Effects of synthesis conditions on the structural features and methane adsorption properties of single-walled carbon nanohorns prepared by a gas-injected arc-in-water method

Abstract

Single-walled carbon nanohorns (SWCNHs) can be easily synthesized via a gas-injected arc-in-water method that is considered to be a cost-effective technique. The electrode configuration and duration of arc discharge were modified in order to enhance the yield and methane-adsorption properties of SWCNHs. As a result, the yield of the SWCNHs was significantly increased by increasing the discharge time and the size of the cathode. Using these modified conditions, the horn units in the SWCNH aggregates increased in size, and the thermal stability of SWCNHs in an oxidative environment increased accordingly. Ab initio molecular orbital calculations were used to explain the trend in the thermal stability. When the conventional conditions were applied, a burn-off of about 40% was necessary in order to achieve the maximum specific surface area and micropore volume. Remarkably, by enlarging the cathode size, the burn-off can be reduced by almost half to achieve the enhanced micropore volume. As a result, SWCNHs obtained using the modified conditions adsorbed a larger amount of methane than did SWCNHs obtained from the conventional synthetic conditions. The effect of a mild oxidation treatment on SWCNHs on their methane adsorption suggested that SWCNHs with micropores would be more flexible than pristine SWCNHs. This tendency was elucidated using a molecular mechanics calculation.