Strengthened Charge Separation And Mechanistic Insights Of Ag₃PO₄-Based Heterojunction Photocatalysts For The Oxygen Evolution Reaction

Semiconductor-based photocatalytic technology achieving the conversion of solar-to-fuel is considered as an effective strategy to solve energy shortage and environmental pollution.Although many semiconductors have been proved to have photocatalytic activity,the recombination of photogenerated electron-hole pair severely limiting the separation and transfer of charge carriers has a negative effect on the photocatalytic efficiency.Therefore,the design and synthesis of highly efficient photocatalyst materials for solar-light-driven photocatalytic water splitting has proven extremely challenging.The researches have indicated that the photocatalytic efficiency can be improved by the effective charge separation through the construction of composite photocatalysts.In addition,in order to achieve photocatalytic overall water splitting,it is necessary to solve the poor oxygen-evolving efficiency originating from intrinsically sluggish oxygen evolution reaction（OER）kinetics.It has been proved that silver phosphate（Ag₃PO₄）with a good visible light response and strong oxidation capacity can be used for photocatalytic oxygen generation from water splitting.However,the photocatalytic efficiency of Ag₃PO₄ is severely limited by self-corrosion of Ag₃PO₄.Herein,we reported the construction of Ag₃PO₄-based composite with a compact contact interface for the application of photocatalytic water oxidation.By effectively assembling with the second phase materials,the self-corrosion phenomenon of Ag₃PO₄ is weakened,and the carrier separation and transmission efficiency is improved,thus the photocatalytic water oxidation activity of Ag₃PO₄ is enhanced.The detailed research contents are as follows:（1）According to the matched energy band structure of graphitic carbon nitride（g-C₃N₄）,Ag₃PO₄/modified g-C₃N₄ composite photocatalyst was constructed by the surface modification of g-C₃N₄ treated by alkali,the electrostatic self-assembly and ion exchange.Compared with pure Ag₃PO₄ and traditional Ag₃PO₄/g-C₃N₄ composites,the water oxidation activity of obtained Ag₃PO₄/fish scale-like g-C₃N₄ composites based on the surface modification has been significantly improved.The results suggested that effective surface alkali treatment of g-C₃N₄ optimizes interfacial interactions between two semiconductors and the strengthened charge separation can accelerate the photocatalytic oxygen evolution efficiency of Ag₃PO₄.（2）Considering the influence of thermal condensation conditions on the synthesis of g-C₃N₄,the synthetical conditions of alkali treated g-C₃N₄ was optimized by adjusting calcining atmosphere and heating rate.The photocatalytic activity of Ag₃PO₄/modified g-C₃N₄ composite photocatalyst was further enhanced.By analyzing the structure and dynamic properties of the optimized g-C₃N₄ material as well as the photoelectrochemical properties of Ag₃PO₄/modified g-C₃N₄ composite,it can be drawn that the optimized calcined conditions could result in g-C₃N₄ with a more complete structure.And the defected site acting as an electron trap was introduced into g-C₃N₄ by the pretreatment of KOH,which promotes the carriers separation in g-C₃N₄ and Ag₃PO₄/modified g-C₃N₄ Z-scheme-type composites,and ensures that more holes participate in the reaction.（3）2D titanium carbide（MXene）nanosheets with abundant metal active sites and higher conductivity were prepared by etching,intercalation and exfoliation of Ti3 Al C2.And the Ag₃PO₄/2D MXene composite photocatalysts with good contact interface were synthesized by the in-situ growth of Ag₃PO₄ particles on MXene nanosheets driven by electrostatic interaction and ion exchange.The photoelectrochemical results indicated that 2D MXene nanosheets could act as a good charge transfer medium in the composite system.Moreover,the excellent metal-like conductivity of MXene can effectively increase the photogenerated charge transmission rate.The unique Schottky-junction electronic configuration in photocatalysts can promote the separation and transfer of electrons from Ag₃PO₄ to 2D MXene nanosheets,which can significantly reduce the photo-corrosion of photogenerated electrons on Ag₃PO₄ semiconductor.Moreover,the strongly oxidized holes retained in the valence band（VB）position of Ag₃PO₄ can adequately participate in the photocatalytic water oxidation reaction.