Solvothermal-Assisted Hybridization between Reduced Graphene

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Solvothermal-Assisted Hybridization between Reduced Graphene Oxide and Lithium Metal Oxides: A Facile Route to Graphene-Based Composite Materials Song Yi Han,† In Young Kim,† Kyung Yeon Jo, and Seong-Ju Hwang* Center for Intelligent Nano-Bio Materials (CINBM), Department of Chemistry and Nano Sciences, Ewha Womans University, Seoul 120-750, Korea S Supporting Information *

ABSTRACT: Hybridization between lithium metal oxide and reduced graphene oxide, or RGO, can be achieved by the solvothermal treatment of the water/ethanol-based suspension of graphite oxide, or GO, nanosheets containing powdery lithium metal oxide. The solvothermal treatment for the mixture suspension of GO and Li4Ti5O12 gives rise not only to the reduction of GO to RGO but also to the attachment of the Li4Ti5O12 particles to the flat surface of RGO 2D nanosheets. The crystal structure and crystal morphology of the Li4Ti5O12 particles remain intact after the composite formation with the RGO nanosheets. The formation of chemical bonds and internal electron transfer between the RGO and Li4Ti5O12 components is evidenced by micro-Raman and X-ray photoelectron spectroscopy, showing the weakening of the carbon−carbon bonds and the formation of carbon−oxygen bonds. In comparison with the pristine Li4Ti5O12 material, the Li4Ti5O12−RGO nanocomposites display better anode performance with a larger discharge capacity of ∼175 mAhg−1, underscoring the merit of RGO hybridization in improving the electrode performance of bulk metal oxide. Diffuse reflectance UV−vis and photoluminescence spectroscopic analyses reveal a strong electrical connection between lithium titanate and RGO, which is mainly responsible for the observed improvement of the electrode performance upon the composite formation. In addition to the electrode performance, the photocatalytic activity of the lithium titanate for the generation of photocurrent can be remarkably enhanced by the coupling with RGO, confirming the usefulness of the present synthetic method in optimizing the photoinduced functionality of metal oxides. The solvothermal strategy presented here is also applicable for the synthesis of LiMn2O4−RGO nanocomposite showing much superior electrode performance over the pristine LiMn2O4. The experimental findings underscore that the present synthetic method can provide a universal way to not only immobilize multicomponent metal oxides on the surface of RGO nanosheets with a strong electrical connection but also improve the electrode and photocatalytic activity of these metal oxides.

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INTRODUCTION A great deal of research effort has been devoted to 2D graphene nanosheets because of their unique physicochemical properties and many valuable functionalities originating from the extremely high anisotropy in their crystal structure and morphology.1−3 Because of the high electrical conductivity and great mechanical strength of the graphene nanosheet, this material can be used as a useful platform to immobilize diverse types of inorganic solids.4−6 As a consequence of the enhancement of their electrical conductivity and structural stability upon their hybridization with graphene, many inorganic materials coupled with graphene show remarkably enhanced catalytic and electrochemical activity.7,8 In one instance, the coupling of an electrode material with highly conductive graphene was quite effective in improving its electrode functionality especially at a high current density.9 There have been many reports regarding the coupling of graphene nanosheets with diverse inorganic solids such as metal oxides and hydroxides.7−9 Because 2D nanosheets of graphene can be prepared in the form of an aqueous colloidal suspension of reduced graphene oxide, or RGO,10,11 most metal oxide− © 2012 American Chemical Society

graphene nanocomposites are synthesized by the direct crystal growth of the metal oxide on the surface of the RGO nanosheets in an aqueous colloidal suspension. Because of the weak thermal stability of the RGO colloidal nanosheets, the synthesis of nanocomposites consisting of RGO and a metal oxide is generally carried out at low temperature.12,13 In this soft-chemical synthetic condition, the metal oxides growing on the surface of the RGO nanosheets are frequently nanocrystalline and poorly crystalline. Since the electrode functionality of metal oxides, especially cathode materials, is closely related to their crystallinity, it is necessary to enhance the crystallinity of the metal oxides coupled with the RGO nanosheets via the heat-treatment at elevated temperature for the optimization of the electrode performance of the resulting nanocomposites.14 However, the poor thermal stability of the RGO component prevents heating the as-prepared metal oxide−RGO nanocomposites at elevated temperature. To circumvent this Received: November 25, 2011 Revised: March 2, 2012 Published: March 6, 2012 7269

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crystal structure and physicochemical properties of the Li4Ti5O12−RGO nanocomposites, different proportions of RGO with respect to Li4Ti5O12 were applied, viz., 0.5, 1, 5, and 10 wt % (the obtained materials are denoted as GLT1, GLT2, GLT3, and GLT4, respectively). To confirm the universal applicability of the present solvothermal method, the LiMn2O4−RGO nanocomposite was also synthesized via the solvothermal treatment at 120 °C of a water/ethanol (volume ratio of 2:1)-based colloidal suspension of GO nanosheets containing lithium manganate nanocrystals. The precursor LiMn2O4 material was prepared by hydrothermal reaction of KMnO4 and organic molecules in the presence of LiOH at 180 °C for 5 h.22 Characterization. The crystal structure and crystal morphology of the pristine lithium metal oxide and lithium metal oxide−RGO nanocomposites were examined with powder X-ray diffraction, or XRD, and field emission-scanning electron microscopy, or FE-SEM, analyses, respectively. The elemental distribution and chemical composition of the nanocomposites were determined with energy dispersive spectrometry, or EDS, elemental mapping, and inductively coupled spectrometry, or ICP, analyses, respectively. Thermogravimetric analysis, or TGA, was performed in ambient atmosphere to estimate the thermal behavior and RGO content of the obtained nanocomposites. The crystal shape and local atomic arrangement of the nanocomposites were examined with high resolution-transmission electron microscopy, or HRTEM, and selected area electron diffraction, or SAED, analyses, respectively. Micro-Raman and Fourier transformed-infrared, or FT-IR, spectroscopic analyses were carried out to study the chemical bonding nature of RGO and lithium titanate. The micro-Raman spectra were collected with a JY LabRam HR spectrometer using an Ar laser with a wavelength of 514.5 nm as the excitation source. The XPS data were recorded with a PHI 5100 Perkin-Elmer spectrometer. The XPS spectrometer that was used adopted a twin source of X-ray beams, leading to the wide spreading of the X-ray beam and the minimization of the charging effect. In addition, all of the present XPS data are collected from a thin layer of the sample loaded on highly conductive copper foil, which suppressed the accumulation of charge during the measurement. Furthermore, any possible shift of the XPS peak caused by the charging effect was calibrated by referencing it to the adventitious C 1s peak at 284.8 eV to rule out the spectral modification by the charging effect. Ti K-edge X-ray absorption spectroscopic, or XAS, analyses were carried out with the extended X-ray absorption fine structure, or EXAFS, facility installed at beamline 7C at the Pohang Accelerator Laboratory, or PAL, in Korea. The XAS experiments were conducted at room temperature in transmission mode using gas-ionization detectors. The energy of the Ti K-edge spectra was referenced by measuring the spectrum of Ti metal foil simultaneously. The data analysis for the experimental spectra was performed by the standard procedure reported previously.23 The diffuse reflectance UV−vis spectra of the present composite materials were obtained on a SINCO 4100 spectrometer equipped with an integrating sphere, using BaSO4 as a reference. The PL spectra of the pristine and lithiated lithium titanates and their nanocomposites with RGO were measured with a Perkin-Elmer LS55 fluorescence spectrometer. Electrochemical Measurements. The electrode functionality of the lithium metal oxide−RGO nanocomposites and the pristine lithium metal oxide was investigated by galvanostatic

problem, it is necessary to develop an effective way to immobilize presintered metal oxide particles on the surface of the RGO nanosheets. Taking into account the presence of many hydroxyl groups on the surface of graphite oxide, or GO,15 the solvothermal treatment of a mixture of GO and the metal oxide can lead to the formation of covalent metal− oxygen bonds between these two materials and also to the reduction of GO, resulting in the synthesis of strongly coupled nanocomposites consisting of RGO and the metal oxide. To date, there have been many reports about the synthesis of lithium metal oxide−graphene nanocomposites like the Li4Ti5O12−graphene nanocomposite via a direct crystal growth of electrode crystal on the surface of graphene nanosheets or a lithiation process of metal oxide−graphene nanocomposite.16−19 However, we are aware of no reports on the direct immobilization of presynthesized lithium metal oxide microcrystals on the surface of RGO 2D nanosheets. Besides, systematic investigation has been carried out neither for the electronic coupling between graphene and composited lithium titanium oxide nor for the effect of RGO coupling on the photocatalytic activity of semiconducting lithium titanate. In the present study, a simple and facile solvothermal route using the mixture suspension of GO nanosheets and lithium metal oxide particle is developed to synthesize intimately coupled nanocomposites of Li4Ti5O12−RGO and LiMn2O4− RGO with electrode and photocatalyst functions. The effects of the composite formation on the crystal structure and chemical bonding nature of the spinel lithium metal oxide and RGO nanosheets are investigated with diffraction and spectroscopic tools. In particular, the formation of chemical bonds between lithium metal oxide and RGO is probed with micro-Raman and X-ray photoelectron spectroscopy, or XPS. To probe the electronic coupling and charge transfer between lithium metal oxide and RGO, the effects of RGO coupling and Li+ insertion on the electronic structure of the lithium metal oxide are investigated with diffuse reflectance UV−vis spectroscopy, photoluminescence, or PL spectroscopy. Charge−discharge cycling tests are carried out for the present nanocomposites in order to estimate the effect of RGO incorporation on the electrochemical activity of the lithium metal oxide materials. In addition to the electrode performance, the evolution of the photocatalytic activity of semiconducting lithium titanate upon the coupling with RGO nanosheets is also examined by measuring photocurrents generated under UV- and visibleilluminations.

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EXPERIMENTAL SECTION Preparation. The pristine Li4Ti5O12 sample was prepared by heating a stoichiometric mixture of Li2CO3 and TiO2 at 850 °C under an ambient atmosphere for 12 h with intermittent grinding.20 Another precursor, that is, an aqueous colloidal suspension of GO nanosheets, was prepared by the acidtreatment of graphite under reflux conditions.21 The nanocomposites of Li4Ti5O12−RGO were synthesized by the solvothermal treatment at 120 °C of a water/ethanol (volume ratio of 2:1)-based colloidal suspension of GO nanosheets containing powdery lithium titanate under vigorous stirring. After allowing the reaction to proceed for 3 h, the powdery precipitates were washed thoroughly with distilled water and then dried. The resulting supernatant solution was almost transparent with only weak turbidity, indicating that most of the RGO nanosheets were incorporated into the precipitate of nanocomposite. To probe the effect of the RGO content on the 7270

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condition is exactly the same as the synthesis condition of the Li4Ti5O12−RGO nanocomposites (the obtained material is hereafter referred to as the solvothermally treated Li4Ti5O12). As plotted in Figure 1, the solvothermally treated Li4Ti5O12 exhibits identical XRD peaks to those of the as-prepared Li4Ti5O12, indicating the negligible influence of the solvothermal treatment on the spinel structure of the pristine Li4Ti5O12. As the amount of RGO incorporated increases, the diffraction peaks corresponding to the Li4Ti5O12 phase reproducibly become stronger, suggesting the enhancement of the crystal order of lithium titanate upon hybridization with RGO. The enhancement of XRD peaks is much more prominent after the immobilization on the RGO nanosheets than after the solvothermal treatment without GO nanosheets. Thus, the observed peak enhancement is attributable to the alignment of Li4Ti5O12 microcrystals with similar orientation on the surface of RGO nanosheets, leading to the reinforcement of X-ray diffraction. FE-SEM/Elemental Mapping and HR-TEM Analyses. The FE-SEM images of the Li4Ti5O12−RGO nanocomposites and the solvothermally treated Li4Ti5O12 are illustrated in Figure 2, as compared to that of the pristine Li4Ti5O12. An irregular polyhedral crystal shape appears for the pristine Li4Ti5O12 with an average size of several hundred nanometers. A very similar morphology is observable for the solvothermally treated Li4Ti5O12, indicating that there is no significant change of the crystal shape upon the solvothermal treatment without the RGO nanosheets. All of the present Li4Ti5O12−RGO nanocomposites display nearly identical morphologies, showing microcrystalline Li4Ti5O12 particles immobilized on the flat surface of the RGO nanosheets. This result provides strong evidence for the intimate coupling of the Li4Ti5O12 microcrystals with the RGO nanosheets. As the mixing ratio of RGO/Li4Ti5O12 increases, the average number of the Li4Ti5O12 crystals per each RGO nanosheet becomes smaller. Thus, in the case of the Li4Ti5O12−RGO nanocomposite with a higher concentration of RGO, smaller numbers of lithium titanate crystals are attached to each RGO nanosheet. The stabilization of the Li4Ti5O12 microcrystals on the surface of the RGO 2D nanosheets is cross-confirmed by the HR-TEM analysis. As presented in Figure 3, polyhedral Li4Ti5O12 particles are immobilized on the flat surface of the RGO nanosheets. The Li4Ti5O12 particles attached to the RGO nanosheets show a typical SAED pattern with a cubic spinel structure, underscoring the maintenance of the original spinel structure after the composite formation. The spatial distribution of the lithium titanate and RGO in the present nanocomposites is probed with EDS−elemental mapping analysis. As illustrated in Figure 4, all of the Li4Ti5O12−RGO nanocomposites show distinct EDS signals corresponding to titanium, oxygen, and carbon elements, clearly demonstrating the composite formation between lithium titanium oxide and RGO species. All of the titanium, oxygen, and carbon elements are homogeneously distributed throughout the sample, confirming the homogeneous hybridization between lithium titanate and the RGO nanosheets. Elemental Analysis, TGA, and N2 Adsorption−Desorption Isotherm Analysis. The cationic composition of the Li4Ti5O12−RGO nanocomposites is determined with ICP analysis. The Li/Ti ratio of the present nanocomposites is evaluated as ∼0.84−0.89 for all of the present nanocomposites, which is similar to the experimentally determined Li/Ti ratio of the pristine Li4Ti5O12 microcrystals (∼0.85). This result clearly

charge−discharge cycling. The electrochemical cycling tests were carried out with a cell consisting of Li/1 M LiPF6 in EC:DEC (50:50 v/v)/active material, which was assembled in a drybox. The composite cathode was prepared by thoroughly mixing the active material (80%) with Super P (10%) and PVDF (10%). FE-SEM analysis of the obtained composite cathode clearly demonstrated that there was no significant destruction of the nanocomposite lattice after mixing them with a Super P conductor and PVDF binder, indicating their high mechanical stability (see the Supporting Information). All of the experiments were carried out in a galvanostatic mode with a WonA Tech multichannel galvanostat/potentiostat in the voltage range 1.0−2.5 V with several current densities. Photocurrent Measurements. The photocurrents were collected as a function of the irradiation time on a Pt plate (2 × 2 cm2) working electrode immersed in an aqueous suspension of Li4Ti5O12−RGO nanocomposite powders with methyl viologen (MV2+, 0.5 mM in 0.95 N NaOH) as an electron shuttle, as described elsewhere.24 A saturated calomel electrode, or SCE, was adopted as the reference electrode and a graphite rod was used as the counter electrode. The working electrode was held at +0.4 V vs SCE for the measurement of the MV2+mediated photocurrent.

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RESULTS AND DISCUSSION Powder XRD Analysis. Figure 1 represents the powder XRD patterns of the Li4Ti5O12−RGO nanocomposites, as

Figure 1. Powder XRD patterns of the Li4Ti5O12−RGO nanocomposites of (a) GLT1, (b) GLT2, (c) GLT3, and (d) GLT4, (e) the as-prepared Li4Ti5O12, and (f) the solvothermally treated Li4Ti5O12.

compared to that of the pristine Li4Ti5O12 microcrystal. The microcrystalline Li4Ti5O12 material shows well-developed XRD patterns of a cubic spinel structure with a lattice parameter a of ∼8.36 Å without any impurity peaks, indicating the formation of a single phase material. After the composite formation with RGO, all of the Bragg reflections of the Li4Ti5O12 phase remain intact and no impurity peaks appear, confirming the maintenance of the spinel lithium titanate lattice. To probe the possible influence of the solvothermal treatment (without the GO nanosheets) on the physicochemical properties of the as-prepared lithium titanate, the precursor Li4Ti5O12 particles were reacted with a water/ethanol (volume ratio of 2:1)-based solution at 120 °C for 3 h under solvothermal conditions. Except for the absence of the GO nanosheets, this reaction 7271

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Figure 2. FE-SEM images of (a) the as-prepared Li4Ti5O12, (b) the solvothermally treated Li4Ti5O12, and the Li4Ti5O12−RGO nanocomposites of (c) GLT1, (d) GLT2, (e) GLT3, and (f) GLT4.

Figure 3. HR-TEM image and SAED pattern of the Li4Ti5O12−RGO nanocomposite.

material. According to the fitting analysis with the Brunauer− Emmett−Teller, or BET, equation, the present nanocomposite possesses a very small surface area of less than 5 m2 g−1. This result sharply contrasts with the much larger surface area of other nanocomposite materials consisting of RGO nanosheets and nanosized metal oxide particles.17,26,27 The large surface area of previously reported materials is attributable to the formation of porous stacking structure between nanosheets and nanoparticles. Conversely, the large particle size of Li4Ti5O12 microcrystals used in this study prevents the formation of a porous stacking structure with the RGO nanosheets. Instead of the formation of porous stacking structure, the RGO nanosheets tend to wrap the surface of larger metal oxide microcrystals, leading to the nonporous structure with small surface area.28 Pore size analysis based on the MP and BJH, or Barrett−Joyner−Halender, methods confirms the negligible formation of micropores and mespores in the present nanocomposite material (see the Supporting Information). Micro-Raman and FT-IR Spectroscopy. The lattice vibration and chemical bonding character of the Li4Ti5O12− RGO nanocomposites were studied with micro-Raman and FTIR spectroscopy. The micro-Raman spectra of the Li4Ti5O12− RGO nanocomposites are plotted in the left panel of Figure 6,

demonstrates the maintenance of the Li4Ti5O12 phases after their hybridization with the RGO nanosheets under solvothermal conditions. The contents of carbon and water in the nanocomposites were examined by TGA. Figure 5 plots the TGA curves of the Li4Ti5O12−RGO nanocomposites. All of the present nanocomposites display only a negligible mass loss of 300 nm), which is much greater than that generated by the pristine Li4Ti5O12 under the same condition. The observed significant enhancement of the UV-induced photocurrent upon the composite formation with RGO provides straightforward evidence for the merit of RGO coupling in enhancing the photocatalytic activity of semi-

very promising as an electrode material. The solvothermalassisted coupling with the RGO colloidal nanosheets is highly effective in enhancing the electrode performance of inorganic solids especially under the conditions of a high current density. Diffuse Reflectance UV−vis, PL, and Photocurrent Measurement. The evolutions of the electronic structure and optical properties of Li4Ti5O12 upon solvothermal treatment, Li+ insertion, and composite formation with the RGO nanosheets were studied with diffuse reflectance UV−vis spectroscopy. To probe the effect of Li+ insertion on the electronic structures of Li4Ti5O12 and the Li4Ti5O12−RGO nanocomposites, the lithiated derivatives of these materials were also prepared by n-BuLi treatment for 5 days at room temperature. The formation of lithiated Li4+xTi5O12 materials with x = ∼4.1 by the reaction with n-BuLi was confirmed for the pristine Li4Ti5O12 and the Li4Ti5O12−RGO nanocomposites by ICP analysis. As plotted in the left panel of Figure 12, the as-prepared Li4Ti5O12 displays a distinct absorption edge at around 3.5 eV and a weak edge at around 1.7 eV, indicating its semiconducting nature. A similar spectral feature was previously reported for nanocrystalline Li4Ti5O12 material.46 The observed UV−vis result is well-consistent with the theoretical calculation of the band structure of lithium titanate.33 A similar UV−vis spectrum can be observed without any significant change of the bandgap energy for the solvothermally treated Li4Ti5O12 material, confirming the negligible influence of the solvothermal treatment on the semiconducting properties of the pristine lithium titanate. The lithiated derivative of Li4Ti5O12 exhibits no clear absorption edge with a strong enhancement of light absorption, strongly suggesting the metallization of the semiconducting nature upon Li+ insertion. This is in good agreement with theoretical calculation on this phase showing the shift of Fermi level upon lithiation and the resulting removal of bandgap separation.34 Similarly, upon the coupling of the RGO nanosheets, an absorption in the visible region becomes stronger and the absorption edge becomes much less prominent, verifying the electronic coupling with metallic RGO nanosheets. The lithiated derivative of the Li4Ti5O12−RGO nanocomposite also shows a strong absorption of visible light, indicating the high electrical conductivity of this material. The present UV−vis results clearly demonstrate that the Li4Ti5O12− RGO nanocomposite retains a metallic nature before and after the insertion of Li+ ions, which contrasts with the case of uncomposited Li4Ti5O12, showing the severe change of electronic property from metallic to semiconducting depending on the content of Li+ ions. The maintenance of the metallic 7277

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spectroscopic analyses clearly demonstrated that the Li4Ti5O12−RGO nanocomposite exhibits metallic nature before and after the insertion of Li+ ions via the strong electrical connection with the metallic RGO nanosheet. The enhancement of the electrical conductivity is mainly responsible for the observed improvement of the electrode and photocatalyst performance upon the composite formation with the RGO nanosheets. In addition, the application of the present solvothermal method for another couple of lithium manganate and RGO leads to the synthesis of LiMn2O4−RGO nanocomposite showing much superior electrode performance over the pristine LiMn2O4. The experimental findings presented here clearly demonstrate that solvothermal treatment with GO is very effective in synthesizing RGO-based nanocomposites containing highly crystalline bulk electrode materials and also in improving the functionalities of lithium metal oxides as a lithium intercalation electrode and photocatalyst. Taking into account the fact that the present method allows us to hybrid presintered metal oxides with the RGO nanosheets, this method is readily applicable for the coupling of RGO with multicomponent metal oxides that can be synthesized only by the sintering process at elevated temperature, not by the solution-based crystal growth reaction at low temperature. Thus, the current project of our group is the application of the present method to other multicomponent metal oxides with diverse functionalities such as electrocatalytic activity, electrode activity for supercapacitor application, and redox catalytic activity.

conducting lithium metal oxide as well as for a strong electronic coupling between Li4Ti5O12 and RGO. As illustrated in the right panel of Figure 13, the electronic connection between Li4Ti5O12 and highly conductive RGO provides an effective pathway for the photoexcited electrons in the CB of lithium titanate to the RGO, i.e., path 1, leading to the efficient flow of the photocurrent under UV-illumination. The notable enhancement of photocurrent upon the coupling of Li4Ti5O12 with RGO is also observable under visible light irradiation (λ > 420 nm); see the middle panel of Figure 13. Like the case of the photocurrent generation under UV illumination, a transfer of photoexcited electrons in the CB of lithium titanate to the RGO via path 1 is also responsible for the observed increase of visible-induced photocurrent. In addition, taking into account the weak absorptivity of the lithium titanate for visible light (Figure 12), an electron transition from the electronic states of the RGO to the empty CB of the lithium titanate via path 2 can make an additional contribution to the remarkable enhancement of photocurrent under visible irradiation, suggesting the role of RGO as a visible-light-absorbing sensitizer.24 The suggested role of RGO as a sensitizer is further supported by the fact that the enhancement of photocurrent upon the coupling with the RGO is more prominent for the visibleinduced photocurrent than the UV-induced one. The present finding provides strong evidence for the positive effect of RGO coupling on the photocatalytic activity of semiconducting lithium metal oxide like Li4Ti5O12. Synthesis and Characterization of LiMn2O4−RGO Nanocomposite. To confirm the universal applicability of the solvothermal route developed in this study, this synthetic method was also applied for the hybridization between nanocrystalline LiMn2O4 and RGO nanosheet. The obtained LiMn2O4−RGO nanocomposite shows a typical XRD pattern of the spinel-structured LiMn2O4 phase, confirming the maintenance of the spinel lithium manganate lattice during the solvothermal treatment (see the Supporting Information). FE-SEM analysis reveals the stabilization of nanocrystalline LiMn2O4 particles on the surface of RGO nanosheets, confirming the formation of the LiMn2O4−RGO nanocomposite. The resulting nanocomposite was tested as a lithium intercalation electrode, in comparison with the pristine LiMn2O4 (see the Supporting Information). The present LiMn2O4−RGO nanocomposite displays a much larger discharge capacity than does the pristine LiMn2O4. This result confirms the universal applicability of the present solvothermal route for synthesizing the lithium metal oxide−RGO nanocomposite electrode but also for improving the electrode performance of lithium metal oxides.

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ASSOCIATED CONTENT

S Supporting Information *

FE-SEM image of the composite cathode of the Li4Ti5O12− RGO nanocomposite mixed with Super P conductor and PVDF binder, N2 adsorption−desorption isotherm data of the Li4Ti5O12−RGO nanocomposites, and discharge capacity plot as a function of cycle numbers for the solvothermally treated Li4Ti5O12. XRD patterns, FE-SEM images, and discharge capacity plots of the LiMn2O4−RGO nanocomposite. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-2-3277-4370. Fax: +82-2-3277-3419. E-mail: [email protected].

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Author Contributions †

CONCLUSION In the present study, strongly coupled Li4Ti5O12−RGO nanocomposites were synthesized by the solvothermal treatment of a GO colloidal suspension containing a microcrystalline Li4Ti5O12 precursor presynthesized at elevated temperature. The combination of diffraction and microscopic tools provided strong evidence for the immobilization of the Li4Ti5O12 microcrystals on the surface of the RGO nanosheets. Of prime importance is that the micro-Raman and XPS results provide strong evidence for the formation of chemical bonds between RGO and Li4Ti5O12. The obtained Li4Ti5O12−RGO nanocomposites show promising electrode performance and photocatalytic activity, which is superior to those of the pristine Li4Ti5O12 microcrystals. The diffuse reflectance UV−vis and PL

These two authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2010-C1AAA001-2010-0029065) and by the Core Technology of Materials Research and Development Program of the Korea Ministry of Intelligence and Economy (grant No. 10041232). The experiments at PAL were supported in part by MOST and POSTECH. The authors thank to Dr. M. G. Kim (PAL, Korea) for helping to collect the XANES spectra. 7278

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(35) Lutz, H. D.; Muller, B.; Steiner, H. J. J. Solid State Chem. 1991, 90, 54. (36) Kovtyukhova, N. I.; Ollivier, B. R.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Chizhik, A. D. Chem. Mater. 1999, 11, 771. (37) Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Carbon 2006, 44, 3342. (38) Li, J.; Jin, Y.-Li.; Zhang, X.-G.; Yang, H. Solid State Ionics 2007, 178, 1590. (39) Treuil, N.; Labrugère, C.; Menetrier, M.; Portier, J.; Campet, G.; Deshayes, A.; Frison, J. C.; Hwang, S.-J.; Song, S. W.; Choy, J.-H. J. Phys. Chem. B 1999, 103, 2100. (40) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558. (41) Stankovich, S.; Piner, R. D.; Chen, X.; Wu, N.; Nguyen, S. T.; Ruoff, R. S. J. Mater. Chem. 2006, 16, 155. (42) Hess, N. J.; Balmer, M. L.; Bunker, B. C.; Conradson, S. D. J. Solid State Chem. 1997, 129, 206. (43) Hur, S. G.; Park, D. H.; Kim, T. W.; Hwang, S.-J. Appl. Phys. Lett. 2004, 85, 4130. (44) Venkateswarlu, M.; Chena, C. H.; Dob, J. S.; Lin, C. W.; Choud, T. C.; Hwang, B. J. J. Power Sources 2005, 146, 204. (45) Ohzuku, T.; Ueda, A.; Yamamoto, N. J. Electrochem. Soc. 1995, 142, 1431. (46) Raja, M. W.; Mahanty, S.; Kundu, M.; Basu, R. N. J. Alloys Compd. 2009, 468, 258. (47) Gunjakar, J. L.; Kim, T. W.; Kim, H. N.; Kim, I. Y.; Hwang, S.-J. J. Am. Chem. Soc. 2011, 133, 14998. (48) Kim, H. N.; Kim, T. W.; Choi, K. H.; Kim, I. Y.; Kim, Y.-R.; Hwang, S.-J. Chem. Eur. J. 2011, 17, 9626. (49) Kim, H. N.; Kim, T. W.; Kim, I. Y.; Hwang, S.-J. Adv. Funct. Mater. 2011, 21, 3111.

REFERENCES

(1) Li, X.; Zhang, G.; Bai, X.; Sun, X.; Wang, X.; Wang, E.; Dai, H. Nat. Nanotechnol. 2008, 3, 538. (2) StanKovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, R. S.; Ruoff, R. S. Nature 2006, 442, 282. (3) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (4) Qiu, J.-D.; Wang, G.-C.; Liang, R.-P.; Xia, X.-H.; Yu, H.-W. J. Phys. Chem. C 2011, 115, 15639. (5) Wang, D.; Choi, D.; Li, J.; Yang, Z.; Nie, Z.; Kou, R.; Hu, D.; Wang, C.; Saraf, L. V.; Zhang, J.; Aksay, I. A.; Liu, J. ACS Nano 2009, 3, 907. (6) Wu, Z.-S.; Ren, W.; Wen, L.; Gao, L.; Zhao, J.; Chen, Z.; Zhou, G.; Li, F.; Cheng, H.-M. ACS Nano 2010, 4, 3187. (7) Kamat, P. V. J. Phys. Chem. Lett. 2010, 1, 520. (8) Lee, Y. R.; Kim, I. Y.; Kim, T. W.; Lee, J. M.; Hwang, S.-J. Chem. Eur. J. 2012, 18, 2263. (9) Zhang, J.; Jiang, J.; Zhao, X. S. J. Phys. Chem. C 2011, 115, 6448. (10) Ji, L.; Tan, Z.; Kuykendall, T.; Aloni, S.; Xun, S.; Lin, E.; Battagliab, V.; Zhang, Y. Phys. Chem. Chem. Phys. 2011, 13, 7170. (11) Gao, Z.; Wang, J.; Li, Z.; Yang, W.; Wang, B.; Hou, M.; He, Y.; Liu, Q.; Mann, T.; Yang, P.; Zhang, M.; Liu, L. Chem. Mater. 2011, 23, 3509. (12) Chen, S.; Zhu, J.; Wang, X. J. Phys. Chem. C 2010, 114, 11829. (13) Chen, S.; Zhu, J.; Wu, X.; Han, Q.; Wang, X. ACS Nano 2010, 4, 2822. (14) Zhang, Y.; Li, H.; Pan, L.; Lu, T.; Sun, Z. J. Electroanal. Chem. 2009, 634, 68. (15) Park, S.; Lee, K.-S.; Bozoklu, G.; Cai, W.; Nguyen, S. T.; Ruoff, R. S. ACS Nano 2008, 2, 572. (16) Shen, L.; Yuan, C.; Luo, H.; Zhang, X.; Yang, S.; Lu, X. Nanoscale 2011, 3, 572. (17) Shen, L.; Yuan, C.; Luo., H.; Zhang, X.; Chen, L.; Li, H. J. Mater. Chem. 2011, 21, 14414. (18) Kim, H.-K.; Bak, S.-M.; Kim, K.-B. Electrochem. Commun. 2010, 12, 1768. (19) Xiang, H.; Tian, B.; Lian, P.; Li, Z.; Wang, H. J. Alloys Compd. 2011, 509, 7205. (20) Wang, G. J.; Gao, J.; Fua, L. J.; Zhao, N. H.; Wu, Y. P.; Takamura, T. J. Power Sources 2007, 174, 1109. (21) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (22) Liddle, B. J.; Collins, S. M.; Bartlett, B. M. Energy Environ. Sci. 2010, 3, 1339. (23) Lee, S. H.; Kim, T. W.; Park, D. H.; Choy, J.-H.; Hwang, S.-J.; Jiang, N.; Park, S. E.; Lee, Y. H. Chem. Mater. 2007, 19, 5010. (24) Kim, I. Y.; Lee, J. M.; Kim, T. W.; Kim, H. N.; Kim, H. I.; Choi, W.; Hwang, S.-J. Small [Online early access]. DOI: 10.1002/ smll.201101703. Published Online: Feb 10, 2012. (25) Yi, T.-F.; Xie, Y.; Shu, J.; Wang, Z.; Yue, C.-B.; Zhu, R.-S.; Qiao, H.-B. J. Electrochem. Soc. 2011, 158, A266. (26) Li, N.; Liu, G.; Zhen, C.; Li, F.; Zhang, L.; Cheng, H.-M. Adv. Funct. Mater. 2011, 21, 1717. (27) Lambert, T. N.; Chavez, C. A.; Hernandez-Sanchez, B.; Lu, P.; Bell, N. S.; Ambrosini., A.; Friedman, T.; Boyle, T. J.; Wheeler, D. R.; Huber, D. L. J. Phys. Chem. C 2009, 113, 19812. (28) Kim, H.; Moon, G.; Monllor-Satoca, D.; Park, Y.; Choi, W. J. Phys. Chem. C 2012, 116, 1535. (29) Wang, G. X.; Shen, X. P.; Yao, J.; Park, J. Carbon 2009, 47, 2049. (30) Casiraghi, C.; Hartschuh, A.; Qian, H.; Piscanec, S.; Georgi, C.; Fasoli, A.; Novoselov, K. S.; Basko, D. M.; Ferrari, A. C. Nano Lett. 2009, 9, 1433. (31) Shi, Y.; Wen, L.; Cheng, H.-M. J. Power Sources 2011, 196, 8610. (32) Cotton, F. Chemical Applications of Group Theory, 2nd ed.; Wiley: New York, 1971; p 127. (33) Liu, D.; Ouyang, C.; Shi, S.; Jiang, J.; Wang, Z.; Chen, L. Phys. Status Solidi B 2006, 8, 1835. (34) Zhong, Z.; Ouyang, C.; Shi, S.; Lei, M. ChemPhysChem 2008, 9, 2104. 7279

dx.doi.org/10.1021/jp301508k | J. Phys. Chem. C 2012, 116, 7269−7279