The Oxidation of Cobalt Nanoparticles into Kirkendall-Hollowed CoO and Co3O4: The Diffusion Mechanisms and Atomic Structural Transformations
The Oxidation of Cobalt Nanoparticles into Kirkendall-Hollowed CoO and Co3O4: The Diffusion Mechanisms and Atomic Structural Transformations
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DOI：
10.1021/jp402939e
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发表时间：
2013-07-11
影响因子：
3.7
通讯作者：
Robinson, Richard D.
中科院分区：
文献类型：
--
作者：
Ha, Don-Hyung;Moreau, Liane M.;Robinson, Richard D.
我们报道了在空气中氧化为空心CoO纳米粒子，然后再转变为Co3O4纳米粒子的过程中，epsilon-Co纳米粒子(NPs)的原子结构变化和扩散过程。通过X射线光电子能谱、X射线衍射仪、透射电子显微镜和密度泛函理论计算，研究了Esisilon-Co向CoO向Co3O4转变的机理。我们的密度泛函计算和实验结果表明，两步扩散过程是导致epsilon-Co空化为CoO纳米粒子的两步扩散过程。这第一步是通过间接交换机制，通过间隙O和esilon-Co相的I型Co位空位实现O的内扩散。这种O的间接交换机制比空位介导的O通过I型位的扩散具有更低的能垒。当形成CoO相时，Co向外扩散的速度快于O向内扩散的速度，从而产生中空的NP。相变过程中的晶格取向表现出单晶之后的择优排列。Epsilon-Co纳米粒子转变为多晶CoO和Co3O4纳米粒子。我们的Co3O4纳米粒子具有很高的{110}表面率，这是众所周知的具有良好催化活性的表面。通过添加表面活性剂，Co3O4纳米粒子可以在有机溶剂中重新分散，从而提供了一种创建可溶液处理的胶体、单分散Co3O4纳米粒子的方法。
We report on the atomic structural changes and diffusion processes during the chemical transformation of epsilon-Co nanoparticles (NPs) through oxidation in air into hollow CoO NPs and then Co3O4 NPs. Through XAS, XRD, TEM, and DFT calculations, the mechanisms of the transformation from epsilon-Co to CoO to Co3O4 are investigated. Our DFT calculations and experimental results suggest that a two-step diffusion process is responsible for the Kirkendall hollowing of epsilon-Co into CoO NPs. This first step is O in-diffusion by an indirect exchange mechanism through interstitial O and vacancies of type I Co sites of the epsilon-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 to 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. epsilon-Co NPs are transformed to polycrystalline CoO and Co3O4 NPs. Our Co3O4 NPs possess a high ratio of {110} surface planes, which are known to have favorable catalytic activity. The Co3O4 NPs can be redispersed in an organic solvent by adding surfactants, thus rendering a method to create solution-processable colloidal, monodisperse Co3O4 NPs.