Structural and Optical Properties of WO3 and its Gas Sensing Application

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.