Role of Oxidation State of Vanadium in Vanadium Oxide-Hematite Nanoparticles
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Role of Oxidation State of Vanadium in Vanadium Oxide-Hematite Nanoparticles Monica Sorescu, Tianhong Xu and Collin Wade
Duquesne University, Department of Physics, Fisher Hall, Pittsburgh, PA 15282-0321, USA
ABSTRACT Single phase, a FeVO4 triclinic crystalline structure was successfully synthesized by annealing the mechanochemically milled xV2O5·(1-x)Į-Fe2O3 composites (x = 0.5) at 550 °C for 1 h. X-ray powder diffraction (XRD) and Mössbauer spectroscopy were combined for a detailed study of the assisting role of the mechanochemical milling process. Mechanochemical milling homogeneously mixed the starting materials of Į-Fe2O3 and V2O5 and substantially decreased their average grain sizes. The partially V5+-substituted Į-Fe2O3 phase and Fe3+-substituted V2O5 could be the important intermediate phases in the production of FeVO4 single phase. In addition, xV2O3.(1-x)α-Fe2O3 (x = 0.1, 0.3, 0.5, and 0.7) solid solutions were successfully synthesized by mechanochemical activation of V2O3 and Į-Fe2O3 mixtures. Complete solid solutions exist after 12 h ball-milling time for all studied x values. The synthesized xV2O3.(1-x)α-Fe2O3 solid solutions with x = 0.5 and 0.7 were mainly paramagnetic at room temperature. The study demonstrates that the transformation pathway is related to the valence state of the metallic specie of the oxide used in connection with hematite. INTRODUCTION Hematite is one of the most used oxides, with various applications in scientific and industrial fields. It can be used as semiconductor compound [1], magnetic material [2], catalyst [3], and gas sensor [4]. The vanadium oxides form a fascinating class of materials that exhibit a large variety of structures with different physical and chemical properties. This diversity makes vanadium oxides technologically relevant and leads to various applications, such as light detectors, electrical and optical switching devices, as well as heterogeneous catalysis [5, 6]. From fundamental research interest and wide application points of view, vanadium oxides possess various vanadium oxidation states ranging from +2 to +5 as well as different oxygen geometries in the crystalline structures. These make vanadium oxides attracting materials due to their rich electronic and magnetic structures as well as phase transitions. Iron (III) vanadate, FeVO4, appears in four polymorphs and has attracted attention because of its interesting physical and chemical properties with wide applications [7-13]. The FeVO4 materials have been synthesized via a surfactant-assisted sol-gel method, hydrothermal method, and a conventional solid state reaction. However, these methods either require expensive chemical reagents or sintering processes at high temperature. The high temperature synthesis leads to aggregation and sintering of the reaction products, which is expected to lower the catalytic activities of FeVO4 due to the reduction of surface area.
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High energy ball-milling is a well established method for preparing extended solid solutions, composite and nanostructure systems
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