Study On The Full Solar Water Splitting Via CdS-Based Nanoarrays

Hydrogen energy is considered as an efficient,clean and pollution-free secondary energy,and has attracted extensive attention.Solar water splitting is an important approach for producing hydrogen.CdS is a typical n-typeⅡ-Ⅵsemiconductor material,and the narrower band gap（2.4 eV）can absorb visible light.Hower,the lower electron-hole separation efficiency and the easy photoetching by hole during the photoelectrocatalysis（PEC）may reduce PEC efficiency.So,the sacrificial agent is generally added to accept holes from CdS to prevent itself oxidization.But,sacrificial agent will increase the hydrogen production cost.In this paper,ZnO/CdS nanoheterojunction arrays（NHAs）was formed to improve the separation efficiency of photogenerated carriers,and oxygen-generating cocatalyst was explored to prevent the photocorrosion of CdS in the solution without sacrificial agent.In this paper,ZnO nanoarrays were synthesized on FTO conductive glass substrate by hydrothermal deposition.Then,a layer of CdS was desposited onto the surface of ZnO nanorods by chemical bath depotion（CBD）method to form ZnO/CdS NHAs.The crystallinity of CdS and the interface quality of ZnO/CdS NHAs were improved by post-annealing.Further,a co-catalyst layer was deposited onto the CdS surface to improve the PEC stability.The morphology,crystalline structure and element composition of the NHAs were characterized by XRD,SEM,TEM and XPS,respectively.The PEC performance of NHAs were characterized by the cyclic voltammetry（CV）and electrochemical impedance spectroscopy（EIS）.The SEM and XRS results showed that CdCl₂-assisting annealing at 430 ^oC improved the crystallinity and the density of CdS layer,and Zn_1-xCd_xS buffer layer was formed at the ZnO/CdS interface.At the same time,the improved density of CdS and the formation of Zn_1-xCd_xS buffer layer by annealing prevented the corrosion of ZnO rods by the electrolyte solution,which contributes to the PEC stability.The effect of loading CoPi onto the surface of CdS on the PEC performance of ZnO/CdS NHAs was studied.The results showed that CoPi layer formed by photoelectrical deposition and electrochemical deposition was extremely thin and had the amorphous structure.CV results indicated that the CoPi-ZnO/CdS NHAs formed by the photoelectrical deposition for 15 min had the higher PEC performance,which had the PEC photocurrent of 2.5 mA.cm^-2 and kept stable for 3 h.This was attributed to the much denser CoPi layer loaded by photoelectric deposition since it effectively prevented the exposure of the CdS layer.Further,EIS results revealed that the improvement of the PEC performance was due to the improved hole transfer rate of CoPi at the heterojunction array/solution interface,which effectively prevented the accumulation of the holes in the CdS and then the oxidization of CdS.However,the PEC efficiency of CoPi ZnO/CdS NHAs in sacrificial agent solution was lower compared to that in the solution without co-catalyst.The main reason was attributed to lowered oxidation potential of full solar water splitting when co-catalyst was applied because the amorphouse CoPi is similar to the surface state which might increase the recombination probability of photon-generated carriers.The further study showed that although the existance of CoPi improved the PEC stability,the photocurrent is also reduced after 3-hour testing.SEM and XRD results confirmed that the damping current was caused by the corrosion of ZnO by phosphate ions in the electrolyte solution.Thus,the borate system co-catalyst（CoBi or NiBi）was explored to improved the PEC stability.The results showed that the oxidation potential sequence of CoBi,NiBi and CoPi was in the order of CoPi<CoBi<NiBi.The the PEC stability of the CoBi-ZnO/CdS NHAs was increased from 3 h to 10 h.It is proved that the borate cocatalysts could provent the corrosion of ZnO and is effective to improve the PEC stability.