The conversion of polyaniline nanotubes to nitrogen-containing carbon nanotubes and their comparison with multi-walled carbon nanotubes

Polymer Degradation and Stability 94 (2009) 929–938

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The conversion of polyaniline nanotubes to nitrogen-containing carbon nanotubes and their comparison with multi-walled carbon nanotubes Miroslava Trchova´ a, *, Elena N. Konyushenko a, Jaroslav Stejskal a, Jana Kova´rˇova´ a, Gordana C´iric´-Marjanovic´ b a b

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic Faculty of Physical Chemistry, University of Belgrade, 11158 Belgrade, Serbia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 January 2009 Accepted 9 March 2009 Available online 19 March 2009

Polyaniline (PANI) nanotubes were prepared by the oxidation of aniline in solutions of acetic or succinic acid, and subsequently carbonized in a nitrogen atmosphere during thermogravimetric analysis running up to 830  C. The nanotubular morphology of PANI was preserved after carbonization. The molecular structure of the original PANI and of the carbonized products has been analyzed by FTIR and Raman spectroscopies. Carbonized PANI nanotubes contained about 8 wt.% of nitrogen. The molecular structure, thermal stability, and morphology of carbonized PANI nanotubes were compared with the properties of commercial multi-walled carbon nanotubes. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Carbon nanotubes Carbonization FTIR spectroscopy Raman spectroscopy Polyaniline Polyaniline nanotubes

1. Introduction Studies of carbon nanotubes (CNT) have greatly stimulated the development of nanotechnologies [1]. The preparation of conducting-polymer nanotubes has been systematically investigated only in recent years [2,3]. This type of morphology, produced especially by polyaniline (PANI) [4–10], belongs now in the class of frequently studied supramolecular polymer structures. It is known that the nitrogen-doped CNT possesses outstanding properties when compared with pure CNT, and could be important in the fabrication of new composites for catalysis, electronic devices, sensors, etc. Several methods for the preparation of nitrogen-containing carbon materials have been used, such as graphitization of nitrogen-containing polymers [11], and exposure of preformed carbons at elevated temperatures to reactive nitrogen-containing gases [12]. The pyrolysis of iron(II) phthalocyanine on nickel substrates leads to effective oxygen-reduction catalysts [13,14]. The carbonization of conducting-polymer nanotubes seems to be a feasible route for the preparation of new materials similar to CNT. The pyrolysis of PANI, which is an aromatic polymer containing nitrogen in the backbone, is a simple way to prepare nitrogendoped carbon-based materials with special chemical and physical properties [15]. * Corresponding author. Tel.: þ420 296 809 381; fax: þ420 296 809 410. E-mail address: [email protected] (M. Trchova´). 0141-3910/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2009.03.001

It has been observed that paper coated with PANI gives a fibrillar residue of a carbonized PANI after burning the cellulose fibres [16]. This principle has been used in the flame-retardation of wood coated with PANI [17], when the porous carbonized PANI prevented the transport of oxygen and heat into the interior of the wood. The morphology of PANI powder remained preserved after carbonization at 1000  C and only shrinkage due to the loss of mass was observed [18]. Experiments in the carbonization of PANI in inert atmospheres followed. Highly carbonized nanotubes have been prepared by heating PANI nanotubes to 500–1100  C [19]. Polypyrrole nano-objects, including nanotubes, were similarly converted to nanocarbons by microwave heating [20], the conductivity of the starting materials being the prerequisite for the successful carbonization in that case. Carbon nanospheres have recently been obtained by using thermogravimetric analysis (TGA) on functionalized PANI nanospheres [21]. The fact that the morphology of conducting polymers remains preserved after carbonization has thus clearly been established. Preparations of CNT and carbonized nanotubes of conducting polymers represent two completely different routes to the preparation of nano-objects which, however, may have some similarities in the structure and properties of products and in the prospects for their applications. This is illustrated, for example, by the ability of PANI to reduce the salts of noble metals to the corresponding metal nanoparticles [22,23]. When applied to PANI nanotubes followed by carbonization, silver–carbon composites could readily be prepared

930

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[20]. Silver nanoparticles could be deposited on the surface, and in the cavities, of nanotubes [20,24]. The direct deposition of metal nanoparticles on CNT would be much more difficult to control and the adhesion of metal to carbon is expected to be inferior. Yet, this is important for the design of catalysts based on noble-metal nanoparticles supported by carbons [25], viz. in the electrodes of fuel cells [26,27]. Thermogravimetry has been used in the micro-synthesis of carbonized PANI nanotubes [21] and is described in the present communication. The milligram quantities of samples used in TGA experiments are sufficient for characterization by microscopic and spectroscopic methods. The scale-up of the carbonization procedure does not seem to present any problem [20]. The comparison of two types of carbonized PANI nanotubes with two samples of commercial CNT is the goal of the present study. 2. Experimental 2.1. Polyaniline and carbon nanotubes Two samples of PANI nanotubes were prepared on the basis of previous experiments [4,28]. The sample denoted as NT-1 was synthesized by the oxidation of 0.25 M aniline with 0.25 M ammonium peroxydisulfate in 0.25 M aqueous solution of succinic acid at 5  C (Figs. 1a and 2a). The sample NT-2 was a result of similar oxidation in 0.4 M acetic acid containing 2 wt.% sodium

bis-(2-ethylhexyl) sulfosuccinate at 20  C (Figs. 1b and 2b). Conducting PANI salts were converted to the corresponding PANI bases by suspension in 1 M ammonium hydroxide. Commercial multiwalled CNT Baytubes C150HP (Bayer AG, Germany), denoted as CNT-1, and those produced by the Tambov Innovative Technological Centre of Mechanical Engineering (Tambov State Technical University, Tambov, Russian Federation), CNT-2, have been used as reference materials. 2.2. Carbonization of PANI nanotubes Thermogravimetric analysis was carried out with a Perkin Elmer TGA7 thermogravimetric analyzer at a heating rate 10  C min1 under 50 cm3 min1 nitrogen or air flow over a temperature range 30–830  C. Deprotonated PANI nanotubes NT-1 and NT-2 were carbonized during TGA in a nitrogen atmosphere and the products of their carbonization were collected as residues. The thus-obtained nitrogen-containing carbon nanotubes (cf. below) were denoted as N-CNT-1 and N-CNT-2, respectively. The TGA was repeated several times to accumulate quantities of samples sufficient for surfacearea determination. 2.3. Spectroscopic characterization Raman spectra excited with HeNe 633 nm and argon-ion 514 nm lasers were collected on a Renishaw inVia Reflex Raman spectroscope. A research-grade Leica DM LM microscope with an objective magnification 50 was used to focus the laser beam on the sample placed on an X–Y motorized sample stage. The scattered light was analyzed by the spectrograph with a holographic grating 1800 lines mm1. A Peltier-cooled CCD detector (576  384 pixels) registered the dispersed light. Fourier-transform infrared (FTIR) spectra were recorded in the range 400–4000 cm1 at 64 scans per spectrum at 2 cm1 resolution using a fully computerized Thermo Nicolet NEXUS 870 FTIR Spectrometer with a DTGS TEC detector. Samples were dispersed in potassium bromide and compressed into pellets. Spectra were corrected for the moisture and carbon dioxide in the optical path. Specific surface areas were determined with a Quantasorb apparatus (Quantachrome) using nitrogen as the sorbate. 3. Results and discussion The motivation of the present study was to prepare novel materials, which would be similar in molecular structure and morphology to multi-walled CNT. Carbonized nanostructures produced by conducting polymers, however, may offer alternative properties and applications compared with classical CNT rather than to be their replacement. Carbonized PANI contains nitrogen atoms. The physical properties of carbon materials are extremely sensitive to the presence of heteroatoms; the nitrogen-doping of carbons was found to exhibit profound effects on surface states, electron-transfer rates, and adsorption phenomena in electro-catalysis [29]. For that reason, carbonized PANI nanotubes, i.e. nitrogen-containing carbon nanotubes (N-CNT), were prepared and their molecular structures were compared with CNT. 3.1. Carbonization

Fig. 1. Scanning electron microscopy of polyaniline nanotubes: (a) NT-1 and (b) NT-2.

The TGA apparatus has been used for the carbonization of PANI nanotubes (Fig. 3). Heating to 830  C in nitrogen left a 45 wt.% residue in both tested samples, which were subsequently characterized as ‘‘carbonized’’ PANI, nitrogen-containing carbon

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Fig. 2. Transmission electron microscopy of polyaniline nanotubes: (a) NT-1 and (b) NT-2.

nanotubes N-CNTs, similarly to experiments reported in the literature [15,21]. In air, PANI nanotubes were completely decomposed after the temperature reached 685  C (NT-1) or 693  C (NT-2) without leaving any discernible residue (Fig. 3). The products of carbonization, N-CNT, were then tested for stability in air in a subsequent TGA experiment (Fig. 4). It was surprising to find that the samples NT-1 and NT-2 lost 10 and 21 wt.%, respectively, already below 100  C. This could be attributed to the substantial absorption of weakly bound moisture, possibly due to the presence of nitrogen in the carbonized PANI (Table 1). The suspicion that N-CNT might be more hydrophilic than CNT would be of importance in many applications. Carbonized PANI nanotubes start to decompose above 400  C, at lower temperature than CNT where a marked decrease in mass was observed only above 500  C (Fig. 4). The process was complete at 650–680  C, while for CNT this limit is close to 700  C. The stability of CNT in air may thus be regarded as better than that of N-CNT.

3.2. Nanotubular morphology The genesis of PANI nanotubes in the course of aniline polymerization has been discussed in the literature [4,28,30]. The formation of nanotubes is based on the adsorption of phenazinecontaining nucleation centres on oligomer nanocrystallites produced in the early stages of aniline oxidation [3]. The polymer chains grow from stacked nuclei to produce nanotubular walls. Hydrogen-bonding and ionic interactions between neighbouring PANI chains stabilize the supramolecular nanotubular structure (Figs. 1 and 2). The nanotubular structure of the PANI was preserved after carbonization, except for some shrinkage (Fig. 5). We suppose that the cross-linking reaction [15,18,31] plays an additional role in the stabilization of supramolecular structure during the carbonization, and that the nitrogen atoms in the polymer participate (Fig. 6). The morphology of the multi-walled CNT, used in the present study for comparison, is different; the CNTs are much longer and produce aggregates (Figs. 7 and 8). Their structure can be visualized as

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Table 1 Elemental compositiona and specific surface area, S, of nanotubes.b

100

C, wt.%

Weight, wt.%

80

nitrogen

40

NT-1 NT-2 air

0

5.1 5.2

13.3 12.0

12.5 17.1

29.4 24.0

Carbonized PANI nanotubes N-CNT-1 72.8 2.4 N-CNT-2 76.5 2.1

8.8 8.7

16.1 12.7

64.3 94.2

Commercial carbon nanotubes CNT-1 98.6 0.72 CNT-2 94.7 0.74

– –

200

400

600

b

800

Temperature, °C Fig. 3. TGA of two types of PANI nanotubes (NT-1 and 2) in air and in nitrogen atmosphere.

being produced by coaxial cylinders [32], i.e. quite different from that of the N-CNT obtained by the carbonization of PANI nanotubes [28]. 3.3. Elemental analysis The theoretical composition of PANI base (Fig. 6, 1) is 79.5% C, 5.0% H, 15.5% N [33]. Depending on the conditions of synthesis, experimental samples always contain also some oxygen, chlorine, and sulfur [33], while the content of carbon is lower than expected. Also, in the present samples NT-1 and NT-2, the carbon content is lower, 69 and 66 wt.% (Table 1), and increases only moderately after TGA experiments to 73 and 76 wt.%. These are much lower values compared with CNT (Table 1), the presence of nitrogen and other elements in the former product being the reason for this difference. One could therefore argue that complete ‘‘carbonization’’ has not taken place and, for that reason, we talk about nitrogen-containing CNT. The molecular structure of PANI base, however, is completely changed after exposure to elevated temperature, as is proved by the spectral analysis reported below. The processes expected to take place are proposed in Fig. 6 [18].

100 CNT-1 80

Weight, wt.%

Others, wt.%

a

20

N-CNT-1

60

CNT-2

N-CNT-2

40

20

0

N, wt.%

Original PANI nanotubes NT-1 69.2 NT-2 65.8

60

0

200

400

600

800

Temperature, °C Fig. 4. TGA of carbonized PANI nanotubes (N-CNT-1 and 2) and of commercial carbon nanotubes (Bayer C150HP and Tambov).

S, m2 g1

H, wt.%

0.72 4.57

153 95.6

An average of two independent analyses. PANI nanotubes in the emeraldine base form.

The atomic concentration of nitrogen in PANI films pyrolyzed at 900  C, reported in the literature, was 5.28% [15]. As expected, in nanotubular PANI the content of nitrogen is higher after pyrolysis extending to 830  C (Table 1). In polyimide, nitrogen atoms almost completely evaporated above 800  C [34], obviously due to the absence of cross-linking reactions. 3.4. Raman spectra of PANI nanotubes Raman spectroscopy is currently used to probe carbons and carbonized samples due to its ability to distinguish between various types of bonding between carbon atoms. Raman spectra of NT-1 and its carbonized version, N-CNT-1, obtained with the 633 nm laser excitation line, are shown in Fig. 9a, and similarly for NT-2 and N-CNT-2 in Fig. 10a. The Raman spectrum of sample NT-1 reflects features corresponding to the molecular structure in the nanotubular morphology of PANI [35]. The main bands assigned to quinonediimine units (Fig. 6, 1) are present at 1592 cm1 and 1610 cm1 with a shoulder at 1632 cm1. The band of C]N vibrations, centred at 1474 cm1, dominates the spectrum. A shoulder at about 1414 cm1 appears in the spectrum of the PANI base. The  position of the band at 1338 cm1, attributed to C–Nþ vibrations, reveals the residual protonation of imine nitrogens, possibly by sulfonic groups occasionally substituting benzene rings [37–39]. Two bands at 1258 and 1222 cm1 of various C–N stretching vibrations and a peak at 1165 cm1 of C–H bending vibration of the quinonoid ring are present in the spectrum (Fig. 9a). A broad structural band with local maxima at 845, 778, and 750 cm1 of vibration modes, corresponding to substituted aromatic rings, is detected. The relatively sharp peak observed at 576 cm1 was proposed to be attributed to the phenoxazine type of units [35,36]. Bands at 528 and 415 cm1 corresponding to out-of-plane ringdeformations are seen in the spectrum. The Raman spectrum of nanotubular sample NT-2 (Fig. 10a) is very close to the spectrum of NT-1 (Fig. 9a), as expected. It is well known that the vibrations of benzenoid or quinonoid constitutional units can be resonance-enhanced, depending on the laser excitation wavelength [40]. It can be expected that vibrations originating from the quinonoid units in PANI should be resonanceenhanced with both 514 and 633 nm excitation wavelengths, but the resonance enhancement of the quinonoid vibrations is most pronounced at 633 nm. The Raman spectra of the NT-1 and NT-2, obtained with the 514 nm laser excitation line, are slightly different (compare the spectra in Fig. 9a and Fig. 10a with Fig. 9b and Fig. 10b, respectively). The relative intensities of the band of C]C vibration in quinonoid units with a maximum at 1592 cm1 and the band of C]N vibrations at 1474 cm1 dramatically increased, and a new intense band and 1563 cm1 appeared. The intensity of the band at w1410 cm1,

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Fig. 5. Polyaniline nanotubes after exposure to 830  C in nitrogen atmosphere: (a) N-CNT-1 (top) and (b) N-CNT-2 (bottom). Two magnifications.

attributed to the ring-stretching vibration of phenazine-like  segments, and the band at 1342 cm1 of C–Nþ vibrations also increased under excitation of 514 nm. It has been shown that phenazine-, safranine- or phenoxazine-like segments are recognized through the bands observed at the following wavenumbers: w1632 (sh), w1570–1560, 1549–1537, 1350, 575 and 417 cm1 [36]. Consequently, the Raman spectra excited with 514 nm strongly support the presence of such units [3,30] in the structure of NT-1 and also in NT-2. 3.5. Raman spectra of carbonized PANI The Raman-active vibration mode of perfectly ordered graphite is situated at 1575 cm1 (labelled ‘‘G’’ for graphite) [41,42]. It originates from the ordered hexagonal rings consisting of conducting sp2-bonded carbon. It corresponds to the stretching vibration of any pair of sp2 C]C sites, whether in linear chains or in aromatic rings [42]. With increasing disorder, this band broadens and shifts to a higher frequency, and a new band appears at

1350 cm1 (labelled ‘‘D’’ for the disordered form) [41,42]. It corresponds to the breathing mode of those sp2 sites located only in rings, not in linear chains. Such a mode is forbidden in perfect graphite and becomes active only in the presence of disorder. The D-peak in carbon-based materials has been observed for disordered graphite, such as in clusters of hexagonal rings. This band has a strongly resonant character, as may be seen by the dependence of the intensity and position on wavenumber [43]. The Raman spectra of the carbonized samples N-CNT-1 and 2, obtained with the 633 nm laser excitation line, are relatively simple (Figs. 9a and 10a). They are composed of two broad bands with intensity maxima at 1596 and 1340 cm1, which can be associated with G- and D-bands [44]. The band of C]N stretching vibrations in quinonoid units, centred at 1474 cm1 in the spectrum of original sample, completely disappeared after carbonization, together with the bands of C–H bending vibration of the quinonoid ring observed at 1165 cm1. Two new broad bands developed from the band of C]C vibrations in quinonoid units with a maximum at 1596 cm1  and from the band of C–Nþ vibrations with a maximum at

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N N 1

1338 cm1. The two bands at 1258 and 1222 cm1 of various C–N stretching vibrations, observed in the spectra of PANI nanotubes, also contribute to the broad wings of the second newly formed band at 1340 cm1.

NH NH

N

NH

N

3.6. The comparison of Raman spectra of carbonized PANI and carbon nanotubes

NH

Δ

2

NH 3

N C

N

N

N NH Fig. 6. The proposed chemistry of carbonization in PANI: Polyaniline base (1) exposed to elevated temperature in inert atmosphere produces cross-linked phenazine-like structures (2) and nitrile units (3).

Raman spectra obtained with the 633 nm excitation line of carbonized PANI nanotubes have been compared with the spectra of two commercial samples of CNT (Fig. 11). The G-band (derived from the graphite-like mode) is situated at 1594 cm1 in the spectrum of sample CNT-1 and at 1591 cm1 in the spectrum of sample CNT-2. In contrast to the graphite Raman G-band, which exhibits a single Lorentzian peak, this band has a marked shoulder at 1615 cm1 in the spectrum of CNT-1. The disorder-induced Dband is situated at 1327 cm1 in the spectrum of sample CNT-1 and at 1333 cm1 in the spectrum of CNT-2, and its second-order harmonic G0 -band [42] can be found at 2662 cm1 in the spectrum of sample CNT-1 and at 2670 cm1 in the spectrum of CNT-2. Two main bands in the Raman spectra of carbonized PANI nanotubes obtained with the 633 nm excitation line are shifted to higher wavenumbers, and are much broader in comparison with the corresponding bands of commercial CNT (Fig. 11a). Two features become more pronounced in the Raman spectra of N-CNT samples. The first, an overlap between the bands G and D, and the second, asymmetric tailing of the peak D extending to lower wavenumbers, may be detected. The broadening is more marked for the band corresponding to the disorder-induced D-band. In nitrogen-containing carbon systems, nitrogen atoms tend to introduce disorder in the graphene plane, and such material shows enhanced oxidation resistance [45]. The ratio of the integrated intensities of the D and G bands is higher for N-CNT samples in comparison to the commercial CNT, which signifies an increase in disorder. All these changes are in accordance with the observations of the Raman spectra of carbon nanotubes enriched with nitrogen (0–10 at.%) prepared through a floating catalyst method using pyridine and ammonia [12]. A blue shift of all bands is the main feature observed in the spectra when going from excitation at 633 nm to 514 nm (Fig. 11b). At the same time, the ratio of the intensities of the two main bands, G/D, in the carbonized PANI nanotubes increased. This increase is much higher in the Raman spectra of the carbonized samples, N-CNT-1 and 2, compared with CNT. This is connected with their different structures containing C–N and C]N bonds, phenazinelike units, and cross-linked structures involving nitrogen atoms. 3.7. FTIR spectra

Fig. 7. Scanning electron microscopy of multi-walled carbon nanotubes: (a) Baytubes C150HP (CNT-1) and (b) Tambov (CNT-2) product.

For the understanding of the bonding configuration in carbonized PANI nanotubes, FTIR spectroscopy is a useful technique because of its greater sensitivity to H, N, and O atom bonding in comparison with that of carbon atoms. FTIR spectra of PANI nanotubes before and after carbonization are presented in Figs. 12 and 13. The molecular structure of nanotubular PANI prepared under mild acidic conditions is different compared with samples prepared in solutions of strong acids [30], which have a granular morphology. In addition to the main absorption bands of the quinonoid and benzenoid ring vibrations located at 1585 and 1503 cm1, a band tail at about 1630 cm1, together with bands at 1445, 1414 and 695 cm1, are present in the spectra. They have been assigned to the presence of ortho-coupled aniline constitutional units and phenazine-like units [30,35]. The presence of the band at 1379 cm1 is attributed to a C–N stretching vibration in the neighbourhood of a quinonoid ring, and is characteristic of PANI base. The band 1299 cm1 assigned to the C–N stretch of

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Fig. 8. Transmission electron microscopy of multi-walled carbon nanotubes: (a) Baytubes C150HP (CNT-1) and (b) Tambov (CNT-2) product.

a

NT-1 D

G

NT-2 D G

N-CNT-2

N-CNT-1

3000

2500

2000

1500

Wavenumber,

1000

3000

500

2000

1000

500

514 nm

Intensity

Intensity

G

1500

b

D

NT-2

N-CNT-1

2500

2000

Wavenumber, cm-1

514 nm

NT-1

2500

cm-1

b

3000

633 nm

633 nm

Intensity

Intensity

a

G

D

N-CNT-2

1500

1000

500

Wavenumber, cm-1 Fig. 9. Raman spectra of original nanotubular PANI (NT-1) and carbonized product (N-CNT-1) obtained with laser excitations at (a) 633 nm and (b) 514 nm.

3000

2500

2000

1500

1000

500

Wavenumber, cm-1 Fig. 10. Raman spectra of original nanotubular PANI (NT-2) and carbonized product (N-CNT-2) obtained with laser excitations at (a) 633 nm and (b) 514 nm.

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a

D 633 nm G

N-CNT-2

Absorbance

Intensity

CNT-1 CNT-2

G

D

NT-2

N-CNT-1 N-CNT-2

3000

2500

2000

1500

1000

4000

500

Wavenumber, cm-1

3500

3000

2500

2000

1500

1000

500

Wavenumber, cm-1 Fig. 13. FTIR spectra of PANI nanotubes (NT-2) and of a carbonized product (N-CNT-2).

b

D

514 nm

G

Intensity

CNT-1

CNT-2

N-CNT-1

N-CNT-2

3000

2500

2000

1500

1000

500

Wavenumber, cm-1 Fig. 11. Comparison of the Raman spectra of multi-walled carbon nanotubes (CNT-1 and 2) with the spectra of carbonized nitrogen-containing PANI nanotubes (N-CNT-1 and 2) obtained with laser excitations at (a) 633 nm and (b) 514 nm.

a secondary aromatic amine has a shoulder at 1335 cm1. The region of 1010–1170 cm1 corresponds to the aromatic C–H inplane bending modes. By analogy with the Raman spectra, the spectrum of a deprotonated sample prepared in the presence of

N-CNT-1

Absorbance

G

3.8. The comparison of FTIR spectra of carbonized PANI nanotubes and carbon nanotubes

D

NT-1

4000

3500

3000

2500

2000

1500

weak acid includes some spectral features (a polaron band above 2000 cm1, a band at 1145 cm1, etc.) typical of protonated structures. It has been shown that the samples with nanotubular structure are more resistant towards deprotonation [5]. The sharp peak of sulfonate groups attached to the aromatic rings, observed at 1040 cm1, but missing from the spectrum of the sample prepared in the presence of strong acid, is well seen in the spectra of nanotubular samples. The band at about 830 cm1, corresponding to the C–H out-of-plane bending vibration of 1,4-disubstituted rings and the additional peaks found at 757, 738, and 695 cm1, corresponding to multiple-substitution on the benzene rings, are present in the spectrum. FTIR spectra of carbonized PANI nanotubes, N-CNT-1 and 2 (Figs. 12 and 13) are composed of two broad bands with maxima at 1585 and 1238 cm1 and a broad absorption over the whole range of 400–4000 cm1, which was assigned to the excitation of free conducting electrons [19]. A small maximum at about 2200 cm1 may correspond to the presence of a C^N triple bond, and the band tail at about 1720 cm1 signifies the presence of carbonyl groups in the structure of sample N-CNT-1. The presence of hydrogen (Table 1) is manifested by a broad maximum around 3440 and 1634 cm1 (O– H stretching and bending vibrations in hydroxyl groups may be present due to adventitious atmospheric moisture in the potassium-bromide pellet). The first broad maximum at 1585 cm1 overlaps the region of the two main bands of quinonoid and benzenoid ring vibrations. The second band at 1238 cm1 is much broader and overlaps mainly the region of various C–N stretching and C–H bending vibrations of the benzenoid and quinonoid rings.

1000

500

Wavenumber, cm-1 Fig. 12. FTIR spectra of PANI nanotubes (NT-1) and of a carbonized product (N-CNT-1).

It is characteristic of carbon-like materials that the Ramanactive D- and G-bands are inactive in FTIR spectra (Fig. 14). In disordered samples, however, they become IR-active because of symmetry-breaking of the carbon network. For that reason, FTIR spectra of neat CNT are practically featureless. The FTIR spectra are widely used to identify the presence of hydrogen and some impurities in samples. The presence of a broad maximum around 3440 and 1634 cm1 in the spectra of CNT-1 and CNT-2 is again due to O–H vibrations from moisture in the potassium-bromide pellet. The small band at about 1724 cm1 corresponds to the carbonyl group in the structure of sample CNT-2.

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In addition to objects having simple one- two- and threedimensional morphologies [6,43], such as nanofibres, nanorods, or nanowires [46–48], dendritic nanofibres [49], nanotubes [4–6,10,30,50,51], rectangular nanotubes [48,52], nanoplates or nanosheets [53,54], nanoflakes and nanospheres [47], microspheres [30,55], and their simultaneous occurrence [24,50] or oriented arrays [56], the PANI world of nanostructures becomes inhabited also by more complex structures, such as chrysanthemum-flower-like [57], rose-like [51] or just flower-like [54] objects, rambutan-like [58] and dandelion-like [59] spheres, nanobrushes [24,60], etc. All of these morphologies can obviously be converted to nitrogen-containing carbons. The variety of materials that can easily be produced by their carbonization is thus immense.

CNT-1

Absorbance

CNT-2

N-CNT-1 G

D

N-CNT-2

4000

3500

3000

2500

2000

1500

937

1000

500

5. Conclusions

Wavenumber, cm-1 Fig. 14. Comparison of the FTIR spectra of carbon nanotubes (CNT-1 and 2) with the spectra of carbonized nitrogen-containing carbon nanotubes (N-CNT-1 and 2).

In the region below 1700 cm1, the FTIR spectra of carbonized PANI nanotubes differ mutually, reflecting some differences in the preparation of PANI. The main G bands are situated at 1577 and 1585 cm1 in the spectra of samples N-CNT-1 and 2, respectively. Broad absorption D bands at about 1259 and 1238 cm1 are observed in the samples N-CNT-1 and 2, respectively. The presence of two broad absorption bands in the FTIR spectra of carbonized nanotubes reflects, in addition to carbon bonding corresponding to disordered carbon-like materials, also the presence of nitrogen, hydrogen and oxygen in their structures. The bonding of nitrogen in PANI films pyrolyzed at 900  C was estimated in the literature from X-ray photoelectron spectra [15]. It was observed that such samples contained nitrogen atoms bound to carbon atoms as C–N, C]N, and C^N, the last two bonds being preferred. According to the present FTIR results, similar bonding is expected in carbonized PANI. 3.9. Specific surface areas The specific surface area of PANI nanotubes increased several times after carbonization and approached values typical of commercial CNT (Table 1). PANI objects having a less organized structure and low surface area are probably less stable during heating and decompose, while more organized objects, such as PANI nanotubes, only convert to a carbonized analogue and constitute a major part of the sample.

(1) PANI nanotubes were ‘‘carbonized’’ in TGA experiments running to 830  C in an inert atmosphere to give nitrogencontaining carbon nanotubes, N-CNT. The content of nitrogen was 8.8 wt.%, this element being absent in commercial CNT. (2) The nanotubular morphology of PANI is preserved following carbonization, leaving about one half of the mass as a residue. The carbonized PANI nanotubes are relatively straight and have a length extending at most to several micrometres. Commercial CNTs are much longer, and produce intertwined aggregates. (3) The thermal stability of carbonized PANI is only slightly lower that that of commercial multi-walled CNT. A specific surface area of the order of 100 m2 g1 may be regarded as comparable in CNT and N-CNT. (4) The molecular structure of carbonized PANI is represented by disordered carbon-like material containing C–N, C]N and C^N bonds, and phenazine-like cross-linked units. It has been demonstrated that both Raman and FTIR spectroscopy are powerful tools in the characterization of carbonaceous materials of this type. (5) The carbonized PANI nanotubes, i.e. nitrogen-containing carbon nanotubes, constitute a novel material which may be considered in relation to potential applications, so far involving CNT. This is especially true when the presence of nitrogen atoms in CNT is of benefit. Acknowledgments The authors thank the Czech Grant Agency (202/08/0686), the Grant Agency of the Academy of Sciences of the Czech Republic (IAA 400500905), and the Ministry of Science and Technological Development of Serbia (Contract No 142047) for financial support.

4. Concluding remarks References Potential applications of CNT are sought in the design of novel catalysts where nanotubes play the role of a chemically inert and a thermally stable support for the deposition of noble-metal catalysts. The morphology, surface area, and ability to affect the particle size of deposited metals are additional parameters to be considered. For this reason, comparison of classical multi-walled CNT and nitrogen-containing CNT produced from PANI makes sense. They are certainly likely to behave in a different manner in these applications, with no obvious preference. The shorter length of carbonized PANI nanotubes may be of benefit in their easier dispersion. Mechanical properties, on the other hand, may be inferior when compared to the classical CNT. It depends on the particular specific applications which type of nanotubes will perform better.

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