Synthesis Of Low Dimensional Semiconductor Photocatalyst For Water Splitting To Produce Hydrogen And Reduction Of Carbon Dioxide

Using photocatalysts to convert solar energy into chemical energy is one of the important means to solve the energy and environmental crisis faced by mankind at present.Currently,Ti-based and g-C₃N₄ photocatalysts are thought to be promising photocatalysts and have been widely studied.Due to the disadvantages of high recombination efficiency,poor conductivity and low utilization rate of visible light,these catalysts suffer from limited applications in the field of photocatalytic energy conversion.In the research work of this thesis,the author mainly focuses on the research of photocatalytic water splitting and photocatalytic carbon dioxide reduction.In the aspect of hydrogen evolution from photocatalytic water splitting,TiO₂ and g-C₃N₄ are selected as the most representative materials,and the effects of cocatalyst and doping on hydrogen production from photocatalytic water splitting are investigated.For CO₂ reduction,SrTiO₃ is taken as the target material,and the influence of crystal plane characteristics and adsorption on photocatalytic CO₂ reduction performance is mainly investigated.In this thesis,some photocatalyst materials with close correlation have been prepared as follows:(W,O)/g-C₃N₄ hollow tubular with high specific surface area and high visible light response;smaller Au particles loadedTiO₂ nanosheets with high-efficiency photolysis of water;AuPt bimetallic loaded sea urchin-likeTiO₂microspheres with high carrier separation efficiency and carrier lifetime;SrTiO₃submicron cubes with high photocatalytic reduction of CO₂ and water splitting performance.Main research results are as follows:1)In Chapter 3,(W,O)/g-C₃N₄ with hollow tubular structure is prepared by one-step pyrolysis method for the first time.The introduction of W and O effectively inhibits the polycondensation of g-C₃N₄,thereby increases the specific surface area of g-C₃N₄ from 10.19 m² g^-1 to 38.39 m² g^-1.At the same time,the light absorption edge of the material is obviously red shifted,the light absorption is enhanced,and the band gap decreases from 2.69 e V to 2.37 e V.The formed W-N bond may act as a channel of electron transport,which plays an important role in promoting the transportation of charge carriers and inhibiting the recombination of carriers.The carrier lifetime of the samples increases from 1.401 ns of g-C₃N₄ to 6.291 ns of 3.0%W/g-C₃N₄.The best hydrogen production performance(403.57?mol g^-1 h^-1)was obtained by 1.0%W/g-C₃N₄,which was 2.5 times higher than that of g-C₃N₄.2)In the fourth chapter of this paper,the highly dispersed Au nanoparticles cocatalyst was obtained on the surface ofTiO₂ by deposition-precipitation method.The presence of high valence Au element indicates that the interaction between Au nanoparticles and the surface ofTiO₂ plays a crucial role in improving the photocatalytic performance.The comparison of hydrogen production performance of the composite catalysts prepared by the two methods shows that the size of Au cocatalyst has a significant effect on the photocatalytic water splitting performance.The hydrogen production ofTiO₂-0.1Au-DP sample(prepared by deposition-precipitation method,Au loaded 0.1 wt%)reaches 1325.9?mol g^-1 h^-1,which is 1.88 times as high as that ofTiO₂-0.1Au-PD(prepared by photodeposition method)sample with the same loading amount.3)In the fifth chapter,AuPt bimetallic cocatalysts with different mixing ratios are prepared by photo-deposition method by accurately controlling the total loading to be1.0 wt%.The loading of AuPt nanoparticles significantly increases the carrier lifetime.ForTiO₂-AuPt23 sample,carrier lifetime reaches 17.524 ns,which is significantly higher than 12.391 ns ofTiO₂-Au and 14.517 ns ofTiO₂-Pt.The photocatalytic hydrogen production rate ofTiO₂-AuPt23 sample is 3318.09?mol g^-1 h^-1,which is 2.2times higher than that ofTiO₂-Au and 2.4 times higher than that ofTiO₂-Pt,respectively,and shows superior stability.The hydrogen production performance per unit specific surface area shows remarkable advantages.TiO₂-AuPt23 sample is 59.32?mol m^-2 h^-1,which is 1.55 times than that of commercial P25 powder(38.25?mol m^-2 h^-1).4)In the sixth chapter of this thesis,single crystalline cubic strontium titanate(SrTiO₃)sub-micron particles are prepared by molten salt method with(100)and(110)plane exposed.During the photocatalytic reduction of carbon dioxide,the experimental results show that both the carbon dioxide reduction and hydrogen evolution occur.However,the reduction of carbon dioxide and the water splitting have an obvious competitive relationship,which is closely related to the loading of Pt nanoparticles.Based on repeated tests,this competition shows recoverability.According to the change of surface state,the author thinks that the reaction process may be related to the selective adsorption of reactants.At the early stage of the reaction,the concentration of CO₂ molecules adsorbed on the surface of SrTiO₃ is high enough for a high rate of CH₄formation during this period.As the reaction continues,the surface chemistry or microstructure of the adsorbed sites of CO₂ changes,making them more inclined to catalyze the generation of hydrogen.