O. D. Jayakumar

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.

Rare examples of fluoride-based multiferroic materials in Mn-substituted BaMgF4 systems: experimental and theoretical studies.

A series of Mn-substituted BaMgF(4) samples have been synthesized by a hydrothermal route. X-ray diffraction study reveals that the products are monophasic in nature. Scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS) studies were carried out to investigate the morphology and stoichiometry for these compounds. X-ray photoelectron spectroscoy (XPS) and electron spin resonance (ESR) studies were done to confirm the oxidation state of dopant ion. Room temperature ferromagnetism was observed on Mn substitution at the Mg site in BaMgF(4) samples. The saturation magnetization increases initially, shows a peaking effect, and then decreases with further increase in Mn concentration in BaMg(1-x)Mn(x)F(4) (0.0 ≤ x ≤ 0.15). However, ferroelectricity was found to decrease with an increase in Mn concentration in the series of investigated BaMg(1-x)Mn(x)F(4) (0.0 ≤ x ≤ 0.15) samples. First-principle calculations, using the projector augmented wave potentials on Mn-substituted BaMgF(4), confirmed the decrease in magnetic moment with an increase in Mn content beyond certain concentration. These samples exhibit very weak magnetocapacitive coupling, which can be attributed to the very small magnetic signal observed in these samples.

Enhancement of ferromagnetic properties in Zn0.95Co0.05O nanoparticles by indium codoping: An experimental and theoretical study

Nanoparticles of Zn0.95−xCo0.05InxO (x=0.0 to 0.07) were synthesized by the pyrolitic reaction of sol-gels obtained from respective metal precursors. X-ray diffraction and high-resolution transmission electron microscopy studies confirm the formation of impurity-free wurtzite type ZnO structure for all the compositions. While pristine ZnO is diamagnetic, Zn0.95Co0.05O nanoparticles show weak paramagnetic behavior at room temperature. When “In.” is codoped with Co with x=0.0 to 0.07 in Zn0.95−xCo0.05InxO, a systematic increase in magnetic moment is observed up to x=0.07. First-principles modeling supports that the ferromagnetic phase become more favorable at higher indium doping concentrations.

Experimental and theoretical investigations on magnetic behavior of (Al,Co) co-doped ZnO nanoparticles.

We present the structural and magnetic properties of Zn(0.95-x)Co(0.05)Al(x)O (x = 0.0 to 0.1) nanoparticles, synthesized by a novel sol-gel route followed by pyrolysis. Powder X-ray diffraction data confirms the formation of a single phase wurtzite type ZnO structure for all the compositions. The Zn(0.95)Co(0.05)O nanoparticles show diamagnetic behavior at room temperature. However, when Al is co-doped with Co with x = 0.0 to 0.10 in Zn(0.95-x)Co(0.05)Al(x)O, a systematic increase in ferromagnetic moment is observed up to x = 0.07 at 300 K. Above x = 0.07 (e.g. for x = 0.10) a drastic decrease in ferromagnetic nature is observed which is concomitant with the segregation of poorly crystalline Al rich ZnO phase as evidenced from TEM studies. Theoretical studies using density functional calculations on Zn(0.95-x)Co(0.05)Al(x)O suggest that the partial occupancy of S2 states leads to an increased double exchange interaction favoring the ferromagnetic ground states. Such ferromagnetic interactions are favorable beyond a threshold limit. At a high level doping of Al, the exchange splitting is reduced, which suppresses the ferromagnetic ordering.

Multiferroic PVDF–Fe3O4 hybrid films with reduced graphene oxide and ZnO nanofillers

Flexible and self-standing polyvinylidene fluoride (PVDF) films loaded with nanofillers, reduced graphene oxide (RGO), zinc oxide (ZnO) and magnetic iron oxide (Fe3O4) nanoparticles, were prepared by a solvent casting method. The crystallinity, morphology and structure of these films were studied using XRD, SEM, FTIR and Raman spectroscopy. FTIR studies reveal a higher percentage of polar ferroelectric β-phase (∼80%) in both pristine PVDF and PVDF–RGO films, whereas the addition of nanofillers, Fe3O4 and ZnO, resulted in a reduced amount of β-phase (∼50%) in the films. Of all the films studied, PVDF–RGO shows an enhanced dielectric constant as well as maximum electric polarization. On the other hand, Fe3O4 loaded–PVDF composite films exhibit reduced values of both dielectric constant and electric polarization. A weak magneto-dielectric behavior is observed in Fe3O4 loaded PVDF nanocomposite films at room temperature with a coupling constant ∼0.04%.

Photocatalytic hydrogen generation from water using a hybrid of graphene nanoplatelets and self doped TiO2–Pd

Nanohybrids of self doped (Ti3+ doped or reduced TiO2–TiO2R) TiO2–graphene nanoplatelets (TiO2R–G) of different compositions are synthesized by a facile soft chemical method. A decrease of bandgap and improved visible light absorption is exhibited by TiO2R–G. Based on current–voltage (I–V) measurements, it is concluded that the hybrid material possesses improved electron transport properties compared to TiO2R and pure TiO2. A detailed characterization of the composites indicated that TiO2R exists as a dispersed phase on graphene nanoplatelets (graphene). Among different compositions of the composites, the catalyst containing 3 weight% of graphene (TiO2R–3G) shows enhanced photocatalytic activity for hydrogen generation from water compared to both TiO2 and TiO2R. When Pd is used as co-catalyst in this composite, a large increase in the activity is observed. The increased efficiency of the nanocomposite is attributed to factors like: (i) improved visible light absorption promoted by G and Ti3+ dopant (ii) increased lifetime of the charge carriers assisted by the enhanced electron transporting properties of G (iii) increased number of active sites for hydrogen evolution provided by the Pd co-catalyst. This work highlights the role of TiO2 based hybrid materials as efficient photocatalysts for solar energy utilization.

Disorder-induced Room Temperature Ferromagnetism in Glassy Chromites

We report an unusual robust ferromagnetic order above room temperature upon amorphization of perovskite [YCrO3] in pulsed laser deposited thin films. This is contrary to the usual expected formation of a spin glass magnetic state in the resulting disordered structure. To understand the underlying physics of this phenomenon, we combine advanced spectroscopic techniques and first-principles calculations. We find that the observed order-disorder transformation is accompanied by an insulator-metal transition arising from a wide distribution of Cr-O-Cr bond angles and the consequent metallization through free carriers. Similar results also found in YbCrO3-films suggest that the observed phenomenon is more general and should, in principle, apply to a wider range of oxide systems. The ability to tailor ferromagnetic order above room temperature in oxide materials opens up many possibilities for novel technological applications of this counter intuitive effect.

A rare defect free 3D ZnO rod structure with strong UV emission

A new class of defect free ZnO rods with racket type handle shape at both the ends has been synthesized via a simple catalyst free method. High resolution transmission electron microscopy and strong ultra-violet emission showed that they are highly crystalline defect free structures. A simple growth mechanism has been suggested and interpreted. This new novel structure could offer opportunities for advanced optoelectronic technological applications.

Kinetic modeling: dependence of structural and sorption properties of ZnO—crucial role of synthesis

Zinc oxide nanoparticles were synthesized by two different routes namely pyrolytic and gel combustion methods and characterized using different techniques like XRD, TGA/DSC, diffuse reflectance spectrophotometry, zeta potential, NMR and IR. The efficiency of the nanoparticles with respect to sorption of different toxic species like chromate and rhodamine 6G was evaluated. The studies showed that the synthesis route adopted affects the characteristics of the nanoparticles, thus leading to the difference in sorption efficiency. The mechanism of sorption proposed was based on the different characterization studies and it was found that the sorption was not only an electrostatic interaction but can occur due to the presence of pores or some binding groups. The highly pH-dependent sorption efficiency of the nanoparticles is evident from the results, whereas their kinetics was described by a pseudo-second-order kinetic model and further explained by using Weber–Morris and Boyd models. The overall rate process appeared to be influenced by both external mass transfer and intraparticle diffusion. Considering the simplicity of the synthetic procedure and the possibility of a cost effective catalyst, the developed nanomaterials have great potential for applications in water treatment technologies.

Bifunctional properties of hydrothermal synthesized BaMF4 (M = Co, Ni and Zn)

BaMF4 (M = Co, Ni and Zn) samples having orthorhombic structure have been synthesized by a mild hydrothermal method and characterized by x-ray diffraction (XRD), magnetic and electrical measurements. Single phase formation of these compounds has been found to be dependent on various factors like reaction time, pH, temperature etc. All the samples showed ferroelectricity, which decreases with increase in temperature. BaMF4 (M = Co, Ni) samples show 10-15 times more leakage current compared to that of BaZnF4 at higher applied field. Absence of multivalent ions in BaZnF4 can be the reason for its minimum leakage current. All the samples except, BaZnF4, exhibit weak room temperature ferromagnetism also.