Preparation Of G-C₃N₄-based Composites For Photocatalytic Production Of Hydrogen Peroxid Under Visible Light Irradiation

Hydrogen peroxide（H₂O₂）is an environment-friendly and green oxidant that has been widely used in the fields of organic synthesis,environmental remediation,disease therapy and fuel cells.Anthraquinone method is conventionally used for H₂O₂production in industry,but suffering from a series of drawbacks,such as a large number of by-products,and high cost due to the use of noble metal（i.e.Pd）based catalysts.What’s more,the H₂and O₂mixtures in feedstock are potentially explosive.Recently,as a green and safe strategy,photocatalytic H₂O₂production has attracted much attention,which only relies on water and oxygen as raw materials.However,most of the commonly used photocatalytic materials have some shortcomings,such as weak ability for catalyzing water oxidation to release protons,single mode for H₂O₂production and low H₂O₂yield.Accordingly,three kinds of g-C₃N₄based photocatalytic materials with Z-scheme heterojunctions were constructed by preparing a,βandγ-g-C₃N₄nanosheets as the carriers,respectively combined with N₂plasma treatment,PVP surfactant assistance and in situ growth for loading Cd S,Fe₂O₃and Bi₂WO₆.Herein,these photocatalysts effectively overcome the current shortcomings of g-C₃N₄nanosheets,such as high carrier recombination rate,then realize the efficient photocatalytic H₂O₂production in pure water under visible light irradiation via the two-step single-electron,one-step two-electron,and ¹O₂-dominated multi-step oxygen reduction pathway,respectively.Cd S/N@α-g-C₃N₄nanosheets with Z-scheme heterojunctions were designed for the photocatalytic H₂O₂production through a series of preparation steps including solvothermal exfoliation ofα-g-C₃N₄,N₂plasma pre-treatment,Cd²⁺and S^2-impregnation.During impregnation,the Cd²⁺could react with S^2-to generate Cd S nanoparticles on N@α-g-C₃N₄nanosheets.In addition,the unsaturated N created by N₂plasma treatment on the nanosheets could act as an electron mediator so as to induce the photoelectrons on the Cd S transferring toα-g-C₃N₄,thus establishing a Z-scheme heterojunction.The energy band structure analysis shows that the protons can be released through the four-hole water oxidation reaction dominated by Cd S.The active species capture experiments show that composite features a two-step single-electron oxygen reduction reaction for H₂O₂production.It was discovered that such composite can present a H₂O₂production rate of 0.96μmol·g^-1·min^-1under visible light irradiation,and its apparent quantum yield at 420 nm is 6.43%.β-g-C₃N₄thin nanosheets were prepared by calcining ethylene glycol-intercalated cyanuric acid through a two-step heating process.Then,carbon quantum dots（CQDs）and Fe₂O₃（serve as two active sites with completely different functions）were decorated onβ-g-C₃N₄nanosheets with the aid of PVP surfactant through a solvothermal process,forming the Fe₂O₃/CQD@β-g-C₃N₄nanosheets with Z-scheme photocatalytic heterojunction.The band structure analysis shows that the protons can be released through the two-hole water oxidation reaction dominated by Fe₂O₃.Active species capture experiments and the rotating ring disk electrode tests both confirm that the composite features an efficient one-step two-electron oxygen reduction for H₂O₂production.Importantly,this study also proposes the design for the Fe₂O₃/CQD@β-g-C₃N₄nanosheets with Z-scheme heterojunction,which employ an optimized two-channel photocatalytic reaction as well as two-electron oxygen reduction and two-hole water oxidation for efficient H₂O₂production in pure water,without any noble metal co-catalyst or organic scavengers.It can be seen that the the composite can present a H₂O₂production rate of 1.81μmol·g^-1·min^-1under visible light irradiation,reaching an apparent quantum yield of 14.29%at 420 nm.P dopedγ-g-C₃N₄thin nanosheets were prepared by calcining phosphonitrilic chloride trimer and ethylene glycol co-intercalated cyanuric acid through a two-step heating process.Sodium tungstate as the W source was impregnated on the surface of P/γ-g-C₃N₄.Then,Bi₂WO₆was in-situ grown at W sites forming the Bi₂WO₆@P/γ-g-C₃N₄nanosheets with Z-scheme heterojunctions through a microwave hydrothermal process.P and Bi₂WO₆were respectively used as the reaction sites for oxygen reduction and water oxidation in Z-scheme heterojunction.Herein,Bi₂WO₆could oxidize water to·OH,then release the protons in water to promote the oxygen reduction reaction.P atoms doped in g-C₃N₄have optimized the pathway of oxygen reduction.Active species capture experiments confirm that the doped P can transform the traditional two-step single-electron oxygen reduction into a novel multi-step oxygen reduction dominated by ¹O₂.Moreover,the composite can present a high H₂O₂production rate of 2.68μmol·g^-1·min^-1under visible light irradiation,reaching an apparent quantum yield of 24.66%at 420 nm,without the aid of any noble metal co-catalyst or organic scavengers.