Controlling by Defects of Switching of ZnO Nanowire Array Surfaces from Hydrophobic to Hydrophilic: First-Principles Calculations

One of the promising applications of nanowires is controlling their hydrophobicity. This can be achieved by modifying the structure and chemical composition of the nanowire surfaces. One-dimensional crystalline nanostructures, such as nanowires, are particularly interesting for wettability control, as superhydrophobic surfaces can be created by precisely adjusting the height and spacing between the nanowires. Zinc oxide (ZnO) nanowires attract significant attention due to their unique properties for superhydrophobic applications. Understanding the interaction between water molecules and ZnO nanowire surfaces is key to investigating their wetting properties and potential use in hydrophobic or hydrophilic coatings. In this study, first-principles calculations were employed to investigate the influence of crystallographic orientation and defects on the wettability of ZnO nanowires, as well as the energy landscapes for water molecule migration along ZnO nanowire surfaces. The results showed that the polar surfaces of ZnO nanowires exhibit hydrophobic properties, while the non-polar side surfaces are hydrophilic. The presence of surface defects in ZnO nanowires, such as oxygen or zinc vacancies, does not qualitatively change the wettability of the surfaces but increases the energy barriers of these processes. It is worth noting that the density of nanowires in the array, which was regulated by varying the distance between the wires, is not a critically important parameter, even when the ZnO nanowire surfaces are modified with defects. The qualitative nature of surface wettability remains unchanged when decreasing the distance between the wires, with only a slight increase in activation energies observed. The critical distances at which water molecules start interacting with the polar surfaces of ZnO nanowires were determined, providing insight into the spatial scale of surface-water interactions. The results of the study deepen the understanding of the mechanisms of reversible wettability of ZnO nanowires.