Synthèse de nanoparticules cristallines en solution : rôle des états transitoires

Abstract : Soft chemistry is attractive thanks to its easy implementation. However, the related phenomena are poorly understood to this day. Usually, crystal size and their nucleation rate are described using classical nucleation theories. By construction, they neglect (i) any potential intermediate state (ii) any consideration of microstructure. In addition, the nucleation rates measured are in disagreement with the prediction, by a factor of at least 1010. Taking into account the intermediate states and investigate their impact on the structure (not only the size) should be a good way to improve crystallisation theories.The characterisation of those intermediate states remains challenging : they are labile, nanometer-sized, and are formed in less than a second. To address our problem, europium-doped yttrium vanadate (YVO4:Eu) is an excellent candidate : it is microstructured and its crystallisation - polycrystalline or monocrystalline depending on the pH – occurs via an amorphous intermediate state.Our work precises the different microstructures observed. We then measure three different nucleation rates in situ X-ray scattering, with different degrees of polycristallinity associated. We propose a simple model predicting the poly/monocrystallinity from the competition between nucleation and crystal growth and the following new idea : the amorphous precipitate confines the reaction. In addition to this role, it also serves as reactor (contains 80% of the reactants) and as template (as it sets the particles’ final size). All three amorphous are structurally similar, its structure alone cannot explain the differences in structural kinetics we observe. We thus focus on chemical processes in play. In particular, we demonstrate that the reaction kinetics depends mainly on the number of hydroxyl ions engaged in the amorphous network.The methods and concepts developed here are independant on the chemical system used, and it is highly probable that they will prove valid for other compounds : other oxide nanoparticles, or crystals in general.
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Submitted on : Saturday, April 6, 2019 - 3:46:28 AM
Last modification on : Thursday, April 11, 2019 - 8:16:12 AM

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  • HAL Id : tel-02091181, version 2

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Alexy de Jesus Almeida Freitas. Synthèse de nanoparticules cristallines en solution : rôle des états transitoires. Chimie théorique et/ou physique. Université Paris-Saclay, 2019. Français. ⟨NNT : 2019SACLX003⟩. ⟨tel-02091181v2⟩

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