Fabrication of Superhydrophobic Surfaces Using CuO Nanoneedles Blended Polymer Nanocomposite Film

Copyright 2013 American Scientific Publishers All rights reserved Printed in the United States of America Nanoscience and Nanotechnology Letters Vol. 5, 1 5, 2013 Fabrication of Superhydrophobic Surfaces Using CuO Nanoneedles Blended Polymer Nanocomposite Film Jeyasubramanian Kadarkaraithangam 1, M. Anthony Raja 1, Karthikeyan Krishnamoorthy 1 2, and Selvakumar Natarajan 1 1 Department of Nanoscience and Technology, Mepco Schlenk Engineering College, Sivakasi, Tamilnadu 626005, India 2 Nanomaterials and System Laboratory, Department of Mechanical System Engineering, Jeju National University, Jeju, 690756, South Korea In this communication, we are reporting the fabrication of super hydrophobic surfaces with water contact angle (WCA) of 153.6 over glass substrate using various stages of surface modifications processes was reported. The transition from hydrophilic nature of glass into super hydrophobic nature was observed in every step of surface modification. The super hydrophobicity was achieved by the impregnation of CuO nanoneedles in the hydrophobic lithium stearate/poly vinyl chloride matrix. The obtained super hydrophobicity is discussed in detail with the Wenzel and Cassie model. Keywords: Super Hydrophobic, Water Contact Angle, Copper Oxide Nanoneedle, Scanning Electron Microscopy. 1. INTRODUCTION Superhydrophobic coatings have been extensively researched due to their potential applications in diverse fields such as self-cleaning, anti-contamination, anti sticking, anti fouling and microfluidic technologies. 1 3 The super hydrophobic behavior of lotus leaves, known as the lotusleaf effect or self-cleaning effect, is found to be a result of the hierarchical rough structure, as well as the wax layer present on the leaf surface. 4 It has been well-established that the wettability of a solid surface is governed by both the surface chemical composition and the surface geometrical microstructures. 5 The interest in self-cleaning surfaces is being driven by the desire to fabricate such surfaces for satellite dishes, solar energy panels, photovoltaic, exterior architectural glass and green houses, and heat transfer surfaces in air conditioning equipment, etc. 6 Many researchers have created artificial super-hydrophobic surfaces by modifying the surface topography of the surfaces by using various techniques like the colloidal assembly, sol gel method, plasma enhanced CVD growth of CNTs and the top-down processes of photolithography, magnetron sputtering, electron beam lithography, interference lithography, pattern transfer of natural surfaces, nano carbon impregnated polymer for fractal microstructures and nanostructures. 7 10 Authors to whom correspondence should be addressed. To fabricate a surface with a water contact angle (WCA) larger than 150, two key factors must be considered viz. surface roughness and surface energy. 11 Micro/nano scale roughness available on the surface improves the amount of air trapped within the pores thereby increasing the nonwettability. 12 And the WCA can be increased by reducing the surface energy of the coating material. Usually, the hydrophobic chemicals like fluorinated hydrocarbons, silicone compounds, etc., are used to reduce the surface energy of the substrates. However, by merely applying a material with the lowest surface energy gives a water contact angle of only around 120. 13 To achieve super-hydrophobic nature, appropriate surface roughness should be created in addition to the low surface energy of the hydrophobic coating material. This two-step process has been widely adopted for the fabrication of superhydrophobic surfaces using several surface modification process. In this letter, we are reporting the fabrication of low cost solution processed superhydrophobic nanocomposite film containing copper oxide (CuO) nanoneedles/lithium stearate (LS)/Poly vinyl chloride (PVC) coated onto hydrophilic glass substrate with aluminium thin film as a buffer layer. A smooth hydrophobic transition from hydrophilic glass substrate to the super hydrophobic nanocomposite film was achieved in every step of surface modification. Nanosci. Nanotechnol. Lett. 2013, Vol. 5, No. xx 1941-4900/2013/5/001/005 doi:10.1166/nnl.2013.1579 1

Fabrication of Superhydrophobic Surfaces Using CuO Nanoneedles 2. EXPERIMENTAL DETAILS 2.1. Synthesis of Copper Oxide The copper nanoparticles have been prepared by following the polyol method. 14 Briefly, 4 gm of copper acetate was ground to fine powder and mixed with 150 ml of ethylene glycol in a dry 500 ml round bottom flask. The resulting mixture is then refluxed at the boiling temperature of ethylene glycol (around 190 C) for about 3 hr. At the end of the reaction, copper nanoparticles were precipitated out as reddish brown powder. The precipitated nanoparticles were collected using filtration and were thoroughly washed with small quantities of ethylene glycol to remove the traces of unreacted elements. The collected particles were stored in an air tight container in vacuum. The isolated copper nanoparticle in required amount (100 mg) on heating at 50 C in hot air oven for 3 hr yields the black colored copper oxide nanostructures. 2.2. Synthesis of Metallic Stearate Lithium stearate has been prepared by precipitation method. Briefly, lithium sulphate (0.1 M, 1.81 g in 50 ml) and stearic acid (0.2 M, 5.68 g in 50 ml) were dissolved in distilled water separately. After mixing these two solutions, a colorless lithium stearate solid was formed and floated on the surface. The obtained material was removed by filtration, washed thoroughly with water to remove the unreacted precursors and then dried in air. 2.3. Fabrication of Super Hydrophobic Surfaces 2.3.1. Preparation of Al Thin Film Aluminium thin films in nanometer thickness were deposited on glass substrates by a magnetron sputtering system. The target used was a 5-cm diameter plate of Aluminium (99% purity). After being evacuated to a base pressure of 1 10 6 mbar, the working chamber was filled with Ar (99.99% purity), the work pressure was held at about 1 10 3 mbar, and an RF power of 120 W 140 W was applied in the sputtering process with substrate temperature of 300 K. The sputter duration time is about 30 sec. The Al coated glass substrate was used as the base plate for the fabrication of superhydrophobic films. 2.3.2. Developing Superhydrophobicity to Al Film (i) PVC film was coated over the Al thin film using spin coating by pipetting 10 L of solution (100 mg of PVC dissolved in 5.4 ml of THF) at 3000 rpm and allowed to dry at room temperature for the solvent evaporation. (ii) PVC/lithium stearate thin film was coated over the spin coating by pipetting 10 L solution (100 mg of PVC and 100 mg of lithium stearate dissolved in 5.4 ml of Kadarkaraithangam et al. THF) into the Al thin film at 3000 rpm and allowed to dry at room temperature for the solvent evaporation. (iii) CuO NP/PVC/lithium stearate thin film was coated over the spin coating by pipetting 10 L solution (100 mg of CuO NP, 100 mg of PVC and 100 mg of lithium stearate dissolved in 5.4 ml of THF) into the Al thin film at 3000 rpm and allowed to dry at room temperature for the solvent evaporation. Before coating, the mixture was thoroughly agitated using a Probe type sonicator (20 W) for homogenous mixing. 15 2.4. Characterization Techniques Phase purity and grain size of the Cu and CuO nanostructures were determined by X-ray diffraction (XRD) analysis recorded on a Siefert X-ray diffractometer (Richard Seifert and Co., Ahrensburg, Germany) using CuK radiation ( = 1 54016 Å) at 60 kev over the range of 2 = 20 80 degrees. The Fourier transformed infrared spectra were analysed using an FTIR (Bruker-Optics, Germany) spectrometer. Aluminium thin film was coated over a glass substrate using planar magnetron RF-DC sputtering unit, Model 12 MSPT, HINDHIVAC, India. Probe type sonicator (SONICS) was used to get a uniform suspension of polymer with other additives. The surface morphology of the samples was observed by a scanning electron microscopy (HITACHI, SU1650, JAPAN). The wettability of the films was measured by a contact angle (CA) measurement system in air at ambient temperature by dropping a distilled water droplet of about 5 L onto the surfaces. The equipment used for measuring CA is a Rame-Hart contact angle goniometer. 3. RESULTS AND DISCUSSION 3.1. X-Ray Diffraction Studies The XRD of Cu nanoparticle synthesized is shown in the Figure 1(A). The peaks appeared at 2 values of 42.6, 51.2 and 74.3 which corresponds to the crystal planes of (111), (200) and (220) respectively. These values are well in agreement with the JCPDS file No. 04-0836 of copper metallic particles. 16 The crystallite sizes are also evaluated using Scherer s formula, 18 D = K / cos where the constant K is taken as 0.94, is the wavelength of X-ray used which is CuK radiation ( = 1 5406 Å), and the full width at half maximum of the diffraction peak corresponding to 2. Using the above equation, the average crystallite sizes are found in the range of 15 nm. Figure 1(B) shows the XRD pattern of copper oxide nanoneedle. The peaks at 2 values 32.6, 35.6, 38.8, 53.5, 58.2, 61.6, 65.8, 66.3, 68.0, 72.4, 75.1 corresponds to the crystal planes of (1 1 0), (0 0 2), ( 1 1 1), (1 1 1), (2 0 0), ( 2 0 2), (0 2 0), (2 0 2), ( 1 1 3), (0 2 2), ( 3 1 1), (3 1 0), (2 2 0) and (1 1 3) respectively. The 2 values 2 Nanosci. Nanotechnol. Lett. 5, 1 5, 2013

Kadarkaraithangam et al. Fabrication of Superhydrophobic Surfaces Using CuO Nanoneedles Fig. 2. Scanning electron microscope of copper oxide nanoneedles. 3.3. FT-IR Spectroscopic Investigation of Lithium Stearate Formation from Stearic Acid Fig. 1. X-ray diffraction pattern of (a) copper nanoparticles and (b) copper oxide nanoneedles. are indexed to their corresponding crystal planes as per the JCPDS (5-0661) data of the copper oxide nanoneedles. Compared to the XRD of Cu nanoparticles, the increase in FWHM of the CuO nanoneedles is observed in the XRD pattern which is due to the needle-like shape of CuO. 3.2. Surface Morphology of Copper Oxide The surface morphology of the as-synthesized copper oxide is studied using scanning electron microscope shown in Figure 2. It shows needle-like structures and densely packed as in lower magnification image (Fig. 2(A)). The higher magnification (Fig. 2(B)) image shows some of the CuO nanoneedles are agglomerated which is probably due to the large surface area and high surface energy. The formation of needle like structures is due to the thermal oxidation of Cu nanoparticles in air. And also it may be due to the intermediate phase of Cu 2 O which formed during the oxidation of Cu into CuO nanoneedles. This is supported by the previous reports of Liu et al. 19 The formation of lithium stearate is analyzed by using the FTIR spectra. The IR spectra of pure stearic acid and lithium stearate are shown in the Figure 3. Pure stearic acid shows a characteristic absorption band at 3460 cm 1 (Fig. 3) attributed to the carboxylic acid OH group frequency. It also shows absorptions at 1696 cm 1 which is due to C O stretching vibration of carboxyl group and a peak around 2900 cm 1 is due to C H stretching vibration. The C O stretching vibration of carboxylate group is also found around 1469 cm 1. Stearic acid on reaction with lithium salt produces lithium stearate through carboxylate condensation. From the spectra of lithium stearate, it is clearly evident that the disappearance of OH group frequency at 3460 cm 1 is due to the carboxylate condensation via lithium metal ion. 20 The C O stretching vibration of carboxylic acid group found in the stearic acid at 1696 cm 1 is shifted to 1682 cm 1 in lithium stearate. These results confirm the formation of lithium stearate on reaction of lithium sulphate with stearic acid. 3.4. Atomic Force Microscopic Analysis of Al Buffer Layer The purpose of coating the Al thin film over glass is to act as protective layer which suppresses the hydrophilic nature of the glass substrate. The AFM image of the 2 nm thick nano aluminium coating in 2D format and in 3D format is shown in Figure 4. The surface morphology is well explored from this image. It shows the aluminium is coated uniformly on the glass substrate with 2 nm thickness. 3.5. Surface Morphology and Superhydrophobic Properties of Nanocomposite Films The superhydrophobic properties of a surface are not only attributed to the surface chemistry, but are also influenced by the micro or nanoscaled surface roughness. 21 In the present work, the super hydrophobicity is achieved by coating various types of polymeric film containing different additives. Figure 5 shows the SEM images and water contact angle of (a) plain glass substrate, (b) Al buffer layer, (c) pure PVC film, (d) PVC/lithium stearate composite film and (e) CuO nanoneedles impregnated PVC/lithium stearate composite film respectively. It clearly Nanosci. Nanotechnol. Lett. 5, 1 5, 2013 3

Fabrication of Superhydrophobic Surfaces Using CuO Nanoneedles Kadarkaraithangam et al. Fig. 3. Fourier transform infra red spectra of stearic acid and lithium stearate. Fig. 4. 2D and 3D image of aluminium thin film of thickness 2 nm using AFM. shows the increase in WCA or the hydrophobicity in each step. The artificial surfaces fabricated here exhibit different wettability ranging from 96.4 to 153.6, which is dependent on the geometry of their microstructures. The hydrophobicity and super hydrophobicity of these different films provides an opportunity for further understanding the relationship between the surface wettability and the morphology. Fig. 5. (a) WCA of plain glass substrate ( = 27 4. (b) SEM of Al thin film (2 nm thickness). (c) SEM of pure PVC film, (d) SEM of PVC/lithium stearate composite film and (e) SEM of CuO nanoneedles impregnated PVC/lithium stearate composite film. The inset of (b) (d) shows their corresponding WCA measured as = 96 4, 101.4, 137.5 and 153.6 respectively. The WCA of plain glass substrate is measured as 27.4 which is more hydrophilic due to the presence of silica groups in the glass substrate. After coating with the Al buffer layer (thickness 2 nm), the WCA improves into 96.4. The increase in WCA of the Al thin film is due to the fact that the Al film acts as a protective layer from the hydrophilic glass substrate and also due to the nanoscaled roughness of the film. The formation of pure PVC film enhances the WCA into 101.4 is attributed to the low surface energy of the polymer. Further improvement in the hydrophobic behaviour is developed by the addiction of lithium stearate into the PVC film. The inclusion of lithium stearate into the PVC film significantly improves the hydrophobicity from 101.4 to 137.5. This improvement in the hydrophobicity is due to the low surface energy and the hydrophobic nature of the metal stearate. Further, superhydrophobicity is achieved in the PVC/LS polymer composite film by the addition of CuO nanoneedles. The WCA of the CuO/PVC/LS nanocomposite film was measured as 153.6. The superhydrophobic nature is achieved in this film due to the nanoscaled roughness of the CuO nanoneedles in the composite film. The dispersion of nanoneedles using the ultrasonication process allows them to form hierarchical surface roughness in the nanocomposite film. The achieved superhydrophobicity may be explained on the basis of Cassie and Baxter model for superhydrophobicity which was relevant to the surface 4 Nanosci. Nanotechnol. Lett. 5, 1 5, 2013

Kadarkaraithangam et al. area between the water and the material. 22 The WCA of the surface can be explained using Cassie-Baxter equation given below: cos = f 1 cos f 2 (1) In Eq. (1), and are the WCA on a rough surface and flat surface, f 1 and f 2 are the ratios of solid/water interface and air/water interface, respectively, and f 1 +f 2 = 1. Given that the WCA of the pure PVC film is 101.4 and the CuO impregnated nanocomposite film is 153.6, f 2 was calculated as 0.8701. This indicated that the air trapped in the rough CuO NP impregnated nanocomposite film was the key factor affecting the superhydrophobicity. 4. CONCLUSION In summary, a facile route for the fabrication of a superhydrophobic nanocomposite film using CuO nanoneedles, PVC and LS mixture on a glass substrate has been developed. The CuO nanoneedles are formed by the thermal oxidation of Cu nanoparticles and are characterized well. The SEM image of the obtained CuO shows the needlelike morphology which is due to the thermal oxidation of Cu. The formation of lithium stearate is confirmed by the FTIR spectroscopy. The coated CuO/PVC/LS nanocomposite film shows good superhydrophobicity behavior with a WCA of about 153. This superhydrophobicity is achieved due to the nanoscaled roughness in the CuO/PVC/LS film. The superhydrophobicity is explained well with the Cassie-Baxter model suggesting that the nanoscaled roughness in created the low surface energy PVC/LS surface were more efficient in enhancing the superhydrophobic behavior. Acknowledgments: The authors thank the management and the principal for their constant encouragement and support. Fabrication of Superhydrophobic Surfaces Using CuO Nanoneedles References and Notes 1. H. S. Lim, D. H. Kwak, D. Y. Lee, S. G. Lee, and K. Cho, J. Am. Chem. Soc. 129, 4128 (2009). 2. L. Feng, Y. Song, J. Zhai, B. Liu, J. Xu, L. Jiang, and D. Zhu, Angew. Chem. Int. Ed. 42, 800 (2003). 3. H. J. Song, X. Q. Shen, H. Y. Ji, and X. J. Jing, Appl. Phys. A 99, 685 (2010). 4. Y. Su, B. Ji, K. Zhang, H. Gao, Y. Huang, and K. Hwang, Langmuir 26, 4984 (2010). 5. C. Neinhuis and W. Barthlott, Ann. Bot. 79, 667 (1997). 6. F. Zhang, L. Zhao, H. Chen, S. Xu, D. G. Evans, and X. Duan, Angew. Chem. Int. Ed. 47, 2466 (2008). 7. E. Hosono, S. Fujihara, I. Honma, and H. Zhou, J. Am. Chem. Soc. 127, 13458 (2005). 8. E. Martines, K. Seunarine, H. Morgan, N. Gadegaard, C. D. W. Wilkinson, and M. O. Riehle, Nano Lett. 5, 2097 (2005). 9. W. Xu, X. Shi, and S. Lu, Mat. Chem. Phys. 129, 1042 (2011). 10. J. Wang, D. Li, R. Gao, Q. Liu, X. Jing, Y. Wang, Y. He, M. Zhang, and Z. Jiang, Mat. Chem. Phys. 129, 154 (2011). 11. A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Blasi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, Thin Solid Films 351, 73 (1999). 12. S. Minko, M. Muller, M. Motornov, M. Nitschke, K. Grundke, and M. Stamm, J. Am. Chem. Soc. 125, 3896 (2003). 13. H. M. Shang, Y. Wang, K. Takahashi, and G. Z. Cao, J. Mat. Sci. Lett. 40, 3587 (2005). 14. B. K. Park, S. Jeong, D. Kim, J. Moon, S. Lim, and J. S. Kim, J. Colloid Interface Sci. 311, 417 (2007). 15. K. Karthikeyan, N. Poornaprakash, N. Selvakumar, and K. Jeyasubramanian, J. Nanostruct. Polym. Nanocompos. 5, 83 (2009). 16. X. Xia, C. Xie, S. Cai, Z. Yang, and X. Yang, Corros. Sci. 48, 3924 (2006). 17. M. Yang, J. He, X. Hu C. Yan, Z. Cheng, Y. Zhao and G. Zuo, Sens. Actuators, B 155, 692 (2011). 18. M. Premanathan, K. Karthikeyan, K. Jeyasubramanian, and G. Manivannan, Nanomedicine: NBM 7, 184 (2011). 19. Y. Liu, L. Liao, J. Li, and C. Pan, J. Phys. Chem. C 111, 5050 (2007). 20. H. Huang, M. Tian, J. Yang, H. Li, W. Liang, L. Zhang, and X. Li, J. App. Poly. Sci. 107, 3325 (2008). 21. A. B. D. Cassie and S. Baxter, Trans. Faraday Soc. 40, 546 (1944). 22. N. J. Shirtcliffe, G. McHale, M. I. Newton, C. C. Perry, and P. Roach, Mat. Chem. Phys. 103, 112 (2007). Received: xx Xxxx xxxx. Accepted: xx Xxxx xxxx. Nanosci. Nanotechnol. Lett. 5, 1 5, 2013 5