Guidelines for Arranging 2D Nanosheets into Neatly Tiled Monolayer Films by a Spin-Coating Process

Neat (dense and nonoverlapped) monolayer tiling of 2D nanosheets on a substrate surface is very important because we can conduct artificial lattice-engineering by repeating the tiling process in a designed sequence to tailor various hierarchical nanostructures, leading to a range of sophisticated functions. It is recently reported that a facile spin-coating technique realizes the neat monolayer tiling of various 2D nanosheets. Establishing universal guidelines to neatly tile 2D nanosheets on substrates of various materials and size/shape is of essential importance to fully apply this technique, but the mechanism of how the nanosheets are tiled has not been clarified yet. In the present study, we have systematically examined the nanosheet deposition process at various rotation speeds by microscopic observations and found that the neat monolayer tiling of nanosheets is attained on the solvent surface during the spin-coating, and then the monolayer film is deposited onto the substrate surface from its center toward the edges upon evaporation of the solvent. Furthermore, we have clarified how the rotation speed and the nanosheet concentration govern the deposition behaviors in terms of neat tiling, overlap, or noncoverage in a such process. On the basis of the guidelines, we can predict the optimum spin-coating conditions for attaining the neat monolayer tiling of various nanosheets over an entire surface of the substrate

Exploration of Mid-Temperature Alkali-Metal-Ion Extraction Route Using PTFE (AEP): Transformation of α-NaFeO₂-Type Layered Oxides into Rutile-Type Binary Oxides

Alkali-metal-ion extraction reactions using poly­(tetrafluoroethylene) (PTFE; AEP reactions) were performed on two kinds of α-NaFeO₂-type layered compounds: Na_0.68(Li_0.68/3Ti_1–0.68/3)­O₂ and K_0.70(Li_0.70/3Sn_1–0.70/3)­O₂. At 400 °C in flowing argon, these layered compounds were reacted with PTFE. By these reactions, alkali-metal ions in the layered compounds were successfully extracted, and TiO₂ and SnO₂ with rutile-type structure were formed. The structural similarity between the alkali-metal-ion-extracted layered compounds and the binary metal oxide products in these unique alkali-metal-ion extraction reactions was interpreted in terms of their interatomic distance distribution by atomic pair distribution function analysis. The results of this study indicate that PTFE is an effective agent to extract alkali-metal ions from layered compounds, and AEP reaction is not limited to the previously reported γ-FeOOH-type layered titania K_0.8(Li_0.27Ti_1.73)­O₄, but is also applicable to other layered titania and other non-titanium-based layered metal oxides. Therefore, it was clarified that AEP reactions are widely applicable routes to prepare various compounds, including those that are difficult to synthesize by other reactions

Modulation of Photochemical Activity of Titania Nanosheets via Heteroassembly with Reduced Graphene Oxide. Enhancement of Photoinduced Hydrophilic Conversion Properties

The heteroassembly of two-dimensional (2D) nanosheets has attracted rapidly increasing attention for designing new materials and nanodevices, in which the properties of the individual components can be modulated through the concerted interaction between the different 2D nanosheets. Here, we report on the layer-by-layer integration of photofunctional titania nanosheets and conductive reduced graphene oxide (rGO) to enhance the photochemical activity of the titania nanosheets. Heteroassembled films were fabricated by sequentially assembling graphene oxide (GO) and titania nanosheets with a cationic polymer and subsequently exposing to UV light to reduce the GO. The films showed an accelerated photoinduced hydrophilic conversion, the rate of which was 2.8 times higher than that of a film solely of the titania nanosheets. This behavior indicates that the rGO worked as an electron transfer mediator and improved the photoinduced charge separation efficiency. The intimate contact between two different 2D nanosheets promotes the efficient utilization of photogenerated carriers

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

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

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

Synthesis and In Situ X‑ray Diffraction Characterization of Two-Dimensional Perovskite-Type Oxide Colloids with a Controlled Molecular Thickness

A series of two-dimensional (2D) niobate nanosheets with a thickness controllable in increments of 0.4–0.5 nm were synthesized by chemically delaminating precursory layered perovskites into their unilamellar layers. The homologous layered compounds of KCa₂Na_<i>n</i>–3Nb_<i>n</i>O_3<i>n</i>+1 for <i>n</i> = 4–6 were prepared as starting materials by repeated solid-state calcination of KCa₂Nb₃O₁₀ (<i>n</i> = 3 analog) and NaNbO₃ at 1573 K. These compounds were then converted into protonic forms and were further reacted with a tetrabutylammonium hydroxide solution to yield a translucent colloidal suspension. Nearly perfect delamination was confirmed by in situ X-ray diffraction (XRD) measurements on glue-like colloids centrifuged from the suspension, which showed total loss of basal diffraction series and an evolution of a broad, wavy pattern instead. The characteristic oscillating profile was dependent on <i>n</i>, or the layer thickness, and could be consistently reproduced by simulation in terms of X-ray scattering from the individual 2D fundamental unit of perovskite-type oxides or diffraction from ultrathin crystallites with a defined repeating (<i>n</i> times) unit composed of NbO₆ octahedra and Ca/Na. Atomic force microscopy (AFM) observation of samples deposited on a Si substrate detected micrometer-sized sheets with a thickness of 2.7 nm (<i>n</i> = 4), 3.1 nm (<i>n</i> = 5), and 3.6 nm (<i>n</i> = 6), results that are compatible with the dimensions for <i>n</i> of corner-shared octahedra. In addition, in-plane XRD data showed sharp lines attributable to a 2D square lattice (<i>a</i> = 0.39 nm) of perovskite structures. These results along with chemical analysis data indicate the formation of a novel class of 2D perovskite oxides, Ca_2−δ(Na,K)_{<i>n</i>−3+δ}Nb_<i>n</i>O_3<i>n</i>+1^– (<i>n</i> = 4–6), with a progressively increasing thickness by 0.4–0.5 nm. These nanosheets showed intense absorption of ultraviolet (UV) light due to their semiconducting nature. The bandgap energy was estimated as 3.96, 3.81, and 3.77 eV, being dependent on <i>n</i>, which may reflect the relaxing degree of size quantization with the nanosheet thickness. Aggregated nanosheets flocculated with a solution containing KCl and trace amounts of RuCl₃ were heated at 773 K to produce photocatalytic materials loaded with RuO₂ as a cocatalyst. The resulting samples underwent water splitting under UV irradiation. The evolution of H₂ and O₂ gas in a 2:1 ratio proceeded on the material from the <i>n</i> = 4 nanosheet, while deviation from stoichiometric decomposition as well as deterioration of activity were observed for the samples from the thicker nanosheets

Soft-Chemical Exfoliation of RbSrNb₂O₆F into Homogeneously Unilamellar Oxyfluoride Nanosheets

Interlayer Rb⁺ of the perovskite-type layered oxyfluoride RbSrNb₂O₆F was ion-exchanged with H⁺, and the protonated phase was reacted with aqueous solution of tetrabutylammonium hydroxide to exfoliate it into nanosheets. The resulting nanosheet suspension exhibits Tyndall scattering of a laser beam, indicating its colloidal nature. Elemental composition of the nanosheet was estimated as Sr_0.98Nb₂O₆F_0.97^δ−, which was quite close to that of the layer unit of the precursor. The homogeneously unilamellar nature of this nanosheet was confirmed by atomic force and transmission electron microscopy observations and X-ray scattering results. The optical absorption edge of the nanosheet suspension was observed around at 293 nm, and two well-defined peaks with their maxima at 229 and 278 nm were observed. Furthermore, the aqueous suspension of the nanosheet exhibits fluorescence emission in the UV-blue region. These properties of the oxyfluoride nanosheets are quite different from those of its oxide analogues without F^–, such as LnNb₂O₇^– nanosheets (Ln = La³⁺, Eu³⁺, Sm³⁺), suggesting that anion-site replacement of oxide nanosheets can be utilized to optimize or induce various properties

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

RbBiNb₂O₇: A New Lead-Free High‑<i>T</i>_c Ferroelectric

RbBiNb₂O₇: A New Lead-Free High‑<i>T</i>_c Ferroelectri