Microplotter Printing of Hierarchically Organized NiCo2O4 Films for Ethanol Gas Sensing

Abstract

Using a combination of chemical coprecipitation and hydrothermal treatment of the resulting dispersed system, a hierarchically organized NiCo2O4 nanopowder was obtained, consisting of slightly elongated initial oxide nanoparticles self-organized into nanosheets about 10 nm thick, which in turn are combined into hierarchical cellular agglomerates of about 2 μm. Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD), selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HR-TEM) allowed to confirm the formation of NiCo2O4 powder with the desired crystal structure via additional heat treatment of the intermediate product. Energy-dispersive X-ray spectroscopy (EDX) was used to confirm the target metal ratio, and the uniform distribution of the elements (Ni, Co and O) was shown by mapping. The resulting nanopowder was employed to prepare functional inks suitable for microplotter printing of the NiCo2O4 film. It was found that an oxide film morphology is fully inherited from the hierarchically organized oxide nanopowder used. Atomic force microscopy (AFM) revealed the film thickness (15 μm) and determined the maximum height difference of 500 nm over an area of 25 μm2. Kelvin probe force microscopy (KPFM) showed that the surface potential was shifted to the depths of the oxide film, and the work function value of the material surface was 4.54 eV, which is significantly lower compared to those reported in the literature. The electronic state of the elements in the NiCo2O4 film under study was analyzed by X-ray photoelectron spectroscopy (XPS). Chemosensor measurements showed that the printed receptor layer exhibited selectivity and high signal reproducibility for ethanol detection. As the relative humidity increases from 0 to 75%, the response value is reduced; however, the sensor response profile and signal-to-noise ratio remain without significant changes.