Synthesis of ordered mesoporous Fe3O4 and gamma-Fe2O3 with crystalline walls using post-template reduction/oxidation

Ordered mesoporous Fe(3)O(4) with crystalline walls (inverse spinel structure) has been synthesized for the first time, representing to the best of our knowledge, the first synthesis of a reduced mesoporous iron oxide. Synthesis was achieved by reducing ordered mesoporous alpha-Fe(2)O(3) (corundum structure) to Fe(3)O(4) spinel then to gamma-Fe(2)O(3) by oxidation, while preserving the ordered mesostructure and crystalline walls throughout. Such solid/solid transformations demonstrate the stability of the mesostructure to structural phase transitions from the hexagonal close packed oxide subarray of alpha-Fe(2)O(3) (corundum structure) to the cubic close packed subarray of Fe(3)O(4) spinel and gamma-Fe(2)O(3). Preliminary magnetic measurements reveal that the spins in both Fe(3)O(4) and gamma-Fe(2)O(3) are frozen at 295 K, despite the wall thickness (7 nm) being less than the lower limit for such freezing in corresponding nanoparticles (>8 nm).     

Size-controlled synthesis of magnetite nanoparticles

Monodisperse magnetite nanoparticles have been synthesized by high-temperature solution-phase reaction of Fe(acac)3 in phenyl ether with alcohol, oleic acid, and oleylamine. Seed-mediated growth is used to control Fe3O4 nanoparticle size, and variously sized nanoparticles from 3 to 20 nm have been produced. The as-synthesized Fe3O4 nanoparticles have inverse spinel structure, and their assemblies can be transformed into gamma-Fe2O3 or alpha-Fe nanoparticle assemblies, depending on the annealing conditions. The reported procedure can be used as a general approach to various ferrite nanoparticles and nanoparticle superlattices.     

Synthesis and characterization of poly(divinylbenzene)-coated magnetic iron oxide nanoparticles as precursor for the formation of air-stable carbon-coated iron crystalline nanoparticles

Maghemite (gamma-Fe2O3) nanoparticles of 15 +/- 3 nm diameter were prepared by nucleation of gelatin/iron oxide followed by growth of gamma-Fe2O3 films onto these nuclei. The gamma-Fe2O3 nanoparticles were coated with polydivinylbenzene (PDVB) by emulsion polymerization of divinylbenzene (DVB) in an aqueous continuous phase containing the gamma-Fe2O3 nanoparticles. The PDVB-coated gamma-Fe2O3 nanoparticles, dispersed in water, were separated from homo-PDVB nanoparticles using the high gradient magnetic field (HGMF) technique. The influence of DVB concentration on the amount of PDVB coating, on the size and size distribution of the coated gamma-Fe2O3 nanoparticles and on their magnetic properties, has been investigated. Air-stable carbon-coated iron (alpha-Fe/C) crystalline nanoparticles of 41 +/- 12 nm diameter have been prepared by annealing the PDVB-coated gamma-Fe2O3 nanoparticles at 1050 degrees C in an inert atmosphere. These nanoparticles exhibit high saturation magnetization value (83 emu g(-1)) and excellent resistance to oxidation. Characterization of the PDVB-coated gamma-Fe2O3 and of the alpha-Fe/C nanoparticles has been accomplished by TEM, HRTEM, DLS, FTIR, XRD, thermal analysis, zeta-potential, and magnetic measurements.     

Systematic control of size,shape, structure,and magnetic properties of uniform magnetite and maghemite particles

As an application of the gel-sol method especially developed for the synthesis of general monodisperse particles in large quantities, uniform hematite (alpha-Fe2O3), magnetite (Fe3O4), and maghemite (gamma-Fe2O3) particles, precisely controlled in size, aspect ratio, and internal structure, have been prepared. For the synthesis of uniform ellipsoidal single-crystal particles of alpha-Fe2O3, a highly condensed suspension of fine beta-FeOOH particles doped with a prescribed amount of PO4(3-) ion in their interiors was aged at 140 degrees C for 24 h with seed particles of alpha-Fe2O3 in an acidic medium containing optimum concentrations of HCl and NaNO3. Systematic control of the aspect ratio and mean size was achieved by regulating the concentration of PO4(3-) ion incorporated into the beta-FeOOH particles and the number of seeds added. The resulting hematite particles were converted into magnetite by reduction in a H2 stream at 330 degrees C for 6 h; the magnetite was then oxidized to maghemite in an air stream at 240 degrees C for 2 h. Magnetite and maghemite thus prepared retained the original shape of the hematite. On the other hand, polycrystalline hematite particles of different sizes and aspect ratios were also prepared by aging a condensed Fe(OH)3 gel in the presence of different concentrations of SO4(2-) ion and seeds. The polycrystalline hematite particles were similarly converted into magnetite and then maghemite. The magnetic properties of these magnetite and maghemite particles were analyzed as a function of their mean particle volume, aspect ratio, and internal structure.     

Doping gamma-Fe(2)O(3) nanoparticles with Mn(III) suppresses the transition to the alpha-Fe(2)O(3) structure

Here we report on a mixed oxide system, gamma-Fe2O3 nanoparticles doped with Mn(III), where the transition from the cubic to the more stable hexagonal alpha-Fe2O3 structure is suppressed. When amorphous Fe2O3 is heated at 300 degrees C for 3 h, ferrimagnetic gamma-Fe2O3 is observed as the sole product. On the other hand, when the temperature is raised to 500 degrees C, one observes only antiferromagnetic alpha-Fe2O3 as the product. However, upon doping with 8.5 wt % Mn(III), the amorphous nanoparticles crystallized to mainly the gamma-Fe2O3 matrix after heating at 500 degrees C for 3 h, and need to be heated to >650 degrees C for the complete transition to the alpha-Fe2O3 structure to take place.     

Iron oxide nanopropellers prepared by a low-temperature solution approach

The alpha-Fe(2)O(3) (hematite) nanopropellers were synthesized via a low-temperature solution-based method using FeCl(2) as a precursor in the presence of urea and glycine hydrochloride. The formation of alpha-Fe(2)O(3) nanopropellers is strongly depended on the addition of glycine hydrochloride, which serves as a pH modulator and affects the oxidation rate of Fe(2+). The structural evolution of the propeller-structured hematite was found to follow dissolution and recrystallization processes. For the structural conformation, each nanopropeller presents a hexagonal central column closed by six equivalent surfaces of {(-)1100} and the six arrays of the nanopropeller structure are a result of growth along +/- [(-)1100], +/- [(-)1010], and +/-[0(-)110]. Preliminary results show that the magnetic maghemite (gamma-Fe(2)O(3)) nanopropellers could also be prepared by a reduction and reoxidation process from the alpha-Fe(2)O(3) (hematite) nanopropeller precursors.     

Nanostructured pure gamma-Fe2O3 via forced precipitation in an organic solvent

Pure maghemite, gamma-Fe(2)O(3), was prepared as ultra fine particles in the nanometer-sized range via the forced precipitation method in an organic solvent. The precipitation of iron(III) ions, from iron(III) chloride in 2-propanol led selectively to highly dispersed particles of ferrihydrite, which upon treatment with temperatures higher than 200 degrees C under dynamic vacuum resulted in high-surface-area particles of gamma-Fe(2)O(3). Precipitation in water also led to ferrihydrite, but the final product, after heating at 300 degrees C, contained a mixture of gamma-Fe(2)O(3) and alpha-Fe(2)O(3) (hematite). The precipitation from iron(III) nitrate in water resulted in goethite which was converted to hematite upon heating. On the other hand, the final product in 2-propanol was a mixture of maghemite and hematite. The products were characterized by FTIR, TGA, XRD, and gas sorption analysis. Nitrogen gas adsorption studies for the pure gamma-Fe(2)O(3) samples revealed mesoporous particles with high surface areas in the range of 70-120 m(2) g(-1) after heat treatment at 300 degrees C. The gamma-Fe(2)O(3) particles retained their gamma-phase as well as their mesoporous structure at relatively high temperatures, as high as 400 degrees C.     

Raman analysis of ancient pigments on a tile from the Citadel of Algiers

A micro-Raman spectroscopy study of a multi-coloured (yellow, blue, white, redish-brown and brown-black) tile shard from the Citadel of Algiers was undertaken. XRD and EDX were used as complementary techniques. The study shows that the heterogeneous three-shade yellow pigment on the tile is composed largely of the ancient ternary (Pb-Sn-Sb) pyrochlore oxide with a dominant Pb-O vibration at 127 cm(-1) consistent with the Pb2SnSbO6.5 structure as verified by XRD. The literature assignment of this band at 132 cm(-1) probably comes from a mixture of pigments. The redish-brown and the brown-black pigments are found to be Naples yellow (Pb2Sb2O7) and lead(II) stannate (Pb2SnO4), respectively, while cobalt blue (CoAl2O4) gives the blue colour and cassiterite (SnO2) is the origin of the white colour. The bulk of the tile body is composed mainly of hematite (alpha-Fe2O3), maghemite (gamma-Fe2O3), magnetite (Fe3O4) and Quartz (alpha-SiO2) with traces of calcite (CaCO3) and amorphous carbon. Micro-Raman spectroscopy proved to be very useful in the characterization of pigments as well as the tile body. These results further establish Raman spectroscopy as a technique of choice for the analysis of pigments on archaeological artifacts. The results obtained here could be used in the restoration and preservation programme of the Citadel itself which stands today as a symbol of pre-colonial Algerian heritage.     

Catalytic efficiency of iron(III) oxides in decomposition of hydrogen peroxide: competition between the surface area and crystallinity of nanoparticles

Various iron(III) oxide catalysts were prepared by controlled decomposition of a narrow layer (ca. 1 mm) of iron(II) oxalate dihydrate, FeC(2)O(4).2H(2)O, in air at the minimum conversion temperature of 175 degrees C. This thermally induced solid-state process allows for simple synthesis of amorphous Fe(2)O(3) nanoparticles and their controlled one-step crystallization to hematite (alpha-Fe(2)O(3)). Thus, nanopowders differing in surface area and particle crystallinity can be produced depending on the reaction time. The phase composition of iron(III) oxides was monitored by XRD and (57)Fe M?ssbauer spectroscopy including in-field measurements, providing information on the relative contents of amorphous and crystalline phases. The gradual changes in particle size and surface area accompanying crystallization were evaluated by HRTEM and BET analysis, respectively. The catalytic efficiency of the synthesized nanoparticles was tested by tracking the decomposition of hydrogen peroxide. The obtained kinetic data gave an unconventional nonmonotone dependence of the rate constant on the surface area of the samples. The amorphous nanopowder with the largest surface area of 401 m(2) g(-1) revealed the lowest catalytic efficiency, while the highest efficiency was achieved with the sample having a significantly lower surface area, 337 m(2) g(-1), exhibiting a prevailing content of crystalline alpha-Fe(2)O(3) phase. The obtained rate constant, 26.4 x 10(-3) min(-1) (g/L)(-1), is currently the highest value published. The observed rare catalytic phenomenon, where the particle crystallinity prevails over the surface area effects, is discussed with respect to other processes of heterogeneous catalysis.     

Analytical utility of valence band X-ray photoelectron spectroscopy of iron and its oxides,with spectral interpretation by cluster and band structure calculations

The valence band and core-level X-ray photoelectron spectroscopy (XPS) of iron and its oxides are reported, and the valence band spectra interpreted by various calculation models. The paper focuses upon the valence band region, which shows significant differences between the metal and the following oxidized iron species: FeO, Fe(3)O(4), alpha-Fe(2)O(3), gamma-Fe(2)O(3), alpha-FeOOH and gamma-FeOOH. The core region is of little analytical value as a means of distinguishing between these species, but the valence band region shows significant differences. These differences are consistent with spectra predicted by cluster and band structure calculations. Cluster calculations are valuable as a means for interpreting the spectra of iron oxides with multiple iron sites and defect characteristics.     

TEM-induced structural evolution in amorphous Fe oxide nanoparticles

Exposure to the high energy electron beam of a TEM changes the morphology of amorphous Fe oxide nanoparticles from solid spheres to hollow shells. Amorphous Fe oxide nanoparticles prepared via high-temperature methods using hexadecylamine and trioctylphosphine oxide surfactants were compared to crystalline gamma-Fe2O3 particles of similar size. Both sets of particles are fully characterized via SQUID magnetometry, X-ray powder diffraction, BET surface analysis, EPR spectroscopy, high-resolution transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). Time-resolved TEM images reveal that the amorphous Fe oxide particles evolve from solid spheres into hollow shells in <2 min, whereas crystalline gamma-Fe2O3 are unaffected by the electron beam. The resulting nanocrystalline Fe oxide shells bear striking resemblance to core-shell nanocrystals, but are a result of a morphology change attributed to restructuring of particle voids and defects induced by quasi-melting in the TEM. These results thus imply that caution is necessary when using TEM to analyze nanoparticle core-shell and heterostructured nanoparticles.     

Conversion of anisotropically phase-segregated Pd/gamma-Fe2O3 nanoparticles into exchange-coupled fct-FePd/alpha-Fe nanocomposite magnets

Exchange-coupled fct-FePd/alpha-Fe nanocomposite magnets were fabricated by converting anisotropically phase-segregated Pd/gamma-Fe2O3 nanoparticles via the interfacial atom diffusion. The magnetically hard fct-FePd phases formed by the interdiffusion between alpha-Fe and fcc-Pd phases nearly preserve their sizes at the nanometer scale because they are surrounded by the alpha-Fe matrix. The VSM measurements reveal that the exchange coupling between the soft and hard phases has been realized.     

Excited carrier dynamics of alpha-Cr2O3/alpha-Fe2O3 core-shell nanostructures

In this work alpha-Cr(2)O(3)/alpha-Fe(2)O(3) core-shell polycrystalline nanostructures were synthesized by using alpha-Cr(2)O(3) nanoparticles as seed crystals during aqueous nucleation. The formation of alpha-Fe(2)O(3) polycrystallites on alpha-Cr(2)O(3) surfaces was confirmed by X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray analysis. The excited-state relaxation dynamics of as-grown core-shell structures and "pure" alpha-Fe(2)O(3) particles of the same size were measured with femtosecond transient absorption spectroscopy. The results show the carrier lifetimes decay within a few picoseconds regardless of sample. This is likely due to fast recombination/trapping of carriers to defects and iron d-states.     

Synthesis of highly crystalline and monodisperse maghemite nanocrystallites without a size-selection process.

The synthesis of highly crystalline and monodisperse gamma-Fe(2)O(3) nanocrystallites is reported. High-temperature (300 degrees C) aging of iron-oleic acid metal complex, which was prepared by the thermal decomposition of iron pentacarbonyl in the presence of oleic acid at 100 degrees C, was found to generate monodisperse iron nanoparticles. The resulting iron nanoparticles were transformed to monodisperse gamma-Fe(2)O(3) nanocrystallites by controlled oxidation by using trimethylamine oxide as a mild oxidant. Particle size can be varied from 4 to 16 nm by controlling the experimental parameters. Transmission electron microscopic images of the particles showed 2-dimensional and 3-dimensional assembly of particles, demonstrating the uniformity of these nanoparticles. Electron diffraction, X-ray diffraction, and high-resolution transmission electron microscopic (TEM) images of the nanoparticles showed the highly crystalline nature of the gamma-Fe(2)O(3) structures. Monodisperse gamma-Fe(2)O(3) nanocrystallites with a particle size of 13 nm also can be generated from the direct oxidation of iron pentacarbonyl in the presence of oleic acid with trimethylamine oxide as an oxidant.     

Facile fabrication of long alpha-Fe(2)O(3), alpha-Fe and gamma-Fe(2)O(3) hollow fibers using sol-gel combined co-electrospinning technology

Long alpha-Fe(2)O(3) hollow fibers have been prepared through a facile sol-gel combined co-electrospinning technique using ferric citrate as precursor, and alpha-Fe and gamma-Fe(2)O(3) hollow fibers have been obtained by reduction and reoxidation at different conditions. The outer diameter of the as-prepared hollow fibers is 0.5-5 microm with wall thickness of 200-800 nm. The obtained tubular fibers were characterized by thermal gravimetric (TG), FT-IR spectra, X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Raman techniques. In addition, magnetic properties of alpha-Fe and gamma-Fe(2)O(3) hollow fibers have also been investigated.     

Formation of protein-metal oxide nanostructures by the sonochemical method: observation of nanofibers and nanoneedles

The sonochemical reaction of iron pentacarbonyl is explored in water and in water with the protein BSA (bovine serum albumen). In water, the reaction is found to produce spherical nanoparticles of magnetite (Fe3O4) with a particle size distribution of <10 to approximately 60 nm. In water with BSA, the reaction produces either nanofibers or nanoneedles, depending on the concentration of BSA. The nanofiber and nanoneedle samples are found to be mixtures of goethite, lepidocrocite, and hematite (alpha-FeOOH, gamma-FeOOH, and alpha-Fe2O3, respectively). The sonochemical reaction of iron pentacarbonyl with BSA in water is thought to proceed through the thermal decomposition mechanism for iron pentacarbonyl with BSA acting as a templating agent.     

Aging effects in the electrokinetics of colloidal iron oxides

We analyze in this contribution the effect of aging on the electrokinetic properties of magnetite (Fe(3)O(4)) and hematite (alpha-Fe(2)O(3)). In both cases, high-purity commercial samples and monodisperse synthetic particles were studied. Commercial magnetite showed a rather erratic dependence of its electrophoretic mobility u(e) with the concentration of NaCl. Furthermore, sufficient concentrations of the latter were able to change the sign of the mobility. When KNO(3) solutions were used, although no such change was observed, no clear effect of [KNO(3)] on the mobility was found, and, in addition, an intense aging effect was detected, as the mobility became increasingly positive in suspensions that were stored over 1 day. The picture was radically different with synthetic magnetite spheres, as the expected overall decrease of u(e) with either NaCl or KNO(3) concentration was measured. However, also in this case the aging effect was clearly observed: u(e) tended in this case to more negative values upon suspension storage, and a steady value of the mobility was reached only after 5 days in NaCl (and even longer in KNO(3) solutions). Because of the crystal structure similarities between magnetite and maghemite (gamma-Fe(2)O(3)), it has been shown that the final step of magnetite oxidation is maghemite. This is confirmed in the present study, as the mobility-pH trends of magnetite progressively approach those of maghemite after about 7 days of storage. Since hematite is chemically more stable than magnetite, our study focused in this case on the comparison between commercial and synthetic particles. The former showed a negative mobility at pH 5.5 under all conditions, suggesting an isoelectric point well below the value accepted for hematite (>/=7). The effect of aging on commercial samples was again very significant, as u(e) decreased in absolute value, apparently without limit as the time since preparation was longer. In contrast, synthetic hematite showed a more predictable dependence on ionic strength, and more limited aging effects, as u(e) reached equilibrium values after around 5 days in NaCl; longer times were required in KNO(3) solutions.     

Nanocrystalline iron oxide aerogels as mesoporous magnetic architectures

We have developed crystalline nanoarchitectures of iron oxide that exhibit superparamagnetic behavior while still retaining the desirable bicontinuous pore-solid networks and monolithic nature of an aerogel. Iron oxide aerogels are initially produced in an X-ray-amorphous, high-surface-area form, by adapting recently established sol-gel methods using Fe(III) salts and epoxide-based proton scavengers. Controlled temperature/atmosphere treatments convert the as-prepared iron oxide aerogels into nanocrystalline forms with the inverse spinel structure. As a function of the bathing gas, treatment temperature, and treatment history, these nanocrystalline forms can be reversibly tuned to predominantly exhibit either Fe(3)O(4) (magnetite) or gamma-Fe(2)O(3) (maghemite) phases, as verified by electron microscopy, X-ray and electron diffraction, microprobe Raman spectroscopy, and magnetic analysis. Peak deconvolution of the Raman-active Fe-O bands yields valuable information on the local structure and vacancy content of the various aerogel forms, and facilitates the differentiation of Fe(3)O(4) and gamma-Fe(2)O(3) components, which are difficult to assign using only diffraction methods. These nanocrystalline, magnetic forms retain the inherent characteristics of aerogels, including high surface area (>140 m(2) g(-1)), through-connected porosity concentrated in the mesopore size range (2-50 nm), and nanoscale particle sizes (7-18 nm). On the basis of this synthetic and processing protocol, we produce multifunctional nanostructured materials with effective control of the pore-solid architecture, the nanocrystalline phase, and subsequent magnetic properties.     

The Raman spectrum of hematite: possible indicator for a compositional or firing distinction among Terra Sigillata wares

The Raman spectrum of hematite (alpha-Fe2O3) is here discussed as a distinctive feature for Terra Sigillata sample wares. The broadening of the Raman lines and the appearance and increasing of the disorder band at 660 cm(-1), while moving from the external slip to the inner body, are proposed as valuation parameters for the study of the mineralogical composition and firing process of pottery productions.     

Homogeneous Precipitation of Uniform alpha-Fe2O3 Particles from Iron Salts Solutions in the Presence of Urea

Uniform alpha-Fe2O3 particles within the nanometer range (100-300 nm) have been obtained by precipitation of iron (III) perchlorate in the presence of urea. Different morphology, from spheres to ellipsoidal particles with axial ratio up to approximately 10, was obtained by adding to the initial solution increasing amounts of phosphate anions up to 7 x 10(-3) M. The main targets of this work are the reduction in particle size and precipitation time and the increase of the particles axial ratio, keeping a narrow particle size distribution, in comparison to other methods previously developed to obtain homogenous alpha-Fe2O3 particles. A detailed analysis of the reaction products and a systematic study of the influence of the different precipitation conditions on the characteristics of the resulting particles have been carried out. Finally, some information on the formation mechanism of the ellipsoidal hematite particles in the iron (III) salt-urea-phosphate system is also given. Copyright 1999 Academic Press.