Acicular α-FeOOH Particles Research Articles

Growth of uniform lath-like α-(Fe,Al)OOH and disc-like α-(Fe,Al) 2O 3 nanoparticles in a highly alkaline medium

The effects of aluminium (Al 3+)-dopant on the precipitation of uniform lath-like α-FeOOH particles, the obtention and growth of α-(Fe,Al)OOH and α-(Fe,Al) 2O 3 solid solutions, particle size and shape were investigated using X-ray powder diffraction, Mössbauer and Fourier transform infrared spectroscopies, field emission scanning electron microscopy and energy dispersive X-ray spectroscopy. Acicular α-FeOOH particles, precipitated in a highly alkaline medium with the addition of tetramethylammonium hydroxide (TMAH), were used as reference material. The influence of Al-dopant was investigated by adding varying amounts of Al 3+ ions to the initial FeCl 3 solution. In the presence of lower concentrations of aluminium ions (up to 11.11 mol%) α-(Fe,Al)OOH as a single phase was formed, whereas higher concentrations led to an additional obtention and growth of α-(Fe,Al) 2O 3. Al-for-Fe substitution in the α-FeOOH and α-Fe 2O 3 structures was confirmed by a decrease in the unit-cell dimensions, a decrease in the hyperfine magnetic field and an increase in the wave number of the infrared absorption bands. The presence of lower concentrations of aluminium ions (up to 11.11 mol%) in the precipitation system did not affect the size and shape of the α-(Fe,Al)OOH particles, whereas higher concentrations influenced a decrease in the length and aspect ratio. In the presence of 42.86 mol% Al 3+ ions fairly uniform disc-shaped α-(Fe,Al) 2O 3 were formed.

Spectroscopic and electron microscopic investigation of iron oxides formed in a highly alkaline medium in the presence of rhodium ions

The effect of the presence of rhodium ions on the formation of iron oxides in a highly alkaline precipitation system was investigated using X-ray powder diffraction (XRD), 57Fe Mössbauer and FT-IR spectroscopies, field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS). Acicular α-FeOOH particles precipitated in a highly alkaline medium with the addition of tetramethylammonium hydroxide (TMAH) were used as reference material. Characterization of α-FeOOH samples formed in the presence of rhodium ions showed a somewhat smaller mean crystallite size, increased unit-cell dimensions, a reduced average hyperfine magnetic field and a slight shift in the position of IR absorption bands in comparison with the reference α-FeOOH sample. By additional heating of the precipitation system, α-FeOOH precipitated in the presence of rhodium ions transformed to α-Fe 2O 3 crystals in the form of hexagonal bipyramids via a dissolution–recrystallization process. Metallic rhodium nanoparticles were formed simultaneously by the reduction of Rh 3+ ions in the presence of the products of TMAH thermal decomposition (trimethylamine and methanol). These rhodium nanoparticles acted as a catalyst for the reductive dissolution of α-Fe 2O 3 particles and the formation of Fe 3O 4 crystals in the form of octahedrons.

Formation of iron oxides in a highly alkaline medium in the presence of palladium ions

The effect of the presence of palladium ions in a highly alkaline precipitation system on the formation of iron oxides was investigated using X-ray powder diffraction (XRPD), Mössbauer and FT-IR spectroscopies, field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS). Acicular α-FeOOH particles precipitated in a highly alkaline medium with the addition of tetramethylammonium hydroxide (TMAH) were used as reference material. The initial addition of palladium ions to that precipitation system had a significant effect on the formation of iron oxide phases and their properties. In the presence of palladium ions, the initially formed α-FeOOH has been transformed to α-Fe 2O 3 crystals in the form of hexagonal bipyramids via a dissolution–reprecipitation mechanism with a simultaneous formation of metallic palladium nanoparticles. These palladium nanoparticles acted as a catalyst for the reductive dissolution of α-Fe 2O 3 particles and the formation of Fe 3O 4 crystals in the form of octahedrons. Increase in the initial concentration of palladium ions in the precipitation system accelerated the transformation process α-FeOOH → α-Fe 2O 3 → Fe 3O 4 and influenced changes in the shape of α-Fe 2O 3 and Fe 3O 4 particles.

The influence of Zn-dopant on the precipitation of α-FeOOH in highly alkaline media

The influence of Zn-dopant on the precipitation of α-FeOOH in highly alkaline media was monitored by X-ray diffraction (XRD), 57Fe Mössbauer and Fourier transform infrared (FT-IR) spectroscopies and field emission scanning electron microscopy (FE SEM). Acicular and monodisperse α-FeOOH particles were precipitated at a very high pH by adding a tetramethylammonium hydroxide solution to an aqueous solution of FeCl 3. The XRD analysis of the samples precipitated in the presence of Zn 2+ ions showed the formation of solid solutions of α-(Fe, Zn)OOH up to a concentration ratio r = [Zn]/([Zn] + [Fe]) = 0.0909. ZnFe 2O 4 was additionally formed in the precipitate for r = 0.1111, whereas the three phases α-FeOOH, α-Fe 2O 3 and ZnFe 2O 4 were formed for r = 0.1304. In the corresponding FT-IR spectra, the Fe OH and Fe O stretching bands were sensitive to the Zn 2+ substitution, whereas the Fe OH bending bands of α-FeOOH at 892 and 796 cm −1 were almost insensitive. The Mössbauer spectra showed a high sensitivity to the formation of α-(Fe, Zn)OOH solid solutions which were monitored on the basis of a decrease in B hf values in dependence on Zn-doping. A strictly linear decrease in B hf for α-FeOOH doped with Zn 2+ ions was measured up to r = 0.0291, whereas for r = 0.0476 and higher there was a deviation from linearity. The presence of α-(Fe, Zn)OOH, α-Fe 2O 3 and ZnFe 2O 4 phases in the samples was determined quantitatively by Mössbauer spectroscopy. Likewise, Mössbauer spectroscopy did not show any formation of the solid solutions of α-Fe 2O 3 with Zn 2+ ions. FE SEM showed a strong effect of Zn-doping on the elongation of acicular α-FeOOH particles (∼500–700 nm in length) up to r = 0.1111. For r = 0.1304 the sizes of ZnFe 2O 4 particles were around 30–50 nm, and those of α-Fe 2O 3 particles were around 500 nm, whereas a relatively small number of very elongated α-(Fe, Zn)OOH particles was observed. A possible mechanism of the formation of α-(Fe, Zn)OOH, α-Fe 2O 3 and ZnFe 2O 4 particles was suggested.

Influence of Ni-dopant on the properties of synthetic goethite

The influence of Ni-dopant on the properties of α-FeOOH was investigated by XRD, FT-IR, 57Fe Mössbauer spectroscopy and transmission electron microscopy. α-FeOOH was synthesized at a highly alkaline pH by precipitation from the FeCl 3 solution with the addition of tetramethylammonium hydroxide and autoclaving at 160 °C. The samples doped with Ni 2+ ions were precipitated in the same way, but in the presence of varying concentrations of NiCl 2. Solid solutions, having the structure of α-FeOOH, were observed in samples with the Ni/Fe ratio up to 0.05. Upon increasing the amount of Ni-dopant the XRD lines were gradually broadened. The sample with the ratio Ni/Fe = 0.10 showed NiFe 2O 4, besides the dominant phase having the structure type of α-FeOOH. Shifts of IR bands at 892 and 796 cm −1 were not observed in all samples doped with Ni. For the ratio Ni/Fe = 0.10, the IR bands centered at 631 and 404 cm −1 were significantly broadened. RT Mössbauer spectrum of undoped α-FeOOH and Ni-doped α-FeOOH showed distributions of hyperfine magnetic fields. B hf decreased from 35.1 T for an undoped α-FeOOH to 32.1 T for α-FeOOH containing Ni 2+ ions (Ni/Fe = 0.05). The saturation of the α-FeOOH structure with Ni 2+ ions in amounts higher than ∼5 mol% was also observed by Mössbauer spectroscopy. The particle size (length) of acicular α-FeOOH particles with a maximum in the interval 180–220 nm was slightly decreased with Ni-doping, but the distribution of the length/width ratio showed no change, having a maximum at 4–5. TEM photographs additionally showed small populations of cubic-shaped or pseudocubic particles of ∼10 nm in size for the ratio Ni/Fe = 0.05 and about 10–20 nm in size for the ratio Ni/Fe = 0.10. These particles were assigned to NiFe 2O 4.

Synthesis of acicular α-FeOOH particles at a very high pH

The acicular α-FeOOH particles were synthesized from FeCl 3 solutions at a very high pH using tetramethylammonium hydroxide as a precipitating agent. Samples were characterized by X-ray powder diffraction, FT-IR spectroscopy and transmission electron microscopy. α-FeOOH precipitated as a single phase from room temperature to 160 °C in a broad aging time interval. The ratio of the length to the width of α-FeOOH particles depended on the temperature and concentration of the starting FeCl 3 solution. Multidomainic character of α-FeOOH particles gradually decreased with an increase in the temperature and time of aging.

Whisker-Like Goethite Nanoparticles Containing Cobalt Synthesized in a Wet Process

Acicular α-FeOOH particles are formed through aging of ferric oxyhydroxide colloidal solution formed by the neutralization of FeCl 3 aqueous solution by NaOH. The effects of cobalt ion addition in the sol-gel-sol process on the formation rate and morphology of α-FeOOH nanoparticles have been investigated. The magnetic properties of metal nanoparticles prepared from the obtained particles have also been examined. The dependence of reaction rate constants in the formation of α-FeOOH in the wet process and the crystallite size of (110) on Co content in α-FeOOH has been made clear. Both the rate constant and crystallite size decrease with increasing Co content in α-FeOOH particles. The negative zeta-potential of the formed α-FeOOH particles increases slowly with increasing pH of the electrolytic solution. The negative zeta-potential of α-FeOOH particles containing Co is larger than that of α-FeOOH particles without Co in the range of pH6-pH12. The relationship between the morphology of the formed α-FeOOH particles and the cobalt content in α-FeOOH particles has been investigated. The length of major axis increases and the length of minor axis decreases with increasing Co content, and the aspect ratio increases remarkably. It is likely that Fe(OH) - 4 complex ion precipitates preferentially on the c plane, and the particles grow remarkably in the direction of c axis. The interplanar spacing of the α-FeOOH decreases with increasing Co content. The formed α-FeOOH grows abnormally like whisker in the direction of major axis: c axis in the case of significantly much Co content: 27 at%, and also the whisker like nanoparticles are observed to grow with curving. The magnetic metal nanoparticles produced have excellent dispersivity and high aspect ratio. The metal nanoparticles have magnetic properties enough for the magnetic recording media.

Heterogeneously Junctioned Nano-Particles of Hematite and Goethite Formed in a Wet Process

Acicular α-FeOOH particles are formed through aging of ferric oxyhydroxide colloidal solution formed by the neutralization of FeCl 3 aqueous solution by NaOH. The effect of foreign ion addition to the colloidal solution on the formation and morphology of a-FeOOH particles has been investigated. The magnetic properties of Fe 3 O 4 particles made from the obtained particles have also been investigated. The rate constant of the formation of α-FeOOH remarkably decreased, but the crystallite size of α-FeOOH particles increased with increasing the quantity of phosphate ion added even with small amounts. These results have been explained as follows: the phosphate ions are selectively adsorbed on the (a) plane of α-FeOOH, cover the (a) plane, and block the crystal growth of the (a) plane of the α-FeOOH. The quantities of the phosphate ion adsorbed on the b and c planes are relatively small. The complex ion of Fe(OH) - 4 is preferentially deposited on both (b) and (c) planes, and the crystal growth of (b) and (c) planes is greatly accelerated. The relationship between the morphology of the formed α-FeOOH particles and the quantity of phosphate ion added has been investigated. The asterisk type particles: α-FeOOH particles heterogeneously junctioned to α-Fe 2 O 3 particles, were formed when a small amount of phosphate was added to the mother liquid. The α-FeOOH crystal epitaxially grew on the junction interface with the α-Fe 2 O 3 crystal. In the case of the aging at the temperature as high as 80°C, the cross type junctioned particles were stably formed at pH below 12.0. The Fe 3 O 4 particles with screw-like unique three-dimensional morphology were produced from the heterogeneously junctioned particles.

Mechanochemical Synthesis of Niobium Pentoxide Nanoparticles

Acicular α-FeOOH particles are formed through aging of ferric oxyhydroxide colloidal solution formed by the neutralization of FeCl 3 aqueous solution by NaOH. The effect of foreign ion addition to the colloidal solution on the formation and morphology of α-FeOOH particles has been investigated. The magnetic properties of Fe 3 O 4 particles made from the obtained particles have also been investigated. The rate constant of the formation of α-FeOOH remarkably decreased, but the crystallite size of α-FeOOH particles increased with increasing the quantity of phosphate ion added even with small amounts. These results have been explained as follows: the phosphate ions are selectively adsorbed on the (a) plane of α-FeOOH, cover the (a) plane, and block the crystal growth of the (a) plane of the α-FeOOH. The quantities of the phosphate ion adsorbed on the b and c planes are relatively small. The complex ion of Fe(OH) 4 - is preferentially deposited on both (b) and (c) planes, and the crystal growth of (b) and (c) planes is greatly accelerated. The relationship between the morphology of the formed α-FeOOH particles and the quantity of phosphate ion added has been investigated. The asterisk type particles: α-FeOOH particles heterogeneously junctioned to α-Fe 2 O 3 particles, were formed when a small amount of phosphate was added to the mother liquid. The α-FeOOH crystal epitaxially grew on the junction interface with the α-Fe 2 O 3 crystal. In the case of the aging at the temperature as high as 80°C, the cross type junctioned particles were stably formed at pH below 12.0. The Fe 3 O 4 particles with screw-like unique three-dimensional morphology were produced from the heterogeneously junctioned particles.

針状α‐ＦｅＯＯＨ粒子の生成に及ぼす炭酸イオン含有コロイド母液のｐＨの影響

The relationship between rate constant, activation energy and growth rate of crystallite for formation of acicular α-FeOOH particles, and pH of colloidal mother liquid containing carbonate ion was discussed for the case, in which addition quantity of CO32- and FeCl3 concentration were constant. Both rate constant and growth rate of crystallite increased with increasing pH, and increased steeply above pH11.6 and pH11.0, respectively. The activation energy decreased and reached minimum values at pH of about 11.6, and increased at higher pH region exceeding 11.6. The supersaturation of solute, Fe(OH)4- increases with increasing pH. At the same time, the singular adsorption of OH- on the side face of acicular particles is restrained by the coating of coexisting CO32- on their surfaces. These may be the reasons for the pH dependencies of rate constant and growth rate of crystallite observed. Both crystallite size and median diameter of the formed α-FeOOH particles increased with increasing pH, then α-FeOOH particles grew larger with keeping needle shape similar according to TEM observation. Moreover, raft-shaped particles which single crystal coalesced with each other, were formed at higher pH region exceeding 11.6. The increase of activation energy may be caused by the formation of raft-shaped particles. The granular hematite particles were also formed with acicular α-FeOOH particles at lower pH region below 11.0.

針状α‐ＦｅＯＯＨ粒子の生成に及ぼすコロイド母液のＦｅＣｌ３濃度の影響

The relationship between rate constant, activation energy and growth rate of crystallite for formation of acicular α-FeOOH particles, and FeCl3 concentration of colloidal mother liquid was discussed. Both rate constant and growth rate of crystallite decreased with increasing FeCl3 concentration at aging above 80°C. At below 55°C it did not almost show the concentration dependence. The activation energy monotonically decreased, and decreased rapidly for the case in which FeCl3 concentration is less than 0.1kmol/m3. The observed dependenCles on FeCl3 concentration can be explained in the following manner: crystal growth of side face parallel to c axis of acicular particles is restrained by singular adsorption of Cl-on the side face. It is considered that activation energy decreases with increasing FeCl3 concentration, because recrystallization process of Fe(OH)4- changes from the surface reaction controlled process to the diffusion controlled process. The crystallite size of acicular α-FeOOH. particles increased steeply with increasing FeCl3 concentration at aging above 80°C. At below 55°C it increased slightly. The particle size grew larger maintainning acicular shape with increasing FeCl3 concentration. The length of major axis and aspect ratio of acicular particles increased especially. It is considered that increasing their size is due to growth of crystallite by coalescence of each nuclei.

Preparation and surface chemistry characteristics of pure and coated acicular α-FeOOH particles

Preparation and surface chemistry characteristics of pure and coated acicular α-FeOOH have been studied. Acicular α-FeOOH particles with narrow size distribution were prepared by a ‘precipitation-oxidation’ process. The electrokinetic behavior of α-FeOOH particles and the specific adsorption of Al(III) ions on α-FeOOH surface were examined. The α-FeOOH particles coated with uniform layers of aluminum hydrous oxide were obtained by dilution hydrolysis of complexed aluminum ions at 25 °C. The effects of pH and surfactants on uniformity and dispersivity of coated α-FeOOH particles were described.

Effect of zinc on the magnetic properties of acicular cobalt-modified ?-Fe2O3 particles

Acicular α-FeOOH particles with a particle length of about 0.35 μm and an axial ratio of about 7 were synthesized by the coprecipitation method using the reaction of FeCl2-NaOH. The (Co, Zn)-modified γ-F2O3 particles were produced by absorbing Co2+ and Zn2+ ions on the surfaces of α-FeOOH particles followed by dehydration, reduction and oxidation. The saturation magnetization and thermal stability of the coercivity of (Co, Zn)-γ-Fe2O3 particles were all higher than those of Co-γ-F2O3 particles. For the same (Co+Zn) content, the saturation magnetization of (Co, Zn)-γ-Fe2O3 particles increased with increasing zinc content but the coercivity decreased.

Preparation and magnetic studies of acicular Fe-Co alloy particles

Acicular α-FeOOH particles with particle length of about 0.2 μm and axial ratio of about 8 were used as the starting material to prepare acicular Fe-Co alloy particles. They were made by adsorbing Co(OH)2 on the surface of α-FeOOH particles, followed by dehydration, annealing, and reduction. In order to prevent sintering at high temperatures, the Co-α-FeOOH particles were first dehydrated at 300 °C and then coated with a thin layer of silica before high-temperature treatments. Furthermore, after reduction, the metallic powder was immersed in toluene to avoid oxidation. The particle structure of Fe-Co was determined from x-ray diffraction. It has been found that, up to 50 at. % Co, all the Fe-Co particles have the same bcc structure and similar lattice constant. The magnetic properties of Fe-Co alloy particles with different Co concentrations were investigated using a vibrating sample magnetometer. The specific saturation magnetizations σs are about 30% below those of bulk materials. The maximum value of σs at room temperature is obtained with 42 at. % Co. The coercivity that mainly originates from shape anisotropy is as high as 1250 Oe for the particles with 33 at. % Co. Furthermore, σs drops sharply with Si content while Hc peaks at 0.6% Si. The coercivities of the particles also depend on the crystallite size. Hc shows a maximum when the crystallite size is about 300 Å.

New approaches to the synthesis of acicular α-FeOOH and cobalt-modified iron oxide particles

Formation of acicular α-FeOOH particles from the reaction system of FeCl2 -NaOH is studied. It is found that acicular α-FeOOH particles with a particle size of about 0.5 μm and axial ratio of about 10 can be obtained by oxidizing the mixed solution of FeCl2 and NaOH with a molar ratio of NaOH/FeCl2 larger than 2. New acicular Co-γ-Fe2 O3 particles are synthesized by absorbing Co2+ ions on the surface of acicular α-FeOOH particles followed by dehydrating, reduction, and oxidation. We find that using N2 as dehydrating atmosphere is better than air or H2. After reduction, variations of the coercivity of Co-Fe3 O4 particles with various cooling rates are investigated. The transformation temperature of Co-Fe3 O4 →Co-γ-Fe2 O3 is about 300 °C for the particles with cobalt content (Co/Co+Fe)<12 mol %. Distributions of Co2+ and Fe2+ ions in the particles are measured by the dissolution method. Coercivity of the acicular Co-γ-Fe2 O3 particles between 400 and 1200 Oe can be controlled by the cobalt content and additional anneal in N2.