Edge-Functionalization of Pyrene as a Miniature Graphene via Friedel-Crafts Acylation Reaction in Poly(Phosphoric Acid)

Abstract

The feasibility of edge-functionalization of graphite was tested via the model reaction between pyrene and 4-(2,4,6-trimethylphenyloxy)benzamide (TMPBA) in poly(phosphoric acid) (PPA)/phosphorous pentoxide (P^sub 2^O^sub 5^) medium. The functionalization was confirmed by various characterization techniques. On the basis of the model study, the reaction condition could be extended to the edge-functionalization of graphite with TMPBA. Preliminary results showed that the resultant TMPBA-grafted graphite (graphite-g-TMPBA) was found to be readily dispersible in N-methyl-2-pyrrolidone (NMP) and can be used as a precursor for edge-functionalized graphene (EFG).

Nanoscale Res Lett (2010) 5:16861691 DOI 10.1007/s11671-010-9697-8

NANO EXPRESS

Edge-Functionalization of Pyrene as a Miniature Graphenevia FriedelCrafts Acylation Reaction in Poly(Phosphoric Acid)

In-Yup Jeon Eun-Kyoung Choi Seo-Yoon Bae

Jong-Beom Baek

Received: 4 June 2010 / Accepted: 1 July 2010 / Published online: 15 July 2010 The Author(s) 2010. This article is published with open access at Springerlink.com

Abstract The feasibility of edge-functionalization of graphite was tested via the model reaction between pyrene and 4-(2,4,6-trimethylphenyloxy)benzamide (TMPBA) in poly(phosphoric acid) (PPA)/phosphorous pentoxide (P2O5)

medium. The functionalization was conrmed by various characterization techniques. On the basis of the model study, the reaction condition could be extended to the edge-functionalization of graphite with TMPBA. Preliminary results showed that the resultant TMPBA-grafted graphite (graphite-g-TMPBA) was found to be readily dispersible in N-methyl-2-pyrrolidone (NMP) and can be used as a precursor for edge-functionalized graphene (EFG).

Keywords Pyrene Graphite Graphene

Edge-functionalization

Introduction

Graphene, a single layer of carbon atom bonded together in a hexagonal lattice, has attracted tremendous attention due to its peculiar electronic and physical properties [16]. However, there are two issues that have to be resolved rst for its use in practice. The one is scalable exfoliation of graphite into graphene and/or graphene-like sheets (less than ten layers) [7]. The other is stabilization of exfoliated graphene suspension in various matrices [8]. Graphite oxide (GO), which is oxidized form of graphite containing oxygenated functional groups on its edge and basal plane,

has been considered the most viable chemical approach for the mass production of graphene [9]. However, GO has inherent problem in reversing to graphene structure, because the reduction conversion from GO into reduced graphene oxide (rGO) is limited to *70%, implying that rGO still contains *30% of oxygenated defects [10]. Thus, an important remaining challenge is still the development of new chemical method to produce large quantity and high quality graphene in large quantities. We believe that one promising chemical approach is the edge-functionalized graphite (EFG) via FriedelCrafts acylation reaction. Unlike GO, the EFG is exclusively functionalized at the edge, where sp2CH is located [11]. As a result, the interior graphene crystalline structure is undamaged and its characteristic properties are preserved. In addition, the EFG is expected to be efciently dispersed and stabilized in common organic solvents to give graphene-like sheets.

Herein, we would like to report the edge-chemistry of graphene via the model reaction between pyrene as a miniature graphene and 4-(2,4,6-trimethylphenyloxy)benzamide (TMPBA) as a molecular wedge. The reaction condition, poly(phosphoric acid) (PPA)/phosphorous pentoxide (P2O5)

medium at 130 C, was previously optimized for the direct functionalization of carbon-based nanomaterials such as carbon nanotubes and carbon nanobers [1220]. The result from the model reaction could give an insight for predicting edge-chemistry of graphene.

Experimental Section

Materials

All reagents and solvents were purchased from Aldrich Chemical Inc. and used as received, unless otherwise mentioned.

I.-Y. Jeon E.-K. Choi S.-Y. Bae J.-B. Baek (&)

Interdisciplinary School of Green Energy, Institute of Advanced Materials & Devices, Ulsan National Institute of Science and Technology (UNIST), 100, Banyeon, Ulsan 689-798, South Koreae-mail: [email protected]

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4-(2,4,6-Trimethylphenyloxy)benzamide (TMPBA) was synthesized by literature procedure [21]. Graphite (Cat#: 496596, type: powder, particle size: \45 lm, purity:99.99?%) was obtained from Aldrich Chemical Inc. and used as received.

Instrumentation

Infrared (FT-IR) and FT-Raman spectra were recorded on a Bruker Fourier transform spectrophotometer IFS-66/ FRA106S. The eld emission scanning electron microscopy (FE-SEM) was performed on FEI NanoSem 200. Matrix-assisted laser desorption ionization time of ight (MALDI-TOF) from Bruker Ultraex III was used for mass analysis. 1H and 13C NMR were conducted with Varian VNMRs 600. Elemental analysis (EA) was conducted with Thermo Scientic Flash 2000. X-Ray photo-electron spectroscopy (XPS) was performed on Thermo Fisher K-alpha.

General Procedure for the Functionalization of Pyrene with 4-(2,4,6-Trimethylphenyloxy)Benzamide (TMPBA) in Polyphosphoric Acid (PPA)/Phosphorous Pentoxide (P2O5)

Into a 250-mL resin ask equipped with a high-torque mechanical stirrer, the nitrogen inlet and outlet, pyrene(0.5 g, 2.47 mmol), 4-(2,4,6-trimethylphenyloxy)benzamide (0.5 g, 1.96 mmol), PPA (83% P2O5 assay: 20.0 g) and

P2O5 (5.0 g) were placed and stirred under dry nitrogen purge at 130 C for 72 h. The initial white mixture became pinkish-white as the functionalization reaction progressed. At the end of the reaction, the color of the mixture turned to violet, and the reaction mixture was poured into distilled water. The resultant brown precipitates were collected by suction ltration, Soxhlet-extracted with water for 3 days to completely remove reaction medium and then with methanol for three more days to get rid of unreacted pyrene and TMPBA. Finally, the sample was freeze-dried under reduced pressure (0.5 mmHg) at -120 C for 72 h to give0.74 g (79% yield) of greenish-brown powder. Anal. Calcd. for C48H38O2 (pyrene-g-TMPBA2): C, 84.93%; H,5.64%; O, 9.43%. Found: C, 84.69%; H, 5.25%; O, 7.58%.

Results and Discussion

As presented in Scheme 1a, pyrene and TMPBA were treated in PPA/P2O5 at 130 C for 48 h. Then, the reaction mixture was poured into distilled water to isolate light greenish-brown powder. The reason for using TMPBA is to prevent self-reaction by blocking 2, 4 and 6 positions to the aromatic ether-activated sites for electrophilic substitution reaction. To avoid unexpected variables, the resultant products were completely worked-up by Soxhlet extraction with water for 3 days to remove reaction medium and with methanol for 3 days to get rid of unreacted TMPBA and low molar mass impurities (see Experimental Section).

OH2N O

+ C

H3C

H3C

O

CH3 PPA

P2O5

C

O

H3C

H3C

CH3

(a)

n

Pyrene 2,4,6-TMPBA

Pyrene-g-(TMPBA)n

H3C

H3C

O

O

O

OHO n

P

+

4 H2N

C

P

O

O

CH3

O O

OH O

P

O

n-4 4

+ 4 H3N

C

O

CH3

H3C

O

H3C

H3C

P

OH O

O

n-4 4

O

P

O

O O

P

+

O

P

4 C

O

CH3

+

+ 4 NH3 O

P

O

O

O P

O

O

H3C

O

O

H3C

NH2

O

(b)

O O

OH O

P

+

4 C

O

CH3

+

n 4

P

O

H3C

10 sp2C-H

C

H3C

H3C

O

NH3

O

CH3

+

O O

OH O

P

n 4

n

P

O

Scheme 1 a The reaction between pyrene and TMPBA in poly(phosphoric acid)/phosphorous pentoxide at 130 C; b proposed mechanism of a direct FriedelCrafts acylation reaction between acylium ion (PhC?=O) of TMPBA and sp2CH of pyrene

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The isolated pyrene-g-(TMPBA)n was freeze-dried (-120 C) under reduced pressure (10-2 mmHg). The proposed mechanism of the electrophilic substitution reaction is a direct FriedelCrafts acylation reaction between acylium ion (PhC?=O) of TMPBA and sp2CH of pyrene to give pyrene-g-(TMPBA)n (Scheme 1b).

FT-IR was used as convenient tool to identify chemical bonds in pyrene-g-(TMPBA)n. If there are free standing

TMPBA and pyrene as residual impurities, there must be trace of carbonyl (C = O) stretching peak at 1,642 cm-1

and amide peaks at 3,215 and 3,386 cm-1 arising frombenzamide, and sp2CH peak at 3,044 cm-1 from pyrene(Fig. 1a). However, pyrene-g-(TMPBA)n does not showbenzamide carbonyl and amine peaks, indicating it doesnot contain residual impurities, while it does show relatively much weaker sp2CH and new sp3CH peaks around2,921 cm-1 due mainly to TMPBA and distinct aromaticcarbonyl (C = O) stretching peak at 1,656 cm-1. Hence, itis evident that most of TMPBA is covalently attached tothe edge of pyrene. However, we cannot reliably calculatethe graft density of TMPBA onto pyrene edges. The covalent attachment of TMPBA onto pyrene could be conrmed by matrix-assisted laser desorption ionization time of ight (MALDI-TOF) analysis (Fig. 2). A series of peak groups appeared, indicating that a mixture of pyreneg-(TMPBA)n (n = 2, 3, 4, 5, 6, 7, 8, 9, 10) is present. The peak groups are separated by 238.1 amu, whose value is exact molecular weight of dehydrated [TMPBA]? (FW = 238.23 g/mol). The strongest peak group contains 679.2 amu, which is exactly matched to the molecular weight of pyrene-g-(TMPBA)2. The highest peak at 615.1 amu corresponds to [CH3]4 losses from pyrene-g-(TMPBA)2. Hence, it can be concluded that the highest population in the mixture of pyrene-g-(TMPBA)n is pyrene-g-(TMPBA)2 (n = 2).

From elemental analysis, experimental CHO contents are 84.69, 5.25 and 7.58% for pyrene-g-(TMPBA)n (Table 1). The values are closest to theoretical CHO values with empirical formula weight of C48H38O4, which agreed

well with those of pyrene-g-(TMPBA)2 (n = 2). Hence,

the bisubstitution of TMPBA onto pyrene could be most likely occurred to pyrene via direct FriedelCrafts acylation reaction.

Although the mixture of pyrene-g-(TMPBA)n contains pyrene-g-(TMPBA)2 as major component, it is still a mixture as referenced by MALDI-TOF analysis. The full assignment of all NMR peaks is technically impossible.Nevertheless, the carbonyl bond (C = O) between pyrene and TMPBA could be clearly assignable from both 1H

(Fig. 3a) and 13C-NMR spectra (Fig. 3b). The results further assure the feasibility of the reaction between pyrene and TMPBA.

On the basis of results from model reaction, the covalent attachment of TMPBA on the edge of graphite can be

Wavenumber (cm-1)

1000

635.184

615.098

609.050

636.186

679.184

650.119

665.108

594.024

623.106

679.184

853.216

600 620 640 660 680 700

1091.309

1329.387

Intensity (a.u.)

600 800 1000 1200 1400 1600 1800 2000

m/z

Fig. 2 MALDI-TOF spectra of pyrene-g-(TMPBA)n. Inset is extended from 500 to 700 amu

(a)

Pyrene

Pyrene-g-(TMPBA)n

3044

2921

1596

1642

1656

2,4,6-TMPBA

3386 3215

Transmittance (a.u.)

4000

3500

3000

2500

2000

1500

(b)

Graphite

1634

2918

Graphite-g-TMPBA

1579

2925

1663

Transmittance (a.u.)

4000

3500

3000

2500

2000

1500

Wavenumber (cm-1)

1000

Fig. 1 FT-IR (KBr pellet) spectra: a pyrene, 4-(2,4,6-trimethylphenyloxy)benzamide and pyrene-g-(TMPBA)n; b graphite and graphite-g-TMPBA

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Table 1 Empirical formula (EF), formula weight (FW), calculated and experimental elemental analysis of samples

Sample EF FW Elemental analysis

C (%)

H (%)

O (%)

Table 2 Elemental analysis of graphite and graphite-g-TMPBA

Sample Elemental analysis

C (%) H (%) N (%) O (%)

As-received graphite Calcd. 100.00 0.00 0.00 0.00

Found 98.81 0.13 BDL* BDL*

Graphite-g-TMPBA Calcd. 92.03 2.56 0.00 5.41

Found 90.41 2.50 BDL* 5.71

* BDL below detection limit

Pyrene C16H10 202.25 95.02 4.98 0.00 Pyrene-g-(TMPBA)1 C32H24O2 440.54 87.25 5.49 7.26 Pyrene-g-(TMPBA)2 C48H38O4 678.82 84.93 5.64 9.43 Pyrene-g-(TMPBA)3 C64H52O6 917.11 83.82 5.71 10.47 Pyrene-g-(TMPBA)4 C80H66O8 1155.39 83.16 5.76 11.08 Pyrene-g-(TMPBA)5 C96H80O10 1393.68 82.73 5.79 11.48 Pyrene-g-(TMPBA)6 C112H94O12 1631.97 82.43 5.81 11.76 Pyrene-g-(TMPBA)7 C128H108O14 1870.25 82.20 5.82 11.98 Pyrene-g-(TMPBA)8 C144H122O16 2108.54 82.03 5.83 12.04 Pyrene-g-(TMPBA)9 C160H136O18 2346.82 81.89 5.84 12.27 Pyrene-g-(TMPBA)10 C176H150O20 2585.11 81.77 5.85 12.38 Pyrene-g-(TMPBA)n CxHyOz Found 84.69 5.25 7.58

anticipated. Hence, graphite was also treated with TMPBA in the same reaction and work-up conditions. For the purpose of having a basic understanding of the starting material, pristine graphite was characterized by elemental analysis (Table 2). When theoretical C H N O contents were calculated, the negligible amount of edge sp2CH contribution was ignored and C content for pristine graphite was assumed to be 100%. However, the elemental analysis of pristine graphite shows C H N O contents of98.81, 0.13, 0.00 and 0.00%, respectively. The result allowed us to estimate the amount of available sp2CH for the FriedelCrafts acylation reaction. The H content, which is most likely from sp2CH at the edges, of graphite, seems minor. However, when it is converted into molar ratio, the C/H ratio becomes 63.8. Thus, the theoretical C H N O values of resultant graphite-g-TMPBA are calculated based on nal yield. For example, assuming the amount of graphite before and after reaction remains constant, the amount of TMPBA grafted onto the edge of graphite can be simply estimated by subtracting the feed amount of

graphite. Considering a low experimental C content of as-received graphite (1.19%), a low experimental C content of graphite-g-TMPBA (1.62%) is expected. As a result, it is fair to say that overall experimental CHNO values obtained from graphite-g-TMPBA are agreed well with theoretically calculated values. In addition, the resultant graphite-g-TMPBA does show aromatic carbonyl (C = O) peak at 1,663 cm-1, indicating covalent linkage between graphite and TMPBA (Fig. 1b).

The scanning electron microscope (SEM) images of graphite-g-TMPBA and pristine graphite display distinct surface morphology. Pristine graphite shows very smooth surface (Fig. 4a), whereas the surface of graphite-g-TMPBA is relatively rough due to the attachment of TMPBA (Fig. 4b).

Both pristine graphite and graphite-g-TMPBA displayed almost identical the XPS peaks with different intensities (Fig. 5a). Pristine graphite showed a predominant C 1-s peak at 285 eV and much weaker O 1-s peak at 530 eV, presumably arising from physically adsorbed oxygen-containing species in pristine graphite [22], whereas graphite-g-TMPBA showed relatively weaker C 1-s peak and stronger O 1-s peak due to oxygen in carbonyl groups (C = O) together with physically adsorbed one.

As expected, the dispersibility of graphite-g-TMPBA was signicantly improved. A red beam from a laser pointer was shined through the graphite-g-TMPBA solution in NMP (0.2 mg/mL) and was able to pass through the dispersed solution, showing Tyndall scattering (Fig. 5b).

0

(a)

(b)

Fig. 3 a 1H NMR (CDCl3) spectrum of pyrene-g-(TMPBA)n; b 13C NMR

(CDCl3) spectrumof pyrene-g-(TMPBA)n

C

O

H C

H C

O

CH

n

C

H C

H C

O

O

CH

n

CH

CDCl3

CH

O

O

O Ar

H2O

TMS

Ar C

Ar C

Ar

TMS

10

9

8

7

6

5

4

3

2

1

210

180

150

120

90

60

30

0

ppm

ppm

CDCl3

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Fig. 4 a SEM image of pristine graphite; b SEM image of graphite-g-TMPBA

Fig. 5 a XPS surveysof pristine graphite and graphite-g-TMPBA;b photograph of graphite-g-TMPBA dispersed in NMP

The resulting solution remained visually unchanged even after months of standing under ambient condition.

Conclusions

The model reaction between pyrene as a miniature graph-ene and 4-(2,4,6-trimethylphenyloxy)benzamide (TMPBA) in polyphosphoric acid (PPA)/phosphorous pentoxide (P2O5) medium was successful for anticipating the edge-chemistry of graphite. The reaction condition was applied for the edge-functionalization of graphite. The resultant graphite-g-TMPBA as an edge-functionalized graphite (EFG) was readily dispersible in N-methyl-2-pyrrodinone (NMP). The result envisions that high quality graphene-like sheets can be synthesized as an alternative approach to problematic graphite oxide (GO).

Acknowledgments This research was supported by World Class University (WCU) and US-Korea NBIT programs through the National Research Foundation (NRF) of Korea funded by the Ministry of Education, Science and Technology (MEST) and US Air Force Ofce of Scientic Research (AFOSR).

Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.

References

1. A.K. Geim, K.S. Novoselov, Nat. Mater. 6, 183 (2007)2. M.S. Dresselhaus, G. Dresselhaus, Adv. Phys. 51, 1 (2002)3. K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306, 666 (2004)

123

Nanoscale Res Lett (2010) 5:16861691 1691

4. K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva, S.V. Dubonos, A.A. Firsov, Nature 438, 197 (2005)

5. Y.B. Zhang, Y.W. Tan, H.L. Stormer, P. Kim, Nature 438, 201 (2005)

6. Y. Zhang, J.P. Small, M.E.S. Amori, P. Kim, Phys. Rev. Lett. 94, 176803 (2005)

7. R.R. Nair, P. Blake, A.N. Grigorenko, K.S. Novoselov, T.J. Booth, T. Stauber, N.M.R. Peres, A.K. Geim, Science 320, 1308 (2008)

8. S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.A. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Nature 442, 282 (2006)

9. S. Stankovich, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Carbon 44, 3342 (2006)

10. D.W. Boukhvalov, M.I. Katsnelson, J. Am. Chem. Soc. 130, 10697 (2008)

11. .. Girit, J. Meyer, R. Erni, M.D. Rossell, C. Kisielowski, L. Yang, C.-H. Park, M.F. Crommie, M.L. Cohen, S.G. Louie, A. Zettl, Science 323, 1705 (2009)

12. J.-B. Baek, C.B. Lyons, L.-S. Tan, J. Mater. Chem. 14, 2052 (2004)

13. J.-B. Baek, C.B. Lyons, L.-S. Tan, Macromolecules 37, 8278 (2004)

14. H.-J. Lee, S.-J. Oh, J.-Y. Choi, J.W. Kim, J. Han, L.-S. Tan, J.-B. Baek, Chem. Mater. 17, 5057 (2005)

15. S.-J. Oh, H.-J. Lee, D.-K. Keum, S.-W. Lee, D.H. Wang, S.-Y. Park, L.-S. Tan, J.-B. Baek, Polymer 47, 1131 (2006)

16. J.-Y. Choi, S.-J. Oh, H.-J. Lee, D.H. Wang, L.-S. Tan, J.-B. Baek, Macromolecules 40, 4474 (2007)

17. H.-J. Lee, S.-W. Han, Y.-D. Kwon, L.-S. Tan, J.-B. Baek, Carbon 46, 1850 (2008)

18. S.-W. Han, S.-J. Oh, L.-S. Tan, J.-B. Baek, Carbon 46, 1841 (2008)

19. S.-W. Han, S.-J. Oh, L.-S. Tan, J.-B. Baek, Nanoscale Res. Lett. 4, 766 (2009)

20. K. Saeed, S.-Y. Park, S. Haider, J.-B. Baek, Nanoscale Res. Lett. 4, 39 (2009)

21. D.-H. Lim, C.B. Lyons, L.-S. Tan, J.-B. Baek, J. Phys. Chem. C 228, 12188 (2008)

22. Q. Chen, L. Dai, M. Gao, S. Huang, A.W.H. Mau, J. Phys. Chem. B 105, 618 (2001)

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