Low temperature fullerene encapsulation in single wall carbon nanotubes: synthesis of N@C60@SWCNT

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Abstract

High filling of single wall carbon nanotubes (SWCNT) with C60 and C70 fullerenes from solvent is reported at temperatures as low as 69 °C. A 2-h long refluxing in n-hexane of the mixture of the fullerene and SWCNT results in a high yield of C60,C70@SWCNT, fullerene peapod, material. The peapod filling is characterized by TEM, Raman and electron energy loss spectroscopy and X-ray scattering. We applied the method to synthesize the temperature sensitive (N@C60:C60)@SWCNT as proved by electron spin resonance spectroscopy. The solvent prepared peapod samples can be transformed to double walled nanotubes enabling a high yield and industrially scalable production of DWCNT.

Introduction

Nanostructures based on carbon nanotubes [1] have been in the forefront of nanomaterial research in the last decade. Single wall carbon nanotube (SWCNT) is an even more exciting material as it represents the perfect, one-dimensional form of carbon. Fullerene encapsulating SWCNTs have attracted considerable interest after the discovery of C60@SWCNT peapods [2]. More recently, several molecules have been successfully inserted into the interior of tubes such as other fullerenes, endohedral metallofullerenes or alkali halides [3]. It is believed that the inside filled structures can alter or enhance the mechanical and electronic properties of the SWCNTs or may allow the fine tuning of these parameters. However, all these synthesis methods required treatment at relatively high temperatures above 400 °C. In particular, the peapod synthesis requires the heat treatment of SWCNT and fullerenes sealed together under vacuum, a method that appears impractical for large scale production purposes. Another important trend is the study of the behavior of the encapsulated materials under special conditions. It was recently shown that fullerene peapods are transformed into a double wall carbon nanotube (DWCNT) structure after high temperature annealing [4]. The fullerenes coalesce into an inner nanotube, which leaves the electronic properties unaffected but is expected to significantly enhance the mechanical properties of the tube system. This enhanced mechanical stability makes DWCNTs promising candidates for applications such as future electronics, probe tips for scanning probe microscopy, field emission devices and many more. Our aim in the current study was twofold: (i) development of a peapod synthesis method that allows the use of low temperatures in order to obtain encapsulated materials which do not survive the usual high temperature synthesis methods, (ii) devising a simple method for the production of peapod starting materials to facilitate the large scale synthesis of DWCNT. In what follows, we describe the synthesis of fullerene peapods from SWCNT mixed with fullerene in solution. We present transmission electron microscopy, Raman spectroscopy, electron energy loss spectroscopy, and X-ray studies to prove that a high filling content of the peapods is achieved. We show electron spin resonance evidence that the temperature sensitive N@C60 survives the filling. We also present the transformation of the solvent prepared peapod samples into DWCNT.

Section snippets

Sample preparation

Commercial SWCNT (NCL-SWCNT from Nanocarblab, Moscow, Russia [5] and Rice-SWCNT from Tubes@Rice, Rice University, Houston, TX) and fullerenes (Hoechst AG, Frankfurt, Germany), and n-hexane (Merck KGaA, Darmstadt, Germany) were used for the low temperature synthesis of fullerene peapods. The NCL-SWCNT material is prepared by the arc-discharge method and is purified to 50% using repeated high temperature air and acid washing treatments by the manufacturer. The Rice SWCNT material had an initial

Results and discussion

We performed TEM studies on our vapor and solvent prepared samples. For both kinds of materials TEM micrographs (not shown) proved that an abundant peapod concentration was achieved. However, it is not representative of the bulk of the sample thus filling efficiency has to be determined from spectroscopic investigations. In Fig. 1, we show the comparison of the Raman spectra of vapor- and solvent-filled C60 peapod samples. The Raman spectra of peapod samples in the plotted frequency range

Conclusion

We presented the preparation of fullerene encapsulated SWCNT at temperatures slightly above room temperature. This method produces very high filling and provides a simple alternative to the commonly used vapor filling method. Its advantage is the relative simplicity and the possibility to scale it up to larger amounts. The mechanism by which C60 enters the tubes from solution at low T’s needs further exploration. At the moment, we argue that the low concentration of dissolved C60 in n-hexane

Acknowledgements

The authors gratefully acknowledge useful discussions with Christian Kramberger and Rudolf Pfeiffer. This work was supported by the Austrian Science Funds (FWF) project No. P16315 and No. 14893, by the EU project NANOTEMP HPRN-CT-2002-00192, by the Deutsche Forschungsgemeinschaft DFG project PI 440, by the Hochschuljubiläumsstiftung der Stadt Wien project H-175/2001 and by the Hungarian State Grant OTKA T043255 and TS040878. The Budapest ESR Laboratory is member of the SENTINEL European

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