Acoustic phonons in multilayer nitride-based AlN/GaN resonant tunneling structures

The study of physical processes associated with acoustic phonons in nitride-based nanosystems is of great importance for the effective operation of modern nanoscale devices. In this paper, a consistent theory of acoustic phonons arising in multilayer nitride-based semiconductor resonant tunneling structures, that can function as a separate cascade of a quantum cascade laser or detector is proposed. Using the physical and geometric parameters of a typical nanostructure, the spectrum of various types of acoustic phonons and the corresponding normalized components of the elastic displacement vector are calculated. It has been established that the spectrum of acoustic phonons of a multilayer nanostructure consists of two groups of the shear phonons dependencies and three groups of dependencies for a mixed spectrum of flexural and dilatational phonons. The dependencies of the acoustic phonons spectrum of the nanostructure and the components of the elastic displacement vector on its geometric parameters are studied. It has been established that for the components of the displacement vector u 2 for shear phonons have a decrease in the absolute values of their maxima with increasing of energy level number. The components u 1 and u 3 of flexural and dilatational phonons behave respectively as symmetric and antisymmetric functions relatively the center of an separate selected layer of the nanostructure. The proposed theory can be further applied to study the interaction of electrons with acoustic phonons in multilayer resonant tunneling structures.

Layered double hydroxides as the next generation inorganic anion exchangers: Synthetic methods versus applicability

This work is the first report that critically reviews the properties of layered double hydroxides (LDHs) on the level of speciation in the context of water treatment application and dynamic adsorption conditions, as well as the first report to associate these properties with the synthetic methods used for LDH preparation. Increasingly stronger maximum allowable concentrations (MAC) of various contaminants in drinking water and liquid foodstuffs require regular upgrades of purification technologies, which might also be useful in the extraction of valuable substances for reuse in accordance with modern sustainability strategies. Adsorption is the main separation technology that allows the selective extraction of target substances from multicomponent solutions. Inorganic anion exchangers arrived in the water business relatively recently to achieve the newly approved standards for arsenic levels in drinking water. LDHs (or hydrotalcites, HTs) are theoretically the best anion exchangers due to their potential to host anions in their interlayer space, which increases their anion removal capacity considerably. This potential of the interlayer space to host additional amounts of target aqueous anions makes the LDHs superior to bulk anion exchanger. The other unique advantage of these layered materials is the flexibility of the chemical composition of the metal oxide-based layers and the interlayer anions. However, until now, this group of "classical" anion exchangers has not found its industrial application in adsorption and catalysis at the industrial scale. To accelerate application of LDHs in water treatment on the industrial scale, the authors critically reviewed recent scientific and technological knowledge on the properties and adsorptive removal of LDHs from water on the fundamental science level. This also includes review of the research tools useful to reveal the adsorption mechanism and the material properties beyond the nanoscale. Further, these properties are considered in association with the synthetic methods by which the LDHs were produced. Special attention is paid to the LDH properties that are particularly relevant to water treatment, such as exchangeability ease of the interlayer anions and the LDH stability at the solid-water interface. Notably, the LDH properties (e.g., rich speciation, hydration, and the exchangeability ease of the interlayer anions with aqueous anions) are considered in the synthetic strategy context applied to the material preparation. One such promising synthetic method has been developed by the authors who supported their opinions by the unpublished data in addition to reviewing the literature. The reviewing approach allowed for establishing regularities between the parameters: the LDH synthetic method-structure/surface/interlayer-removal-suitability for water treatment. Specifically, this approach allowed for a conclusion about either the unsuitability or promising potential of some synthetic methods (or the removal approaches) used for the preparation of LDHs for water purification at larger scales. The overall reviewing approach undertaken by the authors in this work mainly complements the other reviews on LDHs (published over the past seven to eight years) and for the first time compares the properties of these materials beyond the nanoscale.

Influence of cation nature on stabilization of gold nanospecies in mordenites

Gold species deposited on NH4(+)- and H(+)-mordenites were studied by X-ray diffraction, xenon adsorption, NMR of 27Al and 129Xe, UV-Visible spectroscopy and TPR. Mordenite ion-exchange cations exert a significant effect on size distribution and electronic state of gold species supported on the zeolites. AuNH4M contains larger nanoparticles of gold on its external surface than AuHM because protonic form of zeolite has stronger sites for stabilization of gold nanospecies. In the case of NH4(+)-mordenite, Au clusters inside zeolite channels have bigger size and fill the side-pockets completely after gold deposition due to more complete reduction of gold entities, while in H(+)-mordenite only half the side-pockets are filled by gold species. H(+)-mordenite favors the stabilization of both small gold clusters and cations. Even after the reduction of gold by hydrogen flow at 100 degrees C, some gold species are not completely reduced and have certain effective charge (Au(n)delta+ clusters).

Preparation, structure and thermal stability of onium- and amino-functionalized silicas for the use as catalysts supports

We explored and compared several synthetic methods of grafting silica with strong (alkyltriphenylphosphonium, tetralkylammonium, propylpyridinium and dialkylimidazolium) and weak (gamma-aminopropyl, gamma-(N-imidazolyl)propyl) anion-exchanging groups starting with commercially available chloroalkyl- and gamma-aminopropylsilanes. Structure of the intermediate and final materials was investigated by elemental analysis, titration, 13C, 29Si, 31P MAS NMR, DRIFT, and TPD MS. The derivatives of alkyltriphenylphosphonium, propylpyridinium and dialkylimidazolium cation can be prepared with satisfactory quaternisation yields (ca. 30-100%) via the nucleophilic substitution of gamma-chloropropyl groups either in the silane or in chloropropylsilica, resulting in bonded phases with moderate densities: 0.2-1.0 group nm(-2) (onium salts) and 0.2-1.5 group nm(-2) (amines). Parallel one-pot end-capping/hydrophobization can be done if a mixture of target silane with end-capping reagent or gamma-chloropropylsilane is used. The grafted layer is highly stable at the level of Si-C bonds and decomposes at ca. 400 degrees C, while the onium functions begin to decompose at ca. 250 degrees C, lowering the thermal stability of materials. Thus, anion-exchanging silicas can be envisaged for the use as catalyst supports at moderate temperatures.

Chemical shift imaging (CSI) by precise object displacement

A mechanical device (NMR lift) has been built to displace vertically an object (typically an NMR sample tube) inside the NMR probe with an accuracy of 1 microm. A series of single pulse experiments are performed for incremented vertical positions of the sample. With a sufficiently spatially selective radio-frequency (r.f.) field, one obtains chemical shift information along the displacement direction (one-dimensional chemical shift imaging (CSI)). Knowing the vertical r.f. field profile (the amplitude of the r.f. field along the vertical direction), one can reconstruct the spectrum associated with all the slices corresponding to consecutive sample positions and improve the spatial resolution, which is simply related to the accuracy of the displacement device. Beside tests performed on phantoms, the method has been applied to solvent penetration in polymers and to benzene diffusion in a heterogeneous zeolite medium.

129Xe Nuclear Magnetic Resonance Study of Pitch-Based Activated Carbon Modified by Air Oxidation/Pyrolysis Cycles: A New Approach to Probe the Micropore Size

(129)Xe NMR has been used to study a series of homologous activated carbons obtained from a KOH-activated pitch-based carbon molecular sieve modified by air oxidation/pyrolysis cycles. A clear correlation between the pore size of microporous carbons and the (129)Xe NMR of adsorbed xenon is proposed for the first time. The virial coefficient delta(Xe)(-)(Xe) arising from binary xenon collisions varied linearly with the micropore size and appeared to be a better probe of the microporosity than the chemical shift extrapolated to zero pressure. This correlation was explained by the fact that the xenon collision frequency increases with increasing micropore size. The chemical shift has been shown to vary very little with temperature (less than 9 ppm) for xenon trapped inside narrow and wide micropores. This is indicative of a smooth xenon-surface interaction potential.

129Xe NMR study of Xe adsorption on multiwall carbon nanotubes

129Xe NMR spectroscopy has been used to study the adsorption of Xe on multi-wall carbon nanotubes (MWCNT). The results obtained have shown the 129Xe NMR ability to probe the intercrystalline (aggregate) and the inner porosity of CNT. In particular, the effects on porosity of tubes openings by hydrogen exposure and of ball milling were examined. Dramatic changes observed in the 129Xe NMR spectra after moderate ball milling of MWCNTs were attributed to the destruction of the initial intercrystalline pore structure and to the Xe access inside the nanotubes. To examine the exchange dynamics the mixture of as-made and milled MWCNTs was studied with one- and two-dimensional (1D and 2D) 129Xe NMR. The exchange between the interior of milled nanotubes and the aggregate pores of as-made MWCNTs was fast on the NMR acquisition time scale. The Xenon exchange between the interior of the as-made MWCNTs and the large aggregate pores occurred on a longer time scale of 10 ms, as was established by 2D 129Xe NMR exchange spectroscopy. Variable temperature 129Xe NMR data were also discussed and analyzed in terms of the fast exchange approximation.

Synthesis, Structure, and Acidic Properties of MCM-41 Functionalized with Phosphate and Titanium Phosphate Groups

Mesoporous molecular sieves Si-MCM-41 (purely siliceous) and Ti-MCM-41 (partly covered with a surface layer of TiO2) were functionalized with phosphate groups by treatment with POCl3 (denoted -MCM-41(P)and Ti-MCM-41(P), respectively). With the use of TEM, X-ray diffraction, and N2 adsorption, it was shown that the initial hexagonal structure, the high specific surface area, and porosity are retained in the functionalized materials but are not as good as in the starting materials. 1H MAS NMR and 31P MAS NMR revealed that the surface of Si-MCM-41(P) consists of silicon phosphate and pyrophosphate species. That of Ti-MCM-41(P) additionally contains titanium dihydro-, hydro-, and pyrophosphate species, the latter being predominant. TPD of adsorbed ammonia for Si-MCM-41(P) and Ti-MCM-41(P) showed that functionalization leads to the creation of moderate and strong acid sites. A combination of mesoporous structure with acidic properties makes the MCM-41 functionalized with phosphate groups promising for use as solid acid catalysts.