Faceting and Nanostructuring of Gold Surfaces through Cathodic Corrosion: Role of Cationic Electrolyte Species and Water

Abstract

Gold is one of the most frequently employed electrode materials in electrochemistry due to its exceptional properties such as chemical inertness and high resistance to oxidation and dissolution [1-3]. According to Pourbaix diagrams, gold is regarded stable against degradation under cathodic polarization [4]. In contrast, applying relatively high negative potentials in the presence of alkali metal cations induce notable changes on Au electrode surfaces [5].In this study, cathodic corrosion of polycrystalline gold in different electrolytes of alkali metal hydroxides has been systemically investigated. In comparison to the recent work [5,6], this study provides new findings as a function of the applied parameters for the nanostructuring of gold surfaces. Cell design and the distance between the electrodes have carefully been controlled and optimized. The progression of surface restructuring and corrosion features was investigated using cyclic voltammetry (CV) and scanning electron microscopy (SEM). The results show different corrosion features depending on the nature and the concentration of alkali metal hydroxides, on the applied voltage, and on time. Current-potential curves for the modified electrodes in 0.1 M H2SO4 show characteristic features in the double-layer and oxidation regions. Analyzing the cyclic voltamogramms of the corroded electrodes provides evidence about the preferential orientation of facets and the electrochemical active surface area as a function of the applied parameters. The SEM micrographs show that polarizing the gold surfaces at relatively high negative potentials generates different size and orientation of octahedral nanocrystals using NaOH, spherical nanoparticles and triangular pits using KOH and triangular pits using LiOH. Besides the alkali metal cations, the electrolyte concentration and the applied voltage are important parameters. Furthermore, the role of water in cathodic corrosion of gold has been investigated using mixtures of different mole fraction of water and non-aqueous electrolytes. The absence of corrosion features using water-free non-aqueous electrolytes reveal that water plays a key role in the cathodic corrosion process.In addition, various aqueous mixed electrolytes have been systemically investigated. The results show combined corrosion features according to the mole fraction of different cations. For example, a 1:1 NaOH + LiOH mixture leads to the formation of rectangular pits which are attributed to (100)-facets compared to a 1:1 KOH + LiOH mixture which leads to triangular pits revealing the formation of (111)-facets.By applying alternating current (AC) to Au electrodes in contact with the same electrolytes, colloidal gold nanoparticles could successfully be synthesized. These nanoparticles have been used for the functionalization of water-soluble carbon nitride to enhance its activity for specific photocatalytic reactions such as alcohol oxidation.