Deconvolution of the structural and chemical surface properties of carbon nanotubes by inverse gas chromatography

The thermodynamic surface characteristics of carbonaceous nanomaterials (here carbon nanotubes) can be studied by inverse gas chromatography. One potential difficulty is that the effects of surface chemistry and surface topography may become convoluted. To solve this problem, measurements were performed with two distinct sets of probe molecules, each selected to isolate the contribution of either structural or chemical surface features to the vapour retention observed, allowing an unambiguous assessment of solid surface characteristics. The differences in net retention times showed that as-received, high-temperature annealed and thermally oxidised carbon nanotubes are relatively similar in their surface structure but exhibited considerably different concentrations of polar surface groups. The symmetry and tailing of the chromatographic signals was interpreted in terms of structural and chemical surface heterogeneity. The dispersive surface energies, γd, of the as-received and modified carbon nanotubes were found to be very similar while the specific surface energies differed significantly, contributing between 10% and 30% to the total surface energy, depending on the modification treatment. The effect of energetic surface heterogeneity on the IGC results is briefly discussed and assessed quantitatively in terms of γd heterogeneity profiles as a function of surface coverage.