Continuous hollow a-Fe2O3 and a-Fe fibers prepared by the sol–gel

method

Cairong Gong, Dairong Chen,* Xiuling Jiao and Qilong Wang

Department of chemistry, Shandong University, Jinan 250100, P. R. China.

E-mail: cdr@sdu.edu.cn

Received 4th February 2002, Accepted 8th April 2002

First published as an Advance Article on the web 26th April 2002

Continuous iron oxide gel fibers were prepared by the sol–gel method using ferric alkoxide and acetic acid as
starting materials and alcohol as solvent, and continuous hollow a-Fe2O3 fibers produced after the gel fibers
were heat treated at 400 uC for 1 h. The outer diameter of the a-Fe2O3 fibers is between 4–10 mm and the wall
thickness is 1–2 mm, in which the a-Fe2O3 particle size is about 100 nm. 2-Methoxyethanol and acetic acid
played important roles in the formation of the spinnable sol and the possible mechanism for the formation of
the hollow a-Fe2O3 fibers was discussed in detail. The hollow a-Fe fibers could be obtained by reducing the
as-prepared a-Fe2O3 fibers under hydrogen flow and the a-Fe particle size increased to y150 nm.

1 Introduction 2 Experimental
Tubular materials have attracted considerable attention because 2.1 Synthesis
of their peculiar physi-chemical properties and potential
applications in nanofabrication.1 To date, several preparative Iron(III) chloride was purchased from Shanghai Chemical Co.
approaches have been reported for the preparation of oxide (P. R. China), absolute ethanol, benzene, 2-methoxyethanol
tubes by the sol–gel method. Kozuka et al. prepared hollow and acetic acid were purchased from Tianjin Chemical Co.
YBa2Cu3O72x2 and Bi–Pb–Sr–Ca–Cu–O3 superconducting (P. R. China). All reagents were of analytical grade and further
fibers composed of plate-like crystals as large as 5–10 mm dehydrated before utilization.
wide and 0.5 mm thick using acetates as precursors. Aizawa Fe(OC2H5)3 used as precursor was prepared in absolute
et al.4 synthesized hollow TiO2 fibers from titanium tetra- ethanol according to eqn. (1):12
isopropoxide, in which the hollow part was typically 2 mm Refluxing
in length and occupied half of the total length of the fiber, FeCl3 z3NaOC2 H5 ? Fe(OC2 H5 )3 z3NaCl; (1)
N2 atmosphere
and Kobayashi et al.5 synthesized hollow TiO2 fibers using
supramolecular assemblies. Nakamura et al.6 obtained a new The sodium chloride was removed by centrifugation and
type of silica-gel tube by a sol–gel method at room tempera- the solution was distilled under reduced pressure and the
ture and the products had square shapes with an outer remainder was then further purified by extraction with
diameter of 0.8–1.0 mm and 200–300 mm in length. Ono et al.7 benzene, the product was kept in absolute ethanol to prevent
reported the synthesis of hollow silica nanofibers using Fe(OC2H5)3 from hydrolyzing. 30 ml Fe(OC2H5)3/C2H5OH
different amphiphilic molecules as templates. However, the solution with concentration ranging from 0.45–0.96 mol dm23
hollow fibers with short length would impose limitations on was added into a three-necked flask containing some
their potential applications. To the best of our knowledge, 2-methoxyethanol (MOE), the molar ratio (n) of iron(III) to
there have been no papers reported on the preparation of MOE being 1 : 3. After refluxing for 2 h, a red deposit appeared
hollow a-Fe2O3 fibers. In this paper, continuous iron oxide in the solution, which was proved to be oligomeric ferric
green fibers were prepared from ferric alkoxide sol precursor, alkoxides by FT-IR spectroscopic invesitgation. Then acetic
then the hollow hematite fibers were obtained after burning out acid was added into the above system with the molar ratio of
the organic moieties from the green fibers. The length of the iron(III)to acetic acid being 1 : 10, the deposit dissolved and a
as-prepared fibers was as long as 20 cm and the outer diameter clear solution was obtained. The solution was refluxed for an
was between 4–20 mm. Hematite is widely applied in catalysis8 additional 2 h to give the precursor sol. The sol was transferred
and gas sensing9 and the as-prepared micro-tubes have into a 100 ml beaker and conditioned at room temperature for
potential applications in capillary electrophoresis. Further- 7 days and then kept at 40 uC for several days until gel fibers
more, reduction of hematite in hydrogen atmosphere resulted could be drawn from it. The aging time changed with the ferric
in the formation of hollow a-Fe fibers. As absorbing materials, alkoxide and the value of n. Table 1 gives the experimental
a-Fe fibers with large absorbing capacity in a wide frequency conditions for preparing the spinnable sol. The gel fibers drawn
range are extensively applied in obscuring aircraft.10 It is from the spinnable sol were dried at room temperature for 2
proposed that the hollow fibers might be applied as micro- days to remove free organic solvent, during which time the
reactors in catalytic reactions, which exhibit enhanced catalytic gelation continued. Subsequently, they were held at 200 uC for
activity and facility compared with the corresponding bulk 2 h with the aim of removing the residual free organic
counterparts.11 Furthermore, micro-nano devices can also be compounds. Then they were further heated to 400 uC for 1 h to
fabricated using the hollow fibers as host and nano-materials as let the acetates decompose. Finally, the products were held at
guest. either 500, 600 or 750 uC for 1 h. The whole calcination process

1844 J. Mater. Chem., 2002, 12, 1844–1847 DOI: 10.1039/b201243j

Table 1 Description of preparative condition of the spinnable sol

Sample C/mol dm23 VMOE/ml VHAc/ml Aging time/day

1 0.45 3.2 7.7 9

2 0.64 4.5 11 9
3 0.72 5.1 12.4 10
4 0.72 10.2 12.4 12
5 0.72 15.3 12.4 14
6 0.96 6.8 16.5 12

was carried out in air and the heating rate was 100 uC h21. The
as-prepared hollow hematite fibers were reduced in a tubular
furnace under hydrogen flow at temperatures ranging between
340–380 uC for 0.5 h or 1 h; a-Fe hollow fibers were prepared
when the reacting time was 1 h.
The ferric acetate was prepared as follows: 13.3 g
FeCl3?6H2O was dissolved in 50 ml of distilled water, and
5 ml aqueous ammonia (25%) was added dropwise. Then fresh
ferric hydroxide was obtained by filtration and washed with
distilled water several times. The product was dissolved in
200 ml of acetic acid with stirring for 4 h. After evaporation at
60 uC by water bath, the expected products were obtained.

2.2 Characterization Fig. 2 The XRD patterns of hollow fibers reduced by hydrogen flow at
350 uC for (a) 0.5 h and (b) 1 h.
The phase of the fibers was identified by X-ray diffraction
(XRD Rigaku D/Max 2200PC diffractometer with Cu-Ka increasing the heating temperature, there were no obvious
radiation and graphite monochromator) with a scanning rate
changes in the XRD spectra, only the intensity of the peaks
of 8u min21 in the 2h range from 20 to 70u. Microscopic
increased and the full width at half-maximum of the hematite
observation of the fibers was performed by scanning electron
Bragg peak decreased, which suggested the growth of the
microscopy (SEM, Hitachi, Model S-520) under an accelerat-
particles.
ing voltage of 20 kV and by transmission electron microscopy
The XRD patterns of the fibers reduced by hydrogen are
(TEM, JEM-100CX) at an accelerating voltage of 100 kV.
shown in Fig. 2. It can be seen that when the reaction time was
The infrared (IR) spectra of the products were measured using
0.5 h, the product was a mixture of Fe3O4 and a-Fe. Upon
the KBr disc technique in the range 400–4000 cm21. (FTIR,
Nicolet ZOSX). Thermo-gravimetric analysis (TGA) curve was increasing the reaction time to 1 h, phase-pure a-Fe formed.
recorded using a TGA/SDTA851e module from METTLER Obviously, the as-prepared hollow hematite fibers were very
TOLEDO in nitrogen flow in the temperature range 50–500 uC, easy reduced to a-Fe, and the reduction condition was mild
the heating rate being 20 uC min21 and the data was collected compared to the bulk counterparts.13
and analyzed with STARe software. Fig. 3 gives the FT-IR spectra of the gel fibers and those heat
treated at 200, 300 and 400 uC for 1 h, and the FT-IR spectra of
CH3OCH2CH2OH and Fe(OOCCH3)3 are also shown. The IR
3 Results and discussion
As shown in Fig. 1, the fibers which were treated at 200 uC for
1 h still remained amorphous, and when the fibers were held at
300 uC for 1 h, the peaks located at 2h angles of 23, 33, 38, 55
and 66u suggested the occurrence of b-Fe2O3 in addition to
the a-Fe2O3 phase. Upon increasing the heating temperature to
400 uC, b-Fe2O3 completely converted to a-Fe2O3. On further

Fig. 3 IR spectra of (a) MOE, (b) ferric acetate, (c) green gel fibers, and
Fig. 1 XRD patterns of the green gel fibers (a) and the fibers after of hollow fibers after treatment at (d) 200 uC, (e) 300 uC and (f) 400 uC
treatment at 200 uC (b), 300 uC (c), 400 uC (d) and 600 uC (e) for 1 h. for 1 h.

J. Mater. Chem., 2002, 12, 1844–1847 1845

spectrum of the carboxylate group demonstrated the char-
acteristic doublet absorption in the wavenumber range 1200–
1700 cm21 due to the asymmetric and symmetric stretching
vibrations of COO2 (nas(OCO) and ns(OCO)). There are three
modes of coordinated carboxylate ligands: monodentate (I),
chelating (II) and bridging (III).

The values of nas(OCO) and the frequency differences

between nas(OCO) and ns(OCO) offer the most sensitive
indication of the mode of carboxylate coordination.14 Fig. 3b
gives a characteristic IR absorption of a metal acetate. The
band at 3394 cm21 is assigned to the stretching vibration of
OH, and the bands at 1698 and 1348 cm21 are ascribed to
monodentate nas(OCO) and ns(OCO). There are three bands
appearing at 1608, 1519 and 1446 cm21 and the frequency Fig. 4 SEM images of the (a) green gel fibers, (b) fibers after treatment
differences between them are 162 and 73 cm21, respectively, at 400 uC for 1 h, (c) cross-section of a fiber after treatment at 200 uC for
1 h and (d) magnified surface of a fiber after treatment at 400 uC for 1 h.
suggesting the coexistence of chelating and bridging ligands.
The bands between 700–400 cm21 are ascribed to the stretching
vibration of Fe–O. The IR spectrum for gel fibers shown in
Fig. 3c is found to have well-defined doublet peaks at 1576 and
1446 cm21, and the bands of monodentate and chelating OCO
nearly disappear. It can be seen from Fig. 3c that the band
assigned to nOH moves to 3371 cm21, which may result from the
formation of a hydrogen bond. It can be assumed that the main
coordinate mode of the carbonyl oxygen of the acetate to a
ferric atom center is bridging giving the chain structure, and Fig. 5 TEM images of the hollow fibers after treatment at 400 uC for
hydrogen bonds form, which account for the spinnability of the 1 h.
sol. Fig. 3a gives the typical IR spectrum of MOE. Comparing
Fig. 3a and Fig. 3c, it can be concluded that MOE did not bond
to iron(III). The FT-IR spectra of the fibers sintered at 200 uC
have no obvious changes compared with the spectra of the gel
fibers, only the band assigned to nOH reverted to 3396 cm21
which may result from the damage of the hydrogen bonds. The
IR spectrum of the fiber heated at 300 uC indicates that the
organic parts have been removed completely. The peaks at
548 and 472 cm21 are assigned to a-Fe2O3 and the bands of
b-Fe2O3 are located at 620, 570 and 453 cm21. Upon further
increasing the heating temperature, the bands of n(Fe–O)
belonging to b-Fe2O3 completely disappear and pure a-Fe2O3
can be obtained.
Very interestingly, the hollow structure is found after the gel
Fig. 6 SEM images of the fibers reduced by hydrogen flow at 350 uC for
fiber is exposed to air for about one month; even at this stage, (a) 0.5 h and (b) 1 h.
there are a large amount of organic moieties remaining in the
hollow fiber from the IR spectrum. Fig. 4 shows the SEM
images of the gel fibers and the hollow a-Fe2O3 fibers after particles are as large as 150 nm in diameter. Obviously, the
calcination at 200 and 400 uC for 1 h. It can be seen from nanoparticles in the fiber become larger after reduction
Fig. 4b that the outer diameter of the hollow a-Fe2O3 fibers The effect of the molar ratio (n) of Fe(OC2H5)3 to MOE on
is between 4–15 mm with the most frequent ones being around the property of the sol was studied. It was found that spinnable
4–10 mm. Fig. 4c gives the cross-section of the hollow fibers sol could be obtained when n changed between 1 : 3 and 1 : 9,
which have been held at 200 uC for 1 h. It clearly indicates but the weight percent solids of the gel fibers were 64% for
that the tube structure forms and the thickness of the wall is sample 3 and 48% for sample 5 (Fig. 7), respectively. The
between 1–2 mm. The high magnification SEM is used to view thermogravimetric analysis (TGA) curves showed that when
the surface particles of the fiber heat treated at 400 uC for 1 h the temperature ranged from 140–280 uC, the weight loss of the
(Fig. 4d). Obviously, the fiber wall is composed of well-defined gel fibers was changed from 19% to 34% when n changed from
particles, 100 nm in diameter. 1 : 3 to 1 : 9, but a constant weight loss was held from 280–
The TEM image of the hollow fiber fired to 400 uC is shown 400 uC. It can be concluded that the former weight loss may be
in Fig. 5. A tubular structure can be seen, but it is not obvious ascribed to the removal of free organic solvent, and the latter
because of the non-uniform wall thickness, the visible tube is one resulted from the decomposition of the acetate groups,
not located in the middle of the hollow fiber. It is clear that the which is consistent with the IR results (Fig. 3). Replacing the
wall of the fibers is made up of nanoparticles and the particle MOE with other organic compounds such as octanol and 1,2-
size is around 100 nm, which agree with the SEM results. propanediol or MOE being absent, the spinnable sol could not
Fig. 6 gives the SEM images of a-Fe fibers. It shows that the be obtained. It can be sure that MOE did not bond to iron(III)
fiber kept the morphology of the hematite fiber, but the surface in the gel fiber from Fig. 3. What is the role of the MOE in the

1846 J. Mater. Chem., 2002, 12, 1844–1847

 4 Conclusion
Continuous hollow a-Fe2O3 fibers have been prepared by a
sol–gel method using ferric alkoxide containing acetic acid
as starting materials. The outer diameter of the as-prepared
hollow fibers is 4–20 mm with most frequent ones being 4–10 mm
and the wall thickness is between 1–2 mm. Hollow a-Fe fibers
are also prepared by reducing the as-prepared a-Fe2O3 fibers
under hydrogen flow at 350 uC for only 1 h. The possible
mechanism for the formation of the continuous hollow
fiber is given as follows. Ferric acetate formed with the main
coordination mode of the acetate anion to ferric being bridging
and MOE hydrogen-bonded to ferric acetate, which resulted
Fig. 7 DTG curves of the gel fibers with the molar ratio of ferric in the spinnable sol. A dense outer shell came into being as soon
alkoxide to MOE being (a) 1 : 3 and (b) 1 : 9. as the gel fiber was drawn. A tube structure first formed with
the organic compounds vaporizing, and the tube structure was
kept in the decomposed process of acetate anions.
system? In the present synthesis, the molar ratio of Fe(OC2H5)3 The hollow ferric-based fibers have potential applications in
to MOE influences the ageing time (Table 1), and spinnable catalytic and materials science. Currently, detailed studies are
sols can be obtained while n is changed from 1 : 3 to 1 : 9. Here, in progress to modify the hollow fibers with nano-materials to
the possible formation mechanism of the spinnable sol is as organize micro-nano devices.
follows. The –OC2H5 of the Fe(OC2H5)3 molecule can be
replaced by an alkoxy group (–OR) from the other alcohol,
which is termed an alcohol-interchanging reaction (eqn. (2)).12 References
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J. Mater. Chem., 2002, 12, 1844–1847 1847