| The excessive consumption of fossil fuel resources has caused serious energy crisis and environmental pollution.Therefore,it is vital to develop renewable energy,that can replace fossil fuels.Solar energy as a kind of inexhaustible clean energy,meanwhile hydrogen energy is considered the most ideal pollution-free green energy due to the advantages of high combustion value and no pollution of combustion products.Therefore,converting solar energy into hydrogen energy using semiconductor photocatalysis technology is considered to be one of the most important strategies to solve the energy crisis and environmental pollution.Photocatalyst is the core issue of semiconductor photocatalysis technology.Among the many semiconductor catalysts that have been reported,graphite-phase carbon nitride(g-C3N4,CN)has attracted extensive attention in the field of photocatalysis because of its suitable band structure,visible light response,stable physical and chemical properties and readily available raw materials.However,bulk g-C3N4 has low photocatalytic performance due to its narrow light absorption range,low photocarrier separation efficiency,low charge utilization rate and lack of reactive sites.In this paper,aiming at the three key factors that restrict the photocatalytic hydrogen production efficiency of g-C3N4,starting from doping modification and interface structure optimization,taking into account the synergistic effect between them,a series of researches were carried out.Besides,an Ag modified CN-based composite photocatalyst with high photolysis performance for hydrogen production was designed and constructed.Through the nitrate-assisted simultaneous pyrolysis method,the distribution and state of Ag(single atom vs.cluster)within and between g-C3N4 layers can be controlled,which improves its in-plane and interlayer charge transport and overcomes its disadvantage of poor conductivity.At the same time,taking advantage of the improved conductivity of Agdoped CN,Ag nanoparticles were loaded onto the surface of A by means of photodeposition with uniform dispersion and controllable size,and the localized surface plasmon resonance(LSPR)effect was used to significantly improve the light absorption efficiency.Through interfacial chemical bonding(Ag-OOC-and Ag-S bonds),the recombination of CN with cobaltoxime molecular catalyst and MoS2 cocatalyst was optimized,and the CN-based composite photocatalyst was constructed,which significantly improved the photocatalytic cracking activity of hydrogen production in water without Pt co-catalyst.The first chapter summarized the research background of semiconductor photocatalyst technology,the basic principles of photocatalytic hydrogen evolution from water and the structural characteristics and modification strategies of CN.Secondly,the structure characteristics and research status of graphite-like CN materials were analyzed and the main strategies to improve the catalytic activity of CN materials were discussed,including elemental doping modification,cocatalyst,single atom modification,heterojunction construction,etc.Finally,the topic selection and content of this work were described.In the second chapter,we put forward the synthesis strategy of silver nitrate assisted synchronous pyrolysis.The state and distribution of Ag within and between gC3N4 layers can be controlled by a simple one-step calcination method.The single atom Ag were distributed in the conjugate plane of CN as Ag+,and the local D-A structure promoted the in-plane charge transfer by bonding with cyano groups introduced in CN.Ultrafine Ag nanoclusters were encapsulated in CN lamellae.The gold properties were beneficial to improve the charge transfer between CN layers.The single atom Ag and cluster Ag synchronously improved the conductivity of CN,promoted the transport of photogenerated electrons in the CN plane and between layers,and improved the separation efficiency of photogenerated carriers in the bulk phase.On this basis,the cobaltoxime molecular catalyst was loaded on the surface of Ag modified CN by interface regulation,and the covalent bond was formed between the carboxyl group and Ag+,so that the molecular catalyst was anchored on the surface of CN.As a bridge of electron transport,covalent bond can effectively improve the efficiency of photogenerated electron transport across interfaces.As hydrogen evolution sites,cobaltoxime molecular catalyst improved the utilization rate of photogenerated electrons,thus achieving high visible light hydrogen evolution activity without noble metal Pt cocatalyst.In the third chapter,on the basis of Ag single atom and cluster co-modification to improve the conductivity of CN,Ag nanoparticles with uniform dispersion and controball size were deposited on the surface of Ag modified CN by photoreduction.The local surface plasmon resonance(LSPR)effect of deposited Ag nanoparticles was used to enhance the optical absorption intensity of the composites,and the optical absorption range was widened.Therefore,the optical absorption efficiency of CN composites was significantly improved.Subsequently,in order to overcome the lack of active sites for hydrogen production on the surface of CN materials,MoS2 was in situ photodeposition on the surface of CN materials as photocatalytic hydrogen production co-catalyst.The S-Ag bond was formed between the deposited thin layer MoS2 and Ag nanoparticles through interface regulation to achieve close contact.The high electrical conductivity of Ag nanoparticles and the S-Ag bond,which was the "bridge" of electron transfer,can effectively promote the rapid transfer of electrons from CN to MoS2,and promoted the photocatalytic efficiency ofhydrogen evolution by using the unsaturated S site at the edge of MoS2.In this work,the effective coordination between LSPR effect and interface chemical effect(interface Ag-S bond)was realized by using Ag nanoparticles,which comprehensively considered the light absorption efficiency,photocarrier transport efficiency and photoelectron utilization rate of photocatalyst.The fourth chapter summarized the thesis,main conclusions and innovations,and prospectd the next planned work. |
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