Toward Mesoporous 1D Structures: Expanding Interlayer Spaces in Cup-stacked Carbon Nanotubes

Abstract

Tailoring the pore structure in materials is a key to achieving efficient ion/molecular transportation. The interlayer space in graphitic materials can be expanded via the oxidation–exfoliation process. However, since the intercalation-mediated oxidation method does not work for carbon nanotubes (CNTs) that have semi-closed graphitic structures, mesoporous 1D nanostructure has been rarely prepared. In the present study, modified Hummers method-based oxidation was applied to cup-stacked carbon nanotubes (CSCNTs) with open-edge graphitic structure. The degree of oxidation was controlled by the weight ratio of the oxidant and added CNTs. The complete basal-plane oxidation was achieved when the oxidant was 5 times the weight of CSCNTs. Following thermal reduction further expanded the interlayer spaces of the CSCNTs resulting in the formation of mesoporous 1D nanostructure with alternating stacked–expanded graphene layers resembling honeycomb shape, indicating the importance of the unique nanostructure of CSCNTs for pore formation. Heat-treatment of the obtained mesoporous 1D structure helped improve the crystallinity, but at the same time deteriorated the porosity features as the heat-treatment temperature increased. Thermally induced defect repairing flattened the graphitic structure which also induced layer restacking; thus, this revealed the trade-off relationship between the crystallinity and porosity.