Zinc oxide (ZnO) is a versatile inorganic compound with the chemical formula ZnO. It presents as a white powder that is insoluble in water. This compound is widely utilized as an additive in a multitude of materials and products, including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cement, lubricants, paints, sunscreens, ointments, calamine, adhesives, sealants, pigments, foods, batteries, ferrites, fire retardants, semiconductors, and first-aid tapes.
Historically, early humans likely used zinc compounds in both processed and unprocessed forms, such as paint or medicinal ointments, though their exact composition remains uncertain. The Indian medical text, the Charaka Samhita, dating back to 500 BC or earlier, mentions the use of pushpanjan, likely zinc oxide, as a salve for eyes and open wounds. The Greek physician Dioscorides (1st century AD) also noted zinc oxide ointment, and Galen suggested its use for treating ulcerating cancers, a practice echoed by Avicenna in his The Canon of Medicine. The Romans, as early as 200 BC, produced significant quantities of brass, an alloy of zinc and copper, through a cementation process involving the reaction of copper with zinc oxide. It is believed that the zinc oxide was produced by heating zinc ore in a shaft furnace, releasing zinc vapor that condensed as the oxide. Between the 12th and 16th centuries, India recognized and produced zinc and zinc oxide using a primitive direct synthesis process. Zinc manufacturing later moved to China in the 17th century.
The primary historical uses of zinc oxide, known as zinc white, were in paints and as an additive in ointments. Zinc white was recognized as a pigment in oil paintings by 1834, although it did not mix well with oil initially. This issue was resolved through optimized synthesis of ZnO. By 1850, zinc white was being manufactured across Europe, with Edme-Jean Leclaire in Paris being a large-scale producer of the oil paint from 1845. The success of zinc white paint stemmed from its advantages over traditional white lead: it is permanent in sunlight, not blackened by sulfur-bearing air, non-toxic, and more economical. Its "clean" nature made it valuable for tinting with other colors, though it could produce a brittle dry film when used alone. In the late 1890s and early 1900s, some artists used zinc white as a ground layer for their oil paintings.
Properties of Zinc Oxide
Chemical and Physical Properties
Zinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. ZnO is present in the Earth's crust as the mineral zincite. It is an amphoteric oxide, meaning it can react with both acids and bases. ZnO reacts slowly with fatty acids in oils to form corresponding carboxylates, such as oleate or stearate. At approximately 1975 °C under standard oxygen pressure, ZnO decomposes into zinc vapor and oxygen.
Zinc oxide crystallizes in two primary forms: hexagonal wurtzite and cubic zincblende. The wurtzite structure is the most stable at ambient conditions and is therefore the most common. The zincblende form can be stabilized by growing ZnO on substrates with a cubic lattice structure. In both structures, the zinc and oxide centers are tetrahedral, which is characteristic of Zn(II). The hexagonal structure has a point group of 6mm (Hermann-Mauguin notation) or C6v (Schoenflies notation), with the space group P63mc or C6v4. The lattice constants are a = 3.25 Å and c = 5.2 Å, with a c/a ratio of approximately 1.60, close to the ideal hexagonal cell value of 1.633. The bonding in ZnO is largely ionic (Zn2+O2−), with ionic radii of 0.074 nm for Zn2+ and 0.140 nm for O2−. This ionic character contributes to the preferential formation of the wurtzite structure and the strong piezoelectricity of ZnO. Due to the polar Zn−O bonds, the zinc and oxygen planes are electrically charged.
ZnO is a relatively soft material, with a Mohs hardness of approximately 4.5. Its elastic constants are lower than those of relevant III-V semiconductors like GaN. Among tetrahedrally bonded semiconductors, ZnO is noted for having one of the highest piezoelectric tensors, comparable to GaN and AlN. This property makes it technologically significant for many piezoelectric applications requiring substantial electromechanical coupling.
Semiconducting and Optical Properties
Favorable properties of zinc oxide include good transparency, high electron mobility, a wide band gap, and strong room-temperature luminescence. ZnO is a semiconductor belonging to the II-VI semiconductor group, possessing a relatively wide direct band gap of approximately 3.3 eV at room temperature. This wide band gap offers advantages such as higher breakdown voltages, the ability to sustain large electric fields, lower electronic noise, and high-temperature and high-power operation. The band gap of ZnO can be further tuned to approximately 3-4 eV by alloying with magnesium oxide or cadmium oxide. Due to this large band gap, efforts have been made to create visibly transparent solar cells utilizing ZnO as a light-absorbing layer.
Reliable p-type doping of ZnO remains challenging, primarily due to the low solubility of p-type dopants and their compensation by abundant n-type impurities. This issue is also observed with GaN and ZnSe. Current limitations in p-doping restrict the electronic and optoelectronic applications of ZnO, which typically necessitate junctions of n-type and p-type materials. Known p-type dopants include group-I elements (Li, Na, K), group-V elements (N, P, As), as well as copper and silver.
Zinc oxide exhibits strongly nonlinear optical properties, particularly in bulk form. Its semiconductor nature, combined with a large exciton binding energy (~60 meV, approximately 2.4 times the room-temperature thermal energy), leads to bright room-temperature emission from ZnO. This characteristic makes it a promising material for optoelectronic devices, sensors, and UV photodetectors.
Production Methods
There are several methods for producing zinc oxide, each yielding products with different characteristics suitable for various applications.
Indirect (French) 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. The zinc vapor then reacts with oxygen in the air to form ZnO, accompanied by a temperature drop and bright luminescence. The resulting zinc oxide particles are transported into a cooling duct and collected in a bag house. This method, popularized by Edme Jean Leclaire of Paris in 1844, is commonly known as the French process. The product typically consists of agglomerated zinc oxide particles with an average size ranging from 0.1 to a few micrometers.
Direct (American) Process
The direct or American process begins with various contaminated zinc composites, such as zinc ores or smelter by-products. The zinc precursors are reduced (carbothermal reduction) by heating them with a carbon source, like anthracite, to produce zinc vapor. This vapor is then oxidized, similar to the indirect process. A small amount of industrial production involves wet chemical processes, which start with aqueous solutions of zinc salts from which zinc carbonate or zinc hydroxide is precipitated.
Specialized Synthesis Methods
Numerous specialized methods exist for producing ZnO for scientific studies and niche applications. Large single crystals (many cubic centimeters) can be grown using gas transport (vapor-phase deposition), hydrothermal synthesis, or melt growth. However, melt growth can be problematic due to the high vapor pressure of ZnO. Nanostructures of ZnO can be synthesized into various morphologies, including nanowires, nanorods, tetrapods, nanobelts, nanoflowers, and nanoparticles. These nanostructures can be obtained using the techniques mentioned above under specific conditions, as well as the vapor-liquid-solid method. Synthesis is typically carried out at temperatures around 90 °C in an equimolar aqueous solution of zinc nitrate and hexamine, with the latter providing the basic environment.
Aligned ZnO nanowires can be grown on pre-seeded silicon, glass, and gallium nitride substrates using aqueous zinc salts like zinc nitrate and zinc acetate in basic environments. Pre-seeding substrates with ZnO creates nucleation sites for homogeneous crystallization during synthesis. Common pre-seeding methods include in-situ thermal decomposition of zinc acetate crystallites, spin coating of ZnO nanoparticles, and the use of physical vapor deposition methods to deposit ZnO thin films. Pre-seeding can be combined with top-down patterning methods such as electron beam lithography and nanosphere lithography to define nucleation sites before growth.
Applications of Zinc Oxide
The applications of zinc oxide powder are extensive, with most exploiting its reactivity as a precursor to other zinc compounds. For material science applications, zinc oxide offers a high refractive index, high thermal conductivity, binding, antibacterial, and UV-protection properties.
Cosmetics and Personal Care
Zinc oxide is widely recognized for its ability to block both UVA (320-400 nm) and UVB (280-320 nm) rays of ultraviolet light, making it a key ingredient in sunscreens. Many sunscreens utilize nanoparticles of zinc oxide (along with titanium dioxide) because these small particles do not scatter light, thus avoiding a white appearance on the skin. Its UV-blocking capacity and skin compatibility also lead to its use in cosmetics. Zinc oxide has antibacterial and deodorizing properties, leading to its inclusion in medical applications such as baby powder and creams to treat skin irritations like diaper rash, and even dandruff.
Historically, from 1940 to 1980, products containing zinc oxide were primarily used to treat skin conditions like poison ivy or rashes. Zinc oxide is generally considered a safe ingredient in cosmetics and is found in many over-the-counter skin protectants and sunscreen drug products. When used as a sunscreen, it is recommended to apply it 30 minutes before sun exposure, using a sufficient amount and applying it gently to the face, neck, and tops of ears. To treat acne, a small amount of zinc oxide cream can be dabbed onto acne spots after cleansing and moisturizing the face. It is advisable to conduct a patch test before using any new skincare product containing zinc oxide.
Paints and Coatings
Zinc oxide (zinc white) serves as a pigment in paints and offers higher opacity than lithopone but less than titanium dioxide. It is also used in coatings for paper. Paints containing zinc oxide powder have long been employed as anticorrosive coatings for metals, particularly galvanized iron. ZnO's ability to prevent corrosion is valuable because iron's reactivity with organic coatings can lead to brittleness and poor adhesion. Highly n-type doped ZnO with aluminum, gallium, or indium is transparent and conductive (transparency ~90%, lowest resistivity ~10−4 Ω·cm). ZnO:Al coatings are used for energy-saving or heat-protecting windows. Plastics like polyethylene naphthalate (PEN) can be protected by applying a zinc oxide coating, which reduces oxygen diffusion through the PEN. Zinc oxide layers can also be applied to polycarbonate for outdoor applications. Zinc oxide depleted in the 64Zn isotope is used in corrosion prevention within nuclear pressurized water reactors.
Rubber Industry
A significant portion of zinc oxide produced is used in the rubber industry. A key role of zinc oxide in rubber is its function as an activator in the vulcanization process, which improves the strength, elasticity, and durability of rubber. It also enhances the thermal conductivity of rubber compounds, helping to dissipate heat generated during use.
Ceramics and Glass
The ceramic industry consumes a considerable amount of zinc oxide, particularly in ceramic glaze and frit compositions. ZnO's relatively high heat capacity, thermal conductivity, and high-temperature stability, coupled with a comparatively low coefficient of expansion, are desirable properties in ceramic production. ZnO influences the melting point and optical properties of glazes, enamels, and ceramic formulations. As a low-expansion, secondary flux, zinc oxide enhances the elasticity of glazes by reducing viscosity changes with temperature, helping to prevent crazing and shivering. Substituting ZnO for BaO and PbO decreases heat capacity and increases thermal conductivity. Small amounts of ZnO improve the development of glossy and brilliant surfaces, while moderate to high amounts produce matte and crystalline surfaces.
Electronics and Semiconductors
Zinc oxide is a semiconductor of the II-VI semiconductor group with a wide direct band gap of approximately 3.3 eV at room temperature. This wide band gap is advantageous for applications requiring higher breakdown voltages, the ability to withstand large electric fields, lower electronic noise, and operation at high temperatures and powers. The band gap can be further tuned between 3-4 eV by alloying with magnesium oxide or cadmium oxide. Due to these properties, ZnO is explored for use in transparent solar cells, laser diodes, and light-emitting diodes (LEDs). Its large exciton binding energy also contributes to bright room-temperature emission, making it competitive with materials like GaN for optoelectronic applications.
ZnO is also used in semiconductor gas sensors for detecting airborne compounds such as hydrogen sulfide, nitrogen dioxide, and volatile organic compounds. ZnO becomes n-doped upon adsorption of reducing compounds, which lowers the detected electrical resistance. It is often formed into nanostructures like thin films, nanoparticles, nanopillars, or nanowires to maximize surface area for gas interaction. Aluminium-doped ZnO layers are utilized as transparent electrodes, offering a cheaper and less toxic alternative to indium tin oxide (ITO). Transparent thin-film transistors (TTFT) can be produced using ZnO, circumventing the p-type doping challenge by using field-effect transistor configurations that do not require a p-n junction.
Other Applications
Zinc oxide is added to many food products, including breakfast cereals, as a source of zinc, an essential nutrient. In the rubber industry, it is widely used, with over 50% of zinc oxide consumption attributed to this sector. It serves as a crucial buffer layer in CIGS (Copper Indium Gallium Selenide) solar cells. Another significant use is in concrete manufacture. Zinc oxide also finds application in pharmaceuticals, where it is used in ointments to treat skin irritations, cuts, burns, and nappy rash by forming a protective barrier on the skin.
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, as H2S can poison the catalyst. In dentistry, zinc oxide is used in dental cements and fillings, such as zinc oxide eugenol, which has shown a high long-term success rate up to five years in primary molar pulpotomies.
In photocatalysis, ZnO, in both macro- and nano-scales, can be used as an electrode, primarily as an anode, in green chemistry applications. Furthermore, ZnO is a promising anode material for lithium-ion batteries due to its low cost, biocompatibility, and environmental friendliness.