## 3 Results and Discussion
### Gas Sensing Results
(**e**) Current vs. time for ZnO thin film sensor (prepared using PBM nanoninks ground at 400 rpm for 10 min in EG solvent) at different H\({}_{2}\) gas concentrations indicated. Inset shows response as a function of gas concentration. (**f**) Current vs. time for ZnO thin film sensor (sample as in (**e**)) when exposed to different gases (hydrogen, argon, and methane). Inset shows relative response or selectivity to different target gas species. addition to the film thickness), it took some time to reach a stable baseline as displayed in Figure 3c, which shows the sensor stabilizing after approximately 2.5 h. Sensing with dry air as the carrier gas to balance the target species was also possible using our PBM nanoink sensors, as evidenced by the detection of hydrogen in Figure 3d, which shows that sensor current is proportional to target gas concentration. Further study of sensor response was performed for a sequence of hydrogen gas concentrations ranging from 5000 to 20,000 ppm (Figure 3e), which showed a linear response region (Figure 3e, inset) with a sensitivity of approximately 2.4 \(\times\) 10\({}^{-2}\) ppm\({}^{-1}\). In addition, sensor selectivity was tested by exposure to the same concentration for different gas species (Figure 3f), indicating a strong response to methane, followed by hydrogen and argon, which shows potential for selective gas sensing (Figure 3f, inset). Preferential adsorption of different gas molecules [86] likely leads to the sensor selectivity we observed at room temperature. Upon cycling back to the first gas pulse (H\({}_{2}\)), the original sensor signal returned. In general, the gas sensor devices studied gave stable and reproducible responses that stayed within 25% of the original signal after several months. The gas sensing response is usually defined as the ratio of resistances in two different gas atmospheres. In order to compare the performance of the gas sensors prepared using PBM nanoinks from different grinding conditions, the sensor response [87,88] was calculated as  
\[\text{S}=\frac{R_{a}-R_{g}}{R_{a}}, \tag{1}\]  
where \(R_{a}\) and \(R_{g}\) are the sensor resistance in dry air and in the presence of target gas, respectively. Figure 4 compares the effects of different grinding parameters (speed/time/solvent) on gas sensor response in the presence of argon target gas. It can be seen in Figure 4a that sensor response decreases with grinding speed beyond 400 rpm, possibly due to particle agglomeration and porosity variations (see Section 3.3). Response can also be seen to increase with grinding time, peaking near 30 min for a constant grinding speed of 200 rpm (Figure 4b). Grinding of materials using high energy ball milling can produce both surface and bulk defects. It is possible that surface defects would enhance gas sensing performance by exposing adsorption sites, whereas bulk defects could limit the adsorption surface area [55]. The presence of surface defects such as Zn\({}_{i}\)- and V\({}_{0}\)-related defects is indicated in the Raman data of Figure 2g and h as grinding was increased. This could lead to a significant increase in gas sensor response because more oxygen molecules can be easily absorbed and ionized on the surface of ZnO [89]. However, grinding at a higher speed or for a longer time can also give rise to bulk defects; therefore, it is anticipated that there will be a point at which bulk defects arise during grinding, resulting in lower response, which is discussed further below. Figure 4: Response of different gas sensors made from ground PBM ZnO nanoinks towards argon gas. (**a**) Films prepared using EG solvent at different grinding speeds for constant grinding time of 10 min. (**b**) Films prepared using DI water solvent at different grinding times for a constant grinding speed of 200 rpm.