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J. Phys. Chem. C 2008, 112, 13226–13231
Hydroxyl-defect Complexes on Hydrated MgO Smokes F. Finocchi, R. Hacquart, C. Naud, and J. Jupille* INSP, UPMC UniVersity Paris 06, CNRS UMR 7588, 140 rue de Lourmel, 75015 Paris, France ReceiVed: April 15, 2008; ReVised Manuscript ReceiVed: June 14, 2008
The dissociative adsorption of water vapor on MgO smokes is studied by infrared spectroscopy and firstprinciples calculations. The MgO smokes are synthesized in controlled conditions so to display a particularly high concentration of defect-free step edges (4-fold coordinated “4C” sites). Exposure to water vapor results in a set of twinned infrared bands at νA ) 3480 and νB ) 3710 cm-1sthe former having never been observed beforeswith an intensity ratio I(νA)/I(νB) ≈ 2. Simulated configurations include water adsorption, for various coverages, on terraces of distinct orientations, at step edges and corners. It is shown that OH stretching frequencies can be robustly correlated with OH bond lengths. The twinned (νA,RνB) bands are assigned the hydrogen-bonded (Os4C-H, Mgs4C-OH) moiety adsorbed at fully decorated monatomic step edges on (100) terraces. The coordination number of adsorbed OH species is shown not to be sufficient to characterize the physicochemical properties of wet MgO smokes and it is suggested that the hydrated surface can be better described in terms of (OH, MgO) complexes. Introduction Water adsorption on oxides surfaces encompasses many environments,1 and its microscopic description is central in the understanding of adhesion,2 reactivity, dissolution, precipitation,1 and competitive adsorption.3 As in any aspect of the surface chemistry of oxides, defects play a key role in the hydration of oxides. For instance, they drive the interplay between molecular and dissociative adsorption and the modifications that follow the interaction with water.4 On metal and semiconductor surfaces, the role of defects is studied on purposely prepared crystal surfaces on which steps and kinks of defined orientation and controlled concentration are obtained by appropriate cuts. Unfortunately, oxide surfaces can hardly be tailored at will. There are no straightforward recipes to produce a defined population of steps and kinks at the surface of oxide crystals5 and, in common polycrystalline or high surface area samples, both nature and concentration of surface defects usually escape quantitative analysis. Besides the chemical composition of the oxide and the external conditions of temperature and pressure, the variable degree of localization of the electronic states that are associated with specific structural defects6 makes the identification of hydrated complexes on oxide surfaces extremely involved. Therefore, experimental observations of well-defined hydrated oxide complexes are lacking. Moreover, the propensity of water molecules and hydroxyl groups to make hydrogen bonds imposes to disentangle adsorbate-adsorbate interactions from adsorbate-substrate bonds by going beyond the model case of isolated molecules.7,8 The usual picture of oxide surfaces in terms of singular point defects or adsorbate coordination numbers often fails to account for local properties, which commonly involve higher-order neighbors or even global morphological features.9,10 Therefore, a deeper understanding of the reactivity of oxides at the contact of water calls for a direct characterization of complexes formed by extended defects when hydrated. Here we focus on magnesia, which is among the simplest metal oxide surfaces in terms of atomic and electronic structures. * Corresponding author e-mail:
[email protected].
Although its interaction with water is studied as a prototypical case,1,4,11,12 MgO is also involved in several processes of interest such as dissolution,10 catalysis, and air purification.13 Indeed, the bactericidal properties of highly dispersed magnesia are related to the number of hydroxyl groups that could lead to high concentrations of solvated O2- ions.14 The characterization of local configurations on hydroxylated MgO surfaces has been mostly performed by infrared spectroscopy. The OH stretching region of infrared (IR) spectra of hydrated MgO shows a broadband ranging from 3250 to 3650 cm-1 and a narrow band between 3690 and 3750 cm-1, which has also been observed by high-resolution electron energy loss spectroscopy (HREELS) at the surface of bulk MgO crystals.15 In models based on the nature of the surface sites, the narrow band was assigned to Mgs-OH and the broadband to Os-H (“s” denotes a surface atom).16 Classifications relying on the hydroxyl coordination number (C) attributed the broad feature to Os-H on terraces and the high frequency line to isolated OH.17,18 However, firstprinciples simulations showed that dissociative adsorption of the isolated water molecule does not occur on MgO(100) terraces but only on sites with lower coordination numbers (C e 4).11,19 On such sites, the molecule-substrate interaction is strengthened, thus enhancing the stability of the hydroxyl adgroups against desorption well above the room temperature, whenever water pressure is not too low.11 Despite the many attempts to characterize the OH adsorption configurations, unambiguous assignment of IR spectra of hydrated MgO powders has not yet been reached. Under the assumption that the 3690-3750 cm-1 band arises from OH groups adsorbed on surface atoms with C e 4, herein referred to as MgsLC and OsLC, and by comparing H-covered MgO with hydrated MgO, Kno¨zinger et al.20 identified MgsLC-OH (∼3740 cm-1) and OsLC-H (∼3700 cm-1) components. The broad 3250-3650 cm-1 band was suggested to arise from too many surface configurations to be unravelled.20 Recently, the preparation of MgO smokes in controlled conditions was found to strongly favor 4-fold coordinated (4C) sites at step edges.21 The present work aims at identifying the hydroxylated moieties that correspond to those sites by com-
10.1021/jp8032484 CCC: $40.75 2008 American Chemical Society Published on Web 08/05/2008
Hydroxyl-defect Complexes on Hydrated MgO Smokes bining Fourier transform IR spectroscopy with first-principles simulations. We pay a special attention to model several inequivalent (Os-H, Mgs-OH) configurations, for various hydroxyl coverage. A key point of the present work is that mode oscillator strengths (and not only frequencies) are accounted for in order to compare the computed vibrational band intensities to experiments. Experimental and Theoretical Methods MgO smokes were prepared by burning Mg ribbons within a glovebox filled in by dry Ar-O2 mixtures (80%-20%).21 Because exposure of MgO smokes to water may induce the transformation of 4C sites (step edges) into sites with lower coordination number,21 the samples were kept clear of contact with the ambient air at any stage of the preparation. In an original setup, smokes were directly collected on a silicon wafer because silicon is transparent to IR light in the OH stretching frequency range. The wafer was placed above the Mg ribbon during the combustion to collect the smoke. It was then kept under dry Ar-O2 mixture, by transferring it to the cell used for IR analysis within the glovebox. Once mounted on the IR spectrometer, the cell was pumped down to 10-3 Pa. MgO samples were degassed by annealing the doped silicon wafer up to 1200 K via a resistive heating. Water vapor was introduced into the cell from a reservoir of liquid water which had been purified by freeze-pump-thaw cycles. The IR cell was equipped with NaCl windows. The ABB-Bomem spectrometer of DA8 type was run in transmission with the Michelson interferometer at 30°, quartz-iodine source, and InSb detector with cold filter. The setup was under vacuum (20 Pa). To optimize the signal/noise ratio, the aperture was chosen to fit the spot diameter on the sample while avoiding saturating the detector. Each spectrum involved 250 scans. To cancel the interferences arising from the parallel faces of the silicon wafer, the resolution was limited to 4 cm-1. The calculations were performed using the periodic ABINIT code,22 which is based on density functional theory, within the generalized gradient approximation (GGA).23 We simulated several configurations, different in surface morphology (orientation or defects) and water coverage. Once we optimized each configuration, we computed the corresponding dynamical matrix within the density functional perturbation theory (DFPT),24 including up to OH third neighbors, as well as the effective charge tensor, in order to determine the mode eigenvectors and absolute intensities.25 We account for the interaction between the ionic cores and the valence electrons by norm-conserving pseudopotentials (PPs).26 Those calculations are rather time-consuming when the number of the atoms in the unit cell exceeds several tens, which is the case here. A practical recipe to reduce the computational burden is the use of soft PPs,26 which allow a substantial lowering of the number of plane waves that are employed to expand the Kohn-Sham orbitals. However, soft PPs are less accurate than harder PPs, especially as far as vibrational properties are concerned. We generated two O PPs, the first of which (PP1) is rather soft (see details in ref 11); the second PP is the default Martins-Troullier PP (PP2).26 PP1 provides wellconverged water adsorption energies already at 50 Ry cutoff. The computed OH equilibrium bond length in the isolated water molecule is 0.956 Å for PP1 and 0.964 Å for PP2. The eigenvalues of the dynamical charge tensors Z*(H) and Z*(O), which are computed with either PP1 or PP2, compare to each other within 1%. As far as the comparison of the theoretical and experimental vibrational frequencies is concerned, there are three main sources
J. Phys. Chem. C, Vol. 112, No. 34, 2008 13227 TABLE 1: Vibrational Frequencies (cm-1) of the Water Moleculea
asym stretching sym stretching bending
experimental29
our our calculation calculation calculation from (PP1) (PP2) ref 27
3756 3657 1595
3700 (-56) 3772 (+14) 3818 (+62) 3585 (-72) 3663 (+6) 3714 (+67) 1620 (+25) 1608 (+13) 1590 (-5)
a
All theoretical values refer to harmonic frequencies, while the experimental frequencies include anharmonic contributions. Shifts of calculated values with respect to the experiment are given in brackets. The calculations in ref 27 were carried out within the same GGA approximation as ours but without ionic pseudopotentials and free of finite-basis errors.
of error. First, we did not take into account anharmonic effects. This approximation will be justified below. Second, the use of the GGA-PBE approximation23 to the exchange-correlation energy leads to underestimated harmonic frequencies.27 Third, for selected cases, the OH stretching frequencies that are computed using PP1 are down-shifted by 50 to 70 cm-1 with respect to those that are obtained by employing PP2. Such errors are largely systematic, which we verified on some test cases: the isolated water molecule, the hydrated monatomic step on MgO(001), and the brucite crystal.28 When considering the isolated water molecule (Table 1), the systematic errors arising from the use of the GGA in conjunction with PP2 and those connected to anharmonic effects largely compensate each others, thereby giving frequencies in agreement with the experimental values29 within 1%. Such a “fortuitous but almost exact” compensation between anharmonic effects and the use of GGA has already been reported for other hydrous minerals such as kaolinite and lizardite;30 the anharmonicity parameter of OH bonds varies within 10 cm-1 in the 3400-3700 cm-1 frequency range, which is of interest here. Furthermore, the use of the PP1 provides ∼70 cm-1 down-shifted stretching frequencies with respect to PP1, which is still reasonable. To reduce the computational burden, we thus adopted the following strategies: (i) we used the soft PP1 for the simulations of all of the configurations; (ii) we compare, in a first stage, the mode frequencies and intensities to the experimental data, adding a 70 cm-1 upshift; and (iii) for selected configurations, we reoptimize the geometry and compute the OH frequencies and intensities using the more accurate default oxygen PP2.26 Results IR spectra were recorded either after exposure of the MgO smoke to water vapor or after desorption of the adsorbed species. In the first case, the wafer was flashed during 10 s at 1100 K and cooled to room temperature prior to exposure to 20 Pa of water vapor. Data were collected in vacuum after removal of water from the gas phase. Then the wafer was flashed again, cooled, and another spectrum was recorded; the subtracted reference, in this case, was the spectrum of the hydrated surface. It was checked that the spectrum only arises from the hydrated MgO smoke. The spectra remains flat upon water adsorptiondesorption cycles on the bare silicon wafer. IR spectra (Figure 1) reproducibly showed a broad band at 3600 cm-1 and two rather narrow bands at 3480 (νA) and 3710 cm-1 (νB). The νA band had never been reported so far. It only compares to the 3465 cm-1 feature observed upon H2 adsorption on MgO nanoparticles.31 The νA and νB bands invariably appeared together. Their constant intensity ratio IνA/IνB ≈ 2 suggests that they are correlated.
13228 J. Phys. Chem. C, Vol. 112, No. 34, 2008
Figure 1. Infrared spectrum of a MgO smoke sample exposed to water vapor (see text). The infrared bands computed at 60 Ry cutoff of the two bonded MgO-OH and O-H groups that occur at fully covered monatomic steps are shown with their absolute intensity. These groups are schematically represented (MgO substrate: O in orange, Mg in green; water adlayer: O in blue, H in white).
Figure 2. Schematic representations of some hydroxylated configurations of MgO surfaces (same color covention as in Figure 1): (a) MgO(310) vicinal surface with a partially dissociated water (two out of three H2O molecules) monolayer; (b) water dissociation on kinks. Case of a monatomic [021] step on MgO(100).
Simulations were intended to allow a critical comparison of the experiment with all configurations that might be expected to dominate the IR spectra of hydroxylated smokes. Quite obviously in such a context, MgO substrates that contain steps have been simulated by adopting vicinal surfaces as models.11 The monatomic step is modeled via a MgO(310) surface; we studied various water coverage, ranging from the dissociative adsorption of an isolated water molecule at the stepsthe fully hydroxyl-decorated step11sto the complete water ML on MgO(310). Such a system, which was not considered in ref 11, shows mixed dissociative and molecular adsorption, with several metastable minima that differ by tiny amounts of potential energy, and will be described in more detail in a forthcoming publication;32 one of those minima is sketched in Figure 2a. Finally, in order to obtain as much of a complete set of hydrated MgO defects as possible, water adsorption at kinks was also considered. It was chosen to examine periodic kinks on a step of average orientation [021] (see Figure 2b). We simulated water adsorption at diatomic steps by using MgO(220) substrates, which exhibit a sequence of (100) and (010) oriented facets.11 Water dissociation can take place at the step edge or in the valley between the facets (see Figure 4 in ref 6); therefore, we considered the two cases, as well as mixed edge-and-valley adsorption. By doubling water coverage and carrying out a simulated annealing run, we obtained a new configuration in which all of the O (Mg) atoms on the facets are saturated with
Finocchi et al. H (OH), whereas the MgO step edges are strongly distorted, as a consequence of water dissociation. Because the detailed surface morphology of MgO crystallites is largely unknown, we considered also other surfaces as possible candidates to interpret the IR data. On MgO(100), both the adsorption of the isolated water molecule and the (3 × 2) reconstructed water monolayer on MgO(100), where OH groups coexist with H2O ad-molecules,7 were simulated.11 Spontaneous dissociative adsorption takes place on the MgO(110) surface, for either 1/4 water monolayer (ML), or 1/3 ML or the full monolayer.11 Because MgO(111) facets form on hydrated MgO,33 a (1 × 1) OH-covered MgO(111) surface34 was considered since it is expected that the dissociative adsorption of a water monolayer leads to a stabilization of the unreconstructed surface.35 Water adsorption on the (2 × 2)-MgO(111) octopolar reconstruction was also simulated34 with its 3-fold coordinated, corner-like sitesseither Mg or Os, at the top of triangular pyramids with (100), (010), and (001) facets. For each of the above-mentioned configurations, we carried out accurate structural optimization (residual atomic forces did not exceed 5 meV/A) to derive vibrational frequencies and intensities through DFPT. In all calculations, the mode frequencies and intensities that have been computed using the soft O pseudopotential (PP1) and adding an upshift a posteriori. For the sake of clarity, all up-shifted frequencies are noted with νcorr OH from now on; they can be easily evaluated from the correspondcorr ing νOH that are listed in Table 2 as νOH ) νOH + 70 cm-1. The results are collected in Figure 3, where the computed OH stretching frequencies are reported as a function of the bond lengths of the hydroxyl groups on which the mode eigenvector is mostly localized. We found an almost linear regression throughout broad OH frequency and bond length ranges. Such a correlation was pointed out for selected cases on the basis of analytical models and semiempirical36 or first-principles calculations.28,37,38 The present results confirm that dilated OH bonds vibrate at lower frequencies than short ones, throughout adsorption configurations that differ by local geometry, water coverage, and hydroxyl coordination number. Discussion The next step regards the interpretation of the experimental IR spectrum (Figure 1) on the basis of OH adsorption configurations. Appropriate configurations must be stable in the probed (P,T) interval and show stretching νOH frequencies and intensity ratios in agreement with the experiment. The presence of the characteristic twin peaks at 3710 and 3480 cm-1 is clearly a much better guidance to examine the configurations listed in Table 2 than the broad structure between 3500 and 3700 cm-1. A natural candidate would be the water ad-layer formed on flat MgO(100) terraces because (100) facets are expected to dominate the surface of the smoke cubic crystallites.33 In the case of the (4 × 2) reconstruction of H2O/MgO(100), which is stable below 250K,39,40 reproducible and quite narrow bands were observed at 3513, 3626, and 3672 cm-1.40 In good corr agreement, we obtain νOH ) 3520, 3565, and 3700 cm-1 for the (3 × 2) H2O ad-layer.7 In the rather complex energy landscape of water adsorbed on flat MgO(100) terraces; the simulated frequencies corresponding to the many metastable configurations look pretty similar to each other,41 consistently with the coexistence of molecular and dissociated water. Although quantitatively different, all those spectra are characteristic of partially dissociated water and lie in the same energy range. Moreover, the adsorption enthalpy of partially dissociated water on MgO(100) 7,8,41 is about -70 kJ/mol. From the latter
Hydroxyl-defect Complexes on Hydrated MgO Smokes
J. Phys. Chem. C, Vol. 112, No. 34, 2008 13229
TABLE 2: Computed Frequencies for the Configurations That Are Discussed in the Main Texta Frequencies (cm-1) 3000 3100 3200
3300
experiment (Figure 1)
3500
3600
3480
Low-index Surfaces MgO(100) with a full (3 × 2) water monolayer MgO(100) (1 × 1) surface, full hydroxyl coverage (ref 43) MgO(110) (1 × 1) surface, full hydroxyl coverage MgO(111) (1 × 1) surface, full hydroxyl coverage
3700
3800
3710
3450-3460, 3495
3630-3640 3730 3575
Monoatomic Steps monoatomic step: isolated hydroxyl monoatomic step: full hydroxylation of the step edge 3060 monoatomic step: full hydroxylation of both step edge and terrace of the MgO(310) surface (Figure 2a) Kink at the monatomic step (Figure 2b) Diatomic Steps diatomic step: adsorption at the step edge diatomic step: adsorption in the valley diatomic step: full hydroxylation
3400
3325 3415, (3490) 3400
3690 3695 3540 3680
3385
3745 3725
(3765)
3700, 3720 3730
3165 3320, 3360
3550 3555
3705
a They are computed using the PP1 pseudopotential at 50 Ry cutoff, apart from those in brackets, which are obtained by adopting the PP2 pseudopotential at 60 Ry cutoff. Fully covered MgO(310) vicinal surface and water dissociation on kinks on monoatomic steps are sketched in Figure 2. The other configurations have been previously described and schematized (see text).
Figure 3. OH stretching frequencies for different surface configurations as a function of the OH bond length on which the mode is especially localized. For the sake of comparison with experiments, we report νcorr OH , corr that is, νOH ) νOH + 70 cm-1 from the frequencies (νOH) that are computed with the PP1 pseudopotential and reported in Table 2.
quantity, the dehydration temperature Td can be evaluated as a function of the water partial pressure from a phenomenological law.21 We estimated Td at 250 K for the partial water pressure of 20 Pa that was kept during the phase of hydration of the MgO sample, before removing the water vapor from the chamber. Therefore, it is concluded that the water ad-layer on flat (100) terraces, consisting of 5C sites, is unstable in the present experimental conditions and should not dominate the IR spectra. This is fully consistent with the fact that the polarized IR spectra observed in ref 40 for the H2O/MgO(100)-(4 × 2) at 203 K and various water pressures clearly differ from the data collected herein. The (1 × 1) OH structure on MgO(100), evidenced both experimentally42 and theoretically,43 is also considered. It is characterized by two very dissimilar hydroxyl groups, with OH bond lengths of 0.950 and 1.018 Å; they correspond to 3730 and 2740 cm-1 stretching frequencies, respectively, as computed by using PP1. Moreover, such a structure turns out to be unstable with respect to hydroxyl desorption, since the computed water adsorption enthalpy from the gas phase amounts to +25 kJ/ mol. As a consequence, neither partially nor fully dissociated
water on flat MgO(100) terraces with 5C sites can explain the experimental IR spectra in Figure 1. Therefore, water dissociation at C e 4 sites with formation of hydroxyl groups must be invoked to interpret the characteristic νA and νB peaks at 3710 and 3480 cm-1. Hydroxyl groups adsorbed on MgO (110) and (111) surfaces, although very stable,6 do not show any vibrational mode close to the νA frequency (3480 cm -1). Relying upon the adsorption energies, the OH groups adsorbed at the diatomic steps should be almost stable in the experimental conditions.8,11 However, we discard both the decorated edge and the decorated valley because their characteristic frequencies are far away from νA and νB. The only configuration that might qualitatively account for the experiment is the fully covered diatomic step, with peaks predicted around -1 (see Table 2 for ν ). νcorr OH ) 3390, 3430, 3625, and 3775 cm OH However, such a configuration spans a 385 cm-1 frequency range, which is much wider than the experimentally observed counterpart (230 cm-1). Among all the configurations that involve well-defined surface orientations and extended defects, hydroxyls adsorbed at monatomic steps on (100) terraces, running along the direction, are the most appropriate to interpret the IR data. The computed vibrational spectrum corresponding to the fully decorated monatomic step shows two distinct corrected frequen-1 -1 cies, νcorr OH ) 3765 and 3485 cm , very close to νA (3480 cm ) and νB (3710 cm-1), respectively. Notably, the relative intensities agree with the experimental ratio IνA/IνB ≈ 2. The comparison between experiment and simulation is more satisfying for the fully hydroxylated step than for the isolated hydroxyls or water adsorption both on the step and the terrace. Beyond the signature of the fully decorated step, which shows characteristic corr peaks at νOH ) 3750 and 3470 cm-1, the latter configuration corr would also give rise to intense bands around νOH ) 3120 cm-1 (see Table 2). Similarly, kinks at monatomic steps should represent a tiny minority, otherwise the weight of the high-1 would increase frequency peak around νcorr OH ) 3770-3790 cm corr -1 with respect to that at νOH ) 3455 cm , at odds with the experimental evidence. We therefore assign the experimental
13230 J. Phys. Chem. C, Vol. 112, No. 34, 2008 twin peaks at νA (3480 cm-1) and νB (3710 cm-1) to fully decorated monatomic steps of MgO(100) terraces. The previous interpretation is also consistent with the fact that monatomic steps are predicted to be completely hydroxylated for the rather high water vapor pressure at which the smoke was exposed. On such steps, water spontaneously dissociates to form two strongly unlike OH groups, a proton bound to a surface O at the step edge (denoted as OsH), and an OH group (Mgs-OH), where the 2-fold coordinated O ion is in a position close to that of a missing oxygen in the MgO lattice (see the inset of Figure 1). At variance, Mgs-OH can be thought as a “free” hydroxyl. The OH bond length is only slightly smaller than in the water molecule. Its stretching mode takes place quite far from the surface, in a region where the electrostatic potential induced by the MgO crystal is sensitively weakened. As a consequence, the mean effective charge , computed as the average trace of the H effective charge tensor, is 0.25, close to that of the isolated water molecule (0.24). The OsH group is instead considerably stretched (+0.12A) with respect to the isolated water molecule,44 mainly because of the hydrogen bond with the neighboring Mgs-OH group. Because it vibrates in a region of rapidly varying electrostatic potential,45 it shows a large mean effective charge ) 0.425. Such a striking difference between the two unlike OH groups is quite independent of the actual step coverage and explains the differences in vibrational frequency and intensity. In the very low-coverage limit of an isolated dissociated water molecule at the monatomic step, the H bonding between the two hydroxyls is reinforceds the OsH bond length increases6sand the vibration frequency shifts down by about 90 cm-1. Therefore, among the configurations that were examined and on the basis of the close correspondence between the computed and the measured IR spectrum, the fully decorated monatomic step at MgO(100) terraces can be held as the main step responsible for the two characteristic νA and νB peaks. This configuration is sketched in Figure 1. The prominence of these IR fingerprints, evidenced here for the first time, shows that MgO smokes prepared in the controlled conditions contain an enhanced population of hydroxyl-step complexes and fully supports the previous interpretation of photoluminescence spectra.21 Consistently, our findings agree with the assumption that monatomic steps systematically appear on (100) faces of MgO crystallites,46,47 according to the general models of crystal growth.46 Moreover, the computed vibrational spectra of dissociated H2O at kinks on monatomic steps (Table 2) fit IR data less accurately than those relative to straight steps. Therefore, kinks should be rather infrequent on MgO smokes that were prepared following our protocol. One may wonder why similar IR spectra to ours have never been observed so far. A suggestion is that MgO smokes prepared in controlled conditions, with large concentration of defect-free step edges, are needed for the twin narrow bands at 3710 and 3480 cm-1 to show up, in the same way as the 3.8 eV emission band in the photoluminescence spectrum of the smoke after excitation at 5.4 eV.21 On the basis of further simulations, the structure around 3600 cm-1 could be tentatively discussed, although it is too broad to be unambiguously assigned to a specific configuration. First, we find that the presence of a fully decorated step may induce dissociation of the neighboring water molecule on the upper terrace. An example might be provided by the adsorption of a full water monolayer on the MgO(310) vicinal surface, which exhibits a regular array of monatomic steps. The corresponding -1 corrected vibrations shows a band at νcorr OH ) 3610 cm , as well corr as two peaks close to νA and νB (νOH ) 3470 and 3750 cm-1),
Finocchi et al. corr and an intense structure at very low frequency (νOH ) 3130 -1 cm ). The latter low-frequency mode is mainly localized at OH groups on terraces, which are less tightly adsorbed than on steps, and thus might give an overall small contribution to the measured spectrum whenever the actual pressure and temperature favor their desorption. However, we cannot exclude a small contribution from fully hydroxylated diatomic steps, whose characteristic frequencies lie in the experimental range (see Table 2).
Conclusion The vibrational spectra of hydrated MgO smokes, which were prepared following a protocol that favors the formation of straight step edges, are unravelled by singling out defined surface configurations in a joint experimental and theoretical approach. Upon exposure to water vapor, two characteristic twin bands at 3480 and 3710 cm-1 were observed by Fourier transform IR spectroscopy and assigned to the hydrogen-bonded (Os4C-H, Mgs4C-OH) moiety adsorbed at monatomic step edges on MgO(100) terraces. In addition to the frequency values, the overall agreement between measured and computed IνA/IνB intensity ratios (3480 cm-1/3710 cm-1 ≈ 2) was crucial for interpreting the IR spectra. Alternative solutions, such as water structures formed on (100) terraces and ad-hydroxyls at diatomic steps and on other surface terminations apart from the (100) one, were discussed and discarded. Our study suggests that the analysis of the stretching frequency νOH in terms of OH coordination number C can be sometimes misleading. Sites with same coordination numbers may lead to distinct IR spectral features either by varying the coverage, as it can be seen by comparing the fully decorated monatomic step at very low coverage, with the fully decorated one or by changing remote neighbors, as in the case of the mono- versus diatomic steps, which both are 4C sites. Sites with distinct coordination numbers can instead lead to similar IR features, as exemplified by water adsorption at monatomic steps that run parallel to directions with respect to flat (100) terraces. The precise characterization of appropriate (OH, defect) configurations and the computation of both frequencies and intensities is thus crucial to understand hydroxylated surfaces. Moreover, beyond the usual models that link the properties of hydroxyl groups to their local coordination number, we confirm that OH stretching frequencies at hydrated metal oxide surfaces can be robustly correlated with OH bond lengths, thus opening the way to the structural characterization of other water complexes by IR spectroscopy. Acknowledgment. We are grateful to Be´ne´dicte Ealet for the communication of early results about water adsorption on the MgO(220) surface. We thank UPMC for the BQR-2004 grant and IDRIS-CNRS for computer facilities. R.H. acknowledges a Ph.D. fellowship from French Research Ministry and further financial support from INSP. References and Notes (1) Brown, G. E.; Heinrich, V. E.; Casey, W. H.; Clark, D. L.; Eggleston, C.; Felmy, A.; Goodman, D. W.; Gratzel, M.; Maciel, G.; McCarthy, M. I.; Nealson, K. H.; Sverjensky, D. A.; Toney, M. F.; Zachara, J. M. Chem. ReV. 1999, 99, 77. (2) Adamson, A. W.; Gast, A. P. Physical Chemistry, 6th ed.; WileyInterscience: New York, 1997. (3) Kosmulski, M. Surfactant Sci. Ser. 2000, 90, 343. (4) M. A.; Henderson, Surf. Sci. Rep. 2002, 46, 1. (5) Joumard, I.; Torrelles, X.; Lee, T. L.; Bikondoa, O.; Rius, J.; Zegenhagen, J. Phys. ReV. B 2006, 74, 205411.
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