Mechanical Properties of CaO–Al2O3–SiO2 Glass-Ceramics Precipitating Hexagonal CaAl2Si2O8 Crystals

Fracture behavior via a flexural test for a newly found CaO–Al2O3–SiO2 (CAS) glass-ceramic (GC) was compared with that of enstatite GC and mica GC, which are well-known GCs with high-fracture toughness and machinability, respectively. By focusing on the nonelastic load–displacement curves, CAS GC was characterized as a less brittle material similar to machinable mica GC, compared with enstatite GC, which showed higher fracture toughness, KIC. The microcrack toughening mechanism in CAS GC was supported by the nondestructive observation of microcracks around the Vickers indentation using the X-ray microcomputed tomography technique. The CAS GC also showed higher transparency than mica GC due to its low crystallinity. Moreover, the precursor glass had easy formability due to its low-liquidus temperature

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

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

Unusually stable similar to 100-fold reversible and instantaneous swelling of inorganic layered materials

Cells can swell or shrink in certain solutions; however, no equivalent activity has been observed in inorganic materials. Although lamellar materials exhibit increased volume with increase in the lamellar period, the interlamellar expansion is usually limited to a few nanometres, with a simultaneous partial or complete exfoliation into individual atomic layers. Here we demonstrate a large monolithic crystalline swelling of layered materials. The gallery spacing can be instantly increased similar to 100-fold in one direction to similar to 90 nm, with the neighbouring layers separated primarily by H2O. The layers remain strongly held without peeling or translational shifts, maintaining a nearly perfect three-dimensional lattice structure of >3,000 layers. First-principle calculations yield a long-range directional structuring of the H2O molecules that may help to stabilize the highly swollen structure. The crystals can also instantaneously shrink back to their original sizes. These findings provide a benchmark for understanding the exfoliating layered materials

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