Study On The Regulation And Degradation Mechanism Of Bismuth-Based Photocatalyst

With the impact of industrial and population growth,diverse hazardous pollutants were discharged into the natural water sources.These pollutants,especially antibiotics and dyes,which are difficult-to-degrade,led to water contamination and water supply safety issues,and have become a key factor constraining the sustainable development and realizing an overall well-off society.Therefore,it is urgently needed that developing efficient and clean techniques to produce reactive oxygen species for detoxifying water.One promising technique for water pollution treatment was the photocatalytic technology,which have many advantages of strong oxidizing ability,thorough degradation of pollutants,mild reaction conditions and environmental friendliness.The basic principle of semiconductor photocatalytic technique is by converting light energy into chemical energy,by which catalytic produce strong oxidizing reactive oxygen species?ROS?such as hydroxyl radical?^·OH?,superoxide radical(O₂^·-),singlet oxygen?¹O₂?,and hole?h⁺?.However,conventional photocatalysts like TiO₂ with a large band gap of about 3.2 eV that only photo-active in the UV region,which can utilize no more than 4%of the total solar energy.The inefficient use of visible light?more than 43%of the total solar energy?greatly limits the practical application of the photocatalytic technique.To improve the utilization of solar energy,it is important to develop strategy for preparation of novel visible light active photocatalysts,which becomes a research hotspot in recent years.Increasing the visible light response range of the semiconductor through band gap modulation is one of effective means for enhancing the visible light photocatalytic performance of semiconductors with too large or too narrow band gaps.While,for semiconductor materials with strong intrinsic visible light absorption,suppressing the recombination of photogenerated electron-hole pairs by constructing heterojunctions could efficiently promote its visible light utilization efficiency.Thus,design and fabricating nanocomposites based on these two effective strategies would be efficient for developing visible light catalytic technique.In recent decades,bismuth-based semiconductor materials have been widely concerned and studied by researchers due to their unique and adjustable electronic band structure and morphology.A series of bismuth containing semiconductor materials have been reported to have excellent photocatalytic activity.According to their chemical composition and crystal structure,the band gap of bismuth-based semiconductor materials could cover the wide energy range from 0.3 eV to 3-4 e V.These bismuth-based semiconductor materials also have the disadvantages of photogenic carriers being easy to recombination,low catalytic activity and low utilization rate of visible light.Therefore,to regulate bismuth-based photocatalyst to improve its visible light catalytic activity is of great importance.In this thesis,we focused on the modification on bismuth-based semiconductors with different band gaps that to improve its visible light catalytic activity by promoting the separation of photogenic carriers and the regulation of energy band structure.Firstly,by optimizing the preparation method,we decorated highly dispersed nano Au and zero-valent Bi on the Bi₂WO₆,BiVO₄ and phosphate doped Bi₂O₂CO₃,respectively,the recombination of photoinduced electrons and holes were effectively suppressed owning to the surface plasmon resonance effect?SPR?of Au and metallic Bi and their role for trap electrons,which is beneficial for produce active species such as·OH,·O₂^-,¹O₂ and h⁺for efficient degradation of ofloxacin antibiotics.Then,through phosphate doping as well as the construction of the surface oxygen vacancy,we effectively reduce the band gap of Bi₂O₂CO₃ broadband gap semiconductor materials,from 3.3 eV down to 1.5 eV,which boosted its visible light catalytic activity.In addition,we also prepared the Bi₂Se₃/Bi_1.007Se_0.993.993 solid solution,which holds an enlarged band gap from 0.3 eV to1.38 eV compared with the bulk Bi₂Se₃,which proximity to the theoretical value of 1.5eV that can be excited by the visible light part of the solar spectrum and with extremely enhanced visible light photocatalytic activity.The addition of H₂O₂ could further improve the production of·OH,which achieved rapid degradation of methylene blue and the degradation rate was increased 15 times.The main research contents include the following aspects:1.The Bi₂WO₆ nanosheets were decorated with glutathione?GSH?,and then the chloroauric acid was added to form complexes with GSH on the surface of Bi₂WO₆nanosheets.The Au/Bi₂WO₆ nanocomposites were successfully prepared as the surface HAuCl₄-GSH complex self-reduction during the pyrolysis process.The Au was found highly dispersed by energy dispersive spectroscopy?EDS?mapping analysis and high resolution transmission electron microscope?HRTEM?analysis.The photocatalytic activity of the sample was evaluated by degradation of OFX under visible light irradiation.Among all prepared photocatalysts,0.14 wt%Au/Bi₂WO₆ composite was able to depredate 95%of the OFX in 180 min under visible light with degradation rate of 0.0193 min^-1,which was about 3 times faster than pristine Bi₂WO₆ nanosheets.X-ray photoelectron spectroscopy?XPS?measurement shows that the Au was present on the surface of Bi₂WO₆ nanosheets.Ultraviolet-visible diffuse reflection spectrum?DRS?test results show obvious surface plasmon resonance?SPR?absorption peak apparent at574 nm after the loading of gold,and no shift with the increased loading amount of gold.In addition,DRS results show that the Au/Bi₂WO₆ nanocomposites have the band gap of 2.91 eV,consistent with the pristine bismuth tungstate nanosheet.Through the Mott-Schottky test could further determine its band structure,which has not changed for the Au/Bi₂WO₆ nanocomposites compared with the pristine bismuth tungstate nanosheet.Mott-schottky?mott-schottky?tests indicate that the Au/Bi₂WO₆ nanocomposites are an n-type semiconductor with a flat band potential of-0.33 V.The photoluminescence spectrum?PL?test results show that the loading gold can effectively inhibit the electron hole recombination.The experiments of inhibitors show that the main active species in the catalytic process are holes?h⁺?,followed by singlet oxygen?¹O₂?and hydroxyl radicals?·OH?.In addition,superoxide radicals also play a certain role in this system.Based on the above analysis of experimental results,we speculate that the enhanced visible light catalytic performance of Au/Bi₂WO₆ nanocomposites is caused by the efficient separation of photoinduced electron and hole pairs based on the surface plasmon resonance effect of gold nanoparticles and its electronic trapping effect.The separated electron and hole could react with the dissolved oxygen in water to produce the corresponding active species,thus to achieve the effect of high efficient degradation of ofloxacin.This method greatly reduces the load of gold,improves the utilization rate of gold and reduces the cost.2.The BiVO₄ hierarchical structure was thermal conversion from flower-like BiOI through an equivalent-volume impregnation method at first.Then the BiVO₄hierarchical structure was decorated with glutathione?GSH?,and then the chloroauric acid was added to form complexes with GSH on the surface of BiVO₄ hierarchical structure.The Au/S/BiVO₄ nanocomposites were successfully prepared as the surface HAuCl₄-GSH complex self-reduction during the pyrolysis process.The sulfur and Au were found highly dispersed by energy dispersive spectroscopy?EDS?mapping analysis.The band gap of BiVO₄ is 2.52 eV,which is smaller than that of Bi₂WO₆?2.91 eV?and has stronger visible light absorption capability.X-ray photoelectron spectroscopy?XPS?measurement shows that the sulfur was doped into the surface crystal lattice of BiVO₄hierarchical structure by replace some of the lattice oxygen and produce lattice oxygen defect in it,which greatly promotes the close integration and electron interaction between Au and BiVO₄.The photocatalytic activity of the sample was evaluated by degradation of OFX under visible light irradiation.Among all prepared photocatalysts,0.3 wt%Au/S/BiVO₄ composite was able to depredate 80%of the OFX in 15 min under visible light with degradation rate of 0.125 min^-1,which was about 7 times faster than pristine BiVO₄.The catalyst has good stability and no significant reduction in the degradation efficiency after five cycles.DRS test results show that the Au/S/BiVO4nanocomposites have a narrower band gap of 2.46 eV compared with the original bismuth vanadate?2.52 eV?.Combining with the Mott-Schottky?mulder-Schottky?test results,it is speculated that its band gap reduction is caused by sulfur mixed valence band position.Further studies on the degradation mechanism show that the holes are the main reactive species?account for more than 80%?in the process of degradation.The enhanced visible light photocatalytic activity of the Au/S/BiVO₄ nanocomposites was mainly attributed to the electron-absorbing effect of oxygen vacancy and the close coupling between the Au and the BiVO₄,which greatly improved its electron transport properties.Plus,the Au/S/BiVO₄ nanocomposites with narrower band gap could produce more electron-hole pairs under the visible light irradiation.3.For Bi₂O₂CO₃ has a wide band gap?3.3 eV?and with a low visible light response,we doped phosphate into the Bi₂O₂CO₃ through coprecipitation method.The phosphates doped Bi₂O₂CO₃ have enlarged specific surface area,which increased from6.1 m²/g up to 44 m²/g,and narrower band gap of 3.23 eV,effectively improve its adsorption performance and visible light catalytic activity.Furthermore,The Bi⁰ nano particle decorated PO₄ oxoanion doped Bi₂O₂CO₃ nanocomposites?Bi-P-BOC?were successfully prepared through an in-situ thermal reduction method by put the PO₄oxoanion doped Bi₂O₂CO₃ annealing in H₂/Ar mixture gas.XPS and UV-vis DRS test results show that during the PO₄ oxoanion doping and H₂/Ar mixture gas calcination,a large number of oxygen vacancies are produced,result in the reduction of the band gap of Bi₂O₂CO₃,from 3.3 eV down to 1.5 eV,which is more conducive to the absorption of visible light.The X-ray diffraction?XRD?,ex-situ field emission scanning electron microscopy?FE-SEM?,High Resolution Transmission Electron Microscopy?HRTEM?,and X-ray photoelectron spectroscopy?XPS?confirm the production of zero-valent bismuth along with much oxygen vacancy on the surface of the composites after the annealing process.The UV-visible diffuse reflectance spectroscopy?UV-vis DRS?spectrum also shows the enhanced visible light absorption attribute to the SPR effect of Bi⁰.The improved separation of the photoinduced electron and hole pairs were confirmed by photoluminescence?PL?spectrum.Such Bi-P-BOC is able to depredate85%of the OFX in 180 min under visible light with degradation rate of 0.01299 min^-1,which was about 2 times faster than the PO₄ doped Bi₂O₂CO₃ and about 8 times faster than pristine Bi₂O₂CO₃.O₂^·-,¹O₂,and h⁺were detected to be the main ROS species for degradation of OFX in this system,and the possible electron transfer path was proposed.This study provides an effective strategy for regulation the energy band of bismuth-based semiconductor material with broad band gap to improve the visible light utilization.Combining with the load of non-noble metal elemental bismuth,on the basis of the band gap regulation,to promote its light carrier separation,which has a certain guiding significance for the application study of broad band gap bismuth-based photocatalyst.4.The bulk Bi₂Se₃ suffers a narrow band gap?⁰.3 eV?and low visible light photocatalytic efficiency.To solve the above problem,we prepared a novel Bi₂Se₃/Bi_1.007Se_0.993.993 solid solution nanocomposite by controlled solvothermal method with a novel selenium source.SEM shows that the solid solution has a flower-like microsphere morphology assembled by ultra-thin nanosheets,and the thickness of the nanosheets is only about 7.46 nm.By means of XRD,HRTEM and ICP-MS,the solid solution structure was confirmed and its growth mechanism has been discussed.The UV-visible diffuse reflectance spectroscopy?UV-vis DRS?spectrum and??h??^2h?curve fitting shows an enlarged band gap from 0.3 eV to 1.38 eV of this Bi₂Se₃/Bi_1.007Se_0.993.993 solid solution nanocomposite,which is proximity to the theoretical value of 1.5 eV that can be excited by the visible part of the solar spectrum.The Mott-Schottky measurement exhibit the more negative conduction band position of the Bi₂Se₃/Bi_1.007Se_0.993compared with the Bi₂Se₃?-0.28 V?and Bi_1.007Se_0.993?+0.2 V?,about-0.3 V?vs.NHE?,which is beneficial to activate O₂ to produce O₂^·-.What'more,with the assistance of H₂O₂,the photocatalytic generated electron could reduce H₂O₂ to produce^·OH more efficiently,which shows a degradation rate of 15 time faster than the Bi₂Se₃.The photoluminescence?PL?spectrum and electrochemical impedance spectroscopy?EIS?shows improved separation of photoinduced electron and hole pairs and decreased resistance of the nanocomposite.Thus,the Bi₂Se₃/Bi_1.007Se_0.993.993 solid solution nanocomposite shows enhanced visible light photocatalysis performance on degradation of methylene blue?MB?than the Bi₂Se₃ and Bi_1.007Se_0.993.The enlarged band gap and appropriate band position were the main reason that dominated the enhanced visible light photocatalytic performance of the Bi₂Se₃/Bi_1.007Se_0.993.993 solid solution nanocomposite.