Tunable intrinsic strain in two-dimensional transition metal electrocatalysts

Abstract

Strain can modify the electronic properties of a metal and has provided a method for enhancing electrocatalytic activity. For practical catalysts, nanomaterials with high surface areas are needed. However, for nanoparticles, strain is often induced with overlayers (adsorbates or heteroatoms) that can undergo reconstruction during operation that releases the induced strain. Wang et al. show that freestanding palladium nanosheets (three to five monolayers thick) form with an internal compressive strain of 1 to 2% and can be much more active for both the oxygen and hydrogen evolution reactions under alkaline conditions compared with nanoparticles. Science , this issue p. 870 Strain in a two-dimensional palladium nanosheet enhances its activity for the alkaline oxygen and hydrogen evolution reactions. Tuning surface strain is a powerful strategy for tailoring the reactivity of metal catalysts. Traditionally, surface strain is imposed by external stress from a heterogeneous substrate, but the effect is often obscured by interfacial reconstructions and nanocatalyst geometries. Here, we report on a strategy to resolve these problems by exploiting intrinsic surface stresses in two-dimensional transition metal nanosheets. Density functional theory calculations indicate that attractive interactions between surface atoms lead to tensile surface stresses that exert a pressure on the order of 10 5 atmospheres on the surface atoms and impart up to 10% compressive strain, with the exact magnitude inversely proportional to the nanosheet thickness. Atomic-level control of thickness thus enables generation and fine-tuning of intrinsic strain to optimize catalytic reactivity, which was confirmed experimentally on Pd(110) nanosheets for the oxygen reduction and hydrogen evolution reactions, with activity enhancements that were more than an order of magnitude greater than those of their nanoparticle counterparts.