Characterization of metal oxide gas sensors via optical techniques

Abstract

Metal oxide (MOX) sensors are increasingly gaining attention in analytical applications. Their fundamental operation principle is based on conversion reactions of selected molecular species at their semiconducting surface. However, the exact turnover of analyte gas in relation to the concentration has not been investigated in detail to date. In the present study, two optical sensing techniques-luminescence quenching for molecular oxygen and infrared spectroscopy for carbon dioxide and methane-have been coupled for characterizing the behavior of an example semiconducting MOX methane gas sensor integrated into a recently developed low-volume gas cell. Thereby, oxygen consumption during MOX operation as well as the generation of carbon dioxide from the methane conversion reaction could be quantitatively monitored. The latter was analyzed via a direct mid-infrared gas sensor system based on substrate-integrated hollow waveguide (iHWG) technology combined with a portable Fourier transform infrared spectrometer, which has been able to not only detect the amount of generated carbon dioxide but also the consumption of methane during MOX operation. Hence, a method based entirely on direct optical detection schemes was developed for characterizing the actual signal generating processes-here for the detection of methane-via MOX sensing devices via near real-time online analysis. Graphical Abstract.

Keywords: Carbon dioxide; Fluorescence sensor; Gas sensors; Infrared sensors; MOX; Metal oxide sensor; Methane; Oxygen; Substrate-integrated hollow waveguide; iHWG.

Graphical Abstract

Fig. 1

Schematic illustration of the developed 2-step approach for analyzing gaseous species during MOX operation based on direct optical detection via iHWG-coupled IR spectroscopy and luminescence-based sensors

Fig. 2

Cross-sectional view of a the MOX gas cell with gas channel (iv) and MOX sensor (v), and b an open iHWG with top substrate (vi), base substrate (vii) including the gas/light propagation channel, and BaF₂ windows (viii)

Fig. 3

MOX sensor signal from sample injection to purging with N₂ (a). Calibration function of the MOX sensor revealing the typical nonlinear behavior when plotting the mean sensor response R_t,m vs. methane concentration (b)

Fig. 4

Oxygen concentrations of methane samples (i.e., sample concentration) during the reaction at the MOX sensor surface are illustrated. Values of sensor Gas IN: “Start,” “End” and Gas OUT: “Purging”

Fig. 5

Summary of 4000 ppm CH₄ IR measurements. “Only-IR” (blue): direct injection in iHWG. “Diluted-IR” (orange): sample in the setup with deactivated MOX, therefore only dilution and no conversion contribution. “Trapped-IR”: sample trapped for 10 min for reaction at MOX sensor surface (gray)

Fig. 6.

Absolute changes during a typical stopped-flow experiment