Doped Fluorine Regulation And Carrier Separation Efficiency Of Titanium Dioxide Nanosheet

Titanium dioxide(TiO₂)has received extensive attention in the fields of photocatalysis and electrochemical energy storage,and is one of the most widely used semiconductor materials for scientific research and practical applications.However,the low efficiency of photogenerated electron-hole pairs(carriers)separation limits the development of TiO₂.It is extensively accepted that fluorine(F)doping is an effective method to improve the carriers separation and transfer efficiency of TiO₂ materials.Since there are many types of fluorine species in fluorine doped TiO₂,the structure-activity relationship and mechanism of performance have yet to been further studied.Therefore,it is critical important to study the F-doped mechanism regarding to the design of high-performance TiO₂ materials.In this thesis,TiO₂ nanosheets were choosed as model.TiO₂ materials with different types of doped fluorine have been constructed.The transformation of F species along with the formation of Ti³⁺defects under irradiation has been observed.The mechanism of transformation of F species and the formation of Ti³⁺defects have been therefore studied.The separation efficiency of carriers has been improved.The design of high-performance photocatalytic materials has been realized.The main research contents and results are as following:(1)Construction of a three-dimensional microsphere(donated as TiO₂M-S)assembling from F-doped TiO₂ nanosheets.The TiO₂M-S shows a high photocatalytic hydrogen production activity(45 mmol·h^-1·g^-1),and has a high conversion rate up to45%(twice that of defluorinated samples)in selective oxidation of benzyl alcohol to benzaldehyde.Fluorescence spectrum,transient photocurrent response and time-resolved fluorescence spectrum further demonstrate that F-doped sample has the highest carriers separation efficiency and the longest carrier lifetime(1.4-fold of defluorinated TiO₂ in photocurrent density).Solid state NMR and EPR tests indicate that the state of doped fluorine species might be changed with the generation of Ti³⁺defects under light illumination.(2)Synthesis of F-doped TiO₂ nanosheet(donated as F-TiO₂).The two-dimensional F-doped TiO₂ nanosheet was observed and researched.According to the theoretical calculations,among all of the F species(bridged Ti₂-F,tri-coordinate Ti₃-F,surface Ti₁-F,and a small amount of bulk Ti₁-F),the electron cloud density of bulk Ti₁-F is highest,indicating that bulk Ti₁-F more favour of the separation and transfer of carriers.Solid state NMR and EPR tests indicate that irradiation causes the transformation of bulk Ti₁-F from other F species and the generation of corresponding Ti³⁺defects,and thus facilitates the separation and transfer of carriers(1.2-fold of commercial TiO₂ in photocurrent density).(3)Construction of titanium difluorooxide/titanium dioxide composite nanosheet(donated as TiOF₂/TiO₂)with controllable designed interfacial Ti₁-F.No Ti₁-F is found in our sample from the tests.The density functional theory further confirms the Ti₂-F bond at the phase interface will break when capture a photogenerated electron,and then generate the bulk Ti₁-F and corresponding Ti³⁺defects.Solid-state NMR and EPR characterizations show that the content of bulk Ti₁-F and Ti³⁺defects increased over irradiation time.The carriers separation and transfer efficiency of TiOF₂/TiO₂ has been significantly improved by the generation of bulk Ti₁-F.TiOF₂/TiO₂ shows high photocatalytic activity(the photodegradation performance of acetone is 5.5 times that of P25)and carriers separation efficiency(2.6-fold of defluorinated TiO₂ in photocurrent density).