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It is commonly known that when zinc metal is placed in a solution of copper(II) sulfate, a displacement reaction occurs, and elemental copper is deposited onto the decomposing zinc metal. A practical demonstration of this shows the zinc turning black from the deposited copper.

My question is, why is the deposited copper black? I thought metallic copper was red. Moreover, the dark color can be seen instantaneously, meaning the color cannot result from copper oxidizing.

I recently read a paper that claimed the copper is deposited in the form of plasmonic nanoparticles, and said nanoparticles display excellent solar absorption (and low thermal emittance, in fact). Any ideas?

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All metals look black or dark when presented as a sufficiently thin powder. The metals display their color only when the dimension of the grain is greater than the wavelength of the light, namely 400 to 700 nm. This is about 1000 times bigger than the atoms. When prepared from a precipitation reaction, the metal atoms are never arranged neatly in a regular fashion. So they do not reflect the light like a metallic plate.

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