A. Popa

Leibniz Institute for Solid State and Materials Research Dresden, Dresden, Saxony, Germany

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Publications (6)19.7 Total impact

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    ABSTRACT: Anatase TiO2 nanotubes were synthesized via the hydrothermal method followed by annealing at 500 °C in argon for 1 h. The phase structure, morphology, and composition were investigated in detail by means of X-ray diffraction, scanning and high-resolution transmission electron microscopy, infrared spectroscopy, and thermal analysis. The material consists of nanotubes with diameter of 10–15 nm and lengths of several hundred nanometers. The electrochemical properties were investigated by cyclic voltammetry and galvanostatic cycling. The data imply a first cycle irreversible capacity of 385 mAh/g, and capacities of 307 and 265 mAh/g after the second and 50th cycle, respectively, at C/10. The Coulombic efficiency of about 99% after cycle 50 implies excellent cycling stability. Hence anatase TiO2 nanotubes evidence great potential for usage in high-power lithium-ion batteries.
    The Journal of Physical Chemistry C 04/2012; 116(15):8714–8720. DOI:10.1021/jp300955r · 4.84 Impact Factor
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    ABSTRACT: The electrochemical behavior and magnetic properties of electrochemically lithium-doped mixed valence vanadium oxide nanotubes (VOx-NTs) have been studied. The materials were synthesized hydrothermally. Primary amines (CH3−(CH2)11−NH2) were used as spacer molecules aiding to retain the nanotubular shape of the VOx layers. Cyclic voltammograms and potentiostatic measurements indicate that the electrochemical behavior of this material is fairly sensitive to the preparation conditions. Specific capacity values for this material are higher than similar template VOx-NTs. Lithium doping level of x = 0.1 in two different VOx-NTs gives rise to room-temperature ferromagnetism in LixVOx-NTs.
    The Journal of Physical Chemistry C 04/2011; 115(13):5265-5270. DOI:10.1021/jp109327x · 4.84 Impact Factor
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    ABSTRACT: We have studied the interplay of magnetism and superconductivity in ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ and $\text{Ca}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$. While antiferromagnetic spin-density wave formation is suppressed and superconductivity evolves, all samples show a doping-independent strong increase in the normal-state susceptibility upon heating which appears a general feature of iron pnictides. The data provide evidence for robust local antiferromagnetic correlations persisting even in the superconducting regime of the phase diagram.
    Physical review. B, Condensed matter 01/2010; 81(2). DOI:10.1103/PhysRevB.81.024506 · 3.66 Impact Factor
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    ABSTRACT: The nature of a puzzling high temperature ferromagnetism of doped mixed-valent vanadium oxide nanotubes reported earlier by Krusin-Elbaum et al., Nature 431 (2004) 672, has been addressed by static magnetization, muon spin relaxation, nuclear magnetic and electron spin resonance spectroscopy techniques. A precise control of the charge doping was achieved by electrochemical Li intercalation. We find that it provides excess electrons, thereby increasing the number of interacting magnetic vanadium sites, and, at a certain doping level, yields a ferromagnetic-like response persisting up to room temperature. Thus we confirm the surprising previous results on the samples prepared by a completely different intercalation method. Moreover our spectroscopic data provide first ample evidence for the bulk nature of the effect. In particular, they enable a conclusion that the Li nucleates superparamagnetic nanosize spin clusters around the intercalation site which are responsible for the unusual high temperature ferromagnetism of vanadium oxide nanotubes. Comment: with some amendments published in Europhysics Letters (EPL) 88 (2009) 57002; http://epljournal.edpsciences.org
    EPL (Europhysics Letters) 12/2009; DOI:10.1209/0295-5075/88/57002 · 2.27 Impact Factor
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    ABSTRACT: The interaction of Cu(II) with the following secondary N-substituted derivatives of di(2-picolyl)amine (1) are reported: N-cyclohexylmethyl-di(2- picolyl)amine (2), N-benzyl-di(2-picolyl)amine (3), N-(4-pyridylmethyl)-di(2- picolyl)amine (4), N-(4-carboxymethylbenzyl)-di(2-picolyl)amine (5), N-(9-anthracen-8-ylmethyl)-di(2-picolyl)amine (6), 1,3-bis[di(2-picolyl) aminomethyl]benzene (7), 1,4-bis[di(2-picolyl)aminomethyl]benzene (8) and 2,4,6-tris[di(2-picolyl)amino]triazine (9). The solid complexes [Cu(2)(μ-Cl)]2(PF6)2, [Cu(3)(μ-Cl)] 2(PF6)2·0.5CH2Cl 2, Cu(4)(ClO4)2, Cu(5)2(ClO 4)2, [Cu(6)(ClO4)2(H 2O)]·0.5H2O, Cu2(7)(ClO 4)4, [Cu2(8)(Cl)4] and [Cu 2(9+H)(μ-OCH3)2(H2O)](ClO 4)3·C4H10O were isolated and X-ray structures of [Cu(2)(μ-Cl)]2(PF6)2, [Cu(3)(μ-Cl)]2(PF6)2·0.5CH 2Cl2, [Cu(6)2(ClO4) 2(H2O)]·0.5H2O, [Cu2(8) Cl4] and [Cu2(9+H)(μ-OCH3) 2(H2O)](ClO4)3·C 4H10O were obtained. The series is characterised by a varied range of coordination geometries and lattice architectures which in the case of [Cu(6)(ClO4)2(H2O)]·0.5H 2O includes a chain-like structure formed by unusual intermolecular π-interactions between metal bound perchlorate anions and the aromatic rings of adjacent anthracenyl groups. Variable temperature magnetic susceptibility measurements have been performed for [Cu(2)(μ-Cl)]2(PF 6)2 and [Cu(3)(μ-Cl)]2(PF6) 2·0.5H2O over the range 2-300 K. Both compounds show Curie-Weiss behaviour, with the data indicating weak antiferromagnetic interaction between the pairs of copper ions in each complex. Liquid-liquid (H2O/CHCl3) extraction experiments involving 1-3 and 7-9 as extractants showed that, relative to the parent (unsubstituted) dipic ligand 1, substitution at the secondary amine site in each case resulted in an increase in extraction efficiency towards Cu(II) (as its perchlorate salt); at least in part, this increase may be attributed to the enhanced lipophilicities of the N-substituted derivatives.
    Dalton Transactions 07/2009; DOI:10.1039/b901832h · 4.10 Impact Factor
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    ABSTRACT: We have studied the interplay of magnetism and superconductivity in LaFeAsO$_{1-x}$F$_x$ and Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$. While antiferromagnetic spin density wave formation is suppressed and superconductivity evolves, all samples show a doping independent strong increase of the normal state susceptibility upon heating which appears a general feature of iron pnictides. The data provide evidence for robust local antiferromagnetic correlations persisting even in the superconducting regime of the phase diagram. Comment: in print