Synthesis and Anisotropic Electrocatalytic Activity of Covellite Nanoplatelets with Fixed Thickness and Tunable Diameter.

Size- and shape-dependent electrochemical activity of nanostructures reveals relationships between nanostructure design and electrochemical performance. However, electrochemical performance of aspect-ratio-tunable quasi-two-dimensional (2D) nanomaterials with anisotropic properties has not been fully investigated. We prepared monodispersed hexagonal covellite (CuS) nanoplatelets (NPls) of fixed thickness (∼2 nm) but broadly tunable diameter (from 8 to >100 nm). These span a range of aspect ratios, from ∼4 to >50, connecting quasi-isotropic and quasi-2D regimes. Tests of electrochemical activity of the NPls for the oxygen reduction reaction in alkaline solution showed improved activity with increasing diameter. Combining experimental results with density functional theory calculations, we attribute size-dependent enhancement to anisotropy of conductivity and electrochemical activity. The lowest computed oxygen adsorption energy was on Cu sites exposed by cleaving covellite along (001) planes through tetrahedrally coordinated Cu atoms. The specific surface area of these planes, which are the top and bottom surfaces of the NPls, remains constant with changing diameter, for fixed NPl thickness. However, charge transport through the electrocatalyst film improves with increasing NPl diameter. These CuS NPl-carbon nanocatalysts provide inspiration for creating well-controlled layered nanomaterials for electrochemical applications and open up opportunities to design new electrocatalysts using transition-metal sulfides.