Myth or reality? A disquisition concerning the photostability of bismuth-based photocatalysts

Photocatalysis is a well-known and extensively investigated field within advanced oxidation processes. Numerous papers have been reported that the semiconductors applied were photostable and potentially reusable based on the degradation efficiencies observed. However, researchers did not attach enough importance to investigate the residual catalysts in most cases. Here, we report on some important alterations that occurred during photocatalytic experiments, such as acid-induced and light-assisted structure break-down and complete transformation of catalysts, which could place their photostability into a new perspective. In this work, the photostability of five, increasingly popular bismuth-based oxides (Bi2WO6, BiVO4, BiOI, BiOCl, and BiOBr) were investigated in the presence of various carboxylic acids (oxalic acid, formic acid, salicylic acid, malonic acid, and ascorbic acid), focusing on the crystallographic and morphological alterations of the residual photocatalysts. When investigating the resistance of the photocatalysts to oxalic acid, the formation of bismuth oxalate hydroxide was observed in all cases. At the same time, most components of the catalysts appeared in the liquid phase in high amounts. Similar transformations occurred to a lesser extent for the other acids, which was deduced to be related to the acidic strength and proton concentration. We also highlighted that in some instances, during photocatalytic processes, the occurrence of light-assisted acid-induced reactions must be considered as well.