An inorganic nanotube is a cylindrical molecule often composed of metal oxides, or group III-Nitrides and morphologically similar to a carbon nanotube. Inorganic nanotubes have been observed to occur naturally in some mineral deposits.
A few years after Linus Pauling mentioned the possibility of curved layers in minerals as early as 1930, some minerals such as white asbestos (or chrysotile)  and imogolite were actually shown to have a tubular structure. However, the first synthetic inorganic nanotubes did not appear until Reshef Tenne et al. reported the synthesis of nanotubes composed of tungsten disulfide (WS2) in 1992.
In the intervening years, nanotubes have been synthesised of many inorganic materials, such as vanadium oxide and manganese oxide, and are being researched for such applications as redox catalysts and cathode materials for batteries.
History and occurrence
Inorganic nanotubes are morphologically similar to carbon nanotubes and are observed in some mineral deposits of natural origin. Synthetic structures of this type were first reported by the group of Reshef Tenne in 1992.
Typical inorganic nanotube materials are 2D layered solids such as tungsten(IV) sulfide (WS2), molybdenum disulfide (MoS2) and tin(IV) sulfide (SnS2). WS2 and SnS2/tin(II) sulfide (SnS) nanotubes have been synthesized in macroscopic amounts. However, traditional ceramics like titanium dioxide (TiO2), zirconia dioxide (ZrO2) and zinc oxide (ZnO) also form inorganic nanotubes. More recent nanotube and nanowire materials are transition metal/chalcogen/halogenides (TMCH), described by the formula TM6CyHz, where TM is transition metal (molybdenum, tungsten, tantalum, niobium), C is chalcogen (sulfur, selenium, tellurium), H is halogen (iodine), and the composition is given by 8.2<(y+z)<10. TMCH tubes can have a subnanometer-diameter, lengths tunable from hundreds of nanometers to tens of micrometers and show excellent dispersiveness owing to extremely weak mechanical coupling between the tubes.
Properties and potential applications
Inorganic nanotubes are an alternative material to better-explored carbon nanotubes, showing advantages such as easy synthetic access and high crystallinity, good uniformity and dispersion, predefined electrical conductivity depending on the composition of the starting material and needle-like morphology, good adhesion to a number of polymers and high impact-resistance. They are therefore promising candidates as fillers for polymer composites with enhanced thermal, mechanical, and electrical properties. Target applications for this kind of composites are materials for heat management, electrostatic dissipators, wear protection materials, photovoltaic elements, etc. Inorganic nanotubes are heavier than carbon nanotubes and not as strong under tensile stress, but they are particularly strong under compression, leading to potential applications in impact-resistant applications such as bulletproof vests.
The mechanical strength of cellulose fibers can be increased by an order of magnitude by adding only 0.1 wt% of TMCH nanotubes, and measurements of electrical conductivity of polycaprolactone doped with TMCH nanotubes revealed a percolative behavior with an extremely low percolation threshold. The addition of WS2 nanotubes to epoxy resin improved adhesion, fracture toughness and strain energy release rate. The wear of the nanotubes-reinforced epoxy was eight times lower than that of pure epoxy. WS2 nanotubes were also embedded into a poly(methyl methacrylate) (PMMA) nanofiber matrix via electrospinning. The nanotubes were well dispersed and aligned along fiber axis. The enhanced stiffness and toughness of PMMA fiber meshes by means of inorganic nanotubes addition may have potential applications as impact-absorbing materials.
Optical properties of semiconductor quantum dot-inorganic nanotube hybrids reveal efficient resonant energy transfer from the quantum dot to the inorganic nanotubes upon photoexcitation. Nanodevices based on one-dimensional nanomaterials are thought for next-generation electronic and photoelectronic systems having small size, faster transport speed, higher efficiency and less energy consumption. A high-speed photodetector for visible and near-infrared light based on individual WS2 nanotubes has been prepared in laboratory. Inorganic nanotubes are hollow and can be filled with another material, to preserve or guide it to a desired location or generate new properties in the filler material which is confined within a nanometer-scale diameter. To this goal, inorganic nanotube hybrids were made by filling WS2 nanotubes with molten lead, antimony or bismuth iodide salt by a capillary wetting process, resulting in PbI2@WS2, SbI3@WS2 or BiI3@WS2 core–shell nanotubes.
Tungsten disulfide nanotubes have been investigated as reinforcing agents to improve the mechanical properties of biodegradable polymeric nanocomposites for bone tissue engineering applications. Addition of ~0.02 weight % of tungsten disulfide nanotubes significantly improved the compression and flexural mechanical properties of poly(propylene fumarate) nanocomposites, greater than carbon nanotubes. This was attributed to increased dispersion of tungsten disulfide nanotubes in the polymer matrix enabling efficient load transfer from the matrix to the underlying nanostructure.
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