Relative Stability of Small Silver, Platinum, and Palladium Doped Gold Cluster Cations
Relative Stability of Small Silver, Platinum, and Palladium Doped Gold Cluster Cations
Ferrari, Piero;Janssens, Ewald
2019-04-23 00:00:00
applied sciences Article Relative Stability of Small Silver, Platinum, and Palladium Doped Gold Cluster Cations Piero Ferrari * and Ewald Janssens Laboratory of Solid State Physics and Magnetism, KU Leuven, 3001 Leuven, Belgium; ewald.janssens@kuleuven.be * Correspondence: piero.ferrari@kuleuven.be; Tel.: +32-16-377-503 Received: 31 March 2019; Accepted: 17 April 2019; Published: 23 April 2019 Featured Application: This work uses small single-atom doped gold clusters as model systems for understanding fundamental physical aspects of Ag-Au, Pt-Au, and Pd-Au alloy nanoparticles. Understanding their intrinsic properties is highly desirable in view of better designed bimetallic nanoparticles in catalytic and optical applications with properties that are tuned to meet the requirements of each specific application. Abstract: The stability patterns of single silver, platinum, and palladium atom doped gold cluster cations, MAu (M = Ag, Pt, Pd; N = 3–6), are investigated by a combination of photofragmentation N 1 experiments and density functional theory calculations. The mass spectra of the photofragmented clusters reveal an odd-even pattern in the abundances of AgAu , with local maxima for clusters N 1 containing an even number of valence electrons, similarly to pure Au . The odd-even pattern, however, disappears upon Pt and Pd doping. Computed dissociation energies agree well with the experimental findings for the dierent doped clusters. The eect of Ag, Pt, and Pd doping is discussed on the basis of an analysis of the density of states of the N = 3–5 clusters. Whereas Ag delocalizes its 5s valence electron in all sizes, this process is size-specific for Pt and Pd. Keywords: metal cluster stability; doping; electronic structure; photofragmentation 1. Introduction Small metal clusters in the gas phase, produced under conditions where cluster-cluster and cluster-environment interactions are absent, are ideal model systems for a fundamental understanding of the dierent physical and chemical properties of matter. In a gas phase experiment, clusters are produced and characterized as a function of size, composition, and charge state with atomic precision, and their inherent small size allows for direct comparison with detailed quantum chemical calculations. Many examples in the literature can be found in which small clusters are used to elucidate intrinsic properties of matter, such as the stability of alloy complexes [1,2], the reactivity and catalytic properties of metals [3,4], the optical responses of matter [5,6], and the magnetic coupling of dierent elements and their evolution from the atom to the bulk [7,8]. In particular, small gold clusters have been intensively studied over the past few decades due to their fascinating properties. For example, at the nanoscale gold becomes reactive towards dierent molecules [9–12], whereas in bulk it is one of the noblest elements [13]. Moreover, small gold clusters possess unique optical properties [6,14] that are dierent from those of silver clusters, even though both elements have a similar electronic configuration. The structures of small gold clusters are also remarkable; for instance, Au is known to adopt a highly symmetric pyramidal geometry [15], and smaller gold clusters remain planar up to surprisingly large sizes. The size at which clusters adopt three dimensional structures in their lowest Appl. Sci. 2019, 9, 1666; doi:10.3390/app9081666 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 1666 2 of 13 energy configuration depends on the charge state. Whereas cationic clusters are planar up to N = 7, according to ion mobility experiments [16], anions become three-dimensional at size N = 12 [17–19]. The size-dependent stability of small gold clusters is also of interest. In those small metal clusters, each atom delocalizes its 6s valence electron over the entire cluster volume. Electron confinement by the small size of the system results in the development of electronic shells with similar nodal character and degeneracy as those in single atoms [20]. Because of the dierent nature of the confining potential, however, electronic shells in clusters follow a dierent order, with no restrictions between quantum numbers [21]. The order of the so-called superatomic electronic shells depends on the exact shape of the confining potential, but in spherically symmetric potentials it follows the pattern: 1S, 1P, 1D, 2S, 1F, 2P, : : : [22,23]. The filling of these shells explains the famous stability pattern of Na clusters, with intensity maxima at clusters composed of 2, 8, 10, 40, : : : atoms [24]. When a cluster has the precise number of atoms, or in this context of delocalized electrons, that close an electronic shell, stability is enhanced with a concomitantly larger energy separation between the highest occupied and lowest unoccupied molecular orbitals (the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap) [15]. Such patterns in stability, related to the cluster ’s electronic shell structure, have been observed mass-spectrometrically in numerous occasions for Au clusters, with a very pronounced size-by-size dependence, since the low-symmetric structures of the clusters lift all but the spin’s degeneracy [25–27]. The size-dependency of cluster properties can be greatly altered by the introduction of dopant atoms. This holds for their reactivities [28–30], stabilities [2,31,32], optical properties [14,33], and magnetism [34]. The changes in their properties can be related to an interplay between cluster geometry and electronic structure, both being affected by doping. In the case of gold, the transition at which clusters adopt three-dimensional structures can be largely altered by doping; according to theoretical calculations, for example, a single Pd dopant atom reduces the size at which cationic Au clusters adopt three-dimensional geometries to the smallest possible size of PdAu [35]. The electronic structure of a cluster can also be drastically influenced by doping. This is especially the case if the dopant atom has a different number of valence electrons, thus altering the number of itinerant electrons available for filling the superatomic electronic shells [32], or when the dopant atom has a different electronegativity than the host element, inducing significant inter-cluster electron charge transfers [4]. The combination of these effects makes it difficult, a priori, to predict the influence of doping on the stability, even at the very smallest sizes, and requests for a combination of dedicated experiments with theoretical calculations. In this work we combine photofragmentation experiments with density functional theory calculations in order to investigate the effect of doping on the relative stability of small Au (N 6) clusters. Three dopant atoms have been 14 10 1 10 1 selected, with electronic configurations slightly different from Au ([Xe] 4f 5d 6s ): Ag ([Kr] 4d 5s ), 14 9 1 10 Pt ([Xe] 4f 5d 6s ), and Pd ([Kr] 4d ). 2. Methods 2.1. Photofragmentation Experiments Gas phase clusters were produced by laser ablation and inert gas condensation using an experimental setup detailed elsewhere [36]. For the production of M (M = Ag, Pt, Pd) doped gold clusters, two independent nanosecond pulsed Nd:YAG lasers (2nd harmonic, 532 nm; Spectra-Physics, Santa Clara, CA, USA) were focused on a gold and an M target, right after a short pulse of He carrier gas (backing pressure of 7 bar) was introduced in the source. By collisions with the carrier gas and subsequent expansion into a vacuum, the ablated plasma condensated and formed a distribution of clusters of various sizes and compositions. This distribution can be tuned by production conditions, including the relative energy of the ablation lasers, their relative firing time, and the pressure of the He gas [37]. In order to obtain information about the relative stability of the clusters, irrespective of the production conditions of the source, the initially charged clusters were electrostatically deflected from the molecular beam and neutral species were excited by a focused excimer F laser 2 Appl. Sci. 2019, 9, 1666 3 of 13 Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 13 (157 nm). This excitation induces a fast ionization process followed by extensive fragmentation [25]. The abundances of the photofragmented species were then analyzed by time-of-flight mass spectrometry, abundances of the photofragmented species were then analyzed by time-of-flight mass spectrometry, allowing for the identification of relative stability patterns. This approach has been used in the past to allowing for the identification of relative stability patterns. This approach has been used in the past investigate the relative stability of several clusters of dierent sizes and compositions [27,35,38,39]. to investigate the relative stability of several clusters of different sizes and compositions [27,35,38,39]. For Ag and Pd doping, there was no mass overlap between dierent species and the dierent For Ag and Pd doping, there was no mass overlap between different species and the different clusters can be easily distinguished in the mass spectra. This is shown in Figure 1, where fractions of clusters can be easily distinguished in the mass spectra. This is shown in Figure 1, where fractions of + + + + typical mass spectra of photofragmented AgAu and PdAu clusters are presented in panels (a) typical mass spectra of photofragmented AgAu NN − 1 1 and PdAuN N−