Preparation And Photocatalytic Performance Modulation Of ZnO/CdS Nanoarrays With CdTe Layer

Hydrogen energy,as the efficient,clean and non-polluting secondary energy,has attracted extensive attention.Photocatalysis or photoelectrocatalysis?PEC?for water splitting was an significant way to produce hydrogen.CdS is a typical II-VI n-type semiconductor material with a suitable visible light absorption range as well as the narrower band gap?2.4eV?.However,a single CdS semiconductor has lower carrier separation efficiency,and low carrier transport and transfer rates especially for the holes,leading to the photoetching of CdS and the poor stability during photocatalysis.The crystallinity of CdS is consider the key factor for the carrier transport and transfer capacity.In this thesis,CdS was combined with ZnO nano-array to form ZnO/CdS nano-heterojunction array and promote the photo-generated carriers separation.Meanwhile,in order to further increase the light absorption range of CdS,CdTe was deposited on the ZnO/CdS nano-heterojunction arrays?NHAs?via electrochemical deposition.In the research,ZnO nanoarrays were grown on FTO conductive glass substrates by hydrothermal method,and the CdS layer was deposited on the surface of ZnO nanorods by chemical bath deposition?CBD?method to form ZnO/CdS NHAs.The crystallinity of CdS and interface quality between ZnO and CdS were improved by annealing with the assistance of CdCl₂.CdTe layer was electrodeposited on CdS/ZnO NHAs to improve the light absorption range.The morphology,crystalline structure and composition of the NHAs were characterized by X-ray diffraction?XRD?,scanning electron microscope?SEM?,transmission electron microscope?TEM?and X-ray photoelectron spectroscopy?XPS?,respectively.The PEC performance were characterized by the linear scanning voltammetry?LSV?and electrochemical impedance spectroscopy?EIS?,respectively.The experimental results showed that annealing at 430?C with the CdCl₂-assisting heat-treatment could improve the crystallinity of CdS and the densification of CdS layer,and Zn_1-xCd_xS intermediate layer was formed at the ZnO/CdS interface.Further analysis of EIS results showed the increase of PEC photocurrent was due to the improvement of CdS crystallinity which enhances the hole transfer rate.The improved densification of CdS layer caused by annealing and the formation of the Zn_1-xCd_xS intermediate layer effectively prevent the internal ZnO from being corroded by the electrolyte solution,which benefits to the stability of PEC.Considering the limited light absorption range of ZnO/CdS NHAs,CdTe thin film was electrodeposited onto CdS/ZnO NHAs and the annealing process was optimized to improve the light absorption range.It was found that the NTA/Cd²⁺complexing ratio in the electrodepositon solution had deep influence on the chemical etching of the internal ZnO core of CdS/ZnO NHAs.The optimized complexing ratio of 1.8:1 was demonstrated to be suitable for preventing ZnO etching.Then,the interdiffusion between CdTe and CdS was also demonstrated by CdCl₂ assistant-heat treatment,and the formed CdS_1-xTe_x alloy layer was found to improve the light absorption range,resulting in the improved PEC current compared to the previous ZnO/CdS.