Carbon capture, storage and utilization have become familiar terms when discussing climate change mitigation actions. Such endeavors demand the availability of smart and inexpensive devices for CO₂ monitoring. To date, CO₂ detection relies on optical properties and there is a lack of devices based on solid-state devices, which can be miniaturized and easily made compatible with IoT platforms. With this purpose, we present an innovative semiconductor as functional material for CO₂ detection. Nanostructured In₂O₃ films, functionalized by Na, proves to enhance the surface reactivity of pristine oxide so much that even rather stable a molecule as CO₂ is reduced. An advanced operando equipment based on surface-sensitive diffuse infrared Fourier transform is used to investigate its improved surface reactivity. The role of sodium is to increase the concentration of active sites such as oxygen vacancies and, in turn, to strengthen CO₂ adsorption and reaction at the surface. It results in a change in film conductivity, i.e., in transduction of a concentration of CO₂. The films exhibit excellent sensitivity and selectivity to CO₂ over an extra-wide range of concentration (250-5000 ppm), which covers most indoor and outdoor applications with marginal influence by environmental humidity.