Adsorption of H2O on Bi2Sr2CaCu2O8 and graphite studied with HREELS

s u r f a c e science ELSEVIER

Surface Science 340 (1995) 134-140

Adsorption of H20 on Bi2Sr2Cafu208 and graphite studied with HREELS R.B. Phelps

a L.L. Kesmodel a,* R.J. Kelley b

a Physics Department and Materials Research Institute, Indiana University, Bloomington, IN 47405, USA b Department of Physics, University of Wisconsin, Madison, W1 53706, USA

Received 7 February 1995; accepted for publication 24 May 1995

Abstract We report vibrational spectra of H20 adsorbed on the (001) surface of Bi2Sr2CaCu20 8 (Bi 2 : 2 : 1 : 2) single crystals at 88 K. Spectra were measured with high-resolution electron-energy-loss spectroscopy (HREELS) for exposures from 0.5 to 1.5 langmuir, and similar data were measured using a graphite substrate for comparison. On both substrates, the adsorbed H20 exhibited molecular adsorption and formed hydrogen-bonded clusters over the range of exposures studied. These observations confirm that H20 binds weakly to the (001) surface of Bi 2 : 2 : 1 : 2, in agreement with photoemission results for the same system. Keywords: Copper oxides; Electron energy loss spectroscopy; Electron-solid scattering and transmission - inelastic; Graphite; Low index

single crystal surfaces; Physical adsorption; Polycrystalline surfaces; Vibrations of adsorbed molecules

1. Introduction The surface properties of high-temperature superconductors (HTSCs) and the properties of interfaces between H T S C s and other materials are currently of considerable interest. Part of this interest stems from the need to assess the role of the surface in measurements of fundamental parameters (e.g., the energy gap) with surface-sensitive techniques such as photoemission and tunneling. The surface chemistry of HTSCs is also o f interest because of its relevance to the corrosion, passivation, and formation o f electrical

* Corresponding author.

contacts on these materials. The present w o r k focuses on a surface chemistry topic: the adsorption of H z O on BizSr2CaCuzO 8 (Bi 2 : 2 : 1 : 2). At present, photoemission is the chief source of information on the chemistry of well-characterized H T S C surfaces. Many studies have focused on bare superconductors [1] and HTSCs covered with thin metallic or insulating films [2]. To our knowledge, the only prior study of H z O adsorption on a wellcharacterized Bi 2 : 2 : 1 : 2 surface is a photoemission study by Flavell et al. [3]. These authors studied H z O adsorption on the (001) face of Bi 2: 2 : 1 : 2 at a temperature of 90 K. (The (001) face is the surface exposed by cleaving a single-crystal sample parallel to the ab-plane.) They found evidence for molecular adsorption of H 2 0 on Bi 2 : 2: 1 : 2, indicating that H 2 0 forms w e a k bonds to this surface. This result is

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R.B. Phelps et aL / Surface Science 340 (1995) 134-140

consistent with other studies [4] which have shown that atmospheric gases do not react as strongly with Bi 2 : 2 : 1 : 2 as they do with other HTSCs. Relatively few studies of HTSCs have been performed with surface2sensitive vibrational spectroscopies. Such studies generally require the reproducible preparation of clean single-crystal surfaces with an area of several square millimeters. For most HTSCs, this requirement has been difficult to satisfy. Technical difficulties relevant to individual experimental techniques have also been noted. In the case of high-resolution electron-energy-loss spectroscopy (HREELS), it has recently been observed that electron-beam-induced charging often occurs when HTSC samples are cleaved in ultrahigh vacuum (UHV), and that careful mounting procedures are necessary to avoid this charging [5]. Further studies have shown the following: (1) reproducible and mutually consistent HREELS spectra a r e obtained for Bi 2 : 2 : 1 : 2 and YBazCu307 (Y 1 : 2 : 3) when the samples are carefully mounted [6,7], and (2) the spectra for Bi 2 : 2 : 1 : 2 are in quantitative agreement with (bulk-sensitive) infrared spectra for the same material [8]. Motivated by these recent successful applications of HREELS to the HTSCs, we have undertaken

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measurements of H z O / B i 2 : 2 : 1 : 2 with this technique. Water is an important adsorbate to consider because it is known to cause corrosion of some HTSCs, such as Y 1 : 2 : 3 [9]. Our results demonstrate that water is molecularly adsorbed on Bi 2 : 2 : 1 : 2 at 88 K, in agreement with the results of the photoemission work by Flavell et al. To facilitate identification of the features in the H z O / B i 2 : 2 : 1 : 2 spectra, we present spectra for a similar but less complex system, HaO/graphite.

2. Experimental Our experimental apparatus and procedures are described elsewhere in detail [6]. Briefly, the measurements were made in a conventional UHV chamber with a base pressure below 1 × 10 - l ° Torr. The samples were introduced into the chamber by means of a load-lock and were attached to the end of a cold finger with spring clips. The cold finger was cooled with liquid nitrogen and reached a temperature of 88 K. Two types of samples were used, Bi 2 : 2 : 1 : 2 and highly oriented pyrolitic graphite (HOPG). The H O P G samples were cleaved outside the chamber with adhesive tape, immediately transferred into the

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Fig. 1. High-resolution electron-energy-loss spectra for H 2 0 / H O P G at 88 K. Spectrum a corresponds to the bare substrate, while spectra b - d correspond to e = 0.5, 1.0, and 1.5, respectively, where e is the H 2 0 exposure in langmuir. The arrows indicate modes of the adsorbate at the indicated frequencies. The lines serve as guides to the eye.

R.B. Phelps et aL / Surface Science 340 (1995) 134-140

136

load-lock, and transferred into the UHV chamber approximately one hour later. The Bi 2 : 2 : 1 : 2 samples were grown using a flux growth method based on standard techniques described elsewhere [10]. These samples were cleaved in UHV at a temperature of 88 K by pulling off a metal post epoxied to the top of the sample. The samples were exposed to the vapor of distilled, deionized water which was degassed prior to use by repeated freeze-pump-thaw cycles. The vapor was admitted to the chamber through a leak valve, and the exposure was estimated using the pressure readings from the chamber's ion gauge. We report the exposure E in units of langmuir (L), where 1 L = 1 X 10 -6 Torr. s = 4.8 × 1014 molecules / c m -2. The samples were examined with HREELS, and, in some cases, with low energy electron diffraction (LEED). The HREELS spectrometer which was used includes a double-pass monochromator and a 127 ° cylindrical deflection analyzer [11]. The spectra were taken with incident and scattered angles both equal to 65 ° . The kinetic energy of the electrons incident on the sample was 3 eV, and the pass energy of the monochromator was 1.5 eV. The typical energy resolution (FWHM) on both HOPG and Bi 2: 2 : 1 : 2 under these conditions was 65 c m - 1. The peaks in the spectra were fitted with Gaussian

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peaks after subtraction of a linear background chosen separately for each peak.

3. Results The HREELS data for H 2 0 adsorbed on HOPG and Bi 2 : 2 : 1 : 2 are shown in Figs. 1 and 2, respectively. The spectra are plotted in order of increasing H 2 0 exposure in steps of 0.5 L, with the spectra for the bare substrates shown at the bottom. The intensity of the elastic peak decreased by a factor of approximately twenty as the exposure was increased from ~ = 0 to ~ = 1.5, resulting in significant degradation of the signal-to-noise ratio at the higher exposures. The spectra for the dosed samples have been scaled relative to those for the bare substrates to make the elastic peaks of all the spectra for each substrate equal in height. For clarity, only the elastic peak corresponding to the bare substrate is shown in each plot. Also for clarity, the loss spectra for the HzO-dosed Bi 2 : 2 : 1 : 2 have been offset vertically relative to the bare Bi 2 : 2 : 1 : 2 spectrum. No smoothing of the data has been performed. We consider first the data for the HOPG substrate, which are shown in Fig. 1. The spectrum for the bare substrate is featureless except for a weak

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Fig. 2. Measurements similar to those of Fig. 1, but with Bi 2 : 2 : 1 : 2 as the substrate. The peaks at ~ 395 and 645 cm -1 are caused by surface optical phonons of the substrate.

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R.B. Phelps et al. / Surface Science 340 (1995) 134-140

Table 1 Vibrational frequencies for H20 in a gas phase, bulk solid, and selected adsorption systems System Frustrated translations v(M-OH 2) Frustratedrotations

HOH scissor

O-H stretch

H20 (Gas) a

1595

3657 (sym.) 3756 (asymm.) 3000-3600 3400 3370 3350

H20 (Solid) a Pt(111) b Graphite c Bi 2 : 2 : 1 : 2 c

220-240 250 235 228

550

525-1040 700 735 825

1650 1625 1600 1680

From Ref. [14]. b From Ref. [15]. Exposure in the range 1.5-2.0 L. c Present work. Exposure equals 1.5 L. a

shoulder at ~ 200 cm -1 and a barely discernible peak near 2900 c m - 1 , likely due to a small amount o f h y d r o c a r b o n impurity introduced in the cleaving/loading procedure. The low energy shoulder has been previously observed and attributed to a plasmon excitation [12]. The smooth frequency dependence of the bare H O P G spectrum facilitates the identification and analysis of adsorbate-induced features. At the lowest exposure measured ( • = 0.5), adsorbate-induced peaks appear at 235, 735, and 3340 cm -1. As the exposure is increased, these peaks grow in intensity, and signs of a very weak fourth peak at 1600 cm -1 are observed. All of the adsorbate peaks are broad relative to the elastic peak. The peak at 3340 c m - 1 is especially broad, with a Gaussian full width at half maximum intensity (FWHM) approximately five times that of the elastic peak. The frequency and F W H M of the adsorbate peaks are independent of the H 2 0 exposure to within the uncertainty of the fitting procedure. The bare substrate spectrum for Bi 2 : 2 : 1 : 2 , spectrum a in Fig. 2, exhibits prominent peaks at 395 and 645 cm -1 caused by surface optical phonons [6,7]. On exposure to H 2 0 , three peaks are observed at 228, 1680, and 3350 cm -1, in close correspondence with the results for H 2 0 / H O P G . A fourth peak, which apparently corresponds to the peak observed at 735 cm -1 for H 2 0 / H O P G , is also observed. This peak is first observed in spectrum b at ~ 765 c m - 1 as a shoulder on the substrate phonon peak at 645 c m - 1 . It shifts to higher frequency with increasing H20 exposure, reaching 825 + 15 cm -1 at • = 1.5 in spectrum d. Additional measurements, including HREELS at

room temperature and LEED at 88 K, were made to complement the measurements described above. HREELS spectra of Bi 2 : 2 : 1 : 2 taken at room temperature showed no sign of adsorbate modes for exposures as high as 20 L, indicating that the sticking coefficient is negligible at this temperature. LEED measurements of Bi 2 : 2 : 1 : 2 at 88 K yielded the rectangular 5 × 1 pattern which others have reported [13]. Adsorption of H 2 0 caused a substantial increase in diffuse scattering but did not produce additional diffraction spots. By • = 2.0, the diffuse scattering was strong enough to largely obscure the diffraction pattern due to the substrate. These observations are consistent with the strong decrease in intensity of the elastic peak in our HREELS spectra noted earlier, and they suggest that the adsorbed H 2 0 is disordered.

4. Discussion The properties of adsorbed H20 have been studied extensively, and data are available for a variety of metallic, insulating, and semiconducting substrates. Detailed references to this literature and an overview of the fundamental principles which govern the adsorption of H20 can be found in a recent review by Thiel and Madey [14]. The present data, viewed in the context of the results reviewed by these authors, are quite similar to those of H20 adsorbed on metallic single crystals such as Pt(111) [15] and Ag(110) [16]. Specifically, we interpret our results as follows: (1) the adsorption is associative (i.e., the adsorbate remains intact); (2) the adsorbate

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forms hydrogen-bonded clusters rather than uniform overlayers; and (3) the vibrational spectrum of these clusters is essentially equivalent to that of bulk ice, even for coverages as low as 1 - 2 monolayers. All of these points suggest that the bond between H 2 0 and Bi 2 : 2 : i : 2 is weak, in agreement with the photoemission results of Flavell et al. The support for associative or molecular adsorption is based on the frequencies of the four H20-induced peaks in Figs. 1 and 2, all of which are in close agreement with frequencies typically observed for molecularly adsorbed H20. The frequencies of these peaks and their assignments are listed in Table 1. For comparison, data are included for H 2 0 in the gas phase, in bulk ice, and adsorbed on a representative metallic substrate. The data for the adsorbed water are the values reported for exposures of 1.5 L or greater. For H20/Pt(111), Sexton found evidence for monolayer completion at ~ 1.5 L and, at approximately the same exposure, an onset of bulk-like behavior [15]. For exposures of 2 - 4 L, similar vibrational spectra are observed for H 2 0 adsorbed on a variety of metallic substrates [14], indicating that formation of bulk-like ice occurs in this exposure range. I t should be noted, however, that the present data closely resemble those for metallic substrates even at exposures below 1.5 L. The modes with the lowest frequencies are frustrated translations of the H 2 0 molecule. Most authors distinguish between two types of frustrated translations. The first type is the adsorbate-substrate stretch, which is perpendicular to the surface. On metals, the H 2 0 molecule bonds to the surface via the oxygen atom, so this mode is designated as v(M-OH2). This mode is typically observed at frequencies between 300 and 600 cm -1. The second type of frustrated translation, often observed at frequencies between 200 and 250 cm -1, is generally attributed to a stretching vibration of the bond between neighboring water molecules [17]. In bulk ice, the H 2 0 - H 2 0 stretch is observed at frequencies from 180 to 3 0 0 c m -~. On the basis of its frequency, we assign the mode near 230 cm -1 in Figs. 1 and 2 to a frustrated translation of the second type, an H z O - H 2 0 stretch. The adsorbate-substrate stretch is not clearly apparent in our data,, but there may be a hint of this mode in spectra c and d of Fig. 1: there appears to be a

very weak shoulder at ~ 570 cm -1 in the lowfrequency tail of the intense mode at 735 cm -1. Sexton has identified the adsorbate-substrate stretch as a low-frequency shoulder on an intense peak at 700 cm -1 in spectra which are similar to those reported here. The broad peak at 735 cm-1 in Fig. 1 and the coverage-dependent feature which shifts from ~ 7 6 5 to ~ 8 2 5 cm -1 in Fig. 2 are assigned to frustrated rotations or librations. There are three possible librations (generally known as rocking, wagging, and twisting motions), but these three modes often give rise to a single broad peak, such as is observed here. The weak peak at 1600 cm -1 is assigned to the scissoring mode 6(HOH), also known as the bending or intramolecular deformation mode. The presence of this mode is a particularly good indicator of molecular adsorption, since the adsorbed OH (hydroxyl) group produced by dissociation of H 2 0 has no vibrational modes in this frequency range [14]. Since the 1600 cm -1 peak is not clearly observed for E < 1.5, it is unclear whether the 6(HOH) mode is absent at low exposure: or merely too weak to be observed. For example, it is possible that the adsorption is dissociative at low exposure and becomes associative as the exposure is increased. However, the fact that we observe evidence for hydrogen bonding at e = 0.5 (see below) suggests that the adsorption is associative even at submonolayer coverage, and that the 6(HOH) mode is present but too weak to be observed at these exposures. Lastly, the broad peak near 3350 cm -1 is assigned to the symmetric and antisymmetric O - H stretching vibrations, v~(OH) and va(OH). The fact that H 2 0 is associatively adsorbed and yields similar spectra on HOPG and Bi 2 : 2 : 1 : 2 can be understood in elementary terms by considering the similarity in the structure of these two substrates. Both substrates have layered structures with weak interlayer bonds (as is evident from the ease with which they can be cleaved). The weakness of the interlayer bonds suggests that surfaces formed by breaking these bonds will be inert with respect to most adsorbates. Since the adsorbate-substrate bonds are weak, the internal bonds of the adsorbate are not significantly weakened by adsorption, and the adsorbate remains intact. Flavell et al. have proposed [3] that sp-hybrid lone pairs associated with Bi cations on the (001) face of Bi 2: 2 : 1 : 2 are responsible for

R.B. Phelps et al. / Surface Science 340 (1995) 134-140

the inert character of this surface. The fact that neither we nor Flavell et al. observe any sign of H 2 0 adsorption at room temperature confirms that the adsorption bond is weak. Given the weakness of the adsorbate-substrate bond and the well-known strength of the hydrogen bond between water molecules, it is reasonable to expect that hydrogen-bonded clusters of H 2 0 will form if the adsorbate is mobile at 88 K. Water is known to exhibit mobility sufficient for the formation of clusters on metallic substrates at 90 K [14]. Several features of the data indicate the presence of hydrogen-bonded clusters in the present work. The dearest evidence of hydrogen bonding is the frequency and width of the u(OH) peak. In Figs. 1 and 2, this peak is significantly lower in frequency than the corresponding gas phase peaks, and it is also much broader ( ~ 200 c m -1 v e r s u s < 1 cm-1). Both effects are commonly observed for adsorbed H 2 0 and are attributed to hydrogen bonding [14,18]. Other evidence of hydrogen bonding is provided by the LEED observations and the exposure dependence of the elastic peak in HREELS mentioned earlier. Both these observations indicate that there is substantial disorder in the adsorbate. It is likely that hydrogen-bonded clusters of H 2 0 , randomly distributed with respect to size, shape, and location, are the source of this disorder. It is not possible to determine from the present data whether these dusters are two- or three-dimensional. An interesting feature of the data for H E O / B i 2 : 2 : 1 : 2 is the exposure-dependent frequency shift exhibited by the frustrated rotation in this system. The frequency of this mode shifts from ~ 765 cm-1 at e = 0.5 to ~ 825 c m -1 at e = 1.5. Stuve et al. [16] have observed a similar shift for H 2 0 / A g ( 1 1 0 ) . It is plausible that hydrogen bonds to neighboring water molecules could stiffen the effective spring constant for this frustrated rotation, giving rise to the observed coverage dependence. The absence of this shift in the data for H z O / H O P G could be due to several factors, including a lower sticking coefficient, a difference in the relative strength of adsorbate-adsorbate and adsorbate-substrate interactions, or a difference in the structure of the adsorbate. Additional information on the structure of the adsorbate would be needed to understand these observations. For example, measurements at higher substrate

139

temperatures would be of interest. On P t ( l l l ) [19] and Ag(111) [20], H 2 0 yielded LEED patterns between 120 and 155 K, but not at temperatures below 120 K. If H 2 0 / B i 2 : 2 : 1 : 2 were to behave similarly, it is possible that more detailed structural information and greater insight into the H 2 0 - H a O interactions could be obtained at higher temperatures.

5. Summary We have measured vibrational spectra of H 2 0 adsorbed on HOPG and Bi 2 : 2 : 1 : 2 at 88 K. On both substrates, H 2 0 adsorbs without dissociating and forms hydrogen-bonded clusters for exposures from 0.5 to 1.5 L. These observations confirm that H 2 0 forms weak bonds to the (001) surface of Bi 2 : 2 : 1 : 2 . This result is in accord with photoemission data for the same system, and with other studies which have shown that Bi 2 : 2 : 1 : 2 is less reactive with respect to atmospheric gases than other high-Tc cuprates. A comparison of the present data with similar measurements for more highly reactive cuprates such as YBa2Cu307 should help shed light on the surface chemistry of these materials.

Acknowledgements We would like to thank Kieron Burke for helpful conversations. This work was supported by DOE Grant No. DE-FG02-84ER45147.

References [1] A.G. Loeser, Z.-X. Shen, D.S. Dessau and W.E. Spicer, J. Electron Speetrosc. Relat. Phenom. 66 (1994) 359; M.S. Golden, R.G. Egdell and W.R. Flavell, J. Mater. Chem. 1 (1991) 489. [2] H.M. Meyer, III and J.H. Weaver, in: Physical Properties of High Temperature Superconductors II, Ed. D.M. Ginsberg (World Scientific, Singapore, 1990). [3] W.R. Flavell, J.H. Laverty, D.S.-L Law, R. Lindsay, C.A. Muryn, C.F.J. Flipse, G.N. Raiker, P.L. Wincott and G. Thornton, Phys. Rev. B 41 (1990) 11623. [4] W.R. Flavell, D.R.C. Hoad, A.J. Roberts, R.G. Egdcll, I.W. Fletcher and G. Beamson, J. Alloys Compounds 195 (1993) 535, and references therein.

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[5] P. Akavoor, R.B. Phelps and L.L. Kesmodel, J. Vac. Sci. Technol. A 12 (1994) 587. [6] R.B. Phelps, P. Akavoor, L.L. Kesmodel, J.E. Demuth and D.B. Mitzi, Phys. Rev. B 48 (1993) 12936. [7] R.B. Phelps, P. Akavoor, L.L. Kesmodel, A.L. Barr, J.T. Markert, J. Ma, R.J. Kelley and M. Onellion, Phys. Rev. B 50 (1994) 6526. [8] D.L. Mills, R.B. Phelps and L.L. Kesmodel, Phys. Rev. B 50 (1994) 6394. [9] W.R. Flavell, D.R.C. Hoad, A.J. Roberts, R.G. Egdell, I.W. Fletcher and G. Beamson, J. Alloys Compounds 195 (1993) 535. [10] P.D. Han and D.A. Payne, J. Cryst. Growth 104 (1990) 201; D.B. Mitzi et al., Phys. Rev. B 41 (1990) 6564. [11] L.L. Kesmodel, J. Vac. Sci. Technol. A 1 (1983) 1456. [12] E.T. Jensen, R.E. Palmer, W. Allison and J.F. Annett, Phys. Rev. Lett. 66 (1991) 492. [13] P.A.P. Lindberg, Z.-X. Shen, B.O. Wells, D.B. Mitzi and I. Lindau, Appl. Phys. Lett. 53 (1988) 2563. [14] P.A. Thiel and T.E. Madey, Surf. Sci. Rep. 7 (1987) 211.

[15] B.A. Sexton, Surf. Sci. 94 (1980) 435. [16] E.M. Stuve, R.J. Madix and B.A. Sexton, Surf. Sci. 111 (1981) 11. [17] Some authors (e.g., Ref. [15]) describe modes observed between 200 and 250 cm -1 as translations parallel to the surface. Other authors (e.g., ReL [16]) have attributed modes in this frequency range to the adsorbate-substrate stretch. The assignments in the present work are based on the overview of the literature given by Thiel and Madey in Ref. [14]. [18] Non-hydrogen-bonded OH groups within undissociated, adsorbed H20 molecules have been observed in some cases, with typical stretching frequencies between 3500 and 3700 em -1 and very narrow linewidths. (See Ref. [14].) It is possible that such groups could contribute to the highfrequency tail of the 3350 cm -1 peak in the present spectra, but it is clear that hydrogen-bonded OH groups make the dominant contribution. [19] L.E. Firment and G.A. Somorjai, Surf. Sci. 55 (1976) 413. [20] L.E. Firment and G.A. Somorjai, Surf. Sci. 84 (1976) 275.