## 3 Results and Discussion
### Gas Sensing Mechanisms and Data Analysis  
An important parameter for gas sensing performance is the response/recovery time of the sensor. Response time is usually defined as the time required for a sensor to reach 90% of the total response signal, such as resistance or current upon exposure to the target gas [88]. On the other hand, recovery time is typically defined as the time required for a sensor to return to within 10% of the original signal upon removal of the target gas [88]. The response and recovery times depend upon diffusion paths (distance travelled by gas molecules through the nanoparticle sensing surface), film porosity, and particle agglomeration [13; 90; 91; 92]. Based on the results shown in Figure 5a, gas sensors prepared from PBM nanoinks with lower grinding speeds have a faster response compared to sensors prepared from higher grinding speeds, with response time reaching a maximum near 600 rpm before decreasing again at higher speeds. Similarly, increased grinding time also appears to increase response time, with a leveling off near 30 min observed (Figure 5c). Gas sensor recovery time generally increased with grinding speed up to 600 rpm before dipping at 800 rpm (Figure 5b), while a grinding time of approximately 30 min also displayed a dip (Figure 5d), indicating a faster sensor recovery. In general, to have an effective sensor that responds/recovers quickly, one should choose the optimal combination of nanoink grinding conditions, and based on Figure 5, this appears to occur for low to moderate grinding speeds (200-400 rpm) and moderate grinding times (\(\sim\)30 min) for the devices studied, or potentially higher grinding speeds (\(\sim\)800 rpm) with short grinding times (\(\sim\)10 min). As mentioned earlier, the response of nanostructured thin film gas sensors is related to particle size and porosity. Figure 2c-e shows that grinding at a higher speed or for a longer time tends to result in smaller particle sizes. The smaller particle size is ac  
Figure 5: Test gas (argon) response time (**a**) and recovery time (**b**) of sensors prepared from ZnO nanoinks ground at different speeds for constant grinding time of 10 min using EG solvent. Response time (**c**) and recovery time (**d**) of sensors prepared from ZnO nanoinks ground at constant speed of 200 rpm for different grinding times using DI water solvent. Note: Response/recovery time evaluation is constrained by gas flow rates (maximum 500–700 sccm), test chamber volume, and gas sensor film thickness used in the present study. companied by an increase in the total sensing surface area; thus, sensor performance is proportional to the decrease in grain size [13]. This explains the increase in sensor response (Figure 4) as grinding speed approaches 400 rpm and grinding time reaches 30 min. As particle size decreases further (<200 nm), it is possible that the nanoparticles suffer from increased agglomeration, which reduces the effective surface area and thus the free surface energy [93; 94; 95] (in addition, a large particle size dispersion as in the 600 rpm sample of Figure 2d may exacerbate this effect). The influence of agglomeration is consistent with decreased porosity observed for the samples made with nanoinks milled at a higher rpm (see Figure 6a) [96]. Lower porosity could also explain the faster response/recovery times observed for some samples in Figure 5. Further to the material characterization data in Section 3.1, grinding of the ZnO material eventually leads to an increase in surface and bulk defects. In general, the defect density on the surface is considered higher than in the bulk [97]. Surface defects function as (i) charge carrier traps and (ii) adsorption sites improving the electron-hole separation [98]. Bulk defects, on the other hand, serve as recombination sites [99]. Thus, the influence of grinding on the gas sensing behavior is dependent on the ratio of bulk-to-surface defects.