Structural coloration of a colloidal amorphous array is intensified by

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Structural coloration of a colloidal amorphous array is intensified by carbon nanolayers Yukikazu Takeoka, Masanori Iwata, Takahiro Seki, Khanin Nueangnoraj, Hirotomo Nishihara, and Shinya Yoshioka Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00242 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Structural coloration of a colloidal amorphous array is intensified by carbon nanolayers Yukikazu Takeoka1*, Masanori Iwata1, Takahiro Seki1, Khanin Nueangnoraj2,§, Hirotomo Nishihara2, Shinya Yoshioka3 1

Department of Molecular & Macromolecular Chemistry, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan 2

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan 3

Department of Physics, Faculty of Science & Technology, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan §

Present address: School of Bio-Chemical Engineering and Technology (BCET), Sirindhorn International Institute of Technology, Thammasat University, Rangsit Campus,P.O. Box 22, Pathum Thani 12121, Thailand

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ABSTRACT: In this study, we introduce the possibility of applying a colloidal amorphous array composed of fine silica particles as a structural color material to invisible information technology. The appearance of a thick film-like colloidal amorphous array formed from fine silica particles is considerably influenced by incoherent light scattering across the entire visible region. Therefore, regardless of the diameter of the fine silica particles, the thick colloidal amorphous array exhibits a white colour to the naked eye. When carbon is uniformly deposited in the colloidal amorphous array by a pressure-pulsed chemical vapor deposition method, incoherent light scattering in the colloidal amorphous array is suppressed. As a result, coherent light scattering due to the short-range order in the colloidal amorphous array becomes conspicuous, and the array exhibits a vivid structural colour. As structures, such as letters and pictures, can be drawn using this technology, the colloidal amorphous array as a structuralcoloured material may also be applicable for invisible information technology.


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INTRODUCTION In modern advanced technological society, the management of personal information and confidentiality is very important. Therefore, the development of new technologies for information management will become increasingly necessary in the future. The use of invisible information is an important technique for information management.1-18 For example, when drawing an image or character, invisible ink that cannot be recognized with naked eyes but can be visualized by performing specific processing is useful for security and steganography. Technologies capable of visualizing information drawn with invisible ink by light, heat, and chemical reactions have been developed; however, in advanced information management, the development of invisible information technology that can be visualized by different methods is needed. In this paper, we explain the possibility of applying invisible information technology using a structural-coloured material composed of a colloidal particle aggregate. A colloidal amorphous array, in which spherical colloidal particles with uniform diameters have short-range order, can exhibit coherent scattering to strengthen the intensity of light with a wavelength of approximately twice the length of the short-range order.19-47 If the colloidal particles have submicron sizes, the wavelength will be within the visible-light region, and the colloidal amorphous array can exhibit structural colour with low angle dependence. Various methods to form colloidal amorphous arrays from submicron-sized colloidal particles have been studied; rapid drying25, 36 of a colloidal particle suspension and electro-deposition48 are effective methods. Using such methods, a film-like colloidal amorphous array can be easily prepared on a glass substrate or an electrode. When the thickness of the film-like colloidal amorphous array is small, light permeability and the colour of the glass substrate or electrode have a considerable influence on the appearance of the colloidal amorphous array. For example, when a colourless


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and transparent glass substrate is used, the influence of visible light transmitted through the glass substrate becomes large, and the structural colour generated from the colloidal amorphous array becomes difficult to recognize with the naked eye.37, 49 A colloidal amorphous array in the form of a thin film exhibits a vivid structural colour on a black substrate, which does not allow visible light to pass through. Furthermore, to achieve a more vivid structural colour in the colloidal amorphous array, increasing the coherent scattering of light of a specific wavelength is effective. For this purpose, increasing the thickness of the colloidal amorphous array to some extent is desired. As the film thickness of the colloidal amorphous array increases, the influence of visible light transmitted from the back of the substrate diminishes.30 However, as the film thickness of the colloidal amorphous array increases, the influence of incoherent light scattering inside the colloidal amorphous array increases in the entire visible-light region. Due to the increased influence of incoherent light scattering, even if coherent light scattering is present due to the short-range order of the colloidal amorphous array, a clear structured colour cannot be observed with the naked eye. In other words, an extremely thick colloidal amorphous array exhibits a white colour irrespective of the diameter of the colloidal particles constituting the array. If the influence of incoherent light scattering can be suppressed by some method, a vivid structural colour can be observed from the colloidal amorphous array even with a thick film. For example, the introduction of a small amount of a black substance that uniformly absorbs visible light across the entire spectrum into the interior of the colloidal amorphous array is an effective method to suppress incoherent scattering.25, 26, 28, 30, 31 This modification can be accomplished by adding a small amount of black particles, such as carbon black (CB), to the initial colloidal particle suspension in the preparation of the colloidal amorphous array. The obtained colloidal amorphous array can exhibit a vivid structural colour with the naked eye based on the amount of


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added black particles. In addition, a method imparting fine colloidal particles with a black substance in advance has also been studied.38, 41, 45, 46, 50 Until now, films of colloidal amorphous arrays exhibiting vivid structural colours have been obtained by the above-mentioned methods. Introducing a black substance into the colloidal amorphous array after its preparation would certainly obtain a structural-coloured colloidal amorphous array. Here, we report a method that allows the observation of vivid structural colours from colloidal amorphous arrays.

RESULTS AND DISCUSSION Utilizing a pressure-pulsed chemical vapor deposition (p-CVD) method, carbon can be uniformly deposited even in the gap of a three-dimensional microstructure, such as a colloidal amorphous array.51 Under high vacuum, a container containing a colloidal amorphous array was heated to approximately 750 - 800 °C, and then, the system was repeatedly gassed and degassed with acetylene and nitrogen. The gas supply for 1 second and the degassing operation for 60 seconds are set as 1 pulse, and it is repeated for the predetermined number of times. Acetylene was supplied to the gaps between the particles in the colloidal amorphous array, and a carbon film uniformly formed in the aggregation of the colloidal particles (Scheme 1). As an example, Fig. 1 shows the relationship between the p-CVD pulse number and the amount of formed carbon film on the silica particles with sizes from 40 to 50 nm in a pelletized colloidal amorphous array at 800 °C. The amount of carbon coating increases according to the number of repetitions. In addition, transmission electron microscopy (TEM) reveals that carbon uniformly deposited on the surface and inside the pelletized colloidal amorphous array (Fig. S5). Therefore, using the pCVD method, a black substance can be uniformly introduced into the inside of the colloidal


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amorphous array composed of fine silica particles. However, with a large amount of deposited carbon, the carbon layer becomes too thick, and the colour from the colloidal amorphous array become completely black (Fig. S5). Therefore, to ensure a minimal amount of the carbon layer, the experiment was carried out by lowering the temperature of the p-CVD to 750 °C. In this report, using a spray method, we prepared a thick film consisting of secondary particles in a colloidal amorphous array composed of silica fine particles on a glass substrate (Fig. S2b). Formation of the carbon coating using p-CVD controlled the observed structural colour development. Fig. 2 shows optical photographs and scattering spectra of the colloidal amorphous array comprising only silica fine particles with an average size of 277 nm (Fig. S1) and the colloidal amorphous arrays formed with the carbon coating using the p-CVD method. By repeating the supply-degassing cycle of acetylene gas 10 to 300 times in the p-CVD method, the carbon deposition amount on the silica fine particles increases. The scattering spectrum was measured by irradiating light from the direction perpendicular to the surface of the colloidal amorphous array film and placing the detector at 10 ° from the light irradiation direction (Fig. 2b). Regardless of the carbon deposition amount, the scattering spectra of all colloidal amorphous arrays display coherent scattering derived from short-range order with a peak at approximately 680 nm. If the colloidal amorphous array only strongly scatters light of this wavelength, a red structural colour will be observed. However, in the colloidal amorphous array without deposited carbon, light is strongly scattered over the entire visible light region, and therefore, the film appears white with the naked eye. On the other hand, as the carbon deposition amount increases, the light scattering from the colloidal amorphous array decreases overall. Accordingly, coherent scattering is emphasized, and the colloidal amorphous array emits a red colour. The reflection intensity on the lower wavelength side is slightly weakened as compared


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with the system that we have previously reported incorporating carbon black.25 However, since it has little influence on the visible region, it hardly affects the appearance color. Likewise, when colloidal amorphous arrays are formed using silica fine particles with different sizes, the colloidal amorphous array appears white with the naked eye only with silica fine particles; however, when deposited with carbon, the colloidal amorphous array exhibits different vivid structural colours due to coherent scattering (Fig. S8). In Fig. 3, the scattering spectra of the colloidal amorphous arrays prepared using different fine silica particles with average particle sizes from 211 nm and 277 nm were measured with different observation angles and are shown by a false colour map (Figs S9 & S10). The false colour maps show the reflected intensity as function of the observation angle and wavelength.52 As the scattering intensity increases, the colour changes from blue to red. Without deposited carbon, the light scattering due to coherent scattering is not conspicuous because considerable light scattering is exhibited across the entire visible-light region. This tendency is the same regardless of the observation angle. However, light scattering due to coherent scattering becomes emphasized upon carbon deposition, and as a result, a vivid structural colour is observed from the colloidal amorphous array. As the colloidal amorphous array has an isotropic fine structure, coherent scattering can be observed even at changed positions of the detector to measure the scattering spectrum. The peak position slightly changes depending on the angle formed by the light irradiation direction and the observation direction. However, when observing the colloidal amorphous array with reduced incoherent scattering under light irradiation from all directions, such as natural light, the structural colour shows little angle dependence. Next, we will focus on the abovementioned phenomenon regarding application to invisible information technology. Using the spray method, we can draw specific structures, such as letters


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and pictures, composed of the colloidal amorphous array using fine silica particles of different sizes. However, for characters or pictures drawn with only fine silica particles, the appearance of the colloidal amorphous array, which does not depend on the diameter of the silica fine particles, is white with the naked eye. As a result, we cannot depict the drawn letters and pictures (Fig. 4a). However, upon deposition of carbon on the colloidal amorphous array, the characters and pictures drawn by the p-CVD method can be seen with the naked eye (Fig. 4b). Therefore, invisible information drawn with silica fine particles can be visualized by the structural colour generated by carbon deposition. Compared to invisible inks using conventional dyes, structurally colouring materials can be used as superior invisible information technology due to their increased durability.

CONCLUSIONS In this communication, we demonstrated the possibility of forming a new invisible information technology utilizing a film-like colloidal amorphous array comprising fine silica particles with a uniform size. The colloidal amorphous array with a large film thickness exhibits non-coherent scattering in the visible-light region and thus appears white regardless of the size of the silica fine particles. However, when carbon is uniformly deposited in the colloidal amorphous array by p-CVD, incoherent light scattering in the colloidal amorphous array is suppressed. As a result, the coherent scattering in different wavelength regions, which is dependent on the particle size, becomes more apparent, and structures drawn with different sized particles are visible through the structural colour change. Regarding steganography, reversibility is not necessarily required. It is important that materials recorded as invisible information can be visualized in some way.


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Also, it may be important to avoid reading that information again. The method we proposed this time can be used for such applications. For example, by depositing a certain amount of carbon, information is visualized and then a larger amount of carbon is deposited, the information becomes invisible to the naked eye. This method can serve as a new invisible information technology, which is different from previous technologies.

Experimental section Materials The silica particles (Fig. S1: KE-W20, and KE-W30) were purchased from Nippon Shokubai. The average particle diameter of the silica fine particles was measured using a disk centrifugal particle size distribution measuring device (CPS Disc Centrifuge DC 24000 UHR). For the fine spherical particles, a very accurate average particle size and particle size distribution was measured. We used silica particles with the average diameters of 211 nm and 277 nm with low coefficients of variation (CV) (< 5%).

Preparation of colloidal amorphous array We prepared a suspension of the fine submicron spherical silica particles. Methanol was used as the dispersion medium. The membranal colloidal amorphous array was formed on a clear glass plate by spraying the suspension of silica particles (6.0 g) and methanol (9.0 g) using an airbrush system with a 0.2 mm bore at pressures ranging from 0.1 to 0.3 MPa. The suspension was air-


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sprayed onto the glass plate positioned approximately 30 cm from the exit of the spraying nozzle (Fig. S2). Because the solvent rapidly evaporated, the silica particles were dried in air and evenly coated in a powdery state on the glass plate to form a membrane (Fig. S3 & S4); the membrane thickness could easily be controlled up to 1 mm. We prepared the membranal colloidal amorphous arrays to have a thickness of approximately 0.5 mm.

Pressure-pulsed chemical vapor deposition (p-CVD) method Prior to the preparation of brighter structural-coloured membranal colloidal amorphous arrays, the relationship between the carbon deposition amount and the number of pulses as well as uniformity of the carbon coating were examined using thick pelletized colloidal amorphous array composed of silica particles prepared by pressing silica nanoparticles (Snowtex® O-40, Nissan Chemical Industries, Ltd.) (Scheme 1. & Fig. S5.). p-CVD was carried out on these pellets by repeating the evacuation and acetylene feed (20 vol% acetylene with 80 vol% nitrogen gas for 1 s) cycles at 800 °C, followed by a heat treatment in a 100% nitrogen atmosphere for 1 h at 950 °C. Brighter structural-coloured membranal colloidal amorphous arrays were obtained by covering the silica particles that form a colloidal amorphous array with a carbon nanolayer using a modified p-CVD process, in which the CVD temperature was decreased to 750 °C, and the subsequent heat treatment was omitted. After cooling the sample, a carbon-coated silica amorphous array was obtained.

Measurements


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A UV–Vis spectrometer (Nippon Bunko Company, V-670) equipped with an absolute reflectance measurement unit (ARMN-735) was used to measure the relative reflectance spectra. The reflection spectra of the colloidal amorphous arrays varied at different measurement angles. The incident angle normal to the planar surface of the membrane was 0°. The measurement angle, θ, was changed from 10° to 60° relative to the normal of the planar surface of the membrane. To investigate the contribution of the incoherent scattering of light from the colloidal amorphous arrays, polarized reflection spectra were collected (Fig. S6, S7). The arrangement of silica particles in the amorphous colloidal array was investigated with scanning electron microscopy (SEM, Hitachi, Miniscope TM3000 instrument) and TEM (Hitachi, H-800 instrument). The samples were coated with a 10-nm Au-Pd layer, and SEM was operated at 15 kV. The 2D Fourier power spectra were obtained from the SEM images using image analysis software (Image-Pro). A digital camera was used to photograph the structural colours of the samples.


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Figure Captions Scheme. 1 Carbon coating method: Pressure-pulsed chemical vapor depositio Figure 1. Plot of the carbon deposition amount on a pelletized colloidal amorphous array consisting of silica nanoparticles (Snowtex® O-40, Nissan Chemical Industries, Ltd.) versus the CVD pulse number at 800 °C. Figure 2. a) Optical images showing the colour change of the membranal colloidal amorphous array composed of 277 nm spherical silica particles with an increasing amount of adsorbed carbon. CVD pulses: (1) 0, (2) 10, (3) 20, (4) 30, (5) 40, and (6) 50. The scale bar is 1 mm. b) Change in reflection spectra of the membranal colloidal amorphous array of 277 nm spherical silica particles with an increasing amount of adsorbed carbon. The numbers shown in the right show the CVD pulse numbers. Figure 3. False colour maps showing the scattered light intensity as function of the observed angle θ and wavelength from a) the membranal colloidal amorphous array composed of 211 nm spherical silica particles without a carbon layer, b) the membranal colloidal amorphous array composed of 277 nm spherical silica particles without a carbon layer, c) the membranal colloidal amorphous array composed of 211 nm spherical silica particles with a carbon layer from 40 CVD pulses, and d) the membranal colloidal amorphous array composed of 277 nm spherical silica particles with a carbon layer from 40 CVD pulses. Figure 4. a) Star-shaped pattern drawn by the spray method through a mask using 211 nm silica particles on a membrane composed of 277 nm silica particles. Due to strong incoherent light scattering from the thick membrane across the entire visible region, the star-shaped pattern cannot be recognized by the naked eye. The scale bar is 1 mm. b) Following carbon p-CVD, the star-shaped pattern composed of an amorphous array of 211 nm silica particles turned green, while the background composed of an amorphous array of 277 nm silica particles turned red.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Present Addresses Yukikazu Takeoka1, Masanori Iwata1, Takahiro Seki1, Khanin Nueangnoraj2, Hirotomo Nishihara2, Shinya Yoshioka3 1

Department of Molecular & Macromolecular Chemistry, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan 2

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan 3

Department of Physics, Faculty of Science & Technology, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan

Author Contributions Y. T. designed the project. All authors performed experiments. All authors discussed the results and contributed to the data interpretation. Y. T. and H. N. wrote the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was mainly supported by a Grant-in-Aid for Scientific Research on Innovative Areas of “Fusion Materials: Creative Development of Materials and Exploration of Their Function


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through Molecular Control” (no. 2206) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT).

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50. Lee, I.; Kim, D.; Kal, J.; Baek, H.; Kwak, D.; Go, D.; Kim, E.; Kang, C.; Chung, J.; Jang, Y.; Ji, S.; Joo, J.; Kang, Y., Quasi-Amorphous Colloidal Structures for Electrically Tunable FullColor Photonic Pixels with Angle-Independency. Adv Mater 2010, 22, (44), 4973-+. 51. Iwamura, S.; Nishihara, H.; Kyotani, T., Fast and reversible lithium storage in a wrinkled structure formed from Si nanoparticles during lithiation/delithiation cycling. J Power Sources 2013, 222, 400-409. 52. Noh, H.; Liew, S. F.; Saranathan, V.; Mochrie, S. G. J.; Prum, R. O.; Dufresne, E. R.; Cao, H., How Noniridescent Colors Are Generated by Quasi-ordered Structures of Bird Feathers. Adv Mater 2010, 22, (26-27), 2871-2880.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Structural coloration of a colloidal amorphous array is intensified by carbon nanolayers

Yukikazu Takeoka1*, Masanori Iwata1, Takahiro Seki1, Khanin Nueangnoraj2, Hirotomo Nishihara2, Shinya Yoshioka3 1Department

of Molecular Design & Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan 3Department of Physics, Faculty of Science & Technology, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan

2Institute


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Page 19 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Langmuir

!"#$%&'"()*$+(,-(./0($12*3%$45(( Pulse controller Temp. controller

C2H2

Flow monitor

Thermocouple fan

Pressure meter

Evacuation Three-way valve

N2

Pressure meter

Gold furnace

Sample

Gas supply

Vacuum pump

Degassing

Acetylene

!"#$%&'()*&"+,-.),/,&0/&1$& 2,34,5"-65,78%&9:&

Silica particle


Scheme 1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

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Carbon / Silica (g/g)

Langmuir


Fig. 1

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Langmuir

a)

(1)

(2)

(3)

(4) (4)

(5) (5)

(6) (6)

b)


Fig. 2

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

a)

b)

c)

d)


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Fig. 3

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Langmuir

a)

b)

CVD


Fig. 4

Structural coloration of a colloidal amorphous array is intensified by

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