| In the 21st century,with the development of science and technology,the environmental pollution and energy depletion caused by the massive consumption of oil,coal,natural gas and other non-renewable energy sources have become more and more serious.Solar energy is a rich green energy.It is a feasible strategy to solve environmental pollution and energy crisis that use of photocatalytic technology to decompose organic pollutants into small inorganic molecules such as H2O and CO2,and convert solar energy into green,sustainable energy sources such as H2,CH4,CH3OH,etc.With the development of photocatalytic technology,scientific researchers have focused on improving the catalytic efficiency of photocatalysts.To solve the high recombination rate of photogenerated electron-hole pairs in a single semiconductor photocatalyst and its low redox ability,it is urgent to construct a heterojunction.Since Professor Jiaguo Yu of Wuhan University of Technology proposed the S-scheme heterojunction,construction of S-scheme heterojunction photocatalysts has been extensively studied.The S-scheme heterojunction is composed of a reduction photocatalyst and an oxidation photocatalyst with alternating energy bands.A built-in electric field is formed at the interface of the two photocatalysts.The separation and transmission of carriers are accelerated under the built-in electric field.This special structure will endow the whole system with high oxidation and reduction ability and high carrier separation efficiency.This thesis focuses on the research of S-scheme heterojunction,constructing S-scheme heterojunction in different photocatalytic systems,and systematically introduces the effects of S-scheme heterojunctions on the performance and stability of photocatalytic degradation,H2 production and CO2 reduction.And the following research results were obtained:First,the porous graphite carbon nitride(Pg-C3N4)with a large specific surface area was obtained by heating the mixture of urea and thiourea and ultrasonically peeling off.Then Ag3VO4 was grown in situ on Pg-C3N4 by a simple chemical deposition method to construct a S-scheme Pg-C3N4/Ag3VO4 heterojunction and its photocatalytic degradation activity was studied.In addition,parts of the Ag ions are reduced to Ag nanoparticles during the synthesis process.Ag nanoparticles will produce surface plasmon resonance(SPR)when subjected to photons with the same frequency as their natural frequency.Ag SPR can promote the absorption of light of the photocatalysts.Under the action of the S-scheme heterojunction,the separation and transport of carriers are accelerated and the Pg-C3N4/Ag3VO4 system is given strong redox ability,thereby greatly improving the photocatalytic performance and stability.Second,the solid solution of Zn0.2Cd0.8S has good photocatalytic H2 production activity,but a single Zn0.2Cd0.8S has a high carrier recombination probability,so it is necessary to construct a heterojunction to improve the photocatalytic performance and stability-Specifically,Pg-C3N4 was obtained by thermal polycondensation,and then organic-inorganic hybrid Pg-C3N4/Zn0.2Cd0.8S-DETA composites was prepared by solvothermal method after the stripped Pg-C3N4,ZnCl2,CdCl2ยท2.5H2O and diethylenetriamine(DETA)are stirred and mixed.The S-scheme heterojunction was constructed by growing Pg-C3N4 in situ on Zn0.2Cd0.8S-DETA.The introduction of DETA can change the morphology of Zn0.2Cd0.8S,and can be used as a linking agent to make Zn0.2Cd0.8S and Pg-C3N4 form close contact.The stripped Pg-C3N4 nanosheet has a large specific surface area and can provide abundant catalytic active sites.The special separation method based on the S-scheme heterojunction promotes the rapid separation and transmission of photogenerated carriers,thereby improving the activity of photocatalytic H2 evolution.Third,on the basis of the above work,we selected the oxidized semiconductor CoO and the reduced semiconductor Pg-C3N4 to construct an S-scheme heterojunction for photocatalytic CO2 reduction.The successful construction of the S-scheme heterojunction greatly accelerates the separation efficiency of photogenerated carriers,thereby improving the activity and stability of photocatalytic CO2 reduction.Fourth,in addition,ultrathin InVO4 nanosheets with a thickness of about 1.5 nm were prepared and DETA-modified CdSe was grown in situ on InVO4 nanosheet by microwave solvothermal method to construct S-scheme InVO4/CdSe-DETA heterojunction composites,Ultrathin InVO4 nanosheets provide abundant CO2 adsorption sites.As an organic amine molecule,protonated DETA forms an interface chemical bond between the ultrathin InVO4 and CdSe-DETA.Under the action of the interface chemical bond,it promotes the transport of carriers at the interface.Finally,the formation of S-scheme heterojunction is verified by XPS and work function calculation.The S-scheme heterojunction in the presence of interface chemical bonds further accelerates the transport of carriers,thereby greatly improving the performance and stability of photocatalytic CO2 reduction. |
