Reversible Superhydrophobicity to Superhydrophilicity Switching of a

Apr 15, 2010 - (1-4) Various transition-metal oxides, such as ZnO, TiO2, WO3, ... a superhydrophobic CNT film fabricated by one-step spray-coating ...
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Reversible Superhydrophobicity to Superhydrophilicity Switching of a Carbon Nanotube Film via Alternation of UV Irradiation and Dark Storage Jin Yang,†,‡ Zhaozhu Zhang,*,† Xuehu Men,† Xianghui Xu,†,‡ and Xiaotao Zhu†,‡ †

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China, and ‡Graduate School, Chinese Academy of Sciences, Beijing 100039, PR China Received January 25, 2010. Revised Manuscript Received April 2, 2010

We describe a simple method of fabricating a superhydrophobic carbon nanotube (CNT) film without any chemical modification. A remarkable surface wettability transition between superhydrophobicity and superhydrophilicity can be easily observed by the alternation of UV irradiation and dark storage. The adsorption and desorption of surface water molecules on the CNT surfaces account for their tunable surface wettability, which is disclosed by X-ray photoelectron spectroscopy analysis. We also perform a series of comparison experiments to confirm the explanation of its distinctive surface wettability. This switchable wettability on the CNT film could have potential applications in areas requiring multifunctional CNT-based films.

Introduction Stimuli-responsive smart surfaces with dynamically tunable wettability have recently received special attention because of their potential applications.1-4 Various transition-metal oxides, such as ZnO, TiO2, WO3, Fe2O3, and V2O5, are well known to have tunable surface wettability in response to UV light.5 These oxide films with hierarchical nanostructure exhibit photoinduced superhydrophilicity as a result of UV irradiation and recover to the pristine superhydrophobicity state after dark storage.6-10 However, their recovery time from superhydrophilicity to superhydrophobicity is very long, even up to several weeks. This article shows that carbon nanotubes (CNTs) have tunable wettability in response to UV light that is similar to that of transition-metal oxides and have a short recovery time, in contrast to that of these oxides. CNTs have a perfect graphitic network of sp2 carbon, they are inherently somewhat hydrophilic, and further surface *Corresponding author. E-mail: [email protected]. (1) (a) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357–360. (b) Hu, S.; Cao, X.; Song, Y.; Li, C.; Xie, P.; Jiang, L. Chem. Commun. 2008, 2025–2027. (c) Liao, K. S.; Fu, H.; Wan, A.; Batteas, J. D.; Bergbreiter, D. E. Langmuir 2009, 25, 26–28. (2) (a) Motornov, M.; Sheparovych, R.; Lupitskyy, R.; MacWilliams, E.; Minko, S. Adv. Mater. 2008, 20, 200–205. (b) Wang, S.; Liu, H.; Liu, D.; Ma, X.; Fang, X.; Jiang, L. Angew. Chem., Int. Ed. 2007, 46, 3915–3917. (3) (a) Lim, H. S.; Lee, S. G.; Lee, D. H.; Lee, D. Y.; Lee, S.; Cho, K. Adv. Mater. 2008, 20, 4438–4441. (b) Qing, G.; Wang, X.; Fuchs, H.; Sun, T. J. Am. Chem. Soc. 2009, 131, 8370–8371. (4) (a) Xu, L.; Chen, W.; Mulchandani, A.; Yan, Y. Angew. Chem., Int. Ed. 2005, 44, 6009–6012. (b) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C.; Roach, P. Chem. Commun. 2005, 3135–3137. (5) Miyauchi, M.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Chem. Mater. 2002, 14, 2812–2816. (6) (a) Liu, H.; Feng, L.; Zhai, J.; Jiang, L.; Zhu, D. Langmuir 2004, 20, 5659– 5661. (b) Papadopoulou, E. L.; Barberoglou, M.; Zorba, V.; Manousaki, A.; Pagkozidis, A.; Stratakis, E.; Fotakis, C. J. Phys. Chem. C 2009, 113, 2891–2895. (7) (a) Sun, W.; Zhou, S.; Chen, P.; Peng, L. Chem. Commun. 2008, 603–605. (b) Zhang, X.; Jin, M.; Liu, Z.; Nishimoto, S.; Saito, H.; Murakami, T.; Fujishima, A. Langmuir 2006, 22, 9477–9479. (c) Nishimoto, S.; Sekine, H.; Zhang, X.; Liu, Z.; Nakata, K.; Murakami, T.; Koide, Y.; Fujishima, A. Langmuir 2009, 25, 7226–7228. (8) Wang, S.; Feng, X.; Yao, J.; Jiang, L. Angew. Chem., Int. Ed. 2006, 45, 1264–1267. (9) Lim, H. S.; Kwak, D.; Lee, D. Y.; Lee, S. G.; Cho, K. J. Am. Chem. Soc. 2007, 129, 4128–4129. (10) Yan, B.; Tao, J.; Pang, C.; Zheng, Z.; Shen, Z.; Huan, C. H. A.; Yu, T. Langmuir 2008, 24, 10569–10571. (11) Liu, H.; Zhai, J.; Jiang, L. Soft Matter 2006, 2, 811–821. (12) (a) Liao, K. S.; Wan, A.; Batteas, J. D.; Bergbreiter, D. E. Langmuir 2008, 24, 4245–4253. (b) Georgakilas, V.; Bourlinos, A. B.; Zboril, R.; Trapalis, C. Chem. Mater. 2008, 20, 2884–2886.

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functionalization or textured arrangement could facilitate easy control of their wetting properties.11-16 Accordingly, various CNT films with tunable wettability have been fabricated via the modification of nanotube surfaces or the construction of rough surface structures using different methods. For instance, Jiang et al. fabricated poly(N-isopropylacrylamide)-modified aligned CNT films with temperature-sensitive wettability,17 and they obtained superhydrophobicity and hydrophilicity on a 3D anisotropically aligned CNT film.18 Lee et al. reported a facile method of creating transparent, conductive, and superhydrophobic (or superhydrophilic) films from a one-component CNT/silane sol solution.19 Pillai et al. discovered CNT bucky papers with controllable wetting properties by improving the surface functionalization strategies.20 Subsequently, they reported the switching from superhydrophobic to superhydrophilic in ozonolyzed CNT bucky paper induced by an electric field.21 In both cases, the switchable wettability was irreversible. It would be desirable to realize the reversible switching of a CNT film between superhydrophobicity and superhydrophilicity triggered by an external stimulus that could meet the needs of a wide range of applications requiring multifunctional films. In this article, we report a superhydrophobic CNT film fabricated by one-step spray-coating method without any chemical modification on which the wettability can be reversibly switched between superhydrophobic and superhydrophilic by the alternation of UV irradiation and dark storage. The details of the fabrication of CNT films and the switching mechanism are described herein.

(13) Pastine, S. J.; Okawa, D.; Kessler, B.; Rolandi, M.; Llorente, M.; Zettl, A.; Frechet, J. M. J. J. Am. Chem. Soc. 2008, 130, 4238–4239. (14) (a) Sethi, S.; Dhinojwala, A. Langmuir 2009, 25, 4311–4313. (b) Zhang, L.; Resasco, D. E. Langmuir 2009, 25, 4792–4798. (15) Li, Y.; Huang, X. J.; Heo, S. H.; Li, C. C.; Choi, Y. K.; Cai, W. P.; Cho, S. O. Langmuir 2007, 23, 2169–2174. (16) Wang, H. Z.; Huang, Z. P.; Cai, Q. J.; Kulkarni, K.; Chen, C.-L.; Carnahan, D.; Ren, Z. F. Carbon 2010, 48, 868–875. (17) Sun, T.; Liu, H.; Song, W.; Wang, X.; Jiang, L.; Li, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 4663–4666. (18) Sun, T.; Wang, G.; Liu, H.; Feng, L.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2003, 125, 14996–14997. (19) Han, J. T.; Kim, S. Y.; Woo, J. S.; Lee, G. W. Adv. Mater. 2008, 20, 3724–3727. (20) Kakade, B. A.; Pillai, V. K. J. Phys. Chem. C 2008, 112, 3183–3186. (21) Kakade, B.; Mehta, R.; Durge, A.; Kulkarni, S.; Pillai, V. Nano Lett. 2008, 8, 2693–2696.

Published on Web 04/15/2010

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Figure 1. (a, b) FESEM images of the as-prepared CNT film at low and high magnifications, respectively.

Figure 2. Water droplet profile (5 μL) on the CNT film with (a) a CA of 155 ( 1° and (b) an SA of 3.1 ( 2°. (c) Optical image of two water droplets on the as-prepared CNT film surface.

Experimental Section Pristine multiwalled CNTs (MWCNTs) and hydroxylic MWCNTs were purchased from Chengdu Organic Chemicals Co., Ltd., China (synthesized by a CVD process; purity >99.9%) with a diameter of 30-50 nm and a length of about 30 μm. According to the supplier specifications, hydroxylic MWCNTs were obtained via the chemical oxidation of pristine MWCNTs and the content of hydroxyl groups was 5.58 wt %. The preparation process of CNT films is as follows: 20 mg of MWCNTs was dispersed in 10 mL of chloroform using an ultrasonic bath (250 W, 59 kHz) for 30 min to get a homogeneous dispersion. The suspension was then sprayed onto columnar copper substrates with 0.2 MPa nitrogen gas using a spray gun (Shanghai, Lotus brand, no.1). The nozzle diameter of the spray gun was 0.8 mm. The sprayed films were annealed at 40 °C overnight in vacuum to remove residual solvent. The thickness of the coatings was in the range of 40-60 μm. UV light irradiation was carried out with a UV lamp (254 nm, 2 μW/cm2). The surface morphology of the CNT films was examined by using a JSM-6701F field-emission scanning electron microscope (FESEM, JEOL, Japan). The water contact angle (CA) and sliding angle (SA) were measured with a Kr€ uss DSA 100 (Kr€ uss Company, Ltd., Germany) apparatus at ambient temperature. All of the measurements of CA and SA were taken after a 5 μL water droplet was placed on the sample after about 0.5 min. The average CA and SA values were obtained by measuring the same sample in at least five different positions. The chemical composition of the as-prepared film 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 285.0 eV. The takeoff angle of XPS was 90°.

Results and Discussion Normally, CNT films can be created using various techniques such as chemical vapor deposition, spray coating, vacuum filtering, drop casting, and Langmuir-Blodgett deposition.22 In this study, we selected spray coating, which is a simple, low-cost (22) Zhang, X. Adv. Mater. 2008, 20, 4140–4144.

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method and can be applied to large areas. Figure 1 shows FESEM images of the as-prepared CNT film. It is seen that the film exhibits hierarchical nanotextured surface morphology (i.e., multiscale surface roughness) that is self-similar throughout the surface area. This rough structure formed by entangled MWCNTs is due to quick evaporation of the solvent during the flight of the atomized slurry toward the substrate. Surprisingly, without any chemical modification, the CNT film shows superhydrophobic properties with a high CA of 155 ( 1° (Figure 2a) and a low SA of about 3.1 ( 2° (Figure 2b). A water droplet of 5 μL was difficult to attach to such a film with a normal syringe. Moreover, water droplets can exhibit spherical shapes on the CNT film as shown in Figure 2c, and they retain the shapes following water evaporation without infiltration, indicating that such films possess stable superhydrophobicity. This phenomenon on the prepared CNT film is different from the aligned CNT films reported previously, on which water droplets are not stable and will eventually infiltrate the forest voids after a few minutes.23 Therefore, for the aligned CNT films, low-surface-energy materials are commonly used to modify the CNT films.23,24 The most distinctive characteristic of the film is its tunable wettability in response to UV light. When the CNT film was exposed to UV light for 40 min, the water CA was found to be about 0°, that is, it was switched from superhydrophobic to superhydrophilic. A water droplet can thoroughly and immediately spread out on the surface. After the UV-irradiated film had been in the dark for 24 h, its wettability recovered to the pristine superhydrophobic state. In a word, superhydrophobicity and superhydrophilicity on the CNT film can be switched by the alternation of UV irradiation and dark storage (Figure 3a), and this process has been repeated with full reproducibility more than five times (Figure 3b). To determine the effects of UV illumination on the CA of the CNT film surface, some detailed experiments (23) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701–1705. (24) Zhu, L.; Xiu, Y.; Xu, J.; Tamirisa, P. A.; Hess, D. W.; Wong, C. P. Langmuir 2005, 21, 11208–11212.

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Figure 3. (a) Photographs of water droplet shape on the as-prepared CNT film before (left) and after (right) UV irradiation. (b) Reversible superhydrophobic and superhydrophilic transition of as-prepared films by alternating UV irradiation and dark storage. (c) Relationship between the UV illumination time and the water CA of the CNT film. (d) Relationship between the dark storage time and the water CA of the CNT film.

Figure 4. Schematic model of the process used to obtain switchable wettability on the CNT film.

were carried out. Figure 3c shows the influence of the illumination time on the water CA. It is seen that the CA decreased only about 5° after 10 min of irradiation; however, with time increasing from 10 to 40 min, it decreased sharply from 150 to 0°. Figure 3d shows the recovery process of the surface during storage in the dark. The water CA of the UV-irradiated film can gradually recover to its initial value within 24 h. It is noteworthy that the recovery speed of the UV-irradiated CNT film is faster than that for the transition-metal oxides. The surface wettability is determined by the chemical composition of a surface and its microstructure. The morphologies of the CNT film are the same before and after UV irradiation, which indicates that the reversible switching between superhydrophobicity and superhydrophilicity is mainly caused by the surface chemical composition. However, the hierarchical nanotextured surface morphologies have an impact on the surface wettability of the CNT films. According to the Wenzel model, the roughness effect can amplify the inherent wettability of the substrate material.25 If the CNTs are hydrophilic, then a rough CNT film is expected to be superhydrophilic; conversely, if the CNTs are (25) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994.

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Figure 5. X-ray photoelectron spectroscopy spectra of the O 1s level before and after UV irradiation.

hydrophobic, then the Wenzel model predicts that a rough film composed of the CNTs would display superhydrophobic behavior. As noted above, this reversible wettability of the CNT film by the alternation of UV irradiation and dark storage is very similar to that of the transition-metal oxides, and in previous reports, the reversible wettability of these oxides has been explained by considering the adsorption and desorption of surface hydroxyl groups at the outermost layer of oxide films.6-9 Therefore, we can explain the tunable wettability of the CNT film using similar methods, though there are inherent differences between CNTs and metal oxides. A possible model exhibiting the transition Langmuir 2010, 26(12), 10198–10202

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Figure 6. (a, b) FESEM images of the hydroxylic CNT film at low and high magnifications, respectively.

between superhydrophobicity and superhydrophilicity on the CNT film surface is shown in Figure 4. It is well known that oxygen molecules can be absorbed at the surfaces of nanotubes.26 When the CNT film is exposed to UV light, some hydrophilic groups, such as hydroxyl groups, are produced at the lattice defects of CNTs.27 UV light can excite the oxygen molecules absorbed at nanotube lattice defects from a ground spin triplet state to a higher-energy spin singlet state, which can give rise to a significant reduction in the activation energy of oxygen molecule chemisorption and hence enhances the oxidation of CNTs to produce the hydrophilic groups.28 Hydroxyl formation on the defect sites is kinetically favorable, leading to the physical adsorption of water molecules at these sites. The absorbed water molecules render the CNT films superhydrophilic. To confirm the increased adsorption of water, the CNT films were analyzed by carrying out XPS before and after UV irradiation. As shown in Figure 5, the shoulder at the higher binding energy of the O 1s peak is broader after the UV exposure and its intensity is increased markedly. The ratios of the half-peak width and the peak height between the two O 1s peaks are 1.3 and 1.8, respectively. This broader shoulder and higher intensity are ascribed to the physical adsorption of water molecules, which means that the CNT surface is very hydrophilic. Water molecules will fill the apertures of a rough surface, owing to the 3D capillary effect, resulting in a water CA of about 0°. When the UV-irradiated film is placed under air in the dark, water molecules adsorbed on the CNT surfaces are gradually replaced by oxygen molecules and the hydrophobicity of the CNT surface increases; that is, the surface recovers to the superhydrophobic state. This process is similar to that of the transition-metal oxides. Oxygen adsorption is thermodynamically favored, and oxygen is more strongly bonded to the defect sites than are water molecules.6-9 In addition, as noted above, the recovery speed of the UV-irradiated CNT film is faster than that of the transitionmetal oxides. This is due to the special surface properties of the CNTs, which absorb oxygen molecules more easily at the defect sites than do transition-metal oxides. To confirm the above explanation of the tunable wettability of the CNT film, we also performed a series of comparative experiments. First, we prepared a superhydrophobic polystyrenefunctionalized CNT film with a water CA of about 160 ( 1° (26) (a) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801–1804. (b) Tchernatinsky, A.; Desai, S.; Sumanasekera, G. U.; Jayanthi, C. S.; Wu, S. Y.; Nagabhirava, B.; Alphenaar, B. J. Appl. Phys. 2006, 99, 034306. (27) (a) Sham, M. L.; Kim, J. K. Carbon 2006, 44, 768–777. (b) Najafi, E.; Kimb, J. Y.; Hanc, S. H.; Shin, K. Colloids Surf., A 2006, 284, 373–378. (28) (a) Savage, T.; Bhattacharya, S.; Sadanadan, B.; Gaillard, J.; Tritt, T. M.; Sun, Y. P.; Wu, Y.; Nayak, S.; Car, R.; Marzari, N.; Ajayan, P. M.; Rao1, A. M. J. Phys.: Condens. Matter 2003, 15, 5915–5921. (b) Grujicica, M.; Caoa, G.; Raob, A. M.; Trittb, T. M.; Nayak, S. Appl. Surf. Sci. 2003, 214, 289–303.

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according to our previous work29 and then exposed the film to UV light for 1 h. It was found that the CA decreased only from 160 to 136° and could not recover to its initial value under air in the dark; that is, the CNT film lost its tunable wettability from superhydrophobic to superhydrophilic after being functionalized with polystyrene. This observation may support our explanation because polystyrene coated onto the CNT surface inhibits the absorption of oxygen molecules and the interaction between UV light and CNTs, but the evidence is indirect. To prove our explanation directly, we used hydroxylic MWCNTs to fabricate CNT films by the spray-coating method described above. As shown in Figure 6, the surface morphology has no evident differences with respect to the pristine CNT film. However, the hydroxylic MWCNT film was superhydrophilic with a CA of about 0°, and it can also change from superhydrophilic to superhydrophobic (CA ≈ 153 ( 1°) when the film is stored in the dark for several days. The longer switching time may be due to more water molecules adsorbed onto the surfaces of hydroxylic MWCNTs. This changed surface wettability of the hydroxylic CNT film is the same with the UV-irradiated CNT film, which can provide direct evidence for the above explanation. That is, the oxidized CNTs with hydroxyl groups can transform from hydrophilic to hydrophobic when they are placed under air in the dark. The absorption of water molecules formed in the photochemical surface reaction transforms the surface into a superhydrophilic state. When the UV-irradiated film is placed under air in the dark, the water molecules are gradually replaced by atmospheric oxygen and the surface evolves back to the initial state. To confirm the function of atmospheric oxygen, we performed the following experiment: when the UV-irradiated sample was kept under nitrogen in the dark for 48 h, the CA recovered only to 50 ( 3°. This slight recovery may be due to oxygen contamination. When this sample was kept under air instead of nitrogen, it returned to the superhydrophobic state. These results suggest that the dominant role of atmospheric oxygen in the recovery process lead from superhydrophilicity back to superhydrophobicity. This reversible switching proceeds only by the adsorption and desorption of water molecules on the CNT surface. The stability of the surface structures, without changes in the switching process, explains why the reversible wettability can be repeated several times.

Conclusions A superhydrophobic CNT film was fabricated via a simple spray-coating method without any chemical modification. Importantly, reversible switching between superhydrophobicity and (29) Yang, J.; Zhang, Z. Z.; Men, X. H.; Xu, X. H. Appl. Surf. Sci. 2009, 255, 9244–9247.

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superhydrophilicity by alternating UV irradiation and dark storage is observed on the film. The absorption of water molecules formed in the photochemical surface reaction makes the film superhydrophilic. The replacement of water molecules by oxygen during storage in the dark causes the film to revert to being superhydrophobic. This study suggests that the CNT materials

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have tunable surface wettability in response to UV light, which may provide good opportunities for the development of multifunctional CNT-based films. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50773089 and 50835009).

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