Solution-Phase Synthesis of Single-Crystalline Iron Phosphide Nanorods/Nanowires

Article abstract of DOI:10.1021/ja038401c

A solution-phase route for the preparation of single-crystalline iron phosphide nanorods and nanowires is reported. We have shown that the mixture of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP), which are commonly used as the solvents for semiconductor nanocrystal synthesis, is not entirely inert. In the current process, TOP, serving as phosphor source, reacts with Fe precursors to form FeP nanostructures with large aspect ratios. In addition, the experimental results show that both TOP and TOPO are necessary for the formation of FeP nanowires and their ratio appears to control the morphology of the produced FeP structures. A possible growth mechanism is discussed.

Full text of DOI:10.1021/ja038401c

Published on Web 01/06/2004
Solution-Phase Synthesis of Single-Crystalline Iron
Phosphide Nanorods/Nanowires
Cheng Qian,† Franklin Kim,‡ Lei Ma,§ Frank Tsui,§ Peidong Yang,‡ and Jie Liu*,†
Contribution from the Department of Chemistry, Duke UniVersity,
Durham, North Carolina 27708, Department of Chemistry, UniVersity of California,
Berkeley, California 94720, and Department of Physics and Astronomy,
UniVersity of North Carolina, Chapel Hill, North Carolina 27599
Received September 8, 2003; E-mail: j.liu@duke.edu
Abstract: A solution-phase route for the preparation of single-crystalline iron phosphide nanorods and
nanowires is reported. We have shown that the mixture of trioctylphosphine oxide (TOPO) and
trioctylphosphine (TOP), which are commonly used as the solvents for semiconductor nanocrystal synthesis,
is not entirely inert. In the current process, TOP, serving as phosphor source, reacts with Fe precursors to
form FeP nanostructures with large aspect ratios. In addition, the experimental results show that both TOP
and TOPO are necessary for the formation of FeP nanowires and their ratio appears to control the
morphology of the produced FeP structures. A possible growth mechanism is discussed.
As compared to bulk materials, nanoscale materials exhibit
large surface areas and size-dependent quantum confinement
effects. They often have distinct electronic, optical, magnetic,
chemical, and thermal properties. Recently, one-dimensional
(1D) nanostructures such as wires, rods, belts, and tubes with
well-controlled dimensions, composition, and crystallinity have
become the focus of intensive research for investigating struc-
ture-property relationships and related scientific and techno-
logical applications due to their dimensional anisotropy.1-3
Significant efforts have been taken in the synthesis of various
1D nanoscale materials over the past few years. Gas-phase
syntheses such as vapor-liquid-solid (VLS) methods have been
successful for the preparation of various nanowires with well-
controlled diameters and lengths,3-6 especially for key semi-
conducting materials of groups II-VI, III-V, and IV. However,
the nanowire diameters are typically larger than the strong-
confinement limit, and the synthesis temperatures are generally
high. The liquid/solution-phase synthesis of anisotropic nano-
crystals has the potential to become a general synthetic method
because it proceeds at a relatively lower reaction temperature
and the diameters of the produced nanocrystals can be controlled
down to several nanometers. However, as compared to the gas-
phase synthesis, the liquid/solution-phase synthesis has only
been demonstrated for a limited number of examples, including
CdSe7-9 and CdTe,10 Si,11,12 GaAs,13-17 InAs,18 Cu2S,19 and
metal nanowires, etc.1,20,21 Here, we describe the use of liquid/
solution-phase synthesis to prepare single-crystalline metal
phosphide nanorods and nanowires with uniform diameters.
The studies on nanostructures of metal phosphides were much
less advanced in comparison to other semiconductor materials
because of their difficult synthetic chemistry.22,23 Many of the
bulk metal phosphide materials are technically important ma-
terials as phosphorescent, magnetic, and electronic materials.24-26
They are important in the study of magnetism because the
interatomic spacing and the anion electronegativity lie in an
intermediate range between those for metals and for the oxides.26
Traditionally, bulk iron phosphides have been prepared by a
variety of high-temperature methods25,27,28 and sonochemical
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† Duke University.
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Am. Chem. Soc. 2003, 125, 5638.
‡ University of California.
§ University of North Carolina.
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A R T I C L E S
Qian et al.
Figure 1. TEM images of FeP nanowires prepared in 50 wt % TOPO in TOP via (a) single injection and multiple injections (b, 2 times; c, 3 times; d, 6
times; and e, 15 times).
methods.29 Ultrafine powder of iron phosphides was also synthe-
sized by a solvothermal method,30 but the morphology is ill-
defined and the average particle size was large (∼200 nm in
diameter). Over the last year, the preparation of nanostructured
metal phosphides has gained significant interest due to the need
to explore the size-dependent physical properties in these
materials. Nanoparticles of Fe2P, FeP, and other metal phos-
phides have been prepared recently.31,32 However, the diameters
of the produced nanoparticles are not uniform. Additionally, to
the best of our knowledge, high-quality and uniform iron
phosphide nanorods and nanowires have never been prepared.
In this paper, we report a novel one-step solution-phase route
to prepare iron phosphide nanorods and nanowires. Our experi-
ments have used a mixed solvent (trioctylphosphine oxide
(TOPO) and trioctylphosphine (TOP)) that is commonly used
by many groups for the preparation of various nanostructures.
We have shown here that TOP may react with Fe precursors
and act as P source for the formation of iron phosphide
nanostructures rather than behaving as inert solvent in previous
experiments.
In a typical experiment, 5 g of TOPO (99% from Aldrich)
and 6 mL of TOP (97% from Aldrich) were mixed as initial
solvent (TOPO in TOP of 50 wt %) and then heated to a desired
temperature (>300 °C) with vigorous stirring under N2 atmo-
sphere. Next, 0.5 mL of stock solution 1 (made from 1 mL of
Fe(CO)5 dissolving in 4 mL of TOP) was quickly injected into
this rapidly stirred, hot TOPO/TOP solution. The temperature
dropped to about 300 °C after the injection. The temperature
was kept constant at 300 °C for further growth. At each 30 min
interval, a 0.2 mL portion of liquid was extracted from the
reaction solution to monitor the growth process, and 0.2 mL of
stock solution 1 was injected to keep the total volume un-
changed. The removed liquid was dispersed in hexane. A drop
of this solution was deposited on a copper TEM grid and
checked under a transmission electron microscope (TEM).
Figure 1 shows TEM images of the prepared nanorods and
nanowires as a function of reaction time. As shown in the
figures, nanorods and nanowires with uniform diameter of about
5 nm can be prepared. Interestingly, the length of the nanorods
increases as a function of reaction time, while the diameter
remains essentially unchanged. Nanowires with length up to a
few microns (aspect ratio >200) can be prepared by the multiple
injections method (Figure 1e).
Energy-dispersive X-ray spectroscopy (EDX) analysis of the
nanowires showed that Fe and P were the main elemental
components (Figure 2a). Quantitative EDX analysis showed the
mole ratio of Fe:P was in the range of 1:1 to 1:1.5. Analysis of
these nanowires using XPS further confirms the observation.
Oxygen and carbon were also found in the XPS spectrum, which
could come from surface adsorption of oxygen and carbon
dioxide.33 Another source of the oxygen and carbon signals
could be the surfactant TOPO that was not completely removed
by the surface-cleaning step. To reduce the effect of surface
adsorption, we used an ion gun to etch the surface of our samples
for 10 and 20 min to remove approximately 1 and 2 nm of the
surface layer. The resulting binding energy changes were shown
in Table 1. There were slight chemical shifts between the Fe
and P on the surface and those in the inner part of the nanowires.
From the binding energy of Fe and P on the surface, we can
exclude the possibility of any type of iron oxides and phosphates
formation, which should have binding energies larger than 709
and 130 eV, respectively.33
All peaks in the X-ray diffraction (XRD) pattern (Figure 2b)
can be indexed to the orthorhombic cell of the FeP phase with
space group Pnma (lattice constant a ) 5.191 Å, b ) 3.099 Å,
and c ) 5.792 Å).34 High-resolution TEM images and electron
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Bull. 2002, 37, 1101-1105.
(33) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of
X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra
for Identification and Interpretation of Xps Data; Perkin-Elmer: Boston,
1995.
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2003, 125, 4038-4039.
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Nanotechnology 1996, 622, 195-204.
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Synthesis of Iron Phosphide Nanorods/Nanowires
A R T I C L E S
Figure 2. (a) Energy-dispersive X-ray spectroscopy (EDX) of FeP
nanowires. The Cu signal is from copper TEM grid. (b) XRD pattern of
FeP nanowires. All peaks can be indexed according to the orthorhombic
structure of FeP.
Table 1. Binding Energy (BE) of Fe and P in FeP Nanowires
Figure 3. Electron diffraction pattern (a) and HRTEM images of FeP
nanowires (b, c). The nanowires grow perpendicular to (013) and (011)
planes in b, c, respectively.
surface
10 min spattering
20 min spattering
Fe 2p
P 2p
707.3 eV
129.1 eV
707.0 eV
128.8 eV
707.0 eV
128.8 eV
Obviously, the TOP in the mixture not only functions as a
cosolvent but also as the P sources in our experiments. As
reported previously, a variety of phosphine-substituted iron
carbonyl intermediate species could form when Fe(CO)5 is
dissolved in TOP.29 We assume that after being injected into
hot TOPO/TOP solutions at high temperature, these intermediate
species could decompose to produce FeP by cleaving the Fe-C
and P-C bonds. The process could be simply described as
follows:
diffraction pattern further confirmed that the produced nanowires
were single-crystalline FeP nanowires. The electron diffraction
pattern (Figure 3a) was taken on bundles of the FeP nanowires.
Ring diffractions with d values of 2.740, 2.427, 1.993, 1.879,
1.754, 1.624, and 1.536 Å can be identified as (011), (111),
(112), (211), (301), (212), and (020) peaks from the FeP
orthorhombic structure, respectively. A weak diffraction at 2.207
Å cannot be identified using the FeP structure, presumably from
either Fe or Fe2P impurities. High-resolution TEM (Figure 3b,c)
studies indicate most of these nanowires grow perpendicular to
either (011) or (013) planes. In addition, the wires are usually
covered with a thin layer of amorphous coating.
Consistent with the HRTEM results, XPS analysis results also
indicated that the surfaces of FeP nanowires were possibly
coated with the surfactant TOPO, or some other unknown
amorphous Fe-P compounds. Moreover, the quantitative EDX
analysis also showed the mole ratio of FeP was in the range of
1:1 to 1:1.5, which also proved that the nanowires were mainly
FeP; the excess amount of P should come from the amorphous
layer on the surface, most likely the surfactants.
To understand the formation mechanism of these nanowires
and identify the source of phosphor, we have performed a series
of experiments. We have first excluded the possibility that the
phosphor source was the impurities in TOPO and TOP because
the amount of phosphor in the products far exceeds the amount
of impurity in the solvents. We have then found that such
nanostructures can only be prepared in the presence of TOP.
When pure TOPO was used, only spherical Fe nanoparticles
were prepared under identical reaction conditions (Figure 4a).
Fe(CO)5 + TOP f (CO)xFe(P(C8H17)3)y + heat f FeP
Multiple injection of stock solution provided feedstocks for
further growth of FeP structures. However, to grow long and
uniform nanowires, the ratio between TOP and TOPO is very
important. Although FeP nanorods can prepared without TOPO
(Figure 4b), the diameter distribution is much broader and the
aspect ratio is lower. At higher TOPO concentration, although
the diameters are uniform, the lengths are usually only tens of
nanometers (Figure 4c). The optimal ratio of TOPO/TOP is
about 50% (Figure 1) to obtain uniform nanorods and nanowires.
Several recent studies have given some insight into the shape
control of anisotropic nanocrystals.1,35 It is well accepted that
the existence of an appropriate amount of capping reagents can
alter the surface energies of various crystallographic surfaces
to promote selective anisotropic growth of nanocrystals. In our
cases, we believe TOPO played the role of the capping reagent
because the morphology of the produced nanowires is greatly
affected by its concentration. From the experimental results,
(35) Lee, S. M.; Cho, S. N.; Cheon, J. W. AdV. Mater. 2003, 15, 441-444.
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A R T I C L E S
Qian et al.
Figure 4. TEM images of nanocrystals prepared in TOPO (a), TOP (b), and 90 wt % TOPO in TOP (c).
paramagnetic parameters obtained from the fits of the behavior
at fields above 0.1 T are (1.1 ( 0.2) × 10-5 µB-K/G per Fe
and -10 ( 2 K. The measured Curie constant for Fe corre-
sponds to an effective magnetic moment of 0.7 ( 0.2 µB, which
is consistent with the bulk value but nearly a factor of 10 smaller
than the values for free Fe spins. The negative Curie-Weiss
temperature indicates the presence of antiferromagnetic interac-
tions, and the low-temperature peak of ø further suggests the
presence of antiferromagnetic correlation. The observed mag-
netic behavior appears to be independent of the morphology of
the nanostructures. However, the low-field signal from the
nanowires is not detectable, because of the presence of a small
quantity of unknown ferromagnetic clusters (possibly Fe or Fe2P,
<0.1 wt %) in the powder with a TC of ∼270 K, so more
quantitative interpretation of possible ordering is not possible,
which would require quantitative measurements of the behavior
at very low fields (,0.1 T).
In summary, high-quality FeP nanorods/nanowires have been
synthesized in the TOPO/TOP solvent systems via a simple one-
step solution-phase route. The length of the nanorodes/nanowires
can be continuously increased by a multiple injection method.
Unlike previous reports of nanostructure synthesis in the TOP/
TOPO system, TOP acts as both solvent and phosphor source
in our experiment. Interestingly, a recent article also reported
that under the right conditions, dodecanethiol, which is typically
used as a capping ligand, can function as a sulfur source for
nanorod synthesis.19 These findings showed that we need to be
very careful in selecting the solvent and capping agents in
nanoparticle and nanowire synthesis. Inert solvent and capping
ligands for one reaction may be reactive for a similar but
different reaction. Additionally, the produced FeP nanowires
exhibit paramagnetism down to 10 K, below which the presence
of antiferromagnetic correlations was observed.
Figure 5. Magnetic susceptibility ø (≡∂M/∂B) of FeP nanowires at B ≈ 2
T as a function of temperature. Circles are measurements using SQUID
magnetometry. The line is a Curie-Weiss law fit of the data with a Curie
constant of (1.1 ( 0.1) × 10-5 µB-K/G and Curie-Weiss temperature of
-9 ( 1 K. The former corresponds to an effective polarization of 0.7 µB
per Fe, and the latter indicates the presence of antiferromagnetic interactions.
TOPO molecules absorbed on the FeP nanocrystals greatly
increased the growth rate along the direction perpendicular to
(011) and/or (013) planes of the FeP structure relative to all
other crystal planes. The concentration of TOPO in our system
is very important for causing a large difference among the
growth rates of various crystallographic surfaces. Additionally,
TOPO was shown to play a very important role in promoting
atomic exchange between nanoparticles, a requirement for size
distribution focusing and kinetic control.36 A similar effect
may also exist in our system. TOP in the mixture reacts with
Fe(CO)5 for the formation of FeP. Without TOPO, the produced
nanocrystals were a mixture of nanorods and nanoparticles.
Magnetic properties of the FeP nanowire have been studied
using SQUID magnetometry in a temperature range between 2
and 400 K and a field range up to 5 T. Bulk FeP exhibits a
double helical magnetic structure propagating along the c-axis
below a Ne´el temperature TN of 115 K.37,38 The helimagnetic
structure consists of two types of Fe sites with respective
magnetic moments of 0.46 and 0.37 µB. This ordering is sup-
pressed in the nanowires. As shown in Figure 5, the differential
magnetic susceptibility ø of the nanowire powder, defined as
∂M/∂B, exhibits a Curie-Weiss paramagnetism down to 10 K
with
Acknowledgment. The authors thank the Air Force Office
of Scientific Research (Grant #49620-02-1-0188, J.L.) and the
National Science Foundation (F.K., F.T., P.Y.) for support. We
thank the National Center for Electron Microscopy for the use
of their facilities. J.L. acknowledges support from DuPont as a
recipient of the 2002 young professor award.
JA038401C
ø ) C/(T - θ)
(1)
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where C is the Curie constant and θ is the Curie-Weiss
temperature. At lower temperatures, it deviates from the Curie-
Weiss law and rises to a peak around 5 K. The respective
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1198 J. AM. CHEM. SOC. VOL. 126, NO. 4, 2004