In this work, ZnO thin films were investigated to sense NO2, a gas exhausted by the most common combustion systems polluting the environment. To this end, ZnO thin films were grown by RF sputtering on properly designed and patterned substrates to allow the measurement of the electrical response of the material when exposed to different concentrations of the gas. X-ray diffraction was carried out to correlate the material’s electrical response to the morphological and microstructural features of the sensing materials. Electrical conductivity measurements showed that the transducer fabricated in this work exhibits the optimal performance when heated at 200 °C, and the detection of 0.1 ppm concentration of NO2 was possible. Ab initio modeling allowed the understanding of the sensing mechanism driven by the competitive adsorption of NO2 and atmospheric oxygen mediated by heat. The combined theoretical and experimental study here reported provides insights into the sensing mechanism which will aid the optimization of ZnO transducer design for the quantitative measurement of NO2 exhausted by combustion systems which will be used, ultimately, for the optimized adjustment of combustion resulting into a reduced pollutants and greenhouse gases emission.

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Nanowires made of materials with non-centrosymmetric crystal structures are expected to be ideal building blocks for self-powered nanodevices due to their piezoelectric properties, yet a controversial explanation of the effective operational mechanisms and size effects still delays their real exploitation. To solve this controversy, we propose a methodology based on DFT calculations of the response of nanostructures to external deformations that allows us to distinguish between the different (bulk and surface) contributions: we apply this scheme to evaluate the piezoelectric properties of ZnO [0001] nanowires, with a diameter up to 2.3 nm. Our results reveal that, while surface and confinement effects are negligible, effective strain energies, and thus the nanowire mechanical response, are dependent on size. Our unified approach allows for a proper definition of piezoelectric coefficients for nanostructures, and explains in a rigorous way the reason why nanowires are found to be more sensitive to mechanical deformation than the corresponding bulk material.

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