Cadmium sulfide

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.
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.

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. This process requies volatile cadmium and sulfur precursors. A common example is the reaction of dimethylcadmium with diethyl sulfide:

Cd(CH₃)₂ + Et₂S → CdS + CH₃CH₃ + C₄H₁₀

Many other methods have been reported.

Other methods to produce films of CdS include

Sol gel techniques

Sputtering

Electrochemical deposition

Spraying with precursor cadmium salt, sulfur compound and dopant

Screen printing using a slurry containing dispersed CdS

Reactions

Cadmium sulfide is soluble in (actually degraded by) acids, and this conversion has been investigated as a method of extracting the pigment from waste polymers e.g. HDPE pipes:

CdS + 2 HCl → CdCl₂ + H₂S

When solutions of sulfide solutions containing dispersed CdS particles are irradiated with light hydrogen gas is generated:

H₂S → H₂ + S ΔH_f = +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 pairReaction of electronReaction of hole

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 gap semiconductor (gap 2.42 eV). The magnitude of its band gap means that it appears coloured.
As well as this obvious property other properties result:

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

electroluminescence

CdS crystal can act as a solid state laser

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.

Pigment

Synthetic cadmium pigments based on cadmium sulfide are valued for their 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. CdS is used as pigment in plastics.

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-CdSe solutions

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.

Safety

Cadmium sulfide is toxic, especially 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.