SILVER SULFIDE (AG2S): STRUCTURE, PROPERTIES AND USES - CHEMISTRY - 2025

The silver sulfide is an inorganic compound whose chemical formula is Ag ₂ S. It consists of a gray-black solid formed by cation Ag ⁺ and anions S ^2- in a 2: 1. S ^2- is very similar to Ag ⁺, because both are soft ions and they manage to stabilize with each other.

Silver ornaments tend to darken, losing their characteristic luster. The color change is not a product of the oxidation of silver, but of its reaction with hydrogen sulfide present in the environment at low concentrations; This can come from the putrefaction or degradation of plants, animals or foods rich in sulfur.

Source: Rob Lavinsky, iRocks.com - CC-BY-SA-3.0, via Wikimedia Commons

H ₂ S, whose molecule carries a sulfur atom, reacts with silver according to the following chemical equation: 2Ag (s) + H ₂ S (g) => Ag ₂ S (s) + H ₂ (g)

Therefore, Ag ₂ S is responsible for the black layers formed on silver. However, in nature this sulfide can also be found in the minerals Acantite and Argentite. The two minerals are distinguished from many others by their shiny black crystals, like the solid in the image above.

Ag ₂ S has polymorphic structures, attractive electronic and optoelectronic properties, is a semiconductor and promises to be a material for the production of photovoltaic devices, such as solar cells.

Structure

Source: By CCoil, from Wikimedia Commons

The upper image illustrates the crystal structure of silver sulfide. The blue spheres correspond to Ag ⁺ cations, while the yellow spheres correspond to S ^2- anions. Ag ₂ S is polymorphic, which means that it can adopt various crystal systems under certain temperature conditions.

How? Through a phase transition. The ions are rearranged in such a way that the increase in temperature and the vibrations of the solid do not disturb the electrostatic attraction-repulsion balance. When this happens it is said that there is a phase transition, and the solid thus exhibits new physical properties (such as luster and color).

Ag ₂ S at normal temperatures (below 179ºC) has a monoclinic crystalline structure (α-Ag ₂ S). In addition to this solid phase, there are two others: the bcc (cubic centered on the body) between 179 to 586ºC, and the fcc (cubic centered on the faces) at very high temperatures (δ- Ag ₂ S).

The argentite mineral consists of the fcc phase, also known as β-Ag ₂ S. Once cooled and transformed into acanthite, its structural features prevail in combination. Therefore, both crystalline structures coexist: the monoclinic and the bcc. Hence, black solids with bright and interesting overtones emerge.

Properties

Molecular weight

247.80 g / mol

Appearance

Grayish black crystals

Odor

Toilet.

Melting point

836 ° C. This value agrees with the fact that Ag ₂ S is a compound with little ionic character and, therefore, melts at temperatures below 1000ºC.

Solubility

In water only 6.21 ∙ 10 ^-15 g / L at 25ºC. That is, the amount of the black solid that is solubilized is negligible. This, again, is due to the low polar character of the Ag-S bond, where there is no significant difference in electronegativity between the two atoms.

Also, Ag ₂ S is insoluble in all solvents. No molecule can efficiently separate its crystalline layers into solvated Ag ⁺ and S ^2- ions.

Structure

In the image of the structure you can also see four layers of S-Ag-S bonds, which move over each other when the solid is subjected to compression. This behavior means that, despite being a semiconductor, it is ductile like many metals at room temperature.

S-Ag-S layers fit properly due to their angular geometries which are seen as a zigzag. As there is a compression force, they move on a displacement axis, thus causing new non-covalent interactions between the silver and sulfur atoms.

Refractive index

2.2

Dielectric constant

6

Electronic

Ag ₂ S is an amphoteric semiconductor, that is, it behaves as if it were of type n and of type p. It is also not brittle, so it has been studied for its application in electronic devices.

Reduction reaction

Ag ₂ S can be reduced to metallic silver by bathing the black pieces with hot water, NaOH, aluminum and salt. The following reaction takes place:

3Ag ₂ S (s) + 2Al (s) + 3H ₂ O (l) => 6Ag (s) + 3H ₂ S (aq) + Al ₂ O ₃ (s)

Nomenclature

Silver, whose electron configuration is 4d ¹⁰ 5s ¹, can lose only one electron: its outermost orbital 5s. Thus, the Ag ⁺ cation is left with a 4d ¹⁰ electronic configuration. Therefore, it has a unique valence of +1, which determines what its compounds should be called.

Sulfur, on the other hand, has a 3s ² 3p ⁴ electronic configuration, and needs two electrons to complete its valence octet. When it gains these two electrons (from silver), it is transformed into the sulfide anion, S ^2-, with configuration. That is, it is isoelectronic to the noble gas argon.

So Ag ₂ S must be named according to the following nomenclatures:

Systematic

Di- silver mono sulfide. Here the number of atoms of each element is considered and they are marked with the prefixes of Greek numerators.

Stock

Silver sulfide. As it has a unique valence of +1, it is not specified with Roman numerals in parentheses: silver (I) sulfide; which is incorrect.

Traditional

Sulfide ARGENT ico. Since silver "works" with a valence of +1, the suffix -ico is added to its Latin name argentum.

Applications

Some of the novel uses for Ag ₂ S are as follows:

-The colloidal solutions of its nanoparticles (with different sizes), have antibacterial activity, are not toxic, and therefore can be used in the fields of medicine and biology.

-Its nanoparticles can form what is known as quantum dots. They absorb and emit radiation with greater intensity than many fluorescent organic molecules, so they can supplant the latter as biological markers.

-The structures of α-Ag ₂ S make it exhibit striking electronic properties to be used as solar cells. It also represents a starting point for the synthesis of new thermoelectric materials and sensors.

References

1. Mark Peplow. (April 17, 2018). Semiconductor silver sulfide stretches like metal. Taken from: cen.acs.org
2. Collaboration: Authors and editors of the volumes III / 17E-17F-41C () Silver sulfide (Ag2S) crystal structure. In: Madelung O., Rössler U., Schulz M. (eds) Non-Tetrahedrally Bonded Elements and Binary Compounds I. Landolt-Börnstein - Group III Condensed Matter (Numerical Data and Functional Relationships in Science and Technology), vol 41C. Springer, Berlin, Heidelberg.
3. Wikipedia. (2018). Silver sulfide. Taken from: en.wikipedia.org
4. Stanislav I. Sadovnikov & col. (July 2016). Ag ₂ S silver sulfide nanoparticles and colloidal solutions: Synthesis and properties. Taken from: sciencedirect.com
5. Azo Materials. (2018). Silver Sulfide (Ag ₂ S) Semiconductors. Taken from: azom.com
6. A. Nwofe. (2015). Prospects and challenges of silver sulfide thin films: A review. Division of Materials Science & Renewable Energy, Department of Industrial Physics, Ebonyi State University, Abakaliki, Nigeria.
7. UMassAmherst. (2011). Lecture Demonstrations: cleaning tarnished silver. Taken from: lecturedemos.chem.umass.edu
8. Study. (2018). What is Silver Sulfide? - Chemical Formula & Uses. Taken from: study.com