Fabrication and Characterization of Angle-Independent Structurally

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Fabrication and Characterization of Angle-independent Structurally Colored Films based on CdS@SiO2 Nanospheres Fen Wang, Yu Xue, Bo Lu, Hongjie Luo, and Jianfeng Zhu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04193 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 8, 2019

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Fabrication and Characterization of Angle-independent Structurally Colored Films based on CdS@SiO2 Nanospheres

Fen Wang *1, Yu Xue1, Bo Lu1, Hongjie Luo2, Jianfeng Zhu1

1

School of materials science and engineering, Shaanxi University of Science & Technology,

Xi’an 710021. PR China. 2

School of Materials Science and Engineering, Shanghai University, Shanghai 200444, PR China

*Corresponding author Fen Wang, E-mail: [email protected]. Tel.: +8615114805183.

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ABSTRACT Chemical pigments produced from organic chemicals are easy to disappear when exposure to light over time. Recently, structurally colored pigments produced by materials with high indices of refraction such as TiO2 or ZnS have attracted great attention. This study presents that CdS@SiO2 coreshell nanospheres were synthesized through a homogeneous deposition method followed with a modified Stӧber method and a calcination process. Colored film assembled by pigments shows low angle dependence with high stability against degradation under environmental factors. Moreover, the structural color of CdS@SiO2 arrays was bright and tunable according to the size without changing the overall material design. Compared with conventional method, the addition of black substances in colloidal spheres is the general-used method to realize angle independent structural coloration. However, black materials (such as carbon blacks and acetylene black) are not stable because of the high surface energy, and usually reunit together easily, then lead to nonuniform distribution and significant decrease in brightness. So, we report self-assembly colored films with great low angle dependent but not any black substances. Moreover, the refractive index of CdS is higher than generally used PS, PMMA and SiO2, and SiO2 shell is poisonless. CdS@SiO2 structurally colored films have promising non-bleaching pigments and have potential applications for displays, colorimetric sensors, colorful decoration and pigments. KEYWORDS Keywords: CdS@SiO2, Colors, Amorphous colloidal structures, Non-iridescent, High refractive index, Self-assembly.


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INTRODUCTION As for conventional dyes and pigments, color depends on its chemical nature. However, these organic chemical dye molecules are easy to fade over time or upon exposure to light.1 At the same time, chemical stain may cause resource damage and environmental pollution while bringing us beautiful enjoyment of sense. For example, printing and dyeing technology causes dangerous byproducts and waste of water resources, and is creasing huge volumes of chemical-laced waters that risks contaminating food chains if left untreated. 2-4 The structural color presents the color dependent on the size and arrangement of nanostructure through the physical processes such as absorption, birefringence, emission, and numerous other mechanisms,5-7 which does not depend on any chemical dye. Antifading pigments with minimal environmental impact will be widely used in technical and industrial applications.8 Gorgeous structural colors exist in the beautiful nature and our life widely.9-11 On the basis of human vision, the structural colors can be divided into two classes: iridescence and noniridescence.12,13 The iridescent structural color is typically angle dependent from periodic nano- and microstructures, although its color appears considerably brighter than those of dyes and pigments, which could limit its applications in displays, optical devices and sensors, where require broad viewing angles.14-16 Non-iridescence comes from amorphous colloidal nanoparticles, which are short-range ordered but long-range amorphous. This structure endows the materials with angle-independent color, while successfully avoids the problems of chemical bleaching which widely exists in dye and pigments. Varieties of non-iridescence structural colors materials have been fabricated by different colloidal nanoparticles, such as polystyrene, poly(methyl methacrylate) and silica. However, the amorphous


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system usually appears low color visibility and remains a hindrance to practical applications when it is thick because of the incoherent multiple scattering.17 Interestingly, researchers have found the color saturation of these aggregates can be controlled by the incorporation of a small amount of black substances, such as carbon black (CB), acetylene black (AC), magnetite and polydopamine (PDA).1719

However, black substances are not stable for the high surface energy which can usually reunit

together easily, then lead to nonuniform distribution and significant decrease in brightness. To design a system with high-refractive index, increasing the total amount of reflected light is accessible. CdS, an important compound semiconductor with wide energy gap, has a great application prospect in the optical and electrical field. So, it is very attractive for applications in photonic crystal devices operating in the visible and near IR region.9 Moreover, the refractive index of CdS (2.51) is higher than generally used PS (1.59), PMMA (1.49), SiO2 (1.457) and ZnS (2.35).9, 20-22 Recently, TiO2 and ZnO with high indices of refraction have also attracted great attention in vividly structurally colored materials. However, superhydrophilicity of titanium dioxide limits it applications due to the decreased refractive index of films once they are wet, then the colors may disappear.10,21 In order to enhanced color brightness, not only broader bandwidth but higher peak value is important. In this work, we report selfassembly and low angle dependent colored films through CdS@SiO2 microspheres without any black substance. Core-shell NPs is different from those single materials and becoming one of the new building blocks for photonic materials.23 In order to obtain multi-functional and hybrid colloidal NPs, core-shell NPs can provide a simple way to incorporate different materials into the same structure. As a widely used coating material, SiO2 is a primary component of soil and it is very cheap. Moreover, SiO2 has many advantages: First, SiO2 is hydrophilic and negatively charged, which can prevent the aggregation


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of the colloidal particles; second, SiO2 shell is nontoxic and it is one of the best candidates for fabricating the environment friendly materials. The colored films are tunable according to different SiO2 shell thickness, which are easy to control. With the help of SiO2 shell, the core-shell structural coloration films enhanced stability with ethanol as solvent could be applied on different kinds of surface after self-assembly. Generally, self-assembly three-dimensional ordered macroporous (3DOM) structures with high indices of refraction were prepared by a colloidal crystal template method.24,25 Nevertheless, a stable, precise, vibration free and thermostatic environmental system is required for the 3DOM structures. Generally, there are several procedures to prepare 3DOM structures (preparation of template, infiltration the colloidal crystal into the voids of template, removal of the template).26, 27 Delicate and strict process is not suitable for a wide range of structural color. Moreover, it will also lead to iridescent colors. Here, due to the high gravity of CdS@SiO2, the structure provides short-range order and longrange disorder and looks like the “defect” photonic structures through self-assembly process during the interaction among spheres and the evaporation of ink. In recent years, APSs (amorphous photonic structures) have received considerable attentions due to their short-range order and long-range disorder structures, resulted in many interesting optical properties such as unusual light scattering, transportation, and localization.28-32 Aimed to prepare noniridescent structurally colored films with enhanced stability for general use. In this paper, structurally colored films self-assembled by CdS@SiO2 nanospheres through the drying of the prepared colloidal suspension in drying oven have been successfully prepared. The synthetic route is relatively inexpensive and environmentally benign. In addition, with the increase of the CdS@SiO2 NPs’ diameter, the peak values of reflection could be varied in the range of 435–605 nm. Four primary


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RGBV (Red, Green, Bluish green, Violet) colors are produced with low angle-dependence and high brightness.33-34 The morphology of CdS@SiO2 NPs and the CdS@SiO2 films is investigated using SEM and TEM technique. The tuneable behaviour of structurally colored CdS@SiO2 films is discussed. APSs of CdS@SiO2 with angle-independence will enrich the application of core-shell structure in many fields such as bionic colors, displays, colorful decoration and photonic papers. EXPERIMENTAL SECTION Materials. The chemical regents used throughout the experiments included the following. Cadmium nitrate tetrahydrate (Cd(NO3) 2• 4H2O), thiourea (TU), poly(vinylpyrrolidine) (PVP) and Diethylene glycol (DEG) were all purchased from Tianli Chemical Reagent Co., Ltd., China; Ammonium hydroxide (28–30 %),Tetraethyl orthosilicate (TEOS, 98 %), ethanol (EtOH, 99.9 %) were obtained from Sinopharm Chemical Reagent Co., Ltd of China.; and deionized water (18.2 MΩ. cm resistivity) was used in all experiments. All the materials were used as received without any further purification. Synthesis of CdS nanosphere. CdS nanospheres synthesized through the homogeneous nucleation from Cd(NO3)2•4H2O and TU precursors using PVP as a structure directing agent. In a typical experiment, of Cd(NO3)2•4H2O (0.0225 mol) and TU (2 g) were completely dissolved in DEG (80 mL), PVP (0.4 g) was dissolved in another DEG (60 mL) and the solution was heated to 190 ℃. Then 60 μL of Cd and 60 μL of TU previous solution were injected into the round bottom flask quickly while the temperature of the reactor kept to 190 ℃ and the reaction lasted for 6 min. After rapidly cooled to 5 ℃ with stirring maintained, the solution was centrifuged at 8000 r/min for 25 min, and the clear and yellow transparent supernatant was left. In the growth reaction, the supernatant was reheated


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to 165 ℃ and kept for 1 h under magnetic stirring with the addition of Cd(NO3)2•4H2O (1.00 g) and TU (0.23 g), then cooling down to environment temperature naturally. Finally, yellow powder of CdS was purified by centrifuging and re-dispersing first in water and then in ethanol. Synthesis of CdS@SiO2 Core-Shell nanospheres. Deionized water (1 mL) and ammonia (0.5 mL) were added to 30 mL CdS colloidal solution (0.75 to 3 wt%) and stirred for 1 h. TEOS (0.75 to 3 g) was added as a silica coating material and the reaction was carried out for 5 h at room temperature. After that, the reactions were purified by centrifuging and re-dispersed in ethanol several times. Finally, the obtained core–shell particles were dispersed in ethanol. Self-assembly process for angle-independent colored films. 3D APSs films of CdS@SiO2 spheres were fabricated by using a simple vertical deposition method. The yellow suspension was transferred into a clean glass beaker and left in 50°C air blast drying oven to obtain a CdS@SiO2 powder in yellow appearance. And then, the powder pigments were then calcined at 500 °C for 2 h in argon. The samples were heated to 500 °C at a rate of 2 °C/min and kept at 500 °C for 2h. In a typical process, CdS@SiO2 powder was dispersed in 10 mL of ethanol by ultrasonic for 30 min at RT to prepare the self-assembly solutions. The concentration of NPs was kept at 10 wt%. Then, those dispersions were poured into a 25 mL beaker, and glass substrates were carefully placed into the beaker. The whole process was carried out at 50 °C for 20 h. Then, the samples were cooled to room temperature. Glass slides were cleaned by treatment with acetone, deionized water, and ethanol under ultrasonic conditions and dried for use. Characterization. The self-assembly APSs films on a glass were imaged using scanning electron microscopy (SEM) (Hitachi S4800) and Transmission electron microscopy (TEM) (FEI Tecnai G2


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F20 S-TWIN). EDS was recorded on an energy X-ray microanalysis system (JEOL B5-U92), which was attached to the JEOL 2010F electron microscope. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max 2200 pc diffractometer using Cu Kα radiation of wavelength λ = 0.15418 nm at 40 kV and 40 mA. The angle-independent reflection spectra performed using a Cary 5000 UV-visNIR spectrometer (Agilen). Digital optical camera (Canon 200 D) was used to take photographs. The porosity was determined by N2 adsorption measurement (BET Brunauer–Emmett–Teller). The diameter of CdS@SiO2 spheres were determined by a statistical method. For example, the mean diameter of spheres was obtained by statistical diameter (Nano Measure) about 100 spheres from the SEM images. RESULTS AND DISCUSSION In this work, amorphous photonic structurally colored films composed of homogeneous CdS@SiO2 nanospheres have been fabricated, which can produce with high brightness and low angledependent. Different colors could be obtained by adjusting the particle size. Because of the interaction among spheres and the high gravity of CdS@SiO2, the structure provides short-range order and longrange disorder after self-assembly.35-36 In this way, CdS APSs display non-iridescent structural colors. Formation procedures of CdS@SiO2 colored films are shown in scheme1.


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Scheme 1 (a). Schematic illustration of the procedures to synthesize CdS@SiO2 nanoparticles; (b) the angle-independent colored film via a self-assembly process. Characterization of CdS and CdS@SiO2 nanospheres. Formation procedure of CdS and CdS@SiO2 core-shell particles was shown in Scheme 1. CdS nanospheres of 229 ± 10 nm in size were synthesized through the homogeneous nucleation from Cadmium nitrate tetrahydrate and thiourea precursors. Figure S1 displayed the XRD pattern of CdS nanospheres and confirms that pure CdS could be obtained under the current synthetic route after calcination. Obviously seen from the red curve, there were apparent diffraction peaks at the 2θ values of 26.6, 44.3 and 52.4 corresponding to the (111), (220), and (311) crystal planes of the cubic crystal, respectively, which matched well with the reported JCPDS data (JCPDS Card No. 65-2887). CdS@SiO2 nanospheres of 249 ± 10 nm, 289 ± 10 nm, and 350 ± 10 nm in size were also synthesized, respectively. In this work, SiO2 shells were prepared by a


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modified Stöber method, involves the slow hydrolysis and condensation of TEOS. The thickness of SiO2 shell was adjusted by the concentration of TEOS precursor.

Figure1 (a). CdS observed under high-resolution transmission electron microscopic. SEM images of (b) CdS and (c) CdS@SiO2 NPs after calcination at 500 ℃ for 2 h under argon protection, D = 289 ± 10 nm. (d-f) TEM images of CdS@SiO2 NPs, and the shell thickness is about (d) 10 nm, (e) 30 nm, (f) 60 nm, respectively. (g) EDS mappings of O, Si, S and Cd in CdS amorphous colloidal structures after calcination. It could be seen that the particles presented uniform spherical morphology and the diameter of microsphere was about 229 nm (Figure1b). The surfaces of both CdS and CdS@SiO2 microspheres


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(Figure1c) were rough, which attributed to growing of the CdS crystalline particle or amorphous silica gradually. Scanning electron microscope (SEM) images in Figure1c and Figure S2 indicate that the as-prepared CdS@SiO2 nanospheres with perfect spherical morphologies are monodisperse and homogeneous with clear boundaries. Homogeneity of the nanospheres is important to produce structural colors. The SEM and TEM images in Figure1b-f indicate that CdS and CdS@SiO2 still keep a spherical structure after calcination, and these nanospheres were stable enough to endure not only the ultrasonic and centrifugation treatment for several times, but also the calcination process. The border of the microspheres was observed in contrast with its inside, which illustrated that the microspheres were typically coated by a lot of amorphous silica (in Figure1d-f). The high-resolution transmission electron microscopy (HR-TEM) images of the microsphere particles (Figure 2a) provided phase information on the particles. They exhibited crystalline structures and well-ordered lattice fringes. The interplanar spacing was calculated to be about 0.35 nm, belonging to the (002) interplanar spacing of the CdS phase. EDS element mapping of CdS@SiO2 NPs in Figure1e was utilized to show the space distribution of O, Si, S and Cd elements. The intense S and Cd signals throughout the microspheres suggest that the CdS@SiO2 is homogeneous. Characterization of structurally colored films. To reveal the arrangement and distribution of the particles, we performed SEM measurements of the APSs films. Figure 2a and b show SEM images of red film self-assembled by CdS@SiO2 nanoparticles (diameter, 350±10 nm). It’s clearly, SEM images of large-scale arrays of CdS@SiO2 showed that the top views of the films were short-range ordered and long-range disordered structures with length scales as large as nearly 15 μm. On the surface, the structure is homogenous and the dispersed CdS@SiO2 solution obtained were approximately 10 wt% as previously mentioned. Compared with PS and SiO2 NPs, monodispersed


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CdS@SiO2 nanoparticles were too heavy to assemble into highly ordered 3D hexagonal close-packed structures in a large scale (in Table S1), which created amorphous colloidal structures eventually. For suspensions, the Stokes settlement law predicts the settling velocity of small spheres in fluid, either air or water using eqn (1): 𝜈=

2𝑟2(𝜌𝑠 ― 𝜌)𝑔 9𝜂

(1)

where r is radius of microsphere; η is hydrodynamic viscosity; ρs and ρ are density of CdS@SiO2 microsphere and the fluid, respectively. Results showed that settling rate is proportional to the value of density of microsphere. The relative settling rate of PS NPs is 1. The relative settling rate of SiO2 NPs is more than 5 times as fast as the PS NPs. As for CdS NPs, the relative settling rate is 15 times as fast as the pure PS NPs. When the solvent volatilizes, the capillary force is not enough to resist the gravity of CdS@SiO2 NPs (showed in Scheme 1b). Therefore, when the assembly temperature rises to 50 ℃ and the concentration is 10 wt%, ethanol volatilizes faster and will provide stronger upward force.

Figure 2 SEM images of self-assembly CdS@SiO2 nanoparticle film, diameter: 350±10 nm. The insets in (a) and (b) are 2D-FFT image obtained from lower SEM images and the corresponding optical picture of colored film, respectively. (c) cross-sectional view. The insert in Figure 2a shows the two-dimensional Fourier Transform (2D-FFT) of the SEM


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image with a ring-like feature, indicating the structure has a well-defined short-range order and the aggregation is typically amorphous and isotropic. SEM images show the top view of the red CdS@SiO2 films, where some typical hexagonal arrangement of CdS@SiO2 NPs represents the (1 1 1) plane of the face-centered-cubic (FCC) lattice.37, 38 Due to short-ranged ordered arrangement of CdS@SiO2 NPs, a particular non-iridescent color can be observed from the surface of the thin film. Corresponding digital photograph of the film can be seen in the upper right corner of Figure 2b. We can assert that the angle independence of structural colors was therefore attributed to the quasiamorphous NPs assembly. Moreover, the cross-sectional SEM image of this red film was given in Figure 2c, which displays the thickness of amorphous CdS@SiO2 film is about 3.5 μm.

Figure 3 (a) Optical image of beetle A. graafi.32 (b) Microscopic image of a greenish white stripe


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under 500 × magnification.32 (c) Close-up SEM cross-section image of the interior of the green scale.32 (d-g) Size distributions of CdS@SiO2 particles and corresponding insert colored film. In the past few years, the discovery of a structure in non-iridescent scales on the elytra of longhorn beetle Anoplophora graafi (Figure 3a) has caused attention.32 The green stripes are composed of differently colored scales with distinct non-iridescent color (Figure 3b). Interestingly, The SEM crosssection image (Figure 3c) shows that scales has uneven surfaces. Herein, we initially fabricated CdS@SiO2 NPs organized in a way of APSs. Figure 2 also demonstrated the distribution of amorphous CdS@SiO2 spheres in our study. The size distribution of CdS@SiO2 nanospheres measured via SEM follow a Gaussian distribution is relatively narrow and with more than 85 % in the size range of ± 60 nm. And the fabricated red, green, bluish green and violet surfaces have distributions with maxima centered at 350 ± 10 nm, 289 ± 10 nm, 249 ± 10 nm and 229 ± 10 nm, respectively. Optical Properties of the structurally colored films. To investigate the optical properties of asprepared films, four thin films were fabricated through self-assembly on glass substrate and reflected red (λ= 605 nm), green (λ= 520 nm), bluish green (λ= 455 nm) and violet (λ=435 nm) light, corresponding digital photographs can be seen in Figure 4a-d, left. The typical optical pictures of CdS@SiO2 structurally colored films assembled by different sizes cover almost the whole visible range. As the size of CdS@SiO2 nanospheres decreased, the color is shifted to a shorter wavelength. In accordance with ordered structures, the reflection peak of the violet film assembled by pure CdS nanoparticles is believed to depend on the filling fraction, the distance between spheres and the refractive index of the material. First, assuming that the volume filling fraction (fCdS) of CdS nanospheres is 74 % in FCC. Then, the neff of this APSs of CdS nanospheres could be expressed with the effective medium model using eqn (2):


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𝑛𝑒𝑓𝑓 = 𝑓𝐶𝑑𝑆𝑛2𝐶𝑑𝑆 + (1 ― 𝑓𝐶𝑑𝑆)𝑛2𝑎𝑖𝑟

(2)

where nCdS = 2.51 and nair = 1. Next, it is inferred that the relationship between the reflection peak wavelength and incidence angle θ of an ordered structure is according to the Bragg law (eqn (3)): λ = 2D(𝑛2𝑒𝑓𝑓 ― 𝑐𝑜𝑠2 𝜃)

12

(3)

where λ is the wavelength of the reflected light and θ is the Bragg angle of incidence of the light. Here, D refers to the distance between spheres. By comparison, it is found that there is a significant mismatch between the practical and theoretical values of reflection peak of the violet film (diameter: 229±10 nm). As for pure CdS spheres, the neff is 2.28, which is less than nCdS (2.51). The measured value is less than the theoretical value, which could be resolved by taking the porous structure into account. Because CdS microspheres are polycrystalline aggregates formed by the agglomeration of single crystals, and the structure contains air holes. The refractive index of the holes is partially replaced by water or other materials which have the similar refractive index with air. This case from low density in CdS nanospheres lead to the decrease refractive index of CdS microspheres. According to the Bragg law, the reflection peaks at 435 nm are less than the theoretical value. Examples of such aggregate materials with lower refractive index have also been reported in the papers, such as TiO2, ZnO and ZnS polycrystalline microspheres.39-41 Furthermore, the porosity of CdS nanosphere was determined by N2 adsorption measurement. The BET (Brunauer–Emmett–Teller) surface area is nearly 30.2 m2/g. Using the BJH(Barret–Joyner–Halenda) method (showed in Figure S3), the calculated pore-size distribution indicates that the material contains an average pore size of 13.2 nm, indicating CdS nanospheres composed by large amounts of uniform CdS grains, which is coincident with the SEM image (shown


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in Fig. 1b).

Figure 4 Optical images and reflection spectra of red film, green film, bluish green film and violet film with diameters of (a) 350 ± 10 nm, (b) 289 ± 10 nm, (c) 249 ± 10 nm and (d) 229 ± 10 nm. In this work, SiO2 shell is transparent, and the measured diameter of CdS and CdS@SiO2 nanospheres and their corresponding reflection peaks position are summarized as shown in Table S2.


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Figure 5 showed that when the thickness of SiO2 shell increased, the color is shifted to a longer wavelength.

Figure 5 Reflection peak position of CdS films in different shell thickness with the same CdS core of 229 ± 10 nm and the corresponding size parameter, π*r/λ. Colored films assembled by 249 ± 10 nm, 289 ± 10 nm and 350 ± 10 nm CdS@SiO2 NPs. For amorphous colloidal structures, the center position of the reflection peak position is also explained by the equation expressed below in accordance with ordered structures: 42 (4)

𝐷/𝜆 ∝ 1/𝑛𝑒𝑓𝑓 Different from the pure CdS NPs, the effective refractive index of core-shell nanospheres could be expressed as below: 𝑛𝑒𝑓𝑓 =

[𝑛𝑆𝑖𝑂

3 2

2

+ (𝑛𝐶𝑑𝑆 ― 𝑛𝑆𝑖𝑂2)(𝑟 𝑅)

]

+ (1 ― 𝑓𝐶𝑑𝑆)𝑛2𝑎𝑖𝑟

(5)

where r and R represent the radius of CdS and CdS@SiO2, respectively. Figure 5 displays that


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for CdS@SiO2 core-shell nanospheres, as the shell thickness improved, the size parameters is indeed increasing. The size parameter (π*r/λ) is performed by redrawing the reflection peak to represent the ratio of particle radius to light wavelength, in order to eliminate the particle size effects by normalizing the sizes. All the evidences suggest that the coating process has been conducted successfully. Characterization of the angle-independent APSs of CdS@SiO2 nanospheres. Apart from the distinguished optical properties to colored film of as-prepared CdS@SiO2 amorphous colloidal structures, angle-independence is another merit in favour of functional applications. Photographs of the red, green, bluish green and violet colored pieces were collected from the bottom of the beaker assembled by calcination the CdS@SiO2 colloidal particles of 350 ± 10 nm, 289 ± 10, 249 ± 10 nm, and 229 ± 10 nm, respectively. The bright shining color could be seen in Figure 6a, where all of the optical images are taken under the same measurement condition. Figure S4 shows the microscope images of these APSs samples. The size of the small pieces was also measured. It shows that uniform color still can be seen in very small aggregates: several dozens of microns. The oblique views presented in 30°and 60° confirm the angle-independent structural coloration of the surfaces. To further verify the property of angle-independence, angle-resolved reflectance spectra of red film surface assembled from CdS@SiO2 nanoparticles (350 ± 10 nm) have been measured (Figure 6b and c). When the incidence angle was increased from 0° to 45°, the intensity of reflection increased (shown in Figure 6b), while virtually peak positions of the reflectance spectra is equivalent, which suggest that there is nearly none shift of colors when the CdS@SiO2 amorphous structures are observed at different angles. Of course, some performance still needs to be improved in future studies for their application in harsh environment. If the pieces of colored film were damaged by external forces and the amorphous structure of CdS@SiO2 nanoparticles was damaged, the color will be weaken to some extent.


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Figure 6 (a) Photographs of the small pieces of colored film pieces self-assembled from CdS@SiO2 nanoparticles solution under three viewing angles. Particle diameters: 350 ± 10 nm (red), 289 ± 10 (green), 249 ± 10 nm (bluish green), and 229 ± 10 nm (violet). (b) Reflectance spectra of the red films at different incident angles, θ, from 0 ° to 45 °. (c) Peak wavelengths in (b) as a function of incident angles. CONCLUSION In summary, we have fabricated CdS and CdS@SiO2 amorphous photonic structurally colored film by a simple fabrication method, which could demonstrate brilliant structural colors but noniridescence without any black substances. Furthermore, amorphous state of CdS@SiO2 aggregates could be easily obtained in a drying temperature of 50 ℃. The low-angle dependence of colored films


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was confirmed by direct observation and angle resolved reflection spectra method. Moreover, these colored films are good candidates as new light-stable pigments that can be prepared by scalable processes without the limitation of black background. We believe that this structurally colored materials have promising non-bleaching pigments and have great potential applications in displays, colorimetric sensors, colorful decoration and pigments. ACKNOWLEDGEMENTS All authors have given approval to the final version of the manuscript. This work was supported by the National Natural Science Foundation of China (51472153, 51232008). Supporting Information Available: The contents of Supporting Information may include the following: (1)The XRD patterns of CdS nanospheres after calcination , (2) SEM of CdS@SiO2 spheres with the same CdS core of 229 ± 10 nm: a) 350nm ±10; b) 249nm ±10 , (3) Barrett–Joyner–Halenda (BJH) pore size distribution plot (inset) and Nitrogen adsorption/desorption isotherm of the CdS spheres , (4) Microscopic optical images of red, green, bluish green and violet colored pieces assembled by CdS@SiO2 colloidal particles , (5) Calculated relative settling rate of different colloid materials , (6) The measured diameter of CdS and CdS@SiO2 nanospheres and their corresponding reflection peaks position.


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Table of Contents (TOC)


Fabrication and Characterization of Angle-Independent Structurally

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