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.

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

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

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

 

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

Manipulating the growth mode of ice crystals by changing the surface wettability could help design better anti-icing surfaces

Design of anti-icing surfaces assumed significance in aerospace, power systems, marine vessels and automotive sectors. Easy removal of ice from solid surfaces has economic, energy and safety implications. A group of researchers from China and USA have described wettability-dependent ice morphology on the surface of aluminium that had been covered with a hydrophobic, or water-repellent, coating under atmospheric conditions and published their findings recently (Liu et al., Distinct ice patterns on solid surfaces with various wettabilities, www.pnas.org/cgi/doi/10.1073/pnas.1712829114).

The researchers have established a correlation between surface wettability and growth mode of ice crystals and suggested that surface wettabilities dictate the ice growth mode. Accordingly, below a critical value of contact angle, the growth of ice crystals follow along-surface growth mode whereas above this critical value of contact angle, the growth of ice crystals follow off-surface growth mode. It has been demonstrated that the ice crystals grown with off-surface growth mode, having a single point attachment with the surface, can be easily blown away by a breeze whereas those grown with along-surface growth mode, having multiple attachment points, stuck to the solid surface.

The discovery of different ice growth modes on solid surfaces and the feasibility of achieving easy removal of ice crystals grown with off-surface growth mode can be exploited to design better anti-icing surfaces.

The schematic illustrations, snap shots acquired using optical microscopy and video clips will give a better insight about their findings.

Fig. 1 Schematic illustration of the effect of solid surfaces on ice growth; (A) introduction of AgI nanoparticles on solid surfaces to achieve ice nucleation over the entire solid surfaces in the same environment.

Fig. 2 Snapshots acquired at different time periods using an optical microscope coupled with a high-speed camera: (B, D) top-view images; and (C, E) side-view images; (B) growth process of six-leaf clover-like ice on a hydrophobic surface (θ = 107.3°); (C) Off-side growth mode; (D) growth process of sunflower-like ice on a hydrophilic surface (θ = 14.5°); (E) Along-surface growth mode (growth environment: surface temperature is −15 °C; and supersaturation is 5.16)

Video clip demonstrating the growth process of six-leaf clover-like ice on a hydrophobic surface (θ = 107.3°)
http://movie-usa.glencoesoftware.com/video/10.1073/pnas.1712829114/video-1

Video clip demonstrating the growth process of sunflower-like ice on a hydrophilic surface (θ = 14.5°)
http://movie-usa.glencoesoftware.com/video/10.1073/pnas.1712829114/video-2

Fig. 3 Schematic illustration depicting that the ice crystals grown with off-surface growth mode can be easily blown away by a breeze whereas those grown with along-surface growth mode stuck to the solid surface.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Liu et al., Distinct ice patterns on solid surfaces with various wettabilities,  
www.pnas.org/cgi/doi/10.1073/pnas.1712829114).