Unusually small electrical resistance of three-dimensional nanoporous gold in external magnetic fields

The Influence of the Mechanical Compliance of a Substrate on the Morphology of Nanoporous Gold Thin Films

Nanoporous gold (np-Au) has found its use in applications ranging from catalysis to biosensing, where pore morphology plays a critical role in performance. While the morphology evolution of bulk np-Au has been widely studied, knowledge about its thin-film form is limited. This work hypothesizes that the mechanical compliance of the thin film substrate can play a critical role in the morphology evolution. Via experimental and finite-element-analysis approaches, we investigate the morphological variation in np-Au thin films deposited on compliant silicone (PDMS) substrates of a range of thicknesses anchored on rigid glass supports and compare those to the morphology of np-Au deposited on glass. More macroscopic (10 s to 100 s of microns) cracks and discrete islands form in the np-Au films on PDMS compared to on glass. Conversely, uniformly distributed microscopic (100 s of nanometers) cracks form in greater numbers in the np-Au films on glass than those on PDMS, with the cracks located within the discrete islands. The np-Au films on glass also show larger ligament and pore sizes, possibly due to higher residual stresses compared to the np-Au/PDMS films. The effective elastic modulus of the substrate layers decreases with increasing PDMS thickness, resulting in secondary np-Au morphology effects, including a reduction in macroscopic crack-to-crack distance, an increase in microscopic crack coverage, and a widening of the microscopic cracks. However, changes in the ligament/pore widths with PDMS thickness are negligible, allowing for independent optimization for cracking. We expect these results to inform the integration of functional np-Au films on compliant substrates into emerging applications, including flexible electronics.

3D Continuously Porous Graphene for Energy Applications

Constructing bulk graphene materials with well-reserved 2D properties is essential for device and engineering applications of atomically thick graphene. In this article, the recent progress in the fabrications and applications of sterically continuous porous graphene with designable microstructures, chemistries, and properties for energy storage and conversion are reviewed. Both template-based and template-free methods have been developed to synthesize the 3D continuously porous graphene, which typically has the microstructure reminiscent of pseudo-periodic minimal surfaces. The 3D graphene can well preserve the properties of 2D graphene of being highly conductive, surface abundant, and mechanically robust, together with unique 2D electronic behaviors. Additionally, the bicontinuous porosity and large curvature offer new functionalities, such as rapid mass transport, ample open space, mechanical flexibility, and tunable electric/thermal conductivity. Particularly, the 3D curvature provides a new degree of freedom for tailoring the catalysis and transport properties of graphene. The 3D graphene with those extraordinary properties has shown great promises for a wide range of applications, especially for energy conversion and storage. This article overviews the recent advances made in addressing the challenges of developing 3D continuously porous graphene, the benefits and opportunities of the new materials for energy-related applications, and the remaining challenges that warrant future study.

Dealloyed Nanoporous Materials for Rechargeable Post-Lithium Batteries

Nanoporous materials (NPMs) made by dealloying have been well recognized as multifunctional electrodes for lithium-ion batteries (LIBs). In recent years, there are ever-increasing demands on grid-scale energy storage devices composed by earth-abundant elements such as Na, K, Mg, Al, and Zn. Compared to LIBs, these electrochemical cells face critical challenges such as slow kinetics of redox reactions and structural instability owing to large ion size and/or multiple-electron process. Much interest has been focused on NPMs to address these issues with great success. This Minireview discusses the recent research progresses on these novel electrode materials in the emerging post-lithium batteries, including the rational-design of NPMs, structure-performance correlation in each battery system, and insights into future development of this rapidly growing field.

Bimetallic PtAu electrocatalysts for the oxygen reduction reaction: challenges and opportunities

Highly active, durable oxygen reduction reaction (ORR) electrocatalysts have an essential role in promoting the continuous operation of advanced energy technologies such as fuel cells and metal-air batteries. Considering the scarce reserve of Pt and its unsatisfactory overall performance, there is an urgent demand for the development of new generation ORR electrocatalysts that are substantially better than the state-of-the-art supported Pt-based nanocatalysts, such as Pt/C. Among various nanostructures, bimetallic PtAu represents one unique alloy system where highly contradictory performance has been reported. While it is generally accepted that Au may contribute to stabilizing Pt, its role in modulating the intrinsic activity of Pt remains unclear. This perspective will discuss critical structural issues that affect the intrinsic ORR activities of bimetallic PtAu, with an eye on elucidating the origin of seemingly inconsistent experimental results from the literature. As a relatively new class of electrodes, we will also highlight the performance of dealloyed nanoporous gold (NPG) based electrocatalysts, which allow a unique combination of structural properties highly desired for this important reaction. Finally, we will put forward the challenges and opportunities for the incorporation of these advanced electrocatalysts into membrane electrode assemblies (MEA) for actual fuel cells.

A novel application of nanoporous gold to humidity sensing: a framework for a general volatile compound sensor

Volatile organic compounds (VOC) are ubiquitous in industrial applications creating a pressing desire for novel transduction pathways to build a broad family of new gas sensors. Nanoporous gold (NPG) is a material with a vast range of untapped potential applications; offering a high surface area found generally in nanomaterials, while also being comparatively simple to fabricate. NPG based sensors can also leverage the unique physics of gold at the nanoscale. In this work, we leverage the multiple unique nanoscale phenomena associated with NPG to demonstrate two novel transduction mechanisms to sense humidity, a model compound. Through direct electrical measurements of NPG, we were able to sense changes in the electronic properties of NPG induced by ambient humidity. We propose two novel transduction mechanisms: chemoresistive changes induced by surface adsorption and electrochemical capacitive changes induced by the electric double layer to detect humidity. To our knowledge this is the first reported application of both these mechanisms for sensing any volatile compounds utilizing NPG.

Essential role of enhanced surface electron-phonon interactions on the electrical transport of suspended polycrystalline gold nanofilms

The electrical resistivity of suspended polycrystalline gold nanofilms with different lengths has been measured over the temperature range of 2 K to 340 K, which dramatically increases compared with bulk gold and slightly increases with length. Classical size effect theories considering surface and grain boundary scatterings cannot explain the increased film resistivity, especially the temperature dependence of resistivity, over the whole temperature range. Considering the fact that the reduction of the coordination number of atoms at the surface and the interface leads to a decrease of the phonon spectrum frequency and consequently affects the surface phonon spectrum, the electron–phonon interaction as a relatively independent surface effect is taken into account. The theoretical predictions and the experimental measured film resistivity match very well over the whole temperature range and the extracted surface Debye temperature decreases significantly compared to the bulk value, which illustrates the essential role of enhanced surface electron–phonon interactions on the electrical transport of the present gold nanofilms.

Considering the enhanced surface electron–phonon interaction, the electrical resistivity of suspended polycrystalline gold nanofilms with different length can be described very well in a temperature range from 2 K to 340 K.

Three-dimensional porous graphene networks expand graphene-based electronic device applications

In recent years, there has been increasing demand for 3D porous graphene structures with excellent 2D characteristics and great potential. As one avenue, several approaches for fabricating 3D porous graphene network structures have been proposed to realize multi-functional graphene materials with 2D graphene structures. Herein, we overview characteristics of 3D porous graphene for applications in future electronic devices along with physical insights into "2D to 3D graphene", in which the characters of 2D graphene such as massless Dirac fermions are well preserved. The present review thus summarizes recent 3D porous graphene studies with a perspective for providing new and board applications of graphene in electronic devices.

Nanoporous Platinum/(Mn,Al)3O4 Nanosheet Nanocomposites with Synergistically Enhanced Ultrahigh Oxygen Reduction Activity and Excellent Methanol Tolerance

At present, metal/metal oxide composites are considered as potential oxygen reduction reaction (ORR) catalysts for energy-related applications like fuel cells. Here, we fabricated a high-activity, low Pt loading ORR electrocatalyst composed of nanoporous Pt (np-Pt) in intimate contact with lamellar (Mn,Al)₃O₄ nanosheet (NS). In comparison to Pt/C (Johnson Matthey), the np-Pt/(Mn,Al)₃O₄ NS catalyst shows a 11.5-fold enhancement in the mass-normalized ORR activity and much better methanol tolerance because of the metal-support interactions between np-Pt and (Mn,Al)₃O₄. Furthermore, the combination of electrochemical experiments with theoretical calculations verifies that the ORR on the np-Pt/(Mn,Al)₃O₄ NS catalyst is a direct 4e^- pathway in the alkaline solution. In addition, the electrocatalytic mechanisms have also been rationalized by density functional theory (DFT) calculations.

Linear magnetoresistance in gold foams

Classical linear magnetoresistance is observed in ultralow density gold foams with strong spatial inhomogeneity.

In situ electrical modulation and monitoring of nanoporous gold morphology

The ability to fine-tune feature size in nanostructured thin films is critical, as many desirable properties of these materials are dictated by their nanostructure. Accordingly, there is a need for techniques that allow for modifying nanostructure while monitoring the morphological changes in situ. Here, we demonstrate a closed-loop electro-annealing system which enables in situ monitoring of morphology evolution in sub-micron nanoporous gold (np-Au) thin films. Np-Au is produced by a microfabrication-compatible self-assembly process that produces a network of interconnected ligaments with tunable diameter (10 s to 100 s of nanometers), making it a desirable material for numerous applications and fundamental studies alike. We specifically investigate the relationship between np-Au morphology (i.e., ligament diameter) and electrical resistance of the thin film. A strong correlation emerges between ligament size and electrical resistance, which puts forward resistance as an effective parameter for monitoring morphology evolution. Surprisingly, np-Au films with thicker ligaments lead to an increase in electrical resistance, which is unexpected since the extent of charge carrier scattering at the ligament surface should decrease with increasing ligament size. Further examination of np-Au morphology with high-resolution electron microscopy revealed grain growth on the ligaments in highly-annealed np-Au thin films. This suggests that grains act as scattering centers for charge carriers and this becomes the dominant mechanism in dictating electrical resistance in a percolated network of thin conductive ligaments.

Nanostructured transition metal dichalcogenide electrocatalysts for CO2 reduction in ionic liquid

Conversion of carbon dioxide (CO2) into fuels is an attractive solution to many energy and environmental challenges. However, the chemical inertness of CO2 renders many electrochemical and photochemical conversion processes inefficient. We report a transition metal dichalcogenide nanoarchitecture for catalytic electrochemical CO2 conversion to carbon monoxide (CO) in an ionic liquid. We found that tungsten diselenide nanoflakes show a current density of 18.95 milliamperes per square centimeter, CO faradaic efficiency of 24%, and CO formation turnover frequency of 0.28 per second at a low overpotential of 54 millivolts. We also applied this catalyst in a light-harvesting artificial leaf platform that concurrently oxidized water in the absence of any external potential.

A nanoporous metal recuperated MnO2 anode for lithium ion batteries

Lithium-ion batteries (LIBs) have been intensively studied to meet the increased demands for the high energy density of portable electronics and electric vehicles. The low specific capacity of the conventional graphite based anodes is one of the key factors that limit the capacity of LIBs. Transition metal oxides, such as NiO, MnO2 and Fe3O4, are known to be promising anode materials that are expected to improve the specific capacities of LIBs for several times. However, the poor electrical conductivity of these oxides significantly restricts the lithium ion storage and charge/discharge rate. Here we report that dealloyed nanoporous metals can realize the intrinsic lithium storage performance of the oxides by forming oxide/metal composites. Without any organic binder, conductive additive and additional current collector, the hybrid electrodes can be directly used as anodes and show highly reversible specific capacity with high-rate capability and long cyclic stability.

The Role of Computer Simulation in Nanoporous Metals-A Review

Nanoporous metals (NPMs) have proven to be all-round candidates in versatile and diverse applications. In this decade, interest has grown in the fabrication, characterization and applications of these intriguing materials. Most existing reviews focus on the experimental and theoretical works rather than the numerical simulation. Actually, with numerous experiments and theory analysis, studies based on computer simulation, which may model complex microstructure in more realistic ways, play a key role in understanding and predicting the behaviors of NPMs. In this review, we present a comprehensive overview of the computer simulations of NPMs, which are prepared through chemical dealloying. Firstly, we summarize the various simulation approaches to preparation, processing, and the basic physical and chemical properties of NPMs. In this part, the emphasis is attached to works involving dealloying, coarsening and mechanical properties. Then, we conclude with the latest progress as well as the future challenges in simulation studies. We believe that highlighting the importance of simulations will help to better understand the properties of novel materials and help with new scientific research on these materials.

Observation of large positive magneto-resistance in bubble decorated graphene oxide films derived from shellac biopolymer: a new carbon source and facile method for morphology-controlled properties

We report a large positive magneto-resistance (MR) in bubble decorated graphene oxide films that are derived from shellac biopolymer as a carbon source. These films were produced on a quartz substrate by heating the biopolymer coated substrate at 900 °C in an argon atmosphere. The characterization data of the films using Raman, X-ray photoelectron spectroscopy, field emission scanning electron microscopy and transmission electron microscopy reveal that shellac can be used as a new carbon source to produce transparent bubble decorated graphene oxide films. The magneto-resistance results show a 130% change in the resistance of the films at 3 K under a perpendicular magnetic field of 15 T, and the value decreases exponentially up to 50 K. The observed MR properties of the bubble decorated graphene oxide films are explained using a weak anti-localization and quantum interference model in the low magnetic field region, while the Lorentz force accounts for the MR properties well in the high magnetic field region.

Monolayer MoS2 films supported by 3D nanoporous metals for high-efficiency electrocatalytic hydrogen production

The "edge-free" monolayer MoS2 films supported by 3D nanoporous gold show high catalytic activities towards hydrogen evolution reaction (HER), originating from large out-of-plane strains that are geometrically required to manage the 3D curvature of bicontinuous nanoporosity. The large lattice bending leads to local semiconductor-to-metal transition of 2H MoS2 and the formation of catalytically active sites for HER.

Nanofabrication of highly ordered, tunable metallic mesostructures via quasi-hard-templating of lyotropic liquid crystals

The synthesis of metal frameworks perforated with nanotunnels is a challenge because metals have high surface energies that favor low surface area structures; traditional liquid-crystal templating techniques cannot achieve the synthetic control required. We report a synthetic strategy to fabricate metal nanomaterials with highly ordered, tunable mesostructures in confined systems based on a new quasi-hard-templating liquid-crystals mechanism. The resulting platinum nanowires exhibit long range two-dimensional hexagonally ordered mesopore structures. In addition, single crystalline hexagonal mesoporous platinum nanowires with dominant {110} facets have been synthesized. Finally, we demonstrate that the mesostructures of metal nanomaterials can be tuned from hexagonal to lamellar mesostructures.

Self-grown Ni(OH)(2) layer on bimodal nanoporous AuNi alloys for enhanced electrocatalytic activity and stability

Au nanostructures as catalysts toward electrooxidation of small molecules generally suffer from ultralow surface adsorption capability and stability. Here, we report Ni(OH)2 layer decorated nanoporous (NP) AuNi alloys with a three-dimensional and bimodal porous architecture, which are facilely fabricated by a combination of chemical dealloying and in situ surface segregation, for the enhanced electrocatalytic performance in biosensors. As a result of the self-grown Ni(OH)2 on the AuNi alloys with a coherent interface, which not only enhances adsorption energy of Au and electron transfer of AuNi/Ni(OH)2 but also prohibits the surface diffusion of Au atoms, the NP composites are enlisted to exhibit significant enhancement in both electrocatalytic activity and stability toward glucose electrooxidation. The highly reliable glucose biosensing with exceptional reproducibility and selectivity as well as quick response makes it a promising candidate as electrode materials for the application in nonenzymatic glucose biosensors.

Nanoporous metal enhanced catalytic activities of amorphous molybdenum sulfide for high-efficiency hydrogen production

We fabricated a robust electrocatalyst by chemically depositing an ultrathin layer of amorphous molybdenum sulfide on the internal surface of dealloyed nanoporous gold. The catalyst exhibits superior electrocatalysis toward hydrogen evolution reaction in both acidic and neutral media with 2-6 times improvement in catalytic activies compared to other molybdenum sulfide based materials.

Metal corrosion for nanofabrication

The annual cost of corrosion has been increasing globally, and it has now reached beyond 3% of the world's gross domestic product. It remains a challenge to reduce or prevent unwanted corrosion effectively after many decades of effort. Nowadays, more efforts are being made to develop anti-corrosion platforms for decreasing the huge cost of corrosion. In parallel, it is also highly expected to be able to use corrosion for producing useful materials with reduced energy consumption. In this review, recent progress in how methods for controlling metal corrosion can be used to produce structure-diversified nanomaterials are summarized along with a presentation of their applications. As a valuable addition to the scientists' toolbox, metal corrosion strategies can be applied to different metals and their alloys for the production of various nanostructured materials; this also provides insights into how metal corrosion can be further prevented and into how corrosion wastage can be reduced.

Apparent inverse Gibbs-Thomson effect in dealloyed nanoporous nanoparticles

The Gibbs-Thomson effect (the reduction of local chemical potential due to nanoscale curvature) predicts that nanoparticles of radius r dissolve at lower electrochemical potentials than bulk materials, decreasing as 1/r. However, we show here that if the particle is an alloy--susceptible to selective dissolution (dealloying) and nanoporosity evolution--then complete selective electrochemical dissolution and porosity evolution require a higher electrochemical potential than the comparable bulk planar material, increasing empirically as 1/r. This is a kinetic effect, which we demonstrate via kinetic Monte Carlo simulation. Our model shows that in the initial stages of dissolution, the less noble particle component is easily stripped from the nanoparticle surface, but owing to an increased mobility of the more noble atoms, the surface of the particle quickly passivates. At a fixed electrochemical potential, porosity and complete dealloying can only evolve if fluctuations in the surface passivation layer are sufficiently long-lived to allow dissolution from percolating networks of the less-noble component that penetrate through the bulk of the particle.

Nanoporous PtCo surface alloy architecture with enhanced properties for methanol electrooxidation

By selectively dealloying a PtCoAl ternary alloy, a novel nanoporous PtCo (np-PtCo) alloy with a three-dimensional bicontinuous pore-ligament structure is successfully fabricated. X-ray diffraction and electron microscopic characterizations demonstrate the single-crystal nature of the alloy ligament with a ligament size down to ~3 nm. After a mild electrochemical dealloying process, a nanoporous near-surface alloy structure with a Pt-rich surface and a PtCo alloy core is obtained. Electrochemical measurements show that the np-PtCo surface alloy has greatly enhanced catalytic activity and durability toward methanol electrooxidation compared with a state-of-the-art Pt/C catalyst. The peak current density of methanol electrooxidation on a np-PtCo surface alloy is more than 5 times of that on Pt/C. More importantly, continuous potential cycling from 0.6 to 0.9 V (vs RHE) in a 0.5 M H(2)SO(4) aqueous solution demonstrates that a np-PtCo surface alloy has excellent structure stability, with more than 90% of the initial electrochemical active surface area (EASA) retained after 5000 potential cycles. Under the same conditions, the EASA of Pt/C drops to ~70%. With evident advantages of facile preparation as well as enhanced electrocatalytic activity and durability, a np-PtCo surface alloy nanomaterial holds great potential as an anode catalyst in direct methanol fuel cells.

Surface effects on the mechanical properties of nanoporous materials

Using the theory of surface elasticity, we investigate the mechanical properties of nanoporous materials. The classical theory of porous materials is modified to account for surface effects, which become increasingly important as the characteristic sizes of microstructures shrink to nanometers. First, a refined Timoshenko beam model is presented to predict the effective elastic modulus of nanoporous materials. Then the surface effects on the elastic microstructural buckling behavior of nanoporous materials are examined. In particular, nanoporous gold is taken as an example to illustrate the application of the proposed model. The results reveal that both the elastic modulus and the critical buckling behavior of nanoporous materials exhibit a distinct dependence on the characteristic sizes of microstructures, e.g. the average ligament width.

Nanoporous metal/oxide hybrid electrodes for electrochemical supercapacitors

Electrochemical supercapacitors can deliver high levels of electrical power and offer long operating lifetimes, but their energy storage density is too low for many important applications. Pseudocapacitive transition-metal oxides such as MnO(2) could be used to make electrodes in such supercapacitors, because they are predicted to have a high capacitance for storing electrical charge while also being inexpensive and not harmful to the environment. However, the poor conductivity of MnO(2) (10(-5)-10(-6) S cm(-1)) limits the charge/discharge rate for high-power applications. Here, we show that hybrid structures made of nanoporous gold and nanocrystalline MnO(2) have enhanced conductivity, resulting in a specific capacitance of the constituent MnO(2) (~1,145 F g(-1)) that is close to the theoretical value. The nanoporous gold allows electron transport through the MnO(2), and facilitates fast ion diffusion between the MnO(2) and the electrolytes while also acting as a double-layer capacitor. The high specific capacitances and charge/discharge rates offered by such hybrid structures make them promising candidates as electrodes in supercapacitors, combining high-energy storage densities with high levels of power delivery.

Correlation of the thermal and electrical conductivities of nanoporous gold

We measured the thermal and electrical conductivities of nanoporous Au thin foils in the temperature range 93-300 K. Resulting from the nanoscale microstructure, the two types of conductivities are both temperature dependent and significantly lower than those of bulk Au. However, the corresponding Lorenz number is strikingly similar to that of bulk Au, indicating that the Wiedemann-Franz law holds perfectly well for nanoporous metals in this temperature range. Compared to the bulk value, the Debye temperature of nanoporous Au is decreased. We predict the theoretical Debye temperature of nanoporous Au by its relation to the elastic constants. The present results indicate that the nanoporous Au foils should be comprised of macroscopic, single-crystalline porous grains rather than nanocrystals.