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In this work, alkali metal Rb-loaded ZnO/In₂O₃ heterojunctions were synthesized using a combination of hydrothermal and impregnation methods. The morphology and structure of the synthesized samples were characterized by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. The enhancement mechanism of the nitrogen dioxide gas sensing performance of the Rb-loaded ZnO/In₂O₃ heterojunctions was systematically investigated at room temperature using density-functional theory calculations and experimental validation. The experimental tests showed that the Rb-loaded ZnO/In₂O₃ sensor achieved an excellent response value of 24.2 for 1 ppm NO₂, with response and recovery times of 55 and 21 s, respectively. This result is 20 times higher than that of pure ZnO sensors and two times higher than that of ZnO/In₂O₃ sensors, indicating that the Rb-loaded ZnO/In₂O₃ sensor has a more pronounced enhancement in performance for NO₂. This study not only revealed the mechanism by which Rb loading affects the electronic structure and gas molecule adsorption behavior on the surface of ZnO/In₂O₃ heterojunctions but also provides theoretical guidance and technical support for the development of high-performance room-temperature NO₂ sensors.