Evaporated porphyrin films as nitrogen dioxide gas sensors

In this work, metal-free and copper(II) 5,10,15,20-tetrakis(-4-(methyl 2-phenoxyacetate)) porphyrin films were employed as nitrogen dioxide gas sensors. The films were vacuum evaporated and the sensor response was evaluated as changes in the optical absorption spectra, hydrophobic properties and conductivity at different gas concentrations. The morphology of the films obtained with scanning electron and atomic force microscopy before and after gas exposure indicated different interaction mechanisms. For instance, metal-free film exhibited an absorption phenomenon due to gas diffusion towards the bulk film because of the presence of pores at the film interface, while the copper-porphyrin film showed interfacial film degradation. From UV–vis results, differences in the gas content within the film were detected due to the formation of new peaks around 680 nm in agreement with morphological findings. Contact angle results exhibited less hydrophobic films after gas adsorption as a result of changes in the surface roughness. In addition, the conductivity values were not only dependent on the gas concentration but also on the molecular structure. For instance, gas absorption within the metal-free films truncated electron mobility paths decreasing the conductivity response in comparison with the copper system. Finally, the molecular packing arrangement and gas absorption were also investigated by using density functional theory, which provided some insights into the molecular-gas interactions and supported experimental results.