Preparation And Hydrogen Production Performance Of The G-C₃N₄-based Photocatalysts

With the rapid development of the global economy and society,the energy and environmental crises caused by the rapid consumption of fossil energy and the continuous deterioration of environmental problems have aroused people’s urgent attention.Since Honda-Fujishima first discovered that hydrogen can be produced by photoelectric-driven water splitting,the use of solar energy to convert hydrogen fuel on a photocatalyst is considered to be one of the most promising strategies to effectively alleviate problems such as energy shortages.TiO₂ is one of the most studied photocatalysts,but it is limited by its own characteristics,so it cannot meet the needs of practical applications.Carbon nitride（g-C₃N₄） is a metal-free two-dimensional polymer semiconductor material.Because of its non-toxic,non-polluting,stable chemical properties,and simple preparation,it has aroused extensive research interest of scholars,but it absorbs visible light.Insufficiency,the rapid recombination of photo-induced electron-hole pairs greatly hinders its practical application.It is still a huge challenge to develop a long-term stable g-C₃N₄-based photocatalyst with high activity.In this work,g-C₃N₄ as the main research object,single-atom doping and heterojunction building have been investigated.The hydrogen production mechanism of the g-C₃N₄-based photocatalyst was explored from the aspects of single-atom doping and heterojunction construction.The research contents of this work are as follows:1.Single-atom Cu-N charge-transfer bridge:Preparation and photocatalytic hydrogen production performance of Cu single-atom catalysts coordinated with N and anchored on porous g-C₃N₄The highly active and stable Cu single-atom catalysts coordinated with N and anchored on the porous g-C₃N₄ were prepared through a facile one-pot calcination method,successfully.The spherical aberration corrected high-angle circular dark-field scanning transmission electron microscope（AC-HAADF-STEM）,XPS and FTIR have identified C u atomic dispersion and Cu-SA/g-C₃N₄ coordination configuration.Experiments and density functional theory calculations（DFT） further verified that Cu-SA/g-C₃N₄can reduce the hydrogen evolution barrier,enhance the use of visible light,and improve the charge transfer of g-C₃N₄.In the application of visible light-driven water splitting H₂ evolution,when 1.5wt%Cu is introduced,the Cu-SA/g-C₃N₄（CCN-1.5） catalyst has the highest hydrogen production activity under visible light irradiation（λ>420 nm）,and the hydrogen production rate reaches 3594.4μmolh^-1g^-1,much higher than the undoped g-C₃N₄(47.9μmolh^-1g^-1).This can be attributed to the Cu-N charge bridge as the active site,providing a fast electron transfer channel,efficaciously reducing the recombination for photogenerated electrons and holes.Moreover,in the long-term cyclic reaction test,the H₂ production capacity for the Cu-SA/g-C₃N₄ catalyst did not decrease significantly,demonstrating the durability of the Cu-SA/g-C₃N₄ single-atom catalyst is higher.2.Preparation of three-dimensional/two-dimensional MoS₂/g-C₃N₄ heterojunction photocatalysts with spherical MoS₂ grown on g-C₃N₄ and performance study of hydrogen productionFlower spherical MoS₂ was successfully grown on g-C₃N₄,and three-dimensional/two-dimensional MoS₂/g-C₃N₄ photocatalyticc material was constructed through a simple hybrid grinding method.MoS₂ nanoflowers are obtained by hydrothermal reaction,while g-C₃N₄ nanosheets are prepared by a one-step calcination hydrothermal exfoliation method.The morphology,component,and physical and chemical characteristics of the heterojunction catalyst were discussed by using SEM,XPS,EIS and another analytical technique,and the prepared material was subjected to a photo-decomposition water hydrogen production experiment.The results show that when the 3D MoS₂ loading is 35wt%,the prepared 35% MoS₂/g-C₃N₄heterojunction photocatalytic material exhibits the highest hydrogen production rate(5840μmolh^-1g^-1)at the sunlight irradiation（λ>420 nm）,which are 72 and 55 times that of simple g-C₃N₄(52.7μmolh^-1g^-1) and MoS₂(69.4μmolh^-1g^-1),respectively.Due to the formation of the heterostructure of spherical MoS₂and g-C₃N₄,enhanced the absorption and utilization of sunlight,promoted the transmission of photo-excited carriers,and the efficiency of water splitting to produce H₂ is improved.C ycling experiments also show that the catalyst has good photochemical stability.3.Synthesis and photocatalytic hydrogen evolution performance of noble-metal-free Cd_0.5Zn_0.5S@g-C₃N₄ heterostructure catalystsA series of nanocatalytic materials composed of Cd_0.5Zn_0.5S nanocrystals and porous g-C₃N₄ material were synthesized by in-situ growth hydrothermal method,successfully.The ability of visible-light drives water splitting to produce H₂ for these samples was tested under visible light irradiation（λ>420 nm）without precious metal co-catalysts.The characterization analysis display the optimal loading of Cd_0.5Zn_0.5S nanocrystals is 30wt%,and the catalyst 30wt%-Cd_0.5Zn_0.5S@g-C₃N₄（30%CZS@CN） has the highest hydrogen production rate,83360μmolh^-1g^-1,its hydrogen production activity is much higher than pure Cd_0.5Zn_0.5S(6750μmolh^-1g^-1)and g-C₃N₄(51.3μmolh^-1g^-1).And in the stability cycle reaction test,the hydrogen production rate of 30%CZS@CN hardly decreased,indicating its good stability.In addition,we further characterized the samples through SEM,TEM,XRD,XPS,UV-DRS,PL,etc.,and found that the heterojunction formed between the contact interface of Cd_0.5Zn_0.5S and g-C₃N₄ resulted in light-induced electrons and voids.The effective space of the holes is separated,thus exhibiting sharply improved the capacity and stability of water splitting hydrogen production.