Effect of carbon nanotubes’ crystal structure on adsorption kinetics of small molecules

Abstract

The effect of carbon nanotubes’ (CNT) crystal structure on chemical reactivity has been studied in much detail in the liquid phase using CNT suspension. This type of information is pertinent for developing CNT separation strategies. However, few experimental studies are available providing data for gas–CNT interactions utilizing ultra-high vacuum (UHV) surface science techniques. Structure–activity relationships (SAR) for gas–surface interactions are important for sensor designs and heterogeneous catalysis exploring, for example, CNT’s potential as a support for fuel cell catalysts. We report on UHV kinetics experiments with single-wall metallic, semiconducting, and mixed CNTs in order to provide the experimental basis to correlate CNT’s crystal structure and chemical activity. Thermal desorption spectroscopy (TDS), a simple temperature ramping technique, has been used to determine the binding energies of a number of probe molecules including alkanes, alcohols, thiophene, benzene, and water on CNTs at UHV conditions. TDS allows for the identification of adsorption sites of probe molecules in CNT bundles, using gold foil or silica as a support for the drop-and-dry technique. A weak and probe molecule dependent SAR is present for adsorption inside the CNTs but not for the population of external sites by the probe molecules. The experimental data are in part consistent with current theoretical predictions by other groups. In addition, the effect of different solvents (methanol, SDS, and NMP) and cleaning procedures will briefly be discussed using results of spectroscopic (Auger electron spectroscopy) and kinetic techniques. Furthermore, molecular beam scattering techniques were utilized to characterize the adsorption dynamics, i.e., the gas-to-surface energy transfer processes of alkanes on CNTs. For example, opening the CNT tube ends by high temperature annealing, increases the so-called initial adsorption probability, that is, the probability for adsorption in the limit of zero surface concentration (coverage). This result directly illustrates the effect of large surface areas of CNTs, using internal and external surfaces, for gas adsorption.