Oxidation behavior of cobalt nanoparticles studied by in situ environmental transmission electron microscopy

Abstract

a b s t r a c tThe dynamics of oxidation of cobalt nanoparticles were directly revealed by in situ environmental transmission electron microscopy. Firstly, cobalt nanoparticles were oxidized to polycrystalline cobalt monoxide, then to polycrystalline tricobalt tetroxide, in the presence of oxygen with a low partial pressure. Numerous cavities (or voids) were formed during the oxidation, owing to the Kirkendall effect. Analysis of the oxides growth suggested that the oxidation of cobalt nanoparticles followed a parabolic rate law, which was consistent with diffusion-limited kinetics. In situ transmission electron microscopy allowed potential atomic oxidation pathways to be considered. The outward diffusion of cobalt atoms inside the oxide layer controlled the oxidation, and formed the hollow structure. Irradiation by the electron beam, which destroyed the sealing effect of graphite layer coated on the cobalt surface and resulted in fast oxidation rate, played an important role in activating and promoting the oxidations. These findings further our understanding on the microscopic kinetics of metal nanocrystal oxidation and knowledge of energetic electrons promoting oxidation reaction.

Comprehensive study of a versatile polyol synthesis approach for cathode materials for Li-ion batteries

“…Formation of the layered phase, albeit disordered, is visible from the growth of the (015) peak growing past 270 °C. The reaction of the spinel phase is consistent with the previous temperature dependence oxidation study of Mn-and Co-species [64][65][66][67]. Several studies have reported phase transitions from metallic Co nanoparticles to Co3O4 and Mn-glycolate to Mn3O4 at a similar temperature range.…”

Resolving single Cu nanoparticle oxidation and Kirkendall void formation with in situ plasmonic nanospectroscopy and electrodynamic simulations

“…1a). Experimentally, the NKE is traditionally explored using transmission electron microscopy (TEM), since it features the necessary high spatial resolution, 8 as well as enables in situ studies in the few mbar pressure range 9,10 and recently at atmospheric pressure. 11 However, despite these traits, in situ TEM is problematic due to beam effects that may both reduce the oxide or prevent oxidation via the deposition of carbonaceous species.…”

Nanoscale Soft Wetting Observed in Co/Sapphire during Pulsed Laser Irradiation

“…On the other hand, Co atoms in the core diffuse into the surface of the CoO shell and are then oxidized, which results in an increase in the diameter of the NPs, consistent with the SEM results in Figure 2. In contrast to the CoO NPs formed via thermal annealing through the Kirkendall effect [45,46], no voids or CoO domains were present in the core NPs. It is worth noting that void formation during the nanoscale Kirkendall effect is a nucleation-type phenomenon that occurs only when the oxide shell is thicker than its critical thickness [52].…”

“…By analyzing the size distribution of the NPs, it was evident that the average NP diameter decreased with increasing the number of pulses under vacuum but increased under air. The former can be explained by the fragmentation and the evaporation of the NPs [22,44], while the latter case is associated with the effects of oxidation, including the Kirkendall effect [45,46]. The morphology of the Co NPs exhibited signatures of the soft wetting, the wetting phenomena observed on a deformable substrate.…”

“…The thickness of the CoO shell was estimated to be approximately 2 nm. The formation of a CoO shell indicates an oxidation process via the Kirkendall effect [45,46]. The CoO shell passivates the surface, preventing the diffusion of oxygen atoms into the core at an early stage.…”