Preparation and Selected Properties of an Improved Composite of the

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Preparation and Selected Properties of an Improved Composite of the Electrophoretically Deposited Single-Wall Carbon Nanotubes, Electrochemically Coated with a C60-Pd and Polybithiophene Mixed Polymer Film Piotr Pieta,† Ganesh M. Venukadasula,‡ Francis D’Souza,*,‡ and Wlodzimierz Kutner*,†,§ 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, and Faculty of Mathematics and Natural Sciences, School of Science, Cardinal Stefan Wyszynski UniVersity in Warsaw, Dewajtis 5, 01-815 Warsaw, Poland ReceiVed: April 28, 2009; ReVised Manuscript ReceiVed: June 17, 2009

An improved carbon nanotube conducting composite material for electrochemical capacitors has been devised and tested. It was prepared by electrophoretic deposition of a film of noncovalently surface modified with 1-pyrenebutyric acid HiPCO single-wall carbon nanotubes (pyr-SWCNTs), which were then electrochemically coated, under multiscan cyclic voltammetry (CV) conditions, with a mixed film of the fullerene-palladium (C60-Pd) polymer and the polybithiophene (PBT) polymer. Both the electrophoretic deposition of pyr-SWCNTs and electrochemical polymerization of (C60-Pd)-PBT was in situ monitored by piezoelectric microgravimetry (PM) with the use of an electrochemical quartz crystal microbalance (EQCM). Atomic force microscopy imaging of the film revealed that pyr-SWCNTs formed tangles of pyr-SWCNTs bundles surrounded by globular clusters of (C60-Pd)-PBT. X-ray photoelectron spectroscopy provided information on the mole ratio of BT/ C60/Pd ≈ 1:2:4 indicating that, most likely, the C60-Pd and PBT polymers are not interconnected in the composite film. The electrochemical and visco-elastic properties of the pyr-SWCNTs/(C60-Pd)-PBT film were investigated and compared to those of the (C60-Pd)-PBT film by simultaneously performed CV and PM measurements in a blank acetonitrile solution of tetra-n-butylammonium perchlorate. Both films revealed two potential windows of electrochemical activity, that is, one at potentials more negative than ∼-0.40 V versus Ag/AgCl due to fullerene electroactivity and the other at potentials more positive than ∼0.40 V due to PBT electroactivity. Advantageously, both cathodic and anodic currents for the pyr-SWCNTs/(C60-Pd)PBT film were much higher than those for the (C60-Pd)-PBT film due to the more developed electrode area in the presence of pyr-SWCNTs and, hence, higher capacitance. The highest value of specific capacitance (Cs), determined from the CV measurements, for the negative potential range was 100, 75, and 50 F g-1 while for the positive potential range it was 200, 180, and 50 F g-1 for the film of pyr-SWCNTs/(C60-Pd)PBT, (C60-Pd)-PBT, and pyr-SWCNTs, respectively. From the electrochemical impedance spectroscopy measurements, it followed that both for the potential range of 0 to -1.10 V and 0 to 1.0 V, the Warburg-type region was linear and its slope was 45° at high and intermediate frequencies indicating control of the charge transport rate by the rate of the semi-infinite diffusion of counterions within the film. At low frequencies, control of this transport rate by the finite diffusion rate dominated, as characterized by sharp upturns in the -Z″ versus Z′ curves. A modified Randles equivalent circuit was used to determine impedance parameters for the pyr-SWCNTs/(C60-Pd)-PBT film-coated electrode. Constant-current 90 mA charging and discharging at different voltage limits for the pyr-SWCNTs/(C60-Pd)-PBT film-coated Au disk electrode was reversible and pseudolinear for the voltage range not exceeding 1.60 V. 1. Introduction Cell phones and pagers, stand-by power systems, engines of electric/combustion hybrid vehicles, and so forth, all require electrical energy storage.1 Moreover, these devices need to be powered with pulsed electrical energy. Unfortunately, electrostatic capacitors, such as those used in electronic circuits, cannot store enough energy in their available volume and weight configurations. For that purpose, capacitors of high energy * To whom correspondence should be addressed. (W.K.) E-mail: [email protected]. Phone: +48 22 343 32 17. Fax: +48 22 343 33 33. (F.D.S.) E-mail: [email protected]. Phone: +1 316-978-7380. Fax: +1 316-978-3431. † Polish Academy of Sciences. ‡ Wichita State University. § Cardinal Stefan Wyszynski University in Warsaw.

density, such as electrochemical capacitors (called supercapacitors), are being devised and tested.1 Supercapacitors use electrical double-layer capacitance and/or faradaic pseudocapacitance reactions for charge storage.2,3 Recently, various carbon-based materials, for example, activated carbons, aerogels, xerogels,3 are combined with conducting polymers to come up with composites that can serve as charge-storage materials in supercapacitors.2,3 For instance, composites containing π-conjugated polymers, such as polypyrrole (PPY),4-10 polyaniline (PANI),11-13 polythiophene (PTP),14,15 or redox conducting polymers, like those fullerene-based (C60-Pd)16,17 and carbon nanotubes (CNTs), are promising for applications as active materials in supercapacitors, owing to high values of specific-, Cs, and pseudocapacitance, Credox. Modification of the CNT filmcoated electrode either by PPY, PANI, PTP, C60-Pd, or their

10.1021/jp903891v CCC: $40.75  2009 American Chemical Society Published on Web 07/14/2009

SWNTs Coated with C60-Pd and Polybisthiophene Film derivatives significantly enhances electrode capacitance.4,16–18 The unique architecture of entangled nanotubes allows us to reach specific capacitance of 80 F g-1 while specific pseudocapacitance for the pristine polymers is ∼90 F g-1.4,10 The coating surface of CNTs in the film by a thin layer of a conducting polymer not only, typically, doubles specific capacitance of the electrode modified that way but, additionally, improves mechanical stability and conductivity of the electrode thus prepared.4,16–18 Both single-wall (SWCNTs) and multiwall (MWCNTs) carbon nanotubes have already been exploited as active materials for supercapacitors,3,4,10,16-20 fuel cells,21-23 hydrogen storage24 as well as for photovoltaic25-27 and field emission devices.24,28 CNTs are highly flexible featuring low mass density and large aspect ratio, typically of ∼300-1000.29 CNTs uniquely combine mechanical, electrical, and thermal properties that make them excellent candidates for fabrication of multifunctional polymer composites.29 They can be immobilized on solid conducting supports in different ways. For instance, a CNT film-coated electrode can be prepared from highly aligned CNTs arrays6,8,30,31 or randomly spread CNTs.19,32,33 Moreover, it can be prepared by covalent bonding of functional groups chemically attached to the surface of CNTs,34-39 by mixing a CNT powder with a solvent-suspended binder,7 or by electrophoretic deposition,22,25-28,40-44 and so forth. The CNT-modified electrodes prepared that way can be coated with a polymer film by chemical10,11,20,45-48 or electrochemical5,8,10,12,15,49-52 polymerization. Moreover, the CNT composites of this type can be prepared by chemical or electrochemical polymerization in solutions of respective monomers and with suspended CNTs.17,18 One of the main aims of fabrication of the CNTs/polymer composites is to efficiently disperse nanotubes in the polymer matrices for the property improvement of the composites.14 Previously, we devised a composite of an electroactive fullerene-based polymer (C60-Pd) and MWCNTs.16 Moreover, we prepared a composite of this polymer and HiPCO singlewall carbon nanotubes, which were noncovalently surface modified by 1-pyrenebutyric acid (pyr-SWCNTs).17,18 This modification ensures that electrical properties of SWCNTs remain unchanged. In contrast, covalent attachment of functional groups for surface modification of SWCNTs causes changes in their physical properties because of scatter of electrons.29 The pyr-SWCNTs/(C60-Pd) film was electrochemically prepared under multiscan cyclic voltammetry (CV) conditions in solution containing pyr-SWCNTs, C60, Pd(II) acetate, and a supporting electrolyte in the mixed toluene/acetonitrile (4:1, v/v) solution.17,18 As opposed to most conducting polymers, the C60-Pd polymer is electrochemically active in the negative potential range due to reductive electro-activity of its fullerene moiety and concomitant p-doping with counter cations.52 Therefore, the resulting composite of pyr-SWCNTs and C60-Pd revealed superior conducting, capacitance and visco-elastic properties with respect to the blank polymer. The objective of the present work, largely extending our previous research, is to devise and characterize a composite containing pyr-SWCNTs electrophoretically deposited on the Au electrode and the (C60-Pd)-PBT mixed polymer to obtain a material conducting both in the negative and positive potential range due to alternate p- and n-doping with counterions and exhibiting both high specific capacitance and conductivity. 2. Experimental Section 2.1. Chemicals. C60 (99.5% purity) was from the M. E. R. Corp. (Tucson, AZ) or SES Research (Houston, TX). HiPCO

J. Phys. Chem. C, Vol. 113, No. 31, 2009 14047 single-wall carbon nanotubes were from Carbon Nanotechnology, Inc. (Houston TX). 1-Pyrenebutyric acid (97% purity), palladium(II) acetate, Pd(ac)2, (98% purity), and bithiophene (also known as 2,2′-bithienyl or 2,2′-dithienyl) (98% purity), BT, were from Aldrich. The toluene, (anhydrous, 99.8%), dimethylformamide, (anhydrous, 99.8%), 1-methyl-2-pyrrolidone, NMP, from Aldrich as well as acetonitrile, (puriss, absolute) and 1,2-dichlorobenzene, (anhydrous, 99%) from Fluka solvents were used as received. The tetra-n-butylammonium perchlorate, (TBA)ClO4, supporting electrolyte salt was used as received from Fluka. 2.2. Apparatus. For electrophoretic deposition, a twoelectrode system was used, which comprised a spiraled Pt wire and an Au electrode serving as the auxiliary (negative) and working (positive) electrode, respectively. Two different Au electrodes were used, that is, a Teflon shrouded 4 mm diameter Au disk and Au film electrode. The Au disk electrode was used for the CV and electrochemical impedance spectroscopy (EIS) measurements. The Au film electrode was that evaporated over a Ti underlayer onto a 14 mm diameter, 10 MHz resonant frequency, plano-plano quartz resonator with a matt finish of the Institute of Tele- and Radio Communication (Warsaw, Poland). Diameter of this film electrode was 5 mm. This electrode was used for simultaneous piezoelectric microgravimetry (PM) and CV measurements. The dc voltage was applied to the electrodes by using the IZS-5/71 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 surfacecoated with 1-pyrenebutyric acid SWCNTs (pyr-SWCNTs) 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 solutions. Simultaneous CV and PM experiments were performed by using an EP-21 potentiostat of Elpan (Lubawa, Poland) connected to a 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 an Au quartz crystal resonator during potential cycling. An AUTOLAB computerized electrochemistry system of Eco Chemie (Utrecht, The Netherlands) was used for the CV and EIS measurements with the film-coated electrodes. This system was equipped with the expansion cards of the PGSTAT 301N potentiostat and the FRA2 frequency response analyzer and was controlled by the GPES 4.9 software of Eco Chemie. A conventional three-neck one-compartment V-shaped glass electrochemical minicell of the working solution volume less than 0.5 mL was used for the CV experiments in the three-electrode system. A 4 mm diameter Au disk, coiled Pt wire, and Ag/ AgCl/NaCl(satd.) served as the working, auxiliary, and reference electrode, respectively. The EIS measurements were performed for the frequency range 10 kHz to 0.1 Hz at various constant electrode potentials using ac voltage of a 10 mV sine-wave amplitude for all experiments. The composite-film-modified electrode was kept at the selected potential for ∼120 s before performing each EIS measurement. Data were collected and analyzed using the Zplot version 2.9c software for Windows of Scribner Associates, Inc. (Southern Pines, NC).

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Figure 1. Schematic view of the instrumental setup for electrophoretic deposition of a pyr-SWCNT film on the Au/quartz electrode, from pyrSWCNT suspension in NMP, and simultaneous monitoring of the film growth with the use of the electrochemical quartz crystal microbalance.

Atomic force microscopy (AFM) imaging was performed with the use of a Multimode NS 3D instrument of the Digital Instruments/Veeco Metrology Group (U.S.A.). In this case the films were deposited on 7 mm diameter tin-doped indium oxide (ITO) electrodes of Image Optics Components Ltd. (Basildon, U.K.). The X-ray photoelectron spectroscopy (XPS) spectra were recorded with an Escalab-210 spectrometer of VG Scientific (East Grinstead, U.K.) using Al KR (hν ) 1486.6 eV) X-ray radiation. The pressure in the spectrometer chamber was ∼5 × 10-9 mbar. High-resolution scans were recorded with the 20 eV analyzer pass energy at a 0.05 eV increment for the C 1s, Pd 3d, and S 2p core level spectrum. The spectra were analyzed by the VG ECLIPSE data system software of Thermo VG Scientific (East Grinstead, U.K.) using a Gauss/Lorentz constant ratio of 0.3. The background spectrum was accounted for by using the Shirley method. All experiments were performed at ambient temperature, (20 ( 1) °C. 2.3. Procedures. 2.3.1. Electrophoretic Deposition of the pyr-SWCNTs Film. The instrumental setup for bulk electrophoretic deposition of the pyr-SWCNTs films is depicted in Figure 1. The horizontally held and facing up Au disk or Au/ quartz electrode with the auxiliary electrode mounted above the working electrode was positively polarized with the 24 V dc voltage. Distance between the two electrodes was kept constant at ∼6 mm all throughout the experiment. During this time, the pyr-SWCNTs film was deposited and the frequency change simultaneously measured with EQCM. After ∼6 min of deposition, the frequency change became negligibly small. Therefore, the total time of each deposition was set at 6 min. When the electrophoresis was seized, the electrode with the pyr-SWCNT film was rinsed with the abundant acetonitrile solvent and dried in an Ar stream. 2.3.2. Preparation of the Composite, pyr-SWCNTs/(C60-Pd)PBT, Film. (C60-Pd)-PBT mixed polymer was grown from the 0.27 mM C60, 3.56 mM Pd(ac)2, and 2 mM bithiophene, 0.1 M (TBA)ClO4 solution of toluene/acetonitrile (4:1, v/v) on the Au electrode (mixed polymer film) or on the Au/pyr-SWCNTs electrode (composite film) under multiscan CV conditions between 1.20 and -0.80 V at the potential sweep rate of 50

mV s-1. The C60-Pd film was deposited by reductive electropolymerization in the potential range of 0 to -0.8 V while that of PBT by oxidative electropolymerization in the range of 0 to 1.2 V during the same CV cycle. Once the film had been formed, the modified electrode was rinsed with acetonitrile. Next, the film-coated electrode was immersed in a 0.1 M (TBA)ClO4 solution of acetonitrile and simultaneous CV and PM, and then EIS measurements were performed for investigation of electrical and visco-elastic film properties. 3. Results and Discussion First, the pyr-SWCNTs film was electrophoretically deposited under PM and CV control. Then, electrochemical and viscoelastic properties of both the (C60-Pd)-PBT film and the pyrSWCNTs/(C60-Pd)-PBT film were investigated. Subsequently, both films were deposited with the same preparation procedure onto the ITO electrodes and imaged by AFM to unravel topographical details of the studied materials. Chemical composition of these materials was elucidated by performing XPS measurements. Finally, the pyr-SWCNTs/(C60-Pd)-PBT film was deposited on the Au disk electrode and its properties were characterized with the EIS and galvanostatic charging-discharging measurements. 3.1. Electrophoretic Deposition of the pyr-SWCNTs Film As Well As Its Electrical and Visco-Elastic Property Studies with the Use of Cyclic Voltammetry and Piezoelectric Microgravimetry. Electrophoretic deposition of the pyrSWCNTs films was monitored by PM. Curve 1 in Figure 2 shows time dependence of the resonant frequency change, ∆f, during electrophoretic deposition of the film. A cavity of the quartz crystal holder was filled with a fine suspension of pyrSWCNTs in NMP and then, after ∼10 s needed to reach stillness, the 24 V dc voltage was applied. In effect, the resonant frequency commenced to decrease with time as the film was deposited (curve 1 in Figure 2). With the use of the Sauerbrey equation, this frequency decrease was recalculated into the mass change (curve 2 in Figure 2). The total mean mass of this film, deposited after ∼6 min, was ∼30 µg. This amount is justified by negligible concomitant changes in the film visco-elasticity as substantiated by merely ∼10 ohm change in dynamic

SWNTs Coated with C60-Pd and Polybisthiophene Film

Figure 2. Time dependence of (1) the resonant frequency change and (2) the mass change for the electrophoretic deposition of the pyrSWCNT film on the Au/quartz electrode from the 0.4 mg mL-1 suspension of pyr-SWCNTs in NMP.

resistance. So, the entire frequency decrease can practically be ascribed to the mass change. In another study electrophoretic deposition of the surface modified by tetraoctylammonium bromide, TOAB, SWCNTs suspended in THF, resulted in a 6 mg deposition of the material under 50 V applied between two electrodes separated by 6 mm.22 The deposition rate depends on several experimental factors including concentration of the SWNT suspension, height of the dc voltage, and distance between electrodes.43 From the slope of the ∆m versus t curve (curve 2 in Figure 2), the rate of the film deposition was determined. That is, the initial rate was ∼20 µg min-1, being subsequently maintained for about 0.5 min. Then, this rate gradually decreased and it was by around an order of magnitude smaller, being equal to 2 µg min-1, in the final stage of the deposition. Over the 0.5-6.5 min time interval, a mean rate of the mass increase was ∼5 µg min-1. Similar rate changes were observed for electrophoretic deposition of the C60 aggregates.53 This rate decrease with time for pyr-SWCNTs might be due to the surface blocking effect. That is, after ∼4 min of the deposition time, all of the electrode surface was coated by the pyr-SWCNTs film. As a result, the effective dc electric field driving migration of the pyr-SWCNTs in solution toward the electrode surface might be decreased due to the increase of the ohmic potential loss across this film. After termination of the electrophoretic deposition voltage, the quartz resonator with its Au electrode coated by the pyrSWCNTs film was dried in an argon stream for 24 h and then rinsed with the acetonitrile solvent immediately before transferring to a 0.1 M (TBA)ClO4 acetonitrile solution. Figure 3 shows simultaneously recorded curves of the current, resonant frequency change versus potential, and dynamic resistance change versus potential for the Au/pyr-SWCNTs electrode in a blank supporting electrolyte solution of 0.1 M (TBA)ClO4 in acetonitrile. A pseudorectangular shape of the CV curve (curve 1 in Figure 3a) suggests a capacitive nature of the current recorded. The departure from the ideal rectangular shape is ascribed to some faradaic processes that might occur. The total current at both the negative and positive potentials includes capacitive and faradaic contributions, both of which linearly depend upon the potential scan rate (Figure 3b), as expected. For the negative potential range, that is, 0 to -1.50 V, the resonant frequency decreases during the cathodic potential scan (curve 2 in Figure 3a). This frequency decrease is accompanied by the increase of dynamic resistance (curve 3 in Figure 3a).

J. Phys. Chem. C, Vol. 113, No. 31, 2009 14049 When the negative potential is applied to charge the Au/pyrSWCNTs film negatively, most likely, counter cations enter the film in order to maintain its neutrality. This cation ingress makes the film less rigid, as manifested by the increase of dynamic resistance (curve 3 in Figure 3a). Changes in the ∆f versus E and ∆R versus E dependences are similar for the anodic and cathodic potential excursions. That is, the resonant frequency decreases but the dynamic resistance only slightly increases when the potential is scanned positively (curves 2 and 3 in Figure 3a). Presumably, this behavior, which is due to the counteranion ingress to the film for compensation of the positive charge generated, makes the film more viscous and heavy. The value of specific capacitance, Cs, for the pyr-SWCNTs film-coated electrode can be calculated from the following equation

Cs )

i dV m dt

( )

(1)

where i is the measured current, dV/dt is the potential scan rate, and m is the mass of the electroactive material deposited on the electrode surface. Accordingly, the Cs values were determined from slopes of the current dependence on the potential scan rate (Figure 3b). Figure 3c shows the dependence of the specific capacitance of the Au/quartz electrode with electrophoretically deposited pyr-SWCNTs on the potential scan rate. For the pyr-SWCNTs coated electrode, the Cs for the potential window of -0.40 to 0.40 V was lower than that for potentials more negative than -0.40 and more positive than 0.40 V. Interestingly, the pyr-SWCNTs coated electrode revealed higher Cs values for the positive potential range compared to the negative potential range. Moreover, for the potential scan rate of 0.1 V s-1, Cs values are slightly higher than those for higher scan rates while for lower than 0.1 V s-1, they revealed a rapid increase (Figure 3c). This behavior implies that the charge transport rate within the film is limited by the rate of the counterion diffusion and at the potential scan rate lower than 0.1 V s-1 the charge stored during the film charging increases with decreasing the scan rate. For each potential selected from the range of the CV scan, a new equilibrium of the ion dynamics is established. This equilibrium is governed by the film charging or discharging.53 However, if the time constant of the counterion diffusion within the film is slower compared to the potential scan rate, the amount of charge stored in the film would be higher.54 The mean capacitance value obtained from CV measurement was equal to ∼50 F/g. This value falls in the literature range of 35-160 F/g reported for CNT films.39,40 3.2. Simultaneous Piezoelectric Microgravimetry and Cyclic Voltammetry Behavior of the (C60-Pd)-PBT and pyrSWCNTs/(C60-Pd)-PBT Films. Simultaneously recorded with EQCM curves of multiscan CV, the resonant frequency change versus potential, and the dynamic resistance change versus potential corresponding to electropolymeric deposition of the (C60-Pd)-PBT film and the pyr-SWCNTs/(C60-Pd)-PBT film on the Au/quartz resonators are presented in Figure 4a,b, respectively. The film deposition was manifested by the decrease of the resonant frequency with the increase of the CV cycle number (curves 2, 3 and 2′, 3′ in Figure 4a,b). The total frequency shift due to mass loading of the quartz resonator is caused both by changes of its mass and changes of its visco-elastic properties. Therefore, the dynamic resistance was additionally measured as a variable related to visco-elasticity of quartz.18 Both films were deposited to reach nearly the same value of the total

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Figure 3. (a) Curves of (1) multiscan cyclic voltammetry as well as potential dependence of (2) the resonant frequency change and (3) the dynamic resistance change for the film of pyr-SWCNTs in 0.1 M (TBA)ClO4, in acetonitrile. Potential sweep rate was 100 mV s-1. The nanotubes were electrophoretically deposited on the Au/quartz electrode from a 0.4 mg mL-1 suspension of pyr-SWCNTs in NMP. Dependence of (b) current and (c) specific capacitance of the pyr-SWCNT film, electrophoretically deposited on the Au/quartz electrode, on the potential scan rate for different constant potentials indicated with numbers at curves.

Figure 4. Curves of (1 and 1′) multiscan cyclic voltammetry current, (2 and 2′) the resonant frequency change, and (3 and 3′) the dynamic resistance change versus applied potential for electrodeposition of films of (C60-Pd)-PBT in the solution of 0.27 mM C60, 3.56 mM Pd(ac)2, 1 mM bithiophene, and 0.1 M (TBA)ClO4 in toluene/acetonitrile (4:1, v/v) on (a) the bare Au/quartz electrode and (b) the Au-quartz/pyr-SWCNTs filmcoated electrode. Potential sweep rate was 50 mV s-1.

frequency decrease. This decrease was accompanied by the dynamic resistance increase of ∼123 and ∼69 Ω for the (C60Pd)-PBT film (curve 3 in Figure 4a) and pyr-SWCNTs/(C60Pd)-PBT film (curve 3′ in Figure 4b), respectively. This resistance decrease corresponds to frequency change of ∼12.3 and ∼6.9 Hz, respectively. However, the total frequency decrease for the (C60-Pd)-PBT film (curve 2 in Figure 4a) and pyr-SWCNTs/(C60-Pd)-PBT film (curve 2′ in Figure 4b), measured after 10 CV cycles, was ∼7.6 and ∼7.4 kHz,

respectively, being 3 orders of magnitude higher than those corresponding to the visco-elastic changes. Hence, the corresponding mass of the deposited film was ∼6.6 and ∼6.5 µg, respectively. The electropolymeric deposition of the (C60-Pd)PBT film on the Au/pyr-SWCNTs electrode resulted in smaller changes in the dynamic resistance than those corresponding to the deposition of the mixed polymer on the bare Au electrode. Apparently, rigidity changes of this latter film were smaller. Presumably, the deposited mixed polymer is incorporated in


J. Phys. Chem. C, Vol. 113, No. 31, 2009 14051

Figure 5. Curves of (1 and 1′) multiscan cyclic voltammetry current, (2 and 2′) the resonant frequency change, and (3 and 3′) the dynamic resistance change versus applied potential for the films of (a) the (C60-Pd)-PBT and (b) the pyr-SWCNTs/(C60-Pd)-PBT in 0.1 M (TBA)ClO4 in acetonitrile. Potential sweep rate was 500 mV s-1. Both films were deposited by electropolymerization from 0.27 mM C60, 3.56 mM palladium(II) acetate, 1 mM bithiophene, and 0.1 M (TBA)ClO4 in toluene/acetonitrile (4: 1, v/v) on (a) the bare Au/quartz electrode and (b) the Au-quartz/ pyr-SWCNTs film-coated electrode. Curve 1′′ in (b) is the CV curve for the pyr-SWCNTs film deposited onto the Au/quartz electrode.

the network of the pyr-SWCNTs bundles resulting in the formation of a much more tightly packed film. Moreover, capacitance current due to charging and discharging of the pyrSWCNTs film-coated electrode flows for the potential range -0.20 to 0.20 V (curve 1′ in Figure 4b). The quartz resonators with their Au electrodes coated either by the (C60-Pd)-PBT or pyr-SWCNTs/(C60-Pd)-PBT film, were transferred to blank 0.1 M (TBA)ClO4 acetonitrile solutions, and curves of the potential dependence of the current, resonant frequency change, and dynamic resistance change were simultaneously recorded (Figure 5). Each film revealed two potential windows of electrochemical activity. That is, one was that at potentials more negative than ∼-0.40 V due to redox electroactivity of the n-doped C60 moiety and the other at more positive than ∼0.40 V due to metal-like conductivity of p-doped PBT (curves 1 and 1′ in Figure 5). Between -0.40 and 0.40 V, the (C60-Pd)-PBT film is faradaically inactive showing a very low current flow (curve 1 in Figure 5a). However, the CV behavior of the pyr-SWCNTs/ (C60-Pd)-PBT film reveals well-developed capacitance current in this potential range arising from charging and discharging of the pyr-SWCNTs film (curves 1′ and 1” in Figure 5b). In the negative potential range, that is, -0.40 to -1.50 V, both cathodic and anodic currents for the (C60-Pd)-PBT film (curve 1 in Figure 5a) are much lower than those for the pyrSWCNTs/(C60-Pd)-PBT film (curve 1′ in Figure 5b). These higher currents for the composite film are most likely due to higher electrode area in the presence of pyr-SWCNTs and, hence, higher capacitance. In this potential range, there are two steps in the ∆f versus E curves (curves 2 and 2′ in Figure 5) and ∆R versus E (curves 3, and 3′ in Figure 5) both for the (C60-Pd)-PBT and pyr-SWCNTs/(C60-Pd)-PBT film electrode. These steps can be ascribed to two consecutive one-electron reductions of C60. That is, when the potential during its cathodic excursion reaches the value of the first electroreduction of the

C60 moiety, that is, that of ∼-0.50 V, the resonant frequency decreases (curve 2 in Figure 5a and curve 2′ in Figure 5b) indicating the increase of the mass of the films. This mass increase is associated with the TBA+ ingress to the film for compensation of the negative charge of C60- generated. However, both the value and the rate of the resonant frequency decrease versus potential applied is higher for the composite than for the mixed polymer being 0.8 and 0.6 kHz V s-1, respectively (curve 2 in Figure 5a and curve 2′ in Figure 5b). Apparently, TBA+ can penetrate the composite film much easier than that of the mixed polymer. The resonant frequency decrease is accompanied by the increase of the dynamic resistance due to the increase of the film visco-elasticity incurred during the TBA+ ingress. Although more TBA+ cations enter the composite film during the first electroreduction of C60 than the mixed polymer film, the changes in dynamic resistance are smaller for the former (curve 3 in Figure 5a and curve 3′ in Figure 5b). Most plausibly, the film of the composite is more rigid than that of the mixed polymer. For the second one-electron C60 reduction potential located at ∼-1.00 V, there is the second step in the resonant frequency decrease versus potential curve for both films (curve 2 in Figure 5a and curve 2′ in Figure 5b). Interestingly, for the composite film, the slope of the resonant frequency decrease versus potential for the second electroreduction of C60 is nearly twice as high as that for the first step and equal to 1.5 kHz V-1 (curve 2′ in Figure 5b). For the mixed polymer film, however, this behavior is opposite, that is, the slope of the resonant frequency decrease versus potential for the second electroreduction of C60 is equal to 0.5 kHz V s-1 being lower than that for the first step (curve 2 in Figure 5a). Further, the slope of this frequency decrease versus potential is much higher for the composite than that for the mixed polymer like if TBA+ more eagerly entered the former film. Moreover, this higher slope indicates that conductivity of the composite is higher than that of the mixed polymer (curve 2 in Figure 5a and curve 2′ in Figure 5b). The dynamic resistance during the

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Figure 6. Potential dependence of specific capacitance for the film of (1) (C60-Pd)-PBT, (2) pyr-SWCNTs/(C60-Pd)-PBT, and (3) pyrSWCNTs in 0.1 M (TBA)ClO4 in acetonitrile.

second electroreduction of the C60 moiety, accompanied by the TBA+ ingress, is similar for both films (curve 3 in Figure 5a and curve 3′ in Figure 5b). Surprisingly, these similar changes of the dynamic resistance correspond to different amounts of TBA+ entering these two films. That is, this amount is twice as high for the film of the composite as that for the mixed polymer (curve 2 in Figure 5a and curve 2′ in Figure 5b). In the positive potential window covering the potential range of 0.40 to 1.00 V, both films are π-conducting and, interestingly, anodic and cathodic currents for both films are similar (curve 1 in Figure 5a and curve 1′ in Figure 5b). Both for the (C60-Pd)PBT and pyr-SWCNTs/(C60-Pd)-PBT film, the current increases when the potential reaches ∼0.40 V and this growth is maintained with the increase of the positive potential (curve 1 in Figure 5a and curve 1′ in Figure 5b). For both films, this current increase is accompanied by a small decrease of the resonant frequency, presumably due to the ClO4- ingress to both films for compensation of the positive charge generated in the polymer. During this positive potential scan, dynamic resistance is virtually constant (curves 3 and 3′ in Figure 5). The Cs value for the electrode coated either by the (C60-Pd)PBT film or the pyr-SWCNTs/(C60-Pd)-PBT film, calculated from eq 1, strongly depends on the applied potential (curve 1 and 2 in Figure 6, respectively), as expected. That is, Cs is higher for the potential windows of electroactivity of C60-Pd and PBT. There is no such a strong potential dependence of Cs for the pyr-SWCNTs coated electrode (curve 3 in Figure 6). Moreover, the pyr-SWCNTs/(C60-Pd)-PBT film-coated electrode features higher specific capacitance than that of the (C60-Pd)-PBT filmcoated electrode. The determined average Cs value was equal to 71, 60, and 50 F g-1 for the film of pyr-SWCNTs/(C60-Pd)PBT, (C60-Pd)-PBT, and pyr-SWCNTs, respectively. 3.3. Atomic Force Microscopy Imaging of Topography of the Surface of the (C60-Pd)-PBT Film and the pyrSWCNTs/(C60-Pd)-PBT Film. Figure 7 shows the AFM images of the surface of the pyr-SWCNTs films, deposited by electrophoresis on the ITO electrodes during 1 min (Figure 7a,b) and 6 min (Figure 7c) of deposition at 24 V (Figure 7a,c) and 40 V (Figure 7b) of dc voltage applied. At higher voltages applied (>100 V), the SWCNT bundles stretch out into the solution and orient themselves perpendicular to the electrode surface.41 For pyr-SWCNTs, however, such a behavior was herein not observed. Instead, for potential higher than 80 V, pyr-SWCNTs aggregated forming visible objects, which did not migrate toward the electrode. After 1 min of deposition at 24 V dc applied, the bundles of pyr-SWCNTs were seen on the ITO surface (Figure 7a). These

Pieta et al. bundles of ∼35 nm in diameter were randomly distributed all over the surface forming 200 nm thick uniform film covering the entire surface. Similar film was obtained after 30 s of deposition at 20 V dc from SWCNTs and sodium hydroxide solution in DMF.43 For the same time of deposition but 40 V dc applied, more pyr-SWCNTs bundles of ∼50 nm in diameter could be distinguished on the surface (Figure 7b). They formed a 400 nm thick uniform film. Similar bundles of the TOABcoated SWCNTs with ∼50 nm in diameter were electrophoretically deposited by applying 100 V cm-1 between two electrodes being kept ∼6 mm apart in the SWCNTs/TOAB suspension in THF.56 For 6 min deposition at 24 V, the resulting film consisted of much more ∼35 nm diameter densely packet pyr-SWCNTs bundles than for 1 min deposition at the same voltage, forming ∼460 nm thick uniform film. The morphology of this film is similar to the SWCNTs film obtained after 10 min deposition at 20 V cm-1 of SWCNTs suspended in solution of sodium hydroxide in DMF.42 Electrophoretic deposition monitored by PM revealed that for 1 and 6 min deposition at 24 V dc the total determined mean mass of the pyr-SWCNTs film deposited onto 5 mm diameter the Au/quartz electrode was ∼10 and 30 µg (see Section 3.1 above), respectively. Assuming that the density of the SWCNTs film is ∼1.4 g cm-3 56 one can estimate the corresponding nanotube film thickness to be ∼37 and 110 µm for ∼10, respectively. However, thickness of the pyrSWCNTs film deposited on the ITO electrode under the same conditions was about 3 orders of magnitude smaller. The apparent discrepancy may arise from the difference in the nature of the surface of the electrode used. That is, the ITO electrode surface is much smoother than that of the Au/quartz. Therefore, pyr-SWCNTs adhere much better to the latter in the course of electrophoretic deposition. However, high roughness of the Au/ quartz electrode makes imaging of the deposited films impossible. Surface roughness, expressed by the relative surface area, Rsa, and represented as Rsa ) A3D/A2D, that is, by the ratio of area in three-dimensional space, A3D, to that projected into twodimensional space, A2D, was different for different conditions of the deposition. That is, the Rsa value for the pyr-SWCNTs film deposited during 6 min at 24 V dc was higher than those for the films obtained at shorter deposition time (Table 1). The increase of the deposition time results in the increase of the film thickness increasing the surface area of the electrode. However, the increase of the dc voltage causes the deposition of thicker bundles of pyr-SWCNTs. Therefore, the Rsa value for the pyr-SWCNTs film deposited during 1 min at 24 and 40 V dc is nearly the same (Table 1). The AFM images of surfaces of the (C60-Pd)-PBT film and the pyr-SWCNTs/(C60-Pd)-PBT film deposited on the ITO electrodes are shown in Figure 8a,b, respectively. In both cases, the (C60-Pd)-PBT films were deposited during 10 CV cycles. The (C60-Pd)-PBT film is relatively uniform. It is built of microscopically distinguishable globular objects with 50-100 nm diameters sticking together to form clusters with 200-320 nm diameters. These clusters are merged forming an ca. 300 nm thick film that coats entire surface of the ITO electrode. The pyr-SWCNTs/(C60-Pd)-PBT film is built of a tangle of bundles of pyr-SWCNTs. These bundles are coated by the ∼100 nm diameter globules of the (C60-Pd)-PBT polymer. The average thickness of this film is ∼500 nm. Interestingly, the (C60-Pd)PBT film deposits onto the nanotube surface gradually filling the accessible space between pyr-SWCNTs.



Figure 7. Atomic force microscopy images (5 × 5 µm2 area) of the pyr-SWCNT film electrophoretically deposited on ITO electrodes. The films were deposited following the application of (a,c) 24 V dc and (b) 40 V dc for a duration of (a,b) 1 min and (c) 6 min.

TABLE 1: Thickness and Relative Surface Area, Rsa, of the pyr-SWCNTs Films Deposited under Different Electrophoretic Conditions time and dc voltage of the electrophoretic film deposition

film thickness (nm)

relative surface area (Rsa ) A3D/A2D)

1 min, 24 V 1 min, 40 V 6 min, 24 V

200 400 460

1.9 1.8 2.3

3.4. Chemical Analysis of the (C60-Pd)-PBT Film and the pyr-SWCNTs/(C60-Pd)-PBT Film by X-ray Photoelectron Spectroscopy. XPS measurements provided information on chemical composition of both the (C60-Pd)-PBT (Figure 9) and pyr-SWCNTs/(C60-Pd)-PBT (Figure 10) film. For these films, peaks corresponding to the C, Pd, and S atoms are distinguished. The binding energy (EB) range of the Pd 3d electrons reveals the presence of three different forms of palladium for both films (Figures 9a and 10a). That is, for the (C60-Pd)-PBT film (Figure 9a) there are three pairs of peaks with the EB values of the 3d5/2 state at 334.6, 336.6, and 338.0 eV and a spin-orbit splitting of the 3d5/2 and 3d3/2 states of 5.3 eV with relative intensities of 0.4:14:1, respectively. The peaks at 334.6 and 338.0 eV can be assigned to Pd0 and Pd2+, respectively.58 The most intense

peak at 336.6 eV would then correspond to a palladium species with an oxidation state intermediate between 0 and +2 indicating the presence of interaction between the Pd and C60 species57,58 in the C60-Pd polymer network. The EB values for the 3d5/2 state of palladium in the case of the pyr-SWCNTs/(C60-Pd)-PBT film (Figure 10a) are 334.8, 337.0, and 338.7 eV with relative intensities of 0.2:8:1, respectively. The distance of the spin-orbit splitting of the 3d5/2 and 3d3/2 states was 5.2 eV. Similarly to the XPS peaks for the (C60-Pd)-PBT film (Figure 9a), the peaks at 334.8 and 338.7 eV can be assigned to Pd0 and Pd2+, respectively, while the most intense peak at 337.0 eV can be associated with the presence of the palladium species bound in the C60-Pd polymer chain with an oxidation state intermediate between 0 and 2+. The sulfur 2p XPS spectra for the (C60-Pd)-PBT and pyrSWCNTs/(C60-Pd)-PBT film are shown in Figure 9b and 10b, respectively. One pair of peaks with the binding energy of the 2p3/2 state at 163.7 and 163.8 eV and a spin-orbit splitting of the 2p1/2 states of 1.4 eV for the polymer and composite film, respectively, corresponds to covalently bound sulfur atom in the aromatic ring of carbon atoms.60 The XPS spectra in the EB range of the C 1s electrons reveal the presence of five and four different forms of carbon for the (C60-

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Figure 8. Atomic force microscopy images, (5 × 5 µm2 area) of (a) and (a′) the film of (C60-Pd)-PBT as well as (b) and (b′) the pyr-SWCNTs/ (C60-Pd)-PBT film.

Pd)-PBT (Figure 9c) and pyr-SWCNTs/(C60-Pd)-PBT film (Figure 10c), respectively. In the case of the (C60-Pd)-PBT film, there are peaks at EB values of 284.7, 286.5, 288.2, 289.7, and 291.1 eV with relative intensities of 38.2:2.5:1.6:1.5:1, respectively (Figure 9c). The most intense peak at 284.7 eV is ascribed to the fullerene and bithiophene carbon atoms.60 The other four small peaks most likely originate from the tetra-n-butylammonium ions (286.5 eV) entrapped in the polymer network during electropolymerization from carboxy groups of acetate ligands (288.2 eV) and from a shakeup process involving the energy of the πfπ* transition in C60 (289.1 and 291.1 eV).61 For the pyr-SWCNTs/(C60-Pd)-PBT film, the peaks are at EB of 284.7, 286.7, 288.5, and 290.2 eV with relative intensities of 29.6:3.0:1.5:1, respectively, (Figure 10c). As above, the most intense peak at 284.7 eV is ascribed to the pyr-SWCNTs, bithiophene, and fullerene carbon atoms.60 The origin of the other three small peaks could be explained in the same way as that for the (C60-Pd)-PBT film. The mole ratio of BT/C60/Pd ≈ 1:2:4 was determined for the (C60-Pd)-PBT film from the relative integrated intensities of the respective S 2p, C 1s, and Pd 3d core level spectra. This mole ratio indicates that, most likely, two different polymers, that is, C60-Pd and PBT, are simultaneously deposited under CV conditions. 3.5. Electrochemical Impedance Spectroscopy (EIS) Studies of the pyr-SWCNTs/(C60-Pd)-PBT Film. The EIS technique was used to evaluate both low-frequency, CL, and doublelayer, Cdl, capacitance as well as the charge transfer resistance, Rct, and diffusion time constant, τ, for the pyr-SWCNTs/(C60Pd)-PBT film-coated electrode.

The complex-plane impedance plots (Z′-Z′′), called Nyquist plots, for the electrode coated with the pyr-SWCNTs/(C60-Pd)PBT film in 0.1 M (TBA)ClO4 in acetonitrile recorded at different frequencies and for different potentials are shown in Figure 11. Both for the potential range of 0 to -1.10 V (Figure 11a) and that of 0 to 1.0 V (Figure 11b) the Warburg-type plot is linear with π/4 slopes at both high and intermediate frequencies implying control of the rate of charge transport within the composite film by the rate of semi-infinite diffusion of counteranions.62,63 Then, at low frequencies the rate of this transport becomes controlled by the rate of finite diffusion, as characterized by sharp upturns in the curves (Figure 11).62,63 The complex plane impedance plots were analyzed using a modified Randles equivalent electrical circuit61 (Scheme 1), reflecting the electrochemical processes at the Au/(pyr-SWCNTs/ (C60-Pd)-PBT)/solution interface. An abridged transmission-line model was used for quantitative expression of the Warburg-type, Zw, element62,63

ZW ) Z0

coth[(iω ¯ τ)0.5] (iω ¯ τ)0.5

(2)

In this model, the time constant of the counterion diffusion, τ, given by

τ)

l2 D

(3)



Figure 9. X-ray photoelectron spectroscopy spectra for the (C60-Pd)PBT film electropolymerized on an ITO electrode. The binding energy region of electrons of (a) Pd 3d3/2 and Pd 3d5/2, (b) S 2p and S 2p3/2 as well as (c) C 1s. The (C60-Pd)-PBT film was deposited on the ITO electrode under conditions described in Section 3.2.

where ω is angular frequency, l is the diffusion length, and D is the counterion diffusion coefficient. Z0 is given by

Z0 )

l2 /D CL

(4)

where CL is the overall differential pseudocapacitance of the film-coated electrode, that is, that associated with the Faradaic process, and measurable in the low frequency range.62,63 The -Z″ versus Z′ curves at different potentials (Figure 11) were simulated and the equivalent circuit electrical parameters fitted. The determined parameters are plotted versus potential in Figure 12. For the potential range of 0 to -0.30 V, conductivity of the pyr-SWNTs/(C60-Pd)-PBT film is relatively low. However, the CV behavior of this film (curve 1 in Figure 13) reveals well-developed capacitance current, already described into more detail (see above in Section 3.2). Moreover, the presence of the straight line with

Figure 10. X-ray photoelectron spectroscopy spectra for the pyrSWCNTs/(C60-Pd)-PBT film deposited on an ITO electrode. The binding energy region of electrons of (a) Pd 3d3/2 and Pd 3d5/2, (b) S 2p and S 2p3/2 as well as (c) C 1s. The (C60-Pd)-PBT film was deposited on the ITO electrode electrophoretically coated by the pyr-SWCNT film under conditions described in Section 3.2.

the π/4 slope for the high-frequency region (Figure 11a) may suggest that the counterion diffuses inside the film during its charging and discharging even for the potential range in which the polymer is not conducting. For this potential range, both lowfrequency redox capacitance and double-layer capacitance are low (Figure 12a,d, respectively) while the charge transfer resistance is high (Figure 12b). At the potential close to -0.40 V, electroreduction of the C60-Pd moiety of the mixed polymer film commences in the course of the negative potential excursion. As a result, the charge transfer resistance decreases (Figure 12b) and both the low-frequency redox capacitance and the double-layer capacitance increases (Figure 12a,d, respectively) with the potential decrease. Moreover, the diffusion time constant then decreases suggesting the increase of the ion diffusion coefficient. At -0.50 V, the charge transfer resistance reaches its minimum (Figure 12b) while both the low-frequency redox capacitance and the double-layer capacitance attain its maximum (Figure 12a,d, respectively). This behavior is associated with the electrode processes of the C60 moiety present

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Figure 11. Complex-plane impedance plots for the pyr-SWCNTs/(C60-Pd)-PBT film-coated 4 mm diameter Au disk electrode in 0.1 M (TBA)ClO4 in acetonitrile in the potential range (a) 0 to -1.10 V and (b) 0 to 1.0 V. The (C60-Pd)-PBT mixed polymer film was grown under CV conditions in the 0.27 mM C60, 3.56 mM Pd(ac)2, 1 mM bithiophene, and 0.1 M (TBA)ClO4 solution of toluene/acetonitrile (4:1, v/v) on the Au-disk/pyrSWCNTs modified electrode.

SCHEME 1: A Modified Randles Equivalent Circuit of the Electrode-Film-Solution Interfacea

a

RΩ is the ohmic resistance of the electrolyte and Au disk electrode, Rct is the total charge-transfer resistance of the faradaic process, Zw is the impedance of the counter ion diffusion in the film, CPEdl represents experimental double-layer capacitance of the film/(electrolyte solution) interface.

in the mixed polymer film. That is, at ∼-0.50 V the first cathodic peak is observed on the CV curve for C60 in solution (curve 2 in Figure 13). For the potential range of -0.70 to -0.90 V, no current peak is seen on the CV curve for C60 in solution (curve 2 in Figure 13). It means that for the film the entire amount of C60 is electroreduced to its monoanion (C60-). Therefore, the redox capacitance decreases (Figure 12a) while the charge transfer resistance (Figure 12b) and diffusion time constant (Figure 12c) increases. Moreover, the double-layer capacitance slightly decreases (Figure 12d). When the potential, during cathodic excursion, reaches the value more negative than ∼-0.90 V, that is, that characteristic of the second cathodic CV peak of the one-electron C60-/C602reduction (Figure 12a), both the redox capacitance (Figure 12a) and the double-layer capacitance (Figure 12d) increases while both the charge transfer resistance (Figure 12b) and the diffusion time constant (Figure 12c) decreases. The mean diffusion time constant does not exceed 0.03 s suggesting high dynamics of the charge propagation within the film.64 This behavior is in good agreement with the CV results. That is, even for the potential scan rate as high as 500 mV s-1 the shape of the CV curve for the pyr-SWCNTs/(C60-Pd)-PBT film-coated electrode is still that typical for capacitance currents indicating rather rapid charge propagation within the composite. 3.6. Galvanostatic Study of Charge-Discharge Cycling of the pyr-SWCNTs/(C60-Pd)-PBT Film. The 90 µA constantcurrent charging-discharging measurements at different dc voltage limits were performed on the pyr-SWCNTs/(C60Pd)PBT-coated Au electrode (Figure 14) in order to unravel its capacitive properties. The rapid voltage increase and then decrease recorded during charging and discharging, respectively, of the composite film-coated Au electrode indicates its high

Figure 12. Curves of (a) the low-frequency redox capacitance change (b) the charge transfer resistance change, (c) the diffusion time constant, and (d) the double-layer capacitance change vs potential for the film of pyr-SWNTs/(C60-Pd)-PBT, in 0.1 M (TBA)ClO4 in acetonitrile.

conductivity. The voltage increased during the charging semicycle until it reached the value of 1.40, 1.80, and 2.30 V (curve 1, 2, and 3 in Figure 14, respectively). The corresponding stored charge of 0.90, 1.35, and 2.7 mC is associated with the semicycle of charging for 10, 15, and 30 s, respectively. Only the charging curves for the voltage limit of 1.40 and 1.80 V (curves 1 and 2 in Figure 14) were nearly linear. For the 2.30 V limit, however, a pronounced deviation from linearity was observed that might arise from some faradaic process occurring. Upon discharging, the voltage decreased with the same rate as that corresponding to charging down to ∼0.80-0.70 V. Then,


Figure 13. Cyclic voltammograms for (1) the film of the pyr-SWCNTs/ (C60-Pd)-PBT in 0.1 M (TBA)ClO4 in acetonitrile, and (2) 0.34 mM C60 in 0.1 M (TBA)ClO4 in 1,2-dichlorobenzene. The film in (1) was deposited from the 0.27 mM C60, 3.56 mM Pd(ac)2, 1 mM bithiophene, and 0.1 M (TBA)ClO4 solution of toluene/acetonitrile (4:1, v/v) on the Au-quartz/pyr-SWCNT film-coated electrode.

J. Phys. Chem. C, Vol. 113, No. 31, 2009 14057 havior of the composite film is controlled by redox conductivity of C60-Pd in the negative potential range and by π-electron conductivity of PBT in the positive potential range. For the negative potential range, the maximum specific capacitance for the pyr-SWCNTs/(C60-Pd)-PBT composite film (100 F g-1) is higher than that for the (C60-Pd)-PBT mixed polymer film (70 F g-1). Similarly, for the positive potential range, the highest Cs value for the pyr-SWCNTs/(C60-Pd)-PBT composite film (∼200 F g-1) is also higher than that for the (C60-Pd)-PBT mixed polymer film (180 F g-1). The constant-current chargingdischarging of the pyr-SWCNTs/(C60-Pd)-PBT film-coated electrode at the low current applied (90 µA) is relatively rapid and equal to ∼0.12 V s-1 at the voltage limit up to 1.80 V. Moreover, the charging curves for this limit are nearly linear. Acknowledgment. The authors thank M.Sc. Andrzej Bilinski and Dr. Olga Chernyaeva for help with the XPS experiments and acknowledge European Regional Development Fund (ERDF, POIG.01.01.02-00-008/08 2007-2013 to W.K.) and National Science Foundation (CHE 0804015 to F.D.) for financial support. References and Notes

Figure 14. The charge-discharge curves for the pyr-SWCNTs/ (C60Pd)-PBT film-coated Au electrode in 0.1 M (TBA)ClO4 in acetonitrile at constant current of 90 µA for the voltage limit of (1) 1.40, (2) 1.80, and (3) 2.30 V. The film was deposited by electropolymerization in 0.27 mM C60, 3.56 mM Pd(ac)2, 1 mM bithiophene, and 0.1 M (TBA)ClO4, in toluene/acetonitrile (4:1, v/v) onto the Auquartz/pyr-SWCNT film-coated electrode.

the pace of this decrease was lowered until complete discharging was reached. The charge for the discharging semicycle was almost the same as that for the charging semicycle indicating very high efficiency of charging/discharging. 4. Conclusions By electrophoretic deposition under different dc voltage and time conditions, the pyr-SWCNTs film of controlled mass and surface topography was prepared on the electrode surface. This film revealed encouraging capacitive features and appreciable charge transport rate during its charging and discharging. The CV electroreduction of C60 in the presence of Pd(ac)2, combined with the electro-oxidation of the bithiophene, results in the formation of a mixed polymer film onto the pyr-SWCNTs coated electrode. This polymer forms ∼100 nm diameter globules on the pyr-SWCNTs bundles. Presumably, the two components of the mixed polymer, that is, C60-Pd and PBT, presumably, are not mutually connected covalently making up rather a mixture of two independent polymers deposited simultaneously under CV conditions. The pyr-SWCNTs/(C60-Pd)-PBT composite film is electrochemically active in both the positive and negative potential range. However, variation of electrical parameters, such as low-frequency redox capacitance, charge-transfer resistance, and double-layer capacitance is different in the negative and positive potential range. This is because electrochemical be-

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