Nanocarbon production by arc discharge in water

Abstract

Carbon nanostructures (onions, nanotubes and encapsulates) were generated by arc discharge in water between pure and catalyst-doped graphite electrodes. These structures were of fine quality and crystalline morphology, similar to those formed in He arc plasma. Emission spectroscopy was performed to assess the plasma components (H, O, C and C₂) and temperature values. C₂ radicals were determined quantitatively, between 10¹⁵ and 10¹⁶ cm⁻² depending on graphite anode composition.The temperature was between 4000 and 6500 K.

Introduction

Carbon plasma, generated by laser ablation [1] or arc discharge [2], has been used routinely in the production of fullerenes and carbon nanotubes (CNTs) [3]. Large quantities of CNTs were first prepared by Ebbesen and Ajayan [4], using arc discharge in a helium atmosphere at ca. 60 kPa. Discharges in liquid media were also investigated as a route to carbon nanomaterials: e.g., (1) C₆₀ from a spark discharge operating at 10–20 kV in toluene or benzene [5], (2) fullerene fragments via a glow discharge in CHCl₃ vapor [6], and (3) CNTs from a low-voltage contact auto-regulated arc discharge in aromatic hydrocarbons. Other nanoparticles have also been formed in liquid media by electrochemical or plasma discharge, e.g., fine silver powders [7] and CNTs [8]. Sundaresan and Bockris [9] studied the behavior of a carbon arc discharge in water, however they focused their attention solely on cold fusion, rather than carbon nanostructure production.

Hsin et al. [10] reported the first low-temperature, solution-phase production of CNTs in water. Recently, Sano et al. [11] described the large scale formation of carbon nano-onions using relatively low discharge currents (30 A), the graphite electrodes being submerged in deionized water. Their study showed that the resulting nanoparticles, floating on the water surface, consisted of C₆₀ cores wrapped in graphenes (ca. 7–15 layers, ca. 25–30 nm diameter). Under similar discharge conditions Zhu et al. [12] investigated the formation of CNTs using also salt solutions as the medium. The authors obtained nanotube-rich material with the yield between 5 and 7 mg/min.

High yield, large-scale, and cost-efficient approaches to such nanostructures should not only boost fundamental research but will also foster industrial applications, e.g., lubrication technology. Such a reaction environment can effectively lead to cooling of the reactants and intensification of the coalescence processes, leading to novel nanostructures. Therefore, it is of interest to carry out detailed studies of the C arc-discharge process in water. First, by focusing attention on the plasma process, we have successfully determined the temperature distribution and identified the intermediate species, for the first time under such arc discharge conditions. Second, we tried to establish the selectivity of this process in nanomaterial generation, i.e., which types of nanostructure would form concurrently during the bulk production of carbon nano-onions. Pure C electrodes, as well as Y- or Gd-doped electrodes, were employed, because these dopants play a critical catalytic role in CNT formation [13].