Synthesis And Photocatalytic Properties Of Bismuth Based Semiconductor Materials

Since the beginning of the 21st century,with the rapid development of industrial society,a large amount of traditional fossil energy is consumed quickly,resulting energy crisis and environmental pollution problems are increasingly prominent.These problems have greatly restricted the sustainable development of economy and society,which is one of the issues that human society must face and urgently need to solve.Duo to the excellent performance in the degradation of organic pollutants,total water,carbon dioxide reduction,organic synthesis and nitrogen fixation by utilizing sunlight,photocatalytic technology is considered to be one of the most promising technical solutions to solve both environmental pollution problems and energy crises,thus attracting worldwide attention.Compared with traditional water treatment methods,photocatalysis technology has been widely studied because its high processing efficiency and no secondary pollution.In the past few decades,semiconductor photocatalytic materials have exhibited satisfactory photocatalytic activity and great application prospects in the field of photocatalysis.The semiconductor photocatalyst with a suitable band gap is excited by light with an energy higher than band gap value decides the photocatalytic redox reaction occurrence.When was activated by light,the electrons?e^-?on the valence band?VB?are excited to transfer to the conduction band?CB?,meanwhile,holes?h⁺?are produced on the valence band.The h⁺on the VB possess oxidizing capacity and the e^-on the CB has reducing capacity.These photo-generated carriers could form active species such as superoxide radicals?·O₂^-?/hydroxyl radicals?·OH?in the system that can further degrade organic pollutants.Additionally,they could also directly participate in redox reactions to degrade organic contaminants adsorbed on the catalyst surface.During the photocatalytic process,the recombination of photogenerated electron-hole pairs in the bulk phase of photocatalyst is carried out all the time,which directly leads to a reduction of the sctive species in the catalytic process,thereby greatly reducing the photocatalytic efficiency.Generally,the semiconductor catalyst with a wide band gap possess a low recombination rate of photo-generated carriers,and its photo-generated holes and electrons have a stronger redox capacity,but utilization of light tends to be low.In contrast,the semiconductor catalyst with a narrow band gap has a higher recombination rate of photogenerated carriers in the bulk phase,and the photo-generated holes and electrons have a weaker redox capacity,but the utilization of light is relatively high.In order to solve the problems of low light utilization rate and high recombination probability of photogenerated carriers,a large number of effective methods have been developed,including intrinsic defect adjustment,crystal plane adjustment,stoichiometric alteration,metal/nonmetal ion doping,rare metal depositio and construct a heterojunction.As a new type of semiconductor photocatalytic material,BiOCl exhibits excellent photocatalytic activity due to its unique layered crystal structure.Nevertheless,the inherent wide band gap of the BiOCl?about 3.2 e V?limits the absorption of sunlight to the ultraviolet region,which reduce photocatalytic activity greatly.Monoclinic bismuth tetroxide?m-Bi₂O₄?is a new type of bismuth-based semiconductor photocatalytic material with mixed valence states.However,its narrow band gap results in a high probability of photo-generated carrier recombination,which seriously restricts its photocatalytic activity.Based on the above analysis,this thesis aims to synthesize BiOCl graded microsphere with defect level by a novel method,and construct m-Bi₂O₄/Bi₂O_4-xdirect Z-type heterojunctions and m-Bi₂O₄/Bi₂O_4-x/BiOCl three-phase heterojunctions to solve the above problems of bismuth-based semiconductor photocatalytic materials.The main research contents of this paper are as follows:1. Developing a novel route to prepare the BiOCl graded microspheres with highly exposed?110?facets and oxygen vacancies at room temperature.The formation mechanism of graded microspheres based on self-assembly was studied.Under visible light??>420 nm?irradiation,0.1 g of the as-prepared BiOCl completely degraded rhodamine B?Rh B?aqueous solution?100 m L,10 mg/L?within 80s,its photodegradation rate constant reach to 2.18 min^-1,which is about 43times and 11 times for the commercial Ti O₂?P25?and BiOCl prepared by conventional hydrothermal method under the same experimental conditions.Futhermore,the as-prepared BiOCl exhibits satisfactory visible light catalytic activity for various organic dyes and antibiotics.The exceptional enhancement of photocatalytic performance was ascribed to the following two aspects.On one hand,oxygen vacancies form defect levels in the catalyst,which not only effectively separate photogenerated electron-hole pairs but also achieve absorption and utilization of visible light.On the other hand,the[Bi-Cl]layer and[O]layer of?110?facets generate internal electric field,which is beneficial for the efficient separation of photogenerated electron-hole pairs.Reactive species trapping experiments results demonstrate the h⁺and·O₂^-were the main active species during the photocatalytic process.These research results provide a practical guide to the development of BiOCl with intrinsic defects to enhance photocatalytic activity.2. A Z-scheme m-Bi₂O₄/Bi₂O_4-xheterojunction was constructed by a facile one-step hydrothermal strategy.The load rate of the Bi₂O_4-xin m-Bi₂O₄/Bi₂O_4-xcould be effectively regulated.Interestingly,the photocatalytic activity of m-Bi₂O₄/Bi₂O_4-xheterojunction is significantly affected by the load rate of Bi₂O_4-x.Futhermore,we also explored the formation mechanism of m-Bi₂O₄/Bi₂O_4-xheterojunction based on in-situ growth.The as-synthesized m-Bi₂O₄/Bi₂O_4-xexhibited a universality for the degradation of varieties of organic pollutants.In comparison with the pure m-Bi₂O₄,the optimum m-Bi₂O₄/Bi₂O_4-xexhibited a difference in the degradation of different organic compounds.The m-Bi₂O₄/Bi₂O_4-xcomposites exhibited varying degrees promotion for the photodegradation of different organic pollutants,which could be ascribed to the structural discrepancy among the pollutants.More specifically,100m L,10 mg/L of rhodamine B?Rh B?aqueous solution was completely degraded by0.1 g of the optimum m-Bi₂O₄/Bi₂O_4-xwithin 80s.Its photodegradation rate constant reach to 0.1 min^-1,which is 5.4 times for the pure m-Bi₂O₄under visible light??>420 nm?irradiation.Futhermore,the as-prepared composite photocatalyst showed excellent photocatalytic activity for both phenol and methyl orange?MO?.This enhance performance could be ascribed to the formation of efficient Z-scheme heterojunction,which not only effectively promote the charge-separation and transfer efficiency but also lead to stronger redox capacity of photogenerated carriers.Finally,the charge-transfer path was studied according to electrochemical measurement,paramagnetic resonance spectrum?EPR?and reactive species trapping experiments.3. A m-Bi₂O₄/Bi₂O_4-x/BiOCl three-phase heterojunction photocatalyst was constructed through a one-step hydrothermal method.The photocatalytic properties of m-Bi₂O₄/Bi₂O_4-x/BiOCl,m-Bi₂O₄,Bi₂O_4-x,BiOCl and m-Bi₂O₄/Bi₂O_4-xwere compared.As expected,m-Bi₂O₄/Bi₂O_4-x/BiOCl sample demonstrate an enhanced photocatalytic inactivation activity than three pure photocatalysts and m-Bi₂O₄/Bi₂O_4-x.This enhance performance mainly due to the formation of three-phase heterojunctions,which further improve the separation and transfer efficiency of photogenerated carriers.In addition,the prepared m-Bi₂O₄/Bi₂O_4-x/BiOCl exhibited excellent visible light catalytic activity for various organic substances?Bisphenol A,Phenol and Methyl orange?,which indicates its excellent photocatalytic universality.The energy level structure of the m-Bi₂O₄/Bi₂O_4-x/BiOCl was analyzed.The results showed that a Z-type heterojunction was formed between m-Bi₂O₄and Bi₂O_4-x,and a type II heterojunction was formed between m-Bi₂O₄and BiOCl.Finally,based on the three-phase heterojunction,we propose a charge transfer mechanism during the photocatalytic process.