Bulk metallic glass nanowire architecture for electrochemical applications

Kinetically Controlled Synthesis of Metallic Glass Nanoparticles with Expanded Composition Space

Nanoscale metallic glasses offer opportunities for investigating fundamental properties of amorphous solids and technological applications in biomedicine, microengineering, and catalysis. However, their top-down fabrication is limited by bulk counterpart availability, and bottom-up synthesis remains underexplored due to strict formation conditions. Here, a kinetically controlled flash carbothermic reaction is developed, featuring ultrafast heating (>105 K s-1) and cooling rates (>104 K s-1), for synthesizing metallic glass nanoparticles within milliseconds. Nine compositional permutations of noble metals, base metals, and metalloid (M1─M2─P, M1 = Pt/Pd, M2 = Cu/Ni/Fe/Co/Sn) are synthesized with widely tunable particle sizes and substrates. Through combinatorial development, a substantially expanded composition space for nanoscale metallic glass is discovered compared to bulk counterpart, revealing that the nanosize effect enhances glass forming ability. Leveraging this, several nanoscale metallic glasses are synthesized with composition that have never, to the knowledge, been synthesized in bulk. The metallic glass nanoparticles exhibit high activity in heterogeneous catalysis, outperforming crystalline metal alloy nanoparticles.

Model Metallic Glasses for Superior Electrocatalytic Performance in a Hydrogen Oxidation Reaction

Metallic glasses or amorphous alloys, with their excellent chemical stability, disordered atomic arrangement, and ability for thermoplastic nanostructuring, show promising performance toward a range of electrocatalytic reactions in proton-exchange membrane fuel cells. However, there are knowledge gaps and a distinct lack of understanding of the role of amorphous alloy chemistry in determining their catalytic activity. Here, we demonstrate the influence of alloy chemistry and the associated electronic structure on the hydrogen oxidation reaction (HOR) activity of a systematic series of Pt_42.5-xPd_xCu₂₇Ni_9.5P₂₁ bulk metallic glasses (BMGs) with x = 0 to 42.5 at%. The HOR activity and electrochemical active surface area as a function of composition were in the form of volcano plots, with a peak around equal proportion of Pt and Pd. The lower relative electron work function and higher binding energy of the Pt core level explain the reduced charge-transfer resistance and improved electrocatalytic activity due to weakened chemisorption of protons in the mid-range composition. Density functional theory calculations show the lower free energy change and higher hydrogen adsorption density for these Pt_42.5-xPd_xCu₂₇Ni_9.5P₂₁ BMGs, suggesting a synergistic effect from the presence of both noble metals, Pt and Pd.

CuZr Metal Glass Powder as Electrocatalysts for Hydrogen and Oxygen Evolution Reactions

For the practical application of water electrolysis, it is essential to develop cost-effective and high efficiency electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evaluation reaction (OER). In this work, we applied CuZr metallic glass powder, after chemical dealloying treatment, as electrocatalysts. The as-prepared sample had both the increased specific area and optimized surface composition of an efficient catalyst. During the HER and OER processes, the dealloyed CuZr sample displayed overpotential of 195 mV and 310 mV at current density of 10 mA cm−2, respectively. A two-electrode water splitting cell, using the as-prepared CuZr sample, exhibited high stability towards a high current density of 500 mA cm−2, and lower overpotential, compared to a Pt/C//IrO2 cell, during the 10 mA cm−2 constant current density aging test.

Review of Thermoplastic Drawing with Bulk Metallic Glasses

This study summarizes the recent progress in thermoplastic drawing of bulk metallic glasses. The integration of drawing with templated embossing enables the fabrication of arrays of high-aspect-ratio nanostructures whereas the earlier drawing methodologies are limited to a single fiber. The two-step drawing can produce metallic glass structures such as, vertically aligned nanowires on substrates, nanoscale tensile specimens, hollow microneedles, helical shafts, and micro-yarns, which are challenging to fabricate with other thermoplastic forming operations. These geometries will open new applications for bulk metallic glasses in the areas of sensors, optical absorption, transdermal drug-delivery, and high-throughput characterization of size-effects. In this article, we review the emergence of template-based thermoplastic drawing in bulk metallic glasses. The review focuses on the development of experimental set-up, the quantitative description of drawing process, and the versatility of drawing methodology.

Editorial for the Special Issue on "Nanoalloy Electrocatalysts for Electrochemical Devices"

Nanoscale science and technology dealing with materials synthesis, nanofabrication, nanoprobes, nanostructures, nanoelectronics, nano-optics, nanomechanics, nanodevices, nanobiotechnology, and nanomedicine is an exciting field of research and development in Europe, the United States, and other countries around the world [...].

Metal and metal oxide amorphous nanomaterials towards electrochemical applications

Amorphous nanomaterials have aroused extensive interest due to their unique properties. Their performance is highly related with their distinct atomic arrangements, which have no long-range order but possess short- to medium-range order. Herein, an overview of state-of-the-art synthesis methods of amorphous nanomaterials, structural characteristics and their electrochemical properties is presented. Advanced characterization methods for analyzing and proving the local order of amorphous structures, such as X-ray absorption fine structure spectroscopy, atomic electron tomography and nanobeam electron diffraction, are introduced. Various synthesis strategies for amorphous nanomaterials are covered, especially the salt-assisted metal organic decomposition method to prepare ultrathin amorphous nanosheets. Furthermore, the design and structure-activity relationship of amorphous nanomaterials towards electrochemical applications, including electrocatalysts and battery anode/cathode materials, is discussed.

Ordered nanostructures arrays fabricated by anodic aluminum oxide (AAO) template-directed methods for energy conversion

Clean and efficient energy conversion systems can overcome the depletion of the fossil fuel and meet the increasing demand of the energy. Ordered nanostructures arrays convert energy more efficiently than their disordered counterparts, by virtue of their structural merits. Among various fabrication methods of these ordered nanostructures arrays, anodic aluminum oxide (AAO) template-directed fabrication have drawn increasing attention due to its low cost, high throughput, flexibility and high structural controllability. This article reviews the application of ordered nanostructures arrays fabricated by AAO template-directed methods in mechanical energy, solar energy, electrical energy and chemical energy conversions in four sections. In each section, the corresponding advantages of these ordered nanostructures arrays in the energy conversion system are analysed, and the limitation of the to-date research is evaluated. Finally, the future directions of the ordered nanostructures arrays fabricated by AAO template-directed methods (the promising method to explore new growth mechanisms of AAO, green fabrication based on reusable AAO templates, new potential energy conversion application) are discussed.

All-in-Fiber Electrochemical Sensing

Electrochemical sensors have found a wide range of applications in analytical chemistry thanks to the advent of high-throughput printing technologies. However, these techniques are usually limited to two-dimensional (2D) geometry with relatively large minimal feature sizes. Here, we report on the scalable fabrication of monolithically integrated electrochemical devices with novel and customizable fiber-based architectures. The multimaterial thermal drawing technique is employed to co-process polymer composites and metallic glass into uniform electroactive and pseudoreference electrodes embedded in an insulating polymer cladding fiber. To demonstrate the versatility of the process, we tailor the fiber microstructure to two configurations: a small-footprint fiber tip sensor and a high-surface-area capillary cell. We demonstrate the performance of our devices using cyclic voltammetry and chronoamperometry for the direct detection and quantification of paracetamol, a common anesthetic drug. Finally, we showcase a fully portable pipet-based analyzer using low-power electronics and an "electrochemical pipet tip" for direct sampling and analysis of microliter-range volumes. Our approach paves the way toward novel materials and architectures for efficient electrochemical sensing to be deployed in existing and novel personal care and surgical configurations.

Electrochemically dealloyed nanoporous Fe40Ni20Co20P15C5 metallic glass for efficient and stable electrocatalytic hydrogen and oxygen generation

The anion exchange membrane (AEM) in fuel cells requires new, stable, and improved electrocatalysts for large scale commercial production of hydrogen fuel for efficient energy conversion. Fe40Ni20Co20P15C5, a novel metallic glass ribbon, was prepared by arc melting and melt spinning method. The metallic glass was evaluated as an efficient electrocatalyst in water-splitting reactions, namely hydrogen evolution reaction under acidic and alkaline conditions. In addition, oxygen evolution reaction in alkaline medium was also evaluated. In 0.5 M H2SO4, the metallic glass ribbons, after electrochemical dealloying, needed an overpotential of 128 mV for hydrogen evolution reaction, while in 1 M KOH they needed an overpotential of 236 mV for hydrogen evolution. For the oxygen evolution reaction, the overpotential was 278 mV. The electrochemical dealloying procedure significantly reduced the overpotential, and the overpotential remained constant over 20 hours of test conditions under acidic and alkaline conditions. The improved electrocatalytic activity was explained based on the metastable nature of metallic glass and the synergistic effect of metal hydroxo species and phosphates. Based on the excellent properties and free-standing nature of these metallic glass ribbons in electrolyte medium, we propose the current metallic glass for commercial, industrial electrocatalytic applications.

Structured nanoscale metallic glass fibres with extreme aspect ratios

Micro- and nanoscale metallic glasses offer exciting opportunities for both fundamental research and applications in healthcare, micro-engineering, optics and electronics. The scientific and technological challenges associated with the fabrication and utilization of nanoscale metallic glasses, however, remain unresolved. Here, we present a simple and scalable approach for the fabrication of metallic glass fibres with nanoscale architectures based on their thermal co-drawing within a polymer matrix with matched rheological properties. Our method yields well-ordered and uniform metallic glasses with controllable feature sizes down to a few tens of nanometres, and aspect ratios greater than 10¹⁰. We combine fluid dynamics and advanced in situ transmission electron microscopy analysis to elucidate the interplay between fluid instability and crystallization kinetics that determines the achievable feature sizes. Our approach yields complex fibre architectures that, combined with other functional materials, enable new advanced all-in-fibre devices. We demonstrate in particular an implantable metallic glass-based fibre probe tested in vivo for a stable brain-machine interface that paves the way towards innovative high-performance and multifunctional neuro-probes.

Noble-Metal based Metallic Glasses as Highly Catalytic Materials for Hydrogen Oxidation Reaction in Fuel Cells

Electro-catalyst design with superior performance and reduced precious metal content (compared to state-of-the-art Pt/C) has been a challenge in proton exchange membrane fuel cells, preventing their widespread adoption. Metallic glasses have recently shown promising performance and large electrochemical surface area in catalytic reactions. The electro-catalytic behavior of recently developed Pt-, Pd-, and Pt/Pd-based metallic glasses was evaluated in this study using scanning electrochemical microscopy. The influence of chemistry and electronic structure on catalytic behavior was studied using scanning kelvin probe technique. The work function for the metallic glasses was lower by 75 mV to 175 mV compared to pure Pt. This resulted in higher catalytic activity for the amorphous alloys, which was attributed to the ease of charge transfer on the surface. The binding energy for the metallic glasses, measured using X-ray photoelectron spectroscopy, was higher by 0.2 eV to 0.4 eV. This explained easier removal of adsorbed species from the surface of amorphous alloys. The synergistic effect of Pt and Pd in alloys containing both the noble metals was demonstrated towards hydrogen oxidation reaction.

Fast Activating Persulfate by Crystallization of Fe-Based Metallic Glasses with External Energy

Very recently, crystallization of metallic glasses (MGs) has presented promising properties in the catalytic field. This work has investigated enhanced catalytic performance of crystallized Fe78Si9B13 ribbons for fast activating persulfate (PS) with assistance of UV-vis light and heat. The ribbons were obtained by annealing at 750°C (Fe-A750) and cibacron brilliant yellow 3G-P (BY 3G-P) dye was used as pollutant. The results indicated that UV-vis light had limited capability to enhance PS activation efficiency by crystallized ribbons while the reaction rate using heat at 65°C was 7.5 times higher than at 25°C, suggesting an advanced performance with heat assistance of Fe-A750 ribbons. Activation energy ∆E for Fe-A750 was measured as 44.5 kJ mol-1. In addition, 5 times reusability could be achieved for Fe-A750 ribbons under 45°C without catalytic decay. The surface morphologies of glassy ribbons, as-annealed Fe-A750 ribbons, HCl-treated Fe-A750 ribbons and after-reused Fe-A750 ribbons have also been systematically studied. This work provides a novel clue to promote applicability of novel crystallized ribbons from MGs.

Nanocatalysts for Electrocatalytic Oxidation of Ethanol

The use of ethanol as a fuel in direct alcohol fuel cells depends not only on its ease of production from renewable sources, but also on overcoming the challenges of storage and transportation. In an ethanol-based fuel cell, highly active electrocatalysts are required to break the C-C bond in ethanol for its complete oxidation at lower overpotentials, with the aim of increasing the cell performance, ethanol conversion rates, and fuel efficiency. In recent decades, the development of wet-chemistry methods has stimulated research into catalyst design, reactivity tailoring, and mechanistic investigations, and thus, created great opportunities to achieve efficient oxidation of ethanol. In this Minireview, the nanomaterials tested as electrocatalysts for the ethanol oxidation reaction in acid or alkaline environments are summarized. The focus is mainly on nanomaterials synthesized by using wet-chemistry methods, with particular attention on the relationship between the chemical and physical characteristics of the catalysts, for example, catalyst composition, morphology, structure, degree of alloying, presence of oxides or supports, and their activity for ethanol electro-oxidation. As potential alternatives to noble metals, non-noble-metal catalysts for ethanol oxidation are also briefly reviewed. Insights into further enhancing the catalytic performance through the design of efficient electrocatalysts are also provided.

Nanoshaping of glass forming metallic liquids by stretching: evading lithography

Lithography-free nanomanufacturing by elongation and fracture of glass forming metallic liquid is presented. The viscous metallic liquid confined in a cavity is laterally downsized to nanoscale by stretching. The extent of size-reduction can be controlled by tuning the active volume of liquid and the viscous and capillary stresses. Very high aspect-ratio metal nanostructures can be fabricated without using lithography or expensive molds. A systematic study is performed using glass forming Pt-Cu-Ni-P alloy to understand the effects of viscosity, surface tension, pulling velocity, and cavity size on the evolution of cylindrical liquid column under tension. The results are quantitatively described using a phenomenological model based on lubrication theory and surface tension induced breakup of liquid filaments. A new manufacturing approach based on variable pulling velocity and/or spinning of metallic liquid is proposed for fabrication of complex geometries.

Recent Advances in Metallic Glass Nanostructures: Synthesis Strategies and Electrocatalytic Applications

Recent advances in metallic glass nanostructures (MGNs) are reported, covering a wide array of synthesis strategies, computational discovery, and design solutions that provide insight into distinct electrocatalytic applications. A brief introduction to the development and unique features of MGNs with an overview of top-down and bottom-up synthesis strategies is presented. Specifically, the morphology and structural analysis of several examples applying MGNs as electrodes are highlighted. Subsequently, a comprehensive discussion of commonly employed kinetic parameters and their connection with the unique material structures of MGNs on individual electrocatalytic reactions is made, including the hydrogen evolution reaction, oxygen reduction reaction, and alcohol (methanol or ethanol) oxidation reaction. Finally, a summary of the challenges and perspective on the future research and development relevant to MGNs as electrocatalysts is provided.

Compelling Rejuvenated Catalytic Performance in Metallic Glasses

Metallic glasses (MGs) with the metastable nature and random atomic packing structure have attracted large attention in the catalytic family due to their superior catalytic performance. In contrast, their crystalline counterparts are restricted by the highly ordered packing structure, fewer surface active sites, and crystallographic defects for catalytic activity. The uncertainty of the different catalytic mechanisms and the intrinsic characteristics correlated to MGs and their crystalline counterparts become a major impediment to promote their catalytic efficiencies and widespread applications. Herein, it is reported that the excellent catalytic behavior in Fe-based MGs goes through a detrimental effect with the partial crystallization, but receives a compelling rejuvenation in the full crystallization. Further investigation reveals that multiphase intermetallics with electric potential differences in fully crystallized alloys facilitate the formation of galvanic cells. More importantly, extensively reduced grain boundaries due to grain growth greatly weaken electron trapping and promote inner electron transportation. The relatively homogenous grain-boundary corrosion in the intermetallics contributes to well-separated phases after reaction, leading to refreshment of the surface active sites, thereby quickly activating hydrogen peroxide and rapidly degrading organic pollutants. The exploration of catalytic mechanisms in the crystalline counterparts of MGs provides significant insights into revolutionize novel catalysts.

Nanopatterned Bulk Metallic Glass Biosensors

Nanopatterning as a surface area enhancement method has the potential to increase signal and sensitivity of biosensors. Platinum-based bulk metallic glass (Pt-BMG) is a biocompatible material with electrical properties conducive for biosensor electrode applications, which can be processed in air at comparably low temperatures to produce nonrandom topography at the nanoscale. Work presented here employs nanopatterned Pt-BMG electrodes functionalized with glucose oxidase enzyme to explore the impact of nonrandom and highly reproducible nanoscale surface area enhancement on glucose biosensor performance. Electrochemical measurements including cyclic voltammetry (CV) and amperometric voltammetry (AV) were completed to compare the performance of 200 nm Pt-BMG electrodes vs Flat Pt-BMG control electrodes. Glucose dosing response was studied in a range of 2 mM to 10 mM. Effective current density dynamic range for the 200 nm Pt-BMG was 10-12 times greater than that of the Flat BMG control. Nanopatterned electrode sensitivity was measured to be 3.28 μA/cm²/mM, which was also an order of magnitude greater than the flat electrode. These results suggest that nonrandom nanotopography is a scalable and customizable engineering tool which can be integrated with Pt-BMGs to produce biocompatible biosensors with enhanced signal and sensitivity.

Tailoring crystallization phases in metallic glass nanorods via nucleus starvation

Many physical phenomena deviate from their established frameworks when the system approaches relevant length scales governing the phenomena. In crystallization, the relevant length scales are the nucleation length set by the nucleus size and density, and the growth length set by diffusion fields. Here we observe unexpected crystallization phenomena at the nanoscale, using metallic glass (MG) nanorods and in situ transmission electron microscopy. The asymmetry between critical heating and cooling rates disappears for small MG nanorods. Strikingly, an apparent single crystalline phase with its composition similar to the glass composition is observed for very small rods, in contrast to bulk samples. We attribute this to the lack of nuclei in small MG nanorods that approach the nucleation length, thus coined the term, nucleus starvation. By controlling the MG nanorod diameter and crystallization kinetics, we can tune the number of nuclei in a nanorod, thereby tailoring the resulting crystallization phases.

Crystallising a bulk metallic glass usually results in separate phases. Here, the authors use metallic glass nanorods to show that as the sample size approaches the nucleation scale lengths, the crystallization behavior is dictated by the lack of nuclei and nanorods crystallise into a single phase.

New and Efficient Electrocatalyst for Hydrogen Production from Water Splitting: Inexpensive, Robust Metallic Glassy Ribbons Based on Iron and Cobalt

Efficient, stable electrocatalysts are required to promote the hydrogen evolution reaction (HER) to obtain hydrogen as a clean, sustainable fuel via water splitting. In the present work, ribbons of the metallic glass Fe₄₀Co₄₀P₁₃C₇ were produced using a conventional melt-spinning technique and assessed as electrocatalysts for HER. In 0.5 M H₂SO₄, these ribbons generated an overpotential of 118 mV at a current density of 10 mA cm^-2. This overpotential remained essentially constant over 20 h under these conditions. On the basis of the excellent properties, these glassy ribbons represent a new type of highly active, robust HER catalyst suitable for practical applications.

Exploring a wider range of Mg-Ca-Zn metallic glass as biocompatible alloys using combinatorial sputtering

In order to bypass the limitation of bulk metallic glasses fabrication, we synthesized thin film metallic glasses to study the corrosion characteristics of a wide atomic% composition range, Mg(35.9-63%)Ca(4.1-21%)Zn(17.9-58.3%), in simulated body fluid. We highlight a clear relationship between Zn content and corrosion current such that Zn-medium metallic glasses exhibit minimum corrosion. In addition, we found higher Zn content leads to a poor in vitro cell viability. These results showcase the benefit of evaluating a larger alloy compositional space to probe the limits of corrosion resistance and prescreen for biocompatible applications.

Large-Scale Fabrication of Porous Gold Nanowires via Laser Interference Lithography and Dealloying of Gold–Silver Nano-Alloys

In this work, we report on an efficient approach to fabricating large-area and uniform planar arrays of highly ordered nanoporous gold nanowires. The approach consists in dealloying Au–Ag alloy nanowires in concentrated nitric acid. The Au–Ag alloy nanowires were obtained by thermal annealing at 800 °C for 2 h of Au/Ag stacked nanoribbons prepared by subsequent evaporation of silver and gold through a nanograted photoresist layer serving as a mask for a lift-off process. Laser interference lithography was employed for the nanopatterning of the photoresist layer to create the large-area nanostructured mask. The result shows that for a low Au-to-Ag ratio of 1, the nanowires tend to cracks during the dealloying due to the internal residual stress generated during the dealloying process, whereas the increase of the Au-to-Ag ratio to 3 can overcome the drawback and successfully leads to the obtainment of an array of highly ordered nanoporous gold nanowires. Nanoporous gold nanowires with such well-regulated organization on a wafer-scale planar substrate are of great significance in many applications including sensors and actuators.

High-throughput drawing and testing of metallic glass nanostructures

Thermoplastic embossing of metallic glasses promises direct imprinting of metal nanostructures using templates. However, embossing high-aspect-ratio nanostructures faces unworkable flow resistance due to friction and non-wetting conditions at the template interface. Herein, we show that these inherent challenges of embossing can be reversed by thermoplastic drawing using templates. The flow resistance not only remains independent of wetting but also decreases with increasing feature aspect-ratio. Arrays of assembled nanotips, nanowires, and nanotubes with aspect-ratios exceeding 1000 can be produced through controlled elongation and fracture of metallic glass structures. In contrast to embossing, the drawing approach generates two sets of nanostructures upon final fracture; one set remains anchored to the metallic glass substrate while the second set is assembled on the template. This method can be readily adapted for high-throughput fabrication and testing of nanoscale tensile specimens, enabling rapid screening of size-effects in mechanical behavior.

Oxide-derived nanostructured metallic-glass electrodes for efficient electrochemical hydrogen generation

Nanostructured materials with large surface areas are continuing to be at the forefront of catalytic applications.

Tuning apparent friction coefficient by controlled patterning bulk metallic glasses surfaces

Micro-honeycomb structures with various pitches between adjacent cells were hot-embossed on Zr₃₅Ti₃₀Cu_8.25Be_26.75 bulk metallic glass surface. The effect of pitch geometry on the frictional behavior of metallic glass surface was systematically investigated. The results revealed that all textured metallic glass surfaces show a reduction in friction coefficient compared to smooth surface. More intriguingly, the friction coefficient first decreased and then increased gradually with increasing pitches. Such unique behavior can be understood fundamentally from the perspective of competing effects between contact area and local stress level with increasing pitches. This finding not only enhance the in-depth understanding of the mechanism of the significant role of surface topography on the frictional behavior of metallic glass surface, but also opens a new route towards other functional applications for bulk metallic glasses.

A Highly Efficient and Self-Stabilizing Metallic-Glass Catalyst for Electrochemical Hydrogen Generation

A multicomponent metallic glass (MG) with highly efficient and anomalous durability for catalyzing water splitting is reported. The outstanding performance of the MG catalyst contributed by self-optimized active sites originates from the intrinsic chemical heterogeneity and selective dealloying on the disordered surface; thus, a new mechanism for improving the durability of catalysts is uncovered.

Regulation of cell-cell fusion by nanotopography

Cell-cell fusion is fundamental to a multitude of biological processes ranging from cell differentiation and embryogenesis to cancer metastasis and biomaterial-tissue interactions. Fusogenic cells are exposed to biochemical and biophysical factors, which could potentially alter cell behavior. While biochemical inducers of fusion such as cytokines and kinases have been identified, little is known about the biophysical regulation of cell-cell fusion. Here, we designed experiments to examine cell-cell fusion using bulk metallic glass (BMG) nanorod arrays with varying biophysical cues, i.e. nanotopography and stiffness. Through independent variation of stiffness and topography, we found that nanotopography constitutes the primary biophysical cue that can override biochemical signals to attenuate fusion. Specifically, nanotopography restricts cytoskeletal remodeling-associated signaling, which leads to reduced fusion. This finding expands our fundamental understanding of the nanoscale biophysical regulation of cell fusion and can be exploited in biomaterials design to induce desirable biomaterial-tissue interactions.

Guided Evolution of Bulk Metallic Glass Nanostructures: A Platform for Designing 3D Electrocatalytic Surfaces

Electrochemical devices such as fuel cells, electrolyzers, lithium-air batteries, and pseudocapacitors are expected to play a major role in energy conversion/storage in the near future. Here, it is demonstrated how desirable bulk metallic glass compositions can be obtained using a combinatorial approach and it is shown that these alloys can serve as a platform technology for a wide variety of electrochemical applications through several surface modification techniques.

Planar Arrays of Nanoporous Gold Nanowires: When Electrochemical Dealloying Meets Nanopatterning

Nanoporous materials are of great interest for various technological applications including sensors based on surface-enhanced Raman scattering, catalysis, and biotechnology. Currently, tremendous efforts are dedicated to the development of porous one-dimensional materials to improve the properties of such class of materials. The main drawback of the synthesis approaches reported so far includes (i) the short length of the porous nanowires, which cannot reach the macroscopic scale, and (ii) the poor organization of the nanostructures obtained by the end of the synthesis process. In this work, we report for the first time on a two-step approach allowing creating highly ordered porous gold nanowire arrays with a length up to a few centimeters. This two-step approach consists of the growth of gold/copper alloy nanowires by magnetron cosputtering on a nanograted silicon substrate, serving as a physical template, followed by a selective dissolution of copper by an electrochemical anodic process in diluted sulfuric acid. We demonstrate that the pore size of the nanowires can be tailored between 6 and 21 nm by tuning the dealloying voltage between 0.2 and 0.4 V and the dealloying time within the range of 150-600 s. We further show that the initial gold content (11 to 26 atom %) and the diameter of the gold/copper alloy nanowires (135 to 250 nm) are two important parameters that must carefully be selected to precisely control the porosity of the material.

Stretchable, Transparent Electrodes as Wearable Heaters Using Nanotrough Networks of Metallic Glasses with Superior Mechanical Properties and Thermal Stability

Mechanical robustness, electrical and chemical reliabilities of devices against large deformations such as bending and stretching have become the key metrics for rapidly emerging wearable electronics. Metallic glasses (MGs) have high elastic limit, electrical conductivity, and corrosion resistance, which can be promising for applications in wearable electronics. However, their applications in wearable electronics or transparent electrodes have not been extensively explored so far. Here, we demonstrate stretchable and transparent electrodes using CuZr MGs in the form of nanotrough networks. MG nanotroughs are prepared by electrospinning and cosputtering process, and they can be transferred to various desired substrates, including stretchable elastomeric substrates. The resulting MG nanotrough network is first utilized as a stretchable transparent electrode, presenting outstanding optoelectronic (sheet resistance of 3.8 Ω/sq at transmittance of 90%) and mechanical robustness (resistance change less than 30% up to a tensile strain of 70%) as well as excellent chemical stability against hot and humid environments (negligible degradation in performance for 240 h in 85% relative humidity and 85 °C). A stretchable and transparent heater based on the MG nanotrough network is also demonstrated with a wide operating temperature range (up to 180 °C) and excellent stretchability (up to 70% in the strain). The excellent mechanical robustness of these stretchable transparent electrode and heater is ascribed to the structural configuration (i.e., a nanotrough network) and inherent high elastic limit of MGs, as supported by experimental results and numerical analysis. We demonstrate their real-time operations on human skin as a wearable, transparent thermotherapy patch controlled wirelessly using a smartphone as well as a transparent defroster for an automobile side-view mirror, suggesting a promising strategy toward next-generation wearable electronics or automobile applications.

Nanoscale size effects in crystallization of metallic glass nanorods

Atomistic understanding of crystallization in solids is incomplete due to the lack of appropriate materials and direct experimental tools. Metallic glasses possess simple metallic bonds and slow crystallization kinetics, making them suitable to study crystallization. Here, we investigate crystallization of metallic glass-forming liquids by in-situ heating metallic glass nanorods inside a transmission electron microscope. We unveil that the crystallization kinetics is affected by the nanorod diameter. With decreasing diameters, crystallization temperature decreases initially, exhibiting a minimum at a certain diameter, and then rapidly increases below that. This unusual crystallization kinetics is a consequence of multiple competing factors: increase in apparent viscosity, reduced nucleation probability and enhanced heterogeneous nucleation. The first two are verified by slowed grain growth and scatter in crystallization temperature with decreasing diameters. Our findings provide insight into relevant length scales in crystallization of supercooled metallic glasses, thus offering accurate processing conditions for predictable metallic glass nanomolding.

Log-normal diameter distribution of Pd-based metallic glass droplet and wire

We have studied the formation of Pd_42.5Cu₃₀Ni_7.5P₂₀ metallic glass droplets and wires in the gas atomization process. We demonstrate that the sizes of droplets and wires can be distinguished by the Ohnesorge number (O_h), which is the proportion of the spinnability to the capillary instability, and the diameter distributions follow a log-normal distribution function, implying cascade fragmentation. For droplets, the number significantly increases at O_h < 1 but the diameter gradually decreases. For wires, the number greatly increases at O_h > 1 while the diameter steadies below 400 nm. Further, the wire diameter is quadrupled at O_h = 16 due to the high viscosity which suppresses both capillary breakup and ligament elongation.

Metallic glass nanostructures of tunable shape and composition

Metals of hybrid nano-/microstructures are of broad technological and fundamental interests. Manipulation of shape and composition on the nanoscale, however, is challenging, especially for multicomponent alloys such as metallic glasses. Although top-down approaches have demonstrated nanomoulding, they are limited to very few alloy systems. Here we report a facile method to synthesize metallic glass nanoarchitectures that can be applied to a broad range of glass-forming alloys. This strategy, using multitarget carousel oblique angle deposition, offers the opportunity to achieve control over size, shape and composition of complex alloys at the nanoscale. As a consequence, nanostructures of programmable three-dimensional shapes and tunable compositions are realized on wafer scale for metallic glasses including the marginal glass formers. Realizing nanostructures in a wide compositional range allows chemistry optimization for technological usage of metallic glass nanostructures, and also enables the fundamental study on size, composition and fabrication dependences of metallic glass properties.

General nanomoulding with bulk metallic glasses

Bulk metallic glasses (BMGs) are ideal for nanomoulding as they possess desirable strength for molds as well as for moldable materials and furthermore lack intrinsic size limitations. Despite their attractiveness, only recently Pt-based BMGs have been successfully molded into pores ranging 10-100 nm (Kumar et al 2009 Nature 457 868-72). Here, we introduce a quantitative theory, which reveals previous challenges in filling nanosized pores. This theory considers, in addition to a viscous and a capillary term, also oxidation, which becomes increasingly more important on smaller length scales. Based on this theory we construct a nanomoulding processing map for BMG, which reveals the limiting factors for BMG nanomoulding. Based on the quantitative prediction of the processing map, we introduce a strategy to reduce the capillary effect through a wetting layer, which allows us to mold non-noble BMGs below 1 μm in air. An additional benefit of this strategy is that it drastically facilitates demoulding, one of the main challenges of nanomoulding in general.