Chemoresistive CO 2 Gas Sensors Based On La 2 O 2 CO 3 : Sensing Mechanism Insights Provided by Operando Characterization.

Our previous studies demonstrated that rare-earth oxycarbonates Ln ₂ O ₂ CO ₃ (Ln = La, Nd, and Sm) and rare-earth oxides Ln ₂ O ₃ (Ln = Nd, Sm, Gd, Dy, Er, and Yb) are sensitive to CO ₂ and that hexagonal La ₂ O ₂ CO ₃ is the best among them in terms of sensitivity, stability, and selectivity. In this study, we have conducted a comprehensive operando characterization on a hexagonal La ₂ O ₂ CO ₃ based sensor for the basic understanding of the sensing mechanism. This was done by performing under actual operating conditions simultaneous DC resistance and work function changes measurements, AC impedance spectroscopy measurements, and simultaneous DC resistance and DRIFT spectroscopy measurements. The results demonstrate that the double Schottky barriers at grain-grain boundaries are dominant contribution to sensor resistance; there is a competitive adsorption between carbonate species and hydroxyl groups, which depends on both CO ₂ concentration and humidity and leads to the change in height of the Schottky barriers. Finally, we propose a reaction model stating that there are three types of adsorbates, -CO ₃ ^2- , -OH ^- , and -O ^2- , and the relative concentration of which is controlled by a reaction with ambient humidity and CO ₂ . This model is able to consistently explain all our experimental findings.