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 elec-tron-hole recombination.However,ferromagnetic photocatalysts are rare,and the impact of spin is negligible in nonferro-magnetic photocatalysts.In this study,the ferromagnetic properties of BiVO4 are regulated in terms of crystal plane orienta-tion 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=l,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=l,BiVO4-pH=l-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 at-oms 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-N2 and BiVO4-pH=1-N2,respectively.The proportion of oxygen vacancies in the BiVO4 surface heterojunc-tions 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 sur-face 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,indi-cating a larger specific surface area and interfacial region.The contribution of the surface unsaturated spin to the total mag-netic moment in BiVO4-pH=0.5 is higher than that in BiVO4-pH=1.The introduced oxygen vacancies enhance the ferro-magnetic properties of BiVO4-pH=0.5 as well as the photoelectrocatalytic H2 production properties of the BiVO4 surface heterojunctions(BiVO4-pH=0.5 and BiVO4-pH=l).The formal quantum efficiency(FQE)used in this study is synony-mous with the photonic efficiency calculated from the known spectral distribution of the excitation source and known absorp-tion spectrum of the reaction system.The FQE values of BiVO4-pH=1,BiVO4-pH=1-N2,BiVO4-pH=0.5,and BiVO4-pH=0.5-N2 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 photo-generated 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 high-er charge mobility,better water adsorption characteristics,and lower energy barrier compared to the {110} facets.
ferromagnetic photocatalystoxygen vacancyBiVO4 surface heterojunctionhydrogen production
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