Toward electronically coupled bio-inorganic conjugates

Spectroelectrochemical studies of nanocrystalline TiO ₂ films on conductive ITO substrates were performed in conjunction with X-ray absorption spectroscopy studies (XANES, XAFS). Electrochemical reduction of nanocrystalline films in the potential range -0.4 V to -1.9 V (vs. Ag/Ag ⁺) led to the reversible changes in UV-vis absorption of titania films along with the change of Ti K-edge associated with the change of the oxidation state Ti ⁴⁺/Ti ³⁺. Additionally, reduction of Ti ⁴⁺ to Ti ³⁺ was accompanied by the change of pre-edge structure of Ti K-edge indicative of the adjustment of the symmetry of Ti sites. The effect of chemical modification of surface Ti atoms on the optical and structural properties was used to differentiate surface and bulk contributions. At high charging potentials, a spectrum characteristic of shallow or free carriers changes and a new localized trapping site was observed that did not change with surface modification. The intensity of this signal increases with the number of injected electrons suggesting that the additional charges are localizing on the existing electron trapping sites causing deeper distortions. The absorption increase and a spectral shift were found to be a slow processes occurring on a second time scale, indicative of a processes accompanied by structural deformation.