Research On Ion Doping And Composite Modified G-C₃N₄ And Photocatalitic Hydrogen Evolution

Obtaining hydrogen energy via solar water splitting is considered to be one of the effective strategies for solving the excessive dependence on fossil fuels which causes energy crisis and environmental pollution,and semiconductor photocatalysts play the most central role during the solar-to-hydrogen energy conversion.Therefore,finding new-type semiconductor photocatalyst with high performance and low cost has become the core issue in the practical industrial production of hydrogen energy.Among the many reported photocatalysts,graphitic carbon nitride（g-C₃N₄）,as a novel non-metallic polymer semiconductor photocatalyst,has been considered as one of the photocatalysts with great application potential due to its environmental-friendly synthesis method,cheap raw materials,unique physicochemical properties,and ability to response visible light.However,the photocatalytic activity of unmodified g-C₃N₄ is hard to meet the practical application due to the low photogenerated carriers’transport ability.Aiming at this problem,this thesis prepared modified g-C₃N₄ by heteroatom co-doping,porous structure constructing and in situ loading of non-noble metal cocatalysts to enhance the mobility.The internal migration ability and surface transfer of carriers were optimized,thereby realizing the improvement of the photocatalytic hydrogen evolution performance of g-C₃N₄ based catalyst.Aiming at the problem of the weak charge carrier migration in g-C₃N₄,the K/I co-doped porous g-C₃N₄（KI-PCN）was prepared by thermal condensation with porous silica spheres（Si O₂）as hard template,dicyandiamide（DCDA）as the precursor,and potassium iodide（KI）as the heteroatom doping source,respectively.Morphological and structural studies exhibited that the synthesized material process continuous pores of about 40-80 nm,and K/I are uniformly and effectively doped inside g-C₃N₄.Through photo-electrochemical tests,density functional theory（DFT）calculations and finite-difference time-domain（FDTD）optical simulations,the different division of labor and synergistic effects of porous structure and heteroatom doping during the photocatalytic process were discussed.The introduction of I^-ions can optimized the energy band structure of g-C₃N₄,thereby expanding the response range of the material to visible light.K⁺promoted the cross-layer migration of carriers,thereby improved the marrier migration ability in g-C₃N₄.The introduction of porous structure enhanced the transmission of visible light to the interior of the material,which enabled efficient excitation inside g-C₃N₄.Under the condition of loading Pt cocatalyst,the photocatalytic hydrogen evolution rate is significantly increased to 752μmol·g^-1·h^-1,which is about 8.9 times that of unmodified g-C₃N₄.Aiming at the problem of dependence on noble metal cocatalyst Pt to reduce the overpotential of hydrogen evolution during the carriers’transfer of g-C₃N₄ surface to H⁺,based on the in situ gas-solid reaction strategy,the preparation of g-C₃N₄ and the synthesis route of Mo S₂ were optimized and integrated,and the g-C₃N₄ loading with2H-Mo S₂ was synthesized with thiourea and Mo O₃ as precursor.With control experiment,the thesis proved that during the process of thermal condensation of thiourea to form g-C₃N₄,its by-product H₂S was released and Mo O₃ was in situ sulfide to Mo S₂.Morphological and structural characterizations demonstrate that the in-situ reaction can maintain the rod-like morphology of Mo O₃ while realizing the conversion of oxides to sulfides.Based on photoelectrochemical tests and characterizations,it was proved that the carriers generated by g-C₃N₄ would migrate to the surface of the cocatalyst 2H-Mo S₂ with higher work function after migrating to the interface.Due to its lower hydrogen evolution overpotential,electron can rapidly reduce H⁺to produce hydrogen.The composite exhibited a significant enhanced in photocatalytic hydrogen evolution activity in absent of noble metal cocatalyst Pt,reaching 133.1μmol·g^-1·h^-1.With WO₃ and thiourea as precursors,a 1T/2H mixed-phase WS₂ cocatalyst was prepared on the basis of constructing the cocatalyst WS₂ supported g-C₃N₄,and the in situ gas-solid phase synthesis strategy was proved to also be effective for WO₃precursor.Through the morphological and structural characterization,it is proved that the rod-like h-WO₃ can obtain a mixed-phase 1T/2H-WS₂ cocatalyst with a unique segmented hollow morphology through this synthetic route,and it is verified that the synthesis of the 1T phase is affected by the crystal form and precursor ratio.Through photoelectrochemical tests,it is proved that the 1T/2H-WS₂ cocatalyst with higher work function has a good migration ability for photogenerated electrons excited and migrated to the surface of g-C₃N₄,and the metal phase 1T-WS₂ is discussed to reduce the hydrogen evolution process positive contribution to the potential and accelerated carrier migration.Without the participation of the noble metal cocatalyst Pt,the synthesized samples showed a significant improvement in the photocatalytic hydrogen evolution performance,reaching 347.6μmol·g^-1·h^-1.