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

Abstract

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 Bi-VO4-pH＝1, respectively.The density of oxygen vacancies in BiVO4 is regulated by annealing at 450 ℃ for 2 h in a N2 atmosphere inside a tube furnace.The annealed BiVO4 surface heterojunctions are called BiVO4-pH＝0.5-N2 and BiVO4-pH＝1-N2.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-N2, Bi-VO4-pH＝0.5, and BiVO4-pH＝0.5-N2 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 Bi-VO4-pH＝0.5-N2and BiVO4-pH＝1-N2, respectively.The proportion of oxygen vacancies in the BiVO4 surface heterojunctions decreases with increasing {010}/{110} ratio after annealing in the N2 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