Humidity sensor utilizing multiwalled carbon nanotubes coated quartz crystal microbalance

Carbon nanotube (CNT) is a promising material for humidity sensing due to its high specific surface area and large water absorption. In this work, multiwall CNTs are coated on quartz crystal microbalance (QCM) for humidity sensing at room temperature. The MWCNTs were then synthesized by chemical vapor deposition (CVD) with iron catalyst powders. The deposition is conducted under hydrogen, acetylene and argon gases at atmospheric pressure and deposition temperature of 700degC. The obtained CNTs were ground to obtain small CNT fragments and purified by etching with nitric acid. The purified MWCNTs were then separated by sonication in methanol. Finally, the CNTs are coated on QCM by drop coating of CNTs in 5% polyethyleneimine-methanol solution. From scanning electron microscope (SEM) characterization, small clusters of MWCNTs were found to uniformly scatter over QCM electrode. The size and density of CNT clusters on QCM can be varied by adjusting the CNT concentration. With optimum CNT concentration, the humidity sensitivity of CNT coated QCM sensor is increased by more than two orders of magnitude compared to uncoated QCM sensor. In addition, the CNT based QCM sensor has fast humidity detection with short response and recovery times. Thus, the proposed CNTs deposition on the QCM electrode is an effective way to improve humidity-sensing characteristic of QCM sensor.