ARTICLE pubs.acs.org/IECR
A Facile Method To Prepare Superhydrophobic Coatings by Calcium Carbonate Hua Zhang,† Xiaofei Zeng,† Yongfeng Gao,† Feng Shi,† Pengyuan Zhang,*,‡ and Jian-Feng Chen*,† †
Key Lab for Nanomaterials, Ministry of Education, and ‡Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing, 100029, People's Republic of China ABSTRACT: In this work, we have developed a facile and inexpensive method to fabricate a superhydrophobic surface with hierarchical nano- and microstructures. The superhydrophobic surface was prepared through spin coating of a mixture of nano- and microsized calcium carbonate (CaCO3) suspensions on a substrate, followed by modification of a low-surface-energy monolayer of stearic acid. The key parameters of the fabrication of suitable surface morphologies, including the content of CaCO3 powder and the percentage of nano- and microsized CaCO3 powder, were also investigated. Results showed that the optimum CaCO3 content was 40 wt % and the self-cleaning property of the film was achieved only by the addition of 2 wt % microsized CaCO3 powder and 38 wt % nanosized CaCO3 powder. After modification with a self-assembled monolayer of stearic acid, the as-prepared coating showed self-cleaning properties with a water contact angle as high as 152.8° and a sliding angle of 7.8°.
1. INTRODUCTION The self-cleaning property of a superhydrophobic surface, on which the water contact angle (CA) is greater than 150° and the sliding angle is no more than 10°,1-3 has drawn tremendous interest recently because of its potential application in industrial areas and biological processes, such as self-cleaning materials,4-6 antiadhesive coatings,7 corrosion-free coatings,8-10 and so on. The superhydrophobicity of a material depends on not only its surface energy but also its surface morphology. Wenzel11 and Cassie-Baxter12,13 models reveal that suitable surface roughness and low-surface-energy materials are both critical to achieving the self-cleaning property. Commonly, there are two kinds of approaches to prepare superhydrophobic surfaces: enhancing the roughness on the hydrophobic substrate or modifying the rough surfaces to lower the surface free energy.14 To date, many efforts have been made to obtain artificial superhydrophobic surfaces with self-cleaning characteristics. Gao and McCarthy15 prepared a perfectly hydrophobic surface with both advancing and receding water CAs of 180° on a silicon wafer using phase separation of a toluene-swollen covalently attached methylsilicone. The high CA was achieved because of the presence of a random network of cylindrical fibers with diameters of about 40 nm. Jiang and coworkers16 fabricated a superhydrophobic polystyrene film with a novel composite structure consisting of porous microspheres and nanofibers by the electrohydrodynamics method. The CA of the film produced was 160.4 ( 1.2°. In Zhang and co-workers' work,17-19 dendritic gold clusters were deposited by electrochemical deposition onto indium-tin oxide electrodes covered with a polyelectrolyte multilayer film. The surface showed superhydrophobicity after immersion in an n-dodecanethiol/ethanol solution with a tilt angle of 1.5°. Moreover, for the first time, they demonstrated that self-cleaning coatings could be used for drag reduction toward the mimicking of a water strider. Shen et al.20 and Rubner et al.21 both created thin-film coatings with controllable film thickness and chemistry through a layer-by-layer (LbL) assembly method. After modification by chemical vapor r 2011 American Chemical Society
deposition with fluoroalkylsilane, the prepared films exhibited superhydrophicity with very low sliding angles. Sun et al.22,23 prepared a superhydrophobic surface by LbL deposition of poly(diallyldimethylammonium chloride) (PDDA)/sodium silicate multilayer films on a silica-sphere-coated substrate followed with a fluorination treatment. Shi and co-workers24 fabricated superhydrophilic roselike microstructures by a direct in situ hydrothermal synthesis method. After further modification with a selfassembled monolayer of octyltrimethoxysilane, the film changed its wetting properties from superhydrophilicity to superhydrophobicity with a CA as high as 154° and a tilt angle lower than 3°. In order to promote the research from fundamental areas to industrial applications, it is highly desirable to develop a facile and inexpensive method for fabrication of the superhydrophobic coatings. Using commercially available materials and simplifying the fabricating process are possible ways to decrease the cost of forming the superhydrophobic coatings. Calcium carbonate (CaCO3) is very cheap and widely used as surface coatings in the construction industry and as whiting in ceramics. It is also a versatile additive for use in a wide range of plastic and elastomeric applications. Nanosized CaCO3 powder is now commercially available and can be used as a raw material for the preparation of superhydrophobic surfaces. Recently, Hu and Deng25 fabricated a superhydrophobic surface with fatty acid modified precipitated CaCO3, which was pressed onto a doublesided adhesive tape, with one side sticking on a glass slice. The method utilized was simple, and the surface with a water CA as high as 163° and a sliding angle of 1.75° could be obtained with oleic acid modified CaCO3 powder. Herein, there remains the challenge to promote a method to fabricate multilayered and selfadhesive superhydrophobic coatings. In this paper, we have Received: October 29, 2010 Accepted: January 12, 2011 Revised: December 27, 2010 Published: February 18, 2011 3089
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Scheme 1. Schematic Illustration of the Fabrication of a Superhydrophobic Coating
developed a facile and inexpensive method for the preparation of superhydrophobic surfaces with hierarchical nano- and microstructures through spin coating of a mixture of nano- and microsized CaCO3 suspensions on a substrate. After modification with a self-assembled monolayer of stearic acid, the as-prepared coating showed self-cleaning properties with a water CA as high as 152.8° and a sliding angle of 7.8°.
2. EXPERIMENTAL METHODS 2.1. Materials. Anhydrous calcium chloride (CaCl2), sodium carbonate (Na2CO3), and n-hexane were purchased from Beijing Fine Chemical Co., Ltd. Stearic acid and poly(vinylpyrrolidone) (PVP; Mw = 30 000) were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. Nanosized CaCO3 powder was purchased from Ruicheng Xintai Nanomaterials Technology Co., Ltd. All reagents were used as received without further purification. Deionized water was used for all of the experiments. Microsized CaCO3 powder was prepared by the addition of Na2CO3 solutions (1.0 mol/L, 100 mL) to CaCl2 solutions (1.0 mol/L, 100 mL) under moderate magnetic stirring. After aging for 5 min, the suspension was filtered, washed with deionized water, and dried at 60 °C for 4 h to obtain microsized CaCO3 powder. 2.2. Scanning Electron Microscopy (SEM) Characterization of Nanosized and Synthesized Microsized CaCO3 Powders. A Supra 55 Zeiss scanning electron microscope was used to characterize the surface morphology of the coated samples, which were prepared by the following steps. The nanosized and synthesized microsized CaCO3 powders were adhered to the conductive double-sided adhesive tape, consecutively. Afterward, they were coated with a layer of platinum by chemical vapor deposition before observation. 2.3. Treatment of the Substrates. Glass slides were cut into 25.4 mm 20 mm pieces, and then they were immersed in a piranha solution [a 3:7 (v/v) mixture of 30% H2O2 and 98% H2SO4] for 1 h and heated until no bubbles were released. Finally, they were rinsed with a copious amount of water and dried in an oven at 60 °C. 2.4. Fabrication of Nano- and Microstructured Coatings with Superhydrophobicity. The procedure for the fabrication
of a superhydrophobic coating is schematically shown in Scheme 1 and described as follows. First, the CaCO3 powders were well dispersed in deionized water at ambient temperature followed by the addition of PVP under magnetic agitation. The suspension was continuously stirred for 0.5 h to fully dissolve PVP and disperse CaCO3 powders. Second, spin coating was carried out using a KW spin coater (Institute of Microelectronics, Chinese Academy of Sciences) to obtain a thin film on the pretreated glasses. The glass slide was placed on the spin-coater tray, and 500 μL of the suspension was dropped onto the glass sheet at 2000 rpm for 30 s, followed by drying in an oven at 60 °C. Finally, the as-prepared coatings were dipped into an n-hexane solution of stearic acid (20 mM) for 10 days, followed by washing thoroughly with n-hexane and drying in an oven at 60 °C. In order to study the influence of the weight contents of nanosized CaCO3 powders on the surface coverage, different weight contents at 30, 40, and 50 wt % were investigated. After it was found that the optimum CaCO3 content was 40 wt %, the ratios of nano- and microsized CaCO3 powders were explored. The ratios (in percent) of nano- and microsized CaCO3 powders discussed in this paper include 38:2, 36:4, 33:7, and 31:9. All of the percentage data stated above refer to the percentage relative to the whole system. 2.5. Characterization. The static CAs and title angle of the as-prepared surfaces were measured with a CA meter (OCA20, DataPhysics Instruments GmbH, Filderstadt, Germany). Water CA data were obtained using the sessile drop method by averaging the measurements on five different positions of the examined surface. The water droplet size used for the measurements was 4.0 μL. Fourier transform infrared (FTIR) spectra were recorded with a Nicolet 8700 spectrometer in the range 4000-400 cm-1 at 2 cm-1 resolution. The KBr pellet technique, with a typical pellet containing an ca. 1 wt % sample in KBr, was employed.
3. RESULTS AND DISCUSSION 3.1. Morphologies of Nanosized and Synthesized CaCO3 Powders. The morphologies of the commercially available nano-
structured and synthesized CaCO3 powders were characterized by 3090
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Figure 1. SEM images of (a) nanosized CaCO3 powder and (b) synthesized CaCO3 powder. Insets: higher magnification images.
Figure 2. SEM images of different CaCO3 contents in the suspension after spin coating on the substrate: (a) 30 wt %; (b) 40 wt %; (c) 50 wt %.
SEM. From Figure 1a, we can see that the CaCO3 nanoparticles have a spherical shape and show an average size of about 60 nm, while two different shapes exist in the synthesized CaCO3 powder: spherical and cubic shapes in Figure 1b. Also, the size of the synthesized CaCO3 powder ranges from 1 to 2 μm. 3.2. Effect of the Overall CaCO3 Content. The content of CaCO3 powder is an important factor in the influence of the surface coverage. As shown in Figure 2, the coverage of the film surface obviously increases with an increase in the content of CaCO3 powder. When the content of CaCO3 powder is below 40 wt %, the surface is mainly scattered with separated and congregated particles. We considered that a higher CaCO3 powder content may lead to full surface coverage; however, we found that it became more difficult to prepare a CaCO3 suspension because of less water content. The optimum content chosen is 40 wt % because of both the full surface coverage and the convenience of preparing a CaCO3 suspension. 3.3. Importance of Nano- and Microstructures on Superhydrophobicity. After 40 wt % was chosen as the overall CaCO3 content, we tried to prepare self-cleaning coatings with nano- and synthesized CaCO3 powders, respectively. Water CAs on the as-prepared coatings were 142° and 138°, while the water droplet stuck completely to the surface even when the coatings were turned upside down. We believe that nano- and microstructured roughness alone may not create satisfactory roughness for the fabrication of superhydrophobic surfaces. Hierarchical structure may be the prerequisite for the fabrication of superwaterrepellent surfaces. Thus, it is necessary to create nano- and microstructured surface morphologies to realize self-cleaning characteristics. One way to create a favorable surface roughness for the fabrication of a superhydrophobic coating is by mixing CaCO3 nanoparticles and a synthesized CaCO3 suspension directly in appropriate proportions. Thus, we prepared three suspensions with different CaCO3 contents that were then spincoated on the substrates to investigate whether the hierarchical structure is critical to the fabrication of a superhydrophobic
surface: one containing a 10 wt % synthesized CaCO3 powder and a 30 wt % nanosized CaCO3 powder, the second one consisting of a 20 wt % synthesized CaCO3 powder and a 20 wt % nanosized CaCO3 powder, and the third comprising a 30 wt % synthesized CaCO3 powder and a 10 wt % nanosized CaCO3 powder. After the coated substrates were modified by stearic acid, the water CAs and sliding angles were analyzed. The CAs on the three substrates were 146.9°, 143.6°, and 145.4° respectively, while the sliding angles of the modified coatings still could not be measured because of the water droplet’s stickiness to the surface. SEM images of substrates coated with different contents of nanosized and synthesized CaCO3 powders were used to explain high sliding angles. From Figure 3, we can see that, with an increase in the synthesized CaCO3 content, the overall coverage of nanosized CaCO3 on synthesized CaCO3 decreases rapidly, which may result in high sliding angles after surface modification. Because the overall coverage of nanosized CaCO3 on synthesized CaCO3 is an important factor in the influence of surface sliding angles, we have to decrease the content of synthesized CaCO3 in order to obtain superhydrophobic coatings with selfcleaning properties. So, we prepared different CaCO3 suspensions with synthesized CaCO3 content ranges from 2 to 9 wt %. Figure 4 presents the surface morphology of the coatings from different contents of synthesized CaCO3 and the corresponding water CAs after modification. As can be seen from Figure 4a-d, with an increase of the synthesized CaCO3 content, the water CA shows a maximum value. The maximum water CA is 152.8°, corresponding to the optimal content of synthesized CaCO3 of 2 wt %. The overall coverage of nanosized CaCO3 on synthesized CaCO3 is almost full when the synthesized CaCO3 content is at 2 wt %, while with an increase of the synthesized CaCO3 content, the overall coverage of nanosized CaCO3 on synthesized CaCO3 decreases and more bare synthesized CaCO3 particles are exposed on the surface. Table 1 exhibits the changes of the water CAs and sliding angles with the different contents of synthesized CaCO3. It can be seen that the sliding angle was also the lowest at 3091
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Figure 3. SEM images of substrates coated with (a) 10 wt % synthesized CaCO3 and 30 wt % nanosized CaCO3, (b) 20 wt % synthesized CaCO3 and 20 wt % nanosized CaCO3, and (c) 30 wt % synthesized CaCO3 and 10 wt % nanosized CaCO3.
Figure 4. (a-d) Field-emission SEM images of the surface structure of the coatings from different contents of synthesized CaCO3: 2, 4, 7, and 9 wt %. (e-h) Water droplets on the corresponding coatings after suface modification by stearic acid.
the content of 2 wt %. Figure 5 shows the dependency of the water CA on the synthesized CaCO3 content. To determine the effect of pH values on the CAs for the prepared coating, a further experiment was developed. Figure 6 shows the relationship between the pH values and CAs on the asprepared coatings. There is no obvious fluctuation of the CA
value within the errors. All of the CA values are around 150.8°. This result indicates that the pH values of the aqueous solution have little or no effect on the superhydrophobicity of the asprepared coatings. 3.4. Formation of a Hydrophobic Monolayer. The formation mechanism of the structure on the substrate can be identified 3092
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Table 1. Water CAs and Sliding Angles of the Coatings with Different Contents of Self-Synthesized CaCO3 synthesized CaCO3 content (wt %)
CA (deg)
sliding angle (deg)
0
142
1 2
143.8 152.8
>20 15.3 7.8
3
146.1
12.6
4
145.2
12.5
5
146.5
7
145.3
16.9 >20
9
146.6
>20
10
146.9
>20
25 30
143.6 145.4
>20 >20
40
138
>20
Figure 6. CA as a function of the pH value on the surface of the prepared substrates, which were modified with stearic acid. The insets are droplets with different pH values applied on the surface.
Figure 7. FTIR spectra of (a) stearic acid, (b) the initial coating film, and (c) a stearic acid modified coating film. Figure 5. Dependency of the water CA on the synthesized CaCO3 content.
with FTIR. FTIR spectra (Figure 7) of stearic acid (line a), the initial coating film (line b), and the corresponding modified film (line c) provide evidence for the formation of stearic acid selfassembled monolayers on the as-prepared film. Compared to the spectrum of the initial coating film (line b), the asymmetric [νas(CH2)] and symmetric [νs(CH2)] methylene vibration peaks appear clearly at 2918 and 2850 cm-1 in stearic acid modified coating film spectra (line c), which indicates the existence of the long-chain aliphatic groups on the surface. 3.5. Theoretical Analysis of the Superhydrophobicity of the As-Prepared Surface. To fully understand the self-cleaning property of the nano- and microstructured CaCO3 coatings modified with a self-assembled monolayer of stearic acid, the Cassie and Baxter equation13 was used to evaluate the relationship between the CA of a water droplet on a smooth surface (θ) and that on a heterogeneous surface (θr) composed of a solid and air cos θr ¼ f1 cos θ - f2 in which f1 and f2 are the fractions of the solid surface and air in contact with liquid, respectively (i.e., f1 þ f2 = 1). It is easy to deduce from this equation that increasing the fraction of air (f2) increases the CA of the rough surface (θr). Here, the CA on a flat
surface is 108°, and the CA on as-prepared films is 152.8°. We can calculate from this equation that the values of f1 and f2 are 0.16 and 0.84. This means that air occupies about 84% of the contact area between the water droplet and nano/microstructures, which is responsible for the self-cleaning property of the surface.
4. CONCLUSION In this work, we have developed a facile method to fabricate superhydrophobic surfaces with hierarchical structures through spin coating of a layer of nano- and microsized CaCO3 suspensions on the substrates followed by hydrophobic modification. The film containing 2 wt % synthesized CaCO3 with an overall CaCO3 content of 40 wt % maintains the highest CA. It is worth noting that the self-cleaning property of the film is achieved only by the addition of 2 wt % synthesized CaCO3. The preparation technology can be scaled up to the manufacturing process to obtain large-area superhydrophobic surfaces. ’ AUTHOR INFORMATION Corresponding Author
*Tel: þ86-10-64443134 (P.Z.), þ86-10-64446466 (J.-F.C.). Fax: þ86-10-64434784 (P.Z.), þ86-10-64434784 (J.-F.C.). E-mail:
[email protected] (P.Z.),
[email protected]. edu.cn (J.-F.C.). 3093
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’ ACKNOWLEDGMENT The authors thank the National Science Foundation of China (Grants 20821004 and 20990221), the National “863 Program” (Projects 2009AA03Z334 and 2009AA033301), Beijing Nova Program of China (Grant 2009B011), Chinese Universities Scientific Fund (Grant JD0901), and Open Project of State Key Laboratory of Supramolecular Structure and Materials (Project SKLSSM201009) for financial support. ’ REFERENCES (1) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Preparation of Transparent Superhydrophobic Boehmite and Silica Films by Sublimation of Aluminum Acetylacetonate. Adv. Mater. 1999, 11, 1365. (2) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y.; Wang, Z. Superhydrophobic surfaces: from structural control to functional application. J. Mater. Chem. 2008, 18, 621. (3) Neinhuis, C.; Barthlott, W. Characterization and Distribution of Water-repellent, Self-cleaning Plant Surfaces. Ann. Bot. 1997, 79, 667. (4) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Transparent Superhydrophobic Thin Films with Self-Cleaning Properties. Langmuir 2000, 16, 7044. (5) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Bioinspired Surfaces with Special Wettability. Acc. Chem. Res. 2005, 38, 644. (6) Bhushan, B.; Jung, Y. C.; Koch, K. Self-Cleaning Efficiency of Artificial Superhydrophobic Surfaces. Langmuir 2009, 25, 3240. (7) Sun, T. L.; Tan, H.; Han, D.; Fu, Q.; Jiang, L. No Platelet Can Adhere—Largely Improved Blood Compatibility on Nanostructured Superhydrophobic Surfaces. Small. 2005, 1, 959. (8) Liu, H. Q.; Szunerits, S.; Xu, W. G.; Boukherroub, R. Preparation of Superhydrophobic Coatings on Zinc as Effective Corrosion Barriers. ACS Appl. Mater. Interfaces 2009, 1, 1150. (9) Ishizaki, T.; Saito, N. Rapid Formation of a Superhydrophobic Surface on a Magnesium Alloy Coated with a Cerium Oxide Film by a Simple Immersion Process at Room Temperature and Its Chemical Stability. Langmuir 2010, 26, 9749. (10) Guo, X. X.; Xu, S. L.; Zhao, L. L.; Lu, W.; Zhang, F. Z.; Evans, D. G.; Duan, X. One-Step Hydrothermal Crystallization of a Layered Double Hydroxide/Alumina Bilayer Film on Aluminum and Its Corrosion Resistance Properties. Langmuir 2009, 25, 9894. (11) Wenzel, R. N. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem. 1936, 28, 988. (12) Cassie, A. B. D.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday Soc. 1944, 40, 546. (13) Baxter, S.; Cassie, A. B. D. The water repellency of fabrics and a new water repellency test. J. Text. Inst. 1945, 36, 67. (14) Feng, X.; Jiang, L. Design and Creation of Superwetting/ Antiwetting Surfaces. Adv. Mater. 2006, 18, 3063. (15) Gao, L.; McCarthy, T. J. A Perfectly Hydrophobic Surface (θA/ θR = 180°/180°). J. Am. Chem. Soc. 2006, 128, 9052. (16) Jiang, L.; Zhao, Y.; Zhai, J. A Lotus-Leaf-like Superhydrophobic Surface: A Porous Microsphere/Nanofiber Composite Film Prepared by Electrohydrodynamics. Angew. Chem., Int. Ed. 2004, 43, 4338. (17) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q.; Jiang, L.; Li, X. Y. Polyelectrolyte Multilayer as Matrix for Electrochemical Deposition of Gold Clusters: Toward Super-Hydrophobic Surface. J. Am. Chem. Soc. 2004, 126, 3064. (18) Shi, F.; Wang, Z.; Zhang, X. Combining a Layer-by-Layer Assembling Technique with Electrochemical Deposition of Gold Aggregates to Mimic the Legs of Water Striders. Adv. Mater. 2005, 17, 1005. (19) Shi, F.; Niu, J.; Liu, J.; Liu, F.; Wang, Z.; Feng, X. Q.; Zhang, X. Towards Understanding Why a Superhydrophobic Coating Is Needed by Water Striders. Adv. Mater. 2007, 19, 2257. (20) Ji, J.; Fu, J. H.; Shen, J. Fabrication of a Superhydrophobic Surface from the Amplified Exponential Growth of a Multilayer. Adv. Mater. 2006, 18, 1441.
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