Unraveling the Role of Alkali Cations in the Growth Mechanism of Gd₂O₂S Nanoparticles

Alkali cations are required for the colloidal synthesis of Ln₂O₂S nanoplates in organic solvent. Based on a thorough experimental analysis using various lanthanides and three alkali ions, we challenge the commonly accepted scenario of partial lanthanide substitution by the alkali, which is supposed to favor the sulfide insertion over the oxide one. Using SAXS-WAXS on aliquots taken from the synthesis medium, we demonstrate the formation of an alkali-stabilized oleate mesophase acting as a template for nanoparticle nucleation and growth. As a consequence, the alkali is not incorporated into the crystalline structure; nanoparticle growth requires only a low alkali concentration that can be readily washed away from the final nanoparticle powder. We expect that similar mesophases may form in other colloidal syntheses using oleates and small cations.