chemical synthesis of aluminum nanoparticles
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RESEARCH PAPER
Chemical synthesis of aluminum nanoparticles
Sekher Reddy Ghanta Krishnamurthi Muralidharan
Received: 19 December 2012 / Accepted: 9 May 2013 / Published online: 23 May 2013
Springer Science+Business Media Dordrecht 2013
Abstract An alternate synthetic route has been
described for the production of aluminum nanoparti-
cles (Al-NPs). These Al-NPs were obtained through a
reduction of aluminum acetylacetonate [Al(acac)3] by
lithium aluminum hydride (LiAlH4) in mestitylene at
165 C. The side products were removed by repeatedwashing with dry, ice cold methanol and the reaction
mixture was filtered to obtain gray-colored Al-NPs.
The synthesized nanoparticles were characterized by
Powder X-ray diffraction pattern and 27Al-MAS-NMR
spectrum. The X-ray diffraction pattern confirmed the
formation of face-centered cubic (fcc) form of alumi-
num. The size and morphology were investigated by
scanning electron microscope and transmission elec-
tron microscope which showed particle of varying
shapes with size ranging from 50 to 250 nm. The
weight loss from the nanoparticles was studied by
thermo gravimetric analysis which indicated that the
nanoparticles were tightly bound with an unknown
amorphous organic residue which cannot be removed
by simple washing. The carbonaceous residue might be
outcome of the decomposition of acac ligand which
was responsible in stabilizing aluminum nanoparticles.
Keywords Aluminum nano particles Al(acac)3 Lithium aluminum hydride Alanes 27Al-MAS-NMR
Introduction
The synthesis of aluminum nanoparticles (Al-NPs)
have been an interesting endeavor for the researchers
due to its use in various fields. Owing to its high
enthalpy of combustion (31 kJ mol-1), Al-NPs have
been used as a fuel in propellants and pyro techniques
(Cooper 1996). Moreover, Al-NPs readily react with
air and water leading to the evolution of heat and
hydrogen gas. Recently, it was reported that the
hydrogen gas was generated at room temperature from
the reaction of Al-NPs with tap water (Bunker et al.
2010). As a consequence, various methods have been
developed for the synthesis of Al-NPs including,
electrochemical deposition method (Pomfret et al.
2008), chemical aerosol flow method (Helmich and
Suslick 2010), sono electrochemical synthesis (Mah-
endiran et al. 2009), and thermal decomposition of
alane precursors (Haber and Buhro 1998) using Ti(O
iPr)4 catalyst. Recently, Jagirdar et al. reported a
synthesis of mono-dispersed Al-NPs by solvated metal
atom dispersion (SMAD) method (Arora and Jagirdar
2012).
S. R. Ghanta K. Muralidharan (&)Advanced Center of Research in High Energy Materials
(ACRHEM), University of Hyderabad,
Hyderabad 500046, India
e-mail: kmsc@uohyd.ernet.in
K. Muralidharan
School of Chemistry, University of Hyderabad,
Hyderabad 50046, AP, India
123
J Nanopart Res (2013) 15:1715
DOI 10.1007/s11051-013-1715-1
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It is well known that the smaller sized metal
nanoparticles are more reactive subsequently leading
to faster overall energy release (Trunov et al. 2005)
from the oxidation reaction. However, the kinetic
instability of metal nanoparticles makes them unstable
which causes the synthesis and stabilization of Al-NPs
such a challenging problem. Further, when Al-NPs are
exposed to the air, they are oxidized and covered by a
26 nm thick oxide layer (Aumann et al. 1995). For
the micron sized aluminum particles, the oxide layer
accounts for\0.5 % of the particle size, while movingtoward nano size the extent of oxide layer becomes
more significant. Thus the naturally forming oxide
layer results in a much lower energy density of an
energetic composition containing Al-NPs, subse-
quently leading to the elevated ignition temperature
(Trunov et al. 2005). It was also shown that the
activation energy for the oxidation of Al-NPs which
was protected from the oxide layer formation is less
(Aumann et al. 1995) compared to their bulk counter-
parts indicating the necessity of preventing the surface
oxidation and agglomeration of Al-NPs.
The protection of the surface could be achieved by
coating the Al-NPs by various capping agents such as
long chain carboxylic acids (Jouet et al. 2005;
Fernando et al. 2009), epoxides (Chung et al. 2009;
Hammerstroem et al. 2011), and transition metals
(Foley et al. 2005). Bunker et al. reported a method
where in Al-NPs were synthesized inside the cavities of
a perflourinated ionomer membrane which could also
act as an oxidizer (Li et al. 2009). As an alternative
method to address the problem of surface oxidation and
also to take advantage of compatibility of polymers
with energetic composition, we recently synthesized of
Al-NPs stabilized inside polymer matrices (Ghanta and
Muralidharan 2010). In our continued effort in this
direction we have developed new alternative chemical
synthetic method for the production of Al-NPs. Herein
we report the synthesis of Al-NPs from Al(acac)3which was used as one of the precursor materials
(Fig. 1). In this reaction, an unknown form of organic
residue was formed from the decomposition of acac
ligand. This carbonaceous residue was found to be
responsible for the stability of Al-NPs.
Experimental section
Materials and synthesis
All the starting materials were purchased from Aldrich
and were used as received. Aluminum acetylacetonate
[Al(acac)3] (10 mmol) (Fig. 1) was added to the
mesitylene which was already placed in a two neck
roundbottom flask (RBF) equipped with a magnetic
stirring bar. Then, 30 mmol of lithium aluminum
hydride (LiAlH4) was added into the reaction flask. A
reflux condenser connected with a nitrogen inlet was
fitted onto the RBF. The reaction mixture was then
refluxed continuously with stirring. Evolution of a gas
was observed during refluxing. When the gas evolu-
tion was ceased after approximately 72 h, the reaction
mixture was cooled down to room temperature. A
gray-colored precipitate was settled down. Most of the
solvent was distilled out by applying vacuum. The
crude product was further dried under low pressure for
34 h. The crude product obtained was divided into
three equal portions and each portion was washed with
20 ml of dry, ice cold methanol at least for four times.
This division was to avoid any difficulty during the
filtration of Al-NPs and also to avoid any exothermic
mixing of solvent with Al-NPs. Most of the organic
part and the unreacted starting materials were washed
away with methanol. The product was filtered through
a glass frit with grade 3 porous size. The final product
was again dried under low pressure. The entire process
was carried out under a standard nitrogen schlenk line.
Instruments
All the powder X-ray diffraction (PXRD), analyses
were conducted on Bruker D8 X-ray diffractometerFig. 1 Structure of Al(acac)3
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(Cu Ka = 1.54 A). The samples for PXRD wereprepared by coating the fine powder samples on quartz
made sample holder. The isotropic solid state 27Al
(100 % abundant, I = 1/2) NMR spectra were
recorded using a Bruker Avance 400 MHz NMR
spectrometer at MAS rotational frequency of 5 kHz
and an observed frequency of 104.34 MHz. For each
experiment, about 100 mg of dry nanoparticles was
tightly filled in a zirconia rotor under nitrogen atmo-
sphere and capped with KEL-F cap. Chemical shift was
referenced to the AlCl3 as external standard. Similarly,
the solid state 13C spectra were recorded using the same
spectrometer and the chemical shift was referenced to
tetramethylsilane as external standard. Thermo gravi-
metric analyses (TG-DTA) were conducted in open
pan alumina cups using TA instruments SDT Q 600
instrument. The samples were heated from room
temperature to 800 C at a rate of 10 C min-1 undera constant flow (100 ml min-1) of nitrogen gas.
The microscopic investigations were carried out on
ultra 55 of carl Zeiss FE-SEM and FEI technai G2 20
TEM while the EDS analyses were conducted on
Philips XL-30 SEM. Samples for FE-SEM were
prepared by drop casting the uniformly dispersed
solution on a glass slide. In the case of TEM, the
samples were prepared by drop casting the well-
dispersed solution on the carbon-coated copper grids.
The dispersed solutions were prepared by sonicating
the suspension for suitable time based on the solvent.
After drop casting the solution, the copper grids were
allowed to dry under ambient conditions. Enthalpy of
combustion of Al-NPs was determined using Parr
6200 model oxygen bomb calorimeter.
Results and discussion
Many physical methods are known for the production
of Al-NPs. Still the production of Al-NPs in larger
quantities remains challenging because of its oxophi-
licity and the tendency of growing to form larger
particles on the expense of smaller particles. In order
to circumvent this problem many efforts were devoted
to the synthesis of Al-NPs which were protected by
coatings or organic surface passivation under ambient
conditions. Among the chemical synthetic methods
available for Al-NPs, the reduction of AlCl3 by
LiAlH4- and thermal decomposition of alanes are
still remaining as promising methods and hence being
widely practiced in various research laboratories
(Haber and Buhro 1998; Jouet et al. 2005; Fernando
et al. 2009; Chung et al. 2009; Hammerstroem et al.
2011; Foley et al. 2005). However, the sources of Al in
these well-utilized chemical synthetic methods are
AlCl3 and alane. The major concern regarding these
starting materials is their sensitivity to moisture. This
drawback gains more importance when the method is
considered for large scale industrial production. In this
aspect, it is necessary to explore an alternative
chemical synthetic procedure for the production of
aluminum. The interest is also there in establishing a
synthetic method wherein purification of nanoparticles
is easy.
In an effort to find an alternate for Al-NPs
production, we have developed a new synthetic
procedure wherein Al(acac)3 was used, one of the
precursor materials (Reaction 1). Aluminum acetyl-
acetone (acac) precursor was primarily selected due to
its solubility in mesitylene which was required for a
clean reaction. In a typical procedure, Al(acac)3 was
reacted with LiAlH4 at 165 C in mesitylene to obtainAl-NPs. This procedure yielded side products which
were soluble in organic solvent which made easy
purification of Al-NPs. The side products were
removed by repeated washing with dry, ice cold
methanol and the reaction mixture was filtered to yield
gray-colored Al-NPs. It was expected that the LiAlH4should transfer the hydride ions to Al(acac)3 forming
aluminum hydride. Since the aluminum hydride was
unstable over 160 C, the reaction supposed toproceed via decomposition of it and therefore the
evolved gas might be hydrogen.
Alacac3 3LiAlH4 !mesitylene; 164
C
reflux; 72 h4Al
H2 other side products Reaction 1
Characterization
The formation of Al-NPs was primarily confirmed
from the powder X-ray diffraction pattern and solid
state 27Al-MAS-NMR spectrum. The powder X-ray
diffraction pattern of as synthesized Al-NPs was
showed in Fig. 2 which were compared with that of
JCPDS data (#040787) and found to be in good
agreement with the face-centered cubic (fcc) form of
aluminum. The 27Al-MAS-NMR spectrum obtained
for the freshly prepared Al-NPs showed a peak at d
J Nanopart Res (2013) 15:1715 Page 3 of 10
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1,641.9 ppm (Fig. 3a) (Hammerstroem et al. 2011;
Ghanta and Muralidharan 2010) which was an indic-
ative of Al (0) species. It is to be mentioned that the
chemical shifts in an Al-MAS-NMR spectrum corre-
lates with aluminum coordination number for both the
organoaluminum compounds as well as for the
inorganic compounds of aluminum (Smith 1993).
Since the spectrum showed no peaks in the region
0100 ppm it was confirmed that nothing was coor-
dinated to the aluminum or chemically bonded i.e., no
other aluminum complex existed in the sample.
Stabilization of Al-NPs
The powder XRD pattern was very clear and no other
peaks were observed related to the aluminum oxide or
any organic acac ligand. The solid state 13C-MAS-
NMR spectrum (Fig. 4) for the Al-NPs sample was
also recorded. In the spectrum, two peaks were
appearing in the range of 165175 ppm. These
chemical shifts did not match with any carbon atom
of acac ligand. To help to elucidate the structure of the
organic residue, a sample was analyzed with FT-IR
Fig. 2 PXRD pattern of assynthesized Al-NPs
Fig. 3 Solid state 27Al-NMR spectra of a assynthesized Al-NPs
b sample annealed at 450 Cfor 12 h
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spectroscopy (Fig. 5). However, the peaks in FT-IR
spectrum could not be assigned to either acac ligand
or any organic moiety presumably resulted from the
decomposition of acac ligand. The peaks seen around
3,500 and 2,350 cm-1 were related to the air back-
ground (moisture and CO2). The peak around
2,900 cm-1 is probably related to CH stretching.
Since the IR and 13C-MAS-NMR spectra did not give
any conclusive evidence for any known organic
species, it was assumed the presence of a carbona-
ceous organic residue which resulted from the
decomposition of acac ligand. This might be possible
since the reaction was carried out at higher temper-
ature (165 C). Similar formation of an unknown
carbonaceous mass was observed during the synthesis
of PtNi intermetallic nanoparticles by Leonard et al. in
2011. In the view of understanding the nature of the
carbonaceous material, we have recorded the Raman
spectrum of the Al-NPs. There were two peaks at
1,350 and 1,598 cm-1 indicating the presence of some
form of graphitic lattice (Sadezky et al. 2005).
While the carbon composition was more difficult to
accurately identify, the carbonaceous residue contrib-
uted to the maximum of 35 % to the total weight of
sample as seen from the thermogravimetric analyses.
The thermogravimetric curve (TG) (Fig. 6) of the
sample of as synthesized Al-NPs showed a significant
loss of weight in the range of 230260 C. This weight
Fig. 4 Solid state 13C-NMR of Al-NPs embedded
in organic moiety
Fig. 5 FT-IR spectrum ofas synthesized Al-NPs
J Nanopart Res (2013) 15:1715 Page 5 of 10
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loss could be attributed to the volatilization of organic
residue attached on surface of the nanoparticles. The
decomposition was also supported by a broad signal in
the DTA (differential thermal analysis) curve
(250260 C range in Fig. 7). After the volatilizationof organic residue, there was no loss of weight until it
gains weight at melting region of aluminum (660 C)probably because of AlN formation. A strong endo-
thermic peak corresponding to the melting point of
aluminum was also observed in the DTA curve.
When the samples were heated at 550 C for 90 minprior to the analysis, the TGA (Fig. 6) curve showed a
decrease in the extent of weight loss. Similarly, when
the sample was heated at 550 C for 180 min, itshowed only 68 % loss in weight. But in all the cases,
there was no change in endothermic peak (Fig. 7) of
product confirming the existence of pure metallic
aluminum. For further understanding, a sample was
heated at 450 C for 5 h in an open air furnace. ThePXRD pattern of annealed sample showed the devel-
oping of c-phase of aluminum oxide (Fig. 8). Simi-larly, 27Al-MAS-NMR also showed new peaks
matching chemical shifts of aluminum oxides
(Fig. 3b). From these studies, it can be explained as
the presence of the surface bound carbonaceous
species was responsible for the protection of Al-NPs
from the surface oxidation; hence, its removal helped
for the aluminum oxide formation as evidenced the
annealed sample.
Microscopic investigation
The size and morphology of the Al-NPs were observed
by a transmission electron microscopy (TEM) (Fig. 9)
and field emission scanning electron microscopy (FE-
SEM) (Fig. 10). The Al-NPs were appeared as fea-
tureless particles, flakes, and cubes with the sizes in the
range of 50250 nm. The size calculated from PXRD
using Scherrer equation was within the range
(93.7 nm) that obtained from the TEM images. The
TEM images clearly showed that the particles were
bound with an amorphous surface of organic residue
obtained from the decomposition of acac ligand
(Fig. 9a, b). It was also observed that some of the
NPs were flakes as seen in Fig. 9c. The presence of
flakes allowed recording HRTEM for the Al-NPs. The
HRTEM image showed the existence of (1 1 1) plane of
fcc Al with d-spacing 2.34 A (Fig. 9d). The selected
area electron diffraction pattern (SAED) showed a ring
pattern corresponding to the all planes of fcc Al
Fig. 6 TGA curves of Al-NPs bound with a
carbonaceous organic
residue
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(Fig. 8e). The FE-SEM images also showed shapeless
individual nanoparticles (Fig. 10a, b). Those individ-
ual particles possibly further grow to form flakes
(Fig. 10c).
The elemental analysis of Al-NPs was carried out
on the Energy Dispersive X-ray Spectrometry (EDS)
attached with SEM and TEM instruments individu-
ally. The compositions obtained from both the instru-
ments were consistent with each other. The EDS
spectrum (Fig. 11) showed the relative intensity of the
aluminum, oxygen, and carbon. The presence of
oxygen and carbon could be attributed to the existence
of unknown form of carbonaceous organic residue as
well as surface oxidation.
Heat of combustion
The heat of combustion of synthesized aluminum
nanoparticles was determined using Parr 6200
Fig. 7 DTA curves of Al-NPs bound with a
carbonaceous organic
residue
Fig. 8 PXRD pattern of theheated (annealed) Al-NPs at
450 C. The additional twopeaks were the indications
of developing the c-Al2O3
J Nanopart Res (2013) 15:1715 Page 7 of 10
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isoperibol oxygen bomb calorimeter. The method
involved the determination of the heat evolved from
the burning of a weighed sample in oxygen atmosphere
of known pressure. The energy equivalent of the
calorimeter was determined from the heat of combus-
tion of standard benzoic acid supplied by Parr instru-
ments (Holley and Huber 1951). The instrument was
calibrated with benzoic acid before burning the
samples. Samples were made into pellets of equal
weights and burned. Burning of the samples was
carried at approximately 30 atm (420 psi) pressure of
purified oxygen. Nichrome fuse wire was used to ignite
the samples. The heat of combustion obtained for the
Al-NPs was 11.9 kJ g-1 (averaged from six measure-
ments). The decrease in the combustion value when
compared with reported value of bulk Al (31 kJ g-1)
might be attributed to the presence of carbonaceous
layer which is bound with Al-NPs. The residue
Fig. 9 TEM images of Al-NPs (a, b) or flakes (c) and HRTEM (d) of Al-NPs with corresponding SAED pattern (e)
Fig. 10 FE-SEM images of Al-NPs and aluminum flakes scale bar 200 nm
Page 8 of 10 J Nanopart Res (2013) 15:1715
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obtained after the combustion of Al-NPs are analyzed
with X-ray diffraction and observed it was in c-Al2O3form (Fig. 12).
Conclusion
An alternative novel chemical synthetic procedure for
the production of Al-NPs from Al(acac)3 has been
developed. The face-centered cubic (fcc) form of
aluminum nanoparticles were unambiguously charac-
terized from powder X-ray diffraction pattern and 27Al-
MAS-NMR spectral data. Further, the unidentifiable
form of carbonaceous residue which was formed from
the decomposition of acac ligand acted as the stabilizing
agent for the nanoparticles. The presence of the organic
residue was confirmed from microscopic images, ther-
mogravimetric analysis and other spectral data.
Acknowledgments The authors gratefully acknowledge thefunding from DRDO, India in the form of research Grant to
ACRHEM and also thank the School of Chemistry, University
of Hyderabad for the infrastructure and other instrument
facilities.
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Chemical synthesis of aluminum nanoparticlesAbstractIntroductionExperimental sectionMaterials and synthesisInstruments
Results and discussion CharacterizationStabilization of Al-NPsMicroscopic investigation
Heat of combustionConclusionAcknowledgmentsReferences
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