SnO2(Sb) electrodes: Morphological changes due to polarizations in the HERand OER potential regions studied by in situ STM and ex situ AFM imaging

The effect of electrochemical perturbation on the microstructure of transpa rent Sb-doped SnO2 electrode in 0.1 M NaClO4, pH 2, was investigated by in situ scanning tunneling microscopy (STM), ex situ atomic force microscopy ( AFM), scanning electron microscopy (SEM),;md other techniques of microstruc ture analysis. The transparent film of SnO2(Sb) on silica prepared by spray pyrolysis has a uniform and smooth surface with a root-mean-square (rms) r oughness ranging from 1 to 2 nm. Its microstructure is constituted of coale sced particles with an average diameter and height of 30 and 6 nm, respecti vely. Anodic polarization at potentials near the oxygen evolution reaction region (OER) or potential cycling induces small alterations on the microstr ucture of the electrode, observable only in submicron range by AFM analysis . Cathodic polarization at potentials in the region of the hydrogen evoluti on reaction (HER), however, strongly damages the film. AFM and STM examinat ion revealed that the particle boundaries are attached and both vertical an d lateral dimensions of the particles decrease. From the observations, it c an be inferred that during the cathodic polarization SnO is formed, and des truction of the film occurs by dissolution of that more soluble oxide. In s itu STM experiments showed that at the initial stages the particles of the damaged film can acquire dimensions even larger than the initial size by an odic polarization. Finally, ii can be concluded that a smooth and compact f ilm with few defects is less susceptible to degradation. (C) 1999 The Elect rochemical Society. All rights reserved.