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Fabrication of magnetic field induced structural colored films with tunable colors and its application on security materials
Journal of Colloid and Interface Science 485 (2017) 18–24
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Fabrication of magnetic field induced structural colored films with tunable colors and its application on security materials Shenglong Shang a, Qinghong Zhang a, Hongzhi Wang a,⇑, Yaogang Li b,⇑ a b
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China Engineering Research Centre of Advanced Glasses Manufacturing Technology, MOE, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China
g r a p h i c a l a b s t r a c t A new type of structural colored patterns which display colors depending on the background are used for security materials.
a r t i c l e
i n f o
Article history: Received 27 July 2016 Revised 7 September 2016 Accepted 10 September 2016 Available online 12 September 2016 Keywords: Magnetic field Structure color High physical rigidity Chain-like structure photolithography Security material
a b s t r a c t A flexible, magnetic field induced structurally colored films with brilliant colors and high physical rigidity were reported in this article. Using an external magnetic field, the photocurable colloidal suspensions that containing superparamagnetic Fe3O4@C colloidal nanocrystal clusters (CNCs) could polymerize under UV light. After polymerization, the films with different colors (red, green, blue) were obtained. Through combination of suspensions which contains Fe3O4@C CNCs with different sizes, a series of multi-colored films were obtained. Moreover, these structural colors can be patterned easily by photolithography and various structural colored patterns were shown in the article. The structural colored patterns could conceal or display its color according to the changing of background which makes them hold significant potential applications for security materials. Ó 2016 Published by Elsevier Inc.
1. Introduction The color of nature leads to a colorful world and many creatures in nature make use of these colors to adapt to the surrounding ⇑ Corresponding authors. E-mail addresses: [email protected] (H. Wang), [email protected] (Y. Li). http://dx.doi.org/10.1016/j.jcis.2016.09.016 0021-9797/Ó 2016 Published by Elsevier Inc.
environment. The colorations are usually used to transmit information, warn other animals or mislead their natural enemies. For example, the owl butterfly lived in the rainforests and secondary forests of Mexico, Central and South America, own special patterns in their wings which looks like an owl, these patterns were used to warn their native predators. The chameleon and cuttlefish could rapidly change the color of their skin in response to the
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surrounding environment for camouflage to escape. Although there are many mechanisms to produce colors, most of the colorations in natural creatures are from the pigment color and the structural color [1]. The pigment color is achieved by chemical chromophores while the structural color is usually generated by physical structures. Compared with the pigments, the structural color is durable, anti-photobleaching, efficient in energy consumption and bright [2,3]. The structural color which exhibit brilliant color due to the periodic arrangement of dielectric materials has drawn a lot of interests. These periodic nanostructures are configured as one, two, or three dimensional photonic crystals [4]. During the past two decades, many researchers have achieved great progress in developing the photonic crystal materials through gravitational force, centrifugal force, hydrodynamic flow, electrophoretic deposition and capillary force [5–16]. Photonic crystals not only produce iridescent structural colors due to their directionindependent partial photonic bandgaps [17], but also could tune its color under the external stimuli such as temperature, humidity, mechanical force, electrical field and magnetic field [18]. So far, various approaches have been used to make artificial structural colors through these external stimulis, especially the magnetic field induced structural colors. Magnetic field induced structural colors have been widely studied owing to their controllable color displaying under magnetic field in recent years, the origin of it can be traced back to the year of 1993. Bibette reported that emulsions consisting of ferrofluid droplets can form one dimensional chain structures and diffract light in the visible range under an external magnetic field [19]. Since then, many researchers have done excellent works in this area. For example, Ge et al. fabricated Fe3O4@SiO2 monodisperse super-paramagnetic colloidal nanocrystal clusters (CNCs) which can diffract light when an external magnetic field was applied [20,21]. Later, Wang et al. fabricated carbonencapsulated Fe3O4@C superparamagnetic CNCs using hydrothermal reaction [22]. These particles can also diffract light under external magnetic field and the structural color depends on the size distribution of the particles [23]. More recently, Luo et al. synthesized monodisperse Fe3O4@PVP super-paramagnetic particles by a modified polyol process in the presence of glucose and poly(vinyl pyrrolidone) (PVP) [24]. Compared with the former research, the new material shows a long-range steric repulsion which is sufficiently strong to counteract the magnetic attractive force. Because of these superparamagnetic CNCs could display multi-structural color under magnetic field, various applications including humidity sensor, invisible photonic printing, flexible color display photonic film and structurally colored fiber have been researched [25–30]. However, few works have been done in the area of security materials using superparamagnetic CNCs so far. In this work, we propose a facile and practical method for the creation of magnetic field induced structural colored films using photocurable colloidal suspensions. The superparamagnetic Fe3O4@C CNCs were dispersed in a photocurable resin, when an external magnetic field was applied, the suspensions display structural colors immediately. Under a UV lamp, the UVinduced photo polymerization enabled the rapid solidification of the suspension and a homogenous film was fabricated. Through the mixing of the suspensions which consist of superparamagnetic Fe3O4@C CNCs with different sizes, multi-colored films were obtained. Furthermore, various patterns with different colors were synthesized using a mask. The present structural colored films are expected to have some potential applications such as the security materials because these structural colored films possess high stability and do not need any additional devices to provide energy.
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2. Experimental 2.1. Chemicals and materials Ferrocene (Fe(C5H5)2), hydrogen peroxide (H2O2), acetone (C3H6O), were purchased from Shanghai Chemical Factory, China. The photocurable resin Ethoxylated trimethylolpropane triacrylate (ETPTA) was purchased from Sigma–Aldrich and 2-Hydroxy-2methylpropiophenone (HMPP) as a photoinitiator was purchased from TCI Shanghai Chemical Company. All the chemicals were of analytical reagent grade and used directly without any further treatment. 2.2. Synthesis of Fe3O4@C superparamagnetic colloidal nanocrystal clusters with different sizes Fe3O4@C colloidal nanoparticles were synthesized using a hightemperature hydrolysis reaction procedure that had been reported previously [23]. In brief, ferrocene (0.50 g) was dissolved in acetone (60 ml). After intense sonication for 15 min, hydrogen peroxide (2.0, 2.5, 3.0 ml, respectively) was slowly added to the above mixture solution and was vigorously stirred for 1 h by magnetic stirring. Then the precursor solution was transferred to a Teflonlined stainless autoclave with a total volume of 80 ml and was heated to and maintained at 180 °C. After 72 h, the autoclave was cooled naturally to room temperature and the products were transferred into a beaker. Then the products were magnetized for 30 min by a magnet with 0.25 T and the supernatant was discarded under a magnetic field. The precipitates were then washed with acetone three times to remove excess ferrocene. Finally, the products were dried at room temperature in a vacuum oven and re-dispersed in ethoxylated trimethylolpropane triacrylate (ETPTA) solution containing 1 wt.% 2-Hydroxy-2-methylpropiophenone (HMPP) as a concentration of 10 mg/ml for the further use. 2.3. Film casting of suspensions and photolithography To prepare the magnetic field induced structural colored films, we employed two parallel glass slides separated by 3 layer of Scotch type (3 M, 50 lm in thickness). After infiltration of Fe3O4@CETPTA suspension into the gap, the glass was exposed to UV light for 30 s, at the same time, a magnet with magnetic field of 0.25T was placed at the bottom of the glass. The polymerized film was detached from the glass carefully after polymerization. In order to acquire the structural colored film with multi-colors, Fe3O4@CETPTA suspension which contains Fe3O4@C CNCs with different sizes were mixed at different concentration. Then the films were fabricated as above process. To obtain the structural colored patterns, a two-step process was used to generate the pattern. Firstly, graphics are designed on a personal computer and printed on a transparent slide. Then, the slide with graphics was put on the surface of the two parallel glass slides separated by 3 layer of Scotch type. At this step, those parts which have no graphics were firstly polymerized under UV light. Next, removing the slide and placing a magnet at the bottom of the glass slides. Then the polymerized film was detached from the glass carefully after polymerization. 2.4. Characterization Digital photos were obtained by a digital camera (Nikon D7000, Japan). TEM images were obtained using a transmission electron microscope (JEOL-2100F, Japan). SEM images were recorded using a field emission scanning electron microscopy (FE-SEM) (Hitachi S-4800, Japan). X-ray diffraction patterns were characterized by X-ray diffractometere (XRD, Rigaku D/max2550 V X-ray
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Fig. 1. (a–c) Three representative digital photos of Fe3O4@C-ETPTA suspensions under magnet with the magnetic field of 0.25T and (d–f) the corresponding TEM image of Fe3O4@C CNCs, 138 nm (a, d), 158 nm(b, e), and 200 nm (c, f). (g) X-ray diffraction patterns of (a), (b) and (c).
Fig. 2. Schematic illustrations of the procedure for fabricating magnetic field induced structurally colored films (a) and digital images of Fe3O4@C-ETPTA composite films which are composed of Fe3O4@C particles 138 nm (b), 158 nm (c), and 200 nm (d). (e) Reflectance spectra of the three composite films shown in (b–d). (f, g) SEM images of a cross-section along the magnetic field of the structural colored film with red color. (For interpretation to colours in this figure, the reader is referred to the web version of this paper.)
S. Shang et al. / Journal of Colloid and Interface Science 485 (2017) 18–24
diffractometer using Cu Ka irradiation (k = 1.5406 Å). Refection spectra of the structurally colored films were obtained using a fiber-optic spectrometer(G2000-Pro-Ex, China). The incident light was aligned perpendicular to the film for all the optical measurements. 3. Results and discussion The digital photos of Fe3O4@C-ETPTA suspensions which contain Fe3O4@C CNCs with three kinds of particle size at the same magnetic field are shown in Fig. 1(a–c). The position of the magnet and the cuvette is shown in Fig. S1. The diameters of these three kinds of Fe3O4@C CNCs, measured by calculating the average diameter value of 100 particles, are 138 nm (blue), 158 nm (green) and 200 nm (red) respectively, the representative TEM images of these particles are shown in Fig. 1(d–f). It is worth mentioning that the colloidal suspensions with a particle size of 200 nm, 158 nm and 138 nm can just diffract red, green and blue colors respectively under the same external magnetic field. The size dependence of magnetically responsive photonic crystals has been investigated by the former researchers. In brief, the larger colloidal particles can only diffract red light and the smaller colloidal particles can
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only diffract blue light, while only the medium-size colloidal particles can diffract tunable color according to the external magnetic field [31]. In our experiments, we found that the color of the suspension depends on two factors: one is the size distribution of the nanoparticles, the other is the distance between the magnet and the suspension (the magnet intensity will increase with the decreasing of the distance). The suspension which contains the nanoparticles with 138 nm just diffracts blue light even though the distance between the magnet and the suspension changes. The suspension which contains the nanoparticles with 200 nm just diffracts red light. While the suspension contains the nanoparticles with 158 nm diffracts different colors with the changes of the distance, but under the magnet intensity of 0.25 T, it just diffracts green light. Fig. 1(g) shows typical XRD patterns of these three kinds of Fe3O4@C CNCs. From the XRD patterns, both Fe3O4 (JCPDS file 19-0629) and C peaks can be clearly observed. Calculations with the Debye-Scherrer formula for the strongest peak (3 1 1) gave grain sizes of 5.51, 6.88, 9.18 nm for Fe3O4@C CNCs with sizes of 138, 158 and 200 nm, respectively. Compared with the former research [31], there is a C peak in the XRD patterns. This may results from an amorphous carbon formed on the shell of the Fe3O4@C CNCs under low synthesis temperature.
Fig. 3. Digital photos of the three composite films composed of blue plus green (a), blue plus red (b) and green plus red (c) with equal mass and their representative reflectance spectra (d). (For interpretation to colours in this figure, the reader is referred to the web version of this paper.)
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To fabricate the magnetic field induced structural colored films, a photocurable resin of ETPTA as a dispersion medium for Fe3O4@C CNCs were employed in this work. Fe3O4@C CNCs formed homogenous suspension as a concentration of 10 mg/ml in ETPTA. Then the Fe3O4@C-ETPTA suspension was infiltrated into the space between two parallel glass substrates, the suspension exhibited a grey color in the absence of an external magnetic field. However, after placing a magnet on the bottom of the glass substrates, the suspension displayed a uniform reflection color due to formation of one dimensional chainlike structure of Fe3O4@C CNCs along the magnetic field. Then the suspension solidified under UV lamp within 30 s. After polymerization, the polymerized film was detached from the glass carefully. The schematic illustrations of the procedure for fabricating magnetic field induced structurally colored films were shown in Fig. 2(a). Fig. 2(b–d) shows digital photos of Fe3O4@C-ETPTA composite films which consist of Fe3O4@C CNCs with different sizes. The films diffracted brilliant structural colors from red, green and blue with decreasing diameter of Fe3O4@C CNCs. The typical reflection spectra of these three composite films are shown in Fig. 2(e), the diffraction wavelength shit from 600 nm (red) to 490 nm (blue). From the digital photos and the reflectance spectra, it can be clearly observed that the larger particles leading to reflection spectra at longer wavelength. Fig. 2(f) shows the cross-section of the red film which had a thickness of 150 lm. From the perspective of the amplified SEM image (as shown in Fig. 2(g)), it can be clearly observed that the Fe3O4@C CNCs formed one dimensional chain-like structure in the inner of the film. Although the particles were not spanned the overall thickness of the film, it can also exhibit brilliant structure colors. In order to acquire the structural colored films with various colors, the multi-colored films (cyan, yellow and dark yellow) were fabricated by mixing the Fe3O4@C-ETPTA suspensions which composed of Fe3O4@C CNCs with different sizes. Fig. 3 shows the digital
photos of the three composite films composed of blue plus green (a), blue plus red (b) and green plus red (c) with equal mass. The reflection spectra for these three composite structural colored films showed peaks at 521 nm (blue plus green), 557 nm (blue plus red) and 565 nm (green plus red), as shown in Fig. 3(d). Compared to the former obtained structural colored films (red, green and blue), the multi-colored films have a number of differences. The red, green and blue films have only a narrow width of the main peaks which peaks positon located at 600 nm, 530 nm and 490 nm respectively. However, the multi-colored films’ reflection spectra are different from the film with single color. For instance, the peak position of the film displayed cyan located between blue and green, which indicates that the photonic band-gap in cyan film was different from blue film or red film. Previous research has demonstrated that the mixing of Fe3O4@C CNCs with different diameters could result in photonic band-gap hetero-structures in solution. The applicationof a magnetic field to superparamagnetic colloids in solution results in additional magnetic packing forces (Fm = r (lB) (B is the external field strength and l is the induced magnetic moment) [31,32]. The Fe3O4@C CNCs with large diameters have strong additional magnetic packing forces and only need a weak external field to induce enough magnetic packing forces for assembly into ordered structures, while the Fe3O4@C CNCs with small diameters need a strong external field to induce enough magnetic packing forces for assembly into ordered structures. So in mixed suspensions, the Fe3O4@C CNCs with large diameters will firstly assemble into an ordered chain-like structure then the Fe3O4@C CNCs with small diameters with increase of the magnetic field. That’s lead to a double photonic band-gap hetero-structure in the mixed suspension within an external magnetic field and the reflection spectra of the suspension are split into two peaks [3]. However, the reflection spectra of the films which composed of blue plus green, blue plus red and green plus red just have a single peak in our work. That’s may
Fig. 4. (a–g) Digital photos of composite films which composed of blue plus red with the rate of 1:0, 8:2, 6:4, 5:5, 4:6, 2:8, 0:1 and their corresponding reflectance spectra (h). (For interpretation to colours in this figure, the reader is referred to the web version of this paper.)
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Fig. 5. Schematic illustrations of the procedure for fabricating magnetic field induced structurally colored patterns. (For interpretation to colours in this figure, the reader is referred to the web version of this paper.)
Fig. 6. (a–f) Digital photos of the photonic patterns which displayed different colors based on the background, (a–c) the white background and (e–f) the black background. (g) Digital photos of the structural colored pattern composed of small flower’s that show a reflection color that strongly depends on the angle of incidence of the light, which is given in each image. (For interpretation to colours in this figure, the reader is referred to the web version of this paper.)
result from that both of the Fe3O4@C CNCs with large diameters or small diameters assembly into ordered structures simultaneously under a strong magnetic field (0.25T). That is to say, in the inner of the films, the Fe3O4@C CNCs formed an amorphous photonic crystal structures (as shown in Fig. S2). The amorphous photonic crystals can also exhibit unique light scattering and transport, bring about a variety of interesting phenomena such as isotropic photonic bandgaps or pseudogaps, noniridescent structural colors, and light localization [33].
To illustrate the relationship between the composition and the color of the film, a variety of films which composed of blue plus red with different ratio were studied, as shown in Fig. 4. Fig. 4 shows digital photos of composite films which composed of blue plus red with the rate of 1:0, 8:2, 6:4, 5:5, 4:6, 2:8, 0:1 and their corresponding reflectance spectra. The reflection spectrum of these films illustrates that the color of the film could be tuned by changing its composition. Furthermore, compared with the film which polymerized by
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Fe3O4@C-ETPTA suspensions with single component, the diffraction wavelength of the composite films are wider. The photocurable Fe3O4@C-ETPTA suspension could hide its color withdrawal of the external magnetic field while show its color in the presence of magnetic field. This property makes it possible that the suspension could be patterned through photolithography. Fig. 5 shows schematic illustrations of the procedure for fabricating magnetic field induced structurally colored patterns. Firstly, a transparent slide with printed patterns was designed by computer, then the slide with graphics was put on the surface of the two parallel glass slides which separated by Scotch type and the suspension was injected into the gap. Under UV light, those parts which have no graphics were firstly polymerized while the part covered by the graphics is still suspension. Following this, the slide was removed and a magnet was placed at the bottom of the glass slides, after polymerization, the graphics with structural color was fabricated. Finally, the whole polymerized film was detached from the glass carefully. Fig. 6 shows the digital photos of the films with structural colored patterns. Through Fig. 6, an interesting phenomenon can be clearly observed, the patterns hide their color under white background, however, they could show their color after transfer them to black background. This may result from that the black backgrounds could act as a good background for more saturated colors [34,35]. In addition, the color of the pattern depends on the angle we observed, as shown in Fig. 6(g). The structural colored flower exhibited the bright reflection of red color under normally incident light, with the angle of incident light decrease from 90° to 22°, the reflection color of the flower changed from red to green. According to Bragg’s law, mk = 2nd sin h (m is the diffraction order, k is the wavelength of incident light, n is the effective refractive index, d is the lattice spacing, and h is the glancing angle between the incident light and diffraction crystal plane) [36], the value of m, n, d are constant in a same system, so the value of k just depends on the value of h. With the decreasing of h, the value of k decreased which bring about the structural colored patterns have a blue-shift. These properties of the structural colored patterns meet the demands of the security materials which require the patterns should be easy making and not easy mimicking. 4. Conclusions In summary, we have demonstrated a practical method to fabricate magnetic field induced structural colored films using photocurable suspensions. The films with different colors (red, green, blue) can display brilliant structural colors, which cannot be easily mimicked by pigments of chemical dyes. Through combination of suspensions which contains Fe3O4@C CNCs with different sizes, a series of multi-colored films were fabricated. Moreover, these structural colors can be patterned by photolithography and various structural colored patterns were shown in the article. Furthermore, the structural colored patterns could conceal or display its color according to changing of background which makes them hold significant potential application for security materials. In addition, the present structural colored films are expected to have some potential applications such as security materials, optical filters and reflectors because of these structural colored films possess high stability and do not need any additional devices to provide energy.
Acknowledgements We gratefully acknowledge the financial support by Natural Science Foundation of China (Nos. 51672043, 61674028), The Shanghai Natural Science Foundation (15ZR1401200), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, Program of Shanghai Academic Research Leader (16XD1400100), the Program of Introducing Talents of Discipline to Universities (No. 111-2-04) and the Fundamental Research Funds for the Central Universities (2232014A306). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.09.016. References [1] M. Srinivasarao, Chem. Rev. 1999 (1935) 99. [2] Y.J. Zhao, Z.Y. Xie, H.C. Gu, C. Zhu, Z.Z. Gu, Chem. Soc. Rev. 41 (2012) 3297. [3] H.B. Hu, Q.W. Chen, J. Tang, X.Y. Hu, X.H. Zhou, J. Mater. Chem. 22 (2012) 11048. [4] N. Zhou, A. Zhang, L. Shi, K.Q. Zhang, ACS Macro Lett. 2 (2013) 116. [5] Z.Z. Gu, A. Fujishima, O. Sato, Chem. Mater. 14 (2002) 760. [6] Y. Masuda, T. Itoh, M. Itoh, K. Koumoto, Langmuir 20 (2004) 5588. [7] A.C. Arsenault, D.P. Puzzo, I. Manners, G.A. Ozin, Nature Photon. 1 (2007) 468. [8] Y.N. Xia, B. Gates, Y.D. Yin, Y. Lu, Adv. Mater. 12 (2000) 693. [9] C. Lopez, Adv. Mater. 15 (2003) 1679. [10] F. Marlow, P. Muldarisnur, R..Brinkmann. Sharifi, C. Mendive, Angew. Chem. Int. Ed. 48 (2009) 6212. [11] S.H. Kim, S.Y. Lee, S.M. Yang, G.R. Yi, NPG Asia Mater. 3 (2011) 25. [12] G.V. Freymann, V. Kitaev, B.V. Lotschzc, G.A. Ozin, Chem. Soc. Rev. 42 (2013) 2528. [13] P.V. Braun, R.W. Zehner, C.A. White, M.K. Weldon, C. Kloc, S.S. Patel, P. Wiltius, Adv. Mater. 13 (2001) 721. [14] S.K. Lee, G.R. Yi, S.M. Yang, Lab Chip 6 (2006) 1171. [15] Y. Lu, Y.D. Yin, B. Gates, Y.N. Xia, Langmuir 17 (2001) 6344. [16] M. Holgado, F.G. Santamaria, A. Blanco, M. Ibisate, A. Cintas, H. Miguez, C.J. Serna, C. Molpecceres, J. Requena, A. Mifsud, F. Meseguer, C. Lopez, Langmuir 15 (1999) 4701. [17] H.W. Yin, B.Q. Dong, X.H. Liu, T.R. Zhan, L. Shi, J. Zia, E. Yablonovitch, PANS 109 (2012) 10798. [18] J.P. Ge, Y.D. Yin, Angew. Chem. Int. Ed. 50 (2011) 1492. [19] J. Bibette, J. Magn. Magn. Mater. 122 (1993) 37. [20] J.P. Ge, Y.X. Hu, M. Biasini, W.P. Beyermann, Y.D. Yin, Angew. Chem. Int. Ed. 46 (2007) 4342. [21] J.P. Ge, Y.D. Yin, J. Mater. Chem. 18 (2008) 5041. [22] H. Wang, Y.B. Sun, Q.W. Chen, Y.F. Yu, K. Cheng, Dalton Trans. 39 (2010) 9565. [23] H. Wang, Q.W. Chen, Y.F. Yu, K. Cheng, Y.B. Sun, J. Phys. Chem. C 115 (2011) 11427. [24] W. Luo, H.R. Ma, F.Z. Mou, M.X. Zhu, J.D. Yan, J.G. Guan, Adv. Mater. 26 (2014) 1058. [25] H.B. Hu, Q.W. Chen, K. Cheng, J. Tang, J. Mater. Chem. 22 (2012) 1021. [26] H.B. Hu, J. Tang, H. Zhong, Z. Xi, C.L. Chen, Q.W. Chen, Sci. Rep. 3 (2013) 1484. [27] R.Y. Xuan, J.P. Ge, J. Mater. Chem. 22 (2012) 367. [28] J.P. Ge, Y.D. Yin, Adv. Mater. 20 (2008) 3485. [29] Z.F. Liu, Q.H. Zhang, H.Z. Wang, Y.G. Li, J. Colloid Interf. Sci. 406 (2016) 18. [30] S.L. Shang, Z.F. Liu, Q.H. Zhang, H.Z. Wang, Y.G. Li, J. Mater. Chem. A 3 (2015) 11093. [31] J.P. Ge, Y.X. Hu, Y. Yin, Angew. Chem. Int. Ed. 46 (2007) 7428. [32] X.L. Xu, G. Friedman, K.D. Humfeld, S.A. Majetich, S.A. Asher, Chem. Mater. 14 (2002) 1249. [33] L. Shi, Y.F. Zhang, B.Q. Dong, T.R. Zhan, X.H. Liu, J. Zi, Adv. Mater. 25 (2013) 5314. [34] Z. Shen, L. Shi, B. You, L. Wu, D. Zhao, J. Mater. Chem. 22 (2012) 8069. [35] S.Y. Ye, Q.Q. Fu, J.P. Ge, Adv. Funct. Mater. 24 (2014) 6430. [36] F. Leal Calderon, T. Stora, O. Mondain Monval, P. Poulin, J. Bibette, Phys. Rev. Lett. 72 (1994) 2959.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Fabrication of magnetic field induced structural colored films with tunable colors and its application on security materials Shenglong Shang a, Qinghong Zhang a, Hongzhi Wang a,⇑, Yaogang Li b,⇑ a b
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China Engineering Research Centre of Advanced Glasses Manufacturing Technology, MOE, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China
g r a p h i c a l a b s t r a c t A new type of structural colored patterns which display colors depending on the background are used for security materials.
a r t i c l e
i n f o
Article history: Received 27 July 2016 Revised 7 September 2016 Accepted 10 September 2016 Available online 12 September 2016 Keywords: Magnetic field Structure color High physical rigidity Chain-like structure photolithography Security material
a b s t r a c t A flexible, magnetic field induced structurally colored films with brilliant colors and high physical rigidity were reported in this article. Using an external magnetic field, the photocurable colloidal suspensions that containing superparamagnetic Fe3O4@C colloidal nanocrystal clusters (CNCs) could polymerize under UV light. After polymerization, the films with different colors (red, green, blue) were obtained. Through combination of suspensions which contains Fe3O4@C CNCs with different sizes, a series of multi-colored films were obtained. Moreover, these structural colors can be patterned easily by photolithography and various structural colored patterns were shown in the article. The structural colored patterns could conceal or display its color according to the changing of background which makes them hold significant potential applications for security materials. Ó 2016 Published by Elsevier Inc.
1. Introduction The color of nature leads to a colorful world and many creatures in nature make use of these colors to adapt to the surrounding ⇑ Corresponding authors. E-mail addresses: [email protected] (H. Wang), [email protected] (Y. Li). http://dx.doi.org/10.1016/j.jcis.2016.09.016 0021-9797/Ó 2016 Published by Elsevier Inc.
environment. The colorations are usually used to transmit information, warn other animals or mislead their natural enemies. For example, the owl butterfly lived in the rainforests and secondary forests of Mexico, Central and South America, own special patterns in their wings which looks like an owl, these patterns were used to warn their native predators. The chameleon and cuttlefish could rapidly change the color of their skin in response to the
S. Shang et al. / Journal of Colloid and Interface Science 485 (2017) 18–24
surrounding environment for camouflage to escape. Although there are many mechanisms to produce colors, most of the colorations in natural creatures are from the pigment color and the structural color [1]. The pigment color is achieved by chemical chromophores while the structural color is usually generated by physical structures. Compared with the pigments, the structural color is durable, anti-photobleaching, efficient in energy consumption and bright [2,3]. The structural color which exhibit brilliant color due to the periodic arrangement of dielectric materials has drawn a lot of interests. These periodic nanostructures are configured as one, two, or three dimensional photonic crystals [4]. During the past two decades, many researchers have achieved great progress in developing the photonic crystal materials through gravitational force, centrifugal force, hydrodynamic flow, electrophoretic deposition and capillary force [5–16]. Photonic crystals not only produce iridescent structural colors due to their directionindependent partial photonic bandgaps [17], but also could tune its color under the external stimuli such as temperature, humidity, mechanical force, electrical field and magnetic field [18]. So far, various approaches have been used to make artificial structural colors through these external stimulis, especially the magnetic field induced structural colors. Magnetic field induced structural colors have been widely studied owing to their controllable color displaying under magnetic field in recent years, the origin of it can be traced back to the year of 1993. Bibette reported that emulsions consisting of ferrofluid droplets can form one dimensional chain structures and diffract light in the visible range under an external magnetic field [19]. Since then, many researchers have done excellent works in this area. For example, Ge et al. fabricated Fe3O4@SiO2 monodisperse super-paramagnetic colloidal nanocrystal clusters (CNCs) which can diffract light when an external magnetic field was applied [20,21]. Later, Wang et al. fabricated carbonencapsulated Fe3O4@C superparamagnetic CNCs using hydrothermal reaction [22]. These particles can also diffract light under external magnetic field and the structural color depends on the size distribution of the particles [23]. More recently, Luo et al. synthesized monodisperse Fe3O4@PVP super-paramagnetic particles by a modified polyol process in the presence of glucose and poly(vinyl pyrrolidone) (PVP) [24]. Compared with the former research, the new material shows a long-range steric repulsion which is sufficiently strong to counteract the magnetic attractive force. Because of these superparamagnetic CNCs could display multi-structural color under magnetic field, various applications including humidity sensor, invisible photonic printing, flexible color display photonic film and structurally colored fiber have been researched [25–30]. However, few works have been done in the area of security materials using superparamagnetic CNCs so far. In this work, we propose a facile and practical method for the creation of magnetic field induced structural colored films using photocurable colloidal suspensions. The superparamagnetic Fe3O4@C CNCs were dispersed in a photocurable resin, when an external magnetic field was applied, the suspensions display structural colors immediately. Under a UV lamp, the UVinduced photo polymerization enabled the rapid solidification of the suspension and a homogenous film was fabricated. Through the mixing of the suspensions which consist of superparamagnetic Fe3O4@C CNCs with different sizes, multi-colored films were obtained. Furthermore, various patterns with different colors were synthesized using a mask. The present structural colored films are expected to have some potential applications such as the security materials because these structural colored films possess high stability and do not need any additional devices to provide energy.
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2. Experimental 2.1. Chemicals and materials Ferrocene (Fe(C5H5)2), hydrogen peroxide (H2O2), acetone (C3H6O), were purchased from Shanghai Chemical Factory, China. The photocurable resin Ethoxylated trimethylolpropane triacrylate (ETPTA) was purchased from Sigma–Aldrich and 2-Hydroxy-2methylpropiophenone (HMPP) as a photoinitiator was purchased from TCI Shanghai Chemical Company. All the chemicals were of analytical reagent grade and used directly without any further treatment. 2.2. Synthesis of Fe3O4@C superparamagnetic colloidal nanocrystal clusters with different sizes Fe3O4@C colloidal nanoparticles were synthesized using a hightemperature hydrolysis reaction procedure that had been reported previously [23]. In brief, ferrocene (0.50 g) was dissolved in acetone (60 ml). After intense sonication for 15 min, hydrogen peroxide (2.0, 2.5, 3.0 ml, respectively) was slowly added to the above mixture solution and was vigorously stirred for 1 h by magnetic stirring. Then the precursor solution was transferred to a Teflonlined stainless autoclave with a total volume of 80 ml and was heated to and maintained at 180 °C. After 72 h, the autoclave was cooled naturally to room temperature and the products were transferred into a beaker. Then the products were magnetized for 30 min by a magnet with 0.25 T and the supernatant was discarded under a magnetic field. The precipitates were then washed with acetone three times to remove excess ferrocene. Finally, the products were dried at room temperature in a vacuum oven and re-dispersed in ethoxylated trimethylolpropane triacrylate (ETPTA) solution containing 1 wt.% 2-Hydroxy-2-methylpropiophenone (HMPP) as a concentration of 10 mg/ml for the further use. 2.3. Film casting of suspensions and photolithography To prepare the magnetic field induced structural colored films, we employed two parallel glass slides separated by 3 layer of Scotch type (3 M, 50 lm in thickness). After infiltration of Fe3O4@CETPTA suspension into the gap, the glass was exposed to UV light for 30 s, at the same time, a magnet with magnetic field of 0.25T was placed at the bottom of the glass. The polymerized film was detached from the glass carefully after polymerization. In order to acquire the structural colored film with multi-colors, Fe3O4@CETPTA suspension which contains Fe3O4@C CNCs with different sizes were mixed at different concentration. Then the films were fabricated as above process. To obtain the structural colored patterns, a two-step process was used to generate the pattern. Firstly, graphics are designed on a personal computer and printed on a transparent slide. Then, the slide with graphics was put on the surface of the two parallel glass slides separated by 3 layer of Scotch type. At this step, those parts which have no graphics were firstly polymerized under UV light. Next, removing the slide and placing a magnet at the bottom of the glass slides. Then the polymerized film was detached from the glass carefully after polymerization. 2.4. Characterization Digital photos were obtained by a digital camera (Nikon D7000, Japan). TEM images were obtained using a transmission electron microscope (JEOL-2100F, Japan). SEM images were recorded using a field emission scanning electron microscopy (FE-SEM) (Hitachi S-4800, Japan). X-ray diffraction patterns were characterized by X-ray diffractometere (XRD, Rigaku D/max2550 V X-ray
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Fig. 1. (a–c) Three representative digital photos of Fe3O4@C-ETPTA suspensions under magnet with the magnetic field of 0.25T and (d–f) the corresponding TEM image of Fe3O4@C CNCs, 138 nm (a, d), 158 nm(b, e), and 200 nm (c, f). (g) X-ray diffraction patterns of (a), (b) and (c).
Fig. 2. Schematic illustrations of the procedure for fabricating magnetic field induced structurally colored films (a) and digital images of Fe3O4@C-ETPTA composite films which are composed of Fe3O4@C particles 138 nm (b), 158 nm (c), and 200 nm (d). (e) Reflectance spectra of the three composite films shown in (b–d). (f, g) SEM images of a cross-section along the magnetic field of the structural colored film with red color. (For interpretation to colours in this figure, the reader is referred to the web version of this paper.)
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diffractometer using Cu Ka irradiation (k = 1.5406 Å). Refection spectra of the structurally colored films were obtained using a fiber-optic spectrometer(G2000-Pro-Ex, China). The incident light was aligned perpendicular to the film for all the optical measurements. 3. Results and discussion The digital photos of Fe3O4@C-ETPTA suspensions which contain Fe3O4@C CNCs with three kinds of particle size at the same magnetic field are shown in Fig. 1(a–c). The position of the magnet and the cuvette is shown in Fig. S1. The diameters of these three kinds of Fe3O4@C CNCs, measured by calculating the average diameter value of 100 particles, are 138 nm (blue), 158 nm (green) and 200 nm (red) respectively, the representative TEM images of these particles are shown in Fig. 1(d–f). It is worth mentioning that the colloidal suspensions with a particle size of 200 nm, 158 nm and 138 nm can just diffract red, green and blue colors respectively under the same external magnetic field. The size dependence of magnetically responsive photonic crystals has been investigated by the former researchers. In brief, the larger colloidal particles can only diffract red light and the smaller colloidal particles can
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only diffract blue light, while only the medium-size colloidal particles can diffract tunable color according to the external magnetic field [31]. In our experiments, we found that the color of the suspension depends on two factors: one is the size distribution of the nanoparticles, the other is the distance between the magnet and the suspension (the magnet intensity will increase with the decreasing of the distance). The suspension which contains the nanoparticles with 138 nm just diffracts blue light even though the distance between the magnet and the suspension changes. The suspension which contains the nanoparticles with 200 nm just diffracts red light. While the suspension contains the nanoparticles with 158 nm diffracts different colors with the changes of the distance, but under the magnet intensity of 0.25 T, it just diffracts green light. Fig. 1(g) shows typical XRD patterns of these three kinds of Fe3O4@C CNCs. From the XRD patterns, both Fe3O4 (JCPDS file 19-0629) and C peaks can be clearly observed. Calculations with the Debye-Scherrer formula for the strongest peak (3 1 1) gave grain sizes of 5.51, 6.88, 9.18 nm for Fe3O4@C CNCs with sizes of 138, 158 and 200 nm, respectively. Compared with the former research [31], there is a C peak in the XRD patterns. This may results from an amorphous carbon formed on the shell of the Fe3O4@C CNCs under low synthesis temperature.
Fig. 3. Digital photos of the three composite films composed of blue plus green (a), blue plus red (b) and green plus red (c) with equal mass and their representative reflectance spectra (d). (For interpretation to colours in this figure, the reader is referred to the web version of this paper.)
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To fabricate the magnetic field induced structural colored films, a photocurable resin of ETPTA as a dispersion medium for Fe3O4@C CNCs were employed in this work. Fe3O4@C CNCs formed homogenous suspension as a concentration of 10 mg/ml in ETPTA. Then the Fe3O4@C-ETPTA suspension was infiltrated into the space between two parallel glass substrates, the suspension exhibited a grey color in the absence of an external magnetic field. However, after placing a magnet on the bottom of the glass substrates, the suspension displayed a uniform reflection color due to formation of one dimensional chainlike structure of Fe3O4@C CNCs along the magnetic field. Then the suspension solidified under UV lamp within 30 s. After polymerization, the polymerized film was detached from the glass carefully. The schematic illustrations of the procedure for fabricating magnetic field induced structurally colored films were shown in Fig. 2(a). Fig. 2(b–d) shows digital photos of Fe3O4@C-ETPTA composite films which consist of Fe3O4@C CNCs with different sizes. The films diffracted brilliant structural colors from red, green and blue with decreasing diameter of Fe3O4@C CNCs. The typical reflection spectra of these three composite films are shown in Fig. 2(e), the diffraction wavelength shit from 600 nm (red) to 490 nm (blue). From the digital photos and the reflectance spectra, it can be clearly observed that the larger particles leading to reflection spectra at longer wavelength. Fig. 2(f) shows the cross-section of the red film which had a thickness of 150 lm. From the perspective of the amplified SEM image (as shown in Fig. 2(g)), it can be clearly observed that the Fe3O4@C CNCs formed one dimensional chain-like structure in the inner of the film. Although the particles were not spanned the overall thickness of the film, it can also exhibit brilliant structure colors. In order to acquire the structural colored films with various colors, the multi-colored films (cyan, yellow and dark yellow) were fabricated by mixing the Fe3O4@C-ETPTA suspensions which composed of Fe3O4@C CNCs with different sizes. Fig. 3 shows the digital
photos of the three composite films composed of blue plus green (a), blue plus red (b) and green plus red (c) with equal mass. The reflection spectra for these three composite structural colored films showed peaks at 521 nm (blue plus green), 557 nm (blue plus red) and 565 nm (green plus red), as shown in Fig. 3(d). Compared to the former obtained structural colored films (red, green and blue), the multi-colored films have a number of differences. The red, green and blue films have only a narrow width of the main peaks which peaks positon located at 600 nm, 530 nm and 490 nm respectively. However, the multi-colored films’ reflection spectra are different from the film with single color. For instance, the peak position of the film displayed cyan located between blue and green, which indicates that the photonic band-gap in cyan film was different from blue film or red film. Previous research has demonstrated that the mixing of Fe3O4@C CNCs with different diameters could result in photonic band-gap hetero-structures in solution. The applicationof a magnetic field to superparamagnetic colloids in solution results in additional magnetic packing forces (Fm = r (lB) (B is the external field strength and l is the induced magnetic moment) [31,32]. The Fe3O4@C CNCs with large diameters have strong additional magnetic packing forces and only need a weak external field to induce enough magnetic packing forces for assembly into ordered structures, while the Fe3O4@C CNCs with small diameters need a strong external field to induce enough magnetic packing forces for assembly into ordered structures. So in mixed suspensions, the Fe3O4@C CNCs with large diameters will firstly assemble into an ordered chain-like structure then the Fe3O4@C CNCs with small diameters with increase of the magnetic field. That’s lead to a double photonic band-gap hetero-structure in the mixed suspension within an external magnetic field and the reflection spectra of the suspension are split into two peaks [3]. However, the reflection spectra of the films which composed of blue plus green, blue plus red and green plus red just have a single peak in our work. That’s may
Fig. 4. (a–g) Digital photos of composite films which composed of blue plus red with the rate of 1:0, 8:2, 6:4, 5:5, 4:6, 2:8, 0:1 and their corresponding reflectance spectra (h). (For interpretation to colours in this figure, the reader is referred to the web version of this paper.)
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Fig. 5. Schematic illustrations of the procedure for fabricating magnetic field induced structurally colored patterns. (For interpretation to colours in this figure, the reader is referred to the web version of this paper.)
Fig. 6. (a–f) Digital photos of the photonic patterns which displayed different colors based on the background, (a–c) the white background and (e–f) the black background. (g) Digital photos of the structural colored pattern composed of small flower’s that show a reflection color that strongly depends on the angle of incidence of the light, which is given in each image. (For interpretation to colours in this figure, the reader is referred to the web version of this paper.)
result from that both of the Fe3O4@C CNCs with large diameters or small diameters assembly into ordered structures simultaneously under a strong magnetic field (0.25T). That is to say, in the inner of the films, the Fe3O4@C CNCs formed an amorphous photonic crystal structures (as shown in Fig. S2). The amorphous photonic crystals can also exhibit unique light scattering and transport, bring about a variety of interesting phenomena such as isotropic photonic bandgaps or pseudogaps, noniridescent structural colors, and light localization [33].
To illustrate the relationship between the composition and the color of the film, a variety of films which composed of blue plus red with different ratio were studied, as shown in Fig. 4. Fig. 4 shows digital photos of composite films which composed of blue plus red with the rate of 1:0, 8:2, 6:4, 5:5, 4:6, 2:8, 0:1 and their corresponding reflectance spectra. The reflection spectrum of these films illustrates that the color of the film could be tuned by changing its composition. Furthermore, compared with the film which polymerized by
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Fe3O4@C-ETPTA suspensions with single component, the diffraction wavelength of the composite films are wider. The photocurable Fe3O4@C-ETPTA suspension could hide its color withdrawal of the external magnetic field while show its color in the presence of magnetic field. This property makes it possible that the suspension could be patterned through photolithography. Fig. 5 shows schematic illustrations of the procedure for fabricating magnetic field induced structurally colored patterns. Firstly, a transparent slide with printed patterns was designed by computer, then the slide with graphics was put on the surface of the two parallel glass slides which separated by Scotch type and the suspension was injected into the gap. Under UV light, those parts which have no graphics were firstly polymerized while the part covered by the graphics is still suspension. Following this, the slide was removed and a magnet was placed at the bottom of the glass slides, after polymerization, the graphics with structural color was fabricated. Finally, the whole polymerized film was detached from the glass carefully. Fig. 6 shows the digital photos of the films with structural colored patterns. Through Fig. 6, an interesting phenomenon can be clearly observed, the patterns hide their color under white background, however, they could show their color after transfer them to black background. This may result from that the black backgrounds could act as a good background for more saturated colors [34,35]. In addition, the color of the pattern depends on the angle we observed, as shown in Fig. 6(g). The structural colored flower exhibited the bright reflection of red color under normally incident light, with the angle of incident light decrease from 90° to 22°, the reflection color of the flower changed from red to green. According to Bragg’s law, mk = 2nd sin h (m is the diffraction order, k is the wavelength of incident light, n is the effective refractive index, d is the lattice spacing, and h is the glancing angle between the incident light and diffraction crystal plane) [36], the value of m, n, d are constant in a same system, so the value of k just depends on the value of h. With the decreasing of h, the value of k decreased which bring about the structural colored patterns have a blue-shift. These properties of the structural colored patterns meet the demands of the security materials which require the patterns should be easy making and not easy mimicking. 4. Conclusions In summary, we have demonstrated a practical method to fabricate magnetic field induced structural colored films using photocurable suspensions. The films with different colors (red, green, blue) can display brilliant structural colors, which cannot be easily mimicked by pigments of chemical dyes. Through combination of suspensions which contains Fe3O4@C CNCs with different sizes, a series of multi-colored films were fabricated. Moreover, these structural colors can be patterned by photolithography and various structural colored patterns were shown in the article. Furthermore, the structural colored patterns could conceal or display its color according to changing of background which makes them hold significant potential application for security materials. In addition, the present structural colored films are expected to have some potential applications such as security materials, optical filters and reflectors because of these structural colored films possess high stability and do not need any additional devices to provide energy.
Acknowledgements We gratefully acknowledge the financial support by Natural Science Foundation of China (Nos. 51672043, 61674028), The Shanghai Natural Science Foundation (15ZR1401200), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, Program of Shanghai Academic Research Leader (16XD1400100), the Program of Introducing Talents of Discipline to Universities (No. 111-2-04) and the Fundamental Research Funds for the Central Universities (2232014A306). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.09.016. References [1] M. Srinivasarao, Chem. Rev. 1999 (1935) 99. [2] Y.J. Zhao, Z.Y. Xie, H.C. Gu, C. Zhu, Z.Z. Gu, Chem. Soc. Rev. 41 (2012) 3297. [3] H.B. Hu, Q.W. Chen, J. Tang, X.Y. Hu, X.H. Zhou, J. Mater. Chem. 22 (2012) 11048. [4] N. Zhou, A. Zhang, L. Shi, K.Q. Zhang, ACS Macro Lett. 2 (2013) 116. [5] Z.Z. Gu, A. Fujishima, O. Sato, Chem. Mater. 14 (2002) 760. [6] Y. Masuda, T. Itoh, M. Itoh, K. Koumoto, Langmuir 20 (2004) 5588. [7] A.C. Arsenault, D.P. Puzzo, I. Manners, G.A. Ozin, Nature Photon. 1 (2007) 468. [8] Y.N. Xia, B. Gates, Y.D. Yin, Y. Lu, Adv. Mater. 12 (2000) 693. [9] C. Lopez, Adv. Mater. 15 (2003) 1679. [10] F. Marlow, P. Muldarisnur, R..Brinkmann. Sharifi, C. Mendive, Angew. Chem. Int. Ed. 48 (2009) 6212. [11] S.H. Kim, S.Y. Lee, S.M. Yang, G.R. Yi, NPG Asia Mater. 3 (2011) 25. [12] G.V. Freymann, V. Kitaev, B.V. Lotschzc, G.A. Ozin, Chem. Soc. Rev. 42 (2013) 2528. [13] P.V. Braun, R.W. Zehner, C.A. White, M.K. Weldon, C. Kloc, S.S. Patel, P. Wiltius, Adv. Mater. 13 (2001) 721. [14] S.K. Lee, G.R. Yi, S.M. Yang, Lab Chip 6 (2006) 1171. [15] Y. Lu, Y.D. Yin, B. Gates, Y.N. Xia, Langmuir 17 (2001) 6344. [16] M. Holgado, F.G. Santamaria, A. Blanco, M. Ibisate, A. Cintas, H. Miguez, C.J. Serna, C. Molpecceres, J. Requena, A. Mifsud, F. Meseguer, C. Lopez, Langmuir 15 (1999) 4701. [17] H.W. Yin, B.Q. Dong, X.H. Liu, T.R. Zhan, L. Shi, J. Zia, E. Yablonovitch, PANS 109 (2012) 10798. [18] J.P. Ge, Y.D. Yin, Angew. Chem. Int. Ed. 50 (2011) 1492. [19] J. Bibette, J. Magn. Magn. Mater. 122 (1993) 37. [20] J.P. Ge, Y.X. Hu, M. Biasini, W.P. Beyermann, Y.D. Yin, Angew. Chem. Int. Ed. 46 (2007) 4342. [21] J.P. Ge, Y.D. Yin, J. Mater. Chem. 18 (2008) 5041. [22] H. Wang, Y.B. Sun, Q.W. Chen, Y.F. Yu, K. Cheng, Dalton Trans. 39 (2010) 9565. [23] H. Wang, Q.W. Chen, Y.F. Yu, K. Cheng, Y.B. Sun, J. Phys. Chem. C 115 (2011) 11427. [24] W. Luo, H.R. Ma, F.Z. Mou, M.X. Zhu, J.D. Yan, J.G. Guan, Adv. Mater. 26 (2014) 1058. [25] H.B. Hu, Q.W. Chen, K. Cheng, J. Tang, J. Mater. Chem. 22 (2012) 1021. [26] H.B. Hu, J. Tang, H. Zhong, Z. Xi, C.L. Chen, Q.W. Chen, Sci. Rep. 3 (2013) 1484. [27] R.Y. Xuan, J.P. Ge, J. Mater. Chem. 22 (2012) 367. [28] J.P. Ge, Y.D. Yin, Adv. Mater. 20 (2008) 3485. [29] Z.F. Liu, Q.H. Zhang, H.Z. Wang, Y.G. Li, J. Colloid Interf. Sci. 406 (2016) 18. [30] S.L. Shang, Z.F. Liu, Q.H. Zhang, H.Z. Wang, Y.G. Li, J. Mater. Chem. A 3 (2015) 11093. [31] J.P. Ge, Y.X. Hu, Y. Yin, Angew. Chem. Int. Ed. 46 (2007) 7428. [32] X.L. Xu, G. Friedman, K.D. Humfeld, S.A. Majetich, S.A. Asher, Chem. Mater. 14 (2002) 1249. [33] L. Shi, Y.F. Zhang, B.Q. Dong, T.R. Zhan, X.H. Liu, J. Zi, Adv. Mater. 25 (2013) 5314. [34] Z. Shen, L. Shi, B. You, L. Wu, D. Zhao, J. Mater. Chem. 22 (2012) 8069. [35] S.Y. Ye, Q.Q. Fu, J.P. Ge, Adv. Funct. Mater. 24 (2014) 6430. [36] F. Leal Calderon, T. Stora, O. Mondain Monval, P. Poulin, J. Bibette, Phys. Rev. Lett. 72 (1994) 2959.