Synthesis of nanosize-necked structure α- and γ-Fe 2O3 and its photocatalytic activity

Article abstract of DOI:10.1111/j.1551-2916.2006.01424.x

Highly dispersed nanometer-sized α-Fe₂O₃ (hematite) and γ-Fe₂O₃ (maghemite) iron oxide particles were synthesized by the combustion method. Ferric nitrate was used as a precursor. X-ray diffractometer study revealed the phase purity of α- and γ-Fe₂O₃. Both the products were characterized using field emission scanning electron microscope and transmission electron microscope for particle size and morphology. Necked structure particle morphology was observed for the first time in both the iron oxides. The particle size was observed in the range of 25-55 nm. Photodecomposition of H ₂S for hydrogen generation was performed using α- and γ-Fe₂O₃. Good photocatalytic activity was obtained using α- and γ-Fe₂O₃ as photocatalysts under visible light irradiation.

Full text of DOI:10.1111/j.1551-2916.2006.01424.x

J. Am. Ceram. Soc., 90 [2] 412–414 (2007)
DOI: 10.1111/j.1551-2916.2006.01424.x
r 2006 The American Ceramic Society
ournal
J
Synthesis of Nanosize-Necked Structure a- and c-Fe2O3 and its
Photocatalytic Activity
S. K. Apte, S. D. Naik, R. S. Sonawane, and B. B. Kalew
Centre For Materials For Electronics Technology (C-MET), Ministry of Information and Communication Technology,
Goverment of India, Panchawati, Pune 411008 Maharashtra, India
J. O. Baeg
Advanced Chemical Division, Korea Research Institute of Chemical Technology, Daejon, South Korea
Highly dispersed nanometer-sized a-Fe2O3 (hematite) and c-
Fe2O3 (maghemite) iron oxide particles were synthesized by the
combustion method. Ferric nitrate was used as a precursor. X-
ray diffractometer study revealed the phase purity of a- and
c-Fe2O3. Both the products were characterized using field emis-
sion scanning electron microscope and transmission electron
microscope for particle size and morphology. Necked structure
particle morphology was observed for the first time in both the
iron oxides. The particle size was observed in the range of 25–55
nm. Photodecomposition of H2S for hydrogen generation was
performed using a- and c-Fe2O3. Good photocatalytic activity
was obtained using a- and c-Fe2O3 as photocatalysts under
visible light irradiation.
II. Experimental Procedure
(1) Synthesis
Ferric nitrate, glycine, ammonium nitrate, and starch (Aldrich,
Schnelldorf, Germany) were used in the synthesis of iron oxide.
For the preparation of a-Fe2O3, ferric nitrate, glycine, and am-
monium nitrate were taken in a stoichiometric mole ratio (1:1:1)
and mixed thoroughly. The starch (0.1%–0.5%) was added in
the above reaction mixture and homogenized, precisely. The
reaction mixture (paste) was poured slowly in the combustion
reactor at 5501, 6001, and 6501C and kept for 2 h. A very fine
powder of a-Fe2O3 was obtained. Glycine was used as a com-
busting agent. For the preparation of g-Fe2O3, a-Fe2O3 was
further mixed with polyethylene glycol (PEG)–400 and com-
busted at 4001C for 2 h. Nanosize pure-phase g-Fe2O3 was ob-
tained. PEG added in this reaction system acts as an oxidant,
surfactant, and dispersing agent. This ultimately prevents the
particle agglomeration and results in the formation of high sur-
face area free-flowing Fe2O3.
I. Introduction
RON oxide has been extensively studied in the past, owing to
its wide applications as a pigment, in magnetic materials, and
(2) Photodecomposition of H2S
I
as catalysts and sensors.1–5 The motivation for the fabrication of
various morphologies and sizes of Fe2O3 nanoparticles was
driven by the strong interest in their novel properties and po-
tential application in electronics.6–8 Major emphasis is given on
magnetic recording applications-oriented materials. The unique
magnetic properties of nanosized magnetic particles have made
them potentially useful from both technological and theoretical
points of view.9 In the past, many synthesis methods have been
developed for the preparation of a and hematite (g-Fe2O3) in-
cluding sol–gel, hydrolysis of iron salt, solvothermal, and hy-
drothermal synthesis.7–10 As it is well understood that the
electronic properties are dependent on particle morphology
and size, recent research has been focused on the size and mor-
phological control of iron oxide products.7–11 The catalytic
properties depend on the morphology, particle size, and surface
area. In view of this, in the present study, we have synthesized a
novel-necked structure nanosize a- and g-Fe2O3 for the first
time. Additionally, we have performed the photocatalytic activ-
ity of these iron oxides in H2S splitting, i.e. hydrogen generation
under visible light irradiation. The use of iron oxide as a pho-
tocatalyst has been given particular attention due to their ab-
sorption in the visible region, abundance, and low cost. Very
limited literature is available12,13 on the photocatalytic study
of iron oxide. Therefore, we have studied the photocatalytic
activity of necked structured nanosize iron oxide for the first time.
The photocatalyst was introduced as a suspension into a cylin-
drical Pyrex photochemical reactor with a quartz window and a
thermostated water jacket. A Xe-lamp light source (Oriel, New-
port Stratford, Stratford, CT) of intensity 450 W with a cut-off
filter (>420 nm) was used. At a constant temperature of
251711C, the vigorously stirred suspension was purged with
argon for 1 h and then hydrogen sulfide (H2S) was bubbled
through the solution for about 1 h. Each experiment was carried
out using 0.5 g of catalyst in 500 mL of KOH solution (0.5M)
with a H2S flow, 2.5 mL/min. The excess hydrogen sulfide was
trapped in NaOH solution. The amount of evolved hydrogen
was measured using graduated gas burette and a gas chromato-
˚
graph (model Shimadzu GC-14B, Kyoto, Japan; MS-5 A col-
umn, TCD, Ar carrier).
(3) Characterization
Powder X-ray diffractograms were recorded using an X-ray dif-
fractometer (XRD, model Rigaku-D/Max-2200V, Tokyo, Japan)
with CuKp radiation and a Ni filter. The surface morphology
and particle size were determined using a field emission scanning
electron microscope (FESEM model JEOL-JSM6700F, Tokyo,
Japan) and transmission electron microscopy (TEM, model EF-
TEM EM 912 Omega, Carl Zeiss (LeO), Oberkochen, Germany).
Sonicated samples in ethanol were used for the TEM measure-
ment. The spectroscopic study of the catalyst was carried out
using a UV-visible (model Shimadzu UV-2450 diffuse reflectance
model) spectrophotometer. Specific surface area measurements
were performed using a BET surface area analyzer (model BET -
Micromeritics ASAP-2400, Norcross, GA). FTIR (Spectrum—
2000 Perkin Elmer, Wellesley, MA) was used to perceive the
stretching of metal oxide bonding.
P. Gouma—contributing editor
Manuscript No. 21598. Received March 17, 2006; approved July 10, 2006.
wAuthor to whom correspondence should be addressed. e-mail: kbbb1@yahoo.com
412

February 2007
Synthesis of Nanosize-Necked Structure a- and g-Fe2O3
413
Fig. 3. Field emission scanning electron microscope (FESEM) image of
(a) hematite (a-Fe2O3), (b) maghemite (g-Fe2O3).
(2) Particle Morphology
Particle morphology of the samples was studied using FESEM.
Figures 3(a) and (b) represents the FESEM micrographs of a-
Fe2O3 and g-Fe2O3, respectively. The necked structure morph-
ology was observed in both the samples. Such type of a unique
morphology was observed for the first time in iron oxide. The
same samples were characterized using a TEM. Figures 4(a) and
(b) depict the TEM images of a-Fe2O3 and g-Fe2O3, respective-
ly. TEM images clearly showed the micro-structural homoge-
neities and a remarkably unique necked structure morphology.
FESEM and TEM pictures also show the nanocrystalline and
porous nature of the product. From the FESEM and TEM, the
particle size was observed to be in the range of 45–55 nm. There
was no appreciable difference in the particle size of both the iron
oxide samples, i.e., for a-Fe2O3 and g-Fe2O3. FESEM and TEM
images show that g-Fe2O3 is slightly porous and smaller in size.
Fig. 1. X-ray diffractometer (XRD) pattern of hematite (a-Fe2O3).
III. Results and Discussion
(1) XRD Analysis
(3) Spectroscopic Analysis
The a-Fe2O3 obtained at different temperatures was character-
ized using an XRD. Figures 1 (a)–(c) depict the XRD pattern of
a-Fe2O3 prepared at 5501, 6001, and 6501C, respectively. The
a-Fe2O3 diffraction peaks are identified, which are in good
agreement with the reported values (JCPDS no. 33–363).13
XRD data also revealed that the a-Fe2O3 obtained had an
orthorhombic structure. There was no indication of any other
additional phases, except a-Fe2O3 in the XRD pattern. It is also
observed that the a-Fe2O3 is formed at 5501C itself. There was
no appreciable change in structure at a higher temperature viz.
6001 and 6501C, which indicates the good stability of a-Fe2O3.
Figure 2 shows the XRD pattern of g-Fe2O3 prepared using
PEG at 4001C. The diffraction peaks in the XRD pattern are in
good agreement with the reported values given in the literature.13
From the XRD data, it was observed that the XRD peaks of a-
and g-Fe2O3 show good broadening, indicating the ultra fine
nature of the particles. The crystallite size calculated using Sch-
errer’s equation is found to be 27 nm for a-Fe2O3 and 22 nm for
g-Fe2O3. The results are matching with the BET surface area of
the a-Fe2O3 (61 m2/g) and g-Fe2O3 (68 m2/g), respectively. There
was no appreciable change in the particle size of a- and g-Fe2O3.
Figures 5(a) and (b) represents the UV-visible diffused reflect-
ance spectra of a-Fe2O3 and g-Fe2O3, respectively. The absorp-
tion edge of both the samples is at 570–572 nm. The band gap
estimated from the onset of the absorption edge was observed at
2.17–2.18 eV, which is in good agreement with a bandgap value
of 2.2 eV for Fe2O3.14 The steep absorption edge in both the
samples is indicative of the uniform particle morphology and
size with fairly good crystallinity.15 A marginal shift in the ab-
sorption edge for g-Fe2O3 may be due to slightly porous and
small size-necked structure particles.15
Infrared spectroscopic studies were performed aimed to as-
certain the metal oxygen nature of the product and to follow the
dehydration process through the combustion technique. The
FTIR of samples (a) and (b) are shown in Fig. 6. Sample (a)
corresponds to a-Fe2O3 and (b) corresponds to g-Fe2O3. Sam-
ples (a) and (b) showed absorption in the regions 3436, 1630,
1384, 532, 553, 458, and 442, and 374 cmÀl. The general range of
3600–3100 cmÀl (relating to antisymmetrical and symmetrical
O–H stretching) may be assigned for water of hydration. Hy-
drates also absorb in the region of 1670–1600 cmÀl (relating to
O–H bonding).15 This latter bond can be taken as another im-
portant means of identifying water of crystallization and it has
been found to be very useful in the elucidation of the crystal
structure. In sample (a), the band associated with the lattice
water molecule is broad and is observed in the region of 3436
Fig. 4. Transmission electron microscopy (TEM) image of (a) hematite
(a-Fe2O3), (b) maghemite (g-Fe2O3).
Fig. 2. X-ray diffractometer (XRD) pattern of maghemite (g-Fe2O3).

414
Journal of the American Ceramic Society—Apte et al.
Vol. 90, No. 2
g-Fe2O3. However, we believe that a-Fe2O3 and g-Fe2O3 may be
showing good activity due to the nanosize porous and necked
structure. Faust et al.12 have already demonstrated the photo-
catalytic activity of a-Fe2O3 and concluded that surface hydrox-
yl groups as the principal reactive sites on the metal oxide
surface are responsible for the photocatalytic activity. In the
present case, the higher surface area and nanocrystalline-necked
porous structure of a-Fe2O3 and g-Fe2O3 further accelerate the
photocatalytic activity. As iron oxide has a bandgap of 2.2 eV, it
therefore absorbs solar radiation from 295 to 600 nm, which
comprises 38% of the phonons of the solar spectrum. So, the
photocatalytic activity obtained over a naked a-Fe2O3 and
g-Fe2O3 is quite explicable. Owing to the low cost of this cata-
lyst. further work in this context related to parameters like sta-
bility, structure, and regeneration is under progress.
Fig. 5. UV-diffused reflectance spectra (DRS) spectrum of (a) hematite
(a-Fe2O3), (b) maghemite (g-Fe2O3).
IV. Conclusion
Nanosize-necked structure a- and g-Fe2O3 have been prepared
from ferric nitrate by the combustion method. The a-Fe2O3
obtained has an orthorhombic structure, whereas g-Fe2O3 has
a tetragonal structure. The FESEM and TEM study revealed a
unique necked structure with a particle size in the range of 45–
55 nm. The BET surface area was observed to be in the range
of 61–68 m2/gm.
The steep absorption edge was observed at 572 nm (band gap,
–2.18). a- and g-Fe2O3 have shown good photocatalytic activity
for hydrogen generation for H2S under visible light irradiation.
The maximum hydrogen production rate achieved was 50 mL/h
for a-Fe2O3 and g-Fe2O3.
and 1630 cmÀl. The simultaneous presence of these two bonds
indicates that the water of crystallization is likely to be present in
the sample. The same behavior was observed in sample (b), i.e.,
g-Fe2O3. It is reported that the presence of water plays an im-
portant role in the formation and stabilization of g-Fe2O3.16 The
bands at 458 and 432 cmÀl observed in sample (a) and at 442
and 553 cmÀl in sample (b) are due to metal oxygen stretching
vibrational modes. These bands are intense and sharp. There
was a slight shift in the g-Fe2O3 because of absorbed oxygen and
some amount of water being removed from g-Fe2O3.17 The in-
tensity of the broad peaks at 3600–3000 cmÀl is found to be
decreasing from samples (a) to (b). The shifts observed for the
above samples may be due to the particle size effect.
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