The Big Impact of a Small Detail: Cobalt Nanocrystal Polymorphism as a Result of Precursor Addition Rate during Stock Solution Preparation

Abstract

The control of nanocrystal structures at will is still a challenge, despite the recent progress of colloidal synthetic procedures. It is common knowledge that even small modifications of the reaction parameters during synthesis can alter the characteristics of the resulting nano-objects. In this work we report an unexpected factor which determines the structure of cobalt nanoparticles. Nanocrystals of distinctly different sizes and shapes have resulted from stock solutions containing exactly the same concentrations of [Co{N(SiMe(3))(2)}(2)(thf)], hexadecylamine, and lauric acid. The reduction reaction itself has been performed under identical conditions. In an effort to explain these differences and to analyze the reaction components and any molecular intermediates, we have discovered that the rate at which the cobalt precursor is added to the ligand solution during the stock solution preparation at room temperature becomes determinant by triggering off a nonanticipated side reaction which consumes part of the lauric acid, the main stabilizing ligand, transforming it to a silyl ester. Thus, an innocent mixing, apparently not related to the main reaction which produces the nanoparticles, becomes the parameter which in fine defines nanocrystal characteristics. This side reaction affects in a similar way the morphology of iron nanoparticles prepared from an analogous iron precursor and the same long chain stabilizing ligands. Side reactions are potentially operational in a great number of systems yielding nanocrystals, despite the fact that they are very rarely mentioned in the literature.

Size Control of Monodisperse Metal Nanocrystals in Ionic Liquids

“…oleic acid (~0.13 mmol) and oleylamine (0.09 mmol). As discussed herein, these ligands can be involved in molecular complexes formed from the metal precursor [4,[23][24][25][26]. As a result, metallic Pt nanoparticles of 4.5 AE 0.8 nm were prepared at 230 C under argon via the reduction of Pt(acac) 2 (0.07 mmol).…”

“…the formation and evolution of a complex formed from the metal precursor and an organic additive, is crucial for controlling the final properties of the nanocrystals (Figs. 2 and 3) [4,[23][24][25][26][27][28]. Furthermore, the preferential adsorption of organic (ligands, polymers, surfactants) or inorganic C under dihydrogen atmosphere (3 bar) following different addition rates of the metal precursor ([Co{N (SiMe 3 ) 2 } 2 (THF)]) to a solution of conventional ligands (hexadecylamine and lauric acid) at room temperature [23] (ions, gases) agents is an advanced strategy for the selective growth or etching of specific facets.…”

“…2 and 3) [4,[23][24][25][26][27][28]. Furthermore, the preferential adsorption of organic (ligands, polymers, surfactants) or inorganic C under dihydrogen atmosphere (3 bar) following different addition rates of the metal precursor ([Co{N (SiMe 3 ) 2 } 2 (THF)]) to a solution of conventional ligands (hexadecylamine and lauric acid) at room temperature [23] (ions, gases) agents is an advanced strategy for the selective growth or etching of specific facets. This can result in sophisticated nanocrystals via well-controlled nucleation, growth and etching steps [1][2][3][4].…”

Exploring New Synthetic Strategies for the Production of Advanced Complex Inorganic Nanocrystals

“…The kinetic control of NC's morphology has been proved effective to control nucleation and growth [23]. The controlled dosing of reagents (rather than the exact amounts of reagents employed) can kinetically trap NCs into metastable states or modify the formation of the sub-products of the reaction in such a way that the final result depends not only on the amount of reagents used but also on the way the reagents are added [88,89]. Thus, starting from the same amount of precursor one can obtain a large amount of small particles or a fewer larger ones.…”

New generation of magnetic and luminescent nanoparticles for in vivo real-time imaging

“…Benefiting from the crystallographic unicity of the c-axis, intensive research has been devoted to the synthesis of anisotropic Co NPs. Polyol synthesis [46] along organometallic approaches [47,48] yielded single crystalline Co nanorods with a relatively narrow size distribution (figure 4c). Though nanorods have never been tested for MRI purposes, they could potentially present high relaxivity, analogous to elongated Co@Au nanowontons, prepared through physical process [21] (table 1).…”