Electrostatic sprayed Cr-loaded NiO core-in-hollow-shell structured micro/nanospheres with ultra-selectivity and sensitivity for xylene

We report a facile and efficient electrostatic spray route to fabricate novel Cr-loaded NiO core-in-hollow-shell structured micro/nanospheres without any templates or surfactants. On the basis of different influencing factors including solvent evaporation rate and temperature-dependent solute degradation process, a multistage reaction mechanism for the formation of core-in-hollow-shell micro/nanospheres is proposed. The gas sensing properties of Cr-loaded NiO sensors toward xylene gas are systematically investigated. The loading ratio of Cr has been varied and the Cr-loaded NiO sensor prepared with an optimum ratio of Cr (1.5 wt%) is found to exhibit enhanced gas sensitivity (Rg/Ra = 20.9) toward 5 ppm xylene gas at an operating temperature of 220 °C in comparison with the pure NiO sensor (Rg/Ra = 1.1). The sensor based on 1.5 wt% Cr-loaded NiO presents 20 times higher sensitivities than the sensor based on pure NiO, which is important for low concentration (1–10 ppm) xylene gas detection. Moreover, we find that the Cr-loaded NiO sensors have excellent selectivity toward xylene gas over other gases while pure NiO sensors show no selectivity toward any specific gas. The decrease in the hole concentration in NiO and the catalytic oxidation of methyl groups by Cr loading evidently play key roles in enhancing the sensitivity and greatly improving the selectivity for xylene gas. The possible gas sensing mechanism of NiO core-in-hollow-shell micro/nanospheres sensors is discussed. The facile preparation method may provide an easy path to the extendable synthesis of other functional materials with core-in-hollow-shell structured micro/nanospheres.