Preparation,Modification And Photocatalytic Performance Of Visible Light-driven BiVO₄ Photocatalyst

The lack of fresh water resources is one of the main problems that plague human survival and development,and the aggravation of water pollution makes wastewater treatment technology become particularly important.Traditional wastewater treatment technologies have some defects,and the semiconductor photocatalysis technology can effectively solve the problems in wastewater treatment.Bismuth vanadate?BiVO₄,hereinafter referred to as BVO?is an inexpensive visible light responsive photocatalyst with the advantages of narrow bandgap,good stability,non-toxicity and harmlessness,which has great potential in photocatalytic wastewater treatment.However,BVO has several severe drawbacks,such as poor adsorption ability and easy recombination of photogenerated electron-hole pairs,which makes the overall photocatalytic performance relatively poor and thus limits its practical application.In this thesis,different methods were used to prepare BVO with different morphologies,and the photocatalytic performance of the prepared BVO was improved by hydrogenation treatment and heterjunction construction.The mechanism of hydrogenation modification and the photocatalytic mechanism of different BVO samples were then preliminarily investigated.The main research contents of this paper include:1.Photocatalytic performances of BVO synthesized by homogeneous coprecipitation and its hydrogenated samplesBVO was prepared by a homogeneous coprecipitation method,using NH₄VO₃,Bi?NO₃?₃·5H₂O as raw materials and urea as the precipitant.The hydrogenated BVO?hereinafter referred to as H-BVO?was then obtained by hydrogenation treatment which was carried out under pure hydrogen atmosphere?20 bar,4 h?at different temperatures?150,180,200,220??.The photocatalytic properties of different hydrogenated samples were characterized by degrading methylene blue?MB?,and it was found that the hydrogenated sample at 200? showed the best photocatalytic performance.The XRD,SEM,TEM and other measurements were used to characterize the BVO and H-BVO hydrogenated at 200?.The samples were found to be monoclinic scheelite crystal,which was in the form of massive particles with a size of 0.2²?m.The visible light absorption of samples was enhanced and the apparent band gap was decreased after hydrogenation.It was proved that the hydrogenation treatment could produce surface oxygen vacancies and heterovalent metal ions,causing a disordered layer on the surface of the hydrogenated sample.The photocatalytic performances of the prepared samples were evaluated by degrading tetracycline hydrochloride?TCH?.It was found that the TCH degradation rate under visible light irradiation for 30 min increased from 52%to 98%after hydrogenation,but the photocatalytic stability of H-BVO was not good.Through PL and photoelectrochemical tests,it was clarified that the enhanced photocatalytic performance of H-BVO was attributed to the presence of oxygence vacanics induced by hydrogenation,which could promote the separation and transfer efficiency of photogenerated electron-hole pairs.2.Photocatalytic performance of BVO synthesized by ultrasonic-assisted and its hydrogenated samplesBVO was prepared by ultrasonic-assisted using Bi?NO₃?₃·5H₂O and NH₄VO₃ as raw materials,and H-BVO was also obtained by hydrogenation under the same conditions?i.e.,200 ?,and 20 bar?in pure hydrogen atmosphere.The experimental results show that the prepared BVO sample was monoclinic scheelite crystal in a shape of irregular particles with a size of 100?200 nm,which had a relatively poor crystallinity.After hydrogenation,a disordered layer was found on the sample surface,the visible light absorption was significantly enhanced and the apparent band gap was decreased.The photocatalytic degradation of TCH under visible light irradiation shows that the TCH degradation rates of BVO and H-BVO were 39% and 82%,respectively,which were worse than those of the samples prepared by homogeneous coprecipitation method.In our opinions,the worse photocatalytic performance of the BVO and H-BVO prepared by ultrasonic irradiation could be ascribed to their excessive internal defects and partial precipitation of Bi element arising from the relatively poor crystallinity.In addition,the stability problem of the hydrogenated sample still exists.On the basis of photoelectrochemical measurements,the photocurrent and carrier concentration of the sample before and after hydrogenation were larger than those of the samples prepared by the homogeneous coprecipitation method.However,the charge transfer resistance was too large,resulting in poor photocatalytic performance.This proves that poor crystallinity was the cause of poor photocatalytic performance.3.Photocatalytic performance of BVO synthesized by hydrothermal method and its BVO/g-C₃N₄ composite and hydrogenated samplesBVO was prepared by hydrothermal method using Bi?NO₃?₃·5H₂O and NH₄VO₃ as raw materials and sodium dodecylbenzenesulfonate as surfactant,and then BVO/g-C₃N₄ was prepared by one-step calcination method.Hydrogenation was also carried out in a pure hydrogen atmosphere at 200? and 20 bar for 4 h to obtain H-BVO and H-BVO/g-C₃N₄.The experimental results show that the prepared BVO sample was monoclinic scheelite in a shape of granular particles with a size of 100?500 nm,and g-C₃N₄ nanosheets were successfully coupled with BVO.After hydrogenation,the disordered layer appeared on the surface of H-BVO and H-BVO/g-C₃N₄.The MB degradation efficiencies catalyzed by BVO,H-BVO,BVO/g-C₃N₄ and H-BVO/g-C₃N₄ were 78%,97%,91%and 98%,respectively,revealing that the hydrogenation treatment and the heterojunction formation with g-C₃N₄ could both enhance the photocatalytic performance of BVO.However,the stability problem of hydrogenation samples still existed.The photoelectrochemical results demonstrate that the enhanced photocatalytic performance of H-BVO/g-C₃N₄ sample could be attributed to the fact that the hydrogenation treatment and the heterojunction construction between BVO and g-C₃N₄ could effectively inhibit the recombination of photogenerated electron-hole pairs and promote the separation and transfer of photogenerated carriers.