Listing
description
·
Linear Formula ZnO
·
Molecular Weight 81.39
Zinc oxide is an inorganic compound with the formula ZnO. ZnO is a white powder that is insoluble in water, and it is
widely used as an additive in numerous materials and products including
rubbers, plastics, ceramics, glass, cement, lubricants,[5] paints, ointments, adhesives, sealants,
pigments, foods,
batteries, ferrites, fire retardants, and first-aid tapes. Although it occurs
naturally as the mineral zincite,
most zinc oxide is produced synthetically.
Detailed
description
ZnO is a wide-bandgap semiconductor
of the II-VI semiconductor group. The native doping of the semiconductor due to oxygen vacancies or
zinc interstitials is n-type.[7] This semiconductor has several favorable
properties, including good transparency, high electron mobility, wide bandgap,
and strong room-temperature luminescence. Those properties are valuable in emerging applications for:
transparent electrodes in liquid crystal displays, energy-saving or heat-protecting windows, and
electronics as thin-film transistors and light-emitting diodes.
Chemical properties
Pure ZnO is a white powder, but in nature it occurs
as the rare mineral zincite, which usually
contains manganese and other impurities that confer a yellow to red color.[8]
Crystalline zinc oxide is thermochromic, changing from
white to yellow when heated in air and reverting to white on cooling.[9] This color change is
caused by a small loss of oxygen to the environment at high temperatures to
form the non-stoichiometric Zn1+xO,
where at 800 °C, x = 0.00007.[9]
Zinc oxide is an amphoteric oxide. It is nearly insoluble in water, but it
will dissolve in most acids, such as hydrochloric acid:[10]
ZnO + 2 HCl → ZnCl2 + H2O
Solid zinc oxide will also dissolve in alkalis to
give soluble zincates:
ZnO + 2 NaOH + H2O
→ Na2[Zn(OH)4]
ZnO reacts slowly with fatty acids in oils to
produce the corresponding carboxylates, such as oleate or stearate. ZnO forms
cement-like products when mixed with a strong aqueous solution of zinc chloride and these are best
described as zinc hydroxy chlorides.[11]This cement was used
in dentistry.[12]
ZnO also forms cement-like material when treated
with phosphoric acid; related materials
are used in dentistry.[12] A major component of
zinc phosphate cement produced by this reaction is hopeite, Zn3(PO4)2·4H2O.[13]
ZnO decomposes into zinc vapor and oxygen at around
1975 °C with a standard oxygen pressure. In a carbothermic reaction, heating with carbon converts the oxide into zinc vapor at a
much lower temperature (around 950 °C).[14]
ZnO + C → Zn(Vapor) + CO
Zinc oxide can react violently with aluminium and magnesium powders, with
chlorinated rubber and linseed oil on heating causing
fire and explosion hazard.[15][16]
ZnO + H2S
→ ZnS + H2O
Physical properties
Structure
Zinc oxide crystallizes in two main forms, hexagonal wurtzite[17] and cubic zincblende. The wurtzite
structure is most stable at ambient conditions and thus most common. The
zincblende form can be stabilized by growing ZnO on substrates with cubic
lattice structure. In both cases, the zinc and oxide centers are tetrahedral, the most characteristic geometry for Zn(II). ZnO converts to
the rocksalt motif at relatively
high pressures about 10 GPa.[7]
Hexagonal and zincblende polymorphs have no inversion symmetry (reflection of a
crystal relative to any given point does not transform it into itself). This
and other lattice symmetry properties result in piezoelectricity of the hexagonal and
zincblende ZnO, and pyroelectricity of hexagonal ZnO.
The hexagonal structure has a point group 6 mm
(Hermann-Mauguin
notation) or C6v (Schoenflies notation), and the space group is P63mc
or C6v4. The lattice constants are a = 3.25 Å and c = 5.2 Å; their ratio c/a ~ 1.60 is close to the ideal value for
hexagonal cell c/a = 1.633.[18] As in most group II-VI materials, the
bonding in ZnO is largely ionic (Zn2+–O2−)
with the corresponding radii of 0.074 nm for Zn2+ and 0.140 nm for O2−.
This property accounts for the preferential formation of wurtzite rather than
zinc blende structure,[19] as well as the
strong piezoelectricity of ZnO. Because of
the polar Zn-O bonds, zinc and oxygen planes are electrically charged. To
maintain electrical neutrality, those planes reconstruct at atomic level in
most relative materials, but not in ZnO – its surfaces are atomically flat,
stable and exhibit no reconstruction. This anomaly of ZnO is not fully
explained.[20]
Mechanical properties
ZnO is a relatively soft material with approximate
hardness of 4.5 on the Mohs scale.[5] Its elastic
constants are smaller than those of relevant III-V semiconductors, such as GaN. The high heat capacity and heat conductivity, low thermal
expansion and high melting temperature of ZnO are beneficial for ceramics. ZnO exhibits a very long lived optical
phonon E2(low) with a lifetime as high as 133 ps at 10 K.
Among the tetrahedrally bonded semiconductors, it
has been stated that ZnO has the highest piezoelectric tensor, or at least one
comparable to that of GaN and AlN.[23] This property makes
it a technologically important material for many piezoelectrical applications, which
require a large electromechanical coupling.
Electrical properties
ZnO has a relatively large direct band gap of ~3.3 eV at
room temperature. Advantages associated with a large band gap include higher
breakdown voltages, ability to sustain large electric fields, lower electronic noise, and
high-temperature and high-power operation. The bandgap of ZnO can further be
tuned to ~3–4 eV by its alloying with magnesium oxide or cadmium oxide.[7]
Most ZnO has n-type character, even in
the absence of intentional doping. Nonstoichiometry is typically the
origin of n-type character, but the subject remains controversial.[24] An alternative
explanation has been proposed, based on theoretical calculations, that
unintentional substitutional hydrogen impurities are responsible.[25] Controllable n-type
doping is easily achieved by substituting Zn with group-III elements such as
Al, Ga, In or by substituting oxygen with group-VII elements chlorine or iodine.[26]
Reliable p-type doping of ZnO
remains difficult. This problem originates from low solubility of p-type
dopants and their compensation by abundant n-type impurities. This problem is
observed with GaN and ZnSe. Measurement of p-type in "intrinsically" n-type
material is complicated by the inhomogeneity of samples.[27]
Current limitations to p-doping limit electronic and
optoelectronic applications of ZnO, which usually require junctions of n-type
and p-type material. Known p-type dopants include group-I elements Li, Na, K;
group-V elements N, P and As; as well as copper and silver. However, many of
these form deep acceptors and do not produce significant p-type conduction at
room temperature.[7]
Electron mobility of ZnO strongly
varies with temperature and has a maximum of ~2000 cm2/(V·s) at
80 K.[28] Data on hole
mobility are scarce with values in the range 5–30 cm2/(V·s).[29]
Production
Indirect process
In the indirect or French process, metallic zinc is
melted in a graphite crucible and vaporized at temperatures above 907 °C
(typically around 1000 °C). Zinc vapor reacts with the oxygen in the air
to give ZnO, accompanied by a drop in its temperature and bright luminescence.
Zinc oxide particles are transported into a cooling duct and collected in a bag
house. This indirect method was popularized by LeClaire (France) in 1844 and
therefore is commonly known as the French process. Its product normally consists
of agglomerated zinc oxide particles with an average size of 0.1 to a few
micrometers. By weight, most of the world's zinc oxide is manufactured via
French process.
Direct process
The direct or American process starts with diverse
contaminated zinc composites, such as zinc ores or smelter
by-products. The zinc precursors are reduced (carbothermal reduction) by heating with a source of carbon such as anthracite to produce zinc
vapor, which is then oxidized as in the indirect process. Because of the lower
purity of the source material, the final product is also of lower quality in
the direct process as compared to the indirect one.
Wet chemical process
A small amount of industrial production involves wet
chemical processes, which start with aqueous solutions of zinc salts, from
which zinc carbonate or zinc hydroxide is precipitated. The
solid precipitate is then calcined at temperatures around 800 °C.
Laboratory synthesis
Numerous specialised methods exist for producing ZnO
for scientific studies and niche applications. These methods can be classified
by the resulting ZnO form (bulk, thin film, nanowire), temperature
("low", that is close to room temperature or "high", that
is T ~ 1000 °C), process type (vapor deposition or growth from
solution) and other parameters.
Large single crystals (many cubic centimeters) can
be grown by the gas transport (vapor-phase deposition), hydrothermal synthesis,[20][30][31] or melt growth.[2] However, because of
high vapor pressure of ZnO, growth from
the melt is problematic. Growth by gas transport is difficult to control,
leaving the hydrothermal method as a preference.[2] Thin films can be
produced by chemical vapor
deposition, metalorganic vapour
phase epitaxy, electrodeposition, pulsed laser deposition, sputtering, sol-gel synthesis, atomic layer deposition, spray pyrolysis, etc.
Ordinary white powdered zinc oxide can be produced
in the laboratory by electrolyzing a solution of sodium bicarbonate with a zinc
anode. Zinc hydroxide and hydrogen gas are produced. The zinc hydroxide upon
heating decomposes to zinc oxide.
Zn + 2 H2O
→ Zn(OH)2 + H2
Zn(OH)2 → ZnO + H2O
ZnO nanostructures
Nanostructures of ZnO can be synthesized into a
variety of morphologies including nanowires, nanorods, tetrapods,
nanobelts, nanoflowers, nanoparticles etc. Nanostructures can be obtained with
most above-mentioned techniques, at certain conditions, and also with the vapor-liquid-solid
method. The
synthesis is typically carried out at temperatures of about 90 °C, in an
equimolar aqueous solution of zinc nitrate and hexamine, the latter
providing the basic environment. Certain additives, such as polyethylene glycol
or polyethylenimine, can improve the aspect ratio of the ZnO nanowires.[34] Doping of the ZnO
nanowires has been achieved by adding other metal nitrates to the growth
solution. The morphology of
the resulting nanostructures can be tuned by changing the parameters relating
to the precursor composition (such as the zinc concentration and pH) or to the
thermal treatment (such as the temperature and heating rate).
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