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Preparation and properties of electrospun Cr-doped α-Fe2O3 nanofibers

Hematite (α-Fe2O3) is the most stable and the most common iron oxide phase [1]. Due to high stability in aqueous solutions, low toxicity, abundance and low cost, it has been extensively investigated for possible applications. Hematite strongly absorbs UV and visible light with energies above its band gap of 2.2 eV. For this reason, it has been investigated as a photoelectrode for photoelectrochemical water splitting [2] and as a photocatalyst for decomposition of organic pollutants [3]. The performance of hematite in these applications can be improved by using nanostructured hematite samples doped with different metal cations [2, 3]. Electrospinning is an excellent method to produce nanostructured hematite fibers possessing a high specific surface area [4, 5]. This property is essential in many applications (catalysts, photocatalysts, photoelectrodes, battery electrodes, supercapacitors, adsorbents, etc). Due to the same charge and similar effective ionic radii of Fe3+ and Cr3+ ions (64.5 and 61.5 pm, respectively), solid solutions α-(Fe1-xCrx)2O3 can be prepared in the whole Fe-Cr concentration range (0 ≤ x ≤ 1) [6, 7] with a gradual reduction of unit cell as Cr content increases. Optical spectra of Cr-doped hematite showed a reduction of the direct and indirect optical band gap to lower energies compared to pure hematite [7, 8]. In these works the authors prepared Cr-doped hematite thin films using pulsed laser deposition [7] and oxygen plasma- assisted molecular beam epitaxy [8]. Aim of the present work was to prepare hematite nanofibers, Cr-doped hematite and eskolaite (α- Cr2O3) nanofibers using electrospinning method. Viscous solutions containing dissolved polyvinylpyrrolidone PVP (Mw = 1 300 000) and nitrate salts (Fe3+, Cr3+) in ethanol were electrospun, dried and calcined to obtain desired metal oxide nanofibers. Thus obtained fibers were characterized by several methods. XRD patterns shoved the presence of corundum-type structure in all samples. Cr-for- Fe substitution in hematite nanofibers was confirmed by measuring unit cell reduction using XRD and hyperfine magnetic field reduction using 57Fe Mössbauer spectroscopy. Field emission scanning electron microscopy (FE-SEM) showed the presence of 1D nanofibers in all prepared samples. Cr doping caused a shift of hematite absorption edge to higher wavelengths in UV-Vis-NIR spectra. A reduction of the direct and indirect optical band gap by Cr doping was estimated using Tauc plots. Visible light photocatalytic activities of the prepared samples were tested by decomposition of rhodamine B dye in the presence of H2O2 under visible light illumination. Hematite nanofibers showed a much better photocatalytic activity compared to Cr-doped hematite nanofibers and commercial hematite.

electrospinning ; hematite

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