On the Reaction Pathways and Growth Mechanisms of LiNbO3 Nanocrystals from the Non-Aqueous Solvothermal Alkoxide Route

Abstract

Phase-pure, highly crystalline sub-50 nm LiNbO3 nanocrystals were prepared from a non-aqueous solvothermal process for 72 h at 230 °C and a commercial precursor solution of mixed lithium niobium ethoxide in its parent alcohol. A systematic variation of the reaction medium composition with the addition of different amounts of co-solvent including butanol, 1,3-propanediol, 1,4-butanediol, and 1,5-pentanediol resulted in the formation of nanocrystals of adjustable mean size and shape anisotropy, as demonstrated from XRD measurements and TEM imaging. Colloidal stability of ethanol- and water-based suspensions was evaluated from dynamic light scattering (DLS)/zeta potential studies and correlated with FTIR data. Thanks to the evolution in the nanocrystal size and shape distribution we observed, as well as to the available literature on the alkoxide chemistry, the reaction pathways and growth mechanisms were finally discussed with a special attention on the monomer formation rate, leading to the nucleation step. The polar, non-perovskite crystalline structure of LiNbO3 was also evidenced to play a major role in the nanocrystal shape anisotropy.

Solution‐Based Synthesis Routes for the Preparation of Noncentrosymmetric 0‐D Oxide Nanocrystals with Perovskite and Nonperovskite Structures

“…[201] More recently, this ligand exchange strategy initially suggested by Livage et al for metal oxides [70] has been thoroughly investigated with the aim to better control the final morphology and to understand the reaction pathways and crystallization mechanisms after a systematic change of the reaction parameters. [55,[202][203][204] For instance, the solvothermal aqueous route developed by Ali and Gates makes use of Nb(OEt) 5 and benzyl alcohol but with triethylamine as a surfactant and stoichiometric amount of LiOH•H 2 O for the lithium source. [55] For this low hydrolysis rate, time-dependent solvothermal treatments demonstrate that the optimal reaction time is 36 h at 220 °C for the preparation of monocrystalline and uniform LiNbO 3 nanocrystals of mean size ≈30 nm according to TEM images and the broadening of several (hkl) XRD peaks.…”

“…[204] Note that a systematic investigation of the reaction medium composition for a very similar nonaqueous route has also been proposed by Urbain et al for the preparation of sub-50 nm LiNbO 3 nanocrystals of adjustable mean size and shape anisotropy according to the chain length of the different glycols used as co-solvent. [202] The reaction pathway is composed of several steps including ligand exchange with the initially mixed lithium niobium ethoxide diluted in its parent alcohol, thermal dissociation of the modified precursor leading to the formation of a NbO double bond [205] and of positively charged carbocations whose stability directly influences the formation rate of monomers and precipitation of smaller nanocrystals with glycols of longer carbon chain (Figure 18a). The nanocrystal growth assumes addition, by a nucleophilic attack, of new monomers containing a NbO double bond to the primary nanocrystals resulting in particles of very variable shape anisotropy and the release of ethers.…”

“…Both the specific nonperovskite crystalline structure of LiNbO 3 comprising an unfavorable arrangement of face-sharing oxygen octahedra along the polar direction and the build-up of a ferroelectric order lowering the addition of new monomers on (00-1) planes, could account for the slower growth velocity along [001]. [202] The mean size S 012 defined along the [012] direction Small 2022, 18, 2200992 Figure 15. a) SEM imaging of LiNbO 3 particles after thermal decomposition of Nb-ethoxide and lithium acetate in butanediol.…”

“…Both the specific nonperovskite crystalline structure of LiNbO 3 comprising an unfavorable arrangement of face‐sharing oxygen octahedra along the polar direction and the build‐up of a ferroelectric order lowering the addition of new monomers on (00‐1) planes, could account for the slower growth velocity along [001]. [ 202 ] The mean size S 012 defined along the [012] direction and the anisotropic ratio S 110 /S 006 derived from XRD refinements along [110] and [006] indeed significantly decrease with the chain length of the glycol used as cosolvent (Figure 18b–e).…”

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