Titanium disulfide is an
inorganic compound
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...
with the formula
Ti S2. A golden yellow solid with high
electrical conductivity
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allow ...
,
it belongs to a group of compounds called transition metal di
chalcogenides : 220px, Cadmium sulfide, a prototypical metal chalcogenide, is used as a yellow pigment.
A chalcogenide is a chemical compound consisting of at least one chalcogen anion and at least one more electropositive element. Although all group 16 elements ...
, which consist of the
stoichiometry
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...
M E2. TiS
2 has been employed as a
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 whi ...
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, TiS
2 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 occu ...
(hcp) structure, analogous to
cadmium iodide
Cadmium iodide is the 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 compound ...
(CdI
2). 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 Ti
4+.
[ 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 : 220px, Cadmium sulfide, a prototypical metal chalcogenide, is used as a yellow pigment.
A chalcogenide is a chemical compound consisting of at least one chalcogen anion and at least one more electropositive element. Although all group 16 elements ...
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, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic bond, ionic or covalent bonds, these attractions do not result from a Chemical bond, chemical electronic bond; they are c ...
, 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 an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it unchan ...
Pm1. The Ti-S bond lengths are 2.423 Å.
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, , ) 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 d ...
, illustrated in the case of lithium:
:TiS2 + Li → LiTiS2
LiTiS2 is generally described as Li+ 2−">iS2− During the intercalation and deintercalation, a range of stoichimetries are produced with the general formul LixTiS2 (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
Cyclic voltammetry (CV) is a type of potentiodynamic electrochemical measurement. In a cyclic voltammetry experiment, the working electrode potential is ramped linearly versus time. Unlike in linear sweep voltammetry, after the set potential is re ...
. The microstructure of the titanium disulfide greatly affects the intercalation and deintercalation kinetic
Kinetic (Ancient Greek: κίνησις “kinesis”, movement or to move) may refer to:
* Kinetic theory of gases, Kinetic theory, describing a gas as particles in random motion
* Kinetic energy, the energy of an object that it possesses due to i ...
s. 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 very small overlap between the bottom of the conduction band and the top of the valence band.
According to electronic band theory, solids can be classified as insulators, semiconductors, semimetals, or metals. ...
, 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 w ...
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 w ...
.
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 magnetic field which bends the particle beam into its closed p ...
x-ray diffraction
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
(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 angstromEntry "angstrom" in the Oxford online dictionary. Retrieved on 2019-03-02 from https://en.oxforddictionaries.com/definition/angstrom.Entry "angstrom" in the Merriam-Webster online dictionary. Retrieved on 2019-03-02 from https://www.m ...
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 property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...
. 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 gro ...
s and organic disulfide
In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...
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 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 insolubl ...
:
: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
Titanium isopropoxide, also commonly referred to as titanium tetraisopropoxide or TTIP, is a chemical compound with the formula . This alkoxide of titanium(IV) is used in organic synthesis and materials science. It is a diamagnetic tetrahedral mo ...
(Ti(OPri)4) followed by spin coating
Spin coating is a procedure used to deposit uniform thin films onto flat substrates. Usually a small amount of coating material is applied on the center of the substrate, which is either spinning at low speed or not spinning at all. The substrate ...
. 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 nanometer-scale hollow tube-like structure.
Kinds of nanotubes
* BCN nanotube, composed of comparable amounts of boron, carbon, and nitrogen atoms
* Boron nitride nanotube, a polymorph of boron nitride
* Carbon nanotube, includes g ...
s, 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 semico ...
, 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 allotrope of carbon whose molecule consists 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 seven atoms. The molecule may be a hollow sphere, ...
-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. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion 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) are semiconductor particles a few nanometres in size, having optical and electronic properties that differ from those of larger particles as a result of quantum mechanics. They are a central topic in nanotechnology. When the ...
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 captur ...
and very large surface to volume ratios. Nanoclusters can be synthesized using micelle
A micelle () or micella () (plural micelles 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 collo ...
. 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 insulat ...
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, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in ...
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 simultaneous increase in fr ...
for the quantum dots of 0.85 eV.
Nanodisks of TiS2 arise by treating TiCl4 with sulfur in oleylamine
Oleylamine is an organic compound with a molecular formula C18H35NH2. It is an unsaturated fatty amine related to the fatty acid oleic acid. The pure compound is a clear and colorless liquid. Commercially available oleylamine reagents vary in colo ...
.
Applications
:
The promise of titanium disulfide as a 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 whi ...
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 ...
was described in 1973 by M. Stanley Whittingham
Michael Stanley Whittingham (born 22 December 1941) is a British-American chemist. He is currently a professor of chemistry and director of both the Institute for Materials Research and the Materials Science and Engineering program at Binghamton ...
. The Group IV and V dichalcogenides attracted attention for their high electrical conductivities. The originally described battery used a lithium anode
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
and a titanium disulfide cathode. This battery had high energy density
In physics, energy density is the amount of energy stored in a given system or region of space per unit volume. It is sometimes confused with energy per unit mass which is properly called specific energy or .
Often only the ''useful'' or extract ...
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
A hybrid electric vehicle (HEV) is a type of hybrid vehicle that combines a conventional internal combustion engine (ICE) system with an electric propulsion system (hybrid vehicle drivetrain). The presence of the electric powertrain is intended ...
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) to store electrical power within its onboard rechargeable battery packs, which then ...
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
Further reading
*http://authors.library.caltech.edu/5456/1/hrst.mit.edu/hrs/materials/public/Titanium_disulfide.htm
*
*
{{Sulfides
Sulfides
Titanium(IV) compounds
Transition metal dichalcogenides