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Brilliant Structurally Colored Films with Invariable Stop-Band and Enhanced Mechanical Robustness Inspired by the Cobbled Road Xin Zhang,* Fen Wang,* Lei Wang, Ying Lin, and Jianfeng Zhu School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, PR China S Supporting Information *

ABSTRACT: Recently, structural colors have attracted great concentrations because the coloration is free from chemical- or photobleaching. However, the color saturation and mechanical robustness are generally competitive properties in the fabrication of PCs (photonic crystals) films. Besides, the structure of PCs and their derivatives are easy to be invaded by liquids and lead to band gap shifts due to the change of refractive index or periodicity. To solve those problems, we infiltrate polydimethylsiloxane (PDMS) into the intervals between regularly arrayed hollow SiO2 nanospheres, inspired by the cobbled road prepared by embedding stone in the bulk cement matrix. Consequently, the as-prepared PCs films show brilliant colors, invariable stop-bands, and excellent mechanical robustness. Moreover, the water contact angle even reached 166° after a sandpaper abrasion test. The combination of brilliant colors, invariable stop-bands, and excellent robustness is significant for potential application in paint and external decoration of architectures. KEYWORDS: PCs, hollow SiO2−PDMS, stop-bands invariable, brilliant colors, enhanced robustness

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INTRODUCTION Photonic crystals (PCs) with a periodic modulation of refractive index usually provide photonic band gaps, and photons with energy in which are prohibited in the materials.1,2 Structure colors originating from the band gaps of PCs have attracted great attention due to their antifading character.3−18 In order to fabricate invariable structural colors, both the periodicity and refractive index should remain unchanged regardless of external conditions according to the Bragg Law.3,19−21 However, PCs and their derivatives with a large portion of air cavities are easily invaded by liquids and lead to band gap shifts due to the refractive index or possibly periodicity changes.22−25 Previously, opal or inverse opal has been chemically treated to render them hydrophobic or superhydrophobic surfaces by trapping air in cavities.26−28 However, band gaps of such PCs are still vulnerable to liquids with low interfacial tensions because of the oleophilic property of porous structures. Generally, both of the oleophobic and hydrophobic performances could be obtained through a combination of re-entrant geometry and low surface energy materials.29,30 However, such a complex geometry is difficult to prepare over delicate porous PCs structures. Therefore, fabrication of PCs films with invariant band gap is still a great challenge for outdoor applications. Recently, Kang et al.2 reported a practical method to create liquid-impermeable inverse opals with an omniphobic surface by subjecting the top surface of inverse opals to reactive ion etching with SF6 gas, and the resultant architecture can efficiently trap air below the heads of the posts for liquids with a wide spectrum of polarity. However, this method only endows the top surface with the © 2016 American Chemical Society

omniphobic property, and liquids will penetrate into this porous structure once the top layer is damaged under the external force. Moreover, all of the documented methods for preparing omniphobic coatings are unable to get rid of the combination of fluorochemicals and dual-scaled (micro and nano) surface roughness. In addition, fluorochemicals as one main branch of low surface energy chemicals have potential risks to human health and the environment.31 Polydimethylsiloxane (PDMS) with a low surface energy of about 20 mN/m has many excellent properties, such as hydrophobicity, low toxicity, long-term endurance, and transparency, making it gradually become an attractive material for outdoor applications.32−35 However, redundant PDMS will penetrate into the voids between neighboring nanospheres and lead to decreased surface roughness and color saturation since the refractive index of PDMS (1.425 at 632.8 nm)36,37 is similar to SiO2 (1.457 at 632.8 nm).38,39 To obtain brilliant structurally colored films, sufficient refractive index contrast is a prerequisite condition and air with the lowest refractive index plays an indispensable role. If air exists in closed pores instead of open pores in PCs, the liquid is not able to infiltrate into this structure and the brilliant color will remain even on rainy days. Water could not infiltrate into the cobbled road (as shown in Figure S1) on a rainy day due to the existence of cement in the pores between neighboring cobblestones. Similarly, if the Received: June 23, 2016 Accepted: August 10, 2016 Published: August 10, 2016 22585

DOI: 10.1021/acsami.6b07576 ACS Appl. Mater. Interfaces 2016, 8, 22585−22592

Research Article

ACS Applied Materials & Interfaces Scheme 1. Schematic Illustrating the Formation of Stop-Band Invariable PCs Films of Hollow SiO2−PDMS

dispersed in 10 mL of ethanol by ultrasonic for 30 min at ambient temperature. Then, those dispersions were poured into a 25 mL beaker, and glass substrates were carefully placed into the beaker. It is noteworthy that the whole process was carried out at 45 °C. Fabrication of the hollow SiO2−PDMS films. PCs of hollow SiO2 were obtained by calcining PCs of PS@SiO2 at 500 °C with a heating rate of 1 °C/min and kept at 500 °C for 2 h in air. Dow Corning Sylgard 184 silicone elastomer and curing agent were mixed at a weight ratio of 10:2. After degassing, the PDMS precursor was gradually infiltrated into the voids between the neighboring particles by casting, and the redundant PDMS was removed with filters. Finally, the samples were cured at 150 °C for 30 min and formed hollow SiO2−PDMS films. Characterization. Transmission electron microscopy (TEM) (FEI Tecnai G2 F20 S−TWIN) was used for observing the morphology and measuring the geometric parameters of the particles. The morphology of PS@SiO2 nanospheres was observed by using scanning electron microscopy (SEM) (Hitachi S4800). A thin Au layer (10 nm) was sputtered on the film before observing. The reflection spectra of the colloidal crystals were performed using a Cary 5000 UV−vis−NIR spectrometer (Agilen). The optical photographs and microscopy images of the fabricated PCs films were taken by a digital camera and an optical microscope (Leica DM2500 M) connected with a CCD camera, respectively.

homogeneous colloidal nanospheres are embedded in bulk materials without opened air pores, the structurally colored films will be liquid impermeable. Herein, we have fabricated PCs structures with regularly arrayed closed air pores embedding in the bulk PDMS matrix through infiltrating PDMS into the interval of PCs assembled by hollow SiO2. The as-prepared colored films showed enhanced color saturation and invariable stop-bands when exposed to five different liquids, including water and TEOS. Moreover, the mechanical robustness of the film was improved significantly in the presence of PDMS, and the water contact angle even reached 166° after a sandpaper abrasion test. This film combining brilliant colors, invariable stop-bands, and excellent robustness would have a potential application in paint and external decoration of architectures.

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EXPERIMENTAL SECTION

Materials. Polystyrene (PS) NPs with diameter of 216 and 268 nm were synthesized by a soap-free emulsion polymerization method, and PS@SiO2 particles are prepared by a modified Stöber method as we previously reported.40 Dow Corning Sylgard 184 silicone elastomer kit was used for preparing PDMS. Preparation of Monodisperse PS@SiO2 Nanospheres. In this study, PS@SiO2 spheres were prepared by coating SiO2 on amino terminated PS beads via a modified Stöber reaction in ethanol by using ammonia as catalyst. All of the reactions were carried out at the same ammonia concentration under constant magnetic stirring, and the shell thickness of SiO2 particle was increased with the concentration of tetraethoxysilane (TEOS). In a typical synthesis, 0.03 g of PS nanospheres were completely dispersed in a mixture of 2 mL of deionized water and 35 mL of ethanol by ultrasonic. To make PS nanospheres terminated in the −NH2 group, 0.5 mL of ammonia was added and reacted for 60 min under constant stirring. Then, various amounts of TEOS were poured into the flask as quickly as possible. The hydrolysis reaction was allowed to proceed at room temperature for 8 h under constant magnetic stirring. Finally, the colloidal dispersions were collected by centrifugation. The obtained core−shell particles could form stable suspensions in aqueous mediums without adding any surfactant due to the negative charges on the surface of silica shells. Preparation of PS@SiO2 PCs Films. 3D PCs films of PS@SiO2 spheres were fabricated by using a simple vertical deposition method. In a typical process, first, 0.02−0.04 g of PS@SiO2 powder was

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RESULTS AND DISCUSSION The overall procedures for creating the stop-band invariable brilliant structurally colored films are demonstrated in Scheme 1. First, PCs of PS@SiO2 are prepared by capillary wetting; then, the as-prepared PCs were calcined at 500 °C (1 °C/min) for 2 h to completely remove the polymer templates and partially sinter the SiO2 structures. Finally, PDMS solution was casted over the PCs films of hollow SiO2 and followed with a curing process (the as-prepared PCs were defined as hollow SiO2−PDMS). In this way, the stop-band invariable brilliant colored films with enhanced mechanical robustness were realized in the simple structure. Morphology of the As-Prepared Nanospheres and Their PCs Films. PS@SiO2 nanospheres of 258, 270, 280, and 290 nm were obtained by using 216 nm sized PS beads as template, respectively (defined as samples I, II, III, and IV), while PS@SiO2 nanospheres of 305 and 322 nm size were fabricated by using PS of 268 nm as template (defined as 22586

DOI: 10.1021/acsami.6b07576 ACS Appl. Mater. Interfaces 2016, 8, 22585−22592

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the liquids could not be infiltrated into this structure to destroy the periodicity. Optical Property of Different PCs Films. Figure 2a shows the microscopic optical images taken by applying an optical microscope connected with a CCD camera under the conditions of reflection mode. PCs assembled by 258, 270, 280, and 290 nm sized PS@SiO2 nanospheres display green and orange, respectively. The red colored PCs films are assembled by 305 and 322 nm sized PS@SiO2. Figure 2b−d shows the reflection spectra of PCs assembled by PS@SiO2, hollow SiO2, and hollow SiO2−PDMS. The differences in reflection peak positions are originated from the variation in effective refractive index, and photonic stop-band arising from stacked (111) planes of close packed face-centered-cubic (FCC) lattice is significantly influenced by the refractive index of components. The wavelength (λ) of the stop-band can be estimated by the Bragg’s Law: λmax = (8/3)1/2 D(neff2 − sin2θ)1/2, where λmax is the wavelength at the peak position, D is the diameter of nanospheres, and neff is the effective refractive index which can be expressed as neff2 = Σi N= 1ni2φi (n is the refractive index, φ is the volume fraction, subscript i refers to the component, and θ is the angle between the incident light and the reflective surface). In the calculation of λmax, we use nps = 1.59, nSiO2 = 1.46, and nair = 1; for an ideal FCC structure assembled by PS@ SiO2 spheres, φ = 0.74 and nsphere can be calculated by using

samples V and VI, respectively). As shown in Figure 1a,b, PS and PS@SiO2 nanospheres are homogeneous in size and have

Figure 1. SEM images of (a) PS nanospheres, D = 268 nm; (b) PS@ SiO2 nanospheres with shell thickness of about 27 ± 2.5 nm; TEM image of (c) PS@SiO2 nanospheres with shell thickness of about 30 nm after calcination. Large-scale domains of (d) PS@SiO2; (e) hollow SiO2 crystalline arrays and (inset) 2D Fourier transform of the SEM images. SEM images of hollow SiO2−PDMS under (f) top view and (g) cross-sectional view. The inset in (f) is the optical picture of colored film of PCs of hollow SiO2−PDMS.

V

VSiO

V

VSiO

nsphere = nPS VPS + nSiO2 V 2 (VPS and VSiO2 are the volumes of PS and SiO2 spheres; V is the total volume of each component (V = VPS + VSiO2)). When PS in the PCs of PS@SiO2 is removed by calcination, the nsphere should be modified into nsphere = nair Vair + nSiO2 V 2 , and the other φ = 0.26 is occupied with air. After the PDMS is infiltrated into the open pores, the neff is significantly improved and results in a red shift of wavelength and enhanced color saturation compared to PCs of hollow SiO2 (nPDMS = 1.425). The measured diameters of PS, PS@SiO2, hollow SiO2 nanospheres, reflection peaks of PCs composed by PS@SiO2, hollow SiO2, and hollow SiO2−PDMS are summarized in Table S1. In Figure 2b, reflectance peaks at 549, 570, 600, 610, 634, and 688 nm are denoted as black, red, green, blue, cyan, and magenta solid lines for samples I−VI, respectively. Selective removal of PS particles leads to a blue-shift in the stop-bands due to the decreased effective refractivity. For PCs of hollow SiO2 (nPS is replaced with n air), samples I−VI provide λ = 429, 458, 440, 500, 463, and 533 nm instead of 549, 570, 600, 610, 634, and 688 nm in PCs of PS@SiO2, as shown in Figure 2c. After the PDMS solution is casted, enhanced refractive index contrasts are obtained and the reflectivity at the stop-band reached as high as 80% for PCs of hollow SiO2−PDMS (shown in Figure 2d). The enhanced color brightness could also be seen in Figure 2a, where all of the optical microscope images are taken under the same measurement condition. In this study, the volume ratio of high refractive index materials is higher than previously reported methods, in which high refractive index materials only account for 24% of the volume ratio.2,16,41−43 It is also noteworthy that the calculated λmax from the Bragg’s equation has little difference with the measured values in reflection spectra, which can be explained by taking defects in PCs into consideration. As shown in Figure 2b−d, the maximum reflectivity and fullwidth-at-half-maximum (FWHM) are decreased for PCs of hollow SiO2 and increased again after PDMS was penetrated

relatively smooth surfaces. Figure 1c shows the TEM image of hollow SiO2 spheres obtained by removing PS core at 500 °C for 2 h. It can be seen that the as-prepared hollow SiO2 shows an obvious hollow structure with a uniform silica shell thickness of about (27 ± 2.5) nm. Figure 1d,e displays the SEM images of large-scale crystalline arrays of PS@SiO2 and hollow SiO2. It is shown that the top views of both opaline films are highly ordered 3D hexagonal close-packed structures with length scales as large as nearly 30 μm. The corresponding fast Fourier transform (FFT) spot patterns (inset in Figure 1d,e) with 6-fold symmetry and a high degree of crystalline orders could be seen clearly, indicating the long-range order of those samples. In addition, cracks in the PCs of hollow SiO2 are increased due to the evaporation of water molecule and hydroxyl group on the surface of PS@SiO2 nanospheres during the calcination treatment compared with PCs of PS@SiO2. As shown in Figure 1f, the as-prepared PCs film of hollow SiO2−PDMS is very smooth and exhibits very brilliant colors in the digital photograph (as shown in the inset in Figure 1f; the area is as large as 2.5 × 1.5 cm2). Moreover, it is found that the hollow SiO2 nanospheres are out of sight under the top view images due to the presence of the PDMS layer. In order to certify the existence of hollow SiO2 nanospheres, the cross-sectional SEM image of the film is provided as shown in Figure 1g. It is found that the regularly arrayed hollow SiO2 nanospheres can be clearly seen, and PDMS has been successfully infiltrated into the voids through a curing process. In this heterostructure, air pores only exist as close forms and locate in the core of core−shell particles. Thus, 22587

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Figure 2. (a) Microscopic optical images of PCs films assembled by (line 1) PS@SiO2 nanospheres; (line 2) hollow SiO2 nanospheres; (line 3) hollow SiO2−PDMS. Reflection spectra of PCs films assembled by (b) PS@SiO2, (c) hollow SiO2, and (d) hollow SiO2−PDMS. Samples I−VI are denoted with black, red, green, blue, cyan, and magenta solid lines, respectively. (e) The measured and calculated reflection peak positions of different PCs films: (black ●, red line) PS@SiO2, (black ■, blue line) hollow SiO2, and (black ▶, black line) hollow SiO2−PDMS. Solid lines represent the measured values while the dashed lines represent calculated values.

into the PCs films of hollow SiO2. All of those changes are originated from the variations in refractive index contrast,44,45 since FWHM normalized with peak position(Δλ/λ) is roughly proportional to the normalized index contrast (|np − nm|/neff or Δn/neff, where np represents the refractive index of nanospheres and nm is the refractive index of matrix). For example, sample V with reflection peak at 638 nm (Δn/neff = 0.3866) exhibits a value of Δλ/λ = 0.0883. After removing PS, the reflection peak is blue-shifted to 463 nm and leads to decrease of Δn/neff (0.1093) and Δλ/λ (0.0734), resulting in a blue shift and narrowing of the stop-band. When the PDMS was infiltrated into the open pores between neighboring hollow SiO2, the Δn/ neff is increased to 0.2411 and leads to a significant red shift and widening of the reflection spectra compared with PCs of hollow SiO2. For sample V, the peak is located at 614 nm and the value of Δλ/λ is 0.0879 as denoted with a cyan solid line in Figure 2d. A linear relation between Δλ/λ and Δn/neff is roughly conserved throughout the overall procedure, indicating that our

approach is reliable and reproducible for the creation of liquidimpermeable PCs. Invariable Stop-Bands of Colored Films. The stop-band invariable property is certified by observing color variations under an optical microscope after five different liquids dropping onto them. The optical microscopic images of samples IV and VI are captured by releasing a drop of liquids onto the films from a pipetting gun, as shown in Figure 3a,b. When water, acetone, ethanol, isopropanol, and tetraethoxysilane (TEOS) are dropped onto the film, they display identical colors in invading and original areas from left to right in turn. The CAs of different liquids are also measured (as shown in Figure 3c) to manifest that the stop-band invariable property is originated from the closed pores rather than a hydrophobic property that induced by the presence of PDMS. The measured CA is 123°, 28.7°, 28°, 19.2°, and 15.6° for water, acetone, ethanol, isopropanol, and TEOS, respectively. It suggests that the liquids we used are almost completely spreading on the PCs films 22588

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Figure 3. Microscopic optical images of PCs films with different liquids-drop-deposited (a) sample IV with green color and (b) sample VI with red color. (c) The CAs of different liquids on the PCs films of hollow SiO2−PDMS. (d) Optic images of colored films in water. (e) The reflection spectra of samples IVandVI in different liquids and the (inset) reflection peak positions in different liquids.

Figure 4. (a) The structurally colored films under high speed running water and (inset) the same films before (left) and after (right) being impacted by water. Finger-wipe test of (b) hollow SiO2−PDMS and (c) PS@SiO2 to test the robustness. (d) Sandpaper abrasion test. Structurally colored films on glass substrates weighing 100 g were faced down to the sandpaper and moved for (e) 15 cm and (f) 300 cm along the ruler. Microscopic optical images of (e) PCs films of PS@SiO2 and the red colored area are water-drop-deposited; (f) PCs films of hollow SiO2−PDMS with waterdrop-deposited area; (inset) the water CA of the colored film after sand paper abrasion for 300 cm where the CA is 166°.

rather than exhibiting a hydrophobic property except water. Thus, the stop-bands invariability of the as-prepared colored films are originating from the closed hollow SiO2. The reflective spectra are also measured after dipping the PCs films into different liquids for 30 min (Figure 3d). Almost invariable broad peaks in 557 and 616 nm could be seen in Figure 3e for samples IV and VI, which further indicates that the stop-bands

are indeed invariable in this study. Thus, the stop-band invariable property in this study is different from the inverse opal modified by fluoride or bulk materials.2,46,47 In which the regularly arranged hollow SiO2 served as low refractive index materials and worked together with PDMS to realize a periodicity of high and low refractive index contrast instead of vulnerable open pores. 22589

DOI: 10.1021/acsami.6b07576 ACS Appl. Mater. Interfaces 2016, 8, 22585−22592

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ACS Applied Materials & Interfaces Mechanical Robustness of Colored PCs Films. In addition to color variation, the weak mechanical property is another main issue that limits the wide spread use of structurally colored films because the particles are only bonded to each other and the substrate via van der Waals interaction in PCs films. In this work, the mechanical strength of the colored films is significantly improved in the presence of PDMS, which exists in the voids between each nanosphere after a curing process at 150 °C for 30 min. Moreover, the improved mechanical strength is compatible with the improvement of color brilliance, while they are incompatible in previous documents.37,48 Besides, the PDMS surrounded the hollow SiO2 nanoparticles as adhesives, making the nanoparticles bonded to each other and to the substrate via Si−O−Si chemical bonds. Two complementary tests are performed to assess the mechanical robustness of the colored films. First, the colored films are put under a high flowing speed water stream, and the colored films still keep their brilliant appearance even after washing for several minutes (as shown in Figure 4a and Video S1). Next, finger-wiping and sandpaper abrasion are combined to assess the mechanical robustness of the colored films, and untreated films are used for comparison. Figure 4b,c shows the finger-wipe tests of the untreated and hollow SiO2− PDMS structurally colored films, respectively. After the fingerwipe, the untreated film is removed partially, whereas the PCs film of hollow SiO2−PDMS still remains without any change in optical images. Then, the sandpaper abrasion test is used to further demonstrate the enhanced mechanical robustness. Colored films on glass substrate weighing 100 g are placed face-down on sandpaper (standard sandpaper, grit no. 600 mesh) and moved along the ruler (Figure 4d). The untreated film is removed absolutely after moved for only 15 cm and the color is transformed from green to red when contacting with water. However, the PCs film of hollow SiO2−PDMS is still remaining on the glass substrate even though the sandpaper abrasion is repeated for 20 times with a total distance of 300 cm. In addition, the CA of colored films could easily reach 166° after the abrasion test for several times. This should be attributed to the combination of enhanced roughness and the infiltrated PDMS. The current test suggests that the mechanical properties of the hybrid films are strengthened dramatically. Structurally colored films with brilliant color, liquids impermeability, and excellent mechanical robustness properties are useful in painting and decoration. As seen in Figure 4a, it will visually remain unchanged even on a rainy day. Moreover, it will also maintain brilliant appearances in sandstorm weather due to the mechanical robustness (as shown in Figure 4f). Figure 5 shows three different colored films prepared by a vertical deposition method, and different colors could be easily obtained by adjusting the size of the nanospheres or shell thickness. In the future, structurally colored films will be used to fabricate any kinds of decorative pattern, such as the beautiful flower with high resolution patched from different colored films in Figure 5. Thus, the as-prepared PCs films with a lasting liquids-impermeable property and invariable colored appearance have wide applications in external environments.

Figure 5. Optical pictures of three different colored films and a blooming flower taken under an ambient environment.

completely spreading on the films. The casted PDMS can be used as high refractive index material to increase the color saturation rather than destroy the periodicity of PCs structures, and the mechanical strength is also improved significantly in the presence of PDMS. Moreover, the casted PDMS endows those colored films with a superhydrophobic property even though the colored film is abraded for 300 cm with 600 mesh sandpaper. This liquids-impermeable film is useful as a kind of practical, environmentally, friendly display material and will provide new opportunities for various practical photonic applications because of the ease of processing, high reflectivity, high controllability, high physical rigidity, and chemical stability of the polymerized resin.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b07576. Structurally colored films under a water stream of high flowing speed (AVI) Figure S1, picture of Cobbled road. Table S1, measured diameter of PS, PS@SiO2, and hollow SiO2 nanospheres and reflection peaks of PCs composed by PS@SiO2, hollow SiO2, and hollow SiO2−PDMS. Table S2, calculated np, nm, neff, Δn/neff, Δλ, and Δλ/λ of sample V (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Z.). *E-mail: [email protected] (F.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51472153, 51232008).

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CONCLUSIONS In summary, PCs films of hollow SiO2−PDMS have been fabricated by casting PDMS solution over the PCs of hollow SiO2 followed by a curing process at 150 °C for 30 min. By employing this method, stop-bands invariable films with brilliant colors have been obtained even when the liquids are

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DOI: 10.1021/acsami.6b07576 ACS Appl. Mater. Interfaces 2016, 8, 22585−22592

Research Article

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DOI: 10.1021/acsami.6b07576 ACS Appl. Mater. Interfaces 2016, 8, 22585−22592