Electrochemistry at the edge of a van der Waals heterostructure

Abstract

Artificial van der Waals heterostructures, obtained by stacking layered two-dimensional materials, represent a novel material platform for investigating physicochemical phenomena and applications. We report here electrochemistry at the one-dimensional edge of a graphene sheet, which is sandwiched between two hexagonal boron nitride (hBN) multilayer flakes. When such an hBN/graphene/hBN heterostructure is immersed in a solution, the basal plane of graphene is protected and isolated by the hBN stack, and the edge of the graphene sheet is exclusively available in the solution. This forms an electrochemical nanoelectrode, which enabled us to investigate electron transfer using several redox probes, e.g., ferrocene(di)methanol, hexaammineruthenium, methylene blue, dopamine and ferrocyanide. Facilitated by the relatively low capacitance of the van der Waals edge electrode, we demonstrate cyclic voltammetry at very high scan rates (up to 1000 V/s). Using fast scan cyclic voltammetry imaging, we show that redox species can be detected voltammetrically down to micromolar concentrations with subsecond time resolution at the sandwiched graphene edge, promoted by the rapid equilibration of analyte species in the diffusion layer. Furthermore, the nanoband nature of the edge electrode allows us to work directly in water in the absence of added electrolyte. Finally, we show that two adjacent edge electrodes can be realized in a redox-cycling format. In all, the van der Waals edge electrode is unique among nanoelectrodes as it enables investigations of all the above-mentioned phenomena in the same device. Due to its versatility, it constitutes a new avenue for nanoscale electrochemistry, which will be useful for studying electron transfer mechanisms as well as for the detection of analyte species in ultralow sample volumes.