Mechanism of Reductive C60 Electropolymerization in the Presence

Pieta , P.; Grodzka , E.; Winkler , K.; Warczak , M.; Sadkowski , A.; Zukowska , G. Z.; Venukadasula , G. M.; D'Souza , F.; Kutner , W. J. Phys. Chem...
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Mechanism of Reductive C60 Electropolymerization in the Presence of Dioxygen and Application of the Resulting Fullerene Polymer for Preparation of a Conducting Composite with Single-Wall Carbon Nanotubes Piotr Pieta,† Grazyna Z. Zukowska,§ Sushanta K. Das,‡ Francis D’Souza,*,‡ Andreas Petr,| Lothar Dunsch,*,| and Wlodzimierz Kutner*,†,⊥ Department of Physical Chemistry of Supramolecular Complexes, Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland, Department of Chemistry, Wichita State UniVersity, 1845 Fairmount, Wichita, Kansas 67260, Faculty of Chemistry, Warsaw UniVersity of Technology, Noakowskiego 3, 00-664 Warszawa, Poland, Department of Electrochemistry and Conducting Polymers, Leibniz-Institute of Solid State and Materials Research, Helmholtzstr. 20, D-01069 Dresden, Germany, and Faculty of Mathematics and Natural Sciences, School of Science, Cardinal Stefan Wyszynski UniVersity in Warsaw, Dewajtis 5, 01-815 Warsaw, Poland ReceiVed: December 21, 2009; ReVised Manuscript ReceiVed: March 5, 2010

The superoxide anion radical, O2•-, induced C60 electropolymerization mechanism was refined by simultaneous cyclic voltammetric (CV) and vis-NIR spectroelectrochemical as well as mass spectrometric (MALDI-TOF) characterization of the one- and two-electron reduction products of C60 in the presence of O2 in a mixed organic solvent solution. The C60 polymer (C60-O) film was also investigated by Raman spectroscopy and imaged by atomic force microscopy (AFM) both at the early and advanced polymerization stage. While the spectroelectrochemical behavior of the C60/C60•- couple in the presence of O2 was similar to that in its absence, at more negative potentials corresponding to C602- and O2•- formation C602- participated in a chemical followup reaction resulting in a product lacking any diagnostic absorption band in the vis-NIR range. Although the main peak in the MS spectrum of the one-electron reduction product was that of C60 at m/z of 720, several additional peaks in the m/z range of 739-760 appeared, indicating generation of C60O•- and C60O2•- followed by their protonation. Interestingly, the MS spectrum of the product of two-electron reduction of C60 revealed several peaks in the m/z range of 1297-1465. These peaks correspond to the oxygen-containing fullerene protonated dimers, which lose the Cn (n ) 1, 2) or C2n (n ) 2-5) fragments upon ionization. Apparently, products of the C602- and O2•- interaction spontaneously dimerize in the electrode vicinity. Importantly, oxygen is built into the dimeric molecule in the initial stage of electropolymerization. The Raman spectroscopy measurements and AFM surface imaging of the C60-O film revealed that the first CV cycle resulted in an electrodeposition of dimers. With the increase of the number of CV cycles, the extent of polymerization increased and the polymer structure became highly heterogeneous. Finally, an electrophoretically deposited film of the HiPCO single-wall carbon nanotubes (pyr-SWCNTs) noncovalently surface modified with 1-pyrenebutyric acid was coated by electropolymerization with the C60-O film under CV conditions to result in a polymer-CNT composite material. AFM imaging of this film showed that tangles of the pyr-SWCNTs bundles, coated with the C60-O globules, were formed. The electrochemical and viscoelastic properties of the pyr-SWCNTs|C60-O film were unraveled by simultaneously performed CV and piezoelectric microgravimetry (PM) measurements in a blank (TBA)ClO4 acetonitrile solution. Specific capacitance of the electrode coated with this composite film was 184 F g-1, a value comparable to those for other SWNT composite film coated electrodes, suggesting a plausible application of this material in developing supercapacitors. 1. Introduction Fullerene-based polymeric materials have attracted considerable interest since it became apparent that they are mechanically robust and appreciably conductive.1,2 In contrast to nonpolymerized solid fullerenes, they are stable with respect to * To whom correspondence should be addressed. (W.K.) E-mail: [email protected]. Telephone: +48 22 343 32 17. Fax: +48 22 343 33 33. (F.D.S.) E-mail: [email protected]. Telephone: 316-9787380. Fax: 316-978-3431. (L.D.) E-mail: [email protected]. Telephone: +49 351 4659 660. Fax: +49 351 4659 811. † Polish Academy of Sciences. ‡ Wichita State University. § Warsaw University of Technology. | Leibniz-Institute of Solid State and Materials Research. ⊥ Cardinal Stefan Wyszynski University in Warsaw.

dissolution in their highly reduced states.3-11 These materials are promising for applications such as data storage media and electrode materials for batteries, supercapacitors, or photoactive devices.12 From the point of view of their composition and structure, fullerene-based polymeric materials fall at least into two categories. One, to which all-carbon polymers belong, consists of covalently linked fullerenes, while the other involves copolymers of the fullerenes, which are parts of the polymer structures.12,13 Various types of fullerene polymers, such as sidechain polymers, main-chain polymers, fullerenes confined to solid substrates, fullerodendrimers, star-shaped polymers, fullereneend-capped polymers, and so on, were synthesized.12 Many different methods have been used to polymerize fullerenes14,15 including photopolymerization,13 electron beam induced po-

10.1021/jp912033q  2010 American Chemical Society Published on Web 04/19/2010

Reductive C60 Electropolymerization lymerization,16 pressure-induced photopolymerization,16 plasmainduced polymerization,17 and electrochemical reductive polymerization.15,18 The latter involves the presence of either a certain transition metal complex18 or dioxygen.15,18 C60 can also be oxidatively electropolymerized in the presence of a very low level of nucleophiles.19 The C60-based polymers can be exploited as redox conducting materials to prepare the polymer-carbon nanotube (CNT) composite films. High conductivity, mechanical stability, and well developed surface of CNTs combined with highly conducting and stable C60-based polymer result in a composite material suitable for constructing different electronic devices.20,21 The composite film of the C60-palladium (C60-Pd) polymer and single-wall carbon nanotubes (SWCNTs) was prepared by electrochemical polymerization in solutions of C60 and palladium acetate in the presence of suspended SWCNTs.21,22 Another approach used for composite preparation was CNT immobilization onto an electrode surface by using electrophoresis23 or a drop-casting procedure.20 Then, the CNT modified electrode was electrochemically coated with either the C60-Pd film20 or the mixed polymer film of C60-Pd and polybithiophene.23 The electrode modified with the composite film of C60-Pd and CNTs revealed a much higher specific capacitance than that solely modified by the C60-Pd film.21,23,24 Moreover, the presence of CNTs in the composite film improved its viscoelasticity.21,22 The procedure of electrochemical polymerization of C60 in the presence of dioxygen is relatively simple. Advantageously, it offers easy control over (i) the thickness of the polymer film, (ii) the rate of film growth, (iii) the film morphology and its surface topography, (iv) polymer doping with counter- and coions, as well as (v) polymer swelling with a solvent.15 C60 is electropolymerized in the presence of dioxygen if the C60-toO2 concentration ratio in solution exceeds 10:1 and the applied potential is more negative than that of the electroreduction of dioxygen.15 Although electropolymerization readily proceeds under these conditions, its mechanism still bears further scrutiny. Particularly, the role of dioxygen remains to be assessed to a greater detail. Both C60 and its derivatives exhibit high affinity toward radicals.25 That is, reactive free radicals easily add to C60 yielding paramagnetic adducts, RnC60• (n ) 1, 2, 3, ...), which can be detected and identified by ESR spectroscopy. One of the interesting features of these radicals is the formation of fullerene dimers.25 Some C60 derivatives have already been demonstrated to effectively trap free radicals, such as the superoxide anion radical, O2•-, or hydroxyl radical, OH•.25 In one of the conceivable hypotheses accounting for the mechanism of electropolymerization of C60 in the presence of O2, it was postulated that, initially, addition of the electrochemically generated O2•- to C60 resulted in formation of a C60-O-O•- radical.15 Next, this radical reacted with another molecule of C60 to form C60O•- and C60O that were, presumably, responsible for further fullerene polymerization through electroreductive formation of C60O2-. It means that oxygen merely initiated electropolymerization not participating in subsequent growth of the polymer.15 In the present work, we refined this mechanism with our new experimental evidence and, moreover, applied the polymer for preparation of a conducting and relatively stable single-wall carbon nanotube composite. 2. Experimental Section 2.1. Chemicals. C60 (99.5% purity) was from the M. E. R. Corp. (Tucson, AZ) or SES Research (Houston, TX). Toluene (TL, puriss, absolute), 1-methyl-2-pyrrolidone (NMP), and acetonitrile (ACN, puriss, absolute) from Fluka were used

J. Phys. Chem. C, Vol. 114, No. 18, 2010 8151 as received. Tetra(n-butyl)ammonium hexafluorophosphate ((TBA)PF6, puriss, electrochemical grade) and tetra-n-butylammonium perchlorate ((TBA)ClO4, puriss, electrochemical grade) from Fluka were used without additional purification. The HiPCO single-wall carbon nanotubes were from Carbon Nanotechnology, Inc. (Houston, TX). 1-Pyrenebutyric acid (97% purity) was from Aldrich. 2.2. Apparatus. The spectroelectrochemical ESR flat cell and the apparatus setup for in situ ESR-(vis-NIR) spectroelectrochemical investigations are described elsewhere.26,27 A laminated platinum mesh, silver wire, and platinum wire were used as the working, pseudoreference, and counter electrode, respectively. The vis-NIR spectra were recorded with a diode array UV-vis-NIR spectrophotometer TIDAS of J&M (Aalen, Germany). In order to avoid any photochemical interfering processes, only the halogen lamp of the TIDAS spectrophotometer was used. A PG 390 potentiostat-galvanostat of HEKA GmbH (Lambrecht, Germany) served for the control of electrochemical and spectroelectrochemical measurements. The MS spectra were recorded with the use of a Biflex MALDI-TOF mass spectrometer of Bruker (Bremen, Germany) with a 337.1 nm pulsed nitrogen laser. All spectra were recorded in the negative reflection mode, and 100 laser pulses were acquired to produce these spectra. No matrix was used in order to avoid any side reactions of molecules with this matrix. In view of insolubility of the C60 electropolymerization products, a solution in the vicinity of the working electrode was examined by MS instead. The Raman spectra were collected using a Nicolet Almega Raman dispersive spectrometer equipped with a confocal Raman microscope, an 1800 lines/mm holographic grating, and a CCD camera. A diode laser operating at 780 nm was used as the excitation source, and the spectral resolution was ∼2 cm-1. All spectra were recorded at 25 °C. Atomic force microscopy (AFM) imaging was performed with the use of a Multimode NS3D microscope of the Digital Instruments/Veeco Metrology Group (Woodbury, NY). For this imaging, the films were deposited on the 7 mm diameter indium-tin oxide9 electrodes of Image Optics Components, Ltd. (Basildon, U.K.). The apparatus setup for electrophoretic deposition of CNTs is described elsewhere.21 A spiraled Pt wire and a Au film electrode served as the auxiliary (negative) and working (positive) electrode, respectively. The Au film working electrode was that 5 mm in diameter evaporated over a Ti underlayer onto a 14 mm diameter, 10 MHz resonant frequency, plano-plano AT-cut quartz crystal resonator with a matte finish of the Institute of Tele- and Radio Communication (Warsaw, Poland). This electrode was used for simultaneous piezoelectric microgravimetry (PM) and cyclic voltammetry (CV) measurements. A dc voltage was applied to the electrodes by using an IZS-5/ 71 09 stabilized power supply of INCO (Warsaw, Poland). In order to control the time-resolved mass changes of the film being deposited, simultaneously with the electrophoretic deposition of the noncovalently surface-coated with 1-pyrenebutyric acid SWCNTs (pyr-SWCNTs)12 films, the PM experiments were performed by using the EQCM 5710 electrochemical quartz crystal microbalance of the Institute of Physical Chemistry (Warsaw, Poland) under control of the EQCM 5710-S2 software of the same manufacturer. A 160-W IS-3R ultrasonic bath of InterSonic (Olsztyn, Poland) was used for dissolution of C60 and dispersion of pyrSWCNTs in selected electrolyte solutions.

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Figure 1. Voltammograms for 0.38 mM C60 (a) in the deaerated and (b) in the dioxygen-saturated solution (∼1.60 mM O2) of 0.1 M (TBA)ClO4 in TL/ACN (4:1, v:v). Potential scan rate was 5 mV s-1. Panels (c) and (d) show vis-NIR spectra recorded simultaneously with the electrochemical measurements shown in panel (a) and (b), respectively.

Simultaneous CV and PM experiments were performed by using an EP-21 potentiostat of Elpan (Lubawa, Poland) connected to an EQCM 5710 electrochemical quartz crystal microbalance under the EQCM 5710-S2 software control. This microbalance allowed for simultaneous measurement of changes of current, resonant frequency, and dynamic resistance of a Au quartz crystal resonator during potential cycling. 3. Results and Discussion First, C60 at different redox states, in a deaerated electrolyte solution, was investigated by vis-NIR spectroelectrochemistry. Then, the fullerene anions and superoxide anion radical were generated electrochemically in situ and allowed to react with each other component. Products of this reaction were examined by vis-NIR spectroscopy. Next, MS was used to identify the products of both the C60 monoanion and dianion of the reaction with O2•-. Subsequently, Raman spectroscopy and AFM served to investigate structural and topographical properties, respectively, of the polymer films at their initial and advanced stages of growth. Finally, the polymer was applied to make a composite film with electrophoretically deposited pyr-SWCNTs, and selected properties of the composite were characterized. 3.1. Spectroelectrochemical and Mass Spectrometric Investigations of C60 in Solution in the Absence and Presence of Dioxygen. Figure 1 shows simultaneously recorded cyclic voltammograms and vis-NIR spectra for 0.38 mM C60 in 0.1 M (TBA)ClO4 in deaerated (Figure 1a and c) and dioxygensaturated (Figure 1b and d) TL/ACN (4:1, v:v) mixed solvent solutions. The spectroelectrochemical behavior of C60 in a deaerated solution has already been thoroughly investigated and well characterized.28-32 Accordingly, the first cathodic peak was herein observed at -0.48 V (Figure 1a) for the first one-electron reduction of C60. This peak was accompanied by an absorption NIR band at 1080

nm (Figure 1c). As soon as the negatively scanned potential reached ∼-0.75 V, that is, the value characteristic for the foot of the second cathodic peak (Figure 1a), the absorption band at 1080 nm disappeared and a new one at 950 nm, characteristic for C602-, emerged (Figure 1c). After reversal of the potential scan direction, the absorption band at 950 nm disappeared as soon as C602- was electrooxidized back to C60-, resulting in formation of the 1080 nm absorption band again (Figure 1c). Spectroelectrochemical behavior of C60 in the presence of O2 was distinctly different from that in the absence of the latter. That is, the first cathodic peak corresponding to the C60/C60•electroreduction is seen at -0.45 V (Figure 1b). Associated with this CV peak, an absorption band at 1080 nm appeared (Figure 1d) as C60•- was generated, similar to that generated in the absence of O2 (Figure 1c). Under the typical CV time scale, the electroreduction of C60 to C60•- in the presence of O2 is reversible.15 However, a slow reaction of C60•- and O2 was reported, and its mechanism is believed to involve the initial electron transfer from C60•- to O2.33-35 That is, C60•- is just reoxidized to C60, according to reaction 1.

C60- + O2 f r C60 + O2•-

(1)

Equation 1 is thermodynamically disfavored by ∼0.40 V in solutions of aprotic solvents of low electric permittivity but may become favored in highly protic solvent solutions.33 The reaction described by eq 1 can only proceed if the superoxide product is irreversibly removed from the system.33-35 The NIR spectroscopic study of the stability of C60•- in oxygen-saturated dimethyl sulfoxide (DMSO) solution revealed that the absorption band at 1080 nm remained virtually unchanged for 30 s, while in the presence of the ammonium ion (NH4+) it decayed within 10 s. This decay was due to hydrogen bond formation between

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Figure 2. Mass spectra for 0.4 mM C60 and ∼1.60 mM O2 in 0.1 M (TBA)PF6, in TL/ACN (4:1, v:v) after electroreduction at a potential corresponding to the second cathodic peak of C60. (a) Part of the spectrum where peaks corresponding to C60O, C60O2, and some additional peaks with m/z of 740 and 741 are present; (b) part of the spectrum where peaks corresponding to C60 dimers are observed.

O2•- and NH4+, which significantly accelerated electron transfer from C60•- to oxygen.35 This result indicates that C60•- does react with O2 but this reaction is very slow. Moreover, the ESR signal of C60•- was also changed after exposure of the solution to air.36 Then, the characteristic broad ESR signal with g ) 1.998 and ∆Hpp ) 37.0 G of C60•- was slowly quenched, and a narrow band with g ) 2.0008 and ∆Hpp ) 3.32 G appeared within 2 h. This evolution of the narrow ESR signal could be due either to direct attachment of oxygen to the negatively charged C60 or to the electron transfer from C60•- to oxygen followed by an addition of the resulting superoxide to the fullerene cage. In the present investigations, the potential scan rate was 5 mV s-1. That is, it was relatively fast compared to the time of evolution of the ESR and NIR spectroscopy signals. Therefore, the reaction between C60•- and O2 should have no influence on the mechanism of C60 electropolymerization in the presence of O2. The second cathodic CV peak, however, was split into two overlapping peaks at -0.73 and -0.90 V (Figure 1b). These peaks correspond to superimposed electroreduction of C60•-/ C602- and O2/O2•-, respectively.15 Moreover, as soon as the negatively scanned potential reached ∼-0.65 V, the 1080 nm band markedly decreased and no 950 nm band appeared in the vis-NIR spectrum recorded (Figure 1d). This behavior suggests that the C602- electroreduction product is involved in a chemical follow-up reaction. Apparently, this reaction results in a product which is devoid of the characteristic absorption bands in the vis-NIR region. Presumably, the negative charge of this product is then compensated by protons. After a reversal of the cathodic potential scan direction, only one well-defined anodic peak was observed at -0.31 V (Figure 1b) having no absorption band in the vis-NIR spectrum (Figure 1d). This indicates that the peak at -0.31 V may be ascribed to electrooxidation of the reaction product of the chemical follow-up reaction rather than to the electrooxidation of C60•- itself. Figure 2 shows the MS spectra for the air-saturated (∼1.6 mM O2) solutions of 0.4 mM C60 after electroreduction

at the potential corresponding to the second cathodic peak of C60 in 0.1 M (TBA)PF6 in TL/ACN (4:1, v:v). The MS spectrum for the freshly prepared C60 solution (not shown) is characterized by a very intense peak with m/z of 720 accompanied by a weak peak with m/z of 736 corresponding to C60 and C60O, respectively.37 The presence of the latter peak indicates that either the initial C60 powder was slightly contaminated with the C60O impurity, or this impurity was generated by a slow reaction of C60 with dioxygen in the electrolyte solution. However, there was no MS peak characteristic of C120O. So, this dimer cannot be considered as a major impurity of the powder C60 samples exposed to air.38 Moreover, the lack of the NIR band at 940 nm (Figure 1d), characteristic of C120O32 electroreduced during C60 electroreduction in the presence of O2, indicates that this dimer is not being generated. Electroreduction of the freshly prepared C60 solution at -0.45 V, that is, at the potential corresponding to the first cathodic peak of C60, results in small changes in the MS spectrum. The MS spectrum, Figure S1, and the corresponding discussion are available in the Supporting Information. At -0.75 V, that is, at the potential corresponding to the second cathodic peak of C60, the MS spectra show further mass peaks (Figure 2). That is, peaks appeared in the m/z range of 739-760. Moreover, there was a broad MS peak, centered at ∼1430, which is characteristic for the oxygen-containing fullerene protonated dimers, which lose Cn (n ) 1, 2) or C2n (n ) 2-5) fragments upon MALDI-TOF ionization (Figure 2b). Apparently, C60 dimerized at the potential where C60 and O2 were electroreduced to C602- and O2•-, respectively. The abundant presence of the MS peaks with m/z between 736 and 760 indicates that C60O as well as C60O2 were generated and then protonated. Previous investigations of the mechanism of C60 electropolymerization in the presence of O2 suggest that O2•- reacts with C60 to produce a C60-O-O•- anion radical in the diffusion layer at the electrode.15 Then, this radical reacts with another C60 molecule producing C60O and C60O•-, which are considered as precursors of subsequent C60 polymerization.

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Based on our results, we postulate that the studied electropolymerization can also proceed via the reaction of C602- with O2 or O2•-. The lack of the characteristic NIR band at 950 nm for C602- in the presence of O2 (Figure 1d) indicates that this anion is consumed in a chemical reaction following the charge transfer. Moreover, the cathodic CV peak associated with the second C60 electroreduction (Figure 1b) is positively shifted in the presence of O2 compared to that obtained in the absence of O2 (Figure 1a). This positive shift suggests that the chemical reaction follows the electrode process.39 Two hypothetic reaction paths can be taken into consideration. In path 1, first, C602- reacts with O2 according to eq 2. The C60O22- dianion product of this reaction is found in the MS spectrum (Figure 2a).

C602- + O2 f C60O22-

(2)

Next, the C60O22- dianions may either react with each other producing different oxygen-containing fullerene dimers (Figure 2b) or with other C60 molecules forming two C60O•- anion radicals, as shown in eq 3.

C60O22- + C60 f 2C60O•-

(3)

This structure is also observed in the MS spectrum (Figure 2a). After reaching the potential of the O2 electroreduction, O2•can react in the solution with C60 according to eq 4. Molecules of the C60O2•- product of this reaction may react with each other producing different oxygen-containing fullerene dimers (Figure 2b) or with another C60 molecule, eq 5, as it has already been postulated.15

O2•- + C60 f C60O2•-

(4)

C60O2•- + C60 f C60O•- + C60O

(5)

In path 2, C602- may initially react with O2•- according to eq 6.

C602- + O2•- f C60O23-

(6)

As a result, C60O23- may be formed which, most likely, reacts with C60 to produce C60O2- and C60O•- (eq 7).

C60O23- + C60 f C60O2- + C60O•-

(7)

These reaction paths propose that both C602- and C60 are involved in the C60 electropolymerization in the presence of O2. Moreover, C602- reacts with O2 or O2•- resulting in products which are not visible in vis-NIR spectroscopy. It means that the negative charge of the product molecules is compensated by protons. Presumably, this reaction is very efficient, and each produced C602- dianion immediately reacts with O2 or O2•-, and then the proton is attached. 3.2. Structural and Topographical Characteristics of the C60-O Film. Raman spectroscopy is one of the most frequently used methods for examining the local molecular environment of the C60 molecule. Due to the high symmetry (Ih) of its cage, Raman spectra of pristine solid C60 exhibit 10 active (eight Hg

and two Ag) intramolecular lines.40 However, electropolymerization of C60 results in breaking this symmetry. The Raman spectra for the C60 polymer films are much richer than the spectrum for the pristine C60 film. That is, several new broader modes appear along with splitting of the Hg and shifting of the Ag modes.41,42 The strongest signal in the Raman spectrum of pristine C60 is the Ag(2) line at 1470 cm-1. It is identified as the “pentagonal pinch” mode. This band position is sensitive to the number of covalent bonds on the C60 cage,43,44 and therefore, it is used to sense the extent of C60 polymerization.44 For instance, this mode is downshifted to ∼1442 cm-1 both for the photoand thermobaric C60 polymer.43 Moreover, the Hg(1) band for the pristine C60 is assigned to an interball vibration and depends upon the nature of the bonding between the fullerene moieties within the polymer matrix.41,45 The splitting of this band indicates formation of new intermolecular bonds and accompanying changes in the intramolecular C-C bonding.41 Figure 3 shows the room-temperature Raman spectra in the range of the Ag(2) band for the pristine C60 film deposited by drop coating (Figure 3a), and a C60 polymer (C60-O) film deposited during 1, 2, 3, 5, and 10 CV cycles (Figure 3b-f, respectively). For the drop-coated pristine C60 film, the Ag(2) band is positioned at 1470 cm-1. However, it shifts downward, as expected, for the C60 polymer films deposited in consecutive CV cycles. Moreover, the shift is more pronounced at higher CV cycle numbers if electropolymerization becomes more extensive. For the film deposited in the first CV cycle, this band appears at 1464 cm-1 (Figure 3b). This position is typical for bands of C60 dimers44 formed, in accord with the MS results (see above in section 3.1). After completion of the second CV cycle, the Ag(2) band shifts more downward, that is, to 1460 cm-1 (Figure 3c). This new band position is characteristic for the polymer chains of C60.44 Then, position of this band remains constant during the next five CV cycles (Figure 3d). After the fifth cycle is completed, however, not only the Ag(2) band is downshifted even more (Figure 3e) but it becomes broader. Moreover, a new band appears on its lower wavenumber shoulder. This spectral evolution indicates the formation of a more complex structure of the C60-O polymer. For the polymer film deposited in the course of 10 CV cycles, the Ag(2) band is composed of several overlapping bands (Figure 3f) with the most intense one at 1443 cm-1 pointing to formation of a branched C60 polymer.44 Moreover, new bands at lower Raman shifts appear. Most likely, these bands correspond to the 3D polymer structure. The polymer structure of the branched chains features lower symmetry than that of the ordered linear chains. The higher the structure disorder, the more complex is the Hg(1) band.44 This band progressively splits with the number of CV cycles completed (Figure 4). Eventually, the Hg(1) band divides into several bands after 10 CV cycles (Figure 4f). Seemingly, multiscan electropolymerization of C60 in the presence of O2 results in a polymer film of a relatively low order. Figure 5 shows the AFM images of the C60-O films deposited by electropolymerization under CV conditions onto the ITO electrode for different numbers of cycles. Our present MS and Raman spectroscopy experiments revealed that the fullerene dimers are formed during the first cycle. In the AFM picture (Figure 5a), randomly located globules of diameter in the range of 50-200 nm are seen on the electrode surface after the first cycle. These objects do not form any uniform film, but more globules are formed in the second cycle (Figure 5b). Interestingly, their diameter (30-50 nm) is smaller than that of those deposited in the first CV cycle (Figure 5a). Clearly, the rate of

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Figure 3. Raman spectra in the shift range of the appearance of the Ag(2) band for (a) the C60 film drop-coated from the 1,2-dichrorobenezene solution and (b-f) the C60-O film, deposited by electropolymerization, recorded after (b) 1, (c) 2, (d) 3, (e) 5, and (f) 10 CV cycles. The C60-O film was deposited by electropolymerization under CV conditions using the solution of 0.38 mM C60, ∼1.60 mM O2, and 0.1 M (TBA)ClO4 in TL/ACN (4:1, v:v). The potential scan rate was 100 mV s-1.

polymer nucleation is much higher than that of the polymer growth, typical for high relative supersaturation, which is easily attained in solution in the electrode vicinity. Moreover, these globules are mutually separated and randomly distributed over the electrode surface rather than form a dense polymer film. Presumably, this topology evolution illustrates the initial growth of the polymer chains, in accord with the Raman spectroscopy results (see above in this section). After the third cycle, the entire electrode surface is coated by the elongated grains of mean length and width of 110 and 30 nm, respectively, (Figure 5c). Moreover, these grains are predominantly ordered in the same direction, forming a uniform film. Subsequent potential cycling (next five cycles) results in the increase of both dimensions of the grains and thickness of the polymer film (Figure 5d). After 10 cycles, the C60-O film is composed of grains with ∼340 nm

mean diameter merged together to form an ∼80 nm thick film densely coating the entire surface of the ITO electrode (Figure 5e). 3.3. Preparation and Properties of the pyr-SWCMTs|C60-O Film. The composite film of pyr-SWCNTs and C60-O was prepared in two steps. First, the pyr-SWCNTs film was deposited electrophoretically onto the electrode surface by using the previously described procedure.23 In brief, the entire surface of the quartz|Au electrode became coated with a black film of the pyr-SWCNTs bundles of ∼35 nm in diameter after 6 min of deposition from the 0.4 mg mL-1 suspension of pyr-SWCNTs in NMP at 24 V dc applied (AFM image of the pyr-SWCNTs film, Figure S2, can be found in the Supporting Information). This pyr-SWCNT film enhances capacitance properties of the electrode due to an increasing electrode surface. Formerly, we

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Figure 4. Raman spectra in the shift range of the appearance of the Hg(1) band for (a) the C60 film drop-coated from the 1,2-dichlorobenezene solution and (b-f) the C60-O film, deposited by electropolymerization, recorded after (b) 1, (c) 2, (d) 3, (e) 5, and (f) 10 CV cycles. The C60-O film was deposited by electropolymerization under potential cycling conditions using the solution of 0.38 mM C60, ∼1.60 mM O2, and 0.1 M (TBA)ClO4 in TL/ACN (4:1, v:v). The potential scan rate was 100 mV s-1.

determined surface roughness and specific capacitance of an electrode coated with the electrophoretically deposited pyrSWCNTs film.23 After 6 min of deposition at 24 V dc, the mean specific capacitance of the electrode modified by this film was ∼50 F g-1. Moreover, the presence of the pyr-SWCNT film on the electrode surface increased its area by ∼230%.23 The electrode modified by pyr-SWCNTs quartz|Au was coated with the C60-O film by electropolymerization under multiscan CV conditions (curve 1 in Figure 6a). During this electropolymerization, the dependence of the change of both resonant frequency (curve 2 in Figure 6a) and dynamic resistance on potential (curve 3 in Figure 6a) was simultaneously recorded. These changes correspond to respective variations of

the mass and viscosity of the resonator due to deposition of the C60-O film on the pyr-SWCNT film. The C60-O film deposition was manifested by a decrease of the resonant frequency with the increase of the CV cycle number (curve 2 in Figure 6a). The frequency decrease accompanying positive potential scanning corresponds to deposition of the polymer film on the electrode surface. This is because molecules serving as precursors of the polymer film formation are produced in the negative potential excursion covering the potential range of the two-step electroreduction of C60 and electroreduction of O2. In effect, anionic multifullerene species are formed. They are then electrooxidized to their neutral forms in the subsequent positive voltammetric cycles and precipitate on the electrode surface.

Reductive C60 Electropolymerization

Figure 5. Atomic force microscopy images (3.5 × 3.5 µm2 area) of the C60-O film deposited by electropolymerization under potential cycling conditions from the solution of 0.38 mM C60 and ∼1.60 mM O2 in 0.1 M (TBA)ClO4 in TL/ACN (4:1, v:v) onto the ITO electrode after (a) 1, (b) 2, (c) 3, (d) 5, and (e) 10 CV cycles. The potential scan rate was 100 mV s-1.

The total frequency shift is caused both by changes of the mass of the film and by changes of its viscoelastic properties.21 Therefore, the viscoelasticity change corresponding to the simultaneous change of dynamic resistance should be taken into account for determination of the mass of the C60-O film deposited on the quartz|Au|pyr-SWCNT electrode. The frequency decrease (curve 2 in Figure 6a) during electropolymeric deposition of C60-O was accompanied by the dynamic resistance increase of ∼41 Ω (curve 3 in Figure 6a) corresponding to the frequency decrease of ∼4.1 Hz.21 This value is negligibly small as compared to the total frequency decrease of ∼4.3 kHz measured after 10 cycles. Hence, the corresponding mass of the deposited film, determined by using the Sauerbrey equation, was ∼3.73 µg. The electrochemical and viscoelastic behavior in a blank acetonitrile solution of 0.1 M (TBA)ClO4 of the quartz|Au electrode, coated with the pyr-SWCNTs|C60-O film, is shown in Figure 6b. The composite film revealed electrochemical activity at potentials more negative than ∼-0.40 V (curve 1′ in Figure 6b), typical of the C60-based polymer films.10,15,21 Current switching with the potential change in this range is due to the first cathodic peak of the C60 moiety. The high electrochemical activity of the composite film in the first cycle (red part of curve 1′ in Figure 6b) gradually decreases in

J. Phys. Chem. C, Vol. 114, No. 18, 2010 8157 consecutive CV cycles. After 15 cycles, the current-potential curve reaches a steady state. The shape of this curve resembles a pseudorectangle, resulting from the presence of pyr-SWCNTs, with some overlapping peaks due to the C60-O electroactivity, suggesting a capacitive nature of this modified electrode.46 Both the resonant frequency and dynamic resistance change for the first cycle distinctly differs from that for the subsequent cycles (red part of curves 2′ and 3′, respectively, in Figure 6b). For the first cycle, both frequency and resistance initially remained unchanged. At ∼-0.60 V, however, the change of the resonant frequency decreases (red part of curve 2′ in Figure 6b) and that of the dynamic resistance increases (red part of curve 3′ in Figure 6b) because of the ingress of counterions to the film for compensation of the negative charge electroreductively generated in the composite. When the potential during its cathodic scan reaches ∼-0.95 V, that is, the value of the second electroreduction of the C60 moiety, the second step in the frequency versus potential curve is seen. The presence of this step suggests that an additional amount of counterions entered the film, causing a frequency decrease (red part of curve 2′ in Figure 6b) and a resistance increase (red part of curve 3′ in Figure 6b). After the potential scanning was reversed, the frequency gradually increased (red part of curve 2′ in Figure 6b) and the resistance decreased (red part of curve 3′ in Figure 6b). However, both frequency and resistance did not reach their initial values. That is, after the first CV cycle was completed, the final frequency and resistance value was lower and higher, respectively, than its initial one. This behavior may indicate that charging followed by discharging of the composite film during its electroreduction and electrooxidation, respectively, is irreversible and leads to a charge storage in the composite film. This irreversible charging and discharging may also induce changes in the composite structure. The decrease of resonant frequency and the increase of dynamic resistance suggest that the film swells up and becomes more viscous during electrochemical treatment. For subsequent cycles, both resonant frequency (curve 2′ in Figure 6b) and dynamic resistance (curve 3′ in Figure 6b) gradually increase. This behavior is most likely due to both the continued charge storage and slow film dissolution thus accounting for the current decrease with the increase of the CV cycle number (curve 1′ in Figure 6b). The AFM image of the surface of the pyr-SWCNTs|C60-O film deposited onto the ITO electrodes is shown in Figure 7. The C60-O film was deposited during 10 CV cycles. The pyrSWCNTs|C60-O film is built of a tangle of 150-250 nm thick bundles (Figure 7a). These bundles are made up of 45 nm thick bundles of pyr-SWCNTs coated with the 60-100 nm diameter globules of the C60-O polymer (Figure 7b). A similar globular polymer film, deposited on the CNT surfaces, was also observed in case of C60-Pd,20,21 (C60-Pd)-polybisthiophene [(C60-Pd)PBT],23 polyaniline (PANI),47,48 and polypyrole (PPY).49 The CV curves in Figure 6b exhibit a capacitive behavior featuring almost straight and vertical current variations at both limits of the potential range. Similar electrochemical behavior is observed for the composites of CNTs|PPY, CNTs|poly(3,4-ethylenedioxythiophene), CNTs|PEDOT,50 and CNTs|(C60-Pd)-PBT mixed polymers.23 This feature suggests a fast charge/discharge switching resulting from high both electronic and ionic conductivity. For the pristine C60O,15 PPY,50 and PEDOT50 films, the straight and vertical current variations during changing of the potential scan direction was only seen at the positive limit of the potential range while for the pristine (C60-Pd)-PBT mixed polymer films this behavior was observed

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Figure 6. Curves of (1, 1′) the multiscan potential cycling as well as the potential dependence of (2, 2′) the resonant frequency change and (3, 3′) the dynamic resistance change for (a) deposition by electropolymerization of the C60-O film in the 0.4 mM C60, ∼1.60 mM O2, and 0.1 M (TBA)ClO4 solution of TL/ACN (4:1, v:v) on the quartz|Au|pyr-SWCNT electrode, and (b) the quartz|Au|pyr-SWCNTs|C60-O electrode in the deaerated acetonitrile solution of 0.1 M (TBA)ClO4. The potential sweep rate was 100 mV s-1. The nanotubes were electrophoretically deposited on the quartz|Au electrode from a 0.4 mg mL-1 suspension of pyr-SWCNTs in NMP.

both at the negative and positive limit.23 Oxidatively polymerized C60 in the presence of very low levels of nucleophiles forms stable polymer films, which are electroactive both in the positive and negative potential range with the almost vertical current variations recorded upon turning the potential scan direction.51 Based on the CV measurements, the value of specific capacitance, Cs,20,23 for the pyr-SWCNTs|C60-O film coated electrode was determined as 184 F g-1. This value compares well with those for films of other composites of CNTs and conducting polymers, measured with the use of a three-electrode system, such as CNTs|PPY (131-890 F g-1 obtained for aqueous solutions),52-56 CNTs|PANI (183-485 F g-1 determined for aqueous solutions),57-62 CNTs|PEPOD (60 F g-1 determined for an organic solvent solution),63 and CNTs|(C60-Pd)-PBT (218 F g-1 determined for an organic solvent solution).23 However, the Cs value for the herein studied composite gradually decreases with the voltammetric cycle number due to irreversible changes in the polymer structure caused by repeated ingress and egress of counterions during film charging and discharging. This ion dynamics results in volume changes of the film during doping and dedoping, and associated polymer swelling and shrinking. Low mechanical stability during potential cycling is the major drawback of conducting polymers in application as active electrode materials.50,64 To overcome this stability problem, carbon nanotubes are used to mechanically reinforce the polymer films. Electrochemical stability of the pyr-SWCNTs|C60O film is, however, relatively low despite the presence of the nanotubes in it. The C60-based polymer film obtained during C60 electrooxidation revealed appreciable electrochemical stability.51 The presently studied polymer can also be considered for composite

preparation. A high Cs value of this composite film is promising for application as an active material for supercapacitors. However, further attempts aiming at improvement of the mechanical properties of the composite are necessary, and such studies are in progress in our laboratories. 4. Conclusions Under electrode potential cycling, the mechanism of electropolymerization of C60 in the presence of dioxygen involves generation of both the dianion of C60 and the superoxide anion radical. Both C602- and O2•- are formed if a sufficiently negative electrode potential is reached. These two anions react with C60 producing dimers, that way becoming effective precursors of subsequent polymerization. The C120O2 dimer formed loses its Cn (n ) 1, 2) or C2n (n ) 2-5) fragments upon MS ionization. The oxygen-containing C60 dimers are deposited on the electrode surface in the course of the first potential cycle. Subsequent potential cycling results in the growth of 1D polymer chains (three cycles) and then branched chains (five cycles) to form a 2D structure. After 10 cycles, a 3D polymer structure is, presumably, formed. That is, the extent of polymerization increases and, consequently, the structure of the polymer changes as the polymer becomes more and more heterogeneous with the increase of the number of cycles. The electroreduction of C60 in the presence of O2 results in the formation of a polymer film onto the pyr-SWCNTs coated electrode. This polymer forms the 60-100 nm diameter globules on the 45 nm diameter pyr-SWCNT bundles. The pyrSWCNTs|C60-O film is electrochemically active in the negative

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Figure 7. Atomic force microscopy image (a) 5 × 5 µm2 area and (b) 2.5 × 2.5 µm2 area of the pyr-SWCNTs|C60-O film deposited onto the ITO electrode. First, the pyr-SWCNT film was electrophoretically deposited at 24 V dc voltage for 6 min. Then, the C60-O film was deposited by electropolymerization under potential cycling conditions in the course of 10 cycles.

potential range. However, the irreversible behavior of both the resonant frequency and dynamic resistance dependence on the potential sweep suggests that electrochemical charging and discharging of the composite is irreversible and leads to charge storage within the film and permanent structural degradation. The value of specific capacitance for the pyr-SWCNTs|C60-O film coated electrode was determined as 184 F g-1, a value comparable to those for other carbon nanotube composite materials, suggesting its usefulness in building supercapacitors. Acknowledgment. The present research was financially supported by the Ministry of Science and Higher Education of Poland (Project No. N204 046 31/1214 to W.K.) and the National Science Foundation (Grant Nos. 0804015 and EPS0903806) and matching support from the State of Kansas through Kansas Technology Enterprise Corporation. P.P. is thankful to Mr. Frank Ziegs for help in performing UV-vis and MS measurements and the M. Nowicki Foundation and the Deutsche Bundesstiftung Umwelt for financial support of his 10-month research work at IFW in Dresden (Germany). Supporting Information Available: Mass spectra for 0.4 mM C60 and AFM image of the pyr-SWCNT film electrophoretically deposited onto the ITO electrode. This material is available free of charge via the Internet at http://pubs.acs.org.

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