Barrande

The application possibilities of gas sensors such as nitrogen dioxide (NO2) based on organic phthalocyanine layers suffer from a long regeneration time after exposure to gas. This imposes the need to operate the sensors at high temperatures (150-200°C), which shortens their lifetime and increases their consumption. In this project, we are interested in a new method for fast and efficient regeneration of such molecular chemistors by UV-visible illumination at low temperature (55°C).

To this end, we are studying the phenomena of adsorption and desorption of NO2 under illumination by phthalocyanine films and neighboring molecules such as protoporphyrins (macrocycles). At Toulon, we carry out these experiments on thin layers (2 to 20 nm) of phthalocyanines on substrates, using analysis techniques in which we have expertise: local probe microscopy (atomic force AFM and tunneling STM) and UV-Vis, infrared and Raman spectroscopy in SERS mode (exaltation by a nanostructured gold surface). We are also contributing our many years of expertise in a complementary approach to the evaporated thick films of our Czech partners: self-assembled molecular monolayers (SAMs), which have the advantage of providing organized molecular systems enabling us to study the effect of macrocycle organization and orientation on the effects of gas adsorption/desorption (Fig. 1).

AFM allowed us to image the phthalocyanine layers. Zinc phthalocyanine films deposited by vacuum evaporation in Toulon on different substrates (SiO2, glass, Au(111) on mica, and graphite) were exposed to NO2 gas and then studied by AFM and STM to analyze their morphology and electronic properties at the local scale, by UV-Vis absorption spectroscopy, and by Raman spectroscopy in SERS mode. After the first set of characterizations, we exposed the layers to various concentrations of NO2 and remeasured their properties. This procedure was performed to observe the changes related to the phthalocyanine-NO2 interaction. Then we exposed the layers to controlled light intensities and repeated the same series of characterizations to study the changes taking place during the photoregeneration mechanism. Concerning the STM measurements, we developed in Toulon an experiment of illumination of layers on the sample positioned under the microscope tip in order to measure possible modifications of conductance and morphology at the local scale which would be related to the adsorption/desorption of gas. Similarly, SERS was used to detect possible changes in the spectra on prepared phthalocyanine films. Some of these films were prepared in Prague and brought by our partners when they came to Toulon in 2018 - P. Fitl, J. Vlcek, and PhD student D. Tomecek - to perform AFM/STM and Raman SERS analyses. The results are currently being analyzed.

In order to better understand the role of the orientation of molecules within the film on the efficiency of gas adsorption and desorption, we have deposited SAMs of different molecules: phthalocyanines and protoporphyrins on substrates featuring a quartz microbalance available to our Czech partners and carried out series of experiments on site in 2018 and 2019. within the UCT and the Institute of Physics :

• spectroscopic ellipsometry in order to obtain the optical properties of macrocycle SAMs and to be able to detect a change related to gas adsorption/desorption. Unfortunately the signal on these films was too weak.
• measurement with quartz microbalances
• impedance measurement

In parallel, on thicker films prepared by our partners in Prague, the effects of light on the rate of NO2 desorption during the regeneration period were studied in the wavelength and intensity ranges of 375-850 nm and 0.2-0.8 mW/mm2, respectively (Fig. 2). This study was performed for a set of phthalocyanine film-based sensors (ZnPc, CuPc, H2Pc, PbPc, and FePc) operating at slightly elevated temperatures (55-100 °C), and was corroborated by UV-vis and infrared spectroscopic analyses. It was shown that a wavelength below 550 nm significantly accelerates NO2 desorption from ZnPc, CuPc, and FePc films, and reduces the measurement period to less than 2 minutes and decreases the power consumption of the sensor by 75%. Possible mechanisms of the light-stimulated desorption are under investigation.