Controlled Preparation And Photocatalytic Hydrogen Evolution Study Of Novel Photocatalysts

With the continuous development of economy,energy and environment issues have become a hot research topic at present.As a kind of clean and renewable energy,hydrogen energy has attracted increasing attention.Photocatalytic hydrogen evolution can alleviate energy depletion caused by economic development to a large extent.Studies show that there are two factors that seriously restrict the performance of photocatalytic hydrogen evolution in the photocatalytic reaction,that is,the light absorption range and the effective transfer of photogenerated carriers.In this paper,through the synthesis of common novel photocatalysts(g-C₃N₄ and TiO₂)and their reasonable modification,based on theoretical calculations,the relationship between the photoabsorption range and the effective transfer of photogenerated carriers and the hydrogen production performance of the photocatalyst was studied.Specific research contents are as follows:1.We study the the relationship between promoting charge transfer and broadening visible light absorption and enhancing the photocatalytic hydrogen evolution(PHE)properties of asymmetric embedding of benzene ring in g-C₃N₄.Based on theoretical calculation,the local asymmetric embedded benzene destroyed the heptazine part of the g-C₃N₄,which changed the electronic structure,and promoted the charge transfer of g-C₃N₄,and broadened the light absorption range.The benzene was reasonably embedded into g-C₃N₄ experimentally,which changed the local symmetry without changing its long-order structure.Photoluminescence(PL)and time-resolved transient PL spectra confirm that the electron transfer efficiency of the benzene ring embedded g-C₃N₄ is greatly improved.The hydrogen evolution rate of modified g-C₃N₄displays 10.8 times higher than that of original g-C₃N₄.This work provides a new method for the design of efficient two-dimensional photocatalytic water splitting materials.2.We study the relationship between promoting charge transfer and broadening light absorption and PHE in visible light by molecular modified g-C₃N₄.The semiconductor photogenerated carriers'fast recombining severely limits the photocatalytic activity.Here,we improve the catalytic activity by adjusting the electronic structure of the semiconductor photocatalyst.Experimentally,we first synthesized the ultrathin accordion-like g-C₃N₄by thermally polymerizing oligomer modified with small molecules at high temperature.The formed g-C₃N₄ structure changes the electronic structure and the energy band structure of the g-C₃N₄,and the utilization of visible light has been further increased.Also,the modified g-C₃N₄structure also changes the interlayer interaction to form an ultrathin layered two-dimensional structure,which increases its specific surface area and reduces the photogenerated carrier recombination between layers.The resultant ultrathin accordion-like g-C₃N₄ exhibits excellent activity for PHE,which is 20.1 times of that of pristine g-C₃N₄.This work provides a novel strategy for the design of efficient photocatalysis systems for solar energy conversion.3.We study the the relationship between promoting charge transfer and broadening the absorption range and PHE by high carbon doped TiO₂(HC-TiO₂).The HC-TiO₂ was prepared in situ by exfoliated Ti₃C₂.The carbon doping can improve photogenerated carriers'lifetime and broaden spectral response range.The high content of carbon doping leads to the band tail state from the strong electron-withdrawing of carboxylate groups,which promotes the effective transfer of photogenerated carriers and adjusts the energy band position.The HC-TiO₂ was 9.7 times higher than that of P25 without cocatalyst for PHE.This study broadens the preparation strategy of C-TiO₂and broaden the applied possibility of MXene for PHE.4.We study the relationship between promoting charge transfer and enhancing PHE in H-doped TiO₂ by wet chemical method,and realize the switchable defect of photocatalyst.Due to replacing oxygen atoms by hydrogen atoms in the lattice to form Ti-H bond,the electronic structure of TiO₂ material changes significantly,that is,the change of asymmetry of charge distribution around Ti and O atoms and electron density of states,which further increases the absorption of visible light,effectively separates photogenerated carriers and reduces the free energy of hydrogen adsorption after the incorporation of H into TiO₂.The results show that the PHE activity is 60 times that of commercial rutile.In addition,by controlling the annealing conditions,switchable defects can be achieved in H-TiO₂:in Ar atmosphere,high temperature annealing causes the Ti-H bond to be converted to oxygen vacancy by losing water.On the contrary,Ti-H defects are replaced by Ti-O to form defect-free TiO₂ in O₂ atmosphere at high temperature.This work provides a new and powerful method for the preparation of hydrogenated TiO₂,and a possibility to control the defects and improve the photocatalytic activity.