Hybrid Co-Au nanorods: Controlling Au nucleation and location

Article abstract of DOI:10.1002/anie.200702017

Au Co-ntraire: The growth of gold nanoparticles on magnetic cobalt nanorods could be initiated either selectively on the tips of the nanorods (see picture, left) or nonselectively over the whole nanorod surface (right) by the proper choice of ligand conce

Full text of DOI:10.1002/anie.200702017

Angewandte
Chemie
DOI: 10.1002/anie.200702017
Hybrid Nanorods
Hybrid Co–Au Nanorods: Controlling Au Nucleation and Location**
Fabienne Wetz, Katerina Soulantica,* Andrea Falqui, Marc Respaud, Etienne Snoeck, and
Bruno Chaudret*
The synthesis of complex nanoobjects containing different
materials in the same nanoparticle may offer many oppor-
tunities for applications. A new generation of hybrid nano-
crystals accommodating two or more different materials on
the same particle has recently appeared in the literature. As
well as core–shell nanocrystals, the multifunctional nano-
structures so far achieved by wet chemical routes include
this selectivity, it is possible to play on the kinetics of the
reaction: temperature and nature of the reducing agents, on
the choice of the Au precursor,[9] and on the surface chemistry
of the cobalt nanorods. Thus the concentration and nature of
the surface ligands—specifically their affinity for selected
surfaces[10] and their dynamics on these surfaces[11]—may
allow the passivation, or lack thereof, of selected faces.
Functionalization of metal-tipped Co nanorods could facili-
tate their attachment and organization on flat substrates or
their use in biological and medical applications,[5b,12] whereas
selective growth of a noble metal layer on the side of magnetic
nanorods could allow the passivation of the particles.
Following our interest for elucidating the role of surface
organometallic chemistry in nanoparticle synthesis and reac-
tivity,[13] we show herein that a judicious choice of metal
precursor, ligand concentration, and ligand mobility may
orientate the growth of gold nanoparticles, either selectively
on the tips of Co nanorods or on the whole body of the
nanorods.
Co nanorods can be synthesized by decomposition of an
organometallic compound under hydrogen in the presence of
a long-chain amine and a long-chain acid.[14] In the present
work the nanorods were synthesized by decomposition of
[Co{N(SiMe3)2}2][15a] under H2 in the presence of lauric acid
(LA) and hexadecylamine (HDA) (see the Supporting
Information).[16] Two gold precursors were selected:
[AuClPPh3],[15b] and [AuCl(tht)] (tht = tetrahydrothiophe-
ne).[15c] [AuCl(tht)] is a reactive precursor that easily loses
tht and is spontaneously reduced in toluene,[17] whereas
[AuClPPh3] is stable and does not form any nanoparticles in
toluene in the absence of Co nanorods, either alone or in the
presence of ligands (HDA, LA, or both), as verified by blank
experiments.
semiconductor/semiconductor,[1]
metal/semiconductor,[2]
metal or metal oxide/magnetic oxide,[3] semiconductor/
oxide,[4] and metal/metal[3c,5] interfaces. Anisotropically
phase-segregated as well as ternary hybrid nanocrystals
have also been reported.[6] A recent review on heterostruc-
tured nanocrystals summarizes the progress and the perspec-
tives on this emerging field.[7] The preferred positioning of the
second material on the tips of semiconductor nanorods has
been attributed to several factors: surface defects, difference
in the passivation degree between chemically different facets
of the same nanocrystal, lattice mismatch, and the minimiza-
tion of interfacial energies between the different materi-
als.[2,3d] Nanostructures with metal–metal interfaces including
a metal nanorod have not yet been reported. However these
materials, for example, gold-tipped magnetic nanorods, could
be useful for implementation in high-density magnetic
recording devices, as such species display a high magnetic
anisotropy and are ferromagnetic at room temperature.
The selective growth of Au on the Co nanorods requires
the promotion of a) the deposition of Au on Co over galvanic
displacement of Co by Au[8] and b) heterogeneous growth
over homogeneous nucleation. As homogeneous nucleation is
energetically more demanding than growth on preformed
nanocrystals, it is in principle possible to find conditions that
allow the selective growth of Au on Co nanorods. To influence
The reaction of [AuCl(tht)] with a suspension of Co
nanorods (55% Co content) in toluene at 208C leads to
dissolution of the rods after 30 minutes (Figure 1). High-
resolution electron microscopy (HREM) analysis shows the
almost complete destruction of the cobalt structure, in
agreement with galvanic displacement of Co by Au. In the
presence of HDA, [AuCl(tht)] is transformed into [AuCl-
(NH2R)], which is slowly decomposed in solution.[17] How-
ever, this reaction is slower than the galvanic displacement
and no homogeneous nucleation was observed under the
conditions employed. Therefore, [AuCl(tht)] may displace the
HDA from the surface of the nanorods, facilitating the access
of Au for the galvanic reaction.
[*] Dr. F. Wetz, Dr. B. Chaudret
Laboratoire de Chimie de Coordination-CNRS
205 route de Narbonne, 31077 Toulouse (France)
Fax: (+33)5-6133
E-mail: chaudret@lcc-toulouse.fr
Dr. K. Soulantica, Dr. A. Falqui, Dr. M. Respaud
Laboratoire de Physique et de Chimie des Nanoobjets-INSA
135, avenue de Rangueil, 31077 Toulouse (France)
Fax: (+33)5-6155
E-mail: ksoulant@insa-toulouse.fr
Dr. E. Snoeck
Centre d’Elaboration de MatØriaux et d’Etudes Structurales
29 rue Jeanne Marvig, B.P. 94347, 31055 Toulouse (France)
[**] The authors thank the European project SA-NANO (Contract no.
STRP 013698) for financial support and TEMSCAN service for TEM.
The same reaction using [AuCl(PPh3)] as a precursor
results in the slow and specific growth of Au nanoparticles on
the Co nanorods (Figure 2). The Au particles are identified by
their increased contrast. Furthermore, a selected-area elec-
Supporting information (experimental details) for this article is
available on the WWW under http://www.angewandte.org or from
the author.
Angew. Chem. Int. Ed. 2007, 46, 7079 –7081
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7079

Communications
described above, whereas in the presence of extra ligands
(1equiv of amine, acid, or both with respect to Co) the growth
of Au happened exclusively on the rod tips, thus indicating
that full coverage of the nanorods sides prevents gold
nucleation (see the Supporting Information). No trace of
gold was found on the main body of Au-tipped nanorods, as
indicated by several energy-dispersive X-ray (EDX) spectra
collected with the electron microscope operating in scanning
(STEM) mode (see the Supporting Information). Finally, we
checked that once deposited the nanoparticles did not move
either from tips to body or from body to tips.
The dimensions of the Co nanorods before and after Au
growth are not modified, indicating a pure heterogeneous
growth mechanism in which Co is not oxidized but serves
strictly as a seed for the Au growth. In our study, no extra
reducing agent was added; the organic stabilizers present on
the nanorods surface serve as mild reductants.[2b,18] This
experiment also reveals the role of PPh3 to stabilize the gold
complex and avoid galvanic reactions. Figure 2a shows a
TEM micrograph of gold-tipped nanorods grown at 408C.
HREM experiments show that, in the case of gold tipped
nanorods, the (111) gold lattice planes (d(hkl) = 2.35 Š) grow
parallel to the direction of the (002) planes (d(hkl) = 2.02 Š)
of the cobalt structure. Only the two expected angles (08 and
70.58) can be measured between the two planes (Figure 3a).
Figure 1. Gold formation around Co nanorods. Galvanic displacement
almost completely destroys the Co structure. [Co]=1.3”10À3 m,
[Au]=1.3”10À4 m, reaction time 30 min. Inset: HREM micrograph, in
which only gold can be seen.
Figure 3. HREM images of: a) gold-tipped nanorods, with the (111)
gold lattice planes parallel to the (002) of the cobalt nanorod (relaxed
growth) and b) gold-decorated nanorod, in which the Au particles
grow epitaxially on the nanorod sides, and the (002) gold lattice
planes are aligned with the (002) of the cobalt.
The large mismatch of the lattice parameters prevents simple
epitaxial growth. Fourier analysis of the HRTEM images
indicates that the lattice parameters progressively relax to the
Au bulk lattice parameters after a few lattice planes from the
Co nanorod. Interestingly, HREM analysis of the gold-
decorated nanorods showed that the Au nanoparticles have
grown epitaxially on the lateral sides of the Co nanorods. The
gold (002) planes (d(hkl) = 2.04 Š) grew on those (002) (d =
2.02 Š) of the cobalt nanorods (Figure 3b).
In a hypothetical situation in which nanorods are com-
pletely free of stabilizers, we would expect that epitaxial
growth would be thermodynamically favored over con-
strained growth. In that case, increasing the temperature
would tend to favor the “thermodynamic product”, that is, the
preferential deposition of Au on the lateral sides, for which
the lattice mismatch is minimum.[19] In the other extreme case,
namely in the presence of a large excess of ligands (Co
Figure 2. TEM images of a) gold-tipped Co nanorods prepared at 408C
and b) gold-decorated Co nanorods prepared at 208C. For both,
[Co]=1.3”10À3 m, [Au]=1.34”10À4 m, reaction time 48 h.
tron diffraction pattern (SAEDP) shows that well crystallized
gold is present on the nanoparticles (see the Supporting
Information). The reaction is temperature-dependent, as Au
nanoparticles are deposited selectively on the tips of the
nanorods after 2 days at 408C (Figure 2a), whereas at 208C
nonselective deposition of Au particles over the whole
nanorod surface is observed (Figure 2b). The location of the
nanoparticles is also dependent upon the ligand concentra-
tion. Thus, in another set of experiments but using a higher
Co/Au ratio, the same reaction was carried out at 208C with
nanorods of 65% Co content. In the absence of additional
ligand, we observed a nonselective deposition of gold as
7080 www.angewandte.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 7079 –7081

Angewandte
Chemie
Porter, J. P. Zimmer, M. G. Bawendi, J. Am. Chem. Soc. 2006,
128, 12590 – 12591.
content 10%), we checked that the growth of Au particles is
almost completely suppressed. For intermediate situations
but at high ligand concentration, we only observe the growth
of gold on the cobalt tips, in agreement with the hypothesis
that the stabilizing ligands are coordinated preferentially on
the sides of the nanorods. The temperature effects observed at
low ligand concentration (Figure 2) are, however, more
difficult to understand, since by increasing the temperature
the tip position (constrained growth) is favored. At low
temperature, the results can be explained if we consider that
the attack of [AuClPPh3] happens on ligand deficient sites on
the nanorods surface. In such samples, ligand-depleted sites
are found both on tips and on the lateral sides. At higher
temperature and for the lateral sides of the nanorods, fast
exchange of ligands[11] would not permit enough resting time
for the nucleation of Au to take place. This fast exchange of
the ligands on the nanorods sides has no influence on the
nucleation of Au on the tips since these areas are expected to
accommodate a lower concentration of ligands. Another
indication of the importance of the timescale for the
formation of the primary nuclei is that practically no growth
of Au takes place if the reaction mixture is stirred. In an
analogous way to crystal formation, stirring suppresses the
formation of the primary nuclei for further crystal growth.
In conclusion, we report the synthesis of hybrid gold–
cobalt nanorods. The present results support the hypothesis
that the stabilizing ligands are coordinated preferentially on
the sides of the nanorods and that a fast exchange occurs on
the surface of the rods which involves the stabilizing ligands
and also probably PPh3. Moreover, we show that chemical
control of both the gold precursors and the ligands allow
1) control of the gold growth process (heterogeneous nucle-
ation versus galvanic displacement) and 2) control of the
location of the gold nanoparticle growth (tip or whole body)
through kinetic control of the reaction. This result makes
possible the selective production of gold-tipped ferromag-
netic cobalt nanorods. We are currently studying the physical
properties of these new nanoobjects: preliminary optical
measurements show in both cases (gold nanoparticles random
or gold nanoparticles on the tips) the presence of absorptions
near 550 nm attributed to a plasmon absorption as well as a
very weak one near 720 nm,[20] whereas magnetic properties
of the new Au–Co nanorods are currently under examination
to investigate the effect of Au presence and location on the
magnetic anisotropy of these objects.
[2] a) C. Pacholski, A. Kornowski, H. Weller, Angew. Chem. 2004,
116, 4878 – 4881; Angew. Chem. Int. Ed. 2004, 43, 4774 – 4777;
b) T. Mokari, E. Rothenberg, I. Popov, R. Costi, U. Banin,
Science 2004, 304, 1787 – 1790; c) T. Mokari, C. G. Sztrum, A.
Salant, E. Rabani, U. Banin, Nat. Mater. 2005, 4, 855 – 863;
d) A. E. Saunders, I. Popov, U. Banin, J. Phys. Chem. B 2006,
110, 25421– 25429; e) J. Yang, H. I. Elim, Q. Zhang, J. Y. Lee, W.
Ji, J. Am. Chem. Soc. 2006, 128, 11921 – 11926; f) S.-H. Choi, E.-
G. Kim, T. Hyeon, J. Am. Chem. Soc. 2006, 128, 2520 – 2521.
[3] a) H. Gu, R. Zheng, X. Zhang, B. Xu, J. Am. Chem. Soc. 2004,
126, 5664 – 5665; b) H. Yu, M. Chen, P. M. Rice, S. X. Wang,
R. L. White, S. Sun, Nano Lett. 2005, 5, 379 – 382; c) H. Gu, Z.
Yang, J. Gao, C. K. Chang, B. Xu, J. Am. Chem. Soc. 2005, 127,
34 – 35; d) R. Buonsanti et al., J. Am. Chem. Soc. 2006, 128,
16953 – 16970; see the Supporting Information.
[4] K.-W. Kwon, M. Shim, J. Am. Chem. Soc. 2005, 127, 10268 –
10275.
[5] a) T. Pellegrino, A. Fiore, E. Carlino, C. Giannini, P. D. Cozzoli,
G. Ciccarella, M. Respaud, L. Palmirotta, R. Cingolani, L.
Manna, J. Am. Chem. Soc. 2006, 128, 6690 – 6698; b) J. Choi, Y.
Jun, S. Yeon, H. C. Kim, J. S. Shin, J. Cheon, J. Am. Chem. Soc.
2006, 128, 15982 – 15983.
[6] a) T. Teranishi, Y. Inoue, M. Nakaya, Y. Oumi, T. Sano, J. Am.
Chem. Soc. 2004, 126, 9914 – 9915; b) T. Teranishi, M. Saruyama,
M. Nakaya, M. Kanehara, Angew. Chem. 2007, 119, 1743 – 1745;
Angew. Chem. Int. Ed. 2007, 46, 1713 – 1715; c) W. Shi, Y. Sahoo,
H. Zeng, Y. Ding, M. T. Swihart, P. N. Prasad, Adv. Mater. 2006,
18, 1889 – 1894; d) W. Shi, H. Zeng, Y. Sahoo, T. Y. Ohulchan-
skyy, Y. Ding, Z. L. Wang, M. Swihart, P. N. Prasad, Nano Lett.
2006, 6, 875 – 881.
[7] P. D. Cozzoli, T. Pellegrino, L. Manna, Chem. Soc. Rev. 2006, 35,
1195 – 1208.
[8] A. M. Schwarztberg, T. Y. Olson, C. E. Talley, J. Z. Zhang, J.
Phys. Chem. B 2006, 110, 19935 – 19944.
[9] D. V. Goia, E. Matijevic, New J. Chem. 1998, 22, 1203 – 1215.
[10] X. Peng, L. Manna, W. Yang, J. Wickham, E. Scher, A.
Kadanavich, A. P. Alivisatos, Nature 2000, 404, 59 – 61.
[11] C. Pan, K. Pelzer, K. Philippot, B. Chaudret, F. Dassenoy, P.
Lecante, M.-J. Casanove, J. Am. Chem. Soc. 2001, 123, 7584 –
7593.
[12] A. Salant, E. Amitay-Sadovsky, U. Banin, J. Am. Chem. Soc.
2006, 128, 10006 – 10007.
[13] B. Chaudret, C. R. Phys. 2005, 6, 117.
[14] a) F. Dumestre, B. Chaudret, C. Amiens, M.-C. Fromen, M.-J.
Casanove, P. Renaud, P. Zurcher, Angew. Chem. 2002, 114,
4462 – 4465; Angew. Chem. Int. Ed. 2002, 41, 4286 – 4289; b) F.
Dumestre, B. Chaudret, C. Amiens, M. Respaud, P. Fejes, P.
Renaud, P. Zurcher, Angew. Chem. 2003, 115, 5371– 5374;
Angew. Chem. Int. Ed. 2003, 42, 5213 – 5216.
[15] a) R. A. Andersen, K. Faegri, J. C. Green, A. Haaland, M. F.
Lappert, W. P. Leung, K. Rypdal, Inorg. Chem. 1988, 27, 1782 –
1786; b) M. I. Bruce, B. K. Nicholson, O. B. Shawkataly, Inorg.
Synth. 1989, 26, 324 – 328; c) R. Uson, A. Laguna, M. Laguna,
Inorg. Synth. 1989, 26, 85 – 91.
Received: May 7, 2007
Published online: August 8, 2007
[16] F. Wetz, K. Soulantica, M. Respaud, A. Falqui, B. Chaudret,
Mater. Sci. Eng. C DOI:10.1016/j.msec,2006.09.010.
[17] S. Gomez, K. Philippot, V. Colliꢀre, B. Chaudret, F. Senocq, P.
Lecante, Chem. Commun. 2000, 1945 – 1946.
[18] M. J. Richardson, J. H. Johnston, T. Borrmann, Cent. Eur. J.
Chem. 2006, 26, 2618 – 2623.
[19] B. Mutaftschiev, The Atomistic Nature of Crystal Growth,
Springer, Heidelberg, 2001, pp. 223 – 248.
[20] J. Pꢁrez-Juste, B. Rodriguez-Gonzalez, P. Mulvaney, L. Liz-
Marzan, Adv. Funct. Mater. 2005, 15, 1065 – 1071.
Keywords: cobalt · gold · heterostructures · magnetic properties ·
nanorods
.
[1] a) D. J. Milliron, S. M. Hughes, Y. Cui, L. Manna, J. Li, L.-W.
Wang, A. P. Alivisatos, Nature 2004, 430, 190 – 195; b) H. Liu,
Nano Lett. 2004, 4, 2379 – 2401; c) S. Kudera, L. Carbone, M. F.
Casula, R. Cingolani, A. Falqui, E. Snoeck, W. J. Parak, L.
Manna, Nano Lett. 2005, 5, 445 – 449; d) J. E. Halpert, V. J.
Angew. Chem. Int. Ed. 2007, 46, 7079 –7081
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7081