Evolution of Water Structures on Stepped Platinum Surfaces

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Evolution of Water Structures on Stepped Platinum Surfaces YiFan Bu, TingTing Cui, Ming Zhao, Weitao Zheng, Wang Gao, and Qing Jiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10329 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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The Journal of Physical Chemistry

Evolution of Water Structures on Stepped Platinum Surfaces YiFan Bu, TingTing Cui, Ming Zhao, WeiTao Zheng, Wang Gao*, and Qing Jiang*. School of Materials Science and Engineering, Jilin University 130022, Changchun (P.R. China). AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], [email protected]. Phone: +86 431 85095371

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Abstract: Water/solid interfaces are relevant and have been studied extensively, however, with few understandings on stepped surfaces. We use a genetic algorithm method on top of density functional theory to determine water structures on Pt(221) and (553) surfaces. By including screened van der Waals (vdW) forces, we uncover a series of novel 1D and 2D water structures, which are essentially determined by the atomic geometry of Pt surfaces. We find that with increasing water coverage, water-metal vdW interactions, water-metal electrostatic interactions, and water-water interactions in turn dictate the evolution of water structures. In particular, the step feature provides the templating effects for the formation of 1D water chains by modulating water-metal interactions, whereas the terrace is crucial to the formation of 2D water networks by altering H-bonds. These findings rationalize several key experimental observations and provide critical clues for understanding water/solid interfaces.


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1. Introduction Water/solid interfaces are of fundamental importance to a broad range of areas in physical and chemical sciences, including surface chemistry, electrochemistry, heterogeneous catalysis, corrosion, lubricants, and so on.1-4 Thus, the atomistic structures of water molecules on metal surfaces have been extensively investigated in the past decades, yielding the most widely used concept—bilayer adsorption model in the field.1, 5, 6 This model proposes hexagonal ice-like or distorted ice-like structures on metal surfaces,7-19 supported by the combination of experimental measurements and theoretical calculations. However, these studies have been exclusively performed on atomically flat surfaces (close-packed), such as Pt(111),7, 8 Cu(110),9-11 Cu(111),13 and Ru(0001),14 which are far from realistic metal surfaces that consist of steps and terraces. This drawback substantially prohibits our understanding of the geometry and evolution of water/solid interface. Some pioneering attempts have been made to elucidate the behavior of water molecules on metal steps.20-28 The scanning tunneling microscopy (STM) study by Morgenstern et al.20 reported that water preferred to adsorb at step edge and formed quasi-one-dimensional (1D) chains by hydrogen bonds. These water chains were suggested to be a zigzag configuration along the step on Pt(211),21 through the surface X-ray diffraction experiments. Comparable structures were identified by density functional theory (DFT) calculations on Pt(221),22 showing an autocatalytic dissociation of water. Recently, a combination of experimental and DFT study reported that double stranded chains with water tetragons were formed at the (111) step of Pt(553).25 The lack of (111) step on Pt(533) was argued to induce multiply water configurations by DFT calculations.29 Cu(110) surface with chain-like arrangement atoms,9,

11, 12

exhibiting

some step features, enables water forming 1D chains at low coverage (< 0.3 ML). Such water 3 ACS Paragon Plus Environment


chains were built from water pentagons, challenging the conventional hexagonal ice-like bilayer structures.9 Note that this pentagon water chains cannot be formed on Pt flat surface like Pt(110), due to the lattice constant issue. At higher coverage (≥ 0.3ML), water forms an extended 2D structure on Cu(110), indicating a diversity of water/solid interface at steps.30 In addition, the two-dimensional square water structures have drawn a lot of attention based on experimental observations,31 which motivated many theoretical studies.32, 33 To the best of our knowledge, it is still ambiguous how the water structures evolve at Pt stepped surfaces. In particular, the role of steps and terraces in determining the water structures remains elusive. The difficulty of experimental measurements is that a number of atoms involved is extremely small at the interface, limiting the experimental visualization. The main challenge of theoretically modeling water/solid interfaces originates from the infinite configurational space of interface structures that requires random structure searching, with huge computational cost. In addition, the widely adopted semi-local DFT functionals like Perdew-Burke-Ernzerhof (PBE),34 are also insufficient, since these functionals miss long range van der Waals (vdW) interactions that are generally expected to be crucial for water adsorption. Notably, the adsorption on the solid surface is accompanied by the many-body collective response of the substrate electrons that induces pronounced screening effect in the vdW contributions of adsorption. This screening effect is unfortunately absent in the scheme of pair-wise vdW approximation. To determine the potential interface structures with minimum energy, we take advantage of genetic algorithm (GA) that mimics the optimal geometries based on a large pool of atomic structures.35 To accurately describe the vdW interactions for water/solid interfaces, we use screened vdW method vdWsurf, which goes beyond the pair-wise approximation and effectively includes the screening effect of metals.36, 37 We focus on the water structures on Pt(221) and (553) surfaces, using (3×1) 4 ACS Paragon Plus Environment

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unit cell due to the calculation accuracy and cost. Our tests demonstrate that the adsorption energy differences of the water structures between (3×1) and (6×1) are in the range 4–16 meV, while the (3×1) and (6×1) unit cells cannot change the configurations of the stable structures on Pt(221) with the GA search (see table S1). Our calculations uncover a series of novel 1D water chains and 2D water networks, and build a more comprehensive picture for the evolution of water structures on Pt stepped surfaces. Moreover, we find that geometry effects of Pt stepped surfaces substantially affect the water-metal and water-water interactions, consequently determining the water configurations on Pt stepped surfaces. In particular, the step feature provides the templating effects for the formation of 1D water chains by modulating water-metal interactions, whereas the terrace is crucial to the formation of 2D water networks by altering Hbonds. These results not only explain the available experimental observations in ultra-high vacuum conditions, but also serve as an important basis for the future studies of water/solid interfaces.

2. Calculation Methods All calculations have been performed using the DFT code CASTEP38-40 with ultrasoft pseudopotentials and FHI-aims41 with all-electron potential. We use the (genetic algorithm) GA search performed on aims code to search the most stable structure. After the GA search, we use CASTEP with ultrasoft pseudopotentials for the further study, since the CASTEP calculations are cheaper than the all-electron aims calculations. The exchange-correlation functional is Perdew, Burke, and Ernzerhof (PBE)34 functionals augmented with screened vdW correction (vdWsurf)36, 37. Since the previous DFT studies of water on Pt surfaces were mostly performed with PBE functional and the PBE functional can correctly predict the relative stability of water



structures on metal flat surfaces,42-44 we adopt the PBE lattice constant of Pt (3.99 Å) for the comparison purpose. Our calculated PBE and PBE+vdWsurf lattice constants are 3.99 and 3.98 Å45 for Pt. This tiny difference 0.01 Å is unlikely to induce noticeable artificial strain and cannot change the relative stability of different water structures as we identified in Table 1 of the main text. We model Pt(221) and (553) surfaces with four-layer-thick slabs and a vacuum of 25 Å, in which the bottom two layers are fixed and the rest are relaxed. Al l structures are calculated with (3×1) unit cells except periodic water chains [using (2×1) unit cells]. We used a 400 eV plane wave cutoff energy, and a Monkhorst-Pack grid with 2×2×1 k-point sampling. The water structure search on Pt(221) and (553) surfaces is performed utilizing an GA35 on top of density functional theory. We model Pt(221) and (553) surface with (3×1) unit cell and a vacuum of 25 Å (Details have been shown in Supporting Information). The 25 most stable structures of water structures on Pt surfaces were selected (Figure S1 in Supplementary Information). Subsequently, we calculate the Gibbs energies per water molecules at 298K. Based on these results, we obtained the structures of water layer on Pt(221) and Pt(553) at the different coverage, which present accurate and reasonable descriptions of the water/solid interfaces on stepped Pt surfaces. The adsorption energy is defined as follows: Ead = [Etot − N·EH2O − EPt]/N where N is number of water molecules, EPt denotes the energy of free Pt surface, EH2O denotes the energy of isolated water molecule, and Etot denotes the energy of the adsorbed systems. The total adsorption energy is split into the water-metal (w-m) interaction and the water-water (w-w) interaction,8 which are defined as follows: Ewm = [Etot − Elayer − EPt]/N 6 ACS Paragon Plus Environment

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Here, the Elayer denotes the energy of water-layer in its geometry obtained by optimizing the total adsorbed system. The water-water interaction, Eww, is simply taken as the difference between Eads and Ewm. The Gibbs energies per water molecules at 298 K are calculated as follow equation: Gad = [Gtot − N·GH2O − GPt]/N where the individual G (e.g. Pt) are estimated as: GPt = EDFT, Pt + ZPEPt – TSPt, vib in which the S is the vibrational entropy, T is the temperature (298 K), and ZPE is the zero-point vibrational energy. Then the Gibbs energies per water molecules are calculated as follow equation: Gad = Ead + ∆EZPE - ∆TS Where the ∆EZPE and ∆TS are estimated as: ∆EZPE = ZPEtot - ZPEPt - ZPEH2O ∆TS = TStot - TSPt - TSH2O The Etot, EPt, EH2O, ZPEtot, ZPEPt, ZPEH2O, TStot, and TSPt are our calculated results. The values for TSH2O at 298 K is 0.58 eV, which is taken from Ref 46.

3. Results and discussion 3.1 Evolution of Water Configurations on Pt(221). We start from the adsorption of water monomer on Pt(221) (Figure 1a, Figure 1b, and Table 1). The different orientations of the OH bonds of water correspond to the different interaction between water dipole and surface, leading to line-like and zigzag-like structures. The line-like structure is more stable than the zigzag-like structure at PBE level, but less stable than the zigzag-like structure at PBE+vdW level, indicating



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that the vdW forces are critical to the configuration of water monomer on Pt(221). Moreover, we also compare the configuration of water monomer on Pt(221) with those on Pt(111), Pt(100), Pt13 and Pt38 clusters (details are shown in Figure S2 in Supplementary Information), finding that the OH bonds of water monomer on Pt(221) are always pointing to the Pt atoms on the terrace. However, the OH bonds of water monomer on Pt(111) and Pt(100) are nearly parallel to the surface and the OH bonds of water monomer on the clusters are pointing to the vacuum. These results indicate that the stepped surfaces have a significant influence on the orientation of OH bonds of water monomer. In the case of water dimer, the line-like structure is more stable than the zigzag-like structure on Pt(221) (Figure S3a and Figure S3b in Supplementary Information). Subsequently, we study the structures of periodic water chains (linear and zigzag ones in Figure 1c and Figure 1d), finding that the linear chains are surprisingly more stable than the zigzag chains by 49 meV/H2O at PBE+vdW level. However, in previous PBE and PW91 studies

22, 24

,

the zigzag chains were suggested to be more stable than the linear chains on Pt(221), which is also confirmed by our PBE results with the PBE lattice constant 3.99 Å. This difference is found to originate from the artificial effect of PBE lattice constant and the missing of vdW forces. Our tests on the experimental lattice constant 3.92 Å 47 using three different setups (PBE, PBE+vdW, and PBE+vdW with all electron potential in Table 1) always predict that the linear chains are more stable than the zigzag chains. These results allow us to conclude that PBE with the TSsurf vdW correction method is effective for describing the configurations of water structures on Pt stepped surfaces. With five water molecules in (3×1) unit cell, the linear 1D water chains transform into the wider 1D water chains, which is built from water tetragon and OH molecule with an adsorption free energy of 332 meV/H2O (4-OH-4 in Figure 2a). Herein, the partially dissociative adsorption 8 ACS Paragon Plus Environment

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is preferred over the fully molecular adsorption. When the water number is six, water chains become wider and transform into several phases that are consisted of tetragon water rings, pentagon water rings, and hexagon water rings on Pt(221). The corresponding water patterns are 1D periodic water tetragons 4-4-4 structures (the double stranded chains in Figure 2b), 1D periodic water 5-3-5 structures (Figure 3a), and 2D periodic water 6-4-6 structures (Figure 3b). The adsorption free energy of 4-4-4 structure is 361 meV/H2O, comparing to 335 meV/H2O of 53-5 structure and 343 meV/H2O of 6-4-6 structure. Therefore, the 1D tetragons 4-4-4 rings are dominant over the 1D pentagon 5-3-5 rings and even the 2D hexagon water rings on Pt(221), and are likely observed in experiments. Indeed, the similar tetragons 4-4-4 chains (Figure 2b) have been identified experimentally on Pt(553) surface.25 Our results of 4-OH-4 and 4-4-4 water rings demonstrate that tetragon water rings are preferred for the water chains at the step of Pt(221) at low water coverage. As the water number is increased to seven, two phases with 2D networks co-exist on Pt(221) (Figures 2c, Figure 4a, and Figure 4b, Figure 4a is the same as Figure 2c, 320 meV/H2O). Notably, comparing to the 4-4-4 water structures at 1/2 ML, the added water molecules at 7/12 ML are located on the terrace of Pt(221) and connect the two chains, transforming the water structures from 1D chains to 2D networks. Besides, a novel 1D chain (Figure 4c, 311 meV/H2O), which is built from 4-5-4 rings, emerges on the step edge of Pt(221). With eight water molecules in (3×1) unit cell, water molecules form exclusively 2D water networks, with three phases coexisting (Figure 2d, Figure S4a, and Figure S4b in Supplementary Information). The irregular configurations of these structures imply that the water monolayers on Pt(221) exhibit substantially different features with respect to those on flat Pt surfaces. Every upper strand Pt atom is occupied by one water molecule in all the three phases. These strongly 9 ACS Paragon Plus Environment


adsorbed water molecules are connected, by serrated water chains in the most stable structure (forming tetragon, pentagon, and so on), but by armchair water chains in other structures (forming hexagon). Thus, tetragonal and pentagonal rings, instead of hexagonal rings, are the main constituents of the water monolayers on stepped Pt surfaces. These results suggest that (111)-like water structure (hexagonal rings) is rarely formed on Pt(221). Notably, a part of water molecules far from the surface in these structures indicates that the water monolayer begins to transform into double layers. This transition can be further confirmed in the water configurations at higher coverage (see Figure S5 in Supplementary Information, nine and ten water molecules). Our results reveal an interesting dependence of water structures on coverage: with increasing coverage, the stable structures evolve in the order of zigzag-like monomer, line-like dimer, linear 1D chains, 4-OH-4 chains, double stranded 4-4-4 chains, mixture of 1D chains and 2D networks, and 2D networks. Water structures start to nucleate and grow from the step edges of Pt(221), demonstrating that the step edges play an essential role in the evolution of water structures on Pt stepped surfaces, in agreement with the experiments.20, 21, 48 All of these water structures can be formed on Pt(221) by handling the water coverage, providing various interesting candidates for future experimental studies. Moreover, the adsorption free energy of water molecules exhibits two peaks (Figure 2e), which correspond to the linear1D chains and the 4-4-4 chains. Both structures are ought to be observed readily on Pt(221) in experiments, in particular the latter one which is the most stable structure. Indeed, experimenters have found this 4-4-4 structure on a comparable stepped surface Pt(553) recently.25 3.2 Effect of the Terrace Size on water configurations. To reveal how the size of terrace alters water configurations, we study water structures on Pt(553) and compare with those on Pt(221). With six water molecules in (3×1) unit cell, the metastable water structures on Pt(221) 10 ACS Paragon Plus Environment

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are built from 1D 5-3-5 water rings (Figure 3a, 335 meV/H2O) and 2D 6-4-6 water rings (Figure 3b, 343 meV/H2O), respectively. However, the H-bonds in this 2D-water network, which connect 6-4-6 chains (marked in Figure 3b), are gradually stretched and broken (O-O distance between 6-4-6 chains is increased from 2.72 Å on Pt(221) to 6.20 Å on Pt(553)) as the width of terrace is increased from 4.664 Å to 6.996 Å (Figure 3d). Consequently, 1D water chains are always more stable than 2D-water networks on Pt(553), with the three different configurations built from 4-4-4, 6-4-6, and 5-3-5 rings. The adsorption free energies are 358, 323, and 317 meV/H2O respectively, predicting that the 4-4-4 water chains are most likely observed experimentally on Pt(553), in good agreement with STM measurements.25 Notably, we find that the 1D chains consisting of OH and H2O on Pt(553) (Figure 3c) are less stable than the 4-4-4 water chains by 40 meV/H2O. This indicates that no water of the 4-4-4 chains is dissociated on Pt(553), again consistent with the experimental observations.25 With seven water molecules in (3×1) unit cell, three phases including 2D networks and 1D chains are found to co-exist on Pt(221) (Figure 4a, Figure 4b, and Figure 4c, Figure 4a is the same as Figure 2c). In contrast, three 1D water chains 4-5-4, 5-5-5, and reversal 4-5-4 (Figure 4d, Figure 4e, and Figure 4f, 332, 327, and 322 meV/H2O) are found to be stable on Pt(553). This difference also stems from the enlarged (111) terrace on Pt(553) compared to that on Pt(221), which forbids the formation of 2D networks on Pt(553) at this coverage (see Figure 4 where the larger terrace on Pt(553) breaks the H-bonds in marked to form 1D water chains). As the terrace is increased by 2.332 Å from Pt(221) to Pt(553), the water-water H-bonds energy is decreased from 188 meV/H2O (6-4-6 rings in Figure 4a) to 168 meV/H2O (4-5-4 chains in Figure 4d), while the water-water vdW interactions remain almost unchanged (39 VS 35 meV/H2O). Namely, the terrace of Pt(221) and (553) surfaces is most likely to influence water 11 ACS Paragon Plus Environment


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structures by altering the strength and length of H-bonds, which eventually change the H-bonds networks. If the terrace is further increased, these 1D water chains on Pt(553) would remain unchanged. Overall, the formation of water structures on Pt(221) and (553) starts at the step edges with 1D chains, and then extends towards the lower terrace by building up 2D frameworks (with H-bonds connecting the 1D chains), being consistent with the experimental observations.20,

21, 48

The

stepped Pt surfaces with larger terrace can effectively break these connecting H-bonds and recover 1D nature of water structures. Interestingly, 1D chains formed mainly by pentagonal water rings, which have been excluded on Pt flat surface like Pt(110), are preferred or even dominant on Pt(553).7,

9

Our calculations demonstrate that the step edges are essential in

determining the water structures on stepped Pt surfaces while the terraces play an increasingly important role with increasing of water coverage. 3.3 Mechanism of Water Evolution on stepped surfaces. To elucidate the formation mechanism of water structures on Pt(221), we separate the total adsorption energy Ead into watermetal (w-m) EWM and water-water (w-w) EWW interactions, which are composed of w-m electrostatic interactions (EWM,PBE by PBE) and w-m vdW interactions EWM,vdW, and w-w electrostatic interactions (EWW,PBE by PBE) and w-w vdW interactions EWW,vdW. It is noteworthy that with increasing the water coverage, the total adsorption energy Ead experiences almost the same trend as the total adsorption free energy Gad. Overall, the w-m interactions are decreased but the w-w interactions are increased, as the number of water molecules increases (Figure 2e). The contributions of vdW forces are found to account for 21~24% of the total adsorption energies (Figure 2f), which are smaller than the results by two- and three-body dispersion corrections (26~30%).49, 12 ACS Paragon Plus Environment

50

Clearly, the screening

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effect of vdW interactions, which reduces adsorption energy of adsorbates on metal surfaces,50 is critical for water adsorption on Pt surfaces. We find that the vdW interactions mainly affect the w-m interactions (by 146-177 meV/H2O) rather than the w-w interactions (by 15-54 meV/H2O), being consistent with the results by vdW-DF and DFT-D3.49,

50

Interestingly, there is a

significant interplay between the w-w vdW interactions and the w-m vdW interactions: with increasing the number of water molecules, the w-w vdW interactions are increased but the w-m vdW interactions are decreased (Figure 2f). The former is due to the additive nature of vdW forces: the more water molecules certainly yield the more pronounced w-w vdW interactions. The latter is caused by the accompanied increase of distances between water molecules and Pt atoms. The average O-Pt distance is increased from ~2.38 Å in a dimer to ~2.76 Å in 2D water networks. The larger interatomic distance yields the smaller vdW interactions. More importantly, we find that the w-m interactions are dominant in the formation of water clusters and 1D chains, whereas the w-w interactions become competitive with the w-m interactions in the formation of 2D water networks (Figure 2e and Figure 2f). In particular, the w-m vdW forces are essential for water monomer, dimer, and linear 1D chains; the w-m electrostatic interactions dictate the wider 1D water chains; the w-w interactions are crucial for the 2D water networks (Figure 2e). It is known that the vdW interactions are crucial to reproduce the correct wetting behavior of water layer on flat metal surfaces by modifying the competition between w-w interaction within ice and w-m interaction, but have a negligible effect on the relative stability, adsorption sites, and adsorption geometries of water adsorption on flat metal surface. 42,49,51,52 In contrast, we find that the vdW forces effectively change the relative stability, adsorption sites, and adsorption geometries of water adsorption on high-index surfaces at low coverage [Pt(221) surface with one 13 ACS Paragon Plus Environment


and two water molecules]. Usually, the vdW interactions remain almost constant with changing adsorption sites on flat metal surfaces, since atom polarizabilities and interatomic distance remain nearly unchanged. In striking contrast, the average distance between atoms in water and Pt atoms in the linear water chains is smaller than that in the zigzag water chains by ~0.048 Å. The shorter interatomic distance leads to the larger vdW interactions, dramatically altering the relative stability, adsorption sites, and adsorption geometries of water on Pt(221) (Table 1). These results shed light on the relationship between Pt surface morphology and vdW bindings. As the water coverage is increased, the w-m vdW interactions drop steeply from 365 meV/H2O (monomer) to 165 meV/H2O (4-4-4 water chains), whereas the w-w vdW and electrostatic interactions are slowly increased. In contrast, the w-m electrostatic interactions are quickly increased and start to dictate the priority of the water structures, with the contribution reaching the maximum (557 meV/H2O) at the water 4-4-4 chains (Figure 2f). The charge redistribution at the step edges generates a net dipole moment with the vectors pointing from bottom to top step edges.25 In addition, the unsaturated Pt atoms in the upper and lower strand are inclined to bind with oxygen atoms and O-H bonds, respectively. The rectangular geometry of the (110) step edge (marked in Figure S6 in Supplementary Information) enables a water monomer or dimer locating at its long bridge sites to optimize the smoluchwski and unsaturated Pt-atoms effects, which correspond to the w-m electrostatic interactions. The adsorbed monomers or dimers form 1D water chains in an end-to-end connection by H-bonds, generating the linear 1D water chains and the 4-4-4 1D water chains. Therefore, the templating effects most likely stem from the (110) step feature instead of the (111) step feature. Notably, the linear 1D water chains (Figure 1c) likely provide an important basis for the formation of the water 4-4-4 chains. At the high water coverage where 2D networks are dominant, the w-m electrostatic 14 ACS Paragon Plus Environment

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interactions decrease dramatically, and the w-w electrostatic interactions (H bonds) increase significantly, with increasing water coverage. The templating effects of (110) step become invalid: the lower strand of step can no longer be fully occupied by water molecules, whereas only the upper strand of step acts as a template and causes 1D arrangements of O atoms of water (Figure 2d, Figure S4a, and Figure S4b in Supplementary Information). These strongly adsorbed water molecules are combined with those adsorbed on the terrace to form 2D networks, for which H-bonds play a crucial role. Notably, the 1D line-like structures of the water chains optimize w-m interactions at the expense of reducing w-w interactions on Pt(221), whereas the zigzag-like water structures are beneficial to maximize H-bonds with an energy advantage of ~25 meV/H2O compared to the line-like water structures (Table 1). The zigzag-like structures thus become exclusive around steps in 2D networks where H-bonds are crucial (Figure 2d, Figure S4a, and Figure S4b in Supplementary Information).

4. Conclusions In conclusions, our results demonstrate that water structures on Pt(221) and (553) surfaces exhibit substantially diverse configurations of 1D chains and 2D networks, some of which are in good agreement with experimental observations in ultra-high vacuum conditions. With increasing the coverage of water, the determining factor of water structures changes from watermetal vdW interactions, to water-metal electrostatic interactions, and then to water-water interactions. This behavior is essentially determined by the atomic geometry of Pt surfaces. The step feature provides the templating effects for the formation of 1D water chains by modulating water-metal interactions, whereas the terrace is crucial to the formation of 2D structures by altering H-bonds network. The water structures we found not only provide various 1D chains and



2D networks as the base for the future experimental studies, but also contribute to the construction of reaction models in aqueous/electrochemical conditions, both of which promote the understanding of solid-liquid interfaces on stepped metal surfaces.


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ASSOCIATED CONTENT Supporting Information. Computational details, including the details of the GA search, DFT calculations, and additional information on water structures on Pt(221) and Pt(553) at different coverages. AUTHOR INFORMATION Notes The authors declare no competing financial interests. Y.F. Bu and T.T. Cui contributed equally to this work. ACKNOWLEDGMENT The authors thank the support from the Program of the Thousand Young Talents Plan, the National Natural Science Foundation of China (No. 21673095, 51631004), the Program of Innovative Research Team (in Science and Technology) in University of Jilin Province, the Fundamental Research Funds for the Central Universities, and the computing resources of High Performance Computing Center of Jilin University and National Supercomputing Center in Jinan and in Tianjin, China.



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Table 1. Adsorption energy Ead of water structures on Pt(221) in meV/H2O. Line-like (zigzag-like) indicates structures close to linear (zigzag) arrangement of water. EWW (EWM) shows water-water (watermetal) interactions

structure monomer dimer

linear 1D chains

Double stranded 1D chains

methods PBE PBE+vdW PBE PBE+vdW PBE PBE+vdW PBE[a] PBE[b] PBE+vdW[b] PBE[c] PBE+vdW[c] PBE[d]

line-like Gad

320 362

PBE PBE+vdW

361

zigzag-like

Ead 505 789 582 868 667 908 591 629 888 631 884 685

EWW 135 150 204 217 160 134 151 144

EWM 505 789 447 718 463 691 430 495 737 541

738

182

557

940

219

722

Gad 290 276 313

335

Ead 451 817 572 812 655 837 622 603 791 605 803 672

EWW 165 179 162 179 217 205 219 214

EWM 451 817 407 633 483 658 406 398 572 458

714

207

507

908

237

671

[a] PBE results with PBE lattice constant of Pt. [b] PBE+vdW results with experimental lattice constant of Pt. [c] PBE+vdW calculations with experimental lattice constant of Pt by FHI-aims with all-electron potential.[41] [d] PBE results with experimental lattice constant of Pt.


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Figure 1. Water monomer (a, b) and 1D chains (c, d) adsorbed on Pt(221). Colour labels: gray = Pt; red = O; white = H.

Figure 2. Water structures on Pt(221) with different number of water: (a) five, (b) six, (c) seven, and (d) eight water molecules. Insets denote side views of the surface. (e-f) total adsorption energies Gad/Ead of water structures and the corresponding separated compositions as a function of the water number (NH2O).



Figure 3. Water structures on (a-b) Pt(221) and (c-d) Pt(553) for six water molecules in (3×1) unit cells (corresponding to the coverage of 1/2 and 2/5 ML), respectively. (a) 5-3 structure; (b) and (d) 6-4-6 structures; (c) 4-4-4 structure with water dissociation.


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Figure 4. Water structures on (a-c) Pt(221) and (d-f) Pt(553) for seven water molecules in (3×1) unit cells (corresponding to the coverage of 7/12 and 7/15 ML), respectively. (a) 6-4-6 structure; (b) 5-6 structure; (c), (d), and (f) 4-5-4 structures; (e) 5-5-5 structure.



TOC Graphic


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Evolution of Water Structures on Stepped Platinum Surfaces

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