NANO LETTERS
Controlling Nanoparticles with Atomic Precision: The Case of Au144(SCH2CH2Ph)60
2009 Vol. 9, No. 12 4083-4087
Huifeng Qian and Rongchao Jin* Department of Chemistry, Carnegie Mellon UniVersity, Pittsburgh, PennsylVania 15213 Received July 17, 2009; Revised Manuscript Received August 21, 2009
ABSTRACT We report a facile, two-step synthetic method for preparing truly monodiserse Au144(SCH2CH2Ph)60 nanoparticles with their formula determined by electrospray mass spectrometry in conjunction with other characterization. A remarkable advantage of our synthetic approach lies in that it solely produces Au144(SCH2CH2Ph)60 nanoparticles, hence, eliminating nontrivial, postsynthetic steps of size separation, which has proven to be very difficult. This advantage makes the approach and the type of nanoparticles generated by it of broad utility for practical applications. Unlike their larger counterparts, Au nanocrystals (typically >2 nm) that are crystalline and show a prominent surface plasmon resonance band at ∼520 nm (for spherical particles), the Au144(SCH2CH2Ph)60 nanoparticles instead exhibit a stepwise, multiple-band absorption spectrum, indicating quantum confinement of electrons in the particle. In addition, these ultrasmall nanoparticles do not adopt face-centered cubic structure as in Au nanocrystals or bulk gold.
It has long been a major goal for nanochemists to create nanostructures with precise control at the atomic level. This is particularly important for understanding the fundamental scienceofnanostructuresandachievingprecisestructure-property correlations.1-5 For example, nanogaps or nanocavities have been discovered to be capable of enormously enhancing second harmonic signals from the nanostructures1 and Raman scattering signals of molecules within the gap or cavity2,5-7 but in most cases the ill-defined surface structures preclude an atomic level understanding of the enhancing mechanism and the exact “hot spot” structures could not be observed at the atomic level.2,5-8 Similarly, in the field of heterogeneous catalysis involving metal nanoparticle catalysts, the concept of “active sites” had long been proposed many decades ago but the exact nature of the active sites on a metal nanoparticle has not been fully understood, albeit some structural features such as sharp corners, atomic steps, and defects have been described to be active sites for surface catalytic reactions.9-13 With respect to colloidal nanoparticles, the recent advances14-24 have made it possible to synthesize small nanoparticles with precise control of the number of metal atoms and ligands in the particle.25-28 This level of precise control is critical for quantum dot nanostructures because their properties are very sensitive to the number of atoms in the core.26,27 For metals, the de Broglie wavelength of conduction electrons is extremely small, i.e., λ ) h/(mυf), where, υf is the velocity of Fermi surface electron (1.4 × * To whom correspondence to be addressed,
[email protected]. 10.1021/nl902300y CCC: $40.75 Published on Web 09/09/2009
2009 American Chemical Society
106 m/s), λ ∼ 0.5 nm for gold; thus, only for ultrasmall nanoparticles (typically 2-3 nm)? (ii) How does the gold quantum dot structure affect the electronic, optical, and magnetic properties? (iii) At what size does the single electron excitation mode transit to a collective excitation mode (i.e., surface plasmon resonance) in larger nanoparticles? To address the aforementioned fundamental questions as well as to explore a wide range of potential applications of gold quantum dots, it is of paramount importance to first achieve success in synthesizing gold quantum dots with a precise control over size (Aun) and attaining true monodispersity. The latter is critically important as the optical and electronic properties of Au quantum dots are very sensitive to the number of atoms (n).28,30,32,36 We choose thiolateprotected Au nanoparticles as a model system to develop wet chemistry approaches that allow for an atomic level control of gold quantum dots.
Figure 1. (A) MALDI mass spectra of Au nanoparticles before (black profile) and after thiol etching (red profile). trans-2-[3-(4-tertButylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB)37 was used as a matrix. (B) UV-vis absorption spectra of Au nanoparticles before (black profile) and after thiol etching (red profile). Inset: optical absorbance vs photon energy (eV). Abs(E) ∝ Abs(w) × w2. The spectra are shifted for the ease of comparison.
In this work, we report a facile, two-step method that permits the synthesis of truly monodisperse Au nanoparticles with their precise formula unequivocally determined to be Au144(SCH2CH2Ph)60 by electrospray ionization (ESI) mass spectrometry in conjunction with other analyses. A remarkable advantage of our synthetic method lies in that it solely produces Au144(SCH2CH2Ph)60 nanoparticles, hence, it eliminates those nontrivial steps of size separation; the latter has proven to be very difficult and time-consuming. The Au144(SCH2CH2Ph)60 nanoparticle should be relevant to the 29 kDa species (assigned Au140-146(SR)50-60) previously reported by Schaaff et al.14 and Hicks et al.,16a,b and the recent Au144(SC12H25)59 reported by Chaki et al.36 The Au144(SCH2CH2Ph)60 nanoparticles in this work were made via two primary steps. In the first step, Au nanoparticles capped by -SCH2CH2Ph thiolates (denoted as -SR below) were made via a modified Brust method.38 The as-prepared nanoparticles tend to be polydispersed, but by adjusting the experimental conditions including the thiol/Au ratio (∼3× used), reaction temperature, and kinetic control, one can obtain relatively narrow size distributed Au nanoparticles (2 nm), the latter typically show a prominent surface plasmon resonance band at ∼520 nm (for spherical particles). The nature of the absorption bands of the Au144(SCH2CH2Ph)60 nanoparticles and their electronic structure are to be explained until the crystal structure is solved and full scale density functional theory (DFT) calculations based on the crystal structure are done. It is worthy of noting that Hakkinen et al. recently performed DFT calculations on a model nanoparticle Au144(SCH3)60 and predicted its structure to be composed of a Au114 core protected by 30 CH3S-Au-SCH3 staple motifs; the calculated X-ray diffraction pattern based upon the predicted structure seems consistent with experiment.43 This predicted structure is closely relevant to the Pd145 structure40 and features an isosahedral (instead of a cuboctahedral) kernel as was found in other clusters as well.44-46 We have performed X-ray diffraction analysis of the Au144(SCH2CH2Ph)60 nanoparticles prepared in this work, see Figure 4; the XRD pattern indicates that the nanoparticles do not adopt the fcc structure, because the diffraction angles (2θ) at 39.5°, 52°, 67°, and 77° for the Au144(SCH2CH2Ph)60 nanoparticles apparently deviate from those for the fcc structure (see the red stick pattern in Figure 4), the latter includes 38° (hkl ) 111), 44° (200), 65° (220), and 77.5° (311). In particular, the 52° diffraction angle observed in the Au144(SCH2CH2Ph)60 nanoparticles is not present in the fcc structure; hence, the Au144(SCH2CH2Ph)60 nanoparticles should not adopt the fcc structure. The intense diffraction 4086
Acknowledgment. We thank Dr. Zhongrui Zhou for kind assistance in ESI-MS analysis. This work is financially supported by CMU, AFOSR, and NIOSH. Supporting Information Available: Experimental details and supporting Figure S1 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Jin, R.; Jureller, J. E.; Kim, H. Y.; Scherer, N. F. J. Am. Chem. Soc. 2005, 127, 12482. (2) Camargo, P. H. C.; Rycenga, M.; Au, L.; Xia, Y. Angew. Chem., Int. Ed. 2009, 48, 2180. (3) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397. (4) Liu, S. T.; Maoz, R.; Schmid, G.; Sagiv, J. Nano Lett. 2002, 2, 1055. (5) Qin, L.; Zou, S.; Xue, C.; Atkinson, A.; Schatz, G. C.; Mirkin, C. A. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 13300. (6) Bosnick, K. A.; Jiang, J.; Brus, L. E. J. Phys. Chem. B 2002, 106, 8096. (7) Kneipp, K.; Kneipp, H.; Kneipp, J. Acc. Chem. Res. 2006, 39, 443. (8) Campion, A.; Kambhampati, P. Chem. Soc. ReV. 1998, 27, 241. (9) (a) Somorjai, G. A. J. Phys. Chem. B 2000, 104, 2969. (b) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (10) Luo, J.; Jones, V. W.; Maye, M. M.; Han, L.; Kariuki, N. N.; Zhong, C.-J. J. Am. Chem. Soc. 2002, 124, 13988. (11) Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem., Int. Ed. 2005, 44, 7852. (12) Guzman, J.; Gates, B. C. J. Am. Chem. Soc. 2004, 126, 2672. (13) Molina, L. M.; Hammer, B. Phys. ReV. Lett. 2003, 90, 206102. (14) (a) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L.; Cullen, W. G.; First, P. N.; Gutierrez-Wing, C.; Ascensio, J.; JoseYacaman, M. J. J. Phys. Chem. B 1997, 101, 7885. (b) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (15) Hussain, I.; Graham, S.; Wang, Z. X.; Tan, B.; Sherrington, D. C.; Rannard, S. P.; Cooper, A. I.; Brust, M. J. Am. Chem. Soc. 2005, 127, 16398. (16) (a) Hicks, J. F.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 13322. (b) Quinn, B. M.; Liljeroth, P.; Ruiz, V.; Laaksonen, T.; Kontturi, K. J. Am. Chem. Soc. 2003, 125, 6644. (c) Donkers, R. L.; Lee, D.; Murray, R. W. Langmuir 2004, 20, 1945. (d) Kim, J.; Lee, D. J. Am. Chem. Soc. 2006, 128, 4518. (e) Ramakrishna, G.; Varnavski, O.; Kim, J.; Lee, D.; Goodson, T. J. Am. Chem. Soc. 2008, 130, 5032. (17) (a) Shichibu, Y.; Negishi, Y.; Tsukuda, T.; Teranishi, T. J. Am. Chem. Soc. 2005, 127, 13464. (b) Shichibu, Y.; Negishi, Y.; Tsunoyama, H.; Kanehara, M.; Teranishi, T.; Tsukuda, T. Small 2007, 3, 835. (18) Zhu, M.; Lanni, E.; Garg, N.; Bier, M. E.; Jin, R. J. Am. Chem. Soc. 2008, 130, 1138. (19) Ackerson, C. J.; Jadzinsky, P. D.; Kornberg, R. D. J. Am. Chem. Soc. 2005, 127, 6550. (20) Gies, A. P.; Hercules, D. M.; Gerdon, A. E.; Cliffel, D. E. J. Am. Chem. Soc. 2007, 129, 1095. Nano Lett., Vol. 9, No. 12, 2009
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