Cluster-associated filling of water molecules in graphene-based mesopores

Abstract

Graphene monoliths were prepared through unidirectional freeze-drying method of graphene oxide colloids-KOH mixed solution and successive reduction by heating at 573 K in Ar. The porosity- and crystallinity-controlled graphene monoliths were prepared by the KOH activation at different temperature and the post-heating in Ar. These activated graphene monoliths were characterized by N2 adsorption at 77 K, X-ray diffraction and Raman spectroscopy. Water adsorption isotherms show a typical hydrophobicity below P/P 0 = 0.5 and a marked hydrophilicity above P/P 0 = 0.6, which depends on the pore width. In the water adsorption isotherms of porous graphene monoliths activated at different temperature, the higher the activation temperature, the larger the rising P/P 0. No essential change in the shape of the water adsorption isotherm for the post-heated nanoporous graphene monoliths is observed except for the decrease in water adsorption amount with higher post-heating temperature. The linear relationship between the saturated water adsorption and pore volume whose width is smaller than 4 nm indicates clearly that water molecules are adsorbed in small mesopores by the cluster-associated filling mechanism.