Rapid Ion Exchange during Redox Switching of Poly(3

Cynthia M. G. Bach, and John R. Reynolds. J. Phys. Chem. , 1994, 98 (51) ... Corey A. Salzer and C. Michael Elliott , Susan M. Hendrickson. Analytical...
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13636

J. Phys. Chem. 1994, 98, 13636-13642

Rapid Ion Exchange during Redox Switching of Poly(3-methylthiophene) Studied by X-ray Photoelectron Spectroscopy Cynthia M. G. Bachl Alcon Laboratories, Fort Worth, Texas 76134

John R. Reynolds* Center for Macromolecular Science and Engineering, Department of Chemistly, University of Florida, Gainesville, Florida 3261 I Received: June 17, 1994@

The ionic composition of oxidatively doped and redox cycled poly(3-methylthiophene) was analyzed by using X-ray photoelectron spectroscopy (XPS) survey scans, quantitative multiplex experiments, and atomic composition depth profiling to determine ion-exchange properties. Thin films of poly(3-methylthiophene tetrafluoroborate), 1000- 13 500 A thick, were synthesized electrochemically onto a platinum electrode from an acetonitrile solution containing 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) as a supporting electrolyte. The films were subsequently potential cycled in tetrabutylammonium perchlorate (TBAC104) and tetrabutylammonium perrhenate (TBARe04) electrolytes to examine ion exchange. Surface analysis of perchlorate-treated films, 1500 A thick, indicated a complete exchange of dopants on the film surface. The extent of ion exchange into the film could not be determined with perchlorate due to deterioration of the chlorine and fluorine signals upon sputtering with an argon ion beam. Surface analysis of perrhenate-treated films, 1000-13 500 A thick, indicated a complete exchange of dopant ions at the surface, and through the bulk, after one redox cycle. Spontaneous ion exchange studies were carried out at 10 times greater exposure time to electrolyte than in switching experiments. While essentially complete exchange occurs in this time frame, some residual tetrafluoroborate could be detected. This indicates that ion transport in poly(3methylthiophene) during redox cycling is rapid and complete.

Introduction Poly(3-methylthiophene)(P3MeT) is one member of the class of heterocyclic polymers which can be electrochemically synthesized in a highly conductive (doped) form and reversibly redox switched to an insulating (undoped) form.2 During electrochemical polymerization and subsequent doping of P3MeT, dopant anions (A-) from the electrolyte solution are incorporated into the polymer matrix to counterbalance the positive charges delocalized along the polymer backbone. As the polymer is switched between the conductive and neutral form, several events can occur as illustrated in Scheme 1. Upon reducing the polymer in a supporting electrolyte to its neutral form, dopant anions can be expelled from the polymer matrix or cations from the electrolyte solution may enter the neutral polymer. Reoxidation results in the cations (C+) being expelled or the anions reentering the film. The movement of ion pairs and solvent molecules can also occur, leading to potentially complex ion-exchange properties. The incorporation of electrolyte ions into P3MeT and polypyrrole during electrochemical reduction has been studied and indicates that the rates of migration of the cations and anions, their respective sizes, and the original dopant anion’s interaction with the cationic polymer will determine the extent of ion exchange in the p ~ l y m e r . ~ . ~ This ion-exchange process can be used to incorporate new ions into the polymer matrix by preparing the polymer in one electrolyte and placing it into another electrolyte for redox switching. Since the switching potential of P3MeT films is lower than the oxidative peak potential of the m ~ n o m e ruseful ,~ ions that might undergo degradation at the potential required for polymerization can be incorporated into the film during the @

Abstract published in Advance ACS Abstracts, November 1, 1994.

0022-365419412098-13636$04.5010

switching process. The rate and depth of penetration of the dopant ions into the film upon switching of P3MeT has not been studied thoroughly. It is possible that several redox switches are required to fully exchange the ions of interest, especially for thicker films. The study reported here has been directed toward determining the penetration depth of the dopant anions as a function of the number of redox switches and film thickness by exchanging the BF4- dopant anions in P3MeT(BF4-) with ReO4- and (2104- ions. Several spectroscopic techniques such as fluorescence, UVvis absorption, Raman, and Auger electron spectroscopy have been used to determine the identity of the mobile ion during redox switching of conducting An electrochemical quartz crystal microbalance (EQCM) has also been used to examine this charge-transport p r o c e s ~ . ~ ~X-ray J ~ J ~photoelectron spectroscopy ( X P S ) , commonly known as electron spectroscopy for chemical analysis (ESCA), can be used to probe the ion content of dry films and the results used to determine the identity of the ions that are moving in and out of the polymer matrix during redox switching. XPS has become a common tool in the analysis of polymeric materials in that it is a nondestructive surface-sensitive technique. In general, XPS is used to elucidate chemical bond information in polymer films and yields quantitative information on atomic compositions at surfaces (10-100 A). Combining XPS with Ar ion beam etching methods allows a depth profile of the atomic composition through the film thickness. Previous XPS studies of P3MeT determined that both the carbon and sulfur atoms become partially charged during oxidation.12 Hernandez, et al. have used XPS to study the surface characteristics of P3MeT prepared by plasma and electrochemical polymerizations.l3 In addition, XPS has been

0 1994 American Chemical Society

J. Phys. Chem., Vol. 98, No. 51, 1994 13637

Rapid Ion Exchange during Redox Switching SCHEME 1

duction anion expulsion

// oxidation anion inclusion

reduction cation inclusion

cation expulsion Oxidation

used to study the effect of the dopant on the structure and electrical properties of P3MeT14 and the incorporation of the electrolyte cation (alkali metal ions) into P3MeT during electrochemical reduction? In this work, X P S will be used as both a quantitative and a qualitative tool to determine the identity and content of the dopant anions in thin films of P3MeT-(BF4-) before and after switching in a solution containing a second electrolyte. Argon ion beam/XPS depth profiling studies of the atomic composition of the pristine and exchanged films were carried out to obtain qualitative information on the identity of the dopant ion and quantitative information on the dopant level through the film thickness. This technique is not recommended if the intent is to elucidate chemical bond information, as sputtering alters the structure of the material. Scanning electron microscopy (SEM) was used to obtain information on the surface morphology of the films studied for comparison of morphology with dopant level. Experimental Section 3-Methylthiophene (3MeT, Aldrich) was passed over a bed of A1203 until colorless. Tetrabutylammonium tetrafluoroborate (TBABF4, Aldrich), tetrabutylammonium perchlorate (TBAC104, Aldrich), and tetrabutylammonium perrhenate (TBARe04, Aldrich) were recrystallized from ethanol and dried at 70 “C under vacuum for at least 12 h. Acetonitrile (ACN, Baker) was distilled over P2O5 before use. Electrochemical polymerizations and experimentation were performed on an EG&G Princeton Applied Research Model 273 potentiostat/galvanostat. Film thicknesses were determined by using an Alpha-Step 200 profilometer (Tencor Instruments). A Perkin-Elmer 5000C Series X-ray photoelectron spectrophotometer equipped with a monochromatized A1 K a X-ray source, and a base pressure of 5 Torr was used for dopant level determinations and depth profile studies. Samples were positioned at an angle of 68” with respect to a plane parallel to the electron analyzer. The X-ray source was operated at a constant power of 300 W. Voltages varied from 11.5 kV (26 mA) to 13.5 kV (22 mA) as several different sources were used during the course of the study. The Ag3d5/2 line at 367.9 eV used for alignment and calibration had a full width at half-maximum of approximately 0.6 eV and greater than 45 000 countsls at a pass energy of 17.90 eV. Scanning electron microscopy was performed on a Cambridge Stereoscan-120 scanning electron microscope at an energy of 10 kV. 3MeT was polymerized galvanostaticallyon a platinum plate working electrode (0.709 cm2) at a current density of 2 mA/

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cm2. All films were synthesized from 6 mL of a solution 0.1 M 3MeT and 0.1 M TBABF4 in ACN. A three-electrode, single-cell compartment was used with a platinum plate counter electrode surrounding the Ag/Ag+ reference electrode, which in tum was centered over the platinum working electrode. Argon was bubbled through the solution for 5 min prior to polymerization,which was carried out under an Ar blanket. The amount of film deposited was controlled by the total charge passed. After polymerization, the films were rinsed well with ACN, and placed in 6 mL of a 0.1 M TBAC104 or TBAReOd ACN electrolyte. The films were cycled a varied number of times under an Ar blanket between -0.6 and +0.65 V at 100 mV s-l. Doping levels for P3MeT-(BF4-) were obtained by comparing the F 1s signal with the S 2p signal, corrected for the fact that the F 1s signal was arising from four atoms, and applying the appropriate sensitivity factors. After ion exchange with C104and Reo4-, doping levels were determined by comparing the F 1s signal with the C1 2p and Re 4f signals respectively. The sensitivity factors for sulfur, fluorine, carbon, boron, nitrogen, chlorine, and rhenium are 0.463, 1.00, 0.25, 0.13, 0.42, 0.631, and 2.604, respectively. Depth profile studies were performed to determine the identity of the dopant anion and the dopant level at the platinudpolymer interface. The argon ion gun used to etch the surface of the film was operated at a voltage of 4 kV and a current of 25 mA. The film was sputtered 5 min between each analysis and the base pressure of the system was maintained at Ilo-* Torr throughout the etching process. P3MeT-(BF4-) films, 1000, 6500, and 13 500 8, thick, were prepared and scanning electron micrographs were obtained at various magnifications to evaluate the morphology of the film surface. Results and Discussion Cyclic Voltammetry. A cyclic voltammogram of a P3MeT(BF4-) thin film in monomer-free 0.1 M TBABF4 electrolyte in Figure 1 shows that polymer oxidation occurs with a peak at 0.45 V and polymer reduction occurs with a peak at approximately 0.35 V. The polymer showed little decay in electroactivity for up to 50 cycles, as indicated by a minimal reduction in both the anodic and cathodic peak currents. The peak appearing immediately after the reduction peak in the second scan is of unknown origin and is not present in the 25th or 50th scans. No interference from the background is observed as can be seen by the low background current at bare Pt. Figures 2 and 3 show voltammograms of P3MeT-(BF4-) in

13638 J. Phys. Chem., Vol. 98, No. 51, 1994

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the two electrolyte solutions used for the switching studies, TBAC104 and TBARe04. The polymer again showed little decay in electroactivity for up to 50 cycles in both electrolytes, and no interference from background currents was observed. The unidentified peak observed during cycling in the TBABF4 electrolyte was not observed in these electrolytes, which indicates that it was not related to the redox reaction of the polymer. The polymer's oxidative peak potential in both electrolytes was 0.40 V, indicative of no electrolyte effect on the poymer's redox properties. Thickness vs Charge Passed. Figure 4 shows the relation-

ship between the amount of charge passed during galvanostatic polymerization of 3MeT in 0.1 M TEiABF4 and the P3MeT(BF4-) thickness obtained by using profilometry. An almost linear relationship was obtained for thicknesses greater than lo00 A (16 mC/cmz), with the deviation from linearity in the region below 1000 A attributed to the inability to accurately measure the film thickness. Figure 4a demonstrates the reproducibility of the film synthesis for three different films, while Figure 4b shows the film uniformity evaluated by measuring the thickness on three separate regions of the same film. The average value obtained for each film of the uniformity study was within the range of values obtained for the three films measured for the charge versus thickness correlation. XPS Determined Dopant Level. Figure 5 shows an X P S survey scan of a 2000-A (30 mC/cm2)P3MeT-(BF4-) film and is representative of all P3MeT-(BF4-) films studied showing the C Is, S 2s, S 2p, F Is, and F 2s peaks expected. It can be seen that the films are quite clean as made. The survey scan indicates the presence of a small amount of oxygen (532 eV) from an unknown source, which was observed o n some of the other films studied. As there was no correlation between dopant level and oxygen concentration, and the oxygen content was quite low; it is assumed that this is due to a small amount of surface impurities.

Rapid Ion Exchange during Redox Switching 10 9-

J. Phys. Chem., Vol. 98, No. 51, 1994 13639 TABLE 1: Elemental Composition, Dopant Levels, and Carbon to Sulfur Ratios as a Function of Deposition Charge for Pt-Supported P3MeT-(BF*-)

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XPS multiplex scans of Pt-supported, 2000-A P3MeT(BF4-); (a) C Is, (b) S 2p, (c) F 1s. I

Figure 6 shows the high-resolution multiplex scan results for the C Is, S 2p, and F 1s regions for the P3MeT-(BF4-) sample surveyed in Figure 5. The multiplex experiments were used to obtain quantitative information on the amount of carbon, sulfur, and fluorine present, along with the ratio of carbon to sulfur and the dopant level. In this case, the elemental composition was 73.05% carbon, 12.14% sulfur, and 14.81% fluorine, which corresponds to a carbon to sulfur ratio of 6.1:l and a dopant level of 0.30. The dopant level is defined as the molar ratio of charge-balancing counterions (BF4-) to 3MeT repeat units. This dopant level correlates well with the one charge per 3-4 3MeT rings expected. The elemental compositions, ratios of carbon to sulfur (C: S), and the dopant levels (r) for all the films studied are listed in Table 1. The atomic compositions were obtained by dividing the area obtained in the multiplex scan by the appropriate sensitivity factor. The ratio of carbon to sulfur on most of the films studied was in the range 5.9-6.5, with a few films possessing ratios greater than 6.5. This is somewhat higher than the theoretical value of 5:l. There was no evidence of a N 1s signal at approximately 400 eV; therefore, the increased carbon signal was not caused by entrapped ACN or tetrabutylammonium ions. This elevated carbon signal is common in XPS work and can be attributed to trace amounts of hydrocarbon contamination on the film surface. The dopant level was obtained from the elemental composition by comparing the S 2p response to the F 1s response, assuming four fluorines per dopant. The sulfur signal was used as the basis for the polymer content in dopant level determinations due to the elevated carbon signal. The dopant levels obtained ranged from 0.23 to 0.52, which is consistent with previously reported dopant 1 e ~ e l s . lThe ~ dopant level did not change with time in the analysis chamber nor was there any evidence of radiation damage to the film. The dopant levels replicated at each thickness were in good agreement with the

exception of three values at 24 mC/cm2. These deviations could not be explained by variations in the polymerization technique or synthesis conditions. In general, there is a slight increase in dopant level with thickness in the range 1000-6500 A (16-60 mC/cm2) and a slight decrease at 13 500 A (108 mC/cm2). In fact, films in the 1000-3500-A (16-36 mC/cm2) range could be considered equivalent within experimental error. This is interesting in light of a previous work by Gamier et al., which states that increasing the film thickness leads to increased morphological disorder and decreased conductivities. l5 In general, dopant levels are expected to decrease with decreasing conductivity. Reproducible dopant levels could not be obtained on films thicker than 13 500 8, or films thinner than 1000 A. Films thicker than the upper limit formed loose, powdery deposits on the surface, which detached during rinsing. Our group has previously studied the morphology of these powdery deposits on a gold/palladium-supportedP3MeT-(BF4-) film, 5000-10 OOO A in thickness, and reported that the surface morphology was a function of the electrolyte used during the synthesis.16 Films thinner than the lower limit (< 1000 A) undoped spontaneously upon rinsing, which was noted by a color change from blue to red. This spontaneous undoping could be used to explain the dopant level versus thickness relationship. If films in the 10003500-A range are undoping slightly, even though there were no observable color changes, the dopant level obtained would be low. Figure 7 shows a survey scan of a P3MeT-(BF4-) film, 2000 A thick, which was deposited onto a platinum electrode and reduced at a potential of -0.60 V for 15 min in monomer-free 0.1 M TBABFJACN. Only C and S are evident as the major atomic species present, indicating that the BF4- is the dominant mobile ion and is expelled from the film during switching. Counter directional cation movement does not occur, as no N is evident from TBA+. The film surface possessed only trace amounts of fluorine, as evident by the extremely small F 1s signal at 685 eV (unobservable in the survey scan). A multiplex scan experiment of the C, S, and F regions, shown in Figure 8,

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was performed to quantitate the dopant level in the reduced film. The fluorine is present at a dopant level of 10.009, which is 30-55 times lower than in the oxidized film. The reduction was also carried out for 10 min and the same low dopant levels were obtained. The inability to completely remove all of the dopant ions from the reduced polymer has also been observed by Gamier et aZ.,14J7who reports that the reduced film exhibits semiconductor-likebehavior. These results indicate that P3MeT(BF4-), redox switched in TBA+ electrolytes, are excellent candidates for anion exchange. It should be noted that the redox switching ion transport processes carried out in alkali metal ion electrolytes (e.g., Li+) are complicated by cation penetration into the film.4 Ion Exchange. To investigate the electrochemically stimulated ion-exchange process, c104- was initially chosen to replace BF4- due to their similar size, solution properties, and electrochemical stability. P3MeT-(BF4-) films, approximately 1500 A thick, were deposited onto a platinum substrate, rinsed well with ACN, placed in 0.1 M TBAC104 electrolyte solution,

Figure 9. X P S survey scan of Pt-supported, 1500-8, P3MeT-(BF4-) after one cycle in 0.1 M TBAClOdACN.

and cycled once between the potential limits of -0.60 and f0.65 V. The films were subsequently rinsed with ACN and vacuum dried. During this process, the film was exposed to the TBAC104 electrolyte for ca. 20-25 s. A typical survey scan of the film after one cycle in TBAC104 electrolyte is shown in Figure 9. It is evident from the C1 2p and strong 0 1s peaks that the identity of the dopant anion is that of the bathing electrolyte (C104-), with essentially none of the original dopant anion (BF4-) detected (no signal at 685 eV). This indicated virtually a complete exchange of dopant anions at the surface of the film in only one cycle. As described above, multiplex scan experiments were used to estimate the dopant levels. The c104- dopant level at the surface of a 1500-8, film after one cycle was 0.17-0.24, which is slightly lower than the dopant level in an uncycled film. No F could be detected at the film surface by using the multiplex experiment. Attempts were made to depth profile the F content in the P3MeT-(BF4-) film and the C1 content in the film after cycling in C104-, to determine the identity of the dopant anion at the platinumP3MeT interface along with the extent of exchange of the two anions throughout the films. Both the chlorine and fluorine signals disappeared instantly upon sputtering with argon ions at 4 kV. It is suspected that the perchlorate and tetrafluoroborate dopant anions break down into small gaseous molecules (e.g., BF3 and C102) when sputtered with the ion beam. No further switching studies were performed with TBAC104 as the second electrolyte. TBARe04 was subsequently employed as the switching electrolyte since the heavy metal rhenium atom in the perrhenate dopant anion would not be susceptible to the formation of small gaseous molecules in the presence of the ion beam. The films used in the TBARe04 switching study were prepared as specified for the perchlorate cycle study, except the film was placed in 0.1 M TBAReOdACN electrolyte for cycling. Four different film thicknesses were studied in the range 100013 500 A. A survey scan of a typical film (13 500 A) cycled once in a TBARe04 is shown in Figure loa. The survey scans on all films studied indicated that a complete exchange of dopant anions had taken place on the surface of the film as evident by the complete absence of a fluorine peak (685 eV) and the presence of strong rhenium peaks. Multiplex scan experiments were used to determine the dopant level on the surface of the film by comparison of the S 2p and Re 4f peaks. The dopant levels obtained on all of the films studied were in the range 0.09-0.17, as shown in Table 2. These dopant levels are lower than the results obtained for an uncycled film. These lower levels are explained by the fact that a small amount of undoping occurs on the surface of the film upon rinsing and was accompanied by a color change from blue to purple. The color change was more evident in the thinner films as compared with the thicker films.

J. Phys. Chem., Vol. 98,No. 51, 1994 13641

Rapid Ion Exchange during Redox Switching

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Figure 10. X P S survey scan of Pt-supported, 13 500-A P3MeT-(BF4-) after (a) one cycle in 0.1 M TBAReOdACN and (b) 250 s ion exchange in 0.1 M TBAReOdACN. TABLE 2: Dopant Levels as a Function of Thickness for Pt-Supported P3MeT-(BF4-) as Formed, after Cycling or Spontaneous Exchange in TBARe04 and at the Pt/ P3MeT-(ReOd-) Interface after Sputtering: thickness, sputter rate, A y" y" &min Y 1000 0.24-0.28 0.09, 0.10' 21.1,23.5 0.18,0.27 0.07, 0.07d 26.7,21.1 0.32,0.19 2000 0.29-0.31 NA,0.12' 21.6, 20.5 0.21,0.32 0.17,O.lodf 38.1, 38.1 0.39,0.25 0.5 1 6500 0.38-0.41 0.17' 26.8 0.lodf 28.6 0.29 0.49,0.49 13500 0.32-0.36 0.17, 0.18' 32.1, NA 0.17,0.19df 37.8, 35.3 0.43,OSO a Range of P3MeT-(BF4-) dopant levels from Table 1. Dopant level after ion exchange in TBARe04 electrolyte. After one cycle in TBARe04 electrolyte. After 250 s ion exchange in TBARe04 electrolyte. e Dopant level at the WP3MeT interface corrected for the decrease observed in the S 2p signal. f Fluorine detected at the surface.

Spontaneous ion-exchange processes can occur when a doped conducting polymer is immersed in an electrolyte containing a different anion. This has been well documented for polyacetylene18 and p o l y p y r r ~ l e . ~P3MeT-(BF4-) ~.~~ films were exposed to 0.1 M TBAReOdACN solutions for 250 s; a period greater than 10 times the exposure time during the cycling experiments. The films were subsequently rinsed with ACN and dried. A survey spectrum of a 13 500-A-thick film, shown in Figure lob, indicates that, while a significant amount of ion exchange has occurred at the surface, the ion-exchange process is not complete. The fluorine concentration determined from multiplex scan experiments increased with increasing thickness from 0.3% at 2000 A to 1.3% at 13 500 A. The total dopant levels on the surface of the film, shown in Table 2 , ranged from 0.07 to 0.19. As with the cycled films, the dopant levels at the surface were somewhat lower than with an uncycled film. This was expected because the color of the film changed from blue to purple during exchange. Since nitrogen was not detected along with the fluorine, the F Is signal is arising from a small amount of the

original dopant anion remaining within the surface layer and not from residual electrolyte. The bathing electrolyte after ion exchange was evaporated to dryness on a platinum plate and analyzed by X P S . Fluorine was detected on the platinum, which indicated that an exchange of anions had occurred as opposed to simple permeation of the Re04- electrolyte into the film pores. Depth Profiling. An Ar+ beam was used to sequentially etch the films, followed by XPS survey/multiplex scans after each etching, to obtain information on the dopant level and the identity of the dopant anion throughout the film. Figure 11 shows a depth profile of the atomic composition for Re and S for a film of P3MeT-(BF4-) that has been cycled once in 0.1 M TBAReOdACN. Depth profiles for all other film thicknesses were carried out and gave similar results. The rhenium content increases with a corresponding increase in the dopant level as the surface of the film is penetrated, ultimately reaching a constant value. It is likely that the surface-confined undoping that occurs during the rinsing of the films leads to this Re gradient. A slight decrease in the sulfur signal was observed, which also contributed to the increase in the dopant level. The profiling was stopped when the Pt 4f doublet at approximately 70 and 74 eV was detected, indicating that the substrate had been reached. The time required to reach the platinum substrate was used to determine the sputter rates shown in Table 2. Also included in Table 2 are the dopant levels obtained at the Pt/ P3MeT interface, which ranged from 0.18 to 0.51 (corrected for the decrease in the sulfur signal that was observed). Attempts to electrosynthesize P3MeT-(Re04-) directly to determine if the spontaneous undoping evident by the Re gradient observed was a function of the cycling, or if it was an inherent property of the film, were unsuccessful. The average final dopant level for all film thicknesses was similar to, or higher than, the results obtained for the uncycled films doped with BF4-, which indicated a complete exchange of dopant anions down to the platinum surface. After one cycle, and with 125 s of exposure to the TBARe04 electrolyte, all of the films of varied thicknesses studied indicated a complete exchange of the anion in the bathing electrolyte with the original dopant anion. As in the uncycled films, an increase in the average dopant level at the Pt/P3MeT interface with thickness was observed. It is known that thinner films of P3MeT have more compact morphologies and higher conductivities. 15,16 Electron micrographs of three films 1000, 6500, and 13 500 8, thick showed all three films to possess nodular deposits on their surface, with the amount of the deposits increasing with thickness. The 1000 8, film shows that the film beneath the nodular deposits is quite smooth with few irregularities. In addition, STM studies suggest that the initial layers of P3MeT are composed of highly

Bach and Reynolds

13642 J. Phys. Chem., Vol. 98, No. 51, 1994 crystalline domains. The more ordered polymer formed initially at the WP3MeT interface is thus expected to have a higher dopant level than the more disordered material formed later in film growth. As film growth proceeds the film morphology becomes more open, evolving into nodular deposits. These deposits have higher defect densities and lower dopant concentrations. This is supported by the fact that the gradient is observed to a lesser extent in the thinner films, which possess fewer nodular deposits. Therefore, it is apparent that the Re gradient is comprised of a morphology factor and an undoping factor. As in the cycled films, the films spontaneously exchanged for 250 s showed an increase in the rhenium content through the film and a slight decrease in the sulfur content. The dopant levels at the PtP3MeT interface ranged from 0.19 to 0.50, similar to the cycled films. Conclusions P3MeT-(BF4-) undergoes reversible redox switching in all three electrolytes studied. There were no significant decreases in electroactivity after 50 cycles. The dopant level of P3MeT(BF47, as determined by using XPS, was relatively constant in thin films and decreased in thicker films as the morphology began to show nodular deposits. Ion-exchange studies of P3MeT-(BF4-) films, cycled in TBARe04 electrolyte, indicated that a complete exchange of the original dopant anion with the anion in the electrolyte had occurred down to the surface of the substrate with one redox cycle and 1 2 5 s exposure to electrolyte. Spontaneous exchange was observed for P3MeT(BF4-) in TBARe04 electrolyte, but, even with 250 s exposure to electrolyte, incomplete exchange had occurred. Acknowledgment. We gratefully acknowledge technical discussions with Prof. K. Rajeshwar and the hospitality of his research group during the completion of this work. C.M.G.B. thanks Alcon Laboratories for support during the completion of this research. We thank Dr. Mitch McCartney of Alcon Laboratories for SEM analyses.

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