Controlled growth of 3D assemblies of edge enriched multilayer MoS2 nanosheets for dually selective NH3 and NO2 gas sensors

Abstract

Herein, we report on the successful controlled growth of edge enriched 3D assemblies of MoS₂ nanosheets by adjusting the gas flow rate during atmospheric pressure CVD. The results revealed that 30 ml min⁻¹ was the optimal flow rate, in which the growth direction shifts from in-plane nanosheets to out-of-plane 3D assemblies of MoS₂ nanosheets. It is suggested that, at this flow rate, we have an ideal tradeoff between the surface interaction and the mass transport of precursor molecules. Morphological, structural and chemical composition analyses showed the formation of vertically oriented MoS₂ nanosheets with MoO_3−x impurities, resulting from the incomplete sulfurization during the synthesis. Based on the morphological evolution of the studied material, the growth mechanism was explored. The gas sensing properties of the as grown films were tested against NH₃ and NO₂. They exhibited stable and reproducible responses with excellent sensitivity to ppm-level NH₃ (20% response to 2 ppm) and ppb-level NO₂ (11% response to 20 ppb). Additionally, this nanomaterial showed dual selectivity towards NH₃ at room temperature and NO₂ at 100 °C. To the best of our knowledge, none of the reported studies on MoS₂ based gas sensors have described this dual selectivity. The experimental detection limit was below 2 ppm for NH₃ while it was below 20 ppb for NO₂. Besides, the vertical growth of edge enriched MoS₂ bestows the sensors with notable resilience against high levels of ambient humidity. The sensor response was only slightly increased (R = 70%) in a humid environment compared to a dry background (R = 65%) when measuring 800 ppb of NO₂. Therefore, this paper shows for the first time that by adjusting the flow rate it is possible to tune the morphology of AP-CVD grown MoS₂ for achieving a 3D sponge-like assembly of nanoflakes, showing high sensitivity to NO₂ and NH₃ and low humidity cross-sensitivity. In addition, the nanomaterial can be made quite specific for detecting NO₂ or NH₃ by selecting its operating temperature.