Single-Atom Ni-Modified SnO ₂ for Ultrasensitive NO ₂ 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 SnO 2 with tunable surface Ni dispersion, enabling a direct correlation between Ni coordination structure and NO 2 sensing behavior. Hydrogen treatment of SnO 2 (H-SnO 2 ) 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 NiO x clusters at higher loading. Among them, the single-atom-dominated Ni/H-SnO 2 sensor exhibits an ultrahigh response of 13,152 toward 1 ppm NO 2 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 NO 2 adsorption and efficient charge transfer via the single-atom Ni–O–Sn bonding structure. In contrast, the formation of NiO x clusters weakens NO 2 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 NO 2 detection and provides an atomic-level bonding engineering strategy for high-performance MOS-based gas sensors.