Ripening of two-dimensional colloidal CdSe nanocrystals into zero-dimensional nanodots

Abstract

Understanding the ripening of two-dimensional (2D) colloidal nanocrystals (NCs) is important for the controllable synthesis of NCs with desired morphology and properties. In this study, we systematically investigate the ripening behavior of the 2D CdSe NCs in the presence of a short-chain acetate ligand and a long-chain oleate ligand. We find that a low acetate/oleate ratio, a low Cd/Se ratio, and a low monomer concentration help in the ripening of the 2D NCs to form 0D NCs. Moreover, a porous nanosheet intermediate is observed when there is a high Cd/Se ratio, whereas in the case of a low Cd/Se ratio, the ripening starts from the edge of the nanosheets, resulting in a saw-like nanosheet intermediate. These findings provide necessary insights into the growth and ripening of 2D CdSe NCs that allow for the controlled synthesis of 0D and 2D CdSe NCs.

Keywords: Colloids; Nanomaterials; Nanoparticles.

Graphical abstract

Figure 1

Ripening of the CdSe NCs synthesized in the presence of different comibinations of carboxylate ligands (A–I) (A, B, D, and E) TEM images, (G and H) STEM images, and (C, F, and I) UV-vis absorption spectra of CdSe NCs synthesized in the presence of only acetate (A–C), acetate and hexanoate (D–F), and acetate and oleate (G–I) within a reaction time of 1 min (BR; D and G), 8 min (BR or AR; A and E), 64 min (AR; H), and 256 min (AR; B), respectively. The right panel shows the schematic illustrations of the possible surface ligand capping for NCs before ripening in each case. (J) A schematic showing that the kinetically stable CdSe nanotubes and nanosheets, formed under different combinations of carboxylate ligands respectively, turn into thermodynamically stable nanodots after ripening. BR, before ripening; AR, after ripening.

Figure 2

Ripening process of the 2D CdSe nanosheets synthesized in the presence of acetate and oleate (A–L) (A–E) TEM images, (F–J) STEM images, (K) absorption spectra, and (L) powder XRD spectra of the CdSe NCs synthesized in the presence of acetate and oleate within a reaction time of 1 min (A and F), 8 min (B and G), 16 min (C and H), 32 min (D and I), and 64 min (E and J), respectively. The schematic in the yellow panel corresponds to the morphologies of the NCs in the same column.

Figure 3

Ripening of the 2D CdSe nanosheets at a high monomer concentration and different Cd/Se ratios (A–L) (A–H) TEM images and (I–L) absorption spectra of the CdSe NCs synthesized in the presence of acetate and oleate at the quantity of 0.25 mmol and 0.05 mmol (A and E), 0.25 mmol and 0.1 mmol (B and F), 0.25 mmol and 0.25 mmol (C and G), and 0.25 mmol and 0.625 mmol (D and H) between Cd and Se precursors, respectively, within a reaction time of 1 min (A–D) and 8 min (E–H). The molar ratios between acetate and oleate are kept at 1: 0.25 in all cases.

Figure 4

Favorable conditions for the ripening of 2D CdSe nanosheets into 0D nanodots (A) A schematic showing the growth of 2D NCs and their ripening into 0D NCs in terms of Gibbs free energy. (B) A plot showing the obtained main CdSe NCs synthesized within a reaction time of 8 min at different acetate/oleate ratios, Cd/Se ratios, and precursor contents.

Figure 5

A possible mechanism for the formation of porous nanosheet intermediates (A) A schematic showing the ripening processes involved in two different crystal growth mechanisms. In route one, where the anisotropic particle is formed by classical monomer addition, the intraparticle ripening is supposed to start from the edge of active facets. Whereas in route two, where the anisotropic particle is formed via non-classical particle attachment, the defects present in the interior of the nanocrystals could be the ripening starting points. (B) An assembly of nanoplatelets into a tube-like nanostructure observed when the CdSe NCs are synthesized using a quantity of 0.05 mmol and 0.01 mmol for the Cd and Se precursors, respectively, and for a reaction time of 1 min. (C) A schematic showing the possible arrangement of long-chain ligands on the lateral surface and hole surface of the NCs.

Figure 6

A possible mechanism for the formation of saw-like nanosheet intermediates (A) A schematic showing the possible arrangement of long-chain ligands adsorbed on the lateral surface of the NCs synthesized at high and low Cd/Se ratio, respectively. (B) FTIR spectra of the surface ligands including TOP, DOAm, Cd(oleate)₂, and Cd(acetate)₂, as well as the CdSe NCs prepared in the presence of acetate and oleate at the quantity of 0.25 mmol and 0.05 mmol, 0.25 mmol and 0.1 mmol, 0.25 mmol and 0.25 mmol, and 0.25 mmol and 0.625 mmol between Cd and Se precursors, respectively, within a reaction time of 8 min. (C) TGA weight loss of the CdSe NCs prepared at different quantities of Cd and Se precursors within a reaction time of 8 min. The surface ligands started to combust at 305°C and be totally removed at around 600°C.