Preparation And Carrier Transport Mechanisms Of Bismuth Vanadate Electrode

Bismuth vanadate（BiVO₄）is an n-type semiconductor and has been identified as one of the most promising photoanode materials.It is composed of cheap elements with a band gap of2.4 e V,and the valence band edge is about 2.4 V relative to the RHE（reversible hydrogen electrode）,providing sufficient overpotential for hole photooxidation of water,while the position of the conduction band edge is just below the thermodynamic level of H₂.Its bandgap is slightly larger than the required bandgap for photoanodes（approximately 2.0 e V）,but its very negative conduction band position can compensate for this disadvantage.In this paper,the crystal and electronic structure closely related to the photoelectric properties of BiVO₄ are introduced,and the photoelectric properties and limitations of BiVO₄are studied.Subsequently,the latest efforts to address these limitations to improve the optoelectronic performance based on BiVO₄ were discussed.These efforts include morphology control,formation of composite structures,composition adjustment,and loading of co catalysts.The discussions and insights provided in this paper reflect the latest methods and directions in the development of general photoelectrodes,which will be directly applicable to the understanding and improvement of other photoelectrode systems.The main research content of this paper is as follows:1.To understand the factors that affect the photoelectric conversion efficiency of BiVO₄nano thin films.BiVO₄nano films with different thickness and density were prepared by sol-gel method and subsequent calcination method by changing the crystallization temperature and annealing temperature to control the micro morphology of BiVO₄ nano films.The photocurrent density of films with different thicknesses and densities under different illumination directions was compared through photocurrent testing.Through comparison,it was found that the photocurrent of all dense films under front illumination was greater than that under back illumination,while the photocurrent of all porous films under back illumination was greater than that under normal illumination.This indicates that the main reason for the difference in photocurrent density between front and back illumination is the transmission mechanism of different electrode structures,The diffusion length of holes is a factor that limits the efficiency of photoelectric conversion.More importantly,as the thickness of the film continues to increase,the photocurrent under both positive and negative illumination decreases significantly.In order to investigate the reasons for the decrease.Resistance and impedance tests were conducted,and it was found that the potential drop generated by polycrystalline boundaries is the reason for limiting the efficiency of BiVO₄ photoelectrodes.On this basis,we propose a carrier transport model that is more suitable for polycrystalline electrodes.2.In order to improve the carrier transport efficiency inside and at the grain boundaries of polycrystalline BiVO₄,the internal structure and surface of polycrystalline BiVO₄ thin films were optimized and modified.Due to the presence of potential barriers between grains,doping can reduce the width of the barrier,causing electrons to tunnel and increasing conductivity.W:BiVO₄ nano films with different concentrations and thicknesses were prepared by sol-gel method and subsequent calcination method.A carrier transport model suitable for doped semiconductors is proposed based on the results of photoelectrochemical testing.In addition,for interface optimization,co catalysts can reduce the potential barrier of charge transfer,thereby improving the interface separation of photo generated charge carriers and promoting the kinetics of the reaction.Co-Pi（Ni OOH,Fe OOH）/BiVO₄ was prepared by electrodeposition method.3.In order to further verify the influence of grain boundaries on carrier mobility and photoelectrochemical properties,the Vertical Bridgman method was used to grow BiVO₄ single crystals with different concentrations of molybdenum doping（0%,0.3%,0.5%,1%,and 2%Mo:BiVO₄）.Raman spectroscopy analysis shows that Mo replaces V and acts as a donor impurity.The photoelectric property testing shows that the photoelectric properties of Mo doped single crystal BiVO₄ are higher than those of undoped single crystal BiVO₄.When the Mo doping concentration is 0.5%,the photocurrent reaches its maximum value.As the doping concentration further increases,the photocurrent shows a decreasing trend.This is because impurities not only introduce some states to improve carrier density,charge transfer and separation efficiency,but also cause defects in the particles,thereby enhancing charge combination.Further loading of the co catalyst can reduce the initial potential and increase the photocurrent density.By using the time of flight method and Hall effect,the carrier mobility of0.3%Mo single crystal BiVO₄ was obtained to be 360 and 420 cm² v^-1s^-1,respectively.This indicates that the carrier mobility of single crystal BiVO₄ is indeed very high,and the surface voltage measured by the dual workstation system shows that the voltage drop caused by grain boundaries is very small,and applying bias voltage can fall to the surface.