Anodic Oxide Films and Electrochemical Reactions on HgTe

Abstract

Anodic oxidation processes on surfaces in acetate buffer pH 4.9, borate buffer pH 8.4, and pH 13.0, with and without ethylene glycol, have been studied. Pt‐split electrode experiments permit the simultaneous examination of formation and reduction of oxides and of soluble reaction products. The composition and thickness of the anodic oxide layers are evaluated coulometrically. XPS measurements yield the chemical composition of the surface after the different electrochemical treatments. They essentially confirm the electrochemical analysis. Optically homogeneous oxide layers of up to ∼1000Å thickness are grown in acetate buffer, borate buffer, and ethylene glycol. The layers are composed of mixed oxides, , with . The molar ratio levels off to a value of for high oxidation potentials in acetate buffer and reaches almost for the glycol mixture. The missing Hg is dissolved as during anodic oxidation with only a small amount of Te compounds. The first cathodic reduction step of the oxides in the same electrolytes reduces to metallic Hg. As a consequence is dissolved, demonstrating the importance of the for the chemical stability of the mixed oxide layer. Special cathodic reduction conditions (−0.6 VH in acetate buffer) produce a stoichiometrically clean surface, removing any oxide or elemental Te at the surface due to previous chemical treatments. The electrochemical reactions are similar for all three electrolytes mentioned above. However, in pure the disk and ring currents are more than one order of magnitude larger (>1 mA cm−2) than those in acetate and boric borate buffer (∼0.1 mA cm−2), indicating a higher dissolution rate of these oxide layers. The presence of 90% ethylene glycol in reduces the current density again to the low levels (<0.1 mA cm−2).