1120
J . Phys. Chem. 1987, 91, 1120-1 126
Finally, some general remarks concerning the photodissociation of nitrous acid ( H O N 0 3 ) and dimethylnitrosamine (DMN4) as compared to methyl nitrite (MEN) are in order. In all three cases, excitation of the SI electronic state leads to anisotropic photofragment angular distributions characterized by negative &values. Furthermore, the rotational alignment found in HONO, MEN, and DMN implies that the dissociation is planar, Le., the fragment recoil vectors lie in the 0-N=O or N-N=O plane. In the case of HONO, information was gathered by probing the O H fragment. Approximately 77% of E,,, appears as translational energy. The O H fragment is vibrationally cold and its moderate rotational excitation is thought to originate mainly from vibrational motion of the parent molecule. Although the N O fragment was not probed spectroscopically, Zare and coworkers concluded that the coupling between the 0-N and the N=O bonds should be weak, based on the observation that the translational energy does not depend appreciably on the number of N=O stretching quanta produced in the electronic excitation of HONO. In contrast to the small molecule HONO, DMN exhibits many features characteristic of a larger molecule. The (CH3),N
fragment has many internal degrees of freedom that can absorb variable amounts of energy, giving rise to a rather wide translational energy distribution. Nevertheless, the product distributions reflect a rapid and presumably direct dissociation mechanism, with almost half of the available energy being channeled into translation. As might be expected from the structure of DMN, the N O product shows considerable rotational excitation. Despite of the fact that excitation at 363.5 nm is well above the estimated 0-0 transition, the average vibrational excitation in the N O fragment is rather small. In conclusion, methyl nitrite might in many respects be considered as an intermediate case between HONO and DMN. With regard to the vibrational excitation of the N O product, however, it should rather be likened to its smaller counterpart HONO.
Acknowledgment. Support of this work by the Schweizerischer Nationalfonds zur Forderung der Wissenschaftlichen Forschung is gratefully acknowledged. We thank Mr. R. Pfister for synthesizing methyl nitrite and Messrs. P. Rutz and A. Kuhne for constructing the apparatus. Registry No. CH,ONO, 624-91-9; NO, 10102-43-9
Effects of Solution Conditions on the Surface-Enhanced Raman Scattering of Cyanide Species at Ag Electrodes Diane S. Kellogg and Jeanne E. Pemberton* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 (Received: December 17, 1984)
The effect of solution pH and supporting electrolyte anion on the SERS behavior of cyanide species adsorbed at Ag electrodes is described. Signals for the u(CN) are observed in both acidic (pH 2) and basic (pH 11) media in C104-, CI-, and supporting electrolytes. The frequency and intensity of the u(CN) band are a function of electrode potential in all media. The potential at which the SERS signal reaches a maximum intensity is slightly dependent on the anion of the supporting electrolyte in basic media, but is a strong function of the anion in acidic media. In acid solution, HCN is adsorbed. Two unresolved bands occur in the u(CN) region in acidic media and several possible assignments are proposed. The assignment that best explains all of the experimental data involves both end-bonded and side-bonded HCN. The effect of supporting electrolyte anion on the SERS response is explained in terms of the relative abilities of these anions to influence the self-association of HCN in the interface.
Introduction Surface-enhanced Raman scattering (SERS) has proven to be a very powerful and sensitive tool for obtaining vibrational information from adsorbates in electrochemical environments.' A wide variety of adsorbates have been investigated as probe molecules. Seki has recently compiled an extensive list of atomic and molecular species that have been characterized using S E R S 2 The SERS behavior of CN- has been studied by various researchers for as long as the SERS phenomenon has been recogni~ed.~-IODespite previous efforts expended in an attempt to (1) For a recent review of the SERS phenomenon see: Chang, R. K.; Furtak, T. E., Eds. Surface Enhanced Raman Scattering Plenum: New York, 1982. (2) Seki, H. J . Electron Spectrosc. Relat. Phenom. 1986, 39, 289. (3) Otto, A. Surf.Sci. 1978, 7 5 , L392. (4) Furtak, T. E. Solid State Commun. 1978, 28, 903. (5) Shumilova, N . A.; Zhutaeva, G. V. In Encyclopedia ofElectrochemistry of the Elements; Bard, A. J., Ed.; Marcel Defier: New York, 1978; Vol. 8, p 9. (6) Benner, R. E.; Dornhaus, R.; Chang, R. K.; Laube, B. L. Surf. Sci.
1980, 101, 341.
(7) Mahoney, M. R.; Cooney, R. P. J . Raman Spectrosc. 1981, 11, 141. (8) Fleischmann, M.; Hill, I R.; Pemble, M. E. J . Electroanal. Chem. 1982, 136, 361. (9) Blondeau, G.; Zerbino, J.; Jaffrezic-Renault, N. J . Electroanal, Chem. 1980, I 1 2, 127.
0022-3654187 /2091-1120$01.50/0
understand the SERS behavior of CN- at Ag, effects of the solution pH and anion of the supporting electrolyte on the SERS response of this system have not been fully characterized. This study was undertaken with the intent of elucidating the interfacial chemistry responsible for the SERS behavior of simple cyanide species at Ag electrodes in electrolyte solutions of different pH and supporting electrolyte anion. The anions chosen for study respresent two different classes based on the extent to which they are specifically adsorbed at Ag; and S042-15 C1- is a strongly adsorbed anion,11J2and C104represent weakly adsorbed anions. The interfacial chemistry is complicated in acidic Sod2-media due to the presence of a significant concentration of HS04-. However, Heranyi, Rizmayer, and J o o ' ~ have previously shown that HS04- is also weakly adsorbed at Ag electrodes. I39l4
(10) Billman, J.; Otto, A. Surf.Sci. 1984, 138, 1. (1 1) Weaver, M. J.; Hupp, J. T.; Barz, F.; Gordon, J. G.; Philpott, M. R. J . Electroanal. Chem. 1984, 160, 321. (12) Valette, G.; Hamelin, A,; Parsons, R. 2.Phys. Chem. N . F. 1978, 113, 71. (13) Valette, G. J . Electroanal. Chem. 1981, 122, 285. (14) Valette, G. J . Electroanal. Chem. 1982, 138, 37. (15) Valette, G.; Hamelin, A. C. R . Acad. Sci. (Paris) Ser. C 1974, 279, 295. (16) Heranyi, G.; Rizmayer, E. M.; Joo, P. J . Electroanal. Chem. 1983, 152. 211.
0 1987 American Chemical Societv
SERS of CN- at Ag Electrodes Two pH values were chosen for study based on the acid-base behavior of CN-. Basic conditions arise naturally from the strongly basic nature of CN-. The natural pH of an aqueous 0.05 M CNsolution is ca. 11. The [CN-]/[HCN] ratio of these solutions is 54 as calculated from the pK, of 9.21 of this system.I7 Acidic conditions are achieved by acidifying these solutions to pH 2.0. In these solutions, the [HCN]/[CN-] ratio is lo7.*'. The use of these two divergent pH values provides conditions in which the predominant solution species is in one instance CN- and in the other, HCN. This report details the first successful attempt to characterize the SERS vibrational behavior of H C N adsorbed at Ag surfaces in electrochemical environments. The SERS results provide information regarding interfacial interactions between various solution and surface components as a function of pH and supporting electrolyte anion.
Experimental Section The Raman system used for these experiments has been described previously.I8 p-Polarized radiation at 5145 8, from a Coherent Radiation Innova 90-5 Ar+ laser was the excitation source for all spectra reported here. Laser power at the sample was typically 200-220 mW. Spectral points were taken at either 1.O-cm-] intervals over a 0.5-s integration period for acidic media, or 0.5-cm-I intervals over a 0.5-s integration period for basic media. All spectra presented in this report represent single scans. The spectral band-pass was 5 cm-l for basic media and 7.5 cm-I for acidic media. The spectroelectrochemical cell was of a previously reported design.19 The spatial and angular position of the electrode surface can be reproduced to within f l mm and *lo, respectively, with an x-y-x-0 translator. The polycrystalline Ag (Johnson Matthey, 99.999%) electrode of geometric area of ca. 0.3 cmz was mechanically polished to a mirror finish with successively finer grades of alumina (Buehler) down to 0.05 hm and rinsed with copious amounts of distilled water. A Pt wire housed in a compartment separated from the main body of the SERS cell by a medium porosity glass frit served as the auxiliary electrode. A saturated calomel electrode (SCE) was the reference electrode in all experiments. All potentials are reported as values vs. SCE. Potentials during the ORC were controlled with an IBM Model EC/225 potentiostat which also provides the linear potential ramp. The charge passed during the oxidation-reduction cycle (ORC) was monitored with a Princeton Applied Research Model 379 digital coulometer. Potentials during the acquisition of SERS spectra were controlled with an ECO Model 551 potentiostat. Linear potential ramps or constant offset potentials were applied to the working electrode by using a triangle wave generator assembled in-house from a design published elsewhere.20 All solutions were prepared by using doubly distilled water, the second distillation being from basic permanganate. All solutions were deaerated by bubbling with N2 prior to use. Solutions from which SERS spectra were acquired for the anion study were 0.1 M NaX (X = C1-, C104-, or S042-)and 0.05 M NaCN. The and 2.5 concentration experiment included 0.5, 0.05, 1.25 X X lo4 M NaCN in 0.1 M NaC104. All reagents were analytical reagent grade and were used as received. Acidification was accomplished by addition of enough concentrated acid, HX, to bring the pH of a 20-mL aliquot of the basic solution to a value of 2. Solution pH values were measured with an Orion Model 601 pH meter and glass pH electrode. Electrochemical pretreatments of the Ag electrodes were accomplished with a single oxidation-reduction cycle (ORC). In accordance with most of the published literature,6-8 initial experiments involved roughening the Ag surfaces by means of an ( 1 7) Peters, D.G.;Hayes, J. M.; Hieftje, G. M. Chemical Separations and Measurements, Theory and Practice of Analytical Chemistry; W. B. Saunden: Philadephia, 1974; p A.6. (18) Guy, A. L.; Bergami, B.; Pemberton, J. E. Surf. Sri. 1985, 150, 226. (19) Pemberton, J. E.; Buck, R. P. Appl. Spectrosc. 1981, 35, 571. (20) Woodward, W. S.; Rocklin, R. D.; Murray, R. W. Chem. Biomed., Enoiron. Instrum. 1979, 9, 95.
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
1121 I
w v E N u m E R !cm-1:2253
lg5' WAVE NUMBE R (Ck
l
2250
Figure 1. SERS spectra for basic CN- solutions (pH 11) at a Ag electrode. Raman acquisition parameters: band-pass, 5 cm-I, 0.5 cm-I (c) C10.; interval, 0.5 s integration time; (a) Cl-; (b) SO>-;
O R C in the presence of CN-. This method was found to yield surfaces which exhibited a high degree of irreproducibility in both electrochemical and spectroscopic respects. Furtak, Trott, and Loo previously reported that SERS signals for adsorbed CN- could be observed on Ag electrodes which had been externally roughened in solutions containing no CN- species.21 During the present investigation, it was determined that this method of electrode preparation offers a more reproducible means of roughening the electrode surface. Therefore, the ORC was performed in the absence of CN- in 0.1 M NaX at basic pH, or in 0.1 M H X or 0.1 M NaX at acidic pH. In both acidic and basic media, the ORC was accomplished as follows. From an initial potential of -0.20 V, the electrode potential was linearly ramped in a positive direction at 20 mV.s-'. After a predetermined charge of 1.3 X C was passed, the scan direction was reversed, resulting in a total anodic charge density of 40-45 mC.cm-2. In C1- media under both pH conditions, the AgCl formed is insoluble such that upon scan reversal ca. 97-99% of the oxidized Ag is reduced.' In SO4*-and Clod- media at both pH values, the Ag salts formed are soluble and diffuse away from the surface. On scan reversal in these electrolytes, only ca. 25% of the oxidized Ag is reduced. Despite this fact, good quality SERS spectra were obtained in these electrolytes. The electrode was then removed from the electrochemical cell and placed in the SERS cell which contained 0.1 M NaX and the desired N a C N concentration at the desired pH. All SERS spectra were recorded at potentials negative of any faradaic processes. In acidic media, this potential region extends from ca. 0.00 to -1.20 V. In basic media, this potential region is between ca. -0.70 and -1.40 V. I3C N M R data were obtained on a Bruker WH-250 N M R spectrometer.
Results and Discussion General Acid-Base Behauior. Figure 1 shows a series of spectra obtained in basic CN- media (pH 11) at various potentials in the three supporting electrolytes. Potentials positive of ca. -0.70 V are not accessible in basic media due to the oxidation of Ag to soluble Ag(CN),(X-')- complexes.I0 The feature observed in the v(CN) frequency region between ca. 2000 and 2200 cm-' is attributed to CN- adsorbed at the Ag surface. This assignment is in agreement with previous literature reports for similar basic conditions~4,6-S,~0,2Z Figure 2 shows a series of spectra obtained at pH 2 in the three supporting electrolytes as a function of potential. One noteworthy feature of these data is the presence of good signals in acidic media for all three anions. This is in contrast to the work of Mahoney and Cooney in which no signals at pH 6.5 in SO4*-media were ~ b s e r v e d .An ~ explanation for the discrepancy between the data reported here and that of Mahoney and Cooney comes from consideration of the potentials used to acquire the spectra in each case. All of their attempts to record SERS spectra from this (21) Furtak, T. E.; Trott, G . ; Loo, B. H. Surf. Sci. 1980, 101, 374. (22) Fleischmann, M. In Comprehensive Treatise on Electrochemistry; Vol. 8 , O'M. Bockris, J.; Conway, B. E.; Yeager, E.; White, R. E. Eds.; Plenum: New York, 1981, p 373.
1122 The Journal of Physical Chemistry, Vol. 91, No. 5. 1987
1950 .,,2250 WAVENUMBER(cm !
Figure 2. SERS spectra for acidic HCN solutions (pH 2) at a Ag electrode. Raman acqu on parameters: band-pass, 7.5 cm-I, 1 .O cm-I interval, 0.5 s integration time; (a) Cl-: (b) SO4*-; (c) C104-.
system were at potentials equal to or more negative than -0.80 V by analogy to the basic CN- system. No signals in the v(CN) frequency region are detected at these potentials. The results presented here are in agreement with this observation. No SERS signals can be observed at Ag in acidic S042-media if the initial electrode potentials are more negative than ca. -0.50 V. At potentials more positive than -0.50 V, SERS spectra of reasonable quality were recorded in this acidic medium as shown in Figure 2. Signals at potentials more negative than -0.50 V can only be obtained if the initial potential applied to a freshly prepared electrode is positive of ca. -0.50 V and then made more negative. The SERS intensities measured in this acidic solutions vary with time. This may be the result of aggregation or polymerization of H C N at the surface or changes in the electrode surface active sites. Two methods of acquiring spectral data were used to deal with this observed time dependence. The first method requires a freshly prepared electrode for each spectrum and avoids any possible time dependence of the intensity of the v(CN) SERS signal. Furthermore, freshly acidified cyanide solutions were used to acquire each spectrum in order to avoid time-dependent solution compositional changes. The time dependence of the solution composition under acidic conditions will be discussed further below. The principal drawback of this method is the irreproducibility in the intensity of the v(CN) band. The source of the irreproducibility lies in the inability to exactly reproduce the Ag surface morphology. This problem has plagued many SERS experiments performed in electrochemical environments. This is particularly true in media in which the supporting electrolyte anion does not form a Ag precipitate during the ORC. To avoid irreproducibility, a series of spectra were taken at different potentials from one electrode surface. To minimize the effects of time-dependent solution compositional changes, the total time any one electrode and solution were used was less than 1 h. The intensity-time behavior at one potential was recorded in each solution to assess the stability of the SERS signal prior to these experiments. In all cases, the uncertainty in the data due to lack of solution and surface stability at one electrode is substantially less than the irreproducibility obtained by using a freshly prepared electrode. The SERS v(CN) feature in acidic media has a more complex band shape than that observed in basic media, suggesting the existence of more than one surface species. This band exhibits one major peak at ca. 2120 cm-' with a significant asymmetry on the low energy side at ca. 2100 cm-I. Explanations for this band shape will be discussed further below. The frequency at which the maximum intensity of the u(CN) band occurs in acidic media is ca. 30 cm-I higher than in basic media. A similar dependence on pH of the frequency of the v(CN) band is observed in solution Raman experiments. The v(CN) band is observed at 2083 cm-' in a 5 M N a C N / I M NaClO, (basic pH) aqueous solution. When the solution is acidified with concentrated.HCIO,, the peak shifts to 2102 cm-1.23 Based on the K, for this system, the predominant species in these acidified solutions is H C N . The similar shift to higher frequency for the v(CN) SERS band at Ag suggests that HCN is the surface species. (23) Hoff, R. L.; Pemberton, J. E., unpublished results.
Kellogg and Pemberton In fact, SERS spectra of CN--containing species formed on Ag by exposure to HCN under vacuum bear considerable resemblance to the spectra shown here.24 The observation that the intensities are high relative to those observed in base is surprising in view of the limited number of metals for which H C N can act as a ligand.25 There is, however, evidence in the literature for the existence of H C N complexes of Ag. Dove and Hallett have reported that H C N acts as a ligand toward Ag' in anhydrous HF.26 N M R and IR studies on solutions of AgCN in H F suggest that both (HCN)Ag+ and (HCN)*Ag' complexes are formed. These researchers assigned the vibrational bands observed at 2138 and 2147 cm-I to the v(CN) for (HCN)Ag+ and (HCN)2Ag+,respectively. Howard, Sutcliffe, and Mile2' have reported an EPR study of H C N complexes of Ag, Cu, and Au formed by reaction of H C N with metal atoms in adamantane at 77 K. Analysis of the EPR band shapes led these researchers to conclude that two Ag(HCN) complexes were present, namely AgNGC-H and Ag-(HC=N). Intensity Behavior in Acid and Base. The intensity of the u(CN) band in basic solutions depends on the potential applied to the working electrode. The potential at which this intensity reaches a maximum shows a slight dependence on the anion of the supporting electrolyte. These occur at -1.30 V in basic ClO;, and at -1.10 V in basic S042-and CI-. The lack of dependence of the potential of maximum intensity on the supporting electrolyte anion is a result of the relative strengths of adsorption of the anions present and the potentials required to desorb these anions. Differential capacitance experiments conducted by Rogozhnikova and Bek28indicate that the order of anion adsorption at Ag is F = SO4" < C1- < CN-. These authors also found that CN- is not totally desorbed from Ag until a potential of -1 $40V vs. SCE is reached. Vitanov and P O P O Vhave ~ ~ reported that C10; is more weakly adsorbed than F a t polycrystalline Ag. Therefore, the order of adsorption strength for the anions in this study is C10, < SO4-' < C1- < CN-. It can be assumed that at the negative potentials where the SERS intensity for the v(CN) reaches a maximum, the only species adsorbed at the Ag surface to any great extent is CN-. Hence, very little effect of supporting electrolyte anion on the SERS intensity is expected. In contrast, the SERS intensities measured in acidic media are dependent on anion and potential. From the data in Figure 2, it can be seen that the potentials at which the maximum intensity of the high-frequency component of the v(CN) band is observed media, and at -0.30 to -0.40 V in C104- media, -0.40 V in -0.50 V in C1- media. The intensity of the low-frequency component also varies with electrode potential, and increases monotonically relative to the intensity of the high-frequency component as the electrode potential is made negative. At the most negative potentials, the intensities of the two components are nearly equal. Frequency Behavior in Acid and Base. The position of the v(CN) band varies with electrode potential in both basic and acidic media. Figure 3 shows plots of the position of the v(CN) band as a function of potential in basic and acidic media. In all cases, the peak frequency of the primary v(CN) band shifts to lower values as the potential is made more negative. This behavior has been previously explained by Fleischmann and co-workers* and Anderson, Kotz, and Yeager30in terms of the bonding interactions of CN- with the Ag surface as shown in Figure 4. An alternate explanation that has been proposed to account for the potential dependence of the v(CN) frequency is the effect of the electric field in the double layer on the dipole moment of the adsorbed molecule. This is known as the Stark e f f e ~ t . ~ ' . ~ ~ (24) Murray, C. A. J . Electron Spectrosc. Relat. Phenom. 1983, 29, 371. (25) Corain, B. Coord. Chem. Rea. 1982, 47, 165. (26) Dove, M. F. A.; Hallett, J. G. J . Chem. SOC.A 1969, 2781. (27) Howard, J. A.; Sutcliffe, R.; Mile, B. J . Phys. Chem. 1984,88, 5155. (28) Rogozhnikov, N . A,; Bek, R. Yu. Elektrokhimya 1980, 16, 7 6 . (29) Vitanov, T.; Popov, A.; Sevastyanov, E. S . J . Electroanal. Chem. 1982, 142, 289. (30) Anderson, A. B.; Kotz, R.; Yeager, E. Chem. Phys. Lett. 1981, 82, 130. (31) Venkatesan, S.;Erdheim, G.; Lombardi, J. R.; Birke, R. L. Sur5 Sci. 1980, 101, 387.
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987 1123
SERS of CN- at Ag Electrodes
2'40m o A C l D CI-
0
BASE CI -
0 ACID ClO,
AAClD S O : -
H BASE CIOA
A BASE SO:-
44
AA
IA i
++++. . 0
.. , +ei#pLl. ,. +
'
2098.0 -0.4 -0.8 -1.2
-1.6
0.0 -0.4 -0.8 -1.2 -1.6 POTENTIAL, volts v s SCE
Figure 3. Position of the v(CN) band as a function of electrode potential and pH. TABLE I: Possible Assignments of Complex v(CN) Band Shape in Acidic Media I. high-frequency band is end-bonded HCN
Figure 4. Simplified model for interaction of CN- with Ag surface.
Venkatesan, Erdheim, Lombardi, and Birke have proposed a theory for the Stark effect in electrochemical SERS in which the overall force constant of the adsorbate bond depends upon three terms.31 One of these terms is a function of the effect of the discreteness of charge of the specifically adsorbed anion, which is dependent upon the electrode potential. Evidence that the Stark effect may, a t least in part, be responsible for the potential dependence of the u(CN) frequency is seen in Figure 3. The slope of the frequency-potential behavior is dependent on the supporting electrolyte anion. The slope in basic C1- is ca. 21 cm-'.V-', while that for basic C104- and Sod2solutions is ca. 10 cm-'.V-'. When HCN is the adsorbate, a slope of ca. 20 cm-'-V-' is observed for the 2130-cm-' band in all three supporting electrolytes. The independence of this slope on the anion suggests that the mode of interaction of H C N with the Ag surface is not affected by the extent of interaction of the anion of the supporting electrolyte with the Ag surface. The peak position estimated for the low-frequency component of the v(CN) band in acid does not appear to be dependent on the electrode potential. This is in obvious contrast to the behavior of the high-frequency component of the v(CN) band and the single peak observed in base. This result has important implications regarding the assignment of the low-energy shoulder. This apsect will be discussed in greater detail in the next section. Assignment of the Adsorbate in Acidic Media. In order to understand the nature of the adsorbate at the Ag electrode in acidic media, the complex band shape observed in the v(CN) region of the spectra must be assigned. A number of feasible explanations for this complex band shape are presented in Table I. These explanations fall into three classes and will be examined in turn below. The various experiments designed to probe each will also be discussed. (32) Kunimatsu, K.; Seki, H.; Golden, W. G.; Gordon 11, J. G.; Philpott, M. R. Surf. Sci. 1985, 158, 596.
(a) low-frequency band is CN(b) low-frequency band is side-bonded HCN (c) low-frequency band is interfacial HCN (d) low-frequency band is polymerized HCN 11. complex band shape due to HCN end-bonded at two different surface sites 111. complex band shape due to polymerized HCN possessing two different environments The first class of explanations centers on the assignment of the high-frequency peak to H C N end-bonded to the Ag surface through the nitrogen lone pair electrons. This is the mode of bonding previously proposed by Dove and Hallett for the (HCN)2Ag+ and (HCN)Ag+ species observed in liquid HF.26 This is also one of the bonding modes proposed by Howard, Sutcliffe, and Mile for H C N with Ag in adamantane.27 The Ag-N surface bond for H C N is proposed to be largely u in character as shown schematically in Figure 6. a-backbonding between the Ag d-orbitals and the a * antibonding orbitals of H C N is probably also significant as has been proposed for end-bonded organonitrile metal complexes.33 This bonding scheme adequately explains the dependence of the high-frequency component of the v(CN) band on the potential of the Ag electrode in a manner analogous to that described for adsorbed C N - . * Z This ~ ~ scheme is also consistent with the properties of H C N complexed to low oxidation state metals or electron-rich metal centers.25 In this first class of explanations, several possibilities can account for the existence of the low-frequency shoulder. The low-frequency component could possibly arise from small amounts of CN- resulting from the dissociation of H C N in the interface. The presence of the Ag surface might perturb the HCN/CN- equilibrium to produce more CN- at the interface. The position of the low-frequency band, ca. 2100 cm-I, corresponds closely with that obtained in basic media for surface CN-. This explanation is not consistent with the observation that the frequency of this band does not shift to lower values as the electrode potential is made more negative. Therefore, the low-frequency band is probably not C N - at the interface. The second possible source of the low-frequency band in this first class of explanations is that H C N can interact with the Ag ~~~
~~
~
~
(33) Storhoff, B. N.; Lewis Jr., H. C. Coord. Chem Rev. 1977, 23, 1
Kellogg and Pemberton
1124 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
1
H
H
Figure 5. Simplified model for interaction of HCN with Ag surface.
surface in a side-bonding configuration. In this model, interaction between the HCN and the surface occurs through the a electrons of C=N. This interaction is shown schematically in Figure 5. If this type of bonding were to occur to any significant extent, the relative energies of the u nd a levels must be similar. The energies of the two filled H C N levels differ by less than 0.5 eV in the UV photoelectron spectrum,34where the CEN a level is more stable than the lone pair. Both modes of bonding are thus feasible, but the end-on mode would appear to be preferred because the lone pair electron density is more energetically accessible. Independent evidence that supports the side-bonding of H C N also exists. Side-on bonding in organonitrile complexes has been previously reported by S t ~ r h o f f although ,~~ it is not common. Howard, Sutcliffe, and Mile2' also report side-on bonding of HCN with Ag in adamantane. The interaction of H C N at the Ag surface could occur through the a-system of the CEN bond with the empty metal d-orbitals. Consistent with experimental observations in this investigation, one would expect the v(CN) of side-bound H C N to occur at lower frequencies than the end-bound HCN. This change in peak frequency results from electron donation from side-bound H C N to the Ag giving a reduction in the net bond order of the C=N bond. This picture is consistent with existing complexes displaying side-on nitrile c ~ o r d i n a t i o nand ~~ previous SERS results for side-bound a-systems on A u . ~ ~ The difference in the frequency-potential behavior of the two bands is significant and warrants further discussion. The behavior of end-bonded H C N can be explained in a manner analogous to that discussed for CN- above. The frequency of this band decreases as a result of changes in either the extent of a-backbonding with changes in the electron density on the electrode, or as a result of the Stark effect, or a combination of both. The potential independence of the low-frequency band is an argument for the importance of the Stark effect in this system. The contribution to the bond order change from a-backbonding due to changes in electron density of the electrode surface is expected to be the same for both side-on and end-on bonding. On the other hand, the Stark effect, which should operate only normal to the s ~ r f a c e ~ 'predicts ~~*, a lack of frequency shift with potential for this low-frequency band. This is a strong argument for the assignment of the low-frequency band to a side-bonded H C N species. Verification of the assignment of the low-frequency band to side-bonded H C N is not straightforward. The most desirable approach is that recently reported by Moskovits and S U ~These . ~ ~ authors demonstrate that for phthalazine adsorbed on Ag colloids, vibrational modes of different symmetry will generate different SERS excitation profiles for molecules adsorbed in distinct orientations with respect to the Ag surface. Unfortunately, this approach could not be applied to the H C N system, because the only mode observed in the SERS spectrum is the v(CN) mode which is of a l symmetry for both end-bound and side-bound HCN. Another technique commonly employed in Raman spectroscopy for the determination of orientation is the determination of the polarization ratios of the two SERS bands. Such studies are generally useless in electrochemical SERS experiments, because
I
1950 2250 WAVENUMBER (cm-" )
Figure 6. Concentration dependence 0.1 M NaC10, at pH 2, -0.20 V and (a) 0.5 M NaCN; (b) 0.05 M NaCN; (c) 1.25 X IOm3 M NaCN; (d) 2.5 X lo', M NaCN. Raman acquisition parameters: band-pass 7.5 cm-', 1.0 cm-' interval. 0.5 s integration time.
the vibrational bands tend to be highly d e p o l a r i ~ e d . ~ ~ , ~ ~ Previous literature has demonstrated that the orientation of adsorbates with respect to the electrode surface can be a sensitive function of solution concentration. Hubbard and co-workers have shown that, in many cases, as the concentration of the adsorbate is decreased, the preferred orientation is one requiring the greater amount of electrode surface area.38 Applying similar reasoning to this system, one would expect that the intensity of the lowfrequency band would increase relative to the intensity of the high-frequency band as the H C N concentration is decreased if the low-frequency band is side-bonded HCN. This is indeed the trend observed for the SERS spectra in Figure 6. These spectra were obtained from a series of solutions with initial H C N concentrations of 0.5, 0.05, 1.25 X and 2.5 X lo-, M in 0.1 M NaC10, at pH 2 and -0.20 V. The SERS intensities obtained at all concentrations are similar. Given the difficulty in reproducing the absolute SERS intensities measured in this system, this observation suggests that the H C N coverage is probably close to saturation in all cases. Thus, these data are strong evidence that the low-frequency shoulder arises from side-bonded HCN. The third explanation in the first category in Table I is that the low-frequency band is due to interfacial H C N found in the vicinity of the electrode, but which is not in intimate contact with the electrode surface. One would expect the u(CN) frequency of HCN molecules in the interfacial region to be similar to that of solution H C N observed at ca. 2100 Surface enhancement of interfacial species could be the result of electromagnetic contributions to the mechanism which are known to be long range in nature.' One would expect very little dependence of the peak frequency on electrode potential for interfacial H C N species. This explanation is consistent with the trends shown in Figure 3. On the other hand, if the low-frequency band were due to interfacial H C N species, one would expect a decrease in the intensity of this band as the concentration of H C N in solution is decreased. However, as discussed above, the opposite trend is observed in the concentration studies. A final possibility in this first class of explanations is that the low-frequency component of the v(CN) band is due to slow polymerization of H C N on the electrode surface. This could be responsible for the observed increase in the intensity of the complex band with time. Moreover, both u(CN) bands shift slightly to higher frequencies as a function of time. Previous studies of HCN polymerization have demonstrated that the polymerization process causes a shift to higher frequency for the v(CN) band.39 The postulate that the low-frequencyband is due to slow polymerization of H C N at the electrode surface does not explain the intensity -~ ~~~
(34) Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C . R. Molecular Photoelectron Spectroscopy; Wiley: London, 1970, p 345. (35) Patterson, M. L. M . J . Weauer, J. Phys. Chem. 1985, 89, 1331. (36) Moskovits, M.; Suh, J. S. J . Phys. Chem. 1984, 88, 5526.
(37) Jeanmaire, D. L.; VanDuyne, R. P. J . Electroanal. Chem. 1977, 84. 1.
(38) Soriaga, M. P.; Hubbard, A. T.J. A m . Chem. SOC.1982, 104,3397. (39) Volker. Th. Angew Chem. 1960, 1 1 , 379.
SERS of CN- at Ag Electrodes increase of both peaks. One would expect the intensity of the low-frequency component to increase relative to that of the high-frequency component as a function of time if this model were valid. The second class of explanations for the complex band shape involves end-bound H C N interacting with the roughened Ag surface at two energetically distinct surface sites. Patterson and Weaver35 have reported two unresolved bands in both the u(CC) and v(CH) region for ethylene adsorbed on Au electrodes. They attribute the presence of the second bands to two different adsorption sites on the Au surface. The lower frequency component of each unresolved band is attributed to ethylene that is more tightly bound to the Au surface. This species would have a lower v(CC) frequency, because electron density has been removed from the a bonding modes and donated into the metaladsorbate surface bond. Patterson and Weaver cite both the potential dependence and supporting electrolyte dependence of the relative intensities of the two unresolved bands as evidence for two distinct surface-bonding sites. The intensity of the low-frequency component of the ethylene bands (more strongly bound) gains intensity with respect to the high-frequency component as the electrode potential is made more positive. Inspection of Figure 2 shows the same trend if the band due to more strongly bound H C N is the higher frequency band. Evidence against this assignment comes from consideration of the peak frequency-potential dependence of this sytem. One would expect a decrease in frequency for both v(CN) bands as the potential is made more negative if H C N was end-bonded to the Ag at two energetically distinct surface sites. As noted above, only the higher frequency v(CN) band exhibits a change in frequency with potential. The third and final class of explanations involves the possibility that both components of the complex band are due to polymerization of H C N at the electrode surface. It is well-known that H C N polymerizes into a brown amorphous solid. Volker, in a study of H C N polymers formed by different methods, reported that the v(CN) of the polymers containing C=N bonds occurs at 2203 ~ m - ’ .This ~ ~ value is higher in frequency than any observed in this study. Polymerization of gas-phase H C N at solid surfaces has also been observed p r e v i o ~ s l y . Therefore, ~ ~ ~ ~ ~ the aggregation and eventual polymerization of H C N at these Ag surfaces would not be surprising. Initial evidence that polymerization processes may be operating in acid was seen in the time dependence of the ORC voltammetry of acidified cyanide solutions. Early attempts to characterize the SERS behavior of this system were performed on electrodes which had been roughened in the same solution in which spectra were acquired. For pH 2 solutions, 100 mL of a 0.05 M NaCN/O.l M NaX (X = Cl-, or C104-) stock solution were prepared and then acidified with the concentrated acid of the anion of the supporting electrolyte. The initial voltammetry obtained in C10,media is shown in Figure 7a. Two very distinct oxidation waves are evident in this cyclic voltammogram. The first wave at ca. +0.10 V is most likely due to the oxidation of Ag to form soluble Ag(CN)?-’)- or Ag(HCN),+ complexes. The second wave starting at ca. +0.45 V is due to the formation of AgX. The AgX is reduced in the cathodic wave at ca. +0.25 V. The cathodic wave at ca. -0.20 V may be due to the reduction of Ag(HCN),+ complexes. The cathodic process at ca. -0.80 V is probably due to the reduction of small amounts of Ag(CN),(X-l)-. Over the course of bubbling with N, for ca. 2 h, the O R C voltammetry eventually reaches the behavior shown in Figure 7b. All redox processes other than the oxidation and subsequent reduction of AgX are suppressed. The SERS spectra obtained from these solutions also change with the voltammetry. Relatively poor SERS intensities are obtained from Ag electrodes that are roughened in the manner of Figure 7a. The general features of the spectra obtained in these experiments are similar to those obtained from Ag surfaces (40) Kortum, G.; Delfs, H. Specrrochim. Acta 1964, 20, 405. (41) Kreitenbrink, H.; Knozinger, H. 2.Phys. Chem. N . F. 1976, 102, 43.
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987 I
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O”C%
I 0.4
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00
-0.2 -0.4 E. VOLTS vs
-0.6 -0.8
-1.0
-1.2
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Figure 7. (a) Initial cyclic voltammetric behavior for a polycrystalline Ag electrode in a solution of 0.05 M NaCN/O. 1 M NaC10, acidified to pH 2; scan rate 20 mV&. (b) Cyclic voltammetry for the same solution as in (a) after 1-2 h under bubbling Nz;scan rate 20 m V d .
roughened in the pure supporting electrolyte and then placed in a freshly prepared H C N solution. When the voltammetry is similar to that in Figure 7b, higher SERS intensities are obtained, but the u(CN) peak position is much less reproducible and tends to occur at higher frequencies than those obtained from freshly prepared solutions. Attempts to detect the presence of a polymer of HCN in these solutions were unsuccessful. I3C N M R analysis of the yellow liquid that slowly forms during the acidification of 5 M aqueous NaCN solutions shows only the presence of HCN. However, a preliminary step to the polymerization of H C N in aqueous solutions is probably the formation of H C N aggregates. As the H C N molecules become more self-associated, one would expect the v(CN) frequency of the resulting aggregates to increase if it can be assumed that the more “polymerlike” the aggregate, the higher the v(CN) frequency. The observation that both u(CN) bands increase in frequency by 5-7 cm-’ over the course of ca. 1 h at constant potential in C10,- media lends further credence to the concept of self-association in these systems. Such aggregation probably occurs to a greater extent in solutions within which there is no other mode of stabilization of the individual H C N species. This model can be applied to the interfacial chemistry occurring in these systems in the following manner. The most favorable situation for H C N in S042-and C104- electrolytes is a high degree of self-association, with eventual polymerization, because of the poor hydrogen-bonding character of these two anions. This could give rise to the higher frequency observed for the v(CN) seen in Figure 3. The C1- anion has stronger hydrogen-bonding character and can probably hydrogen bond with the H C N molecule. Such interaction in the interface would decrease the extent of self-association and polymerization and lead to lower frequencies for the u(CN) band. Additionally, specifically adsorbed C1- at the interface may serve to further aggregate H C N at the surface through surface Cl*--*+H--C=N hydrogen bonding. It has been proposed that solutions of H C N in HC1 produce the species (HC1)3(HCN)2.42 It is possible that this type of complex may be stabilized at the Ag surfaces in this study, further decreasing the extent of H C N association. The most important inadequacy of the polymer model is the absence of other significant spectral features. If a true polymer develops at the interface, bands for C = C and C=N bonds should be observed.39 The spectral region between 1000 and 1800 cm-I where C=C and C=N stretching vibrations should occur contains (42) Allenstein, E.; Schmidt, A. Chem. Ber. 1964, 97, 1863.
1126
J. Phys. Chem. 1987, 91, 1126-1130
only the well-documented peaks due to surface carbon that are ubiquitous in SERS experiments.22 This observation precludes the theory that polymerization of H C N in the interface causes the complex band shape. In total, the data best support the premise that H C N initially interacts with the surface in end-bonded and side-bonded configurations, and that these orientations then lead to self-association of the HCN. Unfortunately, the inability to detect the H C N bending mode, which should occur at ca. 700-800 cm-I, and a Ag-N surface bond stretch in the low-frequency region prohibits unequivocal assignment of the interfacial processes.
with the Ag surface as proposed previously. Effects of the nature of the supporting electrolyte anion on the SERS behavior of CNhave been identified and are under further investigation in this laboratory. For acidic solutions, the adsorbate is H C N which is proposed to adsorb at the Ag surface in both end-bonded and side-bonded configurations. The time dependence of the SERS signals and the cyclic voltammetry for these systems suggests that slow aggregation of the interfacial H C N is occurring. The effects of the nature of the anion on these responses are consistent with this assignment.
Acknowledgment. We are very grateful for the technical assistance of Rebecca L. Hoff for obtaining solution Raman spectra of HCN. We gratefully acknowledge the National Science Foundation (CHE-8309454) for support of this research. Registry NO. HCN, 74-90-8; Ag, 7440-22-4; Pb, 7439-92-1.
Summary In summary, SERS results are presented for pH and anion effects on the adsorption of cyanide species at Ag electrodes. Under conditions of basic pH, the adsorbate is CN- which interacts
Surface-Enhanced Raman Scattering of HCN at Pb-Modffied Ag Surfaces Diane S. Kellogg and Jeanne E. Pemberton* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 (Received: October 8. 1985)
The surface-enhanced Raman scattering (SERS) behavior of HCN at polycrystalline Ag electrodes modified by monolayer and submonolayer amounts of underpotentially deposited Pb is presented. Changes in SERS intensity as a function of Pb coverage are interpreted in terms of preferential displacement of HCN from active sites by deposited Pb. By comparison with previous results obtained with pyridine and CI- as adsorbates in similar experiments,these data suggest that both large-scale and atomic-scale roughness features contribute significantly to the SERS intensities measured in these systems.
Introduction Surface-enhanced Raman scattering (SERS) has proven to be a very powerful tool for obtaining vibrational information from adsorbates in electrochemical environments.] Surface enhancement of Raman signals occurs at a limited number of pure metals, suggesting that the electronic properties of the metals are critical to the enhancement mechanism. The most notable metals capable of supporting SERS are Ag, Cu, and Au. Other metals claimed to support SERS include Pt,3-5 Cd,6 Pd,’ Ni,*s9Li, K,’O and @-PdH.” Further evidence for the dependence of the SERS response on the electronic properties of the metal is the fact that the enhancement for a given metal depends on the frequency of the exciting laser illumination. Enhancement is observed at Ag surfaces throughout the visible region. Cu and Au can only support enhancement when the excitation is in the red wavelength region.]* (1) For a recent review of the SERS phenomenon see: Chang, R. K., Furtak, T. E., Eds.Surface Enhanced Raman Scattering Plenum: New York, 1982. (2) Seki, H. J . Electron Spectrosc. Relat. Phenom. 1983, 30, 287. (3) Yamada, H.; Yamamoto, Y.; Tani, N. Chem. Phys. Lett. 1982, 86, 397. (4) Benner, R. E.; Von Raben, K. U.; Lee, K. C.; Owen, J. F.; Chang, R. K.; Laube, B. L. Chem. Phys. Lett. 1983, 96, 65. (5) Loo, B. H. J . Phys. Chem. 1983, 87, 3003. (6) Loo, B. H . J . Chem. Phys. 1981, 75, 5955. (7) Loo, B. H. J . Electron Spectrosc. Relat. Phenom. 1983, 29, 407. (8) Chou, C. C.; Reed, C. E.; Hemminger, J. C.; Usioda, S. J . Electron Spectrosc. Relat. Phenom. 1983, 29, 401. (9) Yamada, H.; Tani, N.; Yamamoto, Y. J . Electron Spectrosc. Relat. Phenom. 1983, 30, 13. (10) Moskovits, M.; DiLella, D. P. In Surface Enhanced Raman Scattering Chang, R. K., Furtak, T. E., Eds.; Plenum: New York, 1982; p 243. ( 1 1) Fleischmann, M.; Graves, P. R.; Hill, I. R.; Robinson, J. Chem. Phys. Leu. 1983, 95, 322.
0022-3654/87/2091-1126$01.50/0
Pure metals offer only a discrete set of surface electronic properties for study. To completely study the role of surface electronic properties, a systematic method for altering surface properties must be employed. One such method is the electrochemical deposition of small amounts of foreign (Le., different) metals on a substrate metal. For certain electrodeposition systems, deposition of the first monolayer or first few monolayers occurs at potentials positive of the reversible Nernst potential for the metal couple. This process is documented in the literature13 and is defined as underpotential deposition (UPD). The controlled deposition of one metal onto another in this fashion causes systematic variations in the electronic structure, and hence, the optical properties of the resulting metal electrode surface.I3 The use of electrochemical metal deposition systems in SERS has been quite s u c c e s s f ~ l . ~ However, ~-~~ the interpretation of (12) Pettinger, B.; Wetzel, H. In Surface Enhanced Raman Scattering Chang, R. K., Furtak, T. E., Eds.; Plenum: New York, 1982; p 293. (13) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley: New York, 1978; Vol. 11, p 125. (14) Loo, B. H.; Furtak, T. E. Chem. Phys. Lett. 1980, 71, 68. (1 5 ) Pettinger, B.; Moerl, L. J . Electron Spectrosc. Relat. Phenom. 1983, 29, 383. (16) Moerl, L.; Pettinger, B. Solid State Commun. 1982, 43, 3159. (17) Watanabe, T.; Yanagihara, N.; Honda, K.; Pettinger, B.; Moerl, L. Chem. Phys. Lett. 1983, 96, 649. (18) Kester, J. J. J . Chem. Phys., 1983, 78, 7466. (19) Pemberton, J. E. J . Electroanal. Chem. 1984, 167, 317. (20) Roy, D.; Furtak, T. E. J . Chem. Phys. 1984, 18, 4168. (21) Furtak, T. E.; Roy, D. Surf. Sci. 1985, 158, 126. (22) Furtak, T. E.; Roy, D. Phys. Rev. Lett. 1983, 50, 1301. (23) Guy, A. L.; Bergami, B.; Pemberton, J. E. Surf. Sci. 1985, 150, 226. (24) Guy, A. L.; Pemberton, J. E. Langmuir 1985, I, 518. (25) Pemberton, J. E.; Coria-Garcia, J. C.; Hoff, R. L., Langmuir, in press. (26) Pemberton, J. E.; Coria-Garcia, J. C.; Sobocinski, R. L. J . Electroanal. Chem., in press.
0 1987 American Chemical Society
