Study On Synthesis And Photocatalytic Property Of Bismuth-Based Organic-Inorganic Composite Photocatalysts

In recent years,photocatalytic technology has been regarded as an efficient environmental technology in solar energy conversion and environmental purification.During the process of semiconductor photocatalysis,it changes light energy to chemical energy but not to damage the photocatalysts,further promoting the chemical reaction.Moreover,there is less secondary pollution in the conversion process of chemical energy due to the clean and inexpensive solar energy.Therefore,seeking novel photocatalysts and developing various methods to improve the photocatalytic activity is an inevitable trend for the further practical application of photocatalytic technology,which has caught the extensive attention of researchers at home and abroadAmong the photocatalytic materials,bismuth based semiconductors have caused wide public concerns due to their good structural features and excellent photocatalytic activity.However,there are two main factors of affecting photocatalytic performance of bismuth based semiconductors include narrow scope of light absorption and low carrier separation efficiency.Thus.to expand the light absorption,make use of the sun efficiently and suppress photo-induced electrons and holes has become the research emphasis and hotspot of bismuth based photocatalytic materials.The structures affect the performance of semiconductor materials.Thus,the relationship between structure and photocatalytic performance of semiconductor material is inseparable.From the perspective of structure,controlling the crystal structure,electronic structure and surface structure is an important paths to enhance the photocatalytic property of semiconductors.Surface modification,especially semiconductors coupling is one of the important ways for the enhancement of phtocatalytic activity of bismuth based materials.However,traditional semiconductors coupling is inorganic-inorganic coupling,which still exists several unavoidable issues.One hand,the contact area between different components is limited for the independent contact sites between inorganic particles and the particles are uneven at the interface.On the other hand,there is van der waals force between inorganic particles,which is weak and not facilitates the transfer of photogenerated charge carriers.But.relatively speaking,organic-inorganic semiconductor coupling is based on molecular level.Organic molecules could be dispersed on the surface of inorganic particles,resulting in a large contact areas.Moreover,various chemical bonds or other physical forces formed at the interface of the organic-inorganic semiconductor,which makes the transfer of photogenerated electrons easier.In addition,there are many new properties in composites due to the different interface status.In semiconductor coupling,the selected organic phase is usually organic macromolecular(such as conductive polymer,ion liquids,metal organic compounds etc.),but the steric hindrance and geometric structure maybe affect the organic-inorganic semiconductor coupling.Small organic molecule is an excellent choice for its flexible and controllable structure.Especially,the polar small organic molecule could induce a polar interface electric field in organic-inorganic composites.Therefore,it is reasonable to consider that both the optical absorption behavior and the kinetics of photogenerated charge carriers of a nonpolar photocatalyst can be improved by chemically bonding electron withdrawing organic molecules.In this thesis,starting from the perspective of surface modification of bismuth based photocatalysts,the relationship between the polarity of small organic molecule and the structure of semiconductor has been studied via organic-inorganic semiconductor coupling.The main research contents and important results of this thesis are presented as follows:In chapter one,we first briefly introduced the research background and principal mechanism of semiconductor photocatalysis,summarized the main applications,and analyzed the research progress,and discussed the constraints of semiconductor photocatalysis(narrow scope of light absorption and low carrier separation efficiency)as well as the improvement methods.Then,the research progress of bismuth-based compounds was also introduced.And we discussed the new design ideas for improving the photocatalytic property of bismuth based photocatalysts from the perspective of organic-inorganic semiconductor coupling.Finally,the research significance and the outline of this thesis were summarized.In chapter two we studied a series of bismuth based semiconductors including layered BiOX(X==Cl Br,I)and Bi2O2CO3 by 4-carboxybenzenethiol(4CBT-H)modification.and discussed the effects of bismuth based semiconductor microstructures on the organic-inorganic composites.(1)BiOCl with both(001)and(010)facets were prepared by a hydrothermal procedure.4CBT@BiOCl organic-inorganic composites was constructed by 4CBT modification on the surface of BiOCl.The structure and morphology of the composites were characterized by XRD and SEM.The interface mechanism between 4CBT and BiOCl with different facets were analyzed by Raman,XPS and FT-IR.The optical property was characterized by DRS.photocatalytic acticities of 4CBT@BiOCl{001} and 4CBT@BiOCl{001})were evaluated via the photodegradation of RhB and the photocatalytic decomposition of water into oxygen.The results showed that their absorption edges obviously shift to visible light region and the quantum efficiency is enhanced,which can be due to the Bi-S bonds formed between BiOCl and 4CBT.Moreover,a synergetic effect between the polarity of 4CBT and the internal electric field(IEF)of BiOCl.In addition,we studied the photocatalytic properties of 4CBT@BiOBr and 4CBT@BiOI and obtained the same results.(2)Bi2O2CO3 nanosheets(BOC)with different thickness were prepared via a hydrothermal method.By the surface-modification of 4CBT,4CBT@BOC composites were constructed and studied the effect of semiconductor particle size on the photocatalytic properties of composites.The crystal structure,morphology,optical chemical and physical property of the products were characterized by XRD,SEM,BET,DRS,XPS,FT-IR and PL.RhB was selected as a polutant model to evaluated the visible-light photocatalytic acticity of 4CBT@BOC composites.The increased factor before and after 4CBT modification is larger over BOC(100nm)than that over BOC(200nm).The reason is that the larger BET surface area of BOC(100nm)is,the more reactive sites of[Bi2O22+]are,the more Bi-S bands are formed.Thus,to maximize the synergetic effect,it is better to minimize the size of the corresponding nanoparticles.In chapter three,we studied the photocatalytic activity of Bi2O3 with different crystal phases surface-modified by 4CBT-H.First,α-Bi2O3 and β-Bi2AO3 were synthesized using a precipitate method.Then 4CBT@a-Bi2O3 and 4CBT@β-Bi2O3 were prepared by a hydrothermal procedure.The crystal structure,optical chemical and physical property were characterized.The photocatalytic activities of composites were evaluated via the photodegradation of RhB.The results show that 4CBT@a-Bi2O3 and 4CBT@β-Bi2O3 exhibit higher photocatalytic activity than the pristine Bi2O3.We discussed the relationship between the crystal structure and the enhancement of photocatalytic activity.In chapter four,we prepared BiO(HCOO)via a hydrothermal method and abbreviated it as BFM.The obtained BFM was surface-modified by 4CBT.The crystal phase and crystallinity of 4CBT@BFM were analyzed by XRD patterns.The optical properties of the as-prepared samples were confirmed by the UV-Vis diffuse eflectance spectroscopy.The photocatalytic activity of the products was evaluated by photodecomposition of RhB under visible and ultroviolet light irradiation at room temperature.Meanwhile,BFM was respectively studied by chemically attaching electron-withdrawing 4-substituted thiophenolates-S-C6H4Z(Z= NO2,COOH,Cl,Br,H,CH3,NH2).Deep study on the relationship between the apparent rate constant(kapp)and the Hammett constants σpara of the 4-substituted groups were carried out.The result is that the rate becomes higher as the 4-substituent is more-strongly electron-withdrawing.In addition,to further testify the synergistic mechanism of molecular polarity and photocatalytic property in composites,we probe how the rate of the RhB degradation reaction under visible light is affected by the BiOX samples treated with various 4-substituted thiophenolates.The results are the same as that of-S-C6H4Z@BFM.In chapter five,we studied the synthesis and photocatalytic activity of BiO(HCOO)by 4-fluorobenzoic acid(PFBA)modification via a hydrothermal method.The crystal structure,optical chemical and physical property were characterized.The photocatalytic activities of composites were evaluated via the photodegradation of RhB under UV-Vis light irradiation.We discussed the interface mechanism between PFBA and BiO(HCOO).Although the surface modification of PFBA could not help to extend the light absorption,but the quantum efficiency is improved.The polarity of PFBA is strong due to the high electron-withdrawing ability of-F group and the effect of polarity on the transfer of photogenerated electrons in PFBA@BiO(HCOO)was discussed in detail.In chapter six.we comprehensively summarized the research contents of our work,and discussed the shortages and problems remained to be solved based on the current study combining with the development of organic-inorganic composite photocatalysts.Finally,we made a prospect and an approach to further development..In summary,compared with the fabrication of semiconductors with polar surface,the construction of organic-inorganic composite photocatalysts with polar small organic molecule provides a much easier and simpler way to improve the photocatalytic property of inorganic semiconductors.