The effect of visible light on the dissolution of natural chalcopyrite (CuFeS2) in sulphuric acid solutions

• Three regions identified in current–potential curve
• Photocurrents measured in all three regions
• Interpretation of photoeffects consistent with the proposed model
• New explanation for the passivation of chalcopyrite proposed

The effect of visible light on the anodic dissolution of a natural sample of chalcopyrite in 0.3 M sulphuric acid using a rotating disc technique has been studied. Three potential regions were identified in the anodic range. Between the open circuit potential and about 0.73 V vs Ag/AgCl, the current remained low, in the region of 1 μA/cm2. Between 0.73 and 0.83 V vs Ag/AgCl, the current increases dramatically. This region is regarded as the ‘active’ region, in which the passivation layer breaks down and is no longer present. In the third region, at higher potentials, the current reaches a maximum limiting value, which does not appear to be dependent on diffusional processes in solution. Surface analysis by XPS and Raman spectroscopy did not yield any information on any layers in the ‘passive’ region, but show the accumulation of sulphur products in the active region. The experiments with visible light showed that chalcopyrite responds to light in the entire potential region. The absolute value of the photocurrent increased with increasing potential. The results using visible light were supplemented by the determination of the current–voltage curve and capacitance measurements. The calculation of carrier density from the Mott–Schottky plot is in reasonable agreement with the measurement of the bulk carrier density from the Hall effect, suggesting that the n-type semiconducting behaviour detected is characteristic of the bulk, and not of a surface film. This effect of light over the entire range of anodic potentials, that is between the open circuit potential and 1.2 V vs Ag/AgCl, is difficult to reconcile with passivation model of chalcopyrite behaviour previously postulated. An alternative model of the chalcopyrite interface, based on the semiconductor nature of chalcopyrite, is presented.