The effect of chemisorbed dyes on I-V characteristics of mesoporous TiO2 observed in scanning tunnelling spectroscopy

We report UHV STM/scanning tunnelling spectroscopy (STS) studies on mesoporous TiO2 films in the presence and absence of adsorbed dye sensitisers (chlorophyll and a Ru bipyridyl dye). Interconnected and aggregated TiO2 nanocrystallites with mean diameters typically of 30-80 nm and different pore sizes are observed in STM images. The results show that, under controlled tunnelling conditions, the surface apparent band gap of as-prepared thin films is consistent with the bulk band gap of 3.2 eV, but that a quite different, lower surface apparent band gap is observed when a sensitising dye is adsorbed. Moreover, the characteristic measured in this case is dependent on the choice of dye, showing that the STM I/V characteristic is sensitive to the adsorbed dye, rather than merely to the effect of surface adsorbed water. We discuss possible mechanisms for this effect, and the use of this effect in the optimisation of dye-sensitised photovoltaic cells. (C) 2002 Elsevier Science B.V. All rights reserved.