The rutile TiO2 (110) surface: Obtaining converged structural properties from first-principles calculations

Selective Hydration of Rutile TiO2 as a Strategy for Site-Selective Atomic Layer Deposition

The feasibility of a site-selective hydration strategy that enables site-selective atomic layer deposition (ALD) is investigated among four rutile TiO₂ facets [(110), (100), (101) and (001)] and their most prevalent step edges. First-principles simulations of asymmetric slab models were utilized to create accurate representations of pristine terrace and step edge sites. The adsorption free energies for molecular and dissociative adsorption of H₂O were calculated to evaluate this strategy as a viable route to step edge selectivity. We predict that selective hydroxylation is possible on the 110 and 001 step edges and further computationally evaluate three metalorganic ALD precursors for their compatibility with the selective hydration strategy. Experimental evidence for delayed nucleation of ALD on rutile (001), (110), and (100) TiO₂ single crystals corroborates predictions of the dehydration of the surface and suggests the possibility of site-selective ALD.

General embedded cluster protocol for accurate modeling of oxygen vacancies in metal-oxides

The O vacancy (Ov) formation energy, EOv, is an important property of a metal-oxide, governing its performance in applications such as fuel cells or heterogeneous catalysis. These defects are routinely studied with density functional theory (DFT). However, it is well-recognized that standard DFT formulations (e.g., the generalized gradient approximation) are insufficient for modeling the Ov, requiring higher levels of theory. The embedded cluster method offers a promising approach to compute EOv accurately, giving access to all electronic structure methods. Central to this approach is the construction of quantum(-mechanically treated) clusters placed within suitable embedding environments. Unfortunately, current approaches to constructing the quantum clusters either require large system sizes, preventing application of high-level methods, or require significant manual input, preventing investigations of multiple systems simultaneously. In this work, we present a systematic and general quantum cluster design protocol that can determine small converged quantum clusters for studying the Ov in metal-oxides with accurate methods, such as local coupled cluster with single, double, and perturbative triple excitations. We apply this protocol to study the Ov in the bulk and surface planes of rutile TiO2 and rock salt MgO, producing the first accurate and well-converged determinations of EOv with this method. These reference values are used to benchmark exchange–correlation functionals in DFT, and we find that all the studied functionals underestimate EOv, with the average error decreasing along the rungs of Jacob’s ladder. This protocol is automatable for high-throughput calculations and can be generalized to study other point defects or adsorbates.

Degradation of Fatal Toxic Nerve Agents on Dry TiO2

Despite a recent dramatically increased risk of using chemical warfare agents in chemical attacks and assassinations, fundamental interactions of toxic chemicals with other materials are poorly understood, and micromechanisms of their chemical degradation are yet to be established. This represents an outstanding challenge in both fundamental science and practical applications in combat against chemical weapons. One of the most versatile and multifunctional oxides, TiO₂, has been suggested as a promising material to quickly adsorb and effectively destroy toxins. In this paper, we explore how sarin (also known as GB) adsorbs and decomposes on dry nanoparticles of TiO₂ anatase and rutile phases. We found that both anatase and rutile readily adsorb sarin gas molecules because of a strong electrostatic attraction between the phosphoryl oxygen and surface titanium atoms. The sarin decomposition most likely proceeds via a propene elimination; however, the reaction is exothermic on the rutile (110) surface and endothermic on the anatase (101) surface. High energy barriers suggest that sarin would hardly decompose on pristine dry surfaces of TiO₂, and degradation reactions can be triggered by defects or contaminants under realistic operational conditions.

Formation and stability of small polarons at the lithium-terminated Li4Ti5O12 (LTO) (111) surface

Zero strain insertion, high cycling stability, and a stable charge/discharge plateau are promising properties rendering Lithium Titanium Oxide (LTO) a possible candidate for an anode material in solid state Li ion batteries. However, the use of pristine LTO in batteries is rather limited due to its electronically insulating nature. In contrast, reduced LTO shows an electronic conductivity several orders of magnitude higher. Studying bulk reduced LTO, we could show recently that the formation of polaronic states can play a major role in explaining this improved conductivity. In this work, we extend our study toward the lithium-terminated LTO (111) surface. We investigate the formation of polarons by applying Hubbard-corrected density functional theory. Analyzing their relative stabilities reveals that positions with Li ions close by have the highest stability among the different localization patterns.

Canonical, deprotonated, or zwitterionic? II. A computational study on amino acid interaction with the TiO2(110) rutile surface: comparison with the anatase (101) surface

The adsorption of 11 amino acids (Gly, Leu, Met, Phe, Ser, Cys, Glu, Gln, Arg, Lys, and His) on the TiO₂(110) rutile surface is investigated adopting a theoretical approach, using the PBE-D2* functional as implemented in the periodic VASP code. The adsorption of the amino acids is considered in their canonical, deprotonated and zwitterionic forms. For all cases, the most stable adsorption mode adopts a bidentate (O,O) binding with surface undercoordinated Ti atoms, in agreement with previous experimental and computational studies using glycine as a test case. Such a binding mode is possible due to the surface morphology, because the Ti-Ti distances match very well with the carboxylic O-O distance. The most stable adsorption states are the deprotonated and the zwitterionic ones, the canonical one lying significantly above in energy. The relative stability between the deprotonated and the zwitterionic states results in a delicate trade-off among dative interactions (O, N, and S atoms of the amino acids with Ti atoms of the surface), H-bond interactions, dispersive forces and, to a lesser extent, steric hindrance of the amino acidic lateral chains. Finally, the difference in the amino acid adsorption between the (110) rutile and the (101) anatase surfaces is discussed both from the energetic and surface morphological standpoints, highlighting the larger reactivity of the rutile polymorph in adsorbing and deprotonating the amino acids compared with the anatase one.

Atomic-resolution imaging of rutile TiO2(110)-(1 × 2) reconstructed surface by non-contact atomic force microscopy

The structure of the rutile TiO₂(110)-(1 × 2) reconstructed surface is a phase induced by oxygen reduction. There is ongoing debate about the (1 × 2) reconstruction, because it cannot be clarified whether the (1 × 2) structure is formed over a wide area or only locally using macroscopic analysis methods such as diffraction. We used non-contact atomic force microscopy, scanning tunneling microscopy, and low-energy electron diffraction at room temperature to characterize the surface. Ti₂O₃ rows appeared as bright spots in both NC-AFM and STM images observed in the same area. High-resolution NC-AFM images revealed that the rutile TiO₂(110)-(1 × 2) reconstructed surface is composed of two domains with different types of asymmetric rows.

Electronic properties and Schottky barriers at ZnO-metal interfaces from first principles

First principles calculations were performed to study the interface electronic structure and the Schottky barrier heights (SBHs) of ZnO-metal interfaces. Different kinds of metals were considered with different chemistries on the polar (0 0 0 1) and (0 0 0 1¯) ZnO surfaces. The projection of the density of states on the atomic orbitals of the interface atoms reveals that two kinds of interface electronic states appear: states due to the chemical bonding which appear at well defined energies and conventional metal-induced gap states associated with a smooth density of states in the bulk ZnO band gap region. The relative weight and distribution of the two classes of states depend on both the ZnO substrate termination and on the metal species. SBHs are found to be very sensitive to the specific interface chemical bonding. In particular, it is possible to note the occurrence of either Schottky barriers or Ohmic contacts. Our results have been compared with experiments and with available phenomenological theories, which estimate the SBH from few characteristic material parameters. Finally, the electronic and structural contributions to the SBH have been singled out and related to the different charge transfers occurring at the different interfaces.

Solvation effects on the band edge positions of photocatalysts from first principles

Continuum solvation theories predict large shifts in band positions of photocatalysts from vacuum to solution, in agreement with experiment.

The role of F-dopants in adsorption of gases on anatase TiO2 (001) surface: a first-principles study

Ti³⁺ induced by F-dopants plays an important role in enhancing interaction between gas molecules and TiO₂ surfaces.

Physical origin of general oscillation of structure, surface energy, and electronic property in rutile TiO2 nanoslab

Titanium oxide (TiO(2)) nanostructures have been attracting consistent focus in the past few years because of their enhanced power in solar-energy conversion. Surface and interface play a crucial role in the determination of thermodynamic stability and electronic structure of TiO(2) nanostructures. The rutile (110) nanoslab (NS) has been used as a common subject to investigate the surface relaxation, defect characters, molecule adsorption, and chemically dynamic reaction of TiO(2) nanostructures. Up to date, a long-time standing issue in TiO(2) NS, i.e., the general oscillation of structure, surface energy and electronic property with changing of NS thickness, has not been clear. We have presented a comprehensive investigation on the relationship between surface and oscillation behavior in the TiO(2) (110) NS by the first-principles calculations accompanied with the wave function analysis. We clearly, for the first time, pointed out that the dipoles and surface states bonding induced by the surface-surface interactions are the physical origin of general oscillations in the TiO(2) (110) NS. Our findings not only have a new insight into the basic interactions between surfaces in TiO(2) nanostructures, but also provide useful information for tuning the photocatalytic and photovoltaic properties by surface design.

O2 evolution on a clean partially reduced rutile TiO2(110) surface and on the same surface precovered with Au1 and Au2: the importance of spin conservation

We have used spin-polarized density functional theory (DFT) to study O(2) evolution on a clean partially reduced rutile TiO(2)(110) surface (i.e., a surface having oxygen vacancies) and its interaction with Au(1) or Au(2) cluster adsorbed on it. We assume that the total spin of the electronic wave function is related to the number of unpaired spins (N(s)) and calculate the binding and the activation energies involved in O(2) evolution for fixed values of N(s). In addition to keeping N(s) constant, we assume that reactions in which the N(s) of the reactants differs from that of the products are very slow. The potential energy surfaces obtained in this way depend strongly on N(s). For example, O(2) dissociation at the vacancy site on a clean partially reduced TiO(2)(110) surface is exothermic by 0.85 eV in the triplet state and the highest activation energy in the chain of reactions leading to the O(2) dissociation is 0.67 eV. In the singlet state, O(2) dissociation is endothermic by 0.11 eV and the activation energy leading to dissociation is 1.30 eV. These observations are in qualitative agreement with scanning tunneling microscopy experiment in which O(2) dissociation on a partially reduced rutile TiO(2)(110) surface is observed at temperature as low as 120 K. In contrast, O(2) dissociation is predicted to be endothermic and is prevented by an activation barrier larger than 1 eV in all the previous DFT calculations, in which the DFT program varies N(s) to get the lowest energy state. We find that on a partially reduced rutile TiO(2)(110) with Au(1) and Au(2) preadsorbed on its surface, O(2) dissociates at the vacancy site: One oxygen atom fills the oxygen vacancy and the other becomes available for oxidation chemistry. This means that Au(1) and Au(2) supported on a partially reduced TiO(2)(110) surface is not an oxidation catalyst since the presence of oxygen turns it into a stoichiometric Au(n)/TiO(2)(110) surface. Finally, we find that the evolution of oxygen on Au(1) and Au(2) in the gas phase is very different from the evolution on the same clusters supported on the partially reduced TiO(2)(110) surface. For example, the molecular adsorption of O(2) is favored in the gas phase (except on Au(1) (-) and Au(2) (-) in the quartet state), while the dissociative adsorption is favored by more than 1 eV when Au(1) and Au(2) are supported on the partially reduced TiO(2)(110). Furthermore, the activation energies associated with O(2) dissociation in the gas phase (DeltaE(act)>2.4 eV) are reduced by at least a factor of 2 when the clusters are supported on TiO(2)(110).

Surface protonation at the rutile (110) interface: explicit incorporation of solvation structure within the refined MUSIC model framework

The detailed solvation structure at the (110) surface of rutile (alpha-TiO2) in contact with bulk liquid water has been obtained primarily from experimentally verified classical molecular dynamics (CMD) simulations of the ab initio-optimized surface in contact with SPC/E water. The results are used to explicitly quantify H-bonding interactions, which are then used within the refined MUSIC model framework to predict surface oxygen protonation constants. Quantum mechanical molecular dynamics (QMD) simulations in the presence of freely dissociable water molecules produced H-bond distributions around deprotonated surface oxygens very similar to those obtained by CMD with nondissociable SPC/E water, thereby confirming that the less computationally intensive CMD simulations provide accurate H-bond information. Utilizing this H-bond information within the refined MUSIC model, along with manually adjusted Ti-O surface bond lengths that are nonetheless within 0.05 A of those obtained from static density functional theory (DFT) calculations and measured in X-ray reflectivity experiments (as well as bulk crystal values), give surface protonation constants that result in a calculated zero net proton charge pH value (pHznpc) at 25 degrees C that agrees quantitatively with the experimentally determined value (5.4+/-0.2) for a specific rutile powder dominated by the (110) crystal face. Moreover, the predicted pHznpc values agree to within 0.1 pH unit with those measured at all temperatures between 10 and 250 degrees C. A slightly smaller manual adjustment of the DFT-derived Ti-O surface bond lengths was sufficient to bring the predicted pHznpcvalue of the rutile (110) surface at 25 degrees C into quantitative agreement with the experimental value (4.8+/-0.3) obtained from a polished and annealed rutile (110) single crystal surface in contact with dilute sodium nitrate solutions using second harmonic generation (SHG) intensity measurements as a function of ionic strength. Additionally, the H-bond interactions between protolyzable surface oxygen groups and water were found to be stronger than those between bulk water molecules at all temperatures investigated in our CMD simulations (25, 150 and 250 degrees C). Comparison with the protonation scheme previously determined for the (110) surface of isostructural cassiterite (alpha-SnO2) reveals that the greater extent of H-bonding on the latter surface, and in particular between water and the terminal hydroxyl group (Sn-OH) results in the predicted protonation constant for that group being lower than for the bridged oxygen (Sn-O-Sn), while the reverse is true for the rutile (110) surface. These results demonstrate the importance of H-bond structure in dictating surface protonation behavior, and that explicit use of this solvation structure within the refined MUSIC model framework results in predicted surface protonation constants that are also consistent with a variety of other experimental and computational data.

Density functional study of the interaction between small Au clusters, Aun (n=1–7) and the rutile TiO2 surface. I. Adsorption on the stoichiometric surface

This is the first paper in a series of four dealing with the adsorption site, electronic structure, and chemistry of small Au clusters, Au(n) (n=1-7), supported on stoichiometric, partially reduced, or partially hydroxylated rutile TiO(2)(110) surfaces. Analysis of the electronic structure reveals that the main contribution to the binding energy is the overlap between the highest occupied molecular orbitals of Au clusters and the Kohn-Sham orbitals localized on the bridging and the in-plane oxygen of the rutile TiO(2)(110) surface. The structure of adsorbed Au(n) differs from that in the gas phase mostly because the cluster wants to maximize this orbital overlap and to increase the number of Au-O bonds. For example, the equilibrium structures of Au(5) and Au(7) are planar in the gas phase, while the adsorbed Au(5) has a distorted two-dimensional structure and the adsorbed Au(7) is three-dimensional. The dissociation of an adsorbed cluster into two adsorbed fragments is endothermic, for all clusters, by at least 0.8 eV. This does not mean that the gas-phase clusters hitting the surface with kinetic energy greater than 0.8 eV will fragment. To place enough energy in the reaction coordinate for fragmentation, the impact kinetic energy needs to be substantially higher than 0.8 eV. We have also calculated the interaction energy between all pairs of Au clusters. These interactions are small except when a Au monomer is coadsorbed with a Au(n) with odd n. In this case the interaction energy is of the order of 0.7 eV and the two clusters interact through the support even when they are fairly far apart. This happens because the adsorption of a Au(n) cluster places electrons in the states of the bottom of the conduction band and these electrons help the Au monomer to bind to the five-coordinated Ti atoms on the surface.

Theoretical Study of Adsorption of O(3P) and H2O on the Rutile TiO2(110) Surface

The adsorption of oxygen atoms O(3P) on both ideal and hydrated rutile TiO(2)(110) surfaces is investigated by periodic density functional theory (DFT) calculations within the revised Perdew-Burke-Ernzerhof (RPBE) generalized gradient approximation and a four Ti-layer slab, with (2 x 1) and (3 x 1) surface unit cells. It is shown that upon adsorption on the TiO(2) surface the spin of the O atom is completely lost, leading to stable surface peroxide species on both in-plane and bridging oxygen sites with O-binding energies of about 1.0-1.5 eV, rather than to the kinetically unstable terminal Ti-O and terminal O-O species with smaller binding energies of 0.1-0.7 eV. Changes in O-atom coverage ratios between 1/3 and 1 molecular layer (ML) and coadsorption of H(2)O have only minor effects on the O-binding energies of the stable peroxide configurations. High O-atom diffusion barriers of about 1 eV are found, suggesting a slow recombination rate of adsorbed O atoms on TiO(2)(110). Our results suggest that the TiOOTi peroxide intermediate experimentally observed in photoelectrolysis of water should be interpreted as a single spinless O adatom on TiO(2) surface rather than as two Ti-O* radicals coupled together.