Slow back electron transfer in surface-modified TiO2 nanoparticles sensitized by alizarin

Interfacial electron transfer dynamics of alizarin-sensitized surface-modified and unmodified TiO2 nanoparticles has been studied using femtosecond transient absorption spectroscopy. The surface of TiO2 nanoparticles has been modified by sodium dodecyl benzyl sulfonate (DBS). Electron injection has been confirmed by direct detection of the electron in the conduction band, cation radical of the adsorbed dye and by a bleach of the dye as monitored by transient absorption spectroscopy in the visible and near-IR region. The dynamics of back electron transfer (BET) of the injected electron from TiO2 to the parent cation has been measured by monitoring the decay kinetics of the cation radical of the adsorbed dye, and it is found to be multiexponential. We have compared the BET dynamics of alizarin-sensitized unmodified and surface-modified nanoparticles, and it is found to be slow on the modified surface. Solvent polarity did not have much impact on the BET dynamics. As the molecule forms a strong charge-transfer (CT) complex with the TiO2 nanoparticle, significant change in the electronic coupling is not expected. As the other parameters that govern the ET are not changed as a result of surface modification, the reduction in the BET on the surface-modified nanoparticles has been explained by the fact that on surface modification the Fermi level of the modified nanoparticles is pushed up in energy, increasing the overall free energy of reaction (-DeltaGdegrees) for the BET reaction. The high exoergic BET reaction in dye-sensitized TiO2 nanoparticles surfaces falls in the Marcus inverted regime. As a result, with increasing free energy of reaction the BET rate is decreased on the modified surface.