Preparation of M metal/alumina-pillared mica composites (M = Cu, Ni) by in situ reduction of interlayer M ions of alumina-pillared fluorine micas

Alumina-pillared fluorine micas form micropores in the interlayer region and exhibit cation exchangeability. Reduction of 3d transition metal cations (Cu²⁺ and Ni²⁺) in the interlayer regions of the cation exchanged alumina-pillared micas was attempted using ethylene glycol or diethylene glycol. The Cu²⁺-exchanged pillared mica led to the formation of a pillared mica composite containing interlayer zero-valence Cu⁰ metal clusters along with Cu⁰ metal fine particles on external surfaces of pillared mica flakes when refluxed in either ethylene glycol or diethylene glycol. Ni²⁺ cations in the interlayer region were also found to be reducible by refluxing in diethylene glycol. The zero-valence metal contained in the refluxed products occurs in three forms: M⁰ clusters in the interlayer region, elongated fine particles sandwiched between silicate layers, and submicron spherical particles on external surfaces of mica crystals. The zero-valence metal/alumina-pillared mica composites thus obtained largely retained the micropore properties of the alumina-pillared micas.     

Mesoporous assembly of 2D manganate nanosheets intercalated with cobalt ions

Mesoporous compound of cobalt-layered manganese oxide was synthesized by self-assembly reaction between 2D manganate nanosheets and cobalt ions. It was found that 2D manganate nanosheets are interstratified with cobalt ions, leading to the formation of porous intercalation heterostructure with expanded surface area of ∼ 44-100 m²/g. The mesopores with the average diameters of ∼ 11-17 nm originate from the house-of-cards type stacking of the layered manganate crystallites. Co K- and Mn K-edge X-ray absorption spectroscopy revealed that both the cobalt and manganese ions are stabilized in octahedral symmetry with the mixed oxidation states of Mn³⁺/Mn⁴⁺ and Co²⁺/Co³⁺. Considering high oxidation states of component metal ions, this porous material is expected to be very useful as redox catalysts and/or lithium intercalation electrodes.     

Metal clusters and nanoparticles assembled in zeolites: an example of stable materials with controllable particle size

An ion-exchangeable zeolite (mordenite) is used to control the formation of nanoparticles and clusters within the solid matrix by the hydrogen reduction of metal ions (Ag⁺, Cu²⁺, and Ni²⁺). SiO₂/Al₂O₃ molar ratio in mordenite appears to be an efficient tool to manage the reducibility of the metal ions. Few-atomic silver clusters in line with the larger silver nanoparticles were observed with DRS for the reduced Ag⁺-exchanged mordenites. Cu²⁺-exchanged ones produce the copper nanoparticles with different optical appearance, and Ni²⁺-exchanged mordenites are reduced up to complicated species with no explicit assignment of metal particles under the conditions studied.     

Oxidation of CaSi2 layered crystals with CoCl2 solutions

A layer-structured crystal CaSi₂ was found to be oxidized with CoCl₂ aqueous solutions. The Co²⁺ ions of the solution were reduced to metal at the edge surface of the crystals, while the interlayer calcium atoms of the crystals were removed as cations into the solution. A large amount of hydrogen was evolved during the reaction, which was attributed to a reduction product of water with a reactive silicon network generated by the deintercalation of calcium from CaSi₂. The saturation magnetization of the cobalt metal deposited on the surface increased as the reaction proceeded, approaching that of the bulk metal. The oxidation mechanism of the CaSi₂ is discussed on the basis of the chemical analysis data.     

Ion exchange of intercalation compounds from WOP2O7 and intercalation ofn-alkylamines

Ion exchange of Na _x WOP₂O₇ ·nH₂O (x 1.4) prepared from WOP₂O₇ was attempted, using alkaline and alkaline earth ions. The degree of exchange was observed to be >50% at 90° C except for Mg²⁺. The basal spacing of ion-exchanged materials for the hydrated phase were dependent on the number of water molecules in the interlayer spaces, while those for the dehydrated phase increased with the size of ions in the interlayer spaces. The network of water molecules linked by the hydrogen bond in the interlayer spaces seems to determine the basal spacing. By the ion-exchange reaction,n-alkylammonium ions were intercalated into the interlayer spaces of Na _x WOP₂O₇ ·nH₂O (x 1.4) and Sn _x H _y WOP₂O₇ ·nH₂O (2x +y 0.5). In spite of the difference in the charge density of the host layer, a similar arrangement of alkyl chains in the interlayer spaces resulted, and neutral amines were considered to be intercalated as well as ammonium ions. Direct reaction ofn-alkylamine with WOP₂O₇ produced an intercalation compound without reduction of tungsten. The arrangement of the amines in the interlayer spaces is similar to that supposed to the ion-exchanged derivatives when heated at 140° Cin vacuo.     

Mesoporous nickel (or cobolt)-doped silica-pillared clay: Synthesis and characterization studies

The metals-doped silica-pillared clay (SPC) materials with ordered pore structure in the gallery were obtained by a surfactant-directed assembly of silica species in the interlayer space of natural montmorillonite (MMT). The novel method afforded SPC derivatives with basal spacings of 4.4-4.5 nm, BET specific surface areas of 382.4-472.6 m²/g, pore volumes of 0.64-0.71 cm³ g⁻¹ and uniform pores (3.6 nm) between the layers. The main nickel and cobalt species was tetrahedrally coordinated Ni²⁺ or Co²⁺ in the gallery silica framework. Our results indicate that surfactant plays a decisive role in pore formation, because in acts as a micelle-like template during the hydrolysis of TEOS. In particular, the formation of metal-ammonia complex and rapid adsorption by surfactant in galleries controls metal species outflow from interlayers and contributes to the formation of metal species containing firm silica-pillars.     

Effects of Co content and annealing temperature on the structure and optical properties of CoxMg1−xAl2O4 nanoparticles

Co_xMg_1−xAl₂O₄ (x = 0–0.8) nanoparticles were synthesized by sol–gel method, and characterized by X-ray powder diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy and ²⁷Al solid-state NMR spectroscopy were performed to study the chemical environments of cations in the nanoparticles as a function of cobalt content and annealing temperature. The results show that the crystallite size of the particles is about 20–40 nm. Besides the tetrahedral and octahedral coordinations, the second octahedrally coordinated Al³⁺ ions are observed in the samples. The inversion parameter (two times the fraction of Al³⁺ ions in tetrahedral sites) decreases with the increase of annealing temperature and cobalt content. The fraction of octahedral Mg²⁺ decreases with the increase of Co concentration. The absorption spectra indicate that Co²⁺ ions are located in the tetrahedral sites as well as in the octahedral sites in the nanoparticles. The intensity of the absorption peak corresponding to octahedral Co²⁺ ions (300–500 nm) decreases with increasing annealing temperature.     

Coordination and valence state of transition metal ions in alkali-borate glasses

Borate glasses of the 20R₂O·80B₂O₃ type, where R = Li, Na and K, were colored by doping with transition metal ions (Co, Ni, Cr and Mn). The glasses were obtained by melting at the temperature of 1150 °C. For these glasses optical absorption in UV–VIS–NIR range were recorded. Analysis of the spectra allows to be determined the coordination and oxidation states of the doping transition metal ions. Changes of their coordination or oxidation are presented as a function of the optical basicity Λ after Duffy. Cobalt and nickel are present in examined borate glasses as divalent ions (Co²⁺, Ni²⁺) in octahedral coordination mainly, but the tetrahedral coordination state of cobalt is also possible. Chromium and manganese are present in the borate glasses in various oxidation state, though Cr³⁺ and Mn³⁺ ions in the octahedral coordination are probably dominant. A decrease of the electronegativity of the modifiers (Li → Na → K) and an increase of the glass matrix basicity cause a shift of the oxidation/reduction equilibrium towards higher valences of the transition metals (Cr⁶⁺, Mn³⁺).     

Polyol mediated synthesis of sub-micrometer Bi2O3 particles

Bi₂O₃ particles 70 to 90 nm in size were prepared with the polyol method. According to this method a suitable metal precursor (e.g. halogenide, acetate, alcoholate) and a defined amount of water were heated in a high boiling alcohol (e.g. diethylene glycol). Based on temperatures up to 180°C crystalline -Bi₂O₃ particles were formed. The material was isolated by centrifugation and characterized by XRD and SEM. The size of Bi₂O₃ particles in colloidally stable diethylene glycol suspension as well as in diethylene glycol/water mixtures was determined with laser diffraction methods. These suspensions were used to prepare homogeneous thin Bi₂O₃ particle layers on planar glass substrates.     

Ultrafine magnetic particles dispersed in silica: Characterization of cobalt iron citrate precursor and magnetic properties

Ultrafine magnetic particles dispersed in a silica matrix were successfully obtained by treatment of a cross-linked cobalt iron citrate precursor, synthesized by a modified Pechini route, with 0.001 M K₂Cr₂O₇ at 130 °C. The IR and NMR spectroscopic characterization of the precursor gel containing Co²⁺ and Fe³⁺ shows that the citric acid reacts with the metallic ions by coordination, the ethylene glycol by esterification and the tetraethylorthosilicate by substitution. SQUID measurements of the composite indicate superparamagnetic behavior. The blocking temperature, from the peak of the zero-field-cooled measurements, was 3 K at 1000 Oe. The magnetic diameter calculated using Langevin's equation was 4 nm.     

An XPS and reduction study of PrCoO3

The perovskite-type oxide PrCoO₃ has been studied by means of X-ray photoelectron spectroscopy (XPS), reduction in H₂ and X-ray diffraction. Two types of oxygen were detected: lattice oxygen (binding energy = 528.4 eV) and adsorbed oxygen (binding energy = 530.9 eV). The increase in relative intensity of the peak corresponding to the latter species after reduction of PrCo0₃ to 3e^– per molecule is assigned to the formation of hydroxyl groups. Temperature-programmed reduction (TPR) results showed two reduction steps: to 1 e^– per molecule (Co^3.1 Co²⁺) at 475 to 635 K, and to 3e^– per molecule (Co²¹ Co⁰) at 725 to 815 K. Reduction in the first and second steps occurs according to the contracting sphere model and the nucleation mechanism, respectively. Reduction of Co³⁺ to Co²⁺ causes minimal structural changes in the perovskite. Reduction to 3e^– per molecule yielded Pr₂O₃ and metallic cobalt. After this reduction and reoxidation at 973 K, the perovskite structure was regained. By XPS and TPR it was shown that PrCo0₃ is more easily reducible than LaCo0₃. It is concluded that the cation in the A position of the structure plays a significant role in the bulk and surface properties of LnCo0₃ (Ln, lanthanide elements) oxides.     

Electron spin resonance in phosphate glasses containing mixed transition metal ions

Phosphate glasses containing mixed Cu²⁺/Ni²⁺ and Cu²⁺/Co²⁺ oxides have been examined. A pronounced decrease in the optical absorption at 830 nm due to the Cu²⁺ ions is observed as the CuO in the glasses is gradually replaced by NiO or CoO and the decrease is accompanied by a pronounced decrease in the strength of the electron spin resonance (ESR) signal at 9.52 GHz. By combining the ESR and optical absorption data it is concluded that the decrease in concentration of Cu²⁺ ions in phosphate glasses may be due to an oxidation-reduction mechanism between two valency states of the two different transition metals, of the form Cu²⁺+Ni⁺Cu⁺+Ni²⁺ and Cu²⁺+Co⁺Cu⁺+Co²⁺.     

Synthesis and characterization of rod-like Y2O3 and Y2O3:Eu3+

Y₂O₃ rods 100 to 200 nm in diameter and 10 to 20 m in length are accessible via polyol-mediated synthesis of a precursor material with similar shape. By heating of Y(CH₃COO)₃ · xH₂O and a defined amount of water at 190°C in diethylene glycol, the rod-like precursor material is formed. Infrared spectroscopy (IR), differential thermal analysis (DTA) and thermal gravimetry (TG) evidence that this precursor material still contains acetate. However, the precursor material can be transformed to Y₂O₃ by sintering at 600°C without destruction of the rod-like shape. According to X-ray powder diffraction analysis, the rods are well crystallized. They can be assumed to be with [100] orientation. By doping with Eu³⁺ (5 mol%), red emitting phosphor rods can be realized. With optical spectroscopy the typical line emission of Eu³⁺ is observed. Diffuse reflectance of Y₂O₃:Eu³⁺ rods is determined to be higher than 95% in the visible. While exciting at 254 nm (Hg-discharge), a quantum efficiency of 38.5% is proven for the prepared Y₂O₃:Eu³⁺ rods.     

Optical, electrical and magnetic properties of Co3O4 nanocrystallites obtained by thermal decomposition of sol–gel derived oxalates

Nanocrystallites of tricobalt tetraoxide (Co₃O₄) have been synthesized by sol–gel process using cobalt acetate tetrahydrate, oxalic acid as precursors and ethanol as a solvent. The process comprises of gel formation, drying at 80 °C for 24 h to obtain cobalt oxalate dihydrate (α-CoC₂O₄·2H₂O) followed by calcination at or above 400 °C for 2 h in air. These results combined with thermal analysis have been used to determine the scheme of oxide formation. The room temperature optical absorption spectra exhibits blue shift in both (i) ligand to metal (p(O²⁻) → e_g(Co³⁺), 3.12 eV), and (ii) metal to metal charge transfer transitions (a) t_2g(Co³⁺) → t₂(Co²⁺), 1.77 eV, (b) t₂(Co²⁺) → e_g(Co³⁺), 0.95 eV together with the d–d transitions (0.853 and 0.56 eV) within the Co²⁺ tetrahedra. The temperature dependent ac electrical and dielectric properties of these nanocrystals have been studied in the frequency range 100 Hz to 15 MHz. There are two regimes distinguishing different temperature dependences of the conductivity (70–100 K and 200–300 K). The ac conductivity in both the temperature regions is explained in terms of nearest neighbor hopping (NNH) mechanism of electrons. The carrier concentration measured from the capacitance (C)–voltage (V) measurements is found to be 1.05 × 10¹⁶ m⁻³. The temperature dependent dc magnetic susceptibility curves under zero field cooled (ZFC) and field cooled (FC) conditions exhibit irreversibilities whose blocking temperature (T_B) is centered at 35 K. The observed Néel temperature (T_N  25 K) is significantly lower than the bulk Co₃O₄ value (T_N = 40 K) possibly due to the associate finite size effects.     

Interfacial reaction and adhesion between SiC and thin sputtered cobalt films

Thin sputtered cobalt films on SiC were annealed in an Ar/4 vol% H₂ atmosphere at temperatures between 500 and 1450 °C for various times. The reaction process and the reaction-product morphology were characterized using optical microscopy, surface profilometry, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. The relative adhesive strength between the film and substrate was determined by the scratch test method. Below 850 °C sputtered cobalt with a thickness of 2 m on SiC showed no detectable reaction products. Cobalt initially reacted with SiC at 850 °C producing Co₂Si and unreacted cobalt in the reaction zone. At 1050 °C the first-formed Co₂Si layer reacted to CoSi, and carbon precipitates were formed in the reaction zones. Sputtered thin cobalt layers reacted completely with SiC after annealing at 1050 °C for 2 h. Above 1250°C only CoSi was observed with carbon precipitates having an oriented structure in the reaction zone. Above 1450°C, a significant amount of graphitic carbon in the reaction zone was detected.     

Synthesis,phase formation study and magnetic properties of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanopowder prepared by mechanical milling

In this work we report the phase formation and magnetic properties of CoFe₂O₄ nanopowder prepared by mechanical alloying technique using metallic cobalt and hematite powder (1:1 molar ratio) as the initial raw material in ambient air atmosphere. The formation of single phase cobalt ferrite of (Co _0.18 ²⁺ Fe _0.82 ³⁺ )[Co _0.82 ²⁺ Fe _1.18 ³⁺ ]O₄ stoichiometry was confirmed for the samples milled above 15 h without any heat-treatment by XRD and Mössbauer techniques. The average crystallite size of the sample milled for 30 h was ～13 nm. The highest room temperature value of the magnetization measured at 1.5 T was 51 e.m.u/g for the sample milled for 25 h which was much lower than the corresponding value of the bulk cobalt ferrite (80.8 e.m.u/g at 300 K) due to the size effect.     

Chemical vapour growth of HfC whiskers and their morphology

HfC whiskers were prepared from a gas mixture of HfCl₄ + CH₄ + H₂ + Ar in the presence of metal impurities, and the growth conditions and morphology were examined. The HfC whiskers preferentially grew at an H/Cl ratio of above 8, an HfCl₄ gas flow rate of 10–20 standard cm³ min^–1, a CH₄ flow rate of 10–20 standard cm³ min^–1, and at temperatures above 1050 °C. HfC whiskers, 60–170 m long, with a ball-like tip and periodically varying diameters, were obtained at 1250 °C using a cobalt impurity.     

Electrical conductivity study of oxidation process in manganese-substituted magnetites

During oxidation in air of finely-grained manganese-substituted magnetites (Mn _0.8x ²⁺ Fe _1–0.8x ³⁺ )_A– (Fe _1+0.6x ³⁺ Fe _1–0.8x ²⁺ Mn _0.2x ³⁺ )_BO ₄ ^2– (A=tetrahedral, B=octahedral) the temperature dependence of the electrical conductivity over a temperature range of 100 to 700° C was investigated. Below 500° C the evolution of electrical conductivity might be closely associated with the position and nature of cations in the spinel lattice. The profile of the =f(t) curves show that the mechanism of electrical conduction in the temperature range 150 to 300° C can be explained in terms of the oxidation of Fe²⁺ to Fe³⁺ ions at octahedral sites. For the temperature range 300 to 400° C the conductivity involves the hopping of electrons from tetrahedral-site Mn²⁺ ions to tetrahedral-site Mn³⁺ ions. Above 500° C the oxidation of Mn²⁺ ions leads to an increase in conductivity with the generation of new phases of -Fe₂O₃, Mn₂O₃ and -(MnFe)₂O₃.     

Template-free hydrothermal derived cobalt oxide nanopowders: Synthesis,characterization, and removal of organic dyes

Pure spinel cobalt oxide nanoparticles were prepared through hydrothermal approach using different counter ions. First, the pure and uniform cobalt carbonate (with particle size of 21.8–29.8 nm) were prepared in high yield (94%) in an autoclave in absence unfriendly organic surfactants or solvents by adjusting different experimental parameters such as: pH, reaction time, temperature, counter ions, and (Co²⁺:CO₃^2?) molar ratios. Thence, the spinel Co₃O₄ (with mean particle size of 30.5–47.35 nm) was produced by thermal decomposition of cobalt carbonate in air at 500 °C for 3 h. The products were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FTIR), transmission electron microscope (TEM), scanning electron microscope (SEM), and thermal analysis (TA). Also, the optical characteristics of the as-prepared Co₃O₄ nanoparticles revealed the presence of two band gaps (1.45–1.47, and 1.83–1.93 eV). Additionally, adsorption of methylene blue dye on Co₃O₄ nanoparticles was investigated and the uptake% was found to be >99% in 24 h.     

Complexation of polyethylene glycol with Sr<Superscript>2+</Superscript> and Ba<Superscript>2+</Superscript> cations in extracts containing chlorinated cobalt dicarbollide

The composition and structure of complexes that are formed in the system consisting of chlorinated cobalt dicarbollide (CCD), polyethylene glycol (PEG), and Sr²⁺ or Ba²⁺ in a polar diluent, dichloroethane or phenyl trifluoromethyl sulfone, were studied by IR and NMR spectroscopy. In extraction of Sr²⁺ and Ba²⁺ with solutions of [H₅O ₂ ⁺ PEG]CCD^–, the organic phase contains the ionic associates [M²⁺PEG]CCD ₂ ^– . The Sr²⁺ and Ba²⁺ complexes have similar composition and structure: The oxygen atoms of two OH groups and six COC groups of a PEG molecule fill the first coordination sphere of the metal ions. Also, no more than two water molecules can be coordinated in the second sphere, forming hydrogen bonds with the hydrogen atoms of two OH groups of PEG. The coordination of the OH groups of PEG with the Sr²⁺ and Ba²⁺ ions is preferable over the coordination of the COC groups, as follows from the fact that the extraction of Sr²⁺ and Ba²⁺ with CCD-PEG mixtures gets worse on replacement of the OH groups of PEG by other substituents. A considerable increase in the efficiency of Sr²⁺ and Ba²⁺ extraction with H-CCD solutions in the presence of PEG is due to the fact that all the H₂O molecules in the first coordination spheres of the M²⁺ ions are replaced by the COC and OH groups of PEG with the formation of a hydrophobic complex [M²⁺PEG](H₂O)₂.Translated from Radiokhimiya, Vol. 46, No. 6, 2004, pp. 540–545.Original Russian Text Copyright © 2004 by Stoyanov, Smirnov, Babain, Antonov, Peterman, Herbst, Todd, Luther.