arXiv subjects

Philipp Scheiber

Publications and source records attributed to Philipp Scheiber.

3 recordsLinked to original sources

arXiv2018

Reaction of O2 with Subsurface Oxygen Vacancies on TiO2 Anatase (101)

Oxygen (O2) adsorbed on metal oxides is important in catalytic oxidation reactions, chemical sensing, and photocatalysis. Strong adsorption requires transfer of negative charge from oxygen vacancies (VOs) or dopants, for example. With scanning tunneling microscopy, we observed, transformed, and, in conjunction with theory, identified the nature of O2 molecules on the (101) surface of anatase (titanium oxide, TiO2) doped with niobium. VOs reside exclusively in the bulk, but we pull them to the surface with a strongly negatively charged scanning tunneling microscope tip. O2 adsorbed as superoxo (O2)- at fivefold-coordinated Ti sites was transformed to peroxo (O2)2- and, via reaction with a VO, placed into an anion surface lattice site as an (O2)O species. This so-called bridging dimer also formed when O2 directly reacted with VOs at or below the surface.

cond-mat.mtrl-sci↗

arXiv2018

(Sub)surface mobility of oxygen vacancies at the TiO$_2$ anatase (101) surface

Anatase is a metastable polymorph of TiO$_2$. In contrast to the more widely-studied TiO$_2$ rutile, O vacancies (V$_\mathrm O$'s) are not stable at the anatase (101) surface. Low-temperature STM shows that surface V$_\mathrm O$'s, created by electron bombardment at 105 K, start migrating to subsurface sites at temperatures $\geq$ 200 K. After an initial decrease of the V$_\mathrm O$ density, a temperature-dependent dynamic equilibrium is established where V$_\mathrm O$'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, DFT 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.

cond-mat.mtrl-sci↗

arXiv2010

Observation and destruction of an elusive adsorbate with STM: O$_2$/TiO$_2$

When a slightly defective rutile TiO$_2$(110) surface is exposed to O$_2$ at elevated temperatures, the molecule dissociates at defects, filling O vacancies (\ov) and creating O adatoms (\oad) on {\tifc} rows. The adsorption of molecular O$_2$ at low temperatures has remained controversial. Low-Temperature Scanning Tunneling Microscopy (LT-STM) of O$_2$, dosed on TiO$_2$(110) at a sample temperature of $\approx$ 100 K and imaged at 17 K, shows a molecular precursor at \ov as a faint change in contrast. The adsorbed O$_2$ easily dissociates during the STM measurements, and formation of \oad's at both sides of the original \ov is observed.

cond-mat.mtrl-sci↗