Magnetic and Photoelectrocatalytic Properties of BiVO4 Surface Heterojunctions Controlled by Oxygen Vacancies

In ferromagnetic photocatalysts, most charges have the same spin state, which suppresses photogenerated electron-hole recombination. However, ferromagnetic photocatalysts are rare, and the impact of spin is negligible in nonferromagnetic photocatalysts. In this study, the ferromagnetic properties of BiVO4 are regulated in terms of crystal plane orientation and oxygen vacancy synergism, considering the different formation energies of oxygen vacancies in its {010} and {110} planes. BiVO4 powders with tunable crystal facets are synthesized by using a solvothermal method. The synthesized BiVO4 surface heterojunctions with {010}/{110} crystal plane ratios of 0.17 and 0.93 are referred to as BiVO4-pH=0.5 and BiVO4-pH=1, respectively. The density of oxygen vacancies in BiVO4 is regulated by annealing at 450. for 2 h in a N-2 atmosphere inside a tube furnace. The annealed BiVO4 surface heterojunctions are called BiVO4-pH=0.5-N-2 and BiVO4-pH=1-N-2. X-ray photoelectron spectroscopy is performed to examine the ratios of the number of surface oxygen vacancies to the number of surface oxygen atoms at the intrinsic sites (OV/OL). The ratios for BiVO4-pH=1, BiVO4-pH=1-N-2, BiVO4-pH=0.5, and BiVO4-pH=0.5-N-2 are 18.96%, 20.36%, 16.04%, and 21.42%, respectively. Thermogravimetric analysis is performed to determine the concentration of oxygen vacancies because the oxygen vacancies are refilled with oxygen atoms at high temperatures resulting in weight gain. The OV/OL ratios for the entire material are 0.56% and 0.23% for BiVO4-pH=0.5-N-2 and BiVO4-pH=1-N-2, respectively. The proportion of oxygen vacancies in the BiVO4 surface heterojunctions decreases with increasing {010}/{110} ratio after annealing in the N-2 atmosphere. This is because the formation energy of oxygen vacancies in the {010} plane is lower than that in the {110} plane. The ferromagnetic properties of the BiVO4 surface heterojunctions are correlated to the concentration of oxygen vacancies and the ratio of the {010}/{110} crystal planes. The ferromagnetic properties of BiVO4 with a lower {010}/{110} ratio (BiVO4-pH=0.5) are superior to those of BiVO4 with a higher {010}/{110} ratio (BiVO4-pH=1). BiVO4-pH=0.5 has a smaller particle size and is more three-dimensional, indicating a larger specific surface area and interfacial region. The contribution of the surface unsaturated spin to the total magnetic moment in BiVO4-pH=0.5 is higher than that in BiVO4-pH=1. The introduced oxygen vacancies enhance the ferromagnetic properties of BiVO4-pH=0.5 as well as the photoelectrocatalytic H-2 production properties of the BiVO4 surface heterojunctions (BiVO4-pH=0.5 and BiVO4-pH=1). The formal quantum efficiency (FQE) used in this study is synonymous with the photonic efficiency calculated from the known spectral distribution of the excitation source and known absorption spectrum of the reaction system. The FQE values of BiVO4-pH=1, BiVO4-pH=1-N-2, BiVO4-pH=0.5, and BiVO4-pH = 0.5-N-2 are 0.0086%, 0.045%, 0.0019%, and 0.032%, respectively. The enhanced photoelectrocatalytic performance can be attributed to the significantly increased visible light absorption capacity, rapid transport of electrons and holes, high photogenerated electron-hole separation efficiency, and improved reduction potential. In particular, BiVO4 with a higher {010}/{110} ratio exhibits a higher photocurrent density and a greater H2 production efficiency because the {010} facets have higher charge mobility, better water adsorption characteristics, and lower energy barrier compared to the {110} facets.