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.