Reversible wettability of thermally treated Hanji decorated with supersonically sprayed ZnO nanoparticles

In this study, we aimed at transforming naturally superhydrophilic Hanji, a traditional Korean Han paper into its superhydrophobic version. This change is crucial for applications requiring water damage resistance, such as mold growth, abrasion, and degradation. The resulting superhydrophobic Hanji shows potential for diverse applications, including fabricating wallpaper, traditional window covers, and high-end wrapping paper. To achieve this, ZnO nanoparticles (NPs) were deposited on the surface of Hanji via supersonic spraying, resulting in morphological curvatures capable of accommodating nanoscale air pockets. Additionally, supersonic spraying facilitated the robust adhesion of ZnO NPs to the Hanji cellulose fibers. Notably, the hydroxyl groups from both ZnO and Hanji were removed through subsequent annealing, resulting in superhydrophobic ZnO-Hanji. The annealing-induced shrinkage and densification of Hanji enhanced its superhydrophobicity, a property not achieved using regular A4 paper. This substantiated the unique benefits of using Hanji to realize superhydrophobicity. The superhydrophobicity reverted to superhydrophilicity upon exposure to ultra-violet irradiation, initiating reactions of the –OH group from atmospheric moisture with electron–hole pairs on the ZnO surfaces. Consequently, tunable/reversible wettability of ZnO-decorated Hanji was achieved via simple supersonic spraying and annealing, without any chemical modifications.