Study On Structure Regulation Of Novel Bi-Based Photocatalysts And Their Enhanced Broad Spectrum Light Responsive Photocatalytic Mechanism

Semiconductor photocatalytic technology,because of its advantages of high efficiency and greenness in environmental purification and clean energy synthesis,is promising.However,most photocatalysts have the disadvantages with low photoresponse range,high photo-generated carrier recombination rate and low light utilization rate,which limit their applications.Therefore,this study focused on Bi-based photocatalysts,and using ion doping,semiconductor recombination and other methods to regulate them for improving the spectral response range and accelerating the separation of photogenerated carriers for enhancing their photocatalytic activity.The main research contents and conclusions are as follows:（1）Monolayer BiO_2-x photocatalyst was synthesized by combining hydrothermal and ultrasonic method.XRD,XPS and Raman results show that the obtained sample is mixed valence BiO_2-x,containing Bi³⁺and Bi⁵⁺.TEM and AFM reveal that the thickness of the nearly transparent BiO_2-x is 1.0 nm,which is equal to the thickness of[O-Bi-O]layer,therefore,the obtained BiO2-x nanosheet is monolayer structure.XPS,ESR,TG and positron annihilation results reveal that when the thickness of BiO_2-x-x decreases to 1.0 nm,the oxygen vacancy concentration also increases,and the defect type has changed from oxygen defects into combined defect(V_Bi-O’’’).The UV-vis-NIR DRS and DFT calculations show that compared to BiO_2-x nanosheets,the light absorption of the monolayer BiO_2-x is significantly red-shift and the combination of monolayer structure and defects could enhance light absorption and promote separation of photogenerated carriers in monolayer BiO_2-x.Meanwhile,RhB and phenol were selected as target pollutants to study the inner relationship between microstructure of BiO_2-x and its photocatalytic activity.Under UV,visible and NIR irradiation,monolayer BiO2-x exhibits better photocatalytic activity than BiO_2-x and maintains 85%photocatalytic activity for degradation of RhB even after five cycles of experiments.The trapping experiments confirm that under ultraviolet（UV）and visible light irradiation,the main active radicals are photogenerated holes and·O₂^-,while under the irradiation of near-infrared light（NIR）,the main active radical is photogenerated holes.The main reason is that under UV or visible light irradiation,photogenerated electrons with higher energy can pass through the defect trapping state to reach the conduction band and react with oxygen to form·O₂^-;while under NIR light irradiation,photogenerated electrons with lower energy are captured by the defect state and cannot reach the conduction band.（2）Ni-doped BiO_2-x nanosheet photocatalyst with UV,visible and NIR light responsive property was prepared by hydrothermal method.DFT calculation results show that Ni doping can lead to the generation of defect levels,shorten the band gap width of BiO_2-x and enhance the light absorption property.PL,electrical impedanceand UV-vis-NIR DRS spectra show that Ni doping is not only beneficial to enhance the spectral response range of BiO2-x photocatalyst,but also facilitates the separation of photogenerated carriers.At the same time,Ni doping is beneficial to the adsorption and activation of BiO2-x for oxygen.The results of ESR and trapping experiments show that Ni-doped BiO_2-x nanosheets can achieve the activation of molecular oxygen under UV,visible and NIR lights irradiation,especially in NIR irradiation,Ni doping endows BiO2-x the ability of activating molecular oxygen into·O₂^-.When the doping amount of Ni is equal to 0.4 mmol,Ni-doped BiO2-x nanosheets show better photocatalytic activity than that of BiO2-x under UV,visible and NIR irradiation.Moreover,after five cycles,Ni-doped BiO_2-x-x nanosheets still remains good photocatalytic performance and stability.（3）Bi₂O₄ photocatalyst was synthesized as a precursor by hydrothermal method and used to fabricate a series of Bi₂O₄/Bi₂O₂CO₃ composite photocatalysts by photocatalytic in-situ epitaxy in the process of photocatalytic degradation of organic pollutants.The morphological changes of Bi₂O₄/Bi₂O₂CO₃ heterojunction were observed under different reaction time and different wavelengths of light by SEM and TEM characterizations.The（202^<sub>）plane of Bi₂O₄ and the（110）plane of Bi₂O₂CO₃ are tightly connected and extend in the same direction.XRD,XPS and UV-vis-NIR DRS other characterization methods were used to study the phase composition,optical properties,band structure and photogenerated charge behavior of Bi₂O₄/Bi₂O₂CO₃ composite photocatalyst.¹³C NMR analyses demonstrate that the C source in the Bi₂O₄/Bi₂O₂CO₃ composite is derived from the intermediate state of the degradation process of organic pollutants.DFT calculations show that the（202^<sub>）plane of Bi₂O₄ contains a large amount of unsaturated Bi coordination,which can be used as an active site for epitaxial growth of Bi₂O₂CO₃.Moreover,due to the difference of the work functions between Bi₂O₄ and Bi₂O₂CO₃ at their interfaces,the photogenerated electrons on the conduction band of Bi₂O₄ can transfer to the conduction band of Bi₂O₂CO₃ and the holes retain in the valence band of Bi₂O₄.Transient fluorescence lifetime and photoelectrochemical analysis show that photogenerated electron-hole paries can be effectively separated in the Bi₂O₄/Bi₂O₂CO₃ composite.The typical sample Bi₂O₄/Bi₂O₂CO₃（2 h）composite shows excellent photocatalytic degradation of ciprofloxacin（CIP）.（4）The Bi₂O₄/BiO_2-x composite photocatalyst was synthesized by hydrothermal method by controlling the concentration of NaOH and reaction time.The phase composition,morphology,optical properties and photogenerated charge behavior of Bi2O4/BiO2-x composite were investigated by various characterization techniques.Bi₂O₄/BiO_2-x composite photocatalyst reveals more excellent photocatalytic activity as compared to Bi₂O₄ and BiO_2-x under UV,visible and NIR lights irradiation.ESR spectra,trapping experiments and quantitative analysis demonstrate that compared with Bi₂O₄ and BiO_2-x,Bi₂O₄/BiO_2-x composite photocatalyst could generate more·O₂^-in photocatalytic process under UV,visible and NIR lights irradiation.Photoluminescence（PL）spectroscopy and photoelectrochemical tests prove that Bi₂O₄/BiO_2-x composite has the strongest photoresponse and the minimum impedance value,suggesting it has the best separation ability of photogenerated electrons and holes.The DFT calculation results show that the heterojunction of Bi₂O₄/BiO_2-x is beneficial to the transfer of photogenerated charges from BiO_2-x to Bi₂O₄,accelerating the separation of photogenerated carriers.In summary,this work provides theoretical and practical reference for the development,modificationand photocatalytic mechanism of UV,visible and NIR responsive photocatalysts.