Superhydrophobic Materials
Fly-Eye Inspired Superhydrophobic Anti-Fogging Inorganic Nanostructures Ziqi Sun, Ting Liao, Kesong Liu,* Lei Jiang, Jung Ho Kim,* and Shi Xue Dou In nature, biological species have evolved optimal structures over millions of years with amazing characteristics and swift stimulus-responsive capabilities, which give inspiration to researchers interested in the design of functional materials. Learning from nature takes ideas from natural species and develops novel functional materials based on these concepts, e.g., bio-inorganic materials (biomineralization), bio-inspired multiscale structured materials (chiral morphologies), bionanomaterials (bio-nanoparticles), hybrid organic/inorganic implant materials (bonelike composites), and smart biomaterials.[1–9] Many of these smart materials have surfaces that dynamically alter their physicochemical properties in response to changes in their environmental conditions and to triggered control of interfacial properties. By mimicking the well-ordered multiscale structures of natural interfaces or surfaces, many artificial materials with bio-inspired functions have already been created. For example, there are some reports on lotus leaf inspired self-cleaning surfaces, plant and insect inspired anisotropic fog-collection surfaces, mosquito eye inspired anti-fogging coatings, insect eyes inspired antireflection coatings, rose petal, mussel, and gecko foot inspired highly adhesive surfaces, moth-inspired sensors, etc.[10–18] The difficulty in precisely controlling the synthesis of nanostructures that fully mimic the multiscale structure of natural surfaces, however, has encumbered the development
Dr. Z. Sun, Dr. T. Liao, Prof. K. Liu, Prof. J. H. Kim, Prof. S. X. Dou Institute for Superconducting and Electronic Materials University of Wollongong Innovation Campus North Wollongong, NSW 2500, Australia E-mail: [email protected]; [email protected] Prof. K. Liu, Prof. L. Jiang Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of the Ministry of Education School of Chemistry and the Environment Beihang University Beijing 100191, China Prof. L. Jiang Beijing National Laboratory for Molecular Sciences (BNLMS) Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences Beijing 100190, China DOI: 10.1002/smll.201400516 small 2014, DOI: 10.1002/smll.201400516
of novel bio-inspired nanostructures. Even though the bioinspired nanostructures have attracted extensive attention, and some artificial bio-inspired surfaces and interfaces have been successfully studied for the petrochemical industry,[5,15] more interesting bio-functions of natural species remain to be discovered, and some low-cost and large-scale productive fabrication methods also need to be developed. Fogging occurs when moisture condensation takes the form of accumulated droplets with diameters larger than 190 nm or half of the shortest wavelength (380 nm) of visible light. Fogging of surfaces can scatter light and result in poor optical performance of the surfaces.[19] In some extremely low temperature environments, freezing fog or frozen fog can form on any solid surfaces. “Freezing fog” refers to fog where the water vapour is super-cooled. Freezing fog can quickly stick to and condense on a solid surface to form a dense ice layer.[20,21] Freezing fog induced ice build-up on high-voltage overhead power lines and conductors during winter storms is a very serious problem, which can compromise the reliability of electrical transmission and telecommunication networks. It is also a safety hazard on airplanes by reducing the lift force that keeps the airplane in the air and potentially causing aerodynamic stall.[22,23] The development of anti-fogging coatings that are capable of handling a wide range of environmental challenges will be an effective approach to resolve the fogging of surfaces.[13,24,25] Antifogging coatings with hydrophilic or even superhydrophilic wetting behaviour have received significant attention due to their ability to produce film-like condensation.[19] In the more aggressive environments, however, the hydrophilic anti-fogging coatings cannot resist the freezing-fogging induced ice build-up and accumulation, which finally result in the failure of devices. In this study, to develop effective anti-fogging nanomaterials that can provide protection for such surfaces, even when exposed in extreme environments, we designed bio-inspired superhydrophobic anti-fogging nanomaterials that have a low adherence force to water droplets and thus resist fogging-induced ice build-up. Herein, we propose a strategy based on rational synthesis of bio-inspired multifunctional nanostructures possessing well-defined surface properties by learning from the natural biological structures and functions of the compound eyes of the green bottle fly. The common green bottle fly, Lucilia sericata (Beijing), plays significant roles in the human life and human medicine.[26] It is interesting that the fly eyes can remain functional and uncontaminated in some extremely dusty, miry, and moist environments.[27,28] The unique surface
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humidity of the atmosphere and mimic a mist composed of numerous tiny water droplets with diameters less than 10 µm. It is interesting that the fly compound eyes present ideal superhydrophobic and antifogging properties, so that the fog droplets can only condense on the body, but none of them can condense in the compound eyes. The dry-style anti-fogging properties of the compound eyes with superhydrophobicity can provide clear vision for the insects in highly humid environments. This special wettability of the compound eyes is attributed to the combination of surface chemistry and roughness on multiple scales. Figure 1(b) shows the anatomic structure of the fly eyes. It was found that each ommatidium consists of a cornea lens (the front surface of which makes up a single facet), a transparent crystalline cone, a rhabdom or channel to guide light transmission, pigment cells which separate the ommatidium from its neighbours, and a centre zone containing the optic nervus.[29] The scanning electron microscope (SEM) images in Figure 1(c-e) show the microstructures of the fly compound eye on different scales. The compound eyes of the green bottle fly (Figure 1(c)) are around 5 mm in size and made up of thousands of repeating hexagonal units (Figure 1(d)), or called as ommatidia, each with a diameter on the order of 20 micrometers, in a hexagonal-close-packed (hcp) arrangement. On further enlarging the surface of Figure 1. Microstructures of the fly compound eye and bio-inspired nanostructures. an ommatidium, as shown in Figure 1(e), (a) optical image of a green bottle fly, Lucilia sericata, in a fogging test chamber showing the superhydrophobic and clean surface of the compound eyes, even with drops nucleated in the we observe that each ommatidium is surrounding hairs, (b) schematic illustration of the anatomic structure of the fly compound covered with numerous, near hexagonal, eye, (c) low magnification SEM image of one fly compound eye, (d) high magnification bubble-like protuberances with diamSEM image of the compound eye showing the close packed ommatidium lens surface, eters of ∼100 nm. We suspect that these (e) bubble-like protuberances with diameters of ∼100 nm on the surface of the ommatidium, well-ordered, close packed, hierarchical (f) microstructure of the fly-eye bio-inspired ZnO nanostructures consisting of ommatidium- hexagonal nanostructures are one of the lens-like structures, and (g) a cross-sectional view of the bio-inspired nanostructures, showing origins of the superior superhydrophosimilar structures to the anatomic structure of natural fly eyes. bicity and anti-fogging properties of the green bottle fly eyes. The microstructure of properties of the compound eyes can provide some inspira- the green bottle fly eye presents significant differences from tion towards developing advanced anti-contamination, anti- the previously studied mosquito eye, where the ommatidium fogging, or anti-weathering coatings by taking advantage of surface is covered with nanorod arrays with diameters on the their surface features. In this study, hierarchically-ordered fly- order of 100 nm.[24] Another reason for the superhydrophoeye bio-inspired nanostructures were successfully synthesized bicity of the compound eyes may be the existence of a thin via a facile one-pot solvothermal method. This method allows layer of bio-polymer or wax on the surface of the ommatidia, precise control of the morphology of the nanostructures and which can lower the surface energy of the compound eyes. is also feasible for low-cost, large-scale production. The sur- Therefore, the unique surface structures of the green bottle face properties, including the surface wettability and anti-fog- fly compound eyes give us inspiration on how to design inorging property, of the bio-inspired nanostructures were studied ganic nanostructures to produce inexpensive functional nanostructures and coatings to provide environmentally durable to explore their potential multifunctional applications. Figure 1(a) presents optical image of a common green anti-fogging performances. Recently, different kinds of ZnO nanomaterials have bottle fly after exposure to a fogging testing chamber, in which an ultrasonic humidifier was used to regulate the relative been developed which presented versatile wettability to
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small 2014, DOI: 10.1002/smll.201400516
Fly-Eye Inspired Superhydrophobic Anti-Fogging Inorganic Nanostructures
1H, 1H, 2H, 2H – perfluorooctyltriethoxysilane (PFOTES) molecule.[35–37] To clarify whether the size of the “cornea lens” on the surface of the flyeye bio-inspired ZnO nanostructures has an influence on the surface properties, the bio-inspired nanostructures were synthesized by controlling the synthesis parameters. Bio-inspired ZnO nanostructures with different surface morphologies were obtained by adjusting the aging time of the precursor solutions and the solvothermal temperatures, as shown in Figure 2. Solvothermal treatment was carried out at 90–150 °C for 15 h to improve the organization and induce complete condensaFigure 2. Morphologies of the bio-inspired ZnO nanoparticles synthesized under different tion, solidification, and crystallization. The conditions. (a) bio-inspired ZnO nanostructures synthesized at 90 °C from the reaction synthesis details can also be found in the solution without aging (0 d, 90 °C), (b) bio-inspired ZnO nanostructures synthesized Supporting Information. Figure 2(a-e) at 110 °C from the reaction solution after aging for 3 days (3 d, 110 °C), (c) bio-inspired ZnO nanostructures synthesized at 130 °C from the reaction solution after aging for 3 days presents the morphology of the bio(3 d, 130 °C), (d) bio-inspired ZnO nanostructures synthesized at 150 °C from the reaction inspired nanostructures synthesized from solution after aging for 3 days (3 d, 150 °C), (e) bio-inspired ZnO nanostructures synthesized the solutions after aging of 0–7 days, with at 130 °C from the reaction solution after aging for 7 days (7 d, 130 °C), (f) overall particle solvothermal temperatures ranging from size variation with synthesis temperature of the bio-inspired nanostructures synthesized from 90–150 °C. It is interesting that the microthe solutions aged for 3 days and 7 days, and the temperature-dependent size change of spheres synthesized from the solutions the surface hexagonal lens structure of the bio-inspired nanostructures synthesized from the without aging possessed round, smooth 3-day aged solution. surfaces without any secondary nanostructures, while in the samples obtained from water and showed promise for the design of inorganic bio- the solutions aged for 3 days, hexagonal “cornea lens” nanoinspired nanostructures with special surface properties.[30–35] structures can be clearly distinguished, which showed temFigure 1(g-h) shows fly-eye inspired ZnO microspheres that perature-dependent distribution density and size. Figure 2(f) show a similar structure to the fly compound eyes: the sur- presents the influences of the aging time and synthesis temfaces of the ZnO microspheres consist of hexagonal crystal perature on the overall particle size and the surface hexagonal arrays or nanocones around 200–350 nm in size; and in the lens structures of the bio-inspired nanostructures. Figure S1 middle, there are radial pile-pore structures that are formed in the Supporting Information shows the morphology of under the nanocones and work as light guide channels in the the bio-inspired nanostructures synthesized at temperatures same way as the rhabdom in natural eyes, while the centre is from 90–150 °C from solutions aged for 3 and 7 days, respeca round hole that can allow the incorporation of secondary tively. In general, the higher the synthesis the temperature is, optical elements inside. Herein, these complex hierarchically- the larger the size of the lens structures on the surface and ordered bio-inspired nanostructures were synthesized based the larger the overall particle diameters are. All the solutions on a new concept of “two-step” self-assembly, where the aged for 3 days can produce fly-eye-mimicking nanostrucoligomers or the constituent nanostructures with specially tures with not only similar surfaces, but also inner rhabdomdesigned structures are first formed from surfactant tem- like channels, as shown in Figure S2. On further prolongering plates, and then further assembled into complex morpholo- the aging time, the nanostructures ceased to resemble those gies by the addition of a second co-surfactant.[32,35] In the in fly eyes. The aging-time dependent on morphology varipresent synthesis, triblock copolymer polyethylene oxide- ation reveals that suitable aging of the reaction solutions is polypropylene oxide-polyethylene oxide (PEO20-PPO70- very crucial. Based on the morphology of the obtained nanoPEO20, Pluronic P123) surfactant is first added to form structures, it is concluded that the optimized aging time for laminated micelles, which are combined with the following the synthesis of fly-eye bio-inspired nanostructures is 3 days. addition of ZnO precursor solution to guide the formation of The coatings with bio-inspired nanostructures were prepared radial rhabdom-like structures. The rahbdom-like structures via a simple spin-coating method. Figure S3 presents the were then further assembled into hollow spherical structures SEM image of the bio-inspired nanostructured coatings. It with the addition of a second non-aqueous liquid (ethylene is showed that the coating is not fully dense, but rather has glycol (EG) in this case), and finally, the hexagonal lens struc- numerous pores among the bridge-connected bio-inspired ture nucleate and grow on the surfaces of the microspheres nanostructured particles, which can provide enough roughduring the solvothermal synthesis stage. To simulate the bio- ness for superhydrophobicity. To mimic the bio-polymer polymer layer on the surface of the compound eyes, a sur- layer on the fly eye surface, PFOTES molecules with low face treatment was carried out on the bio-inspired inorganic surface energy were deposited on the bio-inspired nanonanostructures by evaporation deposition of a few layers of structured coatings. small 2014, DOI: 10.1002/smll.201400516
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As the morphologies of the ZnO microspheres changed, the contact angle and the CAH also varied. As shown in Figure 3(a), the bio-inspired ZnO microspheres with hexagonal thick nanocones synthesized at 150 °C had the lowest CAH, indicating that the water droplets slid off more easily from their surfaces, and the highest CA, demonstrating the superhydrophobicity. Therefore, we chose the thin films that consisted of ZnO microspheres that were fully covered with hexagonal nanocones synthesized from the precursor solution with 3 days aging and heating at 150 °C. Figure 3(b-c) respectively shows the static wettability of and the sliding off of a water droplet from the bio-inspired nanostructured surfaces. The surface wettability of the bio-inspired ZnO nanostructured thin films was examined by contact angle measurements, as shown in Figure 3(b). Owing to the existence of the hierarchical ordered nanostructures, the bio-inspired nanostructured thin film presents superhydrophobic behavior with a CA of 162.2o. For an anti-fogging thin film or coating, besides a high contact angle, a low sliding angle is also necessary. The sliding tests Figure 3. Static wettability, contact angle hysteresis, and the results of sliding off tests of were carried out on a bio-inspired nanobio-inspired nanostructures. (a) Contact angle (CA) and contact angle hysteresis (CAH) of the structured thin film by dropping a 4 µL coatings of the bio-inspired ZnO nanoparticles. The values of CAH reflect the difficulty that water droplet on the surface with a tilting water droplets have in sliding off of a surface. (b) Optical images of the static contact angle of o 4 µL water droplets on a bio-inspired nanostructured coating, showing superhydrophobicity angle of around 3 (Figure 3(c) and the to water; and (c) the sliding off of a 4 µL water droplet on the glass with a tilting angle of supporting video). It is surprising that the water droplet rolls off so quickly. The high around 3°. water CA and the low sliding angle of the bio-inspired ZnO nanostructured thin Figure 3(a) displays the influences of the morphology of film indicate that the water droplets do not penetrate into the bio-inspired ZnO nanostructured mcirospheres on the the grooves.[40,41] Based on the static wettability test and the static contact angle (CA) and contact angle hysteresis (CAH) sliding-off test, it is concluded that the bio-inspired thin films of the bio-inspired nanostructure coatings. The CAH is one have a low surface energy and suitable surface roughness, of the most important and classical elements of wetting by which can prevent the nucleation, condensation, and sticking liquid droplets and can reflect the sticking and motion of liq- of even tiny water droplets. Figure 4 presents the dynamic anti-fogging properties of uids on inclined planes.[38,39] Under gravity, the droplet will become asymmetric, but will not move: the top of the droplet the bio-inspired ZnO nanostructures. The anti-fogging perforbecomes thin, with a low contact angle, while the bottom mance of the bio-inspired coatings was tested in an artificial becomes thick, with a high contact angle. In other words, a fogging chamber for 2 min together with bare glass substrate small droplet sticks to a surface due to hysteresis. Experimen- without a coating. Figure 4(a) shows optical images of the tally, the static CAH (θ) can be expressed as the difference reference bare glass and the one with the bio-inspired nanobetween the advancing angle (θa) and receding angle (θr) structured coating. In our experiment, the fogging experiof liquid droplets on a tilted plane when the droplet starts ment was first carried out on samples which were horizontally sliding down, θ =θa – θr. Thus, the quantitative CAH of small placed in the fogging chamber, and then, the fogged samples water droplets on thin films or coatings directly reflects how were titled at an angle around 10o to the testing platform difficult it is for the droplets to slide off the plane. As shown (Figure 4(b)). When the fogging tests were carried out on the in Figure 2, with different synthesis protocols, the surfaces of horizontally placed samples, as presented in Figure 4(c), big the microspheres also changed step by step as aging was pro- droplets were formed, grew, and stuck onto the bare glass longed from being smooth surfaced, to being partly covered surface. On the bio-inspired nanostructured coated samples, with hexagonal thin nanocones, to being fully covered with small fogging droplets (
Fly-Eye Inspired Superhydrophobic Anti-Fogging Inorganic Nanostructures Ziqi Sun, Ting Liao, Kesong Liu,* Lei Jiang, Jung Ho Kim,* and Shi Xue Dou In nature, biological species have evolved optimal structures over millions of years with amazing characteristics and swift stimulus-responsive capabilities, which give inspiration to researchers interested in the design of functional materials. Learning from nature takes ideas from natural species and develops novel functional materials based on these concepts, e.g., bio-inorganic materials (biomineralization), bio-inspired multiscale structured materials (chiral morphologies), bionanomaterials (bio-nanoparticles), hybrid organic/inorganic implant materials (bonelike composites), and smart biomaterials.[1–9] Many of these smart materials have surfaces that dynamically alter their physicochemical properties in response to changes in their environmental conditions and to triggered control of interfacial properties. By mimicking the well-ordered multiscale structures of natural interfaces or surfaces, many artificial materials with bio-inspired functions have already been created. For example, there are some reports on lotus leaf inspired self-cleaning surfaces, plant and insect inspired anisotropic fog-collection surfaces, mosquito eye inspired anti-fogging coatings, insect eyes inspired antireflection coatings, rose petal, mussel, and gecko foot inspired highly adhesive surfaces, moth-inspired sensors, etc.[10–18] The difficulty in precisely controlling the synthesis of nanostructures that fully mimic the multiscale structure of natural surfaces, however, has encumbered the development
Dr. Z. Sun, Dr. T. Liao, Prof. K. Liu, Prof. J. H. Kim, Prof. S. X. Dou Institute for Superconducting and Electronic Materials University of Wollongong Innovation Campus North Wollongong, NSW 2500, Australia E-mail: [email protected]; [email protected] Prof. K. Liu, Prof. L. Jiang Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of the Ministry of Education School of Chemistry and the Environment Beihang University Beijing 100191, China Prof. L. Jiang Beijing National Laboratory for Molecular Sciences (BNLMS) Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences Beijing 100190, China DOI: 10.1002/smll.201400516 small 2014, DOI: 10.1002/smll.201400516
of novel bio-inspired nanostructures. Even though the bioinspired nanostructures have attracted extensive attention, and some artificial bio-inspired surfaces and interfaces have been successfully studied for the petrochemical industry,[5,15] more interesting bio-functions of natural species remain to be discovered, and some low-cost and large-scale productive fabrication methods also need to be developed. Fogging occurs when moisture condensation takes the form of accumulated droplets with diameters larger than 190 nm or half of the shortest wavelength (380 nm) of visible light. Fogging of surfaces can scatter light and result in poor optical performance of the surfaces.[19] In some extremely low temperature environments, freezing fog or frozen fog can form on any solid surfaces. “Freezing fog” refers to fog where the water vapour is super-cooled. Freezing fog can quickly stick to and condense on a solid surface to form a dense ice layer.[20,21] Freezing fog induced ice build-up on high-voltage overhead power lines and conductors during winter storms is a very serious problem, which can compromise the reliability of electrical transmission and telecommunication networks. It is also a safety hazard on airplanes by reducing the lift force that keeps the airplane in the air and potentially causing aerodynamic stall.[22,23] The development of anti-fogging coatings that are capable of handling a wide range of environmental challenges will be an effective approach to resolve the fogging of surfaces.[13,24,25] Antifogging coatings with hydrophilic or even superhydrophilic wetting behaviour have received significant attention due to their ability to produce film-like condensation.[19] In the more aggressive environments, however, the hydrophilic anti-fogging coatings cannot resist the freezing-fogging induced ice build-up and accumulation, which finally result in the failure of devices. In this study, to develop effective anti-fogging nanomaterials that can provide protection for such surfaces, even when exposed in extreme environments, we designed bio-inspired superhydrophobic anti-fogging nanomaterials that have a low adherence force to water droplets and thus resist fogging-induced ice build-up. Herein, we propose a strategy based on rational synthesis of bio-inspired multifunctional nanostructures possessing well-defined surface properties by learning from the natural biological structures and functions of the compound eyes of the green bottle fly. The common green bottle fly, Lucilia sericata (Beijing), plays significant roles in the human life and human medicine.[26] It is interesting that the fly eyes can remain functional and uncontaminated in some extremely dusty, miry, and moist environments.[27,28] The unique surface
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1
communications
Z. Sun et al.
humidity of the atmosphere and mimic a mist composed of numerous tiny water droplets with diameters less than 10 µm. It is interesting that the fly compound eyes present ideal superhydrophobic and antifogging properties, so that the fog droplets can only condense on the body, but none of them can condense in the compound eyes. The dry-style anti-fogging properties of the compound eyes with superhydrophobicity can provide clear vision for the insects in highly humid environments. This special wettability of the compound eyes is attributed to the combination of surface chemistry and roughness on multiple scales. Figure 1(b) shows the anatomic structure of the fly eyes. It was found that each ommatidium consists of a cornea lens (the front surface of which makes up a single facet), a transparent crystalline cone, a rhabdom or channel to guide light transmission, pigment cells which separate the ommatidium from its neighbours, and a centre zone containing the optic nervus.[29] The scanning electron microscope (SEM) images in Figure 1(c-e) show the microstructures of the fly compound eye on different scales. The compound eyes of the green bottle fly (Figure 1(c)) are around 5 mm in size and made up of thousands of repeating hexagonal units (Figure 1(d)), or called as ommatidia, each with a diameter on the order of 20 micrometers, in a hexagonal-close-packed (hcp) arrangement. On further enlarging the surface of Figure 1. Microstructures of the fly compound eye and bio-inspired nanostructures. an ommatidium, as shown in Figure 1(e), (a) optical image of a green bottle fly, Lucilia sericata, in a fogging test chamber showing the superhydrophobic and clean surface of the compound eyes, even with drops nucleated in the we observe that each ommatidium is surrounding hairs, (b) schematic illustration of the anatomic structure of the fly compound covered with numerous, near hexagonal, eye, (c) low magnification SEM image of one fly compound eye, (d) high magnification bubble-like protuberances with diamSEM image of the compound eye showing the close packed ommatidium lens surface, eters of ∼100 nm. We suspect that these (e) bubble-like protuberances with diameters of ∼100 nm on the surface of the ommatidium, well-ordered, close packed, hierarchical (f) microstructure of the fly-eye bio-inspired ZnO nanostructures consisting of ommatidium- hexagonal nanostructures are one of the lens-like structures, and (g) a cross-sectional view of the bio-inspired nanostructures, showing origins of the superior superhydrophosimilar structures to the anatomic structure of natural fly eyes. bicity and anti-fogging properties of the green bottle fly eyes. The microstructure of properties of the compound eyes can provide some inspira- the green bottle fly eye presents significant differences from tion towards developing advanced anti-contamination, anti- the previously studied mosquito eye, where the ommatidium fogging, or anti-weathering coatings by taking advantage of surface is covered with nanorod arrays with diameters on the their surface features. In this study, hierarchically-ordered fly- order of 100 nm.[24] Another reason for the superhydrophoeye bio-inspired nanostructures were successfully synthesized bicity of the compound eyes may be the existence of a thin via a facile one-pot solvothermal method. This method allows layer of bio-polymer or wax on the surface of the ommatidia, precise control of the morphology of the nanostructures and which can lower the surface energy of the compound eyes. is also feasible for low-cost, large-scale production. The sur- Therefore, the unique surface structures of the green bottle face properties, including the surface wettability and anti-fog- fly compound eyes give us inspiration on how to design inorging property, of the bio-inspired nanostructures were studied ganic nanostructures to produce inexpensive functional nanostructures and coatings to provide environmentally durable to explore their potential multifunctional applications. Figure 1(a) presents optical image of a common green anti-fogging performances. Recently, different kinds of ZnO nanomaterials have bottle fly after exposure to a fogging testing chamber, in which an ultrasonic humidifier was used to regulate the relative been developed which presented versatile wettability to
2 www.small-journal.com
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201400516
Fly-Eye Inspired Superhydrophobic Anti-Fogging Inorganic Nanostructures
1H, 1H, 2H, 2H – perfluorooctyltriethoxysilane (PFOTES) molecule.[35–37] To clarify whether the size of the “cornea lens” on the surface of the flyeye bio-inspired ZnO nanostructures has an influence on the surface properties, the bio-inspired nanostructures were synthesized by controlling the synthesis parameters. Bio-inspired ZnO nanostructures with different surface morphologies were obtained by adjusting the aging time of the precursor solutions and the solvothermal temperatures, as shown in Figure 2. Solvothermal treatment was carried out at 90–150 °C for 15 h to improve the organization and induce complete condensaFigure 2. Morphologies of the bio-inspired ZnO nanoparticles synthesized under different tion, solidification, and crystallization. The conditions. (a) bio-inspired ZnO nanostructures synthesized at 90 °C from the reaction synthesis details can also be found in the solution without aging (0 d, 90 °C), (b) bio-inspired ZnO nanostructures synthesized Supporting Information. Figure 2(a-e) at 110 °C from the reaction solution after aging for 3 days (3 d, 110 °C), (c) bio-inspired ZnO nanostructures synthesized at 130 °C from the reaction solution after aging for 3 days presents the morphology of the bio(3 d, 130 °C), (d) bio-inspired ZnO nanostructures synthesized at 150 °C from the reaction inspired nanostructures synthesized from solution after aging for 3 days (3 d, 150 °C), (e) bio-inspired ZnO nanostructures synthesized the solutions after aging of 0–7 days, with at 130 °C from the reaction solution after aging for 7 days (7 d, 130 °C), (f) overall particle solvothermal temperatures ranging from size variation with synthesis temperature of the bio-inspired nanostructures synthesized from 90–150 °C. It is interesting that the microthe solutions aged for 3 days and 7 days, and the temperature-dependent size change of spheres synthesized from the solutions the surface hexagonal lens structure of the bio-inspired nanostructures synthesized from the without aging possessed round, smooth 3-day aged solution. surfaces without any secondary nanostructures, while in the samples obtained from water and showed promise for the design of inorganic bio- the solutions aged for 3 days, hexagonal “cornea lens” nanoinspired nanostructures with special surface properties.[30–35] structures can be clearly distinguished, which showed temFigure 1(g-h) shows fly-eye inspired ZnO microspheres that perature-dependent distribution density and size. Figure 2(f) show a similar structure to the fly compound eyes: the sur- presents the influences of the aging time and synthesis temfaces of the ZnO microspheres consist of hexagonal crystal perature on the overall particle size and the surface hexagonal arrays or nanocones around 200–350 nm in size; and in the lens structures of the bio-inspired nanostructures. Figure S1 middle, there are radial pile-pore structures that are formed in the Supporting Information shows the morphology of under the nanocones and work as light guide channels in the the bio-inspired nanostructures synthesized at temperatures same way as the rhabdom in natural eyes, while the centre is from 90–150 °C from solutions aged for 3 and 7 days, respeca round hole that can allow the incorporation of secondary tively. In general, the higher the synthesis the temperature is, optical elements inside. Herein, these complex hierarchically- the larger the size of the lens structures on the surface and ordered bio-inspired nanostructures were synthesized based the larger the overall particle diameters are. All the solutions on a new concept of “two-step” self-assembly, where the aged for 3 days can produce fly-eye-mimicking nanostrucoligomers or the constituent nanostructures with specially tures with not only similar surfaces, but also inner rhabdomdesigned structures are first formed from surfactant tem- like channels, as shown in Figure S2. On further prolongering plates, and then further assembled into complex morpholo- the aging time, the nanostructures ceased to resemble those gies by the addition of a second co-surfactant.[32,35] In the in fly eyes. The aging-time dependent on morphology varipresent synthesis, triblock copolymer polyethylene oxide- ation reveals that suitable aging of the reaction solutions is polypropylene oxide-polyethylene oxide (PEO20-PPO70- very crucial. Based on the morphology of the obtained nanoPEO20, Pluronic P123) surfactant is first added to form structures, it is concluded that the optimized aging time for laminated micelles, which are combined with the following the synthesis of fly-eye bio-inspired nanostructures is 3 days. addition of ZnO precursor solution to guide the formation of The coatings with bio-inspired nanostructures were prepared radial rhabdom-like structures. The rahbdom-like structures via a simple spin-coating method. Figure S3 presents the were then further assembled into hollow spherical structures SEM image of the bio-inspired nanostructured coatings. It with the addition of a second non-aqueous liquid (ethylene is showed that the coating is not fully dense, but rather has glycol (EG) in this case), and finally, the hexagonal lens struc- numerous pores among the bridge-connected bio-inspired ture nucleate and grow on the surfaces of the microspheres nanostructured particles, which can provide enough roughduring the solvothermal synthesis stage. To simulate the bio- ness for superhydrophobicity. To mimic the bio-polymer polymer layer on the surface of the compound eyes, a sur- layer on the fly eye surface, PFOTES molecules with low face treatment was carried out on the bio-inspired inorganic surface energy were deposited on the bio-inspired nanonanostructures by evaporation deposition of a few layers of structured coatings. small 2014, DOI: 10.1002/smll.201400516
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Z. Sun et al.
As the morphologies of the ZnO microspheres changed, the contact angle and the CAH also varied. As shown in Figure 3(a), the bio-inspired ZnO microspheres with hexagonal thick nanocones synthesized at 150 °C had the lowest CAH, indicating that the water droplets slid off more easily from their surfaces, and the highest CA, demonstrating the superhydrophobicity. Therefore, we chose the thin films that consisted of ZnO microspheres that were fully covered with hexagonal nanocones synthesized from the precursor solution with 3 days aging and heating at 150 °C. Figure 3(b-c) respectively shows the static wettability of and the sliding off of a water droplet from the bio-inspired nanostructured surfaces. The surface wettability of the bio-inspired ZnO nanostructured thin films was examined by contact angle measurements, as shown in Figure 3(b). Owing to the existence of the hierarchical ordered nanostructures, the bio-inspired nanostructured thin film presents superhydrophobic behavior with a CA of 162.2o. For an anti-fogging thin film or coating, besides a high contact angle, a low sliding angle is also necessary. The sliding tests Figure 3. Static wettability, contact angle hysteresis, and the results of sliding off tests of were carried out on a bio-inspired nanobio-inspired nanostructures. (a) Contact angle (CA) and contact angle hysteresis (CAH) of the structured thin film by dropping a 4 µL coatings of the bio-inspired ZnO nanoparticles. The values of CAH reflect the difficulty that water droplet on the surface with a tilting water droplets have in sliding off of a surface. (b) Optical images of the static contact angle of o 4 µL water droplets on a bio-inspired nanostructured coating, showing superhydrophobicity angle of around 3 (Figure 3(c) and the to water; and (c) the sliding off of a 4 µL water droplet on the glass with a tilting angle of supporting video). It is surprising that the water droplet rolls off so quickly. The high around 3°. water CA and the low sliding angle of the bio-inspired ZnO nanostructured thin Figure 3(a) displays the influences of the morphology of film indicate that the water droplets do not penetrate into the bio-inspired ZnO nanostructured mcirospheres on the the grooves.[40,41] Based on the static wettability test and the static contact angle (CA) and contact angle hysteresis (CAH) sliding-off test, it is concluded that the bio-inspired thin films of the bio-inspired nanostructure coatings. The CAH is one have a low surface energy and suitable surface roughness, of the most important and classical elements of wetting by which can prevent the nucleation, condensation, and sticking liquid droplets and can reflect the sticking and motion of liq- of even tiny water droplets. Figure 4 presents the dynamic anti-fogging properties of uids on inclined planes.[38,39] Under gravity, the droplet will become asymmetric, but will not move: the top of the droplet the bio-inspired ZnO nanostructures. The anti-fogging perforbecomes thin, with a low contact angle, while the bottom mance of the bio-inspired coatings was tested in an artificial becomes thick, with a high contact angle. In other words, a fogging chamber for 2 min together with bare glass substrate small droplet sticks to a surface due to hysteresis. Experimen- without a coating. Figure 4(a) shows optical images of the tally, the static CAH (θ) can be expressed as the difference reference bare glass and the one with the bio-inspired nanobetween the advancing angle (θa) and receding angle (θr) structured coating. In our experiment, the fogging experiof liquid droplets on a tilted plane when the droplet starts ment was first carried out on samples which were horizontally sliding down, θ =θa – θr. Thus, the quantitative CAH of small placed in the fogging chamber, and then, the fogged samples water droplets on thin films or coatings directly reflects how were titled at an angle around 10o to the testing platform difficult it is for the droplets to slide off the plane. As shown (Figure 4(b)). When the fogging tests were carried out on the in Figure 2, with different synthesis protocols, the surfaces of horizontally placed samples, as presented in Figure 4(c), big the microspheres also changed step by step as aging was pro- droplets were formed, grew, and stuck onto the bare glass longed from being smooth surfaced, to being partly covered surface. On the bio-inspired nanostructured coated samples, with hexagonal thin nanocones, to being fully covered with small fogging droplets (
