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Superhydrophobicity of electrospray-synthesized fluorinated silica layers
Journal of Colloid and Interface Science 368 (2012) 599–602
Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Superhydrophobicity of electrospray-synthesized fluorinated silica layers Eun-Kyeong Kim, Chul-Sung Lee, Sang Sub Kim ⇑ School of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea
a r t i c l e
i n f o
Article history: Received 27 August 2011 Accepted 19 November 2011 Available online 1 December 2011 Keywords: Silica Electrospray Hydrophobicity Fluorination
a b s t r a c t The preparation of superhydrophobic SiO2 layers through a combination of a nanoscale surface roughness and a fluorination treatment is reported. Electrospraying SiO2 precursor solutions that had been prepared by a sol–gel chemical route produced very rough SiO2 layers. Subsequent fluorination treatment with a solution containing trichloro(1H,1H,2H,2H-perfluorooctyl)silane resulted in highly rough, fluorinated SiO2 layers. The fluorinated rough SiO2 layers exhibited excellent repellency toward various liquid droplets. In particular, water repellency of 168° was observed. On the bases of Cassie–Baxter and Young– Dupre equations, the surface fraction and the work of adhesion of the rough, fluorinated SiO2 layers were respectively estimated. In light of the durability in water, ultraviolet resistance, and thermal stability, the superhydrophobic SiO2 layers prepared in this work hold promise in a range of practical applications. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction SiO2 is environmentally safe and harmless to humans and inert to most chemicals. It is made up of the most abundant elements in the earth’s crust and is therefore economical to prepare. Moreover, with the sol–gel process, SiO2 chemistry can be easily modified into various organic–inorganic hybrid materials. Accordingly, achieving control of the surface properties of SiO2 layers is of great importance, and successful control is sure to yield a wide range of applications. In particular, SiO2 layers with additional surface functions, such as antifingerprinting, antisticking, antifogging, antifouling, self-cleaning, or water repellency, have been the subject of increasing research [1,2]. Much interest has been focused on the development of a superhydrophobic surface on which a high water contact angle (WCA) > 150° and a small sliding angle of <5° are to be created, because there is a close correlation between superhydrophobicity and the surface functions mentioned above. Various approaches have been attempted to produce artificial superhydrophobic surfaces [3,4] since the discovery of the special characteristics of the lotus leaf. According to literature surveys [5–7], the surface chemistry and surface structure are known to determine the nature of final surface properties. Therefore, many different methodologies have been tested to control either the surface chemistry or surface nanoor microstructure [8–10]. The modification of SiO2 surfaces from the point of view of either surface chemistry or surface microstructure has been extensively studied [11–13]. On one hand, various methods are available ⇑ Corresponding author. Fax: +82 32 860 5546. E-mail address: [email protected] (S.S. Kim). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.11.047
to vary the chemical composition of SiO2 surfaces via deliberate chemical and physical treatments, such as hybridization of SiO2 [11], immobilization of functional groups [12], and the phase separation method [13]. In particular, in order to prepare superhydrophobic surfaces, a chemical modification that lowers the surface energy is required. On the other hand, the surface microstructure of SiO2 surfaces has been controlled by using the lithographic technique [14], self-assembly [15], and the sol–gel method [16] to realize micro- and nanopatterned surfaces. Electrospray is a simple, inexpensive, and convenient process for producing coated layers in an ambient environment in a highly reproducible manner from solutions [17]. When the viscosity of the solutions is low enough, the solutions are sprayed in the shape of droplets, usually creating a very rough, particulated layer on a substrate [18,19]. In this regard, electrospray deposition can be an appropriate method for producing rough SiO2 layers, consequently exhibiting excellent hydrophobicity. A fluorination treatment in combination with the rough surface structure of electrospraysynthesized SiO2 layers is more likely to enhance the hydrophobicity. To our knowledge, no study has been performed on the preparation and characterization of electrosprayed, fluorinated superhydrophobic SiO2 layers. In this work, electrospray-synthesized highly rough SiO2 layers were subsequently fluorinated. The fluorinated rough SiO2 layers exhibited excellent superhydrophobicity in light of durability, ultraviolet resistance, and thermal stability. 2. Experimental details Nanoscale rough SiO2 layers were prepared on Si(1 0 0) substrates by electrospray deposition. For the precursor solution of the electrospray deposition, tetraethoxysilane (TEOS, Si(OC2H5)4) and
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E.-K. Kim et al. / Journal of Colloid and Interface Science 368 (2012) 599–602
methyltriethoxysilane (MTES, CH3Si(OC2H5)3) were used as precursor materials for SiO2. To prepare the solution, TEOS and MTES were mixed at a 1:1 molar ratio. After being stirred for 2 h at room temperature, the solution was mixed with a separately prepared solution containing ethanol, deionized water, and HCl at a molar ratio of 6:9:0.03. In a typical electrospraying process, when the syringe needle is connected with a high voltage, the precursor solution is fed from the syringe at a low speed of a few ml/hr, and nanosized droplets are sprayed on to the collector placed at the opposite side. In this work, the feeding rate ranged from 0.01 to 0.4 ml/h. The applied voltage between the needle and the collector was 15 kV. The distance between the needle tip and the collector was approximately 10 cm. As-sprayed coated layers were heat-treated at 400 °C for 2 h in air in order to obtain pure phase SiO2 films. The electrospray-synthesized SiO2 layers were subsequently immersed into hexane with 10 mmol trichloro(1H,1H,2H,2H-perfluorooctyl)silane (PFOS, CF3(CF2)5CH2CH2SiCl3) for 10 min at room temperature and dried in air, and finally they were heated at 100 °C for 1 h to achieve surface fluorination. An illustration conceptually showing the steps to prepare electrospray-synthesized, fluorinated SiO2 layers is presented in Fig. 1. The microstructure of the SiO2 layers was investigated by field emission scanning electron microscopy (FE-SEM). The CAs for various liquids were measured by a contact angle analyzer in a static mode with a 5 ll liquid drop. In order to investigate the effects of ultraviolet (UV) light on the WCA, the coated layers were exposed to a UV lamp with an intensity of 3 mW/cm2. Durability was tested by measuring the change in the WCA periodically while the sample was maintained in a normal laboratory with 5% relative humidity. X-ray photoelectron spectroscopy (XPS) was used to characterize the surface chemistry of coated layers.
Fig. 2. Change in water contact angle for rough, fluorinated SiO2 layers with respect to the feeding rate in the electrospray deposition. Some of the shapes of water droplets created on the rough, fluorinated SiO2 layers are included in the insets.
3. Results and discussion First of all, the WCAs of electrosprayed, fluorinated SiO2 layers were measured in relation to the feeding rate of the electrospray deposition. The results are shown in Fig. 2. The WCAs slightly vary depending on the feeding rate. The best WCA is 168°, which was obtained at a feeding rate of 0.05 ml/h. Some of the water droplet shapes created on the layers are displayed in the insets. It is evident that after the fluorination treatment, the influence of the feeding rate, which is known to be one of the key processing parameters in electrospraying, becomes insignificant with respect to the hydrophobic state of the layer. This indicates that the modification of the surface chemistry of SiO2 layers via fluorination plays a dominant role in exhibiting the outstanding hydrophobicity. Fig. 3a shows the surface microstructure, as observed by FE-SEM, of an electrospray-deposited fluorinated SiO2 layer, which was prepared at a feeding rate of 0.05 ml/h. As shown, the SiO2 nanoparticles in the range of 200–1200 nm in diameter homoge-
Fig. 1. Illustration conceptually showing the steps used to synthesize superhydrophobic rough, fluorinated SiO2 layers in this work.
Fig. 3. (a) Surface microstructure, observed by FE-SEM, taken from the rough, fluorinated SiO2 layer prepared at a feeding rate of 0.05 ml/h. The inset is a lowmagnification FE-SEM image. (b) Typical XPS spectrum of the rough, fluorinated SiO2 layer.
neously cover the substrate, resulting in nanoscale roughness. The root-mean-squared roughness, measured by a 3-dimensional laser scanning microscope, was in the range of 600–700 nm. The inset figure is a low-magnification FE-SEM image that shows the overall surface morphology. The WCA measured on this layer was 168°. Fig. 3b shows a result of XPS taken from the rough, fluorinated SiO2 layer. The spectrum clearly shows the presence of a fluorine species. Accordingly, it is reasonable to conclude that the superhydrophobicity of the electrospray-deposited fluorinated SiO2 layer results from the combination effect of the nanoscale surface roughness imposed by the nature of electrospray and the low surface energy generated by fluorosilane functionalization.
E.-K. Kim et al. / Journal of Colloid and Interface Science 368 (2012) 599–602
Various surface properties of the rough, fluorinated SiO2 layer that exhibited the best WCA of 168° were further investigated to explore the potential applicability of the layer. First of all, the capability of repelling liquids for the layer was tested. The shapes of the droplets of various liquids created on the layer were observed. As shown in Fig. 4a, the surface strongly repels liquids, such as water, glycerol, coffee, juice, and milk. The CAs of the various liquids for the rough, fluorinated SiO2 layer are shown in Fig. 4b. The surface tension of the five liquids ranges from 46 to 72 m Nm1. The work of adhesion can be estimated by applying the Young– Dupre equation [20,21], which is expressed as follows,
xad ¼ fs cL ð1 þ cos hY Þ
ð1Þ
where cL is the liquid surface tension, hY is the CA in the Young model, which is measured on a flat, smooth surface, and fs is the surface fraction, that is, the fraction of the SiO2 solid in contact with the liquid in the Cassie–Baxter equation. The values of fs can be obtained by substituting the CAs measured for the flat and the rough, fluorinated SiO2 layers in Fig. 4b into the Cassie–Baxter equation, which is expressed as [22]
cos hCB ¼ fs ðcos hY þ 1Þ 1
ð2Þ
where hCB and hY are CAs in the Cassie–Baxter and Young models, respectively. In order to estimate the work of adhesion and the surface fraction, one has to know the CAs created on a surface that not only has a surface structure that is sufficiently smooth to satisfy the Young equation, but has also the same surface chemistry as the rough, fluorinated SiO2 layer. For the surface satisfying the Young equation, we have prepared a flat SiO2 layer by using an oxidation process. The surface roughness of the prepared flat SiO2 layer was less than 0.6 nm. The flat SiO2 layer underwent the same fluorination treatment as described in Section 2. Using the CAs measured on the flat SiO2 layer (data not presented), the work of adhesion for the rough, fluorinated SiO2 layer was estimated, and the results are summarized in Fig. 5a. The work of adhesion values was less than 3 103 N/m for all the tested liquids, and this is one order
601
of magnitude smaller than those for the flat SiO2 layer. These exceptionally small works of adhesion are responsible for the strong repellency for liquids. The values of fs are shown in Fig. 5b. They are less than 0.1, again suggesting that a nanoscale roughness was created on the surface of the electrospray-synthesized SiO2 layer. The effects of UV exposure on the superhydrophobicity of the rough, fluorinated SiO2 layer were investigated. Fig. 6 shows the change in the WCA and the sliding angle in relation to UV exposure time. The WCA and the sliding angle of the rough, fluorinated SiO2 layer remain almost unchanged. The deterioration of hydrophobicity according to UV exposure has often been reported [23,24]. The UV light may cause decomposition of organic contaminants [23] or increase the surface energy of solids [24]. This facilitates the adsorption of water droplets, thus resulting in an enhanced hydrophilic state. However, the superhydrophobicity of the SiO2 layer in this work is very resistant to UV exposure. The fluoroalkyl group and the negligible amount of UV-liable groups on the silica surface are likely to be responsible for the excellent UV resistance [25]. The long-term stability of the rough, fluorinated SiO2 layer was examined by measuring the WCA and the sliding angle with time at room temperature in air. The results are shown in Fig. 7. The WCA and the sliding angle almost maintain the same values even after 30 days, demonstrating an outstanding durability in water.
Fig. 5. (a) Works of adhesion and (b) surface fractions of the rough, fluorinated SiO2 layer as a function of surface tensions of the liquids.
Fig. 4. (a) Shapes of liquid droplets, such as water, glycerol, coffee, juice, and milk, created on the rough, fluorinated SiO2 layer. (b) Contact angles of various liquid droplets on the rough, fluorinated SiO2 layer. The surface tensions for the liquids are also noted.
Fig. 6. Water contact and sliding angles measured for the rough, fluorinated SiO2 layer as a function of UV exposure time.
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4. Conclusions
Fig. 7. Change in water contact and sliding angles as a function of time.
Superhydrophobic SiO2 layers were prepared through a combination of the nanoscale surface roughness and the fluorination treatment. To synthesize rough SiO2 layers on the nanometer scale, the electrospraying deposition technique was used. In addition, modification of the surface chemistry was accomplished by the fluorination treatment. By this combination, highly rough, fluorinated SiO2 layers were synthesized. They exhibited excellent repellency toward various liquid droplets. For instance, the water contact angle was 168°. The surface fraction and the work of adhesion of the superhydrophobic SiO2 layers were respectively determined using Cassie–Baxter and Young–Dupre equations. The durability in water, the ultraviolet resistance, and the thermal stability were investigated. All the results strongly supported the potential use of the superhydrophobic SiO2 layers in the fields requiring antisticking, self-cleaning, and antifouling. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0023986). References
Fig. 8. Change in water contact angle as a function of heat treatment temperature. The corresponding shapes of water droplets are shown in the insets. The surface microstructure observed from the SiO2 layer after being heat-treated at 600 °C is also included in the inset.
The thermal stability of the rough, fluorinated SiO2 layer was investigated at various temperatures ranging from 100 to 600 °C for 2 h in air. As shown in Fig. 8, the SiO2 layer maintains its superhydrophobicity up to 400 °C. However, the superhydrophobicity greatly deteriorated by annealing at temperatures higher than 400 °C, leading to a hydrophilic state. The XPS spectrum (data not presented here) taken from the sample after being heat-treated at 600 °C confirmed the absence of fluorine species. The surface microstructure, which is included in the inset, of the sample that was heat-treated at 600 °C shows no significant difference from the microstructure before the heat treatment (see Fig. 3a), while it maintained its nanoscale roughness. Thereby it is reasonable to conclude that the drastic loss of superhydrophobicity was due to the volatilization of fluorine species. With respect to durability, UV resistance, and thermal stability, the superhydrophobic SiO2 layers prepared by the combination of the nanoscale roughness and the fluorination in this work hold potential in practical applications needing antisticking, self-cleaning, and antifouling attributes.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
L.Y.L. Wu, S.K. Ngian, Z. Chen, D.T.T. Xuan, Appl. Surf. Sci. 257 (2011) 2965. J.A. Howarter, J.P. Youngblood, Adv. Mater. 19 (2007) 3838. L. Liu, J. Zhao, Y. Zhang, F. Zhao, Y. Zhang, J. Colloid Interf. Sci. 358 (2011) 277. N.A. Patankar, Langmuir 20 (2004) 8209. D. Bonn, Rev. Mod. Phys. 81 (2009) 739. T. Sun, L. Feng, X. Gao, L. Jiang, Acc. Chem. Res. 38 (2005) 644. P.J. Holloway, Pestic. Sci. 1 (1970) 156. A. Munoz-Bonilla, A. Bousquet, E. Ibarboure, E. Papon, C. Labrugere, J. Rodriquez-Hernandez, Langmuir 26 (2010) 16775. C.-T. Hsieh, W.-Y. Chen, F.-L. Wu, Carbon 46 (2008) 1218. L. Zhang, Z. Zhou, B. Cheng, J.M. DeSimone, E.T. Samulski, Langmuir 22 (2006) 8576. H. Tian, T. Yang, Y. Chen, Appl. Surf. Sci. 255 (2009) 4289. P.K. Jal, S. Patel, B.K. Mishra, Talanta 62 (2004) 1005. A. Nakajima, K. Abe, K. Hashimoto, T. Watanabe, Thin Solid Films 376 (2000) 140. Y. Li, J. Zhang, S. Zhu, H. Dong, F. Jia, Z. Wang, Y. Tang, L. Zhang, S. Zhang, B. Yang, Langmuir 26 (2010) 9842. S. Singh, D.Y. Sasaki, J. Cesarano III, A.J. Hurd, Thin Solid Films 339 (1999) 209. K. Makita, Y. Akamatsu, A. Takamatsu, J. Sol–Gel Sci. Technol. 14 (1999) 175. Y. Dzenis, Science 304 (2004) 1917. A. Jaworek, J. Mater. Sci. 42 (2007) 266. I. Uematsu, H. Matsumoto, K. Morota, M. Minagawa, A. Tanioka, Y. Yamagata, K. Inoue, J. Colloid Interf. Sci. 269 (2004) 336. A.W. Adamson, A.P. Gast, Physical Chemistry of Surfaces, sixth ed., Wiley, New York, 1997. Y. Xiu, L. Zhu, D.W. Hess, C.P. Wong, J. Phys. Chem. C 112 (2008) 11403. A.B.D. Cassie, S. Baxter, Trans. Faraday Soc. 40 (1994) 546. J.R. Vig, J. Vac. Sci. Technol. A 3 (1985) 1027. I. Torchinsky, G. Rosenman, Nanoscale Res. Lett. 4 (2009) 1209. Y. Xiu, D.W. Hess, C.P. Wong, J. Colloid Interf. Sci. 326 (2008) 465.
Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Superhydrophobicity of electrospray-synthesized fluorinated silica layers Eun-Kyeong Kim, Chul-Sung Lee, Sang Sub Kim ⇑ School of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea
a r t i c l e
i n f o
Article history: Received 27 August 2011 Accepted 19 November 2011 Available online 1 December 2011 Keywords: Silica Electrospray Hydrophobicity Fluorination
a b s t r a c t The preparation of superhydrophobic SiO2 layers through a combination of a nanoscale surface roughness and a fluorination treatment is reported. Electrospraying SiO2 precursor solutions that had been prepared by a sol–gel chemical route produced very rough SiO2 layers. Subsequent fluorination treatment with a solution containing trichloro(1H,1H,2H,2H-perfluorooctyl)silane resulted in highly rough, fluorinated SiO2 layers. The fluorinated rough SiO2 layers exhibited excellent repellency toward various liquid droplets. In particular, water repellency of 168° was observed. On the bases of Cassie–Baxter and Young– Dupre equations, the surface fraction and the work of adhesion of the rough, fluorinated SiO2 layers were respectively estimated. In light of the durability in water, ultraviolet resistance, and thermal stability, the superhydrophobic SiO2 layers prepared in this work hold promise in a range of practical applications. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction SiO2 is environmentally safe and harmless to humans and inert to most chemicals. It is made up of the most abundant elements in the earth’s crust and is therefore economical to prepare. Moreover, with the sol–gel process, SiO2 chemistry can be easily modified into various organic–inorganic hybrid materials. Accordingly, achieving control of the surface properties of SiO2 layers is of great importance, and successful control is sure to yield a wide range of applications. In particular, SiO2 layers with additional surface functions, such as antifingerprinting, antisticking, antifogging, antifouling, self-cleaning, or water repellency, have been the subject of increasing research [1,2]. Much interest has been focused on the development of a superhydrophobic surface on which a high water contact angle (WCA) > 150° and a small sliding angle of <5° are to be created, because there is a close correlation between superhydrophobicity and the surface functions mentioned above. Various approaches have been attempted to produce artificial superhydrophobic surfaces [3,4] since the discovery of the special characteristics of the lotus leaf. According to literature surveys [5–7], the surface chemistry and surface structure are known to determine the nature of final surface properties. Therefore, many different methodologies have been tested to control either the surface chemistry or surface nanoor microstructure [8–10]. The modification of SiO2 surfaces from the point of view of either surface chemistry or surface microstructure has been extensively studied [11–13]. On one hand, various methods are available ⇑ Corresponding author. Fax: +82 32 860 5546. E-mail address: [email protected] (S.S. Kim). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.11.047
to vary the chemical composition of SiO2 surfaces via deliberate chemical and physical treatments, such as hybridization of SiO2 [11], immobilization of functional groups [12], and the phase separation method [13]. In particular, in order to prepare superhydrophobic surfaces, a chemical modification that lowers the surface energy is required. On the other hand, the surface microstructure of SiO2 surfaces has been controlled by using the lithographic technique [14], self-assembly [15], and the sol–gel method [16] to realize micro- and nanopatterned surfaces. Electrospray is a simple, inexpensive, and convenient process for producing coated layers in an ambient environment in a highly reproducible manner from solutions [17]. When the viscosity of the solutions is low enough, the solutions are sprayed in the shape of droplets, usually creating a very rough, particulated layer on a substrate [18,19]. In this regard, electrospray deposition can be an appropriate method for producing rough SiO2 layers, consequently exhibiting excellent hydrophobicity. A fluorination treatment in combination with the rough surface structure of electrospraysynthesized SiO2 layers is more likely to enhance the hydrophobicity. To our knowledge, no study has been performed on the preparation and characterization of electrosprayed, fluorinated superhydrophobic SiO2 layers. In this work, electrospray-synthesized highly rough SiO2 layers were subsequently fluorinated. The fluorinated rough SiO2 layers exhibited excellent superhydrophobicity in light of durability, ultraviolet resistance, and thermal stability. 2. Experimental details Nanoscale rough SiO2 layers were prepared on Si(1 0 0) substrates by electrospray deposition. For the precursor solution of the electrospray deposition, tetraethoxysilane (TEOS, Si(OC2H5)4) and
600
E.-K. Kim et al. / Journal of Colloid and Interface Science 368 (2012) 599–602
methyltriethoxysilane (MTES, CH3Si(OC2H5)3) were used as precursor materials for SiO2. To prepare the solution, TEOS and MTES were mixed at a 1:1 molar ratio. After being stirred for 2 h at room temperature, the solution was mixed with a separately prepared solution containing ethanol, deionized water, and HCl at a molar ratio of 6:9:0.03. In a typical electrospraying process, when the syringe needle is connected with a high voltage, the precursor solution is fed from the syringe at a low speed of a few ml/hr, and nanosized droplets are sprayed on to the collector placed at the opposite side. In this work, the feeding rate ranged from 0.01 to 0.4 ml/h. The applied voltage between the needle and the collector was 15 kV. The distance between the needle tip and the collector was approximately 10 cm. As-sprayed coated layers were heat-treated at 400 °C for 2 h in air in order to obtain pure phase SiO2 films. The electrospray-synthesized SiO2 layers were subsequently immersed into hexane with 10 mmol trichloro(1H,1H,2H,2H-perfluorooctyl)silane (PFOS, CF3(CF2)5CH2CH2SiCl3) for 10 min at room temperature and dried in air, and finally they were heated at 100 °C for 1 h to achieve surface fluorination. An illustration conceptually showing the steps to prepare electrospray-synthesized, fluorinated SiO2 layers is presented in Fig. 1. The microstructure of the SiO2 layers was investigated by field emission scanning electron microscopy (FE-SEM). The CAs for various liquids were measured by a contact angle analyzer in a static mode with a 5 ll liquid drop. In order to investigate the effects of ultraviolet (UV) light on the WCA, the coated layers were exposed to a UV lamp with an intensity of 3 mW/cm2. Durability was tested by measuring the change in the WCA periodically while the sample was maintained in a normal laboratory with 5% relative humidity. X-ray photoelectron spectroscopy (XPS) was used to characterize the surface chemistry of coated layers.
Fig. 2. Change in water contact angle for rough, fluorinated SiO2 layers with respect to the feeding rate in the electrospray deposition. Some of the shapes of water droplets created on the rough, fluorinated SiO2 layers are included in the insets.
3. Results and discussion First of all, the WCAs of electrosprayed, fluorinated SiO2 layers were measured in relation to the feeding rate of the electrospray deposition. The results are shown in Fig. 2. The WCAs slightly vary depending on the feeding rate. The best WCA is 168°, which was obtained at a feeding rate of 0.05 ml/h. Some of the water droplet shapes created on the layers are displayed in the insets. It is evident that after the fluorination treatment, the influence of the feeding rate, which is known to be one of the key processing parameters in electrospraying, becomes insignificant with respect to the hydrophobic state of the layer. This indicates that the modification of the surface chemistry of SiO2 layers via fluorination plays a dominant role in exhibiting the outstanding hydrophobicity. Fig. 3a shows the surface microstructure, as observed by FE-SEM, of an electrospray-deposited fluorinated SiO2 layer, which was prepared at a feeding rate of 0.05 ml/h. As shown, the SiO2 nanoparticles in the range of 200–1200 nm in diameter homoge-
Fig. 1. Illustration conceptually showing the steps used to synthesize superhydrophobic rough, fluorinated SiO2 layers in this work.
Fig. 3. (a) Surface microstructure, observed by FE-SEM, taken from the rough, fluorinated SiO2 layer prepared at a feeding rate of 0.05 ml/h. The inset is a lowmagnification FE-SEM image. (b) Typical XPS spectrum of the rough, fluorinated SiO2 layer.
neously cover the substrate, resulting in nanoscale roughness. The root-mean-squared roughness, measured by a 3-dimensional laser scanning microscope, was in the range of 600–700 nm. The inset figure is a low-magnification FE-SEM image that shows the overall surface morphology. The WCA measured on this layer was 168°. Fig. 3b shows a result of XPS taken from the rough, fluorinated SiO2 layer. The spectrum clearly shows the presence of a fluorine species. Accordingly, it is reasonable to conclude that the superhydrophobicity of the electrospray-deposited fluorinated SiO2 layer results from the combination effect of the nanoscale surface roughness imposed by the nature of electrospray and the low surface energy generated by fluorosilane functionalization.
E.-K. Kim et al. / Journal of Colloid and Interface Science 368 (2012) 599–602
Various surface properties of the rough, fluorinated SiO2 layer that exhibited the best WCA of 168° were further investigated to explore the potential applicability of the layer. First of all, the capability of repelling liquids for the layer was tested. The shapes of the droplets of various liquids created on the layer were observed. As shown in Fig. 4a, the surface strongly repels liquids, such as water, glycerol, coffee, juice, and milk. The CAs of the various liquids for the rough, fluorinated SiO2 layer are shown in Fig. 4b. The surface tension of the five liquids ranges from 46 to 72 m Nm1. The work of adhesion can be estimated by applying the Young– Dupre equation [20,21], which is expressed as follows,
xad ¼ fs cL ð1 þ cos hY Þ
ð1Þ
where cL is the liquid surface tension, hY is the CA in the Young model, which is measured on a flat, smooth surface, and fs is the surface fraction, that is, the fraction of the SiO2 solid in contact with the liquid in the Cassie–Baxter equation. The values of fs can be obtained by substituting the CAs measured for the flat and the rough, fluorinated SiO2 layers in Fig. 4b into the Cassie–Baxter equation, which is expressed as [22]
cos hCB ¼ fs ðcos hY þ 1Þ 1
ð2Þ
where hCB and hY are CAs in the Cassie–Baxter and Young models, respectively. In order to estimate the work of adhesion and the surface fraction, one has to know the CAs created on a surface that not only has a surface structure that is sufficiently smooth to satisfy the Young equation, but has also the same surface chemistry as the rough, fluorinated SiO2 layer. For the surface satisfying the Young equation, we have prepared a flat SiO2 layer by using an oxidation process. The surface roughness of the prepared flat SiO2 layer was less than 0.6 nm. The flat SiO2 layer underwent the same fluorination treatment as described in Section 2. Using the CAs measured on the flat SiO2 layer (data not presented), the work of adhesion for the rough, fluorinated SiO2 layer was estimated, and the results are summarized in Fig. 5a. The work of adhesion values was less than 3 103 N/m for all the tested liquids, and this is one order
601
of magnitude smaller than those for the flat SiO2 layer. These exceptionally small works of adhesion are responsible for the strong repellency for liquids. The values of fs are shown in Fig. 5b. They are less than 0.1, again suggesting that a nanoscale roughness was created on the surface of the electrospray-synthesized SiO2 layer. The effects of UV exposure on the superhydrophobicity of the rough, fluorinated SiO2 layer were investigated. Fig. 6 shows the change in the WCA and the sliding angle in relation to UV exposure time. The WCA and the sliding angle of the rough, fluorinated SiO2 layer remain almost unchanged. The deterioration of hydrophobicity according to UV exposure has often been reported [23,24]. The UV light may cause decomposition of organic contaminants [23] or increase the surface energy of solids [24]. This facilitates the adsorption of water droplets, thus resulting in an enhanced hydrophilic state. However, the superhydrophobicity of the SiO2 layer in this work is very resistant to UV exposure. The fluoroalkyl group and the negligible amount of UV-liable groups on the silica surface are likely to be responsible for the excellent UV resistance [25]. The long-term stability of the rough, fluorinated SiO2 layer was examined by measuring the WCA and the sliding angle with time at room temperature in air. The results are shown in Fig. 7. The WCA and the sliding angle almost maintain the same values even after 30 days, demonstrating an outstanding durability in water.
Fig. 5. (a) Works of adhesion and (b) surface fractions of the rough, fluorinated SiO2 layer as a function of surface tensions of the liquids.
Fig. 4. (a) Shapes of liquid droplets, such as water, glycerol, coffee, juice, and milk, created on the rough, fluorinated SiO2 layer. (b) Contact angles of various liquid droplets on the rough, fluorinated SiO2 layer. The surface tensions for the liquids are also noted.
Fig. 6. Water contact and sliding angles measured for the rough, fluorinated SiO2 layer as a function of UV exposure time.
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4. Conclusions
Fig. 7. Change in water contact and sliding angles as a function of time.
Superhydrophobic SiO2 layers were prepared through a combination of the nanoscale surface roughness and the fluorination treatment. To synthesize rough SiO2 layers on the nanometer scale, the electrospraying deposition technique was used. In addition, modification of the surface chemistry was accomplished by the fluorination treatment. By this combination, highly rough, fluorinated SiO2 layers were synthesized. They exhibited excellent repellency toward various liquid droplets. For instance, the water contact angle was 168°. The surface fraction and the work of adhesion of the superhydrophobic SiO2 layers were respectively determined using Cassie–Baxter and Young–Dupre equations. The durability in water, the ultraviolet resistance, and the thermal stability were investigated. All the results strongly supported the potential use of the superhydrophobic SiO2 layers in the fields requiring antisticking, self-cleaning, and antifouling. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0023986). References
Fig. 8. Change in water contact angle as a function of heat treatment temperature. The corresponding shapes of water droplets are shown in the insets. The surface microstructure observed from the SiO2 layer after being heat-treated at 600 °C is also included in the inset.
The thermal stability of the rough, fluorinated SiO2 layer was investigated at various temperatures ranging from 100 to 600 °C for 2 h in air. As shown in Fig. 8, the SiO2 layer maintains its superhydrophobicity up to 400 °C. However, the superhydrophobicity greatly deteriorated by annealing at temperatures higher than 400 °C, leading to a hydrophilic state. The XPS spectrum (data not presented here) taken from the sample after being heat-treated at 600 °C confirmed the absence of fluorine species. The surface microstructure, which is included in the inset, of the sample that was heat-treated at 600 °C shows no significant difference from the microstructure before the heat treatment (see Fig. 3a), while it maintained its nanoscale roughness. Thereby it is reasonable to conclude that the drastic loss of superhydrophobicity was due to the volatilization of fluorine species. With respect to durability, UV resistance, and thermal stability, the superhydrophobic SiO2 layers prepared by the combination of the nanoscale roughness and the fluorination in this work hold potential in practical applications needing antisticking, self-cleaning, and antifouling attributes.
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