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

 

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

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

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

 

Self-folding 3D graphene

Graphene finds widespread application in energy storage, sensors and flexible electronics. For most of the applications, a planar geometry of graphene would suffice while applications such as wearable electronics, biological or dispersible sensors, and actuators demand a curved and folded architectures. Since pristine graphene is highly chemically inert, achieving controlled self-folding in response to external environmental stimuli is very difficult.

Researchers at the Johns Hopkins University and Massachusetts Institute of Technology, USA have developed a method to fold and unfold monolayer graphene into ordered 3D structures so that they can be designed and fabricated in accordance with a predictable shape (Weinan Xu et al., Ultrathin thermoresponsive self-folding 3D graphene, Science Advances  06 Oct 2017: Vol. 3, no. 10, e1701084; 10.1126/sciadv.1701084). The processing involves various stages including surface functionalization of graphene, transfer of the functionalized graphene on patterned Al coated on Si, shape design by photolithography, removal of unwanted graphene by oxygen plasma etching, removal of Al by chemical etching and folding of the functionalized graphene by an increase in solution temperature.

Surface functionalization of graphene

Immersion of monolayer graphene in a dilute aqueous solution of dopamine (2.0 mg/ml) at pH of 8.5 (10 mM tris-HCl) for 2 – 4 h promoted self-polymerization of dopamine, that lead to the formation of a thin layer (~5 nm) of polydopamine (PD) on the surface of graphene. Subsequently, the PD-coated graphene was immersed in a dilute aqueous solution containing amine-terminated poly(N-isopropylacrylamide) (PNIPAM) (2.0 mg/ml) at pH 8.5 (10 mM tris-HCl) at 60 °C for 3 h (Fig. 1(a)). The PD served as an intermediate active layer to graft PNIPAM on PD-coated graphene. The thermoresponsive properties of PNIPAM enables the surface functionalized graphene to behave as an ultrathin shape-changing material.

Fabrication of self-folding microstructures

A patterned sacrificial Al layer was deposited on Si. Subsequently, the PD-PNIPAM functionalized graphene was transferred onto the substrate. The functionalized graphene was patterned into various shapes by photolithography, and the graphene in unwanted areas was removed by oxygen plasma etching. The underlying Al layer was dissolved using 5 mM NaOH + 3 mM sodium dodecyl sulphate. Folding of the functionalized graphene was induced by heating the solution to 45 °C using a hot plate. Selective pinning prevented the folded structures from being washed away (Fig. 1(b)).

Fig. 1 Schematic illustration of surface functionalization, patterning, fabrication and folding process of graphene microstructures

Grafting of the thermoresponsive PNIPAM to the surface of graphene is necessary for folding; neither the pristine graphene nor the PD-graphene exhibit the self-folding behavior with an increase in temperature. Different 3D shapes, including flower, dumbbell, and box can be obtained after folding (Fig. 2(a-c)). The reversible switching behavior of PNIPAM also helps to unfold the structure by reversing the temperature from 45 °C to 25 °C (Fig. 2(d-f). Addition of a rigid polymer layer to the petals (increase in thickness up to 100 nm) reduces the adhesion between the petals and favours easy reversibility (Fig. 2(g-i)).

The flower shaped folding tends to fold its free petals toward the center and go from an open to a closed state, it is possible to encapsulate live cells within the self-folding flower. The temperature employed for cell culture (37 °C) is sufficient to induce folding of the functionalized graphene flowers to encapsulate the cells inside its petals. It is confirmed that the cells are alive after encapsulation, which suggests that the self-folding process is biocompatible and can be used to capture biological cargo.

The process is highly tunable and offer control over the self-folding nature of graphene. The extent of folding is increased with an increase in temperature from 25 °C to 45 °C while reversing the temperature enabled unfolding of the structure. Different 3D shapes such as flower, dumbbell, box, etc., can be achieved. A variety of applications such as encapsulation and delivery of cells, design and fabrication of novel electrical devices and field-effect transistors, formation of a variety of Origami and Kirigami shape-changing structures, etc. are envisioned.

Fig. 2 (a-c) Snapshots of temperature-induced self-folding of ultrathin graphene microstructures with a flower geometry; (d-i) Reversibility of the temperature-induced self-folding. The sequence of images (d, e f) shows folding and unfolding of functionalized graphene flower; The sequence of images (g, h, i) shows folding and unfolding of a flower with rigid SU8 polymer petals, with better stability and reversibility but with increased thickness (100 nm). Scale bars: (a-c) 100 µm; and (d-i) 50 µm.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: Weinan Xu et al., Ultrathin thermoresponsive self-folding 3D graphene, Science Advances  06 Oct 2017: Vol. 3, no. 10, e1701084; 10.1126/sciadv.1701084

Super-formable pure magnesium at room temperature

Magnesium is a promising material that can substitute for steel and aluminium alloys to achieve weight reduction in automobile, aerospace and allied industries, which is considered to contribute for energy efficiency and eco-friendly. However, one of the major impediment is the limited formability of magnesium.

Researchers from the Department of Materials Science and Engineering and Department of Mechanical and Aerospace Engineering, Monash University, Australia and Automotive Steel Research Institute, China have reported a breakthrough in the development of polycrystalline pure magnesium that can be tailored to be super-formable at room temperature by conventional processes.

The study reveals that polycrystalline pure magnesium becomes super-formable at room temperature after it is extruded ≤ 80 °C, exhibit no work hardening and shows no sign of fracture during compression at room temperature and at a strain rate of 10⁻³ s^-1. In contrast, those extruded at 150 to 400 °C exhibit poor formability at room temperature, high work hardening and show clear signs of fracture when compressed by 20–30% reduction in height (Fig. 1).

Fig. 1 Room temperature compression of specimens extruded at 80 and 400 °C. Photographs in the inset show the specimens before and after compression test. Specimens extruded at 400 °C fractures after ~20% height reduction while those extruded at 80 °C can be compressed from 10 to 1.5 mm without fracture.

The super-formability of polycrystalline pure magnesium specimens extruded at 80 °C was demonstrated by rolling them at room temperature (cold rolling) without any intermediate annealing stage. Continuous reduction in their thickness from 3 to 1 mm fails to display any edge cracking (Fig. 2(a)). The ability of 1 mm-thick sheet to bent through 180° (hemming) without any failure (Fig. 2(b)), suggests its suitability for the fabrication of automotive panels. The 1 mm thick sheet can be further cold rolled to 0.5 mm, and even 0.12 mm strips (96 % reduction in total thickness equivalent to a true strain of 3.2). The 0.12 mm strips are amenable for cutting and shaping in the form of letters “m” and “g” (Fig. 2(a)). Folding twice followed by complete unfolding fails to display any visible cracks (Fig.2 (c)), which refute the conventional belief that magnesium would fracture after heavy cold work or bending.

Fig. 2 Cold rolling of extruded specimens: (a) Photograph of 3 mm thick magnesium plate extruded at 80 °C, and after 67 and 96% cold rolling without any trimming of specimen edges along the rolling direction. The strip cold rolled by 96% was cut and shaped in the form of letters “m” and g”; (b) Photograph of cold-rolled 1 mm thick strip bent by ~180° at room temperature; (c) Photographs showing folding and unfolding of 0.12 mm strip without any visible cracks. (Scale bars in a –c:  20, 3 and 5 mm, respectively)

Specimens extruded at 80 and 400 °C posses strong basal texture and contain predominantly equiaxed grains with an average grain size of ~1.3 and ~82 μm, respectively. For the specimen extruded at 400 °C followed by 20% cold compression or rolling, the average grain size is decreased to 56–61 μm. In contrast, there observed to be very little change in the size and shape of grains for the specimen extruded at 80 °C followed by 50% cold compression or rolling. Microstructural evolution reveals the presence of a large number of deformation twins (Fig. 3(a)) and slip traces (Fig. 3 (b)) for the specimen extruded at 400 °C followed by 20% cold compression or rolling. In contrast, these features are not detected in the specimen extruded at 80 °C.

Fig. 3 Secondary electron micrographs showing: (a) deformation twins (T); and (b) slip traces (S) in the specimen extruded at 400 °C and compressed by 20%.

The results of the study reveals the occurrence of significantly different deformation modes during cold forming of specimens extruded at 80 °C, even though they also possess a strong basal texture. The superformability behaviour is due to the occurrence of dynamic recrystallisation during extrusion of pure magnesium specimens at room temperature. In addition, dynamic recrystallisation is also possible during compression/rolling at room temperature, either to accommodate grain boundary sliding or to act as an independent softening mechanism. The study demonstrates that extruded pure magnesium remains superformable, even after substantial plastic deformation at room temperature. The findings of the study provide a new avenue for the design and development of highly formable magnesium products by conventional thermomechanical processes that are cost-effective, efficient and industrially scalable. This attribute assumes significance for the development of light weight materials in automobile, aerospace and allied industries.

T.S.N. Sankara Narayanan

For more detailed information, the reader may kindly refer: Zeng et al., Super-formable pure magnesium at room temperature, NATURE COMMUNICATIONS | 8: 972 | DOI: 10.1038/s41467-017-01330-9

Rapid charging of your smart phones – Are we getting closer to reality?

Supercapacitors are used as an alternative power source for rechargeable batteries due to their efficient operation at high power density, long cycle life and improved safety. Nevertheless, the limited energy density, typically of the order of 5-8 Wh/L, limits their widespread use for many practical applications. Boosting capacitance and extending window of cell voltage are the available options to impart further improvement in their energy density. Researchers at University of Waterloo, Canada and Jain University, Bangalore, India have proposed a novel approach towards the development of high voltage super capacitors with high energy density (ACS Nano, 2017, 11 (10), pp 10077–10087).

Ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) and Tween 20 (nonionic surfactant) were mixed together to obtain a stable microemulsion with nanometer sized particles. Upon mixing it with graphene oxide (GO), the surfactant stabilized microemulsion spontaneously adsorbs on the surface of GO. This dispersion was directly casted onto copper with the formation of a dense nanocomposite film of GO/IL/Tween 20. Subsequent thermal treatment leads to the removal of IL by evaporation and reduction of GO to reduced graphene oxide (rGO). The resultant electrode is referred as IL-mediated reduced graphene oxide (IM-rGO).

Fig. 1 Schematic of fabrication of IM-rGO electrode assembly: (a) spontaneous adsorption of surfactant stabilized microemulsion particles on the surface of GO; (b) enlarged view of EMImTFSI/Tween 20/H₂O microemulsion particle; (c) film structure after drop-casting and water evaporation; and (d) film structure after evaporation of Tween 20 following thermal reduction

The surface morphology of the nanocomposite film reveals the presence of macropores (Fig. 2(a)) due to evaporation of water and Tween 20 during thermal treatment. Morphology at the cross-section indicates a layered structure (Fig. 2(b)), in which the sheets lay parallel to the current collector, thus providing a relatively high bulk density.

Fig. 2 Morphology of the IM-rGO film fabricated using 60% IL: (a) at the surface; and (b) at the cross-section

The electrochemical performance of the IM-rGO electrode fabricated using 60 wt% of IL at RT is depicted in Fig. 3. The formation of a dense film enabled a CV of 218 F/cm³. This electrode offered a maximum energy density of 45 Wh/L at a power density of 571.4 W/L and maintained a high energy density of 21.7 Wh/L at a power density as high as 6.04 kW/L at RT.

Fig. 3 Electrochemical performance of 60% IL electrodes at RT: (a) CVs and (b) GCDs for IM-rGO at RT; (c) specific capacitance at varying current density

Eliminating the macropores of the film still remains a challenge and elimination of macropores would help to achieve even higher bulk density. The easy adoptability of the proposed methodology provides new avenues for the manufacturing of large-scale supercapacitors.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Zimin She et al., ACS Nano, 2017, 11 (10), pp 10077–10087

Smart plasma copolymer coatings with tunable wettability from superhydrophobicity to superhydrophilicity

Stimuli-responsive surfaces with switchable wettability assumed significance  in drug delivery, biomedical engineering, sensors and bio-fuel cells. Most of the responsive surfaces show intrinsic responsive wettability and it is difficult to tune chemical structures with controlled wetting characteristics.

Researchers at the Surface Engineering Laboratory, School of Materials Science and Engineering, Dalian University of Technology, China have demonstrated that it would be possible to fabricate smart surfaces with tunable wettability and reversibly switchable pH-responsiveness using plasma copolymerization technique (Reference: Iqbal Muzammil et al., Plasma Processes and Polymers, 14 (10) (2017), DOI: 10.1002/ppap.201700053).

Plasma copolymerization is an efficient one-step process to fabricate new surfaces. It is a clean, dry, and environmentally benign process that enables conformal deposition of coatings over surfaces with complex geometries. In this perspective, the Chinese researchers have deposited a series of plasma copolymer coatings with various carboxylic acid and fluorocarbon group ratio on nanotextured low-density polyethylene (LDPE) surfaces via capacitively coupled radio frequency plasma (CCP) polymerization technique.

Acrylic acid (AA) and octafluorocyclobutane (C₄F₈) were used as monomers. Low density polyethylene (LDPE) was chosen as the substrate and it was oxygen plasma etched at 200W for 30 min with an oxygen flow rate of 50 sccm to develop a nanotextured surface. The C₄F₈-co-AA plasma polymer coatings was deposited on flat nanotextured LDPE surfaces by radio frequency (RF) capacitively coupled plasma reactor (CCP) mode at 50W for 1 min. The C₄F₈ monomer flow rate was fixed for 40 sccm while the AA monomer flow rates was changed from 5 to 40 sccm.

C₄F₈ plasma polymer coating deposited on LDPE surface shows a static water contact angle (SWCA) of 119°. As the carboxylic acid group concentration increases, the SWCA of C₄F₈-co-AA plasma polymer coatings is decreased. For C₄F₈-co-AA (40:5) plasma polymer coatings, the SWCA is decreased to 97°. C₄F₈-co-AA (40:40) plasma polymer coatings show a rapid decrease in SWCA leading to a lower hysteresis. An increase in AA feed ratio increases the ratio of carboxylic acid group to CF_x group, leading to a lower SWCA. The hydrophilic carboxylic acid group controls the wetting state since the polar carboxylic acid group allows permeation of polar water molecules into plasma copolymer coatings. In contrast, the hydrophobic fluorocarbon group controls the dewetting state since the nonpolar groups like CF₂ and CF₃ of low surface energy repel water.

The oxygen plasma etching treatment enables the formation forest like nano-filaments on the surface of LDPE (Fig. 1(a)). Subsequent deposition of either C₄F₈ (Fig. 1(b)) as well as C₄F₈-co-AA plasma polymer coatings (Figs. 1 (c) and 1(d)) has no significant effect on the surface nanotexture. Nevertheless, the surface nanotextures amplify the surface wettability.

Fig. 1 Scanning electron micrographs (a) nanotextured surface of LDPE after oxygen plasma etching treatment; (b) C₄F₈ plasma polymer coating; (c) C₄F₈-co-AA (40:15) plasma polymer coating; and (d) C₄F₈-co-AA (40:25) plasma polymer coating on nanotextured LDPE surfaces

C₄F₈ plasma polymer coating deposited over nanotextured LDPE surface became superhydrophobic with a SWCA of ~163° and low apparent hysteresis of < 1°. This high SWCA with low hysteresis developed over the nanotextured surface can be explained by the Cassie model. Accordingly, water droplet cannot penetrate into cavities of the nanotextured surface as air is trapped at the interface between the water droplet and sharp corners of nanotextured surface.

C₄F₈ plasma polymer coatings show no significant SWCA change with a change in pH, suggesting the absence of pH-responsive behaviour. In contrast, the C₄F₈-co-AA plasma polymer coatings start to a exhibit pH-responsive behaviour with sufficient increase in pH-sensitive carboxylic acid groups. C₄F₈-co-AA (40:5) plasma polymer coatings in different pH solutions of 1, 4, 9, and 13 for 10 min shows a SWCA of 80, 76, 70, and 65°, respectively. C₄F₈-co-AA (40:10) and (40:15) plasma polymer coatings show a SWCA 72 and 70° at pH 1, 67 and 64° at pH 4, 61 and 58° at pH 9, 54 and 49° at pH 13, respectively.

An increase in concentration of carboxylic acid groups as well as pH leads to a decrease in SWCA. This phenomenon can be explained by the protonation and deprotonation of dangling carboxylic acid groups. These carboxylic acid groups become uncharged due to protonation at low pH. It shrinks and gives an additional surface to fluorinated part of the copolymer thus higher SWCA is observed. Correspondingly, at higher pH’s due to deprotonation of the carboxylic acid groups, it becomes charged and the charged state increases the polarity of the polymeric coatings leading to a lower SWCA.

Fig. 2 Schematic illustration of the change in static water contact angle with pH demonstrating the development of C₄F₈-co-AA plasma polymer coatings with tunable wettability and reversibly switchable pH-responsiveness using plasma copolymerization technique

This methodology opens up a potential door for the fabrication of smart surfaces.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Iqbal Muzammil et al., Tunable wettability and pH-responsiveness of plasma copolymers of acrylic acid and octafluorocyclobutane, Plasma Processes and Polymers, 14 (10) (2017), DOI: 10.1002/ppap.201700053

Fabrication of electrochemical paper based analytical devices (ePADs) by direct laser scribing of paperboard

Researchers at the Institute of Chemistry, University of São Paulo, São Paulo, Brazil have developed a method for the fabrication of electrochemical paper based analytical devices by direct laser scribing (LS-e-PAD) of a paperboard surface without the need for any chemical reagents or controlled atmospheric conditions. Pyrolysis of the paperboard using a CO₂ laser enables the formation of a conductive, porous, non-graphitizing carbon material, which is composed of graphene sheets and aluminosilicate nanoparticles. The high conductivity and enhanced active/geometric area ratio suggest that this material is highly promising for the development of portable electrochemical devices.

Comparison of the performance of the LS-ePAD system with that of the conventional glassy carbon electrode and a commercial screen-printed (DropSens^®) electrode using 5 mM mixture of potassium ferricyanide/ ferrocyanide solution as a redox probe reveals superior performance of the LS-ePAD system. Following their higher sensitivity and better reversibility, the utility of LS-ePAD as portable electrochemical sensors is explored for the detection of ascorbic acid and caffeic acid (important antioxidants present in food and dietary supplements) as well as in the forensic detection of picric acid (a military explosive).

The methodology of fabrication of LS-ePAD is simple, easily automated, and scalable for mass production. It is highly promising for the development of portable electrochemical devices with good reproducibility at low-cost.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer William R. de Araujo et al., Single-step Reagentless Laser Scribing Fabrication of Electrochemical Paper-based Analytical Devices, Angew. Chem. Int. Ed. 10.1002/anie.201708527

Programmable liquid materials

Liquid metals such as eutectic gallium indium alloy (EGaIn), due to their unique attributes such as voltage controlled surface tension, high liquid-state conductivity and liquid-solid phase transition at room temperature, open new avenues towards the development of programmable liquid materials.

Researchers at the University of Sussex and Swansea University, UK have exploited the self-locomotion, self-rotation, voltage controlled surface tension, high liquid-state conductivity and deformation characteristics of EGaIn and modulated its stiffness and density so that it would be possible to program the liquid metal in to a desired shape.

A liquid metal blob (certain quantity) in an electrolyte solution is highly conductive. In the absence of contact with any of the electrodes, external force and an applied voltage, the electrolyte induces a uniform charge distribution on the blob’s surface (Fig. 1(a)). However, when the blob is in contact with the anode (Fig. 1(b)) and a suitable voltage is applied to one or more of the other electrodes, the difference in the conductivity between the electrolyte and the liquid metal alters the charge distribution on the blob’ surface. The formation of an electric double-layer (EDL) at the blob’s interface enables deformation of liquid metal in the direction of the electric field (Fig. 1(b)). Hence, it would be possible to deform the liquid metal from anode (high voltage electrode) to cathode (low voltage electrode) to any desired shape (Fig. 1(c)).

Fig. 1 Influence of electric field on the deformation of liquid metal: (a) A liquid metal blob far from the electrodes and in absence of field is subject to no force; (b) Deformation of the blob upon contact with anode and an electric voltage is applied across it; and (c) Deformation of liquid metal to a desired shape.

The basic electrode array control algorithm to deform liquid metal in to a desired shape is shown in Fig. 2. In this system arrangement, the cathode attracts the liquid metal and the anode keeps the liquid metal in a wet and flat state (having the lowest surface tension). Hence, by switching only one selected electrode as cathode (low voltage) and setting all other electrodes as anodes (high voltage), the movement of liquid metal can be controlled. The relative voltage difference decides the speed of liquid metal deformation.

Fig. 2 Basic electrode array control algorithm to make alphabet letter “S”.

Three main problems that are inherent to liquid metal deformation still remains to be solved:

• H₂ evolution at the cathode causes the liquid metal to branch out as multiple trees
• Higher surface tension of liquid metals at smaller size leads to splitting
• Liquid metal body interference stops its movement towards the cathode

The programmable liquid materials will find applications in soft robotics and shape changing, reconfigurable electronic circuits and display domains.

T.S.N. Sankara Narayanan

For a more detailed information, the reader may kindly refer: Yutaka Tokuda et al., Programmable Liquid Matter: 2D Shape Deformation of Highly Conductive Liquid Metals in a Dynamic Electric Field, Proceedings of the Interactive Surfaces and Spaces on ZZZ -ISS ’17 (2017). DOI: 10.1145/3132272.3134132  

B and N codoped nanodiamond – A novel electrocatalyst for the selective electrochemical reduction of CO2 to ethanol

Ethanol is a clean and renewable liquid fuel with high heating value. Use of ethanol as a fuel is a viable strategy in terms of resource utilization and mitigating problems with regard to global warming. Electrochemical reduction of CO₂ can be considered as a possible route to obtain ethanol. Transition metal oxides and chalcogenides, B doped diamond, N doped carbon nanotubes and graphene were hitherto explored as potential electrocatalysts for CO₂ reduction. In spite of their good activity and durability, they reduce CO₂ to CO, HCHO or HCOO^– as the major products with a Faradic efficiency of 74.0 to 87.0%. Nevertheless, development of durable electrocatalysts for selective conversion of CO₂ to CH₃CH₂OH with high Faradic efficiency remains a big challenge.

Researchers from Dalian University of Technology, China and California Institute of Technology, USA have reported that B and N codoped nanodiamond (BND) could function as an efficient and stable electrode for the selective reduction of CO₂ to ethanol (Yanming Liu et al., Angew. Chem. Int. Ed. 10.1002/anie.201706311)

The BND film was deposited on Si by hot filament chemical vapor deposition method using a gas mixture of CH₄/B₂H₆/N₂/H₂ that had 2.5% CH₄. Three different BNDs with same B₂H₆ content (12.5%) but different N₂ levels (2.5%, 5.0% and 10.0% denoted as BND1, BND2 and BND3) were prepared.

All the three BNDs possess a similar crystal structure, morphological features and B content (Fig. 1(a)). All of them are found to be active for electrocatalytic reduction of CO₂ and they preferentially convert CO₂ to CH₃CH₂OH. The BNDs present a more negative H₂ evolution potential, which is favorable for CO₂ reduction with higher Faradic efficiency. The production rate of CH₃CH₂OH is significantly increased with a negative shift in potential from -0.8 V to -1.1 V (Fig. 1(b)) while the extent of formation of CH₃OH and HCOO^– remains low. The synergistic effect of B and N codoping is considered responsible for the better activity and high selectivity for the conversion of CO₂ to CH₃CH₂OH. Among the BNDs, maximum Faradic efficiency for the conversion of CO₂ to CH₃CH₂OH is achieved on BND3 (93.2% at -1.0 V) (Fig. 1(c)), suggesting that the higher the N content, the greater the electrocatalytic effect. The high durability of BND3 for electrocatalytic reduction of CO₂ is evidenced by its ability to show a Faradaic efficiency of ~93.2% during 16 consecutive experiments (Fig. 1(d)). Since a higher N content is likely to promote H₂ evolution, a balance between N content and H₂ evolution must be maintained to achieve better results.

Fig. 1 (a) Surface morphology of BND3; (b, c) production rates of CH₃CH₂OH, CH₃OH and HCOO^– in CO₂ saturated 0.1 M NaHCO₃ and the corresponding Faradaic efficiencies on BDN3; and (d) Faradic efficiency for CO₂ reduction during 16 consecutive runs on BND3 at -1.0 V

Based on the experimental results and density function theory (DFT) calculations, the possible pathway for the multi-electron reduction of CO₂ to CH₃CH₂OH proceeds as follows: CO₂ → *COOH → *CO → *COCO → *COCOH → *COCHOH → *COCH₂OH → *CHOCH₂OH → *CH₂OCH₂OH → CH₃CH₂OH

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Yanming Liu et al., Selective Electrochemical Reduction of Carbon Dioxide to Ethanol on a Boron- and Nitrogen-Co-doped Nanodiamond, Angew. Chem. Int. Ed. 10.1002/anie.201706311

Molten-salt electrodeposition of Si films – Will it open the gates for cost-effective manufacturing of Si solar cells?

Developments in photovoltaic (PV) technologies is believed to make a significant impact in realizing conversion of sunlight into electricity. Reducing the manufacturing costs of Si solar cells is likely to provide a competitive edge for solar energy conversion. Researchers at the University of Texas at Austin, USA, lead by Prof. Allen J. Bard have developed a simple method for electrodeposition of high quality Si films using a CaCl₂-based molten salt electrolyte (Angew. Chem. Int. Ed. 10.1002/anie.201707635)

The CaCl₂-based molten salt electrolyte was modified with CaO (4.8 mol %) and SiO₂ nanoparticles (NPs) (3.9 mol %). Electrodeposition of Si was performed on graphite at 15 mA/cm² for 1, 3 and 7 h. The mechanism of formation of Si films is depicted in Fig. 1. In the CaCl₂-based molten salt, solid SiO₂ NPs react with O^2- to form soluble Si^IV−O anions, which are reduced to Si atoms on the graphite cathode. Continuous generation of soluble Si^IV−O anions and elimination of suspended SiO₂ particles from the molten salt electrolyte are essential to produce good quality Si films.

Impurities in the Si film, particularly, B and P, which are considered to be the most problematic ones for Si used for solar cells, are limited to 0.9 ppm and 0.6 ppm, respectively. The morphological features of the Si film reveals that it is uniform, dense and crystalline (Fig. 1). Photoelectrochemical measurements indicate that the deposited Si films possess a p-type semiconductor character due to doping of Al during electrodeposition.

Fig. 1 Schematic illustration of the formation mechanism of Si film onto a graphite substrate by electrodeposition in molten CaCl₂−CaO−SiO₂ and SEM images of the Si films deposited on graphite substrates in molten CaCl₂−CaO−SiO₂ (CaO: 4.8 mol%; SiO₂: 3.9 mol%) at 1123 K by electrodeposition at 15 mA/cm² for 1 h

 

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer Xiao Yang et al., Angew. Chem. Int. Ed. 10.1002/anie.201707635

Reducing Charge Carrier Transport Barrier in Lithium Ion Batteries by Adopting a Functionally Layer-Graded Electrode Design Approach

High-performance lithium ion batteries (LIBs) with fast charging capability and high capacity are required from portable electronics to electric vehicles. In LIBs, during charging, Li-ions depleted at the cathode get accumulated at the anode. An increase in Li-ion concentration on the anode surface when compared to the bulk, results in premature discharge, which is termed as concentration polarization induced overpotential. The slow Li-ion diffusion and poor electronic conductivity within the electrode limit the performance of LIBs. Hence, it is imperative to decrease the ionic and electronic resistance at electrolyte/electrode and electrode/current collector interfaces, respectively. In addition, it is important to minimize Li-ion diffusion barrier and maximize the electronic conductivity along the charge carrier transport direction.

Researchers at Nanyang Technological University, Singapore and University of Oslo, Norway have employed a rational design approach and fabricated functionally layer-graded electrodes composing of TiO₂(B) nanotubes and reduced graphene oxide (RGO) to reduce charge carrier transport barrier within the electrode (Angew. Chem. Int. Ed. 10.1002/anie.201707883).

The functionally layer-graded electrodes, composed of cross-linking TiO₂(B) nanotubes and well-dispersed RGO nanosheets with different configurations, were fabricated by layer-by-layer coating (Fig. 1). Among the electrode designs explored, up-graded electrode (Fig. 1(a)) exhibits a remarkable capacity of 128 mAh/g at a high charging/discharging rate at 20 °C (6.7 A/g), which is much higher than that of the traditionally homogeneous electrode (74 mAh/g) (Fig. 1(b)) with a similar composition. The improved performance of up-graded electrode is due to the synergistic effect of decrease in Li-ion diffusion energy barrier and improvement of electronic properties within the electrodes.

Fig. 1 Schematic representation of the multilayered electrode design:  (a) up-graded electrode (proposed design); (b) conventionally homogeneous electrode; and (c) down-graded electrode (reference). Colour mapping scale (in the right side) indicates the weight ratio of RGO in RGO/TiO₂(B) nanotube

The remarkable performance of the up-graded electrode design when compared to the homogeneous electrode design is explained in Fig. 2. In the up-graded electrode design arrangement, a high concentration of RGO nanosheets are available at the bottom layer, which ensures excellent electric contact between RGO/TiO₂(B) films and the current collector. This arrangement could offer a considerable reduction in interface resistance of electrode/current collector and improve electronic conductivity of whole electrode.

Fig. 2 Proposed mechanistic pathway for Li-ion and electron transport in the up-graded electrode and homogeneous electrode design arrangements

It is believed that this concept of functionally graded material can be extended to other hybrid electrodes to minimize charge carrier transport barriers and other kinetically-limited electrochemical reactions.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Yanyan Zhang et al., Reducing the Charge Carrier Transport Barrier in Functionally Layer-Graded Electrodes, Angew. Chem. Int. Ed. 10.1002/anie.201707883