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Zinc oxide
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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 tyres), lubricants, paints, ointments, adhesives, sealants, pigments, foods (source of Zn nutrient), batteries, ferrites, fire retardants, etc. ZnO is present in the Earth crust as a 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.
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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 tyres), lubricants, paints, ointments, adhesives, sealants, pigments, foods (source of Zn nutrient), batteries, ferrites, fire retardants, etc. ZnO is present in the Earth crust as a 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 transistor and light-emitting diode are forthcoming as of 2009.
Chemical properties
ZnO occurs as white powder commonly 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. Crystalline zinc oxide is thermochromic, changing from white to yellow when heated and in air reverting to white on cooling. This 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.
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:
- ZnO + 2 HCl ? ZnCl2 + H2O
Bases also degrade the solid to give soluble zincates:
- ZnO + 2NaOH + 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.. This cement was used in dentistry.
ZnO also forms cement-like products when reacted with phosphoric acid, and this forms the basis of zinc phosphate cements used in dentistry. A major component of zinc phosphate cement produced by this reaction is hopeite, Zn3(PO4)2·4H2O.
ZnO decomposes into zinc vapor and oxygen only at around 1975 °C, reflecting its considerable stability. Heating with carbon converts the oxide into zinc vapor:
- ZnO + C ? Zn + CO
Zinc oxide reacts violently with aluminum and magnesium powders, with chlorinated rubber and linseed oil on heating causing fire and explosion hazard.
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.
Physical properties
Crystal structure
Zinc oxide crystallizes in three forms: hexagonal wurtzite, cubic zincblende, and the rarely observed cubic rocksalt). The wurtzite structure is most stable and thus most common at ambient conditions. The zincblende form can be stabilized by growing ZnO on substrates with cubic lattice structure. In both cases, the zinc and oxide are tetrahedral. The rocksalt NaCl-type structure is only observed at relatively high pressures - ~10 GPa.
The hexagonal and zincblende ZnO lattices 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 Ĺ and c = 5.2 Ĺ; their ratio c/a ~ 1.60 is close to the ideal value for hexagonal cell c/a = 1.633. As in most II-VI materials, the bonding in ZnO is largely ionic, which explains its strong piezoelectricity. Due to this ionicity, 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.
Mechanical properties
ZnO is a relatively soft material with approximate hardness of 4.5 on the Mohs scale. 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 benefitial for ceramics .
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. 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; 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.
Most ZnO has n-type character, even in the absence of intentional doping. Native defects such as oxygen vacancies or zinc interstitials are often assumed to be the origin of this, but the subject remains controversial. An alternative explanation has been proposed, based on theoretical calculations, that unintentional substitutional hydrogen impurities are responsible.
Controllable n-type doping is easily achieved by substituting Zn with group-III elements Al, Ga, In or by substituting oxygen with group-VII elements chlorine or iodine.
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, and it is pertinent not only to ZnO, but also to similar compounds GaN and ZnSe. Measurement of p-type in "intrinsically" n-type material is also not easy because inhomogeneity results in spurious signals.
Current absence of p-type ZnO does limit its electronic and optoelectronic applications 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.
Electron mobility of ZnO strongly varies with temperature and has a maximum of ~2000 cm2/(V·s) at ~80 Kelvin. Data on hole mobility are scarce with values in the range 5-30 cm2/(V·s).
Production
For industrial use, ZnO is produced at levels of 105 tons per year by three main processes :
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 micrometres to a few micrometres. 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).
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
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", i.e. close to room temperature or "high", i.e. 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, or melt growth. 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. Thin films can be produced by chemical vapor deposition, metalorganic vapour phase epitaxy, electrodeposition, pulsed laser deposition, sputtering, sol-gel synthesis, spray pyrolysis, etc. Nanostructures can be obtained with most above-mentioned techniques, at certain conditions, and also with the vapor-liquid-solid method.
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, good thermal, binding, antibacterial and UV-protection properties. Consequently, it is added into various materials and products, including plastics, ceramics, glass, cement, rubber, lubricants, paints, ointments, adhesive, sealants, pigments, foods, batteries, ferrites, fire retardants, etc.
Rubber manufacture
About 50% of ZnO use is in rubber industry. Zinc oxide activates vulcanization, which otherwise may not occur at all . 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. 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.
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 called calamine historically (see: calamine (mineral)).
When mixed with eugenol, a chelate, zinc oxide eugenol is formed which has restorative and prosthodontic applications in dentistry.
Reflecting the basic properties of ZnO, fine particles of the oxide have deodorizing and antibacterial action and for that reason are added into various materials including cotton fabric, rubber, food packaging, etc. Enhanced antibacterial action of fine particles compared to bulk material is not intrinsic to ZnO and is observed for other materials, such as silver.
Zinc oxide is also a component of barrier cream used in nappy rash or diaper rash
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 flavour.
Food additive
Zinc oxide is added to many food products, e.g., breakfast cereals, as a source of zinc, 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 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. Because it absorbs both UVA and UVB rays of ultraviolet light, 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 absorber that is approved for use as a sunscreen by the FDA, and is completely photostable. It is also a main ingredient of mineral makeup.
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.
ZnO highly n-type doped with Al, Ga or nitrogen is transparent and conductive (transparency ~90%, lowest resistivity ~10-4 Ocm), and is used as a transparent electrode. 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.
Another application is ZnO:Al coating 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.
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.
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. Radiation resistance 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.
Transparent thin-film transistors (TTFT) can be produced with ZnO. As field-effect transistors, they even may not need a p–n junction, thus avoiding the p-type doping problem of ZnO. Some of the field-effect transistors even use ZnO nanorods as conducting channels.
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.
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, but it is not clear yet whether it originates from the matrix itself or from Mn-containing precipitates.
Piezoelectricity
The piezoelectricity in textile fibers coated in ZnO have been shown capable of "self-powering nanosystems" with everyday mechanical stress generated by wind or body movements.
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 wires. 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 200-300 micrometres and diameters of three to five micrometres, but the device could be scaled down to nanometer size.
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.
History
It is hardly possible to trace the first usage of zinc oxide - various zinc compounds have been widely used by early humans, in various processed and unprocessed forms, as a paint or medicinal ointment, but their exact composition is uncertain. As mentioned in the production sections, common ways of making zinc oxide require relatively high temperatures of 1000 °C that probably hindered ZnO manufacture in the early ages. The Romans produced considerable quantities of brass (an alloy of zinc and copper) as early as 200 bc. Zinc oxide should have been obtained as by product, but its usage is not documented .
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. Zinc was recognized as a separate metal in Europe in 1546. In 1743, the first European zinc smelter was established in Bristol, United Kingdom .
The main usage of zinc oxide (zinc zhite) 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 paints 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 1890's and early 1900's, some artists used zinc white as a ground for their oil paintings. All those paintings developed cracks over the years
In the recent times, most zinc oxide was used in the rubber industry (see applications above). In the 1970ies, 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 expelled from the market by optimizing the process to use ordinary paper .
Safety
As a food additive, Zinc oxide is on FDA's generally recognized as safe, or GRAS, substances.
Zinc oxide itself is non-toxic; however it is hazardous to breathe zinc oxide fumes. Fumes of zinc oxide are generated when zinc or zinc alloys are melted and oxidized at high temperature. This occurs while melting brass because the melting point of brass is close to the boiling point of zinc. 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.
See also
Reviews
- U. Ozgur et al. "A comprehensive review of ZnO materials and devices" (103 pages) - a very highly cited article (~800 citations in the period 2005-2008 according to Web of Science)
- S. Baruah and J. Dutta "Hydrothermal growth of ZnO nanostructures" (18 pages) Sci. Technol. Adv. Mater. 10 (2009) 013001
- R. Janisch et al. "Transition metal-doped TiO2 and ZnO—present status of the field" (32 pages)
- Y.W. Heo et al. "ZnO nanowire growth and devices" (47 pages)
- C. Klingshirn "ZnO: From basics towards applications" (46 pages)
- C. Klingshirn "ZnO: Material, Physics and Applications" (21 pages)
- J. G. Lu et al. "Quasi-one-dimensional metal oxide materials—Synthesis, properties and applications" (42 pages)
External links
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