Nonclassical Nucleation and Crystallization of LiNbO₃ Nanoparticles from the Aqueous Solvothermal Alkoxide Route

Abstract

The exact molecular reaction pathway and crystallization mechanisms of LiNbO3 nanoparticles under solvothermal conditions are derived through extensive time‐ and temperature‐resolved experiments allowing to track all the transient molecular and solid species. Starting with a simple mixing of Li/Nb ethoxides, water addition is used to promote condensation after ligand exchange with different co‐solvents including alcohols and glycols of variable carbon‐chain length. A nonclassical nucleation scheme is first demonstrated after the identification of new octanuclear complexes with a {Li4Nb4O10} core whose solvophobic interactions mediate their aggregation, thus, resulting in a colloidal gel at room‐temperature. Upon heating, a more or less frustrated aggregation‐mediated crystallization process is then evidenced leading to LiNbO3 nanocrystals of adjustable mean size between 20 and 100 nm. Such a fine control can be attributed to the variable Nb−OR (R = alkoxy/glycoxy ligand) binding interactions at the surface of crystalline intermediates. Demonstration of such a nonclassical nucleation process and crystallization mechanism for LiNbO3 not only sheds light on the entire growth process of multifunctional nanomaterials with non‐perovskite crystalline structures, but also opens new avenues for the identification of novel bimetallic oxoclusters involved in the formation of several mixed oxides from the aqueous alkoxide route.

Dual second harmonic generation and up-conversion photoluminescence emission in highly-optimized LiNbO3 nanocrystals doped and co-doped with Er3+ and Yb3+

“…Because we have recently evidenced that preformed stoichiometric moieties are produced after the initial mixing of Li and Nb ethoxides, i.e. before nucleation, and that the final nanocrystal composition is not affected all along the nucleation and crystallization steps, 35 we here sought to extend this solution approach with only alkoxide precursors and without any co-solvent to keep constant the final nanocrystal size. Synthesis of doped (and co-doped) nanocrystals with similar XRD size and with varying lanthanide contents indeed allows a straightforward comparison and optimization of their resultant optical properties.…”

“…Doped and co-doped nanocrystals are here prepared under solvothermal conditions from a recently developed aqueous alkoxide route 33,34 for which the exact molecular reaction pathway and crystallization mechanisms have just been derived. 35 Interestingly, we demonstrate that this route can be extended for the preparation of highly-doped nanocrystals after simple addition of lanthanide isopropoxides in the precursor solution. Because Er 3+ and Yb 3+ ions are known to be incorporated in the LN matrix close to the Li site in bulk crystals, 36,37 the lithium defect model ([Ln x Li 1−3 x ]NbO 3 , Ln = Er or Yb) was assumed to calculate the starting amounts of each metal and rare earth ions while keeping the electroneutrality.…”

“…38 The synthesis protocol of doped and co-doped LN nanocrystals is based on the aqueous alkoxide route as recently described for the preparation of pure LN nanoparticles. 35 Briefly, the initial mixture of alkoxide precursors is heat-treated, after microhydrolysis at room temperature, under solvothermal conditions for 1 day at 235 °C within a 23 mL Teflon-lined stainless-steel autoclave (Parr Instruments Co.). For a typical synthesis leading to pure, stoichiometric LN nanocrystals, niobium ethoxide (308 μL, 1.2 mmol) was first diluted under argon in 7.5 mL of absolute ethanol before addition of lithium ethoxide (1200 μL, 1.2 mmol) thus yielding the bimetallic lithium niobium ethoxide LiNb(OCH 2 CH 3 ) 6 at 0.133 M in ethanol (transparent yellow solution).…”

“…In this study, note that the hydrolysis rate h was fixed at 1.0 ( h is defined as the ratio between the number of water molecules and of precursor ethoxy groups), that no co-solvent was used to eventually tune the final nanocrystal size and that the volume of the initial precursor solution, namely 10.3 mL, is doubled compared to our previous study. 35 Doubling the quantities was indeed necessary to improve the weight measurements of the Er/Yb isopropoxide powders that are tricky within the glovebox because of the real-time pressure regulation. Finally, only half of the precursor solution is heat treated to keep the amount of synthesized LN nanocrystals at 0.6 mmol (∼90 mg).…”