Multiemission of Ce3+ from a Single Crystallographic Site Induced by Disordering of Ions.

White-light emission with a single activator is an attractive function of phosphors. In this work, we investigated the photoluminescence properties of Ca5.7Y1.3Si7O16.7N3.3, which is a compound denoted as Ca4+xY3-xSi7O15+xN5-x discovered by our group, with Ce-activation using optical measurements and density functional theory (DFT) calculation. Samples showed a tunable emission from purple to white under ultraviolet (UV) light. In this compound, Ca and Y as well as anions are distributed disorderly, and Ca/Y ions occupy two crystallographically distinct sites; those sites are possible sites for Ce substitution. DFT calculation and structural refinement revealed that the tunable emission was generated by Ce at the crystallographically equivalent site but with distinct local structures caused by the disordering of cations and anions. As far as we know, this is the first report about a white-light-emitting phosphor with only Ce activation.

Mechanism investigation of the enhanced oxygen storage performance of YBaCo4O7+δ synthesized by a glycine-complex decomposition method.

A YBaCo4O7 oxygen storage material has been synthesized by the glycine-complex decomposition method at a low temperature of 800 °C and its crystal structure and reaction kinetics were investigated. This sample showed the highest storage/release speed among all the reported YBaCo4O7+δ materials.

Optimizing TiO2 through Water-Soluble Ti Complexes as Raw Material for Controlling Particle Size and Distribution of Synthesized BaTiO3 Nanocubes.

Barium titanate (BaTiO3) nanocubes with a narrow particle size distribution were synthesized using a three-step approach. First, a water-soluble Ti complex was synthesized using a hydrolysis method. Next, the titanium dioxide (TiO2) raw material was synthesized via a hydrothermal method using various water-soluble titanium (Ti) complexes. The TiO2 exhibited various particle sizes and crystal structures (anatase, rutile, or brookite) depending on the water-soluble Ti complex and the hydrothermal conditions used in its synthesis. Finally, BaTiO3 nanocubes were subsequently created through a hydrothermal method using the synthesized TiO2 particles and barium hydroxide octahydrate [Ba(OH)2·8H2O] as raw materials. The present study clarifies that the particle size of the BaTiO3 nanocubes depends on the particle size of the TiO2 raw material. BaTiO3 particles with a narrow size distribution were obtained when the TiO2 particles exhibited a narrow size distribution. We found that the best conditions for the creation of BaTiO3 nanocubes using TiO2 involved using lactic acid as a complexing agent, which resulted in a particle size of 166 nm on average. This particle size is consistent with an average of the width of the cubes measured from corner to corner diagonally, which corresponds to a side length of 117 nm. In addition, surface reconstruction of the BaTiO3 was clarified via electron microscopy observations, identifying the outermost surface as a Ti layer. Electron tomography using high-angle annular dark-field (HAADF)-scanning transmission electron microscopy (STEM) confirmed the three-dimensional (3D) structure of the obtained BaTiO3 nanocubes.

Bottom-up method for synthesis of layered lithium titanate nanoplates using ion precursor.

A facile bottom-up method for the synthesis of lithium titanate nanoplates using a peroxo titanium complex ion precursor is reported. Instead of employing complicated treatment with high alkali concentration, the self-organization reaction between lithium and titanium ions in the prepared ion precursor can enable the formation of layered lithium titanate crystals (Li2-xHxTi2O5, where x = 0.1 and 1.52 for as-synthesise and acid-treated samples, respectively) under low alkaline conditions. We demonstrate that layered lithium titanate crystals can be grown anisotropically into individual nanoplates. Our work presents an easy and useful platform for the production of titanate materials with various morphologies based on the interaction with ionic species.

Improvement of the Oxygen Storage/Release Speed of YBaCo4O7+δ Synthesized by a Glycine-Complex Decomposition Method.

The present study explores the oxygen storage capacity of YBaCo4O7+δ prepared by a glycine-complex decomposition method. We reported for the first time that the YBaCo4O7+δ sample was successfully synthesized at such a low temperature of 800 °C by this method. The YBCO-800 N sample exhibited a faster oxygen absorption/desorption speed than that of high calcination temperature samples, and the time required for complete oxygen storage/release was 5 and 6 min at 360 °C, respectively. Moreover, the superior performance observed for this product in the temperature swing adsorption process makes it a promising candidate in oxygen production technologies. This research demonstrated that the glycine-complex decomposition method is an effective method for improving the oxygen storage property of YBaCo4O7+δ and provides a new insight into designing other novel oxygen storage materials.

Stabilization of Size-Controlled BaTiO3 Nanocubes via Precise Solvothermal Crystal Growth and Their Anomalous Surface Compositional Reconstruction.

Crystal growth of barium titanate (BaTiO3) using a wet chemical reaction was investigated at various temperatures. BaTiO3 nanoparticles were obtained at an energy-efficient temperature of 80 °C. However, BaTiO3 nanocubes with a preferred size and shape could be synthesized using a solvothermal method at 200 °C via a reaction involving titanium tetraisopropoxide [(CH3)2CHO]4Ti for nucleation and fine titanium oxide (TiO2) nanoparticles for crystal growth. The BaTiO3 nanocubes showed a high degree of dispersion without the use of dispersants or surfactants. The morphology of BaTiO3 was found to depend on the reaction medium. The size of the BaTiO3 particles obtained using water as the reaction medium was the largest among the particles synthesized using various reaction media. In the case of alcohol reaction media, the BaTiO3 particle size increased in the order methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol. Furthermore, BaTiO3 powder obtained using alcohol reaction media resulted in cubic shapes as opposed to the round shapes obtained when water was used as the medium. We found that the optimal condition for the synthesis of BaTiO3 nanocubes involved the use of 1-butanol as the reaction medium, resulting in an average particle size of 52 nm, which is the average distance of the cubes measured diagonally from corner to corner, and gives an average side length of 37 nm, and a tetragonal crystal system as evidenced by the powder X-ray diffraction pattern obtained using high-energy synchrotron X-rays. The origin of the spontaneous polarization of the BaTiO3 tetragonal crystal structure was clarified by a pair distribution function analysis. In addition, surface reconstruction of BaTiO3 nanocubes led to an outermost surface comprising two layers of Ti columns.

Effects of Annealing Temperature on the Crystal Structure, Morphology, and Optical Properties of Peroxo-Titanate Nanotubes Prepared by Peroxo-Titanium Complex Ion.

This study addresses the effects of annealing temperatures (up to 500 °C) on the crystal structure, morphology, and optical properties of peroxo groups (-O-O-) containing titanate nanotubes (PTNTs). PTNTs, which possess a unique tubular morphology of layered-compound-like hydrogen titanate structure (approximately 10 nm in diameter), were synthesized using peroxo-titanium (Ti-O-O) complex ions as a precursor under very mild conditions-temperature of 100 °C and alkali concentration of 1.5 M-in the precursor solution. The nanotubular structure was dismantled by annealing and a nanoplate-like structure within the range of 20-50 nm in width and 100-300 nm in length was formed at 500 °C via a nanosheet structure by decreasing the specific surface area. Hydrogen titanate-based structures of the as-synthesized PTNTs transformed directly into anatase-type TiO2 at a temperature above 360 °C due to dehydration and phase transition. The final product, anatase-based titania nanoplate, was partially hydrogen titanate crystal in nature, in which hydroxyl (-OH) bonds exist in their interlayers. Therefore, the use of Ti-O-O complex ions contributes to the improved thermal stability of hydrogen titanate nanotubes. These results show a simple and environmentally friendly method that is useful for the synthesis of functional nanomaterials for applications in various fields.

Two-Dimensional Perovskite Oxynitride K2 LaTa2 O6 N with an H+ /K+ Exchangeability in Aqueous Solution Forming a Stable Photocatalyst for Visible-Light H2 Evolution.

Undoped layered oxynitrides have not been considered as promising H2 -evolution photocatalysts because of the low chemical stability of oxynitrides in aqueous solution. Here, we demonstrate the synthesis of a new layered perovskite oxynitride, K2 LaTa2 O6 N, as an exceptional example of a water-tolerant photocatalyst for H2 evolution under visible light. The material underwent in-situ H+ /K+ exchange in aqueous solution while keeping its visible-light-absorption capability. Protonated K2 LaTa2 O6 N, modified with an Ir cocatalyst, exhibited excellent catalytic activity toward H2 evolution in the presence of I- as an electron donor and under visible light; the activity was six times higher than Pt/ZrO2 /TaON, one of the best-performing oxynitride photocatalysts for H2 evolution. Overall water splitting was also achieved using the Ir-loaded, protonated K2 LaTa2 O6 N in combination with Cs-modified Pt/WO3 as an O2 evolution photocatalyst in the presence of an I3 - /I- shuttle redox couple.

Anisotropy in Stable Conformations of Hydroxylate Ions between the {001} and {110} Planes of TiO2 Rutile Crystals for Glycolate, Lactate, and 2-Hydroxybutyrate Ions Studied by Metadynamics Method.

Control over TiO2 rutile crystal growth and morphology using additives is essential for the development of functional materials. Computer simulation studies on the thermodynamically stable conformations of additives at the surfaces of rutile crystals contribute to understanding the mechanisms underlying this control. In this study, a metadynamics method was combined with molecular dynamics simulations to investigate the thermodynamically stable conformations of glycolate, lactate, and 2-hydroxybutyrate ions at the {001} and {110} planes of rutile crystals. Two simple atom-atom distances were selected as collective variables for the metadynamics method. At the {001} plane, a conformation in which the COO- group was oriented toward the surface was found to be the most stable for the lactate and 2-hydroxybutyrate ions, whereas a conformation in which the COO- group was oriented toward water was the most stable for the glycolate ion. At the {110} plane, a conformation in which the COO- group was oriented toward the surface was the most stable for all three hydroxylate ions, and a second most stable conformation was also observed for the lactate ion at positions close to the {110} plane. For all three hydroxylate ions (α-hydroxycarboxylate ions), the stability of the most stable conformation was higher for the {110} plane than for the {001} plane. At both planes, the stability of the most stable conformation was highest for the 2-hydroxybutyrate ion and lowest for the glycolate ion. Supposing that all three hydroxylate ions serve to decrease the surface free energy at the rutile surface and that a more stable conformation at the rutile surface leads to a greater decrease in the surface free energy, the present results partially explain experimentally observed differences in the changes in growth rate and morphology of rutile crystals in the presence of glycolic, lactic, and 2-hydroxybutyric acids.

Crystal structures of Ca4+x Y3-x Si7O15+x N5-x (0 ≤ x ≤ 1) comprising of an isolated [Si7(O,N)19] unit.

Single crystals of the solid solution series Ca4+x Y3-x Si7O15+x N5-x were obtained by a solid-state reaction method using a flux for x = 0, 0.5 and 1, resulting in Ca4Y3Si7O15N5 (tetra-calcium triyttrium hepta-silicon oxynitride), Ca4.5Y2.5Si7O15.5N4.5 and Ca5Y2Si7O16N4 (penta-calcium diyttrium hepta-silicon oxynitride). Single-crystal X-ray analysis revealed that the three compounds are isotypic and belong to space-group type P63/m. Ca2+ and Y3+ cations are distributed over two crystallographic sites (site symmetry .. and 1) in a disordered manner. The corresponding (Ca,Y)-centred polyhedra are connected by edge-sharing, resulting in the formation of a layer structure extending parallel to (001). Three [Si(O,N)]4 tetra-hedra (two with point group symmetry m.., one with 3.. and half-occupancy) are condensed into an isolated [Si7(O,N)19] unit, in which an [Si(O,N)]4 tetra-hedron is located at the center of a 12-membered oxynitride ring with composition [Si6O15N3]. The present compounds are the first to have such an [Si7(O,N)19] unit in their structures.

Sodium titanium oxide bronze nanoparticles synthesized via concurrent reduction and Na+-doping into TiO2(B).

A mixed valence compound, sodium titanium oxide bronze (NaxTiO2-B), combines intriguing properties of high electric conductivity and good chemical stability together with a unique one-dimensional tunnel crystal structure available for cation storage. However, this compound has not been studied for a long period because of the strongly reductive condition at high temperature required for its preparation, which limits the morphological control such as the preparation of nanocrystals. For the first time in this paper, the topotactic synthesis of nano-sized NaxTiO2-B with high specific surface area (>130 m2 g-1) from TiO2(B) nanoparticles has been demonstrated. The reaction of metastable TiO2(B) with NaBH4 allows carrier electrons to be doped simultaneously with incorporation of Na+ ions into the interstitial sites of the host Ti-O lattice at relatively low temperature. An electrochemical investigation of Li+- and Na+-ion storage behaviors suggests that the incorporated Na+ ions are mainly placed in the 6-fold coordination sites of bronze. In addition, optical measurements including time-resolved transient spectroscopy revealed that the doped electrons in the NaxTiO2-B nanoparticles are predominantly in the Ti3+ state and behave as a small polaron. The pelletized NaxTiO2-B nanoparticles shows a good electronic conductivity of 1.4 × 10-2 S cm-1 at 30 °C with an activation energy of 0.17 eV, which is attributable to the thermal barrier for the polaron hopping.

Low-Temperature-Processed Brookite-Based TiO2 Heterophase Junction Enhances Performance of Planar Perovskite Solar Cells.

In the design of electron-transport layers (ETLs) to enhance the efficiency of planar perovskite solar cells (PSCs), facile electron extraction and transport are important features. Here, we consider the effects of different titanium oxide (TiO2) polymorphs, anatase and brookite. We design and fabricate high-phase-purity, single-crystalline, highly conductive, and low-temperature (<180 °C)-processed brookite-based TiO2 heterophase junctions on fluorine-doped tin oxide (FTO) as the substrate. We test and compare single-phase anatase (A) and brookite (B) and heterophase anatase-brookite (AB) and brookite-anatase (BA) as ETLs in PSCs. The power-conversion efficiencies (PCEs) of PSCs with low-temperature-processed single-layer FTO-B as the ETL were as high as 14.92%, which is the highest reported efficiency of FTO-B-based single-layer PSC. This implies that FTO-B serves as an active phase and can be a potential candidate as an n-type ETL scaffold in planar PSCs. Moreover, the surface of highly crystalline brookite TiO2 exhibits a tendency toward interparticle necking, leading to the formation of compact scaffolds. Furthermore, PSCs with heterophase junction FTO-AB ETLs exhibited PCEs as high as 16.82%, which is superior to those of PSCs with single-phase anatase (FTO-A) and brookite (FTO-B) as the ETLs (13.86% and 14.92%, respectively). In addition, the PSCs with FTO-AB exhibited improved efficiency and decreased hysteresis compared with those with FTO-BA (13.45%) due to the suitable band alignment with the perovskite layer, which resulted in superior photogenerated charge-carrier extraction and reduced charge accumulation at the interface between the heterophase junction and perovskite. Thus, the present work presents an effective strategy by which to develop heterophase junction ETLs and manipulate the interfacial energy band to further improve the performance of planar PSCs and enable the clean and eco-friendly fabrication of low-cost mass production.

Ce4+-Based Compounds Capable of Photoluminescence by Charge Transfer Excitation under Near-Ultraviolet-Visible Light.

Ce4+-based charge transfer phosphor is not common and has been reported mainly in Sr2CeO4 with an excitation band peaking at ∼290 nm, mismatching with the near-ultraviolet light emitting diodes. Herein, we report a new series of Ce4+-based compounds Sr4.4Ce2.6REZnO12 (RE = Y, La, and Eu) capable of photoluminescence induced by O2--Ce4+ charge transfer excitation under near-ultraviolet-visible light. The crystal structure of Sr4.4Ce2.6EuZnO12 was determined by single crystal X-ray diffraction. The RE = La and Y samples were confirmed to be iso-structure compounds of the RE = Eu sample by powder X-ray diffraction. By introducing highly covalent Zn2+-O2- bonds into the framework, the Ce4+-O2- bonds are lengthened due to the effect of the Ce4+-O2--Zn2+ stretch. The lengthened Ce4+-O2- bond weakens the repulsion of the electrons between Ce4+ and O2-, thereby lowering the charge transfer energy to the visible light region. Incorporation of Eu3+ into the present compounds realized red emission under near-ultraviolet-visible excitation by the O2--Ce4+ charge transfer followed by energy transfer to Eu3+.

Photoluminescence Properties of Layered Perovskite-Type Strontium Scandium Oxyfluoride Activated With Mn4.

In this research, we have found that layered perovskite titanate Sr2TiO4 doped with Mn4+ exhibits photoluminescence even at room temperature despite no luminescence from Mn4+-doped SrTiO3 with a three-dimensional bulky perovskite structure. The relative position of t2g orbital of Mn to the valence band is a key factor for appearance of Mn4+-emission in Sr2TiO4:Mn. This result suggested usefulness of layered perovskite-type materials as hosts for Mn4+-activated phosphors than the bulky perovskite-type materials. Our investigation into photoluminescence of Mn4+-doped layered perovskite compounds has revealed that strontium scandium oxyfluoride Sr2ScO3F activated with Mn4+ exhibits Mn4+-emission with a peak at 697 nm under excitation at 300-600 nm and its emission intensity is much stronger than that of Sr2TiO4:Mn. The internal and external quantum yields of Sr2ScO3F:Mn were determined to be 50.5 and 43.5% under excitation at 345 nm, respectively.

Undoped Layered Perovskite Oxynitride Li2 LaTa2 O6 N for Photocatalytic CO2 Reduction with Visible Light.

Oxynitrides are promising visible-light-responsive photocatalysts, but their structures are almost confined with three-dimensional (3D) structures such as perovskites. A phase-pure Li2 LaTa2 O6 N with a layered perovskite structure was successfully prepared by thermal ammonolysis of a lithium-rich oxide precursor. Li2 LaTa2 O6 N exhibited high crystallinity and visible-light absorption up to 500 nm. As opposed to well-known 3D oxynitride perovskites, Li2 LaTa2 O6 N supported by a binuclear RuII complex was capable of stably and selectively converting CO2 into formate under visible light (λ>400 nm). Transient absorption spectroscopy indicated that, as compared to 3D oxynitrides, Li2 LaTa2 O6 N possesses a lower density of mid-gap states that work as recombination centers of photogenerated electron/hole pairs, but a higher density of reactive electrons, which is responsible for the higher photocatalytic performance of this layered oxynitride.

Expansion of the photoresponse window of a BiVO4 photocatalyst by doping with chromium(vi).

Doping of Cr6+ into BiVO4 was examined in this study. A new absorption band with a 1.84 eV energy threshold appeared with Cr-doping. The theoretical band calculation has revealed that the new absorption is ascribed to the electron transition from the valence band to acceptor levels formed by empty Cr 3d orbitals. It was confirmed that photocatalytic water oxidation in the presence of Ag+ or Fe3+ of an oxidizing reagent was induced by excitation of the new absorption although activity under band gap excitation decreased with Cr-doping. Characteristics of carrier dynamics were also investigated by transient absorption spectroscopy.

Effects of the SrTiO3 support on visible-light water oxidation with Co3O4 nanoparticles.

The photocatalytic activity of SrTiO3 modified with Co3O4 nanoparticles for water oxidation under visible light (λ > 480 nm) was investigated with respect to the physicochemical properties of the SrTiO3 support. SrTiO3 was synthesized by a polymerized complex method or a hydrothermal method, followed by calcination in air at different temperatures in order to obtain SrTiO3 particles having different sizes. Co3O4 nanoparticles, which provide both visible light absorption and water oxidation centers, were loaded on the as-prepared SrTiO3 by an impregnation method using Co(NO3)2 as the precursor, followed by heating at 423 K in air. Decreasing the SrTiO3 particle size (that is, improving the crystallinity) enhanced the photocatalytic activity by promoting the formation of Co3O4 nanoparticles that provided optimal light absorption and catalytic sites. However, Co3O4 aggregation occurred on overly large SrTiO3 particles, leading to a decrease in activity. This study demonstrates the possibility of tuning the photocatalytic activity of a Co3O4-loaded wide-gap semiconductor for visible light water oxidation through the appropriate selection of the support material.

Cobalt Oxide Nanoclusters on Rutile Titania as Bifunctional Units for Water Oxidation Catalysis and Visible Light Absorption: Understanding the Structure-Activity Relationship.

The structure of cobalt oxide (CoOx) nanoparticles dispersed on rutile TiO2 (R-TiO2) was characterized by X-ray diffraction, UV-vis-NIR diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, X-ray absorption fine-structure spectroscopy, and X-ray photoelectron spectroscopy. The CoOx nanoparticles were loaded onto R-TiO2 by an impregnation method from an aqueous solution containing Co(NO3)2·6H2O followed by heating in air. Modification of the R-TiO2 with 2.0 wt % Co followed by heating at 423 K for 1 h resulted in the highest photocatalytic activity with good reproducibility. Structural analyses revealed that the activity of this photocatalyst depended strongly on the generation of Co3O4 nanoclusters with an optimal distribution. These nanoclusters are thought to interact with the R-TiO2 surface, resulting in visible light absorption and active sites for water oxidation.

Anomalous Orange Light-Emitting (Sr,Ba)2SiO4:Eu(2+) Phosphors for Warm White LEDs.

Phosphors with sufficient red emission component are necessary for warm white light-emitting diodes. In this work on (Sr,Ba)2(1-x)Eu2xSiO4 phosphors, (Sr,Ba)1.5Eu0.5SiO4 achieved 75% of an internal quantum efficiency under excitation by blue light. Surprisingly, the (Sr,Ba)1.5Eu0.5SiO4 exhibited orange emission, against the well-known traditional green-yellow emission of (Sr,Ba)2SiO4:Eu(2+). Moreover, the concentration quenching of Eu(2+) in (Sr,Ba)2SiO4 was abnormally unobvious. With the help of calculations based on the density functional theory, it was discovered that the distinct local environment of luminescence centers rather than usual explanation such as self-absorption or intensified crystal field splitting, is responsible to the interesting red shifts in excitation and emission spectra. The refinement analysis based on X-ray diffraction revealed that the unequal distribution of Eu(2+) to two crystallographic sites caused low concentration of Eu(2+) at the 9-coordination site, inhibiting the concentration quenching. The (Sr,Ba)1.5Eu0.5SiO4 phosphor has warmer emission than the commercial Y3Al5O12:Ce(3+). This study also promotes research on the effect of site occupancy and the local environment of luminescence centers.

A Facile One-Step Solvothermal Synthesis and Electrical Properties of Reduced Graphene Oxide/Rod-Shaped Potassium Tungsten Bronze Nanocomposite.

Reduced graphene oxide (rGO)/rod-shaped potassium tungsten bronze nanocomposites with the different ratio were successfully synthesized by solvothermal reaction and followed by the reduction in H2(5 vol.%)/N2 atmosphere at 550 degrees C. The coupled samples showed excellent shielding ability of NIR light as well as certain visible lights transparency. The synergistic effects could be observed in the composites, i.e., when 15 wt% and 20 wt% of rGO which was fabricated by chemical reduction of graphene oxide, were composed into K(x)WO3, the composite showed the higher electrical conductivity than those of rGO and potassium tungsten bronze.