The interaction of water with TiO2 is crucial to many of its practical applications, including photocatalytic water splitting. Following the first demonstration of this phenomenon 40 years ago there have been numerous studies of the rutile single-crystal TiO2(110) interface with water. This has provided an atomic-level understanding of the water-TiO2 interaction. However, nearly all of the previous studies of water/TiO2 interfaces involve water in the vapour phase. Here, we explore the interfacial structure between liquid water and a rutile TiO2(110) surface pre-characterized at the atomic level. Scanning tunnelling microscopy and surface X-ray diffraction are used to determine the structure, which is comprised of an ordered array of hydroxyl molecules with molecular water in the second layer. Static and dynamic density functional theory calculations suggest that a possible mechanism for formation of the hydroxyl overlayer involves the mixed adsorption of O2 and H2O on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO2 photocatalysis.

As models for probing the interactions between TiO2 surfaces and the dye molecules employed in dye-sensitized solar cells, carboxylic acids are an important class of molecules. In this work, we present a scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) study of three small carboxylic acids (formic, acetic, and benzoic) that were reacted with the TiO2(110) surface via a dipping procedure. The three molecules display quite different adsorption behavior, illustrating the different interadsorbate interactions that can occur. After exposure to a 10 mM solution, formic acid forms a rather disordered formate overlayer with two distinct binding geometries. Acetic acid forms a well-ordered (2 × 1) acetate overlayer similar to that observed following deposition from vapor. Benzoic acid forms a (2 × 2) overlayer, which is stabilized by intermolecular interactions between the phenyl groups.

Scanning tunneling microscopy (STM) has been used to investigate the adsorption of benzoic acid on the rutile TiO2(110)(1 x 1) and the reconstructed TiO2(110)(1 x 2) surfaces. Benzoic acid binds to both surfaces dissociatively via a bridging geometry to two Ti-5c sites. At a slightly elevated sample temperature during deposition onto the (110) (1 x 1) surface, a well-ordered (2 x 1) overlayer was formed at saturated benzoate coverage. On the reconstructed (110) (1 x 2) surface, benzoate was observed to adsorb between the (1 x 2) strands leading to a (2 x 2) superstructure at higher coverage. Elongation along the [1 (1) over bar0] direction in the STM images indicates a rotation of the benzene ring of 90 degrees relative to the carboxylate group, which is reasonably explained by hydrogen bond interactions between terminating O-atoms on the surface and H-atoms of the ring.

Controlled dual mode scanning tunneling microscopy (STM) experiments and first-principles simulations show that the tunneling conditions can significantly alter the positive-bias topographic contrast of geometrically corrugated titania surfaces such as rutile TiO2(011)-(2×1). Depending on the tip-surface distance, two different contrasts can be reversibly imaged. STM simulations which either include or neglect the tip-electronic structure, carried out at three density functional theory levels of increasing accuracy, allow assignment of both contrasts on the basis of the TiO2(011)-(2×1) structure proposed by Torrelles et al. [Phys. Rev. Lett. 101, 185501 (2008)]. Finally, the mechanisms of contrast formation are elucidated in terms of the subtle balance between the surface geometry and the different vacuum decay lengths of the topmost Ti(3d) and O(2p) states probed by the STM-tip apex.