Facile engineering of TiO2@Pd based hybrid heterogeneous nanostructures for enhanced NO2 gas sensing at room temperature under UV activation

Highly sensitive sub-ppb level NO₂ gas sensors based on hybrid heterostructures made out of metal nanoparticles (NPs) and porous metal oxide semiconductors are of great significance in environmental monitoring and medical diagnosis. Herein, we report the successful synthesis of a Pd nanoparticle decorated porous TiO₂ nanostructure-based hybrid Schottky heterostructure and its application in room temperature (RT ∼ 30 °C) low-ppb level NO₂ detection under UV activation. The optimized TiO₂@Pd 50 hybrid heterogeneous nanostructures (HNs) showed an excellent NO₂ (100 ppm) gas sensing response of 175.42%, excellent cycling stability, outstanding selectivity, low detection limit (∼82 ppb) and fast response/recovery speed (48/72 s) under UV illumination. Moreover, the as-prepared hybrid sensor demonstrates reliable reproducibility and robust sensing responses under various elevated temperatures (30-200 °C) and relative humidity conditions (0-80% RH) towards NO₂. The excellent RT NO₂ gas sensing properties of TiO₂@Pd 50 HNs are primarily attributed to the improved surface area, enhanced oxygen vacancies, low charge carrier recombination, and catalytic effect triggered by the hot electrons induced, from the localized plasmonic resonance effect of Pd NPs, resulting in effective NO₂ adsorption/desorption dynamics. The Schottky junction formation at the hybrid TiO₂@Pd 50 HNs interface effectively aids in the separation of charge carriers via band bending, leading to excellent charge transfer between the NO₂ molecules and the heterointerface under UV light, yielding substantial sensitivity and selectivity to NO₂. The present work is expected to provide a strategic route for designing and fabricating sub-ppb level high-performance NO₂ sensors based on hybrid heterostructures.