Nanoparticle synthesis via the photochemical polythiol process

Article abstract of DOI:10.1021/ja0733639

We introduce the photochemical polythiol process, in which we use ambient light to decompose bismuth, antimony, copper, and lead thiolates to metal nanoparticles. Focusing on bismuth, we produce rhombohedral nanoparticles with narrow size distributions. The approach allows a single-phase synthesis of uniform non-noble metal nanoparticles without a secondary size-selection process. It minimizes use of chemicals and solvents and is more environmentally friendly than competing processes described in the literature. Copyright

Full text of DOI:10.1021/ja0733639

Published on Web 07/27/2007
Nanoparticle Synthesis via the Photochemical Polythiol Process
Scott C. Warren,†,‡ Aaron C. Jackson,‡ Zachary D. Cater-Cyker,‡ Francis J. DiSalvo,† and
Ulrich Wiesner*,‡
Department of Chemistry & Chemical Biology and Department of Materials Science & Engineering,
Cornell UniVersity, Ithaca, New York 14853
Received May 11, 2007; E-mail: ubw1@cornell.edu
Scheme 1. Synthetic Route for the Photochemical Polythiol
Process (Metals Tested Include Bismuth, Copper, Antimony, and
Lead)
Significant quantities of waste are produced in the synthesis of
metal nanoparticles. Charge-stabilized sols made by Faraday1 and
Turkevich2 required 1 L of water to produce 40 and 60 mg of
particles, respectively. Even ligand-stabilized particles, such as those
made by the polyol process,3 phase-transfer,4 or reverse micro-
emulsion,5 use large quantities of ligands, surfactants, polymers,
phase-transfer reagents, and solvents. As increasingly electropositive
metals are fashioned into nanoparticles, stronger reducing agents
are required. Not only are many of these reducing agents spontane-
ously flammable in air but their synthesis is energy intensive. Thus,
“greener” methods of nanoparticle synthesis are needed.
Producing monodisperse nanoparticles of electropositive metals
presents its own challenge. Because strong reducing agents must
be employed, reduction of the metal salt occurs before the reducing
agent can be distributed homogeneously through solution. Local
variations in the rates of nucleation and growth result in polydisperse
particles. To address this problem, strong reducing agents with slow
kinetics are needed to enable homogeneous nucleation and growth.
Such a class of slow but strong reducing agents may provide
sufficient time for ligands to cap the growing nanoparticles, thereby
allowing the relative strength of the ligand-nanoparticle interaction
to slow6 or stop7 further growth. In this way, the production of
monodisperse electropositive metal nanoparticles might be possible.
Biological systems provide a clue for solving this problem:
peptides use the thiol/disulfide redox couple for controlling protein
structure and function.8 Thiols are remarkably strong reducing
agentssdepending on their structure, potentials as negative as -0.38
V vs SHE have been measured.9 Compared to other reducing agents,
thiols are moderately air-stable, safe, and inexpensive ($3.68/mole
for dodecanethiol versus $9.36/mole for NaBH4; see Supporting
Information). Finally, because disulfides form monolayers on
nanoparticle surfaces,10 the thiol/disulfide can act as both the
reducing agent and the stabilizing ligand.
photochemical polythiol process to synthesize bismuth, copper, lead,
and antimony nanoparticles (see Supporting Information). We focus
on the room-temperature synthesis of rhombohedral bismuth
nanoparticles with a narrow size distribution. Although bismuth
nanoparticles have been previously described, the synthesis of
monodisperse, homogeneous particles remains a challenge.17-19
We used a bismuth(III) carboxylate such as acetate, 2-ethylhex-
anoate, or oleate as our bismuth source. In a typical synthesis, a
10 wt% solution of bismuth oleate in dry THF was prepared in a
glass flask under N2 (Figure 1A). Dodecanethiol was added to the
bismuth oleate solution in a 3:1 molar ratio. The solution im-
mediately turned yellow (ꢀ ) 2300 mol-1 cm-1 at 360 nm),
indicating the formation of bismuth thiolate and oleic acid (Figures
1B, 3A, and UV-vis spectra, Supporting Information). Exposure
to ambient light in our laboratory decomposed the bismuth thiolate,
turning the solution black (Figure 1C). Continued irradiation at room
temperature for 24 h (Figure 1D) or longer produced bismuth
nanoparticles in high yield. Near-quantitative conversion to bismuth
nanoparticles was achieved after 2 weeks. The solution was not
stirred during the synthesis, allowing the nanoparticles to precipitate
as colloidal crystals. This accelerated the rate of nanoparticle
formation by minimizing photon absorption by the black bismuth
nanoparticles. At the end of the synthesis, the solution was exposed
to air and transferred to a centrifuge tube. The sample was
centrifuged for 2 min at 9000 rpm. The colorless supernatant was
removed, and the black solid was resuspended in THF by shaking
the centrifuge tube. TEM grids were prepared from this solution,
which contained a small amount of residual ligands. Centrifugation
was repeated, and the black solid was dried at room temperature
under vacuum to determine yield. Upon complete conversion, only
10 g of solvent was needed to produce 0.25 g of nanoparticles
(78 wt% Bi). We achieved similar results when we applied the
same protocol to other bismuth carboxylates.
A challenge with thiols results from their ability to form stable
molecular compounds with many metal ions11sthe resulting metal
thiolate does not decompose further. To overcome this limitation,
elevated temperature reduction/decomposition of Ag,12 Bi,13 and
Te14 has been employed to produce micron-sized aggregates or
nanometer-thick platelets, sheets, or wires. Due to the ease with
which metal sulfides are formed, however, thermal decomposition
may not be broadly applicable.15
Examination of the product by TEM reveals uniform rhombo-
hedral nanoparticles (Figure 2A). Bismuth nanoparticles produced
in this process are 9.2 ( 0.7 nm across (measured as the distance
perpendicular to opposite faces). This degree of uniformity allows
the nanoparticles to self-assemble into a superlattice that adopts
the same symmetry as bismuth’s rhombohedral atomic lattice
(Figure 2C). Even after air exposure for several weeks, electron
diffraction and HRTEM (Figure 2B,D) indicate the presence of
crystalline bismuth. A 1-2 nm amorphous coating on the nano-
particles was observed, suggesting the formation of an oxide layer.
Here, we introduce the photochemical polythiol process (Scheme
1). In this process, the thiol reacts with a metal salt, generating a
metal thiolate. This distributes the reducing agent homogeneously.
Visible light then initiates a ligand-to-metal charge transfer (LMCT)
in the metal thiolate, reducing the metal cation. This process was
motivated by a UV-based decomposition of tin(II) dimethylamine
to tin nanoparticles by Chaudret et al.16 Here, we used the
† Department of Chemistry & Chemical Biology.
‡ Department of Materials Science & Engineering.
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10.1021/ja0733639 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
from a doublet of triplets at 2.51 ppm (Figure 3A) to a triplet at
3.73 ppm (Figure 3B) upon formation of bismuth thiolate. After
exposure to 365 nm light for 4 days at a power of 7 mW/cm2, a
decrease in the triplet at 3.73 ppm and growth of the triplet at 2.66
ppm (Figure 3C) indicated disulfide formation. Under these
synthesis conditions, we achieved a 75% yield of nanoparticles after
4 days. As a control, an NMR tube of identically prepared bismuth
dodecanethiolate kept in the dark for 4 days had a spectrum identical
to that of Figure 3B, showing that no decomposition had occurred.
In summary, absorption of a photon initiates decomposition of
a metal thiolate by a LMCT process. Because the reduction potential
of the metals is more positive than that of the thiol, the resulting
metal nanoparticles are not oxidized by the disulfide (see Figure
S2, Supporting Information). The ability to reduce lead, which has
a reduction potential of -0.125 V in acidic solution, demonstrates
the possibility of reducing modestly electropositive elements. The
photochemical polythiol process decomposes the metal thiolate in
a slow, homogeneous process, affording uniform nanoparticles while
using relatively small volumes of solvent.
Figure 1. Photographs of nanoparticle synthesis. To a 10 wt% solution of
bismuth oleate in THF (A) was added dodecanethiol, generating the yellow
bismuth thiolate (B). The black color of nanoparticles is apparent after 2 h
(C) and after 1 day (D) of exposure to ambient light.
The environmental benefits of this process can be extended
through the use of sunlight to initiate the decomposition. Further
experiments will examine the possibility of applying the photo-
chemical polythiol process to other metals.
Acknowledgment. This work was supported by grants from
the DOE (DE-FG02 87ER45298) and the NSF (DMR 0520404),
through the Cornell Center for Materials Research. S.C.W. ac-
knowledges support from the EPA’s STAR fellowship program.
Figure 2. Superlattice of rhombohedral bismuth nanoparticles collected
after 6 days of exposure to light (A) and corresponding electron diffraction
in which red lines indicate the most intense expected reflections of bismuth
(B). The unusual crystal structure of bismuth (C) can be observed in HRTEM
(D).
Supporting Information Available: Experimental procedures,
characterization of new compounds, and discussion. This material is
available free of charge via the Internet at http://pubs.acs.org.
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Figure 3. 1H NMR of dodecanethiol (A), bismuth dodecanethiolate
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