Pit-Induced Electrochemical Layer Dissolution and Wave Propagation on an Au(111) Surface in an Acidic Thiourea Solution

The mesoscopic evolution of the electrochemical dissolution of an Au(111) facet in acidic thiourea solutions was studied using in situ electrochemical atomic force microscopy (EC-AFM). At the open-circuit potential (OCP, 0.39 V vs RHE) of a gold bead electrode in 1.0 mM thiourea +0.1 M HClO4 solution, the Au(111) plane displays pit-induced layer dissolution, which subsequently transforms into a wave-type dissolution at a rate of 0.05 μm/min. The dissolution of the Au(111) is suppressed in the acidic thiourea solutions as the potential on the negative side of the OCP is varied from 0.37 to 0.28 V (vs RHE) but is promoted as the potential on the positive side of the OCP is varied from 0.42 to 0.58 V (vs RHE). As the applied potential becomes more positive than 0.58 V (vs RHE), the dissolution rate of the Au(111) plane becomes too fast, exceeding the scanning capacity of the EC-AFM. The Au-deposition, starting from 0.25 V (vs RHE) to lower potentials, could be observed by situ-EC-AFM. The reason for the formation of a wave-type dissolution of the Au(111) plane was illustrated through the bonding energy between the sulfur atom of thiourea and gold atoms located at different positions in the Au(111) facet. The dependence of the dissolution of the Au(111) plane on the applied potential in acidic thiourea media is clarified in this work, which provides a new understanding of the kinetics and mechanism for the leaching of gold by thiourea at the mesoscopic level.