Adsorption–desorption noise in gas sensors: Modelling using Langmuir and Wolkenstein models for adsorption

A comparison between adsorption–desorption (A–D) noise models obtained using Langmuir and Wolkenstein theories is presented. This noise is generated by instantaneous fluctuations in the number of the adsorbed molecules, which cause free electron's density fluctuations in the sensing layer, and consequently a fluctuation in the conductance of the gas sensor. Both isotherms lead to a Lorentzian contribution of adsorption–desorption noise with the same cut-off frequency. The noise spectrum obtained using Wolkentein adsorption theory has a low frequency magnitude which is greater than that of the spectrum obtained using Langmuir theory. With both models the power density spectrum (PDS) of the fluctuation of the sensor's conductance has a cut-off frequency and low frequency magnitude which are specifics of the adsorbed gas. This result confirms that noise spectroscopy could be a useful tool for extracting information on the nature of the detected gas.