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

Abstract

Atomic dispersion of dopants and control over their defect chemistry are central goals in the development of oxide nanoparticles for functional materials with dedicated electronic, opticalo rm agnetic properties. We produced highly dispersed oxiden anocubes with atomic distribution of cobalt ions in substitutional sites of the MgO hostl attice via metal organic chemical vapor synthesis. Vacuum annealing of the nanoparticlep owders up to 1173 Kh as no effect on the shape of the individual particles and only leads to moderate particle coarsening.S uch materialsp rocessing, however,g ives rise to the electronic reduction of particle surfaces, which-uponO 2 admission-stabilize anionic oxygenr adicals that are accessible to UV/Vis diffuse reflec-tance and electron paramagnetic resonance (EPR) spectroscopy.M ulti-reference quantum chemical calculations show that the opticalbands observed mainly originate from transitions into 4 A 2g (4 F), 4 T 1g (4 P) states with ac ontribution of transitionsi nto 2 T 1g , 2 T 2g (2 G) states through spin-orbit coupling and gain intensity through vibrational motiono ft he MgO latticeo rt he asymmetric ion field. Relatedn anostructures are ap romising materials ystem for singlea tomic site catalysis. At the same time, it represents an extremely valuable model system for the study of interfacial electron transfer processes that are key to nanoparticle chemistry and photochemistry at room temperature, and in heterogeneous catalysis.

Spectroscopy and photochemistry of copper nitrate clusters

“…Upon considering spin-orbit coupling, they might mix with the intense, spin-allowed excitations deeper in the UV and borrow a part of their oscillator strength. 83,84 However, the theoretical description of the multi-reference character and spin-orbit coupling remains challenging, and our attempts in this direction were thus far not successful. For a meaningful multi-reference calculation, a relatively large active space including copper's active 3d, 4s and 4p orbitals along with all participating nitrate orbitals is required.…”

Conversion of MgO nanocrystal surfaces into ceramic interfaces: Exsolution of BaO as photoluminescent interface probes

“…Changes in particle morphology and size distribution originate from the enhanced ion diffusion inside the Ba 2+ ‐doped MgO lattice in combination with Ba 2+ ‐segregation into the nanoparticle surfaces (Figures 3 and S2). The enhanced ion diffusion of Ba 2+ ions is mainly driven by the size mismatch between octahedrally coordinated Ba 2+ (0.135 nm) and Mg 2+ (0.072 nm) 20,24,26–28 . The onset of Ba segregation can be observed for annealing at 873 K (Figure S3) where slight but significant particle coarsening gives rise to a final median size that still remains in the range below 10 nm.…”

“…The enhanced ion diffusion of Ba 2+ ions is mainly driven by the size mismatch between octahedrally coordinated Ba 2+ (0.135 nm) and Mg 2+ (0.072 nm). 20 , 24 , 26 , 27 , 28 The onset of Ba segregation can be observed for annealing at 873 K (Figure S3 ) where slight but significant particle coarsening gives rise to a final median size that still remains in the range below 10 nm. Annealing in the temperature range between 873 and 1173 K, however, enforces ion diffusion and strongly promotes coarsening and disproportionation in the PSD.…”

“…We produced 18 Ba x Mg 1−x O nanocrystals via a hybrid metal-organic chemical vapor synthesis approach that is described in detail elsewhere. [22][23][24] We used a reactor consisting of two concentrically arranged quartz glass tubes (Figure 2). Two precursor containers were placed at different positions inside an inner tube.…”

Oxygen Radicals Entrapped between MgO Nanocrystals: Formation, Spectroscopic Fingerprints, and Reactivity toward Water

“…The origin of the optical transitions reported in ref has so far remained unresolved, and related fundamental understanding, however, is required to further exploit the underlying effects for catalyst design and for new concepts of materials sintering . Theoretical modeling is one of the tools that can provide insight into optical transitions related to adsorbed oxygen radicals on the MgO surfaces. , Computationally feasible cluster models help us to understand how the charged particles are created and how they interact with the surface.…”