NO2 gas sensor made of porous MoO3 sputtered films

Molybdenum trioxide films were sputter-deposited on quartz substrates at various substrate temperatures and discharge gas pressures. The films were annealed in air at 450 degrees C. Scanning electron microscopy showed that fine flakes were formed in the films deposited at 300 degrees C, while the surface of the films deposited at room temperature was smooth. After annealing, the fine flakes in the former films grew, while in the latter films, a layered structure was formed. X-ray diffraction patterns showed that every film was composed of orthorhombic MoO3 crystallites. The highest sensitivity and the fastest response to NO2 gas were observed in a film with a layered structure, which was deposited at room temperature and a high pressure of 11 Pa. This film was found to be porous and to have a density of 2.8 g/cm(3), which is Much lower than the bulk density of 4.7 g/cm(3).