An Ingenious Super Light Trapping Surface Templated from Butterfly Wing Scales
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Based on the super light trapping property of butterfly Trogonoptera brookiana wings, the SiO2 replica of this bionic functional surface was successfully synthesized using a simple and highly effective synthesis method combining a sol–gel process and subsequent selective etching. Firstly, the reflectivity of butterfly wing scales was carefully examined. It was found that the whole reflectance spectroscopy of the butterfly wings showed a lower level (less than 10 %) in the visible spectrum. Thus, it was confirmed that the butterfly wings possessed a super light trapping effect. Afterwards, the morphologies and detailed architectures of the butterfly wing scales were carefully investigated using the ultra-depth three-dimensional (3D) microscope and field emission scanning electronic microscopy (FESEM). It was composed by the parallel ridges and quasi-honeycomb-like structure between them. Based on the biological properties and function above, an exact SiO2 negative replica was fabricated through a synthesis method combining a sol–gel process and subsequent selective etching. At last, the comparative analysis of morphology feature size and the reflectance spectroscopy between the SiO2 negative replica and the flat plate was conducted. It could be concluded that the SiO2 negative replica inherited not only the original super light trapping architectures, but also the super light trapping characteristics of bio-template. This work may open up an avenue for the design and fabrication of super light trapping materials and encourage people to look for more super light trapping architectures in nature.
KeywordsBio-template fabrication Butterfly wing scales Light trapping materials
scanning electron microscope
field emission scanning electron microscope
energy dispersive spectroscopy
atomic layer deposition
Nature offers a variety of surfaces with brilliant properties and is a source of inspiration for numerous applications and techniques. A number of researchers have paid close attention to the fantastic surfaces triggered by nature, such as the anisotropy of the rice leaves [1, 2], the self-cleaning of the lotus leaves [3, 4, 5], the antireflection of moth eyes [6, 7, 8], the fog collection system in cactus [9, 10], the reversible adhesive of the gecko feet [11, 12], the superhydrophobicity of the water strider legs [13, 14], and the iridescence of the butterfly wings [15, 16, 17]. Mimicking or studying the basic principles of the sophisticated tactics from nature is of significant importance for the design of artificial analogs.
During the last few decades, much effort has been directed toward the brightest and most vivid structure-based colors in nature that arise from the interaction of light with surfaces on the micro- and nanoscale. The architectures on the surface of butterfly wings not only formed the gorgeous appearance, but also made the butterfly wings with appealing properties, such as observable optical response to temperature , highly selective vapor response [19, 20], and light trapping effect [21, 22]. Although many scientists have done a lot of research on the architectures of butterfly wings and their multi-functional features, little attention has been paid to the black color in spite of its ubiquity. Many scientists have ascribed the blackness of butterfly wings to melanin solely, until recent research results indicated that the nanostructures of scales also play a key role in the exhibition of blackness [23, 24, 25]. In addition, seeking for light trapping surfaces has been a great challenge for researchers and now the research on light trapping materials has attracted more and more people’s attention because of the increasing importance of light trapping materials. Considering the above, we chose butterfly Trogonoptera brookiana black wings as a natural model and revealed its properties for super light trapping.
On the other hand, although the color mechanism and structural characterizations have been well investigated for a long time , the studies on bionic preparation of the subtle nanostructures on butterfly wing scales have been greatly restricted. In fact, the exact combination of the three-dimensional (3D) structure and cuticle complex refractive index [n* = (1.55 ± 0.05) + i (0.06 ± 0.01)] is beyond the capabilities of existing nanofabrication techniques . In spite of this, the potential valuable application prospect still inspired scientists to devote themselves to mimicking the distinctive surface nanostructure of butterfly wings. The colorful butterfly wing surface has been fabricated using soft lithography technique , low-temperature atomic layer deposition , colloidal self-assembly, sputtering and atomic layer deposition, even combining all these layer deposition techniques together . Not only sorts of surface processing technology but also a variety of materials were employed to fabricate replicas of the multi-layered scales on the surface of butterfly wing, such as polyelectrolyte multilayer films , carbon nanotube , polypyrrole , oxidizing material include TiO2 , Bi2WO6 , Fe3O4 , SnO2 , and so on. Even so, exact replica of such biological structures by an artificial synthesis route are difficult; what is more, the existing studies about butterflies mainly focus on the structural colors, photonic crystal structures, and the replica of photonic structures in butterfly wings. Few researchers showed solicitude for the bionic fabricating of the super light trapping architectures in butterfly wings. So, it is urgently necessary to develop a high-efficiency and low-cost technique to realize the outstanding light trapping architectures.
In this work, we selected the butterfly Trogonoptera brookiana black wings as the bio-templates, and the super light trapping characteristics were characterized by reflectance spectroscopy. Besides, the super light trapping architectures and morphology of the wing scales were characterized by FESEM. In order to prepare the SiO2 negative replica, a simple and highly effective synthesis method combining a sol–gel process and subsequent selective etching was adopted. What is more, the 3D optimized models were generated by CATIA to illustrate the super light trapping architectures and fabrication process effectively. At last, the reflectance spectroscopy of SiO2 negative replica and a flat plate was measured. The SiO2 negative replica was not only stable but also corrosion resistant due to the complex super light trapping architectures and inert SiO2 material, which made it a promising super light trapping surface for various fields such as photoelectrical devices, photo-induced sensors and solar cells. Moreover, this work sets up a strategy for the design and fabrication of super light trapping materials and may encourage people to look for more super light trapping architectures in nature.
Analytic grade reagents, hydrochloric acid, and tetraethyl ortho-silicate were provided by Beijing Chemical Works. Ethanol absolute, diethyl ether, concentrated nitric acid, and perchloric acid were provided by Tianjin Fine Chemical Co., Ltd.
Although the green wings of butterfly Trogonoptera brookiana possess a light trapping property which was confirmed as reported in our previous work , this study found that the black wing scales had the better light trapping characteristic compared with the previous study. Thus, the black wings were selected as a biological prototype in this work. Butterfly wing samples of uniform size (15 mm in length and 10 mm in width, rectangular) were cut off from the black areas in a perpendicular and parallel direction to the ridge veins, respectively. In order to confirm that the butterfly wing samples were clean, some pre-processing was conducted. Firstly, each sample was soaked by aether for 10 min to remove proteins and fattiness on the samples’ surface. Afterwards, the dehydration treatment in ethanol absolute with duration time of 15 min for each specimen was taken. The purpose of conducting the dehydration pre-processing was to increase the mechanical strength and stability of the treated tissues. Then, the samples were dried naturally in air.
A simple discoloration experiment was carried out to confirm that the color of the black wing scales was structure-based rather than pigment. Firstly, the neat and clean black areas were cut off from the butterfly wings meticulously with a scalpel in perpendicular and parallel directions to the nervure, respectively. Then, the sample was clamped with a tweezer to flatwise place in a petri dish and soaked in a certain amount of diethyl ether and ethanol absolute for 10 min, respectively, for degreasing and increasing the mechanical strength of the wing tissues. The color of the air-dried sample was still black, which was not affected by organic solvents virtually.
Preparation of the SiO2 Negative Replica
Firstly, the butterfly wing samples were sandwiched between two glass slides of which both ends were clamped by clips with proper force. Using a micropipette, a suitable amount of the sol–gel precursor solution, a reaction product of tetraethyl ortho-silicate and hydrochloric acid (3:1 in volume), was added to the edge of the assembly with a volume of 4–8 μL. Then, the assembly was heated at 125 °C for 25 min in an electric vacuum-drying oven to further solidify the precursor solution on the surface of the wing samples. Next, the whole assembly was dipped into a mixture of concentrated nitric acid and perchloric acid (1:1 in volume) while heating at 130 °C for 40 min to remove the original organisms. Then, the whole assembly was washed by ultrasonic oscillation for 5 min in deionized water to get rid of the residue. After drying at room temperature, the SiO2 negative replica was fabricated.
The reflectance spectroscopy of butterfly Trogonoptera brookiana wings and the SiO2 negative replica were measured using a miniature fiber optic spectrometer (Ocean Optics USB4000-VIS-NIR) equipped with a halogen tungsten lamp source (Ocean Optics LS-1-LL). The spot size of the incident beam was ~2 mm, and the wavelength of the reflectance spectroscopy varied in the range of 400–900 nm. In addition, the element types and content analysis of the surface of the SiO2 negative replica were characterized by virtue of the energy dispersive spectroscopy (EDS, OXFORD X-MaxN 150) on the SEM.
Morphology and Dimension Characterization
The 2D morphologies and structures of the scales of the black areas of butterfly Trogonoptera brookiana hind wings and the SiO2 negative replica were obtained with the help of field emission scanning electronic microscopy (FESEM: JEOL JSM-6700 F). These data will be used to analyze the inheritance accuracy of the SiO2 negative replica.
Results and Discussion
Macroscopic Morphology Observations and Reflectance Spectroscopy Analysis of Black Butterfly Wing Scales
Microscopic Morphology Observations of Black Butterfly Wing Scales
The Formation Mechanism of the SiO2 Negative Replica
Microscopic Morphology Observations and EDS Analysis of the SiO2 Negative Replica
Analysis of the Light Trapping Mechanism of the SiO2 Negative Replicas
The SEL calculated from the measured reflectance spectroscopy (seen in Fig. 5b) of the flat plate and the SiO2 negative replica were 491.34 and 110.98 W/m2, respectively. Apparently, the SEL of the flat plate was more than four times the SEL of the SiO2 negative replica, which confirmed that the SiO2 negative replica had the better light trapping characteristics. Given that they shared uniform intensity of the light and material, it could be inferred from the results that the negative quasi-honeycomb-like structure which was borrowed from the black wing scales of butterfly acted as super light trapping architectures. Thus, it could be concluded that the SiO2 negative replica inherited not only the original super light trapping architectures, but also the super light trapping characteristics of bio-template.
In summary, the reflectivity of the black wings of butterfly Trogonoptera brookiana was carefully examined, and the wings showed good optical absorption in the visible spectrum, which confirmed that the black wings of this butterfly possessed structure-based super light trapping property. So, a super light trapping material was then fabricated templated from these black wings by transferring the quasi-honeycomb-like structure from the black wing scales to flat plate. The structural replication fidelity of the process is demonstrated on both the macro- and microscale, and even down to the nanoscale, as evidenced by FESEM and energy dispersive spectroscopy (EDS). The comparison results of feature size between the SiO2 negative replica and original bio-templates were in concordance to some extent. Considering of the final observation result, the obtained SiO2 negative replica preserved the super light trapping architectures successfully from the aspects of morphology, dimensions, and distributions of the scales. At last, the reflectance spectroscopy of the SiO2 negative replica and a flat plate was measured. It was found that the SiO2 negative replica obtained 20 % reflection in visible light spectrum. Reflection of the SiO2 negative replica with the negative quasi-honeycomb-like structure was merely 1/4 of that in the flat plate. Thus, it was obvious that the obtained SiO2 negative replica possessed super light trapping property. From the analysis above, it was proved that the super light trapping surface of original bio-templates was also inherited by the SiO2 negative replica faithfully in terms of structure and function. Thus, it could be concluded that the SiO2 negative replica inherited not only the original super light trapping architectures, but also the super light trapping characteristics of bio-template.
The manufacture of the super light trapping architectures of butterfly Trogonoptera brookiana black wing scales is meaningful. The SiO2 negative replica was not only stable but also corrosion resistant due to the complex super light trapping architectures and inert SiO2 material, which made it a promising light trapping surface for various fields such as photoelectrical devices, photo-induced sensors, and solar cells. Moreover, this work sets up a strategy for the design and fabrication of super light trapping materials and may encourage people to look for more super light trapping architectures in nature.
This work is supported by the National Natural Science Foundation of China (Nos. 51175220, 51505183, 51325501, and 51290292), and China Postdoctoral Science Foundation Funded Project (Project No. 2015 M571360).
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