Size-controlled synthesis of alumina nanoparticles from aluminum alkoxides

Materials Research Bulletin 40 (2005) 1506–1512
www.elsevier.com/locate/matresbu
Size-controlled synthesis of alumina nanoparticles from
aluminum alkoxides
Yong Kap Park a, Erica H. Tadd b, Melissa Zubris b, Rina Tannenbaum b,*
a
Division of Material and Chemical Engineering, Sunmoon University Asan, Chungnam 336-840, Republic of Korea
b
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA
Received 29 September 2003; received in revised form 21 March 2005; accepted 27 April 2005
Abstract
Aluminum oxide nanoparticles were prepared by the hydrolysis of aluminum oxide alkoxides followed by
calcinations, in the presence of surface stabilizing agents, such as Na(AOT) molecules. The size of alumina
precursors (bohemite) was 20–30 nm, yielding aluminum oxide particles with an average size of 80 nm after
calcinations at 1200 8C. The shape of the a-alumina nanoparticles was mainly spherical and the high temperature
inhibited the formation of the hexagonal crystals. The introduction of Na(AOT) during the appropriate processing
step, had the effect of controlling the size of the particles, the degree of aggregation and the particles shapes.
# 2005 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures; A. Oxides; B. Crystal growth; B. Sol–gel chemistry; C. X-ray diffraction
1. Introduction
The recent intensive research in the area of metallic and ceramic nanoparticles stems from the special
properties exhibited by these materials and their potential applications in electronics, optoelectronics,
catalysis and thin film coatings [1–6]. In particular, alumina nanoparticles are expected to play important
roles in a variety of relevant applications, and hence, the field has generated important contributions
regarding the synthesis and processing of such particles [7–11]. The synthesis of alumina nanoparticles
has proven rather difficult, since most common methods of in situ synthesis of metal oxide nanoclusters,
* Corresponding author. Tel.: +1 404 3851235; fax: +1 404 8949140.
E-mail address: rina.tannenbaum@mse.gatech.edu (R. Tannenbaum).
0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2005.04.031
Y.K. Park et al. / Materials Research Bulletin 40 (2005) 1506–1512
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i.e. reduction of metal salts followed by oxidation or oxidative decomposition of organometallic
precursors, could not be applied in this case. These methods are applied successfully to the synthesis
of transition metal oxide clusters, but Al does not possess similar coordination capabilities, and hence, the
choice of precursors typical for transition metals is not available in this case. The aluminum ceramic
nanoparticles can, therefore, be prepared either by physical vapor deposition synthesis or by liquid–solid
phase synthesis, i.e. sol–gel processing [12–22].
The purpose of this work is to synthesize nanosized a-Al2O3 with control of particle size, particle size
distribution and particle shape, despite the fact that they have low sintering temperature and a strong
tendency towards extensive agglomeration, due to their high specific surface area and grain growth
[23–25].
2. Materials and methods
Aluminum isopropoxide, Al(OC3H7)3 (Chattem Chemical, Chattanooga, TN) was hydrolyzed by
adding it into excess deionized water (200 mole water/1 mole alkoxide), under continuous vigorous
stirring for 2 h at 85 8C. For the peptization step of the hydroxide, 0.05 moles of hydrochloric acid/1 mole
alkoxide were added and also vigorously mixed for 1 h. The clear solutions obtained by peptization were
aged for 24 h at 85 8C. In the second step of the experiment, the sodium bis-2-ethylhxyl sulfosuccinate,
Na(AOT), (C20H37NaO7, Sigma–Aldrich Chemie GmbH, Germany) was added and stirred in the
bohemite precursor solution. Three different samples were prepared by adjusting the mole ratio between
alkoxide and Na(AOT), from 0.02 to 0.1. Moreover, the Na(AOT) stabilizing molecules were added at
different stages of the sol–gel reaction, in order to probe the mechanistic aspects of the sol–gel method
(Fig. 1). The sol–gel reaction consisted of the transformation of the aluminum oxy-hydroxide, AlO(OH),
Fig. 1. Schematic representation of the various reaction sequences used in the experiments. Option 1 represents the reaction
without the presence of Na(AOT) as stabilizer. Options 2 and 3 show the reactions in which Na(AOT) was added in the early
stages of the process. Option 4 represents the reaction in which Na(AOT) was added prior to the sol–gel processing stage.
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particles into the aluminum oxide network, via a dehydration process. The alumina slurries were then
dried in an oven at 110 8C, and parts of samples were calcined at 600, 1000 and 1200 8C, to check the
effect of temperature on the size and morphology of the particles. The crystalline phase was determined
by X-ray diffraction techniques (XRD) (Rikagu, XRD 25 kV, Japan). Transmission electron microscopy
(TEM) analysis (JEOL 400 EX and 2000FX) was used to characterize the size and shape of the particles.
TEM samples were prepared by suspending the powder in methanol and evaporating a droplet onto a
carbon-coated copper grid. Differential scanning calorimetry experiments (TG/DSC, Dupont, USA)
were carried out by heating the samples at a heating rate of 10 8C/min in air from room temperature to
1200 8C.
3. Results and discussion
The controlled-size synthesis of a-Al2O3 could be carried out via the hydrolysis of aluminum alkoxide
with the concurrent addition of bis-2-ethylhexyl sulfosuccinate sodium salt, Na(AOT), to the precursor
solution, to prevent excessive grain growth and aggregation of nanoparticles. The overall reaction scheme
is shown below:
NaðAOTÞ
2AlðORÞ3 þ 3H2 O ! a-Al2 O3 þ 6ROH;
ðR ¼ alkyl groupÞ
(1)
This is a multi-step process that involves the transformation of the aluminum alkoxide to an aluminum
hydroxide, Al(OH)3, followed by the dehydration of the latter to form an oxo-hydroxide aluminum
intermediate, AlO(OH) (bohemite), that will constitute the precursor in the sol–gel process [14–16,19].
The Na(AOT) surfactant can be added to the reaction mixture at different stages of the reaction, in order
to probe the critical aggregation step in which the adsorption of this stabilizing agent onto the surface of
the forming clusters will have the largest effect on final equilibrium size of the oxide particles, as shown
in Fig. 1. It is expected that the optimal time for the addition of the surfactant would occur immediately
after the drying step and prior to the onset of the nucleation and growth step (the sol–gel process, Option
4), and that will involve the following chemical transformation:
NaðAOTÞ
2AlOðOHÞ ! a-Al2 O3 þ H2 O
(2)
The crystalline phase was determined by X-ray diffraction, and the size and shape of the alumina
powders were observed by transmission electron microscopy, TEM. Thermogravimetry coupled with
differential scanning calorimetry was used to determine the details of the sol–gel phase transformations
[14].
The X-ray diffraction patterns of alumina precursors that were obtained by the sol–gel process are
shown in Fig. 2a–e. The spectra illustrate the influence of the temperature of the sol–gel processing and
the presence of the stabilizing agent in the reaction mixture on the resulting aluminum oxide particles.
The bohemite intermediate obtained in the absence of surfactant, exhibits a crystalline phase in an
amorphous background, as shown in Fig. 2a [14]. The addition of the surfactant Na(AOT) at the onset of
the reaction sequence (Options 2 or 3) resulted in the formation of an amorphous bohemite phase coupled
with some hydrates of Na(AOT), such as NaAl(CO3)(OH)2 or C8H15AlNa2O16, as shown in Fig. 2b. This
may be the result of the adsorption of an Na(AOT) layer on the surface of the initial bohemite nuclei,
which prevented the aggregation and growth of bohemite grains, despite sufficient aging times and
Y.K. Park et al. / Materials Research Bulletin 40 (2005) 1506–1512
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Fig. 2. XRD spectra of bohemite and aluminum oxide obtained under various conditions: (a) XRD spectrum of bohemite formed
without surface stabilizer and dried at room temperature (Option 1); (b) XRD spectrum of bohemite formed after the addition of
Na(AOT) at the onset of the reaction and dried at room temperature (Options 2 and 3); (c) XRD spectrum of g-Al2O3 formed after
drying at 110 8C and calcined at 600 8C. Na(AOT) was added prior to the sol–gel processing step (Option 4); (d) XRD spectrum of a
mixture of XRD spectrum of g-Al2O3, u-Al2O3 and d-Al2O3, formed after drying at 110 8C and calcined at 1000 8C. Na(AOT) was
added prior to the sol–gel processing step (Option 4); (e) XRD spectrum of a mixture of XRD spectrum of a-Al2O3, formed after
drying at 110 8C and calcined at 1200 8C. Na(AOT) was added prior to the sol–gel processing step (Option 4).
temperatures, that would allow, in the absence of Na(AOT), the formation of crystallites. The subsequent
three XRD spectra in Fig. 2 show alumina nanoclusters derived from bohemite, where Na(AOT) was
added in the final stage of the sol–gel process, and the calcination step was carried out at different
temperatures. At 600, 1000 and 1200 8C, the predominant oxides are g-Al2O3 (Fig. 2c), u-Al2O3 (Fig. 2d)
and a-Al2O3 (Fig. 2e), respectively [14]. Fig. 3 shows the DSC curves of the bohemite, in the absence
(Fig. 3a) and in the presence (Fig. 3b) of Na(AOT). In the absence of Na(AOT), the exothermic peak at
302 8C is due to the equilibrium reaction of AlO(OH) and Al(OH)3, as shown below [13,14,19]:
H2 O
AlOðOHÞ @ AlðOHÞ3
(3)
The endothermic peak at 420 8C represents the transformation of bohemite to g-Al2O3, while the
exothermic peaks at 513, 1069 and 1183 8C are attributed to the pseudomophic phase transformation of
g-Al2O3, to d-Al2O3, u-Al2O3 and a-Al2O3, respectively [8,14]. When Na(AOT) is present in the system
from the initial stages of the reaction (as the conditions for the sample shown in Fig. 2b), a weak
transformation of bohemite to g-Al2O3 is detected at a higher temperature, 470 8C, but no further
transformations to any crystalline form of aluminum oxide is observed (Fig. 3b). This is consistent with
the XRD results for this sample, which seem to indicate that the Na(AOT) molecules stabilize the forming
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Fig. 3. DSC experiments of the alkoxide/oxide transformation process in the absence of stabilizing agents (a) and in the
presence (b) of Na(AOT) molecules added at the onset of the reaction (Options 2 and 3).
nuclei of AlO(OH) early in the process, resulting in an amorphous oxide structure. The formation of the
Na(AOT) capping layer onto the surface of the bohemite retards the aggregation of the oxide amorphous
phase, as evidenced by the occurrence of the AlO(OH)/Al(OH)3 equilibrium at 307 8C (Fig. 3b) and the
higher temperature for the partial formation of the g-Al2O3 phase. TEM micrographs of the various
samples are shown in Fig. 4. The TEM micrograph of the uncapped bohemite particles (Fig. 4a) shows
predominantly spherically shaped particles with an average size of about 30 nm. The particles that were
capped with Na(AOT) surfactant exhibit significantly different shape and size from that of bohemite
(Fig. 4b), since the capping layer of Na(AOT) on the surface has impeded the grain growth of the
bohemite, resulting in the amorphous and porous hydroxide as shown in the XRD pattern (Fig. 2b). The
average particle size under these circumstances is about 5 nm and the particles are organized in a network
of flocculates of about 40 nm. Fig. 4c and d shows the TEM images of Al2O3 calcined at 600 and 1200 8C,
respectively. The nano-sized alumina particles formed at 600 8C exhibited irregular hexagonal shapes,
while those formed at the higher temperature were mainly spherical with an average particle size of
80 nm. Hence, the presence of Na(AOT) as a stabilizing agent, when added during the appropriate
processing step, i.e. before the sol–gel transformation, had the effect of controlling both the size of the
particles, their degree of aggregation and their shapes.
4. Summary
In this work, we have shown that the sol–gel method for the synthesis of aluminum oxide from
aluminum alkoxide complexes coupled with the presence of surface-active molecules, such as Na(AOT),
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Fig. 4. TEM micrographs of the various microstructures obtained in the alkoxide/oxide transformation process under different
conditions: (a) TEM micrograph of bohemite obtained in the absence of Na(AOT); (b) TEM micrograph of bohemite obtained in
the presence of Na(AOT), which was added at the onset of the reaction (Options 2 and 3); (c) TEM micrograph of a mixture of gAl2O3, u-Al2O3 and d-Al2O3, obtained in the presence of Na(AOT) (Option 4), which was calcined at 600 8C; (d) TEM
micrograph of a mixture of a-Al2O3, obtained in the presence of Na(AOT) (Option 4), which was calcined at 1200 8C.
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represent an effective pathway for controlling the size of the resulting nanoparticles. However, it is
important to select and control the precise timing for the introduction of the stabilizing molecules into the
reaction. If the Na(AOT) molecules are present from the onset of the synthesis, the result is an amorphous
product. On the other hand, if the Na(AOT) molecules are introduced during the final processing steps
prior to calcination, the result is the formation of nearly uniform a-Al2O3 nanoparticles.
Acknowledgements
This work was partially supported by the research fund of Sunmoon University and by the AFOSR–
MURI on Energetic Structural Materials. The authors would like to thank Professor Yong Choi at
Sunmoon University for his valuable assistance.
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