Comment on “Structure and dynamics of liquid water on rutile TiO_{2}(110)” (original) (raw)
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Surface Science, 2011
Density Functional Theory (DFT), based on both static and Born-Oppenheimer Molecular Dynamics approaches, has been used to investigate the effect of hydrogen bonds and temperature on the water monolayer adsorption on the rutile TiO 2 (110) face. It was demonstrated that the difference between some previous theoretical results and experimental data is due to too slim slab thickness model and/or too small surface area. According to the present static calculations, water monolayer adsorbs molecularly on the fivefold titanium atoms of an optimised five-layer slab thickness, due to the stabilising lateral hydrogen bonds between molecules. From the molecular dynamics simulations, two adsorption mechanisms were described as a function of temperature. Finally, it was pointed out that the dynamics of water adsorption is strongly influenced by the structural model used. When temperature increases, the monolayer dissociates gradually. However, because of the periodic boundary conditions, the 1 × 1 surface unit needs to be extended to at least 2 × 5 to get an accurate representation of the monolayer dissociation ratio. In these conditions, this ratio is around 20%, 25% and 33% at 270, 350 and 425 K, respectively.
The Journal of Physical Chemistry B, 2004
A recently developed force field for interactions of water molecules with the (110) surface of rutile (R-TiO 2 ) has been generalized for atomistically detailed molecular dynamics simulations of the interfacial structure of the uncharged mineral surface in contact with liquid SPC/E water at 298 K and 1 atm and for negatively charged surfaces in contact with SPC/E water containing dissolved electrolyte ions (Rb + , Sr 2+ , Zn 2+ , Na + , Ca 2+ , Cl -). Both hydroxylated (dissociative) and nonhydroxylated (associative) surfaces are simulated, since both types of water-surface interactions have been postulated from ab initio calculations and spectroscopic studies under near-vacuum conditions. The positions of water molecules at the interface were found to be very similar for both hydroxylated and nonhydroxylated surfaces, with either terminal hydroxyl groups or associated water molecules occupying the site above each terminal titanium atom. Beyond these surface oxygens, a single additional layer of adsorbed water molecules occupies distinct sites related to the underlying crystal surface structure. The water structure and mobility quickly decay to the bulk liquid properties beyond this second layer. The hydrogen-bonding structure and water orientation in these first two oxygen layers are somewhat sensitive to the hydroxylation of the surface, as are the electrostatic profiles. For all simulated properties, including space-dependent diffusivity of water molecules, the influence of the interface is negligible beyond distances of about 15 Å from the surface. Increasing the temperature to 448 K while maintaining the density at the liquid-vapor saturated condition had minimal effect on the interfacial structure and electrostatic properties. These results are foundational to the simulation of dissolved ion interactions with the surface and the comparison of the simulation results with X-ray standing wave and crystal truncation rod measurements of water and electrolyte solutions in contact with rutile (110) single-crystal surfaces presented in Part 2 of this series.
Surface Science, 2007
The rutile (1 1 0)-aqueous solution interface structure was measured in deionized water (DIW) and 1 molal (m) RbCl + RbOH solution (pH 12) at 25°C with the X-ray crystal truncation rod method. The rutile surface in both solutions consists of a stoichiometric (1 • 1) surface unit mesh with the surface terminated by bridging oxygen (BO) and terminal oxygen (TO) sites, with a mixture of water molecules and hydroxyl groups (OH À) occupying the TO sites. An additional hydration layer is observed above the TO site, with three distinct water adsorption sites each having well-defined vertical and lateral locations. Rb + specifically adsorbs at the tetradentate site between the TO and BO sites, replacing one of the adsorbed water molecules at the interface. There is no further ordered water structure observed above the hydration layer. Structural displacements of atoms at the oxide surface are sensitive to the solution composition. Ti atom displacements from their bulk lattice positions, as large as 0.05 Å at the rutile (1 1 0)-DIW interface, decay in magnitude into the crystal with significant relaxations that are observable down to the fourth Ti-layer below the surface. A systematic outward shift was observed for Ti atom locations below the BO rows, while a systematic inward displacement was found for Ti atoms below the TO rows. The Ti displacements were mostly reduced in contact with the RbCl solution at pH 12, with no statistically significant relaxations in the fourth layer Ti atoms. The distance between the surface 5-fold Ti atoms and the oxygen atoms of the TO site is 2.13 ± 0.03 Å in DIW and 2.05 ± 0.03 Å in the Rb + solution, suggesting molecular adsorption of water at the TO site to the rutile (1 1 0) surface in DIW, while at pH 12, adsorption at the TO site is primarily in the form of an adsorbed hydroxyl group.
Surface Science, 2007
XPS and periodic DFT calculations have been used to investigate water sorption on the TiO 2 rutile (110) face. Two sets of XPS spectra were collected on the TiO 2 (110) single crystal 2 clean and previously exposed to water: the first set with photoelectrons collected in a direction parallel to the normal to the surface; and the second set with the sample tilted by 70°, respectively. This tilting procedure promotes the signals from surface species and reveals that the first hydration layer is strongly coordinated to the surface and also that, despite the fact that the spectra were recorded under ultra-high vacuum, water molecules subsist in upper hydration layers. In addition, periodic DFT calculations were performed to investigate the water adsorption process to determine if molecular and/or dissociative adsorption takes place.
physica status solidi (b), 2017
The accuracy of the theoretical description of materials properties in the framework of density functional theory (DFT) inherently depends on the exchange-correlation (XC) functional used in the calculations. Here we investigate the influence of the choice of a XC functional (PBE, RPBE, PW91, and PBE0) on the kinetics of the adsorption, diffusion and dissociation of water on the rutile TiO 2 (110) surface using a combined Kinetic Monte Carlo (KMC)-DFT approach, where the KMC simulations are based on the barriers for the aforementioned processes calculated with DFT. We also test how the adsorption energy of intact and dissociated water molecules changes when dispersion interactions are included into the calculations. We consider the beginning of the water layer formation varying coverage up to 0.2 monolayer (ML) at temperatures up to 180 K. We demonstrate that the dynamics of the simulated water-titania system is extremely sensitive to the choice of the XC functional.
Langmuir, 2008
The detailed solvation structure at the (110) surface of rutile (R-TiO 2 ) 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 Å 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 (pH znpc ) at 25°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 pH znpc values agree to within 0.1 pH unit with those measured at all temperatures between 10 and 250°C. A slightly smaller manual adjustment of the DFTderived Ti-O surface bond lengths was sufficient to bring the predicted pH znpc value of the rutile (110) surface at 25°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°C). Comparison with the protonation scheme previously determined for the (110) surface of isostructural cassiterite (R-SnO 2 ) 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.
Journal of the American Chemical Society, 2005
Recent combined experimental and theoretical studies (Beck et al., Phys. Rev. Lett. 2004, 93, 036104) have provided evidence for TidO double-bonded titanyl groups on the reconstructed rutile TiO2-(011)-(2×1) surface. The adsorption of water on the same surface is now investigated to further probe the properties of these groups, as well as to confirm their existence. Ultraviolet photoemission experiments show that water is adsorbed in molecular form at a sample temperature of 110 K. At the same time, the presence of a 3σ state in the photoemission spectra and work function measurements indicate a significant amount of hydroxyls within the first monolayer of water. At room temperature, scanning tunneling microscopy (STM) suggests that dissociated water is present, and about 30 of the surface active sites are hydroxylated. These findings are well explained by total energy density functional theory calculations and Car-Parrinello molecular dynamics simulations for water adsorption on the titanyl model of TiO 2(011)-(2×1). The theoretical results show that a mixed molecular/dissociative layer is the most stable configuration in the monolayer regime at low temperatures, while complete dissociation takes place at 250 K. The arrangement of the protonated mono-coordinated oxygens in the mixed molecular/dissociated layer is consistent with the observed short-range order of the hydroxyls in the STM images.
Ion Adsorption at the Rutile−Water Interface: Linking Molecular and Macroscopic Properties
Langmuir, 2004
A comprehensive picture of the interface between aqueous solutions and the (110) surface of rutile (R-TiO2) is being developed by combining molecular-scale and macroscopic approaches, including experimental measurements, quantum calculations, molecular simulations, and Gouy-Chapman-Stern models. In situ X-ray reflectivity and X-ray standing-wave measurements are used to define the atomic arrangement of adsorbed ions, the coordination of interfacial water molecules, and substrate surface termination and structure. Ab initio calculations and molecular dynamics simulations, validated through direct comparison with the X-ray results, are used to predict ion distributions not measured experimentally. Potentiometric titration and ion adsorption results for rutile powders having predominant (110) surface expression provide macroscopic constraints of electrical double layer (EDL) properties (e.g., proton release) which are evaluated by comparison with a three-layer EDL model including surface oxygen proton affinities calculated using ab initio bond lengths and partial charges. These results allow a direct correlation of the three-dimensional, crystallographically controlled arrangements of various species (H2O, Na + , Rb + , Ca 2+ , Sr 2+ , Zn 2+ , Y 3+ , Nd 3+ ) with macroscopic observables (H + release, metal uptake, zeta potential) and thermodynamic/electrostatic constraints. All cations are found to be adsorbed as "inner sphere" species bonded directly to surface oxygen atoms, while the specific binding geometries and reaction stoichiometries are dependent on ionic radius. Ternary surface complexes of sorbed cations with electrolyte anions are not observed. Finally, surface oxygen proton affinities computed using the MUSIC model are improved by incorporation of ab initio bond lengths and hydrogen bonding information derived from MD simulations. This multitechnique and multiscale approach demonstrates the compatibility of bond-valence models of surface oxygen proton affinities and Stern-based models of the EDL structure, with the actual molecular interfacial distributions observed experimentally, revealing new insight into EDL properties including specific binding sites and hydration states of sorbed ions, interfacial solvent properties (structure, diffusivity, dielectric constant), surface protonation and hydrolysis, and the effect of solution ionic strength.
In the light of recent intensity-voltage low energy electron diffraction LEED-IV experiments Surf. Sci. 316, 92 1994; Surf. Rev. Lett. 10, 487 2003, the electronic and geometric structure of a water bilayer adsorbed at the Ru0001 surface are investigated through first-principles total energy calculations, using periodic slab geometries and gradient-corrected density functional theory DFT. We consider five possible bilayer structures, all roughly consistent with the LEED-IV analysis three intact structures and two half-dissociated, and a water single layer at Ru0001. Adsorption energies and substrate-adsorbate geometry parameters are given and discussed in the light of the experiments. We also give a comparative analysis of the electron density redistribution and of the dipole moment change induced by water adsorption on the Ru0001 surface. In agreement with Feibelman Science 295, 99 2002, the half-dissociated structures are found to be more stable than the intact ones, and their adsorption geometries in better agreement with the LEED-IV data. However, the analysis shows that a half-dissociated structure induces a 0, which would be incompatible with the experimentally measured decrease of the work function following bilayer adsorption; the latter would be consistent, instead, with the 0 induced by the intact structures. It is the aim of this paper to compare various possible adsorption structures, most of them already considered previously , with one and the same method. For this purpose, thick slabs and restrictive computational parameters are chosen to generally address the accuracy and the limits of DFT in reproducing adsorption energies and bond lengths of water-metal interacting systems.
Physical Review B, 2009
We present evidence for mixed molecular and dissociative water adsorption at monolayer coverage on a rutile TiO 2 ͑110͒ surface free from oxygen vacancies using synchrotron radiation photoemission. At monolayer coverage the OH: H 2 O ratio is close to 0.5 and reducing the coverage by heating yields an increased OH: H 2 O ratio. At room temperature neither species originating from the monolayer on the defect-free surface can be detected. The OH species of the monolayer hence recombines and leaves the surface at much lower temperatures than OH formed by water dissociation on oxygen vacancies.