Oxygen vacancies on metal oxide surfaces have long been thought to play a key role in the surface chemistry. Such processes have been directly visualized in the case of the model photocatalyst surface TiO 2 ð110Þ in reactions with water and molecular oxygen. These vacancies have been assumed to be neutral in calculations of the surface properties. However, by comparing experimental and simulated scanning tunneling microscopy images and spectra, we show that oxygen vacancies act as trapping centers and are negatively charged. We demonstrate that charging the defect significantly affects the reactivity by following the reaction of molecular oxygen with surface hydroxyl formed by water dissociation at the vacancies. Calculations with electronically charged hydroxyl favor a condensation reaction forming water and surface oxygen adatoms, in line with experimental observations. This contrasts with simulations using neutral hydroxyl where hydrogen peroxide is found to be the most stable product.The rutile TiO 2 ð110Þ surface, which we use as a model photocatalytic system here, is displayed as a ball model in Fig. 1A where the reduction of one oxygen atom of O 2 ðgÞ to one bridging oxide species (O 2− b ) is accomplished by oxidation of the two Ti 3þ sites associated with O b -vac to Ti 4þ (3), on the basis of a purely ionic model. (Formal charges are written in reactions 1 and 2 to highlight the redox processes involved.)The interaction of O 2 with OH b , on the other hand, is still a matter of controversy. Following the reaction of these species at temperatures ≤240 K, water is seen to desorb at ∼310 K in temperature programmed desorption (TPD) spectra (3, 4). Henderson et al. (3) concluded that this water evolution is a consequence of the formation of oxygen adatoms (O ad ) at the surface as follows:where the two Ti 3þ species provide the two electrons necessary to reduce one oxygen atom of O 2 ðgÞ to H 2 OðgÞ (3). In stark contrast to the TPD results, previous calculations find H 2 O 2 to be by far the most stable product (5). Moreover, on the basis of these calculations, water desorption is not expected up to the highest temperature computed, 350 K (5). This discrepancy provided the initial motivation for the present work. Results and DiscussionWe use STM to provide an additional experimental test of the picture that has emerged thus far. Fig. 1B shows a surface containing both O b -vac and OH b , alongside the same surface in Fig. 1C after it was exposed to 90 Langmuirs (L) O 2 at 300 K (1 L ¼ 1.33 × 10 −6 mbar · s, 1 mbar ¼ 100 Pa). A number of small, bright spots can be seen on the Ti 5c sites (bright rows) in the latter image. The histogram of the height distribution of these bright spots, shown in Fig. 1D, indicates that these bright spots are almost entirely due to one final product.It should be noted that at lower O 2 exposures we see a number of different types of species on Ti 5c rows that are likely to arise from terminal hydroxyls (OH t ) and other metastable species such as O 2 H. These latter results ...

We report the first theoretical study of hydroxyl vacancies in aluminosilicate and aluminogermanate single-walled metal-oxide nanotubes. The defects are modeled on both sides of the tube walls and lead to occupied and empty states in the band gap which are highly localized both in energy and in real space. We find different magnetization states depending on both the chemical composition and the specific side with respect to the tube cavity. The defect-induced perturbations to the pristine electronic structure are related to the electrostatic polarization across the tube walls and the ensuing change in Brønsted acid-base reactivity. Finally, the capacity to counterbalance local charge accumulations, a characteristic feature of these systems, is discussed in view of their potential application as insulating coatings for one-dimensional conducting nanodevices.

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.