## 3 Results and Discussion
### Sensing Response Analysis as a Function of Temperature  
The sensing response was calculated based on the resistance change of the ZnO nanorod gas sensor relative to the concentration of ethanol vapor, as shown in Equation (4) [14]. Figure 10 shows the (a) resistance change and (b) response change of our novel ZnO nanostructure gas sensor initially after exposure to ethanol target vapor in the test tube at saturation level and optimum operating temperature of 320 \({}^{\circ}\)C, which was followed in the next step by completely exhausting the ethanol from the test set as shown in Figure 8.  
At the onset, our novel nested ZnO nanorod/nanotube gas sensors were tested at a constant saturation-level ethanol vapor concentration of 1000 ppm as a function of temperature from 25 \({}^{\circ}\)C to 340 \({}^{\circ}\)C. A sensing response comparison between our novel ZnO nanorod/nanotube gas sensor design and conventional ZnO nanorod gas sensors was performed, as shown in Figure 11. In both cases, the sensing response results of the novel nested ZnO thin-film nanorod/nanotube gas sensors compared to the conventional ZnO nanorod gas sensors demonstrate that the sensitivity response improves steadily with increasing temperature until a maximum value at 320 \({}^{\circ}\)C is reached, which constitutes the optimum working temperature, with more electrons generated at high temperature. After reaching the maximum peak at 320 \({}^{\circ}\)C, any further increase in temperature causes a decrease in sensitivity. This is attributed to the faster temperature-activated reaction ratio of the reduction reaction compared to the slower oxidation reaction ratio. At higher temperatures, there are more electrons adsorbed on the surface of ZnO from the conduction band, which causes an increase of the resistance and a decrease in the response change. Therefore, the sensing response of ZnO nanostructure gas sensors suffers a decrease at higher temperatures beyond the optimum working temperature.  
Based on the comparison of the response versus temperature data shown in Figure 11, the novel nested ZnO thin-film nanorod/nanotube gas sensor design exhibits a higher sensing response compared to the conventional ZnO nanorod gas sensors. The advantage becomes more pronounced at lower temperatures, while the response difference between the two sensor designs narrows as the temperature approaches the optimum operating temperature. The observed enhancement in the sensing response is primarily attributed to the increased surface-to-volume ratio which was experimentally realized by our novel nested nanotube sensor design. After removing the Al\({}_{2}\)O\({}_{3}\) sacrificial layer, a novel ZnO nanorod sensor was synthesized with coaxial nanotube structure, where the resulting empty annular ring space generated two additional ZnO reaction surfaces. Therefore, the chemical dissolution and removal of the Al\({}_{2}\)O\({}_{3}\) sacrificial layer uncovered more reaction surfaces for ethanol vapor sensing in the same-sized area, which in turn resulted in a sizeable increase of the surface-to-volume ratio.  
Figure 10: Plots of (**a**) resistance change and (**b**) response change as a function of time of nested thin-film ZnO nanostructure gas sensors, delineating the response after introducing and exhausting the ethanol target vapor in a sealed testing chamber.