The gram-scale synthesis of carbon onions Mohammad Choucair a, * , John Arron Stride a,b a School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia b Bragg Institute, Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia ARTICLE INFO Article history: Received 20 July 2011 Accepted 12 October 2011 Available online 19 October 2011 ABSTRACT The combustion of naphthalene has been found to yield gram-scale quantities of carbon onions that are free of impurities and furthermore without the use of catalysts. X-ray dif- fraction (XRD) indicates that the interlayer spacing between concentric shells of the carbon onions is not uniform across the particle; rather it decreases from a graphite-like 0.34 nm and approaches a diamond-like 0.29 nm interlayer spacing towards the inner layers. The dispersion in the interlayer spacing is believed to result from differing external pressures exerted on the individual nanometer-sized graphitic membranes during formation of the onions. Electron microscopy techniques such as high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy demonstrate the extensive forma- tion of carbon onions. The HRTEM indicates that the onions consist of 50–54 shells, found to be in good agreement with the XRD data. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction It has been demonstrated that irradiation of carbon soot un- der high-powered focused electron beams, results in a struc- tural transformation to caged concentric structures of fullerenes [1,2]. The term carbon onions has been coined for these carbon structures, due to their cross sectional appear- ance consisting of rings and hence being reminiscent of an onion. Whilst serendipitous, this ‘proof of concept’ experi- ment was seen as a breakthrough in that it demonstrated reaction mechanisms exist to reduce the extent of so-called dangling bonds, and that the preferred morphology of carbon in its lowest energy state is not planar, but spherical. Carbon onions have been considered as large buckyballs, nested one within another, and despite initially only being observed in very small quantities under the irradiating beam of a trans- mission electron microscope (TEM), they represented a new dimension to the field of fullerene research [3]. Later attempts to synthesize carbon onions concentrated on the thermal annealing of carbon nano-diamonds [4,5] and chemical vapor deposition [6]. More recently however, carbon onions have been synthesized by arc-discharge per- formed in water [7], in which graphitic electrodes are used to produce 7–15 walled carbon onions containing buckmin- sterfullerene (C 60 ) cores, at 3 mg min 1 . Any onions formed were found to simply rise to the surface of the water. Carbon onions have also been found to exist in meteorites [8], how- ever the reliance of outer planetary sources of carbon is not feasible for real applications – and with the difficulties in nano-diamond synthesis, the most widely adopted method of producing carbon onions is that of arc-discharge in water [7]. The methods developed so far to synthesize carbon onions remain in their infancy and suffer issues relating to purity, quantity, and quality. Various synthesis pathways have led to carbon onions with differing physical properties as a result of shell number, core type, and precursor [9–16]. Ugarte [1] showed that irradiating carbon soot could result in carbon onions with closed caged cores that varied from a few layers to microns in size. The size of the carbon onion depended on the quantity of the material irradiated. Larger carbon onions appeared to be structurally imperfect compared to smaller 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.10.023 * Corresponding author: Fax: +61 2 93854672. E-mail address: m.choucair@unsw.edu.au (M. Choucair). CARBON 50 (2012) 1109 – 1115 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon