A study of the initial stages of the electrochemical deposition of thallium on copper, Part V. The potential step results: Overpotential deposition on (111), (110) and (100) oriented copper single cry

Abstract

The over potential deposition of thallium onto carefully chemically polished single crystal copper (111), (110) and (100) electrode from sulfate and perchlorate solutions have been investigated using single and double potential step techniques. It appears that the different anions used did not change the current-time response characteristics significantly. The charge corresponding to UPD monolayers of thallium (?210?10-6 Ascm-2 for the first, and ?400?10-6 Ascm-2 for both, first and second, monolayers on all three orientations of the copper substrates used) were observed under the initial falloff seen on the current-time transients obtained at very short times, preceding the currenttime relationships for the overpotential deposition. Indication of a rising i-t transient reflecting bulk thallium deposition on Cu(111) and Cu(100) were usually observed at ?=-9mVvs.Tl while an over potential of ?=-10mVvs. Tl was needed for similar effects on Cu(110). The observed i vs. t2 linearity is characteristic to 3D instantaneous nucleation and further growth of 3D centres. The results show no appreciable 3D nucleation occurring until the UPD monolayers were completed. It must be concluded, therefore, that thallium over potential deposition on copper single crystal surfaces starts with a charge transfer controlled 3D instantaneous nucleation and proceeds by 3D growth. After some time the individual 3D centres overlap and the rate of increase of current falls off until finally the limiting current is reached. Similarly to the cases of lead deposition on Ag18 and Cu30 crystals, bulk deposition of thallium commences only after the two UPD close-packed thallium monolayers have been completed on the underlying substrate. Formation of the two UPD thallium close-packed monolayers, in the case of copper (111), (110) and (100) substrates, is an essential precursor to thicker thallium deposits their influence on the initiation of the over potential deposition and its growth mechanism, was shown to be very significant.