Preparation of scrolled graphene oxides with multi-walled carbon nanotube templates

Preparation of scrolled graphene oxides with multi-walled carbon nanotube templates

CARBON 4 8 ( 2 0 1 0 ) 4 2 8 3 –4 2 8 8 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon Preparation of scrolle...

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CARBON

4 8 ( 2 0 1 0 ) 4 2 8 3 –4 2 8 8

available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/carbon

Preparation of scrolled graphene oxides with multi-walled carbon nanotube templates Young-Kwan Kim, Dal-Hee Min

*

Department of Chemistry, Institute for the BioCentury, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea

A R T I C L E I N F O

A B S T R A C T

Article history:

A simple and efficient wet-chemical strategy was developed to form graphene oxide scrolls

Received 25 May 2010

around multi-walled carbon nanotube templates through covalent bond formation. Scan-

Accepted 21 July 2010

ning electron microscopy, atomic force microscopy, transmission electron microscopy

Available online 27 July 2010

and dynamic light scattering measurements confirmed the scrolled conformation of graphene oxide sheets while Fourier transform infrared spectroscopy and X-ray photoelectron data indicated the formation of covalent bonds between the scrolled sheets and the nanotubes. Conformational changes of the graphene oxide sheets were also monitored by Raman spectroscopy. All the data suggested successful formation of graphene oxide scrolls with multi-walled carbon nanotube templates.  2010 Elsevier Ltd. All rights reserved.

1.

Introduction

The one-dimensional (1D) structure [1] and unique mechanical, physical, and electrical properties of carbon nanotubes have led to various applications such as reinforcing fillers [2], ultra capacitors [3], field-effect transistors [4], and biosensors [5]. Recently, a two-dimensional (2D) carbon-based nanomaterial, called graphene, has emerged as a novel material with unique thermal, electronic, and physical properties [6]. The scrolled form of graphene, carbon nanoscroll (CNS), is easily distinguishable from graphene based on its electrical and optical properties [7,8] and is recognized as a candidate material for the fabrication of hydrogen storage devices [9], supercapacitors, and batteries [10]. Despite the structural similarities of CNS and multi-walled carbon nanotube (MWCNT), the electronic properties of CNS differ greatly from those of MWCNTs, especially with regard to electron transport pathways. For example, in MWCNT, electric current flows only through the outermost shell. In contrast, current flows through the entire CNS sheet despite the similar interlayer

distances in CNS and MWCNT (0.35 nm). This difference in the conduction pathway as a result of only slight differences in structural conformation produces distinct I–V curves [11]. Therefore, control over the conformation of graphene is an important research goal. To date, many strategies have been developed to control the conformation of graphene, including arc-discharge [12], high-energy ball milling [13], chemical intercalation/exfoliation of graphite [14], and isopropyl alcohol-induced scrolling of graphene on a substrate [11]. Graphene oxide (GO), the oxidized form of graphene, is a convenient material for studying the relationship between structural dimensions (or conformations) and physical properties because it is soluble in water, can be easily converted into its graphene analog by chemical reduction, and is easy to covalently conjugate by reaction with functional groups at its basal plane and edges. Composites of GO with other materials have been engineered and characterized for a variety of applications [15,16]. The present study describes a simple and efficient wet-chemical process for GO scroll formation via covalent bond formation between GO and MWCNT (Fig. 1)

* Corresponding author: Fax: +82 42 350 2810. E-mail address: [email protected] (D.-H. Min). 0008-6223/$ - see front matter  2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.07.039

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Fig. 1 – A scheme showing GO scroll formation around a MWCNT template by covalent conjugation.

Scrolled GO (SGO) composites made in this way may possess characteristics of both MWCNT and CNS and certain physical phenomena may be enhanced by composite formation.

2.

Experimental

2.1.

Materials

Natural graphite (FP 99.95% pure) was purchased from Graphit Kropfmu¨hl AG (Hauzenberg, Germany). MWCNTs, produced by chemical vapor deposition (95% pure, length 5–20 lm, outer diameter 15 ± 5 nm), were purchased from NANOLAB (Massachusetts, USA). Hydrogen peroxide (30% in water) and sodium nitrate were purchased from Junsei (Japan). Ethylenediamine, potassium permanganate, 3-aminopropyltriethoxysilane, and anhydrous dimethylformamide (DMF) were purchased from Sigma–Aldrich (St. Louis, MO, USA). Nitric acid and sulfuric acid were purchased from Samchun (Seoul, Korea). Thionyl chloride was purchased from Tokyo Chemical Industry (Tokyo, Japan). Ethanol was purchased from Merck (Darmstadt, Germany). 500-nm SiO2/P++ Si substrates (500 lm in thickness) and were purchased from STC (Japan). All chemicals were used as received.

2.1.1.

Preparation of the GO suspension

A suspension of GO was prepared from natural graphite using Hummer’s method [17]. Natural graphite (1.5 g) and NaNO3 (0.5 g) were stirred into 23 mL of H2SO4 in an ice bath. Next, 3 g of KMnO4 was slowly stirred into the mixture while maintaining the temperature below 20 C. The temperature of the reaction mixture was then raised to 35 C and the mixture was stirred for an hour. Distilled water (40 mL) was added to the mixture, which was then stirred for 30 min and diluted with an additional 100 mL of water in an ice bath (WARNING: The temperature of this reaction mixture rapidly increased to 95 C). In a final step, 3 mL of H2O2 (30%) was added drop-wise. The mixture was filtered and washed with copious amounts of water until the filtrate was neutral. The filter cake was dried under vacuum for 48 h at room temperature.

2.1.2.

Preparation of carboxylated MWCNT suspensions

MWCNT (60 mg) were suspended in 40 mL of a mixture of HNO3 and H2SO4 (1:3), sonicated for 10 h at 60 C, and filtered through an anodic alumina membrane (200 nm pore size). The filtered cake was thoroughly washed with water until the filtrate was neutral and the cake was dispersed in water by sonication to yield a 50 lg/mL suspension of carboxylated MWCNT.

2.1.3.

Preparation of aminated MWCNT suspensions

Fully aminated MWCNT were prepared by amination of carboxylated MWCNT through amide bond formation [18]. A 30 mL aliquot of the above carboxylated MWCNT suspension was filtered through an anodic alumina membrane, washed with 40 mL of anhydrous DMF, and dispersed by sonication in 20 mL of thionyl chloride containing 1 mL of anhydrous DMF. The suspension was refluxed for 24 h at 70 C to convert the carboxylic acid groups on the surface of MWCNT to acid chlorides. After removal of excess chemicals by rotary evaporation, the acid chloride-activated MWCNT suspension was dispersed in 20 mL of ethylenediamine and refluxed at 125 C for 5 days with stirring. Finally, the reaction mixture was filtered through an anodic alumina membrane, washed with ethanol and water, and dispersed in water to a concentration of 50 lg/mL by sonication.

2.1.4.

Scrolling of GO sheets around aminated MWCNT

GO (10 mg) was suspended in 20 mL of thionyl chloride containing 1 mL of anhydrous DMF and sonicated for 30 min. The suspension was refluxed and stirred for 24 h at 70 C to convert the carboxylic acid groups at the edges of the GO sheets to acid chlorides. After removal of excess chemicals in the reaction mixture by rotary evaporation, the acid chloride-activated GO was dispersed in 20 mL of aminated MWCNT suspended in anhydrous DMF (25 lg/mL) by sonication for 30 min. The mixture was refluxed at 120 C for 5 days in order to form covalent linkages between the GO and the aminated MWCNT. Finally, the reaction mixture was filtered through an anodic alumina membrane (pore size 200 nm), washed with ethanol and water, and dispersed in 10 mL of water by sonication. This suspension was stable for several weeks.

2.2. Characterization nanomaterials

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Elemental analysis of the GO was carried out by EA1110-FISONS (ThermoQuest Italia S.P.A., Italia). Atomic force microscopy (AFM) was performed with an XE-100 (Park system, Korea) with a backside gold-coated silicon SPM probe (M to N, Korea). The physical structures of the SGO were observed using field emission scanning electron microscopy (FE-SEM) (S-4800, Hitachi, Japan) and high-resolution transmission electron microscopy (HR-TEM) (Tecnai G2 F30, FEI Company, The Netherlands). The hydrodynamic radii of these nanomaterials were determined by a Plus90 particle size analyzer (Brookhaven). Raman spectra of the GO, MWCNTs, and SGO

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were recorded on a LabRAM HR UV/vis/NIR (HORIBA Jobin Yvon, France) using a CW Ar-ion laser (514.5 nm) as an excitation source focused through a confocal microscope (BXFM, Olympus, Japan) equipped with an objective lens (50·, numerical aperture = 0.50). Fourier transform infrared (FT-IR) absorption measurements of the graphite and GO were recorded with an EQUINOX55 (Bruker, Germany) in KBr pellets. High-resolution X-ray photoelectron spectroscopy (XPS) was performed on an ESCA 2000 (Thermo VG Science, USA) with a twin X-ray source (Mg/Al target).

3.

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Results and discussion

The GO was prepared by Hummer’s method (C/O ratio: 0.95, determined by elemental analysis). The GO and MWCNT were then chemically modified to tailor acyl chlorides and primary amines, respectively (Fig. S1 in the Supplementary material for characterization of acyl chlorinated GO). Scrolling of GO sheet along the surface of MWCNT was achieved by refluxing a reaction mixture of the prepared GO and MWCNT derivatives. AFM studies revealed the successful exfoliation of GO and formation of SGO on the aminated MWCNT surfaces. The dimension and thickness of exfoliated GO sheets were about a few tens of nm to several lm and 0.97 nm, implying the presence of oxygen functional groups on their basal planes (Fig. 2a). The line profiles of the SGO showed approximately 27, 40, and 50 nm in diameter (Fig. 2c and d), which are significantly greater than that of MWCNT (approximately 18.66 nm) (Fig. 2b). The different diameter of SGO formed on MWCNT surface can be attributed to the broad size distribution of GO sheets.

The detailed conformational changes in the SGO were observed with SEM and HR-TEM. The SGO samples for SEM analysis were prepared by immersing an aminated SiO2/Si substrates in a suspension of SGO/MWCNT hybrid nanomaterials for 1 h, washing with water and ethanol and dried under a stream of nitrogen. This process resulted in the immobilization of the SGO through electrostatic interactions between the SGO and the amine-functionalized surface [19], thereby prohibiting SGO formation as a result of solvent evaporation. Many tubular structures of the SGO were observed in SEM images, indicating successful scrolling reaction (Fig. 3a–c). The lengths and diameters of the SGOs ranged from 800 to 1200 nm and from 50 to 120 nm, respectively, which are much larger than the corresponding dimensions of the MWCNT. As a control, graphene oxide without treatment of thionyl chloride was dispersed in an aqueous suspension of aminated MWCNT and carboxylated MWCNT. No tubular structure was observed in the controls, confirming that covalent conjugation between GO and MWCNT is of importance to induce scrolling of GO (Fig. S2 in the Supplementary material). HR-TEM images show that MWCNTs are wrapped with single GO sheet with approximately 1 nm interlayer distance (Fig. 3d–f). The shell thickness of SGO around MWCNT is closely related to the lateral sheet size distribution of GO sheet, which results in the increased shell thickness of SGO composed of a multi-layered GO sheet with scrolling of large GO sheet on MWCNT (Fig. S3 in the Supplementary material). This interlayer distance concurs with a previous study in which interlayer distance between graphite oxide sheets intercalated with diamino alkanes was revealed as 0.8–1 nm (H2N(CH2)nNH2, n: 4–8) [20], indicating that GO sheets were

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Fig. 2 – AFM images and line profiles of GO sheets (a), MWCNT (b) and SGO (c and d). The thickness of GO sheet and diameter of MWCNT were 1.27 and 18.66 nm, respectively. After scrolling of GO sheet on MWCNT surface, the diameter of MWCNT was significantly increased up to 50 nm.

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Fig. 3 – FE-SEM (a–c), HR-TEM (d–f) images of SGO with different magnifications show that tubular SGO structures formed on MWCNT surface. Interlayer space was observed with 1.0 nm distance between SGO and MWCNT template (f).

scrolled on the surface of MWCNT via covalent conjugation using ethylene diamine as a molecular bridge. As a control, MWCNT presenting carboxylic acids exclusively at the ends (not on sides) were prepared by refluxing MWCNT in nitric acid for 24 h [21]. The MWCNT were then converted into acid chlorides with thionyl chloride and aminated by the same procedure described above for GO. In this case, conjugation of GO to MWCNT was achieved between the ends of the MWCNT and the GO sheets but no tubular scroll conformation was observed (Fig. S4 in the Supplementary material). These results indicate that covalent bond formation and the location of the covalent linkage is important for scroll formation. Dynamic light scattering (DLS) measurements confirmed that the conformational changes of the GO sheets in SGO/ MWCNT hybrid material. The hydrodynamic radius of the GO was approximately 1200 nm (Fig. 4c). After reacting with aminated MWCNT, the hydrodynamic radius decreased to approximately 470 nm (Fig. 4d). This drastic change can be attributed to GO scroll formation around the aminated MWCNT surface. In contrast, the hydrodynamic radius of the GO and aminated MWCNT in a simple mixture was approximately 3000 nm (Fig. 4e) due to electrostatic force-induced aggregation. A mixture of GO and carboxylated MWCNT yielded two distinct DLS distributions centered about 145 and 795 nm (Fig. 4f). The carboxylated MWCNT and GO remained separate due to electrostatic repulsions. Thus, the scrolling of GO around aminated MWCNT occurs only in solution and is induced by covalent bond formation. Chemical changes in the GO upon scrolling were observed with Raman spectroscopy and XPS (Fig. 5a and Fig. S5 in the

Supplementary material). The Raman spectrum of the GO displayed a strong D band at 1351 cm 1 and a G band at 1591 cm 1, shifted from 1566 cm 1 in pristine graphite. These bands indicate substantial defect sites formed during the oxidation and exfoliation processes [19,22]. Aminated MWCNT also showed strong D band at 1345 cm 1 due to the presence of defect sites formed during covalent amine functionalization and G band at 1575 cm 1. After scroll formation around aminated MWCNT, the G band of the GO shifted to 1575 cm 1, i.e., back towards the pristine state, as a result of combination of overlapping with MWCNT signal and partial restoration of the graphitic structure induced by refluxing in anhydrous media [23,24]. Partial restoration of the graphitic structure was also confirmed by XPS analysis (Fig. S5 in the Supplementary material) and achieved by incubating the GO in a reaction mixture containing organic amines (Fig. S6a in the Supplementary material) [25]. Although the increased D/ G intensity ratio of GO from 0.89 to 1.14 after the scroll formation could be attributed to overlapping with signal from aminated MWCNT, it may be also due to covalent bond formation between the GO and aminated MWCNT which could generate a considerable number of defect sites in the GO structure [24]. XPS analysis of the SGO also confirmed the formation of covalent bonds in the SGO/MWCNT hybrid structure (Fig. S5 in the Supplementary material). The FT-IR spectrum of the GO in Fig. 5b shows several characteristic peaks corresponding to O–H vibrations at 3415 cm 1, [email protected] stretching at 1716 cm 1, [email protected] skeletal vibrations of the oxidized graphitic domain at 1627 cm 1, O–H deformation at 1400 cm 1, and C–O stretching at 1079 cm 1. After conversion to SGO, the peaks at 1716 and 1079 cm 1 dis-

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appeared and a peak at 1661 cm 1, corresponding to a [email protected] stretch, appeared. This frequency is lower than would be expected for an amide bond. In addition, a new peak arose at 1244 cm 1 from C–N bond stretching, which suggests the formation of covalent bonds between the GO sheets and the aminated MWCNT [26]. Based on this series of analyses, we propose that the formation of SGO from GO proceeds as follows. Firstly, the carbonyl chloride groups at the edge of the GO react with amine groups on the surface of a MWCNT to form amide bonds, which anchor the GO sheets on the surface of MWCNT. Secondly, the anchored GO sheets curve along the surface of the MWCNT due to shear stress from vigorous stirring and continuous perturbation of the surrounding solvent. Finally, the curved GO sheets form scrolls around the surface of the aminated MWCNT. Despite the higher energy of the curved GO conformation [10], scroll formation

decreases the total free energy of GO [27,28] due to attractive interaction between the oxygen containing reactive functional groups on the basal plane of the GO and the amine groups on the surface of the MWCNT. However, further study is necessary to clearly elucidate the detailed scrolling mechanism.

4.

Conclusions

In conclusion, GO sheets were successfully made to adopt a scroll conformation around the surface of aminated MWCNT in solution by covalent bond formation. To the best of our knowledge, the present study is the first example of a wetchemical strategy for GO scroll formation around MWCNT which would allow large-scale production of SGO/MWCNT hybrid materials. We believe that the SGO/MWCNT hybrid nanomaterial is a good addition to existing MWCNT- or GO-

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based hybrid materials and we expect a suite of new properties associated with this novel composite.

Acknowledgments This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korean Government (MEST) (Grant Nos. KRF2008-313-C00538, R01-2008-000-20301-0), by the Nano R&D program of NRF funded by MEST (2008-2004457), and by the National Honor Scientist Program of the Ministry of Education, Science and Technology in Korea.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2010.07.039.

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