INORGANIC NANOTUBES
Carbon nanotubes (CNTs) rose to prominence during nanotubes (NTs) revolution and
had been recognized as material of importance across academic and industrial laboratories. With
this in mind, scientists who work with inorganic materials developed an approach to explore the
possibility of having nanotubes from other materials. Since the first report on the synthesis of
inorganic WS2 nanotubes in 1992, the numbers of articles on the successful growth of different
inorganic nanotubes (INTs) increased rapidly (1). After all, these developments broadened the
concept of hollow nanostructures beyond that of carbon deep into the realm of inorganic
chemistry.
Six families of inorganic nanotubes have been synthesized so far. The current list is as the
table below (2):
Inorganic Nanotubes Family
Metal chalcogenide
Metal oxide
Metal halogenous
Mixed-phase and metal-doped
Examples
MoS2, MoSe2, WS2, WSe2, NbS2, TaS2, ZrS2, HfS2, TiS2,
ZnS2, NiS2, CdSe, CdS
TiO2, ZnO, GaO/ZnO, VOx, W18O49, V2O5, Al2O3, In2O3,
Ga2O3, BaTiO3, PbTiO3, silicon oxide: SiO2, MoO3, RuO2,
rare earth oxides: (Er, Tm, Yb, Lu) oxide
NiCl2
Boron- and silicon-based
PbNbnS2n+1, Mo1-xWs2, WxMoyCzSz, Nb-WS2, WS2-carbon
NTs, Nb2-carbon NTs, Au-MoS2, Ag-WS2, Ag-MoS2,
Cu5.5FeS6.5
BN, BCN, Si
Metal nanotubes
Au, Co, Fe, Cu, Ni, Te, Bi
Metal chalcogenide nanotubes of MoS2 and WS2 are among the earliest development of
inorganic nanotubes. The recent progress represents the culmination of a concerted 16-year effort
to elucidate the growth mechanism of these nanotubes. Further progress may lead to the full
commercial scale production of the WS2 nanotubes. The process does not require catalyst, and
the precursors (tungsten oxide and H2S or sulfur) are relative inexpensive. Therefore, the
moderate cost of such nanotubes may afford numerous of applications like nanocomposites in
the aerospace industry.
SYNTHESIS
General Synthesis Strategies
The most important methods for growing inorganic nanotubes can be divided broadly into six
basic steps; which are sulfurization, decomposition of precursor crystals, template growth,
precursor-assisted pyrolysis, misfit rolling, direct synthesis from vapor phase. Some nanotubes
can only be grown by combination of several processes. The table below show the growth
methods for some inorganic nanotubes.
Table 2 General synthesis methods for inorganic nanotubes (2)
Synthesis of Metal Calchogenices Nanotubes
Metal chalcogenide nanotubes of MoS2 and WS2 as the earliest development of inorganic
nanotubes are the good examples to explain the synthesis of this family. The preparation method
of MoS2 nanotubes employs the gas-phase reaction between MoO3 and H2S in the presence of
argon. The procedure involves heating solid MoO3 in a stream of forming gas (95% N2 + 5%
H2) to reduce the oxide to some extent, followed by the reaction of the oxide with a stream of
H2S mixed with the forming gas. The product contained nanotubes of MoS2 along with
polyhedral particles (3). Similar reactions were then carried out with ammonium thiotungstate to
obtain WS2 nanotubes.
Fig.1 Scanning electron microscopy (SEM) pictures of a thick mate of WS2 nanotubes (two magnifications)
prepared in the fluidized bed reactor. Figure courtesy of A. Zak. (4)
Synthesis of Metal Halides
Although the structure of layered metal dihalides compounds is not very different from their
metal dichalcogenide analogues, they are appreciably more ionic. The first nanotubes and
fullerene-like nanoparticles of NiCl2 were prepared by sublimation of a NiCl2 powder at 960 0C.
Unfortunately, this reaction does not produce the pure IF phase and NiCl2 platelets are present in
the ablated residue. Laser ablation of a NiCl2 target heated to 940 0C was recently used for the
synthesis of NiCl2 nanotubes, albeit in small quantities. CCl4 vapor was added to the reaction
zone in order to compensate for the loss of chlorine during the ablation process. The reaction was
found to go by the common vapor–liquid–solid (VLS) mechanism. More recently, CdCl2 and
CdI2 nanoparticles with a closed cage structure were obtained in situ through electron-beam
induced processes. Both kinds of nanoparticles were partially filled with cadmium in their core.
The halide deficiency in the ablated e-beam irradiated residue was ascribed to its high volatility.
Figure 2. HRTEM image of a NiCl2 nanotube. Inset: electron diffraction pattern of this nanotube (2).
Synthesis of Metal Oxide Nanotubes
This family of inorganic nanotubes has the most various methods of synthesis as almost each
metal oxide has a different method of synthesis. Prominent among the metal oxides are the multicrystalline titania nanotubes, TiO2, which are prepared by electrochemical anodization. Porous
alumina templates are specially useful for fabricating dense, uniform, aligned arrays of TiO2
nanotubes on substrates such as glass, silicon, and polymers. Free-standing porous alumina
templates have been employed for atomic layer deposition (ALD) of ordered TiO2 nanotube
arrays on various substrates. This appears to be an excellent method wherein anodic oxidation is
carried out in a dimethyl sulphoxide (DMSO) medium that contains hydrofluoric acid, potassium
fluoride, or ammonium fluoride as the electrolyte. On the other hand, most of the metal oxides
nanotubes, includes ZnO, CdO, Al2O3, SnO, Fe2O3 and MgO, nanotubes have been prepared by
Chemical Vapor Deposition (CVD) and thermal evaporation, as well as by hydrothermal and
solution methods (5). In comparison with other growth techniques, the advantage of the CVD
lies in the fact that the nanotubes grown by this technique contain an extremely low density of
structural defects (6).
Synthesis of Boron and Silicon Based Nanotubes
Several methods to synthesize Boron Nitride (BN) nanotubes including CVD and electrical
discharge, as well as templating have been available for many years. Thin BN tubes of less than
200nm diameter were first obtained by arc discharge with hollow tungsten electrodes filled with
h-BN powder. Following this initial report, a variety of methods have been employed to prepare
BN nanotubes. The other methods of synthesis of BN nanotubes include those that are far from
equilibrium, such as the electrical arc method, arcing between h-BN and Ta rods in a N2
atmosphere, laser ablation of h-BN, and continuous laser heating of BN. The last method
produces long ropes of BN nanotubes with thin walls (7).
SiC nanowires, SiC/SiO2 core–shell nanocables, and SiC nanotubes have been synthesized
simultaneously by directly heating Si powder and multiwall carbon nanotubes (MWCNTs).
While Silicon oxycarbide ceramic nanotubes can be obtained by the pyrolysis of polysilicone
nanotubes using a sacrificial AM as a template (8). Large-scale aligned silicon carbonitride
nanotube arrays have been synthesized by microwave-plasma-assisted CVD using SiH4, CH4,
and N2 as precursors. The nanotubes are 6–7mm in length and 100–200nm in diameter. More
recently, self-organized growth of smaller-diameter (_13 nm) SiNTs via hydrothermal synthesis,
using silicon monoxide (SiO) as the starting material (without the use of catalysts) has been
demonstrated. Based on a high-resolution TEM micrograph of the obtained nanotubes, the
authors suggested a multiwalled structure with an interlayer spacing of 0.31 nm, covered with a
thick oxide layer that can be removed by HF treatment (9).
Fig. 7 TEM image of a silicon nanotube grown from silicon monoxide.
PROPERTIES
Chemical Reactivity
INT materials are globally metastable and exist only in the nano regime. Eventually they are
expected to transform into the thermodynamically more stable bulk phase. Nonetheless, in
several cases these seamless structures demonstrate appreciable kinetic stabilization under a
harsh environment. In contrast, the platelets of the bulk material are vulnerable to penetration
and the reaction of water or oxygen from the prismatic edges into the galleries between the
layers.
Physical Properties
Fullerene-like and tubular nanostructures are of current interest, owing especially to their
specific and tunable physical properties, which are different from the properties of the
corresponding bulk structures. This subsection summarizes the characteristic optical, electrical,
mechanical, and thermal properties of these nanostructures.
2a. Optical and Electrical Properties
In contrast to carbon nanotubes, which can be metallic or semiconducting depending on
their chirality, inorganic nanotubes of bulk semiconductor materials, like BN, MoS2, WS2, were
found to be also semiconductors (insulators), independent of their chirality.
Fig. 5. The transmission electron diffraction pattern of electron scattered by both walls of a WS2 nanotube
revealing the main chirality of 6.51 and 131. The horizontal line denotes the direction of the nanotube axis (10).
Bulk BN material has an indirect band gap of 5.8 eV. This is to be contrasted with carbon
nanotubes, which are either metallic or semiconducting, depending on their (n,m) values.
Another point to be noted is that the strain in the nanotubes scales like 1/D2, where D is the
nanotube diameter. The strain effect is predominant for nanotubes with small diameters and
therefore overwhelmingly, the band gap of inorganic nanotubes was found to decrease with a
decreasing diameter of the (inorganic) nanotubes while the band gap of semiconducting carbon
nanotubes increases with a shrinking diameter of the cage. It should be furthermore emphasized
that generically, the bandgap of semiconducting nanoparticles increases with a decrease in the
particle diameter, which is attributed to the quantum size confinement of the electron wave
function. The existence of a direct gap in zigzag nanotubes is rather important, since it suggests
that such nanostructures may exhibit strong (electro) luminescence, which has not been observed
for the bulk material.
2b. Mechanical Properties
The mechanical properties of MS2 (M = Mo,W) nanotubes have been studied
experimentally as well as by theoretical means in detail. These properties are interesting not
merely for academic reasons but also because the inorganic nanotubes show substantial potential
for becoming part of ultrahigh-strength nanocomposite technology. Apparently the nanotube is
very flexible and does not break, even after many cycles axial compression. Tensile tests of
individual WS2 nanotubes within the SEM produced the full strain-stress curve of such
nanotubes. The nanotubes exhibited elastic behavior almost to the failure point. Repeating these
experiments many times provides statistically averaged meaningful values for the Young’s
modulus (E ), the strength, and the elongation to failure: 150 GPa, 16 GPa, and 12%,
respectively, with some 50% of the nanotubes exhibiting the maximum values and beyond. The
value of the tensile strength is 11% of the Young’s modulus, which is rarely observed in bulk
materials.
It is believed that when the nanotube reaches its ultimate elongation, a single chemical
bond in the middle of the nanotube breaks. This failure then leads to a stress concentration in the
adjacent chemical bonds, which become overstrained and consequently fail. This bond failure
initiates a series of similar events, leading eventually to the catastrophic failure of the nanotube.
Thus, the individual nanotubes exhibit an ideal strength behavior, providing strong evidence for
nearly defect-free structures, i.e., the onset of failure of the nanotubes emerges from excessive
distortion of a chemical bond, whereas the role of macroscopic failure mechanisms, like
dislocations, diffusion, and the propagation of cracks along grain boundaries, seems irrelevant
here.
Fig. 6 SEM picture of aWS2 nanotube under axial compression. Figure courtesy of I. Kaplan-Ashiri (11)
Twisting experiments recently demonstrated the high torsional strength of WS2
nanotubes. In analogy to previous experiments with carbon nanotubes, the nanotube was
suspended between two contacts, and a Au pedal was affixed to its outermost wall in the center
between the two contacts. By use of AFM, the pedal was deflected, and the applied torque was
measured. A shear modulus of 77 ± 59 GPa, was obtained from the slope of the linear part of the
torque-angle trace of the nanotube. When the torsion angle surpassed a critical value, a sharp
drop in the torque was observed, indicating a reversible stick-slip behavior of the multiwall WS2
nanotubes.
2c. Thermal Properties
A Very few systematic studies have been reported in the area of thermal properties. The
thermal conductivity of a mat of multiwall BN nanotubes was studied as a function of the
temperature and was found to be similar to that of carbon nanotubes. The thermal conductivity of
an individual multiwall BN nanotube was estimated to be roughly 1620WmK−1. This high
thermal conductivity value, which is a factor of 3–4 higher than the thermal conductivity of bulk
2H-BN, is attributed to the ballistic-type thermal conductivity of the 1-D nanostructure. The
thermal conductivity at low temperatures is dominated by the heat capacity and reflects the size
confinement of the phonons in the nanotubes. In another recent study, the low-temperature
specific heats of bulk (platelets) and nanoparticles of WS2 were measured. Below 9 K, the
specific heat of the nanoparticles deviates from that of the bulk counterpart. It also deviates from
the usual T3 dependence below 4 K, which is attributed to finite size effects that eliminate longwavelength acoustic phonons and interparticle-motion entropy.
APPLICATION
Superior physical properties of some inorganic NTs have been predicted and some of
them have already been confirmed by experiment. These properties include shockwave
resistance of WS2 NTs, use of the WS2 NTs as ultra sharp tips in scanning probe microscopy,
superconductivity in NbSe2 NTs and nanorods, superconductivity in mixed-phase WxMoyCzSz
NTs, enhanced magnetic coercivity in Ni NTs in comparison with bulk nickel, and the unusual
magnetic state in lithium-doped MoS2-xIy NTs. The MoS2-xIy NTs sub-nanometer diameter can
be used as a storage material for reversible lithium batteries or as electron field emitter (12).
MoS2 microplatelets have been used as a solid lubricant or as an additive in oil or grease
for more than 60 years. Cage-like nanostructures, e.g. cylindrical MoS2 nanotubes, represent a
new generation of lubricants with extremely low friction resulting from the size, small enough to
turn microvoids and nanovoids of the objects in mechanical contact into lubricant reservoirs, and
by the curved geometry of the nanoparticles, which put them into constantly parallel orientation
with the counterpart surfaces (Remskar, 2011). Various schemes have been proposed, the first
one being as an additive to lubricating fluids. More recently, a number of studies have indicated
that the IF material can serve also very effectively as a dry solid lubricant.
Fig. 5. Friction coefficient (1,2,3) and temperature (1_,2_,3_) vs. load (in kg) of porous bronze–graphite block
against hardened steel disk (HRC 52). In these experiments, after a run-in period of 10–30 h, the samples were
tested under a load of 30 kg and sliding velocity of 1 m s−1 for 11 h. Subsequently, the loads were increased from
30 kg with an increment of 9 kg and remained 1 h under each load. (1,1_) bronze–graphite sample without added
solid-lubricant; (2,2_) bronze–graphite sample with 2H-WS2 (6%); (3,3_) the same sample with (5%) hollow (IF)
WS2 noparticles (13).
Several possible applications of inorganic NTs can be foreseen. Due to their cylindrical
geometry, these nanomaterials have low mass density, a high porosity and an extremely large
surface to weight ratio. Their potential applications range from high porous catalytic and
ultralight anti-corrosive materials to electron field emitters and non-toxic strengthening fibers.
This may lead more efficient use and increased durability of materials. Doping these
semiconducting nano-structured materials may also further miniaturization of electronic system
leading to new optoelectronic materials. The helical structure of undoped tubes, with their
semiconductor behavior and optical activity, opens up possible application in nonlinear optics
and solar-cell technology.
The great diversity of inorganic nanotubes has considerably enlarge the possible
application already predicted for carbon nanotubes. In spite of their many similarities in
morphology and mechanical properties, pure inorganic nanotubes, or those co-grown with
carbon nanotubes, show their specific physical and chemical properties, which justify their
synthesis and further study.
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