Nanocrystalline tungsten oxide thick-films with high sensitivity to H2S atroom temperature

Ultra-fine powder of tungsten oxide was made by evaporation of tungsten met al by an electric are discharge in a reactive atmosphere. The obtained WO3 powder displayed a mixture of monoclinic and tetragonal phases and had a me an grain size of 40 nm. We investigated the gas sensing properties of nanoc rystalline thick-films made from the powder and studied the influence of th e sintering temperature tau (s) of the films on the gas sensitivity. The WO films showed excellent sensing properties upon exposure to low concentrati ons of H2S in air at room temperature. At exposure to 10 ppm of H2S, the co nductance increased by a factor of about 10(4) for films prepared at tau (s ) = 300 degreesC. A further rise of tau (s) for the films made the enhancem ent of their room temperature conductance upon H2S exposure go down, and th e H2S sensitivity disappeared after treatment at tau (s) > 600 degreesC. Th is loss of sensitivity occurred concomitantly with the disappearance of the tetragonal phase, which may point at a specific crystal structure being re sponsible for the unique gas sensing properties of nanocrystalline WO3. (C) 2001 Elsevier Science B.V. All rights reserved.