One-pot synthesis of bi-disperse FePt nanoparticles and size-selective self-assembly into AB2, AB5, and AB13 superlattices

Article abstract of DOI:10.1039/b515673d

By chemically modifying the nucleation burst that generates monodisperse FePt nanocrystals, a mixture of Pt and Fe_xPt_1-x nanoparticles forms during a one-pot reaction that includes a small amount of Cu as a catalyst; size-selective precipitation yields a bi-disperse population of Fe_xPt_1-x nanoparticles, which can assemble into high-quality AB₂, AB₅, and AB₁₃ superlattice structures. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b515673d

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One-pot synthesis of bi-disperse FePt nanoparticles and size-selective
self-assembly into AB , AB , and AB superlattices{
2
5
13
a
a
b
b
Amandeep K. Sra, Trevor D. Ewers, Qiang Xu, Henny Zandbergen and Raymond E. Schaak*
a
Received (in Berkeley, CA, USA) 3rd November 2005, Accepted 30th November 2005
First published as an Advance Article on the web 6th January 2006
DOI: 10.1039/b515673d
By chemically modifying the nucleation burst that generates
monodisperse FePt nanocrystals, a mixture of Pt and Fe Pt
particle size distributions, and will hopefully motivate continued
synthetic exploration into other complex systems.
x
12x
nanoparticles forms during a one-pot reaction that includes a
small amount of Cu as a catalyst; size-selective precipitation
The synthesis (details in Supplementary Information{) was
carried out using standard air-free techniques. In a typical
3 2
synthesis, a 1 : 1 : 0.05 molar ratio of Fe(acac) , Pt(acac) , and
yields a bi-disperse population of Fe Pt
nanoparticles, which
x
12x
Cu(acac) was dissolved in octyl ether. 1,2-Hexadecanediol was
2
can assemble into high-quality AB
structures.
2
, AB
5
, and AB13 superlattice
added, and the reaction mixture was purged with argon. Oleic acid
and oleylamine were then added to the reaction flask at room
temperature before raising the reaction temperature to 250 uC and
then subsequently lowering it to 210 uC. After 40 min, the solution
was cooled to room temperature, and the nanoparticles were
precipitated with ethanol. The product was isolated by centrifuga-
tion at 4000 rpm and washed several times.
1
The synthesis of uniform magnetic nanocrystals and their self-
2
assembly into periodic superlattices is emerging as an attractive
bottom-up strategy for accessing ordered sub-lithographic arrays
with potential applications that include ultrahigh-density magnetic
3
,4
5
storage architectures and spin-exchange magnets. Binary super-
lattices are particularly attractive, as they can produce structures
that are more complex than the standard close-packed hcp and fcc
arrangements most frequently observed for single-component
systems. In addition, binary superlattices have the potential to
create new ‘‘metamaterials,’’ which display unique collective
TEM analysis of the product indicates the presence of three
distinct types of particles: monodisperse 2.3 and 4.5 nm particles,
along with polydisperse 10–20 nm particles (Fig. 1a). EDS analysis
indicates that the largest particles are Pt, and these can easily be
removed by centrifuging at 1000 rpm. After re-suspending the
purified precipitate in hexane and centrifuging at 1000 rpm for
6 min, the product contains exclusively a bimodal distribution of
the smaller 2.3 and 4.5 nm particles (Fig. 1b). Drop-casting the
colloidal solution containing 2.3 and 4.5 nm particles in hexane,
followed by slow evaporation of the solvent, yields three types of
6–8
properties that differ from their individual components.
To
date, binary AB, AB , AB , and AB superlattices with the NaCl,
13
2
5
2 5
AlB , CaCu , and NaZn13 structure types, respectively, have been
observed for physical mixtures of individually-synthesized nano-
particles with carefully-controlled size ratios and very narrow size
7–12
distributions.
2 5
two-dimensional superlattices (Fig. 1c): AB , AB , and AB13,
Here, we present an alternative approach for forming binary
nanoparticle superlattices. Instead of synthesizing the individual
components and physically mixing them, we discovered that
simple modifications to well-established reactions for generating
which match the AlB , CaCu , and NaZn structure types,
13
2
5
14
respectively. For slower centrifugation speeds (1000 rpm) and
shorter times, the AB and AB superlattices predominate. Higher
2
5
centrifugation speeds (4000 rpm) and longer times significantly
enrich the sample in the smaller 2.3 nm particles, allowing the AB13
superlattice to form preferentially.
3,4,13
monodisperse Fe Pt12x nanoparticles
x
yield a mixture of
phases with several size distributions. This one-pot mixture of
nanoparticles can then be size-focused by controlled centrifugation
to form a bi-disperse population of nanoparticles, which readily
2 5
assemble into high-quality AB , AB , and AB13 superlattices. Such
superlattices are still relatively rare, and the fact that they can be
readily accessed using this unconventional method is important for
gaining a better understanding of the range of conditions necessary
to assemble them, as well as the structural details of the
superlattices that form. In a more general sense, these results
highlight the surprising degree of order that can emerge from
complex one-pot reactions with seemingly disordered multi-modal
a
Department of Chemistry, Texas A&M University, College Station,
TX 77842-3012, USA. E-mail: schaak@mail.chem.tamu.edu
Laboratory for Materials Science, National Centre for HREM, Delft
University of Technology, Rotterdamseweg 137, 2628 AL, Delft,
The Netherlands
Fig. 1 TEM micrographs of (a) as-synthesized (1) 10–20 nm Pt, (2)
b
4
.5 nm Fe
sample (2.3 and 4.5 nm particles) obtained after removing larger Pt
particles by centrifugation, and (c) AB , AB , and AB13 superlattices
x x
Pt12x, and (3) 2.3 nm Fe Pt12x nanoparticles, (b) bi-disperse
2
5
{
Electronic supplementary information (ESI) available: Complete syn-
assembled after further size selection via centrifugation. Scale bars
thesis details, particle size distribution data, and representative EDS data.
See DOI: 10.1039/b515673d
represent 10 nm in (a) and (b) and 5 nm in (c).
7
50 | Chem. Commun., 2006, 750–752
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TEM micrographs of the AB
2
5
and AB superlattices, along with
their intermetallic AlB and CaCu
2
5
prototypes, are shown in Fig. 2.
In both structures, the 4.5 nm particles form a hexagonal lattice
surrounded by the 2.3 nm particles. The primary differences
between the two structures are the layer sequences and the number
of 2.3 nm particles surrounding each 4.5 nm particle, which is
5
greater in the AB lattice. The AB13 superlattice, shown in Fig. 3,
has a cubic arrangement of 4.5 nm particles surrounded by an
icosahedron of 2.3 nm particles, as well as an additional 2.3 nm
particle in the center of the unit cell. The unit cell obtained from
the nanoparticle superlattice matches well with that expected for
the intermetallic NaZn13 structure (Fig. 3e). These AB , AB , and
2
5
AB13 structures have been previously observed in binary
7–12
nanoparticle superlattices,
although only for highly mono-
disperse physically-mixed samples.
Interestingly, we observe both the NaZn13-type AB13 structure
with an icosahedral packing of the smaller B spheres (Fig. 3f), as
well as the recently reported cubic AB13 structure with a unique
cubic arrangement of the B spheres (Fig. 3g). The cubic AB13
structure has a lower packing density than the icosahedral
polymorph and is not known in bulk-scale intermetallic systems,
but was recently observed in physical mixtures of PbSe and Pd
8
Fig. 3 TEM micrographs of AB13 nanoparticle superlattices. Panel (a)
shows a large-area superlattice. (b) The corresponding Fourier transform
highlights the cubic long-range ordering, and (c) the selected-area electron
nanoparticles. Our results highlight the prevalence of this new
structure type, and show that it can exist in thin samples that are
nearly two dimensional (e.g. appear to be only 1–2 unit cells thick)
and for a size ratio that is somewhat smaller than previously
x
diffraction pattern shows the fcc structure of the Fe Pt12x nanoparticles
that comprise the superlattice. An extended lattice of several unit cells is
shown in (d), and a single unit cell is shown in (e), along with the crystal
structure of the intermetallic NaZn13 structure. Panels (f) and (g) highlight
the differences between the standard icosahedral AB13 structure (f) and the
cubic AB13 polymorph (g) that was recently reported by Murray and co-
B A
observed (r /r = 0.511).
The Fourier transforms in Figs. 2c and 3b confirm the long-
range order of the hexagonal AB and cubic AB13 superlattices,
5
respectively. Likewise, the selected-area electron diffraction pattern
in Fig. 3c shows a single-phase fcc structure, which is consistent
8
workers. Both polymorphs appear in our samples.
x
with the Fe Pt12x alloy nanoparticles. The particles that comprise
the AB superlattices were found to be 4.1 ¡ 0.4 nm Fe52(12)Pt48(12)
2
deviations in particle size were determined by statistical analysis
of more than 100 individual nanoparticles for each sample; details
are included as Supplementary Information.{ The values in
parentheses are the standard deviations of the composition
measurements, derived from EDS analysis of more than 50
individual particles of each type.)
5
and 2.1 ¡ 0.2 nm Fe54(12)Pt46(12), while the AB superlattices were
comprised of 4.5 ¡ 0.3 nm Fe52(12)Pt48(12) and 2.3 ¡ 0.2 nm
Fe54(12)Pt46(12) particles. The AB13 superlattices consist of 4.5 ¡
0.4 nm Fe46(9)Pt54(9) and 2.3 ¡ 0.2 nm Fe42(8)Pt58(8). (The
Our synthetic approach, which yields a mixture of several
types of particles that ultimately lead to a bi-disperse sample
of Fe Pt , differs from other routine syntheses of FePt
x
12x
3,4,13
nanoparticles
in two primary ways. First, all of the reagents
are added together at room temperature rather than after heating,
which modifies the nucleation burst that typically leads to
15
2+
monodisperse particles. Second, a small amount of Cu is
added to the reaction, which also likely modifies the nucleation
burst and contributes to the multiple types of particles. Samples
2
+
with no Cu yielded only 2.2 ¡ 0.2 nm FePt particles. We
3+
2
+
speculate that Cu reduces before Fe , and the limited amount
of reduced Cu helps to catalyze the formation of a population of
Fe Pt
x
nanoparticles with a size that is different from the
12x
standard value. This argument is based on reduction potentials,
known kinetics of reduction, and knowledge from similar systems
that excess Fe or a strong reducing agent is needed to form
Fig. 2 TEM micrographs of (a, b) AB
superlattices comprised of 4.5 and 2.3 nm Fe
fringes in (b) highlight the nanoparticle crystallinity. The intermetallic AB
AlB ) and AB (CaCu ) crystal structures are shown in the center for
2
and (c) AB
5
nanoparticle
x
Pt12x particles. Lattice
2
13
Fe50Pt50 nanoparticles. A small amount of Cu was detected in
the nanoparticle samples (see Supporting Information{), but not
reproducibly. Thus, it is difficult to discern whether Cu is actually
incorporated into the FePt nanoparticles. This is not unusual for
systems where small percentages of nanoparticle seeds are used in a
(
2
5
5
comparison. The upper inset in (c) shows the Fourier transform,
highlighting the long-range order of the superlattice, and the lower inset
shows the 4.5 nm central particle surrounded by twelve 2.3 nm particles,
5
which is characteristic of the AB structure.
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1
6
reaction, as their identification often remains elusive. (In many
cases reported in the literature, seeds and low-level metal
impurities, which are not reproducibly detected in the final
Notes and references
1
2
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In summary, binary AB , AB , and AB Fe Pt nanoparticle
3
4
2
5
13
x
12x
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pot reaction that generates a multi-modal distribution of particles.
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nanoparticle superlattices. Furthermore, the observation and
analysis of high-quality binary nanoparticle superlattices in this
system represents an important addition to our growing knowl-
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52 | Chem. Commun., 2006, 750–752
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