This study reveals the atomic-scale mechanism behind a reversible wettability transition in ZnO nanostructures, achieved purely through a simple wetting and ambient air-drying cycle. The surface switches from hydrophilic (contact angle <5^) to hydrophobic (CA 155^). Experiments show the transition is chemical in origin, involves no morphological change, and coincides with the passivation of surface oxygen vacancies, which are highly active sites for water adsorption and dissociation. Reactive force field molecular dynamics (ReaxFF-MD) simulations demonstrate that during slow evaporation, water-derived species form a complete, self-assembled monolayer which fills the oxygen vacancies. This monolayer is predominantly a hydrogen-terminated outer layer, where hydrogen atoms are locked within a robust, internal hydrogen-bonding network, which is a structure confirmed by vibrational spectroscopy. This passive, hydrogen-rich outer layer masks the underlying hydrophilic oxide, drastically reducing the surface’s ability to form new hydrogen bonds with water. The work provides a coherent atomic-scale explanation for achieving tunable hydrophobicity in an intrinsically hydrophilic metal oxide via a simple physical treatment, highlighting the governing role of evaporation-directed molecular reorganization at interfaces.
Akbari et al. (Fri,) studied this question.