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Relative Stability of Small Silver, Platinum, and Palladium Doped Gold Cluster Cations

Relative Stability of Small Silver, Platinum, and Palladium Doped Gold Cluster Cations 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 N1 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 N1 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 di erent doped clusters. The e ect 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 di erent 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 di erent 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 di erent 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 di erent 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 di erent nature of the confining potential, however, electronic shells in clusters follow a di erent 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 di erent 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 di erent species and the di erent 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− 1 1 clusters are presented in panels (a) and (b), respectively. Continuous lines connect the MAu species to visualize the size-dependent and (b), respectively. Continuous lines connect the MAu N N -1 1 species to visualize the size-dependent abundances. This situation is di erent for the case of Pt doping. Platinum has a distinct isotopic abundances. This situation is different for the case of Pt doping. Platinum has a distinct isotopic pattern (192 u (0.008%), 194 u (0.329%), 195 u (0.338%), 196 u (0.253%), and 198 u (0.072%)) and the pattern (192 u (0.008%), 194 u (0.329%), 195 u (0.338%), 196 u (0.253%), and 198 u (0.072%)) and the Au atom mass resides in between (197 u). In addition, the strong binding energy of the Pt dimer Au atom mass resides in between (197 u). In addition, the strong binding energy of the Pt2 dime 2 r (4.63 (4.63 eV compared to 3.15 eV of Au in our calculations) makes it dicult to find production conditions eV compared to 3.15 eV of Au2 in o 2 ur calculations) makes it difficult to find production conditions + + + + + + under which only pure Au and singly doped PtAu clusters are formed. Thus, pure Pt under which only pure AuN N and singly doped PtAuN-1 clusters N1 are formed. Thus, pure PtN clusteN rs clusters were also produced, as well as mixed Pt Au (x = 0–N) species. A part of a typical were also produced, as well as mixed PtxAuN-x (x = 0 x –N N) specie x s. A part of a typical mass spectrum mass spectrum of the Pt Au clusters is presented in Figure 1c. For this reason, a deconvolution of the PtxAuN-x clusters is p x resented Nx in Figure 1c. For this reason, a deconvolution process needed to process needed to be performed to quantify the intensity of each cluster composition. An example be performed to quantify the intensity of each cluster composition. An example is presented in Figure is presented in Figure 1c. Each Pt Au (x = 0–N) intensity profile was assumed to be composed 1c. Each PtxAuN-x (x = 0–N) intensit x yN prof x ile was assumed to be composed of a set of Gaussian of a set of Gaussian functions matching the natural isotopic distribution of the cluster. The width functions matching the natural isotopic distribution of the cluster. The width of a single Gaussian of a single Gaussian function was determined by recording a mass spectrum of photofragmented function was determined by recording a mass spectrum of photofragmented pure AuN clusters (with pure Au clusters (with a single isotope for each size) under the same experimental conditions, a single isot N ope for each size) under the same experimental conditions, in order to account for the in order to account for the mass-dependent resolution of the mass spectrometer. Therefore, the center mass-dependent resolution of the mass spectrometer. Therefore, the center and the width of each and the width of each Gaussian function was fixed, while the total intensities of each Pt Au Gaussian function was fixed, while the total intensities of each PtxAuN-x composition were xused as Nx composition were used as fitting parameters. The analysis was restricted to the size range for which fitting parameters. The analysis was restricted to the size range for which this deconvolution could this deconvolution could be applied satisfactorily (N  6). As a corroboration of the procedure, be applied satisfactorily (N ≤ 6). As a corroboration of the procedure, the extracted abundances of + + + + the extracted abundances of photofragmented pure Au and Pt clusters were compared with photofragmented pure AuN and PtN clusters were compared with results from previous N N results from previous measurements [35,40], showing an almost identical result. measurements [35,40], showing an almost identical result. + + Figure 1. Parts of representative mass spectra of photofragmented clusters: (a) AgAu , Figure 1. Parts of representative mass spectra of photofragmented clusters: (a) AgAuN−1 , (b) PdAuN−1 , N1 + + (b) PdAu , and (c) PtAu . In (a,b) continuous lines are included to aid visualization of and (c) PtAuN-1 . In (a) and (b) continuous lines are included to aid visualization of the different N1 N1 the di erent abundances of the doped species. Pure Au clusters are marked by asterisks, whereas abundances of the doped species. Pure AuN clusters are N marked by asterisks, whereas additional additional peaks correspond to carbon and oxygen contaminations or to doubly doped species of low peaks correspond to carbon and oxygen contaminations or to doubly doped species of low intensity. intensity. (d) Deconvolution of the photofragmented cluster peaks around 580–595 u in panel ( +c): Au + , (d) Deconvolution of the photofragmented cluster peaks around 580–595 u in panel (c): Au3 , PtAu3 2 , + + + PtAu + , Pt Au +, and Pt . Each peak is assumed to be composed of Gaussian functions of fixed width Pt2Au 2 , and 2 Pt3 . Each peak 3 is assumed to be composed of Gaussian functions of fixed width and and matching the natural isotopic distribution of each species. matching the natural isotopic distribution of each species. 2.2. Theoretical Calculations Density functional theory calculations were performed with the ORCA 4.0.1 software package [41]. The lowest energy structures of the clusters were adopted as follows. For the Ag doped clusters, Appl. Sci. 2019, 9, 1666 4 of 13 2.2. Theoretical Calculations Density functional theory calculations were performed with the ORCA 4.0.1 software package [41]. The lowest energy structures of the clusters were adopted as follows. For the Ag doped clusters, the lowest-energy isomers calculated in Reference [42] were assumed. Experimental characterization of the structures of mixed Au-Ag clusters have been performed up to size N = 5 using ion mobility experiments [43], photodissociation spectroscopy measurements [14], and far-infrared multiphoton dissociation studies [44,45]. The structures assigned in these studies agree with those in Reference [42]. For the case of Pt doping, we are not aware of any experimental investigation characterizing the structure of these clusters, nor of a theoretical study searching systematically for low-lying isomers of the cationic species. A search for low-energy structures was performed by manually constructing di erent initial geometries. Finally, with regards to Pd doping, a thorough theoretical investigation + + of the Au and PdAu (N = 2–20) clusters was presented in our previous work [35]. We adopt N N1 the structures found there. All geometries were optimized with the LC-BLYP exchange-correlation functional and the extensive Def2-TZVPP basis set [46]. Long-range corrections in Generalized Gradient Approximation (GGA) functionals have been shown to perform well in Pd and Ag doped Au clusters [14,33,35]. In addition, other DFT functionals have been tested, including the GGA PBE, meta-GGA TPSS, and the hybrid B3LYP. For each cluster those other functionals yielded a similar energetic ordering of the considered isomers as the LC-BLYP functional. Additionally, the even larger Def2-QZVPP basis set was checked, showing no significant di erence. For these optimizations, the e ective core potential Def2-ECP was used, replacing 60 core electrons for Au and Pt, and 28 core electrons for Ag and Pd. For the Ag and Pd doped clusters only the lowest possible spin configurations were calculated, as previous studies showed these have the lowest energy. For the Pt doped clusters, di erent spin configurations were computed for each cluster size. Also, in this case the lowest possible spin states were found to be the lowest in energy. Vibrational frequencies were calculated for all the structures at the same level of theory to corroborate that the geometries corresponded to true minima on the potential energy surface and not to transition states. Relativistic e ects are known to be important for heavy atoms, such as Au and Pt [47]. The cluster sizes investigated here are small enough to allow the implicit inclusion of relativistic e ects by performing single point calculations on the optimized structures using the relativistic Hamiltonian within the zero-order regular approximation (ZORA) [48] and the all-electron ZORA-def2-TZVPP basis set [49]. The size-to-size stability of the clusters was analyzed computationally by the dissociation energies D , defined as + + D = E A + E(A ) E A , (1) N Nm m N + + where E(A ), E(A ), and E(A ) correspond to the total energy of the corresponding cationic N Nm clusters and of the emitted neutral fragment. Many theoretical studies make use of another computed quantity when analyzing the relative stability of clusters, namely the second energy di erence D E [35,40,50,51]. This quantity compares the total energy of size N with that of N + 1 plus N 1. Interpretation of experimental data using computed D E values, however, requires that the fragmentation channel is always the same, irrespective of the cluster size, or that at least one channel dominates the decay, for example the neutral monomer emission. Recent attempts have even tried to combine D and D E , in order to find a better stability descriptor, by defining a second dissociation N N energy di erence, but this definition requires an invariant fragmentation channel with size [52]. Since here we focus on doped species, the favored channel may be size dependent. Simply comparing the dissociation energies of all possible fragmentation channels is a more solid description of stability and was therefore used in this work. Appl. Sci. 2019, 9, 1666 5 of 13 3. Results 3.1. Abundances of Photofragmented Clusters The results of the photofragmentation experiments are summarized in Figure 2, which shows + + + + the cluster abundances (I ) for (a) Au , (b) AgAu , (c) PtAu , and (d) PdAu clusters, N N1 N1 N1 in the size range of N = 2–6. The values of I were calculated by integrating the peak area of each Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 13 corresponding cluster size in the mass spectra. In panel (a) the well-known odd-even oscillation in the abundances of Au is seen, with local maxima for clusters composed of an odd number of [25,27,35]. As discussed, this pattern can be understood by the delocalization of each Au 6s valence atoms [25,27,35]. As discussed, this pattern can be understood by the delocalization of each Au 6s electron over the cluster volume, which, due to quantum confinement, develops energy shells. The valence electron over the cluster volume, which, due to quantum confinement, develops energy shells. cluster has an enhanced stability if all shells are completely filled. Since these clusters are positively The cluster has an enhanced stability if all shells are completely filled. Since these clusters are positively charged, an odd number of atoms corresponds to an even number of electrons, required to close the charged, an odd number of atoms corresponds to an even number of electrons, required to close electronic shells due to spin degeneracy [20]. We point out, however, that this simplified model the electronic shells due to spin degeneracy [20]. We point out, however, that this simplified model assumes a negligible influence of the Au 5d electrons on the relative stability pattern. assumes a negligible influence of the Au 5d electrons on the relative stability pattern. + + + + + + Figure 2. Abundances after photofragmentation for (a) Au , (b) AgAu , (c) PtAu , Figure 2. Abundances after photofragmentation for (a) AuN , ( N b) AgAuN−1 , ( N c1) PtAuN-1 , and ( N1 d) and (d) PdAu clusters; these represent the relative stability of the di erent sizes. The values were PdAuN−1 clust Ne 1rs; these represent the relative stability of the different sizes. The values were calculated by integrating the area of the corresponding signal in the mass spectra. calculated by integrating the area of the corresponding signal in the mass spectra. Upon Ag doping, as presented in Figure 2b, the odd-even oscillation in the abundances of pure Upon Ag doping, as presented in Figure 2b, the odd-even oscillation in the abundances of pure + + gold clusters was not significantly modified. Local maxima in I are found for AgAu and AgAu , N 2 + 4 + gold clusters was not significantly modified. Local maxima in 𝐼 are found for AgAu2 and AgAu4 , with an amplitude of the abundance oscillation similar to that for pure Au . The simple explanation with an amplitude of the abundance oscillation similar to that for pure AuN . The simple explanation 10 1 for this is that the silver atom, with an electronic [Kr] 4d 5s configuration, delocalizes its valence 10 1 for this is that the silver atom, with an electronic [Kr] 4d 5s configuration, delocalizes its valence electron for each cluster size. This observation is not surprising, since pure Ag clusters possess a very electron for each cluster size. This observation is not surprising, since pure Ag clusters possess a very similar odd-even stability pattern as pure Au clusters [32], although it is slightly more pronounced due similar odd-even stability pattern as pure Au clusters [32], although it is slightly more pronounced to the less influential d-states of the clusters in their electronic structure [28]. due to the less influential d-states of the clusters in their electronic structure [28]. The e ect of Pt doping on the Au abundances is shown in Figure 2c. As seen, the odd-even N + The effect of Pt doping on the AuN abundances is shown in Figure 2c. As seen, the odd-even staggering is to some extent preserved upon Pt doping up to N = 4, with an intensity maximum at staggering is to some extent preserved upon Pt doping up to N = 4, with an intensity maximum at + + PtAu . This pattern, however, changes at N = 5, because PtAu is not a local intensity maximum. 2 + 4 + PtAu2 . This pattern, however, changes at N = 5, because PtAu4 is not a local intensity maximum. 14 9 1 Since the electronic configuration of the platinum atom is [Xe] 4f 5d 6s , the observed stability 14 9 1 Since the electronic configuration of the platinum atom is [Xe] 4f 5d 6s , the observed stability pattern suggests the delocalization of the Pt 6s valence electron in the N = 2–4 size range, giving a total of two delocalized electrons in the PtAu2 cluster. This assumption is not trivial in view of the open d-shell configuration of Pt. An understanding of the modified pattern in the N = 4–6 size range upon Pt doping requires further analysis. Finally, the abundances of the PdAuN−1 clusters are shown in Figure 2d. The stability pattern is very similar to that seen for the Pt doped case, with a relative intensity maximum at N = 3 but not at N = 5. For the Pt doped clusters, this pattern in the abundances suggests that Pd delocalizes one electron in PdAu2 , even though this atom has a ground state electronic configuration with a full d- shell and no valence s electrons ([Kr] 4d ). Photoelectron spectroscopy experiments on anionic Appl. Sci. 2019, 9, 1666 6 of 13 pattern suggests the delocalization of the Pt 6s valence electron in the N = 2–4 size range, giving a total of two delocalized electrons in the PtAu cluster. This assumption is not trivial in view of the open d-shell configuration of Pt. An understanding of the modified pattern in the N = 4–6 size range upon Pt doping requires further analysis. Finally, the abundances of the PdAu clusters are shown in Figure 2d. The stability pattern is N1 very similar to that seen for the Pt doped case, with a relative intensity maximum at N = 3 but not at N = 5. For the Pt doped clusters, this pattern in the abundances suggests that Pd delocalizes one electron in PdAu , even though this atom has a ground state electronic configuration with a full d-shell and no valence s electrons ([Kr] 4d ). Photoelectron spectroscopy experiments on anionic PdAu clusters below N = 6 have shown that Pd can promote one of its 4d electrons to the 5s shell, which then participates in the bonding with Au [53]. In addition, in our previous work on the photofragmentation of larger Pd doped Au cluster cations, we demonstrated similar behavior in PdAu [35]. The Pd dopant delocalizes one of its 4d electrons, giving to the cluster a total of six itinerant electrons, a pronounced magic number in 2D systems [54]. Therefore, it is possible that Pd is delocalizing an electron in PdAu in order to fill the cluster ’s 1S electronic shell. This assumption, however, requires a detailed analysis of the electronic structure for confirmation. Similarly, further analysis is needed to understand the + + observation that PdAu (as PtAu ) does not seem to possess any particular stability. These matters 4 4 are discussed later in the text. 3.2. Theoretical Results + + The computed lowest energy structures of Au and MAu (N = 2–7, M = Ag, Pt, and Pd) N N1 clusters are shown in Figure 3. The lowest energy structures of cationic Au clusters are planar up to N = 7 (although Au has a slight out-of-plain distortion), according to ion mobility experiments [16]. In many DFT studies, however, it is predicted that the 2D to 3D transition takes place at N = 8, although the planar and 3D isomers of Au are close in energy [35,55]. The source for this discrepancy can be related to the lack of implicit relativistic e ects in most DFT studies on gold clusters, although that question goes beyond the scope of this study. In the N  7 size range, theory and experimental results agree that all Au clusters are two-dimensional. Upon Ag doping, the 2D-3D transition size is modified. AgAu adopts a 3D twisted X-structure, indicating a decrease of the transition + + size to N = 5, but AgAu is again a two-dimensional cluster. Pure Ag clusters are known 5 N to become three-dimensional at N = 5, as determined recently by far-infrared multiple photon dissociation spectroscopy measurements in conjunction with DFT calculations [56]. Upon Pt doping, calculations predict the 2D-3D transition at the lowest possible size, i.e., at N = 4, with PtAu adopting a tetrahedral geometry. Combined far-infrared multiphoton dissociation spectroscopy and DFT calculations have determined that pure Pt clusters become three-dimensional at N = 4 as well [57], showing the strong tendency of platinum to form 3D structures. For N > 4, all cationic Pt doped gold clusters adopt 3D configurations, except for N = 7, which maintains the Au structure substituting the central Au atom by the Pt dopant (however with a larger out-of-plane distortion). Finally, the case of Pd doping is similar to that of Pt, with 3D geometries from the lowest possible sizes + + of N = 4 onward. For most sizes, except N = 5 and 7, the structures of PdAu and PtAu are N1 N1 similar. At N = 5, both clusters adopt a 3D twisted X-structure, but the Pt dopant takes the central position of the cluster, whereas the Pd atom is at a side. At N = 7 the Pd dopant sits on top of an Au + + triangular structure which is similar to AgAu . The structures of pure Pd clusters have not been 6 N characterized experimentally. Nevertheless, DFT calculations predict 3D structures from the tetrameric cluster onward, showing also the strong tendency of palladium to adopt 3D configurations [58]. Appl. Sci. 2019, 9, 1666 7 of 13 Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 13 + + + + + + ++ Figure 3. Computed lowest energy structures of AuN , AgAuN−1 , PtAuN-1 , and PdAuN−1 (N = 2– 7). Figure 3. Computed lowest energy structures of Au , AgAu , PtAu , and PdAu (N = 2–7). N N1 N1 N1 The The computed lowest-ener computed lowest-energy gy dissoc dissociation iation channels channels an and d the corresp the corresponding onding d dissociation issociation ener energ gies ies + + + + + + of of Au AuN and and MM Au Au N-1 (N = 2– (N 7, =M 2–7, = Ag M , Pt, Pd) = Ag, are Pt, listed in Tab Pd) are listed le 1. inAs is known for Au Table 1. As is known N , the preferr for Au ed , N N1 N the dissoc preferr iation channel ed dissociation oscillates channelbetween neutral monomer and neut oscillates between neutral monomer and ral dim neutral er evapor dimer evaporation, ation, with with even(odd even(odd)- )-N clusters pref N clusterserentia preferlentially ly emittiemitting ng a neut ara neutral l monom monomer(dimer) er(dimer) fragment [35 fragment ,59]. This [35,59]. + + + + This demonstrates demonstrates the higher relative stability the higher relative stability of Aof u3 Au and A and u5 s Au ince tsince hey are they in t ar he e inve in the stinvestigated igated size rasize nge 3 5 range of the pre of the ferr pr ed d eferr aughter c ed daughter lusterclusters. s. This obThis servation observation agrees wi agr th the ees with interpreta the interpr tion of etation stabiliof ty p stability atterns patterns based on v based alence electron d on valence electr elocalization on delocalization and electron andic she electrlonic l closshell ing; A clu osing; 3 has two i Au thas inera two nt el itinerant ectrons, electr closin ons, g the closing 1S she the ll, wher 1S shell, eas wher Au5 eas has four deloc Au has four alized delocalized electrons, electr closing ons,the 1S shell and the closing the 1S shell and 1Px the subshe 1P ll. The subshell. odd-even tre The odd-even nd has trend also be has en found also been in e found xperimentally determined in experimentally determined dissociation e dissociation nergies ener of larger gies of Au lar N ger clusters [39]. In Au clusters Re [ferenc 39]. In e [39], ReferD ence N va [lues were 39], D values determined were determined for N = 7–27 for . This ra N = 7–27. nge N N This only over rangelaps only wit overlaps h that for with Authat 7 in o for ur Au work and o in our work ur computed d and our computed issociation en dissociation ergy of 3.20 e enerV gy lies of 3.20 withi eV n the experi lies withinmental the experimental range of 3.range 1–3.6 eV of 3.1–3.6 . This, ho eV wever, is only the case . This, however, is only a the fter ta case ki after ng in taking to account into account relativistic effects i relativistic n e the comp ects in the utations. computations. Otherwise, the computed Otherwise, the computed DN is only 2. D 9 is 0 only eV. We al 2.90 eV so not . We e also that note a high that 𝐷 a high value D wa value s foun was d fo found r Au3 for . As Au ment . ioned, t As mentioned, his clust this er hcluster as two de has loca two lized delocalized electrons, electr clos ons, ing N 3 closing the first the supe first rat superatomic omic shell 1S shell (mo1S re det (mor ails e details are given are l given ater).later). OveraOverall, ll, the cathe lculcalculate ated channel d channels s of Au of N + + Au agree w agr eee ll with the abund well with the abundances ances after p after hotofra photofragmentation, gmentation, showin showing g higher higher intensit intensities ies for Au for 3Au and N 3 and Au5 Au . . Table 1. Lowest-energy dissociation channels and corresponding dissociation energies (DN) of AuN and MAuN-1 (N = 3–7, M = Ag, Pt, Pd) calculated by density functional theory. Dissociation Channel DN/eV + + Au3 → Au1 + Au2 4.14 + + Au4 → Au3 + Au1 2.25 + + Au5 → Au3 + Au2 2.58 + + Au6 → Au5 + Au1 2.71 + + Au7 → Au5 + Au2 3.20 + + AgAu2 → Au1 + AgAu1 4.69 + + AgAu3 → AgAu2 + Au1 2.18 + + AgAu4 → AgAu2 + Au2 2.44 + + AgAu5 → AgAu4 + Au1 2.51 Appl. Sci. 2019, 9, 1666 8 of 13 Table 1. Lowest-energy dissociation channels and corresponding dissociation energies (D ) of Au N N and MAu (N = 3–7, M = Ag, Pt, Pd) calculated by density functional theory. N1 Dissociation Channel D /eV + + Au ! Au + Au 4.14 3 1 2 + + Au ! Au + Au 2.25 4 3 1 + + Au ! Au + Au 2.58 5 3 2 + + Au ! Au + Au 2.71 6 5 1 + + Au ! Au + Au 3.20 7 5 2 + + AgAu ! Au + AgAu 4.69 2 1 1 + + AgAu ! AgAu + Au 2.18 3 2 1 + + AgAu ! AgAu + Au 2.44 4 2 2 + + AgAu ! AgAu + Au 2.51 5 4 1 + + AgAu ! AgAu + Au 3.41 6 4 2 + + PtAu ! Pt + Au 3.97 2 1 2 + + PtAu ! PtAu + Au 3.36 3 2 1 + + PtAu ! PtAu + Au 2.69 4 3 1 ! PtAu + Au 2.89 2 2 ! Au + PtAu 2.91 3 1 + + PtAu ! PtAu + Au 2.62 5 3 2 + + PtAu ! PtAu + Au 3.61 6 4 2 ! PtAu + Au 3.67 5 1 + + PdAu ! Pd + Au 2.83 2 1 2 + + PdAu ! PdAu + Au 3.19 3 2 1 + + PdAu ! PdAu + Au 2.51 4 3 1 ! PdAu + Au 2.54 2 2 + + PdAu ! PdAu + Au 2.46 5 3 2 + + PdAu ! PdAu + Au 3.32 6 4 2 ! PdAu + Au 3.37 5 1 Following M heteroatom doping the dissociation patterns can become more complicated, since besides Au and Au evaporation channels, M and MAu emissions can compete. The dissociation 1 2 1 energies corresponding to the M and MAu emission channels were computed and with only a few exceptions their energies are significantly higher than those of Au and Au evaporation. For the 1 2 + + AgAu clusters, a similar pattern to that of Au was found; clusters composed of even(odd) N1 N numbers of atoms emit a neutral monomer(dimer) gold fragment, thereby preferentially forming + + AgAu and AgAu . These two clusters have closed electronic shells with two and four itinerant 4 2 electrons, respectively, under the assumption that Ag is delocalizing its 5s valence electron. Also, in this case the three-atom cluster, AgAu , has the highest dissociation energy, 4.69 eV, which is actually even + + + higher than that of Au . The cases of PtAu and PdAu are similar but very di erent from 3 N1 N1 + + + pure Au . The clusters MAu and MAu are not the main products of fragmentation, but instead N 4 2 there is competition to form di erent daughter clusters in the N = 3–6 size range, with some preference for MAu . This result is consistent with the experimental observation that there is no special feature + + in the abundances (N = 4–6 size range) of PtAu and PdAu . The underlying reason for this is N1 N1 discussed in the next section. 4. Discussion The electronic structure of the clusters composed of N = 3–5 atoms was analyzed for the di erent pure and doped clusters via calculations of total density of states (DOS) in order to understand their relative stability. Figure 4 presents this analysis for the clusters composed of three atoms. In the left panel, the DOS of Au is shown, which is projected into atomic d-(black) and sp-states (red). Due to the very small size of the cluster, the DOS is composed of molecular-like states with a strong d-character below the HOMO state. Within this dense region of occupied d-states, one state has a higher sp-character (although with an overall low intensity). A plot of the molecular orbital (MO) of this state Appl. Sci. 2019, 9, 1666 9 of 13 shows a wavefunction that is delocalized over the entire cluster, with a nodal character resembling that of the 1S eigenstate of a particle confined in a 2-dimensional potential well [21]. This doubly occupied MO is the only one of delocalized character below the HOMO, showing that Au has two itinerant electrons as anticipated. Each of the three Au atoms delocalizes its 6s electron, of which one is removed during ionization. The DOS also reveals the presence of two, almost degenerate, unoccupied MOs resembling the 1P and 1P cluster orbitals, which are higher in energy. x y Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 13 Figure 4. Total density of states of the clusters composed of N = 3 atoms. Projections into atomic Figure 4. Total density of states of the clusters composed of N = 3 atoms. Projections into atomic states states are presented in black for Au(d), in red for Au(sp), in blue for M(d), and in green for M(sp). are presented in black for Au(d), in red for Au(sp), in blue for M(d), and in green for M(sp). The The molecular orbitals of delocalized character extending over the entire cluster volume are plotted and molecular orbitals of delocalized character extending over the entire cluster volume are plotted and labeled based on their nodal character (1S and 1P ). The highest occupied molecular orbital (HOMO) labeled based on their nodal character (1S and 1Px,y x,y). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states are labeled as well. In the case that the cluster and lowest unoccupied molecular orbital (LUMO) states are labeled as well. In the case that the cluster is open-shell, both and orbitals are plotted. is open-shell, both α and β orbitals are plotted. The second panel of Figure 4 presents the case of AgAu , in which the DOS is also projected + 2 The second panel of Figure 4 presents the case of AgAu2 , in which the DOS is also projected into into the d-(blue) and the sp-states (green) of the silver dopant. The DOS of this cluster is very similar the d-(blue) and the sp-states (green) of the silver dopant. The DOS of this cluster is very similar to to that of Au , with the region close to the HOMO state having a higher Au than Ag character. + 3 that of Au3 , with the region close to the HOMO state having a higher Au than Ag character. This is This is a consequence of the relativistic nature of gold, which reduces the s-d energy separation [28]. a consequence of the relativistic nature of gold, which reduces the s-d energy separation [28]. As the As the figure reveals, this cluster also has only one doubly occupied MO of delocalized character figure reveals, this cluster also has only one doubly occupied MO of delocalized character (1S + + (1S symmetry). Therefore, as expected, Ag is delocalizing its 5s electron. The PtAu and PdAu + + 2 2 symmetry). Therefore, as expected, Ag is delocalizing its 5s electron. The PtAu2 and PdAu2 cases, cases, which are presented in the right panels, are di erent. In both cases the DOS near the HOMO which are presented in the right panels, are different. In both cases the DOS near the HOMO state is + + state is dominated by the d-states of the dopant atom, and, as for Au and AgAu , only one MO of + + 3 2 dominated by the d-states of the dopant atom, and, as for Au3 and AgAu2 , only one MO of + + delocalized character is found to be doubly occupied. This indicates that PtAu and PdAu are also 2 + 2 + delocalized character is found to be doubly occupied. This indicates that PtAu2 and PdAu2 are also clusters with closed electronic shells, which is not trivial considering the electronic configuration of clusters with closed electronic shells, which is not trivial considering the electronic configuration of 14 9 1 these dopant atoms. Therefore, at N = 3 the Pt dopant delocalizes its 6s electron ([Xe] 4f 5d 6s ), 14 9 1 these dopant atoms. Therefore, at N = 3 the Pt dopant delocalizes its 6s electron ([Xe] 4f 5d 6s ), whereas the Pd dopant ([Kr] 4d ) promotes one of its 4d electrons to an electronic configuration of [Kr] whereas the Pd dopant ([Kr] 4d ) promotes one of its 4d electrons to an electronic configuration of 9 1 4d 5s , which then delocalizes over the entire cluster. This leaves both dopant atoms with an open 9 1 [Kr] 4d 5s , which then delocalizes over the entire cluster. This leaves both dopant atoms with an atomic d-shell. open atomic d-shell. + + In Figure 5, a similar DOS analysis is presented for N = 4. Au and AgAu have very similar + + 4 3 In Figure 5, a similar DOS analysis is presented for N = 4. Au4 and AgAu3 have very similar DOS. In both cases the HOMO corresponds to a singly occupied MO of delocalized character ( -state) DOS. In both cases the HOMO corresponds to a singly occupied MO of delocalized character (α-state) with the 1P symmetry. In addition, there is a doubly occupied MO resembling the 1S eigenstate. with the 1Px symmetry. In addition, there is a doubly occupied MO resembling the 1S eigenstate. The The lowest unoccupied molecular orbital (LUMO) of the clusters is now the -state of 1P symmetry. lowest unoccupied molecular orbital (LUMO) of the clusters is now the β-state of 1Px symmetry. + + + + Therefore, Au and AgAu have three itinerant electrons. The DOS of PtAu and PdAu is + + + + 4 3 3 3 Therefore, Au4 and AgAu3 have three itinerant electrons. The DOS of PtAu3 and PdAu3 is remarkable; both clusters have only the 1S-type delocalized MOs below the HOMO, implying that remarkable; both clusters have only the 1S-type delocalized MOs below the HOMO, implying that there are in total only two delocalized electrons. As shown in the figure, the 1P and 1P MOs are x y there are in total only two delocalized electrons. As shown in the figure, the 1Px and 1Py MOs are empty. For N = 4, neither Pt nor Pd contributes to the electron delocalization, contrary to the N = 3 empty. For N = 4, neither Pt nor Pd contributes to the electron delocalization, contrary to the N = 3 + + + + case. PtAu and PdAu are clusters with closed electronic shells, as PtAu and PdAu are. + + + + 3 3 2 2 case. PtAu3 and PdAu3 are clusters with closed electronic shells, as PtAu2 and PdAu2 are. Figure 5. Total density of states of the clusters composed of N = 4 atoms. Projections into atomic states are presented in black for Au(d), in red for Au(sp), in blue for M(d), and in green for M(sp). The Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 13 Figure 4. Total density of states of the clusters composed of N = 3 atoms. Projections into atomic states are presented in black for Au(d), in red for Au(sp), in blue for M(d), and in green for M(sp). The molecular orbitals of delocalized character extending over the entire cluster volume are plotted and labeled based on their nodal character (1S and 1Px,y). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states are labeled as well. In the case that the cluster is open-shell, both α and β orbitals are plotted. The second panel of Figure 4 presents the case of AgAu2 , in which the DOS is also projected into the d-(blue) and the sp-states (green) of the silver dopant. The DOS of this cluster is very similar to that of Au3 , with the region close to the HOMO state having a higher Au than Ag character. This is a consequence of the relativistic nature of gold, which reduces the s-d energy separation [28]. As the figure reveals, this cluster also has only one doubly occupied MO of delocalized character (1S + + symmetry). Therefore, as expected, Ag is delocalizing its 5s electron. The PtAu2 and PdAu2 cases, which are presented in the right panels, are different. In both cases the DOS near the HOMO state is + + dominated by the d-states of the dopant atom, and, as for Au3 and AgAu2 , only one MO of + + delocalized character is found to be doubly occupied. This indicates that PtAu2 and PdAu2 are also clusters with closed electronic shells, which is not trivial considering the electronic configuration of 14 9 1 these dopant atoms. Therefore, at N = 3 the Pt dopant delocalizes its 6s electron ([Xe] 4f 5d 6s ), whereas the Pd dopant ([Kr] 4d ) promotes one of its 4d electrons to an electronic configuration of 9 1 [Kr] 4d 5s , which then delocalizes over the entire cluster. This leaves both dopant atoms with an open atomic d-shell. + + In Figure 5, a similar DOS analysis is presented for N = 4. Au4 and AgAu3 have very similar DOS. In both cases the HOMO corresponds to a singly occupied MO of delocalized character (α-state) with the 1Px symmetry. In addition, there is a doubly occupied MO resembling the 1S eigenstate. The lowest unoccupied molecular orbital (LUMO) of the clusters is now the β-state of 1Px symmetry. + + + + Therefore, Au4 and AgAu3 have three itinerant electrons. The DOS of PtAu3 and PdAu3 is remarkable; both clusters have only the 1S-type delocalized MOs below the HOMO, implying that there are in total only two delocalized electrons. As shown in the figure, the 1Px and 1Py MOs are empty. For N = 4, neither Pt nor Pd contributes to the electron delocalization, contrary to the N = 3 Appl. Sci. 2019, 9, 1666 10 of 13 + + + + case. PtAu3 and PdAu3 are clusters with closed electronic shells, as PtAu2 and PdAu2 are. Appl. Sci. Figure 5. Figure 2019, 5. 9Total , x FO Total R P density of stat density EER REof VIEW states es of the clust of the clusters ers composed of composedN of = 4 atoms. N = 4 atoms. Projections Projections into atinto omic state atomic 10 of s 13 are presented in black for Au(d), in re states are presented in black for Au(d), d for in red Au(s forp), in b Au(sp), lue for in blue Mfor (d), and in green for M(d), and in green Mfor (sp). The M(sp). molecular orbitals of delocalized character extending over the entire cluster volume are plotted and The molecular orbitals of delocalized character extending over the entire cluster volume are plotted labeled based on their nodal character (1S and 1Px,y). The HOMO-LUMO gap is labeled. In the case and labeled based on their nodal character (1S and 1P ). The HOMO-LUMO gap is labeled. In the x,y that the cluster is open-shell, both α and β orbitals are plotted. case that the cluster is open-shell, both and orbitals are plotted. + + + + The fin The final al an analyzed alyzed c case ase (size (size N N= 5) is = 5) ispr pr esented esentedin in Fi Figur gure 6. As e 6. As exexpected, pected, Au A 5u and and AgAu AgAu 4 are 5 4 are clusters with closed electronic shells, since each atom delocalizes one valence electron. In both clusters with closed electronic shells, since each atom delocalizes one valence electron. In both cases there cases ther are two do e are twoubly doubly occupied occupied MOMOs s of d ofeloc delocalized alized char character acter which which rerse esemble mble the the 1S an 1S and d 1P 1P x + + + + eigenstates. This is similar to the cases of PtAu and PdAu , which also have two doubly occupied eigenstates. This is similar to the cases of PtAu4 and PdAu4 , which also have two doubly occupied 4 4 MOs o MOs of f de delocalize localized ch d character aracter. Fo . For r N N = 5, Pt = 5, Pt an and d Pd Pd d delocalize elocalize one one electron electron in in or order der to to clo close se the the 1P 1Px electronic shell of the clusters, thereby gaining stability. This analysis shows an a priori unexpected electronic shell of the clusters, thereby gaining stability. This analysis shows an a priori unexpected behavior behavior of t of the he Pt an Pt and d P Pd d dopants on dopants on the Au cluster the Au clusterss in in the the N N = = 3–5 3–5 size size r range. ange. The dopant The dopant atoms atoms delocalize one electron at N = 3 and N = 5, but none at N = 4, allowing all the doped clusters to have delocalize one electron at N = 3 and N = 5, but none at N = 4, allowing all the doped clusters to have closed closed electro electronic nic she shells. lls. Figure 6. Total density of states of the clusters composed of N = 5 atoms. Projections into atomic states Figure 6. Total density of states of the clusters composed of N = 5 atoms. Projections into atomic are presented in black for Au(d), in red for Au(sp), in blue for M(d), and in green for M(sp). The states are presented in black for Au(d), in red for Au(sp), in blue for M(d), and in green for M(sp). molecular orbitals of delocalized character extending over the entire cluster volume are plotted and The molecular orbitals of delocalized character extending over the entire cluster volume are plotted labeled based on their nodal character (1S and 1Px,y,z). The HOMO-LUMO gap is labeled. In the case and labeled based on their nodal character (1S and 1P ). The HOMO-LUMO gap is labeled. In the x,y,z that the cluster is open-shell, both α and β orbitals are plotted. case that the cluster is open-shell, both and orbitals are plotted. 5. Conclusions 5. Conclusions In this work, the relative stability of small cationic Ag, Pt, and Pd doped Au (N = 2–7) clusters In this work, the relative stability of small cationic Ag, Pt, and Pd doped AuN (N = 2–7) clusters were investigated using a combination of photofragmentation experiments and density functional were investigated using a combination of photofragmentation experiments and density functional theory calculations. Mass spectra revealed a pronounced odd-even pattern in the abundances of Au theory calculations. Mass spectra revealed a pronounced odd-even pattern in the abundances of AuN and AgAu , which can be rationalized by considering the delocalization of the s-valence electron N1 and AgAuN−1 , which can be rationalized by considering the delocalization of the s-valence electron of each atom in the cluster, including the Ag dopant. Clusters composed of an odd number of atoms of each atom in the cluster, including the Ag dopant. Clusters composed of an odd number of atoms possess an even number of delocalized electrons, closing superatomic electronic shells and gaining possess an even number of delocalized electrons, closing superatomic electronic shells and gaining stability. Dissociation energies calculated for these clusters agree well with this picture, since the stability. Dissociation energies calculated for these clusters agree well with this picture, since the + + + preferred fragmentation channels always produce the more stable fragments: Au3 , Au5 , AgAu2 , and + + AgAu4 . The Pt and Pd doped clusters, however, behave very differently. Experimentally, PtAu2 and + + + PdAu2 correspond to intensity maxima, but PtAu4 and PdAu4 do not. The calculated dissociation energies of these clusters reveal a competition between channels that produce fragments of different sizes, especially in the N = 4–6 size range, explaining the smeared out odd-even pattern in their size- to-size dependent abundances. Analysis of the density of states of the clusters in the N = 3–5 size range reveals an unexpected behavior. At N = 3, both Pt and Pd delocalize one valence electron in order to close the 1S superatomic electron shell. At N = 4, though, the valence electrons of both dopants remain localized, which also allows the closing of the clusters’ 1S shells. Finally, at the size N = 5, Pt and Pd again delocalize one electron, closing the 1S and 1Px shells. Overall, these results illustrate that it is difficult to predict a priori how doping would affect the electronic structure of clusters, even at the very smallest sizes. Appl. Sci. 2019, 9, 1666 11 of 13 + + + preferred fragmentation channels always produce the more stable fragments: Au , Au , AgAu , 3 5 2 and AgAu . The Pt and Pd doped clusters, however, behave very di erently. Experimentally, + + + + PtAu and PdAu correspond to intensity maxima, but PtAu and PdAu do not. The calculated 2 2 4 4 dissociation energies of these clusters reveal a competition between channels that produce fragments of di erent sizes, especially in the N = 4–6 size range, explaining the smeared out odd-even pattern in their size-to-size dependent abundances. Analysis of the density of states of the clusters in the N = 3–5 size range reveals an unexpected behavior. At N = 3, both Pt and Pd delocalize one valence electron in order to close the 1S superatomic electron shell. At N = 4, though, the valence electrons of both dopants remain localized, which also allows the closing of the clusters’ 1S shells. Finally, at the size N = 5, Pt and Pd again delocalize one electron, closing the 1S and 1P shells. Overall, these results illustrate that it is dicult to predict a priori how doping would a ect the electronic structure of clusters, even at the very smallest sizes. Author Contributions: Conceptualization, P.F. and E.J.; formal analysis, P.F.; investigation, P.F. and E.J.; resources, P.F. and E.J.; writing—original draft preparation, P.F.; writing—review and editing, P.F. and E.J.; project administration, E.J. Funding: This research was funded by the Research Foundation-Flanders (FWO) (grant number G0B41.15N) and by the KU Leuven Research Council (grant number C14/18/073). Acknowledgments: P.F. acknowledges the FWO for a postdoctoral grant. Conflicts of Interest: The authors declare no conflict of interest. References 1. Liao, T.-W.; Yadav, A.; Hu, K.-J.; van der Tol, J.; Cosentino, S.; D’Acapito, F.; Palmer, R.E.; Lenardi, C.; Ferrando, R.; Grandjean, D.; et al. Unravelling the nucleation mechanism of bimetallic nanoparticles with composition-tunable core–shell arrangemen. Nanoscale 2018, 10, 6684. [CrossRef] 2. Neukermans, S.; Janssens, E.; Chen, Z.F.; Silverans, R.E.; Schleyer, P.v.R.; Lievens, P. 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Relative Stability of Small Silver, Platinum, and Palladium Doped Gold Cluster Cations

Applied Sciences , Volume 9 (8) – Apr 23, 2019

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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 N1 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 N1 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 di erent doped clusters. The e ect 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 di erent 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 di erent 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 di erent 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 di erent 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 di erent nature of the confining potential, however, electronic shells in clusters follow a di erent 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 di erent 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 di erent species and the di erent 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− 1 1 clusters are presented in panels (a) and (b), respectively. Continuous lines connect the MAu species to visualize the size-dependent and (b), respectively. Continuous lines connect the MAu N N -1 1 species to visualize the size-dependent abundances. This situation is di erent for the case of Pt doping. Platinum has a distinct isotopic abundances. This situation is different for the case of Pt doping. Platinum has a distinct isotopic pattern (192 u (0.008%), 194 u (0.329%), 195 u (0.338%), 196 u (0.253%), and 198 u (0.072%)) and the pattern (192 u (0.008%), 194 u (0.329%), 195 u (0.338%), 196 u (0.253%), and 198 u (0.072%)) and the Au atom mass resides in between (197 u). In addition, the strong binding energy of the Pt dimer Au atom mass resides in between (197 u). In addition, the strong binding energy of the Pt2 dime 2 r (4.63 (4.63 eV compared to 3.15 eV of Au in our calculations) makes it dicult to find production conditions eV compared to 3.15 eV of Au2 in o 2 ur calculations) makes it difficult to find production conditions + + + + + + under which only pure Au and singly doped PtAu clusters are formed. Thus, pure Pt under which only pure AuN N and singly doped PtAuN-1 clusters N1 are formed. Thus, pure PtN clusteN rs clusters were also produced, as well as mixed Pt Au (x = 0–N) species. A part of a typical were also produced, as well as mixed PtxAuN-x (x = 0 x –N N) specie x s. A part of a typical mass spectrum mass spectrum of the Pt Au clusters is presented in Figure 1c. For this reason, a deconvolution of the PtxAuN-x clusters is p x resented Nx in Figure 1c. For this reason, a deconvolution process needed to process needed to be performed to quantify the intensity of each cluster composition. An example be performed to quantify the intensity of each cluster composition. An example is presented in Figure is presented in Figure 1c. Each Pt Au (x = 0–N) intensity profile was assumed to be composed 1c. Each PtxAuN-x (x = 0–N) intensit x yN prof x ile was assumed to be composed of a set of Gaussian of a set of Gaussian functions matching the natural isotopic distribution of the cluster. The width functions matching the natural isotopic distribution of the cluster. The width of a single Gaussian of a single Gaussian function was determined by recording a mass spectrum of photofragmented function was determined by recording a mass spectrum of photofragmented pure AuN clusters (with pure Au clusters (with a single isotope for each size) under the same experimental conditions, a single isot N ope for each size) under the same experimental conditions, in order to account for the in order to account for the mass-dependent resolution of the mass spectrometer. Therefore, the center mass-dependent resolution of the mass spectrometer. Therefore, the center and the width of each and the width of each Gaussian function was fixed, while the total intensities of each Pt Au Gaussian function was fixed, while the total intensities of each PtxAuN-x composition were xused as Nx composition were used as fitting parameters. The analysis was restricted to the size range for which fitting parameters. The analysis was restricted to the size range for which this deconvolution could this deconvolution could be applied satisfactorily (N  6). As a corroboration of the procedure, be applied satisfactorily (N ≤ 6). As a corroboration of the procedure, the extracted abundances of + + + + the extracted abundances of photofragmented pure Au and Pt clusters were compared with photofragmented pure AuN and PtN clusters were compared with results from previous N N results from previous measurements [35,40], showing an almost identical result. measurements [35,40], showing an almost identical result. + + Figure 1. Parts of representative mass spectra of photofragmented clusters: (a) AgAu , Figure 1. Parts of representative mass spectra of photofragmented clusters: (a) AgAuN−1 , (b) PdAuN−1 , N1 + + (b) PdAu , and (c) PtAu . In (a,b) continuous lines are included to aid visualization of and (c) PtAuN-1 . In (a) and (b) continuous lines are included to aid visualization of the different N1 N1 the di erent abundances of the doped species. Pure Au clusters are marked by asterisks, whereas abundances of the doped species. Pure AuN clusters are N marked by asterisks, whereas additional additional peaks correspond to carbon and oxygen contaminations or to doubly doped species of low peaks correspond to carbon and oxygen contaminations or to doubly doped species of low intensity. intensity. (d) Deconvolution of the photofragmented cluster peaks around 580–595 u in panel ( +c): Au + , (d) Deconvolution of the photofragmented cluster peaks around 580–595 u in panel (c): Au3 , PtAu3 2 , + + + PtAu + , Pt Au +, and Pt . Each peak is assumed to be composed of Gaussian functions of fixed width Pt2Au 2 , and 2 Pt3 . Each peak 3 is assumed to be composed of Gaussian functions of fixed width and and matching the natural isotopic distribution of each species. matching the natural isotopic distribution of each species. 2.2. Theoretical Calculations Density functional theory calculations were performed with the ORCA 4.0.1 software package [41]. The lowest energy structures of the clusters were adopted as follows. For the Ag doped clusters, Appl. Sci. 2019, 9, 1666 4 of 13 2.2. Theoretical Calculations Density functional theory calculations were performed with the ORCA 4.0.1 software package [41]. The lowest energy structures of the clusters were adopted as follows. For the Ag doped clusters, the lowest-energy isomers calculated in Reference [42] were assumed. Experimental characterization of the structures of mixed Au-Ag clusters have been performed up to size N = 5 using ion mobility experiments [43], photodissociation spectroscopy measurements [14], and far-infrared multiphoton dissociation studies [44,45]. The structures assigned in these studies agree with those in Reference [42]. For the case of Pt doping, we are not aware of any experimental investigation characterizing the structure of these clusters, nor of a theoretical study searching systematically for low-lying isomers of the cationic species. A search for low-energy structures was performed by manually constructing di erent initial geometries. Finally, with regards to Pd doping, a thorough theoretical investigation + + of the Au and PdAu (N = 2–20) clusters was presented in our previous work [35]. We adopt N N1 the structures found there. All geometries were optimized with the LC-BLYP exchange-correlation functional and the extensive Def2-TZVPP basis set [46]. Long-range corrections in Generalized Gradient Approximation (GGA) functionals have been shown to perform well in Pd and Ag doped Au clusters [14,33,35]. In addition, other DFT functionals have been tested, including the GGA PBE, meta-GGA TPSS, and the hybrid B3LYP. For each cluster those other functionals yielded a similar energetic ordering of the considered isomers as the LC-BLYP functional. Additionally, the even larger Def2-QZVPP basis set was checked, showing no significant di erence. For these optimizations, the e ective core potential Def2-ECP was used, replacing 60 core electrons for Au and Pt, and 28 core electrons for Ag and Pd. For the Ag and Pd doped clusters only the lowest possible spin configurations were calculated, as previous studies showed these have the lowest energy. For the Pt doped clusters, di erent spin configurations were computed for each cluster size. Also, in this case the lowest possible spin states were found to be the lowest in energy. Vibrational frequencies were calculated for all the structures at the same level of theory to corroborate that the geometries corresponded to true minima on the potential energy surface and not to transition states. Relativistic e ects are known to be important for heavy atoms, such as Au and Pt [47]. The cluster sizes investigated here are small enough to allow the implicit inclusion of relativistic e ects by performing single point calculations on the optimized structures using the relativistic Hamiltonian within the zero-order regular approximation (ZORA) [48] and the all-electron ZORA-def2-TZVPP basis set [49]. The size-to-size stability of the clusters was analyzed computationally by the dissociation energies D , defined as + + D = E A + E(A ) E A , (1) N Nm m N + + where E(A ), E(A ), and E(A ) correspond to the total energy of the corresponding cationic N Nm clusters and of the emitted neutral fragment. Many theoretical studies make use of another computed quantity when analyzing the relative stability of clusters, namely the second energy di erence D E [35,40,50,51]. This quantity compares the total energy of size N with that of N + 1 plus N 1. Interpretation of experimental data using computed D E values, however, requires that the fragmentation channel is always the same, irrespective of the cluster size, or that at least one channel dominates the decay, for example the neutral monomer emission. Recent attempts have even tried to combine D and D E , in order to find a better stability descriptor, by defining a second dissociation N N energy di erence, but this definition requires an invariant fragmentation channel with size [52]. Since here we focus on doped species, the favored channel may be size dependent. Simply comparing the dissociation energies of all possible fragmentation channels is a more solid description of stability and was therefore used in this work. Appl. Sci. 2019, 9, 1666 5 of 13 3. Results 3.1. Abundances of Photofragmented Clusters The results of the photofragmentation experiments are summarized in Figure 2, which shows + + + + the cluster abundances (I ) for (a) Au , (b) AgAu , (c) PtAu , and (d) PdAu clusters, N N1 N1 N1 in the size range of N = 2–6. The values of I were calculated by integrating the peak area of each Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 13 corresponding cluster size in the mass spectra. In panel (a) the well-known odd-even oscillation in the abundances of Au is seen, with local maxima for clusters composed of an odd number of [25,27,35]. As discussed, this pattern can be understood by the delocalization of each Au 6s valence atoms [25,27,35]. As discussed, this pattern can be understood by the delocalization of each Au 6s electron over the cluster volume, which, due to quantum confinement, develops energy shells. The valence electron over the cluster volume, which, due to quantum confinement, develops energy shells. cluster has an enhanced stability if all shells are completely filled. Since these clusters are positively The cluster has an enhanced stability if all shells are completely filled. Since these clusters are positively charged, an odd number of atoms corresponds to an even number of electrons, required to close the charged, an odd number of atoms corresponds to an even number of electrons, required to close electronic shells due to spin degeneracy [20]. We point out, however, that this simplified model the electronic shells due to spin degeneracy [20]. We point out, however, that this simplified model assumes a negligible influence of the Au 5d electrons on the relative stability pattern. assumes a negligible influence of the Au 5d electrons on the relative stability pattern. + + + + + + Figure 2. Abundances after photofragmentation for (a) Au , (b) AgAu , (c) PtAu , Figure 2. Abundances after photofragmentation for (a) AuN , ( N b) AgAuN−1 , ( N c1) PtAuN-1 , and ( N1 d) and (d) PdAu clusters; these represent the relative stability of the di erent sizes. The values were PdAuN−1 clust Ne 1rs; these represent the relative stability of the different sizes. The values were calculated by integrating the area of the corresponding signal in the mass spectra. calculated by integrating the area of the corresponding signal in the mass spectra. Upon Ag doping, as presented in Figure 2b, the odd-even oscillation in the abundances of pure Upon Ag doping, as presented in Figure 2b, the odd-even oscillation in the abundances of pure + + gold clusters was not significantly modified. Local maxima in I are found for AgAu and AgAu , N 2 + 4 + gold clusters was not significantly modified. Local maxima in 𝐼 are found for AgAu2 and AgAu4 , with an amplitude of the abundance oscillation similar to that for pure Au . The simple explanation with an amplitude of the abundance oscillation similar to that for pure AuN . The simple explanation 10 1 for this is that the silver atom, with an electronic [Kr] 4d 5s configuration, delocalizes its valence 10 1 for this is that the silver atom, with an electronic [Kr] 4d 5s configuration, delocalizes its valence electron for each cluster size. This observation is not surprising, since pure Ag clusters possess a very electron for each cluster size. This observation is not surprising, since pure Ag clusters possess a very similar odd-even stability pattern as pure Au clusters [32], although it is slightly more pronounced due similar odd-even stability pattern as pure Au clusters [32], although it is slightly more pronounced to the less influential d-states of the clusters in their electronic structure [28]. due to the less influential d-states of the clusters in their electronic structure [28]. The e ect of Pt doping on the Au abundances is shown in Figure 2c. As seen, the odd-even N + The effect of Pt doping on the AuN abundances is shown in Figure 2c. As seen, the odd-even staggering is to some extent preserved upon Pt doping up to N = 4, with an intensity maximum at staggering is to some extent preserved upon Pt doping up to N = 4, with an intensity maximum at + + PtAu . This pattern, however, changes at N = 5, because PtAu is not a local intensity maximum. 2 + 4 + PtAu2 . This pattern, however, changes at N = 5, because PtAu4 is not a local intensity maximum. 14 9 1 Since the electronic configuration of the platinum atom is [Xe] 4f 5d 6s , the observed stability 14 9 1 Since the electronic configuration of the platinum atom is [Xe] 4f 5d 6s , the observed stability pattern suggests the delocalization of the Pt 6s valence electron in the N = 2–4 size range, giving a total of two delocalized electrons in the PtAu2 cluster. This assumption is not trivial in view of the open d-shell configuration of Pt. An understanding of the modified pattern in the N = 4–6 size range upon Pt doping requires further analysis. Finally, the abundances of the PdAuN−1 clusters are shown in Figure 2d. The stability pattern is very similar to that seen for the Pt doped case, with a relative intensity maximum at N = 3 but not at N = 5. For the Pt doped clusters, this pattern in the abundances suggests that Pd delocalizes one electron in PdAu2 , even though this atom has a ground state electronic configuration with a full d- shell and no valence s electrons ([Kr] 4d ). Photoelectron spectroscopy experiments on anionic Appl. Sci. 2019, 9, 1666 6 of 13 pattern suggests the delocalization of the Pt 6s valence electron in the N = 2–4 size range, giving a total of two delocalized electrons in the PtAu cluster. This assumption is not trivial in view of the open d-shell configuration of Pt. An understanding of the modified pattern in the N = 4–6 size range upon Pt doping requires further analysis. Finally, the abundances of the PdAu clusters are shown in Figure 2d. The stability pattern is N1 very similar to that seen for the Pt doped case, with a relative intensity maximum at N = 3 but not at N = 5. For the Pt doped clusters, this pattern in the abundances suggests that Pd delocalizes one electron in PdAu , even though this atom has a ground state electronic configuration with a full d-shell and no valence s electrons ([Kr] 4d ). Photoelectron spectroscopy experiments on anionic PdAu clusters below N = 6 have shown that Pd can promote one of its 4d electrons to the 5s shell, which then participates in the bonding with Au [53]. In addition, in our previous work on the photofragmentation of larger Pd doped Au cluster cations, we demonstrated similar behavior in PdAu [35]. The Pd dopant delocalizes one of its 4d electrons, giving to the cluster a total of six itinerant electrons, a pronounced magic number in 2D systems [54]. Therefore, it is possible that Pd is delocalizing an electron in PdAu in order to fill the cluster ’s 1S electronic shell. This assumption, however, requires a detailed analysis of the electronic structure for confirmation. Similarly, further analysis is needed to understand the + + observation that PdAu (as PtAu ) does not seem to possess any particular stability. These matters 4 4 are discussed later in the text. 3.2. Theoretical Results + + The computed lowest energy structures of Au and MAu (N = 2–7, M = Ag, Pt, and Pd) N N1 clusters are shown in Figure 3. The lowest energy structures of cationic Au clusters are planar up to N = 7 (although Au has a slight out-of-plain distortion), according to ion mobility experiments [16]. In many DFT studies, however, it is predicted that the 2D to 3D transition takes place at N = 8, although the planar and 3D isomers of Au are close in energy [35,55]. The source for this discrepancy can be related to the lack of implicit relativistic e ects in most DFT studies on gold clusters, although that question goes beyond the scope of this study. In the N  7 size range, theory and experimental results agree that all Au clusters are two-dimensional. Upon Ag doping, the 2D-3D transition size is modified. AgAu adopts a 3D twisted X-structure, indicating a decrease of the transition + + size to N = 5, but AgAu is again a two-dimensional cluster. Pure Ag clusters are known 5 N to become three-dimensional at N = 5, as determined recently by far-infrared multiple photon dissociation spectroscopy measurements in conjunction with DFT calculations [56]. Upon Pt doping, calculations predict the 2D-3D transition at the lowest possible size, i.e., at N = 4, with PtAu adopting a tetrahedral geometry. Combined far-infrared multiphoton dissociation spectroscopy and DFT calculations have determined that pure Pt clusters become three-dimensional at N = 4 as well [57], showing the strong tendency of platinum to form 3D structures. For N > 4, all cationic Pt doped gold clusters adopt 3D configurations, except for N = 7, which maintains the Au structure substituting the central Au atom by the Pt dopant (however with a larger out-of-plane distortion). Finally, the case of Pd doping is similar to that of Pt, with 3D geometries from the lowest possible sizes + + of N = 4 onward. For most sizes, except N = 5 and 7, the structures of PdAu and PtAu are N1 N1 similar. At N = 5, both clusters adopt a 3D twisted X-structure, but the Pt dopant takes the central position of the cluster, whereas the Pd atom is at a side. At N = 7 the Pd dopant sits on top of an Au + + triangular structure which is similar to AgAu . The structures of pure Pd clusters have not been 6 N characterized experimentally. Nevertheless, DFT calculations predict 3D structures from the tetrameric cluster onward, showing also the strong tendency of palladium to adopt 3D configurations [58]. Appl. Sci. 2019, 9, 1666 7 of 13 Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 13 + + + + + + ++ Figure 3. Computed lowest energy structures of AuN , AgAuN−1 , PtAuN-1 , and PdAuN−1 (N = 2– 7). Figure 3. Computed lowest energy structures of Au , AgAu , PtAu , and PdAu (N = 2–7). N N1 N1 N1 The The computed lowest-ener computed lowest-energy gy dissoc dissociation iation channels channels an and d the corresp the corresponding onding d dissociation issociation ener energ gies ies + + + + + + of of Au AuN and and MM Au Au N-1 (N = 2– (N 7, =M 2–7, = Ag M , Pt, Pd) = Ag, are Pt, listed in Tab Pd) are listed le 1. inAs is known for Au Table 1. As is known N , the preferr for Au ed , N N1 N the dissoc preferr iation channel ed dissociation oscillates channelbetween neutral monomer and neut oscillates between neutral monomer and ral dim neutral er evapor dimer evaporation, ation, with with even(odd even(odd)- )-N clusters pref N clusterserentia preferlentially ly emittiemitting ng a neut ara neutral l monom monomer(dimer) er(dimer) fragment [35 fragment ,59]. This [35,59]. + + + + This demonstrates demonstrates the higher relative stability the higher relative stability of Aof u3 Au and A and u5 s Au ince tsince hey are they in t ar he e inve in the stinvestigated igated size rasize nge 3 5 range of the pre of the ferr pr ed d eferr aughter c ed daughter lusterclusters. s. This obThis servation observation agrees wi agr th the ees with interpreta the interpr tion of etation stabiliof ty p stability atterns patterns based on v based alence electron d on valence electr elocalization on delocalization and electron andic she electrlonic l closshell ing; A clu osing; 3 has two i Au thas inera two nt el itinerant ectrons, electr closin ons, g the closing 1S she the ll, wher 1S shell, eas wher Au5 eas has four deloc Au has four alized delocalized electrons, electr closing ons,the 1S shell and the closing the 1S shell and 1Px the subshe 1P ll. The subshell. odd-even tre The odd-even nd has trend also be has en found also been in e found xperimentally determined in experimentally determined dissociation e dissociation nergies ener of larger gies of Au lar N ger clusters [39]. In Au clusters Re [ferenc 39]. In e [39], ReferD ence N va [lues were 39], D values determined were determined for N = 7–27 for . This ra N = 7–27. nge N N This only over rangelaps only wit overlaps h that for with Authat 7 in o for ur Au work and o in our work ur computed d and our computed issociation en dissociation ergy of 3.20 e enerV gy lies of 3.20 withi eV n the experi lies withinmental the experimental range of 3.range 1–3.6 eV of 3.1–3.6 . This, ho eV wever, is only the case . This, however, is only a the fter ta case ki after ng in taking to account into account relativistic effects i relativistic n e the comp ects in the utations. computations. Otherwise, the computed Otherwise, the computed DN is only 2. D 9 is 0 only eV. We al 2.90 eV so not . We e also that note a high that 𝐷 a high value D wa value s foun was d fo found r Au3 for . As Au ment . ioned, t As mentioned, his clust this er hcluster as two de has loca two lized delocalized electrons, electr clos ons, ing N 3 closing the first the supe first rat superatomic omic shell 1S shell (mo1S re det (mor ails e details are given are l given ater).later). OveraOverall, ll, the cathe lculcalculate ated channel d channels s of Au of N + + Au agree w agr eee ll with the abund well with the abundances ances after p after hotofra photofragmentation, gmentation, showin showing g higher higher intensit intensities ies for Au for 3Au and N 3 and Au5 Au . . Table 1. Lowest-energy dissociation channels and corresponding dissociation energies (DN) of AuN and MAuN-1 (N = 3–7, M = Ag, Pt, Pd) calculated by density functional theory. Dissociation Channel DN/eV + + Au3 → Au1 + Au2 4.14 + + Au4 → Au3 + Au1 2.25 + + Au5 → Au3 + Au2 2.58 + + Au6 → Au5 + Au1 2.71 + + Au7 → Au5 + Au2 3.20 + + AgAu2 → Au1 + AgAu1 4.69 + + AgAu3 → AgAu2 + Au1 2.18 + + AgAu4 → AgAu2 + Au2 2.44 + + AgAu5 → AgAu4 + Au1 2.51 Appl. Sci. 2019, 9, 1666 8 of 13 Table 1. Lowest-energy dissociation channels and corresponding dissociation energies (D ) of Au N N and MAu (N = 3–7, M = Ag, Pt, Pd) calculated by density functional theory. N1 Dissociation Channel D /eV + + Au ! Au + Au 4.14 3 1 2 + + Au ! Au + Au 2.25 4 3 1 + + Au ! Au + Au 2.58 5 3 2 + + Au ! Au + Au 2.71 6 5 1 + + Au ! Au + Au 3.20 7 5 2 + + AgAu ! Au + AgAu 4.69 2 1 1 + + AgAu ! AgAu + Au 2.18 3 2 1 + + AgAu ! AgAu + Au 2.44 4 2 2 + + AgAu ! AgAu + Au 2.51 5 4 1 + + AgAu ! AgAu + Au 3.41 6 4 2 + + PtAu ! Pt + Au 3.97 2 1 2 + + PtAu ! PtAu + Au 3.36 3 2 1 + + PtAu ! PtAu + Au 2.69 4 3 1 ! PtAu + Au 2.89 2 2 ! Au + PtAu 2.91 3 1 + + PtAu ! PtAu + Au 2.62 5 3 2 + + PtAu ! PtAu + Au 3.61 6 4 2 ! PtAu + Au 3.67 5 1 + + PdAu ! Pd + Au 2.83 2 1 2 + + PdAu ! PdAu + Au 3.19 3 2 1 + + PdAu ! PdAu + Au 2.51 4 3 1 ! PdAu + Au 2.54 2 2 + + PdAu ! PdAu + Au 2.46 5 3 2 + + PdAu ! PdAu + Au 3.32 6 4 2 ! PdAu + Au 3.37 5 1 Following M heteroatom doping the dissociation patterns can become more complicated, since besides Au and Au evaporation channels, M and MAu emissions can compete. The dissociation 1 2 1 energies corresponding to the M and MAu emission channels were computed and with only a few exceptions their energies are significantly higher than those of Au and Au evaporation. For the 1 2 + + AgAu clusters, a similar pattern to that of Au was found; clusters composed of even(odd) N1 N numbers of atoms emit a neutral monomer(dimer) gold fragment, thereby preferentially forming + + AgAu and AgAu . These two clusters have closed electronic shells with two and four itinerant 4 2 electrons, respectively, under the assumption that Ag is delocalizing its 5s valence electron. Also, in this case the three-atom cluster, AgAu , has the highest dissociation energy, 4.69 eV, which is actually even + + + higher than that of Au . The cases of PtAu and PdAu are similar but very di erent from 3 N1 N1 + + + pure Au . The clusters MAu and MAu are not the main products of fragmentation, but instead N 4 2 there is competition to form di erent daughter clusters in the N = 3–6 size range, with some preference for MAu . This result is consistent with the experimental observation that there is no special feature + + in the abundances (N = 4–6 size range) of PtAu and PdAu . The underlying reason for this is N1 N1 discussed in the next section. 4. Discussion The electronic structure of the clusters composed of N = 3–5 atoms was analyzed for the di erent pure and doped clusters via calculations of total density of states (DOS) in order to understand their relative stability. Figure 4 presents this analysis for the clusters composed of three atoms. In the left panel, the DOS of Au is shown, which is projected into atomic d-(black) and sp-states (red). Due to the very small size of the cluster, the DOS is composed of molecular-like states with a strong d-character below the HOMO state. Within this dense region of occupied d-states, one state has a higher sp-character (although with an overall low intensity). A plot of the molecular orbital (MO) of this state Appl. Sci. 2019, 9, 1666 9 of 13 shows a wavefunction that is delocalized over the entire cluster, with a nodal character resembling that of the 1S eigenstate of a particle confined in a 2-dimensional potential well [21]. This doubly occupied MO is the only one of delocalized character below the HOMO, showing that Au has two itinerant electrons as anticipated. Each of the three Au atoms delocalizes its 6s electron, of which one is removed during ionization. The DOS also reveals the presence of two, almost degenerate, unoccupied MOs resembling the 1P and 1P cluster orbitals, which are higher in energy. x y Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 13 Figure 4. Total density of states of the clusters composed of N = 3 atoms. Projections into atomic Figure 4. Total density of states of the clusters composed of N = 3 atoms. Projections into atomic states states are presented in black for Au(d), in red for Au(sp), in blue for M(d), and in green for M(sp). are presented in black for Au(d), in red for Au(sp), in blue for M(d), and in green for M(sp). The The molecular orbitals of delocalized character extending over the entire cluster volume are plotted and molecular orbitals of delocalized character extending over the entire cluster volume are plotted and labeled based on their nodal character (1S and 1P ). The highest occupied molecular orbital (HOMO) labeled based on their nodal character (1S and 1Px,y x,y). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states are labeled as well. In the case that the cluster and lowest unoccupied molecular orbital (LUMO) states are labeled as well. In the case that the cluster is open-shell, both and orbitals are plotted. is open-shell, both α and β orbitals are plotted. The second panel of Figure 4 presents the case of AgAu , in which the DOS is also projected + 2 The second panel of Figure 4 presents the case of AgAu2 , in which the DOS is also projected into into the d-(blue) and the sp-states (green) of the silver dopant. The DOS of this cluster is very similar the d-(blue) and the sp-states (green) of the silver dopant. The DOS of this cluster is very similar to to that of Au , with the region close to the HOMO state having a higher Au than Ag character. + 3 that of Au3 , with the region close to the HOMO state having a higher Au than Ag character. This is This is a consequence of the relativistic nature of gold, which reduces the s-d energy separation [28]. a consequence of the relativistic nature of gold, which reduces the s-d energy separation [28]. As the As the figure reveals, this cluster also has only one doubly occupied MO of delocalized character figure reveals, this cluster also has only one doubly occupied MO of delocalized character (1S + + (1S symmetry). Therefore, as expected, Ag is delocalizing its 5s electron. The PtAu and PdAu + + 2 2 symmetry). Therefore, as expected, Ag is delocalizing its 5s electron. The PtAu2 and PdAu2 cases, cases, which are presented in the right panels, are di erent. In both cases the DOS near the HOMO which are presented in the right panels, are different. In both cases the DOS near the HOMO state is + + state is dominated by the d-states of the dopant atom, and, as for Au and AgAu , only one MO of + + 3 2 dominated by the d-states of the dopant atom, and, as for Au3 and AgAu2 , only one MO of + + delocalized character is found to be doubly occupied. This indicates that PtAu and PdAu are also 2 + 2 + delocalized character is found to be doubly occupied. This indicates that PtAu2 and PdAu2 are also clusters with closed electronic shells, which is not trivial considering the electronic configuration of clusters with closed electronic shells, which is not trivial considering the electronic configuration of 14 9 1 these dopant atoms. Therefore, at N = 3 the Pt dopant delocalizes its 6s electron ([Xe] 4f 5d 6s ), 14 9 1 these dopant atoms. Therefore, at N = 3 the Pt dopant delocalizes its 6s electron ([Xe] 4f 5d 6s ), whereas the Pd dopant ([Kr] 4d ) promotes one of its 4d electrons to an electronic configuration of [Kr] whereas the Pd dopant ([Kr] 4d ) promotes one of its 4d electrons to an electronic configuration of 9 1 4d 5s , which then delocalizes over the entire cluster. This leaves both dopant atoms with an open 9 1 [Kr] 4d 5s , which then delocalizes over the entire cluster. This leaves both dopant atoms with an atomic d-shell. open atomic d-shell. + + In Figure 5, a similar DOS analysis is presented for N = 4. Au and AgAu have very similar + + 4 3 In Figure 5, a similar DOS analysis is presented for N = 4. Au4 and AgAu3 have very similar DOS. In both cases the HOMO corresponds to a singly occupied MO of delocalized character ( -state) DOS. In both cases the HOMO corresponds to a singly occupied MO of delocalized character (α-state) with the 1P symmetry. In addition, there is a doubly occupied MO resembling the 1S eigenstate. with the 1Px symmetry. In addition, there is a doubly occupied MO resembling the 1S eigenstate. The The lowest unoccupied molecular orbital (LUMO) of the clusters is now the -state of 1P symmetry. lowest unoccupied molecular orbital (LUMO) of the clusters is now the β-state of 1Px symmetry. + + + + Therefore, Au and AgAu have three itinerant electrons. The DOS of PtAu and PdAu is + + + + 4 3 3 3 Therefore, Au4 and AgAu3 have three itinerant electrons. The DOS of PtAu3 and PdAu3 is remarkable; both clusters have only the 1S-type delocalized MOs below the HOMO, implying that remarkable; both clusters have only the 1S-type delocalized MOs below the HOMO, implying that there are in total only two delocalized electrons. As shown in the figure, the 1P and 1P MOs are x y there are in total only two delocalized electrons. As shown in the figure, the 1Px and 1Py MOs are empty. For N = 4, neither Pt nor Pd contributes to the electron delocalization, contrary to the N = 3 empty. For N = 4, neither Pt nor Pd contributes to the electron delocalization, contrary to the N = 3 + + + + case. PtAu and PdAu are clusters with closed electronic shells, as PtAu and PdAu are. + + + + 3 3 2 2 case. PtAu3 and PdAu3 are clusters with closed electronic shells, as PtAu2 and PdAu2 are. Figure 5. Total density of states of the clusters composed of N = 4 atoms. Projections into atomic states are presented in black for Au(d), in red for Au(sp), in blue for M(d), and in green for M(sp). The Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 13 Figure 4. Total density of states of the clusters composed of N = 3 atoms. Projections into atomic states are presented in black for Au(d), in red for Au(sp), in blue for M(d), and in green for M(sp). The molecular orbitals of delocalized character extending over the entire cluster volume are plotted and labeled based on their nodal character (1S and 1Px,y). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states are labeled as well. In the case that the cluster is open-shell, both α and β orbitals are plotted. The second panel of Figure 4 presents the case of AgAu2 , in which the DOS is also projected into the d-(blue) and the sp-states (green) of the silver dopant. The DOS of this cluster is very similar to that of Au3 , with the region close to the HOMO state having a higher Au than Ag character. This is a consequence of the relativistic nature of gold, which reduces the s-d energy separation [28]. As the figure reveals, this cluster also has only one doubly occupied MO of delocalized character (1S + + symmetry). Therefore, as expected, Ag is delocalizing its 5s electron. The PtAu2 and PdAu2 cases, which are presented in the right panels, are different. In both cases the DOS near the HOMO state is + + dominated by the d-states of the dopant atom, and, as for Au3 and AgAu2 , only one MO of + + delocalized character is found to be doubly occupied. This indicates that PtAu2 and PdAu2 are also clusters with closed electronic shells, which is not trivial considering the electronic configuration of 14 9 1 these dopant atoms. Therefore, at N = 3 the Pt dopant delocalizes its 6s electron ([Xe] 4f 5d 6s ), whereas the Pd dopant ([Kr] 4d ) promotes one of its 4d electrons to an electronic configuration of 9 1 [Kr] 4d 5s , which then delocalizes over the entire cluster. This leaves both dopant atoms with an open atomic d-shell. + + In Figure 5, a similar DOS analysis is presented for N = 4. Au4 and AgAu3 have very similar DOS. In both cases the HOMO corresponds to a singly occupied MO of delocalized character (α-state) with the 1Px symmetry. In addition, there is a doubly occupied MO resembling the 1S eigenstate. The lowest unoccupied molecular orbital (LUMO) of the clusters is now the β-state of 1Px symmetry. + + + + Therefore, Au4 and AgAu3 have three itinerant electrons. The DOS of PtAu3 and PdAu3 is remarkable; both clusters have only the 1S-type delocalized MOs below the HOMO, implying that there are in total only two delocalized electrons. As shown in the figure, the 1Px and 1Py MOs are empty. For N = 4, neither Pt nor Pd contributes to the electron delocalization, contrary to the N = 3 Appl. Sci. 2019, 9, 1666 10 of 13 + + + + case. PtAu3 and PdAu3 are clusters with closed electronic shells, as PtAu2 and PdAu2 are. Appl. Sci. Figure 5. Figure 2019, 5. 9Total , x FO Total R P density of stat density EER REof VIEW states es of the clust of the clusters ers composed of composedN of = 4 atoms. N = 4 atoms. Projections Projections into atinto omic state atomic 10 of s 13 are presented in black for Au(d), in re states are presented in black for Au(d), d for in red Au(s forp), in b Au(sp), lue for in blue Mfor (d), and in green for M(d), and in green Mfor (sp). The M(sp). molecular orbitals of delocalized character extending over the entire cluster volume are plotted and The molecular orbitals of delocalized character extending over the entire cluster volume are plotted labeled based on their nodal character (1S and 1Px,y). The HOMO-LUMO gap is labeled. In the case and labeled based on their nodal character (1S and 1P ). The HOMO-LUMO gap is labeled. In the x,y that the cluster is open-shell, both α and β orbitals are plotted. case that the cluster is open-shell, both and orbitals are plotted. + + + + The fin The final al an analyzed alyzed c case ase (size (size N N= 5) is = 5) ispr pr esented esentedin in Fi Figur gure 6. As e 6. As exexpected, pected, Au A 5u and and AgAu AgAu 4 are 5 4 are clusters with closed electronic shells, since each atom delocalizes one valence electron. In both clusters with closed electronic shells, since each atom delocalizes one valence electron. In both cases there cases ther are two do e are twoubly doubly occupied occupied MOMOs s of d ofeloc delocalized alized char character acter which which rerse esemble mble the the 1S an 1S and d 1P 1P x + + + + eigenstates. This is similar to the cases of PtAu and PdAu , which also have two doubly occupied eigenstates. This is similar to the cases of PtAu4 and PdAu4 , which also have two doubly occupied 4 4 MOs o MOs of f de delocalize localized ch d character aracter. Fo . For r N N = 5, Pt = 5, Pt an and d Pd Pd d delocalize elocalize one one electron electron in in or order der to to clo close se the the 1P 1Px electronic shell of the clusters, thereby gaining stability. This analysis shows an a priori unexpected electronic shell of the clusters, thereby gaining stability. This analysis shows an a priori unexpected behavior behavior of t of the he Pt an Pt and d P Pd d dopants on dopants on the Au cluster the Au clusterss in in the the N N = = 3–5 3–5 size size r range. ange. The dopant The dopant atoms atoms delocalize one electron at N = 3 and N = 5, but none at N = 4, allowing all the doped clusters to have delocalize one electron at N = 3 and N = 5, but none at N = 4, allowing all the doped clusters to have closed closed electro electronic nic she shells. lls. Figure 6. Total density of states of the clusters composed of N = 5 atoms. Projections into atomic states Figure 6. Total density of states of the clusters composed of N = 5 atoms. Projections into atomic are presented in black for Au(d), in red for Au(sp), in blue for M(d), and in green for M(sp). The states are presented in black for Au(d), in red for Au(sp), in blue for M(d), and in green for M(sp). molecular orbitals of delocalized character extending over the entire cluster volume are plotted and The molecular orbitals of delocalized character extending over the entire cluster volume are plotted labeled based on their nodal character (1S and 1Px,y,z). The HOMO-LUMO gap is labeled. In the case and labeled based on their nodal character (1S and 1P ). The HOMO-LUMO gap is labeled. In the x,y,z that the cluster is open-shell, both α and β orbitals are plotted. case that the cluster is open-shell, both and orbitals are plotted. 5. Conclusions 5. Conclusions In this work, the relative stability of small cationic Ag, Pt, and Pd doped Au (N = 2–7) clusters In this work, the relative stability of small cationic Ag, Pt, and Pd doped AuN (N = 2–7) clusters were investigated using a combination of photofragmentation experiments and density functional were investigated using a combination of photofragmentation experiments and density functional theory calculations. Mass spectra revealed a pronounced odd-even pattern in the abundances of Au theory calculations. Mass spectra revealed a pronounced odd-even pattern in the abundances of AuN and AgAu , which can be rationalized by considering the delocalization of the s-valence electron N1 and AgAuN−1 , which can be rationalized by considering the delocalization of the s-valence electron of each atom in the cluster, including the Ag dopant. Clusters composed of an odd number of atoms of each atom in the cluster, including the Ag dopant. Clusters composed of an odd number of atoms possess an even number of delocalized electrons, closing superatomic electronic shells and gaining possess an even number of delocalized electrons, closing superatomic electronic shells and gaining stability. Dissociation energies calculated for these clusters agree well with this picture, since the stability. Dissociation energies calculated for these clusters agree well with this picture, since the + + + preferred fragmentation channels always produce the more stable fragments: Au3 , Au5 , AgAu2 , and + + AgAu4 . The Pt and Pd doped clusters, however, behave very differently. Experimentally, PtAu2 and + + + PdAu2 correspond to intensity maxima, but PtAu4 and PdAu4 do not. The calculated dissociation energies of these clusters reveal a competition between channels that produce fragments of different sizes, especially in the N = 4–6 size range, explaining the smeared out odd-even pattern in their size- to-size dependent abundances. Analysis of the density of states of the clusters in the N = 3–5 size range reveals an unexpected behavior. At N = 3, both Pt and Pd delocalize one valence electron in order to close the 1S superatomic electron shell. At N = 4, though, the valence electrons of both dopants remain localized, which also allows the closing of the clusters’ 1S shells. Finally, at the size N = 5, Pt and Pd again delocalize one electron, closing the 1S and 1Px shells. Overall, these results illustrate that it is difficult to predict a priori how doping would affect the electronic structure of clusters, even at the very smallest sizes. Appl. Sci. 2019, 9, 1666 11 of 13 + + + preferred fragmentation channels always produce the more stable fragments: Au , Au , AgAu , 3 5 2 and AgAu . The Pt and Pd doped clusters, however, behave very di erently. Experimentally, + + + + PtAu and PdAu correspond to intensity maxima, but PtAu and PdAu do not. The calculated 2 2 4 4 dissociation energies of these clusters reveal a competition between channels that produce fragments of di erent sizes, especially in the N = 4–6 size range, explaining the smeared out odd-even pattern in their size-to-size dependent abundances. Analysis of the density of states of the clusters in the N = 3–5 size range reveals an unexpected behavior. At N = 3, both Pt and Pd delocalize one valence electron in order to close the 1S superatomic electron shell. At N = 4, though, the valence electrons of both dopants remain localized, which also allows the closing of the clusters’ 1S shells. Finally, at the size N = 5, Pt and Pd again delocalize one electron, closing the 1S and 1P shells. Overall, these results illustrate that it is dicult to predict a priori how doping would a ect the electronic structure of clusters, even at the very smallest sizes. Author Contributions: Conceptualization, P.F. and E.J.; formal analysis, P.F.; investigation, P.F. and E.J.; resources, P.F. and E.J.; writing—original draft preparation, P.F.; writing—review and editing, P.F. and E.J.; project administration, E.J. Funding: This research was funded by the Research Foundation-Flanders (FWO) (grant number G0B41.15N) and by the KU Leuven Research Council (grant number C14/18/073). Acknowledgments: P.F. acknowledges the FWO for a postdoctoral grant. Conflicts of Interest: The authors declare no conflict of interest. References 1. Liao, T.-W.; Yadav, A.; Hu, K.-J.; van der Tol, J.; Cosentino, S.; D’Acapito, F.; Palmer, R.E.; Lenardi, C.; Ferrando, R.; Grandjean, D.; et al. Unravelling the nucleation mechanism of bimetallic nanoparticles with composition-tunable core–shell arrangemen. Nanoscale 2018, 10, 6684. [CrossRef] 2. Neukermans, S.; Janssens, E.; Chen, Z.F.; Silverans, R.E.; Schleyer, P.v.R.; Lievens, P. 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