Organic vapor adsorption on in situ grown carbon nanotube films

Organic vapor adsorption isotherms are measured on in situ grown carbon nanotube (CNT) films using piezoelectric GaPO4 crystal microbalances as mass sensing substrates. The isotherms are Type IV and show adsorption/desorption hysteresis, consistent with a porous material. The measured porosity is 2%, a value surprisingly low given an over 90% void volume in the film estimated from density considerations. At low pressures (p/p0 < 0.25) the isotherm is well fit by the Freundlich model and at intermediate pressures (p/p0 = 0.1–0.4) by the Brunauer, Emmett, Teller (BET) model. Monte Carlo simulations show three consecutive adsorption processes: filling of the intratube micropores at low pressures, monolayer coverage of the CNT external surface at intermediate pressures, and capillary condensation in the intertube mesopores at high pressures. The simulation results validate the use of the BET model for surface area analysis in the experimental system. The average total accessible surface area is found to be 180 ± 100 mm2 and the specific surface area is estimated to be 45 ± 25 m2/g. Further engineering of the CNT film microstructure should lead to much higher surface areas.