Photoemission Study of Azobenzene and Aniline Adsorbed on TiO2 Anatase (101) and Rutile (110) Surfaces

The electronic structure of azobenzene and aniline, adsorbed on two TiO2 surfaces, anatase (101) and rutile (110), has been studied with ultraviolet synchrotron-based photoemission spectroscopy (UPS). At saturation coverage, azobenzene and aniline exhibit very similar molecular orbitals in UPS valence band spectra. Angle-resolved UPS exhibits anisotropy of the molecular states along the polar and azimuthal direction, as is expected for highly oriented superstructures. For a low coverage of azobenzene adsorbed on anatase, photon irradiation results in the conversion of the flat-lying molecule into two upright phenyl imide species. An irradiation-induced trans-cis isomer conversion is proposed to facilitate the azobenzene cleavage. These results confirm that the N=N double bond of azobenzene is cleaved by TiO2 in the full-coverage regime and that the resulting intermediate is bonded to the substrate, in agreement with a previous scanning tunneling microscopy study and a proposed reaction scheme for azobenzene <-> aniline conversion at TiO2 surfaces [Li, S.-C.; Diebold, U. J. Am. Chem. Soc. 2010, 132, 64].