Dark dissolution and photodissolution of ZnO

The dissolution of ZnO both in the dark and under illumination at 365 nm and the flat-band potential display a linear pH dependence which can be explained by a thermodynamic model based on the linear relationship between either the rate of the reaction or the current and the electrochemical reaction energy used in a proton attack followed by desorption of Zn²⁺ and OH^- from the charged oxide surface. The experimental data supporting this hypothesis are taken from the literature and from some new investigations of the photodissolution of ZnO which are described in this paper. The photocurrent was studied potentiostatically and its potential dependence was interpreted using Gärtner's theory for illuminated metal-semiconductor contacts. The linear relation between the photocurrent and the pH corresponds to a stationary state under constant illumination in which the current is equal to the hole oxidation rate of H₂O at pH 9.2 and of H₂O plus ZnO at pH less than 9.2. The decomposition of the ZnO lattice is thermodynamically controlled by the hole oxidation of desorbing OH^- ions to OH radicals which are assumed to form H₂O₂ by combination.