Letter pubs.acs.org/JPCL
Au25 Clusters Containing Unoxidized Tellurolates in the Ligand Shell Wataru Kurashige,† Seiji Yamazoe,‡,§ Masaki Yamaguchi,† Keisuke Nishido,† Katsuyuki Nobusada,§,∥ Tatsuya Tsukuda,‡,§ and Yuichi Negishi*,†,⊥,# †
Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan ‡ Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan § Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan ∥ Department of Theoretical and Computational Molecular Science and ⊥Department of Materials Molecular Science, Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585, Japan # Photocatalysis International Research Center, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan S Supporting Information *
ABSTRACT: We report herein the synthesis and characterization of Au25 clusters containing tellurolates (TePh) in the ligand shell ([Au25(TePh)n(SC8H17)18−n]−; n = 1−18). [Au25(TePh)n(SC8H17)18−n]− clusters were synthesized by reacting [Au25(SC8H17)18]− with diphenyl ditelluride ((PhTe)2) in solution. Characterization of the products by mass spectrometry and X-ray absorption fine structure analysis revealed that the tellurolates in [Au25(TePh)n(SC8H17)18−n]−, unlike those in tellurolate-protected gold nanoparticles, were not oxidized. Various experiments on the products and theoretical calculations on related clusters revealed that protection by the tellurolates distorts (expands) the central Au13 core and decreases the HOMO−LUMO gap of the Au25 clusters.
SECTION: Glasses, Colloids, Polymers, and Soft Matter
T
hiolate-protected Au25 clusters (Au25(SR)18)1−18 are among the most widely studied metal nanoclusters. Au25(SR)18 exhibits high stability against degradation in solution and etching reaction by thiols. 19 Moreover, Au25(SR)18 can be selectively synthesized with atomic-level precision,8 and it exhibits size-specific properties, such as photoluminescence, redox behavior, and catalytic activity, that are not observed in bulk gold.1,3,4,20 Furthermore, structures and physical/chemical properties for these clusters can be correlated because the electronic and geometric structures have been determined. 1,3−6,21,22 These characteristics make Au25(SR)18 attractive as a new nanomaterial. Recent studies on Au25 clusters revealed that the use of selenolates as ligands (Au25(SeR)18) results in the creation of Au25 clusters that are more stable than Au25(SR)18 against degradation in solution23,24 and thereby enables us to synthesize heteroatomdoped clusters, which are difficult to synthesize stably with the use of thiolates.25 As demonstrated by these studies, Au25 clusters with new properties differing from those of Au25(SR)18 can be created when other chalcogenides are used as ligands. We report herein the synthesis and characterization of Au25 clusters containing tellurolates (TePh) in the ligand shell, Au25(TePh)n(SC8H17)18−n. The element Te belongs to the chalcogen, which also includes S and Se (Group 16 elements); however, it is heavier and has higher metallicity than the other © 2014 American Chemical Society
two. It has been noted that gold clusters protected by tellurolate (Aun(TeR)m), where Te is used as an anchoring element, exhibit higher conductivity between the gold core and the ligand than Au n(SR)m clusters.26−29 Because high conductivity is important in the creation of molecular electronics based on these gold clusters, several studies on Aun(TeR)m nanoparticles have been reported in recent years,26−30 especially by Tong and coworkers.26−29 However, it is difficult to completely prevent the oxidation of tellurolate in those nanoparticles; therefore, most reported gold nanoparticles are covered by OTeR (Aun(OTeR)m).26 It is essential to study gold clusters protected with unoxidized tellurolate to gain a clear understanding of how tellurolate protection affects the structure and physical properties of gold nanoparticles/ clusters. In this study, we succeeded in synthesizing [Au25(TePh)n(SC8H17)18−n]− (n = 1−18), which has unoxidized phenyl tellurolates (TePh) in the ligand shell. Studies on the products and related clusters revealed the effect of the TePh protection on the geometric and electronic structures of Au25 clusters. Received: May 7, 2014 Accepted: May 27, 2014 Published: May 27, 2014 2072
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[Au25(TePh)n(SC8H17)18−n]− (n = 1−18) clusters were synthesized by reacting [Au25(SC8H17)18]− with diphenyl ditelluride ((PhTe)2) in solution under air. In the initial step, 30 mg of [Au25(SC8H17)18]− was dissolved in 20 mL of dichloromethane; then, (PhTe)2 was added to the solution at the molar ratios of [(PhTe)2]/[Au25(SC8H17)18] = 1.5, 4.5, 6.5, or 7.0. After stirring for 24 h at room temperature, the mixture was evaporated to dryness and then washed five times with methanol to remove the byproducts and unreacted (PhTe)2. The obtained clusters were kept under an Ar atmosphere. Figure 1 shows the electrospray ionization (ESI) mass spectra of the products synthesized with the molar ratios of
such conditions. It has been reported that ditellurides have strong reducing abilities.29 Thus, it is considered that in such conditions, extra (PhTe)2 reduces the clusters, which causes dissociation of the clusters and thereby decreases the cluster yield. In all of the ESI mass spectra, virtually no peaks were observed that could be attributed to clusters containing oxidized TePh such as Au25(OTePh)n(SC8H17)18−n (Figures 1 and S5, Supporting Information). Although we also collected matrix-assisted laser desorption ionization (MALDI) mass spectra, the peaks attributed to oxidized clusters were not observed (Figure S5, Supporting Information). X-ray absorption fine structure (XAFS) measurements of the products also confirmed that TePh in the product was unoxidized. Figure 2a
Figure 1. Negative ion ESI mass spectra of the products formed by the reaction between [Au25(SC8H17)18]− and (PhTe)2. The values in parentheses, for example, (1.5), (4.5), and so forth, indicate the molar ratio of [(PhTe)2]/[Au25(SC8H17)18] used in each reaction; for example, (1.5) means [(PhTe)2]/[Au25(SC8H17)18] = 1.5. In each spectrum, the small peaks between the [Au25(TePh)n(SC8H17)18−n]− peaks are attributed to the dianions of cluster dimers, which are presumed to be formed in the ionization process (Figures S2 and S3, Supporting Information). Insets compare experimental data with the calculated isotope pattern for the main peak in each mass spectrum.
Figure 2. (a) Experimental Te K-edge FT-EXAFS spectra for [Au25(TePh)∼3(SC8H17)∼15]− and [Au25(TePh)∼7(SC8H17)∼11]− with that of TeO2 for comparison purposes. (b) Experimental Au L3-edge FT-EXAFS spectra for [Au 25 (TePh) ∼ 3 (SC 8 H 1 7 ) ∼ 15 ] − and [Au25(TePh)∼7(SC8H17)∼11]− with those of [Au25(SC8H17)18]− and Au foil for comparison purposes. These experiments were conducted at 300 K (Te K-edge FT-EXAFS spectra at 8 K are shown in Figure S7, Supporting Information).
shows the Te K-edge Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectra of [Au 2 5 (TePh) ∼ 3 (SC 8 H 1 7 ) ∼ 1 5 ] − and [Au 2 5 (TePh) ∼ 7 (SC8H17)∼11]− (Figures S6a and S7, Supporting Information). In both FT-EXAFS spectra, almost no peaks attributed to oxides were observed. We found that TePh in [Au25(TePh)n(SC8H17)18−n]− was not oxidized even when the clusters were left in dichloromethane solution under air for 32 h (Figures S8 and S9, Supporting Information). The behavior of this tellurolate differs greatly from that of the tellurolate covering 2.7 nm gold nanoparticles, in which oxidation of tellurolates has been reported.26 Although the reason for this difference is not clear at this stage, these results indicate that oxidation of tellurolates is suppressed in [Au25(TePh)n(SC 8 H 17 ) 18−n ] − , and thereby, studying [Au 25 (TePh) n -
[(PhTe)2]/[Au25(SC8H17)18] = 1.5, 4.5, 6.5, and 7.0 (Figure S1, Supporting Information). All of the main mass spectral peaks are attributed to [Au25(TePh)n(SC8H17)18−n]− (n = 0− 18) (Figure S2, Supporting Information), indicating that these species were synthesized by this method (Figures S3 and S4, Supporting Information). The number of TePh ligands contained in [Au25(TePh)n(SC8H17)18−n]− increased with the molar ratio of [(PhTe)2]/[Au25(SC8H17)18]. [Au25(TePh)18]−, in which all SC8H17 ligands had been replaced by TePh, was also synthesized under the condition of [(PhTe) 2 ]/ [Au25(SC8H17)18] = 7.0 (Figure 1). To selectively synthesize only [Au25(TePh)18]−, we attempted to further increase the molar ratio of [(PhTe)2]/[Au25(SC8H17)18]. However, the [Au25(TePh)n(SC8H17)18−n]− yield decreased strikingly under 2073
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(SC8H17)18−n]− will reveal the effects of tellurolate protection on the structure and physical properties of Au25 clusters. Then, we studied the effects of tellurolate protection on the geometric structure of the Au25 cluster by conducting XAFS measurements on the obtained clusters (Figure S10, Supporting Information). Figure 2b shows the Au L3-edge FT-EXAFS spectra of [Au25(TePh)∼3(SC8H17)∼15]− and [Au25(TePh)∼7(SC8H17)∼11]− with those of [Au25(SC8H17)18]− and Au foil for comparison purposes (Figure S6b, Supporting Information). The peaks at ∼1.9 and 2.3−3.1 Å in the spectrum of [Au25(SC8H17)18]− are assigned to Au−S and Au−Au bonds, respectively.14 The intensity of the peak at ∼1.9 Å decreased with ligand substitution (SC8H17 → TePh) (Figure S11, Supporting Information). This decrease in intensity is caused by a decrease in the number of Au−S bonds in the cluster. Instead, the intensities of the peaks at 2.3−3.1 Å increased with the number of substituted ligands (Figure S11, Supporting Information). The atomic radii of Te (1.44 Å) and Au (1.40 Å) are nearly equal; thus, the peak originating from Au−Te bonds is observed at about the same position as that from Au−Au bonds. The increase in peak intensity in this region is caused by the increasing number of Au−Te bonds in the cluster. We performed curve-fitting analysis for [Au 25 (TePh) ∼3 (SC8H17)∼15]− and [Au25(TePh)∼7(SC8H17)∼11]− (Tables S1−S3, Supporting Information). From these results, we could estimate the Au−Au bond lengths to be ∼2.78 Å in both cases (Table S1, Supporting Information). This value is greater than the ∼2.74 Å estimated for [Au25(SC8H17)18]− (Table S1, Supporting Information), implying that inclusion of TePh in the ligand shell lengthens the Au−Au bond length in the Au25 cluster. To gain a deeper understanding for this lengthening of the Au−Au bond distance, we calculated the optimized structure for [Au25(TeCH3)18]−, in which all ligands had been replaced by TeCH3, using density functional theory (DFT) (Figure 3a and b). Figure 3c and d shows the Au−Au bond distances in the optimized structures for [Au 25 (TeCH 3 ) 18 ] − and [Au25(SCH3)18]− (ref 23), respectively. The distances between Au on the gold core surface and Au in the oligomer (Ausurf.− Auolig.) in [Au25(TeCH3)18]− become longer than that in [Au25(SCH3)18]− (Figure 3c). This lengthening is explained by the difference in atomic radii between Te (1.44 Å) and S (1.02 Å). Interestingly, the Aucent.−Ausurf. and Ausurf.−Ausurf. bond distances also slightly increase with the change in ligands (Figure 3d). These results indicate that lengthening of the oligomer structure also affects the structure of the central Au13 core, and therefore, protection by the tellurolate ligand distorts (expands) the Au13 core of the Au25 clusters. We found that the tellurolate protection also affects the electronic structure of the Au25 cluster. Figure 4 shows the optical absorption spectra of [Au 2 5 (SC 8 H 1 7 ) 1 8 ] − , [Au 2 5 (TePh) ∼ 3 (SC 8 H 1 7 ) ∼ 1 5 ] − , and [Au 2 5 (TePh) ∼ 7 (SC8H17)∼11]− (Figure S12, Supporting Information). In the optical absorption spectrum of [Au25(SC8H17)18]−, the peak structures in the region over 2.0 eV are assigned to the absorptions related to the ligand molecules.5,22,31,32 The interesting point shown in Figure 4 is that the peak in the vicinity of ∼1.8 eV shifts to the low-energy side accompanying the ligand change (SC8H17 → TePh). This peak is attributed to the HOMO−LUMO gap of the central Au13 core.5,22,31,32 The observed phenomenon indicates that the HOMO−LUMO gap of the cluster decreases with this ligand change (Figure S13, Supporting Information). A similar phenomenon was also
Figure 3. (a) Optimized structure for [Au25(TeCH3)18]−, (b) the anatomy of [Au25(TeCH3)18]− highlighting three kinds of bonds, and bond distances together with their standard deviations for (c) Ausurf.− Auolig. and (d) Ausurf.−Ausurf. and Aucent.−Ausurf. in [Au25(TeCH3)18]−. In (c) and (d), bond distances of Ausurf.−Auolig., Ausurf.−Ausurf., and Aucent.−Ausurf. in [Au25(SCH3)18]− (ref 23) are also shown for comparison purposes.
Figure 4. Optical absorption spectra of [Au25(TePh)∼3(SC8H17)∼15]− and [Au25(TePh)∼7(SC8H17)∼11]− with that of [Au25(SC8H17)18]− for comparison purposes. Dotted lines indicate the main peak positions in each spectrum. In the optical absorption spectrum of [Au25(SC8H17)18]−, the peak structures in the region over 2.0 eV are assigned to the absorptions related to the ligand molecules, whereas those at ∼1.8 eV are assigned to the HOMO−LUMO gap of the central Au13 core.5,22,31,32
observed in electrochemical experiments (Figure S14, Supporting Information). These results indicate that the HOMO− LUMO gap of the Au25 cluster can be changed by the distortion of the Au13 core (Figure 3d) or the charge transfer at the Au13 surface (Figure S10, Supporting Information), which can be induced by the introduction of tellurolate into the ligand shell. 2074
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In conclusion, we have successfully synthesized [Au25(TePh)n(SC8H17)18−n]− (n = 1−18) with unoxidized tellurolate in the ligand shell. Studies on the synthesized [Au25(TePh)n(SC8H17)18−n]− (n = 1−18) revealed the effect of tellurolate protection on the geometric and electronic structures of the Au25 cluster. As mentioned in the introduction, the most interesting property of the Aun(TeR)m clusters is the conductivity between the gold core and the ligand.26−29 However, to the best of our knowledge, no experimental results have yet been reported to corroborate the evidence of increased conductivity in Aun(TeR)m clusters. We expect that studies using [Au25(TePh)n(SC8H17)18−n]− enable us to elucidate such an effect experimentally.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Details of experimental procedures and supporting figures, including the chemicals used, the synthesis procedure, characterization, stability against decomposition, calculations, curve fitting analysis, ESI mass spectra of products, assignments of observed peaks, MALDI mass spectra, TEM images of the products, comparison of the experimental ESI and MALDI mass spectra of the products, EXAFS oscillations, XANES spectra, comparison of FT-EXAFS spectra, optical absorption, photoluminescence, and photoexcitation spectra, differential pulse voltammetry curves, and optimized coordinates. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Yoshiki Niihori (TUS) for valuable comments. This work was financially supported by Grants-in-Aid for Scientific Research (Nos. 25288009, 25288012, 25102539, and 26620016), the TEPCO Memorial Foundation, and the Tokyo Ohka Foundation. W.K. expresses his gratitude to the Sasagawa Foundation and the Kato Foundation for partial financial support. A part of this work was performed under a management of “Elements Strategy Initiative for Catalysts & Batteries (ESICB)”.
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REFERENCES
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