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.

Combining the versatility of the electrospinning technique and the hydrothermal growth of nanostructures enabled the fabrication of TiO2/ZnO composite nanostructures. XRD, FESEM, TEM and EDX analysis confirmed the growth of ZnO nanosheets with a diameter of about 500 nm on the surface of TiO2 nanofibers. Gas sensing properties of the sensors fabricated from the as-prepared TiO2/ZnO, TiO2 and ZnO were systematically investigated. The TiO2/ZnO sensor exhibits a response of 15.7 to 100 ppm of ethanol at 280 °C, which is much better than pure TiO2 and ZnO. The enhanced sensing performance to ethanol is mainly attributed to the formation of heterojunctions between TiO2 and ZnO, which leads to the interfacial transport of excess carriers.

Three-dimensional brush-like ZnO–TiO2 hierarchical heterojunctions nanofibers have been successfully obtained by the combination of the electrospinning and hydrothermal process. The FESEM images showed that the brush-like ZnO–TiO2 hierarchical heterojunctions nanofibers are composed of uniform ZnO nanorods layer of approximately 100–300 nm in diameter grown on the side surface of TiO2 core nanofibers. The gas sensing studies revealed that the ZnO–TiO2 sensors exhibited enhanced sensing performance to ethanol compared with the pristine TiO2 nanofibers and pristine ZnO nanorods, which might be attributed to the unique hierarchical structure and great degree of electron depletion of the interface based on the synergistic effect among the two components of TiO2 and ZnO.