Single cobalt atoms with unconventional dynamic coordination mechanism for selective ammonia sensor

Developing gas sensors that can simultaneously achieve high sensitivity and selectivity for the detection of a single-type gas remains a significant challenge. Herein we demonstrate cobalt (Co) single atoms with an unconventional dynamically changing coordination structure that could be used as NH₃-sensing material with superior sensitivity and selectivity. According to the steric effect of 2-methylimidazole (2MI) molecules and carbonyl groups on graphene, the Co single atom is evolved into a bidentate coordinated structure (Co-2MI-G). In-situ characterization and theoretical simulation reveal that the sensing mechanism of Co-2MI-G is the specific chemical adsorption between unsaturated coordinated Co single atoms and NH₃ molecules, causing a reversible switching of coordination number from 2 to 4, a valence state transfer from Co²⁺ to Co³⁺ of Co single atoms, and a band-gap width from 0.14 eV to 0.50 eV. Consequently, the Co-2MI-G-based gas sensor presents a sensing response of 67.598% for 1 ppm NH₃ and a limit of detection of 2.67 ppb, at least 1.8 times higher than that of state-of-the-art NH₃ sensors, together with robust stability and reproducibility. This work provides an innovative perspective on utilizing single atoms for ultra-selective gas sensing by coordination regulation.