2D METAL CHALCOGENIDE AND METAL LAYERED DOUBLE HYDROXIDE (LDH) MATERIALS AND THEIR HETEROSTRUCTURES FOR ELECTROCATALYSIS AND DEVICE APPLICATIONS

摘要

For efficient and practical photoelectrochemical solar energy conversion, such as solar water splitting, we need inexpensive electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with catalytic performance rivaling that of the noble metal catalysts. We have discovered that chemical exfoliation of layered MX2 nanostructures and the simultaneous conversion of the as-synthesized MX2 material from its semiconducting 2H polymorph to the metallic 1T polymorph, grants greatly enhanced electrocatalytic activity for HER. The resulting 1T-MoS2 and 1T-WS2 nanosheets exhibit dramatically enhanced HER electrocatalytic performance as compared to their corresponding 2H polymorphs. Structural characterization and electrochemical studies confirm that the nanosheets of the metallic M0X2 polymorph exhibit facile electrode kinetics, low-loss electrical transport, and possess a proliferated density of catalytically active sites. These distinct and previously unexploited features of 1T-MX2 make these metallic nanosheets highly competitive earth-abundant HER catalysts. Furthermore, we have coupled electrocatalytic 1T-MoS2 to a p-type silicon photoelectrode to enable hydrogen evolution driven by solar light. Due to the excellent HER activity of 1T-MoS2 and the high-quality catalyst-semiconductor interface enabled by the direct CVD growth of MoS2 on silicon (followed by chemical exfoliation), a much higher photocurrent density for hydrogen evolution was achieved as compared to 2H-MoS2 on silicon. Photocurrents up to 17.6 mA/cm~2 at 0 V vs reversible hydrogen electrode (RHE) were achieved under simulated 1 sun irradiation, and good stability was demonstrated over repeated scans and long-time operation. Electrochemical impedance spectroscopy revealed low charge transfer resistances at the semiconductor/catalyst and catalyst/electrolyte interfaces, and surface photoresponse measurements also demonstrated slow carrier recombination dynamics and consequently efficient charge carrier separation, providing further evidences for the superior performance. Our results suggest that chemically exfoliated 1T-MoS2/Si heterostructures are promising earth-abundant alternatives to photocathodes based on noble metal catalysts for solar-driven hydrogen production. We have also discovered that low temperature CVD growth using MoCls and S precursors results in conformal deposition of amorphous MoS_xCl_y HER catalyst that show high electrocatalytic activity and high PEC hydrogen generation performance. These results demonstrate chemically exfoliated 1T-MoS2 and amorphous MoS_xCl_ySi heterostructures are promising earth-abundant alternatives to photocathodes based on noble metal catalysts for solar-driven hydrogen production.