Co₃O₄/MoS₂ Nanostructures\nfor NO_<i>x</i> Sensing

Abstract

2D\ntransition metal dichalcogenides have performed exceptionally\nas the active layer for chemiresistive gas sensors. Combining these\nmaterials with semiconductor oxides of tunable properties has proved\nto improve gas sensing and overall device performance due to the synergizing\neffect of the hybrid nanostructures. In this manuscript, we report\nthe synthesis of a Co₃O₄/MoS₂ nanostructure-based\nhighly sensitive chemiresistive gas sensor selective toward NO_<i>x</i> gases. An increase in air pollution has\ncaused an equal increase in the concentrations of toxic NO_<i>x</i> gases in the atmosphere. Exposure to these gases\nleads to grave health hazards such as pulmonary diseases and cardiovascular\ndiseases. Furthermore, recent studies prove that NO_<i>x</i> gases are also a contributor to COVID-19 fatality. We investigated\nthe effect of the change in precursor concentration of cobalt nitrate\n(CoN₂O₆) and temperature on the gas sensor response.\nThe precursor concentration was varied over an increasing range of\nmolarities (1, 5, 10, and 25 mM), and it was observed that the gas\nsensor with a precursor concentration of 25 mM and an operating temperature\nof 200 °C exhibited the highest response of 145.7% toward NO₂ gas (4.3 ppm) and then 105.37% toward NO (2.75 ppm). It was\nalso noted that the device responded to NO₂ gas of concentration\nas low as 300 ppb. This device was then subjected to an increasing\nrange of temperatures (50, 100, 150, 200, 250, and 300 °C). A\nclear increase in the device performance was observed with an increase\nin temperature. It was found that the gas sensor was the most sensitive\ntoward NO₂ gas (4.3 ppm) and exhibited a response of 186.2%\nat 250 °C followed by NO (2.75 ppm) with a response of 141.6%.\nA stable and excellent response toward a low concentration of 50 ppb\nof NO₂ was observed. Two activation energies (<i>E</i>_a) were calculated from the Arrhenius plot<i>E</i>_a1 (0.846 eV) between 150 and 200 °C and <i>E</i>_a2 (1.316 eV) between 200 and 250 °C, indicating\nmultiple energy trapping. These results pave a way for a plausible\napplication of Co₃O₄/MoS₂ hybrid\nnanostructures for the detection and monitoring of NO_<i>x</i> gases in the air.