5 research outputs found
Hybrid Materials Based on Magnetic Layered Double Hydroxides: A Molecular Perspective
ConspectusDesign of functional hybrids lies at the very
core of synthetic
chemistry as it has enabled the development of an unlimited number
of solids displaying unprecedented or even improved properties built
upon the association at the molecular level of quite disparate components
by chemical design. Multifunctional hybrids are a particularly appealing
case among hybrid organic/inorganic materials. Here, chemical knowledge
is used to deploy molecular components bearing different functionalities
within a single solid so that these properties can coexist or event
interact leading to unprecedented phenomena. From a molecular perspective,
this can be done either by controlled assembly of organic/inorganic
molecular tectons into an extended architecture of hybrid nature or
by intercalation of organic moieties within the empty channels or
interlamellar space offered by inorganic solids with three-dimensional
(MOFs, zeolites, and mesoporous hosts) or layered structures (phosphates,
silicates, metal dichalcogenides, or anionic clays).This Account
specifically illustrates the use of layered double
hydroxides (LDHs) in the preparation of magnetic hybrids, in line
with the development of soft inorganic chemistry processes (also called
āChimie Douceā), which has significantly contributed
to boost the preparation hybrid materials based on solid-state hosts
and subsequent development of applications. Several features sustain
the importance of LDHs in this context. Their magnetism can be manipulated
at a molecular level by adequate choice of constituting metals and
interlayer separation for tuning the nature and extent of magnetic
interactions across and between planes. They display unparalleled
versatility in accommodating a broad range of anionic species in their
interlamellar space that encompasses not only simple anions but chemical
systems of increasing dimensionality and functionalities. Their swelling
characteristics allow for their exfoliation in organic solvents with
high dielectric strength, to produce two-dimensional nanosheets with
atomic thickness that can be used as macromolecular building blocks
in the assembly of nanocomposites.We describe how these advantageous
properties turn LDHs into excellent
vehicles for the preparation of multifunctional materials with increasing
levels of complexity. For clarity, the reader will first find a succinct
description of the most relevant aspects controlling the magnetism
of LDHs followed by their use in the preparation of magnetic hybrids
from a molecular perspective. This includes the intercalation anionic
species of increasing nuclearity like paramagnetic mononuclear complexes,
stimulus-responsive molecular guests, one- and two-dimensional coordination
polymers, or even preassembled 2D networks. This approach allows us
to evolve from ādual-functionā materials with coexistence,
for example, of magnetism and superconductivity, to smart materials
in which the magnetic or structural properties of the LDH layers can
be tuned by applying an external stimulus like light or temperature.
We will conclude with a brief look into the promising features offered
by magnetic nanocomposites based on LDHs and our views on the most
promising directions to be pursued in this context
Interplay between Chemical Composition and Cation Ordering in the Magnetism of Ni/Fe Layered Double Hydroxides
We report the synthesis of a family
of ferrimagnetic NiFe layered
double hydroxides (LDHs) with a variable Ni<sup>2+</sup>/Fe<sup>3+</sup> in-plane composition of [Ni<sub>1ā<i>x</i></sub>Fe<sub><i>x</i></sub>(OH)<sub>2</sub>]Ā(CO<sub>3</sub>)<sub><i>x</i>/2</sub>Ā·<i>y</i>H<sub>2</sub>O
(<i>x</i> = 0.20, 0.25, and 0.33) by following a modified
homogeneous precipitation. These layered magnets display high crystallinity,
homogeneous hexagonal morphologies, and micrometric size that enable
their quantitative exfoliation into single layers by sonomechanical
treatment of the solids in polar solvents. This was confirmed by dynamic
light scattering, UVāvis spectroscopy, high-resolution transmission
electron miscroscopy, and atomic force microscopy methodologies to
study the resulting steady suspensions. Our magnetic study reflects
that the iron content in the LDH layers controls the overall magnetism
of these lamellae. Hence, the gradual replacement of Ni<sup>2+</sup> with Fe<sup>3+</sup> centers introduces a larger amount of antiferromagnetically
coupled FeāOHāFe pairs across the layers, provoking
that the compound with the highest Fe/Ni ratio displays spontaneous
magnetization at higher temperatures (<i>T</i><sub>irr</sub> = 15.1 K) and the hardest coercive field (3.6 kG). MoĢssbauer
spectroscopy confirms that the cation distribution in the layers is
not random and reflects the occurrence of Fe clustering due to the
higher affinity of Fe<sup>3+</sup> ions to accommodate other homometallic
centers in their surroundings. In our opinion, this clarifies the
origin of the glassy behavior, also reported for other magnetic LDHs,
and points out spin frustration as the most likely cause
Hybrid Magnetic Superconductors Formed by TaS<sub>2</sub> Layers and Spin Crossover Complexes
The restacking of charged TaS<sub>2</sub> nanosheets with molecular counterparts has so far allowed
for the combination of superconductivity with a manifold of other
molecule-intrinsic properties. Yet, a hybrid compound that blends
superconductivity with spin crossover switching has still not been
reported. Here we continue to exploit the solid-state/molecule-based
hybrid approach for the synthesis of a layered TaS<sub>2</sub>-based
material that hosts Fe<sup>2+</sup> complexes with a spin switching
behavior. The chemical design and synthetic aspects of the exfoliation/restacking
approach are discussed, highlighting how the material can be conveniently
obtained in the form of highly oriented easy-to-handle flakes. Finally,
proof of the presence of both phenomena is provided by the use of
a variety of physical characterization techniques. The likely sensitivity
of the intercalated Fe<sup>2+</sup> complexes to external stimuli
such as light opens the door for the study of synergistic effects
between the superconductivity and the spin crossover switching at
low temperatures
Vapor-Assisted Conversion of Heterobimetallic TitaniumāOrganic Framework Thin Films
Heterobimetallic MetalāOrganic Frameworks (MOFs)
synergically
combine the properties of two metal ions, thus offering significant
advantages over homometallic MOFs in gas storage, separation, and
catalysis, among other applications. However, these remain centered
on bulk materials, while applications that require functional coatings
on solid supports are not developed. We explore for the first time
the deposition of heterometallic Ti-based MOF thin films using vapor-assisted
conversion on substrates functionalized with a self-assembled monolayer.
Furthermore, metal-induced dynamic topological transformation allows
the conversion of MUV-10(Ca) films into MUV-101(Co) and MUV-102(Cu),
which is not accessible through direct synthesis, without morphologically
altering the films. These nonconventional thin-film deposition techniques
enable homogeneous and crystalline coatings of heterometallic titanium
MOFs that also maintain their corresponding porosity
Single Sublattice Endotaxial Phase Separation Driven by Charge Frustration in a Complex Oxide
Complex
transition-metal oxides are important functional materials
in areas such as energy and information storage. The cubic ABO<sub>3</sub> perovskite is an archetypal example of this class, formed
by the occupation of small octahedral B-sites within an AO<sub>3</sub> network defined by larger A cations. We show that introduction of
chemically mismatched octahedral cations into a cubic perovskite oxide
parent phase modifies structure and composition beyond the unit cell
length scale on the B sublattice alone. This affords an endotaxial
nanocomposite of two cubic perovskite phases with distinct properties.
These locally B-site cation-ordered and -disordered phases share a
single AO<sub>3</sub> network and have enhanced stability against
the formation of a competing hexagonal structure over the single-phase
parent. Synergic integration of the distinct properties of these phases
by the coherent interfaces of the composite produces solid oxide fuel
cell cathode performance superior to that expected from the component
phases in isolation