Topological transformation of bismuth vanadate into bismuth oxychloride: Band-gap engineering of ultrathin nanosheets with oxygen vacancies for efficient molecular oxygen activation

Abstract Herein, a facile in-situ topological transformation where BiVO4 nanosheets (NS) with open instead of closed and inert surface are used as template and original material for the preparation of ultrathin BiOCl NS is proposed. The BiOCl NS developed in this work not only inherits the two-dimensional nanosheet structure of the BiVO4 precursor, but also exhibits improved light-harvesting ability in the meantime as a result of the retention of oxygen vacancies during the topological transformation. The defect state mediated by oxygen vacancies under the conduction band endows BiOCl a narrower forbidden band width as compared to its bulk counterpart, which extends its light absorption range from ultraviolet to visible light region. More importantly, oxygen vacancies can also act as cocatalyst to accept photogenerated electrons excited from the valence band of BiOCl, which greatly suppresses the recombination of electron-hole pairs and prolongs the carrier lifetime, thereby promoting the activation of molecular oxygen into superoxide radicals (·O2-) under visible light irradiation. As expected, the optimal BiOCl-3 NS exhibits a 2.2-fold increase in the yield of ·O2- radicals compared with bulk-BiVO4. Furthermore, a large amount of free H+ in the waste acid wastewater can be tightly adsorbed with the terminal oxygen atoms of the (001) facet of BiOCl to compensate for its dangling bonds, which inhibits the crystal growth along the c axis, leaving highly reactive (001) facets as the dominant facets. Thanks to these merits, BiOCl-3 displays the highest photocatalytic performance towards ciprofloxacin (CIP) degradation among all samples, with the corresponding degradation rate of 4.51 times higher than that of the bulk-BiVO4, and the apparent quantum efficiency of 0.411%. Moreover, the as-developed BiOCl NS has excellent stability even after five successive cycles, with no significant changes in photocatalytic activity and structure, showing its good potential in practical applications.