Electric field induced structural color changes of highly monodisperse hollow Fe3O4@C colloidal suspensions

Colloids and Surfaces A: Physicochem. Eng. Aspects 498 (2016) 74–80

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Electric field induced structural color changes of highly monodisperse hollow Fe3 O4 @C colloidal suspensions Xuanxuan Qiao a,b , Aihua Sun b,∗ , Chongyang Wang b , Chengyi Chu b , Si Ma b , Xiaobing Tang b , Jianjun Guo b , Gaojie Xu b a

The School of Materials Science and Engineering, Shanghai University, Shanghai 20072, PR China Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, & Key Laboratory of Additive Manufacturing Materials of Zhejiang Province, Ningbo 315201, PR China b

h i g h l i g h t s

g r a p h i c a l

• Highly

monodisperse hollow Fe3 O4 @C nanoparticles were prepared with controllable particle sizes. • Hollow Fe3 O4 @C colloidal suspension has a wider tunable range of the optical spectrum and higher diffraction intensity. • Hollow Fe3 O4 @C nanoparticles have a number of advantages as a design material for responsive photonic crystal, like low density, better optical properties and high optical contrast.

In this work, highly monodisperse hollow Fe3 O4 @C nanoparticles were prepared. The responsive photonic crystal properties of hollow Fe3 O4 @C particles were investigated under different electric field. The position and intensity of these reflection peaks varied with the change of the external electric field. Hollow Fe3 O4 @C nanoparticles have a number of advantages as a design material for responsive photonic crystal, like low density, better optical properties and high optical contrast.

a r t i c l e

a b s t r a c t

i n f o

Article history: Received 20 October 2015 Received in revised form 25 February 2016 Accepted 10 March 2016 Available online 14 March 2016 Keywords: Photonic crystal Hollow nanoparticles Optical properties Dielectric spectrum

∗ Corresponding author. E-mail address: [email protected] (A. Sun). http://dx.doi.org/10.1016/j.colsurfa.2016.03.027 0927-7757/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

Highly dispersed hollow Fe3 O4 @C nanoparticles with controllable particle sizes were prepared. Hollow Fe3 O4 @C nanoparticles were chosen not only because of their low density and high optical contrast, but also because of high refractive index, better optical properties. Corresponding colloidal suspension of hollow Fe3 O4 @C photonic crystals in propylene carbonate showed a highly adjustable structure color change upon applying of electric fields. After being stored for 28 days in propylene carbonate solution, these colloidal suspensions can still diffract visible lights when an electric field was applied, which is attributed to low density. Based on the dielectric spectroscopy analysis of hollow Fe3 O4 @C and SiO2 @Fe3 O4 @C colloidal suspensions,a dielectric loss model has been proposed to explain the effect of surface properties on the assembly. © 2016 Elsevier B.V. All rights reserved.

X. Qiao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 498 (2016) 74–80

1. Introduction Coloring in nature mostly comes from the inherent colors of materials, which is called chemical color, but it sometimes has a purely physical origin, such as diffraction or interference interactions of natural light with featured microstructures comparable to visible wavelength, which is called structural color. Over chemical color, structural color has several advantages such as higher saturation, non-fading if microstructures retain unchanged and environmental friendliness. In recent years, a particular collection of creation of creatures, plants and even minerals that can produce color with intricate structures have drawn great attention of an increasing number of researchers. In principle, the external stimulus can be any means that effectively induces a change in the refractive indices of the building blocks or the surrounding matrix and change in the lattice constants or spatial symmetry of the crystalline array [1–3]. These changes will result in a shift in the reflected wavelengths of light and thus in a change of color. The majority of researches on structural color has been focused on incorporating stimulus-responsive materials into the self-assembled photonic crystal structures due to their applications in areas such as color displays, biological and chemical sensors, inks and paints, and many optically active components [4–6]. To obtain the color or color change we desired, monodisperse colloidal nanoparticles has often been applied for fabricating photonic crystal materials prepared by self-assembly using magnetic field, electric field or solvent evaporation. Recently, many inorganic materials with high refractive index and better optical properties, such as Fe3 O4 @SiO2 [4], ZnS@SiO2 [7], SiO2 @TiO2 [8], have been investigated as the responsive photonic crystals materials to tune the structural color under electric field. However, synthesis process of these nanoparticles is complicated, in addition, they have an obvious disadvantage that high density of the nanoparticles (e.g., 4.09 g cm−3 for ZnS and 4.23 g cm−3 for TiO2 , respectively) make the colloidal tends to sediment from the suspension, which hamper the formation of crystalline arrays. Some researchers have synthesized low density polymer nanospheres, like PS [9], PMMA [10], though these nanoparticles suspensions also exhibited photonic colors in response to the applied electric field, there has been little success in creating high quality crystalline colloidal arrays, which may not only be due to optical contrast were very poor, but also due to low refractive index, thus hindering them from practical application. Therefore, there is a strong demand for nanoparticles with low density, high refractive index, better optical properties and high optical contrast. Inorganic hollow microspheres have been attracting wide attention because it not only has the advantages of inorganic materials, but also it has a low density due to hollow structure. Wu et al. has reported the synthesis process of superparamagnetic hollow nanoparticles and self-assembly into 3D colloidal crystals with optical properties [11]. Liu et al. have reported one-pot hydrothermal synthesis of highly monodisperse water-dispersible hollow Fe3 O4 microspheres and the color changes from purple to red by an external magnetic field [12]. However, the magnetic field is often hard to be restricted within a small space without interference each other, and there are much difficulty for magnetic field integrating into existing photonic crystal systems. For the electric field, the disadvantages of the magnetic field can be avoided. In this work, SiO2 @Fe3 O4 @C core-shell particles were synthesized, in which SiO2 as core and Fe3 O4 @C as double shell. Then hollow Fe3 O4 @C particles were obtained by removing SiO2 core. We can easily control the hollow Fe3O4@C particle size. The carbon coating and hydrophilic carboxyl as the major surface group make the hollow Fe3 O4 @C particles disperse in most common nonpolar and polar solutions. Hollow Fe3 O4 @C nanoparticles have a number of advantages as a design material, like low density, better optical properties and high optical contrast. The reflectance

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spectrum of the SiO2 @Fe3 O4 @C and hollow Fe3 O4 @C suspension in propylene carbonate were investigated under different electric field. The results showed that the reasonably adjustable structure color change of high-refractive hollow Fe3 O4 @C suspension could be achieved by a change of the applied electric fields. Meanwhile, we use the dielectric spectrum to explain the effect of surface properties on the assembly because it provides critical insight into the electrokinetic properties of colloid nanoparticles. In addition, the hollow Fe3 O4 @C colloidal suspension shows excellent stability after 28 days. 2. Materials and methods 2.1. Chemicals Tetraethyl silicate (C8 H20 O4 Si, AR), ethanol (C2 H6 OAR, ≥99.7, AR), hydrogen peroxide (H2 O2 , 30%, GR), Acetone (C3 H6 O, 99%,AR) were purchased from Sinopharm Chemicals. NH3 ·H2 O (25–28%, AR), ferrocene (Fe(C5 H5 )2 , ≥99.5%, AR), propylene carbonate (99%, AR) and sodium hydroxide (NaOH, AR) were purchased from Aladdin Chemicals. Indium tin oxide (ITO) coated glasses with a resistivity of 7  cm was purchased from Corning Corp. All chemicals were used as received without further purification. The water used in this work was deionized (DI) water from a Millipor-Q purification system (Millipore, USA) of resistivity 18.2 M  cm. 2.2. Synthesis of SiO2 @Fe3 O4 @C Monodispersed SiO2 nanospheres (about 60, 100, 220 nm) were synthesized following the Stöber method [13], 1 g of ferrocene and 5 mL of silica water solution (5 wt%) were dispersed in 300 mL of acetone. After the solution was intensely sonicated for 30 min, 10 mL of hydrogen peroxide (30 wt%) was dropped in. Then the solution mixture was vigorously stirred for 3 h. Subsequently, the precursor solution was transferred to a teflon-lined stainless autoclave (with a volume of 500 mL). The autoclave was maintained at 100 ◦ C for 24 h and then heated to and maintained at 180 ◦ C for 24 h. After cooling down to room temperature, the purplish grey product was obtained. The product was then separated and washed with deionized (DI) water and ethanol 3 times, respectively. At last, the product was centrifuged at a speed of 6000 rpm for 8 min to separate the final product (SiO2 @Fe3 O4 @C nanoparticles). 2.3. Synthesis of hollow Fe3 O4 @C 0.05 g SiO2 @Fe3 O4 @C nanoparticles were added into 50 mL NaOH (3 mol L−1 ) solution at the temperature of 55 ◦ C for 5 h, the hollow structure particles were obtained. Then the product was centrifuged at a speed of 6000 rpm for 10 min to obtain the pure product with deionized (DI) water and ethanol 3 times, respectively. 2.4. Preparation of devices The photonic display cell consists of transparent top and bottom electrode separated by 200 ␮m thick epoxy spacers [4], and sticking the transparent electrode and 200 ␮m thick epoxy spacers together with UV photoresist. The as-obtained different SiO2 @Fe3 O4 @C suspension and hollow Fe3 O4 @C suspension were injected in between those electrodes by using the conventional injection syringe, then we apply an electric field to the photonic display cell. 2.5. Sample characterization The morphology and particle size distribution of the samples was measure by FEI Tecnai G2 F20 transmission electron micro-

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Fig. 1. (a) TEM images of SiO2 @ Fe3 O4 @C nanoparticles, (b) HR-TEM image of a single SiO2 @ Fe3 O4 @C nanoparticle, (c) TEM images of hollow Fe3 O4 @C nanoparticles, (d) HR-TEM image of a single hollow Fe3 O4 @C nanoparticle.

Fig. 2. Line scanning along the diameter of (a) SiO2 @Fe3 O4 @C nanoparticle and (b) hollow Fe3 O4 @C nanoparticle.

scope (TEM). Field emission scanning electron microscopy images were performed on a S4800 scanning electron microscopy images. The reflection spectra of these colloids suspensions under electric field were measured by an Ocean Optics HR 2000CG-UV-NIR spectrometer coupled with a six-around-one reflection/backscattering probe. Electric field was applied to SiO2 @Fe3 O4 @C core-shell colloids suspensions and hollow Fe3 O4 @C colloids suspensions by a

function generator (Agilent, 33220A) for DC power supply. The dielectric spectra of suspensions was measured by an impedance analyzer (HP 4284A) in the frequency range of 101 –106 Hz using a measuring fixture (HP 16452A) for liquids at room temperature. Photographic images were taken under white fluorescent using a digital camera (Canon, EOS 500D).

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3. Results and discussion Silica nanospheres were coated with Fe3 O4 and carbon by the decomposition of ferrocene. The morphology and structure of the as-obtained nanoparticles was characterized by TEM. Fig. 1 shows that the as-obtained SiO2 @Fe3 O4 @C and hollow Fe3 O4 @C nanoparticles are almost uniform in size, and have a spheroid-like shape. The average diameter is about 150 nm. The HR-TEM images (Fig. 1b) shows the double shelled structure of the colloidal nanoparticle, with an SiO2 diameter of 90 nm, a Fe3 O4 thickness of about 10 nm and a carbon thickness of about 20 nm. The SiO2 @Fe3 O4 @C nanoparticles were added into NaOH solution to remove the silica core in order to obtain low-density hollow nanoparticles. From Fig. 1c, the microspheres have pale center region while the edge is relatively dark, confirming the formation of hollow structure. Fig. 1c also shows that the hollow nanoparticles almost keep their original shapes, which suggests the tightness of the Fe3 O4 shell, but there are also some particles broken which may be because the Fe3 O4 shell is too thin or the carbon shell is missing. A high-resolution TEM image (Fig. 1d) reveals that the Fe3 O4 layer is a secondary nanostructure composed of dozens of primary iron oxide particles and an amorphous carbon shell is distributed on the surface of the Fe3 O4 layer [11]. To further confirm SiO2 is coated by Fe3 O4 and carbon, Fig. 2a is the line scanning along the diameter of SiO2 @Fe3 O4 @C. It turned out that SiO2 @Fe3 O4 @C particles have a three layer construction, with SiO2 core surrounded by magnetite Fe3 O4 and carbon layer. As shown in Fig. 2b, the element silicon is not detected, we can conclude that SiO2 core is completely removed by NaOH solution. As is shown in previous reports [14,15], medium size nanoparticles (ca. 150 nm approximately) self-assemble into stable ordered structures with tunable diffraction covering the entire visible spectrum, thus, the as-obtained 150 nm SiO2 @Fe3 O4 @C and hollow Fe3 O4 @C suspension (10 wt% in propylene carbonate) was injected in photonic display pixel, respectively. Propylene carbonate was selected for our electric tuning experiments due to its low vapour pressure and high electrochemical stability among many choices of solvents. The mechanism for the photonic color tuning is based on the electrophoretic movement of the colloidal nanoparticles to the anode and locally increased particle concentration while maintaining the ordered crystal structure through the electrostatic repulsion between adjacent particles. When potential is applied between the two electrodes, balancing the dominant electrophoretic and electrostatic repulsion forces for certain applied voltages leads to various colors being expressed at those potentials. The reflection peak due to the variation of strength between adjacent particles systematically moves from a longer wavelength to a shorter one with increasing applied voltage, as is shown in Fig. 3a and b. Increasing the applied positive violate to1.5 V, the reflection peak was seen to move from 740 nm to 545 nm. After removing the SiO2 core, the diffraction wavelength range has a little blue shift from 710 nm to 500 nm. For example, the photonic pixel of SiO2 @Fe3 O4 @C was green (peak = 520 nm) at 1.5 V, while the photonic pixel of hollow Fe3 O4 @C was blue (peak = 500 nm) at 1.5 V, which can be explained by the following formula. According to the previous literatures, the diffraction wavelength () of the L-gap associated with reflections from the planes can be estimated from Bragg’s law (Eq. (1)) [16],  = 2dneff

(1)

where d is the plane spacing, the effective refractive (neff ) is calculated from the Maxwell-Garnett average of the refractive indices of nanoparticles and propylene carbonate. The wavelength of the structural color can be adjusted by varying the refractive index of nanoparticles. As is shown in previous reports, further modulation of the refractive index could be effectively obtained by controlling

Fig. 3. (a) Reflection spectra of SiO2 @Fe3 O4 @C, photographs taken at 1.5 V, (b) Reflection spectra of hollow Fe3 O4 @C, photographs taken at 1.5 V.

the volume ratios between the core and the shell data [17]. The value of n can be calculated by equation (Eq. (2)) [18,19]. n = nSiO2 SiO2 + nFe3 O4 Fe3 O4 + ncarbon carbon

(2)

When  is volume fraction, SiO2 is the volume of a SiO2 core divided by the volume of a SiO2 @Fe3 O4 @C nanoparticle. From Fig. 2b, we can calculate the thickness of every layer nanoparticles, so we can calculate the volume of every layer nanoparticles: VSiO2 : VFe3 O4 : Vcarbon = 15:11:224. SiO2 = 15/(15 + 11 + 24) = 0.3. nSiO2 = 1.45, ncarbon = 1.3, nFe3 O4 = 3. Then the result nSiO2 @Fe3 O4 @C is 1.72, while nhollow is 1.51, which is in good agreement with the structure color in Fig. 4. Comparing Fig. 4a and b, we can also notice that the diffraction intensity increases after removing SiO2 core. As is shown in previous reports, well-arranged layers of nanoparticles in suspension can affect the assembly behavior and diffraction intensity. There are a greater number of hollow Fe3 O4 @C nanoparticles in the same display unit due to the lower density when the mass fraction of both SiO2 @Fe3 O4 @C and hollow Fe3 O4 @C suspension are 10 wt% in propylene carbonate, thus hollow Fe3 O4 @C particles can form more particle layers. This conclusion is consistent with our observation in Fig. 4. Our photonic pixel responded very fast to the change of the applied voltage. As shown in Fig. 4, the response time was measured to be 200 ms for on-switching (from 0 V to 1.5 V), while the response time was measured to be approximately 30 s for offswitching (from 1.5 V to 0 V). This is due to the formation of particle aggregates on electrodes attributable to the electrophoretic movement of colloidal particles when a 1.5 V DC electric was applied, and it needs more time to return to its original position for nanoparticles in the display unit after turning off the DC power supply. As shown in Fig. 5, the reflectance peak almost returned to its original position after turning off the DC power supply at 1.5 V after 150 s, which suggests that the structure under the electric field is

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Fig. 4. Response time measurements of a photonic display pixel. a) SiO2 @Fe3 O4 @C suspension, c) hollow Fe3 O4 @C suspension. The square region highlighted with sashed line panel a,c are magnified on panel b,d, respectively.

Fig. 5. Time-dependent reflection spectra of a) SiO2 @Fe3 O4 @C, b) hollow Fe3 O4 @C after turning the DC power supply at 1.5 V.

reversible. From Fig. 6b, we can notice that the after turning off the DC power supply at 1.5 V after 30 s, the reflectance peak of hollow Fe3 O4 @C and SiO2 @Fe3 O4 @C already almost returned to its original position, which is consistent with the conclusion that the response time was measured to be approximately 30 s for offswitching (from 1.5 V to 0 V) in Fig. 4. The dielectric spectroscopy of colloidal suspensions can provide critical insight into the aggregation tendency of the particles, the interface polarization of particles and its response to an applied electric field [20–22]. Fig. 6a and b shows the frequency dependence of the permittivity (␧ ) or SiO2 @Fe3 O4 @C and hollow Fe3 O4 @C suspensions at different bias voltages, respectively. We can see that two suspensions strongly influenced the dielectric spectra at low frequencies(<1000 Hz). The dielectric permittivity (␧ ) of SiO2 @Fe3 O4 @C suspensions have a slower decrease under the increasing voltage than that of hollow Fe3 O4 @C suspensions, which reveals that the polarization rate of the SiO2 @Fe3 O4 @C suspensions is much slower, so there is enough time to assembly. However, too

slow polarization rate is not strong enough to maintain stable interparticle interaction [23], so the position of dielectric permittivity is almost maintained. Fig. 6c and d shows the frequency dependence of the dielectric loss (␧ ) of SiO2 @Fe3 O4 @C and hollow Fe3 O4 @C suspensions at different bias voltages, respectively. With increasing frequency, the dielectric loss (␧ ) first increases and then decreases. At a certain frequency, the suspension has the highest dielectric loss factor. In the low frequency range, the charges have enough time to rearrange because the orientation of dipoles can keep up with the change of frequency. If the frequency is sufficiently high the dipoles become unable to follow the change. Thus the dielectric loss begins to decrease while the dielectric permittivity becomes stable [24,25]. The dielectric loss peak of hollow Fe3 O4 @C suspensions and SiO2 @Fe3 O4 @C suspensions appear at approximately 103 . At the same time, we noticed that the position of the dielectric loss peak moves towards lower frequency with increasing bias voltage. However, the change of the position of the dielectric loss peak for hollow Fe3 O4 @C suspensions is larger than that for SiO2 @Fe3 O4 @C

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Fig. 6. Variation of dielectric permittivity of (a) SiO2 @Fe3 O4 @C suspensions and (b) hollow Fe3 O4 @C suspensions, and dielectric loss of (c) SiO2 @Fe3 O4 @C suspensions and (d) hollow Fe3 O4 @C suspensions at different bias voltages.

Fig. 7. Dispersibility of the nanoparticles (1 hollow Fe3 O4 @C suspension, 2 SiO2 @Fe3 O4 @C suspension) in propylene carbonate: (a) 1 day, (b) 14 day, (c) 28 day.

suspensions. This reveals that the response of hollow Fe3 O4 @C suspensions is stronger, which may be due to the surface properties of hollow Fe3 O4 @C suspensions changed through removing SiO2 core with NaOH solution. Moreover, hydrophilic groups can be generated on the carbon surface of hollow Fe3 O4 @C suspensions, a strong and stable interaction between hollow Fe3 O4 @C suspensions leads to the high electric response activity. After removing SiO2 core, the density of the hollow particles is lower than that of SiO2 @Fe3 O4 @C particles. As is known to us, the density of the material plays an important role to determine the dielectric permittivity, and reducing the density of the material helps to reduce the dielectric permittivity of the material. In addition, the hollow structure of the hollow particles may become an obstacle to the free movement of electrons, thus, the aggregation of free electrons will form charge polarization, which will affect the dielectric loss peak [26]. The as-obtained SiO2 @Fe3 O4 @C particles show excellent solvent dispersibility and water stability in the solution. The carbon coating and hydrophilic carboxyl as the major surface group make the SiO2 @Fe3 O4 @C disperse in most common nonpolar and polar solutions [14]. The stability of SiO2 @Fe3 O4 @C and hollow Fe3 O4 @C

nanoparticles in propylene carbonate was verified as shown in Fig. 7. SiO2 @Fe3 O4 @C and hollow Fe3 O4 @C suspension was placed in two identical transparent bottles. Due to the higher density than that of hollow Fe3 O4 @C suspension, SiO2 coated with Fe3 O4 and carbon was precipitated in propylene carbonate after 28 days, while hollow Fe3 O4 @C particles can still be well dispersed in propylene carbonate. Increasing the applied positive voltage, the reflection peak was seen to move from a longer wavelength to a shorter wavelength. In this case, the average inter-particle distance plays an important role to determine the position of reflection spectra and the structural color. However, SiO2 @Fe3 O4 @C suspensions show severe precipitation after 28 days, the aggregation of nanoparticles make it not used as electric field responsive materials any more. While, hollow Fe3 O4 @C nanoparticles show excellent solvent dispersibility and stabilty after 28 days and can still be used as electric field responsive materials. This kind of electric field induced structural color changes of highly monodisperse hollow Fe3 O4 @C colloidal suspensions with high tenability, low driven voltages, fast response time, high stability and easy fabrication are promising for dynamic photonic displays and sensors [27,28].

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Acknowledgments This research is funded by National Natural Science Foundation of China (Grant no.11374311, 11404347, 11574331), the Ningbo Natural Science Foundation (2014A610123). We also express our gratitude to the aided program for Science and Technology Innovative Research Team of Ningbo Municipality (2015B11002). References [1] S.L. Kuai, G. Bader, P.V. Ashrit, Tunable electrochromic photonic crystals, Appl. Phys. Lett. 86 (2005), 221110. [2] S.O. Lumsdon, E.W. Kaler, J.P. Williams, O.D. Velev, Dielectrophoretic assembly of oriented and switchable two-dimensional photonic crystals, Appl. Phys. Lett. 82 (2003) 949–951. [3] M.K. Maurer, I.K. Lednev, S.A. Asher, Photoswitchable spirobenzopyran-based photochemically controlled photonic crystals, Adv. Funct. Mater. 15 (2005) 1401–1406. [4] I. Lee, D. Kim, J. Kal, H. Baek, D. Kwak, D. Go, E. Kim, C. Kang, J. Chung, Y. Jang, S. Ji, J. Joo, Y. Kang, Quasi-amorphous colloidal structures for electrically tunable full-color photonic pixels with angle-independency, Adv. Mater. 22 (2010) 4973–4977. [5] J.H. Holtz, S.A. Asher, Polymerized colloidal crystal hydrogel films as intelligent chemical sensing materials, Nature 389 (1997) 829–832. [6] J. Ge, Y. Hu, T. Zhang, T. Huynh, Y. Yin, Self-assembly and field-responsive optical diffractions of superparamagnetic colloids, Langmuir 24 (7) (2008) 3671–3680. [7] M.G. Han, C.G. Shin, S.J. Jeon, H.S. Shim, C.J. Heo, H. Jin, J.W. Kim, S.Y. Lee, Full color tunable photonic crystal from crystalline colloidal arrays with an engineered photonic stop-band, Adv. Mater. 24 (48) (2014) 6438–6444. [8] Y. Luo, J. Zhang, A. Sun, C. Chu, S. Zhou, J. Guo, G. Xu, Electric field induced structural color changes of SiO2 @TiO2 core–shell colloidal suspensions, J. Mater. Chem. C 2 (2014) 1990–1994. [9] T.S. Shim, S.H. Kim, J.Y. Sim, J.M. Lim, S.M. Yang, Dynamic modulation of photonic bandgaps in crystalline colloidal arrays under electric field, Adv. Mater. 22 (2010) 4494–4498. [10] H.R. Vutukuri, J. Stiefelhagen, T. Vissers, A. Imhof, A. van Blaaderen, Bonding assembled colloids without loss of colloidal stability, Adv. Mater. 24 (3) (2012) 412–416. [11] K. Cheng, Q. Chen, Z. Wu, M. Wang, H. Wang, Colloids of superparamagnetic shell: synthesis and self-assembly into 3D colloidal crystals with anomalous optical properties, CrystEngComm 13 (17) (2011) 5394–5400. [12] C. Li, Y. Liu, H. Zhang, Q. Zhang, One-pot hydrothermal synthesis of highly monodisperse water-dispersible hollow magnetic microspheres and construction of photonic crystals, Chem. Eng. J. 259 (2015) 779–786.

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