The effects of guanylurea phosphate on the properties of precipitates formed by forced hydrolysis of FeCl3 solutions were investigated using a combination of XRD, 57Fe Mössbauer, FT-IR and FE SEM ...analyses. Samples were prepared at 160 °C by varying the concentration of FeCl3 solutions and the amount of guanylurea phosphate added. This study showed that this simple method can be used to prepare giniite (Fe5(PO4)4(OH)3‧2 H2O) as a single phase with a variety of particle shapes. Three main phases, giniite, akaganeite (β-FeOOH) and hematite (α-Fe2O3), were obtained, where the exact phase composition of precipitates depended on the chemical conditions in the starting solutions. The optimum conditions for the formation of different dendritic nano/microstructures were 2‧10−3 and 5‧10−3 M FeCl3 solutions containing guanylurea phosphate. This proposed method of giniite synthesis has potential in the further investigation of the formation nature of dendritic nano/microstructures.
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Sn-doped hematite (α-Fe2O3) nanoparticles of fairly uniform and Sn-dependent size and shape were synthesized via a simple combination of hydrothermal co-precipitation and calcination. The effects of ...Sn doping on the unit cell size, crystallinity, particle size and shape, as well as the magnetic, optical and photocatalytic properties of hematite nanoparticles were analyzed. The incorporation of Sn4+ ions into the crystal structure of hematite was confirmed by determination of the unit cell expansion due to the replacement of octahedrally coordinated Fe3+ ions by significantly larger Sn4+ ions, as well as a substantially reduced hyperfine magnetic field due to magnetic dilution upon the substitution of non-magnetic Sn4+ ions for magnetic high-spin Fe3+ ions. Sn doping caused a decrease in length and width and an increase in thickness of elongated hematite nanoparticles. Fairly uniform Sn-doped hematite nanoellipsoids or nanocuboids were formed, depending on the Sn content. Temperature dependence of magnetization measurements showed the disappearance of the magnetic phase transition (Morin transition) in hematite upon Sn doping. Magnetic coercivity decreased upon Sn doping due to a decrease in shape anisotropy induced by the change in particle shape from nanorods to nanoellipsoids and nanocuboids. The optical and electronic properties of hematite nanoparticles were significantly affected by Sn doping – the absorption edge was shifted to higher wavelengths, while direct and indirect optical band gaps narrowed with the increasing Sn4+-for-Fe3+ substitution. Sn-doped hematite nanoellipsoids containing 4.3 mol% Sn exhibited a substantial visible light photocatalytic activity in the heterogeneous photo-Fenton process, but this activity significantly decreased with higher Sn doping.
The formation and properties of cobalt ferrite were investigated with XRD, FT-IR, FE-SEM, Mössbauer and magnetometry. Cobalt ferrite samples were prepared (a) by combining coprecipitation ...Co(OH)2/2Fe(OH)3, using NaOH between pH 5.2 and 11.4 and autoclaving, and (b) by autoclaving the Co(OH)2/2Fe(OH)3 coprecipitate in a very strong alkaline medium. XRD and FE SEM showed that both CoFe2O4 crystallites and particles were in the nanosize range. The FT-IR spectra were typical of spinel ferrites. Cobalt ferrite precipitated at pH 7.2 and at 11.4 contained a small fraction of α-Fe2O3, whereas in the sample precipitated at pH 11.4 a very small amount (traces) of α-FeOOH were detected by FT-IR, additionally. Parameters obtained by Mössbauer spectroscopy suggested a structural migration of cobalt and iron ions in prepared cobalt ferrite spinels with the prolonged time of autoclaving. Magnetic measurements showed the magnetic behaviour typical of spinel ferrite nanoparticles.
57Fe Mössbauer spectrum of CoFe2O4 nanoparticles obtained by precipitation of mixed Co(OH)2/2Fe(OH)3 hydroxide at pH 5.2 and subsequent hydrothermal treatment of precipitated hydroxide suspension at 160 °C for 24 h (left). Field dependence of magnetic moments of these CoFe2O4 nanoparticles at different temperatures with the inset of the temperature dependence of remanent magnetization Mrem (M at 0 T) and saturation moment Msat (M at 9 T) (right). Display omitted
•CoFe2O4 were synthesized by autoclaving of mixed hydroxide coprecipitate.•The pH and time of autoclaving were the controlling parameters.•The migration of Co and Fe ions in CoFe2O4 depended on the synthesis conditions.•Magnetic behaviour of CoFe2O4 was in agreement with crystallites size.
The properties of Mn-doped hematites and their Mn-doped precursors were investigated using the 57Fe Mössbauer spectroscopy, XRD, FT-IR, UV/Vis/NIR and FE SEM. Mn-doped goethite precursors were ...synthesized in a high alkaline pH medium starting with iron choline citrate. The XRD analysis of all Mn-doped goethites and Mn-doped hematites showed only the presence of a goethite or hematite crystal structure. The RT Mössbauer spectrum of reference goethite showed typical features of precipitated goethite. With increased Mn-doping the spectral lines of the goethite precursor were broadening, the hyperfine magnetic field decreased and the relative intensity of central quadrupole doublets increased. The RT Mössbauer spectra of Mn-doped hematite showed features of the hematite phase and increased relative intensity of a central quadrupole doublet which disappeared in the spectra recorded at liquid N2 temperature. The presence of this superparamagnetic component in the RT Mössbauer spectra is a direct consequence of Mn-doping. In the FT-IR spectra the characteristic IR bands sensitive to Mn-doping were identified. The UV/Vis/NIR spectra showed shifts of optical absorption bands and a strong absorption increase in the visible part of the spectrum. FE SEM images of Mn-doped goethite precursor showed a change in shape from rod to star-like particles. Upon heating at 300 °C for 4 h in air, the obtained Mn-doped hematite particles preserved the original morphologies of Mn-doped goethite precursors. Taking into account the superparamagnetic spectra (RT and liquid N2 temperature), XRD and the corresponding FE SEM images of Mn-doped hematite particles it can be concluded that these particles contained substructure. In this substructure of Mn-doped hematites the fraction of superparamagnetic crystallites (domains) is given by calculated value for the superparamagnetic doublet present.
α-Fe2O3 doped with 10 mol % of Mn. The star branches consist of very thin lamellas. Display omitted
•A new chemical procedure is used in the synthesis of 1D Mn-doped goethite precursors.•Goethite particles shape changed from rod-to star-like by increasing dopant concentration.•Thermal decomposition yielded Mn-doped hematite of the of precursor' morphology.•Mn-doping significantly influenced the corresponding Mössbauer and UV–Vis–NIR spectra.
Indium-doped hematite samples were prepared by calcination of indium-doped goethite samples and investigated using different instrumental techniques. In3+-for-Fe3+ substitution in the hematite ...structure was confirmed by the determination of the unit cell expansion using X-ray powder diffraction and by the measurement of the hyperfine magnetic field reduction using Mössbauer spectroscopy. Indium substitution in hematite also caused a decrease in the crystallite size, an increase in the particle size, a shift in the position of bands in infrared spectra, a decrease in the relative intensity of absorption bands in UV–Vis–NIR spectra and a disappearance of the Morin transition. Maximum substitution was estimated at about 8 mol%.
•In-doped hematite was prepared by calcination of In-doped goethite.•Maximum In3+-for-Fe3+ substitution in hematite was estimated at about 8 mol%.•Significant changes in hematite properties caused by In substitution were observed.
Nanostructural morphology and cation (divalent or tetravalent) doping are known to be important factors for photocatalytic and photoelectrochemical activities of iron oxides. Modifications of iron ...oxides in an appropriate way could significantly improve these activities. In the present work the effects of Cu doping on the microstructural, thermal, optical and photocatalytic properties of goethite (α-FeOOH) and hematite (α-Fe2O3) 1D nanoparticles were investigated. Goethite and Cu-doped goethite 1D nanoparticles were synthesized by precipitation in a highly alkaline medium. Hematite and Cu-doped hematite 1D nanoparticles were prepared by calcination of goethite samples at 500 °C. Cu2+-for-Fe3+ substitution in goethite and hematite was confirmed by determining the change in unit cell size using X-ray powder diffraction and by measuring the hyperfine magnetic field reduction using Mössbauer spectroscopy. An increased Cu2+-for-Fe3+ substitution in goethite caused a gradual elongation and narrowing of nanorods with the formation of nanoneedles at 3 or 4 mol% Cu. The shape and size of nanoparticles were not changed by thermal transformation of goethite to hematite. Thermal analysis showed a lower temperature of goethite dehydroxylation upon increased Cu doping. Cu2+-for-Fe3+ substitution in goethite and hematite caused increased absorption of visible and near-IR radiation beyond the absorption edge, as well as a slight decrease in direct and indirect optical band gaps compared with pure phases. The visible light photocatalytic activities of Cu-doped goethite and hematite nanoneedles were significantly improved compared with undoped goethite and hematite nanorods, which could be attributed to a combined effect of nanoneedle morphology and Cu doping.
•Pure and Cu-doped α-FeOOH and α-Fe2O3 1D nanoparticles were synthesized.•Cu2+-for-Fe3+ substitution in α-FeOOH and α-Fe2O3 up to 4 mol% was confirmed.•A significant effect of Cu doping on different properties was observed.•Cu2+-for-Fe3+ substitution induced formation of nanoneedles at 3 or 4 mol% Cu.•These nanoneedles exhibited highly improved visible light photocatalytic activity.
► Study of the influence of Ir
3+ ions on the precipitation of iron oxides. ► Ir
3+ doping in α-FeOOH caused significant changes in the microstructural properties. ► Ir
3+ doping in α-Fe
2O
3 caused ...an increase in the Morin transition temperature. ► Ir
3+ ions caused a phase transformation α-(Fe,Ir)OOH
→
α-(Fe,Ir)
2O
3
→
Fe
3O
4
+
Ir
0.
The effect of the presence of Ir
3+ 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). Monodispersed lath-like α-FeOOH (goethite) particles precipitated by hydrothermal treatment in a highly alkaline medium with the addition of tetramethylammonium hydroxide (TMAH) were used as reference material. The presence of Ir
3+ ions in the precipitation system strongly influenced the phase composition, magnetic, structural and morphological properties of obtained samples. The formation of α-Fe
2O
3 (hematite) along with α-FeOOH in the first stage of hydrothermal treatment and the transformation of α-FeOOH and α-Fe
2O
3 to Fe
3O
4 (magnetite) by a longer hydrothermal treatment was caused by the presence of Ir
3+ ions. Ir
3+ for Fe
3+ substitution in the structure of α-FeOOH brought about changes in unit-cell dimensions, crystallinity, particle size and shape, hyperfine magnetic field and infrared bands positions. Ir
3+ for Fe
3+ substitution in the structure of α-Fe
2O
3 led to an increase in the temperature of the Morin transition; Mössbauer spectroscopy showed the presence of α-Fe
2O
3 in the antiferromagnetically ordered state at 293
K.
The effect of sodium dextran sulphate (M
w
= 500,000) on the forced hydrolysis of FeCl
3
solutions was investigated using a microwave-assisted method. Reference samples obtained upon 10 to 120 min ...of forced hydrolysis of the FeCl
3
solution showed the presence of an α-Fe
2
O
3
phase alone. After 10 min of the forced hydrolysis of FeCl
3
in the solution containing sodium dextran sulphate, α-Fe
2
O
3
and additionally β-FeOOH precipitated. The Mössbauer spectra of α-Fe
2
O
3
precipitated in the presence of sodium dextran sulphate showed a significant broadening of spectral lines accompanied by decreased values of the hyperfine magnetic field. This was explained by lower α-Fe
2
O
3
crystallinity. The FESEM of reference α-Fe
2
O
3
samples showed cube-like particles of good uniformity in the submicrometer range. In the presence of dextran sulphate polymer, the α-Fe
2
O
3
particles of varying sizes and close to egg-like shape were obtained as a result of the specific adsorption of sulphates on the oxide surface and steric conditions created by dextran branches.
Graphical abstract
Right side
: Effect of sodium dextran sulphate on the size and shape of α-Fe
2
O
3
particles produced by the forced hydrolysis of FeCl
3
solutions in the presence of microwave irradiation;
Left side
: reference α-Fe
2
O
3
particles.
Forced hydrolyses of FeCl3 solutions in the presence of Cr3+ ions without alkali addition were investigated at 160 and 200 °C. These precipitation systems were compared to corresponding reference ...systems. The isolated precipitates were characterized with XRD, 57Fe Mössbauer, FT-IR, FE SEM and photocatalytic measurements. Although chromium was not detected by EDS (Energy Dispersive X-ray Spectroscopy), the analysis of samples precipitated in the presence of Cr3+ ions showed several effects which can be attributed to the presence of these ions in the starting solutions. These effects are specifically well pronounced for samples obtained at 200 °C. For the highest concentration of Cr3+ ions and upon heating at 200 °C the transformation kinetics β-FeOOH to α-Fe2O3 was impeded. Relative intensities of prominent diffraction lines 104 and 110 of α-Fe2O3 changed as a result of the presence of Cr3+ ions. Changes in the size and formation of lemon-like particles instead of cube-like α-Fe2O3 particles were also noticed. The presence of Cr3+ ions in hydrolysing FeCl3 solution influenced the photocatalytic degradation of rhodamine B. Since no specific adsorption of Cr3+ ions occurred in the acidic pH medium, it was inferred that traces of Cr3+ ions enter into the structural tunnels of β-FeOOH crystals. The associates Cl−-Cr3+ are formed, which in the next step destabilize the crystal structure of β-FeOOH particles. Cr3+ ions leave the crystal structure upon dissolution of β-FeOOH. It can be assumed that α-Fe2O3 crystals start to grow on very fine and thin β-FeOOH crystallites which were not completely dissolved, but were previously affected by Cr3+ ions.
•Cr3+ ions influenced forced hydrolysis of FeCl3 solutions at pH < 2.•Cr3+ ions did not change the unit-cell parameters of β-FeOOH and α-Fe2O3.•Cr3+ ions influenced the crystallite size and particles shape.•Cr3+ ions inhibited α-Fe2O3 crystallization and influenced its photoactivity.•The origin of these effects was suggested.
•A new synthesis of uniform α-GaOOH particles of different shapes was reported.•Crystallographic directions and crystal planes in α-GaOOH particles were identified.•α-GaOOH particles showed ...differences in crystallite size, thermal and IR properties.•Uniform α-Ga2O3 and β-Ga2O3 particles were obtained by calcination of α-GaOOH.•Differences in properties of α-Ga2O3 and β-Ga2O3 samples were observed and discussed.
Uniform α-GaOOH submicron particles of different shapes (spindles, rhombic rods, rhombic prisms, hierarchical particles) were synthesized by simple low-temperature (at 60°C) or hydrothermal (at 160°C) precipitation from the mixture of aqueous solutions of gallium(III) chloride and organic alkali tetramethylammonium hydroxide (TMAH) at various pH values (5, 7 or 9). The growth mechanism of these particles was discussed. Crystallographic directions and crystal planes in α-GaOOH particles were identified. Differences in crystallite size, thermal and infrared properties of α-GaOOH particles were observed and discussed. Uniform α-Ga2O3 and β-Ga2O3 particles of the same or similar shapes, containing holes due to dehydroxylation, were obtained by calcination of the corresponding α-GaOOH particles at 500 or 900°C, respectively. Differences in crystallite size and infrared properties of obtained gallium oxide samples, as well as differences in photoluminescence properties of β-Ga2O3 samples were observed and discussed.
