Surface and bulk studies of iron phthalocyanine based gas sensors.

Phthalocyanines films have been noted for their use as chemical sensors by measuring changes in conductivity when a gas is adsorbed at the surface. The sensing mechanism is not well understood. This work uses iron phthalocyanine as a model in an attempt to understand the gas sensing mechanism of metal phthalocyanines.The alpha and beta polymorphs of iron phthalocyanine (FePc) have been synthesised and then studied using infra-red spectroscopy, scanning electron microscopy (SEM), gas sensing experiments and Mossbauer spectroscopy.Infra red spectroscopy indicates that a phase change occurs at 205°C and SEM work confirms this since the microcrystallite size changes at this temperature.Gas sensing work has shown that FePc responds to NO[2] and Cl[2] at room temperature. Variable temperature transmission mode Mossbauer spectroscopy of both phases showed that the alpha phase has a lower recoil free fraction at room temperature than the beta phase suggesting that the beta phase has a more rigid structure i.e. it is less free to vibrate in the lattice than the alpha phase. This also explains the difficulties experienced in attempts to collect backscatter spectra from alpha FePc. Backscatter Mossbauer spectroscopy has been used in vitro to show a difference at ambient temperature and pressure, of beta FePc before and after exposure to a gas. Conversion electron Mossbauer spectroscopy indicates that NO[2] adsorbs on to the surface of the FePc film preventing the escape of conversion electrons, and conversion x-ray experiments have shown that the NO[2] penetrates the near surface causing a chemical change.