Synthesis of bamboo-like carbon nanotubes by ethanol catalytic combustion technique

CHENG Jin(程 进)^1,2, ZOU Xiao-ping(邹小平) ^1,2, LI Fei(李 飞) ^1,2, ZHANG Hong-dan(张红丹) ^1,2, REN Peng-fei(任鹏飞) ^1,2  

1. Research Center for Sensor Technology, Beijing Information Technology Institute, Beijing 100101, China

2. Beijing Key Laboratory for Sensor, Beijing 100101, China

Received 10 April 2006; accepted 25 April 2006

Abstract: Bamboo-like carbon nanotubes were synthesized by ethanol catalytic combustion (ECC) technique with combustion method. Copper plate was employed as substrate, ethanol as carbon source, and iron chloride as catalyst precursor. The as-grown black powder was characterized by means of scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. The results show that the thinner bamboo-like carbon nanotubes have a relatively good structure that the compartment layers are more regular, while the thicker carbon nanotubes have a relatively irregular bamboo-like structure; the proposed method is simple to synthesize bamboo-like carbon nanotubes and has some advantages, such as flexible synthesis conditions, simple setup, and environment-friendly.

Key words: bamboo-like carbon nanotubes; ethanol catalytic combustion technique; compartment layers

1 Introduction

Since the discovery of carbon nanotubes, there has been great interest in synthesizing carbon nanotubes due to their remarkable electrical and mechanical properties[1-3]. Various shapes and structures of carbon nanotubes have been found, such as single-walled carbon nanotubes[4], multi-walled carbon nanotubes[5], zigzag-shaped carbon nanotubes[6], Y-shaped carbon nanotubes[7] and bamboo-like carbon nanotubes [8]. Some of them have been successfully synthesized by various growth methods, such as arc discharge, chemical vapor deposition and combustion approach[9, 10]. The combustion approach utilizes the energy of flame to synthesize various carbon materials, such as C60/C70[11], single-walled carbon nanotubes[9], multi-walled carbon nanotubes[10] and carbon nanowires[12]. However, bamboo-like carbon nanotubes have not been synthesized by combustion approach up to date.

Bamboo-like carbon nanotubes are tube like bamboos which have some compartment layers between the walls. SU et al[7] observed bamboo structure within Y-junction carbon nanotubes which were synthesized by catalytic chemical vapor deposition at 1 000-1 200 ℃[7]. LIU et al[13] synthesized bamboo-like carbon nanotubes earlier by an ethanol thermal reduction process. This was a method which utilized the reaction of ethanol with magnesium in stainless autoclave at 600℃, that is, this reaction was conducted under high pressure. These methods suffer from complex setup and rigorous synthesis conditions.

In the present work the bamboo-like carobn nanotubes were synthesized by ethanol catalytic combustion(ECC) technique. A copper plate was employed as substrate, ethanol as carbon source, and iron chloride as catalyst precursor. The as-grown products were characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy.

2 Experimental

In a typical experiment, 0.01 mol iron chloride was dissolved in ethanol to form 1 mol/L iron chloride solution. Then the solution were diluted to 0.1 mol/L and 0.01 mol/L solution which was employed as catalyst precursor. A round copper plate with diameter of 1 cm, which was employed as substrate, was ultrasonically washed in acetone for several minutes to clean the surface of plate.

After drying, the catalyst precursor solution was applied to the clean surface of the copper plate. Then the copper substrate was baken at 60 ℃ for several minutes to remove the solvent. After that, the substrate was placed in inner flame for about 10 min. Then black wool-like products were obtained. This process is ethanol catalytic combustion technique mentioned above. The synthesis of carbon nanotubes was conducted directly in atmosphere and produced no pollution.

The black wool-like powder was characterized by JEOL 6500F SEM, JEM-200cx TEM and Rainshaw optical confocal Raman spectrometer.

3 Results and discussion

Fig.1 shows the SEM micrograph of a typical sample of carbon nanotubes grown on copper substrate with 0.01 mol/L iron chloride catalyst precursor for about 10 min synthesis duration, indicating the large quantity of carbon nanotubes were achieved by using this approach. These carbon nanotubes have diameters ranging from about 10 to 60 nm and lengths ranging from hundreds of nanometers to several micrometers.

Fig.1 SEM image of carbon nanotubes grown on copper substrate with 0.01 mol/L iron chloride catalyst precursor for about 10 min

Fig.2 shows the TEM images of carbon nanotubes grown with 0.01 mol/L iron chloride catalyst precursor. Fig.2(a) shows the carbon nanotube has a bamboo-like structure clearly. The black arrows in Fig.2(a) mark the compartment layers. Fig.2(b) shows the compart-

ment layers (see arrows ①) whose curvature is directed toward the growth direction (see arrows ②). The thinner bamboo-like carbon nanotubes have a relatively good structure whose the compartment layers are more regular, while the thicker carbon nanotubes have a relatively irregular bamboo-like structure, as the bamboo-like carbon nanotubes shown in Fig. 2.

Fig.2 TEM images of carbon nanotubes grown on copper substrate with 0.01 mol/L iron chloride catalyst precursor (arrows ① mark compartment layers; arrows ② mark growth direction)

Fig.3 shows the Raman spectrum of carbon nanotubes which was obtained with a REINSHAW optical confocal Raman microscope. The sample excitation was performed using 5 mW of 514.5 nm laser with 3 ?m spot size. The spectrum shows the peak frequency of the graphite mode at 1 591 cm^-1 (G) and contains disordered modes at 1 345 cm^-1 (D). When compared with the Raman spectra of carbon nanotubes synthesized by other methods, such as arc discharge[14] and chemical vapor deposition [15], the present Raman spectrum obviously gives the large ratio of I_D/I_G. This phenomenon illustrates that the carbon nanotubes synthesized by ECC technique have a lot of disorder struc-

Fig. 3 Raman spectrum of carbon nanotubes grown on copper substrate with 0.01 mol/L iron chloride catalyst precursor

ture or defects[16]. We speculate that it is related to its unique synthesis conditions in atmosphere.

Fig.4 shows the TEM images of carbon nanotubes by employing 0.1 mol/L iron chloride catalyst precursors. The carbon nanotubes with diameter of about 40 nm shown in Fig.4 have a clear bamboo-like structure. While for 1 mol/L iron chloride catalyst precursor it is found that there is no carbon nanotubes except solid carbon nanowires with diameter ranging from about 50 to 150 nm.

Fig.4 TEM images of carbon nanotubes grown on copper substrate with 0.1 mol/L iron chloride catalyst precursor

So it can be concluded that the morphology and microstructure are affected by the concentration of catalyst precursor solution. The iron chloride catalyst precursors are aggregated to form nanoparticles of different sizes from catalyst precursor solution of different concentrations because of different viscosities of precursor solutions. The precursor solution with lower concentration has lower viscosity, so it tends to form smaller catalyst precursor particles which tend to form smaller catalyst. The smaller catalysts could produce the thinner carbon nanotubes with more regular bamboo-like structure. While the catalysts with larger size tend to form irregular bamboo-like structure or solid nanowires.

It is not clear that why the carbon nanotubes possess bamboo-like structure, but our observations are consistent with those of LEE et al[8], who demonstrated that carbons were first adsorbed on metal particle and then form graphitic sheets as a cap. As the cap lifts off the particle, a close tip with a hollow inside is produced. When the wall grows up towards, the next compartment layer is produced[8]. A further study of the formation mechanism of bamboo-like carbon nanotubes is underway.

4 Conclusions

Bamboo-like carbon nanotubes are synthesized by ethanol catalytic combustion technique with 0.01 mol/L and 0.1 mol/L iron chloride catalyst precursor. The morphology and microstructure of bamboo-like carbon nanotubes are affected by catalyst size which is related to concentration of catalyst precursor. The thinner bamboo-like carbon nanotubes have a relatively good structure whose the compartment layers are more regular, while the thicker carbon nanotubes have a relatively irregular bamboo-like structure. In summary, this is a simple method to synthesize bamboo-like carbon nanotubes due to synthesizing directly in atmosphere and there are some advantages, such as simple setup, flexible synthesis condition, and enviroment-friendly.

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(Edited by CHEN Wei-ping)

Foundation item: Project(KM200510772013) supported by the Science and Technology Development Program of Education Committee of Beijing City; Project(2005-2007) supported by the Academic Innovative Team Porgram (Novel Sensor and Materials: Nanodevice and Nanomaterials) of Education Committee of Beijing City

Corresponding author: ZOU Xiao-ping; Tel: +86-10-64884673-812; Fax: +86-10-64879486; E-mail: xpzou2005@gmail.com