8 research outputs found
Postsynthetic Exchanges of the Pillaring Ligand in Three-Dimensional MetalāOrganic Frameworks
Metalāorganic frameworks,
[NiĀ(HBTC)Ā(dabco)] (<b>2</b>) and [Ni<sub>2</sub>(HBTC)<sub>2</sub>(bipy)<sub>0.6</sub>(dabco)<sub>1.4</sub>] (<b>3</b>) (where
H<sub>3</sub>BTC is 1,3,5-benzenetricarboxylic
acid, dabco is 1,4-diazabicyclo[2.2.2]Āoctane, and bipy is 4,4ā²-bipyridine),
were prepared via postsynthetic ligand exchanges of [NiĀ(HBTC)Ā(bipy)]
(<b>1</b>). By controlling the concentration of dabco, we could
obtain not only entropically favorable <b>2</b> with completely
exchanged dabco but also enthalpically favorable <b>3</b> with
selectively exchanged bipy/dabco in the alternating layers
Metal-Free Ketjenblack Incorporated Nitrogen-Doped Carbon Sheets Derived from Gelatin as Oxygen Reduction Catalysts
Electrocatalysts facilitating oxygen
reduction reaction (ORR) are
vital components in advanced fuel cells and metal-air batteries. Here
we report Ketjenblack incorporated nitrogen-doped carbon sheets derived
from gelatin and apply these easily scalable materials as metal-free
electrocatalysts for ORR. These carbon nanosheets demonstrate highly
comparable catalytic activity for ORR as well as better durability
than commercial Vulcan carbon supported Pt catalysts in alkaline media.
Physico-chemical characterization and theoretical calculations suggest
that proper combination of graphitic and pyridinic nitrogen species
with more exposed edge sites effectively facilitates a formation of
superoxide, [O<sub>2(ad)</sub>]<sup>ā</sup>, via one-electron
transfer, thus increasing catalytic activities for ORR. Our results
demonstrate a novel strategy to expose more nitrogen doped edge sites
by irregular stacked small sheets in developing better electrocatalysts
for Zn-air batteries. These desirable architectures are embodied by
an amphiphlilic gelatin mediated compatible synthetic strategy between
hydrophobic carbon and aqueous water
Anomalous K-Point Phonons in Noble Metal/Graphene Heterostructure Activated by Localized Surface Plasmon Resonance
The metal/graphene interface has been one of the most important research topics with regard to charge screening, charge transfer, contact resistance, and solar cells. Chemical bond formation of metal and graphene can be deduced from the defect induced D-band and its second-order mode, 2D band, measured by Raman spectroscopy, as a simple and nondestructive method. However, a phonon mode located at ???1350 cm-1, which is normally known as the defect-induced D-band, is intriguing for graphene deposited with noble metals (Ag, Au, and Cu). We observe anomalous K-point phonons in nonreactive noble metal/graphene heterostructures. The intensity ratio of the midfrequency mode at ???1350 cm-1 over G-band (???1590 cm-1) exhibits nonlinear but resonant behavior with the excitation laser wavelength, and more importantly, the phonon frequency-laser energy dispersion is ???10-17 cm-1 eV-1, which is much less than the conventional range. These phonon modes of graphene at nonzero phonon wave vector (q ??? 0) around K points are activated by localized surface plasmon resonance and not by the defects due to chemical bond formation of metal/graphene. This hypothesis is supported by density functional theory (DFT) calculations for noble metals and Cr along with the measured contact resistances
Synthesis and Characterization of Patronite Form of Vanadium Sulfide on Graphitic Layer
With the exploding interest in transition
metal chalcogenides,
sulfide minerals containing the dianion S<sub>2</sub><sup>2ā</sup>, such as pyrite (FeS<sub>2</sub>), cattierite (CoS<sub>2</sub>),
and vaesite (NiS<sub>2</sub>), have recently attracted much attention
for potential applications in energy conversion and storage devices.
However, the synthesis of the patronite structure (VS<sub>4</sub>,
V<sup>4+</sup>(S<sub>2</sub><sup>2ā</sup>)<sub>2</sub>) and
its applications have not yet been clearly demonstrated because of
experimental difficulties and the existence of nonstoichiometric phases.
Herein, we report the synthesis of VS<sub>4</sub> using a simple,
facile hydrothermal method with a graphene oxide (GO) template and
the characterization of the resulting material. Tests of various templates
such as CNT, pyrene, perylene-3,4,9,10-tetracarboxylic dianhydride
(PTCDA), and graphite led us to the conclusion that the graphitic
layer plays a role in the nucleation during growth of VS<sub>4</sub>. Furthermore, the VS<sub>4</sub>/rGO hybrid was proved to be a promising
functional material in energy storage devices
Catalytic Transparency of Hexagonal Boron Nitride on Copper for Chemical Vapor Deposition Growth of Large-Area and High-Quality Graphene
Graphene transferred onto h-BN has recently become a focus of research because of its excellent compatibility with large-area device applications. The requirements of scalability and clean fabrication, however, have not yet been satisfactorily addressed. The successful synthesis of graphene/h-BN on a Cu foil and DFT calculations for this system are reported, which demonstrate that a thin h-BN film on Cu foil is an excellent template for the growth of large-area and high-quality graphene. Such material can be grown on thin h-BN films that are less than 3 nm thick, as confirmed by optical microscopy and Raman spectroscopy. We have evaluated the catalytic growth mechanism and the limits on the CVD growth of high-quality and large-area graphene on h-BN film/Cu by performing Kelvin probe force microscopy and DFT calculations for various thicknesses of h-BN
Macroporous Inverse Opal-like Mo<sub><i>x</i></sub>C with Incorporated Mo Vacancies for Significantly Enhanced Hydrogen Evolution
The hydrogen evolution reaction (HER)
is one of the most important
pathways for producing pure and clean hydrogen. Although platinum
(Pt) is the most efficient HER electrocatalyst, its practical application
is significantly hindered by high-cost and scarcity. In this work,
an Mo<sub><i>x</i></sub>C with incorporated Mo vacancies
and macroporous inverse opal-like (IOL) structure (Mo<sub><i>x</i></sub>C-IOL) was synthesized and studied as a low-cost
efficient HER electrocatalyst. The macroporous IOL structure was controllably
fabricated using a facile-hard template strategy. As a result of the
combined benefits of the Mo vacancies and structural advantages, including
appropriate hydrogen binding energy, large exposed surface, robust
IOL structure and fast mass/charge transport, the synthesized Mo<sub><i>x</i></sub>C-IOL exhibited significantly enhanced HER
electrocatalytic performance with good stability, with performance
comparable or superior to Pt wire in both acidic and alkaline solutions
Nitrogen-Doped Graphene Nanoplatelets from Simple Solution Edge-Functionalization for nāType Field-Effect Transistors
The development of
a versatile method for nitrogen-doping of graphitic
structure is an important challenge for many applications, such as
energy conversions and storages and electronic devices. Here, we report
a simple but efficient method for preparing nitrogen-doped graphene
nanoplatelets <i>via</i> wet-chemical reactions. The reaction
between monoketone (Cī»O) in graphene oxide (GO) and monoamine-containing
compound produces imine (Shiff base) functionalized GO (iGO). The
reaction between Ī±-diketone in GO and 1,2-diamine (<i>ortho</i>-diamine)-containing compound gives stable pyrazine ring functionalized
GO (pGO). Subsequent heat-treatments of iGO and pGO result in high-quality,
nitrogen-doped graphene nanoplatelets to be designated as hiGO and
hpGO, respectively. Of particular interest, hpGO was found to display
the n-type field-effect transistor behavior with a charge neutral
point (Dirac point) located at around ā16 V. Furthermore, hpGO
showed hole and electron mobilities as high as 11.5 and 12.4 cm<sup>2</sup>V<sup>ā1</sup>s<sup>ā1</sup>, respectively
Probing Evolution of Twist-Angle-Dependent Interlayer Excitons in MoSe<sub>2</sub>/WSe<sub>2</sub> van der Waals Heterostructures
Interlayer
excitons were observed at the heterojunctions in van
der Waals heterostructures (vdW HSs). However, it is not known how
the excitonic phenomena are affected by the stacking order. Here,
we report twist-angle-dependent interlayer excitons in MoSe<sub>2</sub>/WSe<sub>2</sub> vdW HSs based on photoluminescence (PL) and vdW-corrected
density functional theory calculations. The PL intensity of the interlayer
excitons depends primarily on the twist angle: It is enhanced at coherently
stacked angles of 0Ā° and 60Ā° (owing to strong interlayer
coupling) but disappears at incoherent intermediate angles. The calculations
confirm twist-angle-dependent interlayer coupling: The states at the
edges of the valence band exhibit a long tail that stretches over
the other layer for coherently stacked angles; however, the states
are largely confined in the respective layers for intermediate angles.
This interlayer hybridization of the band edge states also correlates
with the interlayer separation between MoSe<sub>2</sub> and WSe<sub>2</sub> layers. Furthermore, the interlayer coupling becomes insignificant,
irrespective of twist angles, by the incorporation of a hexagonal
boron nitride monolayer between MoSe<sub>2</sub> and WSe<sub>2</sub>