Experimental evidence of a two-step reversible absorption/desorption process in ruthenium phtalocyanine gas sensing films by in situ energy dispersive x-ray reflectometry

An in situ energy dispersive x-ray reflectivity technique was used to study the morphological changes of gas sensing thin films of ruthenium phtalocyanine (RuPc)(2) induced by gas absorption/desorption processes. The time-resolved collection of reflectivity spectra during the exposure of each film to a gas flux of nitrogen oxides provided the evolution of the morphological parameters (thickness and roughness). The gas absorption process develops in two stages: The first induces morphological changes characteristic of a surface (adsorption) process, while the second is dominated by a bulk effect. This two-step behavior is also observed in the desorption process: When the thermal treatment is performed at 130 degrees C, the gas is released from the bulk only. Conversely, at higher temperatures, the gas is fully released, i.e., also from the surface, and the initial film thickness is regained. Finally, a further in situ study upon a second absorption treatment was carried out: In this case, only the film bulk diffusion process takes place.