Probing the oxidative etching kinetics of metals with the feedback mode of the scanning electrochemical microscope

Abstract

The conditions under which the feedback mode of the scanning electrochemical microscope (SECM) can be used to probe the oxidative etching kinetics of metals most effectively are considered. In this application, the oxidant of interest is electrogenerated at the tip ultramicroelectrode (UME) of an SECM from a solution containing only the reduced form of the redox couple. In this study, tris (2,2′-bipyridyl)ruthenium(III)[Ru(bipy)₃³⁺] is generated in aqueous potassium nitrate solutions through the oxidation of Ru(bipy)₃²⁺, and bromine is formed through the oxidation of bromide in aqueous sulfuric acid solutions. The oxidant diffuses to an unbiased metal substrate, where it may induce oxidative etching, resulting in the anodic dissolution of the metal. In the process, the mediator is converted back to its reduced form and fed back to the tip by diffusion, thereby enhancing the current. When the substrate is effectively infinite in size compared to the tip UME, its potential is fixed by the bulk solution conditions at a value that also promotes diffusion-controlled positive feedback of the mediator. Thus, feedback current measurements under these conditions may not simply reflect the kinetics of etching. SECM imaging experiments with Pt substrate electrodes, which are inert with respect to anodic dissolution with the two oxidants of interest, demonstrate that this complication can be circumvented by decreasing the size of the substrate electrode to the dimensions of the tip UME. Under these conditions, substrate potential-driven feedback at an unbiased metal becomes negligible, allowing etching kinetics to be studied in isolation. Investigations of Cu etching with both Ru(bipy)₃³⁺ and Br₂, under these conditions reveal, unambiguously, that the metal dissolution process is diffusion-controlled under all of the conditions examined, suggesting that the heterogeneous rate constant controlling the etching process is large. In the case of the reaction with Ru(bipy)₃³⁺, a minimum rate constant of 0.4 cm s^–1 is estimated for the interfacial reaction, assuming a first-order process. On a general level, the studies provide new information on the imaging capabilities of SECM in the feedback mode with unbiased substrates.