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Structural Coloration Pigments based on Carbon Modified ZnS @SiO2 Nanospheres with Low-Angle Dependence, High Color Saturation and Enhanced Stability Fen Wang, Xin Zhang, Ying Lin, Lei Wang, and Jianfeng Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11919 • Publication Date (Web): 29 Jan 2016 Downloaded from http://pubs.acs.org on February 6, 2016

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ACS Applied Materials & Interfaces

Structural Coloration Pigments based on Carbon Modified ZnS @SiO2 Nanospheres with Low-Angle Dependence, High Color Saturation and Enhanced Stability Fen Wang*, Xin Zhang, Ying Lin, Lei Wang and Jianfeng Zhu

*School of materials science and engineering, Shaanxi University of Science & Technology, Xi’an 710021. PR China.

AUTHOR INFORMATION

Corresponding Author

(*Fen Wang) E-mail: [email protected].

KEYWORDS Keywords: ZnS@SiO2• carbon modified nanospheres• high refractive index • structural coloration pigments • carbonization •spray coating

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ABSTRACT

Vividly structurally colored pigments produced by employing materials with high indices of refraction such as ZnS, TiO2 or ZrO2 have attracted great attention recently. Generally, pigments with high refractive index based on three-dimensional ordered macroporous (3DOM) structures were prepared by a colloidal crystal template method. However, the preparation of 3DOM structures is time consuming. Moreover, it will also lead to iridescent colors. In this work, structurally colored pigments based on carbon modified core-shell nanospheres of ZnS@SiO2 were prepared by a homogeneous deposition method, followed by a modified stöber method and a calcination process. Compared with conventional method, the pigments prepared in our work shows high color saturation, enhanced stability and low angle dependent. Typical paints composed of pigments and ethanol could be spray coated on any substrates without limitation. This core-shell structural coloration pigments have potential applications for displays, colorimetric sensors and pigments. Graphical abstract


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INTRODUCTION

For conventional dyes and pigments, color is an inherent property of the material that depends on its chemical nature. The coloration mechanism of which involves absorption, birefringence, emission, and numerous others serve to absorb, separate, or emit wavelengths of light based on interactions at a molecular level.

1-3

However, chemical pigments containing organic dye molecules are easy to fade

over time or upon exposure to light.4 Non-fading pigments with low toxicity and minimal environmental impact will be widely used in technical and industrial applications.5 Structural color is arising from interference, diffraction, or scattering of light from periodic arrays composed of low and high dielectric constant. Which can reflects light according to the Bragg equation in the visible region.

4-9

However, photonic crystal structural colors with iridescence are not

desirable in some applications that required to be the same color independent of the viewing angle, such as displays, printed media, building skin, and general-use pigments. 10-16 Commonly observed structural colors could be divided into two classes: iridescent colors from periodic nano- and microstructures, as illustrated in butterfly wings and beetle scales,13,

17-21

and

non-iridescent color from amorphous photonic structures, which are short range ordered and long-range amorphous. As illustrated in feathers of many birds which were produced by quasi-random arrays of air vacuoles in the medullary keratin. 7-8, 22 Inspired by nature, when the opal or inverse opal arrangement of long range order was changed into amorphous state, iridescence would expected to be suppressed. Active research was concentrated on fabricating non-iridescent structurally colored pigments.4-5,

15, 22-28

However, it is noteworthy that

artificial APSs (amorphous photonic structures) usually appears white.29 One method for solving this problem is to enhance the color saturation by using carbon nanoparticles or iron oxide black particles with broadband absorption in the visible range.4-5, 22, 30 Zhang et al. reported non-iridescent structurally ACS Paragon Plus Environment


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colored pigments by using a cuttlefish ink as an additive.28 Light-induced saturation changes by applying a photoelectrochemical reaction of the Ag/Ag+ system was created by Ryoko Hirashima et al.

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Teshima et al. prepared monodisperse submicron-sized spheres assembled by SiO2 and Fe3O4

using a micro-flow-focusing device.30 Theoretically, the total amount of light reflected by a reflective display under ambient light condition is proportional not only to peak value but also to the bandwidth of the reflectance spectrum, practical application require both broader bandwidth and higher peak value for enhanced brightness. One important strategy to increase the total amount of reflected light is to design a system with high-refractive index.32-33 To create vividly structurally colored pigments, it is important to employing materials with high indices of refraction such as ZnS, TiO2 or ZrO2.34 In previous studies, 3DOM ZrO2 powders with brilliant structural color were synthesized using an acetate precursor and heating the colloidal crystal template/precursor composite to 450 °C in an air/nitrogen mixture prepared by Schroden et al.35 Josephson et al.,34 also prepared 3DOM ZrO2 by applying PMMA photonic crystal structures as templates, the carbon black was introduced either directly in the synthesis through the precursor of the polymeric template or by post synthesis addition. However, 3DOM structures will lead to iridescent colors. Moreover, the preparation of 3DOM structure often involves several procedures, including preparation of template, infiltration the colloidal crystal in the voids of template and removal of the template. Every step is extremely strict. Thus, the fabrication process is time consuming. In addition, black material in several nanometers randomly distributed in a colloid system was unstable, which often lead to non-uniform distribution and significant decrease in brightness.4, 22 ZnS is very attractive for applications in photonic crystal devices operating in the visible and near IR region due to its high bulk reflective index (2.35) and wide energy gap.33 Moreover, ZnS


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nanostructure arrays has been a developing material star because of its good thermal stability and high electronic mobility.

36-37

Nano-crystal arrays with high contrast in dielectric values could also

potentially disallow the propagation of photons in all directions through the structure to form complete band-gaps.38-39 Aimed to prepare non-iridescent structurally colored pigments with high color saturation and enhanced stability for general use. Structurally colored pigments assembled by carbon modified ZnS@SiO2 nanospheres have been successfully prepared in this work. The carbon absorbers were generated from in-situ carbonization of organics, in which the amorphous carbon is uniformly distributed in each ZnS@SiO2 nanospheres. The color can be tuned simply by adjusting the size of the microspheres without changing the overall material design. A basic paint prepared by a combination of the colored powders with ethanol as solvent could be applied on several kinds of surfaces and maintained the color consistent with the powders after drying. The synthetic route is relatively inexpensive and environmentally benign, which obviate any complex fabrication step, thereby providing a facile way to prepare structurally colored pigments with low-angle dependence, high color saturation and enhanced stability.

EXPERIMENTAL SECTION

Materials. All chemicals were analytic grade without further purification. zinc nitrate hexahydrate (ZnNO3•6H2O) Thioacetamide (TAA, ACS reagent grade, 99%), poly (vinylpyrrolidine) (PVP) , Tetraethyl orthosilicate (TEOS, 98%,), ammonium hydroxide (28–30%), absolute ethanol (99%), were all purchased from Sinopharm Chemical Reagent Co., Ltd., China. Deionized water (18.2 MΩ. cm resistivity) was used in all experiments. Characterization. Transmission electron microscope (TEM) (FEI Tecnai G2 F20 S-TWIN) was used for observing the morphology and measuring the geometric parameters of the particles. The samples ACS Paragon Plus Environment


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were prepared by dipping a carbon coated copper grid into a diluted ethanol solution of particles, taking out the copper grid, and evaporating the ethanol solvent. The high-resolution electron microscopy (HRTEM) experiments were conducted using a field emission gun (FEG) JEOL 2010F microscope with a point resolution of 0.24 nm. EDS was recorded on an energy X-ray microanalysis system (JEOL B5-U92), which was attached to the HitachiS-4800 &Hiroba electron microscope. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max 2200pc diffractometer using Cu Kα radiation of wavelength λ= 0.15418 nm at 40 kV and 40 mA. The morphology of ZnS nanospheres was observed 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). Synthesis of ZnS nanospheres. The synthesis route of ZnS is based on the homogeneous nucleation from zinc nitrate (ZnNO3•6H2O) and thioacetamide (TAA) precursors in Deionized water. In a typical experiment, 0.02mol of Zn(NO3)2•6H2O and 0.02-0.15g PVP are completely dissolved in 150mL deionized water and 0.04 mol of TAA dissolved in another 50mL deionized water, separately. Under magnetic stirring, TAA solution was added into ZnS solution as soon as possible. The mixture was stirred for another 30 min before increased the reactor to 90 oC, then let the system stirred for another 5-7h. Finally, the white powders were collected and purified by centrifuging and re-dispersing first in water and then in ethanol for several times. Synthesis of ZnS@SiO2 Core-Shell nanospheres. Ammonia (0.5 mL) and deionized water (1 mL) were added to 30 ML ZnS colloidal solution (0.5 to 2 wt%) and stirred for 1 h. TEOS (0.5 to 2 g) was added as a silica coating material and the reaction was carried out for 5 h at room temperature. Then the reactions were finished and the obtained core–shell particle were collected and purified by


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centrifuging and re-dispersed first in water and then in ethanol several times and finally dispersed in ethanol. Synthesis of APSs of carbon modified ZnS@SiO2 nanospheres. The suspension of ZnS@SiO2 nanospheres with high concentration was transferred to a clean glass vial and left in an oven at 70℃ with humidity lower than 30%, after solution were all evaporated, ZnS@SiO2 with APSs was obtained. The white powders of ZnS@SiO2 were then placed in an alumina crucible and heated in a tube furnace under argon flowing at 0.6 L/min to obtain the carbon modified ZnS@SiO2 core-shell nanospheres. The samples were heated to 500 ºC at a rate of 3 ºC/min and kept at 500 ºC for 2h. Then, the samples were cooled to room temperature. RESULTS AND DISCUSSION

Monodisperse ZnS sphere was synthesized through the homogeneous nucleation from zinc nitrate hexahydrate (ZnNO3•6H2O) and thioacetamide (TAA) precursors. Fig. S1 (supporting information) displays the XRD pattern of the ZnS after calcination. The X-ray diffraction pattern in Fig. S1 indicates that the particles synthesized are the ZnS cubic sphalerite phase. The three strong peaks with 2θ values of 28.62, 47.84 and 56.63° correspond to the three crystal plane of (111), (220) and (311) of zinc blende ZnS, respectively. No other characteristic peaks of impurities were observed. ZnS naospheres of 250±5 nm, 295±5 nm, and 325±5 nm in size with a low polydispersity were synthesized, respectively. When the reaction was finished, little PVP still residual in each ZnS nanosphere. SiO2 shells were prepared by a modified Stöber method, involves the slow hydrolysis and condensation of TEOS. The thickness of SiO2 shell was adjusted by the concentration of TEOS precursor. In this work, PVP was used both as a structure-directing agent and carbon resources. Formation procedure of ZnS, ZnS@SiO2 and carbon modified ZnS@SiO2 core-shell particles was ACS Paragon Plus Environment


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shown in scheme 1.

SCHEME1. (a) Schematic illustration of the procedures to synthesize carbon modified ZnS@SiO2 APSs pigments with non-iridescent and enhanced color saturation. (b) Preparation of angle-independent structurally colored films via a spray coating process.

Morphology of ZnS and ZnS@SiO2 nanospheres. ZnS nanospheres display a uniform morphology in this work (Figure S2, supporting information). Scanning electron microscope (SEM) image shows that ZnS nanospheres are monodisperse and homogeneous. In addition, the as-prepared ZnS nanospheres can endure not only the ultrasonic and centrifugation treatments but also the calcination process for several times, indicating that ZnS nanospheres are stable enough to keep a ball feature for a long time. Fig.1a, 1d displays the TEM images of ZnS nanospheres. The ZnS prepared in our experiment are tightly and closely aggregated by large amounts of fairly uniform ZnS grains. Individual ZnS nanospheres like a chrysanthemum and each grain like petals grown up from the center pistil. Fig. 1b-1c shows the TEM images of ZnS @SiO2 nanospheres with shell thickness about 10 nm and 30 nm. The ZnS colloids are uniformly coated with silica layer in ethanol by using ammonia as a catalyst through slow hydrolysis of tetraethyl orthosilicate (TEOS). Surface coating appeared to be almost ACS Paragon Plus Environment

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100% under current synthesis method (shown in Figure 1c). According to previous studies, it is difficult to coat homogeneous SiO2 layer on pure ZnS directly. However, the SEM image proved in this work obviously shows that a homogeneous SiO2 layer is generated. The reason is the structure-directing agent of PVP residual on ZnS nanospheres functioned as a good anchoring material for the deposition of silica.33 SEM images of freshly prepared ZnS nanospheres clearly suggest that PVP still residue on ZnS nanospheres after washed with water and ethanol, and the content of PVP decrease as washing time increased. (Fig. S3 supporting information). In this case, silica sols could easily nucleate on the surface of each ZnS nanoparticles and eventually merge and grow into a thin shell characterized by uniform thickness.

FIGURE 1. The TEM images of ZnS and ZnS@SiO2 nanospheres. 1a, 1d) ZnS nanospheres. D = 320 nm; 1b, 1e) ZnS@SiO2 nanospheres with shell thickness about 10 nm; 1c, 1f) ZnS@SiO2 nanospheres with shell thickness about 30 nm. (1a-1c) observed under high expansion transmission electron microscopy.

Amorphous photonic structures of ZnS@SiO2 nanospheres. SEM images in Fig. 2 shows that the colloidal system do not appear any trace to crystallize. The inset in Fig. 2 shows the 2D Fourier Transform of the SEM image with APSs. A ring-like feature of the Fourier components reveals that the structure has a well-defined short-range order. These results



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clearly demonstrate that the whole structure is quite homogeneous, and the aggregation is amorphous and isotropic. Angle-independent structural color is originated from amorphous structures. In this study, amorphous structures based on ZnS nanospheres have been acquired under an appropriate temperature and humidity environment. when the ZnS suspension was drying at a 70℃ oven with a low humidity of 30%, amorphous ZnS powder in white appearance was acquired finally. When the temperature was set at 70℃, the kinetic energy of the colloidal particles was increased, and the effect of Brownian Motion have to be taken into account in considering the characteristics of a colloidal system prepared at high temperature. The high kinetic energy of colloids at high temperature will yield colloidal crystals with poor quality.

40

The low humidity could also function as decreasing secondary capillary force by

missing of fine tuning the relative position between the neighboring particles and resulted in increased defects. In addition, ZnS with a high density was easier to sediment (e.g., 4.09 g•cm−3 for ZnS, compared with 1.0 and 2.0 g•cm−3 for polystyrene (PS) and silica (SiO2), respectively). Thus, under a higher drying temperature and low humidity, ZnS aggregation with amorphous structures could be easily acquired without any elaborate refining process. In this way, experimental procedure could be greatly simplified.

FIGURE 2. High magnification SEM image of a secondary aggregated ZnS@SiO2 powder. Inset is the 2D Fourier transform of the SEM image.


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Characterization of carbon modified ZnS @SiO2 structurally colored pigments. Fig. 3 shows the microscope images of APSs samples. By using the 250 nm ZnS nanospheres as a core, carbon modified ZnS@SiO2 core-shell nanoshperes of 270 nm, 320 nm and 365 nm display green, yellow and red color, respectively. And uniform color in even very small aggregates still can be seen. APSs with this distinct property are important for some applications, e.g., the microscopic color display. Previous study shows that the color saturation of artificial APSs usually has a low color visibility. As expected, the as-prepared fresh ZnS@SiO2 powders also have a white appearance in our study (shown in Fig. S4, supporting information). After calcination at 500℃for 2 h under argon protection, the powder pigments with enhanced color saturation can be observed without adding any black materials. To manifest this is indeed a structural color rather than a chemical reaction that take place in the calcination process, the digital images of green pigments in reflection and transmission mode are brought out, respectively. The optical images taken in reflection mode show a green appearance, while under transmission mode this pigments is red/brown instead, which is complementary to the reflective color of green (shown in Fig. S5).

FIGURE 3. Microscopic optical images of dried carbon modified ZnS @SiO2 powder after calcinated at 500 ºC for 2



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h under argon protection with different diameter. a)270 nm, b)320 nm, c)365 nm with same ZnS core in 250 nm.

Fig. 4 shows the absorption spectral of carbon that originated from the thermal decomposition of PVP at 500 oC in argon. The average absorption is greater than 90% in the visible range, especially in 270 nm, the absorption is reached to 94 %.

Figure 4. Optical properties of the carbon black originate from PVP.

The space distribution of absorb species. Fig.5 shows the space distribution of Zn, S, C, Si and O. The intense Zn and S signals homogeneously distributed throughout the core confirms the homogeneity of ZnS, and the signals of Si and O spread homogeneous in the shell position indicates that the SiO2 is successfully coated on each individual ZnS core. Moreover, the distribution of C is similar to the ZnS@SiO2 core-shell spheres, indicating the uniform of carbon black.


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FIGURE5. Element mapping images of the carbon modified ZnS@SiO2 nanospheres after calcination at 500 ºC for 2 h under argon protection. The scale bar is 200 nm.

Theoretically, both the wavelength-selective single coherent scattering and the multiple scatters of light throughout the entire visible region contribute to the optical property of APSs. When black particle with high absorption in visible region are incorporated into the colloid system, the multiple scattering of light is considerably reduced, whereas the intensity of reflection peak caused by the single scattering is only slightly decreased. Consequently, the saturation of the structural color is significantly enhanced and non-iridescent structural colors could be observed with naked eyes. In this study, SiO2 shell is transparent, and the carbon originates from the carbonization of PVP function as a black substrate just absorbs the incoherent scattering light and no contribution to the structural colors. In addition, the content of carbon absorber species falls within an ideal range that enhances the coloration without over-darkening. Absorb materials uniformly existed in each ZnS@SiO2 nanospheres greatly improve the stability of this colloidal system and result in a uniform color appearance. To manifest the stability, a dispersion of the green pigment in gray appearance is acquired by ultrasonic process. When solvent is evaporated completely, bright green is appeared again. (Shown in Fig. S6, supporting information). Optical properties of the structurally colored pigments The measured diameter of ZnS and ZnS@SiO2 nanospheres and their corresponding reflection peaks position are summarized as shown in Table S1. As the thickness of SiO2 shell increased, the color is shifted to a longer wavelength, and the shift trend agrees well with others.



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FiGURE6. Reflection peak position of ZnS pigments in different shell thickness and the corresponding size parameter, πr/λ. pigments assembled by 270 nm, 320 nm and 365 nm ZnS@SiO2 particles with the same ZnS core of 250 nm.

In accordance with ordered structures, the PBG of the amorphous photonic structures is also related to the distance between pores, the filling fraction of the material and the refractive index of the material. And it is inferred that for the amorphous structures, the center position of the reflection peak position will be at 41 D/λ∝1/neff

(1)

Here, neff is the effective refractive index of this amorphous photonic structure of carbon modified ZnS@SiO2, D refers to the distance between spheres. The effective refractive index of core-shell nanospheres could be expressed as below:

(

2 neff =  nSi O2 + nZn S − nSi O2 

) ( r / R ) 

3 2

2 f + nair (1 − f )

(2)

where nair is the refractive indice of the air, f is the filling factor by spheres, r and R represent the radius of ZnS and ZnS@SiO2, respectively. For ZnS@SiO2 core-shell nanospheres, effective refractive index is increasing as the shell thickness improved, and yield enhancements in the value of size parameter. The size parameter (π*r/λ) is performed by redrawing the reflection peak to represent


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the ratio of particle radius to light wavelength, eliminate the particle size effects by normalizing the sizes. Fig. 6 shows that the size parameters indeed improve as the shell thickness increase. All those evidences suggest that our coating process has been successful conducted. For the red sample, there are two characteristic peaks at 470 and 620 nm in reflection spectra that contribute to the structural colors, and present color mixing of purple-red due to the dual visible reflections. Reflection peak in 470 might derives from the Mie scattering of individual particles.

FIGURE 7. Photographs of three different low-angle dependent structural coloration pigments assembled by 270 nm, 320 nm and 365 nm ZnS@SiO2 particles with the same ZnS core of 250 nm. The angle between incident light and direction of view is controlled to be 0°, 30°, and 60°. Scale bars: 1 cm.

As a demonstration of the angle independence, three artificial structural coloration pigments namely, green, yellow, and red are used, which are prepared by calcination ZnS@SiO2 colloidal particles of 270 nm, 320 nm and 365 nm in argon atmosphere, respectively. Obviously, the structural coloration pigments of carbon modified ZnS@SiO2 show a low degree of angle-dependence (as shown in Figure 7). Characterization of angle-independent structurally colored coatings ACS Paragon Plus Environment


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To demonstrate this structurally colored powder pigments could be easily applied on rigid substrates without limitation. Typical paints combine pigment with ethanol was utilized to prepare colored films by using a spray coating method. The synthesized structurally colored pigments were dispersed into ethanol, followed by ultra-sonication for 1 h to prepare the spray coating solutions, keeping the concentration of NPs at 10 wt%. And then the NP suspension was loaded into an airbrush with a nozzle size of 0.2 mm. Argon was employed as the operating gas under a pressure of 50 kPa, and the distance between the airbrush and the substrate was kept at 5 cm. The airbrush or the substrate was moved in a line-by-line fashion at a speed of ~5 cm·s-1, and then dried at 50 oC. Under these conditions, the alcohol solvent rapidly volatilized, leading to deposition of the ZnS nanospheres onto the substrates and this spray coating was repeated 5 times. Fig. 8a shows examples of the structurally colored film applied on a piece of glass and black leathers with rough surfaces (Fig. S7, supporting information). It is notable that the effective colors of the films on glasses are virtually identical to those powders (Fig. 7). To quantify angle-independence of the sprayed films, angle-resolved optical properties of the coatings were characterized. The reflection spectra of the green films as shown in Fig. 8b-c, it can be seen that a distinct broad peak appeared at around 520 nm, and the peak appeares at the same position when the incidental angle is varied from 0o to 45o, indicating that the structurally colored films are angle-independence.


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FIGURE 8. (a) Photographs of the colored films spray coated from structurally colored ZnS solution under two viewing angles. Particle diameters: 270 nm, 320 nm and 365 nm carbon modified ZnS@SiO2 particles with the same ZnS core of 250 nm in size. (b)Reflection spectra of the green coating at different incident angles, θ, from 0°to 45°. (c) The relationship between the peak wavelength in reflection spectra and the incidental angle.

The calcination temperature plays an important role in the finally color saturation. Fig. 9a shows the films obtained by varying the calcination temperature. The color saturation is found to decrease with the increased calcination temperature due to the decrease of carbon content at higher calcination temperatures. EDX results acquired from Field emission scanning electron microscopy (FE-SEM, HitachiS-4800 &Hiroba EDX electron microscopy) are used to determine the content of carbon in ZnS particles. The samples are prepared by dropping a diluted water solution of ZnS pigments onto aluminum foil, and evaporating the solvent. Carbon content in different calcination temperature is summarized in the Table S2. It is found that the carbon content is decreased with the increasing of heating temperature due to the decomposition of carbon. Fig. 9b-d shows the SEM images of ZnS nanospheres abtained in different calcination temperature. With the increase of calcination temperatures, ZnS nanospheres deformed gradually. When exposed to 700 °C, all of the ZnS ACS Paragon Plus Environment


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nanospheres were damaged and could hardly keep a spherical morphology. Thus, both the morphology of ZnS and carbon content play important roles on the final color appearance.

Fig. 9 (a) Green colored films obtained by varying the calcination temperature. (b-d) the SEM images of ZnS nanospheres under different calcination temperature: b) 500 oC; c) 600 oC; d) 700 oC.

The structurally colored films with photonic crystals arrays were also been synthesized by using 250 nm ZnS nanospheres as a core, and ZnS@SiO2 core-shell nanoshperes of 270 nm, 320 nm and 365 nm as building blocks. The homogeneous ZnS particles were self-assembled into crystalline colloidal arrays by using a vertical deposition method in ethanol solution. And the corresponding photonic crystal films display green, yellow and red color, respectively (shown in Fig. S8a). Fig. S 8b-c displays the SEM images of sections from glass substrate. The typical image suggests that the ordered packing is characterized by the hexagonal close-packed crystalline arrays with many vacancies and defects. Theoretically, hexagonal close-packed crystalline array is the most closed package style and volume ratio of the microspheres is 0.74, which is larger than APSs. However, According to the Bragg Equation, the volume fraction of ZnS nanospheres will lower than 0.74 in considering that there are many vacancies and defects existed in films. Thus, there is little difference between the finally color appearance for ZnS of different packaging style in this system. CONCLUSIONS ACS Paragon Plus Environment

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In summary, angle-independent pigments composed by carbon modified ZnS@SiO2 nanospheres with enhanced color saturation and uniform carbon distribution were prepared in our study. And amorphous state could be easily acquired in a drying temperature of 70 oC and humidity lower than 30%. The color saturation of this APSs could be greatly improved due to the absorb material originated from the carbonization of remained organics (PVP). The stability and durability of this system are also significantly enhanced because carbon uniformly distributes in each nanospheres. Moreover, the color appearance of this powder pigments does not change along with the viewing angle. Non-iridescence was confirmed by direct observation method. Furthermore, the obtained structural coloration pigments could be coated on any substrates without the limitation of black background. These artificial colors might have potential applications in paints, cosmetics, textile, and displays. Supporting Information

XRD pattern of the as-synthesized ZnS sample; SEM images of monodisperse ZnS nanospheres; digital images of As-prepared ZnS@SiO2 amorphous structures; Emulsion of green structural coloration pigments; The measured diameter of ZnS and ZnS@SiO2 nanospheres and their corresponding reflection peaks position. Funding Sources

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



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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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