Electrosynthesis and Electrodoping of Cab (n = 0, 1, 2, or 3) Films

Oct 12, 1992 - The electrosynthesis, electrodoping/undoping, and electrodissolution of thin films of Cm and its salts were studied in acetonitrile sol...
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J. Phys. Chem. 1993,97, 6871-6879

6871

Electrosynthesis and Electrodoping of Cab (n = 0, 1, 2, or 3) Films: Electrochemical Quartz Crystal Microbalance Study in Acetonitrile Solutions of Alkali-Metal, Alkaline-Earth-Metal, and Tetra-n-butylammonium Cations? Wonyong Koh, Dominique Dubois, Wlodzimierz Kutner,* M. Thomas Jones, and Karl M. Kadish' Department of Chemistry, University of Houston, Texas 77204-5641 Received: February 22, 1993; In Final Form: April 9, 1993

The electrosynthesis, electrodoping/undoping, and electrodissolution of thin films of Cm and its salts were studied in acetonitrile solutions at a Au/quartz electrode by simultaneous electrochemistry and microgravimetry with an electrochemical quartz crystal microbalance. The C60° films were deposited either by electrooxidation of Cm2-in solutions containing CsAsF6, KPF6, or Ca(PF&, or by electrooxidation of Cm3-in solutions containing tetra-n-butylammonium perchlorate, (TBA)C104. The initial C602- or Cm3- anions were prepared by controlledpotential bulk electroreduction of a Cwo suspension. The C6o0 films electrodeposited from a Cm2- solution containing Cs+, K+, or Ca2+cations could not be electrodoped with these cations but a conductive (Cs+),(C&) film could be quantitatively electrosynthesized by electroreduction of C602- in a solution containing Cs+. Attempts to electrodeposit (K+)3(C603-) or (Ca2+)s(C,&)2 films were unsuccessful but a (Ca2+)(Cm-)2 film could be prepared. The composition of a given C60° film electrodeposited by constant potential elvtrooxidation of Cm3in an acetonitrile solution containing (TBA)C104 was controlled by the initial C6O3- concentration and/or the electrodeposition potential. These C60° films could be electrodoped with TBA+ cations from solution upon electroreduction at appropriate potentials and both (TBA+)(C60-) and (TBA+)z(Cm2-) films were obtained. Films of the same compositioncould also be obtained by electrooxidation of concentrated C&- solutions containing (TBA)ClOs at a suitably selected potential.

Introduction The electrochemistry of buckminsterfullerene,Cm, has been studied extensively in solution,l" as Langmuir films,e13 and as solid films at an electrode surface.ls20 In accord with quantum chemistry calc~lations,21-2~ six reversible one-electron c60*/ C,&+l)- (n = 0-5) electroreductionswere found in solution1JJ and the first three of these have been examined in films. The electrochemical properties of the films depend considerably on the nature of the supporting electrolyte cation,l3J4J8-20 and the electroreductions in the films differ markedly from those in solutions where half-wave potentials vary with the nature of the solvent, supporting electrolyte and the t e m p e r a t ~ r e . ~ ~ Single- and mixed-phase fulleridesolid salts have been prepared by vapor doping,2629solution phase synthesis,30crystalli~ation,~~ electrocrystallization,32and electrochemicalintercalation.33 Solid CmO films were prepared by molecular-beam epitaxy,34 by a solution-cast technique4.1c17.19,20,35,36 or by electrodeposition.18 In a preliminary report,18we demonstrated how Cm* films could be electrodeposited from acetonitrile solutions of C,& containing 0.1 M tetra-n-butylammonium perchlorate, (TBA)C104. Simultaneous electrochemistry and microgravimetry with an electrochemical quartz crystal microbalance,EQCM,'741 proved useful in studies of processes involving the electrogeneration, electrodoping or undoping of these films with electroinactive countercations and film electrodissolution.16J* The combined response of current (charge) and the mass change of the film was recorded as the film was formed, the ingress or egress of TBA+ proceeded and the film was removed. The present paper is a continuation of our earlier initial study18 and describes more extensive EQCM results on C& films in acetonitrile containing Cs+, K+,Ca*+, or TBA+ cations. The developed film elect Resented in part at the 182nd Meeting of the Electrochemical Society; Symposium on "Fullerenes: Chemistry, Physics and N e u Directions III"; Electrochemical Society, Toronto, Canada, Oct 12-15, 1992 Abstract No.

784FUL. t On leave from the Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44,Ol-224 Warsaw, Poland. To whom correspondence should be addressed.

0022-3654/93/2097-687 1$04.00/0

trodeposition techniqueallows for a selective, controlled-potential electrosynthesis of C& salts which is also described in the present paper. Electrochemicalstudies on Cm* films become important in view of promising superconductive,42magnetic,31.43.Mnonlinear opti~a1~5.~6 or photochemical15 properties of such films.

Experimental Section Chemicals. c 6 0 was purchased from SES Research, Inc., Houston, TX, and its purity verified by HPLCe4' Acetonitrile (EM Science, Gibbstown, NJ) was distilled over CaH2 under nitrogen. Tetra-n-butylammonium perchlorate, (TBA)C104 (Eastman Kodak, Co., Rochester, NY), was recrystallized twice from absolute ethanol and dried under reduced pressure at 40 OC. CsAsF6 was prepared by precipitation from an aqueous solution containing an equimolar mixture of nearly saturated CsBr (Aldrich Chemical Co., Milwaukee, WI) and LiAsF6 (Aldrich). Ca(PF6)2 was precipitated from an aqueous solution containing a mixture of KPFs (Johnson Matthey Alfa Products, Ward Hill, MA) and CaC12(Aldrich). The KPF6, CsAsF6 and Ca(PF& salts were recrystallized twice from methanol by slow evaporation of the solution and then dried in vacuum. Instrumentation and Procedures. Bulk controlled-potential electrolyses were performed in a conventional H-type cell with a PAR 173 potentiostat/galvanostat (EGCG Princeton Applied Research Corp., Princeton, NJ) equipped with a PAR Model 179 digital coulometer. The working and auxiliary electrode compartments of the cell were separated with a medium pore size sintered glass frit. Both the working and auxiliary electrodes were made of a platinum gauze. Simultaneous electrochemical measurements (cyclic voltammetry (CV), chronocoulometry, and chronoamperometry) and microgravimetrywith an EQCM measurements were performed in a conventional three-electrode electrochemical cell. The electrochemical setup consisted of a PAR 175 universal programmer and a PAR 173 potentiostat/galvanostat equipped with a PAR 179 digital coulometer. A platinum wire served as the auxiliary electrode. A saturated calomel electrode (SCE) was

0 1993 American Chemical Society

6872 The Journal of Physical Chemistry, Vol. 97, No. 26, 1993 used as the reference electrode. It was separated from the working solution with a salt bridge filled with the solution under investigation and separated from the working electrode compartment with a fine sintered glass frit. All cited potentials are referred to SCE. The design of the EQCM setup used in the present work is A nonpolished 5-MHz AT-cut quartz described crystal with 6-mm-diameter gold deposit (Phelps Electronics, Goleta, CA) was used as the working electrode. The projected electrode area, which included contacting gold strips, was 0.323 cm2. Each experimental series was performed with a new Au/ quartz working electrode. The experimentally determined mass sensitivity of the Au/quartz electrode was 19.5 ng H z l The ratio of the electrochemical area to the projected area of the Au/quartz electrode (Le., roughnessfactor), determinedaccording to a recommended method,49JO is equal to 2.3. The films were observed to adhere much better to the non-polished surface of the Au/quartz crystal than to the mirror-finished one. The EQCM frequency change was measured with a Philips PM6680 high-resolution programmable timer/counter (J. Fluke Mfg. Co., Inc., Everett, WA) with an analog output option which was set at a 10-ms sampling time. CV and frequency change versus potential curves were recorded simultaneously with a H P 7090A measurement plotting system (Hewlett Packard Co., Avondale, PA). The high-frequency signal waveform was also monitored with a 100-MHz Model 2230 digital storage oscilloscope (Tektronix, Beaverton, OR). All electrochemical experiments were performed inside a TS4000 inert gas atmosphere drybox (Vacuum Atmospheres Co., Hawthorne, CA) at ambient (22 f 1 "C) temperature. Preparation of C,& and C& Solutions. Acetonitrile solutions of C6O2- were prepared by bulk controlled-potential electroreduction at -1.20 V of several milligrams of a c 6 0 suspension in ca. 10 mL of 0.1 M CSASF6,o.l M KPF6, or 0.03 M Ca(PF6)2 while stirring vigorously. The electroreduction current decayed to zero after two electrons were transferred per one Cm molecule. At this point the suspension was completelydissolved and a purplered solution of c602- was obtained. In a similar manner, brown solutions of Cm3- were prepared in 0.1 M (TBA)C104 at -1.60 V after three electrons were transferred per one Cm molecule.

Results and Discussion The properties of c 6 0 and its salts in films electrodeposited from acetonitrile solutions containing KPF6, CsAsF6, Ca(PF&, and (TBA)C104 are described and compared in the following sections. C& Films in Acetonitrile Containing KPF6. Figure 1displays simultaneous CV and EQCM responses of c602- in a 0.1 M KPF6 acetonitrile solution recorded for various potential ranges. If the positive scan reversal is set at -0.72 V, Le., at the value more negative of the CmO/Cm- process, then neither the c6O2-/C60electrooxidationnor Cao2-/Cm3-electroreduction at El/2equal to -0.96 and -1.45 V, respectively (peaks 2,2', and 3,3' in Figure la, cycle I), involve frequency (Le., mass) changes. However, if the positive potential scan reversal is extended positively to-0.40 V, then the cm-/c60° electrooxidation (peak 1) accompanied by a rapid decrease in frequency (i.e., a rapid mass increase) is observed (Figure la, curve 11). Under these conditions a CmO film is deposited. Also, a large electroreduction stripping peak 2" of half-width equal to A E 1 / 2 = 160 mV is seen during the subsequent negative potential scan. The potential of peak 2" is located at E, = -1 .OOV in Figure 1 a and shifts negatively with the increase of the thickness of the Cmo film. The integrated charge of peak 2" (0.53 mC) is more than twice as large as the integrated charge of peak 1 (ca. 0.25 mC), suggesting that the two-electron c6OoI2- electroreduction takes place. This electroreduction is accompanied by a mass drop to its initial value which indicates that the C60° film is completely dissolved and

Koh et al.

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Figure 1. Simultaneous cyclic voltammetry and microgravimetry with EQCM at a Au/quartz electrode of cu. 0.90 mM CmZ- in 0.1 M KPFs, acetonitrile at potential scan ratcs of (a) 0.05, (b) 0.02, and (c) 0.02 V s-l, where the negative potential scanning was stopped for 30 s at -0.78

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that C&is again formedin solution. Wedefine here the potential at which the mass of the film is decreased by half during electrodissolutionunder CV conditions as an "electrodissolution potential". The value of this potential depends on several factors discussed below and is equal to -0.95 V for curve I1 in Figure la. To examine the nature of peak 2" in more detail, we electrodeposited a thin CmO film by setting the positive scan reversal at -0.66 V. Two adsorption-type spikes 1' and 1"are seen during the negative potential scan (Figure lb), and in this respect the resulting CV curve resembles the one obtained for a solution-cast Cmofilm in acetonitrile containing K+ cations.13In Figure lb, the spikes are located at E, equal to -0.85 and -0.88 V, i.e., at the foot of a semiinfinite diffusion-controlled peak 2'. The electrodissolution potential is equal to -0.85 V for the C"J film and coincides with the potential of spike 1'. Contrary to results in TBA+ solutions (see below), there is no mass increase during the negative potential excursion from -0.66 to -0.9 V and thus no indication that the film is electrodoped with K+ cations from solution. Interestingly, spike 1 'vanishes and the current of spike 1 "more than doubles during the negative scan (Figure IC) if the potential is held for 30 s at -0.78 V (Le., a value positive of spike 1'). In addition, there are no mass changes while the potential is held at -0.76 V but the electrodissolution potential shifts negatively to -0.88 V (Le., to the E, value of spike 1'9, when the negative scan is resumed. This may indicate a rearrangement of the film as the potential is held, as was suggested earlier for fullerene films containing TBA+.13J4 The solution-film electrochemical equilibria in this solution are summarized in Scheme I, where s and f represent "solution" and "film" species, respectively. Two forms of the nonreduced film, labeled as C m o and ~ Cuo,e,as well as at least three soluble CmO"anions can be obtained in a 0.1 M KPF6 acetonitrile solution. C&Films in Acetonitrile Containing CsAsF6. The C&/C,and C60-/Cmo CV electrooxidations in acetonitrile solutions of Cs+ containing Cm2- (Figure 2, peaks 1 and 2, respectively) resemble those observed for K+ solutions of C&- (Figure la, curve 11), and also those for a Cmo solution-cast film in a Cs+ acetonitrile solution.13 However,the Cmz-/C&electroreduction in a Cs+ solution of Cm2- is markedly different (cf. peaks 3 and 3'in Figure l a and 2). Figure2 displays the CV-EQCM responses of ca. 1.30 mM Cm2- in 0.1 M CsAsF6 at a scan rate of 0.1 V s-l. The C602-/Cm-electrooxidation (peak 2) involves no change

Electrosynthesis and Electrodoping of C&

The Journal of Physical Chemistry, Vol. 97, No. 26, 1993 6873

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in mass, but the Cm-/Cmo one is accompanied by a rapid increase in mass due to electrodepositionof a Cmofilm. Similarly as was the case for the K+ solution, a large surface-type electroreduction peak 2'of AElp = 105 mV is observed at E, = -1.00 V during a subsequent negative scan. Also, as for the K+ solution, peak 2' is accompanied by a drop in mass to its initial value. Interestingly, the electrode mass does not remain constant upon further electroreduction of C&- to C& and scanning the potential to more negativevalues results in a rapid mass increase associated with a surface-type electroreduction peak 3'. This process is reversible and a subsequent positive potential scan leads to a surface-type electrooxidation peak 3 which is accompanied by an abrupt mass loss to its initial value. In this way a "butterfly" type curve of the mass change versus potential is obtained (Figure 2). For 100% electrodepositionefficiency and a sufficiently elastic film, the molecular mass and stoichiometry of a deposited compound can be determinedfrom the film mass-to-charge ratio.4 The efficiency for the C&-/Cm3- electrodepositionwas tested in a bulk electrolysis cell. For this purpose, a Cm suspension was first dissolved by electrolysisat -1.20 V which resulted in a purplered Cmz- homogeneous solution when the electrolysis was completed. This color began to fade, and a black solid started to deposit on the platinum gauze working electrode when the electroreduction potential was switched to-1.60 V, Le., to a value at which Cmz- is electroreduced to C&. The solution became colorless when one electron per each C& anion was transferred, indicating complete electrodeposition. The ratio of the total mass increase due to Cas- film electrodeposition(4.18 pg) to the correspondingcharge transferred (0.36 mC), calculated by current integration of peak 3% Figure 2, is 1150 g/mol of electrons. This ratio agrees well with the value of 1119 g/mol of electrons calculated for a (Cs+),(C&) film electrodeposited from the C& solution. The same mass change, but of opposite sign, is observed during the subsequent positive potential scan (peak 3), indicating that the (Cs+),(C,&) film completely electrodissolves as C&- is again produced in solution. Current-time transient characteristics of film electrodeposition have been used in the past to provide qualitative information on the formation of conductive nuclei51-54 and a similar approach wasused in the present study. Figure 3 displaysthe simultaneous current (solid line) and mass (dashed line) changesobtained during electroreduction of a C,&- solution for 2.5 min at -1.60 V as the (Cs+),(C&) film is formed. After an initial, ca. 10-s, abrupt decay, the current increases slowly with time while the mass increaseaccelerates slightly. Such a chronoamperometriccurrent

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Potential,V vs. SCE Figure 4. Simultaneous cyclic voltammetry and microgravimetry with EQCM at a Au/quartz electrode of cu. 0.30 mM Cw2-in 0.1 M CsAsF6, acetonitrile at potential scan rate of 0.05 V s-l (a) without and (b, c) with negative potential scanning being stopped for 30 s at -0.85 and -0.88 V, respectively. increase cannot be rationalized by Cottrell linear diffusion to an electrode of a time-invariant area where a current decay with r O . 5 is predicted. However, the observed current increase can be explained in terms of the nonuniform growth of a conductive film on the Au/quartz electrode. Accordingly, given the conductive nature of the electrodeposited (Cs+)3(Cm3-) film, the electrochemically active electrode area should increase, leading to a current increase. Note, no such current increase is observed in the case of electrodeposition of a nonconductive Cmo film (see below). The nature of peak 2'in the presence of Cs+ was examined in more detail for a thin CmO film. For that purpose, the positive potential scan reversal was set at -0.60 V (Figure 4), Le., at a more negative value than that in the experiment illustrated in Figure 2. Under such conditions, only one surface-type peak I ' at E, = 0.90 V is observed (Figure 4a) which is in contrast to the results in solutions of K+ (cf. Figure lb). Peak I 'is located on the rising portion of peak 2'which, for the thin Cmo film, is controlled by bulkdiffusion (Figure 4a). Peak I 'alsocorresponds to a complete electrodissolution of the Cmofilm, as evidenced by an abrupt drop in mass to its initial value. Thus, the C"J film is not electrodoped with Cs+ cations from solution, as is also the case for K+solutions(seeabove). Theelectrodissolutionpotential of the CmO film in the Cs+ solution is -0.90 V (Figure 4a) and shifts to -0.96 V if the negative potential scan is stopped for 30 s at -0.85 V, i.e., a potential at the foot of the electrodissolution peak I '(Figure 4b). Unlike results for solutions of K+,this shift of the electrodissolution potential in the presence of Cs+ is accompanied by a mass decrease, which is indicative of the film electrodissolution. Also, peak 2' which was controlled by a

Koh et al.

6874 The Journal of Physical Chemistry, Vol. 97, No. 26, 1993

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semiinfinite diffusion in Figure 4a, becomes controlled by adsorption in Figure 4b,c. Moreover, the Cmofilm could not be completely dissolved, even if the potential was held for 30 s at -0.88 V, Le., a potential which is closer to E, of peak 1 ’ (cf. Figure 4b,c). Instead, the mass decreased to a constant value and the electrodissolution potential of the remaining film was shifted even more negatively, Le., to -0.98 V (Figure 4c). However, no shift of the electrodissolutionpotential was observed when the potential was held at a value positive of peak I ’ (not shown). This may indicate some transformation of the film when the potential is held in a range where peak I ’occurs. In summary, three types of films can be obtained in acetonitrile solutionscontaining CsAsF6. Two of these i.e., C m oand ~ Cmo,g, are of a nonreduced fullerene and the third one is of a triply reduced, (cs+)J(c603-) fulleride. At least two soluble fullerides in different oxidation states are also obtained in this solution. The overall solution-film electrochemical equilibria are summarized in Scheme 11, which differs from Scheme I only for the most electroreduced, Cm3-, species. C#, Films in Acetonitrile Containing Ca(PF6)2. The CVEQCM behavior of Ca2- in 0.03 M Ca(PF6)z (Figure 5) is generally similar to what is observed in solutions of K+ (Figure 1). The Cm-/C& and C&/Cm- electrochemicalprocesses occur in solution at E112 equal to -0.95 and -1.45 V, respectively. Similarly as described above for K+(cf. Figure l), neither process involves a mass change if the positive potential scan reversal is set at a sufficiently negative value, such that a Cmo film is not electrogenerated (not shown). However, the Cm-/C”J electrooxidation (peak I) is accompanied by a rapid mass increase upon the positive scan initiated at -1.2 V if the reversal potential

is set at 0.0 V, due to electrodepositionof a thick Cwofilm (Figure 5a). This film is completely dissolved during a subsequentnegative potential scan from0 to-1.20 V. The electrodissolution potential of the c60° film is equal to-1.02 V, a value which almost coincides with the potential of a large electroreduction stripping peak 2’ of AE1p,= 140 mV and E, = -1 .OO V. If the positive potential reversal is set at -0.55 V (i.e., less positively than in the experiment illustrated in Figure sa) in order to prepare a thin, rather than thick, CaO film then only one adsorption type peak 1’is observed at E, = -0.84 V, Le., on the rising portion of peak 2’(Figure 5b). This behavior is in contrast to what is observed for the K+ solution (cf. Figure lb) and is similar to that for the Cs+ solution (cf. Figure 4a). Also, as described above for the K+ and Cs+systems, peak 2’ remains under semiinfinite diffusion control if the film is made sufficiently thin. For such a thin film the electrodissolution potential is -0.84 V, a value coinciding with E,of peak I ’. The mass of the film in the Ca2+solutionincreasescontlnuously in the absence of current flow when the negative potential scan is stopped for 40 s at -0.68 V (Figure 5c). This behavior also differs from that of the film in K+ or Cs+ solutions (cf. Figures I C and 4b). The increase of mass by 0.161 pg (34%) of the film grown in the CaZ+ solution is much larger than the 0.014 pg (3%) increase which would be expected for electrodopingof a CmO film with Ca2+ cations from solution if a (Ca2+)(C& film were formed. Therefore, this mass increase is ascribed to the direct electrodepositionof a (Caz+)(Cm-)z film generated from Cm- in the solution. Theelectrodissolutionpotential of the (Ca2+)(Cm-)2 film is shifted negatively by ca. 60 mV as compared to the electrodissolutionpotential observed when the negative potential scanning is not interrupted. The film could not be completely dissolved when the potential was held at -0.79 V, i.e., at the foot of peak I’, as is the case in solution of Cs+ (Figure 4c). The remaining (Ca2+)(Cm-)2film could be dissolved only when the electrodissolution potential of -0.90 V is attained (Figure 5d) which is more negative than that obtained upon noninterrupted potential scanning. This behavior is similar to that of the C”J film electrodeposited from a C&- solution containing Cs+ (cf. Figure 4c). The solution-film electrochemical equilibria in the Ca2+ acetonitrile system are summarized in Scheme 111. Three fullerene solution species, Le., C60-(s), C6O2-(s), and C,$-(s) (Scheme 111, top row), and two film species can be obtained. The latter are Cmo(f)and presumably also (Caz+)(Ca-)~(f)(Scheme 111, bottom row). C#, Films in Acetonitrile Containing (TBA)CIO,. The CaO” films electrodepositedfrom a C& acetonitrilesolution containing (TBA)C104 were investigated in two different ways. In one, the current and frequency change versus potential curves were simultaneously measured as the films were formed under CV conditions, while in the other, simultaneous chronoamperometry (chronocoulometry) and frequency change measurements were performed as the films were electrodeposited by controlledpotential electrooxidation of the (2,503- anions. Figure 6 shows the simultaneous CV and frequency change responses for electrooxidation and subsequent electroreduction of ca. 1.05 mM Cm3- in 0.1 M (TBA)C104, acetonitrile. The effect of the positive potential scan reversal, scan rate, and holding time and/or potential on the film behavior is illustrated in this figure where the scan is always initiated at -1.6 V. A complete CV cycle in a wide, -0.30 to -1.9 V, potential range is presented in Figure 6a. Four redox processes are seen at a scan rate of 0.02 V s-1. At such a low scan rate, the frequency does not change during the Cm3-/C,& electrooxidatioin (peak 3), but this is not

The Journal of Physical Chemistry, Vol. 97, No. 26, I993 6875

Electrosynthesis and Electrodoping of c60na

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scanning.

the case for the C&-/C,electrooxidation (peak 2) which is accompanied by a mass increase due to the electrodeposition of a (TBA+)(C60-) film (Figure 6a). An even more rapid mass increase is observed during the c60-/c@)0 electrooxidation (peak I), indicating a faster electrodeposition of the C60° film than the (TBA+)(Cm-) one. The electroreduction peaks I ’and 2’, which are obtained upon subsequent negative reversing potential scan, are both accompanied by mass increase. Hence, unlike results for the c60° film in solutions of K+, Cs+,or Ca+ described above, these mass increases can be interpreted mostly in terms of a stepwise electrodoping of the Cmo film with TBA+ cations from solution as the (TBA+)(Cao-) and (TBA+)2(Cm2-)salts are formed in the film.13J4J7J8However, the observed mass increases might also be attributed, in part, to deposition of the same salts from the Ca- and C6o2- anions generated in solution (see below). A large adsorption type electroreduction peak 3” in Figure 6a is associated with a drop in mass to its initial value, thus indicating that the film was completely electrodissolved and that Cao3-was again formed in solution. The latter is reduced in turn to a soluble C& anion upon further negative potential scanning, so that the solution Ca3-/C& couple at Ell2 = -1.78 V25 (peaks 4 and 49 is not accompanied by any mass changes of the electrode. Neither the CV nor the frequency responses indicate electrodeposition of a (TBA+)z(C&-) film at sufficiently high potential scan rates (20.1 V s-1) and only the solution C602-/Cs03and c&)-/c602couples are observed at Ell2 of -1.32 and -0.83 V, respectively25(Figure 6b). However,at lower scan rates (50.02 V s-l) a small mass increase is detected during electrooxidation of C,& to C&- (peak 3) and a mass drop to its initial value is also observed at a electrodissolution potential of ca. -1.45 V (peak 33 during a subsequent negative potential excursion (Figure 6c). This behavior indicates that although most of the C602- electrooxidation product still remains in solution in the electrode vicinity, a (TBA+)2(CaZ-)film is actually electrodeposited during

the positive potential scan. Moreover, this film completely dissolves during a subsequent Ca2-/Cm3- electroreduction. To examine the electroreductive surface behavior of the (TBA+)(Ca-) and (TBA+)2(C602-) films in more detail, the CVEQCM investigations were performed at a higher scan rate, 0.1 V s-l, and also at the positive scan reversal extended to -0.75 V, Le., to a more positive potential than that in the experiments illustrated in Figure 6b,c but less negative than that in Figure 6a. Thus, only a thin ( T B ~ I + ) ( C ~ film, ~ - ) free of was electrodeposited. The relevant results are shown in Figure 6d-f. Figure 6d shows the CV-EQCM behavior without holding the potential, while Figure 6e,f shows the behavior with the potential being held at different values during the negative potential scanning. A small mass increase accompanying the c602-/c@electrooxidation (peak 2) indicates electrodeposition of a thin (TBA+)(C60-) film. However, most of the electrogenerated Castill remains in solution in the electrode vicinity, as evidenced by presence of the semiinfinite diffusion controlled electroreduction peak 2’. Adsorption type electroreduction peaks 2” and 3” are observed at E, equal to -1 .OO and -1.45 V, respectively (Figure 6d-f). These peaks are superimposed on semiinfinite diffusion controlled peaks 2’and 3’, respectively. A negative shift of the adsorption peaks with respect to diffusion peaks, independent of the film thickness, can be at least partially accounted for in terms of Donnan potential equilibria.sss6 The c 6 0 concentration in a film is close to 2.2 M (see below). Hence, theTBA+ concentration in the reduced film is the same and it is thus much larger than that of 0.1 M in solution. As the first approximation, the Donnan potential drops by 59 mV per 10-fold increase of the solutionto-film concentration ratio for mobile cations, as verified, for instance, for films of anionic Prussian blueeS7Therefore, the observed ca. -100 and -80 mV shifts in peak potentials for (TBA+)(cso-) and (TBA+)2(C6~2-)films, are qualitatively in accord with the values of Donnan potentials which are estimated as -79 and -97 mV, respectively. Interestingly, there is no mass change of the (TBA+)(Ca-) film when the potential is held for 10 s at -0.92 V, Le., at a value more positive than the E, of peak 2”(Figure 6e). However, the mass increases rapidly if the potential is held for 10 s at -1.06 V, Le., at a value more negative than E , of peak 2”(Figure 6 0 . In both these cases, (2602- is generated in the electrode vicinity as the potential is held. However, a soluble C6o2-anion is obtained at -0.92 V (Figure 6e), while a (TBA+)2(C602-)film is formed at -1.06 V (Figure 6f). Hence, an initial electrodoping of the (TBA+)(C,-) film during electroreduction with TBA+ cations from solution is crucial for the (TBA+)2(C602-)film formation. Moreover, direct electrodeposition of a (TBA+)2(Cw2-)film on a bare electrode is much slower at the same potential scan rate, as is apparent from comparison of the mass changes at a bare electrode (Figure 6b) and at a thin (TBA+)(C,o-) film coated electrode (Figure 6d) during negative potential scans from -1.10 to -1.45 V. Chronoamperometry and chronocoulometry combined with microgravimetry of film electrodeposition provide useful information on the conductivity of the film and on the stoichiometry of a deposited c~mpound.~OFigure 7a displays simultaneous charge and frequency transients for film electrodeposition from 0.3 mM Cao3-in 0.1 M (TBA)ClOI at 0.0 V. The current decays (Figure 7a, curve I) with rO.5 for t C 30 s, indicating Cottrell linear diffusion control, and this changes to a r0.25dependence for 1 > 30 s which may suggest the onset of deposition of a conducting film (see below). The frequency change (Figure 7a, curve 2) accelerates with time while the charge increase (Figure 7a, curve 3) decelerates. Hence, the slope of the Afvs Q curve (Figure 7b) increases from an initial value of 2.40 to a final value of 4.90 mg C-I. The slope of the initial linear part of the curve in Figure 7b corresponds to 695 g/3 mol of electrons transferred and is close to the calculated value (dashed line) of 721 g/3 mol

6876 The Journal of Physical Chemistry, Vol, 97, No. 26, 1993

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Koh et al.

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of electrons expected for electrodeposition of a c60°film from a CsO3-anion in solution. If one assumes a face-centered cubic (fcc) packing of the c60° film with the center-to-center distance between neighboring c60 pseudospheresequal to ca. 1.O nm,58959then a monolayer coverage is obtained for ca. 1.9 X 10-10 mol cm-2. This corresponds to ca. 158 ng cm-2 of Caoo/nm of the film thickness. The estimated film density would be ca. 1.6 g ~ m - which ~ , corresponds to the concentration of c60° in the film of ca. 2.2 M. The Afvs Q curve in Figure 7b deviates from linearity at the point where 4.4 pg of c60° is electrodeposited. This mass corresponds to ca. 43 equivalent monolayers of a ca. 37-nm-thick c60° film, as estimated for the electrode roughness factor of 2.3. The slope of the final part of the curve in Figure 7b corresponds to 934 g/2 mol of electrons and is in good agreement with the calculated value of 963 g (dashed line)/2 mol of electrons for electrodeposition of a (TBA+)(C60-) film from a C6o3- solution. The data in Figure 7 are self-consistent and suggest that the initial electrodepositionof a c60°film is followed by electrodeposition of an overlaying conductive (TBA+)(C60-) film.13 Such a change in film composition during controlled-potential electrodeposition can be readily explained in terms of an increase of the electrode resistance due to the insulating c60° film growth, as schematically illustrated in Figure 8. Solid C6O0 is nonconductive,13 but a thin c60° film should not contribute much to the overall electrode resistance, and therefore the Galvani potential drop across the film-solution interface, A(PFS, should be almost equal to the externally applied potential difference (Figure 8a). However, the resistance increases with increase of the c60°film thickness and as this occurs the potential drop across the film, iR,also increases (i is current density and R the resistance of the film per unit area). Thus, A(PFS should decreasein the courseof electrolysis,as the film grows. Eventually A(PFS,which is responsible for the oxidation state of a given

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Figure 8. Schematic representation of potential distribution across electrode-film-solutioninterfaces during film electrodepositionat 0.0 V from acetonitrilecontaining Cas and (TBA)C104: (a) initial film growth, (b) final film growth at high current density, and (c) final film growth at low current density; represents film-solution potential difference and iR is ohmic potential loss.

anion in the film, should become too small for oxidation of c603to c60°to occur and, at that point, a (TBA+)(C60-) film should start to grow on top of the Cmofilm (Figure 8b). Obviously, the onset of (TBA+)(C60-) film electrodepositioncan be delayed and much thicker pure C6o0 films should be obtained (Figure 8c) if current is kept low. This could easily be accomplished,for instance at low CSo3-concentration and/or with the use of a small area electrode. In contrast, in highly concentrated c603- solutions, the initial mass-to-charge ratio is larger than the value expected for the electrodepositionof a Cmofilm. Hence, the resulting film must contain foreign species, e.g., supporting electrolyte and/or solvent molecules. Such a film is labeled here as a “rapidlygrown” c60°film while the one which is free of foreign species, Le., the film prepared from a dilute C603- solution, is represented as a “slowly grown” c60°film. Noticeably, such a potential and/or C603-concentration control of the initial film composition is not possible with the solution-cast technique of film preparation. Nucleation of Cmo crystallites most likely occurs at isolated regions of a rough Au/quartz electrode surface since the C a o film is relatively porous.13 This implies that the nucleation is more energetically demanding than the crystal growth. At least two mechanisms of this growth are conceivable: (i) the Cao3anions diffuse from the bulk solution to the electrode surface where the charge is exchanged and followed by surface diffusion of the c60° molecules to nucleation sites, or (ii) the Cao3-/0 electrooxidatioin predominantly occurs at nucleation sites. Resolution between these two should bear further scrutiny. Both “rapidly grown” and “slowly grown” films were investigated in a “transfer experiment” in which the films were first electrodeposited from a 0.1 M (TBA)C104 acetonitrile solution containing C603-,rinsed with acetonitrile, and then the film coated

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The Journal of Physical Chemistry, Vol. 97, No. 26, 1993 6877

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electrode transferred to a blank 0.1 M (TBA)C104 acetonitrile solution for further multicycle CV-EQCM investigations. These results are shown in Figures 9 and 10 for “slowly grown” and “rapidly grown” films, respectively. The negative potential scan reversal was set at -1.20 V in the transfer experiments. This potential value is too positive to form a soluble C d - anion and thus electrodissolve the film (cf. Figure 6a). Cyclic voltammograms of the first two cycles were different from the subsequent ones. Coincidently, the CV behavior of both “rapidly grown” and “slowly grown”films is qualitativelysimilar to that of solutioncast films.13 The first three consecutive CV-EQCM responses for a “slowly grown” C”J film are shown in Figure 9. The film was electrodeposited from ca. 0.09 mM Cas, 0.1 M (TBA)C104 for 6.5 min at 0.0V. The total charge passed during electrodeposition was 4.13 mC and the mass of the film was 10.6 mg which corresponds to 103 equivalent monolayers of a 89-nm-thick C a o film. A straight Q vs Af line was obtained for this electrodeposition (not shown) which indicates that the C a o film is sufficiently elastic. The slope of the line was 2.57 mg C-l and close to the 2.50 mg C-1 value expected for a three-electron c6O3-/C6O0 electrooxidation,thus indicating that the film is free of entrapped foreign species. Noticeably, the described electrodeposition technique of film preparation enables one to prepare fullerene films practically free of entrapped solution. This may not occur if the solution-cast technique of film preparation is used13J4since solvated C& molecules are found in the solid.60 The first CV cycle for a “slowly grown” film initiated at 0.0 V (Figure 9a) shows only one broad electroreduction peak at E, = -1.09 V during the negative potential scan. Such a peak is typical for filmsof high resistance. A total electroreduction charge of 1.6 mC was determined by current integration of this peak. This charge is only 57% of the total charge expected for a twoelectron electroreduction of CaO to C&, as estimated from the mass of the initial Cmofilm. However, there is also a small current

step between -0.80 and -0.97 V, Le., at the foot of the broad electroreduction peak (Figure 9a). This step is associated with a small mass decrease and is presumably due to a partial loss of electrogenerated Ca- anions to solution. The electroreduction peak at E, = -1.09 V is accompanied by an abrupt 4.1 5-mg mass increase, which has been interpreted in terms of electrodopingof the film with TBA+ cations from solution.l6J* If the small mass decrease associated with the current step at the foot of the electroreduction peak were taken into account, then this rapid mass increase would correspond to an ingress of 2.0 TBA+ countercations per each C&- anion in the film, as estimated from the total charge transferred. Note, there is a small mass decrease of 0.30 pg between -1.13 and -1.17 V, i.e., at the tail of the electroreduction peak. This decrease was earlier assigned to a slight electrodissolutionof the (TBA+)2(Ca2-) film,13which is more soluble in acetonitrile than the (TBA+)(Ca-) one. The mass increases slightly between -1.17 and -1.20 V during both the initial negative and subsequent positive scan, suggestingfurther electrodoping of the film with TBA+ cations from the solution. However, this small mass increase is seen only in the first cycle. Three electrooxidation peaks are observed at Ep equal to -0.22, -0.55, and -0.63 V on the first cycle during a positive potential scan and each peak is accompanied by a smallstep of mass decrease (Figure 9a). The total mass decrease during this scan is much smaller than the total mass increase during the preceding negative potential scan. The net mass gain in the first cycle is 3.61 pg and amounts to ca. 34% of the initial film mass. This mass gain can be due to at least two effects. One is that the (TBA+)2(Cw2-) film is incompletely reoxidized while the other that the TBA+ countercations are retained in the film after the film is reoxidized to its CaO form. An incomplete electrooxidation of the (TBA+)z(C&) film seems unlikely because the ratio of the total electrooxidation-to-electroreductioncharge is 0.92 on the first cycle. Moreover,’tbetotal electroreduction on the first cycle (1.5 mC) is almost the same as that for electroreduction on the second cycle (1.6 mC). This suggests that the TBA+ cations are most likely retained in the film, thus implying that perchlorate anions from solution must enter the film for charge compensation. Such an entrapment of the ionic charge by a fullerene film has been postulated earlier and confirmed by mass spectrometry studies of an electroreduced and subsequently electrooxidized solutioncast film.139’6 This entrapment may explain why the CV-EQCM responses during the first cycle are different from those obtained on subsequent cycles (Figure 9). The entrapped ions would decrease the film resistance and hence decrease the iR potential loss. Under these conditions, cyclic voltammograms would become much better developed on the second and subsequent cycles.4* The second cycle (Figure 9b) shows three electroreduction peaks at E, equal to -0.83, -0.98, and -1.05 V during a negative potential scan. The peak at E, = -0.83 V is accompanied by a 1.92-pgmass increasewhich is followed by a small mass decrease at slightly more negative potentials. The two peaks at Epcequal to -0.98 and -1.05 V are both coupled to small mass increases. The mass of the film decreases by 1.11 pg at the tail of the third peak. The latter may be due to a loss of perchlorate and/or C& anions to solution. Two major electrooxidation peaks are observed at E p equal to-0.22 and-0.68 V during a positive potential scan of thesecond cycle and each of them is accompanied by a small mass loss. The mass of the film increases by only 0.25 pg after the second cycle, and this value amounts to ca. 1.8% of the actual film mass. Subsequent cycles show two major electroreduction peaks at E, equal to -0.84 and -1.05 V and two electrooxidation peaks at E, equal to -0.22 and -0.67 V. The accompanying mass changes are similar to those observed on the second cycle except that the mass increase corresponding to the first electroreduction peak is slightly larger than on the second cycle. There is virtually

6878 The Journal of Physical Chemistry, Vol. 97, No. 26, 1993

Koh et al.

SCHEME IV

no further net mass increase in the third and subsequent cycles. Electrooxidation peak currents and widths can be used for rough estimation of the lower limit of apparent redox conductivity of the film,61,62 which exceeds 6 X and 8 X le3Ql- cm-I for the Cao/C,- and C60-/C60Z- couple, respectively. A “rapidly grown” Cm0film was electrodeposited from ca. 0.3 mM C603-in 0.1 M (TBA+)C104for 3.0 min at 0.0 V. The mass of the film was 12.0 mg when the charge of 3.5 1 mC was passed. The initial mass-to-chargeratioduring electrodepositionwas 3.51 mg C-l. This value is larger than the one expected for electrodeposition of a pure C”J film. The ratio increased with time (not shown) and eventually reached 8.28 mg C-1 when electrodeposition was interrupted. This indicates that foreign species are entrapped in the film. After rinsing with acetonitrile, the filmcoatedelectrodewas transferred toa blank0.l M (TBA+)C104solution and the first three consecutiveCV-EQCM responses recorded are shown in Figure 10. The “rapidly grown” film voltammogram of the first cycle (Figure loa) resembles that for the “slowly grown” one (Figure sa), Le., there is one broad electroreduction peak at Epc= -1.13 V,and three electrooxidation peaks at E, equal to -0.61, -0.50, and-0.22 V (Figure loa). However,thecorresponding frequency responses are markedly different. The electroreduction peak at E, = -1.13 V is accompanied by two steps of mass losses equal to 2.41 and 1.77 mg (Figure loa). This may indicate that the charge is counterbalanced by the release of perchlorate anions into solution during electroreduction of the Cmofilm to Ca- and then c60- to C6O2- in the film and/or by some dissolution of the C60- or c602- film. Contrary to results for the “slowly grown” c60° film, the total mass of the “rapidly grown” film decreases by 4.53 pg in the first cycle, Le., by 38%. Small net mass losses are also observed on subsequent cycles, suggesting further slight dissolution of the film. The solution-film electrochemical processes in acetonitrile containingTBA+,summarizedin SchemeIV, involve four solution and three film fullerenespecies. Processes involving the C,-/C& and C,z-/C603- couples are controlled by diffusion from the bulk solution both at high potential scan rates and at low concentrations of the C603- anion in solution. These solution electrochemical equilibria are presented in the upper line of Scheme IV. Films of C6O0, (TBA+)(C,-), and (TBA+)z(C&-) can all be readily electrodeposited from a Cas- solution. The composition of these films is governed by both the Cm3- concentration in solution and by the electrooxidation potential (Figure 7). The film formation processes are represented in the scheme by arrows from the upper to the lower line. The CsoO film is easily electrodoped with TBA+ cations from solution and the (TBA+)(c60-) and (TBA+)z(C&) films can be generated at suitably chosen potentials. Both these films can be reoxidized to a C& film even though the supporting electrolyte ions are retained in the film after an electroreduction-reoxidation cycle. Electrochemical equilibria involving film electrodoping/undoping with TBA+ cations from solution are presented in the lower line of Scheme IV. The films are completely electrodissolved upon electroreduction to the soluble C603-form. Conclusions Solid Cmo is insoluble in acetonitrile. However, the C&-anion is appreciably soluble in acetonitrile solutions containing K+,

Cs+, or CaZ+cations as is the C& anion in the presence of TBA+, K+, or Caz+ cations. Thus, solutions of C& and C& , anions can be readily prepared by bulk electroreduction of a C a 0 suspension. The fullerene films can be conveniently electrodeposited from these solutionsand the electrodepositiontechniques of film preparation explored in the present study compare favorably with the solution-cast techniques in that the film thickness and the composition can be easily controlled with the suitably chosen applied potential and/or concentrationof a soluble C& anion. Simultaneous measurements of the mass, current, and charge transferred during the film electrodeposition allows one to determine the stoichiometry and conductive properties of the films and also to detect possible solution and/or electrolyte entrapment. The CdoOfilms electrodeposited from Cm2-acetonitrilesolutions containing Cs+, K+, or Ca2+ cations are nonconductive. The reason why (M+)(C,g) type solids could not be obtained by electrodoping of these Cwo films with any of the investigated cations could be that such solid compounds might be unstable toward disproportionation.63 Attempts to electrodeposit (C&) or (Ca2+)3(C&)2 wereunsuccessful,but electrodeposition of a (Ca2+)(C,-)z film was accomplished. A conductive (Cs+)3(C&) film was quantitatively electrosynthesized by electroreduction of C&- in a Cs+ solution. Recently, it was reported that the CssC~osolid, prepared by vapor doping, is bodycentered cubic (&),@ as opposed to the K3Cm one, which is face-centered cubic (fix).This structural difference might account for lower solubility in acetonitrile of the former and be the reason why the Cs3Cm solid could be prepared electrochemically. Contrary to the tedious vapor doping procedure,@the electrochemical preparation of C s 3 C ~ irelatively s fast and simple. The electrodepositionrate of the films from the C&- solution containing TBA+ cations increases in the order (TBA+)2(Cw2-) < (TBA+)(C,-) < Cmo,which is opposite to the solubility order of these compounds. The Cwofilm electrodepositedfrom a Ca3solution containing TBA+ cations can be electrodoped with these cations, and (TBA+)(C,-) and (TBA+)z(C&) filmsare produced at suitable potentials. Structural rearrangement must accompany this electrodoping to accommodate the TBA+ cation ingress and egress.13J4 The Cmofilm electrodeposited by controlled-potential electroreduction from a TBA+ solution undergoes an irreversible structural transformation during the first electroreductionoxidation CV cycle during which the electrodoping TBA+ cation is retained in the film, increasing its conductivity. At potentials sufficientlyfar from the electroreduction or electrooxidationpeaks of the Cmo/Cm- or Ca-/C& couples, the (TBA+)(C,-) and (TBA+)z(C&) films display ionic type conductivity and in the vicinity of peak potentials, where films of mixed oxidation states are formed, the conductivities reach maxima as the redox conductivity manifests itself. Acknowledgment. Financial support from the Robert Welch Foundation (Grants No. E-680 (K.M.K.) and E-1208 (M.T.J.)) is gratefully acknowledged. K.M.K. also acknowledges support from the President’s Research Fund (PREF) at the University of Houston.

References and Notes (1) Xie, Q.;Ptrcz-Cordero, E.;Echegoyen, L.J. Am. Chem. Soc. 1992,

I 14,397a-39ao.

Electrosynthesis and Electrodoping of Cab

The Journal of Physical Chemistry, Vol. 97, No. 26, 1993 6879

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