## 3 Results and Discussion
### Gas Sensing Results  
Figure 3 shows current (at +2 V applied bias) vs. time and _I-V_ curves for ZnO thin film sensors formed using the PBM nanoinks. In Figure 3a, the current levels vs. time for a flow sequence of dry air/pure argon/dry air are shown (similar behavior was observed for nitrogen target gas environments). The time-dependent sensor behavior was observed upon exposure to a target gas atmosphere after reaching a minimum in pure dry air flow, whereby output current rises upon injection of the target gas source(s) and falls upon release of the gas (and re-exposure to dry air/oxygen). Accordingly, the resistance of the sensor underwent a decrease as it was exposed to a target and an increase as dry air was reintroduced. Initially, the sensor sample current (in ambient atmosphere) was observed to increase, and started to decrease as soon as dry air was introduced, indicating that the response of the ZnO gas thin film sensors are also strongly dependent on operating atmospheric humidity or RH. The humidity or moisture sensing ability of our ZnO films was confirmed by the resistance data shown in Figure 3b, which shows a large sensor response vs. RH. Figure 2: (**a**) SEM image of milled ZnO nanoparticles ground with DI water at 200 rpm for 30 min. The inset is an optical microscope image of doctor-bladed ZnO thin film on glass substrate. (**b**) AFM images of ZnO film prepared using PBM nanoink with DI water ground at 200 rpm for 10 min. (**c**) Plot of RMS roughness from AFM of ZnO thin films using PBM nanoink ground in EG for 10 min at different speeds. Average particle size of ZnO particles obtained from AFM and SEM data of thin films using PBM nanoink ground in (**d**) EG for 10 min at different speeds and (**e**) DI water at 200 rpm for different grinding time (data at 0 rpm/min denote nominal starting bulk powder particle size). (**f**) Photoluminescence spectra for approximately 450–700 nm of ZnO thin film ground at 200 rpm for 60 min using DI water solvent (inset (left) optical map image of total PL intensity obtained for 415–715 nm and (right) overall range averaged full spectrum). (**g**) Raman spectra of ZnO ground film prepared using DI water ground at 200 rpm for 60 min. (**h**) Raman spectra of different ZnO films ground (EG solvent) and unground: black, bulk sample; blue, ground at 200 rpm for 10 min; red, ground at 750 rpm for 90 min. (**i**) EDX spectrum of ZnO film using PBM nanoink ground at 600 rpm for 10 min in EG solvent. The target gas response of our sensors was repeatable over multiple flow sequences; however, due to the size of the test quartz chamber and maximum flow rates used (in  
Figure 3: (**a**) Time dependence of sensor current upon exposure to dry air (\(\sim\)2000 s mark) followed by pure argon gas (\(\sim\)5500 s mark) and then dry air again (\(\sim\)8000 s mark) (all flows 500 sccm) for ZnO thin film sensors formed using PBM nanoninks ground for 10 min in EG (400 rpm). Inset shows _I-V_ plots as current increases near start of argon flow. (**b**) Resistance vs. RH for a ZnO thin film sensor formed using PBM nanoninks ground at 200 rpm for 30 min in DI water. Inset shows individual _I-V_ plots for different humidity values. (**c**) Gas sensor (prepared using nanoninks ground at 400 rpm for 10 min in EG solvent) showing approach to stable baseline vs. time during repeated exposure to 250 sccm of H\({}_{2}\) pulses. Inset shows sensor current vs. time for a similar sequence of on/off dry air/ argon gas pulses for ZnO thin film sensor formed using PBM nanoninks ground for 10 min in EG (600 rpm). (**d**) Sensor current vs. time for 500 sccm dry air followed by 400 sccm dry air/100 sccm hydrogen (sensor formed using PBM nanoninks ground for 10 min in EG (600 rpm)). Inset shows current vs. time for ZnO sensor (prepared using nanoninks ground at 400 rpm for 10 min in EG solvent) exposed to 450 sccm dry air/50 sccm hydrogen (red curve) and 475 sccm dry air/25 sccm hydrogen (black curve).