Article pubs.acs.org/JPCC
Electronic Phase Transitions of δ‑AgxV2O5 Nanowires: Interplay between Geometric and Electronic Structures Peter M. Marley,† Sujay Singh,‡ Tesfaye A. Abtew,‡ Cherno Jaye,§ Daniel A. Fischer,§ Peihong Zhang,‡ Ganapathy Sambandamurthy,‡ and Sarbajit Banerjee*,∥ †
Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, United States Department of Physics, University at Buffalo, The State University of New York, Buffalo, New York 14260-1500, United States § Materials Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States ∥ Department of Chemistry, Texas A&M University, College Station, Texas 77842-3102, United States ‡
ABSTRACT: Vanadium oxide bronzes, with the general formula MxV2O5, provide a wealth of compositions and frameworks where strong electron correlation can be systematically (albeit thus far only empirically) tuned. In this work, we report the synthesis of single-crystalline δ-Ag0.88V2O5 nanowires and unravel pronounced electronic phase transitions induced in response to temperature and applied electric field. Specifically, a pronounced semiconductor−semiconductor transition is evidenced for these materials at ca. 150 K upon heating, and a distinctive insulator−conductor transition is observed upon application of an in-plane voltage. An orbital-specific picture of the mechanistic basis of the phase transitions is proposed using a combination of density functional theory (DFT) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Structural refinements above and below the transition temperature, angle-resolved O K-edge NEXAFS spectra, and DFT calculations suggest that the electronic phase transitions in these 2D frameworks are mediated by a change in the overlap of dxy orbitals.
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INTRODUCTION Electronic phase transitions accompanied by dramatic changes of electrical conductivity are of great fundamental interest, and while defining design principles remain to be elucidated, such phenomena are often manifested in materials characterized by structural or electronic instabilities.1−4 Beyond the fundamental allure of developing a mechanistic description of abrupt changes in physical properties, such phase transitions are also of great practical interest for designing new computing vectors (such as Mott field-effect transistors) and for applications spanning the range from memristors, electromagnetic modulators, and thermal switches to neural networks and electrochromic/thermochromic coatings.1,3 Electronic phase transitions induced in materials as a result of thermal, electrical, mechanical, or magnetic stimuli can be underpinned by a wide range of mechanisms, such as electron−electron correlation (the Mott−Hubbard picture), electron−phonon coupling (such as Peierl’s distortion of the atomistic structure), and disorder (Anderson’s localization).3,5 While considerable interest has focused on the canonical metal−insulator transition material VO2, the relatively large structural transformation (and concomitant elastic and strain effects), sluggish relaxation dynamics of the metal → insulator transition, and impediments to decoupling the structural progression from the electronic transition in VO2 have spurred increasing interest in the © 2014 American Chemical Society
discovery of other materials exhibiting pronounced electronic phase transitions at relatively high temperatures. Vanadium oxide bronzes, with the general formula MxV2O5, provide a richly diversified set of compositions and compounds, where strong electron correlation can be systematically (albeit thus far only empirically) tuned.6−13 In this work, we report the synthesis of single-crystalline δ-Ag0.88V2O5 nanowires, examine the electronic structure of this material using a combination of density functional theory (DFT) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and evidence electronic phase transitions induced as a result of temperature and voltage. MxV2O5 phases adopt a wide range of relatively open crystal structures constituted from interlinked VO5 and VO6 polyhedra with intercalated cations that nominally reduce V5+ to V4+ within the V2O5 framework.6−8 Two common frameworks often observed include the 2D δ-phase with intercalated cations residing between condensed double-layered sheets of V2O5 and the β-phase, which encloses 1D linear tunnels of metal cations (sometimes denoted as a Wadsley phase).7,8 Indeed, the βphase of AgxV2O5 is a useful material for primary batteries used Received: June 23, 2014 Revised: August 20, 2014 Published: August 22, 2014 21235
dx.doi.org/10.1021/jp506238s | J. Phys. Chem. C 2014, 118, 21235−21243
The Journal of Physical Chemistry C
Article
Figure 1. (a) Synchrotron powder XRD pattern (λ = 0.413 746 Å) of δ-Ag0.88V2O5 nanowires acquired at 295 K. The measured data is depicted using black circles, whereas the red lines denote the calculated diffraction pattern corresponding to the refined structure (see Table 1). The blue line plots the differential between the measured and calculated patterns. (b) The structural refinement yields a composition of δ-Ag0.88V2O5. The refined structure comprises V2O5 double layers with vanadium (red spheres) atoms coordinated to oxygen atoms in a distorted octahedral geometry (green spheres). The two crystallographically distinct vanadium sites are denoted using arrows. Ag+ ions (gray spheres) reside between the vanadium oxide layers with metal vacancies shown as partial white spheres. (c) The local coordination around the vanadium atoms is a distorted octahedron, whereas the Ag ions reside in a trigonal prismatic coordination environment bonded to seven oxygen anions.
in implantable cardiac defibrillators.14,15 For both structure types, the intercalated metal cations reduce a fraction of the vanadium sites in the V2O5 framework to V4+, yielding localized V 3d1 sites.6−8 At low temperatures, the MxV2O5 phases tend to be insulating with discrete charge ordering motifs aligned along the vanadium chains of the V2O5 framework, and it is believed that strong electron correlation precludes metallic transport.9−11,16−18 The specific pattern of charge ordering (localization) depends on the size and polarizability of the intercalated cation.19,20 At high temperatures or with increased carrier density, a classical Mott transition is expected with delocalization of charge, resulting in transition to a metallic state. The charge-ordering → charge-delocalization transition is not just of importance for the metal−insulator transitions observed for vanadium oxide bronzes, but it is also thought to underpin the emergence of a magnetoresistive response and metal−insulator transitions observed for manganites as well as the Verwey transition in Fe3O4.17,21−25 Understanding the electronic structure of mixed-valence vanadium oxide bronze phases is thus imperative to develop rational (and potentially generalizable) design principles for modulating metal−insulator transition temperatures. Strong electron correlation is not readily amenable to theoretical treatment, and the interplay of electronic and geometric structure can be rather complex in these compounds, underlining the need for examination of both aspects in elucidating the mechanistic basis for the transition. Given the intrinsically anisotropic crystal structures of the vanadium oxide bronzes (2D for δ-MxV2O5 and 1D for βMxV2O5), charge ordering and metallic transport in these materials tend to be extremely sensitive to disorder.10,11,13,18 Scaling these materials to nanoscale dimensions allows for fortuitous self-purification and well-defined ordering of cations between the 2D sheets and within the 1D tunnels defined by the V2O5 framework. In recent work, we have discovered thermally induced metal−insulator transitions surpassing 4 orders of magnitude in single nanowires of δ-KxV2O5 and β′CuxV2O5; analogous electric-field-induced metal−insulator transitions have been observed for δ-K xV 2 O 5 and β-
PbxV2O5.9−12,26−28 Here, we turn our attention to nanowires of double-layered δ-AgxV2O5 and evidence both thermal and electric field induced transitions of the electrical transport characteristics. Onoda and co-workers have observed electronic phase transitions in sintered specimens of δ-AgxV2O5 (x ≈ 0.68) at ca. 220 K and have noted that the electronic transition is suppressed for x > 0.8.7,26,27 On the basis of our DFT calculations, structural refinements above and below the phase transition temperature, and NEXAFS measurements, the electronic transitions are thought to be mediated by a change in overlap between dxy orbitals on proximate V sites induced by structural distortions as well as added carrier density.
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MATERIALS AND METHODS Synthesis of δ-AgxV2O5 Nanowires. δ-AgxV2O5 nanowires were synthesized through a hydrothermal reaction by first ball-milling stoichiometric amounts of AgCOOCH3 (SigmaAldrich) and V2O5 (Sigma-Aldrich) and then sealing the intimately mixed reaction mixture within a 23 mL PTFE-lined acid digestion vessel (Parr Instruments) with 16 mL of deionized water (ρ = 18.2 MΩ/cm). The reactions were allowed to proceed for 72 h at 250 °C. The resulting product was vacuum filtered, washed with copious amounts of deionized water and 2-propanol, and allowed to dry in air overnight. Characterization. Powder X-ray diffraction (XRD) data were collected in transmission geometry at 100, 295, and 400 K at beamline 11-BM of the Advanced Photon Source located at Argonne National Laboratory. Rietveld refinements were performed using GSAS/EXPGUI.29 The unit cell parameters, atomic positions, thermal parameters, and site occupancies were refined to yield the best fit to the obtained pattern.9,26 The morphology of the product was examined using scanning electron microscopy (SEM, Carl ZIESS operated at 5 and 20 kV) equipped with an energy dispersive X-ray (EDX) detector. Transmission electron microscopy (TEM) was performed using a JEOL 2010 instrument at an accelerating voltage of 200 kV. NEXAFS spectroscopy was performed at the National 21236
dx.doi.org/10.1021/jp506238s | J. Phys. Chem. C 2014, 118, 21235−21243
The Journal of Physical Chemistry C
Article
Table 1. Atomic Parameters and Interatomic V−O Distances Refined from the Synchrotron XRD Pattern Acquired at 295 K Shown in Figure 1aa
a
atom
x
y
Ag(1) V(1) V(2) O(1) O(2) O(3) O(4) O(5)
0.61768(5) 0.23316(10) 0.93613(9) 0.0763(4) 0.7654(3) 0.3967(4) 0.9442(4) 0.2194(4)
0 0 0 0 0 0 0 0
z 0.02733(9) 0.33838(16) 0.335689(13) 0.3909(4) 0.3884(5) 0.3611(5) 0.1490(5) 0.1452(7) Selected Interatomic Distances
occupancy
Uiso
0.8823(17) 1.0 1.0 1.0 1.0 1 1 1
0.02600(26) 0.00258(24) 0.00258(24) 0.00617(12) 0.00150(10) 0.01515(10) 0.01990(13) 0.03540(16)
V(1)O6 polyhedra
distance (Å)
V(2)O6 polyhedra
distance (Å)
AgO7 polyhedra
distance (Å)
V(1)−O(5) V(1)−O(3) V(1)−O(2) V(1)−O(2) V(1)−O(2) V(1)−O(1)
1.687(5) 1.937(4) 2.376(4) 1.9253(12) 1.9253(12) 1.909(4)
V(2)−O(4) V(2)−O(3) V(2)−O(3) V(2)−O(2) V(2)−O(1) V(2)−O(1)
1.627(4) 1.9081(11) 1.9081(11) 2.067(4) 1.717(4) 2.384(4)
Ag(1)−O(5) Ag(1)−O(5) Ag(1)−O(5) Ag(1)−O(4) Ag(1)−O(4) Ag(1)−O(4) Ag(1)−O(4)
2.447(5) 2.4174(30) 2.4174(30) 2.4985(30) 2.4985(30) 2.4985(30) 2.4985(30)
Space group = C2/m, a = 11.7875(1) Å, b = 3.674 245(28) Å, c = 8.698 63(8) Å, β = 90.4451(5)°; χ2 = 3.68, Rw = 8.85%.
Figure 1b; Figure 1c indicates the local coordination environment around the two vanadium atoms that share edges to give rise to the double-layered 2D V2O5 framework. The vanadyl bonds alternately point up and down into the interlayer space between the 2D sheets where they are coordinated to intercalated Ag+ ions. The Ag+ ions reside between the infinite V2O5 layers and are coordinated solely to oxygen atoms from the 2D framework and not to any additional water molecules, as observed for the hydrated δ-CaxV2O5 phase.9,27,33 The Ag+ ions are coordinated by seven oxide ions in a monocapped trigonal prismatic local coordination environment (Figure 1c). The ordering of Ag+ ions along the V2O5 framework necessarily leads to reduction of an equivalent fraction of V5+ ions in the 2D V2O5 framework, thereby giving rise to charge localization patterns along the infinite sheets. To account for charge balance, an approximate empirical formula of Ag0.88(V5+)1.22(V4+)0.88O5 can be written for the prepared materials. The Ag stoichiometry has also been confirmed by hard X-ray photoelectron spectroscopy (HAXPES) measurements. A comprehensive X-ray emission study of AgxV2O5 bronze phases will be published in future work. The nanowire morphology of the obtained δ-AgxV2O5 nanowires is shown in Figure 2a; the nanowires are greater than 10 μm in length and have rectangular cross sections with widths on the order of 200 nm (as illustrated by the TEM images in Figure 2b). The composition of the δ-AgxV2O5 nanowires was further confirmed by EDX yielding x = 0.86, which is very close to the 0.88 composition determined by the structural refinement. Figure 2c depicts a lattice-resolved HRTEM image with a lattice spacing of 0.585 nm, which corresponds to the interplanar separation between the (200) planes of the refined δ-Ag0.88V2O5 structure. The diffraction spots in the selected area electron diffraction pattern shown in Figure 2d can further be indexed to the δ-phase refined in Figure 1b. Electronic Phase Transitions and Electronic Structure. A wide range of electronic phase transitions has been manifested in vanadium oxide bronzes ranging from semiconductor−semiconductor transitions, metal−insulator transitions, and charge ordering to superconductivity.9−13,26,34,35
Institute of Standards and Technology beamline U7A at the National Synchrotron Light Source of Brookhaven National Laboratory using a toroidal mirror spherical grating monochromator with 1200 lines/mm grating and an energy resolution of ca. 0.05 eV. Data was collected in partial electron yield (PEY) mode using a channeltron multiplier near the surface using an entrance grid bias of −200 V; the PEY signal was normalized by the drain current of a clean Au mesh located along the path of the incident X-rays. The data was collected along with a metallic vanadium reference mesh for energy calibration. The NEXAFS spectra were pre- and postedge normalized using the Athena suite of programs. The transport properties of the nanowires were studied by pressing the nanowires into a pellet and measuring the resistance as a function of temperature and current as a function of voltage; the current was limited to 105 mA to protect the sample. Electronic Structure Calculations. DFT calculations were performed using the Quantum ESPRESSO package using the generalized gradient approximation with Perdew−Burke− Ernzerhof functionals.30 Ultrasoft pseudopotentials were used to describe the electron−ion interactions.31,32
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RESULTS AND DISCUSSION Structural Characterization. Figure 1a shows the synchrotron powder XRD pattern obtained for the as-prepared materials at 295 K. The structure can be refined to monoclinic δ-AgxV2O5 in the C2/m space group with x of 0.88 (the interstitial Ag sites are not completely occupied). The residual plot indicates that the prepared materials are essentially phase pure with some remnant metallic Ag (
