Zinc oxide

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Zinc oxide
Names
Other names
Zinc white, calamine, philosopher's wool, Chinese white, flowers of zinc
Identifiers
3D model (
JSmol
)
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard
 100.013.839 Edit this at Wikidata
EC Number
  • 215-222-5
13738
KEGG
RTECS number
  • ZH4810000
UNII
UN number 3077
  • InChI=1S/O.Zn checkY
    Key: XLOMVQKBTHCTTD-UHFFFAOYSA-N checkY
  • [Zn]=O
Properties
ZnO
Molar mass 81.406 g/mol[1]
Appearance White solid[1]
Odor Odorless
Density 5.6 g/cm3[1]
Melting point 1,974 °C (3,585 °F; 2,247 K) (decomposes)[1][7]
Boiling point 2,360 °C (4,280 °F; 2,630 K) (decomposes)
0.0004% (17.8°C)[2]
Band gap 3.2 eV (direct)[3]
Electron mobility 180 cm2/(V·s)[3]
−27.2·10−6 cm3/mol[4]
Thermal conductivity
 0.6 W/(cm·K)[5]
n1=2.013, n2=2.029[6]
Structure[8]
Wurtzite
C6v4-P63mc
a = 3.2495 Å, c = 5.2069 Å
2
Tetrahedral
Thermochemistry[9]
40.3 J·K−1mol−1
43.65±0.40 J·K−1mol−1
Std enthalpy of
formation
(ΔfH⦵298)
 -350.46±0.27 kJ mol−1
-320.5 kJ mol−1
Enthalpy of fusion (ΔfH⦵fus)
 70 kJ/mol
Pharmacology
QA07XA91 (WHO)
Hazards
GHS labelling:
GHS09: Environmental hazard
Warning
H400, H401
P273, P391, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
2
0
0
Flash point 1,436 °C (2,617 °F; 1,709 K)
Lethal dose or concentration (LD, LC):
240 mg/kg (intraperitoneal, rat)[10]
7950 mg/kg (rat, oral)[11]
2500 mg/m3 (mouse)[11]
2500 mg/m3 (guinea pig, 3–4 h)[11]
NIOSH (US health exposure limits):
PEL (Permissible)
 TWA 5 mg/m3 (fume) TWA 15 mg/m3 (total dust) TWA 5 mg/m3 (resp dust)[2]
REL (Recommended)
 Dust: TWA 5 mg/m3 C 15 mg/m3

Fume: TWA 5 mg/m3 ST 10 mg/m3[2]

IDLH
(Immediate danger)
 500 mg/m3[2]
Safety data sheet (SDS) ICSC 0208
Related compounds
Other anions
 Zinc sulfide
Zinc selenide
Zinc telluride
Other cations
 Cadmium oxide
Mercury(II) oxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Zinc oxide is an

food supplements, rubbers, plastics, ceramics, glass, cement, lubricants,[12] paints, sunscreens, ointments, adhesives, sealants, pigments, foods, batteries, ferrites, fire retardants, semi conductors,[13] and first-aid tapes. Although it occurs naturally as the mineral zincite, most zinc oxide is produced synthetically.[14]

History

Early humans probably used zinc compounds in processed

diaper rashes, calamine cream, anti-dandruff shampoos, and antiseptic ointments.[18]

The Romans produced considerable quantities of

Dioscorides in the 1st century AD.[20] Zinc oxide has also been recovered from zinc mines at Zawar in India, dating from the second half of the first millennium BC.[16]

From the 12th to the 16th century zinc and zinc oxide were recognized and produced in India using a primitive form of the direct synthesis process. From India, zinc manufacturing moved to China in the 17th century. In 1743, the first European zinc smelter was established in Bristol, United Kingdom.[21] Around 1782 Louis-Bernard Guyton de Morveau proposed replacing lead white pigment with zinc oxide.[22]

The main usage of zinc oxide (zinc white) was in paints and as an additive to ointments. Zinc white was accepted as a pigment in oil paintings by 1834 but it did not mix well with oil. This problem was solved by optimizing the synthesis of ZnO. In 1845, Edme-Jean Leclaire in Paris was producing the oil paint on a large scale, and by 1850, zinc white was being manufactured throughout Europe. The success of zinc white paint was due to its advantages over the traditional white lead: zinc white is essentially permanent in sunlight, it is not blackened by sulfur-bearing air, it is non-toxic and more economical. Because zinc white is so "clean" it is valuable for making tints with other colors, but it makes a rather brittle dry film when unmixed with other colors. For example, during the late 1890s and early 1900s, some artists used zinc white as a ground for their oil paintings. All those paintings developed cracks over the years.[23]

In recent times, most zinc oxide has been used in the

photocopying. High-quality ZnO produced by the "French process" was added to photocopying paper as a filler. This application was soon displaced by titanium.[24]

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.[25]

Crystalline zinc oxide is

non-stoichiometric Zn1+xO, where at 800 °C, x = 0.00007.[26]

Zinc oxide is an

hydrochloric acid:[27]

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

oleate or stearate. When mixed with a strong aqueous solution of zinc chloride, ZnO forms cement-like products best described as zinc hydroxy chlorides.[28] This cement was used in dentistry.[29]

Hopeite

ZnO also forms cement-like material when treated with phosphoric acid; related materials are used in dentistry.[29] A major component of zinc phosphate cement produced by this reaction is hopeite, Zn3(PO4)2·4H2O.[30]

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).[27]

ZnO + C → Zn(Vapor) + CO

Physical properties

Wurtzite structure
A zincblende unit cell

Structure

Zinc oxide crystallizes in two main

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.[13]

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 (

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,[33] 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.[34] However, studies using wurtzoid structures explained the origin of surface flatness and the absence of reconstruction at ZnO wurtzite surfaces[35]
in addition to the origin of charges on ZnO planes.

Mechanical properties

ZnO is a wide-band gap semiconductor of the

II-VI semiconductor group. The native doping of the semiconductor due to oxygen vacancies or zinc interstitials is n-type.[13]

ZnO is a relatively soft material with approximate hardness of 4.5 on the

optical phonon in ZnO exhibits an unusually long lifetime of 133 ps at 10 K.[36]

Among the tetrahedrally bonded semiconductors, it has been stated that ZnO has the highest piezoelectric tensor, or at least one comparable to that of

AlN.[37] This property makes it a technologically important material for many piezoelectrical applications, which require a large electromechanical coupling. Therefore, ZnO in the form of thin film has been one of the most studied resonator materials for thin-film bulk acoustic resonators
.

Electrical and optical properties

Favourable properties of zinc oxide include good transparency, high

liquid crystal displays, energy-saving or heat-protecting windows, and electronics as thin-film transistors and light-emitting diodes
.

ZnO has a relatively wide

electronic noise, and high-temperature and high-power operation. The band gap of ZnO can further be tuned to ~3–4 eV by its alloying with magnesium oxide or cadmium oxide.[13] Due to this large band gap, there have been efforts to create visibly transparent solar cells utilising ZnO as a light absorbing layer. However, these solar cells have so far proven highly inefficient. [38]

Most ZnO has

Nonstoichiometry is typically the origin of n-type character, but the subject remains controversial.[39] An alternative explanation has been proposed, based on theoretical calculations, that unintentional substitutional hydrogen impurities are responsible.[40] 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.[41]

Reliable

ZnSe. Measurement of p-type in "intrinsically" n-type material is complicated by the inhomogeneity of samples.[42]

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.[13]

Electron mobility of ZnO strongly varies with temperature and has a maximum of ~2000 cm2/(V·s) at 80 K.[43] Data on hole mobility are scarce with values in the range 5–30 cm2/(V·s).[44]

ZnO discs, acting as a

surge arresters.[45][46]

Zinc oxide is noted for its strongly

nonlinear optical properties, especially in bulk. The nonlinearity of ZnO nanoparticles can be fine-tuned according to their size.[47]

Production

See also: Zinc smelting

For industrial use, ZnO is produced at levels of 105 tons per year[25] by three main processes:[24]

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 Edme Jean LeClaire of Paris 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

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

The red and green colors of these synthetic ZnO crystals result from different concentrations of oxygen vacancies.[48]

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),

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,

hexamine, the latter providing the basic environment. Certain additives, such as polyethylene glycol or polyethylenimine, can improve the aspect ratio of the ZnO nanowires.[52] Doping of the ZnO nanowires has been achieved by adding other metal nitrates to the growth solution.[53] 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).[54]

Aligned ZnO nanowires on pre-seeded

electron beam lithography and nanosphere lithography to designate nucleation sites prior to growth. Aligned ZnO nanowires can be used in dye-sensitized solar cells and field emission devices.[58][59]

Applications

The applications of zinc oxide powder are numerous, and the principal ones are summarized below. Most applications exploit the reactivity of the oxide as a precursor to other zinc compounds. For material science applications, zinc oxide has high refractive index, high thermal conductivity, binding, antibacterial and UV-protection properties. Consequently, it is added into materials and products including plastics, ceramics, glass, cement,[60] rubber, lubricants,[12] paints, ointments, adhesive, sealants, concrete manufacturing, pigments, foods, batteries, ferrites, fire retardants, etc.[61]

Rubber industry

Between 50% and 60% of ZnO use is in the rubber industry.[62] Zinc oxide along with stearic acid is used in the sulfur vulcanization of rubber.[24][63] ZnO additives also protect rubber from fungi (see medical applications) and UV light.

Ceramic industry

Ceramic industry consumes a significant amount of zinc oxide, in particular in ceramic glaze and frit compositions. The relatively high heat capacity, thermal conductivity and high temperature stability of ZnO coupled with a comparatively low coefficient of expansion are desirable properties in the production of ceramics. ZnO affects the melting point and optical properties of the glazes, enamels, and ceramic formulations. Zinc oxide as a low expansion, secondary flux improves the elasticity of glazes by reducing the change in viscosity as a function of temperature and helps prevent crazing and shivering. By substituting ZnO for BaO and PbO, the heat capacity is decreased and the thermal conductivity is increased. Zinc in small amounts improves the development of glossy and brilliant surfaces. However, in moderate to high amounts, it produces matte and crystalline surfaces. With regard to color, zinc has a complicated influence.[62]

Medicine

Skin treatment

Zinc oxide as a mixture with about 0.5% iron(III) oxide (Fe2O3) is called calamine and is used in calamine lotion. Historically the mineral calamine is a mixture of the oxides zincite and hemimorphite.

Zinc oxide is widely used to treat a variety of skin conditions, including

emollient and astringent, zinc and castor oil cream, commonly used to treat infants.[66][67]

It is also a component in tape (called "zinc oxide tape") used by athletes as a bandage to prevent soft tissue damage during workouts.[68]

Antibacterial

Zinc oxide is used in

essential oils and plant extracts.[74][75]
Powdered zinc oxide has deodorizing and antibacterial properties.[76]

ZnO is added to cotton fabric, rubber, oral care products,[77][78] and food packaging.[79][80] Enhanced antibacterial action of fine particles compared to bulk material is not exclusive to ZnO and is observed for other materials, such as silver.[81] This property results from the increased surface area of the fine particles.

Sunscreen

Zinc oxide is used in

comedogenic.[86] Zinc from zinc oxide is, however, slightly absorbed into the skin.[87]

Many sunscreens use nanoparticles of zinc oxide (along with nanoparticles of titanium dioxide) because such small particles do not scatter light and therefore do not appear white. The nanoparticles are not absorbed into the skin more than regular-sized zinc oxide particles are,[88] and are only absorbed into the outermost layer of the skin but not into the body.[88]

Dental restoration

When mixed with eugenol, zinc oxide eugenol is formed, which has applications as a restorative and prosthodontic in dentistry.[29][89]

Food additive

See also: Zinc deficiency and Zinc toxicity

Zinc oxide is added to many food products, including breakfast cereals, as a source of zinc,[90] a necessary nutrient. Zinc sulfate is also used for the same purpose. Some prepackaged foods also include trace amounts of ZnO even if it is not intended as a nutrient.

Pigment

Main article: Zinc white

Zinc oxide (zinc white) is used as a

Manet, Munch and others. It is also a main ingredient of mineral makeup (CI 77947).[94]

UV absorber

Micronized and nano-scale zinc oxide provides strong protection against

JPL).[96]

Coatings

Paints containing zinc oxide powder have long been utilized as anticorrosive coatings for metals. They are especially effective for galvanized iron. Iron is difficult to protect because its reactivity with organic coatings leads to brittleness and lack of adhesion. Zinc oxide paints retain their flexibility and adherence on such surfaces for many years.[61]

ZnO highly n-type doped with

resistivity ~10−4 Ω·cm[97]). ZnO:Al coatings are used for energy-saving or heat-protecting windows. The coating lets the visible part of the spectrum in but either reflects the infrared (IR) radiation back into the room (energy saving) or does not let the IR radiation into the room (heat protection), depending on which side of the window has the coating.[25]

Plastics, such as polyethylene naphthalate (PEN), can be protected by applying zinc oxide coating. The coating reduces the diffusion of oxygen through PEN.[98] Zinc oxide layers can also be used on polycarbonate in outdoor applications. The coating protects polycarbonate from solar radiation, and decreases its oxidation rate and photo-yellowing.[99]

Corrosion prevention in nuclear reactors

Main article: Depleted zinc oxide

Zinc oxide depleted in 64Zn (the zinc isotope with atomic mass 64) is used in corrosion prevention in nuclear pressurized water reactors. The depletion is necessary, because 64Zn is transformed into radioactive 65Zn under irradiation by the reactor neutrons.[100]

Methane reforming

Zinc oxide (ZnO) is used as a pretreatment step to remove hydrogen sulfide (H2S) from natural gas following hydrogenation of any sulfur compounds prior to a methane reformer, which can poison the catalyst. At temperatures between about 230–430 °C (446–806 °F), H2S is converted to water by the following reaction:[101]

H2S + ZnO → H2O + ZnS

Electronics

Photograph of an operating ZnO UV laser diode and the corresponding device structure.[102]
Flexible gas sensor based on ZnO nanorods and its internal structure. ITO stands for indium tin oxide and PET for polyethylene terephthalate.[103]

ZnO has wide

transparent conducting oxide layer and ZnO nanostructures provide better light outcoupling.[106] Other properties of ZnO favorable for electronic applications include its stability to high-energy radiation and its ability to be patterned by wet chemical etching.[107] Radiation resistance[108] makes ZnO a suitable candidate for space applications. ZnO is the most promising candidate in the field of random lasers
to produce an electronically pumped UV laser source.

The pointed tips of ZnO nanorods result in a strong enhancement of an electric field. Therefore, they can be used as field emitters.[109]

Aluminium-doped ZnO layers are used as transparent

liquid crystal displays.[110]

Transparent thin-film transistors (TTFT) can be produced with ZnO. As field-effect transistors, they do not need a p–n junction,[111] thus avoiding the p-type doping problem of ZnO. Some of the field-effect transistors even use ZnO nanorods as conducting channels.[112]

Gas sensors

Zinc oxide is used in semiconductor gas sensors for detecting airborne compounds such as hydrogen sulfide, nitrogen dioxide, and volatile organic compounds. ZnO is a semiconductor that becomes n-doped by adsorption of reducing compounds, which reduces the detected electrical resistance through the device, in a manner similar to the widely used tin oxide semiconductor gas sensors. It is formed into nanostructures such as thin films, nanoparticles, nanopillars or nanowires in order to provide large surface area for interaction with gasses. The sensors are made selective for specific gasses by doping or surface-attaching materials such as catalytic noble metals.[113][114]

Piezoelectricity

The

coated in ZnO have been shown capable of fabricating "self-powered nanosystems" with everyday mechanical stress from wind or body movements.[115][116]

In 2008 the Center for Nanostructure Characterization at the

Georgia Institute of Technology reported producing an electricity generating device (called flexible charge pump generator) delivering alternating current by stretching and releasing zinc oxide nanowires. This mini-generator creates an oscillating voltage up to 45 millivolts, converting close to seven percent of the applied mechanical energy into electricity. Researchers used wires with lengths of 0.2–0.3 mm and diameters of three to five micrometers, but the device could be scaled down to smaller size.[117]

ZnO as anode of Li-ion battery

In form of a thin film ZnO has been demonstrated in miniaturised high frequency thin film resonators, sensors and filters.

Li-ion battery and supercapacitors

ZnO is a promising anode material for lithium-ion battery because it is cheap, biocompatible, and environmentally friendly. ZnO has a higher theoretical capacity (978 mAh g−1) than many other transition metal oxides such as CoO (715 mAh g−1), NiO (718 mAh g−1) and CuO (674 mAh g−1).[118] ZnO is also used as an electrode in supercapacitors.[119]

Safety

As a food additive, zinc oxide is on the U.S. FDA's list of generally recognized as safe, or GRAS, substances.[120]

Zinc oxide itself is non-toxic; it is hazardous, however, to inhale high concentrations of zinc oxide fumes, such as generated when zinc or zinc alloys are melted and oxidized at high temperature. This problem occurs while melting alloys containing brass because the melting point of brass is close to the boiling point of zinc.[121] Inhalation of zinc oxide, which may occur when welding galvanized (zinc-plated) steel, can result in a malady called metal fume fever.[121]

In sunscreen formulations that combined zinc oxide with small-molecule UV absorbers, UV light caused photodegradation of the small-molecule asorbers and toxicity in embryonic zebrafish assays.[122]

See also

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