Preparation of Non-Iridescent Structurally Colored PS@TiO2 and Air

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Preparation of Non-Iridescent Structurally Colored PS@TiO2 and Air@C@TiO2 Core-Shell Nanoparticles with Enhanced Color Stability Yu Xue, Fen Wang, Hongjie Luo, and Jianfeng Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12060 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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ACS Applied Materials & Interfaces

Preparation of Non-Iridescent Structurally Colored PS@TiO2 and Air@C@TiO2 Core-Shell Nanoparticles with Enhanced Color Stability

Yu Xue †, Fen Wang †,*, Hongjie Luo‡, Jianfeng Zhu† † School of Materials Science and Engineering, Shaanxi University of Science &

Technology, Xi'an, 710021, PR China School of Materials Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi’an, 710021, People’s Republic of China.

‡ School of Materials Science and Engineering, Shanghai University, Shanghai

200444, PR China

*Corresponding author. Tel.: +8615114805183. E-mail address: [email protected] (F. Wang).

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Abstract： Natural amorphous photonic crystals benefit from reflectance at selective wavelengths in some specific existing natural systems. Non-iridescence from natural organisms has also attracted great interest for various examples in bionic colors, pigments and paintings. Here, Air@C@TiO2 sphere was obtained by the first calcination of PS@TiO2 core-shell nanoparticles (NPs) in nitrogen to make sure the integrity of the shell structure and followed a low-temperature calcination to obtain appropriate color saturation. We demonstrate that compared with prepared colored PS@TiO2/CB pigments, angle independent hollow Air@C@TiO2 nanoparticles own an enhanced color stability under the action of in situ synthesized carbon black (CB). Our results suggest that these Air@C@TiO2 spheres are easy to change the color by adjusting sphere structure size, which have the potential to show visual signaling. Keywords: carbonization, Air@C@TiO2, secondary calcination, non-iridescence, structural color


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TABLE OF CONTENTS (TOC) GRAPHIC：



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Introduction Color production in nature is quite accomplished by means of structural system. While, chemical dyes that are exposed to natural light for a long time fade easily. Generally, pigments and dyes contain organic dye molecules will not only result in color fading but pollute the environment.

1−2

Structural colors are widespread in

myriads of natural living organisms measuring from tens to hundreds of nanometers, involve materials are colorless themselves but gain a competitive edge.

3,4

The

coloration mechanism of different micro-nanostructures involves refraction, absorption, scattering and many other ways serve to the physical interaction between light and intrinsic nanostructures.5−8 According to the results from visual recognition of human, there are two classes of structural colors: iridescent colors with periodic permutation structures, as illustrated in hummingbird feathers, butterfly wings and indigo snake.

9−12

Despite autonomous

regular permutation being widespread among natural, synthetic iridescent structural color materials require strict, complex, and delicate process. Therefore, iridescent colors suffer from some inherent limitations for their applications in quantity. Noniridescent colors with short-range ordered but long-range disordered microstructure from amorphous photonic structures (APSs) with extraordinary optical properties, which has low angle dependence or even angle independence.13 Therefore, APSs, with a single fixed color for structural color materials, will be of value in the development of printed media, displays, and general-use pigments. 14−17 Inspired by the nature, active researches

have

been

concentrated

particularly


on

fabricating

elaborately

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nanostructured APS materials to enable artificial manufacture. 18−21 Nevertheless, it is worth noting that artificial APS powders materials tend to appear their natural color due to the incoherent multiple scattering. 22 To solve this problem, researchers have found an effective means to enhance the degree of saturation to prepare color productions: the addition of small black materials with broadband absorption in the visible range, such as magnetite, carbon black (CB), and metal microparticles. 23−27 Generally, Micro-nano level black materials distributed in APS systems randomly are unstable and could reunit together easily for the high surface energy, and the nonuniform distribution could lead to significant decrease in brightness. 28−29 Compared with the color of pigmentation and dyes, structurally colors are more visually and vibrant, 30−32 which have been synthesized by using numerous polymeric, inorganic, or some hybrid components.

33−35

Being environmentally friendly and

typically biocompatible, TiO2 also did not show evident cell toxicity and is an overall safe material.36 Anatase TiO2, an abundant, readily available, and nontoxic material,37−38 also has a remarkable application in many fields like solar cells, sensors, and optical devices. Also, TiO2 catches the eye for its wide energy gap and high bulk reflective index (2.52), which is higher than generally used SiO2 (1.457), PMMA (1.49), PS (1.59), and ZnS (2.35) in structurally colored materials.

39−42

To fabricate tunable

structural colors, polymeric organic matters are also widely used in getting structurally colored materials because of the controllability and low-cost.

43

In most of the cases

studied so far, a series of methods have been developed. An anodization method was used by Wang et al. 44 to fabricate colored TiO2/Ti films. The particle size of TiO2 has



a significant influence on the variations of color, and TiO2 particles should be distributed uniformly on the anodized Ti foil. Thus, the substrate of these films is limited. Ma et al.

45

fabricated all-nanoparticle P(MMA-AA)/TiO2 with tunable

structural colors using a spin-assisted LbL assembly method. Luo et al. 46 also reported that size adjustable core–shell SiO2@TiO2 nanoparticles were synthesized using one step approach. And the system needs applied electric fields to show adjustable structure color changes. In previous studies, the LbL and self-assembly process are both complex and time-consuming. In addition, compared with artificially prepared APSs with the addition of small black materials, color distribution from in-situ synthesized carbon black is uniform, and color retention is lasting for a longer time. Therefore, our aim was to acquire angle independent colored pigments with enhanced color stability by rapid spraying and painting for general use. We demonstrate that typical non-iridescent structurally colored materials composed of Air@C@TiO2 core-shell nanoparticles could be sprayed on different substrates. It is also worth noting that some advantages of using hollow spheres are characteristic. First, the nanostructures composed of hollow melanin particles also have effect on the structural color in organisms. 47−49 Second, some potential applications on intelligent materials based on hollow C@TiO2 nanoparticles are worth exploring. Because structural color based on hollow spheres are easy affected by external stimuli, tuning their color effectively, such as special light irradiation (UV, or laser), 50 pH, 51 and energy (or vapor) absorption. 52,53 Here, we describe structural colored materials composed of PS@TiO2 and


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Air@C@TiO2 nanoparticles arranged in an amorphous structure over a wide visible range. The in-situ synthesis of carbon, produced during sintering process, is dispersed in each Air@C@TiO2 nanoparticles uniformly. We also find these randomly scattered spheres with in-situ synthesized carbon black enhanced color visibility and color stability. We use SEM, TEM, optical and reflection spectra microscopy to reveal the morphology and describe the theoretical optical properties. Experimental section Chemicals and materials. Styrene (St) and methyacrylic acid (MAA) were obtained from Sinopharm Chemical Reagent Co., Ltd of China. Both of them were purified by aqueous NaOH (5 wt%) for three times and distillation, and then stored in refrigerator. Titanium butoxide (TBOT), Potassium persulfate (KPS), Acetic acid (99 %), ethanol, sulphuric acid (98 %) and hydrogen peroxide (H2O2) (28−30%) were acquired from Tianli Chemical Reagent Co., Ltd., China. Deionized water was purified using a water purification system with resistivity higher than 18 Mohm·cm used in all experiments. Other reagents of analytically grade were directly utilized without further purification. Preparation of monodispersed PS@TiO2 spheres. These PS@TiO2 spheres were fabricated using soap-free emulsion polymerisation and sol–gel methods. The hydrolysis reaction of TBOT in ethanol is worked to prepare PS@TiO2 spheres. Briefly, PS (0.8 g) made in our lab were redistributed in the mixed solution of acetonitrile (10 mL) and ethyl alcohol (70 mL). Ammonia (0.5 mL) was immersed into the PS dispersion for 30 min (500 r/min). At the same time, a solvent of



prescribed amount of TBOT (2 mL), acetonitrile (10 mL) and absolute alcohol (10 mL) were dropwise added into PS dispersion, and then kept stirring for 2h. In addition. The same reaction was carried out for 5, 10, and 15 h. Finally, the PS@TiO2 core-shell nanoparticles were fabricated. Fabrication of Air@C@TiO2 spheres. APSs of hollow Air@C@TiO2 shells were obtained by calcining PS@TiO2 nanoparticles for twice. In detail, the calcination process was described as follow: the PS@TiO2 nanoparticles were calcined at 500 °C in a rate of 1 °C /min and then kept for 2 hours in nitrogen atmosphere. The samples cooling to room temperature, and the dark Air@C@TiO2 shells were thus obtained. Next, in air, as-prepared cupped shells were calcined at 280 °C in a rate of 1 °C /min and kept for 10 min. Last, the products were cooling in the furnace naturally. Finally, the colored dyes composed of complete spherical Air@C@TiO2 shells with appropriate saturation were obtained. PS@TiO2 spheres of different sizes were used for the calcination under the same conditions. Preparation of monodispersed PS@TiO2/CB and Air@C@TiO2 colored films. The glasses were hydrophilic treated in a mixture containing concentrated 98% H2SO4 and 30% H2O2 (H2SO4/H2O2 = 3:7, v/v) for 6 h and washed with deionised water before use. Also, the PS@TiO2/CB or Air@C@TiO2 nanoparticle solution ethyl alcohol, [3 weight % (wt %)] self-assembled on glass slides (20 × 20 mm) by a spray method at environmental temperature. An airbrush with a nozzle size of 0.2 mm was taken to fill the suspension under the operating gas (Argon, pressure: 30 kPa). The airbrush was operated in the horizontal movement at a slow speed of ∼5 cm s−1. Finally,


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when the absolute ethanol to evaporate completely, PS@TiO2 or Air@C@TiO2 coreshell colloids in a variety of particle sizes self-assembled into APS films. The concentration of carbon black is 0.35 wt% with different size of PS@TiO2 nanoparticles or Air@C@TiO2. Characterization. Reflection spectra were obtained using a UV−vis−NIR spectrometer (Agilen Cary 5000). SEM images of as-prepared particles were taken by a scanning electron microscope (Hitachi S4800). TEM images of the products were obtain from (FEI Tecnai G2 F20 S-TWIN). Microscopy images of the fabricated dyes were obtained by an optical microscope (Leica DM2500 M). Optical images were taken by a digital camera (Canon 200D). The geometric parameters of all the spheres were measured by statistical diameter about 100 spheres from the SEM images using a Nano Measurer 1.2 application. Results and discussion Using PS for the core material and the source of carbon black, TiO2 for the shell material, non-iridescent structurally colored Air@C@TiO2 particles were prepared. Xray diffraction (XRD) total pattern in Figure S1 was used to study the microstructure of the Air@C@TiO2 spheres. The curve presented in pattern showed apparent diffraction peaks at the 2θ values of 25.26, 38.16, 48.17, 54.03, 55.11 and 64.69 were indexed to the (101), (004), (200), (105), (211), and (204) planes of anatase TiO2, respectively. In a typical experiment, Air@C@TiO2 core−shell−type particles were fabricated



after calcination. Scheme 1 showed the formation procedure of PS@TiO2 and carbonmodified Air@C@TiO2 core−shell particles. First, the PS@TiO2 sphere was prepared by PS nanoparticle (with different sizes of 215, 225, and 270 nm). Evenly, the TBOT solution had adhered to the homogeneous PS spheres gradually as nanoparticles. In addition, we also prepared PS@TiO2/CB colored films by a spray method (Scheme 1a). The structure of PS@TiO2 spheres have an influence on the optical performance of the film. Under diffuse reflection conditions, the noniridescent red of film was observed according to the description of diffuse scattering mode. Last, the Air@C@TiO2 core−shell artificial nanoparticles were obtained by the repeated calcining process. (Scheme 1b).

Scheme 1. Schemes of the ACSs pigments with opportune enhanced color visibility: (a) Plus CB method, (b) In situ CB synthesis.


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Characterization of PS@TiO2 spheres and APS films. We characterized the shape, arrangement, and structure of APSs using SEM and TEM (Figure 1). The PS@TiO2 spheres in this work display a uniform morphology and high monodispersity. The formation of TiO2 shell on PS beads can withstand repeated ultrasonic treatment, centrifugal separation and calcining process, which indicate that stable PS@TiO2 spheres can keep a sphere characteristic for a long time. Figure 1a-c show the SEM images of PS@TiO2 spheres of 235 ± 5 nm, 265 ± 5 nm, and 330 ± 5 nm in size, respectively. The results clearly show that the surface of spheres is rough, which attributed to growing of the TiO2 shell gradually and is different from the pure PS sphere surface clearly (Figure S2). Structural color materials with angle-independence are originated from amorphous structures to a certain extent. Here, low-magnification SEM image of PS@TiO2/CB film in Figure 1d shows that the colloidal system appears amorphous structures. Increased kinetic energy of the nanoparticles will yield colloidal crystals with increased defects when the film was heating under 60 °C. 54 Also, example of the 2D-FTT output reveals a ring-like feature. These results demonstrate that a well-defined long-range disorder but short-range order, the aggregation is typically isotropic and the whole structure is quite homogeneous. To further exploit the core-shell structure of PS@TiO2 spheres, the TEM images were also recorded, as shown in Figure 1e. It was observed that PS@TiO2 spheres (size: 265 nm) with the shell thickness about 20 nm. In our experiment, the spheres prepared are closely aggregated by amounts of uniform TiO2 grains. As shown in Figure S3, the TEM images of core−shell particles with different sizes were also clearly visible. For



further explanation, the PS@TiO2 spheres with the core size of 235 nm and 330 nm are coated on TiO2 with thicknesses of 10 nm and 30 nm, respectively.

Figure 1. PS@TiO2 spheres: a) 235 nm ±5; b) 265 nm ±5; c) 330 nm ±5; d) Lowmagnification SEM image of PS@TiO2/CB colored film with amorphous structures. The insert in Fig. 1d: FFT spot pattern of the SEM image; e) TEM image of PS@TiO2 particles with thicknesses of 20 nm. Characterization of Air@C@TiO2 nanoparticles. The point contact among the agglomerate different particles leads to poor structural stability. We prepared uniformly distributed in-situ C by the first calcination under nitrogen protection to obtain more stable structure colors. As-prepared PS@TiO2 spheres need to be fully ground in an agate grinding bowl (Figure S4) before carried out at 500 °C in nitrogen protection and then at 280 °C in air. Energy Dispersive X-Ray Spectroscopy (EDS) of colorful materials indicates the existence of C, Ti, and O (Figure S5). Element maps (Figure 2a) are not only used to show the elements space distribution, but also confirm the existence of homogeneous


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C layer in the Air@C@TiO2 nanoparticles. After the two-step calcination in air, C was only partly remained in the TiO2 shell and function as absorb materials. The intense O and Ti signals throughout these spheres suggest that the Air@C@TiO2 is homogeneous and hollow. TEM of large scale of core-shell microspheres were also recorded based on the images of Figure 2b-c, and it is easy to find the shell structure of TiO2. The thickness of the TiO2 bulb is about 20 nm. Moreover, the distributions of these coreshell spheres are in a large scale. The high-resolution transmission electron microscopy (HR-TEM) images of these spheres (size:265 ± 5 nm, Figure 2d,e) provided phase information on the particles. The thickness of the shell is about 20 nm. Figure 2e showed fringes of well-ordered lattice and crystalline structures. The interplanar spacing was calculated to be about 0.35 nm belonging to the (101) plane of anatase TiO2.55

Figure 2. (a) Element mapping and (b-e) TEM images of the identical Air@C@TiO2 nanoparticles.



To further qualitatively analyse the morphology of the composite particles, SEM was performed by focusing the electron beam on Air@C@TiO2 core-shell particles after one-time and two-step calcining process in Figure 3a and Figure 3b, respectively. In Figure 3a, Air@C@TiO2 spheres carried out at 500 °C for 2 h in nitrogen showed completely shrinking and distorted, which can be attributable to the volume reduction of PS core structure. PS, as an organic core polymer, can be reduced to black carbon in the reductive atmosphere on high temperature. Interestingly, these spheres obtained the shape of sphere again after the two-step calcining process in air, as shown in Figure 3b. Because carbon dioxide is produced by calcining in air and blew round TiO2 shell finally. We also simply calcine the PS@TiO2 nanoparticles at 500 °C in a rate of 1 °C /min in air to prepare Air@C@TiO2 nanoparticles. However, the samples we got were almost broken hemispheric shells (in Figure S6). Compare with SEM image of Air@C@TiO2 particles after one-time calcination in nitrogen (in Figure 3a), the breach of these hemispheric shells is open but still full. The shell broken because it was not strong enough to withstand the pressure of carbon dioxide. In Figure 3a, the vacuum calcined shell of Air@C@TiO2 nanoparticles gains sufficient strength and is almost undamaged. After the first step of vacuum calcining, the microspheres are almost complete without holes. A large number of holes appeared after the second calcination, which is presumed to be caused by internal gas flushing. These pores are so small, and they do not destroy the structure colors.


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Figure 3. SEM images of Air@C@TiO2 particles after (a) one-time calcination and (b) two-step calcining process. The SEM images of core-shell Air@C@TiO2 spheres are displayed in Figure 4. In our experiment, the as-prepared pigments are the collection of countless uniform Air@C@TiO2 spheres closely. Each Air@C@TiO2 spheres is like a millet growing up on the shell structure at nanometer size, showed in Figure 4c, f, i. Using lowmagnification SEM image, we examined the aggregated colloidal Air@C@TiO2 system appear no trace to crystallize (in Figure 4a, d, g). When observed in a large area, the morphology of microspheres was uniform and monodisperse well.



Figure 4. SEM images of (a-c) core-shell Air@C@TiO2 spheres with shell thickness about 30 nm, D = 330±5 nm; (d-f) Air@C@TiO2 spheres with shell thickness about 20 nm, D = 265±5 nm; and (g-i) Air@C@TiO2 spheres with shell thickness about 10 nm, D = 235±5 nm. Characterization of Air@C@TiO2 structurally colored pigments. APSs with non-iridescent structurally color has wide range of some application, e.g., the color display and colorful decoration. Artificial APS colored pigments are usually dim for the influence of incoherent scattering and show poor structural stability. 56 As

expected, the pure PS@TiO2 powders without CB are white in the previous study

(Figure S7). In this work, the color saturation is enhanced because of the high absorption from the carbon black layer in the visible region while calcination process, so that the structural colors could be observed. Even small aggregates of Air@C@TiO2 powder can still achieve the uniform beautiful color. Figure 5a shows the microscope images of colored pigments after onestep calcinated at 500 °C under nitrogen protection and two-step calcination at 400 °C, 280 °C, 120 °C and 0 °C in air, respectively. It was observed from Figure 5aⅣ that the structural color of materials using one-time calcination was too dark to recognize, because almost all the PS core are reduced to black carbon. To solve this problem, we used a tow-step calcinating process to get the right saturation for structurally colored pigments. When the temperature was set at 280 °C, the amaranth is clearly visible to the naked eye (in Figure 5aⅡ). Figure 5b-d displays in situ carbon modified core−shell Air@C@TiO2 spheres of


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330±5, 265±5, and 235±5 nm, respectively, showing distinct colors: amaranth, green, and dark blue color. Reflection spectra of corresponding samples were also recorded, as shown in Figure 5e. It is clear to see that the stop band of hollow C@TiO2 powder exists a blue shift from amaranth to dark blue, as induced by the decrease of particle size. To better understand the role of the CB, we propose the following qualitative explanation for the iridescence in red structural pigment. In Scheme 1, partial range of wavelengths in the “particles” are reflected in many directions, leading to the uniform color of the sample pigments with angle independence. Then, the rejected color from reflected wavelength appears stronger due to the enhanced absorption of the transmitted wavelengths through the particles when the system is in contact with many dark CB shells. Schematic illustration of the light absorption based on a single core-shell particle was also recorded, as shown in Figure 5f. Angle independent hollow Air@C@TiO2 nanoparticles own an enhanced color stability, under the action of in situ synthesized carbon black (CB) layer inside a spherical shell made of a large number of white titanium dioxide nanoparticles. The photonic pseudogap of angle independent structural colors is also mainly caused by coherent scattering from the amorphous structure (Scheme 1a), while Bragg diffraction from ordered lattice arrangement always cause the iridescent color. 57−59



Figure 5. (a) Optical images of dried Air@C@TiO2 powder at different two-step calcining temperature: Ⅰ 400 °C, Ⅱ 280 °C, Ⅲ 120 °C, Ⅳ 0 °C. Microscope images of colored powder with diameters of (b) 330±5, (c) 265±5, and (d) 235±5 nm. (e) Reflection spectra of corresponding samples measured by UV−vis−NIR spectrometer in specular reflection mode. (f) Schematic illustration of the light absorption based on a single core-shell particle. Optical properties of different colored pigments. Table S1 displays the different measured diameters of PS@TiO2 and Air@C@TiO2 nanospheres. Also, their corresponding reflection peaks position are listed. The reflection peak positions of pigments powder assembled by PS@TiO2/CB


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in Figure 6a indicate that obvious discrepancies exist between the theoretical and experimental values. The difference could be explained for the following reasons: On the one hand, the effect of light absorption by the carbon layer is not taken into account. On the other hand, the variation of the distance between spheres needs to be considered, which becomes the defects of the structural arrays instead of the ordered packing in photonic crystals. In this way, the volume of the sphere is replaced by something else partly, such as air, solvent and other gas, which leads to a decrease in the effective refractive index ( ＜ 0.74). In addition, with the increase of nanoparticle sizes, the discrepancies become larger. Reflectance spectra of the samples (Figure 6b) show that the characteristic peak shifts from 502 nm to 730 nm as the PS@TiO2 particle size increases. Similarly, with the increasing Air@C@TiO2 particle size, the peak wavelength shifted to the red from 450 nm to 673 nm. In this case, the large spectral differences between the samples assembled by PS@TiO2 nanoparticles and Air@C@TiO2 nanoparticles were less than 100 nm (Δλpeak = 52 nm, 47 nm, 57 nm). Also, it was observed that hollow Air@C@TiO2 nanoparticles showed a lower characteristic peak value than those of core-shell PS@TiO2 nanoparticles at a 0° incidence angle due to the decrease of effective refractive indices (neff) after calcination. The neff of the core−shell particles could be expressed as below: 59 𝑟 3

[ ( ) ]+𝑛

𝑛𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒 = 𝑛𝑠ℎ𝑒𝑙𝑙 1 ―

𝑅

𝑐𝑜𝑟𝑒(𝑟/𝑅)

3

(1)

where r and R are the radius of core and shell structure, respectively. The nshell is the refractive index of TiO2 shell layer (native melanin: 2.52), and ncore represents the



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refractive index of PS (native melanin: 1.59) for PS@TiO2 particles or air (native melanin: 1.0) for Air@C@TiO2 particles. The center position of the reflection peak position of amorphous colloidal structures is also explained by the equation expressed below: 𝐷/𝜆 ∝ 1/𝑛𝑒𝑓𝑓

(2)

Then, the characteristic reflection peak positions can be approximately expressed as the Bragg-Snell equation, that is 60 8

𝜆2 = 3𝐷2(𝑛𝑎𝑣𝑒𝑟𝑎𝑔𝑒2 ― 𝑐𝑜𝑠2𝜃)

(3)

Here, D presents the distance between the nearest spheres, λ is the wavelength of the light, and θ is the angle between the diffraction crystal planes and the incident light. The value of naverage also can be calculated as a weighted sum of the refractive indices of particles and the gap portion. 61 The results showed that the refractive indices of the core materials play a role in coloration of the artificial particles.

Figure 6. (a) The experimental and theoretical reflection peak positions of pigments powder assembled by PS@TiO2 nanoparticles. (b) All experimental reflective peak positions of PS@TiO2 and Air@C@TiO2 powders, respectively. Exp represent the experimental.


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Characterization of coatings assembled by PS@TiO2/CB and Air@C@TiO2 spheres with angle-independence. Structural coloration materials assembled by Air@C@TiO2 particles with the sizes of 230±5 nm, 265±5 nm and 330±5 nm (Figure 7a) and PS@TiO2/CB particles (Figure 7b) are present. The color hardly changes as the viewer’s perspective changes and a further reflectance spectrum test was performed, as shown in Figure 7c. When the angle of incidence varies from 0° to 60°, reflection peaks positions of dark blue colloidal powder prepared by Air@C@TiO2 particles barely shift. Figure 7d displays the slight blue shift of the peak positions of the sample (Δλ＜10 nm), which was restricted to a limited range, indicating good angle-independent property. Not only core-shell Air@C@TiO2 colored pigments (SEM images in Figure 4g), but PS@TiO2/CB colored films are amorphous (SEM image of PS@TiO2/CB film was shown in Figure S8). In this case, irregular morphology changes in the colored samples were prevented. Therefore, the Air@C@TiO2 colored powders had single main characteristic reflection peak and displayed vivid structural colors. To prove the color stability further, we did comparative experiments to compare with Figure 7a. The experimental method was consistent with the preparation of monodispersed Air@C@TiO2 colored films. The sufficient ultrasonic time must be given (> 1 h) to prepare PS@TiO2/CB ethanol dispersion solution. Otherwise, the agglomeration particles of carbon black will be retained, and the black or white spots will appear in the sprayed film, which lead the uneven color distribution, as shown in the figure 7b (ultrasonic time: 30 min). Enlarge figure is given in Figure S9a-b. Even if



the ultrasonic dispersion of uniform solution is fully given, the color could also be white and black in places when the mixed solution was placed for a long time after the ethanol volatilized, as shown in the Figure S9c. But the color of dry Air@C@TiO2 assembled powder could last a long time. To this day, the pigments have been placed for a year, and no fading occurred. Therefore, benefit from the in-situ synthesized CB, the presence of melanin in the inner TiO2 shell makes itself to be evenly distributed and protected. Our work therefore provides a way to improve color stability of the angle independent colored pigments.

Figure 7. Photographs of the colored coatings assembled by (a) Air@C@TiO2 and (b) PS@TiO2/CB nanoparticles (solution without sufficient ultrasonic time); (c) Reflection spectra of dark blue powder in a, θ, from 0 to 60°; (d) The relationship between the peak wavelength and the direction of view. Conclusion


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In summary, nonfading artificial amorphous structures based on core−shell PS@TiO2 and hollow Air@C@TiO2 particles with low environmental impact were prepared. Instead of adding ex-situ carbon black, we use two-step calcination to acquire the enhanced color stability. Moreover, the color saturation of APSs composed by Air@C@TiO2 microspheres is significantly enhanced with the carbon layer. As Our results suggest that as-prepared pigments were blue shifted from those of the core−shell particles because of the refractive index changing. These examples indicated that hollow APSs are highly capable of a wide variety of applications, such as paintings, printed media, and colorful decoration.



Supporting Information Available: The contents of Supporting Information may include the following: (1) XRD patterns of Air@C@TiO2 microspheres after calcinations in air, (2) SEM image of PS spheres, (3) TEM images of PS@TiO2 nanospheres under high-expansion transmission electron microscopy , (4) Microscopic optical image and SEM image of dark blue sample assembled by Air@C@TiO2 colloidal particles without fully ground before calcination, (5) EDS of colorful powders assembled by Air@C@TiO2 nanoparticles , (6) SEM images of only calcined PS@TiO2 nanoparticles at 500 °C in a rate of 1 °C /min in air, (7) Optical image of as-prepared PS@TiO2 powder with a white appearance , (8) SEM images of PS@TiO2 nanoparticles/CB film. Two-dimensional Fourier Transform of the scanning electron microscope image (Insert in Fig. S8), (9) Enlarged optical images of uneven colored coatings assembled by PS@TiO2/CB nanoparticles and dry powder block assembled by PS@TiO2/CB nanoparticles after the ethanol volatilized naturally, (10) Diameter of PS@TiO2 and hollow TiO2 spheres and their corresponding reflection peaks position. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Fen Wang: 0000-0003-1035-2205 Notes


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Preparation of Non-Iridescent Structurally Colored PS@TiO2 and Air

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