J. Phys. Chem. 1992,96,4163-4165
4163
Simultaneous Cycllc Voltammetry and Electrochemical Quartz Crystal Microbalance Studies of Buckminsterfullerene (Ceo) Film Electrodeposition and Tetra-n-butylammonium Electrodoping in Acetonitrile Wonyong Koh, Dominique Dubois, Wlodzimierz Kutner,? M. Thomas Jones, and Karl M. Kadish* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: March 18, 1992)
A simple and efficient method is presented for preparation of insoluble thin films of Cpoand the (TBA+)(C,) and (TBA+)2(C,2-) salts where TBA+ represents the tetra-n-butylammonium cation. The method consists of bulk electroreducing a fine suspension of solid Csoin acetonitrile solutions under a controlled potential which is sufficiently negative to generate the soluble Cm3anion, followed by electrooxidative deposition of a neutral c 6 0 film on the surface of a gold/quartz crystal working electrode. The properties of the electrodeposited films were examined by simultaneous cyclic voltammetry and electrochemical quartz crystal microbalance experiments which enabled determination of the stoichiometry and mass of the film.
Introduc#ion Buckminsterfullerene, c60, is a precursor of doped solid materials which reveal outstanding properties such as superconductivity'-' or ferromagneti~m.~.~ These properties underscore the importance of determining and controlling the structure and stoichiometry of the doped materials. Single- and mixed-phase fulleride salts have been obtained by vapor doping,' solution-phase electrocrystallization,6and electrosynthesis? ~rystallization,4a*~ chemical intercalation.' CWfilms, cast on an electrode surface by evaporation of c 6 0 solutions, can also be doped with alkali metals or quaternary ammonium cations upon electroreduction in acetonitrile.* However, the stoichiometry of electrogenerated Cm" films has not been fully elucidated nor have the solubility and stability of these fulleridefilms been characterized. This is done in the present communication which demonstrates that c60 films can be electrodeposited from C& solutions onto a Pt or Au electrode surface and subsequently doped with a tetra-n-butylammonium cation (TBA+) upon electroreduction. The mass and stoichiometry of these films are monitored and controlled by simultaneous use of an electrochemical quartz crystal microbalance (EQCM)9 and cyclic voltammetry (CV) or chronocoulometry. In the course of our electrochemical and ESR studies on c 6 0 and C70fullerenes,1° we observed that fulleride anions which had been electrogenerated by bulk electrolysis were generally more soluble than the neutral c60 compound. This suggested that solutions of Cso" anions (n = 1,2, or 3) might be obtained under conditions where C, is insoluble but where the more soluble anions might be electrogenerated and then reoxidized to give a solid film of neutral C, A good example is provided in acetonitrile, where Cm is totally insoluble but where fairly concentrated solutions of Ca3- can be easily obtained as described in the following section. Experimental Section Chemicals and Instrumentation. Acetonitrile (CH3CN) was distilled from CaH2 under nitrogen prior to use. Tetra-n-butylammonium perchlorate (TBA(C10,)) was twice recrystallized from absolute ethanol and dried in vacuum at 40 OC prior to use. Purified CWwas provided by Dr. Rodney Ruoff from SRI. All experiments were carried out in a glovebox (Vacuum Atmospheres Co., Hawthorne, CA) filled with dry and oxygen-free nitrogen. The bulk electroreduction of Cmto CSo3-was carried out in an "H" type cell whose working and auxilliary electrode compartments were separated by a sintered glass frit. Both working and auxilliary electrodes were made of a platinum gauze. The EQCM experiments were carried out using a conventional three-electrode configuration and a custom constructed holder 'On leave from the Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44, 01-224 Warsaw, Poland. To whom correspondence should be addressed.
whose design is described in ref 9b. The 5-MHz Aulquartz electrode (Phelps Electronics, Goleta, CA) had a 0.323-cmZ geometric surface area; its sensitivity was 19.5 ng Hz-' cm-2 as determined with the Ag+/Ag system, and this value allows for controlled deposition of films with a mass as low as ca. 40 ng/cm2. Controlled potential electrolyses were performed with an EG&G PAR 173 potentiostat which was coupled to an EG&G PAR 179 digital coulometer. Cyclic voltammetry experiments were performed using an EG&G PAR 173 potentiostat and an EG&G PAR 175 universal programmer. The same instrumentation was used for simultaneousEQCM experiments,and the frequency was measured with a Philips PM6680 high-resolution programmable timer/counter (J. Fluke Mfg. Co., Everett, WA). Simultaneous cyclic voltammograms and resonance frequency changes (ABvs potential (E) curves were recorded with an HP 7090A measurements plotting system (Hewlett Packard Co., Avondale, PA). (1) (a) Hebard, A. F.; Rosseinsky, M. J.; Haddon, R. C.; Murphy, D. W.; Glarum, S.H.; Palstra, T. T. M.; Ramirez, A. P.; Kortan, A. R. Nature 1991, 350,600. (b) Stephens, P. W.; Mihaly, L.; Lee,P. L.; Whetten, R. L.; Huang, S.-M.; Kaner, R.; Deiderich, F.; Holczer, K. Nature 1991,351,632. (c) Kelty, S.P.; Chen, C.-C.; Lieber, C. M. Nature 1991,352,223. (d) Tanigaki, K.; Ebbesen, T. W.; Saito, S.;Mizuki, J.; Tsai, J. S.;Kubo, Y.; Kuroshima, S. Nature 1991, 352, 222. (2) (a) Wang, H. H.; a n i , A. M.; Savall, B. M.; Carlson, K. D.; Williams, J. M.; Lykke, K. R.; Wurz, P.; Parker, D. H.; Pellin, M. J.; Gruen, D. M.; Welp, U.; Kwok, W.-K.; Fleshler, S.;Crabtree, G. W. Inorg. Chem. 1991,30, 2838. (b) Wang, H. H.; Kini, A. M.; Savall, B. M.; Carlson, K. D.; Williams, J. M.; Lathrop, M. W.; Lykke, K. R.; Parker, D. H.; Wurz, P.; Pellin, M. J.; Gruen, D. M.; Welp, U.;Kwok, W.-K.; Fleshler, S.;Crabtree, G. W.; Schirber, J. E.; Overmyer, D. L. Inorg. Chem. 1991, 30, 2962. (3) Chakravarty, S.;Gelfand, M. P.; Kivelson, S.Science 1991, 254, 970. (4) (a) Allemand, P.-M.; Khemani, K. C.; Koch, A.; Wudl, F.; Holczer, K.; Donovan, S.;Griiner, G.; Thompson, J. D. Science 1991, 253, 301. (b) Stephens, P. W.; Cox, D.; Lauher, J. W.; Mihaly, L.; Wiley, J. B.;Allemand, P.-M.; Hirsch, A.; Holczer, K.; Li, Q.; Thompson, J. D.; Wudl, F. Nature 1992, 355, 331. ( 5 ) Penicaud, A.; Hsu, J.; Reed, C. A.; Koch, A.; Khemani, K. C.; Allemand, P.-M.; Wudl, F.J. Am. Chem. Soc. 1991, 113, 6698. (6) Allemand, P.-M.; Srdanov, G.; Koch, A.; Khemani, K.; Wudl, F.; Rubin, Y.; Diederich, F.; Alvarez, M. M.; Anz, S.J.; Whetten, R. L. J . Am. Chem. SOC.1991, 113, 2780. (7)Chabre, Y.; Djurado, D.; Armand, M.; Romanow, W. R.; Coustel, N.; MacCauley Jr., J. P.; Fisher, J. E.; Smith 111, A. B.J. Am. Chem. Soc. 1992, 114, 164. (8) (a) Jehoulet, C.; Bard, A. J.; Wudl, F. J . Am. Chem. Soc. 1991, 113, 5456. (b) Jehoulet, C.; Obeng,Y. S.;Kim, Y.-T.; Zhou, F.; Bard, J. J. Am. Chem. SOC.,in press. (9) (a) For a recent review of the EQCM technique see: Buttry, D. A. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1991; Vol. 17, pp 1-85. (b) The design of the EQCM utilized in this study is described in: Koh, W.; Kutner, W.; Jones, M. T.; Kadish, K. M. Submitted for publication. (10) (a) Dubois, D.; Kadish, K. M.; Flanagan, S.;Haufler, R. E.;Chibante, L. P. F.: Wilson, L. J. J. Am. Chem. Soc. 1991.113. 4364. (b) Duboi. D.;Kadish, K. M.; Flanagan, S.;Wilson, L. J. J. Am. Chem. S o c . ' l k l , 113, 7773. (c) Dubois, D.; Moninot, G.; Kutner, W.; Jones, M. T.; Kadish, K. M. Submitted for publication. (d) Dubois, D.; Jones, M. T.; Kadish, K. M. J . Am. Chem. Soc., in press.
0022-3654/92/2096-4163%03.00/0 0 1992 American Chemical Society
Letters
4164 The Journal of Physical Chemistuy, Vol. 96, No. 11, 1992
a
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0.0
I
-0.5
1
-1.0
I
-1.5
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i
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0.0
-0.3
-0.6
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Potential (V vs. SCE) Figure 2. Cyclic voltammogram of the Cmo-coatedAu/quartz electrode in 0.1 M TBA(C104), acetonitrile solution (v = 0.1 V/s). The voltammogram was recorded after two cycles between 0.0 and -1.2 V.
Potentlal (V vs. SCE) Figure 1. (a) Cyclic voltammogram of ca. 0.45 mM Cm3-in acetonitrile, 0.1 M TBA(C104) at a 0.785-mmz Pt electrode (v = 0.1 V/s) between -2.50and -0.55 V (solid line) and between -2.5 and 0.0 V (dashed line). (b) Simultaneous cyclic voltammetry (curve 1) and EQCM frequency response (curve 2) of ca. 0.63 mM Cm3-in acetonitrile, 0.1 M TBA(Clod) at a Au/quartz electrode (v = 0.1 V/s). The overall mass changes given by the roman numerals correspond to deposition of the neutral Ca film (I), the stepwise uptake of TBA+ into the reduced film (11), and the electrodissolution of the (TBA+)2(C602-)film (111).
Preparation of Ca3 Solutions. Typically, a few milligrams of solid Ca present as a fine suspension in 8-20 mL of acetonitrile, 0.1 M TBA(C104), was bulk electroreduced, while stirring, at a platinum gauze electrode polarized at -1.6 V vs SCE. After the transfer of 3.0 electrons per Cbomolecule all of the solid eventually dissolved as the electrolysis current decayed to zero. The resulting CW3-solution is dark red-brown.
Results and JNscussion Figure la displays the CV behavior of the electrogeneratedCbo* solution at a Pt disk working electrode. Four reversible diffusion-controlled processes are observed between -0.6 and -2.5 V (solid line) which correspond to the known c60w/c&+’)redox couples (n = 1-4).l0 However, if the positive scan reversal is made at potentials more positive than -0.55 V (where neutral c 6 0 is formed), a surface type CV curve is recorded (dashed line). Also, when the concentration of CW3-is increased and/or the scan rate decreased, the heterogeneous electron transfers involving the C60/C6{, Cm-/Cbo2-,and c60~-/c~~~couples all show characteristic adsorption features similar to those reported earlierSs As shown in Figure lb, a positive potential scan from -1.6 to -0.2 V at the Au/quartz electrode results in a slight frequency decrease upon passing through process 2, and this indicates some electrodepositionof a C,- film. The frequency rapidly decreases upon passing through process 1, and this indicates a fast electrodeposition of neutral c60. The ratio of the number of moles of Cb0determined from the total mass uptake at the Au/quartz electrode (calculated by using the Sauerbrey equationga)to the number of moles of Cbodetermined from the charge transferred during a potentiostaticexperiment at -0.2 V is 1.03. This indicates that virtually no solvent or supporting electrolyte is trapped in the film and that all of the c 6 0 formed from Cm3-is deposited onto the electrode. Four cathodic peaks are seen upon scanning from -0.2 to -1.9 V at the Au/quartz electrode (Figure lb, curve l), the first three
of which (l’, 2’, 3’) correspond to surface processes. The mass uptake upon the first reduction of the Cbofilm amounts to 0.90 fig per 2.57 pg of C, (curve 2, process 1’) and is in excellent agreement with the gain of one TBA’ cation (M = 2423 g/mol) per c 6 0 molecule (M = 720.7 g/mol), indicating a film stoichiometry of (TBA+)(Cbo-). A small mass loss is observed at potentials slightly more negative than peak l’, and this may indicate some dissolution of the electrogenerated (TBA+)(Cw) film. However, the mass again increases upon the second reducbon (process 29, and the value is consistent with the uptake of a second equivalent of TBA’ to form a (TBA+)2(C,z-) film. The mass continues to increase slightly during this reduction which suggests further electrodeposition. A similar slow increase in mass is observed by holding the potential of the electrode (and the film) at -1.35 V, and this is accompanied by a net small anodic current flow. Both facts are consistent with a buildup on the electrode of the (TBA+)2(C,2-) film which originates from electroofidation at this potential of fresh C603-diffusing from the solution to the electrode. Finally, as the potential is swept from ca. -1.4 to -1.6 V, the mass of the electrode quickly drops to its original value, indicating a complete electrodissolutionof the film (process 3’). No further changes in mass are observed as the potential is swept in a negative direction through process 4, and hence, it is not surprising that the C603-/C604-couple invariably displays reversible, diffusioncontrolled behavior under all solution and potential scan rate conditions. The analysis of our data leads to the following overall oxidation/reduction scheme where (s) and (0 represent “solution” and “film”, respectively, and the numbers of each process correspond to the electrode reactions shown in Figure 1.
(Cm2-)(TBA*)2(f)
y
(Cw-)(TBA+)(I)
4
The overall three-electron oxidation of Cbo3-(s)(processes 3, 2, 1) results in the ultimate oxidative deposition of Csoo(f) via process 1, while processes 1 and 2’ involve the stepwise reductive doping of this film by TBA+ prior to its dissolution to give back C603-(s)in process 3I. This cyclic mechanism was elucidated in acetonitrile solutions of c603-9 but a similar mechanism should also hold upon starting from a Cbofilm in the absence of Cbo3-. The films formed upon electrooxidation of Cbo3-to form C, should also behave similarly to those cast from solutions of neutral CW8 This was verified in the present study by cyclic voltammetry of a 13.3-fig C, film which had been electraoxidativelydeposited on the Au/quartz electrode, rinsed with acetonitrile, and then analyzed in a fresh acetonitrile solution containing only 0.1 M TBA(C10J. An example of the resulting cyclic voltammogram obtained after three potential cycles is shown in Figure 2. A p parently, all basic CV features of the Cmocast films are repro-
Letters d u d . Major reduction peaks are seen at E = -0.84 and -1.05 V while major oxidation peaks are obtaindat E, = -0.22 and -0.67 V. The peak-to-peak separation is 0.21 V upon reduction and 0.45 V upon oxidation. These values compare favorably to CV data for Cso films formed by drop coating the electrode with Cmsolutions.* However, we have observed that the behavior of the film appears to be quite dependent upon experimental conditions such as film thickness (i.e., deposition time) and the number of potential cycles. It has been suggested that TBA+ remains trapped in the film even after reoxidation of the doubly reduced Cm3-film.” Preliminary EQCM results obtained in our laboratory under the same experimental conditions also indicate such a phenomenon, but the (1 1) Zhou, F.; Yau, S.-L.;Jehoulct, C.; Laude Jr., D. A.; Guan, Z.; Bard,
A.
J. J . Phys. Chem., preceding paper in this issue.
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4165 exact nature of the film after potential cycling and trapping of TBA+ remains to be elucidated. However, it is clear that a Cm film can be obtained from Csa3 in acetonitrile, 0.1 M TBA(C104), and this film can then be electroreduced in one-electron transfer steps to yield fulleride films of known composition, namely, (TBA+)(C,,-) and (TBA+)*(C6,,”). Partial doping should also be possible by means of “fine tuning” the applied potential and/or the electrolysis time. Films doped with other countercations, particularly metal cations, might also be obtained, and studies along these lines are now under way. Acknowledgment. The support of this work by the Texas Center for Superconductivity, the Robert A. Welch Foundation (E-1208, M.T.J.; E-680, K.M.K.), and the National Science Foundation (CHE-8822881, K.M.K.) is gratefully acknowledged. We also acknowledge Professor A. J. Bard for providing a preprint of refs 8b and 11 and for helpful discussions.
