Superatomic Signature and Reactivity of Silver Clusters with Oxygen: Double Magic Ag 17 – with Geometric and Electronic Shell Closure

Open AccessCCS ChemistryRESEARCH ARTICLE1 Dec 2021Superatomic Signature and Reactivity of Silver Clusters with Oxygen: Double Magic Ag17– with Geometric and Electronic Shell Closure Baoqi Yin†, Qiuying Du†, Lijun Geng, Hanyu Zhang, Zhixun Luo, Si Zhou and Jijun Zhao Baoqi Yin† State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 †B. Yin and Q. Du contributed equally to this work.Google Scholar More articles by this author , Qiuying Du† Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Ministry of Education, Dalian University of Technology, Dalian 116024 †B. Yin and Q. Du contributed equally to this work.Google Scholar More articles by this author , Lijun Geng State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Hanyu Zhang State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Zhixun Luo *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Si Zhou *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Ministry of Education, Dalian University of Technology, Dalian 116024 Google Scholar More articles by this author and Jijun Zhao Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Ministry of Education, Dalian University of Technology, Dalian 116024 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000719 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Understanding the stability and reactivity of silver clusters toward oxygen provides insights to design new materials of coinage metals with atomic precision. Herein, we report a systematic study on anionic silver clusters, Agn− (n = 10–34), by reacting them with O2 under multiple-collision conditions. Mass spectrometry observation presents the odd–even alternation effect on the reaction rates of these Agn− clusters. A few chosen clusters such as Ag13− and Ag17-19− hold up in the presence of excessive oxygen gas reactants. First-principles calculation results reveal that the chemical stability of D4d Ag17− is associated with its symmetric ellipsoidal structure and the electronic shell closure of superatomic orbitals (1S2|1P4|1P2|1D4|1D6||2S0). This results in 17c-2e multicenter bonding and a large highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap, the highest electron detachment energy and incremental binding energy among all the studied Agn− clusters, as well as the smallest O2-binding energy and least charge transfer from Ag to O2. We fully demonstrate the superatomic signature of these silver clusters and emphasize the unique Ag17– with both geometric and electronic shell closure, shedding light on the 18e stability for the coinage of metal clusters. The superatomic characteristics are also disclosed for Ag16−, Ag18−, and Ag32− clusters. Download figure Download PowerPoint Introduction Atomically precise coinage metal clusters have attracted great attention in recent decades due to their appealing structural beauty and fascinating properties, giving rise to promising applications in catalysis,1,2 chemo-sensing,3 biology,4,5 optical materials,6 and energy conversion.7,8 Because the uppermost s-band of coinage metals resembles that of alkali metals, their electron–lattice interaction and electron dispersion relations are similar to each other.9 It is anticipated that the relationship between electronic shell structure and the relative stability of coinage metal clusters can be understood in terms of the jellium model of alkali metals. Within the framework of the near free-electron gas (NFEG) theory10 of metals, the empirical jellium model for metal clusters as introduced by Knight et al.11 unambiguously explains the observation of the magic numbers of 2, 8, 20, 40, 70, and 112 in sodium clusters corresponding to electronic shell closure on the basis of spherical harmonic potential. Furthermore, the Clemenger–Nilsson cluster model allows for prolate/oblate ellipsoidal distortion (or anharmonic oscillator distortion),12,13 and enables us to rationalize stable clusters with altered subshells from prolate to oblate and subsequently a series of magic valence electron counts, that is, 2, 8, 18, 20, 34, 40, 58, 70, 92, 112, and so forth.10,12 As an important coinage metal element, the silver atom has a closed d shell and a single s valence electron, similar to alkali metals. Silver readily donates s-electrons and is susceptible to air oxidization to high oxidation states, due to the large band separation between Ag 4d and 5s states with regard to copper and gold. However, previous theoretical studies have shown a sharp drop at 20-electron (20e) silver clusters in the electron affinities (EAs)14 and ionization energies (IEs).12,15–17 This is distinct from alkali metal clusters of magic 20e that correspond to a full 1D shell plus two 2S electrons and exhibit noticeable abundance in the experimental mass spectra.11,14,16,17 A crucial criterion for the validity of the 18e or 20e stability for the metal clusters is still elusive and needs to be explored further. Besides electron count rules,18,19 geometry is also an important factor that determines the stability of metal clusters. For example, those of favorable geometry such as Mackay icosahedra often find prominent stability.20 Advances in exploring stable clusters not only shed light on magic clusters but also enlighten the development of inert/reactive clusters that mimic atoms in the periodic table of elements, giving rise to the establishment of the superatom concept.19,21 In the ongoing pursuit of other superatoms,22–28 it is important to further unravel the nature of superatom chemistry29 and understand how metal clusters are subject to the fundamental principles that govern their stability and activity, especially the interplay between electronic and geometric factors. In this work, we have prepared Agn– (n = 10–34) clusters using a laser ablation source and carried out an in-depth study of the anionic silver clusters reacting with O2. The superatom stability of Ag17–19– and Ag32– is fully illustrated from both geometric and electronic aspects. In particular, the unique Ag17– cluster is highlighted in view of both its geometric and electronic shell closures within the 18e rule. Besides, the unexpected stability of open-shell superatom clusters Ag18– and Ag32– is addressed in terms of the electromagnetic shielding effect of the core–shell structure and the halogen-like electronic configuration, respectively. Experimental and Theoretical Methods Experimental The experiments were conducted on a customized reflection time-of-flight mass spectrometer (Re-TOFMS) in combination with a laser evaporation (LaVa) source coupled with a fast-flow tube reactor.30,31 A pulsed laser (532 nm, Nd3+:YAG, 15–20 mJ·pulse−1, and 10 Hz) was used to generate the Agn– (n = 10–34) clusters. The cluster ions generated in the gas channel were expanded and grown through a nozzle with an internal diameter of 1.5 mm, and then interacted with the reactant gas (O2 seeded in He) in a compact fast-flow reactor (length ∼60 mm) with an instantaneous gas pressure at ∼55 Pa, that is, multiple collisions up to a few hundreds. The reactants and product ions were sampled by a skimmer (2 mm diameter) after ejecting them from the reactor and were analyzed by Re-TOFMS (more details in Supporting Information). Theoretical A comprehensive genetic algorithm (CGA) code was employed for unbiased global search of the lowest-energy structures of Agn– clusters,32 implemented with density functional theory (DFT) calculations performed with the DMol3 package.33 The all-electron relativistic method was used with the double numerical plus polarization d-function basis (DND) and the general gradient approximation (GGA)-Perdew-Burke-Enzerhof (PBE) functional.34 Totally 5000 iterations were carried out to ensure the global minimum on the potential energy surface for each cluster (more details in Supporting Information). Sixteen low-energy isomers from the CGA-DFT search of each size were reoptimized under the the recommended version of the full Heyd-Scuseria-Ernzerh (HSE06) functional35 accompanied by Stuttgart/Dresden effective core potentials (SDD) basis set36 and implemented in the Gaussian16 program to obtain the lowest-energy structure.37 Validation of the PBE and HSE06 functionals is given in the Supporting Information by comparing the relative energies and vertical detachment energies (VDEs) of selected Agn– clusters, calculated by different functionals. Based on the optimized structures by Gaussian16 within HSE06/SDD/Ag/6-311G*/O level of theory, total-energy calculations considering the scalar-relativistic effect were then carried out by using Amsterdam Density Functional (ADF) 2018.104 software and taking into consideration the zero-order regular approximation (ZORA).38–40 The interaction and chemical bonding nature were characterized by the energy decomposition analysis and natural orbitals for the chemical valence (EDA-NOCV) method.41–43 Results and Discussion Mass spectrometry observation Figure 1a shows the typical TOF mass spectrum of anionic silver clusters in the mass range of 1000–3850 amu, where well-resolved Agn– (n = 10–34) clusters are displayed (high-resolution spectrum in Supporting Information Figure S1) and their mass abundances correspond to a normal Gaussian distribution. By introducing different amounts of O2 gas into the fast-flow tube reactor (6 mm diameter, 6 cm long, ∼50 Pa pressure, and ∼60 μs reaction time), molecular adsorption products AgnO2– appeared in the mass spectra (Figure 1 and Supporting Information Figure S2), suggesting that the dominant reaction process is “Agn– + O2 → AgnO2–” in the present experimental condition. Nevertheless, such oxygen-addition reaction products are only observed for even-numbered silver cluster anions, as marked by green dots (Figure 1b and 1c). Even so, there are a few less reactive even-atom clusters Ag2n–, such as Ag18– and Ag32–, suggesting their relative chemical inertness in the reaction with oxygen. Besides the fast oxygen-addition reactions,the intensity of larger-sized Ag2n+1– clusters is slightly reduced, indicative of minor etching-like reactions involved in such conditions.44 In particular, the original mass distribution is seen to be centered at n = 18, while the mass abundances of Ag17–19– reverse with a sequence of “I(Ag17–) > I(Ag18–) > I(Ag19–)”, and Ag17– becomes dominant among all the studied silver cluster anions. In addition to Ag17–19–, the cluster Ag13– is also seen to be inert with oxygen. This is quite consistent with the previous study of relatively smaller Agn– reacting with oxygen in a larger flow tube reactor (60 mm diameter, 1 m long, 0.7 Torr pressure, and ∼80 ms reaction time).22 The differences of Agn– size distribution and experimental conditions result in oxygen-addition to be dominant pathways in this study, while additional products of Ag2O2– are noted in the previously published study. Figure 1 | Typical TOF mass spectra of the reaction between Agn– (n = 10–34) (a) produced via a laser ablation source and partial pressure of O2 gas at 250 mPa (b), and 354 mPa (c). The numbers of atoms in silver cluster anions are labeled on top of panel (a). The products AgnO2– are labeled as green solid circles. Download figure Download PowerPoint We have plotted the integrated mass peak intensities of the Agn– clusters (n = 12, 14, 16, 20, 22, and 24) and their oxides as a function of the partial pressures of O2 introduced into the flow tube, respectively. All the fitting curves correspond to straight linearity ( Supporting Information Figure S3) showing that the oxygen-addition reaction of “Agn– + O2” in the flow tube follows a pseudo-first-order mechanism. From the slopes of the linear fitting, Ag12– and Ag14– display faster reaction rates than that of Ag16–, while Ag17–19– almost does not react with molecular oxygen. On this basis, we have estimated the relative rate constants for the Agn− (n = 10–34) clusters toward O2, using “ k 1 rel = k 1 ( Ag n − + O 2 ) / k 1 ( Ag 16 − + O 2 ) , where Ag16– was chosen as a reference to exclude possible systematic errors from molecular beam fluctuation (Figure 2). The relative rate constants exhibit a significant odd–even effect with two exceptions at Ag18– and Ag32–. Overall, Agn– clusters with an even number of electrons (i.e., Ag2n+1−) are more inert than those with an odd number of electrons (i.e., Ag2n−). This is consistent with the previous publications,22,45–47 verifying that the open-shell Ag2n− clusters donate electrons to oxygen more easily than that of Ag2n+1−; meanwhile, the singlet states of Ag2n+1− clusters suffer from a spin excitation energy to react with 3O2 to form low-spin oxides. Figure 2 | Experimentally determined relative rate constants k 1 rel = k 1 ( Ag n − + O 2 ) / k 1 ( Ag 16 − + O 2 ) for the reactions between Agn– (n = 10–34) and O2. The error bars show the uncertainty range of values, and the black arrows represent the lowest limit of the estimated pseudo-first-order rate constants. Download figure Download PowerPoint Previous photoelectron spectroscopic studies and theoretical calculations have shown that anionic silver clusters with open-shell systems bear low electronic VDEs,14,48–54 enabling them to facilitate electron transfer toward electronegative substances (e.g., π* orbital of O2)45,52,55; however, those with closed-shell systems generally display high VDE and even a significant vertical spin excitation (VSE) energy when reacting with ground-state oxygen (3O2).22,52 Depending on the gas-phase reaction conditions (e.g., buffer gas pressure, reactant concentration, etc.), the electron transfer of silver clusters can be dominated by either oxygen-addition or -etching reactions, accompanied by superoxide or peroxide intermediate states. However, the underlying mechanism related to stability and reactivity could be associated with the following three factors as a whole: (1) geometric structure (with varied charge distribution and likely Lewis acid/base sites); (2) electronic configuration (including shell closure, highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) (H–L) gap, spin population, and superatom orbital characteristics); and (3) the resultant energetics [e.g., EA and detachment energies, atomic binding and removal energies, orbital level energies, vertical/adiabatic spin excitation energies (VSE or ASE), etc.56–59 Structural determination To achieve a better understanding of the stability and reactivity of Agn– clusters with oxygen, we have conducted DFT calculations to determine the lowest-energy structures (Figure 3), which are consistent with previous reports.48,54,60–62 Moreover, the simulated photoelectron spectra of ground-state structures of Agn– agree well with the experimental data ( Supporting Information Figures S5 and S6), further validating our theoretical ground-state structures. It is noteworthy that a distinct structural evolution from bilayer structures to cage and core–shell structures occurs at n = 16. Interestingly, Ag17– has D4d symmetry with a central Ag atom coordinated by five Ag(6) pentagons, similar to the most stable structure of Na17–.63,64 The highly symmetrical geometry indicates homogeneous charge distribution on the Ag17− cluster ( Supporting Information Figures S15 and S16). As will be discussed later, the geometric shell closure plays a key role in the unique stability of Ag17−. In comparison, Ag18− with C2v symmetry can be viewed as a central Ag atom enclosed by two pentagonal pyramids ([email protected]12) and capped with five additional Ag atoms ([email protected]12©Ag5), similar to the way that Cu18–.23 Ag19– with C3 symmetry has a distorted pyramidal structure like the honeycomb structure of Ag20– with one missing corner atom. Ag32– has a low symmetry and contains a four-atom tetrahedral core ( Supporting Information Figure S4). For the structures of Ag10–19– and Ag32–, we have also simulated their photoelectron spectra, and the satisfactory consistence with previous experimental data ( Supporting Information Figures S5 and S6)14,51 validates our predicted ground-state structures. Figure 3 | The lowest-energy structures of Agn– (n = 7–20, 32) at the HSE06/SDD level of theory. The symmetry and electronic states are given in the brackets. Download figure Download PowerPoint Energetics To evaluate the energetic stability of Agn– clusters, we calculated the incremental binding energies (IBEs) and the second-order differences in binding energy (Δ2E) using the following formula: IBE ( Ag n − ) = E [ Ag ] + E [ Ag n − 1 − ] − E [ Ag n − ] (1) Δ 2 E ( Ag n − ) = E [ Ag n + 1 − ] + E [ Ag n − 1 − ] − 2 E [ Ag n − 1 − ] (2) As shown in Figure 4a, a clear odd–even oscillation is seen for both IBE and Δ2E, and Ag17− has the maximum IBE value ( Supporting Information Figures S9) among all the Agn– (8 ≤ n ≤ 20) clusters. Considering that the silver cluster anions react with oxygen by donating electrons, VDE could play a determining role in the reactivity. As shown in Figure 4b, Ag17− possesses the highest VDE value (up to a theoretical value of 3.06 eV) among all the studied Agn– clusters. It is worth mentioning that the open-shell Ag32− cluster also has a high VDE value (2.90 eV), which is comparable with the previous experimental data,14 followed by Ag18− (2.44 eV) and Ag16− (2.56 eV). Since clusters with a large H–L gap often impede the donation of electrons, the gap is also an indicator of metal cluster reactivity with electronegative substances.48,65,66 Ag17− has a H–L gap up to 1.34 eV ( Supporting Information Figures S8a and S12), which is slightly smaller than those of Ag9− and Ag13−, corresponding to the VSE energy ( Supporting Information Figure S8b),22 stressing the importance to involve integrated considerations in judging cluster stability. Figure 4 | (a) Calculated IBEs and Δ2E. (b) Calculated VDEs are compared with the previous experimental results, where the values for Ag7–20− are derived from the given ref 51. (c) Binding energies of Agn− adsorption with O2 at the HSE06/SDD/Ag/6-311 + G*/O level of theory using Gaussian. Insets are the structures of AgnO2− (n = 17, 18, and 32) complexes. The lowest spin states for AgnO2− are triplet state (odd n) and doublet state (even n). (d) Calculated NPA charge (left scale) and bond length (right scale) of O2 in AgnO2− complexes. The bond length of a free oxygen molecule is 119 Download figure Download PowerPoint Figure presents the calculated O2-binding energies for the anionic silver clusters, on the of = + − which also exhibit alternation and agree well with the reactivity of Agn− observed in our that a superoxide state with an adsorption of the O2 molecule on the Ag17– cluster to the lowest O2-binding energy by calculation results using different Supporting Information which from the other anionic silver cluster oxides AgnO2– within peroxide states (i.e., Supporting Information Figure shows the least to the O2 bond and the charge transfer between Ag17− and O2 is the smallest among all the studied Agn− clusters (Figure The charge of the central atom of Ag17− almost and after O2 adsorption ( Supporting Information Figure Furthermore, the electron spin density of O2 on the cluster is to that of a free oxygen molecule (2 In for the other anionic silver clusters ( Supporting Information and Figure values are only prominent electron transfer between oxygen and the clusters. This can be further by the From the bonding energy as the energy of the AgnO2− relative to the energies of an Agn− cluster and O2 is into three energy to the and energies from and orbital Δ E = Δ E + Δ E + Δ E (3) In the bonding energies for are smaller than ( Supporting Information S1) for while that of the are larger than and It is among all the studied clusters, to the energies, and orbital between the orbitals of the metal and unoccupied π* orbital of oxygen ( Supporting Information and Figure Besides, the electron is the cluster in electron density ( Supporting Information Figure suggesting between Ag17− and O2. the ( Supporting Information S1) between Ag16– and O2 and orbital between Ag18– and O2 are relatively but than those between Ag17– and O2. For the other cluster the orbital between Ag32– and O2 are the as those in the however, the relative and interaction result in a comparable bonding energy of and Electronic configuration and orbital analysis It is that the size of the anionic silver clusters is associated with their varied electronic structures and superatomic orbital Figure presents the molecular orbital and energy of Ag17− and Ag18− clusters. Ag17– highly symmetric superatomic orbital with valence electron configuration of where the single vertical observed the double vertical indicates the gap between occupied and unoccupied It is that the lowest-energy structure of D4d Ag17– ( Supporting Information Figure has an oblate which results in a of the 1D and The of D4d Ag17– of 1D and the 2S of is in energy H–L gap of 1.34 eV). The D4d Ag17– with 18e stability the ellipsoidal distortion of the Clemenger–Nilsson shell The distortion Information is estimated to be on the basis of Ag17– corresponding to the energy of and ( Information Figure Interestingly, the effect related to the oblate distortion of D4d Ag17– cluster from a spherical symmetry does not the energy between the 1D and 2S (LUMO) energy an H–L gap and a energy level of the 2S orbital are published studies have a of or to for the stable clusters with both geometric and electronic shell closure within a magic number of valence As such a Ag17– not only has a closed electronic shell but also has a high symmetry cage structure that favorable energy of orbitals for the valence Ag17– is a superatom with unique stability. In comparison, the H–L gap of the 20e cluster Ag19– is relatively smaller eV), and the does not display a highly symmetric superatom likely due to of the 4d and 5s electrons of Ag Figure | of molecular orbitals for Ag17− and clusters. of clusters predicted by the Clemenger–Nilsson with distortion = where n is the number of oscillator is the number of oscillator in the and is of the orbital the of with = and are and (including respectively. The solid and represent occupied and unoccupied respectively. and and of the respectively. The value is Download figure Download PowerPoint To further the stability of Ag17– and has From the natural charge analysis ( Supporting Information Figures S15 and Ag17– and Ag18– clusters bear and on the central silver respectively. we selected the central atom and cage as two to the in the two clusters and as shown in Figures and For Ag17– the 5s orbital of central atom with a energy level from to form the molecular while 4d orbitals of the central atom with the from the The superatomic orbitals of the Ag17– cluster are dominated by the and orbitals of the results in Supporting Information and reveal that the stability of the Ag17– cluster from the interaction than from the orbital as the interaction between central atom and cage has bonding Supporting Information Figure In with the Ag18– cluster Supporting Information Figure the occupied highest molecular electron (i.e., Supporting Information Figure in the core can the surface chemical properties, while the other valence electrons the energy orbitals of the Ag18– also possesses stability as by the mass spectrometry which is consistent with our recent study on Cu18–.23 Figure 6 | and between cluster with the central atom and the cage under HSE06 functional accompanied with basis of with polarization function using The number in to the energy level for the of the energy The black and between the energy represent the of occupied and unoccupied respectively. The level is to and light and and of the function of occupied and unoccupied respectively. The value is (c) chemical bonding of Ag17− and the orbital and respectively.