The fabrication of carbon nanomaterials usually calls for expensive vacuum systems to generate plasmas and yields are disappointingly low. Here we describe a simple method for producing high-quality spherical carbon nano-'onions' in large quantities without the use of vacuum equipment. The nanoparticles, which have C60 cores surrounded by onion-like nested particles, are generated by an arc discharge between two graphite electrodes submerged in water. This technique is economical and environmentally benign, and produces uncontaminated nanoparticles which may be useful in many applications.

A simple method to fabricate high-quality nanoparticles including spherical carbon onions and elongated fullerene-like nanoparticles similar to nanotubes in large quantities without the use of vacuum equipment is reported. The nanoparticles are obtained in the form of floating powder on the water surface following an arc discharge between two graphite electrodes submerged in water. High-resolution transmission electron microscopy ͑HRTEM͒ and scanning electron microscopy images confirm the presence of spherical carbon onions with diameters ranging from 4 to 36 nm. The specific surface area of the floating powder was found to be very large, 984.3 m 2 /g, indicating that the material is promising for gas storage. From the surface area measurements, the mean particle diameter was calculated to be 3.7 nm. This value is close to the lower limit of the carbon onions observed in HRTEM. However, closer HRTEM observations also reveal that some carbon onions are not well crystallized. The large specific surface area can be attributed to the ''surface roughness'' induced by the defective nature of the carbon onion shells. To explain the formation mechanism of the carbon onions, a model of arc discharge in water with two quenching zones is proposed: ͑1͒ the presence of ion current conducive for elongated nanoparticles growth and ͑2͒ the absence of ion current for isotropic growth of carbon onions. Based on this model, we propose that the physical characteristics of the product can be controlled.

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Ultraviolet-visible absorption measurements of high purity and well separated carbon onion samples are reported. The results show that, after purification, absorption features from carbon onions match well with the interstellar UV spectrum. The measurements show that the absorption peak position remains constant at 4.55+/-0.1 microm(-1), and the width varies from 1.2-1.6 microm(-1), a key feature of the interstellar spectrum. The similarities between the experimental and observed absorption spectra indicate that carbon onions are very strong candidates for the origin of the UV interstellar absorption peak at 4.6 microm(-1).

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