9 research outputs found
Tetramethoxypyrene-Based Biradical Donors with Tunable Physical and Magnetic Properties
Synthesis of 2,7-disubstituted tetramethoxypyrene-based neutral biradical donors is reported. The biradicals were characterized by EPR, UV–vis, CV, SQUID, and single-crystal X-ray diffraction, and their optical, electrochemical, and structural properties were compared and discussed. The experimental results are well supported by DFT calculations. Systematic tuning of magnetic exchange interactions was achieved by varying the radical moieties
Crystal Engineering of Tolane Bridged Nitronyl Nitroxide Biradicals: Candidates for Quantum Magnets
The
tolane bridged nitronyl nitroxide biradicals with hydrogen bond donor
and acceptor functional groups were synthesized. The magnetic measurements
and the DFT calculations were performed to ascertain the influence
of the functional groups on inter- and intramolecular magnetic exchange
interactions. While upon functionalizing the tolane bridge the intramolecular
exchange interactions remained nearly unchanged, the fine-tuning of
intermolecular exchange interactions could be achieved by employing
the crystal engineering approach
Crystal Engineering of Tolane Bridged Nitronyl Nitroxide Biradicals: Candidates for Quantum Magnets
The
tolane bridged nitronyl nitroxide biradicals with hydrogen bond donor
and acceptor functional groups were synthesized. The magnetic measurements
and the DFT calculations were performed to ascertain the influence
of the functional groups on inter- and intramolecular magnetic exchange
interactions. While upon functionalizing the tolane bridge the intramolecular
exchange interactions remained nearly unchanged, the fine-tuning of
intermolecular exchange interactions could be achieved by employing
the crystal engineering approach
Chemical Vapor Deposition of N‑Doped Graphene and Carbon Films: The Role of Precursors and Gas Phase
Thermally induced chemical vapor deposition (CVD) was used to study the formation of nitrogen-doped graphene and carbon films on copper from aliphatic nitrogen-containing precursors consisting of C<sub>1</sub>- and C<sub>2</sub>-units and (hetero)aromatic nitrogen-containing ring systems. The structure and quality of the resulting films were correlated to the influence of the functional groups of the precursor molecules and gas phase composition. They were analyzed with SEM, TEM, EDX, XPS, and Raman spectroscopy. The presence of (N-doped) graphene was confirmed by the 2D mode of the Raman spectra. The isolated graphene films obtained from nitrogen-containing precursors reveal a high conductivity and transparency compared to standard graphene CVD samples. Precursors with amine functional groups (<i>e.g.</i>, methylamine) can lead to a direct formation of graphene even without additional hydrogen present in the gas phase. This is not observed for, <i>e.g.</i>, methane under comparable CVD conditions. Therefore, the intermediate gas phase species (<i>e.g.</i>, amine radicals) can significantly enhance the graphene film growth kinetics. Kinetic and thermodynamic effects can be invoked to discuss the decay of the precursors
General Synthesis of MOF Nanotubes via Hydrogen-Bonded Organic Frameworks toward Efficient Hydrogen Evolution Electrocatalysts
The
application scope of metal–organic frameworks
(MOFs)
can be extended by rationally designing the architecture and components
of MOFs, which can be achieved via a metal-containing solid templated
strategy. However, this strategy suffers from low efficiency and provides
only one specific MOF from one template. Herein, we present a versatile
templated strategy in which organic ligands are weaved into hydrogen-bonded
organic frameworks (HOFs) for the controllable and scalable synthesis
of MOF nanotubes. HOF nanowires assembled from benzene-1,3,5-tricarboxylic
acid and melamine via a simple sonochemical approach serve as both
the template and precursor to produce MOF nanotubes with varied metal
compositions. Hybrid nanotubes containing nanometal crystals and N-doped
graphene prepared through a carbonization process show that the optimized
NiRuIr alloy@NG nanotube exhibits excellent electrocatalytic HER activity
and durability in alkaline media, outperforming most reported catalysts.
The strategy proposed here demonstrates a pioneering study of combination
of HOF and MOF, which shows great potential in the design of other
nanosized MOFs with various architectures and compositions for potential
applications
Graphene Layer Encapsulation of Non-Noble Metal Nanoparticles as Acid-Stable Hydrogen Evolution Catalysts
Acid-stable, non-noble
catalysts are promising for hydrogen evolution
reaction (HER); however, they get easily damaged when used in acidic
electrolytes, thus reducing the HER lifetimes. Moreover, completely
blocking catalysts from acidic electrolytes degrades HER performance.
To achieve a balance between the HER lifetime and performance, we
vary the number of N-doped graphene layers (1–2, 2–3,
and 3–5 layers) encapsulating NiMo nanoparticles as efficient
HER catalysts and obtain the optimal number of protective layers.
Our data show that 3–5 graphene layers achieved the best balance,
with a stable current density of 100 mA cm<sup>–2</sup> for
25 h in 0.5 M H<sub>2</sub>SO<sub>4</sub>. Density functional theory
calculations are performed to show the effect of encapsulating graphene
layer number on the catalytic activity and protection of non-noble
NiMo in acidic electrolytes
Chemical Selectivity at Grain Boundary Dislocations in Monolayer Mo<sub>1–<i>x</i></sub>W<sub><i>x</i></sub>S<sub>2</sub> Transition Metal Dichalcogenides
Grain
boundaries (GBs) are unavoidable crystal defects in polycrystalline
materials and significantly influence their properties. However, the
structure and chemistry of GBs in 2D transition metal dichalcogenide
alloys have not been well established. Here we report significant
chemical selectivity of transition metal atoms at GB dislocation cores
in Mo<sub>1–<i>x</i></sub>W<sub><i>x</i></sub>S<sub>2</sub> monolayers. Different from classical elastic
field-driven dislocation segregation in bulk crystals, the chemical
selectivity in the 2D crystals originates prominently from variation
of atomic coordination numbers at dislocation cores. This observation
provides atomic insights into the topological effect on the chemistry
of crystal defects in 2D materials
Cooperation between holey graphene and NiMo alloy for hydrogen evolution in an acidic electrolyte
The
development of noble-metal-free hydrogen evolution reaction
(HER) materials for electrochemical water splitting is the key to
achieving low-cost and efficient electrocatalysis that drives electrochemical
hydrogen evolution. However, the electrocatalytic activities of most
non-noble metals decrease in acidic electrolytes. Here, we have fabricated
non-noble-metal electrodes using a bicontinuous and open porous NiMo
alloy covered by nitrogen-doped (N-doped) graphene with nanometer-sized
holes. This noble-metal-free HER catalyst exhibits performance almost
identical with that of a Pt/C electrode, while its original catalytic
activity is preserved even in acidic electrolytes. Density functional
theory calculations indicate that the interfacial fringes between
the nanoholes and NiMo surface induce charge transfer and promote
hydrogen adsorption and desorption. The nanometer-sized holes simultaneously
provide minimal surface area for chemical reactions, while delaying
NiMo dissolution in excessive amounts of acidic electrolyte. Our method
for the fabrication of the NiMo alloy provides a route to a promising
class of electrochemical hydrogen-producing electrodes
Synthesis of Bisdesmosidic Oleanolic Acid Saponins via a Glycosylation-Deprotection Sequence under Continuous Microfluidic/Batch Conditions
We
report the first synthesis of a series of bisdesmosidic oleanolic
acid saponins using microflow reactor Comet X-01 via a continuous
flow glycosylation-batch deprotection sequence. The main results of
this study can be summarized as follows: (1) The microfluidic glycosylation
of oleanolic acid at C-28 was achieved in quantitative yield and was
applied to the synthesis of six C-28-monoglycosidic saponins. (2)
The microfluidic glycosylation of oleanolic acid at C-3 was achieved
in good yield without orthoester byproduct formation and was applied
to the synthesis of three bisdesmosidic saponins. (3) The continuous
synthesis of saponins via a microfluidic glycosylation-batch deprotection
sequence was achieved in four steps involving two purifications. Thus,
the continuous microfluidic glycosylation-deprotection process is
expected to be suitable for the preparation of a library of bisdesmosidic
oleanolic acid saponins for in vivo pharmacological studies