Synthesis And Photoelectrochemical Properties Of Low Dimensional Nanostructure TiO₂ Prepared By Hydrothermal Method With Pure Titanium

Nano titanium dioxide (nano-TiO₂) is an important inorganic functional material, which has been widely used as photocatalyst, dye-sensitized solar cells (DSSC), gas sensing, water photolysis, and so on. Recently, the field of nano-TiO₂ has attracted a great deal of interest for enhancing nano-TiO₂ properties and extending their utilities, by constructing the special architecture of the material. Furthermore, the support of the nano-TiO₂ on the electrode for easy recycle is always important in practical applications. Based on this consideration, we synthesized the nano-TiO₂ materials with special structures on the Ti foil via hydrothermal method. These nano-TiO₂ materials included F-TiO₂ sphere, 3D network TiO₂ nanowire thin film, and TiO₂ nanowire/nanorod array. The application properties of the electrodes composed of as-prepared nano-TiO₂ were investigated. Furthermore, using Ti powder as reagent, we prepared TiO₂ nanorod/nanobelt sphere powders, and ultra-long TiO₂ nanowire powder. Specimens were characterized by FESEM, XRD, TEM, UV-vis spectrum, and Raman spectrum respectively. The influences of solution concentration, hydrothermal temperature, and hydrothermal time on the structure and morphology of the products were systematically studied. Then, the primary growth mechanisms of the products were proposed. The specific works is as follows:Firstly, hierarchical structured F-containing TiO₂ (F-TiO₂) sphere was synthesized on Ti foil via hydrothermal method in inorganic acid solution. Studies showed that the F-TiO₂ sphere could be successfully prepared on the surface of pure Ti foil. The high viscosity solvent decreased the growth rate of the products. The solution concentration and the acid radical impacted the structures and morphologies of the products. In 5 mM aqueous solution of NH4F and HCl with molar ratio of 1:1, the hierarchical structured F-containing TiO₂ (F-TiO₂) sphere composed of porous octahedron crystals with one truncated cone could be prepared, which was constituted with anatase crystals and rutile crystals. XPS results indicated that F- anions were just physically adsorbed on the surface of TiO₂ microspheres. The optical properties of the F-TiO₂ were studied by UV-visible light absorption spectrum. The surface fluorination of the spheres,the unique nanostructure induced accessible macropores or mescopores,and the increased light-harvesting abilities were crucial for the high photoelectrochemical activity of the synthesized F-TiO₂ sphere for water-splitting. The results of the photoelectrochemical properties indicated that photocurrent density of the F-TiO₂ sphere thin film was more than two times than that of the P25 thin film.Secondly, 3D network TiO₂ nanowire thin film (W-film) was prepared on the surface of Ti foil in NaOH solution. W-film was composed of many randomly-oriented anatase nanowires, which hadiameters of 10³0 nm and lengths of more than 5μm, respectively. The results of optical properties indicated that the absorbency of the W-film was higher than that of a particulate film (P-film) in the 350-700 nm region and the absorption edge was red-shifted. Meanwhile, the absorbency of the W-film increased as the hydrothermal time increased. The photocurrent density of W-film was 2.05 times larger than that of the P-film. The catalytic efficiency of the W-film to methyl orange was found to be 2.3 times higher than that of the P-film. The dynamic equilibrium among the dissolution rate of the Ti, diffusion rate of titanate in the solution, and the crystal growth rate of titanate dominated structures and morphologies of the products during the hydrothermal processes. Based on the mechanism, we successfully synthesized TiO₂ nanowire array vertically grown on Ti foil in 1 M NaOH solution, at 200℃for 48 h.Thirdly, vertically oriented single crystalline anatase TiO₂ nanorod array was directly prepared on titanium foil through hydrothermal method in tetramethylammonium hydroxide (TMAOH) solution. The single crystalline nanorod had a shape of tetragonal crystallographic planes and tetrahydral bipyramidal tip. It was found that the morphology, size, and orientation of the nanorods were affected by the solution concentration and hydrothermal temperature. Moreover, the special morphology of the TiO₂ nanorod array was caused by the selectively absorption of the tetramethyl ammonium (TMA) through hydrogen bonds on the lattice planes parallel to (001) of anatase TiO₂. Furthermore, less grain boundaries and direct electrical pathway for electron transferring were crucial for the superior photoelectrochemical properties of the single anatase TiO₂ nanorod array.Finally, using Ti powder as reagent, TiO₂ nanorod/nanobelt sphere and ultra-long TiO₂ nanowire powders were prepared via hydrothermal method in NaOH solution. The studies indicated that the growth orientations of the crystals were influenced by hydrothermal temperature and solution concentration. Moreover, the size of the products and the surplus of Ti were affected by hydrothermal time. Furthermore, it was found that the reaction rate of Ti and NaOH, diffusion rate of titanate in the solution, and crystallization rate of titanate were in the charge of the growth orientation of the product. When the dynamic equilibrium built among these three rates in some degree, the crystals would grow along the radial direction of the Ti particle. Then, the TiO₂ nanorod or nanobelt spheres were formed after 48 h. However, if this dynamic equilibrium was broken, the ultra-long TiO₂ nanowires or the smaller size TiO₂ nanobelt spheres were formed. TiO₂ nanorod/nanobelt sphere powders and ultra-long TiO₂ nanowire powder were anatase after heat treatment at 450℃for 1 h. The results of the photocatalytic degradation confirmed that the photocatalytic activity of the products is as follows: TiO₂ nanorod sphere > ultra-long TiO₂ nanowire > TiO₂ nanobelt sphere > P25. These nano-structure TiO₂ powders were convenient for recycling.