SIMPLE, EFFECTIVE AND SCALABLE PROCESS FOR MAKING CARBON NANOTUBES

Abstract

A simple, effective and scalable method for fabricating carbon nanotubes. The method has two simple steps: (a) producing the carbon precursors (i.e., nanotubes of conducting polymer) in water solution via a soft template method involving a fibrillar complex and (b) carbonizing the carbon precursors (i.e. the nanotubes of the conducting polymer) at a temperature between 900-2200° C. in a nitrogen atmosphere or under a vacuum condition.

Description

FIELD OF THE INVENTION

The present invention relates to carbon nanotubes. Particularly, it relates to a simple, low cost, safe, and scalable method for synthesizing carbon nanotubes.

BACKGROUND OF THE INVENTION

Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. These cylindrical carbon molecules have many outstanding properties that make them potentially useful in many applications in nanotechnology, electronics, optics and other fields of materials science, as well as potential uses in architectural fields. For example, they exhibit extraordinary strength and unique electrical properties, and are efficient thermal conductors. Since the discovery of fullerenes and carbon nanotubes (Kratschmer W, Lamb L D, Fostiropoulos K, Huffman R D. Nature 1990; 6291(347):354-8), they have attracted tremendous academic and industrial interest, with thousands of papers on nanotubes being published every year.

However, despite of rapid development in carbon nanotube science, commercial applications have been rather slow to develop, primarily because of the high production costs of the best quality nanotubes. Low cost, large-scale synthetic method of carbon nanostructures has been the subject of many research teams, both in academic research institutions and in industries. The synthesis of well defined carbon nanotubes with 50-100 nm diameters has been a long-standing goal of materials chemistry (Tang N J, Zhong W, Au C T, Gedanken Y Y, Du Y W. Adv Funct Mater 2007; 17(9):1542-50).

The preparation of carbon materials commonly involves pyrolysis of the carbon precursor (such as sucrose, furfuryl alcohol, acetylene, acenaphthene, or phenol resin). Aromatic polyimide film has aroused a great deal of interest as it is one of the attractive precursors for producing carbon and graphite films in recent years. Poly (acrylonitrile) (PAN) is another commonly used precursors (Chae H G, Minus M L, Rasheed A, Kumar S. Polymer 2007; 48(13):3781-9). Xia et al. reported a method of fabrication of carbon nanotubes by combining polyelectrolyte electrospinning with vapor deposition polymerization. The process is high-temperature carbonization of poly (acrylonitrile) nanotubes with poly(styrene sulfonate) sodium nanofibers as cores at 900° C. under a nitrogen flow (McCann J T, Lim B, Ostermann R, Rycenga M, Marquez M, Xia Y. Nano Lett 2007; 7(8):2470-4). Wu et al. prepared carbon nanospheres by the pyrolysis of nanospherical polyacrylonitrile (PAN) (Yang L C, Shi Y, Gao Q S, Wang B, Wu Y P, Tang Y. Carbon 2008; 46(13):1816-8).

In a previous study, the applicants developed a new chemical approach (or a “soft” template method), in which a fibrillar complex of the anionic azo dye MO (methyl orange), sodium 4-[4-(dimethyl-amino)-phenyldiazo]phenylsulfonate ((CH₃)₂NC₆H₄N═NC₆H₄SO₃Na) and the oxidant FeCl₃ was used as a reactive self-degradable seed template directing the growth of polypyrrole (PPy) on its surface and promoting the assembly into hollow nanotubular structures (Yang X M, Zhu Z X, Dai T Y, Lu Y. Macromol Rapid Commun 2005; 26:1736-40).

SUMMARY OF THE INVENTION

One object of the present invention is to provide a low-cost, simple method to fabricate carbon nanotubes. This object is realized by a process having two simple steps: (a) producing the carbon precursors (i.e., nanotubes of conducting polymer) in water solution via a soft template method involving a fibrillar complex and (b) carbonizing the carbon precursors, i.e. the nanotubes, at a temperature between 900-2200° C. in a nitrogen atmosphere or under a vacuum condition. An exemplary scheme embodies the present invention is depicted in FIG. 8.

As shown in the scheme of FIG. 8, the carbon precursors can be conducted in water solution to which an oxidant is added. A typical oxidant is FeCl₃, although other oxidants may also be used to obtain satisfactory results, for example, ammonium persulfate, potassium persulfate, and ferric nitrate. In step (a), any soft-template method may be used and an exemplary system can include just an oxidant and methyl orange (MO) (anionic azo dye). Other substances may also be used in the place of MO, for example, such as methyl red, benzyl orange, and 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin. An exemplary monomer is pyrrole but any conductive monomers, such as, for example, aniline or thiophene, may also be used. It is understood that any conjugated polymers may be used as carbon precursors to practice the present invention. For example, polypyrrole, polyaniline and polythiophene are typical conjugated polymers. In step (b), the carbon precursors, which are nanotubes of conducting polymer, are undergone a heat-treatment process under suitable conditions, which typically are 900-2200° C. and 3-5 hours in a nitrogen atmosphere, or 900-2200° C. and 3-5 hours under a vacuum condition. Although these conditions can be modified by people having ordinary skill in the art so that satisfactory results can be obtained under different circumstances. In summary, the present invention provides a simple, feasible and practical method for effectively producing carbon nanotubes on a large scale.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be made to the drawings and the following description in which there are illustrated and described preferred embodiments of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is the TEM images of the PPy nanotubes fabricated according to the present invention (A) and the PPy granules made for the purpose of comparison (B)

FIG. 2 is the TGA curves of (a) PPy nanotubes and (b) PPy granules (under nitrogen atmosphere).

FIG. 3 is the TEM images of the carbon nanotubes fabricated according to the present invention.

FIG. 4 is the TEM image (A) showing one carbon nanotubes and the HRTEM image (B) showing a curved part of the same carbon nanotubes.

FIG. 5 depicts the XRD pattern of carbon nanotubes fabricated according to the present invention.

FIG. 6 depicts the TEM images of the carbon nanotubes fabricated according to the present invention and the iron (pointed to by the arrow).

FIG. 7 shows the Raman spectrums of the PPy nanotubes (A) and carbon nanotubes (B) produced according to the present invention.

FIG. 8 is a diagram showing the general scheme of making carbon nanotubes according to the present invention.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS OF THE INVENTION

As an example, the following describes a preferred embodiment of the present invention.

In the first step, 0.243 g (1.5 mmol) of FeCl₃ was dissolved in 30 mL of 5 mmol/l methyl orange deionized water solution (0.15 mmol). Then 105 ml (1.5 mmol) of pyrrole monomer (Aldrich) was added into the solution and the mixture was stirred at room temperature for 24 h. The formed polypyrrole (PPy) precipitate was washed with deionized water/ethanol several times until the filtrate was colorless and neutral, and finally dried under a vacuum atmosphere at 60° C. for 24 h. A kind of dry black powder was obtained which are the PPy nanotubes fabricated. As a comparison, the PPy granules were fabricated by similar procedures as described in the above except that methyl orange was not used.

In the next step, the obtained PPy nanotubes from the first step were further carbonized to afford the carbon nanotubes. The carbonization process was carried out in a quartz tubular furnace under nitrogen atmosphere. The sample was first gradually heated up to 900° C. at a heating rate of 3° C. min⁻¹, kept at 900° C. for 5 h and then cooled to room temperature. The carbon nanotubes were then successfully fabricated.

The structure of the carbon nanotubes prepared in the above was investigated by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and X-ray diffraction (XRD). It was shown that these as-prepared carbon nanotubes were mostly amorphous in terms of morphology.

A SDT 2960 thermogravimetric analyzer was used to investigate the thermal stability of the PPy nanotube with nitrogen as pure gas at a flow rate of 20 ml min⁻¹. The heating rate was 3° C. min⁻¹. The morphologies of the samples were examined by TEM (JEM-2000CX) and HRTEM (JEOL-2010) operated at accelerating voltage of 100 and 200 kV, respectively. A few drops of the solvent extracts were placed onto the copper mesh covered with a per-coated carbon film and allowed to evaporate. XRD patterns were obtained on PHILIPS PW 3710 diffractometer in the reflection mode using Cu Kα radiation at room temperature. Raman spectra was recorded in the range of 0-3500 cm⁻¹ at ambient temperature with a labRAM HR 800 (France, Jobin Yvon) using 532 nm laser as the excitation source. The results of these measurements are described in the following and in the drawings.

The TEM images of the PPy nanotubes (A) and the PPy granules (B) were made of the samples produced in the above described fabricating process and are reproduced in FIG. 1. As shown in FIG. 1(A), the PPy nanotubes are hollow, and that their outer and inner diameters are determined as about 70 and 50 nm respectively. Therefore, their wall thicknesses are about 20 nm.

The thermogravimetric (TGA) curves in FIG. 2 show the weight losses of the PPy nanotubes (a) and the PPy granules (b). Both the TGA curves of the PPy nanotubes and the PPy granules show a two-step weight loss during the heating processes. The weight loss (about 5%) that occurred in the first step below 100° C. could be attributed to the loss of residual moisture. In the second step, starting around 260° C., the weight loss could be attributed to the degradation of PPy and decomposition of dopants. The weight loss of the PPy nanotubes appeared to be less than that of the PPy granules. About 47% of the original weight remained for PPy nanotubes while it was only 40% for the PPy granules.

It was demonstrated that the PPy nanotubes were successfully transformed into carbon nanotubes after following the heating process at 900° C. in a quartz tubular furnace under a nitrogen atmosphere for 5 h as described in the above, because they are stable under the extremely high temperature. The chemical compositions of the as-prepared carbon nanotubes and the PPy nanotubes were evaluated by EDS (Energy Dispersive Spectrometer) analysis. The data were summarized in Table 1.

TABLE 1
The compositions of the PPy nanotubes and the carbon nanotubes.
Element	C (%)	N (%)	O (%)	S (%)	Cl (%)	Fe (%)
PPy nanotubes	77.00	11.76	5.48	3.86	1.80	0.10
Carbon nanotubes	88.77	4.59	4.38	1.74	0	0.52

It is evident that the content of carbon was increased after calcinations. It is believed that during the carbonization process of PPy nanotubes, carbonization reactions such as dehydrogenation and denitrogenation occur and produce more compact polycondensed graphitic species.

In order to investigate the structure of the as-prepared carbon nanotubes, the TEM image was obtained, which is shown in FIG. 3. It was evident from FIG. 3 that the tubular structures were obtained after pyrolysis at 900° C. The wall thickness of the carbon nanotube is about 15 nm, which is thinner than that of the original PPy nanotubes (20 nm). The size reduction can be attributed to the weight loss and the formation of more compact structure in the carbonization process.

FIG. 4(B) is the HRTEM image of the as-prepared carbon nanotubes and shows some ordered graphitic layers on the wall of the carbon nanotube. Morphologically, these carbon nanotubes were found to be mostly amorphous in structure. The main activity that occurred at this temperature (900° C.) should be described as carbonization but not graphitization. Graphitization may occur at higher temperatures such as 2200° C.

The XRD pattern of the as-prepared carbon nanotubes is shown in FIG. 5. The wide-angle XRD of the carbonized PPy-nanotubes contain two broad signals at 25.68° and 44.75°, corresponding to (002) and a superposition of the (101) reflections of the graphite structure. The XRD peaks with a maximum at 2θ=25.68°, which is equivalent to the d spacing of about 3.432 Å. In addition to the strong (002) peak, the same XRD pattern also showed peaks at 44.75°, which may be ascribed to the diffraction peak of (101) plane of graphite. This confirms that the structure of as-prepared carbon nanotubes is between the disordered amorphous carbon phase and highly ordered graphitic phase.

After the formation of PPy nanotubes, some residual iron salts (FeCl₂) were left inside the PPy nanotubes because the redox polymerization of pyrrole was conducted by using FeCl₃ as an oxidizing agent. In addition, the chemical oxidation polymerization led to the incorporation of iron-based species like FeCl₄₋ anions, which can be coordinated to the polymer backbone. The iron complexes break away from the PPy nanotubes and form iron when they expose to 900° C. under nitrogen. The darker part of FIG. 6 is believed to be the cluster of iron that had broken away from the PPy nanotubes (arrow in FIG. 6).

The structure of the as-prepared carbon nanotubes can also be observed by the Raman spectra as shown in FIG. 7, which are consistent with those reported in the literature for carbon fibers, displaying a characteristic strong, relatively narrow band around 1583 cm⁻¹, corresponding to a graphitic species (G-band) and a band at 1341 cm⁻¹, corresponding to an sp3 carbon species (D-band). The bands at 1583 cm⁻¹ and 1341 cm⁻¹ may be ascribed to the graphite-like and disordered structure of carbons, respectively. The D-band showed an increased intensity relative to that of the G-band. The results also indicate that the as-prepared carbon nanotubes were the disordered amorphous carbon phase.

While the preferred embodiment of the present invention has been described in conjunction with the drawings, the present invention is not limited to the above embodiment. The above embodiment is only illustrative and not limitative. Without departing from the spirit of the present invention and the scope sought for protection by the claims, a person skilled in the art can further make a lot of forms, all of which belong to the protection scope of the present invention.

REFERENCES

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Claims

1. A method for fabricating carbon nanotubes, comprising the steps of

(a) forming chemically a plurality of carbon precursors, which are nanotubes of conducting polymer, by adding conducting monomers in a water solution having a soft template and

(b) carbonizing said carbon precursors from step (a) at a temperature between 900-2200° C. in a nitrogen atmosphere or under a vacuum condition to afford a plurality of carbon nanotubes.

2. The method of claim 1, wherein said soft template is formed in water solution by an oxidant and methyl orange.

3. The method of claim 2, wherein said oxidant is FeCl3.

4. The method of claim 1, wherein said step (a) is carried out at a temperature within the range between room temperature and 130° C., and step (b) is carried out at a temperature within the range between 900-2200° C.

5. The method of claim 4, wherein said step (a) lasts for 24 hours and step (b) lasts for 3-5 hours.

6. The method of claim 1, wherein said step (b) is carried out by putting said carbon precursors from step (a) in a quartz tubular furnace under a nitrogen atmosphere or in a high temperature vacuum oven.

7. The method of claim 1, wherein said conducting polymer is a conjugated polymer.

8. The method of claim 7, wherein said conjugated polymer is polypyrrole, polyaniline or polythiophene and said monomer is pyrrole, aniline, or thiophene, respectively.

9. The method of claim 4, wherein said step (b) is carried at 900° C.

10. The method of claim 2, wherein said oxidant is ammonium persulfate, potassium persulfate, or ferric nitrate.