Temporal changes in nitrogen adsorption properties of single-walled carbon nanotubes

Temporal evolution of N₂ adsorption (77 K) properties of as-produced and purified single-walled nanotubes (SWNTs) samples is described here. The N₂ adsorption isotherms are used to characterize the samples' surface areas and porosities. The as-produced samples demonstrate a temporal increase in surface area and pore volumes for up to 16 months. The purified samples, however, reached their stable values of surface area and pore volumes within four to seven months. N₂ adsorption capacity of the purified SWNTs also increased when the fresh samples were subjected to thermal pre-processing, with diminishing changes in adsorption capacity with increased age. These observations indicate that the freshly prepared SWNTs, both as-produced and purified, were in an unstable state with their porosity changing with increasing sample age and thermal treatments. It is hypothesized that SWNTs undergo slow but progressive changes in their surface chemistry which causes their N₂ adsorption properties to change over several months.