The role of individual Er and Tb incorporation in CO2 gas-sensing properties of hydrothermally grown ZnO nanorods

This study systematically investigated the role of independently doped Erbium (Er) and Terbium (Tb) incorporation on the structural, morphological, optical, and carbon dioxide (CO2) gas-sensing properties of ZnO nanorods synthesized by the hydrothermal method with dopant concentrations of 0, 1, 3, 5, and 7 wt%. X-ray diffraction (XRD) analysis confirmed the successful substitutional incorporation of Er and Tb ions into the ZnO lattice, preserving the hexagonal wurtzite structure without the formation of secondary phases, except at the highest Tb concentration. These findings are in good agreement with scanning electron microscopy (SEM) results, which revealed vertically aligned ZnO nanorods with uniform spatial distribution and apparent diameters below 100 nm. Further, dopant incorporation significantly influenced nanorod morphology and interfacial characteristics, including variations in diameter uniformity, surface charge homogeneity, and ZnO shell composition and thickness due to differences in ionic radii, valence states, and outer-shell electron configurations of the Er³⁺ and Tb³⁺ dopant ions. This modified local strain fields and defect chemistry within the ZnO matrix. Optical characterization also showed a systematic reduction in optical bandgap energy with increasing dopant content. This is consistent with bandgap narrowing induced by enhanced carrier concentration and defect-related band tailing, accompanied by noticeable changes in optical transmittance. CO2 gas-sensing experiments displayed that rare-earth doping markedly enhanced sensor performance compared to that of undoped ZnO nanorods. In particular, 5 wt% Er-doped semiconductor exhibited the highest sensitivity, along with rapid response and recovery times, indicating an optimal balance between surface reactivity, charge transport, and defect-mediated adsorption-desorption dynamics. To sum up, the experimental results and theoretical approach obtained highlighted Er- and Tb-doped ZnO nanorods as promising low-cost, chemically stable, and high-performance sensing materials for CO2 detection in environmental and industrial monitoring applications.