Cadmium sulfide

Cadmium Sulfide

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Cadmium sulfide

Cadmium
Cadmium

Cadmium is a chemical element with the symbol Cd and atomic number 48. A relatively abundant , soft, bluish-white, transition metal, cadmium is known to cause cancer and occurs with zinc ores....
sulfide
Sulfur

Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant Valence non-metal....
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

Greenockite

Greenockite is a rare cadmium mineral that consists of cadmium sulfide, CdS, in crystalline form. Greenockite crystallizes in the Hexagonal system....
and hawleyite

Hawleyite

Hawleyite is a rare sulfide mineral in the sphalerite group, dimorphous and easily confused with greenockite. Chemically, it is a cadmium sulfide, and occurs as a bright yellow coating on sphalerite or siderite in vugs, deposited by meteoric waters....
. Cadmium sulfide is a direct band gap semiconductor (gap 2.42 eV

Electronvolt

In physics, the electron volt is a unit of energy. By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an Electrostatics potential difference of one volt....
) and has many applications for example in light detectors

Photoresistor

A photoresistor or light dependent resistor or cadmium sulfide cell is a resistor whose electrical resistance decreases with increasing incident light intensity....
. It forms thermally stable pigments and with the addition of e.g. CdTe

Cadmium telluride

Cadmium telluride is a crystalline Chemical compound formed from cadmium and tellurium with a zincblende .In the bulk crystalline form it is a direct bandgap semiconductor....
, HgS colours ranging from deep red to yellow are formed.

ium 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.

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Encyclopedia

Greenockite

Greenockite is a rare cadmium mineral that consists of cadmium sulfide, CdS, in crystalline form. Greenockite crystallizes in the Hexagonal system....
and hawleyite

Hawleyite

Electronvolt

Photoresistor

Cadmium telluride

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, required in e.g. photoresistor

Photoresistor

A photoresistor or light dependent resistor or cadmium sulfide cell is a resistor whose electrical resistance decreases with increasing incident light intensity....
s and chemical bath deposition (CBD), has been investigated using the hydrolysis of thiourea as the source of sulfide anions and an ammonium salt /ammonia buffer solution

Buffer solution

A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of acid or base is added to it....
to control pH:

Cd²⁺ + NH₃ ? [Cd(NH₃)₄]²⁺

NH₂CS + OH^- ? SH^- + H₂O + H₂CN₂

SH^- + OH^- ? S^2- + CdS

Cadmium sulfide can be produced from volatile cadmium alkyls, an example is the reaction of dimethylcadmium with diethyl sulfide

Diethyl sulfide

Diethyl sulfide is a clear flammable chemical compound with a pungent garlic-like odor. It has the chemical formula C4H10S....
to produce a film of CdS using metalorganic vapour phase epitaxy

Metalorganic vapour phase epitaxy

Metalorganic vapour phase epitaxy is a chemical vapour deposition method of epitaxy of materials, especially compound semiconductors from the surface reaction of organic compounds or metalorganics and metal hydrides containing the required chemical elements....
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 ?H_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 + h? ? e^- + hole⁺

Reaction of electron

2e^- + 2H₂O ? H₂ + 2OH^-

Reaction of hole

2hole⁺ + S^2- ? S

Structure and physical properties

Cadmium sulfide has, like zinc sulfide

Zinc sulfide

Zinc sulfide is a chemical compound with the chemical formula ZincSulfur. Zinc sulfide is a white- to yellow-colored powder or crystal. It is typically encountered in the more stable cubic form, known also as zinc blende or sphalerite....
, two crystal forms; the more stable hexagonal wurtzite structure (found in the mineral Greenockite

Greenockite

Greenockite is a rare cadmium mineral that consists of cadmium sulfide, CdS, in crystalline form. Greenockite crystallizes in the Hexagonal system....
) and the cubic zinc blende structure (found in the mineral Hawleyite

Hawleyite

Hawleyite is a rare sulfide mineral in the sphalerite group, dimorphous and easily confused with greenockite. Chemically, it is a cadmium sulfide, and occurs as a bright yellow coating on sphalerite or siderite in vugs, deposited by meteoric waters....
). 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

Semiconductor

A semiconductor is a material that has electrical conductivity between those of a Electrical conductor and an electrical insulation; it can vary over that wide range either permanently or dynamically....
with a bandgap of 2.42 eV

Electronvolt

In physics, the electron volt is a unit of energy. By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an Electrostatics potential difference of one volt....
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
Photoresistor

A photoresistor or light dependent resistor or cadmium sulfide cell is a resistor whose electrical resistance decreases with increasing incident light intensity....
)
when combined with a p-type semiconductor it forms the core component of a photovoltaic (solar
Solar cell

A solar cell or photovoltaic cell is a device that converts sunlight directly into electricity by the photovoltaic effect. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the source is unspecified....
) cell and a CdS/Cu₂S solar cell was one of the first efficient cells to be reported (1954)
when fabricated in thin films, it can form transistor
Transistor

In electronics, a transistor is a semiconductor device commonly used to Electronic amplifier or switch Electronics signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit....
s.
when doped with for example Cu⁺ ("activator")and Al³⁺ ("coactivator") CdS luminesces under electron beam excitation (cathodoluminescence
Cathodoluminescence

Cathodoluminescence is an optical p and electrical phenomenon phenomenon whereby a beam of electrons is generated by an electron gun and then impacts on a luminescent material such as a phosphor, causing the material to emit visible light....
) and is used as phosphor
Phosphor

A phosphor is a substance that exhibits the optical phenomenon of phosphorescence .Phosphors are transition metal compounds or rare earth element compounds of various types....
both polymorphs are piezoelectric and the hexagonal is also pyroelectric
electroluminescence
Electroluminescence

Electroluminescence is an optical phenomenon and electrical phenomenon in which a material emits light in response to an electric current passed through it, or to a strong electric field....
CdS crystal 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
Sol gel

The sol-gel process is a wet-chemical technique widely used recently in the fields of materials science and ceramic engineering. Such methods are used primarily for the fabrication of materials starting from a chemical solution which acts as the precursor for an integrated network of either discrete particles or network polymers....
techniques
MOCVD
Sputtering
Sputtering

Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic ions. It is commonly used for thin-film deposition, etching and analytical techniques ....
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

Selenium

Selenium is a chemical element with the atomic number 34, represented by the chemical symbol Se, an atomic mass of 78.96. It is a nonmetal, chemically related to sulfur and tellurium, and rarely occurs in its elemental state in nature....
as selenide

Selenide

The selenide ion is Se2-.A selenide is a chemical compound in which selenium serves as an anion with oxidation number of −2, much as sulfur does in a sulfide....
it is possible to obtain a range of colors for example CI pigment orange 20 and CI pigment red 108.
Synthetic cadmium pigments

Cadmium pigments

Cadmium pigments are a class of pigments that have cadmium as one of the chemical components.Most of cadmium produced worldwide is used in the production of Nickel-cadmium battery, but about half the remaining consumption, which is about 2,000 tons annually, is used to produce colored 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

Vincent van Gogh

Vincent Willem van Gogh was a Dutch people Post-Impressionism artist. Some of his paintings are now among the world's best known, most popular and expensive works of art....
, Monet

Claude Monet

Claude Monet also known as Oscar-Claude Monet or Claude Oscar Monet was a founder of French impressionism painting, and the most consistent and prolific practitioner of the movement's philosophy of expressing one's perceptions before nature, especially as applied to plein-air landscape painting....
(in his London series and other works) and Matisse

Henri Matisse

Henri Matisse was a France artist, known for his use of colour and his fluid, brilliant and original draftsmanship. As a drawing, printmaking, and Sculpture, but principally as a Painting, Matisse is one of the best-known artists of the 20th century....
(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

External links

Last access November 2005.
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.