Structural and antibacterial behaviors of tin dioxide thin films on copper substrates

Abstract

Tin dioxide (SnO₂) thin films were deposited on copper substrates using a spin-coating technique, where a precursor solution of tin chloride (SnCl₂) in methanol and distilled water was applied. The films were annealed at 300 °C to enhance crystallinity. The study investigates how varying SnO₂ layer thicknesses (3 and 9 layers) on Cu substrate influence structural, optical, and antibacterial properties. Characterization techniques included X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–visible spectroscopy, and Fourier-transform infrared (FTIR). The study specifically examines the effects of varying SnO₂ layer numbers (3 and 9) on the properties of the composite. Results indicate that increasing SnO₂ layers affect surface roughness, grain size, optical characteristics, and antibacterial efficacy. XRD analysis revealed a shift in the diffraction pattern, indicating lattice expansion with more layers. SEM analyses identify increased porosity and decreased density, while AFM confirms an increase in roughness with the number of layers. The multilayer system exhibits anisotropic magnetic properties suitable for magnetic and spintronic applications, as evidenced by SQUID magnetometer measurements that reveal a 100 Oe difference in coercive fields between parallel and perpendicular field orientations. As SnO₂ thickness increases, bandgap energies decrease from 3.93 eV (Cu) to 3.89 eV (3 layers) and 3.81 eV (9 layers), as determined by Tauc plots assuming direct transitions (αhν)² vs hν, indicating tunable optical properties. The FTIR spectra of SnO₂/Cu films display characteristic SnO₂ bands and hydroxyl-related peaks, with increased SnO₂ layers resulting in reduced transmittance and more pronounced peaks. Antibacterial activity (n = 5 replicates) of the SnO₂/Cu composite against Pseudomonas aeruginosa improves with additional SnO₂ layers, showing statistically significant (p < 0.05, ANOVA) inhibition zones of 17 ± 0.81 mm for Cu, 21 ± 0.98 mm for 3 layers, and 32 ± 1.57 mm for 9 layers. The activity is attributed to the synergistic effects of Cu²⁺ ions and Sn⁴⁺ions, which generate reactive oxygen species that disrupt bacterial membranes, DNA, and cellular structures.