Balaji P. Mandal

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 ...

Improved magnetic and ferroelectric properties of Sc and Ti codoped multiferroic nano BiFeO3 prepared via sonochemical synthesis

The room temperature multiferroic properties of bulk BiFeO3 are not exciting enough for its application in devices. Here, we report the sonochemical synthesis of scandium and titanium codoped BiFeO3 nanoparticles which exhibit improved magnetic and ferroelectric properties at room temperature. The nanoparticles have been checked for phase purity and composition using powder X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). The size and morphology of the nanoparticles have been confirmed using scanning electron microscopy (SEM), and both low and high resolution transmission electron microscopy (TEM/HRTEM). The breaking of the spin cycloid due to the smaller size and slight structural distortion caused by the doping has been found to be instrumental for the enhancement of multiferroic properties. The electrical polarization increases significantly in the case of BiFe(0.925)Sc(0.05)Ti(0.025)O3 nanoparticles. A marked reduction in the leakage current was seen compared to undoped BiFeO3. Magnetoelectric coupling was also observed in the BiFe(0.925)Sc(0.05)Ti(0.025)O3 sample. Our results demonstrate that codoping with Sc and Ti ions is an effective way to rectify and enhance the multiferroic nature of BiFeO3.

Inorganic–organic multiferroic hybrid films of Fe3O4 and PVDF with significant magneto-dielectric coupling

Flexible and self-standing inorganic-organic hybrid films of silica coated Fe3O4 nanoparticles and polyvinylidene fluoride (PVDF) polymer with significant magneto-dielectric coupling have been prepared at low temperature by a simple solvent casting method. PVDF films with two different concentrations of silica coated Fe3O4 (4.76 and 9.09 wt%) have been developed and well characterized using different techniques like XRD, TEM and IR. The thin film coating of silica on Fe3O4 could be detected by IR and TEM. The ferroelectric, magnetic and magneto-capacitive measurements at room temperature confirm the multiferroic nature of the composite films with significant magneto-electric coupling between Fe3O4 and PVDF.

Oxygen vacancy enhanced room temperature magnetism in Al-doped MgO nanoparticles

We have measured the room temperature magnetization in Al-substituted magnesium oxide, Mg(Al)O nanoparticles with Al fractions of up to 5 at. %. All samples, including undoped MgO nanoparticles, exhibit weak room temperature ferromagnetism, with the saturation magnetization reaching a maximum of 0.023 emu/g at 2 at. % of Al. X-ray photoelectron spectroscopy identifies the presence of oxygen vacancies in both doped and undoped MgO nanoparticles, with the vacancy concentration increasing upon vacuum annealing of Mg(Al)O, resulting in two-fold enhancement of the saturation magnetization for 2 at. % Al-doped MgO. Our results suggest that the oxygen vacancies are largely responsible for the weak room temperature ferromagnetism in MgO.

Biomimetic synthesis of nanocrystalline silver sol using cysteine: stability aspects and antibacterial activities

The study reports the development of a simple, environmentally benign green chemical route to produce stable silver nanoparticle (Ag-np) sols with excellent antibacterial properties under ambient conditions. The method involves the room temperature reduction of AgNO3 by cysteine (aq) and requires no additional capping/stabilizing agent. It essentially mimics the redox reaction that takes place during incubation of the cell-free extract from Trichoderma asperellum in the presence of AgNO3 (aq) (P. Mukherjee, M. Roy, B. P. Mandal, G. K. Dey, P. K. Mukherjee, J. Ghatak, A. K. Tyagi and S. P. Kale, Nanotechnology, 2008, 19, 075103), wherein cysteine, a biomolecule present in the fungal extract, acts as a potential reducing agent. Additionally, cysteine acts as a capping molecule in the present case. Formation of Ag-nps was evidenced from UV-Vis, TEM, XRD and EDS studies. The stability of Ag sols has been shown to depend strongly on the concentration of cysteine relative to that of AgNO3. Sols obtained by reacting 0.1 mM of cysteine with 1 mM of AgNO3 remained stable for more than one month at 24 °C. The role of cysteine as capping molecule and the possible modes of its linkages with Ag-nps was studied by FT-IR, XPS and Raman spectroscopy. Bonding of Ag with either or all the three, thiolate, amino and carboxylate groups of the cysteine molecule via stable PH configuration is believed to have resulted in the stabilization of the Ag-nps. Antibacterial activity of the cysteine capped Ag sol was studied along with that of the Ag sol obtained by fungal route. Both the sols exhibited excellent and comparable efficacies as bactericidal agents against gram negative bacteria E. coli BW (25113), with one of the lowest minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values published so far.

Crucial role of the reaction conditions in isolating several metastable phases in a Gd-Ce-Zr-O system.

A series of samples with composition Gd(2-y)Ce(y)Zr(2)O(7) (0.0 ≤ y ≤ 2.0) were prepared by the gel combustion method followed by high-temperature reduction. The details of the structural variations as a function of the composition, temperature, and oxygen stoichiometry have been investigated by X-ray diffraction (XRD), high-temperature XRD (HT-XRD), and thermogravimetry. A complete solubility of Gd(3+) in Ce(2)Zr(2)O(7) and Ce(2)Zr(2)O(8) could be achieved by this adaptive preparative method. Analysis of the XRD data revealed a sequential variation of the structural features with oxygen stoichiometry as well as Gd(3+) contents in these compositions. The variation in the unit cell parameter along the compositions has a strong influence on the oxygen uptake behavior in the Gd(2-y)Ce(y)Zr(2)O(7) system, as observed from the thermogravimetric and HT-XRD studies. The preparation and stability of various metastable phases in Gd-Ce-Zr-O have been addressed in detail. The details of the study will be useful for the design and application of a potential redox catalyst and an oxygen storage capacitor.

Enhancement of dielectric permittivity and ferroelectricity of a modified cobalt nanoparticle and polyvinylidene fluoride based composite

Dielectric permittivity and loss of percolative composites of cobalt nanoparticle loaded polyvinylidene fluoride (PVDF) have been investigated. The films have been characterized by X-ray diffraction, infrared spectroscopy, dielectric relaxation spectroscopy and electrical polarization measurements. A remarkable increase in the amount of polar β phase of PVDF has been observed upon dispersion of cobalt nanoparticles in the PVDF matrix. The dielectric constant of the composite increases significantly near the percolation threshold (19 vol%) of filler cobalt nanoparticles in the PVDF matrix. The results have been explained by the space charge polarization at the interfaces between the two phases of the composite and the development of several micro capacitor structures. The electrical field dependent polarization also increases upon addition of cobalt nanoparticles. This could be due to formation of a higher amount of the β phase of PVDF.

Synthesis of uniform gold nanoparticles using non-pathogenic bio-control agent: Evolution of morphology from nano-spheres to triangular nanoprisms

Green synthesis of gold nanospheres with uniform diameter and triangular nanoprisms with optically flat surface was carried out using a non-pathogenic bio-control agent Trichoderma asperellum for reduction of HAuCl(4). Kinetics of the reaction was monitored by UV-Vis absorption spectroscopy. No additional capping/complexing agent was used for stabilizing the gold nanoparticles. Evolution of morphology from pseudospherical nanoparticles to triangular nanoprisms was studied by transmission electron microscopy (TEM). It revealed that three or more pseudospheres fused to form nanoprisms of different shapes and sizes. Slow rate of reduction of HAuCl(4) by constituents of cell-free fungal extract was instrumental in producing such exotic morphologies. Isolation of gold nanotriangles from the reacting masses was achieved by differential centrifugation.

Tunability of structure from ordered to disordered and its impact on ionic conductivity behavior in the Nd2−yHoyZr2O7 (0.0 ≤ y ≤ 2.0) system

The present paper describes a detailed investigation of the nature of the order–disorder transition in the Nd2−yHoyZr2O7 system, over the entire composition range (0.0 ≤ y ≤ 2.0), using a combination of diffraction and spectroscopic analysis techniques. The complete series of solid solutions has been prepared by the gel combustion technique followed by high temperature sintering. Detailed structural characterization of the prepared samples using X-ray diffraction studies confirmed their single-phase solid solution formation throughout the series, as well as identifying the transformation of the system from an ordered pyrochlore structure (0.0 ≤ y ≤ 0.8) to a disordered fluorite-type structure (0.8 < y ≤ 2.0). Micro-Raman studies further supported the structural information. Micro-structural studies by SEM revealed dense products with uniform grain distribution in these materials. Comprehensive studies were performed to investigate the ionic conductivity behavior of these solid solutions in correlation with the structural transformations using AC impedance spectroscopy. A detailed analysis of ionic conductivity and related parameters, such as activation energies, etc. corresponding to individual conduction mechanisms such as bulk and grain boundary conduction, clearly correlated the effect of structural transformation on these charge transport properties. The composition Nd1.2Ho0.8Zr2O7 with the pyrochlore structure has been found to show the highest ionic conductivity in the entire series, which has been attributed to a combined effect of increasing carrier concentration and their preferred movement through an ordered array of vacant anion sites.

Fabrication of flexible and self-standing inorganic–organic three phase magneto-dielectric PVDF based multiferroic nanocomposite films through a small loading of graphene oxide (GO) and Fe3O4 nanoparticles

Flexible inorganic-organic magneto-electric (ME) nanocomposite films (PVDF, PVDF-GO, PVDF-Fe3O4 and PVDF-GO-Fe3O4), composed of well-dispersed graphene oxide (GO 5 wt%) and magnetic Fe3O4 nanoparticles (5 wt%) embedded into a poly(vinylidene-fluoride) (PVDF) matrix, have been prepared by a solvent casting route. The magnetic, ferroelectric, dielectric, magneto-dielectric (MD) coupling and structural properties of these films have been systematically investigated. Magnetic (Ms = 2.21 emu g(-1)) and ferroelectric (P = 0.065 μC cm(-2)) composite films of PVDF-GO-Fe3O4 (PVDF loaded with 5% GO and 5% Fe3O4) with an MD coupling of 0.02% at room temperature (RT) showed a three times higher dielectric constant than that of the pure PVDF film, with a dielectric loss as low as 0.6. However, the PVDF-Fe3O4 film, which exhibited improved magnetic (Ms = 2.5 emu g(-1)) and MD coupling (0.04%) properties at RT with a lower dielectric loss (0.3), exhibited decreased ferroelectric properties (P = 0.06 μC cm(-2)) and dielectric constant compared to the PVDF-GO-Fe3O4 film. MD coupling measurements carried out as a function of temperature on the multi-functional PVDF-GO-Fe3O4 film showed a systematic increase in MD values up to 100 K and a decrease thereafter. The observed magnetic, ferroelectric, dielectric, MD coupling and structural properties of the nanocomposite films are attributed to the homogeneous dispersion and good alignment of Fe3O4 nanoparticles and GO in the PVDF matrix along with a partial conversion of nonpolar α-phase PVDF to polar β-phase. The above multi-functionality of the composite films of PVDF-Fe3O4 and PVDF-GO-Fe3O4 paves the way for their application in smart multiferroic devices.