FIG. 2 Transmission electron micrograph of TiO<sub>2</sub> particles used in thin film production. The scale bar represents 10 nm. Particles were prepared by hydrolysis of titanium tetraisopropoxide<sup>18,19</sup> followed by autoclaving for 12 h at 200 °C. To form films the sol was concentrated by evaporation of water in vacuum at 25 °C until a viscous liquid was obtained. Carbowax M-20,000 (40% by weight of TiO<sub>2</sub>) was added and the viscous dispersion (TiO<sub>2</sub> content 20% by weight) was spread on the conducting glass support (Asahi glass, fluorine-doped SnO<sub>2</sub> overlayer, transmission 85% in the visible, sheet resistance 80/square) to give a membrane of 10 µm thickness. This was heated under air for 30 min at 450 °C. High-resolution scanning electron microscopy revealed TiO<sub>2</sub> films to be composed of a three-dimensional network of interconnected nanoscale particles<sup>19</sup>. Transmission electron micrographs of TiO<sub>2</sub> particles were taken before heat treatment; the annealing of 450 °C did not induce significant changes in particle size. Before dye coating, a few monolayers of TiO<sub>2</sub> were electrodeposited onto the colloidal TiO<sub>2</sub> film from a Ti(111) solution. Detailed description of this procedure will be published elsewhere (L. Kaven, B.O'R., A. Kay and M.G., manuscript in preparation).