Optical excitation of Ru II (2,2′-bipyridyl-4,4′dicarboxylate) 2 (NCS) 2 -sensitized nanocrystalline TiO 2 films results in injection of an electron into the semiconductor. This paper addresses the kinetics of charge recombination which follows this charge separation reaction. These charge recombination kinetics were found to be strongly dependent upon excitation intensity, electrolyte composition, and the application of an electrical bias to the TiO 2 film. For excitation intensities resulting in less than one excited dye molecule/TiO 2 particle, the recombination kinetics were independent of excitation intensity. Increasing the excitation intensity above this level resulted in a rapid acceleration in the charge recombination kinetics. Similarly, for positive electrical potentials applied to the TiO 2 electrode, the recombination kinetics were independent of applied potential. If the applied potential was more negative than a threshold potential V kin , a rapid acceleration of the charge recombination kinetics was again observed, for example from ∼1 ms at +0.1 V vs Ag/AgCl to ∼3 ps at -0.8 V (∼10 8 fold increase in the rate). Moreover, at a constant applied potential the charge recombination kinetics were found to be strongly dependent upon electrolyte composition (up to 10 6 -fold change in rate). This strong dependence upon the electrolyte composition was found to be associated with shifts in the threshold potential V kin . Spectroelectrochemical measurements were used to monitor the shift in the trap/conduction band density of states induced by the electrolyte composition. A direct correlation was observed between the threshold voltage V kin observed from kinetic measurements, and the threshold voltage for electron occupation of conduction band/trap states of the TiO 2 observed from spectroelectrochemical measurements. This direct correlation was observed for a wide range of electrolyte compositions including protic and aprotic solvents and the addition of Li + ions and 4-tert-butylpyridine. We conclude that the charge recombination kinetics in such dye-sensitized films are strongly dependent upon the electron occupation in trap/conduction band states of the TiO 2 film. This occupation may be modulated by variations in light intensity, applied electrical potential, and electrolyte composition. These results are discussed with relevance to the function of dye-sensitized photoelectrochemical devices.

Potential step chronoamperometry is employed to compare the capacitances of nanocrystalline ZnO and TiO 2 electrodes. These capacitance data are complemented by transient optical absorption studies of charge recombination following adsorption of molecular sensitizer dyes to these metal oxide electrodes. Both measurements are conducted as a function of electrochemical bias applied to the metal oxide film in a threeelectrode photoelectrochemical cell. For both metal oxides, a power law dependence was observed between the half times for charge recombination (t 50% ) and the metal oxide electron density n determined from integration of the capacitance data, t 50% ∝ n -1/R , where R ) 0.27 and 0.30 ( 0.05 for ZnO and TiO 2 , respectively. A numerical model for the recombination dynamics based upon a random walk of electrons between localized sub-bandgap states is found to be in good agreement with experimental observations for both metal oxides. At negative applied potentials, the film capacitance, and therefore electron density, is observed to increase more rapidly with increasingly negative applied potential for the ZnO film compared to the TiO 2 film. This observation is quantitatively correlated with a more rapid acceleration of the recombination dynamics observed for dye sensitized ZnO films under negative biases. It is suggested that the faster recombination dynamics observed under negative bias may be the origin of the lower open circuit voltages reported previously for dye sensitized photoelectrochemical cells employing ZnO electrodes relative to comparable devices employing TiO 2 .

The preparation and characterization of thick (9 microns), clear, mechanically robust and photocatalytically active films of nanocrystalline anatase titania are described. XRD and SEM analysis show the films comprise 13 nm particles of anatase TiO2. Thin (54 nm) films of the 'paste' TiO2, along with sol-gel titania films made by a more traditional route are also prepared and characterised. All titania films mediate the photocatalytic destruction of stearic acid with a quantum yield of 0.0016 +/- 0.0003, using either 365 nm (i.e. BLB) or 254 nm (germicidal) light. P25 TiO2 films also appear to mediate the same process with a similar formal quantum efficiency. Of all the films tested, the thick paste TiO2 films are the most ideally suited for use with near UV light, for reasons which are discussed. All the titania films tested exhibit photoinduced superhydrophilicity.

We have investigated the use of nanoporous Ðlms as substrates for protein TiO 2 immobilisation. Such Ðlms are of interest due to their high surface area, optical transparency, electrochemical activity and ease of fabrication. These Ðlms moreover allow detailed spectroscopic study of protein/electrode electron transfer processes. We Ðnd that protein immobilisation on such Ðlms may be readily achieved from aqueous solutions at 4 ¡C with a high binding stability and no detectable protein denaturation. The nanoporous structure of the Ðlm greatly enhances the active surface area available for protein binding (by a factor of up to 850 for an 8 lm thick Ðlm). We demonstrate that the redox state of proteins such as immobilised cytochrome-c (Cyt-c) and haemoglobin (Hb) may be modulated by the application of an electrical bias potential to the Ðlm, without the TiO 2 addition of electron transfer mediators. The binding of Cyt-c on the Ðlms is TiO 2 investigated as a function of Ðlm thickness, protein concentration, protein surface charge and ionic strength. We demonstrate the potential use of immobilised Hb on such TiO 2 Ðlms for the detection of dissolved CO in aqueous solutions. We further show that protein/electrode electron transfer may be initiated by UV bandgap excitation of the TiO 2 electrode. Both photooxidation and photoreduction of the immobilised proteins can be achieved. By employing pulsed UV laser excitation, the interfacial electron transfer kinetics can be monitored by transient optical spectroscopy, providing a novel probe of protein/electrode electron transfer kinetics. We conclude that nanoporous Ðlms may TiO 2 be useful both for basic studies of protein/electrode interactions and for the development of novel bioanalytical devices such as biosensors.