Nanoporous metals

Abstract

In this chapter, we mainly describe the fabrication, properties, and potential applications of nanoporous metals (NPMs) with random porous structure. Nanoporous metals represent an interesting type of nanostructured material with nanosized porosity and ultrahigh specific surface area, and thus possess unique mechanical, physical, and chemical properties associated with their nanoporous structure. Based upon the porosity distribution, nanoporous metals can be classified into two categories: one has a random porous structure, and the other has a regular pore distribution. Nanoporous metals with random porous structure can be synthesized by the dealloying strategy, whereas template methods are normally used to fabricate nanoporous metals with more regular pore distribution. Nanoporous metals date back to the days of Raney (1920s) when high specific surface metal catalysts were prepared by dealloying Al-based alloys in alkaline solutions. In the new century, monolithic nanoporous metals received renewed attention due to the observation of a series of very intriguing structural properties. Nanoporous metals made by dealloying exhibit a three-dimensional bicontinuous interpenetrating ligament (metal)–channel (void) structure with a length scale of several nanometers to hundreds of nanometers, and the characteristic size can be modulated to as large as several microns by treatments such as thermal annealing. In contrast, the template technique can precisely control the pore size and microstructure of nanoporous metals, but dynamic modulation of the dominant length scale is virtually impossible. In addition, nanoporous metals are different from metallic foams, which have a length scale of several microns to more than 1 cm, and are normally used as damping and acoustic materials. Here, we mainly focus on dealloyed nanoporous metals. Firstly, the dealloying method and formation mechanism of nanoporous metals are reviewed based upon previous experimental observations and computer simulation. Secondly, we summarize recent knowledge on microstructures of nanoporous metals and their unique properties (catalytic, electrocatalytic, mechanical, sensing, optical, etc.). Finally, potential applications of nanoporous metals are discussed in the fields of fuel cells, catalysis, sensors, actuators, etc.