Accelerating Carrier Transfer in Dual p–n Heterojunctions by Mo–N Coupling to Gain an Ultrahigh-Sensitive NO₂ Sensing at Room Temperature for Asthma Diagnosis

Sensitive gas detection performed on a semiconductor in the absence of heat and irradiation activation remains a substantial challenge. In this study, an activation-free NO₂ gas sensor was developed by integrating MoO_x and conductive polypyrrole (ppy) onto a TiO₂ nanotube array (TiNT) through a direct electropolymerization method from simple monomer and metallic ion precursors. Thanks to the abundant defects and Mo–N coupling, a sensing chip based on the as-formed double p–n heterojunctions (TiO₂/ppy and ppy/MoO_x) exhibited excellent NO₂ sensing performances in the absence of any activation, such as ultrahigh response (R_g/R_a = 11.96, 1 ppm), rapid response/recovery abilities (9/11 s), reliable repeatability, high selectivity, and storage stability. Importantly, the Mo–N coupling was shown to play a key role in accelerating the carrier transfer across the ppy/MoO_x interface, thus contributing to the outstanding sensing response and kinetics. With a subparts-per-billion theoretical limit of detection (LOD for NO₂ = 0.12 ppb), the proposed system represents the best activation-free NO₂ chemiresistive sensor reported to date. In addition to a pure target gas, the sensor is capable of analyzing trace NO₂ gas in complex exhaled air samples for asthma diagnosis. This study provides new insight for establishing the interface chemistry and tuning the charge transfer involved at semiconductor interfaces, enabling the design of activation-free gas sensors.