Nanostructured aluminium oxide powders obtained by aspartic acid-nitrate gel-combustion routes

Article abstract of DOI:10.1016/j.jallcom.2009.10.042

In this work, two new gel-combustion routes for the synthesis of Al₂O₃ nanopowders with aspartic acid as fuel are presented. The first route is a conventional stoichiometric process, while the second one is a non-stoichiometric, pH-controlled process. These routes were compared with similar synthesis procedures using glycine as fuel, which are well-known in the literature. The samples were calcined in air at different temperatures, in a range of 600-1200 °C. They were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and BET specific surface area. Different phases were obtained depending on the calcination temperature: amorphous, γ (metastable) or α (stable). The amorphous-to-γ transition was found for calcination temperatures in the range of 700-900 °C, while the γ-to-α one was observed for calcination temperatures of 1100-1200 °C. The retention of the metastable γ phase is probably due to a crystallite size effect. It transforms to the α phase after the crystallite size increases over a critical size during the calcination process at 1200 °C. The highest BET specific surface areas were obtained for both nitrate-aspartic acid routes proposed in this work, reaching values of about 50 m²/g.

Full text of DOI:10.1016/j.jallcom.2009.10.042

Journal of Alloys and Compounds 495 (2010) 578–582
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Nanostructured aluminium oxide powders obtained by aspartic acid–nitrate
gel-combustion routes
María Celeste Gardey Merinoa,b,∗, Gustavo E. Lascaleaa, Laura M. Sánchezc, Patricia G. Vázquezd,
Edgardo D. Cabanillase, Diego G. Lamasc
a Laboratorio de Investigaciones y Servicios Ambientales Mendoza (LISAMEN) – CCT – CONICET, Avda. Ruiz Leal s/n, Parque Gral. San Martín, (M5502IRA) Ciudad de Mendoza, Prov.
de Mendoza, Argentina
b Grupo CLIOPE, Universidad Tecnológica Nacional – Facultad Regional Mendoza, Rodríguez 273, (M5502AJE) Ciudad de Mendoza, Prov. de Mendoza, Argentina
c CINSO (Centro de Investigaciones en Sólidos), CITEFA – CONICET, J.B. de La Salle 4397, (B1603ALO) Villa Martelli, Prov. de Buenos Aires, Argentina
d Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. Jorge J. Ronco” (CINDECA), CONICET, Universidad Nacional de La Plata, Calle 47 nro. 257, (B1900AJK) La Plata, Prov.
de Buenos Aires, Argentina
e CONICET and Centro Atómico Constituyentes, Comisión Nacional de Energía Atómica, Gral. Paz 1499, (1650) San Martín, Prov. de Buenos Aires, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, two new gel-combustion routes for the synthesis of Al2O3 nanopowders with aspartic acid
as fuel are presented. The first route is a conventional stoichiometric process, while the second one is a
non-stoichiometric, pH-controlled process. These routes were compared with similar synthesis proce-
dures using glycine as fuel, which are well-known in the literature. The samples were calcined in air at
different temperatures, in a range of 600–1200 ◦C. They were characterized by X-ray diffraction, scanning
electron microscopy, transmission electron microscopy and BET specific surface area. Different phases
were obtained depending on the calcination temperature: amorphous, ␥ (metastable) or ␣ (stable). The
amorphous-to-␥ transition was found for calcination temperatures in the range of 700–900 ◦C, while the
␥-to-␣ one was observed for calcination temperatures of 1100–1200 ◦C. The retention of the metastable
␥ phase is probably due to a crystallite size effect. It transforms to the ␣ phase after the crystallite size
increases over a critical size during the calcination process at 1200 ◦C. The highest BET specific surface
areas were obtained for both nitrate–aspartic acid routes proposed in this work, reaching values of about
50 m2/g.
Received 12 August 2009
Accepted 6 October 2009
Available online 7 November 2009
Keywords:
Nanostructured materials
Oxide materials
X-ray diffraction
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
ous solution containing salts of the desired metals (nitrates are
usually preferred) and an organic fuel, such as glycine, citric acid,
In recent years, increasing attention has been focused on the
development of nanocrystalline alumina powders. ␥-Al2O3 is an
extremely important nanosized material considering its appli-
cations as a catalyst and catalyst support in the automotive
and petroleum industries, in structural composites for spacecraft,
miniature power supplies and abrasive and thermal wear coating.
Ultrafine ␣-Al2O3 powder has considerable potential for a wide
range of applications including high strength materials, electronic
ceramics and catalysts.
Several wet-chemical routes have been proposed for the
synthesis of nanocrystalline oxides. Particularly, gel-combustion
processes proved to be useful for the synthesis of nanomaterials
with high specific surface areas and excellent compositional homo-
geneity by simple and low-cost procedures [1–8]. These methods
are based on the gelling and subsequent combustion of an aque-
etc. The combustion process is due to an exothermic, redox reac-
tion between nitrate ions and the fuel. The large volume of gases
released during the reaction promotes a rapid disintegration of
the precursor gel, yielding the desired nanocrystalline material.
The characteristics of the powder, such as crystallite size, spe-
cific surface area, nature (hard or soft) of agglomeration, etc., are
primarily governed by enthalpy and/or flame temperature gen-
erated during the combustion stage, which is dependent on the
nature of the fuel and the fuel-to-oxidant ratio. This ratio is usually
chosen close to that corresponding to a stoichiometric reaction,
since the combustion process is faster under this condition. How-
ever, in previous works, we have demonstrated that pH-controlled
gel-combustion processes with excess of nitrates render composi-
tionally homogeneous, nanocrystalline powders with high specific
surface areas [3–5]. Unfortunately, these routes have not been
extensively explored yet.
In the present work, we investigated novel gel-combustion
routes for the synthesis of Al2O3 nanopowders using aspartic acid
as fuel by both stoichiometric and pH-controlled gel-combustion
∗
Corresponding author. Tel.: +54 0261 5243000; fax: +54 0261 5244531.
E-mail address: mcgardey@frm.utn.edu.ar (M.C. Gardey Merino).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.10.042

M.C. Gardey Merino et al. / Journal of Alloys and Compounds 495 (2010) 578–582
579
Table 1
Identified phases and average crystallite sizes of Al2O3 powders synthesized by the different gel-combustion routes studied in this work and calcined at different temperatures.
Numbers in parentheses indicate the error in the last significant digit.
Sample
Present phase
Average crystallite size (nm)
Gel-combustion route
Calcination temperature (◦C)
Stoichiometric nitrate–aspartic acid
Stoichiometric nitrate–aspartic acid
Stoichiometric nitrate–aspartic acid
Non-stoichiometric nitrate–aspartic acid
Non-stoichiometric nitrate–aspartic acid
Non-stoichiometric nitrate–aspartic acid
Stoichiometric nitrate–glycine
600
900
1200
600
900
Amorphous
␥
␣
Amorphous
␥
␣
–
4.7 (2)
85 (5)
–
8.5 (4)
1.2 (1) × 102
–
1200
600
Amorphous
Stoichiometric nitrate–glycine
Stoichiometric nitrate–glycine
Non-stoichiometric nitrate–glycine
Non-stoichiometric nitrate–glycine
Non-stoichiometric nitrate–glycine
800
1200
600
800
1200
␥
␣
4.8 (2)
45 (3)
–
Amorphous
␥
␣
6.9 (3)
1.4 (2) × 102
This precursor solution was concentrated on a hot plate at 250 ◦C until a viscous
gel was obtained. Soon after, it ignited and the combustion process proceeded softly,
without flame.
This stoichiometric nitrate–aspartic acid route was compared with the conven-
tional nitrate–glycine route (usually called “glycine–nitrate process”, GNP [1,2]). The
same procedure was used, starting from the same mass of Al(NO3)3·9H2O, but in this
case the stoichiometric reaction requires a glycine/Al molar ratio of 5/3.
routes. The use of aspartic acid as fuel has been recently pro-
posed for the synthesis of other oxides by the stoichiometric
gel-combustion process [8–10], but to our knowledge it has not
been investigated for the synthesis of nanocrystalline Al2O3. This
fuel is interesting due to its complex properties since it is a dicar-
boxylic amino acid. Besides, its enthalpy of combustion is higher
than that of glycine [11]. We have studied different calcination
temperatures, in the range of 600–1200 ◦C, in order to evaluate
the influence of the crystallite size on the crystal structure. For
the purpose of comparison, powders synthesized by similar gel-
combustion processes using glycine as fuel were also analyzed.
2.1.2. Non-stoichiometric pH-controlled nitrate–aspartic acid route
A first solution was prepared from 7.5 g of Al(NO3)3·9H2O (Merck), 50 ml of
distilled water and 50 ml of HNO3 (concentrated, Merck). A second solution was
prepared by dissolving 7.99 g of aspartic acid (Anedra) in 100 ml of distilled water
and both solutions were mixed, thus resulting an aspartic acid/Al molar ratio of
3. This ratio was chosen based on the “oxidative valence criterion” [12], taking
into account previous studies of similar processes, the optimum ratio for the pH-
controlled nitrate–glycine route to synthesize ZrO2–CeO2 solid solutions was found
to be 5 [4,5]. Then, NH4OH (diluted, Merck) was added to obtain 300 ml of a precur-
sor solution with pH 2. The solution pH was not further controlled after this stage.
This solution was then concentrated on a hot plate at 250 ◦C until it turned into
a viscous gel, which finally burned due to a vigorous exothermic reaction with a
flaming stage of about 1 min.
2. Experimental procedure
2.1. Synthesis of Al2O3 nanopowders
Both gel-combustion routes studied in this paper using aspartic acid as organic
fuel are described below. They were compared with similar processes using glycine
as fuel. All the chemical routes were performed using reagents of analytical grade.
The final calcination temperatures were in the range of 600–1200 ◦C.
This non-stoichiometric nitrate–aspartic acid route was compared with a similar
pH-controlled nitrate–glycine process. The same procedure was used, but in this
case the glycine/Al ratio was chosen as 5, the optimum ratio found in previous works
for other oxides [4,5].
2.1.1. Stoichiometric nitrate–aspartic acid route
7.5 g of Al(NO3)3·9H2O (Merck) and 2.66 g of aspartic acid (Anedra) were dis-
solved in distilled water to obtain a homogeneous solution, with a total volume of
400 ml (pH 3). The aspartic acid/metal molar ratio was calculated on the basis of a
stoichiometric combustion reaction:
2.2. Characterization of Al2O3 nanopowders
The phases present in the as-synthesized Al2O3 nanopowders (obtained after
calcination) were identified by X-ray diffraction (XRD) using a Philips PW 3710
diffractometer operated with Cu-K␣ radiation. Our data were compared with those
2Al(NO3)3·9H2O + 2NH2CH2CH(COOH)2 → Al2O3 + 4N2 + 8CO2 + 25H2O
Fig. 2. XRD patterns of Al2O3 nanopowders synthesized by non-stoichiometric
pH-controlled nitrate–aspartic acid gel-combustion routes calcined at different tem-
peratures.
Fig. 1. XRD patterns of Al2O3 nanopowders synthesized by stoichiometric
nitrate–aspartic acid gel-combustion routes calcined at different temperatures.

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M.C. Gardey Merino et al. / Journal of Alloys and Compounds 495 (2010) 578–582
Fig. 3. Comparison between XRD patterns of Al2O3 nanopowders synthesized by
Fig. 4. Comparison between XRD patterns of Al2O3 nanopowders synthesized by
non-stoichiometric pH-controlled nitrate–aspartic acid and nitrate–glycine routes
calcined at 800 ◦C.
stoichiometric nitrate–aspartic acid and nitrate–glycine routes calcined at 800 ◦C.
reported in the inorganic crystal structure database (ICSD). The crystallite size was
determined from the broadening of Bragg peaks using the Scherrer equation [13]:
SEM observations showed that all the samples exhibited a high
degree of agglomeration, with large “flake-like” aggregates, as
observed in Figs. 5 and 6. Amorphous samples did not exhibit any
substructure, while small particles were detected in both ␥ and ␣
samples.
TEM observations confirmed that the ␥ and ␣ samples were
composed of aggregates of small particles. Bright-field TEM micro-
graphs (Fig. 7) showed particles with typical sizes close to the
crystallite size determined from XRD data, that also agrees with
the sizes determined by dark-field TEM observations. In the case
of the nanopowders exhibiting the amorphous phase, we found a
morphology similar to that of ␥ samples with very small particles,
but electron diffraction patterns confirmed their amorphous crystal
structure.
BET specific surface areas resulted in agreement with the above
SEM and TEM observations. For example, Table 2 summarizes
the results obtained by both nitrate–aspartic acid gel-combustion
routes, after calcination at different temperatures. It can be
observed that the highest values were found for samples exhibiting
the ␥ phase, while amorphous or ␣ samples exhibited much lower
values. Table 3 presents the BET specific surface areas obtained
for ␥ samples synthesized for all the routes studied in this work,
calcined at 900 ◦C. The highest values were obtained for both
nitrate–aspartic acid routes proposed in this work, reaching values
of about 50 m2/g.
0.9ꢀ
ˇ cos ꢁ
D =
(1)
where D is the crystallite size, ꢀ is the wavelength of the radiation (1.5418 Å for
Cu-K␣ radiation), ˇ is the corrected peak width at half-maximum intensity and ꢁ
is the peak position. Since the line shape was approximately Lorentzian for all the
samples, the value of b was corrected using the formula ˇ = ˇm − ˇi, where ˇm is the
measured peak width and ˇi is the instrumental broadening, which was determined
using an ␣-Al2O3 powder with a mean particle diameter of 25 ␮m.
The morphology of the powders was analyzed by scanning electron microscopy
(SEM, Philips 515 microscope) and transmission electron microscopy (TEM, Philips
EM 300 microscope). The operation voltage was between 10 and 20 kV for SEM study
and 100 kV for TEM one. In both cases, the preparation of samples was performed
following conventional procedures.
The specific surface areas of the powders were measured by the multipoint
Brunauer–Emmett–Teller (BET) adsorption technique using a Micromeritics Accu-
sorb 2100E equipment.
3. Results and discussion
Al2O3 nanopowders synthesized by the new gel-combustion
routes proposed in this work using aspartic acid as fuel exhib-
ited similar characteristics to those obtained by the conventional
nitrate–glycine routes.
Table 1 summarizes the main results obtained by XRD for all the
gel-combustion routes investigated in this work. XRD data showed
that all powders exhibited the amorphous phase after calcination
at 600 ◦C. The phase transformations, amorphous-to-␥ and ␥-to-
␣, proceeded with increasing calcination temperature, as shown
in Figs. 1 and 2. The metastable ␥ phase was fully retained after
calcination at temperatures in the range of 800–900 ◦C, depend-
ing on the synthesis route. In Figs. 3 and 4, it can be noticed that
nanopowders synthesized by both glycine routes fully transformed
to the ␥ phase after calcination at 800 ◦C, while the sample syn-
thesized by stoichiometric nitrate–aspartic acid route exhibited
a mixture of ␥ and amorphous phases and that synthesized by
non-stoichiometric pH-controlled nitrate–aspartic acid route was
amorphous. The average crystallite size of nanopowders exhibiting
the ␥ phase was in the range of 5–9 nm. The smallest crystallite sizes
were obtained for samples synthesized by stoichiometric routes,
being very similar for both of them.
Table 2
BET specific surface areas of Al2O3 samples synthesized by both nitrate–aspartic acid
gel-combustion routes proposed in this work, calcined at different temperatures.
Sample
BET specific surface
area (m2/g)
Gel-combustion route
Calcination
temperature (◦C)
Stoichiometric
nitrate–aspartic acid
Stoichiometric
nitrate–aspartic acid
Stoichiometric
nitrate–aspartic acid
Non-stoichiometric
nitrate–aspartic acid
Non-stoichiometric
nitrate–aspartic acid
Non-stoichiometric
nitrate–aspartic acid
600
900
22.8
52.3
6.0
1200
600
16.4
51.9
12.7
The ␥-to-␣ transition occurs for calcination temperatures of
1100–1200 ◦C, due to crystallite growth. The average crystallite size
of the samples calcined at 1200 ◦C varied in the range of 45–140 nm.
The smallest average crystallite size (45 nm) was obtained by the
stoichiometric nitrate–glycine route.
900
1200

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581
Fig. 5. SEM micrographs of Al2O3 nanopowders exhibiting the metastable ␥ phase. (a) Non-stoichiometric nitrate–aspartic acid route calcined at 900 ◦C. (b) Stoichiometric
nitrate–aspartic acid route calcined at 850 ◦C.
Fig. 6. SEM micrographs of Al2O3 nanopowders exhibiting the ␣ phase. (a) Non-stoichiometric nitrate–glycine route calcined at 1200 ◦C. (b) Non-stoichiometric
nitrate–aspartic acid route calcined at 1200 ◦C.
Fig. 7. TEM micrographs showing the typical morphology of Al2O3 nanopowders exhibiting the metastable ␥ phase. (a) Stoichiometric nitrate–aspartic acid route calcined
at 850 ◦C. (b) Stoichiometric nitrate–glycine acid route calcined at 800 ◦C.
Table 3
characteristics to those synthesized using glycine as fuel. All pow-
BET specific surface areas of Al2O3 samples exhibiting the ␥ phase, synthesized by
ders exhibited the amorphous phase after calcination at 600 ◦C and
all gel-combustion routes studied in this work and calcined at 900 ◦C.
transform to ␥ and ␣ phases with increasing calcination tempera-
Gel-combustion route
BET specific surface
area (m2/g)
ture. The critical calcination temperature for the amorphous-to-␥
phase transition depends on the synthesis route. The ␥-to-␣ tran-
sition occurs for calcination temperatures of 1100–1200 ◦C, due
to crystallite growth. ␥ samples exhibited the highest BET spe-
cific surface areas, reaching values of about 50 m2/g for both
nitrate–aspartic acid routes.
Stoichiometric nitrate–aspartic acid
Non-stoichiometric nitrate–aspartic acid
Stoichiometric nitrate–glycine
52.3
51.9
13.7
36.5
Non-stoichiometric nitrate–glycine
Taking into account the interesting results found in this work,
a systematic study of the influence of the synthesis parameters on
powder morphology will be performed in the near future.
Acknowledgements
The authors thank Agencia Nacional de Promoción Científica y
Tecnológica (Argentina, PICT 38309) for financial support, CNEA
for the use of their electron microscope facilities and Lic. Mariana
Rosenbusch (CONICET) for her assistance during SEM observations.
M.C. Gardey Merino thanks National Technological University for
the Doctoral Scholarship Res. No. 175/2007 and the authorities of
4. Conclusions
Both nitrate–aspartic acid gel-combustion processes proposed
in this work allow the synthesis of Al2O3 nanopowders with similar

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