Behind the transduction mechanism of a nanostructured functional material for environmental CO 2 monitoring

This work investigated an innovative CO2-sensitive nanostructured semiconductor material through a three-level approach, i.e., materials and electrical characterization, chemisorption with probe molecules analyses, and operando Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy under thermoactivation. By integrating these advanced techniques, a complete scenario of the sensing mechanism can be obtained, paving the way for the development of highly sensitive and selective CO2 sensors. It emerged that doping with alkali metals, such as sodium, has proven to be an effective strategy to improve the performance of CO2 devices. This study presents a sustainable and fast synthesis of three samples with varying sodium content and characterization of each material to explore the role of sodium doping. Therefore, extensive materials characterization confirmed the nanostructured nature of the mesoporous Na-doped In2O3 materials as nanoparticles and the presence of intra-structural sodium. These material surfaces were found to possess different acid and basic sites ratio, which can be rationalized by the presence and the amount of sodium. From electrical characterization, a key feature of Na-doped In2O3 was the marginal influence of humidity, which enables such sensors to a wide range of possible real-world applications. Operando DRIFT spectroscopy clarifies which ones and under what conditions the carbonate species formed upon CO2 adsorption interact with the material as a function of the presence of acidic and basic sites, a crucial factor influencing the mechanism of transduction. This study uniquely captures the balance of active sites to motivate and explain the optimal sensing behaviour.