Research On Defect State Regulation Of BiVO₄ And Its Photoelectrochemical Water Splitting Performance

Photoelectrochemical（PEC）water splitting is considered by researchers to be one of the most promising means of converting solar energy into chemical energy.It can realize a green,renewable and sustainable conversion from solar energy to fuel.However,after more than 40years of development,the water splitting efficiencies of the PEC devices have not yet reached an ideal level that can be widely used.One of the main reasons is the lack of suitable photoanode materials.An ideal photoanode material should have suitable band gap size,favorable carrier dynamics,easy synthesis,good stability and so on.Therefore,selecting the suitable photoanode materials and modifying them appropriately is crucial for efficient solar fuel production.Vacancy engineering for metal oxide semiconductor photoelectrode has been a major strategy but has faced a formidable challenge in bulk charge transport because of the elusive charge self-trapping site.Bismuth vanadate（BiVO₄）has been widely investigated as one of the most promising photoanode for PEC water splitting,as it has several intrinsic advantages such as smaller bandgap（ca.2.4 e V）,deeper valance band（VB）position and easier fabrication.In this paper,BiVO4 films were selected as the research model,and Bi vacancies were successfully introduced into BiVO₄ photoanode for the first time using ionic liquid--deep eutectic solvents（DES）,and the influence of cationic defects on the PEC water splitting performance of BiVO₄photoanode was studied.（1）In order to introduce a suitable concentration of Bi vacancies in the BiVO₄ photoanode,a certain amount of ionic liquid--deep eutectic solvents（DES）was used to soak it,and the temperature was controlled by heating and continuous soaking for a period of time.Bi vacancies with different concentrations were introduced into BiVO₄ by adjusting the heating temperature and soaking time,and the most appropriate treatment parameters were obtained by PEC measurement.The results show that BiVO₄ photoanode soaked in DES at 80℃for 10 h can obtain the highest photocurrent density（2.21 m A/cm²）at the potential of 1.23 V vs.RHE,which is nearly 5 times better than the pure BiVO₄ photoanode.（2）In order to further study the reasons for the performance change of BiVO₄ photoanode PEC caused by the introduction of Bi vacancy,a series of performance and structure characterization of Bi_1-xVO₄ photoanode obtained under the most appropriate parameters were carried out on the basis of the previous chapter.The results show that compared with the pure BiVO₄,the charge diffusion coefficient of Bi_1-xVO₄ increases by 5.8 times,from 1.82×10^-7 to1.06×10^-6 cm² s^-1,and the charge transport efficiencies increase from 26.42%to 96.45%at the potential of 1.23 V vs.RHE.Therefore,the role of Bi vacancies in BiVO₄ photoanode is different from that of O vacancies,and the bulk phase charge transmission capacity is improved more.In addition,theoretical calculation and characterization methods such as low temperature photoluminescence lifetime spectrum（TR-PL）have further proved this conclusion.（3）In order to further improve the PEC water splitting performance of Bi_1-xVO₄photoanode and improve its long-term stability,the oxygen evolution cocatalyst（OEC）Co-Bi was supported on its surface to construct Bi_1-xVO₄/Co-Bi photoanode.The results show that under the action of OEC,Bi_1-xVO₄/Co-Bi photoanode can reach the photocurrent density of4.5 m A/cm² at the potential of 1.23 V vs.RHE,and still maintain the PEC activity of 89.71%after 30 h long stability measurement.