Soyeon An

UV-enhanced NO2 gas sensing properties of SnO2-core/ZnO-shell nanowires at room temperature.

SnO2-core/ZnO-shell nanowires were synthesized using a two-step process: the synthesis of SnO2 nanowires by the thermal evaporation of Sn powders followed by the atomic layer deposition of ZnO. The room temperature NO2 gas sensing properties of the nanowires under ultraviolet (UV) illumination were examined. The cores and shells of the nanowires were primitive tetragonal-structured single crystal SnO2 and wurtzite-structured single crystal ZnO, respectively. The responses of multiple networked SnO2 nanowire sensors were increased 2-3-fold at NO2 concentrations ranging from 1 to 5 ppm by encapsulating the nanowires with ZnO. The SnO2-core/ZnO-shell nanowire sensors showed a remarkably enhanced response under UV illumination. The sensing mechanism of the core/shell nanowires under UV illumination is also discussed.

Synthesis of Nanograined ZnO Nanowires and Their Enhanced Gas Sensing Properties

Polycrystalline ZnO nanowires with grain sizes ranging from 20 to 100 nm were synthesized using a newly designed two-step process: (first step) synthesis of ZnSe nanowires by vapor transportation of a mixture of ZnSe powders; and (second step) thermal oxidation of the ZnSe nanowires at 650 °C. Compared to the single-crystal ZnO nanowire gas sensors and other nanomaterial gas sensors reported previously, the multiple networked nanowire gas sensors fabricated from the nanograined ZnO nanowires showed substantially enhanced electrical responses to NO2 gas at 300 °C. The NO2 gas sensing properties of the nanograined ZnO nanowires increased dramatically with increasing NO2 concentration. The multiple-networked nanograined ZnO nanowire sensor showed a response value of 237,263% at 10 ppm NO2 and 300 °C, whereas the single-crystal ZnO nanowire sensors showed a response of only 6.5% under the same conditions. The recovery time of the nanograined ZnO nanowire sensor was much shorter than that of the normal ZnO nanowire sensor over the NO2 concentration range of 1-10 ppm, even though the response time of the former was somewhat longer than that of the latter. The origin of the enhanced NO2 gas sensing properties of the nanograined ZnO nanowire sensor is discussed.

SYNTHESIS, STRUCTURE, AND LUMINESCENCE PROPERTIES OF ZnSnO3 NANOWIRES

ZnSnO3 nanowires were synthesized on Si substrates by thermal evaporation of a mixture of ZnO, SnO2 and graphite powders. The nanowires were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and photoluminescence spectroscopy. The ZnSnO3 nanowires varied from 10 to 100 nm in diameter and up to a few hundred of micrometers in length. Transmission electron microscopy and X-ray diffraction revealed that the nanowires are multiphase nanostructures containing ZnSnO3, Zn2SnO4, ZnO, and SnO2 phases. Photoluminescence measurements showed that ZnSnO3 nanowires had a sharp ultraviolet emission peak at approximately 375 nm as well as a broad green emission band centered at approximately 510 nm. The violet emission of ZnSnO3 nanowires exhibits a blue shift by approximately 5 nm compared to that of ZnO nanowires and the visible emission of ZnO nanowires shifted from the orange region to the green region, which should be attributed to the narrowing of Eg. Thermal annealing enhanced the green emission but degraded the ultraviolet emission of the ZnSnO3 nanowires. In addition, the origin of the enhanced luminescence of ZnSnO3 nanowires compared to ZnO and SnO2 nanowires is discussed.

NO2 Gas Sensing Properties of Multiple Networked ZnGa2O4 Nanorods Coated with TiO2.

The NO2 gas sensing properties of ZnGa2O4-TiO2 heterostructure nanorods was examined. ZnGa2O4-core/TiO2-shell nanorods were fabricated by the thermal evaporation of a mixture of Zn and GaN powders and the sputter deposition of TiO2. Multiple networked ZnGa2O4-core/TiO2-shell nanorod sensors showed the response of 876% at 10 ppm NO2 at 300 degrees C. This response value at 10 ppm NO2 is approximately 4 times larger than that of bare ZnGa2O4 nanorod sensors. The response values obtained by the ZnGa2O4-core/TiO2-shell nanorods in this study are more than 13 times higher than those obtained previously by the SnO2-core/ZnO-shell nanofibers at 5% NO2. The significant enhancement in the response of ZnGa2O4 nanorods to NO2 gas by coating them with TiO2 can be explained based on the space-charge model.