The conversion of polyaniline nanotubes to nitrogen-containing carbon nanotubes and their comparison with multi-walled carbon nanotubes
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], [5], [6], [7], [8], [9], [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 nitrogen-doped carbon-based materials with special chemical and physical properties [15].
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 [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.
Section snippets
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
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
Concluding remarks
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
Conclusions
- (1)
PANI nanotubes were “carbonized” in TGA experiments running to 830 °C in an inert atmosphere to give nitrogen-containing 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
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.
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