Study On The Doping, Characterization, And Optical Response Properties Of Nano Titanium Dioxide

Due to photoinduced properties,Titanium dioxide(TiO₂) has extensive uses in environment-protection and energy-conversion fields,such as photocatalytic degradation of pollutants,self-cleaning coating,hydregon-production through photochemical water splitting,and fabrication of solar energy cells.However,more widespread practical applications have been hampered by its wide band gap,which requires ultraviolet radiation as the optical excitation source.Shifting the optical response region of TiO₂ towards visible spectral range means that the visible spectrum of sun' rays,even the indoor irradiation,can be used as the optical excitation source of TiO₂.This will extend applications of TiO₂.Therefore,many researches are now focused on the preparation and properties of TiO₂ with visible response.The modification of the electronic structures of TiO₂ by doping is one of important routes for preparing TiO₂ with visible response.In this dissertation,several novel doping methods have been successfully developed for preparing V-doped,V/N co-doped,C-doped,and C/N/F co-doped TiO₂ with visible response.By means of various characterization techniques,the microstructures,ingredients,and the chemical states of doping elements of TiO₂ were analysed.The doping mechanisms were in detail discussed.The optical response properties of doped TiO₂ were evaluated thought optical absorption spectra and the photocatalytic degradation of methylene blue under visible light irradiation(λ＞420 nm).The main results are as follows:1.Based on the mechnsim of the formation of metal oxide thin films by the LPD method,this method was first used for preparing V-doped TiO₂ thin films.The effects of V ions concentration on the microstructures,the chemical states of V,and properties of V-doped TiO₂ thin films were investigated.The in situ doping mechanism was proposed.V-doped TiO₂ thin films consist of anatase nano-crystal TiO₂ with the grain size of 8～20nm.For low concentration of V ions,V is incorporated into the TiO₂ lattice in the form of V⁴⁺ by in situ doping.The red-shift was observed in the UV-Vis absorption spectra of V-doped TiO₂ thin films.V-doped TiO₂ thin films show visible (Vis-) photocatalytic activities.With the increase of V ions concentration,the content of V⁵⁺ increases,and the grain size becomes bigger.The latter two factors result in the decrease of Vis-photocatalytic activities of V-doped TiO₂ thin films.2.Taking the electronic structures of N-doped TiO₂ and V-doped TiO₂ into account,V/N co-doping was put forward as a route for preparing doped TiO₂ with enhanced Vis-response and photoeatalytic activities.V/N co-doped nanocrystal TiO₂ powders were prepared at room temperature by controlled hydrlysis followed by room-temperature nitridation.The powders are composed of anatase TiO₂ with grain size of 4～5nm.V occupies some Ti⁴⁺ sites in the forms of V⁴⁺ and V³⁺,N exists in TiO₂ lattice in the forms of substitutional N and interstitial N.V facilitates room-temperature nitridation.The high reactivity and lots of defects on the surface and in the interiors of nano-erystal TiO₂ contribute to the incorporation of N into TiO₂ lattice at room temperature.V/N co-doped TiO₂(2.76eV)shows more band-gap narrowing compared with mono-doped TiO₂(V-doped:2.91eV;N-doped:2.92eV).The enhanced Vis-photoeatalytie activities of V/N co-doped TiO₂ can be ascribed to more band-gap narrowing and higher quantum efficiency.3.The formation of miero-mesoporous structures and C-doping were first achieved at low temperature by the controlled-nitric-acid-oxidation method at one step.This novel method avoids the collapse of porous structures due to the the high-temperature C-doping,a general route for preparing C-doped TiO₂.Moreover,it is simple and easy to sealing up.C-doped micro-mesoporous bimodal TiO₂ prepared by this novel mothod has the pore walls consisting of nanocrystal anatase TiO₂ with the grain size of 3～5nm.The powders has high BET specific surface area up to 246.9 m²/g.C is incorporated into the TiO₂ lattice in the forms of substitutional C and carboneous species(interstitial C).High content(6.3 at%) of substitutional C causes the optical absorbption edge of 453nm and strong absorption within the whole visible region. C-doped micro-mesoporous bimodal TiO₂ shows high Vis-photocatalytic activities. The content of ethanol influences the specific surface area and the lattice-carbon concentration of C-doped micro-mesoporous bimodal TiO₂ powders.4.A novel method,named PBFT method,based on the bi-function of templates was put forward and successfully used for preparing the nonmetal-doped mesoporous TiO₂.For the methods previously reported for preparing C-doped mesoporous TiO₂, templates work with one function as the supports for the formation of porous structures. Differently,for PBFT method,templates function as both the supports for porous structures and the source for the dopants.The high-temperature calcination simultaneously results in the formation of pores,and C-doping and N-doping from the pyrolysis products of templates.Another N source for N-doping comes from the ammonium ions of(NH₄)₂TiF₆.The optical absorption edge of C/N/F co-doped mesoporous TiO₂ prepared by PBFT method is 481nm,along with strong absorption within the whole vigible region,due to high contents of lattice-carbon and lattice-nitrogen.This provides the powders with high Vis-photoeatalytic activities. About 1.0at%F is incorporated into the TiO₂ lattice due to the fast deposition of TiO₂. F-doping further enhances the Vis-photocatalytic activities of TiO₂.The doping mechanism for PBFT method was discussed.The effects of calcinatin on the porous structures,specicific surface areas,content of dopants,and optical response properties of C/N/F co-doped mesoporous TiO₂ were thoroughly investigated.5.The magnetism of doped nano-crystal TiO₂ was investigated.The nano-crystal TiO₂ powders are room-temperature ferromagnetic.The ferromagnetic essence is not changed after V-doping and N-doping.The correlation between the lattice parameters (c/a) and the saturated magnetization(Ms) was observed as:the bigger c/a,the bigger Ms.The lattice-distortion theory was proposed for understanding the origin of room-temperature ferromagnetism:the lattice distortion is one of the important factors resulting in the room-temperature ferromagnetism of diluted magnetic semiconductor oxide.