The Oxidation of Cobalt Nanoparticles into Kirkendall-Hollowed CoO and Co₃O₄: The Diffusion Mechanisms and Atomic Structural Transformations

Abstract

We report on the atomic structural changes and diffusion processes during the chemical transformation of ε-Co nanoparticles (NPs) through oxidation in air into hollow CoO NPs and then Co 3 O 4 NPs. Through XAS, XRD, TEM, and DFT calculations, the mechanisms of the transformation from ε-Co to CoO to Co 3 O 4 are investigated. Our DFT calculations and experimental results suggest that a two-step diffusion process is responsible for the Kirkendall hollowing of ε-Co into CoO NPs. The first step is O in-diffusion by an indirect exchange mechanism through interstitial O and vacancies of type I Co sites of the ε-Co phase. This indirect exchange mechanism of O has a lower energy barrier than a vacancy-mediated diffusion of O through type I sites. When the CoO phase is established, the Co then diffuses outward faster than the O diffuses inward, resulting in a hollow NP. The lattice orientations during the transformation show preferential orderings after the single-crystalline ε-Co NPs are transformed to polycrystalline CoO and Co 3 O 4 NPs. Our Co 3 O 4 NPs possess a high ratio of {110} surface planes, which are known to have favorable catalytic activity. The Co 3 O 4 NPs can be redispersed in an organic solvent by adding surfactants, thus rendering a method to create solution-processable colloidal, monodisperse Co 3 O 4 NPs.

Tuning ultrafine manganese oxide nanowire synthesis seeded by Si particles and its superior Li storage behaviors

“…37 The radial-structure functions of the Mn K-edge (Figure 8c) Tuning MnO nanowire synthesis seeded by Si particles H Wei et al demonstrate the obvious increase in the peak height at 1.5 angstroms, implying an increase in the ordering of the O 2 À ions in the MnO structure, which induces the fast diffusion of Li ions and a low internal resistance. 38 As for the change in the radial distribution function of the second shell of the MnO@C nanowire, it may be consistent with the increase in the disorder degree of the Mn-Mn shell corresponding to the coordination of the second shell, which was supported by the increases of Debye-Waller factor of Mn-Mn.…”

Molecular modelling of the decomposition of NH3 over CoO(100)

“…The configurations of the transitions states show that the dissociating NeH bonds are stretched by 1.75 Å (171%), 1.61 Å (158%) and 1.86 Å (179%) compared to the NeH bonds in the corresponding initial states for dehydrogenation of NH 3 , NH 2 and NH respectively, indicating that the NeH dissociation proceeds via late transition states. The lateral and vertical displacements of surface atoms are more evident for Co sites than O sites, consistent with a lower energy barrier for Co diffusion compared to the O diffusion in bulk CoO [20]. Co sites mostly undergo outwards relaxations whereas O atoms mainly relax inwards upon interaction with NH 2 and NH.…”

“…However, molecular modelling and substitutional cobalt experimental studies of CO oxidation to CO 2 [17,18] and molecular modelling of N 2 O decomposition to N 2 [19] over Co 3 O 4 ascribe some catalytic role during the adsorptionedesorption process to Co in octahedral sites. The Co sites in CoO tend to have lower diffusion energy barriers than O sites [20], and reconstruction of the surface during NH 3 dehydrogenation could be expected, leading to the possibility of dynamic participation of Co in NH 3 dehydrogenation that needs to be assessed.…”

Undoped and Ni-Doped CoOx Surface Modification of Porous BiVO4 Photoelectrodes for Water Oxidation

“…58,59 The second peak (II in Figure 11c) corresponds to the Co 3+ -Co 3+ distance. 58,60 The third peak (III in Figure 11c) is assigned to Co 3+ -Co 2+ and Co 3+ -Co 2+ distances.…”

“…They are all consistent with the published peaks for Co3O4, indicating the CoOx prepared in this work mostly likely has the short range order of Co3O4. [57][58][59][60] There are two types of cobalt ions with different oxidation states in spinel Co3O4: two Co 3+ ions in the octahedral site and one Co 2+ ion in the tetrahedral site. 61 Reported fitting results…”