Control of Luminescence by Tuning of Crystal Symmetry and Local Structure in Mn4+ -Activated Narrow Band Fluoride Phosphors.

Mn4+ -doped fluoride phosphors have been widely used in wide-gamut backlighting devices because of their extremely narrow emission band. Solid solutions of Na2 (Six Ge1-x )F6 :Mn4+ and Na2 (Gey Ti1-y )F6 :Mn4+ were successfully synthesized to elucidate the behavior of the zero-phonon line (ZPL) in different structures. The ratio between ZPL and the highest emission intensity υ6 phonon sideband exhibits a strong relationship with luminescent decay rate. First-principles calculations are conducted to model the variation in the structural and electronic properties of the prepared solid solutions as a function of the composition. To compensate for the limitations of the Rietveld refinement, electron paramagnetic resonance and high-resolution steady-state emission spectra are used to confirm the diverse local environment for Mn4+ in the structure. Finally, the spectral luminous efficacy of radiation (LER) is used to reveal the important role of ZPL in practical applications.

The Ti3 AlC2 MAX Phase as an Efficient Catalyst for Oxidative Dehydrogenation of n-Butane.

Dehydrogenation or oxidative dehydrogenation (ODH) of alkanes to produce alkenes directly from natural gas/shale gas is gaining in importance. Ti3 AlC2 , a MAX phase, which hitherto had not been used in catalysis, efficiently catalyzes the ODH of n-butane to butenes and butadiene, which are important intermediates for the synthesis of polymers and other compounds. The catalyst, which combines both metallic and ceramic properties, is stable for at least 30 h on stream, even at low O2 :butane ratios, without suffering from coking. This material has neither lattice oxygens nor noble metals, yet a unique combination of numerous defects and a thin surface Ti1-y Aly O2-y/2 layer that is rich in oxygen vacancies makes it an active catalyst. Given the large number of compositions available, MAX phases may find applications in several heterogeneously catalyzed reactions.

Ultrafast Elemental and Oxidation-State Mapping of Hematite by 4D Electron Microscopy.

We describe a new methodology that sheds light on the fundamental electronic processes that occur at the subsurface regions of inorganic solid photocatalysts. Three distinct kinds of microscopic imaging are used that yield spatial, temporal, and energy-resolved information. We also carefully consider the effect of photon-induced near-field electron microscopy (PINEM), first reported by Zewail et al. in 2009. The value of this methodology is illustrated by studying afresh a popular and viable photocatalyst, hematite, α-Fe2O3 that exhibits most of the properties required in a practical application. By employing high-energy electron-loss signals (of several hundred eV), coupled to femtosecond temporal resolution as well as ultrafast energy-filtered transmission electron microscopy in 4D, we have, inter alia, identified Fe4+ ions that have a lifetime of a few picoseconds, as well as associated photoinduced electronic transitions and charge transfer processes.

Enhanced Photoluminescence Emission and Thermal Stability from Introduced Cation Disorder in Phosphors.

Optimizing properties of phosphors for use in white-light-emitting diodes (WLEDs) is an important materials challenge. Most phosphors have a low level of lattice disorder due to mismatch between the host and activator cations. Here we show that deliberate introduction of high levels of cation disorder leads to significant improvements in quantum efficiency, stability to thermal quenching, and emission lifetime in Sr1.98-x(Ca0.55Ba0.45)xSi5N8:Eu0.02 (x = 0-1.5) phosphors. Replacing Sr by a (Ca0.55Ba0.45) mixture with the same average radius increases cation size variance, resulting in photoluminescence emission increases of 20-26% for the x = 1.5 sample relative to the x = 0 parent across the 25-200 °C range that spans WLED working temperatures. Cation disorder suppresses nonradiative processes through disruption of lattice vibrations and creates deep traps that release electrons to compensate for thermal quenching. Introduction of high levels of cation disorder may thus be a very useful general approach for improving the efficiency of luminescent materials.

Facile Surfactant-Free Synthesis of p-Type SnSe Nanoplates with Exceptional Thermoelectric Power Factors.

A surfactant-free solution methodology, simply using water as a solvent, has been developed for the straightforward synthesis of single-phase orthorhombic SnSe nanoplates in gram quantities. Individual nanoplates are composed of {100} surfaces with {011} edge facets. Hot-pressed nanostructured compacts (Eg ≈0.85 eV) exhibit excellent electrical conductivity and thermoelectric power factors (S(2) σ) at 550 K. S(2) σ values are 8-fold higher than equivalent materials prepared using citric acid as a structure-directing agent, and electrical properties are comparable to the best-performing, extrinsically doped p-type polycrystalline tin selenides. The method offers an energy-efficient, rapid route to p-type SnSe nanostructures.

Structural Ordering and Charge Variation Induced by Cation Substitution in (Sr,Ca)AlSiN3:Eu Phosphor.

Nitride phosphors are suitable for white light-emitting diode applications. In this study, the structure of phosphor has been modified through cation substitution to induce charge variation and a rearrangement of neighboring nitride clusters, and consequently enhance its luminescent behavior. Substitution of Ca(2+) by Sr(2+) cations expanded the lattice volume and the bc plane, but shortened the distance between the layers along the a axis. Lattice distortion of the framework introduced high-coordination sites in the Sr/Eu centers and adequate space, thereby facilitating charge variation of activators under reduced atmosphere, as detected through X-ray absorption near-edge structure spectroscopy. As such, the photoluminescent intensity of the phosphors increased by more than 10% and a blue shift occurred. The microstructures of the samples were also analyzed using high-resolution transmission electron microscopy. Cation substitution induced a special change in the anion environment, as indicated in the solid-state Raman spectra. Moreover, typical ordering variations in the SiN4 and AlN4 clusters are generated in the lattice. Meanwhile, neighbor sequence of (Si/Al)N4 around the divalent centers were observed through solid-state nuclear magnetic resonance spectroscopy. The modified ordering distribution resulted in a rigid structure and improved the thermal quenching behavior. Thermal stability has been enhanced by 10% at 473 K when x = 0.9 in SrxCa0.993-xAlSiN3:Eu(2+)0.007 compared with that at x = 0. This study promotes the research of neighbor sequence for selective tetrahedral sites such as Li, Mg, Al, and Si coordinated by N atoms in contact with cation sites.

Reversed Crystal Growth of RHO Zeolitic Imidazolate Framework (ZIF).

RHO zeolitic imidazolate framework (ZIF), Zn1.33 (O.OH)0.33 (nim)1.167 (pur), crystals with a rhombic dodecahedral morphology were synthesized by a solvothermal process. The growth of the crystals was studied over time using scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD) and Brunauer-Emmett-Teller (BET) analyses, and a reversed crystal growth mechanism was revealed. Initially, precursor materials joined together to form disordered aggregates, which then underwent surface recrystallization forming a core-shell structure, in which a disordered core is encased in a layer of denser, less porous crystal. When the growth continued, the shell became less and less porous, until it was a layer of true single crystal. The crystallization then extended from the surface to the core over a six-week period until, eventually, true single crystals were formed.

Domination of second-sphere shrinkage effect to improve photoluminescence of red nitride phosphors.

Red Ca0.99Al(1-4δ/3-x)Si(1+δ+x)N(3-x)C(x):Eu(2+)0.01 (δ = 0.345; x = 0-0.2) nitride phosphors exhibit a blue-shifted emission with increased eye sensitivity function and excellent thermal stability. The variations in the photoluminescence in the Ca0.99Al(1-4δ/3-x)Si(1+δ+x)N(3-x)C(x):Eu(2+)0.01 (δ = 0.345; x = 0-0.2) system are thoroughly investigated. The enhanced emission energy and the improved thermal stability with increasing x are dominated by the second-sphere shrinkage effect via the substitution of small Si(4+) for large Al(3+) with simultaneous charge compensation. Related proofs of the second-sphere shrinkage effect control for photoluminescence are confirmed via high-resolution neutron powder diffraction, EXAFS, and (29)Si solid-state NMR techniques.

Iron ochre - a pre-catalyst for the cracking of methane.

BACKGROUND: Iron ochres are gelatinous sludges that can cause problems in terms of water management. In this work, the application of iron ochre obtained from a river has been applied to catalytically crack methane - another potential waste product - into two useful products, hydrogen and a magnetic carbon-containing composite. RESULTS: The powder X-ray diffraction (XRD) pattern of the iron ochre was found to be consistent with the expected 2-line ferrihydrite, and energy dispersive X-ray (EDX) analysis showed Fe to be a major component although some Si and Ca were present. The sample was observed to contain a fraction with a tubular morphology consistent with the presence of extra-cellular biogenic iron oxide formed by leptothrix. Upon exposure to methane at elevated temperatures, the material was found to transform into an active catalyst for hydrogen production yielding a magnetic carbon-containing composite material comprising filamentous carbon and encapsulating graphite. CONCLUSION: The application of two waste products - iron ochre and methane - to generate two useful products - hydrogen and a magnetic carbon-containing composite - has been demonstrated. Furthermore, the ochre has been shown to comprise tubular morphology extra-cellular biogenic iron oxide which may be of interest in terms of other applications. © 2014 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Zeolites with continuously tuneable porosity.

Zeolites are important materials whose utility in industry depends on the nature of their porous structure. Control over microporosity is therefore a vitally important target. Unfortunately, traditional methods for controlling porosity, in particular the use of organic structure-directing agents, are relatively coarse and provide almost no opportunity to tune the porosity as required. Here we show how zeolites with a continuously tuneable surface area and micropore volume over a wide range can be prepared. This means that a particular surface area or micropore volume can be precisely tuned. The range of porosity we can target covers the whole range of useful zeolite porosity: from small pores consisting of 8-rings all the way to extra-large pores consisting of 14-rings.

A new family of two-dimensional zeolites prepared from the intermediate layered precursor IPC-3P obtained during the synthesis of TUN zeolite.

The crystallization of zeolite TUN with 1,4-bis(N-methylpyrrolidinium)butane as template proceeds through an intermediate, designated IPC-3P, following the Ostwald rule of successive transformations. This apparently layered transient product has been thoroughly investigated and found to consist of MWW monolayers stacked without alignment in register, that is, disordered compared with MCM-22P. The structure was confirmed based on X-ray diffraction and high-resolution (HR)TEM analysis. The layered zeolite precursor IPC-3P can be swollen and pillared affording a combined micro- and mesoporous material with enhanced Brunauer-Emmett-Teller (BET) surface area (685 m(2) g(-1) ) and greater accessibility of Brønsted acid sites for bulky molecules. This mesoporous material was probed with 2,6-di-tert-butylpyridine (DTBP). IPC-3P and its modification create a new layered zeolite sub-family belonging to the MWW family. FTIR data indicate that (Al)MWW materials MCM-22 and IPC-3 with Si/Al ratios greater than 20 exhibit a lower relative ratio of Brønsted to Lewis acid sites than MCM-22 (with Si/Al ratios of around 13), that is, less than 2 versus more than 3, respectively. This is maintained even upon pillaring and warrants further exploration of materials like IPC-3P with a higher Al content. The unique XRD features of IPC-3P indicating misaligned stacking of layers and distinct from MCM-22P, are also seen in other MWW materials such as EMM-10P, hexamethonium-templated (HM)-MCM-22, ITQ-30, and UZM-8 suggesting the need for more detailed study of their identity and properties.

Dipole field guided orientated attachment of nanocrystals to twin-brush ZnO mesocrystals.

Mesocrystals of ZnO were synthesized hydrothermally by using gum arabic as a structure-directing agent. Their hierarchical structure has a unique twin-brush form consisting of vertically aligned nanorods in a single-crystal-like porous form. The formation mechanism of the twin-brush ZnO was investigated by quenching a series of samples at different times and examining them by TEM, SEM, and XRD. The alignment of ZnO crystal units can be modulated by adding simple salts such as KCl to change the units from nanorods to nanoplates. This can be explained by screening the dipolar force of the polar crystal. Local cathodoluminescence of twin-brush ZnO was used to follow the local structure changes.

Disruption of extended defects in solid oxide fuel cell anodes for methane oxidation.

Point defects largely govern the electrochemical properties of oxides: at low defect concentrations, conductivity increases with concentration; however, at higher concentrations, defect-defect interactions start to dominate. Thus, in searching for electrochemically active materials for fuel cell anodes, high defect concentration is generally avoided. Here we describe an oxide anode formed from lanthanum-substituted strontium titanate (La-SrTiO3) in which we control the oxygen stoichiometry in order to break down the extended defect intergrowth regions and create phases with considerable disordered oxygen defects. We substitute Ti in these phases with Ga and Mn to induce redox activity and allow more flexible coordination. The material demonstrates impressive fuel cell performance using wet hydrogen at 950 degrees C. It is also important for fuel cell technology to achieve efficient electrode operation with different hydrocarbon fuels, although such fuels are more demanding than pure hydrogen. The best anode materials to date--Ni-YSZ (yttria-stabilized zirconia) cermets--suffer some disadvantages related to low tolerance to sulphur, carbon build-up when using hydrocarbon fuels (though device modifications and lower temperature operation can avoid this) and volume instability on redox cycling. Our anode material is very active for methane oxidation at high temperatures, with open circuit voltages in excess of 1.2 V. The materials design concept that we use here could lead to devices that enable more-efficient energy extraction from fossil fuels and carbon-neutral fuels.

Growth mechanism and microstructures of Cu2O/PVP spherulites.

Cu2O spherulites are solvothermaly fabricated by using Cu(NO3)2 as the starting material and polyvinylpyrrolidone (PVP) as a multifunctional growth agent. The specimens at different growth stages are investigated by using X-ray diffraction, electron microscopy, energy dispersive X-ray spectroscopy, soft X-ray emission spectroscopy and infrared spectroscopy. The formation mechanism of Cu2O spherulites is proposed accordingly. Hierarchically, the spherulites are composed of needle-like submicron-rods lying along the radial orientations. The submicron-rods are constructed by piling up of small Cu2O/PVP spheres. The embedded Cu2O nanocrystallites can generate a dipolar field in each along the [100] direction. They deposit at the surface of a negatively charged PVP-containing spherical core, and self-oriented along the radial directions. Therefore, all the Cu2O nanocrystallites would have their positively charged (100) facet facing to the core and their negatively charged (1̄00) facet turning towards to the spherulite surface, leading to a negatively charged surface of spherulites. Unlike randomly oriented nanocrystallites embedded in polymer microspheres, the spherulites would not undergo surface recrystallisation into a single crystal shell due to the restricted potential of local shift and rotation of the nanocrystallites by the Coulomb force from the core. This work provides new perspective towards the formation of spherulites and their structural properties.

Accelerated electron beam induced breakdown of commercial WO3 into nanorods in the presence of triethylamine.

Tungsten oxide nanorods were prepared from commercial crystalline WO(3) particles treated with triethylamine under electron beam irradiation in an electron microscope operating at 200 kV. The amine acts as a molecular knife, cleaving the particles under beam irradiation.

Synthesis and characterization of hybrid organic/inorganic nanotubes of the imogolite type and their behaviour towards methane adsorption.

Imogolite-like nanotubes have been synthesised in which SiCH(3) groups have been introduced in place of the SiOH groups that naturally occur at the inner surface of imogolite, an alumino-silicate with formula (OH)(3)Al(2)O(3)SiOH, forming nanotubes with inner and outer diameter of 1.0 and 2.0 nm, respectively. The new nanotubular material, composition (OH)(3)Al(2)O(3)SiCH(3), has both larger pores and higher specific surface area than unmodified imogolite: it forms as hollow cylinders 3.0 nm wide and several microns long, with a specific surface area of ca. 800 m(2) g(-1) and intriguing surface properties, due to hydrophobic groups inside the nanotubes and hydrophilic Al(OH)Al groups at their outer surface. Adsorption of methane at 30 °C has been studied in the pressure range between 5 and 35 bar on both the new material and unmodified imogolite: it resulted that the new material adsorption capacity is about 2.5 times larger than that of imogolite, in agreement with both its larger pore volume and the presence of a methylated surface. On account of these properties and of its novelty, the studied material has several potential technical applications, e.g. in the fields of gas chromatography and gas separation.

Formation mechanism of Mn x Co3-x O4 yolk-shell structures.

Formation of Mn x Co3-x O4 yolk-shell microspheres via a solvothermal reaction of hydrated cobalt and manganese nitrates in ethanol is investigated. Spinel nanocrystals of cobalt oxide or cobalt-rich ternary oxide preferentially develop in the system, while manganese-rich hydroxide form Mn(OH)2-type nanosheets. Instead of continuing to grow individually, the nanocrystallites and nanosheets aggregate into large microspheres due to their strong inter-particle interaction. When the proportion of Mn-rich nanosheets is high, therefore the overall density is low, dehydration of hydroxide nanosheets and a surface re-crystallisation lead to formation of a dense and rigid shell, which is separated from a solid or hollow core via a further Ostwald ripening process. The proposed formation mechanism of the yolk-shell structures based on electron microscopic studies would help us to develop yolk-shell structure based multifunctional materials.

Formation and Near-Infrared Emission of CsPbI3 Nanoparticles Embedded in Cs4PbI6 Crystals.

Cs4PbI6, as a rarely investigated member of the Cs4PbX6 (X is a halogen element) family, has been successfully synthesized at low temperatures, and the synthetic conditions have been optimized. Metal iodides such as LiI, KI, NiI2, CoI2, and ZnI2, as additives, play an important role in enhancing the formation of the Cs4PbI6 microcrystals. ZnI2 with the lowest dissociation energy is the most efficient additive to supply iodide ions, and its amount of addition has also been optimized. Strong red to near-infrared (NIR) emission properties have been detected, and its optical emission centers have been identified to be numerous embedded perovskite-type α-CsPbI3 nanocrystallites (∼5 nm in diameter) based on investigations of temperature- and pressure-dependent photoluminescent properties. High-resolution transmission electron microscopy was used to detect these hidden nanoparticles, although the material was highly beam-sensitive and confirmed a "raisin bread"-like structure of the Cs4PbI6 crystals. A NIR mini-LED for the biological application has been successfully fabricated using as-synthesized Cs4PbI6 crystals. This work provides information for the future development of infrared fluorescent nanoscale perovskite materials.

Formation mechanism of CaTiO3 hollow crystals with different microstructures.

The crystal growth of CaTiO(3) hollow crystals with different microstructures has been investigated. In a water-free poly(ethylene glycol) 200 (PEG-200) solution, CaTiO(3) nanocubes formed first. The nanocubes underwent an oriented self-assembly into spherical particles, enhanced by the surface-adsorbed polymer molecules. Since the growth of nanocubes and their aggregation took place simultaneously, the nanocubes in the outer shells were larger than those in the cores. Disappearance of the small nanocubes in the cores of the spheres during an Ostwald ripening process led to spherical hollow crystals. Addition of a small amount of water (1.25 vol %) in the polymer solution enhanced surface recrystallization of the aggregated spheres, forming a cubic morphology. The orthorhombic distortion of the perovskite CaTiO(3) structure did not have a significant effect on the nanocube aggregation, resulting in a domain structure in the shells. Single-crystalline hollow cubes were produced with a slightly higher water content, e.g., 5 vol %. This process of (1) aggregation of nanocubes and (2) surface crystallization followed by (3) surface-to-core extension of recrystallization gives a good example of the reversed crystal growth route in ceramic materials. The proposed formation mechanism of the hollow CaTiO(3) crystals would enable us to control the microstructures of these materials and to explain the formation of many other hollow crystals.

Assessing molecular transport properties of nanoporous materials by interference microscopy: remarkable effects of composition and microstructure on diffusion in the silicoaluminophosphate zeotype STA-7.

The influence of the chemical composition and of the storage and activation protocol on the diffusion of methanol into strongly chemically zoned crystals of the silicoaluminophosphate zeotype STA-7 has been investigated by interference microscopy. Analysis of the evolution of transient intracrystalline concentration profiles reveals that just-calcined SAPO STA-7 crystals with lower Si content (Si/(Si + P) = 0.18) exhibit higher surface permeability and bulk diffusivity than those with higher Si content (S/(Si + P) = 0.37). Remarkably, crystals with the higher Si content which were stored in the calcined form crack during activation along planes of weakness already present in the as-prepared crystals, creating fresh surfaces through regions of lower Si that are much more easily penetrated by the adsorbing methanol than are the original surfaces.