Carbon-coated Transition Metal Nanoparticles Modified G-C₃N₄ Improve Photocatalytic Activity Effectively

Environmental pollution control and development of green and sustainable energy are the two major global challenges of the times.Solar energy,as an inexhaustible energy source,is considered to be the first choice for human to deal with the energy crisis.The development of semiconductor materials for collecting solar energy to produce cleaner fuel and solve environmental problems has become a hot topic.In recent years,the application of visible light catalysis technology in the field of environment and energy has received great attention.Photocatalytic decomposition of water for hydrogen production is to convert unstable and hard-to-store solar energy into stable and easy-to-use chemical energy to achieve the goal of green and sustainable use of energy.The decomposition of water on nano-photocatalysts is considered to be a low-cost technology,which shows the potential of large-scale solar hydrogen production in the future.Graphite phase carbon nitride（g-C₃N₄）as an organic semiconductor catalyst has been widely used in the field of photocatalysis because of its suitable band gap and optical absorption range.However,in the process of photocatalytic hydrogen production,the reduction potential of g-C₃N₄ is too high and the recombination rate of photogenerated electron holes is high.In this paper,the photocatalytic activity of g-C₃N₄ was improved by the preparation of cheap and easy-to-obtain transition metal cocatalysts.The morphology and structure were analyzed by means of XRD,SEM,TEM,XPS,BET,DRS,UV-vis,and the activity of photocatalytic hydrogen production was compared by photocatalytic hydrogen production test.Finally,the activity of photocatalytic hydrogen production was compared with that of PL,EIS,HER et al inferred the possible mechanism of improving the photocatalytic activity of g-C₃N₄-based composite photocatalyst.The results are as follows:（1）The Cu@C nanoparticles prepared by one-step annealing are used as cocatalysts for g-C₃N₄ photocatalysis to produce hydrogen.For Cu@C/g-C₃N₄ photocatalysts,Cu@C nanoparticles increase the transfer efficiency of photogenerated electrons by capturing photogenerated electrons produced by g-C₃N₄,thus reducing the recombination rate of photogenerated electrons and holes and the optimum photocatalytic efficiency of Cu@C/g-C₃N₄(265.1μmol g^-1 h^-1)is close to that of 0.5%Pt/g-C₃N₄ photocatalyst,which is about 27times of that of bare g-C₃N₄.The stability tests of continuous hydrogen production in triethanolamine solution for 12 hours were carried out through the protective effect of carbon layer on Cu nanoparticles.The results showed that the stability of hydrogen production in triethanolamine solution was very stable.（2）Fe₃C@C nanoparticles were used as co-catalysts for photocatalysis of g-C₃N₄ to produce hydrogen for the first time.The photogenerated carriers produced by g-C₃N₄ are captured by Fe₃C@C nanoparticles,which improves the separation efficiency of photogenerated electrons and holes,and thus improves the photocatalytic activity.The photocatalytic efficiency of hydrogen production under visible light irradiation reached 272.1μmol g^-1 h^-1.Simultaneously,the ferromagnetism of Fe₃C@C nanoparticles makes Fe₃C@C/g-C₃N₄ photocatalyst have the advantages of low cost and high recovery efficiency.In the experiment of hydrogen production by four cycles of photocatalysis for 20 hours,the Fe₃C@C/g-C₃N₄ photocatalyst showed a very stable and efficient hydrogen production efficiency.（3）Graphite-coated Fe Cu alloy nanoparticles were prepared by one-step calcination.Under the optimum conditions,the photocatalytic activity of g-C₃N₄ was significantly increased by Fe Cu@C,which was about 34 times higher than that of pure g-C₃N₄.This is also superior to the photocatalytic activity of Fe@C/g-C₃N₄ and Cu@C/g-C₃N₄ samples.In addition,the graphite carbon layer can increase the electron transfer rate and prevent the Fe Cu nanoparticles from being oxidized or chemically corroded.（4）Fe Ni@NC nanoparticles were obtained by calcination method and Fe Ni@NC/g-C₃N₄pholocatalysts were obtained by mechanical grinding of g-C₃N₄ and Fe Ni@NC nanoparticles.The photocatalytic activity of the optimal Fe Ni@NC/g-C₃N₄ sample was up to 45μmol h^-1,which was about 270 times higher than that of bare g-C₃N₄.By comparing with the photocatalytic activity of Ni@C/g-C₃N₄ and Fe@C/g-C₃N₄,it is concluded that the higher H₂ production of Fe Ni@NC/g-C₃N₄ is due to the high electron density formed after secondary transfer in Fe Ni alloy.Moreover,the coating of N-doped graphite carbon layer can promote the capture and transfer of photogenerated electrons and improve the chemical stability of the alloy.In this paper,Cu@C,Fe₃C@C,Fe Cu@C and Fe Ni@NC nanoparticles were used as active centers for the photocatalytic production of hydrogen from g-C₃N₄ for the first time.By increasing the separation efficiency of photogenerated electrons and holes and reducing the over-potential of hydrogen generation,the hydrogen evolution performance of g-C₃N₄-based composite photocatalysts was significantly improved.Meanwhile,a new strategy to improve the efficiency of electron surface catalysis is proposed.The advantage of alloy nanoparticles over metal as co-catalyst lies in the secondary transfer process of photogenerated electrons in the alloy.The higher the number of electrons and the higher the energy at the active center can effectively improve the photocatalytic hydrogen evolution activity of semiconductors.Finally,the design of carbon coating on the surface of metal nanoparticles can effectively improve the chemical stability of the alloy.At the same time,as a high-speed channel,the carbon layer can accelerate the transfer of photogenerated electrons to the active sites to participate in the surface reaction.