CO2 sensing with gas sensors based on rare-earth compounds: Material exploration

•Ln2O2CO3 (Ln = La, Nd, Sm) and Ln2O3 (Ln = Sm, Gd, Dy, Er, Yb) are sensitive to CO2.•Hexagonal La2O2CO3 exhibits the best performance for CO2 sensors among them.•Enough sensitivity, stability, humidity effect, and selectivity for practical use.•Operando XRD and DRIFTS for basic understanding of the CO2 sensing. Rare-earth oxycarbonates Ln2O2CO3 (Ln = rare-earth element) have been identified as materials for chemoresistive CO2 gas sensors. Among them, previous studies identified monoclinic La2O2CO3 as the best performing one. However, not all rare-earth elements have been investigated and, moreover, La2O2CO3 monoclinic phase is metastable and this can influence the long term performance. In this work, we have synthesized rare-earth oxycarbonates Ln2O2CO3 (Ln = La, Nd, and Sm) including monoclinic and hexagonal La2O2CO3, rare-earth oxides Ln2O3 (Ln = Nd, Sm, Gd, Dy, Er, and Yb) and LnO2 (Ln = Ce) by calcination of oxalate hydrate or the acetate hydrate precursors in air. All the materials, except for CeO2 and Nd2O3, were sensitive to CO2. All CO2 sensitive materials, except for monoclinic La2O2CO3 and Nd2O2CO3, were stable and their performance is sufficient for practical use. Hexagonal La2O2CO3 shows the best overall performance. The results of operando investigations indicate that the origin of CO2 sensing is the competitive adsorption between carbonates and hydroxyl groups.