Stress-Induced Photoluminescence Change of Monolayer Nanosheet Prepared by Delamination of Aurivillius-Phase Layered Perovskite

A comprehensive investigation was performed on the photoluminescence properties of monolayer Bi3+-substituted ATa2O72– (A = Ca, Sr, Bi0.5Na0.5). The Bi3+ center in the A–Ta–O perovskite framework functioned as a luminescence center (λmax = 470–520 nm) under UV-light illumination (λex = 250–340 nm). Comparison of the photoluminescence spectra of Bi3+ in several types of layered ATa2O72– structures, including Bi2ATa2O9 ((Bi2O2)(ATa2O7)), protonated Bi2ATa2O9 (H2ATa2O7·xH2O), and free-standing ATa2O72– nanosheet membranes, revealed that the positions of the emission and excitation bands were greatly influenced by the interlayer cation species. The results implied that the photoluminescence properties of Bi3+ in ATa2O72– were sensitive to changes in the electric field surrounding Bi3+ because of the ultimate thinness of the monolayer ATa2O72– nanosheets (1.1–1.2 nm). To further explore the relationship between mechanical stress in Bi3+-substituted ATa2O72– nanosheets and the emission band of Bi3+, changes in the photoluminescence and Raman spectra in response to the curvature radius of free-standing ATa2O72– nanosheet membranes were compared. The present study confirmed that two-dimensional nanomaterials could help further the understanding of mechanochromic phenomena in Bi-containing inorganic materials

Self-Ordering of Disorderly Arranged 2D Crystal Layers to 3D Regular Arrangement Using a Heat-Induced Chemical Reaction between 2D Crystal Layers

Two-dimensional (2D) materials with a thickness of ∼1 nm are candidate nanobuilding blocks to fabricate electronic devices with a three-dimensional (3D) structure using a bottom-up technology. They can be stacked in a precisely controlled hierarchical structure with a controlled number of building layers. However, the atomic arrangements between individual stacked 2D crystal layers are generally not ordered as in a single crystal. The interface and the disordered atomic arrangements result in decrease in the performance of electronic devices prepared from 2D crystals, because the electron flow between 2D crystals is blocked by the interface and the disordered atomic arrangements. Therefore, ordered atomic arrangement of the stacked layers is one of the most critical challenges in the preparation of 3D electronic devices from 2D materials. Here, a successful example of self-ordering of disorderly arranged 2D crystal layers to 3D regular arrangement is described. The multilayer films of nickel hydroxide 2D crystal with a thickness of one NiO₆ octahedral unit was focused as the disorderly arranged 2D crystal layers. The 2D layered films deposited on a substrate were heated to 400 °C. This heat treatment converted the disordered 2D system to ordered 3D NiO with (111)-orientation. The heat-induced chemical reaction between 2D materials allowed the disordered layers to self-order to 3D regular arrangement. The NiO film exhibited a photocathodic current assigned to reduction of water, and then the photocurrent increased with increasing the number of layers. The improvement of the photocurrent property is due to the ordered atomic arrangements without interface

Doped CeO₂–LaFeO₃ Composite Oxide as an Active Anode for Direct Hydrocarbon-Type Solid Oxide Fuel Cells

Direct utilization of hydrocarbon and other renewable fuels is one of the most important issues concerning solid oxide fuel cells (SOFCs). Mixed ionic and electronic conductors (MIECs) have been explored as anode materials for direct hydrocarbon-type SOFCs. However, electrical conductivity of the most often reported MIEC oxide electrodes is still not satisfactory. As a result, mixed-conducting oxides with high electrical conductivity and catalytic activity are attracting considerable interest as an alternative anode material for noncoke depositing anodes. In this study, we examine the oxide composite Ce(Mn,Fe)O2–La(Sr)Fe(Mn)O3 for use as an oxide anode in direct hydrocarbon-type SOFCs. High performance was demonstrated for this composite oxide anode in direct hydrocarbon-type SOFCs, showing high maximum power density of approximately 1 W cm–2 at 1073 K when propane and butane were used as fuel. The high power density of the cell results from the high electrical conductivity of the composite oxide in hydrocarbon and the high surface activity in relation to direct hydrocarbon oxidation

Controlling Thickness of Aurivillius Phase Perovskite Nanosheets Obtained by Delaminating Bi₂SrNa_<i>n</i>–2Nb_<i>n</i>O_3<i>n</i>+3 (<i>n</i> = 2–5) Layered Crystal

A monolayer Bi-doped SrNan–2NbnO3n+12– (n = 2, 3, 4, 5) nanosheet was obtained by delaminating an Aurivillius-phase layered perovskite Bi2SrNan–2NbnO3n+3. The thickness of the Bi-doped SrNan‑2NbnO3n+12– nanosheet was successfully controlled by changing the n values of the precursor layered crystals by using a simple calcination method. The mechanical property of the Bi-doped SrNan–2NbnO3n+12– nanosheets was evaluated by comparing phase delay by AFM measurement, which indicated that the thicker perovskite crystal was more perpendicularly deformable. HAADF-STEM observation confirmed that the A site of the Bi-doped SrNa2Nb4O132– perovskite nanosheet was partially occupied by Bi. EDS study also proved that 9–13% of the A site in the Bi-doped SrNan–2NbnO3n+12– nanosheet was occupied with Bi. Further investigation on the crystal structure of the Bi-doped SrNan–2NbnO3n+12– nanosheet was performed with in-plane XRD patterns, Raman spectra, and FE-SEM images. The band gap energy, flat-band potential, and photoelectrocatalytic activity for oxidation of methanol were studied with (photo)electrochemical measurements and correlated to the number of n values

pH Dependence of the Photoluminescence of Eu³⁺-Intercalated Layered Titanium Oxide

We investigated the pH dependence of Eu3+ emissions from Eu3+-intercalated layered titanium oxide (Eu/TiO) and evaluated the local structure of the intercalated Eu3+ at varying pH values using lifetime measurements and EXAFS. A red Eu3+ emission was observed under radiation by UV light with a higher energy than the band gap of the host TiOx layer. The emission is based on energy transfer from the host TiOx layer to Eu3+. The emission intensity of Eu/TiO in 0.01 M NaOH aqueous solution was stronger than that in 0.01 M HCl aqueous solution, and the emission response of the Eu/TiO film was relatively stable to pH cycling. Two phenomena may provide a mechanism for the change in emission from Eu/TiO. One is a change in the efficiency of energy transfer from the TiOx nanosheet to Eu3+, and the other is a fine hydration state change of Eu3+ without a change in the total water amount

Synthesis of Hexagonal Nickel Hydroxide Nanosheets by Exfoliation of Layered Nickel Hydroxide Intercalated with Dodecyl Sulfate Ions

Synthesis of Hexagonal Nickel Hydroxide Nanosheets by Exfoliation of Layered Nickel Hydroxide Intercalated with Dodecyl Sulfate Ion

Syntheses of Zinc Oxide and Zinc Hydroxide Single Nanosheets

In this study, we present the preparation of zinc oxide (ZnO) and zinc hydroxide (Zn(OH)2) single nanosheets for the first time. ZnO nanosheet was prepared by delamination of layered ZnO film intercalated with the dodecyl sulfate (DS−) ion which was synthesized by cathodic electrodeposition process. In this case, the addition of La3+ ion into the electrolyte was very critical for the further delamination process. On the other hand, Zn(OH)2 nanosheet was prepared by delamination of layered Zn(OH)2 intercalated with the DS− ion that was prepared by a soft solution process using hexamethylenetetramine (HMT). The thicknesses of the ZnO and Zn(OH)2 nanosheets were about 0.7 and 1.0 nm, respectively. The structures of the nanosheets were discussed with the results of transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images

Synthesis and Photocatalytic Activity of Rhodium-Doped Calcium Niobate Nanosheets for Hydrogen Production from a Water/Methanol System without Cocatalyst Loading

Rhodium-doped calcium niobate nanosheets were synthesized by exfoliating layered KCa2Nb3–xRhxO10−δ and exhibited high photocatalytic activity for H2 production from a water/methanol system without cocatalyst loading. The maximum H2 production rate of the nanosheets was 165 times larger than that of the parent Rh-doped layered oxide. The quantum efficiency at 300 nm was 65%. In this system, the methanol was oxidized to formaldehyde (main product), formic acid, and carbon dioxide by holes, whereas electrons cause the reduction of water to H2. The conductivity of the parent layered oxide was decreased by doping, which indicates the octahedral RhO6 unit in the lattice of the nanosheet functions as an electron trap site. The RhO6 units in the nanosheet probably also act as reaction sites for H2 evolution

N Doping of Oxide Nanosheets

N Doping of Oxide Nanosheet

Potential Gradient and Photocatalytic Activity of an Ultrathin p–n Junction Surface Prepared with Two-Dimensional Semiconducting Nanocrystals

The creation of p–n junction structure in photocatalysts is a smart approach to improve the photocatalytic activity, as p–n junctions can potentially act to suppress the recombination reaction. Understanding the surface conditions of the junction parts is one of the biggest challenges in the development of photocatalyst surface chemistry. Here, we show a relationship between the photocatalytic activity and potential gradient of the junction surface prepared from two-dimensional crystals of p-type NiO and n-type calcium niobate (CNO). The ultrathin (ca. 2 nm) junction structure and the surface potential were analyzed using low energy ion scattering spectroscopy and Kelvin probe force microscopy. The photocatalytic H2 production rate for the n–p (CNO/NiO) junction surface was higher than those for p–n (NiO/CNO) junction, p, and n surfaces. The surface potential of the CNO/NiO junction part (surface: CNO) was lower than that of the CNO crystals in the same CNO crystal surface. These potential gradients result in specially separated reaction sites, which suppress the recombination reaction in the CNO nanosheet. Photo-oxidation and photoreduction sites in the junction structure were confirmed using the photodeposition reaction of MnOx and Ag