Iridescent colloidal crystal coatings with variable structural colors were fabricated by incorporating carbon black nanoparticles (CB-NPs) into the voids of polystyrene (PS) colloidal crystals. The structural color of the colloid crystal coatings was not only greatly enhanced after the composition but also varied with observation angles. By changing the diameter of monodisperse PS colloids in the composites, colloidal crystal coatings with three primary colors for additive or subtractive combination were obtained. After incorporation of the PS/CB-NPs hybrid coatings into polydimethylsiloxane (PDMS) matrix, manmade opal jewelry with variable iridescent colors was made facilely.
© 2013 OSA
The beautiful colors of natural opals are not produced from pigments but from the ordered packing of small particles [1–4]. The diffraction and scattering of light by the nanostructures of this species tell us nature got wise to photonic coatings long ago. The colors generated by the nanostructures are called structural colors, which are important not only in nature but also for the understanding of optical band gaps of photonic crystals [5–7]. Brilliant structural colors produced by the interaction of light with periodic nanostructures in the living world have aroused scientific interest for a long time [8–10]. In natural opals and manmade colloidal crystals, the periodic packing of colloids causes Bragg diffraction of visible lights, and thus generates structural colors [11–13].
Monodisperse polystyrene (PS) colloids or latex particles have been widely used to fabricate colloidal crystals or artificial opals [14–16]. The PS colloids in colloidal crystals usually adopt a face centered cubic (fcc) packing with ~26% voids in the structures [17,18]. However, despite having a general milky white looking, the structural colors of PS colloidal crystals are usually faint, which greatly limits their application in making brilliant photonic coatings.
Recently, it was reported that black carbon materials could be used to enhance the structural color of manmade opals. For example, Yamada et al. fabricated artificial black opals from nanoporous carbon spheres, and clearly observed brilliant structural colors . Aguirre et al. prepared colloidal crystal pigments by mixing the white powders of poly(methyl methacrylate) (PMMA) manmade opals with carbon nanoparticles . In this work, carbon black nanoparticles (CB-NPs) were infiltrated into the voids of PS colloidal crystals to fabricate photonic coatings with improved structural colors. After the composition, the structural color of the coatings was not only greatly enhanced but also varied with observation angles and PS particle diameters. The mechanism of this phenomenon and the application of the coatings in making manmade opal jewelry were studied and discussed preliminarily.
2. Materials and methods
Styrene (St, 99%), methyl methacrylate (MMA, 99%), ammonium persulfate ((NH4)2S2O8, 98%), and ammonium bicarbonate (NH4HCO3, analytical pure) were purchased from Tianjin Chemical Company. 3-Sulfopropyl methacrylate potassium salt (SPMAP, 98%) was purchased from Shanghai Jingchun Chemical Company. CB-NPs (N330, technical grade) were purchased from Dongying Carbon Black Company. PDMS (sylgard 184) was purchased as a two-component kit that contained the vinyl-terminated base and curing agent from Dow Corning. PDMS prepolymer was prepared by mixing the PDMS base and the curing agent in a 10:1 (w/w) ratio. St and MMA were distilled under vacuum before use. The other reagents were used as received.
2.2 Synthesis of monodisperse PS colloids
Monodisperse PS colloids were synthesized with soap-free emulsion polymerization according to the literature with some modifications . The typical procedure can be described as follows: 14.1 ml of St, 0.78 ml of MMA, 100 ml of deionized (DI) water were added into a three-necked flask under magnetic stirring. When the flask was heated to 70 °C, the first batch of mixture consisting of 0.73 g of (NH4)2S2O8, 0.51 g of NH4HCO3, 0.12~0.04 g of SPMAP, and 10 ml of DI water was added into the flask. Polymerization was carried out at 70 °C under nitrogen protection for 4 h, and then the second batch of mixture consisting of 2.81 ml of St, 0.16 ml of MMA, 0.50 g of SPMAP, 0.10 g NH4HCO3, and 10 ml of DI water was added into the flask. Polymerization continued for an additional 6 h to obtain white latex. After polymerization, the PS particles in the latex were separated by centrifugation at 12000 rpm and washed with DI water for several times. Finally, the purified products were dried at 25 °C for 48 h. Monodisperse PS colloids with diameters of 190~310 nm were obtained.
2.3 Fabrication of PS/CB-NPs hybrid colloidal crystal coatings
PS/CB-NPs hybrid colloidal crystal coatings were fabricated by centrifuging the mixture of 0.02 g of CB-NPs and 0.12 g of PS latex particles in 70 ml DI water at 12000 rpm for 30 min. After centrifugation, the coating products were separated from water and dried at 25 °C for 48 h. The dried coating flakes were immersed into PDMS prepolymer in a rounded plastic mold and cured at 60 °C for 48 h to make the manmade opal jewelry.
A scanning electron microscope (SEM, JSM-800, Japan) was used to observe the morphology of PS colloids, CB-NPs and colloidal crystal coatings. To reduce charging effects, samples were sputter-coated with a thin layer of gold (~5 nm) prior to the SEM exam. The structural color of the colloidal crystal coatings was characterized using a UV-Vis reflection spectrometer (Ocean Optics USB-2000, USA) equipped with a 150 W haloid lamp cold light (Lamp-house YN XD-301, China). The UV-Vis spectra were collected under perpendicular irradiation of the cold light.
3. Results and discussion
The size and distribution of the synthesized PS colloids and purchased CB-NPs visualized by SEM are shown in Fig. 1. We can see that the three PS colloids with diameters of 190 nm, 217 nm and 308 nm are almost monodisperse [Figs. 1(a)–1(c)]. The CB-NPs have an average diameter of 33 nm and tend to aggregation [Fig. 1(d)].
Figure 2 shows the photos of PS colloidal crystal coatings before and after compositing with CB-NPs. The pure PS colloidal crystal coatings of different diameter colloids are all milky white with very faint structural colors [Figs. 2(a)–2(c)]. By introducing CB-NPs into the lattice structure, the visual appearance of the colloidal crystal coatings changes quite remarkably from milky white to intense green, red, and blue, respectively [Figs. 2(d)–2(f)]. These pictures are taken under natural lighting conditions and show that the weak and highly-orientation-dependent Bragg diffraction gives way to a new color mechanism which should originate from resonant scattering of light on the embedded CB-NPs as reported elsewhere . In other words, when the photons pass through colloidal crystals, one part of them will be reflected and the other will be transmitted. But in the PS/CB-NPs hybrid colloidal crystals, the CB-NPs play as point scatterers, which will stop the transmission of photons by scattering them resonantly, so the intensity of reflection light is greatly increased, and the hybrid coatings show brilliant structural colors.
SEM image of the 217 nm PS colloidal crystal coatings formed in the absence of CB-NPs is shown in Fig. 3(a). From the sectional image, we can see that the monodisperse PS colloids adopt a hexagonal array in the crystal plane. Figure 3(b) shows the SEM sectional image of the hybrid colloidal crystal coatings formed from 217 nm PS colloids in the presence of CB-NPs. We can see that the CB-NPs are almost fully filled into the interspaces of the colloidal crystal.
The maximum band gaps of the hybrid colloidal crystal coatings can be calculated according to the Bragg law shown in Eq. (1) , where λ is the wavelength of the band gap in the (111) direction, k is an arbitrary integer coefficient, D is the diameter of colloids, n is the refractive index of the colloidal crystal, and θ is the angle of incidence.
The refractive index (n) of the colloidal crystal can be calculated according to Eq. (2) , where Φ represents the void ratio of the colloidal crystal (~26%), nps and ncb represent the refractive indices of the PS colloids (~1.59) and CB-NPs (~2.42).
Figure 4 shows the UV-Vis reflection spectra of the structural colors in the pure PS colloidal crystal coatings and PS/CB-NPs hybrid colloidal crystal coatings measured at normal incidence. Compared Fig. 4(a) with 4(b), we can see that the band gaps of pure PS colloidal crystal coatings shift after incorporation of CB-NPS. The band gaps of 190 nm, 217 nm and 308 nm PS/CB-NPs hybrid colloidal crystal coatings are calculated at 560 nm, 639 nm and 453 nm according to Eqs. (1) and (2) at normal incidence, which are very close to the measured value (550 nm, 631 nm and 459 nm) shown in Fig. 4(b). This indicates that the brilliant structural colors coming from Bragg diffraction are sensitive to the diameters of PS colloids.
Figure 5 shows the structural color variation of the 217 nm PS/CB-NPs hybrid colloidal crystal coatings observed from different view angles. Watched from 30°, 45° and 60°, the same coatings show brilliant green, yellow and red colors, respectively. The view angle is the complement of the incident angle θ.
Figure 6 shows the UV-Vis reflection spectra of the 217 nm PS/CB-NPs hybrid colloidal crystal coatings collected from different view angles. According to Eqs. (1) and (2), the band gaps of the 217 nm hybrid coatings at view angles of 30°, 45° and 60° are calculated at 560 nm, 588 nm and 614 nm, respectively, which are very close to the measured values (551 nm, 592 nm and 616 nm) shown in Fig. 6. This indicates that the brilliant structural colors coming from Bragg diffraction are also sensitive to the view angles. Compared with the colloidal crystal pigments with low angle dependence made by mixing the PMMA colloidal crystal powders with carbon nanoparticles , our hybrid coatings are more brilliant and angle sensitive because large areas of ordered colloidal crystal structures are retained instead of breaking them to powders.
Figure 7 shows that not only the three primary colors for additive or subtractive combination can be achieved using the PS/CB-NPs hybrid colloidal crystal coatings, but also the iridescent derivative colors can be achieved by either altering the diameters of PS colloids or changing the view angles of the coatings. Therefore, the hybrid colloidal crystal coatings have great potentials to show not only panchromatic colors but also holographic colors .
The iridescent colloidal crystal coatings can be used to make manmade opal jewelry facilely. As shown in Fig. 8, after embedding the 217 nm PS/CB-NPs hybrid colloidal crystal coating flakes into PDMS matrix, manmade opal jewelry with variable structural colors are fabricated [Figs. 8(a)–8(c)]. Compared with natural opal jewelry [Figs. 8(d) and 8(e)] made from deposition of silica particles in the long time formation of earth, the color variation of our facilely manmade opal jewelry is more obvious due to having the CB-NPs inside. Additionally, the natural opals are made from inorganic minerals which are inelastic and breakable, while our manmade opal jewelry is made from organic materials and protected by soft PDMS matrix which is elastic and flexible. The research about how to make vivid artificial opals with better qualities by this approach is currently under investigation.
Iridescent PS/CB-NPs hybrid colloidal crystal coatings with variable structural colors are fabricated successfully by infiltration CB-NPs into the voids of PS colloidal crystals. Due to the resonant scattering of light on the embedded CB-NPs, the visual appearance of the colloidal crystal coatings changes quite remarkably from faint milky white to brilliant colors. The brilliant structural colors coming from Bragg diffraction are sensitive to the diameters of PS colloids and view angles. Using the PS/CB-NPs hybrid colloidal crystal coatings, not only the three primary colors for additive and subtractive combination but also the iridescent derivative colors can be achieved by either altering the diameters of PS colloids or changing the view angles. After incorporation of the PS/CB-NPs hybrid coatings into PDMS matrix, manmade opal jewelry with variable iridescent structural colors is made facilely. Compared with the natural opal jewelry, the color variation of the manmade opal jewelry is more obvious due to having the CB-NPs inside.
This work is financially supported by the National Key Basic Research Development Program of China (973 special preliminary study plan, Grant 2012CB722705), the Natural Science Foundation of China (Grant 21004035, 21005042), the Fok Ying Tong Education Foundation of Ministry of Education of China (Grant 131045), Science and Technology Program of Qingdao (1314159jch), the Natural Science Foundation of Shandong Province (Grant ZR2010BQ020), and the Promotive Research Fund for Excellent Young and Middle-aged Scientists of Shandong Province (Grant BS2010CL014).
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