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 TiO2 (101) and rutile TiO2 (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 TiO2 (101) and rutile TiO2 (110) show the phenyl rings in the pyrocatechol molecule to be oriented at 27 +/- 6 degrees and 2.3 +/- 8 degrees, respectively, from the surface normal. Experimental data and computational models of the pyrocatechol-anatase TiO2 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 TiO2 and induces a strong final state effect in the carbon K-edge NEXAFS spectrum.