Design And Properties Of Layered Bismuth-based Semiconductor Photo-catalysts

With the rapid development of the industrialized economy,the problem of environmental pollution is becoming increasingly serious.Especially the organic pollutants and bacteria existing in industrial wastewater have seriously polluted the soil and drinking water,which directly affects our production and life.Among many methods to solve the environmental pollution,the green and efficient semiconductor photocatalytic technology can directly use solar energy to degrade organic pollutants and kill bacteria,which is great significance to solve the water pollution problem.In the process of oxidative degradation of organic pollutants by a semiconductor through a redox reaction,the factors that affect the speed of the redox reaction are mainly the semiconductor's ability to absorb light and the separation rate of photo-generated electron and holes.Semiconductors with large band-gap have a weak ability to absorb light,but the rate of electron and hole separation is high;semiconductors with small band gap can absorb more visible light while their distances of conduction band and valence band are close.Therefore,improving the visible light absorption capacity of traditional wide-band-gap semiconductors,exploring new narrow-band-gap semiconductors and effectively separating their electron-hole pairs have become the focus and hotspots all over the world.Layered Bismuth-based semiconductor is a new type of catalyst material,its wide variety,non-toxic,harmless,stable physical chemistry properties as well as excellent photocatalytic activity have attracted more attention.Layered bismuth compounds are composed of?Bi₂O₂?²⁺layers and different ions between the layers to form a stacked sandwich structure.Electrons and holes are transferred between layers,which effectively separation of electrons and holes to improve the photocatalytic redox ability.In addition,most indirect semiconductors of layered bismuth-based compounds have smaller band gaps,and their valence bands are formed by the hybridization of Bi 6s and O 2p orbitals,which is beneficial to improve the oxidation ability and electron migration efficiency.Therefore,the layered bismuth-based semiconductor catalysts show the excellent photocatalytic performance.In this paper,layered bismuth compounds such as bismuth oxychloride,bismuth titanate,and bismuth sulfide are used as substrates to address the problems of limited visible light absorption capacity and high internal electron-hole recombination rate,through introducing oxygen vacancies,doping rare earth ions and constructing heterojunctions to broaden their visible light response range and increase their electron-hole separation rate,thereby enhancing their photocatalytic performance.The research work of this study is divided into three parts:?1?BiOCl with ultraviolet light response was selected as the research object,the three-dimensional flower-like Er³⁺-Ho³⁺/Yb³⁺-BiOCl were synthesized using a simple hydrothermal method with the assistance of surfactant,which reduces the band gap of BiOCl and broadens its visible light absorption range,but its visible light respond is still very limited.To this end,oxygen vacancies were successfully introduced into BiOCl and a black BiOCl crystal plane homojunction was obtained by a hydrothermal method using ethylene glycol as a solvent.The spectral absorption range of black BiOCl was extended to 800 nm,which greatly improved its response in visible light;the different exposed crystal planes in different morphologies of BiOCl construct a crystal plane homojunction,which effectively increases the separation of photo-generated electrons and holes in BiOCl,thus promoting the process of photocatalytic reaction.At the same time,the photocatalytic performance of the prepared samples were evaluated through photocatalytic degradation and disinfection experiments,and its possible photocatalytic mechanisms were proposed in combination with first-principles calculations.?2?In order to use solar energy more effectively,Bi₂Ti₂O₇ with a small band gap and visible light response was selected as the research object.First Bi₂Ti₂O₇ nanoparticles were grown on BiOCl nanosheets by in-situ growth hydrothermal method to build a BiOCl/Bi₂Ti₂O₇ heterojunction;then Cu ions were doped into Bi₂WO₆?Cu-Bi₂WO₆?and flower ball liked Cu-Bi₂WO₆/Bi₂Ti₂O₇ heterojunction was constructed by in-situ growth hydrothermal method,which effectively broadened the spectral response range and photo-generated carrier separation speed.The photocatalytic performance of heterojunction was assessed through degradation and disinfection experiments under simulated sunlight.The photocatalytic ability and bactericidal properties of samples were combined with first-principles calculations and trapping experiments to study the photocatalytic mechanism of the heterojunction.The results show that BiOCl/Bi₂Ti₂O₇ is a type“II”heterojunction,and the oxygen vacancies introduced in Bi₂WO₆ by copper doping cause a change in the band position,making the"II"type electron transfer mechanism of Bi₂WO₆/Bi₂Ti₂O₇ into"Z"type electron transfer mechanism of Cu-Bi₂WO₆/Bi₂Ti₂O₇,which is more conducive to retain the redox potential of the heterojunction and has a higher photocatalytic ability.?3?From the perspective of constructing photocatalysts with wider spectral response,Bi₂S₃ with narrow band gap and the near-infrared light response was selected as the research object.First,a flower-like Bi@Bi₂O₃/Bi₂S₃ heterojunction was synthesized by solvothermal method.Not only the plasmon resonance effect of Bi metal was used to enhance the absorption of visible light by Bi₂O₃,but also the construction of Bi₂O₃/Bi₂S₃ heterojunction accelerated the separation speed of electrons and holes in Bi₂S₃,which improves the photocatalytic ability of Bi₂S₃.In order to make more effective use of the redox properties of heterojunctions,a multi-step hydrothermal method was used to construct a Bi₂O₃/Bi₂S₃/MoS₂ QDs ternary heterojunction.MoS₂ quantum dots?QDs?with bactericidal and electron trapping properties were used as electron transfer"bridge"to builds a"Z"type heterojunction,which improves the photocatalytic performance of Bi₂S₃.The"Z"type electron transfer mechanism of the Bi₂O₃/Bi₂S₃/MoS₂ QDs heterojunction was further verified by combining the band position with the trapping experiments.