30 research outputs found

    Evaluation of Multivalent Cation Insertion in Single- and Double-Layered Polymorphs of V<sub>2</sub>O<sub>5</sub>

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    Multivalent intercalation batteries have the potential to circumvent several fundamental limitations of reigning Li-ion technologies. Such batteries will potentially deliver high volumetric energy densities, be safer to operate, and rely on materials that are much more abundant than Li in the Earth’s crust. The design of intercalation cathodes for such batteries requires consideration of thermodynamic aspects such as structural distortions and energetics as well as kinetic aspects such as barriers to the diffusion of cations. The layered α-V<sub>2</sub>O<sub>5</sub> system is a canonical intercalation host for Li-ions but does not perform nearly as well for multivalent cation insertion. However, the rich V–O phase diagram provides access to numerous metastable polymorphs that hold much greater promise for multivalent cation intercalation. In this article, we explore multivalent cation insertion in three metastable polymorphs, γ′, δ′, and ρ′ phases of V<sub>2</sub>O<sub>5</sub>, using density functional theory calculations. The calculations allow for evaluation of the influence of distinctive structural motifs in mediating multivalent cation insertion. In particular, we contrast the influence of single versus condensed double layers, planar versus puckered single layers, and the specific stacking sequence of the double layers. We demonstrate that metastable phases offer some specific advantages with respect to thermodynamically stable polymorphs in terms of a higher chemical potential difference (giving rise to a larger open-circuit voltage) and in providing access to diffusion pathways that are highly dependent on the specific structural motif. The three polymorphs are found to be especially promising for Ca-ion intercalation, which is particularly significant given the exceedingly sparse number of viable cathode materials for this chemistry. The findings here demonstrate the ability to define cation diffusion pathways within layered metastable polymorphs by alteration of the stacking sequence or the thickness of the layers

    Type-II CuFe<sub>2</sub>O<sub>4</sub>/Graphitic Carbon Nitride Heterojunctions for High-Efficiency Photocatalytic and Electrocatalytic Hydrogen Generation

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    Solar water splitting has emerged as an urgent imperative as hydrogen emerges as an increasingly important form of energy storage. g-C3N4 is an ideal candidate for photocatalytic water splitting as a result of the excellent alignment of its band edges with water redox potentials. To mitigate electron–hole recombination that has limited the performance of g-C3N4, we have developed a semiconductor heterostructure of g-C3N4 with CuFe2O4 nanoparticles (NPs) as a highly efficient photocatalyst. Visible-light-driven photocatalytic properties of CuFe2O4/g-C3N4 heterostructures with different CuFe2O4 loadings have been examined with two sacrificial agents. An up to 2.5-fold enhancement in catalytic efficiency is observed for CuFe2O4/g-C3N4 heterostructures over g-C3N4 nanosheets alone with the apparent quantum yield of H2 production approaching 25%. The improved photocatalytic activity of the heterostructures suggests that introducing CuFe2O4 NPs provides more active sites and reduces electron–hole recombination. The g-C3N4/CuFe2O4 heterostructures furthermore show enhanced electrocatalytic HER activity as compared to the individual components as a result of which by making heterostructures g-C3N4 with CuFe2O4 increased the active catalytic surface for the electrocatalytic water splitting reaction. The enhanced faradaic efficiency of the prepared heterostructures makes it a potential candidate for efficient hydrogen generation. Nevertheless, the designed heterostructure materials exhibited significant photo- and electrocatalytic activity toward the HER, which demonstrates a method for methodically enhancing catalytic performance by creating heterostructures with the best energetic offsets

    Elucidating the Influence of Local Structure Perturbations on the Metal–Insulator Transitions of V<sub>1–<i>x</i></sub>Mo<sub><i>x</i></sub>O<sub>2</sub> Nanowires: Mechanistic Insights from an X-ray Absorption Spectroscopy Study

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    The substitutional doping of Mo within VO<sub>2</sub> substantially alters the electronic and structural phase diagrams of the host lattice, most notably by bringing the technologically relevant metal–insulator phase transition temperature in closer proximity to room temperature. Here, we have used X-ray absorption fine structure (XAFS) spectroscopy at V and Mo K-edges to examine the local electronic and geometric structure of both the dopant atoms and the host lattice. A nominal Mo oxidation state of +5 has been determined, which implies electron doping of the VO<sub>2</sub> band structure. In addition, XAFS studies suggest that doping with Mo creates locally symmetric domains that are more akin to the high-temperature rutile phase of VO<sub>2</sub>, thereby lowering activation energy barriers for structural transformation to the metallic phase. Substantial rectification of octahedral canting is also observed upon Mo doping, which has the effect of decreasing V 3d–O 2p hybridization and likely assists in closing the characteristic band gap of the low-temperature monoclinic phase. A correlated set of cationic interactions is seen to emerge with increasing Mo doping, which can be ascribed to local dimerization along the rutile <i>c</i> axis as has been proposed to be a characteristic structural feature of a correlated metallic phase with intermediate mass

    Separation of Viscous Oil Emulsions Using Three-Dimensional Nanotetrapodal ZnO Membranes

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    The steam-assisted gravity-drainage (SAGD) method has emerged as among the leading methods of enhanced oil recovery and is predicated on the injection of steam within the wellbore followed by extraction of emulsions of viscous oil and water. The emulsions are stabilized by endogenous surfactants, necessitating extensive processing such as addition of chemical de-emulsifiers and slow gravity-based separation methods. Here, we show that a hierarchically textured membrane exhibiting orthogonal wettability, specifically, superoleophilic but superhydrophobic behavior, allows for effective separation of the water and viscous oil fractions of SAGD emulsions. The membrane is constructed by integrating ZnO nanotetrapods onto stainless steel meshes using a conformal amorphous SiO<sub>2</sub> layer and is both mechanically resilient and thermally robust. The intrinsic surface energy characteristics of the ZnO tetrapods as well as their three-dimensional texture when arrayed atop the stainless steel mesh substrates contribute to the observed differential wettability between water and oil. Water content in permeated bitumen is reduced to as low as 0.69 vol % through a single-pass filtration step with the further advantage of eliminating silt particles. The permeation temperature and water content are tunable based on modulation of the mesh size and ZnO loading. The membranes allow for operation at SAGD temperatures in excess of 130 °C, thereby enabling the thermal disruption of hierarchical emulsions. The membrane-based separation of SAGD emulsions under realistic process conditions paves the way for entirely new process designs for recovering dry viscous oil

    Defining Diffusion Pathways in Intercalation Cathode Materials: Some Lessons from V<sub>2</sub>O<sub>5</sub> on Directing Cation Traffic

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    The invention of rechargeable batteries has dramatically changed our landscapes and lives, underpinning the explosive worldwide growth of consumer electronics, ushering in an unprecedented era of electric vehicles, and potentially paving the way for a much greener energy future. Unfortunately, current battery technologies suffer from a number of challenges, e.g., capacity loss and failure upon prolonged cycling, limited ion diffusion kinetics, and a rather sparse palette of high-performing electrode materials. Here, we discuss the origins of diffusion limitations in oxide materials using V<sub>2</sub>O<sub>5</sub> as a model system. In particular, we discuss constrictions in ionic conduction pathways, narrow energy dispersion of conduction band states, and the stabilization and self-trapping of polarons as local phenomena that have substantial implications for introducing multiscale compositional and phase heterogeneities. Strategies for mitigating such limitations are discussed such as reducing diffusion path lengths and the design of metastable frameworks yielding frustrated coordination and decreased barriers for migration of polarons

    Ferroelastic Domain Organization and Precursor Control of Size in Solution-Grown Hafnium Dioxide Nanorods

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    We demonstrate that the degree of branching of the alkyl (R) chain in a Hf(OR)<sub>4</sub> precursor allows for control over the length of HfO<sub>2</sub> nanocrystals grown by homocondensation of the metal alkoxide with a metal halide. An extended nonhydrolytic sol–gel synthesis has been developed that enables the growth of high aspect ratio monoclinic HfO<sub>2</sub> nanorods that grow along the [100] direction. The solution-grown elongated HfO<sub>2</sub> nanorods show remarkable organization of twin domains separated by (100) coherent twin boundaries along the length of the nanowires in a morphology reminiscent of shape memory alloys. The sequence of finely structured twin domains each spanning only a few lattice planes originates from the Martensitic transformation of the nanorods from a tetragonal to a monoclinic structure upon cooling. Such ferroelastic domain organization is uncharacteristic of metal oxides and has not thus far been observed in bulk HfO<sub>2</sub>. The morphologies observed here suggest that, upon scaling to nanometer-sized dimensions, HfO<sub>2</sub> might exhibit mechanical properties entirely distinctive from the bulk

    Ligand-Mediated Modulation of Layer Thicknesses of Perovskite Methylammonium Lead Bromide Nanoplatelets

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    Organic metal halide perovskites have rapidly emerged as among the leading candidates for the next generation of photovoltaic and light-emitting devices. The band gap, exciton binding energy, and absorption cross-section of these materials are tunable to some extent by compositional variation. Dimensional confinement represents an attractive alternative to compositional variation for tuning these properties via quantum confinement close to the Böhr radius. While the stabilization of few-layered nanoplatelets of methylammonium lead bromide has recently been demonstrated, mechanistic understanding of synthetic parameters resulting in dimensional confinement remains to be developed. Here we show that the layer thickness can be precisely modulated as a function of the chain length and concentration of the added alkylammonium cations. Surface capping ligands bind preferentially to sheets of corner sharing PbBr<sub>6</sub> octahedra and thereby buffer the extent of supersaturation of monomeric units enabling precise modulation of the layer-by-layer growth of 2D nanoplatelets. Crystal growth can be confined to yield nanoplatelets with tunable unit cell thickness spanning between one and six layers, which allows for precise tuning of the emissive properties of 2D perovskite nanoplatelets in the range of 430–520 nm depending on the layer thickness. The results suggest a generalizable strategy for tuning the layer thickness of these materials as a function of the alkyl chain length of the ligands

    Nanostructured Magnesium Composite Coatings for Corrosion Protection of Low-Alloy Steels

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    Corrosion of base metals represents a tremendous problem that has spurred a global search for cost-effective and environmentally friendly alternatives to current corrosion-inhibiting technologies. In this work, we report a novel sustainable hybrid Mg/poly­(ether imide) (PEI) nanocomposite coating that provides corrosion protection to low-alloy steels at relatively low coating thicknesses and with reduced weight as compared to conventional metallic coatings. The coatings are constituted using Mg nanoplatelets dispersed within a polyamic acid matrix that is subsequently imidized on the steel substrate to form PEI. The coatings function through a combination of sacrificial cathodic protection (afforded by the preferential oxidation of the Mg nanoplatelets), anodic passivation through precipitation of corrosion products, and the inhibitive action of the PEI polymeric matrix. The use of nanostructured Mg allows for reduced coating thicknesses and a smoother surface finish, whereas the PEI matrix provides excellent adhesion to the metal surface. Based on potentiodynamic testing and prolonged exposure to saline environments, the novel coating materials significantly outperform galvanized Zn and Zn-rich primer coatings of comparable thickness

    Directional Charge Transfer Mediated by Mid-Gap States: A Transient Absorption Spectroscopy Study of CdSe Quantum Dot/β-Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub> Heterostructures

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    For solar energy conversion, not only must a semiconductor absorb incident solar radiation efficiently but also its photoexcited electronhole pairs must further be separated and transported across interfaces. Charge transfer across interfaces requires consideration of both thermodynamic driving forces as well as the competing kinetics of multiple possible transfer, cooling, and recombination pathways. In this work, we demonstrate a novel strategy for extracting holes from photoexcited CdSe quantum dots (QDs) based on interfacing with β-Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub> nanowires that have strategically positioned midgap states derived from the intercalating Pb<sup>2+</sup> ions. Unlike midgap states derived from defects or dopants, the states utilized here are derived from the intrinsic crystal structure and are thus homogeneously distributed across the material. CdSe/β-Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub> heterostructures were assembled using two distinct methods: successive ionic layer adsorption and reaction (SILAR) and linker-assisted assembly (LAA). Transient absorption spectroscopy measurements indicate that, for both types of heterostructures, photoexcitation of CdSe QDs was followed by the transfer of electrons to the conduction band of β-Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub> nanowires and holes to the midgap states of β-Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub> nanowires. Holes were transferred on time scales less than 1 ps, whereas electrons were transferred more slowly on time scales of ∼2 ps. In contrast, for analogous heterostructures consisting of CdSe QDs interfaced with V<sub>2</sub>O<sub>5</sub> nanowires (wherein midgap states are absent), only electron transfer was observed. Interestingly, electron transfer was readily achieved for CdSe QDs interfaced with V<sub>2</sub>O<sub>5</sub> nanowires by the SILAR method; however, for interfaces incorporating molecular linkers, electron transfer was observed only upon excitation at energies substantially greater than the bandgap absorption threshold of CdSe. Transient absorbance decay traces reveal longer excited-state lifetimes (1–3 μs) for CdSe/β-Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub> heterostructures relative to bare β-Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub> nanowires (0.2 to 0.6 μs); the difference is attributed to surface passivation of intrinsic surface defects in β-Pb<sub>0.33</sub>V<sub>2</sub>O<sub>5</sub> upon interfacing with CdSe

    Topochemically De-Intercalated Phases of V<sub>2</sub>O<sub>5</sub> as Cathode Materials for Multivalent Intercalation Batteries: A First-Principles Evaluation

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    The scarce inventory of compounds that allow for diffusion of multivalent cations at reasonable rates poses a major impediment to the development of multivalent intercalation batteries. Here, we contrast the thermodynamics and kinetics of the insertion of Li, Na, Mg, and Al ions in two synthetically accessible metastable phases of V<sub>2</sub>O<sub>5</sub>, ζ- and ε-V<sub>2</sub>O<sub>5</sub>, with the relevant parameters for the thermodynamically stable α-phase of V<sub>2</sub>O<sub>5</sub> using density functional theory calculations. The metastability of the frameworks results in a higher open circuit voltage for multivalent ions, exceeding 3 V for Mg-ion intercalation. Multivalent ions inserted within these structures encounter suboptimal coordination environments and expanded transition states, which facilitate easier ion diffusion. Specifically, a nudged elastic band examination of ion diffusion pathways suggests that migration barriers are substantially diminished for Na- and Mg-ion diffusion in the metastable polymorphs: the predicted migration barriers for Mg ions in ζ-V<sub>2</sub>O<sub>5</sub> and ε-V<sub>2</sub>O<sub>5</sub> are 0.62–0.86 and 0.21–0.24 eV, respectively. More generally, the results indicate that topochemically derived metastable polymorphs represent an interesting class of compounds for realizing multivalent cation diffusion because many such compounds place cations in “frustrated” coordination environments that are known to be useful for realizing low diffusion barriers
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