| TiO2 nanocrystals are well-known as important multifunctional nanomaterials, and have been widely used in many fields, such as photocatalysts, gas sensors, optical materials and dye-sensitized solar cells. Dye-sensitized solar cells that possess the properties of high conversion efficiency and low cost are considered as a promising alternative for conventional silicon solar cells.To explore a route with simple process and high production yield for synthesis of TiO2 with controllable phase composition and morphology, not only is of significance for scientific research, but also is the key step to promote the large-scale application of TiO2 nanocrystals.In the first part of this dissertation, we studied hydrothermal transformation from titanate one-dimensional nanotubes and nanorods produced by the alkaline hydrothermal method into TiO2 nanocrystals with different phase composition and morphology. The obtained products were characterized by XRD,TEM and HRTEM. For the hydrothermal transition of titanate nanotubes, the effects of the sodium content in titanate nanotubes, the pH value and the hydrothermal temperature on the phase composition and morphologies of TiO2 nanomaterials were investigated. Based on the HRTEM images, the transformation mechanisms from titanate nanotubes to TiO2 nanocrystals were presented. It was shown that, the pH value of the titanate nanotube suspension had a significant influence on the phase composition and morphology of TiO2 nanocrystals.The H-titanate nanotubes were predominately transformed into anatase nanoparticle with rhombic shape when the pH value was greater than or equal to 1.0, whereas primarily turned into rutile nanorod with pyramidal ends at the pH value less than or equal to 0.5.We propose a possible mechanism for hydrothermal transformation of H-titanate nanotubes into single-crystalline TiO2 nanomaterials. While the H-titanate nanotubes transform into tiny anatase nanocrystallites of ca.3 nm in size, the formed nanocrystallites as an intermediate grow into the TiO2 nanomaterials with controlled phase composition and morphology. This growth process involves the steps of protonation, oriented attachment, and Ostwald ripening. The sodium content of titanate nanotubes also has an effect on the phase composition and morphology of the obtained TiO2 nanomaterials. For the hydrothermal transformation of titanate nanorods, a mixture of nanoparticles and nanorods with the phase composition of rutile and anatase coexisting was obtained under the condition of pH 0; the anatase nanorods were produced under the condition of pH 2,4 and 7; when the hydrothermal temperaure was low than 180℃, the titanate nanorods have not completely been transformed to TiO2;when the hydrothermal temperaure was 180℃and 200℃, anatase nanorods were obtained.In the second part of this dissertation, oriented rutile TiO2 films were deposited by low-temperature solution method. The obtained TiO2 films were characterized by XRD,TEM and HRTEM. The results indicates that the TiO2 film is well crystallized and grows perpendicular to the substrate, and a preferential orientation in the [001]direction. The effects of process conditions including the growth time,the amount of tetrabutyl titanate, the concentration of titanium tetrachloride, and the amount of hydrochloric acid on the thickness and morphology of films were investigated. The microstructure of TiO2 films were characterization by SEM, TEM and HRTEM. The UV-vis and photoluminescence spetra of the oriented films were measured. The super-hydrophilic properties of the oriented films were characterized by measuring the contact angles. The results indicated that the morphology and thickness of rutile TiO2 film varied by changing the growth time, initial titanium precursor concentration of tetrabutyl titanate and titanium tetrachloride,and the amount of hydrochloric acid. The diffuse reflectance UV-Vis spectra of the rutile TiO2 film indicated that the maximum absorbance was at 374nm and the band gap Eg for the rutile TiO2 is about 3.06 eV. The line shapes in the PL spectra from all the samples are similar, but the intensities of the features in the PL spectra are different from each other. Rutile TiO2 films show super-hydrophobic feature after UV illumination only for 3 min.In the third part of this dissertation, the single-crystalline anatase nanoparticles prepared by the hydrothermal transformation of titanate nanotubes and the oriented rutile TiO2 films prepaed by the low-temperature solution method were used as the photoanodes of dye-sensitized solar cells (DSSCs).Porous TiO2 films with different thickness were fabricated by using the bipyramidal anatase nanocrystals, and their microstructures and specific surface area were characterized by SEM and N2 adsorption. The I-V curves of the DSSCs, including the open voltage, photocurrent density and conversion efficiency were evaluated as compared with the DSSCs based on the commercially available P25 TiO2 nanopowder. The results indicated that:(1)TiO2 nanocrystalline was pure anatase with good crystallinity, and had larger specific surface area, thus exhibited stronger optical absorption as compared with P25 powder. (2) The mesoporous film made from the nanocrystalline TiO2 absorbed more dye molecules as compared with the film made from P25.(3)The mesoporous film made from the nanocrystalline TiO2 exhibited higher light harvesting efficiency (LHE), thus the photocurrent densities and conversion efficiencies were obviously superior over those of the cells based on the P25 films.DSSCs based on oriented rutile thin films.The I-V properties of the DSSCs were evaluated and photocurrent density Jsc (7.9mA/cm2), open voltage Voc (731mV), fill factor FF (59%) and overall conversion efficiencies of the dye-sensitized solar cellsη(3.41%) were obtained.
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