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

Abstract

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.