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Hydrophobic polymer-incorporated hybrid 1D photonic crystals with brilliant structural colors via aqueous-based layer-by-layer dip-coating
Journal Pre-proof Hydrophobic polymer-incorporated hybrid 1D photonic crystals with brilliant structural colors via aqueous-based layer-by-layer dip-coating Xiaolin Yu, Wei Ma, Shufen Zhang PII:
S0143-7208(20)31658-2
DOI:
https://doi.org/10.1016/j.dyepig.2020.108961
Reference:
DYPI 108961
To appear in:
Dyes and Pigments
Received Date: 31 August 2020 Revised Date:
20 October 2020
Accepted Date: 22 October 2020
Please cite this article as: Yu X, Ma W, Zhang S, Hydrophobic polymer-incorporated hybrid 1D photonic crystals with brilliant structural colors via aqueous-based layer-by-layer dip-coating, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2020.108961. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
author statement
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Yu Xiaolin: Methodology, Conceptualization, Data curation, Writing-original draft preparation. Ma Wei: Investigation, Writing-reviewing and editing, Funding acquisition. Zhang Shufen: Supervision.
Hydrophobic polymer-incorporated hybrid 1D photonic crystals with brilliant structural colors via aqueous-based layer-by-layer dip-coating Xiaolin Yu , Wei Ma *, Shufen Zhang State Key Laboratory of Fine Chemicals，Dalian University of Technology，Dalian, Liaoning 116023，PR China * E-mail: [email protected]; Tel: +86-411-84986506
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ABSTRACT: Preparation and application of photonic crystal (PC) films with
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structural colors have received extensive attention in recent years. Aqueous-based layer-by-layer (LbL) dip-coating is an environmentally friendly and versatile method
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to prepare functional films. This work focuses on using the method to prepare a kind
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of hydrophobic polymer-containing 1D PCs with brilliant structural colors. High and low refractive index stacks are produced by electrostatic assembly of nano TiO2/
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poly(styrene sulfonate) (PSS) and poly(diallyldimethyl ammonium chloride) (PDAC)/ nano poly(methyl methacrylate) (PMMA) multilayers, respectively. Hydrophobic
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PMMA was introduced for the first time in this aqueous-based electrostatically-assisted assembly for expanding the applicable materials. The zeta potential and particle size of
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the PMMA and TiO2 nanoparticles are measured under different pHs, and the effects of
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the bilayer number on both film thickness and refractive index are investigated. The structural colors of the heterostructure 1D PCs can be regulated in the visible range by variation of bilayer number, incident angle and stack number. This hydrophobic polymer incorporated 1D PCs can achieve colorful coatings on flat and curved glass substrates. In addition, upon exposure to organic solvent vapors, the PC film show color change. Thereby, the obtained multilayered PCs exhibit potential applications in coating, decoration and sensing. 1 Introduction Structural colors, also known as physical colors, appear when nanostructures selectively reflect certain wavelengths of visible light through interference, refraction or diffraction.[1, 2] They widely exist in biosphere, such as colors of Morpho rhetenor wings, peacock feathers and opals[3], and show characteristics of high brightness and color saturation. At present, hybrid materials have received extensive attention in a 1
wide range of research field[4-6]. Among them, the man-made structural colors based on these materials are also being studied intensively owing to their potential applications in optoelectronic devices[7-10], coatings[11], sensing[12], decoration[13] and anti-counterfeiting[14]. Photonic crystals (PCs), as the main source of structural colors, have been paid significant attention due to their ability to controllably regulate the photonic bandgaps of the materials[15]. Among PC family, 1D PCs, formed by two materials with different refractive indexes in one dimension of length scale, are particularly promising for commercial usages due to their simple multilayer structure[16,
17]
. When these 1D
architectures are designed to reflect light in the visible region, structural color is
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produced, and the color is facilely tuned by adjusting layer thickness and refractive
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index of the materials based on Bragg’s law[18]. Layer-by-layer (LbL) assembly, such as dip-coating, spin-coating and spray-coating, is a prevalent technique for preparing
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stratified structures[19]. Among them, LbL dip-coating through electrostatic interaction
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is a versatile and environmentally friendly aqueous-based technique for forming uniform thin coating on molecular level[20]. The layer thickness can be influenced by [27, 28]
[29]
and solvent
25, 26]
,
. This assembly method shows great
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polyelectrolyte type
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various parameters, such as pH[21], temperature[22-24], ionic strength[23,
advantages in cost, substrate size and film uniformity. Moreover, it is capable of
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producing conformal films on a diverse range of substrates with complex geometries[30-32].
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1D PC assemblies were also demonstrated with LbL dip-coating by deposition of polyelectrolytes or hydrophilic inorganic nanoparticles. Zhai et al. reported construction of polyelectrolyte 1D PCs with alternating assembly of poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA) and PAH/poly(styrene sulfonate) (PSS) multilayers[33]. The LbL structure showed potential application as vapor sensors with tunable structural colors. Incorporation of noble metal, such as confinement of Ag nanoparticles to the carboxylic-acid containing region of the heterostructure, can manipulate the refractive index of the PAH/PAA stack, forming large index contrast and regulating the reflected light[34]. Inorganic nanoparticles, including TiO2 and SiO2, were also employed to prepare 1D PCs using aqueous LbL deposition by aid of polyelectrolytes[35]. These hydrophilic nanoparticles could uniformly disperse in water due to sufficient surface charges and assembled with polyelectrolytes bearing opposite charges repetitively to form colorful multilayers[36]. Research in this area, however, has exposed the great difficulty associated with 2
introducing hydrophobic molecules within the photonic matrix as they are water-insoluble and do not show surface charge. The ability to introduce hydrophobic polymers within the structural colors will further extend the utility of these photonic structures. Here in this study, we show the possibility to produce 1D PCs by introducing hydrophobic polymer via an aqueous-based LbL dip-coating. In order to disperse the polymer uniformly in water, we designed and prepared the polymer through microemulsion polymerization. With this method, hydrophobic nanopolymers with size lower than 100 nm are prepared in the cavities of the micelles formed by aggregation of surfactants, thus they could uniformly and stably disperse in water[37, 38]. As will be demonstrated in this work, the hydrophobic polymer-polymethyl
of
methacrylate (PMMA) with good film-forming property was incorporated into 1D PC
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through the polyelectrolyte-assisted LbL assembly in water. The processing scheme
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developed for the construction of the novel heterostructure PCs is presented in Fig. 1. PMMA and TiO2 are chosen as the main stack materials, they show large refractive
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index contrast, which is beneficial for obtaining bright structural color. During the
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process, positively charged nano TiO2 is first assembled with negatively charged PSS alternately on glass substrate to obtain (TiO2/PSS)n multilayer as high-index stack; then,
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PMMA nanoparticles with negative charges are deposited with the help of polycation-PDAC layer by layer to yield (PMMA/PDAC)n multilayer as low-index
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stack. The above procedures are repeated to create a kind of novel inorganic/organic hybrid 1D PCs on both sides of glass substrates. Through adjusting the bilayer and
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stack numbers and incident angle, brilliant and tunable structural colors were achieved. In addition, coloration on the curved surfaces and sensing property of the PC towards organic solvent vapors were also studied to show the applicability of these hybrid materials.
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Fig. 1. Scheme of preparing 1DPC with aqueous-based LbL dip-coating. Dip-coating assembly of
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(i) TiO2/PSS multilayers; (ii) PDAC/PMMA multilayers.
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2 Experimental
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2.1 Materials
Methyl methacrylate (MMA, 99.5%) and sodium lauryl sulfate (SDS) were
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purchased from Tianjin Bodie Chemical Co., Ltd. Anatase nano titania (TiO2, 99.8%) was purchased from Xuancheng Jingrui New Materials Company. Poly(styrene
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sulfonate) (PSS) (molecular weight 750,000) was obtained from Alfa Aesar (China) Co., Ltd and poly(diallyldimethyl ammonium chloride) (PDAC) (molecular weight 20,000-350000) was gained from Aladdin Reagent Co., Ltd. Potassium persulfate (KPS) was supplied by Xilong Chemical Co., Ltd. 2.2 Preparation of PMMA, TiO2, PDAC and PSS Preparation of nano PMMA and TiO2 sols: PMMA sols was prepared with a modified microemulsion polymerization. The specific experimental process is as follows: 80.0 g of deionized water, 6.0 g MMA and 2.5 g sodium lauryl sulfonate were added to a three-necked flask. After reacting at 75 °C for 30 min, 0.108 g potassium persulfate dissolved in deionized water was slowly poured to the above mixture. Then 30.0 g MMA was added dropwise to the flask and reacted for 1 h. The progress was continued for 30 min after MMA was completely reacted. During the 4
entire reaction, nitrogen was passed to protect the reaction system. After the solution was cooled to room temperature, suction was used to remove impurities. Finally, the prepared PMMA microemulsion was diluted to 0.05 wt% with water before use. 0.03 wt% TiO2 sol was prepared by dispersing nano TiO2 powder into deionized water with assistance of sonication for 5 min. Preparation of PSS and PDAC solutions: A certain mass of PSS with light yellow powder was dissolved in water to obtain a 0.2 wt% aqueous solution, transparent PDAC was formulated into a 0.8 wt% aqueous solution.
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2.3 Assembly of 1D PC films
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Glass substrate cleaning: The purpose of cleaning the glass substrate is to
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increase the hydrophilicity of its surface and facilitate the adsorption of the first layer. First, the glass pieces were ultrasonically cleaned with deionized water into a beaker
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for 15 min and then they were put into piranha solution (volume ratio of hydrogen
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peroxide: ammonia water: deionized water of 1 : 1 : 5) and heated to 70 °C for 30 min. Afterward, the glass substrates were washed with deionized water using sonication for
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30 min and then put in a plasma cleaner for 3 min to improve its surface hydrophilicity. The obtained substrates were then taken out for dip-coating assembly
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experiments.
PTL-MM02-8P type dipping and pulling coating machine was used for
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dip-coating assembly. Unless otherwise specified, the experimental temperature is kept at 28 °C. First, the glass substrates were alternately immersed in positively charged PDAC and negatively charged PSS solutions for 10 min, with three intermediate steps in deionized water, one 2 min and two 1 min rinses, and dried at room temperature after assembly of four bilayers of PDAC/PSS as the bottom layer[35]. For (TiO2/PSS)n assembly, where n represents the number of bilayers, the substrates were deposited into the TiO2 sol for 10 min, then rinsed in pH = 2.5 deionized water for 2, 1 and 1 min, followed by depositing in PSS solution for 10 min and another 3 rinses as before. The assembly was repeated until the required number of bilayers was obtained and then the multilayers were dried at room temperature. For (PDAC/PMMA)n assembly, the above prepared films were dipped in PDAC solution for 10 min and then washed in deionized water for 2, 1 and 1 min, followed by dipping in PMMA sol for 10 min and other 3 rinses with pH = 4.8 deionized water as before. The process was repeated until the required number of bilayers was obtained 5
and then the multilayers were dried at room temperature. After the one stack (TiO2/PSS)n-(PDAC/PMMA)n assembly, the above procedure can be repeated several times to finally obtain [(TiO2/PSS)n-(PDAC/PMMA)n]m 1D PCs with m number of stacks. 2.4 Characterization Particles hydrodynamic size and zeta potential of PMMA and TiO2 sols were measured using a Zetasizer nano series NanoZS-90 (Malvern instruments, Malvern, UK). The thickness and roughness of (TiO2/PSS)n and (PDAC/PMMA)n bilayers were
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measured by a NV5000 profilometer (Zygo Company, USA). The refractive indexes
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of (TiO2/PSS)n and (PDAC/PMMA)n bilayers were measured by an Ellip-ER-III
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Spectroscopic Ellipsometer at an incident angle of 15° and a wavelength of 632.8 nm. The data were averaged over three tests. The surface and cross-section scanning
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electron microscopy (SEM) images of the films were recorded by an SU8220 SEM
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(Hitachi Company, Japan). The optical properties of the photonic stop band and reflection intensity of 1D PCs were measured by a U-4100 solid ultraviolet
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spectrophotometer (Hitachi Company, Japan) at an incident angle of 15°. Reflectance spectra of 1D PCs at different incident angles were measured by an
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angle-resolved microspectroscopy system (ARM, Ideaoptics, PR China). Unless otherwise specified, an iPhone 7 was used for capturing the optical photos of the 1D PCs
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at an observation angle of 15°. 3 Results and discussion
3.1 Preparation and characterization of PMMA and TiO2 sols To achieve LbL PC films with uniform structural colors, the uniformity of the assembly materials is a prerequisite. The suitable nanoparticle size for assembling colorful 1D PC is less than 50 nm, which is beneficial for controlling the layer thickness and obtaining good optical properties[39]. In this study, modified microemulsion polymerization was used to prepare stable and homogeneous PMMA nanoparticles, and water was employed as the solvent. With this method, the hydrophobic MMA monomer is polymerized within the micelles of the anionic surfactants formed in water. The polymer size is well controlled by the size of the 6
microemulsion droplets and the prepared nano PMMA dispersed uniformly in water
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with enough negative charges on the surface.
Fig. 2. Particle size distribution of (a) PMMA and (b) TiO2 sols, SEMs of (c) PMMA and (d) TiO2
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sols. Effect of pH value on particle size and zeta potential of (e) PMMA and (f) TiO2 sols.
Fig. 2a shows that the average diameter of PMMA nanoparticles is 28.55 nm and
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PDI is 0.146, indicating uniformity and stability of the nanoparticles in aqueous solution. Fig. 2b presents the average particle size of TiO2 is 36.46 nm and PDI is
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0.130. In Fig. 2c, its SEM image of the PMMA nanoparticles exhibits that the particle size is homogeneous. The nanoparticle characteristics is clearly observed in the SEM
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image of TiO2 layer (Fig. 2d). Moreover, the measured zeta potential of PMMA and TiO2 sols is -32.0 and 41.6 mV, respectively. Zeta potential is used to measure the strength of mutual repulsion or attraction between nanoparticles. The smaller the particles and the higher the absolute value of the zeta potential, the more stable the dispersing system will be
[40]
. Based on the above parameters, the nano PMMA and
TiO2 are suitable for aqueous-based LbL assembly of 1D PCs. As pH of the sols could greatly influence the stability of the nanoparticles, we focused on the effect of pH on zeta potential and particle size of both materials, the results are showed in Fig. 2e and 2f. From Fig. 2e, with the increase of pH value from 3 to 8, the particle size and zeta potential of PMMA did not show obvious change. Owing to the good stability of the micelles formed in the solution, the emulsion was stable under acidic to weak alkaline condition. Conversely, for TiO2 nanoparticles Fig. 2f shows that its particle size first drops from 45.16 to 36.46 nm with the pH increases from 1.5 to 2.5, and then grows to 71.40 nm with the pH value further increased to 4.0, 7
which indicates TiO2 nanoparticles would aggregate to a certain extent as the pH value changes. The dispersing system is stable during the pH variation, as the surface charge of the nanoparticles is still large. To obtain a better uniformity in LbL dip-coating, we maintained initial pH 2.5 and 4.8 of TiO2 and PMMA sols for PC assembly, respectively. 3.2 Properties of (PDAC/PMMA)n and (TiO2/PSS)n layers In order to better control the thickness of (TiO2/PSS)n and (PDAC/PMMA)n layers to gain rich structural colors, we investigated the influence of bilayer number n
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on the thickness growth of the film with this LbL dip-coating method. The assembly
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process is carried out under 28 °C. Fig. 3a shows that, when n increases from 10, to
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20, 30, 40, 50, 60 and 70, both bilayer thickness increases gradually. For (TiO2/PSS)n multilayer, the thickness varied from 26.0 to 138.0 nm; and for (PDAC/PMMA)n one,
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the thickness changed from 30.0 to 310.0 nm. It is not difficult to demonstrate that the
lP
growth rate of (PDAC/PMMA)n is quicker than that of (TiO2/PSS)n, which is mainly due to that higher concentrations of PMMA and PDAC were employed in assembly
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compared with that of TiO2 and PSS, respectively. Besides layer thickness, surface roughness of (TiO2/PSS)n and (PDAC/PMMA)n
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films was also measured and the results are shown in Table S1. It can be seen that with the increase of the layer number n, the surface roughness increased. It is clearly
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found that the surfaces of (PDAC/PMMA)n film is much rougher compared with that of (TiO2/PSS)n at the same layer number. Fig. 3b and 3c present the microscopic surface morphology of (PDAC/PMMA)30 and the (TiO2/PSS)30 by SEM. It can be clearly observed that the surface of (PDAC/PMMA)30 is rougher than that of (TiO2/PSS)30. Nanopores appeared evidently on the surface of (PDAC/PMMA)30, while (TiO2/PSS)30 looked denser. As spatial arrangement of electrostatic interaction is affected by charge sites and spatial steric hindrance, with the increase of layer number certain pores would form on the surface and within the nanoparticles assembly. Compared with (TiO2/PSS)n film, (PDAC/PMMA)n shows quicker growth rate and the top layer is composed of PMMA nanoparticles, so the film surface is rougher; conversely the surface layer of (TiO2/PSS)n film is extended polymer - PSS, thus it shows denser and smoother surface morphology.
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Fig. 3. (a) Film thickness as a function of bilayer number for (PDAC/PMMA)n and (TiO2/PSS)n
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multilayers. SEM images of (b) (PDAC/PMMA)30 and (c) (TiO2/PSS)30 multilayers.
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Table 1 Refractive indexes of the four assembly material monolayers and both multilayers with different bilayer number. Material
(TiO2/PSS)n
1.72
neff (PDAC)
1.48
neff (PSS)
1.50
neff (PMMA)
1.41
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Bilayer number n
neff (TiO2) 10
30
50
10
30
50
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neff of monolayer
(PDAC/PMMA)n
1.74
1.76
1.33
1.46
1.43
1.59
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neff of multilayer
Table 1 shows the refractive indexes of the four assembly material monolayers and both multilayers with different bilayer number. According to the data in the table, the refractive index of TiO2 layer is 1.72 and that of PMMA is 1.41, and there is an apparent difference in refractive index between them. PSS and PDAC layers show close refractive index of 1.50 and 1.48, respectively. For (TiO2/PSS)n and (PDAC/PMMA)n multilayers, their refractive indexes varied with the increase of bilayer number n. When n is 10, the refractive index of (TiO2/PSS)10 is 1.59, which is much lower than that of titania; that of (PDAC/PMMA)10 is 1.33, which is lower than that of both assembly materials. As n increased to 30, refractive indexes of both multilayers increased, that of (TiO2/PSS)30 and (PDAC/PMMA)30 reached 1.74 and 1.46, respectively. Further increase to n = 50, the change in refractive index is small. The reason for the changing refractive index is that when the number of stacking is as low as 10, the assembled multilayer is loose and the refractive index is low due to 9
existence of much air within the film; as the stacking number increases, the alternating stack of nanoparticles and polyelectrolytes changes from a loose state to a dense one, which leads to the decrease of the air fraction and the increase of the refractive index. 3.3 Fabrication of 1D PCs During assembly process of 1D PCs, it was found the ambient temperature has a great influence on the uniformity of the PC films, thus affecting the evenness of the structural color. This is because that, after each stack fabrication, the assembled
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multilayers would stand still at ambient temperature for drying, and then new stack
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temperature on the uniformity of the PC film.
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assembly begins. In this section, we chose 18 and 28 °C to study the influence of A one-stack PC sample with bilayer number of 30 was employed as an example.
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The contrast of colorful PC films assembled at both temperatures is shown in Fig. 4a.
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When the temperature was 18 °C, the blue film showed some local gradient color, while when the temperature was 28 °C the color looked much uniform and deep. The
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reason for this phenomenon is that, at 18 °C, the water evaporation rate is low during the drying process, due to the gravity, water containing certain solute on the film will
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flow down, resulting in certain layer thickness difference from the top to the bottom of the substrate. At 28 °C, the very thin film can be dried within short time, so the
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influence of the drying on the layer thickness of different parts of the film is small, and the main part color is uniform. In Fig. 4b, eight different sites were selected and their reflectance spectra were measured. It could be seen that the spectrum curves were almost coincident, indicating good uniform of the colors. In addition, boundary effect of the formed structural colors at both temperatures was observed in Fig. 4a, this is mainly due to thicker film thickness on the boundary resulted from coffee-ring effect[41, 42]. To overcome this problem, further study should be made in the future.
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Fig. 4. (a) 1D PCs prepared at different ambient temperatures. (b) Reflectance spectra of 1D PCs at different positions (size:1cm). Digital photo and cross-section (c) and surface (d) SEM images of a
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three-stack 1D PCs.
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Fig. 4c shows the digital and cross-sectional SEM images of a prepared 1D PC with bilayer number of 30 and stack number of 3. Clear three-stack structure was
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observed, and the bright and dark layers are TiO2 and PMMA parts, respectively. At the same time, it can also be seen that the nanoparticles and polyelectrolytes formed a relatively uniform film surface with certain pore structures in the surface SEM image presented in Fig. 4d. 3.4 Optical properties of 1D PCs The position of the Bragg peak of a 1D PC can be calculated by the Bragg-Snell formula[43, 44]: 2
2
1 2
mλBragg = 2D(neff -sin θ)
(1)
where λBragg is the wavelength of the reflected light, which determines the structural color, m is the diffraction order, D = dh + dl is the period, dh and dl are the thicknesses of the high- and low-refractive-index materials, respectively. θ is the incident angle, neff is the effective refractive index. 11
According to Eqn. 1, the position of the reflection peak can be tuned by varying the thickness and incident angle of the 1D PC. By changing the bilayer number n, one-stack 1D PCs with different structural colors were constructed. Fig. 5a (i) shows that, when n is 20, 30, 40 and 50, the maximum reflection wavelength is 379.9, 411.6, 511.8 and 577.3 nm, respectively. Fig. 5a (ii) shows the observation mode of the 1D PCs and their digital photographs were played in Fig. 5a (iii). The PC films presented purple, blue, green and green yellow color, respectively. Fig. 5b shows the reflectance spectra and color change of the PC film by varying the incident angle. The sample is a two-stack PC with bilayer number n of 40. It showed, that as the incident angle continued to increase from 0° to 60°, the position of the photonic band gap gradually
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blue-shifted from 657.4 to 572.9 nm. The above results demonstrate that, with the
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increase of the bilayer number, the layer thickness increases and the stopband position
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gradually red-shifts; conversely, with the increase of the incident light, the stopband position blue-shifts. These are both in good accordance with the Bragg’s law. For
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reducing experimental errors and achieving reproducibility of the phenomena, we
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need carefully control the assembly parameters, among them, the material concentration, the pH value, the environmental temperature, the cycle and stack
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number are all important factors for assembly. In addition, when the bilayer number n was fixed at 20, the stack number
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increased from 1 to 3, it measured the thickness of the PCs increased from 123 to 491 nm, and it displayed the intensity of the reflection peak gradually increases from
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12.87% to 40.41% (see Fig. S1). This is consistent with Eqn. S1 describing the relationship between the reflectivity and the stack number[35].
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Fig. 5. (a) (i) Reflection spectra, (ii) schematic diagram of observation mode and (iii) digital
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photographs of one-stack 1D PCs with bilayer number of 20, 30, 40 and 50. (b) Reflectance
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spectra and digital photographs of [(TiO2/PSS)40-(PDAC/PMMA)40]2 1D PC at different incident angles. (c) Reflectance spectra and digital photograph of 1D PC on single- and double-side of glass substrates. (d) (i) Digital photos of 1D PCs on the curved substrates, (ii) schematic diagram of spectrum measurement and (iii) reflectance spectra of the 1D PCs on the watch glass.
Moreover, as LbL dip-coating could obtain PC films on both sides of the substrate, it is necessary to figure out the impact of single- or double-sided assembly on the reflection spectrum and color. A one-stack PC sample with bilayer number of 40 was used for comparison, and the results are shown in Fig. 5c. It can be seen from the figure that the color is similar for one- and two-sided assembly, and the positions of the maximum reflection wavelength are very close, while the reflectivity of the spectra is different. For the film with double-sided color, the reflectivity is 19.3%, while that with the single-sided one is only 13.1%. It can be obtained that double-sided assembly can enhance the reflectivity of 1D PCs, therefore the advantage of this LbL dip-coating is clearly presented, that is PC can be fabricated 13
simultaneously on both side of the substrate to increase the brilliancy of the structural color, this cannot be realized by other commonly used assembly method, such as spinand spray-coating. In addition, we also tried to prepare 1D PCs on the curved glass substrates by this aqueous-based LbL dip-coating. Watch glass, test tube, vial bottle and flasket were employed as the substrates, and PC films were successfully constructed on them as shown in Fig. 5d (i). Fig. 5d (ii) is the schematic diagram of spectra measurement of watch glass. The structural colors on the watch glass are relatively uniform and color hue does not change significantly when viewing from 0 to 60°, only the color depth increases gradually, which is quite different from that obtained from the color
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on the flat substrates in Fig. 5d (iii), indicating that the hue of the color from 1D PC
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on the curved surface has angle-independence property.
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Moreover, using the mask, we prepared PC film with heart-shape pattern as shown in Fig.S2. In addition to coloration, we also found that the PC films could be
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used for organic solvent sensing as shown below (see Fig. 6).
Fig. 6. (a) Digital photos and (b) reflectance spectra of the 1D PCs film after exposure to different organic solvent vapors.
The PC films show porous structures, the vapors can diffuse into the pores and act upon the films. They contain hydrophobic polymers which show certain swelling ability upon exposure to vapors. Due to these properties, the as-assembled PC films present response to the solvent vapors. We investigated three solvent vapors, they are 14
carbon tetrachloride (CTC), dichloromethane (DCM), and ethanol (EA). Upon exposure to these saturated solvent vapors at 20 °C, the violet PC film showed color change as displayed in Fig. 6a, blue in CTC, blue-green in DCM, and green in EA; and the corresponding spectrum change is presented in Fig. 6b, the maximum reflection wavelength changed from 377.3 nm to 416.9, 479.6 and 534.2 nm in CTC, DCM and EA vapors, respectively. The hydrophobic polymer- PMMA is sensitive to organic solvents and can swell in the vapors, resulting in layer thickness increase. Although the swelling ability of CTC and DCM to PMMA is stronger than that of EA, PC film did not show bigger color change in CTC and DCM. This is due to that within the film, there are much
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hydrophilic polymers- PDAC and PSS, which restrain the swelling ability of the film in
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CTC and DCM. So, the different color change in CTC, DCM and EA is resulted from
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the comprehensive interaction of the solvent vapors on the polymers within the PC
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film.
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4. Conclusion
We have successfully utilized a facile and aqueous-based LbL dip-coating
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method to fabricate [(TiO2/PSS)n-(PDAC/PMMA)n]m 1D PCs with brilliant structural colors on both flat and curved glass substrates. It is the first time to introduce nano
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PMMA with particle size of 28.2 nm into this inorganic/organic hybrid PC
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microstructure to expand the applicability of material selection. The surface morphology and roughness of the films are determined by the assembly material and the bilayer number, and the assembly temperature is an essential factor for influencing the color evenness. The obtained 1D PCs with uniform and gorgeous structural colors present tunable optical properties in the visible region by changing the bilayer and stack number and the incident angle. The colors show distinct angle-dependent property on flat substrate, while almost angle-independent characteristic on the curved one. It is found the advantage of this assembly method is to achieve double-sided assembly with enhanced reflectivity at one time. In addition, due to introduction of the hydrophobic polymer, the obtained PC films showed obvious color change when exposing to organic solvent vapors. This study provides a basis for building new color system for functional coating, decoration and sensing. Acknowledgements 15
This research was financially supported by the National Natural Science Foundation of China (21878040, 21536002, 21421005), the Fundamental Research Funds for the Central Universities (DUT19TD28), the Natural Science Foundation of Liaoning Province
(2019-MS-037)
and
the
Liaoning
Revitaliztion
Talent
Program
(CLXC1801006). References
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[1] Cai D, Marques MAL, Nogueira F. Accurate color tuning of firefly chromophore by modulation of local polarization electrostatic fields. J Phys Chem B. 2011;115(2):329-32. [2] Lee SM, Uepping J, Bielawny A, Knez M. Structure-based color of natural petals discriminated by polymer replication. ACS Appl Mater Inter. 2011;3(1):30-4. [3] Yue YF, Gong JP. Tunable one-dimensional photonic crystals from soft materials. J Photoch Photobio C. 2015;23:45-67. [4] Guo J, Zhao JL, Huang DZH, Wang YZH, Zhang F, Ge YQ, et al. Two-dimensional tellurium-polymer membrane for ultrafast photonics. Nanoscale. 2019;11(13):6235-42. [5] Pintossi D, Iannaccone G, Colombo A, Bella F, Valimaki M, Vaisanen KL, et al. Luminescent downshifting by photo-induced sol-gel hybrid coatings: accessing multifunctionality on flexible organic photovoltaics via ambient temperature material processing. Adv Electron Mater. 2016;2(11). [6] Scalia A, Bella F, Lamberti A, Gerbaldi C, Tresso E. Innovative multipolymer electrolyte membrane designed by oxygen inhibited UV-crosslinking enables solid-state in plane integration of energy conversion and storage devices. Energy. 2019;166:789-95. [7] Colodrero S, Forneli A, Lopez-Lopez C, Pelleja L, Miguez H, Palomares E. Efficient transparent thin dye solar cells based on highly porous 1D photonic crystals. Adv Func Mater. 2012;22(6):1303-10. [8] Wang L, Li Q. Photochromism into nanosystems: towards lighting up the future nanoworld. Chem Soc Rev. 2018;47(3):1044-97. [9] Zhang W, Anaya M, Lozano G, Calvo ME, Johnston MB, Miguez H, et al. Highly efficient perovskite solar cells with tunable structural color. Nano Lett. 2015;15(3):1698-702. [10] Song L, Chen X, Xie Y, Zhong L, Zhang X, Cheng Z. Non-iridescent, crack-free, conductive structural colors enhanced by flexible nanosheets of reduced graphene oxide. Dyes Pigments. 2019;164:222-6. [11] Kurt P, Banerjee D, Cohen RE, Rubner MF. Structural color via layer-by-layer deposition: layered nanoparticle arrays with near-UV and visible reflectivity bands. J Mater Chem. 2009;19(47):8920-7. [12] Kou DH, Ma W, Zhang SF, Lutkenhaus JL, Tang BT. High-performance and multifunctional colorimetric humidity sensors based on mesoporous photonic crystals and nanogels. ACS Appl Mater Inter. 2018;10(48):41645-54. [13] Zhou CT, Qi Y, Zhang SF, Niu WB, Ma W, Wu SL, et al. Rapid fabrication of 16
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Table 1 Refractive indexes of the four assembly material monolayers and both multilayers with different bilayer number Material
(TiO2/PSS)n
neff of monolayer
(PDAC/PMMA)n
neff (TiO2)
1.72
neff (PDAC)
1.48
neff (PSS)
1.50
neff (PMMA)
1.41
10
30
50
10
30
50
neff of multilayer
1.59
1.74
1.76
1.33
1.46
1.43
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Bilayer number n
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Highlights: 1. We successfully utilized a facile and aqueous-based LbL dip-coating method to fabricate hydrophobic polymer-containing 1D PCs. 2. The organic/inorganic hybrid PCs exhibit brilliant and tunable structural colors by modulating bilayer number, incident angle and stack number. 3. The structural colors could be constructed on both sides of the flat and curved
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substrates to enhance the reflectivity.
Declaration of interests ☑ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0143-7208(20)31658-2
DOI:
https://doi.org/10.1016/j.dyepig.2020.108961
Reference:
DYPI 108961
To appear in:
Dyes and Pigments
Received Date: 31 August 2020 Revised Date:
20 October 2020
Accepted Date: 22 October 2020
Please cite this article as: Yu X, Ma W, Zhang S, Hydrophobic polymer-incorporated hybrid 1D photonic crystals with brilliant structural colors via aqueous-based layer-by-layer dip-coating, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2020.108961. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
author statement
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Yu Xiaolin: Methodology, Conceptualization, Data curation, Writing-original draft preparation. Ma Wei: Investigation, Writing-reviewing and editing, Funding acquisition. Zhang Shufen: Supervision.
Hydrophobic polymer-incorporated hybrid 1D photonic crystals with brilliant structural colors via aqueous-based layer-by-layer dip-coating Xiaolin Yu , Wei Ma *, Shufen Zhang State Key Laboratory of Fine Chemicals，Dalian University of Technology，Dalian, Liaoning 116023，PR China * E-mail: [email protected]; Tel: +86-411-84986506
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ABSTRACT: Preparation and application of photonic crystal (PC) films with
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structural colors have received extensive attention in recent years. Aqueous-based layer-by-layer (LbL) dip-coating is an environmentally friendly and versatile method
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to prepare functional films. This work focuses on using the method to prepare a kind
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of hydrophobic polymer-containing 1D PCs with brilliant structural colors. High and low refractive index stacks are produced by electrostatic assembly of nano TiO2/
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poly(styrene sulfonate) (PSS) and poly(diallyldimethyl ammonium chloride) (PDAC)/ nano poly(methyl methacrylate) (PMMA) multilayers, respectively. Hydrophobic
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PMMA was introduced for the first time in this aqueous-based electrostatically-assisted assembly for expanding the applicable materials. The zeta potential and particle size of
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the PMMA and TiO2 nanoparticles are measured under different pHs, and the effects of
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the bilayer number on both film thickness and refractive index are investigated. The structural colors of the heterostructure 1D PCs can be regulated in the visible range by variation of bilayer number, incident angle and stack number. This hydrophobic polymer incorporated 1D PCs can achieve colorful coatings on flat and curved glass substrates. In addition, upon exposure to organic solvent vapors, the PC film show color change. Thereby, the obtained multilayered PCs exhibit potential applications in coating, decoration and sensing. 1 Introduction Structural colors, also known as physical colors, appear when nanostructures selectively reflect certain wavelengths of visible light through interference, refraction or diffraction.[1, 2] They widely exist in biosphere, such as colors of Morpho rhetenor wings, peacock feathers and opals[3], and show characteristics of high brightness and color saturation. At present, hybrid materials have received extensive attention in a 1
wide range of research field[4-6]. Among them, the man-made structural colors based on these materials are also being studied intensively owing to their potential applications in optoelectronic devices[7-10], coatings[11], sensing[12], decoration[13] and anti-counterfeiting[14]. Photonic crystals (PCs), as the main source of structural colors, have been paid significant attention due to their ability to controllably regulate the photonic bandgaps of the materials[15]. Among PC family, 1D PCs, formed by two materials with different refractive indexes in one dimension of length scale, are particularly promising for commercial usages due to their simple multilayer structure[16,
17]
. When these 1D
architectures are designed to reflect light in the visible region, structural color is
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produced, and the color is facilely tuned by adjusting layer thickness and refractive
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index of the materials based on Bragg’s law[18]. Layer-by-layer (LbL) assembly, such as dip-coating, spin-coating and spray-coating, is a prevalent technique for preparing
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stratified structures[19]. Among them, LbL dip-coating through electrostatic interaction
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is a versatile and environmentally friendly aqueous-based technique for forming uniform thin coating on molecular level[20]. The layer thickness can be influenced by [27, 28]
[29]
and solvent
25, 26]
,
. This assembly method shows great
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polyelectrolyte type
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various parameters, such as pH[21], temperature[22-24], ionic strength[23,
advantages in cost, substrate size and film uniformity. Moreover, it is capable of
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producing conformal films on a diverse range of substrates with complex geometries[30-32].
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1D PC assemblies were also demonstrated with LbL dip-coating by deposition of polyelectrolytes or hydrophilic inorganic nanoparticles. Zhai et al. reported construction of polyelectrolyte 1D PCs with alternating assembly of poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA) and PAH/poly(styrene sulfonate) (PSS) multilayers[33]. The LbL structure showed potential application as vapor sensors with tunable structural colors. Incorporation of noble metal, such as confinement of Ag nanoparticles to the carboxylic-acid containing region of the heterostructure, can manipulate the refractive index of the PAH/PAA stack, forming large index contrast and regulating the reflected light[34]. Inorganic nanoparticles, including TiO2 and SiO2, were also employed to prepare 1D PCs using aqueous LbL deposition by aid of polyelectrolytes[35]. These hydrophilic nanoparticles could uniformly disperse in water due to sufficient surface charges and assembled with polyelectrolytes bearing opposite charges repetitively to form colorful multilayers[36]. Research in this area, however, has exposed the great difficulty associated with 2
introducing hydrophobic molecules within the photonic matrix as they are water-insoluble and do not show surface charge. The ability to introduce hydrophobic polymers within the structural colors will further extend the utility of these photonic structures. Here in this study, we show the possibility to produce 1D PCs by introducing hydrophobic polymer via an aqueous-based LbL dip-coating. In order to disperse the polymer uniformly in water, we designed and prepared the polymer through microemulsion polymerization. With this method, hydrophobic nanopolymers with size lower than 100 nm are prepared in the cavities of the micelles formed by aggregation of surfactants, thus they could uniformly and stably disperse in water[37, 38]. As will be demonstrated in this work, the hydrophobic polymer-polymethyl
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methacrylate (PMMA) with good film-forming property was incorporated into 1D PC
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through the polyelectrolyte-assisted LbL assembly in water. The processing scheme
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developed for the construction of the novel heterostructure PCs is presented in Fig. 1. PMMA and TiO2 are chosen as the main stack materials, they show large refractive
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index contrast, which is beneficial for obtaining bright structural color. During the
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process, positively charged nano TiO2 is first assembled with negatively charged PSS alternately on glass substrate to obtain (TiO2/PSS)n multilayer as high-index stack; then,
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PMMA nanoparticles with negative charges are deposited with the help of polycation-PDAC layer by layer to yield (PMMA/PDAC)n multilayer as low-index
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stack. The above procedures are repeated to create a kind of novel inorganic/organic hybrid 1D PCs on both sides of glass substrates. Through adjusting the bilayer and
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stack numbers and incident angle, brilliant and tunable structural colors were achieved. In addition, coloration on the curved surfaces and sensing property of the PC towards organic solvent vapors were also studied to show the applicability of these hybrid materials.
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Fig. 1. Scheme of preparing 1DPC with aqueous-based LbL dip-coating. Dip-coating assembly of
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(i) TiO2/PSS multilayers; (ii) PDAC/PMMA multilayers.
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2 Experimental
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2.1 Materials
Methyl methacrylate (MMA, 99.5%) and sodium lauryl sulfate (SDS) were
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purchased from Tianjin Bodie Chemical Co., Ltd. Anatase nano titania (TiO2, 99.8%) was purchased from Xuancheng Jingrui New Materials Company. Poly(styrene
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sulfonate) (PSS) (molecular weight 750,000) was obtained from Alfa Aesar (China) Co., Ltd and poly(diallyldimethyl ammonium chloride) (PDAC) (molecular weight 20,000-350000) was gained from Aladdin Reagent Co., Ltd. Potassium persulfate (KPS) was supplied by Xilong Chemical Co., Ltd. 2.2 Preparation of PMMA, TiO2, PDAC and PSS Preparation of nano PMMA and TiO2 sols: PMMA sols was prepared with a modified microemulsion polymerization. The specific experimental process is as follows: 80.0 g of deionized water, 6.0 g MMA and 2.5 g sodium lauryl sulfonate were added to a three-necked flask. After reacting at 75 °C for 30 min, 0.108 g potassium persulfate dissolved in deionized water was slowly poured to the above mixture. Then 30.0 g MMA was added dropwise to the flask and reacted for 1 h. The progress was continued for 30 min after MMA was completely reacted. During the 4
entire reaction, nitrogen was passed to protect the reaction system. After the solution was cooled to room temperature, suction was used to remove impurities. Finally, the prepared PMMA microemulsion was diluted to 0.05 wt% with water before use. 0.03 wt% TiO2 sol was prepared by dispersing nano TiO2 powder into deionized water with assistance of sonication for 5 min. Preparation of PSS and PDAC solutions: A certain mass of PSS with light yellow powder was dissolved in water to obtain a 0.2 wt% aqueous solution, transparent PDAC was formulated into a 0.8 wt% aqueous solution.
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2.3 Assembly of 1D PC films
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Glass substrate cleaning: The purpose of cleaning the glass substrate is to
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increase the hydrophilicity of its surface and facilitate the adsorption of the first layer. First, the glass pieces were ultrasonically cleaned with deionized water into a beaker
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for 15 min and then they were put into piranha solution (volume ratio of hydrogen
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peroxide: ammonia water: deionized water of 1 : 1 : 5) and heated to 70 °C for 30 min. Afterward, the glass substrates were washed with deionized water using sonication for
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30 min and then put in a plasma cleaner for 3 min to improve its surface hydrophilicity. The obtained substrates were then taken out for dip-coating assembly
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experiments.
PTL-MM02-8P type dipping and pulling coating machine was used for
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dip-coating assembly. Unless otherwise specified, the experimental temperature is kept at 28 °C. First, the glass substrates were alternately immersed in positively charged PDAC and negatively charged PSS solutions for 10 min, with three intermediate steps in deionized water, one 2 min and two 1 min rinses, and dried at room temperature after assembly of four bilayers of PDAC/PSS as the bottom layer[35]. For (TiO2/PSS)n assembly, where n represents the number of bilayers, the substrates were deposited into the TiO2 sol for 10 min, then rinsed in pH = 2.5 deionized water for 2, 1 and 1 min, followed by depositing in PSS solution for 10 min and another 3 rinses as before. The assembly was repeated until the required number of bilayers was obtained and then the multilayers were dried at room temperature. For (PDAC/PMMA)n assembly, the above prepared films were dipped in PDAC solution for 10 min and then washed in deionized water for 2, 1 and 1 min, followed by dipping in PMMA sol for 10 min and other 3 rinses with pH = 4.8 deionized water as before. The process was repeated until the required number of bilayers was obtained 5
and then the multilayers were dried at room temperature. After the one stack (TiO2/PSS)n-(PDAC/PMMA)n assembly, the above procedure can be repeated several times to finally obtain [(TiO2/PSS)n-(PDAC/PMMA)n]m 1D PCs with m number of stacks. 2.4 Characterization Particles hydrodynamic size and zeta potential of PMMA and TiO2 sols were measured using a Zetasizer nano series NanoZS-90 (Malvern instruments, Malvern, UK). The thickness and roughness of (TiO2/PSS)n and (PDAC/PMMA)n bilayers were
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measured by a NV5000 profilometer (Zygo Company, USA). The refractive indexes
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of (TiO2/PSS)n and (PDAC/PMMA)n bilayers were measured by an Ellip-ER-III
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Spectroscopic Ellipsometer at an incident angle of 15° and a wavelength of 632.8 nm. The data were averaged over three tests. The surface and cross-section scanning
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electron microscopy (SEM) images of the films were recorded by an SU8220 SEM
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(Hitachi Company, Japan). The optical properties of the photonic stop band and reflection intensity of 1D PCs were measured by a U-4100 solid ultraviolet
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spectrophotometer (Hitachi Company, Japan) at an incident angle of 15°. Reflectance spectra of 1D PCs at different incident angles were measured by an
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angle-resolved microspectroscopy system (ARM, Ideaoptics, PR China). Unless otherwise specified, an iPhone 7 was used for capturing the optical photos of the 1D PCs
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at an observation angle of 15°. 3 Results and discussion
3.1 Preparation and characterization of PMMA and TiO2 sols To achieve LbL PC films with uniform structural colors, the uniformity of the assembly materials is a prerequisite. The suitable nanoparticle size for assembling colorful 1D PC is less than 50 nm, which is beneficial for controlling the layer thickness and obtaining good optical properties[39]. In this study, modified microemulsion polymerization was used to prepare stable and homogeneous PMMA nanoparticles, and water was employed as the solvent. With this method, the hydrophobic MMA monomer is polymerized within the micelles of the anionic surfactants formed in water. The polymer size is well controlled by the size of the 6
microemulsion droplets and the prepared nano PMMA dispersed uniformly in water
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with enough negative charges on the surface.
Fig. 2. Particle size distribution of (a) PMMA and (b) TiO2 sols, SEMs of (c) PMMA and (d) TiO2
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sols. Effect of pH value on particle size and zeta potential of (e) PMMA and (f) TiO2 sols.
Fig. 2a shows that the average diameter of PMMA nanoparticles is 28.55 nm and
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PDI is 0.146, indicating uniformity and stability of the nanoparticles in aqueous solution. Fig. 2b presents the average particle size of TiO2 is 36.46 nm and PDI is
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0.130. In Fig. 2c, its SEM image of the PMMA nanoparticles exhibits that the particle size is homogeneous. The nanoparticle characteristics is clearly observed in the SEM
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image of TiO2 layer (Fig. 2d). Moreover, the measured zeta potential of PMMA and TiO2 sols is -32.0 and 41.6 mV, respectively. Zeta potential is used to measure the strength of mutual repulsion or attraction between nanoparticles. The smaller the particles and the higher the absolute value of the zeta potential, the more stable the dispersing system will be
[40]
. Based on the above parameters, the nano PMMA and
TiO2 are suitable for aqueous-based LbL assembly of 1D PCs. As pH of the sols could greatly influence the stability of the nanoparticles, we focused on the effect of pH on zeta potential and particle size of both materials, the results are showed in Fig. 2e and 2f. From Fig. 2e, with the increase of pH value from 3 to 8, the particle size and zeta potential of PMMA did not show obvious change. Owing to the good stability of the micelles formed in the solution, the emulsion was stable under acidic to weak alkaline condition. Conversely, for TiO2 nanoparticles Fig. 2f shows that its particle size first drops from 45.16 to 36.46 nm with the pH increases from 1.5 to 2.5, and then grows to 71.40 nm with the pH value further increased to 4.0, 7
which indicates TiO2 nanoparticles would aggregate to a certain extent as the pH value changes. The dispersing system is stable during the pH variation, as the surface charge of the nanoparticles is still large. To obtain a better uniformity in LbL dip-coating, we maintained initial pH 2.5 and 4.8 of TiO2 and PMMA sols for PC assembly, respectively. 3.2 Properties of (PDAC/PMMA)n and (TiO2/PSS)n layers In order to better control the thickness of (TiO2/PSS)n and (PDAC/PMMA)n layers to gain rich structural colors, we investigated the influence of bilayer number n
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on the thickness growth of the film with this LbL dip-coating method. The assembly
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process is carried out under 28 °C. Fig. 3a shows that, when n increases from 10, to
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20, 30, 40, 50, 60 and 70, both bilayer thickness increases gradually. For (TiO2/PSS)n multilayer, the thickness varied from 26.0 to 138.0 nm; and for (PDAC/PMMA)n one,
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the thickness changed from 30.0 to 310.0 nm. It is not difficult to demonstrate that the
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growth rate of (PDAC/PMMA)n is quicker than that of (TiO2/PSS)n, which is mainly due to that higher concentrations of PMMA and PDAC were employed in assembly
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compared with that of TiO2 and PSS, respectively. Besides layer thickness, surface roughness of (TiO2/PSS)n and (PDAC/PMMA)n
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films was also measured and the results are shown in Table S1. It can be seen that with the increase of the layer number n, the surface roughness increased. It is clearly
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found that the surfaces of (PDAC/PMMA)n film is much rougher compared with that of (TiO2/PSS)n at the same layer number. Fig. 3b and 3c present the microscopic surface morphology of (PDAC/PMMA)30 and the (TiO2/PSS)30 by SEM. It can be clearly observed that the surface of (PDAC/PMMA)30 is rougher than that of (TiO2/PSS)30. Nanopores appeared evidently on the surface of (PDAC/PMMA)30, while (TiO2/PSS)30 looked denser. As spatial arrangement of electrostatic interaction is affected by charge sites and spatial steric hindrance, with the increase of layer number certain pores would form on the surface and within the nanoparticles assembly. Compared with (TiO2/PSS)n film, (PDAC/PMMA)n shows quicker growth rate and the top layer is composed of PMMA nanoparticles, so the film surface is rougher; conversely the surface layer of (TiO2/PSS)n film is extended polymer - PSS, thus it shows denser and smoother surface morphology.
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Fig. 3. (a) Film thickness as a function of bilayer number for (PDAC/PMMA)n and (TiO2/PSS)n
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multilayers. SEM images of (b) (PDAC/PMMA)30 and (c) (TiO2/PSS)30 multilayers.
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Table 1 Refractive indexes of the four assembly material monolayers and both multilayers with different bilayer number. Material
(TiO2/PSS)n
1.72
neff (PDAC)
1.48
neff (PSS)
1.50
neff (PMMA)
1.41
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Bilayer number n
neff (TiO2) 10
30
50
10
30
50
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neff of monolayer
(PDAC/PMMA)n
1.74
1.76
1.33
1.46
1.43
1.59
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neff of multilayer
Table 1 shows the refractive indexes of the four assembly material monolayers and both multilayers with different bilayer number. According to the data in the table, the refractive index of TiO2 layer is 1.72 and that of PMMA is 1.41, and there is an apparent difference in refractive index between them. PSS and PDAC layers show close refractive index of 1.50 and 1.48, respectively. For (TiO2/PSS)n and (PDAC/PMMA)n multilayers, their refractive indexes varied with the increase of bilayer number n. When n is 10, the refractive index of (TiO2/PSS)10 is 1.59, which is much lower than that of titania; that of (PDAC/PMMA)10 is 1.33, which is lower than that of both assembly materials. As n increased to 30, refractive indexes of both multilayers increased, that of (TiO2/PSS)30 and (PDAC/PMMA)30 reached 1.74 and 1.46, respectively. Further increase to n = 50, the change in refractive index is small. The reason for the changing refractive index is that when the number of stacking is as low as 10, the assembled multilayer is loose and the refractive index is low due to 9
existence of much air within the film; as the stacking number increases, the alternating stack of nanoparticles and polyelectrolytes changes from a loose state to a dense one, which leads to the decrease of the air fraction and the increase of the refractive index. 3.3 Fabrication of 1D PCs During assembly process of 1D PCs, it was found the ambient temperature has a great influence on the uniformity of the PC films, thus affecting the evenness of the structural color. This is because that, after each stack fabrication, the assembled
of
multilayers would stand still at ambient temperature for drying, and then new stack
-p
temperature on the uniformity of the PC film.
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assembly begins. In this section, we chose 18 and 28 °C to study the influence of A one-stack PC sample with bilayer number of 30 was employed as an example.
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The contrast of colorful PC films assembled at both temperatures is shown in Fig. 4a.
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When the temperature was 18 °C, the blue film showed some local gradient color, while when the temperature was 28 °C the color looked much uniform and deep. The
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reason for this phenomenon is that, at 18 °C, the water evaporation rate is low during the drying process, due to the gravity, water containing certain solute on the film will
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flow down, resulting in certain layer thickness difference from the top to the bottom of the substrate. At 28 °C, the very thin film can be dried within short time, so the
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influence of the drying on the layer thickness of different parts of the film is small, and the main part color is uniform. In Fig. 4b, eight different sites were selected and their reflectance spectra were measured. It could be seen that the spectrum curves were almost coincident, indicating good uniform of the colors. In addition, boundary effect of the formed structural colors at both temperatures was observed in Fig. 4a, this is mainly due to thicker film thickness on the boundary resulted from coffee-ring effect[41, 42]. To overcome this problem, further study should be made in the future.
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Fig. 4. (a) 1D PCs prepared at different ambient temperatures. (b) Reflectance spectra of 1D PCs at different positions (size:1cm). Digital photo and cross-section (c) and surface (d) SEM images of a
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three-stack 1D PCs.
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Fig. 4c shows the digital and cross-sectional SEM images of a prepared 1D PC with bilayer number of 30 and stack number of 3. Clear three-stack structure was
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observed, and the bright and dark layers are TiO2 and PMMA parts, respectively. At the same time, it can also be seen that the nanoparticles and polyelectrolytes formed a relatively uniform film surface with certain pore structures in the surface SEM image presented in Fig. 4d. 3.4 Optical properties of 1D PCs The position of the Bragg peak of a 1D PC can be calculated by the Bragg-Snell formula[43, 44]: 2
2
1 2
mλBragg = 2D(neff -sin θ)
(1)
where λBragg is the wavelength of the reflected light, which determines the structural color, m is the diffraction order, D = dh + dl is the period, dh and dl are the thicknesses of the high- and low-refractive-index materials, respectively. θ is the incident angle, neff is the effective refractive index. 11
According to Eqn. 1, the position of the reflection peak can be tuned by varying the thickness and incident angle of the 1D PC. By changing the bilayer number n, one-stack 1D PCs with different structural colors were constructed. Fig. 5a (i) shows that, when n is 20, 30, 40 and 50, the maximum reflection wavelength is 379.9, 411.6, 511.8 and 577.3 nm, respectively. Fig. 5a (ii) shows the observation mode of the 1D PCs and their digital photographs were played in Fig. 5a (iii). The PC films presented purple, blue, green and green yellow color, respectively. Fig. 5b shows the reflectance spectra and color change of the PC film by varying the incident angle. The sample is a two-stack PC with bilayer number n of 40. It showed, that as the incident angle continued to increase from 0° to 60°, the position of the photonic band gap gradually
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blue-shifted from 657.4 to 572.9 nm. The above results demonstrate that, with the
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increase of the bilayer number, the layer thickness increases and the stopband position
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gradually red-shifts; conversely, with the increase of the incident light, the stopband position blue-shifts. These are both in good accordance with the Bragg’s law. For
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reducing experimental errors and achieving reproducibility of the phenomena, we
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need carefully control the assembly parameters, among them, the material concentration, the pH value, the environmental temperature, the cycle and stack
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number are all important factors for assembly. In addition, when the bilayer number n was fixed at 20, the stack number
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increased from 1 to 3, it measured the thickness of the PCs increased from 123 to 491 nm, and it displayed the intensity of the reflection peak gradually increases from
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12.87% to 40.41% (see Fig. S1). This is consistent with Eqn. S1 describing the relationship between the reflectivity and the stack number[35].
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Fig. 5. (a) (i) Reflection spectra, (ii) schematic diagram of observation mode and (iii) digital
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photographs of one-stack 1D PCs with bilayer number of 20, 30, 40 and 50. (b) Reflectance
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spectra and digital photographs of [(TiO2/PSS)40-(PDAC/PMMA)40]2 1D PC at different incident angles. (c) Reflectance spectra and digital photograph of 1D PC on single- and double-side of glass substrates. (d) (i) Digital photos of 1D PCs on the curved substrates, (ii) schematic diagram of spectrum measurement and (iii) reflectance spectra of the 1D PCs on the watch glass.
Moreover, as LbL dip-coating could obtain PC films on both sides of the substrate, it is necessary to figure out the impact of single- or double-sided assembly on the reflection spectrum and color. A one-stack PC sample with bilayer number of 40 was used for comparison, and the results are shown in Fig. 5c. It can be seen from the figure that the color is similar for one- and two-sided assembly, and the positions of the maximum reflection wavelength are very close, while the reflectivity of the spectra is different. For the film with double-sided color, the reflectivity is 19.3%, while that with the single-sided one is only 13.1%. It can be obtained that double-sided assembly can enhance the reflectivity of 1D PCs, therefore the advantage of this LbL dip-coating is clearly presented, that is PC can be fabricated 13
simultaneously on both side of the substrate to increase the brilliancy of the structural color, this cannot be realized by other commonly used assembly method, such as spinand spray-coating. In addition, we also tried to prepare 1D PCs on the curved glass substrates by this aqueous-based LbL dip-coating. Watch glass, test tube, vial bottle and flasket were employed as the substrates, and PC films were successfully constructed on them as shown in Fig. 5d (i). Fig. 5d (ii) is the schematic diagram of spectra measurement of watch glass. The structural colors on the watch glass are relatively uniform and color hue does not change significantly when viewing from 0 to 60°, only the color depth increases gradually, which is quite different from that obtained from the color
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on the flat substrates in Fig. 5d (iii), indicating that the hue of the color from 1D PC
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on the curved surface has angle-independence property.
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Moreover, using the mask, we prepared PC film with heart-shape pattern as shown in Fig.S2. In addition to coloration, we also found that the PC films could be
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used for organic solvent sensing as shown below (see Fig. 6).
Fig. 6. (a) Digital photos and (b) reflectance spectra of the 1D PCs film after exposure to different organic solvent vapors.
The PC films show porous structures, the vapors can diffuse into the pores and act upon the films. They contain hydrophobic polymers which show certain swelling ability upon exposure to vapors. Due to these properties, the as-assembled PC films present response to the solvent vapors. We investigated three solvent vapors, they are 14
carbon tetrachloride (CTC), dichloromethane (DCM), and ethanol (EA). Upon exposure to these saturated solvent vapors at 20 °C, the violet PC film showed color change as displayed in Fig. 6a, blue in CTC, blue-green in DCM, and green in EA; and the corresponding spectrum change is presented in Fig. 6b, the maximum reflection wavelength changed from 377.3 nm to 416.9, 479.6 and 534.2 nm in CTC, DCM and EA vapors, respectively. The hydrophobic polymer- PMMA is sensitive to organic solvents and can swell in the vapors, resulting in layer thickness increase. Although the swelling ability of CTC and DCM to PMMA is stronger than that of EA, PC film did not show bigger color change in CTC and DCM. This is due to that within the film, there are much
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hydrophilic polymers- PDAC and PSS, which restrain the swelling ability of the film in
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CTC and DCM. So, the different color change in CTC, DCM and EA is resulted from
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the comprehensive interaction of the solvent vapors on the polymers within the PC
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film.
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4. Conclusion
We have successfully utilized a facile and aqueous-based LbL dip-coating
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method to fabricate [(TiO2/PSS)n-(PDAC/PMMA)n]m 1D PCs with brilliant structural colors on both flat and curved glass substrates. It is the first time to introduce nano
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PMMA with particle size of 28.2 nm into this inorganic/organic hybrid PC
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microstructure to expand the applicability of material selection. The surface morphology and roughness of the films are determined by the assembly material and the bilayer number, and the assembly temperature is an essential factor for influencing the color evenness. The obtained 1D PCs with uniform and gorgeous structural colors present tunable optical properties in the visible region by changing the bilayer and stack number and the incident angle. The colors show distinct angle-dependent property on flat substrate, while almost angle-independent characteristic on the curved one. It is found the advantage of this assembly method is to achieve double-sided assembly with enhanced reflectivity at one time. In addition, due to introduction of the hydrophobic polymer, the obtained PC films showed obvious color change when exposing to organic solvent vapors. This study provides a basis for building new color system for functional coating, decoration and sensing. Acknowledgements 15
This research was financially supported by the National Natural Science Foundation of China (21878040, 21536002, 21421005), the Fundamental Research Funds for the Central Universities (DUT19TD28), the Natural Science Foundation of Liaoning Province
(2019-MS-037)
and
the
Liaoning
Revitaliztion
Talent
Program
(CLXC1801006). References
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Table 1 Refractive indexes of the four assembly material monolayers and both multilayers with different bilayer number Material
(TiO2/PSS)n
neff of monolayer
(PDAC/PMMA)n
neff (TiO2)
1.72
neff (PDAC)
1.48
neff (PSS)
1.50
neff (PMMA)
1.41
10
30
50
10
30
50
neff of multilayer
1.59
1.74
1.76
1.33
1.46
1.43
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Bilayer number n
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Highlights: 1. We successfully utilized a facile and aqueous-based LbL dip-coating method to fabricate hydrophobic polymer-containing 1D PCs. 2. The organic/inorganic hybrid PCs exhibit brilliant and tunable structural colors by modulating bilayer number, incident angle and stack number. 3. The structural colors could be constructed on both sides of the flat and curved
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substrates to enhance the reflectivity.
Declaration of interests ☑ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: