I have seen few reports for the binding of silver ions to sulfur and forming silver sulfide when depositing silver on a semiconductor nanocrystals such as CdS, PbS, CZTS. Why? What are the factors associated with it? Is it a cation exchange process?
I dealt with the interaction of silver and PbS. You are absolutely right, there is an ion-exchange process. For example, in the instruction for ion-selective electrodes based on PbS, it is strictly forbidden to carry out measurements in the presence of silver ions. This not only gives incorrect results, but also leads to the failure of the electrode. Silver replaces lead in PbS, reducing lead in the form of metal.
With your experience, do you think it behaves the same way if you have Ag nanoparticles instead of no free Ag+ ions or some free ions?
Free ions always have counterions. Therefore, if you measure the free energy of the silver ion in solution with respect to metallic compact silver, you will see that the free energy of compact silver is higher. That is, compact silver is more likely to enter into a chemical reaction than the silver ion in the solution. Nanoparticles have even higher free energy due to excess free surface energy. This, in particular, is the driving force for nanoparticle coagulation. Therefore, silver nanoparticles are an even more effective reducer of lead than silver ions in solution.
Dear Anchal
The Ksp of Ag2S is far more less than the Ksp of these sulfides you are using, that is, Ag2S is less soluble in water than CdS, PbS and other metal sulfides. Therefore, when you add AgNO3 to these sulfides the following reaction is expected to take place:
Ag+ + MS Ag2S + M2+
You can also check the release of Pb2+ or Cd2+ after adding silver nitrate to their sulfides.
Dear Alexander N Titov: What "free energy" do you mean? There is a standard Gibbs energy of the formation of a chemical compound. The standard Gibbs energy of a simple substance A is zero by definition of this thermodynamic parameter. The standard Gibbs energy Ag2S is -40.8 kJ / mol, the same PbS is -99 kJ / mol. Therefore, in "dry" conditions, preference for Ag2S is doubtful. The question is interesting, because I recommended it.
Dear Mohammad Kooti: Perhaps your explanation is correct, but young scientists need to bring in the quantities of the solubility product, so that they are convinced of this answer.
Dear Prof.Migorod
We once have been trying to support Ag NPs on hydroxyapatite by adding silver nitrate and a reducing agent, but we got silver phosphate instead of the Ag NPs and this can be same case for what Anchal observed.
I just wanted to emphasize that in these types of reactions the less soluble compound is formed.
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