Defect Mitigation of Solution-Processed 2D WSe2 Nanoflakes for Solar-to-Hydrogen Conversion.

Few-atomic-layer nanoflakes of liquid-phase exfoliated semiconducting transition metal dichalcogenides (TMDs) hold promise for large-area, high-performance, low-cost solar energy conversion, but their performance is limited by recombination at defect sites. Herein, we examine the role of defects on the performance of WSe2 thin film photocathodes for solar H2 production by applying two separate treatments, a pre-exfoliation annealing and a post-deposition surfactant attachment, designed to target intraflake and edge defects, respectively. Analysis by TEM, XRD, XPS, photoluminescence, and impedance spectroscopy are used to characterize the effects of the treatments and photoelectrochemical (PEC) measurements using an optimized Pt-Cu cocatalyst (found to offer improved robustness compared to Pt) are used to quantify the performance of photocathodes (ca. 11 nm thick) consisting of 100-1000 nm nanoflakes. Surfactant treatment results in an increased photocurrent attributed to edge site passivation. The pre-annealing treatment alone, while clearly altering the crystallinity of pre-exfoliated powders, does not significantly affect the photocurrent. However, applying both defect treatments affords a considerable improvement that represents a new benchmark for the performance of solution-processed WSe2: solar photocurrents for H2 evolution up to 4.0 mA cm-2 and internal quantum efficiency over 60% (740 nm illumination). These results also show that charge recombination at flake edges dominates performance in bare TMD nanoflakes, but when the edge defects are passivated, internal defects become important and can be reduced by pre-annealing.