# The color of the most sulfides of p- and d-block elements

My teacher said that many of the sulfides of d-block and p-block are black. Why?

• chemistry.stackexchange.com/questions/32019/… – Mithoron Aug 1 '20 at 14:34
• In my opinion nobody knows why $\ce{FeS, CoS, NiS, CuS, Ag_2S, PbS}$ are black. – Maurice Aug 1 '20 at 16:07
• There, there, colors of complexes and inorganic compounds seem to be well studied and there was lots of posts about them already. Still, it's not like this knowledge is particularly useful or common, even among chemists... – Mithoron Aug 1 '20 at 18:25
• – Mithoron Aug 1 '20 at 18:33

The band gap between conduction band and the valence band plays a role in color of these compounds. The good example is $$\ce{SnS2}$$, which is bronze color (Image $$\bf{A}$$; $$\ce{SnS2}$$ is used in decorative coating where it is known as mosaic gold) and have a band gap of $$\pu{2.18-2.44 eV}$$ (Image $$\bf{B}$$; Ref.1). When an electron in the conduction band falls down to the valence band, it loses energy and the lost energy is emitted as a photon ($$h\nu$$). This photon has an energy equal to the band gap (Image $$\bf{C}$$). In mathematics, it is: $$E_\mathrm{gap} = h\nu$$. The crystals of $$\ce{SnS2}$$ and its calculated band gap is shown below:

The compounds is colored when the band gap is between $$1.8$$ and $$\pu{3.0 eV}$$ (visible range). The compounds with larger band gaps $$(\gt \pu{3.0 eV})$$ are either white or transparent crystals. Those with smaller band gaps $$(\lt \pu{1.8 eV})$$ give black color crystals. For example, the band gap of $$\ce{ZnS}$$ is $$\pu{3.54 eV}$$, the color of which is white. Meantime, the color of $$\ce{CdS}$$ (Pigment name: Cadmium yellow) is yellow because its band gap is $$\pu{2.45 eV}$$. On the other hand, the colors of $$\ce{CdSe}$$ and $$\ce{CdTe}$$, which have the same structure of $$\ce{CdS}$$, are black because their band gaps are $$\pu{1.74 eV}$$ and $$\pu{1.49 eV}$$, respectively. Also, the color of $$\ce{PbS}$$ is black because its band gap is $$\pu{0.37 eV}$$.

For some crystals, the band gap change with temperature (Ref.1). Hence, for those, the color depend on temperature.

References:

1. Lee A. Burton, Thomas J. Whittles, David Hesp, Wojciech M. Linhart, Jonathan M. Skelton, Bo Hou, Richard F. Webster, Graeme O'Dowd, Christian Reece, David Cherns, David J. Fermin, Tim D. Veal, Vin R. Dhanak, Aron Walsh, “Electronic and optical properties of single crystal SnS2: an earth-abundant disulfide photocatalyst,” J. Mater. Chem. A 2016, 4, 1312-1318 (https://doi.org/10.1039/C5TA08214E).
• @ Mathew. So a compound is black if the band gap is smaller than 1.8 eV. OK. So the real question is : why is the band gap smaller than 1.8 eV in FeS, CoS, NiS, CuS, Ag2S and PbS ? And why not in ZnS ? – Maurice Aug 1 '20 at 19:07
• More effective nuclear charge on Zn lowers the conduction band of ZnS, maybe? – Oscar Lanzi Aug 2 '20 at 13:50

I would leave a comment, but the basics of the answer are as follows. It's hard to give an answer that's complete because the chemistry is very diverse for the different compounds.

What you're basically seeing is the transition corresponding to breaking the metal-sulfur bond (i.e. the bonding orbital/antibonding orbital) or electrons moving between the metal and sulfur. The colours tend to be black for sulfides because sulfur orbitals are relatively high energy (compared e.g. to oxides) and form weak bonds so the energy of the transitions is small.

A complete answer would require more detail about the band structures of the relevant compounds. Some of the oxides are also black.