Achieving of bionic super-hydrophobicity by electrodepositing nano-Ni-pyramids on the picosecond laser-ablated micro-Cu-cone surface
Graphical abstract
Introduction
In the nature, super-hydrophobic surfaces can be found in various animals and plants, including leeches, lotus and the aquatic fern Salvinia molesta [[1], [2], [3], [4]]. Water droplets roll easily on these surfaces as the contact angles (CAs) exceed 150° and the corresponding sliding angles (SAs) are <10°, which yields unique properties such as being water-repellent [5], self-cleaning [[6], [7], [8]] and anti-icing [9]. Therefore, bionic methods for processing super-hydrophobic surfaces have attracted much attention, among which picosecond laser texturing of metal surfaces has drawn an enormous amount of research interest due to its advantages of high precision, limited thermal damage, controllability and low pollution [10,11]. However, roughening a metal surface with a laser makes it more hydrophilic because metals are intrinsically hydrophilic materials and have high-surface-energy. To reduce the surface energy after laser irradiation, a layer of low-surface-energy material is usually introduced via chemical modification [[12], [13], [14]]. However, chemically modified layers are usually associated with poor thermal and mechanical stability and are easily damaged in harsh environments. In addition, they affect the inherent properties of metals, such as the surface conductivity and reflectivity [15]. Therefore, techniques are desired to introduce hydrophobic surfaces without the need of chemical modification.
In the past few years, many studies were conducted on processing metals without chemical modification. Bhattacharya et al. [16] were the first to report roughness-induced hydrophobicity of a metallic surface with hydrophobic clustered Cu nano-wires, in which low-surface-energy materials were not involved. Specifically, a ‘two-scale’ roughness surface was made by depositing Cu-nano-wire arrays through porous anodic Al and the following vacuum-dried step. In addition, Kwon et al. [17] used nano-second laser ablation (LA) combined with insulation and electrodeposition to fabricate a micro-pillar array with a re-entrant structure of Cu on stainless steel, for which the water CA was up to 153°. Recently, Yang et al. [18] prepared the superhydrophobic surface with the CA of 153° and the SA of 5.5° on the Al surface without chemical modification via nanosecond laser ablation. They also systematically investigated the wettability transition mechanism by analyzing surface morphology and surface chemical compositions. Although some papers regarding different methods used for fabricating metallic superhydrophobic surfaces without chemical modification, a practical and efficient processing method for mechanical stable super-hydrophobic surface with Cassie-state stability at low temperature and suitable for most metals is relatively limited.
Herein, we fabricated biomimetic super-hydrophobic metal surfaces using different methods, i.e. LA and a sequential processing (LA, electropolishing and electrodeposition). The mechanism for micro-nano hierarchical structure formation was analysed and the condensation experiments were conducted to compare the hydrophobicity and Cassie-state stability of the LA and sequentially processed surfaces. After the analysis of the condensation mechanism, the durability of the sequentially processed surface was analysed by aging experiments. In addition, the sequential processing technique was applied on the inclined surfaces on which we successfully fabricated a super-hydrophobic layer containing micro-nano hierarchical structures.
Section snippets
Experimental details
The experimental device to prepare the hierarchical micro-nano structures comprised an ultra-fast picosecond laser system for the micro-structures and an electropolishing and electrodeposition system for the nano-structures, as shown in Fig. 1(a). A schematic of the overall sequential processing is shown in Fig. 1(b), including arranging the scanning path, LA, electropolishing and electrodeposition. The sample was a 99.9 wt% Cu plate (15 × 15 × 1 mm3). Before the picosecond LA, the sample was
Surface morphology
To compare the surface morphologies of the micro-nano hierarchical structures progressively formed by the techniques used in this study, three types of structures were prepared according to the processing sequence, namely Structure-1 (LA), Structure-2 (LA and electropolishing) and Structure-3 (LA, electropolishing and electrodeposition).
The sample with the periodic micro-nano structures shown in the top row of Fig. 2 (i.e. Structure-1) was fabricated by LA only. The period of these micro-cones
Conclusion
A sequential fabrication technology has been proposed for bionic super-hydrophobic metallic surfaces via picosecond LA, electropolishing and electrodeposition. Using this technology, super-hydrophobic surfaces with micro-nano hierarchical structures were prepared successfully. The most significant advantages and conclusions of sequential processing technique can be summarized as following:
- (1)
Compared with a surface prepared by LA, a sequentially processed surface is covered with densely
Acknowledgement
The authors would like to thank the support of National Natural Science Foundation of China (No. 51675242; No. 11504144), Six Talent Peaks Project in Jiangsu Province (No. GDZB-019), Natural Science Research Project for Universities in Jiangsu Province (18KJB460005) and National Key Laboratory of Science and Technology on Vacuum Technology and Physics (No. 614220701050817).
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2022, Applied Surface ScienceCitation Excerpt :In contrast, the generation of NiO can be attributed to the easy spontaneous oxidation of Ni atoms on the surface of the deposited coating in the air [39], and this behavior of Ni can replace the oxidation of MoS2 nanoflakes under air storage conditions. At the same time, due to the natural hydrophobicity of NiO generated by Ni oxidation [40,41] can isolate the ambient humidity around the MoS2 nanoflakes and further prevent the oxidation of MoS2 nanoflakes, so the Ni-MoS2 composite coating will have a long storage life in the air. Further quantitative analysis of the Ni-containing substances by the deconvolution results of the components of Ni 2p3/2 yields a Ni:NiO: Ni(OH)2 composition percentage of 3.1:47.0:49.9 outside the wear track, and the Ni peak area inside the wear track is almost reduced to 0, with a NiO:Ni(OH)2 composition percentage of 50.3:49.7.