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 quenchin… Show more
“…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 ).…”
Section: Resultsmentioning
confidence: 71%
“…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 asthat drive phase separation in these samples.…”
Section: Resultsmentioning
confidence: 97%
“…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. ,, …”
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.
“…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 ).…”
Section: Resultsmentioning
confidence: 71%
“…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 asthat drive phase separation in these samples.…”
Section: Resultsmentioning
confidence: 97%
“…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. ,, …”
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.
“…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.…”
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