Enhancing hydrogen evolution reaction activity through defects and strain engineering in monolayer MoS2

Abstract

Molybdenum disulfide (MoS₂) has recently emerged as a promising electrocatalyst for the hydrogen evolution reaction (HER). However, the poor in-plane electrical conductivity and inert basal plane activity pose major challenges in realizing its practical application. Herein, we demonstrate a new approach to induce biaxial strain into CVD-grown MoS₂ monolayers by draping it over an array of patterned gold nanopillar arrays (AuNAs) as an efficient strategy to enhance its HER activity. We vary the magnitude of applied strain by changing the inter-pillar spacing, and its effect on the HER activity is investigated. To capitalize on the synergistic effect of improved ΔG _H via strain engineering and leverage basal plane activation by introduction of sulphur vacancies, we further exposed the strained MoS₂ monolayers to oxygen plasma treatment to create S-vacancies. The strained MoS₂ on AuNAs with optimal inter-pillar spacing is exposed to oxygen plasma treatment for different durations, and we study its electrocatalytic activity towards the HER using on-chip microcell devices. The strained and vacancy-rich monolayer MoS₂ draped on AuNAs with a 0.5 μm inter-pillar spacing and exposed to plasma for 50 s (S_0.5μmV₅₀-MoS₂) is shown to exhibit remarkable improvement in HER activity, with an overpotential of 53 mV in 0.5 M H₂SO₄. Thus, the synergistic creation of additional vacancy defects, along with strain-induced active sites, results in enhancement in HER performance of CVD-grown monolayer MoS₂. The present study provides a highly promising route for engineering 2D electrocatalysts towards efficient hydrogen evolution.

Fig. 1. Schematic representation of (a) gold nanopillar arrays on a Si/SiO₂ substrate, (b) CVD-grown MoS₂ monolayer transferred over the AuNAs (S-MoS₂), and (c) vacancy-rich strained MoS₂ (SV-MoS₂).

Fig. 2. (a) Optical microscopy image and (b) AFM image of CVD-grown MoS₂ monolayer domains; the inset shows the height profile in the marked region. (c) SEM image of AuNAs with a separation of 2 μm, and the inset shows the magnified image. (d) Optical image of S_2μm-MoS₂. (e) AFM image of S_2μm-MoS₂ clearly shows wrinkles on the layers. (f) Raman and (g) PL spectra of S_2μm-MoS₂ recorded from spots corresponding to the locations 1 and 2 marked on the Raman map image (E_2g¹ intensity) shown in (h). The clear shift in the spectra shows the action of biaxial strain. (i) PL map image at 1.82 eV of S_2μm-MoS₂, and (j) Raman map image (E_2g¹ peak position) (scale bars of (h–j) are 2 μm).

Fig. 3. (a) SEM and (b) AFM images of S_0.5μmV₃₀-MoS₂. XPS spectra of (c) Mo 3d and (d) S 2p of MoS₂, S_2μm-MoS₂ and S_2μmV₃₀-MoS₂.

Fig. 4. (a) Schematic illustration of SV-MoS₂ microcell device employed for the electrocatalytic HER studies. (b) Optical microscope image of the fabricated microcell device. (c) Linear sweep voltammetry (LSV) curves recorded for pristine MoS₂ and S-MoS₂ with varying inter-pillar spacings, and (d) the corresponding Tafel plots. (e) LSV curves and (f) the corresponding Tafel plots recorded for S_0.5μmV-MoS₂ with varying plasma exposure durations.