Band gap tuning and structural engineering of TIO₂ for photocatalysis and solar cells.

To tackle with the energetic and environmental problems for sustainable development, titanium dioxide ( TIO₂) has attracted extensive scientific interests for its outstanding performances in photocatalytic decomposition of organic molecules and its application in solar cells. This work aims at a fundamental break through towards significant reduction of TIO₂ band gap via doping, since the functionality for pure TIO₂can be excited only by ultraviolet (UV) light that only accounts for less than 5% of solar irradiance because of the wide band gaps (3.2 eV for anatase and 3.0 eV for rutile), as well as structural engineering of TIO₂ thin films, in order to pave a technological basis for truly sustainable technologies for visible light photocatalytic applications for cleaner environment and novel photovoltaic (PV) cells. By using the technique of pulse laser deposition, pure and ions doped TIO₂ thin films were successfully synthesized. The microstructure and optical absorption properties of doped thin films were systematically characterized and compared with pure one. Through involving Fe, Mn, V-Ga, the band gap of TIO₂ thin films were extensively narrowed, amazingly making the absorption edges shift well into visible region or even infrared region. Contrarily, rare earth dopants Gd, Er, Sm could hardly address the band gap of TIO₂thin film. Fe doped TIO₂ nanocatalysts were prepared using a hydrothermal method at low temperature. Fe doping induces significant red shift in the optical absorption edge of TIO₂ by introducing secondary absorption edges, extending the optical absorption edge well into the visible light region. Accordingly the visible light photocatalytic activity of TIO₂ increased on account of visible light absorption by Fe doped TIO₂, resulting in improved degradation rate of methylene blue (MB) under visible light irradiance. Mn doped TIO₂ nanocatalysts were successfully prepared by means of a sol–gel method using ethanol–water as a solvent. A remarkable red shift of the absorption edge was achievable through Mn doping, giving rise to gigantically narrowed energy gap to permit absorption well into the infrared spectral region. The Mn doped anatase powders exhibited considerable high visible light photocatalytic activity, leading to greatly enhanced degradation rate of MB under visible light irradiance. Single rutile TIO₂ nanorod and nanofibre thin films with preferred [002] orientation were successfully synthesized on fluoride doped tin dioxide (FTO) coated substrate. Through adjusting the reaction duration, alcohol substitution of water as solvent, reaction temperature and tetrabutyl titanate (Ti(OBu)4) concentration, TIO₂ thin films of different packing density and thickness were well controlled for various potential applications. By employing the nanorod TIO₂thin film in hybrid solar cell, considerable efficiencies were obtained and expected to improve furthermore