Studies Of Carrier Transport Behavior Tuning Of BiVO₄ Photoanodes And Their Photoelectrochemical Performance For Water Splitting

As a renewable energy,hydrogen energy has several advantages such as high energy density,clean,various storage and utilization methods,which has become the green energy with great development potential.Photoelectrochemical water splitting technology is an effective way to produce oxygen and hydrogen by solar under the assistance of semiconductor materials with low bias,so that oxygen and hydrogen can achieve no carbon emission in the production process,which has very important practical significance to help realize the goal of“carbon peak and carbon neutrality”.The synthesis of large size,high efficiency and stable photoanode is the key to the photoelectrochemical water splitting.Among many photoanode materials,bismuth vanadate（BiVO₄）is widely considered as an ideal photoanode for photoelectrochemical water splitting due to its narrow band gap（～2.40 e V）,suitable band location,and high theoretical solar to hydrogen conversion efficiency（STH,9.2%）.However,the STH of BiVO₄ photoanode is still far below its theoretical value.The fundamental reason is a series of carrier transport problems such as electron and hole due to the pole-hopping transport mechanism,resulting in low carrier separation efficiency,injection efficiency and transport rate of BiVO₄ photoanode Therefore,in this thesis,the carrier transfer behavior of BiVO₄ photoanode is regulated by constructing composite photoanode to improve interface electron or hole transfer and introducing the photothermal effect,which collaboratively improves the carrier separation efficiency,injection efficiency and transfer rate,solves the problems of severe electron-hole recombination and low carrier mobility of BiVO₄ photoanode,and realizes the efficient photoelectrochemical water splitting.The specific research contents are as follows:（1）Strong Metal-Support Interaction was used to prepare VOx/Au/BiVO₄ composite photoanode,and the carrier separation efficiency was improved by efficient trapping of photogenerated holes.The carrier separation efficiency of VOx/Au/BiVO₄ composite photoanode reaches 96%at 1.23 V vs.RHE.The Applied Bias Photon-to-current Conversion Efficiency is 0.68%at 0.87 V vs.RHE.The Monochromatic Incident Photon-to-electron Conversion Efficiency with fixed wavelength reaches 46%at 450nm and 4.2 times more than the photoanode with BiVO₄.The results show that VOx/Au/BiVO₄ composite photoanode can efficiently capture photogenerated holes by forming electron-rich Au(Au^δ-).Which can effectively avoid the recombination of valence band holes and conduction band electrons of BiVO₄ photoanode,and improving the carrier separation efficiency.（2）Based on the hydrothermal synthesis of Fe:VOPO₄ oxygen evolution electrocatalyst,the Fe:VOPO₄/BiVO₄ composite photoanode was prepared successfully,and the efficiency of carrier injection was improved by rapid surface hole transfer.The problem of electron-hole recombination on the surface of BiVO₄ photoanode was effectively solved by constructing V-O-V bond,and the carrier injection efficiency was improved.The results show that the carrier injection efficiency of Fe:VOPO₄/BiVO₄composite photoanode reaches 75%at 1.23 V vs.RHE,which is 2.6 times that of BiVO₄photoanode The Applied Bias Photon-to-current Conversion Efficiency reaches 1.21%at 0.64 V vs.RHE;and the Monochromatic Incident Photon-to-electron Conversion Efficiency reaches 46%at 450 nm,which is 3.6 times that of the BiVO₄ photoanode（3）Co（OH）_x/Cu₂S/BiVO₄ composite photoanode was prepared by successive ionic layer adsorption and reaction method,and the efficiency of carrier separation and injection was improved by electron-hole directed transfer.The carrier separation efficiency of Co（OH）_x/Cu₂S/BiVO₄ composite photoanode reaches 80%and the injection efficiency reaches 70%at 1.23 V vs.RHE.The Applied Bias Photon-to-current Conversion Efficiency reaches 0.94%at 0.74 V vs.RHE;and the Monochromatic Incident Photon-to-electron Conversion Efficiency reaches 55%at 450nm,which is 5.0 times that of the BiVO₄ photoanode The results show that the photogenerated holes in valence band of BiVO₄ can be transferred to the valence band of p-type semiconductor Cu₂S through the electric field built in p-n heterojunction under illumination,realizing the electron-hole directional transport and improving the carrier separation efficiency of BiVO₄ photoanode In addition,the Co（OH）_x electrocatalyst with the coating structure can improve the carrier injection efficiency while protecting Cu₂S from photocorrosion.（4）NiCo₂O₄/BiVO₄ composite photoanode was prepared by electrodeposition on the basis of the synthesis of NiCo₂O₄ nanosheets,and the carrier transfer rate was improved by the photothermal effect.The carrier transfer time of NiCo₂O₄/BiVO₄ composite photoanode was reduced from 0.90 ms to 0.45 ms.The Applied Bias Photon-to-current Conversion Efficiency reaches 2.21%at 0.67 V vs.RHE;and the Monochromatic Incident Photon-to-electron Conversion Efficiency reaches 60%at 450 nm,which is5.5 times that of the BiVO₄ photoanode The results show that the photothermal effect can significantly reduce the interfacial reaction barrier,increase the photogenerated carrier transfer rate and improve the carrier mobility of NiCo₂O₄/BiVO₄ composite photoanode（5）The NiOOH/FeOOH/Co₃O₄/BiVO₄ composite photoanode was prepared by electrodeposition.The carrier separation efficiency,injection efficiency and transfer rate of BiVO₄ photoanode were improved by electron-hole directed transfer and photothermal effect.The carrier separation efficiency of NiOOH/FeOOH/Co₃O₄/BiVO₄composite photoanode reaches 96%,the injection efficiency reaches 97%,and the carrier transfer time is reduced to 0.02 ms at 1.23 V vs.RHE with assist of the photothermal effect.The Applied Bias Photon-to-current Conversion Efficiency reaches 2.72%at 0.60 V vs.RHE;and the IPCE reaches 80%at 450 nm,which is 7.2times that of the BiVO₄ photoanode The results show that by constructing an electron-hole directional transport channel with p-n heterojunction and utilizing the photothermal effect of Co₃O₄,the problems of electron-hole recombination and low carrier mobility of BiVO₄ photoanode are solved simultaneously,and the carrier separation efficiency,injection efficiency and transfer rate of BiVO₄ photoanode are improved cooperatively.