###### Abstract  
Planetary ball-milled zinc oxide (ZnO) nanoparticle suspensions (nanoinks) were used to produce thin film chemiresistive gas sensors that operate at room temperature. By varying milling or grinding parameters (speed, time, and solvent) different thin film gas sensors with tunable particle sizes and porosity were fabricated and tested with dry air/oxygen against hydrogen, argon, and methane target species, in addition to relative humidity, under ambient light conditions. Grinding speeds of up to 1000 rpm produced particle sizes and RMS thin film roughness below 100 nm, as measured by atomic force and scanning electron microscopy. Raman spectroscopy, photoluminescence, and X-ray analysis confirmed the purity and structure of the resulting ZnO nanoparticles. Gas sensor response at room temperature was found to peak for nanoinks milled at 400 rpm and for 30 min in ethylene glycol and deionized water, which could be correlated to an increased film porosity and enhanced variation in electron concentration resulting from adsorption/desorption of oxygen ions on the surfaces of ZnO nanoparticles. Sensor response and dynamic behavior was found to improve as the temperature was increased, peaking between 100 and 150 \({}^{\circ}\)C. This work demonstrates the use of low-cost PBM nanoinks as the active materials for solution-processed thin film gas/humidity sensors for use in environmental, medical, food packaging, laboratory, and industrial applications. ZnO; nanoparticles; planetary ball milling; gas sensor; thin films; nanoink +
Footnote †: journal: Applied Sciences  
13 October 2021  
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**1. Introduction**  
Thin films and nanostructured coatings find broad application due to their unique properties, which is a result of both size and interface effects [1, 2, 3, 4, 5, 6, 7, 8]. In particular, thin film gas sensing devices based on nanostructures are relevant for monitoring environmental gas concentration, controlling chemical processes, and agricultural and medical applications [9, 10, 11, 12]. For efficient gas sensing, the film properties and structure play a vital role in sensor operation depending on particle size, porosity, and the diffusion time of the gas species [13]. Nanoparticles have a large relative surface area, thereby accelerating exchange with the surrounding atmosphere and leading to a larger detection signal relative to larger or bulk particles. A high porosity nanoparticle sensing film also facilitates the rapid diffusion of the gas species and improves sensing performance. Similarly, gas homogenization time for nanoparticles can be several orders of magnitude smaller than for micron-sized particles. Semiconducting metal oxide nanostructures have shown exceptional chemical, optical, mechanical, and electrical properties for sensing [14, 15, 16]. Metal oxides show a large change in their electrical conductivity in the presence of various gas environments [15, 17], and thus, by monitoring the variation of current (or resistance) through these films, they can function as sensitive gas detectors. Lowering the operating temperature needed for metal oxide gas sensors is an important goal, and room temperature operation, in particular, has attracted wide interest due to low-power consumption, portability, and cost-effectiveness [18]. Approaches such as noble metal functionalization or creation of oxygen vacancies have been proposed to reduce sensor working temperatures [19; 20; 21; 22]. Various types of metal oxide films have been used for gas sensing applications, [11; 16; 23] including TiO\({}_{2}\)[24], SnO\({}_{2}\)[25], and ZnO [26; 27; 28]. Among these, ZnO, a wide band gap (\(\sim\)3.34-3.37 eV) [29] semiconductor, is widely used for applications such as varistors [30], memristors [31], solar cells [32], piezoelectric devices [33], and light emitting diodes [34].