12 research outputs found
In Situ XPS Investigation of Transformations at Crystallographically Oriented MoS<sub>2</sub> Interfaces
Nanoscale
transition-metal dichalcogenide (TMDC) materials, such as MoS<sub>2</sub>, exhibit promising behavior in next-generation electronics
and energy-storage devices. TMDCs have a highly anisotropic crystal
structure, with edge sites and basal planes exhibiting different structural,
chemical, and electronic properties. In virtually all applications,
two-dimensional or bulk TMDCs must be interfaced with other materials
(such as electrical contacts in a transistor). The presence of edge
sites vs basal planes (i.e., the crystallographic orientation of the
TMDC) could influence the chemical and electronic properties of these
solid-state interfaces, but such effects are not well understood.
Here, we use in situ X-ray photoelectron spectroscopy (XPS) to investigate
how the crystallography and structure of MoS<sub>2</sub> influence
chemical transformations at solid-state interfaces with various other
materials. MoS<sub>2</sub> materials with controllably aligned crystal
structures (horizontal vs vertical orientation of basal planes) were
fabricated, and in situ XPS was carried out by sputter-depositing
three different materials (Li, Ge, and Ag) onto MoS<sub>2</sub> within
an XPS instrument while periodically collecting photoelectron spectra;
these deposited materials are of interest due to their application
in electronic devices or energy storage. The results showed that Li
reacts readily with both crystallographic orientations of MoS<sub>2</sub> to form metallic Mo and Li<sub>2</sub>S, while Ag showed
very little chemical or electronic interaction with either type of
MoS<sub>2</sub>. In contrast, Ge showed significant chemical interactions
with MoS<sub>2</sub> basal planes, but only minor chemical changes
were observed when Ge contacted MoS<sub>2</sub> edge sites. These
findings have implications for electronic transport and band alignment
at these interfaces, which is of significant interest for a variety
of applications
Seeded Nanowire and Microwire Growth from Lithium Alloys
Although
vapor–liquid–solid (VLS) growth of nanowires
from alloy seed particles is common in various semiconductor systems,
related wire growth in all-metal systems is rare. Here, we report
the spontaneous growth of nano- and microwires from metal seed particles
during the cooling of Li-rich bulk alloys containing Au, Ag, or In.
The as-grown wires feature Au-, Ag-, or In-rich metal tips and LiOH
shafts; the results indicate that the wires grow as Li metal and are
converted to polycrystalline LiOH during and/or after growth due to
exposure to H<sub>2</sub>O and O<sub>2</sub>. This new process is
a simple way to create nanostructures, and the findings suggest that
metal nanowire growth from alloy seeds is possible in a variety of
systems
Operando Synchrotron Measurement of Strain Evolution in Individual Alloying Anode Particles within Lithium Batteries
Alloying anode materials
offer high capacity for next-generation
batteries, but the performance of these materials often decays rapidly
with cycling because of volume changes and associated mechanical degradation
or fracture. The direct measurement of crystallographic strain evolution
in individual particles has not been reported, however, and this level
of insight is critical for designing mechanically resilient materials.
Here, we use operando X-ray diffraction to investigate strain evolution
in individual germanium microparticles during electrochemical reaction
with lithium. The diffraction peak was observed to shift in position
and diminish in intensity during reaction because of the disappearance
of the crystalline Ge phase. The compressive strain along the [111]
direction was found to increase monotonically to a value of −0.21%.
This finding is in agreement with a mechanical model that considers
expansion and plastic deformation during reaction. This new insight
into the mechanics of large-volume-change transformations in alloying
anodes is important for improving the durability of high-capacity
batteries
Reversible Tuning of the Surface Plasmon Resonance of Indium Tin Oxide Nanocrystals by Gas-Phase Oxidation and Reduction
Heavily
doped oxide nanocrystals exhibit a tunable localized surface plasmon
resonance (LSPR) in the infrared, a property that is promising for
applications in photonics, spectroscopy, and photochemistry. Nanocrystal
carrier density and, thus, spectral response are adjustable via chemical
reaction; however, the fundamental processes that govern this behavior
are poorly understood. Here, we study the time dependence of the LSPR
supported by indium tin oxide (ITO) nanocrystals during O<sub>2</sub> and N<sub>2</sub> annealing with <i>in situ</i> diffuse
reflectance infrared Fourier transform spectroscopy (DRIFTS). We show
that the LSPR red-shifts upon oxidation in O<sub>2</sub> and blue-shifts
to its original position upon reduction in N<sub>2</sub>. A reaction–diffusion
model allows us to rationalize the underlying physicochemical processes
and quantitatively connect nanocrystal redox chemistry, solid-state
diffusion, carrier density, and the LSPR
A Yolk-Shell Design for Stabilized and Scalable Li-Ion Battery Alloy Anodes
Silicon is regarded as one of the most promising anode
materials
for next generation lithium-ion batteries. For use in practical applications,
a Si electrode must have high capacity, long cycle life, high efficiency,
and the fabrication must be industrially scalable. Here, we design
and fabricate a yolk-shell structure to meet all these needs. The
fabrication is carried out without special equipment and mostly at
room temperature. Commercially available Si nanoparticles are completely
sealed inside conformal, thin, self-supporting carbon shells, with
rationally designed void space in between the particles and the shell.
The well-defined void space allows the Si particles to expand freely
without breaking the outer carbon shell, therefore stabilizing the
solid-electrolyte interphase on the shell surface. High capacity (∼2800
mAh/g at C/10), long cycle life (1000 cycles with 74% capacity retention),
and high Coulombic efficiency (99.84%) have been realized in this
yolk-shell structured Si electrode
Common Capacity Fade Mechanisms of Metal Foil Alloy Anodes with Different Compositions for Lithium Batteries
Metal foils are attractive anode candidates for replacing
graphite
in lithium-ion batteries, since metal alloys feature high lithium
storage capacity and their direct use as foils could avoid slurry
coating during manufacturing. Aluminum foil is highly abundant and
low-cost, but aluminum foil anodes have generally shown poor cyclability.
Here, we fabricate aluminum alloy foils (aluminum–tin, aluminum–zinc,
and aluminum–gallium) and examine their electrochemical behavior
to understand how composition and microstructure influence cycling
performance of metal foil anodes. We show that the addition of alloy
components can increase the cycle life of aluminum foil anodes by
up to a factor of 2, and both the composition and microstructure of
foils influence the cycling capability. We find an approximate power-law
dependence of cycle life on the extent of lithiation per cycle for
most aluminum-based foils as well as other metal foil compositions,
suggesting a common “electrochemical fatigue” degradation
mechanism arising from internal porosity formation during alloying/dealloying
that governs the behavior of a wide variety of metal foil-based anodes.
This understanding, as well as the improved cyclability of the alloy
foils, suggests possible pathways to enhance performance of foil anodes
for lithium batteries
Common Capacity Fade Mechanisms of Metal Foil Alloy Anodes with Different Compositions for Lithium Batteries
Metal foils are attractive anode candidates for replacing
graphite
in lithium-ion batteries, since metal alloys feature high lithium
storage capacity and their direct use as foils could avoid slurry
coating during manufacturing. Aluminum foil is highly abundant and
low-cost, but aluminum foil anodes have generally shown poor cyclability.
Here, we fabricate aluminum alloy foils (aluminum–tin, aluminum–zinc,
and aluminum–gallium) and examine their electrochemical behavior
to understand how composition and microstructure influence cycling
performance of metal foil anodes. We show that the addition of alloy
components can increase the cycle life of aluminum foil anodes by
up to a factor of 2, and both the composition and microstructure of
foils influence the cycling capability. We find an approximate power-law
dependence of cycle life on the extent of lithiation per cycle for
most aluminum-based foils as well as other metal foil compositions,
suggesting a common “electrochemical fatigue” degradation
mechanism arising from internal porosity formation during alloying/dealloying
that governs the behavior of a wide variety of metal foil-based anodes.
This understanding, as well as the improved cyclability of the alloy
foils, suggests possible pathways to enhance performance of foil anodes
for lithium batteries
Stable Solar-Driven Water Oxidation to O<sub>2</sub>(g) by Ni-Oxide-Coated Silicon Photoanodes
Semiconductors
with small band gaps (<2 eV) must be stabilized
against corrosion or passivation in aqueous electrolytes before such
materials can be used as photoelectrodes to directly produce fuels
from sunlight. In addition, incorporation of electrocatalysts on the
surface of photoelectrodes is required for efficient oxidation of
H<sub>2</sub>O to O<sub>2</sub>(g) and reduction of H<sub>2</sub>O
or H<sub>2</sub>O and CO<sub>2</sub> to fuels. We report herein the
stabilization of np<sup>+</sup>-Si(100) and n-Si(111) photoanodes
for over 1200 h of continuous light-driven evolution of O<sub>2</sub>(g) in 1.0 M KOH(aq) by an earth-abundant, optically transparent,
electrocatalytic, stable, conducting nickel oxide layer. Under simulated
solar illumination and with optimized index-matching for proper antireflection,
NiO<sub><i>x</i></sub>-coated np<sup>+</sup>-Si(100) photoanodes
produced photocurrent-onset potentials of −180 ± 20 mV
referenced to the equilibrium potential for evolution of O<sub>2</sub>(g), photocurrent densities of 29 ± 1.8 mA cm<sup>–2</sup> at the equilibrium potential for evolution of O<sub>2</sub>(g),
and a solar-to-O<sub>2</sub>(g) conversion figure-of-merit of 2.1%
Passivation Coating on Electrospun Copper Nanofibers for Stable Transparent Electrodes
Copper nanofiber networks, which possess the advantages of low cost, moderate flexibility, small sheet resistance, and high transmittance, are one of the most promising candidates to replace indium tin oxide films as the premier transparent electrode. However, the chemical activity of copper nanofibers causes a substantial increase in the sheet resistance after thermal oxidation or chemical corrosion of the nanofibers. In this work, we utilize atomic layer deposition to coat a passivation layer of aluminum-doped zinc oxide (AZO) and aluminum oxide onto electrospun copper nanofibers and remarkably enhance their durability. Our AZO–copper nanofibers show resistance increase of remarkably only 10% after thermal oxidation at 160 °C in dry air and 80 °C in humid air with 80% relative humidity, whereas bare copper nanofibers quickly become insulating. In addition, the coating and baking of the acidic PEDOT:PSS layer on our fibers increases the sheet resistance of bare copper nanofibers by 6 orders of magnitude, while the AZO–Cu nanofibers show an 18% increase
Comparative Study in Acidic and Alkaline Media of the Effects of pH and Crystallinity on the Hydrogen-Evolution Reaction on MoS<sub>2</sub> and MoSe<sub>2</sub>
Single
crystals of n-type MoS<sub>2</sub> and n-MoSe<sub>2</sub> showed higher
electrocatalytic activity for the evolution of H<sub>2</sub>(g) in
alkaline solutions than in acidic solutions. The overpotentials
required to drive hydrogen evolution at −10 mA cm<sup>–2</sup> of current density for MoS<sub>2</sub> samples were −0.76
± 0.13 and −1.03 ± 0.21 V when in contact with 1.0
M NaOH(aq) and 1.0 M H<sub>2</sub>SO<sub>4</sub>(aq), respectively.
For MoSe<sub>2</sub> samples, the overpotentials at −10 mA
cm<sup>–2</sup> were −0.652 ± 0.050 and −0.709
± 0.073 V in contact with 1.0 M KOH(aq) and 1.0 M H<sub>2</sub>SO<sub>4</sub>(aq), respectively. Single crystals from two additional
sources were also tested, and the absolute values of the measured
overpotentials were consistently less (by 460 ± 250 mV) in alkaline
solutions than in acidic solutions. When electrochemical etching was
used to create edge sites on the single crystals, the kinetics improved
in acid but changed little in alkaline media. The overpotentials measured
for polycrystalline thin films (PTFs) and amorphous forms of MoS<sub>2</sub> showed less sensitivity to pH and edge density than was observed
for single crystals and showed enhanced kinetics in acid when compared
to alkaline solutions. These results suggest that the active sites
for hydrogen evolution on MoS<sub>2</sub> and MoSe<sub>2</sub> are
different in alkaline and acidic media. Thus, while edges are known
to serve as active sites in acidic media, in alkaline media it is
more likely that terraces function in this role