Intercalation of WF6 in the interlayer space of multiwall carbon nanotubes - Structural and morphological aspects

Article abstract of DOI:10.1016/j.ssc.2004.01.025

The reactivity of multiwall carbon nanotubes toward WF₆, a strong Lewis acid, has been studied. A material of nominal composition C ₃₆WF₆ has been obtained and characterized by X-ray diffraction. Intercalation between pseudo-graphitic layers has been evidenced, leading to a staging phenomenon at the nanometer scale. A structural model is proposed and the intercalation chemistry of multiwalled carbon nanotubes is discussed.

Full text of DOI:10.1016/j.ssc.2004.01.025

Solid State Communications 130 (2004) 1–5
www.elsevier.com/locate/ssc
Intercalation of WF6 in the interlayer space of multiwall carbon
nanotubes—structural and morphological aspects
a,
D. Claves , J. Giraudet , M.C. Schouler , P. Gadelle , A. Hamwi
a
b
b
a
*
a
´ ´ `
Laboratoire des Materiaux Inorganiques, UMR CNRS 6002-Universite B. Pascal, 24 av. des Landais, 63177 Aubiere Cedex, France
b
´ ´
Laboratoire de Thermodynamique et de Physico-Chimie Metallurgiques, CNRS-INPG-Universite J. Fourier, BP 75,
`
38402 Saint Martin d’Heres Cedex, France
Received 17 September 2003; received in revised form 24 December 2003; accepted 15 January 2004 by C. Lacroix
Abstract
The reactivity of multiwall carbon nanotubes toward WF6, a strong Lewis acid, has been studied. A material of nominal
composition C36WF6 has been obtained and characterized by X-ray diffraction. Intercalation between pseudo-graphitic layers
has been evidenced, leading to a staging phenomenon at the nanometer scale. A structural model is proposed and the
intercalation chemistry of multiwalled carbon nanotubes is discussed.
q 2004 Elsevier Ltd. All rights reserved.
PACS: 61.10 Nz; 61.48 tc; 61.46 tw
Keywords: A. Carbon nanotubes; A. Inorganic fluorides; B. Intercalation
1. Introduction
ordered in a close-packed lattice of cylinders, i.e. the so-
called bundle of single-wall nanotubes. The occupation of
the corresponding intertubular van der Waals gap by
different kinds of guest-species is also well documented
[12–20]. The last and more conventional situation encoun-
tered involves the available interlayer space inherent to the
regular stacking of concentric graphene layers constituting a
multiwalled carbon nanotube (MWNT). Though the oppor-
tunity of accommodation of intercalated species at this level
of the structure turns out to be a pertinent concept, few
attention has been paid to the latter and much work remains
to be done in this field. Hence, only the chemical doping of
MWNTs with some alkali metals [21–28] and a few
transition metal halides [22,23,29] seems to have been
reported, so far. While most of these earlier studies [24–28]
focused on the electrochemical energy storage for a
potential use of carbon nanotubes in Li-ion batteries, a
detailed description of the lattice properties at the atomic
level of the MWNT-based intercalation compounds has
never really been explicit. However, some aspects inherent
to the effect of an intercalation/de-intercalation process on
the overall morphology of a MWNT have already been
depicted [21–24].
Much work has been devoted to the properly speaking
chemistry of carbon nanotubes, including their synthesis,
purification and chemical reactivity with various com-
pounds. Thus, the covalent chemistry of carbon nanotubes
parallels the functionalization of graphite and halogenated
[1,2], oxygenated [3,4] or alkylated [5] derivatives, have
been obtained. The analogy between the chemical properties
of carbon nanotubes and those of the latter well-known
allotropic form of carbon also extends to intercalation
chemistry, within a somehow more general frame, due to the
particular dimensionality of the host–lattice in this case.
Thus, many examples of a reversible filling of the central
channel by capillarity have been reported in the literature
[6–11]. The illustration can be further extended by
considering the special case of some particular carbon
nanotubes, consisting in one single rolled up graphitic layer,
*
Corresponding author. Tel.: þ33-473-407-647; fax: þ33-473-
407-108.
E-mail address: claves@chimtp.univ-bpclermont.fr (D.
Claves).
0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2004.01.025

2
D. Claves et al. / Solid State Communications 130 (2004) 1–5
We will therein present results concerning the reactivity 3. Results
A batch of MWNTs was synthesized and further purified
of WF6 toward MWNTs. The latter inorganic fluoride, well
known to readily intercalate graphite as an electron
acceptor, may be the source of a prototypical p-type
doped carbon nanostructure. We will show that the
structural order in the new material obtained can be
described in terms of a ‘nanostaging’ phenomenon.
as described in Section 2. A structural study being facilitated
by the presence of an appreciable coherence length in the
material, the average final number of graphene layers was,
therefore, modulated in this sense.
The TEM observations (Fig. 1(a)–(c)) confirm the quasi
total removal of residual alumina and cobalt particles after
the purification process and the effective opening of a
significant part of the initially end-capped nanotubes. The
inner diameter distribution of the MWNTs remains narrow
and the number of layers seems to be in the vicinity of 15 for
most of the tubes (Fig. 1).
2. Experimental
2.1. Synthesis and purification of the MWNTs
The distribution of interlayer spacing, obtained from the
position of the main line in the diffraction pattern (Fig. 2),
which is equivalent to the (002) reflection of graphite, is
centred on 0.342 nm. The average coherence length,
extrapolated from the FWHM of such a Bragg peak, in
which the finite-size effect contribution to the global profile
is supposed to be largely predominant, corresponds to 15
successive stacked layers, in agreement with the HRTEM
observations. Though X-ray diffraction also shows that the
purification technique employed is suitable to obtain
MWNTs with correct purity, the persistence of some small
amplitude signals arising from residual Co catalytic
particles remains visible. In the conditions used, the average
carbon thickness at the tips appears then sufficient to partly
resist the preliminary oxidative treatment in air and, thereby,
hinders a complete opening of all tubes. Some of the metal
particles, located at the extremities and still embedded in a
carbon coating, remain isolated from the subsequent acid
wash and are not eliminated. However, a complementary
TGA up to 600 8C in air, during which only carbon is
consumed, shows that the relative amount of carbon is more
than 99% after purification and in regard to the forthcoming
structural investigation, such impurity traces are not
bothersome.
MWNTs were prepared via a catalytic method, pre-
viously described in Ref. [30] and involving the dispro-
portionation, at 510 8C, of carbon monoxide (Boudouard
reaction) over alumina-supported Co catalyst particles.
In order to avoid the formation of oxygenated addends at
the outer surface, a two-step purification method was
employed, involving relatively soft conditions. An oxidation
treatment in air at moderated temperature was first applied
(450 8C, 20 min), followed by a chemical treatment with a
concentrated HF–HCl mixture (50/50% in volume). The
temperature of 450 8C used in the first step was optimized by
thermogravimetric analysis (TGA) and corresponds to the
starting point of decomposition in air of our MWNTs.
2.2. WF6 elaboration and intercalation conditions
Tungsten hexafluoride was prepared in a first reactor by
direct reaction of a pure fluorine atmosphere with the metal,
heated at 300 8C. In order to increase the reaction rate, the
MWNTs were put in a secondary reactor, cooled at 220 8C
with a salt–ice bath and connected to the main reactor, so
that the gaseous fluoride condense at this level. The
container was then isolated and placed at room temperature
for 24 h.
After exposure of the MWNTs to the gaseous reagent
vapours, according to the aforementioned experimental
procedure, the nominal composition of the reaction product,
determined by weight uptake (within ^0.5 mg), is
2.3. Characterization
C
36^3WF6. It can consequently be noticed that the
accommodation of fluoride into the carbon matrix remains
low, compared to graphite powder in similar conditions for
which a ratio C=W ¼ 18 can be easily reached [32].
The starting MWNTs were characterized by trans-
mission electron microscopy (TEM) with a Jeol microscope
operating at 200 kV
The relevant diffraction pattern (Fig. 2) exhibits a single
dominant Bragg peak, characteristic of some raw MWNTs
or of longitudinal sections remaining unaffected, and apart
from the emergence of a broad shoulder beside this initial
main line, the diffraction data are not sensibly modified. The
latter additional feature observed has been interpreted in
terms of a novel ‘long range’ order in the material, related to
an effective intercalation process. Since a precise determi-
nation of each angular position turns out to be delicate, due
to the poorly resolved signals, the structural analysis of the
TGA were performed in air on raw and purified MWNTs
using a Shimadzu TGA-50, with a heating rate of 2 8C/min.
Samples of pristine MWNTs and of the MWNTs–WF6
material were conditioned in 0.5 mm diameter sealed
capillaries. Powder X-ray diffraction measurements were
performed using a Philips X’Pert Pro diffractometer,
working with the Cu Ka1 radiation. Diffraction patterns
were collected in the transmission mode with a position
sensitive detector.

D. Claves et al. / Solid State Communications 130 (2004) 1–5
3
Fig. 1. Transmission electron micrographs of purified MWNTs, produced by thermal decomposition of CO over Co catalyst, showing (a) some
individual tubes in the sample, (b) the layers organization in a tube and (c) the effective opening of some tubes and removal of cobalt particles at
the tips.
intercalated phase can consequently not be performed
without reference to a pre-existing model.
ing direction of the curved carbon layers. The intercalated
phase is assigned an arbitrary stage number N; with a
corresponding identity period along the previous axis, Ic ¼
Nd0 þ t; where d0 ¼ 0:342 nm is the distance between
adjacent shells in the pristine carbonaceous material and t is
the thickness of the intercalate. The latter can be given the
value 0.49 nm, by referring to the c-axis parameters reported
for homologous WF6-based GICs formed under similar
synthesis conditions [32]. The orientation of the intercalated
species should be suitable with the realization of the most
compact atomic arrangement and with the optimization of
the carbon–fluorine interactions, by putting as many
The diffraction pattern has then tentatively been analysed
from the hypothesis stipulating the presence, beside the raw
material, of a major stage number associated with the
intercalated part of the sample and on the basis of an analogy
with the reflections issuing from the graphite–fluorides
intercalation compounds (GICs) [31–35], among which
(001) lines are largely prominent as a signature of the
lamellar character of these phases. Thus, the structurally
constrained model used here considers a one-dimensional
statistically disordered atomic distribution along the stack-

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D. Claves et al. / Solid State Communications 130 (2004) 1–5
4. Discussion
The admixture of unreacted MWNTs in the sample or the
presence of non-intercalated zones along the tube axis gives
evidence, since the global experimental composition and the
refined composition of the intercalated phase are similar,
that part of the fluorinated molecules are involved in
competing fixation modes to the carbon substrate. Adsorp-
tion at the outer surface, condensation in the central channel
or trapping at local defects sites (edges) are, therefore,
highly probable and cannot be evidenced by XRD.
The FWHM of the broad X-ray signature of the
intercalated domain, determined from the previous refine-
ment, allows to specify that the size of the latter is
equivalent to one single unit cell in thickness (,2 nm)
and that consequently, the staging sequence observed is in
no way a bulk property of the system but remains a surface
phenomenon. The former conclusion seems to be indirectly
confirmed by the HRTEM observations (Fig. 1(b)) which
show discontinuous carbon shells near the outer surface of
the tubes. Indeed, the presence of such wall disjunctions
turns out to be necessary for an intercalation mechanism to
be possible, since the increase in the carbon layers
separation which results could not occur from an organiz-
ation of nested closed cylinders. This fact supports the
limitation of fluoride intercalation near the surface only,
where such pre-existing defects are present.
Fig. 2. X-ray diffraction patterns of the pristine and intercalated
MWNTs ( p denote reflections from some residual Co particles).
fluorine atoms as possible in contact with the carbon sheets.
p
Thus, for an hexacoordinated atom t ¼ 2L= 3 þ 2RF; where
L ¼ 0:1832 nm is the W–F bond length [36] and RF
represents an ad-hoc radius for the fluorine atoms satisfying
the above relation and accounting for some possible
distortions from a regular octahedron. The resulting average
atomic coordinates used are detailed in inset on Fig. 3.
The angular range explored has been restricted to the
only zone where significant modifications are observed in
the pattern and extends over 2u ¼ 10–408; which allows to
consider a constant peak width for the deconvolution. At this
stage, a standard least-squares fit procedure can then be
applied to refine the peak shape parameters and the C/WF6
ratio. The best fit has been obtained for a stage 4 (supposed
largely predominant) model and a related composition
C34WF6. The satisfying matching of the corresponding
extracted intensities to experimental data is shown in Fig. 3.
The aforementioned mechanistic approach of insertion
phenomena in such a medium is not without importance in
regard to further elaboration of CNT-based intercalation
compounds. The relative abundance of local defects
observed in the pristine MWNTs used in this work outlines
the advantage of the catalytic synthesis process retained,
which manifestly favors the formation of the necessary
structural disorder in the stacking of the curved graphene
sheets. Catalytically grown MWNTs elaborated at low
temperature appear, therefore, as privileged precursors to be
exploited in the formation of low dimensional intercalation
compounds of and of their subsequent derivatives.
A last comment may be added concerning the effective
global morphology of the final intercalated product, in
relation with the persistence of the raw CNT signal in the
diffraction pattern. The direct observation of an intercalated
tube is extensively difficult here, owing to rapid de-
intercalation of the volatile halide in a vacuum chamber.
Somehow, rather than the unlikely presence in the sample of
tubes either selectively non-intercalated or completely
intercalated over their whole length, and complying with
earlier microscopy observations [22–24] of a heterogeneous
swelling of doped MWNTs, it seems more advisable to infer
a successive alternation of intercalated and non-intercalated
sections along the longitudinal axis of each tube. An
organization of locally non-closed and superposed graphitic
fragments alternating with non-intercalable perfectly nested
shells seems to be the only configuration allowing to
Fig. 3. X-ray diffraction pattern in the low angle region and after
background subtraction of the MWNTs-WF6 sample. Dots are
experimental data and line is the refined profile obtained from a
stage 4 structural scheme with optimized parametrization. Inset
provides details on the fractional atomic coordinates system used
for the intercalated phase (all symbols refer to text).

D. Claves et al. / Solid State Communications 130 (2004) 1–5
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