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Ag (CN)2− system
Surface Soence 166 (1986) 113-128 North-Holland, Amsterdam
113
EVIDENCE FOR A DOUBLE CHARGE TRANSFER CONTRIBUTION T O S E R S I N T H E A g / A g (CN) z S Y S T E M H. B A L T R U S C H A T a n d J. H E I T B A U M Instttute of Phystcal Cherntst~, Wegelerstrasse12, 5300 Bonn, Fed Rep of Germany
Received 28 March 1985; accepted for pubhcatmn 16 September 1985
Using a square wave potential perturbatmn and momtonng the Raman intensity m parallel, the potential dependence of Ag(CN)2 spectra was studied with ddferent excxtlng wavelengths A double peak structure was found for the C-N stretch frequency both peaks having different excitation characteristics In addition, the quenching rate of ad-complexes was measured at negative potentials together wlt.h the activation energy From EA = 75 kJ m o l - l the energy of an acceptor level can be estimated located about 3 eV above E F. This level presumably corresponds to the Ag 5s orbital which is sl'nfted upwards due to the interaction with the hgands of the ad-complex Since the CN- 5o or Dr orbltals form a donor level, the possibilityof a double charge transfer contribution to the SERS enhancement exists
I. Introduction In spite of the e n o r m o u s w o r k d o n e so far, there is still some c o n t r o v e r s y in the literature c o n c e r n i n g the e n h a n c e m e n t m e c h a n i s m of SERS. T h e significance of an e l e c t r o m a g n e t i c c o n t r i b u t i o n which p r o b a b l y arises f r o m localized p l a s m a resonances, is generally a c c e p t e d [1,2]. But there are n u m e r o u s results which stress the i m p o r t a n c e of chemical short range effects, e.g. the irreversible q u e n c t n n g of the R a m a n intensity at p o t e n t i a l s where the a d s o r b a t e d e s o r b s [1]. These o b s e r v a t i o n s have led to the a s s u m p t i o n of a r e s o n a n c e c o n t r i b u t i o n to S E R S which arises f r o m a charge transfer excitation b e t w e e n the m e t a l a n d an a c c e p t o r or d o n o r level of the adsorbate. This c o u l d i n d e e d be p r o v e n in electrochemacal systems, where the F e r m i level can be shifted relative to the levels in solution b y varying the a p p l i e d potential. Thus, the energy difference b e t w e e n E F a n d the a d s o r b a t e level can b e tuned into r e s o n a n c e with the exciting p h o t o n energy. M e a s u r i n g the R a m a n signal as function of the a p p h e d potential, i n t e n s i t y m a x i m a were o b t a i n e d , whose p o s i t i o n s are d e p e n d e n t of the exciting wavelength [3-6]. If the p h o t o n energy is p l o t t e d versus the p o t e n t i a l at which the m a x i m u m is observed, straight lines are o b t a i n e d whose slope Ohvo/~Ema x p h y s i c a l l y represents the p o t e n t i a l d e p e n d e n c e of the energy o f the a d s o r b a t e level with respect 0 0 3 9 - 6 0 2 8 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b h s h i n g Division)
114
H Baltruschat, J Hettbaum / C T contrlbutlon to S E R S in A g / A g( CN ) 2
to the Fermi energy. Depending on the nature of the adsorbate level, the respective slope should be > - 1 e V / V for a donor and < + 1 e V / V for an acceptor level. But all experiments done so far on Ag yielded slopes which exceeded an absolute value of one (e.g. 1 46 e V / V for pyrldlne in F - [3], 2 e V / V for pyridine in SO~- [4], > 2 e V / V for pyridme in C I - electrolyte [3,4] and even - 4 e V / V for CN [5]). This apparent contradiction to the theory was explained by local potenual effects [5], i,e. by an overshooting of the potential in the inner Helmholtz plane (IHP) due to specific ion adsorpUon. The absolute value, however, of this local potential cannot cause the unexpected high slope, but only its variation with the applied potential. Consequently, the slope can only be larger than one, if the coverage of specifically adsorbed ions changes with the potential. In the case of an ionic adsorbate such as C N - , the intensity-potential curves therefore do not reflect the charge transfer contribution to the SERS effect Gcv(l,o, E) alone, but its product with the surface coverage 0oN ( E ) . Consequently, the intensity m a x i m u m for a given excitation wavelength does not represent the resonance condition any longer. Only in the case of neutral adsorbates, the above interpretation may be valid, if coadsorption of the anions of the supporting electrolyte occurs. But F and SO~ are not specifically adsorbed. In the hght of the above consideration, we tried to ascertain for the case of the A g / C N - system that the high slope is not caused by a change of the cyanide coverage. To do so, a fast square wave potential perturbation was applied to the electrode, thereby hoping that the change of the potential dependent surface coverage was sufficiently slow to allow the R a m a n Intensity to be measured at almost constant coverage. In addition, the activation energy of the quenching rate at very negative potentials was measured. W~th certain plausible assumptions, the energy of an acceptor level of the ad-complex can be estimated from this value following a proposal of Hush [7]. Since the existence of a donor level was assumed by Billmann and Otto [5] already, we explain the apparent high slope In the case of A g / C N by the occurrence of two optical charge transfer excitations wtuch can both contribute to the enhancement of the SER signal.
2. Experimental The electrochemical set up and the optical alignment have been described elsewhere, already [8]. The p-polarized 458 n m and 514 nm lines of an Ar laser and the 647 nm line of a Kr laser were used with a power of about 70 mW at the sample. The Ag working electrode had a geometric surface area of 2 × 5 m m 2 and was embedded in epoxy resin. An Ag wire served as the counter electrode and
H Baltruschat, J Hettbaum / C T contrtbutlon to S E R S m A g / A g ( C N ) f
115
a saturated calomel electrode was used as the reference. It was always held at room temperature during the measurements of the activation energy. The three electrode compartments were separated by glass frits. The electrolyte was prepared from p.a. grade chemicals and Milhpore water and contained 0.01M K C N , 0.2M K O H , and 0.2M KC1. K O H was added to achieve a good conductwity which is needed for potential step experiments. KC1 was used to improve the effectiveness of the oxidation reduction cycle (ORC) necessary for activation. Prior to each experiment, the working electrode was pohshed with 13 and 1 /~m aluminla dispersed on a felt and afterwards subjected to an O R C which consisted of a potentml step from - 0 . 9 5 to 0.05 V, a linear sweep from 0.05 to 0.14 V (in some cases to 0.25 V) and back to - 0.25 V with a sweep rate of 0.25 V s - ~; finally the potential was stepped back to - 0 . 9 5 V and kept constant for 6 - 7 min in order to achieve an equlhbrated surface reconstruction. The potential step was introduced to keep the amount of dissolved Ag(CN)2 as low as possible. The reduced charge was about 11 m C cm -2. In all experiments an ~R compensation was applied. In order to measure the Raman intensity as function of the applied potential at the condition of almost constant surface coverage of C N - , the following method was applied: The potential of the working electrode was switched every 5 ms between a reference value of E r = - 0 . 9 5 V and a potential E m . During the first ms immediately after the step, the laser beam was interrupted by a hght chopper. During this period, the double layer charging was completed as was ascertained from c u r r e n t / t i m e curves and the t R drop measured with the galvanostatlc pulse method. During the following 4 ms, the R a m a n intensity was monitored at the respective potential, using an electronic switch which was synchronized with the potential program and which passed the signal of the photon counter (time constant < 0.1 ms) on to the two channels of an averaging amplifier. Thus, it was possible to record R a m a n spectra almost simultaneously at the two potentials. In the case of the above method, the intensity changes of the R a m a n bands should be mainly due to charge transfer effects as well as to alterations of the double layer structure. These relatively slow changes of the macroscopic surface roughness are completely cancelled out by taking the ratio of the R a m a n intensities at E m and E r. Moreover, on the basis of kinetic measurements [9] a time constant in the ms range ~s estimated for the incorporation of ad-complexes (with l 0 = 0.7 mA cm -2, ac~th = 0.4, a potential difference of 0.2 V and an assumed coverage corresponding to 100 /~C cm -2, ~---6.6 ms is calculated). Due to the averaging mode of detection, the influence of an alteration of the surface coverage is minimized as well. Spectra were recorded at 50 mV intervals starting from E m = E r = - 0 . 9 5 V both in positive and negative direction. The R a m a n intensity a t E r depends only little of E m except at extreme E m values, where the intensity at E r could change by a factor of up to 10.
116
H Baltruschat, J Hettbaum / CT contnbutton to SERS m A g / Ag(CN) f
3. Results 3 1 Dependence o f R a m a n mtenslty on potential and e x c m n g photon energy
Fig. 1 d~splays two R a m a n spectra o b t a i n e d at different p o t e n t t a l s E m t o g e t h e r with the s i m u l t a n e o u s l y m e a s u r e d reference s p e c t r a at E~. T h e followm g o b s e r v a t i o n s are typical for all s p e c t r a a n d i n d e p e n d e n t of the exciting wavelength: (1) T h e p e a k frequencies of the R a m a n b a n d s taken at E r = - 0 . 9 5 V were i n d e p e n d e n t of Em. T h e s p e c t r a r e c o r d e d at E m c o r r e s p o n d to those given in ref. [5] having an i d e n t i c a l f r e q u e n c y - p o t e n t i a l d e p e n d e n c e . This i n d i c a t e s that the c h a n g e of the p e a k posiUon ~s a fast, n a m e l y an electronic effect (shift of the electron density) as it was stated before [10]. (2) The F W H M of the b a n d m e a s u r e d at E m is d e p e n d e n t on the p o t e n h a l as
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H Baltruschat, J. Hettbaum / CT contrlbutton to SERS m A g / A g ( C N ) f
117
it was observed already in ref. [5]. In addition we found that the F W H M of the band measured at E r is also dependent on the value of E m. Consequently, the broadening of the R a m a n band at more positive potentials is due to a slow non-electronic process. (3) At E r n = - 0 . 7 V , the signal evidently is composed of two superimposed bands as is evident from fig. 1. This double band structure is best developed when spectra at E m values are recorded at which dissolution of Ag occurs which is immediately redeposited during the E r phase. The double band is also visible a t E m = - 0 . 8 and - 0 . 6 5 V but assumes the form of a plateau at more positive potentials. If the Ag electrode had not been subjected to a dissolution-redeposition process, instead of the distinctive shoulder only a large F W H M and an asymmetric band shape would have to be seen a t E m > - 0 . 9 V. At E m < - 1 . 0 V, a double band structure was never observed. This second peak, i.e. the shoulder, which was not observed before, gives rise to the band broadening at more positive potentials, to the unsymmetrical band shape reported in the literature [11], and to the obvious scatter of the frequency values measured at these potentials (cf. fig. 9 in ref. [5]). It should be realized from fig. 1 that it is not this shoulder but the higher peak on the left side of the band at E m = - 0.7 V which corresponds to the peak at E m = - 1.3 V, as is revealed from the peak position of the band at E r. It is rather improbable that the shoulder arising at positive potentials belongs to an ad-complex with a different number of C N - ligands since the peak difference is rather small. It seems to be more likely that the two frequencies are due to different surface sites, i.e. a different number of nearest neighbours of the central Ag ÷ ion of the ad-complex. This can be visuahzed by assuming adsorption at ad-atoms on a plane on the one hand and on kinks or steps on the other hand. This v~ew is corroborated by the fact, that the shoulder is clearly discriminated only when Ag is dissolved and redeposited on account of the potential program applied, as well as by the potential dependenctes of the vibration frequencies. The fact, that the peak position of the shoulder is almost independent of the potential, points to a more or less flat adsorption of the linear complex on a plane. On the other hand, C N - adsorbed on steps or kink sites will be more directed into the solution in virtue of sterical reasons and therefore feel the potential drop within the I H P to a larger extent than the flat ad-complex. This implies that O h l ' 0 / O E m a x from the CT enhancement of the shoulder is very small. Since the shoulder disappears only slowly, it cannot be explained by electronic effects such as a band splitting due to interactions of the adsorbed oscillators, or different ~ ( E ) or I ( E ) dependencies of two ad-complexes existing in the whole potential range, or a potential dependence of the transition probability of the asymmetric CN vibration. Instead, the following two explanations can be vahd: either a restructuring of a part of the surface occurs as function of the potential, thus changing the number of nearest
118
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n e i g h b o u r s of the central ion, or the a d s o r p t i o n s~tes r e s p o n s i b l e for the s h o u l d e r v a m s h slowly b u t m o r e or less reversibly at negative potentmls. If the first of these two e x p l a n a t i o n s is valid, n a m e l y the p o t e n u a l d e p e n d e n t r e s t r u c t u r i n g of the a d s o r b a t e , it seems r e a s o n a b l e to p l o t the relative i n t e g r a t e d b a n d intensity, i.e.
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as function of the potential E m assuming the charge transfer characteristic of the two ad-complexes to be similar. This plot is shown in fig. 2 for two different exciting wavelengths. The general shape of the curves is in good agreement to those obtained with a linear sweep method using an O M A system [5]. Our curves are flatter, however, because they do not contain contributxons from changes of the surface structure and the coverage, these effects being cancelled by taking the ratio as indicated above. The obvious hysteresis of the curve cannot be ascribed to a slow response of the detector system. Instead, it shows that two kinds of ad-complexes are involved, one of which is irreversibly quenched at negative potentials. Fig. 3 shows the corresponding h v o versus Urea x relationship and a slope of - 4 to - 5 e V / V is obtained from the diagram. This value is even higher than that reported in the hterature. If the second explanation is assumed to be valid, namely that the part of the adsorbate responsible for the shoulder vanishes at negative potentials, it is suggestive to plot the non-integrated peak heights of the mare peak as function of E m. The result is shown in fig. 4 and mainly reflects the intensity dependence of the main peak on the potential, thereby neglecting the shoulder and assuming the F W H M to be independent of the potential. It is interesting to note that the maxima of the curves shown m fig. 4 are not shifted by the exciting wavelength within the limits of error. We thus have to envisage the possibility that there is no CT contribution to the enhancement of this peak and the wavelength dependence shown in fig. 3 may arise from different wavelength dependencies of peak and shoulder. Since in fig. 4 the shoulder still contributes to the intensity of the peak, it might as well be that the peak has a
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C T characteristic with a positive Oh~,o/OE. . . . . This w o u l d be the case if the a d - c o m p l e x contains an acceptor level as well and evidence for this will be given below. More insight into the above problem could be expected from a c o m p a r i s o n with IR experiments. The IR absorbance should be independent of the potential as long as a change of the surface coverage and a reveberatmn of the adsorbate can be neglected. An IR study of the A g / C N system has been published recently [12] but a quantitative c o m p a r i s o n of the band intensities with the R a m a n results is still impossible due to the w e a k I R bands.
H Baltruschat, J Hettbaum / CT contrzbutton to SERS m Ag/Ag(CN)7
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3.2 The rate of ad-complex quenchmg
An irreversible quenching of the R a m a n intensity at potentials where the adsorbate desorbs has been shown for quite a number of SERS systems [1,5,14]. The rate of the quenching process, however, was quantitatively studied for the A g / S C N - system only [13]. N o such study exists for the A g / C N system as far as we know. We measured both the potential and temperature dependence of the quenching rate by two methods: (1) At moderate quenching potentials, the decrease of the R a m a n intensity could directly be measured after stepping from - 0 . 9 5 V to the selected potential. The initial slope of the curve obtained represents the initial rate of ad-atom quenching. This method ~s restricted to low rates, due to the necessarily large time constant of the amphfier and the high noise level. (2) The potential was stepped from - 0 . 9 5 V to the quenching potential for a time interval of some ms up to 1 s and then back to - 0.95 V. The difference of the R a m a n intensity at - 0 . 9 5 V before and after the quenching pulse was taken as a measure for the number of ad-complexes that had vanished. Applying different pulse times at a constant potential, intensity time curves could thus be constructed which correspond to those obtained with the method described above. In most cases, however, the initial rates reported below were calcultated from single step experiments at pulse times short enough to yield a decrease of intensity of about 10%. At pulse times smaller than 0.6 ms, no intensity change was observed at all. Therefore, this time was taken to be necessary for the double layer charging and all A t values were corrected by this value. In order to obtain reproducible results, the electrode had to be polished and actwated by an ORC to 0.14 V before each potential pulse. In no case, the intensity/time curves could be interpreted by simple first or second order kinetics. This was not to be expected, however, in the light of the heterogeneity of the surface: the least stable ad-complexes are discharged with a high rate, the more stable ones with a slow rate and the intensity time curves may be caused by a superposition of exponentials with different txme constants (cf. the evidence for the heterogeneity of the Ag surface outlined in ref. [15]. The potential dependence of the imtial quenching rate is shown in fig. 5. Two potential regions having different slopes can be discriminated and the change-over from one to the other regton seems to be non-monotonic. The temperature dependence of the quenching rate was measured at constant - 1 . 4 3 V and is represented m fig. 6. This potential was choosen m order to have a large temperature region available. From fig. 6, an activation energy of 75 kJ mol-1 is calculated together with an preexponential factor of 10145 s-1 With the typical value of the activation entropy of 25 J K -1 mo1-1 [16], a frequency factor of 1013 s -1 is obtained. Experiments at - 1 . 5 3 V in a
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necessarily limited temperature range and with poorer accuracy delivered an activation energy of 40-65 kJ m o l - 1. 4. Discussion 4.1. On the nature of ad-complexes Two kinds of Raman bands have been observed, one of them arismg at quite positive potentials, only, where Ag dissolution and redeposition occurs. It seems to be most probable that these bands correspond to different kinds of ad-complexes, namely adsorption at ad-atoms on a plane surface formed by dissolution and redeposition and adsorption on kinks and steps as was discussed above. Different results were obtained concerning a charge transfer contribution to the SERS enhancement from these ad-complexes. If the relatwe integrated intensity is plotted versus potential, this results in a high slope of 8huo/OEmax. If, on the other hand, the peak maximum of the main band is plotted versus potential, the maxima of the curves are independent of the exciting photon energy. Both results can be explained if two charge transfer contributions are involved which have different signs of their potential dependencies, as is dmcussed below.
4.2. The mechanism of ad-atom quenchmg Before coming back to the question of the charge transfer, we have to discuss the process of ad-atom quenching. Two mechanisms can be envisaged; see scheme I.
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The two mechanisms differ by the moment when the ad-complex is discharged. After mechanism A, the complex is first discharged and afterwards diffuses to the surface site where it 1s incorporated. After mechanism B, the ad-complex diffuses first to a free step and ~s afterwards discharged and incorporated. As was shown by measurmg the differential capacity of a smooth Ag-electrode, C N - desorbs between - 0 . 8 and - 1 . 2 V [17], It is likely that C N - desorptlon from steps occurs at more negative potentials. The step density in ref. [17] was probably low. The above mentioned mechanisms were written down in order to decide which step is rate determining on the basis of the Tafel slopes ( 0 E / 0 In t,, with t~ = ( A I / I ) / A t ) given in fig. 5. At potentials more positive than - 1 . 4 3 , the slope amounts - 6 0 m V / d e c a d e which means that potentml dependent equihbria preceed the rate determining step. This excludes reaction (c) in mechanism A to be rate determining at these potentials. Step (b) of mechanism A cannot be rate determining either, since both methods applied to measure the quenching rate, give identical results. In fact, the first method measures the rate of disappearance of Ag+L2, i.e. step (c) of mechanism A, whereas the second method yields the rate of the overall reaction including the irreversible incorporation of the ad-atoms. If on the other hand the Tafel slope is only determined by a pre-equllibrlum, e.g. by step (b) of mechanism B, It should be more negative than - 6 0 mV, since OE
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Here the two T represent the electrosorption valencies of reactions (a) and (c), respectively, and a the transfer coefficient of (c). The Tafel slope of 200 mV measured at potentials more negative than 1.43 V is best explained by a rate determanlng charge transfer with - 3' = X = 0.8 or even less due to the negative potentials applied. Although the high Tafel slope could also be explained by the potential dependence of 0, the abrupt change of the Tafel slope points to a change from mechanism B (step (c) rate determaning) at E > - 1.43 V to mechanism A (step (c) rate determining) at E < - 1.43 V. A more detailed discussion of this is given m ref. [22]. The above discussion shows that it cannot be definitely decided which of the above mechanisms is valid. It seems, however, highly probable that the e -
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2150 2100 2050 2000 1950 RAMAN SHIFT/cm-I Fig 7 CN stretching band at E r = -0.95 V and at E,n = - 1 35 V, step length of square wave potennal 40 ms each transfer is rate d e t e r m i n i n g in the negative p o t e n t i a l region. This a s s u m p t i o n is c o r r o b o r a t e d b y the activation energy m e a s u r e d which is typical for electron transfer processes to a d s o r b e d redox systems [19]. In a d d i t i o n , evidence for the f o r m a t i o n of A g ° L 2 is o b t a i n e d at - 1.35 V b y a new R a m a n b a n d arising at 2020 cm 1 which is shown in fig. 7. This b a n d was a l r e a d y o b s e r v e d b y F l e i s c h m a n n et al. [20] a n d assigned to the A g ° ( C N - ) 2 complex. Because of the irreversible quenching, quantitative m e a s u r e m e n t s c o u l d not be p e r f o r m e d , b u t the b a n d shown in fig. 7 does not vanish at larger umes, which indicates that the c o m p l e x is t h e r m o d y n a m i c a l l y stable at a m i n o r i t y of sites. 4 3. The model of a double charge transfer
T h e p r e c u r s o r of a h o m o g e n e o u s redox reaction can be nicely m o d e l l e d by so-called intervalence c o m p o u n d s in which two metal ions with different o x i d a t i o n states are linked b y a b i d e n t a t e hgand. The activation energy for the t h e r m a l charge transfer between the two redox centers can be c o r r e l a t e d with an optical charge transfer excitation [7,21]:
Eopt = 4 E a .
(1)
The equation holds for intervalence compounds assuming equal force con-
126
H Baltruschat, J Hettbaum / CT contribution to S E R S in A g / Ag(CN):
o)
Lb OC LU
CN- ITf o 56
ha
Ag*L2.
e-(EF)
Hc]b °
L2 Ag+ 2L (sol)
REACTION COORDINATE Fig g Potential energy d~agram for the discharge of the ad-complex, insert energy scheme for the optical translt~ons
stants for the metal to hgand bond and neglecting the level sphtting at the crossing point. Hush [7] has pointed out that this expression should also be valid for metal complexes adsorbed on a metal electrode where the charge transfer occurs between the adsorbed metal ion and the electrode. It ~s probably due to the low optical density of a monolayer of adsorbed complexes that such an optical transition has not been observed as far as we know. For the case of the adsorbed Ag(CN)~ complex, the situauon ~s depicted m fig. 8 in a slmphfied model. An electron transfer occurs - w~th a probablhty of x = 1 m the adiabatic case - if the metal hgand distance has the value of the crossing point of the two potentml curves. The mimma of both curves have about the same energy since at - 1 . 3 V some Ag+L2 is m eqmlibrlum with Ag°L2 and A g + 2L and the entropy differences are small (~< 100 J K -~ M o l i [19,22]). The reactaon coordinate of fig. 8 cannot be simply identified w~th the metal ligand distance, but ~t also contains contributions from the solvation shell and the surface-metal distance. Furthermore, the potential curve on the right side of fig. 8 is somewhat repulsive for the metal ligand distance. All this causes some uncertainty concerning the correct curvature of the potential curve, especially at larger values of the reaction coordinate Having these restrictions in mind, one can nevertheless use eq. (1) to roughly estimate the energy of an optical charge transfer from the measured actlvauon energy which was attributed to the e - transfer. A value of Eopt = 3 eV is obtained which can be taken as an upper limit, since the rtght potential curve is probably flatter than the left one. This energy corresponds to the transiUon of an electron from the Ferrm level of the metal to the Ag 5s orbital.
H Baltruschat, J Hettbaum / CTcontrtbutton to SERS m A g / A g ( C N ) ]
127
The Ag 5s level of the complex has a higher energy than E F because of its interaction with the C N - 50 MOs. The above assumption of an acceptor level is corroborated by the fact that Ag + complexes m solutxon own internal charge transfer bands in the range of 5 - 6 eV [23-26]. The complex KAg(CN)2 has two UV bands at 5.3 and 4.6 eV [27] which presumably are CT bands from an C N - 50 or let orbital to the Ag 5s orbital. Neglecting in a first approximation any differenc~es in the electromc relaxation energies *, we thus obtain the energy level scheme shown as inserted in fig. 8. Two possible CT transitions have to be assumed in the visible. Since they have opposite directions and therefore opposite potential dependence w~th respect to the tuning in and out of the resonance, this situation may account for the weak dependence of the potential of maximum intensity on the exciting photon energy. Actually, two maxima should have been observed which may not be resolved in our case. The charge transfer from adsorbed C N - to the Fermi level of the metal is exactly the one already proposed by Billmann and Otto [5]. Their extrapolatmn, however, of the hv(Em) relationship to zero photon energy seems to be questionable in view of the abovementioned relation between optical and thermal charge transfer. In fact, at a potential at which the energy of the optical transition becomes zero, an activationless thermal charge transfer would set in. Multiple electronic transitions in which an additmnal internal charge transfer in the complex is revolved, were already proposed by Watanabe et al. [28]. According to the arguments of these authors, these transitions are responsible for the high Ohvo/OEmax values observed in all SERS systems. Lombardi and coworkers [6] have found a hnear correlation between the potentml of maximum enhancement and the electron donor power of pyridine derivatives and cychc amines given by their Hammet o-parameter and the pK~ values, respectively. In the hght of the above model this can be understood in the following way: the higher the electron donor strength, the larger ~s the overlap between the metal ad-atom and the lone pair electrons of the pyridine N. This results in an increasing split of the levels since the Ag 5s is shifted to h~gher energies whereas the n-level is shifted downwards.
5. Conclusion
Ad-complexes seem to be involved in most SERS sytems [1,28]. Above it was shown on the basis of the potential dependent position of the Raman bands as well as by measuring the activation energy of the quenching rate that * F o r a C T t r a n s i t i o n to or from an electrode the electronic r e l a x a t m n energy c o r r e s p o n d s to the b u i l t up or loss of c o u l o m b interaction wtth the s c r e e m n g charge of the surface A j u s t i f i c a t i o n for ttus a p p r o x i m a t i o n Is g w e n m ref. [22]
128
H Baltruschat, J Hettbaum / CT contrtbutlon to S E R S m Ag/Ag(CN)2~
t h e r e is s o m e e v i d e n c e f o r t h e e x i s t e n c e o f a n a c c e p t o r level a b o u t 3 e V a b o v e E F. T h e l e v e l w a s a s c r i b e d t o a A g 5s o r b i t a l s h i f t e d u p w a r d s d u e t o t h e interaction with the ligands of the ad-complexes. It seems to be likely that this l e v e l p l a y s a k e y r o l e i n o t h e r S E R S s y s t e m s as well. F o r t h e c a s e o f p y r i d i n e adsorbed on Ag and Au, a &scussion of the electromc levels probably involved is g w e n m ref. [29].
References [1] R K Chang and T.E Furtak, Eds, Surface Enhanced Raman Scattering (Plenum, New York, London, 1982) [2] H Ueba, Surface Scl 131 (1983) 347 [3] T E. Furtak and S.H. Macomber, Chem. Phys Letters 95 (1983) 328 [4] J Thtetke, J Bdlmann and A Otto, in Proc 17th Jerusalem Symp m Quantum Chemistry and B~ochenustry, Intern Symp on Dynarmcs on Surfaces, Eds B Pullmann and J Jortner (Reldel, Dordrecht, 1984) [5] J. Bdlmann and A Otto, Surface Scl. 138 (1984) 1 [6] J R. Lombard1, R L Blrke, L A Sanchez, I Bernard and S C. Sun, Chem Phys Letters 104 (1984) 240 [71 N S Hush, Electrochlm Acta 13 (1968) 1005 [8] H Baltruschat and J Heltbaum, J Electroanal Chem. 157 (1983) 319 [9] H. Baltruschat and W Vlelst~ch, J. Electroanal Chem 154 (1983) 141 [10] R. Kotz, E Yeager, J. Electroanal Chem 123 (1981) 335 [11] M Flelschmann and I R Hill m ref [1] [12] H Selo, K Kunlmatsu and W G Golden, m 35th ISE Meeting Berkeley, CA, 1984, Extended Abstracts, A8-29, p 522. [13] M J. Weaver, F Barz, J.G Gordon II and M.R Pbalpott, Surface Scl 125 (1983) 409 [14] H Wetzel, H Gerlscher and B Pettlnger, Chem Phys Letters 78 (1981) 392 [15] S Farquharson, D Mllner, M.A. Taddayyonl and M J Weaver, J Electroanal Chem. 178 (1984) 143. [16] J T Hupp and M J Weaver, J Electroanal. Chem. 145 (1983) 43 [17] R Waser and K G Well, Ber. Bunsenges. Phys Chem 88 (1984) 714 [18] A.B Anderson, R. Kotz and E Yeager, Chem Phys Letters 82 (1981) 130 [19] T T - T LI, K L Guyer, C W Barr and M J. Weaver, J Electroanal Chem 164 (1984) 27 [20] M Flelschmann, P Graves, I Hdl, A Oliver and J Robinson, J Electroanal Chem 150 (1983) 33 [21] C Creutz, Pogr. Inorg. Chem 30 (1983) 1 [22] H Baltruschat, Ph D Thesis, Bonn (1985) [23] Y Brando and S Nagakura, Theoret. Claim Acta 9 (1968) 210. [24] J N. Murrel and S. Carter, J Chem Soc Suppl 2 (1964) 6185 [25] B Relchman and I Ehezer, J. Chem. Phys. 59 (1973) 5219 [26] D.J Greenslade and M.C R Symons, Trans Faraday Soc. 62 (1966) 307 [27] F. Gallals, Compt. Rend. 214 (1942) 552 [28] T. Watanabe, O Kawanarm, K. Honda and B. Pettmger, Chem Phys Letters 102 (1983) 565 [29] H Baltruschat, E Rach and J Heltbaum, J Electroanal Chem 194 (1985)109
113
EVIDENCE FOR A DOUBLE CHARGE TRANSFER CONTRIBUTION T O S E R S I N T H E A g / A g (CN) z S Y S T E M H. B A L T R U S C H A T a n d J. H E I T B A U M Instttute of Phystcal Cherntst~, Wegelerstrasse12, 5300 Bonn, Fed Rep of Germany
Received 28 March 1985; accepted for pubhcatmn 16 September 1985
Using a square wave potential perturbatmn and momtonng the Raman intensity m parallel, the potential dependence of Ag(CN)2 spectra was studied with ddferent excxtlng wavelengths A double peak structure was found for the C-N stretch frequency both peaks having different excitation characteristics In addition, the quenching rate of ad-complexes was measured at negative potentials together wlt.h the activation energy From EA = 75 kJ m o l - l the energy of an acceptor level can be estimated located about 3 eV above E F. This level presumably corresponds to the Ag 5s orbital which is sl'nfted upwards due to the interaction with the hgands of the ad-complex Since the CN- 5o or Dr orbltals form a donor level, the possibilityof a double charge transfer contribution to the SERS enhancement exists
I. Introduction In spite of the e n o r m o u s w o r k d o n e so far, there is still some c o n t r o v e r s y in the literature c o n c e r n i n g the e n h a n c e m e n t m e c h a n i s m of SERS. T h e significance of an e l e c t r o m a g n e t i c c o n t r i b u t i o n which p r o b a b l y arises f r o m localized p l a s m a resonances, is generally a c c e p t e d [1,2]. But there are n u m e r o u s results which stress the i m p o r t a n c e of chemical short range effects, e.g. the irreversible q u e n c t n n g of the R a m a n intensity at p o t e n t i a l s where the a d s o r b a t e d e s o r b s [1]. These o b s e r v a t i o n s have led to the a s s u m p t i o n of a r e s o n a n c e c o n t r i b u t i o n to S E R S which arises f r o m a charge transfer excitation b e t w e e n the m e t a l a n d an a c c e p t o r or d o n o r level of the adsorbate. This c o u l d i n d e e d be p r o v e n in electrochemacal systems, where the F e r m i level can be shifted relative to the levels in solution b y varying the a p p l i e d potential. Thus, the energy difference b e t w e e n E F a n d the a d s o r b a t e level can b e tuned into r e s o n a n c e with the exciting p h o t o n energy. M e a s u r i n g the R a m a n signal as function of the a p p h e d potential, i n t e n s i t y m a x i m a were o b t a i n e d , whose p o s i t i o n s are d e p e n d e n t of the exciting wavelength [3-6]. If the p h o t o n energy is p l o t t e d versus the p o t e n t i a l at which the m a x i m u m is observed, straight lines are o b t a i n e d whose slope Ohvo/~Ema x p h y s i c a l l y represents the p o t e n t i a l d e p e n d e n c e of the energy o f the a d s o r b a t e level with respect 0 0 3 9 - 6 0 2 8 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b h s h i n g Division)
114
H Baltruschat, J Hettbaum / C T contrlbutlon to S E R S in A g / A g( CN ) 2
to the Fermi energy. Depending on the nature of the adsorbate level, the respective slope should be > - 1 e V / V for a donor and < + 1 e V / V for an acceptor level. But all experiments done so far on Ag yielded slopes which exceeded an absolute value of one (e.g. 1 46 e V / V for pyrldlne in F - [3], 2 e V / V for pyridine in SO~- [4], > 2 e V / V for pyridme in C I - electrolyte [3,4] and even - 4 e V / V for CN [5]). This apparent contradiction to the theory was explained by local potenual effects [5], i,e. by an overshooting of the potential in the inner Helmholtz plane (IHP) due to specific ion adsorpUon. The absolute value, however, of this local potential cannot cause the unexpected high slope, but only its variation with the applied potential. Consequently, the slope can only be larger than one, if the coverage of specifically adsorbed ions changes with the potential. In the case of an ionic adsorbate such as C N - , the intensity-potential curves therefore do not reflect the charge transfer contribution to the SERS effect Gcv(l,o, E) alone, but its product with the surface coverage 0oN ( E ) . Consequently, the intensity m a x i m u m for a given excitation wavelength does not represent the resonance condition any longer. Only in the case of neutral adsorbates, the above interpretation may be valid, if coadsorption of the anions of the supporting electrolyte occurs. But F and SO~ are not specifically adsorbed. In the hght of the above consideration, we tried to ascertain for the case of the A g / C N - system that the high slope is not caused by a change of the cyanide coverage. To do so, a fast square wave potential perturbation was applied to the electrode, thereby hoping that the change of the potential dependent surface coverage was sufficiently slow to allow the R a m a n Intensity to be measured at almost constant coverage. In addition, the activation energy of the quenching rate at very negative potentials was measured. W~th certain plausible assumptions, the energy of an acceptor level of the ad-complex can be estimated from this value following a proposal of Hush [7]. Since the existence of a donor level was assumed by Billmann and Otto [5] already, we explain the apparent high slope In the case of A g / C N by the occurrence of two optical charge transfer excitations wtuch can both contribute to the enhancement of the SER signal.
2. Experimental The electrochemical set up and the optical alignment have been described elsewhere, already [8]. The p-polarized 458 n m and 514 nm lines of an Ar laser and the 647 nm line of a Kr laser were used with a power of about 70 mW at the sample. The Ag working electrode had a geometric surface area of 2 × 5 m m 2 and was embedded in epoxy resin. An Ag wire served as the counter electrode and
H Baltruschat, J Hettbaum / C T contrtbutlon to S E R S m A g / A g ( C N ) f
115
a saturated calomel electrode was used as the reference. It was always held at room temperature during the measurements of the activation energy. The three electrode compartments were separated by glass frits. The electrolyte was prepared from p.a. grade chemicals and Milhpore water and contained 0.01M K C N , 0.2M K O H , and 0.2M KC1. K O H was added to achieve a good conductwity which is needed for potential step experiments. KC1 was used to improve the effectiveness of the oxidation reduction cycle (ORC) necessary for activation. Prior to each experiment, the working electrode was pohshed with 13 and 1 /~m aluminla dispersed on a felt and afterwards subjected to an O R C which consisted of a potentml step from - 0 . 9 5 to 0.05 V, a linear sweep from 0.05 to 0.14 V (in some cases to 0.25 V) and back to - 0.25 V with a sweep rate of 0.25 V s - ~; finally the potential was stepped back to - 0 . 9 5 V and kept constant for 6 - 7 min in order to achieve an equlhbrated surface reconstruction. The potential step was introduced to keep the amount of dissolved Ag(CN)2 as low as possible. The reduced charge was about 11 m C cm -2. In all experiments an ~R compensation was applied. In order to measure the Raman intensity as function of the applied potential at the condition of almost constant surface coverage of C N - , the following method was applied: The potential of the working electrode was switched every 5 ms between a reference value of E r = - 0 . 9 5 V and a potential E m . During the first ms immediately after the step, the laser beam was interrupted by a hght chopper. During this period, the double layer charging was completed as was ascertained from c u r r e n t / t i m e curves and the t R drop measured with the galvanostatlc pulse method. During the following 4 ms, the R a m a n intensity was monitored at the respective potential, using an electronic switch which was synchronized with the potential program and which passed the signal of the photon counter (time constant < 0.1 ms) on to the two channels of an averaging amplifier. Thus, it was possible to record R a m a n spectra almost simultaneously at the two potentials. In the case of the above method, the intensity changes of the R a m a n bands should be mainly due to charge transfer effects as well as to alterations of the double layer structure. These relatively slow changes of the macroscopic surface roughness are completely cancelled out by taking the ratio of the R a m a n intensities at E m and E r. Moreover, on the basis of kinetic measurements [9] a time constant in the ms range ~s estimated for the incorporation of ad-complexes (with l 0 = 0.7 mA cm -2, ac~th = 0.4, a potential difference of 0.2 V and an assumed coverage corresponding to 100 /~C cm -2, ~---6.6 ms is calculated). Due to the averaging mode of detection, the influence of an alteration of the surface coverage is minimized as well. Spectra were recorded at 50 mV intervals starting from E m = E r = - 0 . 9 5 V both in positive and negative direction. The R a m a n intensity a t E r depends only little of E m except at extreme E m values, where the intensity at E r could change by a factor of up to 10.
116
H Baltruschat, J Hettbaum / CT contnbutton to SERS m A g / Ag(CN) f
3. Results 3 1 Dependence o f R a m a n mtenslty on potential and e x c m n g photon energy
Fig. 1 d~splays two R a m a n spectra o b t a i n e d at different p o t e n t t a l s E m t o g e t h e r with the s i m u l t a n e o u s l y m e a s u r e d reference s p e c t r a at E~. T h e followm g o b s e r v a t i o n s are typical for all s p e c t r a a n d i n d e p e n d e n t of the exciting wavelength: (1) T h e p e a k frequencies of the R a m a n b a n d s taken at E r = - 0 . 9 5 V were i n d e p e n d e n t of Em. T h e s p e c t r a r e c o r d e d at E m c o r r e s p o n d to those given in ref. [5] having an i d e n t i c a l f r e q u e n c y - p o t e n t i a l d e p e n d e n c e . This i n d i c a t e s that the c h a n g e of the p e a k posiUon ~s a fast, n a m e l y an electronic effect (shift of the electron density) as it was stated before [10]. (2) The F W H M of the b a n d m e a s u r e d at E m is d e p e n d e n t on the p o t e n h a l as
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it was observed already in ref. [5]. In addition we found that the F W H M of the band measured at E r is also dependent on the value of E m. Consequently, the broadening of the R a m a n band at more positive potentials is due to a slow non-electronic process. (3) At E r n = - 0 . 7 V , the signal evidently is composed of two superimposed bands as is evident from fig. 1. This double band structure is best developed when spectra at E m values are recorded at which dissolution of Ag occurs which is immediately redeposited during the E r phase. The double band is also visible a t E m = - 0 . 8 and - 0 . 6 5 V but assumes the form of a plateau at more positive potentials. If the Ag electrode had not been subjected to a dissolution-redeposition process, instead of the distinctive shoulder only a large F W H M and an asymmetric band shape would have to be seen a t E m > - 0 . 9 V. At E m < - 1 . 0 V, a double band structure was never observed. This second peak, i.e. the shoulder, which was not observed before, gives rise to the band broadening at more positive potentials, to the unsymmetrical band shape reported in the literature [11], and to the obvious scatter of the frequency values measured at these potentials (cf. fig. 9 in ref. [5]). It should be realized from fig. 1 that it is not this shoulder but the higher peak on the left side of the band at E m = - 0.7 V which corresponds to the peak at E m = - 1.3 V, as is revealed from the peak position of the band at E r. It is rather improbable that the shoulder arising at positive potentials belongs to an ad-complex with a different number of C N - ligands since the peak difference is rather small. It seems to be more likely that the two frequencies are due to different surface sites, i.e. a different number of nearest neighbours of the central Ag ÷ ion of the ad-complex. This can be visuahzed by assuming adsorption at ad-atoms on a plane on the one hand and on kinks or steps on the other hand. This v~ew is corroborated by the fact, that the shoulder is clearly discriminated only when Ag is dissolved and redeposited on account of the potential program applied, as well as by the potential dependenctes of the vibration frequencies. The fact, that the peak position of the shoulder is almost independent of the potential, points to a more or less flat adsorption of the linear complex on a plane. On the other hand, C N - adsorbed on steps or kink sites will be more directed into the solution in virtue of sterical reasons and therefore feel the potential drop within the I H P to a larger extent than the flat ad-complex. This implies that O h l ' 0 / O E m a x from the CT enhancement of the shoulder is very small. Since the shoulder disappears only slowly, it cannot be explained by electronic effects such as a band splitting due to interactions of the adsorbed oscillators, or different ~ ( E ) or I ( E ) dependencies of two ad-complexes existing in the whole potential range, or a potential dependence of the transition probability of the asymmetric CN vibration. Instead, the following two explanations can be vahd: either a restructuring of a part of the surface occurs as function of the potential, thus changing the number of nearest
118
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as function of the potential E m assuming the charge transfer characteristic of the two ad-complexes to be similar. This plot is shown in fig. 2 for two different exciting wavelengths. The general shape of the curves is in good agreement to those obtained with a linear sweep method using an O M A system [5]. Our curves are flatter, however, because they do not contain contributxons from changes of the surface structure and the coverage, these effects being cancelled by taking the ratio as indicated above. The obvious hysteresis of the curve cannot be ascribed to a slow response of the detector system. Instead, it shows that two kinds of ad-complexes are involved, one of which is irreversibly quenched at negative potentials. Fig. 3 shows the corresponding h v o versus Urea x relationship and a slope of - 4 to - 5 e V / V is obtained from the diagram. This value is even higher than that reported in the hterature. If the second explanation is assumed to be valid, namely that the part of the adsorbate responsible for the shoulder vanishes at negative potentials, it is suggestive to plot the non-integrated peak heights of the mare peak as function of E m. The result is shown in fig. 4 and mainly reflects the intensity dependence of the main peak on the potential, thereby neglecting the shoulder and assuming the F W H M to be independent of the potential. It is interesting to note that the maxima of the curves shown m fig. 4 are not shifted by the exciting wavelength within the limits of error. We thus have to envisage the possibility that there is no CT contribution to the enhancement of this peak and the wavelength dependence shown in fig. 3 may arise from different wavelength dependencies of peak and shoulder. Since in fig. 4 the shoulder still contributes to the intensity of the peak, it might as well be that the peak has a
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EIV Fig 4 Relauve peak mtensmes as function o f E.1 ( × ) v a r i a t i o n o f E m m anodlc dlrecUon starting from - 0 95 V , ( O ) reverse direction from - 0 4 t o - 1 37 V, (H) back again from - 1 35 to - 0 5
V
C T characteristic with a positive Oh~,o/OE. . . . . This w o u l d be the case if the a d - c o m p l e x contains an acceptor level as well and evidence for this will be given below. More insight into the above problem could be expected from a c o m p a r i s o n with IR experiments. The IR absorbance should be independent of the potential as long as a change of the surface coverage and a reveberatmn of the adsorbate can be neglected. An IR study of the A g / C N system has been published recently [12] but a quantitative c o m p a r i s o n of the band intensities with the R a m a n results is still impossible due to the w e a k I R bands.
H Baltruschat, J Hettbaum / CT contrzbutton to SERS m Ag/Ag(CN)7
121
3.2 The rate of ad-complex quenchmg
An irreversible quenching of the R a m a n intensity at potentials where the adsorbate desorbs has been shown for quite a number of SERS systems [1,5,14]. The rate of the quenching process, however, was quantitatively studied for the A g / S C N - system only [13]. N o such study exists for the A g / C N system as far as we know. We measured both the potential and temperature dependence of the quenching rate by two methods: (1) At moderate quenching potentials, the decrease of the R a m a n intensity could directly be measured after stepping from - 0 . 9 5 V to the selected potential. The initial slope of the curve obtained represents the initial rate of ad-atom quenching. This method ~s restricted to low rates, due to the necessarily large time constant of the amphfier and the high noise level. (2) The potential was stepped from - 0 . 9 5 V to the quenching potential for a time interval of some ms up to 1 s and then back to - 0.95 V. The difference of the R a m a n intensity at - 0 . 9 5 V before and after the quenching pulse was taken as a measure for the number of ad-complexes that had vanished. Applying different pulse times at a constant potential, intensity time curves could thus be constructed which correspond to those obtained with the method described above. In most cases, however, the initial rates reported below were calcultated from single step experiments at pulse times short enough to yield a decrease of intensity of about 10%. At pulse times smaller than 0.6 ms, no intensity change was observed at all. Therefore, this time was taken to be necessary for the double layer charging and all A t values were corrected by this value. In order to obtain reproducible results, the electrode had to be polished and actwated by an ORC to 0.14 V before each potential pulse. In no case, the intensity/time curves could be interpreted by simple first or second order kinetics. This was not to be expected, however, in the light of the heterogeneity of the surface: the least stable ad-complexes are discharged with a high rate, the more stable ones with a slow rate and the intensity time curves may be caused by a superposition of exponentials with different txme constants (cf. the evidence for the heterogeneity of the Ag surface outlined in ref. [15]. The potential dependence of the imtial quenching rate is shown in fig. 5. Two potential regions having different slopes can be discriminated and the change-over from one to the other regton seems to be non-monotonic. The temperature dependence of the quenching rate was measured at constant - 1 . 4 3 V and is represented m fig. 6. This potential was choosen m order to have a large temperature region available. From fig. 6, an activation energy of 75 kJ mol-1 is calculated together with an preexponential factor of 10145 s-1 With the typical value of the activation entropy of 25 J K -1 mo1-1 [16], a frequency factor of 1013 s -1 is obtained. Experiments at - 1 . 5 3 V in a
122
H Baltruschat, J Hettbaum
/
I
C T contnbutwn to S E R S m A g / A g ( C N ) f
i
~
r
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..,._,__,_-,--'--'--'--
I0 -i I
E .~ ~ .,.'~
200 mY/dec
10 -z <3
X
/
/
10-a
/.
x-"
60 mY/dec
/X ,/
/
10 -~
o x
o
X X
10-5
L
~
,
,
1.3
{
~
~
L
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1.~
I
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~
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.B
E/V Fig 5 ImUal quenching rate of the Raman intensity as funcUon of the quenching potential, ( O ) taken from intensity ume curves, ( x ) measured by short pulses
10-1
I
'
'1
i
[
'
I
i
I
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T E 5"10-2
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T-1/ K-1 Fig 6 Temperature dependence of the quenching rate ( x ) electrode was actwated at room temperature, ( O ) electrode was activated at measuring temperature
H Baltruschat, J Hettbaum / CT contrtbutton to SERS m A g / A g ( C N ) ~
123
necessarily limited temperature range and with poorer accuracy delivered an activation energy of 40-65 kJ m o l - 1. 4. Discussion 4.1. On the nature of ad-complexes Two kinds of Raman bands have been observed, one of them arismg at quite positive potentials, only, where Ag dissolution and redeposition occurs. It seems to be most probable that these bands correspond to different kinds of ad-complexes, namely adsorption at ad-atoms on a plane surface formed by dissolution and redeposition and adsorption on kinks and steps as was discussed above. Different results were obtained concerning a charge transfer contribution to the SERS enhancement from these ad-complexes. If the relatwe integrated intensity is plotted versus potential, this results in a high slope of 8huo/OEmax. If, on the other hand, the peak maximum of the main band is plotted versus potential, the maxima of the curves are independent of the exciting photon energy. Both results can be explained if two charge transfer contributions are involved which have different signs of their potential dependencies, as is dmcussed below.
4.2. The mechanism of ad-atom quenchmg Before coming back to the question of the charge transfer, we have to discuss the process of ad-atom quenching. Two mechanisms can be envisaged; see scheme I.
AgSLn
Ia']L Ag
s
Ag+L2 (c)~+e+ Ag°(L 2 ) b'l surface )~d±ffuslon Ag s (+2L)
AgSLn (eli[ Ag s
+ I
Ag+L2 surface
(b) dlffuslon AgSAg+L2 (c) ~ +e(AgSAg°(L2) ) Ag s + 2L
mechanlsm
A
mechan±sm
B
Scheme I Here, the following symbols are used Ag + L 2. Raman acUve Ag v+ ( C N - ) 2 - c o m p l e x with "/ partml charge: Ag s surface s~te where mcorporaUon of ad-atoms can occur, e g. kink or step, L C N - hgand, brackets assign eventual weakly bound structures; n number of hgands, l.e lor2
H Baltruschat, J Hettbaum / C T contribution to S E R S m A g / Ag(CN):
124
The two mechanisms differ by the moment when the ad-complex is discharged. After mechanism A, the complex is first discharged and afterwards diffuses to the surface site where it 1s incorporated. After mechanism B, the ad-complex diffuses first to a free step and ~s afterwards discharged and incorporated. As was shown by measurmg the differential capacity of a smooth Ag-electrode, C N - desorbs between - 0 . 8 and - 1 . 2 V [17], It is likely that C N - desorptlon from steps occurs at more negative potentials. The step density in ref. [17] was probably low. The above mentioned mechanisms were written down in order to decide which step is rate determining on the basis of the Tafel slopes ( 0 E / 0 In t,, with t~ = ( A I / I ) / A t ) given in fig. 5. At potentials more positive than - 1 . 4 3 , the slope amounts - 6 0 m V / d e c a d e which means that potentml dependent equihbria preceed the rate determining step. This excludes reaction (c) in mechanism A to be rate determining at these potentials. Step (b) of mechanism A cannot be rate determining either, since both methods applied to measure the quenching rate, give identical results. In fact, the first method measures the rate of disappearance of Ag+L2, i.e. step (c) of mechanism A, whereas the second method yields the rate of the overall reaction including the irreversible incorporation of the ad-atoms. If on the other hand the Tafel slope is only determined by a pre-equllibrlum, e.g. by step (b) of mechanism B, It should be more negative than - 6 0 mV, since OE
2.3RT
0 In t-----S>~~
> 60 mY,
because the electrosorptlon valency 3' can only be smaller than 1. The measured absolute value of the Tafel slope rather seems to be smaller than larger than 60 m V / d e c a d e , and therefore we assume the e transfer in mechanism B to be rate determining with an overall value for the Tafel slope given by 3E
In t,
>/
2.3RT
F(~,~ + a'yc )
> 60 mY.
Here the two T represent the electrosorption valencies of reactions (a) and (c), respectively, and a the transfer coefficient of (c). The Tafel slope of 200 mV measured at potentials more negative than 1.43 V is best explained by a rate determanlng charge transfer with - 3' = X = 0.8 or even less due to the negative potentials applied. Although the high Tafel slope could also be explained by the potential dependence of 0, the abrupt change of the Tafel slope points to a change from mechanism B (step (c) rate determaning) at E > - 1.43 V to mechanism A (step (c) rate determining) at E < - 1.43 V. A more detailed discussion of this is given m ref. [22]. The above discussion shows that it cannot be definitely decided which of the above mechanisms is valid. It seems, however, highly probable that the e -
H Baltruschat,J Heitbaum / CT contrtbutton to SERS m Ag/Ag(CN)f
125
-095V f -135V >.I.-Z LLI F-Z
I
I
I
I
I
2150 2100 2050 2000 1950 RAMAN SHIFT/cm-I Fig 7 CN stretching band at E r = -0.95 V and at E,n = - 1 35 V, step length of square wave potennal 40 ms each transfer is rate d e t e r m i n i n g in the negative p o t e n t i a l region. This a s s u m p t i o n is c o r r o b o r a t e d b y the activation energy m e a s u r e d which is typical for electron transfer processes to a d s o r b e d redox systems [19]. In a d d i t i o n , evidence for the f o r m a t i o n of A g ° L 2 is o b t a i n e d at - 1.35 V b y a new R a m a n b a n d arising at 2020 cm 1 which is shown in fig. 7. This b a n d was a l r e a d y o b s e r v e d b y F l e i s c h m a n n et al. [20] a n d assigned to the A g ° ( C N - ) 2 complex. Because of the irreversible quenching, quantitative m e a s u r e m e n t s c o u l d not be p e r f o r m e d , b u t the b a n d shown in fig. 7 does not vanish at larger umes, which indicates that the c o m p l e x is t h e r m o d y n a m i c a l l y stable at a m i n o r i t y of sites. 4 3. The model of a double charge transfer
T h e p r e c u r s o r of a h o m o g e n e o u s redox reaction can be nicely m o d e l l e d by so-called intervalence c o m p o u n d s in which two metal ions with different o x i d a t i o n states are linked b y a b i d e n t a t e hgand. The activation energy for the t h e r m a l charge transfer between the two redox centers can be c o r r e l a t e d with an optical charge transfer excitation [7,21]:
Eopt = 4 E a .
(1)
The equation holds for intervalence compounds assuming equal force con-
126
H Baltruschat, J Hettbaum / CT contribution to S E R S in A g / Ag(CN):
o)
Lb OC LU
CN- ITf o 56
ha
Ag*L2.
e-(EF)
Hc]b °
L2 Ag+ 2L (sol)
REACTION COORDINATE Fig g Potential energy d~agram for the discharge of the ad-complex, insert energy scheme for the optical translt~ons
stants for the metal to hgand bond and neglecting the level sphtting at the crossing point. Hush [7] has pointed out that this expression should also be valid for metal complexes adsorbed on a metal electrode where the charge transfer occurs between the adsorbed metal ion and the electrode. It ~s probably due to the low optical density of a monolayer of adsorbed complexes that such an optical transition has not been observed as far as we know. For the case of the adsorbed Ag(CN)~ complex, the situauon ~s depicted m fig. 8 in a slmphfied model. An electron transfer occurs - w~th a probablhty of x = 1 m the adiabatic case - if the metal hgand distance has the value of the crossing point of the two potentml curves. The mimma of both curves have about the same energy since at - 1 . 3 V some Ag+L2 is m eqmlibrlum with Ag°L2 and A g + 2L and the entropy differences are small (~< 100 J K -~ M o l i [19,22]). The reactaon coordinate of fig. 8 cannot be simply identified w~th the metal ligand distance, but ~t also contains contributions from the solvation shell and the surface-metal distance. Furthermore, the potential curve on the right side of fig. 8 is somewhat repulsive for the metal ligand distance. All this causes some uncertainty concerning the correct curvature of the potential curve, especially at larger values of the reaction coordinate Having these restrictions in mind, one can nevertheless use eq. (1) to roughly estimate the energy of an optical charge transfer from the measured actlvauon energy which was attributed to the e - transfer. A value of Eopt = 3 eV is obtained which can be taken as an upper limit, since the rtght potential curve is probably flatter than the left one. This energy corresponds to the transiUon of an electron from the Ferrm level of the metal to the Ag 5s orbital.
H Baltruschat, J Hettbaum / CTcontrtbutton to SERS m A g / A g ( C N ) ]
127
The Ag 5s level of the complex has a higher energy than E F because of its interaction with the C N - 50 MOs. The above assumption of an acceptor level is corroborated by the fact that Ag + complexes m solutxon own internal charge transfer bands in the range of 5 - 6 eV [23-26]. The complex KAg(CN)2 has two UV bands at 5.3 and 4.6 eV [27] which presumably are CT bands from an C N - 50 or let orbital to the Ag 5s orbital. Neglecting in a first approximation any differenc~es in the electromc relaxation energies *, we thus obtain the energy level scheme shown as inserted in fig. 8. Two possible CT transitions have to be assumed in the visible. Since they have opposite directions and therefore opposite potential dependence w~th respect to the tuning in and out of the resonance, this situation may account for the weak dependence of the potential of maximum intensity on the exciting photon energy. Actually, two maxima should have been observed which may not be resolved in our case. The charge transfer from adsorbed C N - to the Fermi level of the metal is exactly the one already proposed by Billmann and Otto [5]. Their extrapolatmn, however, of the hv(Em) relationship to zero photon energy seems to be questionable in view of the abovementioned relation between optical and thermal charge transfer. In fact, at a potential at which the energy of the optical transition becomes zero, an activationless thermal charge transfer would set in. Multiple electronic transitions in which an additmnal internal charge transfer in the complex is revolved, were already proposed by Watanabe et al. [28]. According to the arguments of these authors, these transitions are responsible for the high Ohvo/OEmax values observed in all SERS systems. Lombardi and coworkers [6] have found a hnear correlation between the potentml of maximum enhancement and the electron donor power of pyridine derivatives and cychc amines given by their Hammet o-parameter and the pK~ values, respectively. In the hght of the above model this can be understood in the following way: the higher the electron donor strength, the larger ~s the overlap between the metal ad-atom and the lone pair electrons of the pyridine N. This results in an increasing split of the levels since the Ag 5s is shifted to h~gher energies whereas the n-level is shifted downwards.
5. Conclusion
Ad-complexes seem to be involved in most SERS sytems [1,28]. Above it was shown on the basis of the potential dependent position of the Raman bands as well as by measuring the activation energy of the quenching rate that * F o r a C T t r a n s i t i o n to or from an electrode the electronic r e l a x a t m n energy c o r r e s p o n d s to the b u i l t up or loss of c o u l o m b interaction wtth the s c r e e m n g charge of the surface A j u s t i f i c a t i o n for ttus a p p r o x i m a t i o n Is g w e n m ref. [22]
128
H Baltruschat, J Hettbaum / CT contrtbutlon to S E R S m Ag/Ag(CN)2~
t h e r e is s o m e e v i d e n c e f o r t h e e x i s t e n c e o f a n a c c e p t o r level a b o u t 3 e V a b o v e E F. T h e l e v e l w a s a s c r i b e d t o a A g 5s o r b i t a l s h i f t e d u p w a r d s d u e t o t h e interaction with the ligands of the ad-complexes. It seems to be likely that this l e v e l p l a y s a k e y r o l e i n o t h e r S E R S s y s t e m s as well. F o r t h e c a s e o f p y r i d i n e adsorbed on Ag and Au, a &scussion of the electromc levels probably involved is g w e n m ref. [29].
References [1] R K Chang and T.E Furtak, Eds, Surface Enhanced Raman Scattering (Plenum, New York, London, 1982) [2] H Ueba, Surface Scl 131 (1983) 347 [3] T E. Furtak and S.H. Macomber, Chem. Phys Letters 95 (1983) 328 [4] J Thtetke, J Bdlmann and A Otto, in Proc 17th Jerusalem Symp m Quantum Chemistry and B~ochenustry, Intern Symp on Dynarmcs on Surfaces, Eds B Pullmann and J Jortner (Reldel, Dordrecht, 1984) [5] J. Bdlmann and A Otto, Surface Scl. 138 (1984) 1 [6] J R. Lombard1, R L Blrke, L A Sanchez, I Bernard and S C. Sun, Chem Phys Letters 104 (1984) 240 [71 N S Hush, Electrochlm Acta 13 (1968) 1005 [8] H Baltruschat and J Heltbaum, J Electroanal Chem. 157 (1983) 319 [9] H. Baltruschat and W Vlelst~ch, J. Electroanal Chem 154 (1983) 141 [10] R. Kotz, E Yeager, J. Electroanal Chem 123 (1981) 335 [11] M Flelschmann and I R Hill m ref [1] [12] H Selo, K Kunlmatsu and W G Golden, m 35th ISE Meeting Berkeley, CA, 1984, Extended Abstracts, A8-29, p 522. [13] M J. Weaver, F Barz, J.G Gordon II and M.R Pbalpott, Surface Scl 125 (1983) 409 [14] H Wetzel, H Gerlscher and B Pettlnger, Chem Phys Letters 78 (1981) 392 [15] S Farquharson, D Mllner, M.A. Taddayyonl and M J Weaver, J Electroanal Chem. 178 (1984) 143. [16] J T Hupp and M J Weaver, J Electroanal. Chem. 145 (1983) 43 [17] R Waser and K G Well, Ber. Bunsenges. Phys Chem 88 (1984) 714 [18] A.B Anderson, R. Kotz and E Yeager, Chem Phys Letters 82 (1981) 130 [19] T T - T LI, K L Guyer, C W Barr and M J. Weaver, J Electroanal Chem 164 (1984) 27 [20] M Flelschmann, P Graves, I Hdl, A Oliver and J Robinson, J Electroanal Chem 150 (1983) 33 [21] C Creutz, Pogr. Inorg. Chem 30 (1983) 1 [22] H Baltruschat, Ph D Thesis, Bonn (1985) [23] Y Brando and S Nagakura, Theoret. Claim Acta 9 (1968) 210. [24] J N. Murrel and S. Carter, J Chem Soc Suppl 2 (1964) 6185 [25] B Relchman and I Ehezer, J. Chem. Phys. 59 (1973) 5219 [26] D.J Greenslade and M.C R Symons, Trans Faraday Soc. 62 (1966) 307 [27] F. Gallals, Compt. Rend. 214 (1942) 552 [28] T. Watanabe, O Kawanarm, K. Honda and B. Pettmger, Chem Phys Letters 102 (1983) 565 [29] H Baltruschat, E Rach and J Heltbaum, J Electroanal Chem 194 (1985)109