NiO nanosheets assembled into hollow microspheres for highly sensitive and fast-responding VOC sensors

Uniform, hollow NiO microspheres constituted from nanosheets were synthesized by a simple water bath method through a SiO2 spheres template-assisted approach and applied in an efficient gas sensor towards volatile organic compound (VOC) vapors. The structural characterizations reveal that sub-micrometer NiO hollow microspheres (350-400 nm) were formed by assembling NiO nanosheets of 20-40 nm thickness. The gas responses to the five reductive gases, including isopropanol, acetone, methanol, ethanol and formaldehyde, at a low concentration of 50 ppm are 11.3, 9.9, 8.2, 7.7 and 5.0, respectively. NiO hollow microspheres showed higher responses for VOCs compared to other NiO nanostructures previously reported in the literature. The gas sensor based on NiO hollow microspheres shows a high response and fast response and recovery towards VOCs, making it a promising candidate for a practical detector of VOCs. The improved performance of NiO hollow microspheres was attributed to hollow spaces that offer a high surface-to-volume ratio and an intrinsically large specific surface area of 167.31 m(2) g(-1), leading to an improved surface activity. It was suggested that a layer of adsorbed oxygen over the NiO surface possibly decreases the thickness of the space charge layer in VOC gases and leads to an increase in resistance and therefore response.