chemical synthesis of aluminum nanoparticles

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: [email protected]

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

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

Page 2 of 10 J Nanopart Res (2013) 15:1715

123

(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

123

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


123

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


123

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


123

(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


123

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


123

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

chemical synthesis of aluminum nanoparticles

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