Selective organic functionalization of graphene bulk or graphene edges

Article abstract of DOI:10.1039/c1cc13254g

Graphene sheets have been functionalized with a PAMAM dendron, finding that graphene can be efficiently functionalized all over the surface, or only at the edges, depending on the reactions used in the functionalization process.

Full text of DOI:10.1039/c1cc13254g

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 9330–9332
www.rsc.org/chemcomm
COMMUNICATION
Selective organic functionalization of graphene bulk or graphene edgesw
a
a
a
Mildred Quintana,* Alejandro Montellano, Antonio Esau del Rio Castillo,
b
b
a
Gustaaf Van Tendeloo, Carla Bittencourt and Maurizio Prato*
Received 1st June 2011, Accepted 5th July 2011
DOI: 10.1039/c1cc13254g
Graphene sheets have been functionalized with a PAMAM
dendron, finding that graphene can be efficiently functionalized
all over the surface, or only at the edges, depending on the
reactions used in the functionalization process.
Dendrimers have been conjugated to carbon nanotubes in
1
2
order to render them more easily processible materials. In
this work, we functionalize exfoliated graphene in DMF using
dendrons in two different reactions: 1,3-dipolar cycloaddition
and amide-bond condensation reaction on the preexisting
carboxylic groups, generated during the sonication process.
The free amino groups at the tips of the dendrons act as
Colloidal suspensions of graphene sheets hold great promise
1
because of their many interesting properties. However, in view
1
3
of large scale applicability, several issues arise, which include
2
scalability, dispersibility and stability. Another challenging
ligands in the complexation of gold nanoparticles (Au NPs)
which are introduced as contrast markers for the identification
of the reactive sites.
issue is related to the chemical nature of graphene sheets and
3
their reactivity. Until now graphene has been produced by
4
several methods, including micromechanical cleavage, which
À1
Exfoliated graphene (0.02 mg ml ) was produced as reported
7,11
elsewhere (pristine graphene, p-G).
Condensation of para-
cannot be used in bulk applications as a result of their
5
extremely low yields. Oxidation of graphite layers, followed
formaldehyde (0.33 mmol, 2 equiv. per graphene carbon) with
14
a modified a-amino acid (0.33 mmol, 2 equiv. per graphene
by reduction, has yielded functionalized graphene soluble in
some organic solvents. However, this procedure modifies the
carbon) was accomplished in the presence of graphene colloidal
dispersion (100 ml, 0.16 mmol), Scheme 1. The mixture was
stirred at 130 1C for 72 h with successive similar additions of
the reactants every 24 h. The resulting solution was then filtered
and washed several times with DMF. The functionalized
graphene was dispersed in DMF (60 ml, 1 min sonication),
then trifluoro acetic acid (TFA, 30 ml) was added. The mixture
was kept under magnetic stirring overnight at room temperature.
The suspension was filtered and the filtrate washed with DMF
until neutrality to obtain graphene 1.
6
inherent electrical and thermal properties of graphene. Finally,
graphene can be produced by liquid phase exfoliation, in organic
solvents such as N-methyl pyrrolidone (NMP) or dimethyl
formamide (DMF), without chemical modification, by simple
7
ultrasonication assistance. Also, liquid phase exfoliation yields
low concentration of graphene sheets that can be increased by
longer sonication periods resulting in graphene sheets of limited
size. Several authors have proposed chemical functionalization
3
as a practicable solution to some of these problems: chemical
Graphene 1 (from cycloaddition and deprotection of the
Boc-amino groups) and p-G were condensed with dendron 2,
followed by deprotection of the amino groups, to obtain
functionalized graphenes 3 and 4, respectively, Schemes 1
and 2.
functionalization, in fact, can render graphene dispersible in
1
,2
many solvents. Furthermore, in order to exploit the high
mobility present in graphene, the band gap can be engineered
and controlled by doping semimetal graphene through chemical
8
,9
modifications. Chemical modification of various forms of
1
The reactions were performed by adding polyamidoamine,
PAMAM, dendron 2 (0.8 mmol, 5 equiv. per graphene carbon
atom), EDC (1.6 mmol, 10 equiv.), DMAP (1.6 mmol, 10 equiv.)
0
graphene including reduced graphene oxide, liquid-phase
exfoliated graphene and its multilayers has been successfully
1
1
achieved. For instance, the aryl diazonium salt-based reactions
exhibit a higher reactivity at the edges than the interior of the
8
sheets. Instead, the 1,3-dipolar cycloaddition seems to take
11
place not only at the edges but also at the internal CQC bonds.
a
b
Center of Excellence for Nanostructured Materials (CENMAT),
INSTM UdR di Trieste, Dipartimento di Scienze Chimiche e
Farmaceutiche, University of Trieste, Piazzale Europa 1,
I-34127 Trieste, Italy. E-mail: mquintana@units.it, prato@units.it
Electron Microscopy for Material Science (EMAT),
Physics Department, University of Antwerp, Groenenborgerlaan 171,
B-2020 Antwerp, Belgium
w Electronic supplementary information (ESI) available: XPS analysis,
synthesis of Au NPs in water. See DOI: 10.1039/c1cc13254g
Scheme 1
9
330 Chem. Commun., 2011, 47, 9330–9332
This journal is c The Royal Society of Chemistry 2011

View Article Online
nitrogen at 286.0 eV. The other two components, centered at
2
87.5 eV and 288.8 eV, can be attributed to photoelectrons
emitted from carbon atoms belonging to CQO and O–CQO,
N–CQO, respectively. The C 1s peak recorded on sample 4 is
reproduced with the same components used in 3 (ESIw).
In order to investigate the dispersibility of the derivatives,
UV-vis spectroscopy was used to measure the absorption at
660 nm of 3 and 4 in DMF. The maximum stable concentration
of the dispersions produced by mild sonication (5 min) and
later centrifugation (90 min, 500 rpm) was calculated by using
Scheme 2
À1
7
the absorption coefficient a = 2460 ml mg Ám resulting in
À1 À1
0
.52 mg ml and 0.09 mg ml , respectively. The significant
and HOBT (1.6 mmol, 10 equiv.) as coupling agents in
anhydrous DMF (35 ml). The mixtures were kept under Ar
and magnetically stirred for 48 h. The resulting suspensions
were filtered and washed with DMF and then methanol. For
the cleavage of the Boc protecting groups, the precipitates
were dispersed in dioxane (20 ml) and HCl (12 M, 10 ml) was
finally added. The dispersions were kept under magnetic
stirring overnight at room temperature. The final mixtures were
filtered and washed thoroughly with DMF and methanol.
The quantification of the free amino groups present on the
dendrons was carried out using the Kaiser test. Graphene 3
improvement in the dispersibility behavior of 3 and 4 corrobo-
rates the chemical modification by the two reactions. Additionally,
3
and 4 are partially dispersible in water, though, after a few
hours, precipitation was observed. Nevertheless, this partial
dispersibility in water allowed the addition of Au NPs for the
identification of the sites where reaction has taken place. For
this purpose, Au NPs in water (2 ml, 10 Æ 2 nm) were added to
À1
3
or 4 dispersion in DMF (1 ml, 0.02 and 0.01 mg ml
,
respectively). Au NPs were produced in water (ESIw) since no
strong capping agents are needed for their stabilization facili-
tating in this way the interaction with the amine functions of
À1
resulted in 620 mmol g while 4 possessed an average of
1
7
À1
the dendron moieties. Thus, the resulting material was
investigated by transmission electron microscopy (TEM) and
atomic force microscopy (AFM). The images in Fig. 2a–c
show Au NPs distributed all over the graphene surface. Instead,
for the amidation reaction, NPs seem to be deposited mainly
at the edges of the exfoliated material, Fig. 2d–f. Fig. 2f shows
the presence of aggregated Au NPs out of the graphene sheets
as well as aggregated graphene sheets (ESIw). In the case of 4,
the lack of functional groups in the central part of the sheets
induces fast aggregation. The lower concentration of nitrogen
atoms, the deposition of the Au NPs mainly at the edges of 4,
in addition to its aggregation behavior, are indirect proof that
p-G exfoliated in DMF has a low concentration of carboxylic
groups, which are mostly located at the graphene edges.
In order to further investigate the selectivity of the organic
reactions on graphene sheets, Raman spectroscopy was
performed using 532 nm laser excitation. Fig. 3 shows the
Raman spectra of the graphene products compared with that
of p-G. The second order Raman 2D band is sensitive to the
1
12 mmol g of amino groups.
The functionalization steps on the graphene surface were
1
5
followed via X-ray photoelectron spectroscopy (XPS). The
relative nitrogen atomic composition in 3 was B2 atomic%
while in 4 B0.4, i.e., the amount of nitrogen in 3 is five
times higher than in 4. This result is in good agreement with
the number of amino groups obtained with the Kaiser test
(
3/4 = 5.5). Fig. 1 shows the C 1s and the N 1s peaks recorded
from 3 and the result of the curve fittings performed to explain
the spectra.
The chemical modification of the graphene surface can be
identified by the presence of the N 1s peak in the XPS
spectrum. To reproduce the N 1s peak two components were
used: one centered at 400.6 eV and a second at 398.8 eV. These
components were attributed to photoelectrons emitted from
the protonated amino groups and the pyrrolidine nitrogen
1
1
respectively. Due to the weak intensity of the N 1s peak
recorded on sample 4, it was impossible to perform the fitting
analysis. Additional information on the functionalization can
be obtained from the analysis of the C 1s peak. To reproduce
the C 1s peak recorded on sample 3 four components were
used: a main component centered at 284.7 eV attributed to
1
8
number of layers in graphene. In our Raman spectra we are
2
photoelectrons emitted from sp carbon atoms (CQC) in few
1
6
layer graphenes. A second component is attributed to photo-
electrons emitted from carbon atoms bound to oxygen and to
Fig. 1 Photoemission spectra and fit of the (a) N 1s core level region
and (b) C 1s core level region for graphene 3.
Fig. 2 TEM and AFM micrographs of 3 and 4 after the addition of
Au NPs.
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 9330–9332 9331

View Article Online
20085M27SS) and Regione Friuli Venezia-Giulia, the European
Commission under contract ERC 246791 (COUNTATOMS)
and COST Action MP0901 "NanoTP". A.E.R.C. is grateful to
CONACYT-Mexico for a PhD fellowship.
Notes and references
1
M. Segal, Nat. Nanotechnol., 2009, 4, 612; R. S. Ruoff, Nat.
Nanotechnol., 2008, 3, 10.
2
N. Behabtu, J. R. Lomeda, M. J. Green, A. L. Higginbotham,
A. Snitskii, D. V. Kosynkin, D. Tsentalovich, A. N. Parra-Vazquez,
J. Schmidt, E. Kesselman, Y. Cohen, Y. Talmon, J. M. Tour and
M. Pasquali, Nat. Nanotechnol., 2010, 5, 406.
3
4
S. Park and R. S. Ruoff, Nat. Nanotechnol., 2009, 4, 217.
K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth,
V. V. Khotkeivich, S. V. Morozov and A. K. Geim, Proc. Natl.
Acad. Sci. U. S. A., 2005, 102, 10451.
Fig. 3 Raman spectra of p-G, 3 and 4, normalized at the G-band.
able to identify few layer graphenes, which have a sharp and
À1 13,18
5 W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958,
80, 1339; S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas,
A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff,
Carbon, 2007, 45, 1558.
6 S. Stankovich, D. A. Dikin, G. H. B. Dommet, K. M. Kohlhass,
E. J. Zimney, E. A. Stach, R. D. Pines, S. T. Nguyen and
R. S. Ruoff, Nature, 2006, 442, 282; T. Schwanb, B. R. Burg,
N. C. Schirmer and D. Poulikakos, Nanotechnology, 2009,
symmetric 2D band at about 2700 cm
.
A clear difference
between functionalized and non-functionalized graphene is the
À1
appearance of the disorder D band at about 1350 cm . In p-G,
the D band depends on the size of the sheets, the presence of
defects at the graphene edges and the interaction with the
substrate. As compared to p-G, the D band in 4 was not
considerably modified while in 3 there is an increase in the
intensity band associated with the presence of defects. These
defects are known to be related to the change of C atoms from
2
0, 405704.
7 Y. Hernandez, V. Nicolosi, M. Lotya, F. M. Blighe, Z. Sun, S. De,
I. T. McGovern, B. Holland, M. Byrne, Y. K. GunKo,
J. J. Boland, P. Niraj, G. Duesberg, S. Krishnamurthy,
R. Goodhue, J. Hutchison, V. Scardaci, A. C. Ferrari and
J. N. Coleman, Nat. Nanotechnol., 2008, 3, 563; U. Khan,
A. O’Neill, M. Loyta, S. De and J. N. Coleman, Small, 2010,
2
3
the sp to sp hybridization state as a result of the organic
11
functionalization. The ratio of the intensities of the D and G
6
, 864.
R. Sharma, H. J. Baik, C. J. Perera and M. S. Strano, Nano Lett.,
010, 10, 398.
bands is often used as an indication of the level of chemical
8
1
9
modification. The I
D G
/I ratios in p-G, 3 and 4 are 0.11, 0.13
2
and 0.29, respectively.
9 K. A. Ritter and J. W. Lyding, Nat. Mater., 2009, 8, 235.
0 J. R. Lomeda, C. D. Doyle, D. V. Kosynkin, W. F. Hwang and
J. M. Tour, J. Am. Chem. Soc., 2008, 130, 16201.
1 V. Georgakilas, A. B. Bourlinos, R. Zboril, T. A. Steriotis,
P. Dallas, A. K. Stubos and C. Trapalis, Chem. Commun., 2010,
46, 1766; M. Quintana, K. Spyrou, M. Grzelczak, W. R. Browne,
P. Rudolf and M. Prato, ACS Nano, 2010, 4, 3527.
1
1
In conclusion, we have performed two well-established organic
reactions (1,3-dipolar cycloaddition and amide-condensation
reactions) on graphene sheets. By means of these two reactions,
we have introduced dendron moieties with amino groups at the
center or at the periphery of the graphene sheets, which later
recognize Au NPs used as contrast markers in the identification
of the reactive sites. Computational studies predict that
azomethine ylide reactions are endothermic on the center bonds
1
2 M. A. Herrero, F. M. Toma, K. T. Al-Jamal, K. Kostarelos,
A. Bianco, T. Da Ros, F. Bano, L. Casalis, G. Scoles and
M. Prato, J. Am. Chem. Soc., 2010, 132, 1731.
1
3 A. C. Templeton, S. Chen, S. M. Gross and R. W. Murray,
Langmuir, 1999, 15, 66.
20
of graphene. Our experimental results seem to indicate that
,3-dipolar cycloaddition occurs probably next to defects,
1
1
4 D. Milic and M. Prato, Eur. J. Org. Chem., 2010, 476.
5 S. Hufner, Photoelectron Spectroscopy, Springer-Verlag, 2003.
1
inducing a higher dispersibility in DMF. Instead in the amide-
condensation reaction we confirm that the carboxylic groups are
mainly present at the edges of pristine graphene exfoliated in
DMF. By means of the designed organic functionalization, the
properties of graphene in colloidal suspensions can be selectively
tuned for the preparation of diverse types of materials.
16 H. Hibino, H. Kegeshima, M. Kotsugi, F. Maeda, F.-Z. Gao and
Y. Watanabe, Phys. Rev. B, 2009, 79, 125437.
1
7 S. Yang, Y. Wang, Q. Wang, R. Zhang and B. Ding, Colloids Surf.,
A, 2007, 301, 174.
18 A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri,
F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth and
A. K. Geim, Phys. Rev. Lett., 2006, 97, 187401.
1
9 D. Graf, F. Molitor, K. Ensslin, C. Stampfer, A. Jungen,
C. Hierold and L. Wirtz, Nano Lett., 2006, 97, 187401.
20 Y. Cao and K. N. Houk, J. Mater. Chem., 2011, 21, 1503.
This work was partially supported by the University of
Trieste, Italian Ministry of Education MIUR (Cofin Prot.
9
332 Chem. Commun., 2011, 47, 9330–9332
This journal is c The Royal Society of Chemistry 2011