Study Of The Effects Of Oxygen Vacancy On The Electronic,Photocatalytic And Optical Properties Of Monoclinic BiVO₄（001）

As a narrow band gap photocatalytic material,BiVO₄ can be excited in visible light,which greatly improves the utilization of solar energy.However,its disadvantage is its high photogenerated electron-hole recombination rate.Oxygen vacancies,as a kind of crystal defects,can capture electrons as positive charge centers,thus improving the efficiency of electron migration and reducing the recombination rate of electrons and holes,which has an important impact on the improvement of photocatalytic performance of BiVO₄.However,the specific mechanism of the effect of oxygen vacancies on the photocatalytic performance of BiVO₄ is not clear,so we need to further explore the role of oxygen vacancies and further understand the basic physical and chemical properties of materials.For this reason,a series of computational simulations of oxygen-containing vacancies BiVO₄ were carried out by first-principles method,and BiVO₄ samples with different oxygen vacancy concentrations were prepared.BiVO₄ powders containing oxygen vacancies were prepared by hydrothermal synthesis,and the concentration of oxygen vacancies was controlled by controlling calcination time.ESR oxygen vacancy measurements showed that the prepared materials were in good agreement with the experimental expectations.After that,the characterization and catalytic performance experiments of the samples were carried out,and the results were combined with the simulation results.The following conclusions were obtained:（1）Through the construction of BiVO₄（001）perfect crystal plane model and BiVO₄（001）crystal plane model with oxygen vacancies,we found that oxygen vacancies did not affect the structure of BiVO₄（001）crystal plane,and the lattice vector and volume did not change significantly.In contrast,XRD,SEM,EDS and other characterization analysis of the samples confirmed that the existence of oxygen vacancies did not change the crystal structure,morphology and element composition of BiVO₄.The prepared materials were all in the shape of rice grains with a length of 1μm and a width of 500nm,which was consistent with the diffraction peaks of the standard XRD card of monoclinic BiVO₄.On the other hand,004 crystal plane appears in the XRD peak,which indicates that the simulation calculation of oxygen vacancies on 001 crystal plane is correct.（2）Through the construction of BiVO₄（001）crystal surface water adsorption molecular model and oxygen adsorption model,we found that the oxygen vacancy had no significant effect on the adsorption of water molecules at the interface,but had a significant promotion on the adsorption of oxygen molecules.Its adsorption energy increased greatly and chemical adsorption took place.It was judged that there was electron interaction with V atom and Bi atom at the corresponding position.The longer O-O bond of the adsorbed oxygen molecule indicates that the increased activity of the oxygen molecule is conducive to the further generation of superoxide radicals in the photocatalytic reaction and the improvement of the photocatalytic effect of the material.ESR spectra of free radicals in samples irradiated by visible light confirmed that the formation of superoxide radicals increased with the increase of oxygen vacancy content.（3）By calculating and simulating the density of states of BiVO₄ and analyzing the composition of the density of states,it is found that the top of the valence band of BiVO₄is mainly composed of O 2P and Bi 6s orbitals,while the bottom of the conduction band is mainly composed of V 3d,Bi6p and O 2p orbitals.At the same time,we find that a new energy level appears in the crystal plane of the oxygen vacancy,which is mainly contributed by V atom and Bi atom bonded by O atom in the original position of the oxygen vacancy.The new energy level will act as the electron capture center,accelerate the electron migration rate and reduce the probability of electron-hole recombination.On the other hand,we find that the band gap width of the crystal plane with oxygen vacancy is the same as that of the crystal plane without oxygen vacancy without considering the new energy level.It can be concluded that the oxygen vacancy can not change the overall band gap width,but can only provide a new energy level at the oxygen vacancy center.The calculated results are consistent with the experimental results.The absorption band edges of BiVO₄ are not changed by oxygen vacancies in the UV-Vis diffuse reflectance absorption spectra of the samples.The band gaps of the three samples are 2.48eV.The characterization results are consistent with the first-principles simulation.In the computational simulation in sections 3.3 and 3.4,although the existence of oxygen vacancies will add new energy levels to the original band gap of the material,it will not affect the overall band structure of the material.For the sample,a certain concentration of oxygen vacancies can not change the overall optical properties.（4）By visualizing the electron distribution and electron localization function（ELF）,we find that the electron distribution still exists in the oxygen vacancy position on the BiVO₄（001）crystal plane containing oxygen vacancy,although the shape of the electron cloud changes compared with that of the oxygen atom.The electron localization function（ELF）confirms that the ELF in the oxygen vacancy is close to 1 when it is closer to the V atom.This means that the closer to the V atom,the more localized the electrons are,indicating that the electrons are mainly supplied by the V atom.When oxygen atoms exist,V atoms provide electrons to bind to them.When they do not exist,the redundant electrons of V atom become delocalized,which will contribute to the rate of photocatalytic reaction.Although a certain concentration of oxygen vacancies can not change the overall optical properties of BiVO₄,the new energy level energy introduced by V atom as bonding electrons acts as the electron capture center,which makes the adsorption energy of oxygen molecule in oxygen vacancies increase significantly,and the activity of oxygen molecule increases when it is adsorbed,so it can increase the generation of superoxide radicals,while superoxide radicals contribute to photocatalytic degradation pollution.Things play an important role.On the other hand,the possibility of recombination of electrons and holes decreases due to the existence of electron capture centers.With the increase of the probability of photogenerated electrons migrating to the surface captured by oxygen,the probability of the holes participating in the reaction is greatly increased,which will have a better chance to react with water molecules to produce corresponding hydroxyl radicals.The ESR spectra of free radicals produced by samples under visible light illumination confirm this inference,and the degradation experiments of rhodamine B also confirm this inference from the point of view of pollution degradation.