Mixed-valence Manganese Oxides Research Articles

Tuning the Composition, Crystal Structure and Morphology of Manganese(III/IV) Oxide for High-Power Storage Applications

If inherently intermittent renewable energy technologies are to offer a practical alternative to conventional heat engines, it is critically important that efficient, cost-effective and sustainable energy storage solutions are developed to decouple supply and demand profiles. A promising strategy is to pair the competitive energy densities of batteries with the superior cycling lifetimes and power densities of supercapacitors,[1] employing an appropriate maximum power point tracking (MPPT) system to optimise energy exchange between these complementary devices.[2] Mixed-valence manganese(III/IV) oxides (MnOz, where z ranges between 1.5 and 2.0) have emerged as foremost candidates for the electroactive material in supercapacitor cathodes due to their low toxicity, minimal cost,[3] ready availability (manganese appears at an average concentration of ca. 1,000 ppm in the Earth’s crust[4]), wide electrochemical stability window (ca. 1.0 V in aqueous electrolytes[5]), and high maximum theoretical specific capacitance (ca. 1,370 F g-1, based on the reduction of Mn4+ ions by a single electron[6]). Charge storage in MnOz is predominantly mediated by the pseudocapacitive intercalation of cations from the surrounding electrolyte in each discharge phase, followed by the release of these guest ions during charging.Of the various crystallographic phases of MnOz, cryptomelane (α-MnOz) and birnessite (δ-MnOz) polymorphs are deemed two of the most promising candidates for storage applications because they possess characteristically wide channels for intercalation/de-intercalation:[8,9] α-MnOz consists of ca. 4.6 Å diameter one-dimensional tunnels enclosed between corner-sharing MnO6 octahedra, whereas δ-MnOz comprises two-dimensional layers of MnO6 octahedra separated by ca. 7.0 Å. Herein, a scalable, low-cost synthesis procedure is discussed that enables the production of highly pseudocapacitive birnessite and cryptomelane electrodes with tuneable material characteristics, including particle morphology, crystallographic structure, average Mn oxidation state, and the level of pre-intercalation by Na+ and K+ ions.[10] In addition to comparing the electrochemical properties of these NaxKyMnOz products in aqueous conditions, their application within an asymmetric supercapacitor is showcased: by optimising the relative masses of a NaxKyMnOz cathode and activated carbon anode in a 100 cm2 aqueous full-cell configuration, discharge capacitances in the range 10-30 F g-1 can be achieved over a voltage span of 1.8 V at current densities as high as 10 A g-1, with Coulombic efficiency exceeding 80% up to ca. 20 A g-1. With these competitive performance characteristics, the developed NaxKyMnOz materials are a promising option for high-power supercapacitor cathodes, helping to pave the way towards more efficient and sustainable energy storage.

Grain size effect on the critical behavior of the ferromagnetic transition in electronically phase separated La0.5Ca0.5MnO3

Mixed valence manganese oxides (manganites) are known to exhibit intrinsic inhomogeneous magnetic states, which are influenced by a great variety of factors, including oxygen stoichiometry, nanostructuration and ceramic grain size. In this context, the prototypical half-doped compound La0.5Ca0.5MnO3was extensively studied. Here we reloaded the phase separated scenario, focusing on the thermodynamical critical behavior of this system at the paramagnetic - ferromagnetic transition and its correlation with the evolution of its ordering capabilities controlled through the ceramic grain size.Our findings establish a clear connection between the correlation length and the grain size, shedding light on the role of this parameter in the control of inhomogeneous magnetic ground states in manganites. Therefore it is possible to tailor fundamental physical properties as the scaling laws of the system just tuning the ceramic grain size.

Green Three-Step Electrodeposition of Mn+3 Rich Layered δ-Phase MnO2 Nanofibers on Silicon and Epitaxial Graphene-Silicon Carbide Heterostructures

Mixed valence manganese oxides (MnOx) have attracted significant research interest in recent years due to the easily reversible redox reactions between manganese oxidation states (Mn+2, Mn+3, and Mn+4)1 which enable applications in catalysis2, energy storage3, and gas sensing4. Of the manganese oxides manganese dioxide (MnO2) has been of particular interest due to its wide variety of synthesized structural polymorphs ((β)1x1 tunnel5, (α)1x2 tunnel5 (γ) spinel5, (δ) layered5) which allow for enhanced control over the available surface area and reactive properties of MnO2. Among these structural polymorphs, the α and δ phases exhibit mixed-valence character with Mn+3 defects being found throughout the crystal structure5. Mn+3 defects have been found to increase the catalytic activity of MnO2 making its presence desirable6. Hexagonal δ-MnO2, in particular, contains a large number of Mn+3 ions as the interlayer contains Mn+2/+3 ions to neutralize the layer charge from Mn+3 lattice defects7. However, the controlled synthesis of hexagonal δ-MnO2 has proven challenging and often relies on the use of harsh chemicals1, high temperatures1, and long reaction times1. These methods also rely on permanganate salts1 as the manganese precursor, which leads to the formation of monoclinic δ-MnO2 and significantly reduces the number of Mn+3 defects in the crystal lattice. In this work, we present the green facile synthesis of high Mn+3 content hexagonal δ-MnO2 via electrodeposition on both silicon(Si) and epitaxial graphene/silicon carbide(EG/SiC) substrates. The electrodeposition was carried out in a three-electrode electrochemical cell with an Ag/AgCl reference electrode, a Pt counter electrode, and the substrate (Si or EG/SiC) as the working electrode utilizing a 100mM manganese acetate solution. The electrodeposition process contained three steps, an initial pulse at the oxidation potential to seed the surface with oxygen, a second pulse at the reduction potential to deposit manganese on the surface, and a third pulse at the oxidation potential to oxidize the deposited manganese, with the potentials determined by cyclic voltammetry. The resulting δ-MnO2 thin films were then characterized through Raman spectroscopy, X-Ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy(AFM) to determine its crystalline phase and surface morphology. Raman spectroscopy(Fig. 1) confirmed the formation of MnO2 on both substrates due to the presence of Raman peaks between 650-640cm-1,8 575-585cm-1,8, and 490-510cm-1,8, which are associated with the Mn-O symmetric stretching bond8, the Mn-O stretching bond in the basal plane8, and Mn-O stretching bond in the [MnO6] octehedra8. The formation of the δ phase is confirmed by the presence of a weak peak at 130cm-1,2 consistent with the literature. The observed 9cm-1 redshift of the 490-510cm-1,6 between Si and SiC substrates is associated with the weakening Mn-O stretching bond on Si, which combined with the strengthening of the peak at 600-620cm-1 indicates that the growth on Si contains a higher rate of Mn+3defects2,8. XPS corroborated this result and indicated that the manganese in the δ-MnO2 deposited on EG/SiC had an average oxidation state of 3.3. The AFM images indicated that the surface was made up of nanofibrous nanoparticles, with the deposition on Si exhibiting significantly larger nanofibers (Fig 2.). SEM confirmed this (Fig. 3) and indicated that the deposition on EG/SiC was made up of microplates ~40umx40um in size, which was not observed in the deposition on Si. We believe this variation in surface morphology is due to interactions between SiC and MnO2, which are not present when δ-MnO2 is grown on Si. The reactivity of the deposited δ-MnO2 was then tested by depositing 4 Ti/Au (30nm/120nm) contacts on the δ-MnO2/EG/SiC heterostructures to form a simple gas sensor, which was tested against 5ppm NO2, 5ppm NH3, 1000ppm IPA, and 1000ppm methanol. The sensor displayed a remarkably fast response/ recovery time of 3.8s/2.2s to NH3 and 3.4/6.0s to NO2(Fig. 4), demonstrating the high reactive potential of the δ-MnO2 thin films. This reactivity demonstrates that our electrodeposited δ-MnO2 provides a good candidate for use in other applications of MnO2 such as energy storage and catalysis.

Over-Reduction-Controlled Mixed-Valent Manganese Oxide with Tunable Mn2+/Mn3+ Ratio for High-Performance Asymmetric Supercapacitor with Enhanced Cycling Stability.

Manganese oxides composed of various valence states Mnx+ (x = 2, 3, and 4) have attracted wide attention as promising electrode materials for asymmetric supercapacitor. However, the poor electrical conductivity limited their performance and application. Appropriate regulation content of Mnx+ in mixed-valent manganese oxide can tune the electronic structure and further improve their conductivity and performance. Herein, we prepared manganese oxides with different Mn2+/Mn3+ ratios through an over-reduction (OR) strategy for tuning the internal electron structure of mixed-valent manganese, which could make these material oxides a good platform for researching the structure-property relationships. The Mn2+/Mn3+ ratio of manganese oxide could be precisely tuned from 0.6 to 1.7 by controlling the amount of reducing agent for manipulating the redox processes, where the manganese oxide electrode with the most appropriate Mn2+/Mn3+ ratio, as 1.65 (OR4) exhibits large capacitance (274 F g-1) and the assembling asymmetric supercapacitors by combining OR4 (positive) and the commercial activated carbon (as negative) achieved large 2.0 V voltage window and high energy density of 27.7 Wh kg-1 (power density of 500 W kg-1). The cycle lifespan of the OR4//AC could keep about 92.9% after 10 000-cycle tests owing to the Jahn-Teller distortion of the Mn(III)O6 octahedron, which is more competitive compared to other work. Moreover, a red-light-emitting diode (LED) can easily be lit for 15 min by two all-solid supercapacitor devices in a series.

Mixed-valent manganese oxide for catalytic oxidation of Orange II by activation of persulfate: heterojunction dependence and mechanism

The coexistence of aliovalent cations in manganese oxide catalysts with a suitable mole ratio accelerates the activation process of persulfate for the degradation of organic pollutants.

Enhanced zinc storage performance of mixed valent manganese oxide for flexible coaxial fiber zinc-ion battery by limited reduction control

While manganese-based cathodes have been intensively studied for zinc-ion batteries (ZIBs), the limited rate capability and cycle life have always been a difficult problem to be solved. Here, we report a mixed valent manganese oxide (MnOx) cathode with superior electrochemical performance, which exhibits a high specific capacity of 450 mA h/g at 0.2 C and a satisfactory specific capacity of 158.3 mA h/g at a high rate of 5 C. The mixed cathode system reduces the charge transfer resistance, and show good surface stability and adsorption properties, so it is beneficial for the storage of Zn2+. Meanwhile, coaxial fiber ZIBs (CFZIBs) with splendid flexibility are assembled utilizing the elaborately prepared cathode material. The CFZIBs achieve a reversible capacity of 255.8 mA h/g and the capacity retention rate is as high as 80 % after 1000 bending deformations. This study provides new opportunities for designing ZIBs with high performance and high flexibility.

A-site mixed-valence co-doping to optimize room-temperature TCR of polycrystalline La0.8K0.04Ca0.16-xSrxMnO3 ceramics

Herein, polycrystalline La0.8K0.04Ca0.16-xSrxMnO3 ceramics (A-site = La, K, Ca and Sr, 0.00 ≤ x ≤ 0.16) are successfully synthesized via the sol-gel process and the influence of Sr content on microstructural and electrical transport properties is investigated in detail. The results reveal that the replacement of Ca-ions by Sr-ions resulted in a decreased resistivity and enhanced temperature coefficient of resistance (TCR) peak temperature (Tk). The influence of A-site mixed-valence co-doping, where A-site is occupied by monovalent alkali-metal, divalent alkaline-earth element and trivalent rare-earth element, on electrical transport properties can be explained by using different conduction mechanisms in different temperature regions. At optimal doping content of 0.12, peak TCR of the resistivity was found to be 12.04% K−1 at 290.38 K, which is the best reported TCR for A-site mixed-valence co-doped perovskite manganese oxides. These results confirm that high room-temperature TCR values can be achieved by optimizing A-site mixed-valence co-doping and demonstrate the potential of polycrystalline La0.8K0.04Ca0.04Sr0.12MnO3 ceramic in uncooling infrared bolometers.

S-doped graphene/mixed-valent manganese oxides composite electrode with superior performance for supercapacitors

Sulfur doping is one of the most promising approaches to generate electron-dense active region on the surface of graphene-based materials. This work developed an environmental friendly method for synthesizing sulfur-doped graphene (SG)/mixed-valent manganese oxides (MnOx). Three-dimensional (3D) SG is prepared by thermal treatment, and the uniformly doped sulfur provided more anchor points for the compound of subsequent mixed-valent manganese oxides. The final product SG-MnOx (x stands for mixed valence) composite was obtained by one-pot hydrothermal method in which mixed-valence manganese oxides were synthetized and compounded the SG simultaneously. The specific capacitance of SG-MnOx electrode at 0.5 A g−1 is 1311 F g−1 and the long-life cycle retention after 5000 cycles is 81.2%. The excellent manifestation of supercapacitors primarily attributed to the sulfur doping and synergistic effect with MnOx compound. This allows the SG-MnOx electrode to exhibit much better electrochemical performance than the two individual materials. The asymmetric supercapacitor with SG-MnOx and AC exhibits an excellent energy density (21.23 Wh kg−1) with power density of 1.5 kW kg−1. The above performance indicates that SG-MnOx is one of the materials for supercapacitors with great promise.

Mixed-valence mesoporous manganese oxide spheres from waste manganese nitrate aqueous solution

Abstract The paper presents a novel simple and cost-effective approach to preparation of mixed-valence mesoporous manganese oxide in the form of easy to handle spheres. The synthesis relies on swelling of porous polymer templates (i.e. three selected resins Amberlite® XAD7 HP, XAD4 and XAD16) in a saturated Mn(NO3)2 solution and subsequent removal of the template via calcination in air. The products are porous with the specific surface area ( S B E T ) up to 100 m2 g−1. The detailed characteristic of the materials, thermogravimetric insight into its calcination in air, and the influence of porous polymer (template) on MnOx structure is provided. The final product can be of great significance for electrochemistry, catalysis and is of some importance to the environment protection.

Effect of Potassium Ions on the Formation of Mixed-Valence Manganese Oxide/Graphene Nanocomposites.

One-pot synthesis of mixed-valence manganese oxide (MnOx)/potassium ion-doped reduced graphene oxide (rGO) composites for efficient electrochemical supercapacitors is introduced. Using manganese nitrate and potassium permanganate as co-precursors for the MnOx and by directly annealing the rGO without tedious purification steps, as described herein, MnOx/rGO composites with a high specific capacitance of 1955.6 F g−1 at a current density of 1 A g−1 are achieved. It is found that the presence of potassium ions helps in the development of mixed-valence MnOx on the surface of the rGO.

Toward Low-Cost and Sustainable Supercapacitor Electrode Processing: Simultaneous Carbon Grafting and Coating of Mixed-Valence Metal Oxides by Fast Annealing.

There is a rapid market growth for supercapacitors and batteries based on new materials and production strategies that minimize their cost, end-of-life environmental impact, and waste management. Herein, mixed-valence iron oxide (FeOx) and manganese oxide (Mn3O4) and FeOx-carbon black (FeOx-CB) electrodes with excellent pseudocapacitive behavior in 1 M Na2SO4 are produced by a one-step thermal annealing. Due to the in situ grafted carbon black, the FeOx-CB shows a high pseudocapacitance of 408 mF cm−2 (or 128 F g−1), and Mn3O4 after activation shows high pseudocapacitance of 480 mF cm−2 (192 F g−1). The asymmetric supercapacitor based on FeOx-CB and activated-Mn3O4 shows a capacitance of 260 mF cm−2 at 100 mHz and a cycling stability of 97.4% over 800 cycles. Furthermore, due to its facile redox reactions, the supercapacitor can be voltammetrically cycled up to a high rate of 2,000 mV s−1 without a significant distortion of the voltammograms. Overall, our data indicate the feasibility of developing high-performance supercapacitors based on mixed-valence iron and manganese oxide electrodes in a single step.

Composite Aerogels of Carbon Nanocellulose Fibers and Mixed-Valent Manganese Oxides as Renewable Supercapacitor Electrodes.

Bio-waste derived nanocelluloses show excellent mechanical flexibility and self-aggregated capability, which enable them to be good supporting substrates for the synthesis of electroactive materials. Herein, we present a facile route for fabricating composite aerogels consisting of carbonized nanocellulose fibers (CNF) and mixed-valent manganese oxide (MnOx), toward supercapacitor applications. Mixed solutions of nanocellulose and manganese acetate with different ratios were prepared and freeze-dried into hybrid aerogels. The hybrid aerogels were then transformed into CNF/MnOx composites by a calcination process. The CNF membranes served as porous carbon nano-reservoirs for MnOx and electrolyte. The CNF/MnOx composites also kept a 3D porous aerogel structure with hierarchical pores, which enabled stable transport of both electrolyte ions and electrons to the electrode surface, leading to low a charge-transfer impedance and good electrochemical kinetics. The CNF/MnOx-based symmetric supercapacitor showed a satisfied energy density and power density of 37.5 Wh kg−1 and 2.75 kW kg−1, respectively. All the above results demonstrate the feasibility of using sustainable nanocellulose as a nanoscale carbon substrate for the synthesis of hybrid composite electrodes toward renewable supercapacitor applications.

Electrochemical Tuning of Ordered Mesoporous Oxides with Spinel and Perovskite Structures for Reversible Control of Magnetization

Nanoporous oxides in thin film format have been receiving much attention in recent years, especially because of their potential for next generation device applications. However, the preparation of non-siliceous (mixed-metal) oxides that are structurally and compositionally well-defined is challenging. The primary reason is that both the nucleation and growth of the crystalline phase are usually difficult to control. Here we describe the block-copolymer-templating synthesis (using an evaporation-induced self-assembly process) of a series of high quality large-pore mesoporous oxides with spinel and perovskite structures, including metal ferrites and mixed-valence manganese oxides, and their characterization by various imaging, diffraction, and spectroscopic methods. Besides, we demonstrate that the general idea of combining nanomagnetism and electrochemical energy storage concepts can be applied to block-copolymer-templated mixed-metal oxide thin films. Using such high-surface-area materials as the working electrode in lithium-ion battery cells or supercapacitors and carefully controlling the charge/discharge potential allows tuning (virtually fully reversible) the magnetism at room temperature without compromising the lattice and/or pore structure.[1-4] The results are exciting and clearly show that both techniques hold potential for application in the fields of micromagnetic actuation and spintronics to name a few. [1] C. Reitz, P. M. Leufke, R. Schneider, H. Hahn and T. Brezesinski, Chem. Mater. 2014, 26, 5745. [2] C. Reitz, C. Suchomski, D. Wang, H. Hahn and T. Brezesinski, J. Mater. Chem. C 2016, 4, 8889. [3] C. Reitz, D. Wang, D. Stoeckel, A. Beck, T. Leichtweiss, H. Hahn and T. Brezesinski, ACS Appl. Mater. Interfaces 2017, 9, 22799. [4] L. A. Dubraja, C. Reitz, L. Velasco, R. Witte, R. Kruk, H. Hahn and T. Brezesinski, ACS Appl. Nano Mater. 2018, 1, 65.

Ferromagnetic Peierls insulator state in AMg4Mn6O15(A=K,Rb,Cs)

Using the density-functional-theory based electronic structure calculations, we study the electronic state of recently discovered mixed-valent manganese oxides $A$Mg$_4$Mn$_6$O$_{15}$ ($A=$ K, Rb, Cs), which are fully spin-polarized ferromagnetic insulators with a cubic crystal structure. We show that the system may be described as a three-dimensional arrangement of the one-dimensional chains of a $2p$ orbital of O and a $3d$ orbital of Mn running along the three axes of the cubic lattice. We thereby argue that in the ground state the chains are fully spin polarized due to the double-exchange mechanism and are distorted by the Peierls mechanism to make the system insulating.

High-performance hybrid supercapacitor of mixed-valence manganese oxide/N-doped graphene aerogel nanoflower using an ionic liquid with a redox additive as the electrolyte: In situ electrochemical X-ray absorption spectroscopy

Although electrochemical double layer capacitors (EDLCs) have high specific power, their specific energy is rather low. To address this issue, a composite material between EDLC-type N-doped graphene aerogel (N-rGOae) with an N-doped content of ca. 8 at% and pseudocapacitor-type mixed-valence manganese oxide (MnOx) nanoparticles with a diameter of <20 nm was synthesized and used as the electrode material of hybrid supercapacitors. In addition, a new electrolyte of ionic liquid electrolyte 1-butyl-1-methylpyrrolidinium dicyanamide ([BMP][DCA]) with a redox additive (K4[Fe(CN)6]) was employed to further improve the performance of the hybrid supercapacitor. The as-fabricated supercapacitor exhibits an excellent specific energy and power of 44.7 Wh kg−1 and 4551.4 W kg−1, respectively. In situ electrochemical X-ray absorption spectroscopy (XAS) was also used to investigate the charge storage mechanism of the mixed-valence MnOx in the composite. Interestingly, with the presence of redox additive in the electrolytes, a wide oxidation state variation range of Mn in the MnOx/N-rGOae composite can be observed indicating high redox activities and confirming the faradaic charge transfer interaction between redox additive and MnOx. Furthermore, the in situ XAS result also confirms the reversibility of the redox reaction of MnOx showing that the hybrid supercapacitor using the ionic liquid electrolyte with a redox additive in this work may be practically used in high-energy applications.

Metal-Organic Frameworks Derived Nanocomposites of Mixed-Valent MnOx Nanoparticles In-Situ Grown on Ultrathin Carbon Sheets for High-Performance Supercapacitors and Lithium-Ion Batteries

Manganese oxide and carbon hybrid materials have attracted intensive research attention as advanced electrode materials for energy storage. Herein, starting form metal-organic frameworks (MOFs), the nanocomposites of mixed-valent manganese oxide nanoparticles in-situ grown on ultrathin carbon sheets (MnOx-CSs nanocomposites) have been synthesized via a simple method. Benefiting from the unique structural merits, the nanocomposite obtained at 600°C (MnOx-CSs-600) exhibits excellent performance for both supercapacitors (SCs) and lithium-ion batteries (LIBs). A high specific capacitance of 220Fg−1 is achieved at the current density of 1Ag−1. An asymmetric supercapacitor (ASC) based on the MnOx-CSs-600 nanocomposite cathode and activated carbon anode exhibits a high energy density of 27.5Whkg−1 at the power density of 225Wkg−1. Moreover, the MnOx-CSs-600 nanocomposite displays a high reversible capacity of 1217.7mAhg−1 at 200mAg−1 after 160 cycles as an anode material for LIBs. Remarkably, the discharge capacity is still as high as 612.1mAhg−1 even at high current density of 2000mAg−1, indicating good rate capability. This synthetic strategy is quite simple, cost-effective and environmental friendly, which is highly promising for scaled-up production.

Environment-benign synthesis of rGO/MnOx nanocomposites with superior electrochemical performance for supercapacitors

Chemical oxidation synthesis of graphene oxide (GO) through modified Hummers methods has been widely employed for producing graphene- or reduced GO (rGO)-based advanced functional materials such as rGO/MnOx nanocomposites. However, the manganese species in GO colloids are usually washed out during GO synthesis through modified Hummers methods, causing manganese waste and environmental risk. In this paper, we report preparation of rGO/MnOx nanocomposites, in which MnOx is composed of Mn2O3, Mn3O4, and MnO2 components, through anneal treatment of the precursor counterparts obtained by simple pH tuning of GO colloids. The rGO/MnOx nanocomposites exhibit superior electrochemical performance for supercapacitors. rGO/MnOx-5, derived from GO colloids with pH 5, exhibits a high gravimetric discharge capacitance (Cdis) of 191 F g−1 at 20 A g−1 and a high capacitance retention (82.9%) relative to Cdis at 1 A g−1. Furthermore, typical symmetric supercapacitor cells made from rGO/MnOx-5 show a high areal capacitance (172 mF cm−2) and excellent capacitance retention (96.6%) at 2 A g−1 (10 mA cm−2) for 20,000 cycles, holding great potential for practical applications. The superior electrochemical performance of rGO/MnOx nanocomposites is attributed to multiple charge storage mechanisms in association with the coexistence of mixed-valent manganese oxides.

In Situ Tuning of Magnetization Via Double Layer Charging and Topotactic Li Insertion in Polymer-Templated Mesostructured Thin Films

Mesostructured mixed-metal oxide thin films have received much attention in recent years, as they have great potential for future device applications. Advances in polymer templating over the past years have enabled the preparation of a variety of important materials in the form of thin films. However, despite the progress made, complex metal oxides (other than binary systems) often exhibit an ill-defined nanoscale structure after thermally-induced crystallization and removal of the porogen, mainly for stability reasons. Here, we describe the self-assembling synthesis of large-pore mesoporous mixed-metal oxides, with special emphasis on La1–x Ca x MnO3 (LCMO) and alpha-LiFe5O8 (LFO). Electron microscopy, GISAXS, XRD, XPS, TOF-SIMS, and RBS all show that these materials can be templated to produce high quality thin films that are phase pure and thermally stable to over 600 °C. For LCMO, we demonstrate that the surface charge carrier density can be tuned by double layer charging using an ionic liquid electrolyte.[1] Peak-to-peak magnetization modulation values of up to 10% have been achieved, (to our knowledge) the highest thus far reported for electrolyte-gated mixed-valence manganese oxides. Furthermore, we demonstrate that the topotactic Li insertion into LFO allows for the intriguing possibility of controlling the r.t. bulk magnetic state in a highly reversible manner. Overall, both approaches are promising and should be applicable to other magnetic nanomaterials. [1] C. Reitz, P. M. Leufke, R. Schneider, H. Hahn, and T. Brezesinski, Chem. Mater. 26, 5745 (2014).

Hydration-induced spin-glass state in a frustrated Na-Mn-O triangular lattice

Birnessite compounds are stable across a wide range of compositions that produces a remarkable diversity in their physical, electrochemical, and functional properties. These are hydrated analogs of the magnetically frustrated, mixed-valent manganese oxide structures, with general formula, $\mathrm{N}{\mathrm{a}}_{x}\mathrm{Mn}{\mathrm{O}}_{2}$. Here we demonstrate that the direct hydration of layered rock-salt type \ensuremath{\alpha}-$\mathrm{NaMn}{\mathrm{O}}_{2}$, with the geometrically frustrated triangular lattice topology, yields the birnessite type oxide, $\mathrm{N}{\mathrm{a}}_{0.36}\mathrm{Mn}{\mathrm{O}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}0.2{\mathrm{H}}_{2}\mathrm{O}$, transforming its magnetic properties. This compound has a much-expanded interlayer spacing compared to its parent \ensuremath{\alpha}-$\mathrm{NaMn}{\mathrm{O}}_{2}$ compound. We show that while the parent \ensuremath{\alpha}-$\mathrm{NaMn}{\mathrm{O}}_{2}$ possesses a N\'eel temperature of 45 K as a result of broken symmetry in the $\mathrm{M}{\mathrm{n}}^{3+}$ sublattice, the hydrated derivative undergoes collective spin freezing at 29 K within the $\mathrm{M}{\mathrm{n}}^{3+}/\mathrm{M}{\mathrm{n}}^{4+}$ sublattice. Scaling-law analysis of the frequency dispersion of the ac susceptibility, as well as the temperature-dependent, low-field dc magnetization confirm a cooperative spin-glass state of strongly interacting spins. This is supported by complementary spectroscopic analysis [high-angle annular dark-field scanning transmission electron miscroscopy (TEM), energy-dispersive x-ray spectroscopy, and electron energy-loss spectroscopy] as well as by a structural investigation (high-resolution TEM, x-ray, and neutron powder diffraction) that yield insights into the chemical and atomic structure modifications. We conclude that the spin-glass state in birnessite is driven by the spin frustration imposed by the underlying triangular lattice topology that is further enhanced by the in-plane bond-disorder generated by the mixed-valent character of manganese in the layers.

Reversible Magnetization Modulation in Ordered Mesoporous Mixed Metal Oxide Thin Films By Double Layer Charging and Topotactic Lithium Insertion

Mesostructured metal oxide thin films have received much attention in recent years. The primary reason for this is the fact that many such materials have been shown to outperform their bulk counterparts. Energy storage and conversion is one of the fields where this is very evident. Advances in polymer templating over the past 10 years have enabled the preparation of various thin film materials with cubic and 2D-hexagonal pore structures and different ordering lenghts.[1] Their formation relies in principle on the coassembly of either sol-gel precursors or preformed building blocks with an amphiphilic polymer structure-directing agent. Despite the progress made so far, complex metal oxides (mixed metal oxides) often exhibit an ill-defined mesoporous morphology after removal of the polymer template and thermally-induced crystallization because of their limited stability. Here, we describe the evaporation-induced self-assembly (EISA)[2] synthesis of cubic mesoporous mixed metal oxide thin films with >15 nm diameter pores and nanocrystalline walls by using a large polyisobutylene-block-poly(ethylene oxide) diblock copolymer. We focus specifically on perovskite-type La1–x Sr x MnO3 (LSMO) and La1–y Ca y MnO3 (LCMO) as well as spinel-type LiFe5O8. These materials were characterized by various state-of-the-art techniques, including electron microscopy, GISAXS, RBS, XRD and XPS, and were found to be of high quality after calcination at elevated temperatures. For LSMO and LCMO, we show that the charge carrier density, and therefore the magnetization, can be modulated through double layer charging (i.e., electrostatic hole- or electron-doping).[3] Magnetization modulation values of up to 10% have been achieved, the highest thus far reported for electrolyte-gated mixed-valence manganese oxides. Apart from that, we show that the room temperature magnetization of LiFe5O8 can be tuned by topotactic insertion/extraction of Li-ions into/from the spinel lattice and provide insights into the mechanism. Both processes allow for control of magnetism, are highly reversible, and strongly benefit from the high surface-to-volume ratio of the films. [1] C. Sanchez, C. Boissiere, D. Grosso, C. Laberty, and L. Nicole, Chem. Mater.20, 682 (2008). [2] C. J. Brinker, Y. F. Lu, A. Sellinger, and H. Y. Fan, Adv. Mater. 11, 579 (1999). [3] C. Reitz, P. M. Leufke, R. Schneider, H. Hahn, and T. Brezesinski, Chem. Mater. 26, 5745 (2014).