Growing hydrophobicity on a smooth copper oxide thin film at room temperature and reversible wettability transition

The thin CuO film is acquired by a quick dip of copper in H2O2 solutions at room temperature. The CuO film appears smooth and exhibits superhydrophilic nature. The composition change cannot be verified by X-ray photoelectron spectroscopy but can be manifested by the water contact angle. In the ambient condition, the thickness of the oxidized layer and the surface hydrophobicity grow gradually, while the chemical composition of the overall oxidized film remains essentially unchanged. In the vacuum, the growth rate of the hydrophobicity is significantly elevated, revealing deoxidation on the upmost surface. Our results indicate that growing hydrophobicity on the CuO film is spontaneous and the reversible wettability transition can be observed by H2O2 oxidation and vacuum deoxidation.