One-step double covalent functionalization of reduced graphene oxide with xanthates and peroxides

Article abstract of DOI:10.1002/chem.201405148

Radical functionalization of reduced graphene oxide has been achieved by reaction with a xanthate in the presence of peroxide as a radical initiator. X-ray photoelectron spectroscopy, bulk elemental analyses, and thermogravimetric analyses showed that the

Full text of DOI:10.1002/chem.201405148

DOI: 10.1002/chem.201405148
Communication
&
Graphene Functionalization
One-Step Double Covalent Functionalization of Reduced
Graphene Oxide with Xanthates and Peroxides
Florence Pennetreau, Olivier Riant,* and Sophie Hermans*[a]
ative flatness of its structure, several examples of successful
Abstract: Radical functionalization of reduced graphene
oxide has been achieved by reaction with a xanthate in
the presence of peroxide as a radical initiator. X-ray photo-
electron spectroscopy, bulk elemental analyses, and ther-
mogravimetric analyses showed that the xanthate grafting
is covalent and efficient. The synthesis and use of seven
xanthates and three peroxides showed that the highest
grafting yield is obtained when xanthate and peroxide are
introduced in stoichiometric amounts. It also revealed that
the peroxide used as radical initiator is grafted at the gra-
phenic surface during the functionalization. The method
presented in this contribution therefore allows bifunction-
alized reduced graphene oxide samples to be easily ob-
tained in one single step. This method leads to undam-
aged graphene sheets with higher dispersibility than the
pristine sample.
graphene derivatizations performed by methods initially devel-
oped for CNTs functionalization have been reported.[7–13]
Among all of these methods, the use of radicals arising from,
for example, diazonium salts[14] is presented as one of the best
ways to chemically modify graphene sheets.[8]
In this context, we have developed a method of reduced
graphene oxide (rGO) radical functionalization using xanthates
as radical precursors. Xanthates are indeed known in organic
methodology to react with activated and non-activated olefins
in the presence of peroxides as initiator.[15] In addition, we
have recently shown that xanthates react at CNT surfaces.[16]
The use of xanthates is beneficial compared to other radical
precursors for various reasons. The main reason is that the
grafting mechanism implies a degenerative equilibrium that
self regulates the radical production and avoids excessive de-
activation.[15] Moreover, xanthates bearing diverse functional
groups can easily be synthesized at multi-gram scale. A wide
range of functionalities can therefore be anchored at the gra-
phenic surface. The derivatization reaction is also easy to carry
out as no gaseous reactants nor high temperature or long re-
action times are required.
The isolation of graphene, an atom-thick layer of sp2 carbon
atoms arranged in a honeycomb lattice, is one of the signifi-
cant scientific events of the last decade. Since then, graphene
has attracted much interest in various fields[1] such as material
sciences,[2] electronics,[3] and biomedical sciences.[4] This infatu-
ation is due to its unique physical and electrical properties.
Three main methods of graphene synthesis exist: Exfoliation of
graphite with scotch tape, epitaxial growth/CVD, and reduction
of graphene oxide (GO) to rGO (reduced graphene oxide). The
first two allow high-quality graphene to be obtained whereas
the latter is simple and allows large-scale production of
powder samples.[1,5] A critical limitation is nevertheless worth
noting: as in the case of carbon nanotubes (CNTs), graphene is
insoluble in water and common organic solvents and hardly
reacts with common organic reagents. A preliminary function-
alization step is thus usually necessary to benefit from its full
potential in most applications.[5a,6] Fortunately, research in the
field of carbon nanotubes has speeded-up the development of
graphene functionalization methods. Indeed, in spite of the rel-
In addition, we present here the concomitant radical cova-
lent grafting of both xanthate and peroxide initiator fragments
at the reduced graphene oxide surface (Figure 1). Indeed, per-
Figure 1. Schematic representation of rGO (oxygen groups not shown)
double functionalization with R1 from a xanthate and R2 from a peroxide
(minor S-containing functions omitted for clarity).
oxides, as radicals themselves, could also be grafted at the gra-
phene surface. This has been shown extensively on CNTs, in
particular by thermal activation of dilauroyl peroxide,[17] and
also only once on graphene by photochemical activation.[18] In
judiciously choosing the peroxide, a second useful function
could therefore be grafted at the carbonaceous surface. Never-
theless, peroxide-derived radicals were also used as radical ini-
tiators on graphene without indication of its grafting.[19] To our
knowledge, this is the first report of graphene double covalent
[a] F. Pennetreau, Prof. O. Riant, Prof. S. Hermans
Institute of Condensed Matter and Nanosciences
Molecules, Solids and Reactivity (IMCN/MOST)
Universitꢀ catholique de Louvain
Place Louis Pasteur 1, 1348 Louvain-la-Neuve (Belgium)
Fax: (+32)10-472330
E-mail: olivier.riant@uclouvain.be
sophie.hermans@uclouvain.be
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405148.
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Communication
Figure 3. XPS spectra of p-rGO (c) and f-rGO (a). Inset: Magnification
of the sulfur peaks.
by the presence of fluorine and sulfur in bulk elemental analy-
ses (Table S3 in the Supporting Information). Moreover XPS
measurements show that, as previously reported in the case of
carbon nanotubes,[16] the grafting yield of the sulfur-containing
moiety is far lower than that of the R1 fragment.
To highlight the covalent nature of the bond between xan-
thate and the rGO surface, three samples were analyzed by
thermogravimetric analysis (TGA): rGO before and after func-
tionalization with xanthate X2 and a physical mixture of p-rGO
and xanthate X2. Figure 4 shows that p-rGO suffers a weak
Figure 2. Structures of the synthetized xanthates (X1–X7) and peroxides
(P1–P3) and the commercial dilauroyl peroxide (DLP)
functionalization by two different partners in a single chemical
reaction.
First rGO functionalization experiments were carried out
with a xanthate molecule bearing a pentafluorophenol activat-
ed ester group (X2, Figure 2) and dilauroyl peroxide (DLP,
Figure 2) as radical initiator. These choices have been dictated
by the presence of fluorine atoms in the R1 fragment of the
xanthate X2 that permits a reliable quantification of its grafting
rate and the absence of any functional moiety in the R2 ali-
phatic chain of the peroxide. This absence allows the xanthate
behavior to be studied as clearly as possible.
Figure 4. Thermogram derivatives of p-rGO (c), f-rGO (a) and physical
mixture of p-rGO and xanthate X2 (g). Inset: Corresponding thermo-
grams.
mass loss attributable to the decomposition of oxygenated
functions remaining from the GO. After reaction with xanthate
X2, the thermogram exhibits a larger mass loss corresponding
to the decomposition of the grafted fragments, strengthening
the idea of xanthate grafting at the rGO surface. Beyond the
extent of the mass loss, its position in the thermogram also
gives useful information: the higher decomposition tempera-
ture visible in the derivative of f-rGO thermogram compared
with that of the physical mixture is indicative of the presence
of a covalent bond between rGO and xanthate in the case of f-
rGO.
X-ray photoelectron spectroscopy (XPS) analysis of the pris-
tine reduced graphene oxide (p-rGO) shows the presence of
carbon, oxygen and nitrogen atoms. The high oxygen content
in the p-rGO sample indicates that the reduction of the GO in
rGO is incomplete. After functionalization (f-rGO), XPS spectra
show the expected apparition of both sulfur and fluorine
peaks belonging to the xanthate anchored at the carbona-
ceous surface (Figure 3; for reproducibility, see Table S1 and S2
in the Supporting Information). This result is further confirmed
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Notably, f-rGO forms a longer-lasting stable suspension in
DMF compared with p-rGO samples (stable after 7 days and 3
months; see Figure S1 in the Supporting Information). The
methodology is thus appropriate for the preparation of sam-
ples usable in applications in which good graphene disper-
sions are required. The colloidal nature of the f-rGO dispersion
is also exhibited by the appearance of the Tyndall effect (Fig-
ure S2).[20] The rGO integrity after treatment with xanthate and
peroxide has been verified by TEM (Figure S3). Micrographs of
p- and f-rGO reveal the presence of both graphene sheets and
nanoplatelets. The f-rGO sample has suffered no visible
damage during functionalization.
Table 1. XPS characterization of rGO functionalized with xanthate X1–X7.
Xanthate[a]
Surface concentrations [atomic%]
Heteroatom from R1
S 2p
X1
X2
X3
X4
X5
X6
X7
F 1s
F 1s
N 1s
N 1s
N 1s
F 1s
P 2p
7.85
5.96
0.56
0.17
0.62[b]
1.03
0.29
0.65
0.43
0.32
0.37
0.39
0.63
0.50
[a] Xanthate and DLP concentrations are 1 equivalent with respect to the
C content of rGO. [b] Only the nitrogen content corresponding to nitro
moieties is taken into account.
To increase the grafting efficiency, several optimization ex-
periments have been carried out. The choice of the parameters
to vary was facilitated by a preceding study concerning the be-
havior of xanthates towards carbon nanotubes.[16] It was dem-
onstrated that the two relevant parameters regulating the xan-
thate grafting yield are xanthate and peroxide concentrations.
A maximum in the R1 moiety grafting rate was indeed ob-
served when both xanthate and peroxide were introduced in
stoichiometric amounts on CNTs. Optimization reactions have
then been carried out here by varying both parameters. XPS
results show that, in accordance with previous results on CNT,
a maximum of R1 grafting is reached at the stoichiometric
point where the xanthate concentration equals the peroxide
one (Figure 5). Moreover, the grafting of the sulfur-containing
part is less effective than that of the R1 fragment and barely in-
fluenced by external parameters, as previously observed in the
case of CNTs.
Due to the non-negligible concentration of nitrogen in p-
rGO samples, a reliable estimation of xanthate X3 and X4 graft-
ing yields is hard to achieve. For this reason, xanthate X5 bear-
ing a nitro moiety was synthetized and grafted at the rGO sur-
face. Indeed, nitrogen signals arising from nitro compounds
(N1s peak at 406 eV) can easily be distinguished from the ni-
trogen content in p-rGO (N1s at 400 eV; see Figure S4 in the
Supporting Information).
To prove the covalent grafting of the peroxide in the condi-
tions used for xanthate grafting, a reaction between rGO and
DLP without xanthate was performed. The TGA analysis of the
solid obtained shows a weight loss with a maximum slope at
about 5008C, attributable to DLP (Figure S5). The importance
of the mass loss and the high temperature at which it appears
indicates that the peroxide is indeed covalently grafted at the
rGO surface.
By modifying the R2 fragment of the peroxide used for the
xanthate grafting initiation, it becomes then possible to obtain
bifunctionalized rGO samples. For this purpose, diverse heter-
oatom-containing peroxides were synthesized (P1–P3,
Figure 2) and grafted at the rGO surface along with diverse
xanthates. XPS analyses show the efficiency of the double
functionalization as heteroatoms belonging to both xanthate
and peroxide are detected in the samples after functionaliza-
tion (Figure 6 and Table 2). This is, to our knowledge, the only
graphene functionalization method that allows this kind of
double functionalization to be achieved in one single step.
From these results, the degree of functionalization can be
estimated to be about 1 functional group for 35–70 rGO
carbon atoms depending on the xanthate and peroxide used.
Even if the different types of graphene and functionalization
methods cannot be directly compared, this falls within the
range of values reported in the literature for various derivatiza-
tion pathways such as the Bingel reaction,[21] diazonium graft-
ing,[22] cycloadditions,[23] and reductive alkylation[24] (see
Table S4 in the Supporting Information). In addition, as our
sample of rGO is constituted of a few layers of graphene, the
internal graphene sheets were not functionalized but only the
top and bottom faces. Single-layer graphene is known to be
more reactive, therefore our values would have been even
higher on single-layer graphene.[18] It should be noted, howev-
er, that rGO presents many defects which might increase its re-
activity compared to CVD or exfoliated graphene.
Figure 5. Evolution of fluorine (c) and sulfur (a) concentrations deter-
mined by XPS as a function of DLP (left) and xanthate X2 (right) concentra-
tions. In both cases, the concentration of the non-varying parameter is
1 equivalent with respect to the C content of rGO.
To demonstrate the versatility of the method, xanthates X1
to X7 (Figure 2) bearing different R1 fragments have been syn-
thesized and grafted at the rGO surface using the function-free
DLP as radical initiator. In each case, the xanthate was success-
fully grafted at the rGO surface as shown by XPS (Table 1). For
each xanthate, the peak corresponding to the heteroatom in
the R1 fragment is detected in the XPS spectrum. Reduced gra-
phene oxide samples bearing activated ester, phthalimide, suc-
cinimide, nitro, acetamide and phosphonate functionalities
were therefore obtained. It is worth mentioning that the sulfur
concentration remains low in each case, meaning that the
grafting yield of the sulfur-containing fragment is still lower
than that of the R1 fragment, except for X4.
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Keywords: graphene
·
peroxides
·
radicals
·
surface
functionalization · xanthates
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—
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—
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Acknowledgements
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The authors acknowledge FRS-FNRS , FRIA, Fꢁdꢁration Wallo-
nie-Bruxelles, Loterie Nationale and Universitꢁ catholique de
Louvain for funding. We are also grateful to N. Vandelear, H.
Breton, F. Billard and J.-F. Statsijns for technical support.
Received: September 5, 2014
Published online on October 9, 2014
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