1. Introduction
Controllable wettability was realized on laser-treated copper (Cu) substrate surfaces with postprocessing [1,2]. In these reported methods, laser ablation technology was used for fabricating micro/nano structures and followed by postprocessing with a chemical agent or heat treatment to add the hydrophobic base to the surface. The wettability and morphology of these hydrophobic surfaces have been extensively studied, but the influence of surface chemical composition has not been clarified. In this study, we realized the superhydrophobic surface by simply irradiating the Cu substrate surface with a femtosecond laser beam under low fluence, and without any postprocessing. During the laser irradiation, the laser fluence was kept low to avoid laser ablation, and even the laser-induced periodic surface structures were not observed on the low-fluence treated surface. On the other hand, the low-fluence treated surfaces were confirmed with the formation of hydrophobic cuprous oxide (Cu2O) [2].
2. Experiment and results
The Cu substrate surface was scanned by femtosecond laser pulses following the pattern with a size of 0.5 mm x 0.5 mm with a wavelength of 1030 nm, a pulse duration of 700 fs, and a repetition rate of 100 kHz. The pulse energy is set to 1 µJ which is much lower than the ablation threshold energy obtained experimentally. The discoloration of the irradiated surface was observed by a microscopy image. According to the SEM image, nano protuberances were confirmed instead of the general microstructure due to laser ablation. According to the measurement of Raman spectroscopy (Fig. 1 (a)) and XRD (Fig. 1 (b)), we confirmed three vibrational modes attributed to Cu2O and an XRD peak associated with the lattice spacing of Cu2O (1 1 1), respectively. The depth of the oxide layer was also confirmed by the measurement of STEM-EDX. As shown in Fig. 1(c), a 28.4-nm oxide layer was observed on the laser-irradiated surface, whose thickness is about 3 times the natural oxide film of the untreated substrate. These results suggest that we successfully promoted the surface oxidation of the Cu substrate by the low-fluence laser irradiation. Furthermore, the product of Cu2O (1 1 1) has the lowest surface energy than other facets [3]. Interestingly, the facet of obtained Cu2O surface is the same as the main facet of the unprocessed substrate. Cu substrate surfaces with different Cu2O proportions (ratio of peak area: Cu2O/Cu) were fabricated for evaluating the relationship between hydrophobic base proportion and the surface contact angle. With the low surface energy due to Cu2O, the low-fluence treated Cu surface showed a controllable hydrophobicity with a controllable proportion of Cu2O. The contact angle of laser treated substrate surface almost linearly increases with the Cu2O proportion.
3. Conclusion
By a one-step method of low-fluence laser irradiation, we successfully fabricated hydrophobic surfaces on Cu substrates owing to the formation of low-free-energy Cu2O. The contact angle of the Cu surface exhibited enhanced hydrophobicity after this simple one-step process, showing a linear increase with the Cu2O proportion. This simple one-step technique would promote the application of laser-induced functional surfaces.
4. References
[1] J. Long et al., ACS Appl. Mater. Interfaces 7, 9858–9865 (2015).
[2] A. He et al., Appl. Surf. Sci. 434, 120-125 (2018).
[3] Y. Maimaiti et al., Phys. Chem. Chem. Phys. 16, 3036 (2014).
