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An electron energy loss study of carbon dioxide adsorption on alkali metal predosed silver surfaces
40
Surface Science 225 (1990) 40-46 North-Holland
AN ELECTRON ENERGY LOSS STUDY METAL PREDOSED SILVER SURFACES Kevin J. MAYNARD Department
of Chemistty
* and Martin
OF CARBON
DIOXIDE ADSORPTION
ON ALKALI
MOSKOVITS
and Ontario Laser and Lightwave
Received 23 June 1989; accepted for publication
Centre, Uniuersity of Toronto, Toronto, Canada M55 IA1
5 September 1989
The interactions between carbon dioxide (CO,) and alkali metals on rough silver surfaces have been studied with electron energy loss spectroscopy (EELS). In the absence of alkali metal, CO2 is found to bind weakly to the silver surface. With either lithium, potassium or cesium present, a surface complex of the form M+CO; is formed, in agreement with a past Surface Enhanced Raman Scattering (SERS) investigation [J. Chem. Phys. 90 (1989) 66681 of the same systems. At sufficiently high CO, exposures, CO, adsorption also occurs. EELS and SERS spectra obtained from the same alkali-predosed silver surface, are often found to be dominated by different species leading to the conclusion that each of the two spectroscopies probes different regions of the rough surface with differing sensitivities.
1. Introduction The adsorption of molecules onto alkali metal predosed metal surfaces has received much attention in the past several years [l]. In particular, carbon monoxide (CO) adsorption has been intensely studied because of its catalytic importance. What has emerged is a complicated picture of surface interactions between the CO and alkali metal, leading to highly weakened CO bonds and, in some cases, CO bond scission. In a recent report [2], we extended such studies to the adsorption of CO, and alkali metals on silver surfaces. Strong evidence was obtained for a binary surface compound M+CO; (M = Li, K, Cs) and also for a competitive reaction involving CO, dissociation to CO and 0. The latter reaction was found not to occur in the absence of alkali metal on the silver surface. In the present study, electron energy loss spectroscopy (EELS) has been used in obtaining complementary information to the Surface Enhanced
* Present address: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 0039-6028/90/$03.50 (North-Holland)
0 Elsevier Science Publishers B.V.
Raman Scattering (SERS) results of ref. [2]. While EELS has been useful as a vibrational probe of molecules on well ordered substrates, there have been very few EELS studies reported on disordered metal surfaces such as vapor deposited films. In one such study of benzene adsorption on silver [3], the scattered electron intensity was observed to be several orders of magnitude lower than that from single crystal surfaces, due to the surface roughness. The intensity of the “dipole forbidden” losses was also found to be comparable in intensity with the surface dipole allowed bands. In order to distinguish the contributions of dipole, impact and resonance scattering to the spectra of CO, on silver, we have measured the scattering intensities as a function of electron impact energy. In this report, the EELS and SERS spectra from molecules on rough silver surfaces are compared and evidence that the two spectroscopies probe different fractions of the sites present on rough surfaces is presented. In light of other reports pertaining to the specificity of SERS to certain surface structures such as “pores” [4] or other sites that are interstitial between microscopic metal surface features, the present study
K. J. Maynard, M. Moskovits
41
/ CO, adsorption on alkali metal predosed Ag surfaces
attempts to add to this discussion from the vantage point of EELS as well as of SERS performed on the same rough silver films.
Or
2. Experimental The experimental apparatus has been described in detail elsewhere [2,5]. Briefly, the experiments were performed in a stainless steel UHV chamber with a base pressure of 5 X lo-” Torr. The system was equipped with a high resolution EELS spectrometer, visible optics for Raman measurements, electron optics for Auger analysis and a quadrupole mass spectrometer. The EELS spectra were obtained with a Leybold-Heraeus ELS-22 spectrometer operated at 60” angle of incidence with an electron energy of 4 eV (unless otherwise noted). The intensities obtained for the elastically scattered electrons (= 2000 cps) and the loss peaks (= 100 cps) are a factor of - 10’ lower than the norm for the scattering from a single crystal surface. In addition, the resolution of = 15-20 meV is a factor of 3 worse. The feeble intensities are related to the microscopic roughness of the silver films and do not greatly depend on the specific molecule adsorbed on the surface. The low intensities may briefly be explained as follows. With a single crystal sample, the elastic scattering intensity is distributed in the specular direction and thus the analyzer of the EELS spectrometer, which subtends some fixed but small angle, collects the scattered electrons efficiently. For a rough surface, the angular distribution of the scattered electrons is broader and only a fraction of these electrons may be efficiently collected. The SERS spectra were excited with 50 mW of 514.5 nm argon ion laser radiation. Scattered visible radiation from the sample was collected with f/l.8 optics and analyzed with a Spex Triplemate and PAR 1420 OMA detection system. A resistively heated tantalum filament filled with high purity silver (99.99 + W) was used to deposit the silver films in vacua. A cooled (50 K), highly polished copper plate was as the sample substrate. The alkali metals were dosed with commercial SAES Getters filaments. Auger spec-
400
1200
Wavenumbers
2000
2800
3600
/ cm-l
Fig. 1. EELS spectrum of - 5 L CO, on a vapor deposited silver surface at 50 K. The loss intensities are of the order of 20 cps.
troscopy was used to determine the alkali metal coverage, 0x, determined as a fraction of the saturation monolayer coverage then scaled to the absolute coverage by assuming that at saturation 8, = 0.33 as on single crystal silver [2,5]. The CO, (Matheson Research Purity) was used without further purification. All film depositions and spectral analyses were performed at 50 K.
3. Results and discussion 3.1. EELS of CO, on silver at 50 K An EELS spectrum of CO, adsorbed on a rough silver surface is shown in fig. 1. All three fundamental vibrations appear with frequencies (670,132O and 2360 cm-‘) which are very near to the corresponding gas phase values (667, 1337 and 2349 cm-‘). The small frequency shifts indicate that the CO, is physisorbed on the silver surface. The especially weak losses observed indicate that the saturation CO, coverage is low. This is in agreement with a study of CO, on Ag(ll0) [6] which found little or no adsorption of CO, at 100 K. The EELS surface dipole section rule, in which only vibrations perpendicular to the surface are allowed, would predict that only v2 of physisorbed CO2 would be observed for adsorption with the molecular axis parallel to the surface, and only v3
K.J. Maynard, M. Moskovits
-400
400
I
zoo
2000
/ CO, adrorpiion on alkali metal predosed Ag surfaces
2800
Wavenumber / cm-l Fig. 2. EELS spectra of CO, on a potassium predosed silver surface (9, = 0.26) at CO, exposures of (a) 1 L, (b) 2 L, (c) 4 L, (d) 7 L.
for perpendicular adsorption. A combination of these geometries would yield both v2 and Ye losses. The activity of v1 may be explained in terms of impact scattering, or of a negative ion resonance. A recent study of CO, on Ag(l11) showed a clear resonance for the v1 mode at approximately 5.3 eV [7]. The vi feature seems to contain two components due to 2v, and vl, which are both strongly observed in the infrared spectrum of gas phase CO,, owing to Fermi resonance. 3.2. EELS of CO, on potassium predosed silver The EELS spectra obtained after exposing CO, to a potassium predosed silver surface (13, = 0.26) at 50 K are shown in fig. 2. With an exposure of 1 L, weak losses at 760, 1260 and 1600 cm-’ are present. The SERS spectra of these same systems exhibit two main features at 750 and 1220 cm-’
assigned to the bending and symmetric stretch modes of CO; [2]. These two bands correspond well to the 760 and 1260 cm-’ losses of this study. The 1600 cm-’ loss feature, which was unobserved in SERS, may be the antisymmetric stretch of the CO, ion. At higher CO, exposures, the 1600 cm-’ loss remains clearly evident, while the 760 and 1260 cm-’ CO; bands become obscured by the three fundamentals of CO, which appear and intensify with increasing exposure. At the highest exposure shown (7 L), the CO, losses dominate the spectrum. The CO, spectrum observed at high exposure has similar band frequencies and relative intensities as the spectrum obtained in the absence of potassium. In other words, an initial dose of CO, evidently reacts to form CO; and possibly other species, while further CO, exposure leads to molecular CO, adsorption on silver. The large intensity of the loss features at the high CO, exposure indicates that the presence of potassium increases the saturation coverage of CO, on silver. At high CO, exposures (when the CO, losses are sufficiently intense), additional small loss features at 1280 (sh), 1610, 1930 and 3630 cm-’ are visible. Some of these are likely due to overtones and combinations of CO,. For example, the 1280 and 1930 bands may be 2v2 and 3v2, while the 2960 and 3630 bands may be vg + v2 and v3 + 2v,, respectively. The observation of intense combination bands is a characteristic of impact scattering. On silver surfaces with lower potassium coverages, the above results are similar in all respects except that the CO; losses are of lower intensity, and the exposure at which the CO, losses appear is reduced in proportion to the potassium coverage. 3.3. EELS of CO, with lithium and cesium on silver The EELS spectra of CO, coadsorbed with lithium and cesium were also measured. At low CO, exposures, the EELS spectra of both lithium and cesium predosed surfaces exhibit losses due to species other than CO,. Fig. 3 shows the EELS spectra obtained from a low (0.5 L) CO, exposure with the three different alkali metals studied (the potassium result is shown again for comparison).
K.J. Maynard
M. Moskovits
Or
1
study of Li+CO;. The corresponding K-O and Cs-0 losses in the other spectra are, presumably too low in frequency to be clearly resolved above the intense elastic peak. With high exposures of CO, to lithium or cesium predosed surfaces, EELS spectra are obtained that are very similar to the spectrum shown in fig. 2d, which contains losses primarily due to CO,. As with potassium, an increase in the alkali metal coverage increases the exposure at which CO, losses first appear. This suggests that CO, adsorption occurs only after most of the alkali metal sites are occupied on the surface. 3.4. Primary energy dependence loss intensities
II
L
I
1
-400
400
I200
2000
43
/ CO, arlsorption on alkali metal predosed Ag surfaces
2800
Wavenumber / cm-l Fig. 3. EELS spectra of CO, at 0.5 L exposure on alkali metal predosed silver with (a) cesium (b) potassium (c) lithium. The alkali metal coverage was - 0.4 monolayers (8 = 0.13) in each case.
Losses in the 770 and 1220 cm-’ regions, which are assigned to the corresponding SERS lines belonging to CO; (i.e. the bending and symmetric stretch, respectively) are observed in all three spectra. In addition, the 1600 cm-’ loss clearly appears in the cesium spectrum (as with the potassium) and is tentatively assigned to the asymmetric stretch of CO;. In the lithium spectrum, this band is significantly weaker than in the other two cases studied, suggesting, perhaps, that the geometry of the CO, moiety may not be entirely equivalent in the three cases. The lithium/CO, spectrum (fig. 3c) also shows a prominent loss at = 400 cm- ’ which may correspond to Li-0 vibrations originating from the proposed M+CO; adduct. Kafafi et al. [8] report infrared absorptions at 400-500 cm-’ which were assigned to Li-0 vibrations in a matrix isolation
of carbon dioxide
The dependence of EELS loss intensities on the incident electron energy helps to distinguish contributions from impact, dipole and resonant scattering to the observed spectrum [9]. The integrated intensities for the three observed CO, losses (normalized by dividing by the elastic peak intensity) are plotted in fig. 4 as a function of electron energy for a potassium predosed silver surface at 50 K. The v2 and yj losses decrease in intensity with increasing electron energy. This is the expected trend for dipole excitation of these modes [9]. In contrast, the intensity of the y1 loss has a broad resonance centered at = 5.5 eV which
.06
5.06
I
I
I
1
Electron
Energy
(ev)
Fig. 4. A plot of EELS intensities as a function of incident electron energy for CO2 on a silver surface with 8, = 0.07. The vI(COz) mode displays a resonance at 5.5 eV.
44
K.J. Maynard
M. Moskovits
/ CO, adsorption on alkali metal predosed Ag surfaces
appears to be imposed on top of a dipole scattering contribution. The existence of electron scattering resonances is well known in the gas phase [lo] where the temporary negative ion is usually sufficiently long-lived to produce resolved vibrational fine structure. On metal surfaces, electron scattering resonances have been observed for several molecules on silver surfaces such as N, [11] and CO, and Ag(ll1) [7]. A 5.3 eV resonance observed in the latter report corresponds well to the 5.5 eV resonance of CO2 observed in this study. Since the electron energies used for the vibrational spectral were generally 4-5 eV, the unusually large intensity of the dipole-forbidden v, mode is almost certainly due to this resonance. 3.5. Comparison of EELS on rough silver surfaces
and SERS
results of CO,
The EELS spectra of CO2 exposed to alkali metal predosed silver indicate weak losses at low CO, exposures due to CO;. At higher exposures of CO, (after most of the alkali metal sites have been occupied), strong losses due to the three modes of CO, are observed. The SERS spectra of the very same systems [2] indicate that CO, reacts to form a symmetrical (C,,) M+CO; (M = Li, K, Cs) adduct from which two vibrations of CO; are observed. Under conditions of low alkali metal coverage, features due to CO, and CO are also observed, the latter formed from the dissociation of co,. In most aspects, the EELS and SERS results identify the presence of the same surface species. However, in some instances the two techniques produce spectra of different species or at least spectra dominated by different species. For example, the EELS and SERS spectra of a cesium predosed silver surface after exposure to C’sO, are shown in fig. 5. (Similar results are obtained with other isotopic forms of CO,.) The loss peaks present at 660, 1330 and 2350 cm-’ in the EELS spectrum are due to CO,. These losses dominate the spectrum, although weaker losses due to CO; are also present. In the SERS spectrum, on the other hand, the p2 and + CO, peaks are clearly present at the corresponding positions of the EELS losses, but the bands due to CO; dominate. The
I
-403
IOG
600
II00 ISCXJ WAVENUMBERS
zicxl
2mo
Fig. 5. A comparison of EELS (bottom) and SEW (top) spectra for C”O, on cesium predosed silver. The EELS losses are pr~o~n~tly CO, while the SERS shows mostly CO; as well as CO bands. Similar results are obtained with lithium and potassium as well as with other isotopic forms of CO,. The band at 455 is always observed with Cs but not with other alkalis. The weak band at 866 is unidentified. The band at 1435 may be the pa6 band of C’*O;. The band at 2141 cm-’ is due to a small impurity of “Ct60.
intensity of the vi band (or rather bands, because differs between of the 2v,, y1 Fermi resonance), the EELS and SERS spectra. The relative intensities of these bands is very sensitive to the local environment. Small frequency shifts can alter the intensity ratio of the two bands greatly. The spectra indicate that the CO, observed by EELS is less perturbed from its gas-phase counterpart than the CO, probed by SERS. Even more striking is the observation of a strong band due to Cl80 {from the dissociation of C”O,) in the SERS spectrum when none is observed in the EELS spectrum of the same sample. That SERS and EELS could produce different relative intensities of bands for a set of adsorbed molecules is not surprising, given the different surface selection rules operative in these techniques and the different relative cross-sections of molecules in each spectrum. It is expected, however, that the CO should be observed in the EELS spectrum due to its large dynamic dipole moment. There are several possible reasons for the absence of the CO loss in the EELS spectrum of fig. 5; for example, if the CO
K.J. Maynard
M. M~k~v~ts
/ CO, adwrptim
adopted a parallel adsorption geometry when alkali metal is present then its EELS cross-section would be reduced. (Exposure of CO to an alkalifree silver surface does result in a CO loss at 2130 cm-‘). Although not impossible, this seems an unlikely explanation especially in view of the negligible change in the observed CO SERS frequencies with and without alkali present. Alternatively, there may be too little CO present on the surface to be readily detected by EELS. This possibility also seems unlikely in view of the strong SERS intensity of this band. Taking into consideration the fact that the CO; EELS intensity is weak, while the CO, EELS intensity is very strong (the converse is true in the SERS spectrum), we propose that these effects are consequences of the rough silver surface structure and preferential adsorption of the two species on different portions of the silver surface. Cold-deposited metal films are thought to be rough. This is a consequence of the rapid accommodation of the energy of the condensing atoms restricting their diffusion on the surface. In the limit of ballistic aggregation one expects to form a “roughness region” at the surface of the deposit whose rms thickness is proportional to the square root of the mean metal film thickness. Despite the general acceptance of the existence of roughness, a detailed underst~ding of the structure of the roughness is lacking. There are two broad concepts applied to describe the type of roughness that one may find at a cold condensed surface. The first views the structure of the surface as hills and valleys having similar distributions of heights (depths) and mean transverse dimension. In this view the topology of the hills and valleys are similar, at least for films deposited at very low temperatures. With annealing the valleys fill with metal through the sequential collapse of the most unstable structures followed by the diffusion of metal atoms or clusters. By contrast, the other picture depicts the structure of the rough region as an assembly of rather large metal crystallites separated by very small pores or channels at their grain boundaries. The morphology of the metal and the interstitial vacuum is not equivalent in this model. Although counterintuitive as regards accepted mechanisms of film
on alkali beingpressed
Ag surfaces
45
growth, the “pore model” as this view is often referred to, is based on two types of observation. The first is the report by Albano et al. [4] that molecules adsorbed on cold-deposited silver films were not detectable by XPS even though their SERS spectrum was clearly visible and their presence was further confirmed by temperature-programmed desorption. The non-observance of XPS was attributed to the presence of the adsorbed molecules almost exclusively within pores whose dimensions were so fine that photoel~trons could not easily escape from them. A corollary of this observation is that the SERS effect requires the existence of fine pores for its observance. The second piece of evidence is the observation by scanning tunneling microscopy (STM) [12] that, after annealing, the microscopic structure of vapor deposited silver films consists of relatively flat plateau regions separated by pits or pores of various shapes and depths. The observation that EELS and SERS are probing different regions of the spectrum may, therefore, be due to this sort of separation of the surface into “interior” and “exterior” regions. We do not insist, however, that the interior regions are in the form of fine channels or pore. Indeed there would be considerable difficulty in reconciling the traditional electromagnetic models of SERS with such fine pores not to mention the mechanistic difficulties posed by their formation by ballistic aggregation. Moreover it is unnecessary to postulate fine pores in order to argue our results in terms of interior and exterior effects. The hill and valley model of the rough surface suffices. First it is accepted by many that the SERS effect is highly augmented in the interstices between interacting metal particles, as would be the case, for example, in the regions between the microscopic metallic hills [13]. Hence the SERS spectrum would be dominated by the species that aggregate in the valleys. By contrast the primary electrons in EELS will have considerable difficulty in probing the bottom of the valleys when they are incident on the surface at a non-normal angle. Likewise the inelastically scattered electrons may be impeded from leaving the surface as a result of scattering by metallic surface features. Although this will be especially true of electrons scattered by so called impact scattering or by resonant
46
K.J. Maynard, M. Moskovits
/ CO, adwption
scattering both of which require close approach to the surface, it will also apply to dipolar scabtering which, although longer-ranged [9] (- 50 A) will almost certainly be affected by the surface roughness. As a result one expects the EELS spectrum to reflect the species present on the “exterior” portions of the surface while SERS will be more sensitive to the “interior” portions. (One should keep in mind, however, that the foregoing refers to a propensity rather than an exclusive sensitivity). The fact that EELS and SERS are sensitive to a different subset of surface regions is a necessary but not a sufficient condition in order to account of our observations. Because different species are observed as the dominant contributors to the EELS and SERS spectra one must postulate, in addition, that the two species, CO, and CO; are not adsorbed uniformly on the surface. Specifically, the data suggests that CO; (and therefore the alkali) populates the “interior” sites preferentially while CO, is found predominantly at “exterior” sites. One can suggest several reasons for this division. The interior sites may be more reactive and therefore more attractive to the alkali. This may also account for the observation that alkali appears to form islands [2]; the island formation may be a consequence of the propensity of the alkali to adsorb on the interior regions of the silver surface. If so the abundance of the CO; at interior sites may simply reflect the presence of the alkali there. Likewise the low concentration of CO, at interior sites may be due to the lack of a sufficient number of silver sites at those locations.
4. Conclusions (1) CO, is found to be weakly bound to rough silver surfaces at 50 K, with vibrational frequencies within = 10 cm-’ of the corresponding gas phase values. In addition, the saturation coverage is considerably smaller than monolayer coverage. (2) On silver surfaces predosed with alkali metal, exposure to CO, initially results in formation of M+CO; (M = Li, K, Cs), followed by molecular CO* adsorption after most of the alkali metal sites have been occupied. The saturation CO, coverage at 50 K in these systems approaches monolayer coverage.
on alkali
metalpredosed Ag surfaces
(3) The differences in the EELS and SERS spectra from the same samples leads us to propose that each spectroscopy is sensitive to different regions of the rough surfaces. One possibility is that EELS spectroscopy is better able to detect species adsorbed on hills (“exterior” regions of the surface), while species adsorbed in valleys (“interior” regions) produce a disproportionately stronger SERS signal.
Acknowledgements We wish to thank NSERC and the Connaught Fund for financial support. One of us (K.J.M.) wishes to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for graduate and postdoctoral scholarships.
References PI H.P. Bowel, Surf. Sci. Rep. 8 (1987) 43; D. Lackey and D.A. King, J. Chem. Sot. Faraday Trans. 1, 83 (1987) 2001. PI K.J. Maynard and M. Moskovits, J. Chem. Phys. 90 (1989) 6668. [31 R.A. Wolkow and M. Moskovits, J. Chem. Phys. 84 (1986) 5196. [41 E.V. Alhano, S. Daiser, R. Miranda and K. Wandelt, Surf. Sci 150 (1985) 367, 386; H. Seki, T.J. Chuang, M.R. Philpott, E.V. Albano and K. Wandelt, Phys. Rev. B 31 (1985) 5533. 151 K.J. Maynard, PhD Thesis, University of Toronto, 1988. 161 E.M. Stuve, R.J. Madix and B.A. Sexton, Chem. Phys. Lett. 89 (1982) 48. [71 M. Sakurai, T. Okano and Y. Tuzi, J. Vat. Sci. Technol. A 5 (1987) 431. PI Z.H. Kafafi, R.H. Hauge, W.E. Billups and J.L. Margrave, J. Am. Chem. Sot. 105 (1983) 3886. 191 H. Ibach and D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations (Academic Press, New York, 1982) p. 97. DOI G.J. Schulz, Rev. Mod. Phys. 45 (1973) 423. Ull J.E. Demuth, D.S. Schmeisser and Ph. Avouris, Phys. Rev. Lett. 47 (1981) 1166. v21 J.K. Gimzewski, A. Humhert, J.G. Bednorz and B. Reihl, Phys. Rev. Lett. 55 (1985) 951. P31 P.K. Aravind, A. Nitzan and H. Metiu, Surf. Sci. 110 (1981) 189; N. Liver, A. Nitzan and J.I. Gersten, Chem. Phys. Lett. 111 (1984) 449.
Surface Science 225 (1990) 40-46 North-Holland
AN ELECTRON ENERGY LOSS STUDY METAL PREDOSED SILVER SURFACES Kevin J. MAYNARD Department
of Chemistty
* and Martin
OF CARBON
DIOXIDE ADSORPTION
ON ALKALI
MOSKOVITS
and Ontario Laser and Lightwave
Received 23 June 1989; accepted for publication
Centre, Uniuersity of Toronto, Toronto, Canada M55 IA1
5 September 1989
The interactions between carbon dioxide (CO,) and alkali metals on rough silver surfaces have been studied with electron energy loss spectroscopy (EELS). In the absence of alkali metal, CO2 is found to bind weakly to the silver surface. With either lithium, potassium or cesium present, a surface complex of the form M+CO; is formed, in agreement with a past Surface Enhanced Raman Scattering (SERS) investigation [J. Chem. Phys. 90 (1989) 66681 of the same systems. At sufficiently high CO, exposures, CO, adsorption also occurs. EELS and SERS spectra obtained from the same alkali-predosed silver surface, are often found to be dominated by different species leading to the conclusion that each of the two spectroscopies probes different regions of the rough surface with differing sensitivities.
1. Introduction The adsorption of molecules onto alkali metal predosed metal surfaces has received much attention in the past several years [l]. In particular, carbon monoxide (CO) adsorption has been intensely studied because of its catalytic importance. What has emerged is a complicated picture of surface interactions between the CO and alkali metal, leading to highly weakened CO bonds and, in some cases, CO bond scission. In a recent report [2], we extended such studies to the adsorption of CO, and alkali metals on silver surfaces. Strong evidence was obtained for a binary surface compound M+CO; (M = Li, K, Cs) and also for a competitive reaction involving CO, dissociation to CO and 0. The latter reaction was found not to occur in the absence of alkali metal on the silver surface. In the present study, electron energy loss spectroscopy (EELS) has been used in obtaining complementary information to the Surface Enhanced
* Present address: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 0039-6028/90/$03.50 (North-Holland)
0 Elsevier Science Publishers B.V.
Raman Scattering (SERS) results of ref. [2]. While EELS has been useful as a vibrational probe of molecules on well ordered substrates, there have been very few EELS studies reported on disordered metal surfaces such as vapor deposited films. In one such study of benzene adsorption on silver [3], the scattered electron intensity was observed to be several orders of magnitude lower than that from single crystal surfaces, due to the surface roughness. The intensity of the “dipole forbidden” losses was also found to be comparable in intensity with the surface dipole allowed bands. In order to distinguish the contributions of dipole, impact and resonance scattering to the spectra of CO, on silver, we have measured the scattering intensities as a function of electron impact energy. In this report, the EELS and SERS spectra from molecules on rough silver surfaces are compared and evidence that the two spectroscopies probe different fractions of the sites present on rough surfaces is presented. In light of other reports pertaining to the specificity of SERS to certain surface structures such as “pores” [4] or other sites that are interstitial between microscopic metal surface features, the present study
K. J. Maynard, M. Moskovits
41
/ CO, adsorption on alkali metal predosed Ag surfaces
attempts to add to this discussion from the vantage point of EELS as well as of SERS performed on the same rough silver films.
Or
2. Experimental The experimental apparatus has been described in detail elsewhere [2,5]. Briefly, the experiments were performed in a stainless steel UHV chamber with a base pressure of 5 X lo-” Torr. The system was equipped with a high resolution EELS spectrometer, visible optics for Raman measurements, electron optics for Auger analysis and a quadrupole mass spectrometer. The EELS spectra were obtained with a Leybold-Heraeus ELS-22 spectrometer operated at 60” angle of incidence with an electron energy of 4 eV (unless otherwise noted). The intensities obtained for the elastically scattered electrons (= 2000 cps) and the loss peaks (= 100 cps) are a factor of - 10’ lower than the norm for the scattering from a single crystal surface. In addition, the resolution of = 15-20 meV is a factor of 3 worse. The feeble intensities are related to the microscopic roughness of the silver films and do not greatly depend on the specific molecule adsorbed on the surface. The low intensities may briefly be explained as follows. With a single crystal sample, the elastic scattering intensity is distributed in the specular direction and thus the analyzer of the EELS spectrometer, which subtends some fixed but small angle, collects the scattered electrons efficiently. For a rough surface, the angular distribution of the scattered electrons is broader and only a fraction of these electrons may be efficiently collected. The SERS spectra were excited with 50 mW of 514.5 nm argon ion laser radiation. Scattered visible radiation from the sample was collected with f/l.8 optics and analyzed with a Spex Triplemate and PAR 1420 OMA detection system. A resistively heated tantalum filament filled with high purity silver (99.99 + W) was used to deposit the silver films in vacua. A cooled (50 K), highly polished copper plate was as the sample substrate. The alkali metals were dosed with commercial SAES Getters filaments. Auger spec-
400
1200
Wavenumbers
2000
2800
3600
/ cm-l
Fig. 1. EELS spectrum of - 5 L CO, on a vapor deposited silver surface at 50 K. The loss intensities are of the order of 20 cps.
troscopy was used to determine the alkali metal coverage, 0x, determined as a fraction of the saturation monolayer coverage then scaled to the absolute coverage by assuming that at saturation 8, = 0.33 as on single crystal silver [2,5]. The CO, (Matheson Research Purity) was used without further purification. All film depositions and spectral analyses were performed at 50 K.
3. Results and discussion 3.1. EELS of CO, on silver at 50 K An EELS spectrum of CO, adsorbed on a rough silver surface is shown in fig. 1. All three fundamental vibrations appear with frequencies (670,132O and 2360 cm-‘) which are very near to the corresponding gas phase values (667, 1337 and 2349 cm-‘). The small frequency shifts indicate that the CO, is physisorbed on the silver surface. The especially weak losses observed indicate that the saturation CO, coverage is low. This is in agreement with a study of CO, on Ag(ll0) [6] which found little or no adsorption of CO, at 100 K. The EELS surface dipole section rule, in which only vibrations perpendicular to the surface are allowed, would predict that only v2 of physisorbed CO2 would be observed for adsorption with the molecular axis parallel to the surface, and only v3
K.J. Maynard, M. Moskovits
-400
400
I
zoo
2000
/ CO, adrorpiion on alkali metal predosed Ag surfaces
2800
Wavenumber / cm-l Fig. 2. EELS spectra of CO, on a potassium predosed silver surface (9, = 0.26) at CO, exposures of (a) 1 L, (b) 2 L, (c) 4 L, (d) 7 L.
for perpendicular adsorption. A combination of these geometries would yield both v2 and Ye losses. The activity of v1 may be explained in terms of impact scattering, or of a negative ion resonance. A recent study of CO, on Ag(l11) showed a clear resonance for the v1 mode at approximately 5.3 eV [7]. The vi feature seems to contain two components due to 2v, and vl, which are both strongly observed in the infrared spectrum of gas phase CO,, owing to Fermi resonance. 3.2. EELS of CO, on potassium predosed silver The EELS spectra obtained after exposing CO, to a potassium predosed silver surface (13, = 0.26) at 50 K are shown in fig. 2. With an exposure of 1 L, weak losses at 760, 1260 and 1600 cm-’ are present. The SERS spectra of these same systems exhibit two main features at 750 and 1220 cm-’
assigned to the bending and symmetric stretch modes of CO; [2]. These two bands correspond well to the 760 and 1260 cm-’ losses of this study. The 1600 cm-’ loss feature, which was unobserved in SERS, may be the antisymmetric stretch of the CO, ion. At higher CO, exposures, the 1600 cm-’ loss remains clearly evident, while the 760 and 1260 cm-’ CO; bands become obscured by the three fundamentals of CO, which appear and intensify with increasing exposure. At the highest exposure shown (7 L), the CO, losses dominate the spectrum. The CO, spectrum observed at high exposure has similar band frequencies and relative intensities as the spectrum obtained in the absence of potassium. In other words, an initial dose of CO, evidently reacts to form CO; and possibly other species, while further CO, exposure leads to molecular CO, adsorption on silver. The large intensity of the loss features at the high CO, exposure indicates that the presence of potassium increases the saturation coverage of CO, on silver. At high CO, exposures (when the CO, losses are sufficiently intense), additional small loss features at 1280 (sh), 1610, 1930 and 3630 cm-’ are visible. Some of these are likely due to overtones and combinations of CO,. For example, the 1280 and 1930 bands may be 2v2 and 3v2, while the 2960 and 3630 bands may be vg + v2 and v3 + 2v,, respectively. The observation of intense combination bands is a characteristic of impact scattering. On silver surfaces with lower potassium coverages, the above results are similar in all respects except that the CO; losses are of lower intensity, and the exposure at which the CO, losses appear is reduced in proportion to the potassium coverage. 3.3. EELS of CO, with lithium and cesium on silver The EELS spectra of CO, coadsorbed with lithium and cesium were also measured. At low CO, exposures, the EELS spectra of both lithium and cesium predosed surfaces exhibit losses due to species other than CO,. Fig. 3 shows the EELS spectra obtained from a low (0.5 L) CO, exposure with the three different alkali metals studied (the potassium result is shown again for comparison).
K.J. Maynard
M. Moskovits
Or
1
study of Li+CO;. The corresponding K-O and Cs-0 losses in the other spectra are, presumably too low in frequency to be clearly resolved above the intense elastic peak. With high exposures of CO, to lithium or cesium predosed surfaces, EELS spectra are obtained that are very similar to the spectrum shown in fig. 2d, which contains losses primarily due to CO,. As with potassium, an increase in the alkali metal coverage increases the exposure at which CO, losses first appear. This suggests that CO, adsorption occurs only after most of the alkali metal sites are occupied on the surface. 3.4. Primary energy dependence loss intensities
II
L
I
1
-400
400
I200
2000
43
/ CO, arlsorption on alkali metal predosed Ag surfaces
2800
Wavenumber / cm-l Fig. 3. EELS spectra of CO, at 0.5 L exposure on alkali metal predosed silver with (a) cesium (b) potassium (c) lithium. The alkali metal coverage was - 0.4 monolayers (8 = 0.13) in each case.
Losses in the 770 and 1220 cm-’ regions, which are assigned to the corresponding SERS lines belonging to CO; (i.e. the bending and symmetric stretch, respectively) are observed in all three spectra. In addition, the 1600 cm-’ loss clearly appears in the cesium spectrum (as with the potassium) and is tentatively assigned to the asymmetric stretch of CO;. In the lithium spectrum, this band is significantly weaker than in the other two cases studied, suggesting, perhaps, that the geometry of the CO, moiety may not be entirely equivalent in the three cases. The lithium/CO, spectrum (fig. 3c) also shows a prominent loss at = 400 cm- ’ which may correspond to Li-0 vibrations originating from the proposed M+CO; adduct. Kafafi et al. [8] report infrared absorptions at 400-500 cm-’ which were assigned to Li-0 vibrations in a matrix isolation
of carbon dioxide
The dependence of EELS loss intensities on the incident electron energy helps to distinguish contributions from impact, dipole and resonant scattering to the observed spectrum [9]. The integrated intensities for the three observed CO, losses (normalized by dividing by the elastic peak intensity) are plotted in fig. 4 as a function of electron energy for a potassium predosed silver surface at 50 K. The v2 and yj losses decrease in intensity with increasing electron energy. This is the expected trend for dipole excitation of these modes [9]. In contrast, the intensity of the y1 loss has a broad resonance centered at = 5.5 eV which
.06
5.06
I
I
I
1
Electron
Energy
(ev)
Fig. 4. A plot of EELS intensities as a function of incident electron energy for CO2 on a silver surface with 8, = 0.07. The vI(COz) mode displays a resonance at 5.5 eV.
44
K.J. Maynard
M. Moskovits
/ CO, adsorption on alkali metal predosed Ag surfaces
appears to be imposed on top of a dipole scattering contribution. The existence of electron scattering resonances is well known in the gas phase [lo] where the temporary negative ion is usually sufficiently long-lived to produce resolved vibrational fine structure. On metal surfaces, electron scattering resonances have been observed for several molecules on silver surfaces such as N, [11] and CO, and Ag(ll1) [7]. A 5.3 eV resonance observed in the latter report corresponds well to the 5.5 eV resonance of CO2 observed in this study. Since the electron energies used for the vibrational spectral were generally 4-5 eV, the unusually large intensity of the dipole-forbidden v, mode is almost certainly due to this resonance. 3.5. Comparison of EELS on rough silver surfaces
and SERS
results of CO,
The EELS spectra of CO2 exposed to alkali metal predosed silver indicate weak losses at low CO, exposures due to CO;. At higher exposures of CO, (after most of the alkali metal sites have been occupied), strong losses due to the three modes of CO, are observed. The SERS spectra of the very same systems [2] indicate that CO, reacts to form a symmetrical (C,,) M+CO; (M = Li, K, Cs) adduct from which two vibrations of CO; are observed. Under conditions of low alkali metal coverage, features due to CO, and CO are also observed, the latter formed from the dissociation of co,. In most aspects, the EELS and SERS results identify the presence of the same surface species. However, in some instances the two techniques produce spectra of different species or at least spectra dominated by different species. For example, the EELS and SERS spectra of a cesium predosed silver surface after exposure to C’sO, are shown in fig. 5. (Similar results are obtained with other isotopic forms of CO,.) The loss peaks present at 660, 1330 and 2350 cm-’ in the EELS spectrum are due to CO,. These losses dominate the spectrum, although weaker losses due to CO; are also present. In the SERS spectrum, on the other hand, the p2 and + CO, peaks are clearly present at the corresponding positions of the EELS losses, but the bands due to CO; dominate. The
I
-403
IOG
600
II00 ISCXJ WAVENUMBERS
zicxl
2mo
Fig. 5. A comparison of EELS (bottom) and SEW (top) spectra for C”O, on cesium predosed silver. The EELS losses are pr~o~n~tly CO, while the SERS shows mostly CO; as well as CO bands. Similar results are obtained with lithium and potassium as well as with other isotopic forms of CO,. The band at 455 is always observed with Cs but not with other alkalis. The weak band at 866 is unidentified. The band at 1435 may be the pa6 band of C’*O;. The band at 2141 cm-’ is due to a small impurity of “Ct60.
intensity of the vi band (or rather bands, because differs between of the 2v,, y1 Fermi resonance), the EELS and SERS spectra. The relative intensities of these bands is very sensitive to the local environment. Small frequency shifts can alter the intensity ratio of the two bands greatly. The spectra indicate that the CO, observed by EELS is less perturbed from its gas-phase counterpart than the CO, probed by SERS. Even more striking is the observation of a strong band due to Cl80 {from the dissociation of C”O,) in the SERS spectrum when none is observed in the EELS spectrum of the same sample. That SERS and EELS could produce different relative intensities of bands for a set of adsorbed molecules is not surprising, given the different surface selection rules operative in these techniques and the different relative cross-sections of molecules in each spectrum. It is expected, however, that the CO should be observed in the EELS spectrum due to its large dynamic dipole moment. There are several possible reasons for the absence of the CO loss in the EELS spectrum of fig. 5; for example, if the CO
K.J. Maynard
M. M~k~v~ts
/ CO, adwrptim
adopted a parallel adsorption geometry when alkali metal is present then its EELS cross-section would be reduced. (Exposure of CO to an alkalifree silver surface does result in a CO loss at 2130 cm-‘). Although not impossible, this seems an unlikely explanation especially in view of the negligible change in the observed CO SERS frequencies with and without alkali present. Alternatively, there may be too little CO present on the surface to be readily detected by EELS. This possibility also seems unlikely in view of the strong SERS intensity of this band. Taking into consideration the fact that the CO; EELS intensity is weak, while the CO, EELS intensity is very strong (the converse is true in the SERS spectrum), we propose that these effects are consequences of the rough silver surface structure and preferential adsorption of the two species on different portions of the silver surface. Cold-deposited metal films are thought to be rough. This is a consequence of the rapid accommodation of the energy of the condensing atoms restricting their diffusion on the surface. In the limit of ballistic aggregation one expects to form a “roughness region” at the surface of the deposit whose rms thickness is proportional to the square root of the mean metal film thickness. Despite the general acceptance of the existence of roughness, a detailed underst~ding of the structure of the roughness is lacking. There are two broad concepts applied to describe the type of roughness that one may find at a cold condensed surface. The first views the structure of the surface as hills and valleys having similar distributions of heights (depths) and mean transverse dimension. In this view the topology of the hills and valleys are similar, at least for films deposited at very low temperatures. With annealing the valleys fill with metal through the sequential collapse of the most unstable structures followed by the diffusion of metal atoms or clusters. By contrast, the other picture depicts the structure of the rough region as an assembly of rather large metal crystallites separated by very small pores or channels at their grain boundaries. The morphology of the metal and the interstitial vacuum is not equivalent in this model. Although counterintuitive as regards accepted mechanisms of film
on alkali beingpressed
Ag surfaces
45
growth, the “pore model” as this view is often referred to, is based on two types of observation. The first is the report by Albano et al. [4] that molecules adsorbed on cold-deposited silver films were not detectable by XPS even though their SERS spectrum was clearly visible and their presence was further confirmed by temperature-programmed desorption. The non-observance of XPS was attributed to the presence of the adsorbed molecules almost exclusively within pores whose dimensions were so fine that photoel~trons could not easily escape from them. A corollary of this observation is that the SERS effect requires the existence of fine pores for its observance. The second piece of evidence is the observation by scanning tunneling microscopy (STM) [12] that, after annealing, the microscopic structure of vapor deposited silver films consists of relatively flat plateau regions separated by pits or pores of various shapes and depths. The observation that EELS and SERS are probing different regions of the spectrum may, therefore, be due to this sort of separation of the surface into “interior” and “exterior” regions. We do not insist, however, that the interior regions are in the form of fine channels or pore. Indeed there would be considerable difficulty in reconciling the traditional electromagnetic models of SERS with such fine pores not to mention the mechanistic difficulties posed by their formation by ballistic aggregation. Moreover it is unnecessary to postulate fine pores in order to argue our results in terms of interior and exterior effects. The hill and valley model of the rough surface suffices. First it is accepted by many that the SERS effect is highly augmented in the interstices between interacting metal particles, as would be the case, for example, in the regions between the microscopic metallic hills [13]. Hence the SERS spectrum would be dominated by the species that aggregate in the valleys. By contrast the primary electrons in EELS will have considerable difficulty in probing the bottom of the valleys when they are incident on the surface at a non-normal angle. Likewise the inelastically scattered electrons may be impeded from leaving the surface as a result of scattering by metallic surface features. Although this will be especially true of electrons scattered by so called impact scattering or by resonant
46
K.J. Maynard, M. Moskovits
/ CO, adwption
scattering both of which require close approach to the surface, it will also apply to dipolar scabtering which, although longer-ranged [9] (- 50 A) will almost certainly be affected by the surface roughness. As a result one expects the EELS spectrum to reflect the species present on the “exterior” portions of the surface while SERS will be more sensitive to the “interior” portions. (One should keep in mind, however, that the foregoing refers to a propensity rather than an exclusive sensitivity). The fact that EELS and SERS are sensitive to a different subset of surface regions is a necessary but not a sufficient condition in order to account of our observations. Because different species are observed as the dominant contributors to the EELS and SERS spectra one must postulate, in addition, that the two species, CO, and CO; are not adsorbed uniformly on the surface. Specifically, the data suggests that CO; (and therefore the alkali) populates the “interior” sites preferentially while CO, is found predominantly at “exterior” sites. One can suggest several reasons for this division. The interior sites may be more reactive and therefore more attractive to the alkali. This may also account for the observation that alkali appears to form islands [2]; the island formation may be a consequence of the propensity of the alkali to adsorb on the interior regions of the silver surface. If so the abundance of the CO; at interior sites may simply reflect the presence of the alkali there. Likewise the low concentration of CO, at interior sites may be due to the lack of a sufficient number of silver sites at those locations.
4. Conclusions (1) CO, is found to be weakly bound to rough silver surfaces at 50 K, with vibrational frequencies within = 10 cm-’ of the corresponding gas phase values. In addition, the saturation coverage is considerably smaller than monolayer coverage. (2) On silver surfaces predosed with alkali metal, exposure to CO, initially results in formation of M+CO; (M = Li, K, Cs), followed by molecular CO* adsorption after most of the alkali metal sites have been occupied. The saturation CO, coverage at 50 K in these systems approaches monolayer coverage.
on alkali
metalpredosed Ag surfaces
(3) The differences in the EELS and SERS spectra from the same samples leads us to propose that each spectroscopy is sensitive to different regions of the rough surfaces. One possibility is that EELS spectroscopy is better able to detect species adsorbed on hills (“exterior” regions of the surface), while species adsorbed in valleys (“interior” regions) produce a disproportionately stronger SERS signal.
Acknowledgements We wish to thank NSERC and the Connaught Fund for financial support. One of us (K.J.M.) wishes to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for graduate and postdoctoral scholarships.
References PI H.P. Bowel, Surf. Sci. Rep. 8 (1987) 43; D. Lackey and D.A. King, J. Chem. Sot. Faraday Trans. 1, 83 (1987) 2001. PI K.J. Maynard and M. Moskovits, J. Chem. Phys. 90 (1989) 6668. [31 R.A. Wolkow and M. Moskovits, J. Chem. Phys. 84 (1986) 5196. [41 E.V. Alhano, S. Daiser, R. Miranda and K. Wandelt, Surf. Sci 150 (1985) 367, 386; H. Seki, T.J. Chuang, M.R. Philpott, E.V. Albano and K. Wandelt, Phys. Rev. B 31 (1985) 5533. 151 K.J. Maynard, PhD Thesis, University of Toronto, 1988. 161 E.M. Stuve, R.J. Madix and B.A. Sexton, Chem. Phys. Lett. 89 (1982) 48. [71 M. Sakurai, T. Okano and Y. Tuzi, J. Vat. Sci. Technol. A 5 (1987) 431. PI Z.H. Kafafi, R.H. Hauge, W.E. Billups and J.L. Margrave, J. Am. Chem. Sot. 105 (1983) 3886. 191 H. Ibach and D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations (Academic Press, New York, 1982) p. 97. DOI G.J. Schulz, Rev. Mod. Phys. 45 (1973) 423. Ull J.E. Demuth, D.S. Schmeisser and Ph. Avouris, Phys. Rev. Lett. 47 (1981) 1166. v21 J.K. Gimzewski, A. Humhert, J.G. Bednorz and B. Reihl, Phys. Rev. Lett. 55 (1985) 951. P31 P.K. Aravind, A. Nitzan and H. Metiu, Surf. Sci. 110 (1981) 189; N. Liver, A. Nitzan and J.I. Gersten, Chem. Phys. Lett. 111 (1984) 449.