Boehmite nanoparticles by the two-reverse emulsion technique

Article abstract of DOI:10.1111/j.1551-2916.2005.00600.x

Boehmite (γ-AlOOH) nanoparticles were successfully synthesized by the two-reverse emulsion technique at 90°±1°C under constant agitation with varying Al³⁺ concentrations in the aqueous solution. A mixture of cyclohexane and the surfactant, sorbitan monooleate (Span 80), constituted the support solvent in the reverse emulsions. The synthesized particles were characterized by thermogravimetry, differential thermal analysis, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), particle size analysis, and transmission electron microscopy (TEM). XRD and FTIR results confirmed crystalline boehmite formation at 90°±1°C. The average particle size of boehmite was found to be 10 nm. The spherical morphology of the boehmite nanoparticles was confirmed by TEM.

Full text of DOI:10.1111/j.1551-2916.2005.00600.x

J. Am. Ceram. Soc., 88 [12] 3322–3326 (2005)
DOI: 10.1111/j.1551-2916.2005.00600.x
r 2005 The American Ceramic Society
ournal
J
Boehmite Nanoparticles by the Two-Reverse Emulsion Technique
Milan Kanti Naskar and Minati Chatterjeew
Sol–Gel Division, Central Glass and Ceramic Research Institute, Kolkata 700 032, India
Boehmite (c-AlOOH) nanoparticles were successfully synthe-
sized by the two-reverse emulsion technique at 901711C under
constant agitation with varying Al31 concentrations in the aque-
ous solution. A mixture of cyclohexane and the surfactant,
sorbitan monooleate (Span 80), constituted the support solvent
in the reverse emulsions. The synthesized particles were charac-
terized by thermogravimetry, differential thermal analysis,
Fourier transform infrared spectroscopy (FTIR), X-ray diffrac-
tion (XRD), particle size analysis, and transmission electron
microscopy (TEM). XRD and FTIR results confirmed crystal-
line boehmite formation at 901711C. The average particle size
of boehmite was found to be 10 nm. The spherical morphology of
the boehmite nanoparticles was confirmed by TEM.
synthesis. Keeping in view the aforementioned points, the aim of
the present work was to:
(1) develop a process, based on the two-reverse emulsion
technique, for the synthesis of boehmite nanoparticles from an
aqueous inorganic salt solution at a considerably low tempera-
ture,
(2) examine the process parameters for particle formation,
and
(3) characterize the synthesized powders.
The synthesized particles in the present investigation were
free-flowing and monodispersed in nature.
II. Experimental Procedure
(1) Preparation of Aluminum Nitrate Solutions
I. Introduction
Aluminum nitrate solutions of concentrations 0.05M and 0.1M
were prepared by dissolving the required amount of aluminum
nitrate nonahydrate, Al(NO3)3 ꢀ 9H2O (G.R., Merck, Mumbai,
India, purity 499%) in deionized water of conductivity
1.4 ꢁ 10ꢂ5 O.
HE aluminum oxide-hydroxide boehmite (g-AlOOH) nano-
powder has been known to be used in the preparation of
T
membranes, catalysts, coatings on different substrates, and more
importantly, for alumina and alumina-based materials for var-
ious applications in the field of advanced ceramics.1–5 Because of
the above-mentioned applications, considerable work is still be-
ing carried out on the synthesis of boehmite powders and their
characterization. Although numerous methods are reported for
the synthesis of these particles,1–9 generally hot water hydrolysis
of aluminum alkoxides,1,2,9 hydrothermal synthesis,3,5,10 and
precipitation under hot condition from aqueous inorganic salt
solutions4,11–13 are mainly followed. However, the morphology
of the powders and properties of the final materials have been
found to depend strongly on the chemical compositions and the
experimental conditions used during the synthesis.1,3,5,14 Fur-
ther, very often, the final product becomes associated with other
crystalline phases such as gibbsite and/or bayerite.5 Catone and
Matijevic14 synthesized monodispersed spherical particles of hy-
drous aluminum oxide by hydrolysis of aluminum sec-butoxide
in the presence of sodium, potassium, ammonium, and magne-
sium sulfate under controlled experimental conditions of pH,
temperature, aging time, etc. On the other hand, boehmite fibrils
were obtained after prolonged heat treatment of the basic al-
uminum chloride or nitrate solutions in an autoclave at 1601C or
above.6 Again, formation of plate-like boehmite has also been
reported in the literature.15
(2) Preparation of Support Solvents
Support solvents were prepared by mixing cyclohexane of die-
lectric constant 2.042 at 251C (G.R., Merck, purity 499%) and
2 vol% of the emulsifier (surfactant) with respect to the cyclo-
hexane under agitation. Sorbitan monooeleate (Span 80, Fluka
Chemica AG, Switzerland), having a hydrophilic–lipophilic bal-
ance (HLB) value of 4.3, was used as the emulsifier in the present
study.18,19,24–26
(3) Preparation of the Reverse Emulsion (w/o) of Aluminum
Nitrate Solution (Emulsion A)
The reverse emulsion of aluminum nitrate solution (emulsion A)
was prepared at ambient temperature by adding a measured
volume of the salt solution to the support solvent in a closed
glass container under a mechanical agitation of 500 rpm. The
volume ratio of the salt solution: support solvent was 1:4 in all
the experiments.
(4) Preparation of the Reverse Emulsion of Ammonia
Solution (Emulsion B)
Although various chemical methods have been developed to
control the morphology, size, and distribution of nanoparticles,
because of the large surface-to-volume ratio, the particles un-
dergo aggregation. To overcome these problems, many recent
studies followed emulsions to control the size and morphology
of the nanoparticles.16–24 In such cases, the dispersed aqueous
microdroplets in water-in-oil (w/o)-type emulsions act as micro-
reactors in which reactions are carried out. As the alkoxide route
for the preparation of boehmite nanoparticles in bulk quantity is
quite expensive, one has to look for the inorganic salts for its
The emulsion B was prepared at ambient temperature by adding
a measured volume of the ammonia solution (25 wt%, w/v,
G.R., Merck) to the support solvent in a closed glass container
under a mechanical agitation of 500 rpm. The volume ratio of
the ammonia solution: support solvent was 1:4 in all the exper-
iments.
(5) Preparation of Boehmite Nanopoarticles by the
Two-Reverse Emulsion Technique24
The containers with the emulsion A and emulsion B were kept
separately in an oil bath preheated to 901711C (reaction tem-
perature) and kept at that temperature for 10 min under mag-
netic stirring of 500 rpm to attain the reaction temperature. The
temperature of both the emulsions was verified by thermometers
dipped into the respective containers. After attaining a temper-
ature of 901711C, emulsion B was rapidly added to emulsion A
L. Klein—contributing editor
Manuscript No. 20523. Received November 24, 2004; approved May 18, 2005.
wAuthor to whom correspondence should be addressed. e-mail: minati@cgcri.res.in
3322

December 2005
Boehmite Nanoparticles
3323
The novelty of the developed technique primarily lies in pro-
viding a process that is economical, environmentally friendly,
time-saving, and of high yield from inorganic-based precursors.
Cyclohexane
Span 80
Cyclohexane
Span 80
Stirring
(6) Characterization of the Nanoparticles
Mixing
Mixing
The precursor and the calcined powders were characterized by
differential thermal analysis (DTA) and thermogravimetry (TG;
model STA 409C, Netzsch, Bayern, Germany), X-ray diffrac-
tion (XRD; model 1730, Philips, Almelo, the Netherlands) with
Ni-filtered CuKa radiation, Fourier transform infrared spectros-
copy (FTIR) study (model 5PC, Nicolet, Madison, WI), particle
size analysis (Otsuka Electronics Co. Ltd. model DLS 700,
Osaka, Japan), and transmission electron microscopy (TEM;
model JEJ-200X, JEOL, Tokyo, Japan).
Al(NO ) (aq)
3 3
NH (aq)
Emulsion A
Emulsion B
3
90°C
90°C
90°C, mixing, stirring
Emulsified particles
at 90°C
Cooling to r.t.
III. Results and Discussion
Emulsified particles
at r.t.
Acetone
(1) Stabilization of Aqueous Microdroplets in Reverse
Emulsions
Stirring
In a system composed of two immiscible liquids,25–28 i.e. for the
w/o-type emulsions (reverse emulsion) in the present investiga-
tion, dispersion of the water phase (aqueous Al31 or ammonia
solution) in the oil phase (cyclohexane) as small droplets under
agitation causes an increase in the surface area of the dispersed
phase. Thermodynamically, the increase in the surface area (DA)
of the dispersed phase associated with a high free energy (DG)
can be represented as27,29,30
Flocculated particles
Centrifugation, washing
Washed particles
Drying at 100°C
Boehmite( -AlOOH)
nanopowder
DG ¼ gDA
(1)
Fig. 1. Flow diagram for the synthesis of boehmite (g-AlOOH) nano-
particles.
where g is the interfacial tension. The equation indicates that a
low interfacial tension favors droplet disruption. In fact, the
high interfacial tension between the dispersed water phase and
continuous oil phase is reduced by the addition of a system-
compatible amphiphilic surface-active agent or surfactant
(emulsifier in the present case), i.e. a molecule with a polar
head and a non-polar tail.18,24 The surfactant molecules orient
themselves according to the polarities of the chemical constitu-
ents involved. Thus, because of the high polarity of water, the
polar heads of the surfactant molecules at the water–oil interface
are oriented toward the water droplets and become adsorbed on
their surfaces and prevent the coalescence of the droplets by
steric hindrance occurring between two droplets (steric stab-
ilization). Thus, the emulsifier added to the system: (i) lowers the
interfacial tension between two immiscible liquids, favoring
droplet disruption and (ii) prevents re-coalescence of aqueous
droplets by adsorbing on their surfaces, thus producing a stable
emulsion. In the present investigation, reverse emulsions of
aqueous Al31 and ammonia solutions were used as precursors
for the preparation of boehmite nanoparticls in the presence of
sorbitan monooleate, Span 80, as the emulsifying agent.
under stirring to obtain a single reverse emulsion (w/o) followed
by simultaneous precipitation of the precursor powders. The
aging at 901711C was continued for 10 min, after which the
container was removed from the bath and cooled down under a
water tap to the ambient temperature.
The synthesized particles were obtained from the single re-
verse emulsions by adding a known volume of acetone (G.R.,
Merck). Immediate separation of the particles occurred. The
particles were collected by centrifugation at 6000 rpm. To re-
move the last traces of adhered impurities, the above procedure
was repeated twice, each time collecting the powders centrifu-
gally. The washed particles were dried at 1001C, with a dwell
time of 1 h, in an air oven under static condition. The yield of
the product was found to be greater than 95%. Figure 1 presents
the flowchart for the preparation of boehmite nanoparticles.
Table I summarizes the experimental conditions and results ob-
tained in the present investigation. To examine the effect of cal-
cination temperature on phase transformation, the precursor
powder corresponding to run no. 2 of Table I was calcined at
different temperatures, i.e. from 4001 to 12001C in air, each with
a dwell time of 1 h, under static conditions.
(2) Formation of Spherical Boehmite Nanoparticles
The present method, based on the emulsion (w/o type) precip-
itation, involves emulsification of an aqueous metal salt solution
(water phase) with a high dielectric constant in an organic sol-
vent (oil phase) of low dielectric constant by agitation in the
presence of a surfactant. The dispersed droplets act as microre-
actors for carrying out the precipitation reaction. The metal ions
in the microdroplets are precipitated as hydroxides by increasing
the basicity of the system.19,22,25 The surfactants used for the
stabilization of aqueous droplets are characterized by their HLB
values. The surfactants in the span series, i.e. the Span 80 in the
present case, are basically the fatty acid esters of anhydro so-
rbitols, which are good oil-soluble emulsifying agents.27 In fact,
a high HLB value of the surfactant indicates a strongly hydro-
philic character, while a low value is an indication of a strong
hydrophobic nature. Considering these points, a non-ionic, rel-
atively pH-independent hydrophobic surfactant, i.e. sorbitan
monooleate (Span 80) with an HLB value of 4.3, was selected
Table I. Crystal Phases Developed in Nanoparticles Under
Different Experimental Conditions
Concentration Reaction
Run of Al(NO3)3 temperature Final pH of
no.
solution (M)
(1C)
the emulsion
Crystal phase
1
2
3
4
0.05
0.05
0.1
9071
9071
9071
9071
8.5
8.0
8.5
7.0
Boehmite (g-AlOOH)
Boehmite (g-AlOOH)
Boehmite (g-AlOOH)
Boehmite (g-AlOOH)1
bayerite
0.05
5
6
7
0.05
0.05
0.05
8071
9071
8071
7.0
6.0
8.0
Bayerite
Bayerite
Bayerite1boehmite
(U-AlOOH) (little)
Aging time at 901711C: 10 min.

3324
Journal of the American Ceramic Society—Naskar and Chatterjee
Vol. 88, No. 12
Fig. 2. Molecular structure of Span 80 showing the hydrophilic ‘‘head’’
(sorbitan) and the hydrophobic ‘‘tail’’ (oleic acid) parts.
for the present study. Figure 2 shows the molecular structure of
the amphiphilic Span 8018,25 where the hydrophilic sorbitan
group acts as a ‘‘polar head’’ and the hydrophobic oleic acid
group acts as the ‘‘non-polar tail.’’
Fig. 4. Thermogravimetry and differential thermal analysis of boeh-
mite (g-AlOOH) nanoparticles (product of run no. 2 of Table I), from
301 to 12001C.
For the synthesis of precursor particles in the single-emulsion
technique, ammonia gas/organic amine/ammonia solu-
tion19,23,26 is directly introduced into the emulsion system to
cause precipitation of the metal hydroxide inside the dispersed
aqueous microdroplets. As the main disadvantage of the single-
emulsion technique is the non-uniform distribution of the added
base, resulting in the formation of particles with wide size dis-
tribution, in the present investigation, the two-emulsion tech-
nique was followed. Compared with the single-emulsion system,
the two-emulsion system is more advantageous in the sense that
the precipitation and growth (by ripening) of the particles in the
fused dimmer caused by collision of two reactant droplets is
limited.23,31,32 Figure 3 presents a schematic of the formation of
fused dimmers during particle formation in the two-emulsion
system. The steps involved can be visualized as follows:
Step I: Dispersion of the aqueous microdroplets of Al31
and NH3 solution in the organic phase (matrix) through steric
stabilization.
1.66% for the sample was found from 5001 to 12101C. In the
DTA curve of the same sample (Fig. 4) from 301 to 12101C, a
strong endothermic peak appeared at about 1541, which was
because of the expulsion of loosely bound water molecules and
some organics associated with the materials.34–36 The first strong
exothermic peak at 2281 for the nanoparticles of run no. 2 cor-
responded to the decomposition of nitrates.33,35,36 The second
broad exothermic peak at B6001C for the above sample indi-
cates the formation of g-Al2O3. The weak exothermic peak cor-
responding to 10381C indicates the formation of d-Al2O3.
Finally, the appearance of a small exothermic peak at 11501C
for the sample is explained to be because of the crystallization of
a-Al2O3.4,36 It is to be noted that the precursor powders were
heated separately in an Ar atmosphere to the corresponding
exothermic peak temperatures obtained from DTA results at the
same heating rate (101C/min) and quenching to room temper-
ature as in DTA analysis. The heated samples were analyzed by
XRD, which confirmed the crystallization of the respective
phases as observed from the exothermic peaks of DTA curves.
Step II: Formation of fused droplets by collision because
of Brownian motion followed by particle formation via ex-
change reaction (precipitation).
Step III: Dispersion of emulsified particles, i.e. surfactant-
stabilized-precipitated particles by steric hindrance.
Step IV: Separation of spherical boehmite (g-AlOOH)
nanoparticles, by acetone treatment.
Based on the above mechanism, the size of the particles dur-
ing precipitation, in the present investigation, was tailored by
controlling the concentration of Al31 in the starting solution
and aging time at the reaction temperature. Further, the reaction
temperature as well as the final pH of the system played impor-
tant roles in the development of different crystal phases. This is
clearly discernible from the results of Table I where both the low
pH and low reaction temperature favor the formation of baye-
rite instead of boehmite particles.1,2
(4) Phase Transformation of the Boehmite Nanoparticles
Crystalline phases identified by XRD in the precursor corre-
sponding to run no. 2 of Table I calcined at different temper-
atures have been presented in Fig. 5. It indicates that the
formation of g-AlOOH (boehmite) occurs during its synthesis
at 901C. The peaks observed in the XRD patterns match well
with the g-AlOOH (boehmite) peaks reported in the JCPDS File
no. 21-1307. The phase-pure g-AlOOH was found to persist
even after calcination at 4001C. At 6001C, d-Al2O3 was identi-
fied, which persisted as the only phase at 8001C. At 10001C, d-
Al2O3 co-existed as the major phase with the g-Al2O3. The for-
mation of g-Al2O3 and d-Al2O3 was also indicated in the DTA
curve at B6001 and 10381C, respectively, for the same sample.
At 12001C, a-Al2O3 was identified as the only phase. Therefore,
based on the DTA and XRD results, the phase transformation
of the boehmite nanoparticles during heat treatment from am-
bient temperature to 12001C is represented as follows4:
(3) Thermal Behavior of Boehmite Nanoparticles
The thermal behavior of the boehmite nanoparticles corre-
sponding to run no. 2 of Table I was investigated by TG/
DTA from 301 to 12101C at 101C/min in Ar atmosphere, and
the TG/DTA traces of the samples have been presented in
Fig. 4.
Boehmite ðg-AlOOHÞ ! g-Al2O3 ! d-Al2O3 ! a-Al2O3
The TG study of the precursors corresponding to run no. 2 of
Table I shows that weight losses because of the volatilization of
water, and decomposition of nitrates, and organics mostly oc-
curred up to about 5001C33,34 accompanied by a weight loss of
41.56% (Fig. 4). An insignificant amount of weight loss of
(5) FTIR Study
FTIR spectroscopic analysis was performed for the product of
run no. 2 of Table I in the wavenumber range 4000–400 cmꢂ1
.
The spectra in the wavenumber range 1300–400 cmꢂ1 are shown
in Fig. 6. It is to be noted that the absorption bands appeared at
around 1160, 1067, 895, 730, 624, and 490 cmꢂ1 of the g-AlOOH
in the present investigation match with those obtained in the
earlier studies,5,9,10,15 thus further confirming the formation of
boehmite (g-AlOOH) at 901711C.
(6) Particle Size Distribution and TEM of the Boehmite
Nanoparticles
The average particle size of the product of run no. 2 of Table I
(as-synthesized particle) was found to be the smallest, i.e. 10 nm,
Fig. 3. Schematic for the formation of boehmite (g-AlOOH) nanopar-
ticles.

December 2005
Boehmite Nanoparticles
3325
Fig. 7. Particle size distribution of boehmite (g-AlOOH) nanoparticles
(product of run no. 2 of Table I).
with a size distribution of about 2–85 nm. The distribution of
particle size is monomodal in nature (Fig. 7). The average par-
ticle size of the product of other runs of Table I ranged from 265
to 735 nm.
The morphology of the product of run no. 2 of Table I (as-
synthesized powder) and that calcined at 4001C, with a dwell
time of 1 h, was examined by TEM. The investigation revealed
that the as-prepared g-AlOOH powder was free from agglom-
eration and they were spherical in morphology (Fig. 8(a)). At
4001C, the particles became agglomerated, although their
sphericity was retained (Fig. 8(b)).
IV. Conclusions
1. The two-emulsion technique has been proved to be highly
effective for the synthesis of spherical, agglomerate-free boeh-
Fig. 5. Powder X-ray diffraction patterns of boehmite (g-AlOOH)
nanoparticles (product of run no. 2 of Table I) calcined at different
temperatures (901–12001C). b 5 boehmite (g-AlOOH), g 5 g-Al2O3,
d 5 d-Al2O3, a 5Al2O3. Note: dwell time: 1 h at each temperature.
Fig. 8. Transmission electron microscopy photograph of boehmite
(g-AlOOH) nanoparticles (product of run no. 2 of Table I); (a) dried
at 1001C/1 h dwell time, (b) calcined at 4001C/1 h dwell time.
Fig. 6. Fourier transform infrared spectroscopy spectra of boehmite
(g-AlOOH) nanoparticles (product of run no. 2 of Table I).

3326
Journal of the American Ceramic Society—Naskar and Chatterjee
Vol. 88, No. 12
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3. XRD results confirm the crystallization of boehmite (g-
AlOOH) nanoparticles at 901711C, which retains up to 4001C.
4. FTIR studies indicate the characteristic absorption bands
of boehmite (g-AlOOH) at around 1160, 1067, 895, 730, 624,
and 490 cmꢂ1 in the as-synthesized nanoparticles.
5. The particle size distribution is monomodal in nature.
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6. TEM indicates the spherical morphology of boehmite
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The authors thank Dr. H. S. Maiti, Director of the Institute, for his constant
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