# zinc oxide

Crystal structure
__ Zn 2+      __ O 2−
General
Surname zinc oxide
other names
• Zinc (II) oxide
• Zinci oxidum
• Zinc white
• Chinese white
• CI Pigment White 4
• CI 77947
• Lana philosophica
• ZINC OXIDE ( INCI )
Ratio formula ZnO
Brief description

colorless, hexagonal crystals or white, fluffy powder

External identifiers / databases
 CAS number 1314-13-2 EC number 215-222-5 ECHA InfoCard 100,013,839 PubChem 14806 ChemSpider 14122 DrugBank DB09321 Wikidata Q190077
Drug information
ATC code
properties
Molar mass 81.39 g · mol -1
Physical state

firmly

density

5.61 g cm −3

Melting point

1975 ° C (under pressure)

solubility
• practically insoluble in water (1.6 mg l −1 at 29 ° C)
• soluble in dilute acids
safety instructions
Please note the exemption from the labeling requirement for drugs, medical devices, cosmetics, food and animal feed
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary

Caution

H and P phrases H: 410
P: 273
MAK
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Zinc oxide (ZnO) is a chemical compound of zinc and oxygen , which on the one hand forms colorless, hexagonal crystals or, on the other hand, is present as a loose, white powder due to the refraction of light in very small crystals.

The traditional name zinc white ( Chinese white , eternal white , snow white ) comes from its use as a white colorant in painters' paint.

Medicinal preparations for skin and wound treatment often contain zinc oxide because of its antiseptic effect. It is often used in dentistry (e.g. for root canal treatments) and is considered a biomaterial .

## Occurrence

Natural occurrences of zinc oxide can be found in the form of the mineral zincite ( red zinc ore ).

## Extraction and presentation

Depending on the manufacturing process, zinc white or zinc oxide is obtained. Zinc white is produced from zinc vapor and atmospheric oxygen using the so-called French process .

Zinc oxide, however, is obtained from either the execution of the production according to the so-called American method of zinc ores or Scrap- by roasting, by reduction with coal and direct subsequent re oxidation or wet-chemically by precipitation as hydroxide or carbonate of zinc salt solutions and subsequent calcination .

Zinc oxide produced by the combustion of zinc . The oxide can develop in a fine felted, wool-like form ( Lana philosophica ).

${\ displaystyle \ mathrm {2 \ Zn _ {(s)} + O_ {2 (g)} \ longrightarrow 2 \ ZnO _ {(s)}}}$
In the presence of oxygen, zinc burns to form zinc oxide.

But it can also be obtained by annealing ( pyrolysis ) zinc hydroxide , zinc carbonate or zinc nitrate :

${\ displaystyle \ mathrm {Zn (OH) _ {2 (s)} \ longrightarrow ZnO _ {(s)} + H_ {2} O _ {(g)}}}$
At higher temperatures, zinc hydroxide releases water . Zinc oxide is formed.
${\ displaystyle \ mathrm {ZnCO_ {3 (s)} \ longrightarrow ZnO _ {(s)} + CO_ {2 (g)}}}$
Zinc oxide and carbon dioxide are formed from zinc carbonate during annealing .

When roasting of zinc sulfide also zinc oxide is produced:

${\ displaystyle \ mathrm {2 \ ZnS _ {(s)} + 3 \ O_ {2 (g)} \ longrightarrow 2 \ ZnO _ {(s)} + 2 \ SO_ {2 (g)}}}$

Depending on the area of ​​application, zinc oxide can be produced in different qualities by varying the reaction conditions. Industrially produced zinc oxide pigments often contain lead and must therefore also be labeled with the Xn symbol (harmful to health).

Thin layers of zinc oxide are produced using physical deposition processes ( PVD ) such as sputtering and chemical vapor deposition (CVD). CVD allows the formation of rough surfaces which are characterized in incident light to scatter and z. B. to give solar cells a higher efficiency.

## production

The global production of zinc oxide is estimated at 1.5 million t. In Europe, 240,000 t are used every year.

## properties

zinc oxide

Zinc oxide evaporates from approx. 1300 ° C, the sublimation point under normal pressure is approx. 1800 ° C. A melt of zinc oxide can be observed only under elevated pressure at 1975 ° C. When heated it turns lemon yellow, after cooling it is white again ( see thermochromism ). A faint afterglow can then be observed in the dark. The change in color is due to a small excess of zinc (approx. 0.03%) caused by the release of oxygen during heating. This excess zinc or the concentration of the oxygen defects is heavily dependent on the synthesis method, temperature or contamination of the zinc oxide.

Zinc oxide is a direct semiconductor with a band gap of 3.2… 3.4 eV. It therefore absorbs UV radiation and is transparent to visible light . Due to the non-centrosymmetrical unit cell , zinc oxide is piezoelectric .

The electrical conductivity of zinc oxide can be increased by doping it with suitable elements. Often aluminum (doped aluminum-zinc oxide AZO) or boron is used for this. Other elements for N-doping such as indium or gallium are currently not used. P-doped zinc oxide is more difficult to manufacture and is an area of ​​current research.

Zinc oxide is insoluble in water , while dilute acids dissolve it with salt formation . With alkali , such as. B. sodium hydroxide solution , freshly precipitated, hydrated zinc oxide dissolves again with an excess of base . This creates a soluble zincate .

If the compound is heated with cobalt (II) oxide (CoO), a green solid solution of cobalt oxide in zinc oxide ( Rinman's green ) is created.

## use

### pigment

Zinc oxide is used as a pigment under the name zinc white . There are also terms such as Chinese white , eternal white or snow white . In contrast to white lead paints, it is hydrogen sulfide and lightfast, but less opaque. For this reason, mixtures of lead and zinc white were often used as paints . Zinc white is well compatible with all pigments. Alkaline binders should, however, be avoided as there is a risk of zincate formation. Zinc soaps can form to a limited extent in oil . Zinc white has been widely used in aqueous techniques, such as B. in glue , watercolor and gouache paints . It was already known in antiquity under Cadmea (also used for other zinc compounds) and Pompholyx . The alchemists called it Nix alba (white snow) or Nihilum album (white nothing). In 1782 Bernard Courtois and Louis Bernard Guyton de Morveau in Dijon replaced lead with zinc white, and in 1850 it was commercially processed into oil paints . Zinc white has also been available as a water color since 1834 . Zinc white was only slowly being adopted by artists as a substitute for white lead. There was real argument about its use. Ultimately, since zinc white just seems cooler, it was mainly used to blend in with cool hues, while lead white was blended in with the warmer colors. The use of zinc white as an overcoat is declining more and more in favor of titanium white .

### photography

Paper coated on one side with zinc oxide was used earlier (until around 1970) in the direct electrophotography process (electrostatic copying process).

The so-called ZINK paper of the ("Zero-Ink") zinc printing technology , however, does not contain any zinc.

### Energy storage

Researchers at the Swiss Paul Scherrer Institute are trying to convert solar power into chemical energy . To do this, they focus the sun's rays on a melting pot. The light hits zinc oxide at over 1200 ° C, which then evaporates and is converted to metallic zinc, if care is taken that the resulting zinc does not oxidize again immediately. Since this can be stored and transported, the sun's energy can be stored and later z. B. can be used in zinc-air batteries directly as electrical energy or to produce hydrogen. This is shown in simplified form, because the discharged Zn-ZnO is taken from the zinc-air batteries together with potassium hydroxide .

### semiconductor

As a semiconductor , zinc oxide is used as a transparent conductive layer in the manufacture of blue light-emitting diodes (LED), liquid crystal displays (LCD), varistors (VDR) and thin-film solar cells . For this purpose it is usually doped with aluminum ZnO: Al, (AZO = aluminum zinc oxide ). The doping increases the conductivity by several orders of magnitude. As semiconducting nanowires, zinc oxide is used in measurement technology due to its piezoelectric properties and UV light transparency.

### medicine

As a component of pharmaceutical zinc ointments or pastes (e.g. Penaten cream or Mirfulan ), plasters ( Leukoplast ) and bandages , zinc oxide leads to the surface of the skin drying out. This is used, for example, for intertrigo , diaper rash and other weeping wounds. In Fußpilzen and other Dermatomycoses zinc oxide supports healing.

### Other uses

Zinc oxide is also used in sunscreens and as an additive in the vulcanization of rubber . (It then gets into the environment through tire wear). It is also used as a luminophore in fluorescent lamps . Self-activated zinc oxide (ZnO: Zn) is used as a cathodoluminescent phosphor in magic eyes , where it shows blue-green luminescence . Zinc oxide is added as depleted zinc oxide (DZO) to the cooling water of nuclear reactors ( boiling water reactors ) in order to improve the corrosion behavior of the components that come into contact with the cooling water. Zinc oxide has also been used in deodorants since 1888 because it has a slight antibacterial effect.

Recently, zinc oxide nanoparticles have been used as UV absorbers in food packaging .

## Risk assessment

In 2015, zinc oxide was included in the EU's ongoing action plan ( CoRAP ) in accordance with Regulation (EC) No. 1907/2006 (REACH) as part of substance evaluation . The effects of the substance on human health and the environment are re-evaluated and, if necessary, follow-up measures are initiated. Zinc oxide uptake was caused by concerns about consumer use , environmental exposure, other exposure / risk-based concerns, other hazard-related concerns and widespread use. The reassessment has been running since 2017 and is carried out by Germany .

## Individual evidence

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14. Synthesis of photoconducting ZnO nano-needles
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