DFT Study of Bimetallic Palladium−Gold Clusters Pd_nAu_m of Low Nuclearities (n + m ≤ 14)

Abstract

Bimetallic palladium-gold Pd(n)Au(m) clusters of low nuclearity (n + m ≤ 14) are studied using the density functional theory at B3LYP level with a Lanl2DZ pseudopotential to understand the evolution of various structural, electronic, and energetic properties as a function of size (n + m) and composition (n/m) of the system. The potential energy surfaces have been explored for many different structures, and the minima obtained were then collected and used as a starting point for comparing the selected properties. Theoretical results show a logical evolution of the properties depending on the size and the composition of the system. Pd(n) clusters clearly prefer 3D structures while Au(m) clusters favor planar configurations. The geometry of the bimetallic Pd(n)Au(m) clusters mainly depends on their composition, i.e., clusters enriched in palladium atoms prefer 3D structures while increasing gold contents promotes planar configurations with deviation from planarity near Pd centers. Regarding the electronic properties, NBO analysis reveals that the unique closed-shell electronic structure of Pd atoms (4d(10)) requires a (4d → 5s) promotion to form stable bonds. In contrast, the half-occupied Au 6s AO implies effective Au-Au interaction and the electronic structure of Au atoms remains almost unchanged upon formation of bimetallic bonds. Consequently, clusters enriched in palladium atoms have spin multiplicities that increase with the cluster size while clusters enriched in gold atoms maintain the lowest possible spin multiplicity. Finally, the stability of these systems shows a synergic gain in cohesion for mixed Pd(n)Au(m) clusters compared to their monometallic Pd(n) and Au(m) counterparts. The maximal stabilization effect corresponds to n ≈ m, compositions for which the number of mixed Pd-Au bonds is maximized.

Application of a parallel genetic algorithm to the global optimization of medium-sized Au–Pd sub-nanometre clusters

“…The Pd atoms are located in or close to the centre of the clusters in high-coordination sites. The deviation from planarity generally occurs near this Pd centre, as previously reported for N ≤ 14 clusters [35].…”

“…Substitution of one Au atom by Pd in pure Au clusters is responsible for the shift of the 2D-3D transition point from 14-and 16-atom clusters to 12 atoms (Au 11 Pd 1 ). This is in disagreement with the 2D-3D transition point suggested by Zanti and Peeters at size N = 8 [35]. However, in continuation with the planar structural pattern observed for N = 4-10 Pd-doped Au clusters previously [37], the gas-phase Au 10 Pd 1 global minimum is also planar.…”

“…The smallest 3D ground-state structure was predicted previously to be Au 10 by David and co-workers [58], using the second-order Møller-Plesset perturbation theory (MP2) method. Employing hybrid DFT, Zanti and Peeters [35] arrived at the same conclusion showing a 3D structure for N = 10. This, however, disagreed with the 2D Au 10 structure recently obtained by us [37] and previously by different research groups using semi-local density functional theory (DFT) [59] and coupled cluster singles doubles and perturbative triples [CCSD(T)] calculations [60,61].…”

“…Turning our attention to medium-sized Au-Pd nanoalloys, a comprehensive study of structural motifs for all compositions have, to our knowledge, not been investigated before. However, Au-doped Pd clusters [33], (1:1) compositions [34], and nuclearities lower than 14 atoms [35] have previously been studied theoretically.…”

“…In the Au-Pd system, doping an atom of one metal into a pure cluster of the other metal often yields clusters with non-identical structures and properties [35][36][37]. This can increase the difficulty in finding global minima, in addition to the permutational isomers (homotops) effect [38].…”