Modification Of Photocatalytic Property Of Bismuth Ferrite Oxide

Recently,semiconductor photocatalysis has been regarded as an ideal“green”technology for environmental purification and solar energy conversion.BiFeO₃ and Bi₂Fe₄O₉ are known to be the visible-light-driven bismuth ferrite oxide photocatalysts due to their narrow bandgap?².1 eV?and chemical stability,which has attracted widely attentions.Unfortunately,the high recombination rate of photogenerated electrons and holes limits their practical applications in the field of photocatalysis.Therefore,the purpose of this dissertation is the modification of BiFeO₃ and Bi₂Fe₄O₉photocatalysts for the enhancement of their photocatalytic activities.This dissertation explores the methods for effectively extending the photoresponse range and promorting separation of the photogenerated charges of bismuth ferrite oxide through the introduction of surface oxygen vacancies and decoration of carbon quantum dots,investigates the modification routes for e fficiently promoting the separation of photogenerated charges in bismuth ferrite oxide through the assembling type?II heterojunctions and p-n heterojunctions,develops the bismuth ferrite oxide based composites with high photocatalytic redox capacities by constructing direct Z-scheme heterojunctions.This investigation is expected to provide the feasible modification strategies of bismuth ferrite oxide to meet the different photocatalytic degradation requirements,and offer theoretical and experimental supports for the application of bismuth ferrite oxides in the photocatalytic field.The main contents of this thesis are as follows:?1?Oxygen vacancies were introduced onto the surface of BiFeO ₃ nanoparticles by the simple NaBH₄ reduction method to yield oxygen-deficient BiFeO_3-x samples.Comprehensive analysis on the basis of high-resolution transmission electron microscopy?HRTEM?observation and X-ray photoelectron spectrum?XPS?analysis confirms that the thickness of the surface oxygen vacancies can be controlled by changing reduction time.The photocatalytic activity of as-prepared BiFeO_3-x samples was evaluated by the degradation of rhodamine B under simulated sunlight irradiation,indicating that BiFeO_3-x samples exhibit higher photocatalytic activity than that of BiFeO₃,and the BiFeO_3-x sample reduced for 40 min exhibits the highest photocatalytic efficiency.In addition,the obtained BiFeO_3-x sample exhibits good photocatalytic reusability.It is expected that,when oxygen vacancies are introduced onto the surface of BiFeO₃ sample after NaBH₄ reduction,an oxygen vacancy state appears in the forbidden gap of the BiFeO₃,resulting in the reduction of the bandgap.On the other hand,the surface oxygen vacancies act as photoinduced electrons acceptors promote the seperation of photogenerated charges,leading to an increase of photocatalytic activity.?2?Carbon quantum dots?CQDs?were assembled on the surface of BiFeO₃nanoparticles via a hydrothermal route to obtain CQDs/BiFeO₃ samples.On the other hand,carbon coated BiFeO₃ samples?C@BiFeO₃?were also fabricated by a hydrothermal method.The photocatalytic activity of as-prepared samples was evaluated by the degradtion of rhodamine B under simulated sunlight and visible-light irradiation.Compared with C@BiFeO₃,the photocatalytic activity of BiFeO₃ can be further improved by the decoration of CQDs.The enhanced photocatalytic activity of CQDs-modified BiFeO₃ can be attributed to the fact that the photogeneratd electrons in CQDs can be transferred to the conduction band of BiFeO₃,and the photogeneratd electrons in the BiFeO₃ will migrate to the CQDs as an excellent electron acceptor.This interesting electron transfer process can efficiently suppress the recombination of photogenerated charges.On the other hand,the upconversion photoluminescence emitted from CQDs could induce the generation of additional photogenerated charges in BiFeO₃.Consequently,more photogenerated carriers are able to participate in the photocatalytic reactions,and thus leading to an increased photocatalytic performance of the CQDs/BiFeO₃.?3?The g-C₃N₄ nanoparticles decorated BiFeO₃ microspheres composites?g-C₃N₄/BiFeO₃?were successfully synthesized by hydrothermal treatment of g-C₃N₄nanoparticles together with BiFeO₃ microspheres.The SEM and HRTEM observation indicate that the heterojunction between C₃N₄ and BiFeO₃ are formed.It is found that the composites exhibit an enhanced photocatalytic activity for the degradation of methylene blue under simulated sunlight irradiation compared to BiFeO₃ microspheres,achieving the highest degradation percentage at the g-C₃N₄ nanoparticles content of16%.Moreover,the intrinsical photocataltic property of the composite is further confirmed by the degradation of colorless phenol.The migration and seperation of photogenerated electrons and holes can be promoted by the formation of the t ype?II heterojunction between C₃N₄ and BiFeO₃,leading to the enhancement of photocatalytic activity.The active species trapping experiment reveals that the photogenerated hole and hydrogen peroxide are regarded as the major active species for the degradation of dye,while hydroxyl radicals play a minor role in the photocatalytic reaction.?4?Ag₃PO₄ nanoparticles were decorated onto the surface of BiFeO₃ microcuboids through a precipitation method to obtain p-n heterojunction Ag₃PO₄/BiFeO₃composites.The composites were employed for the degradation of acid orange 7 under visible-light irradiation.It is found that the composites exhibit remarkable enhanced photocatalytic activity compared with bare BiFeO₃.Meanwhile,the intrinsical visible-light driven photocatalytic activity of Ag₃PO₄/BiFeO₃ composites was further demonstrated by the degradation of phenol.In addition,the Ag₃PO₄/BiFeO₃ composite possesses good photo-Fenton-like catalysis property.The obvious enhancement in the photocatalytic and photo-Fenton-like catalytic activity of composite is mainly ascribed to the efficient photogenerated charges separation and interfacial charges migration caused by the formation of Ag₃PO₄/BiFeO₃ p-n heterojunctions.?5?Direct Z-scheme Ag₂S/BiFeO₃ heterojunction composites were successfully prepared through a precipitation method.It is found that Ag₂S nanoparticles are well-decorated on the surface of polyhedral BiFeO₃ particles.The photocatalytic and photo-Fenton catalytic activities of the as-derived Ag₂S/BiFeO₃ composites were evaluated by the degradation of methyl orange under visible-light irradiation.The results indicate that the Ag₂S/BiFeO₃ composites exhibit obviously improved photocatalytic and photo-Fenton catalytic activities.The relationship between catalytic activities and Ag₂S content was investigated,indicating that t he optimum composit e sample was observed to be 15%Ag₂S/BiFeO₃ with an Ag₂S mass fraction of 15%.Furthermore,the effect s o f t he catalyst do sage,hydrogen pero xide co ntent and dye co ncentrat io n o n the photo-Fento n catalyt ic degradat io n o f dye over the composite were systematacially evaluated.The analysis of photogenerated charges behavior suggests that the separation of photogenerated charges in the composite can be promoted by the decoration of Ag₂S.The active species detection experiments reveal that the photogenerated ho le and superoxide radical are considered to be the major reactive species,while hydro xyl radicals play a relatively small role in the photocatalytic degradation.On the contrary,hydro xyl rad icals play a relatively large role in the photo-Fenton catalytic reaction.The enhanced catalyt ic act ivit y o f t he direct Z-scheme Ag₂S/BiFeO₃ het ero junct io n co mpo sites can be exp lained as t he result of efficient separation of photoexcited charges,resulting from the Z-scheme electro n transfer.In addition,the photocatalyt ic redo x capacit y of BiFeO₃ can be strengthened.?6?Ternary all-solid-state Z-scheme g-C₃N₄/CNT/Bi₂Fe₄O₉ composites were prepared by a hydrothermal method.The catalytic activity of the samples for the degradation of acid orange 7 was evaluated under simulated sunlight irradiation.It is found that the ternary composites exhibit enhanced photo catalytic and photo-Fenton catalytic activity when compared with bare Bi₂Fe₄O₉ and g-C₃N₄/Bi₂Fe₄O₉ composites.The photoelectrochemical and photoluminescence measurements indicate that the separation efficiency of photogenerated charges in the ternary composite is much higher than those of Bi₂Fe₄O₉ and g-C₃N₄/Bi₂Fe₄O₉.The photocatalytic mechanism of g-C₃N₄/CNT/Bi₂Fe₄O₉ was proposed according to the active species trapping experiment and energy-band potential analysis,revealing that the introduction of carbon nanotubes as an excellent solid electron mediator into the ternary composite s can effectively accelerate the electron migration between Bi₂Fe₄O₉ and g-C₃N₄.This charge transfer process results in highly-efficient separation of photogenerated charges,thus leading to greatly enhanced photocatalytic activity of g-C₃N₄/CNT/Bi₂Fe₄O₉composites.As a result,the efficient Z-scheme g-C₃N₄/CNT/Bi₂Fe₄O₉ composite with high photocatalytic redox capacity has been obtained.