Formation and Microstructural Regulation of Nanoporous Metals

Abstract

In the past 15 years, dealloying has been growing into the most important method to fabricate nanoporous metals. In this chapter, we will address the dealloying-driven formation of nanoporous metals, the methods to characterize the microstructures of nanoporous metals, as well as the strategies to regulate the microstructures of nanoporous metals. Dealloying is a common corrosion process, during which the less noble element(s) is selectively etched away and the more noble element(s) is retained to form a nanoporous structure. First, we briefly discuss the history of dealloying, including “depletion gilding” at the time of pre-Columbian Central America and the early Middle Ages in European and Near Eastern, Raney® metals dealloyed from Al-based precursors in 1920s, and dealloying to form functionalized nanoporous metals at the beginning of this century. Additionally, in the most time of last century, people were concerned with dealloying mainly from the viewpoint of corrosion/protection. To understand the dealloying mechanisms is crucial to the design/fabrication of nanoporous metals. We then outline the related mechanisms being operated in the dealloying process. Since the 1960s, in situ/ex situ experiments and computer simulations have been performed to unveil the formation mechanism of nanoporous metals during dealloying, considering the selective dissolution of the less noble element(s), the surface diffusion of the more noble element(s), the critical potential, and the parting limit. The influence of anions (like halide ions) and the phase constitution should also be taken into consideration. Nanoporous metals exhibit a three-dimensional bicontinuous ligament (metal)-channel (void) structure. Many techniques can be used to characterize the microstructures of nanoporous metals, including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM, scanning TEM, scanning tunneling microscopy (STM), energy dispersive X-ray analysis, small angle neutron scattering (SANS), and so forth. Three-dimensional tomographic reconstruction is also popular to reveal the interior microstructures of nanoporous metals. In addition, the methods to evaluate the characteristic length scale and the specific surface area of nanoporous metals are also reviewed. In the last section of this chapter, we discuss how to regulate the microstructures/compositions/morphologies of nanoporous metals. First, we talk about the design of precursors for dealloying, considering the composition (elements), phase constitution, crystallinity, and microalloying. Second, the microstructural regulation of nanoporous metals can be achieved by controlling over the dealloying parameters, including chemical/electrochemical dealloying, the dealloying solution, temperature, the applied potential, the dealloying step (two-step or multistep), the effect of atmosphere, and dealloying in nonaqueous media. The post-dealloying treatment has also been briefly outlined. Third, we discuss the strategies which are often adopted to further modify nanoporous metals, based upon their potential applications.