Magic Size Au64(S-c-C6H11)32 Nanocluster Protected by

Apr 2, 2014 - Magic Size Au64(S-c-C6H11)32 Nanocluster Protected by Cyclohexanethiolate ... and will facilitate the understanding of the structure and...
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Magic Size Au64(S‑c‑C6H11)32 Nanocluster Protected by Cyclohexanethiolate Chenjie Zeng, Yuxiang Chen, Gao Li, and Rongchao Jin* Department of Chemistry, Carnegie Mellon University, 4400 Fifth Ave, Pittsburgh, Pennsylvania 15213, United States S Supporting Information *

ABSTRACT: We report a new magic-sized gold nanocluster of atomic precision formulated as Au64(S-c-C6H11)32. The Au64 nanocluster was obtained in relatively high yield (∼15%, Au atom basis) by a two-step size-focusing methodology. Obtaining this new magic size through the previously established “size focusing” method relies on the introduction of a new synthetic “parameter”the type of protecting thiolate ligand. It was found that Au64(S-c-C6H11)32 was the most thermodynamically stable specie of the cyclohexanethiolate-protected gold nanoclusters in the size range from ~5k to 20k (where, k = 1000 dalton); hence, it can be selectively synthesized through a careful control of the size-focusing kinetics. The Au64 nanocluster is the first gold nanocluster achieved through direct synthesis (i.e., without postsynthetic size separation) in the medium size range (i.e., ∼40 to ∼100 gold atoms). This medium-sized Au64(S-c-C6H11)32 exhibits a highly structured optical absorption spectrum, reflecting its discrete electronic states. The discovery of this new Au64(S-c-C6H11)32 nanocluster bridges the gap of the gold nanoclusters in the medium size range and will facilitate the understanding of the structure and property evolution of magic-size gold nanoclusters.

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INTRODUCTION It is well-known that when the size of ligand-protected gold nanoparticles reaches the ultrasmall size range (i.e., less than 2− 3 nm, equivalent to a few hundred gold atoms), a series of discrete sizes (or “magic sizes”) of gold nanoparticles can be obtained.1,2 These “magic-sized” gold nanoparticles are often classified as nanoclusters; they possess higher stability than other sizes and hence can be enriched during the synthetic process and obtained in pure form.3,4 The ultrasmall gold nanoclusters also exhibit unique properties compared to their larger counterparts due to the quantum confinement effect,5,6 such as the multiband absorption spectra, enhanced fluorescence, etc.7−9 This class of gold nanomaterials provides a new platform for applications in catalysis, biosensor, as well as energy harvesting.10−12 Many magic-sized gold nanoclusters protected by thiolate ligands, Aun(SR)m (SR = thiolate), have been identified in recent years, with size ranging from several to hundreds of gold atoms, as shown in Scheme 1.13−21 Among those magic sizes, Au25(SR)18, Au38(SR)24, and Au144(SR)60, where SR = SC2H4Ph or SCnH2n+1, have been achieved through direct synthesis (i.e., without postsynthetic size separation) in molecular purity and relatively high yields (>10%, gold atom basis), Scheme 1 (red dots).22−24 The ease of obtaining these ubiquitous sizes has enabled wide research on these three nanoclusters.10,25−28 A “size focusing” synthetic methodology has been summarized, which has been demonstrated to be quite universal and allows one to achieve controlled synthesis of many atomically precise gold nanoclusters.3 In this method, a © 2014 American Chemical Society

mixture of crude nanoclusters with a controlled, proper size range was synthesized first, and then the subsequent sizefocusing step picks up the most robust nanocluster (i.e., magic size) in the size range.3 Our recent effort focuses on direct and high-yield synthesis of magic-sized gold nanoclusters in the medium-sized range (∼40 < n < ∼100). Compared to the already attained rich discoveries of the structures and properties in the small-sized region (e.g., n < ∼40, Scheme 1) such as Au25(SC2H4Ph)18−, Au38(SC2H4Ph)24, Au36(SPh-t-Bu)24, Au28(SPh-t-Bu)20, and Au23(S-c-C6H11)16−,29−34 the medium-sized gold nanoclusters remain to be pursued. Although several species have been identified, 15−18 such as Au 44 (SR) 28 , Au 55 (SR) 31 , and Au67(SR)35, they are all obtained through postsynthetic separation of a mixture of magic-sized nanoclusters by HPLC or solvent fractionation; that is, no direct synthesis methods have been developed for these medium-sized nanoclusters. The Au55(SR)31 and Au67(SR)35 nanoclusters exhibit decay-like (or rather featureless) optical absorption spectra, in contrast with the smaller ones, which exhibit highly structured optical spectra.5,35 To understand the structure and property evolution of gold nanoclusters, it is highly desirable to attain medium-size Aun(SR)m nanoclusters. In recent work, the molecular structure of the protecting thiolate ligands has been found to play an important role in determining the magic size and structure of Received: January 14, 2014 Revised: March 13, 2014 Published: April 2, 2014 2635

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Scheme 1. Magic Sized Aun(SR)m Nanoclusters in the Size Range from ∼10 to ∼200 Gold Atoms (the Red Asterisk Indicates the Au64(S-c-C6H11) from This Work and the Red Dots Refers to Three Ubiquitous Sizes (i.e., Au25(SC2H4Ph)18, Au38(SC2H4Ph)24, and Au144(SC2H4Ph)60)

Figure 1. Size focusing synthesis of Au64(S-c-C6H11)32 nanocluster. (A) MALDI mass spectra of the crude product (from step 1, black profile) and final product (from step 2, red profile). The asterisk (*) indicates the MALDI fragment. (B) The corresponding UV−vis spectra of products of step 1 and 2.

gold nanoclusters.32,33,36 This is in contrast with the ubiquitous Au25(SR)18, Au38(SR)24, and Au144(SR)60 nanoclusters that are quite insensitive to the R group22−24,35,37 (e.g., R = C2H4Ph or CnH2n+1), in which cases the clusters sizes are preserved with different kinds of thiolate ligands. The important role of thiolate ligand in tailoring the magic sizes and structures has been explicitly demonstrated in the high-yield transformation of Au25(SC2H4Ph)18 to Au28(SPh-t-Bu)20, and Au38(SC2H4Ph)24 to Au36(SPh-t-Bu)24.32,33,38 The solved crystal structures of Au28 and Au36 are drastically different from the “parent” Au25 and Au38 clusters, that is, cuboctahedron-based kernels32,33 vs icosahedron-based kernels.29−31 This newly discovered “parameter” for the synthesis holds potential in expanding the library of magic-sized, thiolate-protected gold nanoclusters. Herein, we report a new magic-sized Au64(S-c-C6H11)32 nanocluster via direct synthesis by adopting the “ligand parameter” strategy in combination with the size-focusing methodology. Unlike other magic nanoclusters in the medium size range, the Au64(S-c-C6H11)32 nanoclusters show rich features in the absorption spectrum, indicating multiple discrete electronic transitions due to quantum confinement effect. The band gap of Au64(S-c-C6H11)32 is ~0.8 eV determined by electrochemical analysis. The nanoclusters also exhibit red photoluminescence.

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0.254 mmol) in 5 mL H2O was mixed with TOAB (0.161 g, 0.294 mmol) in 10 mL toluene. The solution was vigorously stirred for 15 min to allow all the Au(III) salt to transfer into the organic phase with the help of TOA+. The colorless aqueous phase was removed by pipet. HS-c-C6H11 (97 μL, 0.80 mmol) was added. The color of the toluene phase changed from dark orange to light yellow and finally to colorless within 10 min, which indicated that Au(III) was reduced into Au(I) by the cyclohexanethiol to form Au(I)S-c-C6H11 complex/polymer. The solution was kept vigorously stirred for another 20 min. NaBH4 (0.096 g, 2.54 mmol) dissolved in 5 mL cold nanopure H2O was poured into the solution; the colorless solution turned to dark immediately. The reaction was continued for another 4 h. Then, the aqueous phase was removed by a pipet, and the organic phase was dried by rotary evaporation. The black precipitate containing the Aun(S-c-C6H11)m nanoclusters was washed with methanol thoroughly to remove thiol and inorganic salts. Then, the crude Aun(S-c-C6H11)m nanoclusters were extracted from the black residue by toluene. About 50 mg of gold nanoclusters were obtained in this step. Step 2: Size Focusing of 5−20k Nanoclusters into Single-Sized Au64(S-c-C6H11)32. The crude Aun(S-c-C6H11)m nanoclusters obtained from step 1 were dissolved in 1 mL toluene and 1 mL cyclohexanethiol. The solution was heated to 90 °C and maintained at this temperature under gentle stirring and air atmosphere. After 14 h, the initial polydispersed Aun(S-c-C6H11)m nanoclusters transformed to single-sized Au64(S-c-C6H11)32 nanoclusters. The Au64 nanoclusters were separated from the thiol/toluene solution by adding ∼8 mL of methanol and further centrifugation at 3000 rpm for 10 min. The supernatant containing excess thiol was discarded, and the precipitate containing the Au64 nanoclusters was washed with methanol for twice. Finally, Au64 nanoclusters were extracted from the precipitate by 4 mL of CH2Cl2. About 10 mg of Au64(S-c-C6H11)32 was obtained. The yield is ca. 15% based on Au atom. 3. Characterization. UV−vis spectra of the Au nanoclusters were acquired on a Hewlett- Packard (HP) Agilent 8453 diode array spectrophotometer at room temperature. Electrospray ionization (ESI) mass spectra were recorded using a Waters Q-TOF mass spectrometer equipped with Z-spray source. The source temperature was kept at 70 °C. The sample was directly infused into the chamber at 5 μL/min. The spray voltage was kept at 2.20 kV and the cone voltage

EXPERIMENT AND CHARACTERIZATION

1. Chemicals. Tetrachloroauric(III) acid (HAuCl4·3H2O, >99.99% metals basis, Aldrich), tetraoctylammonium bromide (TOAB, ≥98%, Fluka), cyclohexanethiol (HS-c-C6H11, 97%, Aldrich), cyclopentanethiol (HS-c-C5H9), sodium borohydride (NaBH4, Aldrich), tetrabutylammonium hexafluorophosphate (TBAPF6 ≥99.0%, Fluka). Solvents: toluene (HPLC grade, 99.9%, Aldrich), methanol (HPLC grade, ≥99.9%, Aldrich), dimethylene chloride (HPLC grade, ≥99.9%, Aldrich). All chemicals were used as received. 2. Preparation of Au64(S-c-C6H11)32 Nanoclusters. Step 1: Synthesis of Crude 5−20k Da Nanoclusters. HAuCl4·3H2O (0.100 g, 2636

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at 60 V. The ESI sample was dissolved in toluene and diluted (1:2 v) by dry methanol containing 50 mM CsOAc to impart charges to the clusters through the formation of Cs+cluster adducts in ESI. Matrixassisted laser desorption ionization (MALDI) mass spectrometry was performed with a PerSeptive-Biosystems Voyager DE super-STR timeof-flight (TOF) mass spectrometer. Trans-2-[3-(4-tert-Butylphenyl)-2methyl-2-propenyldidene] malononitrile18 (DCTB) was used as the matrix. Typically, 0.5 mg of matrix and 0.005 mg of sample (i.e., a 100:1 mass ratio between DCTB and sample) were mixed in 50 μL of CH2Cl2. A 10 μL portion of solution was applied to the steel plate and air-dried. Differential pulse voltammetry (DPV) measurements were performed on a CHI 620C electrochemical station at room temperature under N2 atmosphere. Pt disk working electrode, Pt wire counter electrode, and Ag/Ag+ quasi-reference electrode were used in the analysis. A 0.03 mM Au64 solution was prepared in an electrolyte solution of 0.1 M TBAPF6 (tetrabutylammonium hexafluorophosphate) in anhydrous CH2Cl2. Photoluminescence measurements were performed on a Fluorolog-3 spectrofluorometer (HORIBA Jobin Yvon). The emission spectra were record at the excitation wavelength of 450 nm (slit width: 10 nm). The concentration of Au25(SC2H4Ph)18 and Au64(S-c-C6H11)32 was adjusted to absorbance at λ = 400 nm of ∼1. CH2Cl2 was used as the solvent.

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Figure 2. MALDI-MS of the size focusing product under different laser powers. The arrow shows increasing laser power. The asterisk (*) indicates the fragment.

note that we observed in the experiment that, when the size of the nanoclusters is relatively large, such as Au144(SR)60 and Au333(SR)79, the 2+ charge peak was often observed in the MALDI spectrum. The atomic−monodispersity nature of the obtained product can also be proved by its UV−vis absorption spectrum (Figure 1B, red profile), in which several distinct peaks are observed at 400, 520, and 960 nm, in contrast with the decaying curve of the mixture in the first step. The rich features in the absorption spectrum indicate that a unique magic-size nanocluster has been obtained. In order to determine the precise composition of the 16.3k specie, that is, the (n, m) value in the Aun(S-c-C6H11)m formula, high resolution electrospray ionization mass spectroscopy (ESIMS) was conducted to obtain the exact molecular weight of the nanocluster. CsOAc was added to the cluster solution to form cationic cesium-cluster adducts. The ESI-MS spectrum shows two distinct peaks at 16425.6 and 8279.1 (Figure 3A),

RESULT AND DISCUSSION

The synthesis of Au64 nanoclusters (yield ∼15%, Au atom basis) involved a two-step process, (1) preparation of a proper size range of Aun mixture and (2) size focusing of Aun mixture into single-sized nanoclusters. Briefly, for an optimized procedure, 0.25 mmol of HAuCl4 was reacted with 0.80 mmol of cyclohexanethiol in toluene under the aid of phase transfer agent TOABr to form the Au(I)-S-c-C6H11 complexes or polymers. The as-formed Au(I)-thiolate intermediate was further reduced by NaBH4 to form a mixture of Aun(S-cC6H11)m nanoclusters. The size distribution of the assynthesized nanocluster mixture was found to be in a range from m/z ∼5 to ∼20 k characterized by MALDI-MS (Figure 1A, black profile). A decaying UV−vis absorption spectrum was observed (Figure 1B, black profile), indicating polydispersity of the crude nanoclusters from the first step. In the second step, the nanocluster mixture obtained from step 1 was incubated in 1 mL of cyclohexanethiol and 1 mL of toluene at 90 °C under air atmosphere. The reaction conditions of high temperature and large excess of thiol were applied to select the most stable size in the 5k to 20k range. It was found that after 14 h of reaction, the initial polydispersed nanoclusters were converted into a product of single magic-size, as indicated by the MALDI-MS and UV−vis absorption spectra (Figure 1AB, red profiles). In MALDI-MS, three clear peaks at m/z = 16.3k, 15.0k and 8.1k are observed after the size-focusing process. Of note, the three m/z peaks come from a single species, instead of three species. The highest peak (16.3k) is assigned to the singly charged molecular ion peak of the cluster product, and the 8.1k peak is assigned to the doubly charged clusters. The 15.0k peak is a fragmentation peak of the 16.3k species as it is not observed in ESI-MS analysis (vide inf ra). The fragmentation effect can be further judged through the laser intensity dependent analysis (Figure 2). With increasing laser power, the intensity of the 15.0k peak increased relative to the 16.3k peak, indicating that the 15.0k peak is a fragmentation peak of 16.3k. The 1.3k mass difference between the fragment and the molecular ion peak corresponds to the loss of Au4(S-cC6H11)4, a commonly observed fragmentation mode in thiolateprotected gold nanoclusters.39 The other peak at m/z = 8.1k is assigned to the doubly charged peak (z = 2) of the 16.3k specie;

Figure 3. ESI mass spectrum of the 16.3 kDa product. CsOAc was added to form Cs+ adducts with the charge-neutral nanoclusters. The low m/z “comb” is due to (CsOAc)nCs+ peaks.

corresponding to the singly charged [M+Cs]+ and doubly charged [M+2Cs]2+ peaks, respectively, where M represents the charge-neutral nanocluster. Thus, the molecular weight of the nanocluster (M) is determined to be 16292.7 (from the singly charged adduct peak, after subtracting one Cs mass) and 16292.4 (from the doubly charged adduct peak), respectively. These two values are apparently identical within the experimental error. We take the average cluster mass (16292.5 Da) for deducing the Aun(S-c-C6H11)m cluster formula. To deduce the exact numbers of gold atoms (n) and 2637

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ligands (m), a series of (n, m) values are compared to the experimentally determined cluster mass (i.e., 16292.5 Da), see Supporting Information (SI) Table S1. Only the value of (n, m) = (64, 32) matches well with the experimental mass (deviation < 0.2 Da). Other (n, m) compositions are ruled out due to large discrepancies. The Au64(S-c-C6H11)32 formula was further confirmed by synthesizing cyclopentanethiolate-protected Au64(S-c-C5H9)32 nanoclusters (see SI, Figure S1). Cyclopentanethiol is one CH2 group less than cyclohexanethiol, hence the SC5H11 protected nanoclusters will have a smaller mass than that of the SC6H11 counterpart, and the exact number of ligands can be calculated by dividing the mass difference with 14 (mass of CH2). We observed a mass peak at 15.8k for the cyclopentanethioalte protected gold nanocluster, as shown in SI Figure S1, and the 0.45k mass difference corresponds to 32 CH2 groups, hence 32 ligands, confirming the Au64(SR)32 formula (R = C6H11 and C5H9). We note that the synthetic procedure for the Au64(SC5H11)32 synthesis was not optimized; hence, several impurities were observed in the MALDI spectrum. Direct synthesis of Au64(S-c-C6H11)32 by the size focusing methodology relies on a careful tuning of the size distribution of the initial nanocluster mixture in step 1 and the etching kinetics in step 2. In order to obtain pure Au64(S-c-C6H11)32, it is important to adjust the initial mixture with size range from ∼5k to ∼20k. A smaller size range (e.g. 5−16k for 4:1 thiol/ gold ratio in step 1) would result in no Au64(S-c-C6H11)32 formation in the size focusing step, possibly due to the lack of larger sizes and suitable gold kernels for conversion into the Au64 cluster. On the other hand, a wider size range (e.g. 5−40k for 2.8:1 thiol/gold ratio) would result in other stable magic sizes larger than 20k. The tuning of the initial size distribution here is achieved through adjusting the thiol/gold ratio. After many trials, we found that a starting HS-c-C6H11 to HAuCl4 ratio of 3.2: 1 can give a good size distribution from ∼5k to ∼20k. With a higher thiol/gold ratio, the resulting crude product will be in a smaller size range, while with a lower ratio (i.e., less thiol), a wider size range was obtained. An interesting aspect in the synthesis of Au64 nanoclusters is the fate of the different intermediate nanoclusters in the sizefocusing step. The reaction progress of size focusing was monitored by MALDI-MS. As shown in Figure 4 (red profile), the starting crude product shows no peak corresponding to the Au64 nanocluster (16.3k); instead, other distinct mass peaks

were observed at 6.4k, 9.8k, and 11.5k, which indicate that several discrete sizes were kinetically “trapped” during the first direct-reduction step. For example, the 6.4k mass value matches the [Au23(S-c-C6H11)16]− nanocluster reported recently.34 When the crude nanoclusers reacted with excess HS-c-C6H11 thiol at 90 °C for just 1 h, the initial kinetically trapped clusters transformed into several thermodynamically stable species, including the Au64 (the intact molecular peak at 16.3k and its fragment at 15.0k). Several smaller nanoclusters with their molecular or fragmentation peaks at 8.0k, 8.5k, 10.0k, 11.3k, and 12.3k were also observed. Those smaller species were less stable compare to Au64, as indicated by their decreasing abundances with increasing reaction time from 1 to 8 h (Figure 4). At 14 h, pure Au64 without any smaller clusters were formed (1+ and 2+ peaks at 16.3k and 8.1k, respectively). It indicates that the Au64 nanocluster is the most thermodynamically stable magic size in the range smaller than 20k with the protection of cyclohexanethiolate. Of note, at a longer reaction time Au64 nanoclusters would gradually decompose under the harsh sizefocusing conditions; we found that after 24 h all the clusters were decomposed, forming a white floccule (the Au(I)-SR polymer/complex). Through this detailed study of the sizefocusing kinetics, the optimum reaction time in step 2 for collecting Au64 nanoclusters is when all the smaller sizes are just decomposed (∼14 h). It is the difference in the decomposing rates of different species that makes the selective synthesis of Au64(S-c-C6H11)32 nanoclusters possible. It is worth noting that the synthetic method reported here for Au64(S-c-C6H11)32 nanoclusters is similar to the previous one used to synthesize Au144(SC2H4Ph)60 nanoclusters.24 However, the interesting difference between the two synthetic routes is the thiol used. Using the primary phenylethylthiolate (i.e., S-CH2−), Au144 was selected by the size focusing step, while using the secondary cyclohexanethiolate (i.e., S-CH