Synthesis of tunable sized capped magnetic iron oxide nanoparticles highly soluble in organic solvents

Surface modification of magnetic nanoparticles by organic surfactants is known to provide them with solubility in organic solvents (ferrofluids), which undoubtedly is an important property in several applications and studies. In this report, the main interest is focused on structural, magnetic and adsorption properties of iron oxide nanoparticles that are derived under water/toluene biphase conditions in the presence of oleic acid or oleylamine as the capping agents. The surfactants provide them with excellent stability and solubility in organic solvents like toluene or chloroform. Furthermore, by adding the appropriate surfactant or altering the temperature of the aqueous phase at the initial stage of the reaction we achieve a size control of the nanoparticles within the range 6–18 nm. The presence of capping agents or high reaction temperatures favours the formation of smaller nanoparticles. The adsorption of the surfactants (chemisorption) was identified with FT-IR spectroscopy, while Mössbauer studies have been performed to representative samples in order to identify the presence of either γ-Fe₂O₃ or Fe₃O₄, depending on the reaction temperature. Finally, the magnetic properties of representative samples have been studied at 5 K and room temperature.     

Oleic-acid-coated CoFe2O4 nanoparticles synthesized by co-precipitation and hydrothermal synthesis

Oleic-acid-coated CoFe₂O₄ nanoparticles were synthesized by co-precipitation and hydrothermal synthesis. The coprecipitation of the nanoparticles was achieved by the rapid addition of a strong base to an aqueous solution of cations in the presence of the oleic acid surfactant, or without this additive. The nanoparticles were also synthesized by a hydrothermal treatment of suspensions of the precipitates, coprecipitated at room temperature in the presence of the oleic acid, or without it. The influence of the synthesis conditions, such as the valence state of the iron cation in the starting aqueous solution, the temperature of the treatment and the presence of oleic acid, on the particles size was systematically studied. X-ray powder diffractometry (XRD) and transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS) revealed that, although spinel forms at room temperature, a substantial amount of Co was incorporated within the secondary, feroxyhyte-like phase when the iron cation was in the 2+ state. In contrast, when iron was in the 3+ state, the spinel forms at elevated temperatures of approximately 60 °C. The presence of the oleic acid further increased the formation temperature for the stoichiometric spinel. Moreover, the oleic acid impeded the particles’ growth and enabled the preparation of colloidal suspensions of the nanoparticles in non-polar organic solvents. The nanoparticles’ size was successfully controlled by the temperature of the synthesis in the region where superparamagnetism dominates to the region where mono-domain ferrimagnetism dominates the magnetic properties.     

Synthesis of Fe x O y nanoparticles during iron oxidation by supercritical water

It is established that iron is oxidized by supercritical water (SCW) with the formation of H₂ and nanoparticles of iron oxides (Fe₃O₄, FeO, and γ-Fe₂O₃). The kinetics of H₂ production and iron oxidation has been studied by SCW injection at T = 673, 723, 773, 823, and 873 K into a reactor with iron particles. Data of X-ray diffraction and transmission electron microscopy show that the phase composition and morphology of synthesized oxide nanoparticles depend on the SCW temperature.     

One-step solution auto-combustion process for the rapid synthesis of crystalline phase iron oxide nanoparticles with improved magnetic and photocatalytic properties

We report the solution auto-combustion (AC) process for the rapid synthesis of Fe₃O₄ nanoparticles derived from the sol–gel (SG) process. The citric acid (CA) and tartaric acid (TA) is used as gelling agents in the SG process, where the citric acid turns into a fuel that combusts the gel and yields a highly magnetic crystalline phase Fe₃O₄ nanoparticles in one step with an average particle size of 50 nm. In contrast, the citric acid at different concentrations and tartaric acid at any concentrations do not lead to any combustion process and yield amorphous iron oxides. Upon annealing, these CA and TA derived iron oxide samples are turned to crystalline phase α-Fe₂O₃ particles. In contrast, the as-synthesized AC sample (i.e. Fe₃O₄) is oxidized to γ-Fe₂O₃ phase, which is confirmed from their respective XRD, Rietveld refinement and XPS studies. All the synthesized iron oxide phases showed broad visible light absorption. The room temperature M?H hysteresis curves obtained from VSM revealed that the Fe₃O₄ and α-Fe₂O₃/γ-Fe₂O₃ phases exhibit super-paramagnetic and ferromagnetic properties, respectively. The photocatalytic efficiencies of the samples are found to be in the order of Fe₃O₄ > γ-Fe₂O₃ > α-Fe₂O₃ with 98, 87, 79/73% degradation of rhodamine B dye at the end of 3 h and H₂ evolution rate over these systems is found to be 2.1, 1.3 and 0.92/0.89 mmol/h/g, respectively under simulated solar light irradiation. The photocatalytic recycle studies demonstrated that all the synthesized photocatalysts possess excellent chemical and photo-stabilities.     

Hydrogen evolution kinetics during transition metal oxide-catalyzed ammonia borane hydrolysis

This paper examines the feasibility of using transition metal oxides (cobalt, iron, copper, molybdenum, and vanadium oxides) as catalysts for ammonia borane (AB) hydrolysis. In our experiments, we used an aqueous solution containing 0.24 wt % AB. The amount of oxide catalysts was 10–40 mg. The hydrolysis process was run in the temperature range from 35 to 80°C. The highest hydrogen evolution rate was observed at a temperature of 80°C in the presence of cobalt and iron oxides (Co₃O₄ and Fe₂O₃ · nH₂O). The data obtained for the cobalt and iron oxides demonstrate that the reaction is first-order in ammonia borane. We determined the rate constants of the process and its apparent activation energy: 47.5 kJ/mol for Co₃O₄ and 60.2 kJ/mol for Fe₂O₃ · nH₂O. The cobalt and iron oxides were shown to be efficient catalysts for hydrogen production from aqueous AB solutions.     

Facile synthesis of porous hollow iron oxide nanoparticles supported on carbon nanotubes

Porous hollow iron oxide nanoparticles (PHNPs) supported on carbon nanotubes (CNTs) were facilely synthesized by etching Fe@Fe_xO_y/CNT with dilute nitric acid aqueous solution at ambient temperature without the assistance of any surfactants and ligands. The mean diameter of hollow iron oxide nanoparticles was about 17 nm, with a wall thickness of about 4 nm. The formation mechanism of PHNPs is discussed based on the characterization results from TEM, XRD and H₂-TPR. The combination of nanoscale Kirkendall effect and selective acid etching is proposed to be responsible for the formation of CNT supported PHNPs, through a transformation from core/void/shell structures to hollow nanoparticles.     

Towards carbon-free flame spray synthesis of homogeneous oxide nanoparticles from aqueous solutions

Flame-assisted spray pyrolysis (FASP) is a versatile process for synthesis of nanoparticles from a broad choice of precursors and solvents. Water is an attractive solvent particularly for inexpensive inorganic precursors (e.g. metal nitrates) as it can effectively reduce the process cost. Furthermore when water usage is combined with a carbon-free fuel (e.g. H₂), nanoparticles can be made without forming CO₂. Here such a FASP process is explored for synthesis of Bi₂O₃ and other oxide nanoparticles from aqueous precursor solutions. The flame temperature was measured by FTIR emission–transmission spectroscopy while powders were characterized by X-ray diffraction and N₂ adsorption. At low FASP fuel gas (H₂ or C₂H₂) flow rates or process temperatures, product powders had a bimodal crystal size distribution. Its large and small modes were made by droplet- and gas-to-particle conversion, respectively. Homogeneous Bi₂O₃ and CeO₂ powders were obtained for sufficiently high flow rates of either C₂H₂ or H₂. Prolonged high temperature residence times promoted precursor evaporation from the spray droplets and yielded homogeneous nanostructured powders by gas-to-particle conversion. In contrast, FASP of aqueous solutions of aluminum nitrate yielded rather large particles by droplet-to-particle conversion at all fuel flows investigated.     

Synthesis of maghemite (γ-Fe2O3) nanoparticles by thermal-decomposition of magnetite (Fe3O4) nanoparticles

In this research work, we prepared γ-Fe₂O₃ nanoparticles by thermal-decomposition of Fe₃O₄. The Fe₃O₄ nanoparticles were synthesized via co-precipitation method at room temperature. This simple, soft and cheap method is suitable for preparation of iron oxide nanoparticles (γ-Fe₂O₃; Fe₃O₄). The samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), vibrating sample magnetometer and differential scanning calorimeter (DSC). The XRD and FT-IR results indicated the formation of γ-Fe₂O₃ and Fe₃O₄ nanoparticles. The TEM images showed that the γ-Fe₂O₃ and Fe₃O₄ were spherical, and their size was 18 and 22 nm respectively. Magnetic properties have been measured by VSM at room temperature. Hysteresis loops showed that the γ-Fe₂O₃ and Fe₃O₄ nanoparticles were super-paramagnetic.     

Metal oxide/polyaniline nanocomposites: Cluster size and composition dependent structural and magnetic properties

Nanocomposites of iron oxide with conducting polymer in the form of powders of varying compositions have been studied to understand the effects of particle size, cluster size and magnetic inter-particle interactions. The sizes of the nanoparticles were estimated to be ∼ 10–20 nm from the X-ray diffraction (XRD) and the transmission electron micrographs (TEM). XRD shows a single crystalline phase for the γ-Fe₂O₃. The presence of conducting polymer was confirmed through Fourier transform infrared (FTIR) spectroscopy. The amount of polymer present in the composite, the transition temperature of iron oxide and the thermal stability of polymer was determined through thermogravimetric and differential thermal analysis (TGA-DTA). The room temperature magnetic hysteresis measurements show reduction in saturation magnetization with increasing polymer concentrations. A low value of coercivity was observed for low polymer composites. On increasing the polymer concentration, the coercivity and remanence become negligible indicating a superparamagnetic phase at room temperature. Beyond a certain composition, the system shows paramagnetic behaviour which is also confirmed through zero field cooled-field cooled (ZFC-FC) measurements. We also report preliminary results on the magnetic properties of self standing sheets prepared using γ-Fe₂O₃ and NiFe₂O₄ nanoparticles and conducting polymers.     

Magnetic Field Dependence of Blocking Temperature in Oleic Acid Functionalized Iron Oxide Nanoparticles

We report the synthesis of phase pure, mono-dispersed Fe₃O₄ nanoparticles of size ??10?nm via chemical co-precipitation of ferrous and ferric ions, under controlled pH and temperature. The nanoparticles are oleic acid functionalized and hence dispersible in organic medium. The structure and morphology of nanoparticles are determined by analyzing XRD pattern and TEM micrographs, confirming the formation of phase pure Fe₃O₄ nanoparticles. The magnetization studies reveal the superparamagnetic behavior of the nanoparticles at room temperature. The changes in blocking temperatures (T _B) of magnetic nanoparticles with applied magnetic fields (H _ap), noted from the cusp of the zero-field-cooled magnetization, the indicate effects of dipole interactions. A decrease in blocking temperature from 95?K to 15?K has been observed on varying the magnetic field from 50?Oe to 5000?Oe. T _B versus H relation follows the equation T _B(H)=T _o(1?(H/H _o)) ^m , i.e. the Néel?CBrown model of magnetic relaxation in nanoparticles.     

Synthesis of bipyramidal gold nanoparticle-[C60]fullerene nanowhisker composites and catalytic reduction of 4-nitrophenol

The seed solution was prepared by dissolving hydrogen tetrachloroaurate(III) trihydrate (HAuCl₄·3H₂O), trisodium citrate dihydrate (C₆H₅Na₃O₇·2H₂O), and sodium borohydride (NaBH₄) in distilled water. The resulting reddish-purple seed solution was stirred for 3 h at room temperature. The growth solution was prepared by mixing hydrogen tetrachloroaurate(III) trihydrate (HAuCl₄·3H₂O), cetyltrimethylammonium bromide (CTAB, (C₁₆H₃₃)N(CH₃)₃Br), silver nitrate (AgNO₃), hydrochloric acid (HCl), and ascorbic acid (C₆H₈O₆) in distilled water. Subsequently, 100 μL of this seed solution was transferred into 10 mL of the growth solution. The mixed solution was maintained at 30 °C for 3 h to obtain a solution of bipyramidal gold nanoparticles. The bipyramidal gold nanoparticle-[C₆₀] fullerene nanowhisker composites were synthesized by applying the liquid-liquid interfacial precipitation (LLIP) method to a saturated solution of C₆₀ in toluene, the solution of bipyramidal gold nanoparticles, and isopropyl alcohol. The prepared bipyramidal gold nanoparticle-[C₆₀] fullerene nanowhisker composites were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. The catalytic activity of these composites was confirmed in the reduction of 4-nitrophenol by UV-Vis spectroscopy.     

A facile aqueous-phase route for the synthesis of silver nanoplates

A new seed-mediated method for the synthesis of silver nanoplates is reported, in which tannin (C₇₆H₅₂O₄₆) is used to reduce silver salt in aqueous solution. This synthesis is performed at room temperature, and doesn't need any surfactant or polymer to direct the anisotropic growth. The obtained nanoplates exhibit polygonal morphologies and their size can be tuned through adjusting the amounts of seeds. Due to the anisotropic shape feature, the absorption spectra of the nanoplates prove to be quite different from that of spherical nanoparticles.     

A facile route to synthesis of superparamagnetic Fe3O4–PDVB nanoworms

Fe₃O₄–polydivinylbenzene (PDVB) nanoworms were firstly synthesized by precipitation polymerization of divinylbenzene in the presence of oleic acid coated iron oxide nanoparticles. The nanoworms had superparamagnetic properties at room temperature, but ferromagnetism at 5 K. Thermogravimetric analysis curves indicated that in comparison with magnetic nanoparticles, the weight percent of iron oxide in nanoworms was slightly declined due to the formation of Fe₃O₄–PDVB nanocomposites. The superparamagnetic nanoworms could be well dispersed in ethanol, and were capable of easy separation by an external magnetic field. Overall, this provided a valuable methodology for preparation of elongated magnetic nanoparticles with high surface-to-volume ratio, which had potential applications in drug delivery/targeting, magnetic resonance imaging, and nanoprobes for diagnosis and disease treatment.     

Hydrothermal synthesis and magnetic property of Cu3(OH)2V2O7·nH2O

Copper vanadium oxide hydroxide hydrate (Cu₃(OH)₂V₂O₇·nH₂O) nanoparticles with mean size of about 100 nm were successfully synthesized by a simple hydrothermal method. The structure and morphology of the as-synthesized products were characterized by X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Raman spectra, and Fourier transform infrared spectra (FTIR). The composition and purity of the as-synthesized Cu₃(OH)₂V₂O₇·nH₂O nanoparticles were characterized by Energy disperse spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The magnetic property of the as-synthesized Cu₃(OH)₂V₂O₇·nH₂O nanoparticles was characterized by vibrant sample magnetometer. Magnetic hysteresis curve indicate that the as-synthesized nanoparticles are of weak ferromagnetic property at room temperature.     

A facile approach to Fe3O4@Au nanoparticles with magnetic recyclable catalytic properties

A novel approach has been developed to synthesize gold-coated iron oxide nanoparticles. Fe₃O₄ nanoparticles were initially prepared by co-precipitation method and subsequently coated with gold layer under oleylamine reduction of AuCl₄⁻ at room temperature. The core-shell nanoparticles showed magnetic recoverable catalytic activity for the reduction of 4-nitrophenol with NaBH₄.     

Water-Soluble Anisotropic Iron Oxide Nanoparticles: Dextran-Coated Crystalline Nanoplates and Nanoflowers

We report a simple phase transfer based synthesis route for two novel anisotropic water soluble iron oxide nanoparticle shapes, namely, nanoplates and nanoflowers. The nanoplates and nanoflowers are initially prepared in an organic solvent via a modified “heat-up” method. Then, the crystalline nanoparticles are rendered hydrophilic via sonication in the presence of dextran and water. These nanoparticles are highly monodisperse and superparamagnetic at room temperature. High resolution transmission electron microscopy indicates that the iron oxides cores are not affected by the phase transfer. Dextran coating is confirmed by dynamic light scattering, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The obtained dextran coverage was 26 wt% for the nanoplates and 37 wt% for the nanoflowers. The nanoplates and nanoflowers were not only water soluble, but also remained stable at different pH (4–7) and in common aqueous buffer solutions. Thorough characterizations of the nonspherical iron oxide nanoparticles indicate that these particles could be useful for potential biomedical applications and magnetic resonance imaging.     

Preparation of ultra-fine cobalt-nickel manganite powders and ceramics derived from mixed oxalate

Co_0.30Ni_0.66Mn_2.04O₄ negative temperature coefficient ceramics were derived from mixed oxalate Co_0.30Ni_0.66Mn_2.04(C₂O₄)₃·nH₂O. The mixed oxalate was synthesized by milling a mixture of cobalt acetate, nickel acetate, manganese acetate, and oxalic acid at room temperature. An ultra-fine Co_0.30Ni_0.66Mn_2.04O₄ powder was obtained by calcining the mixed oxalate in air at 800 °C for 3 h. The oxide powder compact was sintered at a relatively low temperature of 1100 °C for 5 h, achieving a relative density of ∼98%. The specific resistivity ρ_25 °C and the thermal constant B_25/85 °C were 765.2 Ω cm and 3604 K, respectively. The resistance drift after aging at 150 °C for 500 h was 1.5%.     

Inorganic-organic hybrid semiconductor nanomaterials: (ZnSe)(N2H4)x(C5H5N)y

In this paper, inorganic-organic hybrid semiconductor (ZnSe)(N₂H₄)_x(C₅H₅N)_y nanosheets as well as (ZnSe)(C₅H₅N)_y nanoparticles were first synthesized by a solvothermal method in a ternary solution and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectra, ultraviolet-visible (UV-vis) absorption spectra, thermogravimetric analysis (TGA), differential scanning calorimeter (DSC) and photoluminescence (PL) spectra. The results indicated that the morphology and composition of the products were largely influenced by the reaction temperature and the volume ratio of water. When the reaction temperature and the water content were lower, (ZnSe)(N₂H₄)_x(C₅H₅N)_y nanosheets were formed. As the reaction temperature or the content of water was high enough, (ZnSe)(C₅H₅N)_y nanoparticles were formed. Pure hexagonal wurtzite ZnSe nanosheets or zinc blende ZnSe nanoparticles were obtained by extracting the corresponding hybrids. The bandgap absorption of ZnSe nanocrystals blue shifted in comparison with the bulk. The photoluminescent intensity of (ZnSe)(N₂H₄)_x(C₅H₅N)_y nanosheets was much stronger than that of (ZnSe)(C₅H₅N)_y nanoparticles.     

Facile synthesis of capped γ-Fe2O3 and Fe3O4 nanoparticles

Facile methods for the selective preparation of capped iron oxide nanoparticles (γ-Fe₂O₃, Fe₃O₄) are described. The magnetic oxides are obtained via oxidative transformation of an iron hydroxide gel using H₂O₂ or (NH₄)₂S₂O₈ solutions as oxidants. Capping with oleic or other aliphatic acids is established simultaneously in one step by adding a toluene solution of the capping agent and refluxing the resulting biphase system. The method is simple, soft and affords nanoparticles of γ-Fe₂O₃ or Fe₃O₄ of controlled size depending on the reaction conditions. The capped nanoparticles are readily soluble in organic or aqueous media according to the nature of the sheath surrounding the surface of the particles, providing stable and high concentration ferrofluids.     

One-step approach to prepare magnetic iron oxide/reduced graphene oxide nanohybrid for efficient organic and inorganic pollutants removal

An environmentally friendly effective technique was demonstrated to prepare iron oxide/reduced graphene oxide nanohybrid (IO/RGO) at room temperature by using banana peel ash aqueous extract as the base source and Colocasia esculenta leaves aqueous extract as the reducing agent. The nanohybrid was characterized by Fourier transform infrared spectroscopy, X-ray diffractometry, transmission electron microscopy, vibrating sample magnetometry, Raman spectroscopy and thermal studies. The results indicated the decoration of superparamagnetic IO nanoparticles on the surface of the RGO. Both organic and inorganic pollutants were effectively removed from the contaminated water (for Pb²⁺ and Cd²⁺ within 10 min, whereas for tetrabromobisphenol A within 30 min) by IO/RGO. The study revealed that adsorption followed pseudo-second order kinetics and isotherms were well described by the Langmuir model in all the cases. The thermodynamics parameters (ΔG°, ΔS° and ΔH°) were calculated from the temperature dependent isotherms and indicated that the adsorptions were endothermic and spontaneous.