Growth and NO₂-Sensing Properties of Biaxial p-SnO/n-ZnO Heterostructured Nanowires

Biaxial p-SnO/n-ZnO heterostructured nanowires (average length of 10 μm) were grown onto a glass substrate by thermal evaporation in vacuum. These nanowires had spherical ball tips, and the size of the SnO part increased gradually from the top to the bottom of the nanowire, but the corresponding size of ZnO varied slightly. The Sn–Zn alloy formed in the tips resulted in determined as the catalyst of the growth of the ZnO nanowires. The growth process of the p-SnO/n-ZnO biaxial nanowires is discussed based on vapor–liquid–solid (VLS) based on the subsequent growth process: the VLS catalytic growth of the ZnO nanowire and subsequent epitaxial SnO growth on the sidewall of the pregrown ZnO nanowire. An epitaxial relationship, (001)SnO//(110)ZnO and [110]SnO//[002]ZnO, was observed in the biaxial p-SnO/n-ZnO heterostructured nanowires. The gas-sensing properties of the as-synthesized p-SnO/n-ZnO nanowires were investigated. The results show that the device exhibit a good performance to the ppb-level NO2 at room temperature (25 °C) without light illumination. The detection limit of the p-SnO/n-ZnO sensor to NO2 is 50 ppb. Moreover, the NO2-sensing properties of the p-SnO/n-ZnO device were investigated under various relative humidity. Finally, the NO2-sensing mechanism of the p-SnO/n-ZnO nanowires was proposed and discussed.