Thermally-induced formation of atomic Au clusters and conversion into nanocubes

Article abstract of DOI:10.1021/ja0482482

A thermal method for converting Au colloids into atomic Au clusters and subsequent growth of Au nanocubes from clusters is reported. Mass spectral analysis shows that these clusters are Au trimers. The Au clusters show distinct optical absorption at 305 and 250 nm and have extraordinary stability under ambient conditions. Copyright

Full text of DOI:10.1021/ja0482482

Published on Web 07/28/2004
Thermally-Induced Formation of Atomic Au Clusters and Conversion into
Nanocubes
Rongchao Jin,† Shunji Egusa,‡ and Norbert F. Scherer*,†
Departments of Chemistry and Physics, James Franck Institute, and Consortium for Nanoscience Research,
UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
Received March 26, 2004; E-mail: nfschere@uchicago.edu
Metal and semiconductor nanoparticles are having a major impact
on research in materials, chemistry, physics, and biological and
environmental sciences.1 Control over particle size and shape has
been achieved for a variety of compositions, e.g., Au, Ag, Pt, Co,
and PtFe.1-8 Small metal clusters (composed of several to tens of
atoms) with robust quantum effects or molecule-like properties (e.g.,
fluorescence) have also been extensively investigated.9-14 Early
research on metal clusters was focused on gas-phase beam
experiments, including the physical synthesis of clusters (e.g., Nan,
Agn) and spectroscopic studies of their structural and electronic
properties.14 However, these gas-phase clusters are typically short-
lived and difficult to chemically functionalize for applications such
as catalysis or electron microscopy contrast enhancement of
biological samples.15,16 Therefore, since the 1980s, research has
focused on solution-phase chemical synthesis of metal clusters.17-19
A number of improved synthetic methods have been reported for
the preparation of high-quality, relatively monodisperse, and ligand-
stablized nanoclusters in the form of solution dispersions.17-22 Metal
clusters, e.g., Au75, Au55, Au28, Au11, and Au8, as well as differently
sized Ag nanoclusters,17,22,23 have been synthesized, and their
catalytic and photoluminescence properties were extensively
studied.24-26 These synthetic strategies lead to nanoclusters with
greatly enhanced stability and allowed some degree of tailoring of
their physical and chemical properties.27 To fully exploit the
fascinating properties of metal clusters, it is important to develop
new strategies for synthesizing clusters with useful properties and
enhanced stability.
This paper reports a facile thermal method in which Au
nanocolloids are converted into atomic Au clusters under high-
temperature reflux conditions. These clusters show distinct optical
properties and extraordinary stability. In addition, they serve as
building blocks allowing the synthesis of Au nanocubes.
Au clusters are prepared via a two-step process; the synthesis of
∼6 nm (diameter) Au nanocolloids and subsequent conversion into
clusters. Au colloids were synthesized via a modification of a
literature protocol.28 Briefly, AuCl3 (30 mg, 0.1 mmol) was
dissolved in a solution of didodecyldimethylammonian bromide
(DDAB, 20 mM) in anhydrous toluene under inert gas (e.g., N2).
An aqueous solution of NaBH4 (40 µL, 9.0 M, freshly prepared)
was injected into the Au precursor solution under vigorous stirring
(see Supporting Information). Au colloids were further passivated
with dodecylthiol to enhance their stability. The colloids were
precipitated with ethanol and washed with hexane to remove excess
DDAB and possible reaction side-products. In the second step, the
Au nanoparticles (0.025 mmol in atomic Au) were redispersed in
a solution of dodecylthiol (0.4 M) in octyl ether, and the solution
was refluxed at ∼300 °C for ∼50 min. During the initial stages
(<10 min), transmission electron microscopy (TEM) shows that
Figure 1. (A) TEM image of Au nanoparticles. (B) Time-dependent UV-
vis spectra of the Au colloid under high-temperature reflux (∼300 °C),
curve a ) 5 min, b ) 15 min, c ) 25 min, d ) 35 min, and e ) 55 min.
The inset shows the absorption (solid line) and fluorescence spectra (dashed
line, excitation ) 250 nm). (C) Mass spectrum of Au clusters (note that
the m/z ) 178.4 species is assigned to CH3(CH2)10Na+ adducts). (D) TEM
image of Au nanocubes converted from Au clusters.
only Au nanoparticles with a diameter of 6.0 ( 0.5 nm are present
(Figure 1A). The associated UV-vis spectrum shows an absorption
peak at 525 nm, which is the plasmon absorption of small Au
nanoparticles (Figure 1B, curve a). As thermal reflux proceeds, the
ruby red color of the Au colloids gradually fades; the absorption
at 525 nm decreases, and a new peak at 305 nm develops and grows
with time. Finally, the 525 nm absorption completely disappears
after ∼50 min of reflux (Figure 1B, curves b-e). Under TEM, Au
nanoparticles are no longer observed in the final sample. These
observations show that during the high-temperature reflux process,
the initial 6 nm Au nanocolloids are converted into ultrasmall
clusters that cannot be imaged by TEM due to their extremely small
size.
The as-prepared ultrasmall Au clusters are characterized by mass
spectrometry. Whetten and co-workers have shown that Au clusters
passivated with alkanethiols can be directly analyzed with laser
desorption/ionization mass spectrometry in the absence of matrix
molecules.26 Following this method, the as-prepared Au cluster
sample (diluted with hexane in a 1:2 ratio) was deposited on a steel
plate without matrix molecules and, after drying, analyzed with a
MALDI mass spectrometer. Laser (337 nm) irradiation results in
desorption and ionization of clusters due to the breaking of S-C
† Department of Chemistry.
‡ Department of Physics.
9
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J. AM. CHEM. SOC. 2004, 126, 9900-9901
10.1021/ja0482482 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
and Au-S bonds.29 Three major peaks are observed in the mass
spectrum (Figure 1C). All the peaks are singlets, and their mass/
charge ratios (m/z ) 197.2, 394.1, 591.5, respectively) are assigned
to Au monomers (Au+ ) 197, isotope abundance 100%), dimers
(Au2 ) 394), and trimers (Au3 ) 591). No larger clusters are
detected when scanning the spectrum over a wide mass range (10-
100 000 Da). Taken together with the spectroscopic data (see next
paragraph), the clusters prepared from Au colloids are most
probably molecular Au trimers, i.e., Au3(SC12H25)3. The monomers
NSF for an MRI grant (CHE 0216492) that aided this research.
We thank Prof. Norris for use of a UV-vis spectrometer, Prof.
Lee for use of a hood for synthesis, and Dr. Qin for assistance in
MS analysis.
+
+
Supporting Information Available: Detailed experiments for the
synthesis of Au nanoparticles and clusters (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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Previous gas-phase absorption measurements of bare Au trimers
established that Au3(g) shows transitions at 302, 285(292), and 258
nm.32,33 Solution-phase Au trimers show transitions at 305 and 250
nm (measured in hexane, cutoff wavelength 220 nm). The 305-nm
absorption (Figure 1B inset, solid line) should correlate with the
292 nm transition of bare Au3(g) clusters when accounting for the
solution dielectric-induced spectral red-shift. Photoexcitation of
either the 305 or 250 nm absorption band of the clusters leads to
strong fluorescence emission centered at 340 nm (Figure 1B inset,
dashed line). The Au clusters are highly luminescent with both states
having the same emission spectrum, indicating an efficient non-
radiative decay of the 250 nm state to the 305 nm-associated state.
Therefore, the optical transitions at 305 and 250 nm are both
assigned to Au trimers. The long-wavelength (470 nm) absorbance
reported in ref 32 is attributed to aggregates/larger cluster in a more
recent paper.33 Therefore, no such absorption is expected for a pure
Au trimer sample, consistent with our observation.
The as-prepared Au3 clusters show extraordinary stability, as
evidenced by the lack of observable degradation in two months
monitored by UV-vis spectroscopy. This great stability of the Au
clusters is partly due to the alkanethiol ligands.
Another striking feature of the atomic Au clusters prepared here
is that they can serve as building blocks allowing the synthesis of
Au nanocubes on a substrate. To demonstrate this, a diluted cluster
solution was deposited onto a carbon film-coated TEM grid. When
the substrate was heated on a hotplate at ∼100 °C for a few minutes,
Au nanocubes formed (average lateral dimension ∼10 nm, Figure
1D). Note that when the cluster solution was heated in a flask,
formation of Au nanocubes was not observed. Therefore, the growth
of Au nanocubes must involve a nonequilibrium and surface-assisted
nucleation/growth process. Conditions, structures formed, and
proposed growth mechanism(s) are described elsewhere.34 Femto-
second laser two-photon interferometric fluorescence and pump-
probe measurements35 will be carried out to establish the electron
dynamics of the clusters.
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Acknowledgment. This work was supported by the University
of Chicago MRSEC (#DMR0213745), NSF (#CHE0317009), and
the Consortium for Nanoscience Research. We acknowledge the
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