1023-1935/05/4104- © 2005

åÄIä “Nauka

/Interperiodica”0439

Russian Journal of Electrochemistry, Vol. 41, No. 4, 2005, pp. 439–446. Translated from Elektrokhimiya, Vol. 41, No. 4, 2005, pp. 501–509.Original Russian Text Copyright © 2005 by Ovsyannikova, Efimov, Moravsky, Loutfy, Krinichnaya, Alpatova.

INTRODUCTION

In 1991, that is six years after the fullerenes werementioned for the first time [1], a new form of con-densed carbon was reported, namely, cylindrical carbonnanotubes (CNT) [2]. Carbon atoms of a sphericalfullerene molecule, e.g.

ë

60

, are bound into alternatesix- and five-membered condensed cycles (Fig. 1). Theelectronic structure of such macrocycles facilitates adiscrete transfer of up to six electrons to the fullerene[3, 4], yielding an anion.

Single-walled nanotubes (SWNT) are graphitesheets rolled to make cylinders whose typical diameteris 1.4 nm, which is close to the

ë

60

diameter. Multi-walled nanotubes (MWNT) comprise concentric cylin-ders with a wall-to-wall distance of 3.4 nm and a diam-eter of 10 to 20 nm (Fig. 1). The length of CNT rangesfrom hundreds of micrometers to a few centimeters [5].The CNT terminate in fullerene-type hemispheres(“caps”). To open CNT (remove the caps), selectiveoxidation may be used [6]. Unlike fullerenes, CNTentirely consist of six-membered cycles and are notsuitable for a discrete electron transfer. These proper-ties make CNT good objects for studying redox activityof compounds coated on them, in particular, variouspolymer films. Advances in the development of meth-ods for synthesizing CNT [6, 7] stimulated intensestudies of their properties and applications [5, 8–10].

The outstanding strength of CNT, combined withhigh electrical and thermal conductivity, suggests that

one of their most perspective applications is the deve-lopment of polymer–carbon composites (the more so,including conducting polymers) with unique physicalcharacteristics [11–20] and their use as substrates forcatalysts (electrode materials for fuel cells, lithium bat-teries, sensors).

Depending on the method of preparation, CNT canbe grouped as (

i

) those obtained with arc-discharge orlaser heating, with a chaotic (involved) structure[7, 21], and (

ii

) those grown at a substrate surface (cat-alytic pyrolysis) [7, 22]. In the latter case, by choosingthe catalyst composition and pyrolysis conditions, onecan obtain well orientated structures in which CNT arearranged normally or at some angle to a plane substrate(“rye field”).

A hydrocarbon pyrolysis at high-melting substratesin the presence of an iron-containing catalyst is also aconvenient way for electrodepositing polymers andobtaining the applied catalysts [22]. The resultant coat-ing is voluminous and comprises interwoven MWNT;each of them is accessible for the substance to bedeposited, which is a crucial condition for the polymerelectrodeposition.

Numerous attempts were made at using CNT forimproving characteristics of electron-conducting poly-mers (ECP) (Table 1). Composites with polymers solu-ble in common organic solvents are usually producedusing a CNT suspension (Table 1). The latter isobtained in a sonificated polymer solution and thenapplied onto a substrate by drop casting or spin casting.An advantage of this method is the possibility of using

Electrochemical Properties of Thin-Layered Composites Formed by Carbon Nanotubes and Polybithiophene

E. V. Ovsyannikova

a

, O. N. Efimov

b

, A. P. Moravsky

c

, R. O. Loutfy

c

, E. P. Krinichnaya

b

, and N. M. Alpatova

a,

z

a

Frumkin Institute of Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119071 Russia

b

Institute of Problems in Chemical Physics, Russian Academy of Sciences, p/o Chernogolovka, Noginskii raion, Moscow oblast, 142432 Russia

c

MER Corporation, Tucson, USA

Received June 30, 2004

Abstract

—Redox behavior of thin films of polybithiophene deposited on substrates of conducting glass andsingle- or multi-walled carbon nanotubes is studied at positive potentials in a 0.1 M (C

4

H

9

)

4

NBF

4

solution inacetonitrile. The polymer’s formal doping–undoping potentials are nearly the same for all substrates, whichpoints to the absence of any marked donor–acceptor interaction between nanotubes and polybithiophene. Somepolybithiophene electrochemical characteristics (reversibility, doping degree) are improved when depositedonto nanotubes, probably due to the developed surface of the electrode based on carbon nanotubes.

Key words

: carbon nanotubes, polybithiophene, composite, anodic redox behavior

z

Corresponding author, e-mail: alpatova@elchem.ac.ru

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OVSYANNIKOVA

et al

.

C

60

C

70

C

80

Straight Spiral

Open Closed

Single-walled Two-walled Multiwalled

Fig. 1.

Representative fullerenes (C

60

,

C

70

,

C

80

) and carbon nanotubes.

both conducting and nonconducting substrates. Thismethod produced polythiophene-based compositescontaining long-chained alkyl substituents in position 3and composites with derivatives of poly(

p

-phenylene-vinylene); these are characterized by chaoticallyarranged CNT; moreover, small-diameter SWNT formbunches.

Stability is the distinguishing feature of CNT sus-pensions in polymer solutions. It is caused by thedonor–acceptor interaction between the dissolved poly-mer and the CNT surface. By enveloping CNT, thepolymer prevents small CNT bunches from forminglarge aggregates. Due to interaction of the dissolvedpolymer with CNT, such solutions can be used for sep-arating nanotubes from their mixture with graphite andother carbonaceous impurities. Such mixtures occur,for example, when synthesizing CNT by arc discharge.Thanks to the polymer’s interaction with CNT, thedrop-cast composites consist of CNT (individual orbraided) uniformly covered by the polymer.

The spin-cast composite films were thoroughlytested for their electroconductance. As the polymer inthese composites is undoped (nonconducting), thecomposite’s conduction crucially depends on the con-tent of the conducting component (CNT). These sys-tems demonstrate a well pronounced percolation effect.On reaching the percolation threshold (7–10 wt % ofCNT), the conductance increases by 8–10 orders ofmagnitude. The photoconductance also has a percola-tion threshold.

A drop-cast layer of SWNT and poly(3,4-ethylene-dioxythiophene) (PEDT) fills a highly important placeamong the CNT–polymer composites. The PEDTuniqueness is its doped state in the initial state.

Composites for insoluble ECP are known mainly forpolypyrrole and polyaniline and its derivative, poly(

o

-

anisidine). These polymers can be prepared via bothchemical and electrochemical polymerization usingCNT suspended in solution (chaotic structure) andgrown at high-melting substrates (well organized struc-ture). The electrochemical polymerization yields moreuniform coatings with a higher redox activity.

Of particular interest is electrochemical applicationof a composite from a solution containing simulta-neously a monomer and CNT. In the strong donor–acceptor interaction between SWNT and aniline, CNTaccept electrons [46]. Due to the formation of a strongcharge-transfer complex between aniline and SWNT,such tubes “dissolve” in many media (water, acetone,etc.). Under anodic polarization, a monomer–SWNTcomplex undergoes oxidation to form a composite,which contains a dense uniform network of CNT cha-otically distributed in the polymer matrix.

Noteworthy is that the papers cited in Table 1 weremainly done as applications; therefore, use was made ofthick polymer or composite films, which camouflagedthe polymer–CNT interaction.

In this work we study electrochemical behavior ofthin films of unsubstituted polythiophene—poly-bithiophene, electrochemically deposited on three sub-strates: conducting glass (ITO), conducting glass witha drop-cast SWNT film on it, and titanium nitride withMWNT grown on it.

EXPERIMENTAL

The precursor was bithiophene (Kassei, Tokyo)purified by a vacuum sublimation. Acetonitrile (ultra-high purity grade, for chromatography) and the sup-porting salt

(

C

4

H

9

)

4

NBF

4

were used as received.When preparing electrodes, we used SWNT (1.03–

1.32 nm in diameter) purified by an original procedure

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ELECTROCHEMICAL PROPERTIES OF THIN-LAYERED COMPOSITES 441

developed in the Institute of Problems in ChemicalPhysics (IPCP) and samples with oriented MWNTmasses grown on TiN (both types of samples were pre-pared in the MER Corporation) [48].

The SWNT were obtained in the electric arc dis-charge, with a blended Co–Ni catalyst. To separate and

quickly purify SWNT from metal impurities, amor-phous carbon, nanoparticles, and various graphite spe-cies formed as condensation products in an electric-arcsynthesis, we used a procedure developed in IPCP [48].Specifically, SWNT were preliminarily sonificated in con-centrated hydrochloric acid and then subjected to multi-stage high-temperature oxidation in a muffle furnace.

Table 1.

Methods for preparation of CNT–ECP composites

No. System Preparation method Properties (applications) Reference

ECP soluble in common organic solvents

1 MWNT–poly(3-hexy-lthiophene) + C

60

Drop casting on quartz, glass, ITO of CNT suspensions in polymer solutions in toluene, chloroform, and hexane–chlo-roform

Optical absorption spectrum, photoconductivity [23]

2 SWNT–poly(3-oc-tylthiophene)

Optical absorption spectrum, conductivity(photodiodes) [24, 25]

3 MWNT–poly(3-oc-tylthiophene) Conductivity; hardness, thermal stability [26]

4 SWNT (CNT)–substi-tuted phenylene vi-nylene copolymer

(photodiode) [27]

5 Conductivity (polymer solution: CNTseparation from mixture with graphite) [28–33]

ECP producing a stable suspension

6 SWNT–poly(3,4-eth-ylenedioxythiophene)

Drop casting mixture of SCNT and polymersuspensions on ITO

(light-emitting diodes) [34]

ECP insoluble in common organic solvents

Polypyrrole (PP)

7

MWNT–PP

CP* and EP** on chaotic CNT structures (supercapacitors)

[35–37]

8 EP [38]

9 EP from monomer- and CNT-containing solution Spectra (displays) [39]

10CNT–PP

EP on assembled CNT struc-tures

Increase of redox activity and improvement of electrochemical reversibility [40, 41]

11CP on chaotic CNT structures

d

CNT

= 20–30 nm,

d

CNT + PP

= 80–100 nm [42]

12 SWNT–PP Increase of redox activity (supercapacitors) [43]

Polyaniline (PAN) and poly(

o

-anisidine)

13MWNT–PAN CP and EP on the CNT assem-

bled structures

d

CNT

= 20–60 nm,

d

CNT + PAN

= 90–100 nm [44]

14 [45]

15 SWNT–PAN EP from monomer- and CNT-containing solution

Charge-transfer complex between monomer and CNT; CNT+PAN chaotic distribution [46]

16 MWNT–poly(

o

-anisi-dine)

CP in aqueous solution; solving of composite in chloroform and applying to electrode

Photoelectrochemical activity of films [47]

* Chemical polymerization of monomer.** Electrochemical polymerization of monomer.

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et al

.

Thin SWNT films were applied onto ITO by castingan SWNT solution in 1,2-dichlorbenzene (1 mg in3 ml) first sonificated (35 kHz, 100 W) for 20 min. TheSWNT/ITO electrodes were dried at room temperaturefor several days. The SWNT weight was estimated as0.01 mg/cm

2

.

To produce MWNT masses, we used catalyticpyrolysis of hydrocarbons [22, 49], a well knownmethod used with different catalysts, substrates, andcarbon sources [6, 50].

We used thermal decomposition at

700°ë

in aquartz reaction chamber of volatile, at atmosphericpressure, ferrocene + xylene mixture in an Ar–H

2

atmo-sphere. The MWNT thus produced are cylinders con-

centrically arranged on TiN, whose purity was esti-mated as 90% (because the MWNT synthesis at TiNyields practically no side products of catalytic pyrolysisof hydrocarbons) and whose outer and inner diametersare 3–70 and 2–10 nm. Figure 2 shows the scanning elec-tron microscopy (SEM) images of MWNT we used.

Films of polybithiophene (PBT) were electropoly-merized at substrates of ITO, SWNT, and MWNT in a0.1 M

(

C

4

H

9

)

4

NBF

4

solution in acetonitrile, in a gal-vanostatic regime (0.5 mA per cm

2

of visible area). Theprecursor (BT) concentration was 0.02 to 0.04 M. Priorto depositing the polymer, the SWNT and MWNT sub-strates were rinsed with acetonitrile, while the ITO sub-strates were sonificated.

The redox behavior of the films was studied in a 0.1 M

(

C

4

H

9

)

4

NBF

4

solution in acetonitrile containing no pre-cursor. The currents are referred to 1 cm

2

of visiblearea. The electropolymerization and electrochemicalmeasurements were carried out in standard three-elec-trode cells. Platinum wire was the auxiliary electrode.Potentials were measured and are given against a satu-rated aqueous calomel electrode. The solutions werebubbled with argon. Absorption spectra were recordedfor undoped PBT films deposited on ITO and SWNT.

5

µ

m

2

µ

m

(b)

(a)

Fig. 2.

MWNTs grown on TiN plates: (a) top view (decora-ting with palladium is used for emphasizing the carbonmatrix relief) and (b) side view (dark band below is the TiNlayer; the sample is obtained by cutting with a blade); SEMimages are obtained with an Amray 1860 FE electronmicroscope.

–8

–0.3

0

0 0.3 0.6

E

, V

8 (b)

2

1

–100

0

100

(‡)

2

1

i

,

µ

A/cm

2

3

Fig. 3.

EDL capacitance charging currents for (

1

) ITO,(

2

) SWNT, and (

3

) MWNT (scales in a and b differ);

v

= 0.050 V/s.

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ELECTROCHEMICAL PROPERTIES OF THIN-LAYERED COMPOSITES 443

The instruments for electrochemical and spectroelec-trochemical measurements are described in [51].

RESULTS AND DISCUSSION

The charging currents for the MWNT, SWNT, andITO substrates in the electrical double layer (EDL)region increase in the sequence ITO SWNT MWNT (Fig. 3). Roughly assuming the same specificcapacitance of EDL on all substrates, we explain thisincrease by the surface development, caused by CNT.The EDL capacitance, calculated from the curves inFig. 3, was ~0.022 (ITO), 0.095 (SWNT), and 1.65(MWNT) mF per cm

2

of visible area. Taking the rough-ness factor of ITO as unity, we estimated it as ~4.3 and~75 for SWNT and MWNT.

In Fig. 4 we give cyclic voltammograms (CVA) forthe PBT films of different thicknesses and the EDLcapacitance charging current for the MWNT-basedelectrode. The charging-current scatter is due to aninaccurate determination of the true area of the elec-trode’s part dipped into the supporting electrolyte.Nevertheless, we infer from Fig. 4 that the CVA por-tions that characterize the EDL capacitance do notdepend on the film thickness: at these thicknesses thepolymer has no effect on the EDL capacitance causedby MWNT.

The formal doping–undoping potential of PBT (

E

f

)and the potential difference for anodic and cathodiccurrent peaks (

∆

) were calculated for PBT films onMWNT (Table 2). As seen,

E

f

is practically the same(

0.90

±

0.02

V) for all films. For very thin polymerfilms, at potential scan rate 0.010 V/s, the redox processis well reversible. We also see that

∆

increases with thefilm thickness, which points to decreasing reversibility.However, when

∆

increases, the mid-point (correspond-

ing to

E

f

) hardly moves. In theory,

∆

can increase dueto ohmic losses, diffusion limitations (entrance of theanion into the film or its exit), and the charge transferhindrance (detachment/attachment of electron). Thecharge transfer in ECP is usually fast. Diffusion limita-tions manifest themselves in a linear dependence of apeak current (

I

) on

v

1/2

, but in our case the

I

vs.

vdependence is linear. Hence, the degraded reversibilitycan be attributed to ohmic losses.

The effect of the substrate material on the PBTbehavior was estimated by analyzing CVA for PBTdeposited on MWNT, SWNT, and ITO, when passing aconstant polymerization charge (Figs. 5, 6). In a gal-vanostatic synthesis, which occurs with the current effi-ciency (CE) for PBT equal to 100%, the PBT amounton all substrates is nearly identical. For the thinnestPBT film (Fig. 5), where no well pronounced peakrelated to the polymer-on-SWNT doping can beobserved, the Ef value cannot be accurately determined.The Ef values calculated from CVA at large charges(Fig. 6) are close (Table 3) and agree with those inTable 2. That Ef for the CNT-based electrodes does notalter points to the absence of any significant donor–acceptor interaction between SWNT and the polymerthat could alter Ef. Thus, the CNT effect on PBT is dueto the electrode surface development.

Table 2. Characteristics of PBT films on MWNT; v = 0.010 V/s

Synthesis charge, mC/cm2 Ef, V ∆, V

5.0 0.90 0.049

12.5 0.88 0.105

22.5 0.90 0.195

33.0 0.92 0.273

–50

–0.4 0

i, µA/cm2

100

0.4 0.8 1.2E, V

0

501

7

6

53, 42

Fig. 4. (1) EDL capacitance charging current for MWNT;and (2)–(7) CVA for PBT on MWNT synthesized with(2) 2.25, (3) 4.5, (4) 5.5, (5) 12.5, (6) 22.5, and(7) 33 mC/cm2; v = 0.010 V/s.

–20

–0.4 0

i, µA/cm2

40

0.4 0.8 1.2E, V

0

20

1

3

2

Fig. 5. (1) EDL capacitance charging current for SWNT and(2, 3) CVA for PBT on (2) SWNT and (3) ITO synthesizedwith (2) 0.85 and (3) 1.00 mC/cm2; v = 0.050 V/s.

444

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The CVA for all substrates (Figs. 5, 6) have the fol-lowing common features: the accelerated polymer dop-ing on CNT is more pronounced for MWNT than forSWNT (cp. doping currents at initial doping poten-tials); higher reversibility of the redox process atMWNT (cp. ∆; values of ∆ for ITO and SWNT con-verge); and a one-stage cathodic process (PBT undop-ing) for MWNT at all film thicknesses (for SWNT, onlyfor the thinnest film). With increasing film thickness,

the CVA shape for PBT deposited on SWNTapproaches that on ITO.

It is possible to roughly estimate and compare dop-ing degrees of PBT (for PBT films synthesized withequal or nearly equal charges) at the used electrodematerials. It was done for a charge of 20.0–22.5 mC/cm2 (Fig. 6b). The negligible charging cur-rents for ITO and SWNT allowed us to calculate ratheraccurately the “doping” charge by integrating i vs. Ecurves at 0.6−1.15 V (Fig. 6b); for PBT on MWNT, weintroduced a correction for the EDL capacitance charg-ing and then calculated the doping charge (Table 4).The doping degree increases significantly when passingfrom ITO to CNT. Its decrease when passing fromSWNT to MWNT is probably just seeming and can becaused by the lowering of the integration limit to 1.10 Vand by wrong correction for the charging current.

We also recorded visible absorption spectra for dryPBT films on ITO and SWNT and for SWNT on quartz(Fig. 7). Quartz is convenient as a material that does notabsorb light over a wide wavelength range. Therefore,the spectrum for SWNT on quartz characterizes solelyCNT (Fig. 7, curve 1). As seen, the SWNT absorptionmakes no significant contribution to the spectra. Spec-tra for PBT on SWNT (curves 2, 2') demonstrate anabsorption band with a peak at ~478 nm due to undopedPBT. The peak is shifted by ~20 nm to longer wave-lengths relative to PBT on ITO (curve 3). This shift canbe ascribed to increased polyconjugation for PBT onSWNT.

CONCLUSIONS

The formal potentials of redox conversions of thinfilms of PBT deposited on ITO, SWNT, and MWNTsubstrates are nearly identical, pointing to the absence

–600

–0.4

1200

0 0.4 0.8 1.2E, V

600

0

1

2

3

4

(c)

–400

800

400

01

2

3

4

(b)

–200

400

200

01

2

34

(‡)i, µA/cm2

Fig. 6. (1) EDL capacitance charging current for MWNTand (2–4) CVA for PBT on (2) MWNT, (3) SWNT, and(4) ITO synthesized with (a) 5.0, (b) 20–22.5, and (c) 30–32.5 mC/cm2; v = 0.050 V/s.

0.2

400

A1.0

600 800 1000λ, nm

0.8

0.6

0.4

3

2'

2

1

Fig. 7. Absorption spectra for (1) SWNT on quartz, (2, 2')PBT on SWNT, and (3) PBT on ITO.

RUSSIAN JOURNAL OF ELECTROCHEMISTRY Vol. 41 No. 4 2005

ELECTROCHEMICAL PROPERTIES OF THIN-LAYERED COMPOSITES 445

of a donor–acceptor interaction between CNT and PBT(both neutral and oxidized).

Using CNT as a substrate for PBT improves electro-chemical reversibility of anodic doping of PBT andincreases its doping degree. The better reversibility ispresumably caused mainly by the CNT surface devel-opment and, consequently, a decrease in the PBT filmthickness at a constant PBT amount.

ACKNOWLEDGMENTS

This work was supported by the Russian Foundationfor Basic Research, project no. 02-03-32039.

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Table 3. Characteristics of PBT films on various substrates;v = 0.050 V/s

Synthesis charge,mC/cm2 Substrate Ef, V ∆, V

5.0 ITO 0.90 0.360

SWNT 0.88 0.338

MWNT 0.89 0.074

22.5 ITO 0.89 0.432

20.0 SWNT 0.89 0.439

22.5 MWNT 0.91 0.263

32.5 ITO 0.88 0.432

30.0 SWNT 0.90 0.455

32.5 MWNT 0.93 0.342

Table 4. Doping degree for PBT films on various substrates

Synthesischarge,mC/cm2

Substrate Integrationrange, V

Dopingdegree

Doping degreerelative to ITO

22.5 ITO 0.60–1.15 0.11 1.0

20.0 SWNT 0.60–1.15 0.17 1.5

22.5 MWNT 0.60–1.10 0.14 1.3

446

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