Improving the optoelectronic property and photoactivity of nano-structured titanuim dioxide: Effect of particle size, oxygen vacancy, and nitrogen doping

Titanium dioxide (TiO2) has been proven to be one of the most important and widely used photocatalyst for applications such as gas/liquid phase environmental cleaning, solar hydrogen generation, sensitized solar cell, UV filtration, etc. The biggest challenge in the applications of this semiconductor photocatalyst is its large band gap (∼ 3.2 eV) which limits the utilizable spectrum of photons from the solar light (∼ 4 to 5 %). To improve the optical sensitivity of TiO2 in the visible light region, the band gap of TiO2 needs to be tailored. The reduction of TiO2 band gap can be achieved by controlling of its electronic structure via two routes: changing the particle size and doping it with impurities.;To precisely control the size of TiO2 particles, anatase TiO 2 nanocrystallines (17 to 29 nm) were synthesized by metallo-organic chemical vapor deposition (MOCVD) method with moderate control on system parameters (i.e. pressure and gas flow rates). The results of band gap change as a function of particle size agreed well with what was predicted using the Brus' effective mass model (EMM). However, the observations from photocatalytic oxidation of 2-chlorophenol (2-CP) showed that the smaller the particle size, the faster the degradation rate. This is attributed in part to the combined effect of band gap change relative to the spectrum of the light source and the specific surface area (or particle size) of the photocatalysts. Our results indicate that the gain in specific surface area due to the smaller particle size outweighs the improvement on its optical property (e.g. reduction in bandgap) under similar experimental condition. Our observation also showed the secondary particle size to be time dependent due to the aggregation and is highly correlated with its primary particle size.;The nitrogen doped TiO2 thin film synthesis was carried out with two different approaches: (1) oxidation of the titanium nitride (TiN) thin film and (2) reactive pulsed laser deposition (PLD). Nitrogen doping of TiO2 in the former approach was done by the oxidation of TiN thin films at 800 °C in ambient air. The phase transformation of TiN to TiO2 appears to be a function of annealing time. The X-ray photoelectron spectroscopy (XPS) studies have shown that the substitutionally doped TiO 2-xNx (N 1s ∼ 396 eV) remains stable only for the first two minutes of annealing. As the annealing proceeds, the N 1s XPS peak shifts to higher binding energy (N1s ∼ 400 and 402 eV) which indicates that binding energy of N atom shifted from substitutional site (Ti-N) to chemisorbed site (N≡N) when annealing process exceeds five minutes due to the substitution of Ti-N bond by Ti-O bond. Results from both depth profile XPS and cross-section TEM showed that the formation of TiO2-xNx depends on the distance from the film surface. Film structure studied by X-ray diffraction (XRD) showed that mixed TiN and rutile TiO2 phases formed between four and twenty four hours of annealing time. The optical properties were examined using UV-VIS spectroscopy. The most visible light sensitive samples showed ∼65 to 105 nm red-shift on its absorption edges from that of pure TiO2, which is equivalent to ∼0.47 to 0.73 eV reduction in the effective bandgap. Our results suggest that surface oxidation of TiN is an effective method for the synthesis of band gap tailored oxides which have applications in photocatalysis, photovoltaic, etc.;Doping of N into TiO2 lattice was also achieved using the reactive pulsed laser deposition technique. The N concentration was controlled by adjusting the mixing ratio of make-up (N2) and buffer (1:1 O 2 and Ar mixture) gases during laser ablation. (Abstract shortened by UMI.)...