8718
J. Phys. Chem. B 2001, 105, 8718-8724
Electrochemical and Spectroelectrochemical Investigations into the Nature of Charge-Trapping in Electrochemically-Generated Homopolymer Films of Tris(4-vinyl-4′-methyl-2,2′-bipyridine)ruthenium(II)† Scott C. Paulson,‡ Shawn A. Sapp, and C. Michael Elliott* Department of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523 ReceiVed: April 6, 2001; In Final Form: May 31, 2001
Electrochemical and spectroelectrochemical studies were conducted on thin films of poly- tris(4-vinyl-4′methyl-2,2′-bipyridine)ruthenium(II) which were formed by electrochemical reductive-polymerization. These homopolymer films undergo a process whereby redox charge is trapped in the film when the potential is scanned between the formal Ru(I/II) and Ru(II/III) couples of the polymer. This trapped charge is then released at the leading edge of each respective bulk redox process. The results of these studies show that the redox sites responsible for this trapped charge have potentials that lie exclusively in the potential region between the two bulk processes. Additionally, difference spectra obtained during the charge-release indicate that the oxidative and reductive processes have important qualitative and quantitative differences.
Introduction Early in the study of redox active polymer films Murray and co-workers observed and reported a phenomenon that has come to be called “charge-trapping”.1 This phenomenon gives rise to sharp prewaves (or “trapping-peaks”) in the cyclic voltammogram (CV) of certain homopolymers which precede the main redox wave, generally in both the oxidation and reduction scans (Figure 1).1 Many varied types of redox-active polymers exhibit this behavior.1-14 Most commonly, the phenomenon occurs in polymers that have been generated electrochemically and when the polymer has multiple redox processes separated widely in potential (as does the polymer in Figure 1).1,3 There are a number of mechanisms whereby such trappingpeaks could result. Despite the considerable time passed since their original observation, there is still speculation on their exact origin. Basically, the possible causes can be separated into two groups: ones that are primarily kinetic in origin and ones that are predominantly thermodynamic. Before considering either, it is useful to review the general way that conductivity of a redox polymer changes as a function of potential. For that purpose, the polymer responsible for the CV in Figure 1 will serve as a good example. Ignoring for the moment the chargetrapping, within the potential range scanned the polymer exhibits two Nernstian processes which involve most of the redox sites within the polymer (hereafter referred to as “bulk” or “main” redox processes). At approximately 0.87 V there is the formal 3+/2+ couple and at approximately -1.70 V there is the formal 2+/1+ couple. In the vicinity of either of these waves, the polymer is mixed-valent (e.g., either 3+/2+ or 2+/1+) and therefore electronically conductive. In this context, by electronically conductive we mean that the polymer has the ability to conduct electrons by site-to-site electron exchange. The extent of mixed-valency, and therefore conductivity, depends on the potential.15 In the potential region between the two main †
Part of the special issue “Royce W. Murray Festschrift”. * Corresponding author. ‡ Present address: Department of Chemistry, University of Calgary, Calgary, Alberta, Canada T2N 1N4.
Figure 1. Cyclic voltammograms of a poly-[Ru(vbpy)3] film (Γ ≈ 1.7 × 10-8 M cm-2) on a glassy carbon electrode in 0.1 M TBAPF6/ acetonitrile scanned at 100 mV s-1. The outer curve (a) is the steadystate voltammogram obtained by scanning several times between the potential limits of -1.8 and 1.2 V. The two inner curves (b) and (c) were obtained by cycling to steady-state between -1.15 and -1.80 V and between 0.45 and 1.2 V. Potentials are reported vs Ag/Ag+.
processes, several hundred millivolts from either, the polymer is nominally in a single valence state (i.e., 2+) and is essentially insulating. The conductivity of the polymer has a direct effect on the charge-trapping. Any redox site which has a formal potential which lies within one of the two conductivity windows (i.e., the mixed-valent region) and fails to discharge (i.e., change oxidation state) during the scan before that conductivity window closes, must wait until the other conductivity window opens (i.e., the potential is scanned near the other peak). This process would correspond to what might be called “kinetic-trapping”. There is also the possibility that states exist which have formal potentials that lie within the insulating region. These states oxidize within one conduction window and reduce in the other. In principle, one might be able to distinguish between these two types of processes by varying the experimental rate (or by some other similar means). In practice, it is not so simple for reasons such as the insulating region of potential is not infinitely resistive and, furthermore, that resistance changes with potential. Moreover, the CV tells nothing about what the actual redox states are that make up the trapped sites. In other words, in the
10.1021/jp011281f CCC: $20.00 © 2001 American Chemical Society Published on Web 07/18/2001
Electrochemically-Generated Homopolymer Films case of the polymer represented in Figure 1, the reductive trapping peak could be composed of Ru(III) f Ru(II) and/or Ru(II) f Ru(I) conversions and the CV experiment, alone, is silent on these details. Below we present results from an extensive set of electrochemical and spectroelectrochemical studies of one specific polymer: poly(tris-4-vinyl-4′-methyl-2,2′-bipyridine)ruthenium(II) (poly-Ru(vbpy)2+ 3 ). This material is grown as an electrodebound thin film by the electrochemical reduction of the corresponding monomer. Excluding conventional organic conducting polymers (e.g., polypyrroles, polythiophene, and polyaniline), it is probably more studied than any other redox polymer; therefore, understanding the origin of its chargetrapping behavior is relevant to a large body of published work. 1,3,15-41
Experimental Section Materials. Acetonitrile used for electrochemistry was supplied by either Baker (“Photrex” Reagent) or Burdick & Jackson (“Distilled in Glass”). All other chemicals were reagent grade and used as is. Tetra-n-butylammonium hexafluorophosphate (TBAPF6) electrolyte was prepared by metathesis of TBA+Iand NH4PF6 and was recrystallized several times from ethanol and vacuum dried. The compound 4,4′-dimethyl-2,2-bipyridine (dmb) was supplied by Reilly Industries, Inc. (Indianapolis, IN) and was twice recrystallized from ethyl acetate before use. The compound 4-methyl-4-vinyl-2,2-bipyridine (vbpy) was prepared by literature methods.42,43 A silica gel/CH2Cl2/acetone/ ethanol column was used to separate the alcohol intermediates. The THF employed was distilled under nitrogen over sodium/ benzophenone. [Ru(vbpy)3](PF6)2 was prepared using the method of Abruna et al.43 Instrumentation and Measurements. All UV-vis measurements were performed on a Perkin-Elmer 553 or an HP8452 diode array UV-vis spectrometer. Concentrations of [Ru(vbpy)3](PF6)2 in the electrochemical solutions were determined by measuring the solution’s absorbance in a 0.1 mm quartz cell, and by calculating the concentration vs a standard solution which had an extinction coefficient of 1.8 × 104 cm-1 M-1 at 466 nm. Except where noted, all electrochemical measurements were carried out in nitrogen-purged acetonitrile solutions of 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. A conventional three-electrode electrochemical cell was used for cyclic voltammetry work, which included either a 0.13 cm2 glassy-carbon or a 0.018 cm2 platinum disk working electrode, a platinum-wire loop auxiliary electrode, and a fritted compartment containing a 0.1 M AgNO3/ DMSO/Ag (Ag+) reference electrode. All cyclic voltammetry and bulk electrolysis experiments were performed using either a PAR model 173 potentiostat or a BAS-100 electrochemical analyzer. Spectroelectrochemical Experimental Design. The basic optically transparent thin-layer electrochemical (OTTLE) cell design has been described previously.44 A SnO2-In2O3 coated quartz working electrode was used in conjunction with a platinum-wire loop auxiliary electrode and a 0.1 M AgNO3/ DMSO/Ag (Ag+) reference electrode. The cell was designed to be filled with the monomer solution in a nitrogen atmosphere drybox, and then transferred, under nitrogen, to the spectrometer, where it fitted snugly into a 10 mm quartz cell in the optical light path. Spectra were recorded on an HP 8452 diode array spectrophotometer.
J. Phys. Chem. B, Vol. 105, No. 37, 2001 8719
Figure 2. Cyclic voltammogram of a 0.9 mM solution of [Ru(vbpy)3]PF6 on a glassy carbon electrode in 0.1 M TBAPF6/acetonitrile scanned at 100 mV s-1 during a typical electrochemical reductive polymerization. The potential was scanned five times between 0.5 and -1.7 V. The current increase with each sequential scan is due to the buildup of poly-[Ru(vbpy)3] on the electrode surface.
A spectroelectrochemical experiment typically consisted of scanning the working potential of the thin layer electrode at 25 mV/s while taking a spectra, covering a 560 nm window between 260 and 820 nm, every 2 sseach spectrum being integrated for 0.5 s. Consequently, during the collection of each spectrum the potential for a CV was scanned 12.5 mV. Polymerization Procedure. Polymerization solutions varied in concentration from ∼4 to ∼0.2 mM [Ru(vbpy)3](PF6)2 in either 0.1 or 0.2 M TBAPF6/CH3CN. Electropolymerizations were performed by cycling the potential into either the first reduction wave or through the first- and second-ligand-based reductions, depending on the monomer concentration. Results and Discussion Electropolymerization of [Ru(vbpy)3]2+. Reductive polymerization of vinylbipyridine-containing redox complexes has been speculated to go through one, or both, of two major pathways. For the example of [Ru(vbpy)3](PF6)2, the first reducing equivalent of electrons reside on the vinylbipyridine ligand, which creates radical anion character at the vinyl substituent.3 The vinyl groups have been speculated to form an intricate network of cross-linked metal complex sites by way of radical-radical dimerization3,8 and/or radical chain propagation via attack of unreduced vinyl bipyridine ligands.4 Ostensibly, as the molecular weight of these oligomers increase, they precipitate onto the electrode surface to form a polymer film. Figure 2 shows a typical voltammogram during the reductive polymerization of [Ru(vbpy)3](PF6)2 onto a conventional glassy carbon electrode. During each successive sweep the deposited film becomes thicker. Spectroelectrochemical studies (SnO2In2O3 OTTLE cell) performed during the polymerization of [Ru(vbpy)3]2+ showed that all vinyl groups within the thin layer were altered during the first reduction cycle.44 Consequently, the presumption is that films formed by cycling the potential (such as in Figure 2) consist of deposited polymer layers which are not covalently bound to the adjacent layerssat least not via attachment to vinyl groups.44 Figure 1 shows a voltammogram of a typical poly[Ru(vbpy)3]2+ film grown by cycling the electrode potential in a solution of the monomer and then transferring it (in the 2+ form) to a new electrolyte solution free of monomer. The outer steady-state voltammogram is obtained when the potential is cycled over the full range between 1.2 and -1.8 V. The two inner voltammograms are obtained when the potential is swept only through one or the other bulk redox process. Clearly, under the conditions of the experiment illustrated in Figure 1, when the potential is not cycled through both processes the charge trapping prewaves are absent or much reduced.3 While this
8720 J. Phys. Chem. B, Vol. 105, No. 37, 2001
Paulson et al.
Figure 3. Cyclic voltammogram of a poly-[Ru(vbpy)3] film on an ITO electrode in the OTTLE all in 0.1 M TBAPF6/acetonitrile scanned at 25 mV s-1 (no monomer present). The inner curve is for a steady-state scan between 0.5 and 1.2 V and the outer curve is the steady-state scan between the full potential limits.
Figure 5. Difference spectra obtained during a 25 mV/s potential scan of the same poly-[Ru(vbpy)3] film as the CV in Figure 3. Steady-state was first achieved by several scans of potential between 1.2 and -1.8 V. The data were obtained on the positive going potential scan. The procedure for generating the difference spectra in the figure was as follows: A spectrum was acquired at one potential and a second spectrum acquired 50 mV later; the first spectrum was then subtracted from the second. The following list refers to the potential where the spectra were acquired. The minus sign indicates that the first spectrum was subtracted from the second and does not refer to the sign of the potential. Starting with the uppermost spectrum: (a) [0.45-0.40], (b) [0.50-0.45], (c) [0.55-0.50], (d) [0.60-0.55], (e) [0.65-0.60], (f) [0.65-0.70] (dotted line), (g) [0.70-0.75] (dashed line), and (h) [0.750.80] (solid line).
Figure 4. UV-visible spectra of the same poly-[Ru(vbpy)3] film as the CV in Figure 3 in its “pure” oxidation state forms: 1+ (-1.8 V), 2+ (0.4 V), and 3+ (1.2 V). Each spectra was obtained after holding at the respective potential indicated for several minutes. In the case of the 2+ spectrum, the potential was first scanned to the positive limit (1.2 V) and back to 0.4 V.
particular film was grown by potential cycling, the general behavior it exhibits is common to other poly-[Ru(vbpy)3]2+ films irrespective of the exact method of film growth (e.g., cycling vs constant potential). In other words, the existence of trapping peaks is not limited to a layered polymer structure produced by cycling the potential during growth. Spectroelectrochemistry. Figure 3 shows the voltammogram of a poly-[Ru(vbpy)3]2+ film in the OTTLE cell. After this film was grown, the monomer solution was replaced with pure electrolyte (all in the absence of oxygen). While the peaks are not as sharp as those obtained for a polymer on a conventional electrode in a conventional electrochemical cell, all of the same features are present (cf. the voltammogram in Figure 1). Figure 4 shows the spectra of this polymer in its “pure” 1+, 2+, and 3+ formal oxidation states for the film with the CV shown in Figure 3. Our original intent was to try to fit the spectrum of the polymer at each potential with a linear combination of the three spectra shown in Figure 4 and thus determine its composition. This approach was unsuccessful, however, because the three spectra in Figure 4 are not truly of “pure” oxidationstate forms.45 For instance, initial spectroelectrochemical studies employing both slow potential scans and potential hold studies indicated that spectral changes for the Ru(III) state ceased by the point that the potential was >200 mV positive of Epa(Ru(II)fRu(III)). In contrast, because of the existence of “trappedstates,” the spectrum of the “pure” Ru(II) always contains contributions from Ru(III) and/or Ru(I) states.45 These contribu-
tions should be small but not zero. Also, the Ru(I) spectrum could have contributions from Ru(0) due to the close overlap of the formal potentials of the Ru(II/I) and Ru(I/0) couples. Also, if the film contains small amounts of spectroscopically different species (other than the three poly-[Ru(vbpy)3]n oxidation state forms being considered) that contribute to the spectrum, these might be difficult to deconvolute. Despite these various caveats, with appropriate care, each “pure” spectrum can be acquired from films that are predominately a single oxidation state form (>95%). Therefore, attempts to fit experimental spectra from linear combinations of the “pure” spectra would tend to break down when trying to determine the absolute concentrations of minor components. Because the species which are responsible for the trapping peaks are relatively minor components (based on coulometry), a different approach must be taken. Rather than attempting to fit the entire spectrum at each potential in the region of the release of the trapped charge, a series of difference spectra were obtained by subtracting the spectrum taken at one potential during a slow CV scan from the spectrum taken at a potential 50 mV later in the scan. Figure 5 shows a series of difference spectra obtained for the same polymer used to generated the data given in Figures 3 and 4. The top curve (a) in Figure 5 was generated by subtracting the spectrum taken at 0.40 V (after first having reached steady state by scanning the potential several times between -1.8 and 1.2 V) from the spectrum taken at 0.45 V. Curves b-e in Figure 5 (descending order) were generated from spectral differences taken at [0.50V - 0.45V], [0.55V - 0.50V], [0.60V - 0.55V], and [0.65V - 0.60V], respectively. With the peak of the trapping peak located at 0.68 V vs Ag+, this series of five difference spectra represent the differential chemical changes occurring to the polymer as the potential approaches the oxidation trapping peak. The bottom three spectra (f-h) in Figure 5 are for [0.70-0.65 V] (dotted curve), [0.75-0.70 V] (dashed curve), and [0.80-0.75 V] (solid curve), respectively.
Electrochemically-Generated Homopolymer Films
Figure 6. Difference spectra obtained during a 25 mV/s potential scan of the same poly-[Ru(vbpy)3] film as the CV in Figure 3 under the same conditions as Figure 5 except on the negative-going potential scan. In ascending order at 360 nm: (a) [(-1.20 V)-(-1.15 V)], (b) [(-1.25 V)-(-1.20 V)], (c) [(-1.30 V)-(-1.25 V)], (d) [(-1.35 V)(-1.30 V)], (e) [(-1.40 V)-(-1.35 V)], and (f) [(1.45 V)(-1.40 V)].
J. Phys. Chem. B, Vol. 105, No. 37, 2001 8721
Figure 8. Differential change in composition (in arbitrary units) of the poly-[Ru(vbpy)3] film considered in Figures 3-7 during the negative potential sweep. (0) ∆[Ru(I)], (4) ∆[Ru(II)], and (O) ∆[Ru(III)]. The dashed line represents the value of ∆[Ru(I)] + ∆[Ru(II)] + ∆[Ru(III)] at each potential. The x-axis is the final potential over the 50 mV scan interval as described in the legend for Figure 5.
Figure 7. The difference spectra from Figure 6a-f after smoothing (solid lines) along with the linear least-squares fits to the three spectra in Figure 4 (dashed lines).
By the point the final spectrum is acquired the scan is well into the bulk process. Figure 6 contains similar difference spectra obtained on the negative scan approaching the reduction charge trapping process. The bottom three difference spectra in Figure 6 show the spectral changes as the potential approaches the reduction trapping peak. The bottom curve (a) in Figure 6 is generated by subtracting the spectrum taken at -1.20 V from the spectrum taken at -1.15 V. The remaining curves (b-f) in Figure 6 are difference spectra obtained at [(-1.25 V)-(-1.20 V)], [(-1.30 V)-(-1.25 V)], [(-1.35 V)-(-1.30 V)], [(-1.40 V)-(-1.35 V)] and [(-1.45 V)-(-1.40 V)], respectively. In an attempt to understand what components are involved in these changes, the difference spectra were fit to a linear combination of the three spectra given in Figure 4. To accomplish these fits, the data in each spectrum were first smoothed and then converted to digital form. Figure 7 shows the smoothed difference spectra (solid lines) along with the fits (dashed lines) for the negative scan. Similar but modestly poorer fits were obtained for the data acquired on the positive scan. Figures 8 and 9 show the calculated changes in composition for the film as the potential is swept through the trapping region in the positive and negative directions, respectively. Before considering these results in more detail, some critical scrutiny of this whole approach is in order. First, the original data has relatively poor signal-to-noise; moreover, it was
Figure 9. Differential change in composition (in arbitrary units) of the poly-[Ru(vbpy)3] film considered in Figures 3-7 during the positive potential sweep. (O) ∆[Ru(I)], (4) ∆[Ru(II)], and (0) ∆[Ru(III)]. The dashed line represents the value of ∆[Ru(I)] + ∆[Ru(II)] + ∆[Ru(III)] at each potential. The x-axis is the final potential over the 50 mV scan interval as described in the legend for Figure 6.
smoothed and converted from analogue to digital form. Also, as discussed in detail earlier, the spectra used to obtain the fits were probably not pure oxidation-state forms. With respect to this latter point, the data becomes questionable for a given component any time that component is present in ca. >10% relatiVe amount. In other words, while the polymer film might be >90% in the Ru(II) form over most of the potential scan used to generate the data in Figures 5 and 6, the relative contributions to the changes are what is important. Thus, for data in Figures 8 and 9 when the relatiVe change in concentration of a given component drops below ca. 10% of the total change, the data becomes suspect. Also, it is difficult to quantitatively evaluate what effect all of the data manipulations might have had on the results. However, mass-balance considerations can provide some useful insight. The fits used to generate the data plotted in Figures 8 and 9 were carried out
8722 J. Phys. Chem. B, Vol. 105, No. 37, 2001 without any mass-balance restrictions. Since the original spectral differences ostensibly arise only because of changes in the oxidation states of the sites in the polymer, those compositional changes should sum to zero. In other words, since the loss of every Ru(III) should result in the gain of either a Ru(I) or Ru(II), then {∆[Ru(I)] + ∆[Ru(II)] + ∆[Ru(III)]} ) 0. Since the fits were made without imposing this restriction, an internal check on the validity of the whole experimental approach would be given by how close the results come to meeting this criterion. The dashed lines in Figures 8 and 9 are a plot of this residual determined at each potential. While the fitted results do not perfectly obey mass-balance, the deviation is fairly small which gives us some confidence in the results. Moreover, there seems to be a small systematic trend in both sets of datasat every potential, |∆[Ru(I)]| > |∆[Ru(II)] + ∆[Ru(III)]|. It is possible that this apparent deviation arises because of contamination of the “pure” Ru(I) spectrum with some Ru(0), however this is entirely speculation. Returning to the results presented in Figures 8 and 9 the most interesting feature evident from these results is the fact that, the contributing components to the two trapping processes are qualitatively different. During the oxidative scan until about 700 mV, both Ru(III) and Ru(II) are being produced at the expense of Ru(I). In the potential region of the trapping peak, there is a maximum in the rate of conversion of Ru(I) to Ru(III), while the rate of conversion of Ru(I) to Ru(II) remains roughly constant. In contrast, the reductive scan contributions from Ru(III)fRu(I) and Ru(II)fRu(I) are of comparable magnitudes until a potential of about -1.32 V is reached. At that point the Ru(II)fRu(I) conversion becomes dominant. As will be discussed later, the relative contribution of the two processes to the reductive trapping peak in this potential region is scan rate dependent. At slower scan rates Ru(III)fRu(I) conversion becomes much less significant. Potential Reversal and Hold Studies. The spectroelectrochemical results provide information on the types of redox couples that are contributing to the current when the trapped charges are released. What these studies do not address is where on the potential axis these couples reside. In other words, are these trapped charges originating from redox sites having potentials in or near the opposite conduction window (e.g., in the Ru(II/I) mixed valent region for the oxidative trapping process) which simply did not have time to discharge before the conducting window closed; are they redox sites which have formal potentials within the insulating region between the two conduction windows; or are they some combination of each? To try to answer these questions, consider first the data in Figure 10a,b. In Figure 10a poly-[Ru(vbpy)3]2+ film grown on a conventional glassy carbon electrode was scanned at a rate of 100 mV/s. Each scan of the potential was started by cycling from -0.7 V to -1.80 V to release any trapped oxidized sites. The potential was then scanned to differing potentials positive of the Ru(I/II) couple and either reversed or held at that potential for a length of time and then reversed. For curve 1 the potential was simply scanned to -0.8 V and reversed. For curve 2 the potential was scanned to 0.0V and held for 2 min before scanning back. In the first case there is no evidence of any released trapped charge and in the second only a very small shoulder in the leading edge of the main reduction is observed. Curves 3, 4, and 5 correspond to scanning to 0.5 V (just prior to the oxidative trapping process) and holding for 2, 6, and 12 min, respectively. For these three scans the charging peak recovers partially. In the case of the 12 min hold, it recovers approximately half of its full magnitude attained by scanning
Paulson et al.
Figure 10. Cyclic voltammograms of a poly-[Ru(vbpy)3] film on a glassy carbon electrode in 0.1 M TBAPF6/acetonitrile scanned at 100 mV s-1. In A, each CV was scanned to -1.8 V and then to the potential indicated before being scanned back to -1.8 V: (1) scanned to -0.8 V, (2) scanned to 0.0 V and held for 2 min, (3) scanned to 0.5 V and held for 2 min, (4) scanned to 0.5 V and held for 6 min, (5) scanned to 0.5 V and held for 12 min (6). Scanned to 1.2 V. In B, each CV was scanned to 1.2 V and then to the potential indicated before being scanned back to 1.2 V: (1) scanned to -0.25 V, (2) scanned to -0.25 V and held for 2 min, (3) scanned to -0.25 V and held for 6 min, (4) scanned to -0.50 V and held for 6 min, (5) scanned to -0.95 V and held for 2 min, (6) scanned to -1.15 and held for 2 min, (7) scanned to -1.35 V and held for 2 min, (8) scanned to -1.4 V and held for 12 min, (9) scanned to -1.8 V.
to 1.2 V through the bulk oxidation process (curve 6). The fact that at least some charge is trapped in these experiments indicates that the redox sites responsible have potentials which are negative of the bulk Ru(II/III) oxidation process. Figure 10b shows a related experiment for the oxidation trapping peak (see figure legend for details). As in Figure 10a the trapping peak partially recovers as the switching potentials are made more negative and/or held for longer times. Interestingly, the oxidative trapping peak fully recovers when the potential is held for an extended period in the leading edge of the reductive trapping peak. Since this potential is ca. 300 mV positive of E1/2 for the 2+/1+ couple, none of the trapped charge that is released on the oxidative scan can be due to sites which have redox potentials within the bulk reduction regionsi.e., the entire trapping process on the oxidative scan is due to sites which have redox potentials which lie nominally within the insulating region of the polymer. Mediation by Ferrocene/Ferricenium. For the experiment illustrated by curves 3-5 in Figure 10a, the reductive trapping peak was still increasing between 6 and 12 min of holding the
Electrochemically-Generated Homopolymer Films
J. Phys. Chem. B, Vol. 105, No. 37, 2001 8723 ca. 0.0 V. Therefore, it seems likely that the “extra” current evident in curve 4 of Figure 11a is not due to re-reduction of ferricenium entrained in the film. The more likely scenario is that the extra current is due either to reduction of residual ferricenium in the solution diffusion layer at the surface of the reduced polymer and/or to additional trapped sights that are not generally accessible to oxidation by the electrode/polymer alone. The important observation, however, is that it is not necessary to scan the potential of the electrode into the bulk oxidation process to reintroduce most of the trapped oxidized charge. Conclusions
Figure 11. (A.) Cyclic voltammograms of the same polymer as in Figure 1 in 0.1 M TBAPF6/acetonitrile containing ca. 9 mM ferrocene scanned at 100 mV s-1. (1) Scanned without stopping between -0.6 and -1.8 V. (2) Scanned without stopping between 0.25 and -1.8 V. (3) Scanned without stopping between 0.50 and -1.8 V. (4) Scanned without stopping between 1.2 and -1.8 V. (B) Cyclic voltammogram of the same ferrocene solution on a bare glassy carbon electrode of the same dimensions.
potential at 0.5 V. In light of the results from curve 6 in Figure 10a, the question arises about whether all of the sites that should be oxidized at 0.5 V are oxidized in the 12 min hold experiment. In an effort to address this question, the film in Figure 1 was examined in a solution containing ca. 9 mM ferrocene. Denisevich et al. had shown earlier that the presence of ferrocene in solution affected the size of the reductive trapping peak apparently by mediating redox processes in the film.3 Figure 11b shows the CV of the ferrocene solution on a bare glassy carbon electrode identical to the one used in Figure 1. Figure 11a shows the CV of the poly-[Ru(vbpy)3]2+ film in this solution. The quasi-sigmoidal oxidation wave around 0.0 V is due the mass-transport-limited oxidation of the ferrocene which is transported through the film either by diffusion through the polymer or through nanoscopic channels (note the different scale for Figure 11a,b). Because the ferrocene and ferricenium can infuse the polymer, this couple should also be capable of efficiently mediating the oxidation/reduction of sites in the film.3 Curves 2 and 3 in Figure 11a show the effect of this mediation. When the potential is swept positively into the ferrocene oxidation and reversed, there is a marked increase in the reduction trapping peaksmuch more so than that for the scanand-hold experiments illustrated in Figure 10a. The increase in the reductive trapping peak caused by scanning to 0.25 V in the ferrocene solution is considerably greater than for holding the potential in the absence of mediator at 0.5 V for 12 min. Scanning the potential in the presence of ferrocene to 0.5 V restores almost all of the trapped charge (cf. Figure 1). Interestingly, when the potential is scanned out to 1.2 V the trapping peak is actually slightly larger than for the same scan in the absence of ferrocene. Since ferricenium is cationic, it should be expelled from positively charged film which is consistent with the lack of any significant rereduction wave at
These electrochemical results show that, for the specific polymer film considered, both the reductive and oxidative trapped charge arises exclusively from redox sites having formal potentials which lie in the insulating region between the two bulk redox processes. In other words, none of the charge is due to sites which were kinetically unable to discharge during one of the mixed valent conduction windows. The spectroelectrochemical results show that the two trapping release processes are qualitatively different. In the oxidative scan there is a net conversion of Ru(I) sites to both Ru(II) and Ru(III) prior to the scan reaching the bulk oxidation. Also the “peak” in the release process corresponds to a net conversion of Ru(I) to Ru(III). In contrast, the charge released on the reductive scan is due to a net loss of both Ru(III) and Ru(II) at the beginning of the charge release process and becomes dominated by Ru(II) to Ru(I) conversion as the bulk process is approached. Interestingly, other spectroelectrochemical studies in our laboratory suggest that the contribution of Ru(III) reduction to the charge release process ceases to be a major contribution earlier in the scan (i.e., at more positive potentials) as the scan rate is slowed.45 While these spectroelectrochemical studies shed light on the net oxidation state changes occurring during the release of trapped charge, it tells us nothing about the individual redox sites undergoing change. In other words, Ru(I)fRu(III) is indistinguishable from Ru(I)fRu(II) concurrent with Ru(II)′fRu(III)′ where the primed and nonprimed descriptor indicate physically different redox sites. Also, this study provides little insight into why these trappable sites have formal potentials that lie so far from the potentials of the majority sites. There are several possibilities. For example, these sites could be chemically altered. We have observed that exposing a poly-[Ru(vbpy)3]+ film (i.e., electrochemically reduced) to air greatly increases the number of trappable sites at the expense of the charge in the bulk process.45 Another possibility is that the sites have shifted potentials for electrostatic reasons having to do with counterion access. For instance, a Ru(II) site adjacent to an anion physically trapped in the polymer matrix could be more difficult to reduce than one which is not; or a Ru(II) site which cannot be closely accessed by anions should be easier to reduce than one which can. However, we have no data which would allow us to distinguish between any of these options. We can only say that, if the abnormal redox potentials are due to chemically altered sites, the alteration is not so severe as to grossly change the spectrum of the sites in any oxidation state. Otherwise, the attempts at fitting the spectra would likely not have worked. Finally, it is important to appreciate the fact that, to some degree, all of these results are film specific and experimentalcondition specific. Grossly thicker or thinner films, faster or slower scan rates or different film histories could all alter the polymer’s qualitative redox behavior.
8724 J. Phys. Chem. B, Vol. 105, No. 37, 2001 Acknowledgment. The authors acknowledge the support of this work by the National Science Foundation (CHE-971408). We also, acknowledge the many contributions of Professor Royce W. Murray to the area of redox-active materials without which this study would not have been possible. References and Notes (1) Denisevich, P.; Willman, K. W.; Murray, R. W. J. Am. Chem. Soc. 1981, 103, 4727-4737. (2) Borjas, R.; Buttry, D. A. Chem. Mater. 1991, 3, 872. (3) Denisevich, P.; Abruna, H. D.; Leidner, C. R.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1982, 21, 2153-2161. (4) Baldy, C. J.; Morrison, D. L.; Elliott, C. M. Langmuir 1991, 7, 2376-2379. (5) Potts, K. T.; Usifer, D. A.; Guadalupe, A.; Abruna, H. D. J. Am. Chem. Soc. 1987, 109, 3961. (6) Gould, S.; Strouse, G. F.; Meyer, T. J.; Sullivan, B. P. Inorg. Chem. 1991, 30, 2942. (7) Gould, S.; Earl, C.; Sullivan, B. P.; Meyer, T. J. J. Electroanal. Chem. 1993, 350, 143. (8) Calvert, J. M.; Schmehl, R. H.; Sullivan, B. P.; Facci, J. S.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1983, 22, 2151-2162. (9) Meyer, T. J.; Sullivan, B. P.; Caspar, J. V. Inorg. Chem. 1987, 26, 4145. (10) Ellis, C. D. Ph.D. Thesis, Chemistry, The University of North CarolinasChapel Hill, Chapel Hill, 1983. (11) Ellis, C. D.; Margerum, L. D.; Murray, R. W.; Meyer, T. J. Inorg. Chem. 1983, 22, 1283-1291. (12) Bidan, G.; Deronzier, A.; Moutet, J.-C. NouV. J. Chim. 1983, 8, 501. (13) Cosnier, S.; Deronzier, A.; Llobet, A. J. Electroanal. Chem. 1990, 280, 213. (14) Salmon, M.; Diaz, A.; Goitia, J. Chemically Modified Surfaces in Catalysis and Electrocatalysis; ACS Symposium Series 192; American Chemical Society: Washington, DC,, 1982. (15) Chidsey, C. E. D.; Murray, R. W. J. Phys. Chem. 1986, 90, 14791484. (16) Reed, R. A.; Geng, L.; Murray, R. W. J. Electroanal. Chem. Interfacial Electrochem. 1986, 208, 185-193. (17) O’Toole, T. R.; Margerum, L. D.; Westmoreland, T. D.; Vining, W. J.; Murray, R. W.; Meyer, T. J. J. Chem. Soc., Chem. Commun. 1985, 1416-1417. (18) Chidsey, C. E.; Feldman, B. J.; Lundgren, C.; Murray, R. W. Anal. Chem. 1986, 58, 601-607. (19) Kanda, S.; Murray, R. W. Bull. Chem. Soc. Jpn. 1985, 58, 30103015. (20) Feldman, B. J.; Ewing, A. G.; Murray, R. W. J. Electroanal. Chem. Interfacial Electrochem. 1985, 194, 63-81.
Paulson et al. (21) Jernigan, J. C.; Chidsey, C. E. D.; Murray, R. W. J. Am. Chem. Soc. 1985, 107, 2824-2826. (22) Ewing, A. G.; Feldman, B. J.; Murray, R. W. J. Phys. Chem. 1985, 89, 1263-1269. (23) Leidner, C. R.; Schmehl, R. H.; Pickup, P. G.; Murray, R. W. Proc. Electrochem. Soc. 1984, 84-12, 389-402. (24) Leidner, C. R.; Murray, R. W. J. Am. Chem. Soc. 1985, 107, 551556. (25) Ewing, A. G.; Feldman, B. J.; Murray, R. W. J. Electroanal. Chem. Interfacial Electrochem. 1984, 172, 145-153. (26) Pickup, P. G.; Leidner, C. R.; Denisevich, P.; Murray, R. W. J. Electroanal. Chem. Interfacial Electrochem. 1984, 164, 39-61. (27) Pickup, P. G.; Murray, R. W. J. Electrochem. Soc. 1984, 131, 833839. (28) Pickup, P. G.; Kutner, W.; Leidner, C. R.; Murray, R. W. J. Am. Chem. Soc. 1984, 106, 1991-1998. (29) Leidner, C. R.; Murray, R. W. J. Am. Chem. Soc. 1984, 106, 16061614. (30) Margerum, L. D.; Meyer, T. J.; Murray, R. W. J. Electroanal. Chem. Interfacial Electrochem. 1983, 149, 279-285. (31) Westmoreland, T. D.; Calvert, J. M.; Murray, R. W.; Meyer, T. J. J. Chem. Soc., Chem. Commun. 1983, 65-66. (32) Ikeda, T.; Leidner, C. R.; Murray, R. W. J. Electroanal. Chem. Interfacial Electrochem. 1982, 138, 343-365. (33) Schneider, J. R.; Murray, R. W. Anal. Chem. 1982, 54, 15081515. (34) Ikeda, T.; Schmehl, R.; Denisevich, P.; Willman, K.; Murray, R. W. J. Am. Chem. Soc. 1982, 104, 2683-2691. (35) Willman, K. W.; Murray, R. W. J. Electroanal. Chem. Interfacial Electrochem. 1982, 133, 211-231. (36) Ikeda, T.; Leidner, C. R.; Murray, R. W. J. Am. Chem. Soc. 1981, 103, 7422-7425. (37) Umana, M.; Denisevich, P.; Rolison, D. R.; Nakahama, S.; Murray, R. W. Anal. Chem. 1981, 53, 1170-1175. (38) Abruna, H. D.; Denisevich, P.; Umana, M.; Meyer, T. J.; Murray, R. W. J. Am. Chem. Soc. 1981, 103, 1-5. (39) Meyer, T. J.; O’Toole, T. R.; Margerum, L. D.; Westmoreland, T. D.; Vining, W. J.; Murray, R. W.; Sullivan, B. P. In U.S.; Gas Research Institute: DesPlaines, IL, 1987; p 5. (40) Leidner, C. R.; Sullivan, B. P.; Reed, R. A.; White, B. A.; Crimmins, M. T.; Murray, R. W.; Meyer, T. J. Inorg. Chem. 1987, 26, 882-891. (41) Jernigan, J. C.; Murray, R. W. J. Am. Chem. Soc. 1987, 109, 17381745. (42) Spiro, T.; Ghosh, P. J. J. Am. Chem. Soc. 1980, 102, 2. (43) Abruna, H. D.; Walsh, J. L.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1981, 20, 1481-1486. (44) Paulson, S. C.; Elliott, C. M. Anal. Chem. 1996, 68, 1711-1716. (45) Paulson, S. C. Ph.D. Thesis, Chemistry, Colorado State University, Fort Collins, 1994.
