peer journals

Nanocrystallinity and the Ordering of Nanoparticles in 2D Superlattices: Controlled Formation of either Core/Shell (Co/CoO) or Hollow CoO Nanocrystals. 

Here it is demonstrated that the di!usion process of oxygen in Co nanoparticles is controlled by their 2D ordering and crystallinity. The crystallinity of isolated Co nanoparticles deposited on a substrate does not play any role in the oxide formation. When they are self-assembled in 2D superlattices, the oxidation process is slowed and produces either core/shell (Co/CoO) nanoparticles or hollow CoO nanocrystals. This is attributed to the decrease in the oxygen di!usion rate when the nanoparticles are interdigitated. Initially, polycrystalline nanoparticles form core/shell (Co/CoO) structures, while for single-domain hexagonal close-packed Co nanocrystals, the outward di!usion of Co ions is favored over the inward di!usion of oxygen, producing hollow CoO single-domain nanocrystals.

Source : Nanocrystallinity and the Ordering of Nanoparticles in 2D Superlattices: Controlled Formation of either Core/Shell (Co/CoO) or Hollow CoO Nanocrystals. 
Z.Yang, I.Lisiecki, M. Walls, M.P. Pileni ACS Nano. , 2013,  7, 1342–1350.

peer journals

Unusual Effect of an Electron Beam on the Formation of Core/Shell Co/CoO) Nanoparticles Differing by Their Crystalline Structures.

In this study, an unusual eff ect of the electron beam in transmission electron microscopy (TEM) on the formation of Co/CoO core/shell structures is developed through careful in situ TEM/scanning TEM (STEM) analysis. By feature of the nanoscale precision of this approach, the electron beam-irradiated Co nanoparticles reveals remarkable resistance to oxidation compared to those without irradiation treatment. Moreover, the irradiated hcp single domain Co nanocrystals result in Co/CoO core/shell nanoparticles after oxidation, instead of the CoO hollow nanoparticles without irradiation treatment. This study highlights the electron beam can also play a role in nanoscale Kirkendall eff ect, in addition to the nanocrystallinity and 2D ordering eff ect that we have recently demonstrated. By careful in situ STEM-EELS (electron energyloss spectroscopy) studies of the Co nanoparticles, it was found that the deliberately irradiated nanoparticles undergo an outward diffusion process of Co ions, forming an oxide layer with O species produced by the carboxylic group covalently bound to the Co atoms of the surface.

Source : Unusual Effect of an Electron Beam on the Formation of Core/Shell Co/CoO) Nanoparticles Differing by Their Crystalline Structures
 Z .Yang, M ; Walls, I. Lisiecki, and M. P. Pileni. Chem. Mater., 2013, 25, 2372-2377

peer journals

Crystal polymorphism: dependence of oxygen diffusion through 2D ordered Co nanocrystals.

8 nm Co nanoparticles with various crystalline structures called polymorphs were produced using different synthetic procedures, such as using reverse micelles, the thermal decomposition of organometallics approach or the hot injection process. These 8 nm Co nanoparticles differing by their crystalline structures are exposed to oxygen at elevated temperature. The fcc Co polycrystalline nanoparticles produce either Co–CoO yolk–shell or CoO hollow structures whereas amorphous Co nanoparticles produce Co–CoO core–shell nanoparticles. Furthermore, single domains with either hcp or e crystalline structure behave differently upon oxygen diffusion. Co–CoO nanoparticles were produced from the hcp phase while CoO hollow nanoparticles were the product for e-phase Co nanocrystals.

Source : Crystal polymorphism: dependence of oxygen diffusion through 2D ordered Co nanocrystals. Z .Yang, J.Yang, J.Bergström, K.Khazen, and M. P. Pileni Phys. Chem. Chem. Phys., 2014,16, 9791-9796.

peer journals

Control of the oxygen and cobalt atoms diffusion through Co nanoparticles differing by their crystalline structure and size.

The size-dependent Kirkendall effect is studied by using Co nanoparticles.The sizes of Co nanoparticles differing by their crystal structures callednanocrystallinity, namely amorphous, polycrystalline fcc, single crystalline hcp, and single crystalline ε  phase, are modulated from 4 to 10 nm. The nanoparticles self-assembled in 2D superlattices and differing by their nanocrystallinities are subjected to oxygen at 200 ° C for 10 min. With single-domain nanocrystals differing by their crystalline structure (ε  and hcp  phases), marked changes in the fi nal structures are observed: upon increasing the nanocrystal size, the ε  phase favors formation of a hollow structure whereas a transition from single-domain hollow to multidomain core/shell structures takes place with the hcp  phase. With polycrystalline fcc  Co nanocrystals, a transition from a hollow to a yolk/shell structure is observed, whereas with amorphous cobalt, solid CoO nanoparticles are produced at the smaller size and are converted to the core/shell structure at the larger one. These differences in size effect are attributed to the change in the control of the inward fl ow of oxygen atoms and the outward fl ow of Co atoms with the crystalline structure of cobalt nanoparticles. Such a diffusion process described here on the Kirkendall effect can be studied for other metal nanocrystals.

Source : Control of the oxygen and cobalt atoms diffusion through Co nanoparticles differing by their crystalline structure and size. Z. Yang, N. Yang,  J. Yang,  J. Bergström  and M.P. Pileni Adv.Funct.Mater.,2015, 25, 891-897.

peer journals

Nano Supracrystallinity.

It is shown that the chemical and physical properties of a collection of nanocrystals either isolated or self assembled in 3D superlattices called supracrystals markedly depend on the crystalline structure of the nanocrystal.

Source : Nano Supracrystallinity M.P.Pileni EPL, 2015, 109 58001.

peer journals

Nano Kirkendall Effect Related to Nanocrystallinity of Metal Nanocrystals: Influence of the Outward and Inward Atomic Diffusion on the Final Nanoparticle Structure.

The Kirkendall eff ect is a classical phenomenon in materials science, and it is referred to as a nonreciprocal interdiff usion process through an interface of two metals with strikingly diff erent atomic diff usivities, leading to a formation of vacancies called Kirkendall voids. The nanoscale Kirkendall eff ect has been vastly applied in the fabrication of hollow nanostructures after the fi rst report on the synthesis of Co-based hollow nanocrystals. In this Feature Article, we briefl y start with an introduction on the Kirkendall effect concept, followed by the general synthetic strategy toward the production of hollow Kirkendall voids. The overall synthetic strategies are based on the design of diff usion couples at the nanoscale, and then, we discuss the factors that govern the formation of Kirkendall voids at the nanoscale, from the viewpoint of the nanoparticle size, nanoparticle crystallinity, and nanoparticle environment. We conclude with a summary and perspectives on the design of hollow nanostructures governed by the Kirkendall effect.

Source : Nano Kirkendall Effect Related to Nanocrystallinity of Metal Nanocrystals: Influence of the Outward and Inward Atomic Diffusion on the Final Nanoparticle Structure
ZYang, N. Yang and M.P.Pileni J.Phys.Chem.C., 2015,119, 22249-22260