Synthesis and Substitution Chemistry of Redox-Active Manganese/Cobalt Oxide Nanosheets

We report the synthesis and electrochemical properties of Co-substituted manganese oxide nanosheets (Mn_1–<i>x</i>Co_<i>x</i>O₂). Polycrystalline samples of layered Na_0.6Mn_1–<i>x</i>Co_<i>x</i>O₂ (<i>x</i> = 0.2–0.5) were synthesized as starting materials. A linear decrease in the lattice constant <i>a</i> with increasing Co content supported the successful substitution of Co³⁺ ions for Mn³⁺ ions in the host layers. Acid-exchange treatment of the Na_0.6Mn_1–<i>x</i>Co_<i>x</i>O₂ powders resulted in the formation of H–Mn_1–<i>x</i>Co_<i>x</i>O₂ while preserving the Mn/Co ratio and layered structure. Exfoliation of H–Mn_1–<i>x</i>Co_<i>x</i>O₂ was achieved by reaction with tetra–<i>n</i>–butylammonium ions, yielding unilamellar Mn_1–<i>x</i>Co_<i>x</i>O₂ (<i>x</i> = 0.2–0.5) nanosheets with a thickness of 0.8 nm. The optical absorption peak of the obtained Mn_1–<i>x</i>Co_<i>x</i>O₂ nanosheets was continuously blueshifted as the Co content increased. The Mn_1–<i>x</i>Co_<i>x</i>O₂ nanosheets exhibited well-defined redox peaks, which were shifted to a negative potential with increasing Co content. These results suggest that the 3d orbitals of Mn and Co are mixed owing to their statistical distribution in the nanosheets. The Mn_1–<i>x</i>Co_<i>x</i>O₂ nanosheet electrodes showed a capacitance of 700–1000 F g^–1 and improved cycle performance compared to MnO₂ nanosheets

Genuine Unilamellar Metal Oxide Nanosheets Confined in a Superlattice-like Structure for Superior Energy Storage

Two-dimensional (2D) metal oxide nanosheets can exhibit exceptional electrochemical performance owing to their shortened ion diffusion distances, abundant active sites, and various valence states. Especially, genuine unilamellar nanosheets with an atomic-scale thickness are expected to exhibit the ultimate energy storage capability but have not yet achieved their potential. Here, we demonstrate the utilization of genuine unilamellar MnO₂ nanosheets for high-performance Li and Na storage using an alternately stacked MnO₂/graphene superlattice-like structure. Different from previous reports, all unilamellar MnO₂ nanosheets are separated and stabilized between the graphene monolayers, resulting in highly reversible 2D-confined conversion processes. As a consequence, large specific capacities of 1325 and 795 mA h g^–1 at 0.1 A g^–1, high-rate capacities of 370 and 245 mA h g^–1 at 12.8 A g^–1, and excellent cycling stabilities after 5000 cycles with ∼0.004% and 0.0078% capacity decay per cycle were obtained for Li and Na storage, respectively, presenting the best reported performance to date

Highly Enhanced and Switchable Photoluminescence Properties in Pillared Layered Hydroxides Stabilizing Ce³⁺

We have developed pillared layered rare earth hydroxides showing the reversible photoluminescence switching via reducing–oxidizing processes. An air-stable Ce³⁺-based host, Ce₂(OH)₄SO₄·2H₂O, was successfully synthesized via a homogeneous alkalization protocol to precipitate Ce³⁺ ions from a solution of the relevant salt. Structural analysis revealed that the compound consists of cationic layers of {[Ce­(OH)₂(H₂O)]⁺}_∞, linked by sulfate bidentate ligands to construct a layered framework architecture. Tb³⁺ ion was incorporated into this host lattice to form a solid solution across the full compositional range. At an optimized doping of ∼30%, the characteristic green emission was enhanced by ∼20 times, being promoted by the efficient energy transfer from Ce³⁺ to Tb³⁺. The emission could be drastically diminished upon the action of the KMnO₄ oxidizing reagent, which induced the transformation of Ce³⁺ to Ce⁴⁺. Characterizations by X-ray diffraction and X-ray photoelectron spectroscopy showed that the oxidation of Ce³⁺ occurs without degradation of the crystalline framework. The emission could be recovered to its original intensity by the reduction treatment with ascorbic acid. This photoluminescence switching behavior was detectable by the eye and exhibited high reversibility

New Family of Lanthanide-Based Inorganic–Organic Hybrid Frameworks: Ln₂(OH)₄[O₃S(CH₂)_<i>n</i>SO₃]·2H₂O (Ln = La, Ce, Pr, Nd, Sm; <i>n</i> = 3, 4) and Their Derivatives

We report the synthesis and structure characterization of a new family of lanthanide-based inorganic–organic hybrid frameworks, Ln₂(OH)₄[O₃S­(CH₂)_<i>n</i>SO₃]·2H₂O (Ln = La, Ce, Pr, Nd, Sm; <i>n</i> = 3, 4), and their oxide derivatives. Highly crystallized samples were synthesized by homogeneous precipitation of Ln³⁺ ions from a solution containing α,ω-organodisulfonate salts promoted by slow hydrolysis of hexamethylenetetramine. The crystal structure solved from powder X-ray diffraction data revealed that this material comprises two-dimensional cationic lanthanide hydroxide {[Ln­(OH)₂(H₂O)]⁺}_∞ layers, which are cross-linked by α,ω-organodisulfonate ligands into a three-dimensional pillared framework. This hybrid framework can be regarded as a derivative of UCl₃-type Ln­(OH)₃ involving penetration of organic chains into two {LnO₉} polyhedra. Substitutional modification of the lanthanide coordination promotes a 2D arrangement of the {LnO₉} polyhedra. A new hybrid oxide, Ln₂O₂[O₃S­(CH₂)_<i>n</i>SO₃], which is supposed to consist of alternating {[Ln₂O₂]²⁺}_∞ layers and α,ω-organodisulfonate ligands, can be derived from the hydroxide form upon dehydration/dehydroxylation. These hybrid frameworks provide new opportunities to engineer the interlayer chemistry of layered structures and achieve advanced functionalities coupled with the advantages of lanthanide elements

A Superlattice of Alternately Stacked Ni–Fe Hydroxide Nanosheets and Graphene for Efficient Splitting of Water

Cost-effective electrocatalysts based on nonprecious metals for efficient water splitting are crucial for various technological applications represented by fuel cell. Here, 3<i>d</i> transition metal layered double hydroxides (LDHs) with varied contents of Ni and Fe were successfully synthesized through a homogeneous precipitation. The exfoliated Ni–Fe LDH nanosheets were heteroassembled with graphene oxide (GO) as well as reduced graphene oxide (rGO) into superlattice-like hybrids, in which two kinds of oppositely charged nanosheets are stacked face-to-face in alternating sequence. Heterostructured composites of Ni_2/3Fe_1/3 LDH nanosheets and GO (Ni_2/3Fe_1/3-GO) exhibited an excellent oxygen evolution reaction (OER) efficiency with a small overpotential of about 0.23 V and Tafel slope of 42 mV/decade. The activity was further improved <i>via</i> the combination of Ni_2/3Fe_1/3 LDH nanosheets with more conductive rGO (Ni_2/3Fe_1/3-rGO) to achieve an overpotential as low as 0.21 V and Tafel plot of 40 mV/decade. The catalytic activity was enhanced with an increased Fe content in the bimetallic Ni–Fe system. Moreover, the composite catalysts were found to be effective for hydrogen evolution reaction. An electrolyzer cell powered by a single AA battery of 1.5 V was demonstrated by using the bifunctional catalysts

Redox Active Cation Intercalation/Deintercalation in Two-Dimensional Layered MnO₂ Nanostructures for High-Rate Electrochemical Energy Storage

Two-dimensional (2D) layered materials with a high intercalation pseudocapacitance have long been investigated for Li⁺-ion-based electrochemical energy storage. By contrast, the exploration of guest ions other than Li⁺ has been limited, although promising. The present study investigates intercalation/deintercalation behaviors of various metal ions in 2D layered MnO₂ with various interlayer distances, K-birnessite nanobelt (K-MnO₂), its protonated form (H-MnO₂), and a freeze-dried sample of exfoliated nanosheets. Series of metal ions, such as monovalent Li⁺, Na⁺, and K⁺ and divalent Mg²⁺, exhibit reversible intercalation during charge/discharge cycling, delivering high-rate pseudocapacitances. In particular, the freeze-dried MnO₂ of exfoliated nanosheets restacked with the largest interlayer spacing and a less compact 3D network exhibits the best rate capability and a stable cyclability over 5000 cycles. Both theoretical calculation and kinetic analysis reveal that the increased interlayer distance facilitates the fast diffusion of cations in layered MnO₂ hosts. The results presented herein provide a basis for the controllable synthesis of layered nanostructures for high-rate electrochemical energy storage using various single- and multivalent ions

A Superlattice of Alternately Stacked Ni–Fe Hydroxide Nanosheets and Graphene for Efficient Splitting of Water

Cost-effective electrocatalysts based on nonprecious metals for efficient water splitting are crucial for various technological applications represented by fuel cell. Here, 3<i>d</i> transition metal layered double hydroxides (LDHs) with varied contents of Ni and Fe were successfully synthesized through a homogeneous precipitation. The exfoliated Ni–Fe LDH nanosheets were heteroassembled with graphene oxide (GO) as well as reduced graphene oxide (rGO) into superlattice-like hybrids, in which two kinds of oppositely charged nanosheets are stacked face-to-face in alternating sequence. Heterostructured composites of Ni_2/3Fe_1/3 LDH nanosheets and GO (Ni_2/3Fe_1/3-GO) exhibited an excellent oxygen evolution reaction (OER) efficiency with a small overpotential of about 0.23 V and Tafel slope of 42 mV/decade. The activity was further improved <i>via</i> the combination of Ni_2/3Fe_1/3 LDH nanosheets with more conductive rGO (Ni_2/3Fe_1/3-rGO) to achieve an overpotential as low as 0.21 V and Tafel plot of 40 mV/decade. The catalytic activity was enhanced with an increased Fe content in the bimetallic Ni–Fe system. Moreover, the composite catalysts were found to be effective for hydrogen evolution reaction. An electrolyzer cell powered by a single AA battery of 1.5 V was demonstrated by using the bifunctional catalysts

Controllable Fabrication of Amorphous CoNi Pyrophosphates for Tuning Electrochemical Performance in Supercapacitors

Incorporation of two transition metals offers an effective method to enhance the electrochemical performance in supercapacitors for transition metal compound based electrodes. However, such a configuration is seldom concerned in pyrophosphates. Here, amorphous phase CoNi pyrophosphates are fabricated as electrodes in supercapacitors. Through controllably adjusting the ratios of Co and Ni as well as the calcination temperature, the electrochemical performance can be tuned. An optimized amorphous NiCo pyrophosphate exhibits much higher specific capacitance than monometallic Ni and Co pyrophosphates and shows excellent cycling ability. When employing NiCo pyrophosphates as positive electrode and activated carbon as a negative electrode, the fabricated asymmetric supercapacitor cell exhibits favorable capacitance and cycling ability. This study provides facile methods to improve the transition metal pyrophosphate electrodes for efficient electrodes in electrochemical energy storage devices

Osmotic Swelling of Layered Compounds as a Route to Producing High-Quality Two-Dimensional Materials. A Comparative Study of Tetramethylammonium versus Tetrabutylammonium Cation in a Lepidocrocite-type Titanate

Osmotic swelling and exfoliation behaviors in a lepidocrocite-type titanate H_1.07Ti_1.73O₄·H₂O were investigated upon reactions with tetramethylammonium (TMA⁺) and tetrabutylammonium (TBA⁺) cations. The reaction products in various physical states (suspension, wet aggregate, and deposited nanosheets) were characterized by several techniques, including X-ray diffraction under controlled humidity, small-angle X-ray scattering, particle size analysis, and atomic force microscopy. As the ratio of tetraalkylammonium ion in a solution to exchangeable proton in a solid decreased, the predominant product changed from the osmotically swollen phase, having an interlayer spacing <i>d</i> of several tens of nanometers, to the exfoliated nanosheets. The different behaviors of two cations in the osmotic swelling were evident from the slope and the transition point in the <i>d</i> versus <i>C</i>^–1/2 plot, where <i>C</i> is the concentration of the cations. At a short reaction time, crystallites of a few stacks were obtained as a major product in the reaction with TMA⁺. On the other hand, a mixture of those crystallites and a significant portion of unilamellar nanosheets were obtained in the reaction with TBA⁺. In both cases, those stacks were ultimately thinned down at long reaction time to unilamellar nanosheets. The lateral size of the nanosheets could be controlled, depending on the type of the cations, the tetraalkylammonium-to-proton ratios, and the mode of the reaction (manual versus mechanical shaking). The nanosheets produced by TMA⁺ had large lateral sizes up to tens of micrometers, and the suspension showed a distinctive silky appearance based on liquid crystallinity. Our work provides insights into the fundamentals of osmotic swelling and exfoliation, allowing a better understanding of the preparation of nanosheets, which are one of the most important building blocks in nanoarchitectonics

Tuning the Surface Charge of 2D Oxide Nanosheets and the Bulk-Scale Production of Superlatticelike Composites

The surface charge of various anionic unilamellar nanosheets, such as graphene oxide (GO), Ti_0.87O₂^0.52–, and Ca₂Nb₃O₁₀^– nanosheets, has been successfully modified to be positive by interaction with polycations while maintaining a monodispersed state. A dilute anionic nanosheet suspension was slowly added dropwise into an aqueous solution of high molecular weight polycations, which attach on the surface of the anionic nanosheets via electrostatic interaction. Surface modification and transformation to positively charged nanosheets were confirmed by various characterizations including atomic force microscopy and zeta potential measurements. Because the sizes of the polycations used are much larger than the nanosheets, the polymer chains may run off the nanosheet edges and fold to the fronts of the nanosheets, which could be a reason for the continued dispersion of the modified nanosheets in the suspension. By slowly adding a suspension of polycation-modified nanosheets and pristine anionic nanosheet dropwise into water under suitable conditions, a superlatticelike heteroassembly can be readily produced. Characterizations including transmission electron microscopy and X-ray diffraction measurements provide evidence for the formation of the alternately stacked structures. This approach enables the combination of various pairs of anionic nanosheets with different functionalities, providing a new opportunity for the creation of unique bulk-scale functional materials and their applications