The Gibbs free energy of silver nanoparticles has been obtained from the calculations of bulk free energy and surface free energy for both the solid and liquid phase. On the basis of the obtained Gibbs free energy of nanoparticles, thermodynamic properties of silver nanoparticles, such as melting temperature, molar heat of fusion, molar entropy of fusion, and temperature dependences of entropy and specific heat capacity have been investigated. Calculation results indicate that these thermodynamic properties can be divided into two parts: bulk quantity and surface quantity, and surface atoms are dominant for the size effect on the thermodynamic properties of nanoparticles. The method that the intersection of the free-energy curves for solid and liquid nanoparticles decide the melting point of nanoparticles demonstrates that the surface freeenergy difference between the solid and liquid phase is a decisive factor for the size-dependent melting of nanostructural materials.
Monte Carlo simulations were carried out to systematically investigate the effects of composition, size, and temperature on the surface segregation and structural features of Au−Pt nanoparticles in the present paper. A strong surface Au enrichment was observed in all of the nanoparticles, and the surface segregation of Au was promoted by increasing the particle sizes. It is found that the core−shell structure was preferred in the equilibrium Au−Pt nanoparticles with low Au composition, and three-shell onion-like structure was formed at high Au composition. The competitive multisite segregation was predicted in the core−shell nanoparticles in which Au atoms favor sites at the vertices, edges, and facets. The reverse temperature dependency of segregation for different surface sites has also been discussed.
The reduced graphene oxide (RGO)-based composites have attracted intensive attention in experiment due to its superior performance as photocatalysts, but still lacking is the theoretical understanding on the interactions between constituents, and the connection between such interaction and the enhanced photoactivity. Herein, the interaction between the g-C 3 N 4 and RGO sheets is systematically explored by using state-of-the-art hybrid density functional theory. We demonstrate that the O atom plays a crucial role in the RGO-based composites. Compared to the isolated g-C 3 N 4 monolayer, the band gap of composites obviously decreases, and at higher concentration, the levels in the vicinity of Fermi level are much more dispersive, indicating the smaller effective mass of the carrier. These changes are nonlinear on the O concentration.Interestingly, appropriate O concentration alters the direct-gap composite to indirect-gap one.Most importantly, at a higher O concentration, a type-II, staggered, band alignment can be obtained in the g-C 3 N 4 -RGO interface, and negatively charged O atoms in the RGO are active sites, leading to the high hydrogen-evolution activity. Furthermore, the calculated absorption spectra varying with the O concentration shed light on different experimental results. The findings pave the way for developing RGO-based composites for photocatalytic applications.
Analytic modified embedded atom method (AMEAM) type many-body potentials
have been constructed for ten hcp metals: Be, Co, Hf, Mg, Re, Ru, Sc, Ti, Y
and Zr. The potentials are parametrized using analytic functions and fitted
to the cohesive energy, unrelaxed vacancy formation energy, five
independent second-order elastic constants and two equilibrium conditions.
Hence, each of the constructed potentials represents a stable hexagonal
close-packed lattice with a particular non-ideal c/a ratio. In order to
treat the metals with negative Cauchy pressure, a modified term has been
added to the total energy. For all the metals considered, the hcp lattice
is shown to be energetically most stable when compared with the fcc and bcc
structure and the hcp lattice with ideal c/a. The activation energy for
vacancy diffusion in these metals has been calculated. They agree well with
experimental data available and those calculated by other authors for both
monovacancy and divacancy mechanisms and the most possible diffusion paths
are predicted. Stacking fault and surface energy have also been calculated
and their values are lower than typical experimental data. Finally, the
self-interstitial atom (SIA) formation energy and volume have been
evaluated for eight possible sites. This calculation suggests that the
basal split or crowdion is the most stable configuration for metals with a
rather large deviation from the ideal c/a value and the non-basal
dumbbell (C or S) is the most stable configuration for metals with c/a
near ideal. The relationship between SIA formation energy and melting
temperature roughly obeys a linear relation for most metals except Ru and
Re.
Monte Carlo simulations were performed to study systematically the surface segregation behaviors and atomic-scale structural features of AuÀAg nanoparticles for a range of alloy compositions, particle sizes, and temperatures. Segregation of Ag to the surface was observed in all the particles considered. The surface segregation was promoted by increasing the particle sizes or Ag compositions and decreasing nanoparticles' temperatures. It was found that the most stable mixing patterns are the onionlike structure with Ag-rich shell for small particles, and the alloyed-core/layered-shell structure for large particles. Accordingly, the calculated alloying extents based on Monte Carlo simulations are consistent with experimental EXAFS analysis, which indicates more obvious alloying features in nanoparticles with larger sizes or at higher temperatures, and more obvious segregated features in nanoparticles under the opposite conditions. The size distribution of Au ensembles on different coordinated sites was analyzed quantitatively, which presented varied composition-and temperature-dependent effects. The possible effects of size and shape distribution of surface ensembles on tuning the catalytic activity and selectivity of bimetallic nanoparticles were also discussed.
Multilayer van der Waals (vdWs) heterostructures assembled by diverse atomically thin layers have demonstrated a wide range of fascinating phenomena and novel applications.Understanding the interlayer coupling and its correlation effect is paramount for designing novel vdWs heterostructures with desirable physical properties. Using a detailed theoretical study of 2D MoS 2 -graphene (GR)-based heterostructures based on state-of-the-art hybrid density functional theory, we reveal that for 2D few-layer heterostructures, vdWs forces between neighboring layers depend on the number of layers. Compared to that in bilayer, the interlayer coupling in trilayer vdW heterostructures can significantly be enhanced by stacking the third layer, directly supported by short interlayer separations and more interfacial charge transfer. The trilayer shows strong light absorption over a wide range (<700 nm), making it very potential for solar energy harvesting and conversion. Moreover, the Dirac point of GR and band gaps of each layer and trilayer can be readily tuned by external electric field, verifying multilayer vdWs heterostructures with unqiue optoelectronic properties found by experiments. These results suggest that tuning the vdWs interaction, as a new design parameter, would be an effective strategy for devising particular 2D multilayer vdWs heterostructures to meet the demands in various applications.
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