|Jmol-3D images||Image 1|
|Molar mass||144.48 g mol−1|
|Appearance||Yellow-orange to brown solid.|
|Density||4.826 g/cm3, solid.|
|Melting point||1,750 °C (3,180 °F; 2,020 K) 10 MPa|
|Boiling point||980 °C (1,800 °F; 1,250 K) (sublimation)|
|Solubility in water||insoluble|
|Solubility||soluble in acid
very slightly soluble in ammonium hydroxide
|Refractive index (nD)||2.529|
|Crystal structure||Hexagonal, Cubic|
|Std enthalpy of
|EU classification||Carc. Cat. 2
Muta. Cat. 3
Repr. Cat. 3
Dangerous for the environment (N)
|R-phrases||R45, R22, R48/23/25, R62, R63, R68, R50/53|
|S-phrases||S53, S45, S61|
|LD50||7080 mg/kg (rat, oral)|
|Other anions||Cadmium oxide
|Other cations||Zinc sulfide
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
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.
Cadmium sulfide can be prepared by the precipitation from soluble cadmium(II) salts with sulfide ion and this has been used in the past 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 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.
Routes to thin films of CdS
Thin films of CdS are 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. This process requies volatile cadmium and sulfur precursors. A common example is the reaction of dimethylcadmium with diethyl sulfide: Many other methods have been used to deposit these thin films, for example (note: there is a large body of research in this area and only representative references are given):
- Cd(CH3)2 + Et2S → CdS + CH3CH3 + C4H10
Other methods include
- Sol gel techniques
- Electrochemical deposition
- Spraying with precursor cadmium salt, sulfur compound and dopant
- Screen printing using a slurry containing dispersed CdS
- CdS + 2 HCl → CdCl2 + H2S
When sulfide solutions containing dispersed CdS particles are irradiated with light hydrogen gas is generated:
- H2S → H2 + S ΔHf = +9.4 kcal/mol
- Production of an electron hole pair
- CdS + hν → 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.
- the conductivity increases when irradiated with light (leading to uses as a photoresistor)
- when combined with a p-type semiconductor it forms the core component of a photovoltaic (solar) cell and a CdS/Cu2S solar cell was one of the first efficient cells to be reported (1954)
- when doped with for example Cu+ ("activator") and Al3+ ("coactivator") CdS luminesces under electron beam excitation (cathodoluminescence) and is used as phosphor
- both polymorphs are piezoelectric and the hexagonal is also pyroelectric
- CdS crystal can act as a solid state laser
CdS is predominantly used as a pigment. About 2000 tons are produced annually.
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 Cadmium Sulfide can be piezoelectric and have been used as transducers which can operate at frequencies in the GHz region.
CdS is known as cadmium yellow (CI pigment yellow 37). By adding varying amounts of selenium as selenide, it is possible to obtain a range of colors, for example CI pigment orange 20 and CI pigment red 108.
Synthetic cadmium pigments based on cadmium sulfide are valued for their good thermal stability, light and weather fastness, chemical resistance and high opacity. (but with problems of biocompatibility when used as colors in tattoos). 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 is used as pigment in plastics.
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