Salt-assisted growth of rhombohedral 3R-MoS single-crystal with enhanced electrocatalytic hydrogen evolution reaction performance.

Compared to hexagonal 2H-MoS, rhombohedral 3R-MoS exhibits noncentrosymmetric atomic structures and enhanced current density and carrier mobility. Rhombohedral 3R-MoS is considered to possess outstanding electrocatalytic properties comparable to those of 1 T-MoS. However, the preparation of 3R-MoS has been rarely reported owing to its metastable nature and high formation energy. Herein, we perform theoretical calculations to demonstrate that Na adsorption in MoS interlayers can alter the arrangement of adjacent layers. Based on this finding, and assisted by NaSO, a bottom-up synthesis strategy is designed for the in-situ growth of 3R-MoS. In-situ Raman spectroscopy confirms that the electronic configuration of Mo is regulated by the insertion of Na into the MoO lattice, which is facilitated by high-temperature diffusion, thereby guiding the epitaxial growth of 3R-MoS. As hydrogen evolution reaction catalysts, 3R-MoS exhibits an overpotential of 51.4 mV and a Tafel slope of 35.6 mV/dec. Further analyses indicate that this excellent performance can be attributed to the intrinsic activity of 3R-MoS. Additionally, obtaining a product with perfect crystallinity is fundamental to its electrochemical performance. Importantly, the preparation strategy for pure 3R-MoS developed in this study does not rely on substrates, and the resulting products can be easily transferred and integrated. The proposed method is expected to contribute to the practical application of 3R-MoS in energy storage and also provide a platform to explore stacking sequences and correlations between new phenomena and properties.