Physisorption of molecular oxygen on single-wall carbon nanotube bundles and graphite

We present a study on the kinetics of oxygen adsorption and desorption from single-wall carbon nanotube ~SWNT! and highly oriented pyrolytic graphite ~HOPG! samples. Thermal-desorption spectra for SWNT samples show a broad desorption feature peaked at 62 K, which is shifted to a significantly higher temperature than the low-coverage desorption feature on HOPG. The low-coverage O2 binding energy on SWNT bundles ~18.5 kJ/mol! is 55% higher than that for adsorption on HOPG ~12.0 kJ/mol!. In combination with molecular mechanics calculations we show that the observed binding energies for both systems can be attributed to van der Waals interactions, i.e., physisorption. The experiments provide no evidence for a more strongly bound chemisorbed species or for dissociative oxygen adsorption.