Abstract
AbstractThe electrochemical behavior of nanoporous gold (NPG) obtained by dealloying a AgAu alloy has been investigated by means of cyclic voltammetry (CV) in 0.1 M H2SO4 and 0.1 M KOH solutions supplemented by X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) in order to understand different effects of the electrochemical treatment on the development of the surface structure of NPG. In order to reduce the IR drop caused by the high surface area of the bicontinuous network of pores and ligaments in NPG, NPG was transformed to a powder, from which a small portion was filled into a cavity microelectrode (CME). Additionally, this avoided sample‐to‐sample variation from the dealloying process because many fillings could be made from one NPG monolith. The cycling in 0.1 M H2SO4 led to restructuring of the surface to a more faceted one, only after the residual silver on the surface had been removed in the initial scan. The same cycling program in 0.1 M KOH did not cause restructuring. However, a transfer of the sample to 0.1 M H2SO4 could start the process. The ligament size did not change during restructuring. Additionally, it was found that residual Ag in NPG stabilizes the highly curved surfaces of the ligaments containing a high density of surface defects. The dissolution of the residual Ag in acid electrolytes lifts the blockage towards surface restructuring. These findings form a basis for understanding the electrochemical behavior of NPG and to devise appropriate treatments, for instance for their use in electrocatalysis.
Highlights
The adjustable sizes of pores and strutsrange between ten and several hundred nanometers and support effective mass transport in liquid phase
HClO4, during which the residual Ag mole fraction xAg in the bulk of the ligaments is decreased to ca. 1 % as measured by energy-dispersive X-ray spectroscopy (EDX) while at the same time the ligament diameter increases to about 40 nm
After 2–6 weeks storage in air, the nanoporous gold (NPG) monoliths are transformed to a powder
Summary
Range between ten and several hundred nanometers and support effective mass transport in liquid phase. The change of feature sizes is enabled by (surface) diffusion of the metal atoms leading to a state of lower surface energy Those transport processes occur as long as the activation energy for diffusion is provided[11,12] and cause a phenomenon called coarsening, because the overall shape of the structural elements is mainly preserved while the diameters of pores and ligaments grow. The study of Wang et al.[23] appears to be in contradiction to the findings for other gold electrodes, where cycling into the potential regions of Au surface oxidation increased roughness of single crystal electrodes as evidenced by scanning tunneling microscopy.[24] More recently, Ahrens et al.[25] demonstrated that the signature of surface roughness could clearly be detected even for commonly used flat polycrystalline Au electrodes This process could be linked to the growth of Au nanoparticles on the Au surface. These are mainly those grains in direct contact with the side wall or bottom of the cavity and those that form the outer surface as evident from further images in the Supporting Information (SI),
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