J . Phys. Chem. 1986, 90,4057-4063
4057
bridge sites of 2.89 A. The SO3 also differs from the C 0 3 in that for the surface SO4,and it has C, symmetry (Figure sa). At higher it disproportionates to form an SO4 species whereas the C 0 3 temperatures an upright configuration with C,, symmetry is merely dissociates to form gaseous C02and adsorbed oxygen. This suggested with no third Ag-0 interaction (Figure 5b). Again, difference is attributable to the inaccessibility of the +6 oxidation a site on top of the atomic ridges of the (1 10) surface is also state for carbon. consistent with the symmetry and the frequency assignments of this study, but this would result in a lower coordination of the Acknowledgment. We (D.A.0 and R.J.M.) gratefully acoxygen atoms bound to the surface which we feel is unlikely. knowledge the support of the National Science Foundation (CPE The reaction of SO2 with atomic oxygen on Ag( 110) parallels 8320072) for this work. The cooperation of the management of the reaction of C 0 2with atomic oxygen on this same s ~ r f a c e . ' ~ - ~ ~the General Motors Research Laboratories in facilitating this In both cases a trioxide species is formed near 200 K, and the research is also deeply appreciated. structure of both species has been interpreted in terms of monRegistry No. SO2, 7446-09-5; Ag, 7440-22-4. odentate oxygen coordination. The SO3 differs in that an irreversible transformation to a bidentate species is observed, whereas (43) Detoni, S.;Hadzi, D. Spectrochim. Acta 1957, 11, 601. no such transformation occurs for the C03. One could speculate (44) Pauchert, C. J. The Aldrich Library of Infrared Spectra; Aldrich that the smaller size of the C03with its typical 0-0separation Chemical Co.: Milwaukee, WI, 1981. (45) Balvich, J.; Fivizzani, K. P.; Pavkovic, S. F.; Brown, J. N. Inorg. of 2.15 A3' is a poorer match for the span between long twofold (37) Wyckoff, R. W. G. Crystal Structures; Interscience: New York, 1964; Vol. 2, p 365. (38) Stopperka, K. Z. Anorg. Allg. Chem. 1966, 345, 277. (39) Kalder, A,; Maki, A. G.; Dorney, A. J.; Mills, I. M. J. Mol. Spectrosc. 1973, 45, 247. (40) Evans, J. C.; Berstein, H. J. Can. J . Chem. 1955, 33, 1270. (41) Hall. J. P.: Griffith. W. P. Inorp. Chim. Acta 1955. 48. 1270. (42) Larsson, L. 0.; Niinisto, L. AcG Chem. Scand. 1973, 27, 859.
Chem. 1976, 13, 71. (46) Reed, J.; Soled, S.L.; Eisenberg, R. Inorg. Chem. 1976, 15, 71. (47) Benelli, C.; Di Vaira, M.; Nocvidi, G.; Socconi, L. Inorg. Chem. 1977, 16, 182. (48) Lucas, B. C.; Moody, D. C.; Ryan, R. R. Cryst. Struct. Commun. 1977, 6, 57. (49) Muravieskaya, G. S.; Kukina, G. A.; Orlova, V. S.; Eustafeva, 0. N.; Porai-Koshits, M. A. Dokl. Akad. Nauk, SSSR 1976, 226, 76. (50) Cotton, F. A.; Frenz, B. A.; Shive, L. W. Inorg. Chem. 1975,14,649. (51) Ghatak, I.; Mingos, D. M. P.; Hursthouse, M. B.; Abdul Malik, K. M.; Transition Met. Chem. (N.Y.) 1979, 4, 260.
Metal-Adsorbate Vibrational Frequencies as a Probe of Surface Bonding: Halides and Pseudohalldes at Gold Electrodes Ping Gao and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: November 21, 1985; In Final Form: March 13, 1986)
The frequencies of metal surface-adsorbate vibrations for monolayers of chloride, bromide, iodide, thiocyanate, and cyanide adsorbed at gold electrodes as obtained by surface-enhanced Raman scattering are compared along with corresponding data for silver electrodes and for related bulk-phase metal complexes with the aim of elucidating the nature of the surface bonding. The frequencies and derived force constants are generally substantially higher at gold than at silver, especially when the comparisons are made at the same metal charge rather than at a common electrode potential. This is interpreted in terms of a greater degree of bond covalency, Le., adsorbate-surface charge transfer, at gold. Also consistent with this interpretation are the derived force constants for gold that increase from chloride to iodide and the frequency-potential dependencies that decrease in the same sequence. The latter can be understood in terms of the influence of the electrode charge on the ionic component of the bond energy. Greater surface-adsorbate frequencies and smaller frequency-potential dependencies are found also for thiocyanate at gold compared with silver electrodes, again implicating the greater importance of bond covalency at the former surface. Some limitations of 'surface complex" models for describing SERS for such simple anion adsorbates at gold are also noted.
A primary application of surface vibrational spectroscopies at metalsolution as well as other types of interfaces is to gain insight into the nature of the metal-adsorbate bonding. While most attention has been focused on vibrational frequency shifts of internal adsorbate modes upon surface binding, the frequencies of the surfaceadsorbate modes themselves are clearly of particular significance. The paucity of information on the latter is probably due in part to the difficulties in detecting such low-frequency modes from electron energy loss or infrared spectroscopies. Nevertheless, surface Raman spectroscopy can readily detect such modes, even for surface-adsorbate bonds involving heavy atoms where their frequencies should be below ca. 300 cm-I. Of particular electrochemical interest are surface-adsorbate interactions for halides and pseudohalides in view of their structural simplicity and strong adsorption at metal surfaces. A significant amount of information on surface-adsorbate vibrations for these systems has been accumulated for the silveraqueous inteface using
surface-enhanced Raman scattering (SERS).1,2 Similar reports for the other SERS-active surfaces, gold and copper, have been sporadic and usually less reliable (vide infra). However, we have recently reported an ex situ electrochemical roughening procedure for gold that yields unusually stable as well as intense SERS.3 (1) For example: (a) Wetzel, H.; Gerischer, H.; Pettinger, B. Chem. Phys. Lett. 1981, 78, 392. (b) Fleischmann, M.; Hendra, R. J.; Hill, I. R.; Pemble, M. E. J. Electroanal. Chem. 1981, 117, 243. (c) Owen, J. F.; Chen, T. T.;
Chang, R. K.;Laube, B. L. Surj. Sci. 1983, 125, 679. (d) Weaver, M. J.; Hupp, J. T.; Barz, F.; Gordon, J. G.; Philpott, M. R. J. Electroanal. Chem. 1984, 160, 321. (2) For example: (a) Wetzel, H.; Gerscher, H.; Pettinger, B. Chem. Phys. Lett. 1981,80, 159. (b) Weaver, M. J.; Barz, F.;Gordon, J. G.; Philpott, M. R. Surf. Sci. 1983,125,409. (c) Dornhaus, R.; Long, M. B.; Benner, R.E.; Chang, R. K. Surf. Sci. 1980, 93, 240. (d) Fleischrnann, M.; Hill, I. R.; Pemble, M. E. J. Electroanal. Chem. 1982, 136, 361. (3) Gao, P.; Patterson, M. L.; Tadayyoni, M. A.; Weaver, M. J. Lungmuir 1985, I , 173.
0022-3654/86/2090-4057$01 .SO10 0 1986 American Chemical Society
4058 The Journal of Physical Chemistry, Vol. 90, No. 17, I986
Several applications have emerged, including the determination of adsorbate bonding and orientation for substituted benzenes? alkenes and alkyne^,^ and iodide and carbon monoxide6 at gold. Easily detectable SER spectra can be obtained at gold for a variety of adsorbed anions, including halides, pseudohalides, and oxyanions, facilitated by the wide polarizable potential range available together with the excellent temporal stability of the SERS signal^.^ We present here a comparison of SERS frequencies and resulting force constants of metal-adsorbate modes for chloride, bromide, iodide, thiocyanate, and cyanide at gold as a function of electrode potential with corresponding data obtained for silver electrodes and for related bulk-phase metal complexes. These results provide some insight into the roles of covalent and ionic electrodeadsorbate interactions at gold compared with silver surfaces.
Gao and Weaver P
Experimental Section Surface Raman spectra were obtained by using a conventional two-compartment electrochemical cell with the working electrode illuminated through a window in the cell base.’ The gold electrodes are of rotating-disk construction (Pine Instruments Co.), with a 4-mm-diameter metal disk imbeded in a 1.Zcrn-diameter Teflon sheath. The gold surface was polished successively with 1.O-and 0.3-pm alumina and rinsed with distilled water prior to use. The electrochemical roughening so to yield SERS consisted of successive computer-controlled anodic-cathodic potential scans in 0.1 M KCI, followed by rinsing with water and transfer to the solution of interest.g The electrochemical conditions for optimal SERS were found to be 25 cycles at 500 mV s-l from -300 to +1200 mV vs. the saturated calomel electrode (SCE), holding the potential for 1-1.5 s at the positive limit during each cycle. (This last feature was found to be extremely beneficial; a detailed study of the factors influencing SERS activity at gold will be published elsewhere.) The gold surface was positioned in the same configuration when SERS measurements were made in a nitrogen gas-phase, rather than the usual solution, environment,except that the cell was purged instead with dry nitrogen (vide infra). Adsorbed-halide SERS at silver electrodes were obtained similarly, except that in situ anodic-cathodic potential cycling was found to be desirable in order to yield satisfactory spectra. A singlepotential-step cycle was generally employed from -300 to + 150 mV and returned after passing 25 mC cm-* of anodic charge. Raman excitation employed a Spectra Physics Model 165 Kr+ laser operated at 647.1 nm, and the spectra were obtained with a SPEX Model 1403 double monochromator. Further details are given in ref 7. Differential capacitance-potential (Cdl-E)measurements at the gold-aqueous interface were made with a phase-sensitive detection method, employing a PAR Model 5204 lock-in amplifier together with a PAR Model 173/175/179 potentiostat system. The Cdlvalues were extracted from the in-phase and quadrature current components by means of an on-line computer. All measurements were made at mom temperature, 23 f 0.5 OC,and all potentials were recorded and are quoted vs. the SCE.
Results and Discussion Halide Adsorbates. Figure 1 consists of typical SER spectra for chloride, bromide, and iodide at gold electrodes as a function of electrode potential. For chloride, a band centered at 255-275 cm-I, with a full width at half-maximum (fwhm) of 45-50 em-’, was obtained at potentials from +600 to -200 mV using chloride (4) Gao, P.; Weaver, M. J. J. Phys. Chem. 1985, 89, 5040. (5) (a) Patterson, M. L.; Weaver, M. J. J . P h p . Chem. 1985, 89, 1331. (b) Patterson, M. L.; Weaver, M. J. J. Phys. Chem. 1985,89, 5046.
(6) (a) Tadayyoni, M.A.; Gao, P.; Weaver, M.J. J. Efecfroanaf.Chem. 1986,198,125. (b) Tadayyoni, M.A.; Weaver, M. J. Lmgmuir 1986,2, 179. (7) Tadayyoni, M. A.; Farquharson, S.;Li, T. T.-T.; Weaver, M. J. J . Phys. Chem. 1984,88, 4701. (8) (a) Larkin, D.; Guyer, K. L.; Hupp, J. T.; Weaver, M. J. J . Electroanaf. Chem. 1982, 138,401. (b) Hupp, J. T.;Larkin, D.; Weaver, M.J. Surf. Sci. 1983, 125, 429.
-500 mV
u 300 250 200
200
I50
101
Figure 1. SER spectra for (A) chloride, (B) bromide, and (C) iodide adsorbed on gold at various electrode potentials are indicated. Electrolytes are aqueous 0.1 M potassium halides. Laser excitation was 647.1 nm, with 35-mW power spot focused to ca. 2-mm diameter on the surface. Spectral band pass was 5 cm-’, scanned at 0.5 cm d.
concentrations above ca 10 mM. At more negative potentials, the intensity decreased sharply and virtually disappeared by -700 mV (Figure 1A). However, in contrast to the negative potential-induced irreversibility seen at silver,lsd the SERS signal almost entirely recovered upon return to more positive values after such negative potential excursions. This was true even over the time scales (20-30 min) required to obtain a comprehensive series of spectra using a scanning spectrometer. Addition of 10 mM or more bromide or iodide anions entirely displaced the adsorbed chloride, as evidenced by the disappearance of the 265-cm-I band and the appearance of lower frequency modes. For bromide, a strong band is obtained around 185 cm-’ with fwhm of approximately 30 cm-I having approximately equal intensity between +600 and -200 mV (figure lB), whereas for iodide a band at about 120 cm-I (fwhm of 25 cm-I) is obtained along with a weaker feature at 157 cm-’ (Figure 1C). Similarly to the corresponding low-frequency bands at silver,’ these bands are attributed to gold surface-halide stretching vibrations, uAu-x. Both iodide SERS bands exhibited approximately potential-independent intensities between the onset of iodide oxidation at 200 mVh and the negative potential limit at -900 mV set by vigorous hydrogen evolution. Essentially similar results were obtained with either pure 0.1 M KBr or KI electrolytes (as shown in Figure 1) or for more dilute halide solutions in “inert” supporting electrolytes such as 0.1 M NaC104 or 0.1 M KF. No SERS band could be detected for adsorbed fluoride at gold, even at far-positive potentials, consistent with the extremely weak adsorbing properties of this anion. Figure 2 shows representative differential capacitancepotential (Cdl-E)plots for 0.1 M KF, KCI, KBr, and KI at a SERS-active gold electrode. Very similar cd,-E plots were also obtained at a mechanically polished gold surface and when somewhat more dilute (e.g., 10 mM) C1-, Bi,or I- in 0.1 M KF was employed. For chloride and bromide, a pair of broad Cd1-E peaks are obtained, similar to those reported previously for polycrystalline gold.’
Metal-Adsorbate Vibrational Frequencies
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4059
t
250
t
260 -
200
240
-
\
N
' E 150 V
LL
i
-
X I
'0
0
100
'AQ-Br
,501 50
5 00
:---
T I30
0
-500
-1000
E,rnV vs. SCE Figure 2. Differential double-layer capacitance, cdl, for SERS-active gold surface in contact with 0.1 M potassium halide as indicated plotted against electrode potential, E. c d l values obtained by phase-sensitive detection (see Experimental Section) using 93 Hz at 8-mV peak-to-peak oscillation, scanning at 2 mV s-I positive direction. (Cd1-E curves were virtually identical also for negative-going potential scans).
Both features are also usually seen for various oriented singlecrystal gold faces1° and are undoubtedly associated with potential-dependent halide-specific adsorption. The more negative potential peak for iodide is less resolved, lying close to the negative limit. Comparison between the SERS v ~ intensity-potential ~ - ~ dependence (Figure l ) and the Cd1-E plots for chloride and bromide shows that the SERS band intensity remains almost independent of potential from the positive potential limit through the major, more positive, C r E peak, only becoming weaker and disappearing within the potential region corresponding to the second capacitance peak. For iodide, the vAU-, intensities remain unaltered even at the negative potential limit, around -1.0 V. The presence of substantial iodide adsorption even at such negative potentials is consistent with the markedly larger CdI values observed for iodide soluti6ns in comparison with those for nonadsorbing fluoride electrolytes (Figure 2). The halide coverage must be decreasing substantially below a monolayer over the potential regions corresponding to the major Cd1-E peak, since the appearance of such peaks is generally associated with strongly potential-dependent adsorption.8b The SERS signals are therefore presumably associated with another form of adsorbed halide, most reasonably the more strongly bound adsorbate associated with the cd1-E peak seen at more negative potentials. This behavior is somewhat different from that obtained for halides adsorbed at silver electrodes, whereby the reversible potential-dependent vAg+ intensities for bromide and chloride were found to be roughly proportional (9) (a) Clavilier, J.; Huong, N. V.J . Electroanal. Chem. 1973, 41, 193. (b) Hamelin, A. J . Electroanal. Chem. 1982, 142, 299. (10) (a) Braunstein, P.; Clark, R. J. H. J. Chem. SOC.Dalton Trans. 1973, 1845. (b) Bowmaker, G . A.; Whiting, R. Aust. J. Chem. 1976, 29, 1907. (c) Breitinger, D.;Leuchtenstern, H. Z . Noturforsch. 1974, 29b, 1974. (d) Husson, E.;Dao, N.Q.;Breitinger, D.K . Spectrochim. Acta 1981, 38a, 1087. (e) Puddephatt, R. J. The Chemistry of Gold; Elsevier: Amsterdam, 1978.
IIO
1
1000
'-A
r, .\~,AQ
0
-I -1000
E,mV vs. SCE
Figure 3. SERS peak frequencies for surface-halide stretching modes, at gold and silver electrodes plotted against electrode potential under conditions of almost constant SERS band intensities. Data obtained for 0.1 M potassium halide solutions, except for open symbols for iodide and bromide at gold that were obtained following electrode transfer into 0.1 M HCIO4 (see text). Data for gold obtained in part from Figure 1, experimental conditions for both gold and silver as noted in caption to Figure 1.
to the halide coverage as obtained by a thermodynamic analysis of Cd1-E data.1d,8bHowever, in contrast to gold the Cdl-E curves for silver typically exhibit only one major Cd1-E peak with additional partly resolved features a t more negative potentials.8b Figure 3 summarizes peak frequencies for each of the v ~ modes as a function of potential for 0.1 M halide solutions under conditions where the band intensities are roughly constant. The wide (1-2 V) potential ranges over which this condition can be achieved for gold contrasts the situation for silver;Id this enables acceptable estimates of the band frequency-potential slopes, dvAu-x/dE, to be obtained for the former surface from Figure 3. Interestingly, this slope is substantially smaller for bromide (6 cm-' V-I) than for chloride (35 cm-' V I ) . For iodide, the higher frequency band exhibits no detectable potential dependence, whereas a small yet significant value of dvAu-I/dE(5 cm-' V-') is obtained for the lower frequency band. The open symbols plotted for iodide at gold in Figure 3 at potentials positive of 200 mV were obtained by immersing the gold electrode in an iodide-containing solution, rinsing with water, and transferring into 0.1 M HC104. The two surface-iodide bands were obtained with comparable intensity as before, indicating that the iodide desorption rates are very sluggish. Such adsorbed iodide does not undergo electrooxidation even at +1100 mV.6a A similar electrode-transfer experiment for bromide yielded a weaker ca. 185-cm-' band than that obtained in bromide-containing electrolytes, which survived a t far-positive potentials, as for iodide, but was lost irreversibly following a negative potential excursion to -400 mV. No detectable SERS band was seen for adsorbed chloride upon such electrode transfer. The kinetic, as well as
~
-
~
4060
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
TABLE I: Raman Frequencies and Derived Force Constants for Gold- and Silver-Halide Bonds at Electrode Surfaces and in Related Bulk-Phase Environments fM-X,*
105dvn
environ" vib Au/CI-, H20; 500 mV Au-CI Au/CI-, H,O; -400 mV solid [Bu4N][AuC12] solid (AuCI), Ag/CI-, H20; -200 mV Ag-CI AgC12- in TBP gaseous AgCl Au/Br-, H 2 0 ; 500 mV Au-Br Au/Br-, HzO; -400 mV solid [Bu4N][AuBr2] solid (AuBr), Ag/Br-, H 2 0 ; -200 mV Ag-Br Ag/Br2- in TBP
gaseous AgBr Au/I', H20; 500 mV Au/I-, H,O; -400 mV solid [Bu4N][ A d 2 ] solid (AuI), Ag/I-, H 2 0 ; -200 mV Ag12-in TBP gaseous AgI
Au-I
cm-'
Y ~ - ~ ,
275 245 329 238
268
lit. source C
1.25 2.07 1.75 1.2 1.45
ref 10d c, ref la,d ref 10f
C
ref 10a
1.86
ref 10f
186 181 209
1.6 1.55
C C
1.86 1.63
158
1.2
170
1.29 1.65 1.15, 1.87 1.1, 1.87 1.65 1.22
ref 10a ref 10d c, ref la,d ref 10f
124, 158 120, 158
158 Ag-I
cm"
1.6
115 132
1.o 1.17 1.45
ref 10f c c
ref 10a ref 10d c, ref 1a.d ref 10f ref 10f
See literature sources quoted for further experimental details. TBP is tri-n-butyl phosphate (ref 100; the (AuX), data refer to solid-state chain structures (ref 10d). Force constants for surface-halide vibrations obtained from experimental SERS frequencies by using eq 1; those for bulk-phase complexes extracted as outlined in quoted literature sources. 'This work; frequency values taken from Figure 3. thermodynamic, stability of the gold surface-halide bonds therefore increases in the sequence C1- < Br- < I-. Figure 3 also contains corresponding plots of the surfacehalide stretching frequencies at silver, also obtained in this work with 0.1 M potassium halide solutions. The v ~ values ~ are - plotted ~ only within the potential regions where the band intensity remains constant; this corresponds to approximately monolayer halide coverage at silver.'dSb Since this condition corresponds to narrower potential ranges for silver than for gold, the dvAgdx/dEslopes are considerably less reliable, especially for chloride and bromide. Nevertheless, the dvAg-X/dEslopes appear to be comparable to, or higher than, those for gold. Table I contains representative frequency data for the surface-halide bonds a t gold and silver electrodes, together with corresponding estimates of the force constants, fM-X. Also included for comparison purposes are Raman and/or infrared band frequencies and derived force constants for the bulk-phase monovalent complexes AuX2-, AuX, AgX2- and AgX that are culled from the literature.I0 The surfacehalide force constants were obtained by using
where c is the velocity of light and p is the effective reduced mass of the vibrating bond, taken as equal to the halide mass, rn. This simplified procedure is clearly only very approximate. First, the presumption that the surface mass is effectively infinite (so that p = m) is questionable, especially for vibrations involving heavier adsorbates." Unfortunately, the magnitude of the coupling between the metal-adsorbate and metal-metal vibrations is hard to assess. It is, nevertheless, probably smaller than that calculated for an isolated square array of silver atoms in ref 11 since this neglects the damping influence of surrounding metal atoms (vide infra). Second, the effect of vibrational dipolar coupling between nearby adsorbate bonds can be substantial'2 so that the observed frequencies and hence force constants may not really reflect those for isolated surfaceadsorbate vibrations. On (1 1) Moskovits, M. Chem. Phys. Lett. 1983, 98, 498. (12) Nichols, H.; Hexter, R. M. J . Chem. Phys. 1981, 74, 2059.
Gao and Weaver the other hand, the use of force constants for surface bonds as derived from eq 1 provides a convenient means of "normalizing" the observed frequencies for the halide mass, at least in a relative sense. They therefore can provide a valid means of comparing vibrational data obtained for different halides on a given surface and especially for a given halide on different surfaces, provided that the likely uncertainties in this procedure are borne in mind (vide infra). Several trends are seen upon inspecting Table I. Most importantly, the frequencies (and hence the derived force constants) for the gold surface-halide bonds are significantly higher than for the corresponding silver surface-halide bonds. This is true not only when the comparisons are made at the same (or similar) electrode potentials but is especially marked when the data are compared instead at potentials corresponding to comparable electrode charge densities, qm, for the two metal surfaces. The potential of zero charge (pzc) of polycrystalline gold in the absence of specific adsorption is around 0-100 mV vs. SCE, while that for silver is about -900 mV.13 Therefore provided the negative pzc shifts upon halide adsorption, as well as the integral electrode capacitances, are comparable for the two metals,I4 an electrode potential of about 600-700 mV vs. SCE at gold should correspond to roughly similar qmvalues as for an electrode potential of -200 mV at silver. Up to ca. 35% larger force constants at gold relative to silver are estimated from Table I when the comparison is made under such a condition of constant electrode charge. This conclusion should remain essentially unaffected, at least qualitatively, even if the surface mass cannot be regarded as infinite. Thus this effect" will tend to decrease the force constant inferred from a given vibrational frequency and, if significant, should therefore decrease the fM-X estimates in Table I to a smaller extent for gold than for silver on account of the larger atomic mass of the former. Stronger metal-halide bonds for gold compared with silver are also seen for the related bulk-phase complexes.*Ob~c These differences are consistent with the smaller dI0 d9s electron promotion energy for Au(1) vs. Ag(1) and suggest that the gold-halide bonds are more covalent than their silver counterparts.Iob The similar behavior observed for the metal-halide surface bonds therefore strongly suggests that the gold surface also engenders greater coordinate covalency than silver for the halide chemisorption bond. Such covalency effects are also manifested in the observed differences in the dependence of the force constants in the surface and bulk-phase environments on the nature of the halide. While the fM-x values for the bulk-phase A(1) and especially Au(1) complexes decreases monotonically from chloride to iodide, those for the surface Ag-X bonds appear to be roughly independent of the halide and for surface Au-X bonds instead increase substantially from chloride to iodide (Table I). (This is true at least for the high-frequency iodide form.) This can be understood, at least qualitatively, in terms of the differing importance of ionic and covalent interactions. The influence of the former, greatest for chloride, is presumably responsible for the observed trend in fM-x values for the bulk-phase halide complexes.I0 Such electrostatic effects should be smaller for the surface-halide bonds since the effective metal charge density should be markedly less than +1 per surface atom.2b However, the contribution of electrostatic interactions to the surface bonding should increase as the potential (and hence electrode charge) becomes more positive and to the greatest extent for chloride. This argument provides a simple explanation of the observed increase in the dvM-x/dE slopes from iodide to chloride (Figure 3). The larger dvM-X/dE slopes observed at silver relative to gold, at least for iodide, are also consistent with this picture, given the greater covalency of the bonding at the latter surface. Within the accessible potential region at gold the influence of covalent bonding, greatest for iodide, -+
(13) Leikis, D. I.; Rybalka, K. V.;Sevastyanov, E. S.; Frumkin, A. N. J . Electroanal. Chem. 1973, 46, 16 1. (14) Note that the electrode charge will be. substantially positive under the conditions encountered in Table I, even for gold, since large negative shifts in the pzc (ca. 1 V) will generally be brought about by monolayer levels of anionic adsorbates, at least if the bonding is relatively ionic.2b
Metal-Adsorbate Vibrational Frequencies presumably outweighs that of ionic bonding, greatest for chloride, so that fAu-1 > fA,&. On this basis, these two effects therefore appear to be roughly balanced for halide bonding on the silver surface, so that fAeI fAgx1. These considerations, however, are complicated by two factors. First, the two bands seen for iodide at gold correspond to markedly different force constants (Table I). The force constant corresponding to the lower frequency mode is not greatly different from that for the silver surface-iodide bond, suggesting the presence of a relatively ionic Au-I bond in comparison with that responsible for the higher frequency (157-cm-l) band. The covalent nature of the latter is also consistent with the virtual potential independence of the band frequency (Figure 3). The second complication, alluded to above, concerns possible systematic errors in these force constants induced by setting the effective surface mass as infinity. If significant, the fM-X estimates given in Table I will be greater than the actual values by factors that increase from chloride to iodide since the influence of such vibrational coupling will become more important as the frequency decreases. Indeed, treating the surface as a square array of metal atoms as in ref 11 yieldsfM-x values for iodide and chloride vibrations that are roughly 40% and 60%, respectively, as large as the estimates given in Table I. Therefore, for example, recalculating the force constants for the surface Ag-X bonds on the basis of this model yieldsfex values that decrease by ca.40% from chloride to iodide, rather than the mild dependence on the halide given in Table I. Even though the model in ref 11 probably overestimates the influence of metal atom vibrations, these considerations emphasize the uncertainties contained in comparing fM-x values for different halides and between surface and bulkphase environments. It is nevertheless still worthwhile to note that the force constants in Table I for the surface Au-Br and high-frequency Au-I vibrations approach and even surpass those for the bulk-phase AuBr2- and A d 2 - complexes. This suggests that the extent of bond covalency is a t least comparable to that for the gold-halide complexes.'lb The high values of the electrosorption valency (ca. -0.8) reported for bromide at gold15 also reflect the influence of bond covalency for this system. Comparison with Metal SurfaceHalogen Bonding in the Gas Phase. It also is of interest to compare the electrochemical surface metal-halide bonding with that at metal-gas surfaces. Vibrational information for the latter surface bonds is apparently unavailable, presumably due to the difficulties of obtaining such low-frequency data from electron energy loss or infrared spectroscopies. Nevertheless, thermal desorption measurementsi6 indicate that the strength of the metal surfacehalogen bonding in the gas phase is normally in the order C1 > Br > I. This sequence can be understood by noting that adsorption of neutral halogen atoms at an initially uncharged metal surface will generally be accompanied by a degree of surface-to-adsorbate charge transfer to an extent that increases in the order I C Br C C1, so that the metal will acquire a positive charge that also increases in this sequence. The ionic bonding forces thus created, greatest for chlorine, presumably outweigh the effect of covalency, which is largest for iodine. This trend is compatible with the present findings since the metal-gas phase results refer to surfaces that will carry a substantial positive electronic charge if the bonding is relatively ionic, at least for high halogen coverages. It should be noted that regarding the adsorbed species as halide anions or halogen atoms, as for the metalsolution and metal-gas environments, respectively, is purely a matter of formal convenience; the extent of metaladsorbate charge sharing in the two cases may well be comparable. We undertook several SERS experiments involving transfer of gold electrodes with adsorbed halide into the gas phase, with the aim of discerning any differences between the surface bonding in electrochemical and gas-phase environments. In one set of experiments, the electrode was emersed from solution directly into dry nitrogen, and the SER spectra were recorded. Intense bands were observed at 275 and 189 cm-' for chloride and bromide,
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4061
Po0
-
(15) Sedlmaier, H. D.; Plieth, W. J. J. Electroaml. Chem. 1984,180,219. (16) Gruze, M.; Dowben, P. A. Appl. Surf. Sci. 1982, 10, 209.
PC
I \
I/--
2150
237
2050
700
500
'
'
I
350
'
'
'
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Raman shift, cm-I Figure 4. SER spectra for thiocyanate adsorbed at gold at various electrode potentials as indicated. Solution was 2 mM NaNCS in 0.1 M NaC104. Other conditions as in Figure 1.
respectively. Three major bands were seen for iodide, at 108, 144, and 160 cm-'. The addition of a drop of water or?aqueous HC104 resulted in the immediate disappearance of the 188- and 144-cm-l bands. This suggests that these two features are due to adsorbed triiodide produced by iodide air oxidation, supported by the similar electrochemical SERS features observed upon iodide electrooxidation.6a In a second set of experiments the surface was rinsed thoroughly with water before placing in the dry nitrogen atmosphere and recording the SER spectra. No band was detected for adsorbed chloride, indicating that rapid desorption occurred, as also suggested by the electrochemical electrode-transfer experiment noted above. Reasonably strong surfamadsorbate bands were seen, however, under these conditions for bromide a t 189 cm-' and for iodide at 108 and 160 cm-I. These data indicate that the nature of the surface-halide bonding as deduced from the SERS frequencies is similar in the electrochemical and gas-phase environments. The dif'iculty, however, is that the effective surface charge and potential of the surface after transfer into the gas phase is not well-defined in these experiments since such parameters can be influenced by reaction with impurities, such as oxygen. Indeed, an identical gas-phase SERS frequency of 188 f 2 cm-' was obtained for adsorbed bromide irrespective of the potential at which the electrode was emersed from solution, even though a significant potential-dependent frequency is obtained in the electrochemical environment (Figure 3). Pseudohalide Adsorbates. With a substantial influence of covalent, as well as ionic, interactions upon the gold surface-halide bonding having been identified, it is of interest to ascertain if similar effects are apparent for pseudohalide adsorbates at gold. The problem, of course, with such polyatomic adsorbates is that the observed frequencies for the surface bonds will be influenced by coupling to internal adsorbate modes, thereby further complicating the estimation of bond force constants. Nevertheless, the comparison of observed metal-ligand stretching frequencies
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
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E,mV vs. SCE Figure 5. SERS peak frequencies for adsorbed thiocyanate monolayers at gold (closed symbols) and silver (open symbols) as a function of electrode potential: (A,A) C-N stretch; (0,0) s u r f a c e s stretch. Data for gold from present work (Figure 4), for silver from ref 2b, using 0.1 M KCI 0.01 M N a N C S (see Table 2 of ref 2b).
+
for a given adsorbate in different interfacial environments can still yield valuable information on possible variations in the surface bonding. Figure 4 contains representative SER spectra for thiocyanate adsorbed at gold as a function of potential in the low-frequency region as well as at higher frequencies where the internal C-N stretching mode, vCN, is observed. We have recently discussed briefly the form of these spectra in comparison with corresponding surface infrared data." The bands around 455, 705, 2120-2130 cm-' are due to N-C-S bending (6Na), C-S stretching (vCs), and C-N stretching (vCN)modes, respectively (cf. ref 2b). The band at ca. 235 cm-' is most reasonably ascribed to gold surface-sulfur stretching, vAu*CN, while the feature around 300 cm-' appearing at more negative potentials (Figure 4) is probably due to gold surface-nitrogen stretching, VAu-NCS.17 These assignments are made on the basis of the relative masses of sulfur and nitrogen, together with the expectation that binding via the more negatively charged and polarizable sulfur atom should dominate a t least at more positive potential^.'^ Capacitance-potential data indicate that extensive thiocyanate specific adsorption occurs throughout the polarizable potential range, ca. +500 to -900 mV." It is interesting to compare the potential-dependent SERS frequencies of both the Y ~ and vCN ~ modes with ~ those ~ for the ~ corresponding SERS bands obtained at silver electrodes, v ~ and vcN. Such a comparison is shown in Figure 5, the data for ~ and vCN ~ modes ~ silver being extracted from ref 2b. Both the v occur at significantly higher frequencies than for thiocyanate adsorbed at silver, especially at the most negative potentials. As for the halides these frequency differences are accentuated further if the comparison is made a t potentials corresponding to similar electrode charges for the two metals rather than at a common electrode potential. With the assumption again that the extent of the negative pzc shift caused by anion adsorption is not greatly different for the two metals; this comparison involves potentials (17) Corrigan, D. S.; Foley, J. K.; Gao, P.; Pons, S.: Weaver, M. J. Langmuir 1985,1, 616.
Gao and Weaver that are about 900 mV more positive at gold relative to silver. Surface-sulfur frequencies that are about 30 cm-' higher for the former metal are obtained under such conditions. As for the adsorbed halides, the higher frequencies obtained for the v~~~~~ mode at gold are indicative of a greater surface bond covalency for S-bound thiocyanate than at silver. Also consistent with this picture on the basis of the above argument is the smaller value of dvAu-SCN/dE(ca. 10 cm-' V-I) compared to dvAg-SCN/dE(ca. 25 cm-' v-') (Figure 5). The observed larger values of vCN and also the smaller value of dvCN/dEat gold vs. silver (13 and 30 cm-' V-I, Figure 5) are also consistent with greater electron donation from sulfur to the metal surface in the former case.18 Examination of cyanate and azide adsorption are also of interest given their structural similiarities to thiocyanate. We have recently been obtaining both the surface infrared and Raman spectra of these and other pseudohalide adsorbates in order to compare the vibrational features by these two techniques.I9 However, no well-defined low-frequency SERS modes could be obtained for either adsorbate at gold or silver surfaces.z0 The SERS of cyanide is the most extensively studied of any pseudohalide This includes numerous reports for silverz2 and also for gold surfaces.23 However, a complication for the present purpose is that several low-frequency SERS bands are often observed for cyanide,2d the assignments for which are not unambiguous. This complexity is also a feature of the vibrational spectra for the bulk-phase metal-cyano complexes.24 For cyanide at gold, we observed two major low-frequency bands at about 300 and 380 cm-' (cf. ref 23). The frequencies of these bands are largely independent (
