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Sequential Observation of AgnS4- (1 e n e 7) Gas Phase Clusters in MS/MS and Prediction of Their Structures Zhikun Wu,† De-en Jiang,*,‡ Eric Lanni,† Mark E. Bier,*,† and Rongchao Jin*,† †

Carnegie Mellon University, Department of Chemistry, Pittsburgh, Pennsylvania 15213, and ‡Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831

ABSTRACT Recently we reported synthesis and characterization of a monodisperse thiolate-protected Ag7 cluster. Here we show in detail that a unique series of silver sulfide cluster anions (AgnS4-) were observed sequentially from n = 7 to 1 when subjecting the thiolate-protected Ag7 cluster to an MS/MS experiment. Random silver cluster anion distributions were not observed in a wide range of collision energies. This indicates the special structure and stability of these gas phase AgnS4- clusters. Global minimum search based on density functional theoryenabled basin hopping has yielded the most stable structures for AgnS4- (1 e n e 7). The global minima show a transition from three-dimensional to two-dimensional and then to one-dimensional geometry with decreasing n for AgnS4clusters. This joint experimental and computational effort provides a pathway to discover and elucidate metal-sulfide clusters of unique stoichiometry, which are not accessible through conventional methods such as laser ablation of mixed metal and sulfur powders. SECTION Nanoparticles and Nanostructures

M

etal cluster anions have attracted great attention in cluster research. Most studies have focused on properties such as structure,1 photoelectron properties,2,3 and catalytic activity.4-6 Dance et al. extensively studied the redox potential,7 structure,8,9 and reactivity8,10 of metal sulfide clusters, especially for iron sulfide7,8 and copper sulfide.9 But for silver sulfide cluster anions, there are fewer reports. Nakajima et al. identified a wide distribution of AgnSm- (n - m = 0-3) species generated by focusing the second harmonic (532 nm) of a YAG laser onto a rotating and translating sample rod, which is made of mixed metal and sulfur powders with a molar ratio of 1:1, and studied the photoelectron spectra of these clusters.3 Dance et al. found that silver sulfide can yield [Ag2n-1Sn]- anions (n e 14); these cluster anions are generated by laser-ablation and analyzed by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry.11 In our recent work,12-14 we have demonstrated that gas phase metal sulfide clusters of unique compositions can be produced using metal thiolate clusters made by wet chemistry approaches. We found that matrix-assisted laser desorption ionization (MALDI) or LDI of thiolated gold nanoclusters can generate some unique compositions of gold sulfide nanoclusters. For example, LDI of Au25(SR)18- leads to a series of gold sulfide cluster anions with the most abundant species being Au25S12-, followed by Au27S13-, Au23S11- and many others.13,14 The generation of these gold sulfide clusters is attributed to the laser-induced, selective breaking of S-C bonds in the gold thiolate clusters; note that a loss of some

r 2010 American Chemical Society

sulfur atoms is observed in the LDI process of gold thiolate clusters. Global minimum search by the density functional theory (DFT)-enabled basin-hopping method has found that these gold sulfide nanoclusters have a unique core-in-cage structure, distinct from the bulk Au2S structure.15 As wetchemistry synthesis of thiolated metal clusters can produce monodisperse species in a wide range of sizes, these solution phase clusters can be used as precursors to access a wide range of gas-phase metal sulfide clusters in mass spectrometry. We recently synthesized a solution-phase thiolated silver nanocluster that is determined by electrospray ionization mass spectrometry (ESI-MS) and MALDI-MS to be composed of seven silver atoms and four thiolate ligands.16 MS/MS analysis of the cluster dianion confirmed the composition of seven silver atoms with an interesting finding that, with increasing collision energy up to 50 eV, Ag7S4-, Ag6S4-, and Ag5S4- were formed. With collision energies up to 100 eV, we also observe Ag4S4-, Ag3S4-, Ag2S4-, and AgS4- sequentially. These ions are formed as a result of collision-induced fragmentation with argon in the collision cell. With the aid of DFT-based global-minimum research, we have predicted the structures of these clusters observed in gas phase. The predicted structures show that the AgnS4- nanocluster transits from a three-dimensional structure to a planar structure

Received Date: March 11, 2010 Accepted Date: April 7, 2010 Published on Web Date: April 15, 2010

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DOI: 10.1021/jz100317w |J. Phys. Chem. Lett. 2010, 1, 1423–1427

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Figure 1. MS/MS spectrum (collision energy: 10 eV) of dianionic [Ag7L4]2- in ESI mode (L = DMSA). Insets show the isotope patterns of a ([L-H]-, deprotonated) and b (assigned [Ag7L3S]2-). This observation was briefly discussed in ref 16; here we show the figure for description clarity.

with a decreasing number of Ag atoms and then to a ring structure. We first present observations of the fragmentation process from Ag7L42- to Ag7S4-, where ligand L represents meso-2, 3-dimercaptosuccic acid (henceforth abbreviated DMSA). We previously performed MS/MS analysis by selecting the precursor ion in the quadruple with an isolation width of several Thomson (Th = m/z); then one passes the ion through a hexapole transfer/collision chamber filled with Ar for collisioninduced dissociation (CID) and finally into the time-of-flight (TOF) reflectron for mass analysis.16 We chose [Ag7L4-3Hþ 2Naþ]2- (exactly [Ag7C16H21O16S8Na2]2-, abbreviated [Ag7L4]2-, m/z 759.50) as the parent ion (or precursor ion) for MS/MS analysis (Figures 1-3). With a collision energy of 10 eV, we observed a main fragment [Ag7L3S]2- (m/z: 685.58; Figure 1, peak b), which is caused by a loss of neutral SC4O4H4 (147.98 Da); indeed, we found the deprotonated species (SC4O4H3ions, theoretical 146.97) at m/z 146.99 (Figure 1, peak a). Such an S-C breaking mode was also observed in the case of gold thiolate nanocluster.13,14 At higher collision energy (>20 eV), the remaining three thiolate ligands also broke at their S-C bonds, resulting in the Ag7S4- ions observed in the MS/MS spectrum (Figure 2A); note that each DMSA ligand contributes only one sulfur to the silver core. Of another note, it is expected that the resulting Ag7S4- species carries -1 charge, instead of -2 (i.e., overall charge of the parent Ag7L42- ion), since the metal core of the parent ion carries only -1 charge.16 When the collision energy is further increased, fragmentation of the Ag7S4- ions occurs and, interestingly, Ag6S4-, Ag5S4-, Ag4S4-, Ag3S4-, Ag2S4-, and AgS4- are sequentially observed (Figure 2B-G); note that their Na-adducts are also observed (Figure S1). This series of silver sulfide anions are more clearly represented in the composite fragmentation spectrum (Figure S1 in the Supporting Information) of the entire MS/MS experiment (all collision energies 10-180 eV combined). Of particular interest, during the fragmentation process, the silver number decreases sequentially from 7 to 1, but the sulfur number remains at 4. When the number of silver atoms is more than 4 (that is, n > 4), no loss of any of the four


Figure 2. MS/MS spectra of [Ag7L4]2- for different collision energies (A: 25 eV, B: 40 eV, C: 50 eV, D: 70 eV, E: 80 eV, F: 90 eV, G: 100 eV).

Figure 3. MS/MS spectrum of [Ag7L4]2- for collision at 120 eV. Inset shows theoretical isotope distribution (upper profile) and experimental one (lower profile) for S4- species. The intense peak at m/z 102.99 (singly charged) is assigned to SCH(COO-)CH2 radical anion (i.e., a fragment of the ligand after losing CO2).

S atoms was found; when n e 4, we did observe weak signal of AgnS3- species besides AgnS4-, indicating the loss of one S atom. Furthermore, it was found that the ion intensities of AgnS4depend on the collision energy. One example of collision energy dependence of those gas-phase clusters is shown in Figure S2. When the energy was increased to 120 eV, species with an m/z of 127.90 corresponding to S4- was detected, and the isotope distribution also has an excellent match with the theoretical pattern (Figure 3), although S42- (m/z: 63.95) was not observed. It is well-known that sulfur vapor is a

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also be stable as a neutral cluster, which is confirmed by our DFT calculations. Because the six Ag atoms are equivalent in Ag6S4-, we just remove one Ag atom to provide an initial guess for the basinhopping search for Ag5S4-. We found that the global minimum of Ag5S4- has a disulfide bond (that is, some S atoms can not be isolated from each other any more). The Ag5 core has a square-pyramid structure, and the entire cluster has a C2v symmetry. From this structure, there are two ways to lose one silver atom: the pyramid top or the corner. Both ways led to the same global minimum. By comparing with that of Ag5S4-, the global minimum of Ag4S4- indicates that the corner Ag atom is the preferred one to lose. This can be understood by the fact that, in Ag5S4-, the corner Ag atom has fewer nearest neighbors than the top Ag atom. Interestingly, we found a planar global minimum for Ag3S4-; thus, from Ag4S4- to Ag3S4- a transition from three-dimensional to twodimensional geometry occurs, which is reminiscent of the similar changes in gold clusters.26 The global minimum of Ag2S4- has an approximately planar shape with a sulfur trimer formed, while that of AgS4- has a ring structure with an S4 unit and can be considered as a one-dimensional structure. The high symmetry of the Ag6S4- cluster indicates a higher stability of this cluster than its neighbors. To confirm this higher stability, we computed the sequential binding energy of an AgnS4- cluster to form Agnþ1S4- with a silver atom. We found that the binding energy of Ag5S4- with Ag to form Ag6S4- is indeed higher than that of all the other clusters (see Table S1 in the Supporting Information). The global minima we found for AgnS4- can help us understand our MS/MS results. The high-symmetry structures of Ag7S4- and Ag6S4- and to a lesser degree Ag5S4- underpin their stability and high abundances in the MS/MS spectra. The fact that we observed a loss of S atoms with Ag4S4- or smaller ones is also supported by the structures of the global minima. As one can see, S atoms in Ag7S4-, Ag6S4-, and Ag5S4- are all bonded to at least three atoms, while in Ag4S4- two-coordinate S atoms begin to appear. These less coordinated S atoms are relatively weakly bonded in the cluster, and therefore can be lost at a not-so-high collision energy. The joint experimental and computational efforts in this work provide a pathway to discover metal-sulfide clusters of unique compositions, which are not accessible through conventional methods such as laser ablation of bulk metal sulfides11 or mixed metal and sulfur powders.3 The welldefined size of the parent complex provides an effective way to probe the special metal-sulfide clusters generated in MS/MS. Due to the transient nature of these gas-phase clusters, structural information is difficult to obtain experimentally. This then provides a perfect opportunity for a DFTbased basin-hopping search for global minima, which is ideal for simple (such as one-component or binary) clusters with 50 atoms or less. We expect that more unique-composition metal sulfide clusters will be elucidated by this combined approach. In summary, we report the sequential observation of AgnS4- (1e n e 7) species, demonstrating the utility of MS/MS in determining the composition and charge state of nanoclusters. In some cases, it is expected to be useful to

Figure 4. Putative global minima for AgnS4- (1e n e 7) as found by DFT-enabled basin-hopping search. Color labels: Ag, green; S, red.

complex mixture, and all Sn in the range 2 e n e 10 have been detected.17 S4-/ S42- were also identified by cyclic voltammetry (CV), electron spin resonance (ESR),18 and MS.19 Their electronic and geometric structures have been studied theoretically, and a trans-C2h isomer was predicted for S4-.17 In the MS/MS analysis, of particular interest is the sequential observation of AgnS4- (for n from 7 to 1) with increasing collision energy (Figure 2). The initial Ag7S4- species loses silver atoms one by one, and every AgnS4- species is found to be in high abundance in a certain collision energy range (for example, the Ag5S4- can exist in high abundance in 50 eV ∼ 80 eV collision; see Figure 2C-E and Figure S2 in the Supporting Information), indicating high stability of each of the AgnS4- species. Moreover, unlike Nakajima et al.'s observation of a wide distribution of AgnSm (n - m = 0-3) in MS analysis,3 or Dance et al.'s observation of continuous distributions of other metal sulfide clusters,20 we did not observe obvious AgnS3- (5 e n e 7) or AgnS2- (1 e n e 7) species. This fact indicates the particular stability of AgnS4- (1 e n e 7) species. Herein, some interesting questions naturally arise: what are the structures of this series of AgnS4- species? And how does the cluster lose Ag atoms sequentially? To answer the above questions and unravel the structures of the series of AgnS4- clusters we observed, we performed a first principles global minimum search by employing the basin-hopping method21 enabled by DFT-based geometry optimization. DFT-enabled basin hopping has been successfully employed to explore structures of nanoclusters.22-25 Herein we performed an extensive global minimum search started with Ag7S4- from a random initial structure. In the putative global minimum we found for Ag7S4- (Figure 4), the four S atoms in Ag7S4- are isolated from each other and cap the four faces of the approximately face-capped-octahedron Ag7 core. The structure of Ag7S4- indicates a straightforward way to lose one Ag atom to form Ag6S4-; we found that the Td-symmetry, face-capped octahedron is indeed the global minimum of Ag6S4-. This highly symmetric structure should


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probe the structure of nanoclusters by observing how the parent cluster ion fragments in the CID process.27 We show that thiolated metal clusters prepared by wet chemistry can serve as useful precursors to obtain metal sulfide clusters of unique composition and particular stability. The observed series of AgnS4- (1e n e 7) species are unique; these species are different from the previously reported wide distribution of AgnSm (n - m = 0-3)3 or [Ag2n-1Sn]- anions (n e 14).11 DFTenabled basin-hopping search for global minima shows an interesting transition from three-dimensional Ag7S4- to onedimensional AgS4-. Among the putative global minima found for AgnS4- (1e n e 7), Ag6S4- shows the most symmetric structure: a tetrahedral symmetry with S capping four faces of the Ag6 octahedron. By MS/MS analysis, we also demonstrated the existence of S4- species in gas phase.

ref 15 for the details of the basin-hopping process and validation of the methods). The global minimum found by VASP basin-hopping was then reoptimized with Turbomole V5.10, which was used for parallel resolution-of-identity density functional theory (RI-DFT) calculations.33 The def2-TZVP orbital and auxiliary basis sets34 were used for all atoms for structural optimization. Effective core potentials that have 19 valence electrons were used for Ag.35 The force convergence criterion was set at 1.0 10-3 a.u. All structures for the AgxSyclusters reported in the text are from Turbomole results.

SUPPORTING INFORMATION AVAILABLE Combined MS/ MS spectrum for collision energies from 10 to 180 eV, the ion intensity as a function of collision energy, MALDI-TOF of Ag7(DMSA)4, and the sequential binding energies. This material is available free of charge via the Internet at http://pubs.acs.org.

METHODS

AUTHOR INFORMATION

Synthesis of Silver Nanocluster. The details of the synthesis and MS/MS conditions are reported in our recent work.16 Briefly, silver salt (AgNO3, 34.0 mg, 0.2 mmol) was first mixed with 20 mL ethanol; the solution was cooled to ∼0 °C in an ice bath. The magnetic stirring speed was reduced to ∼60 rpm, and DMSA (146.0 mg) was then added. The solution was slowly stirred for an additional 4 h in an ice bath. Then NaBH4 (7.6 mg, powders) was slowly added to the solution under vigorous stirring. The reaction was allowed to proceed for ∼12 h under constant stirring. The product suspension was centrifuged at 13 500 rpm for 10 min, and the resultant black precipitates were collected, washed thoroughly with methanol, then dissolved in water; Na2CO3 (∼ 20 mg) was added to adjust the pH to 9-10 to enhance the solubility of the clusters. The undissolved precipitates were further extracted with water. All the supernatants were combined. The clusters were precipitated again by adding MeOH. Repeating the recrystallization 2-3 times leads to highly pure Ag7(DMSA)4 clusters. MS/MS Analysis. MS/MS was conducted on a Waters Q-TOF II spectrometer. The sample was dissolved in water/methanol (1:1 vol.) with a concentration of ∼0.2 mg/mL, and the solution was infused directly into the source at 10 uL/min by a syringe pump. (The solution was kept at 0 °C until infusion.) Source block temperature was set at 80 °C and desolvation temperature was set at 120 °C. Cone gas was set to 50 L/h and desolvation gas was set to 100 L/h. Sample cone voltage was set at -20 V and electrospray capillary was set at -3.2 kV, the collision energy was varied from 10 to 180 eV. Computational. Global-minimum search was performed by using a Python script (available upon request) to interface the basin-hopping algorithm21 with the Vienna Ab Initio Simulation Package (VASP)28,29 for density functional theory geometry optimization. VASP employs periodic boundary conditions and planewave bases. The Perdew-Burke-Erzonhoff (PBE) form of the generalized-gradient approximation (GGA) was chosen for electron exchange and correlation,30 and the electron-core interaction was described by the projectoraugmented wave (PAW) method within the frozen-core approximation.31,32 The cluster was placed in a cubic box (15 15 15 Å3). A low-precision setup was used for fast geometry optimization (PREC = L and force tolerance at 0.25 eV/Å; see


Corresponding Author: *To whom correspondence should be addressed. E-mail: jiangd@ ornl.gov; [email protected]; [email protected].

ACKNOWLEDGMENT The work at ORNL was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy. R.J. acknowledges the financial support from CMU, AFOSR, and NIOSH. M.E.B acknowledges the financial support for the Waters QTOF2 from Carnegie Mellon.

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