Sodium Doping Controlled Synthesis of Monodisperse Lanthanide Oxysulfide Ultrathin Nanoplates Guided by Density Functional Calculations

Abstract

Doping of nanocrystals is an intriguing field, since the intentional introduction of impurities has long been regarded as a major way of tailoring the properties of materials. Furthermore, it was recently found that the use of dopants in the synthesis of inorganic colloidal nanocrystals can not only introduce novel properties but also influence shape and size evolution during nanocrystal nucleation and growth. For example, Liu and co-workers showed that NaYF4-based nanocrystals can be precisely tuned in size, phase, and upconversion emission through Gd doping. Wang and co-workers reported that lanthanide doping of alkaline earth metal fluoride nanocrystals can lead to a significant increase in monodispersity. On the other hand, the special 4f electron configurations of lanthanides endow their compounds with promising functionalities, such as luminescence, catalytic activity, and permanent magnetism. This has therefore stimulated recent efforts to synthesize colloidal lanthanide-based nanocrystals (Ln-based NCs) with tunable morphologies and unique material properties. For example, both the small size and excellent luminescence properties make Ln-based NCs a potential new type of fluorescent probes. Specifically, lanthanide oxysulfides (Ln2O2S; Ln=La, Gd, Y) can serve as one of the most effective hosts for fluorescence applications, and research on Ln2O2S NCs is therefore highly intriguing. [4] However, the synthesis of monodisperse Ln2O2S NCs remains a challenge, since the theory of hard and soft acids and bases (HSAB) predicts a lack of affinity between the hard Lewis acid Ln and the soft Lewis base S . Previously, Gao and coworkers presented pioneering work in the synthesis of monodisperse lanthanide oxysulfide nanocrystals, yet due to the difficulty in preparing the corresponding single-source precursors, this method was limited to only a few lanthanides (i.e., Sm, Eu, Gd). On the basis of both experimental characterization and DFT calculations, we now demonstrate that introduction of monovalent Na ions as dopants in trivalent Ln host lattices can significantly facilitate the formation of Ln2O2S NCs in oleic acid (OA)/oleylamine (OM)/1-octadecene (ODE) mixed solvent by creating oxygen vacancies in the host lattice during sulfurization reactions. In a typical synthesis, monodisperse and single-crystalline Na-doped La2O2S nanoplates were formed with a diameter of (22.3 2.0) nm (Figure 1a and b). High-resolution transmission electron microscopy (HRTEM) revealed that the morphology of the as-synthesized nanoplates was mainly hexagons with six {100} facets as their side planes (Figure 1b). When the dispersion of nanoplates in cyclohexane was highly