Preparation, Characterization And Properties Of Doped Titanium Dioxide Thin Films

Titanium dioxide (TiO2) is one of important functional inorganic materials. Since Fujishima discovered the photocatalytic splitting of water on a TiO2 electrode under ultraviolet light in 1972, enormous efforts have been devoted to the research of TiO2 materials due to its excellent optical, electrical, photocatalytic and thermal properties, which led to promising applications in the fields of photovoltaics, photocatalysis, sensors, medicine et al. These applications can be roughly divided into "energy" and "environmental" categories. For example, the control of environmental contamination and the investigation of new energy sources depend not only on the properties of the TiO2 material itself but also on the modifications of the TiO2 and on the interactions of TiO2 materials with the environment. The development of photocatalysts under visible light irradiation is one of the major goals for enhancing the efficient utilization of solar energy and realizing the practical industrialization, which is also one of the challenging tasks in the field of photocatalysis.In this thesis, series of TiO2 thin films have been synthesized deposited on microscope slides or indium tin oxides (ITO) glass substrate using sol-gel dip-coating method, including pure anatase TiO2 thin films and (101) crystalline plane oriented TiO2 thin films; nonmetallic ions (N, S and F) doped TiO2 thin films; metallic ions (Li+, Ni2+, Bi3+, Ce4+, V5+ and W6+) doped TiO2 thin films; Ce-Ni, Bi-N and Ce-S co-doped TiO2 thin films; Y2O3/TiO2 nanocomposite thin films and CuO/TiO2 up-graded nanocomposite thin films with gradient bandgap. The as-synthesized samples are characterized using thermo-gravimetric /differential scanning calorimetric (TG/DSC), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-vis), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), cyclic voltametry (CV) and contact angle analysis technologies. The synthesis process of TiO2 thin films and the physicochemical changes of sol have been briefly investigated. The effects of different treatments on structure, optical, electrochemical and photo-induced hydrophilic properties of TiO2 based thin films have been systematically emphasized. The as-synthesized TiO2 based materials have been simulated using the first-principle calculation based on the Density Functional Theory (DFT). The aim is to investigate the intrinsic factor of enhancing visible light activation of TiO2 based thin films from different modified methods. The main works and results are as following.(1) Effects of processes optimization on the structure and properties of TiO2 thin filmsBased on the analysis on the physicochemical changes of sol-gel preparation processes, the effects of 100℃,500℃, freeze drying and microwave heating pretreatment on crystalline structure, surface morphology and optical properties of TiO2 thin films have been primarily investigated. The pretreatment process is important during preparation of thin films and shows obvious effect on the morphology, crystalline size and roughness of the surface of TiO2 thin films due to the different heating mechanisms. The absorption edge of TiO2 thin films after freeze drying and microwave heating pretreatment, whose crystalline size is 30.1 nm and 61.45 nm respectively, has a red shift compared with that of 100℃pretreated sample (15.45 nm) due to quantum size effect. On the other hand, the researches on the different crystalline plane orientation of TiO2 thin films have drawn greater attention gradually. The oriented TiO2 thin films synthesized by modified preparation process have obviously absorbed property in range of visible spectra and the band gap is 2.9 eV for indirect transmission. The value of capacitance for (101) oriented TiO2 thin film electrode decreases with increasing the potential, the flat-band potential is-0.493 V and the donor concentration (ND) is 6.28×1021 cm-3. These results can provide technical support for the development of visible light activity of TiO2 thin films and novel ideas for the realization of the controlled synthesis of TiO2 thin films.(2) Effects of different ions doping on the structure and properties of TiO2 thin filmsThe effects of doping of different ions, including Li+, Ni2+, Bi3+, Ce4+, V5+, W6+ metallic ions and N, S, F non-metallic ions, on the structure, optical and hydrophilic properties of TiO2 thin films have been primarily investigated. The results indicate that the morphology and crystalline size is different on the surface of thin films; the absorption edge produces a red shift to some degree, except for Li+. The band gap of Ni2+ and Ce4+ doped TiO2 thin films are 2.42 eV and 2.51 eV, respectively, and that of doped with V5+ and W6+ are not obvious, while the band gap increases by doping Li+. The absorption edge of S and N doped TiO2 also has red shifts because the band gap of TiO2 thin film decreased obviously and the visible light activity is remarkably improved, while that of F doped sample produces blue shift. The N-Bi,Ni-Ce and S-Ce co-doped TiO2 thin films have been successfully synthesized for the first time. The effective synergetic effects resulted from co-doping have been discussed through analyzing the effects on the structure, optical electrochemical and photo-induced hydrophilic properties of TiO2 thin films doped with the two different dopants. The results shows that the properties of doped TiO2 thin films with different ions have close relations with the intrinsical properties of the doped ions, the effective methods of modification by doping are the conduction band (CB) and valence band (VB) of TiO2 influenced by doping ions and the band gap obviously narrows to enhance the response in visible light region.(3) Effect of composite metal oxides on the structure and properties of TiO2 thin filmsIt is an important technological method to prepare visible-light irradiated TiO2 thin films through compositing with semiconductors. Many researches have focused on sulfide compositing with TiO2. In this thesis, the optical, electrochemical and photo-induced hydrophilic properties of Y2O3/TiO2 (YTF) and CuO/TiO2 nanocomposite thin films in visible light region have been obviously improved compared with that of the pure TiO2 thin film. The binding energy appears chemical shift and relatively more Y and Ti species are present on the surface, indicating that active surfaces of the nanocomposite films have been enhanced with more oxygen vacancies Vo due to doping of Y2O3 to TiO2. The absorption edge has a red shift with the band gap of 2.68 eV. The water contact angle is about 8°after daylight lamp irradiation for 60 min. Based on the Mott-Schottky equation, the donor concentration (ND) for YTF is 1.05×1020cm-3, which enhances one order of magnitude than that for pure TiO2 film (TF), but the flat-band potential (Vfb) and the space charge layer(dsc) obviously decreased. The CuO/TiO2 nanocomposite thin film has successfully been synthesized on the basis of the relation between the chemical composition and band gap in mixed semiconductor systems. From the surface of the up-graded thin films to substrate glass in the direction of depth, the percentage of Cu decreases, that of O increases, that of Ti is slowly enhancing and then quickly decreasing. It can be concluded that the band gap of the as-synthesized film decreases alone the direction of depth due to the graded chemical composition. The anode photocurrent of up-graded nanocomposite thin film electrode obviously increases and the photo-response region is greater than that of TiO2 thin film electrode. These results should provide basic theories for the TiO2 thin film as solar cells and as a novel self-cleaning material in the field of in construction industry, which also have important instructional functions for the development and application of novel structured TiO2 thin films.(4) Computational simulation of visible light activities for nano-TiO2 using the first-principle calculation based on the Density Functional Theory.The structure of energy level, density of states and optical properties of three crystalline models TiO2, and F,S,N,Li+,Ni2+,Bi3+,Ce4+,V5+,W6+ doping TiO2, and N-Bi co-doping TiO2 have theoretically been calculated using the first-principle methods based on plane-wave pseudo-potential theory in detail. The results indicate that:(1) the CB and VB of TiO2 consist of both the Ti 3d and O 2p orbital, the CB distributes is mainly attributed to Ti 3d orbital with a smaller contribution from O 2p orbital, and the VB consists of the O 2p orbital (the main contribution) hybridized with the Ti 3d orbital, the physicochemical properties of rutile, anatase and brookite TiO2 are different due to the difference in the structure of CB and VB; (2) the O 2p and Ti 3d orbital, mixed with F 2p, S 3p, N 2p, Ni 3d, Bi 6p, Ce 5d,Ce5p, V 3d,V 3p and W 5d orbital of doping F, S, N, Ni, Bi, Ce, V and W ions, respectively, causes the changes of constitute of CB and VB and narrows the band gap; (3) the optical properties obtained from calculation are in good agreement with that of experimental results. These computational results discover the intrinsic nature of improving visible light activities of TiO2 through doping ions, which can provide basic theory for developing novel catalyst with visible light irradiation and significative directions to build the relation of composition, structure and property of TiO2 materials.