ESI-MS Identification of Abundant Copper–Gold Clusters Exhibiting

Jan 25, 2015 - The protected noble-metal structures comprising 145 metal-atom sites and 60 ligands are among the frequently identified larger metal-cl...
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ESI-MS Identification of Abundant Copper−Gold Clusters Exhibiting High Plasmonic Character Nabraj Bhattarai,†,∥ David M. Black,‡ Snigdha Boppidi,‡ Subarna Khanal,† Daniel Bahena,† Alfredo Tlahuice-Flores,§ S. B. H. Bach,‡ Robert L. Whetten,*,† and Miguel Jose-Yacaman*,† †

Department of Physics and Astronomy and ‡Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249, United States § CICFIM-Facultad de Ciencias Fisico-Matematicas, Universidad Autonoma de Nuevo Leon, San Nicolas de los Garza, NL 66450, Mexico S Supporting Information *

ABSTRACT: The protected noble-metal structures comprising 145 metal-atom sites and 60 ligands are among the frequently identified larger metal-cluster systems exploited in many avenues of research. Herein we report a comparative electrospray ionizationmass spectrometry (ESI-MS) investigation of the 60-fold thiolated Au144 and CuAu144 clusters, in various positive charge-states, in conjunction with a density-functional theoretical (DFT) analysis based upon the icosahedral Pd145-structure-type applicable to these systems. Samples rich in the hexanethiolate-protected CuAu144 clusters are obtained via a single-phase reduction process. The predicted electronic structure of the vacancy-centered Au144(SR)60 system provided a simple rationale for the limiting [4+] charge-state observed of Au144, whereas the maximal [3+] charge detected on the CuAu144(SR)60 cluster can be explained if the 145th atom occupies the central site. Occupancy of the center-site stabilizes the superatomic 3S-orbital, and thereby shifts the shell-closing count from 82 to 84 free electrons. The DFT-calculated energetics also predicts a strong (0.65 eV) preference for placing the smaller Cu ion in this central site. Remarkably, the optical absorption spectra of dilute tetrahydrofuran (THF) solutions feature a broad band centered near 2.3 eV, in contrast to the previously reported “nonplasmonic” response of sub-2.0-nm all-gold or -copper clusters. Other methods (matrix-assisted laser desorption ionization mass spectrometry and high-resolution electron microscopy) were used to investigate whether aggregation phenomena might account for this observed plasmon emergence. This unusual result points to the need to obtain highly purified samples of copper-doped gold clusters of ca. 145 atoms total.

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INTRODUCTION In the physical chemistry of metal clusters, one prime concern has been to elucidate the rules, or criteria, that can explain the selection of exceptionally stable cluster sizes, or compositional “magic numbers”, and predict the existence of other related special clusters. Typically, such special clusters are first identified by mass-spectrometric analysis, well before definitive structural characterization can be achieved. In the simpler ligand-free metal clusters, the main rules select for (i) the number of free electrons, ne, available to fill the set of angularmomentum shells, according to the universal aufbau of the spherical electron-shell model (or superatom analogy); and (ii) the number of metal atoms, n, required to complete concentric shells of atomic sites, as relating to crystallography and surface structure.1,2 A simple metal cluster of formula An[z] is said to be “doubly magic”, if both counts, n and ne = vAn − z, satisfy the respective shell-closing criteria, where vA represents the metalatom valence and ze is the electrical charge.3 Selected monolayer-protected clusters (MPCs),4 denoted compositionally by AnXp[z], feature an additional, external shell of p anionic ligand-groups, X−, that coordinate specifically to the surface atoms of the metal-cluster core. The ligand-shell © XXXX American Chemical Society

bonding modifies the electron-counting rule (i), in accordance with the valence requirements of all coordinating ligands. For monovalent metals (vA = 1) and simple anionic ligands (X−), the free-electron count, ne = n − p − z, is obtained directly from the interpretation of the formula as A n X p [ z ] = (A+)n(X‑)p(e‑)n−p−z. By analogy to conventional metal-atom centered coordination complexes, one refers here to the superatom coordination-complex (SACC) model.5 The ideal number (p*) of coordinating ligands is given by the saturation coverage of the surface by the ligand head-groups, so as to achieve optimal protection of the cluster’s metallic core and its cargo of free electrons, i.e., the incipient plasma responsible for the interesting and valuable characteristic properties of metallic clusters generally. The complexity introduced by this additional constraint helps explain the rarity of correct predictions and Special Issue: Current Trends in Clusters and Nanoparticles Conference Received: October 30, 2014 Revised: January 24, 2015

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X-ray and electron diffraction measurements and X-ray absorption fine structure (XAFS) spectroscopy,16 established the structure as the conventional model for clusters of this general (144−5,60) formula, across variable ligand R-group identities and metallic constituents, although such general applicability remains far from rigorously proven. Yet the idealized structure-model serves the general purpose of rationalizing the selective high-yield formation in terms of closed, compact shells of metal atoms and protective ligand groups, and thus it has provided the best available guide for planning experiments and analyses.17 The global electronic structure associated with the A145X60 structure has been regarded as a secondary factor, despite the proximity of the metal−ligand excess (n − p ∼ 85)18 to the major shell closing at (90−92) free electrons as well as the prevalence of electron-shell closings in the related gold− thiolate clusters, e.g., the established neutral Au102(pMBA)44 compound, which conforms perfectly to the major 58-electron closing.4,12,13 High-level electronic structure calculations, based on the structure-model, showed clearly that all the frontier orbitals are well classified by their globular angular momentum, or superatom character, but there is an unusually large splitting of the 1H-set under the influence of the icosahedral crystal field.11,14,15 The relevant energy-level structure, pertaining to these orbitals, is depicted in Figure 2. The charge-neutral

highlights the great challenge involved in analyzing the fundamentals involved in already known systems. The present report concerns the chemical selection of certain larger clusters (MPCs) of the noble metals (Au, Ag, Cu, Pd) protected by 60 organo-sulfur thiolate ligands (RS-groups). The relevant Ih-Pd145 structure-type (Figure 1) was solved by

Figure 1. Shows the structure-type comprising 145 metal-atom sites and 60 ligand headgroup sites. The conventional structure-model has a single central site, which is vacant in the case of Au144, surrounded by concentric shells of 12, 42, and 90 metal-atom sites, plus the shell of 60 ligands. This particular representation derives from the structure of the copper-centered structure Cu@Au144 Cl60[3+], which has been optimized using the DFT-based methods discussed in the Methods Section. Here the central site (Cu) is white, the various subshells of Au atom sites are depicted in red, orange, and blue, and the thiolate anions (RS−) are simplified by chloride ions (Cl−, green), via isoelectronic substitution of thiolates by halides. This particular view is aligned along a 3-fold rotation axis of the chiral-icosahedral structure. Cf. ref 15 for further details of this structure-model.

Figure 2. Contrast of the predicted energy levels of the frontier orbitals of the protected Au144 (left, as reported in ref 15) and Cu@ Au144 (right) clusters. Paired arrows indicate the occupancy of these orbitals, when clusters are optimized in the [4+] vs [3+] charge states, corresponding to free-electron counts of 80 vs 82 electrons, respectively, to agree with the maximal charge states observed experimentally. (Arithmetically, these are obtained from 144 − 60 − 4 = 80 vs 145 − 60 − 3 = 82.) The key difference between the two patterns is the position of the superatom 3S level, which is stabilized (and occupied) in the case of the Cu-centered structure-model. The calculated HOMO−LUMO gaps in either case are ca. 0.4 eV.

Tran et al. in 1999 for the eponymous icosahedral palladium− carbonyl Pd145(CO)60 compound they crystallized as an airsensitive product.6 Already a gold−thiolate then known by the various estimates for its core-mass (∼30 kDa), diameter (∼1.7 nm) or total composition (Au144−6(SR)56−60), was found to have an X-ray scattering function fit by this structure-type.7 These ubiquitous gold-cluster substances could be obtained in high yield, and their favorable handling properties led to their use in many notable applications, but their fundamental elucidation began only in 2008, when Chaki et al.8 showed how to electrospray these massive complexes under nearly fragmentation-free conditions. Their groundbreaking electrospray ionization-mass spectrometry (ESI-MS) results led to the first definite composition, Au144(SR)59−60, which was then confirmed as (144,60) by others who employed ionizationenhancement strategies, i.e., exchange of ligands bearing pendant quaternary-ammonium groups or cesium-adduct formation.9,10 The composition determination (144,60) inspired a reconciliation11 between the Au144-substructure, as that of Pd145 but with a central vacancy, and the newly discovered staple-motif principle,12,13 which coordinates the 60 thiolate (−S−) head-groups as 30 pairs of metal-bridging sites. (Figure 1 gives pertinent details for a simplified form of this structure.14,15) Structural refinements, on the basis of further

assumption (z = 0) generates an instability because the highest level depicted is (2/10) occupied.15 The variable filling of this large set of near-degenerate orbitals has been argued to account for the observed electrochemical Coulomb-Staircase series of redox waves,11 as well as features of the optical absorption ranging from the mid-infrared (0.5 eV) to the mid-ultraviolet regions.19 Several recent developments have greatly broadened the range of these clusters: Kumara and Dass found that silver (Ag) may replace gold (Au), to the extent of 50 or even 60 of the 145 sites, without deviation from the overall composition (144,60).20 Here the distribution over metallic compositions {q} in AgqAu144−q(SR)60 is measured by soft ESI-MS. This astonishing invariance suggests, without proving, that the essential structure-type is also preserved. Pradeep and coworkers have identified an all-Ag analogous compound, but it is understood to have not the staple-motif characteristics but rather to form an extended Ag(I)-thiolate bonding network.21 The latest reported work in this series extends the substitution to copper (Cu) and Pd.22,23 Dahl and co-workers had much B

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EXPERIMENTAL METHODS The preparation method followed closely that of Dharmaratne and Dass, ref 22, except that no second-stage (etching) procedure was used, and the ratio of Cu:Au is at the low extreme. Gold and copper salts were combined in a 10:1 ratio (HAuCl4, 0.1418 mmol; Cu2Cl2, 0.01418 mmol) in 6 mL of absolute ethanol and added to a round-bottom flask containing excess tetraoctyl-ammomium bromide (TOA-Br; 0.468 mmol) dissolved in 15 mL of absolute ethanol, then stirred rapidly for 30 min, after which time 55 μL of hexanethiol (0.468 mmol, 3 equiv) was injected, followed by 1 h rapid stirring. Finally, excess NaBH4 (0.3783 g, 10 mmol, in 10 mL of absolute ethanol) was added all at once, causing a color change from turbid white to opaque brown-black. The stirred reaction was allowed to proceed for 2 h, after which the products settled overnight. The ethanol was decanted from the precipitated products, and the byproducts were extracted by repeated washing with methanol. Finally, the soluble product was extracted easily into tetrahydrofuran (THF) with mild sonication, followed by centrifugation to remove insoluble byproducts, and analysis of the abundant red-wine colored supernatant by methods (see below) including optical absorption spectroscopy, MALDI- and ESI-MS, and electron microscopy. The products are stable in THF solution, or as dry forms, over a period of months, without special precautions, as they present the same optical spectral and mass-spectrometric characteristics. The comparative ESI-MS analysis of the Au144(SR)60 system, obtained as described in ref 15, for the ligand phenyl-ethanethiolate (peth), employed toluene as solvent. ESI-MS spectra were acquired on a Bruker time-of-flight (microTOF) mass spectrometer (Bruker Daltronics) in positive mode. Prior to MS analysis, the electrospray source was rinsed with 1 mL of deionized water followed by 1 mL of 1 mM cesium acetate in methanol and finally rinsed with 1 mL of neat methanol to remove the cesium acetate from the source. The Cu−Au sample was dissolved in THF and introduced to the mass spectrometer by loop injection using a 5 μL stainless steel sample loop. After the sample was injected into the loop, neat methanol was used as the carrier solvent to push the injected sample into the source of the mass spectrometer at a flow rate of 0.5 mL/h (8.3 μL/min). The all-Au samples were analyzed similarly, as described in ref 15, except that toluene was used in place of THF as solvent. The UV−visible−near-infrared (UV−vis−NIR) spectra were recorded in variable-concentration THF solution, using a Varian-Cary 5000 spectrophotometer in the double beam mode. Scans were typically run from 1000 to 250 nm at a scan rate of 100 nm/min, a data interval of 1.0 nm and using a full slit-height. The parameters for the mass-spectrometric and STEM analyses are given in the respective Results and Discussion section and the Supporting Information. The synthesis of Au144 (peth)60 compound, the MALDI-MS procedure, and STEM imaging have all been described previously.14 The resolution (x) of the ESI-MS instrument, is 4000 < x < 36 000, implying that we are unable to resolve the individual isotopic peaks, but are well able to resolve the shape of the isotopic distribution.

earlier shown the limited Pt-for-Pd incorporation in their Pd145related compounds.24 The striking changes reported in the optical absorption spectra associated with these alloying or intermetallic substitutions, in the direction of emergent LSPR (plasmon-like) bands, have recently attracted theoretical attempts to account for such phenomena.25,26 In another key development, Wong et al. used 1H NMR spectroscopy to conclude that all 60 thiolate ligands of Au144(pMBA)60 are chemically equivalent, on the NMR time scale, in striking contrast to the low symmetry of other thiolateprotected gold clusters, e.g., in Au102(pMBA)44 the ligands are dispersed over 22 distinct sites.27,28 The observed 60-fold equivalence stimulated an effort, in conjunction with timeresolved electron-diffraction measurements on single clusters, to symmetrize the cluster within the full symmetry-group I, the chiral-icosahedral group of order O(60), resulting in a more coherent picture of the atomic and electronic structure.14 In our recent report,15 we have described the electrospray ion-distribution, up to a limiting positive charge, [z] = [4+], on Au144(SR)60[z] clusters, under essentially electrolyte-free conditions. To model the observed charge-states theoretically, we employed a simplified representation of the structure-model, wherein a halide (X−) replaces each thiolate anion (RS−), this isoelectronic substitution following the Jiang−Walter investigation of the small Au25(SR)18[1−] cluster.29 This tactic facilitates investigation of structural and electronic stability across a wide range of charge-states, from the observed oxidation limit, [4+], to the extreme opposite case of [8−], which may be implicated in the chemical-selection process involving strongly reducing conditions (excess BH4 −). Structurally, only the closed-shell configurations, [z] = [4+], [2+], and [8−], were found to hold I-symmetry within statistical uncertainty. It was proposed that the limiting [4+] charge-state found experimentally is derived from the special stability of its electronic configuration, cf. Figure 2, characterized by a highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO−LUMO) gap of nearly 0.5 eV, as compared to the nearly vanishing gap of the lower charge-states. In this paper, we present the preparation of hexanethiolateprotected Au−Cu clusters by a single-phase reduction process, following the first stage of the procedure reported in ref 22. The products obtained are investigated by matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) and ESI-MS, which indicated a predominant species CuAu144(SR)60, detected in the maximal charge-state [3+]. On the theoretical side, this CuAu144 observation motivates a density functional theory (DFT) analysis, based on the assumption that the lone (Cu) atom resides internal to the structure, i.e., in one of the 145 sites of the structure-model. We find there is a strong energetic preference for the smaller Cu atom to occupy the central vacancy, rather than displacing a larger Au atom into that site; and the energy-level structure obtained with all 145 sites occupied is different in one essential respect, leading to a rather large stabilization of the [3+] state ̈ rather than the naively expected [5+]. Finally, we find that dilute solutions rich in the CuAu144 species exhibit a striking LSPR-like optical absorption band, despite the indications from ESI-MS, MALDI-MS, and high resolution scanning/transmission electron microscopy (HRSTEM) that the substance is likely free of contamination by much larger clusters or aggregates.

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RESULTS AND DISCUSSION The optical absorption spectra of the Cu−Au clusters are studied by UV−vis−NIR spectroscopy and are presented in C

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and MALDI mass spectra of the Cu−Au sample at different pulsed ion extraction (PIE) times are presented in Figure S2. To determine the precise mass and compositions of the clusters, we employ ESI-MS, which is a softer ionization technique. Typical results are presented in Figures 5 and 6, and the summary of assignments of the major peaks is given in Table 1. The results indicate the dominant formation of two different Cu−Au clusters protected by the n-hexanethiolate ligand. The two peaks of greatest signal strength at m/z = 11820 and 11204 Da are consistent with the triply charged species of gold-rich clusters containing a single Cu atom, namely Au144Cu1(SR)60 and Au137Cu1(SR)56, respectively.30 A magnified spectrum is presented in the right frame of Figure 5. Each of these larger peaks have smaller, structurally related, peaks at lower m/z that are likely analogue byproducts formed during the synthesis of the principal components. The peaks at m/z = 11792, 11775, 11748, and 11731 Da are consistent with 3+ charge states for Au143 Cu 1 (SR)61 , Au 143 Cu 2 (SR)60 , Au142Cu2(SR)61, and Au142Cu3(SR)60 respectively. Peaks at m/z = 11177 and 11159 Da are consistent with Au136Cu1(SR)57 and Au136Cu2(SR)56, respectively. Similarly, m/z values for 16797 and 17717 Da correspond to doubly charged states for Au137Cu1(SR)56 and Au144Cu1(SR)60 nanoclusters, respectively. For comparison purposes, Figure 6 presents ESI-MS results for the Cu-free samples contacting Au144 and Au137 clusters, protected by peth ligands; these results show four sets of data points, corresponding to 1+, 2+, 3+ and 4+ charged states. In addition to the primary product, Au144(SR)60[z], major byproducts are in evidence, which can be identified as Au137(SR)56[z] and Au130(SR)50[z], by comparison to the prior literature.30,31 The similar ESI MS result for pure Au144 clusters, with overlay of the spectra at different charge states, is presented in Figure S3. We have used electron microscopy in this work primarily as a means to verify the absence of aggregates (associated dimers, trimers etc.) of the Cu−Au clusters identified by the MALDIMS and ESI-MS analyses. As mentioned in previous reports, obtaining HRSTEM images and chemical analysis is challenging for thiolate-protected gold nanoclusters with ultrasmall sizes.33 To obtain the chemical information (distribution of Au and Cu atoms) in a single nanocluster is difficult because there will be very few Cu atoms present so that the signal will be very weak. The electron micrographs obtained from aberration-corrected STEM are shown in Figure 7, which includes the average size ∼2 nm for Cu−Au nanoclusters. A typical high-angle annular dark field (HAADF) STEM nanocluster image is presented in right frame of Figure 7 where decahedral and icosahedral morphology is observed. It should be noted that it might not represent an actual morphology or crystalline structure of the nanoclusters. A more accurate structure can be predicted from the combination of HRSTEM images and diffraction patterns along with theoretical DFT calculation.14 Insight into the preferred formation of a monodoped CuAu144 cluster may be obtained from theoretical (DFT) calculations of the relative energy of various isomeric forms of CuAu144X60, where an iso-electronic substitution of thiolate by chloride (X = Cl) was carried out (cf. Figure 1). The advantages of following this methodology has been reported previously.15 Moreover, this viewpoint provided a rationale (cf. Figure 2) for the maximum charge state detected during ESIMS measurements of the thiolated Au144 cluster.15 Briefly, we have relaxed a set of initial structural isomers by imposing a 0.01 eV/Å value in force, using the PBE approximation for the

Figure 3. In contrast to the Au144 clusters, which exhibit only a finely stepped structure,18 a single broad LSPR-like feature

Figure 3. Comparison between optical absorption (UV−vis−NIR) spectra for thiolate-protected Au144 clusters and for the CuAu144-rich sample. The latter is diluted in THF solution, at varying dilution, while Au144 is diluted in toluene solution. The spectral density vs photon energy (eV) is presented in Figure S1.

dominates the spectra of the Cu−Au clusters, peaking near 540 nm. The spectral density vs photon energy is inset in Supporting Information Figure S1, which shows a strong maximum centered at 2.25 eV and a smaller feature at 1.45 eV. The monodispersity of the Cu−Au cluster samples were investigated using MALDI MS, as presented in Figures 4 and

Figure 4. A typical mass spectrum of the Cu−Au clusters with hexanethiolate ligands, obtained by MALDI-TOF mass spectrometry. The two features are assigned z = 1+ and 2+ charge states. The peaks are broad and shifted left (slightly low mass) due to the effect of laser fluence on these cluster ions. In this mass spectrum (see also the Supporting Information), the mass-range extends to 10 000 at the lowmass end. As the main concern in this paper is the possible presence of higher mass species (plasmonic clusters), the MALDI-MS range has been chosen carefully.

S2. The mass-spectral results indicate an overwhelming predominance of species in a narrow size (mass) range. The observed spectrum is dominated by a strong feature whose high-mass edge is near the ∼35 kDa mass calculated for Au144(SC6H13)60, plus a secondary feature assigned to the z = 2+ charge-state. The absence of any other major features masses indicates the quality of the sample. Comparative LDI D

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Figure 5. In this ESI-ToF mass spectrum for AuCu clusters (left frame), the dominant features are attributed to CuAu144(SR)60[z] (blue) and CuAu137(SR)56[z] (red), corresponding to the addition of a single Cu atom to the respective Cu-free species identified as in Figure 6. An expanded view of the 3+ region is presented in the right frame, and the assignments are presented in Table 1.

Figure 6. Electrospray mass spectra of all-Au clusters protected by RS = phenylethanethiolate (peth) ligands. (Lower Frame) In addition to the primary product, Au144(SR)60[z] (blue), the major byproducts are in evidence, which can be identified as Au137(SR)56[z] (green) and Au130(SR)50[z], by comparison to the prior literature.30,32 (Upper Frame) This result is analogous to that shown in Figure S3, showing only the more robust Au144(SR)60[z], but it is for the sample prior to the etching treatment.

Figure 7. Shows typical STEM results, AuCu nanoclusters (left), magnified HAADF STEM images of typical clusters (right), obtained when the dilute solution is dropcast on a graphene membrane. At ultrahigh resolution, one observes numerous smaller (∼1.6 nm) cores, evidencing the complex internal structure of the ubiquitous icosahedral structure, along with larger complexes that may be associated with the electron-stimulated mobility, damage, and resulting aggregation (sintering) that may occur under such conditions.

exchange-correlation functional34 and a double-ζ polarized basis set. Troullier−Martins scalar relativistic norm-conserving pseudopotentials35 were used for electronic representation of all atoms. The employed pseudopotentials consider the 3s23p5 valence electrons for Cl atoms, and 6s15d10 and 4s13d10 configurations for Au and Cu atoms, respectively. Selected initial structures have as a parent an I-symmetry structure (Figure 1) that holds one Au atom in the center (Au145Cl60). Subsequently tested were substitutions of a single Au atom by a Cu atom, in the various sites indicated in Table 2. The relative energies of the optimized structures found are also reported in Table 2. It was found that the entering Cu atom

Table 2. Energetic Ordering of Isomers of the CuAu144Cl60 Cluster Holding a 3+ Charge State Erelative, eV

Cu atom location

HOMO−LUMO gap, eV

0.00 0.65 0.94 0.99 1.01

center staple Au60 Au42 Au12

0.38 0.34 0.31 0.32 0.32

Table 1. Assignment of Major Au−Cu-Hexanethiolate Clusters Detected by ESI-MS peak mass (m/Z) Da

z

no. of Au

no. of Cu

no. of SR

calculated mass (Da)

experimental mass (Da)

deviation (Da)

notation for clusters

11820 11204 11177 11159 11792 11775 11748 11730

3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+

144 137 136 136 143 143 142 142

1 1 1 2 1 2 2 3

60 56 57 56 61 60 61 60

35461.58 33613.83 33534.1 33480.36 35381.85 35328.11 35248.38 35194.64

35460 33612 33531 33477 35376 35325 35244 35190

1.58 1.83 3.1 3.36 5.85 3.11 4.38 4.64

Au144Cu1(SR)60 Au137Cu1(SR)56 Au136Cu1(SR)57 Au136Cu2(SR)56 Au143Cu1(SR)61 Au143Cu2(SR)60 Au142Cu2(SR)61 Au142Cu3(SR)60

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demonstrated by Weissker et al.36 for the all-gold Au144(SR)60 system. In any event, the observed dominance, in the mass spectrum, of the single species assigned to CuAu144(SR)60[3+], justifies the attempt to rationalize its high abundance in terms of the closing of structural and electronic shells. The availability of a widely accepted structure-model, one of extraordinarily high symmetry (I) and compactness, makes it possible to justify the preferred accommodation of a single Cu atom in the unique central site, or cavity, as well as providing a satisfying rationale for the preferred charge-state, 3+, in terms of the modifications of the electronic subshell structure that occur only when this vacancy is occupied. The as-yet unknown structure of the Au137(SR)56 cluster precludes a similar analysis of the CuAu137 vs Au137 atomic and electronic structures.30 Clearly required is continued investigation into the Cu−Au intermetallic system, which has historically been of great metallurgical significance,38 at the nanoscale and to obtain a molecular understanding of those systems of greatest chemical relevance and applicability. Two major changes have occurred since this manuscript was completed and first submitted. First, we have succeeded in obtaining CID (collision-induced dissociation) mass spectra from samples of the Cu-doped gold clusters, as obtained in several repeated preparations (syntheses).31 The analysis of these CID patterns shows that Cu atoms (or −ions) are retained, i.e., not lost or fragmented from the cluster, even as multiple thiolates (as RS−SR) and gold−thiolates (e.g., the (AuSR)4 “rings”) are dissociated as neutral species from the main peaks (parent ions). These results and interpretation provide direct support for the hypothesis that the Cu atoms are internal to the structure, rather than being loosely associated in the exterior or bound in the staple units. These and other results will be reported in full detail in a forthcoming publication. Second, a highly relevant new theoretical report39 has come to our attention, demonstrating thatcounter to all expectationsthe incorporation (substitution) of even a few Cu atoms, into the Pd145-class (I-Au144) structure-model, results in a major change (enhancement) in the optical response (absorption spectrum) in the critical region, i.e., the heart of the visible (green−orange), as would be consistent with the emergence of a well-defined collective or plasmonic response. Concerning the limiting case of a single Cu atom addition, the authors write “Even the simplest possible addition (copper atom in the central vacancy of Au144(SH)60) increases the oscillator strength of transitions in the Au144 spectrum at around 540, 440, and 380 nm (for a direct comparison see Figure S1).” This detailed theoretical electronic-structure analysis of the stability, structure, and optical response is of course in striking agreement with the interpretation given in the present report.

prefers to incorporate at the centered site, this being favored by at least 0.65 eV over the other sites. This most-stable structure holds an I symmetry at a tolerance of 0.045 Å, when it is in the closed-shell 3+ charge state (Figure 2). A further analysis of the optimized structure reveals that the inner-12 shell becomes slightly dilated, enlarged by 0.06 Å (2%) radially, to accommodate the central Cu atom.

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CONCLUDING REMARKS The localized surface-plasmon resonance (LSPR) of noblemetal nanostructures is of great interest for both fundamental investigations and diverse applications. There is much uncertainty as to how far the key optical-spectroscopic properties of larger noble-metal nanocrystals may extend into the chemically interesting range below the metal-core diameter of ∼2.0 nm (∼200 metal atoms).36 Herein we have reported that chemical reduction of a 10% Cu:Au solution, containing hexane-thiolate (RS-) ligands, generates an abundant THFsoluble fraction that exhibits a dominant LSPR-like spectral band centered at 540 nm, despite the absence of products with mass above 34 kDa (>150 Au atoms). Analysis by electrospray mass spectrometry reveals the dominant solution-phase species are monocopper derivatives of the known Au144(SR)60, i.e., CuAu144(SR)60, as well as a CuAu137(SR)56 species, analogous to the recently found copper-free cluster.30 Copper-free species contribute negligibly to the spectrum, but a small quantity of multiply substituted (Cu-for-Au) species is indicated. By contrast, in the report of Dharmaratne and Dass,22 variable copper incorporation into the 144−145 atom Cu−Au clusters was found, depending on the starting ratio, up to perhaps 20 or 25 Cu atoms in total, but always as a broad, binomial-like distribution. They also reported optical (UV− vis−NIR) spectra for these samples, and noted the emergence of a profound LSPR-like band centered in the 520−550 nm regions. Similarly, no indication of solution-phase aggregation was found that might account for this unexpected ‘plasmonemergence phenomenon’. Because 144-atom all-gold clusters have been usually considered as “nonplasmonic”, one requires a mechanism whereby Cu atom incorporation drastically alters the optical response. The Cu atom location is considered, at the DFTlevel, for chiral-icosahedral CuAu144(SR)60, modeling thiolate (RS-) as chloride (Cl-) ions. Placing the Cu atom in the central site (vacancy) is strongly preferred, by >0.6 eV). For this structure, the [3+] charge-state shows special stability (HOMO−LUMO gap approaching 0.4 eV), which accounts for the maximal charge detected in the mass spectrum. From a purely structural point of view,37 there is a strong energetic preference for the smaller Cu atom to occupy the central vacancy, rather than displacing a larger Au atom into that site. The energy-level structure obtained with all 145 sites occupied is different in one essential respect, leading to a rather large ̈ stabilization of the [3+] state rather than the naively expected [5+]. Finally, we find that dilute solutions rich in the CuAu144 species exhibit a striking LSPR-like optical absorption band, despite the indications from ESI-MS, MALDI-MS, and HRSTEM that the substance is likely free of contamination by much larger clusters or aggregates. These intriguing findings should stimulate efforts to isolate and completely characterize the major species identified, and to gain a fundamental understanding of the electronic-optical character of these transitional metal-cluster systems, as has recently been

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ASSOCIATED CONTENT

* Supporting Information S

Additional information about the optical absorption spectra (UV−vis), LDI and MALDI mass spectra, and the mass spectrum of highly purified Au144(SR)60[z] obtained from the highly purified sample, using the ESI-ToF-MS. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. F

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The Journal of Physical Chemistry C Present Address

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Division of Material Science and Engineering, Ames DOE Laboratory, Ames, IA 50011, USA. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by grants from the Welch Foundation (AX-1857 to R.L.W. and AX-1615 to M.J.Y.), and from the National Center for Research Resources (5 G12RR013646-12) and the National Institute on Minority Health and Health Disparities (G12MD007591) from the National Institutes of Health. Dr. Alfredo Tlahuice acknowledges the financial support by CONACyT under Grant 206981.

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DOI: 10.1021/jp510893h J. Phys. Chem. C XXXX, XXX, XXX−XXX

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NOTE ADDED IN PROOF After this article was accepted, the new report of Weissker et al.40 became available, who examined the sensitivity of the optical response (spectra) of the Au144 clusters with respect to variations in theoretical parameters, including structuresymmetry, charge-state, ligand type, and DFT parameterizations; in no case did these variations produce an LSPR (“plasmonic”) characteristic such as the ones described above.

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DOI: 10.1021/jp510893h J. Phys. Chem. C XXXX, XXX, XXX−XXX