Porous, 3D-hierarchical α-NiMoO₄ rectangular nanosheets for selective conductometric ethanol gas sensors

Transition metal oxides with unique nanoarchitectures are being appraised as plausible candidates for the highly selective detection of contaminating organic gases due to their earth abundance, high surface area, and outstanding miniaturization potential. Here, two different nanoarchitectures of nickel molybdate (NMO), namely 3D-honeycombs (NMO-HCs) and 3D-rectangular nanosheets (NMO-RNS), are proposed and synthesized via a two-step wet-chemical process and characterized for their potential application in ethanol gas sensors. The successful formation of the different nanostructures and the even distribution of all the elements are demonstrated using various morphological and energy-dispersive X-ray spectroscopic (EDAX) analyses. In addition, N₂-adsorption/desorption analysis is performed to confirm the highly porous nature and maximum surface areas of 30.4 m² g⁻¹ and 91.3 m² g⁻¹ at 77 K for the NMO-HCs and NMO-RNS, respectively. Furthermore, gas sensing analysis reveals that the NMO-HCs-based sensor provides a maximum gas response (R_a/R_g) of ~15.2 while the NMO-RNS-based sensor exhibits a 5.5 times higher maximum gas response of ~78.9 at the optimal operating temperature. Further, using R_a/R_g = 1.2 as the detection limit point, the lower limit of ethanol gas detection is found to be 0.2 ppm for the NMO-RNS-based device. Finally, the probable gas-sensing mechanism of the NMO-RNS is discussed. The present study indicates that the NMO-RNS sensor can sense ppb-levels of ethanol and is suitable for the monitoring of outdoor and indoor air quality in practical applications.