Studies On Preparation And Hydrogen Evolution Performance Of Modified Photocatalysts For Large Band Gap Semiconductors Under Visible Light

Hydrogen is considered as an ideal energy carrier in the future.Because solar energy is inexhaustible,photocatalytic water splitting by solar energy has been considered as one of the ideal hydrogen production processes.In this thesis,we studied the preparation and the photocatalytic activity of B-N-codoped TiO₂,basic Zincoxysulfide(ZnO_xS_1-x-0.5y(OH)_y) solid solutions for hydrogen evolution.The research work is composed of two parts:1.Boron and nitrogen co-doped titania with enhanced visible-light photocatalytic activity for hydrogen evolution.A visible-light boron and nitrogen co-doped titania (B-N-TiO₂) photocatalyst was prepared by sol-gel method with titanium tetra-n-butyl oxide,urea and boric acid as precursors.Photocatalytic activity for hydrogen production from aqueous solution containing sacrificial reagents,EDTA-2Na,over platinized B-N-TiO₂ under visible light(λ＞420 nm) irradiation was investigated.Pt was deposited on B-N-TiO₂ in situ by photochemical deposition under visible light irradiation.The photocatalyst was characterized by Fourier Transform Infrared (FT-IR),UV-Vis diffusive reflectance spectroscopy(DRS),X-ray photoelectron spectroscopy(XPS),X-ray diffraction(XRD) and BET specific surface area, Electrochemistry method.In nitrogen doped titania(N-TiO₂) N-Ti-O bond is formed, which extends the absorption edge to the visible light region.A part of doping boron enters into titania lattice and most of the boron exists at the surface of the catalyst. Compared B-N-TiO₂ with N-TiO₂,the crystallite size of the former decreases,the photocurrent increases,and the content of the surface hydroxyl group increases. Furthermore,doping boron could act as shallow traps for photoinduced electrons to prolong the life of the electrons and holes.Therefore,the visible light activity of B-N-TiO₂ increases greatly compared with that of N-TiO₂.2.Photocatalytic H₂ evolution under visible-light irradiation over basic Zincoxysulfide(ZnO_xS_1-x-0.5y(OH)_y) solid solution was investigated.The ZnO_xS_1-x.0.5y(OH)_y solid solution showed photocatalytic activities for H₂ evolution from aqueous solution containing sacrificial reagents,SO₃^2- and S^2-,under visible-light(λ＞420 nm) irradiation without cocatalyst.The photocatalysts were prepared by general coprecipitation method using Zn(NO₃)₂·6H₂O,Na₂S·9H₂O and NaOH.The photocatalyst was characterized by UV-Vis diffusive reflectance spectroscopy(DRS),X-ray diffraction(XRD),BET specific surface area,Differential Thermal Analysis-Thermogravimetry(DTA-TG) and Electrochemistry method.The composition and coprecipitation temperature greatly influenced the activity of the photocatalysts for hydrogen production.At low coprecipitation temperature, ZnS_1-x(OH)_2x solid solution was formed,whereas at high coprecipitation temperature or after calcination,a part of ZnO separated out from ZnS_1-x(OH)_2x solid solution.At higher calcination temperature,basic Zincoxysulfide(ZnO_xS_1-x-0.5y(OH)_y) solid solution decomposed completely to form ZnO and ZnS.Hydroxide groups(-OH) played a major role in forming solid solution and photocatalytic activity.The active component of the photocatalysts was ZnO_xS_1-x-0.5y(OH)_y.The optimized photocatalyst, with a molar composition of ZnS to ZnO 1:1,coprecipitated at 373 K and calcined at 673 K under N₂ atmosphere,exhibited an apparent quantum yield of ca.2.9%at visible-light irradiation without loaded noble metal.