Preparation And Photocatalytic Hydrogen Evolution Performance Of Step-scheme ZnO/ZnS Composite Materials

Nowadays,semiconductor photocatalytic hydrogen production technology is an effective method to solve the energy crisis and environmental pollution,which can convert solar energy into chemical energy to improve the utilization of solar energy.ZnO is a wide-gap semiconductor,which has attracted wide attention due to its low cost,simple preparation and controllable morphology.However,the practical application of ZnO is limited by its low visible light utilization and the fast recombination of photogenerated charge.Building heterojunction at the interface between two semiconductors is an efficient way to obtain high performance photocatalysts.ZnS possesses a high conduction band position,which can make up for the low reduction ability of ZnO.In this paper,the controllable preparation of hierarchical structure ZnO/ZnS composite materials for tuning morphology and constructing heterojunction have been investigated,and the S-scheme heterojunction mechanism of the enhanced photocatalytic hydrogen evolution has been proposed.The main contents are as follows:1. The hierarchical ZnO/ZnS hollow spheres composite have been successfully prepared by hydrothermal reaction and ion exchange method.The experimental results showed that the hierarchical hollow structure can enhance the light absorption and provides abundant active reaction sites,which can improve the photocatalytic performance.The hydrogen production activity of ZnO/ZnS composite is 3.2 times as high as that of pure ZnO.The enhanced photocatalytic performance can be attributed to the formation of S-scheme heterojunction between ZnO and ZnS,which can reduce the recombination rate of photogenerated charge carriers and improve the redox capacity in the photocatalytic reaction.2. The ZnO nanofibers were first prepared via electrostatic spinning and high temperature calcination method,and then the 1D S-scheme ZnO/ZnS heterojunction photocatalyst was obtained via ion-exchange method by using impregnation of ZnO nanofibers in Na₂S solution.The prepared ZnO/ZnS heterojunction photocatalyst has uniform 1D structure,which can reduce the transfer distance of photogenerated charges and improve the separation rate of photoinduced electrons and holes.The experimental results demonstrateed that the photocatalytic H₂-production performance of the ZnO/ZnS composite was drastically improved,which was mainly due to the formation of S-scheme heterojunction and the enhanced light absorption performance.Meanwhile,the S-scheme transfer mechanism of photogenerated charge carriers can be proved by DFT calculations and XPS analysis.3. The ZnO/ZnS photocatalyst was prepared via the ion-exchange method to incorporate ZnS onto the outer shell of ZnO flower-like spheres.The experimental results showed that content of ZnS greatly influenced the photocatalytic hydrogen production performance of the ZnO/ZnS composites.When the molar ratio of ZnS to ZnO is 1.5,the composite exhibited the best photocatalytic hydrogen evolution activity(9.97 mmol g^-1 h^-1)under solar light irradiation.DRS results show that the introduction of ZnS can broaden the light response of ZnO/ZnS.In addition,the S-scheme charge transfer mechanism of ZnO/ZnS composite was certified by XPS analysis,work function calculation and hydroxyl radical capture experiment,which can promote the efficient separation of photogenerated charge carriers and improve the photocatalytic performance.