Study On Photoelectrochemical Metal Deposition Driven By Semiconductor Photoelectrodes And Its Mechanism Of Photoinduced Charge Transfer

Faced with the gradually depleting global fossil fuel resources,solar energy has been extensively researched due to its advantages of being clean,environmentally friendly,economical,and sustainable.At present,the development and utilization of solar energy have been increased mainly through various approaches such as photothermal conversion,photoelectric conversion,photochemical conversion.Meanwhile,experts in related fields are striving to improve the utilization efficiency of solar energy.Semiconductor photocatalytic technology is widely regarded as one of the promising technologies to solve the problems of environmental pollution and energy crisis.Photodeposition is a photocatalytic technology that irradiates semiconductors in metal salt liquid solutions to deposit metal（oxide）nanoparticles.It has the advantages of energy conservation and environmental protection and is widely used in wastewater treatment,air purification,etc.It combines electrochemical technology with photocatalytic technology to convert solar energy into chemical energy or electric energy,involving the research of fuel cells,photo-assisted metal-air cells,and so on.1.While metal electrodeposition is a highly efficient preparation technology of environmental protection,it is inevitable that metal electrodeposition is confronted with the problem of high energy consumption.Here,we employ three semiconductor photoanodes with excellent photocatalytic properties,Ti O₂,BiVO₄ and Fe₂O₃,to deposit Cu or Bi at the cathode under illumination without external voltage.It is found that when Ti O₂ and BiVO₄ are used as photoanode,Cu,Cu₂O and Bi can be continuously deposited on the cathode,but when Fe₂O₃ photoanode is irradiated,no material comes out.The related mechanism involves the oxygen evolution reaction（OER）of the valence band holes of the photoanode,the collection of conduction band electrons by back electrode,as well as the reduction of metal ions on the counter electrode,the potential of electrons in the conduction band of the semiconductor photoanode determines whether the reaction can proceed.Such deposition can be considered as a light energy storage process that converts solar energy into chemical energy.In addition,we also put forward the concept of using the deposited Cu or Bi as the metal electrode to form a new metal-air battery to release chemical energy of the deposited metal again.Through the above research,we expect to find a new way for long-term storage of solar energy,and further realize a series of conversion perfectly from solar energy to chemical energy and then to electric energy.2.The recovery of noble metals is crucial in terms of resource utilization and ecological environment.Here,we describe a green strategy for the recovery of three noble metals,Au,Ag and Pt,by photoelectric deposition at the cathode without external voltage.ZnO/ZnS composites with special heterostructures derived from metal organic frameworks（MOFs）,as well as ZnO and Fe₂O₃ semiconductors,were used to study and analyze the realizability and mechanism of photodeposition of noble metals from the aspects of energy band structure and metal ion reduction potential.It was found that ZnO/ZnS had preferable photoelectrochemical performance than pure ZnO due to their effective electron-hole separation and appropriate band matching structure.The potential of electrons in the conduction band of ZnO/ZnS should be more negative than the reduction potential of metals such as Au,Ag and Pt,allowing for the deposition of these metals while simultaneously undergoing an oxygen evolution reaction（OER）,mediated by the photogenerated holes on the surface of the photoanode,and the collection of conduction band electrons by the back electrode.