Growth Pathway of CdS Nanoplatelets Investigated by In Situ X-ray Scattering and Optical Spectroscopy

Two-dimensional (2D) CdS nanoplatelets (NPLs) have attracted much attention due to their sharp absorption peaks and unique optical properties, but their formation mechanism has not been clearly explained. In this study, the whole growth process of 2D CdS NPLs from the original precursor compounds to the final products was investigated by in situ small-angle X-ray scattering (SAXS), ultraviolet-visible (UV-vis), and transmission electron microscopy (TEM). It reveals that the growth of CdS NPLs can be divided into three stages, i.e. dissolution of the precursors Cd(OA)(OAc)_2- and sulfur powder accompanied by the formation of CdS precursor compounds (PCs) from 30 to 120 °C (stage I), formation of the CdS magic-sized clusters (MSCs) and their growth into NPLs from 120 to 240 °C (stage II), and curling and uncurling growth of the NPLs at 240 °C (stage III). Dramatically, the shape of the NPLs changes from flat to curly when the temperature reaches 240 °C, but very interestingly, they become flat again after a prolonged stay at 240 °C. The curling and uncurling of NPLs is proposed to be driven by their surface stress imbalance induced by ligands.