## 3 Results and Discussion
### Gas Sensing Mechanisms and Data Analysis
Initially, as the grinding time/speed increases, the ratio of bulk-to-surface defects decreases, which can be attributed to an increase in the total surface area leading to comparatively higher surface defects. However, based on our results at higher grinding speeds and for longer times, this ratio seems to increase due to the formation of excess bulk defects, which may be attributed to bulk crystal distortion caused by excess mechanical attrition. Since ZnO is a chemiresistive sensing material [29], the sensor response mechanism is influenced by the gas-surface absorption-desorption process, surface diffusion-reaction, and redox reactions between active species on the sensor surfaces [100]. In addition, defects of nanostructured ZnO such as oxygen vacancies (see Raman data, Figure 2) can act as electron donors, which enhances conductivity and promotes gas sensing performance [19; 90]. Grinding in the presence of solvents during PBM can also result in surface modification [101], since ball milling is an energy intensive process. The energy from the high speed impact of the beads can result in the formation of reactive radicals [101]. The grinding media, therefore, can influence the baseline resistivity of the ZnO nanostructured films [29; 102; 103]. The sensing mechanisms in our fabricated sensors likely depend on surface interaction and charge transfer between adsorbed gas species, which results in variation of sensor resistance (or conductance) [104]; it has been reported that O\({}_{2}\)  
Figure 6: Porosity of sensors prepared using ZnO nanoinks obtained from AFM and SEM data. (**a**) Sensor prepared using EG solvent for different grinding speeds at constant grinding time of 10 min. (**b**) Sensor prepared from DI water solvent at different grinding times for constant grinding speed of 200 rpm. ZnO film surfaces at room temperature [105; 106], which results in an electron-depletion layer (barrier layer) on the outer surface of the ZnO in air. Between grains, the merging of the two depletion regions creates a Schottky barrier, the magnitude of which determines the conductivity of the materials [96]. The reaction between the adsorbed oxygen species with adsorbed gas molecules can modify the height of the Schottky barrier, resulting in the variation of sensor resistivity; in dark conditions, when the ZnO surface is exposed to an air environment, electrons from the ZnO conduction band ionize atmospheric oxygen to produce negative oxygen ions at the surface of the nanostructured film (Equation (2)), thereby creating a low conductivity depletion layer near the surface [107]. \[\mathrm{O_{2}\ (gas)+\ e^{-}\to\ O_{2}^{-}\ (ads)}, \tag{2}\]  
When light with higher energy than the bandgap of ZnO is incident, electron-hole pairs will be generated. Holes produced by the light absorption migrate to the surface along the potential slope created by band bending and neutralize the negatively charged adsorbed oxygen ions (Equation (3)), which are then desorbed, while the photogenerated electrons reduce the width of the depletion layer, thereby increasing conductivity [107]. \[\mathrm{h^{+}+O_{2}^{-}\ (ads)\to O_{2}\ (gas)}, \tag{3}\]  
Thus, conduction in dark and photoconduction of the ZnO films are strongly dependent on the ambient gas conditions and oxidizing atmosphere. In addition, RH, defined as the amount of water vapor in air, expressed as a percentage of the maximum amount that the air could hold at a given temperature, can also strongly impact a metal oxide surface [108] and thus alter the sensor response [109; 110; 111]. Water molecules adsorbed on the surface tend to dope ZnO with electrons and/or displace previously adsorbed ionized oxygen, releasing electrons back to the conduction band (i.e., the reverse of Equation (2)) [110]. In both cases, sensor conductivity increases as consistent with Figure 3b. For the ZnO thin film gas sensors based on PBM nanoinks reported here, the ultimate performance likely depends on a combination of grain size, porosity, and surface-to-bulk defect ratio within the films.