30 research outputs found
Evaluation of Multivalent Cation Insertion in Single- and Double-Layered Polymorphs of V<sub>2</sub>O<sub>5</sub>
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 Earthâs
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 Îą-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 VâO 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, Îłâ˛, δâ˛, and Ďâ˛
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
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 electronâhole 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 electronâhole
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 MetalâInsulator Transitions of V<sub>1â<i>x</i></sub>Mo<sub><i>x</i></sub>O<sub>2</sub> Nanowires: Mechanistic Insights from an X-ray Absorption Spectroscopy Study
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 metalâinsulator
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 3dâO 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
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
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
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 solâgel 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
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 BoĚhr 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 430â520 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
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/polyÂ(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
For
solar energy conversion, not only must a semiconductor absorb
incident solar radiation efficiently but also its photoexcited electronî¸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 âź2 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 (1â3 Îźs) 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 Îźs);
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
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>, Îś-
and Îľ-V<sub>2</sub>O<sub>5</sub>, with the relevant parameters
for the thermodynamically stable Îą-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 Îś-V<sub>2</sub>O<sub>5</sub> and Îľ-V<sub>2</sub>O<sub>5</sub> are 0.62â0.86 and
0.21â0.24 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 âfrustratedâ coordination
environments that are known to be useful for realizing low diffusion
barriers