J. Phys. Chem. C 2007, 111, 4153-4158
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Formation of Alkanethiolate-Protected Gold Clusters with Unprecedented Core Sizes in the Thiolation of Polymer-Stabilized Gold Clusters Hironori Tsunoyama,† Patricia Nickut,‡ Yuichi Negishi,† Katharina Al-Shamery,‡ Yoshiyasu Matsumoto,† and Tatsuya Tsukuda*,†,§ Research Center for Molecular-Scale Nanoscience, Institute for Molecular Science, National Institutes of Natural Sciences, Myodaiji, Okazaki 444-8585, Japan, Institute for Pure and Applied Chemistry, Carl Von Ossietzky UniVersity of Oldenburg Center of Interface Science, P.O. Box 2503, D-26111 Oldenburg, Germany, and CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan ReceiVed: October 26, 2006; In Final Form: December 28, 2006
Octadecanethiolate-protected gold (Au:SC18) clusters were prepared by the reaction of C18SH and Au clusters stabilized by poly(N-vinyl-2-pyrrolidone) (PVP). Four samples were fractionated by recycling size exclusion chromatography of the as-prepared Au:SC18 clusters, and their core sizes were determined to be 8, 11, 21, and 26 kDa by using laser desorption ionization mass spectrometry. Unexpectedly, the sequence of these core sizes is different from that (8, 14, 22, and 29 kDa) obtained by conventional reduction of Au(I)-SC18 polymers, which is governed by kinetic factors. The present finding shows that the Au:SR (R ) organic group) clusters with a high tolerance to thiol etching can be systematically synthesized by first populating precursory Au clusters in a PVP matrix with subsequent thiolation of the preformed Au clusters.
Introduction
SCHEME 1: Methods for Preparing Au:SR
Thiolate-protected gold (Au:SR, R ) organic group) clusters have provided researchers with the opportunity to study the evolution of stability and properties as a function of cluster size. These clusters are also potential candidates for use as building blocks in functional nanoscale devices.1 It is of the utmost importance to synthesize Au:SR clusters with well-defined chemical compositions for fundamental studies and applications. The chemical preparation method for Au:SR clusters, most widely used since the first report by Schiffrin et al.,2 utilizes the reduction of a Au(I)-SR polymer by a reducing agent, such as sodium borohydride (reaction 1 in Scheme 1). Upon reduction, the nascent Au(0) atoms begin to aggregate in competition with surface passivation by thiolate molecules. Core growth is terminated at the stage when a protective monolayer has enveloped the Au core. Under such circumstances, kinetic factors play a nontrivial role in determining the core size distribution of the Au:SR products. In other words, the Au:SR clusters thus formed may not always exhibit thermodynamic or chemical stability but could possibly correspond to a metastable species. This inference is supported by the recent observation in which the sequence of core sizes was observed to vary with the thiolate structure.3 Another method for the preparation of Au:SR clusters is to exchange the phosphine ligands of gold clusters for thiolates. The stability of the resulting Au:SR clusters may reflect that of the Au cores themselves since the stability of the phosphinestabilized Au clusters can be explained in terms of the electronic and geometric shell closing of the Au cores. Typical examples can be found in Au11(PAr3)7X3,4-9 [Au11(PPh3)8X2]+,10,11 [Au11(BINAP)4X2]+,12 [Au13(PPhMe2)10Cl2]3+,13 [Au20(PPh3)8]2+,14 * Corresponding author. E-mail:
[email protected]. † National Institutes of Natural Science. ‡ Carl von Ossietzky University of Oldenburg. § Japan Science and Technology Agency.
[Au39(PPh3)14Cl6]2+,15 and Au55(PPh3)12Cl6,16,17 where BINAP represents 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl. Hutchison’s group has reported the preparation of Au11:SR by the ligand exchange reactions of [Au11(PPh3)8X2]+.11,18 Murray and co-workers have detected Au55:SC6 clusters, a counterpart of the well-known Au55(PPh3)12Cl6,16,17 by mass spectrometry in the reaction products of Au55(PPh3)12Cl6 with hexanethiol (C6SH).19 These studies hint at the possibility that Au:SR clusters with stable cores can be accessed by thiolation of preformed Au clusters weakly stabilized by a linear polymer (reaction 2 in Scheme 1) or encapsulated within dendrimers.20,21 This hypothesis has recently been verified through the isolation of Au55: SCx using recycling size exclusion chromatography (SEC) from the reaction products of alkanethiol (CxSH, x ) 12 and 18) and Au clusters (1.3 ( 0.3 nm) stabilized by poly(N-vinyl-2pyrrolidone) (PVP).22 As an extension of our previous work, we investigated a sequence of core sizes for a prototypical system, Au:SC18, formed by reaction 2. We first prepared the Au:SC18 clusters by thiolation of polymer-stabilized Au clusters. The as-prepared Au:SC18 clusters were fractionated by recycling SEC,22-31 and the core sizes were then characterized by mass spectrometry. The sequence of core sizes for the Au:SC18
10.1021/jp067025q CCC: $37.00 © 2007 American Chemical Society Published on Web 02/28/2007
4154 J. Phys. Chem. C, Vol. 111, No. 11, 2007 SCHEME 2: Protocols for Preparing Au:SC18
clusters thus determined was compared with that obtained by reaction 1; the Au:SCx clusters with core masses of 8 kDa (ca. 38 atoms), 14 kDa (ca. 75 atoms), 22 kDa (ca. 101 atoms), and 28 kDa (ca. 146 atoms) have been isolated by Whetten et al.32-35 The difference in the core sequence is discussed in terms of the controlling factors for the core formation in reactions 1 and 2. Evolution of the electronic structures as a function of the core size is also probed by measuring the optical gap of the isolated Au:SC18 clusters. Experimental Section Chemicals. All of the chemicals were commercially available and used without further purification. Sodium tetrahydroborate and toluene were obtained from Wako Pure Chemical Industries. Hydrogen tetrachloroaurate tetrahydrate was obtained from Tanaka Kikinzoku Kougyo KK. PVP ((C6H9NO)n) with an averaged molecular weight of approximately 40 kDa (K-30) was purchased from Tokyo Chemical Industry Company, Ltd. Tetraoctylammonium bromide (TOAB) was obtained from Lancaster Synthesis. Gel permeation chromatography (GPC) grade polystyrene (PS) standards (molecular weight ) 29 300, 18 700, and 13 700) and n-octadecanethiol (C18H37SH) were obtained from Aldrich. The PS standards (molecular weight ) 50 000, 25 000, and 5 780) were procured from Chemco Scientific. Deionized water, with a resistivity of > 18 MΩ·cm, was used in the present study. Chemical Preparation of Au:SC18 Clusters. The preparation process (reaction 2) employed consists of two steps (Scheme 2): preparation of PVP-stabilized Au clusters (Au:PVP)36 as precursors (step 1) and thiolation of the Au:PVP clusters (step 2). Details of the protocols used in each step are described as follows: the Au:PVP clusters were prepared by reducing HAuCl4 (1 mM, 50 mL) with NaBH4 (0.1 M, 5 mL) in the presence of PVP (0.5-5 mmol in monomer unit) at temperature T1 (K). A toluene solution of C18SH (5 mM, 50 mL) was placed on top of a hydrosol of the as-prepared Au:PVP (55 mL), and the biphasic mixture was vigorously stirred under atmospheric
Tsunoyama et al. conditions at temperature T2 (K) for a period of t (hours). The organic phase was separated and evaporated in vacuo. The crude Au:SC18 sample was then purified with ethanol. Three samples of Au:SC18 (samples 1-3) were prepared under the conditions summarized in Scheme 2. For comparison, Au:SC18 clusters were prepared by reaction 1 according to the method of Schiffrin et al.2 with slight modifications under ambient conditions. HAuCl4 (30 mM, 15 mL) was transferred to a toluene phase by TOAB (15 mM, 75 mL) and stirred for 15 min. A toluene solution of C18SH (90 mM, 15 mL) was added to the mixture and stirred for an additional 30 min. An aqueous solution of NaBH4 (0.3 M, 15 mL) was rapidly injected into the biphasic mixture, which was then stirred vigorously for another 3 h before the organic phase was separated and evaporated in vacuo. The crude sample was finally purified with ethanol (sample 4). Samples 1-4 in powder form (ca. 60 mg) were mixed with neat C18SH (3 mL), and the mixture was stirred at 353 K for 24 h in an air atmosphere. During this treatment, the core of the metastable Au:SC18 cluster is etched37 to form a stable core. Finally, the excess C18SH and byproducts were removed using warm ethanol (ca. 320 K). The purified Au:SC18 clusters (samples 1′-4′) were stored in a freezer (255 K) in powder form. Isolation and Characterization. The Au:SC18 samples were purified by passing them through SiO2 gel (particle size: 4575 µm) before being fractionated using recycling SEC. The typical throughput of the SEC was estimated by optical spectroscopy to be >95%. The SEC system used (LC-908, Japan Analytical Industry Company, Ltd.) accommodates two columns (W-253, Japan Analytical Industry Company, Ltd.) with a total exclusion limit of 5 × 104 in series and a UV-visible detector operated at 290 nm.22,38 Toluene was used as the eluent, and the flow rate was 3.5 mL/min. Monodisperse polystyrenes were used to obtain a calibration curve from which the hydrodynamic diameter (Dhydro) of the Au:SC18 clusters was estimated. The least-squares fit to the data gave the following conversion equation for our W253 × 2 system
Dhydro (nm) ) 265.2 × exp(-0.1064 × tR (min))
(1)
where tR represents the retention time. Chemical compositions of the clusters fractionated by SEC were studied using a laser desorption ionization (LDI) mass spectrometer that was built in-house.39,40 The mass spectra were obtained in positive ion mode with a laser fluence of 0.3-3.0 mJ/pulse. UV-vis/near infrared (NIR) absorption spectra were recorded with a spectrometer (JASCO, V-670) under ambient conditions in carbon tetrachloride. Absorbance in the wavelength domain, I(λ), was converted to that in the energy domain, I(E), according to the relation I(E) ) I(λ)/(∂E/∂λ) ∝ I(λ) × λ2 so that the integrated spectral areas were conserved. Transmission electron microscopy (TEM) images were taken using an electron microscope operated at 100 kV (Hitachi, H-7500). The typical magnification of the TEM images was 100 000. Results Isolation by Recycling SEC.41 Figure 1a shows typical chromatograms of samples 1′-3′ at the second recycling stage. The retention times of the main components decrease in the order of 1′ > 2′ > 3′, indicating that the average diameter increases in the order of 1′ < 2′ < 3′. We have noticed from experience that samples 1′-3′ contain four components that give highly reproducible retention times (shown by the downward arrows in Figure 1a), although the relative populations vary with each batch. This suggests that there are four distinct Au:SC18
Thiolation of Polymer-Stabilized Gold Clusters
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Figure 1. (a) Typical chromatograms of 1′-3′ at the second recycling stage. Chromatograms of fractions A-D at the seventh recycling stage (b) and at the final stage (c). Fractions a-d were collected for further characterization.
with a known core mass of 14 and 21 kDa were isolated by recycling SEC of sample 4 (see Figure 2).41 Geometric Structures. Figure 3 shows typical TEM images and size distributions of the clusters in fractions a-d. The core sizes are determined to be 1.2 ( 0.2, 1.4 ( 0.2, 1.6 ( 0.2, and 1.8 ( 0.2 nm, respectively. Table 1 lists the core sizes (dcore) calculated by assuming that the cores are spheres with the same density of bulk gold. The core sizes observed by TEM are consistent with the calculated core sizes. The hydrodynamic diameters (Dhydro) of the clusters in fractions a-d are estimated from the retention time (Figure 4) using eq 1 and are summarized in Table 1. The thickness of the monolayer (L) was calculated by the following equation
L ) (Dhydro - dcore)/2
Figure 2. Representative LDI mass spectra of fractions a-d. Red curves superimposed with the spectra for b and c represent mass spectra of Au:SC18 clusters with a core mass of 14 and 21 kDa, respectively, fractionated from sample 4 (see Discussion for details).
clusters with high stability in samples 1′-3′. In order to isolate these stable clusters, the main components of 1′-3′ were fractionated into A-D, which were individually re-injected into the column. At the seventh recycle (Figure 1b), fractions A-D nearly exhibit a single peak with a different hydrodynamic diameter. Finally, central portions of the peaks were collected as fractions a-d (Figure 1c). Characterization by LDI Mass Spectrometry. Figure 2 shows representative LDI mass spectra of fractions a-d. The spectra are composed of progressions of mass peaks assigned to AunSm+ which are typically observed in LDI mass spectra of the Au:SR clusters.22,42-46 The mass spectral profiles reflect the core size distribution in the sample since the profiles were not affected by the laser fluence in the range of 0.3-3.0 mJ/ pulse. The core masses of fractions a-d were determined from the peak positions in the mass spectra as being 8, 11, 21, and 26 kDa, respectively (Table 1). The peak positions are reproducible within a typical accuracy of 1 kDa. The Au:SC18 clusters
(2)
on the assumption that a uniform monolayer is formed on a spherical gold cluster. In this calculation, the dcore values estimated from the bulk density are used. The apparent monolayer thickness increases with the core size and reaches the length of C18SH in the all-trans configuration (ca. 2.5 nm). Electronic Structures. The optical absorption spectra of fractions a-d are shown in Figure 5. A discrete structure was clearly observed for the clusters in fraction a, whereas the structures become less apparent with an increase in core mass. The absorption onsets were determined by linear extrapolation, and the least-squares fitting provided the optical gaps listed in Table 1. Discussion Sequence of Core Sizes of Isolated Clusters. As summarized in Table 1, recycling SEC allows us to isolate Au:SC18 clusters with core masses of 8, 11, 21, and 26 kDa from the crude products of reaction 2 (samples 1′-3′), which are located in the transition region between metallic and nonmetallic states. Probably, the most notable finding was that the sequence of the stable core sizes differed from that (8, 14, 22, and 29 kDa) in reaction 1.32-35 The 11 kDa (fraction b) and 26 kDa (fraction d) clusters have not previously been isolated from the reaction products of reaction 1, whereas Murray et al. have “detected” 10 kDa clusters as a minor product in the ligand exchange reactions of Au55(PPh3)12Cl6.19 The mass spectral peak position of fraction b at 11 kDa is obviously different from that of the 14 kDa clusters (red curve in Figure 2b) that were isolated from sample 4 produced by reaction 1. In order to confirm that the
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TABLE 1: Characterization of Fractionated Au:SC18 Clusters fraction
core mass (kDa)
dcorea (nm)
dcoreb (nm)
tRc (min)
Dhydro (nm)
Le (nm) (approximate)
optical gapf (eV)
a b c d
8 11 21 26
1.2 ( 0.2 1.4 ( 0.2 1.6 ( 0.2 1.8 ( 0.2
1.1 1.2 1.5 1.6
37.8 36.7 35.5 35.0
4.8 5.4 6.1 6.4
1.9 2.1 2.3 2.4
0.9 0.8 0.3
