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Cadmium Sulfide

Cadmium Sulfide

Cadmium sulfide is a compound that has many uses. Given below are details on preparation method, properties and toxicity of this compound...
Dr. Sumaiya Khan
Cadmium sulfide is a chemical compound which has the formula CdS. It is yellow in color and is a semiconductor. It is found in two different minerals, hexagonal greenockite and cubic hawleyite.
Preparation
Cadmium sulfide can be prepared by precipitation from soluble cadmium salts with sulfide ions. This method has been used in the past for the gravimetric analysis of cadmium. Pigment production usually involves the precipitation of this compound, the washing of the precipitate to remove soluble cadmium salts followed by calcination, that is, roasting to convert it to the hexagonal form. This is followed by milling it to produce powder.
Industrially, the production of thin films of this compound has been carried out using the hydrolysis of thiourea as the source of sulfide anions and an ammonium salt /ammonia buffer solution to control pH. These films were used in photoresistors, and chemical bath deposition. This chemical compound can be produced from volatile cadmium alkyls as well, an example being the reaction of dimethylcadmium with diethyl sulfide to produce a film of the compound using metalorganic vapor phase epitaxy techniques.
Properties
  • This chemical compound is soluble in acids. This procedure has been investigated as a method of extracting pigment from waste polymers.
  • The crystals of this compound can act as a solid state laser.
  • CdS is also known as cadmium yellow and by adding various amounts of selenium and selenide, one can obtain a range of colors, like pigment orange and pigment red. These synthetic cadmium pigments are valued for their good thermal stability, light and weather fastness, chemical resistance and high opacity.
  • When sulfide solutions that contain this compound are irradiated with light, hydrogen gas is generated.
  • The conductivity of CdS increases when it is irradiated with light, which is the reason why it is used as a photoresistor.
  • When this compound is combined with a p-type semiconductor, it forms the core component of a photovoltaic (solar) cell.
  • When fabricated into thin films, it can be used as a transistor.
  • It is a direct bandgap semiconductor, having a bandgap of 2.42 eV at 300K, which is the reason why it appears colored.
  • It also has properties of electroluminescence and cathodoluminescence. Cathodoluminescence means that when this compound is mixed with copper acting as the activator and aluminum acting as the coactivator, it luminesces under electron beam excitation. Thus, it is used as phosphor.
  • Polymorph forms of CdS are piezoelectric while the hexagonal types are pyroelectric.
  • It is the presence of cadmium that helps to detect forgeries in paintings that are claimed to have been painted before the 19th century.
  • It is also used as a pigment in plastics.
Cadmium Sulfide Toxicity
There are quite a few risks involved with this chemical compound. CdS is combustible and tends to give off irritating or toxic fumes, hence, needs to be kept away from open flames. On inhalation, it tends to cause cough, redness and pain in the eyes and on ingestion, can lead to severe diarrhea and nausea. This is the reason why this chemical needs to be handled carefully and people coming in contact with it need to wear protective gloves, safety goggles and other eye protection gear.
There also needs to be a local exhaust or some type of breathing protection in the surrounding. The compoud also reacts with strong oxidants and with acids to form toxic hydrogen sulfide. Long term effects of exposure to the compound can lead to adverse effects on the kidneys, bones, respiratory tract, etc. All this can lead to kidney impairment, osteoporosis and chronic inflammation of the respiratory tract. It is also said to be a carcinogen.
Although cadmium sulfide does have many uses for us, it needs to be handled with care because it is very harmful for the body.