Markedly Controllable Adhesion of Superhydrophobic Spongelike

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Markedly Controllable Adhesion of Superhydrophobic Spongelike Nanostructure TiO2 Films Yuekun Lai,†,‡ Changjian Lin,*,†,‡ Jianying Huang,‡ Huifang Zhuang,‡ Lan Sun,‡ and Tinh Nguyen§ State Key Laboratory for Physical Chemistry of Solid Surfaces and College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, China, and National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ReceiVed October 13, 2007. In Final Form: January 28, 2008 A simple electrochemical and self-assembly method was adopted for the fabrication of superhydrophobic spongelike nanostructured TiO2 surfaces with markedly controllable adhesion. Water adhesion ranging from ultralow (5.0 µN) to very high (76.6 µN) can be tuned through adjusting the nitro cellulose dosage concentrations. The detailed experiments and analyses have indicated that the significant increase of adhesion by introducing nitrocellulose is ascribed to the combination of hydrogen bonding between the nitro groups and the hydroxyl groups at the solid/liquid interfaces and the disruption of the densely packed hydrophobic 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PTES) molecule. A mechanism has been proposed to explain the formation of superhydrophobic TiO2 films with distinct adhesion.

1. Introduction In recent years, superhydrophobic surfaces, with a water contact angle (CA) greater than 150°, have attracted considerable interest due to their importance in both fundamental research and practical application.1 It is well-known that the preparation of superhydrophobic surfaces requires not only a suitable rough structure, but also low surface energy materials.2-5 There exists two kinds of extremely superhydrophobic cases in nature, that is, “sliding” superhydrophobic lotus leaves with ultralow water sliding resistance and “sticky” superhydrophobic gecko feet with high adhesive force. These findings have inspired the creation of superhydrophobic functional surfaces with self-cleaning and novel adhesives by mimicking their special structures.6 However, a typical, artificial superhydrophobic surface always has a moderate adhesive force (e.g., approximately 6-8 µN for a surface with * To whom correspondence should be addressed. E-mail:[email protected]. † State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University. ‡ College of Chemistry and Chemical Engineering, Xiamen University. § National Institute of Standards and Technology. (1) (a) Ichimura, K.; Oh, S. K.; Nakagawa, M. Science 2000, 288, 1624. (b) Liu, Y.; Ma, L.; Liu, B. H.; Kong, J. L. Chem.sEur. J. 2005, 11, 2622. (c) Nakajima, A.; Hashimoto, K.; Watanabe, T. Montash. Chem. 2001, 132, 31. (d) Wang, S. T.; Song, Y. L.; Jiang, L. J. Photochem. Photobiol., C 2007, 8, 18. (2) (a) Gao, X. F.; Jiang, L. Nature 2004, 432, 36. (b) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. B. AdV. Mater. 2002, 14, 1857. (c) Gao, X. F.; Yao, X.; Jiang, L. Langmuir 2007, 23, 4886. (d) Feng, X. J.; Zhai, J.; Jiang, L. Angew. Chem., Int. Ed. 2005, 44, 5115. (e) Liu, H.; Zhai, J.; Jiang, L. Soft Matter 2006, 2, 811. (3) (a) Aussillous, P.; Que´re´, D. Nature 2001, 411, 924. (b) Callies, M.; Que´re´, D. Soft Matter 2005, 1, 55. (c) Que´re´, D.; Ajdari, A. Nat. Mater. 2006, 5, 429. (d) Que´re´, D. Rep. Prog. Phys. 2005, 68, 2495. (4) (a) Balaur, E.; Macak, J. M.; Taveira, L.; Schmuki, P. Electrochem. Commun. 2005, 7, 1066. (b) Balaur, E.; Macak, J. M.; Tsuchiya, H.; Schmuki, P. J. Mater. Chem. 2005, 15, 4488. (5) (a) 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. (b) Guo, Z. G.; Fang, J.; Hao, J. C.; Liang, Y. M.; Liu, W. M. ChemPhysChem 2006, 7, 1674. (c) Yu, X.; Wang, Z. Q.; Jiang, Y. G.; Shi, F.; Zhang, X. AdV. Mater. 2005, 17, 1289. (d) Lu, X. Y.; Zhang, C. C.; Han, Y. C. Macromol. Rapid Commun. 2004, 25, 1606. (e) Guo, Z. G.; Zhou, F.; Hao, J. C.; Liu, W. M. J. Am. Chem. Soc. 2005, 127, 15670. (6) (a) Blossey, R. Nat. Mater. 2003, 2, 301. (b) Autumn, K.; Liang, Y. A.; Hsieh, S. T.; Zesch, W.; Chan, W. P.; Kenny, T. W.; Fearing, R.; Full, R. J. Nature 2000, 405, 681. (c) Yabu, H.; Takebayashi, M.; Tanaka, M.; Shimomura, M. Langmuir 2005, 21, 3235. (d) Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H. J. Am. Chem. Soc. 2005, 127, 13458.

a CA of about 160° and sliding angle smaller than 5°). Recently, several attempts have been made to fabricate sticky superhydrophobic surfaces, on which a water droplet does not roll off even with a 180 degree tilt. Jiang and co-workers7 successfully prepared well-ordered superhydrophobic polystyrene nanotubes with alumina membrane templates. They cited that the key factor responsible for the great adhesion is the enhancement of van der Waals forces between the densely packed nonpolar polystyrene nanotubes in close contact with water. Guo et al.8 reported a simple and inexpensive method to fabricate a sticky superhydrophobic surface via etching of an aluminum alloy and eliminating its loose layer. They subsequently demonstrated that the even higher adhesion is due to the capillary forces and van der Waals forces from the micro-orifices and hydrophilic nanoparticle composite structures, respectively. To the best of our knowledge, the fabrication of superhydrophobic films with a wide range of tunable adhesive forces via the control of surface chemical components instead of surface structure is still scarce, and discussion on the adhesion mechanism has been limited.9 It is expected that novel superhydrophobic TiO2 nanostructure films with controllable adhesion to water will have many potential applications, such as sticky tape,10 liquid transportation without loss,11 and microfluidic channels with diminished resistance. Specific TiO2 nanostructures have become a focus of tremendous interest due to their good chemical stability and unique physicochemical properties related to applications including photocatalysis,12 solar cells,13 sensor devices,14 and selfcleaning.15 In this work, we applied a simple electrochemical (7) Jin, M. H.; Feng, X. J.; Feng, L.; Sun, T. L.; Zhai, J.; Li, T. J.; Jiang, L. AdV. Mater. 2005, 17, 1977. (8) Guo, Z. G.; Liu, W. M. Appl. Phys. Lett. 2007, 90, 223111. (9) (a) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818. (b) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (c) Jin, M. H.; Feng, X. J.; Xi, J. M.; Zhai. J.; Cho, K.; Feng, L.; Jiang, L. Macromol. Rapid Commun. 2005, 26, 1805. (d) Boduroglu, S.; Cetinkaya, M.; Dressick, W. J.; Singh, A.; Demirel, M. C. Langmuir 2007, 23, 11391. (10) Geim, A. K.; Dubonos, S. V.; Grigorieva, I. V.; Novoselov, K. S.; Zhukov, A. A.; Shapoval, S. Y. Nat. Mater. 2003, 2, 461. (11) (a) Hong, X.; Gao, X. F.; Jiang, L. J. Am. Chem. Soc. 2007, 129, 1478. (b) Zhao, N.; Xie, Q. D.; Kuang, X.; Wang, S. Q.; Li, Y. F.; Lu, X. Y.; Tan, S. X.; Shen, J.; Zhang, X. L.; Zhang, Y. J.; Xu, J.; Han, C. C. AdV. Funct. Mater. 2007, 17, 2739. (12) Zhuang, H. F.; Lin, C. J.; Lai, Y. K.; Sun, L.; Li, J. EnViron. Sci. Technol. 2007, 41, 4735.

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method16 to fabricate spongelike structured, rough TiO2 films in a large scale directly on a titanium substrate, and then we immersed the as-anodized samples into a mixture of 1H,1H,2H,2Hperfluorooctyltriethoxysilane (PTES) and nitrocellulose (NC) to explore the effects of chemical composition on the surface hydrophobic and adhesive properties. Extensive experimental results indicate that NC concentration has a great effect on both the contact angle and adhesive force for water droplets. This remarkable phenomenon and further mechanism analysis offer us an insight into how to control the surface chemical compositions to regulate the superhydrophobicity with a markedly controllable adhesive force that varies over a wide range (5.0-76.6 µN). This simple method on the control of water adhesion under ambient conditions without resorting to other special coating processes may be conveniently applied to other superhydrophobic surfaces. 2. Experimental Section Preparation of Superhydrophobic TiO2 Nanostructure Films. The spongelike nanostructure TiO2 films were fabricated by electrochemically anodizing titanium sheets (purity 99.5%) in 0.5% (by mass) HF electrolyte with a Pt counter electrode. The anodization was carried out at 50 V for 20 min. The TiO2 film was then rinsed with deionized water and dried in a dry nitrogen stream. The asanodized TiO2 films were treated with a mixed methanolic solution of hydrolyzed 1% by mass PTES (Degussa Co.) with or without NC under basic conditions for 1 h and subsequently baked at 140 °C for 1 h. Characterization of Superhydrophobic TiO2 Nanostructure Films. The surface structure of the spongelike nanostructure TiO2 thin film was examined by using a JSM-6700F field-emission scanning electron microscope (SEM, JEOL, Japan) at 3.0 kV. The crystallinity of the samples before and after annealing treatment was measured using an X-ray diffractometer with Cu KR radiation (XRD, Phillips X’pert-PRO PW3040). The surface roughness of samples before and after PTES-NC modification was measured at ambient conditions by using a Nanoscope IIIa atomic force microscope (AFM, Digital Instruments Inc., CA). The chemical compositions of spongelike nanostructure TiO2 films with or without NC modification were studied by X-ray photoelectron spectroscopy (XPS, VG ESCALAB MK II) using a 300 W Al KR X-ray source (1486.6 eV photons) with a base pressure of about 3 × 10-9 mbar. The binding energies were referenced to 285.0 eV as determined by the locations of the maximum peaks on the C 1s spectra of the hydrocarbon associated with adventitious contamination, while the F 1s peak maximum was set at 689.0 eV for the only PTES and PTES-NC modified surfaces. Peak fit analysis was done using a curve fitting program provided with a Gaussian/Lorentzian curve generator. Water contact angles were measured at ambient temperature using an OCA 20 instrument (Dataphysics, Germany). Drops of 4 µL of various probe liquids were used, and the values reported were the average of five drops per sample at different locations. Deionized water (Millipore, 18 MΩ‚cm) was employed as the source for the CA measurements. The dynamic advancing and receding angles were obtained by the sessile/captive drop method, which were recorded with a high-speed digital camera and analyzed by using commercial software. The advancing and receding angles were measured by averaging the maximum and minimum angles over a large number of measurements while a certain volume of water was gradually added to or withdrawn from the captive drop through the (13) Paulose, M.; Shankar, K.; Varghese, O. K.; Mor, G. K.; Hardin, B.; Grimes, C. A. Nanotechnology 2006, 17, 1446. (14) Varghese, O. K.; Gong, D. W.; Paulose, M.; Ong, K. G.; Grimes, C. A. Sens. Actuators, B 2003, 93, 338. (15) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (16) (a) Gong, D. W.; Grimes, C. A.; Varghese, O. K. J. Mater. Res. 2001, 16, 3331. (b) Beranek, R.; Hildebrand, H.; Schmuki, P. Electrochem. Solid-State Lett. 2003, 6, B12. (c) Lai, Y. K.; Sun, L.; Chen, Y. C.; Zhuang, H. F.; Lin, C. J.; Chin, J. W. J. Electrochem. Soc. 2006, 153, D123. (d) Raja, K. S.; Gandhi, T.; Misra, M. Electrochem. Commun. 2007, 9, 1069.

Lai et al. dispensing needle. The needle remained in contact with the drop during the three-phase boundary moving over the surface. The contact angle hysteresis is defined as the difference between the abovemeasured dynamic advancing and receding contact angles. The surface energy and its component of each sample were estimated using the approximation of Extend Fowkes, a software equipped by DSA 100 (Kru¨ss, Germany), and contact angles were measured with water, diiodomethane, and ethylene glycol. Since the γL, γLd, γLp, and γLh values of the three liquids are known (i.e., for water γL ) 72.8 mN/m, γLd ) 29.1 mN/m, γLP ) 1.3 mN/m, and γLh ) 42.4 mN/m; for diiodomethane γL ) 50.8 mN/m, γLd ) 46.8 mN/m, γLP ) 4.0 mN/m, and γLh ) 0 mN/m; and for ethylene glycol γL ) 48.2 mN/m, γLd ) 29.29 mN/ m, γLP ) 18.91 mN/m, and γLh ) 0 mN/m), the surface energy (γS) of the sample was estimated to be the sum of the calculated γSd, γSP, and γSh values. The superscripts d, p, and h represent the dispersion, polar, and hydrogenbonding forces, respectively. Adhesive Force Measurements. The adhesion of a water droplet on the superhydrophobic spongelike TiO2 thin film was assessed by using a high-sensitivity microelectromechanical balance system (Dataphysics DCAT 11, Germany). A superhydrophobic metal ring suspended with a 3 mg deionized water droplet was connected to the cantilever, and the force of this balance system was first initialized to zero. The nanostructure TiO2 film was then placed on the balance table to approach and retract from the upper water droplet at a constant speed of 0.01 mm/s in ambient air environment with a relative humidity around 40%. The relation between force and distance was recorded automatically by using software when the water droplet contacts and leaves the prepared spongelike TiO2 film. Once the sample contacted the water droplet, the sample kept moving for a 0.05 mm distance to simulate the real situation of a water droplet on the superhydrophobic surface. Adhesive forces were measured at five different locations on each sample, and their average values were reported.

3. Results and Discussion Figure 1 shows typical top and cross-sectional SEM images of the as-anodized samples. It can be seen that the surface has a uniformly distributed spongelike nanostructure and the thickness of the nanostructured TiO2 film is approximately 500 nm. The XRD patterns (Figure 2) reveal that the as-anodized spongelike layers on the titanium substrate contain only the Ti peaks, indicating that such layers have an amorphous structure. After annealing at 450 °C in air for 2 h, the sample has transformed to the anatase phase. On increasing the annealing temperature further to 600 °C, the structure evidently consists of anatase and rutile. Water rapidly spreads and wets this as-anodized spongelike film without PTES modification due to side penetration of the liquid by capillary forces, indicating that such a sample is superhydrophilic. However, it is observed that the droplets with spherical shapes slide spontaneously and hardly come to rest even when they are placed gently onto the PTES modified spongelike TiO2 surface. The water CA on such a spongelike structure film is as high as 160° (shown in the inset of Figure 1a), while that on a regular flat TiO2 surface is only 115° (Figure S1 in the Supporting Information), which is in good agreement with the results of coating fluorocarbon hydrophobic layers on smooth surfaces by self-assembly. To understand the superhydrophobicity of the spongelike TiO2 surface, we describe the contact angle in terms of the Cassie equation:17 cos θr ) f1 cos θ - f2. Here, f1 and f2 are the interfacial area fractions of the spongelike TiO2 surface and of the air in the interspaces surrounding the TiO2 material, respectively (i.e., f1 + f2 ) 1); θr (160°) and θ (115°) are the contact angles on the rough spongelike TiO2 surface and on the self-assembled monolayer of PTES on a flat TiO2 surface, respectively. It is (17) Cassie, A. B. D.; Baxter, S. Discuss. Faraday Soc. 1944, 40, 546.

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Figure 3. (a) Behavior of a water droplet on the PTES modified 50 V spongelike structure surface. The time sequence is shown in the top right corners of the images. (b) Shape of a water droplet on the PTES and 0.2 mg/mL NC modified 50 V spongelike structure surface with different tilt angles: 0°, 90°, and 180°.

Figure 1. (a) SEM image of a typical spongelike structure TiO2 film on titanium substrate by electrochemically anodizing in 0.5 mass % HF solution (50 V for 20 min). (b) Corresponding crosssectional SEM image of the film. The inset of Figure 1a shows the shape of a water droplet on the corresponding PTES modified film.

Figure 4. Advancing and receding angles of the PTES modified spongelike structure TiO2film.

Figure 2. XRD patterns of the sample with and without annealing. A, R, and T represent anatase, rutile, and titanium, respectively.

easy to deduce from this equation that the contact angle of the rough surface (θr) increases with increasing the air fraction (f2). According to the equation, the f2 value of the rough, spongelike TiO2 surface is estimated to be 0.90. Therefore, we can realize that the large fraction of air trapped in the rough surface (i.e., to prevent water from penetrating) is important to the superhydrophobicity. These results demonstrate that the combination of a uniquely rough structure with a large air fraction and a low surface energy coating modification are definitively vital for superhydrophobicity.

The roll-off behavior was recorded via high-speed photography as shown in Figure 3a. The white arrows below each droplet show the sliding direction of the water droplet on the spongelike TiO2 thin film. The sliding behavior shown in the photographs of Figure 3a indicates that this film has an exceptionally low resistance to water droplet rolling. It is also interesting to note that the water droplet bounces off this spongelike structure film when it is dropped from a certain height above the surface. Figure 4 displays the dynamic advancing and receding water contact angles on the spongelike structure TiO2 surface by the sessile/ captive drop method. The very small deviations in both the advancing and receding angles demonstrate that the superhydrophobicity of the as-anodized spongelike surface is uniform. The mean advancing and receding angles of the water droplet on the surface are 160.1° and 159.3°, respectively, indicating that this unique structure film has an ultralow CA hysteresis (only about 0.8°). Up to now, only limited information on the ultralow water hysteresis phenomenon has been reported.2c,18 The reason for such ultralow hysteresis can be attributed to the large fraction of air trapped in the rough spongelike nanostructure TiO2surface, which significantly decreases the contact area. On the other hand, it has been suggested that the unstable discontinuous three-phase (solid-liquid-air) contact line of the (18) Gao, L.; McCarthy, T. J. Langmuir 2007, 23, 9125.

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Figure 5. (a) XPS survey spectra of the spongelike structure TiO2 surface before (1) and after only PTES (2) and PTES-NC modification (3). (b-d) Corresponding high-resolution XPS spectra of C 1s, F1s, and N 1s. The inset of (b) shows the peak fitting of the PTES modified spectrum with six Gaussian components.

rough TiO2surface can lead to a smaller hysteresis than the wellordered two-dimensional surface.19 In contrast to the rolling behavior of a water droplet on the PTES only modified spongelike structure film, the behavior of a water droplet on the surface of the same film that was modified by a mixture of PTES and NC is totally different. Figure 3b shows the shape of a water droplet on the PTES and 0.2 mg/mL NC (PTES-NC) modified spongelike structure film with different tilt angles. The static water CA on the horizontal surface of this material is also very high, approximately 153.6°, indicating that these films retain their superhydrophobicity after the incorporation of NC. For these films, the water droplet adheres firmly to the surface and can resist against its gravitational forces when the sample is tilted vertically (90°) or even turned upside down (180°), indicating that a strong adhesive effect exists between the water droplet and the “sticky” TiO2 spongelike structure surface. From the highly magnified SEM images, we did not see any morphological difference before and after PTES-NC modification. The average surface roughness (Ra) and the root-mean-square roughness (Rrms) values of the as-anodized spongelike TiO2 film (19) (a) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.; Youngblood, J. F.; McCarthy, T. J. Langmuir 1999, 15, 3395. (b) Shiu, J. Y.; Kuo, C. W.; Chen, P.; Mou, C. Y. Chem. Mater. 2004, 16, 561. (c) Marmur, A. Langmuir 2003, 19, 8343.

were estimated from the AFM image to be 6.0 and 7.8 nm, respectively. The values Ra and Rrms change only slightly to 7.2 and 11.0 nm, respectively, after PTES-NC modification. We believe that the drastic increase of adhesion resulting from the incorporation of NC is not due to the small increase in surface roughness, but due to its inherent chemical properties. The surface chemical composition of spongelike samples before and after modification with PTES only or a mixture of PTES and 0.2 mg/mL NC (PTES-NC) was characterized by X-ray photoelectron spectroscopy (XPS). The XPS spectra (Figure 5) indicate that the as-anodized sample is composed of the elements titanium, oxygen, carbon, and some traces of fluorine. The presence of the strong F 1s peak and C 1s peak (due to C-F) together with the C-H appearance and attenuation of the Ti 2p and Ti 2s peaks confirm that PTES has been successfully self-assembled onto the spongelike TiO2 surface (Figure 5a and b). Compared to that of the as-anodized sample, spectra of the sample after PTES modification (inset of Figure 5b) consists of six components centered at binding energies (BEs) of 283.6, 285.0, 286.5, 290.6, 291.8, and 294.1 eV, corresponding to C-Si, CH2-CH2, C-O, CF2-CH2, CF2-CF2, and CF3-CF2 groups, respectively. This is in good agreement with the results reported by Hozumi et al.20 (20) Hozumi, A.; Ushiyama, K.; Sugimura, H.; Takai, O. Langmuir 1999, 15, 7600.

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and Wang et al.21 Another obvious characteristic of the presence of a PTES monolayer is the large intensity increase of the F 1s peak at 689.0 eV and the sharp intensity decrease of the F 1s peak at 684.4 eV (Figure 5c). The main peak corresponding to CFx indicates that the PTES coating is on the outermost surface, while the metal fluoride peak at 684.4 eV is due to anion uptake into the oxide layers during the anodizing process.22 However, when NC is introduced into the film, an obvious decrease of the C-F peak can be observed. This is probably due to the competition between NC and PTES molecules for the hydroxyls on the hydroxylated TiO2 surfaces. More detailed XPS spectra for the corresponding N 1s region of the spongelike TiO2 films with and without 0.2 mg/mL NC are shown in Figure 5d. For the sample containing no NC, there is only one peak in the N 1s spectrum at a BE of 400.1 eV, which could be ascribed to molecularly chemisorbed N2. However, an additional new peak attributed to nitrates is observed at a BE of approximately 407.2 eV with PTES-NC mixed modification. These results confirm that NC has been effectively introduced on the spongelike TiO2 surface.23 Figure 6a shows a typical force-distance curve of the PTES only modified spongelike structure film when it approaches and retracts from the water droplet. From the linear force-distance relationship during the contacting and retracting processes, it reveals that the water droplet always maintains a spherical shape without any noticeable distortion. The adhesive force the water droplet is subjected to pull down by the spongelike structure surface is only approximately 5.0 µN. The low value is attributed to the large fraction of air trapped beneath the water droplet. This effect greatly reduces the solid-liquid interface and the rolling resistance and, thus, cannot create a high adhesive force to the water droplet. The force-distance curve shows a marked change when the spongelike TiO2 nanostructure was coassembled with PTES and 0.2 mg/mL NC under identical experimental conditions (Figure 6b). It is obvious that an attractive force is created as soon as this sticky, superhydrophobic surface makes contact with the water droplet. When the sample is withdrawn from the droplet, the adhesive force gradually increases and the droplet shape changes from spherical to elliptical. Then, just before detachment, it yields the highest adhesive force of approximately of 76.6 µN. To our surprise, no detectable water remained on the PTES-NC modified film surface. The final balance force comes back to nearly zero, also confirming that there is no abrupt water loss, except for faint evaporation of the water droplet during the measurement process under an ambient environment. This high adhesive force is 15 times stronger than that required to remove a 3 mg water droplet from the PTES only modified spongelike TiO2 nanostructure surface. To determine the effects of NC concentration on the adhesion of PTES-NC modified TiO2 nanostructure surfaces, other samples of superhydrophobic, spongelike TiO2 nanostructures modified with different NC concentrations were fabricated. The experimental results indicate that NC concentration has a drastic effect on adhesive force change. Figure 7 shows the influence of NC concentration in the sample on water CA and adhesive force. When the amount of NC increases from nothing to 0.04 and 0.067 mg/mL, the adhesive forces between the water droplet and the spongelike structure film increase sharply from 5.0 to 19.8 and 70.7 µN, respectively. It is noted that the largest deviation of the adhesion values measured in this study is (4.2 µN, (21) Wang, A. F.; Cao, T.; Tang, H. Y.; Liang, X. M.; Black, C.; Sally, S. O.; McAllister, J. P.; Auner, G. W.; Ng, H. Y. S. Colloids Surf., B 2006, 47, 57. (22) Macak, J. M.; Aldabergerova, S.; Ghicov, A.; Schmuki, P. Phys. Status Solidi A 2006, 203, R67. (23) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; Perkin-Elmer: Waltham, MA, 1992.

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Figure 6. Typical force-distance curve of the spongelike structure TiO2 film as it approaches and retracts from a 3 mg water droplet: (a) PTES modification and (b) PTES and 0.2 mg/mL NC mixed modification. The arrows represent the sample’s moving direction relative to the water droplet.

Figure 7. Inter-relationships between water CA and adhesive force with the NC concentration on the spongelike structure TiO2 surface.

indicating that the superhydrophobicity of the modified spongelike surface is uniform and highly reproducible. In contrast, the water CA decreases gradually and then reaches a minimum near 153.5° with an increase of the NC concentration. This difference in the behavior between CA and adhesion suggests that other factors besides wettability may be involved in the adhesion of water of these PTES-NC modified samples.

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Table 1. Dispersive (γSd), Polar (γSP), and Hydrogen-Bonding (γSh) Components of Surface Energies (γS) for the Samples with Only PTES or PTES-NC Modificationa PTES

PTES-NC (0.04 mg/mL)

DI water diiodomethane ethylene glycol

160.0 137.8 139.5

156.6 132.4 137.5

γSd γSP γSh γS b

0.81 0.04 0.20 1.05

1.42 0.01 0.28 1.71

PTES-NC (0.067 mg/mL)

PTES-NC (0.2 mg/mL)

NC

contact angle (deg)

a

154.1 123.2 126.5

surface energy (mN/m) 2.53 0.07 0.65 3.25

153.6 120.2 125.3

119.0 27.9 41.6

3.02 0.10 0.83 3.95

42.65 2.43 7.85 52.93

Values were determined from contact angles using three liquids: water, diiodomethane, and ethylene glycol. bγS ) γSd + γSP + γSh.

To further understand changes of the adhesive force and CA, which may be influenced by the presence or absence of hydrogen bonding, surface energy components of samples with PTES alone and PTES-NC mixed modification were calculated by using the Extend Fowkes approach. The contact angle and surface energy component values are summarized in Table 1, showing that NC concentration has a great effect on both the contact angle and surface energy changes. For samples modified with only PTES, it is expected that a highly oriented monolayer with CF3 terminal groups aligned toward the outer surface has been assembled on the TiO2/Ti surface. The surface energy (γS) values of the PTESNC modified surfaces are all greater than those of the material modified with PTES alone and increase with increasing NC concentration. One reason for the increase of both the dispersion force and polar force components (including hydrogen bonding) with increasing NC concentration is the substitution of the lower energy CF3 groups by the more energetic NO groups on the film surface. Although information on the dispersion and polar forces of the NC material and optically smooth PTES surface (so that true surface free energy values can be obtained) is not available, the surface energy of nitrocellulose has been reported to be 38 mN/m, which is substantially higher than the values ranging from 15 to 24 mN/m for perfluoro- and siloxane-based compounds.24 The substitution of CF3 by NO is also evidenced by the substantial decrease of the F 1s intensity and the appearance of the N peak at a BE of 407.2 eV in the XPS spectra after incorporating NC into the structure (Figure 5c and d). Another reason is the conformation change of the PTES layer resulting from NC incorporation. This comes about because the NC nitro groups had probably formed hydrogen bonds with the silanol groups on the TiO2 surface that were generated from the hydrolysis of the Si-OC2H5 in solution (Figure S2 in the Supporting Information). Such association between NO2 and Si-OH would produce an inhomogeneous, less-ordered, and smaller number of hydrophobic PTES molecules on the nanostructure surface. Such conformation and chemical changes would likely decrease the contact angle and increase the surface energy polar components. A possible model for surface chemical composition changes during the self-assembly process of mixed PTES-NC on hydroxylated spongelike TiO2 surfaces and the formation of hydrogen bonds between the NC nitro group and water hydroxyl is shown in Figure 8. When NC is introduced into the PTES system, competition occurs between the NC and PTES molecules for the hydroxyls on hydroxylated TiO2 surfaces (Figure 8a). This would lead to a disruption of the densely packed hydrophobic (24) Wu, S. Polymer Interface and Adhesion; Marcel Dekker: New York, 1982.

Figure 8. Model of surface chemical composition changes during the PTES-NC mixed layer self-assembly on the hydroxylated spongelike TiO2 surfaces (a) and conformation of hydrogen bonding association between the nitro group and hydroxyl group of the water droplet (b).

PTES molecules and thus a decrease of the CA to some extent.25 The hydrophilic nitro groups on the self-assembled layer surface can readily form hydrogen bonds with water (Figure 8b), which directly provides good adhesion between the PTES-NC modified, spongelike TiO2 layer and water. Therefore, a combination of the hydrogen bonding offered by the nitro groups on the surface and the disruption of the densely packed hydrophobic PTES molecules is the primary factor responsible for the significant increase of adhesion between a PTES-NC modified, spongelike TiO2 film and water.

4. Conclusions By using facile electrochemical oxidation and mixed selfassembly processes with proper control of the nitrocellulose concentration, we have fabricated a spongelike nanostructure of superhydrophobic TiO2 thin films that have a wide range of adhesive forces. The results provide new insights into how to (25) Schondelmaier, D.; Cramm, S.; Klingeler, R.; Morenzin, J.; Zilkens, Ch.; Eberhardt, W. Langmuir 2002, 18, 6242.

Controllable Adhesion of Spongelike TiO2 Films

vary the wettability and adhesion of superhydrophobic surfaces. These stable TiO2 nanostructure superhydrophobic films with exceptional adhesion can potentially be used in many industries and can be further extended to control the adhesion of a wide variety of superhydrophobic functional materials. Acknowledgment. The authors thank the National Nature Science Foundation of China (Grant Nos. 50571085, 20773100, 20620130427) and the Key Scientific Project of Fujian Province, China (Grant No. 2005HZ01-3). The authors would like to thank Prof. Jiang Lei and Dr. Gao Xuefeng of the National Center for

Langmuir, Vol. 24, No. 8, 2008 3873

Nanoscience and Nanotechnology Center for Molecular Science for providing access to facilities for water contact angle and adhesion measurements. Supporting Information Available: Photograph of a water droplet on the PTES modified smooth TiO2 surface and probable situations of the hydrogen bonding association between the nitro groups of the NC and the silanol (Si-OH) groups. This material is available free of charge via the Internet at http://pubs.acs.org. LA7031863