Emptying and filling a tunnel bronze

We report the synthesis of a new tunnel-structured polymorph of V2O5 (ζ-V2O5) synthesized by topotactic ion extraction.


Introduction
It was in 1867 that the British chemist Sir Henry Eneld Roscoe rst outlined the various binary oxides of vanadium resulting from the facile accessibility of multiple oxidation states of this transition metal as part of his Bakerian lecture to the Royal Society; in doing so, he set the record straight on the formula of the end-member "vanadic acid" V 2 O 5 , which had previously erroneously been described by Berzelius to have the formula VO 3 . 1 As it turns out, the facile stabilization of mixed valence vanadium sites and the accommodation of oxygen vacancies through crystallographic shear allows for a much richer phase diagram than he originally anticipated even for just binary vanadium oxides with the occurrence of numerous Magnelitype phases. 2 However, it took more than a century since the initial studies by Roscoe for the rst accurate elucidation of the crystal structure of V 2 O 5 , which with pentavalent vanadium is the "thermodynamic sink" in this system, and not surprisingly, is the most common naturally occurring oxide ore of vanadium (found typically in volcanic craters) and an ubiquitous industrial precursor for the preparation of ferrovanadium. Byström et al. rst established the now classic orthorhombic layered structure of V 2 O 5 with a space group of Pmmn derived from square pyramidally coordinated vanadium-centered building blocks; in this structure, the VO 5 polyhedra are knitted together with shared edges forming a zig-zag chain along the b-axis and are arrayed with shared corners along the crystallographic a axis (Fig. 1a). 3 Galy further established that the long V-O distance was much too long to be a bonding interaction and thus proposed that the 2D innite V 2 O 5 sheets were held together in the c direction by relatively weak van der Waals' interactions. 2c,4 The combination of an open layered framework that is only weakly interacting in the c direction and the facile accessibility of multiple oxidation states underpins the use of V 2 O 5 for a broad range of applications ranging from cathodes for Li-ion batteries, and catalysts for selective oxidation to electrochromic elements, actuators, anti-fouling lms, and photodectors. 5,6 Despite the technological interest derived from this remarkable combination of an open framework, facile redox characteristics, and tolerance to defects, it appears that orthorhombic V 2 O 5 is by far the most thermodynamically accessible phase under ambient conditions. There is only one notable metastable phase of V 2 O 5 (space group ¼ Pmna) that is derived from de-intercalation of Li-ions from a puckered (but still layered) lithiated phase, g-Li x V 2 O 5 (x > 1). 7a This puckered g-V 2 O 5 phase is stable up to ca. 340 C where it transforms to the thermodynamically stable orthorhombic V 2 O 5 structure. 7a A high-pressure b-phase has also been reported. 7b In this work, we report the stabilization of a novel tunnel-structured z-V 2 O 5 (space group: C2/m) phase based on the hydrothermal deintercalation of Ag + ions from nanowires of a b-Ag x V 2 O 5 tunnel structure. The tunnel framework appears to be stable to temperatures of up to 600 C and remains available for subsequent intercalation with Li and Mg-ions.

Synthesis and characterization
b-Ag x V 2 O 5 nanowires were synthesized by reacting stoichiometric amounts of silver acetate (Sigma Aldrich) and V 2 O 5 (Sigma Aldrich) with 16 mL H 2 O (r ¼ 18 MU cm À1 ) in a Teonlined acid digestion vessel at 210 C for 72 h. The resulting solid was washed with water and allowed to dry in air. The leached z-V 2 O 5 structure was synthesized by hydrothermally treating the b-Ag x V 2 O 5 nanowires with 15 mL of 0.71 M HCl at 210 C for 24 h. Aer allowing the reaction to cool the solid was ltered and washed with copious amounts of water and isopropanol and then allowed to dry in air overnight. Reinsertion of Li-ions into the empty z-V 2 O 5 framework was performed by mixing stoichiometric amounts of leached z-V 2 O 5 nanowires and n-butyllithium for 96 h in toluene under an Ar atmosphere. The resulting solid was ltered, washed with copious amounts of water, and allowed to dry in air overnight. b-Mg x V 2 O 5 nanowires were synthesized by stirring Mg nanoplatelets (dimensions of 100-500 nm in diameter, prepared by the reduction of CH 3 MgCl by lithium naphthalide as reported in our previous work) 8 with the leached z-V 2 O 5 nanowires in 20 mL of H 2 O at room temperature for 48 h. The resulting solid was washed with copious amounts of water and allowed to dry in air.
Synchrotron powder X-ray diffraction data were acquired in transmission geometry at beamline 11-BM of the Advanced Photon Source at Argonne National Laboratory. Rietveld renements were performed using the GSAS/EXPGUI soware. 9 In situ heating of the leached z-V 2 O 5 phase was performed using a Rigaku Ultima IV diffractometer with Cu Ka radiation and an Ultima IV HT 1500 temperature attachment with a PTC-30 programmable temperature controller and a platinum sample holder from room temperature to 600 C under an ambient atmosphere. A 10 C min À1 ramp rate was used with a hold time of 30 min before acquiring each pattern. The nanowires were further examined by scanning electron microscopy (SEM, Hitachi SU-70, 20 kV, equipped with an energy dispersive X-ray detector for elemental composition) and transmission electron microscopy (TEM, JEOL 2010, 200 kV). Chemical analysis was performed by inductively coupled plasma mass spectrometry (ICP-MS) aer digesting the samples in 10% aqueous solutions of nitric acid. X-ray photoelectron spectroscopy was performed on a Phi 5000 VersaProbe instrument with monochromatic Al Ka X-rays and with charge neutralization of the samples. Nearedge X-ray absorption ne structure (NEXAFS) measurements were carried out at the National Institute of Standards and Technology beamline U7A of the National Synchrotron Light Source of Brookhaven National Laboratory with a toroidal mirror spherical grating monochromator using a 1200 lines per mm grating and an energy resolution of 0.1 eV. NEXAFS data were collected in partial electron yield (PEY) mode with a channeltron multiplier near the sample surface using the detector at À200 V bias to enhance surface sensitivity. The PEY signal was normalized by the drain current of a clean gold mesh located along the path of the incident X-rays. All data was collected with a standard reference of metallic vanadium foil for energy calibration. DFT calculations were performed using the QUANTUM ESPRESSO package using the generalized gradient approximation with Perdew-Burke-Ernzerhof functionals. 10 Ultraso pseudopotentials were used to describe the electronion interactions. 11,12 Results and discussion Ion-exchange and other topotactic reactions are commonly used to access a variety of compounds that cannot directly be synthesized from their constituent elements owing to insurmountable thermodynamic considerations; examples of the synthetic utility of these methods span the range from novel perovskites to semiconductor quantum dots and chalcogenides. 13,14 Scaling materials to nanoscale dimensions further provides access to crystal structures that may not be readily stabilized at room temperature in bulk form. While some of these structures are metastable, others are in fact thermodynamically stable either due to surface energy considerations or because strain effects can be better accommodated at nanoscale dimensions. 15 Indeed, the z-V 2 O 5 phase isolated here appears to be an unusual thermodynamically stable polymorph.
The novel z-V 2 O 5 structure has been derived from the almost complete leaching of Ag-ions from a ternary vanadium oxide bronze b-Ag 0.33 V 2 O 5 as per: Fig. 1 (a) Crystal structure of the orthorhombic layered structure of a-V 2 O 5 . High-resolution synchrotron powder X-ray diffraction pattern b). Blue squares (top) and black circles (bottom) show the observed reflections for b-Ag x V 2 O 5 and z-V 2 O 5 , respectively. The red and teal lines are the calculated diffraction patterns and residuals, respectively. (c) The refined b-Ag x V 2 O 5 structure indicating the three vanadium atoms (black) bonded to oxygen (orange) and Agions (grey) residing within tunnel sites. (d) The crystal structure of Agleached z-V 2 O 5 refined from the diffraction pattern in (b). Upon reaction with HCl, the Ag-ions are almost entirely removed. The Fourier maps of the observed electron density projected on the (010) lattice plane for b-Ag x V 2 O 5 and z-V 2 O 5 are shown in (e) and (f). The electron density from the Ag-ions is clearly almost completely elimi- The ternary vanadium oxide bronzes of which b-Ag 0.33 V 2 O 5 is an example are important in their own right and exhibit remarkable rstand second-order electronic phase transitions such as colossal metal-insulator transitions (induced as a function of temperature, voltage, or cation concentration), superconductivity, and charge and spin density waves. 16,17 Cation intercalation and concomitant partial reduction of the V 2 O 5 frameworks yields charge ordered networks that extend along the length of the vanadium oxide chains, thereby affording excellent model systems for examination of strong electron correlation. 18 The size, polarizability, extent of covalency of cation-oxygen interactions, and cation concentration dictates the structure adopted by the ternary and quaternary vanadium oxides with single-layered, highly puckered layered, double-layered, and tunnel frameworks being some of the more commonly adopted frameworks. 2a,c,16c,19,20 Indeed, the ability to ll empty tunnels of the novel z-V 2 O 5 phase with other cations as noted below provides a versatile synthetic route for obtaining intercalated ternary vanadium oxide bronzes and for systematically tuning electron correlation.
b-Phase vanadium oxide bronzes have been extensively studied since Wadsley discovered that b-Na x V 2 O 5 has the ability to accommodate a range of sodium concentrations. 19a Nanowires of b-Ag 0.33 V 2 O 5 have been prepared by the hydrothermal reaction between silver acetate and V 2 O 5 as described in the Methods section. The top panel in Fig. 1b shows the rened powder XRD pattern and Fig. 1c illustrates the rened structure (space group: C2/m), which is based on three distinct vanadiumcentered polyhedra: edge-sharing V(1)O 6 distorted octahedra, corner-sharing V(2)O 6 distorted octahedra, and V(3)O 5 square pyramids (see Table S1, ESI † for rened unit cell parameters and atom positions). The polyhedra form innite chains parallel to the crystallographic b-axis and enclose tunnel sites wherein the intercalated Ag + ions reside. A Ag stoichiometry of x ¼ 0.33 (occupancy of 0.4956) yields the best t to the powder pattern and has been further veried by energy dispersive X-ray analysis as well as XPS (Fig. S2 †). Furthermore, ICP-MS analysis of b-Ag x V 2 O 5 yields the relative concentrations of vanadium and silver to be 3.072 upon acid digestion, which results in an x value of 0.31 further corroborating the stoichiometry from the renement.
The open tunnel framework of the b-phase permits removal of cations from interstitial sites while keeping the tunnel structure intact; indeed, for the nanowires this occurs without amorphization. Furthermore, the ability of the vanadium atom to occupy various coordination environments and oxidation states results in formation of the z-V 2 O 5 open tunnel framework without reversion to a-V 2 O 5 . 2c,19a The bottom panel of Fig. 1b plots the rened powder XRD pattern of the leached phase and Fig. 1d shows the structure of the novel z-V 2 O 5 phase. The open tunnel framework comprising V(1)O 6 , V(2)O 6 , and V(3)O 5 polyhedra is still clearly retained with an almost complete removal of the interstitial Ag-ions (see Table S2, ESI † for rened unit cell parameters and atom positions). A comparison of the local vanadium-oxygen coordination environment for the three distinct vanadium atoms is shown in Table S3. † The signicant elimination of the intercalated Ag + cations is veried by the Fourier maps of the charge density along the (010) planes depicted in Fig. 1e and f. The x value determined from the renement (and corroborated via energy-dispersive X-ray analysis and XPS) is much lower than was previously thought to be necessary to stabilize the b-phase framework. 2c,19a ICP-MS analysis of the z-V 2 O 5 structure gives relative concentrations of vanadium and silver to be 16.08; this conrms the stoichiometry from the renement and the removal of the interstitial Agions (x ¼ 0.06). Fig. S1 (ESI †) shows the marked change in color accompanying the leaching of Ag ions from b-Ag 0.33 V 2 O 5 , suggesting recovery of the orange color characteristic of V 5+ cations. Fig. S2 (ESI †) shows V 2p X-ray photoelectron spectroscopy data acquired for b-Ag 0.33 V 2 O 5 and the leached z-V 2 O 5 phase. Upon removal of Ag-ions, it is expected that the number of electrons localized on the vanadium oxide framework will decrease as veried by the pronounced decrease in the intensity of the V 4+ shoulder. The inclusion of some protons in the tunnels cannot be entirely ruled out but the recovery of the orange color and the predominant contributions from V 5+ in both XPS and NEXAFS spectra are suggestive of oxidation of V 4+ sites within the vanadium oxide bronze back to V 5+ .
The thermal stability of z-V 2 O 5 has been studied by temperature-dependent powder X-ray diffraction. Fig. 2a shows a contour plot of the reections as a function of temperature from 20 to 600 C under an ambient atmosphere. z-V 2 O 5 is clearly stable up to at least 490 C (demarcated by a white line for clarity) at which point the low 2q reections (001) and (200) decrease in intensity indicating loss of long-range order. The reections suggest that the leached structure is not merely a metastable phase of V 2 O 5 . Differential scanning calorimetry (DSC) is a sensitive probe of dehydration of intercalated water molecules in hydrated vanadium oxides; in past work, we have observed a pronounced endothermic feature at ca. 250 C corresponding to the removal of interstitial water molecules. 18c However, DSC measurements performed on the leached z-V 2 O 5 structure show no calorimetric signatures corresponding to removal of water molecules from the interstitial tunnel sites. Fig. 2b shows a SEM image of the starting b-Ag x V 2 O 5 nanowires, which have lengths up to and greater than 10 mm and lateral dimensions of 150 AE 9 nm. The nanowire morphology is retained for the leached z-V 2 O 5 phase aer removal of Ag-ions from the interstitial tunnel sites as indicated in Fig. 2c without any discernible fragmentation. The HRTEM image in Fig. 2d along with the corresponding selected area electron diffraction (SAED) pattern indicates that the initial b-Ag x V 2 O 5 nanowires are single crystalline. Fig. 2e and the SAED pattern in the inset further indicate that the leached z-V 2 O 5 remain single crystalline upon removal of Ag-ions. The lattice spacing of 0.942 nm closely matches the spacing between the (001) lattice planes of the rened structure (Fig. 1d). The electron microscopy observations suggest that the shorter diffusion path lengths available at the nanoscale (as compared to the bulk) facilitate rapid deintercalation of the Ag + ions, allowing for stabilization of the empty tunnel framework. The ability of the nanowires to accommodate strain also likely plays a signicant role in permitting the removal of silver cations without destruction of the morphology or amorphization. Intriguingly, the role of nite size here is some-what different than in the case of deintercalation of the Li x V 2 O 5 . Indeed, removal of Li-ions from nanoscale g-Li x V 2 O 5 results in transformation back to a-V 2 O 5 unlike in the bulk wherein the meta-stable g-V 2 O 5 structure is stabilized. 21 From a structural perspective, the ability of the framework to remain intact upon removal of the majority of the Ag + ions is likely mediated by the VO 5 square pyramid chains that act in the same fashion as the PO 4 tetrahedra that hold the iron oxide octahedra together upon delithiation of LiFePO 4 . 22 Such structural stability of the tunnel framework is imperative to realize the potential of topotactic reactions and precisely manipulate the physical properties of vanadium oxide bronzes. In contrast, a control de-intercalation reaction attempted for micron-sized particles of b-Ag 0.33 V 2 O 5 (prepared by the solidstate reaction of Ag 2 O, V 2 O 5 , and V 2 O 3 at 650 C) shows no evidence for deintercalation of Ag-ions from the interstitial tunnel sites. Fig. 3a and b contrasts the calculated density of states of layered orthorhombic a-V 2 O 5 (Fig. 1a) and the novel tunnelstructured leached z-V 2 O 5 phase (Fig. 1d). For both polymorphs, the valence band is mostly O 2p in character, whereas the conduction band is primarily V 3d in character. 23 Eyert and coworkers have calculated a bandgap of ca. 1.7 eV for orthorhombic a-V 2 O 5 ; a notable consequence of the anisotropic layered structure of a-V 2 O 5 is the appearance of a split off conduction band about 0.35 eV below the primary conduction band that is essentially derived from V 3d xy orbitals. 23b The V 3d xy origin of the split-off conduction bands has been veried in Fig. 3 (a) Calculated density of states for a-V 2 O 5 . (b) The density of states for the z-V 2 O 5 structure (the legend on the right illustrates atomprojected density of states derived from the different atoms). The DOS have been normalized per V 2 O 5 formula unit. (c) Top panel: polarized NEXAFS measurements performed on a pressed pellet of z-V 2 O 5 nanowires indicating V L III , V L II , and O K-edge resonances. Although the intensity of the t 2g * feature is seen to increase with increasing incident photon energy, the strong anisotropy characteristic of a and d-phase V 2 O 5 and ternary vanadium oxides is not observed for the z-framework. Bottom panel: NEXAFS spectra acquired for a-V 2 O 5 (red), b-Ag x V 2 O 5 (black), and z-V 2 O 5 (blue).
previous polarized near-edge X-ray absorption ne structure (NEXAFS) spectroscopy studies of a-V 2 O 5 nanowire arrays. 23a In contrast, the bandgap calculated using the rened coordinates for the z-V 2 O 5 structure (Fig. 3b) is ca. 1.1 eV. Owing to the more complex geometric structure of this novel tunnel phase, no distinct split-off conduction band feature is predicted. Fig. S3 (ESI †) shows the atom-projected density of states calculated for the three distinct V(1), V(2), and V(3) atoms of z-V 2 O 5 ; the contributions from each of the ve V 3d orbitals has been deconvoluted and it is apparent that the lowest energy conduction band states in z-V 2 O 5 includes contributions from V(1) 3d xy , V(1) 3d zy , V(2) 3d xy , V(2) 3d zy , and V(3). The complex tunnel structure (compared to the more simple orthorhombic layered structure of a-V 2 O 5 ) thus gives rise to a very different bonding motif and electronic structure. 24 More recent DFT calculations of a-V 2 O 5 from Da Silva and co-workers in the LDA and GGA approximations suggest bandgaps of 2.21 and 2.27 eV for a-V 2 O 5 that are closer to the optically measured values. 25 The novel tunnel-structured phase clearly has a much smaller bandgap. The top panel in Fig. 3c depicts polarized NEXAFS spectra acquired for a pressed pellet of z-V 2 O 5 nanowires and illustrates the reduced anisotropy of the lowest energy conduction band states for both the V L III -edge region and the O K-edge regions; in contrast, orthorhombic V 2 O 5 and other layered d-M x V 2 O 5 phases show a pronounced modulation of t 2g : e g * intensity ratios at the O K-edge and clear delineation of a split-off band at the V L III -edge with varying polarization. 2a,18b The bottom panel in Fig. 3c contrasts the NEXAFS spectra of a-V 2 O 5 (red) with b-Ag x V 2 O 5 (black) and z-V 2 O 5 (blue).
The ability of the novel z-V 2 O 5 phase to serve as a synthon for preparation of ternary vanadium bronzes and for systematic modulation of strong electron correlation therein, as well as its ability to facilitate electrochemical energy storage, is predicated on the further intercalation of ions within the open framework. Here, we demonstrate that the tunnels of the z-V 2 O 5 phase can be topotactically packed with Li and Mg ions as per: z-V 2 O 5 (s) + 0.66C 4 H 9 Li (toluene) / b 0 -Li 0.66 V 2 O 5 (s) + 0.33C 8 H 18 (butane and butane products have also been reported) (2) Fig. 4a shows the synchrotron powder XRD pattern obtained aer reaction of z-V 2 O 5 with a stoichiometric amount of n-butyl-Li for 96 h at room temperature. The rened pattern is shown in red and the rened structure is illustrated in Fig. 4c. Table S4 (ESI †) lists the structural parameters of the b 0 -Li 0.66 V 2 O 5 rened structure. Upon intercalation, the Li-ions (green) occupy the cation sites characteristic of the b 0 -phase owing to their small size (shied by b/2 from the b-phase as illustrated in Fig. 4c). A comparison of the z-V 2 O 5 pattern with that of the lithiated b 0 -Li x V 2 O 5 structure is shown in Fig. S4 (ESI †) and evidences the structural transformation despite the low scattering from Liions. The exibility of the z-V 2 O 5 framework allows for an expansion of the tunnel structure to accommodate Li-ions (as seen in the change in the unit cell volume increasing from 522.271 to 534.581Å 3 ). The nanowire morphology is entirely retained aer insertion of lithium-ions as depicted by the SEM and TEM images of Fig. 4d and e. The SAED pattern in Fig. 4f can be indexed to the rened b 0 -Li x V 2 O 5 structure and illustrates the single crystalline nature is preserved aer lithiation, suggesting a classical topotactic reaction.
Incorporation of Mg 2+ -ions into the open tunnel structure of z-V 2 O 5 further illustrates the control over the electronic structure and properties that can be obtained through the topotactic ion-exchange pathway. Fig. 4b shows the (200) reection for z-V 2 O 5 (black), b 0 -Li x V 2 O 5 (red), and b-Mg x V 2 O 5 (green); the shi to lower 2q values in progressing from z-V 2 O 5 to b 0 -Li x V 2 O 5 to b-Mg x V 2 O 5 is consistent with an increase in the ionic radii of the inserted metal cation and conrms the incorporation of Mg within the interstitial tunnel sites of the b-phase. The retention of the nanowire morphology and the insertion of Mg is further veried by the SEM image and corresponding energy dispersive X-ray pattern depicted in Fig. S5. † Note that a b-phase Mg bronze has not heretofore been prepared and thus the method demonstrated here suggests the utility of the z-V 2 O 5 structures to serve as synthons for preparation of novel bronzes.

Conclusion
In conclusion, a novel polymorph of V 2 O 5 with an open tunnellike framework has been prepared by the topotactic de-intercalation of Ag-ions from b-Ag 0.33 V 2 O 5 nanowires. The ability of the nanostructures to accommodate strain allows for stabilization of the tunnel framework, which is characterized by a distinctive bonding motif and an electronic structure that is signicantly different from orthorhombic a-V 2 O 5 . The facile reinsertion of different metal-ions into the empty tunnel framework provides a facile synthetic route to control the chargeordering network and electronic properties of vanadium oxide bronzes with implications for Mott eld-effect transistors and memristors. Stabilizing the open tunnel structure without amorphization and being able to induce intercalation of cations further suggests applications of these materials in Li-ion and "beyond Li" batteries. 2a,26