Insight into the photocatalytic mechanism of the optimal x value in the BiOBrxI1−x, BiOClxI1−x and BiOClxBr1−x series varying with pollutant type

Abstract

It is known that bismuth oxyhalides (BiOX, X = Cl, Br, I) can easily form solid solutions like BiOBr_xI_1−x, BiOCl_xI_1−x and BiOCl_xBr_1−x (0 ≤ x ≤ 1) and exhibit composition-dependent photocatalytic performance. However, the reported results indicate that the optimal composition changes with pollutant type. That is to say, the specific x value with the best photocatalytic activity towards the degradation of a certain pollutant does not imply that it is an optimum x value for another pollutant. In order to explore the reason behind this, herein, three types of solid solutions with various x values were prepared in ethylene glycol/H₂O (V_EG : V_H2O = 1) solution at room temperature, and their photocatalytic activity towards the degradation of bisphenol A (BPA), tetracycline (TC), malachite green (MG), methyl violet (MV) and rhodamine B (RhB) was assessed under visible-light illumination. Taking BiOBr_xI_1−x as an example, BiOBr_0.5I_0.5 exhibited the best degradation efficiency for BPA, MV and MG, whereas BiOBr_0.95I_0.05 possessed the best photocatalytic activity towards TC and RhB degradation. Detailed characterization suggests that light absorption and charge separation efficiency are not the main factors behind this difference. Given that direct oxidation of the holes was dominant in the degradation process, the oxidation ability of the solid solutions was correlated with the oxidation behavior of the pollutant. The prerequisite condition for degrading a certain pollutant is that the valence band potential of the solid solution should be more positive than the oxidation potential of the pollutant, and yet, too big a difference between these two potentials does not benefit rapid degradation.