A Study of the Interaction of Cu_2-xS Coatings with S₄^2- Ions by the Method of Cyclic Voltammetry

SUMMARY

The interaction between Cu_2-xS (1<(2-x)<2) coatings without elemental sulphur and 0.01 M Na₂S₄ has been studied by cyclic voltammetry in 0.05 M H₂SO₄.

It has been shown that elemental sulphur forms in the Cu_2-xS coating exposed to Na₂S₄ solution. The highest S^o amount (2·10⁻⁸ mol cm² during the first 30 s) has been found to be formed when the coating held in Na₂S₄ solution was Cut_1.25S. This is explained by the fact that during interaction of a Cu_2-xS coating with Na₂S₄ solution the unstable mixed polysulphides of Cu(I) and Cu(II) can be formed, e.g. in the case of Cu_1.25S (Cu₂S₄) the compound Cu₅S₇ can arise:

Cu^I₂Cu^II₃S₄ + Na₂S₄ → Cu^I₂Cu^II₃S₇ + Na₂S

which is again decomposed, initially into Cu₅S₄ and S^o:

Cu^I₂Cu^II₃S ₇ → Cu^I₂Cu^II₃S₄ + 3S^o

It is supposed that this process can recur and due to this four modifications of S^o arise as the time of treatment is increased. Stoichiometric sulphide Cu_l.99S does not interact and CuS interacts insignificantly with Na₂S₄ (after 30 s 0.4·10⁸ mol cm² of S is formed).

In a real process the contribution from the coating is supposed not to exceed several per cent.

Keywords:

• coating of non-stoichiometric copper sulphide
• Cu(I) and Cu(II) polysulphides
• cyclic voltammetry