Madhuri Mandal

Enhancement of ferromagnetism upon thermal annealing in pure ZnO

We report here the enhancement of ferromagnetism in pure ZnO upon thermal annealing with the ferromagnetic transition temperature Tc above room temperature. We observe a finite coercive field up to 300K and a finite thermoremanent magnetization up to 340K for the annealed sample. We propose that magnetic moments can be formed at anionic vacancy clusters. Ferromagnetism can occur due to either superexchange between vacancy clusters via isolated F+ centers or through a limited electron delocalization between vacancy clusters. Isolated vacancy clusters or isolated F+ centers give rise to a strong paramagneticlike behavior below 10K.

Reduction of methylene blue (MB) by ammonia in micelles catalyzed by metal nanoparticlesPresented at the national conference on “Self Aggregating System – Recent Advances” held March 16th, 2002 in Calcutta, India.

The increased encounter probability between ammonia and an immobilized cationic dye in non-ionic micelles demonstrates the reduction capability of ammonia while present even at ppm levels. Catalytic amounts of silver nanoparticles (∼7 nm in size) have been shown to enhance the reaction rate of the reduction reaction, explained by the strong affinity between Ag nanoparticles and the nitrogen atom of the donor ammonia molecule. Furthermore, the preferential participation of the ammonia molecule as reductant has been proven unequivocally by taking micelle-bound dye as the reagent in the presence of phenolphthalein. The proper choice of micelle, influence of different “salting-in” and “salting-out” agents, catalysis involving nanoparticles and the thermodynamic parameters of the reaction support the possible quantification process of ammonia in solution.

Design and synthesis of high performance multifunctional ultrathin hematite nanoribbons.

1D porous ultrathin nanoribbons of hematite (α-Fe2O3) were prepared by controlled annealing of different oxides and hydroxides of iron obtained from a solvothermal synthesis method. It is found that calcination at a temperature of 500 °C for 150 min decomposes these iron hydroxides into their most stable oxide form, i.e., α-Fe2O3. Driven by different attractive forces, these porous α-Fe2O3 nanoparticles get aggregated in an ordered fashion to form an ultrathin 1D nanoribbon structure, as observed by detailed time dependent TEM and HRTEM analysis. It has been found that the high aspect ratio and porous surface morphology of these nanoribbons have significantly improved their electronic and spintronic properties as manifested by their photocatalysis, gas sensing, electrochemical, and magnetic behaviors. These hematite nanoribbons exhibit a weak ferromagnetic behavior due to surface spin disorder and shape anisotropy. Lateral confinement of electrons increases the band gap of the nanoribbons, as evident from the UV absorption analysis, which in turn improves their photocatalytic degradation efficiency (rate constant ∼0.95 h(-1)) by delaying the electron-hole recombination process. However, their liquid petroleum gas sensing properties have been found to be mainly governed by the improved (porous) surface of the hematite nanoribbons that provides huge interaction sites for the analyte gas. Most of all, these hematite nanoribbons show a significantly enhanced pseudocapacitive performance exhibited by their high specific capacitance of about 145 F g(-1) at a current density of 1 A g(-1), high rate capability, and also long cycle stability (nearly 96% of capacity retention after 1600 charging/discharging cycles).

Domain controlled magnetic and electric properties of variable sized magnetite nano-hollow spheres

Here, we report the synthesis of variable sized magnetite (Fe3O4) nano-hollow spheres in one step template free solvothermal method and their size dependent magnetic and electrical properties. Size of the hollow spheres is varied from 100 nm to 725 nm by changing the concentration of capping agent. Trace of Verway transition is found for all sets of spheres and the Verway transition temperature (TV) increases with increasing size of the spheres. The domain structure of these spheres changes from pseudo single domain to multi domain state as the size increases from 100 nm to 725 nm as evident from Day plots. This change in domain structure also changes the magnetic and electric properties of these spheres. Temperature dependent of high field magnetization of the hollow spheres can be well explained by Blochs power law with higher than the bulk value of Bloch constant. The Bloch exponent varies from 1.94 to 1.69 with increasing size of the spheres. Frequency dependence of electrical conductivity (σ) shows ...

Micelles induced high coercivity in single domain cobalt-ferrite nanoparticles

We have prepared CoFe2O4 nanoparticles in micellar medium by wet chemical technique and obtained very high coercivity value of 4.4 kOe at room temperature for particle size ∼16 nm. A large coercivity (∼20 kOe) is observed on cooling down to 2.5 K. We annealed the sample at different temperatures to check the role of micelles and particles size in the change in coercivity value. Here we observed micelles as capping agent playing an important role to enhance the coercivity, as after removal of micelles for the same particle size the coercivity drops from 4.4 kOe to small value ∼350 Oe. But the coercivity again increases due to the increase in particle size with increase in annealing temperature from 873 K and above. To obtain structural information and size of particles, we have taken x-ray diffraction spectra from the samples before and after annealing at different temperatures which confirm the spinel phase only.

Lanthanide cation-induced tuning of surface capping properties in zinc sulfide nanoparticles: an infrared absorption study

This study reveals the tuning of the vibrational characteristics of the capping ligands in lanthanide [Ln = Sm, Eu, Tb, Dy] cation-incorporated zinc sulfide nanoparticles, Zn(Ln)S, as monitored by Fourier transform infrared (FTIR) spectroscopy. Both stearate and trioctylphosphine oxide (TOPO) were found to act as surface-capping ligands for nanoparticles with multiple coordination environments. The vibrational characteristics of the capping ligands in the undoped ZnS nanoparticles exhibited noticeable differences in absorption as compared to those of the pure zinc stearate and TOPO molecules. Lanthanide cation incorporation tunes the corresponding vibrational characteristics to a significant extent, as compared to that in the undoped ZnS nanoparticles. It has been argued that the observed tuning of the capping ligand IR absorption characteristics is induced by the lanthanide cations that are located near or on the surface of the nanoparticles. The IR spectra suggest a probable change of carboxylate coordination environment in the Zn(Ln)S nanoparticles. The results obtained from the ZnS-based nanoparticles were analyzed based on a semi-quantitative analysis and compared with those from ZnSe and Zn(Tb)Se nanoparticles, in order to evaluate the effect of the constituent anion of the nanoparticles in modulating the IR signature.

Tuning of magnetic properties of CoFe2O4 nanoparticles through charge transfer effect

Herein, we report the microscopic origin of surfactant modified magnetic properties of nearly monodispersed CoFe2O4 nanoparticles (NPs). Surface modification is carried out with four surfactants having π-acceptor/π-donor head group along with different chain-length. Upon functionalization, magnetic NPs show a maximum 65.61% increase in coercivity and 78.24% decrease in magnetization as compared to the bare one. Furthermore, π-donor head group surfactant modified CoFe2O4 NPs show higher coercivity and magnetization with respect to π-acceptor head group surfactant modified NPs and with the increase in chain-length of the surfactant, coercivity of NPs enhances slightly. These consequences are explained in context of crystal field splitting energy and steric hindrance offered by the surfactant.

Synthesis of CoxPt1−x alloy nanoparticles of different phase by micellar technique and their properties study

Among many ferromagnetic materials, CoPt alloy nanoparticles with high coercivity and small grain size are one of the potential candidates having its application in high-density storage media manufacturing. CoPt alloy nanoparticles of different compositions have been prepared in TX-100 micellar medium and capped with sodium oleate to prevent the agglomeration during annealing at high temperature. Their structural and magnetic properties were investigated before and after heat treatment. The particles with approximately 50:50 compositions of Co and Pt gives L1(0) phase after annealing at 900 degrees C for 1 h and show a high coercivity (approximately 4.4 kOe at room temperature) and remanence due to its highly anisotropic L1(0) structure.

Research Update: Facile synthesis of CoFe2O4 nano-hollow spheres for efficient bilirubin adsorption

Herein, we report an unprecedented bilirubin (BR) adsorption efficiency of CoFe2O4 (CFO) nanostructures in contrast to the commercially available activated carbon and resin which are generally used for haemoperfusion and haemodialysis. We have synthesized CFO nanoparticles of diameter 100 nm and a series of nano-hollow spheres of diameter 100, 160, 250, and 350 nm using a simple template free solvothermal technique through proper variation of reaction time and capping agent, oleylamine (OLA), respectively, and carried out SiO2 coating by employing Stober method. The comparative BR adsorption study of CFO and SiO2 coated CFO nanostructures indicates that apart from porosity and hollow configuration of nanostructures, the electrostatic affinity between anionic carboxyl group of BR and cationic amine group of OLA plays a significant role in adsorbing BR. Finally, we demonstrate that the BR adsorption capacity of the nanostructures can be tailored by varying the morphology as well as size of the nanostructures. We believe that our developed magnetic nanostructures could be considered as a potential material towards therapeutic applications against hyperbilirubinemia.

Synthesis of DNA templated trifunctional electrically conducting, optical, and magnetic nanochain of Nicore–Aushell for biodevice

Synthesis of tri functional electrically conducting, optical and magnetic nano-chain of Nicore-Aushell has been discussed here. Our Investigation indicates that such material attached with biomolecule DNA in chain form will have great potentiality in medical instrument and bio computer device.