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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 (HRTEM) 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 m2/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: (1) the presence of ion current conducive for elongated nanoparticles growth and (2) 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.