8488
J. Phys. Chem. 1994,98, 8488-8493
Adsorption of 4-Methoxybenzyl Cyanide on Silver and Gold Surfaces Investigated by Fourier Transform Infrared Spectroscopy Dong Hee Son, Sang Jung Ahn, Young Joo Lee, and Kwan Kim' Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul I51 -742, Korea Received: February 25, 1994; In Final Form: May 25, 1994"
The adsorption of 4-methoxybenzyl cyanide (4MBC) on gold and silver surfaces has been investigated by infrared spectroscopy. The molecule was found to adsorb on both surfaces via the CN ?r system. The benzene ring of 4MBC on gold appeared to assume a more perpendicular stance with respect to the surface than that on silver. At the gold electrode, a minimal amount of surface reaction seemed to occur to produce CN- species. Otherwise, the orientation of 4MBC adsorbed on gold was insensitive to the electrode potential. On the other hand, any surface reaction hardly appeared to occur a t the silver electrode. However, upon the potential variation, the adsorbate structure seemed to change greatly by rotating through the NC-CHI and HzC-C~H~ bond axes. Namely, a t higher potentials, the benzene ring of adsorbed 4MBC seemed to take a more parallel stance with respect to the silver surface.
Introduction
Experimental Section
Adsorption of an organic nitrile on a metal surface has been a field of extensive investigation because of its relevance to organometallic chemistry.Iv2 Such surface chemical information is, on the other hand, a prerequisite to elucidating the mechanism of catalytic reactions occurring at transition metal surfaces.3 An organic nitrile can in principle be bound to a metal via either its nitrogen lone pair electrons or its C=N ?r system. Nonetheless, organometallic compounds are usually claimed to have a-type coordinations via the nitrogen lone pair electron^.^ In light of the fact that the nature of surface adsorption is generally much the same as that of metal-ligand interactions,' the surface adsorption of nitriles would be expected to occur similary via the nitrogen lone pair electrons. In recent years, we have investigated the adsorption of various aromatic nitriles on the silver surface by surface-enhanced Raman scattering (SERS).5-10 On the basis of the compiled SERS data, it has been concluded that the majority of aromatic nitriles were adsorbed on silver via the CEN ?r systems. This is obviously in contrast with what was claimed in organometallic chemistry. Although SERS has been regarded as a very useful technique for the spectroscopic investigation of surface adsorbates, its unequivocal selection rule has not been established yet.11J2 Hence, one might not be convinced by the proposed adsorption mechanism of aromatic nitriles since our earlier conclusions were made mainly by referring to the SER spectral shifts of u(CN) bands. Other spectroscopic evidence is needed for a more firm conclusion. The infrared selection rule at metal surfaces has been well established.I3-l5 In conjunction with the above implication, we have thus attempted to investigate the adsorption of aromatic nitriles on silver by infrared reflection-absorption (IRA) spectroscopy. As a first model case, we have examined the IRA spectral pattern of 4-methoxybenzyl cyanide (4MBC) adsorbed on the surfaces of silver films and silver electrodes since 4MBC was concluded by SERS to be the most prototypical aromatic molecule adsorbing at the silver colloidal surface via its C=N ?r system. The infrared spectroscopic study was undertaken also for 4MBC adsorbed on gold surfaces in order todetermine whether the adsorption behavior of 4MBC depended on the specific metal substrate.
The metal substrates were prepared by evaporating silver (Aldrich, 99.9%) or gold (Aldrich, 99.99%) at 10-5-106 Torr on batches of previously sonicated silicon wafers or glass slides. After a deposition of approximately 200 nm, the evaporator was backfilled with nitrogen to reduce the ambient contamination. The metal substrates were immediately placed into 2.0 X 10-2 M 4-methoxybenzyl cyanide (4MBC) in ethanol. The solution was initially bubbled with nitrogen, and the whole self-assembling system was kept in the N2-purged drybox during the film deposition. To investigate the adsorption behavior at metal electrodes, subtractively normalized interfacial FT-IR (SNIFTIR) spectroscopy was employed. The electrochemical cell was designed similarly to that of Seki et a1.16 The cell body was made from Teflon, and a triangular CaFz prism was used as an optical window. The polycrystalline silver or gold electrode attached to a cylindrical Teflon rod was initially polished with 0.3 and 0.05Mm y-alumina, successively, and then sonicated in distilled water before use. The infrared spectra were obtained with a Bruker IFS 113v Fourier transform spectrometer equipped with a globar light source and a liquid N2-cooled mercury cadmium telluridedetector. A specular reflection attachment (Harrick VRA) was used in conjunction with a Harrick KRS-5 wire grid polarizer. The angle of incidence for the p-polarized light was set at 80° when the spectra of 4MBC self-assembled on the silver and gold surfaces were recorded. To reduce the effect of rotational lines of water vapor and C02, the sample and reference interferograms were recorded alternately every 32 scans. Each spectrum was obtained by averaging 2048 interferograms at 4 cm-I resolution. All spectra are reported as the -log(R!Ro), where R and Ro are the reflectivities of the sample and the bare clean metal substrate, respectively. When the electrode spectra were recorded, the angle of incidence of p-polarized light was set at ca. 70' with respect to the electrode surface normal. The SNIFTIR spectra were obtained by recording the reference and sample interferograms at two different potentials alternately every 32 scans. The collection of interferograms was synchronized with the potential change such that the former started 4-5 s after the latter was made. The total scans at each specified potential were 1024 with 4 cm-l resolution. The electrode potential was referenced to the saturated calomel electrode (SCE). The aqueous sample solution was bubbled with N2 gas before the electrochemical cell was
* To whom all correspondence should be addressed.
e Abstract
published in Advance ACS Abstracts, July 15, 1994.
0022-365419412098-8488$04.50/0
0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8489
Adsorption of 4MBC on Silver and Gold Surfaces
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Figure 1. Infrared reflection-absorption spectra of 4MBC self-assembled on (a) gold and (b) silver surfaces. (c) Transmission infrared spectrum of neat 4MBC.
filled. The Happ-Genzel apodization function was used in Fourier transforming all the interferograms. For recording the Raman spectrum, a SPEX 1877 E laser Raman spectrometer equipped with a triple monochromator was used, with a 514.5 nm excitation source (Spectra Physics Model 164-06 Ar+ laser). All the chemicals unless otherwise specified were reagent grade, and triply distilled water (Barnstead Co. Nanopure 11) was used throughout in the preparation of aqueous solutions.
Results and Discussion The infrared reflection-absorption (IRA) spectra of 4-methoxybenzyl cyanide (4MBC) which self-assembled on the gold and silver surfaces are shown in Figure l a and b, respectively. For comparison, the transmission infrared spectrum of neat 4MBC is shown in Figure IC. Although the IRA spectral pattern appears different from that of neat 4MBC, the peaks in the IRA spectra correlate well with those in the transmission spectrum. For instance, the benzene ring modes at 1257 ( ~ 1 3 1515 ) ~ (VIP,),1614 (vsa), and 3005 ( Y C H ) cm-l and the methoxy group modes at 1180 [v(CO)], 1305 (6,(CH3)], and 2840 [v,(CH3)] cm-l in the IRA spectrum in Figure l a correlate with the peaks at 1249, 1513, 1613, 3003, 1180, 1305, and 2840 cm-l in the transmission spectrum in Figure IC, respectively. The corresponding peaks appear at 1259,1512,1608,3006,1175,1308, and 2843 cm-l in Figure 1b, respectively. The band which can be attributed to the CEN stretching vibration appears a t 2222,2220, and 225 1 cm-I, respectively, in Figure 1a-c. Considering that a minute difference in the peak positions between the IRA and transmission spectra arises from the electronic structural change upon surface adsorption, the species responsible for the IRA spectra can be regarded as due to adsorbed 4MBC. The detailed spectral difference will be discussed later.
The 4MBC molecule appeared to adsorb on the gold and silver surfaces very favorably. The IRA spectra were nearly independent of the immersion time used in the preparation of the self-assembled films. Namely, over a period of 1 h to 8 days of self-assembling (SA), the positions and intensities of IRA peaks were observed to barely change. The IRA spectra shown in Figure 1 correspond to what were obtained after S A for 20 h in 4MBC solution. From a separate work, a gold or silver substrate self-assembled with 4MBC seemed not to possess vacant metallic sites available for further chemisorption. When a self-assembled film was spincoated with a solution of stearic acid that could be chemisorbed fairly well onto the metal substrate as carboxylate,17 neither the v,(COO-) nor v ,,(COO-) peak was identifiable in the IRA spectrum. On the other hand, when a gold or silver substrate coated with 4MBC in multilayers was washed with ethanol, the IRA spectrum restored to what was obtainable after SA alone (Figure 1). These suggest that the adsorption of 4MBC on the gold and silver surfaces is energetically very favorable, and thus complete monolayers can be formed readily by contacting the metal substrates with the 4MBC solution. The favorability of self-adsorption of nitriles on the gold and silver surfaces has been reported also by several investigators specifically for long-chain alkyl nitriles.18 The most noteworthy observation in the IRA spectra is that the CGN stretching band red-shifted by ca. 30 cm-1 upon the surface adsorption. Its bandwidth (30 cm-1) is broader than that in the transmission spectrum (20 cm-1). The substantial red shift and band broadening upon surface adsorption indicate direct interaction between the cyano group and the metal surface. From the studies of the metal-nitrile complexes and the nitriles adsorbed on metal surfaces using EELS, XPS, UPS, and other techn i q ~ e s , l ~ -it2 ~has been generally accepted that the linear coordination (u bonding) through the nitrogen lone pair electrons results in an increase in the C=N stretching frequency from that of the free molecule. On the other hand, coordination through the C r N A system is known to result in a decrease in the C=N stretching frequency. The substantial red shift of the v(CN) peaks in the IRA spectra may then indicate that 4MBC is adsorbed on the gold and silver surfaces via the CEN A system. Namely, such a huge red shift can be attributed to the combined effects of A (CN)-to-metal electron donation and metal-to-?r* (CN) backdonation. The broadening of the v(CN) band can be attributed, on the other hand, simply to inhomogeneity effects. We have made a similar conclusion from the SERS study that 4MBC should be bound to the silver sol surface via the A system of the cyanogroup.1° One intriguing aspect of the data is that the v(CN) band in Figure l a is very intense but the corresponding band in Figure 1b is, on the contrary, very weak. Invoking that a near monolayer is formed on both metal surfaces indicates that the adsorbate structure, more specifically its orientation, on the gold surface must be different from that on the silver surface. According to the IRA selection rule,l3-15 thevibrational modes whose dipole moment derivatives possess components normal to the metal surfaces are preferentially excited. On the basis of this, one may think at first glance that the distinct appearance of the u(CN) band in Figure l a is in contradiction to thesuggested adsorption scheme that the surface bonding occurs through the A system of the cyano group. To resolve this difficulty, an attempt has been made to determine the direction of the transition dipole vector accompanying the CEN stretching vibration. Through the assumption that the C=N stretching mode was not coupled with other molecular vibrational modes, the dipole moment derivatives were calculated quantum mechanically by varying the distance of the C=N bond. Initially, the equilibrium geometry of 4MBC was determined by applying a geometry optimization routine using the Gaussian 92 program for Windows25 with a 6-3 1G basis set. The molecular dipole moments were calculated consecutively with an equilibrium geometry and then after the
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6) Figure 2. (a) Transition dipole vector for the CN stretching mode in 4MBC determined from an ab initio quantum mechanical calculation (see text). Plausible geometries of 4MBC adsorbed on (b) gold and (c) silver surfaces. (a)
(b)
CN bond was lengthened by 0.02 A. The difference between the two dipole vectors was taken as the direction of the transition dipole moment of the C=N stretching mode. The transition dipole vector thus obtained is directed 28" away from the C N bond axis as displayed schematically in Figure 2a. The present calculation suggests that the v(CN) band can appear distinctly in the IRA spectrum even when the 4MBC adsorbs on the metal surface via the a system of the cyano group. Although the calculated result on the CEN stretching mode must be helpful for the qualitative interpretation of observed spectral behavior, one should be, nonetheless, cautious in applying the presently adapted method to the determination of direction of the transition dipole vector since the vibrational modes are generally coupled to one another. The ab initio calculations indicate that the transition dipole vectors corresponding to the in-plane benzene ring vibrational modes are aligned mostly in the ring plane. In this sense, it is informative that distinct in-plane ring modes appear in the IRA spectrum obtained on the gold surface (Figure la). This suggests that the benzene ring of 4MBC should assume a rather perpendicular stance with respect to the gold surface. In this orientation, the possibility of direct interaction between the benzene ring and the gold surface will be low. It is useful at this moment to recall the vibrational studies of benzene and alkyl benzenes adsorbed on metal It has generally been known for the ring modes that a 10 cm-I or more red shift as well as substantial band broadening occurs as the benzene ring adsorbs on a metal surface via its a system. Such a red shift is presumed to arise from the bond weakening in the benzene ring system caused by the back-donation of the metald electrons to the benzene ring antibonding a* orbitals. In the case of 4MBC, the ring modes were observed not to red-shift upon adsorption on the gold surface. Instead, the ring modes blue-shifted by 1-8 cm-1 in the IRAspectrum (Figure la). Furthermore, the bandwidths of ring modes are not affected much either. These imply, in fact, that the benzene ring of adsorbed 4MBC does not interact with the gold surface directly. Since the positions of methoxy group vibrational bands were hardly changed upon surface adsorption, the methoxy group of 4MBC was thought also not to interact directly with the gold surface. On the basis of this information, the plausible geometry of 4MBC adsorbed on the gold surface isdrawnin Figure 2b. It is to benoted that the proposed adsorbate struture is also consistent with the observation that the symmetric stretching band of the CH3 group is weaker than others in the IRA spectrum (Figure la). As mentioned previously, the IRA spectrum taken on the silver surface exhibits several different features from that on the gold surface. For instance, the u(CN) band was observed to be very weak on the silver surface, in contrast to that on the gold surface. The ring modes in Figure 1b are also substantially weaker than those in Figure l a in a relative sense. One may argue that the
Son et al. weak band intensities arise from the smaller population of adsorbate on the silver surface than on the gold surface. However, it has already been noted that complete monolayers can be formed readily by contacting the metal substrates to the 4MBC solution. Considering that 4MBC is adsorbed on both surfaces via the C=N a system, the surface density on silver will not be greatly different from that on gold. This implies that the IRA spectral difference arises mainly from the different adsorbate structure rather than the surface density difference. Although the v(CN) bandisvery weak, the V I 3 band at 1259cm-I appearsvery distinctly in Figure lb. This fact supports the fact that the weak intensity of the v(CN) band in Figure 1b has nothing to do with the smaller population of adsorbate on silver than on gold. The near coverage may be inferred further from the fact that the SER spectrum of4MBC can be readily obtained in the silver s01.I~ In a separate experiment, we found also that even the attenuated total reflection infrared spectrum could be readily obtained for 4MBC adsorbed on the silver surface.29 More plausible evidence for the presence of a sufficient amount of 4MBC on the silver surface may be drawn from the infrared spectral pattern at a silver electrode, to be discussed later. The fact that the v(CN) mode of 4MBC has red-shifted also by ca. 30 cm-I as the molecule was adsorbed on the silver surface reflects that adsorption should take place via the a system of the cyano group, as is likely on the gold surface. The same conclusion was made earlier from the SERS study in the silver sol.lO The very weak intensity of the v(CN) band indicates that its transition dipolevector should be directed nearly parallel to the silver surface. This is obviously different from that claimed on the gold surface. In order to accommodate the weak intensity of the v(CN) band with the a (CN) coordination, the benzene ring of the adsorbate on silver is required to possess a more parallel stance with respect to the surface than that on gold. This implies that the adsorbate on silver must be distorted from its structure in the free state by a rotation around the NC-CH2 bond. Namely, molecular reorganization occurs for 4MBC as the molecule is adsorbed on the silver surface. On the basis of this, a plausible geometry on the silver surface is drawn schematically in Figure 2c. It is to be noted that the proposed adsorbate structure is consonant with the relatively weak intensities of the in-plane ring vibrational bands in Figure 1b. Numerous cases have been reported indicating that the adsorption behavior depends on the kind of substrate to which molecular adsorption takes place. For instance, long-chain aliphatic thiols were known to adsorb differently on the gold and silver surfaces, with the alkyl chain being tilted on gold about 30" with respect to the surface but by only 6-7" on silver.30 Hence, it is not surprising to observe different adsorption behaviors for 4MBC on the gold and silver surfaces. In order to obtain further information on the adsorption behavior of 4MBC, we have attempted to measure the infrared spectra at the gold and silver electrodes. Specifically, the potential dependence of the adsorption behavior was examined with SNIFTIRS.31 Figure 3 shows the potential difference infrared (PDIR) spectra of 4MBC obtained by SNIFTIRS at a gold electrode in the potential range from -0.6 to 0.4 V. When the PDIR spectra were recorded, the reference potential was set at -0.8 V. Two distinct bipolar peaks appear around 2100 and 2200 cm-l. In other spectral regions, the PDIR spectra exhibited silent features. Referring to the principle of SNIFTIRS," the positive peaks have to be attributed to chemical species existing a t the reference potential, i.e. -0.8 V, while the negative peaks are attributed to those existing at the specified sampling potential. The positions of positive peaks were insensitive to the potential variation, indicating that the peaks are in fact due to the species existing at -0.8 V. The negative peaks were found, on the other hand, to blue-shift as the applied potential was increased. Since
Adsorption of 4MBC on Silver and Gold Surfaces
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8491
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Figure3. SNIFTIRspectraof4MBCat thegoldelectrode. The reference potential was -0.8 V vs SCE. The sample potential was set at (a) -0.6, (b) -0.4, (c) -0.2, (d) 0.0, (e) 0.2, and (0 0.4 V vs SCE to obtain each spectrum. The positions of positive peaks are insensitive to the potential variation. The negative peaks are blue-shifted upon thepotentiial increase.
no peaks are observed in the PDIR spectra with s-polarized light, the bipolar peaks arise from the adsorbed species. The appearance of two bipolar peaks in the PDIR spectra is intriguing. From the IRA spectrum shown in Figure la, the species responsible for the peak near 2200 cm-l in Figure 3 can be attributed to 4MBC adsorbed at the gold electrode via the a system of the cyano group. The blue shift of the u(CN) mode upon the potential increase can be understood by recalling that the metal-to-r* (CN) electron back-donation becomes less favorable at higher electrode potentials. On the other hand, the origin of the peak around 2100 cm-I is rather uncertain. From the GC/MS analysis, the organic chemicals used in this work were found to be free from impurities that could possibly cause the appearance of a triple-bond stretching band. Several other causes are nonetheless conceivable. In principle, multiple peaks can arise in a surface spectrum from either the presence of more than one ~ o n f o r m a t i o n or 3 ~adsorption ~~~ on more than one type of crystal plane or on adsorption sites with different coordination numbers.34 In the former case, the multiple peaks should appear also in other spectral regions, contrary to the present case. On theother hand, a splitting associated with different crystal planes is generally about 10-15 cm-1, far less than the peak separation, i.e., 100 cm-I, observed in the present case. If one of the latter models is applicable to the present case, a similar peak splitting should have appeared even in the IRA spectrum (Figure la). The relative influence of metal d-band electron density on the CO or C N bond strength versus the coordination number of the adsorption site has been a debate of considerable interest.24For CO adsorbed on nickel surfaces, for instance, two peaks are observable. The first peak at ca. 2060 cm-1 which is red-shifted by 90 cm-l from the peak position for the gaseous carbon monoxide has been ascribed to a linear monodentate structure, while the
second at ca. 1935 cm-I has been assigned to a bridged structure.34 In both cases, the surface bonding occurs via the 5u orbital. From a SNIFTIRS study of CN- a t a palladium electrode, Ashley et al. observed two peaks due to adsorbed cyanide species at 2065 and 198Ocm-l at 0.7 V.35 Thosepeaks wereassigned, respectively, as linear and bridge-bound cyanide species formed through the lone pair electrons of the carbon atom. On the other hand, in a SNIFTIRS study of CN- a t gold and silver electrodes,36 we could not observe any peak attributable to a bridge-bound cyanide species; only one peak assignable to a linearly bonded species was exclusively observed. Besides, in our previous infrared study,3’ acetonitrile was concluded to adsorb on the nickel surface by forming a u bondvia thenitrogen lone pair electrons. In contrast, the molecule was adsorbed on the nickel oxide surface by forming di-ubondswith thenitrilecarboa andnitrogenatoms. Thev(CN) band was attributed in the latter case to appear at 1560 cm-I. The amount of v(CN) peak shift that should occur for a a to di-u structural change is thus estimated to be around 650 cm-I. Since the frequency difference between the two bipolar peaks in Figure 3 is substantially smaller than this, the formation of di-u bonds seems infeasible when 4MBC is adsorbed on the gold electrode surface. It is conceivable that a bridge-bound species is more preferable when thesurface adsorption occurs through the nitrogen lone pair electrons. Since 4MBC is concluded already to adsorb on the gold surface via the r system of the cyano group, the peak near 21 00 cm-I in Figure 3 is presumed to have nothing to do with the bridge-bound species. The bipolar peak a t 2100 cm-‘ in Figure 3 could arise from an electrochemical reaction product. For the justification of such an assignment, it was intended to examine the cyclovoltammetric behavior of 4MBC at the gold electrode. However, Faradaic current was not detectable in the potential range of -0.8 to 0.4 V. Nonetheless, we prefer to assign the 2100 cm-1 peak in Figure 3 to cyanide species formed by the NC-CHI bond scission at the gold electrode. This has been inferred from the similar characteristics of the bipolar peaks observable, respectively, for CN- and 4MBC at the gold electrode. The peak position and its potential dependence in the PDIR spectra of 4MBC were much the same as those in the PDIR spectra of CN-. At this moment, it will be useful to mention that the NC-CH2 bond scission can also occur at the platinum electrode. Namely, in a separate SNIFTIRS study at a platinum electrode, we could identify peaks due to CN- adsorbed on Pt as well as its oxidation product, C 0 2 , dissolved in bulk as suggested by Paulissen and Korzeniewski.38 Furthermore, it should be mentioned that both the benzyl cation and benzyl radical are reported to be fairly stable due to their electron delocalization ~ a p a b i l i t y .In ~ ~addition, the aforementioned surface reaction seems to occur also in a self-assembling process on gold. That is, the very weak peak a t 2124 cm-1 in Figure l a can be attributed to the linearly adsorbed CN- species that has been produced by the decomposition of 4MBC on the gold substrate. In the SNIFTIRS study of CN- on the gold electrode, two different kinds of peaks were identified.36 One peak appearing a t 2106 cm-’ at-1 .OV was observed toshift linearly with potential with a slope of 30 cm-I/V, and the other appearing at 2146 cm-1 was insensitive to the applied potential. The former was assigned to the cyanide species linearly adsorbed on the gold surface but the latter to the electrochemical reaction product, AU(CN)~--, present in the bulk phase.@ The lack of signal from a Au(CN)zspecies in Figure 3 suggests that all the cyanide species formed from 4MBC must exist on the surface. The other surface reaction product, i.e., methoxybenzyl radical, is thought, on the other hand, to diffuse out into the bulk medium, and to react further to form other species like 4-methoxybenzyl alcohol. The lack of such species in Figure 3 can be understood by assuming that, regardless of electrode potential, a certain amount of decomposition occurs immediately after the initial adsorption. In that case,
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Figure 4. SNIFTIR spectra of 4MBC at the silver electrode. Spectrum (a) corresponds to what was obtained with the reference potential at -0.8 Vand the sample potential at -0.4 Vvs SCE. Other spectra were obtained altogether with the reference potential at -0.4 V, along with sample potentials at (b) -0.2, (c) 0.0, and (d) 0.1 V vs SCE. their corresponding peaks will not appear in the PDIR spectra even with s-polarized light, as observed. This view agrees, in fact, with what was observed in the self-assembled film as mentioned at the end of previous paragraph. The other bipolar peak around 2200 cm-I in Figure 3 was attributed already to u(CN) for 4MBC adsorbed on the gold surface. Its blue shift with increasing potential, Le. the peak appearing at 2200,2214,2216,2220,2226,2232, and 2234 cm-I, respectively, a t -0.8, -0.6, -0.4, -0.2, 0.0, 0.2, and 0.4 V, is attributed to the diminuation of metal-to-** (CN) electron backdonation. The observation that no other peaks due to 4MBC appear in Figure 3 indicates that the orientation of adsorbed 4MBC changes only slightly with the applied potential. Namely, regardless of electrode potential, the adsorbate seems to retain almost the same geometry as that shown in Figure 2b. The symmetrical shape of the 2200 cm-1 bipolar peak suggests, on the other hand, that the surface coverage is essentially independent of the potential change. The potential change can thus be concluded to affect only the strength of the CEN bond in adsorbed 4MBC. As shown in Figure 4, a remarkably different PDIR spectral pattern was observed at the silver electrode. Various in-plane benzene ring modes appeared distinctly in this case. Besides, in the CEN stretching region, only one bipolar peak appeared, in contrast with thecaseofthegoldelectrode. The bandattributable to the u(CN) mode of adsorbed 4MBC appeared at 2182,2205, 2216, and 2222 cm-I, respectively, at -0.8, -0.4, -0.2, and 0 V. The band shifted linearly with potential with a slope of ca. 37 cm-l/V. From the similarity of u(CN) peak positions between the IRA spectrum (Figure lb) and the PDIR spectrum (Figure 4), 4MBC is concluded also to bind to the silver electrode via the T system of the cyano group.
Son et al. At this point, it should be mentioned that no peak due to CNis observable in the SERS of 4MBC in a silver sol even though the adsorption strength of CN- is far greater than that of 4MBC.10 This supports the interpretation that the bipolar peak observable at 2100 cm-I in Figure 3 has nothing to do with any chemical contamination. Thecompleteabsenceof a u(CN) band assignable to CN- in Figures 1b and 4 suggests that the decomposition of 4MBC is minimal on the silver surface. However, it is not apparent why the surface reaction is possible at the gold surface but infeasible at the silver surface. It is interesting that ring vibrational bands appear in the PDIR spectra obtained at the silver electrode, in contrast to the case of the gold electrode. The appearance of ring modes can be attributed to either the change in surface coverage or the change in adsorbate orientation, accompanied by the electrode potential variation. The peak positions of the ring modes in Figure 4 were not exactly the same as those in Figure IC. If the surface coverage depends on the applied potential, the concentration of bulk phase near the electrode will change accordingly. In that case, peaks due to bulk phase will be observable in the PDIR spectrum. Since the relative peak intensities in the PDIR spectrum are, however, very different from those in the neat transmission spectrum, the benzene ring vibrational bands such as those appearing at 1259 and 1518 cm-1 in Figure 4 are thought to have nothing to do with the coverage change. Also, from the fact that the IRA spectral pattern for the self-assembled film is insensitive to the surface coverage, it is tempting to attribute the appearance of ring modes in Figure 4 entirely to the orientational change. As can be seen from Figure 4, the spectral intensities of C=N stretching and ring vibrational bands become altogether weakened as the electrode potential increases. This seems to indicate that the benzene ring assumes a more flat stance with respect to the surface with increases in the potential. That is, the adsorbate is thought to assume a more flat geometry by rotating through the NC-CH2 and HzC-CsHd bond axes as the electrode potential increases. Since the positions of ring modes depend only slightly on the applied potential, however, the possibility of direct interaction between the benzene ring and the metal surface looks, however, very scarce. If a direct interaction was possible, a 10 cm-I or more red shift might have occurred for the ring modes upon the potential change. In our previous SERS study in a silver sol, 4MBC was concluded to adsorb on the silver surface via its C N ?r system, based on the appearance of the C=N stretching band at 2204 cm-1.10 When BHd-ion was present in the silver sol, two peaks appeared distinctly in the CEN stretching region, namely at 2204 and 2263 cm-1, in the SER spectrum. Since the position of the latter peak was higher than that of the free species, the species responsible for the 2263 cm-I peak was attributed to 4MBC adsorbed on silver through the nitrogen lone pair electrons. Hence, two different kinds of adsorbates were assumed toco-exist on the silver surface. Since the sol potential measured by the potential of a silver electrode dipped into the sol medium varied as a function of time, the adsorption behavior of 4MBC was conjectured to depend on the surface potential. Namely, 4MBC was concluded to adsorb on silver via the C s N a system at low sol potential and to change to a perpendicular stance as the potential was increased. This conclusion contrasts with that made in the present work. It may be difficult to rationalize the different views on the adsorption mechanism of 4MBC on silver. Nonetheless, we prefer the conclusion made in this work rather than that made from the SERS study. This is based on two independent observations. In the first, it has to be noticed that in the SER spectra the positions of two v(CN) bands were invariant although their relative intensities changed as the sol potential was increased. This contrasts with thegeneral expectation that the peak position should change gradually along with the potential change. Hence, the two SERS peaks seem not to be directly related to the potential
Adsorption of 4MBC on Silver and Gold Surfaces change. In a separate experiment, it has been concluded that 4MBC interacts with Ag+ ions via the nitrogen lone pair electrons since the v(CN) band is observed distinctly at 2275 cm-l in both the infrared and Raman spectra of aqueous solution of 4MBC containing AgNO3. Although not exactly the same, the peak position in the latter spectra is close to that (2263 cm-l) observable in the SER spectra. Hence, the role of BHd- ions in silver sol is thought not only to change the sol potential but also to induce the silver colloidal particles to possess partial positive charges. Considering that the average charge per silver atom in the aggregated silver sol should be different from that of free ions, such a marginal frequency difference, Le., 12 cm-I between 2263 and 2275 cm-1, may not be unexpected. For a more definite explanation, the SERS of 4MBC at the silver electrode needs to be investigated. Unfortunately, the corresponding SER spectra could not be obtained at the moment. In summary, 4MBC has been concluded from an infrared spectroscopic study to adsorb on the silver surface via its C=N .rr system, as is likely in the SERS study. The molecule was adsorbed onto the gold surface also via the C s N .rr system. In contrast to the case of SERS, invaluable structural information on the adsorbate could be extracted from the infrared spectral pattern owing to its well-established selection rule. The benzene ring of 4MBC on gold was thus determined to assume a more perpendicular stance with respect to the surface than that on silver. Furthermore, the adsorbate structure on silver could be concluded to change greatly upon the potential variation while on gold it was insensitive to the applied potential. From this work, one can be convinced of the usefulness of infrared spectroscopy with regard to obtaining information on the nature of chemical species adsorbed on metal surfaces. Infrared data is surely indispensable for a better interpretation of SERS spectral patterns. It is hoped that an unequivocal SERS selection rule could be established by referring to the infrared spectroscopic data. Acknowledgment. This work was supported by the Specified Basic Research Fund, Korea Scienceand Engineering Foundation (Grant 92-25-00-06) and by the Non Directed Research Fund, Korea Research Foundation, 1993. We acknowledge also the Ministry of Education, Republic of Korea, for supporting the participants of the present work through the Basic Science Research Program, 1994. References and Notes (1) Albert, M. R.; Yates, J. T. Jr. The Surface Scientist’s Guide to Organometallic Chemistry; American Chemical Society: Washington, DC, 1987. (2) Rappoport, Z . The Chemistry of Cyano Group; Interscience: New York, 1970. (3) March, N. H. Chemical Bonds Outside Metal Surfaces; Plenum: New York, 1986.
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