Cadmium sulfide is the inorganic compound with the formula CdS. Cadmium sulfide is a yellow salt.[Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier ] 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.
Production
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.
[Fred Ibbotson (2007), The Chemical Analysis of Steel-Works' Materials,Read Books, ]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.
[Paul Klocek (1991), Handbook of Infrared Optical Materials, CRC Press ]
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.
Cadmium sulfide is sometimes associated with sulfate reducing bacteria.[Larry L. Barton 1995 Sulfate reducing bacteria, Springer, ]
Routes to thin films of CdS
Special methods are used to produce films of CdS as components in some photoresistors and solar cells. In the chemical bath deposition method, thin films of CdS have been prepared using
thiourea as the source of sulfide anions and an
ammonium buffer solution to control pH:
- Cd2+ + H2O + (NH2)2CS + 2 NH3 → CdS + (NH2)2CO + 2 NH4+
Cadmium sulfide can be produced using metalorganic vapour phase epitaxy and MOCVD techniques by the reaction of dimethylcadmium with diethyl sulfide:
- Cd(CH3)2 + Et2S → CdS + CH3CH3 + C4H10
Other methods to produce films of CdS include
-
Sol–gel techniques
-
Sputtering
-
Electrochemical deposition
-
Spraying with precursor cadmium salt, sulfur compound and dopant
[ Photovoltaic cells, J.F. Jordan, C.M. Lampkin Issue date: April 25, 1978]
-
Screen printing using a slurry containing dispersed CdS
[, High performance photoresistor, Y.T. Sihvonen, issue date: September 21, 1965]
Reactions
Cadmium sulfide can be dissolved in acids.
- CdS + 2 HCl → CdCl2 + H2S
When solutions of sulfide containing dispersed CdS particles are irradiated with light, hydrogen gas is generated:[Mario Schiavello (1985) Photoelectrochemistry, Photocatalysis, and Photoreactors: Fundamentals and Developments Springer ]
- H2S → H2 + S Δf H = +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 + hν → e− + Electron hole
- Reaction of electron
- :2e− + 2H2O → H2 + 2OH−
- Reaction of hole
- :2h+ + 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.
[Wells A.F. (1984) Structural Inorganic Chemistry 5th edition Oxford Science Publications ] There is also a high pressure form with the NaCl rock salt structure.
Cadmium sulfide is a direct band gap semiconductor (gap 2.42 electronvolt).[D. Lincot, Gary Hodes Chemical Solution Deposition of Semiconducting and Non-Metallic Films: Proceedings of the International Symposium The Electrochemical Society, 2006 ] The proximity of its band gap to visible light wavelengths gives it a coloured appearance.
As well as this obvious property other properties result:
-
the conductivity increases when irradiated,
[ (leading to uses as a photoresistor)
]
-
when combined with a p-type semiconductor it forms the core component of a photovoltaic (solar cell) cell and a CdS/Cu2S solar cell was one of the first efficient cells to be reported (1954)
[Antonio Luque, Steven Hegedus, (2003), Handbook of Photovoltaic Science and Engineering John Wiley and Sons ]
-
when dopant with for example Cu+ ("activator") and Al3+ ("coactivator") CdS luminesces under electron beam excitation (cathodoluminescence) and is used as phosphor
[C. Fouassier,(1994), Luminescence in Encyclopedia of Inorganic Chemistry, John Wiley & Sons ]
-
both polymorphs are piezoelectric and the hexagonal is also pyroelectric
-
electroluminescence
-
CdS crystals can act as a gain medium in solid state laser
-
In thin-film form, CdS can be combined with other layers for use in certain types of solar cells.
-
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.
Applications
Pigment
CdS is used as
cadmium pigments 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).
[R. M. Christie 2001 Colour Chemistry, p. 155 Royal Society of Chemistry ] About 2000 tons are produced annually as of 1982, representing about 25% of the cadmium processed commercially.
[Karl-Heinz Schulte-Schrepping, Magnus Piscator "Cadmium and Cadmium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, 2007 Wiley-VCH, Weinheim. .]
Historical use in art
The general commercial availability of cadmium sulfide from the 1840s led to its adoption by artists, notably Van Gogh,
Claude Monet (in his London series and other works) and
Henri Matisse (
Bathers by a River 1916–1919).
[Sidney Perkowitz, 1998, Empire of Light: A History of Discovery in Science and Art Joseph Henry Press, ] The presence of cadmium in paints has been used to detect forgeries in paintings alleged to have been produced prior to the 19th century.
[W. Stanley Taft, James W. Mayer, Richard Newman, Peter Kuniholm, Dusan Stulik (2000) The Science of Paintings, Springer, ]
CdS-CdSe solutions
CdS and CdSe form solid solutions with each other. 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
(light dependent resistors) sensitive to visible and near infrared light.
Safety
Cadmium sulfide is toxic, especially dangerous when inhaled as dust, and cadmium compounds in general are classified as
carcinogenic.
Problems of
biocompatibility have been reported when CdS is used as colors in
.
CdS has an LD
50 of approximately 7,080 mg/kg in rats - which is higher than other cadmium compounds due to its low
solubility.
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