Room temperature operating formaldehyde sensor based on n-type ZnO functionalized surface acoustic wave resonator

Abstract

In this investigation, Surface Acoustic Wave (SAW) formalin gas sensor is explored for low-level formalin gas sensing applications. The formalin gas sensor consists of ST-cut quartz as a base substrate material, interdigitated transducer (IDT) electrodes, reflectors and gas sensing n-ZnO thin film coated on top of the SAW resonator. Nano-structured optimal n-ZnO thin film was deposited as a gas sensing layer. It is noted that the sensor response was not affected by relative humidity and found stable. Formalin gas concentration was established in the controlled process chamber environment. The sensor response was measured at room temperature (25 °C) and relative humidity was 50 % during the experiment. Experimental data shows that return loss peak was shifted towards down side, return loss was better when formalin content (0–8 ppm) was increased in the process chamber. In order to understand, the underlying electrical sensing mechanism, modified Butterworth-Van Dyke (mBVD) equivalent circuit model was developed using Advanced Design System (ADS) software. This model provides the insights about change in electrical behaviour of sensor when it is exposed to relative gas content. The fitted theoretical model follows the experimental data, which indicate the goodness of the mBVD model and curve fitting. The highest frequency down-shift was observed 10.5 kHz due to formalin gas content of 8.0 ppm. Motional arm is mainly influenced by the formalin gas exposure. Motional arm resistance (Rm) and inductance (Lm) was also influenced by the formalin gas content and found to increase by the formalin gas content and mainly accountable for gas sensing behaviour. It is also noted that static arm is not influenced by gas content and only motional arm is accountable for gas sensing behaviour. Highest sensitivity (S) was 1.3125 kHz/ppm and limit of detection (LOD) was 80 ppb. This new kind of formalin sensor is low cost, reliable, robust sensor and highly useful for room temperature operating formalin gas sensor applications.