Zinc sulfide (or zinc sulphide) is an inorganic compound with the chemical formula of ZnS. This is the main form of zinc found in nature, where it mainly occurs as the mineral sphalerite. Although this mineral is usually black because of various impurities, the pure material is white, and it is widely used as a pigment. In its dense synthetic form, zinc sulfide can be transparent, and it is used as a window for visible light and infrared optics.
Structure
ZnS exists in two main
. This dualism is an example of polymorphism. In each form, the coordination geometry at Zn and S is tetrahedral. The more stable cubic form is known also as zinc blende or
sphalerite. The hexagonal form is known as the mineral
wurtzite, although it also can be produced synthetically.
[.] The transition from the sphalerite form to the wurtzite form occurs at around 1020 °C.
Applications
Luminescent material
Zinc sulfide, with addition of a few ppm of a suitable activator, exhibits strong
phosphorescence. The phenomenon was described by
Nikola Tesla in 1893,
and is currently used in many applications, from
through
X-ray screens to
phosphorescence products. When
silver is used as activator, the resulting color is bright blue, with maximum at 450
. Using
manganese yields an orange-red color at around 590 nanometers.
Copper gives a longer glow, and it has the familiar greenish glow-in-the-dark. Copper-doped zinc sulfide ("ZnS plus Cu") is used also in electroluminescent panels.
[Karl A. Franz, Wolfgang G. Kehr, Alfred Siggel, Jürgen Wieczoreck, and Waldemar Adam "Luminescent Materials" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. ] It also exhibits
phosphorescence due to impurities on illumination with blue or
ultraviolet light.
Optical material
Zinc sulfide is also used as an
infrared optical material, transmitting from visible
to just over 12
micrometre. It can be used planar as an
optical window or shaped into a lens. It is made as
microcrystalline sheets by the synthesis from
hydrogen sulfide gas and zinc vapour, and this is sold as
FLIR-grade (Forward Looking Infrared), where the zinc sulfide is in a milky-yellow, opaque form. This material when hot isostatically pressed (HIPed) can be converted to a water-clear form known as
Cleartran (trademark). Early commercial forms were marketed as
Irtran-2 but this designation is now obsolete.
Pigment
Zinc sulfide is a common
pigment, sometimes called sachtolith. When combined with barium sulfate, zinc sulfide forms
lithopone.
[Gerhard Auer, Peter Woditsch, Axel Westerhaus, Jürgen Kischkewitz, Wolf-Dieter Griebler and Marcel Liedekerke "Pigments, Inorganic, 2. White Pigments" in Ullmann's Encyclopedia of Industrial Chemistry 2009, Wiley-VCH, Weinheim. ]
Catalyst
Fine ZnS powder is an efficient
photocatalyst, which produces hydrogen gas from water upon illumination. Sulfur vacancies can be introduced in ZnS during its synthesis; this gradually turns the white-yellowish ZnS into a brown powder, and boosts the photocatalytic activity through enhanced light absorption.
[
]
Semiconductor properties
Both sphalerite and wurtzite are intrinsic, wide-bandgap . These are prototypical II-VI semiconductors, and they adopt structures related to many of the other semiconductors, such as gallium arsenide. The cubic form of ZnS has a band gap of about 3.54 at 300 , but the hexagonal form has a band gap of about 3.91 electron volts. ZnS can be doped as either an n-type semiconductor or a p-type semiconductor.
History
The phosphorescence of ZnS was first reported by the French chemist Théodore Sidot in 1866. His findings were presented by A. E. Becquerel, who was renowned for the research on luminescence.
Production
Zinc sulfide is usually produced from waste materials from other applications. Typical sources include smelter, slag, and pickle liquors.[ As an example, the synthesis of ammonia from methane requires a priori removal of hydrogen sulfide impurities in the natural gas, for which zinc oxide is used. This scavenging produces zinc sulfide:
]
- ZnO + H2S → ZnS + H2O
Laboratory preparation
Crude zinc sulfide can be produced by igniting a mixture of zinc and sulfur.
- Zn2+ + S2− → ZnS
This reaction is the basis of a gravimetric analysis for zinc.
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