Titanium disulfide is an

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

with the formula Ti S₂. A golden yellow solid with high

electrical conductivity Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...

, it belongs to a group of compounds called transition metal di chalcogenides, which consist of the

stoichiometry Stoichiometry () is the relationships between the masses of reactants and Product (chemistry), products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must ...

M E₂. TiS₂ has been employed as a

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device such as a lead-acid battery. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. Conventional curren ...

material in

rechargeable batteries A rechargeable battery, storage battery, or secondary cell (formally a type of energy accumulator), is a type of electrical battery which can be charged, discharged into a load, and recharged many times, as opposed to a disposable or prima ...

Structure

With a layered structure, TiS₂ adopts a

hexagonal close packed In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement (or lattice). Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occ ...

(hcp) structure, analogous to

cadmium iodide Cadmium iodide is an inorganic compound with the formula CdI2. It is a white hygroscopic solid. It also can be obtained as a mono- and tetrahydrate. It has few applications. It is notable for its crystal structure, which is typical for compounds ...

(CdI₂). In this motif, half of the octahedral holes are filled with a "

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

", in this case Ti⁴⁺. Each Ti centre is surrounded by six sulfide ligands in an octahedral structure. Each sulfide is connected to three Ti centres, the geometry at S being pyramidal. Several metal di chalcogenides adopt similar structures, but some, notably MoS₂, do not. The layers of TiS₂ consist of covalent Ti-S bonds. The individual layers of TiS₂ are bound together by

van der Waals forces In molecular physics and chemistry, the van der Waals force (sometimes van der Waals' force) is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical ele ...

, which are relatively weak intermolecular forces. It crystallises in the

space group In mathematics, physics and chemistry, a space group is the symmetry group of a repeating pattern in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of the pattern that ...

Pm1. The Ti-S bond lengths are 2.423 Å. LiTiS2IntercalationCartoon-en

Intercalation

The single most useful and most studied property of TiS₂ is its ability to undergo intercalation upon treatment with electropositive elements. The process is a

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

, illustrated in the case of lithium: :TiS₂ + Li → LiTiS₂ LiTiS₂ is generally described as Li⁺ iS₂⁻ During the intercalation and deintercalation, a range of stoichimetries are produced with the general formul Li_xTiS₂ (x < 1). During intercalation, the interlayer spacing expands (the lattice "swells") and the electrical conductivity of the material increases. Intercalation is facilitated because of the weakness of the interlayer forces as well as the susceptibility of the Ti(IV) centers toward reduction. Intercalation can be conducted by combining a suspension of the disulfide material and a solution of the alkali metal in anhydrous ammonia. Alternatively solid TiS₂ reacts with the alkali metal upon heating. The Rigid-Band Model (RBM), which assumes that

electronic band structure In solid-state physics, the electronic band structure (or simply band structure) of a solid describes the range of energy levels that electrons may have within it, as well as the ranges of energy that they may not have (called ''band gaps'' or '' ...

does not change with intercalation, describes changes in the electronic properties upon intercalation. Deintercalation is the opposite of intercalation; the cations diffuse out from between the layers. This process is associated with recharging a Li/TiS₂ battery. Intercalation and deintercalation can be monitored by

cyclic voltammetry In electrochemistry, cyclic voltammetry (CV) is a type of voltammetric measurement where the potential of the working electrode is ramped linearly versus time. Unlike in linear sweep voltammetry, after the set potential is reached in a CV expe ...

. The microstructure of the titanium disulfide greatly affects the intercalation and deintercalation kinetics. Titanium disulfide nanotubes have a higher uptake and discharge capacity than the polycrystalline structure. The higher surface area of the nanotubes is postulated to provide more binding sites for the anode ions than the polycrystalline structure.

Material properties

Formally containing the d⁰ ion Ti⁴⁺ and closed shell dianion S²⁻, TiS₂ is essentially diamagnetic. Its magnetic susceptibility is 9 x 10⁻⁶ emu/mol, the value being sensitive to stoichiometry. Titanium disulfide is a

semimetal A semimetal is a material with a small energy overlap between the bottom of the Electrical conduction, conduction Electronic band structure, band and the top of the valence band, but they do not overlap in momentum space. According to Band theory ...

, meaning there is small overlap of the

conduction band In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in ...

and

valence band In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in ...

High pressure properties

The properties of titanium disulfide powder have been studied by high pressure

synchrotron A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed closed-loop path. The strength of the magnetic field which bends the particle beam i ...

x-ray diffraction X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...

(XRD) at room temperature. At ambient pressure, TiS₂ behaves as semiconductor while at high pressures of 8 GPa the material behaves as a semimetal. At 15 GPa, the transport properties change. There is no significant change in the density of states at the Fermi level up to 20 GPa and phase change does not occur until 20.7 GPa. A change in the structure of TiS₂ was observed at a pressure of 26.3 GPa, although the new structure of the high pressure phase has not been determined. The unit cell of titanium disulfide is 3.407 by 5.695

angstrom The angstrom (; ) is a unit of length equal to m; that is, one ten-billionth of a metre, a hundred-millionth of a centimetre, 0.1 nanometre, or 100 picometres. The unit is named after the Swedish physicist Anders Jonas Ångström (1814–18 ...

s. The size of the unit cell decreased at 17.8 GPa. The decrease in unit cell size was greater than was observed for MoS₂ and WS₂, indicating that titanium disulfide is softer and more compressible. The compression behavior of titanium disulfide is

anisotropic Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ver ...

. The axis parallel to S-Ti-S layers (c-axis) is more compressible than the axis perpendicular to S-Ti-S layers (a-axis) because of weak van der waals forces keeping S and Ti atoms together. At 17.8 GPa, the c-axis is compressed by 9.5% and the a-axis is compressed by 4%. The longitudinal sound velocity is 5284 m/s in the plane parallel to S-Ti-S layers. The longitudinal sound velocity perpendicular to the layers is 4383 m/s.

Synthesis

Titanium disulfide is prepared by the reaction of the elements around 500 °C. :Ti + 2 S → TiS₂ It can be more easily synthesized from

titanium tetrachloride Titanium tetrachloride is the inorganic compound with the formula . It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. is a volatile liquid. Upon contact with humid air, it forms thick clouds o ...

, but this product is typically less pure than that obtained from the elements. :TiCl₄ + 2 H₂S → TiS₂ + 4 HCl This route has been applied to the formation of TiS₂ films by chemical vapor deposition.

Thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...

s and organic

disulfide In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inorg ...

s can be employed in place of hydrogen sulfide. A variety of other titanium sulfides are known.

Chemical properties of TiS₂

Samples of TiS₂ are unstable in air. Upon heating, the solid undergoes oxidation to

titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound derived from titanium with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or Colour Index Internationa ...

: :TiS₂ + O₂ → TiO₂ + 2 S TiS₂ is also sensitive to water: :TiS₂ + 2H₂O → TiO₂ + 2 H₂S Upon heating, TiS₂ releases sulfur, forming the titanium(III) derivative: :2 TiS₂ → Ti₂S₃ + S

Sol-gel synthesis

Thin films of TiS₂ have been prepared by the sol-gel process from titanium isopropoxide (Ti(OPrⁱ)₄) followed by spin coating. This method affords amorphous material that crystallised at high temperatures to hexagonal TiS₂, which crystallization orientations in the 01 00 and 01directions. Because of their high surface area, such films are attractive for battery applications.

Unusual morphologoes of TiS₂

More specialized morphologies—

nanotube A nanotube is a nanoscale cylindrical structure with a hollow core, typically composed of carbon atoms, though other materials can also form nanotubes. Carbon nanotubes (CNTs) are the most well-known and widely studied type, consisting of rolled- ...

nanoclusters Nanoclusters are atomically precise, crystalline materials most often existing on the 0-2 nanometer scale. They are often considered kinetically stable intermediates that form during the synthesis of comparatively larger materials such as semic ...

, whiskers, nanodisks, thin films, fullerenes—are prepared by combining the standard reagents, often TiCl₄ in unusual ways. For example, flower-like morphologies were obtain by treating a solution of sulfur in 1-octadecene with titanium tetrachloride.

Fullerene-like materials

A form of TiS₂ with a

fullerene A fullerene is an allotropes of carbon, allotrope of carbon whose molecules consist of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to six atoms. The molecules may ...

-like structure has been prepared using the TiCl₄/H₂S method. The resulting spherical structures have diameters between 30 and 80 nm. Owing to their spherical shape, these fullerenes exhibit reduced

friction coefficient Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...

and wear, which may prove useful in various applications.

Nanotubes

Nanotubes of TiS₂ can be synthesized using a variation of the TiCl₄/H₂S route. According to

transmission electron microscopy Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a g ...

(TEM), these tubes have an outer diameter of 20 nm and an inner diameter of 10 nm. The average length of the nanotubes was 2-5 μm and the nanotubes were proven to be hollow. TiS₂ nanotubes with open ended tips are reported to store up to 2.5 weight percent hydrogen at 25 °C and 4 MPa hydrogen gas pressure. Absorption and desorption rates are fast, which is an attractive for hydrogen storage. The hydrogen atoms are postulated to bind to sulfur.

Nanoclusters and nanodisks

Nanoclusters, or

quantum dots Quantum dots (QDs) or semiconductor nanocrystals are semiconductor particles a few nanometres in size with optical and electronic properties that differ from those of larger particles via quantum mechanical effects. They are a central topic i ...

of TiS₂ have distinctive electronic and chemical properties due to

quantum confinement A potential well is the region surrounding a local minimum of potential energy. Energy captured in a potential well is unable to convert to another type of energy (kinetic energy in the case of a gravitational potential well) because it is captu ...

and very large surface to volume ratios. Nanoclusters can be synthesized using

micelle A micelle () or micella () ( or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension (also known as associated colloidal system). ...

. The nanoclusters are prepared from a solution of TiCl₄ in tridodecylmethyl ammonium iodide (TDAI), which served as the inverse micelle structure and seeded the growth of nanoclusters in the same general reaction as nanotubes. Nucleation only occurs inside the micelle cage due to the insolubility of the charged species in the continuous medium, which is generally a low

dielectric constant The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insul ...

inert oil. Like the bulk material, nanocluster-form of TiS₂ is a hexagonal layered structure. . Quantum confinement creates well separated electronic states and increases the

band gap In solid-state physics and solid-state chemistry, a band gap, also called a bandgap or energy gap, is an energy range in a solid where no electronic states exist. In graphs of the electronic band structure of solids, the band gap refers to t ...

more than 1 eV in comparison to the bulk material. A spectroscopic comparison shows a large

blueshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and increase in frequency and e ...

for the quantum dots of 0.85 eV. Nanodisks of TiS₂ arise by treating TiCl₄ with sulfur in oleylamine.

Applications

The promise of titanium disulfide as a

material in

was described in 1973 by M. Stanley Whittingham. The Group IV and V dichalcogenides attracted attention for their high electrical conductivities. The originally described battery used a lithium

anode An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...

and a titanium disulfide cathode. This battery had high

energy density In physics, energy density is the quotient between the amount of energy stored in a given system or contained in a given region of space and the volume of the system or region considered. Often only the ''useful'' or extractable energy is measure ...

and the diffusion of lithium ions into the titanium disulfide cathode was reversible, making the battery rechargeable. Titanium disulfide was chosen because it is the lightest and cheapest chalcogenide. Titanium disulfide also has the fastest rate of lithium ion diffusion into the crystal lattice. The main problem was degradation of the cathode after multiple recycles. This reversible intercalation process allows the battery to be rechargeable. Additionally, titanium disulfide is the lightest and the cheapest of all group IV and V layered dichalcogenides. In the 1990s, titanium disulfide was replaced by other cathode materials (manganese and cobalt oxides) in most rechargeable batteries. The use of TiS₂ cathodes remains of interest for use in solid-state lithium batteries, e.g., for hybrid electric vehicles and

plug-in electric vehicle A plug-in electric vehicle (PEV) is any road vehicle that can utilize an external source of electricity (such as a wall socket that connects to the power grid) via an detachable power cable to store electrical energy within its onboard rechar ...

s. In contrast to the all-solid state batteries, most lithium batteries employ liquid electrolytes, which pose safety issues due to their flammability. Many different solid electrolytes have been proposed to replace these hazardous liquid electrolytes. For most solid-state batteries, high interfacial resistance lowers the reversibility of the intercalation process, shortening the life cycle. These undesirable interfacial effects are less problematic for TiS₂. One all-solid-state lithium battery exhibited a power density of 1000 W/kg over 50 cycles with a maximum power density of 1500 W/kg. Additionally, the average capacity of the battery decreased by less than 10% over 50 cycles. Although titanium disulfide has high electrical conductivity, high energy density, and high power, its discharge voltage is relatively low compared to other lithium batteries where the cathodes have higher reduction potentials.

Notes