Unleashing the\nPotential of Tailored ZnO–MgO\nNanocomposites for the Enhancement of NO<sub>2</sub> Sensing Performance\nat Room Temperature

Surface functionalization of semiconducting metal oxides\nhas emerged\nas a highly effective approach for enhancing their sensing capabilities.\nIn the present work, the surface of randomly oriented zinc oxide (ZnO)\nnanowires is modified with an optimized thickness (7 nm) of magnesium\noxide (MgO), which exhibits an exceptionally sensitive and selective\nbehavior toward NO<sub>2</sub> gas, yielding a response of approximately\n310 for 10 ppm concentration at room temperature. The synergistic\ninterplay between ZnO and MgO leads to a remarkable 20-fold improvement\nin sensor response compared to a pristine ZnO film and allows the\ndetection of concentrations as low as 50 ppb. The ZnO–MgO composite\nwas characterized using X-ray diffraction (XRD), XPS, and SEM-EDS\nto gain structural, compositional, and morphological insights. The\ninteraction of the NO<sub>2</sub> molecule with the sensor film was\ninvestigated using density functional theory (DFT) simulations, revealing\nthat oxygen vacant sites on the MgO surface are most favorable for\nNO<sub>2</sub> adsorption, with an adsorption energy of −3.97\neV and a charge transfer of 1.74e toward NO<sub>2</sub>. The XPS,\nphotoluminescence (PL), and EPR measurements experimentally verified\nthe presence of oxygen vacancies in the sensing material. The introduction\nof localized levels within the band gap by oxygen vacancies significantly\npromotes the interaction of gas molecules with these sites, which\nenhances the charge transfer toward NO<sub>2</sub> gas molecules.\nThis augmentation has a profound influence on the space charge region\nat the ZnO–MgO interface, which is pivotal for modulating the\ncharge transport in the ZnO layer, resulting in the substantial improvement\nof NO<sub>2</sub> response at room temperature.