Directing Effect Of Halide Ions To The Surface Of Bismuth Vanadate And The Research Of Its Photocatalytic Performance

Photocatalysis which use clean,pollution-free solar is considered to be one of the most potential solutions to solve energy and environmental problems.Photocatalytic water oxidation is not only the half reaction of water splitting but also the half reaction of CO₂ reduction,which is regarded as the rate-determining step of these two reactions.Therefore,the study of photocatalytic water oxidation is of great importance.Bismuth vanadate has a wide range of applications in photocatalytic oxygen evolution due to its visible-light response,high catalytic efficiency and good stability.Previous studies have shown that the?040?crystal plane of monoclinic bismuth vanadate shows superior photocatalytic activity than other crystal planes in the degradation of photocatalytic organic compounds.However,there is still a lack of a simple and method for continuously controlling the?040?crystal plane of bismuth vanadate;on the other hand,many works have been carried out from the perspectives of active facets,surface defects,etc for improving the oxygen evolution activity of bismuth vanadate,however,the underlaying mechanism for improving the photocatalytic activity is not yet clear;and also it lacks a simple and efficient strategy or means to promote the photocatalytic oxygen evolution activity.In this work,we have succeeded in directing the growth of specific crystal plane of bismuth vanadate by introducing halogen anions into the hydrothermal process.XRD phase characterization results show that by changing the molar ratio of chloride ions to bismuth ions,we achieve a continuous control of?040?exposure of bismuth vanadate.SEM results indicate that with the increase of chloride ion concentration,the crystal size also increases.The results of water decomposition illustrates that the bismuth vanadate showed high photocatalytic oxygen evolution activity after being treated by halogen,when Cl:Bi=1:1,the hydro-decomposition oxygen activity is the strongest?357.1?mol?,which is 4.96 times of bismuth vanadate without chlorine ion modification.However,further study found that the activity at higher concentrations began to decline.Based on the results of water oxidation test,comparative experiments and the use of a variety of characterization methods,we systematically discussed and studied various factors that affect the photocatalytic oxygen evolution activity.It is found that valence band position,the crystallinity,the specific surface area and the surface defects are not the main factors affecting the photocatalytic oxygen evolution activity.The simulation results indicate that the incorporation of halogen element significantly enhances the adsorption of water molecules and oxygen desorption ability over bismuth vanadate.It is calculated that the water molecule adsorption energy decreased from-0.6127 eV to-0.7801 eV after modification by chloride ion;the adsorption energy of oxygen molecule increased from 0.97 eV to 1.19 eV.This conclusion was first validated by adsorption of water on the surface of the catalyst with in-situ infrared spectroscopy;what's more,the introduction of bromine and iodine ions also promoted the photocatalytic oxygen evolution over bismuth vanadate but lower than the chloride ion effect,which are consistent with simulation results.On the other hand,the adsorption strength of chloride ions is stronger than the adsorption capacity of water molecules.When there is too much chloride ions adsorbed on the surface,the adsorption of water molecules will be hindered.In addition,the introduction of chloride ion increases the charge transfer resistance.These factors are considered to be the reasons for the observed volcanic activity curve over bismuth vanadate modified by chloride ions.This study is of great significance to the control of crystal planes of bismuth vanadate materials and the development of highly efficient photocatalytic oxygen-producing materials.