The interaction of organic molecules with titanium dioxide surfaces has been the subject of many studies over the last few decades. Numerous surface science techniques have been utilised to understand the often complex nature of these systems. The reasons for studying these systems are hugely diverse given that titanium dioxide has many technological and medical applications. Although surface science experiments investigating the adsorption of organic molecules on titanium dioxide surfaces is not a new area of research, the field continues to change and evolve as new potential applications are discovered and new techniques to study the systems are developed. This tutorial review aims to update previous reviews on the subject. It describes experimental and theoretical work on the adsorption of carboxylic acids, dye molecules, amino acids, alcohols, catechols and nitrogen containing compounds on single crystal TiO(2) surfaces.

The adsorption of dopamine onto an anatase TiO(2)(101) single crystal has been studied using photoemission and NEXAFS techniques. Photoemission results suggest that the dopamine molecule adsorbs on the surface in a bidentate geometry, resulting in the removal of band gap states in the TiO(2) valence band. Using the searchlight effect, carbon K-edge NEXAFS spectra indicate that the phenyl rings in the dopamine molecules are orientated normal to the surface. A combination of experimental and computational results indicates the appearance of new unoccupied states arising following adsorption. The possible role of these states in the charge-transfer mechanism of the dopamine-TiO(2) system is discussed.

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 hybridisation 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.

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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 TiO 2 (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 TiO 2 (101) the phenyl ring is oriented 65 ± 4°away from the surface plane with an azimuthal twist of 57 ± 11°away from the [101] azimuth.