Composition-dependent chemical ordering predicted in Pt-Ag nanoalloys.

Pt-Ag nanoalloys display an astonishing chemical organization depending on their size and composition. Reversed size-dependent stabilization of ordered nanophases [J. Pirart et al., Nat. Commun., 2019, 10, 1982-1989] has recently been shown around equiconcentration. We extend this study by a theoretical investigation on the whole range of compositions showing a significant composition-dependent chemical ordering in Pt-Ag nanoalloys. At a low silver content, the surface exhibits a strong Ag segregation coupled to a (2 × 1) superstructure on the (100) facets. By increasing the silver concentration, the system displays an L11 ordered phase in the core, interrupted in a narrow range of concentrations by a concentric multishell structure characterized by an alternation of Ag-pure/Pt-pure concentric layers starting from the surface shell to the core. Although the L11 ordered phase has been observed experimentally, the concentric multishell structure is lacking due to the difficulty of the experimental characterization.

Melting properties of AgxPt1-x nanoparticles.

At the nanoscale, materials exhibit unique properties that differ greatly from those of the bulk state. In the case of AgxPt1-x nanoalloys, we aimed to study the solid-liquid transition of nanoparticles of different sizes and compositions. This system is particularly interesting since Pt has a high melting point (2041 K compared to 1035 K for Ag) which could keep the nanoparticle solid during different catalytic reactions at relatively high temperatures, such as we need in the growth of nanotubes. We performed atomic scale simulations using a semi-empirical potential implemented in a Monte Carlo code at constant temperature and chemical composition in a canonical ensemble. We observed that the melting temperature decreases with decreasing size (pure systems and alloys) and increasing Ag content. We show that the melting systematically passes through an intermediate stage with a crystalline core (pure platinum or mixed PtAg depending on the composition) and a pure silver liquid skin, which strongly questions the idea of having a faceted solid particle in catalytic reactions for carbon nanotube synthesis.

Interplay between interdiffusion and shape transformations in nanoalloys evolving from core-shell to intermixed structures.

Nanoalloys are often grown or synthesized in non-equilibrium configurations whose further evolution towards equilibrium can take place through complex pathways. In this work, we consider bimetallic systems with tendency towards intermixing, namely AgAu, PtPd and AuCu. We analyze their evolution starting from non-equilibrium initial configurations, such as phase-separated core@shell ones, by means of molecular dynamics (MD) simulations. These systems present some differences, since AuCu bulk alloys make ordered phases at low temperature whereas AgAu and PtPd remain in solid solution. Moreover, Cu, Au and Ag have similar cohesive energies whereas Pt is much more cohesive than Pd. We consider both truncated octahedral and icosahedral initial shapes in the size range between 2 and 3 nm. For each AB system, we consider both A@B and B@A core@shell starting configurations. The evolution is characterized by monitoring the time-dependent degree of intermixing and the evolution of the shape. The simulations are performed up to temperatures close to the melting range. The approach to thermodynamic equilibrium is monitored by MD simulations and compared with the equilibrium chemical configurations obtained by Monte Carlo simulations.

Stress effect on segregation and ordering in Pt-Ag nanoalloys.

We performed a theoretical study of the chemical ordering and surface segregation of Pt-Ag nanoalloys in the range of size from 976 to 9879 atoms (3.12 to 6.76 nm). We used an original many-body potential able to stabilize the L11ordered phase at equiconcentration leading to a strong silver surface segregation. Based on a recent experimental study where nanoparticles up to 2.5 nm have been characterized by high transmission electron microscopy with the L11ordered phase in the core and a silver surface shell, we predict in our model via Monte Carlo simulations that the lower energy configuration is more complicated with a three-shell alternance of Ag/Pt/Ag from the surface surrounding the L11ordered phase in the core. The stress analysis demonstrates that this structure softens the local stress distribution inside the nanoparticle which contributes to reduce the internal energy.

Crossover among structural motifs in Pd-Au nanoalloys.

The crossovers among the most abundant structural motifs (icosahedra, decahedra and truncated octahedra) of Pd-Au nanoalloys have been determined theoretically in a size range between 2 and 7 nm and for three compositions equivalent to Pd3Au, PdAu and PdAu3. The chemical ordering and segregation optimisation are performed via Monte Carlo simulations using semi-empirical tight-binding potentials fitted to ab initio calculations. The chemical configurations are then quenched via molecular dynamic simulations in order to compare their energy and characterize the equilibrium structures as a function of the cluster size. For the smaller sizes (of around 300 atoms and fewer) the structures are also optimized at the electronic level within ab initio calculations in order to validate the semi-empirical potential. The predictions of the crossover sizes for the nanoalloys cannot be simply extrapolated from the crossover of the pure nanoparticles but imply stress release phenomena related to the size misfit between the two metals. Indeed, alloying extends the range of stability of the icosahedron beyond that of the pure systems and the energy differences between decahedra and truncated octahedra become asymptotic, around the sizes of 5-6 nm. Nevertheless, such equilibrium results should be modulated regarding kinetic considerations or possible gas adsorption under experimental conditions.

Aspect-ratio- and size-dependent emergence of the surface-plasmon resonance in gold nanorods--an ab initio TDDFT study.

It is known that the surface-plasmon resonance (SPR) in small spherical Au nanoparticles of about 2 nm is strongly damped. We demonstrate that small Au nanorods with a high aspect ratio develop a strong longitudinal SPR, with intensity comparable to that in Ag rods, as soon as the resonance energy drops below the onset of the interband transitions due to the geometry. We present ab initio calculations of time-dependent density-functional theory of rods with lengths of up to 7 nm. By changing the length and width, not only the energy but also the character of the resonance in Au rods can be tuned. Moreover, the aspect ratio alone is not sufficient to predict the character of the spectrum; the absolute size matters.

Structures of metal nanoparticles adsorbed on MgO(001). I. Ag and Au.

The structure of metal clusters supported on a MgO(001) substrate is investigated by a computational approach, with the aim to locate stable structural motifs and possible transition sizes between different epitaxies. Metal-metal interactions are modeled by a second-moment approximation tight-binding potential, while metal-oxide interactions are modeled by an analytic function fitted to first-principles calculations. Global optimization techniques are used to search for the most stable structural motifs at small sizes (N < or = 200), while at larger sizes different structural motifs are compared at geometric magic numbers for clusters up to several thousand atoms. Metals studied are Ag, Au, Pd, and Pt. They are grouped according to their mismatch to the oxide substrate (lattice constant of the metal versus oxygen-oxygen distance on the surface). Ag and Au, which have a smaller mismatch with MgO, are studied in Paper I, while Pd and Pt, with a larger mismatch, are investigated in Paper II. For Ag the cube-on-cube (001) epitaxy is favored in the whole size range studied, while for Au a transition from the (001) to the (111) epitaxy is located at N=1200. The reliability of the model is discussed in the light of the available experimental data.

Structures of metal nanoparticles adsorbed on MgO(001). II. Pt and Pd.

The structure of metal clusters on MgO(001) is searched for by different computational methods. For sizes N < or = 200, a global optimization basin-hopping algorithm is employed, whereas for larger sizes the most significant structural motifs are compared at magic sizes. This paper is focused on Pt and Pd/MgO(001), which present a non-negligible mismatch between the nearest-neighbor distance in the metal and the oxygen-oxygen distance in the substrate. For both metals, a transition from the cube-on-cube (001) epitaxy to the (111) epitaxy is found. The results of our simulations are compared to experimental data, to results found for Au and Ag in the previous paper (paper I), and to predictions derived from the Wulff-Kaischew construction.

Growth and structural properties of CuAg and CoPt bimetallic nanoparticles.

Core/shell CuAg and alloyed CoPt have been synthesized using two vapor phase deposition techniques. For CuAg prepared by Thermal Evaporation (TE), the size and the morphology of the Cu cores are the key parameters to promote the formation of the core/shell arrangement. For CoPt synthesized by Pulsed Laser Deposition (PLD), the growth kinetics of nanoparticles, depending on the deposition rate, the substrate nature and the temperature, controls the nanoparticle morphology. The competition between the growth and the ordering kinetics governs the nanoparticle structure. By reducing the growth kinetics, as-grown L1(0) ordered nanoparticles are obtained according to the bulk phase diagram.