Cadmium sulfide is a chemical compound with the formula CdS. Cadmium sulfide is yellow in colour and is a semiconductor. It exists in nature as two different minerals, greenockite and hawleyite. Cadmium sulfide is a direct band gap semiconductor (gap 2.42 eV) and has many applications for example in light detectors. It forms thermally stable pigments and with the addition of e.g CdTe, HgS colours ranging from deep red to yellow are formed.

Preparation

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 the gravimetric analysis of cadmium.
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.
Industrially the production of thin films of CdS is required in e.g. photoresistors and CBD, Chemical bath deposition, has been investigated using the hydrolysis of thiourea as the source of sulfide anions and an ammonium salt /ammonia buffer solution to control pH:

Cd²⁺ + NH₃ → [Cd(NH₃)₄]²⁺

NH₂CS + OH⁻ → SH⁻ + H₂O + H₂CN₂

SH⁻ + OH⁻ → S²⁻ + CdS

Cadmium sulfide can be produced from volatile cadmium alkyls, an example is the reaction of dimethylcadmium with diethyl sulfide to produce a film of CdS using MOCVD techniques.

The preparative route and the subsequent treatment of the product, affects the polymorphic form that is produced. It has been asserted in the past that chemical precipitation methods produce the cubic zincblende form however there more recent examples where the hexagonal form is produced, e.g. see

Chemical Properties

Cadmium sulfide is soluble in acids and this has been investigated as a method of extracting the pigment from waste polymers e.g. HDPE pipes:

CdS + HCl → CdCl₂ + H₂S

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

H₂S → H₂ + S ΔH_f = +9.4 kcal/mol

The reaction mechanism proposed 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⁻ + 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 bandgap semiconductor with a bandgap of 2.42 eV at 300 K.The magnitude of its band gap means that it appears coloured.
As well as this obvious property others 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
• solid CdS can act as a solid state laser

Industrial Processes used to produce thin films

Thin films of CdS are required in components such as a photoresistor and solar cells. Various 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):

• Chemical bath deposition, CBD
• Sol gel techniques
• MOCVD
• Sputtering
• Electrochemical deposition
• Spraying with precursor cadmium salt, sulfur compound and dopant
• Screen printing using a slurry containing dispersed CdS

Pigment

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. The general commercial availability of cadmium sulfide from the 1840s lead 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

Biological

Cadmium sulfide is produced by sulfate reducing bacteria. This ability is being investigated as a means of producing nano- crystalline CdS

References

• International Chemical Safety Card 0404
• Cadmium(II) sulphide information at Webelements

External links

• IARC Monograph: "Cadmium and Cadmium Compounds" Last access November 2005.
• National Pollutant Inventory - Cadmium and compounds
• 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.