Langmuir 1996,11, 2777-2786
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Model Electrochemical Interfaces in Ultrahigh Vacuum: Solvent Effects upon Coverage-DependentInfrared Spectra of Carbon Monoxide on Platinum(111) Naushad Kizhakevariam, Ignacio Villegas, and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received January 17, 1995@ The influencesof the solvatingmedium on a model electrochemicalchemisorbatesystem-carbon monoxide on Pt(ll1)-have been explored by means of the infrared spectral responses in the C - 0 stretching (YCO) region to the dosage of various solvents onto CO adsorbed on Pt(ll1) in ultrahigh vacuum at 100 K. Measurements were made for a range of preadsorbed CO coverages (Bco) as a function of the solvent dosage. The molecules selected-methanol, acetonitrile, acetone, benzene, and ammonia-span a range of solvatingand dielectric properties. At low CO coverages (Bco 5 0.25),even submonolayersolvent dosages induced in most cases a near-complete shift in the CO binding geometry from atop to doubly bridging coordination, as seen from a displacement of the sharp YCO band at 2090-2095 cm-l by a markedly weaker feature at ca. 1790- 1820 cm-l. Additional (multilayer) solvent dosing yielded essentially no further spectral changes, other than small additional frequency downshifts. Submonolayer ammonia dosage triggered more substantial frequency decreases, yielding vco bands at ca. 1640 and 1530-1565 cm-l. At saturated (or near-saturated) CO coverages (Bco 0.65), solvent dosages yield only milder (550 cm-l) frequency downshifts in the atop and bridging YCO features that are present in the absence of solvent. The marked vco spectral changes occurring at low (and intermediate) BCO values are interpreted in terms of CO- solvent coadsorption,involving short-range dipolar interactions and “through-metal”charge polarization. The marked solvent-induced attenuation in the YCO band intensities seen at low Bco are discussed in terms of dielectric screening. The conventionaldipole-couplingtreatment is able to describe approximately the nature and solvent-dependent magnitude of the effect, including the severe intensity attenuation observed with the polarizable solvent benzene. The milder YCO frequency downshifts observed at higher solvent dosages, as well as for high CO coverages, are consistent with Stark-tuning effects exerted by overlayer (rather than coadsorbed) solvent. Comparisons are made with in-situ infrared spectra for corresponding electrochemical interfaces. Attention is called to the value of this infrared-based “UHV electrochemical modeling” approach for elucidating interfacial solvation effects.
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Of central interest in electrochemical surface science is elucidating the molecular roles of the interfacial solvent on chemisorbate binding and energetics. In recent years, the development of in-situ spectroscopic techniques, especially infrared reflection-absorption spectroscopy (IRAS),along with the evolution of methods for preparing ordered single-crystal metal electrodes have been yielding detailed information on the surface bonding for selected chemis0rbates.l One intriguing consequence of these insitu studies is the opportunity thus afforded for comparing the vibrational spectral properties of chemisorbates in analogous ultrahigh vacuum (UHV) environments. In particular, this situation provides a new impetus for pursuing so-called “electrochemical-modeling” studies, whereby various components of the electrode-solution interface can be examined, both separately and together, following gas-phase dosing onto a clean surface in UHv.2 We have recently been drawn to undertake such UHVbased measurements, focusing especially on IRAS studies of systems also amenable to in-situ electrochemical examination, especially with a view toward elucidating the various influences of the solvent and ionic components upon the interfacial s t r ~ c t u r e . ~ , ~
Although several stringent factors act to limit the range of model chemisorbates, carbon monoxide turns out to be uniquely suitable for this purpose. This arises partly from the detailed structural information available for carbon monoxide adlayers a t metal-UHV interfaces, along with the well-known sensitivity of the vibrational spectral properties of CO to the electrostatic and chemical environment. We have chosen for initial detailed study CO on Pt(lll1, following earlier extensive in-situ IRAS measurements for this system in both aqueous and various nonaqueous solvent environments.ls5 One facet of this work concerns understanding the observed marked differences in the CO coverage (&o)-dependent form of the infrared C-0 stretching (YCO)bands on a clean Pt(ll1) surface in UHV and in the electrochemical envir0nment.l Substantial insight into this issue can be obtained by means of UHV-based IRAS experiments involving controlled solvent postdosing onto Pt(lll)/CO adlayers, the corresponding changes in the surface potential being monitored by work-function measurements. Such a study undertaken recently for the Pt(111)/CO,D20 system showed that water coadsorption is largely responsible for the marked difference in the Bco-dependent binding site occupancies characterizing the aqueous electrochemical and anhydrous UHV systems.3a The additional presence of double-layer cations, so to mimic the complete elec-
Abstract published in Advance ACS Abstracts, June 1, 1995. (1)For a n overview, see: Chang, S.-C.;Weaver, M. J . J. Phys. Chem. 1991,95,5391. (2)For a erudite recent review, see: Wagner, F. T. In Structure of ElectrifiedZnterfaces; Lipkowski, J.,Ross, P. N., Eds.; VCH Publishers: New York, 1993;Chapter 9. (3) (a)Kizhakevariam, N.; Jiang, X.; Weaver, M. J. J . Chem. Phys. 1994,100, 6750. (b) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. Surf: Sci., in press.
(4)(a)Kizhakevariam, N.; Villegas, I.; Weaver, M. J . J. Phys. Chem. 1996,99,7677.(b)Villegas, I.; Kizhakevariam, N.; Weaver, M. J. Surf: Sci., in press. ( c ) Villegas, I.; Weaver, M. J . J . Chem. Phys., in press. (d) Villegas, I.; Weaver, M. J . Manuscript in preparation. ( 5 ) (a) Chang, S.-C.; Weaver, M. J . J. Chem. Phys. 1990,92,4582. (b) Chang, S.-C.; Weaver, M. J . Surf: Sci. 1990,238,142.( c ) Chang, S.-C.; Jiang, X.; Roth, J. D.; Weaver, M. J. J. Phys. Chem. 1991,95, (e) Villegas, 5378.(d) Jiang, X.;Weaver, M. J. Surf: Sci. 1992,275,237. I.;Weaver, M. J . J . Chem. Phys. 1994,101, 1648.
Introduction
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2778 Langmuir, Vol. 11, No. 7, 1995 trochemical system, has been modeled in subsequent experiments by potassium dosing.4a However, while the solvated cations exert major influences upon the surface potential, encouraging ongoing studies with codosing solvent and ions,4c UHV-based experiments involving dosing with various solvents alone should provide useful insight into the molecular factors that control solvent effects upon CO bonding. To this end, we have recently undertaken a detailed IW-work-function study entailing postdosing a number of solvents onto Pt(1 l l ) K O . The molecules, specifically benzene, acetone, acetonitrile, methanol, and ammonia, were chosen so a s to span a range of polarity and related solvating properties. The results for saturated CO adlayers are described in detail elsewhere;3bthese uncover marked Stark effects on the vco frequencies associated with large solvent-induced surface-potential changes on CO-covered as well as clean Pt(ll1). We provide herein a briefer report of the vco spectral responses incurred a t lower CO coverages under these conditions, with a view toward acquiring a n appreciation of the coadsorbed COsolvent interactions that are present a t analogous electrochemical interfaces. Besides uncovering a marked solvent-induced suppression of the YCO band intensities, attributable to electronic screening effects in broad agreement with earlier work,6 marked YCO frequency downshifts and coordination site switching are observed which imply the presence of short-range CO-solvent interactions.
Experimental Section The experiments were performed using a two-level stainless steel UHV chamber with facilities for IRAS, work-function measurements, temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). The base pressure, 2 x 10-lo Torr, was achieved by a combination of turbomolecular, ion, and titanium sublimation pumps. The infrared spectra were obtained with a Fourier transform infrared spectrometer (RS-1000, Mattson Instruments) equipped with a Globar light source and a MCT narrow-band detector. The infrared beam was incident on the surface at near-grazing incidence through KBr windows. As usual, a reference spectrum on the clean F’t(ll1) crystal was obtained prior t o dosing the surface as desired, thereby subtracting out the spectral features present in the optical path. The work-function changes, A@, induced by adsorbate dosing were measured with a Kelvin Probe (Delta Phi Electronic, Julich, Germany), monitored continuously with a y-t recorder. The infrared spectral fingerprint of a saturated CO adlayer at 100 K, in conjunction with LEED and AES of the bare surface, was used to monitor the cleanliness and order of the Pt(ll1) sample. Carbon contamination was removed by heating the Torr of oxygen for 30 min on a daily sample to 900 Kin 3 x basis. Periodically, sputtering of the surface was undertaken (vide infra); this consisted of heating the sample to 900 K while being bombarded by 10pA of 0.5 kVAr+ ions. Subsequently, the sample was annealed at 1200 K for 2-3 min. All gases and solvent vapors were dosed at 100 K. Line-ofsight dosing of the solvents was undertaken during FTIR experiments. This procedure enabled accurate stepwise dosing while collectinginfrared spectra for increasing solvent coverages. Experiments involving continuous monitoring of the work function during solvent dosing required raising the pressure in the chamber to 1x 10-8Torr. Thermal desorption spectroscopy (TPD) provided the required calibration link between line-ofsight and background dosing. Carbon monoxide (Airco)and ammonia (Matheson)high-purity gases were used as supplied. Acetonitrile (Mallinckrodt),dichloromethane, benzene, methanol (Fisher), and acetone (Aldrich) were of spectral grade and were thoroughly degased by repeated (6) (a) Ehlers, D. H.; Esser, A. P.; Spitzer, A.; Luth, H. Surf Sei. 1987,191,466.(b) Moritz, H.; Luth, H. Vacuum 1990,41, 63.
cycles of freezingand thawing. The solvent mass spectra provided an additional means of assaying their purity. The absolute CO coverages, &o, (referenced to the metal atomic density) quoted herein were calibrated with respect to the saturation value of ca. 0.65 obtained at 100 K, and also from the c(4 x 2) and ( 4 3 x d3)R30°LEED patterns observed for Bco values of 0.5 and 0.33, re~pectively.~
Results Similarly to our recent related s t ~ d i e sa, basic ~ ~ ~tactic employed here entails examining the infrared vco responses for chemisorbed CO to stepwise increasing solvent dosages a t a suitably low temperature (100 K) to avoid subsequent evaporation. Parts A-D of Figure 1 show such data acquired for different predosed CO coverages, 8c0, as indicated (in fractional monolayers, ML). The solvent dosage values, Os*, as indicated above each spectrum are given in “equivalent layers” (EL). The required calibration for each system was obtained from correspondingtemperature-programmed desorption (TPD) data at 4 K s-l by presuming that the maximum area under the higher-temperature partner of the usually observed pair of TPD peaks following dosage onto clean Pt(111)corresponds to Os*= 1. (Thisdefinition of a solvent “monolayer”therefore differs from the alternative assay, employed for chemisorbed CO, which gives the density of adsorbate molecules relative to top-layer Pt atoms. The former yields instead a n approximate estimate of the coverage relative to a supposedly close-packed solvent monolayer, For methanol, for example, this desorption of monolayer and subsequent multilayer adsorbate occurred a t 185 and 135 K, r e ~ p e c t i v e l y .(See ~ ~ ref 3b for further details.) The first spectral sequence, Figure lA, refers to a low CO coverage, 8co = 0.1 ML. The top spectrum, referring to the absence of methanol, shows a single vco band a t 2093 cm-’, characteristic of the presence of only atop CO (i.e. terminal CO, bound to only a single Pt atom). The addition of even small dosages of methanol, e,* 0.20.4, yield dramatic changes in the YCO spectra (Figure 1A). The atop vco band is first markedly attenuated and then is displaced entirely, a much weaker feature appearing a t about 1815 cm-l for 8,” 2 0.4. This feature is attributed to 2-fold bridging CO by comparison with YCO bands at similar frequencies known to arise from this binding g e o m e t ~ - y . ~The ~ , ~ methanol-induced *~ spectral changes 0.5, although the ca. 1815 are largely complete by 8,” cm-’ band appears to downshift in frequency and weaken further toward high solvent exposures. Figure 2B shows a corresponding methanol-induced YCO spectral sequence for a higher 8co value, 0.25. Again, a n essentially complete atop-to-bridging CO site transfer is achieved by Os* 0.5, although the YCO band is now clearly discernible (at 1805 cml) for multilayer methanol dosages. Comparable spectral responses were also observed for the related hydrogen-bound solvent, water.3a The last two methanol-induced sequences, Figure 1C,D, both refer to sequential solvent dosing onto saturated CO adlayers, 8co % 0.65. While the Pt(ll1)crystal underwent oxygen cleaning in both cases, the surface was sputterannealed only before the latter sequence (Figure 1D).The spectra obtained prior to methanol dosage consist of the well-known pair of YCO bands at 2104 and 1855 cm-l, attributable to atop and largely 2-fold bridging CO, respectively.8c
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(7) (a)Stenniger,H., Lehwald, S., Ibach, H. Surf: Sei. 1982,123,264. (b)Ogletree, D.F.; Van Hove, M. A.; Somojai, G . A. Surf: Sci. 1983, 125,787.
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Frequency (cm-l) kequency (cm-1) Figure 1. Effectsof progressively larger solvent dosages on YCO infrared spectra for GO coverages, as indicated (A-D), on Pt(ll1) at 100 K. The solvent dosage values given along each spectrum, Os*,are in units of “equivalentmonolayers”(see text and ref 3b). ( C ) and (D)refer to “saturated GO adlayers on “unsputtered”and freshly sputtered Pt(ll1) surfaces, respectively (see text). In contrast to lower CO coverages, solvent addition onto saturated CO adlayers yields no marked shifts in CO site occupancies; significant spectral changes are nonetheless observed. As noted elsewhere,sbinitial methanol addition yields a progressive attenuation of the 2104 cm-l band and the corresponding appearance of a feature a t slightly lower frequencies (Figure 1C,D). This “titration-like”band rep1acement, seen upon water is largely complete by 8,* 1.5. As detailed in refs 3 and 4a, this
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titration-like response is indicative of the initial formation of hydrogen-bonded solvent clusters which grow upon further solvent dosage to eventually encompass the entire surface. Distinctly different YCO spectral responses are (8)(a) Tushaus, M.; Schweizer, E.; Hollins, P.; Bradshaw, A. M. J. Electron. Spectrosc. Relat. Phenom. 1987,44, 305. (b) Schweizer, E.; Persson, B. N. J.; Tiishaus, M.; Hoge, P.; Bradshaw, A. M. Surf Sci. 1989,213,49,and references cited therein.(c)Persson,B. N. J.;Wshaus, M.; Bradshaw, A. M. J. Chem. Phys. 1990,92, 5034.
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Figure 2. Similar to Figure 1,for the four CO coverages as indicated (A-D), but for the indicated sequence of acetonitrile dosages.
obtained with nonassociating solvents, which presumably are deposited largely as random molecules on the COsaturated surface.4a An unexpected behavioral difference is observed, however, in that the freshly sputtered surface exhibits upon methanol dosing a significantly smaller frequency downshift of the atop vco band, to 2085 cm-’ (Figure lD), compared to that for the “unsputtered crystal, 2072 cm-1 (Figure 1C). This differencewas quite reproducible. (Note that the superior signal-to-noise ratio seen in Figure 1D arises chiefly from more extended nitrogen purging within the spectrometer light path rather than from intrinsically different surface responses.) Evidently, the detailed
nature of the methanol overlayer, or a t least its effect on the CO adlayer, is significantly different on the freshly sputtered Pt surface. It is plausible that this difference arises from a higher degree of chemisorbate order on the newly sputtered versus the unsputtered surface. A similar vco spectral sensitivity to the crystal pretreatment has also been observed for water dosing.4a No significant differences were observed, however, upon solvent postdosing for subsaturated CO coverages-the condition of primary interest in the present work. Experiments similar to those resulting in Figure 1A-D were also undertaken by utilizing deuterated-hydroxyl methanol (i.e. CH30D)in place ofCH3OH. One motivation
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for these experiments was to check the possible involvement of CO-solvent dipole coupling in the solvent-induced vco spectral changes (vide infra). However, essentially the same vco spectra were obtained in the presence of equivalent doses of both CH30H and CH30D (cf. ref 6a). Parts A-D of Figure 2 show vco spectral responses similar to those above for four increasing CO coverages, as indicated (0.08, 0.25, 0.55, 0.651,but now induced by sequential acetonitrile dosing. Similarly to methanol dosing, the addition of acetonitrile at low CO coverages yields a n essentially complete replacement of the atop YCO band by a markedly weaker bridging vco feature, now located a t ca. 1790-1820 cm-l (Figure 2A,B). This displacement requires a higher acetonitrile dosage, 8," 1.5. Also, in the case of acetonitrile a more clearcut frequency downshift of the bridging vco band occurs toward higher multilayer dosages, a t least up to 8," 3.5 (Figures 2A,B). Qualitatively similar behavior of such vco spectra upon acetonitrile dosing has been reported by Moritz and Luth.6b For near-saturated and saturated CO adlayers, acetonitrile dosing again yields band downshifts rather than substantial CO site transfer (Figure 2C,D). However, only a single downshifting atop band is seen upon initial (8," 5 1)acetonitrile dosing, indicative of random monomeric solvent a d d i t i ~ n . ~No " substantial differences in the spectral responses were seen for freshly sputtered and "nonsputtered" Pt surfaces. (Figure 2D was obtained for the former condition.) The third solvent examined here, acetone, yielded vco spectra largely similar to those for acetonitrile. A pair of significant differences was observed, however, which is evident in the spectral sequences shown for 8co = 0.15 and 0.25 in Figure 3A,B, respectively. First, complete attenuation of the atop vco band at the lower Bco value 0.5 (Figure required only a small acetone dosage, 8," 3A), comparable to that seen for methanol (Figure 1A). Second, only incomplete removal of the atop vco band was observed even for multilayer acetone dosages a t &o = 0.25 (Figure 3B), contrasting with the situation for acetonitrile (Figure 2B). Since acetonitrile and acetone have roughly similar polarity and dielectric properties (vide infra), these findings point to a n influence of other molecular properties upon the marked coadsorption effects. Given these observations, it is of interest to broaden somewhat the range of solvents examined. Parts A-C of Figure 4 show such solvent-induced vco spectral sequences a t 8co = 0.14,0.25, and 0.45, respectively, for the markedly less polar (yet more polarizable) solvent benzene. In overall outline, the observed benzene-induced spectral changes roughly mirror those noted above, featuring attenuation andor removal of the atop vco band along with the appearance of bridging VCO. Some differences, however, are noticeable upon closer inspection. First, a t low 8co values the bridging vco band so produced is barely detectable (Figure 4A,B), and markedly weaker than that observed upon dosage of acetonitrile (Figure 2A,B) and especially acetone (Figure 3A,B). While this atop-bridging intensity transfer is incomplete for Bco = 0.25, the bridging YCO band intensity for high (e,* 2 1)solvent exposures is a t least 5-fold weaker for benzene than for acetone. Such marked net vco band attenuation is also observed for higher coverages (such as 8co = 0.45, Figure 4C), although is largely absent for saturated CO adlayers. Another dissimilarity with the solvents considered above is that 0.2-0.3, are required only small benzene dosages, 8," to induce the final marked vco spectral changes. This difference may reflect partly the chemisorbing properties of benzene on Pt(ll1) (vide infra).
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The final solvent considered here, ammonia, also offers properties quite distinct from those of methanol, acetonitrile, and acetone. Although a gas at room temperature, ammonia constitutes an effective polar solvent a t lower temperature^.^ Notable in the present context are the very substantial (ca. 3 eV)work-function decreases induced by ammonia on Pt(lll),3bJ0indicative of an intricate (9) For example: Demortier, A.; Bard, A. J. J.Am. Chem. SOC.1973, 95, 3495 and references quoted therein. (lO)Fisher, G. B. Chem. Phys. Lett. 1981, 79, 452.
2782 Langmuir, Vol. 11, No. 7, 1995
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(Figure 5B). Admittedly, the ammonia-induced downshift of the atop vco frequency, to 2054 cm-l, is somewhat greater than that found for other solvents. As detailed in ref 3b, this frequency downshift can be matched with the solvent-induced potential drop across the saturated GO adlayer and correlates with the observed shift in the work function. In assessing the solvent-induced YCO spectra, it is useful also to consider spectral sequences as a function of the GO coverage for a fxed, appropriately high, solvent dosage. Such Bco-dependent spectra, in particular, constitute a
useful comparison with corresponding in-situ infrared spectra for the appropriate electrochemical i n t e r f a ~ e . ~ " Three of these spectral sequences, for methanol, acetonitrile, and benzene, respectively, are shown in Figure 6A-C. The fixed solvent dosages are Os* = 1.5,3, and 1.5 in these three cases. In addition, a corresponding 8codependent spectral set for water forms Figure 4 of ref 3a. As can be deduced from the foregoing, these &odependent spectra have an appearance very different from those obtained in the absence of postdosed solvent. The differences become markedly more pronounced toward lower Bco; while solvent postdosing causes vco frequency downshifting yet only moderate atop-bridging site transfer a t high eco, the solvent-induced site transfer becomes increasingly dominant for 8co I0.45. As already noted and further evident in Figure 6A-C, the CO coverage corresponding to the onset of complete solvent-induced CO site conversion is somewhat solvent dependent, occurring a t Bco < 0.2, ~ 0 . 3 5 and , 50.1 for methanol, acetonitrile, and benzene, respectively. In addition to the vco vibrations, it is also feasible in most cases to detect intramolecular solvent vibrations a t submonolayer as well as multilayer dosing levels. Such data, however, will be described in detail e l s e ~ h e r e . ~ ~ ~ ~ ~ Also detailed elsewhere are the main (1-3 eV) workfunction decreases that are commonly observed upon dosage of the present solvents onto CO-covered as well as clean Pt(lll).3b As a complement to the UHV-based "electrochemical modeling" studies described above, some attempts were made to acquire corresponding Bco-dependent spectra for several in-situ Pt(ll1)-nonaqueous solvent interfaces. Extensive data along these lines have been obtained previously in aqueous media,5a,bwhereas the acquisition of vco spectra for Pt(ll1)- and other metal-nonaqueous solvents have been limited to saturated CO a d l a y e r ~ . ~ ~ t ~ J ~ The lack of lower-coverage vco spectra in nonaqueous environments arises chiefly from difficulties in quantifylng the CO coverage. While variations in t9co can readily be brought about by adjusting the exposure time of the surface to the CO-containing ele~trolyte,~" evaluation of 8co is most directly obtained from the intensity of the 2343 cm-l band arising from the COZproduced upon electrooxidation of the adsorbed C0.5a While this procedure is reliable in aqueous solution, often only incomplete CO electrooxidation occurs in nonaqueous media since the oxygen required for COZ formation from CO is usually only available from the residual water content of the solvent. The difficulty is alleviated only partly by the deliberate addition of further water. Nevertheless, preliminary &-dependent series of insitu vco spectra were successfully obtained on Pt(ll1) in acetonitrile in this fashion (by X. Jiang of this laboratory), chiefly within the electrode potential range -0.5 to +0.5 V versus saturated calomel electrode (SCE). In qualitative accordance with the corresponding UHV-based spectra in Figures 2A-D and 6B, the intensity of the atop vco band (at 2060-2071 cm-l) declined sharply toward lower CO coverages, so that this feature disappeared for 8co values below ca. 0.25. No bridging vco band, however, was detectable within this low Bco region; this apparent disparity with the UHV-based data, however, could well be due merely to spectral signal-to-noiseconstraints, which tend to be more severe in the in-situ electrochemical environment. (13)(a) Anderson, M. R.;Huang, J. J.Electroanal. Chem. Interfacial Electrochem. 1991,318,335. (b) Roth, J. D.;Weaver, M.J.Langmuir 1992,8,1451.
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frequency (cm-1) Figure 6. Sequences of YCO spectra for varying CO coverages on Pt(lll),as indicated, with fixed high solvent dosages as follows: (A) methanol, Os* sz 1.5; (B) acetonitrile, e.* 3; (C)benzene, Os* FZ 1.5.
Discussion Solvent-InducedCO Site Transfer. Although differing in significant detail, for the most part the five solvents considered (along with water as described in ref 3a) exert similarly marked effects upon the 8co-dependent vco spectra on Pt(ll1). Most strikingly, even submonolayer solvent dosage onto low CO coverages (8co < 0.25) yields replacement of the initially intense sharp atop vco band by a markedly weaker feature at around 1800 cm-l attributable to 2-fold bridging CO. For saturated (or nearsaturated) CO adlayers, however, relatively little site
transfer commonly occurs upon solvent dosage, the chief consequence being to downshift the atop and bridging vco frequencies. “he latter effects are discussed in detail in ref 3b: they are apparently of a “nonspecific”nature, being associated with alterations in the potential drop across the CO adlayer resulting from the solvent dipolar ~verlayer.~~ The former effect, arising at lower CO coverages, most realistically involves solvent/CO coadsorption, given the availability of surface binding sites under these conditions. We have recently discussed the likely origin of the similar
Electrochemical Znterfaces in Ultrahigh Vacuum water-induced CO site transfer occurring under these condition^.^" The observed displacement of atop CO into doubly bridging sites was suggested to be due to a preferential stabilization of the latter geometry via dipolar CO-solvent interactions. Note that the magnitude ofthe W = 0 6 - dipole is probably larger in the latter configuration as a result of the greater &-27c* metal-CO backdonation (which also lowers the vco freq~ency1.l~ This large dipole should offer a more favorable interaction with water, especially in the usual adsorbed configuration with the 6-0-Hd+ dipole tiltedwith the oxygen toward the metal surface.3a Related arguments can account for the comparable effects upon the CO site occupancy seen here to be exerted by the present solvents at low CO coverages. Thus there is evidence from vibrational spectroscopy that the oxygen is bound to the platinum surface for both a c e t ~ n e and ~~J~ m e t h a n ~ l ; ~such ~ , ~orientations J~ would favor a stabilizing interaction with adjacent W-06- dipoles. Such an attractive pairwise interaction can account nicely for the near-completeCO site transfer achieved for these solvents at low (e,* 0.5) dosages. The higher acetonitrile dosages (e,* 1.5) required for CO site transfer under these conditions may be connected with the flat -C=N surface orientation, as deduced from vibrational spectr~scopy,~’ thereby hindering perhaps the -CO/H&-CN pairwise interaction. However, acetonitrile along with methanol display the greatest tendency for CO site transfer, being essentially complete even for 8co x 0.25 (Figure 6A,B). The comparable findings seen also for benzene, however, oblige a modified interpretation, since this solvent is known to be chemisorbed flat on Pt(111),18and offers no permanent molecular dipole. Nevertheless, benzene as well as methanol, acetonitrile, and acetone induces similarly large work-function decreases (-A@ 1.5 eV) when adsorbed on clean P t ( l l l ) , and in the presence of low CO coverages.3bThese comparable -A@ values (along with that for water, -A@ 1 eV 3a) suggest that the stabilization ofthe bridgingversus atop CO arises at least partly from a (‘through-surface” interaction, involving coupled solvent-surface and surface-CO charge transfer. This line of argument has been discussed elegantly by Mate et al. in connection with related vibrational findings, together with the observation of ordered coadsorbate structures for COhenzene and other systems on Rh(lll).19 Both types of CO-solvent interaction considered here-((through-space)’ (dipole-dipole) and “throughsurface” variants-are consistent with the observed occun-ence of complete CO site transfer only at low (or intermediate) CO coverages. For saturated (or nearsaturated) CO adlayers, the solvent will largely be denied direct access to the metal surface, eliminating (or at least muting severely) the occurrence of such “short-range” coadsorbate effects. It is nonetheless worth mentioning that the inducement of complete site transfer for a chemisorbed molecule by relatively weakly adsorbing solvents, while perhaps surprising, is undoubtedly encouraged by the small difference (ca. 1-1.5 kcal mol-l) in CO binding energies between the atop and bridging sites on clean Pt(111).8bEven relatively weak yet site-selective coadsorbate interactions then can readily account for the observed effects.
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(14)Mehandru, S.P.;Anderson, A. B. J.Phys. Chem. 1989,93,2044. (15)Vannice, M. A.;Erley, W.; Ibach, H. Surf Sci.1991,254,1. (16)(a)Ehlers, D.H.; Spitzer, A.; Luth, H. Surf Sci. 1988,160,57. (b) Sexton, B. A. Surf Sci. 1981,102,271. (17)Sexton, B.A.; Avery, N. R. Surf Sci. 1983,129,21. (18)Lehwald, S.;Ibach, H.; Demuth, J. E. Surf Sci. 1978,78, 577. (19)Mate, C. M.; Kao, C.-T.; Somojai, G. A. Surf Sci. 1988,206, 145.
Langmuir, Vol. 11, No. 7, 1995 2785 It remains to consider the rather more dramatic spectral shifts induced by ammonia coadsorption. Marked vco frequency downshifts triggered by ammonia adsorption have also been reported on Ru(001).20 While the magnitude (ca. 450-550 cm-l) ofthe band downshifts observed here (Figure 5A) are suggestive of CO site switching into a 3-fold hollow geometry, at least part of the effect likely arises from specific CO-NH3 interactions (cf. ref 19). Similarly, large vco frequency shifts have also been observed upon water coadsorptionon nickel and rhodium surfaces.21 At least in the present case, however, the markedly large vco shifts induced by ammonia in comparison with the other solvents appear to be connected with the especially large decreases in surface potential (ca. 3 V) accompanying ammonia adsorption on Pt(lll).3b Solvent-Znduced Znfrared Intensity Screening. Along with the complete CO site transfer, the most striking solvent-induced characteristic at low CO coverages is the weak intensity of the ensuing bridging vco feature. At first sight, the markedly (at least 3-5-fold) smaller integrated intensity of the latter in comparison with the displaced atop vco band may be attributed to an intrinsically smaller absorptivity of the bridging vco band. Support for this notion might be garnered from the observation that the 2-fold bridging YCO band is considerably (2.5-3-fold) weaker than the atop feature for CO on Pt(ll1)at 8co = 0.5 (in the absence of solvent),even though the atop and bridging site occupancies are equal under these conditions.8b Closer consideration of these and other findings, however, reveals a rather different picture. At least at a semiquantitative level, an overall understanding of the observations can be achieved by considering the effects of dynamic dipole coupling within the CO adlayers. First, a quantitative fit to the observed infrared spectrum of CO on Pt(ll1)at 8co = 0.25, where the ratio of atop to bridging band integrated intensities ZdZb = 2.7, yields on the basis of conventional dipole coupling theory closely similar values of the vibrational polaezabilities foroCOin these = 0.22 A3,a,,b = 0.18 A3.8bOn this two binding sites: basis, the markedly smaller intensity of the bridging vco band is due chiefly to the phenomenon of “intensity borrowing”, whereby a substantial fraction of the infrared absorption intensity of the lower-frequency(bridging)band is transferred t o the higher-frequency (atop) band ~ a r t n e r . ~Therefore ~ - ~ ~ in the (hypothetical) absence of dynamic dipole couplingwithin the mixed-site CO adlayer, the atop and bridging vco bands would exhibit comparable intensity. [Note that the integrated infrared band intensity is proportional to a,; the latter is related to the square of the so-called “dynamic dipole moment”.] The inference that a,,t a,,b is also consistent with some theoretical expectation^.^^ Consequently, then, the marked vco band attenuation that is seen to accompany solvent-induced site transfer is unlikely to be due merely to a smaller band absorptivity.
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(20)(a)Zhou, Y.;Akhter, S.; White, J. M. Surf: Sci. 1988,202,357. (b) Sasaki, T.; Aruga, T.; Kuroda, H.; Iwasawa, Y. Surf Sci. 1990,240, 223. (21)(a) Ellis, T. H.; Kous, E. J.; Wang, H. Surf Sci.1992,273,73. (b) Wagner, F.T.; Moylan, T. E.; Schmieg, S. J. Surf Sci.1988,195, 403. (22)Physically, this “intensity borrowing“ effect can be considered to arise within such a n intermixed array oftwo types of oscillators from the markedly better dielectric screening exerted by the higher-frequency (atop)oscillator on the lower-frequency (bridging) oscillator rather than vice versa.23 (23)(a) Persson, B.N. J.; Ryberg, R. Phys. Rev. B 1981,24,6954.(b) Persson, B. N. J.; Liebsch, A.Surf Sci. 1981,110,356. (24)For explanative reviews, see: (a)Hollins, P.; Pritchard, J. Prog. Surf Sci.1986,19,275.(b) Hoffman, F.Surf Sci.Rep. 1983,3,107. (25)Hush, N.S.;Williams, M. L. J. Mol. Spectrosc.1974,50, 349.
Kizhakevariam et al.
2786 Langmuir, Vol. 11, No. 7, 1995 Instead, therefore, the results suggest that the low intensities of the bridging vco bands seen upon solvent addition are due to the coadsorbed solvent itself. This issue has been discussed briefly by Moritz and Luth,6b who suggested the occurrence of dielectric screening. In concurring with the assertion, we now consider in semiquantitative terms the manner in which the present findings can be understood on this basis. The conventional treatment of dynamic dipole coupling relates the integrated absorption intensity, I, to the oscillator vibrational polarizability, a”,and the electronic polarizability of the local environment, a,,by23,26
where n(0) is an interaction-potential term which depends upon the adsorbate spatial positions. Equation 1therefore predicts that the infrared absorption intensity will be suppressed to a greater extent in media having larger a, values. This effect can be understood simply in terms of the greater “cancelation” of the dynamic dipole moment of the oscillator envisaged for larger values of the highfrequency (electronic) polarizability of the local interfacial environment. Since typically for adsorbed CO, U(0)% 0.3 even coadsorbate environments having relptively 3 A3, are small electronic polarizabilities, say a, predicted to yield significant (t2-fold) suppression of the infrared band intensity compared with the absence of coadsorbate (whereupon a, 0). Indeed, such dielectric screening by coadsorbed CO is the likely major reason why the coverage-normalized intensity of the atop vco band, (Z/&o), diminishes significantly as Bco increase^.^^^^^ In principle, then, the factor by which the infrared band intensity will be suppressed by molecular coadsorption with respect to the “infinite dilution” limit (i.e. 8co 0) in the absence of solvents, (I$Io), can be estimated from eq 1with knowledge ofthe solvent electronic polarizability, G. This procedure is hampered in practice by the inevitable uncertainties in U(0)and the effective values of G, [The latter difficulty arises from the molecular anisotropy of a,;while average a, estimates are readily obtained from refraction-index data,z7the relevant quantity in eq 1is the “local”a,value along the direction of the C=O vibration.] Nevertheless, by emplopng spatially averaged a,valuesz7and presuming that U(0) 0.3 A-3, we estimate the following (I$Io) values from eq 1: 0.5, water; 0.2, acetonitrile; 0.15, acetone; 0.25, methanol; 0.05, benzene. These (I$Io) estimates can be compared with the extent of solvent-induced band attenuation induced a t low 8co (e.g. t9co 0.11, where the effect of CO- induced screening should be small. Inspection ofthe relevant data in Figures lA, 2A, 3A, and 4A shows that these estimated (I$Zo)values are roughly in accord with (within ca. 2-fold of) the experimental observations. Although the observed extent
w-3,23
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(26) Luo, J.S.;Tobin, R. G.;Lambert, D. K. Chem. Phys. Lett. 1993, 204, 445.
(27) For examplesee: Hill, N. E.;Vaughan,W. E.;Price,A. H.;Davies, M. Dielectric Properties and Molecular Behavior; Van Nostrand Fkinhold: London, 1969; pp 235-40. (28) By using the Lorenz-Lorentz equation,27the following a.values (A3) were estimated from refraction-index data (visible light): 1.45, water; 4.35, acetonitrile; 6.4, acetone; 3.25, methannol; 10.3, benzene. These values can be compared with a, x 2.5 A3 for CO itself.
ofband attenuation induced by acetone is somewhat milder than expected on this basis (Figure 3A), the theory accounts nicely for the very marked band suppression seen in the presence of the polarizable molecule benzene (Figure 4A). Such dipole-coupling considerations can also provide a straightforward interpretation of the roughly linearl-&o relation seen for the atop vco band on Pt(ll1) in aqueous mediasa compared with the markedly nonmonotonic plot seen for the solvent-free UHVenvironment.z6 In the latter case, &-induced band suppression only occurs toward high &o, attributable to depolarization from surrounding CO molecules. In the former environment, however, such band suppression should be present throughout the CO coverage range, arising primarily from coadsorbed solvent and CO at low and high 8co values, respectively. Finally, it is worth mentioning that the above dipolecoupling model is also consistent with the above observation that band intensity suppression as well a s CO site transfer occurs even for submonolayer solvent dosages, typically 8,” 0.5. Thus the “nearest-neighbor solvation”, associated with attractive CO-solvent interactions, apparently is primarily responsible for the dielectric screening that yields the intensity suppression. Dosing additional solvent up to several multilayers typically yields little or no further effect on the band intensities, although significant (20-30 cm-l) frequency downshifts are then observed in some cases, most clearly for acetonitrile (Figure 2A,B). This latter influence is consistent with a nonspecific “Stark tuning” effect, connected with the additional changes in surface potential seen from workfunction data that occur upon multilayer acetonitrile dosing (e,* 1-5).3b
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Concluding Remarks Although the interpretations are inherently semiquantitative or even descriptive in nature, the foregoing provides us with a n apparently satisfactory framework with which to understand (or a t least rationalize) the marked effects exerted by the solvating medium on the infrared spectral properties of our metal-chemisorbate “electrochemical model” system. Perhaps most importantly, the results indicate that the effects of coadsorbed solvent a t low (or intermediate) CO coverages are quite different in both nature and degree from those observed with “overlayer” solvent molecules which control the spectral properties of saturated CO ad1aye1-s.~~ Together, therefore, these distinct “coadsorbed” and “overlayer” solvent effects can account readily for the important differences in infrared spectral behavior between the insitu electrochemical and conventional UHV systems. The utility of the IRAS-based “UHV electrochemical modeling“ approach for elucidating such double-layer environmental effects is bolstered further by its ready applicability to most systems (and issues) that are accessible to in-situ electrochemical I M S . For these reasons, we are expanding such studies in our laboratory to encompass combined ionic chargelsolvation effects on interfacial structure and bonding.* Acknowledgment. Experimental assistance was rendered by Xudong Jiang. This work is supported by the National Science Foundation. LA9500282
