| Photo-electrochemical processes are of special importance in sustainable energy and environmental arenas. However, the efficiency of these processes in sustainable energy production or pollutant abatement has been limited by fundamental constraints such as the chemical stability and the optical and electrical properties of the light absorbing materials. Oxides such as TiO 2 and ZnO that are chemically stable in corrosive electrochemical environments are ineffective because their band gaps are too large for effective harvesting of visible light. There have been numerous attempts to bridge the band gap mismatch by doping TiO2 with a single metallic or non-metallic element such as Fe, V, W, N, and F, or by fabricating composite materials via mixing TiO2 with narrow-band gapped semiconductors. However, to date no major breakthrough has been achieved, limiting the performance of solar energy conversion by photocatalysis well below the 10% target required for commercial applications. The overall objective of this dissertation is to develop a model guided gas-phase synthesis route for high throughput production of visible-light-active TiOx.; Nucleation, the first step in gas-phase synthesis, plays a critical role in determining the particle size, composition and crytallinity and hence its photoactivity. Therefore, development of an accurate atomistic model of the nucleation process is highly desirable. To this end, we have developed a molecular dynamics simulation technique based on thermodynamic path integration that allows accurate determination of the nucleation barrier height over a broad range of supersaturations.; A high throughput flame synthesis technique has been used to produce visible-light-active titanium dioxide photocatalyst. Specifically, various size and morphology substoichiometric TiOx (x<2, determined by Electron Energy Loss Spectroscopy) nanoparticles were synthesized in a diffusion flame reactor. In turn, the optimum processing conditions for synthesis of TiOx particles with maximum visible light photoactivity was determined. An alternative approach for making visible-light-active TiOx, namely, oxidation of TiO and Ti2O3 was explored. Although this technique allowed precise determination of crystalline phase transformation during oxidation of TiO and Ti2O3 to TiOx, the particles synthesized by this approach did not show pronounced visible light photoactivity. |
