Underpotential Deposition of Te Monolayers on Au Surfaces from Perchloric Acid Solution Studied by Chronocoulometry and EQCM

Abstract

We report a study of the underpotential deposition (UPD) of Te monolayers onto Au electrodes from perchloric acid solution using electrochemical quartz microgravimetry (EQCM) and chronocoulometry. We find that tellurium(IV) oxide (as TeO32-, TeO2 or HTeO2+) is adsorbed onto Au at potentials positive of the UPD region. EQCM experiments, in which an Au electrode poised at +0.7 V in pure HClO4 electrolyte was monitored as a function of time before and after the addition of TeO2, showed a slow adsorption of a submonolayer of TeO2. Energy-dispersive X-ray (EDX) measurements confirm the presence of a Te-containing species on the surface. Two distinct reductive UPD features at approximately +0.400 V and −0.100 V vs Ag|AgCl were observed by cyclic voltammetry. EQCM measurements indicate that there is a small, reproducible mass decrease accompanying the first UPD wave, consistent with the loss of H2O from the surface as an adsorbed layer of HTeO2+ is reduced to a submonolayer of Te atoms. Chronocoulometry indicates a charge density of 160 μC/cm2 for the first UPD wave, which, when taken together with the mass change data, is consistent with the four-electron reduction of adsorbed HTeO2+ to Te. In addition, analysis of the current−time data indicates that this process occurs by a two-dimensional instantaneous nucleation and growth mechanism, resulting in an ordered overlayer with a fractional coverage of ca. 0.41. The second UPD peak results in the formation of a dense monolayer by reduction of HTeO2+ from solution. This process also appears to occur by a direct four-electron reduction and exhibits simple Langmuir adsorption behavior as evidenced by exponentially decaying current−time transients. The saturation coverage is approximately 0.9.