Adsorbate-Induced Modification of Surface Electronic Structure: Pyrocatechol Adsorption on the Anatase TiO₂ (101) and Rutile TiO₂ (110) Surfaces

Photoemission and near-edge X-ray absorption fine structure (NEXAFS) techniques have been used to study the adsorption of pyrocatechol on anatase TiO 2 (101) and rutile TiO 2 (110) single crystals. Photoemission results suggest the pyrocatechol molecule adsorbs on both surfaces predominantly in a bidentate geometry. Using the searchlight effect, the carbon K-edge NEXAFS spectra recorded for pyrocatechol on anatase TiO 2 (101) and rutile TiO 2 (110) show the phenyl rings in the pyrocatechol molecule to be oriented at 27 ± 6° and 23 ± 8°, respectively, from the surface normal. Experimental data and computational models of the pyrocatechol–anatase TiO 2 interface indicate the appearance of new occupied and unoccupied states on adsorption due to hybridization between the electronic states of the pyrocatechol molecule and the surface. The atomic character of the new orbitals created facilitates direct photoinjection from pyrocatechol into anatase TiO 2 and induces a strong final state effect in the carbon K-edge NEXAFS spectrum.