(A) study on sensing performance of $NO_2$ gas sensor using QCM and $H_2Pc$ LB film

Abstract

A gas sensor system using quartz crystal microbalance(QCM) has been developed for the purpose of detecting low levels of nitrogen dioxide ($NO_2$) in the air. A 9 MHz (QCM) was used as the transducer and a multilayer thin film of octa-(2ethylhexyloxy) dihydrogen phthalocyanine ($H_2Pc$) was coated on it by using the Langmuir-Blodgett (LB) technique. The monolayer behavior of $H_2Pc$ in air-water interface was investigated by Brewster angle microscopy (BAM) and UV spectroscopy, and its LB film was characterized by ellipsometry, scanning electron microscopy (SEM), and X-ray diffraction (XRD) method. The LB film was quite stable at sensor operating temperature of 35℃, but it became unstable over 70℃. A 20-layer $H_2Pc$ film was deposited onto QCM and its sensing capability was examined in a dry nitrogen environment. It produced a significant frequency change even at very low concentration of 35 ppb. Dry nitrogen was replaced by air to make the experimental condition more realistic. It was observed that the presence of oxygen greatly reduced the sensitivity and thus increased the sensing limit to hundreds of ppb. Results of experiments with moisture added showed that the interference of moisture was quite severe making its removal indispensable.