Peeling off the surface: Pt-decoration of WSe2 nanoflakes results in exceptional photoelectrochemical HER activity

Photoelectrochemical (PEC) hydrogen evolution reaction (HER) was studied on exfoliated, pristine and Pt-decorated tungsten diselenide (p-WSe2) nanoflake samples, using a previously developed microdroplet PEC microscopy approach. The WSe2 nanoflakes had well-defined thicknesses as measured by atomic force microscopy, and the Pt nanoparticles (NPs) were deposited by a variable number of atomic layer deposition (ALD) cycles. An exceptionally high photocurrent density of 49.6 mA cm(-2) (under 220 mW cm(-2) irradiation) and internal-photonto-electron-conversion efficiency (similar to 90% at 550 nm) were demonstrated on these Pt-decorated WSe2 (WSe2-Pt) photocathodes. The Pt NP loading and thickness of WSe2 nanoflakes (in the 24-235 nm range) were used to fine-tune their PEC activity for HER. We found similar charge transfer and surface recombination kinetics of pristine and WSe2-Pt specimens (as assessed by intensity-modulated photocurrent spectroscopy), which indicated significant differences in their bulk properties. X-ray and ultraviolet photoelectron spectroscopies were performed to identify defect states and quantify the density of states around the valence band of WSe2. The elevated temperature of the ALD process and the evolving Pt NP phase conspired to passivate the sub-surface (i.e., bulk) defects in the WSe2 nanoflakes, resulting in their vastly improved PEC performance.