Indium(III) sulfide · Lablypedia

It has a "rotten egg" odor characteristic of sulfur compounds, and produces hydrogen sulfide gas when reacted with mineral acids.

Three different structures ("polymorphs") are known: yellow, α-In₂S₃ has a defect cubic structure, red β-In₂S₃ has a defect spinel, tetragonal, structure, and γ-In₂S₃ has a layered structure. The red, β, form is considered to be the most stable form at room temperature, although the yellow form may be present depending on the method of production. In₂S₃ is attacked by acids and by sulfide. It is slightly soluble in Na₂S.

Indium sulfide was the first indium compound ever described, being reported in 1863. Reich and Richter determined the existence of indium as a new element from the sulfide precipitate.

Structure and properties

In₂S₃ features tetrahedral In(III) centers linked to four sulfido ligands.

α-In₂S₃ has a defect cubic structure. The polymorph undergoes a phase transition at 420 °C and converts to the spinel structure of β-In₂S₃. Another phase transition at 740 °C produces the layered γ-In₂S₃ polymorph.

β-In₂S₃ has a defect spinel structure. The sulfide anions are closely packed in layers, with octahedrally-coordinated In(III) cations present within the layers, and tetrahedrally-coordinated In(III) cations between them. A portion of the tetrahedral interstices are vacant, which leads to the defects in the spinel.

β-In₂S₃ has two subtypes. In the T-In₂S₃ subtype, the tetragonally-coordinated vacancies are in an ordered arrangement, whereas the vacancies in C-In₂S₃ are disordered. The disordered subtype of β-In₂S₃ shows activity for photocatalytic H₂ production with a noble metal cocatalyst, but the ordered subtype does not.

β-In₂S₃ is an N-type semiconductor with an optical band gap of 2.1 eV. It has been proposed to replace the hazardous cadmium sulfide, CdS, as a buffer layer in solar cells, and as an additional semiconductor to increase the performance of TiO₂-based photovoltaics.

The unstable γ-In₂S₃ polymorph has a layered structure.

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Production

Indium sulfide is usually prepared by direct combination of the elements.

Production from volatile complexes of indium and sulfur, for example dithiocarbamates (e.g. Et₂In^IIIS₂CNEt₂), has been explored for vapor deposition techniques.

Thin films of the beta complex can be grown by chemical spray pyrolysis. Solutions of In(III) salts and organic sulfur compounds (often thiourea) are sprayed onto preheated glass plates, where the chemicals react to form thin films of indium sulfide. Changing the temperature at which the chemicals are deposited and the In:S ratio can affect the optical band gap of the film.

Single-walled indium sulfide nanotubes can be formed in the laboratory, by the use of two solvents (one in which the compound dissolves poorly and one in which it dissolves well). There is partial replacement of the sulfido ligands with O²⁻, and the compound forms thin nanocoils, which self-assemble into arrays of nanotubes with diameters on the order of 10 nm, and walls approximately 0.6 nm thick. The process mimics protein crystallization.

Safety

The β-In₂S₃ polymorph, in powdered form, can irritate eyes, skin and respiratory organs. It is toxic if swallowed, but can be handled safely under conventional laboratory conditions. It should be handled with gloves, and care should be taken to keep from inhaling the compound, and to keep it from contact with the eyes.

Applications

Photovoltaic and Photocatalytic

There is considerable interest in using In₂S₃ to replace the semiconductor CdS (cadmium sulfide) in photoelectronic devices. β-In₂S₃ has a tunable band gap, which makes it attractive for photovoltaic applications, and it shows promise when used in conjunction with TiO₂ in solar panels, indicating that it could replace CdS in that application as well. Cadmium sulfide is toxic and must be deposited with a chemical bath, but indium(III) sulfide shows few adverse biological effects and can be deposited as a thin film through less hazardous methods.

Thin films β-In₂S₃ can be grown with varying band gaps, which make them widely applicable as photovoltaic semiconductors, especially in heterojunction solar cells.

Plates coated with beta-In₂S₃ nanoparticles can be used efficiently for PEC (photoelectrochemical) water splitting.

Biomedical

A preparation of indium sulfide made with the radioactive ¹¹³In can be used as a lung scanning agent for medical imaging. It is taken up well by lung tissues, but does not accumulate there.

Other

In₂S₃ nanoparticles luminesce in the visible spectrum. Preparing In₂S₃ nanoparticles in the presence of other heavy metal ions creates highly efficient blue, green, and red phosphors, which can be used in projectors and instrument displays.

General references

WebElements
Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.