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Cadmium sulfide is the inorganic compound with the formula CdS. Cadmium sulfide is a yellow solid. It occurs in nature with two different crystal structures as the rare minerals greenockite and hawleyite, but is more prevalent as an impurity substituent in the similarly structured zinc ores sphalerite and wurtzite, which are the major economic sources of cadmium. As a compound that is easy to isolate and purify, it is the principal source of cadmium for all commercial applications. Its vivid yellow color led to its adoption as a pigment for the yellow paint "cadmium yellow" in the 18th century.
Cadmium sulfide can be prepared by the precipitation from soluble cadmium(II) salts with sulfide ion. This reaction has been used for gravimetric analysis and qualitative inorganic analysis. The preparative route and the subsequent treatment of the product, affects the polymorphic form that is produced (i.e., cubic vs hexagonal). It has been asserted that chemical precipitation methods result in the cubic zincblende form.
Pigment production usually involves the precipitation of CdS, the washing of the solid precipitate to remove soluble cadmium salts followed by calcination (roasting) to convert it to the hexagonal form followed by milling to produce a powder. When cadmium sulfide selenides are required the CdSe is co-precipitated with CdS and the cadmium sulfoselenide is created during the calcination step.
Cadmium sulfide is sometimes associated with sulfate reducing bacteria.
When solutions of sulfide containing dispersed CdS particles are irradiated with light hydrogen gas is generated:
H2S -> H2 + S ?Hf = +9.4 kcal/mol
The proposed mechanism involves the electron/hole pairs created when incident light is absorbed by the cadmium sulfide followed by these reacting with water and sulfide:
Production of an electron hole pair
CdS + h? -> e- + hole+
Reaction of electron
2e- + 2H2O -> H2 + 2OH-
Reaction of hole
2hole+ + S2- -> S
Structure and physical properties
Cadmium sulfide has, like zinc sulfide, two crystal forms. The more stable hexagonal wurtzite structure (found in the mineral Greenockite) and the cubic zinc blende structure (found in the mineral Hawleyite). In both of these forms the cadmium and sulfur atoms are four coordinate. There is also a high pressure form with the NaCl rock salt structure.
Cadmium sulfide is a direct band gapsemiconductor (gap 2.42 eV). The proximity of its band gap to visible light wavelengths gives it a coloured appearance. As well as this obvious property other properties result:
In thin-film form, CdS can be combined with other layers for use in certain types of solar cells. CdS was also one of the first semiconductor materials to be used for thin-film transistors (TFTs). However interest in compound semiconductors for TFTs largely waned after the emergence of amorphous silicon technology in the late 1970s.
Thin films of CdS can be piezoelectric and have been used as transducers which can operate at frequencies in the GHz region.
Nanoribbons of CdS show a net cooling due annihilation of phonons, during anti-Stokes luminescence at ~510 nm. As a result, a maximum temperature drop of 40 and 15 K has been demonstrated when the nanoribbons are pumped with a 514 or 532 nm laser.
Yellow cadmium sulfide- pigment
CdS is used as pigment in plastics, showing good thermal stability, light and weather fastness, chemical resistance and high opacity. As a pigment, CdS is known as cadmium yellow. (CI pigment yellow 37) About 2000 tons are produced annually as of 1982, representing about 25% of the cadmium processed commercially.
Historical use in art
The general commercial availability of cadmium sulfide from the 1840s led to its adoption by artists, notably Van Gogh, Monet (in his London series and other works) and Matisse (Bathers by a river 1916-1919). The presence of cadmium in paints has been used to detect forgeries in paintings alleged to have been produced prior to the 19th century.
CdS and CdSe form solid solutions. Increasing amounts of cadmium selenide, gives pigments verging toward red, for example CI pigment orange 20 and CI pigment red 108. Such solid solutions are components of photoresistors (light dependent resistors) sensitive to visible and near infrared light.
Cadmium sulfide is toxic, especially dangerous when inhaled as dust, and cadmium compounds general are classified as carcinogenic. Problems of biocompatibility have been reported when CdS is used as colors in tattoos.
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 Report by the Academy of Medical Sciences to the Chief Scientific Adviser, Ministry of Defence on the zinc cadmium sulphide dispersion trials undertaken in the United Kingdom between 1953 and 1964.