LAUSR

Our previous studies demonstrated that rare-earth oxycarbonates Ln2O2CO3 (Ln = La, Nd, and Sm) and rare-earth oxides Ln2O3 (Ln = Nd, Sm, Gd, Dy, Er, and Yb) are sensitive to CO2, and that hexagonal La2O2CO3 is the best among them in terms of the sensitivity, stability, and selectivity. In this study, we have conducted a comprehensive operando characteriza-tion on the hexagonal La2O2CO3 based sensor for the basic understanding of the sensing mechanism. This was done by per-forming, 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 re-sistance; there is a competitive adsorption between carbonate species and hydroxyl groups, which depends on both CO2 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, -CO32-, -OH-, and -O2- , the relative concentration of which is controlled by the reaction with ambient humidity and CO2. This model is able to consistently explain all our experimental findings.