Titania nanotubes for solar energy and catalysis

Since their discovery in 2001, anodic titania nanotubes formed from titanium foils have shown potential for use in photocatalytic and catalytic applications. This dissertation investigates the fabrication process in detail. A new kind of titania nanotube that makes use of a chlorine based electrochemistry, as opposed to existing fluorine based chemistry, is introduced. The chlorine based process is very rapid compared to the existing process and yield nanotubes with smaller diameters (∼ 15nm) and high aspect ratios (up to 1000 lengths to diameters). Several experiments that probe and reveal mechanistic aspects of nanotube formation on titanium foils are presented. In particular, field-enhanced dissolution does not occur by dissolving oxide (by a water forming reaction) but by Tix+ cation ejection. Attention is also given to the application of titania nanotube technology to two solar energy applications: hydrogen production by the photoelectrochemical (PEC) splitting of water and to photovoltaic electricity generation using dye sensitized solar cells (DSSC's). The production of H2 by splitting water using solar energy would be an ideal future energy source since it is both a renewable and an environmentally friendly process. The natural bandgap of titania (3.2 eV) however is too wide to efficiently harvest solar energy (less than 5% of solar energy is accessible at this band gap). Hence carbon doping was investigated as a means to narrow the titania bandgap while retaining its water-splitting capability. Finally, in DSSC's a ruthenium dye was adsorbed onto the surface of titania nanotubes. The absorption spectrum of the dye has excellent overlap with the solar spectrum and can efficiently harvest photons. Titania nanotubes with their high surface area and order may be a superior dye support and charge collector in these cells compared to widely used sintered nanoparticulate films. Fabrication protocols are developed and evaluated that are specifically designed for optimal performance in DSSC's.