Synergistic effects of ruthenium doping on the structural and ammonia sensing properties of In₂O₃ thin films

The present work reports the preparation of ruthenium (0, 1, 3 and 5 wt%)-doped indium oxide thin films using the nebulizer spray pyrolysis (NSP) technique and the characterization of their physical properties. The novelty of this work lies in the systematic Ru-concentration-dependent defect engineering of In₂O₃ thin films, in which the synergistic effects of Ru-induced catalytic activation, oxygen-vacancy-related defect states, and interfacial charge transfer are optimized to achieve enhanced room-temperature NH₃ sensing. XRD analysis confirmed the cubic bixbyite structure with a slight increase in crystallite size upon Ru doping, suggesting the successful incorporation of Ru ions into the In₂O₃ lattice. UV-vis analysis confirmed the presence of a direct band gap with a small redshift attributed to Ru-induced defect states. FESEM images showed increased surface roughness and porosity upon doping, which are beneficial for gas adsorption. XPS measurements verified the incorporation of Ru into the In₂O₃ matrix. Photoluminescence spectra showed enhanced charge separation and an increase in oxygen-vacancy concentration at the optimum Ru concentration of 3 wt%. Gas sensing studies revealed that the 3 wt% Ru-doped In₂O₃ thin film exhibits an excellent response toward NH₃ at room temperature, with a maximum response of 1140 for 250 ppm NH₃, along with response and recovery times of 17.3 s and 20.1 s, respectively. The sensing measurements were carried out in a controlled chamber using calibrated NH₃ concentrations, and the reported values represent averaged results from multiple sensing cycles, ensuring good reproducibility and reliability. The improved sensing performance is attributed to the catalytic effect of Ru, defect-state-driven adsorption sites, and optimized surface morphology. The sensor also exhibited excellent selectivity, reproducibility, stability, and enhanced gas response under humid conditions. These findings demonstrate that Ru-doped In₂O₃ thin films are promising candidates for high-performance room-temperature ammonia gas sensors.