3 research outputs found
First-Principles Study of Antisite Defect Configurations in ZnGa<sub>2</sub>O<sub>4</sub>:Cr Persistent Phosphors
Zinc
gallate doped with chromium is a recently developed near-infrared
emitting persistent phosphor, which is now extensively studied for
in vivo bioimaging and security applications. The precise mechanism
of this persistent luminescence relies on defects, in particular,
on antisite defects and antisite pairs. A theoretical model combining
the solid host, the dopant, and/or antisite defects is constructed
to elucidate the mutual interactions in these complex materials. Energies
of formation as well as dopant, and defect energies are calculated
through density-functional theory simulations of large periodic supercells.
The calculations support the chromium substitution on the slightly
distorted octahedrally coordinated gallium site, and additional energy
levels are introduced in the band gap of the host. Antisite pairs
are found to be energetically favored over isolated antisites due
to significant charge compensation as shown by calculated Hirshfeld-I
charges. Significant structural distortions are found around all antisite
defects. The local Cr surrounding is mainly distorted due to a Zn<sub>Ga</sub> antisite. The stability analysis reveals that the distance
between both antisites dominates the overall stability picture of
the material containing the Cr dopant and an antisite pair. The findings
are further rationalized using calculated densities of states and
Hirshfeld-I charges
Determination of the Nature of the Cu Coordination Complexes Formed in the Presence of NO and NH<sub>3</sub> within SSZ-13
Ammonia-selective catalytic reduction
(NH<sub>3</sub>-SCR) using
Cu zeolites is a well-established strategy for the abatement of NO<sub><i>x</i></sub> gases. Recent studies have demonstrated
that Cu is particularly active when exchanged into the SSZ-13 zeolite,
and its location in either the <i>6r</i> or <i>8r</i> renders it an excellent model system for fundamental studies. In
this work, we examine the interaction of NH<sub>3</sub>-SCR relevant
gases (NO and NH<sub>3</sub>) with the Cu<sup>2+</sup> centers within
the SSZ-13 structure, coupling powder diffraction (PD), X-ray absorption
spectroscopy (XAFS), and density functional theory (DFT). This combined
approach revealed that, upon calcination, cooling and gas exposure
Cu ions tend to locate in the <i>8r</i> window. After NO
introduction, Cu ions are seen to coordinate to two framework oxygens
and one NO molecule, resulting in a bent Cu–nitrosyl complex
with a Cu–N–O bond angle of ∼150°. Whilst
Cu seems to be partially reduced/changed in coordination state, NO
is partially oxidized. On exposure to NH<sub>3</sub> while the PD
data suggest the Cu<sup>2+</sup> ion occupies a similar position,
simulation and XAFS pointed toward the formation of a Jahn–Teller
distorted hexaamine complex [CuÂ(NH<sub>3</sub>)<sub>6</sub>]<sup>2+</sup> in the center of the <i>cha</i> cage. These results have
important implications in terms of uptake and storage of these reactive
gases and potentially for the mechanisms involved in the NH<sub>3</sub>-SCR process
Bipyridine-Based Nanosized Metal–Organic Framework with Tunable Luminescence by a Postmodification with Eu(III): An Experimental and Theoretical Study
A gallium 2,2′-bipyridine-5,5′-dicarboxylate
metal–organic
framework, GaÂ(OH)Â(bpydc), denoted as COMOC-4 (COMOC = Center for Ordered
Materials, Organometallics and Catalysis, Ghent University) has been
synthesized via solvothermal synthesis procedure. The structure has
the topology of an aluminum 2,2′-bipyridine-5,5′-dicarboxylate
– the so-called MOF-253. TEM and SEM micrographs show the COMOC-4
crystals are formed in nanoplates with uniform size of 30–50
nm. The UV–vis spectra of COMOC-4 in methanol solution show
maximal electronic absorption at 307 nm. This results from linker
to linker transitions as elucidated by time-dependent density functional
theory simulations on the linker and COMOC-4 cluster models. When
excited at 400 nm, COMOC-4 displays an emission band centered at 542
nm. Upon immersion in different solvents, the emission band for the
framework is shifted in the range of 525–548 nm depending on
the solvent. After incorporating Eu<sup>3+</sup> cations, the emission
band of the framework is shifted to even shorter wavelengths (505
nm). By varying the excitation wavelengths from 250 to 400 nm, we
can fine-tune the emission from red to yellowish green in the CIE
diagram. The luminescence behavior of Eu<sup>3+</sup> cations is well
preserved and the solid-state luminescence lifetimes of τ<sub>1</sub> = 45 μs (35.4%) and τ<sub>2</sub> = 162 μs
(64.6%) are observed