Simulation analysis and experimental verification for sensitivity of IDE-QCM humidity sensors

Abstract

• A facile and effective approach to improve the sensitivity of piezoelectric quartz crystal humidity sensors was presented. • Both IDE and QCM are used as sensitive components to improve humidity sensitivity. • The superposition relationship of the frequency change in IDE-PQC and QCM was proved using equivalent circuit method. • The sensitivity of the IDE-QCM humidity sensor reaches 262.21 Hz/%RH in the humidity range of 11.3–97.3 % RH. This paper proposes a new, simple, and effective approach that uses both an interdigital electrode (IDE) and a quartz crystal microbalance (QCM) as sensitive components to improve humidity sensitivity. First, a simulation analysis of the equivalent electronic circuit was used to verify that the frequency response of the IDE-QCM humidity sensor is the sum of that of the QCM and interdigital electrode-piezoelectric quartz crystal (IDE-PQC) sensors. Then, cellulose nanocrystals (CNCs) were used as humidity-sensitive materials deposited on IDE and QCMs, respectively, to fabricate several PQC sensors. The humidity-sensing performances of all sensors were tested at room temperature, including the dynamic response, sensitivity, response/recovery time, repeatability, and long-term stability. The experimental results prove the superposition relationship and can be used to enhance the humidity sensitivity. The maximum sensitivity of the IDE-QCM sensor reaches 262.21 Hz/%RH in the humidity range of 11.3 %–97.3 %, which is higher than that of all frequency-modulated piezoelectric quartz crystal humidity sensors. Moreover, the humidity hysteresis (∼ 8.1 %RH) and response/recovery time (68/4 s) did not significantly increase compared with the IDE-PQC and QCM sensors. All the sensors exhibited good repeatability and long-term stability. Finally, the humidity-sensing mechanism of the CNC-based IDE-QCM sensor is discussed in detail. This work demonstrates that using both IDE and QCM as sensitive elements is a simple and effective method to improve the humidity sensitivity of frequency-modulated piezoelectric quartz crystal sensors. This superposition relationship of capacitance-inductance frequency modulation to enhance sensitivity also provides an excellent reference for the design of other high-performance sensors.