Ag₃PO₄-based Composite Materials:Controllable Synthesis And Z-Scheme Photocatalytic Water Splitting

Semiconductor photocatalysis is a clean、cyclic utilization and non-pollution energy conversion technology.It can not only transform the renewable solar energy into chemical energy,but also can oxidize and degradation organic pollutants.It is expected to provide an effective solution for the energy shortage and environmental pollution problems.As a result of single component semiconductor catalyst is difficult to achieve good chemical and photo stability、visible light response、photogenerate electron-hole separation fast、appropriate valence band（VB）and conduction band（CB）position and the electrode potential of water splitting reaction.It is difficult to utilize solar energy to realize photocatalytic total water splitting efficiently.Therefore,the key and the only way to realize the photocatalytic decomposition of water is to design the photocatalytic composite with visible light response by building a semiconductor-semiconductor composite catalyst.which is an effective way to realize the photocatalytic decomposition of water by solar energy.The research shows that the semiconductor heterostructures can effectively reduce the photogenerated electron-hole recombination phenomenon.However,the redox ability of the photogenerated electron and hole in the traditional heterostructure catalytic system is further weakened when the energy level is moved to a lower level in the energy level.This will lead to the semiconductors heterojunction not able to reduce the water molecules into hydrogen or oxygen.Inspired by the artificial photosynthesis,when constructing Z-Scheme photocatalytic water splitting system,semiconductors selecting only needs to consider whether it can satisfy the redox potential of the water splitting.Furthermore,the selection of different intermediate electron transport mediums can play a role in the effective transmission and separation of the photogenerated electrons and holes.The oxidation capacity of the cavity retained at the location of the semiconductor VB is stronger,so as to achieve efficient photocatalytic production of oxygen.Herein,three kinds of composite catalytic systems are designed and constructed by using g-C₃N₄ and Ag₃PO₄ as semiconductor materials and using graphene or different morphologies of MoS₂ as intermediate carrier medium.The composite photocatalyst has different chemical composition and micromorphology.We will further study the mechanism of photocatalytic oxygen production performance and mechanism:（1）Based on precursor copolymerization modification method,g-C₃N₄ was prepared by thermal condensation polymerization.By liquid phase synthesis,Ag₃PO₄、monolayer（less layer）graphene and nanostructured g-C₃N₄ were prepared Ag₃PO₄/GR/g-C₃N₄ composite photocatalyst via nano composite technology、electrostatic driven self-assembly and ion exchange process.The composite catalyst constituent、crystal structure and micromorphology of the composite materials were analyzed by different characterization methods.It was found that the interface contact of the composite catalyst was better.The results of photocatalytic oxygen production test showed that the oxygen generation activity of the composite catalyst increased several times than pure Ag₃PO₄ material.（2）Preparation of lamellar MoS₂ by ultrasonic stripping bulk-like MoS₂.Then in situ formation of Ag₃PO₄ particles between g-C₃N₄ and MoS₂ through electrostatic driven ion self-assembly,and preparation of Ag₃PO₄/nanosheet MoS₂/g-C₃N₄ three constituents composite catalyst.Preparation of flower-like MoS₂ by hydrothermal method and used as precursor to prepare Ag₃PO₄/flower-like MoS₂/g-C₃N₄ three constituents composite catalyst.The results of SEM and EDS mapping analysis show that the composite has good interface contact.A variety of spectroscopic tests on composite catalysts and semiconductor monomers show that the optical absorption properties of the composite catalysts are better than pure Ag₃PO₄ or g-C₃N₄,and the composite catalysts have lower electrochemical impedance and higher photocurrent density.The results of photocatalytic oxygen generation test also showed that the oxygen generation performance of the prepared composite catalyst was much higher than that of pure silver phosphate.In addition,the test found that the flower-like MoS₂ has more suitable energy level matching and higher contact area with other semiconductors,and is more suitable to be a carrier carrier transmission medium than the lamellar MoS₂ in the Z-Scheme catalyst system,which makes the prepared composite photocatalyst have a high catalytic activity.（3）The type and concentration of free radicals produced in the process of photocatalytic reaction were detected by ESR test on the composite catalyst and Ag₃PO₄.The results showed that the composite catalyst had a stronger to generate free radicals’ability.The separation efficiency and life of the photogenerated carrier in the catalyst were detected by the PL.It was found that the composite catalyst had higher separation efficiency of photoelectron hole pair.The band gap width of different semiconductor monomers is fitted by using the Kubelka-Munk function with the UV-vis spectrum,and the CB and VB position of different semiconductor monomers are measured by Mott-Schottky spectra and VB XPS spectrum.The photocatalytic reaction mechanism of different series of composite materials is speculated according to the results of various analytical tests.