Balancing the Hydrogen Evolution Reaction, Surface Energetics, and Stability of Metallic MoS2 Nanosheets via Covalent Functionalization

We modify the fundamental electronic properties of metallic (1T phase) nanosheets of molybdenum disulfide (MoS₂) through covalent chemical functionalization, and thereby directly influence the kinetics of the hydrogen evolution reaction (HER), surface energetics, and stability. Chemically exfoliated, metallic 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 that is slightly worse compared 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 of the functional group. 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 nanosheet. The functional groups preserve the metallic nature of the MoS₂ nanosheets, inhibiting conversion to the thermodynamically stable semiconducting state (2H) when mildly annealed in a nitrogen atmosphere. We propose that the electron density and, therefore, reactivity of the MoS₂ nanosheets are controlled by the attached functional groups. Functionalizing nanosheets of MoS₂ and other transition metal dichalcogenides provides a synthetic chemical route for controlling the electronic properties and stability within the traditionally thermally unstable metallic state.