Guided Electro-Optical Switching of Small Graphene Oxide Particles

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Guided electro-optical switching of small graphene oxide particles by larger ones in aqueous dispersion Rana Tariq Mehmood Ahmad, Tian-Zi Shen, Aurangzeb Rashid Masud, Thilini K. Ekanayaka, Bomi Lee, and Jang-Kun Song Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03460 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 25, 2016

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Guided electro-optical switching of small graphene oxide particles by larger ones in aqueous dispersion Rana Tariq Mehmood Ahmad†, Tian Zi Shen†, Aurangzeb Rashid Masud, Thilini K. Ekanayaka, Bomi Lee, Jang Kun Song* School of Electronic and Electrical Engineering, Sungkyunkwan University, Suwon, Gyeonggido 440-746, South Korea ABSTRACT Although the large Kerr coefficient of aqueous graphene oxide (GO) dispersions is quite attractive for electro-optical applications with low power consumption, the maximum birefringence of GO dispersions is not sufficiently high for actual display applications. Here we report that adding a small amount of larger GO particles (about 4 µm) into a high-concentration dispersion of small GO (about 0.2 µm) can improve the electro-optical sensitivity to an electric field and also the maximum birefringence. Large GOs induces the ordering of small particles and enhances the electro-optical switching. Large GOs have higher polarizability and are easily driven under an applied electric field, and the rotational motion of large GO particles leads to switching of surrounding small GO particles, improving the electro-optical performance. The binary mixture can overcome the limitations of pure dispersions of large GO or small GO particles; the former has high interparticle interaction, and the latter has low sensitivity to an electric field.

KEYWORDS. Graphene oxide, Dispersion, Liquid crystal, Electro-optical switching

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Graphene oxide (GO) colloids in water or an organic solvent1-3 exhibit several remarkable characteristics such as self-assembly and liquid crystallinity,4-6 a thick electric double layer,7 photonic crystallinity with regular spacing,8, 9 amphiphilicity,10 light absorber,11, 12 and a large Maxwell–Wagner polarization.13, 14 These unique properties have expanded the potential of GO materials for use in various applications such as functional fiber and film fabrication,15, 16 energy storage,17 nanofiltration,18 and electro-optical devices.13, 19-22 In these applications, control of the ordering of GO particles is among the most crucial factors in realizing the desired properties. Our group recently reported that an aqueous GO dispersion has an extremely large Kerr coefficient, which can make it possible to control the GO particle alignment by applying very low electric fields.13 Electro-optical switching of a GO dispersion demands unique conditions in terms of interparticle interaction and assembly; that is, the weak nematic self-assembly interaction promotes field-induced ordering of particles, but excessive interparticle interaction hinders switching owing to increasing interparticle friction.13 Hence, optimal electro-optical switching was obtained in a biphasic state in which the interparticle interaction is neither inadequate nor excessive. The GO particle size also has a similar effect on the switching behavior. Larger GO particles have larger interparticle friction as well as larger polarizability; consequently, electro-optical switching is observed only in a range of low GO concentrations, compared to that in dispersions of small GO particles. The low GO concentration causes a low maximum birefringence. On the other hand, when GO particles are too small, their polarizability is also low, and a higher voltage is required to control the GO alignment.20 Hence, the optimum size is in the middle range, roughly 0.5 µm.20 Thus, manipulation of the interparticle interaction is the most important factor for improving the electro-optical performance of GO colloids.


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The steric interparticle interaction between particles in lyotropic liquid crystals can be explained by Onsager’s excluded volume model, which describes well the second-order phase transition from isotropic to nematic via a biphasic state.23 The phase sequence is determined by minimizing the free energy of the competing effects of the orientational and translational entropies.24 As the particle alignment improves, the free energy is increased by the loss of orientational entropy and decreased by the translational entropy, which is related to the excluded volume interaction. The nematic phase appears when the contribution of the excluded volume interaction term is more significant than that of the orientational free energy term; hence, the anisotropy of the particle geometry and the concentration are dominant in the calculation. Within the framework of steric interactions between particles and electro-optical switching, a binary mixture of two types of particles with different sizes provides an interesting system, and the interparticle interaction and phase behavior deviate greatly from a simple additive approximation of the two systems before mixing. The coupling term between small and large particles in the excluded volume interaction becomes more important; for example, the third- and fourth-order virial coefficients for the interactions among three and four particles must differ greatly from those in monodispersed GO colloids.25 Jalili et al. reported that adding larger GO particles to a colloid containing small GO in the isotropic phase induces nematic ordering of the small GO particles by the large GO particles.26 Polydispersity reportedly expands the biphasic region in the phase diagram as a function of concentration.23, 27 However, the effect of the polydispersity of GO particles on electro-optical switching has not been explored. The application of electric fields to an isotropic GO colloid induces antinematic ordering of GO particles,28 and the field-induced ordering behavior of the antinematic binary mixture can produce quite new phenomena.29, 30


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In this study, we investigate electro-optical switching of binary mixtures of two GO colloids with small and larger GO particles, where the GO concentration is adjusted to be constant. Interestingly, the addition of a small amount of large GO particles into a colloid containing small GO particles dramatically enhances the electro-optical birefringence. The resulting birefringence is almost 1.5 times that of the colloid containing small GO particles before the large GO particles are added. There is an optimal mixing ratio (about 10% added large GO in our sample) at which the binary mixture simultaneously offers the advantages of the large polarizability of large GO and the low interparticle friction of small GO. The findings can be useful for realizing electro-optical devices using GO liquid crystals and fabricating selfassembled 3D architectures.20, 26, 31

RESULTS AND DISCUSSION We prepared an aqueous GO dispersion following Hummers’ method (see Materials and Methods). We obtained x-ray photoelectric spectroscopy (XPS) spectra of the GO sample to confirm the high oxidation of the GO (see Supplementary Figure S1). The C–O peak was higher than the C–C peak, indicating that the GO was highly oxidized.32 The GO dispersion with a 1 wt% concentration exhibited the nematic phase. We ultrasonicated the sample within a base of ice water for 5 h to reduce the mean size of the GO particles.20 The ice bath was used to prevent the thermal reduction of GO particles during sonication. According to our previous work,20 the GO particles are shattered due to interparticle collisions and collisions with the bottle wall during the ultrasonication, and the mean size of GO particles is decreased exponentially depending on the ultrasonication time. After 5 h sonication, the sample exhibited the isotropic phase.20, 33 As predicted by Onsager’s volume excluded theory,23 small-GO colloid exhibited the isotropic


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phase because of its small aspect ratio. Figure 1a shows scanning electron microscope (SEM) images of each sample; the size distributions were measured using the SEM images. The particle size of the large-GO sample was about 4.06 ± 3.84 µm, and that of the small-GO sample after 5 h of ultrasonication was 0.17 ± 0.11 µm, as shown in Figure 1b. The large GOs were about 20 times larger than the small ones. We also measured the particle size distribution using a laser scattering particle size analyzer (Zetasizer Nano ZS, Malvern Instruments Ltd., UK). The sizes of the large and small GO particles were 2.5 ± 1.4 µm and 0.26 ± 0.17 µm, respectively (see Supplementary Figure S2). In the latter measurement, the size ratio was about 10. The deviation may arise from the nonspherical shape of the GO particles, which may cause an error in the measurement using the laser scattering particle size analyzer. In either case, the large GO particles are one order of magnitude larger than the small GO particles.

Figure 1. Particle size distribution of graphene oxide (GO). (a) Scanning electron microscope (SEM) images of small GO particles (left) and large GO particles (right). (b) Size distributions of the colloids containing small and large GO particles measured from SEM images.


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Then, mixtures of 1 wt% of the large GO dispersion and 1 wt% of the small GO dispersion with varying mixing ratio were prepared. It should be noted that the GO concentration in all of the mixtures is 1 wt% because the GO concentrations in both the small and large GO dispersions are 1 wt%. We simply adding two dispersions in a bottle and shook it for 10 min. Figure 2a shows the macroscopic birefringent optical patterns of GO mixtures with varying mixing ratios under crossed polarizers in a 1-mm-thick cell. The 1 wt% small-GO dispersion was in the isotropic phase, and the cell containing a pure small-GO dispersion appeared dark under the crossed polarizers. With the addition of 10% large GO, the isotropic GO dispersion became biphasic; the birefringent brush pattern is partially visible in the second image in Figure 2a. Further addition of the large-GO colloid made the birefringent brush patterns more distinct; the phase gradually transitioned to the nematic phase. From the macroscopic observation under crossed polarizers, the phase sequence can be roughly determined as follows: the isotropic phase appeared at a mixing ratio of less than 10%, and the biphasic state appeared in mixtures with ratios of 10% to 30% large GO. Beyond a mixing ratio of 30%, the mixtures displayed a clear, bright birefringent texture in the entire cell area, indicating the nematic phase. Thus, the addition of large GO sensitively influences the overall GO assembly and nematic ordering. In particular, the mixtures with 30% and 40% large GO exhibit long, wide, and clear brushes, indicating that the GO alignment exhibits long-range correlation. On the other hand, the texture in the 100% large-GO colloid has unclear and complicated brushes, indicating short-range correlation. This may be related to the large interparticle frictional effect, which complicates the alignment modulation. The viscosity of the GO mixtures was measured as a function of the mixing ratio of


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large GO (Figure 2b; the vertical axis is on a logarithmic scale). The results confirm that the interparticle friction increases exponentially with increasing mixing ratio of large GO.

Figure 2. (a) Macroscopic observation of 1 wt% aqueous GO dispersions with varying mixing ratios of large and small GO particles in a cell under crossed polarizers. (b) Viscosity of aqueous GO dispersions as a function of the mixing ratio of large GO.

Next, the electro-optical switching behavior of the GO mixtures was analyzed, and the results are shown in Figure 3. The cell structure is illustrated in Figure 3a. The gap between the electrodes was 2 mm, and the cell thickness was 1 mm. Figures 3b shows polarized optical microscope images of the cells at 0 and 40 V under crossed polarizers. The top and middle images in Figure 3b show the 100% small-GO dispersion and the mixture with 10% large-GO


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dispersion, respectively. The dark states were more or less similar in both cases, but the bright state of the 10% large-GO mixture was much brighter than that of the small-GO dispersion. The bottom images show the 100% large-GO colloid, in which the birefringent pattern is present from the beginning and does not change under the application of 40 V. The optical intensity (I) was measured as a function of applied voltage across the gap between two electrodes for the cell containing different mixtures (Figure 3c). Pure large GO in the nematic phase exhibited birefringence at zero voltage, and little change was observed under an applied voltage. GO particles were immobilized by strong interparticle friction in the nematic phase. On the other hand, the other GO mixtures in the isotropic or biphasic states exhibited field-induced birefringence. The highest intensity was obtained from the mixture of 10% large GO with 90% small GO, and the optical intensity at 40 V was about twice that of the 100% small-GO dispersion (that is, 0% larger GO). As the mixing ratio of large GO increased further beyond 10%, the optical intensity decreased and became similar to that of the small-GO colloid at a 30% mixing ratio. The deterioration of the electro-optical performance with increasing largeGO concentration is due to the increasing interparticle friction. From the optical transmittance (I/I0), where I0 is the optical intensity of an isotropic small-GO colloid cell under parallel polarizers at zero voltage, the field-induced birefringence (∆n) of the GO dispersion was calculated as ∆n =

λ0 sin −1 I / I 0 . πd

(1)

Here, the wavelength λ0 is 0.55 µm, and the cell thickness d is 1,000 µm. The birefringence is directly proportional to the GO concentration and order parameter. Because all of the GO colloids have the same concentration (1 wt%), the birefringence reflects the average order parameter of the GO particles. Here the average denotes the overall order parameter of large and


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small GO particles. As shown in Figure 3d, the birefringence of the 10% large-GO mixture is about 1.5 times higher at 40 V than that of the small-GO colloid. This result confirms that the average order parameter of field-induced ordered GO particles is increased by 1.5 times at 40 V by adding large GO to the small GO dispersion. Note that 90% of the GO particles in the 10% mixture are small GO particles, and the high birefringence must be attributed mainly to the aligned small GO particles. Figure 3e shows the birefringence at 40 V as a function of the large GO concentration. Thus, the optimum mixing ratio for electro-optical sensitivity was about 10% large-GO dispersion in our sample. The optimal condition may depend on the size of GO sample, the functional group density, Debye length, and the applied voltage. Figures 3f shows the electro-optical switching behavior when turning 40 V signal on and turning off the signal after 100 s. The electro-optical switching behavior may reflect the rotational viscosity of GO particles. As the large GO concentration increased, the rising time increased dramatically, and the falling time increased weakly (see the expanded image in the inset). The result qualitatively confirms that the increasing concentration of large GO particles also influences the rotational viscosity during electrical switching of GO particles.


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Figure 3. (a) Cell structure for electro-optical switching experiments. (b) Polarized optical microscope images of GO mixtures with large-GO mixing ratios of 0%, 10%, and 100%, respectively, under application of 0 V (left) and 40 V (right). (c) Field-induced optical intensity as a function of voltage for different mixing ratios of large GO in the mixtures. (d) Field-induced birefringence (∆n) for the mixtures. (e) The maximum ∆n at 40 V as a function of the large GO concentration. (f) The response of electro-optical switching on applying 40 V (at 4 s) and on switching off (at 102 s).

To visualize the interparticle interaction in the mixtures, we analyzed the GO mixtures using a confocal laser scanning microscope (CLSM), as shown in Figures 4a–d. The CLSM analysis allows one to obtain microscopic image at a certain vertical section in a sample differently from usual optical microscopic analysis that provides a vertically accumulative image.34 In particular, GO particles have fluorescence, and so, direct observation of GO particles is achievable without doping additional fluorescence dye.8 The intensity of fluorescence from


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GO particles depends on the particle orientation; when GO particles are parallel to the cell substrate, they appear bright, and when they are perpendicular to the cell substrate, they appear relatively dark.8, 34, 35 For the CLSM analysis, we used diluted mixtures for each case, where the concentration was adjusted to provide the optimal conditions for CLSM analysis. For the diluted large-GO dispersion (0.1 wt%) in Figure 4a, the orientation of the GO particles is clearly discernible; they were randomly disordered owing to the low concentration. On the other hand, the CLSM image of the small-GO dispersion (1 wt%) exhibited uniform brightness, and one cannot distinguish individual particles due to limited resolution of CLSM used, as shown in Figure 4b. For the mixture of 30% large GO and 70% small GO (overall concentration is 0.25 wt%) in Figure 4c, cloud-like groups of small GO particles are seen overlapping the disordered large GO particles. In the region marked by a blue box, large GO particles align vertically to the substrate, and, in contrast, in other regions (marked by a red box and dotted yellow lines), large GOs are parallel to the substrate; the difference is better discernible in the expanded images in Figure 4c-i and c-ii. The cloud-like groups of small particles are also dark in the ‘i’ region and bright in the ‘ii’. This observation indicates that the small GO particles are aligned along the large GO particles; that is, small GOs are aligned vertically around a large GO standing vertically, and they are aligned horizontally around a large GO lying horizontally. The phenomenon is more obvious in the 10% large-GO mixture in Figure 4d (overall concentration is 0.3 wt%). In the sample, small GO particles make up 90% of the GO particles, and the cloud-like brightness modulation due to the small GO particles is dominant. The left side of the image looks darker than the right side; on the left side, the edges of vertically aligned large GO particles are easily distinguished. Thus, around the vertically aligned large GO particles, small GO particles are also aligned vertically, and the corresponding region appears darker. On the other hand, the


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right side has large GO particles parallel to the substrate, and the small particles are aligned parallel to the large GOs. Thus, the alignment of large GO particles can induce the alignment of small GO particles.

Figure 4. (a–d) Confocal laser scanning microscopy images of (a) 0.1 wt% large-GO dispersion, (b) 1 wt% small-GO dispersion, (c) a mixture of 30% large GO (0.25 wt%), and (d) a mixture of 10% large GO (0.3 wt%). (c-i) and (c-ii) are the expanded images of the blue and red box areas (i and ii) in (c), respectively. (e–g) Schematic illustrations elaborating the effect of large GO particles on the orientation of small GO particles: (e) aqueous dispersion of small GO in isotropic phase (random orientation of GO particles), (f) surface effect of large GO particles on the orientation of nearby small GO particles, (g) effect of surface and electrostatic forces between two large GO particles on the small particles between them.


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Figures 4e–g schematically illustrate the enhancement of the electro-optical performance by the addition of large GO particles. In an aqueous dispersion of 1 wt% small GO particles, the GO particles are randomly oriented, and the dispersion is in the isotropic phase (Figure 4e). Small GOs have low polarizability owing to their low aspect ratio, so they have low electrooptical sensitivity. When a small quantity of large GO particles is added, the small particles are aligned along the large GO particles. Figure 4f shows the interaction of a large GO particle with surrounding small GO particles. The large particle provides an excluded volume for small particles and forces the surrounding small GOs to become rearranged along the large GO. Here the coupling effect between large and small particles in the excluded volume free energy is increased as the size difference increases. Therefore, the biphase starts at a lower concentration when a small amount of large GO particles is added. The ordering of small particles induced by large particles works effectively even under field-induced alignment; when an external voltage is applied across the sample, large GO particles with large polarizability are rearranged by the external field more than the small GO particles with small polarizability, because the fieldinduced torque is proportional to the polarizability. Then, small GO particles around large GO particles are readily aligned by the combined effects of field-induced alignment and the excluded volume effect of large GOs. Therefore, the field-induced order parameter of small particles in the mixture is much higher than that in the pure small-GO colloid.

CONCLUSION The impact of the addition of large GO particles on the electro-optical switching behavior was investigated in 1 wt% GO dispersion mixtures of small and large GO particles. Adding 10% large GO particles to a small-GO dispersion was found to enhance the field-induced optical


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transmittance by about two times. The significant improvement is related to the improved ordering of small GO particles around large GO particles. Thus, a binary mixture of GO particles having sizes that differ by one order of magnitude can offer the advantages of the large polarizability of the large-GO dispersion and the high-concentration biphasic state of the smallGO dispersion; the former increases the electro-optical sensitivity and ordering, and the latter causes a higher maximum birefringence. Further increases in the mixing ratio of large GOs induce nematic ordering of GO particles and increase the interparticle friction, which hinders the electro-optical switching. The large birefringence in a binary mixture of large and small GO particles can be an important requirement for display applications using GO liquid crystals, and the results can be useful for developing such electro-optical devices. Moreover, field-induced order control of GO particles can be used in combination with various wet processes to design self-assembled structures for ordered architectures.

Materials and Methods Preparation of graphene oxide. GO was prepared using Hummers' method.23 H2SO4 (60 ml, 98%) was added to a mixture of 2 g of graphite flakes (7–10 µm, 99%, Alfa Aesar) and 1.5 g of NaNO3 in a bath at 0 °C. Then, 7.0 g of KMnO4 was slowly added to the mixture under stirring for 1 h at a temperature below 20 °C. The mixture was heated to 35 °C and stirred for 45 min. Deionized (DI) water (150 ml) was slowly added at a temperature below 100 °C, and the mixture was stirred for 1 h. Then, 5 ml of 30% H2O2 was added to terminate the reaction. After the mixture was cooled to room temperature, the GO solution was purified using a high-speed centrifuge at 22 krpm for 2 h after 5% HCl was added in the first round of centrifugation and DI water was added in several additional rounds of centrifugation, until the pH of the supernatant


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liquid reached 7. The GO particles had a mean flake size of about 4 µm. We used an ultrasonication bath (SD-250H, Mujigae Co., Korea) to prepare small GO particles, which had a mean size of about 0.17 µm. We prepared the 1 wt% GO sample by adding the required amount of DI water to the GO gel to achieve the desired concentration.

Sample characterization. XPS (Auger Electronics, ESCA20000) was used to analyze the quality of the GO. The samples for XPS were prepared by drop-casting of GO on a cleaned silicon wafer and drying at room temperature. We measured the size distribution of the GO sample using a Zetasizer instrument (Zetasizer Nano ZS, Malvern Instruments Ltd, UK). The size of the GO particles was also confirmed using SEM images. The samples for SEM were prepared on a silicon wafer by spin-coating a highly diluted aqueous GO dispersion and drying it at room temperature. An erect-type CLSM (K1-Fluo, Nanoscope Systems Company, Daejeon, Korea) was used to analyze the GO particles' orientation in a water dispersion. In the CLSM system, a 405 nm laser, 0.5 a.u. pinhole, and 40× objective with a numerical aperture of 1.2 were used.

AUTHOR INFORMATION †

These two authors contributed to this work equally.

*Address correspondence to [email protected]

ACKNOWLEDGMENT This work was supported by the Samsung Research Funding Center of Samsung Electronics under Project No. SRFC-MA1402-03.


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Guided Electro-Optical Switching of Small Graphene Oxide Particles

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