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

Electronic Band Structure of Exfoliated Titanium- and/or Niobium-Based Oxide Nanosheets Probed by Electrochemical and Photoelectrochemical Measurements

Exfoliated two-dimensional (2D) unilamellar nanosheets of Ca₂Nb₃O₁₀^–, TiNbO₅^–, Ti₂NbO₇^–, and Ti₅NbO₁₄^3– were deposited layer-by-layer to produce multilayer films on indium–tin–oxide (ITO)-coated glass electrodes, and their electrochemical and photoelectrochemical properties were explored. The layer-by-layer assembly process via sequential adsorption with counter polycations was monitored by UV–visible absorption spectra and X-ray diffraction measurements, which confirmed the successful growth of films, where nanosheets and polycations are alternately stacked at a separation of 1.6–2.4 nm. Exposure to UV light totally removed polycations, producing inorganic films. Cyclic voltammetry on Ti and/or Nb oxide nanosheet electrodes thus fabricated showed reduction/oxidation (Ti³⁺/Ti⁴⁺ and Nb⁴⁺/Nb⁵⁺) peaks associated with insertion/extraction of Li⁺ ions into/from intersheet galleries of the films. The extent of the redox reaction is found to be governed by the cation density in the nanosheet gallery. Anodic photocurrents of the oxide nanosheet electrodes were observed under UV light irradiation. These action spectra showed close resemblance to optical absorption profiles of the colloidal nanosheets, indicating that the photocurrent was generated from the nanosheets. Their analysis indicates that the nanosheets of Ca₂Nb₃O₁₀^–, TiNbO₅^–, Ti₂NbO₇^–, and Ti₅NbO₁₄^3– are all indirect transition-type wide-gap semiconductors with bandgap energies of 3.44, 3.68, 3.64, and 3.53 eV, respectively. These values are larger than those for corresponding parent layered oxide compounds before delamination, suggesting confinement effects into 2D nanosheet structure. Furthermore, the value was invariable for the films with a different number of nanosheet layers, indicating that quantized nanosheets were electronically isolated with each other. In addition, photocurrent generation was measured as a function of applied electrode potential, and the flatband potential was estimated from the photocurrent onset values as −1.12, −1.33, −1.30, and −1.29 V vs Ag/Ag⁺, for Ca₂Nb₃O₁₀^–, TiNbO₅^–, Ti₂NbO₇^–, and Ti₅NbO₁₄^3– nanosheets, respectively, providing a diagram of electronic band structure for the nanosheets

Bulk Functional Materials Design Using Oxide Nanosheets as Building Blocks: A New Upconversion Material Fabricated by Flocculation of Ca₂Nb₃O₁₀^– Nanosheets with Rare-Earth Ions

A new upconversion (UC) material was designed by flocculating a Ca₂Nb₃O₁₀^– nanosheet, which acts as thermal and structural stabilizer, with Ho³⁺ photoactivator, Yb³⁺ sensitizer, and Y³⁺ space filler. The flocculated product consists of the restacked nanosheets and the rare-earth ions in the internanosheet gallery. The restacked sheet faces of the Ca₂Nb₃O₁₀^– nanosheet building blocks are self-organized in a parallel manner, and their crystallographic coherency extends to three layers on average. On the other hand, the nanosheet building blocks are randomly staggered along the in-layer direction. Chemical composition of the flocculated product was estimated as (Ho_0.096Yb_0.23Y_0.164)­Ca_1.76□_0.24Nb₃O₁₀·1.4H₂O. Heat treatment of the flocculated product at 500 °C was necessary in order to suppress nonradiative energy loss via OH vibration and to induce UC emission. Even after the heat treatment, perovskite-type atomic arrangement of the Ca₂Nb₃O₁₀^– nanosheet building block was retained. Upon laser irradiation at 980 nm, two UC emission bands around 550 and 660 nm were observed, and the emission was visible to the eye. The result from this study suggests that flocculation of nanosheets, as building blocks, with counterions is a promising way to design bulk functional materials that are rather difficult or impossible to prepare by conventional synthetic approaches

High Thermal Robustness of Molecularly Thin Perovskite Nanosheets and Implications for Superior Dielectric Properties

A systematic study has been conducted to examine the thermal stability of layer-by-layer assembled films of perovskite-type nanosheets, (Ca₂Nb₃O₁₀^–)_<i>n</i> (<i>n</i> = 1–10), which exhibit superior dielectric and insulating properties. In-plane and out-of-plane X-ray diffraction data as well as observations by atomic force microscopy and transmission electron microscopy indicated the high thermal robustness of the nanosheet films. In a monolayer film with an extremely small thickness of ∼2 nm, the nanosheet was stable up to 800 °C, the temperature above which segregation into CaNb₂O₆ and Ca₂Nb₂O₇ began. The critical temperature moderately decreased as the film thickness, or the number of nanosheet layers, increased, and reached 700 °C for seven- and 10-layer films, which is comparable to the phase transformation temperature for a bulk phase of the protonic layered oxide of HCa₂Nb₃O₁₀·1.5H₂O as a precursor of the nanosheet. This thermal stabilization of perovskite-type nanosheets should be associated with restricted nucleation and crystal growth peculiar to such ultrathin 2D bound systems. The stable high-<i>k</i> dielectric response (ε_r = 210) and highly insulating nature (<i>J</i> < 10^–7 A cm^–2) remained substantially unchanged even after the nanosheet film was annealed up to 600 °C. This study demonstrates the high thermal stability of 2D perovskite-type niobate nanosheets in terms of structure and dielectric properties, which suggests promising potential for future high-<i>k</i> devices operable over a wide temperature range

Correction to “Synthesis and Atomic Characterization of a Ti₂O₃ Nanosheet”

Correction to “Synthesis and Atomic Characterization of a Ti₂O₃ Nanosheet

Synthesis and Atomic Characterization of a Ti₂O₃ Nanosheet

Titanium oxide nanosheets have been attracting much attention owing to their photocatalytic property. Here, we synthesized a Ti₂O₃ nanosheet by the reduction of a titania nanosheet (Ti_0.87O₂) that was one or two atoms in thickness. The atomic structure of the Ti₂O₃ nanosheet was quantitatively revealed by electron diffraction analysis, electron energy-loss spectroscopy, and high-resolution transmission electron microscopy (TEM). A titania nanosheet (Ti_0.87O₂) consisting of edge-shared TiO₆ octahedra was transformed to a Ti₂O₃ nanosheet consisting of face-shared octahedra by electron beam irradiation. This represents a stable crystal phase of titania nanosheets like the Magneli phase in oxygen-deficient environments. The atomic arrangement of the Ti₂O₃ nanosheet was directly observed by newly developed aberration-corrected TEM

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

Atomic Layer Engineering of High‑κ Ferroelectricity in 2D Perovskites

Complex perovskite oxides offer tremendous potential for controlling their rich variety of electronic properties, including high-<i>T</i>_C superconductivity, high-κ ferroelectricity, and quantum magnetism. Atomic-scale control of these intriguing properties in ultrathin perovskites is an important challenge for exploring new physics and device functionality at atomic dimensions. Here, we demonstrate atomic-scale engineering of dielectric responses using two-dimensional (2D) homologous perovskite nanosheets (Ca₂­Na_<i>m</i>–3­Nb_<i>m</i>­O_3<i>m</i>+1; <i>m</i> = 3–6). In this homologous 2D material, the thickness of the perovskite layers can be incrementally controlled by changing <i>m</i>, and such atomic layer engineering enhances the high-κ dielectric response and local ferroelectric instability. The end member (<i>m</i> = 6) attains a high dielectric constant of ∼470, which is the highest among all known dielectrics in the ultrathin region (<10 nm). These results provide a new strategy for achieving high-κ ferroelectrics for use in ultrascaled high-density capacitors and post-graphene technology