Eco-Bifunctional CuO Thin films: Tuning antibacterial and photocatalytic response via molarity adjustment molarities using spray pyrolysis

Abstract Copper oxide (CuO) thin films were successfully deposited by spray pyrolysis using precursor solutions with different molarities (0.02, 0.05 and 0.1 M) in order to investigate the correlation between growth conditions and multifunctional properties. Structural analysis confirmed the formation of monoclinic CuO with improved crystallinity as the molarity increased. The crystallite size increased to \(\:\sim\)17 nm while the strain and dislocation density decreased, indicating enhanced crystal quality at higher precursor concentrations. In FTIR spectra, it was observed that the bonding at 760 and 912\(\:\:\)cm -1 which attributed to Cu-O stretching bonds which confirms the formation of CuO phase. Surface morphology analysis showed that higher molarity led to increased roughness and grain size to \(\:\sim\:\)61 nm and \(\:\sim\)1.9\(\:\times\:\)10 5 nm respectively, reflecting a smoother and more homogeneous surface. Optical studies revealed a decrease in optical bandgap to 1.41 eV at 0.1 M. The Urbach energy increased to 0.89 eV at the same molarity indicating a low disorder and defect density in the films. Surface morphology analysis showed that higher molarity led to increased roughness and grain size to \(\:\sim\:\)61 nm and \(\:\sim\)1.9\(\:\times\:\)10 5 nm respectively. Electrical measurements demonstrated a significant enhancement in conductivity with increasing molarity reaching to 7.88 x 10 13 cm -3 , driven by the combined effect of higher carrier concentration and improved mobility due to reduced grain boundary scattering. Photocatalytic performance evaluated through methylene blue (MB) degradation under natural sunlight revealed that the 0.02 M film exhibited the highest degradation efficiency ~70.21%, which is attributed to its higher surface area ~ 67.91 m 2 /g and greater density of active sites. In contrast, antibacterial activity tests showed that films prepared at higher molarity 0.1 M exhibited superior bactericidal performance, particularly against Gram-negative bacteria, due to enhanced Cu²⁺ ion release and increased generation of reactive oxygen species (ROS). These findings demonstrate a clear trade-off between photocatalytic and antibacterial performance, governed by precursor molarity. The study highlights molarity control as a simple and effective strategy for tailoring CuO thin films toward eco-friendly multifunctional applications in environmental remediation and antimicrobial coatings.