Effect of ball milling on morphology of cup-stacked carbon nanotubes

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Abstract

The effect of grinding on cup-stacked carbon nanotubes is studied. The application of a bending force on the sidewall of such carbon nanotubes, that is higher than the Van der Waals force acting between the cups, results in the shortening of carbon nanotubes and the formation of nano-barrels exhibiting an increased number of accessible active sites (represented by the ends of graphitic planes) and anisotropic properties at both ends.

Introduction

Carbon nanotubes have acted as a vanguard among nanomaterials with a high potential of applications in various fields of engineering, due to their extraordinary electronic and physical properties [1]. One cause for limiting recent trends toward widespread practical applications of carbon nanotubes is considered to be the difficulty of large-scale production of high purity carbon nanotubes at low cost. In this sense, a catalytic chemical vapor deposition method is considered to be the answer for the large-scale production of carbon nanotubes, especially, using a floating reactant method [2], [3], [4], [5], [6], [7]. In terms of the morphology of carbon nanotubes, it is recognized that conventional carbon nanotubes are made up of single or multiseamless cylinders [1]. Recently, a new type of carbon nanotubes was introduced as a novel functional nanomaterial [8], [9]. One of main features of this carbon nanotube is its stacking morphology, consisting of truncated conical graphene layers (cups), which, in turn, exhibit a large portion of open edges in the outer surface. These structural characteristics and also their possible low-production cost, using a floating reactant production system, may make it possible to use this novel carbon nanotube in the fabrication of absorbent materials, catalyst-supports, field emitters, gas storage components and nano-composites [10], [11], [12], [13], [14].

Mechanical treatments, such as the ball milling process, have previously been applied to obtain tailor-made carbon materials [15], [16], [17], [18]. In this study, novel carbon nanotubes, obtained by a floating reactant method, are ground mechanically using ball mills in order to maximize the utilization of the open edges in the outer surface and also in the hollow channels, and to improve the handling properties due to the reduction of the apparent density of the bulk state, and also to create new carbon nanomaterials, such as nano-barrels.

Section snippets

Experimental

The carbon nanotubes used in this study were synthesized by a floating reactant method, using ferrocene or iron pentacarbonyl as a catalyst precursor, hydrogen sulfide as a co-catalyst, and natural gas as a carbon feedstock in a continuous process [5], [6]. Carbon nanotubes were mechanically ground in an ambient atmosphere using a ball mill, containing an aluminum oxide ball with a diameter of 5 mm. The rolling speed of the milling machine is fixed at 350 rpm. This type of ball milling machine

Results and discussion

Relatively straight and long carbon nanotubes with a hollow core along the tube length exhibit a large distribution of diameters, ranging from 50 to 150 nm (Fig. 1a), which is considered to be an unavoidable phenomenon for large-scale production of carbon nanotubes by using a catalytic CVD synthetic method. The unusual characteristics of these tubes, as compared with those of conventional multiwalled carbon nanotube, are a stacking morphology of truncated conical graphene layers (cups) (Fig. 1b

Conclusion

The effects of ball milling on cup-stacked carbon nanotubes are summarized as follows. Two different cleavage modes are considered, based on TEM observations. It is expected that the bending mode plays the main role in cleaving carbon nanotubes from the viewpoint of the cup-stacked morphology of these carbon nanotubes. It is obvious that the length of the carbon nanotube gets shorter with increasing time. One of the main changes is the increased apparent density of the milled tubes, that is,

Acknowledgments

M.E., T.H. and Y.A.K. would like to acknowledge the Research for the Future (RFTF) Program of the Japan Society for the Promotion of Science (JSPS), `Nanocarbons for advanced energy devices' for supporting this work.

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