Titania

pubs.acs.org/Langmuir © 2009 American Chemical Society

UV-Driven Reversible Switching of a Polystyrene/Titania Nanocomposite Coating between Superhydrophobicity and Superhydrophilicity Weixin Hou†,‡ and Qihua Wang*,† †

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China, and ‡Graduate School, Chinese Academy of Sciences, Beijing 100039, P. R. China Received January 14, 2009. Revised Manuscript Received March 23, 2009 Hydrophilic titania (TiO2) nanoparticles were dispersed in solutions of polystyrene (PS), and the suspensions were cast on glass surfaces. The effect of drying temperature on the hydrophobic character of PS/TiO2 was investigated: the static water contact angle increased with the drying temperature, and the as-prepared coating could be adjusted from superhydrophilicity to superhydrophobicity just by controlling the drying temperature. Moreover, the superhydrophobic coating turning into a superhydrophilic one (CA < 5°) after UV illumination, which can be recovered through being heated.

Introduction In recent years, superhydrophobic surfaces with water contact angles (CA) higher than 150° have attracted considerable interest with respect to both academic research and industrial applications because of their potential applications.1-5 Among these, surfaces with reversible switching between superhydrophobicity and superhydrophilicity get more attention.6-8 Up to now, various methods have been used to fabricate surfaces with reversible wettability, including thermal treatments,9 electric field,10 solvent treatments,11 or light irradiation12 and so on.13-15 Among the methods used to control the surface wettability, light switching has received special attention because it can be controlled quickly *Corresponding author. E-mail: [email protected]. (1) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994. (2) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546–551. (3) Sun, T.; Gao, X.; Feng, L.; Jiang, L. Acc. Chem. Res. 2005, 38, 644–652. (4) Tadanaga, K.; Morinaga, J.; Masuda, A.; Minami, T. Chem. Mater. 2000, 12, 590–591. (5) Woodward, I.; Schofield, W. C. E.; Roucoules, V.; Badyal, J. P. S. Langmuir 2003, 19, 3432–3438. (6) Zhang, X. T.; Jin, M.; Liu, Z. Y.; Nishimoto, S.; Saito, H.; Murakami, T.; Fujishima, A. Langmuir 2006, 22, 9477–9479. (7) Borras, A.; Barranco, A.; Gonzalez-Elipe, A. R. Langmuir 2008, 24, 8021– 8026. (8) Liu, H.; Feng, L.; Zhai, J.; Jiang, L.; Zhu, D. B. Langmuir 2004, 20, 5659– 5661. (9) (a) Sun, T. L.; Wang, G. J.; Feng, L.; Liu, B. Q.; Ma, Y. M.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2004, 43, 357 –360. (b) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C.; Roach, P. Chem. Commun. 2005, 3135–3136. (10) (a) Krupenkin, T. N.; Taylor, J. A.; Wang, E. N.; Kolodner, P.; Hodes, M.; Salamon, T. R. Langmuir 2007, 23, 9128–9133. (b) Dhindsa, M. S.; Smith, N. R.; Heikenfeld, J. Langmuir 2006, 22, 9030–9034. (c) Kakade, B.; Mehta, R.; Durge, A.; Kulkarni, S.; Pillai, V. Nano Lett. 2008, 8, 2693–2696. (d) Verplanck, N.; Galopin, E.; Camart, J. C.; Thomy, V. Nano Lett. 2007, 7, 813–817. (11) (a) Zhu, Y.; Feng, L.; Xia, F.; Zhai, J.; Wan, M. X.; Jiang, L. Macromol. Rapid Commun. 2007, 28, 1135–1141. (b) Yu, X.; Wang, Z.; Jiang, Y.; Shi, F.; Zhang, X. Adv. Mater. 2005, 17, 1289–1293. (c) Jiang, Y. G.; Wang, Z. Q.; Yu, X.; Shi, F.; Xu, H. P.; Zhang, X. Langmuir 2005, 21, 1986–1990. (12) (a) Lim, H. S.; Han, J. T.; Kwak, D. H.; Jin, M. H.; Cho, K. W. J. Am. Chem. Soc. 2006, 128, 14458–14459. (b) Lim, H. S.; Kwak, D. H.; Lee, D. Y.; Lee, S. G.; Cho, K. W. J. Am. Chem. Soc. 2007, 129, 4128–4129. (c) Feng, X. J.; Feng, L.; Jin, M. H.; Zhai, J.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2004, 126, 62–63. (13) Zhang, J. L.; Lu, X. Y.; Huang, W. H.; Han, Y. C. Macromol. Rapid Commun. 2005, 26, 477–480. (14) Zhu, Y.; Li, J. M.; He, H. Y.; Wan, M. X.; Jiang, L. Macromol. Rapid Commun. 2007, 28, 2230–2236. (15) (a) Xu, L. B.; Chen, Z. W.; Chen, W.; Mulchandani, A.; Yan, Y. S. Macromol. Rapid Commun. 2008, 29, 832–838. (b) Zhu, W. Q.; Zhai, J.; Sun, Z. W.; Jiang, L. J. Phys. Chem. C 2008, 112, 8338–8342.

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and the switch is individually addressable. To control the wettability through light irradiation, organic compounds that show a reversible structural change triggered by light are commonly employed,16 such as spiropyrans;17 inorganic materials exhibit better light, thermal, and chemical stabilities. Among photosensitive inorganic compounds, metal oxides6,7,12b,c,18 are mostly used materials. Titania is an important material that can be used in many industrial applications related to photosplitting of water, photocatalysis, photovoltaic devices, etc. Titania-based superhydrophobic surface can be fabricated via two kinds of approaches: modifying a titania rough surface with materials of low surface free energy6 or creating titania surface with a special morphology, for example, lotus-leaf-like structure.19 Such surfaces have shown UV-driven superhydrophobic-superhydrophilic conversion. For instance, Zhang et al.18b reported the preparation and UVstimulated wettability conversion of superhydrophobic titania surfaces as well as the preparation of superhydrophilic-superhydrophobic patterns by use of UV irradiation through a photomask. Feng et al.19 reported on the creation of superhydrophobic surfaces consisting of inorganic nanorods fabricated from hydrophilic TiO2 and further demonstrated that their wettability can be reversibly switched between superhydrophobicity and superhydrophilicity. Most TiO2-based superhydrophobic surfaces need special morphology (lotus-leaf-like structure), especially the nanopillarlike structure; the fabrication process is time-consuming and needs some special experiments, which are not beneficial to the application. There is still a lack of convenient methods to prepare (16) (a) Abbott, S.; Ralston, J.; Reynolds, G.; Hayes, R. Langmuir 1999, 15, 8923–8928. (b) Rosario, R.; Gust, D.; Garcia, A. A.; Hayes, M.; Taraci, J. L.; Dailey, J. W.; Picraux, S. T. J. Phys. Chem. B 2004, 108, 12640–12642. (17) Rosario, R.; Gust, D.; Hayes, M.; Jahnke, F.; Springer, J.; Garcia, A. A. Langmuir 2002, 18, 8062–8069. (18) (a) Yan, B.; Tao, J.; Pang, C.; Zheng, Z.; Shen, Z.; Huan, C. H. A.; Yu, T. Langmuir 2008, 24, 10569–10571. (b) Zhang, X. T.; Jin, M.; Liu, Z. Y.; Tryk, D. A.; Nishimoto, S.; Murakami, T.; Fujishima, A. J. Phys. Chem. C 2007, 111, 14521– 14529. (19) (a) Feng, X. J.; Zhai, J.; Jiang, L. Angew. Chem., Int. Ed. 2005, 44, 5115 – 5118. (b) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 1999, 103, 2188–2194. (c) Wang, L. Q.; Baer, D. R.; Engelhard, M. H.; Shultz, A. N. Surf. Sci. 1995, 344, 237–350.

Published on Web 04/23/2009

DOI: 10.1021/la900151y

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TiO2-based superhydrophobic surface. We previously reported two kinds of nanocomposite superhydrophobic surfaces:20 polystyrene/silica surface and methylsilicone/phenolic resin/silica surface. According to this, here we prepared a PS/TiO2 nanocomposite coating by a simple casting method; the wettability of the as-prepared coating also exhibits UV-driven superhydrophobic-superhydrophilic conversion, even if there is no nanopillarlike structure.

Experimental Section Preparation. Polystyrene (PS, Aldrich, 0.1 g) with a molecular weight of 100 000 g/mol was dissolved in tetrahydrofuran (THF, 20 mL) to prepare stock solutions. Titania (TiO2, 0.3 g) particles with a 20 nm mean diameter were mixed with the PS solutions. The mixture was subsequently stirred for 1 h with a magnetic stirrer. The PS-TiO2 dispersions were cast onto clean glass plates (1  3 cm). The samples were dried at 20, 60, 100, 140, and 180 °C in an oven to form nanocomposite coatings. UV light irradiation was carried out with a UV lamp (254 nm, 1180 μW/cm2). The irradiation time was 1 h. Characterization. The morphologies of the coatings were investigated by a scanning electron microscope (JSM-6701F, JEOL, Japan). Contact angle measurements were carried out by the sessile drop method with a Kyowa contact-angle meter. The sliding angles were also measured using the same apparatus. Water droplets (5 μL) were delivered on different sample spots for each specimen. An average of the five measurements was used for analysis. The chemical composition of the as-prepared surface was investigated using X-ray photoelectron spectroscopy (XPS), which was conducted on a PHI-5702 electron spectrometer using an Al KR line excitation source with the reference of C 1s at 285.00 eV. The takeoff angle of XPS was 90°.

Results and Discussion The water static contact angle and the contact angle hysteresis of PS were studied in our previous report.20a Here, the polystyrene solution was dropped on a glass slide and dried at 20 °C. A transparent film was obtained. The SEM image (Figure 1) shows a relatively smooth surface with a water static contact angle about 92°. The wetting behavior of the coating was first investigated. Figure 2 shows the variation of water contact angle with the drying temperature. The water contact angle of the asprepared coating varied from 0 ( 2° to 158 ( 2° as the drying temperature increased from 20 to 180 °C; a lower sliding angle (∼2°) was also obtained when the coating dried at 180 °C. The wettability of the coating achieved the transition from superhydrophilicity to superhydrophobicity as the drying temperature increased. Further investigating the transition reason, the microstructure and the surface components of the coating were analyzed. Figure 3 shows SEM images of the PS/TiO2 nanocomposite coatings deposited on the glass. The TiO2 nanoparticles aggregated together to form a rough surface. The dimensions of titania particles are nanometers, and the nanoparticle aggregates are beneficial to the formation of superhydrophobic surface. Comparing Figure 3a with Figure 3b, the surfaces showed similar morphology when dried at different temperatures. As we know, the wettability of a surface is determined by two factors: surface compositions and the microstructure. Because of the similarity of the as-prepared surface morphologies, it is believed that the transition from superhydrophilicity to superhydrophobicity is caused by the variation of the surface compositions. XPS was used to investigate the compositions of the as-prepared coatings (20) (a) Hou, W. X.; Wang, Q. H. J. Colloid Interface Sci. 2007, 316, 206–209. (b) Hou, W. X.; Wang, Q. H. Langmuir 2007, 23, 9695–9698.

6876 DOI: 10.1021/la900151y

Figure 1. SEM image of PS film. The inset is photograph of a water droplet on the surface.

Figure 2. Variation of water contact angle with the drying temperature.

dried at 20 and 180 °C. The peaks of Ti 2p, O 1s, and C 1s which appeared on the surface demonstrate that TiO2 and PS coexisted on the as-prepared coating as shown in Figure 4. It can be concluded that the surface makeup is governed by the drying temperature from Figure 4, which may be explained as follows: the PS was dissolved in the THF solvent, and the volatilization rate of the solvent increased with the drying temperature, which will lead to the aggregates of PS molecules on the surface, especially when the temperature is higher than boiling point of THF. The different ratio of Ti, O, and C clarified the variation of the surface components. The ratio of C/O/Ti is 25.2%/50.2%/ 24.6% (Figure 4a) when the drying temperature is 20 °C, but it increased to 49%/37.7%/13.3% (Figure 4b) when the coating dried at 180 °C. The variation of the atom concentration indicates that the main component of the coating is TiO2 when dried at lower temperature (20 °C); however, the hydrophobic PS aggregated gradually on the surface as the drying temperature increased, until it became the main component of the as-prepared coating when dried at higher temperature (180 °C). The main component variation of the coating can explain the wettability transition from superhydrophilicity to superhydrophobicity as the drying temperature increased. The reversible switching of the PS/TiO2 nanocomposite coating between superhydrophobicity and superhydrophilicity was evaluated by the water contact angle measurement. Figure 5a shows a spherical water droplet with a water contact angle of 158 ( 2° when the coating dried at 180 °C. Upon UV (obtained from a 8 W Hg lamp with a wavelength 254 nm) irradiation for 1 h, the water droplet penetrated into the coating, resulting a Langmuir 2009, 25(12), 6875–6879

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Figure 3. SEM images of the as-prepared coatings under different conditions: (a) dried at 20 °C; (b) dried at 180 °C; (c) after UV irradiation for 1 h; (d) UV irradiated coating being heated at 180 °C for 1 h.

Figure 4. XPS spectra of the nanocomposite coatings dried at (a) 20 °C and (b) 180 °C.

water contact angle about 0 ( 2°. The results indicate that the wettability of the as-prepared coating varied from superhydrophobicity to superhydrophilicity. After the UV irradiated coating being heated at 180 °C for 1 h, the superhydrophobicity of the coating was obtained again. This process has been repeated several times, and good reversibility of the surface wettability was observed as shown in Figure 5b. Langmuir 2009, 25(12), 6875–6879

Figure 5. (a) Photographs of water droplet shape on PS/TiO2 coat ings before (left) and after (right) UV illumination. (b) Reversible superhydrophobic-superhydrophilic transition of the as-prepared coatings under the alternation of UV irradiation and heating.

To thoroughly understand the reversible superhydrophobicity to superhydrophilicity transition of the PS/TiO2 nanocomposite coatings, the surface compositions and the microstructure, which DOI: 10.1021/la900151y

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Table 1. Ratio of Atom Concentrations of the As-Prepared Coatings and PS Films at Different Conditions: (a) Dried at 180 °C; (b) after UV Irradiation for 1 h; (c) UV Irradiated Coating/Film Being Heated at 180 °C for 1 h (a) 180 °C (b) 180 °C f UV 1 h (c) 180 °C f UV 1 h f 180 °C 1 h

TiO2/PS C/O/Ti

O% (O-Ti%)

O% (O-C%)

PS C/O

49%/37.7%/13.3% 37.8%/47.9%/14.3% 60.9%/28.6%/10.5%

26.2% 30.1% 19.8%

11.5% 17.8% 8.8%

95%/5% 67%/33% 75.4%/24.6%

are two main factors governing the surface wettability, are considered. TiO2 nanoparticles formed the rough structure as shown in Figure 3b,c. The morphologies of the as-prepared coatings are almost the same before and after UV irradiation, even being heated at 180 °C for 1 h (Figure 3d), which indicate that the reversible switching between superhydrophobicity and superhydrophilicity mainly caused by the surface compositions, not the surface microstructure. XPS results further proved this conclusion. (XPS spectra of the as-prepared coatings under different conditions can be seen in Figure S1 of the Supporting Information.) Table 1 gives the atomic percentages of the coating under different conditions. The ratio of C/O/Ti is 49%/37.7%/13.3% when the coating dried at 180 °C, but it decreased to 37.8%/47.9%/14.3% after UV irradiation for 1 h. The ratio of hydrophobic compositions to hydrophilic compositions will determine the water contact angle of the as-prepared coating, and the decrease of the ratio of C/O/Ti means that the hydrophilic composition became the main component after UV irradiation, which resulted in the transition from superhydrophobicity to superhydrophilicity. However, after the UV irradiated coating be heated at 180 °C for 1 h, the ratio of C/O/ Ti increased to 60.9%/28.6%/10.5%, which means that the hydrophobic composition became the main component again and led to the transition from superhydrophilicity to superhydrophobicity. To further clarify the mechanism of the reversible switching between superhydrophobicity and superhydrophilicity in our study, high-resolution of O 1s peak was further investigated. There are two peaks derive from O 1s peak at high resolution (as shown in Figure 6): O-Ti (belonging to TiO2; 530.6 eV) and O-C (belonging to the oxidation of PS surface; 532.9 eV). The difference from previous study is that the variation of atom concentration of oxygen not only ascribes to O-Ti but also attributes to O-C (as shown in Table 1). Therefore, the reversible switching in the wetting behavior of the coating ascribed to two aspects: the surface of TiO2 and PS film. Moreover, the components variation of the PS surface plays the most important role in the reversible switching between superhydrophobicity and superhydrophilicity as shown in Table 1. The wetting behavior reversible switching of TiO2 surface can be explained by previous studies.19 As reported, UV irradiation will generate electron-hole pairs in these metal oxides surfaces; the holes react with lattice oxygen, leading to the creation of surface oxygen vacancies. The defective sites are kinetically more favorable for hydroxyl adsorption than oxygen adsorption, which leads to dissociative adsorption of water molecules at these sites. As a result, the surface hydrophilicity is improved. While the oxygen adsorptions thermodynamically favored, and it is more strongly bonded on the defect sites than the hydroxyl group, and the hydroxyl groups adsorbed on the defective sites can be replaced gradually by oxygen atoms when the UV-irradiated coating dried at 180 °C, which lead to the hydrophobicity of the surface increases.

Nevertheless, the variation of O-C in PS film needs more investigation. The contact angle inherent for treated PS surfaces was studied in previous reports;21,22 here we also made a similar comparative experiment. The variation of atom concentration in

(21) (a) Klein, R. J.; Fischer, D. A.; Lenhart, J. L. Langmuir 2008, 24, 8187– 8197. (b) Li, Y.; Pham, J. Q.; Johnston, K. P.; Green, P. F. Langmuir 2007, 23, 9785–9793. (c) Klein, R. J.; Fischer, D. A.; Lenhart, J. L. Langmuir 2008, 24, 8187– 8197.

(22) (a) Davies, J.; Nunnerley, C. S.; Brisley, A. C.; Sunderland, R. F.; J. C. et al. Colloids Surf., A 2000, 174, 287-295. (b) Lubarsky, G. V.; Mitchell, S. A.; Davidson, M. R.; Bradley, R. H. Colloids Surf., A 2006, 279, 188-195.

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Figure 6. XPS spectra of O 1s peak under different conditions: (a) dried at 180 °C; (b) after UV irradiation for 1 h; (c) UV irradiated coating being heated at 180 °C for 1 h.

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PS film under the same conditions is also given in Table 1. (XPS spectra of the PS films under different conditions can be seen in Figure S2 of the Supporting Information.) Compared to PS/TiO2 nanocomposite coating, the variation law of C/O in PS film is the same, which indicates that the oxidation and deoxidation of the PS surface can occur, even if there are no TiO2 nanoparticles. And with the addition of TiO2 nanoparticles, as a result of photocatalysis of TiO2,23 the oxidation and deoxidation of the PS surface are much more easier, which is beneficial to the reversible switching between superhydrophobicity and superhydrophilicity of the coating. PS/SiO2 nanocomposite coating was also investigated for comparation (other conditions are the same); it is found that the PS/SiO2 superhydrophobic surface can also turn into a superhydrophilic one (CA < 5°) after UV illumination, which also can be recovered through being heated, but this process exceeds 2 h individually, and this UV/heating cycle cannot be repeated as more as PS/TiO2 nanocomposite coating, which further proving the catalytic effect of TiO2 nanoparticles. (23) Zan, L.; Wang, S.; Fa, W.; et al. Polymer 2006, 47, 8155-8162.

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Conclusion In summary, we have presented a facile method for the fabrication of PS/TiO2 nanocomposite coating. The wettability of the coating can be controlled by the drying temperature. In addition, the superhydrophobic PS/TiO2 surfaces were able to be converted to superhydrophilic surfaces under low-intensity UV illumination, which can be recovered through being heated. Other than previously reported TiO2-based superhydrophobic surface, the as-prepared coating has no special structure. Furthermore, other metal oxide particles may also be applied in this way. Acknowledgment. The authors acknowledge the important direction project for the knowledge innovative engineering of Chinese Academy of Sciences (Grant KGCX3-SYW-205) and the innovative foundation of national defense scientific technology (CXJJ-144). Supporting Information Available: XPS spectra of the asprepared coatings and PS films under different conditions can be seen in Figures S1 and S2 individually. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la900151y

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