Room Temperature Magnetism in Layered Double Hydroxides due to Magnetic Nanoparticles

Two-dimensional magnetic behaviour in hybrid NiFe-layered double hydroxides by molecular engineering

Layered double hydroxides (LDHs) are a class of two-dimensional (2D) anionic materials that exhibit remarkable chemical versatility, making them ideal building blocks in the design of complex multifunctional materials. In this line, a NiFe-LDH is probably one of the most important LDHs due to its interesting electrochemical and magnetic properties. However, no direct magnetic measurements of exfoliated NiFe-LDH nanosheets have been reported so far. Herein, we synthesize a hybrid NiFe-LDH family through anion exchange reactions using surfactant molecules in order to increase the interlayer space (ranging from 8 to 31.6 Å), minimizing the interlayer dipolar interactions. By intercalation with octadecylsulphate, we have managed to obtain the largest interlayer space reported for a NiFe-LDH while keeping a similar size, morphology and composition. This wide interlayer separation results in a decrease in temperatures at which spontaneous magnetization (T_M) occurs and the blocking temperature (T_B), as well as a decrease in the coercive fields (H_C). In fact, an abrupt drop in all these magnetic parameters above 30 Å interlayer distance is observed, evidencing complete magnetic decoupling of the layers. We have further validated our molecular engineering approach by characterizing the hybrid materials by Mössbauer spectroscopy and comparing the magnetic analysis results with those for a liquid phase exfoliated NiFe-LDH sample. Overall, this work provides fundamental insights into the magnetism of NiFe-LDHs, showing the potential of molecular engineering for designing hybrid layered magnetic materials approaching the 2D magnetic limit.

Influence of Fe-clustering on the water oxidation performance of two-dimensional layered double hydroxides

Among the two-dimensional (2D) materials family, layered double hydroxides (LDHs) represent a key member due to their unparalleled chemical versatility. In particular, Fe-based LDHs are distinguished candidates due to their high efficiency as oxygen evolution reaction (OER) electrocatalysts. Herein, we have selected MgFe-based LDH phases as model systems in order to decipher whether Fe-clustering exerts an effect on the OER performance. For that, we have optimized hydrothermal synthesis by using triethanolamine (TEA) as the chelating agent. The magnetic characterisation allows us to identify the Fe-clustering degree by following both magnetic susceptibility as well as magnetization values at 2 K. Thanks to this, we demonstrated that TEA induces an increment in Fe-clustering. Electrochemical OER measurements show that both samples behave identically by using glassy carbon electrodes. Interestingly, when the samples are tested in the most commonly employed electrode, nickel foam, striking differences arise. The sample exhibiting a lower Fe-clustering behaves as a better electrocatalyst with a reduction of the overpotential values of more than 50 mV to reach 100 mA cm^-2, as a consequence of a favoured surface transformation of MgFe-LDHs phases into more reactive oxyhydroxide NiFe-based phases during the electrochemical tests. Hence, this work alerts about the importance of the electrocatalyst-electrode collector interactions which can induce misinterpretations in the OER performance.

The Missing Link in the Magnetism of Hybrid Cobalt Layered Hydroxides: The Odd-Even Effect of the Organic Spacer

A dramatic change in the magnetic behaviour, which solely depends on the parity of the organic linker molecules, has been found in a family of layered Co^II hydroxides covalently functionalized with dicarboxylic molecules. These layered hybrid materials have been synthesized at room temperature using a one-pot procedure through the epoxide route. While hybrids connected by odd alkyl chains exhibit coercive fields (H_c ) below ca. 3500 Oe and show spontaneous magnetization at temperatures (T_M ) below 20 K, hybrids functionalized with even alkyl chains behave as hard magnets with H_c >5500 Oe and display a T_M higher than 55 K. This intriguing behaviour was studied by density functional theory with the incorporation of a Hubbard term (DFT+U) calculations, unveiling the structural subtleties underlying this observation. Indeed, the different molecular orientation exhibited by the even/odd alkyl chains, and the orientation of the covalently linked carboxylic groups modify the intensity of the magnetic coupling of both octahedral and tetrahedral in-plane sublattices, thus strongly affecting the magnetic properties of the hybrid. These findings offer an outstanding level of tuning in the molecular design of hybrid magnetic materials based on layered hydroxides.

Fundamental Insights into the Covalent Silane Functionalization of NiFe Layered Double Hydroxides

Layered double hydroxides (LDHs) are a class of 2D anionic materials exhibiting wide chemical versatility and promising applications in different fields, ranging from catalysis to energy storage and conversion. However, the covalent chemistry of this kind of 2D materials is still barely explored. Herein, the covalent functionalization with silanes of a magnetic NiFe-LDH is reported. The synthetic route consists of a topochemical approach followed by anion exchange reaction with surfactant molecules prior to covalent functionalization with the (3-aminopropyl)triethoxysilane (APTES) molecules. The functionalized NiFe-APTES was fully characterized by X-ray diffraction, infrared spectroscopy, electron microscopy, thermogravimetric analysis coupled with mass spectrometry and ²⁹ Si solid-state nuclear magnetic resonance, among others. The effect on the electronic properties of the functionalized LDH was investigated by a magnetic study in combination with Mössbauer spectroscopy. Moreover, the reversibility of the silane-functionalization at basic pH was demonstrated, and the quality of the resulting LDH was proven by studying the electrochemical performance in the oxygen evolution reaction in basic media. Furthermore, the anion exchange capability for the NiFe-APTES was tested employing Cr^VI , resulting in an increase of 200 % of the anion retention. This report allows for a new degree of tunability of LDHs, opening the door to the synthesis of new hybrid architectures and materials.

Iron oxide magnetic nanoparticles combined with actein suppress non-small-cell lung cancer growth in a p53-dependent manner

Actein (AT) is a triterpene glycoside isolated from the rhizomes of Cimicifuga foetida that has been investigated for its antitumor effects. AT treatment leads to apoptosis in various cell types, including breast cancer cells, by regulating different signaling pathways. Iron oxide (Fe₃O₄) magnetic nanoparticles (MNPs) are nanomaterials with biocompatible activity and low toxicity. In the present study, the possible benefits of AT in combination with MNPs on non-small-cell lung cancer (NSCLC) were explored in in vitro and in vivo studies. AT-MNP treatment contributed to apoptosis in NSCLC cells, as evidenced by activation of the caspase 3-signaling pathway, which was accompanied by downregulation of the antiapoptotic proteins Bcl2 and BclXL, and upregulation of the proapoptotic signals Bax and Bad. The death receptors of TRAIL were also elevated following AT-MNP treatment in a p53-dependent manner. Furthermore, a mouse xenograft model in vivo revealed that AT-MNP treatment exhibited no toxicity and suppressed NSCLC growth compared to either AT or MNP monotherapies. In conclusion, this study suggests a novel therapy to induce apoptosis in suppressing NSCLC growth in a p53-dependent manner by combining AT with Fe₃O₄ MNPs.

Alkoxide-intercalated NiFe-layered double hydroxides magnetic nanosheets as efficient water oxidation electrocatalysts

Alkoxide-intercalated NiFe-layered double hydroxides were synthesizedviathe nonaqueous methanolic route. These magnetic nanosheets can be exfoliated in water and exhibit an outstanding behaviour as OER electrocatalysts.

Self-Assembly of 1D/2D Hybrid Nanostructures Consisting of a Cd(II) Coordination Polymer and NiAl-Layered Double Hydroxides

The preparation and characterization of a novel hybrid material based on the combination of a 2D-layered double hydroxide (LDH) nanosheets and a 1D-coordination polymer (1D-CP) has been achieved through a simple mixture of suspensions of both building blocks via an exfoliation/restacking approach. The hybrid material has been thoroughly characterized demonstrating that the 1D-CP moieties are intercalated as well as adsorbed on the surface of the LDH, giving rise to a layered assembly with the coexistence of the functionalities of their initial constituents. This hybrid represents the first example of the assembly of 1D/2D nanomaterials combining LDH with CP and opens the door for a plethora of different functional hybrid systems.

A high surface area flower-like Ni-Fe layered double hydroxide for electrocatalytic water oxidation reaction

Layered double hydroxide has been used in a variety of areas, including but not limited to catalysis, energy storage, drug or gene delivery, water treatment, etc. Herein, we report a new simple hydrothermal method to prepare a high surface area flower-like Ni-Fe layered double hydroxide (LDH) assembled by nanosheets by using nickel alkoxide and FeSO4 as the only starting materials. It is free of alkaline solution and other additives for directing or supporting in the synthesis procedure. The formation mechanism of this flower-like LDH formed by ultrathin nanosheets is also discussed. Moreover, the as-obtained LDH material shows increased electrocatalytic activity and stability toward WOR in alkaline media compared with the materials prepared without a Ni alkoxide precursor or Fe precursor, namely α-Fe2O3 and Ni(OH)2, respectively. In addition, the electrocatalytic activity is demonstrated to be related to the molar ratio of Fe and Ni in the final Ni-Fe material, and the best activity is achieved when the ratio reaches 0.52 : 1. The phase compositions of the resulting Ni-Fe(x) are discussed. Furthermore, the Ni-Fe LDH material reported herein might be employed as a promising noble-metal-free water oxidation catalyst to replace the IrOx material-the state-of-the-art water oxidation catalyst.

Hybrid Materials Based on Magnetic Layered Double Hydroxides: A Molecular Perspective

Design 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 "Chimie Douce"), 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 "dual-function" 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.

Stimuli-responsive hybrid materials: breathing in magnetic layered double hydroxides induced by a thermoresponsive molecule

A hybrid magnetic multilayer material consisting of CoAl–LDH ferromagnetic layers intercalated with thermoresponsive molecules diluted with a flexible surfactant has been obtained.

A hybrid magnetic multilayer material of micrometric size, with highly crystalline hexagonal crystals consisting of CoAl–LDH ferromagnetic layers intercalated with thermoresponsive 4-(4-anilinophenylazo)benzenesulfonate (AO5) molecules diluted (ratio 9 : 1) with a flexible sodium dodecylsulphate (SDS) surfactant has been obtained. The resulting material exhibits thermochromism attributable to the isomerization between the azo (prevalent at room temperature) and the hydrazone (favoured at higher temperatures) tautomers, leading to a thermomechanical response. In fact, these crystals exhibited thermally induced motion triggering remarkable changes in the crystal morphology and volume. In situ variable temperature XRD of these thin hybrids shows that the reversible change into the two tautomers is reflected in a shift of the position of the diffraction peaks at high temperatures towards lower interlayer spacing for the hydrazone form, as well as a broadening of the peaks reflecting lower crystallinity and ordering due to non-uniform spacing between the layers. These structural variations between room temperature (basal spacing (BS) = 25.91 Å) and 100 °C (BS = 25.05 Å) are also reflected in the magnetic properties of the layered double hydroxide (LDH) due to the variation of the magnetic coupling between the layers. Overall, our study constitutes one of the few examples showing fully reversible thermo-responsive breathing in a 2D hybrid material. In addition, the magnetic response of the hybrid can be modulated due to the thermotropism of the organic component that, by influencing the distance and in-plane correlation of the inorganic LDH, modulates the magnetism of the CoAl–LDH sheets in a certain range.