Neodymium Dioxide Carbonate as a Sensing Layer for Chemoresistive CO₂ Sensing

We report the synthesis of neodymium hydroxide nanoparticles via a nonaqueous and surfactant-free sol−gel process and their subsequent thermal transformation into neodymium dioxide carbonate, which can be applied as a sensing layer for resistive-readout CO2 sensing. The sensors show an increase in resistance when exposed to CO2 in both dry and humid air in the operation temperature range of 250−400 °C, with a maximum sensor signal of 4 in humid air at 350 °C in 1000 ppm CO2. Another important feature of the sensor is the fact that exposure to water vapor leads to a pronounced decrease in resistance (opposite of the CO2 effect), which indicates different charge-transfer mechanisms. The CO2 gas-sensing mechanism was studied via the Operando approach, by performing direct-current (DC) resistance and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements simultaneously under operation conditions. This combination enables the correlation of any concentration changes of specific surface species with electrical effects. The correlation found between the concentration of surface-adsorbed OH and carbonate species and the electrical conductivity suggests that the reaction between CO2 and water-related surface species is responsible for the gas-sensing effect.