59 research outputs found
Electrocatalytically Active Graphene–Porphyrin MOF Composite for Oxygen Reduction Reaction
Pyridine-functionalized graphene (reduced graphene oxide)
can be
used as a building block in the assembly of metal organic framework
(MOF). By reacting the pyridine-functionalized graphene with iron–porphyrin,
a graphene–metalloporphyrin MOF with enhanced catalytic activity
for oxygen reduction reactions (ORR) is synthesized. The structure
and electrochemical property of the hybrid MOF are investigated as
a function of the weight percentage of the functionalized graphene
added to the iron–porphyrin framework. The results show that
the addition of pyridine-functionalized graphene changes the crystallization
process of iron–porphyrin in the MOF, increases its porosity,
and enhances the electrochemical charge transfer rate of iron–porphyrin.
The graphene–metalloporphyrin hybrid shows facile 4-electron
ORR and can be used as a promising Pt-free cathode in alkaline Direct
Methanol Fuel Cell
Periodic Grain Boundaries Formed by Thermal Reconstruction of Polycrystalline Graphene Film
Grain boundaries
consisting of dislocation cores arranged in a
periodic manner have well-defined structures and peculiar properties
and can be potentially applied as conducting circuits, plasmon reflectors
and phase retarders. Pentagon-heptagon (5–7) pairs or pentagon-octagon-pentagon
(5–8–5) carbon rings are known to exist in graphene
grain boundaries. However, there are few systematic experimental studies
on the formation, structure and distribution of periodic grain boundaries
in graphene. Herein, scanning tunneling microscopy (STM) was applied
to study periodic grain boundaries in monolayer graphene grown on
a weakly interacting Cu(111) crystal. The periodic grain boundaries
are formed after the thermal reconstruction of aperiodic boundaries,
their structures agree well with the prediction of the coincident-site-lattice
(CSL) theory. Periodic grain boundaries in quasi-freestanding graphene
give sharp local density of states (LDOS) peaks in the tunneling spectra
as opposed to the broad peaks of the aperiodic boundaries. This suggests
that grain boundaries with high structural quality can introduce well-defined
electronic states in graphene and modify its electronic properties
Whisper Gallery Modes in Monolayer Tungsten Disulfide-Hexagonal Boron Nitride Optical Cavity
There
are strong interests in constructing nanolasers using two-dimensional
transition metal dichalcogenides (TMDs) due to their strong light–matter
interactions and high optical gain. However, most cavity designs based
on transfer of exfoliated TMDs on silicon oxide are not optimized
since monolayer emitters are located far from where the photonic mode
reaches maximum intensity. By taking advantage of the excellent dielectric
properties of hexagonal boron nitride (h-BN), we design a new microdisk
optical cavity fabricated from a van der Waals (VdW) stacked h-BN/WS<sub>2</sub>/h-BN. The heterostructure is patterned into microdisk cavities
characterized by whispering gallery modes (WGMs). The emission intensity
of the WS<sub>2</sub> trion is enhanced by 2.9 times that of exciton
in the heterostructure, giving rise to whisper gallery modes with
resonance intensities that show nonlinear power dependence. A Rayleigh
scatterer directs the cavity emission to vertical collection. Such
VdW heterostructure provides an atomically smooth interface that is
ideal for low loss photon propagation, giving a <i>Q</i> factor of 1200
Thermally Stable Mesoporous Perovskite Solar Cells Incorporating Low-Temperature Processed Graphene/Polymer Electron Transporting Layer
In the short time since its discovery,
perovskite solar cells (PSCs) have attained high power conversion
efficiency but their lack of thermal stability remains a barrier to
commercialization. Among the experimentally accessible parameter spaces
for optimizing performance, identifying an electron transport layer
(ETL) that forms a thermally stable interface with perovskite and
which is solution-processable at low-temperature will certainly be
advantageous. Herein, we developed a mesoporous graphene/polymer composite
with these advantages when used as ETL in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> PSCs, and a high efficiency of 13.8% under AM 1.5G
solar illumination could be obtained. Due to the high heat transmission
coefficient and low isoelectric point of mesoporous graphene-based
ETL, the PSC device enjoys good chemical and thermal stability. Our
work demonstrates that the mesoporous graphene-based scaffold is a
promising ETL candidate for high performance and thermally stable
PSCs
Thermally Stable Mesoporous Perovskite Solar Cells Incorporating Low-Temperature Processed Graphene/Polymer Electron Transporting Layer
In the short time since its discovery,
perovskite solar cells (PSCs) have attained high power conversion
efficiency but their lack of thermal stability remains a barrier to
commercialization. Among the experimentally accessible parameter spaces
for optimizing performance, identifying an electron transport layer
(ETL) that forms a thermally stable interface with perovskite and
which is solution-processable at low-temperature will certainly be
advantageous. Herein, we developed a mesoporous graphene/polymer composite
with these advantages when used as ETL in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> PSCs, and a high efficiency of 13.8% under AM 1.5G
solar illumination could be obtained. Due to the high heat transmission
coefficient and low isoelectric point of mesoporous graphene-based
ETL, the PSC device enjoys good chemical and thermal stability. Our
work demonstrates that the mesoporous graphene-based scaffold is a
promising ETL candidate for high performance and thermally stable
PSCs
Studying Edge Defects of Hexagonal Boron Nitride Using High-Resolution Electron Energy Loss Spectroscopy
Studying the phonons of hexagonal
boron nitride (h-BN) is important
for understanding its thermal, electronic, and imaging applications.
Herein, we applied high-resolution electron energy loss spectroscopy
(HREELS) to monitor the presence of edge defects in h-BN films. We
observed an edge phonon at 90.5 meV with the initial formation of
island-like domains on Ru(0001), which subsequently weakens with respect
to the bulk phonon as the islands congregate into a film. The presence
of a weak edge phonon peak even at full surface coverage of the h-BN
film indicates the sensitivity of HREELS in detecting line defects.
A shoulder peak at ∼160 meV assignable to sp<sup>3</sup> bonded
modes was attributed to grain boundaries arising from misaligned domains.
In addition, the strengths of substrate interaction and the rippling
of the h-BN film can be judged from the shift in the phonon energy
of the out-of-plane TO<sub>⊥</sub> mode
Low Power Consumption Ammonia Electrosynthesis Using Hydrogen-Nitrate Flow Electrolyzer
Ammonia synthesis through electrochemical nitrate reduction
has
emerged as a promising alternative to the conventional Haber–Bosch
process. However, the use of a sluggish oxygen evolution reaction
as the anode reaction leads to high energy consumption in nitrate
reduction. In this study, we directly utilize hydrogen gas to synthesize
ammonia by pairing the hydrogen oxidation reaction with the nitrate
reduction reaction. A significantly lower cell voltage for ammonia
synthesis was realized on a 16 cm2 flow electrolyzer. We
achieved an impressive ammonia yield rate of 16.9 mmol h–1 at a cell voltage of 1.2 V cell voltage. Notably, this approach
exhibits a low power consumption of 17 kWh kg–1 of
NH3. The mechanism study shows hydroxyl ions generated
from water splitting at the cathode cross the anion exchange membrane
to react with protons generated from hydrogen oxidation at the anode.
Through rigorous technical and economic analyses, this approach is
found to be economically viable for industrial synthesis
Analyzing Dirac Cone and Phonon Dispersion in Highly Oriented Nanocrystalline Graphene
Chemical
vapor deposition (CVD) is one of the most promising growth
techniques to scale up the production of monolayer graphene. At present,
there are intense efforts to control the orientation of graphene grains
during CVD, motivated by the fact that there is a higher probability
for oriented grains to achieve seamless merging, forming a large single
crystal. However, it is still challenging to produce single-crystal
graphene with no grain boundaries over macroscopic length scales,
especially when the nucleation density of graphene nuclei is high.
Nonetheless, nanocrystalline graphene with highly oriented grains
may exhibit single-crystal-like properties. Herein, we investigate
the spectroscopic signatures of graphene film containing highly oriented,
nanosized grains (20–150 nm) using angle-resolved photoemission
spectroscopy (ARPES) and high-resolution electron energy loss spectroscopy
(HREELS). The robustness of the Dirac cone, as well as dispersion
of its phonons, as a function of graphene’s grain size and
before and after film coalescence, was investigated. In view of the
sensitivity of atomically thin graphene to atmospheric adsorbates
and intercalants, ARPES and HREELS were also used to monitor the changes
in spectroscopic signatures of the graphene film following exposure
to the ambient atmosphere
Energy Storage Studies on InVO<sub>4</sub> as High Performance Anode Material for Li-Ion Batteries
InVO<sub>4</sub> has attracted much attention as an anode material due to
its high theoretical capacity. However, the effect of preparation
methods and conditions on morphology and energy storage characteristic
has not been extensively investigated and will be explored in this
project. InVO<sub>4</sub> anode material was prepared using five different
preparation methods: solid state, urea combustion, precipitation,
ball-milling, and polymer precursor methods. Morphology and physical
properties of InVO<sub>4</sub> were then analyzed using X-ray diffraction
(XRD), scanning electron microscope (SEM), and Brunauer–Emmett–Teller
(BET) surface area method. XRD patterns showed that orthorhombic phased
InVO<sub>4</sub> was synthesized. Small amounts of impurities were
observed in methods II, III, and V using XRD patterns. BET surface
area ranged from 0.49 to 9.28 m<sup>2</sup> g<sup>–1</sup>.
SEM images showed slight differences in the InVO<sub>4</sub> nanosized
crystalline structures with respect to preparation methods and conditions.
Energy storage studies showed that, among all the preparation methods,
the urea combustion method produced the best electrochemical results,
with negligible capacity fading between the 2nd and 50th cycles and
high capacity of 1241 mA h g<sup>–1</sup> at the end of the
20th cycle, close to the theoretical capacity value. Precipitation
method also showed good performance, with capacity fading (14%) and
capacity of 1002 mA h g<sup>–1</sup> at the 20th cycle. Cyclic
voltammetry (CV) and electrochemical impedance spectroscopy (EIS)
was then used to determine the reaction mechanisms of InVO<sub>4</sub>
Triple-State Liquid-Based Microfluidic Tactile Sensor with High Flexibility, Durability, and Sensitivity
We
develop a novel triple-state liquid-based resistive microfluidic
tactile sensor with high flexibility, durability, and sensitivity.
It comprises a platinum-cured silicone microfluidic assembly filled
with 2 μL liquid metallic alloy interfacing two screen-printed
conductive electrodes on a polyethylene terephthalate (PET) film.
This flexible tactile sensor is highly sensitive ((2–20) ×
10<sup>–3</sup> kPa<sup>–1</sup>) and capable of distinguishing
compressive loads with an extremely large range of pressure (2 to
400 kPa) as well as bending loads. Owing to its unique and durable
structure, the sensor can withstand numerous severe mechanical load,
such as foot stomping and a car wheel rolling over it, without compromising
its electrical signal stability and overall integrity. Also, our sensing
device is highly deformable, wearable, and able to differentiate and
quantify pressures exerted by distinct bodily actions, such as a finger
touch or footstep pressure. As a proof-of-concept of the applicability
of our tactile sensor, we demonstrate the measurements of localized
dynamic foot pressure by embedding the sensor inside the shoes and
high heels. This work highlights the potential of the liquid-based
microfluidic tactile sensing platform in a wide range of applications
and can facilitate the realization of functional liquid-state sensing
device technology with superior mechanical flexibility, durability,
and sensitivity
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