Article pubs.acs.org/Langmuir
Synthesis and Electrochemical and Spectroscopic Characterization of Biicosahedral Au25 Clusters SunYoung Park and Dongil Lee* Department of Chemistry, Yonsei University, Seoul 120-749, Korea S Supporting Information *
ABSTRACT: The synthesis and electrochemical and spectroscopic characterization of biicosahedral Au25 clusters with a composition of [Au25(PPh3)10(thiolate)5Cl2]2+ are described. The biicosahedral Au25 clusters protected with various types of thiol ligands including alkanethiols, 2-phenylethanethiol, 11-mercaptoundecanoic acid, and 11-mercapto-1-undecanol were synthesized in high yields using a onestep, one-phase procedure in which Au(PPh3)Cl is reduced with tertbutylamine−borane in the presence of the thiol ligand in a 3:1 v/v chloroform/ethanol solution. All biicosahedral Au25 clusters prepared exhibit characteristic optical absorption and photoluminescence properties. The emission energy is found to be substantially smaller than the optical absorption energy gap of 1.82 eV, indicating a subgap energy luminescence. The electrochemical HOMO−LUMO gap (∼1.54 eV) of the clusters is also substantially smaller than the optical absorption energy gap but rather similar to the emission energy. These electrochemical and optical properties of the biicosahedral Au25 clusters are distinctly different from those of the Au25(thiolate)18 clusters.
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ties.27 The bi-Au25 cluster is unique in that the cluster is solely constructed from the Au13 unit, and thus one may expect that the resulting electronic structure and properties are predominantly determined by the Au13 core and their coupling. This would set the bi-Au25 clusters apart from the aforementioned Au25 clusters where the electronic structure is controlled not only by the Au13 core but also by the exterior semirings.21 It will therefore be of great interest to investigate the electrochemical and optical properties of bi-Au25 clusters and compare them with those of the Au25 clusters. Herein, we report a facile one-step synthesis procedure for the preparation of bi-Au25 clusters. This synthesis appears to be quite general because bi-Au25 clusters can be prepared from various types of thiol ligands in good yield. All bi-Au25 clusters produced exhibit characteristic optical absorption and photoluminescence properties, which are distinctly different from those of Au25 clusters. Furthermore, voltammetry of the bi-Au25 clusters has revealed that their electrochemical HOMO− LUMO gaps are ∼1.54 V irrespective of the thiol ligand used, which is also clearly different from that of the Au25 clusters (∼1.65 V).
INTRODUCTION Thiolate-protected gold clusters containing a few to hundreds of core atoms have received much attention in recent years because of their importance in both fundamental science and technological applications.1−7 These clusters display unique properties, such as quantized single-electron charging and molecule-like HOMO−LUMO energy gaps, that differ substantially from those of the corresponding atoms and bulk materials.8−13 Among the clusters, thiolate-protected Au25 clusters (Au25) with a composition of Au25(SR)18, where SR is the thiolate, are perhaps the most extensively studied system.14−19 The crystallographic study of the cluster structure revealed that the cluster is composed of a Au13 core and six S− Au−S−Au−S semirings.20,21 Furthermore, electrochemical and optical studies revealed the presence of the molecule-like HOMO−LUMO gap and discrete electronic states.9,21,22 More recently, there has been significant progress in the synthesis and structural characterization of another kind of Au25 cluster.23−26 In a single-crystal X-ray structural analysis,23 Tsukuda and co-workers revealed that this Au25 core is constructed by connecting two Au13 icosahedrons via vertex sharing to form a biicosahedral Au25 clusters (bi-Au25) and the bi-Au25 core is protected with both thiolate and phosphine ligands. These clusters were typically prepared by a two-step process in which Au11 clusters were first prepared and subsequently reacted with excess thiol ligands to form bi-Au25 clusters. Jin et al. also reported the preparation of bi-Au25 clusters via one-phase thiol etching of Au nanoparticles stabilized with the PPh3 ligand.25,26 However, much less is known about their electrochemical and spectroscopic proper© 2012 American Chemical Society
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EXPERIMENTAL SECTION
Chemicals. Triphenylphosphine gold(I) chloride (Au(PPh3)Cl, 99.9%), 1-butanethiol (C4, 99%), 1-hexanethiol (C6, 95%), 1octanethiol (C8, 98.5%), 1-decanethiol (C10, 96%), 1-dodecanethiol Received: February 26, 2012 Revised: April 8, 2012 Published: April 10, 2012 7049
dx.doi.org/10.1021/la300817j | Langmuir 2012, 28, 7049−7054
Langmuir
Article
Figure 1. (A) Positive-ion ESI mass spectrum of [Au25(PPh3)10(SC6H13)5Cl2]2+. The inset shows a comparison between the experimental data and the calculated isotope pattern. (B) Structure of a bi-Au25 cluster (redrawn from its X-ray crystal structure23). For clarity, only S and P are shown: Au (yellow), S (red), P (blue), and Cl (green). (C12, 98%), 2-phenylethanethiol (PhC2, 98%), 11-mercaptoundecanoic acid (MUA, 95%), 11-mercapto-1-undecanol (MU, 97%), tertbutylamine−borane (97%), lithium triethylborohydride (LiEt3BH, 1.0 M in tetrahydrofuran), 3,3′-diethylthiatricarbocyanine iodide (DTTC, 99%), and tetrabutylammonium hexafluorophosphate (Bu4NPF6, 98%) were purchased from Sigma-Aldrich and used as received. Ethanol (J. T. Baker, 99.9%), methanol (Carlo Erba, 99.9%), chloroform (Samchun, 99.5%), ethyl acetate (Burdick and Jackson, 99.9%), ethyl ether (Duksan, 99%), hexane (Duksan, 95%), benzene (Junsei, 99.5%), tetrahydrofuran (THF, Duksan, 99.5%), acetone (Samchun, 99.9%), and 1,4-benzoquinone (Tokyo, 98%) were used as received. Syntheses of Biicosahedral Au25 Clusters. The clusters were synthesized using a one-step, one-phase method. In a typical synthesis, 0.124 g (0.25 mmol) of AuPPh3Cl and 74 μL (0.50 mmol) of hexanethiol were mixed in 20 mL of 3:1 v/v chloroform/ethanol at 25 °C. To this clear solution was added 0.217 g (2.5 mmol) of a tertbutylamine−borane complex. The solution became dark brown immediately, indicating the formation of clusters. The reaction mixture was stirred for 3 h at 25 °C. After the completion of the reaction, the solution was rotary evaporated to near dryness. Four milliliters of methanol was added to the product to dissolve the bi-Au25 clusters. The methanol solution was rotary evaporated, and the dried product was thoroughly washed with copious amounts of ethyl acetate, hexane, and then ethyl ether, providing ∼20 mg of C6 bi-Au25 clusters whose composition was determined to be [Au25(PPh3)10(SC6H13)5Cl2]2+ (Cl− is probably the counterion of the cationic cluster) by positiveion electrospray ionization (ESI) mass spectrometry. bi-Au25 clusters protected with other alkanethiols (C4−C12) and PhC2 were prepared similarly. For bi-Au25 clusters protected with MUA and MU, the clusters were isolated with THF instead of methanol. The THF solution was rotary evaporated, and the dried product was thoroughly washed with copious amounts of benzene. biAu25 clusters were then isolated by dissolving them in CH2Cl2. Au25 clusters with a composition of [Au25(SC6H13)18]− were prepared following a procedure described elsewhere.7 Characterization. ESI mass spectrometry of the synthesized biAu25 clusters was carried out on a mass spectrometer (Synapt HDMS System, Waters). Transmission electron microscopy (TEM) images of clusters were collected on a Jeol JEM 2011 microscope. TEM samples were prepared by drop casting a CH2Cl2 solution of clusters (1 mg/ mL) on a 400 mesh Formvar/carbon-coated copper grid (01814-F, Ted Pella) and drying for 2 h at room temperature before imaging. UV−visible absorbance spectra of clusters dissolved in CH2Cl2 were acquired using a Shimadzu UV−vis (UV-3600) spectrometer. Photoluminescence spectra were collected on dilute methanol cluster solutions with an absorbance of 0.025 ± 0.002 using a Sinco spectrofluorometer (FS-2). Emission spectra were taken with
excitation at 680 nm, and excitation spectra were taken with emission at 827 nm. Quantum yields were measured with respect to DTTC. Electrochemistry. Electrochemical measurements of clusters were performed with an electrochemical workstation (model 660B, CH Instruments) using a Pt working electrode (diameter 0.4 mm), a Pt wire counter electrode, and a Ag wire quasi-reference electrode in 0.1 M Bu4NPF6/CH2Cl2. Cluster solutions were degassed and blanketed with a high-purity Ar atmosphere during measurement. 1,4Benzoquinone (BQ−/0) was used as an internal reference for the Ag quasi-reference electrode. The BQ−/0 couple was found to be −0.97 V versus the ferrocene couple in CH2Cl2. All potentials in this article are reported with respect to BQ−/0. Voltammograms of the cluster solutions were acquired at −78 °C using an acetone/dry ice bath.
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RESULTS AND DISCUSSION Synthesis of [Au25(PPh3)10(SC6H13)5Cl2]2+. It was reported28 that monodisperse nanoparticles could be produced by reducing gold salt precursor AuPPh3Cl with a weak reducing agent such as tert-butylamine−borane in the presence of thiol ligands. In previous work,29 we further revealed that the size of the gold nanoparticles in this synthesis could be systematically controlled by the solvent composition of the reaction media. The solvent-controlled nucleation and growth were found to be the key step in the control of the final size; that is, the particle growth was suppressed and smaller particles were obtained in polar media with increasing CHCl3 content. The gold particles produced in this synthesis were found to be stabilized by both thiol and phosphine ligands, and the thiol/phosphine was found to increase with the particle size. Knowing that smaller particles are preferentially formed in polar media with higher fractions of CHCl3, the one-step, onephase synthesis was conducted in a 3:1 v/v chloroform/ethanol medium to produce ultrasmall clusters as described in the Experimental Section. This synthesis generally produced small (
