A non-carbon nanotube is a cylindrical

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...

often composed of

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

oxides, or group III-Nitrides and morphologically similar to a carbon nanotube. Non-carbon nanotubes have been observed to occur naturally in some mineral deposits. A few years after Linus Pauling mentioned the possibility of curved layers in

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

s 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 non-carbon 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 non-carbon materials, such as vanadium oxide and

manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...

oxide, and are being researched for such applications as

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

s and

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in wh ...

materials for batteries.

History and occurrence

Non-carbon 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.

Materials

Typical non-carbon nanotube materials are 2D layered solids such as tungsten(IV) sulfide (WS₂), molybdenum disulfide (MoS₂) and

tin(IV) sulfide Tin(IV) sulfide is a compound with the formula . The compound crystallizes in the cadmium iodide motif, with the Sn(IV) situated in "octahedral holes' defined by six sulfide centers. It occurs naturally as the rare mineral berndtite. It is useful ...

(SnS₂). WS₂ and SnS₂/

tin(II) sulfide Tin(II) sulfide is a chemical compound of tin and sulfur. The chemical formula is SnS. Its natural occurrence concerns herzenbergite (α-SnS), a rare mineral. At elevated temperatures above 905 K, SnS undergoes a second order phase transition ...

(SnS) nanotubes have been synthesized in macroscopic amounts. However, traditional ceramics like

titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolub ...

(TiO₂),

zirconium dioxide Zirconium dioxide (), sometimes known as zirconia (not to be confused with zircon), is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. A dopant stabi ...

(ZrO₂) and

zinc oxide Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cement ...

(ZnO) also form non-carbon nanotubes. More recent nanotube and

nanowire A nanowire is a nanostructure in the form of a wire with the diameter of the order of a nanometre (10−9 metres). More generally, nanowires can be defined as structures that have a thickness or diameter constrained to tens of nanometers or less ...

materials are

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that ca ...

chalcogen The chalcogens (ore forming) ( ) are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. Group 16 consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the radioac ...

/ halogenides (TMCH), described by the formula TM₆C_yH_z, where TM is transition metal ( molybdenum,

tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...

tantalum Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that ...

, niobium), C is

( sulfur,

selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...

, 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. In 2007, Chinese scientists announced the creation in the laboratory of

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

and

bismuth Bismuth is a chemical element with the symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs ...

nanotubes.

Properties and potential applications

Non-carbon nanotubes are an alternative material to better-explored carbon nanotubes, showing advantages such as easy synthetic access and high crystallinity, good uniformity and

dispersion Dispersion may refer to: Economics and finance * Dispersion (finance), a measure for the statistical distribution of portfolio returns * Price dispersion, a variation in prices across sellers of the same item *Wage dispersion, the amount of variat ...

, predefined electrical conductivity depending on the composition of the starting material and needle-like morphology, good

adhesion Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another). The forces that cause adhesion and cohesion can b ...

to a number of polymers and high impact-resistance. They are therefore promising candidates as fillers for

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

composites with enhanced thermal, mechanical, and electrical properties. Target applications for this kind of composites are materials for heat management, electrostatic dissipators,

wear Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical (e.g., erosion) or chemical (e.g., corrosion). The study of wear and related processes is referred to as tribology. Wear in ...

protection materials, photovoltaic elements, etc. Non-carbon nanotubes are heavier than carbon nanotubes and not as strong under

tensile stress In continuum mechanics, stress is a physical quantity. It is a quantity that describes the magnitude of forces that cause deformation. Stress is defined as ''force per unit area''. When an object is pulled apart by a force it will cause elonga ...

, but they are particularly strong under compression, leading to potential applications in impact-resistant applications such as bulletproof vests. The mechanical strength of

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...

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 Polycaprolactone (PCL) is a biodegradable polyester with a low melting point of around 60 °C and a glass transition temperature of about −60 °C. The most common use of polycaprolactone is in the production of speciality polyureth ...

doped with TMCH nanotubes revealed a

percolative In statistical physics and mathematics, percolation theory describes the behavior of a network when nodes or links are added. This is a geometric type of phase transition, since at a critical fraction of addition the network of small, disconnected ...

behavior with an extremely low

percolation threshold The percolation threshold is a mathematical concept in percolation theory that describes the formation of long-range connectivity in random systems. Below the threshold a giant connected component does not exist; while above it, there exists a ...

. The addition of WS₂ nanotubes to epoxy resin improved

, 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 non-carbon nanotubes addition may have potential applications as impact-absorbing materials. Optical properties of semiconductor quantum dot–non-carbon nanotube hybrids reveal efficient resonant energy transfer from the quantum dot to the non-carbon 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. Non-carbon 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, non-carbon 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.

Biomedical applications

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.

References

External links