Room temperature ppt-level NO₂ gas sensor based on SnO _x /SnS nanostructures with rich oxygen vacancies

Abstract

Abstract In this paper, tin oxidation (SnO x )/tin-sulfide (SnS) heterostructures are synthesized by the post-oxidation of liquid-phase exfoliated SnS nanosheets in air. We comparatively analyzed the NO 2 gas response of samples with different oxidation levels to study the gas sensing mechanisms. The results show that the samples oxidized at 325 °C are the most sensitive to NO 2 gas molecules, followed by the samples oxidated at 350 °C, 400 °C and 450 °C. The repeatabilities of 350 °C samples are better than that of 325 °C, and there is almost no shift in the baseline. Thus this work systematically analyzed the gas sensing performance of SnO x /SnS-based sensor oxidized at 350 °C. It exhibits a high response of 171% towards 1 ppb NO 2 , a wide detecting range (from 1 ppb to 1 ppm), and an ultra-low theoretical detection limit of 5 ppt, and excellent repeatability at room temperature. The sensor also shows superior gas selectivity to NO 2 in comparison to several other gas molecules, such as NO, H 2 , SO 2 , CO, NH 3 , and H 2 O. After x-ray diffraction, x-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscope, and electron paramagnetic resonance characterizations combining first principle analysis, it is found that the outstanding NO 2 sensing behavior may be attributed to three factors: the Schottky contact between electrodes and SnO x /SnS; active charge transfer in the surface and the interface layer of SnO x /SnS heterostructures; and numerous oxygen vacancies generated during the post-oxidation process, which provides more adsorption sites and superior bandgap modulation. Such a heterostructure-based room-temperature sensor can be fabricated in miniaturized size with low cost, making it possible for large-scale applications.