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Zinc sulfide

physical science Maturity 11-13 Next article →

Zinc sulfide is an inorganic compound with the chemical formula ZnS. It is the primary form of zinc found throughout the natural world. In nature, it most often appears as a mineral known as sphalerite. While pure zinc sulfide is actually white, the mineral is usually black due to various impurities. This white material is widely used as a pigment in many industries. When mixed with barium sulfate, it forms a substance called lithopone.

ZnS powders2
ZnS powders2

This compound is a fascinating example of polymorphism. Polymorphism occurs when a substance exists in two or more different crystalline forms. Zinc sulfide has two main types. The first is the cubic form, also called zinc blende or sphalerite. This is the more stable version of the material. The second is the hexagonal form, known as the mineral wurtzite. Both forms share a tetrahedral coordination geometry at the zinc and sulfur atoms. The transition from the sphalerite form to the wurtzite form happens at approximately 1020 °C.

Sphalerite-unit-cell-3D-balls
Sphalerite-unit-cell-3D-balls
Wurtzite-unit-cell-3D-balls
Wurtzite-unit-cell-3D-balls

One of the most famous properties of zinc sulfide is phosphorescence. This is a type of light emission that continues after the source of light is removed. This effect can be enhanced by adding a tiny amount of an activator. An activator is a substance added in parts per million to change how the material reacts to light. For example, adding silver creates a bright blue glow at a maximum of 450 nanometers. Using manganese produces an orange-red color at around 590 nanometers. Copper creates a longer-lasting greenish glow. This copper-doped version is also used in electroluminescent panels.

ZnS powders2
ZnS powders2

History shows how scientists have marveled at these glowing properties. In 1866, a French chemist named Théodore Sidot first reported the phosphorescence of zinc sulfide. His findings were presented by A. E. Becquerel, who was a famous researcher in luminescence. Later, in 1893, Nikola Tesla described this phenomenon. In the early years of nuclear physics, scientists like Ernest Rutherford used zinc sulfide as a scintillation detector. It emits light when excited by X-rays or electron beams. Because of this, it was used in X-ray screens, cathode-ray tubes, and even the dials of radium watches.

Zinc sulfide also serves as a vital optical material. It can transmit light from visible wavelengths to just over 12 micrometers. This makes it useful for infrared optics, which involves light that is invisible to humans. It can be shaped into a lens or used as a flat optical window. Some versions are made as microcrystalline sheets using zinc vapor and hydrogen sulfide gas. This opaque, milky-yellow material is known as FLIR-grade. When it undergoes hot isostatic pressing, or HIPing, it becomes a water-clear form called Cleartran.

ZnS powders2
ZnS powders2

Beyond light and optics, zinc sulfide acts as a powerful photocatalyst. When fine ZnS powder is illuminated, it can produce hydrogen gas from water. During synthesis, scientists can introduce sulfur vacancies into the material. This process turns the white-yellowish powder into a brown powder. These vacancies actually boost the photocatalytic activity by enhancing how the material absorbs light. This ability to use light to drive chemical reactions makes it very important for energy research.

Finally, zinc sulfide is a prototypical II-VI semiconductor. This means it is a material that can conduct electricity under certain conditions. Both the sphalerite and wurtzite forms are intrinsic, wide-bandgap semiconductors. The cubic form has a band gap of about 3.54 electron volts at 300 kelvins. The hexagonal form has a larger band gap of about 3.91 electron volts. It can be doped to become either an n-type or a p-type semiconductor. This flexibility allows it to function in many electronic systems, similar to other semiconductors like gallium arsenide.

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File:Sphalerite-unit-cell-3D-balls.png
Sphalerite-unit-cell-3D-balls.png
File:Wurtzite-unit-cell-3D-balls.png
Wurtzite-unit-cell-3D-balls.png
File:ZnS powders2.jpg
ZnS powders2.jpg
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