Preparation And Doping Of High Crystallinity One Dimensional TiO₂ Nanostructures And The Photoelectric Properties Research

Titanium dioxide (TiO₂) has attracted much attention because of its unique physical, optical, electrical and chemical properties and its many important potential applications, such as photo-electric device, photocatalyst and gas sensors. So it is important and valuable to explore a low cost and simple preparation of one dimensional (1D) TiO₂ nanomaterials and enhance their photoelectricity properties. Presently, there are many reports on fabrication of 1D TiO₂ nanomaterial by wet methods. However, it is difficult to synthesize the 1D TiO₂ nanomaterials by dry method due to the high melting point (1668℃) and low vapor pressure (10^-3 Torr at 1577℃) of Ti. In this dissertation, a simple and low cost method for preparation of the 1D TiO₂ nanostructures with high crystallinity was explored based on a combination of the wet and dry methods. This route shows an advantage that the high temperature condition and high vaccum system are not needed for this newly developed technique. The morphologies, structures, and growth mechanism as well as the photoelectric properties and surface modification of the 1D TiO₂ nanostructures have been studied systematically and the results are summarized below.Firtsly, TiO₂ nanoballs, nanotubes and nanoflowers all with rutile structure have been fabricated by sol-gel, anodization and hydrothermal routes, respectively. The surface photovoltage measurement demonstrated that the TiO₂ nanoballs and nanotubes show higher surface photovoltage response under positive bias, however, the TiO₂ nanoflower have no surface photovoltage response even under external bias.Secondly, investigation on self-organized growth of TiO₂ nanotube arrays by anodic oxidization on the Ti foil and FTO coated conductive glass. It revealed that the thickness of Ti and different electrolytes are the critical experimental parameters for the formation of TiO₂ nanotubes arrays with varied length. TiO₂ nanotube arrays fabricated on the Ti foil were several micrometers in length, but～300 nm for that on the FTO-coated glass. The as obtained amorphous TiO₂ layer became anatase when the annealing temperature below 700℃. However, rutile instead of anatase will be resulted if the annealing temperature over 700℃.Thirdly, the high crystallinity of 1D TiO₂ nanowires/nanobelts have been fabricated on Ti foils using the anodized Ti foils followed by thermal heating at temperature ranging from 480 to 780 oC with the Pd catalyst. The Pd catalyst plays a very important role in the fabrication of metal oxide nanostructures at low temperature. The vapor–liquid–solid (VLS) and vapor–solid (VS) may be the most suitable mechanisms for 1D TiO₂ nanostructure growth.Fourthly, TiO₂ nanowires synthsized by thermal annealing of the anodized Ti foil with Pd catalyst were sensitized with CdS quantum dots (QDs) via chemical bath deposition. Microstructural characterizations show that the CdS nanocrystals with the cubic structure have intimate contact with the TiO₂ nanowires. The amount of CdS QDs can be controlled by adjust of the CBD cycles. The surface photovoltage measurements confirm that TiO₂ NWs can be successfully sensitized by CdS QDs to enhance the surface photovoltage response intensity and broaden the surface photovoltage response region. It proved that the CdS QDs sensitization is an available path for improvement of solar energy conversion efficiency of the TiO₂ NWs.Finally, Ag nanoparticles were deposited onto the TiO₂ nanowires by photocatalysis reducing method. The Raman spectra of the TiO₂/Ag composites exhibited red-shit or/and blue-shift of Raman active mode depending on Ag amount. Moreover, the appropriate amount of Ag can effectively enhance the surface photovoltage.