Effect of pre-intercalation on Li-ion diffusion mapped by topochemical single-crystal transformation and operando investigation.

Limitations in electrochemical performance as well as supply chain challenges have rendered positive electrode materials a critical bottleneck for Li-ion batteries. State-of-the-art Li-ion batteries fall short of accessing theoretical capacities. As such, there is intense interest in the design of strategies that enable the more effective utilization of active intercalation materials. Pre-intercalation with alkali-metal ions has attracted interest as a means of accessing higher reversible capacity and improved rate performance. However, the structural basis for improvements in electrochemical performance remains mostly unexplored. Here we use topochemical single-crystal-to-single-crystal transformations in a tunnel-structured ζ-V2O5 positive electrode to illustrate the effect of pre-intercalation in modifying the host lattice and altering diffusion pathways. Furthermore, operando synchrotron X-ray diffraction is used to map Li-ion site preferences and occupancies as a function of the depth of discharge in pre-intercalated materials. Na- and K-ion intercalation 'props open' the one-dimensional tunnel, reduces electrostatic repulsions between inserted Li ions and entirely modifies diffusion pathways, enabling orders of magnitude higher Li-ion diffusivities and accessing higher capacities. Deciphering the atomistic origins of improved performance in pre-intercalated materials on the basis of single-crystal-to-single-crystal topochemical transformation and operando diffraction studies paves the way to site-selective modification approaches for positive electrode design.

Cation reordering instead of phase transitions: Origins and implications of contrasting lithiation mechanisms in 1D ζ- and 2D α-V2O5.

Substantial improvements in cycle life, rate performance, accessible voltage, and reversible capacity are required to realize the promise of Li-ion batteries in full measure. Here, we have examined insertion electrodes of the same composition (V2O5) prepared according to the same electrode specifications and comprising particles with similar dimensions and geometries that differ only in terms of their atomic connectivity and crystal structure, specifically two-dimensional (2D) layered α-V2O5 that crystallizes in an orthorhombic space group and one-dimensional (1D) tunnel-structured ζ-V2O5 crystallized in a monoclinic space group. By using particles of similar dimensions, we have disentangled the role of specific structural motifs and atomistic diffusion pathways in affecting electrochemical performance by mapping the dynamical evolution of lithiation-induced structural modifications using ex situ scanning transmission X-ray microscopy, operando synchrotron X-ray diffraction measurements, and phase-field modeling. We find the operation of sharply divergent mechanisms to accommodate increasing concentrations of Li-ions: a series of distortive phase transformations that result in puckering and expansion of interlayer spacing in layered α-V2O5, as compared with cation reordering along interstitial sites in tunnel-structured ζ-V2O5 By alleviating distortive phase transformations, the ζ-V2O5 cathode shows reduced voltage hysteresis, increased Li-ion diffusivity, alleviation of stress gradients, and improved capacity retention. The findings demonstrate that alternative lithiation mechanisms can be accessed in metastable compounds by dint of their reconfigured atomic connectivity and can unlock substantially improved electrochemical performance not accessible in the thermodynamically stable phase.

Interface-modulated kinetic differentials in electron and hole transfer rates as a key design principle for redox photocatalysis by Sb2VO5/QD heterostructures.

The efficient conversion of solar energy to chemical energy represents a critical bottleneck to the energy transition. Photocatalytic splitting of water to generate solar fuels is a promising solution. Semiconductor quantum dots (QDs) are prime candidates for light-harvesting components of photocatalytic heterostructures, given their size-dependent photophysical properties and band-edge energies. A promising series of heterostructured photocatalysts interface QDs with transition-metal oxides which embed midgap electronic states derived from the stereochemically active electron lone pairs of p-block cations. Here, we examine the thermodynamic driving forces and dynamics of charge separation in Sb2VO5/CdSe QD heterostructures, wherein a high density of Sb 5s2-derived midgap states are prospective acceptors for photogenerated holes. Hard-x-ray valence band photoemission spectroscopy measurements of Sb2VO5/CdSe QD heterostructures were used to deduce thermodynamic driving forces for charge separation. Interfacial charge transfer dynamics in the heterostructures were examined as a function of the mode of interfacial connectivity, contrasting heterostructures with direct interfaces assembled by successive ion layer adsorption and reaction (SILAR) and interfaces comprising molecular bridges assembled by linker-assisted assembly (LAA). Transient absorption spectroscopy measurements indicate ultrafast (<2 ps) electron and hole transfer in SILAR-derived heterostructures, whereas LAA-derived heterostructures show orders of magnitude differentials in the kinetics of hole (<100 ps) and electron (∼1 ns) transfer. The interface-modulated kinetic differentials in electron and hole transfer rates underpin the more effective charge separation, reduced charge recombination, and greater photocatalytic efficiency observed for the LAA-derived Sb2VO5/CdSe QD heterostructures.

Effect of crystallite geometries on electrochemical performance of porous intercalation electrodes by multiscale operando investigation.

Lithium-ion batteries are yet to realize their full promise because of challenges in the design and construction of electrode architectures that allow for their entire interior volumes to be reversibly accessible for ion storage. Electrodes constructed from the same material and with the same specifications, which differ only in terms of dimensions and geometries of the constituent particles, can show surprising differences in polarization, stress accumulation and capacity fade. Here, using operando synchrotron X-ray diffraction and energy dispersive X-ray diffraction (EDXRD), we probe the mechanistic origins of the remarkable particle geometry-dependent modification of lithiation-induced phase transformations in V2O5 as a model phase-transforming cathode. A pronounced modulation of phase coexistence regimes is observed as a function of particle geometry. Specifically, a metastable phase is stabilized for nanometre-sized spherical V2O5 particles, to circumvent the formation of large misfit strains. Spatially resolved EDXRD measurements demonstrate that particle geometries strongly modify the tortuosity of the porous cathode architecture. Greater ion-transport limitations in electrode architectures comprising micrometre-sized platelets result in considerable lithiation heterogeneities across the thickness of the electrode. These insights establish particle geometry-dependent modification of metastable phase regimes and electrode tortuosity as key design principles for realizing the promise of intercalation cathodes.

Effect of Stereochemically Active Electron Lone Pairs on Magnetic Ordering in Trivanadates.

Stereoactive electron lone pairs derived from filled 5/6s2 states of p-block cations are an intriguing electronic and geometric structure motif that have been exploited for diverse applications such as thermoelectrics, thermochromics, photocatalysis, and nonlinear optics. Layered trivanadates are dynamic intercalation hosts, where the insertion of cations can be used to tune electron correlation, charge localization, and magnetic ordering. However, the interaction of 5/6s2 stereoactive electron lone pairs with layered trivanadates remains unexplored. In this study, we contrast s- and p-block trivanadates and map off-centering in the coordination environment and reduction in symmetry arising from the stereochemical activity of lone pair cations to the emergence of filled antibonding lone-pair 6s2-O 2p hybridized states. The former is studied by high-resolution single-crystal X-ray diffraction studies of TlV3O8 and isostructural RbV3O8 to probe distinct differences in Tl and Rb coordination environments and the resulting modulation of V-V interactions in V3O8 slabs. The latter has been probed by variable-energy hard X-ray photoelectron spectroscopy (HAXPES) measurements, which manifest orbital-specific contributions from bonding and antibonding interactions of stereoactive Tl 6s2 electron lone pairs in TlV3O8. The spectroscopic assignment of valence band states to stereoactive lone pairs is further corroborated by first-principles electronic structure calculations, crystal orbital Hamilton population analyses, and electron localization function maps. The presence of the Tl 6s2 electron lone pair in TlV3O8 brings about the off-centering of Tl+ cations, which leads to anisotropy in Tl-O bonds. The off-centering of Tl ions weakens V-O bonds in one direction, which subsequently strengthens directional V-V coupling. Magnetic measurements reveal ferromagnetic signatures for both RbV3O8 and TlV3O8. However, the differences in V···V interactions significantly affect the energy balance of the superexchange interactions, resulting in an ordering temperature of 140 K for TlV3O8 as compared to 125 K for RbV3O8. The results demonstrate the distinctive effects of stereochemically active lone pairs in modifying electronic structure near the Fermi level and for mediating superexchange interactions.

Emulating synaptic behavior in surface-functionalized MoS2 through modulation of interfacial charge transfer via external stimuli.

Neuromorphic computing requires materials able to yield electronic switching behavior in response to external stimuli. Transition-metal dichalcogenides surfaces covered by partial or full monolayers of molecular species have shown promise due to their potential for tunable interfacial charge transfer. Here, we demonstrate a class of molecules able to position MoS2 surfaces on the cusp of electronic instabilities. Density functional theory (DFT) calculations and ab initio molecular dynamics simulations are used to study the interaction of four reduced pyridinium-derived pi-conjugated molecules with the pristine basal planes of MoS2, by exploring the dynamical evolution of the system at room temperature with regards to the effective band gap, radius of gyration (rog), and charge transfer. Computed rog profiles show that low concentrations of small reduced methyl viologen molecules have high mobilities on top of the surface of the basal plane at room temperature leading to unstable surface deposition, whereas a full monolayer of larger fused-ring molecules deposited on the basal surface shows greater thermal stability. DFT analyses show these larger reduced pyridinium derivatives promote n-type doping on the basal plane due to a built-in electric field, which can be systematically tuned to induce a switching effect, opening and closing a bandgap and providing a fundamental means of driving electronic instabilities needed for emulating neuronal functionality.

Elucidating the Role of Dissolved Organic Matter and Sunlight in Mediating the Formation of Ag-Au Bimetallic Alloy Nanoparticles in the Aquatic Environment.

Elucidating the interactions between metal ions and dissolved organic matter and deciphering mechanisms for their mineralization in the aquatic environment are central to understanding the speciation, transport, and toxicity of nanoparticles (NPs). Herein, we examine the interactions between Ag+ and Au3+ ions in mixed solutions (χAg = 0.2, 0.5, and 0.8) in the presence of humic acids (HAs) under simulated sunlight; these conditions result in the formation of bimetallic Ag-Au NPs. A key distinction is that the obtained alloy NPs are compositionally and morphologically rather different from NPs obtained from thermally activated dark processes. Photoillumination triggers a distinctive plasmon-mediated process for HA-assisted reductive mineralization of ions to bimetallic alloy NPs which is not observed in its dark thermal reduction counterpart. The initial nucleation of bimetallic NPs is dominated by differences in the cohesive energies of Ag and Au crystal lattices, whereas the growth mechanisms are governed by the strongly preferred incorporation of Ag ions, which stems from their greater photoreactivity. The bimetallic NPs crystallize in shapes governed by the countervailing influence of minimizing free energy through the adoption of Wulff constructions and the energetic penalties associated with twin faults. As such, assessments of the stability and the potential toxic effects of bimetallic NPs arising from their possible existence in aquatic environments will depend sensitively on the origins of their formation.

Halide Replacement with Complete Preservation of Crystal Lattice in Mixed-Anion Lanthanide Oxyhalides.

A challenge in anion control in periodic solids is to preserve the crystal lattice while substituting for different anions of widely varying size and hardness. Post-synthetic modification routes that place cations or anions in non-equilibrium configurations are promising; however, such methods remain relatively unexplored for anion placement. Here, we report the synthesis of LaOI nanocrystals by a non-hydrolytic sol-gel condensation reaction and their transformation into LaOBr, LaOCl, and LaOF nanocrystals along hard-soft acid-base principles using post-synthetic metathesis reactions with ammonium halides. Anion displacement proceeds along halide planes, preserving the tetragonal matlockite structure. Energy-variant X-ray excited optical luminesce signatures of alloyed Tb3+ -ions is a sensitive quantum reporter of the preservation of the cation sublattice and hardening of the crystal structure upon anion replacement.

Atomic Hourglass and Thermometer Based on Diffusion of a Mobile Dopant in VO2.

Transformations between different atomic configurations of a material oftentimes bring about dramatic changes in functional properties as a result of the simultaneous alteration of both atomistic and electronic structure. Transformation barriers between polytypes can be tuned through compositional modification, generally in an immutable manner. Continuous, stimulus-driven modulation of phase stabilities remains a significant challenge. Utilizing the metal-insulator transition of VO2, we exemplify that mobile dopants weakly coupled to the crystal lattice provide a means of imbuing a reversible and dynamical modulation of the phase transformation. Remarkably, we observe a time- and temperature-dependent evolution of the relative phase stabilities of the M1 and R phases of VO2 in an "hourglass" fashion through the relaxation of interstitial boron species, corresponding to a 50 °C modulation of the transition temperature achieved within the same compound. The material functions as both a chronometer and a thermometer and is "reset" by the phase transition. Materials possessing memory of thermal history hold promise for applications such as neuromorphic computing, atomic clocks, thermometry, and sensing.

The Middle Road Less Taken: Electronic-Structure-Inspired Design of Hybrid Photocatalytic Platforms for Solar Fuel Generation.

The development of efficient solar energy conversion to augment other renewable energy approaches is one of the grand challenges of our time. Water splitting, or the disproportionation of H2O into energy-dense fuels, H2 and O2, is undoubtedly a promising strategy. Solar water splitting involves the concerted transfer of four electrons and four protons, which requires the synergistic operation of solar light harvesting, charge separation, mass and charge transport, and redox catalysis processes. It is unlikely that individual materials can mediate the entire sequence of charge and mass transport as well as energy conversion processes necessary for photocatalytic water splitting. An alternative approach, emulating the functioning of photosynthetic systems, involves the utilization of hybrid systems wherein different components perform the various functions required for solar water splitting. The design of such hybrid systems requires the multiple components to operate in lockstep with optimal thermodynamic driving forces and interfacial charge transfer kinetics. This Account describes a new class of nanoscale heterostructures comprising M xV2O5 nanowires, where M is a p-block cation with a ( n - 1) d10 ns2 np0 electronic configuration characterized by a stereoactive lone pair of electrons and x is its stoichiometry, interfaced with II-VI semiconductor quantum dots (QDs). Photocatalytic water splitting involves the transfer of excited-state holes from QDs to mid-gap states (derived from the stereoactive lone pairs of p-block cations) of nanowires, hole transport through nanowires, the reduction of protons at a QD-immobilized catalyst, and water oxidation at an anode. The M xV2O5/QD architectures provide a vast design space for evolutionary optimization of function with considerable tunability of composition and structure of the individual components as well as of the interfacial structure, thereby facilitating programmability of absorption spectra, energetic offsets, and charge-transfer reactivity. The available design space spans choice of the p-block cation M, its stoichiometry x, the composition and size of various QDs, and the nature of the nanowire/QD interface. This multivariate parameter space has been navigated by integrating first-principles modeling, diversified synthesis, spectroscopic measurements, and catalytic evaluation to facilitate the rational design of several generations of heterostructures and the systematic improvement of their photocatalytic performance. The electronic structures of the target heterostructures are predicted by DFT calculations in light of the revised lone pair model of stereoactive structural distortions and evaluated by hard X-ray photoelectron spectroscopy such as to systematically tune the interfacial band offsets. Central to this approach is the development of a topochemical "etch-a-sketch" intercalation approach that allows for facile installation of p-block cations in metastable polymorphs of V2O5. The interfacial charge transfer kinetics of M xV2O5/QD heterostructures is further evaluated by transient absorption spectroscopy to measure excited-state charge-transfer dynamics and is found to depend sensitively on interfacial structure and the thermodynamic driving forces in accordance with Marcus theory. The integration of theory and experiment has allowed for the design of viable photocatalytic architectures exemplified by the exceptional catalytic performance of ß-Pb xV2O5/CdX (X= S, Se) architectures, which has subsequently been elaborated to other lone-pair M xV2O5 compounds, demonstrating the effective exploitation of the opportunities for programmability available in the design space.

Optical modulation in hybrid antiresonant hollow-core fiber infiltrated with vanadium dioxide phase change nanocrystals.

We present a study of optical modulation by the effect of temperature-induced insulator-to-metal phase transition of vanadium dioxide (VO2) nanocrystals deposited in an antiresonance hollow-core fiber (AR-HCF). We fabricate such a VO2-coated fiber by embedding alkylsilane functionalized VO2 nanocrystals into the air holes of an AR-HCF. With this fiber, we achieve an optical loss modulation of ∼60% at a temperature above ∼53∘C over an ultrabroad spectral range that encompasses the S+C+L band.

An Atomic View of Cation Diffusion Pathways from Single-Crystal Topochemical Transformations.

The diffusion pathways of Li-ions as they traverse cathode structures in the course of insertion reactions underpin many questions fundamental to the functionality of Li-ion batteries. Much current knowledge derives from computational models or the imaging of lithiation behavior at larger length scales; however, it remains difficult to experimentally image Li-ion diffusion at the atomistic level. Here, by using topochemical Li-ion insertion and extraction to induce single-crystal-to-single-crystal transformations in a tunnel-structured V2 O5 polymorph, coupled with operando powder X-ray diffraction, we leverage single-crystal X-ray diffraction to identify the sequence of lattice interstitial sites preferred by Li-ions to high depths of discharge, and use electron density maps to create a snapshot of ion diffusion in a metastable phase. Our methods enable the atomistic imaging of Li-ions in this cathode material in kinetic states and provide an experimentally validated angstrom-level 3D picture of atomic pathways thus far only conjectured through DFT calculations.

Type-II heterostructures of α-V2O5 nanowires interfaced with cadmium chalcogenide quantum dots: Programmable energetic offsets, ultrafast charge transfer, and photocatalytic hydrogen evolution.

We synthesized a new class of heterostructures by depositing CdS, CdSe, or CdTe quantum dots (QDs) onto α-V2O5 nanowires (NWs) via either successive ionic layer adsorption and reaction (SILAR) or linker-assisted assembly (LAA). SILAR yielded the highest loadings of QDs per NW, whereas LAA enabled better control over the size and properties of QDs. Soft and hard x-ray photoelectron spectroscopy in conjunction with density functional theory calculations revealed that all α-V2O5/QD heterostructures exhibited Type-II band offset energetics, with a staggered gap where the conduction- and valence-band edges of α-V2O5 NWs lie at lower energies (relative to the vacuum level) than their QD counterparts. Transient absorption spectroscopy measurements revealed that the Type-II energetic offsets promoted the ultrafast (10-12-10-11 s) separation of photogenerated electrons and holes across the NW/QD interface to yield long-lived (10-6 s) charge-separated states. Charge-transfer dynamics and charge-recombination time scales varied subtly with the composition of heterostructures and the nature of the NW/QD interface, with both charge separation and recombination occurring more rapidly within SILAR-derived heterostructures. LAA-derived α-V2O5/CdSe heterostructures promoted the photocatalytic reduction of aqueous protons to H2 with a 20-fold or greater enhancement relative to isolated colloidal CdSe QDs or dispersed α-V2O5 NWs. The separation of photoexcited electrons and holes across the NW/QD interface could thus be exploited in redox photocatalysis. In light of their programmable compositions and properties and their Type-II energetics that drive ultrafast charge separation, the α-V2O5/QD heterostructures are a promising new class of photocatalyst architectures ripe for continued exploration.

Hole Extraction by Design in Photocatalytic Architectures Interfacing CdSe Quantum Dots with Topochemically Stabilized Tin Vanadium Oxide.

Tackling the complex challenge of harvesting solar energy to generate energy-dense fuels such as hydrogen requires the design of photocatalytic nanoarchitectures interfacing components that synergistically mediate a closely interlinked sequence of light-harvesting, charge separation, charge/mass transport, and catalytic processes. The design of such architectures requires careful consideration of both thermodynamic offsets and interfacial charge-transfer kinetics to ensure long-lived charge carriers that can be delivered at low overpotentials to the appropriate catalytic sites while mitigating parasitic reactions such as photocorrosion. Here we detail the theory-guided design and synthesis of nanowire/quantum dot heterostructures with interfacial electronic structure specifically tailored to promote light-induced charge separation and photocatalytic proton reduction. Topochemical synthesis yields a metastable ß-Sn0.23V2O5 compound exhibiting Sn 5s-derived midgap states ideally positioned to extract photogenerated holes from interfaced CdSe quantum dots. The existence of these midgap states near the upper edge of the valence band (VB) has been confirmed, and ß-Sn0.23V2O5/CdSe heterostructures have been shown to exhibit a 0 eV midgap state-VB offset, which underpins ultrafast subpicosecond hole transfer. The ß-Sn0.23V2O5/CdSe heterostructures are further shown to be viable photocatalytic architectures capable of efficacious hydrogen evolution. The results of this study underscore the criticality of precisely tailoring the electronic structure of semiconductor components to effect rapid charge separation necessary for photocatalysis.

Ligand-Mediated Control of Dopant Oxidation State and X-ray Excited Optical Luminescence in Eu-Doped LaOCl.

The development of an expanded palette of X-ray phosphors is a critical imperative for applications in medical imaging, radiation detection, and scientific instrumentation. The rational design of X-ray phosphors has been stymied by the absence of fundamental understanding of activation channels, sensitization mechanisms, and recombination pathways induced upon high-energy excitation of luminescent centers. In this article, we describe the preparation of Eu-doped LaOCl nanocrystals based on the condensation of molecular precursors. The synthetic route allows for control of the oxidation state of the incorporated Eu-atoms based on ligand-induced oxidation or reduction of the Eu-precursors. Nanocrystals exhibiting blue and red X-ray excited optical luminescence are developed by tuning the oxidation state of europium ions incorporated within the LaOCl nanocrystal matrix. Pronounced modulation of the intensity of the optical luminescence is evidenced at and near the giant resonance absorption of the host matrix as a result of distinctly divergent recombination channels. Resonant excitation results in recombination via Auger electron ionization and relaxation of a single electron-hole pair, whereas excitation away from the giant resonance results in thermalization of "hot" electron-hole pairs, while launching cascades of energy transfer, excitation, and radiative recombination events at the Eu-luminescent centers. Mechanistic elucidation and the development of a generalizable synthetic route starting from molecular precursors paves the way to an expanded palette of X-ray phosphors.

Stabilization of Ag-Au Bimetallic Nanocrystals in Aquatic Environments Mediated by Dissolved Organic Matter: A Mechanistic Perspective.

Gold and silver nanoparticles can be stabilized endogenously within aquatic environments from dissolved ionic species as a result of mineralization induced by dissolved organic matter. However, the ability of fulvic and humic acids to stabilize bimetallic nanoparticles is entirely unexplored. Elucidating the formation of such particles is imperative given their potential ecological toxicity. Herein, we demonstrate the nucleation, growth, and stabilization of bimetallic Ag-Au nanocrystals from the interactions of Ag+ and Au3+ with Suwannee River fulvic and humic acids. The mechanisms underpinning the stabilization of Ag-Au alloy NPs at different pH (6.0-9.0) values are studied by UV-vis spectrophotometry, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). Complexation of free Ag+ and Au3+ ions with the Lewis basic groups (carbonyls, carboxyls, and thiols) of FA and HA, followed by electron-transfer from redox-active moieties present in dissolved organic matter initiates the nucleation of the NPs. Alloy formation and interdiffusion of Au and Ag atoms are further facilitated by a galvanic replacement reaction between AuCl4- and Ag. Charge-transfer from Au to Ag stabilizes the formed bimetallic NPs. A more pronounced agglomeration of the Ag-Au NPs is observed when HA is used compared to FA as the reducing agent. The bimetallic NPs are stable for greater than four months, which suggests the possible persistence and dispersion of these materials in aquatic environments. The mechanistic ideas have broad generalizability to reductive mineralization processes mediated by dissolved organic matter.

Structure-Induced Switching of the Band Gap, Charge Order, and Correlation Strength in Ternary Vanadium Oxide Bronzes.

Recently, V2 O5 nanowires have been synthesized as several different polymorphs, and as correlated bronzes with cations intercalated between the layers of edge- and corner- sharing VO6 octahedra. Unlike extended crystals, which tend to be plagued by substantial local variations in stoichiometry, nanowires of correlated bronzes exhibit precise charge ordering, thereby giving rise to pronounced electron correlation effects. These developments have greatly broadened the scope of research, and promise applications in several frontier electronic devices that make use of novel computing vectors. Here a study is presented of δ-Srx V2 O5 , expanded δ-Srx V2 O5 , exfoliated δ-Srx V2 O5 and δ-Kx V2 O5 using a combination of synchrotron soft X-ray spectroscopy and density functional theory calculations. The band gaps of each system are experimentally determined, and their calculated electronic structures are discussed from the perspective of the measured spectra. Band gaps ranging from 0.66 ± 0.20 to 2.32 ± 0.20 eV are found, and linked to the underlying structure of each material. This demonstrates that the band gap of V2 O5 can be tuned across a large portion of the range of greatest interest for device applications. The potential for metal-insulator transitions, tuneable electron correlations and charge ordering in these systems is discussed within the framework of our measurements and calculations, while highlighting the structure-property relationships that underpin them.

Fabrication and Electrochemical Performance of Structured Mesoscale Open Shell V2O5 Networks.

Crystalline vanadium pentoxide (V2O5) has attracted significant interest as a potential cathode material for energy storage applications due to its high theoretical capacity. Unfortunately, the material suffers from low conductivity as well as slow lithium ion diffusion, both of which affect how fast the electrode can be charged/discharged and how many times it can be cycled. Colloidal crystal templating (CCT) provides a simple approach to create well-organized 3-D nanostructures of materials, resulting in a significant increase in surface area that can lead to marked improvements in electrochemical performance. Here, a single layer of open shell V2O5 architectures ca. 1 µm in height with ca. 100 nm wall thickness was fabricated using CCT, and the electrochemical properties of these assemblies were evaluated. A decrease in polarization effects, resulting from the higher surface area mesostructured features, was found to produce significantly enhanced electrochemical performance. The discharge capacity of an unpatterned thin film of V2O5 (∼8.1 µAh/cm2) was found to increase to ∼10.2 µAh/cm2 when the material was patterned by CCT, affording enhanced charge storage capabilities as well as a decrease in the irreversible degradation during charge-discharge cycling. This work demonstrates the importance of creating mesoscale electrode surfaces for improving the performance of energy storage devices and provides fundamental understanding of the means to improve device performance.

Contrasting 1D tunnel-structured and 2D layered polymorphs of V2O5: relating crystal structure and bonding to band gaps and electronic structure.

New V2O5 polymorphs have risen to prominence as a result of their open framework structures, cation intercalation properties, tunable electronic structures, and wide range of applications. The application of these materials and the design of new, useful polymorphs requires understanding their defining structure-property relationships. We present a characterization of the band gap and electronic structure of nanowires of the novel ζ-phase and the orthorhombic α-phase of V2O5 using X-ray spectroscopy and density functional theory calculations. The band gap is found to decrease from 1.90 ± 0.20 eV in the α-phase to 1.50 ± 0.20 eV in the ζ-phase, accompanied by the loss of the α-phase's characteristic split-off dxy band in the ζ-phase. States of dxy origin continue to dominate the conduction band edge in the new polymorph but the inequivalence of the vanadium atoms and the increased local symmetry of [VO6] octahedra results in these states overlapping with the rest of the V 3d conduction band. ζ-V2O5 exhibits anisotropic conductivity along the b direction, defining a 1D tunnel, in contrast to α-V2O5 where the anisotropic conductivity is along the ab layers. We explain the structural origins of the differences in electronic properties that exist between the α- and ζ-phase.

Atomic Origins of Monoclinic-Tetragonal (Rutile) Phase Transition in Doped VO2 Nanowires.

There has been long-standing interest in tuning the metal-insulator phase transition in vanadium dioxide (VO2) via the addition of chemical dopants. However, the underlying mechanisms by which doping elements regulate the phase transition in VO2 are poorly understood. Taking advantage of aberration-corrected scanning transmission electron microscopy, we reveal the atomistic origins by which tungsten (W) dopants influence the phase transition in single crystalline WxV1-xO2 nanowires. Our atomically resolved strain maps clearly show the localized strain normal to the (122̅) lattice planes of the low W-doped monoclinic structure (insulator). These strain maps demonstrate how anisotropic localized stress created by dopants in the monoclinic structure accelerates the phase transition and lead to relaxation of structure in tetragonal form. In contrast, the strain distribution in the high W-doped VO2 structure is relatively uniform as a result of transition to tetragonal (metallic) phase. The directional strain gradients are furthermore corroborated by density functional theory calculations that show the energetic consequences of distortions to the local structure. These findings pave the roadmap for lattice-stress engineering of the MIT behavior in strongly correlated materials for specific applications such as ultrafast electronic switches and electro-optical sensors.