Synthesis of carbon nanotubes by electrochemical deposition at room temperature

Abstract

Carbon nanotubes are finding increasing commercial applications in modern technologies, for example, composite materials, electrochemical devices, hydrogen storage, field emission devices, and nanoscale electronic devices [1]. Wide applications of carbon nanotubes are based on their unique physical and mechanical properties, which show the high electrical and thermal conductivities, and high mechanical strength along the tubular axis [2–4]. Carbon nanotubes are normally produced by either a carbon arc-discharge technique [5,6] or a pyrolysis of hydrocarbon gases on particles of transition metals such as Fe, Ni, and Co in a chemical vapor deposition (CVD) reactor [7,8]. For the carbon nanotubes produced from a CVD technique, their diameters are controlled by the sizes of the catalytic particles. The growth mechanism of the carbon nanotubes from a CVD process has been the subject of some controversy, particularly regarding to the active catalytic state of the particles. Key steps include the absorption and decomposition of hydrocarbon molecules on the exposed metal surface to produce carbon species which dissolve into and diffuse through the underlying bulk, ultimately precipitating at the rear surface of the particle to form the carbon nanotubes [9–12]. At the same time, in recent years, more attention has been paid to electrochemical deposition technique for manufacturing thin films and devices due to its simplicity, its low capital equipment cost, and its ability to be scaled up for large production [13–16]. So far, it has not been reported that carbon materials can be deposited or plated through an electrical process using an organic solvent. In this work, an electrochemical deposition technique has been employed to produce carbon nanotubes from organic solvents at room temperature [17]. Transition metal nanoparticles (Ni and/or Fe) were coated on the electrodes to provide the nucleation sites for the formation and growth of carbon nanotubes. The microstructure characterization of the carbonaceous deposits has been carried out with SEM, TEM, and energy dispersive X-ray spectroscopy (EDS). A standard three-component electrochemical cell was used as the electrochemical reactor for the carbonaceous depositions. A saturated calomel electrode was used as reference electrode. Two silicon wafers coated with 50:50 Fe/Ni alloy nanoparticles [12] were employed as counter electrode (anode) and working electrode (cathode), respectively. A mixture of 40 vol% methanol (CH3OH) and 60 vol% benzyl alcohol (C6H5CH2OH) was used as the electrolyte. The sizes of counter and working electrodes were about 2.5 cm · 2.0 cm. The distance between cathode and anode was kept at 5 mm, which was precisely controlled by a SiC spacer. The potential difference applied between the anode and cathode was kept nominally at 1000 V. The depositions were carried out at room temperature and