La-Doped ZnO Nanorods for Photoexcitation-Induced Room-Temperature Ammonia Sensing

Abstract La-doped ZnO nanorods (NRs) were synthesized via a hydrothermal route to develop efficient room-temperature ammonia (NH3) gas sensors for industrial and environmental monitoring applications. Structural and morphological analyses confirmed the formation of vertically aligned wurtzite ZnO NRs, where La doping tailors the nanoscale morphology including nanorod diameter and packing density. Notably, 1 mol % La doping yields uniform, densely distributed nanorods, which are expected to promote accessible surface reaction sites and efficient charge transport pathways. X-ray photoelectron spectroscopy (XPS) and optical studies revealed effective La3+ incorporation, enhanced oxygen-vacancy-related defect states, and Burstein–Moss (BM)-type band gap widening up to 1 mol % doping. The optimized 1 mol % La-doped ZnO sensor exhibited a NH3 sensing response of ∼55 toward 25 ppm at room temperature (RT), with rapid response/recovery times of 30/10 s, and excellent long-term stability. Complementary density functional theory (DFT) calculations indicated that La doping modifies the ZnO electronic structure, introduces defect levels near the Fermi level, and enhances charge redistribution during NH3 adsorption, consistent with experimental trends. Under 365 nm UV illumination, the response further increased to ∼113 due to photoinduced charge carrier generation via both band-to-band and defect-mediated transitions. The combined influence of La-induced defects and photon activation leads to improved charge-transfer kinetics and superior sensitivity. These findings demonstrate a synergistic strategy for optimizing nanoscale surface activity and electronic interactions, offering an effective route toward high-performance, energy-efficient ammonia sensors operating at room temperature for real-time industrial leak detection and environmental monitoring.