Morphological Control of Cu<sub>2</sub>O Particles via NaCl Addition and their Antibacterial Activity

Cuprous oxide (Cu<sub>2</sub>O) particles have attracted significant attention due to their low cost, non-toxicity, and abundance with applications spanning solar energy conversion, photocatalysis, sensing, and antimicrobial fields. The antibacterial activity of Cu<sub>2</sub>O particles is strongly influenced by their morphology, which determines the exposure of specific crystal facets with distinct surface properties. In this study, the morphological evolution of Cu<sub>2</sub>O particles was systematically investigated by varying the water content in an ethanol–water mixed solvent and adjusting the NaCl concentration during synthesis. Increasing the water content (0–20 vol%) favored growth along the [100] direction while exposing the (111) facets, resulting in the formation of octahedra. In contrast, the addition of NaCl (Cl:Cu = 1:70–1:40) led to preferential chemisorption of Cl<sup>-</sup> ions on the (100) planes, inhibiting the growth of these planes and ultimately inducing the formation of cubes. Morphological changes were confirmed by X-ray diffraction and scanning electron microscopy. Antibacterial assays against S. aureus and E. coli at a concentration of 150 µg/mL verified the morphology-dependent antibacterial performance. In particular, octahedral Cu<sub>2</sub>O particles (Cl:Cu = 1:70) exhibited excellent antibacterial activity with an 88.4% reduction of E. coli. These findings demonstrate that the proposed approach provides a scalable and controllable synthesis strategy for precise morphological control that enables the modulation of exposed crystal planes and enhances the functional performance of Cu<sub>2</sub>O particles.