M. S. Anwar

RAPID AND COST EFFECTIVE SYNTHESIS OF ZnO NANORODS USING MICROWAVE IRRADIATION TECHNIQUE

ZnO nanorods assembled in flower shaped morphology have been successfully synthesized using low power microwave irradiation in a very short duration. The diameter and length of the rods were within 150–190 nm (tip diameter ~15 nm) and 2 μm, respectively, with an aspect ratio of 20–22. The synthesized nanorods were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared microscopy (FT-IR), photoluminescence (PL) and magnetization measurements. The XRD and FT-IR results indicate that ZnO nanorods have the pure wurtzite structure with lattice parameters a and c of 3.254 and 5.197 A, respectively. The selected area electron diffraction (SAED) pattern reveals that the ZnO nanorods are single crystal in nature and grow along [001] plane. Room-temperature PL spectrum of the as-grown ZnO nanorods shows a near-band-edge (NBE) emission peak and defect induced emissions. Magnetization measurements indicate that ZnO nanorods exhibit room temperature ferromagnetism with remanent magnetization (Mr) and coercivity (Hc) about 2.92 × 10-4 (emu/g) and 29.75 Oe, respectively, which may be due to the presence of defects in the ZnO nanorods.

Morphological evolution between nanorods to nanosheets and room temperature ferromagnetism of Fe-doped ZnO nanostructures

In this work, undoped and Fe-doped single-crystalline ZnO nanostructures were successfully synthesized by a facile microwave irradiation method. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results showed that Fe-doped ZnO was comprised of a single phase nature with a hexagonal wurtzite structure up to 5% Fe doping, however, secondary phase ZnFe2O4 appeared upon further increasing the Fe dopant concentration. Field emission scanning electron microscopy (FESEM) and TEM micrographs suggested that the length and diameter of the undoped ZnO rods are about ∼2 μm and ∼200 nm, respectively. Interestingly, the morphology of ZnO changed from nanorods (1% Fe) with length ∼500 nm and diameter ∼50 nm to nanosheets (5% Fe) having thickness and lateral dimension of ∼30 nm and ∼400 nm, respectively. NEXAFS and EELS studies revealed the absence of metal clusters up to 5% and Fe is found to be in a mixed (Fe2+/Fe3+) valence state with Fe2+ as the dominant state. Optical studies depicted that the absorption peak of Fe-doped ZnO was blue-shifted as the concentration of Fe increases from 1 to 5%. However, for dopant concentration >5%, the absorption peak was found to be red-shifted with an additional absorption peak of ZnFe2O4. Also, the band gap energy decreased monotonically with the increase of Fe concentration from 1 to 5%, while increasing on further doping, band gap increased. Raman scattering spectra of Fe-doped ZnO revealed the lower frequency shift of Ehigh2 mode with doping. Magnetic studies showed that Fe doped ZnO exhibit room temperature ferromagnetism (RTFM) and the value of magnetization increased up to 5% doping and then decreased for 7 and 10% Fe-doped samples.

Morphological evolution of ZnO nanostructures and their aspect ratio-induced enhancement in photocatalytic properties

This work presented controllable growth of ZnO nanostructures with different aspect ratios by the microwave irradiation method and investigated the photocatalytic degradation of methyl red (MR). X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) measurements showed that all ZnO nanostructures were of a hexagonal phase structure. It was revealed by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images that the morphology of ZnO can be effectively controlled as sheet-like, rod-like, brush-like, flower-like, prism-like, and pyramid-like only by changing the molar ratio (zinc acetate: KOH) and reaction time. With the increase of molar ratio and reaction time, modification in the E2(high) and E1(LO) Raman modes was observed. The energy band gap was found to be tuned by the aspect ratio of ZnO nanostructures. Photoluminescence spectroscopy revealed the low-intensity NBE emission and high and broad defect-related emission for high aspect ratio (14) nanorods. BET surface area porosity analysis confirmed the presence of a mesoporous network in all the nanostructures, showed high surface area and a uniform pore-size distribution for high aspect ratio nanorods. A terephthalic acid assay study confirmed the formation of hydroxyl radicals (OH) in MR dye solution treated with a ZnO nanostructures photocatalyst. The photodegradation of MR under UV light irradiation showed that ZnO nanorods with a high aspect ratio of ∼14 showed superior photodegradation (∼98% degradation of MR within 60 min) than that of the lower aspect ratio nanostructures. The apparent reaction rate constant for high aspect ratio (14) nanorods was higher than that of the lower aspect ratio nanostructures. The enhancement in photocatalytic performance could be due to the high surface area and enhanced charge separation and transfer efficiency of photoinduced charge carriers in the high aspect ratio nanorods.

Study of A-Site Disorder Dependent Structural, Magnetic, and Magnetocaloric Properties in La0.7-xSmxCa0.3MnO3 Manganites

We report studies on the magnetocaloric effect of samarium doped lanthanum manganites with different Sm-concentrations. Polycrystalline La0.7-xSmxCa0.30MnO3 (0≤x≤0.3) samples were prepared using the conventional solid-state reaction method with phase purity and structure confirmed using X-ray diffraction. Temperature dependent magnetization measurements and Arrott analysis reveal first order ferromagnetic transition in parent sample and second order ferromagnetic transition in doped sample with Curie temperature decreasing progressively with increasing Sm concentration from ~182 K for x = 0.05 to 109 K for x = 0.30. A large magnetic entropy change (~1.75 J kg-1 K-1 at 0.5 T) has been observed in La0.7Ca0.3MnO3 sample, and is greatly suppressed as a function of disorder caused by Sm doping. This is ascribed to the gradual loss of first-order ferromagnetic transition. This investigation suggests that La0.7-xSmxCa0.30MnO3 compounds can be used as a potential magnetic refrigerating material with wide range of temperature.

Facile synthesis of single-crystalline rutile TiO2 nano-rods by solution method

Abstract A convenient and scalable technique for the synthesis of rutile titanium dioxide (TiO2) nano-rods was presented by using bulk TiO2 powder, sodium hydroxide (NaOH) and distilled water as raw materials. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) studies indicate that the prepared sample is crystalline and free from any impurities, however, it has no distinct shape and possesses a huge degree of agglomeration, and the average crystal size is around 40 nm. After annealing the sample at 600 °C for 2 h, it is observed through FESEM that nano-rods are formed. And XRD analysis shows that the nano-rods are single crystalline with distinct and smooth surfaces in different sizes with average length of about 1 µm and diameter of about 80 nm. Further UV-visible spectroscopy and Raman studies were conducted for the prepared sample and the band gap of the final product is found to be 3.40 eV.

Large Magnetic Entropy Change in La 0.55 Ce 0.2 Ca 0.25 MnO 3 Perovskite

In this paper, magnetic property and magnetocaloric effect (MCE) in perovskite manganites of the type La (0.75-X) Ce X Ca 0.25 MnO 3 (x = 0.0, 0.2, 0.3 and 0.5) synthesized by using the standard solid state reaction method have been reported. From the magnetic measurements as a function of temperature and applied magnetic field, we have observed that the Curie temperature (T C ) of the prepared samples strongly dependent on Ce content and was found to be 255, 213 and 150 K for x = 0.0, 0.2 and 0.3, respectively. A large magnetocaloric effect in vicinity of T C has been observed with a maximum magnetic entropy change (|ΔSM| max ) of 3.31 and 6.40 J/kgK at 1.5 and 4 T, respectively, for La 0.55 Ce 0.2 Ca 0.25 MnO 3 . In addition, relative cooling power (RCP) of the sample under the magnetic field variation of 1.5 T reaches 59 J/kg. These results suggest that La 0.55 Ce 0.2 Ca 0.25 MnO 3 compound could be a suitable candidate as working substance in magnetic refrigeration at 213 K.

NOVEL AND COST-EFFECTIVE SYNTHESIS OF SILVER NANOCRYSTALS: A GREEN SYNTHESIS

This paper reports the study on the synthesis and characterization of silver nanocrystals by a two-step synthesis procedure. The first step is the solution-free hand grinding of silver foil and sugar at room temperature for few minutes. The second step is the thermal decomposition of silver/sugar composite to form silver nanocrystals. The as-synthesized silver nanocrystals were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), ultraviolet visible (UV/Vis) spectroscopy, atomic force microscopy (AFM) and transmission electron microscopy (TEM) studies. The XRD pattern showed a face-centered cubic structure (single phase) with high crystallinity. The lattice parameters calculated from XRD pattern were found to be a = 4.12 A for silver nanocrystals with average grain size of ~ 19 nm. The energy dispersive analysis of X-rays (EDAX) of silver nanocrystals confirmed the presence of silver and no peak of any secondary phase was detected. FESEM and AFM studies showed that the crystals have cube-like morphology. TEM results showed that the size of silver nanocrystals was found to be ~ 22 nm. This novel synthesis route, not reported earlier, would be a promising candidate for a variety of future applications of silver nanocrystals.

Enhanced refrigeration capacity and magnetic entropy change in La0.55Ce0.15Sr0.3MnO3 manganite

AbstractThe temperature dependent of the isothermal magnetic entropy change, ΔSM(T), and the field dependence of the refrigeration capacity, RC, have been investigated in La0.7−xCexSr0.30MnO3 (x = 0.0 and 0.15). An enhanced RC and ΔSM(T) were observed in La0.55Ce0.15Sr0.30MnO3. Under a magnetic field change of 2 T, the maximum improvement of 20% of ΔSM(T) and 40% of RC, in comparison with La0.7Sr0.30MnO3, was observed. Moreover, Curie temperature of the La0.7Sr0.30MnO3 can tuned by adjusting the Ce concentration and makes this attractive for magnetic refrigeration at desired temperature.

Effect of sintering temperature on structure, magnetic and magnetocaloric properties of La0.6Ca0.4MnO3 manganite

Abstract The effect of sintering temperature on the structure, magnetic transition and magnetic entropy of La 0.6 Ca 0.4 MnO 3 manganite was studied. It was observed that this compound belongs to the orthorhombic structure with the Pnma space group without any impurity phase. The effect of sintering temperature on the Curie temperature ( T C ) was studied. The small increment in T C is found with increasing the sintering temperature. The magnetocaloric study exposes a quite large change of the magnetic entropy, which varies with sintering temperature. For an applied magnetic field of 3 T and sintering temperature of 1300 °C, the relative cooling power (RCP) is 89 J/kg. As a result, the studied compound can be considered as potential material for magnetic refrigeration near and below room temperature.

STRUCTURAL AND MAGNETIC STUDY OF Co-DOPED ZnO NANOPARTICLES SYNTHESIZED BY AUTO COMBUSTION METHOD

In the present work, we have synthesized Zn1-xCoxO(x = 0.0 ≤ x ≤ 0.1) nanoparticles by an auto-combustion method using C2H5NO2 (glycine) as a fuel. The prepared nanoparticles were characterized by using X-ray diffraction, transmission electron microscopy, Photo-luminescence (PL) and magnetization measurements. XRD and TEM results demonstrated that Co-doped ZnO have a single phase nature with wurtzite structure and Co2+ ions were successfully incorporated into the lattice position of Zn2+ ions in ZnO matrix. PL spectra show two emission bands in visible region. Magnetic studies showed that Co-doped ZnO nanoparticles exhibit room temperature ferromagnetism.