Photochemical Deposition Synthesis Of G-C₃N₄-based Composite Catalysts And Their Photocatalytic Hydrogen Evolution Properties

In today's society,the level of human industrialization has been sufficiently developed.However,it also brings harm for the home of human beings.Therefore,the development of new clean energy has become the way out for the continuous progress of human society.Among them,hydrogen fuel has the characteristics of high energy density,rich raw materials and clean,which makes it to be the development trend of new energy in the future.At present,using the effective semiconductor materials to realize photocatalytic water splitting is one of the most effective means of hydrogen preparation,which is expected to relieve the pressure of environment and energy at the same time.The choice of catalyst is the key factor in the photocatalytic water splitting system.The photocatalyst should meet appropriate bandgap and band position,so as to satisfy the thermodynamic and kinetic requirements in photohydrolytic hydrogen evolution reaction.At the same time,in order to meet the actual production,the catalytic system is required to be efficient,stable and green.So far,researchers have found a variety of semiconductor materials that can be used for photocatalytic water splitting.Among them,graphitic carbon nitride(g-C₃N₄)shows great application prospect in the field of photocatalytic hydrogen evolution due to its excellent stability and suitable energy band position.However,the g-C₃N₄ materials obtained by high-temperature thermal polymerization usually have some deficiencies,such as low specific surface area,low carriers transport and separation efficiency,which makes it difficult to obtain the ideal yield of hydrogen evolution.Consequently,how to effectively regulate the spatial structure,enhance the the final yield of hydrogen has become the focus of research.In this paper,the research and modification status of g-C₃N₄-based catalyst is analyzed.Focuses on the imperfection of material(such as low carriers utilization efficiency and small specific surface area),the following strategies are adopted:Firstly,the microstructure,surface chemical state and energy band structure of the g-C₃N₄ were adjusted by thermal oxidation etching method.Then,the photochemical deposition synthesis method is used to realize the precise loading of the co-catalyst,that can significantly enhance the utilization efficiency of the photo-generated electrons.The mechanism of the enhanced hydrogen evolution performance of the composite catalyst is analyzed by PL spectra,photoelectrochemistry test and density functional theory(DFT).The main research content includes the following parts:Firstly,the preparation and properties of g-C₃N₄ nanosheets.In this part,the bulk g-C₃N₄ was firstly synthesized by a calcination way,and then was thinned by thermal oxidation etching to obtain g-C₃N₄ nanosheets.The changes of phase structure,surface chemistry,optical properties of g-C₃N₄ before and after heat treatment were studied systematically.The hydrogen evolution activities of Pt/g-C₃N₄ were evaluated.The results showed that the basic structural units of g-C₃N₄ were preserved after thermal oxidation etching,while the interlamellar space and the content of surface N element were reduced,the specific surface area has been increased by three times at the same time.From the results of electrochemical test and hydrogen evolution activity test,thermal oxidation etching can effectively improve the transport efficiency of photo-generated carriers inside the catalyst.At the same time,the diffuse reflection spectrum(DRS)and DFT calculations show that the conduction band of g-C₃N₄ moves towards a more negative direction after heat treatment.Thus,the photo-generated electrons have more stronger reduction ability and finally improve the performance of photocatalytic hydrogen evolution.The g-C₃N₄nanosheets show enhanced photoelectrochemical performance than the bulk phase,which is more suitable to be used as the matrix catalyst material for further research and modification.Secondly,photochemical deposition synthesis of NiS/g-C₃N₄ and their hydrogen evolution properties.In this study,the robust NiS modified g-C₃N₄ composite were successfully synthesized by a surface photochemical deposition process.Under the visible light illumination(??400 nm),the NiS/g-C₃N₄ composite photocatalyst with 1.0%weight content of NiS co-catalyst exhibits the highest hydrogen evolution rate of 1346.1?mol·h^-1·g^-1 with an apparent quantum efficiency(AQE)value of 7.67%.According to the analysis of the synthesis mechanism,NiS particles could be precisely loaded on the surface of g-C₃N₄ sheets(active sites).This mainly due to the photo-generated electrons are required to participate in the process of photochemical deposition,thus exhibit enhanced hydrogen evolution performance compared to other methods.Combined with the photoelectrochemical test,PL spectrum and theoretical calculation,it can be seen that the contact interface between NiS and g-C₃N₄ nanosheet forms a schottky-like heterojunction,which can effectively realize the rapid separation of photo-generated electrons and holes.The NiS co-catalyst becomes the electron capture center of the material and the target active site of hydrogen evolution,and finally improves the activity of hydrogen evolution ability.Thirdly,the preparation and the hydrogen evolution properties of CoS/g-C₃N₄composite photocatalyst.Using g-C₃N₄ nanosheets as matrix material,the CoS/g-C₃N₄composite were successfully prepared by photochemical deposition.The morphology,structure and optical properties of the composite photocatalyst were characterized by SEM,TEM,XRD,XPS,DRS and other techniques,and the metal-like properties of CoS are revealed by theoretical calculation.Under visible light irradiation,the hydrogen evolution properties of g-C₃N₄ nanosheets and CoS/g-C₃N₄ composite were evaluated.By optimizing the loading weight of CoS co-catalyst,the optimal hydrogen evolution activity can be obtained when the CoS loading weight is 1.0 wt%,the hydrogen evolution rate is up to1862.2?mol·h^-1·g^-1,and the corresponding AQE value was 10.61%.Comprehensive analysis of photoelectrochemical test and PL spectra,it can be found that the CoS-loaded can remarkably reduce the charges transfer resistance in CoS/g-C₃N₄ composite,and then improve the separation efficiency of photo-generated electron-hole pairs in the composite material.Therefore,more photo-generated electrons are involved in the hydrogen evolution reaction of CoS/g-C₃N₄ composite,which makes it show high efficiency of the photocatalytic hydrogen evolution from water splitting.Lastly,the synthesis and hydrogen evolution properties of CoSe₂/g-C₃N₄heterojunction.The CoSe₂/g-C₃N₄ composite photocatalytic system was successfully prepared by a mild photochemical deposition method,to realize the precise loading of CoSe₂ co-catalyst.The optical properties,morphologies and structures of heterojunction materials were measured by various instruments.The results showed that CoSe₂/g-C₃N₄composite embrace a high hydrogen evolution activity,and the loading weight of co-catalyst had a great influence on the hydrogen evolution performance of the catalytic system.When the loading weight of CoSe₂ was 2.5 wt%,the hydrogen evolution performance reached the maximum value(2468.9 mol·h^-1·g^-1),and the AQE value reached14.05%.The CoSe₂ co-catalyst can be deposited precisely at the exit point of photo-generated electrons by photochemical deposition,and a schottky heterojunction can be formed between the two binding interfaces.Therefore,the photo-generated electrons can be rapidly transferred to CoSe₂ co-catalyst,thus realizing the rapid separation of e^-/h⁺pairs.Finally,the photocatalytic hydrogen evolution activity was enhanced.