Fabrication of Superhydrophobic Surfaces with Controllable Electrical

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Fabrication of Superhydrophobic Surfaces with Controllable Electrical Conductivity and Water-Adhesion Lijun Ye, Jipeng Guan, Zhixiang Li, Jingxin Zhao, Cuicui Ye, Jichun You, and Yongjin Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03848 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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Fabrication of Superhydrophobic Surfaces with Controllable Electrical Conductivity and Water-Adhesion Lijun Ye, Jipeng Guan, Zhixiang Li, Jingxin Zhao, Cuicui Ye, Jichun You, Yongjin Li*

College of Material, Chemistry and Chemistry Engineering, Hangzhou Normal University, Hangzhou 310036, People’s Republic of China

ABSTRACT: A facile and versatile strategy for fabricating superhydrophobic surfaces with controllable electrical conductivity and water-adhesion is reported. “Vine-on-fence” structured and cerebral cortex-like superhydrophobic surfaces are constructed by filtering a suspension of multi-walled carbon nanotubes (MWCNTs), using polyoxymethylene nonwovens as the filter paper. The nonwovens with micro- and nanoporous two-tier structures act as the skeleton, introducing a microscale structure. The MWCNTs act as nanoscale structures, creating hierarchical surface roughness. The surface topography and electrical conductivity of the superhydrophobic surfaces is controlled by varying the MWCNT loading. The “vine-on-fence” structured surfaces exhibit “sticky” superhydrophobicity with high water-adhesion. The cerebral cortex-like surfaces exhibit self-cleaning properties with low water-adhesion. The as-prepared superhydrophobic surfaces are chemically resistant to acidic and alkaline environments of pH 2‒ 12. They therefore have potential in applications such as droplet-based microreactors and thin film microextraction. These findings aid our understanding of the role that surface topography 1

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plays in the design and fabrication of superhydrophobic surfaces with different water-adhesion properties.

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1. INTRODUCTION Superhydrophobic surfaces with water contact angles above 150° have attracted much research interest, because of their potential applications,1-5 including self-cleaning surfaces,6-7 anti-icing surfaces,8-9 anti-reflective surfaces,10-12 fluidic drag reduction,13 liquid transportation,14-15 and oil/water separation.16-19 In the natural world, lotus leaves and rose petals are two typical superhydrophobic surfaces but with very different water-adhesion properties.20-21 It is thought that a special Cassie state with extremely low water-adhesion makes water droplets highly mobile on the surface of lotus leaves,21 and that the Cassie impregnating wetting state fixes water droplets to the surface of rose petals.20 Surface roughness and chemical composition are generally considered to be the two main factors that determine surface wettability. Thus, introducing roughness into the surface of hydrophobic materials, and modifying rough surfaces with low surface energy materials, are good guidelines for fabricating superhydrophobic materials.5 Many methods for designing and fabricating superhydrophobic materials with controllable water-adhesion surfaces have been recently developed.22-29 These include, but are not limited to, creating micro-bowl- and micro-lens-structured polydimethylsiloxane arrays by lithography,23 preparing cupric oxide surfaces with micro-flower/nanorod structures by solution immersion processes combined with self-assembly,25 fabricating nanorods, cacti, and mesh-like manganese dioxide surfaces by hydrothermal processes,26 and constructing periodic microstructures on copper surfaces by femtosecond laser irradiation techniques.29 Electrospinning is a simple and versatile way for fabricating nonwoven mats of

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micro-/nanofibers with large length-to-diameter ratios and high specific surface areas.30-31 Nonwoven mats created by electrospinning are highly porous, and have been applied in filtration,32-33 separation,34-35 absorption,36-37 sensing,38-39 tissue engineering,40 electronic and photonic devices,41-42 and catalysis.43 Electrospinning is now widely used to prepare superhydrophobic surfaces, because of its ability to intrinsically provide at least one length scale of roughness.44-45 Various fibrous superhydrophobic materials have been prepared via the electrospinning of hydrophobic polymers.46 Incorporating nanoparticles such as silicon dioxide or titanium dioxide can introduce a second length scale into the fibrous material, to achieve hierarchical micro- and nanostructures.47-48 In this study, polymeric nonwovens are used to prepare robust micro-scale skeletons. A second length scale (nanoscale) is then introduced into the structure, by filtering multi-walled carbon nanotube (MWCNT) suspensions using polymeric nonwovens as the filter paper. This creates a hierarchical roughness. Poly(L-lactide)/polyoxymethylene (PLLA/POM) blended nonwovens are used as the skeleton, because PLLA can be easily removed by selective extraction. This yields POM nonwovens with micro- and nanoporous two-tier structures.49 “Vine-on-fence” structured and cerebral cortex-like superhydrophobic surfaces with electrical conductivity are formed on the POM nonwovens. The surface topography (wettability) and electrical conductivity of the superhydrophobic surfaces is controlled by varying the MWCNT loading. This is a facile and versatile approach for fabricating superhydrophobic surfaces with tunable electrical conductivity and water-adhesion properties. This strategy is potentially applicable to various polymers and nanoparticles, for fabricating superhydrophobic surfaces with 4


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specific functionality.

2. EXPERIMENTAL 2.1 Materials. The PLLA materials (3001 D), including 1.6% D-lactide, Mn = 89 300 g/mol (Mw/Mn = 1.77), were purchased from Nature Works Co., LLC (USA). The POM materials (MC 90), Mw = 174 300 g/mol (Mw/Mn = 2.19), were kindly provided by Shenhua Co., Ltd. (P. R. China). MWCNTs were kindly provided by Nikkiso Co. Ltd. (Japan). 2.2 Sample Preparation. PLLA/POM blends were prepared by solution-blending, using hexafluoroisopropanol as a mutual solvent. PLLA/POM blended mats were fabricated by the electrospinning of an 8 wt.% PLLA/POM (50/50) solution, with a voltage of 14 kV and a volumetric flow rate of 0.1 mL/h (under a relative humidity of 65‒75%). The PLLA/POM blended fibers were collected on an aluminum foil, with a tip-to-collector distance of 15 cm. PLLA/POM blended mats were extracted by chloroform to remove PLLA components, yielding POM nonwovens with an internal hierarchy. MWCNT suspensions were then filtered using the POM nonwovens as the filter paper. MWCNTs suspensions were prepared through ultrasonication in dichlorobenzene at 40 °C for 30 min, which yielded an exfoliated dispersion. After drying in a vacuum desiccator for at least 12 h, superhydrophobic surfaces with different electrical conductivities and water-adhesion properties were obtained. 2.3 Sample Characterization. The morphologies of the as-prepared superhydrophobic surfaces were observed by field-emission scanning electron microscopy (SEM) (Hitachi S-4800). Water contact angles were examined using a drop shape analyzer (DSA 100) at ambient 5


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temperature, and the volume of the test droplet was 10 µL. For each sample, five different positions were analyzed to determine the average contact angle. Solutions of various pH were obtained by adding hydrochloric acid and sodium hydroxide to distilled water. pH values were determined using a pH meter. Electrical conductivities of the superhydrophobic surfaces were measured according to the testing method for resistivity of conductive plastics, with a four-point probe array (JIS K7194).

3. RESULTS AND DISCUSSION 3.1 Surface Topography and Wettability. Sufficient roughness is necessary for fabricating superhydrophobic surfaces.5 In this study, PLLA/POM blended nonwovens are prepared to act as a robust skeleton, and then selectively etched by chloroform to remove the PLLA components. This yields internal micro- and nanoporous two-tier structures. Figure 1a shows that numerous nanopores are generated after removing the PLLA components from electrospun PLLA/POM blended fibers. PLLA/POM blends are miscible crystalline/crystalline polymer systems. The PLLA and POM components have similar melting points, but very different crystallization kinetics.50-52 In the electrospun PLLA/POM blended fibers, POM crystallizes into hexagonal crystals, while PLLA remains in the amorphous state (Figure S1), similar to that of the bulk materials during rapid cooling from the melt state.52 Nanopores in electrospun PLLA/POM blended fibers originate from solvent extraction of the PLLA components incorporated within the amorphous region of POM.

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Figure 1. SEM images of the top surface of the POM nonwovens with the indicated MWCNT loadings. (a) 0; (b) 1.41; (c) 3.54; (d) 7.07; (e) 14.15 and (f) 28.29 µg/cm2.

Figure 1b-f shows the changes in morphology of the top surface of the POM nonwovens with increasing MWCNT loading. Two typical hierarchical surface morphologies are observed, which are “vine-on-fence” structured and cerebral cortex-like surfaces. The “vine-on-fence” structured surface involves POM nonwovens acting as the “fence”, which is comprised of micro-scale fibers. Figure 1b and c show that MWCNTs act as “nano-vines” twining round the porous POM fibers, in samples with low MWCNT loadings. When the MWCNT loading increases to 7.07 µg/cm2, “nano-vines” twined round the “fence” weave into a mesh and form cerebral cortex-like surfaces, as shown in Figure 1d-f. In the cerebral cortex-like surfaces, the POM skeleton does not act as a microscale structure, but as a microscale template for the MWCNT mesh. These interesting surface structures endow mechanical stable of MWCNTs on the POM nonwovens. Moreover, these two types of surface topographies result in different wettabilities and electrical conductivities of the as-prepared superhydrophobic surfaces, as 7


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0 5 10 15 20 25 30 The loading of MWCNTs (µg/cm2) Figure 2. Water contact angles (left) and electrical resistances (right) of the as-prepared superhydrophobic surfaces versus the MWCNT loadings.

Figure 2 shows the dependence of water contact angle and electrical resistance of the as-prepared superhydrophobic surfaces on the MWCNT loading. The water contact angle increases dramatically and then more slightly with increasing MWCNT loading. The transition occurs at about 7.07 µg/cm2. The electrical resistance decreases significantly when the MWCNT loading increases to 7.07 µg/cm2. The electrical resistance varies slightly among samples with MWCNT loadings of higher than 7.07 µg/cm2. Figure 1 shows that the “vine-on-fence” structured superhydrophobic surface transforms into a cerebral cortex-like surface, when the MWCNT loading reaches 7.07 µg/cm2. We therefore conclude that the variations in wettability and electrical conductivity of the as-prepared superhydrophobic surfaces originate from the different surface topographies.

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Figure 3. Wetting properties of the as-prepared superhydrophobic surfaces with the indicated MWCNT loadings. (a) 0 µg/cm2, the water droplet formed a “Wenzel” state on the surface; (b) 3.54 µg/cm2, showing a “sticky” superhydrophobicity and suspending a 10 µL water droplet upside down; (c) 14.15 µg/cm2, showing a “slippy” superhydrophobicity, the water droplet was mobile and formed a “Cassie” state on the surface.

Figure 3 shows the different water wettabilities of the as-prepared superhydrophobic surfaces with different surface topographies. The POM skeleton without MWCNTs is hydrophobic, and has a water contact angle of 142.2 ± 2.4°, as shown in Figure 3a. Adding 3.54 µg/cm2 of MWCNTs onto the surface of the POM skeleton via filtration increases the water contact angle to 153.2 ± 2.6°, which constitutes a superhydrophobic surface. Figure 3b shows

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that a 10 µL water droplet can be suspended on the surface upside down, without roll-off. This implies a high water-adhesion of “vine-on-fence” structured surfaces with MWCNT loadings of less than 7.07 µg/cm2. Cerebral cortex-like surfaces with MWCNT loadings of at least 7.07 µg/cm2 exhibit excellent self-cleaning properties. The water droplet is easily removed by slightly tilting the substrate, as shown in Figure 3c. The measured sliding angles of the cerebral cortex-like surfaces are shown in Table S1. The cerebral cortex-like surfaces exhibit excellent superhydrophobicity with very low water-adhesion. As shown in Figure 4a, a 5 µL water droplet suspended from a syringe gradually moves to contact with the surface of the POM nonwovens containing a 14.15 µg/cm2 MWCNT loading. The water droplet is then easily removed from the surface, implying that the water-adhesion of the cerebral cortex-like surface is low. Figure 4b shows a water droplet moving back and forth on a cerebral cortex-like surface covered with silicon dioxide (SiO2) powder. The SiO2 powder is eventually removed from the surface, indicating that the surface possesses self-cleaning properties.

Figure 4. (a) Sequential images of a 5 µL of water droplet suspended on a syringe contacts and then departs from the top surface of the POM nonwovens with a 14.15 µg/cm2 MWCNT loading. (b) Demonstration of self-cleaning property of the top surface of the POM nonwovens with a 14.15 µg/cm2 MWCNT loading. Please note that the top surface of the POM nonwovens in (b) was covered with SiO2 powders and the arrows show 10


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the moving direction of the syringe.

3.2 Mechanism. Dynamic wetting properties are strongly influenced by surface topography. The Wenzel and Cassie state models are frequently used to describe solid-liquid interactions between water droplets and rough surfaces. A water droplet forming a Wenzel state will completely fill valleys or gaps in a rough surface. A water droplet forming a Cassie state will retain air inside the gaps in a rough surface. Two types of superhydrophobic rough surfaces are prepared in the current study, i.e. “vine-on-fence” structured surfaces with high water-adhesion, and cerebral cortex-like surfaces with low water adhesion to water. The low water-adhesion superhydrophobicity of the cerebral cortex-like surface can be explained by the Cassie model, as shown in Figure 5a. The hierarchical roughness of the surface is formed by microstructures with deep gaps and rich nanostructures. Water droplets are excluded from both the micro- and nanoscale structures, and “slippy” superhydrophobicity is observed for these cerebral cortex-like surfaces. water

(a)

Cassie state

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Figure 5. Schematic illustration of a water droplet in contact with the as-prepared superhydrophobic surfaces. (a) The “slippy” superhydrophobicity, the droplet is excluded from both the micro- and nanoscale structures. (b) The “sticky” superhydrophobicity (Cassie impregnating wetting state), the microscale structures are partially wetted by the droplet whereas the nanoscale structures are inaccessible to the droplet. The protrusions 11


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represents the surface fluctuation provided by POM fibers, and the black lines (or meshes) are MWCNTs.

For the “vine-on-fence” structured surfaces, the interspace between the adjacent microstructures appears much wider than that in the cerebral cortex-like surfaces, as shown in Figure 1c and d, respectively. This results in “sticky” superhydrophobicity. In this case, the microscale spaces within the “fence” are partially wetted by water droplets, whereas the nanoscale gaps between the “nano-vines” are inaccessible to water droplets, as shown in Figure 5b. The transformation from the cerebral cortex-like surface to the “vine-on-fence” structured surface originates largely from the lower MWCNT loading. The water contact angle of the top surface of the POM nonwovens decreases to less than 150° (close to that of the POM skeleton), when the MWCNT loading is further decreased to 1.41 µg/cm2. This is largely attributed to insufficient surface roughness, as shown in Figure 1b. When the MWCNT loading is very low, the water contact angle of the top surface of the POM nonwovens is close to that of the POM skeleton.

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Figure 6. Statistic of contact angles for water droplets with various pH on the top surfaces of the POM nonwovens with MWCNT loadings of 3.54 and 14.15 µg/cm2, respectively.

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3.3 Applications. The “vine-on-fence” structured and cerebral cortex-like superhydrophobic surfaces on the POM nonwovens exhibit excellent resistant to acidic and alkaline conditions. No distinct variation in the contact angles of pH 2‒12 water droplets is observed, as shown in Figure 6. The surfaces with “vine-on-fence” or cerebral cortex-like structures exhibit similar contact angles for acidic and alkaline water droplets to those of neutral water droplets, even after in excess of one month. This implies good chemical resistance and durability. The as-prepared superhydrophobic surfaces with different water-adhesion properties on the POM nonwovens have potential in droplet transportation,14-15, 29 and droplet-based microreactors.27-28 As shown in Figure 7, the as-prepared superhydrophobic surfaces with different water-adhesion properties can be used as reaction substrates. For example, a metal cap is used to transport a water droplet containing 0.1 mol/L CuSO4 deposited on a cerebral cortex-like superhydrophobic surface, to contact with another water droplet containing 0.2 mol/L NaOH located on a “vine-on-fence” structured superhydrophobic surface (Figure 7a-f). Flocculent deposits of Cu(OH)2 are produced by the reaction between Cu2+ and OH‒ when the two droplets coalesce. The coalesced droplet remains on the “vine-on-fence” structured surface, as shown in Figure 7g-l.

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Figure 7. Application of the as-prepared superhydrophobic surfaces with different water-adhesion in the droplet-based microreactor. A metal cap was used as the transporter to transfer water droplets. (a-d) A water droplet containing 0.1 mol/L CuSO4 was transferred from a cerebral cortex-like superhydrophobic surface with low water-adhesion to the metal cap without any loss. (e-f) Coalescence of the transferred water droplet to another water droplet containing 0.2 mol/L NaOH on a “vine-on-fence” structured superhydrophobic surface with high adhesion to water. (g-j) The flocculent deposits of Cu(OH)2 were produced for the reaction between Cu2+ and OH- after the two droplets coalesced. (k and l) The coalesced droplet was finally located on the “vine-on-fence” structured surface.

In addition to the different surface topographies, the POM skeleton possesses micro- and nanoporous two-tier internal structures. It has excellent chemical resistance to common organic solvents. These properties make the POM nonwovens ideal for application in oil absorption,49 and solid phase microextraction (SPME).53 CNTs have been widely used as sorbents for dispersive SPME, because of their unique physicochemical properties and excellent adsorption capacities.54-55 Thin film SPME has attracted much recent attention as a new SPME technique for chemical analysis, because of its higher extraction capacity and shorter equilibrium time compared to other SPME techniques.56-57 In the current study, we obtained “bilayered”

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membranes of POM and MWCNTs (MWCNT content of at least 7.07 µg/cm2). The POM nonwovens serve as a robust self-supporting skeleton. The MWCNTs endow the bilayered membranes with high electrical conductivity and high adsorption capacity (especially for aromatic compounds). Figure 8 shows the application of the as-prepared POM/MWCNTs bilayered membranes in thin film SPME. The sample solution is preconcentrated via SPME, as shown in Figure 8a, and then detected by capillary electrochromatography (CEC). Figure 8b shows that SPME using the as-prepared POM/MWCNTs bilayered membrane significantly enhances the ultraviolet response of the sample in CEC, compared with direct measurement. The as-prepared POM/MWCNTs bilayered membranes therefore have potential in the absorption and detection of trace hazardous substances in water, beverages, edible oils, and foods. (b)

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Figure 8. (a) Schematic illustration of thin film solid phase microextraction. The as-prepared POM/MWCNTs bilayered membranes with 14.15 µg/cm2 MWCNT loading was tubed and soaked into a 100 mL of aqueous solution containing 2.5 × 10-6 mol/L methylbenzene for 30 min under sonication (absorption). And then, the thin film was transferred into 10 mL of acetonitrile for 10 min under sonication (deabsorption). (b) Electrochromatograms of methylbenzene solutions before and after microextraction (preconcentration). Please note that in this case the as-prepared POM/MWCNTs bilayered film used was activated by acetonitrile (to increase the hydrophilicity) before absorption.

4. CONCLUSIONS

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We report a simple, feasible, and versatile method for fabricating superhydrophobic surfaces with controllable electrical conductivity and water-adhesion. Superhydrophobic surfaces are constructed by filtering a suspension of MWCNTs, using POM nonwovens as the filter paper. The surface topography and electrical conductivity are controlled by varying the MWCNT loading. The “vine-on-fence” structured surfaces exhibit “sticky” superhydrophobicity with high water-adhesion. The cerebral cortex-like surfaces exhibit excellent self-cleaning properties. These superhydrophobic surfaces exhibit chemical resistance to acidic and alkaline conditions. The POM skeleton with hierarchical structures exhibits excellent solvent resistance. These properties give the as-prepared superhydrophobic surfaces on the POM nonwovens potential in applications such as no-loss droplet transportation, droplet-based microreactors, and thin film solid phase microextraction. These findings aid our understanding of the role that surface topography plays in the design and fabrication of superhydrophobic materials with different water-adhesion properties. This strategy is potentially applicable to various polymers and nanoparticles, for fabricating superhydrophobic surfaces with specific functionality.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.xxxxxx. Wide-angle X-ray diffraction parttern of electrospun PLLA/POM blended nonwoven (Figure S1); Sliding angles of the as-prepared superhydrophobic surfaces (Table S1) (PDF)

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AUTHOR INFORMATION Corresponding Author *E-mail: (Y.L.) [email protected]. Fax: +86 571 28867899. Telephone: +86 57128867026. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (51173036, 21374027) and Program for New Century Excellent Talents in University (NCET-13-0762).

REFERENCES (1) Feng, X. J.; Jiang, L., Design and Creation of Superwetting/Antiwetting Surfaces. Adv. Mater. 2006, 18, 3063-3078. (2) Li, X.-M.; Reinhoudt, D.; Crego-Calama, M., What do We Need for a Superhydrophobic Surface? A Review on the Recent Progress in the Preparation of Superhydrophobic Surfaces. Chem. Soc. Rev. 2007, 36, 1350-1368. (3) Roach, P.; Shirtcliffe, N. J.; Newton, M. I., Progess in Superhydrophobic Surface Development. Soft Matter 2008, 4, 224-240. (4) Yao, X.; Song, Y.; Jiang, L., Applications of Bio-Inspired Special Wettable Surfaces. Adv. Mater. 2011, 23, 719-734. (5) Wang, S.; Liu, K.; Yao, X.; Jiang, L., Bioinspired Surfaces with Superwettability: New 17


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Insight on Theory, Design, and Applications. Chem. Rev. 2015, 115, 8230-8293. (6) Blossey, R., Self-Cleaning Surfaces——Virtual Realities. Nat. Mater. 2003, 2, 301-306. (7) Lafuma, A.; Quere, D., Superhydrophobic States. Nat. Mater. 2003, 2, 457-460. (8) Cao, L.; Jones, A. K.; Sikka, V. K.; Wu, J.; Gao, D., Anti-Icing Superhydrophobic Coatings. Langmuir 2009, 25, 12444-12448. (9) Bengaluru Subramanyam, S.; Kondrashov, V.; Rühe, J.; Varanasi, K. K., Low Ice Adhesion on Nano-Textured Superhydrophobic Surfaces under Supersaturated Conditions. ACS Appl. Mater. Interfaces 2016, 8, 12583-12587. (10) Min, W.-L.; Jiang, B.; Jiang, P., Bioinspired Self-Cleaning Antireflection Coatings. Adv. Mater. 2008, 20, 3914-3918. (11) Ghymn, Y. H.; Jung, K.; Shin, M.; Ko, H., A Luminescent Down-Shifting and Moth-Eyed Anti-Reflective Film for Highly Efficient Photovoltaic Devices. Nanoscale 2015, 7, 18642-18650. (12) Mehmood, U.; Al-Sulaiman, F. A.; Yilbas, B. S.; Salhi, B.; Ahmed, S. H. A.; Hossain, M. K., Superhydrophobic Surfaces with Antireflection Properties for Solar Applications: A Critical Review. Sol. Energ. Mat. Sol. C. 2016, 157, 604-623. (13) Cottin-Bizonne, C.; Barrat, J.-L.; Bocquet, L.; Charlaix, E., Low-Friction Flows of Liquid at Nanopatterned Interfaces. Nat. Mater. 2003, 2, 237-240. (14) Jin, M.; Feng, X.; Feng, L.; Sun, T.; Zhai, J.; Li, T.; Jiang, L., Superhydrophobic Aligned Polystyrene Nanotube Films with High Adhesive Force. Adv. Mater. 2005, 17, 1977-1981. (15) Hong, X.; Gao, X.; Jiang, L., Application of Superhydrophobic Surface with High Adhesive 18


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Fabrication of Superhydrophobic Surfaces with Controllable Electrical Conductivity and Water-Adhesion Lijun Ye, Jipeng Guan, Zhixiang Li, Jingxin Zhao, Cuicui Ye, Jichun You, Yongjin Li*

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Fabrication of Superhydrophobic Surfaces with Controllable Electrical

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