Tungsten-oxide thin films as novel materials with high sensitivity and selectivity to NO2, O3, and H2S. Part II: Application as gas sensors

• Published in: Journal of materials science. Materials in electronics 2004-07, Vol.15 (7), p.483-493
• Main Authors: BERGER, O, HOFFMANN, T, FISCHER, W.-J, MELEV, V
• Format: Article
• Language: English
• Subjects: Analytical chemistry; Chemistry; Exact sciences and technology; General equipment and techniques; General, instrumentation; Instruments, apparatus, components and techniques common to several branches of physics and astronomy; Physics; Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing; Thin films
• ISSN: 0957-4522; 1573-482X
• DOI: 10.1023/B:JMSE.0000031602.73321.fe

The electrical response of tungsten-oxide thin films as-deposited by electron-beam deposition and annealed (at 350-800 °C for 1-3 h in O^sub 2^) to NO^sub 2^, O^sub 3^ and H^sub 2^S was studied both experimentally and theoretically. In order to interpret the kinetic characteristics of tungsten-oxide thin films on exposure to different gases, a model based on surface adsorption/desorption processes coupled with bulk diffusion was used. A link between the geometrical and chemical heterogeneities of the tungsten-oxide film surfaces and their performance characteristics as gas sensors was established. It was shown that the nature and amount of surface-adsorption sites in the different nonstoichiometric phases (W^sub n^O^sub 3n-2^ or W^sub n^O^sub 3n-1^) and WO^sub 3^ as well as their conduction mechanisms are defined from the phase composition of the film, the crystallographic and electronic structures of the phases, the orientation of the crystallites within the film and the geometrical shape and dimensions of the crystallites. All tungsten-oxide thin films investigated in this work are suitable for detection of very low concentrations of NO^sub 2^ (0.05-0.5 ppm in N^sub 2^ and synthetic air), ozone (25-90 ppb) and H^sub 2^S (3-15 ppm in N^sub 2^ and synthetic air) at very low working temperatures (80-160 °C). The films annealed at 400 °C for 1-2 h are very selective to ozone at 120-160 °C; the films annealed at 400 °C for 1-3 h and at 800 °C for 1 h are very sensitive to NO^sub 2^ (in N^sub 2^).[PUBLICATION ABSTRACT]