Green synthesis of ultrathin WS2 nanosheets for efficient hydrogen evolution reaction

Abstract

Transition metal dichalcogenide (TMD) nanostructures have emerged as promising electrocatalysts for the hydrogen evolution reaction (HER) as there is an increasing demand for cost-effective and sustainable hydrogen production. Despite significant progress, there is still a critical need for developing facile and green methods for synthesizing ultrathin TMD nanostructures. In this study, we introduce a green, top-down synthesis method to produce highly exfoliated WS₂ nanosheets. The process combines the ultrasonication of bulk WS₂ in a binary water-ethanol solvent with a solvothermal treatment. The resulting ultrathin WS₂ nanosheets exhibit clean surfaces free of surface ligands and impurities, high crystallinity in the semiconducting hexagonal phase, and outstanding electrochemical activity for HER. Key performance metrics include a low onset potential of -0.32 V (vs. reversible hydrogen electrode (RHE)) at 10 mA cm^-2 and a low Tafel slope of 160 mV dec^-1 with a catalyst loading of 0.76 mg cm^-2. The promising HER performance is attributed to (1) a high density of exposed edges and defects, (2) enhanced charge transport due to high crystallinity, and (3) clean surfaces enabling efficient interfacial electron transfer. Furthermore, operando Raman spectroscopy using a 3D-printed electrochemical cell identifies the catalytically active sites on WS₂ nanosheets for HER. This work provides a green route to high-performance, low-dimensional electrocatalysts for sustainable hydrogen production.

Scheme 1. The synthesis of ultrathin WS₂ nanosheets.

Fig. 1. (a) Raman spectra of exfoliated WS₂ nanosheets collected from six spots and WS₂ bulk powder. (b) Raman spectra of six spots normalized to the E^l_2g peak maximum. (c) AFM image of exfoliated WS₂ on a mica substrate. Scanned area of the image is 3 μm × 3 μm. Inset: line–scan profiles across several WS₂ nanosheets on the AFM image. (d) XRD spectra of the ultrathin WS₂ nanosheets (top) and bulk WS₂ (bottom). (e) XPS high resolution spectra of W4f and (f) XPS high resolution spectra of S2p.

Fig. 2. (a) SEM image of the ultrathin WS₂ nanosheets. (b) Distribution of particle sizes of ultrathin nanosheets. (c) Low-resolution TEM image of ultrathin WS₂ nanosheets. (d) HAADF-STEM image of an ultrathin WS₂ nanosheet. (e) HR-TEM image of a WS₂ nanosheet showing lattice fringes of 0.27 nm corresponding to (100) plane of WS₂. (f) SAED pattern of WS₂ nanosheets indexed to the (001) zone axis of the 1H phase. (g) EDS elemental maps of W and S. (h) EDS spectrum indicating the elemental composition of WS₂ (Cu peak originates from the TEM sample grid).

Fig. 3. (a) HAADF-STEM image of monolayer WS₂ nanosheets. (b) Enlarged inverse FFT (IFFT) image of the area shown in the pink square of image (a), showing hexagonal-atomic arrangement of the 1H phase of WS₂, overlayed with the WS₂ 1H crystal structure model obtained from the CrystalMaker software. W atoms are given in purple color and S in green color. (c) Intensity profile across the yellow arrow in image (b) indicating the W and S intensities. Inter-atomic distance of 0.54 nm between W atoms in the 1H phase is shown. (d–f) Atomic resolution HAADF-STEM images showing different types of structural defects in the nanosheets. (g–i) IFFT images of the HAADF-STEM images in (d)–(f) respectively, highlighting the defect areas. The intensity profile along the yellow arrow is given on the inset indicating the reduction in interlayer distance of W atomic layers due to S vacancy lines created.

Fig. 4. (a) Polarization curves of three WS₂ catalyst mixture (10 wt%, 50 wt%, 80 wt%), only carbon black, and commercial 20 wt% Pt/C (inset shows a better representation of the onset potentials), (b) Tafel plots for WS₂, carbon black, and Pt/C with the calculated Tafel slopes for HER, (c) Nyquist plots of three catalyst mixture determined by EIS measurements (inset shows an equivalent circuit), and (d) electrochemical active surface areas estimated from the cyclic voltammograms at various scan rate (10–140 mV s⁻¹, Fig. S3†).

Fig. 5. (a–c) Schematic of experimental design and cell of operando Raman analysis (d) operando Raman spectra of the 50% WS₂ catalyst mixture in 0.5 M H₂SO₄ collected at different voltages (e) variation of Raman intensity of three peaks at 722 cm⁻¹, 812 cm⁻¹ and 214 cm⁻¹ with the increase of applied potential. (f) The bending (in-plane) and wagging (out of-plane) modes of H–S–W corresponding to the operando Raman signal and obtained through periodic DFT study.