Metal Oxide/Sulfide Heterojuncted Semiconductors For Photocatalytic Applications

Due to the rapid growth of gobal population and the continuous acceleration of the industrialization process,energy shortages and environmental pollution have become two major challenges facing humanity.Renewable clean energy,such as solar energy,geothermal energy,wind energy,hydroelectric power and tidal energy,are considered to be excellent alternatives to fossil fuels.Among them,solar energy,as a clean energy with abundant reserves,wide distribution and permanent,can meet the current and future human demands.In recent decades,semiconductor photocatalysis technology has shown strong potentials in solving the world-wide energy shortage and reducing environmental pollution,and has become one of the most promising solar energy utilization solutions.So far,there has been a lot of research devoted to developing various photocatalytic materials,systems and equipments,as well as the discussion of related interface chemistry,electronic structure and reaction kinetics.However,at present,the overall photocatalytic efficiency is still low,and the most prominent problem comes down to the low efficiency of sunlight absorption and utilization and the low efficiency of the separation and transportation of photo-generated carriers.How to better understand the reaction mechanism of semiconductor photocatalysis,explore the basic factors that limit photocatalytic activity,and develop a highly efficient photocatalytic material system with visible light response is the key to breaking through the bottleneck.This thesis proposes to construct a homologous Z-scheme heterojunction photocatalyst for the absorption of sunlight and the separation of photogenerated carriers in current semiconductor photocatalytic systems.Starting from a specific structure,a higher-quality heterojunction interface contact is achieved,which broadens the light absorption range of the material and promotes the separation and transport of photogenerated charges without sacrificing its redox ability.In this thesis,two groups of homologous metal oxygen/sulfide were selected,and the corresponding solutions were proposed from the two most widely used energy conversion fields of photocatalysis,namely photocatalytic water decomposition and carbon dioxide reduction.The specific research content is as follows:?1?Application of bismuth oxide/bismuth sulfide heterojunction in photocatalytic overall water splittingBi₂S₃ nanoneedles were grown on the surface of Bi₂O_2.33 nanosheets by electrodeposition,high temperature heat treatment and chemical bath deposition.A homogeneous Z-scheme heterojuncted photocatalyst of Bi₂O_2.33/Bi₂S₃ was achieved.The homogeneous heterojunction avoids the defect states introduced by lattice mismatch and metal ion doping after heat treatment,which significantly reduces the recombination rate of photo-generated carriers,thereby ensuring a higher charge separation efficiency.Finally,the ultra-thin uniform amorphous TiO₂ layer?about 3 nm?was coated to prevent Bi₂S₃ photocorrosion and extend the service life.The results show that the composite photocatalyst exhibits a very efficient photocatalytic activity for over-all water decomposition,in which the evolution rate of H₂ can reach 0.98?mol?h^-1,and the evolution rate of O₂ is about 0.5?mol?h^-1.There was only a slight decrease in performance after five consecutive cycles.?2?Application of tin-doped indium oxide/indium sulfide heterojunction in photocatalytic carbon dioxide reductionSn-In₂O₃/In₂S₃ homogeneous Z-scheme heterostructure was grown on ITO conductive substrate by CVD and vapor phase vulcanization.Thanks to the high-efficiency charge transport capability and heterojunction of Sn-In₂O₃ single crystal nanowires,the light absorption range of the material was widened and the carrier separation and transmission capabilities were enhanced.The composite sample shows extremely excellent photocatalytic carbon dioxide reduction performance.The results show that Sn-In₂O₃/In₂S₃ composites show better catalytic activity than single-component materials.In the case of a composite material with a curing time of 20 minutes,the yield of CH₄ reached 0.52?mol?h^-1,and the yield of CO was as high as 0.85?mol?h^-1.Our results show that the construction of homologous direct Z-scheme heterojunction composite photocatalysts can obtain the following advantages.First,high-quality heterojunctions with fewer defect states may have longer photogenerated carrier lifetimes;in addition,direct Z-scheme heterostructures can provide effective charge separation and transfer without sacrificing redox of carriers ability.This thesis focuses on the core problems in the research of semiconductor photocatalysis,proposes corresponding solutions,and the working mechanism was explored experimentally and theoretically.The research in this paper has greatly enriched the material and structure design of semiconductor photocatalysts,which will promote the development of semiconductor photocatalysts into practical applications.