Green Colloidal Synthesis of MoS Nanoflakes.

Currently, two approaches dominate the large-scale production of MoS: liquid-phase exfoliation, referred to as the top-down approach, and bottom-up colloidal synthesis from molecular precursors. Known colloidal synthesis approaches utilize toxic precursors. Here, an alternative green route for the bottom-up synthesis of MoS nanoflakes (NFs) is described. The NFs were synthesized by colloidal synthesis using [Mo(CHCOO)] and a series of sulfur (S)-precursors including thioacetamide (TAA), 3-mercaptopropionic acid (3-MPA), l-cysteine (L-CYS), mercaptosuccinic acid (MSA), 11-mercaptoundecanoic acid (MUA), 1-dodecanethiol (DDTH), and di--butyl disulfide (DTBD). While TAA, an S-precursor most commonly used for MoS NF preparation, is a known carcinogen, the other investigated S-precursors have low or no known toxicity. High-resolution scanning transmission electron microscopy (HR-STEM) and grazing incidence wide-angle X-ray scattering (GIWAXS) confirmed that in all cases, the syntheses yielded single-layer MoS NFs with lateral sizes smaller than 15 nm and a well-defined crystal structure. Electronic absorption and Raman spectra showed characteristic features associated with the MoS monolayers. The evolution of the absorption spectra of the growth solution during the syntheses reveals how the kinetics of the NF formation is affected by the S-precursor as well as the nature of the coordinating ligands.