THIN OXIDE-FILMS ON INDIUM - IMPEDANCE SPECTROSCOPY INVESTIGATION OF REDUCTIVE DECOMPOSITION

The kinetics of cathodic reduction of an In2O3 film grown on a bare in dium substrate under dynamic conditions was studied by cyclic voltamme try and electrochemical impedance spectroscopy (EIS) in berate buffer, pH 10. Reductive decomposition of In2O3 up to metallic In proceeds in the solid phase, via a solid-state mechanism: In2SO3(s) + 2H(2)O((aq) ) + 6e(-) <-> 2In((s)) + 6OH((aq)(-)). Impedance spectra obtained betw een 40 mHz and 50 kHz, in the potential range of oxide film stability, its decomposition, new metal phase extraction and hydrogen evolution were interpreted on the basis of equivalent electrical circuits using a suitable fitting procedure. EIS has shown to be a valuable tool in t he characterization of the solid/liquid interface: (i) the passive In2 O3 film behaved as a simple dielectric, the interface of a passive ele ctrode was blocking and in Bode and Nyquist plots a capacitive feature appeared; whereas (ii) in the vicinity of the In/In2O3 redox potentia l a Warburg low-frequency feature was obtained, which was associated w ith diffusion processes in the solid phase as the rate-determining ste p; (iii) the interface of the freshly segregated metallic phase was a close-to-perfect RC feature, which was associated with charge transfer as the rds for the hydrogen evolution reaction on the bare metal surf ace. Cyclic voltammetry has revealed more details of the reduction pro cesses giving further insight into the mechanism of reductive decompos ition of In2O3 up to metallic In.