Article pubs.acs.org/Langmuir
Multilevel Hierarchy of Fluorinated Wax on CuO Nanowires for Superoleophobic Surfaces J. Y. Lee,†,‡ S. Pechook,§,∥ B. Pokroy,*,§,∥ and J. S. Yeo*,†,‡ †
School of Integrated Technology, Yonsei University, 162-1, Songdo-dong, Yeonsu-gu, Incheon 406-840, Republic of Korea Yonsei Institute of Convergence Technology, Yonsei University, Incheon 406-840, Republic of Korea § Department of Materials Science and Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel ∥ Russell Berrie Nanotechnology Institute, Technion-Israel Institute of Technology, Haifa 32000, Israel ‡
ABSTRACT: We demonstrate the multilevel hierarchy of nanoscale wax crystals on nanowire (NW) structures that strongly repels not only water but also olive oil and hexadecane. We deposited C24F50-fluorinated wax (F-wax) using thermal evaporation on the surface of CuO NWs. Fluorinated wax crystals are self-assembled on the CuO NWs forming three-dimensional hierarchical structures. The achieved multilevel hierarchy has strongly repelled water, glycerol, ethylene glycol, and olive oil with contact angles (CAs) exceeding 160°. When sufficient F-wax is crystallized on the CuO NWs, crystals that are assembled perpendicularly to the longitudinal NW axis form a re-entrant curvature allowing superoleophobic characteristics with strong repellence of hexadecane with CAs of ∼150° and a small contact angle hysteresis of 150° and lowers the CA hysteresis to 90° as a metastable state.5,21−27 Received: October 11, 2014 Revised: November 27, 2014
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dx.doi.org/10.1021/la5040273 | Langmuir XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic representation of the structural evolution from nanowire to multilevel hierarchy for the improvement of surface wettability. (b) Scanning electron microscopy micrograph of CuO nanowires grown by the solution method. (c) X-ray diffraction data of CuO NWs, FOTScoated CuO NWs, and hierarchical C24F50 wax on CuO NW structures with varying amounts of deposited wax.
strated the formation of triple hierarchy via the self-assembly process of paraffin wax on CuO nanowires (NWs), resulting in a re-entrant curvature. These wax-on-nanowire structures were able to repel liquids with a surface tension lower than that of water, down to 47.7 mN m−1.38 Broadening the range of liquids repelled by our wax-on-NW surfaces requires the replacement of paraffin wax with C24F50fluorinated wax. We fabricate highly ordered hierarchical structures composed of nanoscale fluorinated wax crystalline structures assembled on top of CuO NWs to achieve superoleophobic characteristics. We have analyzed several CuO NW-based structures, unary structures [surface modified with trichloro(1H,1H,2H,2H-perfluorooctyl)silane (FOTS) self-assembled monolayers (SAMs)] and several hierarchical structures with varying masses of evaporated fluorinated wax (Figure 1a), and identified the optimal structure, i.e., crystal density and alignment, for superoleophobic performance.
In recent years, there has been increasing scientific and industrial interest in superoleophobic properties. A superoleophobic surface can repel low surface tension liquids such as oils and alcohols.22,26,28−31 To achieve superoleophobicity, surface structures with convex or re-entrant curvature are required. Previous studies have shown superoleophobic surfaces using highly rough,32 partially convex,22,25,26,28,31 and threedimensional textures27,33 covered by extremely low surface energy materials. Bioinspired superhydrophobic surfaces utilizing wax crystalline structures have been investigated in recent years. Various types of waxes were used for the fabrication of nonwettable surfaces, from natural waxes such as lotus wax34 and wheat wax35 to artificial waxes such as paraffin waxes.8,35,36 Thermally evaporated wax provides additional surface roughness in addition to low surface energy and thus allows surface superhydrophobicity.8 For the formation of superoleophobic surfaces, fluorinated wax (C24F50) was selected because of its extremely low surface energy.33,37 Thermally evaporated perfluorotetracosane-fluorinated wax (F-wax) has strongly repelled various liquids such as alcohols and oils because of its low surface energy and the formation of hierarchical structure with re-entrant curvature.33 We previously demon-
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EXPERIMENTAL SECTION
CuO Nanowire Unary Structure Formation. The first-level hierarchy was composed of copper oxide nanowires. CuO NWs were formed via an oxidation process in an ammonium ambient solution. Cut copper foils (4 cm × 4 cm) were cleaned via ultrasonication in B
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ethanol for 15 min and rinsed with deionized (DI) water. Cu was oxidized in an ammonium ambient solution [2.5 M NaOH and 0.1 M (NH4)2S2O8] at 4 °C for 10 min.39,40 Then, the samples were rinsed with DI water several times and dried on a hot plate at 180 °C for 2 h. While drying, the Cu(OH)2 nanostructure was transformed to CuO nanowires via dehydration.39,40 The as-grown CuO NW surface is a highly wettable surface because of the surface energy. To achieve hydrophobic properties, the as-prepared nanowire surface was chemically modified by a self-assembled monolayer (SAM) of trichloro(1H,1H,2H,2H-perfluorooctyl)silane (FOTS) in a vacuum desiccator for 2 h. FOTS has an extremely low surface energy, thus allowing the formation of superhydrophobic CuO NWFOTS structures. Wettability of these unary structures was assessed via a contact angle measurement system. Deposition of Fluorinated Wax on CuO Nanowires. To fabricate a higher-level hierarchy of fluorinated wax on CuO NW structures, perfluorotetracosane (C24F50) wax (Sigma-Aldrich) was thermally evaporated using a Moorfield MiniLab evaporator. The thermal evaporation procedure was conducted in a vacuum chamber at 6 × 10−6 mbar.33 Samples on a holder were positioned 12 cm above a crucible in which a varying amount (37−110 mg) of fluorinated wax was loaded. The system was slowly heated from 70 to 130 °C, and evaporation occurred at 110−130 °C over a 15 min period. Evaporated specimens were kept at room temperature (25 °C).33 Characterization. Surface morphologies of the fabricated unary and hierarchical structures were characterized using field emission scanning electron microscopy (FE-SEM) (JSM-7100F, JEOL) and high-resolution scanning electron microscopy (Zeiss UltraPlus HRSEM). To confirm the crystallinity of CuO NWs and crystalline wax, X-ray diffraction analysis (XRD) (Smartlab X-ray Diffractometer, Rigaku) was conducted on CuO nanowire surfaces, FOTS-coated CuO NW surfaces, and C24F50 wax on CuO NW hierarchical surfaces. Wettability of the surfaces was measured using contact angle measurement systems (Dataphysics, OCA15EC and DSA30E measuring system with DSA4 Drop Shape Analysis program, KRUSS) with various liquids: ultrapure DI water (72.2 mN m−1), glycerol (63.4 mN m−1), ethylene glycol (47.3 mN m−1), olive oil (32 mN m−1), and hexadecane (27.5 mN m−1) at room temperature. The contact angle (CA) and CA hysteresis (CAH) for each liquid were measured on unary structures and hierarchical structures. Dynamic contact angles and CAH on various surfaces were measured by continuous liquid injection and retraction using a microsyringe (Hamilton) and microneedle (inner diameter of 3 μm) was required to form a multilevel hierarchy because most of the wax was initially deposited to assemble microscale base structures.33 In contrast, wax deposited on the nanowires directly contributes to the formation of additional levels of hierarchy, where the base structure of the CuO nanowire generally provides chemical and thermal stability better than those of wax-based microstructures. The evolution of the shape and size of each structure is schematically illustrated in Figure 3a.
Figure 4. Optical image of a 100 pL water droplet on a FOTSmodified NW surface.
Laplace pressure. The hierarchical structure based on the nanowires allows a nonwetting property for >1000-fold smaller water droplets compared to the previously published work in which F-wax-coated surfaces could not maintain the superhydrophobic property for a droplet volume of 150°) on all examined surfaces (Figure 4), demonstrating the ability of the surfaces to withstand elevated D
dx.doi.org/10.1021/la5040273 | Langmuir XXXX, XXX, XXX−XXX
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Figure 6. (a) CA as a function of surface tension and (b) CAH as a function of surface tension measured on unary and hierarchical structures. CAs and CAHs are averaged values, and the standard deviation of CA is 100°. NW−C24F50110 mg surfaces were able to support both olive oil and hexadecane in an oleophobic wetting state. All samples remained in a Cassie wetting state for water and glycerol. Superoleophobic characteristics appeared on these surfaces with high CA values of ∼150° and low CAH values of
