Olga Dulub

Local ordering and electronic signatures of submonolayer water on anatase TiO2(101)

The interaction of water with metal oxide surfaces is of fundamental importance to various fields of science, ranging from geophysics to catalysis and biochemistry. In particular, the discovery that TiO(2) photocatalyses the dissociation of water has triggered broad interest and intensive studies of water adsorption on TiO(2) over decades. So far, these studies have mostly focused on the (110) surface of the most stable polymorph of TiO(2), rutile, whereas it is the metastable anatase form that is generally considered photocatalytically more efficient. The present combined experimental (scanning tunnelling microscopy) and theoretical (density functional theory and first-principles molecular dynamics) study gives atomic-scale insights into the adsorption of water on anatase (101), the most frequently exposed surface of this TiO(2) polymorph. Water adsorbs as an intact monomer with a computed binding energy of 730 meV. The charge rearrangement at the molecule-anatase interface affects the adsorption of further water molecules, resulting in short-range repulsive and attractive interactions along the [010] and directions, respectively, and a locally ordered (2x2) superstructure of molecular water.

Sub)Surface Mobility of Oxygen Vacancies at the TiO2 Anatase (101) Surface

Anatase is a metastable polymorph of TiO2. In contrast to the more widely studied TiO2 rutile, O vacancies (V(O)s) are not stable at the anatase (101) surface. Low-temperature STM shows that surface V(O)s, created by electron bombardment at 105 K, start migrating to subsurface sites at temperatures ≥200 K. After an initial decrease of the V(O) density, a temperature-dependent dynamic equilibrium is established where V(O)s move to subsurface sites and back again, as seen in time-lapse STM images. We estimate that activation energies for subsurface migration lie between 0.6 and 1.2 eV; in comparison, density functional theory calculations predict a barrier of ca. 0.75 eV. The wide scatter of the experimental values might be attributed to inhomogeneously distributed subsurface defects in the reduced sample.

Electron-Induced Oxygen Desorption from the TiO2(011)-2×1 Surface Leads to Self-Organized Vacancies

When low-energy electrons strike a titanium dioxide surface, they may cause the desorption of surface oxygen. Oxygen vacancies that result from irradiating a TiO2(011)-2×1 surface with electrons with an energy of 300 electron volts were analyzed by scanning tunneling microscopy. The cross section for desorbing oxygen from the pristine surface was found to be 9 (±6) × 10–17 square centimeters, which means that the initial electronic excitation was converted into atomic motion with a probability near unity. Once an O vacancy had formed, the desorption cross sections for its nearest and next-nearest oxygen neighbors were reduced by factors of 100 and 10, respectively. This site-specific desorption probability resulted in one-dimensional arrays of oxygen vacancies.

THE RELATIONSHIP BETWEEN BULK AND SURFACE PROPERTIES OF RUTILE TiO2(110)

We report scanning tunneling microscopy and complementary spectroscopic measurements on TiO2(110) surfaces. We show data on (i) a surface restructuring process that results from annealing in oxygen; (ii) Pt clusters, grown at room temperature and encapsulated upon high temperature annealing; and (iii) adsorption of sulfur. In each case, heavily reduced, dark crystals show a very different behavior than more stoichiometric, light blue ones.