α-Fe2O3 Single Crystals Research Articles

Fabrication of Hematite Photoanode Consisting of (110)-Oriented Single Crystals.

In this work, α-Fe2 O3 photoanode consisted of (110)-oriented α-Fe2 O3 single crystals were synthesized by a facile hydrothermal method. By using particular additive (C4 MimBF4 ) and regulation of hydrothermal reaction time, the Fe-25 consisted of a single-layer of highly crystalline (110)-oriented crystals with fewer grain boundaries, which was vertically grown on the substrate. As a result, the charge separation efficiency and photoelectrochemical (PEC) performance of Fe-25A (Fe-25 after dehydration treatment) have been greatly improved. Fe-25A yields a photocurrent of 1.34 mA cm-2 (1.23 V vs RHE) and an incident photon-to-current conversion efficiency (IPCE) of 31.95 % (380 nm). With the assistance of cobalt-phosphate water oxidation catalyst (Co-Pi), the PEC performance could be further improved by enhancing the holes transfer at electrode/electrolyte interface and inhibiting surface recombination. Fe-25A/Co-Pi yields a photocurrent of 2.67 mA cm-2 (1.23 V vs RHE) and IPCE value of 50.8 % (380 nm), which is 3.67 times and 2.39 times as that of Fe-2A/Co-Pi. Our work provides a simple method to fabricate highly efficient Fe2 O3 photoanodes consist of characteristic (110)-oriented single crystals with high crystallinity and high quality interface contact to enhance charge separation efficiencies.

Micro-Heterogeneous Annihilation Dynamics of Self-Trapped Excitons in Hematite Single Crystals.

The Auger recombination in bulk semiconductors can quickly depopulate the charge carriers in a nonradiative way, which, fortunately, only has a detrimental impact on optoelectronic device performance under the condition of high carrier density because the restriction arising from concurrent momentum and energy conservation limits the Auger rate. Here, we surprisingly observed enhanced Auger recombination in an α-Fe2O3 single crystal, a wide bandgap semiconductor with low carrier mobility. The Auger process was ascribed to the Coulombically coupled self-trapped excitons (STEs), and the relaxation of momentum conservation due to the strong spatial localization of these STEs should account for the enhancement. The STE-density dependent kinetics suggested that the strong polaronic effect could cause a micro-heterogeneous distribution of STEs in a high-quality bulk single crystal, which also gave rise to the micro-heterogeneous annihilation dynamics, and a stochastic recombination model was developed and successfully described the STE annihilation dynamics.

Magnetic circular dichroism in the canted antiferromagnet α-Fe2O3: Bulk single crystal and nanocrystals

Hematite, α-Fe2O3, is a canted antiferromagnet with the Neel temperature of 960 K. The Morin transition, which occurs when the temperature is lowered to ~260 K, when the magnetic moments of the sublattices become strictly antiparallel to each other and the resulting magnetic moment disappears, is a characteristic feature of this compound. Detailed investigations are presented here of the magneto-optical effects in the α-Fe2O3 nanoparticles in the transmitted light and in the α-Fe2O3 single crystal in the reflected light. Two types of magneto-optic features were revealed in the magnetic circular dichroism (MCD) spectrum. Some of them described by the Gauss line shape were associated with one-ion d-d transitions. Others were described by the S-shaped line with the inflection points corresponding to the absorption maxima. Last features were assigned to the pair exciton-magnon transitions. An exceptionally strong S-shaped line at 2.0–2.5 eV appeared to be a unique feature of α-Fe2O3 distinguishing it from other iron oxide compounds. The origin of the high effectiveness of the pair exciton-magnon interaction giving rise to this strong S-shaped feature in the MCD spectrum of α-Fe2O3 is discussed.

Submicron-sized α-Fe2O3 single crystals as anodes for high-performance lithium-ion batteries

Abstract Although Fe2O3 has been widely used as an anode material in lithium-ion batteries, its synthesis is complicated when a porous structure/nanoscale is needed to accommodate large volume variations and low electron conductivity. In this study, a submicron-sized α-Fe2O3 single-crystal (α-Fe2O3(SMSC)) is synthesized by directly annealing Fe(NO3)3⸳9H2O in an argon for the production of anodes for lithium-ion batteries. Unlike previously reported pure Fe2O3 which has poor electrochemical performance, α-Fe2O3(SMSC) exhibits excellent lithium storage properties. At a current density of 400 mA g−1, the discharge capacity is maintained at 970 mAh g−1 after 300 cycles. At a current density of 3200 mA g−1, the prepared anode shows a stable discharge capacity of ∼680 mAh g−1. During delithiation, the Li+ diffusion coefficient in the α-Fe2O3(SMSC) electrode is closely associated with the potential, which varies from 10−8 to 10−11 cm2 s−1. After the electrode was charged to 3.0 V, the products are shown to contain crystalline Li2Fe3O4, Fe3O4, and Li2O, but surprisingly, crystalline FeO is not detected. After combination with 30 wt% graphite, α-Fe2O3(SMSC) shows improved electrochemical properties, confirming that this crystal can be used as the main active component in high-performance anode composites.

Melamine-assisted to fabricate pure α-Fe2O3 polyhedron with high-index facet exposed as an effective photoelectrode

Introducing high-index facet into crystals is a promising way to provide additional catalytically active sites for boosting intrinsic activity. This work presents a simple one-pot annealing way to fabricate highly photoactive pristine polyhedral α-Fe2O3 single crystal, using melamine as solvent and coordination agent. The HR-TEM results realized that the as-prepared sample was enclosed by high-index {116} facet, which was always not presented in traditional hydrothermal method. The photoelectrochemical (PEC) measurements have been carried out and showed that the polyhedron samples exhibited significantly enhancement in the photocurrent density, which is nearly 28 times higher than that of the α-Fe2O3 prepared without melamine under the same conditions. Our work may guide future designs for high-index crystal facet exposed of metal oxides and suggest paths for further improvements to allow technological development and use.

Monocrystalline hematite nanostructures: three-dimensionally oriented aggregation synthesis and their comparative visible-light photocatalytic activities

Intelligent three-dimensionally oriented aggregation of nanobuilding blocks during the formation of α-Fe2O3 single crystals with a nano-saucer structure has been studied. The hematite truncated hexagonal bipyramids assembled together with the aid of dihydrogen phosphate ions and grew along the direction to form a slice structure, then the slices piled up layer by layer along the [006] zone axis to form the brand-new geometry of saucer-like nanoparticles. On the basis of the small size, rough surface and loose, zigzag edge characteristics of the 3D assembly structure, the novel saucer nanoparticles exhibited distinguishable absorption and specific surface area (SSA) properties, thus serving as the possible origin of their comparative visible-light photocatalytic activities on the degradation of cationic dye rhodamine B. These characteristics make them promising candidates for catalysts and sensing materials.

Tetragonal hematite single crystals as anode materials for high performance lithium ion batteries

Understanding the correlation between the desired morphology of nanostructures and its electrochemical properties is a prerequisite for widespread application of advanced energy materials. Herein, two types of tetragonal α-Fe2O3 single crystals with a mean size of ca. 200 nm, including cubic and thorhombic shapes, have been synthesized via a facile hydrothermal approach. The as-obtained shape of α-Fe2O3 nanocrystals depends on the addition of the metal ions precursor, the Zn2+ ions result in the cubic shape and the Cu2+ ions result in the thorhombic shape, respectively. These two different tetragonal α-Fe2O3 single crystals are used as anode materials for lithium ion batteries (LIBs), and the results reveal that cubic α-Fe2O3 single crystal exhibits a better performance than thorhombic α-Fe2O3 single crystal. The discharge capacity of cubic α-Fe2O3 single crystal is up to 1028 mAh/g, and the current density is up to 1000 mA/g (1C) after 222 cycles. Clearly, the α-Fe2O3 single crystal with controlled shapes would improve the electrochemical performance of LIBs as superior anode materials, and this approach could pave a way to develop high performance LIBs.

Morphological and magnetic properties of the hydrothermally prepared α-Fe2O3 nanorods

α-Fe2O3 nanorods were synthesized via hydrothermal method. X-ray powder diffraction revealed the formation of rhombohedral α-Fe2O3 single crystal phase with fiber texture. Scanning and transmission electron micrographs analyses showed that the rhombohedral α-Fe2O3 has nanorods in shape with diameters of 40–85 nm and lengths of 150–45,000 nm. Isothermal magnetization vs. applied magnetic field curves measured at room and liquid nitrogen temperatures displayed a variation on magnetic ordering: from weak ferromagnetism at room temperature to not hysteretic behavior at liquid nitrogen temperature that is well described by a Langevin function. Moreover, the zero field cooling-field cooling curves under applied magnetic field of 100 Oe confirms the decreasing of Morin temperature transition due to nanometric size of the samples.

Experimental and ab initio study of the nuclear quadrupole interaction of 181 Ta-probes in an α-Fe2O3 single crystal

We report perturbed-angular-correlation (PAC) experiments on 181Hf (→181Ta)-implanted corundum α-Fe2O3 single crystal in order to determine the magnitude, symmetry and orientation of the electric-field-gradient (EFG) tensor at Ta donor impurity sites of this semiconductor. These results are analyzed in the framework of ab initio full-potential augmented-plane wave plus local orbital (FP-APW+lo) calculations. This combined analysis enables us to quantify the magnitude of the lattice relaxations induced by the presence of the impurity and to determine the charge state of the impurity donor level introduced by Ta in the band gap of the semiconductor.

Spin-reorientation phase transition on the surface and in the bulk of α-Fe2O3 single crystals

Direct comparison of the properties of a thin surface layer and the bulk of macroscopic hematite (α-Fe2O3) crystals was used to study the magnetic structure of the surface layer and the bulk and the processes attendant on spin-reorientation phase transition (SRT). The investigation tool was simultaneous γ-ray, X-ray, and electronic Mossbauer spectroscopy, which enabled us to study the bulk and surface properties of macroscopic samples simultaneously and to compare them directly. Direct evidence of the existence of a surface “transition layer” on hematite crystals is obtained. The existence of this layer was suggested and described by Krinchik and Zubov [JETP 69, 707 (1975)]. The study in the SRT region showed that (1) the Morin SRT in the crystal bulk occurs in a jump (as a first-order phase transition), whereas in the surface layer of about 200 nm thick, some smoothness appears in the mechanism of magnetic-moment reorientation; (2) SRT in the surface layer, as in the bulk, involves an intermediate state in which low-and high-temperature phases coexist; and (3) SRT in the surface layer occurs at a temperature several degrees higher than in the bulk. Our experimental evidence on the SRT mechanism in the surface layer correlates with the inferences from phenomenological theory developed by Kaganov [JETP 79, 1544 (1980)].

亜鉛浸出残さの脱鉄処理で得られる酸化鉄の精製と磁化特性に及ぼす事前水洗とＮａ２Ｂ４Ｏ７フラックス添加量低減の影響

The purification of raw iron oxide obtained from iron precipitation of the zinc leach residue using the recrystallization method with Na2B4O7 flux and their magnetic properties has been investigated. The main purpose of present study are to examine the effect of the prior water washing and decrease of amounts of Na2B4O7 flux on the purification of raw material. As a result, it was recognized that the prior water washing of raw material are undesirable, because of decrease of desulfurization in the purification by the sub-sequent recrystallization method,and the elimination of Zn, S and As in raw material is the best, when raw material/flux ratio is 70/30 and the prior water washing is not carried out. The magnetic properties is similar to that of α-Fe2O3 single crystal. The synthesis and magnetic properties of Mn-Zn ferrite from purified iron oxide were also examined.

X-ray characterization of alpha-Fe(2)O(3) single crystals containing 2.2% and 95% (57)Fe.

alpha-Fe(2)O(3) single crystals containing 2.2% and 95% (57)Fe isotopes were characterized by means of X-ray topography, the diffraction rocking curve and the integrated intensity. These crystals were revealed to be nearly perfect and suitable for a further nuclear-resonant Bragg-scattering study.

Anisotropy of electrical properties in α-Fe2O3 ceramics

Electrical conductivity and thermoelectric power measurements carried out in a heamatite ceramic showed a strong anisotropy in directions normal and parallel to the uniaxial pressing direction. This behaviour is similar to that verified in α-Fe2O3 single crystal. The results suggest that the extended structural defects, generated during sintering, disturb the magnetic order on the (001) planes of α-Fe2O3 and limit the mobility of n type carriers.

Determination of valence states in Fe3O4 by resonant photoemission

We report resonant photoemission measurements across the Fe 3p→3d photoabsorption threshold from the (110) surface of cleaved single-crystal Fe3O4 using synchrotron radiation. The resonant enhancement effect is used to distinguish the Fe 3d-derived states from the overlapping O 2p states in the valence band. The Fe 3d-derived states are found to extend about 18 eV below the Fermi level. Constant-initial-state spectroscopy across the Fe 3p excitation threshold reveals that this large valence bandwidth is due to a significant amount of cation–ligand hybridization; this result is in accord with recent configuration interaction cluster calculations of the photoemission final states in iron oxides reported by Fujimori et al. The electronic levels that are primarily associated with the different Fe2+ and Fe3+ cation environments in Fe3O4 are identified based on a comparison with photoemission spectra measured from cleaved FexO (x≂0.945) and α-Fe2O3 single crystals. Upon O2 adsorption, the Fe3O4(110) surface is rapidly oxidized, as evidenced by a decrease in the photoemission features associated with the Fe2+ valence state.

Crystal growth and photoelectrochemical study of Zr-doped α-Fe2O3 single crystal

Undoped and zirconium doped α-Fe2O3 single crystals have been grown by CVD. Undoped crystals had a very high electrical resistivity (about 106 Ω cm at room temperature), so that they could not be used as photoelectrochemical anodes. The resistivity of doped crystals ranged from 4 to 75 Ω cm at 300 K. Their photoresponse has been studied. No anodic dissolution was observed in an alkaline solution. Quantum efficiency increases with decreasing doping level. From differential capacitance measurements, flat band potential and donor concentration of the semiconducting electrodes have been determined. The doping concentration, together with resistivity measurements, led to a free carrier mobility of about 0.1 cm2/V · s. The variation of the flat band potential with donor concentration has been discussed.