Langmuir 1989,5, 679-682
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Water Chemisorbed on Ruthenium(100) and Ruthenium(001): Changes in Physical Properties Due to Substrate Morphology+ P. K. Leavitt, P. J. Schmitz, J. S. Dyer, J. A. Polta, and P. A. Thiel**l Department of Chemistry and Ames Laboratory, Iowa State University, Ames, Iowa 50011 Received December 13, 1988 We compare the properties of water chemisorbed on two surfaces of metallic ruthenium. The f i t surface, the (001) surface, is hexagonally close-packed and presents little electronic corrugation. The second surface, the (100) surface, is a highly corrugated, row-and-trough arrangement of atoms. For Ru(Ool), water exhibita three states in thermal desorption: Al, A2, and C. The desorption spectra of H20from Ru(100) are distinctly different, exhibiting only two states, A and C. For both substrates, the A state(s) represent chemisorbed water, and the C state represents sublimation of bulk ice. Thus, the first difference between the two substrates lies in the number of chemisorbed states. A second difference lies in the isotope effect, which is the suppression of the Al state in D20 desorption from Ru(001). There is no isotope effect-no difference in the thermal desorption states of H20 and D20-for the rougher Ru substrate. A third difference lies in the vibrational spectra, particularly in the librational modes, of chemisorbed water. On Ru(001), there are sharp losses due to librations at 700 and 920 cm-1, whereas on Ru(100), a single broad, icelike feature exists at 730-760 cm-'. We attribute these differences to the formation of less stable and/or less extensive hydrogen-bonded clusters on the rougher substrate. Over the last 10 years, there have been a growing number of studies of the structure of molecular water adsorbed at single-crystal, transition-metal surfaces under ultrahigh vacuum.' These studies are motivated, at least in part, by the hope that they may shed light upon the structure of water at metal electrode surfaces in solution. However, water molecules exist in vastly different environments in these two situations. In the ultra-high-vacuum (UHV) experiments, water is typically frozen (cryogenically) at the metal surface in a static configuration, whereas water a t an electrode surface is in a dynamic environment, usually at room temperature. Thus, the extent to which one may actually draw comparisons between results from the two fields is unknown, as yet. However, it is a question under active discussion and investigation (e.g., ref 2-4). Molecular dynamics simulations6~sseem to support the hypothesis that a metal electrode surface can largely determine the structure of the dynamic interface and may even "freeze" a particular water structure in place. In this paper, we examine the way in which substrate morphology affects the properties of water chemisorbed on ruthenium surfaces. Two faces of ruthenium are c h w n for this comparison: the (Ool), which is hexagonally close packed, and the (loo), which is a row-and-trough type of structure. The former is an atomically smooth surface, and the latter is atomically rough. The two morphologies are illustrated in Figure 1. We emphasize the following properties of water chemisorbed on these two surfaces: (1)the features present in the thermal desorption spectra, (2) the presence or absence of an isotope effect in the desorption kinetics, and (3) the appearance of the librational modes in vibrational spectra. We interpret our data largely in terms of models which have been developed previously for water adsorbed at the (001) f a ~ e . ~ - 'On ~ that surface, it is thought that threedimensional, hydrogen-bonded clusters form which are particularly stable because of the good lattice match bet Presented at the symposium entitled "Electrocatalysis",196th National Meeting of the American Chemical Society, Los Angeles, CA, Sept 27-29, 1988. National Science Foundation Presidential Young Investigator (1985-1989) and Camille and Henry Dreyfus Foundation TeacherScholar (1986-1990).
*
tween Ru(001) and the Ib form of crystalline ice: water can adsorb commensurately with less than 4% expansion from the structure of ice Ibl These hydrogen-bonded clusters affect the physical properties of water measured with a wide variety of techniques: thermal desorption spectroscopy (TDS),7-12low-energy electron diffraction (LEED)?DJ3electron energy loss spectroscopy (EELS)&'O and electron-stimulated desorption-ion angular distribution (ESDIAD).799 To the extent that substrate morphology is indeed important in stabilizing such hydrogen-bonded clusters, one would expect these physical properties to be quite different for water chemisorbed on Ru(100). In the present paper, we test that expectation. Experimental details are given elsewhere.12J4 In short, the experiments are performed in two separate, albeit similar, UHV chambers, each with a typical base pressure 5 1X Torr. Water is introduced into each chamber via an effusive molecular beam doser. Each apparatus is equipped for Auger electron spectroscopy (AES)and TDS. Multiple masses can be monitored, effectively simultaneously, in TDS via computer-interfaced quadrupole mass spectrometers. Standard cleaning procedures are employed for the Ru s a m p l e ~ . ' ~ J ~ Coverages of water are obtained by time-integrating the thermal desorption peak traces. We quote absolute coverages of water on Ru(001), based upon the assumption (1) Thiel, P. A.; Madey, T. E. Surf.Sci. Rep. 1987, 7, 211. (2) Wagner, F. T.; Moylan, T. E. ACS Symp. Ser. 1988,378,65. (3) Banae, K.; Straehler,B.: Sasa, J. K.: Parsons, R. J. Electroanul. Chem. 198?, 229,87. (4) Frese, U.; Stimming, U. J. Electroanul. Chern. 1986, 198, 409. (5) Spohr, E.; Heinziger, K. Chem. Phys. Lett. 1986,123, 218. (6) Gardner, A. A.; Valleau, J. P. J. Chem. Phys. 1987,86,4171. (7) Madey, T. E.; Yates, J. T., Jr. Chem. Phys. Lett. 1977, 51, 77. (8) Thiel, P. A.; Hoffmann, F. M.; Weinberg, W. H. J. Chem. Phys. 1981, 75, 5556. (9) Doering, D. L.; Madey, T. E. Surf.Sci. 1982,123, 305. (10) Thiel, P. A,; DePaola, R. A.; Hoffmann, F. M. J. Chem. Phys. 1984,80, 5326. (11) Polta, J. A.; Flynn, D. K.; Thiel, P. A. J. Catal. 1986, 99,88. (12) Schmitz, P. J.; Polta, J. A.; Chang,S.-L.; Thiel, P. A. Surf. Sci. 1987,186, 219; J. Vac. Sci. Technol. A 1987,5, 1086. (13) Williams, E. D.; Doering, D. L. J. Vac. Sci. Technol. A 1983,1, 1188. (14) Leavitt, P. K.; Davis, J. L.; Dyer, J. S.;"hiel, P. A., submitted for publication in Surf. Sci.
0143-1463/89/2405-0679$01.50/Q0 1989 American Chemical Society
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Leauitt et al.
Ru(010) 5 (200) 4.28
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Ru(100)
[TOO] t[OOlI
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Figure 1. Schematic depiction of Ru(001) and Ru(100)surfaces. Note that there are two possible arrangements of atoms in the trough for Ru(100) and two possible trough depths. For a hypothetical (100) surface with a series of monatomic, equally spaced steps, both surfaces are equally exposed. It is not known whether one type of atomic arrangement is preferentially exposed at a real (100)surface.
that eiCe= 2/3.7-9 (8 = 1 monolayer corresponds to one water molecular per surface Ru atom. We use eiCeto indicate the coverage a t which the multilayer state first appears in TDS.) Because no absolute coverage data are available for water on &(loo), we quote relative coverages of water on Ru(100). These relative coverages are defined by ere1 = 8/8ice. In Figure 2A, we show thermal desorption spectra of H 2 0 from Ru(001). The coverage increases from the bottom curve to the top curve, as labeled. There are three features, named Al, A2, and C. The C state represents ice multilayers. The Al and A2 states are attributed to water molecules a t or very near to the ruthenium surface, with different local hydrogen-bonded configurations and different s t a b i l i t i e ~ . ~The > ~ clusters associated with the Al and A2 states are shown in Figure 3.9 Intramolecular hydrogen bonding is quite common in water adsorbed at metal surfaces, presumably because the strength of two hydrogen bonds is comparable to the strength of a single chemisorption b0nd.l However, the Al state observed on Ru(OO1) is somewhat unusual. Most of the hexagonally close-packed transition-metal surfaces exhibit desorption spectra with only a single feature for chemisorbed water, similar in position to the A2 state of Figure 2A. Re(001) is an exception to this statement,16but the chemistry of water on Re is complicated by significant dissociation. Less than 0.05 monolayer of water dissociates on Ru(OO~).~ Figure 2B illustrates the fact that desorption spectra for D20 are fundamentally different for the Ru(OO1) substrate. Schmitz, et d.l2first reported this “isotope effect”, in which the Al state is suppressed. By varying the heating rate over 1order of magnitude, they demonstrated that the AI state forms by conversion from the A2 state, with the conversion favored a t slower heating rates. They proposed that the isotope effect arises because of a slower rate of A2 Al conversion for the heavier isotope. The data obtained from the variation of heating rate experiments provided a quantitative measure of the isotope effect, with A2 Al
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(15)Jupille, J.; Pareja, P.; Fusy, J. Surf. Sci. 1984, 139, 505.
conversion being 5-8 times slower (at 150-180 K) for D20 than for H20. This range of values, 5-8, is consistent with a process by which one or two water molecules (such as those labeled “a” and “b” in the top of Figure 3) rotate within the cluster, as the inner part of the large cluster disintegrates and desorbs. The rotation of one or two molecules causes the smaller, more stable cluster shown in the bottom of Figure 3 to form. This cluster subsequently breaks up and desorbs in the Al state. By simple zero-point energy arguments, rotation should be much slower for D20 than for H20. Thus, the isotope effect on Ru(OO1) is thought to be inextricably linked to the hydrogen-bonded clusters that form on this atomically smooth surface. Desorption data are shown in Figure 4 for H20 and D20 adsorbed on the rougher Ru(100) substrate. There are two main differences, which are obvious upon comparison of Figure 2 and Figure 4. First, for H20 on Ru(lOO), only two features (labeled A and C in Figure 4A) are present in the desorption traces. This contrasts the three-feature spectrum of Figure 2A. As before, the C state in Figure 4 is due to sublimation of bulk ice, whereas the A state represents water which is somehow stabilized by its proximity to the metal. The peak temperature of the A state decreases, from 240 to 180 K, as coverage increases. This range encompasses both ranges of peak temperatures observed for the Al and A2 states on Ru(001). Second, there is no isotope effect when water desorbs from Ru(100).This is seen by comparing Figure 4A with Figure 4B. This suggests that there is no conversion between hydrogen-bonded water clusters on this substrate, or at least none which is manifest in the desorption spectra. Again, these results for the atomically rough substrate are in marked contrast to results for the close-packed substrate. A third aspect of chemisorbed water, which appears quiie sensitive to substrate morphology, is the vibrational spectrum. This is particularly true of the spectral region in which the librations (frustrated rotations) are observed with EELS. Figure 5 reproduces some of the loss spectralo which are associated with water adsorbed on the smooth
Langmuir, Vol.5, No.3, 1989 681
Water Chemisorbed on Ru(100) and Ru(001)
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/A
Figure 4. Thermal desorption spectra of H 2 0 and D20 from Ru(lOO),following exposureat 80 K. The curve8 are labeled with values of Brei. Details of the experiment are given in ref 14.
o=1.1
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Figure 2. Thermal desorption spectra of H20 and D20 from Ru(001),followingexposure at 90 K. Details of the experiment are given in ref 12.
x 300 1
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[iio]
Figure 3. Structures of hydrogen-bonded clusters on Ru(OOl), originally proposed by Doering and Madey? Filled circles represent hydrogen atoms, open circles represent oxygen atoms in the second layer, and crw-hatchedcircles repreaent oxygen atoms in the layer nearest the metal substrate.
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Energy Loss, cm Figure 5. Electron energy loss spectra of H 2 0 on Ru(001),for ice multilayers (a) and for the same surface heated to 190 K (b). Details of the experiment are reported in ref 10. Ru(001)surface. These data represent a surface initially covered with an ice multilayer at 80 K (Figure 5a), and the same sample after it is annealed to ca. 190 K (Figure 5b). The initial spectrum A, is very similar to that of bulk ice.
Leavitt et al.
682 Langmuir, Vol. 5, No. 3, 1989 l
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Figure 6. Electron energy loss spectra of H20 on Ru(100), following adsorption at 80 K. Details of the experiment are given in ref 14. The relative coverage of each spectrum is (a) 0~ = 3.06, (b) Orel = 1.35,and (c) Orel = 0.66. The spectrum changes considerably, however, as temperature rises and coverage falls. After annealing, the loss features at 390,700, and 920 cm-' are particularly sharp and strong. The 390-cm-' peak is the metal-oxygen stretch, and the latter two peaks are librations!JO These features appear to be "fingerprints" for the hydrogen-
bonded clusters shown in Figure 3. This idea is supported by the data of Figure 6, which shows the EEL spectra of a Ru(100) sample, with increasing coverages of water adsorbed at 80 K. One main difference between the vibrational properties of water on the two Ru surfaces lies in the fact that the effect of annealing is much different. That is, on the (100) surface, adsorption to Om, I1at 80 K yields a spectrum similar to that obtained by adsorption to Orel > 1 at 80 K, followed by desorption of the ice multilayer by annealing. This is quite dissimilar to the results for Ru(001), where strong spectral changes are observed upon annealing.sJO Such changes are illustrated by Figure 5.1° This comparison also suggests that there is no major structural transformation of the water adlayer on Ru(100) when the surface is annealed up to desorption of the A state. Another dissimilarity lies in the fact that the structure in the spectral range below lo00 cm-I is much different for Ru(100) than for Ru(001). In Figure 6, a single broad feature, centered at 730-760 cm-', is prominent. This feature is essentially invariant with coverage and is quite similar to that observed for ice multilayers on both substrates (cf. Figure 5a and Figure 6a). More subtle differences are evident also in the loss features above 1000 cm-l, and these are discussed e1se~here.l~ The main point here is that the two strong, sharp librational features of water on Ru(001) are replaced by a single, broad feature at 730-760 cm-I on Ru(100). This further highlights the differences between water chemisorbed on these two substrates. The exact nature of the librational modes remains unknown at this time, however. In summary, surface morphology has a profound effect on the properties of water adsorbed at ruthenium surfaces. We base this statement on comparison of data for water on Ru(001) and Ru(100). The former surface is atomically smooth, and the latter is atomically rough. The thermal desorption states, the isotope effect in thermal desorption kinetics, and the vibrational features (particularly the frustrated rotations) are much different for water chemisorbed on these two faces. We propose that these differences reflect the formation of more extensive and/or more stable hydrogen-bonded clusters on the (001) face than on the (100) face. It will be interesting to see whether surface morphology can play a similarly strong role in determining the properties of water adjacent to electrode surfaces in aqueous environments.
Acknowledgment. This research is supported by the Director for Energy Research, Office of Basic Energy Sciences. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-ENG-82. Registry No. HzO, 7732-18-5;Ru, 7440-18-8.
