Hollow spheres of CoCr₂O₄-Cr₂O₃ mixed oxides with nanoscale heterojunctions for exclusive detection of indoor xylene

As an effective alternative to simple binary oxide chemiresistors, polynary oxides with excellent tunability of the composition and hetero-interfaces are considered as promising material platforms for designing highly selective and sensitive gas sensors. In this study, ternary spinel CoCr₂O₄ hollow spheres and CoCr₂O₄-Cr₂O₃ mixed oxide hollow spheres were prepared via one-pot ultrasonic spray pyrolysis using solutions with different cation compositions (i.e., [Cr]/[Co] = 2, 3, and 4), and their gas-sensing characteristics were investigated. The pure CoCr₂O₄ hollow spheres exhibited an unusually high response to 5 parts per million (ppm) of p-xylene (ratio of resistance to gas and air = 61.4), with negligible cross-responses to 5 ppm of ethanol, toluene, benzene, trimethylamine, ammonia, formaldehyde, and carbon monoxide. When CoCr₂O₄-Cr₂O₃ hollow spheres with discrete Cr₂O₃ nanoclusters were formed using a spray solution with a [Cr]/[Co] ratio of 3, the xylene response was enhanced to 144.1, which allows the sensitive and selective detection of sub-ppm level p-xylene. The unprecedentedly high xylene selectivity and response in the present study are explained by the gas-accessible hollow morphology, the unique catalytic activity of the ternary and mixed oxides, and the enhanced chemiresistivity due to the formation of a heterojunction between CoCr₂O₄ and Cr₂O₃. The novel ternary oxide-based gas sensors with excellent xylene-sensing performance can be used in indoor air-monitoring applications.