Zinc oxide

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Zinc oxide
Identifiers
CAS number 1314-13-2 Yes check.svgY
PubChem 14806
EC number 215-222-5
RTECS number ZH4810000
Properties
Molecular formula ZnO
Molar mass 81.408 g/mol
Appearance White solid
Odor| odorless|- Density 5.606 g/cm3
Melting point

1975 °C (decomposes)[1]

Boiling point

2360 °C

Solubility in water 0.16 mg/100 mL (30 °C)
Band gap 3.3 eV (direct)
Refractive index (nD) 2.0041
Hazards
EU classification Dangerous for the environment (N)
R-phrases Template:R50/53
S-phrases Template:S60, Template:S61
NFPA 704
NFPA 704.svg
1
2
0
W
Flash point| 1436 °C|- Related compounds
Other anions| Zinc sulfide
Zinc selenide
Zinc telluride|-
Cadmium oxide
Mercury(II) oxide
 Yes check.svgY (what is this?)  (verify)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Zinc oxide is an inorganic compound with the formula ZnO. It usually appears as a white powder, nearly insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber (e.g., car tires), lubricants,[2] paints, ointments, adhesives, sealants, pigments, foods (source of Zn nutrient), batteries, ferrites, fire retardants, first aid tapes, etc. ZnO is present in the Earth's crust as the mineral zincite; however, most ZnO used commercially is produced synthetically.

In materials science, ZnO is often called a II-VI semiconductor because zinc and oxygen belong to the 2nd and 6th groups of the periodic table, respectively. This semiconductor has several favorable properties: good transparency, high electron mobility, wide bandgap, strong room-temperature luminescence, etc. Those properties are already used in emerging applications for transparent electrodes in liquid crystal displays and in energy-saving or heat-protecting windows, and electronic applications of ZnO as thin-film transistors and light-emitting diodes are forthcoming as of 2009.

Contents

Chemical properties

ZnO occurs as white powder known as zinc white or as the mineral zincite. The mineral usually contains a certain amount of manganese and other elements and is of yellow to red color.[3] Crystalline zinc oxide is thermochromic, changing from white to yellow when heated and in air reverting to white on cooling.[4] This color change is caused by a very small loss of oxygen at high temperatures to form the non-stoichiometric Zn1+xO, where at 800 °C, x= 0.00007.[4]

Zinc oxide is an amphoteric oxide. It is nearly insoluble in water and alcohol, but it is soluble in (degraded by) most acids, such as hydrochloric acid:[5][6]

ZnO + 2 HCl → ZnCl2 + H2O

Bases also degrade the solid 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.[7] This cement was used in dentistry.[8]

ZnO also forms cement-like products when treated with phosphoric acid; related materials are used in dentistry.[8] A major component of zinc phosphate cement produced by this reaction is hopeite, Zn3(PO4)2·4H2O.[9]

ZnO decomposes into zinc vapor and oxygen only at around 1975 °C, reflecting its considerable stability. Heating with carbon converts the oxide into the metal, which is more volatile than the oxide.[10]

ZnO + C → Zn + CO

Zinc oxide can react violently with aluminium and magnesium powders, with chlorinated rubber and linseed oil on heating causing fire and explosion hazard.[11][12]

It reacts with hydrogen sulfide to give the sulfide: this reaction is used commercially in removing H2S using ZnO powder (e.g., as deodorant).

ZnO + H2S → ZnS + H2O

When ointments containing ZnO and water are melted and exposed to ultraviolet light, hydrogen peroxide is produced.[6]

Physical properties

Wurtzite structure
A zincblende unit cell

Crystal structure

Zinc oxide crystallizes in three forms: hexagonal wurtzite, cubic zincblende, and the rarely observed cubic rocksalt). 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 rocksalt (NaCl-type) structure is only observed at relatively high pressures about 10 GPa.[13]

Hexagonal and zincblende polymorphs have no inversion symmetry (reflection of a crystal relatively 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 in 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.[14] As in most group II-VI materials, the bonding in ZnO is largely ionic, which explains its strong piezoelectricity. Due to the polar Zn-O bonds, zinc and oxygen planes bear electric charge (positive and negative, respectively). Therefore, 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 yet.[15]

Mechanical properties

ZnO is a relatively soft material with approximate hardness of 4.5 on the Mohs scale.[2] 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.[16]

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.[17] This property makes it a technologically important material for many piezoelectrical applications, which require a large electromechanical coupling.

Electronic properties

ZnO has a relatively large direct band gap of ~3.3 eV at room temperature;[13] therefore, pure ZnO is colorless and transparent. 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 from ~3–4 eV by its alloying with magnesium oxide or cadmium oxide.[13]

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.[18] An alternative explanation has been proposed, based on theoretical calculations, that unintentional substitutional hydrogen impurities are responsible.[19] 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.[20]

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.[21]

Current limitations to p-doping does not 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.[22] Data on hole mobility are scarce with values in the range 5–30 cm2/(V·s).[23]

Production

Template:Seealso For industrial use, ZnO is produced at levels of 105 tons per year[3] by three main processes:[16]

Indirect (French) process

Metallic zinc is melted in a graphite crucible and vaporized at temperatures above 907 °C (typically around 1000 °C). Zinc vapor instantaneously 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. Major applications involve industries related to rubber, varistors, sunscreens, paints, healthcare, and poultry nutrients. Recent developments involve acicular nanostructures (rods, wires, tripods, tetrapods, plates) synthesized using a modified French process known as catalyst-free combust-oxidized mesh (CFCOM) process. Acicular nanostructures usually have micrometre-length nanorods with nanometric diameters (below 100 nm).[24]

Direct (American) process

In the direct process, the starting material is various contaminated zinc composites, such as zinc ores or smeleter by-products. It is reduced by heating with a carbon additive (e.g. antracite) 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

Wet chemical processes start with purified zinc solutions, from which zinc carbonate or zinc hydroxide is precipitated. It is then filtered, washed, dried and calcined at temperatures ~800 °C.

Laboratory synthesis

File:Synthetic Zincite Crystals.jpg
Synthetic ZnO crystals. Red and green color are associated with different concentrations of oxygen vacancies.[25]

A large number of ZnO production methods exist for producing ZnO for scientific studies and electronic 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) are usually grown by the gas transport (vapor-phase deposition), hydrothermal synthesis,[15][25][26] or melt growth.[1] 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.[1] Thin films can be produced by chemical vapor deposition, metalorganic vapour phase epitaxy, electrodeposition, pulsed laser deposition, sputtering, sol-gel synthesis, spray pyrolysis, etc.

Zinc oxide may 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

ZnO nanostructures 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.[15][27]

Aqueous solution synthesis of ZnO nanowires

Many ZnO nanostructures can be produced via aqueous methods. They are attractive because of relatively low synthesis temperatures (<300 °C) and absence of complex vacuum setup. Aligned ZnO nanowires on pre-seeded silicon, glass and gallium nitride substrates have been grown in aqueous solutions using aqueous zinc salts such as Zinc nitrate and Zinc acetate in basic environments.[28] The morphology of the resulting nanostructures can be tuned by changing parameters such as temperature, precursor concentrations and pH.

Pre-seeding substrates with ZnO creates sites for homogeneous nucleation of ZnO crystal during the synthesis. Common pre-seeding methods include in-situ thermal decomposition of zinc acetate crystallites, spincoating of ZnO nanoparticles and the use of various physical vapor deposition methods to deposit ZnO thin films.[29][30] Pre-seeding can be performed in conjunction with top down patterning methods such as electron beam lithography and nanosphere lithography to designate nucleation sites prior to growth.

The synthesis is typically carried out at temperature about 90 °C, in an equimolar aqueous solution of zinc nitrate and hexamine, the latter providing the basic environment. The addition of polyethylenimine improves the aspect ratio of the ZnO nanowires.[31] Doping of the ZnO nanowires has been achieved by adding other metal nitrates to the growth solution.[32]

Aligned ZnO nanowires can be used in dye-sensitized solar cells and field emission devices.[33][34]

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 various materials and products, including plastics, ceramics, glass, cement, rubber, lubricants,[2] paints, ointments, adhesive, sealants, pigments, foods, batteries, ferrites, fire retardants, etc.[35]

Rubber manufacture

About 50% of ZnO use is in rubber industry. Zinc oxide along with stearic acid activates vulcanization, which otherwise may not occur at all.[16] Zinc oxide and stearic acid are ingredients in the commercial manufacture of rubber goods. A mixture of these two compounds allows a quicker and more controllable rubber cure. ZnO is also an important additive to the rubber of car tyres. Vulcanization catalysts are derived from zinc oxide, and it considerably improves the thermal conductivity, which is crucial to dissipate the heat produced by the deformation when the tyre rolls.[36][37] ZnO additive also protect rubber from fungi (see medical applications) and UV light.

Concrete industry

Zinc oxide is widely used for concrete manufacturing. Addition of ZnO improves the processing time and the resistance of concrete against water.[36]

Medical

Zinc oxide as a mixture with about 0.5% iron(III) oxide (Fe2O3) is called calamine and is used in calamine lotion. There are also two minerals, zincite and hemimorphite, which have been historically called calamine. When mixed with eugenol, a chelate, zinc oxide eugenol is formed which has restorative and prosthodontic applications in dentistry.[8][38]

Reflecting the basic properties of ZnO, fine particles of the oxide have deodorizing and antibacterial[39] action and for that reason are added into various materials including cotton fabric, rubber, food packaging, etc.[40][41] Enhanced antibacterial action of fine particles compared to bulk material is not intrinsic to ZnO and is observed for other materials, such as silver.[42]

Zinc oxide is widely used to treat a variety of other skin conditions, in products such as baby powder and barrier creams to treat diaper rashes, calamine cream, anti-dandruff shampoos, and antiseptic ointments.[43][44] It is also a component in tape (called "zinc oxide tape") used by athletes as a bandage to prevent soft tissue damage during workouts.[45]

When used as an ingredient in sunscreen, zinc oxide sits on the skin’s surface ie. is not absorbed into the skin, and blocks both UVA (320-400 nm) and UVB (280-320 nm) rays of ultraviolet light. Because zinc oxide (and the other most common physical sunscreen, titanium dioxide) are not absorbed into the skin, they are nonirritating and nonallergenic.[46]

Zinc oxide can be used in ointments, creams, and lotions to protect against sunburn and other damage to the skin caused by ultraviolet light (see sunscreen). It is the broadest spectrum UVA and UVB reflector that is approved for use as a sunscreen by the FDA[citation needed], and is completely photostable.[47].

Cigarette filters

Zinc oxide is a constituent of cigarette filters for removal of selected components from tobacco smoke. A filter consisting of charcoal impregnated with zinc oxide and iron oxide removes significant amounts of HCN and H2S from tobacco smoke without affecting its flavor.[35]

Food additive

Zinc oxide is added to many food products, e.g., breakfast cereals, as a source of zinc,[48] a necessary nutrient. (Other cereals may contain zinc sulfate for the same purpose.) Some prepackaged foods also include trace amounts of ZnO even if it is not intended as a nutrient.

Pigment

Zinc white is used as a pigment[49] in paints and is more opaque than lithopone, but less opaque than titanium dioxide. It is also used in coatings for paper. Chinese white is a special grade of zinc white used in artists' pigments. It is also a main ingredient of mineral makeup.[50]

Coatings

Paints containing zinc oxide powder have long been utilized as anticorrosive coatings for various metals. They are especially effective for galvanised Iron. The latter is difficult to protect because its reactivity with organic coatings leads to brittleness and lack of adhesion. Zinc oxide paints however, retain their flexibility and adherence on such surfaces for many years.[35]

ZnO highly n-type doped with Al, Ga or In is transparent and conductive (transparency ~90%, lowest resistivity ~10−4 Ω·cm[51]). ZnO:Al coatings are being 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.[3]

Various plastics, such as polyethylene naphthalate (PEN), can be protected by applying zinc oxide coating. The coating reduces the diffusion of oxygen with PEN.[52] Zinc oxide layers can also be used on polycarbonate (PC) in outdoor applications. The coating protects PC form solar radiation and decreases the oxidation rate and photo-yellowing of PC.[53]

Corrosion prevention in nuclear reactors

Zinc oxide depleted in the zinc isotope with the 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.[54]

Potential applications

Electronics

ZnO has wide direct band gap (3.37 eV or 375 nm at room temperature). Therefore, its most common potential applications are in laser diodes and light emitting diodes (LEDs). Some optoelectronic applications of ZnO overlap with that of GaN, which has a similar bandgap (~3.4 eV at room temperature). Compared to GaN, ZnO has a larger exciton binding energy (~60 meV, 2.4 times of the room-temperature thermal energy), which results in bright room-temperature emission from ZnO. Other properties of ZnO favorable for electronic applications include its stability to high-energy radiation and to wet chemical etching.[55] Radiation resistance[56] makes ZnO a suitable candidate for space applications.

The pointed tips of ZnO nanorods result in a strong enhancement of an electric field. Therefore, they can be used as field emitters.[57]

Aluminium-doped ZnO layers are used as a transparent electrodes. The constituents Zn and Al are much cheaper and less poisonous compared to the generally used indium tin oxide (ITO). One application which has begun to be commercially available is the use of ZnO as the front contact for solar cells or of liquid crystal displays.[58]

Transparent thin-film transistors (TTFT) can be produced with ZnO. As field-effect transistors, they even may not need a p–n junction,[59] thus avoiding the p-type doping problem of ZnO. Some of the field-effect transistors even use ZnO nanorods as conducting channels.[60]

Zinc oxide nanorod sensor

Zinc oxide nanorod sensors are devices detecting changes in electrical current passing through zinc oxide nanowires due to adsorption of gas molecules. Selectivity to hydrogen gas was achieved by sputtering Pd clusters on the nanorod surface. The addition of Pd appears to be effective in the catalytic dissociation of hydrogen molecules into atomic hydrogen, increasing the sensitivity of the sensor device. The sensor detects hydrogen concentrations down to 10 parts per million at room temperature, whereas there is no response to oxygen.[61][62]

Spintronics

ZnO has also been considered for spintronics applications: if doped with 1-10% of magnetic ions (Mn, Fe, Co, V, etc.), ZnO could become ferromagnetic, even at room temperature. Such room temperature ferromagnetism in ZnO:Mn has been observed,[63] but it is not clear yet whether it originates from the matrix itself or from secondary oxide phases.

Piezoelectricity

The piezoelectricity in textile fibers coated in ZnO have been shown capable of fabricating "self-powered nanosystems" with everyday mechanical stress from wind or body movements.[64][65]

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.[66]

Biosensor

ZnO has high biocompatibility and fast electron transfer kinetics. Such features advocate the use of this material as a biomimic membrane to immobilize and modify biomolecules.[67]

History

It is hardly possible to trace the first usage of zinc oxide – various zinc compounds were widely used by early humans, in various processed and unprocessed forms, as a paint or medicinal ointment, but their exact composition is uncertain.

The use of pushpanjan, probably zinc oxide, as a salve for eyes and open wounds, is mentioned in the Indian medical text the Charaka Samhita, thought to date from 500 BC or before.[68] Zinc oxide ointment is also mentioned by the Greek physician Dioscorides (1st century AD.)[69] Avicenna mentions zinc oxide in The Canon of Medicine (1025 AD), which mentioned it as a preferred treatment for a variety of skin conditions, including skin cancer. Though it is no longer used for treating skin cancer, it is still widely used to treat a variety of other skin conditions, in products such as baby powder and creams against diaper rashes, calamine cream, anti-dandruff shampoos, and antiseptic ointments.[43]

The Romans produced considerable quantities of brass (an alloy of zinc and copper) as early as 200 BC by a cementation process where copper was reacted with zinc oxide.[70] The zinc oxide is thought to have been produced by heating zinc ore in a shaft furnace. This liberated metallic zinc as a vapor, which then ascended the flue and condensed as the oxide. This process was described by Dioscorides in the 1st century AD.[71] Zinc oxide has also been recovered from zinc mines at Zawar in India, dating from the second half of the first millennium BC. This was presumably also made in the same way and used to produce brass.[69]

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 manufacture moved to China in the 17th century. In 1743, the first European zinc smelter was established in Bristol, United Kingdom.[72]

The main usage of zinc oxide (zinc white) was again paints and additive to ointments. Zinc white was accepted as a watercolor by 1834 but it did not mix well with oil. This problem was quickly solved by optimizing the synthesis of ZnO. In 1845, 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 very valuable for making tints with other colors; however, 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.[73]

In the recent times, most zinc oxide was used in the rubber industry (see applications above). In the 1970s, the second largest application of ZnO was photocopying. High-quality ZnO produced by the "French process" was added into the photocopying paper as a filler. This application was however soon displaced.[16]

Safety

As a food additive, zinc oxide is on the U.S. FDA's generally recognized as safe, or GRAS, substances.[74]

Zinc oxide itself is non-toxic; however it is hazardous to breathe zinc oxide fumes, as generated when zinc or zinc alloys are melted and oxidized at high temperature. This problem occurs while melting brass because the melting point of brass is close to the boiling point of zinc.[75] Exposure to zinc oxide in the air, which also occurs while welding galvanized (zinc plated) steel, can result in a nervous malady called metal fume fever. For this reason, typically galvanized steel is not welded, or the zinc is removed first.[76]

See also

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