Thermal conversion of hollow prussian blue nanoparticles into nanoporous iron oxides with crystallized hematite Phase

Article abstract of DOI:10.1002/ejic.201301307

We recently demonstrated that Prussian blue (PB) coordination polymers can be successfully etched by acidic solution for the preparation of hollow PB nanoparticles (Angew. Chem. Int. Ed. 2012, 51, 984). In this paper, by using hollow PB nanoparticles as starting materials, we calcined them under various conditions to prepare nanoporous Fe oxides with a crystallized α-Fe ₂O₃ (hematite) phase. The obtained particles were carefully characterized by scanning electron microscopy, wide-angle X-ray diffraction, nitrogen gas adsorption-desorption isotherms, transmission electron microscopy, and Mo?ssbauer spectroscopy. The morphologies, surface areas, and degrees of crystallinity of the samples were varied by changing the number of hours of calcination. After calcination at 400 °C for 4 h, formation of a crystallized α-Fe₂O₃ phase was confirmed, although some residues of amorphous and/or γ-Fe₂O₃ phases were also present. With a further increase in the calcination time (up to 7 h), the α-Fe₂O₃ phase was predominantly formed. The obtained sample exhibited high surface area, which will be useful for photocatalytic applications. Hollow Prussian blue nanoparticles are calcined under various reaction conditions to prepare nanoporous Fe oxides with a crystallized α-Fe₂O₃ (hematite) phase. The morphologies, surface areas, and degrees of crystallinity of the samples are varied by changing the number of hours of calcination. After calcination at 400 °C for 4 h, formation of a crystallized α-Fe₂O₃ phase is confirmed. Copyright

Full text of DOI:10.1002/ejic.201301307

SHORT COMMUNICATION
DOI:10.1002/ejic.201301307
Thermal Conversion of Hollow Prussian Blue
Nanoparticles into Nanoporous Iron Oxides with
Crystallized Hematite Phase
Mohamed B. Zakaria,[a,b,c][‡] Ming Hu,[b][‡] Naoaki Hayashi,[d]
Yoshihiro Tsujimoto,[e] Shinsuke Ishihara,[b,f] Masataka Imura,[b]
Norihiro Suzuki,[f] Yu-Yuan Huang,[g] Yoshio Sakka,[e]
Katsuhiko Ariga,[b] Kevin C.-W. Wu,[g] and Yusuke Yamauchi*[a,b,h]
Keywords: Mesoporous materials / Nanoparticles / Nanoporous materials / Iron / Prussian blue
We recently demonstrated that Prussian blue (PB) coordina-
tion polymers can be successfully etched by acidic solution
for the preparation of hollow PB nanoparticles (Angew.
Chem. Int. Ed. 2012, 51, 984). In this paper, by using hollow
PB nanoparticles as starting materials, we calcined them un-
der various conditions to prepare nanoporous Fe oxides with
and Mössbauer spectroscopy. The morphologies, surface
areas, and degrees of crystallinity of the samples were varied
by changing the number of hours of calcination. After calci-
nation at 400 °C for 4 h, formation of a crystallized α-Fe2O3
phase was confirmed, although some residues of amorphous
and/or γ-Fe2O3 phases were also present. With a further in-
a crystallized α-Fe2O3 (hematite) phase. The obtained par- crease in the calcination time (up to 7 h), the α-Fe2O3 phase
ticles were carefully characterized by scanning electron mi-
croscopy, wide-angle X-ray diffraction, nitrogen gas adsorp-
tion–desorption isotherms, transmission electron microscopy,
was predominantly formed. The obtained sample exhibited
high surface area, which will be useful for photocatalytic ap-
plications.
broad range of applications, which include their use in ca-
talysis, drug delivery, and chemical storage. Among these
materials, iron oxides are potentially useful for catalysts[1]
and lithium-ion batteries.[2] Iron oxides can exhibit several
crystal structures, including wüstite (FeO), magnetite
(Fe3O4), maghemite (γ-Fe2O3), cubic (β-Fe2O3), hematite
(α-Fe2O3), and orthorhombic (ε-Fe2O3). In particular, the
interest in hematite (α-Fe2O3) has recently increased owing
to its unique optical and magnetic behavior, high catalytic
activity, suitable band gap, and low toxicity.
Developing new routes for the preparation of nano-
porous α-Fe2O3 and investigating its distinct properties are
of considerable interest. In a recent study, α-Fe2O3 hollow
spheres with sheetlike subunits were synthesized by a facile
quasiemulsion-templated method, in which glycerol was
dispersed in water to form oil-in-water quasiemulsion mic-
rodroplets.[3] Porous α-Fe2O3 nanorods with diameters of
30–60 nm were also synthesized through thermal decompo-
sition of FeC2O4·2H2O nanorods prepared by a poly(vinyl
alcohol)-assisted precipitation process. Compared to com-
mercial submicrometer-sized α-Fe2O3 powders, porous α-
Fe2O3 nanorods exhibit significantly enhanced capability as
an electrode material for lithium-ion batteries.[4] As another
example, 3D flowerlike iron oxide showed excellent ability
to remove heavy-metal ions and other pollutants in water
treatment.[5] Thus, nanoporous α-Fe2O3 materials with high
surface areas have exhibited very interesting properties that
Introduction
Various inorganic nanoporous materials have been draw-
ing intense research interest not only for their unique struc-
tural, optical, and surface properties, but also for their
[a] Faculty of Science and Engineering, Waseda University,
3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
[b] World Premier International (WPI) Research Center for
Materials Nanoarchitechtonics (MANA), National Institute for
Materials Science (NIMS),
1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
E-mail: Yamauchi.Yusuke@nims.go.jp
http://www.yamauchi-labo.com
[c] Department of Chemistry, Faculty of Science, Tanta University,
Tanta, Gharbeya 31527, Egypt
[d] Institute for Integrated Cell-Material Sciences, Kyoto
University,
Yoshida-Ushinomiya, Sakyo, Kyoto 606-8501, Japan
[e] Materials Processing Unit, National Institute for Materials
Science (NIMS),
1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
[f] International Center for Young Scientists (ICYS),
National Institute for Materials Science (NIMS),
1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
[g] Department of Chemical Engineering, National Taiwan
University,
No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
[h] Precursory Research for Embryonic Science and Technology
(PRESTO), Japan Science and
Technology Agency (JST),
2-1 Hirosawa, Wako, Saitama 351-0198, Japan
[‡] These authors contributed equally to this work.
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201301307
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Table 1. Summary of nanostructured α-Fe2O3 materials previously reported.
Shape
Fe source
Surface area
[m2 g–1]
Ref.
[3]
[4]
Hollow spheres
Porous nanorods
3D flowerlike
FeSO4·7H2O
FeC2O4·2H2O
FeCl3·6H2O
FeCl3·6H2O
FeC2O4·2H2O
FeCl3
103
11.8
40.0
–
–
–
125
86.8
105
125
22.3
82.6
66.8
190
18.2
–
[5]
[6]
Porous nanorods
[7]
Spherical nanoparticles
Mesoporous films
[8]
[9]
Flutelike porous nanorods
3D urchinlike superstructures
Mesoporous nanorods
Mesoporous nanorods
Porous nanoparticles
Porous nanorods
Spindlelike nanostructures
Mesoporous thin films
Mesoporous nanospheres
Hollow-structured particles
Nanotubes
β-FeOOH
[10]
[11]
[12]
[13]
[14]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
FeSO4·7H2O
Fe(NO3)3·9H2O
α-FeC2O4
FeCl3
FeCl3·6H2O
Fe(NO3)3·9H2O
Fe(NO3)3·9H2O
FeCl3
FeCl3·6H2O
Fe(NO3)3·9H2O
FeCl3·7H2O
ferric potassium oxalate
FeCl3
–
–
41.1
335
Hollow microspheres
Hollow spheres
Mesoporous particles
are not attainable by bulk α-Fe2O3 without nanopores. time durations (4, 5, 6, and 7 h). The samples were heated
Table 1 summarizes the structural parameters of various inside an electric furnace from room temperature to 400 °C
nanoporous α-Fe2O3 previously reported.[6–21]
at a heating rate of 1 °Cmin–1. After that, the powder was
Recently, we demonstrated the synthesis of hollow cooled inside the furnace. Finally, the obtained powders
Prussian blue (PB) coordination polymers by chemical were collected for further characterization. The entire calci-
etching[22] and reported their thermal conversion into nation process was performed in air.
nanoporous iron oxide particles.[23] PB coordination poly-
mers in which iron ions are bridged by cyano groups
(–FeII–CN–FeIII–) are considered potential precursors to
nanoporous iron oxides with high surface areas, because the
iron component can be oxidized and the organic compo-
nents can be removed by calcination in air. This approach
is simple and convenient, which is useful for the further
preparation of several iron oxides with different degrees of
crystallinity and different structures. Herein, we extend this
concept to synthesize nanoporous iron oxides with the α-
Fe2O3 phase. The obtained samples were carefully charac-
terized by scanning electron microscopy (SEM), wide-angle
X-ray diffraction (XRD), nitrogen gas adsorption–desorp-
tion isotherms, transmission electron microscopy (TEM),
and Mössbauer spectroscopy.
Figure 1. SEM images of (a) PB nanoparticles before chemical
etching, (b) PB nanoparticles after chemical etching, and samples
calcined for (c) 4, (d) 5, (e) 6, and (f) 7 h.
Results and Discussion
The morphology of the calcined samples was observed
For the creation of the hollow cavity, the synthesized PB by scanning electron microscopy (SEM), as shown in Fig-
particles (Figure 1, a) were treated with 1.0 m HCl solution ure 1 (c–f). After calcination at 400 °C for 5 h, the surface
in the presence of polyvinylpyrrolidone (PVP). The PVP structure of the hollow particles clearly changed as a result
polymer was on the external surface of the PB particles and of the crystallization of the PB shell to iron oxide (Figure 1,
thus could serve as a protecting agent during the HCl etch- d). With a further increase in the calcination time to 6 and
ing process.[22] Even after etching, the average diameter of 7 h (Figure 1, e,f), the hollow structure completely col-
the PB particles did not change (ca. 100 nm), and the corre- lapsed. In the starting PB crystals, the Fe atoms were stabi-
sponding particle size distribution was very narrow, as lized by cyano bridges. Upon calcination at high tempera-
shown in Figure 1 (b). The diameter of the internal cavity tures under air flow, the C–N bridges were removed to form
was calculated to be approximately 80 nm. To prepare nan- Fe oxides. After calcination, no carbon content was con-
oporous iron oxides with hematite (α-Fe2O3) phase, we fur- firmed in the final product. The result of thermogravimetry/
ther calcined the hollow PB particles at 400 °C for different differential thermal analysis/mass spectrometry (TG–DTA–
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MS) under He/O2 flow (volume ratio = 80:20, flow rate = peak/area ratio of 30:70. On the basis of the obtained
200 mLmin–1) indicated several exothermic peaks at ap- Mössbauer parameters listed in Table 2, the former was as-
proximately 260–310 and 470 °C, accompanied with the signed as α-Fe2O3. The latter exhibits an isomer shift char-
production of CO2 and/or N2O (m/z = 44) as well as N2 acteristic of FeIII, which was caused by the presence of im-
and/or CO (m/z = 28), which is typical for the combustion purities such as amorphous iron oxide and/or γ-Fe2O3
reaction of a CN-containing material. Emission of ad- phases. Increasing the calcination time up to 7 h signifi-
sorbed H2O (m/z = 18) was also detected. Molecules with cantly improved the quality and volume fraction of α-Fe2O3
masses larger than m/z = 50 were not detected by TG– to around 65% (Table 2 and Figure 3, b). This is in good
DTA–MS up to 1000 °C.
agreement with the XRD data above (Figure 2).
To investigate the effect of the calcination time on the
crystalline phases and degrees, we performed wide-angle
XRD measurements, as shown in Figure 2. With an in-
crease in the calcination time, the intensities of the peaks
for the α-Fe2O3 phase gradually increased, which is indica-
tive of an enhancement in crystal growth. The crystalline
sizes were calculated by using the Scherrer equation. The
average sizes varied from approximately 12.2 (for 4 h), 15.4
(for 5 h), and 17.1 nm (for 6 h) to 17.1 nm (for 7 h). Thus,
the increase in the calcination time promoted the crystalli-
zation of α-Fe2O3, which thereby led to a collapse of the
original hollow structure. As clearly seen in Figure 1 (e,f),
the formation of large-sized nanopores was caused by the
gradual crystallization of iron oxides.
Figure 3. 57Fe Mössbauer spectra collected at room temperature
from samples calcined for (a) 5 and (b) 7 h. Red and blue lines rep-
resent the contributions from α-Fe2O3 and some impurity phases
(amorphous and/or γ-Fe2O3).
The hollow structure of the starting hollow PB particles
and the crystalline structure of the calcined iron oxide sam-
ples were examined by TEM, as shown in Figure 4. Solid
PB nanocubes were transformed into hollow cubes after
chemical etching (Figure 4, a). The sample that was cal-
cined for 5 h showed morphology that was similar to that of
the original hollow PB particles with similar particle sizes
(Figure 4, b). The hollow structure was well retained even
after the crystallization of the iron oxides. Selected-area
electron diffraction (ED) showed very intense spots assign-
Figure 2. Wide-angle XRD profiles for samples calcined for (a) 4,
(b) 5, (c) 6, and (d) 7 h.
To further understand the crystalline structure, we col- able to α-Fe2O3, and this indicated the formation of a
lected 57Fe Mössbauer spectra from the calcined PB sam- highly crystallized α-Fe2O3 phase. Because several α-Fe2O3
ples for 5 and 7 h, as shown in Figure 3. The spectroscopic crystals were randomly oriented in different directions, sev-
data was well resolved into a sextet and a doublet with a eral patterns were mixed with each other. From a high-reso-
Table 2. Mössbauer parameters of samples calcined for 5 and 7 h.
Calcination time
[h]
Isomer shift
[mms–1]
Hyperfine field
[kOe]
Quadrupole splitting
[mms–1]
FWHM[a]
[mms–1]
Relative peak area
[%]
Phase
5
5
7
7
0.37
0.34
0.37
0.33
505.5
0
505.8
0
–0.22[b]
0.73
0.34
0.56
0.28
0.57
23
77
64
36
α-Fe2O3
α-Fe2O3
–0.23[b]
0.73
[a] FWHM = full width at half maximum. [b] (S2 – S1)/2; S1 and S2 are differences of the two outer peaks on the lower and higher
velocity side of the sextet, respectively.
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lution TEM image, clear lattice fringes were oriented in the 0.01 mm, 15 mL) and then sonicated in a cool water bath
same direction at the edge of the cube, which confirmed the for 5 min. Afterward, the suspension was magnetically
formation of the α-Fe2O3 crystalline phase (Figure 5, a). stirred in the dark for 4 h to reach the complete adsorption–
The surface area and pore volume were calculated to be desorption equilibrium, which was followed by the addition
163 m2 g–1 and 0.89 cm3 g–1, respectively, from the corre- of hydrogen peroxide solution (30 wt.-%, 0.2 mL); the bot-
sponding N2 adsorption–desorption isotherms. These val- tle was then exposed to UV light irradiation at room tem-
ues were much lower than those of the sample that was perature.
calcined for 4 h (480 m2 g–1 and 1.5 cm3 g–1, respectively).[23]
The time-dependent UV/Vis absorption spectra in the
absence and presence of iron oxides are shown in Figure S1
(Supporting Information). The remaining MB concentra-
tion was analyzed by measuring the peak intensity derived
from MB at 664 nm. With an increase in the irradiation
time, the maximum absorption intensity gradually de-
creased; this suggests the decolorization and decomposition
of the MB molecules (Figure S1, Supporting Information).
In a control experiment without the photocatalyst, the de-
composition rate under UV light was very slow. In the pres-
ence of the photocatalyst, the decomposition rate was sig-
nificantly increased. With an increase in the hematite (α-
Fe2O3) content (i.e., with an increase in the calcination
time), the decomposition rate of the MB molecules in-
creased (Figure 6). The sample calcined for 7 h exhibited
the highest decomposition rate of all the samples as a result
of the high content of the crystallized hematite phase (α-
Fe2O3), which was confirmed by Mössbauer spectroscopy
(Figure 3). It is suggested that the mechanism for the de-
composition of the dye occurs on the photocatalyst surface
rather than in the bulk solution. Some electrons generated
Figure 4. TEM images and the corresponding ED patterns of
(a) PB nanoparticles after chemical etching, and samples calcined
for (b) 5 and (c) 7 h.
Figure 5. High-resolution TEM images and corresponding fast
Fourier transform (FFT) images of samples calcined for (a) 5 and
(b) 7 h.
In contrast, long calcination times of 6 and 7 h destroyed
the original hollow structures (Figure 4, c). However, a con-
tinuous nanoporous structure derived from the original hol-
low structure was well developed. The surface areas and
pore volumes were calculated to be 100 and 0.50 (for 6 h)
and 70.0 m2 g–1 and 0.41 cm3 g–1 (for 7 h), respectively. Al-
though a longer calcination time decreased the surface
areas of the samples upon crystallization of α-Fe2O3, these
values are highly relative to those of other nanostructured
α-Fe2O3 previously reported (Table 1).
Figure 6. Time-dependent change of (A) absorbance at 664 nm and
(B) MB concentration (Co means the initial MB concentration be-
fore UV irradiation): (a) MB (without catalyst), (b) MB and H2O2
(without catalyst), and samples calcined for (c) 5, (d) 6, and (e) 7 h.
Our porous iron oxides with the α-Fe2O3 phase are useful
as photocatalysts. To investigate the photocatalytic activity,
the photocatalyst (5.0 mg, several iron oxide samples cal-
cined for 5, 6, and 7 h) was mixed with methylene blue (MB, The error bars are also shown in Figure (A).
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Supporting Information (see footnote on the first page of this arti-
cle): Time-dependent UV/Vis spectra.
on the surface under UV light are directly trapped by H2O2
to form OH· radicals. As a result, the OH· radicals lead to
the photocatalytic reaction.[24]
Acknowledgments
Mr. Natsuhiko Sugimura, Mr. Takahiro Yatsu, and Dr. Takeo Suga
(Waseda University, Japan) are acknowledged for kind assistance
with the TG-DTA-MS measurements. K. C.-W. W would like to
thank the following institutions for funding support: National Sci-
Conclusions
To conclude, we established a facile route to prepare
nanoporous iron oxides with an α-Fe2O3 crystalline phase
by thermal decomposition of PB coordination polymers. By ence Council of Taiwan (101-2628-E-002-015-MY3), National Tai-
wan University (102R7842 and 102R7740), Center of Strategic Ma-
terials Alliance for Research and Technology (SMART Center),
National Taiwan University (102R104100).
using hollow PB particles as the starting material, we pre-
pared well-organized nanoporous iron oxides with high sur-
face areas and large pore volumes. Their crystalline phases
and degrees were controlled by several calcination condi-
tions. The use of PB as the starting material is a new and
unique technology, and further use of various PB analogues
with different compositions will produce more functional
materials in the future.
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(132 mg) were added to 0.01 m HCl (40.0 mL) under magnetic stir-
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vial was then placed into an electric oven and heated at 80 °C for
20 h. After aging, the precipitate was collected by centrifugation
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drying at room temperature for 12 h, PB nanocubes of approxi-
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added to 1.0 m HCl (40 mL) in a Teflon vessel under magnetic stir-
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Characterization: SEM images were taken with a Hitachi SU8000
scanning microscope at an accelerating voltage of 5 kV. TEM ob-
servation was performed by using a JEM-2100F TEM system that
was operated at 200 kV and equipped for energy-dispersive spec-
trometer analysis. Wide-angle powder XRD patterns were obtained
with a Rigaku RINT 2500X diffractometer by using monochro-
mated Cu-Kα radiation (40 kV, 40 mA) at a scanning rate of
2 °min–1. N2 adsorption–desorption isotherms were measured on
a high precision adsorption measuring apparatus BELSORP-mini
(BEL Japan) and quanta chrome Autosorb1 Automated Gas Sorp-
tion System. Prior to the measurements, the samples were degassed
in vacuo. The surface areas were calculated by the BET method by
using the adsorption branches of the isotherm. 57Fe Mössbauer
spectra were recorded at room temperature in transmission geome-
try by using a 57Co/Rh γ-ray source. The source velocity was cal-
ibrated by using α-Fe as a reference material. TG–DTA–MS were
measured with a Rigaku Thermo Mass Photo TG-DTA-PIMS 410/
S. The detection range of the MS was from 1 to 250 m/z. The
sample and α-Al2O3 standard were separately loaded inside plati-
num pans, and heated from room temperature to 1000 °C at
10 °Cmin–1 under He/O2 flow to analyze the mass spectrometry of
thermally produced compounds.
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Received: October 6, 2013
Published Online: February 6, 2014
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