Preparation Of BiVO₄-based Photoanodes And Their Highly Efficient Photoelectrochemical Water Splitting Behavior

With the consumption of fossil fuels,people are more and more concerned about the development of sustainable energy,photoelectrochemical（PEC）water splitting is one of the most promising way to convert solar energy into H₂,store solar energy to produce clean and renewable hydrogen fuel under light condition.However,the overall efficiency of PEC is still too low to meet the demand for practical applications.In recent years,photocatalysts of metal oxide containing bismuth have become the focus of many researchers,such as Bi₂O₃,BiVO₄and CaBiVMO₈.Bismuth vanadate（BiVO₄）,as an n-type semiconductor,is one of the most promising photoanode materials for PEC.It possesses an appropriate valence band of 2.4 V（vs.RHE）for oxygen evolution reaction（OER）along with a narrow band gap（2.4 eV）,which can absorb sunlight below 516 nm.In addition,BiVO₄ is an ideal photoanode material due to its vast reserves,low cost and high stability in neutral solutions.This thesis comprehensively reviews the basic principles,experimental methods,basic approaches for the modification of semiconductor materials and the recent research of BiVO₄photoanodes.This research mainly focuses on the preparation and modificati of BiVO₄ for improvement of its PEC performance.We further analyze the enhancement mechanism behind,which provides a useful knowledge for improvement of the efficiency of PEC.The details of this thesis are summarized as follows:1.W doping effect on BiVO₄ photoanodeFirstly,BiVO₄ is prepared by dip coating method,followed by drying at room temperature.An orange product is obtained and the effect of W doping and film thickness of BiVO₄ are futher studied.High efficiency of BiVO₄ electrode is prepared by doping,showing that the site of V is replaced by W.Different W doping concentrations of W:BiVO₄ electrodes are prepared and characterized,of which the W:BiVO₄ electrode with 7.5%doping concentration has the best PEC performance.W:BiVO₄ electrodes with different thicknesses are prepared to obtain the best film thickness.UV-visible spectrum shows W:BiVO₄ has a larger wavelength absorption range than plain BiVO₄,corresponding to a narrower band gap.All these ensure that visible light photoactivity of W:BiVO₄ is better than BiVO₄.It is found that the photocurrent of W:BiVO₄ can reach about 1 mA/cm² at 1.8 V（vs.RHE）and 60%increase than BiVO₄,which is only about 0.41 mA/cm².Besides,W:BiVO₄ has more negative onset potential than BiVO₄,which indicates that W:BiVO₄ electrode can effectively improve the OER kinetics.The AC impedance spectroscopy analysis shows that W:BiVO₄ has a lower resistance in the visible region than BiVO₄,which symbols that the electrode effectively promotes the transport of the electron inside the photoelectrode and inhibit the recombination of the electron-hole pair.2.Multi-metal oxide modified W:BiVO₄ electrode synthesis and the study of PECperformanceAfter the optimization of W:BiVO₄,I prepare a multi-metal oxide catalyst based heterojunction W:BiVO₄-FeCoW photoanode by a sol-gel approach and analyze its crystal structure,surface morphology,LSV curve and UV-vis spectrum for exploration of the enhancement mechanism of its PEC.I find a good heteronanostructure formed between FeCoW and W:BiVO₄ by analyzing its morphology.By analyzing the PEC properties,this multimetal oxide catalyst coated W:BiVO₄ photoanode exhibits a greatly enhanced photocurrent by².8 times at 1.23 V（vs.RHE）and significant shift of the flat-band potential by 421 mV in comparison to W:BiVO₄ photoanode.The performance improvement should be attributed to that W:BiVO₄-FeCoW can significantly inhibit the recombination of the electron-hole pairs,enhance charge transport and light absorption,and finally promote the efficiency of PEC reaction.In summary,the OER kinetics of BiVO₄ photoanode can be greatly improved by W doping and the W:BiVO₄-FeCoW heterojunction.I successfully prepared the BiVO₄-based photoanodes and improve the dynamic mechanism of their PEC reaction,significantly broaden the absorption range of visible light for efficient photoelectrochemical water splitting.