9 research outputs found
Direct Investigation of Mg Intercalation into the Orthorhombic V<sub>2</sub>O<sub>5</sub> Cathode Using Atomic-Resolution Transmission Electron Microscopy
Batteries
based on Mg metal anode can promise much higher specific
volumetric capacity and energy density compared to Li-ion systems
and are, at the same time, safer and more cost-effective. While previous
experimental reports have claimed reversible Mg intercalation into
beyond Chevrel phase cathodes, they provide limited evidence of true
Mg intercalation other than electrochemical data. Transmission electron
microscopy techniques provide unique capabilities to directly image
Mg intercalation and quantify the redox reaction within the cathode
material. Here, we present a systematic study of Mg insertion into
orthorhombic V<sub>2</sub>O<sub>5</sub>, combining aberration-corrected
scanning transmission electron microscopy (STEM) imaging, electron
energy-loss spectroscopy (EELS), and energy-dispersive X-ray spectroscopy
(EDX) analysis. We compare the results from an electrochemically cycled
V<sub>2</sub>O<sub>5</sub> cathode in a prospective full cell with
Mg metal anode with a chemically synthesized MgV<sub>2</sub>O<sub>5</sub> sample. Results suggest that the electrochemically cycled
orthorhombic V<sub>2</sub>O<sub>5</sub> cathode shows a local formation
of the theoretically predicted ϵ-Mg<sub>0.5</sub>V<sub>2</sub>O<sub>5</sub> phase; however, the intercalation levels of Mg are
lower than predicted. This phase is different from the chemically
synthesized sample, which is found to represent the δ-MgV<sub>2</sub>O<sub>5</sub> phase
Polarization-Induced pn Diodes in Wide-Band-Gap Nanowires with Ultraviolet Electroluminescence
Almost all electronic devices utilize a pn junction formed
by random doping of donor and acceptor impurity atoms. We developed
a fundamentally new type of pn junction not formed by impurity-doping,
but rather by grading the composition of a semiconductor nanowire
resulting in alternating p and n conducting regions due to polarization
charge. By linearly grading AlGaN nanowires from 0% to 100% and back
to 0% Al, we show the formation of a polarization-induced pn junction
even in the absence of any impurity doping. Since electrons and holes
are injected from AlN barriers into quantum disk active regions, graded
nanowires allow deep ultraviolet LEDs across the AlGaN band-gap range
with electroluminescence observed from 3.4 to 5 eV. Polarization-induced
p-type conductivity in nanowires is shown to be possible even without
supplemental acceptor doping, demonstrating the advantage of polarization
engineering in nanowires compared with planar films and providing
a strategy for improving conductivity in wide-band-gap semiconductors.
As polarization charge is uniform within each unit cell, polarization-induced
conductivity without impurity doping provides a solution to the problem
of conductivity uniformity in nanowires and nanoelectronics and opens
a new field of polarization engineering in nanostructures that may
be applied to other polar semiconductors
Mixed Polarity in Polarization-Induced p–n Junction Nanowire Light-Emitting Diodes
Polarization-induced nanowire light
emitting diodes (PINLEDs) are
fabricated by grading the Al composition along the c-direction of
AlGaN nanowires grown on Si substrates by plasma-assisted molecular
beam epitaxy (PAMBE). Polarization-induced charge develops with a
sign that depends on the direction of the Al composition gradient
with respect to the [0001] direction. By grading from GaN to AlN then
back to GaN, a polarization-induced p–n junction is formed.
The orientation of the p-type and n-type sections depends on the material
polarity of the nanowire (i.e., Ga-face or N-face). Ga-face material
results in an n-type base and a p-type top, while N-face results in
the opposite. The present work examines the polarity of catalyst-free
nanowires using multiple methods: scanning transmission electron microscopy
(STEM), selective etching, conductive atomic force microscopy (C-AFM),
and electroluminescence (EL) spectroscopy. Selective etching and STEM
measurements taken in annular bright field (ABF) mode demonstrate
that the preferred orientation for catalyst-free nanowires grown by
PAMBE is N-face, with roughly 10% showing Ga-face orientation. C-AFM
and EL spectroscopy allow electrical and optical differentiation of
the material polarity in PINLEDs since the forward bias direction
depends on the p–n junction orientation and therefore on nanowire
polarity. Specifically, C-AFM reveals that the direction of forward
bias for individual nanowire LEDs changes with the polarity, as expected,
due to reversal of the sign of the polarization-induced charge. Electroluminescence
measurements of mixed polarity PINLEDs wired in parallel show ambipolar
emission due to the mixture of p–n and n–p oriented
PINLEDs. These results show that, if catalyst-free III-nitride nanowires
are to be used to form polarization-doped heterostructures, then it
is imperative to understand their mixed polarity and to design devices
using these nanowires accordingly
Synthesis and Characterization of MgCr<sub>2</sub>S<sub>4</sub> Thiospinel as a Potential Magnesium Cathode
Magnesium-ion batteries
are a promising energy storage technology because of their higher
theoretical energy density and lower cost of raw materials. Among
the major challenges has been the identification of cathode materials
that demonstrate capacities and voltages similar to lithium-ion systems.
Thiospinels represent an attractive choice for new Mg-ion cathode
materials owing to their interconnected diffusion pathways and demonstrated
high cation mobility in numerous systems. Reported magnesium thiospinels,
however, contain redox inactive metals such as scandium or indium,
or have low voltages, such as MgTi<sub>2</sub>S<sub>4</sub>. This
article describes the direct synthesis and structural and electrochemical
characterization of MgCr<sub>2</sub>S<sub>4</sub>, a new thiospinel
containing the redox active metal chromium and discusses its physical
properties and potential as a magnesium battery cathode. However,
as chromiumÂ(III) is quite stable against oxidation in sulfides, removing
magnesium from the material remains a significant challenge. Early
attempts at both chemical and electrochemical demagnesiation are discussed
Pharmaceutical Formulation Facilities as Sources of Opioids and Other Pharmaceuticals to Wastewater Treatment Plant Effluents
Facilities involved in the manufacture of pharmaceutical products are an under-investigated source of pharmaceuticals to the environment. Between 2004 and 2009, 35 to 38 effluent samples were collected from each of three wastewater treatment plants (WWTPs) in New York and analyzed for seven pharmaceuticals including opioids and muscle relaxants. Two WWTPs (NY2 and NY3) receive substantial flows (>20% of plant flow) from pharmaceutical formulation facilities (PFF) and one (NY1) receives no PFF flow. Samples of effluents from 23 WWTPs across the United States were analyzed once for these pharmaceuticals as part of a national survey. Maximum pharmaceutical effluent concentrations for the national survey and NY1 effluent samples were generally <1 μg/L. Four pharmaceuticals (methadone, oxycodone, butalbital, and metaxalone) in samples of NY3 effluent had median concentrations ranging from 3.4 to >400 μg/L. Maximum concentrations of oxycodone (1700 μg/L) and metaxalone (3800 μg/L) in samples from NY3 effluent exceeded 1000 μg/L. Three pharmaceuticals (butalbital, carisoprodol, and oxycodone) in samples of NY2 effluent had median concentrations ranging from 2 to 11 μg/L. These findings suggest that current manufacturing practices at these PFFs can result in pharmaceuticals concentrations from 10 to 1000 times higher than those typically found in WWTP effluents
Reversible Modulation of Orbital Occupations via an Interface-Induced Polar State in Metallic Manganites
The
breaking of orbital degeneracy on a transition metal cation
and the resulting unequal electronic occupations of these orbitals
provide a powerful lever over electron density and spin ordering in
metal oxides. Here, we use ab initio calculations to show that reversibly
modulating the orbital populations on Mn atoms can be achieved at
ferroelectric/manganite interfaces by the presence of ferroelectric
polarization on the nanoscale. The change in orbital occupation can
be as large as 10%, greatly exceeding that of bulk manganites. This
reversible orbital splitting is in large part controlled by the propagation
of ferroelectric polar displacements into the interfacial region,
a structural motif absent in the bulk and unique to the interface.
We use epitaxial thin film growth and scanning transmission electron
microscopy to verify this key interfacial polar distortion and discuss
the potential of reversible control of orbital polarization via nanoscale
ferroelectrics
Electrochemical Reduction of a Spinel-Type Manganese Oxide Cathode in Aqueous Electrolytes with Ca<sup>2+</sup> or Zn<sup>2+</sup>
In
this report, the feasibility of reversible Ca<sup>2+</sup> or
Zn<sup>2+</sup> intercalation into a crystalline cubic spinel Mn<sub>2</sub>O<sub>4</sub> cathode has been investigated using electrochemical
methods in an aqueous electrolyte. A combination of synchrotron XRD
and XANES studies identified the partial structural transformation
from a cubic to a tetragonally distorted spinel Mn<sub>3</sub>O<sub>4</sub>, accompanied by the reduction of Mn<sup>4+</sup> to Mn<sup>3+</sup> and Mn<sup>2+</sup> during discharge. TEM/EDX measurements
confirmed that practically no Ca<sup>2+</sup> was inserted upon discharge.
However, non-negligible amounts of Zn were detected after Mn<sub>2</sub>O<sub>4</sub> was reduced in the Zn<sup>2+</sup> electrolyte, but
through the formation of secondary phases that, in some cases, appeared
adjacent to the surface of a cathode particle. This report aims to
identify bottlenecks in the application of manganese oxide cathodes
paired with Ca or Zn metal anodes and to justify future efforts in
designing prototype multivalent batteries
Atomic Origins of Monoclinic-Tetragonal (Rutile) Phase Transition in Doped VO<sub>2</sub> Nanowires
There has been long-standing interest
in tuning the metal–insulator phase transition in vanadium
dioxide (VO<sub>2</sub>) via the addition of chemical dopants. However,
the underlying mechanisms by which doping elements regulate the phase
transition in VO<sub>2</sub> 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 W<sub><i>x</i></sub>V<sub>1–<i>x</i></sub>O<sub>2</sub> nanowires.
Our atomically resolved strain maps clearly show the localized strain
normal to the (122Ì…) 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 VO<sub>2</sub> 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
Mechanism of Zn Insertion into Nanostructured δ‑MnO<sub>2</sub>: A Nonaqueous Rechargeable Zn Metal Battery
Unlike
the more established lithium-ion based energy storage chemistries,
the complex intercalation chemistry of multivalent cations in a host
lattice is not well understood, especially the relationship between
the intercalating species solution chemistry and the prevalence and
type of side reactions. Among multivalent metals, a promising model
system can be based on nonaqueous Zn<sup>2+</sup> ion chemistry. Several
examples of these systems support the use of a Zn metal anode, and
reversible intercalation cathodes have been reported. This study utilizes
a combination of analytical tools to probe the chemistry of a nanostructured
δ-MnO<sub>2</sub> cathode in association with a nonaqueous acetonitrile–ZnÂ(TFSI)<sub>2</sub> electrolyte and a Zn metal anode. As many of the issues related
to understanding a multivalent battery relate to the electrolyte–electrode
interface, the high surface area of a nanostructured cathode provides
a significant interface between the electrolyte and cathode host that
maximizes the spectroscopic signal of any side reactions or minor
mechanistic pathways. Numerous factors affecting capacity fade and
issues associated with the second phase formation including Mn dissolution
in heavily cycled Zn/δ-MnO<sub>2</sub> cells are presented including
dramatic mechanistic differences in the storage mechanism of this
couple when compared to similar aqueous electrolytes are noted