pubs.acs.org/JPCL
Thiolate-Protected Au24(SC2H4Ph)20 Nanoclusters: Superatoms or Not? Manzhou Zhu,*,† Huifeng Qian,‡ and Rongchao Jin*,‡ †
Department of Chemistry, Anhui University, Hefei, Anhui, 230039, P. R. China, and ‡Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
ABSTRACT We report a new gold thiolate cluster with molecular purity. Electrospray ionization (ESI) mass spectrometry in conjunction with thermogravimetric analysis (TGA), elemental analysis (EA), and 1H NMR, unambiguously determined the composition of the as-prepared Au nanocluster to be Au24(SC2H4Ph)20. The optical absorption spectrum of this cluster shows a highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO-LUMO) transition at 765 nm, indicating quantum confinement of electrons in the particle. The HOMOLUMO gap (∼1.5 eV) of Au24(SR)20 is much smaller than that of Au20(SR)16 (∼2.1 eV) but slightly larger than that of Au25(SR)18 (∼1.3 eV). The number of valence electrons in Au24(SC2H4Ph)20 is 4e, which is not predicted by the superatom model. SECTION Nanoparticles and Nanostructures
been predicted by Pei et al.17 and Jiang et al.18 in their recent density functional theory (DFT) calculations. Herein, we report a new thiolated gold nanocluster prepared through a one-pot process. The cluster composition is determined to be Au24(SC2H4Ph)20 by electrospray ionization mass spectrometry (ESI-MS) in conjunction with other detailed analyses. This work demonstrates the feasibility of one pot for one size synthesis35 and an atomic level of control of the gold nanocluster size. In a typical experiment for the synthesis of Au24(SR)20 (see Supporting Information (SI) for details), gold salt (HAuCl4 3 3H2O, 0.4 mmol) was phase-transferred from an aqueous solution to the toluene phase with the aid of a phase transfer agent, tetraoctylammoniun bromide (TOAB). The solution was cooled to 0 °C in an ice bath, and the magnetic stirring speed was reduced to ∼50 rpm. Phenylethylthiol (abbreviated as HS-C2H4Ph hereafter) was added to the cold Au(III) solution, and the solution was kept under constant, slow stirring (∼50 rpm) until the complete formation of [Au(I)SC2H4Ph]x aggregates (nearly clear suspension). After that, an aqueous solution of freshly made, ice-cold NaBH4 (1 eqivalent per mole of gold) was added drop-wise to the solution within a ∼15 min period. The growth of gold clusters was allowed to proceed overnight. The crude reaction products (prior to separation) show a distinct absorption band at ∼700 nm and a shoulder band at ∼780 nm (Figure 1, profile a); note that the solution was diluted with toluene before UV-vis measurements. Ethanol was added to the solution of crude products to precipitate gold
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he evolution of the atomic packing structure, electronic, and optical properties of metal nanomaterials from clusters to nanocrystals constitutes an important topic in current nanoscience research.1-5 Among the precious metals, gold and silver nanoparticles are of particular interest due to their relative stability, interesting optical, and catalytic properties.6-13 The ultrasmall gold nanoparticles (
