Metallic Nanocrystallites-Coated Piezoelectric Quartz for Applications in Gas Sensing

This article reports the gas-sensing capability of nanocrystalline (NC) transition metal-coated piezoelectric quartz crystal (PQC). NC transition metal particles of Pd and Ag with particles size of 10–15 nm, while Pt and Au are 20–25 nm are used. The NC particles deposited on the quartz substrate adsorbs gaseous pollutants, thereby increasing the weight of the quartz substrate and decreasing its vibration frequency. We have found that transition metals, Pd, Pt, Au, and Ag in particular, show good sensitivity for NH3-detection; the maximum frequency change occurs at 150°C for Pd and Pt and at 100°C for Au and Ag. The NC Pd- and NC Pt-coated PQC also show good sensitivity for CO2-detection at 150°C. Likewise, the NC Au-coated PQC shows very good sensitivity for NO2-detection but at a higher temperature (180°C). The frequency change as a function of the pollutant gas concentration (Δ f-curve) follows the Langmuir adsorption isotherm (unimolecular adsorption) except in the case of NC Pd-coated PQC under NH3 and NC Au-coated PQC under NO2. The Δ f-curve for the NC Pd-coated PQC is convex with respect to the NH3 concentration axis. The Δ f-curve for the NC Au-coated PQC is convex in the low NO2 concentration region, and concave in the high NO2 concentration region. Both curves indicate multi-molecular adsorption, Type III and Type V adsorption, respectively. Therefore, the good sensitivity and stability of these gas sensors can be attributed to physical adsorption of the pollutant gases as a result of van der Waals attraction.