Superhydrophobic surfaces via electroless displacement of nanometric Cu layers by Ag+
Introduction
The wettability of solids has become an area of intense research. Particularly, strong water repellent surfaces have many potential applications in microfluidics [1]. The wetting properties of a surface are determined, basically, by both its chemical nature and its roughness. Coating with a low surface energy material, such as silane, provides the hydrophobic character. In this way, advancing contact angles between 95° and 113° are obtained for water drops on evaporated Ag films, according to the silane chain length [2]. The presence of additional roughness induces an increase of the repellent character [3]. Superhydrophobic surfaces are defined as surfaces for which the contact angle of a drop of water is at least 150° and the contact-angle hysteresis is less than 10°. Numerous techniques have been reported in the literature for elaborating such surfaces, processing various materials, polymers, oxides or semiconductors [4]. Many chemical methods deal with metallic substrates, in bulk or film forms.
The surface of bulk polycrystalline metals can easily be roughened by wet chemical etching. In this way, Guo et al. [5] have fabricated lotus morphologies at the micro- and nanometer scales with Al and its alloys, exhibiting superhydrophobic surface properties after suitable coating. Qian and Shen have developed chemical etching on Al, Cu and Zn substrates for elaborating surfaces presenting a wide range of contact angles, according to the processing time [6]. The possibility of growing more or less ramified metallic microstructures owing to the diffusion limited aggregation (DLA) process is very attractive and occurs in electro-deposition [7] and electroless Cu deposition [8], [9]. Another way for fabricating complex metallic surfaces is displacement reactions involving metals with appropriate redox potentials. In this way, ramified Cu deposit has been obtained by Al displacement by Cu2+ [10] and Ag “metal forest” has been grown by Cu displacement by Ag+ [11]. These works are devoted to the analysis of DLA growing mechanism, and, later, Larmour et al. have reported how displacement reactions can be used for building superhydrophobic surfaces of Ag and Au layers, respectively, on bulk Cu and Zn substrates [12]. This method has been successfully extended to Ti substrate previously covered with a 30 μm thick Cu layer and further submitted to displacement by Ag+ [13]. A very promising development pointed out by Larmour et al. lies in the easy possibility of coating most substrates with Cu and, then, making a superhydrophobic surface.
In the present work, the possibility of making hydrophobic or superhydrophobic surfaces has been studied from Cu deposition of nanometer thick Cu layers on Si substrates. We report the study, mainly by scanning electron microscopy, of the morphology of the Ag layers obtained by Ag+ displacement of Cu, writing:Cu + 2Ag+ → Cu2+ + 2Ag
The study was performed with the same silver nitrate concentration and processing time as in Larmour's work [12], [13], as a function of the initial Cu thickness between 10 and 80 nm. In this case, the amount of Cu involved in the oxido-reduction reaction (Eq. (1)) is limited, conversely to the case of micron thick Cu layers, all other conditions being the same ones. Therefore, drastic morphological changes are expected for the grown Ag layer. In the purpose to modify some segregation effects that we have observed in the distribution of the Ag particles, the effect of addition of dimethyl sulfoxide (DMSO) in the aqueous solution of Ag+ is reported. DMSO is a dipolar aprotic solvent. Its high viscosity (2 times the one of water) and the specific interaction of Ag+ with the S atom of DMSO [14] are expected to change the reaction conditions of Cu displacement by Ag+. Above the temperature of 18 °C, DMSO and water are fully miscible and the phase diagram of H2O–DMSO present an eutectic point at the composition (H2O)2–(DMSO)1 occurring at −73 °C [15]. This composition has been chosen in this work.
After grafting of octadecanethiol on Ag, the wetting properties of the surfaces have been investigated by the determination of the contact angles of a drop of water of radius about 0.5 mm. The possible wetting regimes, according to Wenzel [16] or Cassie–Baxter [17], are discussed in the light of the morphological analysis.
Section snippets
Experimental
Copper layers of thicknesses between 10 and 80 nm were deposited at room temperature onto optically polished silicon substrates (dimensions 2 cm × 3 cm) by sputtering technique. The copper thickness was monitored by quartz balance and a sample was prepared for thickness verification by profilometry, in each run. The base pressure was about 10−3 Pa in the chamber. After exposure to ambient atmosphere, the copper coated substrates were rapidly dipped (within 1 min to avoid passivation) into a silver
Particle and film morphologies
Both optical and scanning electron microscopy observations show that the silver particles are not scattered totally independently from each others on the copper layer. Indeed, the particles form groups. For most areas of the surface, these groups are randomly scattered as in Fig. 1(a). However, in some areas the groups make aligned bands separated by bands free of particles having roughly the same width (Fig. 1(b)). The reason of such a periodic scattering (period of 17 μm) is not clear, but the
Conclusion
In this paper, the morphology of Ag layers obtained by displacement of Cu, in nanometric layers, by Ag+ has been investigated in view to prepare hydrophobic or superhydrophobic surfaces. The obtained Ag particles are always grouped on the surface, forming more or less dense bands in some places. It has been shown by SEM and EDX mapping that the oxidation of Cu is confined to areas close to the silver particles. This feature explains the particle grouping and limits their growth. Therefore, the
Acknowledgments
The authors are grateful to C. Cottin-Bizonne and C. Duez for very rewarding discussions on the wetting properties of surfaces, and to A. Rivoire, C. Boulé and X. Jaurand for their efficient assistance in SEM characterization.
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2014, Applied Surface ScienceCitation Excerpt :Furthermore, it has been noted that the roughness-induced hydrophobicity can be increased by creating a multiscale hierarchical structures by superimposing submicron features onto the micron-sized patterns [12]. Manufacturing procedures used for obtaining increased surface roughness include interference and conventional photolithography, mechanical roughening (sand blasting of metallic surfaces), electrochemical methods, powder spraying, nano-coatings, reactive ion etching, and electrospinning [13–30], among others. Several attempts have been made to use femtosecond laser ablation as a way to produce surface features that lead to hydrophobic or superhydrophobic behavior [31–36].
Facile fabrication of superhydrophobic surfaces on zinc substrates by displacement deposition of Sn
2013, Applied Surface ScienceCitation Excerpt :Xu et al. have fabricated stable superhydrophobic silver surfaces on zinc substrates by replacement deposition and surface modification with octadecyl mercaptan [25]. Similarly, Brenier et al. [26] and Gu et al. [27] also obtained superhydrophobic silver surfaces on copper or copper alloy substrates by electroless deposition of Ag and subsequent surface modification with octadecanethiol or n-dodecanethiol. Up to date, only a few reports have demonstrated that surface modifications were unnecessary to obtain superhydrophobicity by displacement depositions.
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