Langmuir 2006, 22, 9477-9479
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Preparation and Photocatalytic Wettability Conversion of TiO2-Based Superhydrophobic Surfaces Xintong Zhang, Ming Jin, Zhaoyue Liu, Shunsuke Nishimoto, Hidenori Saito, Taketoshi Murakami, and Akira Fujishima* Kanagawa Academy of Science and Technology, KSP Building West 614, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012, Japan ReceiVed June 30, 2006. In Final Form: September 28, 2006 We present here a facile method for the preparation of TiO2-based superhydrophobic surfaces. It consists of two steps: (1) roughening of the TiO2 surface with a rf (radio frequency) plasma with CF4 as an etchant and (2) modification of the roughened TiO2 surface with an octadodecylphosphonic acid (ODP) monolayer. Plasma etching caused the thinning of the TiO2 film but at the same time enhanced its surface roughness. A discontinuous wedgelike surface microtexture was formed after etching for 30 s, which, after modification with a monolayer of ODP, showed Cassietype water super-repellency with a contact angle (CA) hysteresis smaller than 2°. The state of water super-repellency (water CA >165°) could be converted to the state of superhydrophilicity (water CA ∼0°) by means of ultraviolet (UV) illumination as a result of the photocatalytic decomposition of the ODP monolayer by TiO2. Readsorption of ODP molecules leads directly to the recovery of water super-repellency.
Superhydrophobic TiO2 surfaces interestingly exhibit extraordinary wettability conversion from superhydrophobicity to superhydrophilicity under UV illumination.1-3 In such a photostimulated wettability conversion, there is a sharp wettability contrast of over 160° in the water contact angle (CA) on the surface, which should have applications in site-selective deposition or transfer of functional materials4,5 and surface-tensionconfined microfluidic devices with open-air structures.4,6-7 Despite these interesting possible applications, only a few reports have dealt with the preparation and photostimulated wettability conversion of TiO2-based superhydrophobic surfaces2,3 because of the lack of facile preparation methods. Herein, we report that superhydrophobic TiO2 surfaces can be easily and reproducibly prepared by roughening the TiO2 surface with a CF4 plasma, followed by modification of the rough surface with a hydrophobic monolayer. The surfaces showed good superhydrophobic properties, and these were easily converted to superhydrophilic ones with UV illumination. CF4 plasmas are often used in the semiconductor industry to etch oxides and polymer materials.8 During discharge, CF4 molecules are dissociated into CF2 and F radicals. These radicals, especially free fluorine, are highly efficient for dry etching processes. Although there have been few reports on the dry etching of TiO2,8 we reasoned that a CF4 plasma could etch TiO2 because the possible etching product, TiF4, which is a liquid (boiling point 284 °C), could be removed from the system by evacuation at elevated substrate temperatures. The rf plasma etching system (BP-1, Samco) used in this study has two parallel plate electrodes in a vacuum chamber. The reactive gas, either CF4 or O2, was introduced below the lower electrode. Caution: CF4 in high * Corresponding author. E-mail:
[email protected]. Tel: +8144-819-2020. Fax: +81-44-819-2038. (1) Sun, T.; Gao, X.; Feng, L.; Jiang, L. Acc. Chem. Res. 2005, 38, 644-652. (2) Tadanaga, K.; Morinaga, J.; Masuda, A.; Minami, T. Chem. Mater. 2000, 12, 590-591. (3) Feng, X.; Zhai, J.; Jiang, L. Angew. Chem., Int. Ed. 2005, 44, 5115-5118. (4) Blossey, R. Nat. Mater. 2003, 2, 301-306. (5) Wang, J. Z.; Zheng, Z. H.; Li, H. W.; Huck, W. T. S.; Sirringhaus, H. Nat. Mater. 2004, 3, 171-176. (6) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R. Science 1999, 283, 46-49. (7) Lam, P.; Wynne, K. J.; Wnek, G. E. Langmuir 2002, 18, 948-951. (8) Pearton, S. J.; Norton, D. P. Plasma Processes Polym. 2005, 2, 16-37.
concentrations may cause asphyxiation. TiO2 films (2 × 2 cm2) were placed on the lower electrode and were first treated with a CF4 plasma (100 W, 27 Pa) for various times at 60 °C substrate temperature, followed by a 30 s treatment with O2 plasma (100 W, 20 Pa) to react any residual titanium fluoride. Polycrystalline TiO2 films (∼1 µm thick) were purchased from Tokita CVD Systems Co., Ltd. The films were prepared on silicon wafers by the CVD method; they were well crystallized in the anatase form. Scanning electron micrographs (SEM) (Hitachi S-4500) show that the film surface is made up of well-faceted microcrystals that are several hundred nanometers in size (Figure 1a). The surface is quite rough and contains a large number of pores. Soon after exposure to the CF4 plasma, the film became thinner, and its surface became rougher (Figures 1 and 2), which is clear evidence of the etching function of the plasma. The etching appeared to proceed more easily along grain boundaries because of the higher defect concentrations; the film-thinning rate was 8-10 nm s-1. No appreciable change was observed in the X-ray diffraction patterns of TiO2 film during plasma etching (Supporting Information). Etching times longer than 30 s caused the formation of a discontinuous wedgelike surface microtexture, which can be seen clearly in the cross-section micrograph shown in Figure 2. The wedges became thinner and shorter and lost height gradually with extended etching as a result of the isotropic nature of plasma etching.8 Approximately 2 min of etching was sufficient to remove the entire TiO2 film from the Si substrate. Immediately after etching, the TiO2 surfaces always showed a nearly 0° water CA (Kyowa CA-X) as a result of their enhanced roughness and intrinsic hydrophilic properties.9,10 To impart the etched surface with superhydrophobic properties, hydrophobic surface modification was found to be necessary. We chose octadecylphosphonic acid (ODP) to modify TiO2 surfaces in our work. This molecule can form a dense self-assembled monolayer on the TiO2 surface as a result of the strong chelating bonds between phosphonic acid headgroups and Ti atoms on the surface;11-13 the dense packing of hydrocarbon chains thus made (9) Zhang, X.-T.; Sato, O.; Taguchi, M.; Einaga, Y.; Murakami, T.; Fujishima, A. Chem. Mater. 2005, 17, 696-700. (10) Gu, Z.-Z.; Fujishima, A.; Sato, O. Appl. Phys. Lett. 2004, 85, 5067-5069. (11) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813-824.
10.1021/la0618869 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/13/2006
9478 Langmuir, Vol. 22, No. 23, 2006
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Figure 3. Influence of etching time on the water contact angle of TiO2 surfaces after being modified with an ODP monolayer: 9, static contact angle; O, advancing contact angle; and 2, receding contact angle. Note that the static contact angle data for samples etched for more than 30 s were estimated as the average of the advancing and receding contact angles. (Inset) Dependence of contact angle hysteresis (CAH) on etching time.
Figure 1. Scanning electron micrographs of TiO2 surfaces etched for various times: (a) no etching and (b) 10, (c) 20, (d) 30, (e) 40, and (f) 50 s.
Figure 4. (Left) A water droplet was placed on an ODP-modified TiO2 superhydrophobic surface. (Right) Water completely wets a UV-treated ODP-modified TiO2 superhydrophobic surface. Note that this water-wettable surface was able to be reconverted to the nonwettable state with the readsorption of ODP molecules.
Figure 2. Scanning electron micrographs of the cross sections of TiO2 films: (a) nonetched film and (b) 40 s etched film.
the TiO2 surface hydrophobic.11 Typically, TiO2 surfaces were immersed in 0.5 mM ODP in a mixed solvent of heptane and 2-propanol (1000:7 v/v) for 48 h at room temperature, and afterwards, they were rinsed thoroughly with 2-propanol to remove any physisorbed ODP molecules. An ODP-modified smooth TiO2 surface showed a static water CA of about 108°, whereas the nonetched TiO2 film used in this study showed a static water CA of 130° after ODP modification (Figure 3) due to its rough surface morphology. The plasma-etched TiO2 surfaces showed even larger water CAs after ODP modification. The water CA on a 10 s-etched TiO2 surface was greater than 150° after ODP modification. For the samples etched for more than 30 s, water CA measurements became impossible by the sessile method because the water droplet did not adhere to the surface. Water droplets either bounced or rolled on such surfaces, and the latter occurred when there was a slight tilt (Supporting Information), indicating very good superhydrophobicity. We thus used dynamic water CAs to characterize the ODPmodified surfaces (Figure 3). The nonetched surface showed 147 and 98° for advancing and receding CAs, respectively. The (12) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1996, 12, 6429-6435. (13) Helmy, R.; Fadeev, A. Y. Langmuir 2002, 18, 8924-8928.
contact angle hysteresis (CAH), which is the difference between the advancing and receding contact angles, was 49°. Plasma etching caused an increase in either the advancing or receding CA of water, but the receding one increased faster. As a result, we observed decreases in CAH with increased etching time (Figure 3 inset). The 30 s etched surface showed advancing/ receding CAs of 167/165° and a CAH of 2°, whereas for the 40 s etched surface both advancing and receding water CAs were 168°, with a resulting CAH of 0°. The large receding CA, together with the extremely low CAH, caused the nonadherence phenomenon mentioned above for water.14-16 As often mentioned in the literature, there are two models to describe the wetting of water on a microtextured hydrophobic surface. The first is the Wenzel model, described as eq 1, which emphasizes the role of the roughness of the hydrophobic surface: 17
cos θ* ) r cos θ
(1)
Here, the apparent water CA (θ*) is larger than the intrinsic CA (θ) as a result of the roughness (r) of the surface. If the contact angle is large and the surface is sufficiently rough, then air pockets may be trapped between the solid and liquid phases so as to give a composite surface. In such a case, the wetting of water should (14) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; O ¨ ner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395-3399. (15) Zhang, X.-T.; Sato, O.; Fujishima, A. Langmuir 2004, 20, 6065-6067. (16) Gao, L.; McCarthy, T. J. Langmuir 2006, 22, 2966-67. (17) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988-994.
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Langmuir, Vol. 22, No. 23, 2006 9479
be described by the Cassie model, as given by eq 2:18
cos θ* ) f1 cos θ - f2
(2)
Here, the apparent CA (θ*) is greatly influenced by the surface fraction of the solid (f1) versus air pockets (f2). Even though both models predict the superhydrophobic state, the Wenzel and Cassie states are quite different in terms of liquid adhesion. The Wenzel superhydrophobic state often exhibits a large CAH (i.e., it is sticky for water droplets) whereas the Cassie-type state can exhibit a small CAH (i.e., it can be slippery for water droplets and thus more important for practical superhydrophobic applications).19,20 Our studies showed that the CF4 plasma could effectively roughen the TiO2 surface; the roughened surfaces exhibited a large CA and a small CAH for water after hydrophobic modification. In particular, the wedgelike surface can give an almost 0° CAH after ODP modification. We consider two reasons here that may account for the good superhydrophobic property of wedgelike surfaces. One is that the steep slope of the hydrophobic wedges prevents water droplets from intruding onto the surface and thus makes the surface capable of trapping air pockets effectively;20 the behavior of a water droplet in contact with the composite surface should be described by the Cassie model. The other one is that wedges surrounded by air pockets are discontinuous at the topmost surface; a water droplet in contact with the surface thus should exhibit discontinuous three-phase (air-liquidsurface) contact lines. The discontinuous structure of contact lines may reduce the energy barriers required for contact line movement, as suggested by McCarthy et al.,14,16 thus resulting in extremely small CAHs. Note that the 20-s-etched TiO2 film showed almost the same static and advancing water CAs as the 30-s-etched film, but the CAH of the former is 19° larger than the latter. This suggests that the CAH as well as the receding CA is more sensitive to surface microtexture than the static and advancing CAs; this point should be considered in the design and preparation of superhydrophobic surfaces. The TiO2-based superhydrophobic surface was stable against immersion in acidic or weakly alkaline solution and mild heating below 150 °C. However, the surface gradually lost superhydrophobicity under UV illumination because of the decomposition of the ODP monolayer as a result of photocatalysis by TiO2, as evidenced by in-situ IR measurements (Jasco FT-IR 6100). UV illumination caused the decrease of both advancing and receding CAs of the surface, but the receding one decreased faster. For the 30-s-etched sample, 1 h of UV illumination (5 mW cm-2, Hayashi LA-310) was able to convert the surface from super(18) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546-551 (19) Lafuma, A.; Que´re´, D. Nat. Mater. 2003, 2, 457-460. (20) Que´re´, D. Rep. Prog. Phys. 2005, 68, 2495-2532.
hydrophobic (CA > 165°) to superhydrophilic (CA ≈ 0°). The conversion rate was approximately dependent on the square root of light intensity for our light conditions (1-10 mW cm-2), which is similar to the case with other photocatalytic reactions.21,22 Dipping the superhydrophilic TiO2 surface into the ODP solution again recovered the superhydrophobic state as a result of the readsorption of the ODP monolayer. This UV/readsorption cycle was repeated more than five times without any loss of the superhydrophobicity of the TiO2 surface. Considering the sharp wettability contrast between the superhydrophobic and superhydrophilic states, it should be interesting to fabricate highperformance wettability micropatterns from such TiO2-based superhydrophobic surfaces with UV light; this will be discussed in a separate publication. In summary, the CF4 plasma was found to be a highly effective technique for roughening polycrystalline TiO2 films. The superhydrophobicity of the ODP-modified TiO2 surfaces in terms of water CA and CAH was closely related to the surface microtexture altered by plasma etching. Wedgelike surfaces exhibited large water CAs (>165°) and extremely small CAHs (∼0°) after ODP modification; this was explained by the Cassie model and a consideration of the contact line. One of the particularly interesting aspects of the TiO2-based superhydrophobic surfaces is that they exhibit a UV-stimulated wettability conversion from superhydrophobicity to superhydrophility due to the photocatalytic decomposition of the hydrophobic monolayer. This special property could be utilized to fabricate micropatterns with sharply contrasting wettability. Additional studies are being carrying out to examine the possible applications of such surfaces, for example, in the printing of functional materials or in microfluidic devices. Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government and by the Core Research for Evolutional Science and Technology (CREST) Program of the Japan Science and Technology (JST) Agency. We thank Dr. D. A. Tryk for a careful reading of the manuscript and helpful discussions. Supporting Information Available: X-ray diffraction patterns of a TiO2 film before and after plasma etching. A movie of the nonadherent surface and movies of water droplets bouncing or rolling on the superhydrophobic TiO2 surface. This material is available free of charge via the Internet at http://pubs.acs.org. LA0618869 (21) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C: Photochem. ReV. 2000, 1, 1-21 and references therein. (22) Fujishima, A.; Zhang, X. C. R. Chimie 2006, 9, 750-760 and references therein.
