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VS2 Nanotubes Containing Organic-Amine Templates from the NT-VOx Precursors and Reversible Copper Intercalation in NT-VS2.

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Communications
Nanotubes
VS2 Nanotubes Containing Organic-Amine
Templates from the NT-VOx Precursors and
Reversible Copper Intercalation in NT-VS2**
Helen Annal Therese, Frank Rocker, Andreas Reiber,
Jixue Li, Michael Stepputat, Gunnar Glasser, Ute Kolb,
and Wolfgang Tremel*
Dedicated to Professor Philipp Gtlich
on the occasion of his 70th birthday
Soon after the discovery of carbon nanotubes[1] the layered
chalcogenides WS2 and MoS2 were reported as the first
inorganic analogues of these hollow-cage structures.[2] Subsequently, more fullerene-type structures of inorganic compounds have been made, among them BN,[3] V2O5,[4] Tl2O[5] as
well as layered metal chalcogenides, such as TiS2,[6] NbSe2,[7]
or ReS2.[8] A common feature of most cage-forming compounds are their layered 2D-structures, which are able to
bend or curl and to avoid vacant bonding sites through the
formation of intralayer linkages. Meanwhile a number of
methods for the synthesis of chalcogenide nanotubes have
been devised. The most successful among them are the
reductive sulfidization of oxide nanoparticles using H2S,[9] the
reduction of trichalcogenides with H2,[7] chemical transport
with iodine,[10] and the sulfidization of reactive metal halide
precursors on a carbon nanotube template.[11] A conversion of
preformed oxide nanotubes into their respective sulfide
counterparts with preservation of the tubelike structures,
however, has not been observed to date. Herein we report the
first synthesis of chalcogenide VS2 nanotubes with intercalated organic molecules by the sulfidization of a tubelike VOx
precursor. The VS2 nanotubes are remarkable, as bulk VS2[12]
(and the VSe2 analogue[13]) can only be obtained as AxVS2
(A = alkali metal, Cu) or self-intercalated non-stoichiometric
(metal-excess V1+xS2) compounds.[14]
The starting material for the synthesis of the VS2 nanotubes (NT-VS2) were vanadium oxide nanotubes (NT-VOx,
x 2.3), their synthesis is given in the Experimental Section.
A representative low-resolution transmission electron micro[*] Dr. H. A. Therese, F. Rocker, A. Reiber, Prof. Dr. W. Tremel
Institut fr Anorganische Chemie und Analytische Chemie
Johannes Gutenberg-Universitt
Duesbergweg 10–14, 55099 Mainz (Germany)
Fax: (+ 49) 6131-39-25605
E-mail: tremel@mail.uni-mainz.de
Dr. J. Li, M. Stepputat, Dr. U. Kolb
Institut fr Physikalische Chemie
Johannes Gutenberg-Universitt
Welderweg 11, 55099 Mainz (Germany)
G. Glasser
Max Planck-Institut fr Polymerforschung
Ackermannweg 10, 55128 Mainz (Germany)
[**] This work was supported by the Bundesministerium fr Bildung und
Forschung (BMBF) and by the Deutsche Forschungsgemeinschaft
(DFG). NT = nanotube.
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200461326
Angew. Chem. Int. Ed. 2005, 44, 262 –265
Angewandte
Chemie
scopic (TEM) image of NT-VOx is given in Figure 1 a. It shows
that the NT-VOx is the main product; the tubes are 0.5–5 mm
long and have an outer diameter between 50 and 150 nm.
Upon sulfidization of NT-VOx with H2S at 225 8C, VS2
Figure 1. a) Low-resolution overview TEM image of VOx nanotubes
used as a starting material for the synthesis of NT-VS2. b) Overview
SEM image of NT-VS2 obtained after sulfidization of NT-VOx.
c) HRTEM image of a single VS2 nanotube with a layer separation of
2.8 nm, a flat cap, and a partially crystalline or amorphous coating.
Imperfections are marked by a circle. d) HRTEM image of a NT-VS2
obtained from NT-VOx intercalated with C12-amine. The layer separation is 1.6 nm.
nanotubes (NT-VS2) are formed. A scanning electron micrograph (SEM) of a typical NT-VS2 obtained after sulfidization
(Figure 1 b), shows that the structure and the morphology of
the VOx tubes are preserved during the oxide into sulfide
conversion. The SEM micrograph also indicates the high yield
of NT-VS2 obtained by this method. Full conversion into NTVS2 was observed for NT-VOx intercalated with dodecyl
amine (C12-amine) but not with hexadecyl amine (C16-amine).
Reaction temperatures higher than 250 8C lead to a total
collapse of the VS2 tube structure.
That NT-VOx has been sulfidized to produce NT-VS2 is
evidenced by the high-resolution TEM images (HRTEM) in
Figure 1 c. All the NT-VS2 tubes are hollow, most of them are
open, around 20 % have flat caps, which make a 908
connection with the outer wall of the cylinder. The flat caps
give an additional indication that structure of the NT-VOx
precursor is preserved during the sulfidization. Although the
inner VS2 layers of the tubes are smooth and almost free of
defects, the outermost VS2 layer is partially incomplete
(Figure 1 c), and each VS2 tube is surrounded by a coating
of amorphous VS2. In addition, the outermost VS2 layer
exhibits a number of defects, this could be due to an
incomplete reconstruction of the outer shell of the oxide
precursor during the sulfidization step.
Angew. Chem. Int. Ed. 2005, 44, 262 –265
Figure 1 c and d show HRTEM images of a partially and a
fully sulfidated VS2 nanotube with lattice fringes corresponding to a layer separation of approximately 2.8 nm (partially
sulfidated) and 1.6 nm (fully sulfidated), values which are
significantly larger than the layer separation in bulk VS2
(0.57 nm). The observed layer spacing corresponds to the
separation of the VOx layers in the starting material, which in
turn was determined by the chain length of the template
molecules used in the synthesis of NT-VOx. This result
indicates that the template layer is partially intact in the VS2
product, a hypothesis which is supported by the IR spectra of
NT-VOx before and after sulfidization (Figure 2). The IR
Figure 2. The FT-IR spectra of NT-VOx samples before and after sulfidization. a) NT-VOx with C16-amines and b) NT-VOx with C12-amines,
before conversion, c) NT-VS2 with C16-amines and d) NT-VS2 with C12amines, after conversion.
spectra of alkylamine-templated NT-VOx, given in Figure 2 a
(C16-amine) and Figure 2 b (C12-amine), show strong vibrational bands characteristic of alkyl CC stretching mode at
2920 cm1 and 2850 cm1. The vibrational bands characteristic
of VO bond appear at 991, 791, 721, 575, and 480 cm1. The
bands corresponding to V=O stretching and V-O-V deformation modes are shifted to lower frequencies compared to the
crystalline V2O5.[15, 16] A new band at 721 cm1, which can be
attributed to VNH2 vibration, indicates that the amine
groups of the templates are bound to the vanadium atoms in
NT-VOx. Chemical analysis of the NT-VS2 samples show that
the C16-amine intercalated NT-VOx does not convert fully, as
also indicated by the characteristic broad VO band at
990 cm1 in the IR spectrum of the corresponding NT-VS2.
The products also tend to retain part of the intercalated
templates, which can be deduced from the vibrational
absorption bands at 2920 cm1 and 2850 cm1 (Figure 2 c).
On the other hand, complete sulfidization was achieved when
C12-amine templated NT-VOx was reduced, as indicated by
the absence of all VO bands from the IR spectrum, while
retaining part of the templates (see Figure 2 d). Furthermore,
the results of elemental analyses indicate a partial thermolysis
(NH3 loss) of the template during the sulfidization step.
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
To ascertain whether the VOx !VS2 conversion was
complete, we performed additional energy dispersive X-ray
(EDX) analyses of the products. Figure 3 a shows the EDX
spectrum of a partially sulfidized product (C16-amine tem-
Figure 4. Plot of discharge/charge potential (U) versus the amount of
electrons in mol (n(e))intercalated per formula weight of VS2 (Mr)
during copper-intercalation studies.
Figure 3. EDX spectra of isolated hollow VS2 tubes. The spectra show
signals arising from V, O, and S atoms. Incomplete reaction of C16amine-templated nanotube is indicated by the much greater intensity
of the peak arising from oxygen in (a) than that in the spectrum of the
VS2 nanotube shown in (b) in which the C12-amine template was used.
The peak marked by * is due to the detector.
plate). The EDX analyses show high sulfur content for the
outermost regions of the tube, whereas significant oxygen
content is found in the interior. This result demonstrates that
the VOx nanotubes are converted layer by layer to NT-VS2.
Figure 3 b shows the EDX spectrum of a fully sulfidized VS2
nanotube (C12-amine). The V:S ratio of 1:2 (accuracy 1 %)
is almost constant over the wall cross section, independent of
the beam position. The results indicated that for C12-amineintercalated NT-VOx the conversion is almost complete
whereas for C16-amine-intercalated NT-VOx the sulfidization
is incomplete.
Studies on electrochemical intercalation of copper into a
partially sulfidized NT-VS2 have revealed that the intercalation of copper up to a composition Cu0.77VS2 can be achieved.
NT-VOx intercalates alkali, alkaline-earth, and a number of
transition metals.[17] Studies on copper intercalation of NTVOx by chemically methods, through exchanging the intercalated templates with copper ions in a liquid medium, show
that a small fraction (0.08 mole) of copper could be intercalated, and that the exchange resulted in considerable changes
in the structure and the morphology of the starting material.[18] In contrast, no electrochemical copper intercalation
was possible in NT-VOx. The discharge/charge voltage profile
of copper intercalation into 4.4 mg of NT-VS2 is shown in
Figure 4. The voltage of the working electrode dropped
gradually with the insertion of copper until a limiting
composition, Cu0.77VS2, is reached. This result corresponds
to a total capacity of 360 mA h g1. The copper is reversibly
deintercalated in the subsequent oxidation to regain a
composition of VS2. The cycle can be repeated, although
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
there is a small degree of irreversibility. The capacity of the
reversible copper intercalation corresponds to a specific
capacity of 314 mA h g1, in comparison, graphite has a
theoretical capacity of 372 mA h g1.[19]
The reason for the easy copper intercalation and high
capacity value of NT-VS2 in comparison with the other noncarbon nanotubes could be due to the defects that form
during the conversion of NT-VOx into NT-VS2. This situation
is in line with reports of chemically etched single-walled
nanotubes (SWNTs)[20] and ball-milled MoS2 nanotubes[21]
showing better storage capacity than their untreated (unactivated) starting counterparts. The etching and ball-milling
processes greatly increase the number of defects in the
nanotube structures. In contrast, chemical intercalation of
silver and gold was not possible with NT-VS2, whereas VOx
nanotubes could easily be filled with gold colloids and were
always heterogeneous, irrespective of their diameter and
length. The gold colloids could chemically be connected to
linear chains with the aid of dithiols and mechanically
recovered from the VOx tubes.
In summary, we have demonstrated that oxide nanotubes
containing template molecules intercalated between the
layers can be converted into sulfide nanotubes under
retention of the tubelike structure. The amine-intercalated
NT-VS2 is the first layered chalcogenide nanotube in which
organic molecules are inserted between the layers. In
addition, we have demonstrated the reversible copper intercalation in these chalcogenide nanotubes.
Experimental Section
Vanadium oxide nanotubes (NT-VOx) were prepared from vanadium(v) alkoxides and primary monoamines (CnH2n+1NH2 ; with n =
12 or 16) by a sol–gel reaction[4] and subsequent hydrothermal
treatment. After washing and drying, the NT-VOx was subjected to
H2S treatment in a tubular furnace at 225 8C for 22 h.
The resulting products were characterized by X-ray powder
diffraction in reflection geometry, (Siemens D8 powder diffractometer, CuKa radiation), scanning electron microscopy (LEO 1530 fieldemission SEM, 6 kV extraction voltage), and by high-resolution
transmission electron microscopy (Philips TECNAIF F30 electron
microscope; field-emission gun, 300 kV extraction voltage) equipped
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Angew. Chem. Int. Ed. 2005, 44, 262 –265
Angewandte
Chemie
with a high-angle annular dark-field detector (HAADF), a Gatan
imaging filter (GIF), and an energy dispersive X-ray analysis (EDX)
system. IR studies of the samples were carried out using a Mattson
Instruments 2030 Galaxy FT-IR spectrometer. Elemental analysis
was performed with an Elemental Analyser Vario EL (Elementar
Analysensyteme). The X-ray powder patterns are almost featureless,
which indicates a low degree of long-range order. The elemental
analysis of NT-VS2 revealed that only a fraction (~ 25 %) of the
template is preserved.
VS2 nanotubes were electrochemically intercalated with copper.
For intercalation studies 4–7 mgs of the NT-VS2 sample was ground
well with Teflon powder, pressed between a platinum mesh and used
as a working electrode. A copper ring and a copper wire were used as
counter and reference electrodes. The cell was discharged and
charged in galvanostatic mode, in an aqueous electrolyte under air
(1 mol L1 CuSO4) and at a current density jc = 50 mA cm2.
[19] B. Gao, A. Kleinhammes, X. P. Tang, C. Bower, L. Fleming, Y.
Wu, O. Zhou, Chem. Phys. Lett. 1999, 307, 153.
[20] H. Shimoda, B. Gao, X. P. Tang, A. Kleinhammes, L. Fleming, Y.
Wu, O. Zhou, Phys. Rev. Lett. 2002, 88, 015 502.
[21] J. Chen, N. Kuriyama, H. Yuan, H. T. Takeshita, T. Sakai, J. Am.
Chem. Soc. 2001, 123, 11 813.
Received: July 16, 2004
Revised: October 6, 2004
.
Keywords: nanotubes · sol–gel processes · template synthesis ·
vanadium
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