Fabrication of Superhydrophobic CuO Surfaces with Tunable Water

Feb 25, 2011 - In this Article, superhydrophobic CuO surfaces with different topographies have been fabricated by combining both a simple solution-imm...
9 downloads 0 Views 2MB Size
Download PDF
ARTICLE pubs.acs.org/JPCC

Fabrication of Superhydrophobic CuO Surfaces with Tunable Water Adhesion Jian Li,†,‡,§ Xiaohong Liu,† Yinping Ye,† Huidi Zhou,† and Jianmin Chen*,† †

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China ‡ Graduate School, Chinese Academy of Sciences, Beijing 100039, People’s Republic of China § College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, People’s Republic of China

bS Supporting Information ABSTRACT: In this Article, superhydrophobic CuO surfaces with different topographies have been fabricated by combining both a simple solution-immersion process and self-assembly of fluoroalkylsilane. We regulate the solution-immersion process by changing the immersion time, the growing temperature, and the solution compositions to control different topographies of CuO surfaces. The as-prepared superhydrophobic surfaces possess tunable water adhesion that ranges from extremely low to very high, on which the sliding angle is 3 ( 1, 12 ( 1, 28 ( 2, 39 ( 2, and 90° (the water droplet is firmly pinned on the superhydrophobic surface without any movement at any tilted angles), respectively. Our work provides a facile and promising strategy to fabricate superhydrophobic surfaces with tunable adhesion.

1. INTRODUCTION In recent years, superhydrophobic surfaces with a water contact angle (CA) above 150° have attracted considerable interest due to their importance in both fundamental research and practical application.1-10 Among these superhydrophobic surfaces, two kinds of extremely superhydrophobic cases exist: low adhesion to water and high adhesion to water. Low-adhesion superhydrophobic surfaces are usually inspired by biological organisms; the selfcleaning lotus leaf is the typical example.11,12 Water droplets do not stably remain but spontaneously roll off and remove dust particles on the surfaces.13-15 On the other hand, high-adhesion superhydrophobic surfaces are inspired by the gecko’s attachment system and rose petals.16,17 On these surfaces, the water droplets cannot move at any tilted angles.18-20 The high-adhesion superhydrophobic surfaces may be important in potential application, such as no loss microdroplet transportation.18,19 For most conventional applications, the vast majority of man-made superhydrophobic materials are not particularly useful, as they all show substantial water adhesion. Recently, intense interest has been focused on a new type of superhydrophobic surface with tunable adhesion.21-24 To the best of our knowledge, the fabrication of superhydrophobic surfaces with a wide range of tunable adhesion via the control of surface structure instead of chemical composition is still scarce. Generally, there are three superhydrophobic wettability states on a rough surface: the Wenzel state, the Cassie state, and the Cassie impregnating state (i.e., the metastable state).25,26 In the Wenzel state, the water droplets pin on the surface in a wet-contact mode and cannot slide on the surface.27 Conversely, in the Cassie state, r 2011 American Chemical Society

the water droplets adopt a nonwet contact mode on the solid surface and can roll off easily owing to their low water adehsion.28 Additionally, the Cassie impregnating state is between the Wenzel and Cassie states, and the water droplets partially wet the roughness features that remain to trap air on the composite surface.25,26,29 Generally, the water adhesion on the superhydrophobic surface is mainly governed by the surface geometrical structure and surface composition.5,30-32 Therefore, through dynamically tuning the two factors, the water adhesion could be effectively tuned. In this paper, we tune the water adhesion on the same superhydrophobic surface by controlling the morphology of microstructures without altering the surface composition. Five types of CuO microflower/ nanorod array structures have been directly fabricated on the surface of copper plates by a simple one-step solution-immersion process.33-35 After being modified with fluoroalkylsilane, the asprepared CuO films show an analogous extreme nonwetting property but high contrast water adhesion that ranges from extremely low to very high. The method used here to tune superhydrophobic surface adhesion could deepen insight into the roles of microstructures in tailoring surface adhesive properties.

2. EXPERIMENTAL SECTION The construction of CuO nanorods and microflowers was carried out as follows. A copper plate (33  7  0.5 mm3) was Received: November 28, 2010 Revised: January 7, 2011 Published: February 25, 2011 4726

dx.doi.org/10.1021/jp111296n | J. Phys. Chem. C 2011, 115, 4726–4729

The Journal of Physical Chemistry C

ARTICLE

ultrasonically cleaned in concentrated hydrochloric acid and deionized water sequentially before use. After that, a cleaned copper plate was immersed into a mixture of aqueous solution of KOH and (NH4)2S2O8, and then, the copper plate was taken out of the solution and rinsed thoroughly with ethanol and dried in air. Finally, the plate was dipped into CF3(CF2)7CH2CH2Si(OCH2CH3)3 (PDES) ethanol solution for a period of 24 h and dried at 120 °C for 2 h in order to self-assemble a monolayer of low surface energy material. The morphological structure of the as-prepared surface was examined by field emission scanning electron microscopy (FESEM, JSM-6701F). The phase structure of the as-prepared sample was characterized by an X-ray diffractometer (XRD) (Rigaku Corp., D/max-2400) equipped with graphite monochromatized Cu KR radiation. The chemical composition of the as-prepared surface was investigated using X-ray photoelectron spectroscopy (XPS), which was conducted on a PHI-5702 electron spectrometer using the Mg KR line as the excitation source with the reference of C 1s at 284.80 eV. The water contact angle (CA) and sliding angle (SA) were measured with a Kruss DSA 100 apparatus at ambient temperature. The volume of the individual water droplet in all measurements was 5 μL. The average CA and SA values were obtained by measuring the same sample at least in five different positions.

3. RESULTS AND DISCUSSION X-ray diffraction (XRD) analysis is used to determine the structure and phase of the samples. As a comparison, the XRD pattern of the copper substrate is also recorded and shown in Figure 1, which implies that the substrate contains solely a cubic copper phase with lattice parameters of a = b = c = 3.615 Å (JCPDS Card No. 04-0836). After treatment with the mixed aqueous solution of KOH and (NH4)2S2O8, the atop layer on the substrate is slowly transformed to the monoclinic phase of CuO with lattice constants a = 4.685 Å, b = 3.425 Å, and c = 5.130 Å, which is consistent with the reported data (JCPDS No. 450937).36 It is noted that the peaks of Cu appearing in the asprepared surface are only driven from the origin copper substrate, strongly indicating that the CuO films are very thin. On the basis of XRD analysis of the experimental data in Figure 1, the reaction processes can be formulated as follows:37-39 Cu þ 2OH-1 þ S2 O8 2- f CuðOHÞ2 þ 2SO4 2-

ð1Þ

CuðOHÞ2 f CuO þ H2 O

ð2Þ

XPS analysis is carried out to determine the surface composition of the as-prepared CuO films modified with PEDS. Figure 2a shows the full spectrum of the CuO films modified with PEDS. Figure 2b and c give the higher resolution spectra of the asprepared films. The peaks at 934.4 and 954.4 eV (Figure 2b) are attributed to Cu 2p3/2 and Cu 2p1/2 of Cu2þ, respectively, demonstrating CuO composition.40 The peak located at about 688.8 eV is ascribed to F 1s in the PEDS (Figure 2c), which demonstrates that a stable monolayer of PDES has already come into being on the surface of the CuO films. Figure 3 shows the FE-SEM images of the as-prepared CuO films. Cleaned copper plates were immersed into a mixture of aqueous solution of 3.2 M KOH and 0.12 M (NH4)2S2O8 at 65 °C for 5-60 min. Consequently, CuO films were formed on the substrates, as shown in Figure 3, exhibiting interesting

Figure 1. XRD patterns of copper substrate before and after surface oxidation.

Figure 2. XPS analysis of the as-prepared CuO films modified with PDES: (a) survey region; (b) Cu region; (c) F region.

microflower/nanorod array structures. The immersion processes have been carefully monitored. When the immersion time is short (5 min), the copper substrate is mainly covered by uniform and compact nanorods with a length of over 5 μm and a diameter of about 100-200 nm, and the microflowers are sparsely interspersed on the nanorods with a diameter of 1-2 μm (Figure 3a and b). Increasing the immersion time to 15 min, the nanorods and microflowers coexist on the surface almost half and half, showing the microflower/nanorod array structures (Figure 3c). With the increase of immersion time, the diameter of the nanorods stays almost unchanged; however, the microflowers grow bigger with a diameter of 2-4 μm. The magnified image of a microflower reveals that the thickness of the flower petals is about 50 nm (Figure 3d). Further increasing the immersion time to 30 min, the copper substrate is covered by one-layer nanorod arrays as well as one-layer microflowers (Figure 3e). The microflowers grow bigger and the thickness of the flower petals increases to about 100 nm, while the size of the nanorods stays almost unchanged (Figure 3f). In addition, there are many irregular pits in the partially covered microflower/nanorod array structures (Figure 3e). For immersion times close to 60 min, the nanorod arrays are completely covered (Figure 3g), and compared with the sample obtained at an immersion time of 30 min, the thickness of the flower petals stays almost unchanged (Figure 3h). After being modified with fluoroalkylsilane, the four types of CuO films show 4727

dx.doi.org/10.1021/jp111296n |J. Phys. Chem. C 2011, 115, 4726–4729

The Journal of Physical Chemistry C

Figure 3. FE-SEM images with different magnifications of CuO films prepared in different reaction times: (a, b) 5 min, (c, d) 15 min, (e, f) 30 min, and (g, h) 60 min. Water droplets on the surfaces shown in the inset.

Figure 4. Water CA and SA measurements on the superhydrophobic CuO surfaces as a function of the immersion time.

superhydrophobic property (the apparent CA is 162 ( 1, 160 ( 1, 154 ( 1, and 162 ( 1°, respectively) but high contrast water adhesion (Figure 3 insets and Figure 4). With the increase of immersion time, the SA on the as-prepared superhydrophobic surfaces increases from 3 ( 1 to 12 ( 1° and then to 90° (the water droplet is firmly pinned on the superhydrophobic surface

ARTICLE

Figure 5. (a, b) FE-SEM images of CuO film at low and high magnifications, respectively. (c) Static water contact angle on the surface. (d) A sliding water droplet on the surface tilted at 39 ( 2°.

without any movement at any tilted angles) and then finally decreases to 28 ( 2° (Figure 3 insets and Figure 4). Figure 5 shows the FE-SEM images of the as-synthesized CuO film prepared by immersing the cleaned copper substrate in a mixture of aqueous solution of 2.5 M KOH and 0.12 M (NH4)2S2O8 at room temperature for 60 min. The low magnification FE-SEM image (Figure 5a) shows that CuO nanorods and microflowers cover a large area of the copper substrate uniformly and compactly, exhibiting interesting microflower/ nanorod array structures. Compared with Figure 3c and d, the size of the nanrods stays almost unchanged; however, the microflowers grow bigger and cover the copper substrate more densely with the nanorod arrays under them. Furthermore, some of the microflowers are transfixed by several nanorods (Figure 5b). After being chemically modified with fluoroalkylsilane, the as-prepared superhydrophobic surface shows a high apparent CA of 161 ( 1° and SA of 39 ( 2° (Figure 5c and d). Five types of superhydrophobic CuO surfaces with different SAs of 3 ( 1, 12 ( 1, 28 ( 2, 39 ( 2, and 90°, respectively, have been fabricated by an easy and effective way. Whether a water droplet is pinned on or rolls off the superhydrophobic surface is ascribed to the distinct contact modes. In the Wenzel state,27 a water droplet fully penetrates the grooves of a textured surface; thus, the surface possesses high adhesion that could pin the water droplet on the 4728

dx.doi.org/10.1021/jp111296n |J. Phys. Chem. C 2011, 115, 4726–4729

The Journal of Physical Chemistry C surface without any movement. In contrast, in the Cassie state,28 a water droplet is suspended by the gas layers trapped at the microscales; thus, the adhesion of the surface is extremely low and the water droplet easily rolls off with the surface slightly tilted. In the Cassie impregnating state,25,26 the adhesion of the surface is between that of the above two states and the water droplet rolls off with the surface significantly tilted. In addition, the superhydrophobic state can be artificially tuned between the Wenzel state with high adhesion to the Cassie state with low adhesion through the design of robust microstructures with variable geometric parameters to control certain solid/liquid contact modes.41-46 In our result, by changing the immersion time, the growing temperature, and the solution compositions (Supporting Information) to control the surface morphology, the water adhesion on the superhydrophobic surface is tuned successfully without altering the surface composition.

4. CONCLUSIONS In summary, superhydrophobic surfaces with tunable water adhesion that ranges from extremely low to very high have been prepared by a simple solution-immersion process as well as modification with fluoroalkylsilane. The superhydrophobic surface adhesion could be effectively tuned by the distinct contact modes, which depend on the microstructure configurations on the superhydrophobic surfaces. The present approach could provide a new strategy to prepare superhydrophobic surfaces with tunable water adhesion. ’ ASSOCIATED CONTENT

bS

Supporting Information. FE-SEM images of CuO films prepared with different solution compositions but at the same temperature (65 °C) for the same duration (15 min) and their wettability. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ86 931 4968018. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors thank the National Nature Science Foundation of China (Grant No. 50705094) and the National Basic Research Program of China 973 Program (2007CB60760). ’ REFERENCES (1) Cho, K. L.; Liaw, I. I.; Wu, A. H. F.; Lamb, R. N. J. Phys. Chem. C 2010, 114, 11228. (2) Xiu, Y.; Zhu, L.; Hess, D. W.; Wong, C. P. Nano Lett. 2007, 7, 3388. (3) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Soft Matter 2008, 4, 224. (4) Li, X. M.; Reinhoudt, D.; Crego-Calama, M. Chem. Soc. Rev. 2007, 36, 1350. (5) Liu, M.; Zheng, Y.; Zhai, J.; Jiang, L. Acc. Chem. Res. 2010, 43, 368. (6) Xin, B.; Hao, J. Chem. Soc. Rev. 2010, 39, 769. (7) Liu, K.; Yao, X.; Jiang, L. Chem. Soc. Rev. 2010, 39, 3240. (8) Bhushan, B.; Her, E. K. Langmuir 2010, 26, 8207. (9) Xiu, Y.; Zhu, L.; Hess, D. W.; Wong, C. P. J. Phys. Chem. C 2008, 112, 11403.

ARTICLE

(10) Shi, F.; Niu, J.; Liu, J.; Liu, F.; Wang, Z.; Feng, X. Q.; Zhang, X. Adv. Mater. 2007, 19, 2257. (11) Blossey, R. Nat. Mater. 2003, 2, 301. (12) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (13) Koch, K.; Bhushan, B.; Barthlott, W. Soft Matter 2008, 4, 1943. (14) Chen, X. H.; Yang, G. B.; Kong, L. H.; Dong, D.; Yu, L. G.; Chen, J. M.; Zhang, P. Y. Cryst. Growth Des. 2009, 9, 2656. (15) Bhushan, B.; Jung, Y. C.; Koch, K. Langmuir 2009, 25, 3240. (16) 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. (17) Feng, L.; Zhang, Y.; Xi, J.; Zhu, Y.; Wang, N.; Xia, F.; Jiang, L. Langmuir 2008, 24, 4114. (18) Jin, M.; Feng, X.; Feng, L.; Sun, T.; Zhai, J.; Li, T.; Jiang, L. Adv. Mater. 2005, 17, 1977. (19) Hong, X.; Gao, X.; Jiang, L. J. Am. Chem. Soc. 2007, 129, 1478. (20) Bormashenko, E.; Stein, T.; Pogreb, R.; Aurbach, D. J. Phys. Chem. C 2009, 113, 5568. (21) Lai, Y.; Lin, C.; Huang, J.; Zhuang, H.; Sun, L.; Nguyen, T. Langmuir 2008, 24, 3867. (22) Lai, Y.; Gao, X.; Zhuang, H.; Huang, J.; Lin, C.; Jiang, L. Adv. Mater. 2009, 21, 3799. (23) Zhu, S.; Li, Y.; Zhang, J.; L€u, C.; Dai, X.; Jia, F.; Gao, H.; Yang, B. J. Colloid Interface Sci. 2010, 344, 541. (24) Lee, W.; Park, B. G.; Kim, D. H.; Ahn, D. J.; Park, Y.; Lee, S. H.; Lee, K. B. Langmuir 2010, 26, 1412. (25) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457. (26) Bormashenko, E.; Pogreb, R.; Stein, T.; Whyman, G.; Erlich, M.; Musin, A.; Machavariani, V.; Aurbach, D. Phys. Chem. Chem. Phys. 2008, 10, 4056. (27) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (28) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (29) Marmur, A. Langmuir 2004, 20, 3517. (30) Zheng, Y.; Gao, X.; Jiang, L. Soft Matter 2007, 3, 178. (31) Parka, B. G.; Leeb, W.; Kim, J. S.; Lee, K. B. Colloids Surf., A 2010, 370, 15. (32) Wang, D.; Liu, Y.; Liu, X.; Zhou, F.; Liu, W.; Xue, Q. Chem. Commun. 2009, 45, 7018. (33) Pan, Q. M.; Jin, H. Z.; Wang, H. B. Nanotechnology 2007, 18, 355605. (34) Guo, Z. G.; Liu, W. M.; Su, B. L. Appl. Phys. Lett. 2008, 92, 063104. (35) Chen, X.; Kong, L.; Dong, D.; Yang, G.; Yu, L.; Chen, J.; Zhang, P. J. Phys. Chem. C 2009, 113, 5396. (36) Zhang, X.; Guo, Y.; Zhang, P.; Wu, Z.; Zhang, Z. Mater. Lett. 2010, 64, 1200. (37) Zhang, W.; Wen, X.; Yang, S.; Berta, Y.; Wang, Z. L. Adv. Mater. 2003, 15, 822. (38) Zhang, W.; Wen, X.; Yang, S. Inorg. Chem. 2003, 42, 5005. (39) Hou, H.; Xie, Y.; Li, Q. Cryst. Growth Des. 2005, 5, 201. (40) Haber, J.; Machej, T.; Ungier, L.; Ziokowski, J. J. Solid State Chem. 1978, 25, 207. (41) Bormashenko, E.; Pogreb, R.; Whyman, G.; Erlich, M. Langmuir 2007, 23, 12217. (42) He, B.; Patankar, N. A.; Lee, J. Langmuir 2003, 19, 4999. (43) Barbieri, L.; Wagner, E.; Hoffmann, P. Langmuir 2007, 23, 1723. (44) Bormashenko, E.; Pogreb, R.; Whyman, G.; Bormashenko, Y.; Erlich, M. Appl. Phys. Lett. 2007, 90, 201917. (45) Extrand, C. W. Langmuir 2004, 20, 5013. (46) Patankar, N. A. Langmuir 2004, 20, 7097.

4729

dx.doi.org/10.1021/jp111296n |J. Phys. Chem. C 2011, 115, 4726–4729