Toward mesoporous 1D structures: Expanding interlayer spaces in cup-stacked Carbon nanotubes

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.