Understanding Aqueous Dispersibility of Graphene Oxide and

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Understanding Aqueous Dispersibility of Graphene Oxide and Reduced Graphene Oxide through pKa Measurements Bharathi Konkena and Sukumaran Vasudevan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: The chemistry underlying the aqueous dispersibility of graphene oxide (GO) and reduced graphene oxide (r-GO) is a key consideration in the design of solution processing techniques for the preparation of processable graphene sheets. Here, we use zeta potential measurements, pH titrations, and infrared spectroscopy to establish the chemistry underlying the aqueous dispersibility of GO and r-GO sheets at different values of pH. We show that r-GO sheets have ionizable groups with a single pK value (8.0) while GO sheets have groups that are more acidic (pK = 4.3), in addition to groups with pK values of 6.6 and 9.0. Infrared spectroscopy has been used to follow the sequence of ionization events. In both GO and r-GO sheets, it is ionization of the carboxylic groups that is primarily responsible for the build up of charge, but on GO sheets, the presence of phenolic and hydroxyl groups in close proximity to the carboxylic groups lowers the pKa value by stabilizing the carboxylate anion, resulting in superior water dispersibility . SECTION: Nanoparticles and Nanostructures

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clear correlation between the oxygen functionalities present on GO and r-GO sheets, as detected by spectroscopy,14−18 and the aqueous dispersibilty has yet to be established. Here, we do so by first correlating the charge buildup on GO and r-GO sheets with variation in pH, as derived from zeta potential measurements, with the distribution of pK values of the ionizable groups present on the sheets. We then follow the sequence of ionization events as the pH of the media is changed using infrared spectroscopy and show that a molecular-level understanding of the acid−base chemistry underlying the aqueous dispersibility of graphene sheets can be established. GO and r-GO were prepared by procedures reported in the literature (see Supporting Information).19,20 The materials were characterized by X-ray photoelectron spectroscopy, 13C magic angle spinning NMR, and Raman spectroscopy (details are provided as part of the Supporting Information). The N2 adsorption BET surface areas of the as-prepared GO and rGO were 170 and 530 m2/g, respectively. The zeta (ζ) potentials of GO and r-GO aqueous dispersions at different values of pH are shown in Figure 1. The values are quite similar to those reported previously.8,9 The zeta potential is an important factor for characterizing the stability of colloidal dispersions and provides a measure of the magnitude and sign of the effective surface charge associated with the double layer around the colloid particle. Generally, particles with zeta potentials more positive than +30 mV or more negative than −30 mV are considered to form stable dispersions due to

raphene sheets are of interest because of their excellent electronic, thermal and, mechanical properties, and a wide range of applications are envisaged.1−3 These would require the availability of processable graphene sheets in large quantities. Chemical reduction of graphene oxide (GO) has been considered a viable route for large-scale production of graphene sheets.4,5 The reduced GO (r-GO) sheets, although their conductivities are comparatively lower than that of graphene obtained by physical methods such as mechanical exfoliation, are nevertheless versatile materials for applications in thin films and composites.6,7 A major challenge is that graphene is hydrophobic and easily forms agglomerates irreversibly or even restacks to form graphite in aqueous solutions, in the absence of dispersing agents. It has, however, been shown recently that chemically converted graphene sheets obtained from GO can readily form stable aqueous colloids through electrostatic stabilization over a restricted range of pH values.8 Zeta potential measurements showed that above a pH value of 8, the r-GO sheets developed sufficient negative charge to form stable dispersions. In comparison, GO sheets form stable dispersion only above a pH value of 4. Understanding the chemistry behind the aqueous dispersibility of GO and r-GO nanosheets at different values of pH is of fundamental importance from the viewpoint of processability and also to understand a number of associated phenomena, for example, understanding size fractionation of GO sheets by simply adjusting the pH value of GO dispersions9 or for a molecular-level explanation of the observation of induced voltages when water with differing molarities of hydrochloric acid flows over graphene.10 There is a broad consensus that the reduced water dispersibility of r-GO as compared to that of GO is because of the absence of oxidized groups.11−13 However, a © 2012 American Chemical Society

Received: February 28, 2012 Accepted: March 13, 2012 Published: March 13, 2012 867

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the presence of acid groups with different dissociation constants. To determine the pK values of the different ionizable functionalities present on GO, we differentiate the concentration profile with respect to pH.24 This is shown as an inset in Figure 2a and shows the presence of three broad peaks, indicating the presence of three different acid groups with differing dissociation constants. Considering the heterogeneous nature of GO (and r-GO), ionizable groups with different dissociation constants, each having a distribution of acidities, may clearly be anticipated. Quantitative information was derived by fitting the experimental differential curve assuming a Gaussian distribution of acidities at each pK value. For GO sheets, the best agreement was obtained by considering three different acid groups with pK values of 4.3, 6.6, and 9.8 and their relative contributions in the ratio 3.0:7.4:6.5. In comparison, the concentration profile of the ionized groups on r-GO sheets (Figure 2b) showed a single inflection, indicating the presence of ionizable acid groups with a single acid dissociation constant (pK = 7.9) and a much narrower distribution (inset of Figure 2b). In order to relate the information from the acid−base titration with the zeta potential measurements, we have calculated the charge present on the GO and r-GO sheets from both measurements. The surface charge density, σs, may be related to the zeta potential, ζ, by the Gouy−Chapman equation25,26

Figure 1. Zeta potential of GO and r-GO aqueous dispersion as a function of pH.

interparticle electrostatic repulsion.21 Figure 1 shows that the zeta potential of GO and r-GO dispersions are pH-sensitive. The GO sheets form stable dispersions in the pH range of 4− 11.5; the highest value of ζ is −54.3 mV at pH 10.3. The r-GO sheets form stable dispersions only in more basic media in the pH range of 8−11.5; the highest value of ζ is −44.2 mV at pH 10. The observed increase in ζ above pH 10.5 is because of the compression of the double layer at high ionic strengths.22 The results in Figure 1 are similar to those reported earlier and suggests that the stability of the GO and r-GO dispersions is a consequence of the negative charge on the graphene sheets that develop as a result of the ionization of the different functionalities present.8 If this is true, the charge should be directly related to the concentration of the ionized groups present at different values of pH. The concentrations of the ionized groups present on the GO and r-GO sheets at different values of pH were determined by an acid−base titration. The procedure involved addition of weighed quantities of GO and r-GO to known volumes of a 0.1 M NaOH solution followed by a pH titration (see Supporting Information).23 The results shown in Figure 2a and b represent the concentration of sodium binding acid groups per gram as a function of pH (the sodium binding isotherm) or, alternatively, the concentration of ionized species as a function of pH. It may be seen that this concentration is much larger for GO as compared to that for r-GO sheets. The concentration profile for GO (Figure 2a) shows multiple inflections that clearly indicate

σs =

⎛ ze ζ ⎞ 2εkT κ ⎟ sinh⎜ ⎝ 2kT ⎠ ze

(1)

where ε is the dielectric constant, z the valency of the counterions, e = 1.6 × 10−19 Coulombs, and κ is the reciprocal of the Debye screening length (m−1). The Debye scattering length is given by the expression 0.304/√I, where I is the ionic strength defined as (1/2)∑ z i 2 [ x i ]; x i is the molar concentration of the ith species, and zi its valency. The surface charge density (C/m2) calculated from the data of Figure 1 is shown in Figure 3a for GO and in Figure 3b for r-GO. The charge density may also be obtained from the concentration of ionized species obtained from the pH titration as the charge per gm (C/g), σg = zeNa, where Na is the concentration of ionized species per gm. The variation of σg with pH is shown in Figure 3a for GO and in Figure 3b for rGO. It may be seen that variations of the charge with pH evaluated from the two measurements are very similar. The

Figure 2. Concentration of ionized groups or the sodium binding isotherm as a function of pH for (a) GO and (b) r-GO. The inset shows the pK distribution of the acid groups. The fitted curve is shown in red, and the individual Gaussians are shown as dashed lines. 868

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Figure 3. The surface charge density (C/m2) for (a) GO and (b) r-GO calculated from the zeta potential as a function of pH using the Gouy− Chapman equation (eq 1) (black curve) and the charge density (C/g) as a function of pH (blue curve) calculated from the titration curve.

Figure 4. The infrared spectra of GO at different values of pH. The panel on the right shows a cartoon representation (C, gray; O, red; H, blue) of the sequence of ionization events at the indicated pH values. The ionized groups are highlighted.

cm−1 spectral range, which is the fingerprint region for the different oxygen functionalities (the O−H stretching region that appears between 3000 and 3500 cm−1 is not shown). It may be seen that there are significant changes with pH in this region; these have been highlighted in Figure 4. Pure GO at pH < 2 shows the presence of a number of undissociated oxygen functionalities. The assignment of the IR bands in GO (at pH < 2) are as follows. The intense band at 1730 cm−1 (designated as the α region in Figure 4) is assigned to the carbonyl (CO) stretching mode of the undissociated carboxylic (COOH) group, the band at 1618 cm−1 (designated as the β region) is due to the bending mode of H2O molecules that hydrate the carboxylic group, the band at 1251 cm−1 (designated as the γ region) is due to the phenol/hydroxyl C−OH stretch, and the band at 1060 cm−1 is due to the C−O−C stretch of the epoxide groups.27,28 Corroborative evidence for this assignment was also obtained form XPS and 13C MAS NMR measurements (see Supporting Information). As the pH is raised, the band at 1620 cm−1, due to water, disappears and is completely absent at pH 4. A new band appears at 1584 cm−1 (designated as the δ region in Figure 4) that may be assigned to the asymmetric stretch of the carboxylate anion, COO−. With further increase

deviation at high pH values is because of the change in the zeta potential due to compression of the double layer at high pH values.22 It may be noted in Figure 3a and b that only the trend in the variation of charge with pH has been compared. For actual comparison of values, the charge obtained from the pH titration would have to be multiplied by the electrochemical active surface area. Nevertheless, the comparison (Figure 3a and b) provides compelling evidence that the variation in the zeta potential with pH may be wholly accounted for by the ionization events that occur at different values of pH in the GO and r-GO sheets. Figures 2 and 3 indicate that the enhanced water dispersibility of GO as compared to that of r-GO is because of a larger concentration of ionizable species, as has been broadly understood in the literature.8 However, that is only half of the story; it is the presence of ionizable groups with pK values of 4.3 and 6.6 on GO that are more acidic as compared to those on r-GO that is the key to the enhanced dispersibility of GO over a wider pH range. The next task is to identify these groups. We did so using infrared spectroscopy of the GO and r-GO sheets at different stages of the pH titration. The infrared spectra of GO sheets at different values of pH are shown in Figure 4. Attention is focused on the 800−2000 869

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Figure 5. The Infrared spectra of r-GO at different values of pH. The panel on the right shows a cartoon representation (C, gray; O, red; H, blue) of the sequence of ionization events at the indicated pH values. The ionized groups are highlighted.

in pH, the band at 1730 cm−1 decreases in intensity and is almost absent at pH 7. The carboxylate stretching band also shows a blue shift from 1584 to 1594 cm−1 (δ region). There is also a decrease in intensity of the C−OH (γ region). At pH 9 and above, a new band appears at 1678 cm−1 (designated as the ε region). The position of this band is characteristic of a ketone group.29 The broad trend in the changes of the infrared spectra with pH is more easily discerned by following the changes in the intensities of the bands in the shaded regions of Figure 4. The infrared spectra of GO show a continuous decrease in the intensity of the carboxylic bands (the α region) with increasing pH; at pH 7, this band is no longer seen. Concomitant with this decrease in intensity is the rise in intensity of the carboxylate bands (the δ region). The acid groups with pK values 4.3 and 6.6 are therefore carboxylic groups. However, there are two different carboxylic groups present. It may be seen that the bands due to water (the β region) disappear above a pH value of 4 and that there is a shift to higher wavenumbers of the carboxylate band (the δ region). We assign these features to carboxylic groups that are in close proximity (one-bond removed) to a hydroxyl group. It is wellknown, for example, for the hydroxynapthoic acids, that the presence of the hydroxy group stabilizes the carboxylate anion through intramolecular hydrogen bonding, making it more acidic (see Supporting Information).30 The pK values for 1hydroxy-2-napthoic acid are 2.7 and 11.8, and for 2-hydroxy-3napthoic acid, the values are 2.9 and 11.5.30 The first value corresponds to the ionization of the carboxylic group and the second to that of the hydroxyl group. It is only the stronger acid groups (pK 4.3) on GO that are hydrated, and once these carboxylic groups are ionized, the hydrogen-bond-stabilized carboxylate anions are no longer hydrated, and the bands due to water are no longer seen above a pH value of 4.0. The remaining carboxylic groups (pK 6.6) being weaker are not hydrated.31,32 The position of the carboxylate asymmetric stretch when first seen in the IR spectra appears at 1584 cm−1; the position is identical to that of the νasym stretch of the hydroxyl napthoate anion.30 At higher pH values, this band shifts to 1594 cm−1. At pH >, 9 the phenolic groups ionize (the δ region) to give the phenolate anion (−C−O−) that

subsequently transforms to the ketone (see Supporting Information) with a characteristic symmetric stretching mode at 1678 cm−1 (the ε region).29 To summarize, there is a clear correlation between the changes seen in the infrared spectra and the ionization of the different functionalities present on GO with pK values of 4.3, 6.6, and 9.8. The pK values of 4.3 and 6.6 correspond to ionization of carboxylic groups, but the former have hydroxyl groups present in an ortho position that allows the carboxylate anion to be stabilized by H-bonding, and hence, these groups are more acidic. The pK of 9.8 corresponds to the ionization of the phenolic OH. Here too, the anion is stabilized by a phenolate to ketone transformation. The infrared spectra of r-GO (Figure 5) are relatively simpler to interpret. The spectra are dominated by the CC stretching modes at 1649 and 1406 cm−1 of the aromatic segments of the sp2 network.33 The intensities of the oxygen functionalities are significantly weaker, or absent, as compared to those of the GO sheets. This is not unexpected; it is known that reduction by hydrazine removes a majority of the oxidized groups, including the phenolic and epoxide groups.21 The absence of these groups was also confirmed by XPS measurements (see Supporting Information). The carbonyl (CO) stretching mode of the undissociated carboxylic (COOH) group in r-GO appears as a weak band at 1724 cm−1 (designated as the α region in Figure 5). With increasing pH, this band is no longer seen, but above a pH value of 8, a new band appears at 1532 cm−1 that corresponds to the asymmetric stretching mode of the carboxylate (COO−) anion (designated as the β region in Figure 5). There is a straightforward correspondence between the changes in the infrared spectra with pH and the presence of acid groups with pK 7.9 present on r-GO. The acid groups are carboxylic groups (α region) that dissociate above pH 8, as confirmed by the appearance of the carboxylate asymmetric stretching bands (β region) at this pH. In conclusion, we have shown using zeta potential measurements, pH titrations, and infrared spectroscopy that the underlying acid−base chemistry of the aqueous dispersibility of GO and r-GO sheets at different values of pH can be established. Zeta potential measurements show that r-GO 870

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sheets are dispersible above pH 8 but that GO sheets are dispersible at a much lower value of pH, 4.0. The distribution of pK values of the ionizable groups on GO and r-GO obtained from pH titrations show why this is so. GO sheets have groups that are more acidic (pK 4.3) in addition to groups with pK values of 6.6 and 9.0. In contrast, r-GO sheets have ionizable groups with a single pK value of 8. The observed distribution of pK values of the ionizable groups and their respective concentrations allow for a quantitative explanation of the variation of the zeta potential with pH for the GO and r-GO sheets. Infrared spectroscopy indicates that in both GO and r-GO sheets, it is ionization of the carboxylic groups that is primarily responsible for the build up of charge on the graphene sheets, although on GO sheets, phenolic and hydroxyl groups are also present. However, there is a notable difference. The presence of other oxygen functionalities like −OH in proximity to the carboxylic groups in GO enhances the acidity by stabilizing the carboxylate anion by intramolecular hydrogen bonding. We draw analogy with the hydroxynapthalenes, which the spectra of GO at pH 4 resemble, to assign the acid group with pK = 4 to carboxylic groups that have a −OH group in an ortho position. The remaining carboxylic groups on GO have a pK value of 6.6, not too different from those on r-GO,7.9, especially considering the fact the acid groups on GO with pK = 6.6 exhibit a wide acidity distribution. The phenolic groups on GO (pK 9) ionize at a fairly basic pH, at which value the sheets already have built up sufficient charge to form stable dispersions. The present studies show, however, that they have a significant role to play in establishing the superior water dispersibility of GO as compared to that of r-GO. It is the presence of phenolic OH groups ortho to a carboxylic group on GO that lowers the pK value of the carboxylic group and allows GO to form stable aqueous dispersions at values of pH much lower than that required for stabilizing r-GO dispersions. Our studies show that the superior water dispersibility of GO as compared to r-GO is not just because of the presence of a larger concentration of ionizable oxygen functionalities but also because of the chemical nature of these functionalities.

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ASSOCIATED CONTENT

S Supporting Information *

(S1) Preparation of GO and r-GO. (S2) X-ray photoelectron spectra of GO and r-GO. (S3) 13C CP-MAS NMR spectra of GO. (S4) Raman spectra of GO and r-GO. (S5) pH titration method. (S6) Scheme for the ionization of carboxylic groups on GO. (S7) Scheme for ketone formation upon ionization of hydroxyl groups on GO. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-80-2293-2661. Fax: +91-80-2360-1552/0683. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. S. Subramanian, Department of Materials Engineering, IISc, for help with the zeta potential measurements. 871

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(24) Strelko, V. Jr.; Malik, D. J.; Streat, M. Characterisation of the Surface of Oxidised Carbon Adsorbents. Carbon 2002, 40, 95−104. (25) Hunter, R. J. Electrified Interfaces: The Electrical Double Layer. Foundations of Colloid Science; Oxford University Press Inc: New York, 2001; Vol. 2, pp 317−325. (26) Jimbo, T.; Tanioka, A.; Minoura, N. Pore-Surface Characterization of Poly(acrylonitrile) Membrane having Amphoteric Charge Groups by Means of Zeta Potential Measurement. Colloids Surf., A 1999, 159, 459−466. (27) Si, Y.; Samulski, E. T. Synthesis of Water Soluble Graphene. Nano Lett. 2008, 8, 1679−1682. (28) Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Synthesis and Exfoliation of Isocyanate-Treated Graphene Oxide Nanoplatelets. Carbon 2006, 44, 3342−3347. (29) Brodskii, A. I.; Kotorlenko, L. A.; Samoilenko, S. A.; Pokhodenko, V. D. IR Spectra of Screened Phenols, Phenol Radicals, Quinone, and Cyclohexadienone. J. Appl. Spec. 1971, 14, 633−638. (30) Cooper, E. M.; Vasudevan, D. Hydroxynaphthoic Acid Isomer Sorption onto Goethite. J. Colloid Interface Sci. 2009, 333, 85−96. ́ (31) Smiechowski, M.; Gojło, E.; Stangret, J. Hydration of Simple Carboxylic Acids from Infrared Spectra of HDO and Theoretical Calculations. J. Phys. Chem. B 2011, 115, 4834−4842. (32) Leuchs, M.; Zundel, G. Polarizable Acid−Water Hydrogen Bonds with Aqueous Solutions of Carboxylic Acids. J. Chem. Soc., Faraday Trans. 2 1980, 76, 14−25. (33) Coates, J. Interpretation of Infrared Spectra, A Practical Approach. In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; J. Wiley: Chichester, U.K., 2000; pp 10821−10822.

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