Cooperative Effects in Water Binding to Cuprous Oxide Surfaces

Article pubs.acs.org/JPCC

Cooperative Effects in Water Binding to Cuprous Oxide Surfaces Christoph Riplinger† and Emily A. Carter*,†,‡,§ †

Department of Mechanical and Aerospace Engineering, ‡Program in Applied and Computational Mathematics, and §Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544-5263, United States S Supporting Information *

ABSTRACT: Water adsorption on solid surfaces plays a part in a variety of processes, including renewable energy applications. Water adsorption can occur either dissociatively, monomolecularly, or as clusters. In contrast to metal surfaces, the compositional and structural complexity of metal oxide surfaces has inhibited atomic-scale understanding of their interactions with water. Cu2O is a promising photocatalyst and (photo)electrochemical catalyst. Here, we investigate water adsorption on its (111) surface, using density functional theory + U with dispersion corrections. A number of monomolecular and dissociated adsorbate geometries are considered on the two most stable surface terminations. H2O is found to adsorb most strongly when datively coordinated to an unsaturated surface Cu cation; dissociative adsorption is not as favorable as this dative bonding mode of molecular chemisorption. If these Cu cations are not present, H2O binds via hydrogen bonding and electrostatic interactions in surface cavities. We also examine a large variety of mixed modes of coadsorption. Mixtures of datively bonded and hydrogen-bonded water molecules adsorb most strongly, exhibiting a strong lateral interaction. The resulting water clusters can adapt to the underlying adsorption site template and maintain significant water−surface and water−water interactions at the same time. This is possible through the proximity of the unsaturated cationic and anionic adsorption sites. The combination of dative and hydrogen bonding to the surface enables water clustering even at low temperatures, probably due to rapid surface diffusion of the more weakly bonded monomers. The strong dative and lateral interactions keep water clusters adsorbed up to unusually high temperatures. The water hexamer can still be observed at room temperature under ultrahigh vacuum conditions; we predict that datively bound monomers, hexamers, and other similarly constructed clusters will remain bound to the stoichiometric surface up to quite high temperatures under conditions of high relative humidity. We suggest that metal oxides with similar surface compositions should show similar properties. Finally, we predict vibrational frequencies for the adsorbed water molecules and distinguish between water−water and water−surface vibrations for comparison with future experimental studies.

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INTRODUCTION Adsorption of water on solid surfaces plays a critical role in many areas of chemistry, materials science, and earth science. Water−solid interactions influence technological and physicochemical phenomena such as heterogeneous catalysis, electrochemistry, and lubrication.1−5 Photocatalytic and (photo)electrochemical processes such as CO2 reduction and H2 generation are promising strategies to generate fuels from solar energy or to convert intermittently produced renewable electricity to fuels as a form of energy storage. Water is an essential part of these reactions and characterizing its interaction with the catalyst or electrode surface is crucial.6,7 An essential step in understanding the water−surface interaction is to analyze the position and orientation of water molecules on the surface. Experimentally, this often involves clean, well-characterized surfaces that are investigated under ultrahigh vacuum (UHV) conditions, because it is so difficult to characterize a liquid−solid interface at the atomic scale.8,9 Alternatively, theoretical methods can be used to compute water−surface properties, compare them to available experimental data, and extend predictions to more realistic conditions beyond UHV. © 2015 American Chemical Society

Although the basic adsorption event can occur molecularly or dissociatively, lateral water−water interactions can also be important, and even competitive with water−surface binding.6 Three different bonding scenarios can be defined:6 (i) the lateral interaction is much weaker than water−surface binding, (ii) the lateral interaction is comparable or slightly weaker than water−surface binding, and (iii) the lateral interaction is comparable or stronger than water−surface binding. The first scenario often manifests as water dissociation. The second scenario is characterized by a trade-off between molecular binding and intermolecular hydrogen bonds. In the last scenario, water clusters characterized by hydrogen bonds between neighboring water molecules are observed. Water cluster formation at low coverages must be preceded by facile surface diffusion, especially at low temperatures, since long diffusion lengths are necessary to enable water molecules to find each other.6,10 Received: January 13, 2015 Revised: April 3, 2015 Published: April 6, 2015 9311

DOI: 10.1021/acs.jpcc.5b00383 J. Phys. Chem. C 2015, 119, 9311−9323

Article

The Journal of Physical Chemistry C

and to explain the two temperatures at which desorption occurs. Namely, periodic GGA+U+D calculations suggested that two different terminations of the Cu2O(111) surface could be responsible for the experimental observations. Another DFT study modeling Cu2O(111) with finite clusters and the local density approximation (LDA) predicted, in contrast to the experimental results of Ö nsten et al., that molecular adsorption is more favorable than dissociative adsorption.30 The validity of this LDA study, however, is questionable based on a more recent study, which found the LDA functional to be insufficient for describing ground-state properties of Cu2O.31 Yet another DFT study modeling the polar, copper-terminated Cu2O(111) surface using GGA calculations with dipolar corrections predicted that dissociation occurs on the Cu-terminated surface.32 However, another DFT-GGA study shows that the Cu-terminated surface is very unstable.33,34 In conclusion, at the moment it is still not well understood whether water adsorbs molecularly or dissociatively on Cu2O(111), which mechanisms are responsible for water desorption at different temperatures, and how water clusters form on the Cu2O(111) surface. In this study, we answer these questions and give a detailed mechanism for water cluster formation on Cu2O(111). We investigate water adsorption on the two most stable Cu2O(111) surfaces. Single water molecule adsorption is analyzed in a variety of different binding modes and at different coverages. Water clusters of different sizes and consisting of water molecules adsorbed in different ways are modeled and the balance between water−surface and lateral water−water interactions is analyzed in terms of the underlying adsorption site template. Finally, we use the computed adsorption free energies for the different models to explain available experimental data for water adsorption and desorption on Cu2O(111).

Most progress in understanding water−surface interactions has been made for clean, single-crystal metal surfaces.7 The balance between the metal−oxygen bond strength and lateral interactions determines the structure of adsorbed water layers, and often water clusters are observed in a variety of configurations.7,11−13 Water adsorption on metal oxide surfaces is much less well understood.14 Due to the extra oxide anions that are present in addition to the metal cations, the water−surface interaction is a more complex phenomenon, as water can form hydrogen bonds to the surface oxide ions, besides forming donor− acceptor bonds between water’s oxygen and the metal cations (hydrogen bonding to the metal atoms, which happens on metal surfaces,15 is unlikely as discussed further below). Several different water−surface binding scenarios can be observed on metal oxides. For example, water adsorbs molecularly as a monomer on anatase-TiO2(101), forming a dative bond to Ti and hydrogen bonds to oxide anions at the same time, as revealed by a combined scanning tunneling microscope (STM) and periodic density functional theory (DFT) study.16 These water−surface interactions place water into an adsorption configuration that prevents strong lateral interactions. Much stronger water−surface interaction can lead to dissociative chemisorption. For example, recent theoretical and experimental work on MgO(100) finds that water adsorbs molecularly and dissociatively in a planar two-dimensional network, where all OH groups are involved in water−water hydrogen bonds, and no dangling OH groups can be found.17 Very strong adsorption correlates with high desorption temperatures, which is generally the case for water on metal oxides known for dissociative adsorption.18 The present study is concerned with water adsorption on cuprous oxide (Cu2O). Cu2O has been shown experimentally to photocatalytically convert CO2 to methanol,19 electrochemically reduce CO2 to methanol,20,21 and generate H2 in a photoelectrochemical setup,22 which makes it a promising material for renewable fuel production. Despite many experimental and theoretical studies, the adsorption of water on the Cu2O surface is still not well understood. For the Cu2O(100) surface, ultraviolet photoelectron spectroscopy and thermal desorption spectroscopy indicate that 10% of a water monolayer (ML) dissociates at 110 K, followed by complete dissociation at 300 K.23 Modified coupled pair functional calculations on an embedded cluster model24 and DFT Generalized Gradient Approximation (GGA) computations on a periodic slab model25 also found that water dissociation is more favorable than molecular adsorption on this surface. By contrast, a periodic DFT-GGA calculation of a Cu 2 O(110)/CuO surface predicted that dissociative adsorption is less favorable than molecular adsorption.26 As the Cu2O(111) surface was found to be the most stable among all possible Cu2O surfaces,27 we focus on this surface in the following. For the Cu2O(111) surface, Ö nsten et al. found, using photoemission spectroscopy and STM, that water desorption occurs between 150 and 180 K, and that some waters adsorb dissociatively.28 This is in partial contrast to a study by Kronawitter et al., who found by STM of adsorbed water on a Cu2O(111)/Cu(111) surface, that water desorbs below 180 K, as observed by Ö nsten et al., on some parts of the surface, but that water clusters stay adsorbed up to room temperature on other parts of the surface.29 In the same work, we performed calculations to model adsorbed water clusters

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COMPUTATIONAL METHODS All calculations were performed with the Vienna Ab initio Simulation Package.35 As established in previous work,36 we used DFT+U theory37−39 with Perdew−Burke−Ernzerhof (PBE)40,41 exchange-correlation and semiempirical dispersion corrections42 (denoted as DFT+U+D) to fully relax the structures of each slab + water, the bare slab, and an isolated water molecule, and to calculate their free energies. We used an ab initio U-J value of 3.6 eV for the Cu d states, which was recently determined from electrostatically embedded Hartree− Fock calculations43 using the method developed by Mosey et al.44 A plane-wave kinetic energy cutoff of 800 eV was used, which converges the total energy to within 1 meV/atom. Blöchl’s all-electron, frozen-core projector augmented wave (PAW) method45 was used to represent nuclei and core electrons. All atoms were structurally relaxed, using a force threshold of 0.03 eV Å−1. For geometry optimization, the Brillouin zone was integrated using Gaussian smearing with a smearing width of 0.01 eV, while final energies were calculated using the tetrahedron method with Blöchl corrections.46 A Γpoint-centered k-point mesh of 2 × 2 × 1 was used, which also converged the total energy to within 1 meV/atom. For the calculation of the single water molecule, only the Γ-point was used with a Gaussian smearing width of 0.05 eV. Structures and energies were calculated for water clusters adsorbed on two Cu2O(111) surfaces, the stoichiometric surface and the surface with Cu vacancies. The same procedures were applied here as in previous studies of 9312


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The Journal of Physical Chemistry C Cu2O(111) surfaces.27,29,36 For both surface models, slabs of five O−Cu−O trilayers (each trilayer consisting of two layers of oxygen sandwiching one layer of copper, see side views of Figure 1) were used. A vacuum layer of 16 Å thickness was

terms are determined for the water cluster on the top surface only, in order to save computational time. The difference in zero-point energies of the three systems ΔZPE = ZPE 2·x H2O/Cu 2O − ZPECu 2O − x· ZPE H2O

is calculated from the vibrational frequencies. The vibrational frequencies are obtained from numerical Hessian calculations computed using finite displacements of ±0.02 Å and the corresponding analytic gradients. The Hessians are calculated entirely for the water molecule but only partly for the bare slab and the slab with adsorbed waters. In the latter cases, we took into account only vibrations of the water molecules on the top surface, surface atoms directly involved in bonding to them, and their nearest neighbors in the slab. Earlier work has shown that such partial Hessian computations yield accurate vibrational frequencies.36 The partial Hessian calculations were also used to verify the optimized structures as true minimum structures. Enthalpic and entropic contributions are calculated using the ideal gas, rigid rotor, and harmonic oscillator approximations. They include vibrational, rotational, and translational terms for the isolated water molecule, but only vibrational terms for the bare Cu2O slab and the slab with adsorbed water. The Bader method was used to analyze the partial charges of individual atoms.47

Figure 1. Optimized structures of (a) the ideal stoichiometric Cu2O(111) surface and (b) the Cu2O(111) surface with Cu vacancies. The subscript index indicates whether the ion is coordinatively saturated (CS) or coordinatively unsaturated (CUS). For OCUS, the additional index 1 or 2 indicates whether it belongs to the upper part (1) or to the lower part (2) of the surface trilayer. Side (figures above) and top (figures below) views are displayed for each model. Red and blue spheres represent oxygen and copper, respectively. For the top view, only the surface trilayer and the oxygen of the subsurface trilayer nearest to the surface trilayer are shown.

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∫0

T0

⎤ CpdT − T ΔS ⎥ ⎦

(1)

with the electronic total energy (1/2ΔE), zero point energy (ΔZPE), enthalpic (Δ∫ T0 0CpdT), and entropic (Δ[−TΔS]) differences between the adsorbed water cluster and the bare slab plus isolated water molecules being defined as follows. The total energy difference is calculated as ΔE = E2·x H2O/Cu 2O − ECu 2O − 2·x·E H2O

RESULTS AND DISCUSSION

The aim of this study is to investigate water adsorption on the Cu2O(111) surface. As there exist different possible Cu2O(111) surfaces reported in the literature,27,33,34,48 we focus on the two structures that were identified as the most stable surfaces, namely, the stoichiometric surface and the surface with Cu vacancies. 27,33,34 On both of these surfaces, we have investigated a number of possible water adsorption configurations. After briefly reviewing the structures of both surfaces, we first describe the results obtained for water adsorption on the stoichiometric surface, followed by the results for the surface with Cu vacancies. A. Surface Description. The stoichiometric surface and the surface with Cu vacancies were characterized in detail in ref 49; we give only a brief summary of their properties here. Optimized structures of the stoichiometric surface and the surface with Cu vacancies are depicted in Figure 1. The surface with Cu vacancies was shown by DFT+U calculations to be the most stable surface at all oxygen partial pressures.27 Under oxygen-poor conditions, the stoichiometric surface is very close in energy to the defective one. As can be seen from Figure 1, the stoichiometric surface consists of an equal number of coordinatively unsaturated oxygen (OCUS) and copper (CuCUS) ions, whereas the only unsaturated species on the surface with Cu vacancies, formed by removing the surface copper ions from the stoichiometric surface, are unsaturated oxygens in the surface (OCUS,1) and the subsurface (OCUS,2) trilayer. The Bader charge of CuCUS is less positive than the charge of bulk Cu (respectively, 0.37 vs 0.51). The Bader charge of OCUS is slightly less negative than that of bulk O (−0.95, −0.94, and −0.96 for the stoichiometric surface and for OCUS,1 and OCUS,2 on the surface with Cu vacancies, respectively, vs −1.01 for bulk O).49 Water binds strongly to the unsaturated surface sites. The water hydrogens (HW) can form a hydrogen bond with the unsaturated oxygen atoms and the water oxygens (OW) can form a dative bond with the coordinatively unsaturated copper

introduced in the [111] direction, which is sufficient to prevent interactions between periodic images of the slabs and adsorbed water molecules. In order to avoid net dipole formation in the periodic DFT+U+D calculation, the water clusters were adsorbed with inversion symmetry on both bottom and top surfaces. Two different surface areas were considered. A (1 × 1) unit cell was used to investigate water adsorption at high coverage. A three times larger surface area, the (√3 × √3)R30° surface unit cell, was used to model lower coverage and water cluster formation. The surface vectors were taken from DFT+U+D optimization of the bulk Cu2O structure, corresponding to a length of u = v = 10.515 Å with an angle of 60°. The adsorption free energy of the water cluster is determined from ⎡ ΔGads = 1/2ΔE + ΔZPE + Δ⎢ ⎣

(3)

(2)

with E2·xH2O/Cu2O being the total energy of two water clusters (each consisting of x water molecules) adsorbed on the top and bottom sides of the slab, ECu2O is the energy of the bare slab, and EH2O is the energy of one isolated water molecule. The energy difference is divided by two, as we are interested in the adsorption energy of only one water cluster. The remaining contributions for zero point energy and enthalpic and entropic 9313


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Low Coverage. The two optimized structures from Figure 2 were also structurally relaxed at lower coverage (1/6 ML for the datively bonded water and 1/3 ML for the dissociated water) in order to investigate the effect of coverage (initial structures, see Figure S2). The initial hydrogen-bonded structures (Figure S1i and S1k) also were relaxed again at lower coverage (1/6 ML) in order to test whether they are stable at a lower coverage. Aside from the dissociative adsorption depicted in Figure 2b, three additional modes of dissociative chemisorption were tested on the larger surface unit cell: one in which H binds to OCUS while OH forms a hydroperoxo species by binding to another OCUS, and two modes in which H binds to CuCUS (Figure S2e,f) while OH binds to CuCUS or OCUS, respectively, as was observed by photoemission.50 The hydrogen-bonded structures again relaxed to the datively bonded structure. The datively bonded structure and the dissociated structure of Figure 2b were stable at the lower coverage as well. Their properties are discussed below. Both initial structures with hydrogens adsorbed on CuCUS (see Figure S2e,f) relaxed to stable minima; however, they were much less stable (by 1 and 3 eV) than the more stable dissociated structure, as in Figure 2b, and are thus not discussed in the following. This result is not surprising, given that both the hydrogen in water and the copper ion in the surface layer are positively charged and therefore, unlike a metal surface, will repel each other. Geometries. The optimized structural parameters for the datively bonded and dissociated water species are summarized in Table 1. H2OCuCUS forms a hydrogen bond between H1 and

ions. A 0.5 ML coverage is reached when water is bound to either the one OCUS or the one CuCUS in each unit cell for the stoichiometric surface, and when water is bound to either the OCUS,1 or OCUS,2 for the unit cell of the surface with Cu vacancies. 1.0 ML coverage is reached when both sites are occupied per unit cell. As discussed above, lateral hydrogen bonding between adsorbed waters can stabilize water clusters. However, in order for this lateral interaction to be possible, the adsorbed water molecules must be sufficiently close to each other. This requires that the surface adsorption sites have intersite distances that are compatible with the distances between the two OW in a lateral (OW)H−OW hydrogen bond. The balance between the water−surface and water−water interactions varies, depending on whether the surface sites are too far from or too close to each other, or whether the water−surface interaction imposes geometric constraints on the water orientation that are unfavorable for lateral interactions. Consequently, the morphology and stability of the adsorbed water clusters is strongly dependent on the template of the surface. As viewed from above, both surfaces have hexagons consisting of linear OCUSCuCSOCS units as sides, where CuCS and OCS, respectively, are coordinatively saturated copper and oxygens ions, with diameters of about 7.1 Å (measured as the average distance between OCUS and OCS on opposing sides of the hexagon). Because the unsaturated Cu ion sites in the middle of the hexagon, the average distance between OCUS and CuCUS adsorption sites is about 3.55 Å for the stoichiometric surface. B. Adsorption on Stoichiometric Cu2O(111). 1. Single Water Molecule Adsorption. High Coverage. To investigate different binding modes of single water molecules to the stoichiometric surface, 11 initial geometries were considered in a first set of calculations (hydrogen-bonded, datively bonded, and dissociated water, see Figure S1). The relaxations yielded only two different optimized structures, one molecular and one dissociated. The datively bonded models all relaxed to a molecular structure with OW datively bonded to CuCUS and with one hydrogen bond to OCUS (Figure 2a). The optimized

Table 1. Optimized Structural Parameters and Adsorption Energies for Adsorbed Water on the Stoichiometric Cu2O(111) Surfacea distances (Å) OW−H2

adsorption energies (eV)

1.01

0.98

−1.03

1.74

1.02

0.98

−1.15

1.81

0.98

2.44

0.97

−0.85

1.83

0.98

2.43

0.97

−0.81

structure

CuCUS−OW

OCUS−H1 OW−H1

H2OCuCUS − 1/2 ML H2OCuCUS − 1/6 ML H2ODiss − 1 ML H2ODiss − 1/3 ML

1.98

1.86

1.95

a

Bond distances are given in Å. The atom definitions are given in Figure 2. The adsorption energies are given in eV per water molecule at 0 K.

the nearby OCUS. Its Cu−O bond is significantly longer than that of H2ODiss. For H2ODiss, the hydroxyl group is datively bonded to CuCUS, H1 is now bonded to OCUS, and a hydrogen bond forms between H1 and OW. However, this hydrogen bond is much longer than that of H2OCuCUS. This is due to the constraints from binding to the surface (the CuCUS−OW and OCUS−H1 bonds are approximately perpendicular to the surface plane, preventing a shorter hydrogen bond; also the angle between OCUS−H1−OW, 108°, is quite unfavorable). The effect of different coverages on the structural properties can also be seen in Table 1. The hydrogen bond to a nearby OCUS is shorter for H2OCuCUS in the 1/6 ML coverage case, while there is no change for H2ODiss when going to lower coverage. For H2OCuCUS, lower coverage leads to a shorter Cu− O bond, while it leads to a slightly longer Cu−O bond for H2ODiss. For H2ODiss, this might be due to lateral interactions.

Figure 2. Optimized adsorbate geometries of water on the stoichiometric Cu2O(111) surface with (1 × 1) periodicity: (a) the datively bonded water at 0.5 ML coverage and (b) the dissociated water at 1 ML coverage. The structural parameters are summarized in Table 1.

dissociated water structure is depicted in Figure 2b. The dissociated proton adsorbs on OCUS, and the hydroxyl group, which is datively bonded to CuCUS, is tilted toward the dissociated proton. The hydrogen-bonded (to either OCUS or CuCUS) molecularly adsorbed initial structures were not stable and relaxed to the datively bonded molecular structure. 9314


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Figure 3. Adsorbate geometries considered for selected H2O dimers on the stoichiometric Cu2O(111) surface with (√3 × √3)R30° periodicity, involving combinations of dissociated water (H2ODiss) with datively bonded (H2OCuCUS) or with hydrogen-bonded water (H2OOCUS), shown as initial structures before relaxation: (a) H2OOCUS and H2ODiss, with the hydroxyl group pointing towards the oxygen of water, (b) H2OCuCUS and H2ODiss, and (c) H2OCuCUS and H2OOCUS. Side (figures above) and top (figures below) views are displayed for each model. Red, blue, and gray spheres represent oxygen, copper, and hydrogen, respectively.

thus all possible hydrogen orientations for dimers (Figure S3a− d) and trimers (Figure S3e−l) were investigated. Additionally, for the most stable coadsorption patterns, larger models were constructed: a tetramer, corresponding to an infinite-chain (Figure S3m); a ring-like tetramer (Figure S3n); and a ring-like hexamer (Figure S3o). The initial structures were relaxed with DFT+U+D. The most stable structures for the dimers and trimers, as well as all relaxed structures for the tetramers and the hexamer, are shown in Figure 4. The adsorption free energies for all optimized structures are summarized in Table 2. The values are given per water molecule and, in order to account for the fact that there are two different adsorption sites in each unit cell, on a per unit cell basis. Additionally, we calculated the cooperative effect of water cluster adsorption by comparing the adsorption energy of the whole cluster with the sum of individual single water adsorption energies (see last column of Table 2). The lateral interaction is further discussed in terms of hydrogen bond distances. For the most stable water clusters (dimers, trimers, tetramers, and hexamer) the (O W )H−O W distance of neighboring water molecules and the water−surface bond distances are given in Table 3. Dimers. The coadsorbed dimers of H2OCuCUS and H2OOCUS exhibit adsorption energies of about −1.0 eV per water molecule. This value is less negative than the adsorption energy of a single H2OCuCUS. However, it is also important to compare the adsorption free energies per unit cell, as the OCUS adsorption site is not occupied for single H2OCuCUS binding. Per unit cell, the adsorption free energy of the H2OCuCUS and H2OOCUS dimer is much more negative compared to single H2OCuCUS adsorption. Compared to the single adsorption modes, an additional strong hydrogen bond between H2OCuCUS and H2OOCUS has formed (with a hydrogen bond distance of 1.53 Å, see Table 3), at the expense of the original hydrogen bond of H2OCuCUS to OCUS (OCUS−H1 distance increase of more than 0.6 Å, from 1.74 Å for single H2OCuCUS (Table 1) to 2.38 Å for the H2OCuCUS−H2OOCUS dimer (Table 3)). At the same time, the CuCUS−OW distance shortens from 1.95 to 1.90 Å. Different orientations of hydrogen atoms not involved in lateral hydrogen bonds do not influence the adsorption energies (as can be seen from Table S1). This is due to the fact that the atomic structure around CuCUS is fairly symmetric. H1(H2OCuCUS) (H1 of H2OCuCUS, as defined in Figure 2) binds

In the 1/3 ML case, H2 of the hydroxyl group is electrostatically attracted to OCUS of the neighboring unit cell, while in the 1 ML case this OCUS is saturated by H1. This attraction results in shrinking of the OCUS,2−H distance from 3.7 to 3.3 Å when going from 1 ML to 1/3 ML. This stronger attraction probably also causes the slightly longer CuCUS−OW bond in the 1/3 ML case. Adsorption Energies. Comparing the adsorption energies in Table 1, we see that molecular binding (H2OCuCUS) is significantly stronger than dissociative binding (H2ODiss). The adsorption energies reflect the trends discussed for the structural properties regarding the types of binding (slightly shorter CuCUS−OW bond for H2ODiss, but a much shorter hydrogen bond for H2OCuCUS) as well as with respect to different coverages. Decreasing coverage leads to stronger adsorption for H2OCuCUS (shorter CuCUS−OW bond and shorter OCUS−H1 hydrogen bond), probably due to less competition between lateral and dative bonding, and to weaker adsorption for H2ODiss (longer CuCUS−OW bond) due to the competition for electrostatic attraction to nearby OCUS, which is not saturated by H1 any more in the low coverage case. 2. Mixed Coadsorption. H2OCuCUS is the most stable adsorption mode, followed by H2ODiss, whereas H2OOCUS is not stable at all. We saw that a lateral interaction is not possible in the H2OCuCUS case, as the CuCUS adsorption sites are too distant. When different binding modes are combined, lateral interactions might become important and enhance water adsorption (cooperativity) or compete with water−surface interactions. In this section we investigate how the different types of adsorbed water molecules influence each other and may form mixed clusters. As H2OOCUS could be stabilized by a coadsorbed water molecule, it was also included in the test. A set of initial geometries of water dimers, trimers, and larger clusters was constructed on the surface with (√3 × √3)R30° periodicity. For dimers, all three possible combinations were investigated. These are depicted in Figure 3: (a) H2ODiss with H2OOCUS, (b) H2OCuCUS with H2ODiss, and (c) H2OCuCUS with H2OOCUS. The dimers containing H2ODiss (a, b) show the weakest adsorption energy and little to no lateral interaction (vide infra) and thus are not studied further. Coadsorbed H2OCuCUS and H2OOCUS (c) exhibit a strong lateral interaction, and are further investigated in the following. A large variety of possible hydrogen orientations exists for these clusters, and 9315


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Figure 4. Equilibrium adsorbate geometries for mixed dimers to hexamers of datively bonded (H2OCuCUS), hydrogen-bonded (H2OOCUS) and dissociated (H2ODiss, with OHCuCUS and HOCUS) water on the stoichiometric Cu2O(111) surface with (√3 × √3)R30° periodicity: (a) one H2OCuCUS and one H2OOCUS, exhibiting a hydrogen bond between the waters and from H2OCuCUS to the neighboring OCUS; (b) same as (a), plus an additional H2OCuCUS with an extra hydrogen bond to the neighboring OCUS, with all neighboring waters hydrogen-bonded; (c) same as (b), plus an additional H2OOCUS, with all neighboring waters hydrogen-bonded in a chain-like arrangement of the hydrogen-bond network over the periodic cells; (d) same as (c), except the tetramer hydrogen-bonding network forms a (open) ring-like pattern; (e) three H2OOCUS and three H2OCuCUS in a ringlike formation with each H2OCuCUS forming hydrogen bonds to both neighboring H2OOCUS; (f) one H2ODiss and one H2OOCUS, where OHCuCUS hydrogen bonds to H2OOCUS; (g) one H2OCuCUS and one H2ODiss. Side (figures above) and top (figures below) views are displayed for each model. Red, blue, and gray spheres represent oxygen, copper, and hydrogen, respectively.

very weakly to OCUS, and it can bind similarly to both OCUS that are not involved in H2OOCUS binding. H2(H2OOCUS) (the hydrogen atom of H2OOCUS pointing toward vacuum) does not undergo intermolecular binding at all, and thus H2OOCUS can flexibly adapt to the orientation of H2OCuCUS without influencing the adsorption energy. Dissociation of water on the surface is promoted slightly by additional H2OOCUS nearby, as the hydroxyl group of H2ODiss forms a weak hydrogen bond to H2OOCUS (Figure 4f, sixth row in Table 2, sixth row in Table 3). This lateral interaction enhances the adsorption energy by about 0.1 eV. If dissociative adsorption is combined with datively bonded water adsorption (Figure 4g, last row in Table 2, last row in Table 3), there is no stabilizing lateral interaction between the water molecules. Both H2OCuCUS, and the hydroxyl group, are bound to CuCUS and are thus too far apart to interact. As H2OCuCUS combined with H2OOCUS (vide supra) is much more favorable, we can conclude that dissociative adsorption will not occur. Trimers. Adding one more H2OCuCUS leads to water trimers (adding H2OOCUS is possible as well, but is expected to have a much lower adsorption energy). As these trimers contain two H2OCuCUS and only one H2OOCUS, we now observe more negative adsorption energies per water molecule than for the dimers. However, on a per unit cell basis, the adsorption energy is smaller than for the dimer. Regarding the different hydrogen orientations (see Table S1) we now see a variation of up to 0.1 eV in adsorption energies per water molecule. The most

favorable trimer is depicted in Figure 4b. In this model, both H2OCuCUS form a hydrogen bond to a neighboring OCUS (with bond lengths of 2.29 and 2.23 Å, Table 3). The OWs of the two H2OCuCUS and of H2OOCUS approximately lie on a circle, whose center is a saturated surface oxygen. With respect to this circle, the HWs point in opposite directions, that is, the two HWs of H2OCuCUS point radially inward and HW of H2OOCUS points radially outward. In this way, the OW electron lone pairs of H2OOCUS are oriented such that they can form hydrogen bonds with both neighboring H2OCuCUS. The circle defined by the OWs is slightly compressed, compared to a circle that is defined by the underlying adsorption sites OCUS and CuCUS. This maximizes both the water substrate binding and lateral interactions by maintaining bonding to the surface and shortening the intermolecular hydrogen bonds. The water− water hydrogen bonds are longer than for the dimer (1.72 and 1.74 Å, Table 3), but strong enough to account for a relatively large cooperative effect of −0.2 eV per water molecule (Table 2). The other trimer configurations show less negative adsorption energies, as they do not maximize lateral interactions. For example, the trimer with least adsorption energy (the NSS model in Figure S3 and Table S1) consists of a structure similar to the dimer (with a hydrogen bond distance of 1.52 Å) plus an extra water molecule that does not laterally interact with the dimer. Tetramers. On the basis of the structure of the most stable trimer, we relaxed a tetramer with the same configuration 9316


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H2OCuCUS at one end of the tetramer forms a weak hydrogen bond to OCUS. Due to its upward pointing hydrogen, H2OOCUS at the other end of the tetramer does not form an extra hydrogen bond. Similar to the most stable trimer, the circle defined by the OWs is slightly compressed toward the central OCS in order to maximize the available hydrogen bonds. The total cooperative effect for the cyclic tetramer is slightly larger than it is for the dimer. Hexamer. Extending the cyclic tetramer configuration and forming a cyclic cluster leads to the hexamer shown in Figure 4e. The hexamer has a 3-fold rotational symmetry. Each water molecule has two hydrogen bonding partners. The alternating pattern of hydrogen orientation is again observed. Similar to the most stable trimer and tetramer, the hydrogens of the H2OCuCUS and the H2OOCUS point in opposite directions relative to a circle through the OWs, that is, the HWs of H2OCuCUS point radially inward and the HWs of H2OOCUS point radially outward. As before, the circle through the oxygen atoms is compressed toward the central OCS, as compared to a circle drawn through the underlying adsorption site OCUS. This again ensures maximization of lateral interactions. The hydrogen bonds are all very similar (1.71−1.73 Å), and are similar to those of the trimer but longer than that of the dimer, optimally balancing water-surface and lateral water−water interactions. The overall cooperative effect is −1.98 eV or −0.33 eV/H2O (Table 2). Out of all investigated mixed clusters, this cyclic hexamer is the cluster with the strongest adsorption energy (per water molecule as well as per (1 × 1) unit cell) and has the strongest cooperative effect. C. Adsorption on Cu2O(111) with Copper Vacancies. 1. Single Water Molecule Adsorption. The strongest adsorption site of the stoichiometric surface, CuCUS, is not present on the surface with Cu vacancies. The only available adsorption sites on this defective surface are unsaturated oxygens: OCUS,1 in the surface of the first trilayer and OCUS,2 in the subsurface of the first trilayer. In order to investigate the different binding modes on this surface, five initial structures were considered on the (1 × 1) surface unit cell (see Figure S4). In all these structures, water is hydrogen-bonded primarily (a) to OCUS,1 and (b−e) to OCUS,2. The relaxations yielded only two distinct optimized structures (Figure 5), with the structural parameters summarized in Table 4. Optimization of structure (a) produces a water hydrogen-bonded to the surface layer (H2OOCUS,1, Figure 5a). The H1−OW bond of H2OOCUS,1 does not stay perpendicular to the surface plane, but shows a slight tilt. The

Table 2. Adsorption Free Energies for Mixed Mode Adsorption of Water Clusters on the Stoichiometric Cu2O(111) Surface at 0 Ka structure (see Figure 4)

model from Figure 4

a 1H2OCuCUS + 1H2OOCUS b 2H2OCuCUS + 1H2OOCUS 2H2OCuCUS c (chainlike) + 2H2OOCUS d (ring-like) e (Hexamer) 3H2OCuCUS + 3H2OOCUS f 1H2ODiss + 1H2OOCUS g 1H2OCuCUS + 1H2ODiss

ΔG1 (eV) per H2O

ΔG2 (eV) per (1 × 1) unit cellb

cooperative effect (eV) per H2O

−0.99

−1.98

−0.29

−1.07

−1.61

−0.22

−0.97

−1.94

−0.28

−1.00 −1.03

−2.00 −2.06

−0.31 −0.33

−0.66

−0.66

−0.13

−0.93

−0.93

0.05

a

The values are given in eV per water molecule (ΔG1) and per unit cell (ΔG2, where we only count those unit cells that contain at least one water molecule). The cooperative effect (fifth column) is calculated as the difference between the computed ΔG1 and the theoretical average value for the three different bonding types at low coverage (see Table 1: H2OCuCUS at 1/6 ML: −1.15 eV, H2ODiss at 1/3 ML: −0.81 eV, H2OOCUS at 1/6 ML: −0.24 eV, with the latter taken from the surface with Cu vacancies, Table 4). The naming convention refers to the structures in Figure 4. bThe bonded water molecules in model a extend over one unit cell; in models b−d, f, and g, over two unit cells; and in model e, over three unit cells.

pattern (Figure 4d). The oxygen atoms lie on a circle, and the hydrogens of H2OCuCUS and H2OOCUS point in opposite directions, that is, the HWs of H2OCuCUS point radially inward and the HWs of H2OOCUS point radially outward. We modeled a second configuration that produces a chain-like pattern (Figure 4c). We expected that this configuration might be more stable than the ring-like pattern, as it can form one more hydrogen bond, forming an infinitely long chain over the repeated unit cells. However, the underlying adsorption site template does not allow maximization of the lateral interaction in that case. The relaxed structure exhibits a set of four hydrogen-bonded water molecules (hydrogen bond distances of 1.62 to 1.94 Å) and a break in between these sets (with a “hydrogen bond” distance of 3.29 Å). The ring-like configuration, on the other hand, is more stable (by about 0.03 eV per water molecule). It exhibits three intermolecular hydrogen bonds (hydrogen bond distances of 1.58−1.82 Å) between the four water molecules.

Table 3. Bond Distances for the Most Stable Small Water Clusters (Dimers, Trimer), Both Tetramers, and the Hexamer on the Stoichiometric Surface, as Given in Figure 4a distances (Å) H2OCuCUS/H2OOCUS

H2ODiss/H2OOCUS 1H2OCuCUS/1H2ODiss

structure (see Figure 4)

(OW)H−OW

H1(H2OCuCUS)−OCUS

OW−CuCUS

H1(H2OOCUS/Diss)−OCUS

dimer (a) trimer (b) chain-like tetramer (c) ring-like tetramer (d) ring-like hexamer (e) dimer (f) dimer (g)

1.53 1.72/1.74 1.62/1.94/1.68/3.29 1.68/1.82/1.58 1.71/1.73/1.73/1.72/1.73/1.72 1.85/2.37

2.38 2.29/2.23 2.68 2.26

1.90 1.94/1.94 1.90/1.93 1.93/1.90 1.93/1.93/1.94 1.83 1.95/1.85

1.60 1.41 1.42/1.60 1.44/1.57 1.36/1.37/1.37 1.73 0.98

1.72

a Bond distances are given in Å. The naming convention refers to the structures in Figure 4. (OW)H−OW refers to a lateral (water−water) hydrogen bond. H1(H2OCuCUS) and H1(H2ODiss) are the hydrogen atoms of H2OCuCUS and H2ODiss, respectively, as defined in Figure 2. H1(H2OOCUS) is the hydrogen atom of H2OOCUS that is directly bonded to OCUS.

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energy than H2OOCUS,1 due to its additional electrostatic interaction with CuCS. 2. Mixed Coadsorption. Above we found two types of adsorption modes on this surface, H2OOCUS,1 and H2OOCUS,2. As with the stoichiometric surface, we next consider mixed coadsorption on the surface with Cu vacancies. In the following, we investigate a variety of different coadsorption models. The initial geometries are visualized in Figure S5. Dimers to Nonamers. For all coadsorption models, the relaxed structures are visualized in Figure 6 and the adsorption free energies are given in Table 5. First, we analyze simple coadsorption of both adsorbed waters (model c, see Figure S5c). After relaxation, the two water molecules stay on their respective adsorption sites in the vacancy site and on top of the surface, and do not visibly interact with each other (Figure 6c). This is also reflected in the cooperative effect, which is negligible (see Table 5). Lateral interactions between water molecules adsorbed directly on the surface are prohibited by the location of the adsorption sites, as we saw for model c (see Figure 6c). For the trimer, we thus added an additional water molecule that can act as a bridge between both adsorbates (model d). Indeed, after relaxation, the bridging water molecule forms hydrogen bonds to both H2OOCUS,2 and H2OOCUS,1. The adsorption energy per water molecule becomes significantly more negative, and we see a large cooperative effect of about −0.3 eV. The initial structure of the tetramer (model e in Figure S5) contains an additional H2OOCUS,1, which can also interact with the bridging water molecule. Upon relaxation, the additional water molecule does not stay hydrogen-bonded to OCUS,1, but it gets closer to the bridging water molecule to form a strong hydrogen bond (O − H distance of 1.47 Å), while interacting electrostatically with CuCS. The gained adsorption energy for this fourth water molecule is about −0.6 eV. This value is similar to the adsorption energy contribution of a single H2OOCUS,2. This is not surprising, as the additional water molecule interacts with a saturated Cu atom and forms one hydrogen bond, as H2OOCUS,2 does. The last two considered initial structures are a water hexamer and nonamer (model f and g in Figure S5). The hexamer initially consisted of three H2OOCUS,1 and three bridging water molecules. From above the surface, this cluster would look similar to the hexamer on the stoichiometric surface. However, due to the vacancies on this surface, the CuCUS adsorption sites are missing. The nonamer initial structure is similar to the hexamer, but three additional water molecules (H2OOCUS,2) fill up the vacancies. This structure consists of three subunits, each comprised of one H2OOCUS,1, one H2OOCUS,2, and one bridging water molecule (model d in Figure S5). Upon relaxation, the hexamer is not stable in its initial configuration. The three H2OOCUS,1 stay bonded to OCUS,1, but two out of the three bridging water molecules bind to OCUS,2 (Figure 6f). In the relaxed nonamer structure, one of the H2OOCUS,1 becomes a bridging water molecule, which additionally interacts with a CuCS, gaining some interaction energy. These two structures, the hexamer and nonamer, show comparable adsorption energies of about −0.59 and −0.57 eV. 3. Comparison to Stoichiometric Surface. The water adsorption properties are very different for the surface with Cu vacancies and the stoichiometric surface. The surface with Cu vacancies lacks strong adsorption sites like the unsaturated Cu atoms. Also, it lacks an adsorption site template that enables highly ordered water cluster arrangements. Because of the

Figure 5. Optimized geometries of water on the Cu2O(111) surface with Cu vacancies: (a) H2OOCUS,1 hydrogen-bonded to OCUS in the surface layer (left figure) and (b) H2OOCUS,2 hydrogen-bonded to OCUS in the subsurface layer (right figure).

other four structures (b−e) all relaxed to the same geometry (H2OOCUS,2, Figure 5b). In this structure, the water molecule is in the cavity, H1 forms a hydrogen bond to OCUS,2, and H1−OW is tilted toward one of the CuCS in the cavity, due to electrostatic attraction (CuCS is positively charged, 0.73, and OW is negatively charged, −1.21). The hydrogen bond (see Table 4) between H2OOCUS,2 and OCUS,2 is shorter than between H2OOCUS,1 and OCUS,1, indicating that the additional electrostatic interaction between OW and CuCS enhances H2OOCUS,2’s binding to the cavity. Table 4. Structural Parameters and Adsorption Energies for the Optimized H2O Monomer Configurations on the Cu2O(111) Surface with Cu Surface Vacanciesa distances (Å) structure H2OOCUS,1 − 1/2 ML H2OOCUS,1 − 1/6 ML H2OOCUS,2 − 1/2 ML H2OOCUS,2 − 1/6 ML

OCUS−H1 OW−H1 OW−H2

CuCS−OW

adsorption energies (eV)

1.83

0.98

0.97

−0.24

1.82

0.99

0.97

−0.24

1.65

1.01

0.98

2.40

−0.59

1.66

1.01

0.98

2.57

−0.53

a

Bond distances are given in Å. The atom definitions are given in Figure 5. The adsorption energies are given in eV per water molecule at 0 K.

H2OOCUS,1 and H2OOCUS,2 were again relaxed at 1/6 ML coverage (on the surface with (√3 × √3)R30° periodicity) in order to investigate their stability and properties at lower coverage. The obtained structural parameters are also summarized in Table 4. The only noticeable structural difference between different coverages is a longer distance between OW and CuCS for H2OOCUS,2 in the lower coverage case, concomitant with a slightly less negative adsorption energy (Table 4). The adsorption energies are also given in Table 4. Compared to water adsorption on the stoichiometric surface, adsorption on the surface with Cu vacancies is much weaker. Adsorption via dative bonding is much more favorable than hydrogen bonding to OCUS (adsorption to CuCUS is about 0.8 eV more stable than to OCUS,1, see Tables 1 and 4). As both adsorption sites can compete with each other on the stoichiometric surface, the initial structure with water hydrogen-bonded to OCUS relaxes to the much more stable datively bonded structure. The adsorption energies for both cases reflect the interaction patterns. H2OOCUS,2 shows a much more negative adsorption 9318


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Figure 6. Equilibrium adsorbate geometries considered for H2O monomers to nonamers of hydrogen-bonded (H2OOCUS,1 and H2OOCUS,2) and bridging (H2Obridge) water molecules on the Cu2O(111) surface with Cu vacancies with (√3 × √3)R30° periodicity: (a) one H2OOCUS,1 hydrogenbonded to OCUS,1 in the surface layer; (b) one H2OOCUS,2 hydrogen-bonded to OCUS,2 in the subsurface layer; (c) one H2OOCUS,1 and one H2OOCUS,2, showing no lateral interaction; (d) same as c, with an additional H2Obridge that forms a hydrogen bond to both neighboring H2OOCUS,1 and H2OOCUS,2; (e) same as (d), with an additional H2Obridge that forms one hydrogen bond to the other H2Obridge and electrostatically interacts with CuCS; (f) three H2OOCUS,1, two H2OOCUS,2, and one H2Obridge; and (g) three H2OOCUS,2, two H2OOCUS,1, and four H2Obridge that bridge between the other water molecules. Side (figures above) and top (figures below) views are displayed for each model. Red, blue, and gray spheres represent oxygen, copper, and hydrogen, respectively.

energies per water molecule that we predict for the trimer to nonamer clusters are all in a similar range. Thus, it is reasonable to assume that our models are representative for the general trends that can be observed for water clusters on this surface. We therefore can deduce some general properties for water binding to the surface with Cu vacancies: water molecules can bind to both unsaturated oxygen species on the surface, OCUS,1 and OCUS,2; water molecules can bridge between these H2OOCUS species and can be further stabilized by interacting electrostatically with saturated Cu atoms in the first trilayer. The cooperative effect observed for the models on the surface with vacancies is in a similar range as observed for the stoichiometric surface, from −0.30 to −0.37 eV. However, as adsorption to CuCUS is much more favorable, the adsorption energy of water clusters on the stoichiometric surface is significantly more negative (−0.99 to −1.03 eV per water molecule) than on the surface with Cu vacancies (−0.55 to −0.59 eV per water molecule). D. Comparison to Experimental Results. We next compare our predictions to measurements for water adsorption on a Cu2O(111) surface. Ö nsten et al. found by photoemission spectroscopy that water desorption on the Cu2O(111) surface occurs between 150 and 180 K.28 In our recent collaboration,29 a Cu(111) surface was oxidized, resulting in islands of

Table 5. Adsorption Free Energies for Mixed Mode Adsorption of Water Molecules on the Cu2O(111) Surface with Cu Vacancies at 0 Ka structure 1H2OOCUS,1 + 1H2OOCUS,2 1H2OOCUS,1 + 1H2OOCUS,2 + 1H2Obridge 1H2OOCUS,1 + 1H2OOCUS,2 + 2H2Obridge 3H2OOCUS,1 + 2H2OOCUS,2 + 1H2Obridge 2H2OOCUS,1 + 3H2OOCUS,2 + 4H2Obridge

model number (Figure 6)

ΔG (eV) per H2O

cooperative effect (eV)

c d

−0.43 −0.55

−0.04 −0.30

e

−0.56

−0.37

f

−0.59

−0.30

g

−0.57

−0.34

a

The values are given in eV per water molecule. The cooperative effect is calculated as the difference between the actually computed ΔG and the theoretical average value for the two different bonding types for low coverage (see Table 4: H2OOCUS,1 at 1/6 ML: −0.24 eV, H2OOCUS,2 at 1/6 ML: −0.53 eV, H2Obridge: 0.00 eV).

vacancies, the surface is rougher. As a consequence, more complex water binding scenarios arise, resulting in a vast conformational space for water binding onto this surface. An extensive investigation of all possible adsorbed water clusters is beyond the scope of this work. However, the adsorption 9319


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Figure 7. Adsorption free energies of different configurations of adsorbed water molecules and clusters at a range of temperatures on the stoichiometric surface and on the surface with Cu vacancies. (a) Adsorption free energies at a water partial pressure of 10−12 atm; (b) values for 0.032 atm water partial pressure (water vapor pressure at room temperature).51 For both surfaces, the values of the single water molecule adsorption modes as well as of the most stable mixed mode cluster are shown (the hexamer in both cases). Negative/positive adsorption free energy means that adsorption is favorable/unfavorable, respectively.

network, and therefore desorption could be kinetically inhibited, as we suggested earlier.29 Thus, higher temperatures than those at which the adsorption free energy turns positive are necessary for desorption. At 180 K, the temperature at which the stoichiometric Cu2O/Cu(111) surface experimentally still shows adsorbed water clusters at the edges of the cuprous oxide islands, our computations show that adsorption of datively bonded water, the water hexamer, and similar structures is favorable. As was shown above, the adsorption energy per unit cell can be maximized by coadsorbing H2OOCUS and H2OCuCUS. Thus, rather than datively bonded water alone, we expect that mixed coadsorption of H2OCuCUS together with H2OOCUS occurs, forming hexamers or larger clusters. 2. Pressure Dependence of Water Adsorption. What happens at more ambient pressure conditions? To investigate this, we computed the adsorption energies in the same temperature range at a water partial pressure of 0.032 atm (the water vapor pressure at room temperature;51 see Figure 7b). Compared to the lines in Figure 7a, the lines in Figure 7b have smaller slopes and thus the intersection with the horizontal axis occurs at higher values, which means that desorption occurs at higher temperatures. The relative stability, however, stays similar. At 0.032 atm, it can be expected that water is adsorbed on the surface with Cu vacancies at room temperature, but only in Cu vacancy sites or as clusters. Only H2OOCUS,1 desorbs at lower temperatures (about 140 K). On the stoichiometric surface, water and water clusters stay adsorbed up to much higher temperatures (about 600 K, data not shown here), suggesting that water should easily wet the stoichiometric surface under conditions of high humidity. 3. Vibrational Frequencies. Not many experimental infrared (IR) spectra have been reported for water on surfaces. IR studies of water on silicon oxide surfaces showed that water forms ice-like structures up to three layers thick on the surface.52 An IR investigation of aluminum oxide surfaces revealed that characteristics of the adsorbing water layer change from adsorbed hydroxides to ordered molecular adsorption to disordered molecular adsorption, depending on the applied relative humidity.53 On a vanadium oxide surface, water adsorption was investigated with IR spectroscopy at varying temperatures. The observed features changed with temperature, which indicated that different adsorption patterns prevailed at different temperatures.54 To further assist in elucidating water adsorption on Cu2O surfaces, here we characterize the different water adsorption vibrational modes on the two studied surfaces. We calculated the vibrational frequencies for the OH stretching

Cu2O(111) on the Cu(111) surface. To investigate water adsorption, water was added to the sample at 20 K, and the sample was then heated to different temperatures. At each temperature, the surface was characterized by constant-current STM imaging under UHV conditions. Water was observed to adsorb at 20 K in both the centers and on the edges of the islands (Figure S1b in ref 29). When the sample was heated to 180 K, water desorbed only from the centers of the islands whereas water clusters were observed at the edges (Figure S1c in reference29). Based on trends in predicted cluster binding energies and structural comparisons to STM images, we deduced that the surfaces of these Cu2O islands have Cu vacancies in the center and stoichiometric composition at the edges. We now test this hypothesis further by computing temperature dependent adsorption free energies, estimating desorption temperatures, and predicting vibrational frequencies for comparison with experiment. 1. Temperature Dependence of Water Adsorption. Figure 7a displays the computed stability of the different adsorbed water molecules and clusters for the two investigated surfaces in a temperature range from 0 to 350 K. As already suggested by the adsorption energies at 0 K, water adsorbed on the surface with Cu vacancies is expected to desorb at much lower temperatures than water adsorbed on the stoichiometric surface. We consider the temperature at which the free energy of adsorption crosses through zero to be the temperature above which desorption will proceed. This is a reasonable assumption, given that typically there is no barrier to adsorption and therefore frequently desorption kinetics track the thermodynamics (desorption barriers being essentially equal to desorption endoergicities). Surface with Vacancies. The least stable adsorbed water species is water hydrogen-bonded to OCUS,1 in the upper part of the surface trilayer (H2OOCUS,1), which is stably adsorbed only up to about 66 K under UHV. Water adsorbed on OCUS,2 (H2OOCUS,2) should desorb at about 140 K under UHV, and hexamer cluster adsorption, the most stable mixed cluster on the surface with vacancies, becomes unfavorable at about 160 K under UHV. This means that at 180 K under UHV, none of these adsorbed water molecules or clusters will remain on the surface with Cu vacancies, consistent with the experimental observations of Kronawitter et al. and Ö nsten et al.28,29 Stoichiometric Surface. On the stoichiometric surface under UHV, the datively bonded water (H2OCuCUS) and the water hexamer are expected to desorb in the range between 270 and 300 K. The water hexamer exhibits a strong lateral interaction 9320


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Table 6. Selected Vibrational Frequencies (in cm−1) of Different Configurations of Adsorbed Water Molecules and Clusters on the Stoichiometric Surface and on the Surface with Cu Vacanciesa OH stretching when hydrogen-bonded to structure stoichiometric surface

surface with Cu vacancies

gas liquid solid

surface (to OCUS) (I)

water (II)

H2ODiss H2OCuCUS ring-like hexamer H2OOCUS,1 H2OOCUS,2 hexamer

vacuum (III) 3725, 3653

2926b 1891symm,c 1834asymm,c 1794asymmc

3727 3204, 3198, 3175, 3101, 3088, 3055 3794, 3789symm, 3789asymm 3732 3807 3522, 3288, 3210symm,d 3080asymm,d 3801, 3792 2713

3050 3469 3463, 3367, 3210symm,d 3080asymm,d 2957, 2657 3775, 3656e 3319e 3411,e 3289e

bending 813,f 760,f 725f 1575 1670 and lower 1568 1591 1665 and lower 1594e 1642e 1641e

a

Reference values for gaseous, liquid, and solid water are also given. bOH hydrogen-bonded to neighboring OCUS with an O−H distance of 1.74 Å. OH of H2OOCUS directly hydrogen-bonded to OCUS with O−H distances of 1.36 to 1.37 Å. dConcerted stretching of water−water and water− surface O−H bonds. eTaken from ref 53. fCu−O−H bending. c

surface shows two narrow ranges that are almost 1200 cm−1 apart from each other. The hexamer on the surface with vacancies does not show the OH stretching region around 1800 to 1900 cm−1, but one wide range from 2650 to 3500 cm−1. Our prediction suggests that dissociative adsorption can be detected from OH stretching frequencies. Dissociated water does not show any OH stretching frequency below 3650 cm−1. Besides that, the Cu−O−H bending frequency of dissociated water, the highest bending frequency where the OH groups are involved, is predicted to be 750 cm−1 below that of the other models.

and H−O−H bending modes (Table 6). Computationally, we can distinguish between OH stretching modes that are involved in water−surface bonding versus water−water interactions, whereas distinguishing these data experimentally is much more complicated, as additional bulk water contributes significantly to the vibrational spectrum. The main features of experimental vibrational spectra of water in gas, liquid, and solid phases are given at the bottom of Table 6 for reference. As can be seen in Table 6, the OH groups involved in water−surface bonding (I), water−water hydrogen bonding (II), and water pointing toward vacuum (III) give rise to different ranges of stretching frequencies. Not surprisingly, the stretching modes of type III are similar to those of water in gas phase. Let us next inspect the type I stretching modes. For the monomer on the stoichiometric surface, the frequency is lower than for the monomers on the surface with vacancies. This is probably due to dative bonding of OW to CuCUS, compared to H2OOCUS,1 and H2OOCUS,2, where OW does not form intramolecular bonds. For the hexamer on the surface with Cu vacancies, a wide range (about 800 cm−1) of type I frequencies is predicted, which is due to the relatively disordered structure of the water cluster on the surface. This causes a large variety of binding strengths of water molecules bonding to the surface. For the ring-like hexamer on the stoichiometric surface, the type I frequencies cover a much smaller range and are significantly lower than the type I frequencies for all other models, which is probably due to the special arrangement of this water hexamer, where each H2OOCUS is strongly stabilized by two neighboring H2OCuCUS. Type II frequencies only occur for the water clusters. Type II frequencies for the hexamer on the surface with vacancies cover a range of 800 cm−1, which are shifted upward by about 100 cm−1 compared to type I frequencies. This range coincides with the observed stretching frequencies for liquid and solid water; the intramolecular bonding might be similar to those cases. The type II frequencies for the ring-like hexamer again cover a much narrower range, which at the same time does not overlap with the experimentally observed frequencies for liquid and solid water; again emphasizing the unusually strong interaction in the ring-like hexamer. In summary, the water clusters on the stoichiometric surface and the surface with vacancies are characterized by different stretching patterns. The ring-like hexamer on the stoichiometric

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CONCLUSIONS We have performed a computational investigation of water adsorption on the two most stable Cu2O(111) surfaces: the stoichiometric surface and the surface with Cu vacancies. The most crucial difference between the two surfaces is the availability of adsorption sites. On both surfaces, unsaturated oxygen atoms (OCUS) are present. Unsaturated Cu atoms (CuCUS) are only available on the stoichiometric surface. We investigated monomolecular adsorption and cluster formation on both surfaces. Binding to the Cu sites is possible via dative bonding. This is much stronger than binding to the oxygen sites, which occurs via hydrogen bonding. On the surface with Cu vacancies, because dative bonding is not an option, hydrogen bonding to subsurface OCUS is preferred. In this position, the water molecule adsorbs in the cavity and can further electrostatically interact with a nearby coordinatively saturated Cu atom that is at the edge of the cavity. This significantly enhances binding compared to adsorption on the top layer OCUS (by about 0.3 eV). On the stoichiometric surface, dative bonding is the most stable monomolecular adsorption mode. Dissociative adsorption is possible only on the stoichiometric surface, but it is less favorable than dative adsorption (by about 0.2 eV). Hydrogenbonded adsorption is only possible if there are no unsaturated Cu atoms present nearby to drive the waters toward dative bonding. Water cluster formation on the surface with vacancies is only possible with extra bridging water molecules. The surface adsorption site template is such that water molecules bound to the surface do not interact with each other directly. This is due to the rough structure of the surface, with neighboring 9321



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ACKNOWLEDGMENTS This work was supported by financial support from the Air Force Office of Scientific Research (Grant No. FA9550−14− 1−0254) and supercomputing resources from the DoD High Performance Computing Modernization Program. We thank Dr. Leah Isseroff Bendavid, Prof. Bruce E. Koel, and Dr. Coleman X. Kronawitter for helpful discussions.

adsorption sites being in the subsurface layer in a cavity and on the top surface layer. With extra bridging water molecules, lateral interactions emerge. The gain in adsorption energy for clusters with bridging waters is about 0.3 to 0.4 eV per water. Water cluster formation on the stoichiometric surface is quite favorable for a combination of datively bonded and hydrogenbonded water molecules. Besides water−surface interactions, they also exhibit lateral water−water interactions via hydrogen bonds. The gain in energy for cluster formation is about 0.3 eV per water molecule for the more stable dimers, trimers, tetramers, and hexamer. When the clusters form, a strong dative bond is paired with a weaker hydrogen bond to the surface. Besides strong lateral interactions, facile surface diffusion will be important for water cluster formation. As monomolecular hydrogen-bonded water shows relatively weak water−surface interactions, it is likely to have a much longer diffusion length than monomolecular, datively bonded water. The latter can act as an anchor, while hydrogen-bonded water will likely be more dynamic. We expect that water molecules will move on the surface until finding one or several datively bonded water molecules, whereupon it will get trapped, bridging the more strongly adhered water molecules. This mechanism of cluster formation also may explain why the desorption temperature observed for the water clusters on the Cu2O(111) surface is unusually high. Altogether, datively bonded water acts as an anchor, providing strong adsorption, and addition of hydrogenbonded water to form a water cluster adds more (albeit weaker) water−surface and lateral attractive water−water interactions. Water clusters on both surfaces are predicted to show quite distinct OH stretching frequency patterns. Additionally, the OH stretching and bending frequencies for dissociatively adsorbed water are very different from that of the other adsorbed waters. These predictions can be used to distinguish between the different adsorption types experimentally. The stoichiometric surface exhibits an ideal template for water cluster binding. Oxygen atoms in water molecules on top of these unsaturated Cu and O adsorption sites are not well suited for lateral hydrogen bonding because the adsorption sites are about 3 Å apart. However, water-surface binding via dative and hydrogen bonds is flexible enough to allow for some rearrangement of the water oxygen atoms, which allows the atoms to position themselves to optimize their hydrogen bond distances. The outcome is a compressed circle containing the oxygen atoms of the adsorbed water molecules, relative to a circle defined by the underlying adsorption sites. This principle could hold for other metal oxides as well. Surface structures that show similar termination and adsorption site templates, or one that is slightly compressed or enlarged, should exhibit similar behavior.

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ASSOCIATED CONTENT

S Supporting Information *

Information is provided on initial geometries and on the adsorption free energies for all dimers and trimers adsorbed on the stoichiometric Cu2O(111) surface. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 9322


Article

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