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

Ordered Mesoporous Thin Film Ferroelectrics of Biaxially Textured Lead Zirconate Titanate (PZT) by Chemical Solution Deposition

Lead zirconate titanate (PZT) thin film nanostructures with a high degree of biaxial texturing and good ferroelectric properties have been prepared by facile chemical solution deposition on (001)-oriented STO:Nb and LSMO/STO:Nb substrates using a poly­(ethylene-<i>co</i>-butylene)-<i>block</i>-poly­(ethylene oxide) diblock copolymer as structure-directing agent. The samples were thoroughly characterized by electron microscopy, synchrotron-based grazing incidence small-angle X‑ray scattering, X-ray diffraction (including θ–2θ, ω, and φ scans), X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, and by ferroelectric polarization switching and fatigue measurements. We show that (1) the cubic mesostructured films with 16 nm diameter pores can be crystallized to produce single phase perovskite PZT with retention of nanoscale order, (2) the sol–gel derived material has an in-plane texture of ∼1.9° and an out-of-plane texture of ∼1.5°, and (3) the top surface is Zr-rich (the composition in the interior of the films is closer to the targeted composition of PbZr_0.52Ti_0.48O₃). The coercive field and remanent polarization of approximately 100 nm-thick films derived from dynamic <i>P</i>–<i>E</i> experiments are ∼250 kV cm^–1 and ∼25 μC cm^–2 (∼7 μC cm^–2 after subtracting the non-switching components). Despite the use of Au top electrodes, the nanocrystalline samples show reasonable fatigue performance. All these features render the mesoporous PZT thin films attractive, for example, for producing strain-coupled composite multiferroics

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

Large Magnetoresistance and Electrostatic Control of Magnetism in Ordered Mesoporous La_1–<i>x</i>Ca_<i>x</i>MnO₃ Thin Films

Ferroic nanomaterials have received much attention in recent times because of their potential for novel applications. Here, we report a facile bottom–up synthetic approach to the first ordered mesoporous mixed-valence manganese oxide. Continuous thin films of perovskite-type La_0.68Ca_0.30Mn_1.02O_3−δ have been prepared from common inorganic salt precursors by taking advantage of the superior templating properties of a polyisobutylene-<i>block</i>-poly­(ethylene oxide) diblock copolymer. This novel solution-processed mesostructured material is well-defined at the nanometer and micrometer levels and behaves superparamagnetically above 230 K. Furthermore, it exhibits large magnetoresistance over a wide range of temperatures due to complex percolation pathways for electron transport imparted by the unique pore–solid architecture, and it is ferromagnetic metallic below 180 K. We also demonstrate reversible magnetization modulation of up to 8.5% by electrostatic charge carrier doping in the polymer-templated thin films. This value is the highest thus far reported for electrolyte-gated mixed-valence manganese oxides

Morphology, Microstructure, and Magnetic Properties of Ordered Large-Pore Mesoporous Cadmium Ferrite Thin Film Spin Glasses

Herein, we report the synthesis, microstructure, and magnetic properties of cadmium ferrite (CdFe₂O₄) thin films with both an ordered cubic network of 18 nm diameter pores and single-phase spinel grains averaging 13 nm in diameter. These mesoporous materials were produced through facile polymer templating of hydrated nitrate salt precursors. Both the morphology and the microstructure, including cation site occupancy and electronic bonding configuration, were analyzed in detail by electron microscopy, grazing incidence small-angle X-ray scattering, Raman and X-ray photoelectron spectroscopy, and N₂-physisorption. The obtained data demonstrate that the network of pores is retained up to annealing temperatures as high as 650 °Cthe onset of crystallization is at ϑ = (590 ± 10) °C. Furthermore, they show that the polymer-templated samples exhibit a “partially” inverted spinel structure with inversion parameter λ = 0.40 ± 0.02. This differs from microcrystalline CdFe₂O₄ which shows virtually no inversion. Magnetic susceptibility studies reveal ferrimagnetic spin coupling below 147 K and further point to the likelihood of glassy behavior at low temperature (<i>T</i>_f ≈ 60 K). In addition, analysis of room temperature magnetization data indicates the presence of sub-10 nm diameter superparamagnetic clusters in an otherwise paramagnetic environment

Electrochemical Tuning of Magnetism in Ordered Mesoporous Transition-Metal Ferrite Films for Micromagnetic Actuation

Controlling magnetism in situ and in a reversible manner using battery redox chemistry is a new concept in the wider field of magnetoelectrics, and electrochemical lithium tuning of electrode materials is one such approach. Here we show that the general idea of combining nanomagnetism and electrochemical energy storage concepts can be applied to ordered mesoporous spinel ferrite thin films. Via an evaporation-induced self-assembly method, a series of cobalt and nickel ferrites (CoFe₂O₄, Co_0.5Ni_0.5Fe₂O₄, and NiFe₂O₄) were prepared and characterized by electron microscopy, grazing-incidence small- and wide-angle X-ray scattering, ⁵⁷Fe Mössbauer spectroscopy, and magnetometry. The dip-coated films after heating in air to temperatures beyond 600 °C are single phase with partially inverted spinel structures and exhibit 15–20 nm pores of oblate spheroidal shape arranged on a cubic lattice. Magnetic measurements clearly show stable ferrimagnetic ordering at room temperature, especially for the cobalt-based materials. More importantly, when using the different transition-metal ferrites as insertion electrodes in lithium-ion cells and carefully controlling the cutoff potentials, they allow for the robust tuning of magnetization without compromising both the lattice and pore structure. As a potential application area, micromagnetic actuation technology is considered

Applying Capacitive Energy Storage for In Situ Manipulation of Magnetization in Ordered Mesoporous Perovskite-Type LSMO Thin Films

Mesostructured nonsilicate materials, particularly mixed-metal oxides, are receiving much attention in recent years because of their potential for numerous applications. Via the polymer-templating method, perovskite-type lanthanum strontium manganese oxide (La_1–<i>x</i>Sr_<i>x</i>MnO₃, LSMO, with <i>x</i> ≈ 0.15 to 0.30) with a continuous 3D cubic network of 23 nm pores is prepared in thin-film form for the first time. Characterization results from grazing incidence X-ray scattering, X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and electron microscopy and tomography show that the dip-coated sol–gel-derived films are of high quality in terms of both composition and morphology and that they are stable to over 700 °C. Magnetic and magnetotransport measurements demonstrate that the material with the highest strontium concentration is ferromagnetic at room temperature and exhibits metallic resistivity behavior below 270 K. Besides, it behaves differently from epitaxial layers (e.g., enhanced low-field magnetoresistance effect). It is also shown that carriers (electrons and holes) can be induced into the polymer-templated mesostructured LSMO films via capacitive double-layer charging. This kind of electrostatic doping utilizing ionic liquid gating causes large relative changes in magnetic susceptibility at room temperature and is a viable technique to tune the magnetic phase diagram in situ

Structural, Optical, and Magnetic Properties of Highly Ordered Mesoporous MCr₂O₄ and MCr_2–<i>x</i>Fe_<i>x</i>O₄ (M = Co, Zn) Spinel Thin Films with Uniform 15 nm Diameter Pores and Tunable Nanocrystalline Domain Sizes

Herein is reported the synthesis and characterization of nanocrystalline cobalt chromite (CoCr₂O₄) and zinc chromite (ZnCr₂O₄) thin films with highly ordered cubic networks of open pores averaging 15 nm in diameter. We also show that the synthesis method employed in this work is readily extendable to solid solutions of the type MCr_2–<i>x</i>Fe_<i>x</i>O₄ (M = Co, Zn), which could pave the way for innovative device design. All of these materials can be prepared by facile coassembly of hydrated nitrate salts with an amphiphilic diblock copolymer, referred to as KLE. The as-made materials are amorphous thin films with face-centered-cubic close-packed pore structures. Electron microscopy, X-ray diffraction, grazing incidence small-angle X-ray scattering, krypton physisorption, UV–vis spectroscopy, time-of-flight secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and Raman spectroscopy studies collectively verify that both the transition metal chromites and the solid solutions are well-defined at the nanoscale and the microscale. In addition, the data show that the different thin film materials are nanocrystalline after annealing in air at 600 °C, adopt the spinel structure in phase-pure form, and that the conversion of the initially amorphous frameworks comes at little cost to the ordering of the cubic pore-solid architectures. Magnetization studies as a function of temperature and field further reveal the high quality of the KLE-templated CoCr₂O₄ thin films with both long-range ferrimagnetic order and spiral magnetic order at low temperatures, in agreement with previous findings

Hierarchical Carbon with High Nitrogen Doping Level: A Versatile Anode and Cathode Host Material for Long-Life Lithium-Ion and Lithium–Sulfur Batteries

Nitrogen-rich carbon with both a turbostratic microstructure and meso/macroporosity was prepared by hard templating through pyrolysis of a tricyanomethanide-based ionic liquid in the voids of a silica monolith template. This multifunctional carbon not only is a promising anode candidate for long-life lithium-ion batteries but also shows favorable properties as anode and cathode host material owing to a high nitrogen content (>8% after carbonization at 900 °C). To demonstrate the latter, the hierarchical carbon was melt-infiltrated with sulfur as well as coated by atomic layer deposition (ALD) of anatase TiO₂, both of which led to high-quality nanocomposites. TiO₂ ALD increased the specific capacity of the carbon while maintaining high Coulombic efficiency and cycle life: the composite exhibited stable performance in lithium half-cells, with excellent recovery of low rate capacities after thousands of cycles at 5C. Lithium–sulfur batteries using the sulfur/carbon composite also showed good cyclability, with reversible capacities of ∼700 mA·h·g^–1 at C/5 and without obvious decay over several hundred cycles. The present results demonstrate that nitrogen-rich carbon with an interconnected multimodal pore structure is very versatile and can be used as both active and inactive electrode material in high-performance lithium-based batteries