Enhanced strain in functional nanoporous gold with a dual microscopic length scale structure

What Drives Deformation of Smart Nanoporous Materials During Adsorption and Electrosorption?

Nanoporous solids have high surface area, so processes at the surface affect the sample as a whole. When guest species adsorb in nanopores, be they molecules adsorbing from the gas phase, or ions adsorbing from solution, they cause material deformation. While often undesired, adsorption- or electrosorption-induced deformation provides a potential for nanoporous materials to be used as actuators. Progress in this direction requires understanding the mechanisms of adsorption- or electrosorption-induced deformation. These two processes are rarely discussed together, and this Perspective aims to fill this gap to some extent, focusing on driving forces for both processes. Typically the main driving force for both is the solvation (disjoining) pressure, acting normally to the pore walls. However, in some cases, solvation pressure is not sufficient to describe the effects even qualitatively. We highlight examples in which the surface stress acting along the solid surface is an additional driving force for deformation.

Electrochemically-driven actuators: from materials to mechanisms and from performance to applications

Soft actuators, pivotal for converting external energy into mechanical motion, have become increasingly vital in a wide range of applications, from the subtle engineering of soft robotics to the demanding environments of aerospace exploration. Among these, electrochemically-driven actuators (EC actuators), are particularly distinguished by their operation through ion diffusion or intercalation-induced volume changes. These actuators feature notable advantages, including precise deformation control under electrical stimuli, freedom from Carnot efficiency limitations, and the ability to maintain their actuated state with minimal energy use, akin to the latching state in skeletal muscles. This review extensively examines EC actuators, emphasizing their classification based on diverse material types, driving mechanisms, actuator configurations, and potential applications. It aims to illuminate the complicated driving mechanisms of different categories, uncover their underlying connections, and reveal the interdependencies among materials, mechanisms, and performances. We conduct an in-depth analysis of both conventional and emerging EC actuator materials, casting a forward-looking lens on their trajectories and pinpointing areas ready for innovation and performance enhancement strategies. We also navigate through the challenges and opportunities within the field, including optimizing current materials, exploring new materials, and scaling up production processes. Overall, this review aims to provide a scientifically robust narrative that captures the current state of EC actuators and sets a trajectory for future innovation in this rapidly advancing field.

Mechanically Robust Self-Organized Crack-Free Nanocellular Graphene with Outstanding Electrochemical Properties in Sodium Ion Battery

Crack-free nanocellular graphenes are attractive materials with extraordinary mechanical and electrochemical properties, but their homogeneous synthesis on the centimeter scale is challenging. Here, a strong nanocellular graphene film achieved by the self-organization of carbon atoms using liquid metal dealloying and employing a defect-free amorphous precursor is reported. This study demonstrates that a Bi melt strongly catalyzes the self-structuring of graphene layers at low processing temperatures. The robust nanoarchitectured graphene displays a high-genus seamless framework and exhibits remarkable tensile strength (34.8 MPa) and high electrical conductivity (1.6 × 104 S m-1). This unique material has excellent potential for flexible and high-rate sodium-ion battery applications.

Dealloying-Derived Self-Supporting Nanoporous Zinc Film with Optimized Macro/Microstructure for High-Performance Solar Steam Generation

Solar steam generation (SSG) is a promising technology for the production of freshwater that can help alleviate global water scarcity. Nanostructured metals, known for their localized surface plasmon resonance effect, have generated significant interest, but low-cost metal films with excellent water evaporation properties are challenging. In this work, we present a one-step dealloying route for fabricating self-supporting black nanoporous zinc (NP-Zn) films with a bicontinuous ligament/channel structure, using Al-Zn solid solution alloys as the precursors. The influence of alloy composition on the formation and macro/microstructure of NP-Zn was investigated, and an optimal Al98Zn2 was selected. Additionally, in situ and ex situ characterizations were conducted to unveil the dealloying mechanism of Al98Zn2 and phase/microstructure evolution of NP-Zn during dealloying, including the phase transition of Al(Zn) → Zn, significant volume shrinkage (89.8%), and the development of high porosity (81.3%). The nanoscale ligament/channel structure and high porosity endow the NP-Zn films with good broadband absorption and superior hydrophilicity and, more importantly, give them excellent SSG performance. The NP-Zn2 film displays high evaporation efficiency, superior stability, and good seawater desalination performance. The efficient SSG performance, material abundance, and low cost suggest that NP-Zn films have promising applications in metal-based photothermal materials for SSG.

Hierarchically Structured Black Gold Film with Ultrahigh Porosity for Solar Steam Generation

Plasmonic metals demonstrate significant potential for solar steam generation (SSG) because of their localized surface plasmon resonance effect. However, the inherently narrow absorption spectra of plasmonic metals significantly limit their applications. The fabrication of nanostructures is essential to achieve broadband solar absorption for high-efficiency vapor generation. Herein, a self-supporting black gold (Au) film with an ultrahigh porosity and a hierarchically porous structure is fabricated by formulating an extremely dilute Cu₉₉ Au₁ precursor and controlling the dealloying process. In situ and ex situ characterizations reveal the dealloying mechanism of Cu₉₉ Au₁ in a 1 m HNO₃ solution as that involving the phase transformation of Cu(Au) → Au(Cu) → Au, giant volume shrinkage (≈87%), structural evolution/coarsening of ligaments, and development of ultrahigh porosity (86.2%). The multiscale structure, consisting of ultrafine nanoporous nanowires, aligned nanogaps, and various microgaps, provide efficient broadband absorption over 300-2500 nm, excellent hydrophilicity, and continuous water transport. In particular, the nanoporous black Au film shows high SSG performance with an evaporation rate of 1.51 kg m^-2 h^-1 and a photothermal conversion efficiency of 94.5% under a light intensity of 1 kW m^-2 . These findings demonstrate that the nanoporous Au film has great potential for clean water production and seawater desalination.

Ionic Liquid-Assisted 3D Printing of Self-Polarized β-PVDF for Flexible Piezoelectric Energy Harvesting

Three-dimensional (3D) printing technologies have unparalleled advantages in constructing piezoelectric devices with three-dimensional structures, which are conducive to improving the efficiency of energy harvesting. Among them, fused deposition modeling (FDM) is the most widely used thanks to its low cost and wide range of molding materials. However, as the best piezoelectric polymer, a high electroactive β-phase poly(vinylidene fluoride) (PVDF) piezoelectric device cannot be directly obtained by FDM printing because the β-crystal is unstable at the molten state. Herein, we develop for the first time ionic liquid (IL)-assisted FDM for direct printing of β-PVDF piezoelectric devices. An IL can induce and maintain β crystals during melt extrusion and FDM printing, ensuring that the β-crystal in the printed PVDF device is as high as 98.3%, which is the highest in 3D-printed PVDF as far as we know. Furthermore, the shearing force provided by the FDM facilitates the directional arrangement of the dipoles, resulting in the printed PVDF device having self-polarization characteristics without poling. Finally, the piezoelectric output voltage of the 3D-printed PVDF device is 4.7 times that of the flat PVDF device, and its area current density (17.5 nA cm^-2) is more than that of the reported 3D-printed PVDF piezoelectric device in the literature by two orders of magnitude. The one-step 3D printing strategy proposed in this paper can realize the rapid preparation of complex-shaped and lightweight self-polarized β-PVDF-based piezoelectric devices for energy harvesting.

The Mechanical Effect of MnO2 Layers on Electrochemical Actuation Performance of Nanoporous Gold

This study investigated the electrochemical actuation behavior of nanoporous material during the capacitive process. The length change of nanoporous gold (npg) was in situ investigated in a liquid environment using the dilatometry technique. The mechanical effect of MnO₂ layers was introduced in this work to improve the actuation characteristics of the npg samples. Our work found that the actuation behavior of npg sample could be significantly modulated with a covering of MnO₂ layers. The electrochemical actuation amplitude was efficiently improved and strongly dependent on the thickness of MnO₂ layers covered. Aside from the amplitude, the phase relation between the length change and the electrode potential was inverted when covering the MnO₂ layer on the npg samples. This means the expansion of the npg samples and the contraction of samples covered with the MnO₂ layer when electrochemical potential sweeps positively. A simple finite element model was built up to understand the effect of the MnO₂ layer. The agreement between the simulation result and the experimental data indicates that the sign-inverted actuation-potential response of nanoporous gold contributes to the mechanical effect of MnO₂. It is believed that our work could offer a deep understanding on the effect of the MnO₂ layer on the electrochemical actuation and then provide a useful strategy to modulate the actuation performance of nanoporous metal materials.

Atomic-scale phase separation induced clustering of solute atoms

Dealloying typically occurs via the chemical dissolution of an alloy component through a corrosion process. In contrast, here we report an atomic-scale nonchemical dealloying process that results in the clustering of solute atoms. We show that the disparity in the adatom-substrate exchange barriers separate Cu adatoms from a Cu-Au mixture, leaving behind a fluid phase enriched with Au adatoms that subsequently aggregate into supported clusters. Using dynamic, atomic-scale electron microscopy observations and theoretical modeling, we delineate the atomic-scale mechanisms associated with the nucleation, rotation and amorphization-crystallization oscillations of the Au clusters. We expect broader applicability of the results because the phase separation process is dictated by the inherent asymmetric adatom-substrate exchange barriers for separating dissimilar atoms in multicomponent materials.

High Entropy Alloys: Ready to Set Sail?

Over the past decade, high entropy alloys (HEAs) have transcended the frontiers of material development in terms of their unprecedented structural and functional properties compared to their counterpart conventional alloys. The possibility to explore a vast compositional space further renders this area of research extremely promising in the near future for discovering society-changing materials. The introduction of HEAs has also brought forth a paradigm shift in the existing knowledge about material design and development. It is in this regard that a fundamental understanding of the metal physics of these alloys is critical in propelling mechanism-based HEA design. The current paper highlights some of the critical viewpoints that need greater attention in the future with respect to designing mechanically and functionally advanced materials. In particular, the interplay of large compositional gradients and defect topologies in these alloys and their corresponding impact on overall mechanical response are highlighted. From the point of view of functional response, such chemistry vis-à-vis topology correlations are extended to novel class of nano-porous HEAs that beat thermal coarsening effects despite a high surface to volume ratio owing to retarded diffusion kinetics. Recommendations on material design with regards to their potential use in diverse applications such as energy storage, actuators, and as piezoelectrics are additionally considered.

Designing Multiscale Porous Metal by Simple Dealloying with 3D Morphological Evolution Mechanism Revealed via X-ray Nano-tomography

Designing materials with multiscale, hierarchical structure is critical to drive the advancement of new technology. Specifically, porous metals with multiscale porosity from nanometer to micrometer sizes would lead to enhanced physical and chemical properties-the micron-sized pores can increase the effective diffusivity of ion transport within the porous media, and the nano-sized pores provide high specific surface area, enabling functionalities that are unique to nanoporous metals. A new ternary precursor alloy selection concept utilizing the different mixing enthalpies is demonstrated in this work for the design of multiscale, bimodal porous copper from a simple, one-step dealloying of Cu-Fe-Al ternary alloy. The nanoporosity in the bimodal porous structure is formed from dealloying of the Cu-rich phase, whereas the microporosity is controlled by dissolving the Fe-rich phase, determined by both the initial Fe particle size and sintering profile. In addition to advancing the materials design method, the multiscale pore formation during dealloying was directly visualized and quantified via an interrupted in situ synchrotron X-ray nano-tomography. The 3D morphological analysis on tortuosity showed that the presence of the microporosity can compensate the increase of the diffusion path length due to nanoporosity, which facilitates diffusion within the porous structure. Overall the focus of the work is to introduce a new strategy to design multiscale porous metals with enhanced transport properties, and sheds light on the fundamental mechanisms on the 3D morphological evolution of the system using advanced synchrotron X-ray nano-tomography for future materials development and applications.

Dendritic core-shell nickel-iron-copper metal/metal oxide electrode for efficient electrocatalytic water oxidation

Electrochemical water splitting requires efficient water oxidation catalysts to accelerate the sluggish kinetics of water oxidation reaction. Here, we report a promisingly dendritic core-shell nickel-iron-copper metal/metal oxide electrode, prepared via dealloying with an electrodeposited nickel-iron-copper alloy as a precursor, as the catalyst for water oxidation. The as-prepared core-shell nickel-iron-copper electrode is characterized with porous oxide shells and metallic cores. This tri-metal-based core-shell nickel-iron-copper electrode exhibits a remarkable activity toward water oxidation in alkaline medium with an overpotential of only 180 mV at a current density of 10 mA cm-2. The core-shell NiFeCu electrode exhibits pH-dependent oxygen evolution reaction activity on the reversible hydrogen electrode scale, suggesting that non-concerted proton-electron transfers participate in catalyzing the oxygen evolution reaction. To the best of our knowledge, the as-fabricated core-shell nickel-iron-copper is one of the most promising oxygen evolution catalysts.

Enhancing the free corrosion dealloying rate with a catalytically driven reaction

Despite its high popularity, chemical dealloying that is widely used for the fabrication of nanoporous metals is a relatively slow process: dealloying a few milligrams of bulk material may take from several hours up to a few days, depending on the material system. Raising the temperature of the corroding medium is a common approach to speed up the dealloying process. However, high temperatures cause undesired ligament growth in dealloyed materials. Here we report for the first time the use of a catalytically driven reaction to speed up the dealloying process at ambient temperature and pressure. To demonstrate the concept, we show that the free corrosion dealloying of a silver-aluminum alloy is significantly faster with the help of a platinum catalyst. More importantly, the corresponding characteristic nanostructured size is much smaller than that without a catalyst. Our finding is expected to play a central role in scaling up the dealloying process from the laboratory to the industrial scale.

Semiordered Hierarchical Metallic Network for Fast and Large Charge-Induced Strain

Nanoporous metallic actuators for artificial muscle applications are distinguished by combining the low operating voltage, which is otherwise reserved for polymer-based actuators with interesting values of strain amplitude, strength, and stiffness that are comparable of those of piezoceramics. We report a nanoporous metal actuator with enhanced strain amplitude and accelerated switching. Our 3D macroscopic metallic muscle has semiordered and hierarchical nanoporous structure, in which μm-sized tubes align perpendicular with the sample surface, while nm-sized ligaments consist of the tube walls. This nanoarchitecture combines channels for fast ion transportation with large surface area for charge storage and strain generation. The result is a record reversible strain amplitude of 1.59% with a strain rate of 8.83 × 10^-6 s^-1 in the field of metallic based actuators. A passive hydroxide layer is self-grown on the metal surface, which not only contributes a supercapacitive layer, but also stabilizes the nanoporous structure against coarsening, which guarantees sustainable actuation beyond ten-thousand cycles.

Using X-ray Microscopy To Understand How Nanoporous Materials Can Be Used To Reduce the Large Volume Change in Alloy Anodes

Tin metal is an attractive negative electrode material to replace graphite in Li-ion batteries due to its high energy density. However, tin undergoes a large volume change upon alloying with Li, which pulverizes the particles, and ultimately leads to short cycling lifetimes. Nevertheless, nanoporous materials have been shown to extend battery life well past what is observed in nonporous material. Despite the exciting potential of porous alloying anodes to significantly increase the energy density in Li-ion batteries, the fundamental physics of how nanoscale architectures accommodate the electrochemically induced volume changes are poorly understood. Here, operando transmission X-ray microscopy has been used to develop an understanding of the mechanisms that govern the enhanced cycling stability in nanoporous tin. We found that in comparison to dense tin, nanoporous tin undergoes a 6-fold smaller areal expansion after lithiation, as a result of the internal porosity and unique nanoscale architecture. The expansion is also more gradual in nanoporous tin compared to the dense material. The nanoscale resolution of the microscope used in this study is ∼30 nm, which allowed us to directly observe the pore structure during lithiation and delithiation. We found that nanoporous tin remains porous during the first insertion and desinsertion cycle. This observation is key, as fully closed pores could lead to mechanical instability, electrolyte inaccessibility, and short lifetimes. While tin was chosen for this study because of its high X-ray contrast, the results of this work should be general to other alloy-type systems, such as Si, that also suffer from volume change based cycling degradation.

Nanoporous Tin with a Granular Hierarchical Ligament Morphology as a Highly Stable Li-Ion Battery Anode

Next generation Li-ion batteries will require negative electrode materials with energy densities many-fold higher than that found in the graphitic carbon currently used in commercial Li-ion batteries. While various nanostructured alloying-type anode materials may satisfy that requirement, such materials do not always exhibit long cycle lifetimes and/or their processing routes are not always suitable for large-scale synthesis. Here, we report on a high-performance anode material for next generation Li-ion batteries made of nanoporous Sn powders with hierarchical ligament morphology. This material system combines both long cycle lifetimes (more than 72% capacity retention after 350 cycles), high capacity (693 mAh/g, nearly twice that of commercial graphitic carbon), good charging/discharging capabilities (545 mAh/g at 1 A/g, 1.5C), and a scalable processing route that involves selective alloy corrosion. The good cycling performance of this system is attributed to its nanoporous architecture and its unique hierarchical ligament morphology, which accommodates the large volume changes taking place during lithiation, as confirmed by synchrotron-based ex-situ X-ray 3D tomography analysis. Our findings are an important step for the development of high-performance Li-ion batteries.

Anomalous low strain induced by surface charge in nanoporous gold with low relative density

The surface charge induced macroscopic strain decreases dramatically with decreasing relative density of NPG, in contrast to the theoretical prediction.

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.

Converting Water Adsorption and Capillary Condensation in Usable Forces with Simple Porous Inorganic Thin Films

This work reports an innovative humidity-driven actuation concept based on conversion of chemical energy of adsorption/desorption using simple nanoporous sol-gel silica thin films as humidity-responsive materials. Bilayer-shaped actuators, consisting of a humidity-sensitive active nanostructured silica film deposited on a polymeric substrate (Kapton), were demonstrated as an original mean to convert water molecule adsorption and capillary condensation in usable mechanical work. Reversible solvation stress changes in silica micropores by water adsorption and energy produced by the rigid silica film contraction, induced by water capillary condensation in mesopores, were finely controlled and used as energy sources. The influence of the film nanostructure (microporosity, mesoporosity) and thickness and the polymeric substrate thickness on actuation force, on movement speed and on displacement amplitude are clearly evidenced and discussed. We show that the global mechanical response of such silica-based actuators can easily be adjusted to fabricate tailor-made actuation systems triggered by humidity variation. This study provides insight into hard ceramic stimulus-responsive materials that seem to be a promising alternative to traditional soft organic materials for surface-chemistry-driven actuation systems.

Nanoporous Gold for Enzyme Immobilization

Nanoporous gold (NPG) is a material of emerging interest for immobilization of biomolecules, especially enzymes. The material provides a high surface area form of gold that is suitable for physisorption or for covalent modification by self-assembled monolayers. The material can be used as a high surface area electrode and with immobilized enzymes can be used for amperometric detection schemes. NPG can be prepared in a variety of formats from alloys containing between 20 and 50 % atomic composition of gold and less noble element(s) by dealloying procedures. Materials resembling NPG can be prepared by hydrothermal and electrodeposition methods. Related high surface area gold structures have been prepared using templating approaches. Covalent enzyme immobilization can be achieved by first forming a self-assembled monolayer on NPG bearing a terminal reactive functional group followed by conjugation to the enzyme through amide linkages to lysine residues. Enzymes can also be entrapped by physisorption or immobilized by electrostatic interactions.

Fast and Reversible Actuation of Metallic Muscles Composed of Nickel Nanowire-Forest

Surface-charge-induced reversible and millimeter-scale deflection is found in a bilayered Ni cantilever upon cyclic potential triggering. The nanowire-forest structure, in which unidirectional primary nanowires are evenly separated by cross-linking subnanowires, ensures fast ion transport leading to a record-high strain response time ≈0.1 s. The actuation is sustainable beyond 800 cycles; the strain energy is compatible with human skeletal muscles.

Weakened Flexural Strength of Nanocrystalline Nanoporous Gold by Grain Refinement

High density of grain boundaries in solid materials generally leads to high strength because grain boundaries act as strong obstacles to dislocation activity. We find that the flexural strength of nanoporous gold of grain size 206 nm is 33.6% lower than that of grain size 238 μm. We prepared three gold-silver precursor alloys, well-annealed, prestrained, and high-energy ball-milled, from which nanoporous gold samples were obtained by the same free-corrosion dealloying process. Ligaments of the same size are formed regardless of precursor alloys, and microstructural aspects of precursor alloys such as crystallographic orientation and grain size is preserved in the dealloying process. While the nanoindentation hardness of three nanoporous golds is independent of microstructural variation, flexural strength of nanocrystalline nanoporous gold is significantly lower than that of nanoporous golds with much larger grain size. We investigate weakening mechanisms of grain boundaries in nanocrystalline nanoporous gold, leading to weakening of flexural strength.

Dealloying-driven nanoporous palladium with superior electrochemical actuation performance

Metal-hydrogen (in particular, Pd-H) interactions have been receiving considerable attention over the past 150 years within the scope of hydrogen storage, catalytic hydrogenation, hydrogen embrittlement and hydrogen-induced interfacial failure. Here, for the first time, we show that the coupling of hydrogen adsorption and absorption could trigger giant reversible strain in bulk nanoporous Pd (np-Pd) in a weakly adsorbed NaF electrolyte. The bulk np-Pd with a hierarchically porous structure and a ligament/channel size of ∼10 nm was fabricated using a dealloying strategy with compositional/structural design of the precursor. The np-Pd actuator exhibits a giant reversible strain of up to 3.28% (stroke of 137.8 μm), which is a 252% enhancement in comparison to the state-of-the-art value of 1.3% in np-AuPt. The strain rate (∼10(-5) s(-1)) of np-Pd is two orders of magnitude higher than that of current metallic actuators. Moreover, the volume-/mass-specific strain energy density (10.71 MJ m(-3)/3811 J kg(-1)) of np-Pd reaches the highest level compared with that of previously reported actuator materials. The outstanding actuation performance of np-Pd could be attributed to the coupling of hydrogen adsorption/absorption and its unique hierarchically nanoporous structure. Our findings provide valuable information for the design of novel high-performance metallic actuators.

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.

Mesoporous Ni60Fe30Mn10-alloy based metal/metal oxide composite thick films as highly active and robust oxygen evolution catalysts †

A major challenge in the field of water electrolysis is the scarcity of oxygen-evolving catalysts that are inexpensive, highly corrosion-resistant, suitable for large-scale applications and able to oxidize water at high current densities and low overpotentials. Most unsupported, non-precious metals oxygen-evolution catalysts require at least ~350 mV overpotential to oxidize water with a current density of 10 mA/cm2 in 1 M alkaline solution. Here we report on a robust nanostructured porous NiFe-based oxygen evolution catalyst made by selective alloy corrosion. In 1 M KOH, our material exhibits a catalytic activity towards water oxidation of 500 mA/cm2 at 360 mV overpotential and is stable for over eleven days. This exceptional performance is attributed to three factors. First, the small size of the ligaments and pores in our mesoporous catalyst (~10 nm) results in a high BET surface area (43 m2/g) and therefore a high density of oxygen-evolution catalytic sites per unit mass. Second, the open porosity facilitates effective mass transfer at the catalyst/electrolyte interface. Third and finally, the high bulk electrical conductivity of the mesoporous catalyst allows for effective current flow through the electrocatalyst, making it possible to use thick films with a high density of active sites and ~3×104 cm2 of catalytic area per cm2 of electrode area. Our mesoporous catalyst is thus attractive for alkaline electrolyzers where water-based solutions are decomposed into hydrogen and oxygen as the only products, driven either conventionally or by photovoltaics.

Formation of Nanoporous Gold Studied by Transmission Electron Backscatter Diffraction

Transmission electron backscatter diffraction (t-EBSD) was used to investigate the effect of dealloying on the microstructure of 140-nm thin gold foils. Statistical and local comparisons of the microstructure between the nonetched and nanoporous gold foils were made. Analyses of crystallographic texture, misorientation distribution, and grain structure clearly prove that during the dealloying manufacturing process of nanoporous materials the crystallographic texture is enhanced significantly with a clear decrease of internal strain, whereas maintaining the grain structure.

Dealloying of Cu-Based Metallic Glasses in Acidic Solutions: Products and Energy Storage Applications

Dealloying, a famous ancient etching technique, was used to produce nanoporous metals decades ago. With the development of dealloying techniques and theories, various interesting dealloying products including nanoporous metals/alloys, metal oxides and composites, which exhibit excellent catalytic, optical and sensing performance, have been developed in recent years. As a result, the research on dealloying products is of great importance for developing new materials with superior physical and chemical properties. In this paper, typical dealloying products from Cu-based metallic glasses after dealloying in hydrofluoric acid and hydrochloric acid solutions are summarized. Several potential application fields of these dealloying products are discussed. A promising application of nanoporous Cu (NPC) and NPC-contained composites related to the energy storage field is introduced. It is expected that more promising dealloying products could be developed for practical energy storage applications.

Reversible electrochemical actuation of metallic nanohoneycombs induced by pseudocapacitive redox processes

Current metallic-based electrochemical actuators are limited to nanoporous gold/platinum with randomly distributed pores, where the charge-induced reversible strain is mainly due to the nonfaradic charging/discharging processes along the capacitive electrochemical double layer. Here, we report an electrochemical actuating property of nanohoneycomb-structured nickel, with the actuation mechanism mainly due to a pseudocapacitive behavior by means of reversible faradic redox reactions. By using a dual-template synthesis method, a bilayered cantilever, comprising a nanohoneycomb layer backed by a solid layer of the same metal, was fabricated. Reversible bending of the cantilever upon cyclic potential triggering was observed. The strain of the cantilever increases nonlinearly with both potential and charge due to redox reactions. The maximum strain that can be achieved under a certain scan rate complies with a linear relationship with the capacity. Benefiting from the stable Ni(II)/Ni(III) redox couples at the electrode surface, the reversible actuation is very stable in hydroxide solutions.

Unusual dealloying effect in gold/copper alloy thin films: the role of defects and column boundaries in the formation of nanoporous gold

Understanding the dealloying mechanisms of gold-based alloy thin films resulting in the formation of nanoporous gold with a sponge-like structure is essential for the future design and integration of this novel class of material in practical devices. Here we report on the synthesis of nanoporous gold thin films using a free-corrosion approach in nitric acid applied to cosputtered Au-Cu thin films. A relationship is established between the as-grown Au-Cu film characteristics (i.e., composition, morphology, and structure) and the porosity of the sponge-like gold thin films. We further demonstrate that the dealloying approach can be applied to nonhomogenous Au-Cu alloy thin films consisting of periodic and alternate Au-rich/Au-poor nanolayers. In such a case, however, the dealloying process is found to be altered and unusual etching stages arise. Thanks to defects and column boundaries playing the role of channels, the nitric acid is found to quickly penetrate within the films and then laterally (i.e., parallel to the film surface) attacks the nanolayers rather than perpendicularly. As a consequence to this anisotropic etching, the Au-poor layers are etched preferentially and transform into Au pillars holding the Au-rich layers and preventing them against collapsing. A further exposure to nitric acid results in the collapsing of the Au-rich layers accompanied by a transition from a multilayered to a sponge-like structure. A scenario, supported by experimental observations, is further proposed to provide a detailed explanation of the fundamental mechanisms occurring during the dealloying process of films with a multilayered structure.

Gold microsheets having nano/microporous structures fabricated by ultrasonic-assisted cyclic galvanic replacement

Ultrasonic-assisted cyclic galvanic replacement is proposed to produce nano/microporous gold microsheets that can potentially be used as surface-enhanced Raman scattering (SERS) substrates.

Electrical conduction of nanoparticle monolayer for accurate tracking of mechanical stimulus in finger touch sensing

A flexible strain gauge is an essential component in advanced human-machine interfacing, especially when it comes to many important mobile and biomedical appliances that require the detection of finger touches. In this paper, we report one such strain gauge made from a strip of nanoparticle monolayer onto a flexible substrate. This proposed gauge operates on the observation that there is a linear relationship between electrical conduction and mechanical displacement in a compressive state. Due to its prompt temporal response, the gauge can accurately track various mechanical stimuli running at the frequencies of interest. Experiments have confirmed that the proposed strain gauge has a strain detection limit as low as 9.4 × 10(-5), and its gauge factor can be as large as 70, making this device particularly suitable for sensitive finger touch sensing. Furthermore, negligible degradation in the gauge's output electrical signal is observed even after 9000 loading/unloading cycles.

Electrocapillary coupling during electrosorption

The electrocapillary coupling coefficient, ς, measures the response of the electrode potential, E, to tangential elastic strain at the surface of an electrode. Using dynamic electro-chemo-mechanical analysis, we study ς(E) simultaneously with cyclic voltammetry. The study covers extended potential intervals on Au, Pt, and Pd, including the electrosorption of oxygen species and of hydrogen. The magnitude and sign of ς vary during the scans, and quite generally the graphs of ς(E) emphasize details which are less obvious or missing in the cyclic voltammograms (CVs). Capacitive processes on the clean electrode surfaces exhibit ς < 0, whereas capacitive processes on oxygen-covered surfaces are characterized by ς < 0 on Au but ς > 0 on Pt and Pd. The findings of ς < 0 during the initial stages of oxygen species adsorption and ς > 0 for hydrogen electrosorption agree with the trend that tensile strain makes surfaces more binding for adsorbates. However, the large hysteresis of oxygen electrosorption on all electrodes raises the question: is the exchange current associated with that process sufficient for its measurement by potential response during small cyclic strain?

Characterization of nanoporous gold disks for photothermal light harvesting and light-gated molecular release

Nanoporous gold disks (NPGDs) with 400 nm diameter, 75 nm thickness, and 13 nm pores exhibit large specific surface area and effective photothermal light harvesting capability with a conversion efficiency of 56%. A potential application is demonstrated by light-gated, multi-step molecular release of the pre-adsorbed R6G fluorescent dye on arrayed NPGDs.

Microporous Co@CoO nanoparticles with superior microwave absorption properties

Nanoporous metal materials with many potential applications have been synthesized by a chemical dealloying approach. The fabrication of nanoporous metal nanoparticles (NPs), however, is still challenging due to the difficulties in producing suitable nanoscale precursors. Herein, nanoporous Co NPs of 31 nm have been successfully prepared by dealloying Co-Al NPs, and surprisingly they possess micropores in a range from 0.7 to 1.7 nm and a large surface area of 50 m(2) g(-1). The crystalline size of the microporous NPs is 2-5 nm. Through the passivation process, the microporous Co NPs covered with CoO (Co@CoO) are generated as a result of the surface oxidation of Co. They exhibit better microwave absorption properties than their nonporous counterpart. An enhanced reflection loss (RL) value of -90.2 dB is obtained for the microporous Co@CoO NPs with a thickness of merely 1.3 mm. The absorption bandwidth corresponding to the RL below -10 dB reaches 7.2 GHz. The microwave absorption mechanism is discussed in terms of micropore morphology, core@shell structure and nanostructure. This novel microporous material may open new routes for designing high performance microwave absorbers.

Sign-inverted response of aluminum work function to tangential strain

We have investigated the response of the work function, W, of low-index aluminum surfaces to tangential strain by using first-principles calculations based on density functional theory. This response parameter is a central quantity in electrocapillary coupling of metal electrodes relating to the performance of porous metal actuators and surface stress based sensing devices. We find that Al surfaces exhibit a positive response for all orientations considered. By contrast, previous studies reported negative-valued response parameters for clean surfaces of several transition metals. We discuss separately the response of W to different types of strain and the impact of the strain on the Fermi energy and the surface dipole. We argue that the reason for the abnormal positive sign of the Al response parameter lies in its high valence electron density.

Metallic muscles at work: high rate actuation in nanoporous gold/polyaniline composites

Metallic muscles made of nanoporous metals suffer from serious drawbacks caused by the usage of an aqueous electrolyte for actuation. An aqueous electrolyte prohibits metallic muscles from operating in dry environments and hampers a high actuation rate due to the low ionic conductivity of electrolytes. In addition, redox reactions involved in electrochemical actuation severely coarsen the ligaments of nanoporous metals, leading to a substantial loss in performance of the actuator. Here we present an electrolyte-free approach to put metallic muscles to work via a metal/polymer interface. A nanocoating of polyaniline doped with sulfuric acid was grown onto the ligaments of nanoporous gold. Dopant sulfate anions coadsorbed into the polymer coating matrix were exploited to tune the nanoporous metal surface stress and subsequently generate macroscopic dimensional changes in the metal. Strain rates achieved in the single-component nanoporous metal/polymer composite actuator are 3 orders of magnitude higher than that of the standard three-component nanoporous metal/electrolyte hybrid actuator.

Nanoporous Gold Electrodes and Their Applications in Analytical Chemistry

Nanoporous gold prepared by dealloying Au:Ag alloys has recently become an attractive material in the field of analytical chemistry. This conductive material has an open, 3D porous framework consisting of nanosized pores and ligaments with surface areas that are 10s to 100s of times larger than planar gold of an equivalent geometric area. The high surface area coupled with an open pore network makes nanoporous gold an ideal support for the development of chemical sensors. Important attributes include conductivity, high surface area, ease of preparation and modification, tunable pore size, and a bicontinuous open pore network. In this paper, the fabrication, characterization, and applications of nanoporous gold in chemical sensing are reviewed specifically as they relate to the development of immunosensors, enzyme-based biosensors, DNA sensors, Raman sensors, and small molecule sensors.