Design Of S-scheme Bismuth Phosphate-based Heterojunction For Simultaneous Redox Removal Of Pollutants

In recent years,S-scheme heterojunction photocatalytic systems have attracted much attention,which has been widely used for hydrogen production,carbon dioxide reduction and pollutant removal.BiPO₄ with low cost and high activity has become an ideal material for the construction of S-scheme photocatalysts due to its high oxidation potential.However,few studies related to bismuth phosphate S-scheme heterojunctions have been reported.In this paper,from the energy band engineering,polypyrrole（PPy）and cadmium sulfide（CdS）with similar energy band structure were deposited on bismuth phosphate-based to design S-scheme heterojunction composites to investigate their mechanism of simultaneous removal of heavy metals and organic pollutants.（1）S-scheme PPy/BiPO₄ composite photocatalysts were prepared by one-step solvothermal method,and the rate constant of PPy/BiPO₄ was enhanced to 0.0308min^-1 compared with that BiPO₄ for the degradation of tetracycline hydrochloride（TCH）at 60 min under visible light(0.0107 min^-1).The great photocatalytic performance was attributed to the S-scheme heterojunction,which could improve charge separation efficiency,inhibit photogenerated electron-hole pair complexation,and retain the strongest redox ability of the material to the maximum extent.The results of masking experiments and characterization tests show that·OH、·O₂^-and h⁺were the major active species.The reaction mechanism of S-scheme PPy/BiPO₄photocatalyst for pollutant removal was clarified by PL spectrum,instantaneous photocurrent response,EIS,ESR and other testing methods.However,S-scheme PPy/BiPO₄ photocatalyst was unable to reduce Cr（VI）.DFT calculations showed that the bonding ability at the contact interface between BiPO₄ and PPy was weak,and PPy contributed little e^-to the conduction bottom of the system,so that S-scheme PPy/BiPO₄ photocatalyst could not have great heavy metal reduction performance.（2）S-scheme PPy/BiPO₄ composite photocatalysts were prepared,and the rate constant of CdS/BiPO₄ was enhanced to 0.0556 min^-1 compared with the rate constant of TCH degradation by BiPO₄ at 60 min under visible light(0.0172 min^-1).The rate constant of Cr（VI）reduction was increased from 0.0007 min^-1 to 0.0293min^-1.In the TCH/Cr（VI）system,compared with the rate constant of TCH degradation by CdS/BiPO₄ alone(0.0556 min^-1),the rate constant of TCH degradation in the synergistic system increased to 0.0680 min^-1.The rate constants for Cr（VI）reduction increased from 0.0293 min^-1 to 0.0336 min^-1.The results of masking experiments and instrumental tests indicated that e^-and h⁺were the main active species.PL spectroscopy,instantaneous photocurrent response,EIS,ESR and other tests were used to reveal the reaction mechanism of S-scheme CdS/BiPO₄photocatalyst for simultaneous redox of pollutants.Among them,S-scheme CdS/BiPO₄ photocatalyst had a strong ability to reduce Cr（VI）.Theoretical calculations showed that the bonding was intense and the interaction force extremely strong at the contact interface between CdS and BiPO₄,and CdS contributed abundant e^-to the conduction band bottom of the system,which made S-scheme CdS/BiPO₄photocatalysts have extremely strong oxidation and reduction ability in the process of pollutant removal.