Facet‐Engineered (110)‐BiOI/Bi Interface on TiO ₂ for Ultra‐Sensitive and Fast Room‐Temperature Formaldehyde Detection

ABSTRACT Facet engineering offers an effective strategy to tailor atomic configurations and interfacial electronic structures in semiconductor‐based gas sensors, yet its role in modulating room‐temperature VOC detection remains insufficiently understood. Here, we report a facet‐dependent interface design that integrates (110)‐exposed BiOI with metallic Bi sensitization on TiO 2 (P25), forming a highly active ternary sensing composite (110‐TB@B) for ultra‐low‐concentration formaldehyde (HCHO) detection at room temperature. The (110) facets of BiOI expose abundant Lewis acid sites that significantly enhance the adsorption and activation of HCHO and oxygen species, while the incorporation of metallic Bi creates strong interfacial electronic coupling at the BiOI/Bi junction. In situ DRIFTS reveals that Bi sensitization accelerates the rate‐determining oxidation step from dioxymethylene to formate, thereby boosting overall redox kinetics. Benefiting from the synergistic effects of facet‐engineering, Bi‐mediated sensitization, and dual interfacial junctions (BiOI/Bi and BiOI/TiO 2 ), the 110‐TB@B sensor delivers an exceptional response of 4.5 to 1 ppm HCHO, rapid response/recovery times of 65 s/60 s, and an experimentally measured detection limit of 50 ppb. The sensor further exhibits excellent selectivity, humidity tolerance, reproducibility, and mechanical stability. This work provides a mechanistic understanding of facet‐dependent interfacial regulation and offers a generalizable strategy for designing high‐performance room‐temperature VOC sensors.