EPR Study of Fe 3+ - and Ni 2+ -Doped Macroporous CaSiO 3 Ceramics

Thermally stable macroporous CaSiO 3 , Fe 3+ - and Ni 2+ -doped (0.5 to 5 mol%) ceramics have been prepared by solution combustion process by mixing respective metal nitrates (oxidizers), fumed silica. Diformol hydrazine is used as a fuel. The combustion products were identified by their X-ray diffraction and thermal gravimetry/differential thermal analysis. Single phases of β-CaSiO 3 and α-CaSiO 3 were observed at 950 and 1200 °C, respectively. The phase transition temperatures of combustion-derived CaSiO 3 were found to be lower compared to those obtained via solid-state reaction method. It is interesting to note that with an increase in the calcination temperature the samples become more porous with an increase in the pore diameter from 0.2 to 8 µm. The electron paramagnetic resonance (EPR) spectrum of Fe 3+ ions in CaSiO 3 exhibits a weak signal at g = 4.20 ± 0.1 followed by an intense signal at g = 2.0 ± 0.1. The signal at g = 4.20 is ascribed to isolated Fe 3+ ions at rhombic site. The signal at g = 2.0 is due to Fe 3+ coupled together with dipolar interaction. In Ni 2+ -doped CaSiO 3 ceramics the EPR spectrum exhibits a symmetric absorption at g = 2.23 ± 0.1. This deviation from the free electron g -value is ascribed to octahedrally coordinated Ni 2+ ions with moderately high spin–orbit coupling. The number of spins participating in resonance and the paramagnetic susceptibilities have been evaluated from EPR data as a function of Fe 3+ as well as Ni 2+ content. The effect of alkali ions (Li, Na and K) on the EPR spectra of these ceramics has also been studied

Solution combustion derived nanocrystalline Zn2SiO4 : Mn phosphors: A spectroscopic view

Manganese doped nanocrystalline willemite powder phosphors Zn2-xMnxSiO4 (0.1less than or equal toxless than or equal to0.5) have been synthesized by a low-temperature initiated, self-propagating, gas producing solution combustion process. The phosphors have been characterized by using x-ray diffraction (XRD), energy dispersive spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR), electron paramagnetic resonance (EPR), and photo luminescence (PL) spectroscopic techniques. The lattice parameters calculated from XRD confirm that Zn2-xMnxSiO4 has a rhombohedral space group R (3) over barH. The XRD patterns confirm that Zn2-xMnxSiO4 phosphor samples undergo a phase transformation from beta-willemite to alpha-willemite phase at 950 degreesC. The EPR spectra of Mn2+ ions exhibit resonance signals at gcongruent to3.24 and gcongruent to2.02, with a sextet hyperfine structure centered around gcongruent to2.02. The EPR signals of Mn2+ give a clear indication of the presence of two different Mn2+ sites. The magnitude of the hyperfine splitting (A) indicates that the Mn2+ is in an ionic environment. The number of spins participating in resonance (N), the paramagnetic susceptibility (chi), and the zero-field splitting parameter (D) have been evaluated as function of x. It is interesting to observe that the variation of N with temperature obeys Boltzmann. The paramagnetic susceptibility is calculated from the EPR data at various temperatures and the Curie constant and Curie paramagnetic temperature was evaluated from the 1/chi versus T graph. The luminescence of Mn2+ ion in Zn2SiO4 shows a strong green emission peak around 520 nm from the synthesized phosphor particles under UV excitation (251 nm). The luminescence is assigned to a transition from the upper T-4(1)-->(6)A(1) ground state. The mechanism involved in the generation of a green emission has been explained in detail. The effect of Mn content on luminescence has also been studied. (C) 2004 American Institute of Physics

Synthesis, characterization and TL studies of porous CaSiO3 ceramic powders

Nanocryst. porous CaSiO3 ceramic powders have been synthesized by a novel low temp. initiated self-​propagating, gas producing soln. combustion process and characterized by XRD, SEM, EDS (energy dispersive spectroscopy)​, porosity, surface area and thermoluminescence (TL) studies. The effect of temp. on cryst. phase formation, amt. of porogens and particle size of porous CaSiO3 have been investigated. Single phase β-​CaSiO3 and α-​CaSiO3 were formed at 950° and 1200°C resp. The phase transformation temps. of combustion derived CaSiO3 were found to be low compared to the powders obtained via solid state reaction method. The microstructure and morphol. were studied by SEM and it was noted that with increase in calcination temp., the samples became more porous and the pore diam. increased from 0.25 to 8 μm. The samples calcined at 950°C for 3 h had 17.5​% porosity, however, the porosity increased to 31.6​% on calcination at 1200°C for 3 h. The surface areas of the as-​formed and calcined (at 950° and 1200°C) CaSiO3 samples were found to be 31.93, 0.585 and 3.48 m2·g-​1 resp. The TL intensity in powder sample was more intense when compared to the pelletized CaSiO3 and it was further obsd. that there was a shift in glow peak temps. in pelletized sample. This is attributed to the interparticle spacing and pressure-​induced defects

Combustion synthesis, characterization and Raman studies of ZnO nanopowders

Spherical shaped ZnO nanopowders (14-50 nm) were synthesized by a low temperature solution combustion method in a short time <5 min. Rietveld analysis show that ZnO has hexagonal wurtzite structure with lattice constants a = 3.2511(1) , c = 5.2076(2) , unit cell volume (V) = 47.66(5) () 3 and belongs to space group P63mc. SEM micrographs reveal that the particles are spherical in shape and the powders contained several voids and pores. TEM results also confirm spherical shape, with average particle size of 14-50 nm. The values are consistent with the grain sizes measured from Scherrer's method and Williamson-Hall (W-H) plots. A broad UV-vis absorption spectrum was observed at �375 nm which is a characteristic band for the wurtzite hexagonal pure ZnO. The optical energy band gap of 3.24 eV was observed for nanopowder which is slightly lower than that of the bulk ZnO (3.37 eV). The observed Raman peaks at 438 and 588 cm -1 were attributed to the E 2 (high) and E 1 (LO) modes respectively. The broad band at 564 cm -1 is due to disorder-activated Raman scattering for the A 1 mode. These bands are associated with the first-order Raman active modes of the ZnO phase. The weak bands observed in the range 750-1000 cm -1 are due to small defects. © 2011 Elsevier B.V. All Rights Reserved

Supported Vanadium Oxide Catalysts: Quantitative Spectroscopy, Preferential Adsorption of V^4+/5+, and Al2O3 Coating of Zeolite Y

A series of supported vanadium oxide catalysts were prepared by the incipient wetness method as a function of the support composition (Al2O3, SiO2, and USY), the metal oxide loading (0-1 wt %), and the impregnation salt (vanadyl sulfate and ammonium vanadate). These catalysts have been studied by combined DRS-ESR spectroscopies in order to quantify the amount of V^4+ and V^5+ and to unravel their coordination geometries. These spectroscopic fingerprints have been used to study the preferential adsorption of V^4+/5+ ions on SiO2, Al2O3, and USY. Both V^4+ and V^5+ were preferentially adsorbed on Al2O3 and showed a much smaller pref-erence for USY and SiO2. The observed preference orders are discussed in relation with the properties of the support. In addition, a novel method is proposed to coat the external surface of USY with a thin film of Al2O3. The method is based on the deposition of USY with the so-called Keggin ion, [Al13O4(OH)24(H2O)12]7+ , which is too big to enter the USY channels or pores. The obtained Al2O3/USY material showed a preferential adsorption of V^4+ onto the Al2O3 film, suggesting that this method could be useful for vanadium passivation of FCC catalysts

EPR and photoluminescence studies of ZnO:Mn nanophosphors prepared by solution combustion route

Nanocrystalline ZnO:Mn (0.1 mol%) phosphors have been successfully prepared by self propagating, gas producing solution combustion method. The powder X-ray diffraction of as-formed ZnO:Mn sample shows, hexagonal wurtzite phase with particle size of ∼40 nm. For Mn doped ZnO, the lattice parameters and volume of unit cell (a = 3.23065 Å, c = 5.27563 Å and V = 47.684 (Å)3) are found to be greater than that of undoped ZnO (a = 3.19993 Å, c = 5.22546 Å and V = 46.336 (Å)3). The SEM micrographs reveal that besides the spherical crystals, the powders also contained several voids and pores. The TEM photograph also shows the particles are approximately spherical in nature. The FTIR spectrum shows two peaks at ∼3428 and 1598 cm−1 which are attributed to O–H stretching and H–O–H bending vibration. The PL spectra of ZnO:Mn indicate a strong green emission peak at 526 nm and a weak red emission at 636 nm corresponding to 4T1 → 6A1 transition of Mn2+ ions. The EPR spectrum exhibits fine structure transition which will be split into six hyperfine components due to 55Mn hyperfine coupling giving rise to all 30 allowed transitions. From EPR spectra the spin-Hamiltonian parameters have been evaluated and discussed. The magnitude of the hyperfine splitting (A) constant indicates that there exists a moderately covalent bonding between the Mn2+ ions and the surrounding ligands. The number of spins participating in resonance (N), its paramagnetic susceptibility (χ) have been evaluated

EPR, thermo and photoluminescence properties of ZnO nanopowders

Nanocrystalline ZnO powders have been synthesized by a low temperature solution combustion method. The photoluminescence (PL) spectrum of as-formed and heat treated ZnO shows strong violet (402, 421, 437, 485 nm) and weak green (520 nm) emission peaks respectively. The PL intensities of defect related emission bands decrease with calcinations temperature indicating the decrease of Zn i and V o + caused by the chemisorptions of oxygen. The results are correlated with the electron paramagnetic resonance (EPR) studies. Thermoluminescence (TL) glow curves of gamma irradiated ZnO nanoparticles exhibit a single broad glow peak at �343 °C. This can be attributed to the recombination of charge carriers released from the surface states associated with oxygen defects, mainly interstitial oxygen ion centers. The trapping parameters of ZnO irradiated with various γ-doses are calculated using peak shape method. It is observed that the glow peak intensity increases with increase of gamma dose without changing glow curve shape. These two characteristic properties such as TL intensity increases with gamma dose and simple glow curve structure is an indication that the synthesized ZnO nanoparticles might be used as good TL dosimeter for high temperature application. © 2011 Elsevier B.V. All Rights Reserved

Nd2O3:Gd3+ nanocrystalline phosphor: gamma-Induced thermoluminescence, EPR and structural properties

Preparation of nanocrystalline Nd2O3:Gd3+ (2 mol%) phosphor was prepared by propellant combustion route at low temperature. Powder X-ray diffraction (PXRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques were used to study the structural and morphological features. A and C type hexagonal phase of Nd2O3 was obtained in as-formed and calcined product at 900 degrees C for 3 h. SEM images show the product was highly porous, agglomerated and irregular shaped particles. The average crystallite size estimated from Scherrer's and W-H plots were found to be in the range similar to 20-50 nm. Combination of F-g and (F-g + E-g) modes were observed from Raman studies. A single TL glow peak at 409 K was recorded at a warming rate 5 degrees C s (1). TL response as a function of gamma-irradiation showed linear response up to 1.6 kGy and later it followed exponential growth. The phosphor show simple glow curve structure with less fading over a period of 30 days. Trap parameters were estimated from Chen's peak shape method. The activation energy (E) and frequency factor (s) was found to in the range 1.377-2.934 eV and 2.01 x 10(11)-8.70 x 10(22) Hz respectively. (C) 2013 Elsevier B.V. All rights reserved

Structural, EPR, optical and Raman studies of Nd 2O 3:Cu 2+ nanophosphors

Nanocrystalline Nd2O3:Cu2+ (2 mol %) phosphors have been prepared by a low temperature solution combustion technique. Powder X-ray diffraction (PXRD) results confirm that hexagonal A-type Nd2O3 (900 °C, 3 h) and the lattice parameters have been evaluated by Rietveld refinement. Surface morphology of as-formed and Cu2+ doped Nd2O3 phosphors show that the particles are irregular in shape and porous in nature. TEM results also confirm the nature and size of the particles. The EPR spectrum exhibits two resonance signals with effective g values at gǀǀ ≈ 2.12 and g⊥ ≈ 2.04. The g values indicate that the site symmetry of Cu2+ ions is octahedral symmetry with elongated tetragonal distortion. Raman studies show major peaks, which are assigned, to Fg and combination of Ag + Eg modes. It is observed that the Raman peaks and intensity have been reduced in Cu2+ doped samples. UV–Visible absorption spectra exhibit a strong and broad absorption band at ∼240 nm. Further, the absorption peak shifts to ∼14 nm in Cu2+ doped samples. The optical band gap is estimated to be 5.28 eV for Cu doped Nd2O3 nanoparticles which are higher than the bulk Nd2O3 (4.7 eV). This can be attributed to the quantum confinement effect of the nanoparticles