3D and <i>in situ</i> electron microscopy studies of the nucleation and growth processes of cobalt-based nanoparticles synthesized <i>via</i> thermal decomposition on carbon nanotubes

Herein, we investigated the confinement effect of carbon nanotubes (CNTs) on the synthesis of cobalt-based nanoparticles (NPs) <i>via</i> the thermal decomposition method. Using an <i>ex situ</i> synthesis approach, the microstructural properties of typical nanoparticles, either confined within or localized on the external surface of CNTs, were first examined using electron tomography (ET) and high-resolution transmission electron microscopy (HR-TEM). The obtained results showed that the "inner" NPs exhibited a Co-CoO crystalline structure, homogeneous size (∼50 nm) and octahedral morphology. In contrast, NPs anchored to the external surface of CNTs exhibited random morphologies and were composed of small particles of ∼20 nm with an oxidized Co₃O₄ layer. A quantitative analysis of the surface faceting of NPs using a geometrical approach revealed that the NPs confined within the CNTs did not adopt a regular octahedral morphology (with eight equal facets) but an elongated morphology, indicating an anisotropic growth along the CNT direction during the synthesis. In the second part of this study, the nucleation and growth mechanisms of both types of NPs were studied <i>in situ</i> by reproducing the solvothermal reaction for the first time using an environmental-cell TEM (EC-TEM) approach. Outside the CNT channel, the direct visualization of the NP formation mechanism as a function of temperature enabled us to observe that their nucleation did not occur homogeneously in the synthesis medium, as expected. Instead, the nucleation was initiated within the vesicle-like structures that appeared in the solvent in the temperature range of the precursor decomposition. The first clusters and subsequent NPs were formed at the liquid-gas interface in the vesicle "walls", which were characterized by a higher monomer concentration. Before leaving the walls, their size grew rapidly until a critical value of 4-5 nm and formed chain-like structures. The NPs close to the CNTs were adsorbed onto the carbon surface owing to the presence of oxygen functions, and their size increased until ∼20 nm by sintering. In the confined channels of the CNTs, the reaction mixture was incorporated <i>via</i> capillarity at low temperatures. Then, a porous micellar aspect of the liquid was observed in relation with the increasing supply of coalescent precursor from the CNT tip. At higher temperatures (∼300 °C), the structure was densified, and the first separated entities formed the Co-based NPs.