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

The coverage-dependent mode of adsorption of terephthalic acid [C6H4(COOH)(2), TPA] on rutile TiO2(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 similar 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 similar to 0.8 and 1 ML: (i) a well-ordered (2 x 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 it saturation of the corresponding signal in the NEXAFS spectra, revealing that the growth of TPA oil TiO2(110) at room temperature is self-limiting.