Electrochemical nanogravimetric studies of sulfur/sulfide redox processes on gold surface

Electrochemical quartz crystal microbalance, combined with cyclic voltammetric, chronoamperometric, and potentiostatic measurements, was used to study electrodeposition/dissolution phenomena at a gold electrode in solutions containing Na2S. Spontaneous, open-circuit deposition processes as well as dissolution of the deposits in sulfide-free solutions have also been investigated. The potential range, scan rate, sulfide concentration, and pH have been varied. The results of the piezoelectric nanogravimetric studies are elucidated by a rather complex scheme involving underpotential deposition of sulfur at approximately -0.85 V vs. sodium calomel electrode, reductive dissolution of the deposited sulfur-containing layer at potentials more negative than approximately -0.9 V, and formation of a sulfur-containing multilayer at potentials more positive than -0.2 V. During the reduction of sulfur deposited on Au, a mass increase due to the formation of polysulfide species in the surface layer, accompanied by incorporation of Na+ counterions, can be observed that starts at approximately -0.4 V. This is a reversible process, i.e., during the reoxidation, counterions leave the surface layers. Frequency excursions during the electroreduction and reoxidation processes reveal existence of several competitive dissolution-deposition steps. Spontaneous interaction between Au and HS- species results in a surface mass increase at the open-circuit potential, and it also manifests itself in the substantial decrease of the open-circuit potential after addition of Na2S to the supporting electrolyte.