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 3–8 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 metal–water 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 “ordered water monolayer that does not completely wet water” 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 A–T 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 Watson–Crick 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 TiO₂ 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 ∼44°. 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 TiO₂ Overcoat on Supported Au Catalysts Using Atomic Layer Deposition for Understanding the Reaction Mechanism and Improved Activity in CO Oxidation

For TiO₂ supported Au catalysts, the Au particle size and the interfacial perimeter sites between Au particles and the TiO₂ 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 TiO₂ overcoat to Au/Al₂O₃ and Au/SiO₂ catalysts using atomic layer deposition (ALD) where the new Au–TiO₂ 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 TiO₂ overcoat preferentially decorates the low-coordinated sites of Au nanoparticles and generates Au–TiO₂ interfaces. In CO oxidation, we demonstrated a remarkable improvement of the catalytic activities of Au/Al₂O₃ and Au/SiO₂ catalysts by the ALD TiO₂ overcoat. More interestingly, the activity as a function of TiO₂ ALD cycles obviously showed a volcano-like behavior, providing direct evidence that the catalytic activities of TiO₂ 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 Cu₂O(100): Nature of Defects

The Cu₂O­(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 nm²) 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 Organic–Metal–QD 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 organic–metal–quantum 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 (NH₄)₃PO₄. 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 Suzuki–Miyaura 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