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Study of chemisorption on silver surfaces using ultraviolet photoelectron spectroscopy (UPS)
Surface Science D North-Holland
99 (1980) 2.59-268 Publishing Company
STUDY OF CHEMISORPTION ON SILVER SURFACES USING ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY (UPS) L.J. GERENSER
and R.C. BAETZOLD
Research Laboratories, Eastman Kodak Company, Rochester, Received
11 March
1980; accepted
for publication
New York 14650,
USA
25 April 1980
IJltraviolet photoelectron spectroscopy (UPS) was used to study the chemisorption of halogens on stepped [3(111) X (loo)] and low-index (111) silver surfaces. The initial rate of halogen adsorption using CHCls exposure on the silver stepped surface is approximately twice that on the low-index surface. This indicates that steps play an important role in chemisorption even on metals with a low density of states at the Fermi level. The adsorbate-induced levels on silver were correlated with halogen p valence orbitals using model extended Htickel calculations. Changes in the silver d band are interpreted as due to p-d orbital interactions.
1. Introduction Studies of chemisorption on stepped and low-index crystal surfaces have been pioneered by Somorjai [ 11. In a study of over 25 hydrocarbons, he found distinctly different chemisorption characteristics for stepped Pt surfaces. In contrast with the chemisorption behavior on low-index Pt surfaces, breaking of C-H and C-C bonds can readily take place at stepped surfaces of Pt. This enhanced reactivity for stepped Pt surfaces is associated with the fact that the stepped surface contains atoms of low coordination number, much the same as is found in metal clusters. Studies of chemisorption on stepped surfaces may, therefore, be models for reactions on metal clusters. Somorjai also studied stepped and low-index Au surfaces and found no special reactivity for the stepped surfaces. Au has a density of states (DOS) at the Fermi that of Pt, and model calculations have indicated that this energy, D(EF), -l/10 property is important in determining step activity [2,3]. Ag, like Au, has a low D(E,), and a study of chemisorption on stepped Ag surfaces may resolve whether the activity at steps is dependent on D(EF) or simply a function of step geometry. UPS studies of chemisorption on Cu, which also has a low D(EF), have been reported previously [4]. It was found that hydrocarbons reacted weakly with Cu surfaces with no evidence of C-H bond breakage. However, the rate of adsorption of electron-withdrawing adsorbates such as 0 or Cl was roughly proportional to the step density. 259
260
L..f. Cerenser,
R.C. Bnetzold
/ Che~~isor~tion
on Ag using UPS
A great deal of research interest has been directed to studies of chlorine chemisorption on a number of different Ag crystal planes [5-l 11. This interest is due to the fact that silver is used as a catalyst for the oxidation of ethylene, and adsorbed chIorine enhances the selectivity of the silver catalyst [12]. In addition, the study of the adsorption of chlorine and other halogens on silver may be useful as models for silver halide surfaces. Most studies employed a combination of low-energy electron diffraction (LEED), Auger spectroscopy, and thermal desorption measurements. Ethylene dichloride [5-71 or Cl, [S-l I] was used as the reacting gas. In all cases, strong chemisorption bonds were formed as indicated by the high temperatures required for desorption. In fact, several workers report evidence for epitaxial AgCl layers formed on Ag [6,7,10]. Briggs et al. [ll] were the first to report UPS measurements on Cl adsorbed on a Ag(ll0) surface. They found a single chlorineinduced feature 3.5 eV below the Fermi energy, which they attribute to the Cl 3p levels. In the present work, we will use LIPS and LEED to probe differences in reactivity and electronic structure of stepped and low-index Ag surfaces, before and after chemisorption. Adsorbateinduced energy levels can readily be observed using UPS. Comparisons can then be made to calculated energy levels using extended Htickel calculations. Thus, it is possible to assign adsorbate-induced energy levels and obtain a better understanding of the Ag-adsorbate bonding interactions. Our study of chemisorption on Ag surfaces has also included Br and I. Several workers have studied iodine adsorption on Ag, but no UPS results have been reported. Forstmann et al. [ 141 have studied iodine adsorbed on a Ag(l I 1) surface using LEED. They found that iodine adsorbs as the atom forming a (43 X ~‘3) R30” structure with the same Ag-I distance as in the AgI crystal. Citrin et al. [15] have reported EXAFS studies of iodine adsorbed on Ag(ll1) and found a Ag-I bond length 0.07 A longer than in the AgI crystal.
2. Exp~~rnent~ All experiments were carried out in an ultra-high vacuum system having a base pressure of 5 X 10 -lo Torr . The chamber is equipped with a PHI rare-gas discharge lamp as the UV source. In the present work, all spectra were taken using helium in the discharge lamp and the He(I) (21.22 eV) resonance as the excitation source. During operation of the lamp, the pressure in the chamber rises to about 3 X lo-’ Torr. The silver single crystals were prepared using standard methods [13]. Prior to exposure, the silver single crystals are alternately xenon ion etched and heated to x400-500°C until well-defined LEED patterns are observed and no impurities are found with Auger electron spectroscopy (AI%). This procedure may take as long as several days. During exposure of the crystals, all electron guns are turned off and the helium lamp is valved off from the system. However, the ionization gauge is left
L.J. Gerenser, R.C. Baetzold / Chemisorption on Ag using UPS
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on during exposure to accurately control the amount of gas introduced into the chamber. fdl spectra were run at a constant energy resolution of 0.1 eV. The photon beam is incident on the sample at an angle of 73” from the crystal plane normal. In all cases the crystal plane normal is aligned along the (CMA) analyzer axis to minimize angular effects. This alignment is reproducible to an accuracy of a few degrees. For a given exposure series, UPS and Auger spectra are recorded for the clean silver surface and immediately after each exposure; the sample is kept in the same position during the entire exposure series. The adsorbates used were all liquids at room temperature with relatively high vapor pressure (e.g., CHCla, CH2Br2, CHJ). The liquids were degassed by repeated cooling to liquid-nitrogen temperatures and pumping with a cryo-pump. The vapors (in equilibrium with the room-temperature liquids) were then admitted into the system at a controlled pressure, generally 10 -6-1O-5 Torr, for a definite amount of time. A variable-rate leak valve was used to accurately control the amount of gas admitted into the chamber.
3. ResuIts The ultraviolet He(I) photoemission spectra of clean Ag(ll1) and Ag[3(111) X (loo)] surfaces, before and after exposure to saturation coverages of CHCla, are shown in fig. 1. In the case of the clean surfaces (solid curves), the most intense portion of the spectrum at 4-8 eV below EF corresponds to the d band.
CHC13 ADSORPTION
Fig. 1. Photoemission spectra for clean (solid line) and CHC13covered (dotted line) silver (111) and [3( 111) x (loo)] surfaces. The difference curves for various levels of exposure 0-j are shown below.
262
L.J. Gerenser, R.C. Baetzold f Chemisorption
on Ag using UPS
The very weak s band begins at EF and extends about 4 eV to the top of the d band. The most striking difference between these two spectra is the greater intensity of the two peaks at ~5 eV in the d band of the Ag[3(111) X (loo)] surface. Other than this feature, the overall number and shape of the peaks in the two spectra are identical. This difference is partly due to the fact that the terraces on the Ag[3(111) X (loo)] surface are at an angle of 20” to the crystal plane, and angular effects have been shown to be large in photoemission spectra of single crystals [16]. We will not try to relate these experimental spectra to calculated band structures; however, we will note that the number and position of the peaks in both spectra are in reasonable agreement with the DOS calculated by Christensen [17] for bulk silver. Different UPS spectra for Cu stepped and low-index surfaces have been reported [ 181 and interpreted in terms of different surface states. Fig. 1 also shows the difference spectra at various exposures to CHCla. In both cases, the principal adsorbateinduced features are a strong, rather narrow peak (FWHM ~1 eV) at 3.5 eV below E,, an attenuation near the top of the Ag d band, and an increase in intensity near the bottom of the Ag d band. These results are consistent with the work of Briggs et al. [ 111, who observed a very similar difference spectrum after adsorption of Cl2 on the Ag(ll0) surface. Although they primarily discuss the strong adsorbate-induced peak above the Ag d band, their difference spectrum also shows the weaker peak near the bottom of the d band. Their results, using Cl2 as the adsorbate, along with the fact that Auger spectroscopy indicates the presence of only Cl after exposure, lead us to conclude that only Cl atoms are adsorbed on the Ag surfaces. Weeks and Rowe [19] have also reported photoemission studies of Cl* on a Ag(OO1) surface. Their angle-integrated difference curves are very similar to what we observe: new peaks above and below the Ag d band, which they attribute to chlorine p orbitals, and an attenuation at the top of the Ag d band. They also find an attenuation in the Ag s-p band which we do not observe. This effect may be due to differences in photon energies or the type of analyzer. The most obvious difference after exposure between the Ag(ll1) and the Ag[3(111) X (loo)] surfaces is the degree of attenuation near the top of the Ag d band. The attenuation is much greater for the stepped surface. The attenuation is partly the result of bonding interactions, and the greater attenuation found on the stepped surface may be due to an angular effect. The adsorbate-induced peaks above and near the bottom of the Ag d band are associated with the Cl 3p levels. We will discuss this assignment in greater detail in a later section. and CHaI onto clean Ag(ll1) and We have also adsorbed CH,Br, surfaces. The He(I) spectra for these adsorbates are shown in Ag[3(111) X (loo)] figs. 2 and 3. Similar difference curves are found for these adsorbates. In all cases, Auger spectroscopy indicates that only the halogen is adsorbed. The only difference between the three halogens adsorbed on Ag is found in the position of the strong peak above the d band. This peak shifts toward BF in going from Cl to I. Also, in the case of I, this peak is split into a doublet separated by -0.6 eV.
L.J. Gerenser, RX. Baetzold / ~~e~i~rptio~ on Ag using UPS CH2Br2
263
ADSORPTION Ag [3(lll~~(l~O)~
Ag(lli)
Fig. 2. Photoem~~on spectra for clean (solid line) and CHzBrzcovered (dotted line) silver (111) and [3(111) X (IOO)] surfaces. The difference curves at saturation coverage are shown below.
Work-function changes can readily be measured from the UPS spectra. Workfunction changes occur during adsorption because the adsorbed atoms or molecules cause a redistribution of the charge density at the crystal surface. If the adsorbed atoms transfer electrons into the crystal surface, the work function decreases, and, conversely, the formation of adsorbed negative ions increases the work function. Table 1 lists the work-function changes measured for adsorption of the halogens at saturation coverage on Ag(l11). Identical work-function changes were measured for
Energy (eV)
Energy
(eb’)
Fig. 3. Photoemission spectra for clean (solid line) and CWsI-covered (dotted line) silver (111) and [ 3(111) X (loo)] surfaces. The difference curves at saturation coverage are shown below.
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L.J. Gerenser, R.C. Baetzold / Chemisorption
on Ag using UPS
Table 1 Work-function changes for adsorption on Ag( 111) -Adsorbate A0 (eV) CHC& CHzBr2 CH31 ~.._
1.0 0.7 0.3 --
0
2000
5000 6CX33
CHC1, exposure (L)
Fig. 4. Change in chlorine to silver Auger signal ratio and work function (A@) on the silver (111) and [3(111) X (IOO)] surfaces asa function of exposure to CHC13.
L.J. Gerenser, R.C. Baetzold / Chemisorption
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265
the Ag[3(111) X (loo)] crystal surface. As can be seen from table 1, the work-function change, A@, decreases in going from Cl to I. This is in agreement with the decrease in electron-withdrawing ability in going from Cl to I. A difference in the rate of adsorption of Cl is found with exposure of the stepped and low-index Ag surface to CHCla. This can be seen by the difference spectra at various exposures in fig. 1 and the increase in the surface concentration of Cl as determined by Auger spectroscopy in fig. 4. The stepped surface reaches saturation coverage at an exposure of 2400 L compared to 4800 L for the Ag(ll1) surface. Similar results are also found for the rate of adsorption of Br and I. In all cases, the initial rate of adsorption is approximately twice as fast on the stepped surface as compared to the low-index surface. This is in agreement with the results found on stepped and low-index Cu surfaces [4] _The differences in rate of adsorption can also be followed by measuring the change in work function, A#, as a function of exposure as shown in fig. 4. Again, A@ reaches saturation at 2400 L exposure for the stepped surface compared to 4800 L for the Ag(ll1). Adsorption of CHCla and CH,Br2 on either the Ag(ll1) or Ag[3(111) X (loo)] surface at room temperature resulted in a LEED pattern unchanged from the clean surface, except for an increase in background. However, CHsI adsorbed on the Ag(ll1) surface at room temperature produces a (d3 X ~‘3)R30” type LEED pattern. The adsorbed halogens can be desorbed by heating to sufficiently high temperatures. For the case of Cl adsorbed on silver, some desorption begins to take place at 300~-4OO”C, as indicated by the decrease in Auger/Cl signal intensity. Desorption of the Cl is complete at 500-SSO’C. These high temperatures required for desorption indicate a strong chemisorption bond. Both Br and I were found to desorb completely in the same temperature range. These results agree well with the thermodynamic studies of Tu and Blakely [lo] for Cl* adsorption on Ag(l11). In their work an epitaxial AgCl layer was formed and the heat of adsorption measured was consistent with AgCl formation.
4. Discussion Halogens were found to adsorb readily on Ag surfaces using halogenated hydrocarbons. The initial rate of halogen adsorption on Ag stepped surfaces is approximately twice that on the low-index surfaces. Thus, for Ag, which has a low D(Er), definite differences in the rate of adsorption were found for stepped and low-index surfaces. UPS spectra indicate interactions between the adsorbate p levels and the Ag d levels, indicating that electronic as well as structural effects may influence the chemisorption process. The UPS spectra show strong similarities for all halogens adsorbed on both stepped and low-index Ag surfaces. In each case, the halogen induces electronic states above and near the bottom of the Ag d band. Some enhancement in the Ag s
266
L.J. Gerenser, R.C. Baetzold / Chemisorption
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band region beginning at the Fermi level is observed. An attenuation near the top of the Ag d band is observed, indicating a shift in these Ag d levels due to Ag d orbital interactions with the adsorbate. The only difference between the three halogens adsorbed on Ag is in the position of the strong adsorbate-induced peak above the Ag d band. The onset of this peak occurs at 2.9 eV, 2.4 eV, and 1.9 eV below EF for Cl, Br, and I, respectively. In each case, this peak is assigned to the halogen p levels. The shift toward the Fermi level in going from Cl to I is consistent with the variations in valence-shell ionization energy of the free atoms, although the differences are of a smaller magnitude. The splitting of 0.6 eV observed for the strong
As,,*, . .
.
;,
.
.
.
l
4
ox
.
0
;
decreasin separatlan ha9aaen D level d ievdl
.
1 between
f
Fig. 5. Calculated density of states for (A) the AgI a plane of atoms and (B)-(D) the halogen p component. The halogen p atomic level is placed at -9, -10 and -11 eV, respectively, for curves B, C and D. Silver atomic s and d levels are -7.56 and -11.58 eV, respectively. All curves are normalized to the same peak amplitude.
L.J. Gerenser, R.C. Baetzold / Chemisorption
on Ag using UPS
267
peak above the Ag d band in the case of I is consistent with the spin-orbit splitting of 0.66 eV found for the 5p,,,,,,, doublet in HI’ [20]. This splitting is reported as 0.33 eV and 0.08 eV for HBr’and HCl’, respectively. We were not able to resolve this splitting for Cl and Br. We have used extended Htickel calculations to help interpret the adsorbateinduced levels on Ag surfaces. Fig. 5 shows the model used for our calculations. It consists of 18 Ag atoms in a plane at the bulk Ag-Ag distance with a 5-atom halogen overlayer. This corresponds to the (d3 X 43)-R30” structure observed for iodine adsorption on the Ag(ll1) surface. Using this model for our calculations, we obtained the plots of the DOS as shown in fig. 5. In all cases, the energy-level distribution has been broadened using a 0.5 eV FWHM Gaussian. Plot (A) is simply the DOS for the Agrs plane of atoms. Plots (B)-(D) contain only the halogen component of the DOS. In effect, what we have done in going from plot (B) to (D) is increased the ionization potential of the halogen p valence orbitals by adjusting the appropriate parameters in our calculations. Such adjustments are necessary since we have no a priori knowledge of the charge on halogen which determines this value. This results in a changing separation between the halogen p level and the Ag d level, which reflects a changing interaction between silver d and halogen p levels. Plot (C) qualitatively agrees with our experimental difference curves. We find a pattern of orbital mixing of the halogen p orbitals with silver s and d orbital% which is consistent with perturbation theory arguments. Mixing of the halogen p level with the silver s level leads to the halogen component near the Fermi energy in fig. 5. Interactions with d orbitals of silver lead to the two peaks on either edge of the silver d band. As the interactions between halogen p and silver d orbitals increase (by decreasing their energy separation), two peaks of equal intensity form. At weaker interactions, the peak above the d band is primarily halogen and the peak below the d band is primarily shifted d orbitals. This picture gives strong evidence for the ability of a filed d shell to interact with adsorbates through a backbonding-type mechanism. The high binding energy peak is due to a bonding p + d orbital interaction while the low binding energy peak is due to an antibonding p-d orbital interaction. This computed result agrees with the conclusions from experimental measurement of the dispersive nature of the two adsorbate induced peaks for CIZ adsorbed on Ag(OO1) [19].
5. Conclusions (A) Rate of halogen adsorption on Ag-stepped surfaces is approximately twice that on low-index surfaces. D(EF) is not the only factor determining special activity at step surfaces. (B) Adsorbate-induced levels on Ag can be correlated with halogen p valence orbitals. Changes in the Ag d band are interpreted as due to Ag d-halogen p orbital interactions. Evidence for interaction of halogen p with silver s levels is also found.
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/ Chemisorption
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(C) Adsorption of Cl and Br on the Ag(ll1) surface using halogenated hydrocarbons results in a LEED pattern unchanged from the clean surface except for an increase in background; however, I forms an ordered (43 X d/3)-R30” structure at room temperature. (D) The halogen chemisorption bond is strong as evidenced by the high temperature for desorption.
Acknowledgement We are grateful to Henry Luss for X-ray alignment
of silver crystals.
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo]
[ 111 [12] [13] [14] [ 151 [16] [17] [18] [19] [20]
G.A. Somorjai, Act. Chem. Res. 9 (1976) 248. L.L. Kesmodel and L.M. Falicov, Solid State Commun. 16 (1975) 1201. R.C. Baetzold, J. Phys. Chem. 82 (1978) 738. R.C. Baetzold, Surface Sci., submitted. G. Rovida, F. Pratesi, M. Maglietta and E. Ferroni, Japan. .I. Appl. Phys. Suppl. 2, Pt. 2 (1974) 117. G. Rovida and F. Pratesi, Surface Sci. 51 (1975) 270. E. Zanazzi, F. Jona, D.W. Jepsen and P.M. Marcus, Phys. Rev. B14 (1976) 432. P.J. Goddard and R.M. Lambert, Surface Sci. 67 (1977) 180. R.A. Marbrow and R.M. Lambert, Surface Sci. 71 (1978) 107. Y.-Y. Tu and J.M. Blakely, J. Vacuum Sci. Technol. 15 (1978) 563; Surface Sci. 85 (1979) 276. D. Briggs, R.A. Marbrow and R.M. Lambert, Chem. Phys. Letters 53 (1978) 462. P.A. Kilty, N.C. Rol and W.M.H. Sachtler, in: Catalysis, Ed. J.W. Hightower (NorthHolland, Amsterdam, 1973) p. 929. R.A. Marbrow and R.M. Lambert, Surface Sci. 61 (1976) 329. F. Forstmann, W. Berndt and P. Biittner, Phys. Rev. Letters 30 (1973) 17. P.H. Citrin, P. Eisenberger and R.C. Hewitt, Phys. Rev. Letters 41 (1978) 309. S. Htifner, in: Topics in Applied Physics, Vol. 27, Photoemission in Solids II, Eds. L. Ley and M. Cardona (Springer, Berlin, 1979) p. 199. N.E. Christensen, Phys. Status Solidi (b) 54 (1972) 551. R.S. Williams, P.S. Wehner, S.D. Kevan, R.F. Davis and D.A. Shirley, Phys. Rev. Letters 41 (1978) 323. S.P. Weeks and J.E. Rowe, J. Vacuum Sci. Technol. 16 (1979) 470. D.C. Frost, C.A. McDowell and D.A. Vroom, J. Chem. Phys. 46 (1967) 4255.
99 (1980) 2.59-268 Publishing Company
STUDY OF CHEMISORPTION ON SILVER SURFACES USING ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY (UPS) L.J. GERENSER
and R.C. BAETZOLD
Research Laboratories, Eastman Kodak Company, Rochester, Received
11 March
1980; accepted
for publication
New York 14650,
USA
25 April 1980
IJltraviolet photoelectron spectroscopy (UPS) was used to study the chemisorption of halogens on stepped [3(111) X (loo)] and low-index (111) silver surfaces. The initial rate of halogen adsorption using CHCls exposure on the silver stepped surface is approximately twice that on the low-index surface. This indicates that steps play an important role in chemisorption even on metals with a low density of states at the Fermi level. The adsorbate-induced levels on silver were correlated with halogen p valence orbitals using model extended Htickel calculations. Changes in the silver d band are interpreted as due to p-d orbital interactions.
1. Introduction Studies of chemisorption on stepped and low-index crystal surfaces have been pioneered by Somorjai [ 11. In a study of over 25 hydrocarbons, he found distinctly different chemisorption characteristics for stepped Pt surfaces. In contrast with the chemisorption behavior on low-index Pt surfaces, breaking of C-H and C-C bonds can readily take place at stepped surfaces of Pt. This enhanced reactivity for stepped Pt surfaces is associated with the fact that the stepped surface contains atoms of low coordination number, much the same as is found in metal clusters. Studies of chemisorption on stepped surfaces may, therefore, be models for reactions on metal clusters. Somorjai also studied stepped and low-index Au surfaces and found no special reactivity for the stepped surfaces. Au has a density of states (DOS) at the Fermi that of Pt, and model calculations have indicated that this energy, D(EF), -l/10 property is important in determining step activity [2,3]. Ag, like Au, has a low D(E,), and a study of chemisorption on stepped Ag surfaces may resolve whether the activity at steps is dependent on D(EF) or simply a function of step geometry. UPS studies of chemisorption on Cu, which also has a low D(EF), have been reported previously [4]. It was found that hydrocarbons reacted weakly with Cu surfaces with no evidence of C-H bond breakage. However, the rate of adsorption of electron-withdrawing adsorbates such as 0 or Cl was roughly proportional to the step density. 259
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L..f. Cerenser,
R.C. Bnetzold
/ Che~~isor~tion
on Ag using UPS
A great deal of research interest has been directed to studies of chlorine chemisorption on a number of different Ag crystal planes [5-l 11. This interest is due to the fact that silver is used as a catalyst for the oxidation of ethylene, and adsorbed chIorine enhances the selectivity of the silver catalyst [12]. In addition, the study of the adsorption of chlorine and other halogens on silver may be useful as models for silver halide surfaces. Most studies employed a combination of low-energy electron diffraction (LEED), Auger spectroscopy, and thermal desorption measurements. Ethylene dichloride [5-71 or Cl, [S-l I] was used as the reacting gas. In all cases, strong chemisorption bonds were formed as indicated by the high temperatures required for desorption. In fact, several workers report evidence for epitaxial AgCl layers formed on Ag [6,7,10]. Briggs et al. [ll] were the first to report UPS measurements on Cl adsorbed on a Ag(ll0) surface. They found a single chlorineinduced feature 3.5 eV below the Fermi energy, which they attribute to the Cl 3p levels. In the present work, we will use LIPS and LEED to probe differences in reactivity and electronic structure of stepped and low-index Ag surfaces, before and after chemisorption. Adsorbateinduced energy levels can readily be observed using UPS. Comparisons can then be made to calculated energy levels using extended Htickel calculations. Thus, it is possible to assign adsorbate-induced energy levels and obtain a better understanding of the Ag-adsorbate bonding interactions. Our study of chemisorption on Ag surfaces has also included Br and I. Several workers have studied iodine adsorption on Ag, but no UPS results have been reported. Forstmann et al. [ 141 have studied iodine adsorbed on a Ag(l I 1) surface using LEED. They found that iodine adsorbs as the atom forming a (43 X ~‘3) R30” structure with the same Ag-I distance as in the AgI crystal. Citrin et al. [15] have reported EXAFS studies of iodine adsorbed on Ag(ll1) and found a Ag-I bond length 0.07 A longer than in the AgI crystal.
2. Exp~~rnent~ All experiments were carried out in an ultra-high vacuum system having a base pressure of 5 X 10 -lo Torr . The chamber is equipped with a PHI rare-gas discharge lamp as the UV source. In the present work, all spectra were taken using helium in the discharge lamp and the He(I) (21.22 eV) resonance as the excitation source. During operation of the lamp, the pressure in the chamber rises to about 3 X lo-’ Torr. The silver single crystals were prepared using standard methods [13]. Prior to exposure, the silver single crystals are alternately xenon ion etched and heated to x400-500°C until well-defined LEED patterns are observed and no impurities are found with Auger electron spectroscopy (AI%). This procedure may take as long as several days. During exposure of the crystals, all electron guns are turned off and the helium lamp is valved off from the system. However, the ionization gauge is left
L.J. Gerenser, R.C. Baetzold / Chemisorption on Ag using UPS
261
on during exposure to accurately control the amount of gas introduced into the chamber. fdl spectra were run at a constant energy resolution of 0.1 eV. The photon beam is incident on the sample at an angle of 73” from the crystal plane normal. In all cases the crystal plane normal is aligned along the (CMA) analyzer axis to minimize angular effects. This alignment is reproducible to an accuracy of a few degrees. For a given exposure series, UPS and Auger spectra are recorded for the clean silver surface and immediately after each exposure; the sample is kept in the same position during the entire exposure series. The adsorbates used were all liquids at room temperature with relatively high vapor pressure (e.g., CHCla, CH2Br2, CHJ). The liquids were degassed by repeated cooling to liquid-nitrogen temperatures and pumping with a cryo-pump. The vapors (in equilibrium with the room-temperature liquids) were then admitted into the system at a controlled pressure, generally 10 -6-1O-5 Torr, for a definite amount of time. A variable-rate leak valve was used to accurately control the amount of gas admitted into the chamber.
3. ResuIts The ultraviolet He(I) photoemission spectra of clean Ag(ll1) and Ag[3(111) X (loo)] surfaces, before and after exposure to saturation coverages of CHCla, are shown in fig. 1. In the case of the clean surfaces (solid curves), the most intense portion of the spectrum at 4-8 eV below EF corresponds to the d band.
CHC13 ADSORPTION
Fig. 1. Photoemission spectra for clean (solid line) and CHC13covered (dotted line) silver (111) and [3( 111) x (loo)] surfaces. The difference curves for various levels of exposure 0-j are shown below.
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L.J. Gerenser, R.C. Baetzold f Chemisorption
on Ag using UPS
The very weak s band begins at EF and extends about 4 eV to the top of the d band. The most striking difference between these two spectra is the greater intensity of the two peaks at ~5 eV in the d band of the Ag[3(111) X (loo)] surface. Other than this feature, the overall number and shape of the peaks in the two spectra are identical. This difference is partly due to the fact that the terraces on the Ag[3(111) X (loo)] surface are at an angle of 20” to the crystal plane, and angular effects have been shown to be large in photoemission spectra of single crystals [16]. We will not try to relate these experimental spectra to calculated band structures; however, we will note that the number and position of the peaks in both spectra are in reasonable agreement with the DOS calculated by Christensen [17] for bulk silver. Different UPS spectra for Cu stepped and low-index surfaces have been reported [ 181 and interpreted in terms of different surface states. Fig. 1 also shows the difference spectra at various exposures to CHCla. In both cases, the principal adsorbateinduced features are a strong, rather narrow peak (FWHM ~1 eV) at 3.5 eV below E,, an attenuation near the top of the Ag d band, and an increase in intensity near the bottom of the Ag d band. These results are consistent with the work of Briggs et al. [ 111, who observed a very similar difference spectrum after adsorption of Cl2 on the Ag(ll0) surface. Although they primarily discuss the strong adsorbate-induced peak above the Ag d band, their difference spectrum also shows the weaker peak near the bottom of the d band. Their results, using Cl2 as the adsorbate, along with the fact that Auger spectroscopy indicates the presence of only Cl after exposure, lead us to conclude that only Cl atoms are adsorbed on the Ag surfaces. Weeks and Rowe [19] have also reported photoemission studies of Cl* on a Ag(OO1) surface. Their angle-integrated difference curves are very similar to what we observe: new peaks above and below the Ag d band, which they attribute to chlorine p orbitals, and an attenuation at the top of the Ag d band. They also find an attenuation in the Ag s-p band which we do not observe. This effect may be due to differences in photon energies or the type of analyzer. The most obvious difference after exposure between the Ag(ll1) and the Ag[3(111) X (loo)] surfaces is the degree of attenuation near the top of the Ag d band. The attenuation is much greater for the stepped surface. The attenuation is partly the result of bonding interactions, and the greater attenuation found on the stepped surface may be due to an angular effect. The adsorbate-induced peaks above and near the bottom of the Ag d band are associated with the Cl 3p levels. We will discuss this assignment in greater detail in a later section. and CHaI onto clean Ag(ll1) and We have also adsorbed CH,Br, surfaces. The He(I) spectra for these adsorbates are shown in Ag[3(111) X (loo)] figs. 2 and 3. Similar difference curves are found for these adsorbates. In all cases, Auger spectroscopy indicates that only the halogen is adsorbed. The only difference between the three halogens adsorbed on Ag is found in the position of the strong peak above the d band. This peak shifts toward BF in going from Cl to I. Also, in the case of I, this peak is split into a doublet separated by -0.6 eV.
L.J. Gerenser, RX. Baetzold / ~~e~i~rptio~ on Ag using UPS CH2Br2
263
ADSORPTION Ag [3(lll~~(l~O)~
Ag(lli)
Fig. 2. Photoem~~on spectra for clean (solid line) and CHzBrzcovered (dotted line) silver (111) and [3(111) X (IOO)] surfaces. The difference curves at saturation coverage are shown below.
Work-function changes can readily be measured from the UPS spectra. Workfunction changes occur during adsorption because the adsorbed atoms or molecules cause a redistribution of the charge density at the crystal surface. If the adsorbed atoms transfer electrons into the crystal surface, the work function decreases, and, conversely, the formation of adsorbed negative ions increases the work function. Table 1 lists the work-function changes measured for adsorption of the halogens at saturation coverage on Ag(l11). Identical work-function changes were measured for
Energy (eV)
Energy
(eb’)
Fig. 3. Photoemission spectra for clean (solid line) and CWsI-covered (dotted line) silver (111) and [ 3(111) X (loo)] surfaces. The difference curves at saturation coverage are shown below.
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Table 1 Work-function changes for adsorption on Ag( 111) -Adsorbate A0 (eV) CHC& CHzBr2 CH31 ~.._
1.0 0.7 0.3 --
0
2000
5000 6CX33
CHC1, exposure (L)
Fig. 4. Change in chlorine to silver Auger signal ratio and work function (A@) on the silver (111) and [3(111) X (IOO)] surfaces asa function of exposure to CHC13.
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265
the Ag[3(111) X (loo)] crystal surface. As can be seen from table 1, the work-function change, A@, decreases in going from Cl to I. This is in agreement with the decrease in electron-withdrawing ability in going from Cl to I. A difference in the rate of adsorption of Cl is found with exposure of the stepped and low-index Ag surface to CHCla. This can be seen by the difference spectra at various exposures in fig. 1 and the increase in the surface concentration of Cl as determined by Auger spectroscopy in fig. 4. The stepped surface reaches saturation coverage at an exposure of 2400 L compared to 4800 L for the Ag(ll1) surface. Similar results are also found for the rate of adsorption of Br and I. In all cases, the initial rate of adsorption is approximately twice as fast on the stepped surface as compared to the low-index surface. This is in agreement with the results found on stepped and low-index Cu surfaces [4] _The differences in rate of adsorption can also be followed by measuring the change in work function, A#, as a function of exposure as shown in fig. 4. Again, A@ reaches saturation at 2400 L exposure for the stepped surface compared to 4800 L for the Ag(ll1). Adsorption of CHCla and CH,Br2 on either the Ag(ll1) or Ag[3(111) X (loo)] surface at room temperature resulted in a LEED pattern unchanged from the clean surface, except for an increase in background. However, CHsI adsorbed on the Ag(ll1) surface at room temperature produces a (d3 X ~‘3)R30” type LEED pattern. The adsorbed halogens can be desorbed by heating to sufficiently high temperatures. For the case of Cl adsorbed on silver, some desorption begins to take place at 300~-4OO”C, as indicated by the decrease in Auger/Cl signal intensity. Desorption of the Cl is complete at 500-SSO’C. These high temperatures required for desorption indicate a strong chemisorption bond. Both Br and I were found to desorb completely in the same temperature range. These results agree well with the thermodynamic studies of Tu and Blakely [lo] for Cl* adsorption on Ag(l11). In their work an epitaxial AgCl layer was formed and the heat of adsorption measured was consistent with AgCl formation.
4. Discussion Halogens were found to adsorb readily on Ag surfaces using halogenated hydrocarbons. The initial rate of halogen adsorption on Ag stepped surfaces is approximately twice that on the low-index surfaces. Thus, for Ag, which has a low D(Er), definite differences in the rate of adsorption were found for stepped and low-index surfaces. UPS spectra indicate interactions between the adsorbate p levels and the Ag d levels, indicating that electronic as well as structural effects may influence the chemisorption process. The UPS spectra show strong similarities for all halogens adsorbed on both stepped and low-index Ag surfaces. In each case, the halogen induces electronic states above and near the bottom of the Ag d band. Some enhancement in the Ag s
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band region beginning at the Fermi level is observed. An attenuation near the top of the Ag d band is observed, indicating a shift in these Ag d levels due to Ag d orbital interactions with the adsorbate. The only difference between the three halogens adsorbed on Ag is in the position of the strong adsorbate-induced peak above the Ag d band. The onset of this peak occurs at 2.9 eV, 2.4 eV, and 1.9 eV below EF for Cl, Br, and I, respectively. In each case, this peak is assigned to the halogen p levels. The shift toward the Fermi level in going from Cl to I is consistent with the variations in valence-shell ionization energy of the free atoms, although the differences are of a smaller magnitude. The splitting of 0.6 eV observed for the strong
As,,*, . .
.
;,
.
.
.
l
4
ox
.
0
;
decreasin separatlan ha9aaen D level d ievdl
.
1 between
f
Fig. 5. Calculated density of states for (A) the AgI a plane of atoms and (B)-(D) the halogen p component. The halogen p atomic level is placed at -9, -10 and -11 eV, respectively, for curves B, C and D. Silver atomic s and d levels are -7.56 and -11.58 eV, respectively. All curves are normalized to the same peak amplitude.
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267
peak above the Ag d band in the case of I is consistent with the spin-orbit splitting of 0.66 eV found for the 5p,,,,,,, doublet in HI’ [20]. This splitting is reported as 0.33 eV and 0.08 eV for HBr’and HCl’, respectively. We were not able to resolve this splitting for Cl and Br. We have used extended Htickel calculations to help interpret the adsorbateinduced levels on Ag surfaces. Fig. 5 shows the model used for our calculations. It consists of 18 Ag atoms in a plane at the bulk Ag-Ag distance with a 5-atom halogen overlayer. This corresponds to the (d3 X 43)-R30” structure observed for iodine adsorption on the Ag(ll1) surface. Using this model for our calculations, we obtained the plots of the DOS as shown in fig. 5. In all cases, the energy-level distribution has been broadened using a 0.5 eV FWHM Gaussian. Plot (A) is simply the DOS for the Agrs plane of atoms. Plots (B)-(D) contain only the halogen component of the DOS. In effect, what we have done in going from plot (B) to (D) is increased the ionization potential of the halogen p valence orbitals by adjusting the appropriate parameters in our calculations. Such adjustments are necessary since we have no a priori knowledge of the charge on halogen which determines this value. This results in a changing separation between the halogen p level and the Ag d level, which reflects a changing interaction between silver d and halogen p levels. Plot (C) qualitatively agrees with our experimental difference curves. We find a pattern of orbital mixing of the halogen p orbitals with silver s and d orbital% which is consistent with perturbation theory arguments. Mixing of the halogen p level with the silver s level leads to the halogen component near the Fermi energy in fig. 5. Interactions with d orbitals of silver lead to the two peaks on either edge of the silver d band. As the interactions between halogen p and silver d orbitals increase (by decreasing their energy separation), two peaks of equal intensity form. At weaker interactions, the peak above the d band is primarily halogen and the peak below the d band is primarily shifted d orbitals. This picture gives strong evidence for the ability of a filed d shell to interact with adsorbates through a backbonding-type mechanism. The high binding energy peak is due to a bonding p + d orbital interaction while the low binding energy peak is due to an antibonding p-d orbital interaction. This computed result agrees with the conclusions from experimental measurement of the dispersive nature of the two adsorbate induced peaks for CIZ adsorbed on Ag(OO1) [19].
5. Conclusions (A) Rate of halogen adsorption on Ag-stepped surfaces is approximately twice that on low-index surfaces. D(EF) is not the only factor determining special activity at step surfaces. (B) Adsorbate-induced levels on Ag can be correlated with halogen p valence orbitals. Changes in the Ag d band are interpreted as due to Ag d-halogen p orbital interactions. Evidence for interaction of halogen p with silver s levels is also found.
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(C) Adsorption of Cl and Br on the Ag(ll1) surface using halogenated hydrocarbons results in a LEED pattern unchanged from the clean surface except for an increase in background; however, I forms an ordered (43 X d/3)-R30” structure at room temperature. (D) The halogen chemisorption bond is strong as evidenced by the high temperature for desorption.
Acknowledgement We are grateful to Henry Luss for X-ray alignment
of silver crystals.
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[ 111 [12] [13] [14] [ 151 [16] [17] [18] [19] [20]
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