Design And Synthesis Of Cobalt-based Phosphide/Carbon Nitride Composite Photocatalysts And Their Photocatalytic Properties For Hydrogen Production

Photocatalytic water splitting is a technology to convert solar energy into hydrogen energy,which can alleviate the energy crisis and environmental problems in today’s society.Among many photocatalysts,graphitic-phase carbon nitride（g-C₃N₄）has been widely studied because of its unique layered structure,suitable band structure and simple preparation method.However,because g-C₃N₄ have the disadvantages of high recombination rate of photogenerated electrons-holes and low light absorption efficiency.It is of great significance to explore the co-catalyst that can replace precious metals to realize efficient and low-cost hydrogen production from carbon nitride materials.In recent years,transition metal phosphates（TMPs）have been considered as one of the candidates to replace precious metal-based cocatalysts in the field of photocatalytic hydrogen production.However,there are still great challenges in the application of TMPs,mainly including:developing safer and faster synthetic TMPs strategies,optimizing the loading method of TMPs on g-C₃N₄,synthesizing highly active TMPs with different morphologies,and applications in overall water splitting,etc.Based on the above problems and challenges,this paper focuses on the design and synthesis of different cobalt-based phosphides as co-catalyst modification of g-C₃N₄,and the influences of morphology and crystal structure of cobalt phosphide on hydrogen evolution performance and stability of g-C₃N₄ were investigated.Specifically,it includes the following three research contents:1.The 1D/2D Co₂P/g-C₃N₄ composite photocatalyst was prepared in situ by solvothermal and low temperature phosphating processes using urea as raw material.The optimum phosphating condition（200℃,1h）was obtained by adjusting the temperature and time during the phosphating process.The hydrogen evolution test results showed that the hydrogen production rate of the 5%Co ₂P/g-C₃N₄ composite photocatalyst was up to1155μmol·h^-1·g^-1 under visible light（λ>420 nm）,which was 96.3 times that of the pure g-C₃N₄ nanosheets,respectively.The photocatalytic activity was also stable in the cycle test.This is mainly due to the formation of tight1D/2D heterojunction between Co ₂P nanorods and g-C₃N₄ nanosheets successfully prepared.Co ₂P in close contact with g-C₃N₄ can expand the light absorption range,promote the interfacial charge transfer significantly,inhibit the recombination of photogenerated electrons and holes,and thus greatly improve the photocatalytic hydrogen evolution activity of g-C₃N₄.2.The pure g-C₃N₄ was synthesized by thermal polymerization with urea as raw material,and the hollow Co-ZIF was synthesized by solvothermal method.The Co-ZIF and g-C₃N₄ were mixed uniformly by physical grinding method,and the ratio of Co-ZIF and Na H₂PO₂ was adjusted and calcined at low temperature to prepare Co P/g-C₃N₄ composite photocatalysts.The experimental results revealed that the Co P nanoparticles derived from Co-ZIF can enhance the hydrogen evolution activity of g-C₃N₄.Under visible light,the hydrogen production rate of 5%Co P/g-C₃N₄composite photocatalyst is 2336μmol·h^-1·g^-1,which is about 156 and 9times that of pure g-C₃N₄ and Pt/g-C₃N₄,respectively.It also showed good stability in the cycle test.It was found by photoelectric characterization that the coupling of Co P and g-C₃N₄ enhanced the visible light absorption capacity,promoted the migration and separation of photogenerated carriers,and thus improved the hydrogen evolution capacity of photocatalytic decomposition of water.3.Pure g-C₃N₄ was synthesized from urea,and a regular core-shell Co（OH）₂@Ni Co-LDH/g-C₃N₄ precursor was prepared using dodecahedron ZIF-67 as a sacrificial template.This special core-shell precursor prevented the aggregation of Ni Co-LDH nanosheets.Finally,a series of Ni Co P/g-C₃N₄photocatalysts were prepared by phosphating treatment,and the effects of different mass ratios of Ni sources on the photocatalytic performance were investigated.The experimental results showed that the hydrogen evolution rate reache d 992μmol·h^-1·g^-1 under visible light,which was about 43 times that of pure g-C₃N₄ when the Ni Co P loading was 5%.At the same time,compared with the precious metal（Pt）modified g-C₃N₄ photocatalyst(259.5μmol·h^-1·g^-1),the hydrogen evolution performan ce was about 3.8times higher.It also show ed good stability in cycle test.The hydrogen evolution rate of the photocatalyst prepared by ZIF-67@Ni Co-LDH/g-C3N4precursor system is 396.8μmol·h^-1·g^-1.The results of photoelectric characterization revealed t hat the introduction of Ni Co P could effectively promote the transfer and separation of photogenerated carriers and provide abundant reactive sites,thus enhancing the activity of photocatalytic hydrogen evolution.