Radical addition of xanthates on carbon nanotubes as an efficient covalent functionalization method

Article abstract of DOI:10.1002/chem.201203207

Radical-initiated support: Xanthates were used as chemical reagents for the sidewall covalent functionalization of carbon nanotubes (see figure). The best grafting yields were obtained with stoichiometric ratios of xanthate and the radical initiator lauroyl peroxide. One grafted function was used as a tether for bimetallic cluster compounds, which were converted into very small (1-2nm) supported nanoparticles upon heating. Copyright

Full text of DOI:10.1002/chem.201203207

DOI: 10.1002/chem.201203207
Radical Addition of Xanthates on Carbon Nanotubes as an Efficient
Covalent Functionalization Method
Bꢀatrice Vanhorenbeke, Charles Vriamont, Florence Pennetreau, Michel Devillers,
Olivier Riant,* and Sophie Hermans*[a]
Carbon nanotubes (CNTs) attract wide interest because
of their outstanding properties, leading to applications in
various domains such as electronics, material engineering,
gas storage, drug delivery and catalysis.[1] CNTs are also
used as nanoparticle (NP) supports for applications in sen-
sors, fuel cells and composite materials.[2] However, to im-
plement carbon nanotubes in such applications, chemical
modification is required. As a consequence, many efforts
have been driven during the last decade by developing CNT
chemical functionalization processes.[3] These are usually
grouped into three main routes: non-covalent, defect-site,
Scheme 1. Functionalization of CNTs by xanthates 1 and 2.
and covalent sidewall functionalization. Among them, the
most promising seems to be covalent functionalization be-
cause it produces highly functionalized nanotubes exhibiting
good dispersion of strongly bonded moieties. Reported side-
wall grafting reactions are mainly fluorination, cycloaddi-
tion, carbene or nitrene addition, and reactions involving
radicals.[4] The most frequently used radicals for CNT func-
tionalization are obtained from the decomposition of diazo-
nium salts or organic peroxides.[5]
We report here a new approach for carbon nanotubes co-
valent functionalization by using xanthates as radical precur-
sors. These compounds are known to react with non-activat-
ed olefins and arenes to create new carbon–carbon bonds
through activation by a radical initiator.[6] We based our hy-
pothesis on the fact that CNTs may be regarded as polyole-
fins, capable of reacting with xanthates (Scheme 1). This ap-
proach is advantageous compared with the other radical
methods cited above, since the mechanism is self-regulated
due to a degenerative equilibrium; therefore, it allows the
used to optimize reaction parameters, whereas the second
one allowed us to obtain the activated-ester-functionalized
CNTs, which may be further derivatized.
Covalent functionalization of single-walled nanotubes
(SWNTs) involving fluorinated functions have already ap-
peared elsewhere. Kawasaki et al.[7] reported grafting yields
of open and closed SWNTs by fluorination, that is, by react-
ing SWNTs samples with elemental fluorine gas at different
temperatures (300–523 K) for different time periods (5 h–1
month). However, xanthate functionalization is advanta-
geous because it is an easy process compared with fluorina-
tion since it does not imply gaseous reagents, high tempera-
tures, or long reaction times. Moreover, xanthate functional-
ities are easily tuned, offering a large scope of products,
whereas applications of fluorinated-SWNTs require post-
functionalization to implement them in practical devices.
Another typical example of SWNTs covalent fluorine-con-
taining functionalization has been reported by Bahr et al.[5a]
This implies the well-known aryl diazonium salts reaction in-
itiated by electrochemical reduction. It should be noted that
this was performed on very small amounts of SWNTs (1–
2 mg) of a small diameter selected for their higher reactivity.
The prime materials to be functionalized in the present
study were multi-walled nanotubes (MWNTs) because they
offer the advantages of being available to functionalize the
external layer while being stabilized by the internal layers,
and also resilient enough to be used in devices. However,
we also used SWNTs as a tool because they allow spectro-
scopic characterization and distinct applications. TEM
images of the pristine (p)-MWNTs used in this work re-
vealed an average of 10 concentric graphene layers per tube
(see Figure S1a and b in the Supporting Information).
production of functionalized radicals in
a controlled
manner. Moreover, xanthate reagents are easily prepared
and offer a wide range of functionalities. In this work, two
model xanthates 1 and 2, both fluorinated, have been select-
ed for initial studies (Scheme 1), in combination with lauroyl
peroxide (DLP) as the radical initiator. The first model was
[a] B. Vanhorenbeke, C. Vriamont, F. Pennetreau, Prof. M. Devillers,
Prof. O. Riant, Prof. S. Hermans
IMCN Institute, Universitꢀ catholique de Louvain
Place L. Pasteur 1, 1348 Louvain-la-Neuve (Belgium)
Fax : (+32)10-47-23-30
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.201203207.
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COMMUNICATION
For the initial optimization experiments, each reaction
was carried out on a 24 mg amount (2 mmol C atoms) of
MWNTs with varying amounts of xanthates and DLP. TEM
images (see Figure S1c and d in the Supporting Information)
showed that the nanotube integrity was preserved after the
functionalization procedure and XPS revealed non-negligi-
ble amounts of fluorine arising from the xanthate surface
functionalization. Four different parameters were modified
to optimize the grafting yield of xanthate 1 on MWNTs,
namely the 1) xanthate concentration, 2) radical initiator
concentration, 3) portionwise additions of radical initiator
and 4) reaction time. Only the two former parameters were
actually found to influence the functionalization level: the
higher the xanthate and initiator concentrations, the higher
the grafting yield (Figure 1). These experiments also showed
Figure 2. Fluorine and sulfur surface concentrations determined by XPS
as a function of the amount of DLP for xanthates 1 and 2 (1 equiv).
F/C ratios obtained by XPS were higher (Table 1), which is
in line with the absence of internal non-functionalizable
carbon layers. Reported surface F/C ratios ranged from 0.23
to 0.51 in the case of direct fluorination of SWNTs,[7] which
is approximately 10–25 times higher than here (0.02). Fluo-
rophenyl-functionalized SWNTs obtained by using aryl di-
azonium salts[5a] presented a surface F/C atomic ratio of
0.037; this is very similar to our results.
Table 1. Comparison of F/C ratios obtained by XPS and EA.
F/C molar ratios
f-MWNTs (1:1:1)[a]
f-SWNTs (1:1:1)[a]
XPS
EA
0.0131
0.0076
0.0189
0.0198
Figure 1. Fluorine surface concentrations determined by XPS as a func-
tion of the amount of xanthate 1.
[a] (Xanthate 2/DLP/C from nanotube) equivalents.
that the highest fluorine surface concentrations are obtained
when xanthate and DLP are introduced in stoichiometric
amounts. The number of DLP additions or the reaction time
had negligible influence on the functionalization yield (see
Figure S2 and Tables S1 and S2 in the Supporting Informa-
tion). Taking into account the sulfur content of p-MWNTs,
it should be noted that only the R1 group of the starting
xanthate is grafted onto the CNT, whereas the sulfur-con-
taining moiety is not significantly present on the surface of
the functionalized CNT (see Table S3 in the Supporting In-
formation for numerical values). Measured F/S ratios are
indeed very different from the theoretical value of 1.5 which
would be observed if R1 and R2 moieties were grafted in
stoichiometric proportions.
Similar experiments were carried out with xanthate 2 on
MWNTs (Figure 2 and Tables S4 and S5 in the Supporting
Information) and they led to the same conclusions, that is,
negligible sulfur contents and highest fluorine rates for stoi-
chiometric DLP/xanthate ratios. The fluorine amounts are
slightly lower with xanthate 2 than with 1, whereas F/S
ratios are also different from the theoretical value (here
2.5).
XPS is used as a routine technique for the quantitative
analysis of functionalized nanotubes throughout the litera-
ture. However, its information depth is of the same order of
magnitude than MWNT diameters. Therefore, a bulk ele-
mental analysis (EA) is required to obtain more reliable
quantitative concentrations in each heteroelement within
the sample. EA indeed revealed a smaller F/C ratio than
XPS in the case of MWNTs, meaning that XPS only probes
a fraction of the concentric carbonaceous layers. This hy-
pothesis is confirmed by the fact that in the case of SWNTs,
which comprise only one graphene layer, the F/C ratios ob-
tained by XPS and EA are in agreement.
The covalent nature of the bond between the R1 moiety
and the nanotube framework was proven using Raman spec-
troscopy and thermogravimetric analysis (TGA). Indeed the
Raman spectrum of functionalized (f)-SWNTs exhibits a
large increase of the so-called D band, attributed to sp3-hy-
bridized carbon atoms, located at 1277 cmÀ1, compared with
the Raman spectrum of pristine nanotubes (Figure 3). TGA
revealed a mass loss at higher temperature in the f-SWNTs
case than with a p-SWNTs/xanthate physical mixture (see
Figure S3 in the Supporting Information).
The same methodology was applied to SWNTs under the
optimized conditions for further characterizations by using
spectroscopic techniques. In the case of SWNTs, the surface
The grafted fragments were used as tethers for clusters to
probe their reactivity. These clusters can be used as molecu-
lar precursors of NPs. Molecular clusters can be modified by
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O. Riant, S. Hermans et al.
Scheme 2. Reaction scheme for the post-functionalization of carbon nanotubes. Filled sphere=metallic core, empty circle=ligand shell.
served. The final NP/CNT nanocomposite was obtained by
treatment at 3508C; this temperature being determined by
TGA analysis of the pure cluster. After such a treatment,
the material is converted into bimetallic NPs, stripped off
the ligands, and supported on CNTs.
The samples at each step in the preparation of the nano-
structured material were characterized by XPS and EA
(Table 2). Before activation, the cluster was indeed tethered
to the tube surface, as evidenced by the decrease of fluorine
content with concomitant appearance of Ru and Pt. The Ru/
Pt ratio being close to the theoretical value of 5 indicates
that the cluster keeps its integrity. After activation, no more
fluorine remains, meaning that the organic moiety has been
released upon heating. The Ru/Pt ratio obtained by XPS
after activation demonstrates some surface rearrangements;
whereas the Ru/Pt ratio close to 5 obtained by EA shows
that the global composition is preserved. Finally, also ac-
cording to the EA results, the final sample of NP/CNT con-
tains about 2 wt% of metal (Ru+Pt), which is sufficient for
most applications.
Those samples were also characterized by secondary ion
mass spectrometry (SIMS, Figure S4a–d in the Supporting
Information). The pure cluster SIMS spectrum (Figure S4a)
revealed a typical fragmentation pattern for clusters (succes-
sive CO losses). Before heating (Figure S4b), the supported
cluster revealed peaks arising from the metallic core of the
cluster. Finally, after activation, only a few peaks arising
Figure 3. Raman spectra (l=1064 nm) of pristine- (c) and functional-
ized- (b) SWNTs. Inset shows a magnification of the radial breathing
modes.
ligand exchange to introduce in their coordination sphere a
reactive fragment capable of covalently binding to a func-
tionalized surface. Moreover, when multimetallic, they are
excellent precursors of bimetallic nanoparticles. Given that
xanthate 2 contains an activated ester function, an amine-
containing ligand was selected (Scheme 2). The cluster
[Ru5PtC(CO)16] was treated with 2-(diphenylphosphino)-
A
2ACHTUNGTNERUN(GN CH2)2PPh2Ru5PtC(CO)15 adduct
This adduct was then reacted with MWNTs functionalized
with xanthate 2, to covalently attach the precursor to the
CNT. This was performed in solution followed by filtering.
A control experiment was carried out with pristine-MWNT
in the presence of a Ru5Pt cluster solution: 0% loading of
the cluster on the non-functionalized carbon surface was ob-
À
À
from the cluster core still appear, namely Ru Ru and Ru
Pt fragments.
After thermal treatment the supported clusters were also
characterized by TEM. The micrograph (Figure 4) showed
Table 2. Characterization by XPS and EA of samples at each step of NP/CNT preparation.
Element content
Molar ratios
Ru/C[c]
C
F
P
N
Ru
Pt
Ru/Pt
Pt/C[c]
f-MWNTs
cluster
95.10[a]
95.12[a]
1.25[a]
0.72[a]
N/A
N/A
N/A
N/A
N/A
4.7
N/A
0.30
N/A
0.06
0.06[a]
0.28[a]
0.28[a]
0.06[a]
supported
activated
NP/CNT
97.55[a]
89.24[b]
0.01[a]
N/A
0.05[a]
0.14[b]
0.23[a]
0.60[b]
0.34[a]
1.35[b]
0.05[a]
0.62[b]
6.8
4.2
0.35
0.18
0.05
0.04
[a] XPS (at%). [b] EA (wt%). [c]ꢂ100.
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Radical Addition of Xanthates on Carbon Nanotubes
COMMUNICATION
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Figure 4. TEM image of nanoparticles supported on CNT.
that homogeneously dispersed metallic nanoparticles of 1–
2 nm in size were obtained. This is lower than previous pub-
[8]
À
lished results of Ru Pt NPs supported on CNTs. Retro-
spectively, this also proves the homogeneity of xanthate
functionalization.
In conclusion, we have proven that xanthate is capable of
reacting with the CNT surface thanks to a radical mecha-
nism. Only the moiety containing no sulfur is grafted on the
tube surface. Two different xanthates were used and have
shown a similar behavior when varying the reaction parame-
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nature of such functionalization. The activated ester groups
introduced on CNTs from xanthate reactants were further
implemented as an anchor for molecular clusters, which
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Acknowledgements
We acknowledge the FRS-FNRS, FRIA, Fꢀdꢀration Wallonie-Bruxelles,
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Keywords: cluster compounds · covalent functionalization ·
nanoparticles · nanotubes · radicals · xanthate
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Received: September 9, 2012
Published online: December 12, 2012
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