We modify solution-exfoliated MoS2 nanosheets to control the phase and surface energetics, which directly impacts the catalytic activity for the hydrogen evolution reaction (HER). HER is enhanced for the metallic (1T) phase relative to the thermodynamically stable semiconducting (2H) phase. We show that covalent functionalization of the 1T phase of MoS2 nanosheets prohibits the nanosheets from converting back to the 2H phase. Our results demonstrate a strong correlation between the electron donating strength of substituents and the surface energetics, electron transfer resistance, and the HER catalytic activity of functionalized MoS2 nanosheets. Specifically, 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. To gain additional insight, we employ scanning electrochemical microscopy (SECM) for high-resolution mapping (&lt;20 nm spatial resolution) of surface reactivity for mixed-phase and pure 2H-only MoS2 nanosheets. For mixed-phase MoS2 nanosheets, we find major differences in reactivity of the two phases for electron transfer involving ferrocenemethanol, allowing us to locate 1T and 2H regions and directly map the corresponding HER activity. The correlations between the topography/phase and surface reactivity for H2 generation were further elucidated by additional microscopies, where the lower energy, local workfunction measured on the mixed-phase nanosheets should provide a greater driving force to generate H2. In addition, we also perform SECM, scanning Kelvin probe microscopy, and transmission electron microscopy-electron energy loss spectroscopy on functionalized MoS2 nanosheets to understand where the functional groups attach to the MoS2 and how this influences HER on a microscopic level. These results provide a framework for understanding and controlling the balance between catalytic activity and stability for these unique 2D materials.

Controlling the phase and stability of metallic MoS2 nanosheets for hydrogen generation

2019 Spring National Meeting

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