A study on the Langmuir adsorption for quartz crystal resonator based low pressure CO2 gas sensor

Quartz Crystal Resonator (QCR) has been proved to be effective for CO2 gas sensing, however, the mechanism of this sensing process remains unclarified. This paper proposes an innovative modeling for a QCR-based CO2 gas sensor. The proposed modeling is based on the Langmuir Adsorption Theorem and the Sauerbrey equation of QCR. The Langmuir Adsorption Theorem assumes that a monolayer of gas molecules is formed at the maximum gas concentration while each adsorbate molecule is assumed to take only one active site on the adsorbent surface. Therefore, the Sauerbrey equation is modified by combining Langmuir Adsorption Theorem and then the effect of both surface concentration and Langmuir constant are investigated and analyzed. Different from traditional analysis, the proposed modeling works well on not only the linear relationship between CO2 concentration and frequency shift at low CO2 concentration but also the non-linear saturation stage at higher CO2 concentration. Moreover, a lab experiment is carried out to verify the Langmuir-Sauerbrey adsorption modeling. Surface concentration constant and Langmuir constant are retrieved from the experiment and considered as material properties for gas sensing capability. The proposed modeling could also be applied to most QCR-based gas sensor, in which physical adsorption is mainly involved. Additionally, determination of surface concentration constant and Langmuir constant will benefit the understanding of many other gas sensing process.