Room-temperature chemical synthesis of shape-controlled indium nanoparticles

Article abstract of DOI:10.1021/ja801949c

Shape-controlled metal nanoparticles are of interest because of their wide range of properties that are useful for applications that include optics, electronics, magnetism, and catalysis. Indium metal is an attractive target for nanoparticle synthesis because it is superconducting, plasmonically active, and is a component in low-melting solders and solid-state lubricants. Indium nanoparticles are typically synthesized using harsh physical or chemical techniques, and rigorous shape control is difficult. Here we present a simple and robust kinetically controlled process for synthesizing shape-controlled indium nanoparticles. By controlling the rate of dropwise addition of a solution of NaBH₄ in tetraethylene glycol to an alcoholic solution of InCl₃ and poly(vinyl pyrrolidone), indium nanoparticles are formed with shapes that include high aspect ratio nanowires and uniform octahedra and truncated octahedra. The zero-dimensional indium nanoparticles exhibit an SPR band centered around 400 nm, and all morphologies are superconducting (T_c = 3.4 K) with higher critical fields than bulk indium. Copyright

Full text of DOI:10.1021/ja801949c

Published on Web 06/10/2008
Room-Temperature Chemical Synthesis of Shape-Controlled Indium
Nanoparticles
†
‡,§
‡,§
,†,§
Nam Hawn Chou, Xianglin Ke, Peter Schiffer, and Raymond E. Schaak*
Department of Chemistry, Department of Physics, and Materials Research Institute,
The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802
Received March 16, 2008; E-mail: schaak@chem.psu.edu
The synthesis of shape-controlled metal nanocrystals is motivated
by their wide range of properties that are useful for applications
1
that include optics, electronics, magnetism, and catalysis. As a
target for shape-controlled nanocrystal synthesis, indium metal is
2
attractive because it is superconducting, is active for surface
3
plasmon resonance (SPR), and is used as a component in low-
4
5
melting solders and solid-state lubricants. Nanocrystalline In with
predominantly spherical shapes has been synthesized using a variety
of physical and chemical methods. Representative physical methods
6
2
5
include laser ablation, evaporation, ultrasonication, and dispersing
7
molten In into paraffin oil. Chemical methods have typically
required the use of strong reducing agents such as sodium metal,
zinc powder, and alkalides and electrides. Decomposition of low-
valent organometallic complexes has also been used to synthesize
In nanoparticles and nanowires.
While In + should be reducible to In by BH
reduction potentials (E In3+/In ) -0.338 eV vs SHE, E B(OH)3/BH4-
3
8
9
1
0
11
3
0
-
4
based on standard
o
o
Figure 1. TEM micrographs of indium (a) nanowires, (b) octahedra, and
3
+
)
-0.481 eV vs SHE), there have been no reports describing the
(c) truncated octahedra synthesized by borohydride reduction of In .
synthesis of In nanocrystals using borohydride reduction, polyol
reactions, or related mild solution methods that are among the most
1
2
common strategies for generating shape-controlled nanocrystals.
Here we describe a simple kinetically controlled borohydride
reduction process for synthesizing shape-controlled In nanocrystals
at room temperature.
In nanoparticles were synthesized by the dropwise addition of a
solution of NaBH
InCl and poly(vinyl pyrrolidone) (PVP, MW ) 630000) in
isopropyl alcohol (IPA). In a typical synthesis, InCl (10 mg) and
4
in tetraethylene glycol (TEG) to a solution of
3
3
PVP (0.10 g) were dissolved in 10 mL of IPA. After purging with
Ar, 15 drops (13 µL/drop, ∼0.2 mL total) of a freshly prepared
4 4
NaBH /TEG solution (0.07 g of NaBH in 5 mL of TEG) were
Figure 2. (Left) Powder XRD patterns for In nanoparticles and simulated
XRD data for In metal; (right) higher-angle XRD data for all samples; (inset)
representative SAED pattern for In nanowires.
added at ambient temperature. After 10 min of stirring, the
precipitate was separated by centrifugation and washed several times
with ethanol. The isolated product was easily dispersible in water
drops at the same rate. The selected area electron diffraction (SAED)
pattern in Figure 2 confirms that the nanowires are composed of
In metal. The powder X-ray diffraction (XRD) pattern for this same
sample (Figure 2) shows evidence of preferred orientation, with
the 011, 002, and 022 peaks exhibiting relative intensities much
higher than predicted for an isotropic sample. Figure 2 also shows
the XRD pattern for irregularly shaped In nanocrystals synthesized
without controlling the reduction rate (Supporting Information,
Figure S1), which shows no preferred orientation.
and ethanol. It is important to note that if an IPA solution of NaBH
is used, In nanocrystals do not form, presumably because of a faster
4
3
+
reaction of NaBH
4
with the alcohol than with In . Using TEG as
maintains its reactivity long enough to reduce
the solvent for NaBH
4
3
+
0
In to In after it is added to IPA.
4
The rate of dropwise addition of the NaBH /TEG solution to
the InCl
influenced the shape of the In nanocrystals. For example, Figure
a shows transmission electron microscope (TEM) images of In
nanowires (50-70 nm diameter, 3-4 µm long, 95% yield)
synthesized by initially adding 10 drops of NaBH /TEG at a rate
3
/IPA solution, and therefore the reduction kinetics,
1
When the reduction rate is increased, other nanocrystal mor-
phologies can be accessed. For example, upon adding 15 drops of
4
NaBH
4
/TEG solution at a rate of 1 drop/s, 80 ( 7 nm In octahedra
of 1 drop every 2 s, then waiting 50 s before adding 5 additional
are formed in 85% yield (Figure 1b). When 10 drops of NaBH
4
solution are added at a rate of 1 drop/s, followed by an 8-s pause
and the addition of the remaining 5 drops at a rate of 1 drop/s, 95
( 5 nm truncated octahedra are formed in 80% yield (Figure 1c).
†
Department of Chemistry.
‡
Department of Physics.
§
Materials Research Institute.
8
140 9 J. AM. CHEM. SOC. 2008, 130, 8140–8141
10.1021/ja801949c CCC: $40.75  2008 American Chemical Society

C O M M U N I C A T I O N S
1
4
to that observed for Sn and Pb nanowires. Interestingly, while
measurements at 100 Oe significantly decrease T in bulk In as
expected because of its low critical field, T remains almost
c
c
unchanged for all of the In nanoparticle samples (Figure S2). This
suggests that the In nanoparticles have a much higher critical field
than bulk In, similar to recently reported results on In nanopowder
2
synthesized via thermal evaporation of bulk In.
In conclusion, shape-controlled In nanoparticles have been
synthesized using a room-temperature NaBH
In nanowires, octahedra and truncated octahedra can be accessed
simply by changing the rate of dropwise addition of NaBH /TEG
into a solution of InCl /PVP/IPA. These nanoparticles exhibit the
4
reduction strategy.
4
3
Figure 3. TEM images from intermediate stages of the reaction: (a) In
nanowires growing off of octahedral seeds; (b,c) nanowires becoming thicker
as the reaction progresses. (d) Enlarged TEM image showing the nanowire
tips, which maintain the points defined by the octahedral seeds.
expected SPR properties, as well as superconductivity with a higher
critical field than bulk In. It is anticipated that the shape-controlled
In nanoparticles could also serve as templates for conversion into
1
0d,15
In-based nanomaterials, including InP and In intermetallics.
Acknowledgment. This work was supported by the U.S.
Department of Energy (DE-FG02-06ER46333), the Petroleum
Research Fund (administered by the American Chemical Society),
a DuPont Young Professor Grant, a Beckman Young Investigator
Award, a Sloan Research Fellowship, and a Camille Dreyfus
Teacher-Scholar Award. X.K. and PES thank the Penn State
MRSEC for funding. Electron microscopy was performed at the
Materials Characterization Facility at the Penn State Materials
Research Institute. The authors also acknowledge use of facilities
at the PSU site of the NSF NNIN.
Figure 4. (a) Representative UV-visible absorption spectrum of In
nanoparticles (octahedra) and (b) temperature dependent magnetization at
a 8.5 Oe applied field (black diamonds, bulk In; red circles, In nanowires;
blue triangles, In octahedra; green squares, In truncated octahedra).
Supporting Information Available: Additional experimental and
characterization details, additional TEM images, supporting data for
the proposed nanocrystal formation mechanism, and M vs T data for
bulk In and In nanoparticles at an applied field of 100 Oe. This material
is available free of charge via the Internet at http://pubs.acs.org.
Preliminary evidence suggests that other shapes, including triangles
and decahedra, can also be accessed.
Shape control in this system appears to be driven by reduction
kinetics, rather than by the influence of different types or concentra-
tions of surface stabilizers or additives that are typically used for
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JA801949C
J. AM. CHEM. SOC. 9 VOL. 130, NO. 26, 2008 8141