752
Langmuir 1986, 2, 752-757 matsu for sending us a copy of ref 22 prior to publication. This work is supported in part by grants from the National Science Foundation, the Air Force Office of Scientific Research, and the Office of Naval Research. D.S.C. received fellowship support from the Eastman Kodak Co. as part of the Purdue Chemistry Industrial Associates Program.
exception, carbon monoxide, is interesting and deserves further investigation. Further detailed comparisons of this type are needed to clarify the situation, especially using polarization modulation as well as potential-difference infrared techniques.
Acknowledgment. We are grateful to Dr. K. Kuni-
Investigations of Silver Electrode Surfaces in Propylene Carbonate/Alkali Halide Electrolytes by Surface-Enhanced Raman Scattering I. R. Hill, D. E. Irish,* and G. F. Atkinson Department of Chemistry, Uniuersity of Waterloo, Waterloo, Ontario N2L 3G1, Canada Received July 3, 1986 Surface-enhanced Raman scattering from a silver electrode in solutions of 1 m LiI or NaI in propylene carbonate (PC) has been analyzed. In addition to bands of the solvent, the characteristic band of carbonate ion has been observed (Na2C03or Li,C03). The observation of the spectrum of PC depends on the presence of strongly adsorbed Br- or I-; it is not observed when the anions are NO; or Clod-. The detection of solvated cations on the surface is obscured by Fermi resonance interactions in the C=O stretching region. When trace water is present, signals from adsorbed water and from OH- were observed. The conversion of the former into the latter could be followed with time. At quite negative potentials (-1.5 and -1.8 V) new bands arise in the spectrum when under laser illumination, suggesting new surface species formed by photoelectrochemical processes. A carbon-oxygen-containing species is suggested.
Introduction The identification and characterization of protective films on lithium are the subjects of much current research because of relevance to the fabrication of ambient temperature, high energy density, rechargeable Li batteries. Propylene carbonate (PC), HzC-0
I
HC-0
>o
I
H 3C
a dipolar aprotic solvent with a high dielectric constant (64), a wide liquid range, and efficient solvating powers for both inorganic and organic solutes, has been extensively studied as an electrolyte solvent.' Recently surface layers formed on Li in PC/LiC104 or LiAsF, solutions have been investigated by electrochemical studies,, ellip~ometry,~ X-ray d i f f r a ~ t i o n ,and ~ , ~IR, SIMS, ESCA, and SEM.5 In this study we present details of surface-enhanced Raman scattering (SERS) from silver electrode surfaces in LiI and NaI solutions of PC and then report on the effect of trace water on these spectra. In a previous paper we reported the SER spectra from silver electrode surfaces in acetonitrile (AN) solutions of lithium and sodium iodide., It is of interest to compare the results from PC solutions with those from AN solu(1)Yeager, H. L.; Fedyk, J. D.; Parker, R. J. J. Phys. Chem. 1973, 77, 2407. (2) Geronov, Y.;Schwager, F.; Muller, R. H. J. Electrochem. Soc. 1982, 129, 1422. (3) Schwager, F.; Geronov, Y.; Muller, R. H. J. Electrochem. SOC.1985, 132, 285. (4) Nazri, G.; Muller, R. H. J . Electrochem. Soc. 1985, 232, 1385. (5) Nazri, G.; Muller, R. H. J. Electrochem. Soc. 1985, 132, 2050. (6) Irish, D. E.; Hill, I. R.; Archambault, P.; Atkinson. G. F. J. Solution Chem. 1985, 24, 221.
0743-7463/86/2402-0'752$01.50/0
tions. For the latter, bands characteristic of the solvated Li+ and Na+ cations bonding to iodide ion on the silver surface were identified, as well as bands of adsorbed CH&N. In the presence of trace water, additional bands were seen from the species HzO, OH-, LiOH, and LiOH.H20. In ref 6 the SER spectrum of wet NaI solutions in the OH stretching mode region was not fully interpreted. Following further studies using D,O we can now say, in the case of HzO, that a t -1.5 V the splitting of the band a t 3604 cm-l into two components a t 3633 and 3588 cm-' results from forming crystalline NaOH at the surface (3633 cm-') and from OH- ions (3588 cm-') forming ion pairs with [Na.xH,O]+ where 3c = 0-3. When we reported the photoelectrochemical reduction of acetonitrile to CN- ions (which has also been reported by Mernagh and Cooney7 for Cu electrodes, but in the presence of dissolved 0,) a t potentials negative of the zero charge potential, we also briefly reported a similar photoelectrochemical reduction of propylene carbonate (PC) to carbonate ions, which formed a compound with the cation. In this paper we present a more extensive study of this phenomenon.
Experimental Section The spectroelectrochemical cell, electrochemical equipment, Raman spectrometer, and lasers were described in ref 6. The anhydrous salts used in this work, lithium iodide (Alfa products) and sodium iodide (BDH, AnalaR grade),were dried by heating under vacuum up to 160 "C. To avoid decomposition of the lithium salt, the temperature needed to be slowly increased over a 12-h period. The propylene carbonate (BDH, GLC grade) was stirred over 5A molecular sieve for 2 days and then distilled at a reflux ratio of 101 with the still head temperature at 80 "C and a pressure of 1 mmHg. Water content of the reagents was determined by Karl Fischer analysis. The reagents were stored in (7) Mernagh, T. P.; Cooney, R. P. J. Electroanal. Chem. 1984, 177,
139.
0 1986 American Chemical Society
Langmuir, Vol. 2, No. 6, 1986 753
Investigations of Silver Electrode Surfaces A
849
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SOLUTION
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I
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1809
1786
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2000 2700
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Figure 1. Normal Raman spectrum of a very dry solution of 1
m NaI in PC compared with the SERS spectrum obtained at a silver electrode (514.5-nm excitation, 5-cm-' band-pass).
a Vacuum Atmospheres Dri-Lab glovebox, in which the solutions were also prepared and the cell was filled and sealed. All potentials quoted in this paper are relative to the tungsten wire reference electrode."'0 It is assumed that the tungsten electrode with Li is an intercalation-type electrode.ll Solution concentrations were 1mol kg-' solvent (1m). SERS activation of the Ag electrode surface was achieved by AgI formation and reduction. SERS was not obtained as readily as in aqueous solutions, owing to the much lower conductivity of these solutions in PC and dissolution of AgI as Ag12-. However, good quality surface-enhanced spectra were obtained after up to four potential steps between -0.8 and +0.2 V, remaining at +0.2 V for 15 s and -0.8 V for 30 s. SER spectra were obtained by using both the 514.5-nm line of an Argon ion laser and the 647.1-nm line of a Krypton ion laser, the laser power at the sample being 100 mW in both cases. Spectra were recorded by using a 5-cm-' bandpass and a 1-s photon counting period, unless stated otherwise. The contribution of the normal Raman (NR) spectrum of the electrolyte solution to the measured surface spectra was minimized by positioning the electrode as close as possible to the cell window. Any residual intensity from the solvent (characterized by remaining at constant intensity and frequency at all potentials and also differentiated from SERS bands by having different degrees of depolarization and, usually narrower bandwidths) was a minor component of the SER spectrum.
Results and Discussion (i) The SER Spectrum of PC. SERS obtained from a roughened silver electrode in 1 m NaI/PC is first considered. A t -0.8 V a band was seen a t 115 cm-' which progressively decreased in intensity when the potential was made more negative. This band arises from the Ag-Istretching mode of specifically adsorbed I- and has been observed in aqueous studies and also in experiments using AN as solventa6 At the same time SER bands from PC were observed; intensity was high a t -0.8 V but progressively decreased as the potential was made more negative, following the behavior of the adsorbed I-. Hence an association between PC and adsorbed I- is suggested. The NR spectrum of PC is compared with the SER spectrum obtained at -1.0 V in Figure 1. Bands in the CH stretching mode region of the SER spectrum are significantly lowered in frequency compared with the liquid, which parallels the behavior for AN: again indicating a (8) Burke, L. D.; Whelan, D. P. J . Electroanal. Chem. 1982, 132,55. (9) Burke, L. D.; Twomey, T. A. M.; Whelan, D. P. J. Electroanal. Chem. 1980, 107, 201. (10) Reichman, B.; Bard, A. J. J. Electrochem. SOC.1979, 126, 583. (11) Silbernagel, B. G.Solid State Commun. 1975, 17,361.
I785
A 1720
A;,c, -l
1870
Figure 2. Potential dependence of SERS in the C=O stretching
mode region for very dry (top three rows of this figure) and wet (1 m H,O, bottom two rows) solutions of PC with (A) 1 m NaI and (B) 1 m LiI (647.1-nm excitation, 5-cm-' band-pass). weak I--- -H interaction for the adsorbed PC. PC generally solvates cations strongly and anions weakly, although 'H NMR data have shown that there can be a weak interaction with certain anions such as Br- and I-.' Our measurements of the NR spectra of solutions of iodide in PC reveal shifts in the CH stretching mode region of ca. 2 cm-l, which are not observed when using anions such as Ad?,-. This is clearly much less than the shifts of 20 cm-' seen for concentrated solutions of I- in AN and obviously reflects a weaker interaction. The C=O stretching mode in the SER spectrum is quite different from that of the liquid and this will be discussed in detail in the next section. The band at 1075 cm-' in the SER spectrum is assigned to the CO symmetric stretching mode of sodium carbonate (or an ion pair resembling that compound), formed following photoelectrochemical reduction of PC. If LiI is dissolved in the PC, this band is shifted to 1085 cm-l. Similar shifts have been observed for the crystalline species, Le., 1079/1083 cm-l for Na2C0, and 1091 cm-' for Li2C03.12 Lithium carbonate has recently been detected by in situ X-ray diffraction of a lithium electrode (deposited on a nickel substrate) operating with 1.5 M LiC10, in PC with Li counter and reference electrode^.^,^ Other bands of adsorbed PC are little changed from those seen in the NR spectrum. SERS of PC was also observed when using LiBr as supporting electrolyte but not when using NO3- or Clod-; these anions do not specifically adsorb to the silver electrode surface. This is in keeping with previous studies using AN.6 (ii) The C==O Stretching Mode Region of PC. In our previous study of SERS using AN as solvent we reported two fundamental CN stretching modes, one being unshifted from that of the liquid and the other being cation-dependent.6 The SER bands had counterparts in infrared and Raman bands of concentrated solutions. PC strongly solvates cations through the carbonyl oxygen and (12) Brooker, M.H.; Bates, J. B. J. Chem. Phys. 1971,54, 4788.
Hill et al.
754 Langmuir, Vol. 2, No. 6,1986 weakly through the ring;' two bands in the C=O stretching mode region were observed in the SER spectra, a t 1792 and 1811 cm-' at -0.8 V (Figure 2). These might also be expected to reflect cation-free and cation-solvated PC species, but other factors obscure this interpretation. The C=O stretching mode region of PC is complicated by Fermi resonance and by association of PC molecules which we will now discuss. Janz and c o - ~ o r k e r s have ' ~ assigned the Raman spectrum of PC; the weak shoulder (1806 cm-') seen on the high-frequency side of the C=O stretching mode (1786 cm-') (also visible in Figure 1) was assigned as a combination band (852 + 959) in Fermi resonance with the fundamental. Fermi resonance has been confirmed by the use of the 13C=0 isotope of PC.14 The same combination band in the SERS spectrum should occur at ca. 1804 cm-l, but the band we have observed is ca. 1811 cm-'. Interpretation of this region of the spectrum requires consideration of the infrared and Raman spectra of both the pure liquid and solutions of PC. Fini et al.15 observed that the C=O stretching mode is located at 1784 cm-' in the Raman spectrum of liquid PC but at 1800 cm-' in the infrared spectrum. Both spectra show a shoulder at about 1805 cm-', which was assigned to the combination band. For dilute solutions of PC in benzene both the infrared and Raman spectra contain broad asymmetric bands, with the main component at 1815 cm-' and a shoulder at 1798 cm-I. The unperturbed frequencies of both fundamental and combination were inferred to occur at ca. 1806 cm-'.15 Dielectric studies16 have suggested little intermolecular association in PC and this convergence was attributed to a reduction of short-range order upon dissolution in the benzene, which would lead to uncoupling of transition dipoles. The presence of Fermi resonance complicates the SER spectra because a shift to higher frequency of the fundamental will lead to increased Fermi resonance and hence increased separation of the perturbed levels. In the SER spectrum obtained by using 1 m NaI/PC these bands occur at 1792 and 1811 cm-' at -0.8 V (Figure 2). The combination band at 1811 cm-' decreases in relative intensity as the potential is made more negative while both bands shift to lower frequency so Fermi resonance is inferred to be reduced; thus the C=O stretching mode is expected to have shifted to a lower frequency. In comparison with the earlier SERS work with acetonitrile,' a larger local concentration of sodium ions is expected at more negative potentials and one might expect to see a C=O stretching mode shifted due to the presence of Na+ ions. For PC dissolved in nitromethane a second C=O stretching mode occurs in the infrared spectrum when Na+ ions are added, shifted to 1783 cm-' from 1798 cm-l.' Similar bands are seen at 1758 cm-' for Ag+ solutions and 1773 cm-' for Li+ solutions.' The observed SER shift is compatible with such cation solvation although the signal to noise ratio of the spectra does not allow any resolution of possible components, which themselves may also shift with potential. The effect of water on this region of the SER spectrum was also investigated. In Figure 2 we see that at -1.0 V the band contour, 1793-1815 cm-', is broad and shifted to higher frequency compared with results from the dry solution, whereas at -1.2 V the spectra are similar. At -1.0 V the results are consistent with dilution of PC, which (13) Janz, G. J.; Ambrose, J.; Coutts, J. W.; Downey, J . R., Jr. Spectrochim. Acta, Part A 1979. 35A. 175. (14) Ai-Jallo, H. N.; Al-Azawi, F. N. Spectrochim. Acta 1978,34A, 819. (15) Fini, G.; Mirone, P.; Fortunato. B. J. Chem. SOC.,Faraday Truns. 2 1973, 69. 1243. (16) Payne, R.; Theodorou, I. E. J. Phys. Chem. 1972, 76, 2892.
A
B -0.8Y
1
-0.8V
-1.2v
-I ov
3
x 3480
3200
A
v/ c mI
I .
3750
Figure 3. Potential dependence of SERS in the OH stretching mode region for 1 m NaI/PC solutions containing trace water: (A) 5 X lo4 m H20;(B)9.0 X m H20. The bottom spectrum in column B was obtained after prolonged reduction of H20 at the electrode surface (514.5-nmexcitation, 5-cm-' band-pass).
causes decoupling of adjacent carbonyl groups; at -1.2 V it appears that solvation of the cation may be more important. Further work was done with 1 m LiI solutions in PC (Figure 2). For the dry solutions, no frequency shift is apparent with potential although there is a decrease in intensity of the overtone at 1803 cm-' at more negative potentials, indicating less Fermi resonance, and thus a shift of the C=O stretching fundamental mode to lower frequency (offset by decreased Fermi resonance interaction). The general lowering of frequencies probably indicates relatively high and constant solvation of Li+ by adsorbed PC at all potentials measured. The components of the combination band in the SER spectra using LiI are at 847 and 954 cm-l at -1.0 V compared with 850 and 957 cm-' for NaI; hence the combination band is also expected to be shifted to lower frequencies. These bands are presumably shifted by the stronger interaction between the Li+ ion and the carbonyl group. For a LiI solution containing 1 mol of H20 per kg of PC, the Fermi resonance doublet occurred at 1784 and 1809 cm-', with the combination band becoming relatively weaker in intensity. The SER spectrum would appear to reflect the carbonyl group of adsorbed PC interacting with water rather than the cation, although no SERS from water was observed. In previous aqueous solution work only water associated with a cation has been observed. li (iii) SERS in the OH Stretching Mode Region. SERS in the OH stretching mode region was readily observable for undried 1 m NaI solutions in PC. Typical spectra can be seen in Figure 3 for 5 X and 9.0 X m water content, as determined by Karl Fischer analysis. Considering the drier solution, the two bands observed at -1.4 V are accompanied by a single band in the OH bending mode region and addition of equimolar D,O showed that the band at 3517 cm-' can be assigned to H20 whereas that at 3616 cm-l arises from OH- ions.6 (17) Fleischmann, M.; Hill, I. R. J. Electround. Chem. 1983, 146, 367.
Langmuir, Vol. 2, No. 6, 1986 755
Investigations of Silver Electrode Surfaces Another species containing OH groups which could be present is propane-1,2-diol which is produced by hydrolysis of PC (see below). However, in CC14 solution there are two OH stretching modes for this moleculels whereas we see only one. There is also no sign in the SER spectrum of other bands attributable to propane-1,2-diol. Therefore we are confident that the band we see arises from OH- ions. The band contour is appreciably broadened and shifted to lower frequency with an increase in water content (Figure 3B, -1.4 V) because of increased hydrogen-bonding interactions with the increase in concentration of water molecules in the double-layer region. Such behavior is expected; however, the spectrum of the wetter solution at -1.5 V is changed. Bands at 3527 and 3600 cm-' appeared within a short time while reduction of water was occurring at the silver surface. The reduction of water has apparently led to an increase in the surface concentration of OHions, manifested by an increase in relative intensity of the 3600-cm-' band, while depletion of water in the doublelayer region has led to the appearance of a sharper band arising from H20, a t 3527 cm-', superimposed on the broader band near 3480 cm-'. This spectrum, therefore, suggests the presence of water molecules which are hydrogen bonded to a lesser degree than is bulk water. Over a period of 1 h the spectrum steadily changed until that a t the bottom of Figure 3B was obtained. Clearly little water is now present in the double-layer region or in the small volume of electrolyte present between the Ag electrode and the cell window, while a large amount of OHhas been formed. Small bubbles believed to be hydrogen were also present. We believe that the OH- ions are present as a quasi-crystalline form of NaOH, possibly present as a monolayer film. The OH stretching mode is broader than expected for polycrystalline NaOH and shifted 30 cm-' to lower frequency. For drier solutions of NaI in PC the OH stretching mode of the OH- ion increased in intensity relative to that from HzO. This may reflect a fast rate of photoelectrochemical reduction of H 2 0 to OH- ions compared with the migration rate of trace water to the surface. At a water content of 6 X 10" m,no bands were detected in the OH stretching mode region. Chen et al.I9 have reported SERS from OH- ions adsorbed to Ag electrodes from aqueous metal chloride solutions made basic with hydroxide. In that work two bands were identified as arising from OH- ions: the first, at 3680 cm-', was assigned to OH- ions specifically adsorbed to Ag via the oxygen, whereas the second band, at 3595 cm-' and only visible at potentials more negative than -1.2 V (SCE), was assigned to OH- in the inner Helmholtz plane with the opposite orientation, the oxygen interacting with a cation in the outer Helmholtz plane. This second band corresponds to the OH- vibration observed in the present investigations, although we differ from Chen's interpretation by assuming that the metal ion of the Na+OH- ion pair is closer to the electrode surface. Because, in previous studies, the H2O detected in SERS experiments has always been associated with the cation of the supporting electrolyte, we expected to see changes in the OH stretching mode region of the spectrum when using 0.1 m H,0/1 m LiI/PC. Indeed a difference was seen-no SER spectrum from H20, nor OH-, was detected at all, in spite of the observation of SERS from the PC and I- ion. Observation of the C=O stretching region of the spectrum (see earlier) shows that H20 is present in the double-layer region, so why were signals from it not de(18) Kuhn, L. P. J . Am. Chem. SOC.1952, 74, 2492. (19) Chen, T. T.; Chang, R. K.; Laube, B. L. Chem. Phys. Lett. 1984, 108, 39.
f 2000 c t s
340
250
A V / cm-'
560
Figure 4. SERS from a Ag electrode at -1.4 V in 1 m H20/1m NaI/PC. This band was not observed in the absence of water, see text (647.1-nm excitation, 5-cm-' band-pass).
tected? NMR studies have shown that when trace amounts of water are present in PC solutions of lithium and sodium salts, the water selectively hydrates the cation, the water being strongly bound to Li+ and less strongly bound to Na+.20 In ref 6 we reported observing bands of trace water in AN solutions of LiI. SERS from water is not present a t -0.8 V for NaI solutions and its appearance follows that of the carbonate species. It is, therefore, relevant to note that although hydrated forms of Na2C03exist (mono-, hepta-, and decahydrates) this is not the case for Li2C03,which is not hygroscopic. Hence, we conclude that the observed water is associated with both Na+ and CO:-. The OH stretching mode of Na2C03.H20is seen at 3250 cm-l while for heptaand decahydrates two bands are seen, at 3125 and 3480 cm-1.21 These polycrystalline hydrates are not present on the electrode surface as judged from the observed frequencies and the water being observed is not as strongly interacting with the anion (we have already talked of photoelectrochemical reduction of this water leading to formation of Na+OH-). A band was also detected at ca. 340 cm-' in SER spectra when using wet NaI solutions. This band was not seen when using dry NaI and dry or wet LiI. The band became intense near -1.0 V and increased in intensity when the potential was changed to -1.4 V, whereas the intensity arising from PC fell. Figure 4 shows the spectrum obtained at -1.4 V, after the subtraction of the contribution from the background electrolyte and the cell window given by the SER spectrum obtained with the dry electrolyte. The band appears to involve either H20 or OH- ions. Concentrated aqueous solutions of NaOH give rise to a broad Raman band at ca. 300 cm-' which has been assigned to vibrations of Na+ OH- ion pairsz2and recently more accurately assigned to the NaO stretching mode of the hydrated ion pair Na+OH--H20.23 Although the band observed in this work is at higher frequency we note that the NaO stretching mode of NaOH isolated in an Ar matrix is located at 431 ~ m - ' thus ; ~ ~the surface species could have a frequency intermediate between 300 and 431 cm-l. Therefore, this band is tentatively assigned to the NaO stretching mode of the hydrated ion pair. The SER spectrum in the OH stretching mode region supports this assignment. The "anhydrous" Li+/COS2-ion pair apparently does not give a similar band in the LiI/PC system. (20) Cogley, D. R.; Butler, J. N.; Grunwald, E. J . Phys. Chem. 1971, 75, 1477. (21) Buijs, K.; Schutte, C. J. Spectrochim. Acta 1961, 17, 917. (22) Sharma, S. K.; Kashyap, S. C. J. Inorg. Nucl. Chem. 1972, 34, 3623. (23) Moskovits, M.; Michaelian, K. H. J . Am. Chem. SOC.1980, 102, 2209. (24) Acquista, N.; Abramowitz, S. J . Chem. Phys. 1968, 51, 2911.
756 Langmuir, Vol. 2, No. 6, 1986
Hill et al.
B I
1368
-2 4v
l0000 c t s
A
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I
10300cls
1233
1
7
/I
100
A V /cm-'
1800
Figure 5. (A) SERS from a very dry solution of 1 m NaI/PC. These bands were observed between -1.5 and -2.0 V (514.5-nm, excitation, 5-cm-' band-pass). (B) SERS from a very dry solution of 1 m LiI/PC. These bands were only observed between -2.4 and -2.6 V and increased in intensity with irradiation time (647.1-nm excitation, 5-cm-' band-pass).
(iv) SERS from Photoelectrochemically Formed Products of PC. When using very dry 1m NaI or LiI/PC solution at -1.5 V the SER spectrum contained new bands; these became much more intense a t -1.8 V (Figure 5A). The intensity of the band a t 1239 cm-' was monitored over a period of time and it was observed that, when using 514.5-nm radiation, the band intensity rose only when under laser irradiation. Also, relative to the intensity of the unchanged band of carbonate at 1062 cm-', the intensity of the 1239-cm-' band was found to vary with excitation frequency (measurements were recorded within 5 min): for a power of 100 m W the approximate intensity ratio was 1:1020 for wavelengths of 647.1,514.5, and 488.0 nm, respectively. These facts suggest that a new surface species is formed by a photoelectrochemical process. The bands at 275, 772, and 1239 cm-l (Figure 5A) probably arise from the same species. The diffuse band at 1462 cm-' did not increase in intensity as fast as these three bands and may arise from another species. No bands were detected in the CH stretching region of the spectrum. A t open circuit the band intensity a t 1239 cm-' decreased with time, while the band intensity at 1566 cm-' increased. This broad band corresponds to that which has been assigned to carbon.25 When the potential was taken to -0.8 V the band at 1239 cm-' disappeared while those at 1062 and 1566 cm-' remained. Assignment of the bands has proven to be difficult. The absence of CH stretching modes suggests that the species may contain only carbon and oxygen. The symmetric 0-C-0 bend of the formate ion (NaHCOO) gives a very strong Raman band a t 770 cm-1.26 The symmetric and asymmetric C-0 stretching modes give bands at 1355 (vs) and 1580 (w) cm-', respectively. Although the latter band would not necessarily be detected one would expect to see (25) Cooney, R. P.; Mahoney, M. R.; McQuillan, A. J. In Aduances in Infrared and Raman Spectroscopy; Hester, R. E., Clark, R. J. H., Eds.; Heyden: London, 1982; Vol. 9, p 190. (26) Tajima, I.; Takahashi, H.; Machida, K. Spectrochim. Acta, Part A 1981, 37A, 905.
a C-0 stretching band; 1239 cm-' seems low for this frequency unless the C-0 bond has been weakened and in that case the bending vibration would also be expected to shift to lower frequencies. The 275-cm-l band can tentatively be attributed to either a Ag-C or a Na-C vibration. When lithium atoms and carbon dioxide are cocondensed in argon five products are formed, namely, LiC02 (C2"),LiC02 (CJ, Li2C02,LiC204,and LizCz04.27Their infrared spectra have been measured and assigned. One would not anticipate exact frequency correspondence between the argon matrix spectra and SERS from an Ag/ NaI, PC system, but it is interesting to note that there is some similarity of frequencies. Recently Maynard and Moskovits28have reported SER bands a t 757 and 1260 cni-' from potassium-predosed silver surfaces exposed to CO in UHV experiments. An intense band in the region 1240-1280 cm-I is characteristic of an epoxy (oxirane) ring-breathing vibration; a second ring-breathing mode occurs in the region 770-850 cm-' and is substituent-dependent.29 These correlations also point to the existence of a carbon-oxygen-containing species. The existence of CO or C02i n t e r m e d i ~ n s ~does *~' not appear to be sufficiently well established to be invoked in this situation. However, a carbon-oxygen-containing species is strongly suggested. For a dry solution of 1 m LiI in PC, when the electrode potential was quickly taken to -2.4 V under 647.1-nm irradiation, a different type of SER spectrum was obtained (Figure 5B). A broad band a t 1068 cm-* (fwhm ca. 190 cm-') and a weak band a t 700 cm-' were observed. The intensities of these bands also increased with time during laser irradiation. When the beam was moved to a fresh part of the electrode surface the bands were absent but reappeared over a 2-min period. A t -2.6 V the band intensities diminished with time. The 1068-cm-l band suggests a species that contains carbonate, but the breadth of the band appears to rule out the possibility that the spectrum arises from a thick film of LizC03. It is possible that the spectrum has been surface enhanced by lithium which was deposited a t an underpotential by the action of the laser beam. Evidence in favor of this is the analogous surface enhancement of the band of Nz adsorbed to Li under UHV condition^.^^ The SER band of N2 was about 5 times broader than that of a polycrystalline film. Other Raman-active vibrations of C032-are the doubly degenerate antisymmetric stretching (- 1440, 1380 cm-') and the in-plane bending (-700 cm-') modes; these are not expected to be as strongly enhanced as the totally symmetric stretching mode. The band at 700 cm-' can be assigned to the in-plane bending mode of COS2-. (v) Mixed PC and AN. Experiments were carried out by using a 50 mol % mixture of PC and AN, with NaI, in order to see which solvent was preferentially adsorbed in the double-layer region. SERS of both solvents was seen at -1.0 V and the relative intensities of the bands of the two solvents were similar to those seen in the normal Raman spectrum of the solution. Therefore, neither solvent appeared to be preferentially adsorbed.
-
(27) Kafafi, Z. H.; Hauge, R. H.; Billups, W. E.; Margrave, J. L. J. Am. Chem. SOC.1983, 105, 3886. (28) Maynard, K. J.; Moskovits, M. Surface Canada '8612th Canadian seminar on surfaces, London, Ontario, Canada, June 8-11, 1986. (29) Dollish, F. R.; Fateley, W. G.; Bentley, F. F. Characteristic Raman Frequencies of Organic Compounds; Wiley: New York, 1974;pp 191 ff. (30) Gardner, R. A.; Petrucci, R. H. J. Phys. Chem. 1963, 67, 1376. (31) Gardner, R. A. J . Catal. 1972, 25, 254. (32) Moskovits, M.; Dilella, D. P. In Surface Enhanced Raman Scattering; Chang, R. K., Furtak, T. E., Eds.; Plenum: New York, 1982, p 243.
Langmuir 1986,2, 757-760 The potential dependence of the bands in the CN stretching mode region was similar to that with AN alone. However, the C=O stretching mode region was different; a single band was seen a t 1788 cm-l a t -0.8 V, with no shoulder present near 1805 cm-l. This again indicates that the carbonyl group is interacting with other solvent molecules as well as the cation and decoupling of intermolecular interactions has occurred with dilution.
Summary SERS from the solvent PC at a silver electrode has been observed. Intensities of PC and adsorbed I- diminish as the electrode potential is progressively changed to negative values. Evidence for an X--PC interaction at the surface is presented for X- = I- or Br- but not the weakly adsorbed anions NO3- or C104-. There is some evidence for Li+ or Na+ solvated by PC at the silver surface, but the evidence is obscured because of the Fermi resonance between the C=O stretching mode and a combination band. The effect of water on this region of the spectrum is reported.
757
A band characteristic of carbonate ion has been observed at 1075 cm-l for 1 m NaI/PC and 1085 cm-' for 1 m LiI/PC. Bands due to OH- and H20 were observed at 3616 and 3517 cm-l, respectively, when the PC solution contained water. On changing to more negative potentials the surface population of OH- increased and the population of H20 decreased. Water associated with Na+OH- and Na2C03 has also been detected. At more negative potentials bands arising from species formed photoelectrochemically were observed. The species probably involves the COz moiety. At -2.4 V the carbonate spectrum suggests the possible codeposition of lithium. For a 50 mol % PC-AN mixture, preferential adsorption was not detected. PC molecules were decoupled.
Acknowledgment. This research was supported by a contract from the Defence Research Establishment, Ottawa, and grants from the Natural Sciences and Engineering Research Council of Canada. The assistance of P. Archambault in the early stages of the work and helpful discussions with W. A. Adams are gratefully acknowledged.
Effects of an Acoustic Wave on the Aerosol Collection Efficiency of a Packed Bed of Spheres H. Tavossi Uniuersitt? Paris Val-de-Marne, U.F.R. de Sciences et Technologie, 94010 Crt?teil, Cedex, France Received April 7, 1986. I n Final Form: August 19, 1986 The collection efficiency of a granular media consisting of a randomly packed bed of glass spheres is investigated for submicronic aerosol particles in the presence of an intense acoustic wave. A significant increase in the collection efficiency is obtained at high acoustic intensities and low frequencies, for aerosol particles of intermediary dimensions, in the range 0.1-2 Mm in diameter, which are most difficult to remove under normal conditions. An expression is found for the collection efficiency of a single spherical collector for the acoustic turbulent diffusion of submicronic part.icles, which fits the experimental results at high acoustic intensities and low frequencies.
Introduction The efficiency of all filter media depends on aerosol particle size and passes through a minimum, for the aerosol particles of diameters in the range 0.1-2 Mm. These intermediary size aerosols are difficult to remove from the carrier gas and have a long residence time in the air. In addition, being respirable, they can transport into the lungs the gaseous pollutants adsorbed on their surface, becoming an important risk factor in the environment and for the public health. The physical properties of the acoustic wave are used in order to increase the collection efficiency of a granular media for the aerosol particles of intermediary sizes at low concentrations, where acoustic agglomeration of aerosols is ineffective.
For a single spherical collector, the total efficiency due to all the principal collection mechanisms is given by
1. Background The global collection efficiency of the media is defined by E , = 1- ( C f / C i )where , Ci and C f are initial and final concentration of aerosol particles. This efficiency, measured for a given particle size and flow conditions, can be regarded as the probability that an aerosol particle is collected during its passage through the media.
(1) Hidy, G. M.; Brock, J. R. The Dynamics of Aerocoloidal Systems; Oxford: Pergamon Press, 1970. (2) Mednikov, E. P. Acoustic Coagulation and Precipitation of Aerosol. A Special Research Report; translated from Russian by Larrick V. Consultants Bureau: New York, 1965. (3) Mercer, T. T. Hazard Eualuation; Academic Press: London, 1973. (4) Tavossi, H. "Performance et Effcaciti de Fixation des ABrosols par un Lit Granulaire en Prgsence d'0ndes Acoustiques," 1982. RapportCommissariat B l'Energie Atomique-R-5259, 1984.
77tot
=
771
+ 772 + 773 + ... + 17n
(1.1)
These collection mechanisms are supposed to be separable and the probability of simultaneous actions of two or more mechanisms to be very small. The global collection efficiency of the packed bed of spheres, for an aerosol of a given size, is then
0743-7463186/2402-0757$01.50/0 0 1986 American Chemical Society
