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Ab initio simulations of the structure of thin water layers on defective anatase TiO 2 ( 101 ) surfaces
| Content Provider | Semantic Scholar |
|---|---|
| Author | Aschauer, Ulrich Tilocca, Antonio Selloni, Annabella |
| Copyright Year | 2016 |
| Abstract | Titania-water interfaces are important in various fields of science, from geophysics to photocatalysis and biochemistry. Here we use ab initio molecular dynamics simulations to investigate the structure of thin water overlayers on the (101) surface of TiO2 anatase in the presence of oxidizing defects. For comparison, results of our previous studies of water layers on defect-free and reduced anatase (101) are also reviewed. On the stoichiometric defect-free surface ordered structures are formed at one and two monolayer coverage, and the order in the first bilayer is largely maintained when a third water layer is adsorbed. By contrast, the vertical and in-plane ordering of the water layers is strongly perturbed in the presence of both oxidizing and reducing defects. As a result, the structure of the water layer is much more diffuse, and frequent exchanges of water molecules between different layers are observed. s o u r c e : h t t p s : / / d o i . o r g / 1 0 . 7 8 9 2 / b o r i s . 7 2 9 8 8 | d o w n l o a d e d : 1 3 . 3 . 2 0 1 7 1Introduction The interaction of water with titania surfaces has attracted broad interest and stimulated intensive studies for decades. In particular, interest in the structure of adsorbed water layers on TiO2 is motivated by their relevance to essentially all applications of this material, ranging from photocatalysis and biocompatible devices to more traditional applications like pigments and coatings. TiO2 has two main crystalline phases, rutile, which is the most stable polymorph of TiO2, and anatase, which is metastable but is predominant in nano-sized crystals. Anatase is also considered the TiO2 polymorph most efficient in photocatalysis, which provides further motivation for the study of water adsorption on its surfaces. Anatase (101) is the most abundant surface of anatase. It is strongly corrugated, with ridges of twofold coordinated bridging oxygen ions (O2c) along the [010] direction (Figure 1a). It was shown theoretically, and later confirmed by experiment, that water adsorbs in molecular form on this surface, with the water oxygen (Ow) binding to the unsaturated five-fold coordinated Ti cations (Ti5c), while the water hydrogen atoms (Hw) form H-bonds with two O2c atoms on the next ridge (Figure 1b). It was also theoretically predicted that surface oxygen vacancies (VO’s) on anatase (101) would lead to water dissociation, as observed in many studies of the rutile TiO2(110) surface. Unlike rutile (110), however, anatase (101) was experimentally found to have a very low concentration of surface VO’s and in subsequent studies the predominance of subsurface oxygen vacancies was clearly established. The influence of subsurface VO’s and Ti interstitials (another typical TiO2 defect) on water adsorption in the dilute limit was investigated by Aschauer et al.. The calculations showed a preference for water to adsorb in the vicinity of the subsurface defects, as well as a stronger tendency for water to dissociate on the defected surface than on the defect-free one. For higher water coverages, the structure and reactivity of thin water layers on defect-free and defective anatase (101) surfaces were investigated by Tilocca and Selloni using ab initio molecular dynamics (AIMD) simulations. Compared to the defect-free surface, subsurface defects were found to enhance the surface reactivity and to lead to a more disordered structure of the first water layers adsorbed on the reduced surface. In particular, no water dissociation took place in rather long simulations for a water monolayer, bilayer and trilayer on the defect-free surface, whereas dissociated water was observed on the defected surface. While oxygen vacancies and Ti interstitials (Tiint’s) are almost invariably present in TiO2, causing the material to be reduced and conductive (both VO’s and Tiint’s are electron donors), recently oxidizing defects with the character of “bridging oxygen dimers” have been reported to exist on the anatase (101) surface. It was also suggested that these defects – denoted (O2)O’s − play an important role in water oxidation, which makes their study particularly interesting. The main purpose of this work is thus to investigate how the oxidizing (O2)O defects affect the structure of adsorbed water layers on the anatase (101) surface by AIMD simulations analogous to those we previously reported for the stoichiometric and reduced surfaces. Another aim is to compare the present results for oxidized (i.e. electron-poor) anatase (101) to our previous studies for the stoichiometric and reduced (i.e. electron-rich) surfaces in order to explore similarities and differences between different types of defects as well as possible correlations between the surface electronic structure and the structure of adsorbed water. To make the present work more self-contained, a review of our previous studies on the stoichiometric and reduced anatase surfaces is included in the following. 2Computational methods All calculations of this work were performed using Density Functional Theory within the Generalized Gradient Approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) approximation as implemented in the Quantum ESPRESSO package. The adequacy of this approximation for discussing the structure of adsorbed water on the reduced anatase (101) surface has been tested and discussed in detail in Ref. . It was shown there that the DFT+U and DFT-GGA descriptions of the structural and dynamical features of a water monolayer on the reduced surface are sufficiently close to justify the use of the standard DFT-GGA approach to investigate the structure of thin water films. Electron-ion interactions were described using ultrasoft pseudopotentials, with Ti(3s, 3p, 3d, 4s), O(2s, 2p) and H(1s) electrons treated explicitly as valence electrons. Wavefunctions were expanded in plane waves up to a kinetic energy cutoff of 25 Ry, while a cutoff of 200 Ry was used for the augmented density. The anatase (101) surface was modeled as a periodically repeated slab of three TiO2 layers, with vacuum regions of 10 Å separating consecutive water-covered slabs along the surface normal. Water molecules were adsorbed on one side of the slab only. Different water coverages were considered, from submonolayer up to three monolayers, where one monolayer (ML) corresponds to one molecule per Ti5c site. For static calculations we used a 1×3 surface supercell of dimensions 10.262 Å x 11.310 Å exposing six Ti5c sites, while for molecular dynamics a 1×4 surface supercell of dimensions 10.262 Å x 15.080 Å exposing eight Ti5c sites was considered. The atoms at the bottom of the slab were kept fixed in all cases. Reciprocal space sampling was restricted to the Γ-point. Structural relaxations were carried out until forces converged below 0.05 eV/Å and reaction barriers were determined using the climbing-image nudged-elastic band (NEB) method. Molecular dynamics calculations were carried out using the Car-Parrinello method with a timestep of 5 a.u., a fictitious electron mass of 500 a.u. and deuterium masses for the hydrogen atoms. The temperature was kept at 160 K using a Nosé-Hoover thermostat with an oscillation frequency of 20 THz. All configurations have been equilibrated for 1.5 ps prior to recoding data for 20 ps. 3Review of previous results 3.1 Thin water layers on stoichiometric anatase (101) The structures of one (ML), two (BL), and three (TL) water layers on the defect-free surface were investigated by AIMD simulations at T = 160 K, the temperature at which desorption of adsorbed water is observed to start in temperature programmed desorption (TPD) experiments. Snapshots from these simulations (Figure 2a) suggest the presence of a partially layered structure. A more detailed analysis of this structure is reported in Figure 3a, which shows the time evolution of the vertical distances of the water molecules from the surface along with their probability distribution. We can see that the structures of the ML and BL are characterized by all molecules being at well-defined distances from the surface and corresponding sharp peaks in the distance distributions at Δz = 2 and 3 Å. The left and middle panels in Figure 2a show that water molecules in the first layer (Δz = 2Å) are coordinated to Ti5c, whereas the second layer (Δz = 3Å) is formed by strongly oriented molecules, all arranged with one hydrogen pointing upward and the other toward the surface and forming a strong H-bond with an O2c . The vertical ordering of the first two layers in contact with the surface is maintained also in the TL, but the presence of the additional water molecules introduces some disorder. The peak corresponding to the third layer in the height distribution is at Δz ~ 4.5 Å but has a tail extending up to 6 Å, suggesting a higher mobility and a more disordered arrangement within the top layer. The pair correlation functions are shown in Figure 4a, where the upper and lower panels refer to the water-water and water-surface interactions, respectively. In the upper panel, we can see that Ti5c-coordinated water molecules in the ML do not significantly interact with each other, due to the large Ti5c−Ti5c distances. Clear signatures of a H-bonded system (intermolecular Ow−Hw and Ow−Ow peaks at 1.7 and 2.7 Å, respectively) emerge for the BL, and are maintained for the TL, with a generally more disordered arrangement evident from the broader peaks. In the lower panel, the persistent Ti−Ow peak around 2.3 Å indicates that water molecules remain strongly coordinated to Ti5c at the different coverages. Water molecules in the ML are anchored to the Ti5c sites and can only form weak H-bonds with the surface O2c. On the other hand, the BL and TL also include molecules that are not coordinated to Ti5c and are thus able to form stronger Hbonds with the surface. In particular, water−surface H-bonds are strongest in the BL due to the contribution from strongly oriented molecules pointing a hydrogen downw |
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| Alternate Webpage(s) | https://core.ac.uk/download/pdf/33089859.pdf |
| Alternate Webpage(s) | https://boris.unibe.ch/72988/8/IJQC_final.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
