Adsorption induced phase transition in Ti(Pc)₂ gas sensing films: A joint AFM-EDXR study

The morphological properties of thin metal phthalocyanine films, used as active material in gas sensing devices, are studied. Morphological changes, such as bulk or surface phase transitions, are key phenomena, having a determinant influence on the final properties and stability of the materials and devices. In this work, we successfully prove that joint in-situ atomic force microscopy and Energy Dispersive X-ray Reflectivity can be used, not only to monitor the sensing activity of bi[phthalocyaninato(2-)]titanium(IV) films, but also as a powerful tool to disclose the physical-chemical process beneath the sensing activity. Indeed it is shown that NO_x adsorption activates a phase transition in the TiPc₂ film, characterized by an oscillating behavior between two metastable states. Such phenomena endure as long as the gas molecules are present, finally evolving into the thermodynamically most stable phase. Fourier transform infrared spectroscopy analysis supports the above conclusions. - Highlights: • The NO_x gas/TiPc₂ sensing film interaction is investigated in real time. • Simultaneous EDXR/AFM techniques disclose a phase transition in the sensing material. • An oscillating α to β₁ phase transition is induced by the presence of NO_x molecules. • When the sensing process is concluded, the material turns to the most stable phase