Article pubs.acs.org/Langmuir
Redox-Triggered Coloration Mechanism of Electrically Tunable Colloidal Photonic Crystals Ho-Sung Yang,†,‡ Jinhyeok Jang,†,‡ Byoung-Sun Lee,§ Tae-Hyung Kang,†,∥ Jong-Jin Park,⊥ and Woong-Ryeol Yu*,† †
Department of Materials Science and Engineering (MSE) and Research Institute of Advanced Materials (RIAM), Seoul National University, 599 Gwanangno, Gwanak-gu, Seoul 151-742, Republic of Korea § Department of Nanoengineering, University of California, San Diego, California 92093, United States ⊥ School of Polymer Science & Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea S Supporting Information *
ABSTRACT: Electrically tunable colloidal photonic crystals (ETPCs) have been investigated because of several merits such as easy color tunability, no discoloration, and clear color. The coloration mechanism of ETPCs has been explained in terms of only the electric field. Herein, we report on a new mechanism: electric field plus redox reaction. Specifically, the coloration behavior of ETPCs was investigated under electrically conductive or insulated conditions using current− voltage, cyclic voltammetry, and zeta potential measurements, as well as scanning electron microscopy. Electrophoretic movement of ETPC particles toward the positive electrode was caused by the electric field due to the particles’ negative surface charge. At the positive electrode, ETPC particles lost their electrons and formed a colloidal crystal structure. Finally, an ETPC transparent tube device was constructed to demonstrate the coloration mechanism.
1. INTRODUCTION Since the concept of photonic crystals was first suggested by John and Yablonovitch,1,2 much research on them has been carried out because of their many merits, including easy color tunability, no discoloration, and clear color. Photonic crystals consist of periodic dielectric nanostructures that form photonic band gaps, preventing certain wavelengths of light from propagating through the crystal; thus, light is selectively reflected by Bragg optical diffraction in the visible wavelength regime.3 As a result, the colors formed from photonic crystals are called structural colors, distinguished from artificial colors from dyes or pigments. Photonic crystals have been used in the form of thin films in lenses and mirrors, allowing easy control of the light flow.4 Recently, photonic crystals have also appeared as colorimetric sensors for detecting various organic solvents.5,6 On the other hand, photonic crystals have attracted much attention as energy-saving alternatives in the display field,7 promoting various investigations for several decades. The fabrication of three-dimensional (3D) photonic crystals has been a particular focus because of the high stability of their photonic band gap and the high convenience of light control. Among a wide variety of forms, the ordered assembly of colloidal spheres has been studied as potential 3D photonic crystals8−11 because such a colloidal crystal with a periodicity on the order of visible © XXXX American Chemical Society
wavelengths exhibits a structural color similar to that of natural opal.12 Furthermore, colloidal crystals can be constructed from low-cost materials, such as silica7,13,14 and copolymers.15,16 Colloidal crystals are preferred over film or composite forms15 mainly because they can be assembled spontaneously over a wide range of lengths. Various stimuli have been used for the coloration of colloidal crystals, including redox with metallopolymer,17,18 temperature,19,20 pH,21,22 stress,23−25 magnetic fields,26,27 and electric fields.28−31 Among these stimuli, electric fields have commonly been used because they can effectively modulate the periodicity of colloidal crystals with a fast electrophoretic rate of particles and easy control of the intensity of the stimuli.32 Negatively or positively charged particles are well-dispersed in colloidal solution with electrostatic repulsive forces between particles. Once the electric field is applied, the electrophoretic movement of the particles to the oppositely charged electrode is initiated. The higher the applied voltage, the closer the distance between particles. The periodicity of the particles is achieved once a balance is established between the electrostatic repulsive and electrophoretic forces. As the electric field increases, the color Received: June 12, 2017 Revised: August 11, 2017 Published: August 14, 2017 A
DOI: 10.1021/acs.langmuir.7b01919 Langmuir XXXX, XXX, XXX−XXX
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Photographic images of ETPC were obtained under white fluorescent lamp light conditions with a digital camera (CMOS 16.0 MP; Samsung, Seoul, Korea) and a digital camcorder (DCR-SR85; Sony, Tokyo, Japan). Image processing was carried out using Adobe Photoshop CC 2014 imaging software (Adobe Inc., Mountain View, CA). The tone and chroma values of the images from tube-type device increased by 20 and 30, respectively, to show the color difference of extreme cases. Reflectance spectra in the visible wavelength range were also obtained using a USB 4000 spectrometer (Ocean Optics, Dunedin, FL) in reflection mode. The surface morphologies and surface chemistry of the raw (before experiment) and colored (after experiment) colloidal particles were investigated using field-emission scanning electron microscopy (FE-SEM) (JSM-7600F; JEOL, Japan) and X-ray photoelectron spectroscopy (XPS) (Axis-HIS, Kratos Inc.). All samples were dried in a vacuum oven at 150 °C for 2 h to remove the solvent. Finally, the ETPCs were measured for their current−time (I−t) behavior using an electrometer (6517B; Keithley Instruments, Cleveland, OH) and cyclic voltammetry (WBCS3000S; WonATech, Seoul, Korea) using a planar device. Cyclic voltammetry was conducted over five cycles at a 0.5 V/s scan rate under conductive conditions.
of the photonic crystal changes from red to blue, as a result of a reduction in the interparticle distance and lattice constant of colloidal crystals. This is a commonly mentioned mechanism of the coloration of photonic crystal colloids. In this mechanism, the driving force inducing the coloration caused by electrophoretic movement is only the electric field.29,33 Note that almost all of the experiments with photonic crystal colloids have been conducted between two electrodes of indium tin oxide (ITO) glass. In this article, we report a new mechanism regarding the coloration of electrically tunable colloidal photonic crystals (ETPCs). Experiments were performed to investigate how colloidal particles behave in electric fields and how colloidal crystals form at the positive electrode under electrically conductive and insulated conditions. Through these experiments, we found that the coloration mechanism of ETPCs is not only an effect related to the electric field, but is also a consequence of the redox reaction of colloidal particles at the electrode of the ETPCs. Finally, to expand the observed mechanism for photonic crystal fibers, experiments were performed using a tube-type transparent electrode.
3. RESULTS AND DISCUSSION 3.1. Basic Results on ETPCs. The movement of the colloidal particles in the planar device (Figure S1a) as a function of the applied electric field was recorded in a video (Movie S1). (The video recorded the cross section of the device between two ITO glasses.) Figure 1 shows a captured
2. EXPERIMENTAL SECTION 2.1. Materials. An ETPC solution was obtained from a commercial source (ETX, Nanobrick, Gyeonggi-do, Korea). In the solution, polystyrene (PS) particles (average diameter = 245 nm) having phenoxide ion groups with negative charges on their surfaces (zeta potential = −68 mV) were dispersed in a propylene carbonate (PC) solvent at a concentration of 25 wt % with 2 wt % NaOH·H2O as an additive. There was no surfactant in the ETPC solutions, and the PS particles were stabilized by only their surface charges. ITO-coated glass (ITO glass; surface resistivity = 8−12 Ω/sq, Sigma-Aldrich, St. Louis, MO) and ITO-coated poly(ethylene terephthalate) film (ITO-PET film; surface resistivity = 60 Ω/sq, Sigma-Aldrich) were used for the electrodes as received. A coverslip (22 × 22 × 0.1 mm, Duran Group, Wertheim, Germany) was used as a spacer. 2.2. Preparation of Sample Devices. Two types of ETPC devices were prepared: planar and tube devices. For the planar devices, two ITO glasses were used as negative and positive electrodes, as shown in Figure S1a. The ETPC solution was injected into the space between the two ITO glasses. The spacer between the ITO glasses at the end of the device was introduced using the coverslip (0.1 mm). Two assembly types were prepared to simulate (1) conductive (ETPC solution between ITO glasses) and (2) insulated (ETPC solution between insulator/ITO glasses) conditions. An electric field was applied to the devices using a direct-current (dc) power supply to investigate the color change of the ETPC solutions. A tube-type device containing ETPCs was also prepared as shown in Figure S1b. ITO-PET film was rolled into a cylindrical shape (diameter = 6 mm, length = 10 cm). To form the tube, super glue and double-sided tape were used in the overlapped parts of the film. Both sides of the cylinder were filled with glue using a glue gun. Before the hardening of the glue, a conductive (iron) wire was inserted into the middle of the tube. After the glue had hardened, ETPC solutions (25 and 2.5 wt %) were injected using a syringe needle (15 GP). The hole made from the syringe needle was again filled with glue. Then, the ITO-PET film and the conductive wire were connected with positive and negative sockets, respectively, to the dc power supply for the application of an electric field. 2.3. Characterization. The particle size of the ETPCs was measured using a dynamic light scattering spectrophotometer (DLS700; Otsuka Electronics, Osaka, Japan). To compare the surface charges of the ETPCs before and after the colorations, the zeta potentials of the particles were measured at the positive electrode with an electrophoretic light-scattering spectrophotometer (ELS-8000; Otsuka Electronics).
Figure 1. Captured side images of the ETPC solution between ITO glasses at (a) 0 and (b) 5 V. A movie is provided in the Supporting Information, showing a continuous color change as the electric potential increases.
image from the video. The entire ETPC solution without an applied electric field exhibited various colors mixed with iridescent white light (Figure 1a). When an electric field was applied, the ETPC particles moved toward the positive electrode; a distinct boundary line was visible between the ETPC particles and the PC solvent (Figure 1b). Before application of an electric field, the colloidal particles were initially in a free state and experienced only repulsive forces in the proximity of other particles. In this state, the interparticle distance was determined by the volume fraction of colloidal particles having no regularity.32 Under the electric field, B
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color), implying that the ETPC particles had crystallized. The peak was broad, which was probably due to randomly arranged particle layers below the crystallized uppermost layer. At 2.7 V, the blue shift was observed, and the peak became sharp and split. The main peak at 500 nm represents a reduced interparticle distance in the uppermost crystallized layers. A shoulder peak appeared at about 650 nm. We believe that this is due to crystallized particle layers in the bottom side, which were once randomly arranged at 1.9 V. Overall, the spectrum at 2.7 V was broad, implying that randomly arranged particle layers still existed. At 3.5 V, the peak became sharp and clearly split into peaks at 575 and 420 nm. As explained for the 2.7 V case, two crystallized layers were formed at the uppermost layer (420 nm) and the bottom layer (575 nm) with different interparticle distances. Note that the shoulder peak at 2.7 V became strong and sharp at 3.5 V, implying that more crystallization occurred throughout the ETPC solution; that is, multiple scattering no longer occurred, and noniridescent color was observed (Figure 2a). The shoulder peaks or split peaks as discussed above can be confirmed phenomenologically in Figure 1b, showing different colors according to the positions of the layers. 3.2. Coloration Mechanisms of ETPC Solution. The same experiment as described in section 3.1 was carried out under insulated conditions [i.e., insulator (coverslip glass) inserted between the ITO glass and ETPC solution]. Prior to experiments, simulation studies were performed using COMSOL Multiphysics to confirm that the electric field was properly applied to the ETPCs under the conductive and insulated conditions used in this study (Figures S2−S4). The applied electric field was not much different (i.e., 1.5 × 104 and 0.6 × 104 V/m for conductive and insulated conditions, respectively). The electric field intensity can be obtained from the derivative of the electric potential with respect to the position (constant slope of Figure S4b), showing that there was no electric field distribution. Figure 3 shows that the iridescent color (Figure 3a) and the reflection spectra did not show any
negatively charged colloidal particles moved toward the positive electrode (electrophoretic movement) and became concentrated as a result of interactions from both adjacent particles and numerous surrounding particles toward the positive electrode.33−35 The color change of the planar device (Figure S1a) was observed as a function of the voltage (Figure 2a). As the voltage
Figure 2. (a) Color change and (b) reflection spectra of the ETPC solution with increasing applied voltages between ITO glasses under conductive conditions.
increased, the color changed from iridescent (angle-dependent) white (0 V) to red (1.9 V), green (2.7 V), and noniridescent (angle-independent) blue (3.5 V). The color change occurred momentarily upon application of a voltage (time elapsed for color change was not perceptible by the human eye). This color change was attributed to the change in the interparticle distance and the balance between the electrophoretic force of individual particles and their repulsive force. As the electric field increased, the interlayer distance between colloidal particles gradually decreased because of the electrophoretic movement of the colloidal particles. The changes in the interparticle distance can be explained in terms of the electric potential. We approximately calculated the net charges of particles from the zeta potential of the particles and the point electrode by the applied electric potential using the Q−V relationship (Q = 4πε0rV) and the Columbic law. We estimated that the attractive force between the particles and the point electrode was roughly at least 30 times larger than the repulsive force between two particles. In addition, the oxidation reaction occurs, and the repulsive force decreases. In fact, the attractive force between the electrode and the particle becomes much larger, resulting in decreased interparticle distances. From Bragg’s law combined with Snell’s law,36 decreased interparticle distances reflect light with shorter wavelengths; that is, the reflected light blue-shifted. This mechanism can be indirectly confirmed through reflectance spectra. Reflectance spectra in the visible wavelength range were obtained with continuously increasing voltage applied to the planar device (Figure 2b). The peak (480 nm) observed at 0 V represents the original color of the ETPC solution, mainly weak blue with various mixed colors (iridescent color due to multiple scattering28,37), and its intensity was low. As the voltage was increased to 1.9 V, a peak with high intensity appeared at about 560 nm (reddish
Figure 3. (a) Color change and (b) reflection spectra of the ETPC solution with increasing applied voltages between ITO glasses under insulated conditions (with an insulator insert between the ITO glass and the ETPC solution). C
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Figure 5. Current vs time plot for 25 wt % ETPC solution and PC solvent. The inset shows current vs time for a short time (50 s).
Figure 5 can be misunderstood like I−t curves of capacitors (mainly composed of non-Faradaic charges). The actual behavior of Figure 5, however, is very different from that of capacitors because charging and discharging in a capacitor are completed within only several seconds,38,39 whereas the current in Figure 5 was saturated over 1200 s. This fact strongly supports the conclusion that Faradaic current mostly accounted for the total current in Figure 5. Initially, the current through the ETPCs was 1 order of magnitude larger than the current through the PC solvent. However, as the experiment progressed (over 1 h), the current through the ETPC solution and PC solvent converged to the similar value. This indicates that colloidal particles in ETPC play an important role in additional current flow and that as the time progressed, the charges of the colloidal particles disappeared. This phenomenon was also investigated using cyclic voltammetry measurements of the ETPC solution and PC solvent (Figure 6).
Figure 4. Scanning electron microscopy (SEM) images of colloidal particles (a,b) before and (c−f) after application of an electric field (3 V) under (c,d) insulated conditions (with an insulator insert) and (e,f) conductive conditions (without an insulator insert).
images of the colloidal particles for raw ETPCs (Figure 4a,b) and ETPCs after electric field application under insulated (Figure 4c,d) and conductive (Figure 4e,f) conditions. The colloidal particles under insulated conditions were located randomly under an applied electric field (Figure 4c,d). The formation of colloidal crystals with electric field application only, generally known as the major factor causing electrophoretic movement of ETPC particles, did not occur, that is, no geometric crystallinity was observed in this case. This phenomenon was also evident in the recorded video (Movie S2). In contrast, a 3D colloidal crystal structure is clearly visible under conductive conditions, as shown in Figure 4e,f. Therefore, we conclude that a different parameter, in addition to the electric field, plays an important role in the coloration of ETPCs. Considering conductive and insulated boundary conditions, we easily deduced that the electric current would definitely be different in the two cases. Because the electric current was the only different parameter under the two sets of conditions, it can be claimed that the electric current brought about different coloration. I−t experiments were carried out on the ETPC solution and PC-only solvent to investigate which part of the solution (particles or solvent) contributes to the current, in an attempt to reveal the coloration mechanism. Two planar devices including ETPC solution and pure solvent were prepared to have the same volume and contract area with the electrodes. Figure 5 compares the current flow through the ETPC solution and PC solvent at 5 V for 1 h. The total current is the sum of non-Faradaic (capacitive) and Faradaic currents.
Figure 6. Cyclic voltammetry behavior of 25 wt % ETPC solution and a PC sovlent: comparison of ETPC and PC at the first and fifth cycles (scan rate = 0.5 V/s).
Similarly to the I−t results, the current in the ETPC solution was larger than that in the PC solution for every voltage step, again supporting the conclusions that colloidal particles in ETPC solutions play an important role in additional current flow and, furthermore, that redox reactions occurred. Moreover, in the first cycle of ETPC solution, the current started to D
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Figure 7. Redox mechanism of ETPC solution: (a) oxidation of colloidal particles and (b) reduction of propylene carbonate solvent.
increase at about 2 V, which is similar to the voltage of the first color change of the ETPCs (Figure 2a). The I−t and cyclic voltammetry measurements provide convincing evidence that the coloration of ETPCs accompanies the oxidation of the surface charge on colloidal particles. From these results, the redox reaction of ETPC particles during coloration is suggested in Figure 7. The ETPC particles in this study consist of polystyrene with phenolic groups on the surface. The phenolic groups on the surface of the polystyrene nanoparticles exist as phenoxide ions in propylene carbonate solvent. The phenoxide ions oxidize over two steps near the positive electrode; the H2O in the first step comes from the additives in the ETPC solution (section 2.1 and Figure 7a).40 At the same time, the propylene carbonate solvent reduces near the anode electrode, as shown in Figure 7b.41 An XPS study was carried out to check the redox reactants of ETPC particles without and with the application of an electric field (applied voltage = 5 V). Figure 8 shows four major peaks corresponding to sp2 carbon−carbon bonding of the phenyl ring (284.5 eV), sp3 carbon−carbon bonding (285.3 eV), CO bonding (286.6 eV), and CO bonding (288.7 eV).40 The CO peak in Figure 8a resulted from the fact that propylene carbonate was not dried perfectly because of its high boiling point (242 °C). (If the temperature had been increased to 242 °C, polystyrene would have decomposed.) As the electric field was applied to the ETPC solutions, the intensity fractions of sp3 carbon− carbon bonding and the CO peak increased, whereas the intensity of sp2 carbon−carbon bonding of the phenyl ring decreased (Figure 8b), suggesting that the benzene rings were broken and more carbonyl groups were formed by oxidation (as suggested in Figure 7a). In addition, XPS spectra of the valence bands of the same samples were obtained to investigate changes in the carbon−oxygen bonding due to the electric field applied to the ETPC solution (Figure 8c). Peaks at 17 and 25 eV are attributed to electrons of C 2s (phenyl ring) and oxygen-related bonding [O(2p)−H(1s) and O(2p)−C(2p) molecular orbitals], respectively.40,42,43 The fraction I25 eV/I17 eV can be defined, the value of which represents the damage level of the benzene rings (in polystyrene in this study). The calculated fractions were 0.95 and 1.06 for rETPCp (raw ETPC particles) and eETPCp (ETPC particles experiencing the electric field), respectively, implying that more phenyl rings were broken in the eETPCp sample. A similar result was also obtained in the FTIR spectra (Figure S8). A peak at about 1800 cm−1 was
observed only from ETPC particles experiencing the electric field. The peak was from carbonyl group, which was formed by oxidation (as described in Figure 7a), confirming the redox reactants. These results again support the conclusion that the redox reaction of ETPC particles occurred during coloration, as suggested in Figure 7. On the other hand, no coloration of the ETPC solution under insulated conditions can be explained; the oxidation of the colloidal particles was blocked by the insulator, and the corresponding structural color change did not occur, despite application of an electric field. Another notable point is that the current response was irreversible with cycling; this was also attributed to the oxidation of the colloidal particles. The preformed colloidal particle arrays from the previous cycle partially remained in the subsequent cycle, such that the crystalline structure of the ETPCs formed more quickly; this contributed to a higher current at low voltage (up to 3 V) compared with the first cycle. However, above 3 V, the particles that were quickly oxidized at low voltage did not have additional surface electrons to be oxidized at higher voltage; thus, the current at high voltage decreased as the cycle progressed. This can explain the current behavior in Figure 5, when the currents of the ETPC solution and PC solvent had nearly the same value. Repeated on−off switching tests of photonic crystals were also carried out. Figure S7a shows similar peak wavelengths upon switching (3.5 V) as the number of cycles increased. The switch-off (0 V) state showed a gradual decrease of the peak wavelength with increased number of cycles. This implies that preformed colloidal particle layers from the previous cycle partially remained in the following cycle, which was probably due to a decreased repulsive force between particles because of oxidation. Figure S7b shows that the reflectance of the main peak decreased upon switching on as the number of cycles increased. This was caused by various colloidal particle layers (having different interparticle distances) that were formed because of the different degrees of oxidation. The main peak wavelength and its reflectance did not return to the original state (the first cycle at 0 V), meaning the irreversibility of colloidal particle formation due to oxidation. Additional evidence for the proposed mechanism can be found in the zeta potential (or electrokinetic potential) of the colloidal particles. The zeta potential of ETPCs after application of an electric field was lower (−35 mV) than the E
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particles were not impurities (Figure S6). These large colloidal particles were created by direct fusion.44 Colloidal particles were kept apart by negative-to-negative repulsive interactions with high surface charges. However, after a sufficient time (∼1 h), the ETPC particles, gradually losing their charge, grew through direct fusion with other particles under high-solubility conditions upon the application of an electric field. The growth of the colloidal particles indirectly supports the conclusion that the repulsive interactions of the colloidal particles decreased as a result of the oxidation. As a result, it can be concluded that not only colloidal particle migration as a result of an electric field but also oxidation of the colloidal particles is necessary to form colloidal crystal structures. As evidenced above, the coloration mechanism of ETPCs can be explained by both electric field (electrophoretic movement of colloidal particles) and oxidation (see Figure 9 for schematic explanation). Particle oxidation is initiated in the top layer closest to the positive electrode, concurrently with the electrophoretic movement due to the electric field. When a voltage is applied, the degree of oxidation is more intensified; the layer moves closer to the electrode because of the decreased repulsive force between particles. The uppermost particle layer shows a bluish color under intensified oxidation, whereas the bottom layer just above the boundary line between ETPC particles and PC solvent shows a reddish color with moderate oxidation (Figure 1b), resulting in two peaks for the 3.5 V case in Figure 2b. The colloidal particles lose their electrons at the interface between the ETPCs and the positive electrode. The repulsive force within colloidal particles is reduced because of the decrease of the negative surface charge in colloidal particles. At higher voltages, the crystal lattice distance becomes much shorter as oxidation proceeds on the positive electrode. As a result, by virtue of their continuously tunable state of oxidation using the electric field, colloidal crystals display voltagedependent continuous shifts in their reflected colors. The redox-triggered coloration mechanism was also confirmed using polystyrene (PS) nanoparticles without surface charges. Although an electric potential was applied to the solution containing the PS nanoparticles, no color change was observed because the PS nanoparticles without surface charges were not influenced by the electric field (Figure S10). 3.3. Application of Redox-Triggered Coloration. Using the coloration mechanism of ETPCs, the feasibility of developing photonic crystal fibers was investigated. For simplicity, a tube-type device was constructed and tested. For electrophoretic movements and the oxidation of colloidal particles, a conductive wire was inserted through a tube with a conductive inner surface (Figure S1b). We expected the tube device to show color changes when a higher voltage was applied because of the large distance between the electrodes. Despite the aggregation of colloidal particles at the positive electrode, unfortunately, no coloration was observed up to ∼200 V (Figure 10a−d). We determined that this was due to the thickness of the colloidal layers formed in the tube. The thickness of the periodic colloidal layer stacked on the electrode surface is proportional to the number of colloidal particles. The width of the photonic band gaps becomes dramatically narrower according to the increased number of colloidal particles. Thus, the diffraction efficiency per layer decreases as the layer thickness increases.45,46 In addition, the sharp increase in the thickness of the colloidal layers brings about multiple scattering events because the colloidal particles in thick layers are difficult to secure uniform periodicity in a crystalline
Figure 8. XPS spectra of (a) raw ETPC particles (rEPTCp) without an applied electric field, (b) ETPC particles (eETPCp) with an applied electric field, and (c) the valence bands for rEPTCp and eEPTCp.
zeta potential of raw ETPC (before the electric field was applied) (−68 mV). In other words, oxidation reduced the surface charge of the colloidal particles. The large colloidal particle, which was never observed in raw ETPCs (Figure 4) or the insulator-insert experiments, also provides evidence of oxidation (see Figure S5). The average number of large colloidal particles per unit square was 4.25/100 μm2, and EDS mapping image of the large particles showed that these large F
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Figure 9. Schematic diagram explaining the coloration mechanism of ETPCs by their electrophoretic movement and the redox reaction and showing the photonic crystal (a) before and (b) after application of an electric potential.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01919. Schematic diagram of the experimental device, calculation of electric field in ETPC solution, simulation geometry and boundary conditions of a planar-type ETPC device, electrical potential of ETPC solution upon application of a voltage, SEM and wide-scan EDS images of colloidal particles after application of a high voltage, variations of the peak wavelength and the reflectance at the peak wavelength during repeated voltage sweeps from 0 to 3.5 V, FTIR spectra of ETPC particles without and with an applied electric field, and results of coloration experiments for PS nanoparticles without surface charge (PDF) Movement of the colloidal particles in the planar device as a function of the applied electric field (AVI) Observation of the colloidal crystals with electric field application only (AVI)
Figure 10. Color changes of ETPC solution in a tube-type device according to the applied voltage: (a−d) 25 wt % and (e−h) 2.5 wt % ETPC at (a,e) 0, (b,f) 70, (c,g) 120, and (d,h) 200 V.
structure and reduces the reflection intensity.47,48 These two factors can explain why no color change of the ETPC solution was observed in the tube device, suggesting the reduced thickness of the colloidal layers in the tube device. To reduce the thickness of the colloidal layers, the colloidal particle concentration was reduced to 2.5 wt %. As shown in Figure 10e−h, colloidal crystals with a proper layer thickness were then formed, allowing an ETPC color change in the tube device with an applied voltage.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +82 2 880 9096. Fax: +82 2 883 8197. E-mail:
[email protected].
4. CONCLUSIONS The coloration mechanism of ETPCs was investigated to explain how ETPC particles form colloidal photonic crystals and why the diffraction wavelength changes as a function of the applied voltage. Through a series of experiments, we observed that ETPC particles cannot form a photonic crystal structure without the redox reaction, despite the application of an electric field, concluding that the coloration mechanism of ETPCs is not only an effect related to the electric field but also a consequence of the redox reaction. Also, controlling the layer thickness of the ETPC crystal is important for coherent and uniform light scattering. The tunable color mechanism of ETPCs and the tube-type device demonstrated here with colloidal particles, will be applicable to several emerging applications, such as thin photonic crystal fibers for sensors, portable electronics, and photonic displays.
ORCID
Woong-Ryeol Yu: 0000-0001-5916-3339 Present Address ∥
Post-Silicon Semiconductor Institute, Korea Institute of Science and Technology, Seoul 136.791, Republic of Korea.
Author Contributions ‡
H.-S.Y. and J.J. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Ministry of Education Science and Technology (MEST) as part of the “Space Core Technology Development Program” project (NRFG
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2015M1A3A3A02027377). The authors also thank for the financial support from the Industrial Fundamental Technology Development Program (10051440, Development of fiber-based transistors for wearable integrated circuit device applications) funded by the Ministry of Trade, Industry and Energy (MOTIE), Korea.
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DOI: 10.1021/acs.langmuir.7b01919 Langmuir XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.langmuir.7b01919 Langmuir XXXX, XXX, XXX−XXX
