Triarylamine on nanocrystalline TiO2 studied in its reduced and oxidized state by photoelectron spectroscopy

Abstract

The electronic and molecular structures of an electrochemically reduced and oxidized triarylamine layer (((3-ethyl(p-N,N-dimethylamino)phenyl)amino)propyl-1-phosphonic acid) adsorbed on a nanocrystalline TiO2 film have been investigated. Photoelectron spectroscopy (PES) measurements on electrochemically prepared surfaces and density functional theory (DFT) calculations for the free triarylamine molecule were performed. The major electronic difference between the reduced and oxidized state of the triarylamine molecules appears on the nitrogen atom. In the PES measurement a shift of the N 1s signal of 1.5 eV toward higher binding energies was measured between triarylamine in its reduced and oxidized states. The molecular structure of the surface layer was found to depend on the electrolyte used in the electrochemical preparation. The oxidized triarylamine molecule is positively charged and must therefore be accompanied by a negative counterion. The identity of the counterion was found to influence electron transfer between adjacent molecules, as explained by geometrical differences of the surface layer. During the measurements the PES N 1s signal was affected by the continuous X-ray illumination. Interestingly, a reversible photoreduction of the oxidized triarylamine was observed. The effect is accounted for by X-ray induced electron hole-pair generation in the TiO2 and subsequent electron transfer from the TiO2 conduction band to the singly occupied molecular orbital of the adsorbed molecule.