Construction Of Bismuth Vanadate-based Photocatalyst And Enhancement Of Their Photoreduction Of CO₂ Process

The excessive consumption of fossil fuels in modern society has brought about serious climate change problems.Photocatalytic technology can convert CO2 into valuable hydrocarbons with mild reaction conditions,low energy consumption,and green features.However,the problems of low solar light quantum efficiency and poor product selectivity will in turn limit the practical application of photocatalytic conversion of CO2 technology.The development of photocatalysts with visible light responsiveness,high carrier separation efficiency and high stability is the key to breakthrough this technology.As a typical semiconductor photocatalyst,bismuth vanadate is of great interest in photocatalysis because of its good visible light absorption,narrow band gap width and safety and non-toxicity,but there are still drawbacks such as limited reactive sites and slow carrier migration rate in bulk bismuth vanadate.In this thesis,ultra-thin bismuth vanadate nanosheets were prepared by size modulation,heterogeneous structure construction and elemental doping,and further constructed coal-based graphene oxide/bismuth vanadate composites to form a metal-zinc doped bismuth vanadate photocatalytic reaction system,and investigated the catalytic activity and strengthening mechanism of the series of photocatalysts for the photocatalytic reduction of CO2 to synthesize methanol.The main research results are as follows:（1）Ultrathin bismuth vanadate nanosheets（BiVO4-NSs）of uniform size were prepared by the solvothermal method using bismuth chloride and sodium vanadate as substrate materials and cetyltrimethylammonium bromide（CTAB）as surfactant.It is found that BiVO4-NSs have narrower band gap width and more negative conduction band potential than bulk bismuth vanadate（BiVO4-NBs）,and the tuning of its energy band structure originates from the change of photocatalyst morphology and size.During the sample synthesis,the BiCl4--CTA+ mixed precursors formed by the combination of CTA+cations and BiCl4-anions interacted with VO43ions to gradually exfoliate the originally stacked nanosheet structures into ultrathin nanosheets.The photocatalytic performance test showed that the methanol yield of the reduced product of the photocatalyst BiVO4-NSs could reach 83.13 μmol-g-1-h-1 under the continuous reaction of 300 W xenon lamp irradiation for 8 h,which was about 1.2 times of the yield of BiVO4-NBs.（2）Two-dimensional/two-dimensional CGO/BiVO4 composites were prepared by coupling ultrathin bismuth vanadate nanosheets with two-dimensional pleated morphology of coal-based graphene oxide（CGO）through electrostatic self-assembly technique.It is further found that the loading of CGO can effectively increase the photocurrent density of the composite photocatalyst,reduce the charge transfer resistance at the heterojunction interface,and decrease the rate of electron-hole complexation.The product methanol production rate can reach 537.78μmol·g-1·h-1 when reacting under 300 W xenon lamp irradiation for 8 h with CGO/BiVO4,which is about 6.47 times that of ultrathin bismuth vanadate nanosheets.The mechanism of enhanced photocatalytic performance shows that the reason for the enhanced photocatalytic performance of CGO/BiVO4 composites comes from the large specific surface area of CGO,which can be used to receive photoexcited electrons and make the catalyst obtain more active sites,on the other hand,the large π-conjugated structure of CGO and CO2 molecules can produce a uniqueπ-π conjugated structure and increase the CO2 concentration on the surface of the sample,thus enhancing the photocatalytic performance of the material.（3）The nano-sheeted Zn-BiVO4 composite photocatalysts were prepared by a solvent thermal in situ doping technique using zinc nitrate as the metal element source.It was further found that the composite photocatalyst exhibited strong light capture ability,high photocurrent density and low interfacial charge transfer resistance after doping with transition metal zinc.The methanol yield of the reduced product reached 583.35 μmol·g-1·h-1 under the reaction of 300 W xenon lamp light for 8 h with Zn-BiVO4,which was 7.02 times higher than that of the ultrathin bismuth vanadate nanosheets.When the reaction was cycled three times,the methanol yield of the sample 5%-Zn-BiVO4 remained the same as the yield of the initial reaction.The enhanced photocatalytic performance mechanism study shows that the enhanced photocatalytic performance of Zn-BiVO4 originates from the built-in electric field and hybridization energy level formed by Zn within the BiVO4 semiconductor,which improves the electrical conductivity of the sample as well as the electron density on the catalyst surface and helps to promote the rapid reaction of carriers with CO₂.