Quantum Chemical Study of Photoinjection Processes in Dye-Sensitized TiO₂ Nanoparticles

Abstract

Results from quantum chemical INDO/S-CI calculations of the absorption properties of anatase TiO 2 nanoparticles sensitized by catechol and benzoic acid are reported. Experimental observations that catechol causes a strong shift in the TiO 2 absorption threshold, while benzoic acid does not, are explained from the calculations which indicate that only catechol introduces electronic levels in the TiO 2 band gap. The 420 nm excitation in the TiO 2 + catechol system responsible for the shift is seen to be a charge-transfer excitation from the catechol π HOMO orbital to Ti(3d) levels at the bottom of the conduction band. A direct photoinjection scheme which does not involve excited states of the adsorbate is proposed to account for the electron injection process.