Size- and shape-selective synthesis of metal nanocrystals and nanowires using CO as a reducing agent

Article abstract of DOI:10.1002/anie.201003383

Putting the pedal to the metal: A facile strategy for the synthesis of metal nanocrystals is demonstrated that employs carbon monoxide as a reducing agent. Highly monodisperse platinum nanocubes, spherical palladium nanocrystals, and ultrathin gold nanowires can be produced within 15 minutes.

Full text of DOI:10.1002/anie.201003383

Communications
DOI: 10.1002/anie.201003383
Metal Nanocrystals
Size- and Shape-Selective Synthesis of Metal Nanocrystals and
Nanowires Using CO as a Reducing Agent**
Yijin Kang, Xingchen Ye, and Christopher B. Murray*
Colloidal metal nanocrystals (NCs) have been extensively
studied because of their various applications in catalysis,[1,2]
sensors,[3,4] surface-enhanced spectroscopy,[5] and biological
imaging.[6] A variety of wet chemical approaches have been
developed to prepare both monodisperse spherical[7,8] and
nonspherical[9–14] metal NCs; the latter often exhibit proper-
ties unobtainable simply by tuning the size of the spheres. The
synthesis of metal NCs typically employs the reduction or
decomposition of metal precursors in the presence of ligands,
which prevent aggregation and improve the colloidal stability
of the NCs. Among the wide spectrum of reducing agents that
have been used, gases such as hydrogen under pressure have
proven effective in delicately manipulating the growth
kinetics and thus tailoring the size and morphology of the
metal NCs.[15,16] Despite these efforts, a one-pot synthesis of
highly monodisperse metal NCs at ambient pressure using
gaseous reducing agents generated at point-of-use is still an
important advance.
ure S1) as a mild reducing agent. Pt nanocubes with straight
edges (edge length = 8.9 nm, s = 6%) and sharp corners are
produced after 15 min of reaction at 2008C (Figure 1). The X-
Herein we report the size- and shape-selective formation
of metal nanostructures including Pt nanocubes, Pd spherical
NCs, and Au nanowires (NWs) using carbon monoxide (CO,
generated at point-of-use) as a reducing agent. We also
discuss the implications of our observation on several recent
reports of the preparation of PtNCs utilizing metal carbonyls.
In catalysis, it is well-known that particle shape (the facets
exposed) can be as important as the particle surface area in
activity and selectivity. For example, Pt(100) exhibits higher
electrocatalytic activity than Pt(111) for the oxygen reduction
reaction in H2SO4 electrolyte.[17,18] Pt(100) also shows differ-
ent selectivity from Pt(111) towards hydrogenation reac-
tions.[19] Thus Pt nanocubes with well-defined {100} facets
provide a model system for understanding microscopic sur-
face phenomena in many catalytic processes. We report the
synthesis of Pt nanocubes employing CO (generated by
dehydration of formic acid; Supporting Information, Fig-
Figure 1. a,c) TEM images, b) HRTEM image, and d) XRD pattern of
Pt nanocubes synthesized using gaseous CO. Inset in (a): fast-Fourier-
transform pattern of the self-assembled Pt nanocube superlattices with
simple cubic symmetry. Scale bars: a) 100 nm, b) 2 nm, c) 30 nm.
ray diffraction (XRD) pattern (Figure 1d) of the as-synthe-
sized Pt nanocubes confirms the face-centered cubic (fcc)
crystal structure. High-resolution TEM (HRTEM) image of a
single Pt nanocube reveals an interplanar distance of about
0.20 nm, which is consistent with the lattice spacing of the
{100} planes of the fcc platinum structure (Figure 1b).
Furthermore, study of the shape evolution (Figure 2) reveals
that NCs formed at the early stage (2–4 min) of the reaction
typically adopt a polyhedral shape. As the reaction proceeds,
the particles transform to cubes. The reaction exhibits a
minimum size- and shape dispersity at about 15 min, after
which the size distribution broadens and other shapes (rods,
truncated cubes, and polyhedra) start to appear. To confirm
the role of CO as a reducing agent, the following control
experiments were performed: the reaction mixture was
bubbled with CO for 10 min at room temperature and then
purged with N2 for 20 min. After that, the solution was
brought to 2008C and kept at this temperature for 15 min.
This process only produces large aggregates of irregular
shaped NCs (Supporting Information, Figure S2c), similar to
the products synthesized in the absence of CO (Supporting
Information, Figure S2a,b). This observation implies that in
the present case, CO neither serves as ligand nor forms
complex with platinum, which is different in mechanism from
[*] Y. Kang,[+] X. Ye,[+] Prof. C. B. Murray
Department of Chemistry, University of Pennsylvania
Philadelphia, PA 19104 (USA)
E-mail: cbmurray@sas.upenn.edu
Prof. C. B. Murray
Department of Materials Science and Engineering
University of Pennsylvania (USA)
[+] These authors contributed equally to this work.
[**] The synthesis of PtMn nanocrystal magnets was developed with
partial support from the Army Research Office through MURI Award
W911NF-08-1-0364 (Y.K.), while studies of plasmonic gold nano-
wires were funded by the Department of Energy’s Division of Basic
Energy Sciences through Award: DE-SC0002158 (X.Y. & C.B.M.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003383.
6156
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 6156 –6159

Angewandte
Chemie
PdNCs as primary catalysts or a co-catalyst for fuel-cell
applications, with enhanced performance and greater stability
compared to pure Pt catalysts.[1,30–32] Despite the recent
advances in the shape-controlled synthesis of Pd nanostruc-
tures,[33] one-pot synthesis of monodisperse spherical PdNCs
has been limited. Importantly, previous studies showed that
trioctylphosphine (TOP) can serve as an effective ligand in
the synthesis of monodisperse PdNCs.[34] Herein we demon-
strate that by using the Pd–TOP precursor, monodisperse
PdNCs can be produced under a CO blanket within 10 min at
1958C. TEM images (Figure 3a,b) show that 5.8 nm PdNCs
Figure 2. TEM images taken at different stages of Pt nanocube
syntheses using CO. a) 2 min, b) 4 min, c) 6 min, d) 8 min, e) 10 min,
f) 15 min, g) 20 min, h) 30 min, i) 90 min. Scale bars: 20 nm.
previous studies using [Pt(dba)2] precursors (dba = dibenz-
ylideneacetone).[20,21] Furthermore, the FTIR spectra (Sup-
porting Information, Figure S3) of the Pt nanocubes show no
peaks in the region of 1800–2300 cmÀ1, suggesting that there is
no Pt–CO species in the products.
Figure 3. TEM images of a,b) 5.8 nm and c) 5.0 nm PdNCs. d)
HRTEM image of the 5.8 nm PdNCs. Scale bars: a,c) 50 nm, b)
20 nm, d) 5 nm.
Recently we synthesized MnPt nanocubes with a compo-
sition of Mn/Pt ꢀ 1:3.[22] However, by reducing the amount of
[Mn2(CO)10], uniform Pt nanocubes (> 99.0% Pt content by
elemental analyses) can be produced rather than the MnPt
nanocubes (Supporting Information, Figure S5). Sun et al.
have reported the synthesis of Pt nanocubes by using
[Fe(CO)5], demonstrating that the presence of [Fe(CO)5]
facilitates the nucleation and controls the growth of Pt
nanocubes.[23] Several subsequent reports also focus on the
synthesis of Pt nanocubes employing metal carbonyls.[24,25]
Some researchers claim that the metal (W, Co) decomposed
from carbonyl compounds plays the key role (serving as the
nucleation center or as the in-situ-generated reducing agent)
during the synthesis of Pt nanocubes. However, based on our
observations, the presence of neither foreign metals (W, Co)
nor metal carbonyls are necessary for the growth of uniform
Pt nanocubes. Furthermore, metal carbonyls can introduce
impurities either as alloy incorporated into Pt or as adatoms
on Pt surfaces, which could result in surface properties distinct
from those of “clean” Pt NCs.[26] In contrast, CO-assisted
synthesis produces Pt nanocubes with high chemical purity,
crystallinity, and shape uniformity.
self-assemble into close-packed superlattices. A HRTEM
image of the 5.8 nm PdNCs reveals that most NCs are
twinned, which is commonly seen in small metal NCs
(Figure 3d). Transmission small-angle X-ray scattering
(TSAXS) pattern (Supporting Information, Figure S11) of
the 5.8 nm PdNCs is fitted to the spherical model with a
diameter of 6.05 nm and a size distribution of about 9%,
which is consistent with the TEM and HRTEM observations.
The powder XRD pattern confirms the fcc crystal structure
(Supporting Information, Figure S6). By changing the reac-
tion medium from a mixture of oleic acid and oleylamine to
trioctylamine, 5.0 nm PdNCs are produced (Figure 3c).
Recognizing the potential of CO to accelerate reaction
rates, we target the use of CO to the synthesis of ultrathin
AuNWs. Several groups have developed colloidal methods to
synthesize uniform AuNWs. However, these recipes usually
require hours or even days.[35–38] As a final example, we
demonstrate that the ultrathin AuNWs can be synthesized
within 10 min by reducing AuCl–oleylamine complexes with
CO. The AuNWs form within 2 min after introducing CO,
growing longer as the reaction proceeds (Supporting Infor-
mation, Figure S14). TEM images of different magnifications
show that the AuNWs obtained tend to assemble into bundles
(Figure 4; Supporting Information, Figures S12, S13). Indi-
vidual AuNWs typically terminates with enlarged spherical
ends (Figure 4c). Similar wires have been observed and
proposed to result from oriented attachment during the
growth.[39] The AuNWs are of about 2.5 nm in width and of
In recognition of the fact that foreign metals (Fe, W, Co,
etc.) are not essential for the formation of well-faceted metal
NCs, we extended the synthetic use of CO as a reducing agent
to other metal NCs. Palladium is widely used as catalysts in
catalytic converters[27] and in a range of organic reactions,
including cross-coupling reactions[28] and hydrogenation.[29]
Several research groups have also demonstrated the use of
Angew. Chem. Int. Ed. 2010, 49, 6156 –6159
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6157

Communications
and was diluted with hexane. The NWs were precipitated by
centrifugation at 6000 rpm.
Characterization: Transmission electron microscopy (TEM) was
carried out with a JEOL1400 operating at 120 kV. High-resolution
TEM (HRTEM) images and energy dispersive X-ray spectra (EDX)
were obtained with a JEOL2010F (200 kV). X-ray diffraction (XRD)
and transmission small-angle X-ray scattering (TSAXS) patterns
were obtained on Rigaku Smartlab diffractometer with CuKa
radiation (l = 1.5418 ꢀ). For TSAXS measurement, a hexane solu-
tion of PdNCs was loaded into a capillary tube. The TSAXS pattern
was fitted using NANO-Solver (Rigaku software). Quantitative
elemental analyses were carried out with both EDX and inductively
coupled plasma optical emission spectrometry (ICP-OES) on SPEC-
TRO GENESIS ICP spectrometer.
Received: June 3, 2010
Figure 4. TEM images of ultrathin Au nanowires. Inset in (d): photo-
Published online: July 26, 2010
graph of Au nanowires dispersed in hexane. Scale bars: a) 5 mm,
b) 0.5 mm, c) 100 nm, d) 20 nm.
Keywords: carbon monoxide · gold · nanocrystals · palladium ·
platinum
.
several micrometers in length (Figure 4d). Furthermore,
ultrathin AuNWs can also be produced with AuCl3, albeit
in lower yield (Supporting Information, Figure S17). These
one-dimensional metal nanostructures are promising building
blocks for plasmonic waveguides[40] and nano-electronic
devices.[41]
In summary, we have developed a general approach for
the shape-selective synthesis of different metal NCs and
AuNWs with a narrow size distribution. The use of CO was
found to play an important role in controlling the reaction
kinetics and the morphology of the metal nanostructures.
These synthetic routes are being extended to the design of
other colloidal metal and metal alloy NCs.
[1] B. Lim, M. J. Jiang, P. H. C. Camargo, E. C. Cho, J. Tao, X. M.
Lu, Y. M. Zhu, Y. N. Xia, Science 2009, 324, 1302.
[2] S. E. Habas, H. Lee, V. Radmilovic, G. A. Somorjai, P. Yang, Nat.
Mater. 2007, 6, 692.
[3] S. J. Park, T. A. Taton, C. A. Mirkin, Science 2002, 295, 1503.
[4] A. J. Haes, R. P. Van Duyne, J. Am. Chem. Soc. 2002, 124, 10596.
[5] A. Tao, F. Kim, C. Hess, J. Goldberger, R. R. He, Y. G. Sun, Y. N.
Xia, P. D. Yang, Nano Lett. 2003, 3, 1229.
[6] M. S. Yavuz, Y. Y. Cheng, J. Y. Chen, C. M. Cobley, Q. Zhang, M.
Rycenga, J. W. Xie, C. Kim, K. H. Song, A. G. Schwartz, L. H. V.
Wang, Y. N. Xia, Nat. Mater. 2009, 8, 935.
[7] S. Stoeva, K. J. Klabunde, C. M. Sorensen, I. Dragieva, J. Am.
Chem. Soc. 2002, 124, 2305.
[8] N. F. Zheng, G. D. Stucky, J. Am. Chem. Soc. 2006, 128, 14278.
[9] Y. G. Sun, Y. N. Xia, Science 2002, 298, 2176.
[10] R. C. Jin, Y. W. Cao, C. A. Mirkin, K. L. Kelly, G. C. Schatz, J. G.
Zheng, Science 2001, 294, 1901.
[11] N. R. Jana, L. Gearheart, C. J. Murphy, J. Phys. Chem. B 2001,
105, 4065.
[12] C. J. Murphy, T. K. Sau, A. M. Gole, C. J. Orendorff, J. X. Gao,
L. Gou, S. E. Hunyadi, T. Li, J. Phys. Chem. B 2005, 109, 13857.
[13] J. Perez-Juste, I. Pastoriza-Santos, L. M. Liz-Marzan, P. Mulva-
ney, Coord. Chem. Rev. 2005, 249, 1870.
[14] T. K. Sau, A. L. Rogach, Adv. Mater. 2010, 22, 1781.
[15] F. Dumestre, B. Chaudret, C. Amiens, P. Renaud, P. Fejes,
Science 2004, 303, 821.
[16] J. T. Ren, R. D. Tilley, J. Am. Chem. Soc. 2007, 129, 3287.
[17] N. M. Markovic, H. A. Gasteiger, P. N. Ross, J. Phys. Chem.
1995, 99, 3411.
Experimental Section
Gaseous CO was generated at point-of-use by dehydration of formic
acid. Briefly, formic acid was delivered into warm concentrated
sulfuric acid (608C) at a constant rate using syringe pump (Caution:
CO is toxic and all operations should be carried out inside a well-
ventilated hood!). All syntheses were performed using standard
Schlenk techniques.
Synthesis of Pt nanocubes: [Pt(acac)2] (0.04 g) was dissolved in
benzyl ether (5 mL), oleylamine (3.68 mL), and oleic acid (0.63 mL)
under an N2 atmosphere. Gaseous CO was then introduced to the
solution (ca. 10 cm3 minÀ1) at room temperature. The reaction flask
was then placed into a preheated oil bath. After 15 min of reaction at
2008C, the solution was allowed to cool and the products were
isolated by adding ethanol and centrifugation. The NCs were then
redispersed in hexane.
[18] C. Wang, H. Daimon, T. Onodera, T. Koda, S. H. Sun, Angew.
Chem. 2008, 120, 3644; Angew. Chem. Int. Ed. 2008, 47, 3588.
[19] K. M. Bratlie, H. Lee, K. Komvopoulos, P. D. Yang, G. A.
Somorjai, Nano Lett. 2007, 7, 3097.
Synthesis of Pd spherical NCs: The Pd–TOP complex was
prepared by dissolving [Pd(acac)2] (0.05 g) in TOP (0.6 mL). The
resulting precursor was mixed with TOP (0.15 mL), oleic acid (2 mL),
and oleylamine (13 mL) at room temperature. Under a constant
heating rate of 158CminÀ1, the solution was brought to 1958C, at
which point CO was introduced to the reaction flask (ca.
15 cm3 minÀ1). The reaction was conducted for 10 min and the
products were purified as described above.
Synthesis of AuNWs: AuCl (0.046 g) was dissolved in chloroform
(20 mL) and oleylamine (1.84 mL). The mixture was then heated to
608C and CO was introduced to the reaction flask (ca. 15 cm3 minÀ1).
After 10 min, the solution was allowed to cool to room temperature
[20] A. Rodriguez, C. Amiens, B. Chaudret, M. J. Casanove, P.
Lecante, J. S. Bradley, Chem. Mater. 1996, 8, 1978.
[21] C. Amiens, D. Decaro, B. Chaudret, J. S. Bradley, R. Mazel, C.
Roucau, J. Am. Chem. Soc. 1993, 115, 11638.
[22] Y. J. Kang, C. B. Murray, J. Am. Chem. Soc. 2010, 132, 7568.
[23] C. Wang, H. Daimon, Y. Lee, J. Kim, S. Sun, J. Am. Chem. Soc.
2007, 129, 6974.
[24] J. Zhang, J. Y. Fang, J. Am. Chem. Soc. 2009, 131, 18543.
[25] S. I. Lim, I. Ojea-Jimenez, M. Varon, E. Casals, J. Arbiol, V.
Puntes, Nano Lett. 2010, 10, 964.
[26] M. E. Grass, Y. Yue, S. E. Habas, R. M. Rioux, C. I. Teall, P.
Yang, G. A. Somorjai, J. Phys. Chem. C 2008, 112, 4797.
6158
www.angewandte.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 6156 –6159

Angewandte
Chemie
[27] Y. Nishihata, J. Mizuki, T. Akao, H. Tanaka, M. Uenishi, M.
Kimura, T. Okamoto, N. Hamada, Nature 2002, 418, 164.
[28] S. W. Kim, M. Kim, W. Y. Lee, T. Hyeon, J. Am. Chem. Soc. 2002,
124, 7642.
[29] M. Q. Zhao, R. M. Crooks, Angew. Chem. 1999, 111, 375;
Angew. Chem. Int. Ed. 1999, 38, 364.
[35] Z. Y. Huo, C. K. Tsung, W. Y. Huang, X. F. Zhang, P. D. Yang,
Nano Lett. 2008, 8, 2041.
[36] X. M. Lu, M. S. Yavuz, H. Y. Tuan, B. A. Korgel, Y. N. Xia,
J. Am. Chem. Soc. 2008, 130, 8900.
[37] C. Wang, Y. J. Hu, C. M. Lieber, S. H. Sun, J. Am. Chem. Soc.
2008, 130, 8902.
[38] N. Pazos-Perez, D. Baranov, S. Irsen, M. Hilgendorff, L. M. Liz-
Marzan, M. Giersig, Langmuir 2008, 24, 9855.
[39] H. J. Feng, Y. M. Yang, Y. M. You, G. P. Li, J. Guo, T. Yu, Z. X.
Shen, T. Wu, B. G. Xing, Chem. Commun. 2009, 1984.
[40] A. L. Pyayt, B. Wiley, Y. N. Xia, A. Chen, L. Dalton, Nat.
Nanotechnol. 2008, 3, 660.
[30] H. J. Lee, S. E. Habas, G. A. Somorjai, P. D. Yang, J. Am. Chem.
Soc. 2008, 130, 5406.
[31] Z. M. Peng, H. Yang, J. Am. Chem. Soc. 2009, 131, 7542.
[32] C. Bianchini, P. K. Shen, Chem. Rev. 2009, 109, 4183.
[33] Y. J. Xiong, Y. N. Xia, Adv. Mater. 2007, 19, 3385.
[34] S. W. Kim, J. Park, Y. Jang, Y. Chung, S. Hwang, T. Hyeon, Y. W.
Kim, Nano Lett. 2003, 3, 1289.
[41] Y. Lu, J. Y. Huang, C. Wang, S. Sun, J. Lou, Nat. Nanotechnol.
2010, 5, 218.
Angew. Chem. Int. Ed. 2010, 49, 6156 –6159
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6159