ITO thin films coated quartz crystal microbalance as gas sensor for NO detection

A quartz resonator with a thin indium tin oxide (ITO) deposited film was investigated as a gas sensor for detecting the presence of nitric oxide (NO) in air. The sensor consists of ITO thin films, with thickness of ∼100 nm, as the receptor to sense the toxic gas NO and quartz crystal, with frequency of ∼10 MHz, as the transducer to convert the surface adsorption/reaction into the frequency shift. Both the frequency shift and the motional resistance were measured. The experimental results had indicated that the ITO-coated crystals had the approximately linearly negative frequency shifts when the sensors were exposed to the NO-doped atmosphere. Considerable variations of the resonance frequency were observed depending on both the NO concentrations and the processing time as well as the ITO material properties. These variations were non-reversible. The electrical impedance analysis in the gas sensing process revealed that with the frequency shift, a small motional resistance variation, less than 1%, was observed, which implies the small energy dissipation and the good stability. The frequency response is also dependent on the ITO material properties that can be varied by adjusting the ITO sputtering conditions. At last, the possible sensing mechanism is also discussed.