Preparation of iron phosphide particles from an aqueous solvent of oxygen-contained precursors using spray pyrolysis

Article abstract of DOI:10.1111/j.1551-2916.2007.02038.x

Iron phosphides were prepared by an economically feasible spray pyrolysis method, in which nontoxic and inexpensive precursors are used in an aqueous solution. The addition of NaCl to the precursor solution promoted crystal growth at a low temperature and helped to avoid sintering during posttreatment. The key conclusion is that the stoichiometry, shape, and size of the particles are related to the NaCl/Fe molar ratio and the posttreatment temperature. Below the melting temperature of NaCl, the shape and stoichiometry of the particles depended on the NaCl/Fe molar ratio, while above the melting temperature, they were dominated by the posttreatment temperature. Wire-like particles were formed at a NaCl/Fe molar ratio over 15.

Full text of DOI:10.1111/j.1551-2916.2007.02038.x

J. Am. Ceram. Soc., 90 [12] 3767–3772 (2007)
DOI: 10.1111/j.1551-2916.2007.02038.x
r 2007 The American Ceramic Society
ournal
J
Preparation of Iron Phosphide Particles from an Aqueous Solvent of
Oxygen-Contained Precursors Using Spray Pyrolysis
Kee Young Koo and Seung Bin Parkw
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology,
Daejeon 305-701, Korea
Iron phosphides were prepared by an economically feasible
spray pyrolysis method, in which nontoxic and inexpensive pre-
cursors are used in an aqueous solution. The addition of NaCl to
the precursor solution promoted crystal growth at a low tem-
perature and helped to avoid sintering during posttreatment. The
key conclusion is that the stoichiometry, shape, and size of the
particles are related to the NaCl/Fe molar ratio and the post-
treatment temperature. Below the melting temperature of NaCl,
the shape and stoichiometry of the particles depended on the
NaCl/Fe molar ratio, while above the melting temperature, they
were dominated by the posttreatment temperature. Wire-like
particles were formed at a NaCl/Fe molar ratio over 15.
ability of cheap and nontoxic precursors, controllability of
composition, and the high purity of the products.
2
6–29
Nevertheless, spray pyrolysis has never been used to prepare
iron phosphides, because it is not commonly accepted that
nonoxide particles are prepared from oxygen-contained precur-
sors and processed under an oxygen-rich condition such as an
aqueous solution. Another reason is that the residence time in
the conventional spray pyrolysis is too short to produce crys-
30,31
talline nonoxide particles.
This means that a posttreatment
step is required to produce fully crystallized and reduced parti-
cles. However, posttreatment makes the particles agglomerated
and produces particles of irregular shape. To prevent the ag-
glomeration of particles and improve the crystallization at a low
temperature over short times, sodium chloride was added to the
3
2
precursor solution before atomization. Xia et al. reported that
the addition of salt to the aqueous solution in the spray pyrolysis
helped to keep the particles segregated inside a droplet and to
I. Introduction
3
3
RANSITION metal phosphides are considered to be one of the
candidates for application as magnetic, optical, and elec-
produce nanosize particles.
T
trolled by stoichiometry
tronic semiconductors, because physical properties are con-
and the shape and size of
1
–5
6
II. Experimental Procedure
1) Solution Preparation
Iron(II) nitrate hexahydrate (Fe(NO
Tokyo, Japan) and ammonium hydrogenphosphate ((NH
HPO , 98%, Aldrich, St. Louis, MO) were used as precursors of
iron and phosphorus, respectively. Nitric acid (HNO , Junsei,
0%–62%) was added to the precursor solution until a clear
phosphide particles. In particular, iron phosphides are one of
the most important groups of transition metal phosphides due
to their applications as low band gap semiconductor materials,
(
7
)
3 2
Á 6H
2
O, 98%, Junsei,
8
9
4
)
2
Á
electrode materials, catalysts, magnetic storage media, and di-
luted magnetic semiconductors when doped with magnetic ions
4
1
0,11
3
(
V, Cr, Mn, Co, and Ni).
that the magnetic properties of iron phosphides are sensitive to
Previous works have demonstrated
6
1
2,13
solution was obtained. The concentration of the Fe and P pre-
cursors was fixed at 0.3M, while the NaCl/Fe molar ratio was
varied from 0 to 20. Sodium chloride (NaCl, Junsei, 99.5%) was
used as the segregating agent.
their stoichiometry, size, and shape.
Iron phosphides are synthesized by various conventional
methods such as reduction of iron phosphate,
9
,14,15
solvother-
1
0,16
mal process,
complexes.
and thermal decomposition of metal–organic
Typically, these conventional methods require
1
1–13,17
9
,14,15
(
2) Spray Pyrolysis
prolonged heat treatment at a high temperature and cau-
tious handling of toxic precursors and organic solvents such as
A schematic diagram of the experimental apparatus for the
spray pyrolysis system is shown in Fig. 1. In the conventional
spray pyrolysis method, droplets are generated by an atomizer
and transported into the furnace by a carrier gas. The particles
are directly formed after the decomposition of precursors in a
7,18–21
trioctylphosphane (TOP), trioctylphosphine oxide (TOPO),
2
2
and triethylphosphine. Moreover, these precursors are com-
plex and expensive. It is difficult to control the stoichiometry of
the final iron phosphides in these conventional preparation
routes.
2
1,32
furnace-type reactor.
2 2
The H (50% vol%)/N mixed carrier
In this report, we propose an economically feasible method
for preparing iron phosphides via spray pyrolysis at a low tem-
perature using nontoxic and inexpensive precursors such as iron
nitrate and ammonium phosphate. The dependence of the mag-
netic properties of iron phosphides on the stoichiometry and
shape is reported. Spray pyrolysis is an aerosol process that
produces solid particles via reactions of precursors within aero-
gas flow rate was 1 L/min and the residence time in the tubular
furnace-type reactor was 4–5 s. The particles produced were then
posttreated at 6001–10001C for 2–6 h under H (20% vol%)/N
mixed gas (1 L/min) for further crystallization and reduction of
iron phosphates within the particles. In salt-assisted spray
pyrolysis, the salt (NaCl) was removed by washing with
deionized water and then the particles were dried at room
temperature.
2
2
23–25
sol droplets.
The advantages of spray pyrolysis are avail-
T. Vanderah—contributing editor
(3) Characterization
The crystal structure and compositional homogeneity of the
particles were examined by X-ray diffraction (XRD, Rigaku
D/Max-b C, Tokyo, Japan, CuKa radiation), and the lattice
constants of Fe P were refined by a least-square method using
2
the Lattice Constant Refinement Program of the Rigaku soft-
ware. Energy-dispersive X-ray spectroscopy (EDS, EDAX,
Manuscript No. 22961. Received March 20, 2007; approved August 2, 2007.
This work was supported by Center for Ultramicrochemical Process Systems sponsored
by KOSEF.
wAuthor to whom correspondence should be addressed. e-mail: SeungBinPark@
kaist.ac.kr
3
767

3
768
Journal of the American Ceramic Society—Koo and Park
Vol. 90, No. 12
:
:
:
FeP
Fe2P
Fe P O
7
2
2
Post-treatment
(
(
d)
c)
(b)
(a)
Spray Pyrolysis
60
30
40
50
2ꢀ (°)
Fig. 2. X-ray diffraction patterns of particles prepared by conventional
spray pyrolysis at (a) 9001C and then posttreated at (b) 7001C, (c) 9001C,
or (d) 10001C under 20% H /N mixed gas for 2 h.
2 2
ment temperature and time. However, SEM images of the
particles (Fig. 3) revealed severe agglomeration after posttreat-
ment, which can be attributed to the sintering of particles.
(2) Salt-Assisted Spray Pyrolysis: Effect of Salt
Concentration on the Stoichiometry and Shape in Iron
Phosphides
Fig. 1. Schematic diagram of the spray pyrolysis system.
Addition of salt in the salt-assisted spray pyrolysis solves prob-
lems of conventional spray pyrolysis with regard to the agglom-
eration of particles and crystallization at a low temperature over
short times during posttreatment. Figure 4 shows the XRD pat-
terns of particles prepared with NaCl/Fe molar ratios of 5, 10,
Genesis, Mahwah, NJ) was performed to identify the elements
present in the powder. The morphology and size of the particles
were determined by scanning electron microscopy (SEM, Philips
XL30SREG, Eindhoven, the Netherlands) and transmission
electron microscopy (TEM, TECNAI F30 S-TWIN, FEI, Eind-
hoven, the Netherlands). The hysteresis curves (M–H curve) of
the iron phosphides were obtained at room temperature using a
vibrating sample magnetometer (VSM, RIKEN CENSHI VT-
1
2
5, and 20. At all NaCl/Fe molar ratios, Fe P was formed after
posttreatment at 6001C for 6 h. However, Fe P and sodium iron
2
phosphate coexisted at the NaCl/Fe molar ratio of 5. All XRD
2
patterns of the Fe P phase can be indexed to a hexagonal struc-
ture with the space group P-62m (PDF: 85–1727), and the re-
˚
fined lattice constants are a 5 5.8664 A and c 5 3.4624 A. We
˚
8
00, Tokyo, Japan). The temperature dependence of the mag-
suggest that salt plays the role of a matrix in keeping the inter-
mediates of Fe P particles apart above an NaCl/Fe molar ratio
2
netization curve (M–T curve) during zero-field-cooling (ZFC)
and field-cooling (FC) procedures was examined by a Squid
magnetometer (Quantum Design, MPMS-XL, San Diego, CA)
between 5 and 400 K.
of 10, whereas at a low salt concentration, the crystallites remain
close to each other and agglomerate. SEM pictures (Fig. 5) of
33
the particles prepared with various NaCl/Fe molar ratios show
that they are wire-like and spherical in shape. These images
show that, as the salt concentration is increased, the particle size
decreases and wire-like particles are formed. The wire-like par-
ticles were examined further by means of TEM. TEM images
III. Results and Discussion
(1) Formation of Iron Phosphides by Conventional Spray
Pyrolysis
show that a wire-like Fe P with a high aspect ratio was formed
2
(
Fig. 6(a)). Select area electron diffractions (SAED) confirm that
In conventional spray pyrolysis, iron phosphides FeP and Fe2P
were formed after posttreatment at temperatures of 7001C and
above. The XRD patterns of the iron phosphides prepared by
spray pyrolysis are shown in Fig. 2. At 9001 and 10001C, the
spray-pyrolyzed particles had an amorphous structure
2
Fe P nanowire is a single crystal that grows perpendicular to the
(
001) plane of a basal plane of a hexagonal system (inset of
Fig. 6(a)). Fe P nanowire shows (001), (101), and (100) reflection
2
(
(
9
Fig. 2(a)). After posttreatment at 7001C, FeP and Fe
ferrous diphosphate) formed, with FeP only being present at
001C and Fe P and FeP coexisting at 10001C. EDS analysis
2 2 7
P O
Table I. EDS Data of Particles Prepared by Conventional
Spray Pyrolysis and then Posttreated at Various Temperatures
2
was used to identify the elemental composition of the powders
(
Table I). This analysis demonstrated that iron, phosphorus,
Spray
Iron
Phosphorus
(at.%)
Oxygen
(at.%)
and oxygen are present in the particles, and that the atomic ratio
of oxygen decreases with increasing posttreatment temperature
and time. EDS data show that particles are composed of higher
atomic percent of Fe and O. This is due to the fact that iron
oxide and iron phosphate are present in the amorphous phase,
which is not detected in XRD analysis. The XRD and EDS
analysis results indicate that the particles require posttreatment
for further crystallization and reduction of particles. The stoic-
hiometry of iron and phosphorus depended on the posttreat-
pyrolysis
Posttreatment
(at.%)
9
001C
—
23
35
54
62
68
21
20
30
37
38
33
20
57
35
9
0
0
59
7001C, 2 h
9001C, 2 h
9001C, 6 h
1
0001C, 2 h
1
0001C
—
EDS, energy-dispersive X-ray spectroscopy.

December 2007
Spray Pyrolysis
3769
Fig. 3. Scanning electron microscopy images of particles prepared by conventional spray pyrolysis at (a) 9001C and posttreated at (b) 7001C, (c) 9001C,
or (d) 10001C under 20% H /N mixed gas for 2 h.
2
2
characteristics of the [010] orientation of an Fe P hexagonal
2
ichiometric ratios of iron and phosphorus are in the range of
1.5:1–2:1 in the wire-like particles (Fig. 7). It seems likely that an
excess amount of phosphorus exists in the amorphous phase,
and that oxygen is present due to surface oxidation. On the
other hand, sodium remains in spherical particles and slightly
more oxygen exists. From the EDS data, we found that the un-
confirmed amorphous phase was sodium iron phosphate. We
were unable to remove sodium iron phosphate selectively by
washing.
˚
structure (a 5 5.867 A, c 5 3.458 A, PDF: 85–1727). High-res-
˚
olution transmission electron microscopy image of particles
˚
shows that the lattice fringe distances are 3.5 and 5.1 A, consis-
˚
tent with Fe P interplanar spacing (d001 5 3.4581 A,
2
˚
d100 5 5.0813 A) (Fig. 6(b)). EDS analysis reveals that the sto-
(
111)
:
:
Fe P
2
(3) Magnetic Property: Comparison of Magnetic Property
Due to Shape Anisotropy and Stoichiometry
NaFePO4
The magnetic property of iron phosphides was investigated by
changing the salt concentration and posttreatment temperature:
the magnetic properties were measured by means of VSM and
SQUID analysis. We confirmed that the salt concentration
affected both the stoichiometry and crystalline growth in the
particles and that wire-like particles were formed and the vol-
umetric ratio of wire-like particles increased as the salt concen-
tration increased in SEM images (Fig. 5). Hence, we expected
that the magnetization of particles during ZFC and FC pro-
cesses and transition temperature depended on the transforma-
tion of particle shape and the decrease of particle size owing to
effect of the addition of salt. However, diamagnetization was
observed, which we ascribe to the compositional inhomogeneity
caused by the presence of sodium iron phosphate.
(
201)
(
210)
(
300)
(
002)
(
211)
(
a)
b)
c)
d)
(
(
(
In the present study, we observe that Fe
hysteresis at room temperature. It is known that Fe
a first-order ferromagnetic–paramagnetic transition between
2
P shows magnetic
2
P exhibits
1
2,13
À581 and À71C.
We presume that the magnetic hysteresis
P) or iron
40
45
50
55
arises from the formation of iron-rich phosphides (Fe
3
2
ꢀ (°)
oxide with a structure that is an amorphous phase, whose
detection is not possible by XRD. Figure 8 shows the XRD
patterns of particles posttreated at various temperatures. We
Fig. 4. X-ray diffraction patterns of particles prepared by salt-assisted
spray pyrolysis at 7001C with various NaCl/Fe molar ratios of (a) 5, (b)
0, (c) 15, or (d) 20, and then posttreated at 6001C under 20% H
1
2
/N
2
2 3
identified Fe P and Fe P coexisting after posttreatment at 8001
mixed gas for 6 h.
or 9001C, whereas only Fe P existed after posttreatment at
2

3
770
Journal of the American Ceramic Society—Koo and Park
Vol. 90, No. 12
Fig. 5. Scanning electron microscopy images of particles prepared by salt-assisted spray pyrolysis at various NaCl/Fe molar ratios, (a) 5, (b) 10, (c) 15,
or (d) 20, and then post treated at 6001C for 6 h.
Fe Ka
O Ka
P Ka
Fe La
Fe Kb
(a)
(b)
1
2
3
4
5
6
7
8
Energy (keV)
Fig. 7. Energy Dispersive X-ray spectroscopy data of wire-like particles
prepared at a NaCl/Fe molar ratio of 20. On the (a) surface of particles,
the actual atomic composition is Fe 37%, P 15%, and O 48%, whereas
Fe 60%, P 40%, and O 0% at the (b) center.
1
0001C. In Table II, the EDS result shows that no sodium is left
in the particle at the posttreatment temperature of 10001C.
Some iron component exists as pure iron. This result is consis-
tent with Park et al.’s report in which they claimed that there
Fig. 6. (a) Bright field image (inset: select area electron diffraction pat-
tern) and (b) high-resolution transmission electron microscopy image of
2
Fe P particles prepared at 7001C using a NaCl/Fe molar ratio of 20 and
then posttreated at 6001C for 6 h.

December 2007
Spray Pyrolysis
3771
(
a)
(b)
16
1
1
1
4
2
0
8
6
4
2
0
FC(500Oe)
1
000°C
1000°C
ZFC(500Oe)
900°C
900°C
800°C
800°C
FC(100Oe)
700°C
700°C
600°C
600°C
ZFC(100Oe)
40
45
50
θ (°)
55
40
45
50
55
0
100
200
300
400
2
2θ (°)
T (K)
Fig. 8. X-ray diffraction patterns of particles prepared by SASP with
NaCl/Fe molar ratios of (a) 15 and (b) 20, posttreated at 6001 to 10001C.
2
Fig. 10. Magnetization versus temperature for Fe P prepared by SASP
with a NaCl/Fe molar ratio of 20 and posttreated at 10001C for 6 h.
ꢀ
2 3
( ) Fe P, (J) Fe P.
Table II. EDS Data of Particles Prepared by Salt-Assisted
Spray Pyrolysis and then Posttreated at 10001C
NaCl/Fe molar ratio
Iron (at.%)
Phosphorus (at.%)
Oxygen (at.%)
1
2
5
0
76
74
24
26
0
0
EDS, energy-dispersive X-ray spectroscopy.
6
4
2
0000
0000
0000
0
9
00°C
00°C
8
1000°C
Fig. 11. Shape and stoichiometry of iron phosphides prepared by spray
pyrolysis.
6
00°C
7
00°C
−
−
−
20000
40000
60000
exists a small fraction of pure iron or other Fe-rich phase in the
amorphous phase. The M–H curve (Fig. 9) shows that the
change in the composition of iron phosphide with increasing
posttreatment temperature influenced the magnetic behavior of
(
a)
21
s
the particles. The saturated magnetization (M ) value is the
highest for the systems posttreated at 8001 or 9001C, in which
Fe P and Fe P coexisted. The M–T curve of Fe P formed at
−
10000
−5000
0
5000
10000
2
3
2
H [Oe]
10001C, obtained by means of SQUID analysis, shows a block-
6
4
2
0000
0000
0000
0
ing temperature close to the bulk ferromagnetic transition tem-
perature of 215 K (Fig. 10). There is no characteristic behavior
of pure iron or Fe P in the SQUID measurement.
Combining the results of EDS (Table II), XRD (Fig. 8), and
2
SQUID measurement (Fig. 10), we conclude that Fe P is the
3
8
00°C
9
1
00°C
000°C
6
00°C
major phase at 10001C of posttreatment temperature.
700°C
IV. Conclusions
−
−
−
20000
40000
60000
Iron phosphide particles were prepared by spray pyrolysis from
nontoxic and inexpensive precursors in an aqueous solution. In
conventional spray pyrolysis, the particles were severely agglom-
erated during posttreatment. In salt-assisted spray pyrolysis, by
contrast, homogeneous and isolated Fe
were formed at a low temperature (6001C).
Two key variables affecting shape and stoichiometry were
identified, and the results were summarized on a plot of NaCl/
Fe molar ratio versus posttreatment temperature as shown in
Fig. 11. When salt was not added (conventional spray pyrolysis),
(
b)
2
P wire-like particles
−
10000
−5000
0
5000
10000
H [Oe]
Fig. 9. Magnetization versus applied field (M–H curve) for particles
prepared by SASP with NaCl/Fe molar ratios of (a) 15 and (b) 20,
posttreated at 6001, 7001, 8001, 9001, and 10001C.

3
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Journal of the American Ceramic Society—Koo and Park
Vol. 90, No. 12
12
¨ ¨ ¨ ¨
R. Wappling, L. Haggstrom, S. Rundqvist, and E. Karlsson, ‘‘Mossbauer
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homogeneous FeP particles were formed above 8001C, while the
formation of Fe P required a high temperature of 10001C. How-
2
13
ever, Fe P particles were formed at a low temperature of 6001C
2
when salt was added (salt-assisted spray pyrolysis).
As the salt concentration was increased at a fixed temperature
Magnetic and Catalytic Materials,’’ Chem. Eur. J., 10, 3364–71 (2004).
K. L. Stamm, J. C. Garno, G. Liu, and S. L. Brock, ‘‘A General Methodology
14
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of 6001C, wire-like Fe P particles were formed and the size of
2
the particles decreased. Fe P and Fe P coexisted after posttreat-
2
3
15
2
ment at 8001 or 9001C, whereas only Fe P existed after post-
J. Gopalakrishnan, S. Pandey, and K. K. Rangan, ‘‘Convenient Route for the
Synthesis of Transition-Metal Pnictides by Direct Reduction of Phosphate, Ar-
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3
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16
17
In summary, below the melting temperature of the NaCl (To
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2
2
2
2
P
5 2
2
8
001C), the stoichiometry and shape of iron phosphides depend-
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perature of NaCl, the posttreatment temperature determined the
stoichiometry and shape of iron phosphides.
(
18
S. C. Perera, P. S. Fodor, G. M. Tsoi, L. E. Wenger, and S. L. Brock,
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19
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Acknowledgments
‘
3 4
O
The authors thank the National Nanofab Center for TEM examination and
Korea Basic Science Institute for SQUID measurement.
20
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21
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