Integrated super-hydrophobic and antireflective PDMS bio-templated from nano-conical structures of cicada wings

Abstract

Inspired by cicada wings that can greatly minimize the reflectivity on their surfaces over broad angles or frequency ranges due to the presence of tapered pillar arrays, we fabricated a polydimethylsiloxane (PDMS) positive replica of cicada wings, which demonstrated antireflective and super-hydrophobic characters. Firstly, the cicada wings were selected as a template to duplicate a SiO₂ negative replica. Then, the SiO₂ negative replica was used as a secondary template to prepare a PDMS positive replica. The resultant PDMS replica inherited the nano-conical structures and thus exhibited outstanding antireflective effect. A suppression of reflectance to a minimum of 0.7% that benefits from nano-cones has been achieved, since the path length and quantity of the incident light irradiated onto PDMS could be increased by incorporating anti-reflective and light scattering patterns. In the meantime, the PDMS replica demonstrates super-hydrophobicity due to the presence of nano-cone structures. The PDMS replica of cicada wings that possesses both anti-reflective and superhydrophobic properties may hold great promise for applications in anti-reflection and self-cleaning windows, in photovoltaic cells, telescopes, camera lenses, glass windows and beyond.

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1. Introduction

In recent years, scientists have devoted great efforts to the surfaces with a contact angle (CA) over 150° (so-called super-hydrophobicity) owing to their potential applications in fundamental research and practical applications.¹ Super-hydrophobic properties are desirable for many industrial applications including deicing,² self-cleaning of lotus,³ anti-corrosion,⁴ and outdoor windows,⁵ as well as for enhanced antibacterial and anti-biofouling surfaces.⁶ Although the preparation of super-hydrophobic surfaces has been extensively studied, only a few scientists have studied antireflective films with super-hydrophobicity, which is crucial for their practical applications particularly in window treatments and solar cell panels.

Furthermore, the anti-reflective properties have also played an important role in the development of optical devices, for instance, telescopes, gratings, or photovoltaic cells. Coating multilayer thin films or texturing the substrate surfaces with sub-wavelength structures on substrates are the main ways to suppress reflection. However, thin film coatings exhibit some problems such as material selection, adhesion, thermal mismatch or diffusion of one material into another.⁷ Fortunately, sub-wavelength structures on various substrates could overcome these aforementioned limits. These years, many different structures, such as nano-rods, nano-pillars,^8–10 gratings,¹¹ porous structures,¹² and nanotubes,¹³ have been created to restrain the surface reflection. However, these surfaces of various structures haven't possessed self-cleaning super-hydrophobicity facing up to outer door dusts when used in light-emitting diodes (LEDs) or, camera lenses and outdoor windows. To date, continued efforts in endowing novel materials with unique de-wetting properties and lower reflectance are still highly required to realize their structural and functional integrity.

In fact, natural biological systems have developed many nano/micro structures to produce striking multifunctional effects,^14–16 for instance, the anisotropy surface of the rice leaves,^17,18 the self-cleaning surface of the lotus leaves,^19–21 the antireflective and antifogging surfaces of butterfly wings,^22,23 the antireflective and super-hydrophobic surface of insect wings.^24–26 In particularly, the cicada wings possess excellent antireflective properties due to the presence of hexagonal arrays of non-close-packed (NCP) cones.²⁷ As a matter of fact, the conical structures with a feature size smaller than the propagating wavelength could form an effective refractive index gradient between air and the wings, which would suppress the Fresnel reflection.^28,29 Furthermore, it was also demonstrated that the cicada wings could prevent water drops from being trapped in the voids between nanostructures due to the presence of NCP nanoscale cones.^30–32 Accordingly, it appears super-hydrophobicity and self-cleaning functionality on the wing surface.^33,34 Thus, the multifunctional materials inspired from cicada wings would be a novel materials with unique de-wetting properties and lower reflectance and are promising in many optical devices such as camera lenses, glass windows and telescopes etc. In order to mimic the cicada wing scales, more and more scientists have devoted their energy to develop various advanced nanotechnologies including chemical etching,³⁵ soft imprint lithography technique,³⁶ and reactive ion etching.³⁷ For instance, Chen et al. coated two-dimensional non-close-packed colloidal crystals as etching masks to pattern subwavelength-structured cone arrays directly on polymer substrates and the reflectance was lower than 1%.³⁷ Leem et al. fabricated the biomimetic nanogratings (NGs) on the surface of UV-curable NOA63 polymer films using a soft imprint lithography and the NOA63 NGs/glass exhibits a hydrophobic surface with a water contact angle of 112°.³⁶ Despite the rapid development of synthetic methods, the current design methodologies still face serious problems with respect to expensive apparatus, the complex software operation and incontrollable morphology and size. Meantime, the presence of local defects due to soft imprint lithography technique also posed challenges to achieving uniform antireflection replicating these complex arrays on an industrial scale.

In this work, inspired by the broadband antireflection performance and excellent de-wetting properties of cicada wings, the cicada flower wings were selected as the bio-template. An economical and effective method called twice bio-template synthesis method was adopted to fabricate the artificial multifunctional surface of cicada wings. PDMS was used to copy the nano-cones structures of cicada wings due to lower surface energy. By comparing the morphologies and reflectance of PDMS replica and original template, it was confirmed that the positive PDMS replica not only inherited the nano-conical structures but the outstanding antireflective effect of the cicada wings. Meantime, CA of the positive PDMS replica can be up to 152°, which is higher than planar PDMS. Thus, it was confirmed that the mimic cicada-wing structures also demonstrated super-hydrophobic characteristic. Overall, the multifunctional materials inspired from cicada wings would be potential in applications of photovoltaic cells, telescopes, camera lenses, and glass windows, which require both lower reflection and excellent water resistant ability in humid environment.

2. Results and discussion

Cicada flower owns the nano-cones attributing excellent antireflective capacity due to a gradient change in refractive index.^28,29 Meantime, the rough sub-microstructures confer cicada wings the super-hydrophobic effect and self-cleaning capability.^30–32 As a result, in this work, we select the cicada flower wings as the bio-template. Fig. 1a showed the overall view of the original cicada wings, and Fig. 1b and c demonstrated the details of the top appearance and a tilted image respectively. Meantime, the macroscopic details of cicada wings under low magnification were shown in Fig. S1.† From these images, it was confirmed that the microscopic structures of the cicada wings consisted of ordered hexagonal non-close packed arrays with a spacing of 720 nm. As well as, the heights of the pillars ranged between about 800 nm and 900 nm, and the diameters at the pillar top and bottom were about 100 nm and 250 nm respectively. According to the optical properties and wetting characters of cicada wings, we measured the reflectance and WCA of original template shown in Fig. 1d. On the basis of the measurements, the WCA of original template (Fig. 1d, inset) equally came up to 155.5°. Furthermore, the reflectance of cicada wing arrived as low as 0.05%. Thus, cicada flower is the appropriate template to use in this research.

Fig. 1 The macroscopic morphology of the original cicada flower wings and the SEM images of original cicada scales with certain magnification. (a) Photograph of cicada flower. (b) Provide a top appearance of cicada wings under higher magnification. (c) Schematic image containing a tilted SEM image of cicada wings. (d) The reflection spectra of cicada wings (water drop profile of cicada wings inset in d).

2.1 The overall process of bio-template method

As shown in Fig. 2, it was the overall process of the twice bio-template synthesis method: (a) cleaned cicada wing was fasten with double-sided adhesive tape on the glass slide after pretreatment by diethyl ether and ethanol absolute for 15 min respectively; (b) a layer of thick precursor liquid was dispersed on the cicada wing (about 1 μm higher than the height of the pillars on the wing). Then, a cover glass was slowly put above it (removal of small bubbles); (c) in order to solidify the precursor solution, the glasses were heated at 90 °C for 120 min; (d) the original template was removed with a tweezer directly, and we will get the SiO₂ negative replica on glass; (e) the prepared PDMS was poured onto the nano-pores of negative replica and we got the PDMS/SiO₂ negative replica assembly; (f) with the aim of evacuating the air bubbles, the glass was placed in a vacuum oven to heat at 80 °C for 120 min; (g) after cooling at room temperature and picking the PDMS off the SiO₂ negative replica, we would obtain the PDMS positive replica.

Fig. 2 Fabrication process of the twice bio-template synthesis method. Cicada flower wings was the original biology template. (a) The 3D nanostructured model of the original biology template. (b) The precursor solution filled the space between nano-conical shapes. (c) The precursor solution became solidified during heating, making the place that used to be cones became nano-wells. (d) The solid negative replica was obtained after removing the original template. (e) and (f) The prepared PDMS was poured onto the SiO₂ negative replica and the assembly was placed in a vacuum oven to evacuate the air bubbles. (g) The prepared PDMS was heated to solidify. (h) The PDMS positive replica was fabricated after separating from the SiO₂ negative replica.

2.2 Morphology and composition analysis

SEM was used to depict the morphology of the negative SiO₂ replica, and the results were illustrated in Fig. 3. Fig. 3a and b demonstrated the scales structures of the negative SiO₂ replica. From these data, the microscopic structures of the negative SiO₂ replica consisted of ordered hexagonal close-packed nano-wells with a distance of 720 nm and the diameter of the nano-well was about 450 nm, which were similar to the Fig. 1b. According to the results, it can be obviously found that the negative structures of the stamp have been successfully fabricated by bio-template. Thus, it proves that the prepared negative SiO₂ replica have inherited the nanostructure of cicada wing scales successfully. The negative porous structures were formed from the conical shapes on the surface of the original template. The inter-conical shapes spacing of the original scales were filled with sol–gel precursor and the sol–gel precursor became solidified, making the space that used to be nano-conical shapes became negative porous scales. At the same time, the spaces between conical shapes of the original scales became humps. It could be confirmed that the SiO₂ negative replica were the negative structured replica of the original template from the aspect of the sizes and shapes. In a word, it was obvious that the original functional nanostructure of the original scales were replicated with a high fidelity from the aspect of the size, shape and arrangement of the scales, including the nano-conical units, negative pores and humps. On the other hand, when a water droplet contacted with porous SiO₂ negative replica, it spread flat instantaneously on the surface, indicating a great super hydrophilic nature due to the porous structures and hydrophilic silica combination keys (Video. V1 in the ESI† shows the process). Thus, the SiO₂ negative replica possesses the super hydrophilic capability as shown in Fig. 3c. Lastly, the spectra of silicon (2p) region shown in Fig. 3d verified the silicon oxygen compounds was SiO₂. The single peak at an electron binding energy of 103–105 eV was assigned to Si⁴⁺ oxidation states. Besides, ∼104 eV was deem to 2p^1/2 spin–orbit partner peak and ∼103 eV stand for 2p^3/2 spin–orbit partner peak.

Fig. 3 (a) and (b) SEM images represent lower and higher magnification images of SiO₂ negative replica. (c) Water drop profile of SiO₂ negative replica. (d) XPS spectra of SiO₂ negative replica.

The details of the prepared PDMS positive replicas were observed under SEM, as shown in Fig. 4a–c. Fig. 4a and b provided a top appearance of the final replica and showed the nano-conical morphology of the PDMS positive replica under different magnification, the shapes of which were similar to those of the scales shown in Fig. 1b. Fig. 4c showed the schematic image containing a tilted SEM image of PDMS positive replica, which confirmed that the replica maintained basically the conical shapes of the original scales. Meantime, the microscopic structures of positive replica were shown in Fig. S2† under lower magnification. The nano-conical shapes were formed from the porous structures on surface of the SiO₂ negative replica. The inter-pores spaces of the SiO₂ negative replica were filled with PDMS and the PDMS became solidified after heating at 80 °C for 120 min, which made the spaces that used to be pores become conical shapes. Meantime, tilted SEM image of PDMS positive replica (Fig. 4c) revealed that the top diameter is about 150 nm and the bottom diameter is about 250 nm. Meantime, the height of nano-nipples was measured by AFM shown in Fig. S3.† Apparently, the height of conical shapes is much lower than original template because that it is hard for larger molecular weight of PDMS to permeate into the porous structures entirely. In addition, the pitch between inter-pillars is about 720 nm. These results indicated that the overall dimension of the conical shapes units in the PDMS positive replica was similar to those in the original scales. At last, the WCA water drop profile (Fig. 4d) confirms definite super-hydrophobic characters of PDMS positive replica and demonstrate a higher WCA about 152°. Meantime, the PDMS positive replica had a low adhesion to water droplets which was similar to the cicada wings.

Fig. 4 SEM photograph of PDMS positive replica. (a) and (b) Demonstrate a top appearance image of PDMS positive replica under different magnification. (c) The tilted SEM image of PDMS positive replica. (d) Water drop profile of PDMS positive replica.

In order to verify the unmodified PDMS after a series of treatments, we measured the FTIR spectra of the PDMS positive replica shown in Fig. 5a. The peaks at 1060 cm⁻¹ and 1635 cm⁻¹ were assigned to Si–O–Si stretching vibration bond. Peaks at around 1263 and 803 cm⁻¹ were attributed to the Si–C bond. Other diagnostic peaks in the spectra were assigned to C–H, C–H₂ and C–H₃ groups of the original PDMS (2964 cm⁻¹, 2919 cm⁻¹ and 1415 cm⁻¹, respectively). It could be observed that the FTIR spectrum of the PDMS positive replica was consistent with the general pattern of FTIR spectrum of the PDMS. Meantime, the element compositions of PDMS positive replica were determined by XPS. Spectra of carbon (1s) region shown in Fig. 5b and the single peak at an electron binding energy of 285 eV verified that the carbon in unmodified PDMS was predominantly in the form of methyl groups. In addition, the atomic percentages of these three elements (Si C O) were 21.71%, 54.53% and 23.75% respectively. It matched with the standard of unmodified PDMS no considering of some carbon pollution from original template. It can be confirmed that the composition of the bionic PDMS positive replica was not changed after a series of treatment. Meantime, the unmodified PDMS assures that the conical structures attribute more to optical performance and super-hydrophobicity than polymer materials.

Fig. 5 (a) FTIR spectrum of the PDMS positive replica. (b) XPS spectra of PDMS positive replica.

2.3 Optical property of conical shapes

Although the PDMS positive replica morphology is not as ordered as the cicada wing's structure, similar antireflective and super-hydrophobic performance can be achieved since that the structure of the cicada wings will lead to the decrease of the refractive index and the increase in the roughness of the PDMS. The reflectivity of the SiO₂ negative replica, clean slide glass and PDMS with and without the conical structures has been carefully examined (Fig. 6a). It could be obtained that the bionic PDMS positive replica owns the least reflectivity, which is very similar to the bio-template. It is obvious that the average reflection of the PDMS positive replica is just 1/4 of that of the flat PDMS film, which confirms that the bionic PDMS positive replica possesses a good antireflective property. Given that they share uniform intensity of the light and material, it could be inferred that the nanostructures on PDMS surface endow itself excellent antireflective characteristic. What is more, the PDMS positive replica attained lower reflectivity than SiO₂ negative replica and the slide glass because of conical structures on the surface. The SiO₂ negative replica possesses lower reflectivity than slide glass due to that the nano-wells provide a multi-layers reflection. In addition, the simplified model of the PDMS positive replica was built as shown in Fig. 6b. Incident light travelled much longer distance after multiple reflections, resulting less light loss. Thus, it could be concluded that the bionic PDMS positive replica not only inherits the nano-conical architectures of the bio-template but also the antireflective characteristics. Furthermore, we have measured the transmission spectra of PDMS positive replica shown in Fig. 6c and the transmittance came into 67%, which was because nano-conical shapes might inhibit the travel of incident light. Lastly, to investigate the effect of the incident angle on the reflectivity, we measured the angle-dependent reflective spectra of the PDMS positive replica shown in Fig. 6d. No obvious change can be observed in the reflective spectra when the incident angle is smaller than 30° in the wavelength region of 400–1000 nm. The reflection remains below 0.7% at the wavelengths of 400–1000 nm. Based on the results, there exists potential application in photovoltaic cells or other optoelectronic system about PDMS positive replica.

Fig. 6 (a) Comparison of reflectance of slide glass, SiO₂ negative replica, and PDMS positive replica for light whose wavelength varied between 400 nm and 1000 nm. (b) Schematic illustration of the multiple reflection and refraction occurred in the PDMS positive replica. (c) The transmission spectra of PDMS positive replica. (d) Measured angle-dependent reflective spectra of the PDMS positive replica.

2.4 Super-hydrophobic property of conical shapes

Contact angle is one of a state of wettability. The measurement about water contact angle of the surfaces was performed by the sessile drop technique and an average of five measurements on each sample was effective. Fig. 7a shows the increase in water contact angle (WCA) as the surface texture became progressively multiscale, namely, from planar films to rough arrays. The WCA gradually increase in the order of SiO₂ negative replica, slide glass, planar PDMS and PDMS positive replica. When a water droplet contacted with porous SiO₂ negative replica, it spread flat instantaneously on the surface, indicating a great super hydrophilic nature due to the porous structures and hydrophilic silica combination keys. Thus, the SiO₂ negative replica possesses the super-hydrophilic capability, while slide glass composed of silica shows super hydrophilicity. The enhanced water repellence of the PDMS positive replica maybe attributed to both the multiscale texture and the presence of the hydrophobic PDMS. Meantime, the comparison about surface energy of the cicada wings, negative replica and positive replica shown in Table ST1† and supported the changes in WCA. Fig. 7b demonstrates a photograph of a spherical water droplet on the clean PDMS positive replica. The average value of the static water CA is 152°, which indicated that the nanoscale structure could greatly enhance the water-resistant property of the surface. According to Cassie's principle for surface wettability,³⁸ such nanostructure may be considered as complex surfaces composed of air and solids. Similar to the surfaces of moth eyes, air may be trapped in the spaces between nano-nipples to form a stable air cushion. It acts as effective water barriers.³⁹ The simulated wetting model from the PDMS positive replica was shown in Fig. 7c. However, it was noted that the artificial cicada wing is less hydrophobic than the original template because the nanostructure on the surface of the PDMS was not organized as the cicada wings shown in Fig. 1a. The conical scales exhibited some local defects, which could reduce the fraction of air trapped in the voids between nano-nipples. Thus, it would decrease the water CA and surface hydrophobicity.

Fig. 7 Super-hydrophobicity characteristics. (a) Water contact angle (WCA) on surfaces with various materials and nanostructures. The PDMS positive replica shows the highest increase in WCA. (b) Water drop profile of PDMS positive replica. (c) Simulated wetting model from the PDMS positive replica.

3. Experimental section

3.1 Materials

In this work, the hydrochloric acid, PDMS and tetraethyl orthosilicate (TEOS) were provided by Beijing Chemical Works. The ethanol absolute and diethyl ether were provided by Tianjin Fine Chemical Co., Ltd. All chemicals used for synthesis were analytic grade reagents.

The cicada flower wings with nano-conical shapes possess excellent antireflective capability due to the gradient change in refractive index.^28,29 Meantime, the arrays of tapered pillars in cicada wings show not only the excellent antireflective effect but also super capability of self-cleaning due to the rough nanostructure, which increases the air volume between water and wings.^30–32 In addition, comparing with other rare species, cicada flower appears in wider distribution and is available to obtain. As a result, we choose cicada flower wings as the bio-template to do further research.

3.2 Preparation of the SiO₂ negative replica

Firstly, cicada's wing was washed by the concentration of 0.65% NaCl at least three times or more, and the dried wing was soaked in alcohol and ethanol for 10 min respectively. Then, using a micropipette, a suitable amount of the sol–gel precursor solution, a reaction product of TEOS, hydrochloric acid and ethanol (volume: 30 ml, 0.2 ml, 10 ml), was dipped on the cicada wing, which was fasten with double-sided adhesive tape on the glass slide. Afterwards, a cover glass was slowly put above it (removal of small bubbles) and the set of device would be set in the oven to heat at 90 °C after 120 min. At last, we can get relatively complete negative mold.

3.3 Preparation of the PDMS positive replica

Firstly, the pre-polymer and the crosslinking agent were uniformly mixed and stirred (10 : 1 in weight) in a glass beaker to synthesize the PDMS. Then, the glass beaker with PDMS was placed in a vacuum oven to evacuate the air bubbles. The PDMS was poured onto the SiO₂ negative replica, and we got the PDMS/SiO₂ negative replica assembly. After that, the PDMS/SiO₂ negative replica assembly was placed in a vacuum oven to evacuate the air bubbles again and heated at 80 °C for 120 min to complete heating curing process. After cooling at room temperature, the PDMS was taken off the SiO₂ negative replica and the PDMS positive replica was get.

3.4 Measurements

The morphologies and structures of cicada wings, SiO₂ negative replica and PDMS positive replica were obtained by scanning electron microscopy (SEM) observations after gold coating (approximately 10 nm thickness). These data would be used to analyse the inherited accuracy of the SiO₂ negative replica and the PDMS positive replica. Reflection spectra in the wavelength range of 200–1100 nm were recorded by a Varian Cary 5000 UV-VIS-NIR spectrophotometer. The PDMS replica was examined by Fourier transform infrared spectroscopy (FTIR) measurements using a Bruker EQUINOX 55 instrument. WCA of species were measured with a Kruss contact angle instrument (DSA 100, Germany). In addition, X-ray photoelectron spectroscopy (XPS) analysis was performed by an AXIS multifunctional X-ray photoelectron spectrometer (ULTRA DLD, Shimadzu Ltd, Japan) at a power of 450 W. Lastly, an atomic force microscope (AFM; Park Systems, XE-100) was used to measure the height of PDMS positive replica.

4. Conclusions

In summary, the PDMS positive replica bio-templated from cicada wings was successfully fabricated by a twice bio-template synthesis method. Firstly, the cicada wings, owning excellent antireflective effect, were selected as template and the SiO₂ negative replica was firstly duplicated. Then, the SiO₂ negative replica was used as a secondary template and the PDMS positive replica was also successfully synthesized. Meanwhile, the surfaces functional structures in cicada wings were transcribed onto the surface of the replicas. Based on kinds of measurement including the morphology, optical property and wetting characters, it was concluded that the positive PDMS replica inherited the nano-conical structure and outstanding antireflective effect with a minimum reflectance lower than 0.7% because of increased path length and quantity of the incident light onto PDMS. Interestingly, it not only has antireflective properties but also demonstrates excellent super-hydrophobic performance with a WCA up to 152° higher than planar PDMS film, which was caused by nano-cones preventing water drops from being trapped in the voids between nanostructures. Based on the discussion, it was anticipated that this method would offer a convenient and scalable way for inexpensive and high-efficiency organic optoelectronic designs. In addition, the integrated super-hydrophobic and antireflective PDMS would be a potential application material for photovoltaic cells, windows, automobile windshields, and outdoor textiles that need super-hydrophobicity for self-cleaning and antireflective capacities for clear image.

Acknowledgements

The authors thank the National Natural Science Foundation of China (No. 51275555, 51475200, 51325501 and 51505183), Science and Technology Development Project of Jilin Province (No. 20150519007JH), and 111 project (B16020) of China.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23811d

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