Quartz crystal microbalance coated with sol–gel-derived indium–tin oxide thin films as gas sensor for NO detection

This paper presents the possibilities and properties of indium–tin oxide (ITO)-covered quartz crystal microbalances (QCM) as a NOx toxic gas sensors. The starting sol–gel solution was prepared by mixing indium chloride dissolved in acetylacetone and tin chloride dissolved in ethanol (0–20% by weight). The ITO thin films were deposited on the gold electrodes of quartz crystals by spin-coating and annealing and subsequently followed by a standard photolithography to pattern the derived films to ensure all sensors had the same sensing areas. All heat treatment was controlled below 500 °C in order to avoid the degradation of the piezoelectric characteristics of quartz crystal (Quartz will lose its piezoelectricity at ∼573 °C due to the phase change from α to β). The electrical and structural properties of ITO thin films were characterized with Hall analysis system, TG/DTA, XRD, XPS, SEM, etc. The gas sensor featured ITO thin films of ∼100 nm as the receptor to sense the toxic gas NO and a quartz crystal with frequency of 10 MHz as the transducer to transfer the surface reactions (mass loading, etc.) into a frequency shift. A homemade setup had been employed to measure the sensor response under the static mode. The experimental results indicated that the ITO-coated QCM had good sensitivity for NO gas, ∼12 Hz/100 ppm within 5 min. These results prove that the ITO-covered quartz crystals are usable as a gas sensor and as an analytical device.