1980
Langmuir 1993,9,1980-1982
Synchrotron Infrared Spectroscopy of H20 Adsorbed on Polycrystalline Gold? C. A. Melendres,*J B. Beden,e G. Bowmaker,ll C. Liu,t and V. A. Maroni* Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, Laboratoire de Chimie I, U A CNRS 350, Universite de Poitiers, Poitiers, France, and Department of Chemistry, University of Auckland, Auckland, New Zealand Received February 16,1993. In Final Form: May 28,1993 Synchrotron infrared spectra of HzO adsorbed on a polycrystalline gold surface have been obtained for the first time. Water was found to adsorb molecularly and associativelyon gold at low temperatures, Le., 100-140 K. For example, at 117 K and exposure of 0.3 langmuirs, the 0-H stretching vibration was observed at 3387 cm-l, which is indicativeof hydrogen-bondedsurfacewater. Bands attributed to frustrated translation were observed at about 230 and 290 cm-1; a band at 817 cm-1 was assigned to frustrated rotational motion. The assignments were confirmed through isotopic shift measurements with DzO. The observed spectra are suggestive of the formation of (HOH), clusters at low coverage; with increasing coverage, the formation of an ordered multilayer with an icelike structure is apparent. Introduction The interaction of molecules with solid surfaces is of considerableinterest not only from a theoretical standpoint but also from a practical one. Numerous industrial and technological processes depend on the efficacy of such interactions, e.g., heterogeneous catalysis, corrosion, etc. The structure of adsorbed molecules on surfaces has been the object of a great deal of study and many techniques have been developed for such investigations, among them XPS,AES, LEED, EELS,etc. Infrared spectroscopy has also been used to obtain structural information from a study of the vibrational motions of molecular adsorbates on different substrates. The use of a synchrotron as an IR source is, however, of fairly recent origin and affords great advantage in the far-infrared region.l12 Recent work by Hirschmugl et aL3 on the adsorption of CO on Cu has nicely demonstrated the capability of the technique for providing new information on adsorbent-adsorbate interactions. The high brightness of the synchrotron source in the far-IR (-100 to lo00 times brighter than a conventional globar source) is a distinct advantage, and it may be anticipated that more new systems and problems will now be amenable to study than had been previously possible. In the present work, we have employed synchrotron IR radiation to examine the adsorption of HzO on a polycrystalline gold surface. Our interest in the long term is the elucidation of the structure of the interface between a solid metal electrode and its aqueous solution environment under the influence of an applied electrical potential. We thought it worthwhile to begin with a study of the interaction of water with a commonly used noble metal electrode material (Au) under UHV conditions and wish to report here some interesting results of this preliminary work. Experimental Section The polycrystalline gold sample (supplied by Princeton Scientific Corp., Princeton, NJ) was 99.999% pure, measured about 2.54 cm long X 6.1 mm wide X 3.2 mm thick, and was + Research supported by the Office of Basic Energy Sciences, Division of Materials Sciences, US Department of Energy under Contract W-31-109-ENG-38. Argonne National Laboratory. Universite de Poitiers. 11 University of Auckland. (1)Stevenson, J. R.; Ellis, H.;Bartlett, R. Appl. Opt. 1973,12,2884. (2) Meyer, P.; Lagarde, P. J. Phys. (Paris) 1976, 37, 1387. (3) Hirschmugl, C. J.; Williams, G. P.; Hoffmann, F. M.; Chabal, Y. J. Phys. Reu. Lett. 1990,65, 480.
0743-7463/93/2409-1980$04.00/0
polished to an opticalfinishon one face. The sample was initially cleaned by heating it under high vacuum to about 900 K, cooling to ambient temperature, and sputtering the optically finished face with Ne+ (0.5-1 keV energy). The main contaminantfound on the surface, carbon (asdeterminedby Auger electron analysis), was removed by the Ne+sputtering. No annealing of the surface was carried out after sputteringsinceroughnessand heterogeneity are less of a consideration for electrode surfaces as much as chemical purity is. Moreover, we found that far-IR spectra of water onannealed, single crystalPt(lll),albeit this is adifferent metal, were very similar to that obtained on unannealed polycrystalline Au surfaces. Our cryogenic sample holder was similar in design to that of Wu and Ignatiev.‘ The sample could be brought to 900 K by radiativeheating from a tungsten filament and cooled to 100 K with liquid nitrogen. Infrared measurementswere carried out in an ultrahigh vacuum (UHV) chamber at the U4IR beamline of the National SynchrotronLight Source (Brookhaven National Laboratory, Upton, NY). This infrared facility has been described elsewhere.*#A Nicolet 20F interferometer was used for spectral acquisition.A liquid-helium-cooled, boron-doped silicon bolometer was used as detector in the far-IR,while a Cu-doped Ge detector was used at higher frequencies (320-4000 cm-I). Results and Discussion Spectra of water adsorbed on a Ne+ sputtered, polycrystalline gold surface in the near-IR region are shown in Figures 1 and 2 as a function of exposure (dose in langmuirs). At relatively low exposures (0.1 to 0.4 langmuirs, Figure 1curves a to d) prominent bands are observed at about 3400 cml, which is due to the 0-H stretching ( U O H ) mode and a t about 811 cm-l, which is generally assigned to the frustrated rotation (i.e., “libration”) of surface water.6 We note that even at the presumably submonolayer coverage, the VOH band appears at a frequency which is significantly lower than that for isolated water molecules of about 3600 cm-l. This is generally taken to be indicative of hydrogen bonding of the surface water molecules and their tendency to cluster together.6 Knochenmuss and Leutwyler’ have carried out ab initio molecular orbital calculations on the structure and infrared vibrational frequencies of small water clusters which are in good agreement with our present measurements. The shift to lower frequency of the UOH band with increasing N. J.; Ignatiev, A. Reu. Sci. Inatrum. 1985, 56, 752. (5)Williams, G. P.; Hirschmugl,C. J.; Kneedler, E. M.; Sullivan, E. A,; Siddons, D. P. Rev. Sci. Inatrum. 1989,60,2176. (6) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (7) Knochenmuss, R.; Leutwyler, S. J. Chem. Phys. 1992,96, 5233. (4) Wu,
0 1993 American Chemical Society
Letters
Langmuir, Vol. 9, No. 8, 1993 1981
99
91
-
I
_ 1
loo
-
97
"
E
s
96-
-
95
nquan
ICs-$
Figure 1. Mid-IR spectra of HzO adsorbed on polycrystalline Au at 117 K and low coverages: (a) 0.1;(b) 0.2;(e) 0.3; (d) 0.4 langmuir. 97 96 95
-
92 91 90 19 88 17 66 94
85
13 82
111 80
250
430
350
rlquacr
330
IC.-!)
Figure 3. Far-IR spectra of HzO adsorbed on polycrystalline Au at 119 K and various coverages: (a) 0.3; (b) 1.0; (e) 1.8; (d) 2.7 langmuirs.
I
93
14
I50
IW
I
-
-
nwwn (r.41
Figure 2. Mid-1R spectra of HsO adsorbed on polycrystalline Au at 114K and high coverages: (a) 1;(b)2;(e) 3; (d)4 langmuirs. coverage (at low exposures) may be a consequence of the increasing interaction between the adsorbed water molecules.* However, at higher dosage, Le., about l langmuir (Figure 21, the YOH band position become nearly independent of exposure. There also appears to be a shift in the frequency of restricted rotation band from about 817 cm-l at 0.1 langmuir to 890 cm-l at 4 langmuirs. Such an increase in frequency is predicted by the theoretical calculation of Knochenmuss' as the cluster size of the molecules increases. The band at 1663 cm-l is the characteristic H-0-H bending or scissoring mode ( 8 ~ 0 ~ ) . The appearance of this band reinforces the suggestion that H2O is adsorbed molecularly (i.e., nondissociatively) on the gold surface and with the oxygen end of the molecule directed toward the surface.s This is similar to that observed for other metals like Pt,eJORu,ll Cu,12etc., where bonding of H2O with the surface is through the oxygen lone pair electrons, and HzO acts as an electron donor or Lewis base molecule. The appearance of librations (restricted rotations) with dipole components perpendicular to the surface indicates a lower symmetry than 12% for the adsorbed water; thus the molecular symmetry axis (8) Dumas, P.; Tobin, R. G.; Richards, P. L. Surf. Sci. 1986,171,579. (9) Sexton, B. A. Surf. Sci. 1980,94,435. (10)Ibach, H.; Lehwald, 5.Surf. Sci. 1980, 91, 187. (11)Madey, T.E.;Yates, J. T.,Jr. Chem. Phys. Lett. 1977, 51, 77. (12) Andemon, S.;Nybe.rg,C.; Tengstal,C . G. Chem.Phys. Lett. 1984, 104, 305.
1 0
IOOQ
1000
I000
4000
FnC9UINCI h-')
Figure 4. Mid-IR spectra of deuterated water adsorbed on polycrsytalline Au at 116 K and various coverages: (a) 0.6; (b) 1.2;(e) 1.8; (d) 2.4 langmuirs. appears tilted with respect to the surface normal. Thus, it appears that even at submonolayer coveragesthe adsored HzO molecules form clusters or islands on the gold surface. The broad width of the YOH band at 0.4 langmuir (fwhm about 313 cm-') reflects some disorder in the initial water layer. As the coverage is increased, the formation of a more ordered multilayer is observed, as indicated by the sharper VOH band (Figure 2) with a half width of about 240 cm-l a t 4 langmuirs. The icelike structure is manifested by the appearance of a shoulder at about 3200 cm-l in the VOH region and the 8 ~ band 0 appearing ~ at -1660 cm-1, as in ice.13 Figure 3 shows far-infrared spectra of HzO adsorbed on gold at different exposures. Quite prominent are the bands at about 236 and 290 cm-l, which are attributed to the restricted translational motion of the HzO molecules. The intensity of the 236-cm-l band increases with exposure while that a t 290 cm-l stays essentially constant and becomes a shoulder at 2.7 langmuirs. The 290-cm-l band is probably due to the initial layer of H2O on the surface, (13)Thiel, P.A.; DePaola, R. A.; Hoffmann, F. M. J. Chem. Phys.
1984,80,5326.
1982 Langmuir, Vol. 9, No. 8, 1993
93
I I50
Letters
, 250
350
rawucncv
450
550
(cm-'~
Figure 5. Far-IR spectra of deuterated water adsorbed on polycrystalline Au at 111 K and various coverages: (a) 0.3; (b) 0.6; (c) 0.9; (d) 1.2; (e) 1.5 langmuirs.
while the one at 236 cm-l is generally attributed to multiple layers of H20 or ice.13J4 The results of experiments using D20 are shown in Figures 4 and 5. The band at 2481 cm-l is due to O-D stretching while that a t 1480is due to the H-O-D bending mode. These are close to the values measured by Sexton14 in deuteration experiments on Pt(ll1). The frequency of the frustrated rotational mode is shifted from 892 to 832 cm-l while that of the OH mode from 3393 to 3352 cm-l. It is interesting to note this effect since one might expect that deuterium substitution should not significantly affect the electronic properties of the molecule and therefore should not change the force constant of the OH band. It is possible that the shifts may have its origin from intramolecular interaction. The intense O-H stretching vibration at -3400 cm-l (Figure 3) indicates a rapid exchange of DzO with H2O desorbing from the UHV chamber walls or with that presumably arising from momentary exposure to the air of the D2O sample during filling of the doser reservoir. (14)Sexton, B. A. Appl. Phys. A 1981,26, 1.
In the far-IR region, a new band at about 215 cm-l appears in the spectra (Figure 4) upon dosing with D20. The shift from 236 cm-1 (with H2O) to 215 cm-l (with DzO) is qualitatively consistent with that expected for deuterium substitution. The shift is somewhat greater than that calculated on the basis of a simple approximation that the frequency varies inversely as the square root of the mass of the molecules. However, the broad overlapping bands leave much uncertainty in the exact band positions; moreover,the effect of anharmonicity could also contribute to the discrepancy. There seems little doubt, however, from the reproducibility of the spectra that the lowfrequency band a t 236 cm-' is due to the restricted translation of H2O on the gold surface. There is good reproducibility of the observed spectra as a function of coverage and temperature. The richness of the spectra of adsorbed water presumably arises from the many structural configurations of the water molecules as they hydrogen bond and form clusters of various sizes.' A more detailed interpretation of the spectra should lead to a greater understanding of the structure of this important molecule on metal surfaces.
Acknowledgment. Financial support for this research was provided by the Division Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under Contract W-31-109-ENG-38. The work was carried out a t the National Synchrotron Light Source a t Brookhaven National Laboratory. Partial support to B. Beden by the Ministry of Foreign Affairs (France) and through a NATO CollaborativeResearch Grant No. 920512 is also gratefully acknowledged. G. Bowmaker was supported by a grant from the University of Auckland Research Committee. R. Cooney of the University of Auckland was involved in the initial conception of the ideas leading to the work described here. We thank G. P. Williams and C. J. Hirschmugl for their assistance in the operation of the U41R beamline facility and for many useful discussions. We thank S. Johnson and M. Pankuch of Argonne for their help in designing and testing the UHV sample holder.
