Adsorption of water vapor on mesoporosity-controlled singe wall carbon nanohorn

The adsorption mechanism of water vapor in hydrophobic carbon mesopores is not clearly understood. Single wall carbon nanohorns (SWCNH) having different tube diameters in the range of 2 to 6 nm were oxidized at a lower temperature than the widely used temperature of 823 K. Oxidation at 633 K can selectively open thinner tubes of SWCNH and then the oxidation at different times can control the mesoporosity. The porosity of thus-oxidized SWCNH was evaluated from the nitrogen adsorption isotherms at 77 K. The pore volume ratio of micropores to mesopores was in the range of 0.50 to 0.90. The linear relationship between water adsorption amount and the pore volume of micropores and small mesopores (< 5 nm) was observed, showing that water molecules are adsorbed even in small mesopores by the cluster-mediated filling mechanism as well as in micropores.