Investigation of the reaction mechanisms in work function type sensors at room temperature by studies of the cross-sensitivity to oxygen and water: the carbonate–carbon dioxide system

Thin films and screen-printed films of BaCO3 react to changes of CO2 in wet synthetic air at room temperature with a change of the work function as measured by the Kelvin method. The reaction is due to the formation of hydrogen carbonate ions on the surface. In this work, the reaction mechanism is investigated by studying the cross-sensitivities to oxygen and humidity in the temperature range between 25°C and 250°C. At low temperatures between 25°C and 100°C, the reaction involves water whereas at temperatures above 200°C, the reaction is driven by the presence of oxygen. In both cases, the reaction can be described in terms of a Nernst equation. In the low-temperature region, however, the (thickness-independent) potential type of reaction is superimposed by a (thickness-dependent) capacitance type of reaction to humidity like in the well-known capacitive humidity sensors.