Enhanced vacuum sensing performance of multiwalled carbon nanotubes: role of defects and carboxyl functionalization

Defect controlled multiwalled carbon nanotubes (MWCNTs) were synthesized using a new modified one step pyrolysis method using ferrocene and xylene as effective precursors. Growth was performed at different pyrolysis temperatures (770–970 °C). Morphological and structural characterization shows the formation of nanotubes with diameter and length of about 10–80 nm and 0.2 to 2.0 μm, respectively. MWCNTs grown at 870 °C show a higher degree of graphitization (76%) with a lower defect density compared to other growth temperatures. Thermogravimetric and Fourier transform infrared spectroscopy results revealed the formation of nanotubes with good structural quality and a high quantity of carboxyl functional groups. The drop casted MWCNTs network based sensors were fabricated and studied for their vacuum sensing properties. MWCNTs grown at 870 °C show a vacuum sensitivity of 26%. MWCNT vacuum sensors showed excellent reversibility with fast response and recovery time (20 s) at room temperature. Our observations show that stopping the carrier flow in the MWCNTs using atmospheric oxygen adsorption at the carboxyl functional sites and subsequent desorption of oxygen during the vacuum decreases the sensor resistance. Investigations showed the presence of surface active carboxylic sites and that the nature of the (ordered and defective) stacking of graphite layers in the nanotubes determined the vacuum sensing characteristics.