Research On Preparation And Photoelectrochemical Performance Of ZnO-based Heterojunction Composite Structures

Photoelectrochemical water splitting to produce hydrogen is an effective way to alleviate the energy crisis.And it is the key for improving the solar energy conversion efficiency and photoelectrochemical performance to expand the light response range,accelerate the charge separation,and improve the utilization rate of photogenerated carriers of semiconductor photoelectrodes.This paper focuses on the construction and photoelectrochemical properties of ZnO and its composites.Firstly,ZnO nanorod array films were prepared by hydrothermal method.The chemical reaction process and growth mechanism of ZnO nanorods were analyzed,and the influences and related mechanisms of preparation factors on the microstructure and photoelectrochemical properties of ZnO were also discussed.Subsequently,the ZnO/Cu₂O heterojunction photoelectrode was fabricated by electrochemical deposition method and modified by Co-Pi cocatalyst afterwards.The influence of process parameters on the microstructure and photoelectric properties of composite photoelectrodes were analyzed.And the effects of heterojunction and cocatalyst on the light absorption efficiency,photogenerated carriers separation and the utilization efficiency were studied.The results show that ZnO is epitaxially grown by seed layer to form a one-dimensional ZnO nanorod arrays with preferential orientation along the c-oriented on the ITO conductive substrate during hydrothermal reaction.ZnO nanorods with different microstructures can be prepared by controlling the concentration of the growth solution,the reaction temperature and time in the chemical reaction process.The above process parameters play an important role in the photocurrent values of ZnO nanorod electrode in the photoelectrochemical water splitting.When the growth solution concentration is 0.05 M,the growth temperature is 90°C,and the growth time is 2 h,the photocurrent density of the ZnO nanorod electrode can reach 0.99 mA/cm²（1.23 V vs.RHE）.Then,ZnO nanorod electrode was modified by Cu₂O which is a narrow bandgap semiconductor and Co-Pi cocatalyst to construct ZnO/Cu₂O heterojunction and ZnO/Cu₂O/Co-Pi photoelectrodes,respectively.The ZnO/Cu₂O heterojunction and ZnO/Cu₂O/Co-Pi display improved photocurrent density of 1.46 mA/cm² and 1.58mA/cm² at 1.23 V vs RHE,which is 1.475 and 1.596 times compared to that of pure ZnO,respectively.The results of UV-vis spectrum,surface photovoltage spectrum,and impedance spectrum indicate that the enhanced photoelectrochemical property of ZnO/Cu₂O/Co-Pi is attributed to two factors:（1）Cu₂O as a narrow bandgap semiconductor can extend the absorption range;（2）ZnO/Cu₂O heterojunction can improve the separation efficiency of photogenerated carriers;（3）Co-Pi cocatalyst can capture holes,which further improves the separation efficiency of photogenerated carriers.