H2O Interaction with the Polar Cu2O(100) Surface: A Theoretical

Juan-Jesús Velasco-Vélez , Katarzyna Skorupska , Elias Frei , Yu-Cheng Huang , Chung-Li Dong , Bing-Jian Su , Cheng-Jhih Hsu , Hung-Yu Chou , Jin-Mi...
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J. Phys. Chem. 1996, 100, 1874-1878

H2O Interaction with the Polar Cu2O(100) Surface: A Theoretical Study Martin A. Nygren and Lars G. M. Pettersson* Department of Physics, UniVersity of Stockholm, Box 6730, S-113 85 Stockholm, Sweden ReceiVed: August 21, 1995; In Final Form: October 16, 1995X

The adsorption of water on the polar Cu2O(100) surface has been studied theoretically. Both molecular and dissociative adsorption are studied. Dissociative adsorption is found to be energetically favorable, and good qualitative agreement is achieved between experimental and simulated UPS spectra for low coverages. The simulated spectrum allows for an assignment of the peaks in the experimental spectra. For higher coverages the model used is found to be insufficient; we believe the main reason for this is adsorbate-induced reconstruction of the surface.

Introduction Cutting an ionic crystal such that only one type of ions is exposed at either surface leads to a polar surface, which would have a resulting dipole across the crystal. This is an electrostatically unstable situation and the surface must by necessity reconstruct in one of several different ways. For instance, the dipole across the crystal may be removed by having (for a rocksalt structured crystal) half the ions go with either surface; this would result in two equivalent cation or anion terminated surfaces, each with half occupancy of the surface sites. A second possibility is to have the surface relax electronically, i.e., a change of charge state of the ions at the surface. A third possibility is for the surface to become chemically stabilized by decomposing, e.g., water leading to (M2+OH-)+ and OHoccupancy at the surface, which gives the required reduction of the charges. We have recently studied the structural reconstruction of the Cu2O(100) surface and its reaction with hydrogen.1 In the present work we will continue our studies with an investigation of the reactivity of the reconstructed surface toward H2O, both dissociative and molecular adsorption. Cox and Schulz have published a series of experimental investigations of the polar Cu2O(100) surface.2-7 Among other adsorbates they considered atomic hydrogen,3 propene,4 alcohols,5 and carboxylates6 in addition to water.7 For the water adsorbate both temperature-programmed desorption (TPD) and UPS spectra were obtained. The TPD experiments for small coverages gave two main peaks in the spectrum at 210 and 465 K, which were interpreted as due to molecularly and dissociatively chemisorbed water, respectively. At the lowest coverage investigated only the high-temperature peak was observed. At high coverage the high-temperature peak becomes negligible compared to the low-temperature peak. The low-temperature peak shifted to lower temperatures with increasing coverage and at the highest (2 langmuirs) coverage was found at 183 K with a new peak split off from the main one at 170 K. Only H2O was observed as desorption product in the experiments. At 110 K both dissociatively and molecularly adsorbed water was observed, but with dissociation corresponding to only 10% of a monolayer. At 300 K only dissociative adsorption was obtained. The active sites and the structure and composition of the final products were, however, not unambiguously established. In our earlier work we investigated different possible reconstructions of the (100) surface; the most stable structure * Author to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, December 15, 1995.

0022-3654/96/20100-1874$12.00/0

of those investigated turned out to be a (1 × 1) missing-row reconstruction, which will be used as model of the surface also in the present work. We have performed a complete geometry optimization, including dissociation, of the water molecule interacting with embedded cluster models of the surface. A special procedure was implemented to automatically and efficiently optimize the structure without gradient information; this procedure can be advantageous for systems where a large number of atoms are to be frozen as in the present cluster models and for cases where analytical gradients are not available. Computational Details Cu2O has a cuprite structure with a lattice parameter of 4.267 Å.8 The unit cell is cubic with oxygen ions at the center and corners and four of the eight interstitial positions occupied by the Cu+ cations. Several different reconstructions of the polar (100) surface were studied and reported in ref 1 and here we will only summarize the procedure used in that work. The reconstructed and relaxed surface was obtained using the MARVIN9 program and a two-body potential (Buckingham type) description of the interatomic interactions. The anion polarizability was described with a harmonic spring shell model. The main result of the relaxation of the reconstructed surface is a more compact surface where the cation sinks in and the oxygen anion becomes somewhat more exposed. The motions are quite large within the surface unit cell; the top-layer Cu+ moves downwards by 1.36 Å, the second layer copper ions by 0.28 Å, while in the third layer one copper moves down by 0.32 Å and the other up by 0.44 Å. Similarly, the topmost oxygen moves downwards by 0.90 Å, while the second-layer oxygen moves upwards by 0.24 Å. The quantum chemical models of the surface consisted of a CuO, Cu2O, or Cu3O unit embedded in a surrounding of total ion ab initio model potentials (AIMP)10 out to a radius of 10.5 a0 (counted from each cluster center); the positions of cluster and embedding ions were generated from the reconstructed and relaxed surface unit cell obtained from MARVIN. The AIMP formalism gives an approximate representation, without basis functions, of all interactions between the cluster and the surrounding frozen charge distributions; thus long-range (pointcharge) and short-range (incomplete screening) Coulomb interactions, and Pauli repulsion through level-shift operators, as well as a spectral resolution of the exchange interaction are included. Allowing a water molecule to interact directly with a Cu+ center represented by either an all-electron Cu ion or the AIMP representation gave very similar results. The difference © 1996 American Chemical Society

H2O Interaction with the Polar Cu2O(100) Surface in interaction energy between using an AIMP description for Cu+ and an all electron description was 0.07 eV and the distance between the copper ion and the oxygen decreased from 4.3 to 3.7 a0 when changing from an all-electron description to an AIMP description of the copper ion. These differences were small enough for the AIMP description to be trusted within the required accuracy. The crystal potential was included by directly evaluating the one-electron integrals over the Ewald sums using a newly written extension of the ECPAIMP program.11 The geometry optimizations were performed at the SCF level using a (5s 1p)/[3s 1p] basis12 for hydrogen, the Wachters (15s 12p 6d)/[6s 5p 3d] basis for copper,13 and a (10s 6p 1d)/[5s 4p 1d] basis for oxygen.14 At the optimized geometry the total energy was evaluated at the modified coupled pair functional (MCPF)15 level and the Cu basis extended to also include a contracted f-function (3f/1f). Relativistic effects were obtained through a first-order perturbation theory treatment of the mass-velocity and Darwin terms and have been added to all MCPF results. In order to find the optimal geometries, a numerical minimizer was implemented. The method used is Powell’s method as described in ref 16 with a parabolic interpolation for the line minimizations. Thus, pointwise calculations were performed in a first step minimizing the energy along each degree of freedom. After all coordinates have been separately optimized once, linear combinations of these are used in the subsequent optimization steps. Typically this required 70-80 points for 5-6 degrees of freedom giving an optimized geometry after three complete sets of line minimizations. The present implementation thus corresponds to 20-30 gradient evaluations in cost, but has clear advantages in cases with a large number of frozen centers (e.g. cluster models) and when complicated model Hamiltonians, such as the present AIMP formalism combined with the Madelung potential, are being employed. Furthermore, since numerical derivatives could be generated from the computed points, the implementation can be extended to include update procedures to accelerate convergence. In order to compare with the experimental UPS data from Schulz and Cox7 a very simple procedure to convert the Koopmans ionization potentials (i.e., orbital energies) to a synthetic, qualitative spectrum was applied. Thus, the contribution to the spectrum from each orbital was estimated based on a Mulliken population analysis and estimated relative transition probabilities of 1:3:10 in the order s, p, and d; the energy dependence of the cross sections was thus not accounted for. The individual peaks were then broadened by a Gaussian of full width at half-maximum of 1 eV after shifting each peak by a constant of 6 eV in order to align the computed and experimental spectra. The positions of the s- and p-dominated states should be well represented by this procedure, since, for the valence, relaxation and correlation effects tend to cancel. Furthermore, final state effects can be expected to be small due to the low polarizability of the ionic substrate. The ionization potentials of the 3d states can be expected to be somewhat underestimated due to the neglect of the large initial state dynamical correlation contribution. The difference spectra, comparing clean and covered surfaces, are expected to give a good representation of the shifts of the 3d states as well as the s,p-states, however, Furthermore, the difference spectra correspond to differences in populations using the same basis sets, such that uncertainties in the underlying Mulliken population analysis are expected to be largely eliminated. Results and Discussion Dissociative Adsorption. Experimentally, water adsorption on Cu2O(100) is dissociative at 300 K.7 At 110 K both

J. Phys. Chem., Vol. 100, No. 5, 1996 1875

Figure 1. Dissociatively adsorbed water with the OaH- group on top. Largest spheres, Cu+; smallest spheres, H.

Figure 2. Dissociatively adsorbed water with the OaH- group in bridge position. Largest spheres, Cu+; smallest spheres, H.

dissociative and molecular adsorptions are observed at low coverages, about 10% of a monolayer being dissociated. There is no dependence on the concentration of defects, thus the dissociation must be assumed to occur on regular sites of the reconstructed surface. However, which sites are active and what the positions of the dissociated products are has not been known. The calculations have been performed by keeping the substrate ions fixed and varying all degrees of freedom for the two hydrogens and the oxygen in the approaching H2O molecule. Only the final minimum-energy positions and the resulting binding energy were determined in the present work. Transition state searches will be the subject of future work. Two resulting stable, dissociated structures were obtained with the dissociated OH group either on top of one surface Cu+ ion (Figure 1) or bridging between two cations (Figure 2). The computed binding energies relative to surface and H2O were 0.64 and 0.01 eV, respectively. Clearly, the on-top position is preferred. The dissociation leads to the formation of two OH- groups at the surface and a further redistribution of charge. In the cluster before adsorption we have a charge distribution according to Mulliken population analysis of +0.6 for the Cu and -1.6 for the oxygen. For the on-top dissociation the Cu-OH unit has an internal charge distribution of Cu +0.8, oxygen -1.1, and hydrogen +0.2 leading to an overall charge for the CuOH of -0.1, instead of the initial +0.6. Similarly, the charge on the surface oxygen ion is reduced from -1.6 to -1.2 and with

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Figure 4. Molecularly adsorbed water on top of an oxygen ion, hydrogen downwards. Largest spheres, Cu+; smallest spheres, H. Figure 3. Simulated UPS spectra for (a) cluster model of clean Cu2O(100) missing-row reconstructed surface; (b) with dissociated water molecule (on-top; see text); and (c) difference spectrum.

the hydrogen charge included the resulting charge for this OH group is -0.9. Thus, the dissociation of water would be expected to affect the surface dipole through the redistribution of charge. Experimentally, a reduction of the surface dipole is observed when the surface is covered by dissociated water.7 Since hydroxy ions on the surface are expected to lower the surface dipole, this is interpreted as the dissociation products being hydroxy groups. This is thus in accordance with our calculations. The optimized Cu-O distance is 3.9 a0 which is 0.4 a0 longer than the Cu-O distance in the surface layer. The OH distance in the Cu-OH group is 1.79 a0, while the surface oxygen derived OH group has an internal distance of 1.83 a0, indicating a somewhat weaker OH bond. In the latter case it is noteworthy that the hydrogen is sufficiently close (3.12 a0) to the oxygen in the CuOH unit that some contribution from hydrogen bonding can be expected. UPS Spectra. The UPS results were obtained from the Koopmans’ theorem valence ionization potentials (IP) as described in the methods section; the form given in Figure 3 provides an easy representation to discuss the changes upon dosing H2O onto the surface. Beginning with the clean surface we find two peaks in the valence: the peak at lower binding energy consists mainly of oxygen p-contributions with some antibonding contribution from the 3d-manifold. The dominating peak arises from ionization from the 3d orbitals with a high binding energy shoulder from bonding O(2p) combinations. Upon dissociative chemisorption of H2O the changes may be understood from the basic picture of a proton interacting with the surface oxygen (Os) which increases the oxygen orbital binding energies uniformly by about 5 eV. For the Cu 3d spectrum, on the other hand, the shift is to lower binding energy due to the interaction with the OH- unit. The spectrum now shows a set of three major peaks, where the original Os low binding energy peak has been replaced by a mainly adsorbate derived, Oa, shoulder on the 3d main peak. The 3d peak is now shifted by about 1 eV to lower binding energy. The second peak consists of contributions from mainly surface oxygen (Os) and adsorbed hydroxyl group (OaH- σ), while the OsH- σ is found at about 4 eV higher binding energy. The peak structure, for both the clean and adsorbate-covered surfaces, is in good qualitative agreement with the experiment.7 In particular, the difference spectrum demonstrates the shifts also observed experimentally: the initial peak is narrowed due

to the shift of the 3d states to lower binding energy and at the same time the large shift of the Os(2p) antibonding states to higher binding energy. The latter are replaced by the OsH-(2p) states, but at somewhat higher binding energy. The increase observed experimentally at about 2 eV higher binding energy than the main 3d-peak corresponds to the OaH- σ-bond and the downwards shifted surface oxygen states. The increase at about 7 eV higher energy than the main peak thus corresponds to the OaH- σ-bond. It should be pointed out that the two highenergy peaks are assigned to the 3σ and 1π molecular orbitals of the OH- ion in ref 7. In the calculated spectra both peaks are from σ-orbitals, but different hydroxy groups. The highest energy peak corresponding to the surface oxygen and the second highest to the adsorbed oxygen. Experimentally, an additional structure between the two higher energy peaks is observed; this is not found using the present cluster model. Molecular Adsorption. The adsorption of molecular water was investigated both on the clean surface and on a cluster model of the OH-covered surface. In both cases, a restricted optimization, maintaining equal O-H distances, was performed in order to avoid dissociation. We will begin with the results for the clean surface. For the clean surface we have found two stable adsorption sites: on top of a surface oxygen anion with the hydrogens pointing downwards (Figure 4) and adsorption on a Cu+ ion at the step in the surface formed by the missing-row reconstruction (Figure 5). In the latter case the hydrogens point away from the surface. The structures show very similar stabilities with computed binding energies (MCPF) of 0.49 and 0.48 eV for oxygen down and hydrogen down, respectively. The experimental TPD results as function of coverage show no indication of additional stability due to hydrogen bonding; this is in contradiction to what is found on atomically smooth metal surfaces and has been suggested7 to be due to either a lower surface mobility on the oxide or that the surface, through the structure of bonding sites, inhibits the formation of extensive hydrogen bonding. The two structures for the molecular adsorption obtained in the present work would be in accord with the latter assumption; having alternating geometries for the water molecules with respect to the surface allows for at most pairwise hydrogen bonding and should exclude the formation of larger areas or regions of hydrogen-bonded water. For the adsorption on top of a cation we find very small effects on the internal H2O geometry. The O-H distance is 1.8 a0 and the angle is 105.3°, which is very close to the gas phase values. The distance to the Cu+ cation is 4.3 a0 which is

H2O Interaction with the Polar Cu2O(100) Surface

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Figure 6. Molecular water adsorbed in a threefold hollow position on a surface with two dissociated water molecules. Largest spheres, Cu+; smallest spheres, H. Figure 5. Molecularly adsorbed water on top of a copper ion, oxygen downwards. Largest spheres, Cu+; smallest spheres, H.

similar to what was found for the dissociated OH group (3.94.2 a0). When the adsorption is on top of an oxygen, the effects on the H2O are somewhat larger with the angle decreasing to 98.6°, but still with an OH distance of 1.8 a0. The distance of the hydrogens to the surface oxygen is 4.8 a0. The binding energies obtained for the molecularly adsorbed waters are slightly lower than the computed binding energy of 0.64 eV for the dissociation and so the driving force for dissociating the H2O would be expected to be weak from the present models. It should, however, be kept in mind that all optimizations have been performed assuming a fixed cluster geometry; adsorbate-induced reconstruction of the surface, which would tend to increase the binding energy, has not been included and it is our expectation that inclusion of this would favor the dissociation thus leading to a larger difference in the binding energies. This is furthermore in agreement with the observation that heating to 300 K is required for all adsorption to be dissociative; a higher binding energy for the dissociated products than what has been computed here is also indicated by the high (465 K) desorption temperature found for the recombination peak in the TPD spectrum. The recombination barrier should be lower than the binding energies so that, even if considering a barrier to the recombination, the computed binding energies are still too small to give the TPD hightemperature peak. We believe that the main discrepancy in the computed binding energy is due to the neglected stabilization obtained from adsorbate-induced substrate relaxation. This should also be expected to affect the stabilities and structures of water adsorbed in the presence of previously dissociated water molecules. Thus, the results for the molecularly adsorbed water should be regarded as preliminary and should be confirmed in calculations including additional surface relaxation in the presence of hydroxy groups. A further indication of the importance to consider additional surface relaxation is obtained from the results for the model where water is adsorbed molecularly in the presence of two previously dissociated molecules. The structure is shown in Figure 6 where the H2O is found to adsorb into a hollow site formed by three Cu+ cations between the two sets of OHgroups. The distances of the oxygen to the cations are in the range 3.84-3.93 a0 so that the binding is equally to all three centers. The computed binding energy at the SCF level is 0.98 eV to be compared with the SCF results of 0.3 eV for the two molecularly adsorbed structures without hydroxy groups. Even allowing for the error (3 × 0.07 eV) due to using an AIMP description for the nearest Cu+ cations and adding 0.2 eV as the average effect of correlation, this would still be the most

Figure 7. Simulated UPS spectrum for molecular water adsorbed in a threefold hollow position: (a) cluster model with preadsorbed OH groups; (b) H2O adsorbed into threefold hollow site; and (c) difference spectrum.

strongly bound structure with a binding energy that would correspond well with the TPD peak at 465 K. Comparing the simulated UPS spectrum (Figure 7) for this situation with the experiment for molecular water adsorption7 (note that the 3d-band is absent since only oxygen anions are explicitly included in this cluster model), we see that this situation must be ruled out, however. The experimental UPS spectrum was generated by starting with a multilayer ice structure on the crystal and successively removing adsorbates by a heat treatment adjusted to remove the different states observed in the TPD. The most prominent feature is the disappearance of the 1b2 signal already after heating to 165 K. In Figure 7 the 1b2 orbital of the adsorbed H2O is very much in evidence. Thus, the computed, very high binding energy for the molecularly adsorbed H2O on the hydroxylated surface must be reinvestigated using larger cluster models and in particular including relaxation of the surface upon adsorption. This will require the construction of pair- or three-body potentials describing the interaction of the hydroxy groups with the reconstructed and relaxed Cu2O(100) surface. This will be the subject of future investigations. Conclusions The present calculations show that the dissociation of water on the Cu2O(100) missing-row reconstructed polar surface is

1878 J. Phys. Chem., Vol. 100, No. 5, 1996 indeed energetically possible. The dissociation is found to occur heterolytically and is best described as resulting in an OHgroup interacting with an exposed Cu+ cation and a proton interacting with a surface oxygen to yield a second OH- group. The charge distribution, and consequently the surface dipole, is strongly affected in accordance with experiment. A simple analysis of the Koopmans’ theorem based ionization potentials shows that the cation levels are destabilized and the anion levels stabilized through the interaction with the negatively charged hydroxy group and the proton, respectively; the resulting difference spectrum is in good qualitative agreement with experiment and allows an assignment of the peaks. The surface models should be fair for low coverages but at higher coverages it would be desirable to include surface relaxation due to the hydroxylation. The neglect of this further relaxation is believed to be the reason that the calculated binding energies for dissociated water are too small compared with the observed high-temperature peak at 465 K in the TPD spectra. Furthermore, this relaxation will stabilize the surface and the binding energy for molecular water is expected to decrease. The effects of the adsorbate induced reconstruction/relaxation will be the subject of future work. Acknowledgment. This work was partly supported by funds from the Swedish Consortium on Oxidic Overlayers.

Nygren and Pettersson References and Notes (1) Nygren, M. A.; Pettersson, L. G. M.; Freitag, A.; Staemmler, V.; Gay, D. H.; Rohl, A. L. J. Phys. Chem., in press. (2) Schulz, K. H.; Cox, D. F. Phys. ReV. 1991, B43, 1610. (3) Schulz, K. H.; Cox, D. F. Surf. Sci. 1992, 278, 9. (4) Schulz, K. H.; Cox, D. F. Surf. Sci. 1992, 262, 318. (5) Schulz, K. H.; Cox, D. F. J. Phys. Chem. 1993, 97, 647. (6) Schulz, K. H.; Cox, D. F. J. Phys. Chem. 1992, 96, 7394. (7) Schulz, K. H.; Cox, D. F. Surf. Sci. 1991, 256, 67. (8) Restori, R.; Schwarzenbach, D. Acta Crystallogr. 1986, B 42, 201. (9) Gay, D. H.; Rohl, A. L. J. Chem. Soc., Faraday Trans. 1995, 91, 925. (10) (a) Barandiara´n, Z.; Seijo, L. J. Chem. Phys. 1988, 89, 5739. (b) Barandiara´n, Z.; Seijo, L. In Computational Chemistry: Structure, Interactions and ReactiVity; Fraga, S., Ed.; Studies in Physical and Theoretical Chemistry, Vol. 77(B); Elsevier: Amsterdam, 1992; pp 435-461. (11) ECPAIMP is an integral program for ECP and AIMP calculations written by L. G. M. Pettersson and L. Seijo. Integrals over the Ewald sum have been written by M. A. Nygren. (12) Huzinaga, S. J. Chem. Phys. 1965, 42, 1293. (13) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033. (14) Nygren, M. A.; Pettersson, L. G. M.; Barandiara´n, Z.; Seijo, L. J. Chem. Phys. 1994, 100, 2010. (15) Chong, D. P.; Langhoff, S. R. J. Chem. Phys. 1986, 84, 5606. (16) Press, W. H.; Teukolsky, S. A.; Vetterling, W. T.; Flannery, B. P. Numerical recipes in FORTRAN: the art of scientific computing; 2nd ed.; Cambridge University Press: London, 1992; p 410.

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