Anchoring Transition Metal Hydrogen-Evolving Sites Onto Graphitic Carbon Nitride For Photocatalytic Hydrogen Generation

Rapid development of the modern industry consumed a huge amount of fossil fuels,which not only causes energy shortage,but also harms the natural environment.Hydrogen energy is a clean and renewable energy with a high combustion calorific value,which is expected to replace the traditional fossil energy in the future and meet the growing energy demand of human beings.The photocatalytic technology based on semiconductors can realize the conversion of solar energy into hydrogen energy,which is one of the development trends of hydrogen energy.Graphitic carbon nitride（g-C₃N₄）is an organic polymer semiconductor material that is responsive to visible light,but it exhibits low photocatalytic activity due to high photocarrier recombination rate and poor visible light utilization.The photocatalytic activity of g-C₃N₄ can be improved by doping and construction of heterojunction with other materials.However,most g-C₃N₄ based photocatalysts must use the expensive and rare noble metals as co-catalysts to reduce the overpotential of hydrogen evolution reaction in order to achieve hydrogen production.It is a hot research direction to search for new non-precious metal co-catalysts for hydrogen evolution.Therefore,four kinds of novel photocatalysts in this paper were prepared for photocatalytic hydrogen production by building transition metal hydrogen-evolving sites onto g-C₃N₄ and combining with functional nanomaterials.The main contents are as follows:（1）An inexpensive CoL₂（SCN）₂-CN catalyst was obtained by in-situ grafting cobalt molecule catalyst（Co L₂（SCN）₂）from g-C₃N₄ nanosheets（CN）through amide reaction between the abundant amine groups of CN and the carboxyl groups of organic ligands.The Co L₂（SCN）₂sites improve the light absorption of CN to some extent,and serves as a cocatalyst to accelerate separation of photoexcited charge carriers and reduce overpotential for H₂ evolution.The optimized Co L₂（SCN）₂-CN catalyst without any precious metals exhibits excellent performance of H₂ evolution(272mmol g^-1 h^-1)and high stability during photocatalytic reaction.（2）A superior ternary NiL/NiO_x/CN photocatalyst was rationally prepared for highly efficient H₂ production by post-grafting a nickel complex（Ni L）cocatalyst onto g-C₃N₄nanosheets（CN）with nickel oxides（Ni O_x）as a hole transport material.During photocatalytic reaction,the Ni L could replace platinum nanoparticles to effectively decrease the overpotential for water reduction,and the introduction of Ni O_x nanoparticles could significantly improve the separation between photoexcited electrons and holes.Eventually,the final product Ni L/Ni O_x/CN without noble metals exhibited an excellent photocatalytic performance over H₂evolution of 289 mol g^-1 h^-1 in TEOA solution.（3）A low-cost photocatalyst was easily prepared by binding nickel complex hydrogen-evolving catalyst that was covalently grafted from primary amine of chitosan（Ni L）onto CN through electrostatic interaction at room temperature.The introduction of Ni L results in more efficient utilization of solar energy,faster photocarrier transfer and lower overpotential for water reduction.The strategically developed Ni L-CN photocatalyst shows superior H₂evolution rate of 346mmol g^-1 h^-1 under visible light irradiation and exhibits a high stability during stability test.（4）An efficient CuL/CNTs/CN photocatalyst was easily prepared by covalently grafting copper complex（Cu L）from CN modified with carbon nanotubes（CNTs）through a bimolecular nucleophilic substitution reaction.The Cu L co-catalysts uniformly dispersing on CN could enhance decrease the overpotential of water reduction.Besides,the CNTs were employed to separate the photogenerated charge carriers rapidly.The optimized Cu L/CNTs/CN hybrid exhibited a superior photocatalytic H₂ evolution rate of 931mmol g^-1 h^-1 in TEOA aqueous solution.