Magneto-optical properties of α-Fe2O3@ZnO nanocomposites prepared by the high energy ball-milling technique

Magnetic–fluorescent nanocomposites (NCs) with 10 wt% of α-Fe₂O₃ in ZnO have been prepared by the high energy ball-milling. The crystallite sizes of α-Fe₂O₃ and ZnO in the NCs are found to vary from 65 nm to 20 nm and 47 nm to 15 nm respectively as milling time is increased from 2 to 30 h. XRD analysis confirms presence of α-Fe₂O₃ and ZnO in pure form in all the NCs. UV–vis study of the NCs shows a continuous blue-shift of the absorption peak and a steady increase of band gap of ZnO with increasing milling duration that are assigned to decreasing particle size of ZnO in the NCs. Photoluminescence (PL) spectra of the NCs reveal three weak emission bands in the visible region at 421, 445 and 485 nm along with the strong near band edge emission at 391 nm. These weak emission bands are attributed to different defect – related energy levels e.g. Zn-vacancy, Zn interstitial and oxygen vacancy. Dc and ac magnetization measurements show presence of weakly interacting superparamagnetic (SPM) α-Fe₂O₃ particles in the NCs. ⁵⁷Fe-Mössbauer study confirms presence of SPM hematite in the sample milled for 30 h. Positron annihilation lifetime measurements indicate presence of cation vacancies in ZnO nanostructures confirming results of PL studies.     

Photocatalytic reduction of carbon dioxide with water using InNbO4 catalyst with NiO and Co3O4 cocatalysts

InNbO₄ was prepared by the solid-state reaction method. Various cocatalysts were added on InNbO₄ by the incipient-wetness impregnation method. The effects of co-catalyst and pretreatment conditions on the photocatalytic activity of InNbO₄ for photoreduction of carbon dioxide were investigated. NiO–InNbO₄ and Co₃O₄–InNbO₄ were pretreated by reduction at 500 °C for 2 h and subsequent oxidation at 200 °C for 1 h. The catalysts were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and UV–vis diffuse reflectance spectroscopy. The characterization results of NiO–InNbO₄ catalysts after pretreatment showed the presence of highly crystalline NiO and monoclinic Nb₂O₅. NiO–InNbO₄ with reduction–oxidation pretreatment exhibited the highest activity due to the presence of core–shell type Ni⁰ and NiO on the surface and the presence of a small amount of Nb₂O₅ as a promoter.     

X-ray photoelectron spectroscopy of LiM0.05Mn1.95O4 (M = Ni,Fe and Ti)

LiMn₂O₄ and LiM_0.05Mn_1.95O₄ (M = Ni, Fe and Ti) were synthesized by using solid-state reactions and their surface stoichiometries were confirmed by XPS data. The crystal and electronic structures were investigated by using X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). XRD data suggested that LiM_0.05Mn_1.95O₄ possesses nearly no any variations in lattice parameters compared with LiMn₂O₄ for slight substitution of Ni, Fe and Ti; the substituted Ni, Fe and Ti ions were located on the 16d octahedral sites in the spinel crystal lattice. The XPS results suggested that Fe and Ti ions were at + 3 and + 4 oxidation states, respectively; while Ni ions are mixed with + 2 and + 3 oxidation states. The normal oxidation state of Mn ions in the above four materials is almost the same and calculated as + 3.55 according to the splitting energies of Mn3s states.     

A comparative study of different concentrations of pure Zn powder effects on synthesis,structure, magnetic and microwave-absorbing properties in mechanically-alloyed Ni–Zn ferrite

In this study, a powder mixture of Zn, Fe₂O₃ and NiO was used to produce different compositions of Ni_1−xZn_xFe₂O₄ (x=0.36, 0.5 and 0.64) nanopowders. High-energy ball milling with a subsequent heat treatment method was carried out. The XRD results indicated that for the content of Zn, x=0.64 a single phase of Ni–Zn ferrite was produced after 30 h milling while for the contents of Zn, x=0.36 and 0.5, the desired ferrite was formed after sintering the 30 h-milled powders at 500 °C. The average crystallite size decreased with increase in the Zn content. A DC electrical resistivity of the Ni–Zn ferrite, however, decreased with increase in the Zn content, its value was much higher than those samples prepared by the conventional ceramic route by using ZnO instead of Zn. This is attributed to smaller grains size which were obtained by using Zn. The FT-IR results suggested two absorption bands for octahedral and tetrahedral sites in the range of 350–700 cm⁻¹. The VSM results revealed that by increasing the Zn content from 0.36 to 0.5, a saturation magnetization reached its maximum value; afterwards, a decrease was observed for Zn with x=0.64. Finally, magnetic permeability and dielectric permittivity were studied by using vector network analyzer to explore microwave-absorbing properties in X-band frequency. The minimum reflection loss value obtained for Ni_0.5Zn_0.5Fe₂O₄ samples, about −34 dB at 9.7 GHz, making them the best candidates for high frequency applications.     

Identification of iron oxide phases in thin films grown on Al2O3(0 0 0 1) by Raman spectroscopy and X-ray diffraction

We report on the identification of Fe₃O₄ (magnetite) and α-Fe₂O₃ (hematite) in iron oxide thin films grown on α-Al₂O₃(0 0 0 1) by evaporation of Fe in an O₂-atmosphere with a thickness of a few unit cells. The phases were observed by Raman spectroscopy and confirmed by X-ray diffraction (XRD). Magnetite appeared independently from the substrate temperature and could not be completely removed by post-annealing in an oxygen atmosphere as observed by X-ray diffraction. In the temperature range between 400 °C and 500 °C the X-ray diffraction shows that predominantly hematite is formed, the Raman spectrum shows a mixture of magnetite and hematite. At both lower and higher substrate temperatures (300 °C and 600 °C) only magnetite was observed. After post-annealing in an O₂-atmosphere of 5 × 10^?5 mbar only hematite was detectable in the Raman spectrum.     

Spectroscopic studies of the structural properties of Ni substituted spinel LiMn2O4

Microstructure and local structure of spinel LiNi_xMn_2 − xO₄ (x = 0, 0.1 and 0.2) were studied using X-ray diffraction (XRD) and a combination of X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge spectroscopy (XANES) and Raman scattering with the aim of getting a clear picture of the local structure of the materials responsible for the structural stability of LiNi_xMn_2 − xO₄. XRD study showed that Ni substitution caused the changes of the materials’ microstructure from the view of the lattice parameter, mean crystallite size, and microstrain. XPS and XANES studies showed the Ni oxidation state in LiNi_xMn_2 − xO₄ was larger than + 2, and the Mn oxidation state increased with Ni substitution. The decrease of the intensity of the 1s → 4p_z shakedown transition on the XANES spectra indicated that Ni substitution suppressed the tetragonal distortion of the [MnO₆] octahedron. The Mn(Ni)–O bond in LiNi_xMn_2 − xO₄, which is stronger than the Mn–O bond in LiMn₂O₄ was responsible for the blue shift of the A_1g Raman mode and could enhance the structural stability of the [Mn(Ni)O₆] octahedron.     

Sonochemical assisted synthesis of SrFe12O19 nanoparticles

We present the synthesis of M-type strontium hexaferrite by sonochemistry and annealing. The effects of the sonication time and thermal energy on the crystal structure and magnetic properties of the obtained powders are presented. Strontium hexagonal ferrite (SrFe₁₂O₁₉) was successfully prepared by the ultrasonic cavitation (sonochemistry) of a complexed polyol solution of metallic acetates and diethylene glycol. The obtained materials were subsequently annealed at temperatures from 300 to 900 °C. X-ray diffraction analysis shows that the sonochemical process yields an amorphous phase containing Fe³⁺, Fe²⁺ and Sr²⁺ ions. This amorphous phase transforms into an intermediate phase of maghemite (γ-Fe₂O₃) at 300 °C. At 500 °C, the intermediate species is converted to hematite (α-Fe₂O₃) by a topotactic transition. The final product of strontium hexaferrite (SrFe₁₂O₁₉) is generated at 800 °C. The obtained strontium hexaferrite shows a magnetization of 62.3 emu/g, which is consistent with pure hexaferrite obtained by other methods, and a coercivity of 6.25 kOe, which is higher than expected for this hexaferrite. The powder morphology is composed of aggregates of rounded particles with an average particle size of 60 nm.     

Intermediate formation in the reduction of Ni-ferrite with irradiation of high-flux infrared beam up to 1823 K

Ni-ferrite (NiFe₂O₄) has a lower reaction temperature for the O₂-releasing reaction when irradiation by a high-flux solar beam than that predicted by the estimation based on the thermodynamic data. The reaction mechanism of Ni-ferrite in the O₂-releasing reaction of a two-step water-splitting process at high temperatures (1,273–1,823 K) was clarified by means of X-ray diffractometry (XRD), extended X-ray absorption fine structure (EXAFS) analysis, Mössbauer spectroscopy and Magnetization measurement. The analysis of the EXAFS and Mössbauer spectra for Ni-ferrite before and after the O₂-releasing reaction shows that a lattice defect (Fe³⁺ (B-site)?Fe³⁺ (interstitial A-site)) intermediate with a spinel-type structure was formed in the early stage of the O₂-releasing reaction (up to 1,723 K). It is suggested that irradiation by a high-flux infrared beam resulted in the formation of the lattice defect intermediate with a Frenkel defect. The formation of the lattice defect intermediate with a Frenkel defect was due to the high reactivity of Ni-ferrite in the O₂-releasing reaction, as compared with other ferrites.     

Rapid synthesis of LiCr0.15Mn1.85O4 by glycine–nitrate method

LiCr_0.15Mn_1.85O₄ spinel has been successfully synthesized by glycine–nitrate method (GNM). The presence of pure spinel phase was confirmed by long term XRPD measurements and the Rietveld structural refinement. Lattice parameter was estimated to be 8.2338 Å. Average particle size of prepared powder material is below 500 nm. The BET surface area is 9.6 m² g^− 1. As a cathode material for lithium batteries LiCr_0.15Mn_1.85O₄ shows initial discharge capacity of 110 mA h g^− 1 and capacity retention of 83% after 50 cycles.     

Study of Schottky contact between Au and NiO nanowire by conductive atomic force microscopy (C-AFM): The case of surface states

In this work, NiO nanowires have been synthesized by a hydrothermal reaction of NiCl₂ with Na₂C₂O₄ in the presence of ethylene glycol at 180 °C for 12 h, then calcinated at 400 °C for 2 h. The NiO nanowires were analyzed by means of scanning electron microscope (SEM), atomic force microscope (AFM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The resulting current–voltage (I–V) characteristics of the NiO nanowires exhibited a clear rectifying behavior. This rectify behavior was attributed to the formation of a Schottky contact between Au coated atomic force microscopy (AFM) tip and NiO nanowires (nano-M/SC) which was dominated by the surface states in NiO itself. Photo-assisted conductive AFM (PC-AFM) was used to demonstrate how the I–V characteristics are influenced by the surface states. Our I–V results also showed that the nano-M/SCs had a good photoelectric switching effect at reverse bias.     

Effect of Fe doping on the room temperature ferromagnetism in chemically synthesized (In1−xFex)2O3 (0 ⩽ x ⩽ 0.07) magnetic semiconductors

Observation of room temperature ferromagnetism in Fe doped In₂O₃ samples (In_1−xFe_x)₂O₃ (0 ⩽ x ⩽ 0.07) prepared by co-precipitation technique is reported. Lattice parameter obtained from powder X software shows distinct shrinkage of the lattice constant indicating an actual incorporation of Fe ions into the In₂O₃ lattice. X-ray diffraction data measurements show that the entire sample exhibits single phase polycrystalline behavior. SEM micrographs showed the prepared powder was in the range 25–36 nm. SEM EDS mapping showed the presence of Fe and In ions in the Fe doped In₂O₃ sample. The highest remanence magnetization moment (6.624 × 10⁻⁴ emu/g) is reached in the sample with x = 0.03.     

Preparation of phase pure and well-crystallized Li4Ti5O12 nanoparticles by precision control of starting mixture and calcining at lowest possible temperatures

In an attempt to obtain spinel Li₄Ti₅O₁₂ with smallest possible grain size and highest possible phase purity via a solid state route, we tried to elevate reactivity of the reactant mixture by mechanical activation and appropriate choice of the starting materials. From the stoichiometric mixture comprising Li₂CO₃ and 150 nm anatase, we needed to heat at 950 °C for 1 h to obtain 81–88% phase purity (PhP) of Li₄Ti₅O₁₂ with its average grain size ca 600 nm. After mechanical activation with a multi-ring mill for 30 min, 850 °C was enough to obtain 85–87% pure 500 nm spinel. From a combination of LiNO₃ and 50 nm anatase, 90–91% phase pure product with its grain size 240 nm was obtained at 750 °C due to fusion of the nitrate and shorter diffusion path. By using CH₃COOLi.2H₂O and 50 nm anatase we obtained 130 nm Li₄Ti₅O₁₂ with its PhP ca 90% by milling the mixture preliminarily calcined at 500 °C for 1 h and heating subsequently at 700 for 1 h.     

Solid state interactions in nano-sized oxides

Comparative study of reactivity of nano- and micro-sized alumina and nickel oxide, obtained by the electrical explosion of metal wires in oxidizing atmosphere, was carried out for the reactions NiO + MoO₃, NiO + Al₂O₃, and Al₂O₃ + Bi₂O₃ by coupled anneals of ceramics, measurements of the conductivity of individual oxides and raw oxide mixtures, X-ray diffraction and differential thermal analysis. The total conductivity of nano-structured oxides was found lower than that of micro-structured ceramics. Mixing bismuth oxide with nano-structured alumina leads to stabilization of the low temperature polymorph α-Bi₂O₃ up to 780 °C. The diffusion permeability of NiMoO₄ layer grown at the surface of NiO ceramics, having submicron grains, was found 2 times lower if compared to NiMoO₄ grown at micro-sized NiO ceramics. NiO and Al₂O₃ nano-powders preserve the high reactivity even when heated up to 1000 °C. The results are discussed in terms of size effects on the solid state reactivity of oxides.     

Synthesis and characterization of spinel type high-power cathode materials Li MxMn2−x O4 (M=Ni,Co, Cr)

The transition metal-doped spinel cathode materials, LiM_0.5Mn_1.5O₄ (M=Ni. Co, Cr) were prepared by solid-state reaction. The structure and morphology of the samples were investigated by X-ray diffraction, Rietveld refinement and scanning electron microscopy (SEM). The diffraction peaks of all the samples corresponded to a single phase of cubic spinel structure with a space group Fd3m. Field-emission SEM shows octahedron like shapes and the primary particles size was between 500 nm and 2 μm. Oxidation states of Ni, Co and Cr were found to be 2+, 2+ and 3+ as revealed by X-ray photoelectron spectroscopy. During discharging, LiNi_0.5Mn_1.5O₄ and LiCo_0.5Mn_1.5O₄ sample shows more than 130 mAh/g between 3.5 and 5.2 V at a current density of 0.65 mA/cm² and well developed plateau around 5 V, respectively.     

Structural,magnetic and electrical study of nano-structured α-Fe1.4Ti0.6O3

Nano-structured α-Fe_1.4Ti_0.6O₃ has been synthesized using a simple technique of mechanical alloying. Doping of Ti atoms into α-Fe₂O₃ structure has been characterized by XRD, SEM-EDX, FTIR, VSM and impedance spectroscopy. Magnetic and electrical studies have revealed important changes at the interfaces of grains and grain boundaries, and at rhombohedral planes during formation of α-Fe_1.4Ti_0.6O₃ alloy. The alloy has shown the properties of ferromagnetic semiconductor with substantial increase of magnetic moment and electrical conductivity with the decrease of grain size. The 100 h milled sample has been air annealed to study the effects of thermal activated grain size, lattice structure, magnetism and electrical properties. This work has demonstrated some recent issues of the synthesis of ferromagnetic semiconductor, e.g., structural phase stability, grain size effects, magnetic ordering and electrical conductivity at different stages of the mechanical alloying and subsequent annealing at 700 °C.     

Preparation and characterization of Fe3O4(1 1 1) nanoparticles and thin films on Au(1 1 1)

Fe₃O₄ nanoparticles and thin films were prepared on the Au(1 1 1) surface and characterized using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Fe₃O₄ was formed by annealing α-Fe₂O₃(0 0 0 1) structures on Au(1 1 1) at 750 K in ultrahigh vacuum (UHV) for 60 min. Transformation of the α-Fe₂O₃(0 0 0 1) structures into Fe₃O₄ nanoparticles and thin films was supported by XPS. STM images show that during the growth procedure used, Fe₃O₄ initially appears as nanoparticles at low coverages, and forms thin films at ～2 monolayer equivalents (MLE) of iron. Two types of ordered superstructures were observed on the Fe₃O₄ particles with periodicities of ～50 and ～42 Å, respectively. As the Fe₃O₄ particles form more continuous films, the ～50 Å feature was the predominant superstructure observed. The Fe₃O₄ structures at all coverages show a hexagonal unit cell with a ～3 Å periodicity in the atomically resolved STM images.     

Effect of ball milling time on the substitution of carbon in glucose doped MgB2 superconductors: Dispersion behavior of glucose

The effect of the ball milling time (BMT) on the substitution of the carbon in the glucose doped MgB₂ samples is investigated here. Using in situ solid state reaction, four different doped samples of Mg(B_.98C_.02)₂ were prepared by mixing powders of Mg, boron and glucose for 2 h, 4 h, 8 h and 12 h using planetary ball milling. A reference sample of un-doped MgB₂ was also prepared under same conditions. The particle size distribution of the un-reacted samples show a decrease in the particle size as the BMT is increased. Both the average particle size as well as the standard deviation show a substantial decrease with the increase in the milling time up to 8 h. After 8 h, the size reduction is rather insignificant. From the XRD data, the crystallite size of the doped MgB₂ computed using the Scherrer formula was found to decrease with the increasing BMT, showing a saturation level after 8 h of the milling time. TEM images also confirm the crystallite size obtained from the XRD data. The substitution of the C in the MgB₂ lattice, measured from the change in the c/a ratio, increases with increasing BMT. The maximum carbon substitution is achieved at approximately 8 h of BMT. Moreover, a systematic enhancement of the residual resistivity and a decrease in T_C with an increasing BMT further confirms a progressive substitution of the carbon in the MgB₂. These results suggest that a minimum ball milling time is necessary to disperse the glucose uniformly for a maximum substitution of nano C in the B plane of MgB₂ lattice. The optimum BMT is found to be 8 h. Thus, the decrease in the particle size due to the ball milling enhances the dispersion of the constituent materials thereby favoring a greater substitution of the dopant in the MgB₂ during the solid-state reaction.     

Ultrasonic assisted-Fenton-like degradation of nitrobenzene at neutral pH using nanosized oxides of Fe and Cu

Ultrasonic-assisted heterogeneous Fenton reaction was used for degradation of nitrobenzene (NB) at neutral pH conditions. Nano-sized oxides of α-Fe₂O₃ and CuO were prepared, characterized and tested in degradation of NB (10 mg L⁻¹) under sonication of 20 kHz at 25 °C. Complete degradation of NB was effected at pH 7 in presence of 10 mM H₂O₂ after 10 min of sonication in presence of α-Fe₂O₃ (1.0 g L⁻¹), (k = 0.58 min⁻¹) and after 25 min in case of CuO (k = 0.126 min⁻¹). α-Fe₂O₃ showed also effective degradation under the conditions of 0.1 g L⁻¹ oxide and 5.0 mM of H₂O₂, even though with a lower rate constant (0.346 min⁻¹). Sonication plays a major role in enhancing the production of hydroxyl radicals in presence of solid oxides. Hydroxyl radicals-degradation pathway is suggested and adopted to explain the differences noted in rate constants recorded on using different oxides.     

Synthesis of magnetic γ-Fe2O3-based nanomaterial for ultrasonic assisted dyes adsorption: Modeling and optimization

γ-Fe₂O₃ nanoparticles were synthesized and loaded on activated carbon. The prepared nanomaterial was characterized by field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), Fourier transforms infrared spectroscopy (FT-IR) and X-ray diffraction (XRD). The γ-Fe₂O₃ nanoparticle-loaded activated carbon (γ-Fe₂O₃-NPs-AC) was used as novel adsorbent for the ultrasonic-assisted removal of methylene blue (MB) and malachite green (MG). Response surface methodology and artificial neural network were applied to model and optimize the adsorption of the MB and MG in their individual and binary solutions followed by the investigation on adsorption isotherm and kinetics. The individual effects of parameters such as pH, mass of adsorbent, ultrasonication time as well as MB and MG concentrations in addition to the effects of their possible interactions on the adsorption process were investigated. The numerical optimization revealed that the optimum adsorption (>99.5% for each dye) is obtained at 0.02 g, 15 mg L⁻¹, 4 min and 7.0 corresponding to the adsorbent mass, each dye concentration, sonication time and pH, respectively. The Freundlich, Langmuir, Temkin and Dubinin–Radushkevich isotherms were studied. The Langmuir was found to be most applicable isotherm which predicted maximum monolayer adsorption capacities of 195.55 and 207.04 mg g⁻¹ for the adsorption of MB and MG, respectively. The pseudo-second order model was found to be applicable for the adsorption kinetics. Blank experiments (without any adsorbent) were run to investigate the possible degradation of the dyes studied in presence of ultrasonication. No dyes degradation was observed.     

Influence of Zn–Zr ions on physical and magnetic properties of co-precipitated cobalt ferrite nanoparticles

Cobalt ferrite nanoparticles having the chemical formula CoFe_2−2xZr_xZn_xO₄ with x ranging from 0.0 to 0.4 were prepared by chemical co-precipitation method. The powder X-ray diffraction pattern confirms the spinel structure for the prepared compound. The particle size was calculated from the most intense peak (3 1 1) using Scherrer formula. The particle size of the samples was found within the range of 12–23 nm for all the compositions. The magnetic and electrical properties of these materials have been studied as a function of temperature. Activation energy and drift mobility have been calculated from the DC electrical resistivity measurements. Dielectric properties such as dielectric constant and dielectric loss tangent were measured at room temperature in the frequency range 100 Hz–1 MHz.