Flexoelectricity enhanced water splitting and hydrogen evolution reaction on grain boundaries of monolayer transition metal dichalcogenides

Our extensive first-principles calculations reveal that the chemical activities of monolayer transition metal dichalcogenides(TMDs)MX₂(M=Mo or W,and X=Te,Se,or S)for water splitting and hydrogen evolution are modified and promoted on their grain boundaries(GBs)when in-plane tensile loadings are applied.Compared with monolayer TMDs without GBs,the flexoelectricity induced by nonuniform deformation and strain gradient significantly enhances the charge polarizations of X and M atoms at the GB sites of monolayer TMDs,which facilitates the dissociation of water molecules on the GB sites and reduces the reaction barrier of hydrogen evolution reaction.The energy barriers of splitting water molecules and hydrogen adsorption free energies on the GB sites decrease with increasing the flexoelectric effect.These results highlight an attractive way of utilizing the flexoelectric effect of GB-containing TMDs to improve their surface catalytic capability for hydrogen generation.     

Microanalysis on the Hydrogen Ion Irradiated 50 wt pct TiC-C Films

The 50 wt pct TiC-C films were prepared on stainless steel substrates by using a technique of ion beam mixing. These films were irradiated by hydrogen ion beam with a dose of 1×10^18 ions/cm^2 and an energy of 5 keV. Microanalysis of X-ray photoelectron spectroscopy （XPS） and secondary ion mass spectroscopy （SIMS） were used to analyze the films before and after hydrogen ion irradiation and to study the mechanism of hydrogen resistance.     

Size-dependent dissociation of small cobalt clusters on ultrathin NaCl films

Mass-selected anionic cobalt clusters of two different sizes (Co2 and Co13) were deposited onto ultrathin NaC1 films grown on an Au(111) substrate.Using scanning tunneling microscopy experiments and density functional theory simulations,we show that the deposited Co2 cluster dissociates and that the resulting Co atoms dope the NaCl surface by substituting Na ions.In contrast,the larger Co13 cluster does not dissociate and remains stable on top of the NaC1 film.The sizedependent fragmentation of clusters is an important aspect in the understanding of the chemical interaction between size-selected small aggregates of atoms and supporting surfaces.     

Highly Improved Gaseous Hydrogen Storage Characteristics of the Nanocrystalline and Amorphous Nd-Cu-added Mg2Ni-type Alloys by Melt Spinning

The nanocrystalline and amorphous Mg-Nd-Ni-Cu quaternary alloys with a composition of （Mg24Ni10Cu2）loo-xNdx （x = 0-20） were prepared by melt spinning. The X-ray diffraction and transmission electron microscopy inspections reveal that, by varying the spinning rate and the Nd content, different microstructures could be obtained by melt spinning. Particularly, the as-spun Nd-free alloy holds an entire nanocrystalline structure but the as-spun Nd-added alloy has a nanocrystalline and amorphous structure, which implies that the addition of Nd facilitates the glass forming in the Mg2Ni-type alloy. Also, the degree of the amorphization in the as-spun Nd-added alloys clearly increases with increasing the spinning rate and the Nd content. The H-storage capacity and the hydrogenation kinetics of amorphous, partially and completely nanocrystalline alloys were investigated and it was found that they are dependent on the microstructure and the phase composition of the alloys. Specially, enhancing the spinning rate from 0 （the as-cast was defined as the spinning rate of 0 m/s） to 40 m/s makes the hydrogen absorption saturation ratio （R5a） （a ratio of the hydrogen absorption quantity in 5 min to the saturated hydrogen absorption capacity） increase from 35.2% to 90.3% and the hydrogen desorption ratio （R10d） （a ratio of the hydrogen desorption quantity in 10 min to the saturated hydrogen absorption capacity） rise from 12.7% to 44.9% for the （x = 5） alloy. And the growing of the Nd content from 0 to 20 gives rise to the R5a and R10d values rising from 85.7% to 94.5% and from 36.7% to 54.8% for the as-spun （30 m/s） alloys, respectively.     

Photovoltaic Properties of the Modified n-Si（111） Electrodes with Ethyl and Carboxyls

n-Si（111） surface tailed -C2H5, -C2H4COOH, -C2H2COOH were prepared by the reactions among Si-H to ethyl-Grignard, methyl acrylate and ethyl propionate, and the carboxyls were formed under the existence of trifluoroacetic acid. The composite n-Si（111） electrodes were obtained by depositing Pt nanodots and the photovoltaic characteristics for these electrodes were studied in I^-/I3^- redox electrolyte. The j-U （photo current density-potential） behaviors of photo-voltage and photocurrent densities to the electrodes under solar illumination varied regularly with groups of -C2H2COOH〉-C2H4COOH〉-H〉-C2H5. The photo-voltage and photocurrent density of the electrode tailed -C2 H2COOH were -0.641 V and 5.25 mA/cm^2, respectively, more negative than those of the non-conjugated modification.     

Electrochemical Fabrication of Pd-Ag Alloy Nanowire Arrays in Anodic Alumina Oxide Template

The synthesis of Pd-Ag alloy nanowires in nanopores of porous anodic aluminum oxide （AAO） template by electrochemical deposition technique was reported. Pd-Ag alloy nanowires with 16%-25% Ag content are expected to serve as candidates of useful nanomaterials for the hydrogen sensors. Scanning electron microscopy （SEM） and energy dispersed X-ray spectroscopy （EDX） were employed to characterize the morphologies and compositions of the Pd-Ag nanowires. X-ray diffraction （XRD） was used to characterize the phase properties of the Pd-Ag nanowires. Pd-Ag alloy nanowire arrays with 17.28%-23.76% Ag content have been successfully fabricated by applying potentials ranging from -0.8 to -1.0 V （vs SCE）. The sizes of the alloy nanowires are in agreement with the diameter of AAO nanopores. The underpotential deposition of Ag＋ on Pd and Au plays an important role in producing an exceptionally high Ag content in the alloy. Alloy compositions can still be controlled by adjusting the ion concentration ratio of Pd^2＋ and Ag＋ and the electrodeposition processes. XRD shows that nanowires obtained are in the form of alloy of Pd and Ag.     

Formation and Transport of Atomic Hydrogen in Hot-Filament Chemical Vapor Deposition Reactors

In this paper we focus on diamond film hot-filament chemical vapor deposition reactors where the only reactant is hydrogen so as to study the formation and transport of hydrogen atoms. Analysis of dimensionless numbers for heat and mass transfer reveals that thermal conduction and diffusion are the dominant mechanisms for gas-phase heat and mass transfer, respectively. A simplified model has been established to simulate gas-phase temperature and H concentration distributions between the filament and the substrate. Examination of the relative importance of homogeneous and heterogeneous production of H atoms indicates that filament-surface decomposition of molecular hydrogen is the dominant source of H and gas-phase reaction plays a negligible role. The filament-surface dissociation rates of H2 for various filament temperatures were calculated to match H-atom concentrations observed in the literature or derived from power consumption by filaments. Arrhenius plots of the filament-surface hydrogen dissociation rates suggest that dissociation of H2 at refractory filament surface is a catalytic process, which has a rather lower effective activation energy than homogeneous thermal dissociation. Atomic hydrogen, acting as an important heat transfer medium to heat the substrate, can freely diffuse from the filament to the substrate without recombination.     

A stepwise-designed Rh-Au-Si nanocomposite that surpasses Pt/C hydrogen evolution activity at high overpotentials

Hydrogen evolution by electrocatalysis is an attractive method of supplying clean energy.However,it is challenging to find cheap and efficient alternatives to rare and expensive platinum based catalysts.Pt provides the best hydrogen evolution performance,because it optimally balances the free energies of adsorption and desorption.Appropriate control of these quantities is essential for producing an efficient electrocatalyst.We demonstrate,based on first principles calculations,a stepwise designed Rh-Au-Si ternary catalyst,in which adsorption (the Volmer reaction) and desorption (the Heyrovsky reaction) take place on Rh and Si surfaces,respectively.The intermediate Au surface plays a vital role by promoting hydrogen diffusion from the Rh to the Si surface.Theoretical predictions have been explored extensively and verified by experimental observations.The optimized catalyst (Rh-Au-SiNW-2) has a composition of 2.2∶28.5∶69.3 (Rh∶Au∶Si mass ratio) and exhibits a Tafel slope of 24.0 mV·dec-1.Its electrocatalytic activity surpasses that of a commercial 40 wt.％ Pt/C catalyst at overpotentials above 0.19 V by exhibiting a current density of greater than 108 mA·cm-2.At 0.3 V overpotential,the turnover frequency of Rh-Au-SiNW-2 is 10.8 times greater than that of 40 wt.％ Pt/C.These properties may open new directions in the stepwise design of highly efficient catalysts for the hydrogen evolution reaction (HER).     

Effect of Interstitial Hydrogen on Cohesive Strength of Al Grain Boundary with Mg Segregation

The effect of interstitial hydrogen on the cohesion of the Al ∑ =11（113） grain boundary （GB） is investigated based on the thermodynamic model of Rice-Wang using the first-principles density tunction calculation. I he results indicate that interstitial H behaves as an embrittler from ＂strengthening energy＂ analysis. The reduced GB cohesion due to the presence of H at the GB is attributed to the low affinity between H and Al, and the weakened bonding of Al atomic pairs perpendicular to GB plane.     

Steric and electronic selectivity in the synthesis of Fe- 1,2,4,5-tetracyanobenzene （TCNB） complexes on Au（111）： From topological confinement to bond formation

A study of the surface assisted self-assembly of 1,2,4,5-tetracyanobenzene （TCNB） acceptor molecules and Fe atoms on an Au（111） surface is presented. While conditions to get the two-dimensional arrays of stable Fe（TCNB）4 complexes are clearly identified, ultrahigh vacuum scanning tunneling microscopy and spectroscopy （STM/STS） coupled with first-principles calculations reveals that situations may occur where Fe and TCNB survive on the surface （as Fe-4TCNB entities） at a higher density than the original molecular monolayer without forming coordination bonds with each other. It is found that the square planar coordination of the Fe（TCNB）4 monomer complexes cannot fully develop in the presence of lateral strain due to growth-induced confinement. A phenomenon similar to steric hindrance involving a strongly modified chirality with a Fe-N-C bond angle of 120° compared to the 180° for the stable complex may then explain why the Fe atom keeps its metallic bond with the surface. The competition between steric and electronic effects, not reported before, may arise elsewhere in surface chemistry involved in the synthesis of new and potentially useful organic nanomaterials.     

Enhanced π-π interactions between a C60 fullerene and a buckle bend on a double-walled carbon nanotube

In situ low-voltage aberration corrected transmission electron microscopy (TEM) observations of the dynamic entrapment of a C₆₀ molecule in the saddle of a bent double-walled carbon nanotube is presented. The fullerene interaction is non-covalent, suggesting that enhanced π-π interactions (van der Waals forces) are responsible. Classical molecular dynamics calculations confirm that the increased interaction area associated with a buckle is sufficient to trap a fullerene. Moreover, they show hopping behavior in agreement with our experimental observations. Our findings further our understanding of carbon nanostructure interactions, which are important in the rapidly developing field of low-voltage aberration corrected TEM and nano-carbon device fabrication.      

Tuning the structures of two-dimensional cuprous oxide confined on Au(111)

Two-dimensional (2D) cuprous oxide (Cu₂O) nanostructures (NSs) of monolayer thickness were synthesized on Au(111) and characterized using atomic-resolution scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. The surface and edge structures of 2D Cu₂O were resolved at the atomic level and found to exhibit a graphene-like lattice structure. Cu₂O NSs grew preferentially at the face centered cubic (fcc) domains of Au(111). Depending on the annealing temperature, the shapes and structures of Cu₂O NSs were found to vary from elongated islands with a defective hexagonal lattice (mostly topological 5–7 defects) to triangular NSs with an almost-perfect hexagonal lattice. The edge structures of Cu₂O NSs also varied with the annealing temperature, from predominantly the arm-chair 56 structure at 400 K to almost exclusively the zig-zag structure at 600 K. DFT calculations suggested that the herringbone ridges of Au(111) confined the growth and structure of Cu₂O NSs on Au(111). As such, the arm-chair edges of Cu₂O NSs, which are less stable than the zig-zag edges, could be exposed preferentially at 400 K. Cu₂O NSs developed into the thermodynamically-favored triangular form and exposed zig-zag edges at 600 K, when the Au(111) substrate became mobile. The confined growth of 2D cuprous oxide on Au(111) demonstrated the importance of metal-oxide interactions in tuning the structures of supported 2D oxide NSs.

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From zero to two dimensions: supramolecular nanostructures formed from perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) and Ni on the Au(111) surface through the interplay between hydrogen-bonding and electrostatic metal-organic interactions

Supramolecular self-assembly of the organic semiconductor perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) together with Ni atoms on the inert Au(111) surface has been investigated using high-resolution scanning tunneling microscopy under ultrahigh vacuum conditions. We demonstrate that it is possible by tuning the co-adsorption conditions to synthesize three distinct self-assembled Ni-PTCDI nanostructures from zero-dimensional (0-D) nanodots over one-dimensional (1-D) chains to a two-dimensional (2-D) porous network. The subtle interplay among non-covalent interactions responsible for the formation of the observed structures has been revealed from force-field structural modeling and calculations of partial charges, bond orders and binding energies in the structures. A unifying motif for the 1-D chains and the 2-D network is found to be double N-H??O hydrogen bonds between PTCDI molecules, similar to the situation found in surface structures formed from pure PTCDI. Most interestingly, we find that the role of the Ni atoms in forming the observed structures is not to participate in metal-organic coordination bonding. Rather, the Ni adatoms acquire a negative partial charge through interaction with the substrate and the Ni-PTCDI interaction is entirely electrostatic.      

Negative supracrystals inducing a FCC-BCC transition in gold nanocrystal superlattices

The growth of nanocrystal superlattices of 5 nm single domain Au nanocrystals at an air-toluene interface induces formation of well-defined thin films （300--400 nm） with large coherence lengths. High-resolution electron microscopy showed that polyhedral holes （negative supracrystal） were formed on the nanocrystal superlattice surface. Formation of negative supracrystals is attributed to inclusion in the superlattice of organic molecules （dodecanethiol）, which are present in concentrated zones at the air-toluene interface. The coexistence of two supracrystalline structures （bcc/fcc） is attributed to diffusion of dodecanethiol molecules resulting in a Bain deformation of the nanocrystal array.     

Superfluidity in Hydrogen-Deuterium Mixed Clusters

Path-integral Monte Carlo calculations have been performed on para-H₂ clusters mixed with several ortho-D₂ molecules, (D₂) $_{N_{D}}$ (H₂) $_{N_{H}}$ with 1≤N _D ≤5 and 10≤N _H ≤20, to study their superfluid behavior at low temperatures. It is found that heavier D₂ molecules are located near the center of the mixed cluster, and radial density distributions of D₂ and H₂ tend to separate from each other as the number of D₂ molecules increases. We have also found significant suppression of superfluidity at specific cluster sizes when compared to those of the neighboring sizes. This can be understood in terms of the magic number stability previously reported in hydrogen clusters. Finally we present the local superfluid density distributions in the mixed clusters, which shows uniform local superfluidity of H₂ except near D₂ molecules.     

Preparation, characterization, and application of electrochemically functional graphene nanocomposites by one-step liquid-phase exfoliation of natural flake graphite with methylene blue

Electrochemically functional graphene nanocomposites have been directly prepared by one-step liquid-phase exfoliation of natural flake graphite with methylene blue (MB). UV-visible spectra of the obtained aqueous dispersions of graphene-methylene blue (G-MB) nanocomposite at different exfoliation time indicate that the concentration of graphene dispersion increased markedly with increasing exfoliation time. Atomic force microscopy (AFM) and Raman spectroscopy verified that the graphene was exfoliated into single-layer or bilayer states. FT-IR spectroscopy of G-MB suggests that a ??-?? stacking interaction is involved in the structure-associated interactions between graphene and adsorbed MB molecules. A G-MB nanocomposite modified glassy carbon (GC) electrode exhibits excellent electrochemical properties and good electrochemical stability. Additionally, the G-MB/GC modified electrode shows more favorable electron transfer kinetics for potassium ferricyanide and potassium ferrocyanide probe molecules, which are important electroactive compounds, compared with reduced graphene oxide (RGO)-MB/GC, RGO/GC, bare GC and graphite/GC electrodes. Furthermore, the G-MB/GC modified electrode exhibits good electrocatalytic activity toward hydrogen peroxide (H₂O₂) and ??-nicotinamide adenine dinucleotide (NADH). The excellent electroactivity, electrochemical stability and electrocatalytic activity of the G-MB nanocomposites prepared in this work are potentially very useful for basic electrochemical studies and for the practical development of electronic devices such as biosensors and photovoltaic cells.      

Mesocrystal Co3O4 nanoplatelets as high capacity anode materials for Li-ion batteries

Faceted crystals with exposed highly reactive planes have attracted intensive investigations for applications. Herein, we demonstrate a general synthetic method to prepare mesocrystal Co3O4 with predominantly exposed {111} reactive facets by the in situ thermal decomposition from Co（OH）2 nanoplatelets. The mesocrystal feature was identified by field emission scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and N2 isotherm analyses. When applied as anode material in lithium-ion batteries, mesocrystal Co3O4 nanoplatelets delivered a high specific capacity and an outstanding high rate performance. The superior electrochemical performance should be ascribed to the predominantly exposed {111} active facets and highly accessible surfaces. This synthetic strategy could be extended to prepare other mesocrystal functional nanomaterials.     

Pentacene-based nanorods on Au(111) single crystals: Charge transfer, diffusion, and step-edge barriers

We investigate nanorod assemblies of two δ ₄-substituted pentacenes, namely (2,3-X ₂-9,10-Y ₂)-substituted pentacenes with X = Y = OCH₃ (MOP) and with X = F, Y = OCH₃ (MOPF), grown on Au(111) single crystals. By using a multi-technique approach based on ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption, we find evidence for charge transfer screening at the interface with gold. Furthermore, the MOP and MOPF nanorods show a rough surface morphology, which was investigated with atomic force microscopy. We use molecular simulation techniques to investigate the energetic barriers to diffusion and to traverse step-edges to estimate their influence on the nanorod roughness. We find that barriers to surface diffusion on a terrace are anisotropic and that their direction favors the formation of nanorods in these materials.      

Anomalous moire pattern of graphene investigated by scanning tunneling microscopy： Evidence of graphene growth on oxidized Cu（111）

The growth of graphene on oriented （111） copper films has been achieved by atmospheric pressure chemical vapor deposition. The structural properties of as-produced graphene have been investigated by scanning tunneling microscopy. Anomalous moir6 superstructures composed of well-defined linear periodic modulations have been observed. We report here on comprehensive and detailed studies of these particular moir6 patterns present in the graphene topography revealing that, in certain conditions, the growth can occur on the oxygen-induced reconstructed copper surface and not directly on the oriented （111） copper film as expected.