Electro-optical Characteristics of Aqueous Graphene Oxide Dispersion

Oct 22, 2014 - We can depict the phenomenon by a simple illustration shown in Figure 9. GO flakes supply the dispersion with H+ ions, which form an ...
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Electro-optical Characteristics of Aqueous Graphene Oxide Dispersion Depending on Ion Concentration Seung-Ho Hong, Tian-Zi Shen, and Jang-Kun Song* School of Electronic & Electrical Engineering, Sungkyunkwan University, Jangan-Gu, Suwon, Gyeonggi-do 440-746, Korea ABSTRACT: Facile alignment control of graphene oxide (GO) particles in aqueous dispersions is a highly useful technique for various potential applications. Aqueous GO dispersions were recently reported to have an extremely large Kerr coefficient, which may provide a valuable pathway to the alignment control of GO particles. We investigated the electro-optic responses of GO dispersions in various ionic solutes and with varying ionic concentrations. We found that the addition of NaOH actually improved the electro-optic sensitivity of the GO dispersion, while other ionic additives resulted in desensitization. We experimentally and theoretically elucidated the underlying mechanism of the phenomena. The mechanism is closely related to the acidic nature of the GO dispersion, which is neutralized by the addition of NaOH. The addition of ionic solutes caused only a mild change in the surface conductivity of GO particles, but it brought about a large variation in the bulk solvent conductivity. The electro-optical sensitivity agreed well with the variation in solvent conductivity. Thus, the electro-optic response of GO dispersion was influenced more by the electric properties of solvent rather than by those of the GO particle itself. We also found that the cation-exchange capacity for H+ ions in the electrical double layer is quite high; i.e., H+ ions are not likely to be replaced by other ions.

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INTRODUCTION Recent increases in scientific activity related to graphene oxide (GO) are partially attributed to the tremendous attention paid to graphene since Geim and Novoselov reported an easy graphene sheet preparation method in 2004.1 GO sheets can be reduced to graphene-like sheets by removing functional groups on the basal plane and recovering a conjugated structure, although the electrical performance of these sheets is still greatly inferior to that of graphene.2 However, as several interesting features of GO itself have been reported recently, the intrinsic properties of GO have also attracted research interest, and a wide range of new applications have been suggested in mechanical, optical, and biological devices. Moreover, GO flakes disperse well in nontoxic solvents like water, which is a significant benefit for solution processes in flexible electronics and next-generation devices.3−5 A useful property of GO is that aqueous GO exhibits liquid crystalline behavior even at low concentrations.6−11 Since Onsager’s theory was developed for the phase-transition behavior between the isotropic and nematic phase in monodisperse and rodlike colloids,12 the theory has been further developed to be applicable to other types of materials such as disklike and polydispersed colloids.13−15 According to those models, the aspect ratio (diameter/thickness) of particles in a colloid directly influences the phase-transition concentrations. Since GO flakes in aqueous dispersions have a large aspect ratio of more than 1000,6,9,16−18 the liquid crystalline behavior of GO flakes is naturally expected even in low concentrations. Many researchers have reported liquid crystalline properties of GO and potential applications, especially after aligning GO flakes © 2014 American Chemical Society

with an external stimulus, such as a surface or magnetic field.6,8,19 Meanwhile, the effect of ionic addition on liquid crystal behavior or phase separation has been widely studied in different colloidal systems.20,21 An electric field is the easiest and most widely used method to dynamically control the alignment of liquid crystals in most liquid crystal devices. However, GO flakes cannot be simply aligned by applying an electric field because of several critical issues such as insensitivity to electric fields, electrolysis of water, and electrophoretic migration of GO flakes toward the anode.6 We recently reported that these issues can be partially overcome by applying high-frequency AC fields and reducing the interparticle interactions between GO flakes.18,22 Interflake friction decreases significantly with decreasing GO concentration, and the optical Kerr coefficient of GO is the highest reported for all Kerr materials. The extremely large optical Kerr coefficient is attributed to the high aspect ratio of GO flakes and the electrical double layers on the surface. Functional groups on the GO basal plane, such as hydroxyl and carboxyl groups, are negatively charged, supplying H+ ions to water, and decrease the pH of GO dispersions. The counterions in electrical double layers are mainly H+ ions, with some other positive ions also present. In 1960, O’Konski modified the Maxwell−Wagner model for the polarizability of dispersed particles in a colloid, assuming that the thin ionic layers near the surface form an electrical double layer and enhance the Received: May 18, 2014 Revised: October 21, 2014 Published: October 22, 2014 26304

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effective electric polarizability.23 Based on the Maxwell− Wagner-O’Konski model, the electric field-induced birefringence has been investigated in several colloidal materials such as beidellite and gibbsite colloids.24,25 Although these approaches provided useful insight in the system, the relevant research has mostly emphasized the effect of neutral species like NaCl or KCl, but the effect of increased acidity on materials like aqueous GO dispersions has not been considered. It is known that the characteristic contrast of conductivity between dispersed particles and solvent is an important factor in the electro-optic properties of colloids,26 but the degree of (de)protonation depending on H+ concentration, which could be another important factor, has not been considered. Recently, we reported that the number of centrifugal cleaning cycles during GO preparation significantly influence the electrical sensitivity of GO dispersions due to the concentration of the residual salts of oxidizing reagents and the degree of the exfoliation of graphite oxide.27 In the study, only the ions that were used in Hummers method were considered, but the effect of various types of ions including basic ions was not studied. In this paper, we investigated the material properties and electro-optic response of aqueous GO dispersions with varying ion types and ion concentrations. The material properties included the zeta potential, pH, and conductivity. We found a clear contrast between the NaOH−GO dispersion and GO dispersions with other added ions. Other ions drastically desensitized the electro-optic response of GO dispersions, but the addition of NaOH slightly enhanced the electrical sensitivity of GO dispersions. We investigated the underlying mechanisms of the results and clarified the ionic effect on both the characteristic contrast between dispersed particles and solvent and the surface conductivity of GO. We found that solvent conductivity is important for the electrical sensitivity of GO dispersions, which influences the characteristic contrast between particles and solvent. Additionally, we investigated the surface electrical characteristic of GO depending on ions of solvent. These results will help to understand the electrochemical and liquid crystalline characteristics of GO dispersions and to develop new electro-optic devices using these materials.

Figure 1. (a) Normalized size distribution of the GO flakes in the GO dispersion used. The size was measured by analyzing SEM images (inset). (b) Thickness of GO flakes was measured using dry samples on Si wafer, and the mean thickness was roughly 1 nm.

hard platelets with polydispersity in the size of the particles,15 and their model was confirmed to accord well with the phase behavior of GO dispersions.18,29 In the model, the thickness of platelets is an important parameter. In many literature citations, the physical thickness of dried particles, which was usually measured by AFM analysis, was used for the theoretical analysis of the phase behavior.6,9,18,19,29 However, it was suggested that the thickness of electrical double layer should be taken into account in addition to the physical thickness of particles, because the slippery surface on the electrical double layer may act as the surface in the steric interaction between particles.21 The Debye thickness of our sample was calculated to be approximately 22 nm (see the Methods), so the total effective thickness of GO particles with the electrical double layer can be approximately estimated to be 55 nm.21,30 According to Bates and Frenkel’s model, the biphasic transition concentration of the GO dispersion with the effective thickness of 55 nm was calculated to be 0.17 wt % (= 2.09 vol %) considering the electrical double layer, which accorded well with the experimental biphasic transition concentration that was roughly 0.25 wt %. Interestingly, although the effective GO thickness increases from 1 to 55 nm in the calculation, the biphasic transition concentration in weight percentage increases only 2fold compared to the value estimated without considering the electrical double layer (∼0.08 wt %).6,9,18,19,29 We used a 0.1 wt % GO dispersion for all experiments in this study in order to ensure that the GO dispersion was in the isotropic phase with weak interparticle interactions. Thereby, we can focus on the ionic effect on the anisotropic polarizability of GO particles and the resulting electro-optic response,

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EXPERIMENTS AND RESULTS Material Properties of Ion-Containing GO Dispersions. We prepared an aqueous GO dispersion using the Hummers method.28 In order to eliminate the residual ions that may come from the oxidizing agents added in the Hummers method, we performed a rigorous cleaning process using a dialysis bag for 2 weeks and then centrifuged the dispersion 15 times. In each centrifuge step, the supernatant liquid was replaced with fresh deionized distilled water. After completing the centrifuge processes 15 times, the pH of the replaced supernatant liquid was in the level 7, and the residual ionic concentration measured through a combustion chromatography analysis was less than 10−7 M, which is smaller than the density of H+ ions in the GO dispersions with pH 6 or less. Thus, we can neglect the residual ions from the oxidizing agents. The size distribution of our GO sample was measured using scanning electron microscopy (SEM) and atomic force microscopy (AFM), as shown in Figure 1. The GO sample was largely polydispersed. The mean size was 1.9 μm, the standard deviation was 1.7 μm, and the thickness was approximately 1 nm. Bates and Frenkel calculated the phase behavior of model colloidal systems composed of infinitely thin 26305

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excluding the influences of interparticle interactions, which become dominant in nematic phase with high concentrations.18 As ionic additives, we selected neutral NaCl, acidic HCl, and basic NaOH and prepared GO dispersions with the ionic additives at varying concentrations. Addition of NaCl and HCl at concentrations greater than 10−2 M into a GO dispersion caused partial flocculation of the GO contents, accompanied by a decreased absolute value of zeta potential. Addition of NaOH at a concentration greater than 10−2 M brought about a noticeable color change from bright brown to dark brown, indicating partial reduction of GO. Hence, the concentrations of ionic additives varied from zero to 10−3 M in most experiments. Typical material properties, such as pH, solution conductivity (κe), and zeta potential (ζ), of the GO dispersions were measured. For comparison, we also calculated the solution conductivities for NaCl, HCl, and NaOH solutions in water in the absence of GO.31 The pH of a pure GO dispersion was 3.7, as shown in Figure 2a, corresponding to 2 × 10−4 M H+ ions. The pH of GO dispersions may vary depending on the GO concentration and density of the functional groups on the GO basal plane. The pH varied largely from 3.7 to 6.1 upon addition of up to 10−3 M NaOH, but HCl caused relatively little variation because of the already low initial pH. Addition of NaCl did not cause meaningfully change of pH, as expected. The solution conductivities of GO dispersions with various ionic additives and the theoretical conductivities of the electrolytes without GO are shown in Figure 2b. The GO solution without any ionic additives had a conductivity comparable to that of solutions with 10−4−10−3 M electrolyte concentrations. This result indicates that 0.1 wt % GO in pure water has the same effect on conductivity as 10−4−10−3 M ions, which is well-matched with the pH of the pure GO dispersion in Figure 2a. In addition, the brines have the lowest solution conductivities among the three electrolytes without GO, but the trend was opposite in the GO−ion solutions. That is, the NaOH−GO solution had the lowest conductivity of the three sets of GO solutions across the entire range of ionic concentrations. Since H+ ions have the highest mobility, the observed trend for the pure electrolytes was naturally expected. Addition of NaCl or HCl to a GO solution increased the total number of ions. However, addition of NaOH to a GO dispersion suppresses ion generation because the OH− ions supplied by NaOH are mostly neutralized by H+ ions from the GO functional groups, and as a result, H+ ions in solution are mostly replaced by Na+. Considering that the mobility of Na+ is lower than that of H+, the conductivity of the GO solutions to which NaOH was added was slightly decreased, unlike the other solutions, as indicated in Figure 2b. In order to confirm the result, we repeated the same experiment using another GO dispersions and the same decreasing trend of bulk conductivity with increasing ionic strength was observed (the data are not shown here). The absolute values of zeta potentials decreased slightly upon addition of ions, although there was some variation depending on the additive, as shown in Figure 2c. The zeta potentials were high enough to sustain stabile colloidal states of the samples.32,33 An Electric Field Induced Birefringence Depending on Ions Added. Application of an electric field with 10 kHz induces optical birefringence in a GO dispersion due to fieldinduced ordering of GO flakes,18 as indicated in the macroscopic photo images in Figure 3a. The initial dark state

Figure 2. PH values (a), solution conductivities (b), and zeta potentials (c) for GO solutions with various ionic solutes. The solid lines in (a) are the conductivities of the pure electrolyte without GO, which were determined theoretically.

with no electric field was changed into a bright state by activation of the voltage. Figures 3b−d show the birefringence with increasing voltage of the three sets of GO solutions with different ions added. Interestingly, addition of HCl or NaCl to GO dispersions decreased the electro-optic response, but adding up to 10−3 M NaOH slightly increased the sensitivity of the GO dispersion to external fields. We measured the dynamic response of the field-induced birefringence in order to clarify the possible increase in interflake frictional upon addition of ions. As shown in Figure 4a, when the voltage was applied, the normalized birefringence increased within 0.3 s. Addition of up to 10−3 M NaOH did not cause any significant change in the dynamic response, but addition of just 10−4 M HCl slowed the dynamic response. The corresponding relaxation time constant increased 3-fold compared to that of the solution with no added ions, as indicated in Figure 4c. On the other hand, the off response did not show any meaningful variation with the addition of ions 26306

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Figure 3. Electro-optical experimental results. (a) Cell configuration: l = 500 μm, w = 10 μm, and d = 300 μm. Photographs with and without applied voltage taken without the green filter. (b−d) Birefringence as a function of applied voltage for NaOH−GO dispersions (b), HCl−GO dispersions (c), and NaCl−GO dispersions (d).

Maxwell−Wagener−O’Konski model. We calculated the anisotropy of polarizability (Δα) of the pure GO solution with no added ions. In our previous work (see the Supporting Information in ref 18 and references therein), we explained the relevant theory in detail; here, we briefly summarized the theory to further study the ionic effects.23−26 According to the Maxwell−Wagener−O’Konski model, the electric polarizability of a spheroidal particle in a colloidal solution can be calculated. By assuming that the shape of a GO particle is a uniaxial spheroid with two long principle axes within the GO basal plane and one very short principle axis normal to the plane, we can use the model for investigating the polarizability of GO particles. The kind of approximation has been commonly used in 2D colloidal systems.18,24−26 Here, the diameter and thickness of a GO particle can be assigned as 2b and 2a, respectively, where b and a are the radiuses along the long and short axes of the model spheroid, respectively. The dielectric constant of GO flake (εp) is given as approximately 2.4 in the dispersion. The dielectric constant (εe) and conductivity (κe) of water depend on the concentration and the chemical structures of ions that exist in the dispersions.34,35 When the spheroidal particle has an electrical double layer, the electrical polarizabilities (α) parallel and perpendicular to the normal direction of the GO basal plane can be represented as

(see Figure 4b,c). This result indicates that the slow onresponse time of the HCl−GO solution is not responsible for the increased viscosity but for a weak driving force. Considering that the driving force of the rotation of GO particles arises from the anisotropic polarizability of GO flakes, it is concluded that addition of HCl weakens the anisotropy of GO polarizability, although the same is not true for addition of NaOH.

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THEORY AND DISCUSSION We calculated the Kerr coefficient, which reflects the sensitivity of electro-optic response to external fields, using the electrooptic response in low electric fields (Figure 3b−d), where the linear fitting is possible as a function of the square of the electric field (E) at a given wavelength (λ). K=

Δn λE 2

(1)

The calculated Kerr coefficients in Figure 5 were 1 order smaller than those reported in the previous paper,18 possibly due to different cell geometries and the smaller GO size. The cell used in this experiment (Figure 3a) had a distorted field distribution and a larger surface effect that decreased the effective birefringence compared with the cuvette cell used previously, and the average GO size decreased from 3.2 to 1.9 μm. As indicated in Figure 5, the sensitivity to electric fields decreased with increasing ionic concentration for the HCl−GO and NaCl−GO solutions but increased slightly for the NaOH− GO solutions. To determine the underlying mechanism for the clear contrast in electrical sensitivity between the NaOH−GO solutions and the other solutions, we considered the microscopic electrical polarization of GO particles using the

α|| , ⊥ = α||∞, ⊥ +

α||0, ⊥ − α||∞, ⊥ 1 + ω 2τ||2, ⊥

(2)

where the subscripts ∥ and ⊥ indicate the directions parallel and perpendicular to the normal of the GO basal plane, respectively, and the superscripts ∞ and 0 are the infinite and zero angular frequency (ω), respectively. τ is the dielectric relaxation time. The variables α∞, α0, and τ in eq 2 can be 26307

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expressed using the depolarization factor (L) and the effective conductivity (κp) of GO particles as τ|| , ⊥ = ε0 α||∞, ⊥ = α||0, ⊥ =

(1 − L|| , ⊥)εe + L|| , ⊥εp (1 − L|| , ⊥)κe + L|| , ⊥κp , || , ⊥

εp − εe 4πab2 ε0εe εe + (εp − εe)L|| , ⊥ 3

κp , || , ⊥ − κe 4πab2 ε0εe κe + (κp , || , ⊥ − κe)L|| , ⊥ 3

(3)

where ε0 is the permittivity of the vacuum and κp , ⊥ =

2κ s a a κs , κp , = , L ≅ 1 − π , and L⊥ ≅ π a b 2b 4b (4)

Here, the surface conductivity κ (= μs × σs) near the GO surface is introduced using the electrical mobility (μs) and the surface charge density (σs) by assuming that the electrical double layer is sufficiently thin. Once the physical properties εe, κe, and κs (= μs × σs) are determined, the anisotropic polarizability, Δα (= α⊥ − α∥), can be calculated using eqs 2 and 3. For a pure GO dispersion with no added ions, κs (= μs × σs) can be determined as follows. Our pure GO dispersion without added ions exhibits pH 3.7, which corresponds to a H+ concentration of 2 × 10−4 M. The H+ ions are supplied by the negatively charged surfaces of GO flakes. Dividing the total charge of H+ ions in the water by the total surface area of GO flakes in a unit volume of a 0.1 wt % GO dispersion, the surface charge density of GO flakes that supply the bulk H+ ions was approximately 0.02 Cm2−. In the pure GO dispersion without added ions, μs is the mobility of H+ ions and was 3.6 × 10−7 m2 V−1 S1−. Using the values of κs (= μs × σs), Δα for the pure GO dispersion without added ions was 1.8 × 10−27 Fm2. The relationship between the Kerr coefficient (shown in Figure 5) and the anisotropy of polarizability (Δα) can be expressed as18,25 s

Figure 4. Dynamic electro-optic response of the GO dispersions: (a) turn-on response of the field-induced birefringence; (b) turn-off response of the field-induced birefringence; and (c) corresponding relaxation time constant s, which was calculated by fitting the curves in (a) and (b) using exponentially increasing and decreasing functions, respectively.

K=

ΔnsatφwDS Δn e (E ) = 2 λE λDGOE2

(5)

Here, ϕw, DGO, De, and S(E) are the weight concentration of GO in the sample, the density of GO particles, the density of water, and the order parameter as a function of electric field. Δnsat is the specific birefringence when S = 1 and ϕw = 1. By assuming that the interparticle interaction is negligible (note that we used a dilute dispersion with low concentration), the order parameter can be obtained using the Boltzmann distribution function of GO particles ( f(θ, E)) with a potential, U(θ) = −1/2ΔαE2 cos2 θ, as S(E ) =

∫0

π

⎛3 2 1 ⎞⎟ ⎜ cos θ − f (θ , E) sin θ dθ ⎝2 2⎠ ≈−

Δα 2 E 15kT

(6)

Here, k and T are the Boltzmann constant and the absolute temperature, respectively. From eqs 5 and 6, the Kerr coefficient can be simplified as18

Figure 5. Optical Kerr coefficient of GO dispersions with various ionic additives.

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ΔnsatφwDe 15λDGOkT

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decreased sharply as the ionic strength increases (Figure 6(a)), because of the increased bulk conductivity that enhances the characteristic contrast between the particle conductivity and solvent conductivity. Thus, we conclude that the Kerr coefficient of GO dispersions is desensitized by the addition of ions due to the increased bulk conductivity rather than the modified electrical double layer on the GO surface. The slight increase in Kerr coefficient of GO dispersions added by NaOH can be explained by the slight decrease in the bulk conductivity as shown in Figure 2b. Directly calculating the surface charge density using the Grahame equation gives the same conclusion. The Grahame equations is expressed as30,37

Δα (7)

It is difficult to measure Δnsat experimentally due to the difficulty in obtaining a perfectly aligned sample. However, using the experimental value of K and the theoretical value of Δα for the pure GO dispersion, we can estimate Δnsat from eq 7, which was calculated to be 9.27 × 10−2. Since the optical refractive index of GO particles is not influenced by the ionic concentration in dispersion, Δnsat is supposed to be the same in all the GO dispersions with different ionic strengths as long as the degree of oxidation of GO particles does not change. Hence, based on the linear relationship between the Kerr coefficient and Δα, we determined Δα as a function of concentration of ion added, as shown in Figure 6a. Although we

σs = (2ε0εrkTN0 × 103 ∑ |A±|s )1/2

(8)

where the molar concentration of ion A near the surface is |A±|S = |A±|∞ exp(∓(eV0)/kT) − |A±|∞. The A± variable was Na+, Cl−, H+, or OH− in our experiments, and the subscripts S and ∞, respectively indicate the surface area and bulk area. e, εr, N0, and V0 represent the elementary positive charge, dielectric constant of pure water, Avogadro’s number, and the surface potential, respectively. All of the variables were known except V0, which is difficult to determine. Usually, the absolute value of V0 is larger than that of the zeta potential, but the relationship may depend on the distance between the GO surface and the slipping plane and on the charge distribution in that area. We already determined σs for the pure GO dispersion, and by substituting the value of σs and A = H+ in eq 8, we obtained V0 ≈ 2.55 × ζ for the pure GO dispersion. Assuming this relationship is valid for the other dispersions, we calculated σs for the other dispersions as shown in Figure 7a. Using the surface conductivity from Figure 6b and the surface charge density from Figure 7a, the charge mobility was simply calculated as shown in Figure 7b. The result shows that the surface charge densities increased and the mobility decreased with a similar slope, compensating for each other and giving rise to the nearly fixed surface conductivity. In parts a and b of Figure 7, the changes in surface charge density and mobility appeared at lower concentrations in the HCl− and NaCl−GO solutions than in the NaOH−GO solution, although the data error seems too large to draw a definite conclusion. Such behavior also supports the occurrence of ionic compensation upon addition of NaOH. The mobility seems to be only weakly dependent on the type of ion added. Addition of NaOH was expected to quickly replace H+ ions in the electrical double layer with Na+ ions due to recombination of H+ and OH−, and the mobility was expected to decrease accordingly. However, addition of NaOH did not demonstrate a meaningful difference from addition of other ions. This result may have arisen because the cation exchange capacity for H+ ions in the electrical double layer is quite high. That is, H+ ions are not likely to be replaced by other ions. Since κp,∥ is much smaller than κp,⊥ from the definition shown in eq 4, α⊥ is always much greater than α∥, and as a result, Δα ∼ α⊥, approximately. When the frequency is lower than the resonance frequency and κe ≪ κp,⊥, the anisotropic polarizability can be approximately reduced as

Figure 6. Anisotropy of polarizability (a) and the surface conductivity (b) of GO dispersions as a function of concentration.

followed a long calculation process to determine Δα, the results in Figure 6a are intuitively predictable from the Kerr coefficient (Figure 5) because the GO dispersion is desensitized when ions are added, mainly due to the decrease in Δα, the driving force of GO reorientation. Using eqs 2 and 3 and the values of Δα (Figure 6a) and bulk conductivity (κe shown in Figure 2b), we calculated the surface conductivity, κs, of all GO dispersions with added ions, as shown in Figure 6b. Interestingly, the surface conductivity on GO dispersion was almost constant with varying ionic concentration. Mantegazza et al. also reported that the surface conductivity in the electrical double layer is insensitive to the ionic strength when the surface charge density is in the order of 10−1 S/m, which our GO samples belong to.36 However, Δα

Δα ≈ α⊥0 ≈ − 26309

⎛ ⎞ κp , ⊥ 4πab2 ⎟⎟ ε0εe⎜⎜ 3 ⎝ κe + L⊥κ p, ⊥ ⎠

(9)

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mainly due to the variation of the bulk conductivity in our sample. We can depict the phenomenon by a simple illustration shown in Figure 9. GO flakes supply the dispersion with H+ ions, which form an electrical double layer around GO flakes and acidify the dispersion (Figure 9a). Because a large difference between the surface and bulk conductivities causes dipolar charge distribution on a GO particle, a large polarization is transiently achieved under the application of an electric field (Figure 9b). When acidic HCl or neutral NaCl ions are added, the bulk charge density and the bulk conductivity will also increase (Figure 9c,d), resulting in the decrease in the polarization of the GO particle. The zeta potential of the dispersions just slightly changes when the ions are added up to 10−3 M, indicating that the electrical double layer of GO is just weakly influenced by the ionic addition. Hence, the surface conductivity is nearly maintained irrespective of the ion concentration up to 10−3 M. The addition of NaOH does not increase the total number of ions in the solvent (Figure 9e), and H+ ions with high mobility in water are replaced by Na+ ions with low mobility, resulting in the slight decrease in the bulk conductivity. As a result, the addition of NaOH slightly enhances the electro-optical sensitivity, unlike the addition of NaCl or HCl.

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CONCLUSION Liquid crystalline behavior of GO dispersions and its control using electric fields are quite interesting from fundamental and practical points of view. Therefore, we measured the optical Kerr coefficients of GO dispersions with varying ion types and concentrations. We also determined the zeta potential, conductivity, and pH of these GO dispersions, as they are important parameters in the electro-optic effect of GO dispersions. The pH of the pure GO dispersion was 3.7, indicating a H+ ion concentration of 2 × 10−3 M in the solvent. The solvent conductivity was slightly decreased by addition of NaOH, which may result from the replacement of H+ ions in the solvent with Na+ ions due to the recombination of H+ with OH−. This result is directly reflected in the Kerr coefficient. Although we added 10−3 M NaOH, the electro-optic sensitivity increased rather than decreased. The calculated surface charge density and mobility indicate that H+ ions are more attracted to the surface than are Na+ ions, and that H+ ions persist in the electrical double layer instead of being replaced by Na+ ions.

Figure 7. Surface charge density (a) and mobility (b) of GO particles as a function of ionic concentration.

In our sample, κp,⊥ (= ∼ 6) ≫ κe (= 6 × 10−3 − 7 × 10−2) > L⊥κp,⊥ (= ∼1.5 × 10−3). Hence, the bracket in eq 9 is roughly reduced to (κp,⊥/κe). Thus, Δα ∼ (κp,⊥/κe), approximately. Since κp,⊥ is almost constant in our sample as shown in Figure 6b, it is expected that K ∼ Δα ∼ (1/ κe). Using the Kerr coefficient (K) and the bulk conductivity (κe) of the GO dispersions that were experimentally obtained, we compared K and 1/κe for various GO dispersions with different ionic strength. Figure 8 shows more or less good correlation between K and 1/κe, as expected in the theoretical analysis, which confirms that the variation of K as a function of ionic strength is

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MTHODS GO samples were prepared by the Hummers method.28 Residual ions such as Mn+, K+, and H+ were carefully removed by centrifuging the sample and refreshing the solvent 15 times. Complete removal of residual ions was confirmed by combustion chromatography analysis to the level of 10−7 M.18 The Debye thickness (TD) was calculated using the formula30 TD =

ε0εkT N0e 2M

(10)

Here, ε is the dielectric constant of solvent, and M is the total mole concentration of ions in the bulk. A GO cell was fabricated using two substrates with patterned ITO (indium tin oxide) electrodes with an electrode width (w) of 10 μm, 500 μm between electrodes (l), and a cell thickness (d) of 300 μm, as shown in Figure 3a. The electro-optical properties were measured under crossed polarizers and a 550

Figure 8. Correlation between the Kerr coefficient (K) and the bulk conductivity (κe) for the GO dispersions with varying ionic concentration. 26310

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Figure 9. Simple illustration that explains the phenomenon observed in an ion-added GO dispersion. (a) GO flakes supplies H+ ions to water, which forms the electrical double layer (EDL). (b) Large difference between the surface and bulk conductivities causes a large polarization. (c, d) In the HCl−GO and NaCl−GO dispersions, the bulk ionic strength increases. (e) In the NaOH−GO dispersion, H+ ions in the solvent are replaced by Na+ ions.

nm narrow band-pass filter (the photo in Figure 3a was taken without the filter). The effective birefringence (Δn) of the cell was calculated by measuring the optical intensity of the system using the following relationship. ⎛ πdΔn ⎞ ⎟ I = I0 sin 2⎜ ⎝ λ ⎠

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(11)

Here, I and I0 are the light intensity under crossed polarizers with voltage and parallel polarizers without voltage, respectively. A square wave AC voltage at 10 kHz was applied. The conductivity, κe, of the bulk solvent and zeta potential were measured simultaneously using a zeta potential analyzer (Zetasizer Nano ZS90, Malvern Company, UK). The pH of the dispersions was measured with a pH meter (Cyberscan PC300 by Eutech Instrument Company, Singapore).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

J.-K.S. planned and supervised the project. S.-H.H. and T.-Z.S. contributed equally to this work by performing all the experiments. All authors analyzed the data and participated in writing the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIP) (Nos. 2012R1A1A1012167 and 2013R1A1A2057455).

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