Study On Preparation,characterization And Mechanism Of Heterojunction Photocatalyst

Photocatalysis utilizes solar energy to evolve hydrogen from water,convert CO2 into carbon fuels,or directly synthesize targeted organics,which are significant for the sustainable development of energy and the environment.However,due to weak redox ability of a single photocatalyst,the low utilization of photogenerated electrons and holes,and the high surface catalytic reaction energy barrier,the current photocatalytic activity is far from reaching the demand for industrialization.It is an important means to construct composite for boosting charge separation,enhancing redox capacity and reducing the energy barrier of surface reactions.In this article,CdS/TiO2,BiOBr/NiO and Bi2MoO6/BiOI S-scheme heterostructures and ZnCdS/Pt Schottky junction composites were constructed for studying their photocatalytic mechanism through a combination of in-situ characterizations and density functional theory(DFT)calculations.First,TiO2 nanoparticles were grown directly in the pores of the shell of CdS hollow sphere to prepare CdS/TiO2 hollow spheres with an intimate heterojunction and high mechanical strength.As a result,the CdS/TiO2 composite exhibited better CO2 photoreduction activity than pure CdS and TiO2.In-situ X-ray photoelectron spectroscopy(ISI-XPS)results show that before light illumination,electrons transfer from CdS to TiO2.When illuminated,electrons transfer from TiO2 to CdS.This result indicates that an S-scheme heterojunction will be formed between CdS and TiO2.The DFT calculations also show that a built-in electric field(IEF)directed from CdS to TiO2 will be formed during the formation of CdS/TiO2 heterojunction to drive photogenerated charge separation,which is a key factor for improving the photocatalytic performance of CdS/TiO2.Then the p-p BiOBr/NiO composite was synthesized for studying the CO2 photoreduction activity.The results of work function and differential charge density indicate that the BiOBr/NiO composite belongs to the S-scheme heterojunction.The ISI-XPS results further confirmed this conclusion.Electron paramagnetic resonance spectroscopy(EPR)verified the strong redox ability of BiOBr/NiO composite through the detection of superoxide radicals and hydroxyl radicals.DFT calculations show that the rate-determining step(RDS)for the formation of CO and CH4 is the step of formation of*COOH.S he preferential production and easy desorption of CO were significant reasons why the yield of CO was greater than that of CH4.Moreover,Bi2MoO6/BiOI 2D/2D van der Waals heterojunction was constructed.The 2D/2D heterojunction enlarges the interface area and exposes more active sites.In-situ diffuse reflectance infrared spectra(In-situ DRIFTS)results show that massive carbonate is adsorbed on the BiOBr/NiO surface under dark conditions.Under light conditions,various CO2 reduction intermediate products,such as formate,methoxy,and formaldehyde groups can be detected.DFT calculations revealed that the RDS for the production of CO and CH4 is the step of hydrogenation and reduction of CO2 molecules to generate*OCHO.The composite of Bi2MoO6 does not affect the RDS,but the energy barrier of the RDS is reduced by 0.35 eV by changing the charge density distribution on the BiOI surface.Subsequently,metal Pt nanoclusters were photo-deposited on the surface of ZnCdS solid solution to study the performance of photocatalytic hydrogen production and furfuryl alcohol oxidation.The contribution of Schottky junction to charge transfer and separation was confirmed through ISI-XPS.In addition,organic reaction intermediates were investigated by EPR and In-situ DRIFTS,and the changes of adsorption free energy of these intermediates on ZnCdS were calculated through DFT.It was found that the energy barrier for the conversion of furfuryl alcohol to furoic acid was significantly higher than that to furfural,elucidating the photocatalytic selective oxidation of furfuryl alcohol to synthesize furfural by ZnCdS/Pt composite.In conclusion,the combination of in situ characterization techniques with DFT can provide insight into photocatalytic reaction intermediates,pathways,and mechanisms,which is a powerful approach for the development of photocatalysis.