Synchrotron-Induced Photoelectron Spectroscopy of the Dye-Sensitized Nanocrystalline TiO₂/Electrolyte Interface:  Band Gap States and Their Interaction with Dye and Solvent Molecules

Abstract

The adsorption of Ru dye N3 [Ru II (2,2‘-bipyridyl-4,4‘-dicarboxylate) 2 (NCS) 2 ] from ethanol solution and gas-phase adsorption and coadsorption of the solvent acetonitrile on liquid nitrogen cooled nanocrystalline (nc) TiO 2 films have been investigated using highly surface sensitive synchrotron-induced photoelectron spectroscopy. Resonantly excited Ti 3+ -related band gap states form a peak 2.2 eV above the leading edge of the valence band, and additional states up to the Fermi level are found. The intensities of the gap states and related Ti 3+ 2p emissions of pristine samples depend strongly on sample preparation and history. Gap-state-related emissions are partially quenched by dye as well as acetonitrile adsorption. Quenching of gap states due to acetonitrile is reversible. The maximum of the highest occupied molecular orbital (HOMO) emission of the dye is found 1.6 eV above the leading edge of the nc-TiO 2 valence band. The binding energies of dye orbitals are increased upon acetonitrile coadsorption. The HOMO emission is shifted by 150 meV and that of N 1s of the dye NCS group by approximately 200 meV. These shifts are reversed upon acetonitrile desorption.