J. Phys. Chem. 1995,99, 17431-17436
17431
A Study of Fullerene Epoxide Electroreduction and Electropolymerization Processes Krzysztof Winkler? David A. Costa, Alan L. Balch,* and W. Ronald Fawcett" Department of Chemistry, University of California, Davis, California 95616 Received: June 21, 1995@
The electrochemical behavior of CmO at a gold electrode in o-dichlorobenzene, dichloromethane, and a mixture of toluene and acetonitrile was studied in the potential range from 0.4 to - 1.5 V. Three one-electron transfer steps are observed for CmO within this potential window. The first and the third electron transfers are attributed to the reduction of the fullerene cage. The product of the first electron transfer is found to undergo a solventand rate-dependent decomposition to Cm. The irreversible transfer of the second electron, attributed to the presence of the epoxide oxygen, leads to a substantial change in fullerene structure and initiates polymer formation for which the kinetics of growth are found to be strongly solvent dependent.
Introduction The redox behavior of fullerenes and fullerene derivatives has been intensely studied in recent years, and the electrochemistry of both solution and solid phase fullerene systems has been reported. Due to the potential technological applications of solid fullerene layers, their redox properties have received considerable attention.'-I0 It has been demonstrated that the electronic properties of fullerene films are dependent on the nature and degree of cation doping.'-6s'o A significant problem with the electrochemical study of fullerene films in solution has been the increased solubility of fullerene anions which leads to dissolution of the film upon reduction. Recently, we have focused our attention on the development of conductive fullerene polymers. A polymer layer would tend to have a lower solubility than its monomeric counterpart, and the ability to electrochemically form a fullerene polymer on an electrode surface would be expected to enhance stoichiometric control over the film. Fullerene polymers have been reported to be formed via high-energy UV irradiation" and thermal,I2 high-pre~sure,'~-'~ electron-indu~ed,~~ and radio-frequency plasmal6 techniques. The last method is reported to produce amorphous films of very low conductivity. In a previous paper we reported the synthesis of a fullerene polymer by electrodeposition during the reduction of CmO." This is a rare case of reductively initiated polymerization. The process allows the film to be readily doped with cations which increase its conductivity. A (-CmO-CmO-)n structure was proposed for the polymer film. The use of a fullerene polymer rather than a fullerene multilayer on the electrode surface has greatly enhanced our ability to study the electrochemistry of this system. To more fully understand the nature of the fullerene polymer and the mechanism of its formation, the electrochemistry of CmO in the potential window from 400 to - 1.500 mV has been investigated. The results of this study are the focus of the present article.
Experimental Section Reagents. CmO was obtained by the m-chloroperoxybenzoic acid oxidation of Cm.'s*19Cm was purchased from MER Corp. (Tucson, AZ) and used without additional purification. Tetrabutylammonium perchlorate (TBAP) (Sigma Chemical Co.) was dried under reduced pressure at 70 O C for 24 h. Anhydrous On leave from the Institute of Chemistry, University of Warsaw, Bialystok Branch. Abstract published in Advance ACS Abstracts, September 15, 1995. +
@
0022-365419512099-17431$09.0010
acetonitrile (99.8%) was used as received from Aldrich. The o-dichlorobenzene, toluene, and dichloromethane (Aldrich Chemical Co.) were purified by distillation under argon atmosphere over CaH2. Apparatus. Cyclic voltammetry was performed using the EG&G/PAR 273 potentiostat in a three-electrode cell. The working electrode was a gold wire (BioanalyticalSystems) with a diameter of 1.5 mm. A gold ultramicroelectrode with a 50 mm diameter (Goodfellow Metal Ltd.) sealed into a soft glass capillary in a Bunsen flame was used as the working electrode in voltammetry experiments at sweep rates higher then 1 VIS. The ultramicroelectrode was also used for diffusion coefficient determinations on the basis of steady state currents. Before each experiment both electrodes were polished with fine Carborundum paper and a 0.5 p m alumina slurry in sequence. Next, the electrodes were sonicated in order to remove the traces of alumina from the gold surface, washed with water, and dried. A silver wire immersed in 0.01 M AgC104 and 0.09 M TBAP in acetonitrile and separated from the working solution by a ceramic tip (Bioanalytical Systems) served as the reference electrode. All potentials given in this paper are expressed with respect to this electrode potential. The counter electrode was a Pt tab with an area of -0.5 cm2, A large platinum electrode (1 cm2) was used as the working electrode in bulk electrolysis experiments. In these studies the counter electrode was immersed in a solution containing a supporting electrolyte, but no CmO, and separated from the working solution by a ceramic tip. Electronic absorption spectra were obtained using a HP 8452A W-vis spectrometer.
Results and Discussion The Study of (2600Electroreduction Processesin Solution. The electrochemistry of Cm epoxide (CmO) in the potential window from 400 to - 1500 mV was examined in order to gain a more thorough understanding of the nature of the observed electropolymerization. A comparison of the voltammetric curves of Cm and C600 (4: 1 toluenelacetonitrile, 0.1 M TBAP) is shown in Figure 1. The voltammogram of C a O is similar to voltammetric curves obtained by Suzuki20 in a o-dichlorobenzene solution. Two electrochemically reversible reduction waves, C1 and C3, are found for both Cm and C ~ O O and are assigned to fullerene cage reductions. A totally irreversible peak (C2) at - 1100 mV is observed only for CmO and is attributed to a process that involves the epoxide oxygen. The irreversibility of the second electron transfer process indicates substantial changes in reactant structure during the electroreduction. 0 1995 American Chemical Society
Winkler et al.
17432 J. Phys. Chem., Vol. 99, No. 48, 1995
mV/s. The observation that the limiting value for this ratio is less than one is probably related to the differences in the diffusion coefficients of the oxidized and reduced forms of the redox couple. The dependence of the peak currents on sweep rate was also analyzed. The linear relationship between the anodic peak current (iA1P)and VI'^ and the deconvoluted peak current (IAIP) and v over the entire range of sweep rates studied indicates a diffusion-controlled oxidation process. However, a similar analysis for the C1 reduction peak shows a significant deviation from linearity for sweep rates lower than about 200 mV/s. The plot of the normalized peak of deconvoluted currentz1 (Inor) as a function of log v (Figure 3b) clearly demonstrates the degree of departure from linearity. The normalized peak current was calculated as the ratio of the observed peak current to the theoretical diffusion-controlled peak current, which is linearly AI 1 I I 1 I 1 dependent on sweep rate. This ratio was taken to be one at the 4.4 4 4 -1.1 E [VI highest sweep rates studied. For v equal to 5 mV/s the observed Figure 1. Cyclic voltammogram of 0.25 mM Cm (a) and 0.25 mM reduction current is nearly twice that expected under diffusionCaO (b) in 0.1 M TBAP/4:1 toluene/acetonitrile mixture (v = 100 controlled conditions. The slight decrease in the anodic mV/s). normalized current observed for low sweep rates is related to the minor influence of the chemical reaction coupled with the The AI and A3 peaks observed in the anodic sweep correspond second charge transfer process, which is described later in this to the C1 and C3 peaks in the forward sweep, respectively. The paper. The diffusion coefficients for CmO and CmO-obtained distance between the anodic and cathodic peak is equal to 65 from the slope of iP vs VI'^ or P vs v are 1.55 x loT5and 1.40 mV and is very close to that expected for a reversible electrode x cm2/s, respectively. The diffusion coefficient of CmO process. However, the ratio of anodic to cathodic current is in toluene-acetonitrile determined from the steady state vollower than expected, which indicates that a more complex tammetric current recorded for a 25 pm radius gold microelecmechanism is involved. The broad A2 oxidation peak appears trode is 1.45 x cm2/s. only if the potential in the cathodic sweep approaches the In Figure 4 the dependencies of the ZC~PIZCIPand ZC~PIZCIP potential window for the second reduction process (CZpeak). ratios on the log v are presented. The changes of Zc2P/ZcIP with The dc current was semidifferentiated with respect to time using sweep rate correspond to the changes in the IAIp/ICIp against the procedure proposed by Dalrymple-Alford and co-workers.zl log v relation. The poor separation of CZand C3 peaks for Semidifferentiationallows for the quantitative analysis of poorly higher scan rates leads to a rather low precision of peak current separated reduction peaks which are difficult to analyze from determination. This can explain the fact that the limiting value standard cyclic voltammetric curves. For a reversible electrode for the ZCZP/IC~P ratio observed for high sweep rates is signifiprocess the deconvoluted peak current is proportional to the cantly lower than 1. The ZC~PIZCIPratio for all studied sweep sweep rate, and the peak potential is equal to the formal potential rates is higher than one, and it increases with decreasing scan of the redox system. In Figure 2 the I-E deconvoluted rate. The broad AZ peak observed on the deconvoluted voltammetric curves recorded in the potential range from -0.3 voltammetric curves in the anodic cycle for higher sweep rates to -1.45 V for different sweep rates are presented. (Figure 2 ) is probably related to the oxidation of the product of To characterize the first electron transfer step (CI), the ratio of the deconvoluted anodic to cathodic peak currents (IAIp/ICIp) the second electron transfer to CmO and indicates partial electrochemical reversibility of this process at high sweep rates. as a function of sweep rate ( v ) was analyzed. Peak currents were deconvoluted in order to allow for a precise determination To explain the electrode processes that occur at the CI of the background current for the oxidation process. The reduction peak, several bulk electrolysis experiments (Pt decrease in the deconvoluted current ratio with decreasing sweep electrode, 2 cm2) were performed in 4:1 toluene/acetonitrile in rate (Figure 3a) indicates the presence of a chemical reaction the potential window from -0.75 to -0.95 V. The charge associated with the first electron transfer process. The ZAIP/ associated with the Cl reduction was determined from the area under the current-time curves. The number of electrons IclP ratio is constant (0.96) for sweep rates greater than 200 I
I
I
1
‘
A
-
I
.
-3.0 -0.6
-
-
-/-
.2.0
“E
.o.z
I
0 0.2
1.0
Figure 2. Deconvoluted cyclic voltammogram of 0.25 mM CmO in 0.1 M TBAP/4:1 toluenelacetonitrile mixture. u = 50 (a), 100 (b), and 500 mV/s (c). The deconvoluted current is expressed by d([Red])/dr at the surface of the electrode.
Electrochemical Behavior of CmO
J. Phys. Chem., Vol. 99, No. 48, 1995 17433
I
0.8
0.8
2< 0.6
0.6
--
I
0.4 0.4
0.2 -2.2
-1.2
0.8
-0.2
0.2
LOG ( ! IV/Sl )
-0.9
-1.1
-1.3
-1.s
E' IVl
Figure 5. Dependence of the ratio of anodic to cathodic deconvoluted peak current (ZAIWCIP) on the switching potential (E*) for the first electron transfer process. Sweep rate v = 100 mV/s. The solution contained 0.25 mM CmO in a 4:1 toluene/acetonitrile mixture containing 0.1 M TBAP.
"..I
~
~
-2.2
-1.2
-0.2
LOG (
0.8
[V/SI )
Figure 3. (a) Dependence of the ratio of anodic to cathodic deconvoluted peak curent of the first electron transfer process (ZAIp/Zcp)on the logarithm of the sweep rate. (b) Dependence of normalized anodic on log v . The solution and cathodic deconvoluted peak current (ZnOr) contained 0.25 mM Cs00 in a 4:1 toluene/acetonitrile mixture containing 0.1 M TBAP.
2.4
I
'
.2
.I
b.1
0
LOG I V
1 VI5 1)
Figure 4. Dependence of (a) the ratio of deconvoluted peak currents of the CZ and CI peaks (ZcP/Zcp) (solid line) and the ratio of deconvoluted peak currents of the CZ to the expected diffusioncontrolled deconvoluted current of CI peak (Zc2pIZcpd,ff) (broken line) and (b) the ratio of deconvoluted peak currents of the C3 and C1 peaks (IC~P/ZCIP) (b) and on the logarithm of the sweep rate, v. The solution contained 0.25 mM Cs00 in a 4:1 toluene acetonitrile mixture containing 0.1 M TBAP.
transferred per molecule of CmO was found to be in the range from 1.5 to 1.8. During the electrolysis an insoluble brown
powder formed. The solution that remained was examined by UV-vis spectroscopy, cyclic voltammetry, and HPLC. Analysis showed absorption peaks at 334 and 408 nm in the UV-vis spectrum, an HPLC retention time equivalent to that of a C60 control sample, and a cyclic voltammogram identical to that of Cm. No C a O was detected in this solution. The experiments on the electrolyzed CmO solution suggest that the formation of CmO- at the C I peak is followed by a decomposition reaction which produces Cm. In the potential range required to reduce CaO, Cm would also be reduced. This added reduction leads to the observed increase in the reduction current above that expected for a diffusion-controlled process. Further, the decomposition of CmO- explains the observed dependence of the IC2P/ICIP(diff)ratio on sweep rate (Figure 4). CmO- decomposition occurs to a greater extent at low sweep rates and causes a diminution in the current associated with the reduction of the epoxide. The observation that the C3 cage reduction peak does not disappear at low sweep rates indicates that the cage structure of the fullerene remains intact during the chemical process of CmO- decomposition. The dependence of IAIPIIcIP on the CV switching potential (E*) was also examined. The results of this study are presented in Figure 5. For an E* potential less negative than -900 mV the IAIPIIcAPratio is almost constant. A decrease in the AI peak current is observed when the E* potential reaches the potential window of the C2 peak. This decrease in AI suggests that an additional chemical reaction is occuning between the product of the first reduction and the product of the second irreversible electron transfer. The products of this reaction are probably responsible for the growth of the broad oxidation wave (A2). However, the reaction between the product of the second electron transfer step and CmO is also possible. Solvent effects on the CmO electrochemistry have also been studied. Dichloromethane and o-dichlorobenzene solvent systems were chosen for this study, owing to the relatively good solubility of both the fullerene and the supporting electrolyte in these solvents. For sweep rates greater than 500 mV/s the shape of the voltammetric curves is solvent independent. However, for lower sweep rates significant differences arising from the different rates of the following chemical reaction were detected. At lower sweep rates the dichloromethane system exhibits a much greater decrease in the deconvoluted ZAIP/ICIP and ICZp/ICl(diff)ratios as compared to the original toluene/ acetonitrile system (Figures 3 and 4). The rates of the chemical reactions in o-dichlorobenzene and toluenelacetonitrile are similar. In dichloromethane the C2 reduction peak is almost not observed at sweep rates lower than 50 mV/s (Figure 6a).
11434 J. Phys. Chem., Vol. 99,No. 48, 1995
-2
-1
0
Winkler et al.
1
LOG f" I v/s I) Figure 6. (A) Cyclic voltammograms of 0.15 mM CWO in 0.1 M TBAPIdichloromethane solution. u = 5M) (a) and 50 mV1s (b). (B)
Dependence of the ratio of anodic to cathodic deconvoluted peak current (IAIpIIc14 (broken line) and normalized cathodic deconvolured pak current (ino,)on the logarithm of the sweep rate for the first electron transfer process. The solution contained 0.15 mM CMOin dichloromerhane with 0.1 M TBAP.
SCHEME 1 prcducts I
Further, at this low sweep rate the I A I p / I C I p ratio and the normalized current (inw)reach limiting values of 0.4 and 2.1, respectively (Figure 6b). To establish further the nature of the CmO decomposition, the dependence of the ratio of the C3 peak current to the CI peak current (ic3PlicIP) was studied for low sweep rates in a dichloromethane solution. Under these conditions the C2 peak is almost not observed on the voltammetic curves, allowing for the accurate determination of the ic+%cIP ratio. As opposed to the results obtained for toluene1 acetonitile mixture (Figure 5). the ic3PIicIP ratio decreases with decreasing sweep rate. A limiting value of 0.5 for the ic3Plic1P ratio obtained for low sweep rates is predicted by the mechanism for formation of Cm from CmO- (see Scheme 1). These results also show that during CmO decomposition the fullerene cage structure remains intact. The following scheme is proposed to account for the Cm reduction features: The results of a more detailed study of the chemical reaction between product and reactant of second
Figure 7. Multicyclic voltammograms of 0.25 mM CwO in 4 1 toluene/ acetonitrile mixture containing 0.1 M TBAP recorded for different switching potentials at a sweep rate v = 100 mV/s for 40 scans. Switching potentials: (a) -0.95, (b) -1.05, (c) -1.15, and (d) -1.25 V.
electron transfer process will be presented in the next section. It is reasonable to suggest that trace amounts of water in the reaction solution are responsible for the observed decomposition CmO- to Cm. Further, differences in the concentration of water in the samples studied could account for the observed differences in the rate constant for this reaction. However, the addition of small amounts of water to a toluenelacetonitrile solution of CmO caused no change in the kinetics of CmO- decomposition. From the slope of a linear plot of the deconvoluted reduction peak current (IcIP)versus sweep rate obtained for high sweep rates, diffusion coefficients of CmO equal to 0.66 x and 1.75 x cm21s were calculated in o-dichlorobenzene and dichloromethane, respectively. Electropolymerization. In a previous paper," it was reported that multicyclic polarization of an electrode in CmO containing toluenelacetonitrile leads to the formation of an insoluble, yellow-brown film on the surface of the electrode. The results of laser desorption time-of-flight mass spectrometry, scanning tunneling and electron microscopy, and infrared spectroscopy suggested that this film is polymeric. In this paper a more detailed description of the formation mechanism of the polymer and the solvent effects on the kinetics and structure of the polymer layer are presented. Figure 7 shows the effect the switching potential (E*) on the process of polymer formation. For values of E* less negative than the potential of the second reduction process (C2). no film formation is observed at the electrode surface. Film formation is initiated only if the switching potential is set at least as negative as the second CaO reduction. This potential dependence, and the dependence of the IAIPIIcIP ratio on E* (Figure 5 ) , is consistent with a mechanism for film formation based on a reaction between the product of the irreversible transfer of the second electron and the product of the first reduction process
or cmo.
Electrochemical Behavior of CmO
J. Phys. Chem., Vol. 99, No. 48, 1995 11435
"I
h
+o.,
I 0
1 -11.4
-0."
E
1\11
Figure 9. Multicyclic voltammogram of 0.35 mMCMO in a odichlorobenzene solution containing 0.1 M TBAP ( u = 100 rnV/s. 100 SCLl"S).
I I ~1 ~
m.4
0
-11.1
-0."
-1.2
F I\'I
Figure 8. Multicyclic voltammograms of 0.2 mM CmO in a dichloromethane solution containing 0.1 M TBAPL (a) v = 500 mV/s, 50 scans: (b) v = 50 mV/s. 120 scans.
To further our insight into the mechanism of the electrode film growth, a study of this process in dichloromethane and o-dichlorobenzene was undertaken. The cyclic voltammetric curves recorded for a dichloromethane solution of CmO for two sweep rates over several cycles are presented in Figure 8. The results of this study demonstrate the relatively fast kinetics of CmO- decomposition in dichloromethane. For low sweep rates (
