Predictive Modeling and Experimental Correlation of SnO2 Thin Films Synthesized by Ultrasonic Spray Pyrolysis for Gas Sensing Applications

Tin dioxide (SnO2) thin films were synthesized using ultrasonic spray pyrolysis at a fixed substrate temperature of 400°C and a nozzle-to-substrate distance of 5 cm. SnCl2·2H2O was used as the precursor with molarities ranging from 0.01 to 0.1 mol/L. The influence of precursor molarity on the structural, morphological, optical, and electrical properties of the films was systematically investigated using X-ray diffraction, scanning electron microscopy, UV-Vis spectroscopy, and four-point probe measurements. As molarity increased, a clear evolution in film structure was observed: crystallite size decreased, surface morphology transitioned from porous to dense, and resistivity showed non-linear variation. Lower molarity films were found to have larger grains, higher porosity, and higher resistivity features known to enhance gas sensing performance due to increased surface reactivity and charge carrier modulation. To predict and interpret these variations, two multivariable models were proposed to correlate the optical band gap the first based on crystallite size and molarity, and the second on resistivity and molarity. The models exhibited strong agreement with experimental values. The resistivity-based model proved more robust, with prediction errors below 1.8%, and as low as 0.08% in some cases, while the crystallite-size-based model exhibited larger deviations, particularly at higher concentrations (up to 4.2%). Despite the absence of direct gas sensitivity measurements, the structural and electrical insights obtained provide a solid foundation for anticipating sensor behavior. These results demonstrate that combining experimental characterization with predictive modeling offers an effective strategy for tailoring SnO2 thin films for chemiresistive gas sensor applications prior to fabrication.