Influence of thiourea on the nucleation of copper on polycrystalline platinum

The influence of thiourea on the nucleation of copper from a 0.30 M CuSO4-1 M H2SO4 solution on polycrystalline platinum electrodes covered by a coppe r adlayer was investigated. In the case of diffusion controlled nucleation and growth the conditioning potential, that is, the potential of the electr ode prior to the application of a large negative potential step, has a stro ng influence on the nucleation transients. This can result in either a prom otion or an inhibition of the nucleation (which is characterized by a chang e in the nucleation rate constant and/or the site density) depending on the applied potential and the concentration of thiourea. In the region of mixe d kinetics and for a fixed value of the conditioning potential (0.175 V vs Cu2+\Cu, that is, in the region of strongest inhibition), a new and rather unexpected effect was observed. Thus, after an induction period, which is p roportional to the concentration of thiourea, the current increases sharply to a much higher value, but after reaching a maximum drops again to its or iginal value. At present there is no ready explanation for this phenomenon, which has been called 'nucleation outbursts', but it deserves more investi gation because the linearity between the induction time and the concentrati on of thiourea might have practical applications.