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The growth of ordered films during the cathodic reduction of a tetraalkylammonium ion
91
J. Eiectroanal Chem., 209 (1986) 91-100 Elsevter Sequoia S.A., Lausanne - Printed
in The Netherlands
THE GROWTH OF ORDERED FILMS DURING REDUCTION OF A TETRAALKYLAMMONIUM
VESNA
SVETLICIC
and ESSIE KARIV-MILLER
Department of Chemrsty. (Received
16th January
THE CATHODIC ION
Vnwersity of Mmnesota, Mmneapohs. MN 55455 (US A.) 1986; m revtsed form 8th April 1986)
ABSTRACT The reduction of dimethylpyrrolidinmm tetrafluoroborate in DMF on a mercury cathode was studied and it was shown that a new ordered phase 1s formed. At times of the order of tens of milliseconds, chronoamperometry gave evidence for a phase transitton involvmg progressive three-dimensional nucleation and hemispherical growth. At longer times the film growth was limited by dtffusion in solution. Quantitative oxidative dtssolutton of the film was possible. The rate of reoxidation was rapid and independent of the thtckness of the film, over the range studted (Q Q 8 mC cm-*).
INTRODUCTION
Since 1911 several researchers have reported the formation of black, mercurycontaining precipitates from the reduction of simple tetraalkylammonium cations on mercury electrodes [l]. Recently we have observed the formation of similar, colored products from the reduction of tetraalkylammonium ions on a variety of metals [2]. The precipitates dubbed “ tetraalkylammonium-metals” * have been shown to participate as electrocatalysts in a number of organic electroreductions. Because they are not very stable and they are difficult to isolate, the structures of these materials are yet unknown. Two of the more stable tetraalkylammonium-mercury examples are formed by reduction of tetramethylammonium [3] and dimethylpyrrolidinium [4,5] (DMP) cations. The DMP reduction process [4,5] is of interest here. It is attractive for study because DMP reduces at rather positive potentials compared to other tetraalkylammonium ions. Thus, polarography of 0.005 M (DMP)BF, in DMF containing 0.1 M tetrabutylammonium tetrafluoroborate [(TBA)BF,] as the supporting electrolyte, gives rise to a well defined wave for DMP which is
* The term “amalgam” has previously been employed. but since that term is associated with mercury and tmphes certain structures, the term “ tetraalkylammonium-metal” seems more appropriate..“›
92
clearly separated from the background. Analysis of the wave shape and coulometric experiments have shown that the process involves a reversible one-electron transfer. DMP and mercury. The Reoxidation of the product (DMP-Hg) *, regenerates half-life of DMP-Hg on mercury in DMF at 5°C is about 6 h and a simplistic formulation of the reduction of DMP is: DMP+ + 1 e- + n Hg @ DMP-(Hg). The present study was initiated because we detected some unusual behavior in the cyclic voltammograms of DMP. Polarographic measurements have indicated that DMP-Hg is insoluble in both DMF and mercury and it seemed that the unusual cyclic voltammograms might reveal more about the process if considered in the context of phase transitions. The DMP-Hg formation can be considered as growth (deposition) of a new phase. If that phase is ordered, the electrodeposition of DMP would be similar in character to processes like metal electrodeposition and electrocrystallization [6] which involve nucleation and growth as phenomenological components [7]. In order for the new phase to develop, single molecules must first aggregate to small clusters (nucleation) before these clusters can expand to large dimensions (growth). The kinetics of film growth naturally depend on both of these phenomena and can be studied by electrochemical techniques. Indeed electrochemical techniques are especially useful for studying phase transitions, because the observed current is an exact measure of the combined rate of nucleation and growth, and the degree of supersaturation and interfacial conditions are easily controlled by the electrode potential. A variety of current-potential-time relationships can be explored, and experiments can be conveniently and exactly repeated many times. Some of the typical effects of phase transitions on cyclic voltammograms are a characteristic crossover upon switching of the scan direction, appearance of sharp spikes and enhanced peak separation of otherwise normal mass transport controlled voltammograms [8]. We observed such effects while studying the reduction of DMP and other tetraalkylammonium ions and developing their use as catalysts for organic electroreductions [5,9]. The goal of the present work was to investigate the hypothesis that a new ordered phase is generated during the reduction of DMP on mercury cathodes and to make initial steps toward understanding the events at the mercury/solvent interphase during the process. EXPERIMENTAL
The preparation of (DMP)BF,, (TBA)BF, and the SCE reference electrode, the purification of DMF, the cell and the apparatus used for cyclic voltammetry have been described elsewhere [5]. The working electrodes were a sessile mercury drop electrode [5] 0.0215 cm2 and
* The abbrewation
DMP-Hg
IS not mtended
to imply stoichiometry
93 a planar platinum based mercury film electrode 0.008 cm2. The latter was prepared by deposition of mercury at constant current from solutions of Hg,(NO,)* (analytical reagent “Mallinckrodt”). Optimal mercury films were formed by depositions using a charge of 200 mC cmp2 and all the results reported were obtained using films prepared in that manner. The counter electrode was a platinum wire and the reference electrode was a SCE. All potentials reported are vs. SCE. Experiments were carried out at 24 &-0.2”C under an argon or nitrogen atmosphere. Most experiments reported were carried out using 10 -3 M (DMP)BF,, but concentrations up to 1o-2 M were studied and found to behave similarly. It is of interest to note that we were unable to obtain discernible cyclic voltammograms using (DMP)BF, in concentrations lower than 5 X lop4 M. The techniques used to study the electrodeposition of DMP and the phase transitions involved are largely based on experimental methods developed for the study of supramolecular phenomena in methylene blue/leucomethylene blue redox films at mercury [lo], platinum and gold [11,12] electrodes. Linear sweep voltammetry was one of the methods used. In these experiments the DMP deposit was accumulated at constant potential (k 1 mv> at various generation times (0.25-6 min; Figs. 3 and 5). The charge transferred during the positive scans (Fig. 4) was calculated by integrating the appropriate areas of the LSV curves using a planimeter. Chronocoulometric experiments with pulse duration G 1 s (Fig. 2) were performed using a BAS-100 Electrochemical Analyzer (Bioanalytical Systems). Potential step experiments were performed using a PAR 173 potentiostat in combination with a PAR 175 universal programmer (both are Princeton Applied Research instruments). The resulting current-time transients (in the ms range, Figs. 6 and 7) were recorded by means of a storage oscilloscope (Tektronix 5111) and photographed (Tektronix C5A Polaroid camera). RESULTS
AND
DISCUSSION
The initial experiments were done on a mercury drop cathode and a typical cyclic voltammogram of lop3 M (DMP)BF, is shown in Fig. 1. The particular shape of
E/V
vs. SCE
Fig. 1. Cyclic voltammogram of 10K3 M (DMP)BF,, mercury drop electrode m DMF. 0.1 M (TBA)BF, as the supporting electrolyte. CJ= 50 mV s-‘; areas corresponding to reduction and oxidation are schematically indicated.
94
TIME/s
Fig. 2. The charge as a function of time in a double potential Dotted line: 0.1 M (TBA)BF,; solid line: 10d3 M (DMP)BF,
step expenment, mercury drop electrode. +O.l M (TBA)BF, in DMF.
the sharp rising cathodic current and the stripping anodic peak are similar to those observed during metal deposition onto foreign substrates [13,14] and are an indication that the reduction product forms a film on the cathode. The cathodic charge * (Qred) was calculated and found to be equal to the anodic charge (Q,,) implying that the reduction product is stable and remains on the surface (on the time scale of the experiment). The values for u = 50 mV s-* were Qred = Q,, = 186 PC cm-‘. The film thickness is of interest, but since the exact structure of the film is not known it cannot be calculated. However, using space filling models, the area occupied by one DMP was estimated to be 0.4 nm2 and a close packed monolayer composed of DMP only, was calculated to have a surface concentration (I,,,) of 4.2 x lo-” mol cm -*. The charge required to form such a monolayer is 40 /.LC cm-‘. Considering the amount of charge transferred, (Qred and Q,,) the formation and dissolution of 4-5 hypothetical close packed DMP layers takes place during the recording of one cyclic voltammogram at 50 mV s -‘, as the one shown in Fig. 1. Obviously, this is a limiting value since the incorporated mercury (0.07 nm*) and any other constituents will increase the thickness per coulomb. Figure 2 shows the charge as a function of time recorded during a double potential step experiment using 1O-3 M (DMP)BF, and a mercury drop electrode. The potential was stepped from - 2.000 V where there is no reaction of DMP to the region where reduction is diffusion controlled (- 2.760 V). After one second the potential was stepped back to -2.000 V. The dotted line in Fig. 2 represents the same experiment performed, using the background solution of the supporting electrolyte alone (0.1 M (TBA)BF,). It shows that the background consumes charge, forming a product which does not reoxidize under the experimental conditions. In order to verify that the dotted line represents quantitatively the background reaction, experiments like the one described in Fig. 2 were performed using higher concentrations of DMP. The charge (Qced) increased proportionally, but at all
l It is noted that the cathodic charge mcludes charge part of the return half cycle. as schematically mdlcated
transferred in Fig. 1.
during
the first half cycle as well as
95
E/V
vs. SCE
Fig. 3 The effect of generation time (I~) on the accumulation of the DMP-deposit. 10K3 M (DMP)BF, m DMF [5x lo-* M (TBA)BF,]; mercury drop electrode; E, = -2.760 V; u = 50 mV SC’. tG: (0) 0 min. (1) 0.25 mm. (2) 2 mm, (3) 3 min. (4) 4 min. (5) 5 min. (6) 6 min.
concentrations, it decayed to the same value of 40 PC cm2 characteristic of the electrolyte solution. Taking into account the background contribution, the chronocoulometric curves for DMP show that Qred = Q,,. As expected from the similar time scale, the values of of Pox and Qred from Fig. 2 are compatible with those calculated by integration the cyclic voltammogram in Fig. 1. The dominant characteristics of the chronocoulometric transients of DMP, recorded in the time range of a few seconds, are diffusion controlled reduction followed by “instantaneous” oxidation. It shows that the reduction product is deposited on the cathode and that the oxidation which releases DMP back into solution is a fast surface reaction, In order to determine if thick films could be grown, and to learn about the reoxidation process, we performed linear negative sweeps and stopped at - 2.760 V for various generation times (to) before reversing for the positive sweep. The LSV curves recorded during the sweep to more positive potentials are shown in Fig. 3. They are characterized by a smoothly increasing current, followed by an instantaneous drop to the base line. This can be rationalized by an oxidation rate which increases exponentially with potential until the film is completely oxidized. All indications are that the film is completely removed by the stripping process. The rising portions of all reoxidation curves, regardless of their to, are identical indicating that the rate is independent of the amount of deposit. With increasing to the peak currents are higher and are shifted to more positive potentials because there is more material to oxidize before the current can drop to the base line. These curves are analogous to the oxidation curves found for dissolution of certain metal
Fig. 4. The oxidation charge as a function of the generation time. 10e3 M (DMF)BF, in DMF[S M (TBA)BF,]; mercury drop electrode, E, = - 2.760 V. (a) quiet solution. (b) stirred solution.
x
lo-’
films [15] and of methylene blue/leucomethylene blue redox film [11,12]. The anodic stripping peaks contain information about the thickness and conductivity of the film. The dependence of the oxidation charge (calculated by integration of the corresponding curves) on to characterizes the film growth at the potential of generation (Eo). Figure 4a shows the dependence of the oxidation charge on to at -2.760 V. Assuming the presence of only DMP molecules at the electrode surface and one electron reduction, the oxidation charge corresponds to multilayer films of more than 200 hypothetical close packed DMP layers, with no indications of limitation to film growth. This observation and the independence of the oxidation rate on the film thickness suggests that thick films are conductive to the extent that conductivity does not control either the growth or the dissolution. Except during the first minute, the charge increases in an approximately linear manner. This is expected at these longer times where convection is important and the bulk concentration is not substantially depleted. It was of interest in this regard, to stir the solution during an accumulation-stripping experiment and the results are shown in Fig. 5. As expected, stirring causes larger Q values and thicker films (Fig. 4b). It, however, does not affect the rate of dissolution and the rising portion of the anodic currents is identical in stirred and unstirred solutions (Fig. 5). This demonstrates again that the oxidation is a surface phenomenon and is independent of the solution conditions. For a variety of reasons it was preferable to conduct further experiments on mercury film rather than on mercury drop working electrodes. To assure that the results were comparable, experiments like those described above were repeated using mercury films deposited on platinum. The results were similar to those shown in Figs. l-5. From the data of a potential step experiment, exactly like the one depicted in Fig. 2, a plot of Qred vs. t’/‘2 was constructed. Using the integrated Cottrell equation the resulting slope gave D = (5 + 0.5) X 1O-5 cm’ s-‘. This value is in satisfactory agreement with D = (4.9 + 0.5) x lo-’ cm’ s-’ calculated from polarographic measurements. The experiments described above characterize the film growth and dissolution processes on a time scale of one second to several minutes. It seemed possible that
97
P
E/V
4
vs SCE
Fig. 5. The effect of stirring on the accumulation of the DMP-deposit. 10W3 M (DMP)BF, m DMF [5 x10-* M (TBA)BF,]; Eo = -2.760 V; u = 50 mV s-l. (a) t G = 0.5 min. quiet solution; (a’) same as (a) with starring: (b) to = 1 min, quiet solutron; (b’) same as (b) with stirring.
by using a smaller time window, information could be collected about the initial stages of this process. An appropriate technique is double potential step chronoamperometry, where a potential step is applied from the region where there is no electrochemical reaction to various potentials in the reduction region and the resulting current is measured during and after the pulse. The short time i-t transients show the same characteristics on drop or film electrodes. It was decided to analyze those recorded on the mercury film because the electrode is planar and its surface area is better defined. Figure 6 shows the data from a typical set of experiments, where the potential is stepped from - 2.500 V to values in the range of - 2.720 to - 2.760 V for 10 ms and then returned to - 2.500 Consider first the transients observed after the negative step. At each potential
TIME/m5
Fig. 6. The effect of the reduction potential on the cathodtc and anodtc 1-t transients of DMP on a mercury-film electrode. 10m3 M (DMP)BF, f0.1 M (TBA)BF, in DMF. Double potential step experiment: to = 10 ms; Et = -2.500 V; E,/V: (1) -2.720, (2) -2.730, (3) -2.740, (4) -2.750, (5) -2.760.
98
0
10
5 TIME/m5
Fig. 7. Cathodic z-t transients -2.730. (3) -2.735, (4) -2.740,
recorded under the same condmons (5) -2.745, (6) -2.750. (7) -2.760.
as Rg.
6. El/V.
(I)
- 2.720, (2)
the initial decay, due to the charging current, is followed by a rising transient. After reaching a maximum, the current decreases slowly with time. This behaviour is similar to that observed for other reactions governed by nucleation and growth kinetics [8,16.17]. Thus, the current due to DMP reduction is initially small because nucleation is just beginning. As nucleation and growth proceed, the current rises until it becomes limited by the diffusion of some reactant. As expected, the current after the maximum corresponds to t”’ decay. The anodic transients show rapid oxidation but indicate a phase transition in the process of film dissolution which seems to be confined to the layers next to the electrode surface. Such transients deserve more detailed study; however in the present work we were more interested in the film formation. Figure 7 illustrates the cathodic transients only, recorded on an expanded time scale to facilitate analysis. Over a great part of the range of rising portions, the current-time relationship is linear for i vs. t312 as shown in Fig. 8. A similar i-t relationship has been observed in other cases. including recently the deposition of cadmium on vitreous carbon [14]. Linear dependence of i vs. t”j2 is expected from processes involving three dimensional nucleation and growth, with the growth of nuclei controlled by localized hemispherical diffusion [S]. It is thus
3,~ _ -2.760
0
/
I
10 +3/2,53/Z
Fig. 8. The dependence
of the current
on time in the rising portlons
of the r-t
transients
in Fig. 7.
99 TABLE
1
Dependence of surface coverage 19on potential during a double potential step experiment V, fG = 10 ms). Mercury film electrode; 10m3 M (DMP)BF, +O.l M (TBA)BF, m DMF E, /V vs. SCE
Qred/K cm-2
ea
-
6.9 10.6 14.8 20.0 23.7
0.17 0.27 0.37 0.50 0.59
2.720 2.730 2.740 2.750 2.760
,’ Hypothetical
condensed
(E, = - 2.500
DMP layers.
reasonable to conclude that like the cadmium deposition, the observed cathodic phase transition of DMP corresponds to progressive three dimensional nucleation and growth. The charge, measured during chronocoulometric experiments performed under conditions identical to those used for the i-t transients shown in Fig. 8, was used to calculate the surface coverage 0 (19 = Q,,,/Q,,,,,,,,,) during the nucleation and growth process. The results shown in Table 1 show that, assuming a hypothetical condensed DMP structure, less than a monolayer is formed. It is now appropriate to use the results presented to formulate a coherent picture of the reduction of DMP. It is first noted that the DMP is incorporated into the surface layer in a chemically reversible manner. After many experiments in which thick layers were formed and reoxidized, analysis of the solution demonstrated that the DMP concentration was unchanged. This is in accord with coulometric experiments on larger electrodes which have shown [4,5] a decrease in the concentration of DMP in solution, during layer formation, followed by reappearance of DMP in solution after reoxidation. The present set of experiments extends from a time scale of a few milliseconds to several minutes. The results obtained at longest times allow the conclusion that thick films (capacity at least up to 8 mC cmp2) could be formed and quantitatively reoxidized. Independence of the oxidation rate on the film thickness suggests that thick films are conductive and conductivity is not rate determining for either growth or dissolution. Experiments on the time scale of seconds showed that reduction involves a diffusion controlled process in solution, while reoxidation is fast and independent of solution conditions like stirring. On the shortest time scale, clear evidence for a phase transition was obtained for both the cathodic and anodic processes. The linear increase in the rate (current) with t3/2 during the film formation indicates that it involves progressive three dimensional nucleation and hemispherical growth. The results and the conclusions presented here for DMP, resemble those for metal electrodeposition. The closest analogs involving organic molecules which have been carefully investigated are viologens. It has been found [17], for example, that heptyl viologen (HV2’) bromide in aqueous solution forms an insoluble, electronically conductive film of HV’+Br-. In parallel to DMP the one electron reduction of HV2+ on mercury shows three dimensional growth and nucleation kinetics at short
100
times. It is however, important to remember that the DMP-Hg process must be rather different. First, the reduction of DMP is not expected to lead to a stable neutral product. The electron must be added to a u* orbital which is high in energy and this is expected to cause cleavage. This situation contrasts with the known stability of the conjugated aromatic m-system of the reduction product HV’+. In addition, DMP-Hg film formation, unlike the HV’+Br _, involves incorporation of mercury into the film. In a kinetic-mechanistic sense then, it may be useful to consider the DMP-Hg film formation as a cathodic analog of anodic film formation where the metal enters the oxidized film [15,18]. It thus seems that further study of tetraalkylammonium-metal film formation will reveal phenomena similar to those in the anodic reactions. Clearly, these cathodic corrosion processes are intriguing and the current studies seem to open up an entirely new field. REFERENCES 1 (a) H.N. McCoy and W.C. Moore, J. Am. Chem. Sot., 33 (1911) 273; (b) L. Homer in M.M. Baizer and H. Lund (Eds.), Organic Electrochemistry, Marcel Dekker, New York, 1983, p. 397 and refs. therein. 2 E. Kariv-Miller, P.B. Lawin and Z. Vajtner. J. Electroanal. Chem., 195 (1985) 435. 3 J.D. Littlehailes and B.J. Woodhall, Discuss. Faraday Sot., 45 (1968) 187. 4 E. Kariv-Miller, C. NanJundiah, J. Eaton and K.E. Swenson, J. Electroanal. Chem., 167 (1984) 141. 5 E. Kariv-Miller and R. Andruzzi, J. Electroanal. Chem., 187 (1985) 175. 6 (a) M. Fleischmann and H.R. Thirsk m P. Delahay (Ed.), Advances in Electrochemtstry and Electrochemical Engineering, Vol. 3, Interscience, New York, 1963, pp. 123-210; (b) J.A. Harrison and H.R. Thirsk in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 5, Marcel Dekker, New York. 1971, pp. 67-148; (c) E.B. Budevski in D.A. Cadenhead and J.F. Danielli (Eds.). Progress in Surface and Membrane Science. Vol. 11, Academic Press, New York, 1976, pp. 71-116. 7 E. Bosco and S.K. Rangarajan, J. Electroanal. Chem.. 129 (1981) 25. 8 G. Gunawardena, G. Hills, I. Montenegro and B. Scharifker, J. Electroanal. Chem., 138 (1982) 225. 9 E. Kariv-Miller and Z. VaJtner, J. Org. Chem., 50 (1985) 1394, E. Kariv-Miller and T.J. Mahacht, ibid., 51 (1986) 1041. 10 V. SvetliEiC, J. Tomaic, V. ZutiC and J. Chevalet, J. Electroanal. Chem., 146 (1983) 71. 11 V. SvethEic, V. ZutiC, J. Chevalet and J. Clavilier, Mol. Cryst. Liq. Cryst., 113 (1984) 93. 12 V. SvetliEiC, V. ZuttC. J. Clavilier and J. Chevalet, J. Electroanal. Chem.. 195 (1985) 307. 13 G. Gunawardena, G. Hills and I. Montenegro, J. Electroanal. Chem., 184 (1985) 357. 14 G. Gunawardena, G. Hills and I. Montenegro. J. Electroanal. Chem., 184 (1985) 371. 15 Kh.2. Braimna, Stripping Voltammetry in Chemical Analysis. Wiley, New York, 1974, pp. 16-35, 132-141. 16 S. Asavapuiyanon, G.K. Chandler. G.A. Gunawardena and D. Pletcher, J. Electroanal. Chem.. 177 (1984) 229. 17 B. Scharifker and C. Wehrmann, J. Electroanal. Chem., 185 (1985) 93. 18 D.A. Vermilyea in ref. 6a, pp. 211-286.
J. Eiectroanal Chem., 209 (1986) 91-100 Elsevter Sequoia S.A., Lausanne - Printed
in The Netherlands
THE GROWTH OF ORDERED FILMS DURING REDUCTION OF A TETRAALKYLAMMONIUM
VESNA
SVETLICIC
and ESSIE KARIV-MILLER
Department of Chemrsty. (Received
16th January
THE CATHODIC ION
Vnwersity of Mmnesota, Mmneapohs. MN 55455 (US A.) 1986; m revtsed form 8th April 1986)
ABSTRACT The reduction of dimethylpyrrolidinmm tetrafluoroborate in DMF on a mercury cathode was studied and it was shown that a new ordered phase 1s formed. At times of the order of tens of milliseconds, chronoamperometry gave evidence for a phase transitton involvmg progressive three-dimensional nucleation and hemispherical growth. At longer times the film growth was limited by dtffusion in solution. Quantitative oxidative dtssolutton of the film was possible. The rate of reoxidation was rapid and independent of the thtckness of the film, over the range studted (Q Q 8 mC cm-*).
INTRODUCTION
Since 1911 several researchers have reported the formation of black, mercurycontaining precipitates from the reduction of simple tetraalkylammonium cations on mercury electrodes [l]. Recently we have observed the formation of similar, colored products from the reduction of tetraalkylammonium ions on a variety of metals [2]. The precipitates dubbed “ tetraalkylammonium-metals” * have been shown to participate as electrocatalysts in a number of organic electroreductions. Because they are not very stable and they are difficult to isolate, the structures of these materials are yet unknown. Two of the more stable tetraalkylammonium-mercury examples are formed by reduction of tetramethylammonium [3] and dimethylpyrrolidinium [4,5] (DMP) cations. The DMP reduction process [4,5] is of interest here. It is attractive for study because DMP reduces at rather positive potentials compared to other tetraalkylammonium ions. Thus, polarography of 0.005 M (DMP)BF, in DMF containing 0.1 M tetrabutylammonium tetrafluoroborate [(TBA)BF,] as the supporting electrolyte, gives rise to a well defined wave for DMP which is
* The term “amalgam” has previously been employed. but since that term is associated with mercury and tmphes certain structures, the term “ tetraalkylammonium-metal” seems more appropriate..“›
92
clearly separated from the background. Analysis of the wave shape and coulometric experiments have shown that the process involves a reversible one-electron transfer. DMP and mercury. The Reoxidation of the product (DMP-Hg) *, regenerates half-life of DMP-Hg on mercury in DMF at 5°C is about 6 h and a simplistic formulation of the reduction of DMP is: DMP+ + 1 e- + n Hg @ DMP-(Hg). The present study was initiated because we detected some unusual behavior in the cyclic voltammograms of DMP. Polarographic measurements have indicated that DMP-Hg is insoluble in both DMF and mercury and it seemed that the unusual cyclic voltammograms might reveal more about the process if considered in the context of phase transitions. The DMP-Hg formation can be considered as growth (deposition) of a new phase. If that phase is ordered, the electrodeposition of DMP would be similar in character to processes like metal electrodeposition and electrocrystallization [6] which involve nucleation and growth as phenomenological components [7]. In order for the new phase to develop, single molecules must first aggregate to small clusters (nucleation) before these clusters can expand to large dimensions (growth). The kinetics of film growth naturally depend on both of these phenomena and can be studied by electrochemical techniques. Indeed electrochemical techniques are especially useful for studying phase transitions, because the observed current is an exact measure of the combined rate of nucleation and growth, and the degree of supersaturation and interfacial conditions are easily controlled by the electrode potential. A variety of current-potential-time relationships can be explored, and experiments can be conveniently and exactly repeated many times. Some of the typical effects of phase transitions on cyclic voltammograms are a characteristic crossover upon switching of the scan direction, appearance of sharp spikes and enhanced peak separation of otherwise normal mass transport controlled voltammograms [8]. We observed such effects while studying the reduction of DMP and other tetraalkylammonium ions and developing their use as catalysts for organic electroreductions [5,9]. The goal of the present work was to investigate the hypothesis that a new ordered phase is generated during the reduction of DMP on mercury cathodes and to make initial steps toward understanding the events at the mercury/solvent interphase during the process. EXPERIMENTAL
The preparation of (DMP)BF,, (TBA)BF, and the SCE reference electrode, the purification of DMF, the cell and the apparatus used for cyclic voltammetry have been described elsewhere [5]. The working electrodes were a sessile mercury drop electrode [5] 0.0215 cm2 and
* The abbrewation
DMP-Hg
IS not mtended
to imply stoichiometry
93 a planar platinum based mercury film electrode 0.008 cm2. The latter was prepared by deposition of mercury at constant current from solutions of Hg,(NO,)* (analytical reagent “Mallinckrodt”). Optimal mercury films were formed by depositions using a charge of 200 mC cmp2 and all the results reported were obtained using films prepared in that manner. The counter electrode was a platinum wire and the reference electrode was a SCE. All potentials reported are vs. SCE. Experiments were carried out at 24 &-0.2”C under an argon or nitrogen atmosphere. Most experiments reported were carried out using 10 -3 M (DMP)BF,, but concentrations up to 1o-2 M were studied and found to behave similarly. It is of interest to note that we were unable to obtain discernible cyclic voltammograms using (DMP)BF, in concentrations lower than 5 X lop4 M. The techniques used to study the electrodeposition of DMP and the phase transitions involved are largely based on experimental methods developed for the study of supramolecular phenomena in methylene blue/leucomethylene blue redox films at mercury [lo], platinum and gold [11,12] electrodes. Linear sweep voltammetry was one of the methods used. In these experiments the DMP deposit was accumulated at constant potential (k 1 mv> at various generation times (0.25-6 min; Figs. 3 and 5). The charge transferred during the positive scans (Fig. 4) was calculated by integrating the appropriate areas of the LSV curves using a planimeter. Chronocoulometric experiments with pulse duration G 1 s (Fig. 2) were performed using a BAS-100 Electrochemical Analyzer (Bioanalytical Systems). Potential step experiments were performed using a PAR 173 potentiostat in combination with a PAR 175 universal programmer (both are Princeton Applied Research instruments). The resulting current-time transients (in the ms range, Figs. 6 and 7) were recorded by means of a storage oscilloscope (Tektronix 5111) and photographed (Tektronix C5A Polaroid camera). RESULTS
AND
DISCUSSION
The initial experiments were done on a mercury drop cathode and a typical cyclic voltammogram of lop3 M (DMP)BF, is shown in Fig. 1. The particular shape of
E/V
vs. SCE
Fig. 1. Cyclic voltammogram of 10K3 M (DMP)BF,, mercury drop electrode m DMF. 0.1 M (TBA)BF, as the supporting electrolyte. CJ= 50 mV s-‘; areas corresponding to reduction and oxidation are schematically indicated.
94
TIME/s
Fig. 2. The charge as a function of time in a double potential Dotted line: 0.1 M (TBA)BF,; solid line: 10d3 M (DMP)BF,
step expenment, mercury drop electrode. +O.l M (TBA)BF, in DMF.
the sharp rising cathodic current and the stripping anodic peak are similar to those observed during metal deposition onto foreign substrates [13,14] and are an indication that the reduction product forms a film on the cathode. The cathodic charge * (Qred) was calculated and found to be equal to the anodic charge (Q,,) implying that the reduction product is stable and remains on the surface (on the time scale of the experiment). The values for u = 50 mV s-* were Qred = Q,, = 186 PC cm-‘. The film thickness is of interest, but since the exact structure of the film is not known it cannot be calculated. However, using space filling models, the area occupied by one DMP was estimated to be 0.4 nm2 and a close packed monolayer composed of DMP only, was calculated to have a surface concentration (I,,,) of 4.2 x lo-” mol cm -*. The charge required to form such a monolayer is 40 /.LC cm-‘. Considering the amount of charge transferred, (Qred and Q,,) the formation and dissolution of 4-5 hypothetical close packed DMP layers takes place during the recording of one cyclic voltammogram at 50 mV s -‘, as the one shown in Fig. 1. Obviously, this is a limiting value since the incorporated mercury (0.07 nm*) and any other constituents will increase the thickness per coulomb. Figure 2 shows the charge as a function of time recorded during a double potential step experiment using 1O-3 M (DMP)BF, and a mercury drop electrode. The potential was stepped from - 2.000 V where there is no reaction of DMP to the region where reduction is diffusion controlled (- 2.760 V). After one second the potential was stepped back to -2.000 V. The dotted line in Fig. 2 represents the same experiment performed, using the background solution of the supporting electrolyte alone (0.1 M (TBA)BF,). It shows that the background consumes charge, forming a product which does not reoxidize under the experimental conditions. In order to verify that the dotted line represents quantitatively the background reaction, experiments like the one described in Fig. 2 were performed using higher concentrations of DMP. The charge (Qced) increased proportionally, but at all
l It is noted that the cathodic charge mcludes charge part of the return half cycle. as schematically mdlcated
transferred in Fig. 1.
during
the first half cycle as well as
95
E/V
vs. SCE
Fig. 3 The effect of generation time (I~) on the accumulation of the DMP-deposit. 10K3 M (DMP)BF, m DMF [5x lo-* M (TBA)BF,]; mercury drop electrode; E, = -2.760 V; u = 50 mV SC’. tG: (0) 0 min. (1) 0.25 mm. (2) 2 mm, (3) 3 min. (4) 4 min. (5) 5 min. (6) 6 min.
concentrations, it decayed to the same value of 40 PC cm2 characteristic of the electrolyte solution. Taking into account the background contribution, the chronocoulometric curves for DMP show that Qred = Q,,. As expected from the similar time scale, the values of of Pox and Qred from Fig. 2 are compatible with those calculated by integration the cyclic voltammogram in Fig. 1. The dominant characteristics of the chronocoulometric transients of DMP, recorded in the time range of a few seconds, are diffusion controlled reduction followed by “instantaneous” oxidation. It shows that the reduction product is deposited on the cathode and that the oxidation which releases DMP back into solution is a fast surface reaction, In order to determine if thick films could be grown, and to learn about the reoxidation process, we performed linear negative sweeps and stopped at - 2.760 V for various generation times (to) before reversing for the positive sweep. The LSV curves recorded during the sweep to more positive potentials are shown in Fig. 3. They are characterized by a smoothly increasing current, followed by an instantaneous drop to the base line. This can be rationalized by an oxidation rate which increases exponentially with potential until the film is completely oxidized. All indications are that the film is completely removed by the stripping process. The rising portions of all reoxidation curves, regardless of their to, are identical indicating that the rate is independent of the amount of deposit. With increasing to the peak currents are higher and are shifted to more positive potentials because there is more material to oxidize before the current can drop to the base line. These curves are analogous to the oxidation curves found for dissolution of certain metal
Fig. 4. The oxidation charge as a function of the generation time. 10e3 M (DMF)BF, in DMF[S M (TBA)BF,]; mercury drop electrode, E, = - 2.760 V. (a) quiet solution. (b) stirred solution.
x
lo-’
films [15] and of methylene blue/leucomethylene blue redox film [11,12]. The anodic stripping peaks contain information about the thickness and conductivity of the film. The dependence of the oxidation charge (calculated by integration of the corresponding curves) on to characterizes the film growth at the potential of generation (Eo). Figure 4a shows the dependence of the oxidation charge on to at -2.760 V. Assuming the presence of only DMP molecules at the electrode surface and one electron reduction, the oxidation charge corresponds to multilayer films of more than 200 hypothetical close packed DMP layers, with no indications of limitation to film growth. This observation and the independence of the oxidation rate on the film thickness suggests that thick films are conductive to the extent that conductivity does not control either the growth or the dissolution. Except during the first minute, the charge increases in an approximately linear manner. This is expected at these longer times where convection is important and the bulk concentration is not substantially depleted. It was of interest in this regard, to stir the solution during an accumulation-stripping experiment and the results are shown in Fig. 5. As expected, stirring causes larger Q values and thicker films (Fig. 4b). It, however, does not affect the rate of dissolution and the rising portion of the anodic currents is identical in stirred and unstirred solutions (Fig. 5). This demonstrates again that the oxidation is a surface phenomenon and is independent of the solution conditions. For a variety of reasons it was preferable to conduct further experiments on mercury film rather than on mercury drop working electrodes. To assure that the results were comparable, experiments like those described above were repeated using mercury films deposited on platinum. The results were similar to those shown in Figs. l-5. From the data of a potential step experiment, exactly like the one depicted in Fig. 2, a plot of Qred vs. t’/‘2 was constructed. Using the integrated Cottrell equation the resulting slope gave D = (5 + 0.5) X 1O-5 cm’ s-‘. This value is in satisfactory agreement with D = (4.9 + 0.5) x lo-’ cm’ s-’ calculated from polarographic measurements. The experiments described above characterize the film growth and dissolution processes on a time scale of one second to several minutes. It seemed possible that
97
P
E/V
4
vs SCE
Fig. 5. The effect of stirring on the accumulation of the DMP-deposit. 10W3 M (DMP)BF, m DMF [5 x10-* M (TBA)BF,]; Eo = -2.760 V; u = 50 mV s-l. (a) t G = 0.5 min. quiet solution; (a’) same as (a) with starring: (b) to = 1 min, quiet solutron; (b’) same as (b) with stirring.
by using a smaller time window, information could be collected about the initial stages of this process. An appropriate technique is double potential step chronoamperometry, where a potential step is applied from the region where there is no electrochemical reaction to various potentials in the reduction region and the resulting current is measured during and after the pulse. The short time i-t transients show the same characteristics on drop or film electrodes. It was decided to analyze those recorded on the mercury film because the electrode is planar and its surface area is better defined. Figure 6 shows the data from a typical set of experiments, where the potential is stepped from - 2.500 V to values in the range of - 2.720 to - 2.760 V for 10 ms and then returned to - 2.500 Consider first the transients observed after the negative step. At each potential
TIME/m5
Fig. 6. The effect of the reduction potential on the cathodtc and anodtc 1-t transients of DMP on a mercury-film electrode. 10m3 M (DMP)BF, f0.1 M (TBA)BF, in DMF. Double potential step experiment: to = 10 ms; Et = -2.500 V; E,/V: (1) -2.720, (2) -2.730, (3) -2.740, (4) -2.750, (5) -2.760.
98
0
10
5 TIME/m5
Fig. 7. Cathodic z-t transients -2.730. (3) -2.735, (4) -2.740,
recorded under the same condmons (5) -2.745, (6) -2.750. (7) -2.760.
as Rg.
6. El/V.
(I)
- 2.720, (2)
the initial decay, due to the charging current, is followed by a rising transient. After reaching a maximum, the current decreases slowly with time. This behaviour is similar to that observed for other reactions governed by nucleation and growth kinetics [8,16.17]. Thus, the current due to DMP reduction is initially small because nucleation is just beginning. As nucleation and growth proceed, the current rises until it becomes limited by the diffusion of some reactant. As expected, the current after the maximum corresponds to t”’ decay. The anodic transients show rapid oxidation but indicate a phase transition in the process of film dissolution which seems to be confined to the layers next to the electrode surface. Such transients deserve more detailed study; however in the present work we were more interested in the film formation. Figure 7 illustrates the cathodic transients only, recorded on an expanded time scale to facilitate analysis. Over a great part of the range of rising portions, the current-time relationship is linear for i vs. t312 as shown in Fig. 8. A similar i-t relationship has been observed in other cases. including recently the deposition of cadmium on vitreous carbon [14]. Linear dependence of i vs. t”j2 is expected from processes involving three dimensional nucleation and growth, with the growth of nuclei controlled by localized hemispherical diffusion [S]. It is thus
3,~ _ -2.760
0
/
I
10 +3/2,53/Z
Fig. 8. The dependence
of the current
on time in the rising portlons
of the r-t
transients
in Fig. 7.
99 TABLE
1
Dependence of surface coverage 19on potential during a double potential step experiment V, fG = 10 ms). Mercury film electrode; 10m3 M (DMP)BF, +O.l M (TBA)BF, m DMF E, /V vs. SCE
Qred/K cm-2
ea
-
6.9 10.6 14.8 20.0 23.7
0.17 0.27 0.37 0.50 0.59
2.720 2.730 2.740 2.750 2.760
,’ Hypothetical
condensed
(E, = - 2.500
DMP layers.
reasonable to conclude that like the cadmium deposition, the observed cathodic phase transition of DMP corresponds to progressive three dimensional nucleation and growth. The charge, measured during chronocoulometric experiments performed under conditions identical to those used for the i-t transients shown in Fig. 8, was used to calculate the surface coverage 0 (19 = Q,,,/Q,,,,,,,,,) during the nucleation and growth process. The results shown in Table 1 show that, assuming a hypothetical condensed DMP structure, less than a monolayer is formed. It is now appropriate to use the results presented to formulate a coherent picture of the reduction of DMP. It is first noted that the DMP is incorporated into the surface layer in a chemically reversible manner. After many experiments in which thick layers were formed and reoxidized, analysis of the solution demonstrated that the DMP concentration was unchanged. This is in accord with coulometric experiments on larger electrodes which have shown [4,5] a decrease in the concentration of DMP in solution, during layer formation, followed by reappearance of DMP in solution after reoxidation. The present set of experiments extends from a time scale of a few milliseconds to several minutes. The results obtained at longest times allow the conclusion that thick films (capacity at least up to 8 mC cmp2) could be formed and quantitatively reoxidized. Independence of the oxidation rate on the film thickness suggests that thick films are conductive and conductivity is not rate determining for either growth or dissolution. Experiments on the time scale of seconds showed that reduction involves a diffusion controlled process in solution, while reoxidation is fast and independent of solution conditions like stirring. On the shortest time scale, clear evidence for a phase transition was obtained for both the cathodic and anodic processes. The linear increase in the rate (current) with t3/2 during the film formation indicates that it involves progressive three dimensional nucleation and hemispherical growth. The results and the conclusions presented here for DMP, resemble those for metal electrodeposition. The closest analogs involving organic molecules which have been carefully investigated are viologens. It has been found [17], for example, that heptyl viologen (HV2’) bromide in aqueous solution forms an insoluble, electronically conductive film of HV’+Br-. In parallel to DMP the one electron reduction of HV2+ on mercury shows three dimensional growth and nucleation kinetics at short
100
times. It is however, important to remember that the DMP-Hg process must be rather different. First, the reduction of DMP is not expected to lead to a stable neutral product. The electron must be added to a u* orbital which is high in energy and this is expected to cause cleavage. This situation contrasts with the known stability of the conjugated aromatic m-system of the reduction product HV’+. In addition, DMP-Hg film formation, unlike the HV’+Br _, involves incorporation of mercury into the film. In a kinetic-mechanistic sense then, it may be useful to consider the DMP-Hg film formation as a cathodic analog of anodic film formation where the metal enters the oxidized film [15,18]. It thus seems that further study of tetraalkylammonium-metal film formation will reveal phenomena similar to those in the anodic reactions. Clearly, these cathodic corrosion processes are intriguing and the current studies seem to open up an entirely new field. REFERENCES 1 (a) H.N. McCoy and W.C. Moore, J. Am. Chem. Sot., 33 (1911) 273; (b) L. Homer in M.M. Baizer and H. Lund (Eds.), Organic Electrochemistry, Marcel Dekker, New York, 1983, p. 397 and refs. therein. 2 E. Kariv-Miller, P.B. Lawin and Z. Vajtner. J. Electroanal. Chem., 195 (1985) 435. 3 J.D. Littlehailes and B.J. Woodhall, Discuss. Faraday Sot., 45 (1968) 187. 4 E. Kariv-Miller, C. NanJundiah, J. Eaton and K.E. Swenson, J. Electroanal. Chem., 167 (1984) 141. 5 E. Kariv-Miller and R. Andruzzi, J. Electroanal. Chem., 187 (1985) 175. 6 (a) M. Fleischmann and H.R. Thirsk m P. Delahay (Ed.), Advances in Electrochemtstry and Electrochemical Engineering, Vol. 3, Interscience, New York, 1963, pp. 123-210; (b) J.A. Harrison and H.R. Thirsk in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 5, Marcel Dekker, New York. 1971, pp. 67-148; (c) E.B. Budevski in D.A. Cadenhead and J.F. Danielli (Eds.). Progress in Surface and Membrane Science. Vol. 11, Academic Press, New York, 1976, pp. 71-116. 7 E. Bosco and S.K. Rangarajan, J. Electroanal. Chem.. 129 (1981) 25. 8 G. Gunawardena, G. Hills, I. Montenegro and B. Scharifker, J. Electroanal. Chem., 138 (1982) 225. 9 E. Kariv-Miller and Z. VaJtner, J. Org. Chem., 50 (1985) 1394, E. Kariv-Miller and T.J. Mahacht, ibid., 51 (1986) 1041. 10 V. SvetliEiC, J. Tomaic, V. ZutiC and J. Chevalet, J. Electroanal. Chem., 146 (1983) 71. 11 V. SvethEic, V. ZutiC, J. Chevalet and J. Clavilier, Mol. Cryst. Liq. Cryst., 113 (1984) 93. 12 V. SvetliEiC, V. ZuttC. J. Clavilier and J. Chevalet, J. Electroanal. Chem.. 195 (1985) 307. 13 G. Gunawardena, G. Hills and I. Montenegro, J. Electroanal. Chem., 184 (1985) 357. 14 G. Gunawardena, G. Hills and I. Montenegro. J. Electroanal. Chem., 184 (1985) 371. 15 Kh.2. Braimna, Stripping Voltammetry in Chemical Analysis. Wiley, New York, 1974, pp. 16-35, 132-141. 16 S. Asavapuiyanon, G.K. Chandler. G.A. Gunawardena and D. Pletcher, J. Electroanal. Chem.. 177 (1984) 229. 17 B. Scharifker and C. Wehrmann, J. Electroanal. Chem., 185 (1985) 93. 18 D.A. Vermilyea in ref. 6a, pp. 211-286.