© Copyright 2002 by the American Chemical Society
VOLUME 106, NUMBER 7, FEBRUARY 21, 2002
LETTERS Dissociation of Water on MgO(100) Y. D. Kim, J. Stultz, and D. W. Goodman* Department of Chemistry, Texas A & M UniVersity, College Station, Texas 77842-3012 ReceiVed: June 19, 2001; In Final Form: NoVember 28, 2001
The adsorption of water on well-ordered MgO(100) surfaces has been studied using metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS), and temperature-programmed desorption (TPD). Experimental evidence is presented that water adsorbs dissociatively and molecularly within the first monolayer on well-ordered MgO(100) surfaces. These results support recent theoretical predictions [Giordano et al. Phys. ReV. Lett. 1998, 81, 1271] that water partially dissociates on MgO(100) surfaces.
The interaction of water with oxide surfaces has received considerable attention because of its prominence in geochemistry, atmospheric chemistry, electrochemistry, and catalysis. Because of this pivotal role, the adsorption of water on oxide surfaces has been extensively studied experimentally and theoretically;1 however, the details of the interaction of water with oxides is still in debate. In particular, many issues remain unresolved related to the molecular details of the interaction between water and MgO(100), despite the fact that many studies have been conducted on this system. It is generally believed that water dissociation occurs only at defects on oxide surfaces,1 in particular, for water on MgO. Evidence for such includes low-energy electron diffraction (LEED) studies that attribute the reversibility of adsorption isotherms of water on MgO to molecular adsorption.2 Infrared reflection absorption spectroscopy (IRAS) data show no evidence for an O-H stretching feature, suggesting that H2O is adsorbed coplanar with the MgO surface at monolayer coverages.3 Three distinct absorption features between 3500-3700 cm-1 were observed using polarization Fourier transform infrared spectroscopy (PIR), and assigned to molecular H2O.4 Metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS) also corroborated molecular adsorption, since no hydroxyl-derived peaks were detected even * To whom correspondence should be addressed. E-mail: goodman@ mail.chem.tamu.edu.
at very low water coverages.5 Earlier theoretical works predicted molecular adsorption of water on MgO(100); only defect sites were found to facilitate the dissociation of water.6-9 Recent theoretical calculations, however, have contradicted the experimental results that show evidence for molecular adsorption of water on MgO(100), suggesting that additional experimental studies of this system are warranted.10 The reversibility of the adsorption isotherms also can be explained by the dissociative adsorption of water.11 The interpretation of the IR-spectra of ref 4 is also controversial.10 Furthermore, recent experimental results indicate that the (100) plane of MgO very well may stabilize the formation of hydroxyl groups.12 Several recent theoretical calculations have predicted that a mixed (H2O + OH) monolayer can form on MgO(100).10,13,14 These new theoretical predictions question the accepted notion that the dissociation of water occurs only at defect sites on oxides. On the other hand, no clear experimental evidence has shown that water can dissociate on flat, well-ordered MgO(100) surfaces. Therefore, a careful experimental investigation to explore the interaction of water with MgO(100) at the molecular level is needed. In this paper, we report MIES results for D2O adsorbed on MgO(100). For the first time, we provide experimental evidence that a mixed (D2O + OD) layer is formed on a well-ordered MgO(100) surface. These results support recent theoretical
10.1021/jp012306d CCC: $22.00 © 2002 American Chemical Society Published on Web 01/29/2002
1516 J. Phys. Chem. B, Vol. 106, No. 7, 2002
Figure 1. D2O TPD spectra collected as a function of D2O coverage for the MgO(100) thin film on Mo(100). D2O was dosed at a sample temperature of 100 K.
predictions that water partially dissociates on a smooth, lowdefect, MgO(100) surface.10,13,14 Experiments were carried out in an ultrahigh vacuum (UHV) system with a base pressure of 1 × 10-10 Torr. Briefly, this system is equipped with LEED, temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), MIES, and UPS. MIES and UPS were measured simultaneously using a cold-cathode discharge source15,16 that provides both ultraviolet photoelectrons and metastable He 23S/21S (E* ) 19.8/20.6 eV) atoms with thermal kinetic energy. Metastable and photon contributions to the signal were separated by time-of-flight using a mechanical chopper. MIES/UPS data were acquired with an incident photon/ metastable beam at 45° with respect to the surface normal in a constant pass energy mode using a double pass cylindrical mirror analyzer (CMA). The energy denoted by EF in the spectra corresponds to electrons emitted from the Fermi level of the Mo(100) substrate. In the following spectra, all binding energies are referred to EF. MgO(100) films on a Mo(100) substrate were prepared by deposition of Mg in an O2 atmosphere of 1 × 10-7 Torr at a sample temperature of 600 K. Subsequently, the sample was annealed at 1150 K. The thickness of the MgO(100) films was estimated to be ca. 15 monolayers. It is noteworthy that by employing techniques such as LEED, scanning tunneling microscopy (STM), and MIES, we have found that annealing to 1150 K is essential to create a flat and very well-ordered MgO(100) surface.17 To determine the defect densities of MgO(100) films, the films were exposed to CO at a sample temperature of 90 K. At this temperature, CO selectively adsorbs on defect sites of MgO(100) surfaces.18 No CO adsorption was observed on the MgO(100) films used in our experiments, indicating that the defect densities of the MgO(100) films are negligibly low.17 D2O (Aldrich 99.9%), used to minimize the TPD background, was purified by several freeze-pump-thaw cycles and was dosed via back-filling the chamber. To investigate the adsorptive properties of D2O on MgO(100) on Mo(100), TPD spectra were acquired as a function of D2O coverage (Figure 1). At lower coverages, a sharp feature at 240 K appears and is assigned to correspond to the first monolayer of D2O. After saturation of the 240 K feature, two additional peaks appear at 180 and 150 K. In previous LEED studies, the 180 K peak was assigned to the partial desorption
Letters
Figure 2. MIES spectra of a well-ordered MgO(100) film on Mo(100) as a function of the D2O exposure. The energy positions of the three molecular orbitals of water and the 1 - π peak of an hydroxyl group are denoted in the pictures.
of water during the transition from the c(4×2) overlayer to the p(3×2) phase.2 The growth of the feature at 150 K does not saturate and corresponds to the formation of multilayer D2O. That the multilayer feature does not appear until saturation of the monolayer feature is consistent with a layer-by-layer growth mode. There has been justified concern that the adsorption of water on MgO(100) thin films may differ from water adsorption on MgO(100) single crystals.9 However, the shapes and peak temperatures of the D2O TPD spectra of Figure 1 are identical to those found for MgO(100) single crystals with atomically flat and very well-ordered surfaces.19,20 Therefore, we conclude that adsorption of water on the MgO(100) thin films used in this work yields essentially identical results to those found for well-ordered MgO(100) single crystals. The adsorption properties of D2O on MgO(100) at a sample temperature of 95 K were studied using MIES (Figure 2). The MIES spectrum of the clean MgO(100) surface exhibits a sharp O(2p) feature at 5.0 eV. With increasing D2O coverage, four new features appear at 5.8, 7.3, 9.4, and 13.0 eV, while concomitantly the intensity of the O(2p) peak attenuates. Following exposure of the MgO(100) surface to 1.4 langmuirs of D2O, the O(2p) feature completely disappears, while the four D2Oinduced peaks continue to grow. Further exposure to D2O leads to an increase in the features at 7.3, 9.4, and 13.0 eV, and a decrease in the intensity of the 5.8 eV feature. At higher D2O coverages, the feature at 5.8 eV completely disappears, and three features at 7.3, 9.4, and 13.0 eV grow in intensity. The UPS spectra from the MgO(100) surface show changes similar to those of MIES upon D2O adsorption. It is important to mention that the D2O TPD spectra collected after the MIES/UPS measurements are identical to those collected without MIES/ UPS experiments. Thus, we believe that the behavior of D2O adsorption on MgO(100) is not influenced by UV or metastable He beams. By comparing our results with those of previous MIES studies on water/MgO, water/Na/MgO,5,21,22 and water/TiO2 systems,23 we can assign the three features at 7.3, 9.4, and 13.0 eV to the 1b2, 3a1, and 1b1 molecular orbitals of D2O. The peak at 5.8 eV cannot be produced by a linear combination of the bare MgO(100) and D2O multilayer spectra. Thus, the feature at 5.8 eV does not arise from D2O. Previous MIES and UPS investi-
Letters gations have shown that OD groups on MgO and TiO2 surfaces give a feature at about 6 eV.5,21-23 The peak at 5.8 eV in Figure 2 most likely originates from the 1 - π orbital of OD groups. Consequently, D2O molecules adsorb dissociatively and molecularly simultaneously, forming a mixed (D2O + OD) phase at submonolayer coverages. After dosing with 2.4 langmuirs of water, the OD feature disappears, whereas the D2O features increase in intensity. The attenuation of the OD feature is caused by the D2O multilayer formation on top of the (D2O + OD) mixed monolayer. Note that MIES is exclusively sensitive to the outermost surface layer. TPD spectra (Figure 1) indicate that the D2O multilayer feature appears at D2O exposures exceeding 1.3 L, consistent with the MIES and UPS results. It is noteworthy that the ratio of the OD(1 - π) and the D2O (1b2) feature implied by the MIES intensity is ca. 1:1. Thus, it is not likely that defect sites alone are responsible for the dissociation. These results are inconsistent with previous TPD and MIES investigations,5,22 where no evidence for layer-by-layer growth and the dissociation of water on MgO(100) was found. This discrepancy is most likely related to the relative high defect densities of the MgO(100) films used in the previous studies. In fact, the MgO(100) films used in ref 5 were prepared by deposition of Mg in O2 ambient at 550 K, followed by an anneal at 750 K in O2. In the present work, an anneal temperature of 1150 K was used in the synthesis of the MgO(100) thin films. Atomic force microscopy (AFM) studies have shown that a high-temperature anneal (>1250K) is essential to form a flat MgO(100) surface.24 In the synthesis of the MgO thin films used in this study, the annealing step was crucial for creating a flat and well-ordered MgO(100) surface. Therefore, it is probable that the MgO(100) films of ref 5 were more defective than ours and are responsible for the discrepancy found in the adsorption properties. Sputtering the as-prepared MgO(100) thin films in this study to create a more defective surface led to MIES and TPD results that showed precisely those changes seen previously as a function of water coverage.25 The MIES data in Figure 2 demonstrate that flat and wellordered MgO(100) surfaces readily dissociate water. In contrast, on defective surfaces, molecular adsorption preferentially takes place at 95 K. These results do not agree with the accepted picture that dissociation of water occurs only at defect sites1 but do correspond with recent theoretical calculations that predict that a mixed (H2O + OH) monolayer is more stable than a pure water monolayer.10,13,14 The thermodynamic stability of the mixed phase can be a driving force for water dissociation on flat and well-ordered MgO(100) surfaces. It is probable that a flat and well-ordered MgO(100) surface is required to allow the formation of the two-dimensional (water + hydroxyl) network, maintained via hydrogen bonding. On rough surfaces, in contrast, a distortion of the two-dimensional distribution is needed to maintain the (water + hydroxyl) phase. The highenergy penalty induced by the distortion of the (water + hydroxyl) phase may result in the preferential formation of the 3D water islands on defective MgO(100) surfaces.
J. Phys. Chem. B, Vol. 106, No. 7, 2002 1517 In conclusion, for the first time, we have demonstrated experimental evidence that D2O can partially dissociate on wellordered, flat MgO(100) surfaces. In contrast, no water dissociation was found for defective MgO(100) at 95 K. These results agree with theoretical predictions that a mixed (H2O + OH) layer is energetically favorable on a flat MgO surface compared to pure water monolayers.10,13,14 Similar to MgO, theoretical studies for water on TiO2(110) also suggest that partial dissociation of water can take place.26 Thus, it is likely that the results of this study may also be relevant to other oxide surfaces. Acknowledgment. Funding for this work was provided by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences and the Robert A. Welch Foundation. References and Notes (1) Thiel, P. A.; Madey, T. E. Surf Sci. Rep. 1987, 7, 211-385. (2) Ferry, D.; Picaud, S.; Hoang, P. N. M.; Girardet, C.; Giordano, L.; Demirdjian, B.; Suzanne, J. Surf. Sci. 1998, 409, 101-116. (3) Xu, C.; Goodman, D. W. Chem. Phys. Lett. 1997, 265, 341-346. (4) Heidberg, J.; Redlich, B.; Wetter, D. Ber. Bunsen-Ges. Phys. Chem. 1995, 99, 1333-1337. (5) Johnson, M. A.; Stefanovich, E. V.; Truong, T. N.; Gunster, J.; Goodman, D. W. J. Phys. Chem. B 1999, 103, 3391-3398. (6) Langel, W.; Parrinello, M. Phys. ReV. Lett. 1994, 73, 504-507. (7) Russo, S.; Noguera, C. Surf. Sci. 1992, 262, 259-270. (8) Russo, S.; Noguera, C. Surf. Sci. 1992, 262, 245-258. (9) Scamehorn, C. A.; Harrison, N. M.; Mccarthy, M. I. J. Chem. Phys. 1994, 101, 1547-1554. (10) Delle Site, L.; Alavi, A.; Lynden-Bell, R. M. J. Chem. Phys. 2000, 113, 3344-3350. (11) Giordano, L.; Goniakowski, J.; Suzanne, J. Phys. ReV. B 2000, 62, 15406-15408. (12) Abriou, D.; Jupille, J. Surf. Sci. 1999, 430, L527-L532. (13) Odelius, M. Phys. ReV. Lett. 1999, 82, 3919-3922. (14) Giordano, L.; Goniakowski, J.; Suzanne, J. Phys. ReV. Lett. 1998, 81, 1271-1273. (15) MausFriedrichs, W.; Dieckhoff, S.; Kempter, V. Surf. Sci. 1991, 249, 149-158. (16) MausFriedrichs, W.; Wehrhahn, M.; Dieckhoff, S.; Kempter, V. Surf. Sci. 1990. (17) Kim, Y. D.; Santra, A. K.; Stultz, J.; Goodman, D. W. Unpublished results, 2001. (18) Dohnalek, Z.; Kimmel, G. A.; Joyce, S. A.; Ayotte, P.; Smith, R. S.; Kay, B. D. J. Phys. Chem. B 2001, 105, 3747-3751. (19) Ahmed, S. I. U.; Perry, S. S.; El Bjeirami, O. J. Phys. Chem. B 2000, 104, 3343-3348. (20) Stirniman, M. J.; Huang, C.; Smith, R. S.; Joyce, S. A.; Kay, B. D. J. Chem. Phys. 1996, 105, 1295-1298. (21) Gunster, J.; Krischok, S.; Stultz, J.; Goodman, D. W. J. Phys. Chem. B 2000, 104, 7977-7980. (22) Gunster, J.; Liu, G.; Stultz, J.; Krischok, S.; Goodman, D. W. J. Phys. Chem. B 2000, 104, 5738-5743. (23) Krischok, S; Hoefft, O; Guenster, J; Stultz, J.; Goodman, D. W.; Kempter, V. Surf. Sci., in press. (24) Perry, S. S.; Kim, H. I.; Imaduddin, S.; Lee, S. M.; Merrill, P. B. J. Vac. Sci. Technol. A 1998, 16, 3402-3407. (25) Kim, Y. D.; Stultz, J.; Goodman, D. W. Unpublished results, 2001. (26) Lindan, P. J. D.; Harrison, N. M.; Gillan, M. J. Phys. ReV. Lett. 1998, 80, 762-765.
