Moderate valence band of bismuth oxyhalides (BiOXs, X = Cl, Br, I) for the best photocatalytic degradation efficiency of MC-LR

Abstract BiOXs (X = Cl, Br, I) were prepared and their abilities to degrade microcystin-LR (MC-LR) under UV light were compared. They degraded MC-LR in the order of BiOBr > BiOCl > BiOI and the oxygen consumption was in the order of BiOBr > BiOI > BiOCl, which demonstrated that BiOBr exhibited the highest degradation rate and O 2 consumption. The significant differences in the degradation rate and net O 2 consumption between three photocatalytic systems containing BiOCl, BiOBr and BiOI are attributed to different valence band potentials ( E vb ) of BiOXs. BiOCl ( E vb  = 3.85 eV) oxidizes H 2 O to produce O 2 and this reaction is primary, rather than directly degrades MC-LR by positive valence band holes ( h vb + ). BiOI ( E vb  = 2.66 eV), displays the slowest degradation rate due to its very low oxidative potential towards MC-LR. BiOBr is the most effective photocatalyst because its valence band potential ( E vb  = 3.02 eV) is intermediate, which helps to avoid occurrence of side-reaction of H 2 O splitting to O 2 . The most important results of our investigation demonstrate that h vb + can be tuned to optimize photocatalysis.