Abstract:
The development of sensitive, selective, stable and suitable gas sensors for
monitoring toxic gases in mineshafts and the environment in general, requires the
basic understanding of the active sensing material. This dissertation focuses on the
conductometric-type sensor which uses semiconducting metal oxides as the active
sensing material. In particular, the focus was on tungsten trioxide semiconducting
metal oxide, which amongst its “smart-material” capabilities, is known to exhibit
several temperature-dependent phases viz. triclinic, monoclinic, orthorhombic and
tetragonal WO3. The phases influence the physical properties, which in turn
influence the sensing capabilities towards certain toxic gases. To better understand
the phase transitions and structural changes in this compound, these properties
were investigated in light of the Landau-Lifshitz-Lyubarskii theory for second-order
phase transitions. These structural changes are observed experimentally by Raman
spectroscopy, and so the Raman-active modes for the corresponding Raman
spectra of each phase are derived here. Reactive-sputtering with subsequent
annealing was used here to synthesize triclinic-phase WO3 film, and the film was
characterized structurally, optically and electrically. The film was used to show the
sensing of parts per million concentrations of nitrogen dioxide and ammonia at 30oC
and 100oC, as well as the sensing of these gases simultaneously to show that pure
triclinic WO3 has better selectivity to NO2. To speed up the recovery time of the film
after exposure to the target gas, light was shone onto the film to remove any residual
species on the surface. This dissertation is one of the first steps into an attempt to
investigate the phases of WO3 for the application of gas sensing.
