Mapping Electron Transfer at MoS2 using Scanning Electrochemical Microscopy
Nicole L. Ritzert, Veronika A. Szalai, Thomas P. Moffat
Understanding the role of macroscopic and atomic defects in the interfacial electron transfer properties of layered transition metal dichalcogenides is important in optimizing their performance in energy conversion and electronic devices. Means of determining the heterogeneous electron transfer rate constant, k, have relied on deliberate exposure of specific electrode regions or additional surface characterization to correlate proposed active sites to voltammetric features. Few studies have investigated the electrochemical activity of surface features of layered dichalcogenides under the same experimental conditions. Herein, MoS2 flakes with well-defined features were mapped using scanning electrochemical microscopy (SECM). At visually flat areas of MoS2, k of hexacyanoferrate(III) ([Fe(CN)6]3−) and hexacyanoferrate(II) ([Fe(CN)6]4−) was typically smaller and spanned a larger range than that of hexaammineruthenium(III) ([Ru(NH3)6]3+), congruent with current literature. However, in contrast to previous studies, reduction of [Fe(CN)6]3− and oxidation of [Fe(CN)6]4− exhibited similar rate constants, attributed to the dominance of charge transfer through surface states. Comparison of SECM with optical and atomic force microscopy images revealed that while most of the flake was electroactive, edge sites associated with freshly exposed areas that include both macrosteps consisting of several monolayers and recessed areas exhibited the highest reactivity, consistent with reported results.