An Experimental Investigation of the Adsorption of a Phosphonic Acid on the Anatase TiO2(101) Surface

A combination of synchrotron radiation photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy has been used to study the adsorption of phenylphosphonic acid (PPA) on anatase TiO2(101) single crystal at coverages of 0.15 monolayer (ML) and 0.85 ML. The photoelectron spectroscopy data suggest that at 0.15 ML coverage PPA adsorbs in a bidentate geometry following deprotonation of both phosphonate hydroxyl groups, leaving the P-O group unbound. At 0.85 ML there is a shift to a mixed bidentate/monodentate binding mode. The carbon K-edge NEXAFS spectra were recorded at two azimuths. Our calculations show that for PPA on anatase TiO2(101) the phenyl ring is oriented 65 +/- 4 degrees away from the surface plane with an azimuthal twist of 57 +/- 11 degrees away from the [101] azimuth.