The effect of pH on the photochemical reactivity of BaTiO3

Abstract

When particulate photocatalysts promote a reaction, the overall reaction rate is limited by the constraint that the photoreduction and photooxidation reactions must occur on the surface of the same particle and at the same solution potential. The optimized solution potential would be the one where the photoanodic and photocathodic reactions proceed at the same maximum rate. Here, the relative photochemical reactivity of BaTiO3 was investigated as a function of the surrounding aqueous solution pH. Topographic atomic force microscopy (AFM) was used to quantify the amount of metallic silver produced from the photochemical reduction of Ag+ (Ag+→Ag) under UV light illumination. The overall reaction rate was observed to increase with pH. A similar pH-dependence existed for both high concentration (0.1 M) and low concentration (10−4 M) Ag+ solutions. When the pH is changed, the adsorbed charge at the surface is changed and this has the same effect as applying an external electric field. The observations are consistent with the idea that increasing the pH increases the net negative charge adsorption on the surface, increasing the upward band bending and promoting the oxidation half reaction, which is the rate-limiting factor for the overall reaction. As a result, the rate of Ag+ reduction increases with pH. The results show that adjusting the aqueous solution pH has the potential to control the rates of the two half reactions and increase the overall photochemical reaction rate.