Anal. Chem. 2000, 72, 1598-1603
Open Circuit Reactions Complicating the Electroprecipitation of Poly(vinylferricinium) Films from Methylene Chloride Stanley Bruckenstein* and Ewa Pater
Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14260-3000 A. Robert Hillman
Department of Chemistry, University of Leicester, Leicester LE1 7RH, U.K.
We describe an electrochemical quartz crystal microbalance interfacial gravimetric study of the electroprecipitation and dissolution of PVF+ClO4- (PVF ) poly(vinylferrocene)) films exposed to methylene chloride solutions. Film deposition is diffusion controlled by the supply of PVF from the solution. Film solvent content is markedly dependent upon the deposition potential. The selfexchange process between surface-bound ferricinium sites and solution-phase ferrocene sites, which is responsible for film deposition under conditions of positive applied potential, is also responsible for film dissolution under open circuit conditions. Dependent upon the deposition potential, releasing the electrode from potential control may (not) result in transient deposition processes that temporarily increase surface coverage. Under all conditions, opening the circuit ultimately leads to reductive stripping of the film by solution-phase PVF. The dissolution rate is independent of applied potential, and is not limited by electron transfer at the outer film interface or by electron (and coupled ion) transport within the film; we speculate that polymer-based processes, such as chain disentanglement, are rate limiting. Electroprecipitation from a poly(vinylferrocene) (PVF) solution in methylene chloride (CH2Cl2) is commonly used to make thin films of PVF+X- (where X- is a counterion) on metal electrodes.1 The reaction for total oxidation of the polymer in perchlorate medium is
PVF + nClO4- a PVFn+(ClO4-)n + ne
(1)
Films obtained in this way from various solvents2-4 have been the subject of detailed studies. However, scant attention has been paid to the electroprecipitation process itself.5 We believe that a (1) Merz, A.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 3222. (2) Smith, T. W.; Kuder J. E.; Wychick, D. J. Polym. Sci. 1976, 14, 2433. (3) Peerce, P. J.; Bard, A. J. J. Electroanal. Chem. 1980, 114, 89. (4) Bandey; H. L.; Gonsalves, M.; Hillman, A. R.; Glidle. A.; Bruckenstein, S. J. Electroanal. Chem. 1996, 410, 219 and references therein. (5) Hillman, A. R.; Loveday, D. C.; Bruckenstein, S. Langmuir 1991, 7, 191.
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more detailed understanding of this process may, at least partially, explain the reported diversity of PVF film characteristics and provide the key to tighter control thereof. Here we demonstrate, for PVF, that potentiostatic deposition yields a film of homogeneous composition, but that this composition is a function of deposition potential. Consequently, material deposited at different potentials will have different characteristics that reflect these varying compositions. This may obviously be anticipated for different films deposited potentiostatically, but also, less obviously, within different regions of the same film deposited by galvanostatic or potentiodynamic (i.e., varying potential) protocols. In the past few years many PVF films were made in our laboratory by applying the electrodeposition technique.6-8 In our experience such films varied appreciably from each other (as judged by subsequent characterization in water) even if prepared by the same person, on the same day, from the same solution, and by apparently following exactly the same procedure. Frequently films with different thicknesses were obtained. Most of the time the qualitative behaviors of the films (e.g., shape of the current voltage curves in water) were similar, but even in this respect we observed some variations. In this paper we focus on one particular phenomenon which impacts this issue significantly, but which has not received attention to date: the spontaneous open circuit reaction of the PVF+ClO4- film with solution-phase PVF. This process occurs during the time a film is released from potentiostatic control prior to removal from the “plating” solution, here methylene chloride. It can, in some cases, increase the film thickness before it dissolves the film in methylene chloride solution. These changes in film thickness during the electroprecipitation process are readily studied via film mass. In this study, we follow such changes using the electrochemical quartz crystal microbalance (EQCM) technique. (6) Mensah, E. A. Conducting Polymer: Experimental Aspects of Piezoelectric Quartz Crystal Oscillator and Electrodeposited Polyvinylferrocene (PVF) Film System. Ph.D. Thesis (Part 1), SUNY at Buffalo, 1993. (7) Holt, R. J. Electrodeposition and Characterization of Polyvinylferrocene Films. Ph.D. Thesis (Part II), SUNY at Buffalo, 1993. (8) Fensore, A. T. Development of the Dual Crystal Microbalance and EQCM Studies of Polymer Modified Electrodes. Ph.D. Thesis, SUNY at Buffalo, 1995. 10.1021/ac990990a CCC: $19.00
© 2000 American Chemical Society Published on Web 03/08/2000
EXPERIMENTAL SECTION Reagents. PVF was supplied by Polysciences, and was purified as described before.9 PVF (20-40 mg/mL of solvent) was dissolved in methylene chloride (CH2Cl2) (distilled over CaH2 and the fraction boiling at 39-40 °C collected). The solution was centrifuged and decanted, and a few milliliters of CH2Cl2 was poured over the solid residue. This mixture was shaken to suspend the solid PVF, and then centrifuged again. The supernatant solutions with dissolved PVF were, if cloudy, filtered using Fisher Q2 filter paper. CH2Cl2 was evaporated at room temperature from the clarified solution, using a gentle stream of N2, to produce solid particles of PVF. This PVF was dissolved in the minimum possible volume of benzene and reprecipitated by slow addition of methanol until no more precipitation was observed. Tetrabutylammonium perchlorate (TBAP) (Eastman Fine Chemicals) was used as supplied. Instrumentation. The electrodeposition process of PVF+ClO4was studied in 0.1 M TBAP/CH2Cl2 as a supporting electrolyte using the EQCM. The concentration of PVF was 0.50-0.75 mM (in monomer units). The EQCM circuitry and cell configuration have been described previously.10,11 The quartz crystals (International Crystal Manufacturing Co., Inc., Oklahoma City, OK) were AT-cut, 10 MHz, 5 µm finish plates with gold electrodes (900 Å) evaporated on both sides of the plate. The piezoelectrically and electrochemically active areas were 0.22 and 0.25 cm2, respectively. Although thick films of PVF+ClO4- under deposition conditions in 0.1 M TBAP/CH2Cl2 have been shown to be viscoelastic,4,5 the films deposited and studied here are acoustically thin. Consequently, we are able to interpret the data in gravimetric terms, using the Sauerbrey equation.12 A home-built three-electrode potentiostat employing a currentto-voltage converter for current measurements was used. Experiments were performed at room temperature in a two-compartment cell containing four electrodes. The thin layer of gold on the quartz crystal (EQCM) served as a working electrode, a platinum wire was used as a counter electrode, and the potential of the working electrode was measured using an aqueous saturated calomel reference electrode (SCE). The SCE was separated from the main compartment by a salt bridge consisting of a glass tube with a glass frit on its end. The tube was filled with 0.1 M TBAP without PVF. A fourth electrode installed in the cell, the “security” electrode, was a small grounded Pt wire connected to the input of the current-to-voltage converter. It prevented the potentiostat from “open looping” by providing a path to ground when the working electrode was disconnected from the input of the currentto-voltage converter during our open circuit experiments. Procedure. Deposition and Electrochemical Stripping of PVF+ClO4-. The cell was filled with the PVF/0.1 M TBAP/CH2Cl2 solution, and the gold working electrode was held at 0.00 V for 2 min. Then a selected potential (between 0.45 and 0.70 V) was applied for another 2 min. Next the potential was switched back to 0.0 V; after at least 1-2 min the whole procedure was repeated. The charge and frequency changes accompanying the (9) Hughes, N. A Study of Ion, Solvent and Salt Transfer in Polyvinylferrocene Films. B.Sc. Thesis, Department of Chemistry, University of Bristol, Bristol, U.K., 1991. (10) Bruckenstein, S.; Shay, M. Electrochim. Acta 1985, 30, 1295. (11) Bruckenstein, S.; Michalski, M.; Fensore, A.; Li, Z. F.; Hillman, A. R. Anal. Chem. 1994, 66, 1847. (12) Sauerbrey, G. Z. Z. Phys. 1959, 155, 206.
potential (E) steps were recorded as functions of time. No trend depending on the order of an experiment was observed. These experiments were performed many times, on two different crystals (designated no. 1 and no. 2), on several different days, using several PVF solutions. Before the above procedure was adopted, some measurements involving an E-step to 0.6 V were performed with variable holding times (45-150 s) at that potential. Those measurements were done on crystal no. 1, on three different days and using two different solutions. Deposition and Open Circuit Reaction between PVF and PVF+ClO4-. All open circuit experiments on a crystal were performed during a single day. The solution used in these measurements contained 7.5 mg of PVF/50 mL of 0.1 M TBAP/ CH2Cl2 and was about three weeks old. The working electrode was held at 0 V vs SCE (a potential where no PVF+ClO4- forms). Then an E-step (to a potential where PVF+ClO4- forms) was applied to the working electrode and maintained for 45 s before the circuit was opened. The E-steps used were from 0.00 to 0.60, 0.50, 0.475, 0.46, and 0.45 V. The working electrode’s frequency and charge changes were recorded as functions of time on strip chart recorders. During the open circuit part of the experiment, the working electrode’s potential was also determined with a digital voltmeter. The working electrode’s potential was stepped from 0.00 V and held for 120 s at 0.60, 0.46, and 0.45 V in two experiments, at 0.5 V in three experiments, and at 0.475 V (with a holding time of 105 s) in one experiment. RESULTS AND DISCUSSION Deposition and Electrochemical Stripping of PVF+ClO4-. Identifying Maximally Oxidized PVF. We set out to determine the applied potential that would produce a maximally oxidized PVF+ClO4- film. The reaction of the total oxidation of polymer may be written as
(VF)n + nClO4- + mnSolv a (VF+ClO4-mSolv)n + ne (2) (VF)n represents reduced poly(vinylferrocene) made up of n monomer units, and (VF+ClO4-mSolv)n represents the totally oxidized form, poly(vinylferricinium perchlorate) with m solvent molecules associated with each redox center. As will be seen below, the experimental values of the molar mass (MM ) ma/ (q/F), where ma is the change in electrode mass accompanying passage of a faradaic charge, q, and F is the Faraday constant) indicate that substantial solvent is incorporated in the film, i.e., m . 1. Consequently, the absolute value of the deposited mass alone could not be used to establish the film’s level of oxidation (based on perchlorate incorporation alone). Therefore, as a practical approach, we defined a maximally oxidized film as one whose molar mass was not a function of the potential applied to the film. The potential we used was 0.6 V (see Table 1). Potential steps from 0 V to selected positive potentials electroprecipitated PVF+ClO4- films of various oxidation levels. This step was followed after 120 s by a reverse E-step to 0 V, which reductively removed the film from the electrode. Figures 1 and 2 show typical examples of the mass (ma) and charge vs time obtained in these experiments involving E-steps from 0.0 to 0.70, Analytical Chemistry, Vol. 72, No. 7, April 1, 2000
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Table 1. Average Molar Masses for the Deposition (MMup) and Dissolution (MMdown) Steps, Their Standard Deviations, and the Results of the t-Test at the 95% Confidence Interval (95% ci)
E (V) Nup 0.70 0.60 0.50 0.45 0.60 0.70 0.60 0.50 0.45 0.60 a
3 3 4 4 3 3 4 3 4 2
comparison MMup std dev MMdown std dev of “up” and (av) “up” Ndown (av) “down” “down” MM 1452 1458 785 232 1216 1634 1699 633 179 1326
26 76 46 8 50 50 202a 13 16 0
3 3 3 4 3 3 4 3 4 2
1450 1468 817 323 1255 1700 1658 681 274 1376
20 78 62 3 35 37 148a 12 14 13
same same same different same same same different different same
Standard deviation much larger than for other measurements.
Figure 2. Charge vs time plots for the experiments shown in Figure 1.
Figure 1. Mass change vs time plots following a positive potential step from 0.00 V to the indicated potentials, and a reverse potential step after the positive potential was held for 120 s.
0.60, 0.50, and 0.45 V and in each case then back to 0.0 V. Potential step experiments that formed and dissolved oxidized PVF+ClO4films were performed numerous times to provide enough data for statistical analysis. The mass change (ma) vs time behavior (Figure 1) depends on the potential applied in the E-step. The 0.60 and 0.70 V curves show a sharp linear increase during the first 10-15 s. Then a slower growth is observed, and finally both curves bend down toward the time axis and continue to rise at a slower rate. The initial slopes for the 0.50 and 0.45 V curves are smaller than for those at more positive deposition potentials. (Note that the 0.45 V curve is displayed with 10 times higher sensitivity.) After 30 s for the 0.50 V trace, and after 45 s for the 0.45 V trace, both responses increase linearly. For the first 2-3 s after the potential is stepped to a more positive value, there is a sharp rise in the charge curves (Figure 1600 Analytical Chemistry, Vol. 72, No. 7, April 1, 2000
2). Then a slower increase is observed, and after 45-60 s the charge changes linearly with time. When the potential is switched back to 0.00 V, both ma and q decrease rapidly. Values of the charge change for the reverse E-step were found by extrapolation back to the time at which the reversed E-step was imposed. The ma value returns (very close) to its original value before the positive E-step. Nearly all of the oxidation charge is immediately recovered, and with time, a small amount of missing charge is subsequently recovered. Effect of the Deposition Potential on the Average Molar Mass. Molar mass data calculated from E-step experiments to various positive potentials are given in Table 1, and are listed in the chronological order the data was obtained. We use the notation “up” and “down” to signify quantities associated, respectively, with a positive- and negative-going potential step (corresponding, in broad terms, to overall film deposition and dissolution). The means of the “up” and “down” molar masses obtained in all 0.60 and 0.70 V E-step experiments are the same, within experimental error, according to a t-test at the 95% confidence interval. The anodic and cathodic mean molar masses for both 0.45 V experiments are different. In one 0.50 V E-step experiment, “up” and “down” mean molar masses are the same (785 ( 46 g mol-1 vs 817 ( 62 g mol-1), while in the second one they are different (633 ( 13 g mol-1 vs 681 ( 12 g mol-1). Additional data (not presented here) show that films deposited at 0.60 and 0.70 V have the highest (and the same) molar masses associated with film deposition (MMup) and dissolution (MMdown) over the time course of the experiment. Consequently, we selected 0.60 V as the optimum potential for forming films. Using the mean value of the molar masses at 0.60 V (1456 ( 200 g mol-1), the number of solvent molecules associated with a charge center (unsolvated molar mass 311.5 g mol-1) is ca. 13 ((2). Films formed at less positive
Table 2. Molar Masses Calculated from Initial Slopes of ma vs q Plots during PVF+ClO4- Deposition at Different Potentials molar mass (g mol-1)
Figure 3. Test for diffusion control. Mass vs t1/2 plots for the positive potential steps from 0.00 V to the indicated potentials.
Figure 4. Test for diffusion control. Charge vs t1/2 plots for the positive potential steps from 0.00 V to the indicated potentials.
Figure 5. Mass vs charge plots following positive potential steps from 0.00 V to the indicated potentials. The lower solid line represents the theoretical value for a solvent-free redox center undergoing complete oxidation. The upper solid line represents a 10× magnification of the lower line.
potentials were less oxidized. These results show that even incompletely oxidized PVF+ClO4- films are insoluble. Test for Diffusion Control during Deposition. Figures 3 and 4, respectively, show plots of ma and q vs t1/2 (using values of ma and q from Figures 1 and 2) as a test of diffusion control of the deposition process. Both plots are linear, but the ma vs t1/2 plot has an intercept on the t-axis of ca. 1 s, which we attribute to the time for nucleation to produce a growing film on the electrode. For all the deposition potentials, ma and q change linearly with t1/2 up to t ≈ 25-30 s. After that time, their plots bend up slightly, a phenomenon that could be explained by natural convection becoming significant at longer times. The ma and q values obtained at 0.45, 0.50, 0.60, and 0.7 V, and used in Figures 3 and 4, are plotted against each other in Figure 5. After a short time delay, while the solution adjacent to the electrode becomes supersaturated with the oxidized polymer so that film growth can occur, the ma-q plots for all deposition potentials are linear. This important result shows that the stoichiometric composition of the film precipitated at each constant potential is independent of film thickness. The linear relationship between the ma and q values demonstrates that film solvent
molar mass (g mol-1)
E (V)
crystal no. 1
crystal no. 2
E (V)
crystal no. 1
crystal no. 2
0.70 0.60
1083 ( 43
1483 ( 66 1305 ( 57
0.50 0.45
533 ( 5 213 ( 5
698 ( 19 186 ( 12
population varies linearly with q. The latter conclusion is a consequence of the following argument. During the deposition process, PVF molecules become attached to (precipitate on) the gold electrode when they are only partially oxidized (for E g ca. 0.45 V). At more positive potentials, individual molecules (polymer chains) become more oxidized, but their number on the electrode surface does not increase. Consequently, the mass plotted in Figure 5 represents that of the mobile species involved in the redox process, namely, perchlorate ion and the solvent (methylene chloride). Since the mass of incorporated perchlorate ions changes proportionally with q (on electroneutrality grounds), the mass of solvent must also change linearly with q so that the total mass change will vary linearly with q. The lower solid straight line in Figure 5 corresponds to the theoretical mass vs charge plot assuming that each charge center in the film is oxidized but unsolvated, and thus has the theoretical molar mass of 311.5 g mol-1. The upper solid line was multiplied by 10 for comparison with the 0.45 V series. The molar masses obtained from these lines are listed in Table 2, as are data obtained in a duplicate experiment using a fresh crystal and fresh methylene chloride solutions. The agreement between the molar mass results obtained in these two experiments is the typical reproducibility for the deposition of PVF+ClO4- in our laboratory. At all potentials greater than 0.45 V, the molar mass exceeds 311.5 g mol-1 because methylene chloride, the solvent, is present in the oxidized film at all potentials. The 0.45 V molar masses are less than 311.5 g mol-1, a result that can be rationalized if this partially oxidized film contains little, if any, solvent. Also, we note the possibilitysindeed likelihoodsthat here the deposition of PVF+ClO4- is much less than 100% efficient, because of the need to supersaturate the solution adjacent to the electrode before nucleation, leading to film formation. Open Circuit Experiments. These experiments differ from those in the preceding section in one important respect. After the E-step from 0 V to the selected deposition potential, the polymercovered working electrode was disconnected from the potentiostatic circuit, rather than the potential being stepped back to 0.00 V. Figure 6 shows typical examples of mass vs time changes during open circuit experiments which followed E-steps from 0.00 to 0.45, 0.50, and 0.60 V. Values of the potential in the E-step are given on each curve. Note that the mass scale for the 0.45 V curve is 10 times greater than those for the 0.50 and 0.60 V curves. The open circuit mass-time behavior depends markedly on the holding potential. On opening the circuit at 0.45 V, the mass decreases immediately, demonstrating that film dissolution predominates. After ca. 60 s, the mass reaches the value it had before the E-step from 0.00 V. With time, a further slight decrease in Analytical Chemistry, Vol. 72, No. 7, April 1, 2000
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Table 3. Experimental Data for the Open Circuit Experiments Preceded by the E-Step rd/MMupc E-step (V) q (µC) ma,up (ng) ∆m (ng) tm rd (ng/s) (pmol s-1) 0.60a 0.60 0.50b 0.50a 0.50 0.475 0.46 0.46 0.45 0.45a
481 477 471 348 348 348 245 247 215 213
4112 4151 2832 1610 1649 1125 582 566 341 330
194 213 39 58 58 39 0 0 0 0
18 19 21 15 15 13 0 0 0 0
44.4 42.4 20.8 22.2 21.6 16.2 13.9 13.5 7.1 10.3
53.8 50.4 35.9 49.8 47.3 51.9 60.7 61.1 46.4 68.7
a Data for these experiments are presented in Figure 6. b After the E-step the working electrode was held at 0.50 V for 105 s (for all other cases it was 45 s). c Mean value 52 ( 9.2 pmol s-1.
Figure 6. Mass change vs time plots following positive potential steps from 0.00 V to the indicated potentials, and the open circuit responses after the positive potential was held for 120 s.
mass is observed. At more positive potentials the film does not dissolve immediately on being open circuited. At more positive deposition potentials, the mass of the electrode initially increases with opening of the potentiostatic circuit. This increase becomes more rapid as the deposition potential becomes more positive. Also, the maximum mass reached at the end of the “holding” time increases with the applied potential: 330 ng for the 0.45 V experiment, 1610 ng for the 0.50 V experiment, and 4112 ng for the 0.60 V experiment. In all cases, after remaining for some time at open circuit, the electrode mass decreases and ultimately returns to the value it had before any PVF+ClO4- was deposited. When the circuit is opened at 0.50 or 0.60 V, three regions are apparent in the mass-time curves. First, a mass increase is observed: 58 ng for the 0.50 V experiment and 194 ng for the 0.60 V experiment. Second, there is a “flat” region in the masstime curve. During this time the electrode potential is gradually decreasing. The mass stays constant: for 15 s in the 0.50 V experiment and 18 s in the 0.60 V experiment. Third, the mass decreases again as the open circuit potential approaches a value between 0.44 and 0.45 V, again signaling the dissolution of the films. Eventually (t ) 84 s for the 0.50 V deposited film and t )114 s for the 0.60 V deposited film), the electrode mass returns to its predeposition value. The open circuit potential for which film dissolution is completed is the same in all three cases, E ) 0.41 V. The potential continues to decrease for another 4 min, after which it reaches 0.36 V. Experiments similar to those just described were performed with E-steps to 0.46 and 0.475 V. The mass-time curve for the last one resembles the 0.50 V experiment in Figure 1, although the maximum mass recorded in this case was smaller and the time for which the mass stayed constant was shorter. Mass-time curves for the 0.46 V experiments are very similar to the 0.45 V response in Figure 6. All experimental values for those cases, as well as more data for replicate 0.60, 0.50, and 0.45 V experiments, are summarized in Table 3. 1602
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The first column of Table 3 lists the potentials applied in the E-step part of the experiment. The next two columns give the values of charge (q) used to deposit the film, and the apparent mass (ma,up) of the film obtained in the E-step. ∆m designates the apparent mass increase after the circuit is opened. tm denotes the time during which mass remains constant (see Figure 6 and its description above). ∆m and tm increase with increasing magnitude of the potential applied in the E-step. The next to the last column of Table 3 shows the average dissolution rate (rd) of the film in each experiment, calculated as
rd ) (ma,up + ∆m)/t95
(3)
where ma,up denotes the mass of the film deposited during the E-step and t95 designates the time at which 95% of the film had dissolved (the mass at t95 equals 0.95% × (ma,up + ∆m)), measured from the moment the mass started to decrease. The higher the potential applied in the E-step, the faster was the film dissolution rate under open circuit conditions. This result is consistent with the 0.60 V film being the most oxidized film. Discussion of Open Circuit Results. Under open circuit conditions, no external source of electrons is available; thus, no currentinduced electrochemical process may occur. All observed mass changes during this part of the experiment must be a result of a purely chemical redox reaction between the PVF+ClO4- film deposited on the electrode and reduced, dissolved PVF present in the solution:
PVF+ClO4-(film) + yPVF(solution) a [(1 + y)PVF(1/(1+y))+]ClO4- (4) where the product becomes completely soluble at a large enough y value. We tested our hypothesis that a self-exchange reaction involving the dissolved, unoxidized PVF was causing the oxidized PVF film’s removal from the QCM electrode as follows. First, while maintaining potential control at 0.6 V, we exchanged the methylene chloride/tetra-n-butylammonium perchlorate solution containing PVF for supporting electrolyte free of PVF. Then we opencircuited the electrode and followed the mass of the oxidized film
on the QCM with time. For two films we studied this way, the mass remained constant for between 15 and 70 min. After 2 h, the film’s mass had decreased less than 2% of the total film dissolved under these open circuit conditions. The total film mass was established by electrochemical stripping. We ascribed the small dissolution that occurred under these open circuit conditions to trace PVF that remained in the electrochemical cell because of incomplete drainage while removing the original deposition solution. This experiment conclusively demonstrated that the presence of PVF in the supporting electrolyte was required for spontaneous film dissolution at open circuit. If the circuit is opened at E g 0.475 V, a mass increase first occurs. We ascribe this to PVF in the solution being oxidized by the PVF+ClO4- film, to form an insoluble film that contains more PVFsthat originally present, plus additional material precipitated as a consequence of reaction 4. Next, for a short time, the film mass remains constant, because the rates of PVF+ClO4- deposition and dissolution are equal and these processes balance each other. Finally, an excess of the neutral PVF from the solutionsan effectively infinite reservoir of reductantsis incorporated into the PVF+ClO4- film to produce a barely oxidized film, which is soluble. This result confirms and supplements unexplained effects observed by Holt7 in open circuit experiments performed on a rotating ring disk electrode. There the potential of a disk electrode was stepped from 0.0 to 1.2 V, and after 300 s the circuit was opened. The potential of the ring was kept at 0.0 V throughout the whole experiment. Immediately after the circuit was opened, an increase in the cathodic current on the ring was observed. Next, the ring current was seen to increase linearly with time, and after about 20-25 s the rate of this increase became even larger. Holt’s result shows that some partially oxidized and soluble PVF is produced by reaction between the more oxidized film and completely reduced, dissolved PVF. This partially oxidized, soluble PVF can be reduced at the ring electrode. The subsequent increase in the ring current occurred when the film’s oxidation level decreased to the point where it became completely soluble and left the disk electrode. The potential at which a PVF+ClO4- film starts to dissolve upon opening of the circuit is ca. 0.44-0.45 V. In cyclic voltammetric experiments (not shown), for a potential cycle from 0 to 0.6 V and back to 0 V, an abrupt mass decrease occurred between 0.47 and 0.42 V during the cathodic half-cycle. Both cyclic voltammetry and open circuit methods give the same minimum potential required to maintain a PVF+ClO4- deposit on the electrode. These observations explain why the film dissolves immediately when the circuit is opened at 0.45 or 0.46 V. The average dissolution rate, in gravimetric terms, of the film after the circuit is opened increases with the potential at which the circuit is opened. This occurs even though the total time required to remove the film is longer for the higher potential experiment, because more oxidized PVF has deposited via reaction 4. This dissolution rate does not depend on film thickness, as seen in the 0.5 V experimental data in Table 3, where different film deposition times were used. The last column in Table 3 normalizes the dissolution rate of the film at the various potentials with respect to the molar mass of the film. It demonstrates that this chemical rate, expressed in molar terms (of redox sites), does not depend on the deposition potential used for the film. The mean value of
the normalized rate, 52 pmol s-1, corresponds to a self-exchange current of ∼20 µA cm-2 for reaction 4. It is interesting to explore the nature of reaction 4 a little further. It is essentially a heterogeneous self-exchange reaction between (surface-confined) oxidized and (solution-phase) reduced ferrocene moieties. As a reversible redox couple, it would be associated with a heterogeneous electron-transfer rate constant in excess of 10-2 cm s-1. Viewing reaction 4 as a (pseudo-)firstorder process, this leads to an exchange current density in excess of 1 A cm-2. Hence, interfacial electron transfer cannot be rate limiting. Alternatively, we may consider diffusion of charge within the polymer (coupled electron/ion transfer) as a possible ratelimiting step. The effective diffusion coefficient for this process is on the order of 10-10 cm2 s-1. The diffusion-limited flux across a film on the order of 1000 Å thick would be 10-8 mol cm-2 s-1, corresponding to a current density of ∼10-3 A cm-2sagain far greater than observed. We therefore conclude that polymer-based processes are rate limiting; we speculate that these could be associated with chain disentanglement. CONCLUSIONS The composition of electroprecipitated PVF+ClO4- films in methylene chloride is dependent upon the deposition potential. Even rigid (acoustically thin) films contain a substantial amount of solvent, to an extent that increases at low deposition potentials, and then becomes approximately constant at E ≈ 0.60 V. At these more anodic deposition potentials, oxidation of solution-phase PVF is diffusion controlled. After an induction period, attributed to solution supersaturation and interfacial nucleation processes, the film mass increases with the square root of time, until convection processes become significant at longer times. There is continual self-exchange between surface-bound and solution-phase ferrocene/ferricinium moieties. This is evidenced by the behavior upon release of the polymer-coated electrode from potential control. At short times thereafter, oxidized surface-bound sites are able to oxidize solution-phase PVF, resulting in precipitation of the latter; this is manifested via an increase in film mass. This increase in surface coverage is greater for more oxidized films, formed at more positive applied potential before open circuit. It is, however, a transient process, since the effectively infinite solution reservoir of PVF ultimately reduces all the film sites to the (soluble) ferrocene state. When expressed in molar (cf. gravimetric) termssnormalized to take into account potentialdependent solvationsthe dissolution rate is independent of applied potential. Simple arguments show that the rate of this selfexchange-type dissolution process is not limited by electron transfer at the outer film interface or by electron (and coupled ion) transport within the film, in agreement with experiment. The fact that the rate is potential independent points to a polymer film process. We speculate that polymer chain disentanglement is rate limiting. ACKNOWLEDGMENT We thank the National Science Foundation (Grant CHE9616641) for financial support. Received for review August 27, 1999. Accepted December 2, 1999. AC990990A Analytical Chemistry, Vol. 72, No. 7, April 1, 2000
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