## 2 Materials and Methods
### Sensing Response Testing System  
To investigate the sensing performance of our novel designed ZnO gas sensors, a gas sensor testing system was designed and custom-built by using a sealed-reaction quartz chamber and accurate temperature controller. Figure 4 shows the schematic diagram of the testing circuit built in the gas sensor testing system. The nested thin-film ZnO nanostructure gas sensor and a reference resistor were installed in a series electrical circuit. A voltage meter was used to measure the voltage changes on the reference resistor. The sensing response of the novel ZnO gas sensor in this research is related to its DC resistance change and can be measured with an electric circuit. The resistance change is caused by redox surface reactions starting with an initial oxidation reaction between oxygen in atmospheric air and the ZnO sensing surfaces during the set operation heating up the sensor material to the optimum operating temperature. During the oxidation reaction, the majority carriers in n-type ZnO, electrons, are transferred from the conduction band to the oxidizing agent and thereby help to form one layer of negative oxygen ions on the surface of the ZnO nanotubes for the subsequent sensing reduction reaction with the target gas molecules [11]. This oxidation-induced loss of majority carriers results in a decrease of conductivity and a concomitant increase in resistivity [12]. The resistance change of the ZnO nanorod gas sensor was detected and recorded based on Equations (1)-(3), once the ZnO sample was connected to a simple electric circuit design. \[V_{Sample}=V_{Source}-V_{Reference} \tag{1}\]  
Figure 3: Cross-sectional schematic of (**a**) ZnO nanorod with ALD Al\({}_{2}\)O\({}_{3}\) sacrificial film and ZnO film, (**b**) top ALD coatings removed by PIPS and (**c**) final completed nested ZnO nanotube structure after NaOH etch removal of the sacrificial alumina film exposing additional ZnO sensing surfaces. \[I=\frac{V_{Reference}}{R_{Reference}} \tag{2}\]  
\[R_{Sample}=\frac{V_{Sample}}{I}=\frac{V_{Source}-V_{Reference}}{V_{Reference}/R_{Reference}} \tag{3}\]  
\[Response=\frac{R_{Air}-R_{Gas}}{R_{Air}}\times 100\% \tag{4}\]  
where \(V_{Source}\) is the source voltage of the testing circuit, \(V_{Reference}\) is the voltage of the measurable reference resistor, \(I\) is the current of the testing circuit, \(R_{Sample}\) is the resistance of the prepared sample, \(R_{Air}\) is the resistance of the prepared sample at the testing temperature and \(R_{Gas}\) is the resistance of the prepared sample at the same testing temperature after introducing the target gas. To measure the sensing response of the ZnO nanostructures, the gas sensor testing system was developed, which included a sealed reacting chamber, a simple resistance monitoring circuit, mass flow controllers and a stable temperature controller, as shown in Figure 5.  
The controller of the gas sensor testing system was based on the National Instruments Compact Rio 9068 with corresponding modules and has been programmed with LabVIEW. For the temperature control, the controller reads the real-time temperature signal from a resistance temperature detector (RTD) and provides the corresponding control signal to the heater to form a closed-loop temperature control. The controller also provides power to the electrical testing circuit and monitors the resistance change of the ZnO gas sensor [12]. To accurately control the concentration of the injected ethanol vapor in the test tube, the controller communicates with the two MFCs by providing an input volume signal and reading the real-time input volume signal, which can precisely control the concentration of the ethanol vapor on a ppm scale. Also, the humidity has been measured and was 68% during the experiments. The Dubowski equation was used to calculate the injected ethanol vapor concentration [13], as shown in Equation (5).