Kalyan Mandal

Synthesis of a α-Fe2O3 nanocrystal in its different morphological attributes: growth mechanism, optical and magnetic properties

Nanospindle and nanorhombohedron and nanocube structured α-Fe2O3 was synthesized by the solvothermal method. An intermixing of ethylenediamine (EN) either with ethanol (EtOH) or water in different volume ratios (either 15:85, 50:50 or 85:15 in particular) was used to generate the structural forms of α-Fe2O3. The study showed that, during synthesis, EN functioned as a ligand and facilitated the growth of nanostructured samples. The probable growth mechanism is discussed in this paper. Field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) investigations revealed that the nanostructures were formed through oriented attachment of primary nanocrystals. Fourier transform infrared spectroscopy (FTIR) results showed the presence of Fe–O or Fe–O–Fe vibrational bands whereas UV–vis–NIR optical absorbance spectra showed two prominent absorption bands around 540–560 and 670–680 nm. The room temperature magnetization measurement revealed that the remanence and coercivity depend on the morphological attributes of the nanocrystals. The magnetic hysteresis measurement also revealed that α-Fe2O3 nanostructures displayed weak ferromagnetic behaviour at room temperature.

Hydrogenated NiO nanoblock architecture for high performance pseudocapacitor.

Supercapacitor electrodes are fabricated with the self-organized 3D architecture of NiO and hydrogenated NiO (H-NiO) nano-blocks (NBs) grown by the facile electrodeposition and high temperature annealing of the Ni foil on Cu substrate. The unique architecture of H-NiO NBs electrode exhibits excellent cycling stability (only 5.3% loss of its initial specific capacitance after 3000 cycles at current density of 1.1 A g(-1)) along with the high specific and areal capacitance of ∼1272 F g(-1) and 371.8 mF cm(-2), respectively at scan rate of 5 mV s(-1) compared with the pure NiO NBs electrode (∼ 865 F g(-1) and 208.2 mF cm(-2), respectively at scan rate of 5 mV s(-1)). H-NiO NBs electrode also exhibits excellent rate capability; nearly 61% specific capacity retention has been observed when the current density increases from 1.11 to 111.11 A g(-1). This electrode offers excellent energy density of 13.51 Wh kg(-1) and power density of 19.44 kW kg(-1) even at a high current density of 111.11 A g(-1). The superior pseudocapacitive performance of the H-NiO NBs electrode is because of the high electron and ion conductivity of the active material because of the incorporation of hydroxyl groups on the surface of NiO NBs.

Vacancy-induced intrinsic d0 ferromagnetism and photoluminescence in potassium doped ZnO nanowires

Cation vacancy-induced d0 room temperature ferromagnetism was observed in nonmagnetic potassium (K) doped ZnO nanowires (NWs) synthesized within the pores of the anodic aluminum oxide template. The ferromagnetic signature was found to be significantly enhanced in the K-doped ZnO NWs with respect to the pristine ZnO NWs. The photoluminescence studies clearly indicated the presence of a large concentration of zinc vacancies in the K-doped ZnO NWs. An interesting correlation between the saturation magnetization and green luminescence intensity with the increase of K-doping has suggested that the magnetic moment originates due to Zn vacancy defects. It is expected that the incorporation of K-related defects at the Zn site might promote the formation of zinc vacancies in the system and introduce holes to stabilize the hole-mediated room-temperature ferromagnetism. For the doped ZnO NWs the ferromagnetic response was found to be a maximum at an optimum K-concentration of 4 at. %. This study demonstrates that th...

Unique hydrogenated Ni/NiO core/shell 1D nano-heterostructures with superior electrochemical performance as supercapacitors

This study demonstrates a scheme to design and fabricate a novel 1D core/shell Ni/NiO nano-architecture electrode as a pseudocapacitor with significantly improved capacitive performance through hydrogenation. The specific capacitance of the as prepared 1D core/shell Ni/NiO nanoheterostructure (717 F g−1 at a scan rate of 2 mV s−1) is nearly 1635 F g−1 after the hydrogenation. The improved pseudocapacitive properties of hydrogenated Ni/NiO nano-heterostructures are attributed to the incorporation of the hydroxyl groups on the NiO surface due to hydrogenation, where the metallic Ni nanowire core of this unique 1D core/shell heterostructure serves as the efficient channel for the fast electron conduction to the current collector. The H–Ni/NiO nanoheterostructures exhibit good rate capability (retaining nearly 60% of their initial charge) and good long-term cycling stability with an excellent specific energy and power density of 49.35 W h kg−1 and 7.9 kW kg−1, respectively, at a current density of 15.1 A g−1. This study demonstrates that the H–Ni/NiO nano-heterostructure is very promising for next generation high-performance pseudocapacitors.

The study of magnetocaloric effect in R2Fe17 (R = Y, Pr) alloys

Magnetocaloric effect (MCE) in low-cost iron-based binary alloys R2Fe17 (R = Y, Pr) has been investigated by measuring their magnetic properties. The Curie temperature of these alloys is found to be close to room temperature. The specific heat measurement of these materials indicates a second order ferromagnetic/paramagnetic phase transition. The MCE properties in these alloys are comparable to that of gadolinium, which is very expensive for domestic use. Therefore these binary alloys are suitable as room temperature MCE materials. The effect of pulsed magnetic field on the MCE properties has also been studied up to 25 T.

Enhanced ferroelectric, magnetoelectric, and magnetic properties in Pr and Cr co-doped BiFeO3 nanotubes fabricated by template assisted route

Arrays of single phase perovskite-type polycrystalline pure, Pr and Cr doped, and Pr-Cr co-doped BiFeO3 (BFO) nanotubes (NTs) (∼50 nm wall thickness) have been synthesized using simple wet chemical liquid phase deposition template assisted technique. Spontaneous enhancement in the ferroelectricity, magnetoelectricity, and ferromagnetic ordering are evidenced in the Pr and Cr co-doped BFO NTs. Significant increase in the ferroelectric characteristics in co-doped BFO NTs suggests the lower leakage current due to the reduction of the oxygen vacancies in the structure. Strong magnetoelectric coupling is observed in co-doped BFO NTs, where the increase of the dielectric constant is noticeable with the increase of the applied magnetic field. Substantial increase in the ferromagnetic signature in the co-doped BFO NTs is believed to be due to the collapse of the space-modulated spin structure.

A study of magnetic flux-leakage signals

Magnetic flux-leakage (MFL) measurement is the most widely used non-destructive technique for in-service inspection of oil and gas pipelines. The effect of line-pressure-induced hoop stress on MFL signals has been studied for an electrochemically milled pit (50% penetration) in a 9 mm thick steel pipe wall. The existing theoretical models for flux-leakage signals are discussed. Among them the Zatsepin-Shcherbinin and Edwards-Palmer models have been fitted to the axial and radial MFL signals. Both the leakage-flux components under various stresses are scaled to make them stress independent.

Size-dependent magnetic properties of Mn0.5Zn0.5Fe2O4 nanoparticles in SiO2 matrix

Mn0.5Zn0.5Fe2O4 ferrite nanoparticles (<100 nm) in SiO2 matrix have been prepared by the sol-gel method. The particle size was varied by changing the duration of heat treatment above crystallization temperature. An x-ray diffraction study indicates the presence of single-phase spinel ferrite in the sample. The particle size was estimated by the x-ray diffraction method as well as from the micrograph taken by a transmission electron microscope. The magnetic properties of the samples were studied by a vibrating sample magnetometer. The samples show superparamagnetic behavior when the particle size is below 20 nm, which is confirmed by Mossbauer spectroscopy measurements. The average particle size in the superparamagnetic state was also estimated from the low-field magnetization measurement by considering the samples as consisting of noninteracting single domain particles.

Surface Modification of MnFe2O4 Nanoparticles to Impart Intrinsic Multiple Fluorescence and Novel Photocatalytic Properties

The MnFe2O4 nanoparticle has been among the most frequently chosen systems due to its diverse applications in the fields ranging from medical diagnostics to magnetic hyperthermia and site-specific drug delivery. Although numerous efforts have been directed in the synthesis of monodisperse MnFe2O4 nanocrystals, unfortunately, however, studies regarding the tuning of surface property of the synthesized nanocrystals through functionalization are sparse in the existing literature. Herein, we demonstrate the emergence of intrinsic multicolor fluorescence in MnFe2O4 nanoparticles from blue, cyan, and green to red, upon functionalization and further surface modification with a small organic ligand, Na-tartrate. Moreover, we have found an unprecedented photocatalytic property of the functionalized MnFe2O4 nanoparticles in the degradation of a model water contaminant. Detailed characterization through XRD, TEM, and FTIR confirms the very small size and functionalization of MnFe2O4 nanoparticles with a biocompatible ligand. Proper investigation through UV-visible absorption, steady-state and time-resolved photoluminescence study reveals that ligand-to-metal charge-transfer transition from the tartrate ligand to the lowest unoccupied energy level of Mn(2+/3+)or Fe(3+) of the NPs and Jahn-Teller distorted d-d transitions centered over Mn(3+) ions in the NPs play the key role behind the generation of multiple fluorescence from the ligand-functionalized MnFe2O4 nanoparticles. VSM measurements indicates that the superparamagnetic nature of MnFe2O4 nanoparticles remains unchanged even after surface modification. We believe that the developed superparamagnetic, multicolor fluorescent MnFe2O4 nanopaticles would open up new opportunities as well as enhance their beneficial activities toward diverse applications.

Defect-driven magnetism in luminescent n/p-type pristine and Gd-substituted SnO2 nanocrystalline thin films.

The effects of rare-earth-element Gd doping on the intrinsic magnetic ordering, photoluminescence, and electrical-conducting properties of the pristine SnO(2) nanocrystalline thin films fabricated by radio-frequency (RF) sputtering are investigated. The pristine SnO(2) thin film exhibits significant ferromagnetism while Gd doping results in an absence of intrinsic ferromagnetism. The presence of large amounts of singly ionized oxygen vacancies (V(O)(+)) is traced by photoluminescence spectroscopic analysis and they are found to be responsible for the observed ferromagnetism in pristine SnO(2) thin films. A significant reduction of oxygen vacancies is observed after Gd doping, and that might be insufficient to mediate long-range ferromagnetic ordering between V(O)(+) defects in a Gd-doped SnO(2) system. Although the associated magnetic moment is increased by 1 order of magnitude, because of the insertion of Gd(3+) ions, which have localized f-shell paramagnetic moment, there is no intrinsic FM ordering. Hall measurement reveals that the pure SnO(2) exhibits n-type behavior whereas Gd-doped SnO(2) films show the p-type conductivity with higher resistivity. The studies demonstrate that only structural defects such as V(O)(+) defects, not magnetic ions such as Gd(3+), are responsible for inducing ferromagnetism in SnO(2) thin films.