Highly selective and sensitive xylene sensors using Cr₂O₃-ZnCr₂O₄ hetero-nanostructures prepared by galvanic replacement

Cr₂O₃/ZnCr₂O₄ nanocomposite powders and phase-pure ZnCr₂O₄ powders were prepared by the galvanic replacement of Zn in ZnO hollow spheres by Cr and subsequent heat treatment. The Cr₂O₃/ZnCr₂O₄ nanocomposite powders prepared by galvanic replacement consisted of nanocrystalline Cr₂O₃ and ZnCr₂O₄ particles, which showed a high response (resistance ratio) of 69.2 to 5 ppm xylene at 275 °C and excellent xylene selectivity. In contrast, both Cr₂O₃ and ZnO powders showed selectivity to ethanol, and no significant selectivity to a specific gas was found for the ZnCr₂O₄ powders. The Cr₂O₃/ZnCr₂O₄ composite powders consisting of coarse particles prepared by solid-state reaction showed relatively low response and selectivity to xylene. The high selectivity and response to xylene of the Cr₂O₃/ZnCr₂O₄ nanocomposite powders were attributed to the synergistic promotion of the methylbenzene-sensing reaction by two sensing materials with different catalytic activities and the high chemiresistive variation of the small particles, respectively, both of which result from the intimate and uniform mixing of nanocrystalline Cr₂O₃ and ZnCr₂O₄ particles.