9914
J. Phys. Chem. 1991, 95,9914-9919
aromatics. This appears reasonable, in view of the much higher reducing ability of this sample. Probably the oxygen from T H F ends as lithium oxide or carbonate. The nonappearance of surface aromatics in this case suggests that these arise from thermal evolution of 0-containing species, possibly the initial aldehyde.
served at temperatures below 150 O C , which is above the range of operation of lithium/organic electrolyte battery systems. The observed electrolyte decomposition in batteries must therefore have an explicitly electrochemical origin, or must involve reactions with some other battery component.
Conclusion While we have exhibited a range of hitherto unexplored surface chemistry, we have not illuminated the question of electrolyte decomposition in Li-MoS2 batteries. Our results show clearly that T H F is stable for long periods of time in contact with MoS2 or lithiated MoS2 at room temperature. No reactivity was ob-
Acknowledgment. This work was funded by an operating grant from the Natural Sciences and Engineering Research Council of Canada. M.W.J. thanks Moli Energy Ltd. for financial support during the course of this work. Registry No. THF, 109-99-9;McS2, 1317-33-5; LiMd2, 12201-18-2; Li,MoS2, 136676-60-3.
Surface-Enhanced Infrared Absorptlon of p -Nltrobenzolc Acid Deposited on Silver Island Fllms: Contrlbutlons of Electromagnetic and Chemical Mechanisms Masatoshi Osawa**+and Masahiko Ikeda' Department of Molecular Chemistry and Engineering, Faculty of Engineering, Tohoku University, Aramaki, Aoba- ku, Sendai 980, Japan, and Application Laboratory, Horiba, Ltd., 2- 12-5 Iwamoto-cho, Chiyoda- ku, Tokyo 101, Japan (Received: October 12, 1990)
We have studied the infrared transmission spectra of thin organic films of p-nitrobenzoic acid deposited on silver island films evaporated on CaF2 The absorption of the molecule is enhanced remarkably by the presence of silver island films. When mass thickness of the silver film is very small, the enhancement is restricted only to the molecule adsorbed directly onto the silver surface. As the mass thickness increases, the absorption intensity of the chemisorbed molecule increases and, in addition, the absorption of the molecules that were condensed over the chemisorbed molecule becomes to be enhanced. The largest enhancement factor is 500-600 for the chemisorbed molecule and is 20-30 for the overlayer. It has been suggested that the enhancement of electromagnetic field and the increase in vibrational polarizabilities of the molecule caused by a chemical interaction with the metal surface contribute independently to the absorption enhancement. Orientation of the chemisorbed molecule provides further additional small enhancement. The observed quite large enhancement is explained by a product of these three contributions.
due to a strong electromagnetic (EM) field amplified through the Introduction excitation of collective electron resonance (localized plasma osOptical properties of molecules are altered dramatically when they are adsorbed on or near some rough metal surfaces. Surface-enhanced Raman scattering (SERS) is the most prominent (1) Chang, R. K., Furtak, T. E., Eds. Surjuce Enhanced Rumun Scut. example.'-" Recently, SERS was observed for near-infrared teringr Plenum Press: New York, 1982. (2) Metiu, H. frog. Surface Sci. 1984, 17, 153. (near-IR) excitation (1 -06pm from a Nd:YAG laser) as strong (3) Moskovits, M. Reu. Mod. Phys. 1985,57, 783. as for the SERS for visible excitation^.^-' Similar surface-en(4) Otto, A. In Light Scattering in Solids; Cardona, M., Guntherodt, G., hanced phenomenon is observed also in the infrared (IR) region. Eds.; Springer: Berlin, 1983; Vol. IV, Chapter 6. Hartstein et ale8found that IR absorption of organic thin films (5) Crookell, A.; Fleischmann, M.; Hanniet, M.; Hendra, P. J. Chem. Phys. Lett. 1988, 149, 123. is enhanced remarkably by the presence of very thin overlayer (6) Chase, D. B.; Parkinson, B. A. Appl. Specrrosc. 1988, 42, 1186. or underlayer of silver or gold with a use of attenuated total (7) Angel, S. M.; Katz, L. F.; Archibald, D. D.; Honigs, D. E. Appl. reflection (ATR) technique. Since the discovery, the surfaceSpectrosc. 1989, 43, 367. enhanced IR absorption (SEIRA) has been investigated by several (8) Hartstein, A.; Kirtley, J. R.; Tsang, T. C. Phys. Rev. Lett. 1980, 45. 201. groups.g-20 The SEIRA was observed also in the external re(9) Hatta, A,; Suzuki, Y.; Sultaka, W. Appl. Phys. A 1984, 35, 135. flection and transmission geometries."*" (10) Sigarev, A. A.; Yakovlev, V. A. Opt. Spectrosc. (Engl. Trunsl.) 1984, The SEIRA spectroscopy in the ATR geometry was successfully 56, 336. applied to in situ characterization of molecules adsorbed at (1 I ) Hatta, A.; Chiba, Y.; Su&taka,W. Appl. Surfuce Sci. 1986,25, 327. electrode/electrolyte interfaces" and of thin organic l a y e r ~ . ' ~ J ~ J ~ (12) Nakao, Y.; Yamada, H. Surfuce Sci. I%, 176, 578. (13) Kamata, T.; Kato, A,; Umemura, J.; Takenaka, T. Langmuir 1987. Recently, its potential for trace organic analysis has been ex3, 1150. plored:" Nanogram quantities of molecules were detected in the (14) Osawa, M.;Kuramitsu, M.; Hatta, A.; SuEtaka, W.; s k i , H. Surfuce transmission measurements by spotting sample solution onto the Sci. 1986, I 75, L787. (15) Suzuki. Y.:Osawa.. M.; D D ~Surface . Sci. . Hatta.. A.:. Sultaka. W. A.silver-coated IR transparent substrates. If an FT-IR microsI 9 d , 33/34, 815. pectrometer is available, the detection limit can be reduced to (16) Hatta, A.; Suzuki, N.; Suzuki, Y . ;%*taka, W. Appl. Surfuce Sci. subpicogram ~rder.~'-*IHowever, the mechanism of SEIRA is 1989, 37, 299. not fully understood. (17) Nishikawa, Y . ;Fujiwara, K.; Shima, T. Appl. Spectrosc. 1990, 44, 691. Island structure of the very thin metal film plays an important (18) Wadayama, T.; Sakurai, T.; Ichikawa, S.;Sultaka, W. Surfuce Sci. role in SEIRAS8 It has been proposed that the enhancement is 1988, 198, L359. To whom correspondence should be addressed. 'Tohoku University. 1 Horiba. Ltd.
0022-3654/91/2095-9914%02.50/0
(19) Badilescu, S.;Ashrit, P. V.; Truong, V.-V. Appl. Phys. Lett. 1988, 52, 1551. ( 2 0 ) Badilescu, S.;Ashrit, P. V.; Truong, V.-V.; Badilescu, 1. I. Appl. Spectrosc. 1989, 43, 549.
0 1991 American Chemical Society
Surface-Enhanced IR Absorption of PNBA Deposited on Ag cillation) of the small metal island^,^-^^*'^-'^ as in the case of SERS.'-' The resonance has been studied in detail in the visible region.'" However, no experimental evidence of the resonance has been reported in the IR region. It is well-known that the IR absorption intensity of molecules increases when they are adsorbed on metal surfaces. Dumas et a1.,22for example, showed that the absorption intensity of C O chemisorbed on coldly deposited silver is 4 times stronger than that for further condensed CO. There exist several experimental results suggesting that chemical interactions between the molecule and metal surface play a role also in the SEIRA on metal island fiIms.18-20 An example of such interactions would be charge transfer between the molecule and the metal, which can lead to an enhanced vibrational polarizability of the molecules adsorbed directly onto metal surface^.^^-^^ However, the extent of the chemical contribution to the SEIRA has not yet been attributed definitely. The present investigation was conducted for further understanding of the mechanism of SEIRA on silver island films. The SEIRA spectra were measured in the conventional normal incident transmission geometry by using CaF2 as an IR-transparent substrate and p-nitrobenzoic acid (PNBA) as a model organic compound. The absorption intensity of the molecule is presented as functions of thicknesses of the metal and organic films. We show both EM and chemical mechanisms are responsible for the IR enhancement effect. The contributions of these mechanisms to the overall enhancement are discussed.
Experimental Section The substrates (CaF2 disks of 20-mm diameter and 1-mm thickness) were cleaned with acetone in an ultrasonic cleaner. Silver films of 5-20-nm mass thickness (dAg)were evaporated on the substrate surface from a tungsten boat by electron-beam heating under a pressure of 7 X IOd Torr. The film thickness was monitored by a quartz thickness gauge. The deposition rate was kept at 0.1 nm/s. A dilute acetone solution of PNBA was dropped onto the silver films with use of a microsyringe, and then the solvent was evaporated in the air. The average thickness of the organic film (tiPNBA) was estimated from the volume and concentration of the solution by assuming the density of 1.61 g/cm3 for the crystalline PNBA.9 All the chemicals used in the present study were analytical grade and were used without further purification. Infrared spectra were recorded on a Horbia FT-300 Fourier transform infrared spectrometer operating at 4-cm-I resolution. Coaddition of 60 scans was used. A Shimadzu UV-3100 UVvis-near-1R recording spectrometer was used for spectral measurements in the visible and near-IR regions. All the spectra were obtained in the conventional transmission geometry. Results It is well-known that very thin silver films evaporated on nonconducting substrates consist of small i~lands.~'The islands grow with increasing dAgand eventually become continuous film due to the coalescence of them. Scanning electron microscopic observations of our silver films showed that it is also the case on CaF2. Our silver films of dAg= 10 nm consisted of closely crowded islands roughly 30 nm in average diameter. The films of dAg greater than 12 nm were nearly continuous ones. Surface-Enhanced IR Spectra. Figure l a shows the IR transmission spectra of thin PNBA films of 1.7-nm average thickness (ca. 270 ng/cm2)'deposited on thin silver films for increasing dAgfrom 0 to 14 nm. The spectra of the metal films measured before the deposition of the organic films were used as reference spectra. The organic films are almost monolayer (2!) Osawa, M.; Ataka, K.; Ikeda, M.;Uchihara, H.; Namba, R. Anal. Sci., in press. (22) Dumas, P.;Tobin, R. G.; Richards, P. L.SurfaceSci. 1986, 171, 555. (23) Persson, 9. N. J.; Ryberg, R. Phys. Reu. B 1981, 24, 6954. (24) Devlin, J. P.; Consani, K. J . Phys. Chem. 1981, 85, 2597. (25) Sennett. R. S.;Scott, G.D. J . Opr. Soc. Am. 1950, 40, 203.
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 9915
(b)PWA( 132.8 nm)/CaFn
(a) M A ( 1.7 nm)/Ag/CaFz
m
Figure 1. Infrared transmission spectra of the organic films of p-nitrobenzoic acid (PNBA)deposited on silver-coatedCaF2 (a) and on clean CaF2 (b). The mass thicknesses of the silver films are shown in the figure. The average thickness of the organic films is 1.7 nm (a) or 132.8 nm (b).
equivalent as described below. N o microcrystallines of PNBA were observed by an optical microscope. The spectrum of a PNBA layer of 132.8-nm average thickness deposited on a CaFz plate without silver film is also shown in the Figure Ib for comparison. The relative intensities of the bands in spectrum b are the same as those in the spectrum of PNBA in a KBr pellet. Therefore, PNBA is oriented randomly on CaF2. The spectra of PNBA deposited on silver films are different from the spectrum of PNBA and rather resemble the spectrum of p-nitrobenzoate (PNBA-) except for the difference in relative intensities. According to the normal-coordinate analysis by Ernstbrunner et ai.,% most of the bands observed here are assigned to the symmetric modes of PNBA- (a, modes in C, symmetry). The strong band at 1390 cm-I is assigned to the symmetric C02stretching mode, indicating that PNBA was adsorbed onto the silver surface through the carboxyl group and dissociated the proton. The strongest band at 1350 cm-' is assigned to the symmetric NO2 stretching mode. The 1170-, 1107-, 1014-, 864-, and 825-cm-I bands are also assigned to the a l modes of PNBA-. However, most of the antisymmetric (b2) modes (some of which are strong in the spectrum of PNBA- in KBr pelletM) are missing in the enhanced spectra. Only one of the antisymmetric C 0 2 - / N 0 2 stretching combination modes26 is observed weakly at 1528 cm-l. In addition to these several bands of PNBA-, the C = O stretching mode of the molecule, which does not dissociate a proton, is observed very weakly at 1690 cm-I. This band disappeared completely when the sample plates were washed with acetone, while other bands were almost unchanged. Hartstein et aL8 observed three. bands around 2900 cm-1 in the ATR spectra of PNBA adsorbed on silver island films. They (26) Ernstbrunner, E. E.; Girling, R. 9.; Hester, R. E. J . Chem. SOC., Faraday Trans. 2 1978, 74, 1540.
Osawa and Ikeda
9916 The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
?=1350
cm"
I
0.0
f
0
111111111111111
0)
z
5
10
15
Slver Thickness / nm
Figure 2. Peak intensity of the 1350-cm-l band of PNBA deposited on silver films evaporated on CaFz (open circles) and the base-line level of the spectrum at 1350 cm-' (filled circles) as a function of the mass thickness of silver. The data are average values in several experiments. Thickness of the PNBA film is a constant of 1.7 nm.
assigned these bands to the C H stretching modes of PNBA. The spectral range below 2800 cm-l was not reported. We also observed three bands at 2955,2924, and 2854 cm-'. However, these bands were observed even in the reference spectra of the silver films and were reduced in intensity by the deposition of the organic films, resulting in the negative peaks in Figure la. The assignments by Hartstein et al. are questionable because the C H stretching modes of PNBA and PNBA- should be observed above 3000 ~ m - ' . ~ , The ~ ' bands observed here are assigned undoubtedly to the symmetric CH, and to the antisymmetric and symmetric CH2 stretching modes (from higher to lower wave number^).'^*^' Weak bands assignable to the asymmetric and symmetric CH3 deformation and CH2 deformation modes were observed at 1435-1385 cm-' in the reference spectra of the silver films. Therefore, we attribute these bands to saturated hydrocarbon contaminants. The enhancement of the 1R absorption by the presence of silver is obvious in Figure I . For dAg= 10 nm and ~ P N B A= 1.7 nm, the peak intensity of the symmetric NO2 stretching mode at 1350 cm-' is 0.1 in absorbance scale. From a comparison of this value with an absorbance of 0.016 for the same mode in the spectrum of a thick PNBA film of dpNBA = 132.8 nm on CaF2 shown in Figure lb, an enhancement factor of 500 is calculated. The enhancement factor varied from sample to sample and was between 500 and 600. I t must be noted here that the 1350-cm-' peak contains contributions from both chemisorbed and physisorbed molecules (PNBA- and PNBA, respectively). However, the latter contribution can be neglected because this band did not change in intensity when the physisorbed molecules were washed out from the sample plate with acetone. Therefore, the actual enhancement factor for the symmetric NO2 stretching mode of chemisorbed PNBA is slightly greater than 500-600. We cannot attribute the absorption enhancement for the chemisorbed molecule to the conformational change (proton dissociation) because the absorption coefficients of this mode for solid PNBA and its potassium salt (KBr pellets) were almost equal. Furthermore, as described below, the absorption of a PNBA molecule, which does not dissociate a proton, is also enhanced by a factor of 20-30 on a silver film of dA, = 10 nm. The absorption enhancement depends greatly on dAgas shown in Figure 2, where the peak intensity of the 1350-cm-' band is plotted as a function of dAg(open circles). The intensity increases with dAs: It reaches a maximum around dA, = 10 nm and then decreases. The decrease in the intensity at d- > 12 nm is connected with the loss of the island nature of the film.8 Other (27) Colthup, N. B.;Daly, L. H.; Wibcrley, S.E. Inrroducrion to Infrared and Roman Specrroscopy; Academic Press: New York, 1964.
{ 0.1 y1
a
/ /*
0 0 135oUn-1
A
1390
O .
1690
0.0
0
Figure 3. Peak intensity of the 1350-, 1390-, and 1690-cm-I bands in the transmission spectra of PNBA deposited on silver island films of 4- or IO-nm mass thickness as a function of the average thickness of the organic layer (filled symbols). The open symbols are obtained without silver.
absorption bands in Figure 1 showed similar dAgdependences. Coverage Dependence of Absorption Intensity. The peak intensities of three absorption bands of PNBA deposited on the silver film of dAg= 4 or 10 nm are plotted in Figure 3 with filled symbols as a function of dpNBA. The data were obtained by sequential deposition of PNBA on the same silver films. For dAe = 4 nm (upper half of the figure), the symmetric C02- stretching mode at 1390 cm-' rises very rapidly and saturates at dpNBA < 1.7 nm, suggesting that full monolayer coverage is reached. The gradual increase in intensity of the C - 0 stretching mode at 1690 cm-' observed after the saturation of the 1390-cm-l band is clearly attributed to condensation of PNBA, which does not associate a proton. The symmetric NO2 stretching mode at 1350 cm-l rises rapidly in the small dpNBA range as is the case of the 1390-cm-' band but continues to grow at a much slower rate. Since this band contains contributions from both chemisorbed and further condensed PNBA molecules, the initial rapid and later gradual increases in intensity are attributed to the increase in numbers of chemisorbed and further condensed molecules, respectively. The absorption enhancement factor for the 1350-cm-' band at dpp,BA 5 1.7 nm (i.e,, enhancement factor for the chemisorbed molecule) was calculated to be ca. 30 on an average with use of the method described above. We note here that the enhancement is restricted only to the chemisorbed molecule. No enhancement is observed for the 1690-cm-' band of the condensed PNBA layer as can be seen in the figure. Similar behavior is observed for dA, = 10 nm (lower half of Figure 3), but two differences are seen. First, the 139O-cm-' band saturates at a larger dmB, (about 3.5 nm) compared with the case of dAs= 4 nm. This is probably because the total silver surface area IS roughly twice as large as that for dAg= 4 nm. Second,
Surface-Enhanced IR Absorption of PNBA Deposited on Ag
The Journal of Physical Chemistry, Vof. 95, No. 24, 1991 9917
1znm 14rm
0
1
I
1
I
I
I
I
I
1
I
absorption of the further condensed PNBA layer is also enhanced on this silver film. The gradients of the peak intensity versus d m B A curves for the 1350- and 1690-cm-' bands are 20-30 times larger at dpNBA = 3-5 nm than those in the comparative measurements without silver film. The gradients decrease gradually with increasing dpNBA and asymptotically approach values that are the same as those observed without the silver film. This indicates that the enhancement for the physisorbed PNBA overlayer is relatively long-ranged, extending several monolayers. The enhancement factor for the overlayer is estimated to be 20-30 at dpNBA = 3-5 nm from the difference in the gradients. From these results, it is suggested that two distinct mechanisms contribute to the total enhancement: one that applies to the first chemisorbed layer and another that extends several monolayers away from the surface. Background Absorption by Silver Island Film. The enhanced absorption bands of PNMA are superposed on an extremely broad background absorption by silver as shown in Figure 4. These spectra were obtained relative to a clean CaFz plate. The deposition of the organic film did not change the background spectra 0 0.05 0.1 greatly. The dashed trace in the figure shows the transmission spectrum of a hypothetical smooth and continuous silver film of Absorption by silver IAbS. dpg= 10 nm calculated with use of the Fresnel formulaz8and the F'igure 5. Linear relation between peak intensity of the 1350-cm-' band dielectric constants of bulk silver.z9 Similar spectra were calof PNBA of 1.7-nm average thickness and the background absorption culated for different de,, except for intensity. by silver at 1350 cm-I. The spectra of silver island films of dAg< 15 nm are markedly different from the calculated spectrum. The strong and sharp starts to increase around dAg= 4 nm. The reflectivity of the metal band around 500 nm has been attributed to the excitation of film was small when dAg,is small, and therefore, the increase in collective electron resonance of the metal p a r t i ~ l e s . I - ~No * ~ ~ ~ ~the base-line level is mainly due to the absorption by the metal absorption is observed in the near-IR and IR regions when dAg islands. We see that the IR absorption of the molecule increases is small. As dAgincreases, a tail appears in the longer wavelength with increasing absorption by the metal. In spite of the decrease region of the 500-nm band and increases in intensity. A band in the absorption by the molecule in the d range above 10 nm, maximum is observed clearly around 2000 nm for dAg= 10 and the background level continues to increase.%owever, most of the 12 nm. The longer wavelength band is extremely broad and increase in background level is due to the absorption not by metal extends well into the IR region. The nearly continuous silver film islands but by bulk metal. The increase in the reflectivity of the of d A s = 15 nm absorbs in the low-frequency region more strongly silver films with increasing dAgalso results in the increase in the than in the high-frequency region, which resembles relatively the background level. calculated spectrum. In order to show the relation mentioned above more clearly, We note an intimiate correlation between the absorption enthe peak intensity of the 1350-cm-' band is plotted in Figure 5 hancement of the organic film and the background absorption by as a function of the absorption by the metal films at 1350 cm-l the metal. The base-line level at 1350 cm-l (in absorbance scale) ( A = 1 - T - R, where T and R represent the transmittance and is compared with the peak intensity of the 1350-cm-' band in reflectance of the silver film, respectively). The reflectance was Figure 2 as a function of dAg(filled circles). The base-line level measured relative to a thick silver mirror at the incident angle of 10'. The data for the silver films of db > 12 nm were omitted here because the absorption by the metal islands and continuous (28) Heavens, 0.S.Optical Properties of Thin Solid Films; Dover: New metal film cannot be separated from each other. This figure shows York, 1965. that the 1R absorption enhancement is almost linear with the (29) Lynch, D. W.; Hunter, W. R. In Handbook of Optical Constants of Solids; Palik, E. D., Ed.; Academic Press: Orlando, 1985; p 350. absorption by the metal islands. (30) (a) Creighton, J . A.; Blatchford, C. G.; Albrecht, M.G. J . Chem. Soc.,Faraday Trans. 2 1979,75,790; (b) Creighton, J. A. Reference 1, p 315. Discussion ( 3 I ) Yamaguchi, T.; Yoshida, S.;Kimbara, A. Thin Solid Films 1974,21, Electromagnetic Mechanismof the IR Absorption Enhancement. 171. .. _. As shown in Figure 3, the absorption enhancement extends several (32) Laor, U.; Schatz, G. C . Chem. Phys. Lett. 1981,82, 566; J . Chem. phys. 1982,76,2aaa. monolayers away from the surface for dAg= 10 nm. The relatively
9918 The Journal of Physical Chemistry, Vof. 95, No. 24, 19I91
Osawa and Ikeda
electron resonance in the IR region, the EM enhancement will long-ranged enhancement is obviously of EM origin. We first be relatively long-ranged and the absorption of the molecules discuss the EM mechanism contributing to SEIRA. embedded in the voids will be enhanced. The island nature of the metal film plays an important role in The coverage dependence of the peak intensity of the 1690-cm-' both SERSI4 and SEIRA.* When visible light is incident on a band on the silver film of dA = 10 nm (lower half of Figure 3) silver island film, collective electron resonance of the islands is shows that the long-ranged enaancement decreases with increasing excited and a strong local EM field is produced around the isdpNeA and disappears around 7 nm. However, this does not land~.'-~ It is well established that this strong local EM field plays a major role in SERS.14,30*32When dAgis extremely small ( 1 4 represent the actual moleculesurface separation dependence of the enhancement because the molecules will condense preferennm), the resonance is observed around 5 0 0 nm but no absorption tially in the voids between metal islands rather than on metal is seen in the IR region (Figure 4). Therefore, the enhanced EM field of the resonance is hardly expected for IR radiation. It has islands. Since the packing density (or volume fraction) of silver in the island film of dAg= 10 nm is 0.6-0.7,38 the voids will be been suggested, however, that the absorption band of the resonance is red-shifted well into the near-IR region and becomes broad due filled with PNBA molecules at dpNeA= 4-7 nm (density of 1.61 to the couplings of the induced dipoles or higher multipoles when g/cm3 is assumed). The disappearance of the enhancement around metal particles are This is observed in Figure dpNBA = 7 nm is, therefore, well understood by considering that the IR absorption of the molecules embedded in the voids is 4. It is obvious that the broad background absorption in the IR enhanced. region is the tail of the collective electron resonance. Therefore, the enhanced EM field associated with the resonance can also be Other Mechanisms Contributing to the Enhancement. As deexpected in the IR region when metal islands are aggregated. scribed above, we observed the absorption enhancement of ca. 30 The contribution of collective electron resonance to SERS on for the 1350-cm-l band on the silver island film of dAg= 4 nm. metal island films and metal colloids was demonstrated from an Since this very thin silver film has no absorption of collective intimate correlation between the SERS excitation profile and electron resonance in the IR region (Figure 4), the long-ranged absorption spectrum of the metal particle^.^^*^^ According to the EM field enhancement mentioned above is hardly expected in this EM models of SERS,'-4 the EM field around a metal particle case. In fact, the enhancement is restricted only to the chemiincreases linearly as the absorption by the metal particle increases. sorbed molecule and no enhancement is observed for the overlayer Since both incident and Raman-scattered EM fields are enhanced (upper half of Figure 3). Therefore, the short-ranged enhancement through the resonance in SERS, the SERS intensity is proportional observed on the very thin silver film must be attributed to other to the square of the absorption.30b If collective electron resonance mechanism(s). If the short-ranged enhancement is independent is responsible for SEIRA, the absorption intensity of the molecule of the long-ranged EM enhancement, the observable total enwill be directly proportional to the background absorption by the hancement factor (etotal) is a product of these two enhancement Ag islands. This is observed in Figure 5. We thus conclude that factors (eEM and eShon). If we further assume that the short-ranged the EM field of the resonance is responsible for SEIRA. enhancement is independent of dAg,since zEM is 20-30 on a silver Laor and S c h a t calculated ~~~ the EM field enhancement on film of d+ = 10 nm, the enhancement factor of 500-600 (or more) two-dimensionally distributed metal particles at visible and near-IR for chemisorbed PNBA on this silver film is well explained by frequencies. The enhancement factor is almost independent of (cEM = 20-30) X (tshort = 30). the frequency and is calculated to be 10'-103. Chew and Kerker33 One of the possible explanations of the short-ranged effect is calculated the local field enhancement in small cavities within the orientation effect of dipoles. It is known that PNBA chemmetal films to be 104-106 at near-IR frequencies. If the spaces isorbed on silver is oriented with its C, axis normal to the surfaceO9 surrounded by metal islands are modeled as cavities, their model Since the EM fields around metal particles are normal to the local will be applicable to metal island films. These strong EM fields surface^,^^,^^^^' the absorption intensity of the symmetric modes in the near-IR region probably explain the near-IR-SERS as of the oriented molecule is 3 times stronger than that for randomly strong as for the SERS for visible excitation^.^-^ However, the oriented molecule. However, this is not strong enough to explain enhancement of the EM field in the IR region has not been the short-ranged enhancement of 30, and therefore, a factor of calculated yet. We calculated the average EM field enhancement IO must be attributed to further additional short-ranged effect(s). using the model of Bergman and N i t ~ a n , which '~ was proposed Greenler et al.39suggested theoretically that the EM field is to calculate average EM enhancement in the visible region. The enhanced by a factor of -2 around step sites of metal surfaces. calculated enhancement factor was an order of 10 for dAg= 10 Even if this EM field enhancement were effective, however, its nm and is in agreement with the observed long-ranged absorption contribution should be negligibly small when it is averaged over the surfaces of the metal islands. enhancement factor of 20-30. In theoretical studies of the EM field enhancement, metal When a molecule is chemisorbed on a metal surface, its viislands are modeled by prolate or oblate ellipsoids whose major brational polarizability will be changed. There exists evidence of the increase in vibrational polarizability for CO chemisorbed axes are parallel to the The amplitude of the enhanced EM field around an ellipsoid is proportional to ( ~ / r ) ~ , on metal surface^^^^^^ (chemical mechanism). Since chemical interactions between molecule and surface are restricted only to where a is the local radius of curvature of a metal island and r the first monolayer, the chemical mechanism is favorable to explain is the distance from the center of the curvature.'-3 Since the EM the short-ranged enhancement. Hartstein et aL8 reported that enhancement is largest at the region of highest curvature, the the enhancement does not depend on the kind of molecules (PNBA enhanced IR spectra would be dominated by molecules adsorbed near the tip of the ellipsoid. Murray35estimated the average radius and its relatives) and concluded that the chemical contribution of curvature, a, at the tip to be 0.9 nm for a silver island film of is negligible in SEIRA. However, the enhanced IR spectra they dAg= IO nm. This implies that the EM enhancement falls within observed are not of the aromatic molecules but of hydrocarbon almost one monolayer distance from the surface, which does not contaminants as described above. Therefore, their conclusion is agree with our experimental results. Gersten et a1.36937suggested, not reliable. We obtained an enhancement factor of 100 for however, that the enhanced EM field is strongest in the voids PNBA chemisorbed on the silver island film of dAg= 5 nm in the between adjacent metal islands when the metal particles are present study. Smaller enhancement factors have been reported aggregated. This is due to the concentration of the flux lines of for other molecules on silver films of the same thickness. Kamata the EM fields around each island. Considering that the aggreet aI.l3 reported a factor of 30 for the symmetric C 0 2 - stretching gation of the islands is necessary for the excitation of collective mode of steric acid chemisorbed on silver. Hatta et aler6reported a 10-fold enhancement for poly(cyanoacrylate), which is not
-
(33) (34) (35) (36) (37)
Chew, H.; Kerker, M. J. Opr. Soc. Am. B 1985, 2, 1025. Bergman, D. J.; Nitzan, A. Chem. Phys. Leu. 1982,88, 409. Murray, C. A. In Reference I , p 203. Liver, N.; Nitzan, A.; Gersten, J. 1. Chem. Phys. Lert. 1984, 111,449. Gersten, J. 1.; Nitzan, A . Surface Sci. 1985, 158, 165.
(38) Yamaguchi, T.; Yoshida, S.; Kimbara, A. J. Appl. Phys. Jpn. 1969, 8. 559. (39) Greenler, R.; Duke, J. A,; Beck, D. E. Surface Sci. 1984, 145, L435.
J. Phys. Chem. 1991, 95, 9919-9924 chemisorbed on silver. Badilescu et al.'9*20reported that the absorption enhancement is highly dependent on the chemical structure of the molecules. All these results suggest that chemical interactions between molecules and metal surface play a role in the absorption enhancement. We thus suppose that the additional short-ranged enhancement by a factor of 10 is due to the change in vibrational polarizability of the molecule caused by chemical interactions with the metal surface. Devlin and C ~ n s a n have i ~ ~ suggested that the IR absorption intensity of molecules can be enhanced through a vibronic coupling of their vibrational modes with the charge-transfer (CT) excitation between the molecules and metal surfaces. The C T mechanism suggests that the IR absorptions of totally symmetric modes are enhanced prefer en ti all^.^^ This is in agreement with the observation in Figure 1 . However, the vibrational selectivity can be explained even by the EM mechanism because the PNBA molecule is oriented with its C, axis normal to the s u r f a ~ e . ~The ,'~ image dipole induced in the metal may also prevent the antisymmetric modes from being observed." Therefore, it is not clear (40) Pearce,
H.A.; Sheppard, N. Surface Sci. 1976.59, 205,
9919
at present whether the chemical enhancement is due to the CT mechanism.
Conclusion We have studied surface-enhanced infrared absorption of pnitrobenzoic acid on silver island films. It is suggested that at least three effects contribute to the enhancement independently. The first is a long-ranged enhancement extending several monolayers (on average) away from the metal surface. From the linear relationship between the absorption intensity of molecular vibrational modes and the background absorption due to the excitation of collective electron resonance of silver islands, the long-ranged enhancement has been attributed to the electromagnetic field enhanced through the excitation of the resonance. The second is due to the orientation of vibrational dipoles of the molecule. The third is probably due to the change in vibrational polarizability of the molecule caused by chemical interactions with the metal surface, although the details are still not known with certainty. The total enhancement is well explained by the product of the three contributions. Registry No. PNBA, 62-23-7; Ag, 7440-22-4.
Formation of Hexasilver at the Center of the Large Cavity. Three Crystal Structures of ( x = 2, 3, and 4) Dehydrated Ag+- and Ca2+-Exchanged Zeolite A, Treated with Rubidium Vapor Seong Hwan Song, Yang Kim,* Chemistry Department, Pusan National University, Pusan 609- 735, Korea
and Karl Seff* Chemistry Department, University of Hawaii, Honolulu, Hawaii 96822-2275 (Received: April I, 1991)
Three crystal structures of dehydrated Ag+- and Ca2+-exchangedzeolite A (A&Ca4-A, A&Ca3-A, and Ag8Ca2-A) treated at 250 "C with 0.1 Torr of Rb vapor have been determined by single-crystal X-ray diffraction techniques in the cubic space group Pm3m at 21 (1) "C (a = 12.271 ( I ) A, 12.255 ( I ) A, and 12.339 (1) A, respectively). Their structures were refined to the final error indices, R(weighted), of 0.072 with 130 reflections, 0.050 with 110 reflections, and 0.083with 86 reflections, respectively, for which I > 340. In each structure, Rb species are found at three different crystallographic sites: 3 Rb+ ions per unit cell are located at 8-ring centers, ca. 5.6-6.4 Rb+ ions are found opposite 6-rings on threefold axes in the large cavity, and ca. 2.5-3.0 Rb+ ions are found on threefold axes in the sodalite unit. Also, Ag species are found at two different crystallographic sites: ca. 0.7-2.1 Ag+ lie opposite 4-rings and ca. 2.2-4.8 Ag atoms are located near the center of the large cavity. In these structures, the numbers of Ag atoms per unit cell are 2.2, 2.4, and 4.8, respectively, and these are likely to have formed hexasilver clusters at the centers of the large cavities. The Rb+ ions, by blocking 8-rings, may have prevented silver from migrating out of the structure. Each hexasilver cluster is stabilized by coordination to up to 13 Rb+ ions. An excess absorption of about 0.8 Rb atom per unit cell indicates the presence of a triangular symmetric (Rb3)2+cation in the sodalite cavity. At least one large-cavity 6-ring Rb+ ion must necessarily approach this cluster and may be viewed as a member of it to give (Rb4)3+,(RbS)4+,or (Rb$+.
Introduction Ag+ ions in zeolite A can be reduced by heating,'.2 by reaction with reducing agents,' or by sorption of metal vapor^.^ Tsutsumi and Takahashi found that Ag+ ions in zeolite Y can be reduced to bulk clusters of Ago by treatment with alcohol and alkylbenzenes.s Ag+ ions in Ag-A, Ag-Y, Ag-mordenite, and Agchabazite can also be reduced by hydrogen.&8 Beyer et al. ( I ) Kim, Y.; Seff, K. J . Am. Chem. SOC.1977, 99, 7055-7057. (2) Kim, Y.; Seff, K. J. Am. Chem. Soc. 1978, 100, 6989-6997. (3) Kim, Y.; Seff, K. Bull. Korean Chem. SOC.1984, 5, 135-140. (4) McCusker, L. B. Ph.D. Thesis, University of Hawaii, 1980. (5) Tsutsumi, H.; Takahashi, H. Bull. Chem. Soc. Jp. 1972, 45, 2332-2337. (6) Beyer, H. K.; Jacobs, P. A. In Metal Microstructures in Zeolites; Jacobs, P. A., Ed.; Elsevier Scientific: Amsterdam, 1982; pp 95-102.
0022-3654/91/2095-99l9$02.50/0
reported that the silver clusters (Ag3)+ and (AB5)+ are formed at temperatures up to 150 "C, but that silver crystallites external to the zeolite are created at higher temperatures (350 "C) in A g Y and Ag-mordenite treated with H,.6 Also, Ag+ ions in zeolite A can be easily reduced by H2, and reduced Ag atoms or clusters An (A&)'+ cluster was can be readily reoxidized to Ag+ by 02.8 found crystallographically in the large cavity of partially Ag+exchanged zeolite A treated with H2.9 The neutral cluster (Ags)O, stabilized by coordination to 8 Ag+ ions, has been found by X-ray diffraction methods.'S2 This cluster may alternatively be viewed as (Ag14)*+. Hermerschmidt and (7) Beyer, H. K.; Jacobs, P.A.; Uytterhoeven, J. B. J . Chem. Soc., Faraday Trans. I 1976, 72, 674-685. (8) Kim, Y.;Seff, K. J. Phys. Chem. 1978, 82, 921-924. (9) Kim, Y.;Seff, K. J. Phys. Chem. 1987, 91, 668-671.
0 1991 American Chemical Society
