Photocatalytic Hydrogen Evolution Over Transition Metal Complexes Anchoring To Graphitic Carbon Nitride

Energy crisis has always been a huge challenge in the progress of population growth and industrialization.Current energy consumption is mainly based on non-renewable fossil energy,and the produced greenhouse gases cause global warming.As a renewable energy,hydrogen energy holds the advantages of high calorific value,green and clean.Continuous solar-to-hydrogen conversion can effectively solve the increasingly severe energy crisis and environmental damage.Among them,photocatalytic hydrogen evolution technology has been proven to be the greenest,safest and efficient approach for obtaining hydrogen energy,in which the key is to develop a cost-effective,environmentally friendly,stable and efficient visible light photocatalyst.Graphitic carbon nitride（g-C₃N₄）,as a two-dimensional（2D）layered metal-free conjugated polymer,has been widely studied due to its narrow band gap,high stability and easy availability.However,in view of its shortcomings of fast carrier recombination and low visible light utilization,modification methods such as doping with other elements,tuning microscopic morphologies and constructing heterojunctions have been developed to improve its photocatalytic activity.However,due to the existence of overpotential during hydrogen evolution reaction（HER）,noble metals are needed as co-catalysts during the reaction.Therefore,designing a novel and efficient noble-metal-free hydrogen production photocatalyst is a major challenge.In view of this,this study aims at replacing precious metals to produce hydrogen by splitting water.To do that,in-situ and post-chemical modification methods are used to create hydrogen evolution active site on g-C₃N₄ to reduce protons to hydrogen by integrating transition metal complex as a cocatalyst.Meanwhile,π-electron-rich aromatic molecules are introduced to g-C₃N₄ to expand itsπ-conjugated system,resulting in a wider visible light response and faster charge separation.The obtained composite catalyst exhibits excellent hydrogen production activity.The main research contents are as follows:（1）Metal-functionalized photocatalyst was successfully synthesized by in-situ constructing metal coordination sites in g-C₃N₄ framework,so as to realize the construction of transition metal complex co-catalysts and efficient hydrogen production.Among them,pyrimidinone derivative ligand is first covalently bonded to g-C₃N₄ framework by vapor diffusion,and then transition metal ions are fixed under strong coordination interaction.The coordination-introduced metal ions not only serve as catalytic reaction sites,but also improve the utilization of visible light through the metal ion to ligand charge transfer（MLCT）pathway.The obtained Ni/Ph-CN catalyst can produce hydrogen continuously and efficiently without precious metals.The highest hydrogen evolution rate on Ni/DBM（0.12%）-CN is 68.6μmol h^-1（λ≥420 nm）,and the quantum conversion efficiency at 450 nm is 5.57%.（2）An extremely active molecular complex cocatalyst Ni L₂（Cl）₂ was successfully designed through bimolecular nucleophilic substitution reaction.After being covalently anchoring to thiophene-embedded g-C₃N₄ nanosheets through post-chemical modification,the resulting composite catalyst Ni L₂（Cl）₂/TPCN shows excellent hydrogen evolution activity without Pt.A hydrogen evolution rate of 95.8μmol h^-1（λ≥420 nm）and an apparent quantum yield（AQY）of 6.68%（λ=420 nm）are achieved.This exceeds the most reported values under the same conditions.Among them,the embedded thiophene ring not only expands theπ-conjugated system to enhance visible light absorption,but also accelerates the separation of photogenerated hole-electron pairs through the electron withdrawing effect.The C-N bond formed between Ni L₂（Cl）₂ and TPCN significantly promotes the transfer of photogenerated electrons to the active center of metallic Ni,which is the key to achieving high activity and high stability in the process of hydrogen production.