Prediction of the structures of free and oxide-supported nanoparticles by means of atomistic approaches: the benchmark case of nickel clusters.

The structures of Ni/MgO nanoparticles are studied by means of global optimization searches. The results from four different model potentials, sharing the same functional forms but different parametrizations, are reported and compared. Two parametrizations over four give qualitatively correct results, and one of them is also quantitatively satisfactory. The other models fail to explain some qualitative features observed in the experiments, such as the formation of hcp nanodots at small sizes or the transition to fcc structures at large sizes. The important features that an atomistic potential must present for the correct prediction of Ni cluster structures are discussed and generalized.

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.

Electronic and structural shell closure in AgCu and AuCu nanoclusters.

The structures of AgCu clusters containing 40 atoms are investigated. The most promising structural families (fcc clusters, capped decahedra, and two types of capped polyicosahedra) are singled out by means of global optimization techniques within an atom-atom potential model. Then, representative clusters of each family are relaxed by means of density-functional methods. It is shown that, for a large majority of compositions, a complex interplay of geometric and electronic shell-closure effects stabilizes a specific polyicosahedral family, whose clusters are much lower in energy and present large HOMO-LUMO gaps. Within this family, geometric and quantum effects concur to favor magic structures associated with core-shell chemical ordering and high symmetry, so that these clusters are very promising from the point of view of their optical properties. Our results also suggest a natural growth pathway of AgCu clusters through high-stability polyicosahedral structures. Results for AuCu clusters of the same size are reported for comparison, showing that the interplay of the different effects is highly material specific.

Interface-stabilized phases of metal-on-oxide nanodots.

The control of the structure of oxide-supported metal nanoparticles is crucial in determining their properties and possible applications. Here, building principles are derived for predicting the epitaxies of metal nanoparticles on square-symmetry oxide surfaces. Unusual phases are found for an appropriate choice of the metal-oxide pair, where nanoparticles with hcp structure are stabilized for fcc metals such as Ni, Pd, and Pt, or for Co in a size range in which Co has typically nonhcp arrangements. These predictions are supported by a comparison with available experimental data on Ni/MgO(100) nanodots, and generalized to a whole class of metal-oxide systems of great potential interest, such as Pd and Pt on CaO, Ni on CoO, and Co on MgO. The atomistic features of the nanoparticles in turn suggest that these materials should possess peculiar properties; in particular, the facets exposed by the nanodots reveal adsorption sites with unusual geometry of possible effect on their catalytic properties, while the destabilization of stacking faults and the structural deformations observed for these particles are expected to influence their magnetic behavior.