Post on 16-May-2023
Structural phase transformations in radiolytically synthesizedAl–Cu bimetallic nanoparticles
Farhad Larki1 • Alam Abedini1 • Md. Shabiul Islam1• Sahbudin Shaari1 •
Sawal Hamid Md Ali1 • P. Susthitha Menon1• Azman Jalar1
• Elias Saion2•
Jahariah Sampe1• Burhanuddin Yeap Majlis1
Received: 15 December 2014 / Accepted: 20 March 2015 / Published online: 2 April 2015
� Springer Science+Business Media New York 2015
Abstract The structural changes of radiolytically pre-
pared aluminium–copper (Al–Cu) bimetallic nanoparticles
by adjusting the precursors’ mole ratio and gamma ra-
diation dose were investigated by transmission electron
microscopy, field emission scanning electron microscopy/
energy dispersive spectroscopy, X-ray diffraction, Fourier
transform infrared spectroscopy (FTIR), and X-band con-
tinuous wave electron paramagnetic resonance (EPR). The
EPR spectrum was also analysed through the simulation of
the powder-like EPR spectra. The results note that in pre-
pared samples with higher Al contents, formation of core–
shell structure is dominant, whereas in Cu-rich samples, the
final structures are primarily in alloy and oxide forms.
According to the analysis of data obtained from X-ray
diffraction, FTIR, and EPR, we found that the unpaired
electron of the Cu2? ion in various phases play the main
role in structural phase transformation of Al–Cu nanopar-
ticles. Additionally, based on the information extracted
from simulated EPR peaks of Cu–Cu, the diameter of the
Cu core in core–shell structures was obtained. We showed
that by increasing the gamma radiation dose from 80 to
120 kGy, the overall size of nanoparticles decreases from
9.47 to 3.75 nm, but the contribution of copper core in-
creases from 11 to 22 % of overall particle size.
Introduction
Aluminium–copper (Al–Cu) bimetallic nanoparticles have
been recently studied for improving the material properties
of aluminium in terms of a lower density, higher strength
and stiffness at elevated temperatures, outstanding catalytic
activity with decreasing of particle size, and stronger re-
sistance to corrosion [1, 2]. Laser ablation [3], electro-
chemical deposition [4], and radiolytic reduction [5, 6] are
some of the synthesis techniques that have been used for
producing Al–Cu bimetallic nanoparticles. Among these
techniques, the radiolytic technique is excellent because it
is a simple, clean, environmentally benign, and low-cost
method for preparation of a large quantity of size- and
structure-controllable metal nanoparticles [7–10]. More-
over, homogeneous formation of nuclei and elimination of
excessive chemical reducing agents through the radiolytic
reduction process allow for the creation of uniformly dis-
persed and highly stable nanoparticles without unwanted
by-products of the reductants [11–13]. The synthesis of Al–
Cu bimetallic nanoparticles using the radiolytic technique
has already been reported in our previous works [6, 14].
However, a precise explanation of the variation of local
structures of various components due to the modification of
precursors’ mole ratio and radiation dose has not yet been
obtained.
Because the analysis of electron paramagnetic resonance
(EPR) spectra can provide useful information on the local
structures of components, which help in understanding the
role of each compound in final structure, in parallel to other
characterization techniques studies on the EPR results are
of particular scientific and practical significance. There-
fore, in this work, we present a detailed investigation on the
impact of the precursors’ mole ratio and radiation dose
variation on the structural phase transformation of
& Farhad Larki
farhad.larki@gmail.com
1 Institute of Microengineering and Nanoelectronics (IMEN),
Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor,
Malaysia
2 Department of Physics, Universiti Putra Malaysia,
43400 Serdang, Selangor, Malaysia
123
J Mater Sci (2015) 50:4348–4356
DOI 10.1007/s10853-015-8988-y
radiolytically synthesized Al–Cu bimetallic nanoparticles.
We probed the structural properties of the Al–Cu
nanoparticles using transmission electron microscopy
(TEM), field emission scanning electron microscopy
(FESEM)/energy dispersive spectroscopy (EDS), X-ray
diffraction (XRD), Fourier transform infrared spectroscopy
(FTIR), and continuous wave electron paramagnetic reso-
nance (CW-EPR). A simulation of the EPR spectra was
also performed using the EasySpin software [15] to un-
derstand the influence of mole ratio and gamma radiation
dose on different phase formation through the nucleation
and growth process.
Methodology
Experimental procedure
All of the precursors, purchased from System, were of
analytically pure grade and used without further purifica-
tion. Aluminium and copper chloride were used as precur-
sors and were dissolved in a stock solution of 3 % polyvinyl
alcohol (PVA 88 % hydrolysed, MW = 22000) at various
mole ratios (Al3?/Cu2? = 2/8, 3/7, 5/5, 7/3, 8/2) in the
presence of isopropyl alcohol (IPA) which was used to
scavenge H and OH radicals (the volume ratio of isopropyl
alcohol/deionized water is 1/5). The solution was stirred for
1 h at constant 70 �C and, before sending for irradiation,
was bubbled with nitrogen gas. The final solution was ir-
radiated by a Co-60 gamma-ray source at different doses
from 80 to 120 kGy with dose rate of 2.9 kGy/h.
The samples were characterized by TEM, FESEM/EDS,
XRD, FTIR, and CW-EPR spectrometer. A ZEISS MERLIN
compact field emission scanning electron microscope cou-
pled with Oxford X-MaxN 50 silicon drift detector (SDD) is
implemented to obtain the EDS elemental mapping of the
samples. A Shimadzu diffractometer (model XRD 6000)
with Cu Ka (0.154 nm) was used for XRD characterization to
obtain the micro-structure of the Al–Cu nanoparticles. TEM
was performed using a HITACHI transmission electron
microscope (TEM; H-7500) with an accelerating voltage of
100 keV. FTIR spectra were recorded using a Perkin Elmer
FTIR model 1650 spectrometer. The X-band CW-EPR
spectra were recorded on a JEOL FA200 spectrometer (mi-
crowave frequency, 9194.4 MHz). All the spectra were
recorded at room temperature (RT) with a microwave power
of 0.99 mW, a modulation frequency of 100 kHz, and
modulation amplitude of 0.35 mT.
Simulation procedure
The computer simulation of the EPR spectra was per-
formed using the EasySpin package (version 4.5.5, option
Pepper) [15]. This software numerically solves the spin
Hamiltonian of the component spin state, which is ran-
domly oriented in the solid phase. Principally, the obtained
EPR spectrum can be simulated by a diagonalization of the
spin Hamiltonian to determine the allowed transitions for
the unpaired system with adjustments to the following
anisotropic parameters: g-tensor, surrounding magnetic
nuclei with specified hyperfine interaction and line width.
Accordingly, the information for elements of the EPR
signal for Al–Cu nanoparticles synthesized with different
precursor mole ratios and radiation doses were constructed
using EasySpin.
Results and discussion
Impact of mole ratio and irradiation dose
During the irradiation of an aqueous system by gamma
rays, various active intermediates were generated during
the radiolysis of water. The main active products are listed
in the following reaction:
H2O ��������!Gammairradiatione�aq;OH
�;H2O2;H
:;H2;HO2
�ð1Þ
The radiolysis yield of species (x) noted G(x) which is
defined as:
G xð Þ ¼ #of species created or destroyed
100 eV deposited energyð2Þ
G values for mentioned species are as follow: G (e�aq) =
2.50, G (OH�
) = 2.50, G (H2O2) = 0.70, G (H�) = 0.56, G
(H2) = 0.45, and G (HO2
�) = 0.027. As it can be seen, the
strongest reducing agent is electron aqueous (e�aq) which
easily reduce all metal ions to their neutral state metal
atoms. It should be noted that the hydroxyl radicals that
formed during radiolysis of water are able to oxidize the
ion or the atoms into higher oxidation state. In order to
prevent this stage, the radicals were scavenged by adding
isopropyl alcohol.
Figure 1 schematically illustrates the proposed forma-
tion mechanism of Al–Cu bimetallic nanoparticles in dif-
ferent Al3?/Cu2? mole ratio. The solvated electrons (e�aq),
which formed in the radiolysis of water under gamma ir-
radiation, can reduce Cu2? and Al3? ions from higher
valences to lower or zero-valent state atoms. Copper ions
are more easily reducible than aluminium ions under irra-
diation due to their higher redox potential. Therefore, the
reduction process starts by reducing copper ions, which
ultimately merge to form aggregation of metal nanoparti-
cles. Then, the unreduced ions adsorb on the surface of the
newly formed clusters. The formation of bimetallic clusters
J Mater Sci (2015) 50:4348–4356 4349
123
is a direct consequence of this sequential reduction and
adsorption process. In further reduction steps, in the sam-
ples with higher Al content (Al3?/Cu2? = 8/2 and 7/3),
when Cu2? ions are exhausted, the reduction of Al3? in-
creases which exclusively occurs at the surface of the
newly formed Cu particles. Accordingly, the final particles
are mostly in the form of core–shell bimetallic nanoparti-
cles with Cu concentrated in the core and Al mostly con-
centrated in the shell.
For better understanding of the core–shell formation
mechanism by variation of gamma radiation dose, FESEM
image and the corresponding EDS elemental mapping
images of Cu and Al at different gamma dose for sample
with Al3?/Cu2? = 7/3 are presented in Fig. 2a–l. Fig-
ure 2a–d displays a high content of Cu at the surface of
sample at 80 kGy. This result is illustrating when the
mixture of bivalent Cu and trivalent Al ions are irradiated.
Under this condition, the reaction of solvated electrons
with Cu2? has higher possibility than that of Al3? ions. By
increasing gamma dose from 80 to 100 kGy, when Cu2?
ions are depleted, the reduction of Al3? increased, which
occurs exclusively at the surface of the Cu particles. Ac-
cordingly, at 120 kGy by increasing the reduction of Al3?
ions on the surface the resulting particles become bilayered
with a core of Cu and a shell made of the Al or Cu–Al
cluster.
In the following discussions, these statements will be
approved by XRD, FTIR, and EPR results. XRD patterns
of Al–Cu nanoparticles at mole ratio of Al3?/Cu2? = 7/3
in various radiation doses are presented in Fig. 3a–c. The
XRD peaks at 43�, 50�, and 74� are corresponded to the
crystal planes of (1 1 1), (2 0 0), and (2 2 0) of the face-
centered cubic (fcc) crystalline Cu nanoparticles, respec-
tively, which increased by increasing dose [16]. The ob-
tained pattern at 80 kGy proves the formation of oxide
form of Al and Cu nanoparticles which is due to incom-
plete reduction of Cu and Al ions under lower irradiation
dose. The intensity of these oxide peaks decreases as the
dose increases.
The oxidation form of CuAl in Fig. 3b and c may result
in a slight inevitable surface oxidation during the process
of polymer capping, washing, or drying. The XRD analyses
of resultant particles in three different mole ratio of Al3?/
Cu2? (7/3, 5/5, and 3/7) at the fixed irradiation dose of
120 kGy are presented in Fig. 4a–c. It is clear from Fig. 4b
and c for samples with mole ratio of Al3?/Cu2? = 5/5 and
3/7 that no peak attributable to the pure Cu is observed. In
samples with higher Cu2? mole fraction, the Al and Cu
elements coexist in bimetallic nanoparticles and form an
alloy structure of CuAl rather than a core–shell structure.
While the mixed solution of Al3? and Cu2? is irradiated,
the Al3? ions which are not reduced at early irradiation
stage adsorb on the surface of the newly formed Cu clus-
ters. These Al3? ions are then reduced in situ by solvated
electrons. The result of this successive adsorption and re-
duction process is the formation of CuAl alloy structures.
The FTIR spectroscopy in the region 400–4000 cm-1 is
used to characterize the functional groups and obtain the
nature of compounds in the samples. Figure 5a–c shows
the FTIR spectra of pure PVA, PVA-capped Al-rich (Al3?/
Cu2? = 7/3), and PVA-capped Cu-rich (Al3?/Cu2? = 3/7)
nanoparticles at 120 kGy. The pure PVA spectrum are
Fig. 1 Schematic illustration of Al–Cu nanoparticles formation in
different Al–Cu mole ratio. In the samples with higher Al content the
final particles are mostly in the form of core–shell and in samples with
higher Cu contents nanoparticles are mostly alloy or oxide forms. The
corresponding TEM image of each structure is indicated at the bottom
of proposed formation mechanism
4350 J Mater Sci (2015) 50:4348–4356
123
Fig. 2 FESEM and corresponding element mapping images for sample with Al3?/Cu2? = 7/3 at different doses, a–d 80 kGy, e–h 100 kGy, and
i–l 120 kGy, EHT = 10 kV and Oxford X-MaxN 50 silicon drift detector (SDD) is used as the EDS detector
Fig. 3 X-ray diffraction patterns of sample with Al3?/Cu2? = 7/3 at
different dose of: a 80, b 100, and c 120 kGy
Fig. 4 X-ray diffraction patterns of Al–Cu nanoparticles in three
different mole ratio of Al3?/Cu2?, a 7/3, b 5/5, and c 3/7 at 120 kGy
J Mater Sci (2015) 50:4348–4356 4351
123
mainly assignable to the hydrogen bound O–H vibration at
3300 cm-1, stretching vibration of C–H or C–H2 at
2920 cm-1, stretching of C=O at 1725 cm-1, bending vi-
bration of CH or CH2 (symmetric) at 1427 cm-1, vibration
stretching mode of CO at 1290–1000 cm-1, and bending
vibration of C–H (out of plane) at 834 and 595 cm-1 [17,
18]. The comparison of FTIR spectra of pure PVA
(Fig. 5a) and gamma-irradiated samples (Fig. 5b, c) indi-
cates that the absorption bands for most of the functional
groups were disappeared or weakened which is mainly a
consequence of the cross-linking of PVA which results in a
decrease of the intermolecular hydrogen bonds.
The FTIR spectra of irradiated samples exhibited a very
broad band between 3000 and 3450 cm-1 corresponds to
the O–H stretching frequency. The stretching C=O band at
1725 cm-1 of pure PVA moved to the lower wavelength in
Fig. 5b and c due to the cross-linking of polymer under
irradiation [18]. The intensity of the peaks associated to the
stretching mode of CO in 1248 and 1080 cm-1 decreased
in PVA-capped particles compared to the pure PVA,
indicating on the formation of intermolecular bonds be-
tween polymer and surface of nanoparticles. Accordingly,
it is expected that PVA coordinate particles at the surface
of particle through C–O bands [19]. This carbonyl group of
PVA plays main role in preventing the nanoparticles ag-
gregation within the matrix. The comparison of peaks as-
sociated to the CO stretching mode indicates that the
suppression of the peak intensity is more significant in Cu-
rich (Fig. 5b) than the Al-rich samples (Fig. 5c). This
might be due to the stronger interaction of PVA with higher
content of Cu atoms at the surface of the alloy structure
nanoparticles.
In further analysis, EPR spectroscopy is used for iden-
tifying the metal ion localization and coordination site.
Figure 6a–e shows the EPR spectra and the respective
simulation results of the Al–Cu nanoparticles at different
precursors’ mole ratios at the fixed irradiation dose
(120 kGy). The asymmetry spectra of Cu-rich samples
(Al3?/Cu2? = 2/8 and 3/7) in Fig. 6a and b indicate the
overlapping of two Cu2? signals (signal a and b). Signal a(g\ = 2.15) is associated with Cu2? ion in CuO clusters
and signal b derived from Cu2? ions, which strongly in-
teract with the aluminium nuclei with nuclear spin 5/2 and
show g\ = 2.10. A blow-up of Fig. 6a shows signal a,
signal b, and a superposition of two signals. The broad line
width of signal a is associated with the dipolar broadening
effects caused by mutual interactions between paramag-
netic Cu2? ions.
This indicates that the corresponding ions are located in
the Cu2?-containing aggregated phase of CuO clusters
[20–22]. The presence of various ratios of Cu2? ions in
CuO and Al–Cu compositions could result in different EPR
spectra. Accordingly, in Fig. 6c, due to the equal ratio of
Al and Cu ions, a complete overlap of the related signals
appears. It should be noted that Cu? ions, which exist in
some compounds such as CuAlO2, are non-magnetic ions
and cannot be recorded by the EPR method [23–25]. On the
other hand, a strong interaction of Cu–Cu due to the for-
mation of the Cu core in the Al3?/Cu2? = 8/2 compound
and a partial formation in the Al3?/Cu2? = 7/3 compound
creates a broad signal ascribed to dipolar coupling from
interaction of neighbouring copper ions when they are too
close [26].
Impact of radiation dose on EPR spectra
The simulation of the EPR spectra with the respective
experimental traces of irradiated Al–Cu samples in differ-
ent gamma doses of 80, 100, 110, and 120 kGy are pre-
sented in Fig. 7a–d.
It can be seen that by increasing the gamma dose the
intensity of the resonance peaks decreases. The difference
in the peak intensity is mainly due to the signal that cor-
responds to the unreduced copper ions. As it is noted, the
reduction process starts by reducing copper ions to lower or
zero-valent atoms. At a low dose (80 kGy), the concen-
tration of the unreduced Cu2? ions is significantly high. By
increasing radiation dose the reduction rate of Cu2? ions
increases which appears as decrease in the intensity of EPR
peaks.
In further steps, the reduction of Al3? increases which
can either leads to the formation of Al–Cu or the alu-
minium can be assaulted by oxygen at the surface of the Cu
which mostly appears in the form of AlCuO2. It is note-
worthy that, by increasing the radiation dose, the reduction
rate of Al3? on the surface of Cu increases. This higher rate
of Al3? reduction increases the interaction of Cu with theFig. 5 FTIR spectra of a Pure PVA, b Al3?/Cu2?= 3/7, and c Al3?/
Cu2?= 7/3
4352 J Mater Sci (2015) 50:4348–4356
123
neighbouring Al nuclei on the surface. This trend clearly
observed in the simulated EPR spectrum. The simulation
data illustrate that all spectra can be interpreted as the
superposition of at least three sub-spectra (Fig. 8). The first
signal (signal A) is attributed to the isolated or nearly
isolated Cu2? ions in axial symmetry with g\ % 2.11,
which can be seen mostly in samples that irradiated with
lower radiation doses. Signal B with g\ approximately
2.15 can be assigned to the dipolar interaction of Cu ions
that are sufficiently close to one another, mostly in the Cu
centers in core–shell nanoparticles [27]. The third signal
(signal C) with g\ approximately 2.10 appears as a result
of interaction of Cu2? ions on the surface with aluminium
nuclei (I = 5/2). With increasing the gamma dose, due to
the reduction of Cu2? ions to the EPR-silent low-valence
Cu? or Cu0, the intensity of signal A decreases and the
EPR spectra seem more asymmetric and broader as a result
of dominating signals B and C. Asymmetry of peaks after
100 kGy indicates that two or more magnetically coupled
copper (II) centers are present in the nanoparticles [27].
Fig. 6 Experimental EPR spectrum along with the respective simulation images of Al–Cu bimetallic nanoparticles in different Al3?/Cu2? mole
ratios of a 2/8 b 3/7 c 5/5 d 7/3 e 8/2. Inferior of Fig. 4a shows signal a, signal b, and the superposition of the two signals
J Mater Sci (2015) 50:4348–4356 4353
123
Simulated signals clearly show the change in the resonance
linewidth when the dose increases. The resonance line-
width of samples on the x-axis (DHpp) is generally deter-
mined by measuring the distance between the highest and
lowest points of the first derivative curve.
The main factor in the modification of these resonance
linewidths is the relaxation time of the excited electron,
which itself relates to the interactions of the spins with their
environment and to their motion. According to Kawabata’s
theory [28], for a Gaussian or approximately Gaussian line
shape DH1/2 = DHpp, the resonance linewidth of very
small metal particles is given by:
DHpp ffi VFðDgÞ2h#e=ddce ð3Þ
where VF is the Fermi velocity of the conduction electrons,
Dg is the bulk metal conduction EPR g-shift from
ge = 2.0023 that can determined from the experimental
results, h#e is the electron Zeeman energy, d is the particle
Fig. 7 Experimental EPR spectrum along with the respective simulation images of Al–Cu nanoparticles for Al3?/Cu2?=7/3 at different radiation
doses of a 80, b 100, c 110, and d 120 kGy
Fig. 8 EPR simulation image of sample with molar ratio of Al3?/Cu2?=7/3 in radiation doses of 80 kGy along with the EPR signals A, B, and
C. The inferior part shows the blow-up of the signals
4354 J Mater Sci (2015) 50:4348–4356
123
diameter, d is the average electronic energy level spacing
of the metal (units of energy), and ce is the electron mag-
netogyric ratio.
By decreasing the size of the metal particles, quantum
size effects can alter the electronic properties. Kubo has
shown that the average electronic energy level spacing of
the metal can be presented as d ¼ 4EF=3N, where EF is the
Fermi energy of the metal and N is the number of atoms in
the particle [29]. In the Kubo equation, the Fermi energy of
the metal is EF ¼ 1=2meV2F and N is the number of atoms
in the metal particle. N is given by N ¼ qNAp6
� �d3=M
where q is density, M is atomic mass, NA is Avogadro’s
number, and p6
� �d3 is the volume of a spherical particle
with diameter d. By substitution of these parameters, we
can simplify the Kubo equation to
d ¼ 4ðmeMV2FÞ=ðqNApd3Þ ð4Þ
Therefore using Eqs. 3 and 4, and values of NA; h#e;me,
and ce, the diameter of a small metal particle can be cal-
culated by
d(nm) ¼ ½0:56� 10�11DHppðGÞVFðms�1ÞMðgmol�1Þ=ðDgÞ2qðgcm�3Þ�1=2 ð5Þ
where DHpp is determined by simulation. In Table 1, the
resonance linewidth of copper core and calculated core
diameter and shell thickness in different radiation dose are
presented.
As the dose is increased from 80 to 120 kGy, the reso-
nance linewidth of the Cu core decrease. This linewidth of
the Gaussian shape directly reflects the diameter of the Cu
core, which depends on the difference in the nucleation rate
and growth processes of Cu centre. At a low radiation dose
(80 kGy), the reduction rate of Cu ions was slow and only
few nuclei were formed. Accordingly, the unreduced Cu
ions were used mainly to collide with the newly formed Cu
nuclei instead of forming new nuclei and therefore led to
form of larger Cu core (1.11 nm) in the presence of large
amount of unreduced Al ions. In the next step, the Al ions
adsorb on the surface of the Cu core and form the particle’s
shell. At higher doses, the enhanced reduction rate favours
the generation of much more nuclei. When the number of
Cu nuclei increases faster than amount of unreduced ions,
smaller core (0.83 nm) and thinner shell (2.92 nm) are
obtained.
Conclusion
We have investigated the impact of the precursors’ mole
ratio and gamma radiation dose effect on the formation
mechanism and structural phase transformation of Al–Cu
nanoparticles using TEM, FESEM/EDS, XRD, FTIR, and
(CW)-EPR. By EPR spectroscopy and simulation of the
respective spectra, we are able to individually distinguish
the effect of each compound on the structural formation of
Al–Cu nanoparticles. We have found that in samples with
higher contents of Al, core–shell structure formation is the
most probable result and the unpaired electron in Cu2? is
the most effective factor in modifying the EPR output
characteristics. In the EPR spectrum of core–shell struc-
tures, three different signals play the main roles in the
spectrum formation: the first signal comes from isolated
Cu2? ions while the other two signals belong to the reduced
Cu in the core structure and the Cu in interaction with Al
nuclei in the shell structure. The calculation of the Cu core
dimension according to its corresponding signal in the EPR
spectrum reveals that by increasing the radiation dose from
80 to 120 kGy the core diameter varies from 1.11 nm to
0.83 nm, which are 11 and 22 % of total nanoparticles
diameter, respectively.
Acknowledgements The authors gratefully acknowledge that this
work was financially supported by the High Institution Centre of
Excellence (HiCoE) research fund (AKU95) from the Ministry of
Education, Malaysia. We also would like to thank to the centre of
research and instrumentation management (CRIM) Universiti Ke-
bangsaan Malaysia for provision of laboratory facilities.
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Irradiation
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