Deprotonated Water Dimers: The Building Blocks of Segmented Water

Aug 10, 2015 - Despite the importance of RuO2 in photocatalytic water splitting and catalysis in general, the interactions of water with even its most...
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Deprotonated Water Dimers: The Building Blocks of Segmented Water Chains on Rutile RuO2(110) Rentao Mu,∥,†,§ David C. Cantu,∥,†,§ Vassiliki-Alexandra Glezakou,†,§ Igor Lyubinetsky,‡,§ Roger Rousseau,*,†,§ and Zdenek Dohnálek*,†,§ †

Fundamental and Computational Sciences Directorate, ‡Environmental Molecular Sciences Laboratory, and §Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: Despite the importance of RuO2 in photocatalytic water splitting and catalysis in general, the interactions of water with even its most stable (110) surface are not well understood. In this study we employ a combination of high-resolution scanning tunneling microscopy imaging with density functional theory based ab initio molecular dynamics, and we follow the formation and binding of linear water clusters on coordinatively unsaturated ruthenium rows. We find that clusters of all sizes (dimers, trimers, tetramers, extended chains) are stabilized by donating one proton per every two water molecules to the surface bridge bonded oxygen sites, in contrast with water monomers that do not show a significant propensity for dissociation. The clusters with odd number of water molecules are less stable than the clusters with even number and are generally not observed under thermal equilibrium. For all clusters with even numbers, the dissociated dimers represent the fundamental building blocks with strong intradimer hydrogen bonds and only very weak interdimer interactions resulting in segmented water chains.

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INTRODUCTION Water solid interfaces are of great importance in many diverse areas including catalysis, electrochemistry, corrosion, atmospheric science, geology, astrophysics, and others.1−7 In this article, we focus on understanding the interactions of water with ruthenium dioxide, RuO2, which is generally employed as a cocatalyst in many photocatalytic water-splitting systems.8,9 Understanding such interactions is also essential for other RuO2 applications, such as HCl oxidation (Deacon process),10,11 Chlor-Alkali electrolysis,8 and low-temperature CO oxidation in the presence of water vapor.12,13 Despite that, very little is known about how water adsorbs, diffuses, and aggregates on different RuO2 surfaces.8,14,15 In the study by Lobo et al.14 the authors showed that water desorbs from RuO2(110) at 400 K. While such high temperature is generally suggestive of recombinative desorption from dissociatively adsorbed water,16 the observed high-resolution electron energy loss spectroscopy (HREELS) spectra were indicative of the molecular adsorption. In our previous study,17 we employed scanning tunneling microscopy (STM) and density functional theory (DFT) based ab initio molecular dynamics (AIMD) to follow the interactions of isolated water molecules with the thermodynamically most stable rutile RuO2(110) surface. We showed that water molecules adsorb on top of coordinatively unsaturated Ru sites (Rucus), begin to diffuse along the Rucus rows at ∼230 K, and dimerize upon encountering other monomers. Surprisingly, water dimers were found to readily deprotonate, forming Rubound H3O2 and bridging hydroxyl (HOb) species. Pairs of H3O2 and HOb were found to be 0.3 eV lower in energy than molecular water dimers. This is in a sharp contrast with the © 2015 American Chemical Society

behavior of water molecules adsorbed on the isostructural rutile TiO2(110), where molecularly bound and deprotonated water dimers are isoenergetic.18 The onset of measurable diffusion of the H3O2 species on Rucus rows was observed above ∼277 K, and the diffusion barrier was determined to be 0.7 eV. In this study, we focus on the formation of larger water clusters, up to a full monolayer, on the stoichiometric RuO2(110) using a combination of STM imaging and AIMD DFT calculations. We find that clusters of all sizes are stabilized by the deprotonation of water dimers on the surface. Further, the clusters with odd number of water molecules are less stable than even number water clusters. For all even-numbered clusters, dissociated dimers are the fundamental building blocks, with strong intradimer hydrogen bonds and only weak interdimer hydrogen bonds resulting in segmented water chains. This behavior is different from the water chains observed on TiO2(110), where molecularly bound and dissociated water clusters are close in energy and no preference for chains with even number of water molecules is observed.19

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METHODS Experiments were carried out in an ultrahigh vacuum (UHV) system combining variable-temperature STM (Omicron), lowenergy electron diffraction (LEED), Auger electron spectroscopy (AES), and a growth chamber for the preparation of thin oxide films.20,21 Clean stoichiometric RuO2(110) surfaces were prepared by a two-step process.22 First, the RuO2(110) thin film is grown by oxidation of a clean Ru(0001) surface Received: July 23, 2015 Published: August 10, 2015 23552

DOI: 10.1021/acs.jpcc.5b07158 J. Phys. Chem. C 2015, 119, 23552−23558

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The Journal of Physical Chemistry C (Supporting Information (SI), Figure S1a) in 2.5 × 10−5 Torr O2 at the temperature of 600 K for 5 h. Generally, the freshly prepared films are covered with clusters and adsorbates (SI, Figure S1b) as observed in prior studies.23−25 To remove these imperfections, several cycles of O2 adsorption (exposure of ∼2 × 10−6 Torr) at 300 K and UHV annealing at 750 K (SI, Figure S1c) were performed. Water was dosed via a retractable tube doser directly in the STM stage. Water coverage is given in monolayers (MLs), where 1 ML is defined relative to the coverage of Rucus sites, which have a surface density of 5.04 × 1014 cm−2. All STM images were collected using a tungsten tip in a constant current mode (10−20 pA, unless stated otherwise) with a sample bias voltage (∼−1.0 V). Spin-polarized DFT calculations were carried out using the gradient-corrected functional of Perdew, Burke, and Ernzerhof (PBE) as implemented in the CP2K package.26 The wave functions of the valence electrons were expanded in terms of a molecularly optimized (MOLOPT) double-ζ Gaussian basis set, designed to minimize basis set superposition errors.27 An additional, auxiliary plane wave basis of 500 Ry energy cutoff was used28 for the calculation of the electrostatic energy terms. Core electrons have been modeled by scalar relativistic normconserving pseudopotentials.29,30 The Γ-point approximation was employed for Brillouin zone integration. Dispersion interactions were included by means of the third generation of semiempirical van der Waals correction proposed by Grimme31 (DFT-D3). All calculations were carried out on (6 × 2) RuO2(110) six RuO2 trilayer thick slabs with a 30 Å vacuum layer between periodic images. All energies reported in the manuscript were corrected for zero-point energy (ZPE). Computed properties of RuO2 bulk and RuO2(110) surface were reported in our previous study.17 Static structures of adsorbed (H2O)n (n = 3, 4, 12), for both molecular and deprotonated (dissociated) conformations, were prepared by short AIMD trajectories where the structure was first equilibrated at 200 K for 2 ps followed by a slow simulated cooling to 0 K. A full water monolayer corresponds to 12 water molecules on the slab surface. Static structures of adsorbed 1 and 2 water molecules are reported in our previous work.17 Additionally, longer AIMD simulations (∼20 ps) of 1, 2, and 12 waters (full monolayer) on the RuO2(110) slab, at 200 and 400 K, were performed for this work. To assess the nature of surface interactions among the clusters, we carried out Monte Carlo simulations by populating the surface lattice with different dimer coverages using distinct probabilities for placing a cluster next to (Padjacent) and/or spaced apart (Papart) from clusters already on the surface. Clusters showing no (Padjacent = Papart), attractive (Padjacent > Papart), or repulsive (Padjacent < Papart) interactions were all considered. Free energy differences were evaluated as ΔG = RT ln(Papart/Padjacent), where R is the universal gas constant and T the temperature. Attractive interaction energy values are negative, in line with the notation for the relative adsorption energies obtained from DFT.

Figure 1. STM images (5 × 5 nm) of (a) a clean stoichiometric RuO2(110) and (b−d) surface after water adsorption at 300 K. The local H2O coverages in the STM images are (b) 0.06 ML, (c) 0.30 ML, and (d) 0.67 ML. The overlaid ball model in (a) illustrates the corrugated structure of RuO2(110) composed of high-lying rows (ridges, imaged bright) of bridging oxygen (Ob) anions (red balls) and low-lying rows (troughs, imaged dark) of coordinatively unsaturated ruthenium (Rucus) cations (cyan balls). The inset in (b) shows a highresolution image of the H3O2 species.

Figure 1b shows RuO2(110) after dosing 0.06 ML of H2O at 300 K. Two dominant types of features are observed, large bright features (labeled H3O2) centered on the dark Rucus rows and the smaller less bright features (labeled HOb) centered on top of the Ob rows. The high resolution inset further reveals two maxima, within the H3O2 feature, positioned on top of the Rucus ions. In our prior study17 we demonstrated that at 300 K water monomers diffuse and readily form dimers. Further, upon dimerization one of the two water molecules dissociates (deprotonates) yielding two species, the Rucus-bound H3O2 and Ob-bound HOb species: 2H2O + Ob → H3O2 + HOb. The H3O2 is a hydrogen-bonded H2O−HOt pair, where HOt denotes the terminal hydroxyl group. The equivalent brightness of the two maxima in the inset of Figure 1b suggests fast exchange of the proton between the hydrogen-bonded H2O and HOt species: H2O−HOt ⇄ HOt−H2O.17 Further, the spatial separation between the H3O2 and HOb species is a result of facile H3O2 diffusion at 300 K.17 Larger H2O coverages of 0.30 and 0.67 ML are shown in Figure 1c and 1d. The images reveal the formation of longer linear water clusters (chains), extending along the Rucus rows. A more detailed inspection shows that all observed chains cover an even number of Ru cus sites suggesting a (H 3 O 2 ) n composition, where n is an integer. Unfortunately, at high water coverage, where extended chains are present, we are unable to determine whether one out of every two water molecules remains dissociated. We have further analyzed the cluster size distributions for the extensive set of experiments obtained at coverages of 0.25 and 0.35 ML and plotted the results in Figure 2a. For both coverages, dimers remain the most common species, and

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RESULTS AND DISCUSSION Coverage-Dependent Cluster Size Distributions. The evolution of water-covered RuO2(110) as a function of increasing coverage is shown in Figure 1. The atomically resolved image of clean RuO2(110) in Figure 1a (also in SI, Figure S1c) reveals the corrugated structure composed of highlying bridging oxygen rows (ridges, imaged bright) and lowlying Rucus rows (troughs, imaged dark). 23553

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Figure 2. (a) Experimentally determined water cluster size distributions resulting from the analysis of ∼1000 molecules at coverages of 0.25 and 0.35 ML. (b,c) The distributions determined from Monte Carlo simulations assuming (a) no interactions (ΔGint = 0 eV) and (b) attractive interactions (ΔGint = −0.04 eV) between the dimers.

dissociated states are favored over the molecular ones.17 As shown in Figure 3, this is true for larger clusters as well. While the monomer energy difference between the dissociated and molecular configuration is very small (0.03 eV), the energy stabilization (per water molecule) provided by dissociation is significant in all clusters: 0.27 eV for dimers, 0.10 eV for trimers, 0.22 eV for tetramer, and 0.15 eV for the monolayer coverage. The lowest energy dissociated configurations for monomer, dimer, and trimer have one proton removed (as HOb), whereas the tetramer and complete monolayer have one proton removed for every two water molecules (dissociated dimer unit). Cluster-size dependency trends of dissociated low-energy configurations (ΔEdis) further reveal that, on a per water molecule basis, odd-numbered clusters (monomer, trimers) are less stable (∼0.1 eV) than even-numbered ones (dimers, tetramers). This explains their absence in the STM images acquired at 300 K (see Figure 2) since at this temperature facile diffusion of both monomers and dimers allows for the equilibration of the distribution of the cluster sizes. It is notable that the molecularly bound clusters all lie within 0.1 eV in energy/water molecule for n > 2 which would provide a more uniform cluster size distribution than that observed experimentally. Conversely, calculations show that the energies of dissociated clusters, both odd- and even-numbered, are dis approximately size independent: ΔEdis trimer ≈ ΔEmonomer and dis dis ΔEtetramer ≈ ΔEdimer; note that our DFT analysis finds that tetramers are slightly destabilized, 0.02 eV relative to dimers which we interpret as isoenergetic as it is unlikely that the current level of theory is reliable to this degree of accuracy. This implies that it is the number of proton donations per water molecule which drives the energetics and that even number clusters are able to donate up to 1/2 protons per water molecule, while odd-numbered clusters yield total average value of 1/3, 2/5, and 3/7 for n = 3, 5, and 7. This simple description is in qualitative agreement with the above Monte Carlo analysis showing that tetramers are only slightly more stable (∼−0.04 eV) with respect to isolated dimers since the main energetic driver in both cases is approximately the same. Water Trimers. As observed experimentally (Figures 1 and 2) and validated with DFT calculations (Figure 3), water trimers are not stable at room temperature. To follow their formation and dissociation dynamics, we carried out experiments at lower temperatures (220−240 K) to track the motion of water monomers and limit the diffusion of dissociated water dimers (H3O2 + HOb).17 Under these conditions, H3O2 species remain paired with HOb ’s generated from the dimer dissociation, (H2O)2 + Ob → H3O2 + HOb. The formation and dissociation of water trimers from water monomers and

cluster abundance decreases with increasing size. As expected, for the larger coverage (0.35 ML, red), higher abundance is observed for larger clusters as compared with the lower coverage (0.25, blue). Monte Carlo simulations were performed to assess if the observed cluster size distributions indicate the presence of attractive interactions. The cluster size distribution for noninteracting dimers (Figure 2b) decays significantly faster than the experimentally observed distribution and clearly overestimates the populations of small clusters (dimers and tetramers), indicating attractive interactions. As shown in Figure 2c, an interaction free energy of −0.04 ± 0.01 eV reproduces the experimentally observed cluster size distribution fairly well. It should be noted that the interaction energy value is significantly smaller than the value of a typical hydrogen bond of ∼0.2 eV suggesting that these interactions are rather weak. To further understand the structure, binding, and stability of different sized clusters, we carried out DFT-based AIMD calculations. We first focus on comparing the energetics of water molecules in various structural arrangements (monomer, dimer, trimer, tetramer, complete monolayer) in both molecular and dissociated (deprotonated) configurations. These are schematically compared in Figure 3. In our previous work, we focused on the structure and energetics of water monomers and dimers, and found that the

Figure 3. (a) Relative energies (per water molecule) of molecularly (ΔEmol) and dissociatively (ΔEdis) bound water clusters with respect to the energy of a molecularly bound water monomer (ΔEmol monomer ≡ 0 eV). The lowest energy dissociated configurations for monomer, dimer, and trimer have one proton removed as HOb, tetramer, and a compete monolayer has one proton removed per every two water molecules (dimer unit). 23554

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higher stability of the clusters with even number of water molecules. As noted above, DFT calculations also support that trimers are an energetically unfavorable configuration (Figure 3), as they are higher in energy than dissociated monomers, dimers, and tetramers. The molecular structure of the dissociated trimer is illustrated in Figure 4e. Although a hydrogen bond (2.62 Å) is present between the bottom H2O and top H3O2 species, it does not provide any significant energy stabilization with respect to the dissociated monomer. Additional trimer conformations with the dissociated water (HOt−HOb) on the edges, as opposed to the middle as in Figure 4e, were also explored (SI, Figure S3). These were found to be energetically unfavorable (∼0.1 eV/water higher) with respect to the dissociated trimer with the central water deprotonated. Water Tetramers. We continue our discussion with water tetramers. Their formation and subsequent separation is illustrated in the time-lapsed sequence of images obtained at 300 K (Figure 5a−5d). While Figure 4 illustrates that tetramer

dissociated dimers is illustrated in Figure 4, which shows the time evolution of the same area on RuO2(110) following water

Figure 4. (a−c) Time-lapsed sequence of STM images from the same area (3.5 × 5 nm2) obtained at 238 K. Image (a) is obtained 20 min after the water dose. Images (b) and (c) are obtained 3 and 58 min after (a). Circles highlight the positions of water monomers, dimers, trimers, and tetramers that are discussed in the text. (d) Line profiles of water monomer, dimer, trimer, and tetramer along the Rucus row. (e) The top view of a water trimer consisting of a water monomer adjacent to the dissociated water dimer (H3O2--HOb + H2O). The dashed cyan lines indicate the hydrogen-bonded network. Cyan is Rucus, red lattice O, purple water O, white H. Figure 5. (a−d) Time-lapsed sequence of STM images from the same area (4.5 × 4.5 nm2) obtained at 300 K. The white grid indicates the Ob lattice, circles the positions of water molecules. Images (b), (c), and (d) are obtained 4, 8, and 10 min after (a). The inset in (c) shows the high-resolution image revealing an internal structure within a tetramer and a dimer. (e,f) The top and side views of the DFToptimized dissociated tetramer structure. The dashed cyan lines indicate the hydrogen-bonded network. Cyan is Rucus, red is lattice O, purple water O, white H.

adsorption at 238 K. Figure 4a shows water monomers and dimers. Their identities are distinguished by their brightness and width along the Rucus row (see line profiles in Figure 4d). Two water dimers with nearby monomers on the same Rucus rows are highlighted with circles in Figure 4a. In the subsequent frame (Figure 4b), the monomer on the right has diffused to a position adjacent to the dimer: H2O + (H3O2 + HOb) → (H5O3 + HOb). We follow the same area for another 55 min (Figure 4c) and observe that the trimer falls apart, releasing a water monomer from the opposite end, effectively shifting the position of the original dimer by one lattice constant. Furthermore, the dimer that was on the left side (Figure 4a,b) sequentially combined with two monomers (one from top, one from bottom) forming a stable tetramer (Figure 4c). The time-dependent cluster size distribution (SI, Figure S2) further reveals the relative stability of different clusters. Analysis of three sets of data, corresponding to the images shown in Figure 4, illustrates that, as the time after the water dose increases, the fraction of water trimers decreases from 0.10 (at 20 min) to 0.06 (34 min) and 0.05 (48 min). Similarly, the fraction of water monomers decreases from 0.22 (20 min) to 0.15 (34 min) and 0.11 (48 min). In contrast, the fraction of water dimers increases from 0.54 (20 min) to 0.63 (34 min) and 0.68 (48 min). The fraction of tetramers remains mostly unchanged, as their formation at low temperatures requires the sequential attachment of two monomers to a dimer, a lowprobability occurrence. The observed trends further confirm

formation via addition of a monomer to a trimer is feasible, kinetically the dominant mechanism at higher temperatures, where dimers are mobile, will be the recombination of two dissociated dimers (Figure 5). It should be noted that at 300 K the H3O2 species readily separate from their nascent HOb’s, as discussed in detail in our prior study.17 Generally, due to the lower brightness of HOb species, we are not able to distinguish whether they are next to a bright H3O2 species. Figure 5a shows two distant dimers present on the same Rucus row (highlighted with circles). In the subsequent frame (Figure 5b) these two dimers are closer to each other as they diffuse along the Rucus row and finally (Figure 5c) combine to form a tetramer. We note that dimer species always diffuse as a single unit, without decomposing, a further indication of their high stability. We have studied the diffusion mechanism of the H3O2 species via DFT in our previous study17 and found that the transition state geometry displays HOt species bound to a Rucus atom with the water molecule moving over it at a 23555

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back to a molecular water dimer, the H atom remains tightly bound to the Ob throughout all AIMD simulations (SI, Dimer AIMD analysis, Figures S6−S9). Extended Water Chains at High Coverages. As the water coverage approaches a complete monolayer, long water chains are observed on Rucus rows (Figure 6). This is illustrated

hydrogen bonding distance. For a subsequent hop by a similar rollover mechanism, the H2O in the H2O···HOt pair is required to first reorient its H-bonding pattern and transfer the shared proton to the Ru-bound OH. The inset in Figure 5c shows a high-resolution STM image (from a different area) that reveals an internal structure within a tetramer, as well as a nearby dimer. Three minima can be distinguished: the two within the original dimers are shallow, and the one between them is deeper. The line scan along the Rucus row (SI, Figure S4) further supports this observation and yields an intra- and interdimer distance of 2.96 and 3.25 Å, comparable to the Rucus−Rucus lattice spacing of 3.11 Å. The larger separation between the dimer fragments within the tetramer suggests a rather weak interaction. This assertion is also supported by the fact that the tetramers can fall apart to two spatially separated dimer species as shown in Figure 5d. Nonetheless, it is notable that the current DFT-D approach underestimates the stability of the tetramer by 0.06 eV relative to our Monte Carlo results suggesting that the dispersion interactions from the weak hydrogen bond are not fully accounted for. Figure 3 shows that dissociated tetramers are practically isoenergetic with separated dissociated dimers. The DFToptimized static structure of the low-energy configuration of the dissociated tetramer (H3O2−H3O2 + 2HOb) is shown in Figures 5e,f. The structure, with two dissociated water molecules, consists of the H2O-HOt--H2O-HOt sequence with the HOb species neighboring the HOt’s. The intra- and interdimer O−O distances of 2.67 and 3.73 Å further demonstrate that only weak interactions with extremely long interdimer hydrogen bonds (>3.0 Å) are present. In contrast, the length of the intradimer hydrogen bond is significantly shorter, 1.68 Å. Water tetramer structures with only a single dissociated water molecule were also considered (SI, Figure S5) and found to be energetically very unfavorable (∼0.2 eV/water higher) with respect to the dissociated tetramer with two deprotonated water molecules described in Figure 3 and shown in Figure 5e,f, supporting that dissociated dimer species are energetically favored on RuO2(110) and that the tetramer consists of neighboring dissociated dimers. AIMD simulations provide further details about the dynamics of H3O2. Due to a weak interaction between the dimers in the tetramer, relevant processes are confined within each H3O2 unit. Simulations at 400 K show rapid switching of the H that is shared between the two oxygens of the H3O2: HOH−OH ⇄ HO−HOH with the average hydrogen bond distance of 1.68 Å. In contrast, simulations at 200 K show very little evidence of H transfer within the H3O2 species (SI, Dimer AIMD Analysis, Figures S6−S9). Additionally, the free energy of proton transfer was estimated to be 0.05 eV at 400 K and 0.07 eV at 200 K, from the AIMD simulation (SI, Dimer AIMD Analysis, eq 2). This implies that proton scrambling within the dimer will be extremely fast even at low temperatures in accord with our previous conclusions regarding the mechanism of dimer diffusion.17 Although internal dimer proton transfers occur, the internal H atom prefers to remain bound with the oxygen atom away from the neighboring HOb (Figure 5e). The strong H-bond within H3O2 also makes the intradimer O−O bond length smaller than the Rucus−Rucus lattice spacing. The values calculated using DFT-optimized structure and AIMD average are 2.67 and 2.68 Å, respectively, which are very close to the previously determined gas-phase value of 2.70 Å.32−34 Supporting that H3O2 species are stable and do not revert

Figure 6. (a−d) STM images (1.7 × 3.5 nm2) of segmented water chains observed on Rucus rows at high coverages at 300 K. (b,d) Overlaid schematic dimer structure on top of images displayed in (a,c), respectively. The overlays highlight the segmented nature of the chains composed of weakly interacting dissociated water dimers. Images display areas with predominantly straight (a, b) and zigzag (c, d) water chains.

on two different areas, displayed in Figure 6a and 6c. Statistical analysis reveals that all chains contain an even number of water molecules, similar to shorter chains observed at lower coverages (see Figure 2). This is demonstrated by overlaying the chains (Figure 6a and 6c) with the schematics of water dimers (Figures 6b and 6d). Clear boundaries can be observed between the dimers, indicating that intradimer O−O distances are smaller than interdimer O−O distances (see also the line profile in Figure S4). While we are unable to determine experimentally whether the dimers are bound molecularly or dissociatively, DFT calculations discussed above (see Figure 3) clearly show that stable configurations are composed of dissociated water dimers (H3O2 + HOb)n, analogous with the tetramers discussed in the previous section. Careful inspection of Figure 6c reveals that the O−O axes in the dimers in this image are misaligned with respect to the underlying Rucus rows. The misalignment of neighboring dimers in a chain alternates forming the zigzag arrangement as highlighted in Figure 6d. This can be attributed to the formation of a H-bond between H3O2 species and Ob atoms. DFT provides further insight into the structure of the water chains. Figure 7 shows the energy-optimized static structure of water chains composed of dissociated water dimers, (H3O2 + HOb)n. The calculated structure shows the dissociated dimerlike character similar to that observed for dimers and tetramers. A closer inspection further reveals an arrangement of interdimer O−O bonds that are tilted slightly away from the Rucus row direction, albeit with different periodicity than that observed in Figures 6c and 6d. This difference originates from the size of the simulation super cell which accommodates three water dimers per each Rucus row and does not allow for the periodicity with the even number of dimer units (two here). Nonetheless it is clear that the rotation of the O−O axis in the H3O2 away from the Rucus row direction results from the strong hydrogen bonding between the H atom in HOb and the HOt oxygen. As such, the overall long-range periodicity of the tilted H3O2 species within each Rucus row is dictated by the positions 23556

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H-bond network rapidly, and deprotonated species are not the major component of the equilibrium population. As a result, there is no even/odd preference in the resulting water chain length. In contrast on RuO2(110), deprotonated clusters dominate the equilibrium population, and their fundamental building blocks are H3O2 species.

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SUMMARY

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ASSOCIATED CONTENT

We carried out a detailed study of water adsorption and clustering on the RuO2(110) surface as a function of water coverage. We find that linear water clusters (chains) forming on Rucus rows show a strong preference for an even number of water molecules. Clusters with an odd number of water molecules are less stable (by ∼0.1 eV) than even-numbered clusters and are generally not observed under thermal equilibrium. The fundamental building blocks of evennumbered clusters are dissociated water dimers which have one proton irreversibly reacted away with the adjacent bridging oxygen sites: (H2O)2 + Ob → H3O2 + HOb. This preferential dissociation is a result of the high Lewis acidity of the Rucus sites. This in turn polarizes the O−H bonds within the water molecules making them easier to dissociate and engage in stronger H-bonds. The strong intradimer hydrogen bonding on the Rucus−Rucus rows with the spacing of 3.11 Å allows only very weak interdimer hydrogen bonding and results in segmented water chains.

Figure 7. Structure of the H2O monolayer obtained from the DFT calculations displaying the segmented water chains composed of the dissociated water dimers (H3O2 + HOb). The O−O axes of the H3O2 species are tilted slightly away from the direction of the Rucus rows. The O−O tilt is indicated by the green lines. The dashed cyan lines indicate the hydrogen-bonded network in the left water chain. Cyan is Rucus, red is lattice O, purple water O, white H.

and periodicity of the HOb species. On the basis of these observations we speculate that the experimentally observed zigzag structure results from a different well-ordered arrangement of HOb’s that has not been further explored in this study. (Additional details about the dynamic behavior of this structure are described in the SI, Full Monolayer AIMD Analysis, Figures S10−S12). As with the tetramer, adjacent H3O2 species interact only weakly. This is clearly evident from the large differences between the intra- and interdimer O−O distances that were calculated to be 2.63 and 3.66 Å from the DFT-optimized structures, 2.65 and 3.67 Å from the AIMD ensemble average at 200 K, and 2.69 and 3.63 Å from AIMD average at 400 K (see SI Figures S10−S12). The fact that the intradimer O−O distances in the tetramer and the monolayer are almost identical and that the hydrogen bond between adjacent H3O2 species remains >3.0 Å shows that chain length does not change the stability of H3O2 species. As in the tetramer, the H remains bound to Ob, and the central H in H3O2 forms a strong hydrogen bond that at times switches the O−H bond and H− O hydrogen bond identities (SI, Full Monolayer AIMD Analysis, Figures S10−12). It is instructive to further compare water adsorption on RuO2(110) with that on the isostructural TiO2(110) where continuous water chains with no preference for even/odd number of water molecules has been observed.19 Our analysis suggests that this difference is rooted in two different underlying causes: (1) The higher acidity of RuO2(110) (H2O binding energy is ∼1.3 times higher relative to TiO2(110)),17 which leads to high preference for deprotonation of dimers and further limits the number of strong hydrogen bonds that can be formed in a continuous water chain. (2) The larger Rucus−Rucus distances of 3.11 Å relative to Ti−Ti distances of 2.95 Å which does not allow for the formation of strong hydrogen-bonded continuous chains on RuO2(110). These factors lead to the overall scenario that on TiO2(110) water chains are formed predominantly of undissociated H2O molecules that are able to reorient their

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07158. Details of the preparation of a clean stoichiometric RuO2(110) surface, a plot of the distribution of water cluster sizes at 238 K, additional trimer and tetramer conformations, line profiles derived from high-resolution STM images, as well as dimer and full monolayer AIMD analysis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Roger Rousseau: [email protected]. *Zdenek Dohnalek: [email protected]. Author Contributions ∥

R.M. and D.C.C. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences and performed in EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle. The authors also acknowledge Mal-Soon Lee for discussions of the AIMD trajectory analysis and Cortland Johnson for the artistic rendering of the cover image. 23557

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DOI: 10.1021/acs.jpcc.5b07158 J. Phys. Chem. C 2015, 119, 23552−23558