J. Phys. Chem. 1993,97, 12067-12072
12067
Electrochemical Microgravimetry of C a Films Tetsu Tatsuma, Susumu Kikuyama, and Noboru Oyama. Department of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan Received: July 23, 1993”
Mass changes of CWfilms, which are prepared on electrodes by casting, during redox processes in acetonitrile were examined by means of quartz crystal microbalance technique. In the case where tetra-n-butylammonium perchlorate was used as a supporting salt, frequency decrease indicating mass increase corresponding to cation doping was observed during the first and second reduction processes. These doping processes were found to be reversible, though the irreversible and gradual mass decrease owing to C60 dissolution was observed during continuous potential cycling. A fraction of the dissolved C60 anion was deposited on the electrode again by the reoxidation. In the third reduction process, dramatic increase in the frequency indicating dissolution of C6o was observed. In the case where lithium perchlorate or sodium perchlorate was used as a supporting salt, dramatic and irreversible frequency increase and deterioration of electrochemistry were observed even in the first reduction process. Therefore, c 6 0 anion is soluble in those solutions. Dependence of the stability of the film electrochemistry on cation species in acetonitrile was thus found to arise predominantly from difference in solubility of the cation fulleride salt.
1. Introduction
Electrochemistry of fullerenes has recently attracted much attention1-’ and Cm or c 7 0 were found to accept 1-6 electrons to be fulleride anions, CmO” or C706 (n = 1-6), respectively.’ Kinetics of the electrochemistry has also been studied and kinetic parameters such as heterogeneous rate constants and diffusion coefficients were evaluated.2 The heterogeneous rate constants for CmO/-, c60+-, Cu2-/’-, and Cm3-/& were found to possess similar values.% Further, spectroelectrochemicaltechniques were used for characterization of fullerene electrochemistry.3-5 Films of Cm,- which may be prepared on the electrode by casting, exhibit unique voltammetric behavior, namely large peak separation in spite of narrow peak width and so forth. Characterization of the films have been conducted by means of quartz crystal microbalance (QCM),’ laser desorption mass spectrometry? scanning tunneling microscopy,6b and scanning electrochemical microscopy,6bas well as conventionalelectrochemical techniques.6 As a result, the followinghave been found as general characteristics of a cast film of Cm in acetonitrile: (1) The electrochemistry of the film does not depend on the nature of anion but cation. (2) The electrochemistryof the film is relatively stable in solutions of tetraalkylammonium salts but unstable in solutions of alkali metal salts. (3) In a solution of tetra-nbutylammonium (TBA) salt, electrochemistry of the first and second reduction-reoxidation processes is relatively stable, while that of the third and fourth is unstable. To date, however, it has not been revealed what is responsible for these characteristics. In view of this, h&e we intend to perform microgravimetric analysis of the c60 film by the QCM technique to elucidate processes of dissolution and deposition of c 6 0 and injection and ejection of ions. Although Bard et aL7a reported their preliminary study on the Cm film using QCM technique, their cyclic voltammogram showed that at least 95% of Ca in their film had lost its redox activity and that the current peaks were broadened, unlike the typical voltammograms for the Cm film. This might be because the experiments were performed out of a glovebox, though thecell was sealed. They examinedonly the first potential cycle and some potential step measurements in a TBAPF6 solution alone, and considered only cation injection and c60 dissolution as factors for the resonant frequency changes. Kadish et Abstract published in Advance ACS Absrracrs, October 15, 1993.
0022-3654/93/2097-12067$04.00/0
also reported preliminary study on electrodeposition and dissolution of c 6 0 films in acetonitrile containing C&. They showed some very significant results, though information about behavior in acetonitrile containing no CsO3-was not provided. In the present work, we made careful experiments in a glovebox filled with dry argon to give typical voltammograms and reproducible and reliable frequency behavior. Further, not only the first cycle but also the following cycles, and not only TBA salts but also alkali metal salts as supporting salts, were examined. 2. Experimental Section 2.1. Materials. Cm (>98%, Texas Fullerene Corp.) was used as obtained. Acetonitrile used as a solvent for electrochemical measurements was distilledand stored under nitrogen. TBAC104 and TBAPF6 were recrystallized from ethanol. AT-cut quartz crystal resonators with Pt electrodes (9 MHz, Seiko Instruments, Japan) were used for all piezoelectricmeasurements. Geometrical area of electrodes was 0.196 cm2. Mass sensitivity of this quartz crystal based on Sauerbrey equation is 1.05 X 10-9 g HE’.Aucoated glass plate was used for conventional electrochemical measurements. 2.2. Procedures. A 5-clL aliquot of toluene containing Cm (1 mM) was cast on an electrode surface and dried up to give surface coverage of about 2.6 X 10-8 mol cm-2. Measurements were performed in acetonitrile containing 0.1 M supporting salt. Electrode potential was controlled with a potentiostat/function generator (PS-07, Toho Technical Research, Japan). An Ag wire and a platinum wire were used as pseudoreferenceand counter electrodes. Potential was calibrated with respect to ferrocene/ ferrocenium ion couple. TTL oscillator was used to oscillate the quartz crystal resonator, and the frequency was measured by a frequency counter (5334B, Hewlett Packard). Admittance of a quartzcrystal oscillatorwas measured by the use of an impedance analyzer (4192A, Hewlett Packard) and all parameters set and data obtained were transferred to a personal computer PC-9801 (NEC, Japan) via GP-IB interface for subsequentdata processing and analysis. All the electrochemical measurements were performed in a glovebox filled with dry argon.
3. Results and Discussion 3.1. Quartz Crystal Microbalance Analysis. First, we must summarize what causes a decrease or increase in the resonant 0 1993 American Chemical Society
12068 The Journal of Physical Chemistry, Vol. 97, NO.46, 1993
frequency of a film-coated quartz crystal os~illator.~ As is wellknown, a mass increaseof the film results in a frequency decrease. If the film is sufficiently rigid and thin, the frequency decrease is proportional to the mass increase. An increase in internal stress of the film also causes a frequency decrease, in the case of AT-cut quartz crystal. Therefore, doping a chemical species into a rigid film will eventually cause a frequency decrease. An increasein surface roughness is additional cause for the frequency decrease. Further, increases in elasticity, viscosity, density, and thickness result in the frequency decrease. Swelling behavior of the film owing to solvation, however, involves increases in thickness, decrease in elasticity and viscosity, and some change in density; hence the frequency change is difficult to predict. Similarly, it is difficult to predict a frequency change caused by some change in a film structure. Admittance Measurements. To examine the rigidity of the C a film, piezoelectricadmittanceof the Cwfilm-coatedquartz crystal was measured. From the conductancefrequency and susceptance-frequency spectra, parameters of the electrical equivalent circuit for the film-coated quartz crystal can be evaluated. The equivalent circuit consists of inductance L1, which reportedly corresponds to mass of the vibrating body (a quartz plate, electrode, and the film), capacitance CI corresponding to mechanical elasticity of the vibrating body, resistance R I corresponding to a loss of mechanical energy dissipated to a surrounding medium, and capacitance CO between the two electrodeson the quartz plate. In the present stage, independent evaluation of LI and CIis not necessarily easy, so that it is difficult to estimate a mass increase of the film from the admittance data. Evaluation of mass changes of a film on a quartz crystal from the resonant frequency is possible on the basis of Sauerbrey equation, if the film is sufficiently rigid. Since rigidity of the film can be estimated from the parameter R1, values of the parameter for a bare and the C~CJfilm-coated crystal in air and in acetonitrile containing TBAClO, were measured. Here we must note that only these admittance measurements were made out of a glovebox. In air, R1 value was not changed significantly by coating the quartz crystal with the Cm film; R1values before and after the coating were 10.4 and 10.2 Q,respectively. Since westuck the quartz crystal onto the bottom of a tubed electrolytic cell with silicone sealant, it was difficult to coat the crystal with the Cm film after measurements in a liquid phase. Therefore, comparison of R1values of the film-coated and the bare crystal in contact with the acetonitrile solution had to be made using a different quartz crystal. In the acetonitrile solution, R1 values were evaluated as 190.8 and 167.1 Q for the coated and bare crystal, respectively. Considering that R1 values for thosecrystal in air were 23.9 and 17.6 i l for the coated and bare ones, respectively, the difference observed in the solution can probably be attributed to the individual difference of quartz crystal. The Cmfilm may thus be rigid enough toapply thesauerbrey equation, though the C a film must be oxidized to a large extent because the measurements were performed out of a glove box. 3.1.1. TBAClOd Solution. First, TBAC104 was employed as a supporting salt for electrochemical measurements. All the electrochemical measurements were performed in a glove box filled with dry argon. Figure 1A shows a typical cyclic voltammogram for the Cm film-coated quartz crystal oscillator obtained in acetonitrile containing 0.1 M TBAClO, at 10mV s-1. The shape of this voltammogram is well-known to be specific to C ~ films. O Broadening of the peaks, which was observed out of a glovebox by Bard et al.,7a was not observed in the present experimentsmade in a glovebox. Peak currents linearly depended on the potential scan rate up to 100 mV s-l; this result is similar to that described by Bard et a1.6b Thus, we observed thermodynamics of electrochemistry of the film at 10 mV s-I. Figure 1B depicts a typical frequency-potential curve obtained in the course of cyclic voltammetry. Althouh shape of the curve
Tatsuma et al. e
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Figure 1. Typical cyclic voltammogram (A) and frequency-potential curve (B) obtained for the Cm film-coated quartz crystal resonator (with Pt electrodes whose area is 0.196 cml) in acetonitrile containing 0.1 M TBAClO4 in the course of potential cycling (10 mV s-l) over the first and second redox waves.
for the first scan was different from those for the following scans in detail, the frequencydecreased during reduction and increased during oxidation in every potential cycle. The First Potential Cycle. In the first potential cycle, the resonant frequency did not change until the reduction current (peak a in Figure 1A) started to flow. A frequency decrease was observed soon after the reduction occurred. Even after the first reduction peak was passed, the frequency continued to decrease gradually, and then dropped again corresponding to the second reduction peak (peak b). These frequency decreases corroborate the cation doping behavior as has been evidenced by laser desorption mass spe~trometry,~~ because cation injection into the fairly rigid Cm film will give rise to an increase in mass and internal stress of the film, both of which cause a frequency decrease. In the reverse scan,the frequency decrease faded away together with the reduction current. Then, the frequency increased during oxidation (peak c), which is indicative of cation ejection. After the oxidation was completed, the frequency decreased to some extent, at around the potential where a small oxidation peak (peakd) was observed. Similar results were obtained also for the potential cycling over the first redox waves alone, though both frequency change and peak height were smaller. Bard et a1.6b assigned the small peak to the oxidation of Cm- dissolved in the solution. Our results suggest that Cw- (and Ca2-) dissolved in the reduction process(es) may be deposited onto the film again by reoxidation. The frequency increased again swn after the oxidation current (peak e) started to flow, and continued to increase during the time oxidation current flowed. In Figure lB, one can find the final frequency in the first cycle was lower than the initial
The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 12069
Electrochemical Microgravimetry of C60 Films
1
1
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Figure 2. Typical steady-state frequency-potential curve obtained for the C a film-coatedquartz crystal resonator in acetonitrile containing0.1 M TBAClOd in the course of potential cycling (10 mV s-l) over the first redox waves.
frequency. However, the final frequency was higher than the initial one in some experiments. This low reproducibility indicates that the first cycle involves at least two factors causing irreversible frequency changes, such as irreversible cation doping, reorganization of the film, dissolution of Cm- and Cd-,and ejection of toluene from the film. Toluene was used as the solvent for casting Cm and is known to be trapped in the Cm film even after evaporation.6b The frequency behavior shown here is not consistent with that given by Bard et al.,7athough their measurements were conducted at 200 mV s-'. They observed a frequency increase in the first reduction process and no frequency changes in the reoxidation processes. From peak area of their cyclic voltammogram, one will find that their film lost at least 95% of the electrochemical activity; most of Cm in their film must be deactivated. The broadened peaks in their voltammogram also evidenced denaturation of the film. Oxidation of Cm by oxygen might be responsible for this deactivation, because their experiments were made out of a glovebox, though they stated that the cell was sealed. Further, the frequency increase in the first reduction process suggests that more than 5% of the initial film mass (calculated from the amount of cast Cm) was lost. Thus, in their experiments, the reduction of the residual active Ca in their film might have caused flaking off of a part of the film, and mass changes arising from ion injection and ejection could not be observed because the amount of active Cm was small. In contrast, in our experiments, almost all of Cm in the film retains its redox activity because all the experimentswere performed in a glovebox filled with dry argon. FoUowhgCycles. In the second and followingcycles, a gradual frequency increase before the first reduction peak (peak a) was observed. This frequency increase may correspond to a small reduction current (shoulder f ) , and may arise from dissolution of Cm adsorbed weakly on the film, which may be Cm deposited with peak d because shoulder f and the frequency increase are almost negligible in the first cycle. After the frequency decrease corresponding to peak a was completed, a relatively small increase in the frequency was observed. Thisincreaseisrelated topeaka becauseit wasobserved even in the case of potential scans over the first redox waves alone (Figure 2). During the second reduction process (peak b), the frequencydecreased, then increased, and finally decreased again; two small frequency peaks were observed. However, sometimes, these peaks are not clearly split. This complicated behavior of the frequencyimplies that the first and second reduction processes partially overlap each other, that these processes involve not only injection of cation but also dissolution of Cm- and C&-, ejection of anion (if any), injection or ejection of solvent, or reorganization of the film, and that these processes do not necessarily proceed simultaneously. In the reverse scan, the frequency behavior was similar to that in the first cycle. However, the frequency gradually and
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Figure 3. Typical frequency-potential curve obtained for the C a filmcoated quartz crystal resonator in acetonitrilecontaining 0.1 M TBAClOd in the course of potential cycling (10 mV s-l) over the first to third redox waves.
irreversibly increased during continuous potential cycling accompanied by a gradual decrease in electrochemical activity. The activity almost disappeared after the 20th cycle. The difference between the initial and final frequencies (about 2600 Hz) agreed with that between the frequencies before and after the casting of Cm (2500-2900 Hz, in a gas phase). Further, these frequency changes corresponding to mass changes of (2.63.0) X 10-6 g roughly agree with amount of CW cast on the electrode (3.6 X 10-6 g). Therefore, the observed frequency increase and the loss of electrochemical activity were caused by dissolution of Cm- and/or Ca2-,most probably. Although Bard et al." stated that an insulating layer remains on the electrode surface even after the electrochemical activity was lost, this layer might have consisted of oxidized and deactivated Cm or some ion complex of C60 anion and TBA+, which is not electrochemically active. Potential Cycling over the Third Reduction Wave. Figure 3 shows a typical frequency-potential curve obtained in potential cycling over the first to third redox waves. In the third reduction process, dramatic and irreversible increase in the frequency was observed. Dissolutionof Cas into the solutionmust be responsible for this frequency increase, because Cm3-is known to be soluble in acetonitrile.7b In the reverse scan, three oxidation peaks c', d', and e' were observed at around the potentials of peaks c, d, and e, respectively, though the height of peak d' was about half of peak e', and that of peak c' was about half of peak d'. Peaks c' and d' were accompanied by frequency decreases, and hence these peaks correspond to reoxidation and deposition of C a s (and Ca2- and Cm-). Peak e' was accompanied by a frequency increase so that it proceeded from oxidation of Cm- in the film. In the second scan toward the negative direction, the behavior of the frequency was similar to that in Figure lB, though a large increase was observed again in the third reduction process. The differencebetween the initial and final frequenciesroughly agreed with that between the frequencies before and after the casting of Cm again; hence almost all of Cm was verified to be dissolved. These results do not contradict results obtained by Kadish et a1.,7b though their experiments were carried out in acetonitrilecontaining Cm3--. Quantitative Analysis. It is very difficult to evaluate apparent mass number of ion injected into and ejected from the film, on the basis of the frequency changes accompanying the redox reactions, because contribution of changes in internal stress and surface roughness to the frequency changes is unclear and there proceeds some dissolution of Cm. However, we dare to do this because estimated values are expected to give much information. Since mass changes for the reduction processes are fairly difficult to estimate owing to overlappingof the first and second processes, mass changes for the oxidation processes were estimated.
Tatsuma et al.
12070 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993
I500 Hz
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Figure 4. Typical cyclic voltammogram (A) and frequency-potential curve (B) obtained for the C a film-coatedquartz crystal resonator (with Pt electrodes whose area is 0.196 cm2) in acetonitrile containing 0.1 M NaC104 in the courseof potential cycling (10 mV s-l) over the first redox waves. Apparent mass numbers of monovalent cation ejected in the oxidation processes are calculated to be about 93 g mol-' for peak e and about 154 g mol-' for peak c (error is about *20 g mol-'). Thesevaluesare much smaller than mass number of TBA+ (242.5 g mol-'). The following are possible reasons for this: (1) Not only TBA+ ejection but also C104- (99.5 g mol-') injection accompanies the oxidation process. (2) Some solvent molecules (molecular weight of acetonitrile is 41.1) is injected into the film in place of ejected TBA+. (3) Deposition of Cm which has been dissolved in the reduction processes proceeds. (4) Surface roughness of the film increases with oxidation. Among these, transport of C104-willbe discussed below in the sectionof Donnan potential analysis. From the frequency decrease accompanying peak d, apparent mass number of deposited species was estimated as 400-1000 g mol-' on the assumption that the oxidation is one-electron reaction. Although the error is very large because both the frequency change and peak area are too small, this value does not contradict our speculation that peak d corresponds to deposition of c 6 0 which has been dissolved in the reduction processes. 3.1.2. TBAPFs Solution. QCM analysis in the cause of potential cycling over the first and second redox waves in a TBAPF6 solution gave similar frequency-potential curves to those in a TBAC104 solution, except for the first cycle. In the first cycle, a frequency increase was observed immediately after the potential sweep was turned over from negative to positive direction. Although this increase is probably caused by reorganization of the film, dissolution of C&, or ejection of toluene from the film, it is not clear how the anion dependence arose. Almost all other behavior qualitatively agreed with that in a TBAC104 solution, indicating that anion does not play an essential role in the redox processes or that ClO4- and PF6- behave similarly in the present system. 3.1.3. NaC104 Solution. Electrochemistry of the C a film is reportedly unstable in a solutioncontaining an alkali metal cation.6 Bard et a1.6band Compton et a1.a inferred that alkali metal cations form electrochemically inactive salt with a Cm anion. Actually, Bard et al." stated that an insulating film remains on the electrode surface even after the electrochemical activity was lost. Figures 4 and 5 show cyclic voltammograms and frequency-potential
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Figure 5. Typical cyclic voltammogram (A) and frequency-potential curve (B) obtained for the C a film-coated quartz crystal resonator in acetonitrile containing 0.1 M NaC104 in the course of potential cycling (10 mV s-l) over the first and second redox waves. curves obtained in a NaC104 solution. Electrochemical activity of the c 6 0 film was rapidly suppressed during the potential cycling. As can be seen in Figure 4, the first reduction and reoxidation waves were accompanied by a frequency increase and decrease, respectively, and the frequency a t the oxidized state increased with decreasing peak current, while the frequency at the reduced state changed little. These observations reflect that a Cm anion is dissolved in the first reduction process and a fraction of the dissolved Cm is deposited again in the reoxidation process. By extending the sweep range toward the negative direction, the second reduction and reoxidation waves were obtained (Figure 5). The frequency increase was substantially small for the second reduction process, and the frequency did not change in the reoxidation process. Thisimpliesthat thesecond waves correspond to redox reactions of the Cm anion dissolved in the first reduction process. Assuming that the first reduction process is one-electron reduction of c 6 0 to Ca- and c60- is dissolved, a mass change expected from the charge passed in the first reduction process is about 1.3-fold larger than that evaluated from the frequency change (Figure 4). If the assumption is valid, some other electrochemical reactions proceed with the first reduction process simultaneously, such as further reduction of the dissolved Ca(i.e., the second reduction process) or reduction of Cm to form insoluble salts with Na+. Otherwise, the reduction process is two-electron reaction, though the mass decrease is larger than that expected from the charge in this case. The excess mass decrease may be ascribed to solvent and/or supporting salt having been entrapped in the film before the potential scan. In either event, c 6 0 anion, 0-or c602-, neither of which is very soluble in acetonitrile containing TBAC104, may be more soluble in a NaC104 solution. Flaking off of a part of the filmcould be another possible reason for the excess mass decrease in the case where two- or three-electron reduction is assumed. The charge passed in the second reduction process seems to be larger than that in the first process, hence the second reduction process may be twoor three-electron reaction.
The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 12071
Electrochemical Microgravimetry of C60 Films
TABLE I: Slopes of the Plots of Peak Potentials for Ca Film-Coated Electrodes against log[TBA+r peakb slope (mV per decade) estimated t+ first reduction (peak a) 51 0.98 first reoxidation (peak e) 40 0.84 second reduction (peak b) 43 0.86 second reoxidation (peak c) 53 0.95 Measured in acetonitrile containing TBAClO4 at 10 mV s-I. See Figure 1A. Transference number of TBA+ estimated from the slope (see text).
B
molecule. The subscripts f and s represent the film and solution phases. The number of water transported with ions, rn, is not necessarily an integer and sign of it is not necessarily plus. Thus, the apparent formal potential of a C"+/(n+l)- couple in the film, E:pp,is formulated as follows:
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= E'+ (RT/F)(t-In a(A-,) - t+ In a(C+,) t (t+ - t)In a(C+,) + m In a@)) (2)
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Figure 6. Typical cyclic voltammogram (A) and frequency-potential curve (B) obtained for the Cm film-coatedquartz crystal resonator (with Pt electrodes whose area is 0.196 cm*) in acetonitrile containing 0.1 M LiC104 in the course of potential cycling (10 mV s-l).
The difference between the initial and final frequencies (25003300 Hz) roughly agreed with that between the frequencies before and after the casting of c 6 0 (2500-2900 Hz) again. However, some residue of the film was left on the electrode surface, even after the electrochemistry was disappeared. Since this residue cannot be dissolved to toluene, it is not initial Cm but c 6 0 oxidized and deactivated by oxygen or some ion complex of a c 6 0 anion and Na+ which is not soluble to acetonitrile. 3.1.4. LiClO&dution. In the case where LiC104 was employed asa supporting salt (Figure 6), threecathodicpeakswereobserved in the first scan while only one cathodic peak was observed in the following scans and two anodic peaks were observed in every scan. The frequency increased during the reduction process(es), and a frequency decrease was observed for the oxidation process proceeding at a more anodic potential, while a frequency change was not observed for the oxidation process proceeding at a more cathodic potential. These observations are in line with those for the case of NaC104, if the two reduction processes observed in the case of NaC104 were regarded to be overlapped in the case of LiC104. 3.1.5. Supporting Salt Dependence. Thus, the c 6 0 film was found to be dissolved in acetonitrile by electrochemical reduction in both NaC104 and TBAC104 solutions, though the stability is higher in the TBAClOl solution. That is, solubility of a salt NanCa may be higher than that of TBAnC60(n = 1, 2). 3.2. Donnan Potential Analysis. To examine ion transport process more precisely, Donnan potential analysis10was conducted in acetonitrile containing TBAC104. The redox processes of the Cm film can be formulated as follows:
+
C60'+f t-A-,
+ ?+e+,+ mS, + e- e Cao(n+l)-f+ t- A-s + + mS, ( I )
where n = 0 or 1, A- and C+ are anion and cation, respectively, t- and t+ are transference numbers of anion and cation, respectively, at the film/solution interface, and S represents solvent
where Efis the formal potential of Cm in the film and a represents activity. Then, dependencies of peak potentials (not in the first cycle) on the concentration of the supporting salt (TBAC104 here) were examined. The plots of peak potentials against common logarithm of the salt concentration gave roughly straight lines, and slopes of the plots in mV per decade are summarized in Table I. Although activities of C6o and solvent in the film might depend on the salt concentration, the observed dependencies of Eippare caused predominantly by a difference in transference numbers of the cation and anion, most probably (see eq 2). In the case where ion activities in the film do not change or are much smaller than the activities in the solution, the plus slopes mean that ion transport during redox reactions at the film/solution interface is rather selective toward cation. In this case, the transference number of TBA+ ion can be estimated as listed in Table I. On the other hand, in the case where ion activites in the film are much larger than those in the solution, the plus slopes mean that ion transport is rather selective toward the anion. However, ion activities in such a condensed film as the present one are unlikely to be higher than those in a solution. Further, as described above, the results of QCM measurements suggest cation exchange behavior, and electrochemical behavior of the c 6 0 film strongly depends on cation species in a solution while it is almost independent of anion species. Considering all these observations, we conclude that the ion transport at the film/solution interface during the first and second redox reactions is rather selective toward TBA+, in acetonitrile containing TBAC104. Similar results were obtained also in TBAPF6 solutions. On the assumption that the values of transference numbers estimated above are valid, expected values of apparent mass numbers (Mapp) of ion ejected from the film during oxidation are calculated to be 188 and 225 for peak e and c, respectively, from the following equation:
Mapp= t+M+ - t-M(3) where M+ and M- are mass numbers of cation (TBA+ here) and anion (C104- here), respectively. As described above, experimentally evaluated apparent mass numbers are 93 g mol-1 for peak e and about 154 g mol-' for peakc. These values are smaller than the expected values, so that solvent injection instead of the cation ejection, deposition of c60, and/or a surface roughness increase accompany the film oxidation even if anion is injected. Assuming that the solvent exchange alone is responsible for the lower experimental mass numbers, the numbers of exchanged acetonitrile (Le,, m in eq 1) are about 2.3 for peak e and 1.7 for peak c. In the present stage, however, validity of anion transport has not been examined by another means.
12072 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993
Acknowledgment. This work was supported in part by the MitsubishiScienceFoundation and a Grant-in-Aid for Scientific Research (No. 05233210) from the Ministry of Education, Science and Culture of Japan.
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