Scanning Electrochemical Microscopy for the Electroless Deposition of Gold on Natural Pyrite: Effect of Ferric Ions

The local electroless deposition of gold on pyrite is studied by scanning electrochemical microscopy (SECM). The SECM and additional SEM experiments provide kinetic and mechanistic insights on the local galvanic replacement of pyrite by gold. We evidence that individual gold nanoparticles or full coverage of gold on the pyrite surface can be obtained by controlling either the deposition time or the flux of gold ions electrogenerated at the ultramicroelectrode. We also studied the influence of ferric ions, known to be implied on gold etching and pyrite corrosion and therefore, on the overall process of gold deposition on pyrite. We then demonstrate that the presence of ferric ions significantly modifies i) the thermodynamic of the gold electrodissolution, ii) the process of adsorption of auric ions on pyrite in the first instants, iii) the number of nucleation sites on the pyrite surface and, iv) the structure of the deposited gold layer.