Tantalum(V) ethoxide is a
metalorganic compound with formula Ta
2(OC
2H
5)
10, often abbreviated as Ta
2(O
Et)
10. It is a colorless solid that dissolves in some organic solvents but
hydrolyzes
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolysi ...
readily.
It is used to prepare films of tantalum(V) oxide.
Structure
Tantalum(V) alkoxides typically exist as
dimers with octahedral six-coordinate tantalum metal centres. Subsequent
crystallographic analysis established that the methoxide and isopropoxides of niobium adopt bioctahedral structures.
From a geometric perspective, the ten ethoxide
ligand
In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elec ...
oxygen atoms of the Ta
2(OEt)
10 molecule in solution define a pair of octahedra sharing a common edge with the two tantalum atoms located at their centres.
From a bonding perspective, each tantalum centre is surrounded octahedrally by four
monodentate
In coordination chemistry, denticity () refers to the number of donor groups in a given ligand that bind to the central metal atom in a coordination complex. In many cases, only one atom in the ligand binds to the metal, so the denticity equals ...
and two
bridging ethoxide ligands. The oxygen atoms of the bridging ethoxides are each bonded to both tantalum centres, and these two ligands are
''cis'' to one another within the
coordination sphere
In coordination chemistry, the first coordination sphere refers to the array of molecules and ions (the ligands) directly attached to the central metal atom. The second coordination sphere consists of molecules and ions that attached in various ...
. The formula
EtO)4Ta(μ-OEt)sub>2 more comprehensively represents this dimeric structure, although the simplified formula is commonly used for most purposes.
Preparation
Several approaches are known for preparing tantalum(V) ethoxide.
Salt metathesis
A salt metathesis reaction, sometimes called a double displacement reaction, is a chemical process involving the exchange of bonds between two reacting chemical species which results in the creation of products with similar or identical bonding a ...
from
tantalum(V) chloride is generally the most successful.
Tantalum pentachloride
Tantalum(V) chloride, also known as tantalum pentachloride, is an inorganic compound with the formula TaCl5. It takes the form of a white powder and is commonly used as a starting material in tantalum chemistry. It readily hydrolyzes to form tant ...
, Ta
2Cl
10, provides a convenient starting point. To avoid the generation of mixed chloride-ethoxide species, a base such as
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
is usually added to trap liberated HCl:
:10 EtOH + Ta
2Cl
10 + 10 NH
3 → Ta
2(OEt)
10 + 10 NH
4Cl
Salt metathesis using an
alkali metal alkoxide can be used as well:
:10 NaOEt + Ta
2Cl
10 → Ta
2(OEt)
10 + 10 NaCl
The same compound can be prepared
electrochemically.
The two
half-equations and the overall equation
for this reaction are:
:''
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 ...
'': 2 EtOH + 2 e
− → 2 EtO
− + H
2
:''
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 ...
'': Ta → "Ta
5+" + 5 e
−
:''overall'': 2 Ta + 10 EtOH → 2 "Ta
5+" + 10 EtO
− + 5 H
2 → Ta
2(OEt)
10 + 5 H
2
Commercial production of tantalum(V) ethoxide using this electrochemical approach has been employed in Russia.
The compound can also be prepared by direct reaction of tantalum metal with ethanol, in which case the overall equation is the same as that shown above for the electrochemical approach.
Since the 1970s, Bayer of Germany had been producing tantalum (V) ethoxide in Leverkusen, however following the break-up of Bayer, production moved to Heraeus.
Meanwhile, Inorgtech (later MultiValent), started production in 1974 in Cambridge, UK.
Both routes involved the direct reaction of the metal chloride with alcohol in the presence of solvents to give a product of 99.999%+ purity.
Reactions
The most important reaction of tantalum alkoxides is
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolys ...
to produce films and gels of tantalum oxides. Although these reactions are complex, the formation of a
tantalum(V) oxide film by hydrolysis
can be described by this simplified equation:
:Ta
2(OC
2H
5)
10 + 5 H
2O → Ta
2O
5 + 10 C
2H
5OH
Tantalum(V) ethoxide
optical coatings can be produced by low pressure
chemical vapour deposition
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (substra ...
.
At pressures as low as 1.33
mPa
MPA or mPa may refer to:
Academia
Academic degrees
* Master of Performing Arts
* Master of Professional Accountancy
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* Master of Public Affairs
Schools
* Mesa Preparatory Academy
* Morgan Park Academy
* Mou ...
and temperatures of 700 °C, a
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
film of the desired depth is first deposited by the decomposition of
tetraethoxysilane
Tetraethyl orthosilicate, formally named tetraethoxysilane (TEOS), ethyl silicate is the organic chemical compound with the formula Si(OC2H5)4. TEOS is a colorless liquid. It degrades in water. TEOS is the of orthosilicic acid, Si(OH)4. It ...
, Si(OEt)
4, or di-''t''-butyoxydiacetoxysilane, Si(OC(CH
3)
3)
2(OOCCH
3)
2, then tantalum(V) ethoxide is introduced.
As in the case of
niobium(V) ethoxide, the ethoxide precursor thermally decomposes to produce the oxide layer with the associated release of
diethyl ether
Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable li ...
:
:Ta
2(OEt)
10 → Ta
2O
5 + 5 Et–O–Et
Pyrolysis
The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...
also produces a tantalum(V) oxide film by chemical vapor deposition in which case the tantalum(V) ethoxide is completely oxidised, producing
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
and
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
vapor:
:Ta
2(OC
2H
5)
10 + 30 O
2 → Ta
2O
5 + 20 CO
2 + 25 H
2O
Amorphous tantalum(V) oxide films can also be prepared by
atomic layer deposition or by a pulsed chemical vapour deposition technique in which tantalum(V) ethoxide and tantalum(V) chloride are applied alternately.
At temperatures approaching 450 °C the films produced have
refractive indices
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
and
permittivity
In electromagnetism, the absolute permittivity, often simply called permittivity and denoted by the Greek letter ''ε'' (epsilon), is a measure of the electric polarizability of a dielectric. A material with high permittivity polarizes more in ...
properties similar to those produced from conventional approaches.
The preparation of these films occurs with the loss of
chloroethane:
:Ta
2(OC
2H
5)
10 + Ta
2Cl
10 → 2 Ta
2O
5 + 10 C
2H
5Cl
Sol-gel processing also produces thin films of tantalum(V) oxide
using a similar chemical approach. Sol-gel routes using tantalum(V) ethoxide to generate layered
perovskite
Perovskite (pronunciation: ) is a calcium titanium oxide mineral composed of calcium titanate (chemical formula ). Its name is also applied to the class of compounds which have the same type of crystal structure as (XIIA2+VIB4+X2−3), known a ...
materials have also been developed.
Applications
It is mainly used for the manufacture of
tantalum(V) oxide thin-film materials by approaches including
chemical vapor deposition,
,
and
sol-gel processing.
These materials have
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
,
electrochromic
Electrochromism is a phenomenon in which a material displays changes in color or opacity in response to an electrical stimulus.
In this way, a smart window made of an electrochromic material can block specific wavelengths of ultraviolet, vis ...
,
and
optical
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...
applications.
Tantalum(V) oxide films have a variety of applications including as optical films with refractive indices as high as 2.039
and as a thin-film
dielectric material
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mater ...
in
dynamic random access memory and
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
field-effect transistor
The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs ( JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs cont ...
s.
The approach chosen for preparation of these materials is determined by the desired properties. Direct hydrolysis is appropriate when the presence of residual water or the use of high temperatures for drying is acceptable.
Micropatterns can be produced by site-selective deposition using the hydrolysis approach by forming a
self-assembled monolayer followed by high temperature
annealling. Chemical vapour deposition allows control of the thickness of the film on a nanometre scale, which is essential for some applications. Direct
pyrolysis
The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...
is convenient for optical applications,
where transparent materials with low light loss due to absorption is important,
and has also been used to prepare nitride
read-only memory
Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing sof ...
.
Electrochromism
Electrochromism is a phenomenon in which a material displays changes in color or opacity in response to an electrical stimulus.
In this way, a smart window made of an electrochromic material can block specific wavelengths of ultraviolet, vis ...
is the property of some materials to change color when charge is applied, and is the means by which so-called
smart glass
Smart glass or switchable glass (also called a smart window or switchable window) is a glass or glazing whose light transmission properties dynamically alter to control the passage of solar irradiation into buildings. In general, the glass chan ...
operates. Films produced by tantalum(V) ethoxide hydrolysis has been used to prepare amorphous tantalum(V) oxide films suitable for electrochromic applications.
Mixed-metal thin-films have also been prepared from this compound. For example, lithium tantalate, LiTaO
3, films are desirable for their
non-linear optical properties and have been prepared by first reacting tantalum(V) ethoxide with lithium dipivaloylmethanate, LiCH(COC(CH
3)
3)
2, to prepare a precursor suitable for
metalorganic vapour phase epitaxy
Metalorganic vapour-phase epitaxy (MOVPE), also known as organometallic vapour-phase epitaxy (OMVPE) or metalorganic chemical vapour deposition (MOCVD), is a chemical vapour deposition method used to produce single- or polycrystalline thin films. ...
(a form of chemical vapor deposition). Films of strontium tantalate, Sr(TaO
3)
2, have also been prepared using atomic layer deposition approaches and their properties investigated.
Tantalum(V) ethoxide condenses with carboxylic acids to give oxo-alkoxide-carboxylates, e.g., Ta
4O
4(OEt)
8(OOCCH
3)
4.
The Ta
4O
4 core of such compounds form a
cubane-type cluster
A cubane-type cluster is an arrangement of atoms in a molecular structure that forms a cube. In the idealized case, the eight vertices are symmetry equivalent and the species has Oh symmetry. Such a structure is illustrated by the hydrocarbon ...
.
References
{{Tantalum compounds
Tantalum compounds
Ethoxides