pH-Dependent photocatalytic performance of faceted BiOBr semiconductor particles in degradation of dyes

Bismuth oxyhalides, such as bismuth-oxy-bromide (BiOBr), show high performance in photocatalytic oxidation of water pollutants. Process conditions, and in particular the pH of the solution, largely affect the photocatalytic efficacy, which is fundamentally poorly understood. We prepared {001} faceted bismuth-oxy-bromide (BiOBr), and determined by advanced AFM analysis that the surface charge of the {001} facet of BiOBr is slightly positive at acidic pH (pH 3) and significantly negative and increasing in negative surface charge in the order pH 6 < pH 9. Decomposition of MB or RhB by illumination of BiOBr is strongly favored by basic conditions (pH 9-10), as evident from discoloration experiments, and determination of the TOC as a function of time of illumination. Reference experiments show the pH dependent degradation profiles cannot be explained by differences in quantities of adsorption of the dye, but rather by the lifetime and/or quantity of BiOBr related photoexcited (surface) charge carriers (holes and electrons) – correlating with surface charge. Using MeOH and other scavengers, we show oxidation of MB or RhB is mainly induced by superoxide anions (•O2− radicals) at low pH and by holes or hydroxyl radicals at high pH – in agreement with surface charge induced band bending. This study provides novel understanding of the pH dependence of the rates of photocatalytic degradation of dyes using BiOBr photocatalysts.