The need to improve the sensitivity, selectivity and stability of ozone gas sensors capable of monitoring the environment to prevent hazard to humans has sparked research on binary metal oxides. Here we report on a novel ozone gas sensor made with ca. 0.5 m yolk-shelled ZnCo 2 O 4 microstructures synthesized via an eco-friendly, co-precipitation method and subsequent annealing. With these ZnCo 2 O 4 microspheres, ozone concentrations down to 80 parts per billion (ppb) could be detected with a.c. and d.c. electrical measurements. The sensor worked within a wide range of ozone concentrations, from 80 to 890 ppb, being also selective to ozone compared to CO, NH 3 and NO 2. The high performance could be attributed to the large surface area to volume ratio inherent in yolk-shell structures. Indeed, ozone molecules adsorbed on the ZnCo 2 O 4 surface create a layer of holes that affect the conductivity, as in a p-type semiconductor. Since this mechanism of detection is generic, ZnCo 2 O 4 microspheres can be further used in other environment monitoring devices.

The gas sensor response of tin monoxide micro-disks, functionalized with noble metal nanoparticles (Pd and Ag), to NO 2 , H 2 and CO were studied by monitoring changes in their resistance upon exposure to the various gases. The tin monoxide, with unusually low Sn oxidation state, was synthetized by carbothermal reduction. Surface modification by Pd and Ag catalysts was achieved by coating the micro-disks by metallic nanoparticle dispersions, prepared by the polyol reduction process, followed by thermal treatment. SEM and TEM analysis showed nanoparticles to be well-dispersed over the SnO surfaces. The decorated SnO micro-disks exhibited high sensor response to reducing gases such as H 2 and CO. On the other hand, the catalytic particles tended to reduce the sensor response to oxidizing

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