Thin-layer electrochemical studies of the underpotential deposition of cadmium and tellurium on polycrystalline Au, Pt and Cu electrodes

Abstract

Thin-layer electrochemical studies of the underpotential deposition (UPD) of Cd and Te on polycrystalline Au, Pt, and Cu substrates have been performed. These studies were done in order to investigate the initial stages of the electrodeposition of CdTe. Tellurium deposition on Cu electrodes is very rapid at potentials between hydrogen evolution and Cu dissolution; as a result, the amount of electrodeposited Te cannot be suitably controlled with potential. Subsequent removal of Te on Cu also proved difficult by standard electrochemical cleaning procedures. Tellurium UPD and stripping on Pt occurred simultaneously with Pt oxide reduction and Pt oxidation, respectively. In addition, cadmium UPD on Pt is ill-defined, resulting in three peaks overlapping with the hydrogen wave voltammetry. It is presumed that the hydrogen waves are suppressed by the deposited Cd, although this is not definite. The most informative results were obtained with Au because of its broad double-layer window, which is free of complications from surface-specific faradaic reactions. Deposition of Te from TeO 2 solutions on Au was shown to require 3.9 ± 0.1 electrons per deposited Te. A substantial loss of electrodeposited Te was observed when the potential was cycled into the oxidation region on both the Au and Pt electrodes. Quantitation of the charge on Au indicated this loss was the result of oxidation of the Te(IV) to a Te(VI) compound which is difficult to reduce back to Te(IV) prior to hydrogen evolution on Au or Pt. At potentials between Au oxidation and Te UPD, the soluble Te species, HTeO + 2, was shown to adsorb reversibly on the Au surface. In studies of the alternated deposition of Cd and Te, initially deposited Cd is displaced by Te at potentials positive of −0.35 V. Subsequent UPD of Cd on the Te-covered Au resulted in one Cd atom for each deposited Te.