Among various Metal Oxide Semiconductor (MOS) materials for in gas sensor applications, ZnO has been widely used in gas detection due to its wide band gape of 3.37 eV, large exciton binding energy (~60 meV), good electrical conductivity, and high mechanical stability. Moreover, owning to the high surface-to-volume ratio, ZnO gas sensors based on nanorod structure are extensively applied into gas specie detection and gas concentration detection. Therefore, a large number of methods and techniques have been applied to enhance the sensing performance of ZnO nanorod gas sensors. In this work, the ZnO nanorods with coaxial nanotubes gas sensors were fabricated to detect the concentration of ethanol vapor under the different temperatures and concentration conditions. In addition, Al doped ZnO (AZO) thin films were deposited on the surface of ZnO nanorods by Atomic Layer Deposition (ALD). The additional oxygen vacancies deficiency from the Zn-O and Al-O bands in the Zn-Al-O interface could improve the sensing performance of prepared ZnO gas sensors.

In this study, ZnO nanorods were synthesized by hydrothermal method with ZnO seed layers deposited on Si wafer by ALD, as shown in Figure 1(a). The solutions used for ZnO nanorods hydrothermal growth were prepared with zinc nitrate hexahydrate (Zn(NO3)6H2O) and hexamethylenetetramine ((CH2)6N4) dissolved in DI water. To synthesize ZnO nanotubes around nanorods, one additional Al₂O₃ sacrificial layer was deposited on the surface of ZnO nanorods by ALD with TMA (Al₂(CH₃)₆) and DI water as precursors. Then ZnO thin films were deposited on ZnO nanorods with Al₂O₃ sacrificial layer wrapped around, as shown in Figure 1(b). To eliminate the sacrificial layer, Precision Ion Polishing System (PIPS) was used to remove the top cover of ZnO nanorods. Figure 1(c) and Figure 1(d) show the Al₂O₃ sacrificial layer was exposed for Sodium hydroxide (NaOH) etching. After the sacrificial layer was removed by NaOH, ZnO nanorod/nanotube gas sensors were synthesized, as shown in Figure 1(e). To further enhance the sensing performance of ZnO nanorod/nanotube to the ethanol, AZO thin films were deposited on the surface by ALD as shown in Figure 1(f).

A testing system of gas sensors was developed with a sealed reaction chamber and control system based on CompactRio from National instrument to investigate the sensing performance of ZnO nanorod/nanotube to ethanol vapor. The sensing response of ZnO nanorods to ethanol vapor was recorded and analyzed from the resistance change of ZnO nanorods. The sensing performances of ZnO nanorods, ZnO nanorod/nanotube, and ZnO nanorod/nanotube after coated with AZO thin films were measured and analyzed under various volumes of ethanol vapor at different temperatures. Base on the results, the sensing performance of ZnO nanorod gas sensor was enhanced by increase surface-to-volume ratio and oxygen vacancies.

Figure 1. FESEM results of (a) ZnO Nanorods, (b) ZnO Nanorods with Al₂O₃ Coating and ZnO thin films, (c) and (d) Al₂O₃ sacrificial layer exposed after PIPS, (e) ZnO Nanorod/Nanotube after NaOH Etching, (f) ZnO Nanorod/Nanotube with AZO Coating.