Lattice small polarons and magnetic interactions drive preferential nanocrystal growth in silicon doped hematite (original) (raw)
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Electronic, structural, and magnetic effects of 3d transition metals in hematite
Journal of Applied Physics, 2010
We present a density-functional theory study on the electronic structure of pure and 3d transition metal ͑TM͒ ͑Sc, Ti, Cr, Mn, and Ni͒ incorporated ␣-Fe 2 O 3 . We find that the incorporation of 3d TMs in ␣-Fe 2 O 3 has two main effects such as: ͑1͒ the valence and conduction band edges are modified. In particular, the incorporation of Ti provides electron carriers and reduces the electron effective mass, which will improve the electrical conductivity of ␣-Fe 2 O 3 . ͑2͒ The unit cell volume changes systematically such as: the incorporation of Sc increases the volume, whereas the incorporation of Ti, Cr, Mn, and Ni reduces the volume monotonically, which can affect the hopping probability of localized charge carriers ͑polarons͒. We discuss the importance of these results in terms of the utilization of hematite as a visible-light photocatalyst.
Materials Research Bulletin, 2016
The crystallization and physical properties of hematite (α-Fe 2 O 3) nanocrystals with a rhombohedral shapeand with rounded edges,obtained by thermally induced hydrolysis of iron (III) solutions under acidic conditions and a fast nucleation,have beenrevisited in the present work. In particular, the morphological and themicrostructural properties of such nanocrystals have beeninvestigatedin detail as a function of aging time using severalcharacterization techniques, including X-ray diffraction, conventional and high resolution transmission electron microscopyand selected area electron diffraction. Also different spectroscopies were employed to study the vibrational, optical and semiconductor properties of the obtained materials; concretely, studies of Fourier transform infrared and Raman spectroscopies confirmed the hematite phase of the rhombohedral nanocrystals, whose vibrational bands are shifted to lower frequencies relative to the bulk hematite onesas the aging time is reduced due to phonon confinement effects. Also, the indirect and direct transition band gaps were estimated from the UV-visible spectra using Tauc´s plot analysis, finding interesting dependences on the crystal size arising from quantum confinement and surface effects.
Symmetry and nonstoichiometry as possible origins of ferromagnetism in nanoscale oxides
Physical Review B, 2012
We show through density functional theory calculations that extended magnetic states can inherently occur in oxides as the size of the crystals is reduced down to the nanometer scale even when they do not explicitly include intrinsic defects. This is because in nanoscale systems crystallographically perfect crystallites paradoxically result in nonstoichiometric compositions owing to the finite number of constituting atoms. In these structurally perfect but stoichiometrically imperfect nanocrystallites, the spin-triplet state is found to be more stable than the spin-singlet state, giving rise to an extended spin distribution that expands over the entire crystal. According to this picture, long-range magnetic order arises from the combined effect of crystal symmetry and nonstoichiometry that can coexist exclusively in nanoscale systems. The idea can also give reasonable explanations for the unprecedented ferromagnetic features observed commonly in nanoscale oxides, including ubiquity, anisotropy, and diluteness.
Electronic and magnetic structure of transition-metal-dopedα-hematite
Physical Review B, 2005
We investigate the electronic and magnetic structure of transition-metal-doped ␣-hematite using the local density approximation with local correlations ͑LDA+ U͒. The dopants in this study are the 3d transition metals Sc-Zn and Ga and are assumed to substitute on an Fe site. The calculated net moment per substitutional impurity is found to be ͉z D − z Fe ͉, opposite to that of the replaced Fe, where z D and z Fe are the numbers of valence electrons of the dopant and Fe, respectively. The dopants, D, substitute in an effective charged state D 3+ except for Ti 4+ and Zn 2+. In the case of Ti, the extra electron converts a neighboring Fe 3+ atom to Fe 2+. In the case of Zn, the missing electron generates a relatively diffuse hole at the top of the valence band spread over neighboring O atoms.
We propose a simple method for the efficient and rapid synthesis of one-dimensional hematite ( Fe 2 O 3 ) nanostructures based on electrical resistive heating of iron wire under ambient conditions. Typically, 1-5 μm long -Fe 2 O 3 nanowires were synthesized on a time scale of seconds at temperatures of around 700 °C . The morphology, structure, and mechanism of formation of the nanowires were studied by scanning and transmission electron microscopies, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Raman techniques. A nanowire growth mechanism based on diffusion of iron ions to the surface through grain boundaries and to the growing wire tip through stacking fault defects and due to surface diffusion is proposed.
Size dependence of the spin-flop transition in hematite nanoparticles
Physical Review B, 2003
The spin-flop transition of acicular hematite nanoparticles of different size synthesized by chemical route and annealed at different temperatures has been investigated as a function of temperature. Transmission electron microscopy ͑TEM͒ measurements show that particles have an ellipsoidal shape, with a major axis of 330Ϯ50 nm and a minor axis of 70Ϯ10 nm. TEM and x-ray diffraction experiments show that nanoparticles are made of hematite crystallites, which grow on increasing the annealing temperature. Both the Morin transition temperature (T M ) and the spin-flop transition field (H s f ) have been found to increase for increasing crystallite size (d): for instance, for dϭ36 nm particles T M ϭ164 K and the value of H s f extrapolated at T ϭ0 (H s f 0 ) is 1.7 T, whereas for bulk hematite T M ϭ263 K and H s f 0 ϭ6.5 T. Both H s f 0 and T M follow a 1/d dependence ͑at a faster rate for H s f 0 ), indicating that their variation is mainly driven by surface effects.
Current Nanoscience, 2021
Background: In nano-size α-Fe2O3 particles, the Morin transition temperature was reported to be suppressed. This suppression of the TM in nano-size α-Fe2O3 was suggested to be due to high internal strain and to the enhanced role of surface spins because of the enhanced surface to volume ratio. It was reported that for nanoparticles of diameters less than 20 nm, no Morin transition was observed and the antiferromagnetic phase disappears. In addition, annealing of samples was reported to result in both an increase of TM and a sharper transition which were attributed to the reduction in defects, crystal growth, or both. Objective: In this work, we investigated the role of applied magnetic field in TM, the extent of the Morin transition, thermal hysteresis, and the spin-flop field in synthetic α-Fe2O3 nanoparticles of diameter around 20 nm. Methods: Hematite nanoparticles were synthesized using the sol-gel method. Morphology and structural studies of the particles were done using TEM, a...