Interface−Defect−Morphology Multivariable Engineering of Two-Dimensional BiOBr Nanosheets on TiO ₂ Nanotube Arrays for ppb-Level NO ₂ Detection at Room Temperature

Two-dimensional (2D) metal oxide semiconductor-based gas sensors usually suffer from limited sensitivity and sluggish recovery at ppb levels due to the intrinsic interlayer restacking and poor out-of-plane charge transport. Herein, TiO₂ nanotubular arrays (NTAs) acting as a scaffold were applied to grown 2D highly dispersed curved BiOBr nanosheets to form heterojunctions, thereby achieving full surface utilization in gaseous sensing reactions. Positron annihilation lifetime spectroscopy verified the presence of three types of defects (V_O, V_Br, and V_BrBiBr) in the as-formed BiOBr nanosheets; Monte Carlo simulations further revealed that these curved nanosheets exhibited a substantially increased target collision frequency and higher adsorption probability compared to planar structures. Using gaseous NO₂ molecules as the model target, the interface-defect-morphology synergistic effect enabled the resulting BiOBr/TiO₂ NTA composite to exhibit high activity in NO₂ sensing reactions even at room temperature, with a detection linear range from 1 ppb to 10 ppm (LOD = 0.12 ppb), sensitive response, excellent selectivity, satisfactory humidity tolerance, and superior operational stability (>60 days). <i>In situ</i> Raman analysis demonstrated that vacancy-mediated NO₂ adsorption contributed to the excellent sensing performance, which was further confirmed by strong NO₂ adsorption energy (-2.553 eV) and midgap defect state-triggered efficient charge transfer. This work not only provides an effective route for designing gas sensing materials but also paves a new way for preparing 2D materials with abundant active surfaces for catalytic applications.