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 °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.
Highlights
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]
Several films of each of the ground zinc oxide (ZnO) trials were prepared for characterization via doctor blading on substrates as follows: A few μL of ZnO nanoink was used to coat the surface by sweeping the blade across the substrate along with the suspension
The gas sensing studies were conducted using the setup shown in Figure 1b, under ambient pressure and temperature, which consisted of a 3-inch diameter quartz tube chamber connected to a precision electrical source-measure system (Keithley 4200-SCS) and compressed gas source(s) through mass flow controllers (MFCs)
Summary
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]. 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]. Nontoxicity, large exciton binding energy, different nanostructured geometries, along with high surface-to-volume ratios make ZnO nanoparticles an excellent choice for optoelectronic and gas/vapor sensing applications [35,36]. The grinding parameters and solvent used influence the properties and size distribution of the resulting nanoparticle inks and thin films [60], which can be optimized for different applications, including gas sensing. By varying grinding parameters and examining the effect on nanoparticle structure and electrical characteristics of the resultant films, we are able to tune the response signal magnitude and response/recovery times of the ZnO gas sensor devices. Tests conducted in dry/humid air and different target gas environments allowed us to study the ZnO film fabrication conditions required for optimal gas sensing and validate the feasibility of using PBM nanoinks as the active material for thin film gas sensors
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