Reaction of silver and sulphur

[osskall]

I was thinking about the reaction of silver and sulphur.

I've read that when copper is oxidised by oxygen, after the formation of the first film of Cu2O, it is actually the copper atoms diffusing out of the bulk copper through the Cu2O layer to the air, where they are oxidised.

Does anyone know what happens when silver is in contact with pressed sulphur tablets? I guess Ag2S forms instantaneously (we did this in a lab session and saw indeed black circles, but we didn't manage to interpret the X-ray diffractograms in a way that made sense), but what happens then? Can we exspect silver to diffuse through the Ag2S layer, or will sulphur diffuse through Ag2S towards silver and oxidise it? Of course, I can put a tablet on a silver plate and leave it for a year and see if the reaction has stopped because of complete formation of an Ag2S film, or if the silver plate seems porous or hollow (or vice versa the sulphur tablet). Could I deduce from ionic radiuses and silver sulphide's structure which species should diffuse through which medium?

 

[Gokul43201]

You can make a guess from the atomic radii (S is much smaller and lighter than Ag, so is more likely to diffuse across) and the crystal structure of Ag2S...but it would only be a very crude guess. The dynamics are more complex because they depend on lattice energies, atomic masses, the nature of the Ag - Ag2S - S interfaces and much more. So, my advice would be to not try to deduce the mechanism simply from such data.

The best way to determine what - if at all - was the diffusing species, is through experimental methods. Something like Auger Electron Spectroscopy, or Secondary Ion Mass Spectroscopy should work very well for this, but such instruments are often expensive, and hence uncommon. SEM and XRD are more common, and will give you pretty good info as well.

What was the problem with the XRD data ? I'm assuming you took powder (from different depths) diffractograms. Were you unable to identify the Ag2S phase itself ? I know that Ag2S is distinctly crystalline, with some kind of cubic structure - unlike say, Al2O3, which under some conditions is quite amorphous. Could you see peaks that corresponded to Ag (by comparing with the diffractogram from deep within the Ag plate) , S (likewise) and Ag2S (from some XRD database) ? I can't see what exactly the problem here might have been.

 

[osskall]

What was the problem with the XRD data ? I'm assuming you took powder (from different depths) diffractograms.

No, much more crude than so.
I really can't see how I would succeed in accessing different depths, scraping a powder is one thing.
But, no, we just put the silver plate with the black Ø~0.5 cm spot in the diffractometer and looked for suitable PDF files. Some peaks could possibly be explained by some elementary sulphur phases but the data were too bad to draw any safe conclusion. I recall that one time (I think I did the XRD analysis twice) (different samples) I maybe found Ag2S among other peaks that I could not explain.

I was just thinking about the Cu-Cu2O-O2 interface, where copper actually diffuses out to the oxygen. Should't the radius of oxygen also be smaller than that of Cu?

 

[Gokul43201]

It certainly is, if you consider only oxygen atoms. But oxygen exists as a diatomic molecule. The size of the molecule is only marginally smaller than that of a Cu atom. Still, this shows why you can't simply go by the size.