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The role of bulk defects in the oxygen chemistry on reduced rutile TiO(2)(110)-(1 × 1) has been studied by means of temperature-programmed desorption spectroscopy and scanning tunneling microscopy measurements. Following O(2) adsorption at 130 K, the amount of O(2) desorbing at ∼410 K initially increased with increasing density of surface oxygen vacancies but decreased after further reduction of the TiO(2)(110) crystal. We explain these results by withdrawal of excess charge (Ti(3+)) from the TiO(2)(110) lattice to oxygen species on the surface and by a reaction of Ti interstitials with O adatoms upon heating. Important consequences for the understanding of the O(2)-TiO(2) interaction are discussed., (© 2011 American Chemical Society)

Calcite is a mineral of fundamental importance that plays a crucial role in many fields of research such as biomineralization, biomolecule adsorption, and reactivity as well as industrial and daily life applications. Consequently, the most stable cleavage plane of calcite has been studied extensively using both direct imaging techniques such as atomic force microscopy as well as spectroscopic and diffraction techniques. Several surface structures have been reported for the (1014) cleavage plane of calcite differing from the simple bulk-truncated structure and an ongoing controversy exists in literature whether the cleavage plane exhibits a (2 x 1) reconstruction or not. We study the (1014) cleavage plane using high-resolution noncontact atomic force microscopy (NC-AFM) under ultrahigh vacuum conditions and obtain a clear signature of the (2 x 1) reconstruction. This reconstruction is observed in very narrow tip-surface distance ranges only, explaining why in some experiments the reconstruction has been observed and in others not. Moreover, as all sample preparation is performed in ultrahigh vacuum, the possibility of the (2 x 1) reconstruction being adsorbate-induced appears rather unlikely. Additionally, tip-induced surface changes are ruled out as origin for the observed reconstruction either. In conclusion, our study suggests that the (2 x 1) reconstruction is a true surface property of the (1014) cleavage plane of calcite.

Non-contact atomic force microscopy (NC-AFM) at true atomic resolution is used to investigate the (110) surface of rutile TiO(2). We are able to simultaneously resolve both bridging oxygen and titanium atoms of this prototypical oxide surface. Furthermore, the characteristic defect species, i.e. bridging oxygen vacancies, single and double hydroxyls as well as subsurface defects, are identified in the very same frame. We employ density functional theory (DFT) calculations to obtain a comprehensive understanding of the relation between the tip apex structure and the observed image contrast. Our results provide insight into the physical mechanisms behind atomic-scale contrast, indicating that electrostatic interaction can lead to a far more complex contrast than commonly assumed.

Titanium dioxide represents a very important wide bandgap photocatalyst that is known to be sensitized to visible light by transition metal doping. Antimony doping has been demonstrated to provide photocatalytic activity when codoped with chromium at an optimum dopant ratio [Sb]/[Cr] of about 1.5. Here, the role of antimony doping on the surface structure of rutile TiO(2)(110) is studied using non-contact atomic force microscopy (NC-AFM) under ultra-high vacuum conditions. At first glance, the surface structure of antimony-doped TiO(2)(110) resembles the structure of pristine TiO(2)(110). However, in contrast to what is found in pristine TiO(2)(110), a dense layer of protruding features is observed upon antimony doping, which is tentatively ascribed to antimony-rich clusters. Moreover, homogeneously distributed holes are found on the surface, which differ in depth and shape depending on the preparation conditions. Holes with depths ranging from a few up to more than a hundred monatomic steps are observed. These holes are explained by surface segregation of antimony during annealing, as the ionic radius of Sb(3+) is considerably larger than the ionic radius of Ti(4+). Our finding provides an indication of why an antimony concentration larger than the optimum ratio results in decreased photocatalytic activity. Moreover, controlling annealing temperature seems to constitute a promising strategy for creating nanosized holes on TiO(2) surfaces.

Non-contact atomic force microscopy is used to study C(60) molecules deposited on the rutile TiO(2)(110) surface in situ at room temperature. At submonolayer coverages, molecules adsorb preferentially at substrate step edges. Upon increasing coverage, ordered islands grow from the decorated step edges onto the lower terraces. Simultaneous imaging of bridging oxygen rows of the substrate and the C(60) island structure reveals that the C(60) molecules arrange themselves in a centered rectangular superstructure, with the molecules lying centered in the troughs formed by the bridging oxygen rows. Although the TiO(2)(110) surface exhibits a high density of surface defects, the observed C(60) islands are of high order. This indicates that the C(60) intermolecular interaction dominates over the molecule-substrate interactions that may cause structural perturbations on a defective surface. Slightly protruding C(60) strands on the islands are attributed to anti-phase boundaries due to stacking faults resulting from two islands growing together.