XRD, TEM and thermal analysis of Fe doped boehmite nanofibres and nanosheets

Article abstract of DOI:10.1007/s10973-006-8248-0

Iron doped boehmite nanofibres with varying iron content have been prepared at low temperatures using a hydrothermal treatment in the presence of poly(ethylene oxide) surfactant. The resultant nanofibres were characterized by X-ray diffraction (XRD), and transmission electron microscopy (TEM). TEM images showed the resulting nanostructures are predominantly nanofibres when Fe doping is no more than 5%; in contrast nanosheets were formed if Fe doping was above 5%. For the 10% Fe doped boehmite, a mixed morphology of nanofibres and nanosheets were obtained. Nanotubes instead of nanofibres were observed in samples with 20% added iron. The Fe doped boehmite and the subsequent nanofibres/nanotubes were analysed by thermogravimetric and differential thermogravimetric methods. Boehmite nanofibres decompose at higher temperatures than non-hydrothermally treated boehmite and nano-sheets decompose at lower temperatures than the nanofibres.

Full text of DOI:10.1007/s10973-006-8248-0

Journal of Thermal Analysis and Calorimetry, Vol. 90 (2007) 3, 755–760
XRD, TEM AND THERMAL ANALYSIS OF Fe DOPED BOEHMITE
NANOFIBRES AND NANOSHEETS
Y. Zhao, R. L. Frost*, W. N. Martens and H. Y. Zhu
Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO
Box 2434, Brisbane Queensland 4001, Australia
Iron doped boehmite nanofibres with varying iron content have been prepared at low temperatures using a hydrothermal treatment
in the presence of poly(ethylene oxide) surfactant. The resultant nanofibres were characterized by X-ray diffraction (XRD), and
transmission electron microscopy (TEM). TEM images showed the resulting nanostructures are predominantly nanofibres when Fe
doping is no more than 5%; in contrast nanosheets were formed if Fe doping was above 5%. For the 10% Fe doped boehmite, a
mixed morphology of nanofibres and nanosheets were obtained. Nanotubes instead of nanofibres were observed in samples with
20% added iron. The Fe doped boehmite and the subsequent nanofibres/nanotubes were analysed by thermogravimetric and differ-
ential thermogravimetric methods. Boehmite nanofibres decompose at higher temperatures than non-hydrothermally treated
boehmite and nano-sheets decompose at lower temperatures than the nanofibres.
Keywords: acicular, boehmite, nanofibre, nanosheets, nanotube
Introduction
from boehmite can keep the original size and mor-
phology after calcination, great effort has been de-
voted to the investigation of nanoscale boehmite ma-
terials, especially 1D nanostructures, such as
nanofibres and nanotubes [16, 17].
Recently, synthesis of inorganic nanoscale materials
with special properties have been of great interest in
material science [1, 2], because their intrinsic proper-
ties of nanoscale materials are mainly determined by
their composition, structure, crystallinity, size and
morphology [3]. In particular, one dimensional (1D)
nanoscale inorganic materials including nanofibers,
nanowires and nanotubes have attracted intensive in-
terest due to their distinctive geometries, novel physi-
cal and chemical properties and potential applications
in numerous areas [4].
Because of its high surface area, chemical and
thermally stable properties and mesoporous proper-
ties, alumina has been extensively used as carrier and
support for a variety of industrial catalysts at high
temperature as well as low temperature. Alumina can
be employed as catalyst [5], adsorbent [6, 7], compos-
ite materials [8, 9] and ceramics [10–12]. Boehmite
(g-AlOOH), a principal oxo hydroxide of aluminium,
is a crucial precursor in sol–gel technique for prepar-
ing high-purity and high-strength monolithic a-alu-
mina ceramics for use as substrates for electronic cir-
cuits, abrasive grains, high-temperature refractory
materials, fibres and thin films [13]. Boehmite
nanofibres were first synthesised by Bugosh [14] in
1961. Nanofibres can be used as molecular building
units in the preparation of core/shell materials
[10, 15]. Also, since the resulting alumina prepared
Latterly, Zhu et al. [18] reported an interesting new
synthesis method by using a surfactant. Rather than act-
ing as templates as for the synthesis of mesoporous ma-
terials, the surfactant was able to direct formation of
boehmite fibres [17]. It was also reported that a much
higher Al concentration and lower temperatures can be
used compared to traditional methods for the synthesis
of boehmite nanofibres. This is an efficient approach of
producing nanofibres in large quantity. The growth of
boehmite nanofibres can be improved by a regular inter-
val supply of fresh precipitate of aluminium hydrate and
the fibres can grow to over 100 nm long when reaction
conditions are well-controlled. It is well known that the
necessity of later catalyst or absorbent separation is an
important issue in industrial catalytic process and water
treatment. Except for filtration technique, magnetic sep-
aration is another good method [19, 20]. To achieve
magnetic property, doping with iron would be an ideal
way. Besides, doped with iron, the resulting boehmite
nanostructure may have special optical properties which
will enable it to further industrial applications.
Thermal analysis has proved most useful for the
analysis of minerals and related materials [21–30]. In
this work, boehmite nanofibres based on Zhu’s meth-
odology were synthesised by introducing iron as dop-
*
Author for correspondence: r.frost@qut.edu.au
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2007 Akadémiai Kiadó, Budapest

ZHAO et al.
TEM analysis
ant and a series of iron doped boehmite nanofibres
with varying iron content have been systematically
studied using thermogravimetric techniques.
A Philips CM 200 transmission electron microscopy
(TEM) at 200 kV was used to investigate the mor-
phology of the boehmite nanofibres. All samples were
dispersed in absolute ethanol solution and then
dropped on copper grids coated with carbon film,
dried in an oven at 60°C for 10 min for TEM studies.
Experimental
Synthesis of Fe-doped boehmite nanofibres
The detailed experimental procedure is as follows. A
total amount of 0.25 mol aluminium nitrate and ferric
nitrate were mixed before being dissolved in ul-
tra-pure water. To make a comparison, mixtures with
iron molar percentage of 0, 1, 2, 3, 4, 5, 10 and 20%
were prepared separately and then dissolved in ul-
tra-pure water to form a solution with a metal ion to
H2O molar ratio of 1:100 and heated to 80°C.
5 mol L–1 NaOH solution was then added dropwise at
a constant rate of 5 mL min–1 to form precipitate. Af-
ter that it was aged for three hours with constant stir-
ring at 80°C, the resulting precipitate was recovered
by centrifugation, washed with pure water four times
to remove sodium nitrate. The fresh precipitate was
then split into five parts evenly for hydrothermal
treatment and characterization. Water and non-ionic
Thermal analysis
Thermal decomposition of the Fe-doped boehmite
was carried out in a TA® Instruments incorporated
high-resolution thermogravimetric analyzer (se-
ries Q500) in a flowing nitrogen atmosphere
(60 cm3 min–1). Approximately 35 mg of each sample
underwent thermal analysis, with a heating rate of
5°C min–1, with resolution of 6 from 25 to 1000°C.
With the isothermal, isobaric heating program of the
instrument the furnace temperature was regulated pre-
cisely to provide a uniform rate of decomposition in
the main decomposition stage.
Results and discussion
PEO surfactant Tergitol 15-S-7 (C12–14H25–29
O
X-ray diffraction
(CH2CH2O)7H, Aldrich) with average molecular mass
of ~508 were mixed with the first part of precipitate at
a metal:PEO:H2O molar ratio of 1:0.4:16. The vis-
cous mixture is stirred for 1 h at room temperature and
then transferred into an autoclave and kept in oven at
120°C. The remaining three parts of fresh precipitate
and the same amount of ultra-pure water were added
into autoclave every two days. Accordingly, the mo-
lar ratio of metal:PEO:H2O changed to 2:0.4:32,
3:0.4:48 and 4:0.4:64, respectively, after 2, 4, 6 days.
The final product was washed by water first and then
acetone and dried in air at 80°C.
The X-ray diffraction of the precipitated boehmite and
the hydrothermally treated boehmite are shown in
Figs 1a and b. The XRD patterns show the precipitates
Methods
X-ray diffraction
XRD analyses were performed on a PANalytical
X’Pert PRO X-ray diffractometer (radius: 240.0 mm).
Incident X-ray radiation was produced from a line fo-
cused PW3373/10 Cu X-ray tube, operating at 45 kV
and 35 mA, wavelength of 1.540596 . The incident
beam passed through a 0.04 rad, Soller slit, a 1/2° di-
vergence slit, a 15 mm fixed mask and a 1° fixed
anti-scatter slit. After interaction with the sample, the
diffracted beam was detected by an X’Celerator
RTMS detector. The detector was set in scanning
mode, with an active length of 2.022 mm. Samples
were analysed utilising Bragg–Brentano geometry
over a range of 3–75° 2q with a step size of 0.02° 2q,
with each step measured for 200 s.
Fig. 1 a – Powder X-ray diffraction patterns of various % Fe
doped boehmite and b – nanofibres
756
J. Therm. Anal. Cal., 90, 2007

Fe DOPED BOEHMITE NANOFIBRES AND NANOSHEETS
Fig. 2 TEM images of a – 3%, b – 5% and c– 10% Fe doped boehmite nanofibres
to be boehmite g-AlOOH except for a minor amount of
doped sample; however, nanotubes of 40–100 nm in
length and 5–10 nm in diameter were formed. The Fe
content affects not only the growth of the nanofibres
but also the aggregation of nanostructures. At low Fe
content, nanofibres with regular shape were formed;
at a higher iron content, nanofibres aggregated paral-
lel resulting in the formation of nanosheets with
pointed ends; at high Fe content, no fibres are formed
and some of the sheets rolled to form nanotubes.
bayerite formed in 0 and 1% samples. By using the
Debye–Scherrer equation the crystallite sizes vary initial
from 15.5 nm in pure boehmite precipitates, to a con-
stant value of ~22 nm with Fe doping. The XRD peaks
in the patterns for 1% Fe doped boehmite nanostructures
show decreased peak width with the increase in hydro-
thermal time, indicating
a gradual increase in
crystallinity of the nano-structures during the soft chem-
ical treatment. The bayerite phase converts into
boehmite with hydrothermal treatment.
Thermogravimetric analysis
The thermogravimetric analysis and the differential
thermal analysis of boehmite and Fe doped boehmite
and their nanostructures with varying amounts of Fe
from 0 to 20% are shown in Figs 3–5. The results of the
Transmission electron microscopy
Typical transmission electron microscopy images for
3, 5 and 10% Fe doped nanofibres and nanosheets are
shown in Figs 2a–c. The growth of pure boehmite
nanofibres at 100°C has been systematically studied
by Zhu et al. [17, 18, 31]. The average length of
boehmite nanofibres after 8 days hydrothermal treat-
ment was found to be ~92 nm. Hydrothermal treat-
ment at 150°C resulted in the formation of 6 nm wide
and 30 to 70 nm long lath like nanostructures [31]. In
this work, boehmite nanostructures at 120°C were
grown. As can be seen from Fig. 1a pure boehmite
nanofibres with length up to 300 nm were synthe-
sised, which is around three times long as that re-
ported in a previous study [17].
As for iron doped samples, nanofibres were
formed in samples with added iron content up to 10%.
The iron doped boehmite fibres were up to 250 nm
long while their width varied. Significant amounts of
nanosheets were formed in samples with added iron
percentage above 5%. Interestingly, most of the
nanosheets formed have bevelled ends. As the iron
doping was increased, more sheets formed; in particu-
lar, nanosheets are dominant in 10 and 20% iron
doped samples. The nanofibres in these two samples
aggregated in a parallel fashion forming bundle-like
nanostructures resulting from increased surface
charge of the nanostructure with increasing amounts
of Fe. No nanofibres were observed in the 20% iron
Fig. 3 Thermal analysis patterns of boehmite and boehmite
nanofibres
J. Therm. Anal. Cal., 90, 2007
757

ZHAO et al.
thermal analyses are reported in Table 1, including the
boehmite nanofibre the peak at 207°C is diminished in
intensity with a 1.49% mass loss. This decomposition is
attributed to the combustion of the surfactant molecules
used to template the nanofibres. The boehmite nanofibre
decomposes over the 343 to 383°C. There is a 13.90%
mass loss over this temperature range. It is concluded
that boehmite nanofibres decompose at temperatures
higher than for untreated boehmite.
results form other Fe doped boehmite compositions.
Pure boehmite shows four decomposition steps .
The step which is characteristic of boehmite is the
sharp DTG step at 397°C with a mass loss of 4.55%.
The first mass loss occurs at around 50°C with a mass
loss of 8.11%. This mass loss step is attributed to ad-
sorbed water. The second mass loss step is observed
at around 152°C and accounts for the major mass loss
step with 18.15% mass loss. This mass loss is attrib-
uted to interstitial water trapped between the
boehmite layers. The theoretical mass loss based upon
the equation:
For the 2% Fe doped boehmite four decomposi-
tion steps are observed at 50°C (7.25%), 169°C
(14.21%), 302°C (11.44%) and 447°C (1.90%).
These thermal decomposition steps are attributed to
(a) the loss of adsorbed water, (b) the decomposition
of the surfactant, (c) the thermal decomposition of the
goethite, (d) the decomposition of boehmite. The
thermal decomposition at temperatures above 400°C
is attributed to impurities. Step (c) is different to that
of this step observed for the 1% Fe doped boehmite. It
is possible that the goethite is an Al substituted
goethite [32–34]. For the 2% Fe doped boehmite
nanofibres two decomposition steps are observed at
245 and 335°C with mass losses of 7.35 and 13.98%.
The thermal decomposition of the 3% Fe doped
boehmite and 3% Fe doped boehmite nanofibres are
similar to that of the 2% Fe doped materials. For the
3% Fe doped boehmite nanofibre two mass loss steps
are observed at 289 and 340°C with mass losses of
10.55 and 8.44%. For the 4% Fe doped boehmite ther-
mal decomposition takes place over a wide tempera-
ture range from 269 to 374°C. Such a wide tempera-
ture range is indicative of ill defined and even amor-
2AlOOH®Al2O3+H2O
is 11.5%. In order to synthesize the boehmite
nanofibres, the boehmite (precipitate) is hydrother-
mally treated at 120°C for 8 days. In comparison the
thermal analysis of the boehmite nanofibres (Fig. 3b)
shows a large thermal decomposition step at 247 and
312°C with a mass loss of 18.87%. It is concluded
that the boehmite nanofibres thermally decompose at
a higher temperature than boehmite. The sharp band
at 403°C is attributed to boehmite in a non-fibre form.
The 1% Fe doped boehmite thermal decomposi-
tion shows an additional sharp mass loss of 12.15% is
observed at 195°C. This mass loss is attributed to the
decomposition of goethite according to the equation:
2FeOOH®Fe2O3+H2O
The boehmite precipitate decomposes over a tem-
perature range from 229 to 364°C. In the 1% Fe doped
Fig. 4 Thermal analysis patterns of 4% Fe doped boehmite
and boehmite nanofibres
Fig. 5 Thermal analysis patterns of 5% Fe doped boehmite
and boehmite nanofibres
758
J. Therm. Anal. Cal., 90, 2007

Fe DOPED BOEHMITE NANOFIBRES AND NANOSHEETS
Table 1 Results of the thermal analysis of boehmite and Fe doped boehmite and the equivalent nanofibres
Decomposition step
Added Fe %
First
Second
Third
Fourth
0
1
8.11%/55°C
7.34%/50°C
7.25%/50°C
6.88%/55°C
4.43%/46°C
6.69%/50°C
6.20%/50°C
3.96%/50°C
18.15%/152°C
12.15%/195°C
14.21%/169°C
11.51%/171°C
4.17%/147°C
4.92%/318°C
25.52%/348°C
11.44%/302°C
13.74%/313°C
14.98%/327°C
14.92%/351°C
10.43%/337°C
7.73%/350°C
4.55%/397°C
–
2
1.90%/447°C
3
1.91%/702°C
Precipitate
4
–
–
–
–
5
8.74%/148°C
10
20
10.15%/139~250°C
7.99%/143~268°C
0
1*
1
4.13%/50°C
1.78%/50°C
1.30%/50°C
1.82%/50°C
1.78%/50°C
1.06%/50°C
12.78%/120°C
1.00%/50°C
1.14%/50°C
8.70%/247°C
1.37%/213°C
1.49%/207°C
7.35%/245°C
10.55%/289°C
7.09%/272°C
12.19%/292°C
3.62%/230°C
–
10.17%/312°C
14.42%/384°C
13.90%/383°C
13.98%/335°C
8.44%/340°C
6.16%/353°C
10.08%/376°C
4.04%/295°C
–
1.19%/403°C
–
–
2
–
Nanofibre
3
–
4
8.04%/379°C
5
10
20
11.09%/376°C
17.40%/380°C
*hydrothermally treated for 6 days
Conclusions
phous boehmite materials (Figs 4a and b). The ther-
mal decomposition at 241 and 272°C is attributed the
combustion of the surfactant templating molecules.
The temperature may be compared with the tempera-
tures of 147 and 210°C for the 4% doped boehmite
precipitate. For the 4% Fe doped boehmite nanofibres
two decomposition steps at 272 and 379°C with mass
losses of 7.09 and 14.20%. A similar set of results is
observed for the 5% Fe doped boehmite precipitate
and nanofibres. Three decomposition steps are ob-
served for the 5% Fe doped boehmite precipitate at
around 50, 150 and 351°C. For the 5% Fe doped
boehmite nanofibres thermal decomposition steps are
observed at 55, 120, 292 and 376°C.
As the concentration of Fe is increased the hydro-
thermally treated boehmite becomes a mixture of
nanofibres and nanosheets. The 10% doped boehmite
precipitate shows three decomposition steps at 139 to
179, 250 and 304 to 367°C. The hydrothermally treated
boehmite shows three peaks at 230, 295 and 376°C. One
possible assignment of these peaks is to (a) first decom-
position step to goethite, (b) the decomposition of the
surfactant and the third decomposition step to boehmite
nanosheets. For the 20% Fe loaded boehmite the DTG
patterns are similar to those of the 10% Fe doped sam-
ple. For the 20% Fe loaded boehmite nanomaterials a
broad thermal decomposition is observed from 200 to
400°C with a maximum at 380°C.
Fe doped boehmite nanofibres 250 nm in length have
been prepared at 120°C using soft chemical hydro-
thermal methodology using PEO as a surfactant di-
recting agent. Fibres or needles were formed at low Fe
doped samples. With the increasing iron content
nanosheets are formed. Nanotubes were formed when
added Fe content is increased to 20%.
Thermal analysis shows the non-treated
boehmite decomposes in three steps at around 50,
150, 318 and at 397°C. The boehmite nanofibres ther-
mally decompose at 50, 247, 312 and 403°C (Ta-
ble 1). The first decomposition step is attributed to the
loss of adsorbed water, the second to the loss of inter-
stitial water and the third step to the dehydroxylation
of the boehmite. In general the Fe doped boehmite
nanofibres thermally decompose at higher tempera-
tures than undoped boehmite. This is observed for
both the loss of interstitial water and the
dehydroxylation. For the Fe doped boehmite with Fe
concentration above 5%, the thermal decomposition
is of a mixed system consisting of both boehmite
nanofibres and boehmite nanosheets.
J. Therm. Anal. Cal., 90, 2007
759

ZHAO et al.
17 H. Y. Zhu, D. Y. Song, Y. Q. Bai, S. P. Ringer, Z. Gao,
Acknowledgements
Y. X. Xi, W. Martens, J. D. Riches and
R. L. Frost, J. Phys. Chem. B, 108 (2004) 4245.
18 H. Y. Zhu, J. D. Riches and J. C. Barry, Chem. Mater.,
14 (2002) 2086.
The financial and infra-structure support of the Queensland
University of Technology Inorganic Materials Research Pro-
gram of the School of Physical and Chemical Sciences is
gratefully acknowledged. The Australian Research Council
(ARC) is thanked for funding the Thermal Analysis facility
through a LIEF grant. One of the authors (YZ) is thankful for a
Queensland University of Technology international doctoral
scholarship (QIDS).
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Received: November 7, 2006
Accepted: February 22, 2007
OnlineFirst: June 28, 2007
DOI: 10.1007/s10973-006-8248-0
760
J. Therm. Anal. Cal., 90, 2007