Zinc oxide is aninorganic compound with theformulaZnO. It is a white powder which is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics,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 mineralzincite, most zinc oxide is produced synthetically.[14]
Early humans probably used zinc compounds in processed[14] and unprocessed forms, as paint or medicinal ointment; however, their composition is uncertain. The use ofpushpanjan, probably zinc oxide, as a salve for eyes and open wounds is mentioned in the Indian medical text theCharaka Samhita, thought to date from 500 BC or before.[15] Zinc oxide ointment is also mentioned by the Greek physicianDioscorides (1st century AD).[16]Galen suggested treating ulcerating cancers with zinc oxide,[17] as didAvicenna in hisThe Canon of Medicine. It is used as an ingredient in products such asbaby powder and creams againstdiaper rashes,calamine cream, anti-dandruffshampoos, andantiseptic ointments.[18]
The Romans produced considerable quantities ofbrass (an alloy ofzinc andcopper) as early as 200 BC by a cementation process where copper was reacted with zinc oxide.[19] 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 byDioscorides in the 1st century AD.[20] Zinc oxide has also been recovered from zinc mines at Zawar inIndia, 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 inBristol, United Kingdom.[21] Around 1782,Louis-Bernard Guyton de Morveau proposed replacinglead 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; by 1850, zinc white was being manufactured throughout Europe. The success of zinc white paint was due to its advantages over the traditionalwhite 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. These paintings developed cracks over time.[23]
In recent times, most zinc oxide has been used in therubber industry to resistcorrosion. In the 1970s, the second largest application of ZnO wasphotocopying. High-quality ZnO produced by the "French process" was added to photocopying paper as a filler. This application was soon displaced bytitanium.[24]
Pure ZnO is a white powder. However, in nature, it occurs as the rare mineralzincite, which usually containsmanganese and other impurities that confer a yellow to red color.[25]
Crystalline zinc oxide isthermochromic, changing from white to yellow when heated in air and reverting to white on cooling.[26] This color change is caused by a small loss of oxygen to the environment at high temperatures to form thenon-stoichiometric Zn1+xO, where at 800 °C, x = 0.00007.[26]
Solid zinc oxide will also dissolve in alkalis to give soluble zincates:[27]
ZnO + 2 NaOH + H2O → Na2[Zn(OH)4]
ZnO reacts slowly with fatty acids in oils to produce the correspondingcarboxylates, such asoleate orstearate. When mixed with a strong aqueous solution ofzinc 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 withphosphoric acid; related materials are used in dentistry.[29] A major component of zinc phosphate cement produced by this reaction ishopeite, Zn3(PO4)2·4H2O.[30]
ZnO decomposes into zinc vapor and oxygen at around 1975 °C with a standard oxygen pressure. In acarbothermic reaction, heating with carbon converts the oxide into zinc vapor at a much lower temperature (around 950 °C).[27]
Zinc oxide crystallizes in two mainforms, hexagonalwurtzite[31] and cubiczincblende. 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 aretetrahedral, the most characteristic geometry for Zn(II). ZnO converts to therocksalt motif at relatively high pressures about 10 GPa.[13]
Hexagonal[32] and zincblende polymorphs have noinversion symmetry (reflection of a crystal relative to any given point does not transform it into itself).[33] This and other lattice symmetry properties result inpiezoelectricity of the hexagonal[32] and zincblende[33] ZnO, andpyroelectricity of hexagonal ZnO.[34]
The hexagonal structure has a point group 6 mm (Hermann–Mauguin notation) or C6v (Schoenflies notation), and thespace group is P63mc or C6v4. The lattice constants area = 3.25 Å andc = 5.2 Å; their ratioc/a ~ 1.60 is close to the ideal value for hexagonal cellc/a = 1.633.[35] As in mostgroup II-VI materials, the bonding in ZnO is largelyionic (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,[36] as well as the strongpiezoelectricity 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.[37] However, studies using wurtzoid structures explained the origin of surface flatness and the absence of reconstruction at ZnO wurtzite surfaces[38] in addition to the origin of charges on ZnO planes.
ZnO is a wide-band gap semiconductor of theII-VI semiconductor group. The nativedoping 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 theMohs scale.[12] Its elastic constants are smaller than those of relevant III-V semiconductors, such asGaN. The high heat capacity and heat conductivity, low thermal expansion and high melting temperature of ZnO are beneficial for ceramics.[24] The E2optical phonon in ZnO exhibits an unusually long lifetime of 133 ps at 10 K.[39]
Among the tetrahedrally bonded semiconductors, it has been stated that ZnO has the highest piezoelectric tensor, or at least one comparable to that ofGaN andAlN.[40] This property makes it a technologically important material for manypiezoelectrical applications, which require a large electromechanical coupling. Therefore, ZnO in the form ofthin film has been one of the most studied and used resonator materials forthin-film bulk acoustic resonators.[41]
ZnO has a relatively widedirectband gap of ~3.3 eV at room temperature. Advantages associated with a wide band gap include higherbreakdown voltages, ability to sustain large electric fields, lowerelectronic noise, and high-temperature and high-power operation. The band gap of ZnO can further be tuned to ~3–4 eV by its alloying withmagnesium oxide orcadmium 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.[44]
Most ZnO hasn-type character, even in the absence of intentionaldoping.Nonstoichiometry is typically the origin of n-type character, but the subject remains controversial.[45] An alternative explanation has been proposed, based on theoretical calculations, that unintentional substitutional hydrogen impurities are responsible.[46] 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 elementschlorine oriodine.[47]
Reliablep-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 withGaN andZnSe. Measurement of p-type in "intrinsically" n-type material is complicated by the inhomogeneity of samples.[48]
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.[49] Data on hole mobility are scarce with values in the range 5–30 cm2/(V·s).[50]
Zinc oxide is noted for its stronglynonlinear optical properties, especially in bulk. The nonlinearity of ZnO nanoparticles can be fine-tuned according to their size.[53]
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,[54] 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.[citation needed]
The direct or American process starts with diverse contaminated zinc composites, such aszinc ores or smelter by-products. The zinc precursors are reduced (carbothermal reduction) by heating with a source of carbon such asanthracite 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.[54]
A small amount of industrial production involves wet chemical processes, which start with aqueous solutions of zinc salts, from whichzinc carbonate orzinc hydroxide is precipitated. The solid precipitate is then calcined at temperatures around 800 °C.[citation needed]
The red and green colors of these synthetic ZnO crystals result from different concentrations of oxygen vacancies.[55]
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.[citation needed]
Zinc oxide can be produced in bulk byprecipitation from zinc compounds, mainlyzinc acetate, in various solutions, such as aqueoussodium hydroxide or aqueousammonium carbonate.[60] Synthetic methods characterized in literature since the year 2000 aim to produce ZnO particles with high surface area and minimal size distribution, including precipitation,mechanochemical, sol-gel,microwave, andemulsion methods.[61]
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 thevapor–liquid–solid method.[37][62][63] The synthesis is typically carried out at temperatures of about 90 °C, in an equimolar aqueous solution ofzinc nitrate andhexamine, the latter providing the basic environment. Certain additives, such as polyethylene glycol or polyethylenimine, can improve the aspect ratio of the ZnO nanowires.[64] Doping of the ZnO nanowires has been achieved by adding other metal nitrates to the growth solution.[65] 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).[66]
Aligned ZnO nanowires on pre-seededsilicon,glass, andgallium nitride substrates have been grown using aqueous zinc salts such as zinc nitrate andzinc acetate in basic environments.[67] 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 ofzinc acetate crystallites,spin coating of ZnO nanoparticles, and the use ofphysical vapor deposition methods to deposit ZnO thin films.[68][69] Pre-seeding can be performed in conjunction with top down patterning methods such aselectron beam lithography and nanosphere lithography to designate nucleation sites prior to growth. Aligned ZnO nanowires can be used indye-sensitized solar cells and field emission devices.[70][71]
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 highrefractive index, high thermal conductivity, binding, antibacterial and UV-protection properties. Consequently, it is added into materials and products including plastics, ceramics, glass, cement,[72] rubber, lubricants,[12] paints, ointments, adhesive, sealants,concrete manufacturing, pigments, foods, batteries,ferrites, and fire retardants.[73]
Between 50% and 60% of ZnO use is in the rubber industry.[74] Zinc oxide along withstearic acid is used in thesulfur vulcanization of rubber.[24][75] ZnO additives in the form of nanoparticles are used in rubber as a pigment[76] and to enhance its durability,[77] and have been used in composite rubber materials such as those based onmontmorillonite to impartgermicidal properties.[78]
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.[74]
Zinc oxide as a mixture with about 0.5%iron(III) oxide (Fe2O3) is called calamine and is used incalamine lotion, a topical skin treatment.[79] Historically, the namecalamine was ascribed to a mineral that contained zinc used in powdered form as medicine,[80] but it was determined in 1803 that ore described as calamine was actually a mixture of the zinc mineralssmithsonite andhemimorphite.[81]
ZnO is added to cotton fabric, rubber, oral care products,[95][96] and food packaging.[97][98] Enhanced antibacterial action of fine particles compared to bulk material is not exclusive to ZnO and is observed for other materials, such assilver.[99] The mechanism of ZnO's antibacterial effect has been variously described as the generation ofreactive oxygen species, the release of Zn2+ ions, and a general disturbance of the bacterial cell membrane by nanoparticles.[100]
Zinc oxide is used insunscreen to absorbultraviolet light.[82] It is the broadest spectrum UVA and UVB absorber[101][102] that is approved for use as a sunscreen by the U.S.Food and Drug Administration (FDA),[103] and is completely photostable.[104] When used as an ingredient in sunscreen, zinc oxide blocks bothUVA (320–400 nm) andUVB (280–320 nm) rays ofultraviolet light. Zinc oxide and the other most common physical sunscreen,titanium dioxide, are considered to be nonirritating, nonallergenic, and non-comedogenic.[105] Zinc from zinc oxide is, however, slightly absorbed into the skin.[106]
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[107] and are only absorbed into theoutermost layer of the skin but not into the body.[107]
Zinc oxide (zinc white) is used as apigment inpaints and is more opaque thanlithopone, but less opaque thantitanium dioxide.[14] It is also used in coatings for paper. Chinese white is a special grade of zinc white used in artists' pigments.[111] The use of zinc white as a pigment in oil painting started in the middle of 18th century.[112] It has partly replaced the poisonouslead white and was used by painters such asBöcklin,Van Gogh,[113]Manet,Munch and others. It is also a main ingredient of mineral makeup (CI 77947).[114]
Micronized and nano-scale zinc oxide provides strong protection againstUVA andUVBultraviolet radiation, and are consequently used insunscreens,[115] and also in UV-blockingsunglasses for use in space and for protection whenwelding, following research by scientists at Jet Propulsion Laboratory (JPL).[116]
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.[73]
ZnO highly n-type doped withaluminium,gallium, orindium is transparent and conductive (transparency ~90%, lowestresistivity ~10−4 Ω·cm[117]). 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 aspolyethylene naphthalate (PEN), can be protected by applying zinc oxide coating. The coating reduces the diffusion of oxygen through PEN.[118] Zinc oxide layers can also be used onpolycarbonate in outdoor applications. The coating protects polycarbonate from solar radiation, and decreases its oxidation rate and photo-yellowing.[119]
Zinc oxide (ZnO) is used as a pretreatment step to removehydrogen sulfide (H2S) fromnatural gas followinghydrogenation of anysulfur compounds prior to amethane reformer, which can poison the catalyst. At temperatures between about 230–430 °C (446–806 °F), H2S is converted towater by the following reaction:[121]
ZnO has widedirect band gap (3.37 eV or 375 nm at room temperature). Therefore, its most common potential applications are in laser diodes andlight emitting diodes (LEDs).[124] Moreover, ultrafast nonlinearities and photoconductive functions have been reported in ZnO.[125] Some optoelectronic applications of ZnO overlap with that ofGaN, which has a similar band gap (~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. ZnO can be combined with GaN for LED-applications. For instance, atransparent conducting oxide layer and ZnO nanostructures provide better light outcoupling.[126] 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.[127] Radiation resistance[128] makes ZnO a suitable candidate for space applications. Nanostructured ZnO is an effective medium both in powder and polycrystalline forms inrandom lasers,[129] due to its highrefractive index and aforementioned light emission properties.[130]
Zinc oxide is used in semiconductorgas sensors for detecting airborne compounds such ashydrogen sulfide,nitrogen dioxide, andvolatile organic compounds. ZnO is a semiconductor that becomesn-doped by adsorption ofreducing compounds, which reduces the detected electrical resistance through the device, in a manner similar to the widely usedtin oxide semiconductor gas sensors. It is formed into nanostructures such as thin films,nanoparticles,nanopillars, ornanowires to provide a 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.[131][132]
Aluminium-doped ZnO layers are used as transparentelectrodes. The components Zn and Al are much cheaper and less toxic compared to the generally usedindium 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 ofliquid crystal displays.[42]
Transparent thin-filmtransistors (TTFT) can be produced with ZnO. As field-effect transistors, they do not need a p–n junction,[133] thus avoiding the p-type doping problem of ZnO. Some of the field-effect transistors even use ZnO nanorods as conducting channels.[134]
Thepiezoelectricity intextile fiberscoated in ZnO have been shown capable of fabricating "self-powered nanosystems" with everyday mechanical stress from wind or body movements.[135][136]
ZnO, both in macro-[137] and nano-[138] scales, could in principle be used as an electrode inphotocatalysis, mainly as ananode[139] ingreen chemistry applications. As a photocatalyst, ZnO reacts when exposed toUV radiation[137] and is used inphotodegradation reactions to remove organic pollutants from the environment.[140][141] It is also used to replace catalysts used inphotochemical reactions that would ordinarily require costly or inconvenient reaction conditions with lowyields.[137]
ZnO is a promising anode material forlithium-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).[143] ZnO is also used as an electrode in supercapacitors.[144]
Zinc oxide itself is non-toxic; it is hazardous, however, to inhale high concentrations of zinc oxide fumes, such as those generated when zinc or zinc alloys are melted and oxidized at high temperature. This problem occurs while melting alloys containingbrass because the melting point of brass is close to the boiling point of zinc.[146] Inhalation of zinc oxide, which may occur when welding galvanized (zinc-plated)steel, can result in a malady calledmetal fume fever.[146]
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.[147]
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