NO₂ Gas Sensor Made of Porous MoO₃ Sputtered Films

Abstract

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°C. Scanning electron microscopy showed that fine flakes were formed in the films deposited at 300°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 MoO 3 crystallites. The highest sensitivity and the fastest response to NO 2 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 .