Organisation and Reactivity of Nanoparticles at Molecular Interfaces. Part II. Dye Sensitisation of TiO_2 Nanoparticles Assembled at the Water | 1,2-Dichloroethane Interface

摘要

Dye sensitisation of wide bandgap semiconductors has been extensively studied in the fields of photography and photovoltaics. Strong electronic coupling between dyes featuring suitable anchoring groups and metal oxides can lead to electron injection from the excited state into the solid in the subpicosecond domain. Time-resolved transient absorption studies of TiO_2 and ZrO_2 nanoparticles sensitised by alizarin have demonstrated that ultrafast injection may occur not only into the conduction band but also into empty surface states located in the bandgap. These studies also showed that the lifetime of the charge separated state in the TiO_2-alizarin system exhibits a multiexponential relaxation with a fast component of the order of 430 fs. However, a substantial fraction of the charge separated state feature lifetimes beyond the nanosecond time scale. Previous studies by Moser and Gratzel have shown that the slower component is of the order of 500 μs. This slow back electron transfer has been rationalised in terms of the inverted Marcus region for electron transfer. On the other hand, Haque et al, have indicated that population/depopulation dynamics in traps levels can be the determining factor in the rate of back electron transfer in dye-sensitised mesoporous TiO_2 nanocrystaline photoelectrodes.