Temperature-controlled Synthesis of ZnO Nanolayers and Nanocrystals: Photocatalytic and Antibacterial Activities

The present work describes the synthesis of zinc oxide nanoparticles (ZnO NPs) using Baccaurea macrocarpa peel extract and their potential applications as photocatalysts and antibacterial agents. The synthesis was conducted at different temperatures ranging from 25 to 120 °C. The chemical composition, crystal structure, optical characteristic, and surface morphology of the as-synthesized ZnO NPs, without subsequent calcination, were investigated using various spectroscopic and imaging techniques. Characterization results revealed that ZnO NPs synthesized at 25 and 40 °C formed semicrystalline nanolayers, whereas those synthesized at 90 and 120 °C exhibited a hexagonal wurtzite structure. Despite their difference in crystallinity, all the ZnO NPs exhibited comparable bandgap energies in the range of 3.26–3.45 eV and contained organic compounds likely originating from the B. macrocarpa peel extract. Among the synthesized samples, ZnO-90, consisting of wurtzite ZnO nanocrystals synthesized at 90 °C, exhibited the highest photocatalytic and antibacterial performance. They effectively degraded both anionic and cationic dyes, including methyl violet, indigo carmine, and methyl orange, under UV light irradiation and inhibited the growth of the Gram-positive bacterium Staphylococcus aureus and the Gram-negative bacterium Escherichia coli. These results indicate that the photocatalytic and antibacterial activities of ZnO NPs are strongly governed by the long-range atomic arrangement within their crystal structure.