Aging of nanocarbons in ambient conditions: Probable metastability of carbon nanotubes

We studied the physicochemical properties of several commercially available single- and multi-walled carbon nanotubes (SWNTs and MWNTs) and fullerenes stored in normal ambient conditions for 24 months. We found that SWNTs exhibit a trend of decreasing surface area and pore volume up to 7-15 months but then stabilized, no longer being impacted by sample age or outgassing temperatures. Using X-ray Photoelectron Spectroscopy. we also observed a trend of decreasing surface oxygen in all samples from the beginning with much lower % oxygen observed after 12-15 months of aging under ambient conditions. The surface oxygen then stabilized for the duration of this study. There was also evidence that the total structural-defect concentration, estimated from Raman spectroscopy, was somehow lowered during the aging process. The decrease in surface oxygen is an unexpected phenomenon because most other carbons, such as activated carbons or carbon molecular sieves, either oxidize or remain unaffected by age. We believe that nanocarbons are meta-stable materials (in pseudo-thermodynamic equilibrium), and that their aging in ambient conditions makes them more thermodynamically stable with fixed properties. This new information about the properties of nanocarbons should be further explored as it can help resolve some of the conflicting reports such as those about the environmental impacts of nanomaterials. (C) 2009 Elsevier Inc. All rights reserved.