Gases adsorption on single-walled carbon nanotubes measured by piezoelectric quartz crystal microbalance

A piezoelectric quartz crystal microbalance (PQCM) was used to demonstrate an easy method for examining the gas adsorption behavior of O2, N2 and Ar on single-walled carbon nanotubes (SWCNTs). The coated PQCM was set in a vacuum chamber, and its temperature was controlled by a heater. Before each test, the nanotube bundles were exposed to a high vacuum for at least 2 h, to ensure that the gases were completely desorbed. The temperature of the chamber was set to 300, 323 or 343 K while the vacuum chamber was filled with a gas to increase the pressure stepwise. At the beginning of each step of the increase in pressure, the frequency of the PQCM gradually decreased and finally reached a stable value. The change in frequency is proportional to the amount of adsorption on the PQCM. According to Sauerbrey theory, the changes in frequency are expressed as adsorption isotherms. These isotherms can be used to determine experimentally the isosteric heat of adsorption (qst) of O2, N2 and Ar on the SWCNT bundles. The binding energies (ε) of these gases were also calculated from the average values of qst on the SWCNT bundles. These results indicate that these values of ε on the SWCNTs samples exceed those obtained for the same adsorbents on planar graphite, by approximately 41–52%. The adsorption isotherms of O2, N2 and Ar at 77 K were also investigated by the Brunauer–Emmett–Teller (BET) method to verify these results.