Effect of Copper Doping on Electronic Structure, Geometric Structure

Letter pubs.acs.org/JPCL

Effect of Copper Doping on Electronic Structure, Geometric Structure, and Stability of Thiolate-Protected Au25 Nanoclusters Yuichi Negishi,*,†,‡ Kenta Munakata,† Wataru Ohgake,† and Katsuyuki Nobusada*,§ †

Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan ‡ 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: Several recent studies have attempted to impart [Au25(SR)18]− with new properties by doping with foreign atoms. In this study, we studied the effect of copper doping on the electronic structure, geometric structure, and stability of [Au25(SR)18]− with the aim of investigating the effect of foreign atom doping of [Au25(SR)18]−. CunAu25−n(SC2H4Ph)18 was synthesized by reducing complexes formed by the reaction between metal salts (copper and gold salts) and PhC2H4SH with NaBH4. Mass analysis revealed that the products contained CunAu25−n(SC2H4Ph)18 (n = 1−5) in high purity. Experimental and theoretical analysis of the synthesized clusters revealed that copper doping alters the optical properties and redox potentials of the cluster, greatly distorts its geometric structure, and reduces the cluster stability in solution. These findings are expected to be useful for developing design guidelines for functionalizing [Au25(SR)18]− through doping with foreign atoms. SECTION: Glasses, Colloids, Polymers, and Soft Matter atoms with the aim of producing [Au25(SR)18]− with new properties. Such studies have revealed that doping Au25(SR)18 with a single palladium atom increases the cluster stability,24,27,30 whereas doping with silver continuously changes the electronic structure and luminescence properties of the cluster.25 Moreover, doping with chromium, manganese, or iron has been predicted to make the cluster paramagnetic.28,31,32 Foreign atom doping has been extended to larger stable clusters. The results reveal that foreign atom doping alters the stability and the electronic structure of Au38(SR)2433,34 and Au144(SR)60.35,36 Because foreign atom doping modifies the electronic structure and physical properties of [Au25(SR)18]−, it is expected to be widely used to functionalize [Au25(SR)18]− in the future. However, to the best of our knowledge, successful synthesis of foreign-atomdoped Au25(SR)18 has been limited to [PdAu24(SR)18]0 and [AgnAu25−n(SR)18]−. Consequently, little is known about how doping affects the fundamental properties of [Au25(SR)18]−. It is important to gain more knowledge about foreign atom doping in order to develop guidelines for functionalizing [Au25(SR)18]−.

R

ecently, functional nanomaterials have been fabricated and investigated in the fields of nanoscience and nanotechnology. Thiolate-protected gold clusters (Aun(SR)m) have attracted considerable attention as nanomaterials. Among them, [Au25(SR)18]− is the most frequently studied cluster, since it is more stable against degradation in solution1 and thiol etching2 than thiolate-protected gold clusters of other sizes. Additionally, this cluster can be synthesized with an atomic resolution by controlling the preparation conditions.3−5 Moreover, this cluster exhibits unique physical and chemical properties not observed in bulk gold, including redox properties,6−10 photoluminescence,1,6,11−15 and catalytic activity.16−18 These characteristics make [Au25(SR)18]− very attractive as a functional nanomaterial. The high stability of [Au25(SR)18]− is due to both geometric and electronic factors. X-ray crystallography19,20 and theoretical calculations21 have shown that [Au 25 (SR) 18 ] − has an icosahedral Au13 core that is surrounded by six [−SR−Au− SR−Au−SR−] oligomers. [Au25(SR)18]− is considered to be geometrically stabilized because it has a symmetric metal core, and such a core is completely covered by [−SR−Au−SR−Au− SR−] oligomers. Additionally [Au25(SR)18]− is calculated to have eight valence electrons.22 Since this value coincides with the number of electrons in the electron shell, [Au25(SR)18]− is considered to be electronically stable. In recent years, many experimental23−26 and theoretical27−32 studies have investigated doping of [Au25(SR)18]− with foreign © 2012 American Chemical Society

Received: July 6, 2012 Accepted: July 27, 2012 Published: July 27, 2012 2209

dx.doi.org/10.1021/jz300892w | J. Phys. Chem. Lett. 2012, 3, 2209−2214

The Journal of Physical Chemistry Letters

Letter

Figure 1 (Figure S4). These results indicate that CunAu25−n(SC2H4Ph)18 was synthesized in high purity by the present experimental method. The number of copper atoms in CunAu25−n(SC2H4Ph)18 varied slightly with the initial [HAuCl4]/[CuCl2] ratio (Figure 1). However, CunAu25−n(SC2H4Ph)18 containing many copper atoms (n ≥ 6) was hardly obtained for any initial [HAuCl4]/ [CuCl2] ratio (Figure S5). Similar results were obtained when C8H17SH was used as the ligand (Figure S6), when the clusters were synthesized by a two-phase procedure (the Brust method39) (Figure S7), and when other initial copper salts (Cu(C5H8O2)2, Cu(NO3)2, or CuSO4) were used (Figures S8− S10). Thus, CunAu25−n(SC2H4Ph)18 (n ≥ 6) was hardly synthesized for any of the experimental conditions used in this study. These experimental results contrast with those for doping cognate silver, which forms AgnAu25−n(SR)18 (R = C12H25 or C2H4Ph) with up to 11 silver atoms doped in Au25(SR)18.25 The atomic radius of silver (1.44 Å) is almost identical to that of gold (1.44 Å), whereas that of copper (1.28 Å) is smaller. Thus, copper doping is considered to significantly distort the cluster structure, causing the cluster to become unstable. Consequently, only up to five copper atoms can be doped in Au25(SR)18. Figure 2a shows optical absorption spectra of Cu ∼1 Au ∼24 (SC 2 H 4 Ph) 18 , which was synthesized with

We report herein the effect of copper doping. Copper belongs to the same group as gold in the periodic table. Copper (1.28 Å) has a smaller atomic radius than gold (1.44 Å), and copper−gold is a stronger bond than gold−gold.37 In this study, we investigated how copper doping affects the electronic st r uc t u re , g e o m e t r ic st r u c t u r e , a n d s t a b i l i t y o f Au25(SC2H4Ph)18. To this end, CunAu25−n(SC2H4Ph)18 (n = 1−5) was synthesized in high purity. Experimental and theoretical analysis of the synthesized clusters revealed that copper doping alters the optical properties and redox potentials of the cluster, greatly distorts its geometric structure, and reduces the cluster stability in solution. CunAu25−n(SC2H4Ph)18 was synthesized by reacting a gold salt (HAuCl4) and a copper salt (CuCl2) with PhC2H4SH in methanol and reducing the resulting complexes with NaBH4. After extracting clusters that were soluble in acetonitrile from the as-prepared clusters, CunAu25−n(SC2H4Ph)18 was separated from the extract by gel permeation chromatography (Figure S1, Supporting Information (SI)).38 Transmission electron microscopy analysis of the products revealed that they are clusters with diameters of ∼1 nm (30 atoms) (Figure S2). X-ray photoelectron spectra of the products reveal that these clusters contain gold and copper (Figure S3). Figure 1 shows negative-ion matrix-assisted laser

Figure 1. Negative-ion MALDI mass spectra of the products synthesized with [HAuCl4]/[CuCl2] = 24:1, 22:3, and 20:5. All peaks are assigned to the series of CunAu25−n(SC2H4Ph)18 (n = 0−5).

desorption/ionization (MALDI) mass spectra of the products synthesized with [HAuCl4]/[CuCl2] = 24:1, 22:3, and 20:5. Multiple peaks were observed at 133−134 Da intervals in each mass spectrum. These intervals are consistent with the mass difference (133.5 Da) of gold (197.0 Da) and copper (63.5 Da); the series of observed peaks was assigned to CunAu25−n(SC2H4Ph)18 (Table 1). No other peaks were observed in the mass spectrum outside the region shown in Figure 2. (a) Optical absorption spectra of Cu∼1Au∼24(SC2H4Ph)18 and [Au25(SC2H4Ph)18]−. Dotted lines indicate the main peak positions in the absorption spectrum of [Au25(SC2H4Ph)18]−. (b) DPV curves of Cu∼1Au∼24(SC2H4Ph)18 and [Au25(SC2H4Ph)18]−. The red shaded regions indicate the peaks derived from the redox potentials of −2/−1 and −1/0.

Table 1. Yield of Each CunAu25−n(SC2H4Ph)18 in the Product Synthesized with [HAuCl4]/[CuCl2] = 22:3

a

cluster

yield (%)a

Au25(SC2H4Ph)18 Cu1Au24(SC2H4Ph)18 Cu2Au23(SC2H4Ph)18 Cu3Au22(SC2H4Ph)18 Cu4Au21(SC2H4Ph)18 Cu5Au20(SC2H4Ph)18

18.5 37.2 25.9 11.5 4.3 2.6

[HAuCl4]/[CuCl2] = 22:3, and [Au25(SC2H4Ph)18]−. The peak structure of Cu∼1Au∼24(SC2H4Ph)18 is shifted to a lower energy relative to that of Au25(SC2H4Ph)18, indicating that copper doping modifies the optical properties of the cluster.40 On the basis of density functional theory calculations of

Estimated from the relative ion intensities in the mass spectrum. 2210



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[Au25(SCH3)18]−,20−22,41 peaks in the vicinity of 1.2−2.0 eV in the optical absorption spectrum of [Au25(SCH3)18]− can be attributed to a transition from the highest occupied molecular orbital (HOMO) (occupied Au 6sp orbital in Au13 core) to the lowest unoccupied molecular orbital (LUMO) (unoccupied Au 6sp orbital in Au13 core). Peaks in the vicinity of 2.2−3.0 eV are attributed to the transition from HOMO to LUMO+1 (unoccupied Au 6sp orbital in Au13 core) and the transition from ligand-based orbitals to LUMO.20−22,41 The shift of the peaks to a lower energy on copper doping indicates that copper doping affects the transition energy of these transitions. Figure 2b shows differential pulse voltammetry (DPV) curves of Cu∼1Au∼24(SC2H4Ph)18 and [Au25(SC2H4Ph)18]−. The peaks at −282 and −1942 mV in the voltammogram of [Au25(SC2H4Ph)18]− are considered to originate from the redox potentials of [Au25(SC2H4Ph)18]−1/0 and [Au25(SC2H4Ph)18]−2/−1, respectively (Figure 2b and Table 2).6,42,43 These peaks were observed at slightly higher energies Table 2. Formal Potentialsa of Cu∼1Au∼24(SC2H4Ph)18 and [Au25(SC2H4Ph)18]− in 0.1 M (C4H9)4NPF6 CH2Cl2 Solution redox potential (mV) cluster

−1/0

−2/−1

Cu∼1Au∼24(SC2H4Ph)18 [Au25(SC2H4Ph)18]−

−276 ± 5 −282 ± 5

−1896 ± 5 −1942 ± 5

a

Formal potentials are averages of reduction and oxidation peak potentials in DPV potential scans.

(−276 and −1896 mV) in the voltammogram of Cu∼1Au∼24(SC2H4Ph)18 (Table 2). This indicates that copper doping reduces the HOMO and LUMO energies of the cluster. The shift of LUMO (46 mV) is larger than that of HOMO (6 mV), indicating that copper doping reduces the HOMO− LUMO gap.6 These results are consistent with the optical absorption analysis results presented above. To investigate the copper doping sites of Cu∼1Au∼24(SC2H4Ph)18, the optimized structure and absorption spectrum of the cluster were calculated for clusters in which a single copper atom is doped in [Au25(SCH3)18]− at the following positions: the center of the Au13 core (C), the surface of the Au13 core (S), and the [−SR−Au−SR−Au−SR−] oligomer (O). All the theoretical results presented in this study were obtained by density functional theory calculations utilizing the TURBOMOLE package of ab initio quantum chemistry programs. The calculation method is described in the SI. Figure 3a,b shows the optimized structures and absorption spectra of C, S, and O, respectively. Figure 3b also shows the optical absorption spectrum of [Au25(SCH3)18]− for comparison. The overall absorption spectrum of C is shifted to a lower energy relative to that of [Au25(SCH3)18]−, and thus the HOMO− LUMO gap is reduced (red line in Figure 3b and Table 3). On the other hand, the peak structure in the absorption spectrum of S is shifted to a higher energy relative to that of [Au25(SCH3)18]−, indicating that the HOMO−LUMO gap is increased (blue line in Figure 3b and Table 3). For O, there is almost no change in the absorption spectrum compared to that of [Au25(SCH3)18]−, indicating that the HOMO−LUMO gap has barely changed (green line in Figure 3b and Table 3). As shown in Figure 2a, the optical absorption spectrum of Cu∼1Au∼24(SC2H4Ph)18 is shifted to a lower energy relative to that of Au25(SC2H4Ph)18, indicating that the HOMO−

Figure 3. (a) Optimized structures for C, S, and O. (b) Calculated optical absorption spectra for [Au25(SCH3)18]−, C, S, and O. (c) Calculated bond distances in [Au25(SCH3)18]−, C, S, and O; red: central atom to surface atom; blue: surface atom to surface atom. The numbers indicate averages and standard deviations for each bond distance.

Table 3. Relative Stabilities and HOMO−LUMO Gaps Calculated for C, S, and Oa

a

structure

stability

HOMO−LUMO gap

C S O

0.40 0 0.13

2.21 2.53 2.48

All energies are in eV.

LUMO gap is reduced. On the basis of these results, all or most of the main product, CuAu24(SC2H4Ph)18, is expected to have a core−shell structure (C). It is noteworthy that S is more energetically stable than C and O in our calculation (Table 3). This implies that the most energetically stable cluster could not be synthesized under the present experimental conditions, which contrasts with the results for AgAu24(SR)18. Theoretical calculations predict that a 2211



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chemical composition when Cu∼1Au∼24(SC2H4Ph)18 is placed in a toluene solution at 80 °C. Hardly any clusters besides Au25(SC2H4Ph)18 are observed in the mass spectrum after 48 h. Similar results were obtained when dichloromethane and neat thiol were used as solvents (Figures S17 and S18, respectively). These results indicate that copper doping reduces the stability of the cluster in solution45 (i.e., the stabilities of the cluster against degradation in solution, thiol etching, and/or metal atom replacement in solution44). As mentioned above, gold− copper is a stronger bond than gold−gold.37 A similar result was also obtained from density functional theory calculations for core−shell [CuAu24(SCH3)18]−. The interaction energy between the central atom and the Au24(SCH3)18 cage structure has been reported to increase when the central gold atom is replaced with copper.27 Nevertheless, CunAu25−n(SC2H4Ph)18 is less stable than Au25(SC2H4Ph)18. This reduction in stability is considered to be related to structural distortion of CunAu25−n(SC2H4Ph)18 due to the different atomic radii of gold and copper.46 Such a significant reduction in stability was not observed for silver doping (Figure S19). These results suggest that the geometric structure strongly affects the stability of [Au25(SR)18]−. Thus, it is recommended to select a doping element that has a very similar atomic radius to that of gold when it is desired to functionalize [Au25(SR)18]− while maintaining its stability. In summary, CunAu25−n(SC2H4Ph)18 (n = 1−5) was successfully synthesized in high purity. Optical absorption spectroscopy and electrochemical measurements of the product revealed that doping with copper alters the electronic structure. Investigation of the geometric structure implies that the main product, CuAu24(SC2H4Ph)18, has a structure in which copper is doped in the center of the metal core and copper doping significantly distorts the metal core. Experiments revealed that copper doping reduces the cluster stability in solution. These findings are expected to be useful for developing design guidelines for functionalizing [Au25(SR)18]− through foreign atom doping.

structure in which the Ag atom is located on the surface of the M13 core will be more energetically stable than one in which the Ag atom is located in the central position or oligomer.31 Our experimental and theoretical results strongly imply that the most energetically stable cluster was actually synthesized in our synthesis of AgAu24(SR)1825 (Figures S11−13). The preferential formation of the C isomer of CuAu24(SC2H4Ph)18 is considered to be due to the kinetics of the growth mechanism. Unlike silver, copper has a smaller atomic radius than gold. On the other hand, in icosahedral structures, the distances between the central and surface atoms are shorter than those between surface atoms. It is thus considered that copper with a small atomic radius is readily trapped in the center of the icosahedral metal core (Figure 3a), leading to the preferential formation of the C isomer of CuAu24(SC2H4Ph)18. The second, third, and fourth Cu atoms are presumed to be doped on the surface of the metal core since the calculation predicts that S will be more energetically stable than C and O. The exact structures of these clusters are expected to be determined in a future study based on the present results. The effect of copper doping on the cluster structure was also examined. Figure 3c shows the bond distances between metal atoms in the metal core of [Au25(SCH3)18]−, C, S, and O. For C, both the bond distances between the central atom and surface atoms (red) and those between the surface atoms (blue) are much smaller than those in [Au25(SCH3)18]−, indicating that the metal core contracts in C (Figure 3a). For S, some of the bonding between central atoms and the surface atoms are significantly reduced, and the bond distances between the surface atoms are also uneven. These results indicate that the symmetry of the metal core is reduced in S (Figure 3a). For O, there is barely any distortion of the metal core. However, the distances between the metal core and the metal atoms in the oligomer differ (Figure S14), indicating that the total symmetry of the cluster is reduced. Thus, the geometric structure of the cluster is significantly distorted when copper is doped at any cluster site. These results differ from those for silver doping. Silver doping distorts the structure little, regardless of the doping site (Figures S15 and S16). Because copper doping significantly distorts the cluster structure, the number of doped atoms is considered to be limited to a relatively low value (∼5) for CunAu25−n(SC2H4Ph)18 (Figure 1). Finally, the effect of copper doping on cluster stability in solution was investigated. Figure 4 shows the changes in the

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ASSOCIATED CONTENT

* Supporting Information S

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

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]; [email protected]. jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. Wataru Kurashige and Mr. Ukyo Kamimura 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) and the “Nanotechnology Network” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

Figure 4. Time dependence of chemical composition of Cu∼1Au∼24(SC2H4Ph)18 in toluene at °80 C. 2212



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(22) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Grönbeck, H.; Häkkinen, H. A Unified View of Ligand-Protected Gold Clusters as Superatom Complexes. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9157−9162. (23) Fields-Zinna, C. A.; Crowe, M. C.; Dass, A.; Weaver, J. E. F.; Murray, R. W. Mass Spectrometry of Small Bimetal MonolayerProtected Clusters. Langmuir 2009, 25, 7704−7710. (24) 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. (25) 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. (26) Qian, H.; Barry, E.; Zhu, Y.; Jin, R. Doping 25-Atom and 38Atom Gold Nanoclusters with Palladium. Acta Phys.-Chim. Sin. 2011, 27, 513−519. (27) Jiang, D.-E.; Dai, S. From Superatomic Au25(SR)18− to Superatomic M@Au24(SR)18q Core−Shell Clusters. Inorg. Chem. 2009, 48, 2720−2722. (28) Jiang, D.-E.; Whetten, R. L. Magnetic Doping of a ThiolatedGold Superatom: First-Principles Density Functional Theory Calculations. Phys. Rev. B 2009, 80, 115402. (29) Kacprzak, K. A.; Lehtovaara, L.; Akola, J.; Lopez-Acevedo, O.; Häkkinen, H. A Density Functional Investigation of ThiolateProtected Bimetal PdAu24(SR)18z Clusters: Doping the Superatom Complex. Phys. Chem. Chem. Phys. 2009, 11, 7123−7129. (30) Walter, M.; Moseler, M. Ligand-Protected Gold Alloy Clusters: Doping the Superatom. J. Phys. Chem. C 2009, 113, 15834−15837. (31) Reveles, J. U.; Clayborne, P. A.; Reber, A. C.; Khanna, S. N.; Pradhan, K.; Sen, P.; Pederson, M. R. Designer Magnetic Superatoms. Nat. Chem. 2009, 1, 310−315. (32) Zhou, M.; Cai, Y. Q.; Zeng, M. G.; Zhang, C.; Feng, Y. P. MnDoped Thiolated Au25 Nanoclusters: Atomic Configuration, Magnetic Properties, and a Possible High-Performance Spin Filter. Appl. Phys. Lett. 2011, 98, 143103. (33) 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. (34) Kumara, C.; Dass, A. AuAg Alloy Nanomolecules with 38 Metal Atoms. Nanoscale 2012, 4, 4084−4086. (35) Kumara, C.; Dass, A. (AuAg)144(SR)60 Alloy Nanomolecules. Nanoscale 2011, 3, 3064−3067. (36) Malola, S.; Häkkinen, H. Electronic Structure and Bonding of Icosahedral Core−Shell Gold−Silver Nanoalloy Clusters Au144−xAgx(SR)60. J. Phys. Chem. Lett. 2011, 2, 2316−2321. (37) Ackerman, M.; Stafford, F. E.; Drowart, J. Mass Spectrometric Determination of the Dissociation Energies of the Molecules AgAu, AgCu, and AuCu. J. Chem. Phys. 1960, 33, 1784−1789. (38) See the Supporting Information. (39) 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. (40) The retention time in reverse phase columns24,25 strongly imply that Cu ∼ 1 Au ∼ 2 4 (SC 2 H 4 Ph) 1 8 has a charge state of −1 ([Cu∼1Au∼24(SC2H4Ph)18]−), indicating that copper doping modulates the optical properties. (41) Aikens, C. M. Origin of Discrete Optical Absorption Spectra of M25(SH)18− Nanoparticles (M = Au, Ag). J. Phys. Chem. C 2008, 112, 19797−19800. (42) Guo, R.; Murray, R. W. Substituent Effects on Redox Potentials and Optical Gap Energies of Molecule-like Au38(SPhX)24 Nanoparticles. J. Am. Chem. Soc. 2005, 127, 12140−12143. (43) Choi, J.-P.; Fields-Zinna, C. A.; Stiles, R. L.; Balasubramanian, R.; Douglas, A. D.; Crowe, M. C.; Murray, R. W. Reactivity of [Au25(SCH2CH2Ph)18]1− Nanoparticles with Metal Ions. J. Phys. Chem. C 2010, 114, 15890−15896.

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(1) 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. (2) Shichibu, Y.; Negishi, Y.; Tsunoyama, H.; Kanehara, M.; Teranishi, T.; Tsukuda, T. Extremely High Stability of GlutathioneProtected Au25 Clusters Against Core Etching. Small 2007, 3, 835− 839. (3) 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. (4) 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. (5) 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. (6) 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. (7) Murray, R. W. Nanoelectrochemistry: Metal Nanoparticles, Nanoelectrodes, and Nanopores. Chem. Rev. 2008, 108, 2688−2720. (8) 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. (9) Negishi, Y.; Chaki, N. K.; Shichibu, Y.; Whetten, R. L.; Tsukuda, T. Origin of Magic Stability of Thiolated Gold Clusters: A Case Study on Au25(SC6H13)18. J. Am. Chem. Soc. 2007, 129, 11322−11323. (10) Negishi, Y.; Kamimura, U.; Ide, M.; Hirayama, M. A Photoresponsive Au25 Nanocluster Protected by Azobenzene Derivative Thiolates. Nanoscale 2012, 4, 4263−4268. (11) Link, S.; Beeby, A.; FitzGerald, S.; El-Sayed, M. A.; Schaaff, T. G.; Whetten, R. L. Visible to Infrared Luminescence from a 28-Atom Gold Cluster. J. Phys. Chem. B 2002, 106, 3410−3415. (12) Wang, G.; Huang, T.; Murray, R. W.; Menard, L.; Nuzzo, R. G. Near-IR Luminescence of Monolayer-Protected Metal Clusters. J. Am. Chem. Soc. 2005, 127, 812−813. (13) Negishi, Y.; Takasugi, Y.; Sato, S.; Yao, H.; Kimura, K.; Tsukuda, T. Magic-Numbered Au n Clusters Protected by Glutathione Monolayers (n = 18, 21, 25, 28, 32, 39): Isolation and Spectroscopic Characterization. J. Am. Chem. Soc. 2004, 126, 6518−6519. (14) Shibu, E. S.; Muhammed, M. A. H.; Tsukuda, T.; Pradeep, T. Ligand Exchange of Au25SG18 Leading to Functionalized Gold Clusters: Spectroscopy, Kinetics, and Luminescence. J. Phys. Chem. C 2008, 112, 12168−12176. (15) Wu, Z.; Jin, R. On the Ligand’s Role in the Fluorescence of Gold Nanoclusters. Nano Lett. 2010, 10, 2568−2573. (16) Liu, Y.; Tsunoyama, H.; Akita, T.; Tsukuda, T. Efficient and Selective Epoxidation of Styrene with TBHP Catalyzed by Au25 Clusters on Hydroxyapatite. Chem. Commun. 2010, 46, 550−552. (17) Zhu, Y.; Qian, H.; Drake, B. A.; Jin, R. Atomically Precise Au25(SR)18 Nanoparticles as Catalysts for the Selective Hydrogenation of α,β-Unsaturated Ketones and Aldehydes. Angew. Chem., Int. Ed. 2010, 49, 1295−1298. (18) Zhu, Y.; Qian, H.; Jin, R. Catalysis Opportunities of Atomically Precise Gold Nanoclusters. J. Mater. Chem. 2011, 21, 6793−6799. (19) 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. (20) 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. (21) 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. 2213



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(44) Song, Y.; Huang, T.; Murray, R. W. Heterophase Ligand Exchange and Metal Transfer between Monolayer Protected Clusters. J. Am. Chem. Soc. 2003, 125, 11694−11701. (45) Considering the cluster stability, CunAu25−n(SC2H4Ph)18 is considered to be the metastable species. The formation of such clusters is related to the kinetics of the growth process.1 The present experimental conditions are considered favorable for forming CunAu25−n(SC2H4Ph)18 (n = 1−5). (46) To enclose the central atoms, the distance between the 12 surface atoms in an icosahedral structure consisting of identical atoms is 5% longer than that between the central atom and the surface atoms. Therefore, the cluster is expected to become more stable when the central atom is replaced by an atom with a smaller atomic radius (an atom that is approximately 95% of the size of the original central atom). However, the atomic radius of copper (1.28 Å) is only about 89% of the atomic radius of gold (1.44 Å) so that copper is too small for the central atom. CuAu24(SC2H4Ph)18 is considered to be unstable for this reason. On the other hand, previous studies have revealed that [Au25(SR)18]− becomes more stable when the central atom is replaced with palladium, despite the distortion of the metal core.24 Palladium doping is expected to make the cluster more stable because the effect of the increased interaction energy between the central atom and the Au24(SR)1827 is greater than that of the instability due to metal core distortion.

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Effect of Copper Doping on Electronic Structure, Geometric Structure

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