Langmuir 1994,10, 723-729
723
Formation of Two-Dimensional Phases of 4,4'-Bipyridine Cation Radical over Mercury in the Presence of Iodide Ions Manuel S h c h e z Maestre, Rafael Rodrfguez-Ammo, Eulogia Mufioz, Juan Jose Ruiz, and Luis Camacho* Department of Physical Chemistry and Applied Thermodynamics, Faculty of Sciences, University of Cbrdoba, Avda Sun Albert0 Magno sln, E-14004 Cbrdoba, Spain Received July 16, 1993. In Final Form: November 18,199P
The behavior of 4,4'-bipyridine cation radical (bpyHz'+) over mercury in the presence of iodide ions at pH 1.5 was studied by cyclic voltammetry, chronoamperometry, and capacity measurements. The results showthe occurrence of three nucleationor phase-change processes below 15"C,all of which can be ascribed to the formation of the cation radical at the electrode. The surface concentration of the cation radical obtained is consistent with the occurrence over a given potential range of a mixed bpyHz*+-I- phase over the electrode at low temperatures. At more negative potentials than those of the above-mentioned range, a new phase consisting solely of bpyHz*+is formed that covers the electrode throughout. The molecular area obtainedis intermediate between those of molecules lying paralleland normal to the electrode surface. Introduction The discovery by Eddowesand that the adsorption of 4,4'-bipyridyl (bpy) a t gold electrodes promotes the quasi-reversible electrochemistry of cytochrome c was followed by much study aimed at determining the orientation of the bpy molecule in an adsorbed state at various electrodes,s12 as well as a potential relationship between suchan orientation and the catalytic action of the molecule. The surface behavior of bpy is extremely complex, as reflected in the wide variety of structures proposed for its adsorbed form. Thus, at Au electrodes, bpy seemingly rearranges from a normal configuration at potentials below 0.2 V (us SCE) to one parallel to the electrode above 0.2 V.3 On the other hand, the results obtained with optical techniques are in contradiction with these assertions.6J At a Pt(ll1) electrode with a potential of 0.2 V (us Ag/ AgCl), the packing density of bpy has been found to increase with its concentration over the range 1-10 mM.8 This was ascribed to a gradual transition of the average molecular conformation of the adsorbed molecules from one with twisted aromatic rings to a coplanar conformation.819 By using surface enhanced Raman spectroscopy (SERS) and an Ag electrode, Lu et al.1° studied the influence of pH and the supporting electrolyte used on the adsorption of the three redox forms of bpy. As regards the acid zone-the one dealt with in this work-Lu et al."Jfound ~
~
* Author to whom correspondence should be addressed. e Abstractpublishedin Advance ACSAbstracts, January 15,1994.
(1) Eddowes, M. J.; Hill, H. A. 0. J. Chem. SOC.,Chem. Commun. 1977,771. (2) Eddowes, M. J.; Hill, H. A. 0. J. Am. Chem. SOC.1979,101,4461. (3) U d ,K.; Hill, H. A. 0. J. Electroanol. Chem. 1981,122, 321. (4) Cotton, T.M.; Kaddi, D.; Iorga, D. J. Am. Chem. SOC.1983,105, 7462. (5) Cotton, T.M.; Vavra, M. Chem. Phys. Lett. 1984,106, 491.
(6)Taniguchi, I.; Iseki, M.; Yamaguchi,H.; Yasukouchi,K. J. Electroanal. Chem. 1986,186,299. (7)Niwa, K.;Furukawe, M.; Niki, K. J.Electroanul. Chem. 1988,245,
275.
(8) Chaffiis, S. A.; Gui, J. Y.;Kahn, B. E.; Lin, C.; Lu,F.; Salaita, G. N.; Stem, D. A.; Zapien, D. C.; Hubbard, A. T.;Elliot, C. M. Langmuir 1990,6,957. (9) Munavalli, 5.;Poziomek,E. J.;Day, C. S. J.Mol. Struct. 1987,160, 311. (10) Lu, T.;Cotton, T. M.; Birke, R. L.;Lombardi, J. R.Langmuir 1989,5,406. (11) Heyrovsky, M.;Novotny,L. Collect.Czech. Chem. Commun.1987, 52, 54. (12) Heyroveky, M.; Pospisil, L.J.Electroanal. Chem. 1988,255,291.
0743-7463/94/2410-0723$04.50/0
the adsorption of doubly protonated bpy ( b p ~ H 2 ~and +) ita cation radical (bpyH$+) to depend markedly on the counterion. Thus, the spectra of b ~ y H 2 ~ and + bpyH2'+ obtained in the presence of I- anions, which posses a high adsorption capacity, are believed to be the result of the species being adsorbed normal to the electrode via ionpairs with I- ions. In the presence of weakly adsorbed anions such as Sod2-, bpyHz2+and bpyH2'+ are adsorbed parallel to the electrode. Anions with an intermediate adsorption capacity (e.g.C1-) give rise to also intermediate situations.10 In an acidicsulfate medium, bpyHz2+is adsorbed parallel to an Hg e1ectrode.l1J2 At the highest concentrationtested (0.62 mM),11the surface excess of bpyHz2+,I',is markedly dependent on the applied potential. Thus, under such conditions, I'is minimal a t potentials close to -350 mV (us SCE)." Above this potential, I'increasessharply, probably through formation of ion-pairs with adsorbed anions. On the other hand, below -350 mV, I' again increases, though in a more gradual way. In any case at the above-mentioned concentration (0.62 mM) and a potential of -600 mV, the experimentally determined surface concentration I' is only one-third of that needed for the electrode to be fully covered with a monolayer of molecules arranged parallel to the surface.ll The results of earlier studies1-12 seemingly suggest a marked dependence of the molecular orientation on such variables as the metal support used, the presence of certain anions in the medium, and the concentration of the adsorbed molecule itself. On the other hand, the effect of temperature on molecular arrangement and orientation in surface electrochemistryhas so far scarcelybeen studied. As shown in this work, relatively small temperature decrements can reveal otherwise unnoticed intermediate surface states. Below pH 2, 4,4'-bipyridine is doubly protonated (bpyHZ2+),with two pKvalues of 2.7 and 4.8, respectively.ll Therefore, under these conditions, b ~ y H 2 ~ can + be considered the first member of N,N'-dialkyL4,4'-bipyridiniums or vi01ogens.l~ In this sense, the electrochemical behavior of bpyHz2+over Hg is very similar to that of all other viologens.11J3-16 Thus, this species undergoes two (13) Bird, C. L.; Kuhn, A. T.Chem. SOC.Rev. 1981,101, 49. (14) Wan,A. B.; Linnel, R. H. J. Am. Chem. SOC.1956, 77,6207.
0 1994 American Chemical Society
S6nchez Maestre et al.
724 Langmuir, Vol. 10, No. 3, 1994
one-electron transfers that are distant enough to allow its cation radical (bpyHz'+) to be the species produced at the electrode over a given potential range. Such a cation radical is fairly stable;lShowever,its reductionmechanism, like that of viologens in general, is more complex as it can involve various dimerization or disproportionation reactions.11JSJ6J8 In this work we concentrated on the first reduction process, Le. that of the bpyHz2+/bpyHz'+couple since one of the aims was to study the adsorption and orientation of the cation radical bpyHz'+ over mercury. We chose to use I- as counterionon the grounds of ita strong adsorption in order to compare the results with those previously obtained by Lu et aL10 over Ag. In addition, we used a pH of 1.5 throughout on account of the seemingly close relationship between bpyHzz+and viologens under these acidity conditions. All viologens give rise to a very narrow peak by cyclic voltammetry and at an Hg electrode that corresponds to the reduction of adsorbed molecules. Such a peak appears at more positive potentials than does the diffusion peak arising from the electrode process itself.17 The distance between the adsorption and diffusion peak potentials (Ep) increases with increase in the chain length of the alkyl substituent17-it is only 100mV for methyl viologen.These adsorption peaks resemble nucleation peakP and must be related to the formation of a two-dimensional phase by the radical cation over the electrode. Though previously unreported, this adsorption peak can be observed in the voltammetric recordings for bpyHz2+ over Hg in the presence of any supporting electrolyte provided Cbpy 1 0.3 mM.19 In addition, the peak is only observed in acid media, where bpy is doubly protonated, the distance between ita peak potential and that of the diffusion peak being substantially greater than for methyl viologen.19 The above-mentioned adsorption peak for bpy is not observed with other types of electrodes,'-l0 not even at bpy concentrations of 1 mM or higher, which provides even strongersupport for the influenceof the metal support on the orientation of the adsorbed species. One additional aim of this work was to describe and analyze the two-dimensional nucleation peaks observed in acid media in the reduction of bpyHz2+ at an Hg electrode. Such peaks suggest that, in a certain potential zone, the electrode is covered with a mixed bpyHz*+-Iphase. Experimental Section bpy was supplied by Lancaster. All solutions were made in HI (Fluka) at pH 1.5. The final I- concentration was adjusted to 0.1 M with KI (Merck r.a. grade). The working bpy concentration used were 1 and 4 mM. Voltammetric recordings were obtained by using an HQ Instnunenta 305 programmable function generator, an HQ Instrumenta 105 potentiostat, a Prowler N400 digital memory oscilloscope,and a Houston Instrumentarecorder. The working electrode was a Metrohm EA-290/1 HMDE with a surface area of 2.20 0.06 mmz. i-t curves were recorded with the aid of a PAR M273 potentiostat with automatic correction for the iR drop. The (15) Volke, J.; Volkova, V. Collect. Czech. Chem. Commun. 1972,37, 3686. (16) Cotton, T.M.; Kim, J. H.; Uphasus,R.A. Microchem. J. 1990,42, 44. (17) Kitamurn, F.;Oheaka,T.;Tokuda,K. J. Electroanal. Chem. 1993, 947, 371. (18) Scharifker, B.;Wehrmann, C. J. Electroanal. Chem. 1985,185, 93.
(19) SAnchez Maeatre, M.; Rodriguez-Amaro, R.; Ruiz, J. J.; Camacho,
L.Unpublished renults.
500
600
700
800
-E (mV) Figure 1. i-E curves (voltammogram) obtained for 1 mM bpyH22+in the presence of 0.1 M I- at pH 1.5, u = 160 mV/s and T = 25 O C . curves were acquired with the above-mentioned digital oscilloscope. The HMDE used (a PAR 303A Model) had a surface area of 1.22 f 0.05 mm2. Capacity-potential curves were recorded on an Inelecsa Instrum. Electro. S.A. Platinum and saturated calomel were used as the auxiliary and reference electrode, respectively. All measurements were made in a nitrogen atmosphere, the temperature being measured to within f0.1 O C in every case.
Rssults Figure 1 shows a voltammogram recorded under the above-described experimental conditions, with u = 160 mV/s and T = 25 "C. The peak observed at the more positive potentials (c) and that appearing at the more negative potentials correspond to the reduction (positive current)-oxidation (negative current) of the bpyHzz+/bpyHz*+ couple. Peak c is very narrow and lacks a diffusion tail. This is typical of electrode processes corresponding to the oxidationreduction of immobilizedmolecules at the electrode. The peak at the more negative potentials corresponds to the reagent diffusion-controlledelectrode process and is not analyzed in this work; in any case, it is consistent with a one-electron process. Figure 1also shows a small peak between the other two, peak d, the nature of which is discussed below. Figure 2 showsseveralvoltammetric recordings obtained at different temperatures, Cbpy = 4 mM and u = 37 mV/s over the potential range from -550 to -625 mV, where the diffusion peak cannot be observed. As can be seen, below 15 "C,peak c splits into two that shall henceforward be referred to as peak a (the one appearing at the more positive potentials) and b (that observed at the more negative potentials). Peaks a-d can only be observed below pH 3 and bpy concentrations above 0.3 mM. In addition, while peaks c and d are also observed in other supporting electrolytes, peaks a and b only appear in an I- medium.'9 We studied the influence of the scan rate, u, on peaks a-d and found the peak current (Ip)to vary nonlinearly with both u and u1I2 for all. Also, the peak half-widths (the width in millivolts at the peak half-widths)was found to decreasewith decreasingu. For example,the half-height for peak c at u = 5 mV/s was ca. 3 mV. Rather surprisingly, at u = 0.5 mV/s, the peaks become so narrow that their half-width is smaller than the resolution of our experi-
Behavior of bpyH$+ over Mercury
Langmuir, Vol. 10, No. 3, 1994 725 540
I
', ,
580
560
I
, 111
0
540
575
550
I
600
625
-E (mV) Figure 2. Voltammograms recorded over the potential range from -550 to -625 mV at C b = 4 mM, v = 37 mV/s, and temperatures of 8 "C (solid line), 10 "C (dabhed line) and 12 "C (dotted line). All other conditions as in Figure 1.
-E
P
625
-
600
-
(mv)
575
t
550 I 0
I
?r"
10
, 40
(OC)
I 50
30
Figure 3. Variation of E (forward scan) with temperature for peake a-d. Experimendconditione as in Figure 2.
mental systems. One other interesting feature of peaks a-d is that the differences between the oxidation and reduction peak potentials for a given process are never zero however low the scan rate may be. The magnitude of the hysteresis between the forward and reverse scan in the voltammogram is also specific to each peak and anomalously large for peak b. Integration of these peaks with respect to time allows one to calculate the charge exchanged with the electrode in each process. The values thus obtained were ea. 12 1, 20 1, 27 1, and 1.5 0.5 pC/cm2 for peaks a, b, c, and d, respectively. All these values were independent of the temperature (in the zone of appearance of each), scan rate, concentration (1or 4 mM), and whether the reduction or oxidation peak of the corresponding process was integrated. Figure 3 shows the variation of the peak potentials (E,) with temperature (2') for peaks a-d at v = 37 mV/s. As can be seen, the Ep values for peaks a and d are similarly dependent on T, whereas that of peak b is seemingly T-independent. Finally, the E, of peak c, a combination of peaks a and b, exhibits a T dependence that is intermediate between those of its two parent peaks. Figure 4.1 shows the differential capacity-potential curves obtained for a 0.1 M solution of I- at pH 1.5 in the absence of bpy (dotted line) and the presence of 4 mM bpy (solid line), at 100 Hz, AE = 10 mV, v = 2 mV/s, and T = 5 "C. Figure 4.2 shows another voltammogram
*
*
*
580
560
I
IV
600
-E (mV)
I
I
600
*
Figure 4. Plot of C, (1) and the current (2) as a function of E at T = 5 O C . C, was determined at a frequency of 100 Hz and a scan rate of 2 mV/s by using a 10-mV pulse. The broken line corresponds to a solution containing no bpy while the solid line corresponds to a solution containing 4 mM bpy. The voltammogram was run with the same bpy solution at v = 46 mV/s. All other conditione as in Figure 3.
(forward scan only) obtained at the same temperature and concentration but v = 46 mV/s. The zone of potentials where peaks a and d appear also gives rise to two peaks in the capacity-potential curves. Obviously, such peaks cannot be interpreted in terms of capacity measurements (C,) owing to the simultaneous occurrence of Faradaic processes. Anomalously, the appearance of peak b does not give rise to a maximum in the Cp-E curves, which may be the result of the interface capacity change being quantitatively more significant than the potential Faradaic effect. Only in the absence of Faradaic current do the curves in Figure 4.1 provide a correctmeasurement of the interface capacity, at least at a given frequency. Thus, at more positive potentials than those of appearance of peak a, zone I, C, is somewhat higher than the capacity of the supporting electrolyte. At intermediate potentials between those of appearance of a and b, zone 11,the capacity decreases slightly relative to the previous zone but still exceeds that of the supporting electrolyte. However, in the potential zone between peaks b and d, zone 111, the capacity decreases sharply, even below that of the supporting electrolyte. Finally, at more negative potentials than those of appearance of peak d, zone IV, C, is hardly altered with respect to the previous potential zone. Figure 5 shows the i-t potentiostatic curves obtained at T = 25 "C, where only peaks c and d are observed in the voltammetric recordings. Experimentally, the curveswere obtained by applying a potential of -500 mV for 5 s, followed by a potential pulse up to the desired value. The final potential of the potentiostatic jump is given in the figure. The curves were corrected for the iR drop. The curves show an initial falling portion that corresponds to the interface charging current, which rapidly falls to zero. After that, the current reaches a maximum that is typical of nucleation processes.*22 Also, the curve shapes are quite complex as a result of the two nucleation phenomena taking place at very close times. Integration of these curves after elimination of the interface charging current allowed us to calculate the charge exchanged throughout the process, viz. 27 1 pC/cm2, which is consistent with the value obtained by integrating the corresponding voltammetric peak. This type of i-t curve was also obtained at T = 5 "C, where peaks a and b were observed. Figure 6 shows such
*
726 Langmuir, Vol. 10, No. 3, 1994
20
100
1
a) -600 mV b ) -601 m V c ) -602 mV d) -603 mV e) -605 mV
i (PA)
J
1
,
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a) -568 mV b ) -570 m V c ) -572 m V d) -574 mV
f)
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e ) -578 mV f ) -586 mV
25
1
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0 025
0 050
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600
figure^ 6. i-t curves obtained at T = 26 "C.The potentialapplied before the potentialjump was -600 mV and was held for 6 s. The final potential for each curve is shown in the figure. The curves shown correspond to the appearance of peak c in the voltammogram. All other conditions as in Figure 2.
a) -558 m V b) -559 mV c ) -560 m V d) -561 mV e) -562 mV f ) -564 m V
LA
0
0 I
0 01
0 03
0.02
t (s) Figure 6. i-t curves obtained at T = 6 O C , similarly to Figure 6. The curves shown correspond to the appearance of peak a in the voltammogram. All other conditions as in Figure 1. The dashed line is the extrapolationto zero current after elimination of the charging current.
curvesfor pulse potentials correspondingto the appearance of peak a under the same experimental conditions as in Figure 5. As can be seen, the curves still show a current maximum but are more simply shaped. Integration of these curves allowed the charge exchanged in the process to be calculated: 12 f 1 pC/cm2,which, as in the previous case, was consistent with the value obtained by integrating the corresponding voltammogram. The i-t curves were extrapolated after eliminating the contribution of the interfacecharging current; the resulting intercepts (dotted lines) were all zero. Figure 7showsthe i-t curvesobtained at pulse potentials correspondingto the appearance of peak b under the same conditions as in Figure 6. Again, the curves had simple shapes and a current maximum that is typical of a nucleation or phase-change process.M23At very short times, the curves included peak a, but virtually masked by the charging current drop. ~~
002
t
003
004
(SI
Figure 7. i-t curves recorded at T = 6 O C , similarly to Figure 6. The curves shown correspond to the appearance of peak b in the voltammogram.
The curves in Figure 7were obtained by eliminating the interface charging current and the current of peak a and extrapolating the results to zero current (dotted lines). Unlike Figure 6, the intercepts were nonzero. Nucleation at peak b thus calls for a given waiting time. Integration of this peak leads to an exchanged charge of ca. 20 f 1 pCIcm2. Finally, curve f i n Figure 7 shows a small peak with a maximum. Such a peak must be related to peak d in the above-described voltammograms.
50
25
001
~~
(20) Fleiechmann,M.;Thirsk, H. R. In Aduanceu in Electrochemistry
and Electrochemical Engineering; Delahay, P., Ed.; Interscience: New York, 1963; Vol. 3.
Discussion The shape of peaks a-d in Figures 1 and 2 is typical of electrode processes involving immobilized molecules at an electrode. There are several theoretical treatmenta for the oxidation-reduction of monolayers accountingfor the appearance of very narrow peaks in voltammetry. Such treatments are based on the use of Frumkin isotherms with attractive interactionparameters26or in more complex models involving side interactions and configurational effects between molecules.26 In any case, none of such treatment~~ accounts ~ p ~ for some of the above-described experimental properties for these voltammetric peaks. Thus, according to the aforementioned models, the halfwidth of the adsorption peak should be independent of the scan rate, contrary to our experimental observations. Also, any of these treatmenta261asreported in the literature predict that applicationof a constant overpotential results in the rate (current) of the process being maximal at time zero. This is clearly not so, as can be seen from the potentiostatic transitions shown in Figures 6-7. The chief reason why the above-mentionedtreatments are inapplicable to the behavior of the adsorption peaks of bpyHz2+is that they assume the occurrence of no phase or orientational change in the molecules upon their oxidation-reduction or that, if such a change occurs, it is instantaneous. An appropriate treatment for our results (21) Harrison, J. A.; Thirsk, H. R. In Electroanalytical Chemistry; Bard, A. J., Ed.;Marcel Dekker: New York, 1977; Vol. 5, p 67. (22) Obretenov,W.; Petrov, I.; Nachev, I.; Staikov, G. J . Electroanal. Chem. 1980,109,195. (23) Fletcher, S . J. Electroanal. Chem. 1981,118, 419. (24) B u m Herman,C1. J. Electroanal. Chem. 1986,186,27 and 41. (25) Laviron, E. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1982; Vol. 12, p 53. (26) Matauda, H.; Aoki, K.; Tokuda, K. J. Electroanal. Chem. 1987, 217, 1 and 15.
Behavior of bpyH2*+over Mercury
Langmuir, Vol. 10, No. 3,1994 727
should therefore take account of the kinetics of formation I I of the new phase. The sole mathematic tools available at present to interpret our results are mathematicalnucleation-growthcollision (NGC) models for metal ions over supports of i/im the same metal or inert supports.20-29 This type of process is a special case of the electrochemical phase-change, of a high conceptual simplicity and ideal in many aspects. NGC models should be applied reservedlyto phase-change phenomena involvingorganicmolecules owing to the vast amount of simplification it entails. Thus, all nucleation sites are assumed to be identical irrespective of whether the solvent or supporting electrolyteis previously adsorbed and which orientation the molecules adopt. In any case, NGC models have been widely used to study surface phase changes in organicmolecules i n v ~ l v i n g ~ ~ ~ ~ - ~ ~ 0 0 or not involvingz4~30*s1 a Faradaic current exchange. 0 1 2 3 4 Cyclic voltammetry is not the most suitable technique utm for characterizing two-dimensional (2D)nucleation processes owing tq their mathematic complexity. However, Figure 8. Plot of i/, V 8 t / t m for instantaneous(curve I, eq 1) and progressive nucleation (curve 11, eq 2), as well as for the curves some simplifiedtreatments32pmpredict the decreasein the qm in Figure 6 (0)E -668 mV, t m 10.6 ms, i, = 7.61 4, peak half-width with v that was experimentally observed 0.141 pC; (v) E -669 mV, t m 4.36 IIU, i, = 11.2 4, qm in this work. 0.149 PC;( 0 )E -660 mV, t m 3.87 IIU,i, = 11.2 MA,q, 0.146 Chronoamperometryis better suited to the study of these PC. processes2lsincethe use of a constant overpotential allows the mathematical problems to be substantially simplified. 1 1 .o If the 2D nucleation rate tends to infinity, then nucleation is said to be instantaneous,21 in which case i-t curves conform to the following equation I
I
I
i
2
3
i/im
where i, and t , are the maximum current and its corresponding time. If nucleation is progressive (i.e. the nucleation rate is very low), thenz1
;);(
i[);(
]
0.5
(2)
= ex,{- 13 Let us call qm the overall charge exchanged in the potentiostatic jump. This quantity is related to im and t m in such a way that i m t d q m = 0.606 for instantaneous nucleation and i m t d q m= 1.027for progressivenucleation. Equations 1and 2,and these analytical criteria, can be
used to study the curves in Figures 5-7. Fitting these models to the curves in Figure 5, which correspond to voltammetric peak c, is impossible owing to their complexity. The recordings obtained under these conditions are the result of two overlapped processes, as shown at temperatures below 15 O C , where the two processes are resolved. The complex shape of these curves was preserved even up to 45 "C,the highest temperature assayed. Figure 8 shows the variation of ili, with tit, based on the predictions of eq 1 (curve I, instantaneous nucleation) and eq 2 (curve 11, progressive nucleation). The figure also shows the same plot for some of the experimental i-t curves in Figure 6, which correspondto voltammetricpeak a. (27)Ward, M. D.In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 16,p 181. Ward, M. D. In Supramolecular Architecture; Bein, T . , Ed.; American Chemical Society Washington, DC, 1992; p 231. (28)Li, F. B.; Alhry, W. J. Langmuir 1992,8,1645. (29)Huckaby, D.A.;Blum, L. J. Electroanul. Chem. 1991,315,255. (30)Retter, U.J. Electroanul. Chem. 1984,179, 25. (31)P o ~ ~ i eL. i l .J. Phvs. Chem. 1988.92.2501;J. Electroanul. Chem. 1986,206,iS9.' (32)Boeco, E.;Rangarajan, S. K.J. Electroanal. Chem. 1981,129,25. (33)Noel,M.;Chandrasekaran, S. J. Electroanal. Chem. 1987,225, 93.
0.0 0
t/tm Figure 9. Plot of i / i , v8 t / t m for instantaneous (curve I, eq 1) and progressive nucleation (curve 11, eq 2), as well as for the curves in Figure 7: ( 0 )E = -670 mV, t, = 9.4 ms, i, = 16.33HA, qm 0.2432 pC, t r = 6.1 m ~ (V) ; E -672 mV, t, 6.37 me, i, = 22.09 HA, qm 0.239 pC, t r 3.3 ms; ( 0 )E = -674 mV, t , 4.66 ms, i, = 29.296 pA, q, = 0.233 IC, t, = 2.6 me. All the curves were shifted by a time tr in order to superimpose i = 0 and t = 0.
As can be seen, the experimental data fit none of the above-described NGC models. In addition, the i m t d q , ratio is dependent on the applied overpotential. In fact, the ratio decreases below 0.6, which indicates that the curves would not fit a model involving nucleation rates lying between those of the previous theoretical cases. The fact that the our experimental data do not conform to the NGC models can be ascribed to the great simplifications of the latter. In our case, the electrode surface was previously covered with strongly adsorbed I- ions, which must be at least partly displaced from the electrode.18tm Figure 9 shows a plot similar to that in Figure 8 for some of the i-t curves in Figure 7, which correspond to voltammetric peak b. In this case, the curves obtained conform to an instantaneous nucleation model with an i,tJqm ratio of 0.59,
728 Langmuir, Vol. 10, No. 3, 1994 i.e. very close to theoretical 0.606. In addition, this ratio is virtually independentof the applied overpotential. (The i,, t m and qm values are given in the figure caption.) In order to run the graph in Figure 9, experimental data were shifted by an amount of time tr (also given in the figure caption) on the time scale so as to superimposetime zero with the extrapolation to zero current in Figure 7 (dottedline). Thisisquitesignificant and, asnotedearlier, indicates the need for a waiting time trfor the phase change in process b to occur. Such a waiting time is no doubt the time required for the nucleation process represented by peak a to occur. One possible explanation for this phenomenon is provided below. Assigning and interpreting the nature of each of the peaks observed in the voltammograms of Figures 1 and 2 is quite complex and could probably be done only incompletely with the experimental data obtained in this work, which, however, allow one to draw a number of interesting conclusions that are discussed below. First, as stated above, the appearance of peak a implies the exchange of ca. 12 pC/cm2. This charge value must be ascribed to a Faradaic process which thus involves the exchange of electrons between bpyHz2+and the electrode. The possibility of this peak being due to a non-Faradaic process arising from the charging current produced by a relatively slow change in the orientation can be ruled out since the charges associated with these processes are of only ca. 1 or 2 pC/cm2 at the The i-t curves obtained for peak a (Figures 6 and 8) conform to none of the NGC models analyzed before. In addition, 12 pC/cm2is equivalent to ca. 133 A2/molecule, which is much greater than the theoretical value for a full monolayer of bpyH$+ units lying parallel to the electrode. In fact, such species would take ca. 72 A2/moleculesin a configuration with the rings parallel to the electrode and ca. 23 A2/molecules8J6in one with the rings normal to the electrode. The above results can be related to the C,-E curve in Figure 4.1. As can be seen, the capacity in zone I (prior to the appearance of peak a) is only slightly greater than in zone I1 (intermediate between the appearance of peaks a and b). In addition, C, is greater in both zones than the capacity of the supporting electrolyte (dotted line), which suggests a increase in the interface structure thickness despite the experimentally observed nucleations (Figures 6 and 8). In the potential zone I, adsorption must preferentially involve I- ions, as is the case in the absence of b ~ y H 2 ~ + ; the outer I- ions may form ion-pairs with bpyHz2+thereby increasingthe thickness of the layer of adsorbed molecules at the electrode and hence the capacity. This is consistent with the conclusions of Lu et al.1° All the experimental data obtained in this work seem to suggest that a mixed bpyH$+-I- phase of unknown structure might be formed in the potential zone I1 (Figure 4). In such a phase, bpyHz'+ would be in direct contact with the electrode and I- would be in excess owing to the high surface concentration of bpyHz'+. The corresponding excess charge might be neutralized by bpyHz2+ ions through the formation of ion-pairs. Further I- ions might join the structure to increase the interface thickness as in the previous case, thereby giving rise to the high capacity observed. The charge exchanged during appearance of peak b was 20pC/cm2. The appearance of this peak raises the question of whether b p y H P units previously deposited during the (34)Peter, L.M.;Reid, J. D.;Scharifker, B.R.J. Electroanul. Chem. 1981, 119, 73.
S6nchez Maestre et al.
appearanceof peak a remain on the electrode at potentials above the appearance of peak b (potentialzone 111in Figure 4). This question must be answered in order to calculate the effective electrode area occupied by each molecule in the potential zone 111. In principle, every molecule deposited during appearance of peak a might remain at the electrode, which would result in 32 pC/cm2 for the depoeited monolayer; altematively, the molecules deposited during appearance of peak b might completelydisplace those depoeited during appearanceof peak a, thus resulting in 20 pC/cm8 for the deposited monolayer. Answering this question requires relating some of the above-described experimental results. First, as noted earlier, for the phase transition of peak b to occur, a given waiting time t r must elapse (Figure 7). In addition, when peaks a and b overlap at T > 15 O C to yield peak c, the overlap does not give rise to the phase change observed in peak b; rather, the new peak formed, c, has a charge of 27 pC/cm2 which is intermediate between those of the overlapping peaks. The shape of the i-t curves for this peak is always complex, even at 45 OC, which seemingly indicates that peak c always arises from overlap of two different phenomena. In other words, the formation of peak b or, at a higher temperature, peak c, always requires the prior formation,at least partly, of peaka. The bpyH$+ cation radicals formed during the appearance of this peak a may act as nucleation sites in the formation of the new phase produced after peak b or c disappears. This would account for the delay time observed in the i-t curves for peak b (Figure7), as well as the fact that the curves conform to an ideal instantaneous nucleation model (Figure 9). If 27 pC/cm2 is used as the charge exchanged by the molecules retained at the electrode after the appearance of peak b or c, indifferently, the area occupied by each molecule on the assumption of a full monolayer is ca. 69 A2, which is consistent with a tilted orientation of the molecules (a 5 6 O angle between the principal molecular axis and the electrode plane)-a tilted structure was also put forward for the adsorption of bpy over PtLor an edge-on orientation as proposed for other bipyridines.le In any case, the formation of the new phase involves a deep change in the interface structure,as seen in analyzing the capacity at more negative potentials than the appearance of peak b or c (potential zone I11in Figure 4). In fact, the capacity in this potential zone decreases markedly relative to the previous zones to the point of falling below that of the supporting electrolyte. This must be ascribed to the displacement of I- ions directly bound to the electrode by bpyH2*+;Le. it is in this potential zone that a compact monolayer of bpyH2*+is formed, even though with the concourse of an equivalent number of I- ions to neutralize the electric charge. Let us finally analyze peak d, which involved the exchange of 1.5 pC/cm2. This value might correspond to a non-Faradaic charging current process arising from a slow change in the molecular orientation.N*MHowever, this type of process involves a deep change in the interface capacity in potential zones preceding and following its appearance (zones I11 and IV in our case).%*" This was not the case, so the peak must be related to a redox Faradaic process of b ~ y H 2 ~ One + . explanationfor this phenomenon may be the uptake of new bpyHz'+ units in the already formed surface film to yield a more compact monolayer. The surface behavior of bpyHz'+ in the presence of Iions is rather surprising and different from that inferred from the results obtained at an Ag electrode,1° where the surface heterogeneity may preclude the formation of compact two-dimensional phases over the electrode. In
Behavior of bpyH$+ over Mercury
isolation, bpyHz*+is unable to displace surface-bound Iions, even though it can form ion-pairs that would take a normal configuration relative to the electrode.10 Benzyl and heptyl viologen exhibit two-dimensional nucleation and growth of monomolecular phases over an Ag electrode in the presence of bromide ions.18 The fact that no nucleation is observed in bpyHz2+lounder similar conditions may be the result of the different treatments to which the electrodes were subjected. To the authors'
Langmuir, Vol. 10, No. 3, 1994 729
minds, displacement of I- ions by bpyH2*+only seem to take place when the electrode surface is homogeneous enough for the monomolecular phase of the cation radical to be formed. Acknowledgment. The authors wish to express their gratitude to the Spanish DGICyT for financial support awarded for the realization of this work as part of Project PB91-0834.
