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A simple way to achieve tunable adhesion in superhydrophobic nanostructured Co3O4 films
Materials Letters 67 (2012) 334–337
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A simple way to achieve tunable adhesion in superhydrophobic nanostructured Co3O4 films Qunbing Zhang, Jun Wang ⁎ Department of Physics, Faculty of Science, Ningbo University, Ningbo, 315211, China
a r t i c l e
i n f o
Article history: Received 30 July 2011 Accepted 24 September 2011 Available online 8 October 2011 Keywords: Microstructure Surfaces Superhydrophobic Tunable adhesion
a b s t r a c t Tunable adhesive forces on the superhydrophobic nanostructured Co3O4 films have been achieved by simply rubbing the as-prepared films. Rubbing makes the change of morphologies of the films and leads to the tuning of adhesion. This morphology-dependent adhesive property is attributed to the difference in kinetic barriers resulting from the contact-state change. This simple and practical method can provide an important strategy for the adhesion adjustment on superhydrophobic surfaces. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Superhydrophobic surfaces have recently attracted much attention due to their potential industrial and biomedical applications [1–5]. Nature presents us with at least two types of surfaces: highadhesion superhydrophobic and low-adhesion superhydrophobic surfaces. For example, lotus leaves [6] possess a water contact angle (CA) of greater than 150° and a critical slide angle (SA) of less than 5°. And the petals of the red rose show high-adhesion superhydrophobic surfaces, where water drops get pinned to the petals surfaces [7]. Recently, special attention has been focused on the strong adhesive superhydrophobic, or more properly, superhydrophobic-like surfaces that enable a nearly spherical water droplet to be firmly pinned on the surface [8–12]. They are expected to have particular applications in open microdroplet devices [10,13]. Here we present a simply way to acquire superhydrophobic Co3O4 nanostructured films with tunable water adhesion. Our aim is to explore a simple way to achieve tunable adhesion for superhydrophobic surfaces. To the best of our knowledge, the fabrication of superhydrophobic surfaces with a wide range of tunable adhesion via this simple physical control of surface structure is still scarce.
2. Experiment In a typical process, 1.4552 g Co(NO3)2 6H2O, 0.3704 g NH4F, and 1.515 g CO(NH2)2 were dissolved in 50 mL of water under stirring. After 10 min of moderate stirring, the homogeneous solution ⁎ Corresponding author. Tel.: + 86 547 87600952; fax: + 86 574 87600744. E-mail address: [email protected] (J. Wang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.09.091
prepared above was transferred into Teflon-lined stainless steel autoclaves. Then, a piece of clean glass substrate was immersed into the reaction solution with a slant angle of about 60°. Then, the autoclave was sealed and maintained at 120 °C for 5 h and allowed to cool down to room temperature naturally. After the reaction, the glass substrate was dried at 60 °C for 6 h, and then calcinated at 400 °C in air for 2 h. To achieve superhydrophobicity, three same Co3O4 films were immersed in a methanol solution of hydrolyzed 1 wt.% PFOTES (CF3 (CF2)5CH2CH2Si(OCH2CH3)3 for 1 h at room temperature. The samples were rinsed with ethanol and subsequently heated to 80 °C for 2 h to remove nonbonded PFOTES molecules. The samples were characterized by XRD spectrum, which was taken using an X-ray diffractometer with high-intensity Cu Kα radiation (λ = 1.54178 Å). A FE-SEM Hitachi SU-70 was employed to investigate the morphology of the samples. Speedy video was taken by a high-speed camera (Hitachi camera). Water CA was measured by Contact Angle Meter(Data physics, OCA20)at the ambient temperature.
3. Result and discussion The crystal structures of the products were examined by the XRD spectrum. As shown in Fig. 1(a), the diffraction peaks, typical of Co3O4, are clearly observed and agree with those of the standard cubic-structure Co3O4 (JCPDS Card No. 42–1467), with the lattice constant a = 8.078 Å. No impurities such as precursor compounds have been detected, indicating the formation of pure cobalt oxides. The width of the (311) reflection is used to estimate the crystallite size using the Scherrer equation. The crystallite average size of the Co3O4 films was about 14.5 nm.
Q. Zhang, J. Wang / Materials Letters 67 (2012) 334–337
Fig. 1. The X-ray powder diffraction pattern of the as-synthesized Co3O4 films.
Fig. 2(a, b) show the typical SEM images of the Co3O4 products obtained after calcination. The glass substrate is mainly covered by uniform and compact nanorods with a length of about 5 μm and a diameter of about 150 nm. The nanorods self-assembled into microflowers with
335
the diameter of about 10 μm. After being moderately rubbed, the microflowers on the surface of the Co3O4 film were broken into pieces as shown in Fig. 2(c, d). When the prepared Co3O4 film was rubbed severely, the microflowers of the surface were severely damaged with no visible nanorods as shown in Fig. 2(e, f). Although these three Co3O4 films with different morphologies, they all exhibit the superhydrophobic feature after surface treatment with PFOTES with static CA larger than 150°, that is, 162 ± 1°, 155 ± 1°, and 150 ± 1°(inserts in Fig. 3a, c and e). Such superhydrophobicity can be explained by Cassie's theory [14]. The water droplet is supported by a composite surface composed of solid and air, where the air parts hide in the micro-structures of the solid parts to prevent contact from the solid and liquid, which leads to this perfect unwetting. The different nanostructures of the surfaces give rise to different adhesive forces which can be shown intuitively in the form of SA (slide angle). After the film being rubbed slightly and then severely, the SA of the film increased from 2 ± 1° to 68 ± 1° and then to 90° (inserts in Fig. 2b, d, f). It is proved that the adhesion of the films increases substantially. This difference in adhesion of the patterned films can be explained by Cassie's theory [14] and Wenzel's theory [15]. In Cassie state, water droplets float on the preiection of the surface. The contact area between the surface and the droplet was so small that the droplet almost has no wetting of the space between the nanopaticles so the
Fig. 2. FE-SEM images with different magnifications of Co3O4 films: (a, b) no rub, (c, d) rubbed slightly, and (e, f) rubbed severely.
336
Q. Zhang, J. Wang / Materials Letters 67 (2012) 334–337
Fig. 3. Schematic illustration of the contact state on Co3O4 films with different patterns: (a) not rubbed (Cassie state), (b) rubbed slightly (metastable state), and (c) rubbed severely (Wenzel state).
Fig. 4. Selected snapshots of the impact and rebound of millimetric water droplet.
Fig. 5. The progress of a water drop rolling on the partly rubbed superhydrophobic Co3O4 film.
adhesion was small (Fig. 3a). After the film was rubbed severely, the contact state is changed into Wenzel state. Microflower films were broken into small nanoparticals so that the water droplet can wet the contact area completely. The adhesion becomes very strong to be able to firmly pin the water droplet at any tilted angles (Fig. 3c). However, being rubbed slightly, microflowers of the film were not completely damaged, so the droplet just wets the contact area partially. The adhesion was between Cassis state and Wenzel state (Fig. 3b). This is normally called metastable state. In Fig. 4, the process of a 5 μl water droplet free-falling on the unrubbed surface of the superhydrophobic Co3O4 film with low adhesion was studied by a visualization method with a high-speed camera. The water droplet impacts the surface with a low velocity. In particular, we can observe that water droplet shape can change significantly during the impact because its kinetic energy transforms into stored energy. Fig. 5 shows the progress of a water droplet rolling on the partly rubbed superhydrophobic Co3O4 film. The rear half of the surface was rubbed and has strong adhesion. The water droplet will roll on the front half of the surface when the sample was tilted a little. The velocity of the water droplet is sharply reduced when it gets to the rear half of the surface. Its kinetic energy transforms into stored energy which is shown in the form of sharply change of the shape of the water droplet. Finally, the water droplet is pinned on the surface motionlessly.
Furthermore, the environmental stability and durability of the superhydrophobic property of the sample were checked after exposure to the atmosphere for 3 months. The measurement result shows that the CA decreased with the maximum variable quantity less than 2° and the SA almost had no change, which indicates that the as-prepared films possess high stability of superhydrophobicity. 4. Conclusion In this study, we have described a facile and inexpensive technique for preparing the microflower-shaped Co3O4 structures by a hydrothermal approach. We try to achieve tunable water adhesion of the superhydrophobic surfaces with the CA ranging from extremely low to very high by a simple rubbing method. The superhydrophobic surface adhesion could be tuned effectively by the distinct contact state between Cassie, metastable and Wenzel state. It depends on the microstructure configurations on the superhydrophobic surfaces. This approach is so practical that could provide a new strategy to prepare superhydrophobic surfaces with tunable adhesion. Acknowledgments This work was supported by the National Natural Science Foundation of China (10874095, 11174165), the K. C. Wong Magna Foundation,
Q. Zhang, J. Wang / Materials Letters 67 (2012) 334–337
Foundation (XRZL1001) and the Excellent Dissertation Cultivation Fund of Ningbo University (2010). References [1] [2] [3] [4] [5]
Xiu Y, Zhu L, Hess DW, Wong CP. Nano Lett 2007;7:3388. Bhushan B, Her EK. Langmuir 2010;26:8207. Feng XJ, Jiang L. Adv Mater 2006;18:3063. Blossey R. Nat Mater 2003;2:301. Cho KL, Liaw II, Wu AHF, Lamb RN. J Phys Chem C 2010;114:11228.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
337
Xi J, Jiang L. Ind Eng Chem Res 2008;47:6354. Feng L, Zhang YA, Xi JM, Zhu Y, Wang N, Xia F, et al. Langmuir 2008;24:4114. Bormashenko E, Stein T, Pogreb R, Aurbach DJ. Phys Chem C 2009;113:5568. Guo ZG, Liu WM. Appl Phys Lett 2007;90:223111. Hong X, Gao X, Jiang L. J Am Chem Soc 2007;129:1478. Song X, Zhai J, Wang Y, Jiang L. J Phys Chem B 2005;109:4048. Winkleman A, Gotesman G, Yoffe A, Naaman R. Nano Lett 2008;8:1241. Yuan JJ, Jin RH. Nanotechnology 2010;21:065704. Cassie ABD, Baxter S. Trans Faraday Soc 1944;40:546. Wenzel RN. J Phys Colloid Chem 1936;28:988.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A simple way to achieve tunable adhesion in superhydrophobic nanostructured Co3O4 films Qunbing Zhang, Jun Wang ⁎ Department of Physics, Faculty of Science, Ningbo University, Ningbo, 315211, China
a r t i c l e
i n f o
Article history: Received 30 July 2011 Accepted 24 September 2011 Available online 8 October 2011 Keywords: Microstructure Surfaces Superhydrophobic Tunable adhesion
a b s t r a c t Tunable adhesive forces on the superhydrophobic nanostructured Co3O4 films have been achieved by simply rubbing the as-prepared films. Rubbing makes the change of morphologies of the films and leads to the tuning of adhesion. This morphology-dependent adhesive property is attributed to the difference in kinetic barriers resulting from the contact-state change. This simple and practical method can provide an important strategy for the adhesion adjustment on superhydrophobic surfaces. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Superhydrophobic surfaces have recently attracted much attention due to their potential industrial and biomedical applications [1–5]. Nature presents us with at least two types of surfaces: highadhesion superhydrophobic and low-adhesion superhydrophobic surfaces. For example, lotus leaves [6] possess a water contact angle (CA) of greater than 150° and a critical slide angle (SA) of less than 5°. And the petals of the red rose show high-adhesion superhydrophobic surfaces, where water drops get pinned to the petals surfaces [7]. Recently, special attention has been focused on the strong adhesive superhydrophobic, or more properly, superhydrophobic-like surfaces that enable a nearly spherical water droplet to be firmly pinned on the surface [8–12]. They are expected to have particular applications in open microdroplet devices [10,13]. Here we present a simply way to acquire superhydrophobic Co3O4 nanostructured films with tunable water adhesion. Our aim is to explore a simple way to achieve tunable adhesion for superhydrophobic surfaces. To the best of our knowledge, the fabrication of superhydrophobic surfaces with a wide range of tunable adhesion via this simple physical control of surface structure is still scarce.
2. Experiment In a typical process, 1.4552 g Co(NO3)2 6H2O, 0.3704 g NH4F, and 1.515 g CO(NH2)2 were dissolved in 50 mL of water under stirring. After 10 min of moderate stirring, the homogeneous solution ⁎ Corresponding author. Tel.: + 86 547 87600952; fax: + 86 574 87600744. E-mail address: [email protected] (J. Wang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.09.091
prepared above was transferred into Teflon-lined stainless steel autoclaves. Then, a piece of clean glass substrate was immersed into the reaction solution with a slant angle of about 60°. Then, the autoclave was sealed and maintained at 120 °C for 5 h and allowed to cool down to room temperature naturally. After the reaction, the glass substrate was dried at 60 °C for 6 h, and then calcinated at 400 °C in air for 2 h. To achieve superhydrophobicity, three same Co3O4 films were immersed in a methanol solution of hydrolyzed 1 wt.% PFOTES (CF3 (CF2)5CH2CH2Si(OCH2CH3)3 for 1 h at room temperature. The samples were rinsed with ethanol and subsequently heated to 80 °C for 2 h to remove nonbonded PFOTES molecules. The samples were characterized by XRD spectrum, which was taken using an X-ray diffractometer with high-intensity Cu Kα radiation (λ = 1.54178 Å). A FE-SEM Hitachi SU-70 was employed to investigate the morphology of the samples. Speedy video was taken by a high-speed camera (Hitachi camera). Water CA was measured by Contact Angle Meter(Data physics, OCA20)at the ambient temperature.
3. Result and discussion The crystal structures of the products were examined by the XRD spectrum. As shown in Fig. 1(a), the diffraction peaks, typical of Co3O4, are clearly observed and agree with those of the standard cubic-structure Co3O4 (JCPDS Card No. 42–1467), with the lattice constant a = 8.078 Å. No impurities such as precursor compounds have been detected, indicating the formation of pure cobalt oxides. The width of the (311) reflection is used to estimate the crystallite size using the Scherrer equation. The crystallite average size of the Co3O4 films was about 14.5 nm.
Q. Zhang, J. Wang / Materials Letters 67 (2012) 334–337
Fig. 1. The X-ray powder diffraction pattern of the as-synthesized Co3O4 films.
Fig. 2(a, b) show the typical SEM images of the Co3O4 products obtained after calcination. The glass substrate is mainly covered by uniform and compact nanorods with a length of about 5 μm and a diameter of about 150 nm. The nanorods self-assembled into microflowers with
335
the diameter of about 10 μm. After being moderately rubbed, the microflowers on the surface of the Co3O4 film were broken into pieces as shown in Fig. 2(c, d). When the prepared Co3O4 film was rubbed severely, the microflowers of the surface were severely damaged with no visible nanorods as shown in Fig. 2(e, f). Although these three Co3O4 films with different morphologies, they all exhibit the superhydrophobic feature after surface treatment with PFOTES with static CA larger than 150°, that is, 162 ± 1°, 155 ± 1°, and 150 ± 1°(inserts in Fig. 3a, c and e). Such superhydrophobicity can be explained by Cassie's theory [14]. The water droplet is supported by a composite surface composed of solid and air, where the air parts hide in the micro-structures of the solid parts to prevent contact from the solid and liquid, which leads to this perfect unwetting. The different nanostructures of the surfaces give rise to different adhesive forces which can be shown intuitively in the form of SA (slide angle). After the film being rubbed slightly and then severely, the SA of the film increased from 2 ± 1° to 68 ± 1° and then to 90° (inserts in Fig. 2b, d, f). It is proved that the adhesion of the films increases substantially. This difference in adhesion of the patterned films can be explained by Cassie's theory [14] and Wenzel's theory [15]. In Cassie state, water droplets float on the preiection of the surface. The contact area between the surface and the droplet was so small that the droplet almost has no wetting of the space between the nanopaticles so the
Fig. 2. FE-SEM images with different magnifications of Co3O4 films: (a, b) no rub, (c, d) rubbed slightly, and (e, f) rubbed severely.
336
Q. Zhang, J. Wang / Materials Letters 67 (2012) 334–337
Fig. 3. Schematic illustration of the contact state on Co3O4 films with different patterns: (a) not rubbed (Cassie state), (b) rubbed slightly (metastable state), and (c) rubbed severely (Wenzel state).
Fig. 4. Selected snapshots of the impact and rebound of millimetric water droplet.
Fig. 5. The progress of a water drop rolling on the partly rubbed superhydrophobic Co3O4 film.
adhesion was small (Fig. 3a). After the film was rubbed severely, the contact state is changed into Wenzel state. Microflower films were broken into small nanoparticals so that the water droplet can wet the contact area completely. The adhesion becomes very strong to be able to firmly pin the water droplet at any tilted angles (Fig. 3c). However, being rubbed slightly, microflowers of the film were not completely damaged, so the droplet just wets the contact area partially. The adhesion was between Cassis state and Wenzel state (Fig. 3b). This is normally called metastable state. In Fig. 4, the process of a 5 μl water droplet free-falling on the unrubbed surface of the superhydrophobic Co3O4 film with low adhesion was studied by a visualization method with a high-speed camera. The water droplet impacts the surface with a low velocity. In particular, we can observe that water droplet shape can change significantly during the impact because its kinetic energy transforms into stored energy. Fig. 5 shows the progress of a water droplet rolling on the partly rubbed superhydrophobic Co3O4 film. The rear half of the surface was rubbed and has strong adhesion. The water droplet will roll on the front half of the surface when the sample was tilted a little. The velocity of the water droplet is sharply reduced when it gets to the rear half of the surface. Its kinetic energy transforms into stored energy which is shown in the form of sharply change of the shape of the water droplet. Finally, the water droplet is pinned on the surface motionlessly.
Furthermore, the environmental stability and durability of the superhydrophobic property of the sample were checked after exposure to the atmosphere for 3 months. The measurement result shows that the CA decreased with the maximum variable quantity less than 2° and the SA almost had no change, which indicates that the as-prepared films possess high stability of superhydrophobicity. 4. Conclusion In this study, we have described a facile and inexpensive technique for preparing the microflower-shaped Co3O4 structures by a hydrothermal approach. We try to achieve tunable water adhesion of the superhydrophobic surfaces with the CA ranging from extremely low to very high by a simple rubbing method. The superhydrophobic surface adhesion could be tuned effectively by the distinct contact state between Cassie, metastable and Wenzel state. It depends on the microstructure configurations on the superhydrophobic surfaces. This approach is so practical that could provide a new strategy to prepare superhydrophobic surfaces with tunable adhesion. Acknowledgments This work was supported by the National Natural Science Foundation of China (10874095, 11174165), the K. C. Wong Magna Foundation,
Q. Zhang, J. Wang / Materials Letters 67 (2012) 334–337
Foundation (XRZL1001) and the Excellent Dissertation Cultivation Fund of Ningbo University (2010). References [1] [2] [3] [4] [5]
Xiu Y, Zhu L, Hess DW, Wong CP. Nano Lett 2007;7:3388. Bhushan B, Her EK. Langmuir 2010;26:8207. Feng XJ, Jiang L. Adv Mater 2006;18:3063. Blossey R. Nat Mater 2003;2:301. Cho KL, Liaw II, Wu AHF, Lamb RN. J Phys Chem C 2010;114:11228.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
337
Xi J, Jiang L. Ind Eng Chem Res 2008;47:6354. Feng L, Zhang YA, Xi JM, Zhu Y, Wang N, Xia F, et al. Langmuir 2008;24:4114. Bormashenko E, Stein T, Pogreb R, Aurbach DJ. Phys Chem C 2009;113:5568. Guo ZG, Liu WM. Appl Phys Lett 2007;90:223111. Hong X, Gao X, Jiang L. J Am Chem Soc 2007;129:1478. Song X, Zhai J, Wang Y, Jiang L. J Phys Chem B 2005;109:4048. Winkleman A, Gotesman G, Yoffe A, Naaman R. Nano Lett 2008;8:1241. Yuan JJ, Jin RH. Nanotechnology 2010;21:065704. Cassie ABD, Baxter S. Trans Faraday Soc 1944;40:546. Wenzel RN. J Phys Colloid Chem 1936;28:988.