Preparation And Photoelectrochemical Properties Of Doped BiVO₄ Thin Films

In recent years,the photoelectrochemical anticorrosion property of semiconductor photocatalyst has been drawn more attention besides its wide applications in light-electricity conversion,air purification and waste water treatment.TiO₂ has been a research focus of researchers due to its high photocatalytic performance,low cost,non-toxicity,and easy fabrication.However,the development and utilization of TiO₂ is greatly limited due to its narrow light absorption region and lower quantum efficiency.Therefore,it is necessary to develop visible-light-driven photocatalysts from the viewpoint of the efficient utilization of solar energy.Monoclinic scheelite bismuth vanadate（Bi VO₄）is one of the excellent n-type semiconductor photocatalysts which shows well visible light response.The valence band（VB）of BiVO₄ composed of hybridized Bi 6s and O 2p orbitals.This hybridization makes the VB largely dispersed,which favors the mobility of photogenerated holes in the VB and is beneficial to the oxidation reaction.In addition,the holes of the BiVO₄ have strong oxidation ability due to its more positive position of the VB.Herein,BiVO₄ thin films were prepared by modified sol-gel method.The photoelectrochemical properties and photoelectrochemical anticorrosion properties of these films were studied,and improved the properties of these films by doping modification.In this dissertation,the following several parts of work have been done:（1）BiVO₄ thin films coated on F-doped SnO₂（FTO）conductive glass were prepared by modified sol-gel method.The effects of the calcinations temperature to the surface morphology,crystalline type,grain size,optical and photoelectrochemical property of the BiVO₄ thin films were investigated.The results shown that the crystallinity and grain size of BiVO₄ thin films were closely related to the calcinations temperature,while the crystalline type and optical absorption property of the thin films was not affected by the calcinations temperature.Under the simulated solar irradiation,the photocurrent density of the BiVO₄ thin film calcined at 500℃ was 0.91 mA·cm^-2 at a bias of 1.23 VRHE,which obviously was higher than others.Moreover,photoelectrochemical anticorrosion performance of the BiVO₄ photoanodes against copper was best under light irradiation.（2）On the basis of the first work,W-doped BiVO₄ thin films were prepared by using Na₂WO₄·2H₂O as doping source and calcining at 500℃.The research shown that the incorporation of W did not change the crystalline type of the BiVO₄ thin films,but the higher Mo doping content decreased the crystallinity of BiVO₄.After W doping,the absorption band edge of the BiVO₄ was shifted,which was attributed to the electronic structure change caused by the substitution of W6+ for the V5+ in the BiVO₄ lattice.When the content of W doping is 4 at.%,the photoelectrochemical property of the BiVO₄ thin film was optimal.Its photocurrent density was 1.78 mA·cm^-2 under the simulated sunlight irradiation,which was 1.95 times of that of the pure BiVO₄ film.Meanwhile,the thin film shown excellent photoelectrochemical anticorrosion performance.（3）Mo-BiVO₄ thin films with different Mo doping were prepared coated on FTO conductive glass by using Na2MoO₄·2H₂O as doping source and calcining at 500℃.The results shown that the doping of Mo did not change the crystalline type of the BiVO₄ thin films,remained monoclinic scheelite type.The doping of Mo⁶⁺ could form the shallow potential well in the BiVO₄ thin films,which can effectively separate photo-generated electron-hole pairs,reduce the recombination rate of photo-generated electron-hole.Moreover,doping of Mo⁶⁺ into the V5+ site of BiVO₄ strengthens the n-type characteristics by supplying additional free electrons,resulting in an increase in the electric conductivity and carrier concentration of BiVO₄.Therefore,the photoelectrochemical performance and photoelectrochemical anticorrosion performance of the BiVO₄ thin film was improved by Mo doping.As the content of Mo doping is 4 at.%,the photoelectrochemical performance of the Mo-doped BiVO₄ thin film was the best,whose the photocurrent density was 2.52 times of that of the pure BiVO₄ thin film,and was 1.29 times of 4 at.% W-doped BiVO₄ thin film under the simulated sunlight irradiation.Meanwhile,the 4 at.% Mo-doped BiVO₄ thin film not only exihibited the excellent photoelectrochemical anticorrosion performance on copper under light irradiation,which made the potential of copper reduced 0.228 VRHE from its corrosion potential,but also shown the best anticorrosion performance on copper after light irradiation.