Influence of Sn and Al Doping on ZnO thin films: A Study of Structural, Optical, and Langmuir Adsorption Properties for Photocatalytic Applications

Zinc oxide (ZnO) thin films have garnered significant interest for their applications in optoelectronics and environmental remediation due to their exceptional optical, electrical, and photocatalytic properties. However, the high resistivity and rapid charge recombination of pure ZnO necessitate doping to enhance its performance. In this study, ZnO thin films doped with tin (Sn) and aluminum (Al) were synthesized via a cost-effective pneumatic spray technique. The structural, optical, and morphological properties of the films were systematically characterized using X-ray diffraction (XRD), UV-Vis spectrophotometry, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). The results indicate that Sn and Al doping significantly influence ZnO’s crystallinity, bandgap energy, and surface morphology. The optimal crystallite size was obtained for 1 wt.% Sn (37.98 nm) and 5 wt.% Al (48.63 nm), while excessive doping (>3 wt.%) introduced microstrain (10.41 × 10⁻⁴ for Sn and 7.13 × 10⁻⁴ for Al), reducing crystallinity. The optical bandgap decreased from 3.254 eV (pure ZnO) to 3.142 eV (1 wt.% Sn) and 3.152 eV (5 wt.% Al), accompanied by increased Urbach energy (0.34 eV for 5 wt.% Al). The highest optical transmittance (86%) was observed for 3 wt.% Al-doped ZnO. Pure ZnO exhibited the highest photocatalytic efficiency, achieving 85% methylene blue degradation under solar irradiation. Langmuir adsorption modeling revealed that Sn-doped ZnO exhibited the highest adsorption capacity (1.422 mg/g), followed by Al-doped ZnO (0.617 mg/g) and pure ZnO (0.495 mg/g). These findings emphasize the critical role of doping concentration in optimizing ZnO thin films for advanced photocatalytic and optoelectronic applications.