Transition of Molecule Orientation during Adsorption of Terephthalic Acid on Rutile TiO2(110)

The coverage-dependent mode of adsorption of terephthalic acid [C 6 H 4 (COOH) 2, TPA] on rutile TiO 2 (110) was investigated by means of noncontact atomic force microscopy (NC-AFM) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy under ultrahigh vacuum conditions at room temperature. Individual molecules are observed to adsorb in an disordered, flat-lying geometry at low coverages up to ∼0.3 monolayer (ML). The molecules are immobile at room temperature, implying a diffusion barrier larger than 0.8 eV. This rather high value might be explained by anchoring to surface defect sites. A transition from flat-lying to upright-oriented molecules is revealed by NEXAFS when saturation coverage is achieved. High-resolution NC-AFM images reveal two different structures at coverages between ∼0.8 and 1 ML: (i) a well-ordered (2 × 1) structure and (ii) a structure of single and paired rows oriented along the [001] crystallographic direction. The latter structure might originate from a pairwise interaction of two neighboring molecules through the top carboxyl groups. Further increase in the exposure results in a saturation of the corresponding signal in the NEXAFS spectra, revealing that the growth of TPA on TiO 2 (110) at room temperature is self-limiting.