Single-Atom Ni-Modified SnO2 for Ultrasensitive NO2 Gas Sensing through Enhanced Molecular Adsorption and Efficient Charge Transfer.

Precisely introducing single-atom sites into metal oxide semiconductors (MOS) is essential for achieving ultrasensitive and selective detection of trace gases; however, the structure-dependent role of these atomically dispersed sites remains insufficiently understood. Here, we report a defect-assisted strategy to construct Ni-modified SnO2 with tunable surface Ni dispersion, enabling a direct correlation between Ni coordination structure and NO2 sensing behavior. Hydrogen treatment of SnO2 (H-SnO2) produces abundant surface defects that stabilize isolated Ni atoms through Ni-O-Sn coordination. Aberration-corrected scanning transmission electron microscopy and X-ray absorption spectroscopy unambiguously confirm the formation of atomically dispersed Ni sites and their aggregation into NiOx clusters at higher loading. Among them, the single-atom-dominated Ni/H-SnO2 sensor exhibits an ultrahigh response of 13,152 toward 1 ppm NO2 at 175 °C, together with excellent selectivity, good long-term stability, and a detection limit down to 10 ppb. Experimental results combined with density functional theory calculations reveal that the superior sensing performance originates from the synergy of strong and selective NO2 adsorption and efficient charge transfer via the single-atom Ni-O-Sn bonding structure. In contrast, the formation of NiOx clusters weakens NO2 adsorption and deteriorates charge-transfer efficiency, leading to reduced sensing performance. This work identifies the single-atom Ni-O-Sn bonding as the decisive active structure for ultrasensitive NO2 detection and provides an atomic-level bonding engineering strategy for high-performance MOS-based gas sensors.