Nanoporous metals (NPMs) made by dealloying represent a class of functional materials with the unique structural properties of mechanical rigidity, electrical conductivity, and high corrosion resistance. They also possess a porous network structure with feature dimensions tunable within a wide range from a few nanometers to several microns. Coupled with a rich surface chemistry for further functionalization, NPMs have great potential for applications in heterogeneous catalysis, electrocatalysis, fuel cell technologies, biomolecular sensing, surface-enhanced Raman scattering (SERS), and plasmonics. This article summarizes recent advances in some of these areas and, in particular, we focus on the discussion of microstructure, catalytic, and optical properties of nanoporous gold (NPG). With advanced electron microscopy, three-dimensional tomographic reconstructions of NPG have been realized that yield quantitative characterizations of key morphological parameters involved in the intricate structure. Catalytic and electrocatalytic investigations demonstrate that bare NPG is already catalytically active for many important reactions such as CO and glucose oxidation. Surface functionalization with other metals, such as Pt, produces very efficient electrocatalysts, which have been used as promising fuel cell electrode materials with very low precious metal loading. Additionally, NPG and related materials possess outstanding optical properties in plasmonics and SERS. They hold promise to act as highly active, stable, and economically affordable substrates in high-performance instrumentation applications for chemical inspection and biomolecular diagnostics. Finally, we conclude with some perspectives that appear to warrant future investigation.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0883769400003407

Nanoporous Metals for Catalytic and Optical Applications

Nanoporous Pt-Co alloy nanowires were synthesized by electrodeposition of Co-rich Pt(1)Co(99) alloy into anodic aluminum oxide (AAO) membranes, followed by a dealloying treatment in a mild acidic medium. These nanowires consist of porous skeletons with tiny pores of 1-5 nm and crystalline ligaments of 2-8 nm. Morphological and compositional evolutions of the porous Pt-Co nanowires upon dealloying were investigated, and their formation mechanism is discussed. The nanoporous Pt-Co alloy nanowires are found to exhibit distinctly enhanced electrocatalytic activities toward methanol oxidation as compared to the current state-of-the-art Pt/C and PtCo/C catalysts, thus showing substantial promise as efficient anode electrocatalysts in direct methanol fuel cells.

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Nanoporous Pt−Co Alloy Nanowires: Fabrication, Characterization, and Electrocatalytic Properties

Nanostructured materials with designable microstructure and controllable physical and chemical properties are highly desired for practical applications in nanotechnology. In this article, it is reported that nanoporous copper with a tunable nanopore size can be fabricated by controlling the dealloying process. The influence of acid concentration and etching potential on the formation of nanoprosity is systematically investigated. With optimal etching conditions, the nanopore sizes can be tailored from ∼15 to ∼120 nm by controlling the dealloying time. It is found that the tunable nanoporosity leads to significant improvements in surface-enhanced Raman scattering (SERS) of nanoporous copper and peak values of SERS enhancements for both rhodamine 6G and crystal violet 10B molecules are observed at a pore size of ∼30–50 nm. This study underscores the effect of complex three-dimensional nanostructures on physical and chemical properties and is helpful in developing inexpensive SERS substrates for sensitive instrumentations in molecular diagnostics.

Nanoporous Copper with Tunable Nanoporosity for SERS Applications

A nanoporous PdNi (np-PdNi) bimetallic catalyst fabricated by electrochemically dealloying a Pd20Ni80 alloy in an acid solution is reported. Residual Ni in the nanoporous alloy can be controlled by tuning dealloying potentials and the electrocatalysis of the np-PdNi shows evident dependence on Ni concentrations. With ∼9 at.% Ni, the np-PdNi bimetallic catalyst presents superior electrocatalytic performances in methanol and formic acid electro-oxidation as well as oxygen reduction in comparison with commercial Pd/C and nanoporous Pd (np-Pd). The excellent electrocatalytic properties of the dealloyed np-PdNi bimetallic catalyst appear to arise from the combined effect of unique bicontinuous nanoporosity and bimetallic synergistic action.

Nanoporous PdNi Bimetallic Catalyst with Enhanced Electrocatalytic Performances for Electro-oxidation and Oxygen Reduction Reactions

Stronger than steels but able to be shaped and molded like plastics, bulk metallic glasses are the quintessential engineering materials.

Bulk Metallic Glasses

We report the fabrication of bimodal nanoporous palladium with pore sizes of ∼50 and 5 nm by electrochemically dealloying a ternary Pd30Ni50P20 metallic glass. The nanostructured palladium is expected to have superior performances in functional applications because of its nanoporous channels for fast mass transport and chemical reactions. This study paves a new way for fabricating novel nanoporous metals by electrochemically dealloying multicomponent metallic glasses.

Nanoporous metals by dealloying multicomponent metallic glasses

PROBLEM TO BE SOLVED: To solve the problem that a nano-size structure such as a nano-wire and a nano-tube is indispensable for a technology constructing a nano-scale device, and a metal nano-structure in an amorphous state is required from theoretical research. SOLUTION: In a careful examination on the destructed face of a metal glass produced by a compression test, the fact that a nano-size structure of a metal glass such as a metal glass nano-wire and a metal glass nano-tube is formed has been found. Even in the nano-scale structure, regarding a bulk metal glass, the fact that the structure of an amorphous state is held has been confirmed. A metal glass nano-wire, a metal glass nano-tube or the like in which the excellent characteristics of a metal glass are made the most of can be provided, and can be applied to various fields in an electrode material, a motor material, a nano-electronics material, a nano-medical device, a nano-sensor, an optical material or the like. COPYRIGHT: (C)2010,JPO&INPIT

Toshio Sakurai

Papers

Nanoporous metals by dealloying multicomponent metallic glasses

Nano-size metal glass structure