capacity retention, which exceeds performance of benchmark limestone-derived CaO by ≈500%. FIB cross-sections of the CO₂ sorbent confirm that the hollow, spherical structure is largely preserved after 30 cycles of operation (Fig. 3).

Fig. 3 (a) CO₂ uptake performance of uncoated and Al₂O₃ coated CaO sorbents; (b, c) FIB cross-sections of Al₂O₃ coated CaO sorbent: (b) calcined state; and (c) carbonated state after exposed for 30 cycles of calcination and carbonation.

The improved performance of the synthesized sorbents is due to the ability of (i) central void to accommodate the volumetric changes during cyclic operation; (ii) porous shells to favour transport of CO₂; (iii) shell-comprising nanoparticles (~100 nm) that ensure occurrence of carbonation reaction in the kinetically controlled regime; and (iv) homogeneous coating of Al₂O₃ that increases the thermal stability and enables long-term utilization of CaO-based CO₂ sorbents.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer to Andac Armutlulu et al., Adv. Mater. 2017, 1702896, DOI: 10.1002/adma.201702896

Formation of Luminescent Carbon Nitride Nanosheets by Spontaneous Liquid Phase Dissolution

2D materials due to their unique physical and chemical properties assume significance in a wide variety of applications. Among the various strategies employed for the synthesis of 2D materials, liquid phase exfoliation from layered crystalline precursors (bottom-up route) is considered to be beneficial. Nevertheless, use of aggressive chemicals and formation of fragmented or chemically modified nanosheets limit the applicability of this methodology. Researchers at University College London, Imperial College London, University of Bristol, United Kingdom and École Polytechnique Fédérale de Lausanne, Switzerland have demonstrated a liquid phase dissolution route for the synthesis of 2D carbon nitride (CN) nanosheets using poly(triazine imide)-lithium bromide (PTI-LiBr) as the crystalline precursor and aprotic polar solvents as the liquid phase. The spontaneous dissolution of PTI-LiBr in organic solvents yield solutions containing defect-free, crystalline, 2D CN nanosheets.

Dicyandiamide (DCDA), lithium bromide (LiBr) and potassium bromide (KBr) were used as the starting materials. 2 g of DCDA was mixed with 10 g of the LiBr/KBr (52%:48%) and thoroughly ground. 7 g of the ground homogeneous powder was heated to 400 °C under flowing N₂ and soaked at 400 °C for 6 h. 4 g of this pretreated mixture was placed inside a quartz tube sealed at one end. The quartz tube was evacuated to < 10^-6 mbar and sealed. The quartz ampoule was heated to 600 °C for 12 h. The resultant brown coloured  material was removed from the ampoule, repeatedly washed with hot deionized water, centrifuged at 4000 rpm and the retrieved PTI-LiBr was washed with methanol. The structural and morphological properties of PTI-LiBr are shown in Fig. 1

Fig. 1 (a) XRD pattern of crystalline PTI·LiBr (Inset: one unit cell of a PTI·LiBr); (b) SEM image of an aggregate of hexagonal prismatic PTI·LiBr crystallites (Inset: TEM image of hexagonal PTI·LiBr crystallites).

Dissolution of as-synthesized PTI-LiBr crystals in N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) indicates a change in the color of the liquid over time (Fig. 2(a)). The extent of dissolution of PTI-LiBr crystals is enhanced under UV-light illumination (Fig. 2(b)).

Fig. 2 Time-lapse photographs depicting spontaneous dissolution of PTI-LiBr in DMSO up to 48 h under visible and UV illuminations.

The high-resolution TEM images of CN nanosheets deposited from solutions containing PTI-LiBr dissolved in NMP (Figs. 3(a)-3(c)) indicate that the CN nanosheets are atomically intact with well-defined edges and maintained the hexagonal shape with its lateral dimensions close to that of the precursor bulk crystals. No evidence of any dislocations or point defects could be observed.

Fig. 3 (a-c) HR-TEM images of CN nanosheets deposited from solutions containing PTI-LiBr dissolved in NMP

Both bulk and exfoliated CN exhibit luminescence in the UV/visible range. The normalized photoluminescence (PL) emission spectra of CN nanosheets dissolved in DMF exhibit a broad peak ∼380 nm, which slightly shift toward blue-green range with an increase in wavelength excitation from 260 to 330 nm (Fig. 4(a)). The PL spectra of stacked or aggregated films of CN nanosheets deposited from dissolved solution also exhibit a broad peak centered ∼480 nm (red-shift when compared to PL spectra of dissolved CN nanosheet) (Fig. 4(b)). The broadening of the PL spectra of CN nanosheets dissolved in DMF as well as the stacked or aggregated CN film deposited from dissolved solution indicates that they could be composed of 9 to 40 layers in thickness. These inferences indicate that depending on the thickness of CN nanosheets, it would be possible to tune the PL wavelength from narrow UV to broad-band white.

Fig. 4 PL spectra of CN nanosheets at varying excitation wavelength: (a) CN nanosheets dissolved in DMF; (b) stacked or aggregated CN film deposited from dissolved nanosheets

The methodology employed for the synthesis of 2D CN nanosheets is simple  and easily scalable. The spontaneous dissolution of PTI-LiBr crystals in NMP, DMF, and DMSO results in the formation of stable solutions of pristine, defect-free CN nanosheets with well-defined functional properties. The luminescence property of dissolved as well as stacked film of CN nanosheets indicate that they can be explored as potential next-generation materials for photocatalysis. The tunability of PL spectra depending on the stack thickness of CN nanosheets makes them as suitable candidate materials for UV-blue and white LED emitters. The CN nanosheets prepared by this method can be used for a wide range of optoelectronic devices.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Thomas S. Miller et al., Single Crystal, Luminescent Carbon Nitride Nanosheets Formed by Spontaneous Dissolution, Nano Lett. 2017, 17, 5891−5896.

Selective Laser-Induced Hydrothermal Growth of Hierarchical Heterogeneous Nanowire arrays for Nanoelectronics

Metal and semiconductor nanowires (NWs) become the core components of electronics. Integration of NWs involves the use of electrically driven Joule heating, which is not site-selective. In addition, Joule heating increases the complexity during the fabrication of nanoelectronics. Researchers at Seoul National University, Korea, University of Michigan, USA, University of California, Berkeley, USA, Ajou University, Korea, Hanyang University, Korea and Kyungpook National University, Korea have demonstrated a selective laser-induced hydrothermal growth (LIHG) process for integrating hierarchical heterogeneous nanowire-on-nanowire structure which can be used on-demand without the need for conventional photolithography or vacuum deposition. They have also fabricated an all-nanowire UV sensor using this methodology.

Single-crystalline Ag NW (length: ∼300 μm; diameter: >200 nm)  was prepared by modified polyol synthesis. The Ag NW was deposited on a clean glass substrate with a help of a fluidic channel and post-treated at 150 °C for 30 min to completely remove the polyol on the surface of Ag NW. Subsequently the Ag NW coated glass was wetted with ZnO quantum dot (QD) seed solution (prepared by mixing 10 mM Zn(OAc)₂ in 60 mL of ethanol with 30 mM NaOH in 30 mL of ethanol and heating the mixture at 60 °C for 2 h). Then, the ZnO seeded Ag NW coated glass was immersed in ZnO precursor solution (prepared by mixing 25 mM Zn(NO₃)₂·6H₂O, 25 mM hexamethylenetetramine, 5−7 mM polyethylenimine and 100 mL of deionized water and, heating the mixture at 95 °C for 1 h) and subjected to LIGH process (Fig. 1).

Fig. 1 Schematic of the LIHG process with hybrid background heating; Nd:YAG laser (532 nm) is focused at a specific spot on the ZnO seeded Ag NW coated glass immersed in ZnO precursor solution. As the temperature within a confined region (at the center of the laser focused area) rises above the threshold temperature, growth of ZnO NW is initiated and the growth continues only within the laser heated spot.

Scanning electron micrograph of the hierarchical ZnO NW branched on a Ag NW backbone that is suspended on an etched Si substrate (Fig. 2(a)) confirms selective growth of ZnO NW on Ag NW by the LIHG process. The micrograph of ZnO NW grown after 10 min of laser irradiation possesses a crystalline structure with a hexagonal cross section (Fig. 2(b)), which is further confirmed by transmission electron microscopy (inset of Fig.2(b)).

Fig. 2 (a) SEM image of selective growth of ZnO NW branches on Ag NW suspended on an etched Si substrate by LIGH process; (b) Magnified view of ZnO NW array with a hexagonal cross section (Inset: TEM image)

Scanning electron micrographs of ZnO NW grown at different polarization angles of 45°, 60°, and 90° on Ag NW at 1 W laser power for 6 min (Fig. 3) indicate that size of the secondary ZnO NW branch array is increased as the laser polarization becomes perpendicular to the Ag NW and the lateral size can be elongated up to 2.5 μm.

Fig. 3 SEM images of ZnO NW arrays grown on a single Ag NW at various polarization angles: (i) 45°; (ii) 60°; and (iii) 90°, after 6 min of laser irradiation

An all-nanowire UV sensor is fabricated by placing two Ag NWs adjacent to each other so that the ZnO NW grown from them can be connected as a photoconductive channel network. Electrical pads are attached at each end by laser sintering (Fig. 4(a)). The photocurrent measured by switching UV illumination under 0.1 V bias indicates that the current is around 0.7 nA, which is much higher than the dark current of <0.3 nA (Fig. 4(b)). In spite of a small on/off ratio, the rise in photocurrent and decay time are relatively fast when compared to other UV sensors with a similar configuration.

Fig. 4 (a) Schematic illustration of the fabrication of all-nanowire UV sensor; and    (b) Photocurrent measurement with switching UV illumination (0.1 V bias).

The LIGH process seems promising for the bottom-up fabrication of next-generation all-nanowire electronics and multifunctional environmental sensors.

 T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Habeom Lee et al., Nanowire-on-Nanowire: All-Nanowire Electronics by On-Demand Selective Integration of Hierarchical Heterogeneous Nanowires, ACS Nano, Article ASAP DOI: 10.1021/acsnano.7b06098

Designing All-Weather Flexible Electrically Conductive Paper with Superhydrophobic and Flame-Retardant Properties

Flexible electronic devices are gaining momentum in various applications including touch screen panels, solar cells, wearable devices, etc. Susceptibility to the environmental conditions limits their performance. Researchers at Shanghai Institute of Ceramics, Chinese Academy of Sciences, China have suggested a novel strategy for the design of all-weather flexible superhydrophobic, electrically conductive paper with flame-resistant property.

Hydroxyapatite nanowires (HNs), Ketjen black (KB), and polydimethylsiloxane (PDMS) were used to fabricate the all-weather flexible electrically conductive paper. The HNs were prepared by mixing an aqueous solution (500 ml) containing 2.2 g of CaCl₂, 10 g of NaOH and 2.8 g of NaH₂PO₄·2H₂O with an ethanolic solution (140 g) containing 100 g of oleic acid under stirring followed by hydrothermal treatment at 180 °C for 24 h in a Teflon-lined stainless steel autoclave. The HNs were dispersed in ethanol to form a colloidal suspension. KB was dispersed in ethanol under ultrasonication to obtain a KB colloidal suspension. The colloidal suspension of KB was mixed with the colloidal suspension of HNs under stirring for 10 min. Vacuum-assisted filtration was adopted to fabricate the KB + HNs paper and it was peeled off after drying at 90 °C for 5 min. The KB + HNs paper was immersed in dilute PDMS solution (ratio of PDMS:curing agent:ethylacetate is 10:1:100) at room temperature for 30 min and subsequently cured at 100 °C for 1 h (Fig. 1(a)). The as-prepared KB+HNs+PDMS paper (KHP paper) exhibits a rough morphology (Fig. 1(b)), higher water contact angle (>150°) and low sliding angle (<10°) (Fig. 1(c)) high flexibility (twisted and bent without breaking for 500 cycles) (Fig. 1(d)) and electrically conductive (Fig. 1(e)).

Fig. 1 (a) Schematic illustration of various stages involved in the preparation; (b) morphology; (c) water contact angle; (d) flexibility; and (e) electric conductivity of flexible electrically conductive KB + HNs + PDMS paper.

The high water contact angle (>150°) and a low sliding angle (<10°) enables water droplets to bounce off from the surface and automatically rolled away even at a small tilting angle, thus keeping the KHP paper to keep dry (Fig. 2(a)). The water repellent ability of KHP paper is retained under highly corrosive conditions (pH: 2-13), when heating up to 300 °C for 12 h and when exposed to humid conditions (50 -90% RH) for 24 h. The KHP paper also exhibits self-cleaning ability, which is evidenced by the easy removal of soil by water droplets (Figs. 2(b-e)). Real-time electrical performance of the KHP paper upon wetting, monitored by measuring the resultant current upon applying a potential of 3 V indicates that the water droplets remains stable on the surface without wetting (Figs. 2(f) and 2(g)) and the electrical current was steady from 0 to 10 s (Fig. 2 h and 2(i)). Real-time electrical performance underwater (Fig. 2(j)) also indicates the ability of the KHP paper to exhibit a good stability, as evidenced by the brightness of the LED lamp from 0 to 120 s (Figs. 2(k) and 2(l)).

Fig. 2 (a) Bouncing-off of water droplets; (b-e) self-cleaning ability; (f-l) Real-time monitoring of electrical conductivity: (f-i) with a few water droplets; (j-l) after total immersion in water

The electrothermal effect of the KHP paper is ascertained by applying a direct voltage to the paper covered by copper foils at the edges and measuring change in surface temperature using an infrared thermal imaging camera (Fig. 3(a)). The surface temperature is increased quickly within 10 s and then leveled off (Fig. 3(b)); the higher the applied voltage, the higher the surface temperature of KHP paper. The ability of the KHP paper to retain the rapid thermoresponsive behavior for five cycles indicated its recyclability (Fig. 3(c)). Due to its electrothermal effect, the KHP paper in capable of quickly evaporating tiny water droplet within 128 s (Fig. 3(d)) and deicing of ice within 23 s (Fig. 3(e)).

Fig. 3 (a) Schematic illustration of the surface temperature measurement of the KHP paper; (b) Change in surface temperature with time; (c) stability upon repeated heating/cooling cycles; (d) evaporation of tiny water droplet (3 µL); and (e) deicing of ice.

The KHP paper also exhibits flame retarding characteristics and its electrical conductivity is increased from 11.92 mA to 13.39 mA after exposure to flame for 60 s and stabilized to 13.34 A after 7 min (Fig. 4(a)). Real-time monitoring of the electrical current and the brightness of an LED lamp up to 7 min confirm the ability of the KHP paper to retain electrical conductivity even  under extreme condition of combustion (Fig. 4(b)).

The KHP paper exhibits superhydrophobicity, better flexibility, enhanced mechanical properties, good electrical conductivity, high thermal stability, suitable electrothermal effect and good flame retardancy. Due to its ability to perform well under extreme conditions (underwater as well as in flame), the KHP electrically conductive paper seems to be promising for applications in flexible electronic devices.

T.S.N. Sankara Narayanan

Fig. 4 Real-time monitoring of electrical conductivity of the KHP paper in flame up to 7 min : (a) change in current; and (b) brightness of LED lamps.

For more information, the reader may kindly refer: Fei-Fei Chen et al., Hydroxyapatite Nanowire-Based All-Weather Flexible Electrically Conductive Paper with Superhydrophobic and Flame-Retardant Properties, ACS Appl. Mater. Interfaces, DOI: 10.1021/acsami.7b09484

 

 

Fabrication of biomorphic SiO2 with nano-nipple array structures inspired from cicada wings

Antireflective structures (ARSs) reduce Fresnel reflection to boost light transmission or absorption and improve the performance of optical devices over a wide range of wavelengths. Researchers at Shanghai Jiao Tong University, China, have fabricated biomorphic SiO₂ with ARSs that are inspired from cicada wings using a simple and inexpensive sol-gel ultrasonic method combined with calcination.

Black cicada (Cryptotympana atrata Fabricius) wings were chosen as the biological prototype due to the nano-nipple array structure on their wings. The cicada wings were cleaned with absolute ethanol followed by deionized water, dried in air and pretreated with 8% NaOH. Ethanol/water/TEOS/HCl mixture at a molar ratio of 3:12:1:0.03 modified with Triton X-100 was used as a precursor sol for SiO₂. The pretreated cicada wings were immersed in the precursor sol and sonicated using high-intensity ultrasonic irradiation (20 kHz; 100W/cm²) at room temperature for 3 h. Subsequently, the cicada wings were kept in the precursor sol for 12 h for solidification, cleaned with ethanol and dried at 60 °C under vacuum. The SiO₂ coated wings were calcined in vacuum at 500 °C for 2 h to remove the organic template, leaving behind SiO₂ with the surface structure of the cicada wing (biomorphic SiO₂).

Fig. 1 Schematic of the synthesis process for the fabrication of biomorphic SiO₂

The morphological features of the biomorphic SiO₂ indicate replication of the nano-nipple array structures (Figs. 2(a) and 2(b)) similar to that of the cicada wing (inset of Fig. 2(a)). The nano-nipple arrays increased the surface roughness and decreased the water contact angle to 16° (inset of Fig. 2(b)), thus imparting hydrophilic properties for the biomorphic SiO₂. The reflectance spectra of biomorphic SiO₂ gradually changed from 0.3% to 3.3% as the angle of incidence is changed from 10° to 60° (Figs. 2(c) and 2(d)). The excellent antireflection property is due to the formation of ARS on the surface of biomorphic SiO₂. The gradation in the refractive index between air and SiO₂ introduced by the ARS causes a dramatic reduction in the reflectance in the visible wavelength range (450–800 nm) over a wide range of incident angles.

Fig. 2 SEM images: (a) top view; (b) side view; and (c) angle dependent; and (d) counter map angle dependent antireflection properties of biomorphic SiO₂

The antireflective properties of biomorphic SiO₂ are promising and suitable for applications in photovoltaic devices and solar cells. Similarly, the hydrophilic properties of biomorphic SiO₂ will be useful for the development of self-cleaning and antifogging optical materials.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer to: Imran Zada et al., Multifunctional, angle dependent antireflection, and hydrophilic properties of SiO₂ inspired by nano-scale structures of cicada wings, Appl. Phys. Lett. 111, 153701 (2017); doi: 10.1063/1.4986133

Designing high capacity cation-disordered cathode materials for lithium ion batteries

Li-excess disordered rock-slat transition metal oxides (LEX-RS), have received considerable attention as high-capacity cathode materials. The excess Li reduces the transition metal (TM) content and increases the average TM oxidation state, leading to a decrease in TM-based redox capacity. Hence, the high capacity of such materials relies on oxygen redox processes wherein delivery of high capacity cold trigger oxygen loss and formation of high-impedance layers that limit the performance. Researchers at University of California, Berkeley and Lawrence Berkeley National Laboratory, USA have demonstrated that fluorine substitution is a viable option to overcome this limitation. Since partial substitution of fluorine in place of oxygen lowers the average anion valance, more Ni²⁺ ions could be incorporated. This strategy helps to increase the Ni redox reservoir, limits oxygen redox process and prevents oxygen loss.

Li-Ni-Ti-Mo based metal oxides with suitable stoichiometric ratios were synthesized by a solid-state method using Li₂CO₃, NiCO₃, TiO₂, MoO₂, and LiF as precursors. The precursors (suitable stoichiometric ratios) were dispersed in acetone and ball milled for 15 h, dried overnight in an oven, pelletized, calcined at 700-750 °C for 2-10 h in air, furnace cooled and ground to form fine powders. Li_1.15Ni_0.375Ti_0.375Mo_0.1O₂ (LN15), Li_1.2Ni_0.333Ti_0.333Mo_0.133O₂ (LN20) and Li_1.15Ni_0.45Ti_0.3Mo_0.1O_1.85F_0.15 (LNF15) were evaluated.

The voltage profile of LN15, recorded during galvanostatic cycling between 1.5 and 4.6 V, exhibits a large hysteresis (voltage gap, polarization) between charge and discharge cycles with a discharge plateau at ~2.2 V (Fig. 1(a)). In contrast, LNF15 exhibits a much reduced polarization, delivering high discharge capacities, in which this discharge plateau is hardly noticed (Fig. 1(b)).

Fig. 1 Comparison of voltage profiles of (a) LN15; and (b) LNF15 when cycled between 1.5 and 4.6 V at 20 mA/g (Inset: capacity retention – first 20 cycles)

Differential electrochemical mass spectrometry (DEMS) measurements performed on LN15 and LNF15 indicate that LNF15 has experienced a lower oxygen loss than LN15. Upon first charge to 4.8 V, O₂ gas could be detected from ~4.35 V (~185 mAh/g) for LN15 (Fig. 2(a)) whereas detection of O₂ gas is delayed up to ~4.5 V (~220 mAh/g) for LNF15 (Fig. 2(b)). Upon charging, reaction between the oxygen radicals generated at the cathode and the carbonate-based electrolytes could lead to the formation of CO₂ gas. Irrespective of the type of cathode materials, the evolution of  CO₂ gas occurs > ~4.4 V for both LN15 and LNF15.  The total amount of O₂ evolved for LN15 and LNF15 is 0.30 and 0.09 μmol/mg, respectively. Similar to O₂, the amount of CO₂ gas evolved is also low for LNF15 (0.05 μmol/mg) than for LN15 (0.14 μmol/mg). If all of the O₂ is presumed to be originated from the cathode, then the amount of O₂ evolved corresponds to a loss of 2.3 and 0.7% of the total oxygen content for LN15 and LNF15, respectively.

Fig. 2 Comparison of the DEMS study of LN15 and LNF15 when charged to 4.8 V and discharged to 1.5 V at 20 mA/g

The combined effect of fluorine substitution along with an increase in nickel content has enabled LNF15 to achieve a decrease in O₂ loss from the cathode, better capacity retention and improved performance. Fluorine substitution opens up new avenues for designing high-capacity cathode materials with transition metal ions.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: Jinhyuk Lee et al., Mitigating oxygen loss to improve the cycling performance of high capacity cation-disordered cathode materials, Nature Communications, 8: 981  DOI: 10.1038/s41467-017-01115-0