Photocurrent lead corrosion

In a subsequent work [182], it was shown that the photoelectrochemical performance of InSe can be considerably improved by means of selective (photo)electrochemical etching. Interestingly, whereas the cleavage vdW plane showed little improvement, the photocurrent in the face parallel to the c-axis was doubled. Note that, in contrast to InSe crystals cleaved in the plane perpendicular to the c-axis that are almost defect free, the crystals cut in the plane parallel to the c-axis contain a high density of defects on their surface which leads to a high rate of electron-hole recombinations and inferior quantum efficiency. The asymmetry in the role of electrons and holes, as manifested, e.g., in the fact that surface holes carry out the selective corrosion of the semiconductor surface in both cleavage orientations, was discussed. 

One interpretation presumes that the photocurrent onset in the absence of sulfide is determined by electron-hole recombination. The sulfide ions on the surface are then supposed to be bound to these surface recombination levels rendering them unavilable for recombination reactions. The charge transfer reactions could then proceed at lower voltages. In this case the corrosion suppression role of the sulfide ions would be to reduce the oxidized corrosion site before a cadmium ion could go into solution. A variation on this theme is to consider the corrosion site to be the recombination state, i.e., the site on the surface that normally leads to corrosion when oxidized by a photoexcited hole can be... 

To summarize, the semiconduetor/electrolyte interfaee presents two types of currents in the dark this is a current of majority carriers whereas the photocurrent is a current of minority carriers. The same reactions can be monitored at n- and p-type electrodes but under different conditions. Hole accumulation corresponds to corrosion, since holes are trapped in surface bonds. Electron accumulation is generally not destructive for the surface unless cathodic reduction leads to decomposition. The band diagrams of Fig. 5 indicate that a downward shift of the flat band potential is expected at an illuminated n-type electrode. At negative bias, conversely, the shift is upward since electrons are accumulated in a thin surface layer (metallic-like behavior). 

The growth of PbO films on lead in H2S04 has been discussed in the previous section, where it was shown that the tetragonal modification of the oxide is formed. The corrosion of lead in sulphuric acid is more rapid at elevated temperatures and a systematic study of the rate of oxide growth as a function of temperature has been made by photocurrent spectroscopy [17]. 

The photocurrent due to the photo-generation of minority carriers is controlled by the discharge of the minority carriers from the surface of the electrode into the electrolyte and by competing recombination reactions of the minority charge carriers with the majority charge carriers. Of course, there are also competing side reactions of the minority carriers at the crystal surface, e.g., those leading to the corrosion of the electrode... 

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