Subsurface Oxygen Vacancies Research Articles

CO2 adsorption on TiO2(101) anatase: A dispersion-corrected density functional theory study

Adsorption, diffusion, and dissociation of CO(2) on the anatase (101) surface were investigated using dispersion-corrected density functional theory. On the oxidized surface several different local minima were identified of which the most stable corresponds to a CO(2) molecule adsorbed at a five-fold coordinated Ti site in a tilted configuration. Surface diffusion is characterized by relatively small activation barriers. Preferential diffusion takes place along Ti rows and involves a cartwheel type of motion. The presence of a bridging oxygen defect or a surface interstitial Ti atom allows creation of several new strong binding configurations the most stable of which have bent CO(2) structures with simultaneous bonding to two surface Ti atoms. Subsurface oxygen vacancy or interstitial Ti defects are found to enhance the bonding of CO(2) molecules to the surface. CO(2) dissociation from these defect sites is calculated to be exothermic with barriers less than 21 kcal/mol. The use of such defects for catalytic activation of CO(2) on anatase (101) surface would require a mechanism for their regeneration.

The Role of Surface and Subsurface Point Defects for Chemical Model Studies on TiO2: A First‐Principles Theoretical Study of Formaldehyde Bonding on Rutile TiO2(110)

We report a systematic investigation of the effects of different surface and subsurface point defects on the adsorption of formaldehyde on rutile TiO(2)(110) surfaces using density functional theory (DFT). All point defects investigated--including surface bridging oxygen vacancies, titanium interstitials, and subsurface oxygen vacancies--stabilize the adsorption significantly by up to 56 kJ mol(-1) at a coverage of 0.1 monolayer (ML). The stabilization is due to a decrease of the coordination (covalent saturation) of the surface Ti adsorption sites adjacent to the defects, which leads to a stronger molecule-surface interaction. This change in the Ti is caused by the removal of a neighboring atom (oxygen vacancies) or substantial lattice relaxations induced by the subsurface defects. On the stoichiometric reference surface, the most stable adsorption geometry of formaldehyde is a tilted η(2)-dioxymethylene (with an adsorption energy E(ads)=-125 kJ mol(-1)), in which a bond forms to a nearby bridging O atom and the carbonyl-O atom in the formaldehyde binds to a Ti atom in the adjacent fivefold coordinated lattice site. The η(1)-top configuration on five-coordinate Ti(4+) is much less favorable (E(ads)=-69 kJ mol(-1)). The largest stabilization is exerted by subsurface Ti interstitials between the first and second layers. These defects stabilize the η(2)-dioxymethylene structure by nearly 40 kJ mol(-1) to an adsorption energy of -164 kJ mol(-1). Contrary to popular belief, adsorption in a bridging oxygen vacancy (E(ads)=-86 kJ mol(-1)) is much less favorable for formaldehyde compared to the η(2)-dioxymethylene structures. From these results we conclude that formaldehyde will bind in the η(2)-dioxymethylene structure on the stoichiometric surface as well as in the presence of Ti interstitials and bridging oxygen vacancies. In the light of these substantial effects, we conclude that it is essential to include all the types of point defects present in typical, reduced rutile samples used for model studies, at realistic concentrations to obtain correct adsorption sites, structures, energetic, and chemi-physical properties.

Exchange between sub-surface and surface oxygen vacancies on CeO2(111): a new surface diffusion mechanism

Using first principles calculations for O vacancy diffusion on CeO(2)(111), we locate a surface diffusion mechanism, the two-step O vacancy exchange one, which is more favored than the most common hopping mechanism. By analyzing the results, we identify quantitatively the physical origin of why the two-step exchange mechanism is preferred.

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

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.

Influence of Subsurface Defects on the Surface Reactivity of TiO2: Water on Anatase (101)

The adsorption of water on a reduced TiO2 anatase (101) surface is investigated with scanning tunneling microscopy and density functional theory calculations. The presence of subsurface defects, which are prevalent on reduced anatase (101), leads to a higher desorption temperature of adsorbed water, indicating an enhanced binding due to the defects. Theoretical calculations of water adsorption on anatase (101) surfaces containing subsurface oxygen vacancies or titanium interstitials show a strong selectivity for water binding to sites in the vicinity of the subsurface defects. Moreover, the water adsorption energy at these sites is considerably higher than that on the stoichiometric surface, thus giving an explanation for the experimental observations. The calculations also predict facile water dissociation at these sites, confirming the important role of defects in the surface chemistry of TiO2.

Miniaturized pH Sensors Based on Zinc Oxide Nanotubes/Nanorods.

ZnO nanotubes and nanorods grown on gold thin film were used to create pH sensor devices. The developed ZnO nanotube and nanorod pH sensors display good reproducibility, repeatability and long-term stability and exhibit a pH-dependent electrochemical potential difference versus an Ag/AgCl reference electrode over a large dynamic pH range. We found the ZnO nanotubes provide sensitivity as high as twice that of the ZnO nanorods, which can be ascribed to the fact that small dimensional ZnO nanotubes have a higher level of surface and subsurface oxygen vacancies and provide a larger effective surface area with higher surface-to-volume ratio as compared to ZnO nanorods, thus affording the ZnO nanotube pH sensor a higher sensitivity. Experimental results indicate ZnO nanotubes can be used in pH sensor applications with improved performance. Moreover, the ZnO nanotube arrays may find potential application as a novel material for measurements of intracellular biochemical species within single living cells.

Stability and morphology of cerium oxide surfaces in an oxidizing environment: A first-principles investigation

We present density functional theory investigations of the bulk properties of cerium oxides (CeO2 and Ce2O3) and the three low index surfaces of CeO2, namely, (100), (110), and (111). For the surfaces, we consider various terminations including surface defects. Using the approach of “ab initio atomistic thermodynamics,” we find that the most stable surface structure considered is the stoichiometric (111) surface under “oxygen-rich” conditions, while for a more reducing environment, the same (111) surface, but with subsurface oxygen vacancies, is found to be the most stable one, and for a highly reducing environment, the (111) Ce-terminated surface becomes energetically favored. Interestingly, this latter surface exhibits a significant reconstruction in that it becomes oxygen terminated and the upper layers resemble the Ce2O3(0001) surface. This structure could represent a precursor to the phase transition of CeO2 to Ce2O3.

Energetics and diffusion of intrinsic surface and subsurface defects on anatase TiO2(101)

We report on density functional theory (DFT) calculations of the formation energies and diffusion pathways of oxygen vacancies and Ti interstitials at and near the (101) surface and in the bulk of the anatase polymorph of TiO(2). At the generalized gradient approximation level, both defects are found to be energetically more stable by approximately 0.5 eV or more at bulk and subsurface sites than on the surface. Moreover, the energy barriers to diffuse from the surface to the bulk are rather low, while the opposite is true for the barriers to diffuse from the bulk to the surface. This indicates that similar to Ti interstitials, also oxygen vacancies should preferentially occur at subsurface rather than at surface sites. To substantiate these findings, additional DFT + U calculations have been performed using different values of U in the range 2.5 < or = U < or = 4.5 eV. These show small differences in the relative stabilities of surface and subsurface oxygen vacancies with subsurface vacancies being more stable at low U values and a crossover in stability taking place around U approximately 3 eV. Analogous calculations for the TiO(2)(110) surface of rutile show that surface bridging oxygen vacancies are largely favored with respect to subsurface vacancies for all values of U. Altogether, our results provide evidence of important differences between reduced anatase and rutile surfaces in agreement with recent experimental observations.

Structure and Local-Equilibrium Thermodynamics of thec(2×2)Reconstruction of RutileTiO2(100)

We resolve the structure of a c(2x2) reconstruction of the rutile TiO2 (100) surface using a combination of transmission electron diffraction, direct methods analysis, and density functional theory. The surface structure contains an ordered array of subsurface oxygen vacancies and is in local thermodynamic equilibrium with bulk TiO2, but not the with oxygen gas-phase environment. The transition into a bulklike (1x1) reconstruction offers insights into the time-dependent local thermodynamics of TiO2 surface reconstruction under global nonequilibrium conditions.

Evidence of Subsurface Oxygen Vacancy Ordering on ReducedCeO2(111)

Surface and subsurface oxygen vacancies on the slightly reduced CeO(2)(111) surface have been studied by atomic resolution dynamic force microscopy at 80 K. Both types of defect are clearly identified by the comparison of the observed topographic features with the corresponding structures predicted from recent first-principles calculations. By combining two simultaneously acquired signals (the topography and the energy dissipated from the cantilever oscillation), we are able to unambiguously locate subsurface oxygen vacancies buried at the third surface atomic layer. We report evidence of local ordering of these subsurface defects that suggests the existence of a delicate balance between subtle interactions among adjacent subsurface oxygen vacancy structures.

ZnO Nanotetrapods: Controlled Vapor‐Phase Synthesis and Application for Humidity Sensing

AbstractA systematic study on controlled synthesis of ZnO nanotetrapods by combining metal‐vapor transport, oxidative nucleation/growth, fast‐flow quenching, and water‐assisted cleaning is reported. The technique developed in this work makes possible the fabrication of ZnO nanotetrapods with different morphologies, with arm diameters down to 17 nm, and with arm lengths ranging from 50 nm up to a few micrometers. The octa‐twin model is verified for the growth of the ZnO nanotetrapods. Photoluminescence (PL) studies indicate a higher level of surface and subsurface oxygen vacancies for smaller ZnO nanotetrapods. The ZnO nanotetrapods are first used for the fabrication of resistor‐type humidity sensors, which show high sensitivity, quick response/recovery, long lifetime, and a wide range of humidity response. These favorite characteristics of the humidity sensors are ascribed to the unique morphology of the nanotetrapods, which can create a film with faceted pores and large internal surfaces.

Electron Localization Determines Defect Formation on Ceria Substrates

The high performance of ceria (CeO2) as an oxygen buffer and active support for noble metals in catalysis relies on an efficient supply of lattice oxygen at reaction sites governed by oxygen vacancy formation. We used high-resolution scanning tunneling microscopy and density functional calculations to unravel the local structure of surface and subsurface oxygen vacancies on the (111) surface. Electrons left behind by released oxygen localize on cerium ions. Clusters of more than two vacancies exclusively expose these reduced cerium ions, primarily by including subsurface vacancies, which therefore play a crucial role in the process of vacancy cluster formation. These results have implications for our understanding of oxidation processes on reducible rare-earth oxides.

Origin of defect states on the surface of TiO2.

We have investigated the electronic structure of oxygen vacancies on ${\mathrm{TiO}}_{2}$(110), including atomic relaxation. The study of different defect sites shows that the experimentally observed gap state at 0.7 eV below the conduction-band edge is indeed due to an oxygen vacancy. This state, however, is not due to the removal of an O atom from a surface bridging site, as frequently proposed, but results from a subsurface oxygen vacancy. In this configuration, there is a maximum reduction of the screening between surface cations.