Regulating Band Structure of BiOBr Via Rare Earth (La,Yb) Doping to Enhance Photocatalytic Performance

Author:Fan Qi Zhe

Supervisor:liao chun fa


Degree Year:2019





The oxidation ability of holes and the reduction ability of electrons seemed unsatified,though the energy gap of BiOBr is moderate and it has good visible light absorbtion.Heterojunction,can become a effective way to regulate BiOBr due to its characteristic lamellar structure and anisotropy,which is advantageous to form heterojunction and regulate photoactivity.However,it’s still to face two main problems.On the one hand,the oxidation ability of holes and the reduction ability of electrons can not be improved by means of formation heterojunction.Moreover,the invalid heterojunction would lead the formation of holes-electrons recombination centers,which ascribed to the unmatched energy gap.Therefore,to solve aforementioned problems,the methods of regulating the position of valence band and conductive band can be used.In present work,the mechanism of rare earth-doped regulating the structure of energy gap,distribution of electrons and the mechanism of improved-photoactivity were investigated in detail.Solvothermal method was first used for synthesis of photocatalyst.Here,cetyltrimethyl ammonium bromide served as organic bromine source and surfactant and NaBr served as inorganic bromine source.The nano-flake structure BiOBr and La-BiOBr with different content of La(0.4%,0.8%,1.6%,3.2%)were obtained.The characterization of materials contains XRD,SEM,TEM,TG-DTA,N2 adsorption and etc.The photoelectric property was detected by Uv-vis DRS,PL,XPS.Electrochemical workstation was used to characterize band-gap energy,potential of conductive band and photochemical property.Combine with Materials Studio(MS),based on first-principle calculation,the mechanism of regulating band-gap structure of BiOBr by use of La doping was discussed.The results showed that,La-doped hindered the growth of grains,promoted the grains heap up and formed thylakoid structure.More importantly,the band-gap structure of BiOBr was changed.Compared with pure BiOBr,much positive potential of valence band and much negative potential of conductive band appear in La-BiOBr.La would act as a sink for electrons,thus caused the improvement of oxidation ability and weakness of reduction ability.Based on the property of heat-driven-self-transformation and the formation of BiOBr/Bi24O31Br10 heterojunction,we regulated thermo-stability and band-gap structure of BiOBr by La doping during the process of heat-driven-self-transformation.The results showed that BiOBr would convert to Bi24O31Br10 at high temperature calcination and form BiOBr/Bi24O31Br10 type I heterojunction.However,type I heterojunction did not benefit to improve photoactivity.La3+doping would improve the thermo-stability of BiOBr at low temperature(200℃-400℃),but make BiOBr transform easily at high temperature.More importantly,the position of valence band and conductive band can be regulated by usage of La doping,making BiOBr form BiOBr/Bi24O31Br10 typeⅡheterojunction during heat treatment,and it benefit to improve photoactivity.Furthermore,using photoreduction deposition method,we prepared x%Pd/BiOBr and x%Pd/0.8%La-BiOBr(x=1%,2%,4%,8%).The results showed that BiOBr is stable under ultraviolence irradiation,but during the process of Pd deposition,the stability of BiOBr was reduced,making BiOBr easily take place photocorrosion.Hence,the nano-flaky structure of BiOBr was corroded,and the crystallinity of BiOBr decreased.Aforementioned phenomenon did not benefit to deposit Pd.However,La-doping would improve the stability of BiOBr during the process of Pd deposition,and improve the exposure of active phase of BiOBr.Hence,La doping makes Pd deposite on active phase of BiOBr effectively,forming Schottky barrier,and initiate SPR effect to improve photoactivity in a significant degree.Based on research results of La doped BiOBr,Yb-doping on regulating the band gap and the mechanism of improved-photoactivity of Schottky barrier-basis BiOBr photocatalysts were studied.We studied the influences of photoreconversion between Yb3+and Yb2+on band structure,physicochemical properties and photocatalytic performance of BiOBr.And adoption of photoreduction deposition method,the Yb doped BiOBr with Pd deposition photocatalyst were synthesized.The synergistic effect between Yb doping and SPR effect produced by Pd for enhancing photocatalytic performance were studied.The results showed that Yb-doping would increase the crystallinity of BiOBr and lead to much exposed active phase,in favoring of Pd deposition,Schottky barrier formation,and band gap energy increase.Moreover,Yb3+and Yb2+would convert constantly under light irradiation,and emit visible light during the process of accepting and donating electrons,also improving the generation of SPR effect.The utilization efficiency of light was improved in a significant degree,and photoactivity was promoted.