Article pubs.acs.org/JPCC
Important Role of Nanopore Morphology in Superoleophobic Hierarchical Surfaces Takashi Fujii, Hina Sato, Etsushi Tsuji, Yoshitaka Aoki, and Hiroki Habazaki* Division of Materials Chemistry, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan ABSTRACT: This work reports the importance of nanopore morphology in designing super liquid-repellent submicropillar/ nanopore hierarchical surfaces. The hierarchical surfaces were fabricated using a combined process of oblique angle sputter deposition of aluminum with subsequent anodizing, and the surfaces were coated with a fluorinated alkyl phosphate layer to reduce the surface energy. The size of the nanopores, the interpore distance, and the porosity of the anodic films on the submicrometer pillars were controlled by varying the anodizing and pore-widening conditions. The present study demonstrates that super liquid repellency can be achieved on intrinsically oleophilic surfaces by introducing hierarchical submicropillar/nanopore morphology even for oils with surface energies as low as ∼25 mN m−1. The porosity in the submicrometer pillars was a key factor in influencing the contact angle hysteresis; higher porosity is needed to reduce the contact angle hysteresis.
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reduced to less than ∼6 mN m−1 for oils, which have a γl value in the range 20−30 mN m−1. It is known that a CF3-terminated surface (γs = 6 mN m−1) possesses the lowest surface energy, which only just satisfies the above requirement for making the surface superoleophobic by introducing surface roughness (θflat > 90°). However, experimentally observed contact angles on CF3-terminated surfaces for oils with γl values of less than 30 mN m−1 have been shown to be less than 90°. Thus, the superoleophobic surfaces reported in the literature so far are mostly limited for oils with γl values of more than 30 mN m−1.9−12 Recently, it has been demonstrated that superoleophobic surfaces can be obtained by the introduction of specially designed surface geometries, such as re-entrant and overhang surface structures, even if the material surface is intrinsically oleophilic.13,14 Such structures prevent the oils from penetrating the cavities as a consequence of capillary forces, making a composite interface of air, oil, and solid. Such a state is often referred to as the Cassie−Baxter state. It has also been reported that the Cassie−Baxter state is stabilized on solid surfaces with a hierarchical roughness.15−17 There have been many reports of artificial hierarchical surfaces that exhibit superhydrophobicity.18−25 We have recently demonstrated that hierarchically structured dual pillar surfaces coated with a fluorinated alkyl phosphate monolayer are superoleophobic even for hexadecane (γl = 27.5 mN m−1), although their θflat values for hexadecane are less than 90°.26 The study revealed the potential of such hierarchical structures for making superoleophobic surfaces. In addition, the
INTRODUCTION Inspired by the superhydrophobic surfaces that occur in nature, such as lotus leaves,1 cicadas’ wings,2 and water striders’ legs,3 artificial superhydrophobic surfaces have attracted extensive attention recently. It is well-known that the wettability of a surface is influenced by surface roughness, in addition to the surface energy of the solid material. By combining appropriate surface microstructures and materials with low surface energies, many artificial superhydrophobic surfaces have successfully been fabricated.4−7 However, the surfaces that effectively repel water are not usually oil repellent, too. Thus, these superhydrophobic surfaces must be contaminated with oil and lose their superior properties in the process. According to Wenzel model,8 the contact angle of a flat surface, θflat, needs to be greater than 90° in order for the introduction of surface roughness to cause an increase in the contact angle for a liquid. If θflat is less than 90°, then surface roughening will reduce the contact angle. The θflat value for a liquid on a solid surface can be correlated with the surface energies of the solid (γs) and the liquid (γl), and the interface energy between the solid and the liquid (γsl), in accordance with Young’s equation cos θflat =
γs − γsl γl
(1)
where γsl can be expressed as γsl = γs + γl − 2 γsγl
(2)
These correlations indicate that a γs value of less than ∼20 mN m−1 is needed to get a θflat value of less than 90° for water, which has a γl value of 72.8 mN m−1. The corresponding γs value will be © 2012 American Chemical Society
Received: May 24, 2012 Revised: September 20, 2012 Published: October 4, 2012 23308
dx.doi.org/10.1021/jp305078t | J. Phys. Chem. C 2012, 116, 23308−23314
The Journal of Physical Chemistry C
Article
Figure 1. Scanning electron micrographs of submicrometer aluminum pillar films deposited by OAD on a scalloped substrate with a cell size of ∼900 nm (a) and then anodized in sulfuric acid electrolyte at 25 V (b) and immersed in phosphoric acid for 0.9 ks for pore-widening (c).
phosphoric acid at 303 K for selected periods of time for porewidening of the nanoporous anodic oxide films. Next, the specimens were coated with a layer of mono-[2-(perfluorooctyl)ethyl] phosphate (FAP) to reduce the surface energy by immersing them in a 2 mass % solution of FAP in ethanol for 2 days at room temperature. The FAP was synthesized by a previously described approach.31 The surface morphology of the films was observed using a field emission scanning electron microscope (SEM) (JEOL, JSM-6500F). The pore size, pore wall thickness, and porosity of the nanopores formed by anodizing and subsequent pore-widening of aluminum pillars were calculated from SEM images of the surfaces with dual-scale morphology using the Image J software, a public domain for image processing and analysis developed by the National Institutes of Health. The porosity is defined as the surface fraction of nanopores on aluminum pillars without considering the gaps between the pillars. The wettability of the obtained specimens was evaluated using dynamic contact angle measurements for various liquid droplets (∼2 μL), including water, rapeseed oil, and hexadecane, using an optical contact angle meter (Kyowa Interface Science Co., DM-CE1). For the dynamic advancing and receding contact angle measurements, liquids were added to and removed from the surface with a syringe.
size of the gaps between submicrometer pillars on the submicrometer−nanometer dual pillar surfaces was demonstrated to have an effect on the superoleophobicity of the surface. In the present study, we demonstrate that superoleophobicity can be achieved on novel submicrometer pillar/nanopore hierarchical surfaces. The submicrometer pillars were prepared by oblique angle deposition (OAD) of aluminum, while the nanopores were developed through anodizing to form a nanoporous anodic alumina layer. Similar hierarchical surface morphology comprising micrometer-scale asperities and nanopores was developed on silicon, and the surface coated with a selfassembled fluoroalkylsilane monolayer revealed superoleophobicity.27 Although such hierarchical surface geometry was demonstrated to be able to induce superoleophobicity on a intrinsically oleophilic surface, necessary geometrical parameters of nanopores, such as porosity, pore size, and pore wall thickness, for superoleophobicity have not been examined, probably because of difficulty controlling such parameters. It is known that the size of nanopores in porous anodic alumina is controlled by anodizing voltage.28 The pore-widening treatment is also possible to change the porosity and pore size.29 Thus, in this work, the size of pores and porosity of the nanoporous anodic alumina layer on submicrometer aluminum pillars were controlled by varying the anodizing voltage and subsequent pore-widening treatment. The correlation between the surface wettability for oils and the nanopore morphology of the submicrometer pillars was examined in detail for optimization of hierarchical geometry necessary for superoleophobicity.
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RESULTS AND DISCUSSION Figure 1 shows SEM images of the submicrometer pillar aluminum deposits obtained by OAD on a scalloped substrate. The pillars were developed at the triple-point ridges of the concave cell structure with an approximately hexagonal arrangement because of a shadowing effect of the substrate during OAD.30 Each pillar had a faceted morphology, probably as a consequence of surface diffusion of aluminum adatoms to a stable surface crystallographic plane. After anodizing of the aluminum pillars at 25 V in a sulfuric acid electrolyte (Figure 1b), the morphology of the pillars was similar to that seen before anodizing (Figure 1a). Although volume expansion during the conversion of aluminum metal to anodic alumina is expected,32 no obvious volume expansion can be seen in Figures 1a and 1b, which is likely due to formation of a relatively thin anodic alumina layer (
