Neodymium Dioxide Carbonate as a Sensing Layer for Chemoresistive CO₂ Sensing

Abstract

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.

Structure-Dependent CO2 Gas Sensitivity of La2O2CO3 Thin Films

“…It can be seen that the relative gas response to 1500 ppm of CO 2 is about 2.2; that is, the measured current decreased from 2. [2][3][4][5]. It is especially notable in the case of recovery times, which have been found to be over 30 min in [2][3][4][5].…”

“…The measurements were made using the electrodes with 48 m gap width and 0.6 mm gap length. The current through the sample was monitored at the applied constant voltage of 3 V. Previously, it is reported that the sensor structures based on La and Nd oxycarbonate nanorods have high electrical resistivity and that restrict their use in commercial sensors [2][3][4]. In our sensor structures, the resistivity followed the Arrhenius law with an activation energy of 0.56 eV between 250 and 450 ∘ C. It shows that the conductivity is determined by deep trap levels, as the bandgap of the material, as concluded from X-ray absorption spectroscopy and X-ray emission spectroscopy studies by Hirsch et al [6], is approximately 3.7 eV in the case of La 2 O 2 CO 3 nanoparticles.…”

“…Adsorbing CO 2 acts as an electron acceptor, thus reducing the concentration of free electrons and causing an increase of resistivity of the sensor material. Also, recent studies indicate that surface hydroxyl groups play important role in CO 2 adsorption at La 2 O 2 CO 3 , but exact mechanism and the role of OH groups are not known [4,6].…”

“…Current industrially available CO 2 sensors are relatively complex and expensive, so it is desirable to develop simple and low-cost methods for monitoring CO 2 level [1]. La and Nd oxycarbonates have recently been studied as promising candidates for chemiresistive CO 2 sensor materials [2][3][4][5][6]. However, in these reports, La and Nd oxycarbonates have been studied in a form of nanorods and there are no investigations of thin film deposition of these promising materials for CO 2 gas sensor application.…”

“…However, in these reports, La and Nd oxycarbonates have been studied in a form of nanorods and there are no investigations of thin film deposition of these promising materials for CO 2 gas sensor application. In [2][3][4][5], La and Nd oxycarbonate materials were made using respective precursor hydroxides in a form of nanoparticles prepared in a sol-gel process. Conversion of La and Nd hydroxide nanorods to respective oxycarbonates was carried out by heating the materials in the atmosphere containing CO 2 gas.…”