Atomic‐Level Built‐in Electric Field Drives Sub‐ppb NO₂ Detection in Fe‐Doped BiOCl at Room Temperature

Chemiresistor sensors are essential for real‐time environmental monitoring, yet conventional devices suffer from persistent limitations: high operating temperatures, insufficient sensitivity, poor selectivity, and long‐term instability. To overcome these challenges, an atomic‐level built‐in electric field (BIEF) strategy is proposed through Fe‐doped BiOCl (Fe‐BOC), which drives directional electron migration within a single‐component material with hierarchical nanoflower structures. This atomic‐level BIEF, generated by asymmetric Fe─O─Bi sites, synergizes with dynamic Fe 2+ /Fe 3+ valence transitions to construct an “electric field engine”. This engine promotes cyclic electron flow and enhances oxygen activation, achieving ultra‐sensitive NO 2 detection at 117 ppt with a rapid response time (<10 s) and outstanding stability (>6 months) at room temperature. The universality of this design is demonstrated by extending it to Cu and Sn dopants, providing a scalable platform for next‐generation gas sensors. A wireless sensing system further validates its real‐world applicability.