Introduction to Nanoporous Metals

Abstract

In this chapter, we briefly give an introduction to nanoporous metals (NPMs). First, we show “what are NPMs.” The definition of NPMs is given, considering the characteristic length scale and porous structure. NPMs are such a kind of metallic materials with interconnected backbones (ligaments) and pores (channels) on the nanoscale. Here, the term “nanoporous” is different from “mesoporous,” which is defined by the International Union of Pure and Applied Chemistry (IUPAC). Moreover, the length scale of nanopores (several to hundreds of nanometers) in NPMs is several orders of magnitude smaller than that (above tens of microns) of pores in normal metal foams. The pore distribution in NPMs could be ordered, or random, or the combination of the former two. Many methods could be used to fabricate NPMs, and dealloying is the most important one. Second, the microstructural characteristics of NPMs are outlined. Besides the prototype nanoporous gold (NPG), many pure elements (transition metals, elements from IIIA-VA groups, and even semiconductor elements) and alloys could be fabricated into a nanoporous structure. Both bulk (up to centimeters) and nanosized (zero-dimensional (0D), 1D, and 2D) NPMs have been reported. Metallic ligaments and nanopore channels in dealloying-driven NPMs are topologically and morphologically equivalent, i.e., they are inverses of each other in three-dimensional space. The microstructure of NPMs may be homogeneous, and NPMs with multiscale or multilevel porous structures can also be prepared. In addition, the crystalline orientation and lattice defects of NPMs depend upon the microstructure of the precursor alloys and the dealloying process. Third, the properties of NPMs are summarized. Due to their unique microstructures, nanoporous metallic materials combine the properties of both metals and nanostructured materials. Thus NPMs show the structure-related electrical, magnetic, mechanical, optical, catalytic, and electrocatalytic properties. Moreover, the microstructures and the related properties of NPMs could be facilely designed and modulated. Last, we discuss the potential applications of NPMs. Owing to their unique microstructures and related properties, NPMs show promising applications in sensors, actuators, fuel cells, lithium-ion batteries (LIBs), supercapacitors, metal–air batteries, water splitting, synthesis of chemicals, hydrogen storage, automobile exhaust treatment, drug loading and release, bonding materials, and so forth.