Synthesis of ultrathin palladium and platinum nanowires and a study of their magnetic properties

Article abstract of DOI:10.1002/anie.200704707

(Chemical Equation Presented) Thin and wiry: Ultrathin palladium and platinum nanowires with widths of 2.4 ± 0.2 (Pd) and 2.3 ± 0.2 nm (Pt) and lengths of over 30 nm are synthesized by a modified phase-transfer method (see picture). Both nanowires show ferromagnetic properties and unusual shifts in their hysteresis loops at low temperature. ODA = octadecylamine, DTAB = n-dodecyltrimethylammonium bromide.

Full text of DOI:10.1002/anie.200704707

Angewandte
Chemie
DOI: 10.1002/anie.200704707
Metallic Nanowires
Synthesis of Ultrathin Palladium and Platinum Nanowires and a Study
of Their Magnetic Properties**
Xiaowei Teng, Wei-Qiang Han,* Wei Ku, and Markus Hücker
Careful control of the size, shape, and composition allows the
properties of various noble metals, particularly palladium and
platinum, to be varied, which leads to a wide range of
applications for these metals in fields such as catalysis and
magnetism.[1,2] In confined nanoscale systems such as ultra-
thin nanowires or nanoclusters, which are often only a few
atoms thick, the reduced degree of coordination and bonding
favors more localized electronic states as well as narrower
Scheme 1. Formation of Pd and Pt nanowires.
bands and larger densities of states, which often lead to rather
exotic magnetic behavior.[3] Several 4d and 5d noble metals
such as Pd, Pt, and Au have been shown experimentally to be
ferromagnetic in two-dimensional (2D) atomic layers[4–6] or
zero-dimensional (0D) nanoclusters,[7–12] whereas their bulk
counterparts are diamagnetic or paramagnetic. Although
theoretical studies have predicted that a one-dimensional
(1D) nanowire should show different magnetic properties
from a 0D nanocluster,[13,14] no experimental confirmation has
yet been reported for the magnetism of 1D nanowires of 4d or
5d metals, mainly because of the limited number of synthetic
approaches to monodisperse ultrathin nanowires.
Herein we report the synthesis of ultrathin Pd and Pt
nanowires by a modified phase-transfer method in which
inorganic palladium nitride or platinum chloride is trans-
ferred into a mixture of octadecylamine (ODA) and toluene
using a new type of phase-transfer agent, namely n-dodecyl-
trimethylammonium bromide (DTAB).[15,16] Subsequent
addition of a large excess of the reducing agent sodium
tetrahydroborate (NaBH4) in aqueous solution reduces the
palladium or platinum cations rapidly to form thermodynami-
cally unstable elongated primary nanostructures (PNs).
Secondary growth of these PNs apparently takes place in
preferred equivalent directions and also along h111i, which
leads to the growth of thread-like quasi-nanowires
(Scheme 1).
resolution TEM images (Figures 1b and 1d) show that both
nanowires are polycrystalline with well-defined lattice planes;
both twinning boundaries and stacking faults are also
observed. The lattice spacings of 2.7 and 2.8 Š can be
assigned to the {111} planes of Pd and Pt, respectively. X-
ray diffraction measurements show that both nanowires
belong to the Fm3m space group.[17]
Crystal structure anisotropy is believed to be the main
driving force for the anisotropic growth of nanostructures.[18]
However, this 1D growth of Pt and Pd is rather unexpected
owing to their symmetric cubic lattices. HRTEM reveals that
each nanowire is composed of several single-crystalline,
elongated PNs. Similar PN-to-wire transformations have
already been observed for the oriented growth of different
types of nanocrystals.[19] Unlike quantum dots, however,
where the dipole moment is the driving force, the oriented
attachment in these Pd and Pt nanostructures is most likely
due to the specific binding of a capping agent to certain crystal
surfaces. In general, the shape of nanomaterials is highly
dependent on the competitive growth rate along the low index
crystal planes in the presence of surfactants.[18a] Bases, for
example, seem to preferentially adsorb on the {100} surfaces
of Pd and Pt and facilitate the growth of PNs along the h111i
directions.[20]
Our results show that the oxygen content is critical to
nanowire formation. Since the PNs are interconnected at
various angles, twin boundaries and stacking faults are readily
observed along the nanowires. These defects are the primary
sites for oxygen absorption and remain susceptible to
oxidative etching.[21] Elongated nanoparticles (ENP) 3.5 ꢀ
0.8 nm wide and 5–25 nm long were formed after two hours
in an oxidative etching experiment upon exposure to air;
nearly spherical nanoparticles 27 ꢀ 4.3 (Pd) and 31 ꢀ 3.9 nm
(Pt) in diameter were formed after 12 h (Figures 1g,h). We
believe that the dissolved oxygen inhibits the secondary
growth of PNs by oxidatively etching the nanowires at the
twin boundary defects. Subsequent Ostwald ripening
accounts for the shape evolution from ENPs to nearly
spherical nanoparticles.
Figure 1 shows transmission electron microscopy (TEM)
images of these thread-like Pd and Pt nanowires. The width of
the Pd and Pt nanowires is 2.4 ꢀ 0.2 and 2.3 ꢀ 0.2 nm,
respectively, and their length is over 30 nm. The high-
[*] Dr. X. Teng, Dr. W.-Q. Han
Center for Functional Nanomaterials
Brookhaven National Laboratory,
Upton, NY 11973 (USA)
Fax: (+1)631-344-3093
E-mail: whan@bnl.gov
Dr. W. Ku, Dr. M. Hücker
Condensed Matter Physics & Materials Science Department
Brookhaven National Laboratory, Upton, NY 11973 (USA)
[**] This work was supported by the U.S. DOE under contract DE-AC02-
98CH10886 and the Laboratory Directed Research and Develop-
ment Fund of Brookhaven National Laboratory (W.H.).
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Figure 2. a) Temperature-dependent magnetization of Pd (black) and
Pt (red) nanowires obtained by zero-field cooling in an external field of
1000 Oe. b–d) Field-dependence of the magnetization of Pd and Pt
nanowires at 5 (b), 300 (c), and 2 K (d). e,f) The coercivity (HC) and
biased field (HB) of Pd (e) and Pt (f) nanowires as a function of
temperature.
materials are stabilized by covalent bonds to thiol-derivatized
alkenes or adsorbed oxygen, the increase in the density of
holes localized by the bonded S and O atoms might give rise
to the ferromagnetism, whereas there are no covalent bonds
between the metal core and the protective surfactant in
alkylamine-protected nanomaterials, therefore less ferromag-
netic order might be expected.[8,22]
The magnetization curves of Pd and Pt nanowires in an
applied field of up to 50 kOe (5 T) are shown in Figure 2b and
the coercivity (HC) in the hysteresis loop at 300 K can be seen
in Figure 2c. These combined results suggest that both
nanowires are ferromagnetic rather than superparamagnetic.
A similar unsaturated magnetization at high field has been
reported for Pd nanoparticles, thereby indicating the presence
of adsorbed molecules on the surface.[8]
A remarkable “magnetic memory” effect is observed for
these nanowires at low temperatures. Thus, instead of a
hysteresis loop with regular symmetry, the loop exhibits a
distinct shift that is biased in the direction of the cooling field
and which remains even after removing the field. These loops
show that HB, the shift of the hysteresis loop along the fieldꢀs
axis, emerges at about 40 K for the Pd nanowires and at about
60 K for the Pt nanowires. The value of HB increases rapidly at
lower temperatures up to 40 ꢀ 4 Oe for Pd and 53 ꢀ 3 Oe for
Pt at 2 K (Figures 2e,f).
Figure 1. TEM images of as-synthesized Pd (a) and Pt (c) nanowires
and their high-resolution TEM images (b and d, respectively). TEM
images of the Pd (e) and Pt (f) elongated nanoparticles (ENP)
obtained after 2 h in air and the nearly spherical Pd (g) and Pt (h)
nanoparticles obtained after 12 h.
SQUID magnetometry measurements revealed a strong
temperature-dependent magnetization in the as-synthesized
nanowires (Figure 2a). The magnetizations are roughly con-
stant above a temperature of about 60 K for Pt and 30 K for
Pd nanowires, whereas below those temperatures the magnet-
izations increase sharply. This steep increase of the magnet-
ization at low temperatures is similar to the behavior of
stabilizer-capped Au and Pt nanoparticles and is mainly due
to the temperature-independent Pauli-paramagnetic parts of
the magnetization in the low-temperature range.[9,11] The low
magnetic moments observed indicate that the overall ferro-
magnetism of the wire is suppressed significantly, as in the
bulkform of these material, owing to the enhanced itinerancy
associated with the macroscopic length scale of the wires. The
nature of the adsorbed capping agents might also account for
the change of magnetic moment. For example, when nano-
We propose a microscopic model whereby the memory
effect observed in nanowires is a consequence of magneto-
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elastic coupling. In the course of field cooling the local
magnetic moments become aligned by the strong field. This
alignment is associated with a relaxation of the local lattice
that is coupled to the magnetic moment. As illustrated in
Figure 3 some of the “vulnerable” local lattice structure (e.g.,
materials. However, because of the long length of these wires,
the large number of atoms in this system stabilizes the
ferromagnetism and, in particular, the memory effect, and
uniquely distinguishes the long wires from ENPs and nearly
spherical nanoparticles.
Besides the memory effect mentioned above, the
observed coercivity shows an unusual temperature depend-
ence that is contrary to that of typical itinerant ferromagnetic
materials: it becomes weaker at lower temperature (Fig-
ure 2e,f) regardless of the significant enhancement of the
magnetic moment. This result strongly suggests an enhanced
electron localization in these systems at low temperature,
which enhances the local magnetic moment and decreases the
ferromagnetic exchange interactions between them. This
effect again illustrates the uniqueness of wires (or ensembles
of nanoparticles) in the field of nanomagnetism and deserves
further scientific investigation.
In summary, we have presented the synthesis of Pd and Pt
nanowires with widths of less than 2.5 nm and lengths of over
30 nm. Both nanowires are ferromagnetic up to room temper-
ature, although with unusual shifts in the hysteresis loop at
low temperature, instead of being paramagnetic as their bulk
forms. A microscopic model suggests that this shift is a
consequence of magneto-elastic coupling. We consider that
these nanomaterials are an ideal model for studying the
magnetism of quasi-1D nanomaterials and also offer oppor-
tunities for designing nanodevices with memory-effect-
related spintronics.
Figure 3. Moment configuration at different stages in a shifted hyste-
resis loop. a) Random regular moments (black arrow) and “localized”
moments (red arrow) seen at “vulnerable” lattice structures (red spot)
in the nanowire without a magnetic field; b) all moments (black and
red arrows) are aligned after cooling to a certain temperature in the
presence of a field; c) the regular moments (black arrow) rotate in the
opposite direction and the localized moments remain “locked” when
the field is reversed. Some moments (purple arrow) align preferentially
in the direction of adjacent “locked” moments (red arrow) due to
interfacial interactions. In other words, “locked” moments exert a
microscopic torque on the adjacent moments; d) an extra field (HB) is
needed to overcome the microscopic torque and completely reverse
the moments (both black and purple arrows); e) when the field returns
to its original direction, the aligned moments are reversed in the
smaller field (HCꢁHB) because the “localized” moments exert a torque
in the same direction as the field.
Experimental Section
All solvents were degassed by bubbling argon through them for at
least ten minutes before use. A mixture of Pd(NO3)2 (Alfa Aesar,
99.95%; 13 mg, 0.06 mmol) or PtCl2 (Alfa Aesar, 99.9%; 16 mg,
0.06 mmol), ODA (Aldrich, 97%; 0.4 g, 1.5 mmol), and DTAB
(Aldrich, 98%; 60 mg, 0.2 mmol) was dissolved in toluene (7 mL)
by sonicating for 20 min under argon. Reduction was then performed
by the dropwise addition of a freshly prepared solution of NaBH4
(Alfa Aesar, 99%; 13 mg, 0.34 mmol) in 2 mL of distilled water.
Stirring was stopped after one hour and then distilled water (2 mL)
and chloroform (2 mL) were added to the solution. The aqueous
solution was discarded and the organic phase containing the alkyl-
amine-stabilized Pt or Pd nanowires was collected. Further separation
was conducted by adding ethanol (10 mL) and centrifuging the
mixture at 8000 rpm for 10 min. The supernatant was discarded and
the collected precipitate was redispersed in chloroform. This process
was repeated three times using ethanol and chloroform alternately.
The Pd and Pt ENPs and nearly spherical nanoparticles were
synthesized in a similar manner except that air was bubbled through
the solvent and the reaction was left open to the air. ENPs and nearly
spherical nanoparticles were obtained after 2 and 12 h, respectively.
High-resolution electron microscopy was performed with a field-
emission JEM 3000FEG equipped with an energy-dispersive X-ray
spectrometry (EDS). The magnetic measurements were conducted on
a Superconducting Quantum Interference Device (SQUID; model:
MPMS XL Quantum Design). Diamagnetic contributions due to the
sample holder were measured previously and removed from the total
magnetization. The hysteresis loop was measured in a field-cooled
process with a field of ꢁ1000 Oe. The temperature-dependent
magnetization was measured following the standard zero field
cooled (ZFC) procedure in which the sample is cooled from 300 K
to 5 K without a field and then heated to 300 K in a field of 1000 Oe.
XRD spectra were obtained with a Philips diffractometer equipped
twin boundaries, stacking faults along the nanowires) become
“locked” into a meta-stable configuration when various
phonon modes are removed upon cooling. These locked
“defect spots” fix the corresponding local magnetic moments
even after reversing the external field and thereby create a
bias in the neighboring moments in the direction of the field
during cooling. This magneto-elastic effect should persist until
the temperature is high enough to create a large number of
phonon modes that can release the “locked” local lattice
structure. We presume that the higher bias onset temperature
of Pt compared with that of Pd reflects its heavier atomic
mass, which modifies the phonon properties to a greater
extent.
While it is not possible to unambiguously identify the
origin of the magnetism from our experimental analysis, the
very small moment observed suggests that the magnetic
moment resides primarily in “defects” in the structure, such as
twinning boundaries and stacking faults, as well as changes of
“boundary conditions” such as twists and bending of the wire.
Local magnetic instability near these defects is enhanced by
the collapse of the density of states resulting from localization
of the electrons, which is a necessary condition considering
the lackof itinerant magnetism in the bulkform of these
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Communications
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Keywords: magnetic properties · materials science ·
nanostructures · palladium · platinum
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