Surface characterization of oxygen-functionalized multi-walled carbon nanotubes by high-resolution X-ray photoelectron spectroscopy and temperature-programmed desorption

Multi-walled carbon nanotubes were exposed either to nitric acid or to an oxygen plasma to synthesize oxygen-containing functional groups which were characterized by high-resolution X-ray photoelectron spectroscopy (XPS). The C 1s spectra revealed that the treatment with nitric acid mainly resulted in the formation of carboxylic (COOR) and phenolic (COR) groups, whereas the plasma treatment led to a higher amount of carbonyl (CO) groups. Furthermore, the nitric acid treatment yielded a 60% higher surface oxygen concentration compared to the plasma treatment, and created a minor amount of nitrogen-containing functional groups. Thus, the nitric acid treatment was found to be more effective in creating acidic functional groups. The presence and the thermal stability of these groups was also investigated by temperature-programmed desorption (TPD). The release of carbon dioxide was detected at about 350 and 450 °C, indicating the decomposition of COOR groups. The CO groups were more stable decomposing even above 600 °C. In addition, ammonia was adsorbed as probe molecule followed by TPD to derive the amount and the acidity of the carboxylic and phenolic groups.