Study of the gold-thiosulfate interface under leaching conditions
Sweep voltammetry, chronocoulometry and Surface Enhanced Raman Spectroscopy (SERS) were employed in this research program to characterize the gold-electrolyte interface under leaching conditions in alkaline thiosulfate solution. A new methodology was established that allowed for the measurement of the leaching currents from current-potential curves with minimal interference of the applied potential on the structure of the passive layer. In this thesis, a gold nanorod array electrode is proposed as a new substrate for SERS studies of these passive layers. This SERS-active substrate allowed time dependent studies in solutions where leaching of gold is promoted. Important information regarding the composition of the passive layer that is formed on the gold surface when in contact with thiosulfate solution was obtained. Tetrathionate and trithionate were identified for the first time as main components of the passive layer. The role of thiourea or ammonia in the enhancement of the leaching reaction in thiosulfate-based solutions was investigated. The presence of either of these two additives in the leaching solution prevents formation of the oxygen-containing sulfur species responsible for passivation of the gold surface. The effect of other additives that show potential to improve the gold dissolution reaction in thiosulfate electrolyte was investigated. Preliminary studies in the presence of 3-mercaptopropionic acid or 2-thiouracil indicate that these two compounds produce an increase in the gold leaching current in thiosulfate solution. A new experimental procedure was developed to overcome the limitations encountered when measuring leaching current from current-potential curves obtained with sweep voltammetry. The method uses a rotating ring-disk electrode with the glassy carbon-gold configuration to determine the leaching current without the interference of side reactions such as oxidation of electrolyte components.