Cobalt and Iron Ions in MgO Nanocrystals: Should They Stay or Should They Go

Abstract

Identification and manipulation of transition-metal ion impurities in oxide nanoparticles require an in-depth understanding of their stability, segregation behavior, and, at the same time, knowledge about their surface reactivity. Powders of magnesium oxide nanoparticles with admixtures of iron or cobalt ions, as two next neighbors in the periodic table, were synthesized in the gas phase via injection of metal−organic precursors into the magnesium combustion flame followed by temperature quenching of resulting nanocrystals in argon. In these model systems of cubic nanocrystals, we explored the distinct stability of these impurities in great detail. While Co 2+ ions keep their divalent valence state and substitute the host ions in the cationic sublattice, Fe 3+ ions emerge due to the energy gain provided by charge compensation and impurity−vacancy complex formation. The very different behavior of Co and Fe ions in the MgO host lattice, their changes in the local environment, and the different trends in segregation have been investigated by means of X-ray absorption and photoelectron spectroscopies and structure characterization techniques. Abundance and energetics of the defects and defect complexes were determined within the framework of the density functional theory and enabled us to explain consistently the reported experimental observations. Oxidation state and nature of the defect cluster have a significant impact on particle size and annealing-induced morphology evolution, which determine their material properties as components in heterogeneous catalysis and functional ceramics.

Isolated Cobalt Ions Embedded in Magnesium Oxide Nanostructures: Spectroscopic Properties and Redox Activity

“…The production of transition metal oxide doped MgO nanocubes in the gas phase utilizes the decomposition of cobaltocene inside the strongly exothermic Mg combustion flame (Figure 1 a). Such an approach provides high control over the integral concentration and homogeneous distribution of the admixed cobalt ions over the metal oxide nanoparticles [6] . A dedicated annealing protocol with vacuum annealing at base pressures as low as p (O 2 )<10 −6 mbar (Figure 1 b) was developed for nanoparticle purification, that is, for the generation of particles with surfaces that are exempt from synthesis related carbonaceous species, adsorbed water or hydroxyls as remnants.…”

“…Our results of a recent X‐ray spectroscopy study on these nanoparticle powders revealed that the Co 2+ ions in MgO remain within the highly symmetric octahedral environment and do not change their (+2) valence state and are perfectly in line with the here presented results. A high definition of the cubic particle habit with a high abundance of sharp edges and corners is retained after annealing at 1173 K. Thus, annealing in vacuum and oxygen‐rich environment does not affect the hierarchy of surface energies and Co‐doped MgO(100) surfaces remain significantly lower in energy as compared to all other crystal planes [6] …”

“…Moreover, segregation effects or dopanti nduced surface energyc hanges can occur. [6] The chemical nature of the impurity ions, their ionic radius as substitutionald opantsa nd their redox activity are indispensable for the predictiono fa ssociated property changes the nanoparticles undergo in the course of annealing and surfacec hemicalr eactions. Prototypicalm odel systemsf or the study of doping effects on insulating metal oxidesa re related to transition metal ions in micro-or single-crystalline MgO.…”

“…This study makes use of as ystem of doped MgO nanocubes where recent XAS andX PS studies in combination with results from ab initio calculations [6,15] provided detailed information about the uniformd istributiono fC o 2 + ions over the cationic sublattice. Here, we report the spectroscopic changes Codoped MgO nanocube powders can undergo during interfacial redox reactions at room temperature.…”

“…The admixture of impurities to introduce additional properties to the functional particle system, such as redox‐activity, [4] color or magnetism, [5] complicates the structural situation due to changes in the defect chemistry of the particle. Moreover, segregation effects or dopant induced surface energy changes can occur [6] . The chemical nature of the impurity ions, their ionic radius as substitutional dopants and their redox activity are indispensable for the prediction of associated property changes the nanoparticles undergo in the course of annealing and surface chemical reactions.…”