Controlled synthesis, efficient purification, and electrochemical characterization of arc-discharge carbon nano-onions

Arc-produced carbon nano-onions (A-CNOs), with a hollow core surrounded by multilayered sp² carbon shells, possess unique structural and electronic properties. While nanodiamond derived CNOs (5-7 nm) are attracting significant attentions, for A-CNOs (20-50 nm) controlling the growth and separating them from carbon impurities are the major challenges that impede further study and application of these materials. We have addressed these issues; first by designing an in-house automated underwater arc discharge apparatus to control the arc plasma that produces homogeneous A-CNOs (diameter 25-35 nm) with minimal carbon impurities. Secondly, for further purification we have developed a very efficient method by utilizing the strong preferential adsorption of polyoxometalates. A-CNO growth and purification were investigated using thermogravimetric analysis (TGA), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and X-ray diffraction (XRD). Furthermore, the electrochemical properties and electrocatalytic activities of purified A-CNOs were investigated with various redox species including neurotransmitter molecules. Compared to glassy carbon (GC) electrode, A-CNO showed excellent electrochemical performances including larger faradaic currents and facilitated electron-transfer kinetics. Controlled synthesis, efficient purification and the excellent electrocatalytic activities of A-CNOs reported herein will enable the utilization of these materials for various applications including biosensing, fuel-cell catalysts and energy-storage devices.