In this work, In 2 O 3 nanospheres were synthesized using urea as a carbon template and modified with Co ions. The synthesized materials were characterized using various instruments to obtain information about the elemental composition and morphology of the materials. The results showed that Co ions replaced the lattice position of In ions and synthesized nanospheres aggregated from 20 nm particles. The results of gas sensitivity tests on all the samples showed that the response to 50 ppm n-butanol at 250 °C reached 540 when Co was doped up to 5 at%. In comparison, the sensor showed very low response to benzene gas with good selectivity. The high response of the sensor to n-butanol is due to the increase in vacancy oxygen content and redox reactions due to the multivalent nature of Co ions. The article concludes with a detailed analysis of the sensing mechanism.
Using metal–organic frameworks (MOF) materials as a filter membrane is an innovative method to improve the selectivity of gas sensors by adjusting the size of the pores and cavities; however, the active surface and dynamic sensing processes are inevitably negatively affected. A facile but effective strategy was implemented in this work by integrating Co ions as catalysts into the inner three-dimensional ZnO skeleton and outer MOF filter membrane. The appropriate incorporation of Co cations was found to increase the surface area, facilitate the adsorption of oxygen species onto the zeolitic imidazolate framework (ZIF) surface, and promote the catalytic oxidation of H 2 S. In the optimised gas sensor, an enhanced response of H 2 S (260 to 5 ppm) and a low detection limit (70 ppb) at 180 °C were obtained because of the catalytic action of appropriate amounts of Co. In addition, beyond the molecular sieve effect to exclude interference from larger gas molecules, the incorporation of Co catalysts can adjust the number of base sites in the ZIF filter (from 1.592 to 3.325 mol/g), which further assists in avoiding basic-type interference from gas molecules with smaller kinetic diameters. As a result, a 130-fold increase in H 2 S selectivity (among various interfering gases) and a 54-fold increase (compared to the control sensor without Co) were obtained. Moreover, the sensor was demonstrated to be a powerful platform for assessing trace amounts of H 2 S in pesticide volatile gas identification and protein food quality evaluation.
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