Gogotsi Y. (Ed.) Nanotubes and Nanofibers
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ACKNOWLEDGMENT
This work was supported by the U.S. Department of Energy grant DE-FJ02-01ER45932.
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134 Nanotubes and Nanofibers
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4
Inorganic Nanotubes and
Fullerene-Like Materials of
Metal Dichalcogenide and
Related Layered Compounds
R. Tenne
Department of Materials and Interfaces,
Weizmann Institute, Rehovot, Israel
CONTENTS
4.1 Preface ..................................................................................................................................135
4.2 Synthesis of Inorganic Nanotubes .......................................................................................138
4.3 Inorganic Nanotubes and Fullerene-Like Structures Studied by
Computational Methods ......................................................................................................144
4.4 Study of the Properties of Inorganic Nanotubes in Relation to
Their Applications ...............................................................................................................148
4.5 Conclusions ..........................................................................................................................150
Acknowledgments .........................................................................................................................150
References ....................................................................................................................................150
4.1 PREFACE
The discovery of carbon fullerenes [1] and later carbon nanotubes [2] stimulated research on car-
bonaceous nanomaterials and also the search for nanotubes and fullerene-like structures from other
layered compounds. Numerous inorganic compounds possess layer (2-D) structure, including metal
dichalcogenides (sulfides, selenides, and tellurides), metal dihalides (chlorides, bromides, and
iodides), metal oxides, and numerous ternary or quaternary compounds, resembling thereby
graphite. The metal dichalcogenides, MX
2
(M = Mo, W, Nb, Ta, Hf, Ti, Zr, Re; X = S, Se) contain
a metal layer sandwiched between two chalcogen layers with the metal in a trigonal pyramidal or
octahedral coordination mode [3,4]. Analogous to graphite
(Figure 4.1a), weak van der Waals forces
are responsible for the stacking of the MX
2
layers along the c-axis (Figure 4.1b). The molecular
sheets of inorganic layered compounds consist of multiple layers of different atoms chemically
bonded together.
The stimulus for the formation of fullerenes and nanotubes can be understood on the basis of the
structure of the parent bulk compound, graphite, which consists of atomically flat carbon (graphene)
sheets. Here, each carbon atom is bonded to its three nearest-neighboring carbon atoms via sp
2
135
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hybridization, forming a hexagonal (honeycomb) network. The layers are stacked together via weak
van der Waals forces. Carbon atoms that are disposed on the rim of the sheet are only twofold-
bonded, each one thus having a dangling bond. When the graphene sheet size shrinks, the ratio of
rim/bulk atoms increases as 2
n
, where n is the number of cutting cycles of the sheet into smaller
pieces. The large increase in the ratio of rim/bulk atoms with shrinking size of the graphene sheet
makes the planar nanostructure unstable. This instability results in folding of the graphite layers and
the formation of carbon fullerenes and nanotubes, which are seamless structures. While the nan-
otubes involve winding of the graphene nanosheet in one axis, the fullerenes are obtained by folding
the nanosheet along two axes. Since the latter process requires much higher elastic energy, the fold-
ing is realized by disposing 12 pentagons into the hexagonal carbon network. It is important to real-
ize that folding of the graphene nanosheet involves overcoming an elastic energy activation barrier,
which is provided by heating or irradiation. The invested energy is more than compensated once the
fullerene is closed and the dangling bonds at the periphery of the nanosheet disappear.
In analogy to that, one notes that the chalcogen atoms of MX
2
compounds on the basal (0 0 0 1)
plane are fully bonded and therefore nonreactive. In contrast the metal and chalcogen atoms on the
rim of the nanosheet are not fully bonded. While the metal atom within the MX
2
layer is sixfold-
coordinated to the nearest chalcogen atoms, it is only fourfold-coordinated at the edge of the MS
2
nanocluster. Similarly, the chalcogen atom in the rim is only twofold-bonded to the metal atoms
instead of being threefold-coordinated as in the bulk (Figure 4.1b). Therefore, planar nanosheets of
the dichalcogenide compounds become unstable, forming structures reminiscent of nested carbon
fullerenes and nanotubes
(Figure 4.2), referred hereafter as inorganic fullerene-like (IF) structures
136 Nanotubes and Nanofibers
(a) Bulk
Rim
S bulk
Mo bulk
S rim
Mo rim
(b)
FIGURE 4.1 (a) Schematic structure of graphite. (b) Schematic presentation of MoS
2
layer with both bulk
and rim atoms delineated.
Copyright 2006 by Taylor & Francis Group, LLC
[5,6]. Generally, the fullerene-like structures are less regular in their shape than the respective nan-
otubes and contain many defects (see below). Other related structures are nanoscrolls, which are
obtained by winding molecular sheets several times around their axial direction. In the energy land-
scape, nanoscrolls can be considered to be an intermediate stage between a perfectly crystalline the
stable nanotubular structure and the unstable flat nanosheets. Nanoscrolls are preferably produced by
“chemie douce” (low temperature) processes, like sol–gel, intercalation–exfoliation, hydrothermal
synthesis, etc., in which the thermal energy is not sufficiently large to induce growth of the ultimately
more stable nanotubular structure.
Over the past few years, considerable progress has been achieved in the synthesis of nanotubes
from layered metal dichalcogenides and several other layered compounds. Nanotubes of various
layered transition metal compounds, including halides like NiCl
2
[7], and various layered-type
oxides like V
2
O
5
[8,9] and H
2
Ti
3
O
7
[10], have been synthesized using different methodologies. The
advent of the synthesis, structural characterization, properties, and applications of inorganic nan-
otubes have been covered by a number of recent review articles, see, e.g., [11,12].
Perhaps, the most well-known example of an early tube-like structure with diameters in the
submicrometer range is formed by the asbestos minerals, like chrysotile, kaolinite, etc. The folding of
kaolinite sheet was studied by Pauling [13], and was attributed to an asymmetry of the layered struc-
ture, which consists of fused silica tetrahedra and alumina octahedra (
Figure 4.3). The larger alumina
octahedra occupy the outer face of the scrolling sheet, whereas the smaller silica tetrahdra are found
Inorganic Nanotubes and Fullerene-Like Materials of Metal Dichalcogenide 137
FIGURE 4.2 TEM image of a WS
2
nanotube.
Copyright 2006 by Taylor & Francis Group, LLC
at the inner face of the asbestos sheet. More recently, a systematic effort to synthesize “misfit” com-
pounds, in which cubic crystals like PbS are intercalated between layers of, e.g., NbS
2
, forming
thereby an artificial superlattice, is well documented [14]. Owing to lattice mismatch between the two
compounds, an asymmetry arises in the “misfit” lattice, which induces folding of the layers and the
formation of tubular structures [15]. The synthesis of mesoporous silica with well-defined nanopores
in the range 2 to 20 nm was reported by Beck and Kresge and their coworkers [16]. The syntheses
strategy involved the self-assembly of liquid crystalline templates. The pore size in zeolitic and other
inorganic porous solids is varied by a suitable choice of the template. In contrast with the IF phases,
which consist of isolated nanoparticles, mesoporous materials form an interconnected and extended
lattice.
Using various templates, recent efforts have resulted in the synthesis of nanotubes from isotropic
3-D (nonlayered) compounds also, further widening the scope of inorganic nanotubes. Even folding of
sheets of 3-D compounds, like silica or AlN into nanotues with below 100 nm diameter requires very
large amounts of elastic energy and is therefore prohibitive. Even higher elastic energy is required for
the formation of quasi-spherical polyhedral (fullerene-like) nanostructures from 3-D compounds. One
way for a 3-D compound to overcome this energy barrier is to introduce grain boundaries and disloca-
tions, leading thereby to polycrystalline nanotubes. Nanotubes of 3-D compounds can be obtained
through a templated growth using an artificial scaffold. Earlier, amorphous silica nanotubes were
obtained by hydrolysis of tetraethylorthosilicate (TEOS) in a mixture of water, ammonia, ethanol, and
tartaric acid [17,18]. Later on, polycrystalline silica, alumina, and vanadia nanotubes were obtained by
using carbon nanotubes as a template and subsequent calcination of the product at elevated tempera-
tures [19]. Recently, however, highly faceted and crystalline nanotubes of isotropic (3-D) compounds
were obtained by growth along an easy axis, as demonstrated for AlN [20]
(Figure 4.4) and for micro-
tubules of Ga
2
O
3
(Figure 4.5) [21]. Here, the c-axis
具
0 0 0 1
典
serves as the fast growth axis for this
wurzite-type lattice, with (1 0 1 0) and (1 1 2
苶
0) faces making the facets of the tubes. The atomic struc-
ture of the edges between two such facets has not been studied in detail so far. Nanotubes of 3-D com-
pounds are inherently unstable and their surface is generally very reactive, a property which can be very
useful for catalyzing chemical reactions.
4.2 SYNTHESIS OF INORGANIC NANOTUBES
Even before IF materials were recognized as a universal phenomenon, tubular and fullerene-like
forms of layered compounds were reported. Perhaps the earliest among them were nanoscrolls of
MoS
2
obtained by reacting MoCl
4
with Li
2
S in tetrahydrofurane (THF) solution, and subsequent
annealing at 400°C [22]. In another study, NiS–MoS
2
core–shell structures, in which closed MoS
2
layers enfold NiS nanoparticles, were reported [23]. Over the past few years, a number of synthetic
strategies have been successfully used for the production of nanotubes from layered (2-D) as well
138 Nanotubes and Nanofibers
a-axis
= 105°
715 pm
b -axis
6(OH)
4Al
4O + 2(OH)
4Si
6O
Kaolinite
FIGURE 4.3 Schematic representation of the mineral kaolinite.
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Inorganic Nanotubes and Fullerene-Like Materials of Metal Dichalcogenide 139
FIGURE 4.4 SEM image of faceted AIN nanotubes. (Adapted from Wu, Q. et al., J. Am. Chem. Soc., 125,
10176, 2003.)
FIGURE 4.5 Faceted microtubules of Ga
2
O
3
. (Adapted from Sharma, S. and Sunkara, M.K., J. Am. Chem.
Soc., 124, 12288, 2002.) Note the clear hexagonal cross section of the tubes in both cases.
Copyright 2006 by Taylor & Francis Group, LLC
140 Nanotubes and Nanofibers
as nonlayered (3-D) compounds. Table 4.1 brings a concise summary of the various metal
dichalcogenide nanotubes that have been reported over the last few years, and the synthetic proce-
dures which have been used for their production. This summary shows the large versatility of the
chemical apparatus that has been developed for the synthesis of these nanotubes. It further suggests
that these nanostructures are thermodynamically stable, with the sole constraint that their diameter
is smaller than approximately 100 nm. Most commonly, the synthesis of metal dichalcogenide nan-
otubes is done at elevated temperatures. “Chemie douce” (low-temperature) processes like
hydrothermal synthesis, which are very useful for the synthesis of metal-oxide nanotubes, have not
been developed so far for chalcogenide nanotubes. Heating is not only important for accelerating
TABLE 4.1
Summary of the Inorganic Nanotubes Reported in the Literature and the Synthetic
Procedures Used for Their Production
Compound Synthetic Approach References Comments
MS
2
(M=W, Mo) Reaction with the respective oxide [5,24–26] ReS
2
is a 2-D
ReS
2
at elevated temperature compound with
Re-Re bonds
MS
2
(M=W, Mo) Chemical vapor transport (CVT) [27,28]
Single wall MoS
2
CVT(I
2
)+C
60
catalyst [29]
nanotubes
MoS
2
Ammonium thiometallate (ATM) solution [30,31] Not perfectly
in porous alumina template, reaction with H
2
S crystalline
MS
2
(M=Nb, Ta, Ti, Zr, Hf) Decomposition of MS
3
at elevated temperature [32,33]
MoS
2
Heating MoS
2
powder in a closed Mo crucible [34]
NbS
2
, ReS
2
Depositing NbCl
2
(ReCl
3
) from solution onto [35,36]
carbon nanotubes; firing in H
2
S at elevated
temperature
NbSe
2
Direct reaction of the elements at 800°C [37]
MoS
2
Hydrothermal reaction of ATM; [38] Nonperfect
crystallization from acetone; crystallinity
firing in H
2
S at 600°C
WS
2
Firing of ATM in thiophene/hydrogen [39]
at 360–450°C
TiS
2
CVT(I
2
) [40]
MoS
2
Firing of ATM in H
2
[41]
MoS
2
Firing of MoO
3
nanobelts in the [42]
presence of sulfur
WS
2
Hydrothermal synthesis with organic amines [43]
and a cationic surfactant and firing at 850°C
WS
2
Growth of WO
3⫺x
nanowhiskers; annealing in [44,45]
H
2
S/H
2
atmosphere at elevated temperature
NbS
2
/C Deposition of NbCl
4
on carbon nanotubes [46]
template/firing in H
2
S
MX
2
(M=Mo, Nb; Electron beam irradiation of MX
2
powder [47–50]
X⫽S, Se, Te)
MS
2
Arc discharge of submerged electrodes [51]
MoS
2
, WS
2
Microwave plasma [52]
SnS
2
/SnS (misfit) Laser ablation of SnS
2
[54]
InS Reacting t-Bu
3
In with H
2
S in aprotic [88] Metastable phase
solvent at 203°C in the presence of
benzenethiol
Copyright 2006 by Taylor & Francis Group, LLC
the reaction kinetics, but also to endow sufficient thermal energy to the nanoclusters. This thermal
energy induces structural fluctuations in the lamella, leading to folding of the planar nanosheet into
a crystalline nanotubular structure. Obviously, ionizing or nonionizing irradiation can serve as a
source of energy to elicit the chemical reaction and the structural transformation [47–49]. This
effect was first recognized in the electron beam synthesis of fullerene-like MoS
2
nanoparticles [50].
However, such a procedure is not likely to become a useful way to grow macroscopic amounts of
inorganic nanotubes. Arc-discharge of MoS
2
electrode submerged in water led to the synthesis of
fullerene-like nanoparticles of this compound [51]. Microwave plasma has also been used to syn-
thesize fullerene-like nanoparticles of WS
2
and MoS
2
[52]. Sonochemical reaction has been used to
synthesize nanoscrolls of the layered compound GaO(OH) [53].
Laser ablation has been successfully used for the growth of carbon nanotubes and nanowires of
Si, GaAs, and similar compounds. Recently, very short nanotubes and nested fullerne-like nanopar-
ticles of “misfit” compound SnS
2
/SnS have been obtained by laser ablation of an SnS
2
pellet [54].
Here an ordered superstructure with one or a pair of SnS layers sheathed between two SnS
2
layers
was observed. The formation of these unusual nanostructures can be attributed to the volatility of
sulfur during the laser ablation process. Unfortunately, the “misfit” nanotubes are rather short
(<1 µm) and cannot be produced in large amounts. Laser ablation of MoS
2
and WS
2
targets yielded
nanoparticles with fullerene-like structure [55,56]. Growth of NiCl
2
nanotubes by a laser ablation
process was demonstrated [57]. The nanotubes were shown to grow in a vapor–liquid–solid (VLS)
mechanism. However, little control over the growth mode could be established, and the yield of the
nanotubes was extremely low.
Hexagonal boron nitride (BN) crystallizes in a graphite-like structure. Here, the iso-electronic
B–N pair replaces a C–C pair in the graphene sheet. Partial or entire replacement of the C–C pairs by
the B–N pairs in the hexagonal network of graphite leads to the formation of a wide array of two-
dimensional phases, like BC
2
N. BN tubes with turbostratic structure were reported by Hamilton et al.
[58]. The first synthesis of crytalline BN nanotubes was reported shortly afterwards [59]. BN nanotube
phases have been produced by several synthetic approaches [60–62]. Double-wall BN nanotubes were
obtained by plasma arc reaction using boron anodes impregnated with nickel and cobalt catalysts [63].
Likewise, BC
x
N nanotubes were synthesized by reacting aligned CN
x
nanotubes with B
2
O
3
in the pres-
ence of CuO in N
2
atmosphere and at 1800°C [64]. Sheated carbon/BN nanotubes were synthesized
as well [65]. Here a graphite cathode was arced with an HfB
2
anode in a nitrogen atmosphere. In this
configuration, the three constituents have different sources: the anode for boron, the cathode for car-
bon, and the chamber atmosphere for nitrogen. In addition, hafnium present in the plasma may be act-
ing as a catalyst. Reactive magnetron sputtering of graphite target in the presence of nitrogen
atmosphere led to the synthesis of carbon fullerenes with 20% N content. The aza-fullerene C
48
N
12
(Figure 4.6) is considered to be the most stable cluster belonging to the CN
x
family of compounds [66].
Each of the 12 pentagons is decorated by a single nitrogen atom.
In summary, the plurality of methods used to synthesize B
x
C
y
N
z
nanotubes suggests that these
nanostructures, which are derived from macroscopically stable 2-D compounds are themselves
thermodynamically stable phasses, given the sole constraint that their diameter is smaller than
approximately 0.1 µm.
Systematic attempts to grow nanotubular structures from quasi-isotropic (3-D) oxide and
chalcogenide compounds have been undertaken [19,67]. By coating carbon nanotubes (CNTs) with
the respective metal-oxide gels and subsequently burning the carbon, polycrystalline nanotubes and
nanowires of a variety of metal oxides, including ZrO
2
, SiO
2
, and MoO
3
have been obtained [19].
Using a surfactant (Triton 100-X) as scaffold at low temperatures, polycrystalline CdSe and CdS
nanotubes were obtained after firing at 300 to 400°C [67].
The synthesis of numerous kinds of oxide nanotubes through “chemie douce” processes have
been recently documented. Sol–gel synthesis of oxide nanotubes is also possible in the pores of alu-
mina membranes [68]. An interesting case is that of the titania nanotubes, which are hypothesized to
exhibit excellent photocatalytic and photoelectrochemical properties for self-cleaning surfaces or
Inorganic Nanotubes and Fullerene-Like Materials of Metal Dichalcogenide 141
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