M. A. Karakassides

Infrared reflectance spectra of lithium borate glasses

Abstract The infrared reflectance spectra of lithium borate glasses (0–73 mol% Li 2 O) have been measured over a broad and continuous spectral range (30–4000 cm −1 ) in an attempt to study systematically their structure. The reflectance data have been analyzed by Kramers-Kronig inversion to obtain the optical and dielectric properties of the materials. The study of the deconvoluted mid-infrared spectra has revealed a glass network built up of various boron-oxygen groups, with structure and concentration strongly dependent on the Li 2 O content. Of particular interest was the far-infrared asymmetric absorption profile, which could be well simulated by two broad bands attributed to vibrations of lithium cations in two distinct network environments. The presence of these two Li + -host site distributions was found to be independent of the details of the borate network structure and has been taken to imply a more general characteristics of ionic conducting oxide glasses.

Nanoscale zero-valent iron supported on mesoporous silica: Characterization and reactivity for Cr(VI) removal from aqueous solution

MCM-41-supported nanoscale zero-valent iron (nZVI) was sytnhesized by impregnating the mesoporous silica martix with ferric chloride, followed by chemical reduction with NaHB4. The samples were studied with a combination of characterization techniques such as powder X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) and Mössbauer spectroscopy, N2 adsorption measurements, transmission electron microscopy (TEM), magnetization measurements, and thermal analysis methods. The experimental data revealed development of nanoscale zero-valent iron particles with an elliptical shape and a maximum size of ∼80 nm, which were randomly distributed and immobilized on the mesoporous silica surface. Surface area measurements showed that the porous MCM-41 host matrix maintains its hexagonal mesoporous order structure and exhibits a considerable high surface area (609 m(2)/g). Mössbauer and magnetization measurements confirmed the presence of core-shell iron nanoparticles composed of a ferromagnetic metallic core and an oxide/hydroxide shell. The kinetic studies demonstrated a rapid removal of Cr(VI) ions from the aqueous solutions in the presence of these stabilized nZVI particles on MCM-41, and a considerably increased reduction capacity per unit mass of material in comparison to that of unsupported nZVI. The results also indicate a highly pH-dependent reduction efficiency of the material, whereas their kinetics was described by a pseudo-first order kinetic model.

Synthesis and Characterization of γ-Fe2O3/Carbon Hybrids and Their Application in Removal of Hexavalent Chromium Ions from Aqueous Solutions

Magnetic Fe(2)O(3)/carbon hybrids were prepared in a two-step process. First, acetic acid vapor interacted with iron cations dispersed on the surface of a nanocasted ordered mesoporous carbon (CMK-3). In the second step, the primarily created iron acetate species underwent pyrolysis and transformed to magnetic iron oxide nanoparticles. X-ray diffraction, Fourier-transform infrared, and Raman spectroscopies were used for the chemical and structural characterization of the hybrids, while surface area measurements, thermal analysis, and transmission electron microscopy were employed to determine their physical, surface, and textural properties. These results revealed the preservation of the host carbon structure, which was homogenously and controllably loaded (up to 27 wt %) with nanosized (ca. 20 nm) iron oxides inside the mesoporous system. Mössbauer spectroscopy and magnetic measurements at low temperatures confirmed the formation of γ-Fe(2)O(3) nanoparticles exhibiting superparamagnetic behavior. The kinetic studies showed a rapid removal of Cr(VI) ions from the aqueous solutions in the presence of these magnetic mesoporous hybrids and a considerably increased adsorption capacity per unit mass of sorbent in comparison to that of pristine CMK-3 carbon. The results also indicate highly pH-dependent sorption efficiency of the hybrids, whereas their kinetics was described by a pseudo-second-order kinetic model. Taking into account the simplicity of the synthetic procedure and possibility of magnetic separation of hybrids with immobilized pollutant, the developed mesoporous nanomaterials have quite real potential for applications in water treatment technologies.

Catalytic synthesis of carbon nanotubes on clay minerals

Novel clay–carbon tube composites were synthesized by catalytic decomposition of acetylene over iron-catalyst centers supported on montmorillonite surfaces by ion-exchange. TEM and SEM micrographs show the growth of carbon tubes rooted to the clay surfaces, while the iron-nanoparticles (which catalyze the formation of carbon-nanotubes) are detected and characterized by Mossbauer spectroscopy, mainly as ferromagnetic cementite (Fe3C). In the hybrid materials the clay retains its exchange properties making possible the preparation of clay–carbon nanotube derivatives that are valuable for various technological applications.

The devitrification of lithium metaborate: polymorphism and glass formation

Abstract The ambient pressure devitrification of glassy lithium metaborate has been studied by Raman and infrared spectroscopies. Of the three different polymorphs obtained, two are identified as α- and γ-LiBO2, while a third one, β′-LiBO2, is thought to consist of alternating borate triangles and tetrahedra respecting the metaborate stoichiometry (BO2-. The network of glassy LiBO2 prepared by melt quenching consists mainly of local arrangements similar to those encountered in all three polymorphs in accordance with the ideas of Krogh-Moe. It is argued that the glass structure exhibits the memory of structural transformations occuring in the vicinity of Tg, and may be described by models implying the existence of structural ‘granularity’.

Synthesis, bioactivity and preliminary biocompatibility studies of glasses in the system CaO–MgO–SiO2–Na2O–P2O5–CaF2

New compositions of bioactive glasses are proposed in the CaO–MgO–SiO2–Na2O–P2O5–CaF2 system. Mineralization tests with immersion of the investigated glasses in simulated body fluid (SBF) at 37°C showed that the glasses favour the surface formation of hydroxyapatite (HA) from the early stages of the experiments. In the case of daily renewable SBF, monetite (CaHPO4) formation competed with the formation of HA. The influence of structural features of the glasses on their mineralization (bioactivity) performance is discussed. Preliminary in vitro experiments with osteoblasts’ cell-cultures showed that the glasses are biocompatible and there is no evidence of toxicity. Sintering and devitrification studies of glass powder compacts were also performed. Glass-ceramics with attractive properties were obtained after heat treatment of the glasses at relatively low temperatures (up to 850°C).

Magnetic Fe2O3–Al2O3 composites prepared by a modified wet impregnation method

Fe2O3–Al2O3 composites were prepared by interaction of acetic acid vapors with iron oxides dispersed on the surface of a sol–gel derived porous alumina. Upon pyrolysis the created iron acetate species were transformed to magnetic iron oxide nanoparticles. The atmosphere which is used during the synthetic procedure affects significantly the nature of the nanoparticles which could be either γ-Fe2O3 or magnetite, or non-magnetic such as α-Fe2O3. X-Ray diffraction, surface area measurements and scanning electron microscopy (SEM) were used for the structural characterization and determination of the sorption properties of the composite material properties. The development of magnetic phases decreases the specific surface area of alumina by seeding of the alumina particles and, in parallel, the coverage of their free surface. Mossbauer spectroscopy, magnetic measurements and transmission electron microscopy (TEM) provide evidence for the formation, size and type of magnetic iron oxide phases.

Spectroscopic study of carbonate retention in high-basicity borate glasses

Abstract Raman and infrared spectroscopic techniques have been employed to investigate the carbonate retention in high alkali content sodium- and lithiumborate glasses: xM2O·(1 − x)B2O3 (x = 0.65, M = Li; x = 0.65, 0.71, M = Na. Spectroscopic evidence for carbonate retention was found only in sodiumborate glasses for three glass compositions studied here. It was shown that carbonate species are present in the glass as CO32− ions which are perturbed through interactions with the glassy environment. Decreasing the concentration of carbonates in the glass, by longer melting times and higher melting temperatures, was found to induce significant structural changes in the borate network. These were manifested by the destruction of certain borate groups and the creation of new ones, which are favoured under conditions of higher glass basicity. The specific structural rearrangements were found to depend strongly on the alkali oxide content. The concentration of carbonate and various borate species has been investigated as a function of melting time, and found to follow a two-step exponential dependence. Modification schemes describing the changes of the borate network have been proposed on the basis of the spectroscopic results.

Heterogeneous clay-manganese(II) oxidation catalyst

A manganese(II)-Schiff base complex has been heterogenized by its intercalation into clay minerals. The incorporation of the homogeneous manganese(II) complexes in the interlayer space of aluminosilicate mineral is accomplished by a cation exchange process. The obtained clay-manganese(II) composite has been studied by chemical analysis, X-ray diffraction and FT-IR spectroscopy. The new catalytic material has been evaluated as oxidation catalyst. Our results showed that this composite exhibits significant catalytic activities for alkene epoxidation using hydrogen peroxide as oxidant.

Synthesis and characterization of hollow clay microspheres through a resin template approach

Hollow clay microspheres of high thermal stability were easily fabricated by the adsorption of colloidal clay layers onto the surfaces of a spherical anion exchange resin and calcination of the resulting resin–clay composite.