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

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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

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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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