Impurity Segregation and Nanoparticle Reorganization of Indium Doped MgO Cubes

Abstract

Metal oxide nanocomposites are non‐equilibrium solids and promising precursors for functional materials. Annealing of such materials can provide control over impurity segregation and, depending on the level of consolidation, represents a versatile approach to engineer free surfaces, particle‐particle interfaces and grain boundaries. Starting with indium‐magnesium‐oxide nanoparticle powders obtained via injection of an indium organic precursor into the magnesium combustion flame and subsequent particle quenching in argon, we investigated the stability of the trivalent In 3+ ions in the host lattice of MgO nanoparticles by determining grain growth, morphology evolution and impurity segregation. The latter process is initiated by vacuum annealing at 873 K and can be tracked at 1173 K on a time scale of minutes. In the first instance the surface segregated indium wets the nanoparticle interfaces. After prolonged annealing indium evaporates and leaves the powder via the gas phase. Resulting MgO nanocubes are devoid of residual indium, regain their high morphological definition and show spectroscopic fingerprints (UV Diffuse Reflectance and Photoluminescence emission) that are characteristic of electronically unperturbed MgO cube corner and edge features. The results of this combined XRD, TEM, and spectroscopy study reveal the parameter window within which control over indium segregation is used to introduce a semiconducting metal oxide component into the intergranular region between insulating MgO nanograins.

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

“…A similar situation was observed for Zn–Mg–O nanocubes where the Zn 2+ ions diffuse from the bulk into low-coordinated surface sites of the MgO particles, as the energy of the resulting substitutional impurity decreases with coordination number of the site . In the case of Fe–Mg–O, where trivalent Fe impurity ions have an ionic radius that is smaller than that of Mg 2+ , the situation is different, and it is helpful at this point to also include insights about the stability of trivalent In 3+ ions in the MgO nanocrystal lattices . Apart from its aliovalent oxidation state, In 3+ is around 10% larger than the Mg 2+ cations of the host lattice (Figure ).…”

“…Apart from its aliovalent oxidation state, In 3+ is around 10% larger than the Mg 2+ cations of the host lattice (Figure ). Mismatch of both size and oxidation state was found to promote In 3+ exsolution from the lattice and In 2 O 3 segregation into the particle surface . As the Fe 3+ ion is smaller than Mg 2+ (Figure ), it must be its trivalent oxidation state and, consequently, the introduction of defect clusters such as that drive phase separation in these samples.…”

“…On the contrary, for the Co–Mg–O samples, we observed no change in the crystallite behavior: the cubic shape is retained in the course of annealing to 873 or 1173 K (Figure d,f). Previous studies revealed that although residual carbon may remain from synthesis in the as-synthesized nanoparticle powders, this type of impurity can be perfectly eliminated by thermal treatment at 873 K in alternating atmospheres switching from high vacuum to oxygen and back. ,, …”

Catalytic activity, water formation, and sintering: Methane activation over Co- and Fe-doped MgO nanocrystals

“…Details are given elsewhere. 28,29 The metalorganic chemical vapor synthesis (MOCVS) process was adapted to obtain good control over the concentration and distribution of iron and cobalt in Fe-Mg-O and Co-Mg-O nanocomposite samples. The two-hot-zone reactor system consists of two quartz glass tubes, which are mounted concentrically inside a heating coil (first heating zone: operation temperature T 1 ) followed by a ceramic tube furnace (second heating zone: operation temperature T 2 ).…”

“…1,27 The situation becomes more complex for aliovalent ions that give rise to cation vacancies as intrinsic and charge compensating defects. 28 Substitutional Fe 3+ cations in the MgO lattice, for example, can associate with Mg vacancies. These impurity-vacancy complexes were found to segregate in the course of annealing into the particle surface, where they form nuclei of a newly formed magnesioferrite phase.…”