Heather F. Greer

Early stage reversed crystal growth of zeolite A and its phase transformation to sodalite.

Microstructural analysis of the early stage crystal growth of zeolite A in hydrothermal synthetic conditions revealed a revised crystal growth route from surface to core in the presence of the biopolymer chitosan. The mechanism of this extraordinary crystal growth route is discussed. In the first stage, the precursor and biopolymer aggregated into amorphous spherical particles. Crystallization occurred on the surface of these spheres, forming the typical cubic morphology associated with zeolite A with a very thin crystalline cubic shell and an amorphous core. With a surface-to-core extension of crystallization, sodalite nanoplates were crystallized within the amorphous cores of these zeolite A cubes, most likely due to an increase of pressure. These sodalite nanoplates increased in size, breaking the cubic shells of zeolite A in the process, leading to the phase transformation from zeolite A to sodalite via an Ostwald ripening process. Characterization of specimens was performed using scanning electron microscopy and transmission electron microscopy, supported by other techniques including X-ray diffraction, solid-state NMR, and N(2) adsorption/desorption.

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.

Nanosegregation and Neighbor‐Cation Control of Photoluminescence in Carbidonitridosilicate Phosphors

Phosphor materials play a key role in white-light emitting diode (LED) devices based on gallium indium nitride (GaInN). Phosphors for white LEDs should have good thermal stability and conversion efficiency and an excitation wavelength range in the UV to blue region (370–460 nm). The yellow-emitting phosphor (Y,Gd)3(Al,Ga)5O12:Ce 3+ is a well-known commercial example, but the lack of red emission results in cold white light and a low color-rendering index (CRI). 9, 10] Nitride-based materials are more covalent than oxides, which improves thermal stability, and their greater crystal field splitting increases the red emission leading to warmer white light. 11,12] The chemistry of doped nitride phosphors is often complex and it may be difficult to understand how substitutions tune photoluminescence properties, as the local environment of activator ions may be quite different to the structural average. For example, local O/N ordering driven by the size mismatch between the Eu activator and the host cations was found to be an important effect in M1.95Eu0.05Si5 xAlxO8 xNx (M=Ca, Sr, Ba) phosphors. Herein we demonstrate a new approach to controlling phosphor properties through segregation of activator cations on the nanoscale, as applied to Sr1 xY0.98+xCe0.02Si4N7 xCx carbidonitridosilicate phosphors, and we show that overall trends evidence a significant neighboring-cation influence. Two structure types are encountered in the studied system. The SrYSi4N7 (1147) type structure with hexagonal (space group P63mc) symmetry contains a network of cornerlinked N(SiN3)4 structural units. [13–16] Sr and Y sites are coordinated by 12 and 6 nitrides, respectively. The related Y2Si4N6C (2461)-type structure has monoclinic P21/c symmetry with C substituted only at the 4-connected positions to give C(SiN3)4 units. [17–19] The 5-coordinate Y1 and 6-coordinate Y2 sites are derived from the Y and Sr sites respectively in the SrYSi4N7 type. Photoluminescence properties of doped SrYSi4N7 and Y2Si4N6C types have previously been reported. Euand Cedoped SrYSi4N7 respectively emit 548–570 nm yellow light and 450 nm blue light when excited at 390 nm. Photoluminescence of Ceand Tb-doped Y2Si4N6C materials, [18] and the underlying Ce– Tb energy transfer process, have also been investigated, and Ln1.99Ce0.01Si4N6C for Ln=Gd and Lu have emission peaks of 610 nm and 540 nm. Several studies have proposed that higher covalence is expected in the Y2Si4N6C network owing to the introduction of carbon, so that the thermal stability of photoluminescence in carbidonitridosilicates is expected to be better than that of corresponding SrYSi4N7 type nitrides. However, no studies of the evolution of phosphor properties within Sr1 yY1+ySi4N7 yCy hosts have been reported. Powder X-ray diffraction (Supporting Information, Figure S1) shows that the Sr1 xY0.98+xCe0.02Si4N7 xCx samples appear to be solid solutions throughout the x= 0–1 range, although more highly resolved synchrotron data reveal a phase-segregated region (see below). Excitation and emission spectra for excitation by 380 nm light are shown in Figure 1a, and the emission positions in the standard CIE chromaticity diagram are indicated on Figure 1b. Both excitation and emission spectra show a substantial red-shift as x increases from 0 to 1. A notable range of useful emissions

Facile Surfactant‐Free Synthesis of p‐type SnSe Nanoplates with Exceptional Thermoelectric Power Factors

Abstract 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 (E g≈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.

The origin of ZnO twin crystals in bio-inspired synthesis

A fabrication of uniform nacre-like hierarchical nanostructures of faceted ZnO twin-crystals was established by a hydrothermal route using gelatin as the structure-directing agent, zinc nitrate hexahydrate as the Zn source, and hexamethylenetetramine to control alkalinity. Early stage growth of ZnO twin-crystals was investigated by powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy. A new formation mechanism is proposed. In the bio-inspired synthesis, Zn5(NO3)2(OH)8·2H2O nanoplatelets (10 to 20 nm in size) undergo orientated aggregation with gelatin to form Zn5(NO3)2(OH)8·2H2O/gelatin mesocrystalline nanoplates (150 to 400 nm in diameter and 20 to 50 nm in thickness). Surface re-crystallization of these nanoplates leads to two thin layers of ZnO separated by gelatin molecules. These double-layer nanoplates, negatively charged on both outer surfaces, are the cores of the twin-crystals. The dipolar Zn5(NO3)2(OH)8·2H2O nanoplates then stack on both sides of the double-layer nanoplates, followed by a phase transformation to ZnO. Eventually, twin-crystals are constructed in a manner reminiscent to that of an hourglass. The hexagonal morphology of the twin-crystals resulted from a late re-crystallization. The microstructure of the ZnO twin-crystals is very similar to the brick and mortar arrangement found in nacre. The present study is expected to shed light on the formation mechanism of many naturally occurring biominerals, as well as many other synthetic twin-crystals.

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.

Reversed crystal growth of ZnO microdisks

Hexagonal ZnO microdisks are grown and then selectively dissolved to form microstadiums. Analysis of the growth and dissolution of the microdisks revealed that they follow a reversed crystal growth mechanism, i.e. aggregation of precursors followed by surface crystallization and extension of crystallization from the surface to the core.

Microstructural study of the formation mechanism of metal–organic framework MOF-5

Metal–organic framework, MOF-5, is re-synthesised using an established method, which reveals an extraordinary formation mechanism. The earliest detected crystalline phase is Zn5(OH)8(NO3)2·2H2O, in the form of nanoplatelets 5 to 10 nm in diameter, which aggregate with surface adsorbed organic molecules into a layered inorganic–organic composite. Multiple nucleation of MOF-5 takes place inside the composite via intercalation of 1,4-benzenedicarboxylate molecules and phase transformation from Zn5(OH)8(NO3)2·2H2O. The as-formed MOF-5 nanocrystallites aggregate into cubic polycrystalline particles, which undergo surface re-crystallisation followed by extension of re-crystallisation from the surface to the core. This newly established formation mechanism may shed light on the crystal growth of many other MOFs. It may enable scientists to precisely control the microstructures and morphologies of these materials and gain a better understanding of their properties for future applications.

Ru/TiO2-catalysed hydrogenation of xylose: The role of the crystal structure of the support

Effective dispersion of the active species over the support almost always guarantees high catalytic efficiency. To achieve this high dispersion, a favourable interaction of the active species with the support is crucial. We show here that the crystal structure of the titania support determines the interaction and consequently the nature of ruthenium particles deposited on the support. Similar crystal structures of RuO2 and rutile titania result in a good lattice matching and ensure a better interaction during the heating steps of catalyst synthesis. This helps maintain the initial good dispersion of the active species on the support also in the subsequent reduction step, leading to better activity and selectivity. This highlights the importance of understanding the physico-chemical processes during various catalyst preparation steps, because the final catalyst performance often depends on the type of intermediate structures formed during the preparation.

Electron diffraction and HRTEM imaging of beam-sensitive materials

The high-resolution transmission electron microscopic investigation of electron beam-sensitive materials is a challenging research field. Applying a low-temperature specimen holder can reduce the sample damage rate. However, both the specimen holder and the running costs are expensive. Alternatively, it has been found that some treatments are helpful to overcome the beam damage problem of operating transmission electron microscope at room temperature. In this article, we review some typical examples of electron diffraction and high-resolution transmission electron microscopic imaging of beam-sensitive specimens carried out at St Andrews University and in other institutions. The samples in question include C60/trimethylbenzene nanowires, zeolites, metal organic frameworks, etc. These developed techniques can certainly be used for many other beam-sensitive materials.