508
Langmuir 1991, 7, 508-513
Direct Comparison of the Chemical Properties of Single Crystal Ag( 11 1) and Electrochemically Roughened Ag as Substrates for Surface Raman Scattering S. Byahut and T . E. Furtak' Physics Department, Colorado School of Mines, Golden, Colorado 80401 Received August 10, 1990. In Final Form: September 20, 1990 Surface plasmon assisted Raman scattering spectroscopy has been used to characterize flat single crystal silver under electrochemical control. By monitoring the vibrational modes of p-nitrosodimethylaniline (pNDMA), we show that a Ag(ll1) surface grown on a mica substrate is devoid of active sites that are responsible for the electronic component of enhancement in surface enhanced Raman scattering (SERS). This surface has been used as a reference condition in a study of roughening by oxidation-reduction cycles (ORC) in C1- containing solutions. We show that active sites, which are produced with as little as one monolayer of Ag restructuring, and which are associated with more intense Raman scattering from pNDMA, have a higher electron accepting character than sites on Ag(ll1). We provide direct evidence for the adatoms or small clusters being the active sites. On the basis of the spectra obtained on a single crystal and electrochemically roughened surfaces, we discuss the role of the adsorbate's excited-state stability in the determination of the lineshape of the observed spectra.
Introduction Surface enhanced Raman scattering (SERS) has proven to be a veryuseful tool to identify and study the vibrational spectrum of adsorbates and is routinely being used to characterize metal-adsorbate Since its discovery in 1974 by Fleishmann et al.,4 many theoretical and experimental efforts have been directed toward understanding its nature and clarifying the underlying mechanisms giving rise to the phenomen0n.'-3+5-'~Despite these efforts, a complete quantitative understanding has not yet been reached.' The current consensus is that there are at least two different mechanisms contributing simultaneously on most surfaces-electromagnetic and e l e c t r ~ n i c . ~ -In ~ Jthe ~ first case, enhancement of the Raman scattered intensity arises because of the large enhancement of the electromagnetic (both incident laser and the Raman scattered) fields on the surface." Large scale surface features and special experimental geometries (such as surface plasmon excitation on gratings or in thin films) are responsible for this type of enhancement. Calculated fields and Raman scattered intensities on such surfaces quantitatively agree with the experimental results with uncertainty introduced by statistical distribution of the surface roughness features. The second enhancement mechanism involves electronic resonance due to charge transfer excitation (CTE) associated with chemisorption.2 It has been suggested by Otto12 and Ueba'3 that atomic (1)Surface Enhanced Raman Scattering; Chang, R. K., Furtak, T. E., Eds.; Plenum: New York, 1982. (2) Otto, A. In Light Scattering in Solids; Cardona, M., Guntherodt, G., Eds.; Springer: Berlin, 1984; Vol. IV. (3)Moskovits, M. Reo. Mod. Phys. 1985,57(3),783. (4)Fleishmann, M.; Hendra, P. J.; McQuillan, A. J. Chem.Phys. Lett. 1974,26,163. (5)Jeanmaire, D. L.; Van Duyne, R. P.J. Electroanal. Chem.Interfacial Electrochem. 1977,84,1. ( 6 ) Albrecht, M. G.; Creighton, J. A. J . Am. Chem. SOC.1977,99,5215. (7)Chang, R. K.; Laube, L. In CRC Critical Reuiews in Solid State and Materials Science; Chemical Rubber Co.: Boca Raton, FL, 1984, Vol. 12,pp 1-73. (8)Metiu, H.Prog. Surf. Sci. 1984,17,153. (9)Chang, R. K. In Proceedings of the International Conference on Ellipsometry and Other Optical Methods of Surface and Thin Film Analysis; Paris, 1983. (10)Furtak, T.E.J . Electroanal. Chem.InterfacialElectrochem. 1983, 150, 375. (11)Wang, D.-S.; Kerker, M. Phys. Reo. B 1981,24, 1777. (12)Billmann, J.; Kovacs, G.; Otto, A. Surf. Sci. 1980,92,153.
scale defect sites are more important in creating effective conditions for the charge transfer excitations than are large scale roughness features. There are two main drawbacks of SERS that have prevented it from becoming quantitative and predictive. SERS has been traditionally associated with surface roughness. In addition, the SERS intensity strongly depends on the nature of the active sites which, in turn, depend on the surface preparation methods. The second drawback is that several enhancement mechanisms work a t the same time. It is difficult to isolate them and investigate their respective contributions independently. Surface vibrational spectroscopies provide information on the changes in the vibrational properties of the adsorbate brought about by the presence of the surface.14 From the knowledge of the shifts in peak positions and line width broadening, adsorption sites and geometries can be inferred which provide information about the dynamical interactions between the adsorbate and the substrate and between adsorbates themselves.15J6 This is possible only when the experiments are performed on a well-defined single crystal surface, free of site inhomogeneity. The microscopic and macroscopic features and their influence on SERS intensity are important both for the electronic and electromagnetic enhancement mechanisms. The experiments in SERS point toward the existence of special sites-active sites-on the surface, which are atomic scale roughness features (or defects). Their contribution on the overall Raman intensity is much more than the molecules adsorbed at other sites.2 Moskovits estimates their size to be no bigger than five or six atoms.3 In AgKC1 systems Ag-adatom complexes proposed by Roy and Furtak are Ag4+ clusters stabilized by C1- ions." Several research groups have concentrated their efforts on investigating the exact nature of the surface that produces enhancement. In most experiments under electrochemical control, the starting point was a poly(13)Ueba, H. Surf. Sci. 1983,129,L267. (14)Gadzuk, J. W. In Vibrational Spectroscopy of Molecules on Surfaces; Yates, J. T., Madey, T. E., Eds.; Plenum: New York, 1987;pp 50-51. (15)Gadzuk, J. W.;Luntz, A. C. Surf. Sci. 1984,144,429. (16)Ueba, H.Prog. Surf. Sci. 1986,22, 181. (17)Roy, D.; Furtak, T. E. J. Chem. Phys. 1984,81,4168.
0 1991 American Chemical Society
Comparison of Ag(111) and Roughened Ag for SERS
crystalline surface, which was then roughened by using an oxidation-reduction cycle (ORC) and investigated for SERS. These surfaces were later investigated by SEM and other techniques.18-20 Only large scale roughness features could be detected because of the involved procedure and experimental resolution. A totally different approach was taken by Campion and his group and some other groups, who used single-crystal (both low and high index) surfaces to investigate the role of surface defects in the electronic enhancement. They did not find any enhancement (except for the factor of 4 due to the reflection from the metallic surface) and concluded that kinks and steps do not contribute to the e n h a n ~ e m e n t . ~ lThere -~~ has been no attempt, to our knowledge, in which an atomically flat, well-characterized surface was used as a substrate to start with; an unenhanced Raman signal from the surface was observed and then, with the same sample, surface roughness features of different scale were added on and investigated for active sites. Recently, we have developed an efficient device, in which the “Kretschmann geometry” provides the enhancement of the surface electromagnetic fields via surface plasmon polariton excitation. We have used it to obtain surface plasmon assisted Raman scattering (SPARS) spectra from a smooth or single crystal surface.24This device, combined with electrochemical control, gives a unique way to do surface Raman spectroscopy on a single crystal or a smooth surface. The single crystal surface was then gradually roughened to produce atomic scale defects. In this paper, we report the Raman investigation of a Ag(ll1) surface usingp-nitrosodimethylaniline (pNDMA) as a probe molecule. This molecule shows a well documented sensitivity to its chemical e n ~ i r o n m e n t . ’ ~By ,~~ monitoring the changes in the vibrational modes of the molecule when a Ag(ll1) surface is modified, we show that there are no active sites on a single crystal surface. Extended surface defects such as dislocations and domain boundaries, which exist an epitaxially grown Ag(lll), do not provide electronic enhancement. Further, by analyzing the electrochemical stripping and deposition process on a silver surface, we present direct evidence indicating that the adatoms are the active sites. An analysis of the lineshapes of different modes shows that the narrowing of the phenyl-nitroso mode of pNDMA on an electrochemically roughened surface is connected to the stabilization of the excited state of the molecule upon adsorption on an active site.
Background A SERS-active surface has been loosely defined by Otto as “any metal surface for which surface enhancement of Raman scattering is observed”.2 This definition is not precise because it is necessary to separate “surface features”, which give rise to the electromagnetic enhancement and are always present on a rough surface, and “sites”, which participate specifically in the electronic enhancement. Its importance can be appreciated from the fact that a SERS-active surface may show enhancement for some adsorbates and not for the others. The atomic scale roughness associated with the electronic part of the (18)Bryant, Mark, A.; Pemberton, Jeanne E. Langmuir 1990,6,751. (19) Jaya, S.; Rao, T. Prasada; Rao, G. Prabhakara J. Appl. Elec-
trochem. 1987, 17,635. (20) Devine, T. M.; Furtak, T. E.; Macomber, S. H. J. Electroanal. Chem. Interfacial Electrochem. 1984, 164, 299. (21) Campion, A.; Mullin, D. R. Chem. Phys. Lett. 1983, 54, 576. (22) Campion, A.; Mullin, D. R. Surf. Sci. 1985, 158, 263. (23) Campion, A. J. Vac. Sci. Technol., B 1985, 3 (5), 1404. (24) Byahut, S.; Furtak, T. E. Reu. Sci. Instrum. 1990, 62, 27. (25) Brazdil, J. F.; Yeager, E. B. J. Phys. Chem. 1981,85, 995.
Langmuir, Vol. 7, No. 3, 1991 509
enhancement in SERS is often called “SERS-active site” or simply “active site”. We will adopt this definition. There are three main criteria that have been used to investigate the active sites on a s u r f a ~ e : ~(1) J ~chemical specificity; (2) charge transfer excitation effects; and (3) reversible and irreversible effects in the observed surface Raman intensity. A SERS-active site has chemical specificity. Cooperative adsorption and enhancement of the Raman signal of both metal-ligand and intraligand modes is another important aspect of this phenomenon. It is connected with the dynamic charge transfer excitation between the metal and the adsorbate. This charge transfer involves the excitation of an electron from below the Fermi surface to one of the unoccupied states of the adsorbed molecule.2 An opposite process of charge transfer from the adsorbate to the metal may also take place. In this respect, electrochemical experiments offer some distinct advantage over UHV experiments. With a fixed energy of the incident photons, the CTE can be tuned in and out by varying the applied ~ o t e n t i a l . ~Therefore, J~ CTE provides a suitable means to distinguish between the electromagnetic and the electronic effects. The lifetime of the charge transfer states determines the magnitude of enhancement.2 The strength of the enhancement is greater when the initial states are confined to a narrow energy range. Therefore, small changes in the configurations of adsorbed molecules and the presence of other neighbors could lead to drastic changes in the enhancement. Active sites can be created by roughening the surface and they can be destroyed by raising the temperature of the surface or by applying a negative charge to Once destroyed, they do not contribute to the Raman intensity. These tests lead to irreversible effects in SERS and can be used to investigate the presence of the active sites on a surface.I7 In addition to influencing the SERS intensity, the nature of the active site is also related to the vibrational signature of the probe molecule. A molecule, which shows a wellbehaved sensitivity to changes in its environment through changes in its vibrational frequencies, line widths, and relative intensities of different modes, can be used to characterize the nature of the surface itself. Bond formation between a probe molecule and an active site may be influenced by the electron accepting (Lewis acidic) or donating (Lewis base) character of the site. This frequently leads to electron redistribution within the molecule itself and, consequently, frequencies and intensities of vibrational modes are affected by it.25 The degree of acidity of an adsorption site is dictated by the amount of influence exerted by the coadsorbed anions in the electrolyte. Adsorption and desorption of anions may lead to reduced or increased acidity and stabilization or destabilization of the active site.17 All these processes can be controlled electrochemically. Thus, it is possible to study the detailed character of the active sites by carefully studying the vibrational spectra provided by such molecules. p-Nitrosodimethylaniline (pNDMA) is such a molecule. It has two resonance forms (Figure l),which has been attributed by Tsuzuki et al. to T-T* transitions involving considerable intramolecular charge transfer from the amino nitrogen to the nitroso oxygen.27 In a polar environment, a large stabilization of the excited state of the molecule is realized due to the increased dipole moment caused by the allowed electronic transitions. The overall effect is that the basicity of the nitroso oxygen increases (26) Macomber, S.; Furtak, T. E. Solid State Commun. 1983,45,267. (27) Tsuzuki, Y.; Vemura, T.; Hirasawa Ber. Dtsch. Chem. Ges. 1941, 74, 616.
Byahut and Furtak
510 Langmuir, Vol. 7, No. 3, 1991
Figure 1. Unpolarized (left) and polarized (right) forms of
eV incident photon energy and that excited state charge transfer occurred from the molecule to the metal. As the potential was made more negative, the intensity was observed to drop dramatically but recovered only partially when the potential was brought back to more positive values, showing the irreversible effect. The extent of the loss of the Raman intensity depended upon the extreme value of the negative potential. Thus, pNDMA satisfies the three criteria required to investigate the presence of the active sites: it shows sensitivity to the chemical environment; when adsorbed on a SERS active site in the electrochemical environment, it shows reversible and irreversible effects, and it shows sensitivity of the SERS intensity to the applied voltage indicating a CTE effect.
and a significant charge redistribution occurs in the benzene ring. This is manifested by resonant absorption of light and the absorption peak shifts to the red with increasing polari~ation.~5 This is also accompanied by characteristic changes in the Raman spectrum, which can be used to quantify the relative acidity of the adsorption site.25 Normal Raman,%surface resonant RamaqZ5and SERS17 spectra of pNDMA in different environments have been investigated thoroughly. There are four important vibrational modes that serve as indicators of the extent of polarity of its chemical environment: (A) the symmetric stretch mode of the benzene ring at -1613 cm-l; (B) symmetric deformation of the dimethyl amino group at 1400 cm-'; (C) the phenyl-nitroso stretch mode a t 1165 cm-'; and (D) nitroso stretch mode at 1335 cm-'. The basic trend, as this molecule transforms from unpolarized to polarized form, is as follows (Figure l).17125 1. The bond order between the phenyl and nitroso groups changes from single to double. The vibrational frequency of this mode (mode C ) should increase with increasing polarity. 2. The nitroso bond order changes from double to single. The frequency of this mode (mode D) should decrease with increasing polarity. 3. The electron distribution in the benzene ring changes. This increases the dipole moment of the molecule and causes a considerable change in the scattering intensity of mode A. 4. The electron distribution in the bonds of the dimethylamino group does not change. Therefore, the scattering intensity of mode B remains unchanged. Brazdil and Yeager have established two independent measures for the extent of polarization: the relative peak positions of modes C and D, and the intensity ratio of modes A and B.25 The first criterion is not valid in the case of Raman scattering from surfaces because the surface induces shifts in the vibrational modes of adsorbates due to several reasons other than surface acidity.28 Hence, we will use only the intensity ratio of modes A and B as a measure of the extent of polarization in the presence of a surface. pNDMA has been used as a probe to characterize an ORC roughened Ag surface and to investigate the nature of the active site by Roy and Furtak.17 In their experiment, in which 150monolayers of silver surface were re-formed, they reported a CTE resonance at -0.3 V vs SCE for 2.41
Experimental Section The device for the SPARS spectroscopy has been described elsewhere.24 It involves an internal reflection element (hemispherical prism) to couple the incoming and outgoing radiation to surfaceplasmon polaritons (SPP). For p-polarized light, SPP's are excited at a definite value of the incident angle determined by the dispersion relation for them in the prism-metal film (silver, thickness 500 &dielectric (airor water) configurationinvolved in the Kretschmann geometry.8 The resulting large electromagnetic fields stimulate Raman transitions in the molecule adsorbed at the dielectric side of the metal. Since the Raman emission process is not coherent with the incoming light, the outgoing SPP's travel in all directions parallel to the surface. The backcoupled light within the prism follows a cone whose apex angle is associated with the SPP-light coupling condition at the Raman shifted frequency. The maximum enhancement of the Raman scattered light is of the order of 10'. In the SPARS device, a parabolic mirror has been used to collect the Raman scattered light and focus it to the entrance slit of the monochromator. (For the details of the theory, construction, and operation of the device, see refs 24 and 28.) Silver single crystal films (thickness 500 A) were grown by electron beam evaporation of 5 N silver on mica substrates in a diffusion pumped vacuum chamber at a base pressure of 4 X 10" Torr and subsequently annealed at 250 "C for -20 min. The orientation of the films was checked by 8-28 X-ray diffraction and found to be [lll]. The scanning tunneling micrograph of the films showed an atomic scale smoothness with domains of crystallinity of -500 A. The polycrystalline silver sample was a disk micropolished successively with 1-, 0.3-, and 0.05-pm alumina and ultrasonically cleaned in triply distilled water. The silver surface was electrochemicallycontrolled by a potentiostat (RDE4, Pine Instruments). The Raman spectra were obtained by using a Triplemate monochromator (Spex) and a cooled intensified photodiode array (1420R, Princeton Applied Research). In all experiments, a 2 x lo4 M solution of pNDMA in triply distilled water was used. The above solution was deaerated with ultrazero grade nitrogen for -20 min and pumped into the electrochemical cell. The optics were optimized by observing the spectrum of pNDMA in the range 1000-1700 cm-l. Whenever electrochemical control over the surface was required, C1- or Fwas used separately as electrolytes by adding NaCl or NaF in adequate amount in the solution to get 0.025 and 0.05 M, respectively. In those cases, the solutions were first purged with ultrazero grade nitrogen for -20 min and pumped into the electrochemical cell and an open circuit (no electrochemical control) spectrum was taken in each case. Thereafter, electrochemical control was established and several spectra were obtained at the values of the applied potential between +0.085 and -0.5 V (vs SCE). Care was taken not to apply potentials more positive than 85 mV as stripping of silver would begin in that case. Special attention was paid to the region of applied potential near -0.3 V as CTE resonance has been reported to occur at this voltage in the ORC roughened Ag-pNDMA systems.'7
(28) For a detailed discussion, see Byahut, S. Ph.D. Thesis, Physics Department, Colorado School of Mines, Golden, CO, 1990.
Surface Plasmons;
II
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pNDMA molecule. The transformation of the molecule from unpolarized to the polarized form changes the electron distribution in the benzene ring (A), has no effect on the dimethyl group (B),and changes the character of the phenyl-nitroso (C) and nitroso (D) bonds. (See text for details.)
-
-
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(29) For a detailed discussion of surface plasmons, see Raether, H., Springer-Verlag: Berlin, 1988.
Comparison of Ag(ll1) and Roughened Ag for SERS 11
Langmuir, Vol. 7, No. 3, 1991 511
,
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Figure 2. SPARS of pNDMA from Ag(ll1) film grown on mica with NaF as electrolyte at Vapp1=(a) 0, (b) -0.3, (c) -0.4, and (d) 0 V. The signal is almost unobservable at V,, 1 = -0.4 V and almost completely recovers when the applied vojtage is brought back to 0 V. To investigate the role of surface defects, the Ag( 111)electrode was gradually roughened by allowing the electrode potential each time to cycle to more positive values, thus stripping silver from the surface and redepositing it during the ORC. The amount of redeposited charge was determined by coulometry and then converted to an equivalent number of layers of Ag by using the densityof bulk Ag. Roughening by this procedure is the standard way of achieving an "active" surface for enhanced Raman spectroscopy. It has been shown through scanning tunneling microscopythat this leads to uniformly random ro~ghness.3~ Raman spectra were acquired at 0,2,7,9,13, 19,33,42,50,and 59 monolayers of the re-formed surface. The spectra obtained from the roughened surface showed maximum intensity at V,,,l = -0.3 V. The film cracked and peeled off after 59 monolayers, and it was not possible to continue the experiment. To investigate the role of large scale roughness features vs atomic scale features, we repeated the experiment first done by Roy and Furtakl' on a polycrystalline silver surface in a conventional SERS geometry and using pNDMA as a probe molecule. To compare with the resultsobtainedon a single crystal surface, a spectrum of pNDMA from the surface was obtained without C1- in the solution. Then, an adequate amount of NaCl was added to make the concentration 0.025 M. Silver deposition on the polycrystalline surface was comparable to the deposition on the single crystal surface. The spectra were obtained at V,,,l = -0.3 V.
Results and Discussion The electronic part of the SERS enhancement is intimately connected with the existence of the SERS-active sites on the silver surface, which manifest themselves through a large increase in Raman intensity due primarily t o the CTE.2J7 We demonstrate here that these are absent on Ag(ll1). Figure 2 shows the Raman intensity of different modes of pNDMA a t V,,l = 0, -0.3, -0.4, and again a t 0 V on flat Ag(ll1) in the SPARS geometry with NaF as electrolyte. The intensity of 1613 cm-l mode vs applied potential is plotted in Figure 3. The intensity of all modes decreased as the potential was scanned to more negative values. This is related to the desorption of the pNDMA molecules from the surface. As the potential was scanned back to 0 V, the signal was recovered almost fully. The spectra obtained from a single crystal silver surface using the SPARS device have a very low, essentially featureless background. We did not see the large background always present in SERS and associated with a rough surface (also see ref 24). Thus, the absence of any resonance related to the charge transfer excitation and (30) Sakamaki, K.; Itoh, K.; Fujishima, A.; Gohshi, Y. J. Vac. Sci. Technol. 1990,8 (l),525.
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Raman S h i f t (cm-') Figure 4. SPARS spectra of pNDMA from Ag(l1l) grown on mica after gradual roughening of the Ag surface. Spectra were taken after (a) 0, (b) 19, (c) 42, and (d) 59 monolayers of the surface were re-formed. Applied voltage in each case was -0.3 V.
the reversible nature of the Raman intensity confirms that there are no active sites on the Ag(ll1) surface. Several groups have studied the surface morphology of Ag films grown under conditions similar t o ours. They report a very good orientation of the films in the [ l l l ] direction with domains of crystallinity -500 A.31932 A SEM study by Pascard e t a1.31reports dislocations and twinning as surface defects. Similarly, a LEED study by Welkie e t al.32 reports dislocations and grain boundary (domain boundary) as the only surface defect. They did not find any steps on the surface. A STM study in our laboratory showed a very flat (rms height variation -3 A) surface with domain size -500 A. It did not show any steps in the domain of crystallinity.28 The variation in height may be due to isolate kink sites. The results of the SPARS s t u d y given above show t h a t extended surface defects-dislocations and grain boundaries ending a t the surface-and twinning are not the active sites. This observation is also in tune with the results of Mullin and Campion, where they did not find enhancement on kinks and ~ t e p s . ~ l - ~ ~ Figure 4 shows the pNDMA spectra after a certain number of monolayers were re-formed. These spectra were taken a t V = -0.3 V as the Raman signal of pNDMA gave a maximum on an electrochemically roughened surface. There is a very small, barely recognizable signal a t 0 monolayer after C1- was added to the solution (Figure 4a). Most anions in the solution compete favorably with pNDMA (31) Pascard,H.;Quintana, C.;Hoffmann,F.;Sella, C.J.Cryst. Growth 1972, 13/14, 225. (32) Welkie, D. G.; Legally. M. G.: Palmer. R. L. J.Vac. Sci. Technol. 1980, 17, 453.
512 Langmuir, Vol. 7, No. 3, 1991
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for adsorption sites.33 The C1- used in the solution displaced the adsorbed pNDMA from the surface, which leads to a weak Raman signal even with the aid of the SPARS device. Also, specifically adsorbed C1- decreased the electron-accepting character of the surface. As a consequence the intensity ratio of the ring stretch and phenyl-nitroso modes decreased. As the surface was gradually roughened, the overall magnitude of the signal increased. The Raman intensity is plotted against the number of re-formed monolayers in Figure 5. It has contributions from two competing sources: (1)the surface plasmon excitation whose efficiency decreases with increasing roughness due to shift in the surface plasmon optimum coupling angle and radiation losses and (2) active sites. Thus, the Raman intensity is expected first to increase due to creation of active sites and then decrease when deteriorating surface plasmon coupling efficiency and increasing radiation losses take over. The AgCl formation during oxidation and deposition of Ag during reduction cycles plays a very important role in creating the active sites. Figure 6 shows the Raman intensity as a function of the number of re-formed monolayers on a polycrystalline surface in a conventional SERS geometry. The intensity, which is proportional to the number of active sites, is linear with the number of re-formed monolayers. Both electrochemists and SERS spectrocopists have extensively studied the process of nucleation and growth of AgCl and Ag with a goal to reveal the underlying mechanisms which determine the microscopic surface morphology after an oxidation reduction cy~le.~"3' The (33) Popp, Carl J.; Ragsdale, Ronald 0.Inorg. Chem. 1968, 7, 1845. (34) Katan, T.; Szapak, S.;Bennion, Douglas N. J.Electrochem. SOC. 1974, 121, 757. (35) Tu, Ying-Yi; Blakely, J. M. Surf. Sci. 1979,85, 276. (36) Birss, V. I.; Smith, C. K. Electrochim. Acta 1987, 32 (2), 259. (37) Krebs, W. M.; Roe, D. K. J.Electrochem. SOC.1967,114 (9), 892.
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and Benni0n3~suggested a mechanism by which sites of high potential energy, such as dislocations or grain boundaries, were attacked electrochemically (in a C1environment) during the oxidation cycle and Ag was deposited at nucleation sites. These nuclei subsequently grew and coalesced during reduction by several diffusion mechanisms. In another study by T u and B l a k e l ~on ,~~ a single crystal silver surface, a AgCl(111) layer formation was shown to occur. Birss and Smith36 gave a detailed account of their investigation on initial stages of AgCl formation and its reduction on a polycrystalline silver surface. The initial formation of AgCl was in a noncontinuous, two-dimensional layer. The specific locations at which AgCl grew at this stage were not identified. At later stages, they showed that three-dimensional nucleation and growth occurred when more positive potentials were attempted. In the cathodic sweep, the reduction of AgCl occurred in two stages, which was manifested by the appearance of two peaks in the voltammogram. One of the peaks, which Birss and Smith linked to the creation of active sites in SERS (peak c2 in ref 36),was shown to be particularly sensitive to the conditions under which the ORC proceeded. It was small at low sweep rates (it appears as a shoulder in our voltammogram, not shown) and developed as a prominent peak only when a large number of scans at larger scan rates were performed. It is this peak that is specifically connected to the silver chloride nuclei formation during the anodic scan and to the deposition of Ag adatoms during the cathodic scan. These adatoms seem to be a particularly active (photochemically) form of silver. When the potential is maintained at a large negative value, adatoms are "deactivated" and peak c2 disappears. Similar time-dependent and potential-dependent behavior has also been shown to occur in SERS.293," The intensity ratio of modes A and B on a single crystal surface without electrolyte is 1.94. It shows that pNDMA is significantly polarized and is in the excited state after adsorption on the Ag(1ll) surface (Figure 1). However there is very little contribution in the Raman intensity from the internal resonances of the molecule because the 2.41-eV laser excitation energy lies just near the tail of the absorption band.25 As the intensity ratio increases, the absorption band maximum moves to lower energy and the contribution from the internal resonance of the molecule increases. We have already noted that anions in the solution displace the pNDMA molecule from the surface and reduce the acidity of the surface.17 The extent to which the two effects are observed depends upon the bulk concentration and the degree of specific adsorption of the anions and the applied potential across the interface. This behavior is reflected in the fact that the Raman signal almost disappears when NaCl is added to the solution at open circuit. The adsorption of pNDMA on active sites, if they are different than other adsorption sites on the surface, must also be reflected in the mode A to mode B intensity ratios. In Figure 7 the intensity ratio is plotted against the number of re-formed monolayers on the silver surface. A t 0 monolayer, the error bar is large because the signal is small. Despite the large error bar the increase in the intensity ratio is distinct for 2 monolayers. This is a clear indication that single-crystal Ag( 111) is chemically different from
Comparison of Ag(ll1) and Roughened Ag for SERS I -2.6
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1
Langmuir, Vol. 7, No. 3, 1991 513
remains below 40. The acidity of the surface is not changed at this stage and the intensity of the Raman signal increases with the number of re-formed monolayers of the surface. Beyond 40 monolayers, the acidity increases, which is associated with the appearance of large scale roughness features. Because the small scale roughness features do not additionally change the acidity of the surface, we conclude that only large scale roughness features produce a chemically different surface. While surface acidity, as manifested by the intensity ratio, is an averaged property of the surface, minute change in the mode frequency and line width are indicative of specific adsorption sites on the surface. Table I compares the peak positions and the line widths of a benzene ring stretch mode at 1613 cm-l (mode A) and phenyl-nitroso stretch mode a t 1163 cm-' (mode C)on different surfaces. While one would except that the line width of a mode would broaden on arough, heterogeneous surface, the phenyl-nitroso mode shows the opposite trend-the lineshape actually narrows. (Brazdil and Yeager found similar broadening behavior of pNDMA on a silica and alumina surface.25) Thus, on a single-crystal surface the line width is 30 cm-1 (same as on a mechanically polished polycrystalline surface) and on an ORC roughened surface it drops down to 21 cm-l. The line width of the 1613-cm-' mode is not affected by the roughening. The other modes are weaker in intensity and overlap. Therefore, their line width changes are harder to determine. The narrowing of the phenyl-nitroso mode shows that (1)the pNDMA molecule is adsorbed via the excess electron density on the nitroso oxygen and (2) the resulting configuration after adsorption on an active site is more ~ t a b 1 e . lAdsorption ~~~~ on an ordinary site (single crystal or polycrystalline) does not produce a stable excited-state configuration. The result is a broader density of states for the nitrosoelectrons. The adsorption of pNDMA on an active site (or Ag cluster stabilized by anions) makes the nitroso density of states narrower and brings the excited states closer to the Fermi level of the silver. Thus, the CTE from a narrow density of states of the molecule to the metal states results in a reduced line width of the relevant phenyl-nitroso mode.
; 1.1
0
10
20
30
40
50
60
70
Number o f M o n o l a y e r s
Figure 7. Intensity ratio as a function of the number of the re-formed monolayers in the SPARS geometry. Table I. Intensity Ratios and Full Width at Half Maximum (fwhm) of the Ring Breathing Mode (A) and the Phenyl-Nitroso Stretch Mode (C) experimental conditions
fwhm(A), fwhm(C), Z(A)/Z(B) cm-I (f2.6) cm-l (f2.6)
Ag(1 ll)/solution SPARS geometry 0 ML,' no electrolyte
1.94 f 0.06
19.6
27.5
AG(1ll)/solution SPARS geometry 42 ML re-formed
2.07 f 0.02
21.4
21.7
Ag/solution 3.14 f 0.1 SERS, conventional optics 0 ML, no electrolyte
21.5
30.2
Ag/solution 2.54 f 0.02 SERS, conventional optics 43 ML re-formed
19.3
22.6
Ag island films/air conventional optics
18.5
21.5
a
2.6 f 0.1
ML, monolayer.
even a mildly roughened surface. The intensity ratio does not change appreciably between 2 and approximately 40 monolayers, after which it again increases. This means that the gradual re-formation of the silver surface takes place in two stages. For the re-formation of the surface below 40 monolayers, the resulting active sites are the same. Beyond 40 monolayers, large scale roughness features appear on the surface and the intensity ratio increases again. An investigation of this effect in conventional SERS shows intensity ratios of 3.14 on a polycrystalline surface (no ORC, no electrolyte) and 2.54 (0.025 M of NaC1, 43 monolayers re-formed). On a Ag island film the intensity ratio is 2.6. These surfaces differ substantially from each other with regard to surface roughness. A single crystal surface has no roughness features and is the least acidic; an island film has roughness on the scale of 300 A and is more acidic; a mechanically polished Ag surface has roughness of the order of 3000 A and is the most acidic. Thus a correlation is clearly seen in the progression from a single crystal to a rough polycrystalline surface. Table I summarizes this trend. The acidity of the surface increases as the scale of the surface roughness features increases. We have identified three different surfaces during the roughening process by ORC. A single-crystal surface has no active site on it. A reformation of the surface by as little as one monolayer creates active sites which make that surface chemically different than a single-crystal surface. Beyond this, additional ORC's create roughness of small scale as long as the number of re-formed monolayers on the surface
Conclusions We have shown that a single-crystal silver surface has no active sites. This type of surface shows no active site enhancement for the Raman process. By performing controlled and gradual roughening of the single crystal surface, we give direct evidence of adatoms on the surface being the active sites. The roughened surfaces show larger Raman scattering but are also chemically different. Our experimental procedure also identifies two distinct stages in the change in morphology of the surface when a single crystal silver surface is electrochemically re-formed. Even a slight roughening creates active sites on the surface and changes its chemical nature. After that, a re-formation up to -40 monolayers does not change its chemical nature. Surface morphology is dominated by the active sites. After 40 monolayers, the morphological changes on the surface are substantial and large scale surface features are created. The narrowing of the phenyl-nitroso mode is explained by adsorption induced narrowing and localization of the nitroso electron density of states in the adsorbed pNDMA molecule. Acknowledgment. This work was supported by the
U.S.Department of Energy through the Office of Basic Energy Sciences, Grant No. DE-FG02-86ER45253.
