Thermal Stability and Reducibility of Oxygen-Containing Functional Groups on Multiwalled Carbon Nanotube Surfaces: A Quantitative High-Resolution XPS and TPD/TPR Study

The thermal stability and the reducibility of oxygen-containing functional groups on the surface of nitric acid-treated multiwalled carbon nanotubes (CNTs) have been studied using temperature-programmed desorption and reduction (TPD and TPR) and high-resolution X-ray photoelectron spectroscopy (XPS). The thermal treatments up to 720 °C were carried out in the XPS setup, either under ultrahigh vacuum (UHV) or in diluted hydrogen. Deconvoluted XP spectra were used for the quantitative determination of the amount of the different functional groups on the CNT surfaces as a function of the pretreatment. The number of the oxygen atoms per unit surface area was obtained from the oxygen to carbon (O/C) ratio derived from the corresponding peak areas in the XP spectra. The results obtained by XPS agree quantitatively with the observations by TPD and TPR. The acid treatment not only introduced carboxyl, carbonyl, and phenol groups on the surface but also generated ether-type oxygen groups between the graphitic layers as indicated by the oxygen balance. Generally, the presence of hydrogen decreased the thermal stability of the oxygen-containing functional groups. Both XPS and TPR provided evidence for the reduction of carboxylic groups to phenolic groups at 300 °C in hydrogen. Heating in hydrogen was found to be more effective in removing the oxygen-containing functional groups compared to heating in UHV but did not allow either to remove all oxygen species even at 720 °C.