Magnetic properties in Fe-doped NiO synthesized by co-precipitation

The magnetic properties of 1.5 at% Fe-doped NiO bulk samples were investigated. The samples were prepared by sintering the corresponding precursor in air at temperatures between 400 and 800 °C for 6 h. The synthesis was by a chemical co-precipitation and post-thermal decomposition method. In order to allow a comparison, a NiO/0.76 at% NiFe₂O₄ mixture was also prepared. The X-ray diffraction pattern shows that the samples that were sintered at 400 and 600 °C remain single phase. As the sintering temperature increased to 800 °C, however, the sample becomes a mixture of NiO and NiFe₂O₄ ferrite phases. The samples were investigated by measuring their magnetization as a function of magnetic field. The samples sintered between 400 and 800 °C and the one mixed directly with NiFe₂O₄ nanoparticles show a coercivity value of H_c≈200, 325, 350 and 110 Oe, respectively. The magnetic properties of the samples depend strongly on the sintering temperature. Simultaneously, the field-cooling hysteresis loop shift also observed after cooling the sample sintered at 600 °C to low temperature suggests the possibility of the existence of a ferromagnetic/antiferromagnetic exchange coupling.     

Preparation and characterization of multi-functional CoFe2O4-ZnO nanocomposites

Multi-functional magnetic, photoluminescent and photocatalytic CoFe₂O₄-ZnO nanocomposites were successfully synthesized by a collosol method. The average diameter of the prepared CoFe₂O₄-ZnO nanocomposites was 30±5 nm, and a diffusion layer was formed to link CoFe₂O₄ and ZnO. The saturation magnetization of the CoFe₂O₄-ZnO nanocomposites was 8.99 emu/g. Generation of ZnO from Zn(OH)₂ collosol was nearly complete after thermal decomposition at about 380 °C. A photoluminescence emission peak was observed at 443 nm when excitated at 350 nm. Degradation of methyl orange is performed by CoFe₂O₄-ZnO nanocomposites under ultraviolet radiation, with a degradation rate of up to 93.9%.     

Effect of annealing temperature on structure, magnetic properties and optical characteristics in Zn0.97Cr0.03O nanoparticles

The Cr-doped zinc oxide (Zn_0.97Cr_0.03O) nanoparticles were successfully synthesized by sol-gel method. The relationship between the annealing temperature (400 °C, 450 °C, 500 °C and 600 °C) and the structure, magnetic properties and the optical characteristics of the produced samples was studied. The results indicate that Cr (Cr³⁺) ions at least partially substitute Zn (Zn²⁺) ions successfully. Energy dispersive spectroscopy (EDS) measurement showed the existence of Cr ion in the Cr-doped ZnO. The samples sintered in air under the temperature of 450 °C had single wurtzite ZnO structure with prominent ferromagnetism at room temperature, while in samples sintered in air at 500 °C, a second phase-ZnCr₂O₄ was observed and the samples were not saturated in the field of 10000 Oe. This indicated that they were mixtures of ferromagnetic materials and paramagnetic materials. Compared with the results of the photoluminescence (PL) spectra, it was reasonably concluded that the ferromagnetism observed in the studied samples was originated from the doping of Cr in the lattice of ZnO crystallites.     

The structure and magnetic properties of Zn1−xNixFe2O4 ferrite nanoparticles prepared by sol–gel auto-combustion

Zn_1−xNi_xFe₂O₄ ferrite nanoparticles were prepared by sol–gel auto-combustion and then annealed at 700 °C for 4 h. The results of differential thermal analysis indicate that the thermal decomposition temperature is about 210 °C and Ni–Zn ferrite nanoparticles could be synthesized in the self-propagating combustion process. The microstructure and magnetic properties were investigated by means of X-ray diffraction, scanning electron microscope, and Vibrating sample magnetometer. It is observed that all the spherical nanoparticles with an average grain size of about 35 nm are of pure spinel cubic structure. The crystal lattice constant declines gradually with increasing x from 0.8435 nm (x=0.20) to 0.8352 nm (x=1.00). Different from the composition of Zn_0.5Ni_0.5Fe₂O₄ for the bulk, the maximum M_s is found in the composition of Zn_0.3Ni_0.7Fe₂O₄ for nanoparticles. The H_c of samples is much larger than the bulk ferrites and increases with the enlarging x. The results of Zn_0.3Ni_0.7Fe₂O₄ annealed at different temperatures indicate that the maximum M_s (83.2 emu/g) appears in the sample annealed at 900 °C. The H_c of Zn_0.3Ni_0.7Fe₂O₄ firstly increases slightly as the grain size increases, and presents a maximum value of 115 Oe when the grains grow up to about 30 nm, and then declines rapidly with the grains further growing. The critical diameter (under the critical diameter, the grain is of single domain) of Zn_0.3Ni_0.7Fe₂O₄ nanoparticles is found to be about 30 nm.     

Micro-structural investigations and paramagnetic susceptibilities of zinc oxide, europium oxide and their nanocomposite

Nanoparticles of zinc oxide (ZnO), europium oxide (Eu₂O₃) and their nanocomposite system {(ZnO)_0.55(Eu₂O₃)_0.45} have been prepared by pyrophoric reaction and chemical co-precipitation methods. The precursor materials used for the synthesis were Zn(NO₃)₂·6H₂O and bulk Eu₂O₃. For nanocrystallization, the as-prepared samples were annealed at 500 and 600 °C for 6 h. The X-ray diffractograms (XRD) confirmed the formation of desired phases of the nanoparticles of ZnO, Eu₂O₃ and nanocomposite of {(ZnO)_0.55 (Eu₂O₃)_0.45}. Particle sizes of all the samples have been estimated from the width of the XRD peaks using the Debye-Scherrer equation. Particle sizes, crystallographic phases, etc. extracted from the high resolution transmission electron microscopy of a few selected samples are in agreement with those obtained from the XRD. Field emission scanning electron microscopy showed that ZnO nanoparticles are more-or-less spherical in shape. Average magnetic susceptibilities of all the annealed samples measured in the temperature range of 300-14 K indicate that all the samples including the zinc oxide, which is normally diamagnetic in the bulk state, are paramagnetic and the data are tried to analyze by the Curie-Weiss law. Photo-luminescence data recorded at room temperature of all the samples indicate that the optical property of the ZnO nanoparticles are not affected by Eu₂O₃ nanoparticles in the nanocomposite system though its bulk magnetization is substantially enhanced by incorporating the Eu₂O₃ nanoparticles.     

Nanoparticles of the giant dielectric material, CaCu3Ti4O12 from a precursor route

A method of preparing the nanoparticles of CaCu₃Ti₄O₁₂ (CCTO) with the crystallite size varying from 30 to 200 nm is optimized at a temperature as low as 680 °C from the exothermic thermal decomposition of an oxalate precursor, CaCu₃(TiO)₄(C₂O₄)₈·9H₂O. The phase singularity of the complex oxalate precursor is confirmed by the wet chemical analyses, X-ray diffraction, FT-IR and TGA/DTA analyses. The UV-vis reflectance and ESR spectra of CCTO powders indicate that the Cu(II) coordination changes from distorted octahedra to nearly flattened tetrahedra (squashed) to square-planar geometry with increasing annealing temperature. The HRTEM images have revealed that the evolution of the microstructure in nanoscale is related to the change in Cu(II) coordination around the surface regions for the chemically prepared powder specimens. The nearly flattened tetrahedral geometry prevails for CuO₄ in the near surface regions of the particles, whereas square-planar CuO₄ groups are dominant in the interior regions of the nanoparticles. The powders derived from the oxalate precursor have excellent sinterability, resulting in high-density ceramics which exhibited giant dielectric constants upto 40,000 (1 kHz) at 25 °C, accompanied by low dielectric loss <0.07.     

Magnetic properties of nanosized γ-Fe2O3 and α-(Fe2/3Cr1/3)2O3, prepared by thermal decomposition of heterometallic single-molecular precursor

Thermal decomposition of the trinuclear complex [Fe₂CrO(CH₃COO)₆(H₂O)₃]NO₃ at 300, 400 and 500 °C gave γ-Fe₂O₃ nanoparticles along with amorphous chromium oxide, while decomposition of the same starting compound at 600 and 700 °C led to the formation of α-(Fe_2/3Cr_1/3)₂O₃ nanoparticles. Size of γ-Fe₂O₃ nanoparticles, determined by X-ray diffraction, was in the range from 9 to 11 nm and increased with formation temperature growth. Average size of α-(Fe_2/3Cr_1/3)₂O₃ nanoparticles was about 40 nm and almost did not depend on the temperature of its formation. γ-Fe₂O₃ nanoparticles possessed superparamagnetic behavior with blocking temperature 180-250 K, saturation magnetization 29-35 emu/g at 5 K, 44-49 emu/g at 300 K and coercivity 400-600 Oe at 5 K. α-(Fe_2/3Cr_1/3)₂O₃ nanoparticles were characterized by low magnetization values (2.7 emu/g at 70 kOe). Such magnetic properties can be caused by non-compensated spins and defects present on the surface of these nanoparticles. The increase of α-(Fe_2/3Cr_1/3)₂O₃ formation temperature led to decrease of magnetization (being compared for the same fields), which may be caused by decrease of the quantity of defects or non-compensated spins (due to decrease of particles' surface).     

Photoluminescence observations of hydrogen incorporation and outdiffusion in ZnO thin films

ZnO thin films were deposited with the addition of H₂ to the reaction gas using the atmospheric-pressure metal organic chemical vapor deposition method. The incorporation and outdiffusion of hydrogen in ZnO films were investigated by comparing the intensity of the hydrogen-related bound-exciton peak (I₄: 3.363 eV) in the photoluminescence spectrum. The intensity of I₄ peak was found to be the strongest in the ZnO film deposited at 680 °C with H₂ present. However, for the ZnO films prepared at the same temperature 680 °C but without H₂ present and at the higher temperature of 900 °C with H₂ present, respectively, the I₄ peak was just a minor shoulder of another bound-exciton peak (I₈: 3.359 eV). The intensity of I₄ peak in the ZnO films deposited with H₂ present was found to decrease with the increasing of annealing temperature. These results suggest that it is difficult for hydrogen to incorporate into ZnO thin films grown at high temperatures even in the hydrogen-present ambient.     

Surface of Zn-Mn-O and its role in room temperature ferromagnetism: An XPS analysis

The existence of ferromagnetism in Zn-Mn-O semiconductor samples and dependence on the preparation condition were investigated. We systematically examined the samples with manganese concentration ranging from 0 to 10 at.%, prepared by a solid state reaction route using (ZnC₂O₄·2H₂O)_1−x and (MnC₂O₄·2H₂O)_x as precursors. Thermal treatment was carried out in air at temperatures ranging from 400 to 900 °C. The samples were investigated by X-ray diffraction, transmission electron microscopy, magnetization measurements and XPS spectroscopy. XPS surface composition, chemical analysis and depth profiling were successfully employed on powder revealing the chemical composition at the surface of the grains and underneath. The present investigation suggests that physical properties and observed room temperature ferromagnetism might be due to grain surface effects. It seems that the ferromagnetic phase is correlated with oxygen build up at the surface.     

Hydrothermal synthesis and characterization of nanocrystalline Zn-Mn spinel

Hydrothermal method had been used to successfully synthesize the nanocrystalline spinel zinc manganese oxide (ZnMn₂O₄) directly from Zn(CH₃COO)₂·2H₂O, NaOH, Mn(NO₃)₂ and H₂O₂ at 170 °C for the reaction time of 48 h. The effects of the synthesis conditions, such as the Zn/Mn molar ratio, the reaction temperature, the reaction time, the zinc source and the concentrations of NaOH and H₂O₂, on the formation of the Zn-Mn spinel were investigated. The products were characterized by means of X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The results indicated that the compositions of the Zn-Mn spinel with the tetragonal structure were Zn_1.14Mn_1.86O₄. Scanning electron microscope (SEM) and transmission electron microscopy (TEM) images showed that the products at 170 °C were with square-shaped nanocrystalline spinel with the particle size of about 20-50 nm. The thermal behaviors of the products were investigated by thermogravimetric analysis (TG).     

Single step thermal decomposition approach to prepare supported γ-Fe2O3 nanoparticles

γ-Fe₂O₃ nanoparticles supported on MgO (macro-crystalline and nanocrystalline) were prepared by an easy single step thermal decomposition method. Thermal decomposition of iron acetylacetonate in diphenyl ether, in the presence of the supports followed by calcination, leads to iron oxide nanoparticles supported on MgO. The X-ray diffraction results indicate the stability of γ-Fe₂O₃ phase on MgO (macro-crystalline and nanocrystalline) up to 1150 °C. The scanning electron microscopy images show that the supported iron oxide nanoparticles are agglomerated while the energy dispersive X-ray analysis indicates the presence of iron, magnesium and oxygen in the samples. Transmission electron microscopy images indicate the presence of smaller γ-Fe₂O₃ nanoparticles on nanocrystalline MgO. The magnetic properties of the supported magnetic nanoparticles at various calcination temperatures (350-1150 °C) were studied using a superconducting quantum interference device which indicates superparamagnetic behavior.     

The gas response enhancement from ZnO film for H2 gas detection

ZnO thin films were prepared by thermal oxidation of Zn metal at 400 °C for 30 and 60 min. The XRD results showed that the Zn metal was completely converted to ZnO with a polycrystalline structure. The sensors had a maximum response to H₂ at 400 °C and showed stable behavior for detecting H₂ gases in the range of 40 to 160 ppm. The film oxidized for 60 min in oxygen flow exhibited higher response than that of the 30 min oxidation which was approximately 4000 for 160 ppm H₂ gas concentration. The sensing mechanism was modeled according to the oxygen-vacancy model.     

On the synthesis and optical absorption studies of nano-size magnesium oxide powder

Magnesium oxide (MgO) nano-size powder is synthesized using magnesium nitrate hexahydrate and oxalic acid as precursors with ethanol as a solvent. The process involves gel formation, drying at 100 °C for 24 h to form magnesium oxalate dihydrate [α-MgC₂O₄·2H₂O] and its decomposition at 500, 600, 800, and 1000 °C for 2 h to yield MgO powder (average crystallite size ∼6.5-73.5 nm). The sol-gel products at various stages of synthesis are characterized for their thermal behaviour, phase, microstructure, optical absorption, and presence of hydroxyl and other groups like OCO, CO, C-C, etc. MgO powder is shown to possess an f.c.c. (NaCl-type) structure with lattice parameter increasing with decrease in crystallite size (t_av); typical value being ∼4.222(2) Å for t_av∼6.5 nm as against the bulk value of 4.211 Å. Infrared absorption has shown MgO to be highly reactive with water. Also, a variety of F- and M-defect centres found in MgO produce energy levels within the band gap (7.8 eV), which make it attractive for application in plasma displays for increasing secondary electron emission and reducing flickering effects. The possible application of the intermediate sol-gel products, viz., α-MgC₂O₄·2H₂O and anhydrous magnesium oxalate (MgC₂O₄) in understanding the plants and ESR dosimetry, respectively, has also been suggested.     

Cation distribution dependence of magnetic properties of sol-gel prepared MnFe2O4 spinel ferrite nanoparticles

MnFe₂O₄ nanoparticles have been synthesized with a sol-gel method. Both differential thermal and thermo-gravimetric analyses indicate that MnFe₂O₄ nanoparticles form at 400 °C. Samples treated at 450 and 500 °C exhibit superparamagnetism at room temperature as implied from vibrating sample magnetometry. Mössbauer results indicate that as Mn²⁺ ions enter into the octahedral sites, Fe³⁺ ions transfer from octahedral to tetrahedral sites. When the calcination temperature increases from 450 to 700 °C, the occupation ratio of Fe³⁺ ions at the octahedral sites decreases from 43% to 39%. Susceptibility measurements versus magnetic field are reported for various temperatures (from 450 to 700 °C) and interpreted within the Stoner-Wohlfarth model.     

Shape-controlled synthesis of CuS nanocrystallites via a facile hydrothermal route

Without the use of any extra surfactant, template or other additive, shape-controlled synthesis of sphere-, urchin- and tube-like CuS nanocrystallites has been realized just via hydrothermal treatment of different amounts of CuSO₄·5H₂O and equimolar Na₂S₂O₃·5H₂O in water at 150 °C for 12 h. The possible mechanism for the formation of the various nanostructures of CuS in this system was discussed.     

The influence of temperature (20-1000 °C) on binary mixtures of solid solutions of CH3COOLi·2H2O-MgHPO4·3H2O

Thermally induced phase transitions (20-1000 °C) in the substrates and binary mixtures of CH₃COOLi·2H₂O(1)-MgHPO₄·3H₂O(11) have been analysed. Changes taking place on dehydration and thermal dissociation of binary mixtures prepared with percent molar ratios of 90-10% were studied by differential thermal analysis (TG, DTG, DTA), IR-spectroscopy and WAXS.The above-mentioned substrates changed their structure when heated for 1 h at 500 or 1000 °C. CH₃COOLi·2H₂O(1) (ID: 23-1171) changed the structure at 500 °C to that of Li₂CO₃ (ID: 22-1141), while at 1000 °C the structure was impossible to analyse as the compound reacted both with porcelain and with platinum (crucible materials). MgHPO₄·3H₂O(11) (Newberyite, ID: 35-780, 19-762) changed its structure at 500 °C to amorphous phase and at 1000 °C to Mg₂P₂O₇ (ID: 32-626).The following compounds were assayed in the respective binary mixtures heated at 500 °C for 1 h: 70% (1)-30%(11): LiMgPO₄ (ID: 18-735), MgO (ID: 4-829); 50%(1)-50%(11): LiMgPO₄ (ID: 18-735), Li₃PO₄ (ID: 25-1030); 30%(1)-70%(11): LiMgPO₄ (ID: 32-574); binary mixtures heated at 1000 °C contained the following compounds: 70%(1)-30%(11): LiMgPO₄ (ID: 32-574,18-735), Li₃PO₄ (ID: 15-760,25-1030), MgO (ID: 4-829); 50%(1)-50%(11): LiMgPO₄ (ID: 32-574, 18-735), MgO (ID: 4-829); 30%(1)-70%(11): LiMgPO₄ (ID: 18-735, 32-574), Mg₂P₂O₇ (ID: 22-1152, 8-38), Li₄SiO₄ (37-1472).     

Octanethiolated Cu and Cu2O nanoparticles as ink to form metallic copper film

The synthesis and spectroscopic characterizations of size-controlled Cu and Cu₂O nanoparticles forming self-assembled 2D superlattices with hexagonal packing are described. The scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), and X-ray photoelectron spectroscopy (XPS) were used to characterize the octanethiol-protected copper nanoparticles. Analysis of XPS confirms the formation of oxidized copper nanoparticles. Conductivity of copper metal film (0.1 μm) on the Si wafer can be improved simply by thermal annealing of copper monolayer protected clusters (MPCs) film (4.8 ± 0.5 × 10² μΩ cm) under air at 300 °C for 1 h, and then for another 5 h under a protective atmosphere of 90% N₂-10% H₂.     

Synthesis of magnetite nanoparticles via a solvent-free thermal decomposition route

Superparamagnetic nanoparticles have been widely applied in various bio-medical applications. To date, it is still a challenge to synthesize nanosized Fe₃O₄ particles with controlled size and distribution. In this paper, a novel solvent-free thermal decomposition method is reported for synthesizing Fe₃O₄ nanoparticles. Size and morphology of the nanoparticles are determined by TEM while the structure of the nanoparticles is identified by FTIR, XPS and TGA measurements. Magnetic properties of the obtained particles are determined using VSM and SQUID measurement. The particle size of the Fe₃O₄ can be tailored by adjusting either reaction temperature or time. When the reaction temperature is increased to 330 °C and the reaction time is extended to 4 h, the average particle size of the obtained nanoparticles is ∼9 nm, while Ms value reaches ∼76 emu/g. The as synthesized Fe₃O₄ nanoparticles show well-established superparamagnetic properties with the blocking temperature at around 100 K.     

Effect of thermal annealing on the structure and magnetism of Fe-doped ZnO nanocrystals synthesized by solid state reaction

High purity Fe₂O₃/ZnO nanocomposites were annealed in air at different temperatures between 100 and 1200 °C to get Fe-doped ZnO nanocrystals. The structure and grain size of the Fe₂O₃/ZnO nanocomposites were investigated by X-ray diffraction 2θ scans. Annealing induces an increase of the grain size from 25 to 195 nm and appearance of franklinite phase of ZnFe₂O₄. Positron annihilation measurements reveal large number of vacancy defects in the interface region of the Fe₂O₃/ZnO nanocomposites, and they are gradually recovered with increasing annealing temperature. After annealing at temperatures higher than 1000 °C, the number of vacancies decreases to the lower detection limit of positrons. Room temperature ferromagnetism can be observed in Fe-doped ZnO nanocrystals using physical properties measurement system. The ferromagnetism remains after annealing up to 1000 °C, suggesting that it is not related with the interfacial defects.     

The n-butyl amine TPD measurement of Brönsted acidity for solid catalysts by simultaneous TG/DTG-DTA

The method for Brönsted acidity measurement based on TPD of alkyl amines desorption by gas-chromatography or thermogravimetry was adapted for simultaneous TG/DTG-DTA analysis. The acidity measurements were focused on the 12-tungstophosphoric acid (H₃PW₁₂O₄₀) and its salts, especially with Cesium since these posses the highest Brönsted acidity and they are among the most interesting catalysts. The n-butyl amine (NBA) desorption takes place in three steps for Cs_xH_3−xPW₁₂O₄₀, x = 0-2, and four steps for the Cs_2.5H_0.5PW₁₂O₄₀. The steps of desorption correspond to the release of NBA molecules in stages, as NBA or butene molecules resulted from the Hofmann elimination reaction and NH₃ + H₂O formed by decomposition of ammonium salt. The quantities of desorption products, C₄H₈ and NH₃ + H₂O, corresponding to the stages with the maximum desorption rates at 400-420 °C, respectively 560-600 °C, are in the stoichiometric ratio with the Brönsted acidity.