Transparent Surface with Reversibly Switchable Wettability between

Aug 5, 2013 - In the present work, we have successfully fabricated a polyelectrolyte-tethered transparent surface on which superhydrophobicity and ...
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Transparent Surface with Reversibly Switchable Wettability between Superhydrophobicity and Superhydrophilicity Zan Hua,† Jun Yang,† Tao Wang,† Guangming Liu,*,† and Guangzhao Zhang*,‡ †

Department of Chemical Physics, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, PR China, 230026 ‡ Faculty of Materials Science and Engineering, South China University of Technology, Guangzhou, PR China, 510640 S Supporting Information *

ABSTRACT: In the present work, we have successfully fabricated a polyelectrolytetethered transparent surface on which superhydrophobicity and superhydrophilicity can be reversibly switched via counterion exchange between the chloride ion (Cl−) and perfluorooctanoate ion (PFO−). The stable superhydrophobic state can be obtained only when a certain extent of fluorine is chemically incorporated into the grafted polyelectrolyte. The counterion exchange does not have any influence on the transmittance of the transparent surface. The superhydrophobicity and superhydrophilicity can be reversibly switched on the surface for many cycles without any apparent damage to the wetting properties. Additionally, the transparent surface can be applied to prepare smart glass displays to hide and convey information by patterning the counterion distribution on the surface on the basis of the different antifogging properties between superphydrophobic and superhydrophilic surfaces.

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INTRODUCTION Smart solid surfaces with reversibly switchable wettability between superhydrophobicity and superhydrophilicity are vital in many fields such as microfluidics and biosensors.1,2 It is reported that the reversible switching between superhydrophobic and superhydrophilic states can be achieved on rough surfaces by varying the external conditions including temperature, pH, solvency, electrical potential, chirality, counterions, and so forth.3−14 However, all of the surfaces mentioned above are opaque because roughness and transparency are a pair of competitive properties.15 Specifically, both superhydrophobicity and superhydrophilicity are strengthened by increasing surface roughness, whereas transparency decreases significantly as surface roughness increases.16−18 To overcome this drawback, some transparent superhydrophobic or superhydrophilic surfaces have been prepared,19−24 but the reversible switching between superhydrophobicity and superhydrophilicity on transparent surfaces has never been achieved, which would limit their applications in various fields including solar cells, optical devices, and smart displays. To achieve the reversible wetting transition via external stimuli, the transparent surfaces need to be modified by stimuliresponsive materials to a certain degree, which may strongly reduce the transparency of the surfaces. This might be the reason that reversible switching between the extreme conditions of superhydrophobicity and superhydrophilicity has never been achieved on transparent surfaces until now. Herein, we report a method to fabricate a polyelectrolyte-tethered transparent glass surface on which superhydrophobicity and superhydrophilicity can be reversibly switched via counterion exchange. Furthermore, the transparent surface can be used to prepare smart glass displays to hide and convey information based on the different © XXXX American Chemical Society

antifogging properties between superhydrophobic and superhydrophilic surfaces.

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EXPERIMENTAL SECTION

Preparation of Silicone Nanofilament-Coated Glass Substrates. The silicone nanofilaments coated glass substrate was prepared according to the procedure reported previously.25,26 Briefly, the glass slide was cleaned in piranha solution (3:1 v/v H2SO4/H2O2) at 80 °C for 30 min, rinsed with water, and dried under a flow of nitrogen. The activated glass slide was immersed in 60 mL of anhydrous toluene with 9 μL of water in a flask. Subsequently, trichloromethylsilane (TCMS, 75 μL) was injected into the flask at 25 °C to trigger the growth of silicone nanofilaments on the glass surface for a certain time (Figure S1, Supporting Information). Afterward, the glass slide was successively rinsed with toluene, ethanol, and water/ethanol (1:1 v/v) and then dried under a flow of nitrogen. Preparation of the Poly[2-(methacryloyloxy)ethyltrimethylammonium chloride-co-trifluoethyl methacrylate] (poly(METAC-co-TMA))-Tethered Surface. The poly(METAC-coTMA)-tethered nanostructured transparent surface was prepared as follows. First, the TCMS-coated glass surface was activated with water plasma treatment at a power of 18 W for ∼30 min. The activated substrate was immersed in a toluene solution of 10 mM 3-(2-bromoisobutyryl)propyl triethoxysilane (BPE) and 15 mM triethylamine for 24 h in a sealed flask. Then, the surface was washed with toluene and ethanol and dried with N2. Afterward, the substrate was immersed in a solution containing 10.0 mL of METAC/trifluoroethanol (METAC/ TFE) solution (2.0 M) and 0.75 mL of isopropyl alcohol in a flask. Ethyl-2-bromoisobutyrate (0.06 mmol) and 2,2′-bipyridyl (0.10 mmol) diluted with 5 mL of TFE was injected into the above solution. After bubbling with argon at room temperature for 2 h, CuBr Received: July 8, 2013 Revised: August 4, 2013

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(0.05 mmol) was introduced into the solution and the flask was sealed. The mixture was stirred at 60 °C for a prescribed time to conduct the surface-initiated atom-transfer radical polymerization (SI-ATRP). SI-ATRP was halted by exposing the solution to air at 20 °C. The copolymer-tethered surface was successively rinsed with TFE, ethanol, and water and dried with N2. Counterion Exchange. In the counterion exchange experiments, the copolymer-tethered substrate was immersed in a methanol solution containing 0.1 M NaCl or NaPFO for 5 min. To exchange the counterion selectively on a certain area of the surface, the area was covered with superhydrophilic paper with the same shape, and then counterion exchange was achieved by adding the selected counterioncontained solution onto the paper. More detailed information regarding our experiments can be found in the Supporting Information.

surface as initiators for the following grafting of poly(METACco-TMA) by SI-ATRP. Figure 1c shows that the grafting of poly(METAC-co-TMA) does not influence the nanostructures on the surface. It is known that surface wettability can be switched by exchanging the counterions of positively charged groups of grafted polyelectrolyte chains.10−14,27 Here, PMETAC chains were grafted on the surface by SI-ATRP in a methanol/water mixture as in previous work,28 enabling the wettability to be switched between superhydrophilic with the chloride ion (Cl−) as the counterion and superhydrophobic with the perfluorooctanoate ion (PFO−) as the counterion (Figure S2, Table S1). However, the superhydrophobic state is unstable because of the slow penetration of water toward the ion pairs (Figure S3). To solve this problem, a certain extent of hydrophobic segments should be chemically incorporated into the grafted polyelectrolyte to stabilize the superhydrophobicity, but one should make sure that this modification has little effect on the superhydrophilicity. Consequently, the SI-ATRP grafting of METAC was conducted in a TFE solution. Because of the alcoholysis of METAC by TFE, some hydrophilic ethyltrimethylammonium chloride groups were substituted by the hydrophobic trifluoethyl groups (Figure 1a), which was confirmed by X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance (NMR) measurements (Table S1, Figure S4). That is, the grafted polymer now becomes a copolymer. Nonetheless, the superhydrophobic state was still unstable if SI-ATRP was conducted in a freshly prepared METAC/TFE solution because of the low atomic concentration (0.9%) of covalently bound fluorine (CBF) in the copolymer (Figure S3, Table S1). Thus, the METAC/TFE solution was stored at 8 °C for 10 days before SI-ATRP to enable sufficient alcoholysis to increase the content of CBF (Table S1). In Figure 2a, the water contact angle (CA) is 39° on the copolymer-tethered flat surface with Cl− as the counterion after the modification by SI-ATRP for 1 h. The CA increases to 98° when Cl− is substituted by PFO−. On the rough surface with the same nature, the CA decreases to 3° with Cl− as the counterion but increases to 164° with PFO− as the counterion. This is understandable because both hydrophobicity and hydrophilicity can be strengthened by increasing the surface roughness according to the classical Cassie and Wenzel equations.17,18 Clearly, the superhydrophobicity/superhydrophilicity can be obtained on the rough surface with different counterions. Moreover, the superhydrophobicity of the surface is quite stable with a self-cleaning property (Figures S3 and S5). As the ATRP time increases from 1 to 16 h, the CA increases from 39 to 61° on the flat surface and increases from 3 to 58° on the rough surface with Cl− as the counterion, which is due to the increasing content of CBF in the copolymer induced by the increasing alcoholysis (Table S2). In other words, the rough surface gradually loses its superhydrophilicity with the increasing ATRP time. With PFO− as the counterion, the CA remains almost constant at 98° on the flat surface but decreases from 164 to 154° on the rough surface as the ATRP time increases from 1 to 16 h. The weakening of superhydrophobicity with the ATRP time may be because the areal fraction of air in the contact area between the water droplet and the rough surface decreases with the increase in the thickness of the grafted polymer layer (Figure S6). Note that the superhydrophilicity cannot be achieved with Cl− as the counterion when the surface is modified by ATRP with a shorter time (e.g., 30 min) though the superhydrophobicity can be achieved on

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RESULTS AND DISCUSSION It is generally thought that surface roughness and transparency are opposing properties.15 Namely, the transparency would significantly decrease with the increasing surface roughness because of the increasing Mie scattering.21 Thus, the size of surface roughness should be smaller than ∼100 nm to reduce the Mie scattering from the surface and to achieve a high surface transparency in the visible light region.16,21 In Figure 1a,

Figure 1. (a) Schematic illustration of the fabrication of the transparent coating on a glass substrate. (b) SEM image of the silicone nanofilaments deposited on the glass surface. The inset shows that the fabricated surface has a high transparency and a good superhydrophobicity. (c) SEM image of the silicone nanofilaments after the modification by SI-ATRP with poly(METAC-co-TMA) for 1 h.

silicone nanofilaments were deposited on a glass surface by immersing the glass slide in a toluene solution containing TCMS and trace water for 6 h.26 The scanning electron microscopy (SEM) image shows nanofilaments with a diameter of 30−50 nm and a length of several micrometers densely interconnecting on the surface, which creates a nanostructured rough surface with good transparency (Figure 1b). Meanwhile, the anchored methyl groups render the surface superhydrophobic (Figure 1b). Subsequently, the surface was activated using water plasma treatment to generate abundant hydroxyl groups. Afterward, BPE was immobilized on the B

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Figure 2. (a) Change in the contact angle as a function of ATRP time for the copolymer-tethered flat silicon wafer surfaces with Cl− (Cl−/F) or PFO− (PFO−/F) as the counterion and for the copolymer-tethered rough glass surfaces with Cl− (Cl−/R) or PFO− (PFO−/R) as the counterion. (b) Transmittance spectra of the copolymer-tethered rough glass surfaces as a function of ATRP time with Cl− as the counterion. The transmittance spectra of the bare, flat glass surface (glass) and the bare, rough glass surface (glass/R) are also plotted for comparison. (c) XPS spectra of the rough glass surface in the presence of different counterions, where the surface is modified by SI-ATRP with the copolymer for 1 h.

this surface with PFO− as the counterion (Figure S7). However, Figure 2b shows that the glass surface is highly transparent after the deposition of nanofilaments. The modification by SI-ATRP for 1 h has almost no influence on the transmittance. However, the transmittance significantly decreases with the further increases in ATRP time as a result of the increasing thickness of the grafted polymer layer. Obviously, the surface that is modified by SI-ATRP for 1 h is the best one to demonstrate the switching between superhydrophobicity and superhydrophilicity on the transparent surface. In Figure 2c, the F 1s peak is observed in the XPS spectrum with Cl− as the counterion, indicating the presence of F within the grafted polymer resulting from the alcoholysis of METAC by TFE. Substituting Cl − with PFO − results in the disappearance of the Cl 2p peak accompanied by an increase in the atomic concentration of F from 5.1 to 43.6% (Table S1), suggesting that Cl− can be completely substituted by PFO− during the counterion exchange, which makes the transition from superhydrophilic to superhydrophobic possible. Likewise, PFO− can also be completely substituted by Cl− when immersing the substrate in a NaCl/methanol solution (Table S1). Thus, it is anticipated that superhydrophobicity and superhydrophilicity can be reversibly switched on the transparent surface via counterion exchange.10−14 Figure 3a shows that the wettability can be switched between superhydrophobic (164°) and superhydrophilic (3°) states via counterion exchange on the transparent surface. Also, the wettability can be reversibly switched many cycles without any apparent damage to the wetting properties (Figure 3b). Moreover, the counterion exchange does not have any influence on

the transmittance of the surface (Figure 3c). The reversible switching between superhydrophobicity and superhydrophilicity can be more clearly visualized in Figure 3d. The wettability is sequentially changed from superhydrophilic to superhydrophobic (left) and superhydrophilic (right), to superhydrophobic, and finally to superhydrophilic (left) and superhydrophobic (right) by selectively choosing Cl− or PFO− as the counterion on localized areas of the transparent surface. It is known that superhydrophilic transparent surfaces can resist fogging because the moisture forms a continuous water film on the surfaces.29 In contrast, humid air condenses into discrete droplets on superhydrophobic surfaces, resulting in a significant decrease in the transparency.30 Fogging experiments were conducted by exposing glass slides to steam (Figure 4a). The boxed areas, which are uncovered, contain the same words as those under the glass slides for comparison. The rough glass surface is superhydrophilic with Cl− as the counterion, thus the surface remains transparent and the words beneath it can be clearly seen. For the bare, flat glass surface, fogging obscures the words beneath the slide. When Cl− is substituted by PFO−, the rough glass surface is fogged heavily, strongly distorting the words beneath. This feature may be used to provide privacy. More interestingly, on the basis of the different antifogging properties between superhydrophobic and superhydrophilic surfaces, one can hide information on the transparent surface by patterning the counterion distribution between Cl− and PFO−. The information is hidden at room temperature in air (Figure 4b, middle). However, the hidden information (e.g., the letters USTC and the “happy” and “unhappy” images) is clearly presented when the surface is exposed to steam or to moisture C

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Figure 3. (a) Photographs of water droplets on the copolymer-tethered rough glass surface. The wettability can be switched between superhydrophobicity and superhydrophilicity via counterion exchange between Cl− and PFO−. (b) Reversible change in wettability on the copolymer-tethered rough glass surface via counterion exchange for many cycles. (c) Transmittance spectra of the copolymer-tethered rough glass surface in the presence of different counterions. The transmittance spectrum of the bare, flat glass surface is also plotted for comparison. (d) Selective change in the wettability by selectively choosing Cl− or PFO− as the counterion on localized areas of the transparent surface. The water droplets are dyed with rhodamine B (red) or methylene blue (blue) to improve the visualization of the wettability.

Figure 4. (a) Antifogging property of the copolymer-tethered rough glass surface with (top) Cl− or (bottom) PFO− as the counterion compared to the bare, flat glass surface (middle). The words in the boxed areas are uncovered and are shown for comparison. (b) Photographs of the copolymer-tethered rough glass surface containing a pattern. The pattern is hidden at room temperature in air (middle) but is clearly seen when the surface is exposed to steam (top and bottom). D

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(4) Yu, X.; Wang, Z.; Jiang, Y.; Shi, F.; Zhang, X. Reversible pHResponsive Surface: From Superhydrophobicity to Superhydrophilicity. Adv. Mater. 2005, 17, 1289−1293. (5) Motornov, M.; Minko, S.; Eichhorn, K.-J.; Nitschke, M.; Simon, F.; Stamm, M. Reversible Tuning of Wetting Behavior of Polymer Surface with Responsive Polymer Brushes. Langmuir 2003, 19, 8077− 8085. (6) Lahann, J.; Mitragotri, S.; Tran, T.-N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. A Reversibly Switching Surface. Science 2003, 299, 371−374. (7) Xu, L.; Chen, W.; Mulchandani, A.; Yan, Y. Reversible Conversion of Conducting Polymer Films from Superhydrophobic to Superhydrophilic. Angew. Chem., Int. Ed. 2005, 44, 6009−6012. (8) Lim, H. S.; Han, J. T.; Kwak, D.; Jin, M.; Cho, K. Photoreversibly Switchable Superhydrophobic Surface with Erasable and Rewritable Pattern. J. Am. Chem. Soc. 2006, 128, 14458−14459. (9) Qing, G.; Sun, T. Chirality-Triggered Wettability Switching on a Smart Polymer Surface. Adv. Mater. 2011, 23, 1615−1620. (10) Wang, L.; Lin, Y.; Peng, B.; Su, Z. Tunable Wettability by Counterion Exchange at the Surface of Electrostatic Self-Assembled Multilayers. Chem. Commun. 2008, 5972−5974. (11) Wang, L.; Lin, Y.; Su, Z. Counterion Exchange at the Surface of Polyelectrolyte Multilayer Film for Wettability Modulation. Soft Matter 2009, 5, 2072−2078. (12) Jiang, C.; Wang, Q.; Wang, T. Tunable Wettability via Counterion Exchange of Polyelectrolyte Brushes Grafted on Cotton Fabric. New J. Chem. 2012, 36, 1641−1645. (13) Lim, H. S.; Lee, S. G.; Lee, D. H.; Lee, D. Y.; Lee, S.; Cho, K. Superhydrophobic to Superhydrophilic Wetting Transition with Programmable Ion-Pairing Interaction. Adv. Mater. 2008, 20, 4438− 4441. (14) Wang, L. M.; Peng, B.; Su, Z. H. Tunable Wettability and Rewritable Wettability Gradient from Superhydrophilicity to Superhydrophobicity. Langmuir 2010, 26, 12203−12208. (15) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Preparation of Transparent Superhydrophobic Boehmite and Silica Films by Sublimation of Aluminum Acetylacetonate. Adv. Mater. 1999, 11, 1365−1368. (16) Yabu, H.; Shimomura, M. Single-Step Fabrication of Transparent Superhydrophobic Porous Polymer Films. Chem. Mater. 2005, 17, 5231−5234. (17) Wenzel, R. N. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem. 1936, 28, 988−994. (18) Cassie, A. B. D.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday Soc. 1944, 40, 546−551. (19) Deng, X.; Mammen, L.; Butt, H. J.; Vollmer, D. Candle Soot as a Template for a Transparent Robust Superamphiphobic Coating. Science 2012, 335, 67−70. (20) Zhang, L.; Li, Y.; Sun, J.; Shen, J. Mechanically Stable Antireflection and Antifogging Coatings Fabricated by the Layer-byLayer Deposition Process and Postcalcination. Langmuir 2008, 24, 10851−10857. (21) Xu, L. B.; Karunakaran, R. G.; Guo, J.; Yang, S. Transparent, Superhydrophobic Surfaces from One-Step Spin Coating of Hydrophobic Nanoparticles. ACS Appl. Mater. Interfaces 2012, 4, 1118−1125. (22) Lai, Y.; Tang, Y.; Gong, J.; Gong, D.; Chi, L.; Lin, C.; Chen, Z. Transparent Superhydrophobic/Superhydrophilic TiO2-Based Coatings for Self-Cleaning and Anti-Fogging. J. Mater. Chem. 2012, 22, 7420. (23) Lin, J.; Chen, H.; Fei, T.; Zhang, J. Highly Transparent Superhydrophobic Organic−Inorganic Nanocoating from the Aggregation of Silica Nanoparticles. Colloids Surf., A 2013, 421, 51−62. (24) Zhang, X.; Kono, H.; Liu, Z.; Nishimoto, S.; Tryk, D. A.; Murakami, T.; Sakai, H.; Abe, M.; Fujishima, A. A Transparent and Photo-Patternable Superhydrophobic Film. Chem. Commun. 2007, 4949−4951. (25) Gao, L.; McCarthy, T. J. A Perfectly Hydrophobic Surface (θA/ θR = 180°/180°). J. Am. Chem. Soc. 2006, 128, 9052−9053.

exhaled with a breath (Figure 4b, top and bottom). After the moisture evaporates from the surface, the information is again hidden. The information can be hidden and presented many times on the basis of the method described above (Movies S1−S4). More importantly, the information can be erased and rewritten on the transparent surface via counterion exchange,10−14 which paves the way for applications in the field of smart glass displays. It should be noted that the information can be easily written on the transparent surface by patterning the counterion distribution, which is difficult to realize using other methods, for example, by fluorinated silane or thermal treatment, even though the transparent surfaces can be switched irreversibly between superhydrophobic and superhydrophilic.22,23

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CONCLUSIONS The copolymer-tethered rough glass surface presented here represents the first report of a transparent surface with reversibly switchable wettability between superhydrophobic and superhydrophilic states. The reversible wetting transition is induced by counterion exchange between Cl− and PFO−. The stable superhydrophobic state can be obtained only when a certain extent of fluorine is chemically incorporated into the grafted polyelectrolyte. On the basis of the different antifogging properties between superhydrophobic and superhydrophilic surfaces, the transparent surface can be used to prepare smart glass displays to hide and convey information. The surface may also find applications in other fields such as solar cells and smart windows.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details about the materials and methods, the preparation of nanostructured surfaces, the contact angle measurements, the spectroscopic ellipsometry measurements, the XPS and NMR measurements, and the movies for hiding and presenting information on the transparent surface. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.M.); [email protected] (G.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the National Program on Key Basic Research Project (2012CB933800), the National Natural Science Foundation of China (21004058, 91127042, and 21234003), and the Scientific Research Startup Foundation of the Chinese Academy of Sciences is acknowledged.

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REFERENCES

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(26) Zhang, J.; Seeger, S. Superoleophobic Coatings with Ultralow Sliding Angles Based on Silicone Nanofilaments. Angew. Chem., Int. Ed. 2011, 50, 6652−6656. (27) Azzaroni, O.; Brown, A. A.; Huck, W. T. S. Tunable Wettability by Clicking Counterions into Polyelectrolyte Brushes. Adv. Mater. 2007, 19, 151−154. (28) Moya, S.; Azzaroni, O.; Farhan, T.; Osborne, V. L.; Huck, W. T. S. Locking and Unlocking of Polyelectrolyte Brushes: Toward the Fabrication of Chemically Controlled Nanoactuators. Angew. Chem., Int. Ed. 2005, 44, 4578−4581. (29) Grosu, G.; Andrzejewski, L.; Veilleux, G.; Ross, G. G. Relation between the Size of Fog Droplets and Their Contact Angles with CR39 Surfaces. J. Phys. D: Appl. Phys. 2004, 37, 3350−3355. (30) Chen, Y.; Zhang, Y.; Shi, L.; Li, J.; Xin, Y.; Yang, T.; Guo, Z. Transparent Superhydrophobic/Superhydrophilic Coatings for SelfCleaning and Anti-Fogging. Appl. Phys. Lett. 2012, 101, 033701.

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