Colloidal preparation and electrocatalytic hydrogen production of MoS2 and WS2 nanosheets with controllable lateral sizes and layer numbers

Although layered transition metal dichalcogenide (TMD) nanosheets have attracted great attention due to their unique properties, it still remains challenge to develop a facile strategy for the precise control of the lateral sizes and layer numbers of TMD nanosheets. In this study, we demonstrate a solution-phase synthetic protocol to prepare colloidal MS2 (M = Mo, W) nanosheets which possess extremely small lateral dimensions from 15 to 40 nm and well-controlled odd numbers of layers, such as 1, 3, and 5 layers, as characterized by transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The size-and layer-dependence of the optical properties of colloidal MS2 (M = Mo, W) nanosheets are revealed by Raman and absorption spectra for the first time. These colloidal nanosheets, especially the single-layer ones, possess a large number of edge sites that serve as active sites for the hydrogen evolution reaction (HER). The catalysts exhibit a small HER overpotential and low Tafel slope of approximately 100 mV and 52 mV per decade for MoS2, and 80 mV and 46 mV per decade for WS2, respectively. Importantly, these products show enhanced stability after 500 potential cycles, and the current density remains almost unchanged during the test.