Angle-Independent Structurally Colored PS@TiO2 Film with Excellent

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Angle-Independent Structurally Colored PS@TiO2 Film with Excellent Underwater Superoleophobicity in Harsh Environments Yu Xue, Fen Wang, Yi Qin, Bo Lu, Lei Wang, and Jianfeng Zhu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04194 • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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For Table of Contents Only

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Angle-Independent Structurally Colored PS@TiO2 Film with Excellent Underwater Superoleophobicity in Harsh Environments Yu Xue, Fen Wang*, Yi Qin, Bo Lu, Lei Wang, 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. *Corresponding author Fen Wang, E-mail: [email protected]. Tel.: +8615114805183.

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ABSTRACT

Chemical pigments are damaging to the environment, however, no-toxic substance (including TiO2, SiO2) often appears white because of the incoherent multiple scattering. The addition of small black substances can enhance the structural color of the amorphous system. Herein, colored amorphous photonic crystals (APCs) with angle-independence were fabricated by TiO2-coated polystyrene core-shell nanoparticles (PS@TiO2 NPs) via a healthy and time-saving sprayed method. Compared with generally used collide materials, such as PS and SiO2 NPs with smooth surfaces, the rough TiO2 NPs shell structure has significant advances for the underwater oilrepellent property. Thanks to the TiO2 NPs adhered to PS spheres, the multi-scale roughness was enhanced and the PS@TiO2/CB film was endowed with excellent underwater superoleophobicity without other extremely hydrophilic chemical composition. Furthermore, the waterborne polyurethane (WBPU) improves the robustness of the film. This film is free from the adhesion of oil in the water, which can be employed into diverse undersurface systems, and may boost the promising applications of APCs. KEYWORDS Keywords: Angle-independent, Structural color, Self-assembly, PS@TiO2, Underwater superoleophobicity, Spray coating.


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INTRODUCTION Photonic crystals (PCs) present a long-range order on an optical length scale.1-3 Recently, many studies confirmed that amorphous photonic crystals (APCs) with short-range order but longrange amorphism can also display structural color.4 Structural colors avoids the problems of chemical bleaching successfully which widely exists in dye and pigments and can be divided into two classes: iridescence and noniridescence.5 The iridescent structural color is typically angle dependent from periodic micro/nanostructures, which could limit its applications in displays and optical devices, where require broad viewing angles.6-8 Non-iridescence comes from APCs. This structure endows the materials with angle-independent color. Compare with iridescent structural color, non-iridescence is much easier to be identified underwater. To create angle-independent structural color, spray film could offer the benefits of rapid patterning, simplicity, and mass production over multifarious surfaces. The amorphous system appears almost white when it is thick, because of the incoherent multiple scattering.9 Researchers have found a way to enhance the recognizable beautiful structural colors of the APCs system: the addition of small black substances with the character of a high absorption across the entire visible region, such as carbon black (CB), polydopamine (PDA), Polypyrrole (PPy) and magnetite.10-13 Small black substances can be easily mixed into the spray film solution. In the past few decades, self-cleaning surfaces with extreme wettability are causing special interesting. Among them, underwater superoleophobic materials have been extensively studied.1417

With the processes of nature evolution, various micro/nanostructure surfaces protect some

creatures from oil pollution in aqueous environment, and display performances of anti-fouling, such as fish scales.18, 19 Brilliant structurally colored film is easily contaminated with oils. This 3 ACS Paragon Plus Environment

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disadvantage limits their applications in outdoor environment, such as oil-contaminated water. Taking inspiration from the nature is an exciting approach for designing smart and functional materials.20, 21 Generally, these underwater superoleophobic materials have two keys: surface with the high surface energy comes from micro-/nanoscale roughness, and the extremely hydrophilic chemical composition.22 Hydrophilic composition can cause layers of water molecules on the materials to reduce the contact between the oil and the material surface.15 Usually, hydrophilic chemical composition is not enough to prevent the underwater adhesion of oily pollutants. To achieve subaqueous superoleophobic surface, micro/nanostructured surfaces with high surface energy were effective.23-29 Recently, inorganic NPs have already been widely used to provide rough structures in many reports. For instance, Zhang et al. prepared a flowerlike ZnO/epoxy resin superhydrophobic film by a mixing and heating method.14 Chen et al. prepared an underwater oil-repellent film with 3Dordered structure via the complexing interaction between SiO2 and MAA.15 However, hybrid composites are prone to be abraded because of the weak interaction between NPs and substrates, which will restrict their practical application to some extent.17 Su and coworkers prepared the robust underwater superoleophobic film through MPS (methacryloxy propyl trimethoxyl silane)-SiO2/PNIPAM (N-isopropylacryamide) hybrid NPs and epoxy resin.16 It should be mentioned that the above method is time-consuming. In this article, we first report the angle-independent structurally colored films with excellent underwater superoleophobic property by spray film of monodisperse core-shell NPs (PS@TiO2 NPs)/CB and WBPU solution. Compared with single-sized NPs, TiO2 dual-scale NPs can enhance 4 ACS Paragon Plus Environment

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surface roughness to achieve extreme interfacial wettability. The as-prepared film showed a high static oil contact angle (OCA>150 °) in water. More importantly, the wetting properties show no apparent variation after friction on sand paper and immersion in corrosive solutions (pH from 1 to 13), indicating that the as-prepared film has an excellent mechanical and chemical stability. And the stop-bands could be tuned by adjusting the size of PS NPs easily. Our preparation process is more time-saving, low-cost, and the reaction conditions are more moderate. EXPERIMENTAL SECTION Materials. Styrene (St), acetonitrile, ammonia, methyl methacrylate (MMA), titanium butoxide (TBOT) and potassium peroxydisulfate (KPS) were purchased from Tianjin Kermel Chemical Reagents. Waterborne polyurethane (WBPU) were purchased from Shenzhen Jitian Chemical Reagents. Styrene was first washed with NaOH aqueous solution (5 wt %), followed by washing with deionized water until neutral. After drying overnight, styrene was stored in a refrigerator before use. Deionized water was used throughout the experiments. All the other reagents above were used as received without further purification. Synthesis of monodispersed polystyrene NPs. Monodispersed PS NPs were synthesized via an emulsifier-free emulsion polymerization method. Briefly, 90 mL deionized water, an appropriate amount of St (6-10 mL) and MMA (0.5 ml) were added to a 250 mL threenecked round-bottomed flask accompanied with mechanical stir (300r/min) and a coiled condenser. At the same time, KPS (0.5 g) dissolved in 10 mL deionized water was quickly injected into the reactor then the mixture was heated to 80 °C for 8 h. After the reaction, the PS latex was obtained. Finally, the PS NPs were collected by centrifugation and stored 5 ACS Paragon Plus Environment

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in a hot-air drying oven for the subsequent experiment. Preparation of PS@TiO2 core-shell NPs. PS (0.8 g) were redistributed in the mixed solution of absolute ethyl alcohol (75 mL) and acetonitrile (5mL). Then ultrasonicated for 30 min, and ammonia (0.5 mL) was immersed into the PS dispersion and maintained magnetic stirring (500 r/min) for 30 min. At the same time, TBOT (1 mL) dissolved in the mixed solution of absolute alcohol (15 mL) and acetonitrile (5 mL). As-prepared PS solution was added dropwise of the TBOT solution at ambient temperature and then kept stirring for 5h. Finally, the PS@TiO2 core-shell NPs were fabricated. After separated by a centrifuge and rinsed 3 times with deionized water, the PS@TiO2 NPs were ready for the spraying process. Fabrication of colored APC films with underwater superoleophobicity. CB under a certain ratio mixed with the PS@TiO2 hybrid solution was obtained. A simple spraying method was used to attain the colored APC films with underwater superoleophobicity. Prior to use, the glass slides (24 mm × 24 mm and 25 mm × 76 mm) were immersed in hydrogen peroxide for 2 days, cleaned by ethanol in an ultrasonic bath for 20 min and dried in a vacuum oven. Then, the homogeneous PS@TiO2/CB hybrid solution was sprayed onto the glass substrates with air gas. Furthermore, the self-assembly process ended by placing the glass slides at 40 °C to allow the ethanol to evaporate completely. Finally, 2.0 wt% WBPU was dissolved in deionized water and stirred magnetically for at least 1 h to obtain WBPU solution. After spraying the WBPU solution onto the colored PS@TiO2/CB films and put into a drying oven for 30 min under 60 °C, the films with underwater superoleophobicity was fabricated. 6 ACS Paragon Plus Environment

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Characterization and Spectra Measurement. The surface morphologies of the APCs were imaged by using a field emission scanning electron microscope (FESEM) (Hitachi, FE-SEM S4800) and Atomic force microscopy (AFM). AFM tests were carried out by using an atom force microscope (AFM, Agilent 5500) to observe the 3D microstructure of WBPU-treated sample surface. Transmission electron microscope (TEM) (FEI Tecnai G2 F20 STWIN) was used for observing the morphology and measuring the geometric parameters of the particles. The reflection spectra of the colored films were performed by using a Cary 5000 UV-vis-NIR spectrometer (Agilen). The static contact angle (CA) on the surfaces was measured with a contact angle meter (OCA 20, Dataphysics, Germany). Droplets of doubledistilled water (10 μL) and different oils (1-5 μL) were placed on the films. The CA was measured on five different sites for each sample. The mean value was taken as the final result. RESULTS AND DISCUSSION Morphological analysis of PS@TiO2 NPs and PS@TiO2 NPs/CB films. In this study, PS templates of 275 nm, 225 nm and 185 nm in size have been synthesized via soap-free emulsion polymerization of PS, MMA and KPS. From the sol-gel process of TBOT under constant stirring, PS@TiO2 spheres were prepared by coating TiO2 nanoparticles on those PS beads (Figure 1a). TiO2 particles provided the hierarchical structure of the coming surface material. PS@TiO2 NPs and CB (0.35 wt%) were then sprayed on the glass substrate and coated by a layer of WBPU solution. This system was cured at 60 °C to obtain the robust films. Then the energy spectrum (EDS) of PS@TiO2 NPs/CB is carried out and can be confirmed by the element contents of Ti, O and C (in Figure S1). According to the EDS element mapping of the identical spheres, element C 7 ACS Paragon Plus Environment

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was detected. SEM images (Figure 1b) show spheres have a good dispersity, which conforms the demand of assembling high quality APCs.30 It is obviously that there are large spaces between neighboring TiO2 particles, which resulted in a roughened surface and favoring the formation of underwater superoleophobic coatings after an increase in surface energy. More importantly, it can be observed obviously that the TiO2 NPs had an average diameter around 8 nm. Figure S2 shows the SEM image of PS@TiO2 NPs in 290 nm and carbon black around 50 nm are assembled into APCs under higher magnification. The carbon black nanoparticles with high surface energy merely attached to the surface of colloidal crystal grains in limited regions instead of distributed in the whole colloidal system homogeneously.31,32 Additionally, the TEM images (Figure 1c-d) manifest that lumpy TiO2 shell layers have been successfully formed around the PS spheres, which is consistent with the result of previous research. The thickness of TiO2 shell is about 15 nm.


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Figure 1. (a) Schematic illustration of the fabrication of PS@TiO2 NPs/CB amorphous structures. (b) SEM image of PS@TiO2 NPs/CB films with the nanoparticle size of 290 nm. (c), (d) TEM images of PS@TiO2 NPs. (e) The concentration of CB is: 0.1 wt%; 0.15 wt%; 0.2 wt%, 0.25 wt%; 0.3 wt%; 0.35 wt% with different size of PS@TiO2 NPs: (i)290 nm, (ii)240 nm, (iii)200 nm. Scale bars: (b) 500 μm (inset: 200 nm); (c) 100 nm; (d) 20 nm; (e) 1 cm. The measured diameter of PS, PS@TiO2 NPs and corresponding reflection peaks are summarized in Table S1. For photonic crystal, the changes in reflection peak positions are resulted 9 ACS Paragon Plus Environment

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from the variation of effective refractive index and inter-planar spacing. The Bragg law is given in Eq (1): 𝜆𝑝𝑒𝑎𝑘 = 2𝑑ℎ𝑘𝑙 𝑛2𝑒𝑓𝑓 ― sin2 𝜃

(1)

where λpeak is the wavelength of the reflection peak, dhkl is the inter-planar spacing between (h k l) planes, neff is the effective reflective index of the crystalline lattice, θ is the light incident angle, θ= 0. Generally, in the FCC structure, the inter-planar spacing of d1 1 1 is shown in Eq. (2): 12

𝑑111 = (2 3)

(2)

𝐷𝑠𝑝ℎ𝑒𝑟𝑒

The neff could be expressed with the effective medium model using eqn (2): 𝑛𝑒𝑓𝑓 = 𝑓𝑠𝑝ℎ𝑒𝑟𝑒𝑛2𝑠𝑝ℎ𝑒𝑟𝑒 + (1 ― 𝑓𝑠𝑝ℎ𝑒𝑟𝑒)𝑛2𝑎𝑖𝑟

(3)

where nsphere and nair are the refractive index of PS spheres and air, respectively, fsphere is the volume fraction of spheres, and f = 0.74 for a closed-packed FCC structure. nsphere can be calculated by using Eq. (4) 𝑉𝑃𝑆

𝑉𝑇𝑖𝑂2

𝑛𝑠𝑝ℎ𝑒𝑟𝑒 = 𝑛𝑃𝑆 𝑉 + 𝑛𝑇𝑖𝑂2

𝑉

V = VPS + VTiO2. We assume that nTiO2 = 2.52, nPS = 1.59. Thus, for PS NPs, λ𝑝𝑒𝑎𝑘 = 2.3822𝐷𝑠𝑝ℎ𝑒𝑟𝑒

And for PS@TiO2 PCs, 3

𝑛𝑃𝑆@𝑇𝑖𝑂2 = 2.52 ― 0.93(𝑟 𝑅) 10 ACS Paragon Plus Environment

(4)

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By comparison, it is found that there is a mismatch between the practical and theoretical values of reflection peak. And the difference could be resolved by taking defects of the PCs into account. Interestingly, this amorphous photonic crystals of PS@TiO2 NPs/CB film can be considered as the “defect” photonic structures. There are just some ordered structures characterized by hexagonal close-packed arrays (marked out in red line in Figure S3). Thus, the volume of the sphere is partly replaced by the air, solvent or few CB and the effective refractive index changed from 0.74. In order to gain more insight about the effect of CB among different sizes (290 nm, 240 nm, 200 nm) of PS@TiO2 granules, the abrasive powders of pristine PS@TiO2 NPs are displayed in a set of photographs (Figure 1e), which definitely show that the abrasive powders can produce diverse colors while PS@TiO2 powders is white without CB because of the light scattering. Morphology of WBPU-treated PS@TiO2/CB films. The surface morphology of the WBPUtreated PS@TiO2/CB films was investigated by SEM (Figure 2 a-b). This coating remained a multi-scale rough and highly dense surface structure, which can reduce the contact between the surface and the oil droplets. In order to further study the surface’s structure, the roughness of the surface was measured qualitatively through AFM. The 2D (Figure 2c) and 3D AFM (Figure 2d) images of the film indicate dense spherical papillae structures on the WBPU-treated PS@TiO2/CB films which are mainly caused by PS@TiO2 NPs. Due to the micro/nanometer structure, the maximum reduction in oil droplets and surface contact can be guaranteed.


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Figure 2. SEM and AFM of WBPU-treated PS@TiO2/CB films. Scale bars: (a) 10 μm and (b) 500 nm. Structural colors with angle-independent property. Generally, amorphous APCs would appear white to naked eyes, like TiO2. Incorporate black substances into colloid system is identified as an effective way to improve the color saturation of APCs. CB is harmless to the environment that has a wide-band absorption in visible region. Angle-independence is a merit in favor of pigments and functional applications. In this study, the colored films were fabricated by spray film the ethanol suspension containing PS@TiO2 NPs (10 wt%) and CB (0.35 wt%), and the APC films own wide viewing angles. By using a method of direct observation (Figure 3a), the optical images of three different APC films displayed virtually identical structural colors, which were formed by PS@TiO2 NPs/CB hybrid with the PS@TiO2 particle sizes of 290 nm, 240 nm, and 200 nm, respectively. To 12 ACS Paragon Plus Environment

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further verify the property of angle-independence, angle-resolved reflectance spectra (Figure 3bd) of APCs films have been measured. Red (λmax = 610 nm), green (λmax = 520 nm) and blue (λmax = 430 nm) colors were obtained using PS@TiO2 NPs/CB hybrid particles, clearly indicating that the structural colors could be easily controlled by altering the size of the PS@TiO2 NPs. As the size of PS@TiO2 NPs decreased, the color is shifted to a shorter wavelength. The detection angle was varied from 0° to 55° relative to the normal planar surface. Virtually equivalent peak positions of the reflectance spectra suggest that there was nearly none shift of colors when the films were observed at different angles. Plots of peak wavelength for the reflection spectra (Figure 3e) shows the peak wavelengths of samples appeared at almost the same position, which gives more confirmation to the angle-independent quality of amorphous structural colors in the light of the three approximately parallel horizontal lines.


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Figure 3. (a) Optical images of the colorful films sprayed from PS@TiO2 NPs/CB ethanol solution with different viewing angles, 0°, 45°, 90°, respectively, the scale bar is 1cm. Particle diameters: 290 nm (red), 240 nm (green), and 200 nm (blue). (b), (c), (d) Reflectance spectra of red, green and blue APC coatings at different incident angles. (e) Plots of peak wavelength for the reflection spectra as a function of incident angles. Colorful film with underwater superoleophobicity. In air, the surface of hybrid film is typically oleophilic with the OCA of 26.5 ° (Figure 4a). However, when the film is submerged in water (video S1), the surface is extremely nonwetting to methylene iodide (2 14 ACS Paragon Plus Environment

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μL) (video S2) with the OCA about 168.9 °. That is because the unique hierarchical structure of microgel spheres, and the space among spheres connected by hydrophilic polymeric WBPU, which caused layers of water molecules to reduce the connect between the oil and the material surface. The increase of water contact angles (Figure 5b-c) without and with WBPU for the surfaces indicates that WBPU has a certain hydrophilic effect. Besides methylene iodide, this colored film also exhibits excellent underwater superoleobicity against other oily liquids (Figure 4b), including n-Hexane, rap oil, liquid paraffin, 1-Bromotetradecane, 1-Bromodecane, n-Octyl bromide and 1-Bromododecane (Figure 4c). When WBPU concentration increased to 2.0 wt%, a little WBPU penetrated into the spaces between particles act as interconnectors (Figure 2a). Too much WBPU will decrease in surface roughness. Moreover, structural colored films gradually become transparent (in Figure S4) due to the refractive index of WBPU is similar with the SiO2 (1.457 at 632.8 nm).33, 34 Compared with WBPU-treated PS@TiO2/CB films, an underwater OCA of PS@TiO2 film without CB is obtained. In water, the surface of PS@TiO2 film is also typically oleophobic with the OCA of 156.1 ° (Figure 4d). It shows that the CB nanoparticles with high surface energy is favoring the underwater superoleophobic property of colored films. All above results indicated that surface roughness plays an important role in achieving extreme underwater oil-repellent property.35-37


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Figure 4. (a) Optical image of oil droplets (methylene iodide) and OCA on the WBPUtreated PS@TiO2/CB films in the air and water (methylene iodide = 3.325 g/mL at 25°C). (b), (c) Underwater OCAs of different oily liquids on the surface. (d) Underwater OCA of PS/TiO2 film. Scale bars: (a) 1 cm. Chemical stability and mechanical properties. One of the largest challenges of oilrepellent materials is the chemical stability under high ionic strength and high stability for marine application. Next, we further investigated the oil-repellent behavior of the surface film in highly acidic and alkaline environment (pH from 1 to 13). The underwater OCAs 16 ACS Paragon Plus Environment

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(Figure 5a) still maintain above 150° at acid or base solution immersion, especially under alkaline conditions. These results implied that the film can be served in a variety of harsh environment. Abrasion repellence is an important standard to evaluate the value of underwater superolephobicity. Furthermore, we evaluated the effect with a piece of sandpaper. The result showed that the surface can still hold its underwater superolephobicity (156.6 °→151.2 °) by mechanical abrasion, but increased oil adhension after abrasion. The WBPU-treated PS@TiO2/CB film was placed face-down to sandpaper and weighted 500g. After moving for 10 cm along the ruler (video S3, Figure S5b), the surface of the sample was abraded fully without WBPU, and the scratches are deeper (in Figure S5a). In consideration of the good resistance to mechanical abrasion, two main reasons can be advanced. 15,38-41 Firstly, polymers are more resistant to mechanical abrasion than inorganic substances, which protect surface from being abraded. For our films, the WBPU interlayer provides physical support to the PS@TiO2 NPs. Secondly, the presence of hierarchical structures of NPs under the initial surface can retain underwater superoleophobicity despite abrasion and the removal of substantial amounts of NPs from the film. In order to further compare the chemical strength of between PS@TiO2/CB films and WBPU-treated PS@TiO2/CB films, a water resistance test was processed collaboratively by using a water jet.2,

42

Under the impact of water flushing, the former was broken

immediately. While WBPU-treated films kept almost intact even under water blast many minutes (Figure 5e, left). On the whole, the robustness of the PS@TiO2/CB films can be enhanced by introducing WBPU. As a comparison, the film scraping (video S4) and water flushing 17 ACS Paragon Plus Environment

experiments

(video

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S5) without WBPU are also given in the red frames (Figure 5).

Figure 5. (a) Underwater OCAs of colored films in different acid or base solution, the scale bar is 100 nm. (b) and (c) Water contact angles for treated surfaces. d) Optical images of colored films before and after scraping and oil droplets of 1-Bromotetradecane on the original surface and abraded surface at 20 °C. (e) Optical imagines of colored films. Images shows that oil droplets of methylene iodide (1 μL and 2 μL) can either adhere to the surface or be nonadhesive on the surface of WBPU-treated PS@TiO2 /CB film. As a comparison, the film scraping and water flushing experiments without WBPU are given in the red frame. Scale bars: (d, e) 1 cm. 18 ACS Paragon Plus Environment

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CONCLUSION In brief, based on this study, it is the first reported technique to combine colored APCs film with underwater oil-repellent properties. A facial and environment friendly synthesis of PS@TiO2 NPs was used to prepare WBPU-treated PS@TiO2/CB films with noniridescent effect by means of a simple solution-spraying method, which are highly stable in acidic, alkaline environments, scratching and water jetting. Of course, some performance still needs to be improved in future studies. Low concentration of WBPU did not change the color of the film, but the color saturation will slightly decrease. This colored film could expand the potential applications of APCs in industrial fields, such as underwater instruments, underwater signs and film on the ship. ACKNOWLEDGMENTS 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) EDS of colorful films sprayed from PS@TiO2 NPs/CB, (2) SEM of spray coated films under higher magnification, (3) SEM images of PS@TiO2 NPs/CB film under low magnification and Two-dimensional Fourier Transform of the scanning electron microscope image, (4) images of changes of structurally colored films with WBPU increased, (5) SEM of colored films after scraping experiments (a) without and (b) with WBPU, and (6) Table of diameter and reflection peaks of PS and PS@TiO2 photonic crystals.


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Table of Contents (TOC) PS@TiO2 NPs was used to prepare PS@TiO2/CB films by a simple solution-spraying method. TiO2 dual-scale NPs can enhance surface roughness to achieve extreme interfacial wettability. The as-prepared film showed a high static oil contact angle (OCA>150°) in water and robust resistance to mechanical abrasion.


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Angle-Independent Structurally Colored PS@TiO2 Film with Excellent

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