CHEMPHYSCHEM COMMUNICATIONS DOI: 10.1002/cphc.201301084
Robust Nonsticky Superhydrophobicity by the Tapering of Aligned ZnO Nanorods Jian Tian,[a, b] Yaping Zhang,[a, b] Jie Zhu,[a] Zhongzhen Yang,[a, b] and Xuefeng Gao*[a] The robust nonsticky superhydrophobicity of aligned nanoneedle films is reported. A facile, efficient, cheap, and available method based on the diffusion-limited crystal growth principle is proposed for controlling the tapering of ZnO nanorods, the profiles of which can be tuned effectively by synergetic control over reaction time and temperature in an extremely strong alkaline reaction system. The synthesized nanoneedle, nanopencil, and nanorod arrays are chosen for studying the effects of nanoscale topography on anti-droplet-sticking ability. After silanization, all of them show excellent quasi-static anti-dropletstickiness, and water adhesion along the normal and lateral directions can be greatly reduced by the tapering of nanorods and eliminated by sharp nanoneedles. However, their antisticking stability is distinct under the droplet impact: the nanoneedle sample is still nonsticky but the nanorod sample loses its antisticking ability. Only ensuring the liquid/air interface is in the suspended nonwetting state is insufficient to obtain robust nonsticky surfaces, which also require extremely low solid–liquid van der Waals attraction.
With the discovery of novel superhydrophobic phenomena in nature, such as the self-cleaning of lotus leaves,[1] the swift aquatic walking of water strider legs,[2] the dry-style antifogging of mosquito eyes,[3] and the directional self-cleaning of butterfly wings,[4] surfaces with special wetting and adhesion have attracted intensive interest due to their significance in basic research and technological innovation.[5] However, it is still a great challenge to obtain robust nonsticky surfaces that can fully avoid the pinning of water droplets under quasi-static and dynamic conditions, without any aid from an external force. It should be pointed out that the majority of reported artificial superhydrophobic surfaces with water contact angles above 1508,[6–8] for example, smart responsive surfaces with switchable superhydrophobicity and superhydrophilicity,[8] are highly adhesive, and the adhered droplets are hard to fully remove even under external force. So far, it has not been easy to obtain really perfect nonsticky surfaces,[9–12] although
[a] Dr. J. Tian, Y. Zhang, J. Zhu, Z. Yang, Prof. X. Gao Suzhou Institute of Nano-Tech and Nano-Bionics Chinese Academy of Sciences Suzhou 215123 (China) E-mail: [email protected] [b] Dr. J. Tian, Y. Zhang, Z. Yang University of Chinese Academy of Sciences Chinese Academy of Sciences Beijing 100049 (China) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201301084.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
a great breakthrough has been made in creating low-adhesive[13] and adhesion-switchable surfaces,[14] in which water droplets can be adhered but easily removed by external force, such as by airflow or tilting the samples. Further, a more rigid design of the nanoscale superficial morphology would be required to obtain robust nonsticky surfaces without both clearly detectable van der Waals attraction and capillary adhesion, especially during the droplet impact process.[12] Therefore, it is significant to uncover the crucial structural features of robust nonsticky surfaces and develop novel nanofabrication technologies. Herein, we report a facile, time-saving, and cheap method of fabricating robust nonsticky aligned ZnO nanoneedle films. By adopting an extremely violent alkaline reaction system with suitable concentration and cooperatively controlling both reaction time and temperature to adjust the diffusion rate of growth units, we can control the tapered growth of aligned ZnO nanorods, which can induce perfect quasi-static and dynamically stable nonstickiness after silanization. These tapered nanoneedles with minimized top diameters minimize both the solid–liquid van der Waals attraction and the edge-induced pinning force, to ensure the superior rebounding ability of dynamically stable nonwetting liquid/air interfaces suspended upon nanotips with nanoscale interspaces at the moment of retraction. Figure 1 a shows the basic principle of ZnO nanocrystal growth from the rod to the tapered shape. It is well known that, in the case of normal diffusion of growth units, the growth rates of ZnO crystal facets obey the relationship: (0001) {011¯1} @ {011¯0},[15] which results in the formation of nanorods. If the growth rates of the facets (0001) and {011¯1} were differentiated by the diffusion-limited transport of growth units, aligned ZnO nanopencils or nanoneedles would form. To verify the feasibility of this strategy and offer a facile, efficient, and inexpensive synthesis method available to most researchers, we designed a specific wet-chemical route. A strong alkali was used to rapidly consume zinc ions by violent homogeneous reactions in the bulk solution, which offers the chance of saving time and adjusting the diffusion rate of growth units transported to crystal facets by the synergetic control of growth temperature and time at a suitable concentration condition, for example, the chosen isopycnic 4 mol L 1 NaOH and 0.5 mol L 1 Zn(NO3)2 mixed solution (Figure S1 in the Supporting Information). To avoid the influence of ZnO deposits, the reaction surface coated with ZnO seeds must be upside down. Figure 1 b exhibits typical scanning electronic microscopy (SEM) tilted views of the as-synthesized ZnO nanostructures under different reaction conditions. At a higher temperature ChemPhysChem 2014, 15, 858 – 861
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Figure 2. a) Measured normal-direction adhesive forces (left) and top diameters (right) of nanosamples as-obtained at an identical reaction time (1 h) and different temperatures followed by identical silanization treatment. b) Optical images of aligned nanorod (left) and nanoneedle (right) films separating instantaneously from the suspended droplet after squeezing. The droplet is almost fully nonsticky to the nanoneedles, but has apparent drag deformation (i.e. stickiness) to the nanorods. Figure 1. a) Schematic showing the principle of controlling the growth of ZnO nanocrystals from the rod to the tapered shape by diffusion-limited transport of growth units, which can be realized by synergetic control of both reaction temperature and time in an extremely strong alkaline system. b) SEM tilted views of the ZnO nanorods, nanopencils, and nanoneedles assynthesized under various reaction conditions. Black dividing lines denote the morphological switch from the rod to the tapered shapes.
range (e.g. T 80 8C), the formation of nanorods dominates as t > 30 min due to a higher diffusion rate (Figure S2). As the temperature decreases to 70 8C, nanoneedles, nanopencils, and nanorods form at t = 0.5, 1, and 2 h, respectively. At T = 60 8C, nanoneedles are formed within a longer duration ( 3 h). We found that the morphological transition occurred at approximately 65 8C (Figure S3) and the fading of the tapering profile increases with increasing reaction temperature (Figure S4). The aligned nanotapers cannot be formed below 60 8C due to lower growth and diffusion rates (Figure S5). Evidently, the tapering of aligned nanorods can be realized by synergetic control of the thermodynamic and kinetic conditions of crystal growth in the diffusion-limited strong alkaline reaction system. Compared with previous wet-chemical crystal growth methods for synthesizing aligned ZnO nanoneedles,[16] our method is more efficient and unlimited to substrate materials. After grasping the ability of modulating the tapered profiles of aligned nanorods, we could offer insight into the significance of rigidly tailoring the nanoscale tiplike topography to achieve robust nonstickiness. Five example superhydrophobic samples from nanorods to nanoneedles were obtained at different temperatures of crystal growth for 1 h followed by identical silanization treatment. The adhesive forces along the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
normal direction were probed by a 3 mL droplet suspended on a high-sensitivity micro-electromechanical balance. Figure 2 a shows the measured adhesive forces and top diameters of the as-synthesized nanocrystals. With the temperature decreasing, the profiles of the nanocrystals switch from nanorods (as-obtained at 90 and 80 8C) to nanopencils (at 70 8C) and nanoneedles (at 60 and 65 8C), which resulted in the gradual decrease of both the top diameters and surface adhesion. The surface adhesion ( 3 mN) of the nanoneedle sample obtained at 60 8C is almost negligible, far less than that ( 28 mN) of the nanorod sample obtained at 90 8C. To more intuitively exhibit the antidroplet-stickiness difference caused by the tapering of nanorods, we obtained snapshots of the representative aligned nanorod and nanoneedle samples instantly separating from the suspended droplet after squeezing, as shown in Figure 2 b, which consists of clips from Movies S1 and S2. By observing the droplet bottom indicated by dotted rectangles in Figure 2 b, we found that the nanorod sample has apparent stickiness but can fully separate from the suspended droplet, thus showing a good quasi-static anti-droplet-stickiness nature (Movie S1). Strikingly, the nanoneedle sample appears fully nonsticky to the droplet without detectable deformation (Movie S2). Subsequently, the lateral-direction antisticking abilities of the five samples were evaluated by the critical tilted angle (TA), at which the 4 mL droplet can roll off the sample surface once falling under gravitation. The measured results are shown in Figure 3 a. With the growth temperature decreasing, the lateral-direction adhesion can greatly decrease, for example, TA = ChemPhysChem 2014, 15, 858 – 861
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Figure 4. Sequential optical images showing the dynamic behavior of a 4 mL water droplet impacting the aligned nanoneedle sample at a velocity of 2.7 m s 1, corresponding to a release height of 38 cm. The sample shows striking nonstickiness not only to the impacted millimeter-scale droplet but also to the cracked micrometer-scale droplets.
Figure 3. a) Measured tilted angles (TAs) of five nanosamples as shown in Figure 2 a. The volume of the probed droplet is 4 mL. b) Optical images showing that the same volume of droplet can be pinned on the horizontally placed nanorod sample (left, TA = 08), but can easily roll off the surface on tilting the sample up to TA = 108 (right). c) Optical image showing that a droplet can instantly roll off the surface of horizontally placed nanoneedle samples (TA = 08) once falling under gravitation.
108 (78) for the nanorod samples as-obtained at 90 8C (80 8C), TA = 38 for the nanopencil samples as-obtained at 70 8C, and TA = 08 for the nanoneedle samples as-obtained at 60 and 65 8C. As shown in Figure 3 b and Movie S3, the droplet can be pinned on the surface of horizontally placed nanorod samples but rolls off the samples tilted up to 108. In contrast, the droplets instantly roll off the horizontally placed nanoneedle samples obtained at 60 8C (Figure 3 c and Movie S4). Thus, the lateral stickiness can be greatly reduced by the tapering of nanorods and eliminated by the sharp nanoneedles. We further studied the nonstickiness robustness of the aligned nanoneedle films at the dynamic conditions. As shown in Figure 4 and Movie S5, a 4 mL droplet falling from a height of 38 cm (with an impact velocity of 2.7 m s 1, dynamic pressure of 3.7 kPa, and hammer pressure of 0.8 MPa) was instantly spread under the huge impact action and then cracked into many micrometer-scale droplets, all of which cannot adhere to the sample surface. Among others, partial droplets can reunite and then rebound. Note that aligned nanorod films lose their antidroplet-sticking ability at the dynamic conditions, no matter whether under slight impact (0.44 m s 1, see Movie S6) or violent impact (2.7 m s 1, see Movie S7). No doubt the tapering of aligned nanorods ensures the dynamically stable nonstickiness. Why can the tapering of nanorods induce robust droplet nonstickiness? Firstly, it can be well understood that surface adhesion at the quasi-static state results from the solid–liquid van der Waals attraction, which is only related to the top-layer nanostructure morphologies. In contrast to aligned nanorods, aligned nanoneedles have extremely low solid–liquid contact 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
area and negligible van der Waals attraction, thus rendering their surfaces perfect nonstickiness along the normal and lateral directions.[12] The instantly formed solid–liquid–air contact lines are highly discrete, which is advantageous to both the normal separation and lateral movement of water droplets.[6a, 12] However, the cases become intricate as the droplet impacts. The nonwetting liquid/air interfaces do not mean that the antisticking abilities can be ensured during the impact process, which also depends on the nanoscale topographies. These assynthesized nanorods are characterized by relatively larger average diameters (105 nm) and interspaces (153 nm), hexagonal crystal faces, and uneven height. Although their very high capillary pressure ( 105 Pa),[17] far larger than the exerted hammer pressure and dynamic pressure, can effectively prevent the contact lines from sliding downward along the sidewall of nanorods, the droplet impact would inevitably bring about the additional contact of water films with the top sidewalls of some protruding nanorods, thus further greatly increasing the van der Waals force due to huge density, for which newly added force is not negligible. These ragged nanorods with hexagonal edges somewhat play a role of “re-entrant curvature”,[18] which can effectively resist the advancing (i.e. inward wetting) of water films by enhancing the pinning of contact lines, but necessarily bring about the droplet adhesion due to edge-induced pinning force during their retracting process.[10a, 19] As a result, the impacting droplets are firmly pinned on the surface of aligned nanorods. In contrast, the aligned nanoneedles have far smaller top diameters (17 nm) and interspaces (69 nm), circular cross sections, and sparser contact sites at their top, which generates a critical capillary pressure of approximately 106 Pa so as to avoid penetrating the water films under the current impacting speeds, thereby showing dynamically stable nonstickiness. We suspect that, even if the tips of some protruding nanoneedles may penetrate water films, the pinning resistance of contact lines during the retracting process, in comparison with released dynamic energy, may be neglected. The tapered profiles with gradual curvature can facilitate the retraction of contact lines whereas the minimized top diameters with both negligible solid–liquid van der Waals force and pinning force can ensure the effortless rebound of contact lines.[10a, 19] ChemPhysChem 2014, 15, 858 – 861
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CHEMPHYSCHEM COMMUNICATIONS In summary, we have reported a facile, efficient, and cheap method for synthesizing aligned ZnO nanoneedle films with perfect quasi-static and dynamically stable droplet nonstickiness. The tapering of aligned ZnO nanorods is realized by a diffusion-limited wet-chemical crystal growth strategy for emphasizing the importance of controlling nanoscale tiplike topography in ensuring robust nonstickiness. Only keeping the nonwetting state of liquid/air interfaces is still insufficient to achieve robust nonstickiness, which also requires minimal solid– liquid van der Waals force and edge-induced pinning force. These findings are significant to the development of robust nonsticky surfaces and the exploration of new applications. For example, the superhydrophobic aligned nanoneedles with nanoscale characteristic interspaces have typical anti-microdrop-sticking advantage, which can find significant applications such as dry-style antifogging[3] and enhanced dropwise condensation heat transfer.[20]
Experimental Section Materials Glass slides of size 2 cm 2 cm were rinsed to remove contaminants by sonification in acetone, ethanol, and deionized water for about 5 min, respectively, and then immersed in hot piranha solution overnight for hydroxylation. Subsequently, cleaned glass slides were coated with crystal seeds by the three-cycle treatment of dipcoating in a solution (0.005 mol L 1) of zinc acetate in ethanol, rinsing with ethanol, and annealing at 350 8C for 20 min. The glass slides with the seeded face down were immersed in a chemical bath of isopycnic NaOH (0.5 mol L 1) and Zn(NO3)2 (4 mol L 1) at 60 8C for 15–180 min. The as-synthesized nanoarrays were taken out, rinsed with deionized water, and dried with a nitrogen airflow. Finally, anti-droplet-sticking superhydrophobic surfaces were obtained by immersing the as-synthesized nanoarrays in an ethanol solution of heptadecafluorodecyltrimethoxysilane (2 vol %) at 25 8C for 2 h and then heating at 80 8C for 1 h in an oven.
Characterization The nanostructure morphologies were observed by scanning electronic microscopy (SEM, Hitachi S4800, Japan) after sputtering a layer of Au film 15 nm in thickness. The normal-direction adhesive force was measured by a micro-electromechanical balance (Dataphysics DCAT21, Germany). The lateral-direction adhesion was characterized by the critical tilted angles by means of optical contact angle meters (OCA 20, DataPhysics, Germany). The optical images shown in Figures 2 b and 3 b,c were recorded by CCD camera in the OCA 20 apparatus. The droplet impacting processes shown in Figure 4 were observed by a high-speed high-resolution motion analysis microscope (Keyence VW-9000, Japan) at 50 magnification and a rate of 4000 frames per second (fps).
Acknowledgements This work was supported by the National Basic Research Program of China (2012CB933202), National Natural Science Foundation of China (20773135), and Key Research Program of the Chinese Academy of Sciences (KJZD-EW-M01).
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org Keywords: crystal growth · hydrophobic effect nanostructures · superhydrophobicity · surface chemistry
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[1] a) W. Barthlott, C. Neinhuis, Planta 1997, 202, 1 – 8; b) L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang, D. Zhu, Adv. Mater. 2002, 14, 1857 – 1860. [2] X. Gao, L. Jiang, Nature 2004, 432, 36. [3] X. Gao, Y. Yan, X. Yao, L. Xu, K. Zhan, J. Zhang, B. Yang, L. Jiang, Adv. Mater. 2007, 19, 2213 – 2217. [4] Y. Zheng, X. Gao, L. Jiang, Soft Matter 2007, 3, 178 – 182. [5] a) M. Liu, Y. Zheng, J. Zhai, L. Jiang, Acc. Chem. Res. 2010, 43, 368 – 377; b) B. Bhushan, Y. C. Jung, K. Koch, Philos. Trans. R. Soc. A 2009, 367, 1631 – 1672. [6] a) D. ner, T. J. McCarthy, Langmuir 2000, 16, 7777 – 7782; b) M. Callies, D. Qur, Soft Matter 2005, 1, 55 – 61; c) N. Zhao, Q. Xie, X. Kuang, S. Wang, Y. Li, X. Lu, S. Tan, J. Shen, X. Zhang, Y. Zhang, J. Xu, C. C. Han, Adv. Funct. Mater. 2007, 17, 2739 – 2745. [7] a) H. Y. Erbil, A. L. Demirel, Y. Avci, O. Mert, Science 2003, 299, 1377 – 1380; b) H. Yan, K. Kurogi, H. Mayama, K. Tsujii, Angew. Chem. 2005, 117, 3519 – 3522; Angew. Chem. Int. Ed. 2005, 44, 3453 – 3456; c) X. Hong, X. Gao, L. Jiang, J. Am. Chem. Soc. 2007, 129, 1478 – 1479. [8] a) S. Wang, X. Feng, J. Yao, L. Jiang, Angew. Chem. 2006, 118, 1286 – 1289; Angew. Chem. Int. Ed. 2006, 45, 1264 – 1267; < lit b > X. Feng, L. Feng, M. Jin, J. Zhai, L. Jiang, D. Zhu, J. Am. Chem. Soc. 2004, 126, 62 – 63. [9] a) J. Zhang, S. Seeger, Angew. Chem. 2011, 123, 6782 – 6786; Angew. Chem. Int. Ed. 2011, 50, 6652 – 6656; b) I. A. Larmour, S. E. J. Bell, G. C. Saunders, Angew. Chem. 2007, 119, 1740 – 1742; Angew. Chem. Int. Ed. 2007, 46, 1710 – 1712; c) V. Zorba, E. Stratakis, M. Barberoglou, E. Spanakis, P. Tzanetakis, S. H. Anastasiadis, C. Fotakis, Adv. Mater. 2008, 20, 4049 – 4054; d) S.-J. Choi, K. Y. Suh, H. H. Lee, J. Am. Chem. Soc. 2008, 130, 6312 – 6313; e) L. Gao, T. J. McCarthy, J. Am. Chem. Soc. 2006, 128, 9052 – 9053; f) K. Acatay, E. Simsek, C. Ow-Yang, Y. Z. Menceloglu, Angew. Chem. 2004, 116, 5322 – 5325; Angew. Chem. Int. Ed. 2004, 43, 5210 – 5213; g) M. Miwa, A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Langmuir 2000, 16, 5754 – 5760. [10] a) K. C. Park, H. J. Choi, C. H. Chang, R. E. Cohen, G. H. McKinley, G. Barbastathis, ACS Nano 2012, 6, 3789 – 3799; b) J. Shieh, F. J. Hou, Y. C. Chen, H. M. Chen, S. P. Yang, C. C. Cheng, H. L. Chen, Adv. Mater. 2010, 22, 597 – 601; c) C. Dorrer, J. Rhe, Adv. Mater. 2008, 20, 159 – 163. [11] a) E. Hosono, S. Fujihara, I. Honma, H. Zhou, J. Am. Chem. Soc. 2005, 127, 13458 – 13459; b) X. Yao, Q. Chen, L. Xu, Q. Li, Y. Song, X. Gao, D. Qur, L. Jiang, Adv. Funct. Mater. 2010, 20, 656 – 662. [12] a) Y. Lai, X. Gao, H. Zhuang, J. Huang, C. Lin, L. Jiang, Adv. Mater. 2009, 21, 3799 – 3803; b) X. Gao, X. Yao, L. Jiang, Langmuir 2007, 23, 4886 – 4891. [13] a) X. Liu, Y. Liang, F. Zhou, W. Liu, Soft Matter 2012, 8, 2070 – 2086; b) A. Lafuma, D. Qur, Nat. Mater. 2003, 2, 457 – 460. [14] a) C. Li, R. Guo, X. Jiang, S. Hu, L. Li, X. Cao, H. Yang, Y. Song, Y. Ma, L. Jiang, Adv. Mater. 2009, 21, 4254 – 4258; b) J. Seo, S. Lee, H. Han, H. B. Jung, J. Hong, G. Song, S. M. Cho, C. Park, W. Lee, T. Lee, Adv. Mater. 2013, 25, 4139 – 4144. [15] a) R. A. Laudise, A. A. Ballman, J. Phys. Chem. 1960, 64, 688 – 691; b) W.-J. Li, E.-W. Shi, W.-Z. Zhong, Z.-W. Yin, J. Cryst. Growth 1999, 203, 186 – 196. [16] a) Y.-J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, J. W. P. Hsu, Nano Lett. 2008, 8, 1501 – 1505; b) X. Wu, H. Bai, C. Li, G. Lu, G. Shi, Chem. Commun. 2006, 1655 – 1657; c) J. Liu, X. Huang, Y. Li, X. Ji, Z. Li, X. He, F. Sun, J. Phys. Chem. C 2007, 111, 4990 – 4997. [17] a) D. Richard, D. Qur, Europhys. Lett. 2000, 50, 769 – 775; b) M. Reyssat, A. Ppin, F. Marty, Y. Chen, D. Qur, Europhys. Lett. 2006, 74, 306 – 312; c) T. Deng, K. K. Varanasi, M. Hsu, N. Bhate, C. Keimel, J. Stein, M. Blohm, Appl. Phys. Lett. 2009, 94, 133109. [18] A. Tuteja, W. Choi, M. Ma, J. M. Mabry, S. A. Mazzella, G. C. Rutledge, G. H. McKinley, R. E. Cohen, Science 2007, 318, 1618 – 1622. [19] M. Nosonovsky, B. Bhushan, Microsyst. Technol. 2005, 11, 535 – 549. [20] X. Chen, J. Wu, R. Ma, M. Hua, N. Koratkar, S. Yao, Z. Wang, Adv. Funct. Mater. 2011, 21, 4617 – 4623. Received: November 19, 2013 Published online on March 3, 2014
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Robust Nonsticky Superhydrophobicity by the Tapering of Aligned ZnO Nanorods Jian Tian,[a, b] Yaping Zhang,[a, b] Jie Zhu,[a] Zhongzhen Yang,[a, b] and Xuefeng Gao*[a] The robust nonsticky superhydrophobicity of aligned nanoneedle films is reported. A facile, efficient, cheap, and available method based on the diffusion-limited crystal growth principle is proposed for controlling the tapering of ZnO nanorods, the profiles of which can be tuned effectively by synergetic control over reaction time and temperature in an extremely strong alkaline reaction system. The synthesized nanoneedle, nanopencil, and nanorod arrays are chosen for studying the effects of nanoscale topography on anti-droplet-sticking ability. After silanization, all of them show excellent quasi-static anti-dropletstickiness, and water adhesion along the normal and lateral directions can be greatly reduced by the tapering of nanorods and eliminated by sharp nanoneedles. However, their antisticking stability is distinct under the droplet impact: the nanoneedle sample is still nonsticky but the nanorod sample loses its antisticking ability. Only ensuring the liquid/air interface is in the suspended nonwetting state is insufficient to obtain robust nonsticky surfaces, which also require extremely low solid–liquid van der Waals attraction.
With the discovery of novel superhydrophobic phenomena in nature, such as the self-cleaning of lotus leaves,[1] the swift aquatic walking of water strider legs,[2] the dry-style antifogging of mosquito eyes,[3] and the directional self-cleaning of butterfly wings,[4] surfaces with special wetting and adhesion have attracted intensive interest due to their significance in basic research and technological innovation.[5] However, it is still a great challenge to obtain robust nonsticky surfaces that can fully avoid the pinning of water droplets under quasi-static and dynamic conditions, without any aid from an external force. It should be pointed out that the majority of reported artificial superhydrophobic surfaces with water contact angles above 1508,[6–8] for example, smart responsive surfaces with switchable superhydrophobicity and superhydrophilicity,[8] are highly adhesive, and the adhered droplets are hard to fully remove even under external force. So far, it has not been easy to obtain really perfect nonsticky surfaces,[9–12] although
[a] Dr. J. Tian, Y. Zhang, J. Zhu, Z. Yang, Prof. X. Gao Suzhou Institute of Nano-Tech and Nano-Bionics Chinese Academy of Sciences Suzhou 215123 (China) E-mail: [email protected] [b] Dr. J. Tian, Y. Zhang, Z. Yang University of Chinese Academy of Sciences Chinese Academy of Sciences Beijing 100049 (China) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201301084.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
a great breakthrough has been made in creating low-adhesive[13] and adhesion-switchable surfaces,[14] in which water droplets can be adhered but easily removed by external force, such as by airflow or tilting the samples. Further, a more rigid design of the nanoscale superficial morphology would be required to obtain robust nonsticky surfaces without both clearly detectable van der Waals attraction and capillary adhesion, especially during the droplet impact process.[12] Therefore, it is significant to uncover the crucial structural features of robust nonsticky surfaces and develop novel nanofabrication technologies. Herein, we report a facile, time-saving, and cheap method of fabricating robust nonsticky aligned ZnO nanoneedle films. By adopting an extremely violent alkaline reaction system with suitable concentration and cooperatively controlling both reaction time and temperature to adjust the diffusion rate of growth units, we can control the tapered growth of aligned ZnO nanorods, which can induce perfect quasi-static and dynamically stable nonstickiness after silanization. These tapered nanoneedles with minimized top diameters minimize both the solid–liquid van der Waals attraction and the edge-induced pinning force, to ensure the superior rebounding ability of dynamically stable nonwetting liquid/air interfaces suspended upon nanotips with nanoscale interspaces at the moment of retraction. Figure 1 a shows the basic principle of ZnO nanocrystal growth from the rod to the tapered shape. It is well known that, in the case of normal diffusion of growth units, the growth rates of ZnO crystal facets obey the relationship: (0001) {011¯1} @ {011¯0},[15] which results in the formation of nanorods. If the growth rates of the facets (0001) and {011¯1} were differentiated by the diffusion-limited transport of growth units, aligned ZnO nanopencils or nanoneedles would form. To verify the feasibility of this strategy and offer a facile, efficient, and inexpensive synthesis method available to most researchers, we designed a specific wet-chemical route. A strong alkali was used to rapidly consume zinc ions by violent homogeneous reactions in the bulk solution, which offers the chance of saving time and adjusting the diffusion rate of growth units transported to crystal facets by the synergetic control of growth temperature and time at a suitable concentration condition, for example, the chosen isopycnic 4 mol L 1 NaOH and 0.5 mol L 1 Zn(NO3)2 mixed solution (Figure S1 in the Supporting Information). To avoid the influence of ZnO deposits, the reaction surface coated with ZnO seeds must be upside down. Figure 1 b exhibits typical scanning electronic microscopy (SEM) tilted views of the as-synthesized ZnO nanostructures under different reaction conditions. At a higher temperature ChemPhysChem 2014, 15, 858 – 861
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Figure 2. a) Measured normal-direction adhesive forces (left) and top diameters (right) of nanosamples as-obtained at an identical reaction time (1 h) and different temperatures followed by identical silanization treatment. b) Optical images of aligned nanorod (left) and nanoneedle (right) films separating instantaneously from the suspended droplet after squeezing. The droplet is almost fully nonsticky to the nanoneedles, but has apparent drag deformation (i.e. stickiness) to the nanorods. Figure 1. a) Schematic showing the principle of controlling the growth of ZnO nanocrystals from the rod to the tapered shape by diffusion-limited transport of growth units, which can be realized by synergetic control of both reaction temperature and time in an extremely strong alkaline system. b) SEM tilted views of the ZnO nanorods, nanopencils, and nanoneedles assynthesized under various reaction conditions. Black dividing lines denote the morphological switch from the rod to the tapered shapes.
range (e.g. T 80 8C), the formation of nanorods dominates as t > 30 min due to a higher diffusion rate (Figure S2). As the temperature decreases to 70 8C, nanoneedles, nanopencils, and nanorods form at t = 0.5, 1, and 2 h, respectively. At T = 60 8C, nanoneedles are formed within a longer duration ( 3 h). We found that the morphological transition occurred at approximately 65 8C (Figure S3) and the fading of the tapering profile increases with increasing reaction temperature (Figure S4). The aligned nanotapers cannot be formed below 60 8C due to lower growth and diffusion rates (Figure S5). Evidently, the tapering of aligned nanorods can be realized by synergetic control of the thermodynamic and kinetic conditions of crystal growth in the diffusion-limited strong alkaline reaction system. Compared with previous wet-chemical crystal growth methods for synthesizing aligned ZnO nanoneedles,[16] our method is more efficient and unlimited to substrate materials. After grasping the ability of modulating the tapered profiles of aligned nanorods, we could offer insight into the significance of rigidly tailoring the nanoscale tiplike topography to achieve robust nonstickiness. Five example superhydrophobic samples from nanorods to nanoneedles were obtained at different temperatures of crystal growth for 1 h followed by identical silanization treatment. The adhesive forces along the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
normal direction were probed by a 3 mL droplet suspended on a high-sensitivity micro-electromechanical balance. Figure 2 a shows the measured adhesive forces and top diameters of the as-synthesized nanocrystals. With the temperature decreasing, the profiles of the nanocrystals switch from nanorods (as-obtained at 90 and 80 8C) to nanopencils (at 70 8C) and nanoneedles (at 60 and 65 8C), which resulted in the gradual decrease of both the top diameters and surface adhesion. The surface adhesion ( 3 mN) of the nanoneedle sample obtained at 60 8C is almost negligible, far less than that ( 28 mN) of the nanorod sample obtained at 90 8C. To more intuitively exhibit the antidroplet-stickiness difference caused by the tapering of nanorods, we obtained snapshots of the representative aligned nanorod and nanoneedle samples instantly separating from the suspended droplet after squeezing, as shown in Figure 2 b, which consists of clips from Movies S1 and S2. By observing the droplet bottom indicated by dotted rectangles in Figure 2 b, we found that the nanorod sample has apparent stickiness but can fully separate from the suspended droplet, thus showing a good quasi-static anti-droplet-stickiness nature (Movie S1). Strikingly, the nanoneedle sample appears fully nonsticky to the droplet without detectable deformation (Movie S2). Subsequently, the lateral-direction antisticking abilities of the five samples were evaluated by the critical tilted angle (TA), at which the 4 mL droplet can roll off the sample surface once falling under gravitation. The measured results are shown in Figure 3 a. With the growth temperature decreasing, the lateral-direction adhesion can greatly decrease, for example, TA = ChemPhysChem 2014, 15, 858 – 861
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Figure 4. Sequential optical images showing the dynamic behavior of a 4 mL water droplet impacting the aligned nanoneedle sample at a velocity of 2.7 m s 1, corresponding to a release height of 38 cm. The sample shows striking nonstickiness not only to the impacted millimeter-scale droplet but also to the cracked micrometer-scale droplets.
Figure 3. a) Measured tilted angles (TAs) of five nanosamples as shown in Figure 2 a. The volume of the probed droplet is 4 mL. b) Optical images showing that the same volume of droplet can be pinned on the horizontally placed nanorod sample (left, TA = 08), but can easily roll off the surface on tilting the sample up to TA = 108 (right). c) Optical image showing that a droplet can instantly roll off the surface of horizontally placed nanoneedle samples (TA = 08) once falling under gravitation.
108 (78) for the nanorod samples as-obtained at 90 8C (80 8C), TA = 38 for the nanopencil samples as-obtained at 70 8C, and TA = 08 for the nanoneedle samples as-obtained at 60 and 65 8C. As shown in Figure 3 b and Movie S3, the droplet can be pinned on the surface of horizontally placed nanorod samples but rolls off the samples tilted up to 108. In contrast, the droplets instantly roll off the horizontally placed nanoneedle samples obtained at 60 8C (Figure 3 c and Movie S4). Thus, the lateral stickiness can be greatly reduced by the tapering of nanorods and eliminated by the sharp nanoneedles. We further studied the nonstickiness robustness of the aligned nanoneedle films at the dynamic conditions. As shown in Figure 4 and Movie S5, a 4 mL droplet falling from a height of 38 cm (with an impact velocity of 2.7 m s 1, dynamic pressure of 3.7 kPa, and hammer pressure of 0.8 MPa) was instantly spread under the huge impact action and then cracked into many micrometer-scale droplets, all of which cannot adhere to the sample surface. Among others, partial droplets can reunite and then rebound. Note that aligned nanorod films lose their antidroplet-sticking ability at the dynamic conditions, no matter whether under slight impact (0.44 m s 1, see Movie S6) or violent impact (2.7 m s 1, see Movie S7). No doubt the tapering of aligned nanorods ensures the dynamically stable nonstickiness. Why can the tapering of nanorods induce robust droplet nonstickiness? Firstly, it can be well understood that surface adhesion at the quasi-static state results from the solid–liquid van der Waals attraction, which is only related to the top-layer nanostructure morphologies. In contrast to aligned nanorods, aligned nanoneedles have extremely low solid–liquid contact 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
area and negligible van der Waals attraction, thus rendering their surfaces perfect nonstickiness along the normal and lateral directions.[12] The instantly formed solid–liquid–air contact lines are highly discrete, which is advantageous to both the normal separation and lateral movement of water droplets.[6a, 12] However, the cases become intricate as the droplet impacts. The nonwetting liquid/air interfaces do not mean that the antisticking abilities can be ensured during the impact process, which also depends on the nanoscale topographies. These assynthesized nanorods are characterized by relatively larger average diameters (105 nm) and interspaces (153 nm), hexagonal crystal faces, and uneven height. Although their very high capillary pressure ( 105 Pa),[17] far larger than the exerted hammer pressure and dynamic pressure, can effectively prevent the contact lines from sliding downward along the sidewall of nanorods, the droplet impact would inevitably bring about the additional contact of water films with the top sidewalls of some protruding nanorods, thus further greatly increasing the van der Waals force due to huge density, for which newly added force is not negligible. These ragged nanorods with hexagonal edges somewhat play a role of “re-entrant curvature”,[18] which can effectively resist the advancing (i.e. inward wetting) of water films by enhancing the pinning of contact lines, but necessarily bring about the droplet adhesion due to edge-induced pinning force during their retracting process.[10a, 19] As a result, the impacting droplets are firmly pinned on the surface of aligned nanorods. In contrast, the aligned nanoneedles have far smaller top diameters (17 nm) and interspaces (69 nm), circular cross sections, and sparser contact sites at their top, which generates a critical capillary pressure of approximately 106 Pa so as to avoid penetrating the water films under the current impacting speeds, thereby showing dynamically stable nonstickiness. We suspect that, even if the tips of some protruding nanoneedles may penetrate water films, the pinning resistance of contact lines during the retracting process, in comparison with released dynamic energy, may be neglected. The tapered profiles with gradual curvature can facilitate the retraction of contact lines whereas the minimized top diameters with both negligible solid–liquid van der Waals force and pinning force can ensure the effortless rebound of contact lines.[10a, 19] ChemPhysChem 2014, 15, 858 – 861
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CHEMPHYSCHEM COMMUNICATIONS In summary, we have reported a facile, efficient, and cheap method for synthesizing aligned ZnO nanoneedle films with perfect quasi-static and dynamically stable droplet nonstickiness. The tapering of aligned ZnO nanorods is realized by a diffusion-limited wet-chemical crystal growth strategy for emphasizing the importance of controlling nanoscale tiplike topography in ensuring robust nonstickiness. Only keeping the nonwetting state of liquid/air interfaces is still insufficient to achieve robust nonstickiness, which also requires minimal solid– liquid van der Waals force and edge-induced pinning force. These findings are significant to the development of robust nonsticky surfaces and the exploration of new applications. For example, the superhydrophobic aligned nanoneedles with nanoscale characteristic interspaces have typical anti-microdrop-sticking advantage, which can find significant applications such as dry-style antifogging[3] and enhanced dropwise condensation heat transfer.[20]
Experimental Section Materials Glass slides of size 2 cm 2 cm were rinsed to remove contaminants by sonification in acetone, ethanol, and deionized water for about 5 min, respectively, and then immersed in hot piranha solution overnight for hydroxylation. Subsequently, cleaned glass slides were coated with crystal seeds by the three-cycle treatment of dipcoating in a solution (0.005 mol L 1) of zinc acetate in ethanol, rinsing with ethanol, and annealing at 350 8C for 20 min. The glass slides with the seeded face down were immersed in a chemical bath of isopycnic NaOH (0.5 mol L 1) and Zn(NO3)2 (4 mol L 1) at 60 8C for 15–180 min. The as-synthesized nanoarrays were taken out, rinsed with deionized water, and dried with a nitrogen airflow. Finally, anti-droplet-sticking superhydrophobic surfaces were obtained by immersing the as-synthesized nanoarrays in an ethanol solution of heptadecafluorodecyltrimethoxysilane (2 vol %) at 25 8C for 2 h and then heating at 80 8C for 1 h in an oven.
Characterization The nanostructure morphologies were observed by scanning electronic microscopy (SEM, Hitachi S4800, Japan) after sputtering a layer of Au film 15 nm in thickness. The normal-direction adhesive force was measured by a micro-electromechanical balance (Dataphysics DCAT21, Germany). The lateral-direction adhesion was characterized by the critical tilted angles by means of optical contact angle meters (OCA 20, DataPhysics, Germany). The optical images shown in Figures 2 b and 3 b,c were recorded by CCD camera in the OCA 20 apparatus. The droplet impacting processes shown in Figure 4 were observed by a high-speed high-resolution motion analysis microscope (Keyence VW-9000, Japan) at 50 magnification and a rate of 4000 frames per second (fps).
Acknowledgements This work was supported by the National Basic Research Program of China (2012CB933202), National Natural Science Foundation of China (20773135), and Key Research Program of the Chinese Academy of Sciences (KJZD-EW-M01).
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www.chemphyschem.org Keywords: crystal growth · hydrophobic effect nanostructures · superhydrophobicity · surface chemistry
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[1] a) W. Barthlott, C. Neinhuis, Planta 1997, 202, 1 – 8; b) L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang, D. Zhu, Adv. Mater. 2002, 14, 1857 – 1860. [2] X. Gao, L. Jiang, Nature 2004, 432, 36. [3] X. Gao, Y. Yan, X. Yao, L. Xu, K. Zhan, J. Zhang, B. Yang, L. Jiang, Adv. Mater. 2007, 19, 2213 – 2217. [4] Y. Zheng, X. Gao, L. Jiang, Soft Matter 2007, 3, 178 – 182. [5] a) M. Liu, Y. Zheng, J. Zhai, L. Jiang, Acc. Chem. Res. 2010, 43, 368 – 377; b) B. Bhushan, Y. C. Jung, K. Koch, Philos. Trans. R. Soc. A 2009, 367, 1631 – 1672. [6] a) D. ner, T. J. McCarthy, Langmuir 2000, 16, 7777 – 7782; b) M. Callies, D. Qur, Soft Matter 2005, 1, 55 – 61; c) N. Zhao, Q. Xie, X. Kuang, S. Wang, Y. Li, X. Lu, S. Tan, J. Shen, X. Zhang, Y. Zhang, J. Xu, C. C. Han, Adv. Funct. Mater. 2007, 17, 2739 – 2745. [7] a) H. Y. Erbil, A. L. Demirel, Y. Avci, O. Mert, Science 2003, 299, 1377 – 1380; b) H. Yan, K. Kurogi, H. Mayama, K. Tsujii, Angew. Chem. 2005, 117, 3519 – 3522; Angew. Chem. Int. Ed. 2005, 44, 3453 – 3456; c) X. Hong, X. Gao, L. Jiang, J. Am. Chem. Soc. 2007, 129, 1478 – 1479. [8] a) S. Wang, X. Feng, J. Yao, L. Jiang, Angew. Chem. 2006, 118, 1286 – 1289; Angew. Chem. Int. Ed. 2006, 45, 1264 – 1267; < lit b > X. Feng, L. Feng, M. Jin, J. Zhai, L. Jiang, D. Zhu, J. Am. Chem. Soc. 2004, 126, 62 – 63. [9] a) J. Zhang, S. Seeger, Angew. Chem. 2011, 123, 6782 – 6786; Angew. Chem. Int. Ed. 2011, 50, 6652 – 6656; b) I. A. Larmour, S. E. J. Bell, G. C. Saunders, Angew. Chem. 2007, 119, 1740 – 1742; Angew. Chem. Int. Ed. 2007, 46, 1710 – 1712; c) V. Zorba, E. Stratakis, M. Barberoglou, E. Spanakis, P. Tzanetakis, S. H. Anastasiadis, C. Fotakis, Adv. Mater. 2008, 20, 4049 – 4054; d) S.-J. Choi, K. Y. Suh, H. H. Lee, J. Am. Chem. Soc. 2008, 130, 6312 – 6313; e) L. Gao, T. J. McCarthy, J. Am. Chem. Soc. 2006, 128, 9052 – 9053; f) K. Acatay, E. Simsek, C. Ow-Yang, Y. Z. Menceloglu, Angew. Chem. 2004, 116, 5322 – 5325; Angew. Chem. Int. Ed. 2004, 43, 5210 – 5213; g) M. Miwa, A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Langmuir 2000, 16, 5754 – 5760. [10] a) K. C. Park, H. J. Choi, C. H. Chang, R. E. Cohen, G. H. McKinley, G. Barbastathis, ACS Nano 2012, 6, 3789 – 3799; b) J. Shieh, F. J. Hou, Y. C. Chen, H. M. Chen, S. P. Yang, C. C. Cheng, H. L. Chen, Adv. Mater. 2010, 22, 597 – 601; c) C. Dorrer, J. Rhe, Adv. Mater. 2008, 20, 159 – 163. [11] a) E. Hosono, S. Fujihara, I. Honma, H. Zhou, J. Am. Chem. Soc. 2005, 127, 13458 – 13459; b) X. Yao, Q. Chen, L. Xu, Q. Li, Y. Song, X. Gao, D. Qur, L. Jiang, Adv. Funct. Mater. 2010, 20, 656 – 662. [12] a) Y. Lai, X. Gao, H. Zhuang, J. Huang, C. Lin, L. Jiang, Adv. Mater. 2009, 21, 3799 – 3803; b) X. Gao, X. Yao, L. Jiang, Langmuir 2007, 23, 4886 – 4891. [13] a) X. Liu, Y. Liang, F. Zhou, W. Liu, Soft Matter 2012, 8, 2070 – 2086; b) A. Lafuma, D. Qur, Nat. Mater. 2003, 2, 457 – 460. [14] a) C. Li, R. Guo, X. Jiang, S. Hu, L. Li, X. Cao, H. Yang, Y. Song, Y. Ma, L. Jiang, Adv. Mater. 2009, 21, 4254 – 4258; b) J. Seo, S. Lee, H. Han, H. B. Jung, J. Hong, G. Song, S. M. Cho, C. Park, W. Lee, T. Lee, Adv. Mater. 2013, 25, 4139 – 4144. [15] a) R. A. Laudise, A. A. Ballman, J. Phys. Chem. 1960, 64, 688 – 691; b) W.-J. Li, E.-W. Shi, W.-Z. Zhong, Z.-W. Yin, J. Cryst. Growth 1999, 203, 186 – 196. [16] a) Y.-J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, J. W. P. Hsu, Nano Lett. 2008, 8, 1501 – 1505; b) X. Wu, H. Bai, C. Li, G. Lu, G. Shi, Chem. Commun. 2006, 1655 – 1657; c) J. Liu, X. Huang, Y. Li, X. Ji, Z. Li, X. He, F. Sun, J. Phys. Chem. C 2007, 111, 4990 – 4997. [17] a) D. Richard, D. Qur, Europhys. Lett. 2000, 50, 769 – 775; b) M. Reyssat, A. Ppin, F. Marty, Y. Chen, D. Qur, Europhys. Lett. 2006, 74, 306 – 312; c) T. Deng, K. K. Varanasi, M. Hsu, N. Bhate, C. Keimel, J. Stein, M. Blohm, Appl. Phys. Lett. 2009, 94, 133109. [18] A. Tuteja, W. Choi, M. Ma, J. M. Mabry, S. A. Mazzella, G. C. Rutledge, G. H. McKinley, R. E. Cohen, Science 2007, 318, 1618 – 1622. [19] M. Nosonovsky, B. Bhushan, Microsyst. Technol. 2005, 11, 535 – 549. [20] X. Chen, J. Wu, R. Ma, M. Hua, N. Koratkar, S. Yao, Z. Wang, Adv. Funct. Mater. 2011, 21, 4617 – 4623. Received: November 19, 2013 Published online on March 3, 2014
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