Ulrich Johannes Aschauer

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

Oxide Chemistry Below the Surface Although metal oxides, such as titanium dioxide (TiO2), are used for catalytic oxidation reactions and photocatalysis, the O2 does not react directly with substrates. Vacancies in the surface region of the TiO2 rutile phase can transfer a negative charge to adsorbed O2 to create more reactive species. By contrast, in anatase—the phase associated with nanoscale TiO2 particles—subsurface vacancies form. Setvin et al. (p. 988) used a scanning tunneling microscopy tip to pull these vacancies to the surface in a niobiumdoped anatase crystal and followed the transformation of adsorbed O2− into a peroxo species and a bridging O2 dimer. Subsurface oxygen vacancies created at an anatase surface play a key role in forming a bridging oxygen (O2) dimer from adsorbed O2. 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 (O22–) 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.

Strain-controlled oxygen vacancy formation and ordering in CaMnO3

We use first-principles calculations to investigate the stability of biaxially strained Pnma perovskite CaMnO

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

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Peroxide and superoxide states of adsorbed O2 on anatase TiO2 (101) with subsurface defects

towards the formation of oxygen vacancies. Our motivation is provided by promising indications that novel material properties can be engineered by application of strain through coherent heteroepitaxy in thin films. While it is usually assumed that such epitaxial strain is accommodated primarily by changes in intrinsic lattice constants, point defect formation is also a likely strain-relaxation mechanism. Our first-principles calculations of oxygen vacancy defect formation energy indeed show a strong strain dependence: We find that tensile strain lowers the formation energy, consistent with the established chemical expansion concept that oxygen deficiency increases the molar volume in oxides. In addition, we find that strain differentiates the formation energy for different lattice sites, suggesting its use as a route to engineering vacancy ordering in epitaxial thin films.

Hydrogen interaction with the anatase TiO2(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.

Adsorption and Reactions of O2 on Anatase TiO2

Density Functional Theory (DFT) calculations within the Generalized Gradient Approximation (GGA) and the GGA + U approach are carried out to investigate the adsorption of O(2) on anatase (101) surfaces having subsurface oxygen vacancies. Our results show that O(2) adsorption is strongly enhanced at sites close to the subsurface defect, whereas dissociation is unfavorable at all sites. The adsorption is accompanied by the transfer of the defect electrons to O(2)-derived electronic states in the anatase surface band gap. Peroxide species (O(2)(2-), O-O = 1.48 Å) are stable when the number of adsorbed O(2) molecules is less or equal the number of defects, whereas superoxide species (O(2)(-), O-O = 1.33 Å) become more favorable at coverages exceeding approximately 1.5 O(2) molecules per oxygen vacancy.

Adsorption and Reactions of O₂ on Anatase TiO₂

The interaction of atomic hydrogen with the majority (101) surface of anatase TiO(2) is studied using density functional theory calculations both with a standard semi-local functional and with the inclusion of on-site Coulomb repulsion terms. We investigate the energetics of different adsorption configurations at surface and subsurface sites and different coverages, from low to one monolayer, as well as diffusion pathways among the different sites and recombinative H(2) desorption barriers. While H(2) desorption is the energetically most favorable process, the diffusion of H into the subsurface is found to be at least equally favorable kinetically. It is further shown that subsurface oxygen vacancies on reduced anatase are favorable adsorption sites for hydrogen atoms.

Growth modification of seeded calcite using carboxylic acids: Atomistic simulations

CONSPECTUS: The interaction of molecular oxygen with titanium dioxide (TiO2) surfaces plays a key role in many technologically important processes such as catalytic oxidation reactions, chemical sensing, and photocatalysis. While O2 interacts weakly with fully oxidized TiO2, excess electrons are often present in TiO2 samples. These excess electrons originate from intrinsic reducing defects (oxygen vacancies and titanium interstitials), doping, or photoexcitation and form polaronic Ti(3+) states in the band gap near the bottom of the conduction band. Oxygen adsorption involves the transfer of one or more of these excess electrons to an O2 molecule at the TiO2 surface. This results in an adsorbed superoxo (O2(-)) or peroxo (O2(2-)) species or in molecular dissociation and formation of two oxygen adatoms (2 × O(2-)). Oxygen adsorption is also the first step toward oxygen incorporation, a fundamental reaction that strongly affects the chemical properties and charge-carrier densities; for instance, it can transform the material from an n-type semiconductor to a poor electronic conductor. In this Account, we present an overview of recent theoretical work on O2 adsorption and reactions on the reduced anatase (101) surface. Anatase is the TiO2 polymorph that is generally considered most active in photocatalysis. Experiments on anatase powders have shown that the properties of photoexcited electrons are similar to those of excess electrons from reducing defects, and therefore, oxygen on reduced anatase is also a model system for studying the role of O2 in photocatalysis. Experimentally, the characteristic Ti(3+) defect states disappear after adsorption of molecular oxygen, which indicates that the excess electrons are indeed trapped by O2. Moreover, superoxide surface species associated with two different cation surface sites, possibly a regular cation site and a cation close to an anion vacancy, were identified by electron paramagnetic resonance spectroscopy. On the theoretical side, however, density functional theory studies have consistently found that it is energetically more favorable for O2 to adsorb in the peroxo form rather than the superoxo form. As a result, obtaining a detailed understanding of the nature of the observed superoxide species has proven difficult for many years. On reduced anatase (101), both oxygen vacancies and Ti interstitials have been shown to reside exclusively in the susbsurface. We discuss how reaction of O2 with a subsurface O vacancy heals the vacancy while leading to the formation of a surface bridging dimer defect. Similarly, the interaction of O2 with a Ti interstitial causes migration of this defect to the surface and the formation of a surface TiO2 cluster. Finally, we analyze the peroxo and superoxo states of the adsorbed molecule. On the basis of periodic hybrid functional calculations of interfacial electron transfer between reduced anatase and O2, we show that the peroxide form, while energetically more stable, is kinetically less favorable than the superoxide form. The existence of a kinetic barrier between the superoxo and peroxo states is essential for explaining a variety of experimental observations.

Polaronic metal state at the LaAlO 3 /SrTiO 3 interface

CONSPECTUS: The interaction of molecular oxygen with titanium dioxide (TiO2) surfaces plays a key role in many technologically important processes such as catalytic oxidation reactions, chemical sensing, and photocatalysis. While O2 interacts weakly with fully oxidized TiO2, excess electrons are often present in TiO2 samples. These excess electrons originate from intrinsic reducing defects (oxygen vacancies and titanium interstitials), doping, or photoexcitation and form polaronic Ti(3+) states in the band gap near the bottom of the conduction band. Oxygen adsorption involves the transfer of one or more of these excess electrons to an O2 molecule at the TiO2 surface. This results in an adsorbed superoxo (O2(-)) or peroxo (O2(2-)) species or in molecular dissociation and formation of two oxygen adatoms (2 × O(2-)). Oxygen adsorption is also the first step toward oxygen incorporation, a fundamental reaction that strongly affects the chemical properties and charge-carrier densities; for instance, it can transform the material from an n-type semiconductor to a poor electronic conductor. In this Account, we present an overview of recent theoretical work on O2 adsorption and reactions on the reduced anatase (101) surface. Anatase is the TiO2 polymorph that is generally considered most active in photocatalysis. Experiments on anatase powders have shown that the properties of photoexcited electrons are similar to those of excess electrons from reducing defects, and therefore, oxygen on reduced anatase is also a model system for studying the role of O2 in photocatalysis. Experimentally, the characteristic Ti(3+) defect states disappear after adsorption of molecular oxygen, which indicates that the excess electrons are indeed trapped by O2. Moreover, superoxide surface species associated with two different cation surface sites, possibly a regular cation site and a cation close to an anion vacancy, were identified by electron paramagnetic resonance spectroscopy. On the theoretical side, however, density functional theory studies have consistently found that it is energetically more favorable for O2 to adsorb in the peroxo form rather than the superoxo form. As a result, obtaining a detailed understanding of the nature of the observed superoxide species has proven difficult for many years. On reduced anatase (101), both oxygen vacancies and Ti interstitials have been shown to reside exclusively in the susbsurface. We discuss how reaction of O2 with a subsurface O vacancy heals the vacancy while leading to the formation of a surface bridging dimer defect. Similarly, the interaction of O2 with a Ti interstitial causes migration of this defect to the surface and the formation of a surface TiO2 cluster. Finally, we analyze the peroxo and superoxo states of the adsorbed molecule. On the basis of periodic hybrid functional calculations of interfacial electron transfer between reduced anatase and O2, we show that the peroxide form, while energetically more stable, is kinetically less favorable than the superoxide form. The existence of a kinetic barrier between the superoxo and peroxo states is essential for explaining a variety of experimental observations.

Quantum Critical Origin of the Superconducting Dome in SrTiO3

Molecular dynamics simulations were used to investigate possible explanations for experimentally observed differences in the growth modification of calcite particles by two organic additives, polyacrylic acid (PAA) and polyaspartic acid (p-ASP). The more rigid backbone of p-ASP was found to inhibit the formation of stable complexes with counter-ions in solution, resulting in a higher availability of p-ASP compared to PAA for surface adsorption. Furthermore the presence of nitrogen on the p-ASP backbone yields favorable electrostatic interactions with the surface, resulting in negative adsorption energies, in an upright (brush conformation). This leads to a more rapid binding and longer residence times at calcite surfaces compared to PAA, which adsorbed in a flat (pancake) configuration with positive adsorption energies. The PAA adsorption occurring despite this positive energy difference can be attributed to the disruption of the ordered water layer seen in the simulations and hence a significant entropic contribution to the adsorption free energy. These findings help explain the stronger inhibiting effect on calcite growth observed by p-ASP compared to PAA and can be used as guidelines in the design of additives leading to even more marked growth modifying effects.