Controlled synthesis and chemical conversions of FeO nanoparticles

Article abstract of DOI:10.1002/anie.200701694

(Figure Presented) Shaping up for conversion: Monodisperse FeO nanoparticles were synthesized by reductive decomposition of iron(III) acetylacetonate with oleic acid (OA) and oleylamine (OAm). The nanoparticle sizes were tunable from 14 to 100 nm and thei

Full text of DOI:10.1002/anie.200701694

Angewandte
Chemie
DOI: 10.1002/anie.200701694
Reactions of FeO Nanoparticles
Controlled Synthesis and Chemical Conversions of FeO
Nanoparticles**
Yanglong Hou, Zhichuan Xu, and Shouheng Sun*
Transition metal oxide nanoparticles of type MO, where M is
Mn, Co, Ni, or Fe, have attracted tremendous interest recently
because of their potential as electrode materials for recharge-
The nanoparticles were grown from the reaction mixture
([Fe(acac) ] in a mixture of OA and OAm) by controlled
3
heating at 2208C and 3008C. In the presence of an excess
[1]
able solid-state batteries, as efficient catalysts for fuel-cell
amount of OAm, spherical nanoparticles were formed,
whereas in the presence of equivalent amounts of OA and
OAm, truncated octahedral nanoparticles were obtained. The
size of both kinds of nanoparticles was tuned by simply
controlling the period of heating at 2208C and 3008C. For
example, 14-nm spherical nanoparticles were synthesized by
[
2]
reactions, and as nanoscale magnetic models for under-
[
3]
standing nanomagnetism. Wüstite (FeO) is one form of the
common iron oxides, a group that also includes hematite (a-
Fe O ), maghemite (g-Fe O ), and magnetite (Fe O ). It has a
2
3
2
3
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4
rock-salt structure with Fe and O forming nonstoichiometric
Fe O (x = 0.83–0.96) and Fe vacancies in an ordered distri-
bution. The structure is not chemically stable and is prone to
treating [Fe(acac) ] with OA (8 mL) and OAm (12 mL) at
2208C and 3008C, each for 30 min. Extended heating at
3008C for 1h gave 22-nm nanoparticles. Heating of a reaction
x
3
[
4]
decomposition into a-Fe and inverse spinel Fe O through a
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two-step disproportionation process or to oxidation to form
mixture of [Fe(acac) ], OA (10 mL), and OAm (10 mL) at
3
[
4]
Fe O , g-Fe O , and/or a-Fe O . This chemical reactivity
2208C and 3008C, each for 30 min, led to the formation of 32-
nm truncated octahedral nanoparticles, while heating of the
mixture at 2208C for 1h and at 300 8C for 30 min gave 53-nm
nanoparticles and heating at 2208C for 30 min and at 3008C
for 1h yielded 100-nm truncated octahedral nanoparticles.
Figure 1shows transmission electron microscopy (TEM)
images of representative FeO nanoparticles. The truncated
octahedral shape of the particles can be better seen in the
scanning electron microscopy (SEM) image of Figure 1d.
These size- and shape-controlled syntheses suggest that
1) the use of OA and OAm both as solvents and surfactants
facilitates the formation and stabilization of FeO nano-
particles; 2) the presence of an excess of OAm facilitates
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4
2
3
2
3
makes FeO nanoparticles difficult to make and those
prepared from the high-temperature solution-phase decom-
[
5]
position of iron salt have not been fully characterized.
Herein we report a facile organic-phase synthesis of
monodisperse FeO nanoparticles through high-temperature
reductive decomposition of iron(III) acetylacetonate ([Fe-
(
acac) ]) with oleic acid (OA) and oleylamine (OAm) both as
3
surfactants and solvents. The sizes of the particles are tuned
from 14 to 100 nm by controlling the heating conditions and
the shapes of the particles are controlled to be either spherical
or truncated octahedral depending on the volume ratio of OA
and OAm used in the reaction. Thermal annealing under an
argon atmosphere converted these FeO nanoparticles into
composite FeꢀFe O nanoparticles, while controlled oxida-
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tion of the FeO nanoparticles resulted in the formation of
Fe O , g-Fe O , or a-Fe O nanoparticles. These monodis-
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2
3
2
3
[
2c–f]
perse FeO nanoparticles have great potential for catalysis
[
2g]
and gas-sensor applications. The chemical conversions of
the paramagnetic FeO nanoparticles may also be considered
as an alternative, yet better, approach to the synthesis of
various magnetic iron oxide or iron nanoparticles with sizes
that are difficult to achieve from previous organic-phase
[
6]
syntheses.
[
*] Dr. Y. Hou, Z. Xu, Prof. S. Sun
Department of Chemistry
Brown University
Providence, RI 02912 (USA)
Fax: (+1)401-863-9046
E-mail: ssun@brown.edu
[**] This work was supported by ONR/MURI under grant no. N00014-
05-1-0497, NSF/DMR under grant no. 0606264, and INSIC. We
thank Dr. Y. Ding at the Georgia Institute of Technology for his help
with the high-resolution transmission-electron-microscopy analysis
of the FeO nanoparticles.
Figure 1. TEM images of FeO nanoparticles (the sizes refer to the
average lengths of one side in the projected images): a) 14-nm
spherical, b) 32-nm, and c) 53-nm truncated octahedral. d) SEM image
of 100-nm truncated octahedral nanoparticles.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2007, 46, 6329 –6332
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6329

Communications
growth in all crystal planes on the surface of the FeO nuclei,
tion after purification and separation in air during the
synthesis. This is different from the previous observations
that FeO nanoparticles are not stable and are prone to
oxidation. At elevated temperatures, however, the as-
synthesized FeO nanoparticles undergo chemical and struc-
tural conversions into various iron oxide nanoparticles.
Figure 3 shows the XRD patterns of the 32-nm FeO nano-
thereby giving spherical nanoparticles—this may be
explained by the fact that extra OAm acts as a weaker
ligand during the reaction and there is negligible energy
difference for OAm to dissociate from different crystal planes
on the surface of the nuclei during the high-temperature
growth process; and 3) the 1:1 volume ratio of OA:OAm
presents enough OA to regulate the competitive growth in the
h100i direction over the h111i direction, a process leading to
the formation of the truncated octahedron. Although the
detailed growth mechanism is not completely understood, it is
worth noting that the shape-controlled synthesis reported
herein bears some similarity to the synthesis of rare-earth
metal oxide nanoparticles in the presence of oleic acid and
oleylamine. The binding difference between oleate and
oleylamine on the crystal planes decides the final shape of
[
5]
[
7]
the nanoparticles.
An X-ray diffraction (XRD) study shows that the as-
synthesized FeO nanoparticles have a face-centered cubic
(
fcc) structure and the peak width of the diffraction pattern is
size dependent. Figure 2 shows the XRD patterns of the 14-,
Figure 3. XRD patterns of iron oxide nanoparticles: a) FeO, b) Fe O ,
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c) g-Fe O , and d) a-Fe O .
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2
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particles and the iron oxide nanoparticles obtained from their
controlled oxidation. When annealed under atmospheric
pressure and air, the FeO nanoparticles (Figure 3a) are
converted into Fe O nanoparticles (at 1208C for 90 min,
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4
Figure 3b), g-Fe O3 nanoparticles (at 2008C for 30 min,
2
Figure 3c), or a-Fe O nanoparticles (at 5008C for 120 min,
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3
Figure 3d). The g-Fe O nanoparticles are characterized by a
2
3
peak shift to higher angles and the appearance of two
characteristic superlattice diffractions from the (210) and
Figure 2. XRD patterns of as-synthesized FeO nanoparticles with sizes
of a) 14, b) 32, c) 53, and d) 100 nm.
(213) planes at lower angles (around 248 and 268, respec-
tively). These nanoparticles are also characterized by their
[
4,5b]
conversion into a-Fe O at 5008C under Ar.
TEM image
2
3
3
2-, 53-, and 100-nm FeO nanoparticles, with the (111), (200),
analysis indicates that the annealed nanoparticles show
negligible morphology change during the oxidative annealing
processes.
(
220), (311), and (222) peaks from the fcc structure clearly
identified. The shoulder at 34.58, which becomes more
apparent in the patterns from the smaller nanoparticles, is
due to surface oxidation of the FeO nanoparticles and the
formation of a layer of Fe O coating. The average (200)
The FeO nanoparticles are also subject to disproportio-
nation under controlled annealing conditions. For example,
the 53-nm FeO nanoparticles can be converted into composite
nanoparticles of Fe and Fe O after annealing at 5008C under
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correlation size was estimated with Scherrerꢀs formula to be
3.5 nm in Figure 2a, 31 nm in Figure 2b, 52 nm in Figure 2c,
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ꢀ
1
1
an argon atmosphere for 1h followed by 1Kmin
cooling.
and 98 nm in Figure 2d. These sizes are consistent with those
measured from the TEM images, a result indicating that the
nanoparticles have a single crystal structure. The structure of
a single nanoparticle was studied by high-resolution TEM
The XRD pattern of the composite nanoparticles shows
clearly the body-centered cubic (bcc) Fe and Fe O peaks
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(Figure S2 in the Supporting Information). Figure S3 in the
Supporting Information shows the TEM image of the 53-nm
FeO nanoparticles after the disproportionation reaction, with
smaller dark dots indicating Fe and lighter large particles
indicating Fe O . The interfringe spacing is 0.21nm within the
(
HRTEM), as shown in Figure S1in the Supporting Informa-
tion. The nanoparticle has a core/shell FeO/Fe O structure
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4
with both the core and shell being well crystallized.
The as-synthesized FeO nanoparticles do not undergo fast
oxidation under ambient conditions. An XRD study on the
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smaller dot (Figure S3c in the Supporting Information) and
0.29 nm in the larger particle (Figure S3d in the Supporting
Information). The 0.21-nm spacing is close to 0.203 nm, the
5
3-nm nanoparticle assembly shows no apparent deep oxida-
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Angew. Chem. Int. Ed. 2007, 46, 6329 –6332

Angewandte
Chemie
interplane distance of the (110) planes of bcc Fe, while the
spacing of 0.29 nm matches with the interplane distance of
magnetic properties for various nanoscale magnetic and
catalytic applications.
0
.296 nm for the (220) planes in Fe O .
3 4
The formation of various iron oxide and iron nano-
particles from controlled chemical conversions of the FeO Experimental Section
nanoparticles indicates that the magnetic properties of the
nanoparticles prepared from these FeO nanoparticles can be
readily tuned. Figure 4 shows a series of hysteresis loops for
Synthesis of 14-nm spherical FeO nanoparticles: [Fe(acac)
mmol), OA (8 mL, 25 mmol), and OAm (12 mL, 35 mmol) were
mixed and stirred in a three-necked flask. The mixture was heated at
208C with vigorous stirring for 2 h. During this time, a partial
3
] (1.4 g,
4
1
vacuum was applied to the system to remove trace moisture and
oxygen trapped in the reaction system, thereby giving a clear dark-
brown solution. Under an Ar blanket, the solution was heated to
2
3
208C and kept at this temperature for 30 min before it was heated to
008C at a heating rate of 2 Kmin and kept at 3008C for 30 min. The
ꢀ1
solution was cooled down to room temperature by removing the
heating mantle and the FeO nanoparticles were separated upon
addition of ethanol (20 mL); this was followed by centrifugation. The
nanoparticles were redispersed into hexane and precipitated out by
addition of ethanol to further purify them. The final product was
dispersed in hexane and stored under a nitrogen atmosphere.
Under similar reaction conditions, spherical FeO nanoparticles
with sizes up to 22 nm could be produced by simply extending the
heating time at 3008C.
Synthesis of 32-nm truncated octahedral FeO nanoparticles:
[
Fe(acac) ] (1.4 g), OA (10 mL), and OAm (10 mL) were mixed and
3
stirred in a three-necked flask. The mixture was heated to 1208C with
vigorous stirring and kept at that temperature for 2 h. During this
time, a partial vacuum was applied to the system to remove trace
moisture and oxygen trapped in the reaction system, thereby giving a
Figure 4. Room-temperature hysteresis loops of a) as-synthesized 32-
nm truncated octahedral FeO nanoparticles (inset a1: a close look at a
clear dark-brown solution. The solution was heated at 2208C for
section of trace (a)), b) Fe O nanoparticles, c) g-Fe O nanoparticles,
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4
2
3
ꢀ1
30 min before it was heated to 3008C at a heating rate of 2 Kmin
and d) a-Fe O nanoparticles (inset d1: a close look at a section of
2
3
and kept at 3008C for 30 min. The workup procedures were the same
as those in the synthesis of the 14-nm spherical FeO nanoparticles.
The nanoparticles were dispersed in hexane and stored under a
nitrogen atmosphere.
trace (d)).
iron oxide nanoparticles made from the FeO nanoparticles.
The as-synthesized 32-nm FeO nanoparticles are almost
paramagnetic (Figure 4a and the corresponding inset); the
Fe O coating results in a slight deviation from paramagnetic
Under similar reaction conditions, the sizes of the octahedral FeO
nanoparticles were tuned from 32 to 100 nm by controlling the
heating time. For example, the mixture was heated at 2208C for 1h
and at 3008C for 30 min to prepare 53-nm nanoparticles, while
heating at 2208C for 1h and at 300 8C for 1h led to 10 0-nm
nanoparticles.
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behavior. Both Fe O (Figure 4b) and g-Fe O (Figure 4c)
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4
2
3
nanoparticles exhibit superparamagnetic behavior, with the
ꢀ1
magnetic moments reaching 75 (Fe O ) and 51emug (g-
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4
Received: April 17, 2007
Fe O ), respectively. In contrast to Fe O and g-Fe O , the a-
Published online: July 23, 2007
2
3
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4
2
3
Fe O nanoparticles show a much lower magnetic moment at
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3
ꢀ
1
less than 1emug , which is close to that from the bulk
hematite (0.4 emug ), and a hysteresis behavior at room
Keywords: disproportionation · iron · nanoparticles · oxidation ·
.
ꢀ
1
oxides
[
4]
temperature, which indicates a dramatic structural change
within the particles. As a comparison, the magnetic moment
of the composite a-Fe and Fe O nanoparticles reaches
20 emug .
We have reported a simple process for preparing mono-
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