engleski

Electrodeposition of Thin Sulfide Films: Nucleation and Growth Observed for Bi-sulfide

The sulfide film of interest was made purposely much thicker than a mono-molecular layer. The deposition was, therefore, actually a macrogrowth occurring in three dimensions. Mechanistically, however, it could be achieved practically and described theoretically in two alternative ways as for the case of metal deposition: three-dimensional (3D) nucleation and growth or two-dimensional (2D) layer-by-layer nucleation and growth. The kinetics and mechanism of electrochemical deposition of the Bi-sulfide film on bismuth, in the presence of sulfide ions, have been investigated in detail under potentiostatic conditions by a single potential step technique and impedance spectroscopy in combination with cyclic voltammetry. Spectroscopically pure Bi disc (Johnson-Matthey) was used as the working electrode. All experiments were carried out in 1.0 M NaOH solution containing sulfide ions, using the electrochemical set-up consisting of a potentiostat/galvanostat and a Lock-in Amplifier, controlled by a PC. Impedance measurements were performed in the frequency range from 50 kHz to 30 mHz, with ac voltage amplitude of 5 mV. The essential characteristics of the Bi-sulfide film formation and growth on Bi have been studied using cyclic voltammetry at various sweep rates. The potential at which electronucleation of Bi-sulfide starts was determined in the absence of polarization effects. The obtained value of - 0.940 V is very close to the equilibrium potential of the Bi (III) sufide couple. The feature of current-time curves, together with the current cross-over in the cyclic voltammetry during Bi-sulfide deposition under the swept potential control, is typical of deposition via a nucleation under diffusion control and a growth mechanism of the nuclei. According to the three-dimensional (3D) nucleation model, a progressive nucleation and growth mechanism accounts for this current-time behaviour. Using a fitting procedure, the steady state nucleation rate, the number density of active sites and the diffusion coefficient were determined. The results of impedance spectroscopy have been discussed in terms of nucleation and growth features of the Bi-sulfide films. Impedance results showed that the diffusion of sulfide species controls the overall rate of Bi-sulfide electrodeposition, which is consistent with potential step results.

Bismuth; sulfide films; nucleation; electrodeposition; chronoamperometry; impedance spectroscopy; cyclic voltammetry

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