Investigations of NanoBud formation

doi:10.1016/j.cplett.2007.08.050

Chemical Physics Letters

Volume 446, Issues 1–3, 26 September 2007, Pages 109-114

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Abstract

The formation of a novel hybrid material, NanoBuds, single-walled carbon nanotubes (CNTs) with covalently attached fullerenes was investigated in a ferrocene–carbon monoxide system. Fullerenes and CNTs were simultaneously formed by carbon monoxide disproportionation on the surface of iron particles in the presence of etching agents such as H₂O and CO₂. On the basis of parametric investigations and in situ sampling of the product from different locations in the reactor the mechanisms for NanoBud formation are discussed.

Graphical abstract

Schematic presentation of a novel carbon nanomaterial, NanoBud, grown by continuous transportation of a carbon layer from a catalyst particle to a CNT.

Introduction

Fullerenes and carbon nanotubes (CNTs) have attracted great interest from both fundamental and application points of view due to their remarkable physical and chemical properties [1], [2], [3]. Fullerenes and CNTs are usually produced in similar conditions and, in fact, were notoriously observed in a carbon arc discharge method [4] intended to produce only fullerenes [5]. In addition to arc discharge, another physical method for carbon vaporization, high-energy laser ablation, has also been shown to generate inhomogeneous mixtures of fullerenes and CNTs. Simultaneous formation of fullerenes and CNTs was also shown to occur in a HiPco reactor [6], where the produced fullerenes were separable in a solvent indicating weak bonding to the CNTs [7]. Transmission electron microscopy (TEM) observations often suggest that the surface of many CNTs is not clean and they are covered with some coating. It has been proposed that this coating can be converted to fullerenes in the intense electron beam of the microscope [8], [9]. Also there have been several papers, where the presence of fullerene-like structures (or perhaps even fullerenes) on the surface of CNTs can be distinguished, but are not discussed [10], [11], [12].

There have been published very few experimental works attempting to combine fullerenes and CNTs into a single structure. Fullerene and CNT physical merging by means of solid phase mechanochemical reactions was proposed by Li et al. [13]. Theoretically, the combination of a fullerene and a CNT was first shown in a dynamic process of fullerene penetration into a CNT [14]. Recently, we have reported a one-step continuous process for the synthesis CNTs with covalently attached fullerenes [15]. This hybridization of fullerenes and CNTs resulted in the creation of a new material, NanoBuds, with interesting properties [15]. For instance, this structural arrangement of highly curved (chemically reactive) fullerenes and inert, but thermally and electrically conductive, CNTs, was shown to exhibit enhanced cold electron field emission properties [15]. In this paper, on the basis of parametric investigations and in situ sampling of the product from different locations in the reactor we investigate the conditions and mechanisms for NanoBud formation.

Section snippets

Experimental setup

The experimental method used for the continuous synthesis of NanoBuds is based on ferrocene vapor decomposition in a CO atmosphere (Fig. 1a). The catalyst precursor was vaporized by passing ambient temperature CO (with a flow rate of 300 cm³/min) through a cartridge filled with ferrocene powder [16]. The flow containing ferrocene vapor was then introduced into the high temperature zone of a ceramic tube reactor through a water-cooled probe and mixed with additional CO (100 cm³/min). The partial

Chemical reactions

As has been shown, the fullerene concentration on the surface of CNTs can be altered by changing the concentration of H₂O and CO₂ in the system. It is known from the literature that fullerenes (or even fullerite) can be formed under the effect of acid treatment of single-walled CNTs [17]. Since oxidation may be important for tailoring carbon structures [18] one can expect a similar effect during the CNT growth in our flow reactor when extra water or carbon dioxide are present as oxidizing

Conclusion

We have synthesized NanoBuds, single-walled CNTs covered by covalently bonded fullerenes, in a one-step continuous process by ferrocene vapor decomposition in a carbon monoxide atmosphere. Fullerenes and carbon nanotubes were simultaneously formed by carbon monoxide disproportionation on the surface of iron particles in the presence of etching agents H₂O and CO₂. Varying the introduced H₂O and CO₂ in the reactor revealed that the optimal reagent concentrations were between 45 and 245 ppm for H₂O

Acknowledgements

The authors thank Dr. S. Shandakov for fruitful discussions. Financial support from the Academy of Finland and the Creative Research Initiatives Program supported by the Korean Ministry of Science and Technology are gratefully acknowledged.

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Investigations of NanoBud formation

Abstract

Graphical abstract

Introduction

Section snippets

Experimental setup

Chemical reactions

Conclusion

Acknowledgements

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Chem. Eng. Sci.

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Chem. Phys. Lett.

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Carbon

Physica B

Chem. Phys. Lett.

Carbon

Science of Fullerenes and Carbon Nanotubes