- Email: [email protected]
Simultaneous raman and cyclic voltammetry study of the redox behavior of 4,4′-bipyridine on silver
Volume
106.
number
6
CHEMICAL
SIMULTANEOUS
Therese hi. COTTON
Received
The advantage
1983;
OF 4,4’-BWYRIDINE
STUDY
ON SILVER
and Mark VAVRA
of Chetnistry.
9 January
LETTERS
RAMAN AND CYCLIC VOLTAMMETRY
OF THE REDOX BEHAVIOR
Deportment
PHYSICS
Hiinois Institute of Tecltnology. Chicago, Illinois 606 16, USA in final form
of using an opriwl
29 February
multichannel
i983
Raman
spectrometer
to record
vihr~tlonal
taneously with elrctrochcmicsl studies is considerable. In this study. AelI)-?,3’-bipyridinc tored during the anodization of s~lvcr. At negative porcnrxds the reducrion of adsorbed upon
the bulk bipyridine
tinguished
from
concenlraIion.
those of bipyrtdine
in
Surface
enhanced
Raman
sc;lttrrmr
enabled
spectrd oi ;idsorbJrrs
simul-
complex formarlon was monibrpyridine 1~~s found IO depend the suriacc
redox
reJcrions
to be dir;-
solution.
I. introduction
of Ag4,4’-bipyridine complexes during the ORC of an Ag electrode. We have also used this approxh to unravel the comples redos behavior of bipyridine in the potential region from -0.6 V to -I .4 V versus SSCE.
There have been numerous studies of the experimental variables which contribute to the ma~itude of surface enhanced Raman scattering (SERS) from molecules adsorbed onto silver electrodes [ 11. Two important variables are the anodhation procedure or the oxidation reduction cycle (ORC) used to roughen the electrode surface and the electrode potential. Investigations of the ORC have shown that laser irradiation of the electrode during the procedure can result in significantly greater enhancement factors as compared to anodizations performed in the dark [34]. The formation of Ag adatom complexes with the electrolyte or adsorbate appears to increase the enhancement due to the formation of siIver microstructures. The second variable, the potential dependence of SERS signals, has been studied in detail for a large number of adsorbates. However, there have been relatively few studies of reversible electron transfer reactions in the ndsorbates as a function of electrode potential [S]. Yet, the potential of SERS for providing a molecular interpretation of voltammetric data is considerable as illustrated in the study by Farquharson et al!. [S] and the results reported herein. Through the use of a multich~nel analyzer (OMA U) we have monitored the formation and reduction 0 009-2614/84/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
2. Experimental Chemicals used in the experiments were the purest grades availably. Tha 4,3’-bip>Gdino was obtained from Aldrich Chemical Company and further purified by recrystallization from ethanoifwatzr (I/l _v/v). followed by vacuum sublimation. Burdick and Jackson HPCL grade water was used to prepare the solutions, which wzre degassed for 30 min with prepurified nitrogen. The electrolyte used in al1 of the experiments was 0.1 M Na,SO,. The electrochemical cell used for the cyclic voltammetry (CV) and SERS esperiments has been described [6] _The Ag electrode was constructed from flattened Ag wire, which was sealed into $ass tubing
with Torr-Seal. The exposed surface was approximately 2 X 10 mm. The silver was polished with 0.3 micron alumina on a mechanical polishing wheel prior to immersion in the electrolyte solution. A Pr wire constituted the auxiliary electrode and a satursted NaCl calomel (SSCE) electrode served as a reference. B.V
491
Vohxnc 106. number 6
CHEhilCAL PHYSICS LETTERS
The ORC used in these experiments consisted of a voltage ramp from -0.4 V to +0.45 V versus SSCE. The scan rate was IO mVls and the electrode was irradiated during the ORC. An argon ion laser (INNOVA 90-5) was used to excite Raman scattering. The SERS spectra were recorded in the backscattering geometry and an f/1.2 camera lens was used to focus the Raman scattered light onto the slits of the monochromator/spectrograph (Spex Triplemate 1877). Two 600 grlmm gratings were used in the mo~~hromator stage and a I200 gr/mm grating was used in the spectrograph
11 May 1984
triangular
----
A0
ELECTRODE
0.02 s -c 5
0.00
z 2 ii! I0
0.02’
stage.
Raman and SERS spectra were acquired on an OMA II equipped with a model 1420 intensified SiPD detector which was cooled to -10°C. The detector Integration time was 1 ~(60 delays) for ail of the specua reported here. The progr~abie Timing Mode 6 supplied In the OMA If softwan? was used to acquire data simultaneously with the CV scan. Data collection was synchronized through the external start
-0.04
’
-0.06
ii r0.4
I *0.2
I 0.0
I -0.2
1 -0.4
mode. Fig. 1. Cyeticvoitammo~r~m of a Ag electrode in 0.1 hf Na$OQ (dashed line) and with 1 X IO-* M 4,4’-bipytidineadded (solid
3. Results The presence of 4,4’-bipyridine during the ORC has a pronounced effect upon the oxidation of Ag as may be seen in fig. 1. The dotted curve depicts the oxidation and reduction of Ag in electrolyte only, whereas the solid curve was recorded with 1 X IO-’ M bipyridine in the electrolyte solution. A peak near +0.3 V is observed in the latter, which is attributed to the formation of Ag(I)4,4’-bipyridine complex In the diffusion layer and on the electrode surface_ The complex is reduced at potentials near +O.l V and -0.06 V. The second reduction peak may arise from a different structure and increases in intensity with repeated cycling, with a concomitant decrease in the height of the first reduction peak. The Ag(l)4,4’-bipyridine complex is highly insoluble and a white precipitate cari be observed near the surface of the electrode during the CV experiment. Raman spectra recorded simultaneously with the CV data are displayed in fig. 2. Spectra were acquiied every IO0 mV during the CV scan, but for simplicity not aII spectra are shown. Based upon the integration of charge passed during the oxidation cycle; 492
line). Scan rate = 10 mV/s; ausiliary clcctrode: Pt; refcrcnce electrode: wuratcd NaCl nlomel.
appro~mateIy 100 monoIayers of Ag are oxidized. From the estimated diffusion layer thickness (~1 X IO-* cm [7]) at this scan rate and the diffusion coefficient of bipyridine (7 X IOW6 cm2/s IS]), it can be calculated that there is sufficient bipyridine in the diffusion layer to complex essentially all of the Ag+ which is formed. Thus, there are undoubtedly
multilayers of the complex formed on the electrode surface and it is not possible to estimate the amount of enhancement resulting from the SERS effect. As may be seen in fig. 2, the Raman intensity decreases as the potentiai is scanned into the Ag oxidation region. In addition, shifts are observed in several bands (table 1) and these are characteristic of those observed on preparing the Ag(I)-4,4’-bipyridine com-
pfex in the solid state [9]. The structure of the complex has been described as a Iinear poiymer of Ag(1) by bipyridine molecules [IO]. As the potential is scanned into the region where the complex is reduced, the adsorbed bipyridine spectrum reverts to that of the uncomplexed moiecuIe. As noted for pyridine [I], small band shifts and relative Intensity ions bridged
Vofumc
106, number
CHESfICAL
6
1700
‘I 500
PHYSICS LETTERS
1300 WAVENUMBER
SO0
1100 SHIFT
CM
-1
Fig. 2. Surface Raman spectra of 1 X lo-’ $1 4,4’-bipyridine in 0.L M Nzr2S04 rrcordzd si~~ul~~~euu~l~ with cyclic voltsmmugram in fii. 1. Raman spectra were recorded cvrry 100 mV (not all sp~tra are shown); inrcpration time 1 s: lx+rr rucitarion line 51f.5 nm; potvcr 40 mW. Spccu~! shown are for the second scan.
differences occur for adsorbed bipyridine as compared to the solution species. Following the anodization cycle, the redox behavior of bipyridine was investigated. Fig. 3 shows the CV obtained for a 1 X IO- 2 M solution. Only a single, irreversible reduction peak is manifest near - 1.25 V, with the ~orre~ond~ng reoxidation peak occurring at -0.75 V. The voltammogram is reminiscent of rhe behavior of quinones in the presence of acids. Thus, the -1.25 V peak is assigned to the two-electron reduction of bipyridine coupled with the addition of two protons to form BiPyH?. The pK, values of bipyridine are reported as 3.5 and 4.9 in a solution of
0.3 M ionic strength [ 111. Under these experimental conditions (pfY = 6.4). only about 2.6% of the bipyridine is monoprotona:ed. However, upon reduction the anion radical is a stronger base and abstracts a proton from water. The unstable baseline fol’towing the reoxidation peak at --O-75 V is due to capacitance changes accompanying tie desorp tion of mu1 tilayers of bipyridine which are formed near the cathodic limit. Maintaining the elecrrode potential ~II -I .4 V for about 1 min results in an increase in the oxidation peak current and even greater baseline instability. Moreover, a purplish colored product (the protonated anion radical) can be seen streaming from 493
Cii~~il~AL
Volume 106, number 6 Table I Raman and SERS frequencies BiPy(solid)
for 4,4’-bipyridine
df V) =)
1295 1290 f29O
998 1013 1020
1529,
IS04 1502 153a.1499
1336.1290 1336, 1297(sh) 1334.1288
1039.1009 1039,100o 1040
990
1532.1499
1334.1295
1040
990
BiPy2-(ads) (- 1..4 V) e, BiPyH.(ads) (-1.2 V) f)
1644
BiPvH-(ads)
1641
I.591
V) f)
1512 IS07 1535.1517
1617
1599 1596 1592
(-1.4
1 I May 3984
a) 1615&h), 1606 1602 1613
b)
BiPy(ads) (-0.4 V) c) Biiy(ads) (+0.4 V) c) As(l)-BiPy(solid) BiPy-(ads) (-1.2
PHYSICS LETTERS
1515
1290
1024
frequencies are in cm-t _ b) Spectrum recorded in a 5 mm glass tube.
a) Vibrational
c) Spectru,n recorded during ORC d, A& complex prepared according e) Spectrum recorded during redor f&pectrum recorded during rcdos the
electrode
cycle (figs. 1.2). Numbers in parentheses refer to clectrodc potcnthl. to ref. 191. Spectrum recorded on solid in 5 mm glass tube. scan for mudified electrode in absence of bulk bipyridine. scan (i&s. 3.4). Bulk bipyridine concentration is 1 x 10m2 hf.
surface at about -0.75
the spectrum
V.
AG
of the adsorbed
tion of potential.
Support for the protonation of bipyridine upon reduction is provided by the changes in the Raman spectrum accompanying the reduction. Fig. 4 shows
Near
to the uncomplexed potential isscanned
-0.5
bipy~d~e
as a func-
V the spectrum
is similar
bipyridine in solution. As the to -0.9 V distinct spectral
ELECTRODE
4.4’-BIPYRDINE
1
x
to
-2
M
+
t
100
UAMP
z” ,” 0 3 0
VOLTS
VS
SCE
Fig. 3. Cyclic voltammogram of a Ag electrode in 0.1 hl NazSO;r i-n the presence of 1 x low2 ht 4,4!-bipyridinc. Experimental conditions were: scan rate IO mV/s~auGiiary electrode: Pt;reference electrode: saturated NaCI calomef;stmting potential -0.5 V and initial scan direction: cathodic. The voltammo~am was recorded following the anodization cycle shown in fig. I. 494
Volume
106. number
Fig. 4. Surfkcc simultoncously
6
CHEMICAL
PHYSICS
LETTERS
enhanced Raman spcctrs of 4,4’-bipyridine on Ap as a function of &xtrodc porent~~l with rhe cyclic voltammogram shown in fig. 3. The Raman pammeters wcrc idcntkal
changes occur, including especially the formation of shoulders on the high-frequency side of the 1602, 1507 and 1290cm-1 bands (table 1). Both the 1602 and 1507 cm- 1 band are comprised of mixed C=C and C=N stretching modes and the shoulders are assigned to the protonation of the ring nitrogens. The 1290 cm-l band results from the C-C inter-ring stretch and undergoes a shift to higher frequency upon reduction due to the increased bond order resulting from electron delocalization between the aromatic rings. A similar change is observed for the oneand two-electron reduction of biphenyl [12]. A detailed rationalization of these assignments will appear elsewhere [ 131. As the potential is scanned to a value near -1.2 V, the three shoulders increase in intensity together with the overall Raman intensity. Ar -1.4 V a decrease is observed in the bands associated with the protonated, reduced bipyridine and bands near 1500 and 1040 cm-t increase in intensity_ These are attributed to the two-electron reduced bi-
pyridine. AU on rescanning there are two on reduction BPy,,, &PYH;~,
fhc sprtctra were recorded to those gwcn in fig. 1.
of the changes are completely reversed the electrode potential IO -0.5 V. Thus, distinct spectral changes which occur of the adsorbed bipyridine:
+ e+ e-
+ Hi
w
u
BiPyH,, s BiPyH& ,
7
(1) (‘1
where BiFy,ds is adsorbed bipyridine. In addition to these reactions, adsorption of reduced bulk bipyridine also occurs at the electrode surface near the cathodic limit, which leads to multilayer formation. The effect of multilayer fomlation on tie Raman intensity is discussed below. Changes in the vibrational spectrum of adsorbed bipyridine occur at a potential considerably less negative than the reduction of bipyridine as indicated by CV. This is not surprising since the CV response is indicative of the heterogeneous electron transfer reaction between bulk bipyridine and the electrode at this 495
Volume 106, number 6
CHEMICAL PHYSICS LETTERS
Therefore, to eliminate the voltammetric response of the bulk bipyridine, the electrode and cell were washed with four aliquots of electrolyte solution following tie cathodic scan. The CV response was measured in the presence of fresh 0.1 M Na2S04 with no bipyridine added. A reversible, one-electron transfer peak was observed near -1 .O V and an irreversible electron transfer near -1.35 V. The current
tion in 1 X 10m4 M solutions of bipyridine and the surface spectra showed only slight protonation of the adsorbed bipyridine. Furthermore, variation of the solution pH supported the spectral interpretation. At a pH near 5.6 an increase in the protonated C=N mode at 1644 cm- I was observed. When thepH was
was proportional to scan rate in the case of the first peak, verifying the presence of adsorption. Moreover, the modified electrode was stable during repeated cycling. Integration of the area under the first reduction peak provided a value of 0.5 nmole/cm’ which is reasonable for a monolayer of adsorbed bipyridine. The Raman spectra of the bipyridine modified electrode were recorded in the electrolyte solution as a function of potential as well. The spectra were found to be as iniense as those shown in fig. 4. Thus, it appears that the spectra originate from a monolayer of bipyridine, i.e. the presence of multilayers when the bulk concentration is high does not increase the signal inteiizity. The overall enhancement produced by adsorption of bipyridine on silver can be calculated by a comparison of the 1290 cm- * peak intensity as a function of concentration at the surface with the similar ratio for bipyridine in solution. An enhancement factor of 1.5 X 106 was obtained. An important difference between the spectra of the modified electrode and that observed in the presence of bulk bipyridine is the lack of protonation on reduction in the case of the former. From this result it can be concluded that protonation occurs on multilayer formation. Based upon the pK, value for the fist protonation. it can be calculated that the con. centration of monoprotonated bipyridine is about 2.6 X 10m4 M in a solution containing an overall concentration of 1.0 X lo-? M. This is sufficiently concentrated for the formation of a monolayer of adsorbed bipyridine, which upon reduction interacts with subsequent layers of bipyridine through hydrogen bonding interactions:
strongly adsorbed [ 13 ] . The results of this study show that simultaneous Rarnan/cyclic voltammetry experiments can provide detailed molecular information concerning electron transfer reactions. Of course, the role of SERS in these experiments is crucial. in summary, the following conclusions can be drawn: (I) The presence of 4,4’-bipyridine during the anodization of Ag results in the formation of Ag(I)4,4’-bipyridine complex in the diffusion layer and on the surface of the silver electrode. The concentration of bipyridine during anodization has a significant effect on the enhancement factor observed in SERS [ 131. At high concentrations, the enhancement is about ten times greater than observed for low concentrations(i.e. lo-’ M versus IO-6 M). This may be related to the formation of microstructures as discussed in rzfs. [Z-l]. (2) Cyclic voltarnmetry and SERS indicate that when the bulk pyridine concentration is high (IO- 2 M), the reduction of surface bound bipyridine proceeds through two one-electron steps. A protonation reaction is involved in the first step when the bulk bipyridine concentration is high, but is absent at low concentrations. In the former case, multilayers of bipyridine are adsorbed onto the electrode during the reduction and are desorbed during oxidation of bipyridine. (3) Bipyridine interacts strongly with the silver surface. Following the reduction and reoxidation cycle in concentrated solution, the electrode can be removed, washed, and still exhibits spectra comparable to those seen when the bulk bipyridine concentration is high. However, no protonation of the anion radical occurs under these conditions. Thus, it appears the strong surface spectra result from the first layer of adsorbed bipyridine. The increase in signal intensity with reduction probably results from a resonance Raman effect, since the anion radical is colored. (4) Cyclic voltammetry measurements on the
concentration.
dNC,
H, -C, H,NH..
. NC, H, -C, H, N ,
where the hatched area indicates the electrode surface. It should be noted that 1 X IO- 2 M is near the solubility limit of bipyridine in water at pH 6.4 and multilayer adsorption is probably favored under these conditions. We saw no evidence of multilayer forma496
lowered to 2.9, however, only very weak spectra resulted, indicating that the diprotonated species is not
Volume 106, number 6
CHEhIlCAL PHYSICS LETTERS
modified electrode indicate a surface coverage of which is consistent with a about 0.5 nmole/cm2, monolayer of adsorbed bipyridine. The SERS enhancement factor was estimated at 1.5 X 106.
Acknowledgement
[ 21
S.H. Macombcr. T.E. Furt3k and TA. Dcvine. Chcm. Phys. Letters 90 (1967-j 439. 13) F. Barz. J.C. Gordon II. M.R. Phiipott and M.J. Wesvcr, Chcm. Phys. Letrers 9 1 (1982) 19 I. 141 T.T. Chcn, K.U. “on Rabcn, J.F. O\vcn. R.R. Chzmg and B.L. Lxrbe. Chem. Phys. Lcrtrrs 91 (1982) 494. [S] S. Farquharson. h1.J. Wcaver.?.A. Lay. R.H. !Il+uson and H. Taubc, J. Am. Chem. Sot. 105 (1983) 3350. 161 D.L. Jcnnmdirc and R.P. van Duyne, J. Elcctro~nal. Chcm.
The authors appreciate the helpful comments and suggestions of Richard P. van Duyne. Support of this research by the National Institutes of Health (GM30240) is gratefully acknowledged.
[S] 191 [IO]
References
[ I1 ] [ l]
R.P. wn Duyne, in: Chemical and biochemical applintions of lasers, Vol. 4, cd. C.B. Moore (Academic Press, New York. 1979) ch. 5.
81
(I 977)
1.
[ 71 A.J. Bard 3nd L.R. Faulkner,
[ 121 [ 131
Electrochcminl methods (Wiley, New York, 1980) p. 17-9. J. Volke and V. Volkova, Coil. Czech. Chem. Commun. 37 (197’) 3686. W.J. Peard and R.T. Pilaum. J. Am. Chum. Sot. 80 (1958) 1593. IS. Ahuja, R. Singh and C.P. Rai. J. Inorg. Nucl. Chem. 40 (1978) 921. T.R. Musswe and C.E. Mattson. Inor:. Chum. 7 (1968) 1433. C. Tskuhashi and S. Maeda, Chcm. Phys. Letters 21 (1974) 581. TM. Cotton, manuscript in preparation.
497
106.
number
6
CHEMICAL
SIMULTANEOUS
Therese hi. COTTON
Received
The advantage
1983;
OF 4,4’-BWYRIDINE
STUDY
ON SILVER
and Mark VAVRA
of Chetnistry.
9 January
LETTERS
RAMAN AND CYCLIC VOLTAMMETRY
OF THE REDOX BEHAVIOR
Deportment
PHYSICS
Hiinois Institute of Tecltnology. Chicago, Illinois 606 16, USA in final form
of using an opriwl
29 February
multichannel
i983
Raman
spectrometer
to record
vihr~tlonal
taneously with elrctrochcmicsl studies is considerable. In this study. AelI)-?,3’-bipyridinc tored during the anodization of s~lvcr. At negative porcnrxds the reducrion of adsorbed upon
the bulk bipyridine
tinguished
from
concenlraIion.
those of bipyrtdine
in
Surface
enhanced
Raman
sc;lttrrmr
enabled
spectrd oi ;idsorbJrrs
simul-
complex formarlon was monibrpyridine 1~~s found IO depend the suriacc
redox
reJcrions
to be dir;-
solution.
I. introduction
of Ag4,4’-bipyridine complexes during the ORC of an Ag electrode. We have also used this approxh to unravel the comples redos behavior of bipyridine in the potential region from -0.6 V to -I .4 V versus SSCE.
There have been numerous studies of the experimental variables which contribute to the ma~itude of surface enhanced Raman scattering (SERS) from molecules adsorbed onto silver electrodes [ 11. Two important variables are the anodhation procedure or the oxidation reduction cycle (ORC) used to roughen the electrode surface and the electrode potential. Investigations of the ORC have shown that laser irradiation of the electrode during the procedure can result in significantly greater enhancement factors as compared to anodizations performed in the dark [34]. The formation of Ag adatom complexes with the electrolyte or adsorbate appears to increase the enhancement due to the formation of siIver microstructures. The second variable, the potential dependence of SERS signals, has been studied in detail for a large number of adsorbates. However, there have been relatively few studies of reversible electron transfer reactions in the ndsorbates as a function of electrode potential [S]. Yet, the potential of SERS for providing a molecular interpretation of voltammetric data is considerable as illustrated in the study by Farquharson et al!. [S] and the results reported herein. Through the use of a multich~nel analyzer (OMA U) we have monitored the formation and reduction 0 009-2614/84/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
2. Experimental Chemicals used in the experiments were the purest grades availably. Tha 4,3’-bip>Gdino was obtained from Aldrich Chemical Company and further purified by recrystallization from ethanoifwatzr (I/l _v/v). followed by vacuum sublimation. Burdick and Jackson HPCL grade water was used to prepare the solutions, which wzre degassed for 30 min with prepurified nitrogen. The electrolyte used in al1 of the experiments was 0.1 M Na,SO,. The electrochemical cell used for the cyclic voltammetry (CV) and SERS esperiments has been described [6] _The Ag electrode was constructed from flattened Ag wire, which was sealed into $ass tubing
with Torr-Seal. The exposed surface was approximately 2 X 10 mm. The silver was polished with 0.3 micron alumina on a mechanical polishing wheel prior to immersion in the electrolyte solution. A Pr wire constituted the auxiliary electrode and a satursted NaCl calomel (SSCE) electrode served as a reference. B.V
491
Vohxnc 106. number 6
CHEhilCAL PHYSICS LETTERS
The ORC used in these experiments consisted of a voltage ramp from -0.4 V to +0.45 V versus SSCE. The scan rate was IO mVls and the electrode was irradiated during the ORC. An argon ion laser (INNOVA 90-5) was used to excite Raman scattering. The SERS spectra were recorded in the backscattering geometry and an f/1.2 camera lens was used to focus the Raman scattered light onto the slits of the monochromator/spectrograph (Spex Triplemate 1877). Two 600 grlmm gratings were used in the mo~~hromator stage and a I200 gr/mm grating was used in the spectrograph
11 May 1984
triangular
----
A0
ELECTRODE
0.02 s -c 5
0.00
z 2 ii! I0
0.02’
stage.
Raman and SERS spectra were acquired on an OMA II equipped with a model 1420 intensified SiPD detector which was cooled to -10°C. The detector Integration time was 1 ~(60 delays) for ail of the specua reported here. The progr~abie Timing Mode 6 supplied In the OMA If softwan? was used to acquire data simultaneously with the CV scan. Data collection was synchronized through the external start
-0.04
’
-0.06
ii r0.4
I *0.2
I 0.0
I -0.2
1 -0.4
mode. Fig. 1. Cyeticvoitammo~r~m of a Ag electrode in 0.1 hf Na$OQ (dashed line) and with 1 X IO-* M 4,4’-bipytidineadded (solid
3. Results The presence of 4,4’-bipyridine during the ORC has a pronounced effect upon the oxidation of Ag as may be seen in fig. 1. The dotted curve depicts the oxidation and reduction of Ag in electrolyte only, whereas the solid curve was recorded with 1 X IO-’ M bipyridine in the electrolyte solution. A peak near +0.3 V is observed in the latter, which is attributed to the formation of Ag(I)4,4’-bipyridine complex In the diffusion layer and on the electrode surface_ The complex is reduced at potentials near +O.l V and -0.06 V. The second reduction peak may arise from a different structure and increases in intensity with repeated cycling, with a concomitant decrease in the height of the first reduction peak. The Ag(l)4,4’-bipyridine complex is highly insoluble and a white precipitate cari be observed near the surface of the electrode during the CV experiment. Raman spectra recorded simultaneously with the CV data are displayed in fig. 2. Spectra were acquiied every IO0 mV during the CV scan, but for simplicity not aII spectra are shown. Based upon the integration of charge passed during the oxidation cycle; 492
line). Scan rate = 10 mV/s; ausiliary clcctrode: Pt; refcrcnce electrode: wuratcd NaCl nlomel.
appro~mateIy 100 monoIayers of Ag are oxidized. From the estimated diffusion layer thickness (~1 X IO-* cm [7]) at this scan rate and the diffusion coefficient of bipyridine (7 X IOW6 cm2/s IS]), it can be calculated that there is sufficient bipyridine in the diffusion layer to complex essentially all of the Ag+ which is formed. Thus, there are undoubtedly
multilayers of the complex formed on the electrode surface and it is not possible to estimate the amount of enhancement resulting from the SERS effect. As may be seen in fig. 2, the Raman intensity decreases as the potentiai is scanned into the Ag oxidation region. In addition, shifts are observed in several bands (table 1) and these are characteristic of those observed on preparing the Ag(I)-4,4’-bipyridine com-
pfex in the solid state [9]. The structure of the complex has been described as a Iinear poiymer of Ag(1) by bipyridine molecules [IO]. As the potential is scanned into the region where the complex is reduced, the adsorbed bipyridine spectrum reverts to that of the uncomplexed moiecuIe. As noted for pyridine [I], small band shifts and relative Intensity ions bridged
Vofumc
106, number
CHESfICAL
6
1700
‘I 500
PHYSICS LETTERS
1300 WAVENUMBER
SO0
1100 SHIFT
CM
-1
Fig. 2. Surface Raman spectra of 1 X lo-’ $1 4,4’-bipyridine in 0.L M Nzr2S04 rrcordzd si~~ul~~~euu~l~ with cyclic voltsmmugram in fii. 1. Raman spectra were recorded cvrry 100 mV (not all sp~tra are shown); inrcpration time 1 s: lx+rr rucitarion line 51f.5 nm; potvcr 40 mW. Spccu~! shown are for the second scan.
differences occur for adsorbed bipyridine as compared to the solution species. Following the anodization cycle, the redox behavior of bipyridine was investigated. Fig. 3 shows the CV obtained for a 1 X IO- 2 M solution. Only a single, irreversible reduction peak is manifest near - 1.25 V, with the ~orre~ond~ng reoxidation peak occurring at -0.75 V. The voltammogram is reminiscent of rhe behavior of quinones in the presence of acids. Thus, the -1.25 V peak is assigned to the two-electron reduction of bipyridine coupled with the addition of two protons to form BiPyH?. The pK, values of bipyridine are reported as 3.5 and 4.9 in a solution of
0.3 M ionic strength [ 111. Under these experimental conditions (pfY = 6.4). only about 2.6% of the bipyridine is monoprotona:ed. However, upon reduction the anion radical is a stronger base and abstracts a proton from water. The unstable baseline fol’towing the reoxidation peak at --O-75 V is due to capacitance changes accompanying tie desorp tion of mu1 tilayers of bipyridine which are formed near the cathodic limit. Maintaining the elecrrode potential ~II -I .4 V for about 1 min results in an increase in the oxidation peak current and even greater baseline instability. Moreover, a purplish colored product (the protonated anion radical) can be seen streaming from 493
Cii~~il~AL
Volume 106, number 6 Table I Raman and SERS frequencies BiPy(solid)
for 4,4’-bipyridine
df V) =)
1295 1290 f29O
998 1013 1020
1529,
IS04 1502 153a.1499
1336.1290 1336, 1297(sh) 1334.1288
1039.1009 1039,100o 1040
990
1532.1499
1334.1295
1040
990
BiPy2-(ads) (- 1..4 V) e, BiPyH.(ads) (-1.2 V) f)
1644
BiPvH-(ads)
1641
I.591
V) f)
1512 IS07 1535.1517
1617
1599 1596 1592
(-1.4
1 I May 3984
a) 1615&h), 1606 1602 1613
b)
BiPy(ads) (-0.4 V) c) Biiy(ads) (+0.4 V) c) As(l)-BiPy(solid) BiPy-(ads) (-1.2
PHYSICS LETTERS
1515
1290
1024
frequencies are in cm-t _ b) Spectrum recorded in a 5 mm glass tube.
a) Vibrational
c) Spectru,n recorded during ORC d, A& complex prepared according e) Spectrum recorded during redor f&pectrum recorded during rcdos the
electrode
cycle (figs. 1.2). Numbers in parentheses refer to clectrodc potcnthl. to ref. 191. Spectrum recorded on solid in 5 mm glass tube. scan for mudified electrode in absence of bulk bipyridine. scan (i&s. 3.4). Bulk bipyridine concentration is 1 x 10m2 hf.
surface at about -0.75
the spectrum
V.
AG
of the adsorbed
tion of potential.
Support for the protonation of bipyridine upon reduction is provided by the changes in the Raman spectrum accompanying the reduction. Fig. 4 shows
Near
to the uncomplexed potential isscanned
-0.5
bipy~d~e
as a func-
V the spectrum
is similar
bipyridine in solution. As the to -0.9 V distinct spectral
ELECTRODE
4.4’-BIPYRDINE
1
x
to
-2
M
+
t
100
UAMP
z” ,” 0 3 0
VOLTS
VS
SCE
Fig. 3. Cyclic voltammogram of a Ag electrode in 0.1 hl NazSO;r i-n the presence of 1 x low2 ht 4,4!-bipyridinc. Experimental conditions were: scan rate IO mV/s~auGiiary electrode: Pt;reference electrode: saturated NaCI calomef;stmting potential -0.5 V and initial scan direction: cathodic. The voltammo~am was recorded following the anodization cycle shown in fig. I. 494
Volume
106. number
Fig. 4. Surfkcc simultoncously
6
CHEMICAL
PHYSICS
LETTERS
enhanced Raman spcctrs of 4,4’-bipyridine on Ap as a function of &xtrodc porent~~l with rhe cyclic voltammogram shown in fig. 3. The Raman pammeters wcrc idcntkal
changes occur, including especially the formation of shoulders on the high-frequency side of the 1602, 1507 and 1290cm-1 bands (table 1). Both the 1602 and 1507 cm- 1 band are comprised of mixed C=C and C=N stretching modes and the shoulders are assigned to the protonation of the ring nitrogens. The 1290 cm-l band results from the C-C inter-ring stretch and undergoes a shift to higher frequency upon reduction due to the increased bond order resulting from electron delocalization between the aromatic rings. A similar change is observed for the oneand two-electron reduction of biphenyl [12]. A detailed rationalization of these assignments will appear elsewhere [ 131. As the potential is scanned to a value near -1.2 V, the three shoulders increase in intensity together with the overall Raman intensity. Ar -1.4 V a decrease is observed in the bands associated with the protonated, reduced bipyridine and bands near 1500 and 1040 cm-t increase in intensity_ These are attributed to the two-electron reduced bi-
pyridine. AU on rescanning there are two on reduction BPy,,, &PYH;~,
fhc sprtctra were recorded to those gwcn in fig. 1.
of the changes are completely reversed the electrode potential IO -0.5 V. Thus, distinct spectral changes which occur of the adsorbed bipyridine:
+ e+ e-
+ Hi
w
u
BiPyH,, s BiPyH& ,
7
(1) (‘1
where BiFy,ds is adsorbed bipyridine. In addition to these reactions, adsorption of reduced bulk bipyridine also occurs at the electrode surface near the cathodic limit, which leads to multilayer formation. The effect of multilayer fomlation on tie Raman intensity is discussed below. Changes in the vibrational spectrum of adsorbed bipyridine occur at a potential considerably less negative than the reduction of bipyridine as indicated by CV. This is not surprising since the CV response is indicative of the heterogeneous electron transfer reaction between bulk bipyridine and the electrode at this 495
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CHEMICAL PHYSICS LETTERS
Therefore, to eliminate the voltammetric response of the bulk bipyridine, the electrode and cell were washed with four aliquots of electrolyte solution following tie cathodic scan. The CV response was measured in the presence of fresh 0.1 M Na2S04 with no bipyridine added. A reversible, one-electron transfer peak was observed near -1 .O V and an irreversible electron transfer near -1.35 V. The current
tion in 1 X 10m4 M solutions of bipyridine and the surface spectra showed only slight protonation of the adsorbed bipyridine. Furthermore, variation of the solution pH supported the spectral interpretation. At a pH near 5.6 an increase in the protonated C=N mode at 1644 cm- I was observed. When thepH was
was proportional to scan rate in the case of the first peak, verifying the presence of adsorption. Moreover, the modified electrode was stable during repeated cycling. Integration of the area under the first reduction peak provided a value of 0.5 nmole/cm’ which is reasonable for a monolayer of adsorbed bipyridine. The Raman spectra of the bipyridine modified electrode were recorded in the electrolyte solution as a function of potential as well. The spectra were found to be as iniense as those shown in fig. 4. Thus, it appears that the spectra originate from a monolayer of bipyridine, i.e. the presence of multilayers when the bulk concentration is high does not increase the signal inteiizity. The overall enhancement produced by adsorption of bipyridine on silver can be calculated by a comparison of the 1290 cm- * peak intensity as a function of concentration at the surface with the similar ratio for bipyridine in solution. An enhancement factor of 1.5 X 106 was obtained. An important difference between the spectra of the modified electrode and that observed in the presence of bulk bipyridine is the lack of protonation on reduction in the case of the former. From this result it can be concluded that protonation occurs on multilayer formation. Based upon the pK, value for the fist protonation. it can be calculated that the con. centration of monoprotonated bipyridine is about 2.6 X 10m4 M in a solution containing an overall concentration of 1.0 X lo-? M. This is sufficiently concentrated for the formation of a monolayer of adsorbed bipyridine, which upon reduction interacts with subsequent layers of bipyridine through hydrogen bonding interactions:
strongly adsorbed [ 13 ] . The results of this study show that simultaneous Rarnan/cyclic voltammetry experiments can provide detailed molecular information concerning electron transfer reactions. Of course, the role of SERS in these experiments is crucial. in summary, the following conclusions can be drawn: (I) The presence of 4,4’-bipyridine during the anodization of Ag results in the formation of Ag(I)4,4’-bipyridine complex in the diffusion layer and on the surface of the silver electrode. The concentration of bipyridine during anodization has a significant effect on the enhancement factor observed in SERS [ 131. At high concentrations, the enhancement is about ten times greater than observed for low concentrations(i.e. lo-’ M versus IO-6 M). This may be related to the formation of microstructures as discussed in rzfs. [Z-l]. (2) Cyclic voltarnmetry and SERS indicate that when the bulk pyridine concentration is high (IO- 2 M), the reduction of surface bound bipyridine proceeds through two one-electron steps. A protonation reaction is involved in the first step when the bulk bipyridine concentration is high, but is absent at low concentrations. In the former case, multilayers of bipyridine are adsorbed onto the electrode during the reduction and are desorbed during oxidation of bipyridine. (3) Bipyridine interacts strongly with the silver surface. Following the reduction and reoxidation cycle in concentrated solution, the electrode can be removed, washed, and still exhibits spectra comparable to those seen when the bulk bipyridine concentration is high. However, no protonation of the anion radical occurs under these conditions. Thus, it appears the strong surface spectra result from the first layer of adsorbed bipyridine. The increase in signal intensity with reduction probably results from a resonance Raman effect, since the anion radical is colored. (4) Cyclic voltammetry measurements on the
concentration.
dNC,
H, -C, H,NH..
. NC, H, -C, H, N ,
where the hatched area indicates the electrode surface. It should be noted that 1 X IO- 2 M is near the solubility limit of bipyridine in water at pH 6.4 and multilayer adsorption is probably favored under these conditions. We saw no evidence of multilayer forma496
lowered to 2.9, however, only very weak spectra resulted, indicating that the diprotonated species is not
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CHEhIlCAL PHYSICS LETTERS
modified electrode indicate a surface coverage of which is consistent with a about 0.5 nmole/cm2, monolayer of adsorbed bipyridine. The SERS enhancement factor was estimated at 1.5 X 106.
Acknowledgement
[ 21
S.H. Macombcr. T.E. Furt3k and TA. Dcvine. Chcm. Phys. Letters 90 (1967-j 439. 13) F. Barz. J.C. Gordon II. M.R. Phiipott and M.J. Wesvcr, Chcm. Phys. Letrers 9 1 (1982) 19 I. 141 T.T. Chcn, K.U. “on Rabcn, J.F. O\vcn. R.R. Chzmg and B.L. Lxrbe. Chem. Phys. Lcrtrrs 91 (1982) 494. [S] S. Farquharson. h1.J. Wcaver.?.A. Lay. R.H. !Il+uson and H. Taubc, J. Am. Chem. Sot. 105 (1983) 3350. 161 D.L. Jcnnmdirc and R.P. van Duyne, J. Elcctro~nal. Chcm.
The authors appreciate the helpful comments and suggestions of Richard P. van Duyne. Support of this research by the National Institutes of Health (GM30240) is gratefully acknowledged.
[S] 191 [IO]
References
[ I1 ] [ l]
R.P. wn Duyne, in: Chemical and biochemical applintions of lasers, Vol. 4, cd. C.B. Moore (Academic Press, New York. 1979) ch. 5.
81
(I 977)
1.
[ 71 A.J. Bard 3nd L.R. Faulkner,
[ 121 [ 131
Electrochcminl methods (Wiley, New York, 1980) p. 17-9. J. Volke and V. Volkova, Coil. Czech. Chem. Commun. 37 (197’) 3686. W.J. Peard and R.T. Pilaum. J. Am. Chum. Sot. 80 (1958) 1593. IS. Ahuja, R. Singh and C.P. Rai. J. Inorg. Nucl. Chem. 40 (1978) 921. T.R. Musswe and C.E. Mattson. Inor:. Chum. 7 (1968) 1433. C. Tskuhashi and S. Maeda, Chcm. Phys. Letters 21 (1974) 581. TM. Cotton, manuscript in preparation.
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