Iridium(IV) oxide, IrO
2, is the only well-characterised oxide of iridium. It is a blue-black solid, used with other rare oxides to coat
anodes
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 devi ...
.
Synthesis
As described by its discoverers, it can be formed by treating the green form of
iridium trichloride
Iridium(III) chloride is the inorganic compound with the formula IrCl3. The anhydrous compound is relatively rare, but the related hydrate is much more commonly encountered. The anhydrous salt has two polymorphs, α and β, which are brown and r ...
with oxygen at high temperatures:
:2 IrCl
3 + 2 O
2 → 2 IrO
2 + 3 Cl
2
A hydrated form is also known.
Structure
The compound adopts the TiO
2 rutile structure
Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite.
Rutile has one of the highest refractive indices at visible ...
, featuring six coordinate iridium and three coordinate oxygen.
It forms a
tetragonal lattice with lattice parameters of 4.5Å and 3.15Å.
Mechanical properties
Oxide
An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...
materials are typically hard and
brittle
A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. ...
. Indeed, iridium oxide does not easily deform under stress,
instead cracking easily. Measured deflections of a thin,
cantilevered
A cantilever is a rigid structural element that extends horizontally and is unsupported at one end. Typically it extends from a flat vertical surface such as a wall, to which it must be firmly attached. Like other structural elements, a cantilev ...
iridium oxide film indicate a
Young’s modulus of 300 ± 15 GPa,
substantially lower than the Young's modulus of metallic iridium (517 GPa).
Applications
Iridium dioxide can be used to make coated electrodes for industrial
electrolysis
In chemistry and manufacturing, electrolysis is a technique that uses Direct current, direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of c ...
or as
microelectrodes
A microelectrode is an electrode used in electrophysiology either for recording neural signals or for the electrical stimulation of nervous tissue (they were first developed by Ida Hyde in 1921). Pulled glass pipettes with tip diameters of 0. ...
for
. In electrolytic applications, IrO
2 films evolve
O2 efficiently.
Electrode manufacture typically requires high-temperature
annealing.
Fracture and
delamination
Delamination is a mode of failure where a material fractures into layers. A variety of materials, including Lamination, laminate Composite material, composites and concrete, can fail by delamination. Processing can create layers in materials, suc ...
are well-known problems when fabricating devices that incorporate iridium oxide film. One cause of delamination is
lattice mismatch between iridium oxide and the substrate.
Sputtering
In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and c ...
iridium oxide on a
liquid crystal polymer
Liquid crystal polymers (LCPs) are polymers with the property of liquid crystal, usually containing aromatic rings as mesogens. Despite uncrosslinked LCPs, polymeric materials like liquid crystal elastomers (LCEs) and liquid crystal networks (LC ...
has been proposed to avoid mismatch, but sputtered films spontaneously delaminate during
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 ...
if the maximum potential bias exceeds 0.9 V.
References
{{Authority control
Iridium compounds
Transition metal oxides