High-Performance QCM Humidity Sensors Using Carbon Nanostructure Material as Sensing Film
Authors: Wang Rui , Qi Pengjia , Zhang Tong 
Published: May 2020

Introduction

Nowadays, sensors have become increasingly indispensable. It is particularly worth mentioning that the humidity has gradually become the standard for measuring environmental state and physical health. From the perspective of manufcturing, the performance and technical index of humidity sensors can vary greatly on materials, structures and processes. For instance, multiple sensing technologies including acoustic [1], resistance [2], optical [3], impedance [4],capacitance [5], thermal [6], and resonance [7], etc. have been explored to monitor the humidity. quartz crystal microbalance (QCM) has been proven to be an excellent mass sensing platform for humidity detection according to Sauerbrey’s equation [8].
Sensitive material is the key of a QCM sensor. Among various materials, a variety of sensitive materials has been deposited on the electrode surface of QCM for detecting moisture, such as polymer, metal oxides, carbonic materials and so on. In this study, a series of high performance QCM humidity sensors were developed which using multi-walled CNTs (MWCNTs), Graphene quantum dots(GQDs), Chitosan(CS) and their composites as sensing film. The humidity sensing characteristics, such as dynamical response, reproducibility, and hysteresis characteristics were measured and discussed. Furthermore, the adsorption dynamics of water vapor molecules mechanism was also analyzed.

Method

Characterization

Raman spectra were obtained on J-YT64000 Raman spectrometer with 633 nm wavelength incident laser light. The X-ray photoelectron spectroscopy (XPS) analysis was measured on an ESCALAB MK II Xray photoelectron spectrometer using Mg as exciting source. The Fourier transform infrared spectroscopy (FT-IR) spectra of as-prepared were obtained on a WQF-510AFTIR spectrometer, using KBr pellets as the reference. The transmission electron microscopy (TEM) images were taken by a JEM-2100F electron microscope (JEOL, Japan).

Fabrication of QCM humidity sensor

The QCM substrates used in this work were purchased from Beijing Chenjing Electronic Co., Ltd. China. The QCM was consisted of AT-cut quartz crystal (8 mm diameter) with a fundamental frequency of 10 MHz and silver electrodes (5 mm diameter) were covered on both sides, the electrode leaded two pins for testing purposes. As a typical procedure, the QCM substrates were ultrasonic cleaned by acetone, ethanol and deionized water for 10 min successively, and then dried in an air-tight container which filled with inert nitrogen gas for the whole night at room temperature. The QCM humidity sensor was obtained via dropping the as-prepared composites dispersion by micropipette on the Ag-electrodes surface of the QCM. Then, the modified QCM sensor was put into the foregoing container for at least 12 h.

Results and Conclusions

The humidity sensing characteristics of prepared QCM sensors were quantified by transforming the RH level ambient and measuring the shift of the vibration frequency. The humidity hysteresis performance, reproducibility and reversibility were also examined respectively. After further study, a plausible mechanism analysis for the above excellent sensitivity was given. In conclusion, an excellent QCM humidity sensor was obtained to detect full RH range. The oscillating circuit and impedance analysis methods have been adopted to examine the dynamic response and recovery behaviors and static humidity sensing performances, respectively. In addition, the intrinsic hydrophilicity is able to provide more water adsorption sites. The optimized QCM sensor demonstrated the highest humidity sensitivity and long-term stability, and tiny humidity hysteresis. The prepared QCM sensor is reliable for monitoring water vapor content in wide humidity range.

References

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