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Zinc oxide

 

Zinc oxide

Zinc oxide
Names
Other names
Zinc white, Calamine, philosopher's wool, Chinese white, flowers of zinc
Identifiers
 YesY
ATCvet code QA07
ChEBI  YesY
ChEMBL  N
ChemSpider  YesY
EC number 215-222-5
PubChem
RTECS number ZH4810000
Properties
ZnO
Molar mass 81.38 g/mol
Appearance White solid
Odor odorless
Density 5.606 g/cm3
Melting point 1,975 °C (3,587 °F; 2,248 K) (decomposes)[1]
Boiling point 1,975 °C (3,587 °F; 2,248 K) (decomposes)
0.0004% (17.8°C)[2]
Band gap 3.3 eV (direct)
2.0041
Structure
Wurtzite
C6v4-P63mc
a = 3.25 Å, c = 5.2 Å
Tetrahedral
Thermochemistry
43.9 J·K−1mol−1
-348.0 kJ/mol
Hazards
Safety data sheet ICSC 0208
Dangerous for the environment (N)
R-phrases R50/53
S-phrases S60, S61
NFPA 704
1
2
0
W
Flash point 1,436 °C (2,617 °F; 1,709 K)
Lethal dose or concentration (LD, LC):
LD50 (Median dose)
240 mg/kg (intraperitoneal, rat)[3]
7950 mg/kg (rat, oral)[4]
2500 mg/m3 (mouse)[4]
2500 mg/m3 (guinea pig, 3-4 hr)[4]
US health exposure limits (NIOSH):
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]

500 mg/m3[2]
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  (: YesY/N?)

Zinc oxide is an 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. It occurs naturally as the mineral zincite, but most zinc oxide is produced synthetically.[6]

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 used in emerging applications for transparent electrodes in liquid crystal displays, in energy-saving or heat-protecting windows, and in electronics as thin-film transistors and light-emitting diodes.

Contents

  • Chemical properties 1
  • Physical properties 2
    • Structure 2.1
    • Mechanical properties 2.2
    • Electrical properties 2.3
  • Production 3
    • Indirect process 3.1
    • Direct process 3.2
    • Wet chemical process 3.3
    • Laboratory synthesis 3.4
    • ZnO nanostructures 3.5
  • History 4
  • Applications 5
    • Rubber manufacture 5.1
    • Ceramic industry 5.2
    • Medicine 5.3
    • Cigarette filters 5.4
    • Food additive 5.5
    • Pigment 5.6
    • UV absorber 5.7
    • Coatings 5.8
    • Corrosion prevention in nuclear reactors 5.9
    • Methane reforming 5.10
  • Potential applications 6
    • Electronics 6.1
    • Zinc oxide nanorod sensor 6.2
    • Spintronics 6.3
    • Piezoelectricity 6.4
  • Safety 7
  • See also 8
  • References 9
  • Reviews 10
  • External links 11

Chemical properties

Pure ZnO presents as 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 and in air 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 is soluble in (degraded by) most acids, such as hydrochloric acid:[10]

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.[11] This cement was used in dentistry.[12]

Hopeite

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]

It reacts with hydrogen sulfide to give the sulfide. This reaction is used commercially.

ZnO + H2S → ZnS + H2O

Physical properties

Wurtzite structure
A zincblende unit cell

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 yet.[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.[21] ZnO exhibits a very long lived optical phonon E2(low) with a lifetime as high as 133 ps at 10 K.[22]

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 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.[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

For industrial use, ZnO is produced at levels of 105 tons per year[8] by three main processes:[21]

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 purified zinc salts, from which zinc carbonate or zinc hydroxide is precipitated. The precipitate is then filtered, washed, dried and calcined at temperatures around 800 °C.

Laboratory synthesis

Synthetic ZnO crystals. Red and green color are associated with different concentrations of oxygen vacancies.[30]

A large number of 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), 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.[20][32]

  • Zincite properties
  • International Chemical Safety Card 0208.
  • NIOSH Pocket Guide to Chemical Hazards.
  • Zinc oxide in the Pesticide Properties DataBase (PPDB)
  • Zinc white pigment at ColourLex

External links

  • U. Ozgur et al. "A comprehensive review of ZnO materials and devices" (103 pages) J. Appl. Phys. 98 (2005) 041301 doi:10.1063/1.1992666
  • A. Bakin and A. Waag "ZnO Epitaxial Growth" (28 pages) Chapter in “Comprehensive Semiconductor Science and Technology“ 6 Volume Encyclopaedia, ELSEVIER, edited by Pallab Bhattacharya, Roberto Fornari and Hiroshi Kamimura, ISBN 978-0-444-53143-8
  • S. Baruah and J. Dutta "Hydrothermal growth of ZnO nanostructures" (18 pages) Sci. Technol. Adv. Mater. 10 (2009) 013001 doi:10.1088/1468-6996/10/1/013001 (free download)
  • R. Janisch et al. "Transition metal-doped TiO2 and ZnO—present status of the field" (32 pages) J. Phys.: Condens. Matter 17 (2005) R657 doi:10.1088/0953-8984/17/27/R01
  • Y.W. Heo et al. "ZnO nanowire growth and devices" (47 pages) Mater. Sci. Eng. R 47 (2004) 1 doi:10.1016/j.mser.2004.09.001
  • C. Klingshirn "ZnO: From basics towards applications" (46 pages) Phys. Stat. Solidi (b) 244 (2007) 3027 doi:10.1002/pssb.200743072
  • C. Klingshirn "ZnO: Material, Physics and Applications" (21 pages) ChemPhysChem 8 (2007) 782 doi:10.1002/cphc.200700002
  • J. G. Lu et al. "Quasi-one-dimensional metal oxide materials—Synthesis, properties and applications" (42 pages) Mater. Sci. Eng. R 52 (2006) 49 doi:10.1016/j.mser.2006.04.002
  • S. Xu and Z. L. Wang "One-dimensional ZnO nanostructures: Solution growth and functional properties" (86 pages) Nano Res. 4 (2011) 1013 doi:10.1007/s12274-011-0160-7
  • S. Xu and Z. L. Wang "Oxide nanowire arrays for light-emitting diodes and piezoelectric energy harvesters" (28 pages) Pure Appl. Chem. 83 (2011) 2171 doi:10.1351/PAC-CON-11-08-17

Reviews

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  6. ^ a b Marcel De Liedekerke, "2.3. Zinc Oxide (Zinc White): Pigments, Inorganic, 1" in Ullmann's Encyclopdia of Industrial Chemistry, 2006, Wiley-VCH, Weinheim. doi:10.1002/14356007.a20_243.pub2
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  16. ^ Zinc oxide MSDS. hazard.com. Accessdate January 25, 2009.
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  45. ^ "zinc." Encyclopædia Britannica. 2009 Encyclopædia Britannica. 10 March 2009
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  47. ^ General Information of Zinc from the National Institute of Health, WHO, and International Zinc Association. Retrieved 10 March 2009
  48. ^ Zinc zhite. Retrieved 10 March 2009
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  50. ^ a b c Nav Bharat Metallic Oxide Industries Pvt. Limited. Applications of ZnO. Access date January 25, 2009.
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  72. ^ Quaker cereals content. quakeroats.com
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  74. ^ Kuhn, H., Zinc White, in Artists’ Pigments. A Handbook of Their History and Characteristics, Vol. 1, L. Feller, Ed., Cambridge University Press, London 1986, p. 169 – 186
  75. ^ Vincent van Gogh, 'Wheatfield with Cypresses, 1889, pigment analysis at ColourLex
  76. ^
  77. ^ US Environment Protection Agency: Sunscreen What are the active Ingredients in Sunscreen – Physical Ingredients:"The physical compounds titanium dioxide and zinc oxide reflect, scatter, and absorb both UVA and UVB rays." A table lists them as providing extensive physical protection against UVA and UVB
  78. ^ (2006). Look Sharp While Seeing Sharp. (Originating Technology/NASA Contribution).no author Spinoff 2006: NASA Scientific and Technical Information (STI). Retrieved 17 October 2009. JPL scientists developed UV-protective sunglasses using dyes and "zinc oxide, which absorbs ultraviolet light"
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References

See also

Zinc oxide itself is non-toxic; however it is hazardous to inhale 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.[99] 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.[100]

As a food additive, zinc oxide is on the U.S. FDA's list of generally recognized as safe, or GRAS, substances.[98]

Safety

[97] at the Center for Nanostructure Characterization In 2008 the

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.[95][96]

Piezoelectricity

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,[94] but it is not clear yet whether it originates from the matrix itself or from secondary oxide phases.

Spintronics

Zinc oxide nanorod sensors are devices detecting changes in electric 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.[92][93]

Zinc oxide nanorod sensor

Transparent thin-film transistors (TTFT) can be produced with ZnO. As field-effect transistors, they even may not need a p–n junction,[90] thus avoiding the p-type doping problem of ZnO. Some of the field-effect transistors even use ZnO nanorods as conducting channels.[91]

Aluminium-doped ZnO layers are used as a transparent electrodes. The constituents Zn and Al are much cheaper and less toxic 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.[89]

The pointed tips of ZnO nanorods result in a strong enhancement of an electric field. Therefore, they can be used as field emitters.[88]

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).[84] 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. ZnO can be combined with GaN for LED-applications. For instance as TCO layer and ZnO nanostructures provide better light outcoupling.[85] Other properties of ZnO favorable for electronic applications include its stability to high-energy radiation and to wet chemical etching.[86] Radiation resistance[87] 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.

Electronics

Potential applications

The zinc sulfide (ZnS) is replaced with fresh zinc oxide when the zinc oxide has been consumed.[83]

H2S + ZnO → H2O + ZnS

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:

Methane reforming

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.[82]

Corrosion prevention in nuclear reactors

Plastics, such as polyethylene naphthalate (PEN), can be protected by applying zinc oxide coating. The coating reduces the diffusion of oxygen with PEN.[80] Zinc oxide layers can also be used on polycarbonate (PC) in outdoor applications. The coating protects PC from solar radiation and decreases the oxidation rate and photo-yellowing of PC.[81]

ZnO highly n-type doped with Al, Ga, or In is transparent and conductive (transparency ~90%, lowest resistivity ~10−4 Ω·cm[79]). 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.[8]

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.[50]

Coatings

Micronized and nano-scale zinc oxide and titanium dioxide provide strong protection against UVA ultraviolet radiation, and are used in suntan lotion,[77] and also in UV-blocking sunglasses for use in space and for protection when welding, following research by scientists at Jet Propulsion Laboratory (JPL).[78]

UV absorber

Zinc white is used as a pigment in paints and is more opaque than lithopone, but less opaque than titanium dioxide.[6] It is also used in coatings for paper. Chinese white is a special grade of zinc white used in artists' pigments. The use of zinc white (zinc oxide) as a pigment in oil painting started in the middle of 18th century.[74] It has partly replaced the poisonous lead white and was used by painters such as Böcklin, Van Gogh,[75] Manet, Munch and others. It is also a main ingredient of mineral makeup (CI 77947).[76]

Pigment

Zinc oxide was linked to dioxin contamination in pork exports in the 2008 Chilean pork crisis. The contamination was found to be due to dioxin contaminated zinc oxide used in pig feed.[73]

Zinc oxide is added to many food products, including breakfast cereals, as a source of zinc,[72] 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.

Food additive

Zinc oxide is a constituent of cigarette filters. A filter consisting of charcoal impregnated with zinc oxide and iron oxide removes significant amounts of hydrogen cyanide (HCN) and hydrogen sulfide (H2S) from tobacco smoke without affecting its flavor.[50]

Cigarette filters

Zinc oxide nanoparticles can enhance the antibacterial activity of Ciprofloxacin. It has been shown that nano ZnO which has the average size between 20 nm and 45 nm can enhance the antibacterial activity of Ciprofloxacin against Staphylococcus aureus and Escherichia coli in vitro. The enhancing effect of this nanomaterial is concentration-dependent against all test strains. This effect may be due to two reasons. First, Zinc Oxide nanoparticles can interfere with NorA protein. NorA is a protein which is developed for conferring resistance in bacteria and has pumping activity that mediate the effluxing of hydrophilic fluroquinolones from a cell. Second, Zinc Oxide nanoparticles can interfere with Omf protein. Omf is a membrane protein that is responsible for the permeation of quinolones into the cell.[71]

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. There has been concern that they might be absorbed into the skin.[67][68] A study published in 2010 found a 0.23% to 1.31% (mean 0.42%) of blood zinc levels in venous blood samples could be traced to zinc from ZnO nanoparticles applied to human skin for 5 days, and traces were also found in urine samples.[69] In contrast, a comprehensive review of the medical literature from 2011 says that no evidence of systemic absorption can be found in the literature.[70]

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,[63] and is completely photostable.[64] When used as an ingredient in sunscreen, zinc oxide blocks both UVA (320–400 nm) and UVB (280–320 nm) rays of ultraviolet light. Zinc oxide and the other most common physical sunscreen, titanium dioxide, are considered to be nonirritating, nonallergenic, and non-comedogenic.[65] Zinc from zinc oxide is, however, slightly absorbed into the skin [66]

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.[44][61] It is also a component in tape (called "zinc oxide tape") used by athletes as a bandage to prevent soft tissue damage during workouts.[62]

Reflecting the basic properties of ZnO, fine particles of the oxide have deodorizing and antibacterial[55] properties and for that reason are added into materials including cotton fabric, rubber, oral care products,[56][57] and food packaging.[58][59] Enhanced antibacterial action of fine particles compared to bulk material is not exclusive to ZnO and is observed for other materials, such as silver.[60] This property is due to the increased surface area of the fine particles.

Zinc oxide as a mixture with about 0.5% iron(III) oxide (Fe2O3) is called calamine and is used in calamine lotion. Two minerals, zincite and hemimorphite, have been historically called calamine. When mixed with eugenol, a ligand, zinc oxide eugenol is formed, which has applications as a restorative and prosthodontic in dentistry.[12][54]

Medicine

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.[51]

Ceramic industry

Between 50% and 60% of ZnO use is in the rubber industry.[51] Zinc oxide along with stearic acid is used in the vulcanization of rubber[21][52][53] ZnO additive also protect rubber from fungi (see medical applications) and UV light.

Rubber manufacture

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,[49] rubber, lubricants,[5] paints, ointments, adhesive, sealants, concrete manufacturing, pigments, foods, batteries, ferrites, fire retardants, etc.[50]

Applications

In recent times, most zinc oxide was used in the rubber industry (see applications below). In the 1970s, the second largest application of ZnO was photocopying. High-quality ZnO produced by the "French process" was added to photocopying paper as a filler. This application was soon displaced.[21]

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, 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.[48]

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.[47]

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.[45] 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.[46] 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.[43]

Zinc compounds were probably used by early humans, in processed and unprocessed forms, as a paint or medicinal ointment, but their 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.[42] Zinc oxide ointment is also mentioned by the Greek physician Dioscorides (1st century AD.)[43] 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.[44]

History

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.[37] 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 physical vapor deposition methods to deposit ZnO thin films.[38][39] 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. Aligned ZnO nanowires can be used in dye-sensitized solar cells and field emission devices.[40][41]

[36] 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).[35] Doping of the ZnO nanowires has been achieved by adding other metal nitrates to the growth solution.[34], the latter providing the basic environment. Certain additives, such as polyethylene glycol or polyethylenimine, can improve the aspect ratio of the ZnO nanowires.hexamine The synthesis is typically carried out at temperatures of about 90 °C, in an equimolar aqueous solution of zinc nitrate and [33]

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