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Author: Subject: Cu/Cu2O Lightsenser/Cell
chemkid
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[*] posted on 19-11-2008 at 18:42
Cu/Cu2O Lightsenser/Cell


This problem has been on my mind for a while now. Electricity is produced by this apparatus. A copper plate which has Cu2O (the red oxide) on it by heating and brushing off the CuO is immersed in a NaCl solution. Another Cu plate which is totally clean but of equal size is also immersed such that it is not touching the other plate. A voltage will run between the two of about .5V at .25 micro amperes when this apparatus is exposed to light.

The system seems extremely complex to me, but i am no good with electrochemistry. Here is what i have got: Cu2O is a semiconductor and when plated on copper forms a diode. I also found a standard reduction potentials for this half reactions:

Cu2+ + H2O --> Cu2O + H+ = .2

But there is no acid in the solution! I thought perhaps the Cu2+ complexed with Cl- to form CuCl3- or the like, but i don't know where to go from there. I am kind of stuck on figurig out this reaction.

I know that there is some sort of light based reaction involving Cu2O and Cu as i have read abstracts for papers about it.

Any help appreciated.
Chemkid




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Ozone
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[*] posted on 19-11-2008 at 20:11


Cool experiment, a nice write up is here:

http://scitoys.com/scitoys/scitoys/echem/echem2.html

Photoelectric effect knocks the electron free. It then can move though the salt solution. Because, I suppose, the Cu2O is a semiconductor, it can only move one way; this produces a small current.

Pondering:

It seems possible (though the solution in the experiment above was not green) that Cu° can be attacked by concentrated solutions of NaCl (you did not specify the concentration):

http://www.springerlink.com/content/08451888587n24v6/

This can yield CuCl2-, which can be oxidized to CuCl2, which will hydrate immediately in water. For each hydroxyl ligand (via hydrolysis of the hydrated metal complex) put onto the metal, you get a proton (H3O+) out. This is why CuCl2 (likewise, FeCl3) solutions are quite acidic and make for nice circuit board etchant.

The formation of CuCl2-, however, could also be accompanied by the formation of some NaOH? It might be interesting to monitor the pH of the electrolyte solution, over time, with constant light flux (say, a Hg vapor bulb) in a jacketed (cooled) beaker or photochemical reactor. Relating this to the current output should be instructive (I use a relatively cheap Radio Shak meter with a serial output; it comes with logging software and was ~$70 when last I checked).

Cheers,

O3




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chemkid
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[*] posted on 25-11-2008 at 11:46


I apologize for my late response, i have been ill.

I read up on the photoelectric effect and very much agree with your conclusion as well as the conclusion provided on the website. It is totally consistent with my results in that my Cu2O plate was positive and the Cu negative.

I don't think that the CuCl2- complex was produced as no green/blue color was observed in the solution, thought the pH was never checked and it was a saturated solution of NaCl.

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