A. K. Tyagi

Ultrafine ceria powders via glycine-nitrate combustion

The ultrafine ceria powders have been synthesized by the combustion technique using glycine as a fuel and nitrate as an oxidizer. The auto-ignition (at ≈200°C) of the viscous liquids containing cerium nitrate and glycine resulted in voluminous ceria powders. An interpretation based on an adiabatic flame temperature, for different fuel-to-oxidant ratios, has been proposed for the nature of combustion and its correlation with the powder characteristics. The combustion synthesized ceria powders have been characterized by XRD, HRTEM, surface area analysis, and sinterability. Specific surface area and primary crystallite size of the ceria powder obtained through fuel-deficient precursor was found to be ≈75 m2/g and 2.5–12 nm, respectively. The powder, when cold pressed and sintered in air at 1250°C for 1 h, attained the sintered density ≈94% of its theoretical density, with submicron grain size.

Particle size dependence of magnetization and phase transition near TN in multiferroic BiFeO3

We report results of a comprehensive study of the phase transition at TN (∼643K) as a function of particle size in multiferroic BiFeO3 system. We employed electrical, thermal, and temperature dependent x-ray diffraction studies in order to characterize the transition in a host of samples. We also carried out detailed magnetic measurements over a temperature regime of 2–300K under a magnetic field of 100–10000Oe both on bulk and nanocrystalline systems. While in the bulk system a sharp endothermic peak at TN together with a broad feature, ranging over nearly ∼100K (ΔT), could be observed in calorimetry, the nanoscale systems exhibit only the broad feature. The characteristic dielectric anomaly, expected at TN, is found to occur both at TO and TN across ΔT in the bulk sample. The Maxwell-Wagner component due to interfaces between heterogenous regions with different conductivities is also present. The magnetic properties, measured at lower temperature, corroborate our observations in calorimetry. The metasta...

Theoretical and experimental evidence of enhanced ferromagnetism in Ba and Mn cosubstituted BiFeO3

Ba and Mn doped BiFeO3 prepared through the pyrolysis of xerogel precursors are characterized by powder x-ray diffraction, high resolution transmission electron microscopy, superconducting quantum interference device magnetometry, and polarization measurements. Structural studies by x-ray diffraction and transmission electron microscopy show a tetragonal lattice for Ba substituted BiFeO3 and a rhombohedral lattice for Mn substituted BiFeO3. A large ferromagnetic hysteresis loop is observed for Ba doped BiFeO3. Coexistence of distorted rhombohedral and tetragonal phases is observed in Ba and Mn codoped BiFeO3, where enhanced ferroelectric and ferromagnetic properties are produced by the internal strain. Density functional calculations are used to substantiate the results.

SnO2:Eu3+ nanoparticles dispersed in TiO2 matrix: Improved energy transfer between semiconductor host and Eu3+ ions for the low temperature synthesized samples

SnO2:Eu3+ nanoparticles uniformly dispersed in TiO2 matrix were prepared at 185°C in ethylene glycol. Unlike in SnO2:Eu3+, significant improvement in the exciton mediated energy transfer between SnO2 and Eu3+ ions was observed when SnO2:Eu3+ nanoparticles are dispersed in TiO2 matrix, and this is attributed to effective shielding of surface Eu3+ ions present in SnO2:Eu3+ nanoparticles from the vibrations of stabilizing ligand by TiO2 matrix. Annealing the samples at high temperatures leads to formation of Sn1−xTixO2, without significantly affecting the energy transfer process between Eu3+ ions and semiconductor host.

Coexistence of sign reversal of both magnetization and exchange bias field in the core-shell type La0.2Ce0.8CrO3 nanoparticles

We report an extraordinary coexistence of sign reversal of both magnetization and exchange bias field in the La0.2Ce0.8CrO3 nanoparticles. From the high resolution transmission electron microscopy image, and field dependence of thermoremanent and isothermoremanent magnetization measurements, the nanoparticles are found to be of core-shell nature. The core-shell configuration with an antiferromagnetic core of the Cr3+ and Ce3+ spins and a disordered shell with the uncompensated spins, explains the sign reversal of both magnetization and exchange bias field. The present study shows an excellent way of tuning the sign of both magnetization and exchange bias field in a single magnetic system.

Development of a nano-zirconia based 68Ge/68Ga generator for biomedical applications.

INTRODUCTION Most of the commercially available (68)Ge/(68)Ga generator systems are not optimally designed for direct applications in a clinical context. We have developed a nano-zirconia based (68)Ge/(68)Ga generator system for accessing (68)Ga amenable for the preparation of radiopharmaceuticals. METHODS Nano-zirconia was synthesized by the in situ reaction of zirconyl chloride with ammonium hydroxide in alkaline medium. The physical characteristics of the material were studied by various analytical techniques. A 740 MBq (20 mCi) (68)Ge/(68)Ga generator was developed using this sorbent and its performance was evaluated for a period of 1 year. The suitability of (68)Ga for labeling biomolecules was ascertained by labeling DOTA-TATE with (68)Ga. RESULTS The material synthesized was nanocrystalline with average particle size of ~7 nm, pore-size of ~4 Å and a high surface area of 340±10 m(2) g(-1). (68)Ga could be regularly eluted from this generator in 0.01N HCl medium with an overall radiochemical yield >80% and with high radionuclidic (<10(-5)% of (68)Ge impurity) and chemical purity (<0.1 ppm of Zr, Fe and Mn ions). The compatibility of the product for preparation of (68)Ga-labeled DOTA-TATE under the optimized reaction conditions was found to be satisfactory in terms of high labeling yields (>99%). The generator gave a consistent performance with respect to the elution yield and purity of (68)Ga over a period of 1 year. CONCLUSIONS The feasibility of preparing an efficient (68)Ge/(68)Ga generator which can directly be used for biomedical applications has been demonstrated.

Ag incorporated nano BiPO4: sonochemical synthesis, characterization and improved visible light photocatalytic properties

We report an efficient route for the sonochemical synthesis of undoped BiPO4, Ag3PO4 and silver doped BiPO4:Ag(x%) (x = 2, 5, 10 and 20) nanostructures using bismuth/silver nitrate and ammonium dihydrogen phosphate as precursors. The products obtained have been characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (UV-vis DRS) and X-ray photoelectron spectroscopy (XPS). The size and morphology of BiPO4 exhibited drastic changes on Ag doping. The surface areas of the samples have been estimated using the Brunauer–Emmett–Teller (BET) method. The catalytic activities of all the samples for the rhodamine-B degradation were investigated systematically under UV and visible-light irradiation. Undoped BiPO4 exhibited excellent photocatalytic activity under UV light but the degradation of RhB was only ∼60% under visible light, while Ag doped BiPO4 samples showed almost complete degradation of the dye under visible light. Amongst all of them, BiPO4:Ag(10%) exhibited the best photocatalytic activity. Furthermore, after photocatalysis, the nanoparticles could be readily separated from the reaction system by low-speed centrifugation and reused. Stability of the photocatalysts was ascertained using FT-IR and Raman spectroscopy. After five recycles, the nanoparticles did not exhibit any apparent loss in activity, confirming its stability despite recycling. By tuning the band gap and measuring the surface area of the nanoparticles using BET tests, we found that the combined effect of these two factors resulted in good performance of the BiPO4:Ag(10%) photocatalyst under visible light irradiation.

La1-xCexCrO3 (0.0 ≤ x ≤ 1.0): A New Series of Solid Solutions with Tunable Magnetic and Optical Properties

A new series of La(1-x)Ce(x)CrO(3) (0.0 <or= x <or=1.0) compounds in nanocrystalline form were synthesized using a two-step synthesis route, involving an initial combustion reaction followed by vacuum heating in the presence of a Zr sponge, which acted as an oxygen getter. For the first time, a homogeneous solid solution formation throughout the entire range was obtained in this series. These compounds were characterized using X-ray diffraction, diffuse reflectance UV visible spectrophotometry, and superconducting quantum interference device magnetometry. The crystallite size for the phase-pure products was confirmed to be approximately 42-44 nm by high-resolution transmission electron microscopy. All compounds (nanocrystalline) in this series are found to be predominantly antiferromagnetic in nature with a remarkable linear increasing trend in Neel temperature from 257 to 281.5 K as a function of decreasing Ce(3+) content. Interestingly, the band gap also shows a linear decrease from 3.21 to 3.02 eV as a function of increasing Ce(3+) concentration in the La(1-x)Ce(x)CrO(3) series.

X-ray diffraction, μ-Raman spectroscopic studies on CeO2−RE2O3 (RE=Ho, Er) systems: Observation of parasitic phases

The phase relations in CeO2−Ho2O3 and CeO2−Er2O3 systems have been established under the slow-cooled conditions. As per x-ray diffraction (XRD), in both the series a single-phasic solid solution forms up to the nominal composition Ce0.6RE0.4O1.8 (RE=Ho, Er) retaining the F-type structure of parent ceria. In Ce1−xErxO2−x/2 system the presence of microdomains of C-type phase have been revealed by Raman spectroscopy for composition x=0.4, which has been identified as single phasic by XRD. Photoluminescence studies also show that biphasic region commences from x=0.4 for Ce1−xHoxO2−x/2 series. The biphasicity continues until x=0.7 for both the series. From x=0.8 the solid solutions exist as C-type single phasic, which is isotypic to another end member RE2O3 (RE=Ho, Er) and as revealed by both XRD and Raman spectroscopy. High temperature XRD studies show that no temperature induced phase change has been observed in either of the series until 1273 K. In this work photoluminescence data was used to delineate the ...

Gd2O3:Eu3+ particles prepared by glycine-nitrate combustion: Phase, concentration, annealing, and luminescence studies

Eu3+ doped Gd2O3 particles have been prepared by glycine-nitrate combustion route. Eu–O charge transfer peak shifts to higher wavelength from 260 to 273 nm with increasing Eu3+ concentration, whereas the linewidth at half maximum intensity varies with Eu3+ concentrations and annealing temperatures as well. Lifetime for D50 level of Eu3+ increases with annealing temperature from 500 to 900 °C, and this is related to decreasing surface to volume atomic ratio of particles. Asymmetric ratio defined by the ratio of intensity of electrical dipole transition to that of magnetic dipole transition is found to be 10, which is more than the reported values (3–4) indicating enhancement in luminescence by this synthesis route.