Alloying Mo0.5W0.5S2 Quantum Dots for Selective Detection of ppb-Level Ammonia Near Room Temperature

The detection of low-concentration ammonia is necessary to avoid harm to workers exposed to ammonia atmosphere for long period in the poultry farming and fertilizer industry. Transition metal dichalcogenides (TMDs) nanomaterials have shown good ability in ammonia sensing, while alloying TMDs (two chalcogenide or/and metal elements) with tunable electronic structures have drawn little attention in gas sensing. Herein, alloying Mo0.5W0.5S2 quantum dots (QDs) were prepared by a facile sonication exfoliation method and then for the first time tested for ammonia sensing. The crystal structure, geometric morphology, and elemental composition all proved well-alloyed Mo0.5W0.5S2 QDs. The gas sensing measurements demonstrated Mo0.5W0.5S2 QDs with good response to ammonia even at part per billion (ppb)-level near room temperature. The sensor also displayed good stability as well as superb selectivity to ammonia in the presence of potential interferences, such as methanol, acetone, benzene, and cyclohexane. The theoretical calculations revealed Mo and W atoms at edges (such as Mo0.5W0.5S2 (010)) of sheet-like QDs as the active sites for ammonia adsorption. Electrons donated by the adsorbed ammonia were combined with holes in p-type Mo0.5W0.5S2 QDs, and the concentration of the main charge carrier was reduced, resulting in resistance enhancement.