Elisa Miller-Link1 Eric Benson1 Hanyu Zhang1 Samuel Schuman1 Sanjini Nanayakkara1 Noah Bronstein1 Suzanne Ferrere1 Jeffrey Blackburn1

1, National Renewable Energy Laboratory, Lakewood, Colorado, United States

Transition metal dichalcogenides are being studied due to their unique optoelectronic properties, which can be utilized in the hydrogen evolution reaction (HER). In particular, metallic molybdenum disulfide (MoS2) nanosheets have been studied for HER due to their higher reactivity for HER, earth abundance, low-cost, and non-toxicity, which makes MoS2 a candidate to replace platinum for HER. In this study, we modify the fundamental electronic properties of metallic (1T phase) MoS2 nanosheets through covalent chemical functionalization, and thereby directly influence the HER kinetics, surface energetics, and stability. We also explore the degradation mechanism for HER of the unfunctionalized 1T MoS2 nanosheets. Chemically-exfoliated, metallic (1T) MoS2 nanosheets are functionalized with organic phenyl rings containing electron donating or withdrawing groups. We find that MoS2 functionalized with the most electron donating functional group (p-(CH3CH2)2NPh-MoS2) is the most efficient catalyst for HER in this series, with initial activity similar to the pristine metallic phase of MoS2. The p-(CH3CH2)2NPh-MoS2 is more stable than unfunctionalized metallic MoS2 and outperforms unfunctionalized metallic MoS2 for continuous H2 evolution within 10 min under the same conditions. With regards to the entire studied series, the overpotential and Tafel slope for catalytic HER are both directly correlated with the electron donating strength (Hammett parameter) of the pendant group on the phenyl ring. The results are consistent with a mechanism involving ground-state electron donation or withdrawal to/from the MoS2 nanosheets, which modifies the electron transfer kinetics and catalytic activity of the MoS2 sheet. We show that the functional groups preserve the metallic feature of the MoS2 films, inhibiting conversion to the thermodynamically stable semiconducting state (2H) when annealed at 150 °C for 24 h in a nitrogen atmosphere. We propose that this protection is critical to maintaining the catalytically active state of 1T MoS2 nanosheets. To test this hypothesis, we measure via X-ray photoelectron spectroscopy the chemical environment of the MoS2 electrode after HER to determine how the p-(CH3CH2)2NPh-MoS2 electrode changes compared to the unprotected MoS2 electrode.