Hydrophobic coating of carbon nanotubes by CH4 glow plasma at low pressure, and their resulting wettability

This study demonstrates the hydrophobic coating of carbon nanotubes (CNTs) by methane (CH4) glow discharge plasma at low pressure and investigates the superhydrophobic behavior of the treated CNTs. Carbonaceous species produced from the CH4 glow plasma were identified by optical emission spectroscopy. The treated CNTs floated on water for several months, exhibiting hydrophobic properties. Contact angles (CAs) with test liquids were measured from the capillary rise method based on the Washburn equation, to identify the total surface free energies of CNTs with and without CH4 plasma treatment. The total surface free energy of the CNTs can be determined by making use of the measured CA and the van Oss–Chaudhury–Good equation. It was shown that the surface free energy of the plasma-treated CNTs decreased drastically from 27.04 mN m−1 to 1.32 × 10−7 mN m−1, revealing super-hydrophobic modification of CNTs by the CH4 plasma.