Ultra-selective detection of sub-ppm-level benzene using Pd-SnO₂ yolk-shell micro-reactors with a catalytic Co₃O₄ overlayer for monitoring air quality

Ultra-selective and sensitive detection of carcinogenic benzene vapor with negligible interferences from other major indoor pollutants is not only critical but also challenging because the BTX gases (benzene, toluene, and xylene) are generally less reactive to a majority of n-type oxide semiconductor gas sensors and the similar chemical structures of BTX gases hamper their discrimination by chemiresistive variation. Through this paper, we propose a new strategy to detect sub-ppm-level benzene vapor in a highly selective manner using oxide semiconductor chemiresistors. A Pd-loaded SnO₂ yolk-shell sensing film coated with a thin catalytic Co₃O₄ overlayer showed an ultrahigh response (resistance ratio = 88) to 5 ppm benzene with negligibly low cross-responses to the other representative and ubiquitous indoor pollutants such as toluene, xylene, HCHO, CO, and ethanol. The response to benzene vapor was significantly enhanced by reforming of highly stable benzene into more active and smaller species. The reforming was synergistically assisted by the Co₃O₄ catalytic overlayer and sensing layer consisting of Pd-loaded SnO₂ micro-reactors, while the cross-responses to the other indoor pollutants became low because of the catalytic oxidation of the gases into less- or non-reactive species. This method will pave a new way to the precise monitoring of critically harmful benzene in both indoor and outdoor atmospheres.