Thermodynamic and Photoelectrochemical Behavior of the n ‐ TiO2 Electrode in Fluoride‐Containing Solutions

Abstract

Diagrams of the stable and metastable electrochemical equilibria for the system at 25°C have been constructed. Increased solubility of titanium dioxide in fluoride‐containing solutions has been found to occur over a narrow range of solution pH near 3.2. On the basis of the thermodynamic diagrams, appropriate conditions for etching semiconductor electrodes were chosen. The etching solutions prepared for the semiconductor electrodes showed very good surface‐cleansing properties. A photoetching process under anodic bias is recommended for the preparation of the surface for the purpose of use in solar energy converters. The doping profile in the space‐charge layer which forms during the photoetching treatment is more effective for splitting the electron‐hole pair created in a working electrochemical solar cell. Photocurrent‐voltage characteristics obtained for electrodes in F− solutions do not fit the Butler‐Gärtner equation, since the Butler‐Gärtner equation does not correctly describe photoeffects on medium and heavily doped semiconductor electrodes. From the chronovoltamperometric and coulometric study of the reactions taking place at electrode potentials negative of the flatband potential, only the first step of the reduction to lower oxides is reversible.