Nearly monodisperse hollow hierarchical CoO nanocages of four different sizes (∼0.3, 1.0, 2.0, and 4.0 μm) consisting of nanosheets were prepared by controlled precipitation of zeolitic imidazolate framework-67 (ZIF-67) rhombic dodecahedra, followed by solvothermal synthesis of CoO nanocages using ZIF-67 self-sacrificial templates, and subsequent heat treatment for the development of high-performance methylbenzene sensors. The sensor based on hollow hierarchical CoO nanocages with the size of ∼1.0 μm exhibited not only ultrahigh responses (resistance ratios) to 5 ppm p-xylene (78.6) and toluene (43.8) but also a remarkably high selectivity to methylbenzene over the interference of ubiquitous ethanol at 225 °C. The unprecedented and high response and selectivity to methylbenzenes are attributed to the highly gas-accessible hollow hierarchical morphology with thin shells, abundant mesopores, and high surface area per unit volume as well as the high catalytic activity of CoO. Moreover, the size, shell thickness, mesopores, and hollow/hierarchical morphology of the nanocages, the key parameters determining the gas response and selectivity, could be well-controlled by tuning the precipitation of ZIF-67 rhombic dodecahedra and solvothermal reaction. This method can pave a new pathway for the design of high-performance methylbenzene sensors for monitoring the quality of indoor air.

Xylene is a hazardous volatile organic compound, which should be measured precisely for monitoring of indoor air quality. The selective detection of ppm-level xylene using oxide semiconductor chemiresistors, however, remains a challenging issue. In this study, NiO/NiMoO nanocomposite hierarchical spheres assembled from nanosheets were prepared by hydrothermal reaction, and the potential of sensors composed of these nanocomposites to selectively detect xylene gas was investigated. The sensors based on the NiO/NiMoO nanocomposite hierarchical spheres exhibited high responses (maximum resistance ratio =101.5) to 5 ppm p-xylene with low cross-responses (resistance ratios <30) to 5 ppm toluene, benzene, CHOH, CHCOCH, HCHO, CO, trimethylamine, and NH. In contrast, a sensor based on pure NiO hierarchical spheres exhibited negligibly low responses to all 9 analyte gases. The gas-sensing mechanism underlying the high selectivity and response to xylene in the NiO/NiMoO nanocomposite hierarchical spheres is discussed in relation to the catalytic promotion of the xylene-sensing reaction by synergistic combination between NiO and NiMoO, gas-accessible hierarchical morphology, and electronic sensitization by Mo addition. Highly selective detection of xylene can pave the road toward a new solution for precise monitoring of indoor air pollution.

We report the kilogram-scale, simple, and cost-effective synthesis of Pd-loaded quintuple-shelled Co3O4 microreactors by spray drying of aqueous droplets containing cobalt nitrate, palladium nitrate, citric acid, and ethylene glycol and subsequent heat treatment. Highly viscous gel spheres containing Co and Pd salts were successfully converted into multi thin-shelled Co3O4 reactors uniformly loaded with Pd catalysts by the sequential combustion of carbon and decomposition of the metal salts from the outer to the inner regions during one-step heat treatment. The responses (resistance ratio) of the Pd-loaded quintuple-shelled Co3O4 microreactors to 5 ppm toluene and p-xylene were 30.8 and 64.2, respectively, and the selectivity values to toluene and p-xylene against ethanol interference (response ratio) were 14.5 and 30.1, respectively. The unprecedented high response and selectivity were attributed to the effective dissociation of less reactive methylbenzenes into more active smaller species assisted both by catalytic Co3O4 and Pd during the prolonged retention within the microreactors. Kilogram-scale preparation of noble metal-loaded multishelled microreactors and their unique gas-sensing characteristics based on a novel microreactor concept can pave a new way to design of high-performance gas sensors for practical applications.

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CoO sensors with a nanoscale TiO or SnO catalytic overlayer were prepared by screen-printing of CoO yolk-shell spheres and subsequent e-beam evaporation of TiO and SnO. The CoO sensors with 5 nm thick TiO and SnO overlayers showed high responses (resistance ratios) to 5 ppm xylene (14.5 and 28.8) and toluene (11.7 and 16.2) at 250 °C with negligible responses to interference gases such as ethanol, HCHO, CO, and benzene. In contrast, the pure CoO sensor did not show remarkable selectivity toward any specific gas. The response and selectivity to methylbenzenes and ethanol could be systematically controlled by selecting the catalytic overlayer material, varying the overlayer thickness, and tuning the sensing temperature. The significant enhancement of the selectivity for xylene and toluene was attributed to the reforming of less reactive methylbenzenes into more reactive and smaller species and oxidative filtering of other interference gases, including ubiquitous ethanol. The concurrent control of the gas reforming and oxidative filtering processes using a nanoscale overlayer of catalytic oxides provides a new, general, and powerful tool for designing highly selective and sensitive oxide semiconductor gas sensors.

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