40 research outputs found
Shape Evolution of Metal Nanoparticles in Water Vapor Environment
The
structures of the metal nanoparticles are crucial for their catalytic
activities. How to understand and even control the shape evolution
of nanoparticles under reaction condition is a big challenge in heterogeneous
catalysis. It has been proved that many reactive gases hold the capability
of changing the structures and properties of metal nanoparticles.
One interesting question is whether water vapor, such a ubiquitous
environment, could induce the shape evolution of metal nanoparticles.
So far this question has not received enough attention yet. In this
work, we developed a model based on the density functional theory,
the Wulff construction, and the Langmuir adsorption isotherm to explore
the shape of metal nanoparticle at given temperature and water vapor
pressure. By this model, we show clearly that water vapor could notably
increase the fraction of (110) facets and decrease that of (111) facets
for 3â8 nm Cu nanoparticles, which is perfectly consistent
with the experimental observations. Further investigations indicate
the water vapor has different effects on the different metal species
(Cu, Au, Pt, and Pd). This work not only helps to understand the water
vapor effect on the structures of metal nanoparticles but also proposes
a simple but effective model to predict the shape of nanoparticles
in certain environment
Nanoscale Hydrophilicity on Metal Surfaces at Room Temperature: Coupling Lattice Constants and Crystal Faces
It
is generally accepted that the metalâwater interface
tensions are quite high; thus, the metal surfaces are usually regarded
as hydrophilic. Using the molecular dynamics simulations, we have
investigated the microscopic wetting behaviors of a series of metal
surfaces at room temperature, including Ni, Cu, Pd, Pt, Al, Au, Ag,
and Pb with three crystal faces of (100), (110), and (111). We have
found that the wetting of the metals is greatly dependent on both
the lattice constants and crystal surfaces. Particularly, stable water
droplets are found forming on the first ordered water layer, serving
as an evidence of room temperature âordered water monolayer
that does not completely wet waterâ on Pd(100), Pt(100), and
Al(100) surfaces, while water films without ordered water monolayer
are found on (110) and (111) faces of all metal surfaces and even
(100) face of other metal surfaces (Ni, Cu, Au, Ag, and Pb). The formation
of water droplets is attributed to the rhombic ordered water layers
on the surfaces, reducing the number of hydrogen bond formation between
the monolayers and other water molecules atop the water monolayer.
These results demonstrate a tight correlation among the lattice constant,
the crystal faces, and the surface wetting behaviors. Our findings
of the novel wetting behavior may have potential applications in the
surface friction reduction at the metal surfaces, design of the anti-ice
materials, and the nonfouling materials
DNA Base Pair Hybridization and Water-Mediated Metastable Structures Studied by Molecular Dynamics Simulations
The base pair hybridization of a DNA segment was studied
using
molecular dynamics simulation. The results show the obvious correlation
between the probability of successful hybridization and the accessible
surface area to water of two successive base pairs, including the
unpaired base pair adjacent to paired base pair and this adjacent
paired base pair. Importantly, two metastable structures in an AâT
base pair were discovered by the analysis of the free energy landscape.
Both structures involved addition of a water molecule to the linkage
between the two nucleobases in one base pair. The existence of the
metastable structures provide potential barriers to the WatsonâCrick
base pair, and numerical simulations show that those potential barriers
can be surmounted by thermal fluctuations at higher temperatures.
These studies contribute an important step toward the understanding
of the mechanism in DNA hybridization, particularly the effect of
temperature on DNA hybridization and polymerase chain reaction. These
observations are expected to be helpful for facilitating experimental
bio/nanotechnology designs involving fast hybridization
Friction Reduction at a Superhydrophilic Surface: Role of Ordered Water
Low-friction but superhydrophilic
materials are urgently needed
in biomedical and engineering fields because of their nonfouling property
and biocompatibility, particularly when the surfaces are definitely
superhydrophilic, such as metal or TiO<sub>2</sub> as the surface
coatings of the intravascular stents. However, generally, there is
a higher friction coefficient on the superhydrophilic surfaces than
on the hydrophobic surfaces. On the basis of molecular dynamics simulations,
we show that the friction on the superhydrophilic surface with appropriate
charge patterns is evidently reduced, where the lower friction is
similar to that of a rather hydrophobic surface with a contact angle
of water droplet of âŒ44°. This reduction is attributed
to the existence of an ordered water monolayer on the superhydrophilic
surface with appropriate charge patterns, and the friction between
this ordered water monolayer and the water molecules above is small
Precisely Applying TiO<sub>2</sub> Overcoat on Supported Au Catalysts Using Atomic Layer Deposition for Understanding the Reaction Mechanism and Improved Activity in CO Oxidation
For TiO<sub>2</sub> supported Au
catalysts, the Au particle size
and the interfacial perimeter sites between Au particles and the TiO<sub>2</sub> support both play important roles in CO oxidation reaction.
However, changing the Au particle size inevitably accompanied by the
change of the perimeter length makes it extremely difficult to identify
their individual roles. Here we reported a new strategy to isolate
them by applying TiO<sub>2</sub> overcoat to Au/Al<sub>2</sub>O<sub>3</sub> and Au/SiO<sub>2</sub> catalysts using atomic layer deposition
(ALD) where the new AuâTiO<sub>2</sub> interfacial length was
precisely tuned to different degrees while preserving the particle
size. High resolution transmission electron microscopy (HRTEM), atomic
force microscopy (AFM), and diffuse reflectance infrared Fourier transform
spectroscopy (DRIFTS) measurements of CO chemisorption all confirmed
that the TiO<sub>2</sub> overcoat preferentially decorates the low-coordinated
sites of Au nanoparticles and generates AuâTiO<sub>2</sub> interfaces.
In CO oxidation, we demonstrated a remarkable improvement of the catalytic
activities of Au/Al<sub>2</sub>O<sub>3</sub> and Au/SiO<sub>2</sub> catalysts by the ALD TiO<sub>2</sub> overcoat. More interestingly,
the activity as a function of TiO<sub>2</sub> ALD cycles obviously
showed a volcano-like behavior, providing direct evidence that the
catalytic activities of TiO<sub>2</sub> overcoated Au catalysts strongly
correlate with the total length of perimeter sites. Finally, our work
suggests that this strategy might be a new method for atomic level
understanding the reaction mechanism and high performance catalyst
design
The Surface Structure of Cu<sub>2</sub>O(100): Nature of Defects
The Cu<sub>2</sub>OÂ(100) surface is most favorably terminated by
a (3,0;1,1) reconstruction under ultrahigh-vacuum conditions. As most
oxide surfaces, it exhibit defects, and it is these sites that are
focus of attention in this study. The surface defects are identified,
their properties are investigated, and procedures to accurately control
their coverage are demonstrated by a combination of scanning tunneling
microscopy (STM) and simulations within the framework of density functional
theory (DFT). The most prevalent surface defect was identified as
an oxygen vacancy. By comparison of experimental results, formation
energies, and simulated STM images, the location of the oxygen vacancies
was identified as an oxygen vacancy in position B, located in the
valley between the two rows of oxygen atoms terminating the unperturbed
surface. The coverage of defects is influenced by the surface preparation
parameters and the history of the sample. Furthermore, using low-energy
electron beam bombardment, we show that the oxygen vacancy coverage
can be accurately controlled and reach a complete surface coverage
(1 per unit cell or 1.8 defects per nm<sup>2</sup>) without modification
to the periodicity of the surface, highlighting the importance of
using local probes when investigating oxide surfaces
SERS-Fluorescence Joint Spectral Encoding Using OrganicâMetalâQD Hybrid Nanoparticles with a Huge Encoding Capacity for High-Throughput Biodetection: Putting Theory into Practice
A new concept of optical encoding approach, surface enhanced
Raman
scattering (SERS)-fluorescence joint spectral encoding method (SFJSE),
was demonstrated by using organicâmetalâquantum dot
(QD) hybrid nanoparticles (OMQ NPs) with a nanolayered structure.
This method has two distinct characteristics, which make it more feasible
to achieve enormous codes in practice, compared with a sole fluorescence-
or SERS-based encoding protocol. One of the two characteristics is
to use the joint SERS and fluorescence spectra as the encoding elements
instead of an individual optical signal, resulting in a broadened
optical spectrum range for efficient encoding. The other is to assemble
SERS reporters and fluorescent agents onto different layers of OMQ
NPs, leading to an easier fabrication protocol when a large number
of agents need to be involved into encoding carriers. By conjugating
different antibodies to OMQ NPs with varied codes, the potential application
of such an encoding system in high-throughput detection has been investigated
by multiplex sandwich immunoassays. The high specificity and sensitivity
of the assays suggest that the SFJSE method could be developed as
a powerful encoding tool for high-throughput bioanalysis with the
use of OMQ NPs
Impeded Mass Transportation Due to Defects in Thermally Driven Nanotube Nanomotor
A thermally
driven nanotube nanomotor provides linear mass transportation
controlled by a temperature gradient. However, the underlying mechanism
is still unclear, as the mass transportation velocity in experiment
is much lower than that resulting from simulations. Considering that
defects are common in fabricated nanotubes, we use molecular dynamics
simulations to show that the mass transportation would be considerably
impeded by defects. The outer tube of a double-walled carbon nanotube
transports along the coaxial inner tube subject to a temperature gradient.
While encountering the defects in the inner tube, the outer tube might
be bounced back or trapped at some specific sites due to the potential
barriers or wells induced by the defects. The stagnation phenomenon
provides a probable picture to understand the low transportation velocity
at the microscopic level. We also show that a similar stagnation phenomenon
holds in mass transportation of a fullerene encapsulated in a defective
carbon nanotube. Our result is expected to be helpful in designing
nanotube nanomotors
N/P-Codoped Thermally Reduced Graphene for High-Performance Supercapacitor Applications
Graphene
relative materials for supercapacitors have incurred intense interest
due to their high electrical and thermal conductivity, large surface
area, and good chemical stability. N/P-codoped thermally reduced graphene
oxide (N/P-TRGO) with high density of surface groups was synthesized
by a simple way through thermal annealing of thermally exfoliated
graphene oxide in the presence of (NH<sub>4</sub>)<sub>3</sub>PO<sub>4</sub>. The extreme low C/O atom ratio of 5.9 was reached after
thermal treatment at high temperature of 800 °C for 2 h. N/P-TRGO
exhibits high specific capacitance, high rate capability, and excellent
cycle performance. The effect of codoping on the surface, structural,
and electrochemical capacitive properties was investigated and elucidated
in detail. These results demonstrated that N, P codoping is a convenient
and efficient way for the improving the supercapacitive performance
of thermally reduced graphene oxides
Synthesis of Fmoc-Protected Arylphenylalanines (Bip Derivatives) via Nonaqueous Suzuki-Miyaura Cross-Coupling Reactions
A one-step synthesis
of Fmoc-protected aryl/heteroaryl-substituted
phenylalanines (Bip derivatives) using the nonaqueous palladium-catalyzed
SuzukiâMiyaura cross-coupling (SMC) reaction of Fmoc-protected
bromo- or iodophenylalanines is reported. This protocol allows for
the direct formation of a variety of unnatural biaryl-containing amino
acids in good to excellent yield, which can be readily used in subsequent
Fmoc solid-phase peptide synthesis. The synthetic utility of this
method is also demonstrated by the SMC reaction of bromophenylalanine-containing
tripeptides