Nanomaterials: Application & Properties, 2021 IEEE 11th International Conference on "Nanomaterials: Applications & Properties" (NAP-2021)

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Experimental and theoretical evidence for oriented aggregation crystal growth of oxide particles in polyol
Souad Ammar

Last modified: 2021-09-05


Transition metal oxides (TMOs) form an important class of functional materials. They are widely used in various technological fields: catalysis, battery-based energy storage, ferroelectricity and microelectronics among others. In the form of nanoparticles (NPs), their average particle size and their aggregation state affect also their properties and consequently their applicative potential. Even if several synthesis methods exist for the production of size-controlled TMO NPs, colloidal chemistry remains the most appropriate. By colloidal chemistry, the crystal growth proceeds through two mechanisms: diffusion and aggregation. In water, without any dispersing agent in the reaction medium, the initially formed nanocrystals, also called primary particles, exhibit hydroxyl groups at their surface, which lead to their aggregation and the formation of polycrystals, water acting as a binder [1]. In non-aqueous media aggregation is slower due to there being fewer surface hydroxyl groups and higher solvent viscosity. In this case, the operating conditions may give the nanocrystals enough freedom to rotate and to find a low-energy configuration interface, forming aggregates and sometimes oriented aggregates [2]. In the presence of a dispersing agent, mainly surfactants and alternatively organic solvents, when the solvents may play the role of both reactant and control agent for particle growth, the aggregation process can be avoided and single crystals grow by diffusion. In fact, the control of the final morphology is achieved by fine-tuning the crystal surface energy via preferential adsorption of the additives onto specific crystallographic facets [3]. Usually such morphological control allows reducing the particle size by quenching the growth of the crystal nuclei formed [4], leading to the formation of small isotropic single crystals. In some favourable cases, the primary particles achieve crystallographic alignment despite spatial separation from one to another, leading to mesocrystals, which can fuse into oriented aggregates, also called pseudo-single crystals [5]. This crystal growth mechanism is commonly called oriented aggregation. It attracts a great deal of attention because it is a promising method for creating advanced materials with distinct micro- and nano-structures. Indeed, very often the resulting pseudo-single crystals exhibit physical properties corresponding to larger crystals, different from both those of their bulk counterpart and their primary nanocrystals [6]. So an advanced control of this mechanism would allow a smart way to design by colloidal chemistry advanced functional nanomaterials. In this context, we want to focus on this crystal growth mechanism in polyol media giving representative examples, highlighting the role of the polyol itself.