Preparation, Characterization Of Bi_1-xLn_xVO₄（Ln=La, Sm, Nd, Gd, Eu, Dy, Y） Photocatalyst And Its Photocatalytic Activity For Water Splitting

Nowadays, the world is facing serious energy and environmental problems, so it isvery urgent to produce a clean and sustainable hydrogen energy using renewable energysource to solve such problems. Among many methods of hydrogen production,photocatalytic water splitting using solar energy is one of the most promising and idealways considering energy cost, resource and environmental issues. In the course ofphotocatalysis, the most important and urgent work is to develop semiconductorphotocatalysts responding to visible light with high quantum efficiency quantum and highstability.BiVO4was an excellent photocatalyst for producing O2from AgNO3aqueoussolution under visible light. But it was unable to reduce H₂O to H₂because of its propertyof band structure. In this dissertation, several kinds of photocatalysts Bi1-xLnxVO4（Ln=La,Sm, Nd, Gd, Eu, Dy, Y） were prepared and were discovered to have photocatalytic activityto split water into H2and O2under UV and visible light irradiation. Their physical andchemical properties were characterized by XRD, SEM, XPS, DRS, BET and so on. Theeffects of cocatalysts and pH value on the activity of photocatalysts were investigated. Therelationship between the structure and photocatalytic performance was also discussed.These study indicated that forming solid solution was a feasible method to simultaneouslyadjust the band gap to obtain stable metal oxide photacatalysts for overall water splitting.In order to adjust the band structure and photocatalytic property, a solid solutionphotocatalyst Bi0.5Y0.5VO4（BYV） was synthesized, which could split water into H2and O2under visible light irradiation. BYV, was prepared by conventional solid solution reaction,rheological phase reaction, polymerizble complex and hydrothermal method respectively,while all of the samples showed photacatalytic activities for water decomposition undervisible light.1wt%Pt-Cr₂O₃/BYV prepared by solid solution reaction showed higherphotocatalytic activities. The amounts of the produced hydrogen and oxygen were about35.63μmol/h and18.05μmol/h respectively under irradiation of λ>420nm light.The solid solution Bi1-xDyxVO4（BDV,0≤x≤1） were prepared by solid phase reactionat high temperature. With increasing of Dy in BDV, its crystal structure would transformfrom I2/b to I41/amd. When0.3<x<1.0, BDV were able to split water into H2and O2 under UV light irradiation with cocatalysts0.3wt%Pt, which the amounts of H2and O2produced by BDV firstly increased and then decreased. When x=0.5, BDV had the bestphotocatalytic activity and absorb the visible light up to450nm which band gap wasestimated to be2.76eV. The quantum efficiency was0.15%under420nm visible lightirradiation. BDV（0.5） was discovered to the best photocatalytic activity when pH=7tocompletely split water into H2and O2under λ>300nm or λ>420nm irradiation when itwas loaded1wt%Pt-Cr₂O₃. The results of XRD and XPS indicated the stability ofBDV（0.5） in the photocatalytic reaction. BDV（0.5）) with the monoclinic structure wereprepared by hydrothermal and microwave synthesis. It could absorb visible light up to560nm and split water into H2and O2under UV and visible light. However, it had relativelypoor photocatalytic activity. BDV had different color, absorption property andphotocalytic activities, which was probably due to the different crystal structure andelectron struture..The solid solution Bi1-xLnxVO₄（0≤X≤1；Ln=La, Sm, Nd, Gd, Eu, Y） were preparedby solid state reaction. With increasing of Ln in Bi1-xLnxVO₄, its crystal structure wouldtransform from monoclinic to tetragonal structure. When0.3≤X≤1, Bi1-xLnxVO₄weretetragonal structure, but Bi1-xLaxVO₄had different change. Bi0.5La0.5VO₄was tetragonalstructure and LaVO₄was monoclinic structure. Photacatalytic activities of Bi1-xLnxVO₄indicated Bi_0.5Ln_0.5VO₄had the best activities. The solid solution Bi0.5Ln0.5VO₄（Ln=La,Sm, Nd, Gd, Eu, Y,） were able to split water into H2and O2under UV or visible lightirradiation with cocatalysts. Comparing with the results of Bi0.5Ln0.5VO₄（Ln=La, Sm, Nd,Gd, Eu, Y）, Bi_0.5Y0.5VO₄had the best photocalytic activity.Bi0.5Ln0.5VO₄（Ln=La, Sm, Nd, Gd, Eu, Dy, Y） were found to act as photocatalystsfor water splitting when loading loaded with0.5wt%Au、0.5wt%Pt、0.5wt%RuO2、0.5wt%Rh、1wt%Pt-Cr₂O₃or1wt%Rh-Cr₂O₃.1wt%Pt-Cr₂O₃/Bi0.5Ln0.5VO₄（Ln=La, Sm,Eu, Dy, Y） exhibited the better photocatalytic property, while Bi0.5Ln0.5VO₄（Ln=Nd, Gd）had the best photocatalytic activities when co-loading1wt%Rh-Cr₂O₃. Moreover it wasindicated that the suitable band gaps of Bi0.5Ln0.5VO₄for overall water splitting might beattributed to the formation of new VB due to Bi6s participating in the electronic structure.The addition of Y and Lanthanide （La, Sm, Nd, Gd, Eu, Dy） to tetragonal BiVO₄,making the Bi6s electron into the process of CB, improving the position of VB, thusforming semiconductors which are suitable for water splitting. Bi0.5Ln0.5VO₄solidsolutions had the photocatalytic activity to split water under UV and visible light whenloading cocatalysts, which was probably attributed to the adverse effect of4f electrons andthe ability to absorb visible light. This study indicated that band-gap engineering of solidsolution metal oxides was the feasible method to simultaneously adjust the CB and VB toobtain stable metal oxide photocatalysts and achieve overall water splitting under visible light irradiation.