Among various Metal Oxide Semiconductor (MOS) gas sensors, ZnO nanostructure gas sensors have been widely used due to its large exciton binding energy, wide band gape of 3.37 eV, large exciton binding energy of 60 meV at room temperature, good electrical conductivity, low cost, and high mechanical stability. Furthermore, a variety of morphology has been developed to improve the properties and applications. In this work, ZnO nanotube nanostructures were fabricated as gas sensors to sense ethanol vapor due to its high electrochemical stability, nontoxicity, and, especially, high surface-to-volume ratio. Several coaxial ZnO nanotubes were synthesized on porous substrate to further enhance the sensing response to ethanol vapor with increased reaction surfaces in ZnO nanostructures.

In this study, ZnO nanotubes were synthesized by Atomic Layer Deposition (ALD) on Anodic Aluminum Oxide (AAO) templates with sacrificial Al₂O₃ layers. The second Al₂O₃ sacrificial layer followed by ZnO layer was synthesized on AAO samples by ALD with TMA (Al₂(CH₃)₆) and DI water as precursors. In addition, the third ZnO thin film layer were deposited on the surface of the sacrificial layer of Al₂O₃ with precisely controlled thickness by ALD. The entire procedure outlined above for fabricating ZnO-Al₂O₃-ZnO structures is considered as one super cycle. More super cycles were applied to synthesize additional coaxial ZnO nanotubes. To eliminate the sacrificial layer, a Precision Ion Polishing System (PIPS) was used to remove the top cover as shown in Figure 1. Therefore, the Al₂O₃ sacrificial layers were exposed for Sodium Hydroxide (NaOH) wet etching. After the sacrificial layers were removed by NaOH, ZnO nanotube gas sensors were formed as shown in Figure 1 (b).

To investigate the sensing performance of ZnO nanotube gas sensors to ethanol vapors, a gas sensor testing system was developed with a sealed reaction chamber and a control system with stable temperature control and accurate gas concentration control. The sensing performance of the MOS gas sensors consisting of only core single crystal ZnO nanotubes, was compared to sensors consisting of double nested coaxial ZnO nanotubes, and further compared to three nested tube-in-tube coaxial ZnO nanotube structures and analyzed under various ethanol vapor concentrations at different temperatures. The ZnO nanostructures sensing results on ethanol vapor demonstrated the sensing performance was considerably enhanced due to the increased surface to volume ratio in the ZnO tube structures.