The Innermost Three Gold Atoms Are Indispensable To Maintain the

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The Innermost Three Au Atoms are Indispensable to Maintain the Structure of Au18(SR)14 Cluster Yong Yu, Qiaofeng Yao, Tiankai Chen, Guo Xing Lim, and Jianping Xie J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07795 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 3, 2016

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The Journal of Physical Chemistry

The Innermost Three Au Atoms are Indispensable to Maintain the Structure of Au18(SR)14 Cluster Yong Yu,† Qiaofeng Yao,* Tiankai Chen, Guo Xing Lim, and Jianping Xie* Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260

ACS Paragon Plus Environment

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ABSTRACT: The physical and chemical properties of thiolate-protected metal nanoclusters are highly dependent on their cluster composition. Such physicochemical properties could be further enriched by incorporating two noble metals inside one cluster. Here we report an efficient synthesis of thiolate-protected (AuAg)18 clusters (or Au18-xAgx(SR)14 clusters) by adopting a simple CO-reduction method. The composition and electronic/optical properties of the Au18xAgx(SR)14

clusters are rationally tuned by adjusting the feeding ratio of Ag-to-Au. The

experimental data also suggest that the incoming Ag atoms would first replace the Au atoms in the core and a maximum of six Ag atoms can be incorporated into the structure of Au18(SR)14. More interestingly, doping more than six Ag atoms leads to the collapse of the Au9 core, and the incoming Ag atoms would only go into the motif sites. A detailed investigation on the compositional and optical properties of the as-doped AuAg clusters finally highlights the crucial role of the innermost Au3 core in maintaining the intact structure of Au18-xAgx(SR)14 clusters.


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INTRODUCTION Thiolate-protected gold nanoclusters or thiolated Au NCs, generally described as Aun(SR)m (where n and m are integers), are a promising class of ultrasmall particles with size below 2 nm, which typically contain several to a hundred of Au atoms in the core and a certain number of thiolate ligands (-SR) on the surface.1 Due to the strong quantum confinement effect in this sub2-nm size range, thiolated Au NCs possess discrete electronic structures and feature with some unique molecular-like properties, such as quantized charging2-3 and strong photoluminescence.4-6 The physical and chemical properties of Aun(SR)m clusters (e.g., optical7-8 and catalytic9-10) are highly sensitive to their cluster composition (or the value of n and m). Therefore the precise control of cluster composition at atomic level is central to realize their practical applications, which has also emerged as a hot research topic in NC community in the past decade.11-17 The pursuit of the precise control of cluster composition has greatly motivated the design and development of efficient synthesis methods, and several kinetic control and thermodynamic selection methods have been recently demonstrated in the direct synthesis of thiolated Au NCs with atomic precision, including Au15(SR)13,18-19 Au18(SR)14,18-20 Au25(SR)18,21-24 Au38(SR)24,25-26 Au144(SR)60,27-28 and many others.29-38 In addition, the large scale synthesis of atomically precise Aun(SR)m further facilitates the resolution of the structure of Aun(SR)m by X-ray crystallography, 32, 36, 39-42

which has largely advanced the cluster chemistry including the understanding of the

stability origin and formation process of thiolated Au NCs. More recently, doping a foreign metal (Ag, Cu, Pd or Pt) into the host Aun(SR)m clusters to form a bimetallic cluster has emerged as a new direction of cluster research.43 In principle, the as-synthesized bimetallic NCs could inherently carry the physicochemical properties of both constituting metal species, and the intrinsic properties of Aun(SR)m might be further enriched due


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to the possible synergistic effects. For example, Au25(SR)18 doped with a single Pd or Pt shows improved colloidal stability and enhanced catalytic activity.44-46 While doping Au25(SR)18 with Pd or Pt only introduces one foreign metal atom in the structure of host Au NCs, doping with Ag47-48 or Cu49 into Au25(SR)18 could produce bimetallic NCs with multiple foreign metal atoms. For example, up to eleven Ag atoms have been successfully doped into Au25(SR)18 using a coreduction method.47 Heavily doped (AuAg)25 with up to nineteen Ag atoms has also been reported by treating preformed Au23(SR)16 with Ag(I)-SR complexes.50 Although doping with foreign metals to form bimetallic NCs represents a versatile means to tailor the physicochemical properties of Aun(SR)m, most of these studies are focused on relatively large and stable NCs such as Au25(SR)18,44, 48 Au38(SR)24,51-52 and Au144(SR)60.53 There are very few successful attempts on doping foreign metal atoms into those relatively smaller (n < 20) and less stable NC species such as Au18(SR)14.54 The efforts in doping foreign metals inside small Au NCs like Au18(SR)14 may further enrich the family of bimetallic NCs, and help unravel some fundamental puzzles related to their structures and physicochemical properties. Au18(SR)14 was first obtained by polyacrylamide gel electrophoresis (PAGE) separation,55 and its formula was determined by electrospray ionization mass spectrometry (ESI-MS).7 Since then, theoretical efforts have been made to predict its structure,56-57 while the direct synthesis of Au18(SR)14 was not very successful mainly due to its poorer stability compared to Au25(SR)18.58 Recently, several research groups including us

18-20

recognized the key role of creating a mild

reduction environment in achieving a precise and large-scale synthesis of Au18(SR)14. By using a mild reducing agent and deliberately optimizing the reaction conditions, Au18(SR)14 could be precisely synthesized in a large quantity. Very recently, Au18(SR)14 has also received increasing attention from the applied research. For example, its light harvesting property in solar energy


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conversion

59-60

has been recently investigated, and the synthesis of its selenolate-protected

analogues 61 has also been reported. More recently, two independent groups led by Jin 62 and Zhu 63

have successfully grown the single crystal of Au18(SR)14 and resolved its structure by X-ray

crystallography. Their results reveal that Au18(SR)14 has a Au9 core surrounded by one Au4(SR)5 tetramer, one Au2(SR)3 dimer, and three Au(SR)2 monomers. The Au9 core can be viewed as two Au6 octahedrons fused by sharing one triangular face or three layers of Au3 piled up in a hexagonal close packing (HCP) manner. In addition, there is one Au-Au bond formed within the Au4(SR)5 motif between the two center Au atoms. As Au18(SR)14 is considered as the second smallest thiolated Au NC species that have been identified so far (the smallest one is Au15(SR)13), these findings provide new insights into the nucleation and size evolution of Aun(SR)m. Despite these exciting progresses, doping Au18(SR)14 with foreign metals like Ag to form Au18-xAgx(SR)14 clusters was rarely reported.48 The synthesis of Au18-xAgx(SR)14 clusters, however, is straightly relevant to several fundamental questions, such as how the incoming foreign metal atoms will affect the geometric and electronic structures of Au18(SR)14, and thus their molecular-like properties; what are the smallest amount of Au atoms required to maintain the structure of Au18(SR)14, or in other words, what are the largest number of foreign metal atoms that could be incorporated into Au18(SR)14 without destroying its structure; and which Au site will be first replaced by foreign metal atoms during the formation of bimetallic NCs. The present work aims to answer the above fundamental questions by designing a COreduction method24 to synthesize Au18-xAgx(SR)14 clusters. By introducing Ag salts as the source of foreign metal in the synthesis of thiolated Au18 cluster, and varying the feeding ratio of Ag-toAu (R[Ag]/[Au]) (please refer to the Experimental Section for details), the composition and electronic/optical properties of resultant Au18-xAgx(SR)14 clusters were continuously tuned.


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Moreover, a critical value of R[Ag]/[Au] was observed for maintaining the initial structure of Au18(SR)14. When R[Ag]/[Au] was less than 0.50, the incoming Ag atoms mainly entered the core of Au18(SR)14 while these Ag atoms started to go to the motif sites when this ratio exceeds 0.50. As a result, the initial structure of Au18(SR)14 no longer existed in the latter case. Presented below are the details of this study.

EXPERIMENTAL SECTION Materials. Hydrogen tetrachloroaurate (III) hydrate (HAuCl4·3H2O) and L-glutathione (GSH) from Sigma-Aldrich, silver nitrate (AgNO3) from Merck, carbon monoxide (CO, 99.9%) from Singapore Oxygen Air Liquide Pte Ltd (SOXAL), and all other chemicals were used as received. All glassware were washed with Aqua Regia (HCl : HNO3 volume ratio = 3 : 1), and rinsed with ethanol and copious ultrapure water. Ultrapure water with a specific resistance of 18.2 MΩ was used throughout the experiment. CO-reduction synthesis of bimetallic NCs. The bimetallic NCs were synthesized using a previously reported CO-reduction method,24 with HAuCl4 and AgNO3 as metal precursors. In a typical synthesis, aqueous solutions of GSH (0.15 mL, 50 mM) were added into various 20-mL glass vials each containing 4.6 mL of ultrapure water, followed by adding a certain amount of aqueous solutions of HAuCl4 (20 mM) and AgNO3 (20 mM). The total amount of metal ions was kept as a constant (1 mM) with a GSH-to-metal ratio (R[GSH]/[M]) of 1.5, while the adding amount of Ag was adjusted to keep the Ag-to-Au ratio (R[Ag]/[Au]) as 0 (no Ag, denoted as NC-1), 0.05 (NC-2), 0.10 (NC-3), 0.20 (NC-4), 0.50 (NC-5), 0.75 (NC-6), 1.0 (NC-7), and ∞ (no Au, NC-8), respectively. After 2 min of vigorous stirring, ~30 µL of 1 M NaOH was introduced to each of


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the above reaction solutions, which were all subjected to bubbling with 1 bar of CO gas for 2 min. After that, the reaction solutions were sealed airtight and the reaction was allowed to proceed under gentle stirring (500 rpm) at room temperature for 24 h. The final solutions were purified by a PD-10 column and stored at 4 °C in refrigerator for further characterizations. Materials Characterizations. UV-vis absorption and photoluminescence (PL) spectra were recorded on a Shimadzu UV-1800 spectrometer and a PerkinElmer LS-55 fluorescence spectrometer, respectively. Electrospray ionization (ESI) mass spectra were obtained on a Bruker Micro TOF-Q system. The samples were injected directly into the chamber at 120 µL·min-1. Typical instrument parameters: capillary voltage, 4 kV; nebulizer, 0.4 bars; dry gas, 2 L·min-1 at 120 °C; m/z range, 100–4000. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG ESCALAB MKII spectrometer.

RESULTS AND DISCUSSION Figure 1 shows the results using Au or Ag salts only as the metal precursor. The sample with pure Au salts was in dark green color (NC-1, Figure 1A inset). The characteristic peak at ~562 nm and the shoulder peak at ~630 nm observed on its UV-vis spectrum (Figure 1A, red line) indicates the fomation of Au18(SG)14 cluster.7, 18 Its compositional fomula is also confirmed by the ESI-MS analysis (Figure 1B). The first significant peak located at m/z 1957 (n = 0) represents the ionized [Au18(SG)14 − 4H]4− (1957 × 4 = 7828) as its isotope pattern indicates the ion carries four negative charges (peak spacing ∆(m/z) = 0.25 thus charge = 1/0.25 = 4). The rest peaks are corresponding to loss of H+ and addition of Na+, and can be expressed as [Au18(SG)14 − (4 + n)H + nNa]4− (n = 1 − 6). On the contrary, the sample with pure Ag salts was colorless throughout the


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reaction (“NC-8”, Figure 1A inset), implying no NC was formed. Its UV-vis spectrum has an onset of ~400 nm with no absorptions observed above 400 nm (Figure 1A, blue line). Note that the unique color and molecular-like absorption in the near UV to visible range have been widely implemented as a fast indicator of formation of few-atom metal NCs.7, 64

Figure 1. (A) UV-vis absorption spectra and digital photos (inset) of final products with pure Au (i.e., Au18(SG)14, red line) and pure Ag (blue line). (B) ESI-MS spectra of Au18(SG)14. (Top) peaks of ionized Au18(SG)14 with four negative charges bearing different amount of Na+. (Bottom) observed (black line) and simulated (red line) isotope patterns of [Au18(SG)14 − 4H]4−.

It is interesting that the reaction proceeded when Ag salts were introduced along with Au salts in the precursor, as confirmed by the vivid color observed for the resultant reaction solutions. As shown in Figure 2A, the solution color of NCs 2− −5 changed from reddish brown to red, which is in sharp contrast to characteristic dark green color of Au18(SG)14 (NC-1, Figure 1A), indicating the successful doping of Ag in the final product. Accordingly, the UV-vis absorption peaks of NCs 2−5 gradually shifted from the characteristic peaks of Au18(SG)14 at 562 nm to a lower wavelength (i.e., 540 nm (NC-2) → 526 nm (NC-3)→ 522 nm (NC-4)→ 518 nm (NC-5)) when R[Ag]/[Au] was increased from 0.05 to 0.5 (NCs 2−5, Figure 2A).


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Figure 2. (A) UV-vis absorption spectra and digital photos (inset) of final products with a feeding R[Ag]/[Au] of 0.05 (NC-2), 0.10 (NC-3), 0.20 (NC-4), and 0.50 (NC-5). (B) ESI-MS spectra of NCs 2−5 showing a progressive replacement of various amount of Au atoms by Ag.

ESI-MS was conducted to study the amount of Ag atoms doped into Au18(SG)14. As shown in Figure 2B, multiple peaks were observed for all samples, largely because the resultant NCs are featuring with: i) different charge states; ii) various amount of Ag atoms incorporated; and iii) extensive deprotonation and/or addition of sodium ions. Here only peaks with four negative charges (determined by the isotope analysis, similar to the one shown in Figure 1B) are shown for the convenience of discussion. These peaks can be divided into several different sets according to the number of incorporated Ag atoms. For example, three sets of peaks can be identified for NC-2 with R[Ag]/[Au] = 0.05 (red line). As previously discussed, set one (the peak labeled as x = 0) can be attributed to Au18(SG)14. Similarly, the second set of peaks (the peak labeled as x = 1 and the three peaks on its higher m/z side) correspond to species with one Au


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substituted by one Ag. The first peak (x = 1) observed at m/z 1935 has a ∆m/z of ~22 with respect to that of Au18(SG)14, which is close to the mass difference of +Ag − Au (22 × 4 = 88),53 therefore this peak can be assigned as [Au17Ag(SG)14 − 4H]4−. The other three peaks above m/z 1935 are due to −H + Na (∆m/z = 5.5, z = 4−, 5.5 × 4 = 22).24, 65 Therefore, this set of peaks can be assigned in a generalized form as [Au17Ag(SG)14 − (4 + n)H + nNa]4− (n = 0−3). The third set of peaks (the peak labeled as x = 2 and those peaks at its higher m/z side) can be assigned in a similar way as [Au16Ag2(SG)14 − (4 + n)H + nNa]4− (n = 0−3). Therefore, it can be summarized that up to 2 Au atoms were replaced by Ag when the feeding R[Ag]/[Au] was 0.05. Similarly, up to 3, 4 and 6 Au atoms were replaced by Ag when the feeding R[Ag]/[Au] was 0.10, 0.20, and 0.50, respectively (NCs 3-5, Figure 2B). The above observations thus suggest a unique formation mechanism of Au18-xAgx(SG)14 clusters (Scheme 1). Due to the redox potential difference of Au3+ and Ag+ (~1.0 V for Au3+ vs. ~0.8 V for Ag+) as well as the weak reducing capability of CO, Au3+ alone can be reduced to form Au18(SG)14 at given conditions (route A) while Ag+ alone cannot at the same reaction conditions (route B). When both Au3+ and Ag+ are present (route C), CO would selectively reduce Au(I)-SG complexes first, giving rise to tiny Au NCs formed in the vicinity of Ag(I)-SR complexes. These tiny Au NCs can then serve as nucleation sites and catalyze the reduction of Ag(I)-SR complexes. The exceptional catalytic capability of few-atom Au NCs have been welldocumented in literature.66-67 As a consequence the co-deposition of in-situ reduced Ag and Au atoms spontaneously forms the bimetallic NCs.


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Scheme 1. The proposed formation mechanism of Au18-xAgx(SR)14 clusters in the CO-reduction method, which includes (i) preferential reduction of Au(I)-SR complexes; (ii) catalytic reduction of Ag(I)-SR complexes; and (iii) co-deposition growth of bimetallic NCs.

However, when the feeding R[Ag]/[Au] was further increased to 0.75 (hereafter this sample is referred to as NC-6), the color of the reaction solution changed to brown (Figure 3A top, inset). In addition, the major absorption peaks observed in NCs 2-5 (Figure 2A) became less prominent and two new absorption peaks at ~409 and ~467 nm, respectively, emerged in its UV-vis absorption spectrum (Figure 3A, red line), which might mirror an abrupt change of the M18S14 framework (M = Au or Ag) in resultant NC-6. ESI-MS spectra (Figure 3B, top) further confirmed this assumption. In particular, the ESI-MS spectrum of NC-6 did not show any peaks of Au18-xAgx(SR)14 clusters, but only Au(SG)2 (a, see its isotope pattern in Figure 4A), AuAg(SG)2 (b, Figure 4B), AuAg2(SG)2 (c, Figure 4C) and Au4Ag3(SG)5 (d, Figure 4D and 4E). Note that these species are basic motifs of pure Au18(SG)14 incorporated with a certain amount of Ag atoms. The detailed assignment of Au4Ag3(SG)5 is shown in Figure 4D and 4E. The expanded spectrum shows a set of peaks due to deprotonation of (n + 1)H and addition of nNa+


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(n = 0 − 8) as the spacing between any two adjacent peaks is 11 (Figure 4D), while the isotope analysis shows these peaks bear 2− charges (Figure 4E). It should be mentioned that Au4Ag3(SG)5 carries one intrinsic negative charge, similar to Au25(SR)18 reported previously.68 Therefore, these peaks can be assigned in a generalized form as [Au4Ag3(SG)5 − (n + 1)H + nNa]2− (n = 0 − 8). When the feeding ratio R[Ag]/[Au] was further increased to 1 (NC-7), a similar UV-vis spectrum as NC-6 was observed (Figure 3A, blue line) and its mass spectrum was superimposible to that of NC-6 (Figure 3B, bottom), again suggesting the collapse of Au18xAgx(SR)14

cluster structure and the formation of same products as the consquence.

Figure 3. (A) UV-vis absorption and (B) ESI-MS spectra of final products with a feeding R[Ag]/[Au] of 0.75 (NC-6) and 1.0 (NC-7). Inset in (A) are digital photos of representative NC samples. The dash lines in (A) highlight the three characteristic absorption peaks of NC-6 and NC-7. The wide range spectra in (B) show the presence of Au(SG)2 (a), AuAg(SG)2 (b), AuAg2(SG)2 (c), and Au4Ag3(SG)5 (d). The enlarged spectra in (B) show peaks corresponding to ionized Au4Ag3(SG)5 with two negative charges bearing different amount (n = 0 − 8) of Na+. Detailed peak assignments are shown in Figure 4.


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(A)

−

808

810

812

814

−

(B)

Observed Predicted

[GS-Au-SG]

816

914

916

918

m/z

1

920

922

(C)

[(GS-Ag-Au-Ag-SG) + 2H]−

1020

1022

1024

m/z

[Au4Ag3(SG)5 − (1 + n)H + nNa]

(D)

Observed Predicted

[(GS-Au-Ag-SG) − H]

2−

1026

Observed Predicted

1028

1030

m/z

(E)

2

[4, 3, 5]

2−

3 0

1−

[4, 3, 5]

4

0

5

[4, 3, 5]

6

1320

1330

1340

1350

1360

1370

1380

1390

7 1400

observed

8 1410

1318

1320

m/z

1322

1324

m/z

Figure 4. Observed (black lines) and simulated (colored lines) isotope patterns of (A) [Au(SG)2]−, (B) [AuAg(SG)2 − H]−, (C) [AuAg2(SG)2 - 2H]−, and (D) [Au4Ag3(SG)5 − (1 + n)H + nNa]2−. (E) Enlarged spectra of [Au4Ag3(SG)5 − H]2− showing Au4Ag3(SG)5 alone carries one negative charge, where the motif formula of [AuxAgy(SG)z]q was abbreviated as [x, y, z]q for ease of identification.

On the basis of the above experimental observations and recalling the structure information of Au18(SR)14, a deeper understanding of how Au18-xAgx(SR)14 clusters are formed could be obtained, as illustrated in Scheme 2. In particular, when the feeding R[Ag]/[Au] was increased from 0.05 to 0.50, the incoming Ag atoms first replace the Au atoms in the Au9 core, which could also be verified by the XPS study of NCs 2-5 (Figure S1 and S2). The Au4f peaks gradually shift to a lower energy side as R[Ag]/[Au] was increased from 0.05 to 0.50 (Figure S1), indicating the Au atoms are more negatively charged with the increase of Ag atom number in the bimetallic NCs. This phenomenon suggests the charge transfer from Ag(0) to Au(0) (electronegativity: 1.93 for Ag vs. 2.54 for Au), which is only possible when these Ag atoms are chemically bonded with its neighboring Au atoms.47 The Ag-doping also affects the electronic structure of Au18-xAgx(SR)14 clusters, which is also reflected from their photoluminescence properties (Figure S3). On the


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other hand, as the feeding R[Ag]/[Au] goes beyond 0.50, the structure of Au18(SG)14 collapses and Ag atoms would only go into the motif sites to form AuAg(SG)2, AuAg2(SG)2, and Au4Ag3(SG)5. The formation of these complexes was also confirmed by XPS study. The Au4f and Ag3d peaks shift to the high energy side when R[Ag]/[Au] is 0.75 and 1.00 (NC-7 and NC-8, Figure S1 and S2), which is in line with the higher oxidation states of Au and Ag in these complexes. The possible structures of these alloy motifs could be predicted according to the symmetry and stability consideration. For example, the three Ag atoms could bridge any two adjacent Au atoms of Au4Ag3(SG)5 via one Au-Ag-Au bond, leading to the formation of a highly symmetric 4-unit ring structure repeated twice (see bottom of Scheme 2). Our observations are also in line with a recent theoretical study69 which suggests that Ag doping prefers to occur at the Au core, followed by monomer, tetramer and finally dimer motifs. Note that the alloy-type motif of Au2(SR)3 was indeed not observed in the current study. A very recent study of determining the crystal structure of Au15Ag3 alloy cluster also showed the incoming Ag atoms could replace the Au atoms in the Au9 core.48 The observation of no more than six Ag atoms doped into the structure of Au18(SR)14 also implies that the incoming Ag atoms would most likely replace those Au atoms in the Au3 layers at the two ends of Au9 core, as any attempts to replace the seventh Au atom in the Au9 core turn out in vain. Therefore, it is reasonable to suggest that the Au atoms within the middle Au3 layer of the Au9 core are critical for maintaining the structure of Au18-xAgx(SR)14 clusters. It should be mentioned that a recent study reported that Ag atoms could replace the central triangle Au3 atoms to form Au15Ag3 alloy cluster.54 However, this result was achieved by a two-step ligand exchange method other than a direct reduction method. The difference in the solubility, polarity and size of the two selected thiolates during the ligand exchange process could lead to the Ag centered structure as a more


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preferred one at given experiment conditions. As Au experiences a more significant relativistic effect than Ag,70 the Au-Au is stronger than Au-Ag and Ag-Ag (the bond strength follows the sequence of Au-Au > Au-Ag > Ag-Ag), and thus the Au-Au bond is able to form a more stable middle Au3 layer to “glue” together the end Ag3 layers in the Au9 core, which is crucial for the intactness of M9 core. A recent study reported (AuAg)18 with more than nine Ag atoms that was separated from electrophoresis showed enhanced chiroptical activity, but it should also be mentioned that the formula of Au8.5Ag9.5(SR)14 was measured by XPS.71 The different extents of doping suggest the unique enviroment of CO in our study allow the progressive doping of Ag and discrimination of Ag atoms according to their relative stability. Scheme 2. Diagrammatic illustration of the formation of Au18-xAgx(SR)14 clusters with the increase of the feeding ratio of Ag-to-Au (i.e., R[Ag]/[Au]: 0.05 → 0.50; only the M9 (M = Au, Ag) are shown for clarity) and their decomposed structure (R[Ag]/[Au] ≥ 0.75; the possible structures of these alloy motifs are suggested according to the symmetry and stability consideration, only Au, Ag and S atoms are shown for clarity).


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CONCLUSIONS In summary, the bimetallic analogues (Au18-xAgx(SR)14 clusters) of a less stable Au18(SG)14 cluster have been successfully synthesized in a one-pot manner by using a CO-reduction method. By varying the initial amount of Ag ions, maximum of six Au atoms could be replaced by Ag with a feeding R[Ag]/[Au] of 0.50, and these Ag atoms were identified to be located at the vertex of the Au9 core. As R[Ag]/[Au] was further increased to 0.75 and above, the structure of Au18 (SG)14 collapsed and a number of alloy-type motifs of Au18(SG)14, such as AuAg(SG)2, AuAg2(SG)2, and Au4Ag3(SG)5, were observed as by-products. The critical role of the three innermost Au atoms in maintaining the structure of Au18(SG)14 is thus highlighted. This work is interesting not only because it presents a new method of synthesizing bimetallic NCs in the less stable size regime, but also because it gives some mechanistic insights into the doping sequence and location of foreign Ag atoms in thiolated AuAg NCs.

ASSOCIATED CONTENT Supporting Information. XPS results and photoemission spectra. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (J.X.), [email protected] (Q.Y.). Tel: (65) 6516 1067 Present Addresses


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† Institute of Materials Research and Engineering, 2 Fusionopolis Way. Innovis, #08-03, Singapore 138634. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was financially supported by the Ministry of Education, Singapore, under Grant R279-000-481-112. Y.Y. acknowledges the National University of Singapore for his research scholarship.

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Table of Content


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The Innermost Three Gold Atoms Are Indispensable To Maintain the

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