Langmuir 2004, 20, 5659-5661
5659
Reversible Wettability of a Chemical Vapor Deposition Prepared ZnO Film between Superhydrophobicity and Superhydrophilicity Huan Liu, Lin Feng, Jin Zhai, Lei Jiang,* and Daoben Zhu Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received December 4, 2003. In Final Form: February 19, 2004 A superhydrophobic ZnO thin film was fabricated by the Au-catalyzed chemical vapor deposition method. The surface of the film exhibits hierarchical structure with nanostructures on sub-microstructures. The water contact angle (CA) was 164.3°, turning into a superhydrophilic one (CA < 5°) after UV illumination, which can be recovered through being placed in the dark or being heated. The film was attached tightly to the substrate, showing good stability and durability. The surface structures were characterized by scanning electron microscopy and atomic force microscopy.
Introduction The wettability of the solid surface is a very important property depending on the surface free energy as well as the geometric structures of the surface.1-5 Superhydrophobic surfaces with a water contact angle (CA) higher than 150°6 and superhydrophilic surfaces with a water CA smaller than 5°7 have prompted extensive interests for both fundamental research and practical applications. For the preparation of superhydrophobic surfaces or films, the combination of creating a rough structure on a hydrophobic surface (CA > 90°) and lowering the surface energy with a rough surface by chemical modification is required. Recently, superhydrophobic surfaces with various surface roughnesses have been fabricated successfully in our group, such as the aligned polymer nanofiber8 and aligned carbon nanotubes.9 However, there is still little work reported on the surface, on which the reversible wettability between superhydrophobicity and superhydrophilicity can be realized except for ZnO nanorod alignment and the roughness-enhanced thermal-responsive wettability of the poly(N-isopropylacrylamide)-modified surface reported very recently by our group.10 Such a surface is very important in many fields such as microfluid devices. * Corresponding author. Tel.: +86-10-82627566. Fax: +86-1082627566. E-mail:
[email protected]. (1) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R. Science 1999, 283, 46. (2) Lenz, P. Adv. Mater. 1999, 11, 1531. (3) Kenneth, R. S.; Tom, E. K. Langmuir 1994, 10, 334. (4) Abbott, S.; Ralston, J.; Reynolds, G.; Hayes, R. Langmuir 1999, 15, 8923. (5) Jiang, L.; Wang, R.; Yang, B.; Li, T. J.; Tryk, D. A.; Fujishima, A.; Hashimoto, K.; Zhu, D. B. Pure Appl. Chem. 2000, 72, 73. (6) (a) Aussillous, P.; Que´re´, D. Nature 2001, 411, 924. (b) Bico, J.; Tordeux, C. Que´re´, D. Europhys. Lett. 2001, 55, 214. (c) Bico, J.; Thiele, U.; Que´re´, D. Colloids Surf., A 2002, 206, 41. (d) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31. (e) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanade, T. Adv. Mater. 1999, 11, 1365. (f) Tadanaga, T.; Morinaga, J.; Matsuda, A.; Minami, T. Chem. Mater. 2000, 12, 590 (7) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (8) (a) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2002, 41, 1221. (b) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. Adv. Mater. 2002, 14, 1857. (c) Feng, L.; Song, Y.; Zhai, J.; Xu, J.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2003, 42, 800. (9) (a) Li, H.; Wang, X.; Song, Y.; Liu, Y.; Li, Q.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2001, 40, 1743. (b) Li, S.; Li, H.; Wang, X.; Song, Y.; Liu, Y.; Jiang, L.; Zhu, D. J. Phys. Chem. B 2002, 106, 9274.
The preparation of ZnO films has aroused great interest because of their special optical and electrical properties. Many methods have been used to produce a ZnO thin film on a solid substrate, such as electrochemical deposition,11-13 self-assembly growth,14,15 vapor-phase transport,16 and thermal evaporation.17 However, their wettabilities have seldom been investigated.18,19 The ZnO film prepared by spray pyrolysis showed the water CA of 109°, turning into a superhydrophilic one after UV illumination.18 The conductive hydrophobic ZnO film with the porous surface structure fabricated by the electrochemical deposition method19 exhibits the water CA of 128.3° without any modification of the low-surface-free-energy compound. Very recently, the superhydrophobic ZnO nanorod alignment was also prepared by a two-step wet-chemistry method,10 on which the wettability can change to superhydrophilic after the UV illumination. On the basis of this research, we further fabricated a superhydrophobic ZnO film with hierarchical surface structure directly by the chemical vapor deposition (CVD) method. Different from the alignment mentioned above which can only show the superhydrophobicity several days later, the fresh film here shows the water CA of 164.3°, turning into a superhydrophilic (CA < 5°) one after UV illumination, which can be recovered through being placed in the dark or being heated. Importantly, the surface roughness caused by the special structure is the key factor to affect the hydrophobicity. The fresh film is superhydrophobic as a result of the high temperature applied in the whole process, which can shorten the time of a cycle from superhydrophbic to superhydrophilic and vice versa. This (10) (a) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2004, 126, 62. (b) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357. (11) Izaki, M.; Omi, T. Appl. Phys. Lett. 1996, 68, 2439. (12) Liu, R.; Vertegel, A. A.; Bohannan, E. W.; Sorenson, T. A.; Switzer, J. A. Chem. Mater. 2001, 13, 508. (13) Peulin, S.; Lincot, D. Adv. Mater. 1996, 8, 166. (14) Yoshida, T.; Tochimoto, M.; Schlettwein, D.; Wohrle, D.; Sugiura, T.; Minoura, H. Chem. Mater. 1999, 11, 2657. (15) Saito, N.; Haneda, H.; Sekiguchi, T.; Ohashi, N.; Sakaguchi, I.; Koumoto, K. Adv. Mater. 2002, 14, 418. (16) Reynolds, D. C.; Look, D. C.; Jogai, B. Solid State Commun. 1999, 99, 873. (17) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (18) Sun, R. D.; Nakajima, A.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 2001, 105, 1984. (19) Li, M.; Zhai, J.; Liu, H.; Song, Y.; Jiang, L.; Zhu, D. J. Phys. Chem. B 2003, 107, 9954.
10.1021/la036280o CCC: $27.50 © 2004 American Chemical Society Published on Web 06/02/2004
5660
Langmuir, Vol. 20, No. 14, 2004
Letters
Figure 1. (a) Large-area SEM image of the film catalyzed by Ni; only submicrometer scale structures can be seen. (b) Enlarged view of part a. (c) Large-area SEM image of the superhydrophobic ZnO thin film, catalyzed by Au. (d) Enlarged view of several protuberances from part c, showing hierarchical structure.
Figure 2. Shape of the water droplet on the corresponding surface described in Figure 1. (a) The water CA was 110.6° on the initial surface of the structure in Figure 1a. (b) The water CA was 164.3° on the surface of the structure in Figure 1c. After UV illumination, water drop spread on it quickly.
can certainly further extend the application scopes of ZnO films largely. Experimental Section Films were generally deposited on sapphire substrates. Substrates were rinsed with acetone and air-dried in preparation for deposition before being coated with a layer of catalyst thin film using magnetic sputtering. Equal amounts of ZnO powder and carbon powder were ground together and transferred to a quartz boat. The Au-coated or Ni-coated substrates and the quartz boat were placed into a small quartz tube. The substrates were typically placed about 15-19 cm from the center of the boat. This quartz tube was then placed inside a furnace with the substrates placed downstream of an argon flow (0.0391 SLM). The temperature of the furnace was ramped to 600-650 °C at a rate 1 °C‚s-1 and typically kept at that temperature for 60 min under a constant flow of Ar. After the furnace was cooled to room temperature, a light gray material was found on the surface of the substrate. Different catalysts were presputtered to control the surface morphology of the as-prepared film; thus, the surface wettability was decided. The Au catalyst can yield the superhydrophobic surface. The wettability was characterized by the measurement of the water CA on its surface, using a commercial contact angle meter (Dataphysics, OCA-20). The CAs were measured at five different points of each sample. The surface morphology was examined by scanning electron microscopy (SEM, JEOL JSM6700F, 3.0 kV).
Results and Discussion Parts a and b of Figure 1 are typical field-emission SEM top images of the film catalyzed by the metal Ni at low and high magnifications, respectively, exhibiting the homogeneous sub-microstructures on a large scale. Only sub-micrometer-scaled ZnO slices with the sizes from 130 to 360 nm are distributed on the whole film. The slices are lying on the surface compactly and overlapping each other, reducing the quantum of the trapped air in the grooves of water on them. The roughness is about 86-244.7 nm from the atomic force microsopy (AFM) measurement. Such a small roughness is insufficient to construct a composite surface when the water droplet is placed on it. It more easily makes wetted contact,20 that is, the water can fill the grooves on the surface. The water CA on it is measured as 110.6° (Figure 2a), exhibiting hydrophobicity. Therefore, the roughness structure simply induced by this homogeneous sub-microstructure cannot give the superhydrophobic surface. Interestingly, when Au is used as the catalyst accompanied by no other variations of the experimental conditions, the film exhibits completely different morphology and properties. Figure 1c shows the typical largescale topography SEM image. Many block-shaped pro(20) Patankar, N. A. Langmuir 2003, 19, 1249.
Letters
tuberances ranging from 320 to 560 nm are distributed on the surface densely. The blocks are separated from each other, although they overlap in some areas. The highresolution SEM image (Figure 1d) shows that these protuberances are almost perpendicular to the sapphire substrate. Many ZnO nanopapillae with a size from 21.6 to 41.0 nm are inserted very densely on these protuberances, which exhibit hierarchical structure composed of the sub-micro- and nanostructures similar with those of the lotus leaf8b at a microscopic level. The surface roughness is much higher than the one mentioned above within the range of 600-1191 nm, measured by AFM, which is rough enough to construct a composite surface consisting of ZnO and air upon the water droplet placed on it. The surface with these hierarchical structures (fractal structures) has been well-reported theoretically.21 The fresh film here gives the high water CA of 164.3° (Figure 2b), showing a superhydrophobic property. This is also in accord with the previous work of our group that this kind of surface can give a superhydrophobic surface.8b The wettability of a surface is known as a function of its roughness for a certain material. The smaller surface roughness of the Ni-catalyzed film can give the comparatively small ability to trap air between the interface of the water droplet and the film surface. Therefore, the water can very easily fill the grooves on the rough surface. In this case, the effective contact area enlarged greatly, while the trapped air decreased, which directly results in a decrease of the value of the water CA. Whereas on the Au-catalyzed surface with hierarchical structure, the water drop was inclined to remain in the spherical shape; that is, the water does not fill the grooves on the roughness surface. Upon the water drop on the surface, the typical composite surface can form. So increasing the air fraction can lead to the increasing of the CA because of the air being known as a hydrophobic medium. Different roughnesses can yield different surface wettabilities.22 These results indicate that the superhydrophobicity of the film can be well-attributed to its surface special roughness, that is, the composite surface both with the sub-microand nanostructures. This further makes certain that the composite hierarchical surface structure of the as-prepared film is the indispensable condition for its superhydrophobicity. More importantly, after UV illumination (500-W Hg lamp is used as the light source with a filter centered at 365 nm), the water CAs on both films decrease greatly to superhydrophilic one (CA < 5°). The water droplet spread immediately upon the water drop on it (Figure 2b). After depositing the UV-irradiated film in the dark for several days, the superhydrophobicity can be reversed again, fulfilling a cycle from superhydrophobic to superhydrophilic and vice versa. This process can also be substituted by the heating that can shorten the time of a cycle.23 As is known, the electron and hole generated by ZnO under band gap illumination24 will move to the surface to react with the lattice metal ions Zn2+ to form Zn+ defective sites and the lattice oxygen to form the surface oxygen vacancies, respectively. Water molecules may very easily coordinate into the oxygen vacancy sites, leading to the increase of the water adsorption. When it was placed in the dark or was heated, oxygen atoms that result from the recovery of its initial state can replace these oxygen (21) (a) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, T. Langmuir 1996, 12, 2125. (b) Hazlett, R. D. J. Colloid Interface Sci. 1990, 137, 527. (22) O ¨ ner, D.; McCarthy, T. Langmuir 2000, 16, 7777. (23) Ma, J.; Li, M.; Liu, H.; Feng, L.; Fu, Q.; Jiang, L.; Zhu, D. J. Phys. Chem. B, submitted for publication. (24) Wang, R.; Sakai, N.; Fujishima, A.; Watanable, T.; Hashimoto, K. J. Phys. Chem. B 1999, 103, 2188.
Langmuir, Vol. 20, No. 14, 2004 5661
Figure 3. Time dependence of water contact angle on both films.
vacancies gradually; that is, the wettabily is reconverted from superhydrophilic to superhydrophobic. Further information was acquired from the measurements of the time dependence of the water CA of the asprepared ZnO film under UV illumination (Figure 3). Along with the exposure time under the UV illumination being prolonged, the static water CA is deduced abruptly at the beginning, because of the high-speed yielding of electron-hole pairs on the initial surface. When the water CA turned into about 50°, the change in the velocity is largely slowed, which turned into a superhydrophilic surface finally on both films. Therefore, no matter what the water CA is on its initial surface, it can turn into a highly hydrophilic one under the UV illumination, while the time exposed under the UV is different. The superhydrophobic one needs more time than the other, it probably because the functional surface area is different. It suggests that the photoinduced wettability conversion is independent of its surface structure but depends on its surface chemical composition. Conclusions In conclusion, just as in the nature world, the hierarchical structures similar with the lotus leaf created by nanostructures on sub-microstructures can give a credible approach to constructing superhydrophobic surfaces. Here, the CVD method was used to prepare superhydrophobic ZnO thin films, which proposes a very simple and relatively inexpensive method to produce a good quality film. The fresh film can show the superhydrophobicity, and the film is attached to the substrate tightly. The results confirm that only the composite surface both with the submicrostructure and the nanostructure on it can exhibit superhydrophobicity. This is because it can ensure that there is enough roughness for trapping enough air in it. The photoinduced wettability of its surface from superhydrophobic to superhydrophilic was also studied. The result here can extend to new applications of the ZnO film, such as a self-cleaning field. Acknowledgment. Financial support came from the State Key Project for Fundamental Research (G1999064504) and the Special Research Foundation of the National Natural Science Foundation of China (29992530,20/25/02). Supporting Information Available: The Auger parameter of the prepared film is 2009.7 eV, measured by X-ray photoelectron spectroscopy, which coincides with that of ZnO, indicating the surface property of ZnO. This material is available free of charge via the Internet at http://pubs.acs.org. LA036280O
