Hierarchical nanoporous metals as a path toward the ultimate three-dimensional functionality

Structure/Composition/Phase Regulations of Dealloying-Derived Nanoporous Metals and Their Solar Steam Generation Performances

Structural regulation is of primary importance in structure-property/application studies of dealloyed nanoporous metals. Three aspects are mainly considered to affect the microstructure of nanoporous metals: design of precursor alloy, choosing of dealloying parameter, and annealing treatment. Herein, through the combination of the above three strategies, the regulation of structure, composition and phase in nanoporous metals are simultaneously achieved. With a dilute Cu99Ag0.75Au0.25 as the precursor, three kinds of nanoporous films are fabricated, including bi-phase nanoporous Cu-Ag-Au (B-NP-CuAgAu), hierarchically nanoporous Au (H-NPG) and single-phase homogeneously nanoporous Au (S-NPG). In situ X-ray diffraction and ex situ characterizations are utilized to reveal the structure/composition/phase evolutions during dealloying of Cu99Ag0.75Au0.25, as well as the macroscopic changes of the dealloyed samples. Notably, the ultrafine ligaments/channels of B-NP-CuAgAu and the two-level nanoporous structure of H-NPG endow them with good broadband light absorption and excellent hydrophilicity, which contribute to their outstanding solar steam generation (SSG) performances. Specially, the B-NP-CuAgAu film shows a more efficient SSG performance with water evaporation rate of 1.49 kg m-2 h-1 and photothermal efficiency of 93.6% at 1 kW m-2, and good seawater desalination ability.

Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

Facile fabrication of hierarchically nanostructured gold electrode for bio-electrochemical applications

Nanoporous gold (NPG) is one of the most extensively investigated nanomaterials owing to its tunable pore size, ease of surface modification, and range of applications from catalysis, actuation, and molecular release to the development of electrochemical sensors. In an effort to improve the usefulness of NPG, a simple and robust method for the fabrication of hierarchical and bimodal nanoporous gold electrodes (hb-NPG) containing both macro-and mesopores is reported using electrochemical alloying and dealloying processes to engineer a bicontinuous solid/void morphology. Scanning electron microscopy (color SEM) images depict the hierarchical pore structure created after the multistep synthesis with an ensemble of tiny pores below 100 nm in size located in ligaments spanning larger pores of several hundred nanometers. Smaller-sized pores are exploited for surface modification, and the network of larger pores aids in molecular transport. Cyclic voltammetry (CV) was used to compare the electrochemically active surface area of the hierarchical bimodal structure with that of the regular unimodal NPG with an emphasis on the critical role of both dealloying and annealing in creating the desired structure. The adsorption of different proteins was followed using UV-vis absorbance measurements of solution depletion revealing the high loading capacity of hb-NPG. The surface coverage of lipoic acid on the hb-NPG was analyzed using thermogravimetric analysis (TGA) and reductive desorption. The roughness factor determinations suggest that the fabricated hb-NPG electrode has tremendous potential for biosensor development by changing the scaling relations between volume and surface area which may lead to improved analytical performance. We have chosen to take advantage of the surface architectures of hb-NPG due to the presence of a large specific surface area for functionalization and rapid transport pathways for faster response. It is shown that the hb-NPG electrode has a higher sensitivity for the amperometric detection of glucose than does an NPG electrode of the same geometric surface area.

Catalysis of Alloys: Classification, Principles, and Design for a Variety of Materials and Reactions

Alloying has long been used as a promising methodology to improve the catalytic performance of metallic materials. In recent years, the field of alloy catalysis has made remarkable progress with the emergence of a variety of novel alloy materials and their functions. Therefore, a comprehensive disciplinary framework for catalytic chemistry of alloys that provides a cross-sectional understanding of the broad research field is in high demand. In this review, we provide a comprehensive classification of various alloy materials based on metallurgy, thermodynamics, and inorganic chemistry and summarize the roles of alloying in catalysis and its principles with a brief introduction of the historical background of this research field. Furthermore, we explain how each type of alloy can be used as a catalyst material and how to design a functional catalyst for the target reaction by introducing representative case studies. This review includes two approaches, namely, from materials and reactions, to provide a better understanding of the catalytic chemistry of alloys. Our review offers a perspective on this research field and can be used encyclopedically according to the readers' individual interests.

Nanoporous ultra-high-entropy alloys containing fourteen elements for water splitting electrocatalysis

High-entropy alloys (HEAs) are near-equimolar alloys comprising five or more elements. In recent years, catalysis using HEAs has attracted considerable attention across various fields. Herein, we demonstrate the facile synthesis of nanoporous ultra-high-entropy alloys (np-UHEAs) with hierarchical porosity via dealloying. These np-UHEAs contain up to 14 elements, namely, Al, Ag, Au, Co, Cu, Fe, Ir, Mo, Ni, Pd, Pt, Rh, Ru, and Ti. Furthermore, they exhibit high catalytic activities and electrochemical stabilities in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acidic media, superior to that of commercial Pt/graphene and IrO₂ catalysts. Our results offer valuable insights for the selection of elements as catalysts for various applications.

Fabrication of Nanoporous Al by Vapor-Phase Dealloying: Morphology Features, Mechanical Properties and Model Predictions

The physical and chemical properties shown by nanoporous metals, related to their unique structure, make them very promising for application in several fields. Recently, vapor-phase dealloying has been reported as a method for the preparation of several non-noble nanoporous metals, alternatively to dealloying in aqueous solutions. Using this approach, we have successfully fabricated nanoporous Al starting from an Al20Zn80 nanocomposite obtained by ball milling. The nanocomposite was annealed at 550 °C under high-vacuum conditions, and the difference in the vapor pressures allowed the selective removal of Zn by vapor-phase dealloying. The morphology of the resulting nanoporous material was analyzed by Scanning Electron Microscopy showing pores from few to thousands of nm; moreover, the nanoporous 3D structure was observed through Serial Block Face-Scanning Electron Microscopy. A specific surface area as high as 73 m2 g−1 was estimated by N2 physisorption measurements. In addition, a fractal model able to well reproduce the morphology of nanoporous Al was built. This model has been used for predicting mechanical properties which are in good agreement with experimental data obtained by nanoindentation.

Nanoporous Metals: From Plasmonic Properties to Applications in Enhanced Spectroscopy and Photocatalysis

The field of plasmonics is capable of enabling interesting applications in different wavelength ranges, spanning from the ultraviolet up to the infrared. The choice of plasmonic material and how the material is nanostructured has significant implications for ultimate performance of any plasmonic device. Artificially designed nanoporous metals (NPMs) have interesting material properties including large specific surface area, distinctive optical properties, high electrical conductivity, and reduced stiffness, implying their potentials for many applications. This paper reviews the wide range of available nanoporous metals (such as Au, Ag, Cu, Al, Mg, and Pt), mainly focusing on their properties as plasmonic materials. While extensive reports on the use and characterization of NPMs exist, a detailed discussion on their connection with surface plasmons and enhanced spectroscopies as well as photocatalysis is missing. Here, we report on different metals investigated, from the most used nanoporous gold to mixed metal compounds, and discuss each of these plasmonic materials' suitability for a range of structural design and applications. Finally, we discuss the potentials and limitations of the traditional and alternative plasmonic materials for applications in enhanced spectroscopy and photocatalysis.

NiCo2O4 Nano-/Microstructures as High-Performance Biosensors: A Review

Non-enzymatic biosensors based on mixed transition metal oxides are deemed as the most promising devices due to their high sensitivity, selectivity, wide concentration range, low detection limits, and excellent recyclability. Spinel NiCo2O4 mixed oxides have drawn considerable attention recently due to their outstanding advantages including large specific surface area, high permeability, short electron, and ion diffusion pathways. Because of the rapid development of non-enzyme biosensors, the current state of methods for synthesis of pure and composite/hybrid NiCo2O4 materials and their subsequent electrochemical biosensing applications are systematically and comprehensively reviewed herein. Comparative analysis reveals better electrochemical sensing of bioanalytes by one-dimensional and two-dimensional NiCo2O4 nano-/microstructures than other morphologies. Better biosensing efficiency of NiCo2O4 as compared to corresponding individual metal oxides, viz. NiO and Co3O4, is attributed to the close intrinsic-state redox couples of Ni3+/Ni2+ (0.58 V/0.49 V) and Co3+/Co2+ (0.53 V/0.51 V). Biosensing performance of NiCo2O4 is also significantly improved by making the composites of NiCo2O4 with conducting carbonaceous materials like graphene, reduced graphene oxide, carbon nanotubes (single and multi-walled), carbon nanofibers; conducting polymers like polypyrrole (PPy), polyaniline (PANI); metal oxides NiO, Co3O4, SnO2, MnO2; and metals like Au, Pd, etc. Various factors affecting the morphologies and biosensing parameters of the nano-/micro-structured NiCo2O4 are also highlighted. Finally, some drawbacks and future perspectives related to this promising field are outlined.

A Dewetted-Dealloyed Nanoporous Pt Co-Catalyst Formed on TiO2 Nanotube Arrays Leads to Strongly Enhanced Photocatalytic H2 Production

Pt nanoparticles are typically decorated as co-catalyst on semiconductors to enhance the photocatalytic performance. Due to the low abundance and high cost of Pt, reaching a high activity with minimized co-catalyst loadings is a key challenge in the field. We explore a dewetting-dealloying strategy to fabricate on TiO2 nanotubes nanoporous Pt nanoparticles, aiming at improving the co-catalyst mass activity for H2 generation. For this, we sputter first Pt-Ni bi-layers of controllable thickness (nm range) on highly ordered TiO2 nanotube arrays, and then induce dewetting-alloying of the Pt-Ni bi-layers by a suitable annealing step in a reducing atmosphere: the thermal treatment causes the Pt and Ni films to agglomerate and at the same time mix with each other, forming on the TiO2 nanotube surface metal islands of a mixed PtNi composition. In a subsequent step we perform chemical dealloying of Ni that is selectively etched out from the bimetallic dewetted islands, leaving behind nanoporous Pt decorations. Under optimized conditions, the nanoporous Pt-decorated TiO2 structures show a>6 times higher photocatalytic H2 generation activity compared to structures modified with a comparable loading of dewetted, non-porous Pt. We ascribe this beneficial effect to the nanoporous nature of the dealloyed Pt co-catalyst, which provides an increased surface-to-volume ratio and thus a more efficient electron transfer and a higher density of active sites at the co-catalyst surface for H2 evolution.

Fabrication of Porous Gold Film Using Graphene Oxide as a Sacrificial Layer

An original and simple fabrication process to produce thin porous metal films on selected substrates is reported. The fabrication process includes the deposition of a thin layer of gold on a substrate, spin coating of a graphene oxide dispersion, etching the gold film through the graphene oxide layer, and removing the graphene oxide layer. The porosity of the thin gold film is controlled by varying the etching time, the thickness of the gold film, and the concentration of the graphene oxide dispersion. Images by scanning electron and metallurgical microscopes show a continuous gold film with random porosity formed on the substrate with a porosity size ranging between hundreds of nanometers to tens of micrometers. This general approach enables the fabrication of porous metal films using conventional microfabrication techniques. The proposed process is implemented to fabricate electrodes with patterned porosity that are used in a microfluidic system to manipulate living cells under dielectrophoresis. Porous electrodes are found to enhance the magnitude and spatial distribution of the dielectrophoretic force.

Synthesis and characterization of size controlled bimetallic nanosponges

Metallic and bimetallic nanosponges with well-defined size and form have attracted increasing attention due to their unique structural properties and their potential for many applications. In this chapter, the recently developed methods for the synthesis and preparation of metallic and bimetallic nanosponges are presented. These methods can be mainly cataloged in two groups: dealloying-based methods and reduction reaction-based methods. Different topographical reconstruction methods for the investigation of their structural properties are then reviewed briefly. The optical properties of the metallic nanosponges are clearly different from those of the solid counterparts due to the tailored disordered structure. The recent advances in the exploration of the distinct linear and non-linear optical properties of the nanosponges are summarized.

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Hierarchical Nanoporous Copper Architectures via 3D Printing Technique for Highly Efficient Catalysts

Nanoporous metals represent a class of functional materials with unique bicontinuous open porous structural properties, making them ideal candidates for various catalyst applications. However, the pursuit of nanoporous properties, extremely small pores, and high surface area, results in the restriction of mass transport. Herein, a free-standing hierarchical nanoporous Cu material, prepared by a selective laser melting 3D printing technique and a one-step dealloying process, is presented as a highly efficient electrocatalyst for methanol oxidation. It is demonstrated that the digitally controlled hierarchical structure with macro- and nano-scaled pores can be utilized for promoting and directing mass transport as well as for the enhancement of catalytic properties. This work highlights a facile, low-cost, and alternative strategy for hierarchical nanoporous structure design that can be applied to binary, ternary, and quaternary metal alloys for various functional applications.

Nanoporous Nickel Composite Catalyst for the Dry Reforming of Methane

The development of efficient catalysts with high activities and durabilities for use in the dry reforming of methane (DRM) is desirable but challenging. We report the development of a nanoporous nickel composite (nanoporous Ni/Y₂O₃) via a facile one-step dealloying technique, for use in the DRM. Focusing on the low-temperature DRM, our composite possessed remarkable activity and durability against coking compared with conventional particle-based Ni catalysts. This was attributed to the aluminum oxides present on the Ni surface, which suppress pore coarsening. In addition, the inert bundled Y₂O₃ nanowires are suitable for use as substrates for nanoporous Ni.

Multi-Scale Modeling for Predicting the Stiffness and Strength of Hollow-Structured Metal Foams with Structural Hierarchy

This work was inspired by previous experiments which managed to establish an optimal template-dealloying route to prepare ultralow density metal foams. In this study, we propose a new analytical–numerical model of hollow-structured metal foams with structural hierarchy to predict its stiffness and strength. The two-level model comprises a main backbone and a secondary nanoporous structure. The main backbone is composed of hollow sphere-packing architecture, while the secondary one is constructed of a bicontinuous nanoporous network proposed to describe the nanoscale interactions in the shell. Firstly, two nanoporous models with different geometries are generated by Voronoi tessellation, then the scaling laws of the mechanical properties are determined as a function of relative density by finite volume simulation. Furthermore, the scaling laws are applied to identify the uniaxial compression behavior of metal foams. It is shown that the thickness and relative density highly influence the Young’s modulus and yield strength, and vacancy defect determines the foams being self-supported. The present study provides not only new insights into the mechanical behaviors of both nanoporous metals and metal foams, but also a practical guide for their fabrication and application.