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J . Phys. Chem. 1986, 90, 5300-5306
Coverage-Dependent Orientation of Adsorbates As Probed by Potential-Difference Infrared Spectroscopy: Azide, Cyanate, and Thiocyanate at Silver Electrodes Dennis S. Corrigan and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayctte, Indiana 47907 (Received: March 13, 1986)
Potential-difference infrared spectra obtained with an FTIR spectrometer are reported for azide, cyanate, and thiocyanate anions adsorbed at the polycrystalline silver-aqueous interface. For the first two adsorbates, conditions could be chosen so that the coverage could be varied from very low values at the negative reference potential to those approaching a monolayer at the more positive sample potentials. Bipolar infrared band shapes within the mid-IR (N-N, C-N stretch) region were generally obtained with the negativegoing (Le., decreased transmittance) feature, associated with the formation of the interfacial species, being accompanied by a positive-going band at a frequency equal to that for the solution-phasevibration. The latter is identified with a corresponding loss in the quantity of solution species within the spectral thin-layer cavity resulting from specific adsorption. Although this latter band constitutes a bulk-phase interference, the integrated intensity, IsOl, nevertheless provides a measure of the potential-dependent surface concentration. Comparisons between the integrated intensity of the negative-going “surface” band, I,,,, with Isolyield direct information on the changes in the infrared absorptivity of a given internal vibrational mode induced by specific adsorption. For the asymmetric N-N-N stretching mode of azide, I,,, 0.5. This is interpreted in terms of the surface dipole selection rule, the azide orientation changing from flat to “vertical” at higher coverages. In contrast, for the C-N stretching modes of cyanate and thiocyanate, I,,, = 1.2IsOl and I,,, O.lIsOI,respectively, throughout the accessible coverage ranges. These changes in infrared absorptivities together with the observed frequency shifts upon adsorption are consistent with ”end-on”surface attachment for cyanate and thiocyanate occurring exclusively via the nitrogen and sulfur atoms, respectively, on the basis of comparison with infrared intensity and frequency data for related bulk-phase metal complexes. These results illustrate the importance of both surface chemical bonding and adsorbate orientation effects upon surface infrared intensities.
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Introduction The current development of infrared reflection-absorption spectroscopies (IRRAS) as applied to the in-situ characterization of electrode-solution interfaces’ is starting to provide a wealth of molecular vibrational information to complement that being obtained with surface-enhanced Raman spectroscopy (SERS).* The former approaches have several disadvantages, including lower sensitivity, often-serious bulk spectral interferences, and more restricted vibrational frequency ranges than are readily amenable to SERS. On the other hand, they have the important advantage of not relying on a surface-enhancement mechanism and thereby are applicable to a greater variety of electrode surfaces. Of central interest is the manner i;l which the spectral intensities as well as frequencies for adsorbate vibrational modes are perturbed from those for the bulk-phase species, especially as a function of coverage. Such information should, when combined with surface selection rules, yield detailed insight into the modes of surface bonding, including adsorbate orientation. This issue can be addressed most straightforwardly for infrared spectroscopy since surface enhancement effects (and the attendant dependence of spectral intensity upon surface morphology) should be absent, and the surface vibrational selection rules appear to be more ~ l e a r - c u t .Nevertheless, ~~~~ most attention in infrared spectra of electrochemical adsorbates has been devoted so far to potential-dependent frequency measurements; little information on quantitative intensity-adsorbate coverage relations has become available. We have recently been employing potential-difference infrared spectroscopy as configured for an FTIR spectrometer (so-called “subtractively normalized interfacial Fourier transform infrared spectroscopy”, SNIFTIRS’) to examine a number of anionic and molecular adsorbates on silver and gold electrodes. One objective is to compare corresponding infrared and Raman spectra in order (1) For a recent review, see: Benvick, A.; Pons, S. Ado. Infrared Raman Spectrosc. 1985, 12, 1. (2) For recent reviews, see: (a) Chang, R. K.; Laube, B. L. CRC Crif.Rev. Solid State Mater. Sci. 1984, 12, 1. (b) Otto, A. Top. Appl. Phys. 1983, 54. (c) Moskovits, M. Reu. Mod. Phys. 1985, 59, 783. (3) (a) Greenler, R. G. J . Chem. Phys. 1966, 44, 310; 1969.50, 1963. (b) Pearce, H. A.; Sheppard, N. Surf. Sci. 1976, 59, 205. (c) Moskovits, M. J . Chem. Phys. 1982, 77, 4408. (d) Moskovits, M.; Suh, J. S . J . Phys. Chem. 1984, 88, 5 5 2 6 .
0022-3654/86/2090-5300$01.50/0
to ascertain if the vibrational properties of the species sensed by the latter differ from those of the preponderant adsorbate presumably detected with the former probe.4 Some of these systems, however, also lend themselves well to examinations of the coverage dependence of the infrared spectra. This is especially true of pseudohalide anionic adsorbates whose coverage can readily be varied from very low values to approaching monolayer levels by altering the electrode potential within the polarizable potential range.5 Such potential-induced alterations in the adsorption-desorption equilibria bring about the changes in the surface composition and hence spectral features that enable potential-difference infrared techniques to be employed. Systems of this type have been examined only cursorily so far, it being more usual to utilize the substantial potential-induced shifts in the surface infrared frequencies that occur for some adsorbates at constant coverage. (The most extensively studied example of the latter is CO adsorbed on platinum metals.’) We report here potential-difference infrared (PDIR) spectra6 for azide, cyanate, and thiocyanate anions adsorbed at the polycrystalline silver-aqueous interface. The objective is to compare the coverage-dependent surface infrared absorptivities for suitable internal adsorbate modes with the corresponding solution-phase molar absorptivities in order to explore the perturbations to the adsorbate spectral intensities brought about by surface binding. A virtue of silver electrodes is that quantitative surface concentration-potential data can readily be obtained for comparison with the infrared spectra by means of differential capacitance measurements in mixed electrolytes of constant ionic ~ t r e n g t h .We ~ describe here a simple approach by which the PDIR spectra may additionally be utilized to obtain quantitative coverage-potential ~~
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(4) (a) Corrigan, D. S.; Foley, J. K.; Gao, P.; Pons, S.; Weaver, M . J. Langmuir 1985, I , 616. (b) Corrigan, D. S.; Leung, L.-W.; Gao, P.; Weaver, M. J. Langmuir, in press. ( 5 ) (a) Hupp, J. T.; Larkin, D.; Weaver, M. J. Surf. Sci. 1983, Z25, 429. (b) Larkin, D.; Guyer, K. L.; Hupp, J. T.; Weaver, M. J. J . Elecrroanal. Chem. 1982, 138, 401. (6) The acronym ‘PDIR” is used here rather than “SNIFTIRS” to em-
phasize the fact that a complete normalization of the surface with respect to the solution-phase infrared signals does not occur for systems featuring potential-dependent adsorption-desorption equilibria, as are encountered here, since the extent of infrared absorption by solution as well as adsorbed species eithin the thin-layer cavity will depend upon the electrode potential.
0 1986 American Chemical Society
Coverage-Dependent Orientation of Adsorbates
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5301
information for systems featuring potential-dependent adsorption equilibria. Azide and cyanate adsorbed at silver are of particular interest in this regard since the differential capacitance data indicate that these anions are virtually desorbed at potentials approaching the negative limit,5 enabling difference spectra to be obtained from a negative reference potential corresponding to little or no adsorption.
Experimental Section Details of the surface infrared measurements are in large part similar to those given in ref 7. The infrared beam was reflected from a 9.5-mm-diameter silver electrode const5ained in a thin-layer cell geometry by pushing the surface up against a calcium fluoride optical window. The electrode was mounted on a horizontal glass plunger arrangement for this purpose. The light was incident on the window at about 65" to the normal. This condition was chosen in part to minimize the fraction of radiation that was reflected off the optical window. This fraction was calculated to be small, as confirmed experimentally from the residual light intensity after the electrode was pulled back from the optical window. The glass electrochemical cell also contained a platinum coil counter electrode and a Luggin capillary leading to a silver/si]ver chloride reference electrode containing KCl, although all potentials reported here are vs. the saturated calomel electrode (SCE). The cell was contained in an external sample compartment that was attached to the side of the optics bench. The infrared spectrometer used was a Bruker-IBM IR 98-4A Fourier transform instrument, with a globar light source and a liquid nitrogen cooled M C T narrow-band detector (Infrared Associates Model HCT-18A). The incident light was p-polarized by means of a wire-grid AgBr polarizer (Perkin-Elmer). The polarizer was placed in the sample compartment of the optics bench so to avoid exposure to light and was positioned such that the infrared beam was polarized prior to reflection into the electrochemical cell. Since a large number (500-1000) of interferometer scans were normally acquired in order to obtain a suitable signal-to-noise ratio, periodic alteration of the potential between the "reference" and "sample" values was undertaken to minimize the deleterious effects of any base line drift upon the resulting difference spectra. While such potential-modulation frequencies are much smaller for SNIFTIRS than those commonly employed (ca. 10 Hz) with potential-modulated infrared spectroscopy using a grating-based scanning instrument,' altering the potential at least every 30-60 interferometer scans (Le., every minute or less) was found to be required for satisfactory results. The synchronization of the potential alteration with the interferometer scans was accomplished by employing the arrangement described in ref 8. A device was constructed that utilizes the circuitry that is normally available in the FTIR spectrometer to drive the autosampler. After the collection and storage of a fixed number of scans (commonly 32) at one potential, a pulse is generated and used to trigger the potentiostat. A Hi-Tek Model DT2101 potentiostat was employed for this purpose. The scans are therefore collected sequentially at the two potentials until the desired number of co-added scans has been acquired. Triggering the potential steps in this fashion via the system software allows suitable time delays to be implemented immediately following each potential step in order to allow for the changes in adsorption equilibrium to be completed. Time delays of 2 s were usually used here (vide infra). Aqueous-phase integrated molar absorptivities for infrared modes of interest here were measured with a transmission IR cell having CaF2 windows and a Teflon spacer. The path length was about 0.018 mm and the solute concentration was usually comparable to that employed for the surface reflectance measurements. The integrated molar absorptivities were calculated with the Ramsay method.g The differential capacitancepotential (Cd-E)
u -520 mV
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2300 u(crn-9 1900 Figure 1. Potential-difference infrared (PDIR)spectra for adsorbed azide at the silver-aqueous interface in the asymmetric N-N-N stretch region. The reference (base) potential is -970 mV vs. SCE; sample potentials are as indicated (vs. SCE). The solution contained 0.01 M NaN3 0.49 M NaCIO.,. Spectra are an average of 1024 interferometer scans a t each potential.
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data were obtained with phase-sensitive detection essentially as described in ref 5. The C ,values were extracted from the in-phase and quadrature current components by using an on-line computer; an ac frequency of 100 Hz was employed with an 8-mV peakto-peak amplitude, the dc potential being swept at 5 mV The silver electrode was pretreated by mechanically polishing successively with 3-, 1-, and 0.3-pm alumina on a pad using water as a lubricant and then underwent electrochemical polishing in cyanide as described in ref 5a immediately prior to immersion in deaerated electrolyte in the electrochemical infrared cell. The NaN,, NaOCN, and NaC104 salts used here were reagent grade and recrystallized from water prior to use; NaOCN was purified as in ref 10. The water was purified by means of a Milli-Q system (Millipore, Inc.). All measurements were made at room temperature, 22 f 1 OC.
Results and Discussion Interpretation of Bipolar Band Spectra for Adsorption-Desorption: Azide. Figure 1 shows a representative series of PDIR spectra obtained for 0.01 M NaN, + 0.49 M NaC104 at silver in the frequency region ca. 2000-2100 cm-I, characteristic of the asymmetric N-N-N stretching vibration, ua(N3-). These spectra were obtained by altering the potential to a series of more positive values from a base value of -970 mV. (The use of more negative base potentials yielded less reproducible spectra, probably due to the slow accumulation of hydrogen at the electrode surface.) As is conventiona1,l the spectra are reported as relative changes in the reflected infrared intensities, ARIR. It is seen that a single positive peak centered a t 2048 cm-I is obtained for sample potentials of -100 mV or more negative, whereas at more positive
~
(7) (a) Pons, S. J . Electroanal. Chem. 1983, 150, 495. (b) Pons, S.; Davidson, T.; Bewick, A. J . Electroanal. Chem. 1984, 160, 63. (8) Corrigan, D. S.; Milner, D.; Weaver, M. J. Rev. Sci. Instrum. 1985, 56, 1965.
(9) Ramsay, D.A. J . Am. Chem. SOC.1952, 74, 12. (IO) Fernelius, W. C. Inorganic Synthesis; McGraw Hill, New York, 1946, Vol. 11, p 88.
5302 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
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l
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h
i
I
0
I
-400
-800
E,mV vs. SCE
Figure 2. Plots of azide surface concentration, r, against electrode potential E for the silver electrode in 0.49 M NaC104 + 0.01 M NaN,. The solid curve is as determined by differential capacitance-potential measurements and the dashed curve from integrated infrared intensities of positive-going bands in Figure 1 (see text for details).
values an additional negative-going band at about 2075-2085 cm-' is observed, its frequency increasing toward more positive potentials. Both the positive- and negative-going bands increase in intensity as the step potential is made more positive. (No significant Faradaic current is passed within the potential region over which the infrared data were obtained.) N o readily detectable PDIR bands were obtained in the frequency region, 1250-1 350 cm-I, corresponding to the symmetric N-N-N stretch. This behavior indicates that the species responsible for the positive-going vibrational band is depleted at more positive potentials, whereas another species with higher characteristic frequencies is formed that is only detectable at sample potentials positive of -100 mV. Such bipolar vibrational band shapes are commonly observed in PDIR spectra' and may originate from several phenomena. These include potential-induced changes in the adsorbate orientation as well as frequency shifts due to adsorbate-surface interactions. The interpretation of these results is aided considerably by considering plots of azide surface concentration, I? (mol cm-2), against electrode potential extracted independently from differential capacitance-potential (C,-E) data." Figure 2 shows such a plot (solid curve) corresponding to the conditions of Figure 1, 10 mM N a N 3 0.49 M NaCIO,. It is seen that the azide surface concentration is only small, ca. 7 X lo-'' mol cm-*, at -970 mV yet increases to values close to mol cm-2, at the most the anticipated monolayer limit, 1.6 X positive potentials. This very low r value at the reference potential for Figure 1, -970 mV, makes it implausible that the large positive-going band in the PDIR spectra is due directly to adsorbed azide. Several lines of evidence indicate that the band is due instead to the loss of bulk solution azide in the thin-layer cavity resulting from additional azide adsorption when the potential is altered from -970 mV to more positive values. Thus the band peak frequency, 2048 cm-I, is independent of potential and identical with v, measured for solution azide in the same electrolyte. This explanation infers that the solution in the thin-layer cavity is essentially "trapped"
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(1 1) The r-E plot for N3-in Figure 2 obtained from Cd-E measurements was obtained from data given in ref 5b using the "double integration" method (labeled "Method I" in ref 5b) and is a further refined version12 of the results summarized in Figure 10 of ref 5b. (12) Hupp. J . T.; Weaver, M. J., to be submitted.
Corrigan and Weaver on the time scale of the potential modulation and therefore unable to equilibrate with the surrounding electrolyte in the rest of the electrochemical cell. Consequently the total quantity of solution azide in the cavity will be altered by an amount equal to the quantity of azide adsorbed or desorbed by the potential alteration. The following experiment verified that significant equilibration of the solution in the thin-layer cavity and surrounding electrolyte does not occur in response to the potential modulation. l'he thin-layer cavity was formed in a solution containing only supporting electrolyte (0.1 M NaC10.J and sufficient azide was added to the cell to yield a 10 mM solution. The ensuing diffusion of N3- into the thin-layer cavity was monitored spectrophotometrically via the integrated absorbance of the solution azide band at 2048 cm-]. The potential of the silver electrode was held constant at -1000 mV to minimize anionic adsorption. As expected, the intensity-time (I-t) plot displayed first-order kinetics in that log I was linear in time, with a characteristic half-life of 55 min. Extrapolating such curves to a time scale (30-60 s) on which the potential modulation occurred showed that only an infinitesimal fraction (
