Mechanistic insight into photochemical and photoelectrochemical degradation of organic pollutants with the use of BiVO4 and BiVO4/Co-Pi

In this work we clarify the role of hydroxyl radicals (•OH), superoxide anion radicals (O2•−) and the holes in photocatalytic and photoelectrocatalytic degradation of organic pollutants on BiVO4, and critically discuss different mechanisms of the process reported in the literature. The generation of O2•−anion radicals was monitored by the reaction with Nitro Blue Tetrazolium chloride (NBT), while the formation of •OH was controlled by transformation of terephthalic acid into fluorescent 2-hydroxyterephtalic acid. The band diagram of BiVO4 was constructed on the base of the Efb determined from the measurements of open circuit potential (OCP) and photocurrent onset potential, and the mechanism of photodegradation of caffeine (CAF) on BiVO4 was proposed. It was shown that the photodegradation rate constant of CAF may be increased from 2.1·10−3 to 4.6 10−3 min−1 by modification of BiVO4 surface with cobalt phosphate (Co-Pi) co-catalyst. The presence of Co-Pi resulted in the increase of the electron life-time and suppression of the electron-hole recombination in BiVO4. The 26-fold increase of the degradation rate constant, to the value 5.5·10−2 min−1, was achieved by application of an external potential of 0.6 V vs Ag/AgCl to the BiVO4 electrode and the mechanism of photoelectrocatalytic process was proposed.