Hierarchical Dense Array of ZnO Nanowires Spatially Grown on ZnO/TiO₂ Nanofibers and Their Ultraviolet Activated Gas Sensing Properties

A three-dimensional heterostructure using selective growth of ZnO nanowires on TiO₂/ZnO nanocomposite nanofiber by a novel combination of a postseeding method in an electrospinning process and a hydrothermal technique is presented in this work. A hierarchical dense array of ZnO nanowires with a diameter of 70–120 nm and an aspect ratio of about 10 on TiO₂/ZnO composite nanofibers (ZnO@TiO₂/ZnO) was fabricated. XRD, UV–vis, FESEM, HRTEM, TEM, EDX, and XPS analyses were used for characterization of nanofibers. The results demonstrate about 4 times enhancement in UV spectrum absorption in the range of 240–300 nm compared to the nonhierarchical structures. It was found that the optimized operating temperatures for TiO₂/ZnO nanofibers were 260 and 240 °C in the dark and under UV illumination. Results showed that ZnO@TiO₂/ZnO exhibit about 3 times improvement in gas sensitivity with UV irradiation for 10–200 ppm of ethanol gas with reduced response and recovery times. UV-illuminated ZnO@TiO₂/ZnO had about 3.81 and 4.30 times selectivity on ethanol with respect to acetone and toluene.