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Topic: Etching rates of elements in solids  (Read 2854 times)

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Offline Corribus

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Etching rates of elements in solids
« on: March 06, 2015, 10:46:00 AM »
I need to know if cadmium or selenium is likely to be dissolved into solution more quickly if cadmium selenide is exposed to dilute acid. I can do this experiment, but wanted to know if there's any reason to suspect that the two elements won't be dissolved at the same rate. I.e., will the surface of the solid become enriched with one element or the other?

Any thoughts appreciated.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Enthalpy

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Re: Etching rates of elements in solids
« Reply #1 on: March 08, 2015, 07:20:33 PM »
Similar things happen in semiconductor processing. Each microcrystal of the material gets one element etched faster so etching pauses at the crystal planes that contain the other element (...if the crystal contains such planes). You obtain nicely defined crystal planes with one element.

My unjustified guess is that cadmium is etched more quickly, as it's the soluble electrode ni NiCd accumulators, and selenium isn't even a metal, so the surface would consist most time at most locations of selenium atoms.

Wiki tells for CdSe: wurtzite (hexagonal), sphalerite (cubic) and rock-salt (cubic). I'd say that all three have such planes. Plese take with mistrust.

Offline Corribus

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Re: Etching rates of elements in solids
« Reply #2 on: March 08, 2015, 09:34:04 PM »
Thanks for your thoughts, Enthalpy. I'm still searching around for a reference on acid etching of CdSe but can't find one so far. If selenium etches faster, this may help explain our results, which otherwise are rather perplexing.  But I've always said that experiment beats speculation any day of the week so I bought some bulk cadmium selenide and we're going to let it sit in some dilute acid for a few days, filter, and analyze the filtrate by ICP and see whether we have more dissolved cadmium or selenide. I'll report back.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Corribus

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Re: Etching rates of elements in solids
« Reply #3 on: March 18, 2015, 09:55:12 AM »
Don't know if anyone is interested, but the experimental results confirm Enthalpy's guess. ~ 0.6 g chunk of CdSe stored in dilute acetic acid solution for ~ 1 week resulted in Cd and Se concentrations in the solution of ~ 1440 and < 30 ppb, respectively (based on 50 mL analyzed sample volume). (The concentration of selenium was below the detection limit.)
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Enthalpy

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Re: Etching rates of elements in solids
« Reply #4 on: March 19, 2015, 12:35:35 PM »
Don't know if anyone is interested [...]

Yes! Thanks!

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