Hydrothermal synthesis and characterization of Bi2Fe 4O9 nanoparticles

Article abstract of DOI:10.1246/cl.2004.502

Sheet-like Bi₂Fe₄O₉ nanoparticles were successfully synthesized by a rather facile hydrothermal process. Transmission electron microscopic (TEM) and selected area electron diffraction (SAED) studies show that the particles are sheet-like, about 30 nm in the direction of thickness. X-ray diffraction (XRD) analysis indicated that the as-prepared particles were pure Bi₂Fe₄O₉, but after storing at room temperature for half a year, γ-Fe₂O₃ appeared. The magnetic properties of the nanoparticles before and after storing were characterized by a vibration sample magnetometer (VSM).

Full text of DOI:10.1246/cl.2004.502

502
Chemistry Letters Vol.33, No.5 (2004)
Hydrothermal Synthesis and Characterization of Bi2Fe4O9 Nanoparticles
Ying Xiong, Mingzai Wu, Zhenmeng Peng, Nan Jiang, and Qianwang Chenꢀ
Structure Research Laboratory and Department of Materials Science and Engineering,
University of Science and Technology of China, Hefei 230026, P. R. China
(Received January 20, 2004; CL-040071)
Sheet-like Bi2Fe4O9 nanoparticles were successfully syn-
products was collected by filtration, and washed several times
with distilled water and ethanol, and then dried at 60 ꢁC for 6 h
in air. The as-synthesized sample was characterized by X-ray
powder diffraction (XRD) using a Rigaku D/max-rA diffrac-
thesized by a rather facile hydrothermal process. Transmission
electron microscopic (TEM) and selected area electron diffrac-
tion (SAED) studies show that the particles are sheet-like, about
30 nm in the direction of thickness. X-ray diffraction (XRD)
analysis indicated that the as-prepared particles were pure
Bi2Fe4O9, but after storing at room temperature for half a year,
ꢀ-Fe2O3 appeared. The magnetic properties of the nanoparticles
before and after storing were characterized by a vibration sample
magnetometer (VSM).
ꢀ
tometer with Cu Kꢁ radiation (ꢂ ¼ 1:54178 A.), Transmission
electron microscopic (TEM) and selected area electron diffrac-
tion (SAED) studies were carried out by a Hitachi Model H-
800 instrument with a tungsten filament using an accelerating
voltage of 200 kV. Room temperature M–H loops were meas-
ured on a vibration sample magnetometer (VSM, BHV-55) up
to H = 10 kOe for magnetization measurements.
The XRD pattern of the newly prepared Bi2Fe4O9 powders
was shown in Figure 1(a). All diffraction peaks in Figure 1(a)
can be perfectly indexed to the orthorhombic Bi2Fe4O9 [space
In the 1960s, the crystalline and magnetic structures of
Bi2Fe4O9 had been investigated by neutron diffraction and
¨
nia oxidation to NO is of current interest because it will possibly
replace the high-cost, deficiency, and unrecoverable loss of com-
mercial catalysts (platinum, rhodium, and palladium alloys) in
the industrial process of nitric acid manufacturing.5,6
Mossbauer measurements.1–4 Moreover, its catalysis for ammo-
ꢀ
group: Pbam] with lattice constants of a ¼ 7:995 A, b ¼
ꢀ
ꢀ
8:440 A and c ¼ 5:994 A (JCPDS 25-0090). The average parti-
cle sizes are 27, 23 and 22.6 nm calculated by using the De-
bye–Scherrer formula from the reflection of (211), (121), and
(130), respectively. Also, we find that the relative intensity of
the reflection of (211) to other peaks is much stronger than those
of bulk materials in the JCPDS card, indicating the presence of
(211) orientation in the sample. The XRD pattern of the
Bi2Fe4O9 sample placed for six months under ambient condi-
tions is shown in Figure 1(b). All diffraction peaks can be in-
During the past decades, nanoscale materials have sparked a
worldwide interest for their unique electronic, optic, catalytic,
and magnetic properties and their potential application in the
next generation nanodevices.7–10 Early in the 1964, solid-state
sintering at temperatures over 850 ꢁC has been used to synthe-
size Bi2Fe4O9.1 However, for its disadvantages such as high
temperature, impurity readily present with other compounds,
large dimension, and condition not to be easily controlled, new
methods of the preparation have been investigated until now.
It is well known that hydrothermal processing, for its low cost,
simple process, low temperature synthesis and high monodisper-
sion of product, has been developed for the synthesis of various
nanocrystals, which have a narrow distribution of particle size,
phase homogeneity, and controlled particle morphology.11–14
Since the catalytic activity increases with the surface-to-volume
increase, the nanocrystals obtained by mild hydrothermal condi-
tions are expected to have more application in the field of the cat-
alysis. In our present work, Bi2Fe4O9 nanoscale powders were
prepared at 180 ꢁC via a hydrothermal process.
dexed to the pure Bi2Fe4O9, but
a very weak peak
ꢀ
(d ¼ 2:9396 A) in the diffraction can be indexed of ꢀ-Fe2O3.
We conjecture that the impurity results from the slow decompo-
sition of a small quantity Bi2Fe4O9. The possible reaction is
shown as following:
Bi2Fe4O9 ! Bi2O3 þ 2ꢀ-Fe2O3
ð1Þ
Bismuth nitrate pentahydrate (Bi(NO3)3ꢂ5H2O) and iron(III)
nitrate nonahydrate (Fe(NO3)3ꢂ9H2O) were used as starting ma-
terials. The typical synthesis condition was briefly described as
following: 5 mmol of bismuth nitrate pentahydrate and 5 mmol
of iron nitrate nonahydrate were dissolved into 5 mL dilute nitric
acid (1 M) together to make solution A. Then, 6.20 g of sodium
hydroxide (NaOH) was dissolved into 20 mL distilled water to
make solution B. The solution B was dropwise added to the solu-
tion A with vigorous stirring. After 10 min, the reaction mixture
was transferred to a Teflon-lined stainless autoclave till 80% of
its volume was filled. Then, the autoclave was sealed and main-
tained at 180 ꢁC for 24 h. After the reaction completed, the auto-
clave was cooled to room temperature on standing. The saffron
Figure 1. XRD patterns of the Bi2Fe4O9 via a hydrothermal
process, (a) fresh sample; (b) sample placed six months at ambi-
ent conditions. A very weak peak in (b) was assigned to ꢀ-
Fe2O3, which resulted from the decomposition of Bi2Fe4O9.
Figure 2 shows a typical TEM image of the as-prepared
sample. The TEM image reveals that the sample consists of a
large number of sheet-like particles and the selected area elec-
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.5 (2004)
503
tron diffraction (SAED) of one typical particle confirms the sin-
gle crystal feature of the particles. The above results reveal that
the orientation growth of the particles occurs in the process of
the synthesis, which agree with the results of the X-ray diffrac-
tion.
Figure 4. The XRD patterns of the samples formed at different
conditions. (a) N ¼ 9, [M3þ] ¼ 0:1 M and 180 ꢁC. (b) N ¼ 15,
[M3þ] ¼ 0:1 M and 150 ꢁC.
Figure 2. A typical TEM image (a) and select area electron dif-
fraction (SAED) (b) of an as-prepared sample.
ions to the sum of metal ions (N=OHꢄ/(Bi3þ + Fe3þ)) in the
precursor solution should be kept very high; otherwise
Bi2Fe4O9, BiFeO3 and Bi25FeO40 would coexist (Figure 4a).
The lower N in the solution, the more quantities of the Bi25FeO40
would form. The pure Bi25FeO40 could not be prepared until the
pH decreased to 8.5, and pure Bi2Fe4O9 would be formed if the
value of N was up to 15. The influence of the phase components
was not evidenced when the value of N exceeded 15. Further-
more, the experiments were carried out at 120, 150, and
180 ꢁC for 24 h. When the precursor solution in an autoclave
was treated at 120 and 150 ꢁC (Figure 4b), no Bi2Fe4O9 was
formed and the product was almost pure Bi25FeO40. As the tem-
perature increases to 180 ꢁC, the pure well-crystallized Bi2Fe4O9
could be obtained. When the molar ratio of the Fe3þ and Bi3þ
ions (m = Fe3þ/Bi3þ) was in the range of 1 and 2 and the other
conditions were the same, no impurities were formed, which was
different from the previous results in Ref. 1.
The magnetic measurement results of the samples before
and after storing for half a year were shown in Figure 3. It
showed that the fresh sample is paramagnetic in agreement with
previous neutron diffraction and Mossbauer measurements.3,4
¨
The permeability ꢃ of the products is equal to 6:99 ꢃ 10ꢄ6 H/
m. But it is surprising that we could observe the magnetic hyte-
resis loop in the sample placed for six months under ambient
conditions, as shown in the inset of Figure 3.
This work is supported by the National Natural Science
Foundation of China (NSFC) under grant numbers 20125103
and 90206034.
Figure 3. The magnetization curves versus external magnetic
field of the fresh sample, showing a distinct paramagnetism.
The inset shows a magnetic hysteresis loop for the sample placed
for six months at ambient condition, which arised from the im-
purity of ꢀ-Fe2O3.
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Published on the web (Advance View) March 30, 2004; DOI 10.1246/cl.2004.502