INORGANIC NANOTUBESCarbon 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 Examples

Metal chalcogenide MoS2, MoSe2, WS2, WSe2, NbS2, TaS2, ZrS2, HfS2, TiS2, ZnS2, NiS2, CdSe, CdS

Metal oxide 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

Metal halogenous NiCl2

Mixed-phase and metal-doped PbNbnS2n+1, Mo1-xWs2, WxMoyCzSz, Nb-WS2, WS2-carbon NTs, Nb2-carbon NTs, Au-MoS2, Ag-WS2, Ag-MoS2, Cu5.5FeS6.5

Boron- and silicon-based 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 StrategiesThe 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 NanotubesMetal 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 HalidesAlthough 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 NanotubesThis 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 multi-

crystalline 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 NanotubesSeveral 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 ReactivityINT 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 PropertiesIn 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 PropertiesThe 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 PropertiesA 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 long-

wavelength acoustic phonons and interparticle-motion entropy.

APPLICATIONSuperior 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.

REFERENCE

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Nanotubes, Advanced materials 13, No 44. Zak A, Sallacan-Ecker L, Margolin A, Genut M, Tenne R., 2007 Insight into the growth

mechanism of WS2 nanotubes in the scaled-up fluidized bed reactor. Manuscript submitted

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6. Remskar M. and Mrzel A., 2004, High-temperature fibres composed of transition metal inorganic nanotubes, Solid State and Materials Science 8 (2004) 121–125

7. Chopra N.G., Luyken R.J., Cherrey K., Crespi V.H., Cohen M.L., Louie S.G., and Zettl A, 1995, Boron nitride nanotubes, Science Vol 269, 966-967

8. Chen H., Elabd D.H., and Palmese G.R., 2006, Plasma-aided template synthesis of inorganic nanotubes and nanorods, Journal of material chemistry

9. Perephichka D.F. and Rosei F., 2006, Silicone Nanotubes10. Rao C.N.R. and Nath M., 2002, Inorganic nanotubes, The royal society of chemistry11. Remskar M. and Mrzel A., 2002, Inorganic nanotubes: self-assembly and geometrical

stabilisation of new compounds, Josef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia

12. Kaplan-Ashiri I, Cohen SR, Apter N,Wang Y, Seifert G, et al. 2007. Microscopic investigation of shear in multiwalled nanotube deformation. J. Phys. Chem. C 111:8432–36

13. Tenne R., 2001, Fullerene-like materials and nanotubes from inorganic compounds with a layered (2-D) structure, Department of Materials and Interfaces, Weizmann Institute, Reho_ot 76100, Israel