From Superhydrophilic to Superhydrophobic: Controlling Wettability of

Feb 10, 2009 - Hydroxide zinc carbonate (HZC) films with different morphologies were deposited on zinc plates through a convenient chemical-bath depos...
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Langmuir 2009, 25, 3640-3645

From Superhydrophilic to Superhydrophobic: Controlling Wettability of Hydroxide Zinc Carbonate Film on Zinc Plates Bin Su, Mei Li, Zhengyu Shi, and Qinghua Lu* School of Chemistry and Chemical Technology, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong UniVersity, Shanghai 200240, P. R. China ReceiVed NoVember 30, 2008. ReVised Manuscript ReceiVed January 11, 2009 Hydroxide zinc carbonate (HZC) films with different morphologies were deposited on zinc plates through a convenient chemical-bath deposition (CBD) process using urea solution. By altering deposition conditions, the structure of HZC crystals could be tuned from vertically aligned nanosheets to flowerlike microstructures. The upright-standing HZC nanosheets were developed from interconnected nanorods growing up on the zinc plates in aqueous urea solution and led to superhydrophilic properties because of the hydrophilic upside edges consisting of hydroxyl groups and large distance. In contrast, flowerlike microstructures formed in N,N-dimethylformamide-water solution and the exposed hydrophobic crystal planes resulted in superhydrophobic properties with a water contact angle as high as 155.2°. The final surface wettabilities could be ascribed to both the atomic composition and hydrophilicity of HZC crystal planes exposed to the water-solid interface. All the surfaces with specific wettabilities can be one-step fabricated without subsequent modification, and tunable wetting properties can provide zinc substrates extending applications.

* To whom correspondence should be addressed: Fax +86-21-54747535; e-mail [email protected].

philic nature on titanium substrates.19 By adding poly(ethylene glycol), Amal et al. fabricated nanoporous titania films showing superwetting properties without UV illumination.20 On the other hand, superhydrophobic metallic surfaces were often achieved by modifying microstructured surfaces with low surface free energy materials. For example, Shi et al. fabricated a superhydrophilic surface with lotus-leaf-like binary micronanostructures on a copper plate, and subsequent self-assembly with ndodecanethiol led to dramatic superhydrophobic properties.21 Guo et al. prepared well-aligned single-crystalline zinc oxide films. After modifying the films with octadecanethiol, they obtained a superhydrophobic surface with a water CA of 156.2°.22 However, when exposed to organic solvents or atmosphere for a long time, the self-assembled thiol monolayer tends to lose hydrophobic property because of dissolution or destruction.23 Therefore, fabricating superhydrophobic surfaces without modification is of practical importance. Although current techniques enable researchers to generate superhydrophilic and superhydrophobic surfaces on the same metal substrate, to date, it still remains a great challenge to design and generate both of the extreme surface wettabilities on the same material surface just by rendering different geometric microstructures without following modification.

(1) Cebeci, F. C.; Wu, Z. Z.; Zhai, L.; Cohen, R. E.; Rubner, M. F. Langmuir 2006, 22, 2856–2862. (2) (a) Hong, X.; Gao, X. F.; Jiang, L. J. Am. Chem. Soc. 2007, 129, 1478– 1479. (b) Suh, K. Y.; Jon, S. Langmuir 2005, 21, 6836–6841. (3) Blossey, R. Nat. Mater. 2003, 2, 301–306. (4) (a) Ferrari, M.; Ravera, F.; Liggieri, L. Appl. Phys. Lett. 2006, 88, 203125. (b) Gao, L. C.; McCarthy, T. J. J. Am. Chem. Soc. 2006, 128, 9052–9053. (5) Genzer, J.; Efimenko, K. Science 2000, 290, 2130–2133. (6) Yang, S. Y.; Chen, S.; Tian, Y.; et al. Chem. Mater. 2008, 20, 1233–1235. (7) (a) Teare, D. O. H.; Spanos, C. G.; Ridley, P.; et al. Chem. Mater. 2002, 14, 4566–4571. (b) Tsoi, S. F.; Fok, E.; Sit, J. S.; et al. Chem. Mater. 2006, 18, 5260–5266. (c) Zhang, L. B.; Chen, H.; Sun, J. Q.; et al. Chem. Mater. 2007, 19, 948–953. (8) (a) Callies, M.; Que´re´, D. Soft Matter 2005, 1, 55–61. (b) Lafuma, A.; Que´re´, D. Nat. Mater. 2003, 2, 457–460. (9) Otten, A.; Herminghaus, S. Langmuir 2004, 20, 2405–2408. (10) Zhang, X.; Shi, F.; Yu, X.; et al. J. Am. Chem. Soc. 2004, 126, 3064–3065. (11) Wang, Z.; Koratkar, N.; Ci, L.; Ajayan, P. M. Appl. Phys. Lett. 2007, 90, 143117. (12) Lenz, P. AdV. Mater. 1999, 11, 1531–1534. (13) Kuiper, S.; Hendriks, B. H. W. Appl. Phys. Lett. 2004, 85, 1128–1130. (14) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R. Science 1999, 283, 46–49.

(15) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature (London) 1997, 388, 431– 432. (16) (a) Aussillous, P.; Que’re’, D. Nature (London) 2001, 411, 924–928. (b) Bico, J.; Tordeux, C.; Que’re’, D. Europhys. Lett. 2001, 55, 214–220. (c) Bico, J.; Thiele, U.; Que’re’, D. Colloids Surf., A 2002, 206, 41–46. (d) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31–41. (e) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanade, T. AdV. Mater. 1999, 11, 1365– 1368. (f) Tadanaga, T.; Morinaga, J.; Matsuda, A.; Minami, T. Chem. Mater. 2000, 12, 590–592. (17) Yoshida, T.; Tochimoto, M.; Schlettwein, D.; Wohrle, D.; Sugiura, T.; Minoura, H. Chem. Mater. 1999, 11, 2657–2667. (18) Vogler, E. A. AdV. Colloid Interface Sci. 1998, 74, 69–117. (19) (a) Hosono, E.; Matsuda, H.; Honma, I.; Ichihara, M.; Zhou, H. S. Langmuir 2007, 23, 7447–7450. (b) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 1999, 103, 2188–2194. (20) Gan, W. Y.; Lam, S. W.; Chiang, K.; Amal, R.; Zhao, H. J.; Brungs, M. P. J. Mater. Chem. 2007, 17, 952–954. (21) Wu, X. F.; Shi, G. Q. Nanotechnology 2005, 16, 2056–2060. (22) Guo, M.; Diao, P.; Cai, S. M. Thin Solid Films 2007, 515, 7162–7166. (23) Xi, J. M.; Feng, L.; Jiang, L. Appl. Phys. Lett. 2008, 92, 053102.

1. Introduction Wettability of a solid surface is important in our daily life as well as in many industrial processes. Especially, surfaces with extreme wettability such as superhydrophilic (water contact angle, CA 150°) have attracted great interest in fundamental research fields of biology,2 chemistry,3-7 physics,8,9 and materials science10,11 in recent years because of their widely practical applications. For example, superhydrophilic surfaces can serve as antifogging windows or bacteria-resistant coatings,12 and superhydrophobic surfaces can be employed in tunable optical lenses,13 low flow resistance coatings in microfluidic systems,14 and nonadhesive coatings on outdoor antennae.15-17 Surface wettability is believed to be governed by both the chemical composition and geometric structure of the surface material. Conventionally, superhydrophilic metallic surfaces are mainly produced by introducing hydrophilic compounds with microstructures onto metal substrates.18 For instance, Zhou et al. prepared upright-standing titania nanosheets with superhydro-

10.1021/la803948m CCC: $40.75  2009 American Chemical Society Published on Web 02/10/2009

From Superhydrophilic to Superhydrophobic

Recently, Yang et al. have reported that the surface energies of anatase titania (001) surface terminated with H, B, C, or N atoms are higher than those of the (101) surface due to different bonding energies.24 Williams et al. measured contact angles on the different facets of macroscopic form I paracetamol crystals, and they found that the hydrophilicity order, as determined from the water contact angle, was (001) > (011) > (201) > (110), which correlated well with the concentration of exposed polar groups and the predicted surface hydroxyl group density.25 These interesting discoveries suggest a possible strategy to manipulate surface wettability through preferential growth of crystal planes with different hydrophilicities. In this work reported herein, we attempt for the first time to take advantage of this strategy and present a facile method for controlling surface wettabilities resulting from different morphologies of hydroxide zinc carbonate (HZC) films deposited on zinc plates through a chemical-bath deposition (CBD) process. By altering deposition conditions, the structure of HZC crystals can be easily tuned from vertically aligned nanosheets to mircoflowers, leading to the transition from superhydrophilic to superhydrophobic surfaces without following chemical modification. We believe that the atomic composition and hydrophilicity of HZC crystal planes exposed to the water-solid interface contribute to the final surface wettabilities. This simple and inexpensive approach is feasible for operation in laboratory and industry. Considering wide applications of zinc in mechanical and electroplating industries, tunable wetting properties can provide zinc substrates extending functions.

2. Experimental Section 2.1. Deposition of HZC Surface Layer. All chemicals (Sinopharm Chemical Reagent Co.) were of analytical grade and used without further purification. The HZC films were fabricated through a CBD process, and 2 × 2 cm2 zinc plates (99.5%) were used as substrates for the deposition. The substrates were immersed in an aqueous solution of urea or a 1.5 M urea solution prepared with deionized water and organic solvent (v/v ) 2:1) including acetone or N,N-dimethylformamide (DMF) in a sealed bottle and kept at 80 °C for 1-72 h. Then the zinc plates were washed several times with deionized water and dried at ambient temperature. 2.2. Characterization. Contact angle measurements were performed using the sessile drop method on a Contact Angle System OCA 20 (DataPhysics Instruments GmbH, Germany) in air. The contact angles reported here were the mean values measured with a 3 µL water droplet at five different positions on each sample. Advancing and receding contact angles were measured by increasing or decreasing the volume of the water drops sitting on the surfaces. Sliding angles were determined by slowly tilting the sample stage until a 12 µL water drop started moving. The topography of HZC films was observed using a field emission scanning electron microscopy (FE-SEM, JEOL JSM-7401F). The microstructures of HCZ nanosheets were examined using transmission electron microscopy (TEM, JEOL TEM-2100) operating at an accelerating voltage of 200 kV. The HZC films were scraped directly onto a copper grid for TEM observation. Powder X-ray diffraction (XRD) patterns were taken on an X-ray polycrystalline diffractometer (D8 ADVANCE, Bruker) using Cu KR radiation. X-ray photoelectron spectra (XPS) were collected in a Kratos Axis Ultra DLD system, and an Al anode (KR radiation at 1486.6 eV) was operated at 15 kV with a current of 10 mA. The C 1s orbital at a binding energy of 284.8 eV was used to correct for sample charging. Reflective Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Equinox 55 spectrometer. (24) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Nature (London) 2008, 453, 638–641. (25) (a) Heng, J. Y. Y.; Bismarck, A.; Lee, A. F.; Wilson, K.; Williams, D. R. Langmuir 2006, 22, 2760–2769. (b) Heng, J. Y. Y.; Bismarck, A.; Lee, A. F.; Wilson, K.; Williams, D. R. J. Pharm. Sci. 2007, 96, 2134–2144.

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Figure 1. A water droplet spreading on the upright-standing hydroxide zinc carbonate (HZC) film.

3. Results and Discussion 3.1. Morphology and Chemical Composition of Superhydrophilic Zinc Surface. A zinc plate was deposited with an off-white film when immersed in a 2 M aqueous urea solution. As shown in Figure 1, after deposited for 24 h, the zinc plate shows superhydrophilic property. The water contact angle on this highly wettable surface decreases to 0° in 1 s. The top-view and cross-sectional SEM images in Figure 2A,C elucidate that this film consists of bundled upright-standing nanosheets. The thickness of the nanosheets is about 30-80 nm, and the distance between the nanosheets ranges from 50 nm to 15 µm (Figure 2B). The thickness of the as-deposited film is approximately 10-15 µm. XRD was employed to analyze chemical structure of the vertically aligned nanosheets deposited on zinc substrates. As shown in Figure 3A, compared with the pure zinc plate (pattern m), the deposited film (pattern n) shows specific diffraction peaks in the 2θ range from 12° to 34°, which can be indexed to hydroxide zinc carbonate (HZC), Zn5(CO3)2(OH)6 (in Joint Committee on Powder Diffraction Standards, JCPDS card No. 19-1458). The space group of HZC crystal is monoclinic lattice C2/m, and the lattice parameters are calculated to be a ) 13.54 Å, b ) 6.34 Å, c ) 5.25 Å, and β ) 94° according to the XRD patterns. (Detailed information on HZC crystal is illustrated in Figure S1.) The formation of HZC crystals is initiated by the slow reaction between urea and water. The hydrolysis of urea not only provides carbonate but also gradually turns the solution to weak alkaline with increasing hydroxyl.26,27 Then a slow reaction between zinc atoms on the surface of the substrate and carbonates in the solution leads to the formation of HZC.28 We also compare the XRD pattern n with l (the powder scraped from the same deposited film). The relative intensity of peaks (200) and (020) in pattern n is much weaker, while intensity of peak (002) is stronger than that of pattern l. The intensity ratio R of I(200) to I(002) was 0.49 for pattern n and 3.23 for pattern l. These results imply that the a- and b-axes of the HZC crystals are parallel, while c-axis is vertical to the zinc surface.29 On the basis of the above analysis, we constructed a theoretical crystal model of HZC (Figure 3B). The (002) crystal plane (labeled (26) Kakiuchi, K.; Hosono, E.; Kimura, T.; Imai, H.; Fujihara, S. J. Sol-Gel. Sci. Technol. 2006, 39, 63–72. (27) Xu, L. P.; Ding, Y. S.; Chen, C. H.; Zhao, L. L.; Rimkus, C.; Joesten, R.; Suib, S. L. Chem. Mater. 2008, 20, 308–316. (28) Zhou, X. F.; Hu, Z. L.; Fan, Y. Q.; Chen, S.; Ding, W. P.; Xu, N. P. J. Phys. Chem. C 2008, 112, 11722–11728.

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Figure 2. (A, B) Top-view and (C) cross-sectional SEM images of the upright-standing HZC film deposited in a 2 M aqueous urea solution. Scale bar is 2 µm in (A, C) and 100 nm in (B).

Figure 3. (A) XRD patterns of (m) pure zinc plate, (n) HZC film grown on zinc plate, and (l) HZC powder scraped from the deposited film. (B) Theoretical structure of hydroxide zinc carbonate showing (002) crystal plane (atom colors: O, red; Zn, pink; C, gray). The arrow shows red oxygen atoms in the (002) plane. (C) Nucleation and crystal growth model of HZC showing the hydrophilic (002) crystal plane and hydrophobic (200) crystal plane. (D) Refective FT-IR spectra of (a) pure zinc plate and (b) the HZC film deposited in aqueous urea solution. (E) XPS survey spectrum of the HZC film deposited in aqueous urea solution.

with green, corresponding to the upside edge of vertical HZC nanosheets in Figure 3C) comprises six oxygen atoms, indicating overrich hydroxyl groups. (Crystal planes of HZC are also shown in Figure S1.) It has been reported that higher concentration of surface polar groups such as hydroxyl groups leads to a more hydrophilic behavior,25 even to superhydrophilicity for UVirradiated titania films;19 therefore, the upside edges of HZC nanosheets can exhibit quite hydrophilic properties. However, the sides of these nanosheets, mainly composed of zinc atoms (labeled with pink), will show relative hydrophobicity. Zhou’s group deposited similar HZC films with a vertical b-axis on glass substrates using zinc nitrate and urea solution and proposed that the nanosheets grew perpendicular to the substrate surface after the nucleation because of the concentration gradient around a growing crystal.29 In our case, when a water droplet is released onto the as-deposited HZC films, it is inclined to reach every upside edge of vertical nanosheets mainly containing hydrophilic groups. Moreover, the water drop tends to penetrate into the microgrooves between nanosheets due to the large distance.19 As a result, the water droplet spreads out quickly over the whole surface. In addition, reflective FT-IR and XPS spectra were also employed to analyze the chemical composition of the deposited layers. As shown in Figure 3D, compared with pure zinc plate, (29) Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H. S. AdV. Mater. 2005, 17, 2091–2094.

a broad absorption band coming from -OH groups appears at around 3294 cm-1, which agrees well with the predicted model of hydrophilic (002) crystalline plane (Figure 3B). The absorption bands corresponding to the CO32- group can also be observed for the deposited sample: the absorption band at 1043 cm-1 is assigned to the symmetric stretching ν1 mode, the strong bands at 1498 and 1394 cm-1 come from the asymmetric stretching ν3 modes, and the sharp band at 833 cm-1 should be attributed to the out-of-plane deformation ν2 mode. The presence of the “Zn-OH” bonding is also observed at 955 cm-1.26 Figure 3E shows the XPS survey spectrum of the HZC film. All signals of Zn, C, and O elements appear in this spectrum, and the atomic ratio of O to Zn is 2.24, which is in accordance with 2.4 of ideal hydroxide zinc carbonate considering defects in the as-prepared HZC nanosheets. These results further confirm the formation of HZC crystals on a zinc plate. 3.2. Evolution of Superficial Morphology and Wettability of Deposited Surface Layer. The morphological evolution of zinc surface during the CBD process in 2 M aqueous urea solution was monitored using SEM, and the contact angle were measured. As shown in Figure 4A-C, the water CA on a pure flat zinc plate is 70.7°. When the substrate is deposited for 1 h, sparse nanorods appear on the surface and some cumulate, leading to a decrease of water CA to 69.0° (Figure 4D-F). After a deposition for 3 h, numerous nanorods grow up on the zinc plate and result in a uniform distribution over the whole surface. Because these

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Figure 5. SEM images at different magnifications of HZC films deposited in (A, B) 0.3 M, (D, E) 1 M, and (G, H) 3 M aqueous urea solution and (C, F, I) corresponding profiles of water droplets. Scale bar is 10 µm in (A, D, G) and 2 µm in (B, E, H). (J) XRD patterns of HZC films deposited in 2 and 3 M aqueous urea solutions. (K) Schematic illustrations of HZC microspheres and microflowers, corresponding to (E) and (H), showing the crystal planes with different wettabilities.

Figure 4. SEM images at different magnifications of HZC films deposited in a 2 M aqueous urea solution for (A, B) 0 h, (D, E) 1 h, (G, H) 3 h, (J, K) 6 h, (M, N) 24 h, and (P, Q) 72 h. (C, F, I, L, O, R) corresponding profiles of water droplets. Scale bar is 10 µm in (A, D, G, J, M, P) and 2 µm in (B, E, H, K, N, Q).

nanorods pile up to form clusters (Figure 4G,H), the surface became more undulant with a continuous decrease of water CA to 40.4°. Upon a further increase of deposition time, longer paratactic nanorods begin to connect with each other, leading to the formation of nanosheets in some areas (Figure 4J). As a result, the water CA fell down to 32.1°. It is worth noting that in Figure 4K atelic HZC nanosheets with irregular edges evidently disclose the transformation process from nanorods to nanosheets. When the deposition process is extended to 24 h, the uprightstanding nanosheets are maturely developed. A water droplet spreads out on the surface within 1s (Figure 4O), and this superhydrophilic behavior is mainly due to continuous growth of hydrophilic edge of vertical HZC nanosheets and the large distance between the nanosheets. Elongation of deposition time to 72 h shows no obvious influence on the surface morphology as well as the resulted superhydrophilic properties (Figure 4P-R).

The concentration of aqueous urea solution also plays an important role in shaping the topography of deposited HZC films through altering the crystal nucleation and the concentration gradient. As shown in Figure 5A,B, when the zinc substrate is deposited in a 0.3 M urea solution for 24 h, HZC sea urchins consisting of nanorods appear on the whole surface and lap over each other. The surface roughness is so high that the surface looks undulant even in the image of CA measurement (Figure 5C). When the concentration of urea solution is increased to 1 M, thousands of compacted HZC microspheres grow up (Figure 5D). A further increment of the urea concentration to 2 M leads to the formation of upright-standing HZC nanosheets. Despite various morphologies of the deposited films, all these surfaces show superhydrophilic properties due to the uncovered hydrophilic upside edges of HZC crystals and large distance between the microstructures (see schematic illustration in Figure 5K). An interesting phenomenon was observed when the concentration of urea solution reached 3 M. HZC microflowers, about 5-10 µm, appear on the top of the upright-standing nanosheets, and the water CA sharply increases to 82.7° (Figure 5G). Although XRD patterns discover that all the deposited films are HZC crystal (Figure S2), the intensity ratio R of I(200) to I(002) deposited in a 3 M urea solution is 1.56, which is higher than that deposited in a 2 M urea solution (0.49) (Figure 5J). This result indicates that hydrophobic (200) crystal planes in the HZC microflowers grow preferentially in the horizontal plane, and increased R value

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Figure 6. SEM images at different magnifications of HZC films deposited in urea solution with addition of (A, B) acetone and (E, F) DMF. (C, G) Schematic illustrations. Corresponding profiles of water drops (D, H) residing and (J) sliding on HZC films. (I) Cross-sectional SEM image of (E). Scale bar is 10 µm in (A, E, I) and 2 µm in (B, F).

corresponds to improved hydrophobic property. As illustrated in Figure 5H, the exposure of hydrophobic crystal planes in HZC microflowers to the water-solid interface (see schematic illustration in Figure 5K) prevents water droplets from protruding into the bottom hydrophilic edges of HZC nanosheets, to some degree. Therefore, the flowery surface shows a more hydrophobic behavior. 3.3. Superhydrophobic Zinc Surface. As analyzed above, the experimental results verified that the surface hydrophobicity could be efficiently improved by rendering different morphologies with preferential growth of crystal planes. So we adopted watersoluble organic solvents into the chemical bath considering the organic solvents will affect the nucleation and crystal growth of HZC by changing concentration gradient in the solution and effectively reactive area between zinc surface and urea. Acetone and DMF were proven to be effective in improving the hydrophobicity. As shown in Figure 6A,B, owing to the addition of acetone, the deposited surface is covered with interconnected microspheres with diameter of around 10-15 µm. Crooked hydrophobic crystal plane are discovered on the surface of HZC microspheres. In contrast, thousands of microflowers are arrayed compactly over the whole zinc surface with the adoption of DMF in the chemical bath (Figure 6E,F). The thickness of the flowerlike layer was approximate 15-20 µm, estimated from the crosssectional SEM image in Figure 6I. The microspheres and microflowers are confirmed consisting of HZC crystals by XRD, FT-IR, and XPS analysis (Figures S3, S4, and S5). The reflective FT-IR spectra show no absorption around 1700 cm-1, which is the typical absoption of a -CdO group in acetone and DMF. And the XPS elemental analysis indicates that the atomic concentration of C does not increase when organic solvents are adopted into the solution (Table S1 and Figure S5). These results suggest that there are no organic solvents chemically bonded or physically adsorbed on the surfaces of nanostructures. The intensity ratio R of I(200) to I(002) is 2.11 and 1.76 for HZC films deposited in solutions with addition of acetone and DMF, respectively, indicating more exposed hydrophobic crystal planes consisting of zinc atoms in microspheres and microflowers. These hydrophobic crystal planes could serve as supporting points (Figure 6G) and trap plenty of air to effectively prevent the penetration of water droplet into the interstices, resulting in a

Table 1. Contact Angles, Contact Angle Hysteresis, and Sliding Angles of HZC Films Deposited in Urea Solutions with Different Solvents static advancing receding CA hysteresis sliding solvent CA (deg) CA (deg) CA (deg) (deg) angle (deg) wateracetone waterDMF

147.9

151.1

99.7

51.4

35.5

155.2

160.5

146.8

13.7

9.5

Cassie-Baxter wetting state. The contact angle θr agrees with the Cassie-Baxter equation30

cos θr ) f cos θ - (1 - f)

(1)

where θ represents the contact angle for the corresponding smooth surfaces and f is the area fraction of the solid surface really wetted. Therefore, the specific surface corresponding to Figure 6G can display a highly hydrophobic property. The advancing contact angle, receding contact angle, and sliding contact angle of the obtained surfaces were also measured, and the contact angle hysteresis was calculated (Table 1). The superhydrophobic surface with water contact angles as high as 155.2° prepared in water-DMF solution showed a low contact angle hysteresis (13.7°) and a sliding angle lower than 10°, indicating that a water droplet could easily roll off this surface. Contrastly, the hydrophobic surface prepared in water-acetone solution displayed a water contact angle 147.9°, a contact angle hysteresis of 51.4°, and a sliding angle of about 35.5°, implying a little sticky surface. These results confirm that altering geometric microstructures of HZC crystals can efficiently transform the zinc surfaces from superhydrophilic to superhydrophobic. After stored in air for 6 months or immersed in heptane for 3 h, the zinc plates kept superhydropobic properties, indicating good durability and stability. Actually, other water-soluble organic solvent, i.e., ethanol, diaminoethane, and pyridine, were also employed in the CBD process. They showed effects on the morphology of deposited HZC layers and the water CAs ranged from 71.7° to 121.7°. The mechanism for that still requires more investigation. (30) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouque´rol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603–619.

From Superhydrophilic to Superhydrophobic

4. Conclusions In summary, we have demonstrated a simple but powerful method to tune surface wettability of zinc substrates by rendering different morphologies with preferential growth of HZC crystal planes in a CBD process. Upright-standing HZC nanosheets with hydrophilic upside edges and large distance can lead to superhydrophilic properties, while flowerlike microstructures displaying hydrophobic crystal planes can greatly enhance the surface hydrophobicity, resulting in a water contact angle as high as 155.2°. Both of these specific surfaces can be one-step fabricated without subsequent modification, avoiding the instability caused by the destruction of modifying reagents. Our experiments initiate a new method to manipulate surface wettability of metal substrates and provide a good basis for designing many other functional surfaces.

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Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 60577049), Shanghai Leading Academic Discipline Project (B202), and Fundamental Key-Project (08JC1412300) of the Science and Technology Commission of Shanghai Municipal Government. We also thank Mr. H. B. Han (Instrumental Analysis Center, Shanghai Jiao Tong University) for XRD measurement and valuable technical discussions. Supporting Information Available: Figures showing theoretical structure of Zn5(CO3)2(OH)6, XRD patterns of HZC films, FT-IR spectra of HZC films, and XPS spectra of HZC films; table of atomic concentrations of HZC films. This material is available free of charge via the Internet at http://pubs.acs.org. LA803948M