3D and in situ electron microscopy study of the nucleation and growth processes of cobalt-based nanoparticles synthesized by thermal decomposition on carbon nanotubes

Herein, we investigate the confinement effect of carbon nanotubes (CNTs) on the synthesis of cobalt-based nanoparticles (NPs) by thermal decomposition method. From an ex situ synthesis, the microstructural properties of typical nanoparticles, either confined within or localized on the external surface of CNTs, were first studied using electron tomography (ET) and high-resolution transmission electron microscopy (HR-TEM). The obtained results show that the “inner” NPs display a Co-CoO crystalline structure, homogeneous size (~50 nm) and octahedral morphology. In contrast, NPs anchored to the external surface of CNTs exhibit random morphologies and are made of small particles of ~20 nm with an oxidized layer of Co3O4. The quantitative analysis of the surface faceting of NPs, using a geometrical approach, show that the NPs confined within the CNTs do not adopt regular an octahedral morphology (with eight equal facets) but rather a elongated one, revealing an anisotropic growth along the CNT direction during the synthesis. In the second part of this paper, the nucleation and growth mechanisms of both types of NPs were in situ studied by reproducing the solvothermal reaction for the first time using Environmental-Cell TEM (EC-TEM) approach. Outside the CNT medium, the direct visualization of the NPs formation mechanism as a function of temperature allows to observe that their nucleation does not occur homogeneously in the synthesis medium, as expected, being initiated in the vesicles-like structures that appear in the solvent at the temperature range of the precursor decomposition. The first clusters and the subsequent NPs are formed at the liquid-gas interface in the vesicle “walls”, which are characterized by a higher monomer concentration. Their size grow rapidly until a critical value of 4-5 nm before leaving the walls and form chain-like structures. The NPs close to the CNT were adsorbed at carbon surface due to presence of oxygen functions, and their size increase until ~20 nm by sintering. Inside the confined channel of CNTs, the reaction mixture is incorporated by capillarity at low temperature. Then, a porous micellar aspect of the liquid was observed with an important supply of coalescent precursor from the CNT tip. At higher temperature (~300°C), the structure is densified and form the first separated entities forming the Co-based NPs.