Magnetic Properties of Vanadium Oxide Nanotubes, Nanourchins, and Nanorods.
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Magnetic Properties of Vanadium Oxide Nanotubes, Nanourchins, and Nanorods. Chris Jacobs, Megan Roppolo, Kristin Butterworth, Chunmei Ban, Natasha A. Chernova, and M. Stanley Whittingham Institute for Materials Research, State University of New York at Binghamton, Binghamton, NY, 13902-6000 ABSTRACT A better understanding of magnetic interactions, charge distribution, and redox properties of vanadium oxide nanotubes (VONTs) is necessary for accurate structure and mechanism of formation determination. Magnetic properties have been determined for pristine and lithiated VONTs and for the VONTs arranged in nanourchin morphology. Presence of paramagnetic V4+ ions and V4+ ions coupled in magnetic dimers is found, and their amounts are estimated. Both lithiation and change of morphology to nanourchin destroy the spin-gap behavior, which indicate changes in charge distribution. No ferromagnetic response is observed in lithiated VONTs. Magnetic properties of vanadium oxide nanorods with δ-V4O10 structure are also characterized. INTRODUCTION In the past two decades vanadium oxides and subsequent intercalates have received much attention because of their possible secondary cathode and oxidative catalysis applications [1,2]. Vanadium oxides show a very rich crystal chemistry due to diversity of vanadium oxidation states (3+, 4+, and 5+) and oxygen coordination, which changes from octahedral to square pyramidal and tetrahedral with the increase of vanadium oxidation state [1]. This allows for the formation of many layered and open-framework compounds, which makes them prospective materials for ion intercalation, exchange and electrical energy storage [1,3]. Vanadium oxide nanotubes, VO2.4[C12H28N]0.31⋅nH2O, have gained interest due to their unique physical properties. The multi-walled nanotubes consist of double layer vanadium oxide sheets with alternating layers of surfactant, which bend to form discrete cylinders or scrolls [4,5]. The structure of vanadium oxide sheets is believed to be related to BaV7O16 with double layers of VO6 octahedra and VO4 tetrahedra in the voids (Fig. 1) [6]. The average oxidation state of vanadium is around V4.5+, so that both magnetic V4+ (d1, S=1/2) ions and non-magnetic V5+ are present. Earlier report indicates presence of both paramagnetic and strongly magnetically coupled V4+ ions, as well as a transition to ferromagnetic state upon either reduction of oxidation [7]. However, the charge distribution and magnetic exchange pathways are unknown. The mechanisms of scrolling, redox and ion-exchange reactions are of great interest as the tubular morphology allows for four distinct reaction points: the outer surface, inner surface, interstitial layers, and the open ends [8]. In this work the structure, redox reactions and magnetic properties of various nano forms of double-sheet vanadium oxides are studied with the goal to understand charge distributions, magnetic interactions, and their variations with morphology and oxidation/reduction.
Relative Intensity
001
VONTs a = 6.151(2)Å c=
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