Operating temperature effect in WO3 films for gas sensing

WO₃ film sputtered on Al₂O₃ was used to sense ppm concentrations of NO₂ and NH₃ gas. The optimum working temperature of the amorphous film was found to be 200°C for both the gases, in accordance with literature. However, thermo-gravimetric analysis of the films predicts a different mechanism from that in literature which speculates that there is loss of water molecules from WO₃ at 200°C and therefore better sensing response. Thermo-gravimetric analysis shows that there is an increase in weight percentage in air at 200°C, which we speculate to be due to optimum surface oxygen content which leads to better sensing response. Raman spectroscopy at 200°C supports our speculation by showing no structural change in the WO₃ compound and no shifting of the ~700 cm¹ peak which is indicative of water loss. Furthermore, there was a marked change in the heating-cooling hysteresis at 200°C, which could result from the optimum surface oxygen content changing the electron transport properties The 200°C may also be regarded as a new transition temperature in WO₃ although the transition is not structural (electron-phonon coupling) but is electronic in nature (electron-electron correlation) and this transition temperature could be linked to the optimum sensing temperature of WO₃.