Rapid synthesis of rhombohedral In₂O₃ nanoparticles via a microwave-assisted hydrothermal pathway and their application for conductometric ethanol sensing

Rhombohedral In₂O₃ exhibits high gas sensing potential. However, its rapid synthesis remains challenging, and it is yet to be revealed whether it has a higher gas sensing capability than the cubic counterpart. Herein, we report a facile and rapid synthesis of rhombohedral In₂O₃ nanoparticles (NPs) via a microwave-assisted hydrothermal pathway, and compare their gas sensing characteristics with those of other In₂O₃ polymorphs (i.e., cubic NPs and mixed rhombohedral and cubic phases). To investigate the polymorphs for optimized gas sensing, the gas sensing properties of rhombohedral In₂O₃ NPs are compared with those of mixed-phase In₂O₃ NPs prepared via a conventional hydrothermal pathway and commercial cubic In₂O₃ NPs. The pure cubic In₂O₃ NPs and the mixed-phase In₂O₃ NPs show similar gas responses (resistance ratios) to 100 ppm ethanol in the temperature range of 150–400 °C. In contrast, the rhombohedral In₂O₃ NPs exhibit much higher ethanol responses at all the sensor temperatures. In particular, their ethanol response at 300 °C (43.1) is four times higher than the values obtained for the other polymorphic NPs (9.44–11.6) at the same temperature. Furthermore, the rhombohedral In₂O₃ NPs possess excellent ethanol selectivity with low cross-responses to 100 ppm NH₃, CH₄, H₂, CO, CO₂, and NO₂ and long-term stable ethanol sensing properties for 21 days. The excellent gas sensing characteristics of rhombohedral In₂O₃ NPs are strongly related to their gas adsorption, particle sizes, and electrical conductivity. The rhombohedral In₂O₃ NPs can be potentially used in high-performance breathalyzers for screening drunk drivers.