Construction And Performance Study Of Bismuth-rich Based Bismuth Oxyhalides Photocatalysts

Semiconductor photocatalytic technology is regarded as an efficient,green and promising solution to solve environmental problems and energy crises.It can utilize solar energy for splitting water into hydrogen gas,degradation of harmful pollutants,selective transformations of organics,CO₂ conversion to hydrocarbon fuels and N2 fixation.Exploring highly efficient photocatalysts is the most important part in photocatalysis.Bismuth oxyhalide?BiOX;X=Cl,Br,I?photocatalyst has become one of the most widely studied photocatalysts.By increasing the bismuth content in BiOX,a Bi_xO_yX_z photocatalyst with a layer-like structure and higher conduction band position can be obtained.It is necessary to conduct in-depth studies on Bi_xO_yX_z.At present,photocatalytic reactions mainly focus on carrier photocatalysis,while exciton photocatalysis has been overlooked for a long time.Exploring the role of excitons in photocatalysis is important for understanding the photocatalytic mechanism.In this paper,Bi_xO_yX_z is selected as the research object to regulate the charge separation and exciton generation from the perspective of hollow morphology,doping and surface oxygen vacancies construction.Then the influence of morphology and defect structure on the behavior regulation of photo-induced species and photocatalytic performance is studied in detail.Bi₄O₅I₂ photocatalyst containing surface oxygen vacancy?Bi₄O₅I₂-OV?was prepared with ethylene glycol and ethanol as solvents.The O₂ adsorption energy was obtained by DFT cauclations and it was found that O₂ preferred to adsorb on the defective surface.Further analysis of the density of state after adsorption of oxygen molecules by Bi₄O₅I₂-OV revealed that an energy level mismatched with the bulk and surface occurs.This level mismatch made the surface and bulk phases of the material form a transient homogenous junction,thus promoting the transport and separation of carriers.Bi₄O₅I₂-OV with oxygen-bubbling exhibited significantly enhanced photocurrent intensity compared to that with argon treatment,which inferred that oxygen adsorption promotes carrier separation.Photocatalytic activation of molecular oxygen showed that Bi₄O₅I₂-OV could effectively activate molecular oxygen to produce a large number of superoxide radicals,and the production of superoxide radicals was affected by oxygen vacancy concentration.The higher oxygen vacancy concentration induced the higher molecular oxygen activation.This explained why oxygen vacancy promotes carrier separation from the perspective of molecular adsorption.Carbon-doped Bi₂₄O₃₁Cl₁₀ photocatalyst was prepared by using oxytetracycline hydrochloride as chlorine source and carbon source.The effect of carbon doping oncarrier separation and exciton formation was studied.The result of theoretical calculation indicated that carbon dopants led to charges around the conduction band and the valence band of Bi₂₄O₃₁Cl₁₀ more local,which would cause strong electron-hole interactions.Molecular oxygen activation experiments showed that carbon-doped Bi₂₄O₃₁Cl₁₀ could activate molecular oxygen more effectively,producing singlet oxygen in the exciton photocatalytic process.At the same time,theoretical calculations and experimental analysis proved that an impurity level existed in carbon-doped Bi₂₄O₃₁Cl₁₀,which provided a channel for charge transport and thus promotes charge separation.Photocurrent and electrochemical impedance tests showed that the carbon-doped Bi₂₄O₃₁Cl₁₀ photocatalyst possesses stronger photo-generated carrier separation ability than that of undoped sample.Ascribing to the excellent exciton generation ability,carbon doping significantly improved the photocatalytic degradation activity of Bi₂₄O₃₁Cl₁₀ with a good versatility.At the same time,carbon-doped Bi₂₄O₃₁Cl₁₀ also showed high efficiency of photocatalytic reduction Cr???due to the increase of charge separation.It was the first report that systematically analyzing the regulation of photon-induced species on Bi_xO_yX_z from doping perspective.Oxygen vacancies were introduced on the surface of bulk Bi₂₄O₃₁Cl₁₀(Bi₂₄O₃₁Cl₁₀-OV)to adjust its carrier separation efficiency by hydrogenthermal reduction.By calculating the charge density difference of Bi₂₄O₃₁Cl₁₀ surface before and after introducing oxygen vacancies,it was found that oxygen vacancies could capture electrons and increase charge density.Besides,the oxygen vacancy could effectively adsorb and activate CO₂ molecules.The in situ infrared analysis revealed that COOH*was the main reaction intermediate during the conversion process of photocatalytic CO₂ reduction by Bi₂₄O₃₁Cl₁₀-OV.The energy for the formation of COOH* on Bi₂₄O₃₁Cl₁₀ was calculated to be much higher than that of Bi₂₄O₃₁Cl₁₀-OV.The lower energy for COOH* generation was in favor of the photocatalytic reduction of CO₂.In addition,the defect level provided a charge transport channel that promoted the separation of photogenerated charges.Therefore,Bi₂₄O₃₁Cl₁₀ with oxygen vacancies exhibited a significantly enhanced photocatalytic CO₂ reduction performance than that of bulk Bi₂₄O₃₁Cl₁₀.Bismuth-rich Bi₄O₅Br₂ photocatalyst with hollow morphology was prepared by solvent thermal method as an advanced photocatalyst.The electronic band structure and density of states of Bi₄O₅Br₂ and BiOBr were calculated.The results showed that the band distribution of was more dense and the valence band was more hybrid over Bi₄O₅Br₂,which was beneficial to the migration of photogenerated carriers.At the same time,macroscopic simulation showed that the hollow Bi₄O₅Br₂ produced an obvious resonance phenomenon under illumination,which could induce local electric field andaccelerate carrier separation and transport.The results of photocatalytic CO₂ reduction showed that the CO and CH4 yield by hollow Bi₄O₅Br₂,solid Bi₄O₅Br₂ and solid BiOBr reduced in turn,which proved the cooperation effect of hollow and bismuth-rich structure in promoting the activity.The CO and CH4 generation rate of hollow Bi₄O₅Br₂ achieved 3.16 and 0.500 ?mol g^-1 h^-1,respectively.To the best of our knowledge,this conversion rate was also superior to many previously reported Bi-based and other representative photocatalysts.These results may deepen our understanding of the application of hollow bismuth-rich photocatalysts in energy photocatalysis.