Nanoscale Island Formation during Oxidation of Carbon Monoxide

island formation, described in several earlier reports, is based on the observation of C-O stretching ... at least rough estimates of nanoscale island...
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Langmuir 1998, 14, 5603-5611

5603

Nanoscale Island Formation during Oxidation of Carbon Monoxide Adlayers at Ordered Electrochemical Interfaces: A Dipole-Coupling Analysis of Coverage-Dependent Infrared Spectra Mark W. Severson† and Michael J. Weaver*,‡ Department of Chemistry, Oakland University, Rochester, Michigan 48209, and Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received April 28, 1998. In Final Form: July 14, 1998 Model dipole-dipole coupling calculations of infrared spectra for various ordered CO packing arrangements on Pt(111) have been undertaken as a function of the spatial dimensions of isolated nanoscale adlayer domains. Comparisons of some of these predictions with coverage (θCO)-dependent infrared spectra obtained upon partial adlayer oxidation on low-index Pt-group surfaces in aqueous electrochemical environments enable approximate estimates of densely packed CO island sizes to be obtained. The evidence for CO island formation, described in several earlier reports, is based on the observation of C-O stretching bands, νCO, whose frequency, band shape, and relative intensity exhibit markedly smaller alterations upon progressive electrooxidative removal in comparison with corresponding νCO frequency-coverage data obtained for increasing θCO by means of dosing with dilute CO solutions. The dipole-coupling calculations indicate a significant sensitivity of the νCO frequency and related parameters to island size even for domains containing sizable numbers (100-200) of CO molecules, suggesting the value of the procedure for providing at least rough estimates of nanoscale island dimensions. While the domain size-dependent νCO spectra should be dependent inevitably upon the microscopic CO packing geometry, the (relative) changes in the νCO frequency for atop (i.e. terminal) adsorbate are predicted to be dependent on the island dimensions, yet relatively insensitive to the adlayer structure, suggesting the practical utility of this parameter for island size diagnosis. The resulting estimates of average island dimensions, along with their stability as gleaned from experimental time-dependent spectra, are seen to vary widely on different low-index Ptgroup electrodes. Interestingly, the metastable CO domains formed on Pt(100), estimated to contain 20-50 atop CO’s, have dimensions roughly compatible with the restructured substrate “mesas” seen to form by scanning tunneling microscopy.

Introduction The marked influences exerted by dynamic dipoledipole coupling upon the frequencies and relative intensities of adsorbate vibrational bands, especially for carbon monoxide on ordered metal surfaces, are now widely recognized and reasonably well understood in theoretical terms.1 While most attention in the literature has been devoted to surfaces in ultrahigh vacuum (UHV), a substantial body of information has emerged recently for dipole coupling within ordered electrochemical adlayers.2-7 Although the magnitude of dipole-coupling interactions * Corresponding author. Tel: (765) 494-5466. fax: (765) 4940239. email: [email protected]. † Oakland University. ‡ Purdue University. (1) For reviews, see (a) Hollins, P.; Pritchard, J. Prog. Surf. Sci. 1985, 19, 275. (b) Willis, R. F.; Lucas, A. A.; Mahan, G. D. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1983; Chapter 2. (c) Hoffmann, F. M. Surf. Sci, Rep. 1983, 3, 107. (d) Hollins, P. Surf. Sci. Rep. 1992, 16, 51. (2) For reviews, see (a) Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5391. (b) Korzeniewski, C.; Severson, M. W. Spectrochim. Acta 1995, 51A, 499. (c) Iwasita, T.; Nart, F. C. In Advances in Electrochemical Science and Engineering; Gerischer, H., Tobias, C. W., Eds., VCH Publishers: Weinheim, 1995; p 123. (d) Korzeniewski, C. Crit. Rev. Anal. Chem, in press. (3) (a) Chang, S.-C.; Weaver, M. J. J. Chem. Phys. 1990, 92, 4582. (b) Chang, S.-C.; Roth, J. D.; Weaver, M. J. Surf. Sci. 1991, 244, 113. (4) (a) Kim, C. S.; Tornquist, W. J.; Korzeniewski, C. J. Chem. Phys. 1994, 101, 9113. (b) Kim, C. S.; Korzeniewski, C. Anal. Chem. 1997, 69, 2349. (5) Villegas, I.; Weaver, M. J. J. Chem. Phys. 1994, 101, 1648. (6) Severson, M. W.; Stuhlmann, C.; Villegas, I.; Weaver, M. J. J. Chem. Phys. 1995, 103, 9832.

for the C-O stretching (νCO) vibrations within CO adlayers in electrochemical and UHV systems should often be similar, their effects in the former environment can be manifested in distinct ways. Thus electrochemical CO adlayers are often highly compressed and feature different binding-site arrangements than for the corresponding metal-UHV adlayers due to the combined effects of the lower surface potentials and higher CO effective pressures that typify the former systems.5-8 The kinetically controlled oxidation of such CO adlayers that can readily be undertaken electrochemically in aqueous media at ambient temperatures (but not in UHV or gas-phase systems) also yields interesting coverage-dependent dipole-coupling characteristics.3 Specifically, the strong dipole-dipole coupling that characterizes such densely packed adsorbates is commonly seen to be retained upon partial CO adlayer electrooxidation, as deduced most readily from the relatively invariant νCO frequencies even when the CO coverage, θCO, is diminished to low values.3 (An archetypical example of this phenomenon is the Pt(111)/ CO system, as seen, for example, in Figures 1 and 3 of ref 3a.) This survival of the θCO-induced νCO frequency blueshifts induced by dipole coupling during adsorbate removal contrasts the large (20-40 cm-1) νCO frequency increases which are commonly observed for electrochemical adlayers with progressively higher coverages formed by dosing with (7) Zou, S.; Villegas, I.; Stuhlmann, C.; Weaver, M. J. Electrochim. Acta 1998, 43, 2811. (8) Go´mez, R.; Feliu, J. M.; Aldaz, A.; Weaver, M. J. Surf. Sci. 1998, 410, 48.

S0743-7463(98)00495-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/25/1998

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Figure 1. Simulated νCO spectra for various near-circular CO domain sizes, given as arrays of unit cells, for the following four microscopic adlayer structures: A, (x3 × x3) R30°; B, c(4 × 2)-2CO; C, (x19 × x19) R23.4°-13CO; D, (2 × 2)-3CO. See text for details.

dilute CO solutions.2a,3 These differences signal clearly the occurrence of CO “island” (or “cluster domain”) formation during CO electrooxidation, where the local (microscopic) CO coverages within such patches, θCOloc, remain high, and hence retain largely dipole-coupling

effects characterizing the saturated-adlayer case even when the “macroscopic average” coverages, θCOav, are substantially diminished. This behavior indicates that CO electrooxidation proceeds via a “nucleation-growth” mechanism, where the oxidant (water, hydroxyl) reacts

Dipole-Coupling Analysis of Coverage-Dependent IR Spectra

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Figure 2. (A) The peak frequency of the atop νCO band for the four adlayer structures in Figure 1, as indicated, plotted against the total number of atop (and near-atop) molecules in each CO domain, Na. (B) As for A, but y-axis is now the “relative coupling shift” parameter, δ, as defined by eq 1.

Figure 3. Ratio of the simulated integrated intensities of the “atop” and “multifold” bands, Ia/Im, for the c(4 × 2), (x19 × x19) and (2 × 2) structures plotted against the number of atop (and near-atop) molecules in each CO domain, Na.

only upon coadsorption on empty sites within the CO adlayer.3 Although some CO cluster formation can also occur for progressively higher coverages obtained by gradually increasing θCOav by dosing CO from dilute solutions,3a the common observation of θCO-induced enhancements in dipole-dipole coupling, especially νCO frequency upshifts, indicates that θCOloc usually increases at least roughly in tandem with θCOav under these conditions. Such marked dissimilarities in the form of the θCOavdependent νCO spectra commonly observed under these

so-called “electroxidative stripping” and “direct dosing” conditions, where the adlayer is faradaically removed and nonreactively formed, respectively, suggest that insight into the underlying differences in the spatial adlayer configurations might be forthcoming by undertaking comparisons between the experimental data and the predictions of theoretical dipole-coupling models. Undertaking such calculations strictly requires knowledge of the microscopic adlayer structure as well as dipoleoscillator parameters. The potential usefulness of this approach has been bolstered, however, by the acquisition of detailed spatial adlayer structures for the Pt(111)/COaqueous electrochemical system extracted from in-situ scanning tunneling microscopy (STM) along with infrared reflection-absorption spectroscopy (IRAS).5 Furthermore, we have recently undertaken dipole-dipole calculations for the compressed CO structures occurring in this electrochemical system, which show that these effects largely account for the observed infrared spectral behavior, including that for mixed 13CO/12CO isotopic adlayers.6 While the dipole-dipole interactions responsible for the observed coupling are inherently short range in nature, their effects may be propagated over longer distances so that the form of the νCO spectra can depend significantly upon the adlayer domain size even for nanoscale dimensions. Several dipole-coupling calculations demonstrating qualitatively such spectral sensitivity to the CO island dimensions have been reported earlier.9-12 Interestingly, the adlayer νCO frequencies are predicted to be sensitive (9) Hollins, P. Surf. Sci. 1981, 107, 75. (10) (a) Olsen, C. W.; Masel, R. I. Surf. Sci. 1988, 201, 444. (b) Olsen, C. W.; Masel, R. I. J. Vac. Sci. Tech. 1988, A6, 792. (11) Ippolitova, S. F.; Kumpanenko, I. V.; Entelis, S. G. Surf. Sci. 1987, 188, 301. (12) (a) Leilsle, F. M.; Sorbello, R. S.; Greenler, R. G. Surf. Sci. 1987, 179, 101. (b) Brandt, R. K.; Sorbello, R. S.; Greenler, R. G. Surf. Sci. 1992, 271, 605. (c) Sorbello, R. Phys. Rev. B. 1985, 32, 6294. (d) Greenler, R. G.; Leilsle, F. M.; Sorbello, R. S. Phys. Rev. B. 1985, 32, 8431.

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to the island size even for close-packed domains containing 50 or more CO molecules.10 The demonstrated usefulness of dipole-coupling analyses, together with the availability of spatial structural as well as IRAS data for CO adlayers on several low-index Pt-group electrodes,8 has led us to undertake calculations of νCO spectra for specific close-packed structural arrangements of anticipated relevance to electrochemical systems. The predicted sensitivity of the spectra to the domain sizes (and shapes) has been scrutinized in order to assess the degree to which such nanoscale spatial information might be extracted by comparisons with the experimental θCOdependent νCO spectra. The salient results are outlined in the present communication. While there are inevitable uncertainties in the actual microscopic CO packing arrangements present within the islands formed at lower coverages, the analysis provides useful semiquantitative insight into the size and nature of nanoscale assemblies formed during adlayer electrooxidation. Dipole-Coupling Calculations We have previously undertaken detailed calculations6 of dipole-dipole coupling effects upon νCO spectra for extended terrace planes composed of two compressed CO adlayer arrangements that have been deduced by STM to be present on Pt(111) electrodes.5 These adlattice structures, (2 × 2)-3CO and (x19 × x19)R23.4°-13CO, both involve hexagonal CO packing, with fractional coverages, θCO, equal to 0.75 and 0.685 (13/19), respectively.5 The former, more densely packed, unit cell contains a single atop CO along with a pair of 3-fold hollow CO’s; the latter, more complex, arrangement features predominantly asymmetric, including “near-atop”, binding geometries. Significantly, the dipole coupling calculations can account at least semiquantitatively for the otherwise-puzzling predominant appearance of an “atop” νCO band at 20602075 cm-1 accompanied by 3-fold-hollow and “bridging” νCO features in the (2 × 2) and (x19 × x19) adlayers, respectively, appearing at ca. 1770 and 1850 cm-1.5,6 The 2-3-fold higher atop band intensities than expected from their relative site occupancy, along with the surprisingly simple form of the infrared spectra for the complex (x19 × x19) binding-site arrangement, can be explained on the basis of “intensity-stealing” associated with dielectric screening, where the infrared absorption is influenced to a disproportionately large extent by the higher-frequency oscillator.6 In addition to such band intensity-transfer effects, the most well-known manifestation of dipole coupling is the often-substantial (ca 10-30 cm-1) νCO frequency blueshifts (∆νCO) observed as the oscillator packing density increases. The magnitude of this effect can readily be examined by means of isotopic dilution experiments, whereby the oscillator coupling yielding the ∆νCO upshifts is progressively attenuated. Fitting the results of such experiments with isotopic mixtures on Pt(111) in the framework of dipole-coupling models provides a straightforward means of assessing the relevant dynamic oscillator parameters.6,13 This approach, employing 12CO/13CO mixtures, was applied in ref 6 to analyze coupling parameters associated with the (2 × 2) and (x19 × x19) electrode structures. Armed with such information for these saturated adlayers on extended Pt(111) terraces, we have calculated from the same dipole-coupling formalism the corresponding νCO spectra predicted instead for various finite-size (13) Schweizer, E.; Persson, B. N. J.; Tushaus, M.; Hoge, D.; Bradshaw, A. M. Surf. Sci. 1989, 213, 49.

Severson and Weaver

CO domains. The calculational procedure was identical to that described in ref 6, employing the (essentially equivalent) formalisms described by Persson and Ryberg14 and Leibsle, Sorbello, and Greenler,12 but altering the array size and removing the periodic boundary conditions. As in ref 6, the effect of stochastic (disorder) broadening of the multifold CO oscillators on the coupling in both the (2 × 2) and (x19 × x19) structures was considered, although the possible effects of “short-range vibrational” coupling within the latter structure was not included. [While incorporating this effect, anticipated in view of the very high adsorbate packing density, which yields a more consistent fit between theory and experiment for the (2 × 2) adlattice, additional adjustable parameters are required6 which provide no further insight into the issues at hand here.] In addition to the above calculations for compressed adlayers observed for the Pt(111)-aqueous electrochemical system, we have undertaken parallel computations for the lower-coverage, (x3 × x3)R30° and c(4 × 2), adlayers (for which θCO ) 0.33 and 0.5, respectively) which are observed by low-energy electron diffraction to form at the Pt(111)-UHV interface.13,15,16 These two adlayers consist of exclusively atop, and equal occupancies of atop and 2-fold-bridging CO, respectively.15,16 Together with the corresponding island size-dependent simulated spectra for the (2 × 2) and (x19 × x19) structures, the observed behavioral patterns for these distinctly different adlattices provide some insight into the manner and extent to which the observed effects of CO domain formation on θCOdependent νCO spectra depend on the binding-site preferences and spatial adsorbate distribution. While the specific calculations refer to structures observed for CO on Pt(111), the overall trends should also be relevant to CO adlayers formed on other Pt-group surfaces. Figure 1, parts A-D, display representative νCO spectra for the aforementioned (x3 × x3), c(4 × 2), (x19 × x19), and (2 × 2) CO adlayer structures, respectively, simulated for near-circular island arrays containing (n × n) unit cells, as indicated. The simulation parameters were chosen so to yield a reasonable fit to experimental data on Pt(111) as a function of the CO coverage and isotopic composition. These parameters are specifically the νCO singleton frequencies ωo (i.e. in the absence of dipole coupling), for CO in each binding site, the corresponding vibrational polarizability Rv, and the electronic polarizability Re, the last taken to be 2.5 Å3. Also as in ref 6, the atop (and near-atop), 2-fold bridging, and 3-fold hollow CO’s were presumed to lie 0.8, 0.4, and 0.2 Å, respectively, above the metal image plane. The former parameters selected for each structure are (A) (x3 × x3): ωo(atop) ) 2060 cm-1, Rv ) 0.3 Å3; (B) c(4 × 2): ωo(atop) ) 2088 cm-1, ωo(bridge) ) 1850 cm-1, Rv ) 0.3 Å3; (C) (x19 × x19): ωo(atop) ) 2040 cm-1, ωo(near-atop) ) 2025 cm-1, ωo(nearbridge) ) 1820 cm-1, Rv ) 0.32 Å3; (D) (2 × 2): ωo(atop) ) 2038 cm-1, ωo(hollow) ) 1748 cm-1, Rv ) 0.65 Å3. [Note that the parameters for the first two structures were selected so as to yield a reasonable fit to the experimental data in UHV,13,15 whereas those for the latter were obtained from fits to νCO spectra for the electrochemical systems.6 This procedure justifies the smaller ωo(atop) values chosen for the latter adlayers, which refer neces(14) Persson, B. N. J. Ryberg, R, Phys. Rev. B 1981, 24, 6954. (15) Biberian, J. P.; Van Hove, M. A. Surf. Sci. 1984, 138, 361. (16) Ogletree, D. F.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1986, 173, 351.

Dipole-Coupling Analysis of Coverage-Dependent IR Spectra

sarily to lower surface potentials,17 and also the (artificially high) Rv value for the (2 × 2) structure, due probably to the especially dense adsorbate packing (vide supra).6] While the choice of Rv and the differences in ωo between distinct binding sites for a given adlayer affects the extent of dipole coupling, and hence the predicted spectral sensitivity to the CO domain size, the absolute ωo values chosen exert little influence on this dependence. The form of the νCO spectra in Figure 1, parts A-D, each demonstrate the increasing effects of dipole-dipole coupling that occur toward larger domain sizes. A common behavioral feature is the progressive marked wavenumber upshifts in the higher-frequency (“atop”) νCO band seen as the domain size is enlarged. This effect, along with the predicted band shape changes, can be understood most simply for the (x3 × x3) structure in Figure 1A in view of the exclusive atop CO binding. The overlapping, lowerfrequency shoulder seen on the single νCO band with increasing island size (Figure 1A) arises physically from CO’s located toward the domain edge, which experience weaker dipole coupling and therefore exhibit red-shifted absorption frequencies compared with adsorbate situated within the island interior. However, the intensity-transfer effect that also arises from dipole coupling results in the latter molecules exerting a disproportionately large influence upon the infrared absorption, so that these higherfrequency oscillators tend to dominate the overall spectrum. Nevertheless, dipole-dipole coupling is a sufficiently long-range phenomenon so that even the intermediatesize nanoscale CO clusters, for example the (5 × 5) domain for the (x3 × x3) adlayer shown in Figure 1A (corresponding to ca. 75 CO molecules), exhibit a peak νCO frequency that is significantly (about 5 cm-1) lower than for the “infinite array” case. Comparable variations in the nature and extent of dipole coupling with the island size for such purely atop CO arrays have been discussed previously.9 Similar comments apply to the island sizedependent atop νCO behavior for the other three adlayer structures, even though the spectral form is also influenced by the presence of multifold CO (Figures 1, parts B-D). Interestingly, the functional form of plots of the atop νCO band frequency versus the number of atop CO’s in each domain, Na, is surprisingly similar for all four adlayer arrangements (Figure 2A), despite their markedly dissimilar packing arrangements and binding-site preferences. This similarity is perhaps more clearly evident when plotted in a normalized fashion as in Figure 2B, where the y-axis is now the “relative coupling shift”, δ, given by

δ ) (ωd - ωo)/(ω∞ - ωo)

(1)

where ωd is the atop band frequency for a given domain size, ω∞ is the corresponding frequency for the extendedlattice adlayer, and ωo is the singleton frequency. (Although partly resolved multiple νCO peaks are seen in Figures 1, parts C and D, single predicted “peak frequencies” were obtained from an intensity-weighted average of the individual peaks.) The only clear departure from a common functionality in Figure 2B is seen for the (x19 × x19) adlayer at small island sizes. However, this deviation is due partly to the large unit-cell size (13 CO’s) engendering “edge” effects even for moderate (∼25) (17) The dependence of the vibrational frequencies on the surface potential, φ, the so-called “Stark effect”, is well documented for electrochemical systems; the characteristic differences in φ between these and conventional UHV systems largely account for the observed νCO frequency differences.18.

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numbers of atop CO’s. [Note that the (x19 × x19) unit cell contains six “near-atop” and six “near-bridging” CO’s as well as one in a symmetric atop site;5 the near-atop as well as atop molecules are counted in assessing Na in Figure 2A.] Consequently, the experimental value of the atop νCO frequency for a given intermediate coverage adlayer in relation to those for the “infinitely dilute” and saturated values as prescribed by eq 1 should provide at least a semiquantitative estimate of CO island size. Indeed, we utilize below this functional relationship in order to analyze the experimental θCO-dependent νCO data. Another parameter that might be expected to be diagnostic in this regard is the relative intensity of the atop to multifold bands. Most high-θCO packing arrangements feature CO’s in both these site geometries. As already mentioned, the degree of intensity transfer from the latter to the former oscillator should attenuate markedly as the island size, and hence the extent of dipole coupling, decreases. Close inspection of the spectra in Figures 1B, and especially 1D, indeed shows that the relative integrated intensities of the “atop” to “multifold” νCO bands, Ia/Im, decrease noticeably as the CO island size, and hence Na, diminishes. These dependences are shown in the form of (Ia/Im) Na plots in Figure 3. The predicted effect is substantial for the c(4 × 2) structure only when Na < 20, whereas noticeable Na-dependent changes in Ia/Im are also evident for the (2 × 2) case, for larger CO clusters, even when Na ∼ 100. However, the quantitative reliability of the Nadependent predictions is perhaps questionable for the (2 × 2) adlayer, given the artificially large vibrational polarizabilities required to fit the mixed isotopic data for this densely packed adlayer (vide supra).6 Moreover, the (x19 × x19) adlayer exhibits a qualitatively different, albeit mild, predicted dependence of Ia/Im upon Na (Figure 3). The latter behavior may be attributed to more complex dipole-coupling effects involving asymmetric “near-atop” as well as atop binding.6 Regardless of such details, however, the sensitivity of the (Ia/Im) - Na dependence to the adlayer structure renders this criterion largely unsuitable as a means of estimating CO domain size. Given the relatively uniform functional appearance of the atop νCO frequency - Na relationships already presented (Figure 2, parts A,B), it is also worth considering the corresponding νCO - Na (or related domain-size behavior) of the multifold νCO bands. However, in contrast to the atop case it turns out that the functional relationships for multifold CO are both complex and adlayer structure-sensitive and therefore not useful for islandsize estimation. This can be gleaned from the multifold νCO frequency - Na plots shown for the c(4 × 2), (2 × 2) and (x19 × x19) adlayers in Figure 4, where the band frequency for the last structure exhibits a markedly different, indeed nonmonotonic, dependence on the CO domain size. These more complex trends are due to the varying influence of two competing effects. Although increased coupling with other multifold CO’s in larger clusters tend to blueshift the observed νCO band (as for atop νCO), the enhanced coupling with atop CO tends to decrease the multifold band frequency, yielding smaller (and less diagnostic) νCO frequency-domain size dependencies. So far, we have considered CO islands having roughly circular shapes. Since the degree of dipole-dipole coupling can be anticipated to be different for other island shapes that are likely to be formed during island electrooxidation, we have also examined the size-dependent behavior of

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Severson and Weaver Table 1. Coverage-Dependent Infrared Frequencies for Atop C-O Stretch in Electrochemical CO Adlayers on Ordered Transition Metals surfacea

E vs SCE

θCOsat b

Pt(111)g

-0.25 0.1 -0.25 0.1 -0.25 0.05 -0.25 0.1 0.15 -0.26

0.65

Pt(100)h Pt(110)j Rh(110)k Ir(111)l Ir(100)m

Figure 4. The peak frequency of the multifold νCO band in the c(4 × 2), (x19 × x19) and (2 × 2) structures in Figure 1, parts B-D, as indicated, plotted against the number of atop (and near-atop) molecules in each domain.

0.85 1.0 1.0 0.6 0.62

δνCOs,c cm-1 7 5 16i 10i 3 3 3 2 2 10

δνCOd,d cm-1

∆νCOd,e cm-1

27 11 30 20 50 n ∼60 ∼60 40 40

∼40 ∼40 ∼60 n ∼70 ∼70 ∼60 ∼50

Navf ∼50 ∼80 ∼20 ∼40 >150 >150 >150 >150 >150 ∼30

a Ordered surface in aqueous 0.1 M HClO at electrode potential 4 indicated in adjacent column. Infrared spectral data extracted from references indicated. b Saturated CO coverage, obtained from cited reference for each surface and/or from ref 8. c Shift in atop band frequency (taken with positive sign) between saturated coverage, θCOsat, and (θCOsat/4), following rapid electrooxidative removal (see text for details). d Shift in atop band frequency (taken with positive sign) between saturated coverage, θCOsat, and (θCOsat/4), for adlayers formed by dosing with dilute CO solutions (see text). e Approximate shift in atop band frequency from low θCO (∼0.05) to θCOsat under “adlayer dosing” conditions (see text). f Average number of atop CO molecules in adlayer islands, extracted from listed δνCOs and ∆νCOd values by means of eq 2 and Figure 2B (see text). g Reference 3a. h Reference 19 (data from Figure 2). i Metastable value. See text. j Reference 20. k Reference 21. l Reference 22b,c (also see text). m Reference 23. n Evaluation precluded by apparent substrate phase transition.20

the extent of dipole coupling, and hence the magnitude of the atop νCO blueshifts, is attenuated for a given array size as the shape is altered from a less circular to a more oblong form. This reflects the greater fraction of CO molecules situated in “near-surface” to “interior” regions as the island geometry becomes more stringlike. Comparisons with Experimental Data

Figure 5. The peak frequency of the atop νCO band in the (2 × 2) structure as a function of the number of atop molecules in each domain, Na, for near-circular (n × n), and oblong island array shapes, as indicated (see text).

long oblong (2 × 2) and (x19 × x19) adlayer arrays, limiting the width to only a few unit cells. Such calculations should also be relevant to diverse island shapes, including meandering (“snakelike”) domains. Figure 5 shows a plot of the atop νCO frequency versus Na for (n × n) arrays of the (2 × 2) structure, as before, compared with those for oblong-shaped domains formed by limiting the width to 3, 2, or 1 unit cells, as indicated. The effect of restricting the island width in this fashion is clearly to limit the extent of dipole coupling, and hence the magnitude of the atop νCO blueshift, to progressively smaller asymptotic values. This effect can be understood simply from the inevitably smaller extent of dipole-dipole coupling that will be propagated across the narrower oblong widths, regardless of the overall length of the array. Roughly similar effects upon the atop νCO - Na behavior are seen for the (x19 × x19) structure. More generally,

Based on the foregoing model calculations, the most useful parameter for island-size diagnosis is the coveragedependent νCO frequency for atop CO. Collected in Table 1 are relevant quantities for electrochemical CO adlayers on low-index Pt, Rh, and Ir surfaces, culled from the literature sources indicated in the far left-hand column. While the CO structural arrangements differ significantly on these surfaces, the atop binding geometry is an important contributor to the hexagonal packing in each case.8,21 The parameters δνCOs and δνCOd both refer to the decreases in the atop νCO frequency observed between that for the saturated adlayer (having coverages, θCOsat, listed alongside8) and for an “average” coverage, θCOav, that is 4-fold smaller, at the indicated electrode potentials. (Note that all the electrode potentials quoted herein are versus the saturated calomel electrode, SCE.) The “adlayer stripping” parameter δνCOs refers to the frequency decreases measured for irreversibly adsorbed layers formed upon partial electrooxidation, engendered by short (1-2 s) potential-step pulses to ca. 0.3-0.4 V before returning to the potential indicated. (For example, see refs 3a and 3b). The “adlayer dosing” parameter δνCOd is the corresponding frequency change observed between the same two θCOav values, but now for adlayers (having progressively higher coverages) formed at the indicated potential (18) (a) Weaver, M. J. Appl. Surf. Sci. 1993, 67, 1476. (b) Villegas, I.; Weaver, M. J. J. Phys. Chem. B. 1997, 101, 5842. (19) Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1990, 94, 5095. (20) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 230, 222. (21) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 238, 142.

Dipole-Coupling Analysis of Coverage-Dependent IR Spectra

by dosing with dilute (ca 10- 5M) CO-containing electrolyte.3 The parameter listed in the far right-hand column, ∆νCOd, also refers to such dosing conditions, but now is the “total” νCO frequency change between very low θCO values (typically ca. 0.05) and the saturated adlayer, θCOsat. The last quantity therefore provides an approximate measure of the “overall” νCO frequency upshift due to adsorbateadsorbate interactions within the saturated CO adlayer, arising primarily (if not exclusively) from dipole-dipole coupling. The corresponding δνCOd values tend to be ca. 20% smaller than ∆νCOd, reflecting the lower-limit coverage, θCOsat/4, chosen for the former. This selection of the lower-limit coverage for δνCOs and δνCOd, although semiarbitrary, was made since θCOsat/4 is small enough to engender the formation of near-isolated adlayer islands, yet large enough to facilitate reliable νCO spectral measurements. In any case, since the νCO - θCO plots are very roughly linear for both adlayer “stripping” and “dosing” conditions, the corresponding δνCOs and δνCOd values provide at least a crude measure of their relative slopes. Comparison between the corresponding δνCOs and δνCOd values listed in Table 1 reveals the former to be significantly, and in some cases substantially (2-20-fold), smaller than the latter quantities. At least on a qualitative level, these differences provide a clear indicator that densely packed CO islands are formed upon partial electrooxidative removal of the adlayers. Comparison with the above dipole-coupling calculations, moreover, can yield semiquantitative estimates of the CO island dimensions. For simplicity, we will assume initially that the νCO decreases observed during electrooxidative adlayer stripping are due entirely to dipole coupling-induced diminutions in the average island size. (We will consider further and also relax this restriction below.) Referring back to Figure 2B, a rough estimate of the average island size, expressed in terms of the average number of atop CO’s contained in each domain, Nav, for θCOav ) θCOsat/4, can be extracted by comparing the experimental data with these plots. In terms of the above experimental parameters, we can express the dimensionless parameter δ (cf. eq 1) as

δ ) 1 - (δνCOs/∆νCOd )

(2)

Estimates of Nav (at θCOsat/4) can then be extracted from the observed (δνCOs/∆νCOd ) ratios by extracting δ values means of eq 2 and comparing them with the δ - Na plots in Figure 2B. An obvious limitation of this procedure is that the precise form of the δ - Na plots is somewhat dependent on the microscopic adlayer structure. In the case of Pt(111), considered specifically above, a likely structure present during adlayer electrooxidation is the (x19 × x19) arrangement, along with a lower-coverage (x7 × x7) form (θCO ) 0.57), which is also observed by STM.5 [These structures appear to be stable for irreversibly adsorbed CO, as encountered during adlayer electrooxidation. The reproducible occurrence of the densely packed (2 × 2) layer requires both low potentials and the presence of solution CO.5] While we have not undertaken dipole-coupling calculations for the (x7 × x7) structure in view of fluxionality in the precise binding-site distribution,5 the δ - Na behavior should be comparable to that for the (x19 × x19) adlayer (Figure 2B). Undertaking this simple analysis, based on eq 2 and Figure 2B, for the Pt(111)/CO system at -0.25 and 0.1 V (Table 1) leads to the deduction that the average CO island present upon adlayer electrooxidation down to coverages

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around by θCOsat/4 has a moderate size, containing roughly 50-80 atop (and near-atop) CO’s. Given that the (x19 × x19) unit cell contains 7(atop + near-atop) CO’s, this analysis suggests the formation of average island diameters, di, around say 12-16 Å by (θCOsat/4). Supporting evidence that the νCO frequency downshifts arise chiefly from diminished dipole coupling, presumed in this analysis, is seen from the attenuation in the δνCOs values observed upon isotopic dilution with 13CO (see Figures 6 and 7 in ref 3a). Pursuing a similar analysis for the Pt(100)/CO system (Table 1) leads to the deduction that smaller adlayer islands (Na ∼ 20-40, corresponding to di ∼ 8-10 Å) are formed by this point, whereas large domains (Na > 150) are evidently formed on both Pt(110) and Rh(110) under these conditions. Admittedly, the adlayer structures for Pt(100) and (110) are not known with certainty, unlike Pt(111). However, atop binding appears to dominate under electrooxidative removal conditions, as gleaned from the infrared spectra,19,20 so that the appropriate δ - Na dependencies should approximate those calculated for the (x3 × x3) and c(4 × 2) adlayers shown in Figure 2B. Measurements on Rh(111) are complicated by difficulties in oxidizing CO adlayers,21,24 and CO electrooxidation on Ir(110) exhibits complex behavior indicative of θCOinduced changes in microscopic adlayer structure.25 These systems are therefore omitted from Table 1. The corresponding data for the other two systems included in Table 1, Ir(111) and (100), are of interest here since both systems indicate the dominant presence of atop CO during both dosing and electrooxidative-stripping conditions, as deduced from the appearance of only a single νCO band at ca. 1960-2060 cm-1.22,23 In both cases, large (g50 cm-1) νCO frequency blueshifts are observed with increasing dosed CO coverages, indicative of strong dipole coupling. An earlier study of CO oxidation on Ir(111)22a yielded substantial (∼45 cm-1) δνCOs values for CO dosed in the double-layer region, indicative of adlayer dissipation into small islands (Nav ∼ 5). However, a more recent examination in our laboratory using an improved crystal flameannealing procedure [and hence probably forming large (111) terraces] produced markedly different findings.22b,c At lower coverages (150) CO domains evidently formed during CO electrooxidation on Rh(110). [Note that the saturated CO adlayer on Rh(110) has been deduced by STM to form an ordered (4 × 3)-12CO structure, again featuring CO bound in atop, near-atop, and near-bridging sites.28] It remains to account for the remarkable stability of some densely packed local domains, such as those on Pt(111),3a Pt(110),20 and Rh(110),21 even on longer time scales (at least 30 min) following partial CO adlayer oxidation. Such clusters are expected to be thermodynamically stable with respect to the alternative random distribution of chemisorbate monomers across the substrate terraces if the anticipated repulsive interactions between CO and the water and/or hydrogen coadsorbate present at the electrochemical interfaces outweigh the combined interactions within the CO and water/H domains. (Note that the predominant coadsorbates present at the two electrode potentials selected for Table 1, -0.25 and 0.05-0.1 V vs SCE, are atomic hydrogen and water, respectively.) Evidence for the presence of net repulsive CO-water interactions on Pt(111) has been obtained from the observed thermal destabilization of adsorbed water by coadsorbed CO in UHV.29 Indeed, clustering of adsorbed CO even upon CO dosing on Pt(111) electrodes at potentials (g0 V) where coadsorbed water rather than

(26) Chang, S.-C. Unpublished results. (27) (a) Villegas, I.; Weaver, M. J. J. Electroanal Chem. 1994, 373, 245; (b) Zou, S. Unpublished results.

(28) Gao, X.; Chang, S.-C.; Jiang, X.; Hamelin, A.; Weaver, M. J. J. Vac. Sci. Technol. A. 1992, 10, 2972. (29) Wagner, F. T.; Moylan, T. E.; Schmieg, S. J. Surf. Sci. 1988, 195, 403.

Dipole-Coupling Analysis of Coverage-Dependent IR Spectra

hydrogen predominates has been deduced from the curvilinear νCO - θCOav behavior observed under these conditions.3a Even in the absence of such thermodynamic factors, the kinetic stability of CO islands during adlayer electrooxidation is likely to be driven by the very nature of the nucleation-growth mechanism, featuring reaction propagation only at the edges of the CO (and adsorbed water) domains.30 Nevertheless, we have already mentioned examples of adlayers where the CO domains, although seen to form spectroscopically, dissipate almost entirely within minutes. The Pt(100) system is of particular significance given the unique availability of nanostructural STM data. However, roughly similar CO island dissipation kinetics are evident from time-dependent infrared spectra on Pd(110) in acidic aqueous media, and have been ascribed to θCO-dependent substrate reconstruction/deconstruction.31 Examples where CO island dissipation apparently proceeds on shorter time scales are also available, including the Ir(110)/CO-aqueous interface.25 This system may well also be influenced by substrate reconstruction,25 although the direct structural evidence is so far lacking. However, the Ir(110)/CO system, along with the Pd(111)/CO-aqueous and Pd(100)/CO-aqueous interfaces recently examined by electrochemical IRAS at Purdue,32 constitute examples where little hysteresis between coverage-dependent νCO spectra obtained upon CO solution dosing and electrooxidative stripping is typically observed. For such systems, apparently, any CO islands formed during electrooxidative removal are largely dissipated (30) Love, B.; Lipkowski, J. ACS Symp. Ser. 1988, 378, 484. (31) Zou, S.; Gomez, R.; Weaver, M. J. Surf. Sci. 1998, 399, 270. (32) Zou, S.; Gomez, R.; Weaver, M. J. Manuscript in preparation.

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within the time scales (efew seconds) over which individual infrared spectra can readily be acquired. At this juncture, then, the factors determining the formation, stability, and size of such CO islands at ordered Pt-group electrochemical interfaces are inadequately understood. It would clearly be desirable to acquire more complete substrate structural information as provided by STM, particularly under in-situ reactive conditions. It would also be of particular interest to examine time- and coverage-dependent kinetic data for CO electrooxidation in terms of nucleation-growth models,30,33 with the objective of deducing spatial domain information for comparison with related CO island size parameters extracted from infrared dipole-coupling analyses. Such parallel kineticspectral analyses may well contribute importantly to the attainment of a firm understanding of the microscopic nature of this archetypical adlayer reaction. Especially when applied with a knowledge of the CO microscopic adlayer structure from STM/IRAS data,5 such dipolecoupling analyses should also yield valuable insight into nanoscale structure of stable as well as reactive coadsorbate systems on a broader front. Acknowledgment. This work was supported by a grant from the Analytical and Surface Chemistry Division of the National Science Foundation (to M.J.W.). LA980495U (33) (a) Korzeniewski, C.; Huang, J. Presented at the American Chemical Society National Meeting, Dallas, TX, April 2, 1998, Abstract No. 387. (b) Petukhov, A. V.; Akemann, W.; Friedrich, K. A.; Stimming, U. Surf. Sci. 1998, 402-404, 182.