Ligand-Induced Stability of Gold Nanoclusters: Thiolate versus

Letter pubs.acs.org/JPCL

Ligand-Induced Stability of Gold Nanoclusters: Thiolate versus Selenolate Wataru Kurashige,† Masaki Yamaguchi,† Katsuyuki Nobusada,‡ and Yuichi Negishi*,†,§ †

Department of Applied Chemistry, Faculty of Science and §Research Institute for Science and Technology, Energy and Environment Photocatalyst Research Division, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan ‡ Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585, Japan S Supporting Information *

ABSTRACT: Thiolate-protected gold nanoclusters have attracted considerable attention as building blocks for new functional materials and have been extensively researched. Some studies have reported that changing the ligand of these gold nanoclusters from thiolate to selenolate increases cluster stability. To confirm this, in this study, we compare the stabilities of precisely synthesized [Au25(SC8H17)18]− and [Au25(SeC8H17)18]− against degradation in solution, thermal dissolution, and laser fragmentation. The results demonstrate that changing the ligand from thiolate to selenolate increases cluster stability in reactions involving dissociation of the gold− ligand bond but reduces cluster stability in reactions involving intramolecular dissociation of the ligand. These results reveal that using selenolate ligands makes it possible to produce gold clusters that are more stable against degradation in solution than thiolate-protected gold nanoclusters. SECTION: Glasses, Colloids, Polymers, and Soft Matter

S

number of gold atoms and ligands as the most-studied thiolateprotected gold cluster, [Au25(SR)18]− (refs 3−5), making it an ideal cluster for clarifying the correlation between ligand type and cluster stability. In this study, we compare [Au25(SC8H17)18]− and [Au25(SeC8H17)18]− in terms of stability against degradation in solution, thermal dissolution, and laser fragmentation. The results clarify the effect of changing the ligand on cluster stability. [Au25(SC8H17)18]− and [Au25(SeC8H17)18]− used in this study were atomically, precisely synthesized on an atomic level (Figures S1 and S2, Supporting Information). Regarding their structures, [Au25(SR)18]− consists of a Au13 core protected by six [−S(R)−Au−S(R)−Au−S(R)−] oligomers.30−35 Results of our previous studies imply that [Au25(SeC8H17)18]− has a similar framework structure as [Au25(SR)18]−.29 To confirm this, in this study, we first calculated the optimized structure and absorption spectrum of [Au25(SeCH3)18]−. All of the theoretical results in the present study were obtained by density functional theory calculations utilizing the TURBOMOLE package of ab initio quantum chemistry programs36 (see the Supporting Information). The results indicate that the optimized structure for [Au25(SeCH3)18]− is similar to the structure of [Au25(SR)18]− (Figures 1a and S3, Supporting Information). The optical absorption spectrum of this structure closely resembles that of [Au25(SeC8H17)18]− (Figure 1b).

table and functional nanomaterials are very promising as building blocks for nanotechnology. Thiolate (RS)protected gold nanoclusters (Aun(SR)m) have attracted considerable attention as such nanomaterials and have been extensively researched.1−6 Aun(SR)m nanoclusters have the following characteristics: they are very stable compared with other gold clusters; they can be precisely synthesized at the atomic level by various high-resolution separation methods;7−14 clusters with specific chemical compositions can be sizeselectively synthesized on a large scale by controlling the preparation conditions;15−18 and they exhibit unique physical and chemical properties that are not observed in bulk gold, including redox behavior,19,20 photoluminescence,9,21 and paramagnetism.22 These characteristics make Aun(SR)m attractive as a functional nanomaterial. In recent years, it has been reported that more stable gold clusters can be produced by changing the ligand of these gold clusters from thiolate to selenolate (SeR).23,24 Compared with sulfur (S), the electronegativity and atomic radius of selenium (Se) are closer to those of gold. Consequently, the Au−SeR bond is more covalent and has a higher bond energy than Au− SR.23,25−28 This is expected to make Aun(SeR)m more stable than Aun(SR)m. However, to clarify the correlation between ligand and cluster stability, it is essential to compare the stabilities of clusters with the same number of gold atoms and ligands. We have recently precisely synthesized an alkaneselenolate (SeCnH2n+1; n = 8,12)-protected Au25 cluster ([Au25(SeCnH2n+1)18]−).29 [Au25(SeCnH2n+1)18]− has the same © 2012 American Chemical Society

Received: August 16, 2012 Accepted: September 4, 2012 Published: September 4, 2012 2649

dx.doi.org/10.1021/jz301191t | J. Phys. Chem. Lett. 2012, 3, 2649−2652

The Journal of Physical Chemistry Letters

Letter

S6, Supporting Information) and in tetrahydrofuran at 60 °C (Figure S7, Supporting Information). Furthermore, a similar result was obtained when the alkyl chain length was changed from C 8 H 17 to C 12 H 25 (Figures S8−S11, Supporting Information). These results indicate that changing the ligand from thiolate to selenolate increases the cluster stability against degradation in solution.37 As mentioned above, Au−SeR bonds are more covalent and have a higher bond energy than Au− SR.23,25−28 Our previous study revealed that even in Au25 clusters, the covalency of the Au−ligand bond increases when the ligand is changed from thiolate to selenolate.29 Due to this change in the nature of the bond, the Au−ligand bond energy is considered to increase even in Au25 clusters. In a previous study, we found that clusters degrade in solution through detachment of thiolates and/or gold−thiolate oligomers.9 The higher bond energy of [Au25(SeC8H17)18]− is considered to suppress this detachment, making it more stable in solution than [Au25(SC8H17)18]−. On the basis of these results, we conclude that changing the ligand from thiolate to selenolate increases the gold−ligand bond energy in clusters and thus increases the cluster stability against reactions involving dissociation of these bonds. We next studied the stability of solid [Au25(SC8H17)18]− and [Au25(SeC8H17)18]− against thermal dissociation. Figure 3

Figure 1. (a) Optimized structure of [Au25(SeCH3)18]−. The R moieties are omitted for clarity. (b) Optical absorption spectra calculated for [Au25(SeCH3)18]− and [Au25(SeC8H17)18]− (ref 29).

Compared with the optical absorption spectrum of [Au25(SC8H17)18]−, that of [Au25(SeC8H17)18]− has a very similar absorption peak at 1.2−2.0 eV but with shifts to lower energies at 2.5−3.0 eV (Figure S4, Supporting Information). The absorption spectrum of the structure in Figure 1a also exhibits these characteristics (Figure S4, Supporting Information). These results suggest that synthesized [Au25(SeC8H17)18]− has the geometric structure shown in Figure 1a, strongly supporting the interpretation that the framework structure of the synthesized [Au25(SeC8H17)18]− is the same as that of [Au25(SC8H17)18]− and that they only have different ligands. To clarify how changing the ligand affects cluster stability, we first compared the stabilities of [Au25(SC8H17)18]− and [Au25(SeC8H17)18]− against degradation in solution. Figure 2a

Figure 3. TGA curves of [Au25(SC8H17)18]− and [Au25(SeC8H17)18]−.

shows thermogravimetric analysis (TGA) results for both clusters. The point where the TGA curve starts to fall indicates the temperature at which the ligands start to evaporate. This point was observed at 165 °C for [Au25(SC8H17)18]− and 136 °C for [Au25(SeC8H17)18]−, indicating that the ligands of [Au25(SeC8H17)18]− begin to dissociate at a lower temperature than those of [Au25(SC8H17)18]−. In this type of evaporation, the alkyl chains begin to evaporate first.38 The Se−C bond energy (590.4 kJ/mol) is lower than the S−C bond energy (713.3 kJ/mol). This lower bond energy is considered to cause the ligand to dissociate at a lower temperature for [Au25(SeC8H17)18]− than that for [Au25(SC8H17)18]−. On the basis of these results, we conclude that changing the ligand from thiolate to selenolate reduces the stability of the ligand against intramolecular dissociation and thus reduces the stability of clusters against reactions involving these dissociations. In contrast, in experiments using [Au25(SC12H25)18]− and [Au25(SeC12H25)18]− (Au25 clusters protected by ligands with longer alkyl chains), barely any difference was observed at the point where the TGA curves start to fall (Figure S12, Supporting Information). With [Au25(SC12H25)18]− (189 °C) and [Au25(SeC12H25)18]− (186 °C), this point was observed at higher temperatures than those for [Au25(SC8H17)18]− (165 °C) and [Au25(SeC8H17)18]− (136 °C) (Figure S12, Supporting Information). Lengthening the alkyl chain from C8H17 to

Figure 2. Time dependences of absorption spectra of (a) [Au25(SC8H17)18]− in toluene (1 × 10−5 M) at 60 °C and (b) [Au25(SeC8H17)18]− in toluene (1 × 10−5 M) at 60 °C.

and b shows the temporal variations of the optical absorption spectra of [Au25(SC8H17)18]− and [Au25(SeC8H17)18]− in toluene at 60 °C. The absorption spectrum of [Au25(SC8H17)18]− (Figure 2a) changed gradually over time, whereas that of [Au25(SeC8H17)18]− (Figure 2b) exhibited only a slight change, even after two days (Figure S5, Supporting Information). These results indicate that [Au25(SeC8H17)18]− is more stable than [Au25(SC8H17)18]− in toluene at 60 °C. The same phenomenon was observed in toluene at 80 °C (Figure 2650

dx.doi.org/10.1021/jz301191t | J. Phys. Chem. Lett. 2012, 3, 2649−2652

The Journal of Physical Chemistry Letters

Letter

[Au25(SC8H17)18]−, whereas total Irel is lower at 89% for [Au25(SeC8H17)18]− (Table 1). The same phenomenon was observed at different laser fluences (Figure S13, Supporting Information). Changing the ligand from thiolate to selenolate is expected to increase the bond energy between gold and the ligand. This increase in bond energy is considered to suppress the detachment of Au4(ligand)4 and hence reduce the amount of fragment ions generated from [Au25(SeC8H17)18]− relative to [Au25(SC8H17)18]−. In conclusion, this study has clarified the effect of changing the ligand of gold clusters from thiolate to selenolate on cluster stability using the atomically, precisely synthesized [Au25(SeC8H17)18]− and [Au25(SC8H17)18]−. The results are summarized as follows: (1) The stability increases in reactions involving dissociation of the gold−ligand bond. (2) The stability decreases in reactions involving intramolecular dissociation of the ligand. However, this reduction in stability (due to intramolecular dissociation in the ligand) can be suppressed using selenolate with a long alkyl chain These results indicate that Au25 clusters protected by selenolates with long alkyl chains have similar stabilities against thermal dissolution and greater stabilities against degradation in solution than Au25 clusters protected by thiolates with the same alkyl chain length. Highly stable metal clusters are promising nanomaterials. It is thus expected that such stable selenolateprotected gold clusters will be extensively investigated in the future, just as thiolate-protected gold clusters are currently being researched.

C12H25 increases the intermolecular interactions between ligands. This increase in interactions is considered to suppress evaporation of alkyl groups for both thiolate and selenolate, increasing the evaporation temperature of the alkyl groups for both clusters39 and giving rise to the small difference in these temperatures for [Au25(SC12H25)18]− and [Au25(SeC12H25)18]−. These results indicate that even if selenolate is used as a ligand, a reduced stability due to intramolecular dissociation can be suppressed using a selenolate with a long alkyl chain. Finally, we investigated the stabilities of [Au25(SC8H17)18]− and [Au25(SeC8H17)18]− against light (laser) fragmentation. Figure 4a and b shows matrix-assisted laser desorption/

■

S Supporting Information *

Figure 4. Negative-ion matrix-assisted laser desorption/ionization mass spectra of (a) [Au25(SC8H17)18]− and (b) [Au25(SeC8H17)18]− recorded at a high laser fluence. Both spectra were measured at the same laser fluence. I−IV are assigned to laser fragments (Table 1 and Figure S13, Supporting Information).

Details of experimental procedures, calculations, and characterization of the products. This material is available free of charge via the Internet at http://pubs.acs.org.

■

ionization mass spectra of [Au 25 (SC 8 H 17 ) 18 ] − and [Au25(SeC8H17)18]− measured with a high laser fluence. For both spectra, the main dissociation products (I and II) were assigned to [Au21(SC8H17)14]− and [Au21(SeC8H17)14]−, which were formed by detachment of Au 4 (ligand) 4 from [Au25(SC8H17)18]− and [Au25(SeC8H17)18]−, respectively.40 For [Au25(SeC8H17)18]−, dissociation of the Se−C bond in some ligands (II−IV) was also observed in addition to detachment of Au4(SeC8H17)4 (Figure 4b and Table 1). Similar dissociation was also observed for [Au25(SeC12H25)18]− (Figure S8b, Supporting Information). As mentioned above, Se−C has a lower bond energy than S−C. This lower bond energy is considered to result in the formation of such fragments for [Au25(SeR)18]−. Comparing the relative ionic strengths (Irel) of the fragment ions against parent ions, Irel is 150% for

a b

chemical composition

Irel

I II III IV

Au21(SC8H17)14 Au21(SeC8H17)14 Au21(SeC8H17)12Se2 Au21(SeC8H17)10Se4

1.50a 0.41b 0.31b 0.17b

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Mr. Ukyo Kamimura and Mr. Kenta Munakata for technical assistance and Mr. Yoshiki Niihori for valuable comments. This work was financially supported by a Grant-inAid for Scientific Research (No. 21685003 and 21350018).

■

REFERENCES

(1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Synthesis of Thiol-Derivatised Gold Nanoparticles in a Two-Phase Liquid−Liquid System. J. Chem. Soc., Chem. Commun. 1994, 801−802. (2) Häkkinen, H.; Whetten, R. L. Protected Metallic Clusters, Quantum Wells and Metal-Nanocrystal Molecules. J. Phys. Chem. C 2010, 114, 15877−15878. (3) Parker, J. F.; Fields-Zinna, C. A.; Murray, R. W. The Story of a Monodisperse Gold Nanoparticle: Au25L18. Acc. Chem. Res. 2010, 43, 1289−1296. (4) Maity, P.; Xie, S.; Yamauchi, M.; Tsukuda, T. Stabilized Gold Clusters: From Isolation Toward Controlled Synthesis. Nanoscale 2012, 4, 4027−4037. (5) Qian, H.; Zhu, M.; Wu, Z.; Jin, R. Quantum Sized Gold Nanoclusters with Atomic Precision. Acc. Chem. Res. 2012, DOI: 10.1021/ar200331z.

Table 1. Chemical Composition and Relative Ion Intensity of I−IV peak

ASSOCIATED CONTENT

Relative ion intensity against the ion peak of [Au25(SC8H17)18]−. Relative ion intensity against the ion peak of [Au25(SeC8H17)18]−. 2651

dx.doi.org/10.1021/jz301191t | J. Phys. Chem. Lett. 2012, 3, 2649−2652

The Journal of Physical Chemistry Letters

Letter

(6) Dolamic, I.; Knoppe, S.; Dass, A.; Bürgi, T. First Enantioseparation and Circular Dichroism Spectra of Au38 Clusters Protected by Achiral Ligands. Nature Commun. 2012, 3, 798−803. (7) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Nanocrystal Gold Molecules. Adv. Mater. 1996, 8, 428−433. (8) Wolfe, R. L.; Murray, R. W. Analytical Evidence for the Monolayer-Protected Cluster Au225[(S(CH2)5CH3)]75. Anal. Chem. 2006, 78, 1167−1173. (9) Negishi, Y.; Nobusada, K.; Tsukuda, T. Glutathione-Protected Gold Clusters Revisited: Bridging the Gap between Gold(I)−Thiolate Complexes and Thiolate-Protected Gold Nanocrystals. J. Am. Chem. Soc. 2005, 127, 5261−5270. (10) Qian, H.; Zhu, Y.; Jin, R. Isolation of Ubiquitous Au40(SR)24 Clusters From the 8 kDa Gold Clusters. J. Am. Chem. Soc. 2010, 132, 4583−4585. (11) Negishi, Y.; Kurashige, W.; Niihori, Y.; Iwasa, T.; Nobusada, K. Isolation, Structure, and Stability of a Dodecanethiolate-Protected Pd1Au24 Cluster. Phys. Chem. Chem. Phys. 2010, 12, 6219−6225. (12) Negishi, Y.; Iwai, T.; Ide, M. Continuous Modulation of Electronic Structure of Stable Thiolate-Protected Au25 Cluster by Ag Doping. Chem. Commun. 2010, 46, 4713−4715. (13) Negishi, Y.; Igarashi, K.; Munakata, K.; Ohgake, W.; Nobusada, K. Palladium Doping of Magic Gold Cluster Au38(SC2H4Ph)24: Formation of Pd2Au36(SC2H4Ph)24 with Higher Stability than Au38(SC2H4Ph)24. Chem. Commun. 2012, 48, 660−662. (14) Negishi, Y.; Sakamoto, C.; Ohyama, T.; Tsukuda, T. Synthesis and the Origin of the Stability of Thiolate-Protected Au130 and Au187 Clusters. J. Phys. Chem. Lett. 2012, 3, 1624−1628. (15) Shichibu, Y.; Negishi, Y.; Tsukuda, T.; Teranishi, T. Large-Scale Synthesis of Thiolated Au25 Clusters via Ligand Exchange Reactions of Phosphine-Stabilized Au11 Clusters. J. Am. Chem. Soc. 2005, 127, 13464−13465. (16) Zhu, M.; Lanni, E.; Garg, N.; Bier, M. E.; Jin, R. Kinetically Controlled, High-Yield Synthesis of Au25 Clusters. J. Am. Chem. Soc. 2008, 130, 1138−1139. (17) Qian, H.; Zhu, Y.; Jin, R. Size-Focusing Synthesis, Optical and Electrochemical Properties of Monodisperse Au38(SC2H4Ph)24 Nanoclusters. ACS Nano 2009, 3, 3795−3803. (18) Dharmaratne, A. C.; Krick, T.; Dass, A. Nanocluster Size Evolution Studied by Mass Spectrometry in Room Temperature Au25(SR)18 Synthesis. J. Am. Chem. Soc. 2009, 131, 13604−13605. (19) Lee, D.; Donkers, R. L.; Wang, G.; Harper, A. S.; Murray, R. W. Electrochemistry and Optical Absorbance and Luminescence of Molecule-like Au38 Nanoparticles. J. Am. Chem. Soc. 2004, 126, 6193−6199. (20) Negishi, Y.; Kamimura, U.; Ide, M.; Hirayama, M. A Photoresponsive Au25 Nanocluster Protected by Azobenzene Derivative Thiolates. Nanoscale 2012, 4, 4263−4268. (21) Wu, Z.; Jin, R. On the Ligand’s Role in the Fluorescence of Gold Nanoclusters. Nano Lett. 2010, 10, 2568−2573. (22) Zhu, M.; Aikens, C. M.; Hendrich, M. P.; Gupta, R.; Qian, H.; Schatz, G. C.; Jin, R. Reversible Switching of Magnetism in ThiolateProtected Au25 Superatoms. J. Am. Chem. Soc. 2009, 131, 2490−2492. (23) Yee, C. K.; Ulman, A.; Ruiz, J. D.; Parikh, A.; White, H.; Rafailovich, M. Alkyl Selenide- and Alkyl Thiolate-Functionalized Gold Nanoparticles: Chain Packing and Bond Nature. Langmuir 2003, 19, 9450−9458. (24) Meng, X.; Xu, Q.; Wang, S.; Zhu, M. Ligand-Exchange Synthesis of Selenophenolate-Capped Au25 Nanoclusters. Nanoscale 2012, 4, 4161−4165. (25) Huang, F. K.; Horton, R. C., Jr.; Myles, D. C.; Garrell, R. L. Selenolates as Alternatives to Thiolates for Self-Assembled Monolayers: A SERS Study. Langmuir 1998, 14, 4802−4808. (26) Sato, Y.; Mizutani, F. Formation and Characterization of Aromatic Selenol and Thiol Monolayers on Gold: In-Situ IR Studies and Electrochemical Measurements. Phys. Chem. Chem. Phys. 2004, 6, 1328−1331.

(27) Weidner, T.; Shaporenko, A.; Müller, J.; Höltig, M.; Terfort, A.; Zharnikov, M. Self-Assembled Monolayers of Aromatic Tellurides on (111)-Oriented Gold and Silver Substrates. J. Phys. Chem. C 2007, 111, 11627−11635. (28) de la Llave, E.; Scherlis, D. A. Selenium-Based Self-Assembled Monolayers: The Nature of Adsorbate−Surface Interactions. Langmuir 2010, 26, 173−178. (29) Negishi, Y.; Kurashige, W.; Kamimura, U. Isolation and Structural Characterization of an Octaneselenolate-Protected Au25 Cluster. Langmuir 2011, 27, 12289−12292. (30) Heaven, M. W.; Dass, A.; White, P. S.; Holt, K. M.; Murray, R. W. Crystal Structure of the Gold Nanoparticle [N(C8H17)4][Au25(SCH2CH2Ph)18]. J. Am. Chem. Soc. 2008, 130, 3754−3755. (31) Zhu, M.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R. Correlating the Crystal Structure of A Thiol-Protected Au25 Cluster and Optical Properties. J. Am. Chem. Soc. 2008, 130, 5883−5885. (32) Akola, J.; Walter, M.; Whetten, R. L.; Häkkinen, H.; Grönbeck, H. On the Structure of Thiolate-Protected Au25. J. Am. Chem. Soc. 2008, 130, 3756−3757. (33) Wu, Z.; Gayathri, C.; Gil, R. R.; Jin, R. Probing the Structure and Charge State of Glutathion-Capped Au25(SG)18 Clusters by NMR and Mass Spectrometry. J. Am. Chem. Soc. 2009, 131, 6535−6542. (34) Jiang, D.-e. Staple Fitness: A Concept to Understand and Predict the Structures of Thiolated Gold Nanoclusters. Chem.Eur. J. 2011, 17, 12289−12293. (35) In [Au25(SR)18]−, the only cluster whose structure has been elucidated by single-crystal X-ray diffraction is [Au25(SC2H4Ph)18]−. However, numerous experiments (e.g., ref 33) and theoretical calculations strongly indicate that [Au25(SR)18]−, which is protected by different thiolates, has a similar framework structure. (36) TURBOMOLE, version 6.0; TURBOMOLE GmbH: Karlsruhe, Germany, 2007. (37) These results are consistent with the findings of Meng et al.24 They reported that Au25(SePh)18 is more stable than Au25(SC2H4Ph)18 against degradation in solution. However, their experiment compared the stabilities of Au25(SePh)18 and Au25(SC2H4Ph)18; thus, as they point out, it is difficult to rule out the possibility that the difference in the stability is due to the difference in the interaction energy between ligands and/or between the ligand and the metal core. In contrast, in this study, the ligands of both clusters have the same alkyl groups (C8H17 and C12H25), so that any observed difference in stability is not due to such effects. (38) Xie, S.; Tsunoyama, H.; Kurashige, W.; Negishi, Y.; Tsukuda, T. Enhancement in Aerobic Alcohol Oxidation Catalysis of Au25 Clusters by Single Pd Atom Doping. ACS Catal. 2012, 2, 1519−1523. (39) Qian, H.; Jin., R. Ambient Synthesis of Au144(SR)60 Nanoclusters in Methanol. Chem. Mater. 2011, 23, 2209−2217. (40) Dass, A.; Stevenson, A.; Dubay, G. R.; Tracy, J. B.; Murray, R. W. Nanoparticle MALDI-TOF Mass Spectrometry without Fragmentation: Au25(SCH2CH2Ph)18 and Mixed Monolayer Au25(SCH2CH2Ph)18−x(L)x. J. Am. Chem. Soc. 2008, 130, 5940−5946.

2652

dx.doi.org/10.1021/jz301191t | J. Phys. Chem. Lett. 2012, 3, 2649−2652