2-Mediated Reduction of Graphene Oxide at Room ... - ACS Publications

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Rate and Mechanistic Investigation of Eu(OTf)2‑Mediated Reduction of Graphene Oxide at Room Temperature Tufan Ghosh, Sandeepan Maity, and Edamana Prasad* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India S Supporting Information *

ABSTRACT: We describe a fast, efficient, and mild approach to prepare chemically reduced graphene oxide (rGO) at room temperature using divalent europium triflate {Eu(OTf)2}. The characterization of solutionprocessable reduced graphene oxide has been carried out by various spectroscopic (FT-IR, UV−visible absorption, and Raman), microscopic (TEM and AFM), and powder X-ray diffraction (XRD) techniques. Kinetic study indicates that the bimolecular rate constants for the reduction of graphene oxide are 13.7 ± 0.7 and 5.3 ± 0.1 M−1 s−1 in tetrahydrofuran (THF)−water and acetonitrile (ACN)−water mixtures, respectively. The reduction rate constants are two orders of magnitude higher compared to the values obtained in the case of commonly used reducing agents such as the hydrazine derivative, sodium borohydride, and a glucose−ammonia mixture. The present work introduces a feasible reduction process for preparing reduced graphene oxide at ambient conditions, which is important for bulk production of GO. More importantly, the study explores the possibilities of utilizing the unique chemistry of divalent lanthanide complexes for chemical modifications of graphene oxide.

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the presence of ammonia at 95 °C to synthesize graphene nanosheets.24 A few more reagents such as NaBH4, zinc powder−H2SO4 mixture, and sulfur-containing compounds (e.g., NaHSO3, Na2SO3, Na2S2O3, Na2S.9H2O, SOCl2, and SO2) have been utilized as reducing agents for converting GO to graphene.21,25,26 While Fe powder, L-ascorbic acid, and hydroquinone in aqueous solution are also known to reduce GO, the reaction time was more than 6 h.18−20,27,28 Among all of the reductants, hydrazine derivatives have attracted considerable attention compared to other reducing agents. Nonetheless, use of this reagent is greatly discouraged due to the extremely toxic and explosive nature of hydrazine and its derivatives. In this paper, we have provided alternate ways of preparing rGO under mild conditions using divalent lanthanide complexes. Divalent-lanthanide-based reductants have been widely utilized for various functional group transformations in organic chemsitry.29−36 While most of the divalent-lanthanide-mediated reactions are promoted by Sm(II) salts, studies have shown that Eu(II) salts can also be utilized for selective reduction processes.37−39 Hence, the mild nature of Eu(II) as a reductant is utilized in the present work for reducing the functional groups in GO, keeping the carbon− carbon network intact. Other divalent lanthanides such as Sm(II), Yb(II), Tm(II), and so forth are found to reduce C

INTRODUCTION Graphene is a relatively new member in the carbon family with a 2-D network of sp2 hybridized carbon atoms of single atomic layer thickness.1,2 Recent research in this field has generated immense curiosity due to the unique material properties of graphene and its potential applications in the fields of optoelectronics, energy storage, catalysis, drug delivery, and biosensors.3−9 While isolation of single-layer graphene by micromechanical cleavage of graphite has been known for a decade, the method fails during scale up. As a consequence, the production of highly pure graphene on a large scale remains a challenge.1 Various methods such as chemical vapor deposition, liquid-phase exfoliation, epitaxial growth, and so forth have been reported in the literature for the synthesis of graphene.10−14 Reduced graphene oxide (rGO), which has identical properties to those of graphene, has been prepared by inexpensive and potentially scalable chemical reduction methods.15,16 This method has also been widely used for the preparation of chemically modified graphene-based composite materials.17 Currently, a handful of reagents have been used to prepare rGO in the solution phase.15,16,18−28 For example, Ruoff and co-workers have observed that colloidal suspension of reduced graphene could be prepared using hydrazine as a reducing agent.18 In a separate attempt, Li et al. have demonstrated that aqueous processable graphene nanosheets can be formed by reduction of a suspension of graphene oxide (GO) by hydrazine hydrate in the presence of ammonia at 95 °C.19 Also, Dong and co-workers have treated GO with glucose in © 2014 American Chemical Society

Received: February 25, 2014 Revised: April 30, 2014 Published: May 1, 2014 5524

dx.doi.org/10.1021/jp501994k | J. Phys. Chem. B 2014, 118, 5524−5531

The Journal of Physical Chemistry B

Article

Scheme 1. Schematic Representation of the Possible Reduction of GO to rGO by Eu(OTf)2 at Room Temperature

ments, and various microscopic (for examples, SEM, TEM, and AFM) techniques. The details of the synthesis procedure and characterization of GO are given in the Supporting Information (Figure S1−S6). Preparation of Eu(OTf)2. Europium(II) triflate was prepared by slightly modifying a reported procedure.44 A typical procedure involves stirring a mixture of europium(III) triflate (0.02 g, 0.03 mmol) and magnesium metal (0.20 g, 0.016 mol) in anhydrous THF at room temperature for 3−4 h. Magnesium metal was taken in excess for complete conversion of europium(III) triflate to europium(II) triflate. The UV− visible absorption and steady-state emission spectra of the reaction mixture were recorded to confirm the product formation. (Figure S7 in the Supporting Information). Eu(OTf)2-Mediated Room-Temperature Reduction of GO. In a typical procedure, 100 mg of graphite oxide powder was added to 50 mL of water in a beaker, and the mixture was ultrasonicated for 45 min using an ultrasonic bath cleaner (Toschon Industries Pvt. Ltd., 40 kHz and 150 W). The mixture was then centrifuged at 2000 rpm for 30 min, solid unexfoliated graphite oxide was discarded, and the supernatant containing a few-layer GO was utilize for further steps. Reduction of GO has been carried out by adding a solution of Eu(OTf)2 in THF (or ACN) to 20 mL of an aqueous solution of GO (0.20 mg/mL) in a vial at room temperature. The color of the solution turned black in 5 min, and a black precipitate was obtained after 10 min of stirring. The black precipitate was thoroughly washed with doubly distilled water (8 × 15 mL) followed by washing with methanol (2 × 15 mL) to remove any traces of unreacted starting materials or other byproducts and was dried at room temperature for 24 h. The product (rGO) was characterized using various spectroscopic (FT-IR, UV−visible, and Raman), microscopic (TEM and AFM), and powder XRD techniques. Performance of the Kinetic Experiments. Kinetic experiments have been carried out with a Jasco V-660 spectrophotometer. A THF (or ACN) solution of Eu(OTf)2 was taken (10−3 M) in a cuvette and closed with a septum and Teflon tape. A thoroughly degassed aqueous solution of GO (0.20 mg/mL) was then injected into the Eu(OTf)2 solution in a cuvette. The conversion from GO to rGO was monitored by recording the growth of absorbance at 600 nm (absorbance increases due to formation of rGO) as a function of time. The concentration of substrate (GO) was kept high (>10 times) compared to Eu(OTf)2 in order to maintain the pseudo-firstorder conditions. The observed rate constants were obtained by fitting the growth curve at 600 nm, and then kobs values were plotted against different concentrations of Eu(OTf)2 to obtained the bimolecular rate constant. Similarly, the bimolecular rate constants of reduction for GO to rGO by a hydrazine hydrate−ammonia mixture, sodium borohydride, and a glucose−ammonia mixture were determined.

C, which is a highly undesired process in the reduction of GO.40,49 In the present investigation, we have also determined the rate and mechanistic study of GO reduction by divalent europium(II). Although there are reports focusing on the chemical synthesis of graphene, limited studies are documented in the literature about the mechanistic investigation involved in such synthetic processes.41 Quite surprisingly, data regarding the rate studies of the reduction processes of GO is lacking in the literature.42 The rate constants values for the reduction reactions by Eu(OTf)2 were also compared with the values determined for reactions mediated by commonly utilizing reducing agents. The as-synthesized rGO was characterized by various spectroscopic (FT-IR, UV−visible absorption, and Raman), microscopic (TEM and AFM), and powder X-ray diffraction (XRD) techniques.

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EXPERIMENTAL SECTION Materials. Natural graphite powder (300 mesh) was purchased from Alfa Aesar and has been used without further purification. Europium(III) triflate was purchased from SigmaAldrich. Magnesium turnings were purchased from Burgoyne Burbidges & Co., India and used without further purification. Experiments are performed in dry solvents (tetrahydrofuran (THF) and acetonitrile (ACN)) and a thoroughly degassed water mixture. THF was dried by the sodium/benzophenone method, and ACN was dried using the calcium hydride method. Instruments. The UV−visible absorption spectra were recorded in a Jasco V-660 spectrophotometer. The sample solutions were taken in a quartz cuvette with a path length of 1 cm. The IR spectra were recorded in a Jasco V-660 spectrophotometer at room temperature (298 K). The solid samples were mixed with anhydrous potassium bromide to make the IR pellets. AFM images were taken in a Park System XE-100 (Korea) under noncontact mode. The samples for the AFM were prepared by the drop casting a few drops of the solution on a silicon wafer and drying at room temperature overnight. Transmission electron microscopic (TEM) analysis was carried out in a JEOL 3010 instrument operating at 200 kV. The samples were prepared by drop casting from a diluted solution on a carbon-coated copper grid. Powder XRD experiments were performed in a Bruker D8 Advance powder X-ray instrument using Cu Kα radiation. Scanning electron microscopic (SEM) images were taken in a Quanta FEG 400 instrument. The Raman spectrum was recorded in a Brukar FTRaman (MultiRAM) spectrometer equipped with a 1064 nm laser (Nd:YAG diode pumped laser). Synthesis of GO. GO has been synthesized by oxidation of natural graphite powder by a H2SO4/KMnO 4 mixture according to Hummers’s method,43 followed by exfoliation in water. GO was characterized by using UV−visible absorption spectroscopy, powder XRD, TGA, zeta potential measure5525



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Figure 1. (a) The UV−visible absorption spectra of GO (blue line) and rGO (red line). The peak at 270 nm (red spectrum) originated due to the π−π* transition of CC bonds in rGO. (b) TEM image of rGO showing the crumpled graphene sheets. (c) Tapping mode AFM image of rGO with the corresponding height profile. The height profile analysis shows the layer thickness in the range of 1.54−2.69 nm and the presence of bi- and trilayer rGO sheets.

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RESULTS AND DISCUSSION GO contains four kinds of oxygen functional groups, for example, hydroxyl (−OH), epoxy (−O−), carbonyl (−CO), and carboxyl (−COOH) functional groups.45,46 While Eu(II) salts have been considered as weak reductants for epoxy and carbonyl groups, addition of a solution of Eu(OTf)2 (1 mL, 10−3 M, in THF) to GO solution (20 mL, 0.2 mg/mL, in water) was accompanied by immediate color change of the solution from brown to black, indicating a possible reduction reaction of epoxy, hydroxyl, and carbonyl groups in GO (Scheme 1). A black precipitate was obtained within a few minutes, presumably due to the aggregation of rGO sheets. The photographs of a solution of GO before and after addition of Eu(OTf)2 are provided in the Supporting Information (Figure S8). Characterization of rGO. To confirm the reduction of the functional groups (e.g., carbonyl, epoxy, and hydroxyl) of GO, we have carried out different spectroscopic investigations using FT-IR, UV−visible, and Raman spectroscopy. The FT-IR spectra of rGO show a substantial decrease in peak intensities for the stretching frequencies of CO (1720 cm−1), epoxy C− O (1227 cm−1), alkoxy C−O (1048 cm−1), and O−H (3400 cm−1) compared to that of GO. Also, the peak intensity corresponding to the O−H deformation vibration (1395 cm−1) was decreased remarkably upon reduction with divalent europium, confirming that a large number of epoxy, hydroxyl, and carbonyl groups have been reduced by Eu(OTf)2 (Figure S9, Supporting Information).19 Figure 1a shows the UV−visible

absorption spectra of GO before and after reduction. GO shows characteristics absorption peaks at 230 and 300 nm. The peak at 230 nm has been assigned to a π−π* transition of the CC bonds, whereas the peak at around 300 nm has been assigned to the n−π* transition of CO bonds. After the reduction of GO by Eu(OTf)2, both of the peaks at 230 and 300 nm disappeared, and a new peak at 270 nm originated. The peak at 270 nm is assigned to the π−π* transition of CC bonds in rGO, which is in good agreement with literature reports.19 The red shift in absorbance from 230 to 270 nm for the π−π* transition suggests that reduction of GO restores the conjugation in the graphene framework. The graphene sheets were also characterized by electron microscopic techniques. A dilute solution of rGO was drop cast on a carbon-coated copper grid for TEM study. A TEM image of rGO is shown in Figure 1b. The TEM analysis revealed the presence of crumpled graphene sheets. Atomic force microscopic (AFM) studies were also performed to characterize the morphology of rGO. A diluted solution of rGO was drop cast on a fresh silicon wafer and dried for 24 h at room temperature. The AFM image shows the presence of sheets of graphene. The height profile analysis suggests that the layer thickness of rGO is in the range of 1.54−2.69 nm, indicating the existence of biand trilayer graphene sheets (Figure 1c). The rGO was also characterized by powder XRD technique. The layer-to-layer distance (d-spacing) was calculated from the XRD pattern for pristine graphite, GO, and rGO. The powder XRD pattern of GO shows larger interlayer spacing compared 5526



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Scheme 2. Proposed Mechanism for the Reduction of GO by Eu(II)

to graphite (the calculated d-spacing between the layers of graphene oxide was 8.41 Å, whereas the d-spacing between the layers of graphite was 3.36 Å). The increase in interlayer spacing in GO is presumably due to the presence of oxygen functional groups (hydroxyl, epoxy groups etc.) at the basal plane and also the presence of intercalated water molecules between the planes. In the XRD patterns of rGO (which are exfoliated in individual sheets and finally agglomerated to powder form), the major peak was obtained when the d-spacing between the planes of rGO was 3.70 Å. The slight increase in the value of the d-spacing in rGO compared to that in graphite (3.36 Å) is presumably due the steric encumbrance by the presence of hydroxyl or carboxylic acid groups in rGO, which are absent in graphite (Figure S10, Supporting Information).47 We then recorded the Raman spectrum of the rGO (Figure S11, Supporting Information). The Raman spectrum of rGO shows a disordered-induced D-band at 1295 cm−1 and the Gband at 1589 cm−1. The relative intensity ratio of D-band to Gband, ID/IG, provides valuable information for determination of the in-plane crystallite size or the amount of disorder.48 We have determined the ID/IG ratio for rGO as 1.15, which is in good agreement with literature reports.18 The ID/IG ratio of 1.15 indicates the presence of defect sites in rGO. A plausible mechanism of reduction of ketone and epoxy functionalities of GO by Eu(OTf)2 has been proposed in Scheme 2. In brief, the first step involved an electron transfer from Eu(II) to the electron-deficient carbon center of the ketone or epoxy group in GO, generating a radical on the carbon atom. The second step could be a proton transfer from the water molecules, which are coordinated to Eu(III) (step II in Scheme 2).49 Next, the second electron transfer results in the formation of a carbanion, followed by a proton transfer from another molecule of bound water (steps III and IV in Scheme 2). The final step involves elimination of a water molecule, which is driven by the gain in stability of the aromatic carbon network in rGO sheets and also, presumably, catalyzed by the Lewis acid {EuII(H2O)n}. Rate and Mechanistic Investigation of Eu(OTf)2Mediated Synthesis of GO. Next, we performed a detailed kinetic study of the reduction process. It is clear from the UV− visible absorption spectra of GO and rGO (Figure 1a) that reduction of GO to rGO results in increased absorption at 600 nm. Because Eu(II) or Eu(III) has significantly low absorption in this region, the growth of absorbance at 600 nm is assigned to the product formation. The rate of growth at 600 nm was measured in the presence of various concentration of Eu(OTf)2 at pseudo-first-order conditions. The observed rate constants of reduction, kobs, were obtained by fitting the exponential growth trace with a first-order curve fitting. Figure 2a shows the plot of

Figure 2. (a) Plot of growth of absorbance at 600 nm versus the time for the reduction of GO in the THF−water mixture; [Eu(OTf)2] = 0.2−0.5 mM; [GO] = 173.9 μg/mL. (b) Plot of the observed rate constant versus [Eu(OTf)2]; rate constant = 13.7 M−1 s−1.

absorbance versus time for the reduction of GO in a THF− water mixture. The bimolecular rate constant of reduction was obtained from the slop of a plot of observed rate constants (kobs) versus concentration of Eu(OTf)2 (Figure 2b). The bimolecular rate constant for the reduction of GO by Eu(OTf)2 was obtained as 13.7 M−1 s−1 in the THF−water mixture. We also determined the bimolecular rate constant in the ACN− water mixture, and the value obtained was 5.3 M−1 s−1 (Figure S12, Supporting Information). In order to compare the rate constant values of the present methods to existing methods of rGO preparation, we have determined the bimolecular rate constants for the reduction of GO by conventional reducing agents, such as a hydrazine hydrate−ammonia mixture, sodium borohydride, and a glucose−ammonia mixture (Figures S13−S15, Supporting Information). Table 1 presents the reaction condition and the calculated bimolecular rate constant for the reduction of GO by four different reducing agents. It is interesting to note that the rate of Eu(OTf)2-mediated reduction of GO in the THF−water mixture is 8 times faster than that of the hydrazine−ammonia reduction, 42 times faster than that of sodium borohydride, and 105 times faster than that of the glucose−ammonia reduction of GO. It is very intriguing to observe that while Eu(II) is unreactive toward carbonyl or epoxy functionalities of common organic molecules, it has reduced such functional groups in GO with rate constants of 6−14 M−1 s−1. The relatively fast reduction of GO by Eu(OTf)2 could be due to three possibilities: (a) the radical anion formed after an electron transfer from Eu(II) to GO is highly stabilized through extensive charge delocalization in an aromatic network of the graphene surface, which can shift the equilibrium of the step involving the first electron transfer toward the intermediate; (b) the presence of water might increase the reducing power of Eu(II);50 and (c) after the first electron transfer, water bound to europium acts as a proton 5527



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Table 1. Comparison of Measured Bimolecular Rate Constants for the Reduction of GO by Various Approachesa Sl. no.

reducing agent

solvent

temperature (oC)

1 2 3 4

hydrazine hydrate + ammonia sodium borohydride glucose + ammonia Eu(OTf)2

H2O H2O H2O ACN−water THF−water

95 125 95 RT RT

time 1 3 1 5 5

bimolecular rate constant, M−1 s−1

ref

1.7 3.3 × 10−1 1.3 × 10−1 5.3 13.7

19 21 24 present work present work

h h h m m

Error: ±5%; the reaction conditions have been adopted from the given references, and the bimolecular rate constants have been calculated under identical reaction conditions.

a

donor to capture the radical anion (or anion) formed after reduction, leading to an irreversible step through the formation of a Eu-hydroxide (Scheme 2). The first aspect can be addressed by calculating the freeenergy change of the ground-state electron transfer from the electrochemical reduction potential of the Eu3+/Eu2+ redox couple, GO, and the ketone/aldehyde group present in other organic molecules. The redox potential for Eu3+/Eu2+ is −1.21 V (with respect to SCE) in THF (Figure S16, Supporting Information). The reduction potential of GO is reported as −0.87 V (with respect to SCE).51 The calculated free-energy change (ΔGet = ED+/D − EA/A−) of the reduction process is −7.84 kcal/mol, which is reflected in the fast rate of reduction of GO by Eu(OTf)2. The reduction potentials for a ketone (for example, acetophenone) and an aldehyde (for example, benzaldehyde) are −1.85 and −1.74 V (with respect to SCE), respectively.52 Thus, the free-energy changes for groundstate electron transfer from Eu(OTf)2 to acetophenone and benzaldehyde are 14.76 kcal/mol are 12.22 kcal/mol, respectively. The free-energy value change suggests that while ground-state reduction of acetophenone/benzaldehyde by Eu(OTf)2 is energetically unfeasible, the reduction of GO by Eu(OTf)2 is energetically feasible. To elucidate the role of water, cyclic voltammetry of a solution of Eu(OTf)2 in THF was performed in the presence and absence of water. Surprisingly, it has been observed that the presence of water shifts the peak potential of divalent europium from −1.21 to −0.70 V. The shift of the reduction potential of the Eu3+/Eu2+ couple to more positive values suggests that the presence of water makes Eu2+ less reactive (Figure S16, Supporting Information). Next, we performed reaction progress kinetic analysis (RPKA) to understand the mechanistic aspects of the reduction. RPKA is a useful methodology that has been employed for the determination of the rate equation of many complex chemical reactions.53 The main advantages of this method are that (a) it enables us to perform the kinetic analysis at the synthetically relevant concentrations and (b) the rate equation can be derived with a fewer number of experiments, unlike classical kinetic technique. In RPKA, we often define a parameter called “excess”, which is the difference between the initial concentrations between two substrates and which does not change as the reaction progresses for a fixed volume reaction. Thus, to have a better insight into the mechanism of the reduction process, rate orders with respect to each substrate have been determined through RPKA by performing “different excess” experiments. In the different excess experiment, the excess is varied by changing the concentration of one substrate in two different runs, keeping the concentration of the other substrates constant. The conditions for the different excess experiment in the THF−water mixture are provided in Table 2.

Table 2. Conditions for the Different Excess Experiment for Reduction of GO by Eu(OTf)2 in THFa run 1 2 a

[1], μg/mL 17.39 17.39

[2], M −4

2 × 10 4 × 10−4

[3], M 4.83 4.83

1: GO; 2: Eu(OTf)2; and 3: H2O.

The order with respect to Eu(OTf)2 (2) was determined by normalization of d[4]/dt according to following equation (d[4]/dt ) = kobs[1]y [3]z [2]x

(1)

where x is the order with respect to 2. Overlap of two reaction profiles was observed when x = 0.46, suggesting that the reaction is approximately half order with respect to Eu(OTf)2 in the THF−water mixture (Figure 3).

Figure 3. Plot of rate versus [4] for the different excess experiment for the rate order of Eu(OTf)2 (2). The inset shows the normalized rate versus [4] plot. Run-2 was performed by doubling the concentration of 2 and keeping the concentration of other components the same as that of run-1. The rate order of 2 is 0.46.

The rate orders with respect to other components have been calculated using the same method as that explained above. The rate order with respect to GO (1) is negative, which suggests that a greater amount of GO suppresses the rate of reaction. The calculated rate order for GO is approximately −1 in the THF−water mixture and −0.5 in the ACN−water mixture. Interestingly, we have observed a significant enhancement of the rate of reaction upon addition of water to the reaction mixture. The rate order with respect to water is close to 1 in both THF−water and ACN−water solvent mixtures. Table 3 contains the values of the calculated rate order for Eu(OTf)2, water, and GO in THF−water and ACN−water mixtures. Plots for obtaining the rate orders of different components in 5528



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for the reduction of GO is being demonstrated for the first time. The reduction of GO was confirmed by FT-IR, UV− visible absorption, and XRD studies. We have also carried out a detailed kinetic study and presented the rate constants values for the reduction of GO in the presence of various reducing agents. The work reveals the role of divalent lanthanide chemistry for generating rGO with a few-layer thickness. The work also provided pathways that are potentially capable of generating rGO in bulk amount under mild conditions. Currently, attempts have been initiated in our laboratory to develop a catalytic cycle for the reduction process, and the results will be published in due course.

Table 3. Observed Rate Orders Calculated by the Different Excess Method for Reduction of GO by Eu(OTf)2 in THF and ACN rate order with respect to various components solvents

GO (1)

Eu(OTf)2 (2)

H2O (3)

THF ACN

−1.03 ± 0.1 −0.68 ± 0.05

0.46 ± 0.05 0.44 ± 0.05

0.76 ± 0.05 0.89 ± 0.05

different solvents are given in the Supporting Information (Figures S17−S32). Water has a rate order close to unity, which indicates that water plays a key role in controlling the overall rate of reduction processes. This is feasible if the proton transfer becomes the rate-determining step. To verify whether proton transfer is the rate-determining step or not, we have carried out the reduction reaction in H2O and D2O. We observed that the ratio kH/kD is 1.33, where kH and kD represent observed rate constants in H 2 O and D 2 O, respectively (Figure S33, Supporting Information). While the ratio of the rate constant is close to 1, protonation of O could still be the rate-limiting step according to the Eigen mechanism.54 This also corroborates our hypothesis that the low concentration of the radical generated in the rate-limiting step will undergo further reduction by a second Eu(II), rather than dimerization of the radicals. Partial order for a reactant can be obtained under the following conditions: (i) existence of parallel reaction paths where the said reactant participates only after the ratedetermining step in one of the pathways and (ii) if the reactant dimerizes and only monomers participate in the reaction. Experimental results suggest that the fractional rate order of Eu(OTf)2 could be mainly due to dimerization of Eu(II) in THF. The UV−visible absorption spectra of Eu(II) in THF exhibited a blue shift as a function of concentration of Eu(II), which indicates aggregation (dimerization) of Eu(II) (Figure S34, Supporting Information). Negative rate orders of GO are intriguing and presumably due to the formation of an increased number of nonreacting GO aggregates at higher concentration of GO. The results taken together suggests that the initial step of electron transfer from Eu(II) to a carbonyl or epoxy group will be followed by a slow protonation step, involving water molecules that are coordinated to Eu(III). This ratedetermining step will be followed by a fast electron transfer from another Eu(II), resulting in the formation of a carbanion. Further, second proton transfer is followed by elimination of a water molecule, resulting in the formation of the CC bond of the aromatic carbon networks in rGO. While divalent lanthanide chemistry has been widely utilized for a number of reduction and reductive coupling reactions in synthetic organic chemistry, attempts have not been made to use divalent lanthanides to modify or functionalize GO. The present study opens up novel possibilities of using divalent lanthanides for chemical modification of GO. The present study also shows that divalent europium can be efficiently used for the reduction of GO at room temperature and under mild conditions, which suggests that the process can be scaled up for bulk production of rGO.

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ASSOCIATED CONTENT

* Supporting Information S

Details of the synthesis and characterization of graphene oxide, detailed kinetic data of the reduction of graphene oxide in the presence of Eu(OTf)2 and other reducing agents, cyclic voltammetric analysis data, and other experimental data. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.G. and S.M. are grateful to Council of Scientific and Industrial Research, India for a fellowship. E.P. acknowledges the Nano Mission of Department of Science and Technology (SR/NM/NS-115/2010), India for financial support. We thank the Department of Metallurgical and Materials Engineering (MME), IIT Madras for the SEM facility, and the Department of Chemistry, IIT Madras for others research facilities. We also thank Prof. T. Pradeep, DST-Unit of Nanoscience, Department of Chemistry, IIT Madras for the TEM facility. We are also grateful to Prof. M. V. Sangaranarayanan, Department of Chemistry, IIT Madras for the cyclic voltammetry facility.

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REFERENCES

(1) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324, 1530−1534. (2) Rao, C. N. R.; Sood, A. K.; Subrahmanyam, K. S.; Govindaraj, A. Graphene: The New Two-Dimensional Nanomaterial. Angew. Chem., Int. Ed. 2009, 48, 7752−7777. (3) Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. C. Graphene Photonics and Optoelectronics. Nat. Photonics 2010, 4, 611−622. (4) Dai, L. Functionalization of Graphene for Efficient Energy Conversion and Storage. Acc. Chem. Res. 2013, 46, 31−42. (5) Wan, X.; Huang, Y.; Chen, Y. Focusing on Energy and Optoelectronic Applications: A Journey for Graphene and Graphene Oxide at Large Scale. Acc. Chem. Res. 2012, 45, 598−607. (6) Scheuermann, G. M.; Rumi, L.; Steurer, P.; Bannwarth, W.; Mülhaupt, R. Palladium Nanoparticles on Graphite Oxide and Its Functionalized Graphene Derivatives As Highly Active Catalysts for the Suzuki−Miyaura Coupling Reaction. J. Am. Chem. Soc. 2009, 131, 8262−8270. (7) Pyun, J. Graphene Oxide As Catalyst: Application of Carbon Materials beyond Nanotechnology. Angew. Chem., Int. Ed. 2011, 50, 46−48.

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CONCLUSION In summary, we have developed a fast and efficient method for the reduction of GO for the synthesis of solution-processable rGO under mild conditions. Utilization of divalent europium 5529



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