Unveiling the Effect of Shape Anisotropy of Plasmonic Tungsten Oxide Nanostructures for Enhanced Electrocatalytic Hydrogen Evolution

Abstract

Platinum is the most efficient electrocatalyst for the hydrogen evolution reaction (HER); however, the lack of earth abundance and high cost restrict its widespread use. Herein, we report electrocatalytic HER efficacy of oxygen-deficient, localized surface plasmon resonance (LSPR)-active tungsten-oxide (WO_3–x) nanocrystals (NCs) as a function of free conduction electron density and metal oxidation states. These plasmonic, anisotropic WO_3–x NCs display ∼160 mV overpotential at a current density of 10 mA/cm² and a Tafel slope of 54 mV/dec. Ultraviolet photoelectron spectroscopic measurements support the HER electrocatalytic properties attributed to the Fermi energy pinning as a function of the NC shape (i.e., nanowires, nanorods, and nanoplatelets) and chemical composition of the inorganic WO_3–x lattice. Furthermore, our experimental data show that increasing the number of coordinatively unsaturated tungsten sites, which are generated from the ionization of surface oxygen atoms in WO_3–x NCs, improves the surface reactivity and thus, enhances the catalytic performance. Together, structure–property relationship delineating the NC morphology and the resulting optoelectronic properties studied here provide the opportunity to unravel the correlation between the NC shape, free conduction electron density and the HER kinetics of this unique class of NC and to potentially initiate the designer principle of noble metal-free, highly active, but not limited to HER catalysts to produce an energy-dense fuel.