J. Phys. Chem. 1993,97, 12047-12050
12047
Adsorption of Thiocyanate on Polycrystalline Silver and Gold Electrodes Studied In Situ by Sum-Frequency Spectroscopy T. Hui Ong and Paul B. Davied University Chemical Laboratories, Lensfield Road, Cambridge CB2 1 EW U.K.
Colin D. Bain’ Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ U.K. Received: July 12, 1993”
In situ vibrational spectra of thiocyanate ions adsorbed on smooth silver and gold electrodes have been obtained by infrared-visible sum-frequency spectroscopy (SFS). SCN- was detected on silver in the potential range 0.0 to -0.3 V vs SCE and on gold in the potential range +0.4 to -0.4 V vs SCE. In both cases, a peak due to the C-N stretching mode of S-bound thiocyanate was observed. This peak moved to lower frequencies as the potential was made more negative. There was no evidence for N-bound species. The absence of peaks at large negative potentials is consistent with SCN- ions lying parallel to the surface.
Introduction The adsorption of thiocyanate (SCN-) onto the surfaces of silver and gold has been something of a testing ground for in situ vibrational spectroscopy in recent years.1-16 Thiocyanate has a rich homogeneous coordination chemistry.” It binds readily to most complex-forming metal ions through the nitrogen atom, the sulfur atom, or both. The sulfur atom can also form a bridge between two metal ions. At a metal surface, the thiocyanate ion can, in addition, adopt various orientations with respect to the surface-almost endless possibilities exist. The relative ease of detecting thiocyanate ions by infrared spectroscopy has undoubtedly also contributed to the interest in this system. The C-N stretching mode has a large infrared and Raman cross section and falls conveniently in one of the transmission “windows”in the infrared absorption spectrum of water. The frequency of the C-N stretch is sensitive both to adsorption geometry and to electrode potential and is well removed from the absorption frequency of thiocyanate in solution. Characteristic low-frequency modes in adsorbed thiocyanate provide an ideal signature in Raman spectroscopy. Many different techniques have yielded in situ vibrational spectra of thiocyanate on silver or gold electrodes, including potential difference infrared spectroscopy (PDIR),2,4*6.8 polarization-modulation reflection absorption infrared spectroscopy (PM-RAIRS),s attenuated total internal reflection (ATR) infrared spectroscopy (both with3qg and without’ surface plasmon excitation), and surface-enhanced Raman spectroscopy ( S E R S ) . ~ , ~ S Hubbard ~ J ~ J ~ has employed electron energy loss spectroscopy (EELS) to study thiocyanate on Ag electrodes that have been removed from the electrolyte and placed in a UHV chamber,” and even inelastic electron tunneling spectroscopy (IETS) has been attempted.10 These spectroscopic studies have been supported by differential capacitance measurements1J2J6 and by ab initio calculations.5J1J2 The electrochemistry of thiocyanate thus provides an excellent opportunity for comparing the merits of these various approaches to in situ vibrational spectroscopy and for assessing exactly what each of these techniques is measuring. Here we report vibrational spectra of thiocyanate on polycrystalline gold and silver electrodes obtained by the nonlinear optical technique known as sum-frequency spectroscopy (SFS). This is the first example of in situ SFS of molecules on silver and 0
Abstract published in Advance ACS Abstracts, October 15, 1993.
gold electrodes. In the pioneering work of Guyot-Sionnest and Tadjeddine, SFS was used to detect carbon monoxide, cyanide and thiocyanate adsorbed on platinum electrodes.18Jg To date, no other electrochemical studies by SFS have been reported. If sum-frequency spectroscopy is to make a major contribution to electrochemistry, we must be able to interpret sum-frequency spectra with confidence. An important part of this paper is a comparison of sum-frequency spectra of SCN- on Au and Ag electrodes with vibrational spectra obtained by other techniques. A brief description of sum-frequency spectroscopy is provided here: a more detailed introduction to the technique may be found in the paper by Tadjeddine et al.19 SFS has also been reviewed recently.20 Sum-frequency spectroscopy relies on a nonlinear optical effect called sum-frequency generation (SFG). SFG is a close relative of second harmonic generation (SHG), which has been exploitedextensively for characterizing basic electrochemical processes at electrode surfaces.21 SFG differs from SHG in requiring two lasers. A fixed frequencyvisible laser and a tunable infrared laser are pulsed simultaneously onto an interface and the light emitted at the sum of the input frequencies is detected.22 The tunability of the infrared laser provides molecular specificity that is lacking in SHG: scanning the infrared laser yields a vibrational spectrum of the molecules adsorbed on the electrode surface. Although SFS may seem a needlessly esoteric way of obtaining in situ vibrational spectra, a sum-frequency spectrometer is in reality little more complex than a modern Raman spectrometer, and the technique does have several appealing features. First, it has unusual selection rules: for a vibrational mode to be sum-frequency active it must be both infrared- and Raman-active. Second, isotropic or centrosymmetric phases are sum-frequency inactive: there are no interfering signals from molecules in solution. This feature is particularly useful for distinguishing surface and solution species when there is no shift in vibrational frequency on absorption. Third, the temporal resolution of SFS is limited only by the pulse length of the lasers-typically nanoseconds or picoseconds. Fourth, the polar orientation of the adsorbates can be determined from the phase of the emitted SF signa1.18J9q23.24 For the case of thiocyanate, N-bound and S-bound molecules may be distinguished. SFG differs from infrared absorption or Raman scattering in being a coherent effect. The simultaneous intense laser pulses in the visible and infrared induce an oscillating surface polar-
0022-365419312097-12047$04.00/0 0 1993 American Chemical Society
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Figure 2. Variation in the frequency of the C-N stretch with potential for thiocyanate adsorbed on silver (filled circles) and gold (open circles).
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Figure 1. Sum-frequency spectra of thiocyanate adsorbed on silver for various electrode potentials within the double-layer region. Potential in volts vs SCE. Spectra have been offset virtually for clarity: the zero of the ordinate refers to the spectrum acquired at -0.1 V.
ization, P@),at the sum of the two input frequencies:22
P(2)(~8um) = X ( ~ ) ( O , , , = U&
+ uIR):&E~R
(1)
where ~ ( 2 is) the second-order susceptibility of the surface and &and EIRare the visible and infrared electric fields, respectively. This oscillating polarization emits light at the sum frequency with an intensity proportional to IP(2)12. The sum-frequency emission is coherent and highly directional. ~ ( 2can ) be separated into two terms: a nonresonant contribution from the electrode surface, x(*)NR, which is largely independent of the frequency of the infrared laser; and a resonant contribution, x(~)R, which carries the information on the vibrational characteristics of the adsorbed molecules. The intensity of the sumfrequency signal thus has the form
For Pt electrodes, X(~)NR is very small and the line shapes in sum-frequency spectra resembleconventional infrared absorption or Raman spectra. For Au and Ag electrodes, X(’)NR is large. since both and x(z)R are complex quantities, the line shapes in S F spectra of Au and Ag electrodes depend on the relative phase of the two susceptibilities.25 As a consequence, sumfrequency spectra can take on unfamiliar forms (Figure 1). All the usual spectral parameters, such as frequency, intensity, and line width, can, however, still be obtained from these spectra.25
Experimental Section Polycrystalline Au and Ag electrodes (Goodfellow, 99.99%) were mounted on the tip of a differential micrometer and polished successivelywith 6,1, and 0.1-pm diamond paste. The electrodes were cleaned electrochemically by linear sweep potential cycles in 0.15 M NaC104 (-0.7 to +0.3 V vs SCE for Ag; -0.7 to + 1.2 V for Au) until reproducible cyclic voltammograms of the clean metal were obtained.26 A simple three-electrode electrochemical cell, similar in design to conventional spectroelectrochemical cells,27 was constructed from Teflon and Kel-F, with a Pt wire counter electrode and SCE reference electrode.28 All potentials in this paper are quoted with reference to the SCE. Water was triply distilled, including one distillation from KMn04. Oxygen was not rigorously excluded from the cell, but electrolytesolutions were purged with N2 before use. The working electrode was mounted on a differential micrometer and aligned parallel to a CaFz prism, through which the laser beams passed. Before
acquiring a sum-frequency spectrum, the electrode was allowed to stabilize at a fixed potential with a large (- 1 mm) gap between the electrode and the prism. The electrode was then advanced in order to trap a thin film of electrolyte (- 1 pm) between the electrode surface and the CaFz prism. Although no sumfrequency signal arises from the electrolyte, a thin film is still required to ensure that most of the infrared radiation reaches the electrode surface. Details of the sum-frequency spectrometer have been reported previo~sly.28~2~ A frequency-doubled Nd:YAG laser provided the visible beam (532 nm, 6 ns, 10 Hz, 1 mJ/pulse). Tunable infrared (2000-2200 cm-1, -5 ns, 1 mJ/pulse) was generated by third-order stimulated Stokes scattering of the output of a tunable dye laser in high-pressure hydrogen (35 bar of Hz). Photons emitted at the sum frequency were detected with a photomultiplier tube and gated integrator. For spectra of silver electrodes, the infrared laser was scanned between 2000 and 2220 cm-1, and for gold the range was 2000-2200 cm-I. Each spectrum took 30 min to acquire. The spectra shown in Figures 1 and 3 were reproducibleupon reversing the electrode potential provided that the potential remained within the double-layer region.
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Results and Discussion Thiocyanate Adsorption on Siver. Figure 1 shows sumfrequency spectra of silver electrodes in an aqueous solution containing 25 mM NaSCN and 0.15 M NaC104. At potentials more positive than 0 V (vs SCE), bulk Ag(SCN) appeared as a visible grey film. At potentials more negative than 4 . 4 V, no resonant features assignable to thiocyanate were observed. In each spectrum there is a constant nonresonant background. The resonant features arise principally from the cross term between the resonant and nonresonant susceptibilities (eq 2). The intensity, line width, and resonant frequency can be calculated from these spectra by a procedure that we have described previously.25 The variation in the peak frequency with potential is shown in Figure 2. This peak has previously been assigned to S-bound thiocyanate.1+7J5 The peak frequency decreases from 2120 cm-I at 0.0 V to 2104 cm-1 at -0.3 V. Our results are very similar to those obtained recently by Samant et al.5 by PM-RAIRS of silver electrodes that had been prepared in a similar manner. The resonant frequencies obtained by SFS and RAIRS agree to within experimental error. The infrared absorption intensity was largest at the most positive potentials and decreased as the potential was made more negative. A weak peak was still detected by RAIRS5 at potentials more negative than -0.4 V, which may reflect the better signal-to-noise ratio in the RAIRS experiments compared to our sum-frequency measurements. Within the noise limit of our experiments, there is no evidence for an N-bound species on silver. The conventional description of line widths in terms of the full width at half-maximum (fwhm) is clearly inappropriate for S F spectra of SCN- on silver. Homogeneous and inhomogeneous
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Adsorption of Thiocyanate on Polycrystalline Silver
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broadening affect the line shape in different ways in SFS.25 Broadeningof the thiocyanateresonance on silver is predominantly homogeneous and is equivalent to a fwhm of 30 cm-1 in an IR or Raman experiment. Differential capacitance measurementslJ6 suggest near-monolayer coverage of thiocyanate on the silver electrode throughout the polarizable potential range, even at potentials as low as -1 .O V vs. SCE. The absence of a S F signal at potentials below -0.4 V has two possible origins. First, the infrared transition moment or the Raman transition moment for the C-N stretch may be zero in this potential range. The C-N infrared transition moment is certainly very sensitive to the coordination of the thiocyanate ion, decreasing by a factor of 10 when free thiocyanate in solution is adsorbed on silver at potentials near the positive end of the double-layer region.4 (This sensitivity has also been observed in homogeneous complexes. For example,30 the extinction coefficient of Au(SCN)~-is 10 times weaker than that of free SCN-.) There is, however, no evidence for the infrared transition dipole vanishing at cathodic potentials. One would expect the Raman intensity to be less sensitive to adsorption, and indeed the C-N stretch has been observed by SERS between -0.8 and -0.1 V.1,6J5 An alternative explanation is that the thiocyanate ions are adsorbed with their C, axis parallel to the surface. RAIRS and SFS are only sensitiveto transition dipoleswith a component perpendicular to the surface: no signal would arise from ions lying flat on the surface. A highly bent conformation is favoured for S-bound thiocyanate on silver,sJ1but the potential surface with respect to the polar angle is very flat, and the energetic penalty for reorientation to an IR-active upright configuration at higher coverages (less negative potentials) is small. Both SERSlJS and ATR infrared spectroscopy3.7 detect thiocyanate ions at cathodic potentials where we observe nothing by SFS, and show a weaker dependenceof the resonant frequency on potential (see below). SuBtaka has suggested3 that cavity sites may dominate the signal in ATR and SERS. Such sites will be infrequent on our mechanically polished electrodes. Thiocyanate Adsorption on Gold. The polarizable potential range is wider on gold than on silver. Adsorbed thiocyanate ions were detected by SFS between +0.4 and -0.4 V vs SCE (Figure 3). The phases of X(')NR of silver and gold are not the same,
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giving rise to different peak shapes in the spectra of adsorbates on the twoelectrodes. AswithSCN-onsilver, thepeakfrequency shifts to lower values as the electrode potential is scanned cathodically (from 2127 cm-1 at +0.4 V to 2097 cm-1 at -0.4 V; see Figure 2) and the intensity of the peak decreases. At the most positive potentials the peak is asymmetric, probably due to the presence of multiple adsorption sites on the gold surface. There is general agreement69J4 that thiocyanate is bound to the gold through the sulfur atom at positive potentials, and we therefore assign the peak in our S F spectra to S-bound thiocyanate. Weseenoevidence, however, for N-bound thiocyanateat negative potentials, which has been reported by several a u t h o r ~ . ~ One .~J~ feature of SFS is that x ( ~ ) Rchanges sign if a molecule is inverted. It is highly probable that x ( ~ ) Rhas opposite signs for S-bound and N-bound thiocyanate. Since S-bound thiocyanate gives rise to a peak in the sum-frequency spectrum (constructive interference between x ( ~ ) Rand x(')NR), N-bound thiocyanate would give rise to a dip below the level of the nonresonant background signal (destructive interference between X@)R and x(~)NR). The absence of any negative-going features in our spectra militates against the presence of N-bound thiocyanate on our electrodes. (On silver the converse is true: for S-bound thiocyanate, the phase of X(')NR (-7r/3 for Ag compared to +7r/2 for Au) leads to predominantly destructive interference between the two susceptibilities. An N-bound species would therefore give rise to a peak.) As with silver, differential capacitance measurements12 imply near-monolayer coverage of thiocyanate on gold throughout the double layer region. The absence of any resonant S F signal for E C - 0 . 5 V is consistent with SCN- ions adsorbed flat on the surface in this potential region, as discussedabove. Both SERS8.14 and surface-enhanced ATR9 detect strong C-N stretches in the potential range -0.5 to -1.0 V, where no sum-frequency signal was observed. The surfaces used for these SER and ATR experiments are extremely rough on a nanometer scale. Suetaka9 has assigned the peaks at large negative potentials to adsorbates at specific sites where the transition probabilities are strongly enhanced by electromagnetic or charge-transfer mechanisms, which would explain their absence in the S F spectra. The SERS selection rule is less strict than the IR selection rule, so SERS may also detect SCN- in a flat orientation. The proposal by Korshin et a1.12 that SCN- adopts a perpendicular geometry at negative potentials and a parallel geometry at positive potentials is not consistent with the S F spectra. A characteristic feature of adsorbed thiocyanate is the sensitivity of the frequency of the C-N stretch to the electrode potential. InSFspectraof SCN-ongold, the potentialdependence of the resonant frequency, dv/dE = 40 cm-I V-I, Much smaller values of dv/dE were found in SERS (12 cm-1 V-'),I4 ATR (14 cm-l V-1),7and PDIR (10-16 cm-l V-1).8 In ATR with surface plasmon excitation? dv/dE is closer to the value we measured (20-40 cm-1 V-I), but the frequencies and intensities of the peaks are in poor agreement with the S F data. For thiocyanate on silver, both PM-RAIRS and SFS yielded potential tuning rates of approximately 50 cm-l V-1. A strong dependence of the resonant frequency on potential has also been observed19by SFS for thiocyanate on platinum: dv/dE = 60 cm-1 V-I. Thus, the adsorbed thiocyanate ions detected by SFS seem to be more sensitive to electrode potential than those sensed by SERS and ATR. Conclusion
Sum-frequencyspectroscopypresents a relatively simplepicture of thiocyanate adsorption on silver and gold electrodes. We have detected thiocyanate on silver electrodes over a narrow potential range (0.0 to -0.3 V vs SCE) and on gold over a wider potential range (+0.4 to -0.4 V vs SCE). For each electrode, a peak due to the C-N stretching mode of S-bound thiocyanate was observed.
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12050 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993
This peak moved to lower frequencies as the potential was made more negative. Differential capacitancemeasurementslJf'6 imply that extensive adsorption occurs on both metals throughout the polarizable potential range. The absence of resonant features in SFS at potentials more negative than -0.4 V suggests that the thiocyanate ions are lying flat on the electrode surface at these potentials. There was no firm evidenceinour spectra for N-bound thiocyanate on either electrode. We note, however, that an N-bound ion oriented parallel to the surface would not be observed by SFS. There are considerabledifferences among the spectra obtained by different techniques,particularly on gold electrodes. For SCNon silver, the sum-frequency spectra are in close agreement with PM-RAIRS spectra obtained from electrodes that had been prepared in a similar manner but differ substantially from SERS and ATR spectra obtained on roughened surfaces. Unfortunately, a detailed study by PM-RAIRS of thiocyanateon gold electrodes has not yet been carried out. Quantitative agreement between sum-frequency spectra and SER and ATR spectra was poor. Some of these differences undoubtedly arise from the variable concentrationsof thiocyanateused and from thedifferent electrode preparations. However, SERS and ATR spectroscopy do appear to be sampling a different set of adsorbed thiocyanate species from SFS.
Acknowledgment. We thankunilever Research (Port Sunlight Laboratory), the SERC, and the Royal Society for their generous support, and the Singapore Public Service Commission for a Singapore Overseas Postgraduate Scholarship to T.H.O.We are very grateful to Dr. Trevor Rayment for the load of the electrochemical equipment and to Mr. Nick Levinos for advice on the construction of the Raman Cell. References and Notes (1) Weaver, M. J.; Barz, F.; Gordon, J. G. 11; Philpott, M. R. Surf. Sci. 1983,125,409. (2) Foley, J. K.; Pons, S.;Smith, J. J. Langmuir 1985, 1, 697.
(3) Hatta, A.; Sasaki, Y.; Su€taka, W. J. Electroanal. Chem. 1986,215, 93.
(4) Corrigan, D. S.;Weaver, M. J. J. Phys. Chem. 1986, 90, 5300. (5) Samant, G.; Kunimatsu, M. K.; Viswanathan, R.; Seki, H.; Pacchioni, G.; Bagus, P. S.;Philpott, M. R. Longmuir 1991, 7, 1261. (6) Corrigan, D. S.;Gao, P.; Leung, L.-W. H.; Weaver, M. J. Longmuir 1986, 2, 744. (7) Parry, D. B.; Harris, J. M.; Ashley, K. Longmuir 1990, 6, 209. (8) Comgan,D.S.;Foley,J.K.;Gao,P.;Pons,S.;Weaver,M.J.Longmuir 1985, I, 616. (9) Wadayama, T.; Sakurai, T.; Ichikawa, S.;Sultaka, W. Sur$ Sci. Lett. 1988, 198, L359. (10) Hipps, K. W.; Mazur, U. J. Phys. Chem. 1992, 96, 1160. (1 1) Pacchioni, G.; Illas, F.; Philpott, M. R.; Bagus, P. S.J. Chem. Phys. 1991, 95, 4678. (12) Korshin,G. V.;Nazmutdinov, R.R.;Saifullin,A. R.Sw. Electrochem. 1991, 27, 1299. (13) Cao, E. Y.; Gao, P.; Gui, J. Y.; Lu, F.; Stern, D. A.; Hubbard, A. T. J. Electroanal. Chem. 1992, 339, 31 1 . (14) Gao, P.; Weaver, M. J. J. Phys. Chem. 1986, 90,4057. (15) Huang, Y.; Wu, G. Spectrochim. Acta 1989, 45A, 123. (16) Larkin, D.; Guyer, K. L.;Hupp, J. T.; Weaver, M. J. J. Electroanal. Chem. 1982, 138, 401. (17) Bailey, R. A.; Kozak, S.L.;Michelson, T. W.; Mills, W. N. Coord. Chem. Rev.1971,6, 407. (18) Guyot-Sionnest, P.; Tadjeddine, A. Chem. Phys. Lett. 1990, 172, 341. (19) Tadjeddine, A.; Guyot-Sionneat,P. Electrochim. Acta 1991,36,1849. (20) Eisenthal, K. B. Annu. Rev.Phys. Chem. 1992,43, 627. (21) Richmond, G. L. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1991; Vol. 17, p 87. (22) Shen, Y. R.; Principles of Non-linear Optics; Wiley: New York, 1984. (23) Superfine, R.; Huang, J. Y.;Shen,Y. R. Opr. Lett. 1990,15,1276. (24) Ward, R. N.; Davies, P. B.; Bain, C. D. J. Phys. Chem., 1993, 97, 7161. (25) Bain, C. D.; Davies, P. B.; Ong, T. H.; Ward, R. N.; Brown, M. A. Longmuir 1991, 7, 1563. (26) Sawyer, D. T.; Roberts, J. L. Experimental Electrochemistry for Chemists; Wiley: New York, 1974. Stolberg, L.; Richer, J.; Lipkowski, J. J. Electroanal. Chem. 1986, 207, 213. (27) Foley, J. K.; Korzeniewski, C.; Daschbach, J. L.; Pons, S. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1986; Vol. 14, p 309. (28) Ong,T. H. Ph.D. Thesis, Cambridge, 1993. (29) Ong, T. H.; Daviea, P. B.; Bain, C. D. Longmuir, 1993, 9, 1836. (30) Bailey, R. A.; Michelson, T. W.; Mills, W. N. J. Inorg. Nucl. Chem. 1971, 33, 3206.
