In situ tuning of magnetization via topotactic lithium insertion in ordered mesoporous lithium ferrite thin films

The synthesis and characterization of cubic mesostructured lithium ferrite (α-LiFe5O8) with 20 nm diameter pores and nanocrystalline walls is reported. The material is prepared in the form of thin films by sol–gel dip-coating using a poly(isobutylene)-block-poly(ethylene oxide) amphiphilic diblock copolymer as the porogen. Electron microscopy, X-ray scattering and diffraction, time-of-flight secondary ion mass spectrometry, Raman and X-ray photoelectron spectroscopy all show that α-LiFe5O8 can be templated to produce high-quality films that are chemically and phase-pure and thermally stable to over 600 °C. Magnetometry measurements indicate ferrimagnetic behavior below 300 K, with the coercivity exhibiting a T1/2 dependence. This novel mesoporous spinel material – when used as an electrode in secondary battery cells – can reversibly store charge via topotactic Li insertion, which allows for the intriguing possibility of tuning the magnetization at room temperature in a facile and controlled manner. The general approach is simple and should be applicable to a variety of other magnetic materials that are capable of reacting electrochemically with Li to produce reduced phases

Microwave synthesis of high-quality and uniform 4 nm ZnFe₂O₄ nanocrystals for application in energy storage and nanomagnetics

Magnetic nanocrystals with a narrow size distribution hold promise for many applications in different areas ranging from biomedicine to electronics and energy storage. Herein, the microwave-assisted sol–gel synthesis and thorough characterization of size-monodisperse zinc ferrite nanoparticles of spherical shape is reported. X-ray diffraction, 57Fe Mössbauer spectroscopy and X-ray photoelectron spectroscopy all show that the material is both chemically and phase-pure and adopts a partially inverted spinel structure with Fe3+ ions residing on tetrahedral and octahedral sites according to (Zn0.32Fe0.68)tet[Zn0.68Fe1.32]octO4±δ. Electron microscopy and direct-current magnetometry confirm the size uniformity of the nanocrystals, while frequency-dependent alternating-current magnetic susceptibility measurements indicate the presence of a superspin glass state with a freezing temperature of about 22 K. Furthermore, as demonstrated by galvanostatic charge–discharge tests and ex situ X-ray absorption near edge structure spectroscopy, the as-prepared zinc ferrite nanocrystals can be used as a high-capacity anode material for Li-ion batteries, showing little capacity fade – after activation – over hundreds of cycles. Overall, in addition to the good material characteristics, it is remarkable that the microwave-based synthetic route is simple, easily reproducible and scalable

Long cycle life of CoMn2O4 lithium ion battery anodes with high crystallinity

CoMn2O4 nanomaterials are prepared by a low temperature precipitation route employing metal acetates and NaOH. Structural changes, induced by different annealing temperatures, are comprehensively analyzed by X-ray powder diffraction and Raman spectroscopy. With rising annealing temperature the crystal lattice of CoMn2O4 undergoes changes ; AO4 tetrahedra expand due to thermally induced substitution of Co2+ by larger Mn2+ metal ions on the A-site of the spinel structure, while in contrast, BO6 octahedra shrink since the B-site becomes partially occupied by smaller Co3+ metal ions on account of the migrated Mn ions. CoMn2O4 particle sizes are easily fine-tuned by applying different annealing temperatures ; the particle size increases with increasing annealing temperature. During the battery operation, pulverization and reduction of particle sizes occurs regardless of the initial size of the particles, but the degree of division of the particles during the operation is dependent on the initial particle properties. Thus, contrary to the common assumption that nanostructuring of the anode material improves the battery performance, samples with the largest particle sizes exhibit excellent performance with a capacity retention of 104% after 1000 cycles (compared to the 2nd cycle)

Large-Pore Mesoporous Ho3Fe5O12Ho_{3}Fe_{5}O_{12} Thin Films with a Strong Room-Temperature Perpendicular Magnetic Anisotropy by Sol–Gel Processing

We report the evaporation-induced self-assembly synthesis of large-pore mesoporous thin films of ferrimagnetic holmium iron garnet($Ho_3Fe_5O_{12}$) from nitrate salt precursors and a polyisobutylene-blockpoly(ethylene oxide) polymer structure-directing agent. The phase composition, atomic bonding configuration, and pore structure of the top surface and the interior of the films were investigated by microscopy, scattering, and spectroscopy techniques, including synchrotron-based grazing incidence small-angle X-ray scattering, X-ray photoelectron spectroscopy, X-ray diffraction (including Rietveld refinement), and others. The data provide evidence that the sol−gelderived material is single-phase garnet with 27 nm diameter crystallites and few defects after being heated to 850 °C in air, and the continuous network of pores averaging 23 nm in diameter is preserved to a large extent, despite a solid−solid conversion from metastable h-HoFeO$_3$ to $Ho_3Fe_5O_{12}$ during the crystallization process. Furthermore, dc magnetometry measurements show the thin films are magnetically stable with a room-temperature coercivity of ∼170 Oe and exhibit an out-of-plane easy axis with a significant perpendicular magnetic anisotropy. A strong preference for out-of-plane magnetic alignment in solution-processed mesostructured films is unique, making them attractive for application in spintronics and nanomagnetics

Facile and General Synthesis of Thermally Stable Ordered Mesoporous Rare-Earth Oxide Ceramic Thin Films with Uniform Mid-Size to Large-Size Pores and Strong Crystalline Texture

We describe the general synthesis of submicrometer-thick rare-earth/lanthanide sesquioxide (RE₂O₃) films with tailorable pore and grain sizes via polymer templating of hydrated chloride salt precursors. Mesostructured RE₂O₃ (RE = Sm, Tb–Lu) ceramics with cubic pore symmetry and high surface area (<i>S</i>_BET ≥ 50 m² g^–1) were prepared using different diblock copolymer structure-directing agents and were characterized by a combination of electron microscopy, in situ and ex situ grazing incidence small-angle X-ray scattering, N₂ physisorption, X-ray photoelectron spectroscopy, X-ray diffraction including Rietveld refinement, and ultraviolet–visible spectroscopy. In the present work, we specifically focus on Dy₂O₃ and Yb₂O₃ and use both of these materials as model systems to study, among other things, the film formation and microstructure. Our research data collectively demonstrate that (1) record pore sizes of up to 42 nm in diameter can be achieved without the need for swelling agents, (2) the nanostructure can be preserved up to 1000 °C for the heavier oxides, (3) the sizes of the optical band gaps (4.9–5.6 eV) are comparable to those reported for single crystals, (4) the sol–gel-derived materials are single phase and adopt the <i>C</i>-type crystal structure, and (5) the grain growth is virtually linear, with domain sizes in the range of 3–16 nm. We also show that, except for Yb₂O₃, all of the samples have a fiber texture and the preferred orientation is significant in Sm₂O₃ and Lu₂O₃ films (March parameter <i>G</i>₂ < 0.1). Overall, the synthesis parameters described in this work provide a blueprint for the preparation of thermally stable rare-earth oxide ceramics with both a mesoporous morphology and iso-oriented nanocrystalline walls

Microwave synthesis of high-quality and uniform 4 nm ZnFe2O4 nanocrystals for application in energy storage and nanomagnetics

Magnetic nanocrystals with a narrow size distribution hold promise for many applications in different areas ranging from biomedicine to electronics and energy storage. Herein, the microwave-assisted sol–gel synthesis and thorough characterization of size-monodisperse zinc ferrite nanoparticles of spherical shape is reported. X-ray diffraction, 57Fe Mössbauer spectroscopy and X-ray photoelectron spectroscopy all show that the material is both chemically and phase-pure and adopts a partially inverted spinel structure with Fe3+ ions residing on tetrahedral and octahedral sites according to (Zn0.32Fe0.68)tet[Zn0.68Fe1.32]octO4±δ. Electron microscopy and direct-current magnetometry confirm the size uniformity of the nanocrystals, while frequency-dependent alternating-current magnetic susceptibility measurements indicate the presence of a superspin glass state with a freezing temperature of about 22 K. Furthermore, as demonstrated by galvanostatic charge–discharge tests and ex situ X-ray absorption near edge structure spectroscopy, the as-prepared zinc ferrite nanocrystals can be used as a high-capacity anode material for Li-ion batteries, showing little capacity fade – after activation – over hundreds of cycles. Overall, in addition to the good material characteristics, it is remarkable that the microwave-based synthetic route is simple, easily reproducible and scalable

Room Temperature Magnetic Rare-Earth Iron Garnet Thin Films with Ordered Mesoporous Structure

Amphiphilic polymers are very attractive as porogens for the preparation of ordered mesoporous thin films and powders with pore sizes ranging from 40 down to a few nanometers in diameter because they are capable of both forming different superstructures and interacting with sol–gel precursors. In the present work, we report for the first time the synthesis of a series of highly crystalline rare-earth iron garnet (RE₃Fe₅O₁₂, RE = Y, Gd–Dy) thin films with cubic networks of interconnected pores averaging 17 nm in diameter through facile polymer templating of hydrated nitrate salts. Despite intricate crystallization pathways, e.g., Y₃Fe₅O₁₂ via Y₄Fe₂O₉ and <i>h</i>-YFeO₃, the nanoscale architecture of all these materials is only affected to a limited extent by solid–solid conversions at elevated temperatures. We specifically focus on the characterization of the morphology, microstructure, and magnetic properties of polymer-templated Y₃Fe₅O₁₂. This novel mesoporous material is single phase after heating to 900 °C, free of major structural defects, and is also well-defined on the atomic level, as evidenced by a combination of in situ and ex situ scattering/diffraction techniques, electron microscopy, Raman and X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. The high quality of the nanocrystalline Y₃Fe₅O₁₂ thin films with overall soft magnetic properties and moderate anisotropy is further confirmed by SQUID magnetometry measurements. The magnetization behavior in the temperature range 5–380 K describes Bloch’s <i>T</i>^3/2 law for a 3D Heisenberg-type ferromagnet well

Large-Pore Mesoporous Ho₃Fe₅O₁₂ Thin Films with a Strong Room-Temperature Perpendicular Magnetic Anisotropy by Sol–Gel Processing

We report the evaporation-induced self-assembly synthesis of large-pore mesoporous thin films of ferrimagnetic holmium iron garnet (Ho₃Fe₅O₁₂) from nitrate salt precursors and a polyisobutylene-<i>block</i>-poly­(ethylene oxide) polymer structure-directing agent. The phase composition, atomic bonding configuration, and pore structure of the top surface and the interior of the films were investigated by microscopy, scattering, and spectroscopy techniques, including synchrotron-based grazing incidence small-angle X-ray scattering, X-ray photoelectron spectroscopy, X-ray diffraction (including Rietveld refinement), and others. The data provide evidence that the sol–gel-derived material is single-phase garnet with 27 nm diameter crystallites and few defects after being heated to 850 °C in air, and the continuous network of pores averaging 23 nm in diameter is preserved to a large extent, despite a solid–solid conversion from metastable <i>h</i>-HoFeO₃ to Ho₃Fe₅O₁₂ during the crystallization process. Furthermore, dc magnetometry measurements show the thin films are magnetically stable with a room-temperature coercivity of ∼170 Oe and exhibit an out-of-plane easy axis with a significant perpendicular magnetic anisotropy. A strong preference for out-of-plane magnetic alignment in solution-processed mesostructured films is unique, making them attractive for application in spintronics and nanomagnetics