High-yield Synthesis of Multiwalled Carbon Nanotube by Mechanothermal Method
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This study reports on the mechanothermal synthesis of multiwalled carbon nanotube (MWCNTs) from elemental graphite powder. Initially, high ultra-active graphite powder can be obtained by mechanical milling under argon atmosphere. Finally, the mechanical activation product is heat-treated at 1350°C for 2–4 h under argon gas flow. After heat-treatment, active graphite powders were successfully changed into MWCNTs with high purity. The XRD analyses showed that in the duration 150 h of milling, all the raw materials were changed to the desired materials. From the broadening of the diffraction lines in the XRD patterns, it was concluded that the graphite crystallites were nanosized, and raising the milling duration resulted in the fineness of the particles and the increase of the strain. The structure and morphology of MWCNTs were investigated using scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The yield of MWCNTs was estimated through SEM and TEM observations of the as-prepared samples was to be about 90%. Indeed, mechanothermal method is of interest for fundamental understanding and improvement of commercial synthesis of carbon nanotubes (CNTs). As a matter of fact, the method of mechanothermal guarantees the production of MWCNTs suitable for different applications.
KeywordsCarbon nanotubes Mechanothermal Nanotechnology Advanced materials Outstanding structure
Since the time of discovery by Iijima , there has been much interest in the synthesis and physical properties of carbon nanotubes (CNTs) due to their important applications. For example, CNTs can be used as electrochemical devices , for hydrogen storage , field emission devices , and nanotweezers . Various methods have been developed for the synthesis of carbon nanotubes, including metalcatalyzed chemical vapor deposition (CVD) [6, 7, 8], arc evaporation , laser ablation of carbon , catalytic decomposition , HiPCO process  or pulsed laser vaporization (PLV) . There are growing experimental evidences, showing that the formation of both multiwalled and single-walled nanotubes involves a solid-phase transformation in the gas-phase synthesis processes [14, 15, 16]. It implies that a direct synthesis of CNTs by a transformation of solid carbons under mild conditions is possible; if accessible, then it would be quite beneficial for a large-scale synthesis due to the intrinsic high-feeding- density characteristic of the solid-phase reaction process.
Recently, successful syntheses of CNTs by the solid-phase transformation of granular carbon materials, such as carbon black, amorphous carbon, and fullerene soot, achieved at extremely high temperatures (2000–3000°C) have been reported [15, 16, 17, 18, 19, 20, 21, 22, 23]. However, further technical improvement for practical access and clear understanding of the transformation mechanism for rational process design and control are still necessary and challenging tasks. Zhenping Zhu et al. recently synthesized MWCNTs by the solid-phase transformation of metal-containing glass-like carbon nanoparticles by heating at temperatures of 800–1000°C . More recently, we have suggested that using washable supported catalysts is accompanied by valuable advantages and with an extraordinary structure [25, 26].
Herein, we study mechanothermal method for synthesizing MWCNTs that consists of mechanical milling (for obtaining amorphous carbon nanostructure using ultra-high purity graphite powders) and thermal annealing processes (for transforming into nanotubes via carbon nanostructure and structural crystallization). The latest finding of this article demonstrates that this simple technique is a promising tool to synthesize the MWCNTs with ultra-high purity and high yield without a need for specialized equipment and or a multi-step purification process to eliminate the amorphous carbon and MWCNTs.
where λ is the wavelength of the X-ray, β the full width at half-maximum (FWHM), θ the Bragg angle, and ε is the microstrain.
Results and Discussion
Characteristics of different samples used for investigation during milling
Milling time (h)
Crystalline size = D(nm)
The milled powders had an average crystalline size of about 5–10 nm as determined by the Williamson-Hall method as shown in Table 1. Crystalline size values determined in this way may be low when the concentration of defects in the sample is higher compared to that in the reference large-particulate powder. The BET areas are vastly different for all the samples ranging between 5.5 and 211.2 m2/g as presented in Table 1. In the steady state, the BET surface area of the mechanically activated powders was determined to be about 211.2 m2/g for several samples (C200, C210, C220and …). Measuring the surface area of carbon nanostructures via nitrogen adsorption by the Brunauer–Emmet–Teller (BET) method revealed a specific surface area of 211.2 m2/g which seems relevant for surface area-dependent applications such as diffusion process. Assuming that all the particles are spherical and have the same theoretical density, and form:dBET = 6/S · ρ, whereS is the surface area and ρ is the particle density (2.1 g/cm3for graphite), a BET particle diameter,dBET, of about 20 nm is found for these nanoparticles. These results are also consistent with the HRTEM image observations. Therefore, the obtained results of specific area (SA) and crystalline size (D) for milled graphite indicate that graphite particles are highly chemically active.
Figure 6b shows HRTEM images of individual MWCNTs (C150). The average diameter of resulting MWCNTs with a length of about several millimeters is in the range of 30–70 nm at the open and closed end. Also, we found that the carbon nanotube has a spring-like shape. The SAED pattern (not shown) exhibits a pair of small but strong arcs for (002), together with a ring for (100), and a pair of weak arcs for (004) diffractions. The appearance of (002) diffractions as a pair of arcs indicates some orientation of the (002) planes occurring in the carbon tubes .
In summary, we have postulated a simple method for producing high-yield MWCNTs under mechanothermal conditions. Elemental graphite powder was milled in a planetary ball mill at atmospheric pressure and room temperature. Finally, after annealing at 1350°C, we obtained high-yield MWCNTs. This method also presents a facile route to high-yield MWCNTs without complex purification processes. The yield and good quality of MWCNTs obtained by mechanothermal makes it a suitable promising method of synthesis for the production of MWCNTs or other graphitic nanocarbons. Indeed, because of the simplicity and high yield of this route, it may potentially be applied on the scale of industrial production.
The authors thank the Tarbit Modarres University for access to Raman spectroscopy and their technical support. In addition, the authors would like to acknowledge Dr. Hesari for investigating TEM image, Professor Torabi for helping in the preparation of this article, and Mr Jabbari for performing the experimental tests.
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