This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. Article Cite This: J. Phys. Chem. C 2019, 123, 13545−13550 pubs.acs.org/JPCC Water-Induced Reversal of the TiO2(011)-(2 × 1) Surface Reconstruction: Observed with in Situ Surface X‑ray Diffraction Hadeel Hussain,†,‡ Mahmoud H. M. Ahmed,†,‡ Xavier Torrelles,§ David C. Grinter,∥ Gregory Cabailh,⊥ Oier Bikondoa,#,¶ Christopher Nicklin,∇ Ulrich Aschauer,○ Robert Lindsay,†,‡ and Geoff Thornton*,∥ † Corrosion and Protection Centre, School of Materials, The University of Manchester, Sackville Street, M13 9PL Manchester, U.K. ‡ Photon Science Institute, The University of Manchester, M13 9PL Manchester, U.K. § Institut de Ciència de Materials de Barcelona (CSIC), Campus UAB, 08193 Bellaterra, Spain ∥ London Centre for Nanotechnology and Department of Chemistry, University College London, 20 Gordon Street, WC1H OAJ London, U.K. ⊥ Sorbonne Université, UMR CNRS 7588, Institut des NanoSciences de Paris, 4 place Jussieu, 75252 Paris Cedex 05, France # XMaS, The UK-CRG Beamline. ESRF, The European Synchrotron, 71, Avenue des Martyrs, CS40220, F-38043 Grenoble cedex 09, France ¶ Department of Physics, University of Warwick, Gibbet Hill Road, CV4 7AL Coventry, U.K. ∇ Diamond Light Source Limited, Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, U.K. ○ Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland * S Supporting Information ABSTRACT: The (011) termination of rutile TiO2 is reported to be particularly effective for photocatalysis. Here, the structure of the interface formed between this substrate and water is revealed using surface X-ray diffraction. While the TiO2(011) surface exhibits a (2 × 1) reconstruction in ultra-high vacuum (UHV), this is lifted in the presence of a multilayer of water at room temperature. This change is driven by the formation of Ti−OH at the interface, which has a bond distance of 1.93 ± 0.02 Å. The experimental solution is in good agreement with density functional theory and first-principles molecular dynamics calculations. These results point to the important differences that can arise between the structure of oxide surfaces in UHV and technical environments and will ultimately lead to an atomistic understanding of the photocatalytic process of water splitting on TiO2 surfaces. ■ INTRODUCTION Understanding the interaction of water on solid surfaces is of TiO2(011) surface. We have previously determined the quantitative structure of the clean substrate, which exhibits a both fundamental and practical interest. Because many (2 × 1) reconstruction, in ultra-high vacuum (UHV) by means materials oxidize in ambient conditions, metal oxide studies of surface X-ray diffraction (SXRD),11 revealing a saw-tooth- are particularly pertinent for applications. Titanium dioxide, in like morphology with fivefold coordinated titanium atoms and particular the rutile (110) surface termination, has emerged as twofold coordinated surface oxygen atoms arranged in rows the prototypical metal oxide surface for fundamental studies.1 along the [011̅] direction (Figure 1a). This structure has since After considerable effort, water adsorption on reduced rutile been corroborated by other quantitative structural studies,12,13 (110) surfaces is reasonably well understood: water dissociates as well as density functional theory (DFT) calculations.14 at oxygen vacancies as well as ⟨111⟩ oriented steps at room As for water adsorption on TiO2(011)-(2 × 1), this has been temperature.1−5 Terminal hydroxyls are reported to form at investigated with ex situ scanning tunneling microscopy the interface with liquid water without reconstruction of the (STM). The results show that quasi-one-dimensional clusters substrate,6 although a recent paper has reported the role of populate the surface after exposure to 10−3 Torr of water at carboxylates in the contact layer.7 Whether water dissociates room temperature.15 Repeated scans led to partial removal of on the oxidized, stoichiometric surface remains a contentious issue.8−10 Received: May 8, 2019 In this letter, we focus on the interface formed between Revised: May 12, 2019 water and the reportedly more photocatalytically active rutile- Published: May 13, 2019 © 2019 American Chemical Society 13545 DOI: 10.1021/acs.jpcc.9b04383 J. Phys. Chem. C 2019, 123, 13545−13550 The Journal of Physical Chemistry C Article for the collection of SXRD data. The chamber was mounted to I07’s diffractometer, where data were collected using a 2D Pilatus photon detector. Measurements were carried out at room temperature with an incidence angle of 1° and a photon energy of 17.7 keV (=0.7 Å). The sample was mounted such that the surface was in the horizontal plane. A systematic series of X-ray reflections were acquired in a stationary geometry (l- scans), that is, a full profile, including the background, is measured without scanning. For selected (h, k), the intensity profile at regular intervals of l-values is measured. The spot intensity distribution on the 2D detector of the measured reflection is then integrated and corrected,20 enabling profiles of scattered intensity versus perpendicular momentum transfer for both crystal truncation rods (CTRs) and fractional order rods (FORs). A reference reflection, (−1, 0, 0.97), was recorded for both the clean and water covered surfaces at regular intervals to monitor surface degradation. No significant changes were observed throughout the duration of the experiment. A sizeable dataset was taken for the UHV prepared sample [TiO2(011)-(2 × 1)], comprising 10 419 reflections from 21 Figure 1. Ball and stick models of (a) the optimum TiO2(011)-(2 × CTRs and 23 FORs that after averaging, using pg symmetry, 1) geometry and (b) the ideal bulk terminated TiO2(011) structure. were reduced to 5991 non-equivalent reflections. Subsequent Large light blue (small red) spheres are oxygen (titanium) atoms. The to this, the chamber pump was switched off and the sample numerical labeling of the atoms is employed in Tables 1 and S1 for was exposed to a static pressure of ∼30 mbar of ultra-pure the identification purposes. Symmetry paired atoms are denoted by an H2O via back filling of the chamber, which equates to about asterisk (*). twelve monolayers.21 The chamber pump remained switched off for the duration of measurements. A further 4978 these clusters, revealing a modified morphology. A first- reflections from 22 CTRs were measured from this surface principles molecular dynamics (FPMD) investigation16 that reduced to 3507 non-equivalent reflections after averaging provides some insight as to what the structure may be. It by the same symmetry. Water was thoroughly degassed using suggests that there is a significant rearrangement of surface several freeze−pump−thaw cycles, with cleanliness confirmed atoms upon exposure to H2O, whereby the (2 × 1) by mass spectrometry (MKS Microvision Plus). Here, the reconstruction reverts to a (1 × 1) termination. It was also partial pressure of water was increased to ∼10 mbar in the concluded that water dissociates at the interface to form a baby chamber and, through a precision leak valve, the mixed molecular and dissociated overlayer (on average 3:1 differentially pumped mass spectrometer was exposed to a H2O/OH) at saturation coverage, with adsorption at surface maximum of ∼1 × 10−6 mbar H2O. Several masses were Ti sites (Ti5c). More recently, a room-temperature STM recorded simultaneously (O = 16, H2O = 18, CO = 28, O2 = experiment investigated the effect of dipping TiO2(011) into 32, CO2 = 44, CH2O2 = 29, 46 and CH3COOH = 43, 45, 60). liquid water.17 This ex situ experiment identified a c(2 × 1) While there was an increase of CO and CO2 to ∼1 × 10−8 half monolayer overlayer of OH by comparison with DFT mbar and ∼5 × 10−9 mbar, respectively, there was no calculations, with removal of the underlying (2 × 1) indication of formic or acetic acid evolution. reconstruction. Here, we employ SXRD to provide the first The measured data were indexed with reference to a quantitative structure determination of the TiO2(011)/water monoclinic unit cell defined by lattice vectors (a1, a2, a3). interface, performing in situ data acquisition at room Lattice vectors a1 and a2 are directed along the [1̅00] and temperature with an approximate H2O coverage of 12 [011̅] directions, respectively, and a3 lies in the plane defined monolayers. by the [011̅] and [011] directions at an angle of θ = 2 × ■ EXPERIMENTAL AND THEORETICAL METHODS Experimental work was performed at the Diamond Light [tan−1(a/c)] with respect to a2. The magnitudes of these lattice vectors are a1 = 2 × a and a 2 = a3 = (a 2 + c 2) , where a = Source, employing the Surface Village’s off-line UHV chamber 4.593 Å and c = 2.958 Å are the lattice constants of the for sample preparation and beam line I07 for SXRD tetragonal rutile TiO2 bulk unit cell.22 Such a unit cell, rather measurements.18 In situ preparation of rutile TiO2(011) than an orthorhombic unit cell with a1, a2, and a3 (all involved repeated cycles of Ar+ bombardment and annealing orthogonal to one another), was employed for structure in vacuum to ∼1150 K. A clean, well-ordered single-phase (2 × determination in order to minimize the number of parameters 1) termination was achieved, in agreement with previous varied during refinement. Corresponding reciprocal lattice studies.11,13,19 This was determined using low energy electron vectors are denoted by h, k, and l. The best-fit analysis of the diffraction, X-ray photoelectron spectroscopy, and STM. SXRD data recorded from TiO2(011)-(2 × 1) in UHV is Once prepared, the sample was transferred under vacuum shown in Figure S1. There is generally good agreement using a custom-built vacuum suitcase and UHV baby chamber between the current optimum structure and that emerging combination (base pressure ∼1.5 × 10−9 mbar) incorporating from our previous study of this surface,11 as demonstrated in a dome-shaped X-ray-transparent beryllium window suitable Table S1. 13546 DOI: 10.1021/acs.jpcc.9b04383 J. Phys. Chem. C 2019, 123, 13545−13550 The Journal of Physical Chemistry C Article For surface structure elucidation, the usual approach was where the asymmetrical line shape is no longer present after adopted whereby theoretical SXRD data for a potential exposure to H2O, suggesting more bulk-like relaxation of the structure are generated. The structure is then iteratively surface layers. Furthermore, the FOR intensity, which was refined to determine the best fit between experiment and observed for TiO2(011)-(2 × 1), is not present for theory. A modified version of the ROD software23 was used for TiO2(011)/H2O. This can be construed as a complete absence this task, in which the goodness-of-fit is measured by reduced of the substrate reconstruction (i.e., that it is lifted in the χ2.24 presence of water) or that the domain size has reduced Our DFT calculations were performed using the Quantum sufficiently to render the intensity of the FORs to background ESPRESSO package.25 Main results were obtained using the levels. To rigorously test for the presence of (2 × 1) Perdew−Burke−Ernzerhof (PBE) gradient corrected func- reconstructed domains, refinement of atomic coordinates tional,26 while additional calculations were performed using the along with various nonstructural parameters of a model similar DFT + U methodology27 using a U value of 3.5 eV, which is in to the optimized TiO2(011)-(2 × 1) structure was undertaken. the range of values typically used for TiO2.28−30 Wavefunctions By including an occupancy parameter to quantify the surface were expanded in planewaves up to a kinetic energy of 25 Ry coverage of the (2 × 1) reconstruction, the model can together with a cutoff of 200 Ry for the augmented density. We determine if there is a “mixed” (2 × 1) reconstruction, in used ultrasoft31 pseudopotentials with Ti(3s, 3p, 3d, 4s), O(2s, which both (1 × 1) and (2 × 1) domains exist or that there is a 2p), and H(1s) shells treated as valence electrons and modeled complete removal of the (2 × 1) termination. It was found the unreconstructed rutile (011) surface using a 9.213 Å × during the fitting procedure that the occupancy of the (2 × 1) 10.922 Å (2 × 2) surface supercell using the gamma point only domains tended to a value close to zero, that is, the χ2 reduces for reciprocal space integration. Our slab model contained 4 as the surface coverage of the (2 × 1) reconstruction reduces. TiO2 tri-layers for a total thickness of 9.07 Å and a vacuum gap This indicates that the presence of water induces more than of 10.96 Å. For all calculations, the atoms in the bottom-most just relaxations of substrate atoms at the interface and points to trilayer were kept fixed at their bulk positions. All other a model in which the substrate exhibits a bulk-like (1 × 1) coordinates were relaxed until forces converged below 0.05 termination. This is consistent with both theoretical (DFT) eV/Å. Water/OH was adsorbed at monolayer coverage on all and experimental (STM) studies that have suggested a phase under-coordinated Ti5c sites on the surface. transition from (2 × 1) to (1 × 1) in the presence of water.16,17 ■ RESULTS AND DISCUSSION A comparison between three experimental CTRs for the clean These studies concluded that the surface is decorated with OH/H2O tightly bound at all under-coordinated Ti5c sites in the presence of an aqueous environment,16 while the surface surface and water covered interface of TiO2(011) is shown in adopts a configuration in which every second Ti5c row carries Figure 2. The black and blue error bars represent the an OH when imaged in vacuum after exposure to liquid experimental data for the TiO 2 (011)-(2 × 1) and water.17 TiO2(011)/H2O surfaces, respectively. For ease of compar- First, we consider the structural model resulting from DFT + ison, the CTRs for both data sets have been indexed with U calculations in this work (see Table S2) for hydroxyls bound reference to the (1 × 1) bulk unit cell. Clear qualitative to every Ti5c site. The best χ2 achieved (4.10) suggests poor differences can be seen, particularly in the anti-Bragg regions of agreement between experiment and this structure. This was the [1,1,l] and [−4,0,l] CTRs (as indicated with red lines), found to be due to the relatively large atomic relaxations of Ti(1) and Ti(2) induced by the adsorption of OH forming “tight” bonds with Ti5c sites, when a monolayer coverage is assumed. When considering a model in which alternate Ti5c sites are occupied17 where the large displacements of surface titanium atoms are suppressed (Figure 3), a χ2 of 0.83 is Figure 3. Ball and stick model of the local OH geometry for the optimized TiO2(011)/H2O substrate, indicating values for the bond angle and distance from best-fit structural parameters derived from SXRD. The atom color scheme and labels are the same as Figure 1. The large black atoms are oxygen atoms associated with OH. achieved, which is indicative of excellent agreement between the experimental and calculated data (Figure S2). A total of 93 Figure 2. Comparison of experimental CTRs for TiO2(011)-(2 × 1) parameters were optimized during the structure refinement (bottom curve, black error bars) and TiO2(011)/H2O (top curve, process, incorporating 78 atomic displacements, 10 Debye blue error bars), respectively. Both are indexed using the same (1 × 1) Waller (DW) factors, a scale factor, a surface roughness unit cell for ease of comparison. For data analysis, the TiO2(011)-(2 parameter, a surface fraction parameter, and 2 occupancy × 1) dataset was re-indexed as (2*H,K). CTRs are offset for clarity. parametersone for the (2 × 1) reconstructed domain and Red lines on X-axis indicate the anti-Bragg regions. one for the adsorbate. The surface fraction parameter 13547 DOI: 10.1021/acs.jpcc.9b04383 J. Phys. Chem. C 2019, 123, 13545−13550 The Journal of Physical Chemistry C Article confirmed that the entire surface adopts the structure obtained coverage and structure of the surface could be sensitive to the in this analysis. The optimum roughness parameter was β = pH of the aqueous phase, which affects the H coverage of the 0.15, increasing from β = 0.09 for the TiO2(011)-(2 × 1) surface. structure. This is believed to be due to a combination of the A question arises regarding the interaction between water change in the substrate termination and a manifestation of the molecules and the 0.5 ML of substrate not bound to OH. dynamic nature of the water overlayer.16,32 It should be noted Insight can be garnered from recent ab initio molecular that the presence of ordered water molecules in contact with dynamics simulations on the adsorption of liquid water on the the substrate or above the OH contact layer was tested for (110) surface of titania, where a similar (2 × 1)-OH overlayer during data analysis. However, unlike the (110) surface of is formed.6 Under aqueous conditions, the overlayer was found titania,6,33 the occupancies of such molecules always tended to be stabilized by diffusion of water molecules between the toward zero, suggesting that water layers are completely contact and multilayer such that the average occupation of Ti5c disordered. sites adjacent to the adsorbed OH molecules is 0.18 ML over Based on this best-fit model, adsorbate (O(1′)) molecules the total length of the simulation. It is likely that the same preferentially adsorb to surface Ti5c sites similar to that derived adsorption/desorption process of the water molecules occurs from STM and DFT + U calculations.16,17 Removal of O(1′) on the (011) termination. Although partial occupation of water from our model increases the χ2 to 1.1 (i.e., Figure 1b), on the substrate is not observed in the (011) and (110) SXRD indicating that the quality of the fit is sensitive to the presence data, this is expected, given the large associated DW factors of this adsorbate. As mentioned above, there are two and likely relatively low occupancy. unresolved questions in the literature regarding this Table 1 lists the atomic positions, expressed as displace- adsorbate-decorated surface in an aqueous environment: (i) ments from the bulk-terminated TiO2(011)-(1 × 1) surface, of whether adsorption is mixed or fully dissociated and (ii) if the surface coverage is full or half monolayer. With regard to issue Table 1. Optimized Atomic Displacements of the (i), the chemical identity of O(1′) and thus confirmation of TiO2(011)/H2O and TiO2(011)-(2 × 1) Structures our best-fit model can be distinguished with SXRD by the Resulting from Analysis of SXRD Data length of the Ti−O(1′) bond. displacements (Å) Figure 3 depicts the local adsorption geometry of atom O(1′), indicating values for the bond angle and distance from atom a Δx [H2O: UHV] Δy [H2O: UHV] Δz [H2O: UHV] best-fit structural parameters derived from SXRD. A Ti−O(1′) O(1′) N/A N/A N/A bond distance of 1.93 ± 0.02 Å is obtained in the current O(1) 0.05 ± 0.02 0.02 ± 0.03 0.16 ± 0.02 study, which agrees well with the DFT-calculated bond length −2.48 ± 0.02 2.91 ± 0.03 0.59 ± 0.03 at the PBE level of theory (1.87 Å) and the Ti−OH bond Ti(1) 0.03 ± 0.01 0.00 ± 0.01 0.01 ± 0.01 length (1.95 ± 0.03 Å) found in a recent investigation of the −0.56 ± 0.01 0.32 ± 0.01 0.42 ± 0.01 interface formed between anatase TiO2 (101) and H2O34 as O(2) −0.01 ± 0.02 −0.15 ± 0.02 0.15 ± 0.02 well as the TiO2(110)/water interface.6 PBE DFT calculations −0.12 ± 0.02 −0.31 ± 0.02 −0.06 ± 0.03 suggest a length of 2.24 Å for the Ti−H2O bond. This suggests O(3) −0.01 ± 0.02 −0.09 ± 0.02 −0.03 ± 0.02 that the contact layer consists of OH only rather than a mixed −0.05 ± 0.01 0.27 ± 0.03 −0.04 ± 0.03 dissociated/molecular overlayer. Ti(2) −0.04 ± 0.01 −0.01 ± 0.01 0.01 ± 0.01 As a check to better define the chemical nature of O(1′), the −0.08 ± 0.01 −1.76 ± 0.01 −0.77 ± 0.01 Ti−O(1′) bond distance was fixed at values indicative of Ti− O(4) 0.02 ± 0.02 −0.02 ± 0.02 0.05 ± 0.02 OH bond distances (1.85 Å) up to values typically associated −0.11 ± 0.01 0.43 ± 0.03 0.01 ± 0.03 with the Ti−H2O bond distance (2.25 Å) at intervals of 0.05 O(5) 0.02 ± 0.02 −0.08 ± 0.02 0.00 ± 0.02 Å, and the structure was optimized. This analysis in Figure S3 0.05 ± 0.01 0.02 ± 0.02 −0.08 ± 0.02 shows that varying the bond distance from 1.85 to 2.25 Å Ti(3) −0.01 ± 0.01 −0.01 ± 0.01 0.01 ± 0.01 results in a Δχ2 of 0.1, providing further evidence that the −0.17 ± 0.01 −0.03 ± 0.01 −0.07 ± 0.01 a contact layer contains only OH (i.e., 1.93 Å). All other bond Figures 1 and 3 provide a key to the identity of the atoms. A positive distance values can be ruled out based on the uncertainty of value for x, y, and z indicates a displacement in the [1̅00], [011̅], and the χ2 (1/(N − p) = 0.0003), as described elsewhere.32,35 With [011] directions, respectively. regard to issue (ii), our best-fit structure derived from SXRD reveals an occupancy of 0.5 ± 0.02 for O(1′), that is, half monolayer coverage of OH, in agreement with ref 17. Similar the best-fit structures for TiO2(011)-(2 × 1) and TiO2(011)/ to issue (i), to better define this value, the occupancy of O(1′) H2O (atomic coordinates listed in Table 2). It can be clearly was varied at intervals of 0.1 ranging from no occupancy (0) to seen that after exposure to H2O practically all atoms in the full occupancy (1), and the structure was optimized. The selvedge appear to have bulk-like displacements, a phenomen- lowest χ2 is achieved at an occupancy of 0.5 (Figure S4). All on not uncommon to metal oxide surfaces.6,32,34,35 This is other occupancy values can be ruled out because of the small especially the case for the surface layer [atoms O(1) and χ2 uncertainty. In contrast to the full monolayer mixed H2O/ Ti(1)] and explains why the asymmetrical line shape in the OH structure found in FPMD calculations,16 the present anti-Bragg regions of the CTRs for TiO2(011)-(2 × 1) are no experiments are best described by a half-monolayer of a purely longer present for TiO2(011)/H2O (Figures S1 and S2). OH structure. This difference could, besides the delicate and Finally, to rule out adventitious formate (and carbonate) highly functional-dependent energetics of water dissociation, adsorption, which has been reported at the bulk water interface be related to the fact that in the FPMD calculations, the H with TiO2(110),7,36 we carried out a trial fit of our data with fragment remained bound to the surface, enabling stabilization formate/carbonate replacing OH as the adsorbate in a through H-bonding. This indicates that the precise adsorbate bidentate configuration to adjacent fivefold coordinated Ti 13548 DOI: 10.1021/acs.jpcc.9b04383 J. Phys. Chem. C 2019, 123, 13545−13550 The Journal of Physical Chemistry C Article Table 2. Optimized Atomic Coordinates of the TiO2(011)/ water interface; χ2 against O(1′) occupancy from the H2O Structure Resulting from Analysis of SXRD Data optimized TiO2(011)/water interface; ; tabulated positional parameters from the best-fit model of the bulk terminated (x, y, z) TiO2(011)/H2O (x, y, z) atoma coordinates (Å) coordinates (Å) SXRD data from TiO2(011)-(2 × 1); and tabulated O(1′) N/A −1.44, 3.41, 1.89 positional parameters from the best-fit model obtained O(1) 0.89, 1.90, 0.83 0.94, 1.92, 0.99 from DFT + U calculations for TiO2(011)/water interface at 1 ML coverage (PDF) ■ Ti(1) 0.00, 0.00, 0.00 0.03, 0.00, 0.01 O(2) −1.40, 0.83, −0.83 −1.41, 0.68, −0.68 O(3) 1.40, 1.90, −1.90 1.39, 1.81, −1.93 AUTHOR INFORMATION Ti(2) 0.00, 2.73, −2.73 −0.04, 2.72, −2.72 Corresponding Author O(4) −1.40, 3.57, −3.56 −1.38, 3.55, −3.51 *E-mail: g.thornton@ucl.ac.uk. O(5) 0.89, 1.90, −4.63 0.91, 1.82, −4.63 ORCID Ti(3) 0.00, 0.00, −5.46 −0.01, −0.01, −5.45 a Hadeel Hussain: 0000-0002-1322-261X Figures 1 and 3 provide a key to the identity of the atoms. A positive value for x, y, and z indicates a displacement in the [1̅00], [011̅], and Xavier Torrelles: 0000-0002-6891-7793 [011] directions, respectively. Gregory Cabailh: 0000-0002-8053-2132 Oier Bikondoa: 0000-0001-9004-9032 sites. The best-fit χ2 achieved was 1.30, an increase of 57% Ulrich Aschauer: 0000-0002-1165-6377 compared with that achieved with OH. Robert Lindsay: 0000-0001-5050-669X Finally, in comparison to previous work on the TiO2(011)/ Geoff Thornton: 0000-0002-1616-5606 H2O system in UHV, we observe both similarities and Notes differences.15,37,38 Combined theoretical and experimental The authors declare no competing financial interest. studies37,38 concluded that molecular water adsorption is only possible at temperatures below 200 K, where a mixed dissociative overlayer is favored. At room temperature, a fully ■ ACKNOWLEDGMENTS We thank Annabella Selloni for useful discussions. This work dissociated overlayer of submonolayer coverage exists. was supported by the European Research Council Advanced Reference 37 assumes the (2 × 1) reconstruction remains Grant ENERGYSURF (G.T.), EPSRC (UK) (EP/D068673/ after water adsorption. An ex situ STM study15 has shown that 1), EU COST Action CM1104, the Royal Society (UK) exposing the surface to approximately 10−3 Torr H2O leads to through a Wolfson Research Merit Award to G.T., and M.E.C. a modified surface morphology and concluded that a full (Spain) through project MAT2015-686760-C2-2-P, Severo adsorption layer of water is necessary for the restructuring to Ochoa SEV-2015-0496 grant, and project EFA194/16/TNSI be energetically favorable. ■ funded by interreg POCTEFA program. M.H.M.A. acknowl- edges financial support from EPSRC through the Advanced CONCLUSIONS Metallic Systems Centre for Doctoral Training (EP/G036950/ In summary, we have used SXRD to determine the structure of 1) and from BP. H.H. would like to acknowledge funding from the photocatalytically active TiO2(011)/water interface. Our BP through the BP International Centre for Advanced results point to the formation of half-monolayer OH in the Materials (BP-ICAM). U.A. was supported by the SNF contact layer, which causes the substrate to revert to a bulk-like Professorship grant PP00P2_157615. Calculations were termination from the (2 × 1) reconstruction adopted in UHV. performed on UBELIX (http://www.id.unibe.ch/hpc), the These results compare well with recent experimental and HPC cluster at the University of Bern. theoretical investigations in the literature and will aid efforts to provide an atomic scale understanding of the enhanced photocatalytic activity displayed by the TiO2(011) surface in ■ REFERENCES (1) Pang, C. L.; Lindsay, R.; Thornton, G. Structure of Clean and aqueous environments. ■ Adsorbate-Covered Single-Crystal Rutile TiO2 Surfaces. Chem. Rev. 2013, 113, 3887−3948. ASSOCIATED CONTENT (2) Henderson, M. A. A Surface Science Perspective on TiO2 * S Supporting Information Photocatalysis. Surf. Sci. Rep. 2011, 66, 185−297. The Supporting Information is available free of charge on the (3) Brookes, I. M.; Muryn, C. A.; Thornton, G. 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