A R T I C L E S
Lacroix et al.
amine is added. Using the amido precursor Fe[N(SiMe
3 2 2
) ] (THF)
(
Me ) CH
3
, THF ) tetrahydrofuran) in the presence of oleic
3
a
acid and hexadecylamine (HDA), Dumestre et al. have
prepared 7 nm iron nanocubes organized into superlattices. The
growth process and control of size (in this case limited to 7
nm) and shape (spherical, cubic or anisotropic) were not
achieved on this system, however. We have therefore extended
the study in order to understand and possibly propose a model
for the size and shape control of iron NPs prepared in the
presence of such surfactant mixtures. The route we have chosen
3 2 2 2
is the decomposition of {Fe[N(SiMe ) ] } in the presence of
HDA and palmitic acid (PA) under mild conditions. Several
experimental parameters have been examined and tuned with
the aim of understanding their influence on the mean size and
shape of the final NPs in order to produce high-quality colloidal
samples that could cover the superparamagnetic range of interest.
Supported by a complete M o¨ ssbauer study of the colloid
composition at different reacting stages, this work has lead to
the proposition of an environment-dependent growth mechanism.
Figure 1. TEM pictures of micrometric superstructures observed (a) prior
to reaction under H and (b) after 48 h of reaction.
2
Synthesis of Nanocube Superlattices. A colorless solution of
PA (384 mg, 1.5 mmol, 1.5 equiv per mol of iron) in mesitylene
(
3 2 2 2
10 mL) was added to a green solution of {Fe[N(SiMe ) ] } (376
mg, 0.5 mmol) in mesitylene (5 mL). The mixture was manually
stirred, and its color immediately changed to yellow. After 1 min,
a solution of HDA (482 mg, 2.0 mmol, 2.0 equiv) in mesitylene (5
mL) was added to it. The mixture started to darken and became
black after 10 min of magnetic stirring at room temperature. The
solution was then put under 3 bar of H and allowed to react in an
2
oil bath at 150 °C for 48 h.
The monitoring of this reaction was performed on the same
sample equally distributed into five Fisher-Porter bottles, each of
which was placed in the same oil bath (150 °C) for a specified
time (0.5, 2, 6, 12, or 25 h).
Characterization of the NPs. Microscopy samples were pre-
pared by deposition of a drop of diluted colloidal solution onto a
carbon-coated copper grid and observed on a JEOL 6700F
microscope for scanning electronic microscopy (SEM), a JEOL
Materials and Methods
General Procedures. Mesitylene (Fluka, g99%) was distilled
over sodium according to standard procedures. PA (Sigma, g99%)
and HDA (Fluka, g99%) were used without any additional
purification. Reactants and products were stored and manipulated
in an argon glovebox exclusively. The reactants were mixed together
at room temperature, and all of the syntheses were performed in
Fischer-Porter bottles.
NPs were obtained by decomposition of the iron dimer {Fe[N-
1
2
(
SiMe
3
)
2
]
2
}
2
.
Both kinds of reactions were performed in the
presence of HDA and PA in mesitylene under a reductive
atmosphere of dihydrogen (3 bar). The general conditions were
adjusted to study the reaction kinetics (reaction time varying from
1
011 microscope for bright-field transmission electronic microscopy
3
0 min to 48 h), the influence of temperature (from 100 to 150
(
TEM), or a JEOL-2100F field-emission microscope for high-
°
C), and surfactant concentration effects.
resolution TEM (HRTEM), the latter two working at 100 and 200
kV, respectively. In the majority of cases, size histograms were
1
3
(
8) (a) Finney, E. E.; Finke, R. G. J. Colloid Interface Sci. 2008, 317,
51–374. (b) Dumestre, F.; Chaudret, B.; Amiens, C.; Fromen, M.-
C.; Casanove, M.-J.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed.
obtained by an automatic counting process over 500 particles;
3
on particular samples (stars and large cubes), counting was done
manually for at least 100 nanoparticles. Size distributions were fitted
by the Gaussian law; the results are expressed in terms of the
calculated mean size and the standard deviation (σ). Magnetic
studies were carried out on powder samples by SQuID (Quantum
Design MPMS 5.5), and the iron state and environment were
2
002, 41, 4286–4289. (c) Puntes, V. F.; Zanchet, D.; Erdonnez, C. K.;
Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 12874–12880. (d)
Shevchenko, E. V.; Talapin, D. V.; Schnablegger, H.; Kornowski, A.;
¨
Festin, O.; Svedlindh, P.; Haase, M.; Weller, H. J. Am. Chem. Soc.
003, 125, 9090–9101. (e) Ung, D.; Soumare, Y.; Chakroune, N.; Viau,
G.; Vaulay, M.-J.; Richard, V.; Fi e´ vet, F. Chem. Mater. 2007, 19,
084–2094. (f) Pei, W.; Kakibe, S.; Ohta, I.; Takahashi, M. IEEE
2
5
7
analyzed by M o¨ ssbauer spectroscopy (WISSEL, Co source).
Samples were prepared in the glovebox, and extreme care was taken
to avoid oxidation during transfer to the apparatus. Flame-sealed
glass tubes of powder were prepared under argon to determine the
iron composition from microanalysis measurements performed by
inductively coupled plasma (ICP).
2
Trans. Magn. 2005, 41, 3391–3393. (g) Liang, X.; Wang, X.; Zhunang,
J.; Chen, Y.; Wang, D.; Li, Y. AdV. Funct. Mater. 2006, 16, 1805–
1
813. (h) Casula, M. F.; Jun, Y.-W.; Zaziski, D. J.; Chan, E. M.;
Corrias, A.; Alivisatos, A. P. J. Am. Chem. Soc. 2006, 128, 12675–
682.
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(
9) (a) Huber, D. L. Small 2005, 1, 482–501. (b) Huber, D. L.; Venturini,
E. L.; Martin, J. E.; Provencio, P. P.; Patel, R. J. J. Magn. Magn.
Mater. 2004, 278, 311–316. (c) Sun, S.; Zeng, H.; Robinson, D. B.;
Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. J. Am. Chem. Soc. 2004,
Results
3
a
On the basis of the results obtained by Dumestre et al., we
used a combination of PA and HDA to grow well-defined NPs.
We successively monitored the kinetics of the reaction, the
influence of the temperature, and finally the influence of the
acid/amine ratio, as presented hereafter. The first observation
that actually started this study was made before any NPs were
formed: a mixture of PA/HDA in a 1.5:2 ratio mixed with
1
26, 273–279. (d) Peng, S.; Wang, C.; Xie, J.; Sun, S. J. Am. Chem.
Soc. 2006, 128, 10676–10677. (e) Farrell, D.; Majetich, S. A.;
Wilcoxon, J. P. J. Phys. Chem. B 2003, 107, 11022–11030. (f) Farrell,
D.; Cheng, Y.; McCallum, R. W.; Sachan, M.; Majetich, S. A. J. Phys.
Chem. B 2005, 109, 13409–13419. (g) Hyeon, T.; Lee, S. S.; Park, J.;
Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798–12801.
(
h) Kim, D.; Park, J.; An, K.; Yang, N.; Park, J.; Hyeon, T. J. Am.
Chem. Soc. 2007, 129, 5812–5813. (i) Yang, H. T.; Ogawa, T.;
Hasegawa, D.; Takahashi, M. Phys. Status Solidi A 2007, 204, 4013–
3 2 2 2
{Fe[N(SiMe ) ] } at room temperature led immediately to the
4
016. (j) Shavel, A.; Rodriguez-Gonzales, B.; Spasova, M.; Farle, M.;
formation of organic superstructures in mesitylene, as revealed
by TEM (see Figure 1a). Size measurements by dynamic light
scattering from the starting solution (after heating at only 50
Liz-Marzan, L. M. AdV. Funct. Mater. 2007, 17, 3870–3876.
(
(
10) Yang, H.; Ito, F.; Hasegawa, D.; Ogawa, T.; Takahashi, M. J. Appl.
Phys. 2007, 101, 09J112.
11) Lacroix, L.-M.; Lachaize, S.; Falqui, A.; Blon, T.; Carrey, J.; Respaud,
M.; Dumestre, F.; Amiens, C.; Margeat, O.; Chaudret, B.; Lecante,
P.; Snoeck, E. J. Appl. Phys. 2008, 103, 07D521.
°
C for 10 min) also confirmed the formation of micrometric
(
12) Olmstead, M. M.; Power, P. P.; Shoner, S. C. Inorg. Chem. 1991, 30,
(13) Rasband, W. S. ImageJ; National Institutes of Health: Bethesda, MD,
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2
547–2551.
5
50 J. AM. CHEM. SOC. 9 VOL. 131, NO. 2, 2009