Switching between Z-scheme and type-II charge separation mechanisms in ZnO/ZnS composite photocatalyst by La doping

In order to study the photo-generated carrier transfer mechanism in ZnO/ZnS-based composite photocatalyst, a series of ZnO/ZnS composite samples were prepared by two-step method. Firstly, La-doped ZnO nanorods were grown on silica substrates with hydrothermal method, and then the nanorods were sulfurized in situ to form ZnO/ZnS heterojunctions. The effect of La doping concentration on the structural properties of ZnO nanorods was investigated. With the increase in La doping concentration, the diameter and density of ZnO nanorods increase, but when the doping concentration reaches 2.0%, ZnO crystal quality degrades. Therefore, the ZnO nanorods with La doping concentration of 1.5% were used as matrix to study the effect of sulfurization time on the properties of ZnO/ZnS-based composite. With the increase in sulfurization time, the UV–vis absorbance of the samples first increases and then decreases. The composite photocatalyst with sulfurization time of 60 min shows the best UV–vis absorbance, and its photocatalytic performance is also the best. Then, the photo-generated carrier transfer mechanism in the ZnO/ZnS-based composite was studied. Because the Fermi level of ZnS is higher than that of ZnO, the electrons in ZnS would be transferred to ZnO when they form a heterojunction, then a built-in electric field is constructed from ZnS to ZnO. Generally, the photo-generated carrier transfer mechanism in ZnO/ZnS heterojunction follows a Z-scheme mechanism. However, when La element is doped at a concentration of 1.5%, the Fermi level ZnO is increased and higher than that of ZnS. Then, the electrons in ZnO are transferred to ZnS to form a new built-in electric field from ZnO to ZnS in the heterojunction, and its direction is opposite to that of the undoped ZnO/ZnS composite sample. Then, the photo-generated carrier transfer mechanism in the composite is changed to type-II. Therefore, the photo-generated carrier transfer mechanism in the ZnO/ZnS-based composite can be tailored by La doping concentration.