Rare-earth based Chemoresistive CO2 Sensors

Abstract

CO2 sensing is of paramount importance for monitoring the state of the atmosphere, controlling indoor air quality, and cultivating crops in greenhouses or plant factories. Obtaining low cost, simple and good performance chemoresistive CO2 gas sensors has the potential to be a game changer. Rare-earth oxycarbonates Ln2O2CO3 have been proposed as promising chemoresistive materials for CO2 sensors. The already published results indicate monoclinic La2O2CO3 as the most suitable material. On the other hand, there are no reports about the sensing properties of more stable hexagonal La2O2CO3 and the other rare-earth oxycarbonates than La and Nd. In my master study, I have succeeded for the first time in synthesizing monoclinic La2O2CO3 and hexagonal La2O2CO3 separately by heat treatment of La oxalate hydrate and showing that hexagonal La2O2CO3 possesses better properties as a CO2 sensing material. Here, the heat treatment conditions have been optimized for stabilizing the synthesis and sensing properties of La2O2CO3. In order to obtain the other rare-earth oxycarbonates, heat treatments of the rare-earth organic acid salts hydrate were implemented. Rare-earth oxycarbonates Ln2O2CO3 (Ln = La, Nd, and Sm) and rare-earth oxides Ln2O3 (Ln = Nd, Sm, Gd, Dy, Er, and Yb) and LnO2 (Ln = Ce) have been studied. All the materials, except for CeO2 and Nd2O3, were sensitive to CO2. This is a remarkable new finding that rare-earth oxides Ln2O3 (Ln = Sm, Gd, Dy, Er, and Yb), that crystalize in cubic structures, also exhibited a chemoresistive effect for CO2. All the CO2 sensitive materials, except for Nd2O2CO3, showed sufficient performance for practical use in terms of the stability, influence of humidity, selectivity, and the linearity of sensor signal up to 10,000 ppm. Hexagonal La2O2CO3 was the best among them. For basic understanding of the sensing mechanism, operando characterization including AC impedance spectroscopy, work function, X-ray Diffraction (XRD), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) has been conducted mainly on the best performing hexagonal La2O2CO3 based sensor. From the results, it seems reasonable to conclude that the competitive adsorption between carbonates and hydroxyl groups on the surface of rare-earth based CO2 sensitive material is responsible for the sensor effect.