The prototypical photocatalyst TiO2 exists in different polymorphs, the most common forms are the anatase- and rutile-crystal structures. Generally, anatase is more active than rutile, but no consensus exists to explain this difference. Here we demonstrate that it is the bulk transport of excitons to the surface that contributes to the difference. Utilizing high –quality epitaxial TiO2 films of the two polymorphs we evaluate the photocatalytic activity as a function of TiO2-film thickness. For anatase the activity increases for films up to ~5 nm thick, while rutile films reach their maximum activity for ~2.5 nm films already. This shows that charge carriers excited deeper in the bulk contribute to surface reactions in anatase than in rutile. Furthermore, we measure surface orientation dependent activity on rutile single crystals. The pronounced orientation-dependent activity can also be correlated to anisotropic bulk charge carrier mobility, suggesting general importance of bulk charge diffusion for explaining photocatalytic anisotropies.

Titanium dioxide is the prototypical transition metal oxide photocatalyst. However, the larger than 3 eV bandgap of common bulk phases of TiO₂ limits its light absorption to UV light, making it inefficient for solar energy conversion. Attempts at increasing visible light activity by narrowing the bandgap of TiO₂ through doping have proven difficult, because of defect-induced charge trapping and recombination sites of photo-excited charge carriers. Here, we report the existence of a dopant-free, pure TiO₂ phase with a narrow bandgap. This new pure TiO₂ phase forms on the surface of rutile TiO₂(011) by oxidation of bulk titanium interstitials. We measure a bandgap of only ~2.1 eV for this new phase, matching it closely with the energy of visible light.

Acetic acid adsorption on the rutile TiO 2 (110) and (011)-2 Â 1 surfaces is compared by temperature-programmed desorption, ultraviolet photoemission spectroscopy, and scanning tunneling microscopy. In contrast to the wellestablished bidentate adsorption of carboxylic acids on the (110) surface, we find a monodentate adsorption on the (011)-2 Â 1 surface as the most likely adsorption geometry. The adsorption dynamics is also very different for the two surfaces. While acetic acid adsorbs homogeneously on the (110) surface, acetic acid adsorbs as quasi-one-dimensional clusters on the (011)-2 Â 1 surface. Initially, acetate adsorbs only at surface defects, but subsequent acetic acid can adsorb along these nucleated acetate clusters and spread to the defect-free terraces. Therefore, the preadsorbed acetic acid facilitates further acetic acid adsorption in a self-catalyzed adsorption mechanism. The acetic acid clusters preferentially grow along the [011] crystallographic surface direction forming up to tens of nanometers long clusters with a usual width of three acetates. In TPD studies acetate reacts in a unimolecular dehydration reaction yielding ketene as the main desorption product at ∼585 K for both surfaces.

Grazing incidence low energy ion beams preferentially erode steps with directional components normal to the azimuthal direction of the beam, thus generating step edges aligned along the beam direction. With this kinetic method, the fabrication of thermodynamically metastable low index step-edge orientations is demonstrated on TiO2(110). The 11[over ]0 step edge is prepared, enabling its atomic structure determination by scanning tunneling microscopy and density-functional theory. A reconstructed atom configuration is revealed, which is reminiscent of the structure of the rutile-TiO2(001)-(2 x 1) face.

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