An attempt to prepare carbon nanotubes by carbonizing polyphosphazene nanotubes with high carbon content

An attempt to prepare carbon nanotubes by carbonizing polyphosphazene nanotubes with high carbon content

Materials Letters 62 (2008) 4130–4133 Contents lists available at ScienceDirect Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i...

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Materials Letters 62 (2008) 4130–4133

Contents lists available at ScienceDirect

Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t

An attempt to prepare carbon nanotubes by carbonizing polyphosphazene nanotubes with high carbon content Jianwei Fu, Yawen Huang, Yang Pan, Yan Zhu, Xiaobin Huang ⁎, Xiaozhen Tang School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China

A R T I C L E

I N F O

Article history: Received 15 April 2008 Accepted 10 June 2008 Available online 13 June 2008 Keywords: Nanomaterials Carbon nanotubes Carbonization Polymer Electron microscopy

A B S T R A C T Polyphosphazene nanotubes with about 20 nm in inner diameter and 100–200 nm in outer diameter were fabricated easily and then carbonized at 800 °C in a nitrogen atmosphere. Scanning electron microscopy and transmission electron microscope results showed that the bulk morphology of polyphosphazene nanotubes was retained after carbonization. The carbon content of the carbonized samples reached 93.28%. X-ray diffraction and Raman spectrum showed that the carbonized samples had low graphitization state. The present method can be used for a mass production of carbon nanotubes. © 2008 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental part

Carbon nanotubes (CNTs) have received much attention over the past decade due to their unique physical and chemical properties and potential technological applications, and therefore the development of a mass-production method is necessary. To date, many methods have been reported for the preparation of carbon nanotubes, such as electric arc discharge [1,2], laser vaporization [3], thermal chemical vapor deposition (CVD) [4,5], hydrothermal processing [6,7], and microwave synthesis [8]. But the production amounts of these methods are too small to meet the needs of applications. Recently, Oya and co-workers developed a novel CNT preparing method using polymer blending, spinning, and carbonizing techniques [9,10]. This method has great potential in the mass production of CNTs, but special requirements on raw materials and complicated manufacture process are needed by the technique. Therefore, developing a simple and low-cost mass production method for carbon nanotubes remains a challenge. More recently, an in situ template method was developed to prepare polyphosphazene (PPZ) nanotubes with high carbon content by our group [11]. Based on this method, in this study, PPZ nanotubes with closed end were fabricated easily and cheaply. When the PPZ nanotubes were heated at 800 °C in a nitrogen atmosphere, the bulk morphology of these nanotubes was retained and carbon nanotubes with low crystallinity were obtained. Herein, the preparation procedure and the morphologies of carbon nanotubes were briefly reported.

2.1. Raw materials

⁎ Corresponding author. Tel.: +86 21 54747142; fax: +86 21 54741297. E-mail address: [email protected] (X. Huang). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.06.020

Hexachlorocyclotriphosphazene (HCCP) (synthesized as described in the literature [12]) was recrystallized from dry hexane followed by sublimation (60 °C, 0.05 mmHg) twice before use (mp = 112.5–113 °C). 4,4′-Sulfonyldiphenol (BPS) and triethylamine (TEA) were purchased from Shanghai Chemical Reagents Corp. (Shanghai, China) and used without further purification. Tetrahydrofuran (THF, 99.9%, Sinopharm Chemical Regent co., LTD.) was distilled over sodium under dry nitrogen atmosphere prior to use. 2.2. Preparation of the PPZ nanotubes and carbon nanotubes Poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PPZ) nanotubes with high carbon content were successfully prepared via an in situ template method. In a typical synthesis (Fig. 1), 2 mL TEA was added to a solution of HCCP (0.5 g, 1.44 mmol) and BPS (1.08 g, 4.32 mmol) in 100 mL THF. The reaction mixtures were stirred in an ultrasonic bath (40 kHz, 50 W) at 45 °C for 8 h. The produced solids were centrifugated and then washed three times respectively using THF and deionized water. Finally, the resulting products were dried under vacuum to yield PPZ nanotubes. Synthesis yield was about 85 wt.%, calculated from HCCP (the structure characterization of as-synthesized PPZ nanotubes was seen in the supporting information). The carbonization process was performed by heating the PPZ nanotubes at a heating ratio of 3 °C min− 1 to 800 °C and holding at this temperature for 2 h under a nitrogen atmosphere.

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Fig. 1. Synthesis of the PPZ nanotubes with high carbon content. (P3N3) indicated other phosphazene cores.

2.3. Characterization

3. Results and discussion

The microstructures of the PPZ nanotubes and the carbonized samples were characterized by field emission scanning electron microscopy (JEOL JSM-7401F) and transmission electron microscope (JOEL JEM-100CX). The transmission electron microscope was equipped with an energy dispersive X-ray analyser (EDX). X-ray diffraction (XRD) pattern was recorded at a Bruker D8 Advance instrument. The Raman spectrum was investigated with a French Jobin Yvon High resolution 800 UV confocal laser micro Raman spectrometer; an argon-ion laser at 514.5 nm was used.

Fig. 2(a) and (b) shows typical SEM images of as-synthesized products used as carbon precursors. It can be easily found that the products are nanorods with diameters ranging from 100 to 200 nm and lengths of 2–5 μm, and few impurities are observed. TEM images of as-synthesized products reveal that the nanorods are hollow and the inner diameter ranges from 20 to 30 nm, as shown in Fig. 2(c) and (d). Closer observations reveal that most of the PPZ nanotubes have closed ends. Fig. 3(a) depicts a typical SEM image of the carbonized samples from the PPZ nanotubes after heating at 800 °C. The bulk morphology of the PPZ nanotubes was retained in the carbonized samples, although there was an overall reduction in size dimensions due to mass-transfer flow during carbonization. The inset TEM image in Fig. 3(a) shows that the carbonized samples have an inner diameter of ca. 25 nm and a wall thickness of ca. 60 nm. A similar structure of the closed end was also observed in

Fig. 2. SEM (a,b) and TEM (c,d) images of the PPZ nanotubes.

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the carbonized sample. Fig. 3(b) displays a clear contrast of the energy dispersive X-ray spectroscopy patterns of the PPZ nanotubes and carbon nanotubes. The elemental composition of carbon increased dramatically upon carbonization. For example, the carbon percentage of the PPZ nanotubes (around 45.53%, consistent with the result of elemental analysis, see the supporting information) increased to 93.28%. X-ray diffraction and Raman spectrum were used to prove the crystalline state of the carbonized samples. A typical XRD pattern of the carbonized samples is shown in Fig. 4(a). It contains two characteristic peaks at about 26.0 ° and 43.5°, indexed with 002 and 100 diffraction planes of hexagonal graphite (JCPDS card files, no. 41-1487), respectively. The fact that 002 diffraction peak is relatively low in intensity and broad in shape, suggests that as-prepared carbon nanotubes possess low graphitization and crystallization, which is quite reasonable in light of polyphosphazene precursor resin. Raman spectroscopy is a standard nondestructive tool for the characterization of crystalline, nanocrystalline, and amorphous carbon. As shown in Fig. 4(b), the band at 1600 cm− 1 is assigned to the E2g in-plain vibration mode (G band), which is associated with a graphitic carbon with a sp2 electronic configuration. Relative broad band at 1350 cm− 1 is attributed to the A1g in-plain breathing vibration mode (D band), which is a common feature of all disordered graphitic carbon [13]. Generally, the position of G band will shift toward higher wave number when graphitization degree decreases. Thus, compared with peak position 1582 cm− 1 of G band of a highly oriented pyrolytic graphite sample [14], the data we obtained is higher, which indicates that the carbon nanotubes exhibit relatively low degree of graphitization. The results are identical with the above XRD display.

4. Conclusions We have demonstrated that carbon nanotubes can be successfully prepared by carbonizing the polyphosphazene nanotubes with high carbon content. X-ray diffraction and Raman spectrum have proved that the carbon nanotubes possessed a low crystalline state. The

Fig. 4. (a) X-ray diffraction pattern of the PPZ nanotubes before and after carbonization. (b) Raman spectrum of the PPZ nanotubes after carbonization.

present method is a new route to preparing carbon nanotubes and will offer the following advantages when compared with previously developed methods: (1) it will be suitable for mass-production based on the facile synthesis of the PPZ nanotubes, leading to a supply of cheap carbon nanotubes; (2) it will be possible to prepare various morphology carbon nanomaterials through controlling the morphology of polyphosphazene nanomaterials; (3) it is similar to that of conventional carbon fibers, so the high crystalline carbon nanotubes might be obtained through improving the carbonization process. Work in these aspects is currently in progress. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matlet.2008.06.020. References

Fig. 3. (a) SEM and TEM (inset) images of the carbonized samples; scale bar of the inset presents 100 nm. (b) EDX patterns of the PPZ nanotubes and the carbonized samples.

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