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The development of gas sensors based on two-dimensional (2D) layered materials has received lots of focus attributing to their excellent gas sensitivity. Here, a black phosphorus (BP) gas sensor device is fabricated based on high-quality few-layered BP microribbons using a facile route. Although BP is well known to oxidize in ambient conditions, energy dispersive spectroscopy (EDS) mapping manifests that the few-layered BP microribbons undergo slight oxidation and contamination during the grinding process. It is interesting that the surface and side of BP microribbons have nanoscale thin films and step-like nanoscale thin films, respectively, owing to the in-plane slip of the few-layered BP microribbons in the process of grinding, which are different from the conventional BP bulk crystals. The layered BP microribbon gas sensor demonstrated a high response to lowconcentration NO 2 and a very low limit of detection (LOD) of 0.4 ppb of NO 2 under N 2 and air conditions, which is the lowest LOD for NO 2 detection reported so far. The mechanisms for excellently sensitive detection of NO 2 for the BP microribbons have been investigated by first-principles calculations combined with experiment results, revealing that the sensitization mechanisms of the BP microribbon sensor are abundant nanoscale thin films, an optimum bandgap range with optimal carrier concentration, a hierarchical homojunction structure, and strong adsorption energy to NO 2 . In addition, the BP microribbon sensor demonstrated high selectivity to NO 2 , a low LOD under a high relative humidity, and good repeatability. The reported results of the BP sensor may provide great promise for improving the performance of other 2D material-based gas sensors and may expand sensing applications.

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