Reduction of metallic ions by molten gallium under ultrasonic irradiation and interactions between the formed metals and the gallium

Article abstract of DOI:10.1016/j.jallcom.2015.02.171

Metallic gallium can reduce ions of silver, copper or gold in a slow spontaneous reaction to form the free metals. However, when the reduction is performed with molten gallium under ultrasonic irradiation, the gallium is dispersed into micrometric spheres and the reduction rate is enhanced dramatically. This is due to the large surface area of unoxidized gallium that is formed, on which the heterogeneous reduction occurs. Each of these metals formed also a certain amount of an intermetallic compound with the gallium: Ag₂Ga, CuGa₂ and AuGa₂. Zinc has a more negative reduction potential than gallium and therefore no reduction of zinc ions was expected. Nevertheless we conducted an experiment using a solution of zinc ions to check whether the extremely high temperature that develops near the surface of the particles during cavitation can overcome the energetic barrier for such reduction.

Full text of DOI:10.1016/j.jallcom.2015.02.171

Journal of Alloys and Compounds 637 (2015) 538–544
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Reduction of metallic ions by molten gallium under ultrasonic
irradiation and interactions between the formed metals and the gallium
Vijay Bhooshan Kumar , Ilana Perelshtein , Giora Kimmel , Ze’ev Porat b,c, Aharon Gedanken a,d,
a
a
b
⇑
a
Bar-Ilan Institute for Nanotechnology and Advanced Materials, Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel
Institute of Applied Research, Ben-Gurion University of the Negev, Be’er Sheva 84105, Israel
Division of Chemistry, Nuclear Research Center-Negev, Be’er Sheva 84190, Israel
National Cheng Kung University, Department of Materials Science & Engineering, Tainan 70101, Taiwan
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Metallic gallium can reduce ions of silver, copper or gold in a slow spontaneous reaction to form the free
metals. However, when the reduction is performed with molten gallium under ultrasonic irradiation, the
gallium is dispersed into micrometric spheres and the reduction rate is enhanced dramatically. This is
due to the large surface area of unoxidized gallium that is formed, on which the heterogeneous reduction
occurs. Each of these metals formed also a certain amount of an intermetallic compound with the gal-
Received 12 January 2015
Received in revised form 22 February 2015
Accepted 23 February 2015
Available online 13 March 2015
2 2 2
lium: Ag Ga, CuGa and AuGa . Zinc has a more negative reduction potential than gallium and therefore
Keywords:
no reduction of zinc ions was expected. Nevertheless we conducted an experiment using a solution of
zinc ions to check whether the extremely high temperature that develops near the surface of the particles
during cavitation can overcome the energetic barrier for such reduction.
Ultrasonic cavitation
Nanoparticles
Gallium
Intermetallic compounds
Reduction
Ó 2015 Elsevier B.V. All rights reserved.
1
. Introduction
Sonication of molten metals is used mainly to obtain alloys with
into particles, forming a suspension in the oil. After cooling to
ambient temperature, the solid particles were separated by cen-
trifugation, rinsed with hexane and dried in a glove box under N
2
improved properties, as well as achieving homogeneous dispersion
of additives or grain refinement. For example, Yang et al. [1] per-
formed ultrasonic dispersion of nano-sized SiC particles in molten
aluminum alloy to form lightweight metal matrix nano-compos-
ites (MMMC). Eskin et al. [2] treated molten and solidifying metals
by sonication to disperse intermetallic particles in Al ingots. Grain
refinement in various alloys by ultrasonic treatment was reported
recently by several groups [3–5] A different use of ultrasonic
energy in connection with molten metals was reported by Han
et al. [6] They irradiated molten indium and molten Field’s metal
atmosphere. This procedure was tested with several pure metals
(Zn, In, Sn. Pb, Bi, Ga) and two eutectic alloys (Au–Si and Ag–Ge)
and in all cases micron-size spheres were obtained. In the cases
of the gallium and mercury this procedure could be performed in
water, due to their very low melting temperatures. The formations
of gallium spheres in water and organic solvents [9] and in aque-
ous solutions of organic compounds [10] were reported. In the lat-
ter study it was found that organic compounds could be entrapped
within the gallium spheres and then leach out when the spheres
were immersed in pure water.
(
Bi–In–Sn alloy) in silicone oil containing a surfactant to form
Ultrasonic irradiation caused dispersion of the molten metals
into particles, but different interactions were found to occur
between the various pairs of metals: Bi–Sn and Bi–Zn formed metal
matrix composites, where the spheres were composed of the two
metals, each bearing its crystalline structure. Bi–In formed the
stable nanoparticles of these metals.
Recently we reported on the reactions and interactions between
some low melting-point metals in the molten state under ultra-
sonic irradiation [7,8]. The procedure included melting a granule
of the metal in a quartz test tube containing silicone oil at ca.
2
intermetallic compounds BiIn and BiIn , where the quantitative
4
00 °C, thus forming two separate liquids. Dipping an ultrasonic
ratio between them was a function of the initial ratio between
the metals. The Bi–Ga pair formed an alloy in which the Bi was
crystalline and the gallium was amorphous.
horn in the oil and irradiation for 2–3 min dispersed the metals
Performing our procedure with metals of higher melting points
>400 °C) is a challenge, because of the limitation in heating liquids
above these temperatures. However, it is known that in the
nanometric scale the physical properties of materials change
⇑
Corresponding author at: Bar-Ilan Institute for Nanotechnology and Advanced
Materials, Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel.
Tel.: +972 3 5318315; fax: +972 3 7384053.
(
E-mail address: gedanken@mail.biu.ac.il (A. Gedanken).
http://dx.doi.org/10.1016/j.jallcom.2015.02.171
0
925-8388/Ó 2015 Elsevier B.V. All rights reserved.

V.B. Kumar et al. / Journal of Alloys and Compounds 637 (2015) 538–544
2.3. Analysis
539
dramatically, including a prominent decrease in their melting
points [11]. Thus, starting from nanometric particles of such metals
could perhaps enable their melting together with other metals
such as gallium or bismuth. A possible way of doing that was to
form nanoparticles of metals having high reduction potentials
X-ray diffraction (XRD) measurements were performed with a Bruker D8
Advance X-ray diffractometer using Cu K radiation operating at 40 kV/30 mA with
a
a 0.02 step size and a 1 s step. High resolution scanning electron microscope
(HRSEM) images were obtained with a FEI Megallon 400L microscope, operated
at 20 kV. Samples were applied on a sample holder covered with carbon tape,
and dried at ambient atmosphere. No gold coating of the sample was needed due
to the conductivity of the samples. Elemental analysis and elemental mapping were
performed using HRSEM Energy Dispersive X-ray Spectroscopy (EDS). The
Transmission Electron Microscope (TEM) was a Tecnai G2, FEI High Contrast/Cryo
TEM, Oregon USA, equipped with a Bottom CCD camera 1 K ꢁ 1 K. Samples for
TEM were prepared by making a suspension of the particles in isopropanol, using
an ultrasonic cleaning bath. Two small droplets were applied on a C-coated TEM
copper grid and dried in a covered Petri dish. For the Cu–Ga system a Ni grid was
used. High Resolution TEM (HRTEM) was done with a JEOL 2100 microscope, at
200 keV. Inductively coupled plasma-optical emission spectroscopy (ICP–OES)
analysis was done with the Horiba instrument model Ultima 2.
(
such as gold and silver) via heterogeneous reduction of their ions
by metallic gallium. If gallium is present in stoichiometric excess,
some of it would be left after the reduction and the two metals
could be melted together and irradiated with ultrasonic energy.
In practice, it was found that the heterogeneous reduction by
metallic gallium was very slow, whereas melting the gallium fol-
lowed by ultrasonic irradiation enhanced dramatically the reduc-
tion rate. This paper describes the results of sonication of molten
gallium in aqueous solutions of four metals. According to the stan-
dard reduction potentials, three of them (Ag, Cu, Au) should be
spontaneously reduced by gallium whereas one (zinc) should not.
The overall standard potentials of these reactions are given here:
2.4. Data processing
GaðsÞ þ 3Agþ ! Ga þ 3AgðsÞ E ¼ 1:360 V
3þ
0
ð1Þ
ð2Þ
ð3Þ
ð4Þ
Rietveld’s method [12–14] was applied, using the public domain programs
FullProf [15]. The FullProf performs refinement of the crystal data (atomic positions,
cell parameters and phase amounts).
ðaqÞ
ðaqÞ
2
GaðsÞ þ 3Cu þ ! 2Ga þ 3CuðsÞ E ¼ 0:902 V
2
ðaqÞ
3þ
0
ðaqÞ
3
ðaqÞ
GaðsÞ þ Au þ ! Ga
3þ
0
3. Results
þ AuðsÞ E ¼ 2:252 V
ðaqÞ
3.1. Silver–gallium
2
GaðsÞ þ 3Zn þ ! Ga þ 3ZnðsÞ E ¼ ꢀ0:202 V
2
ðaqÞ
3þ
0
ðaqÞ
The spontaneous reduction of silver ions by gallium was exam-
ined first without stirring or applying ultrasonic energy, at room
temperature. A granule of gallium (0.5 g, 7.2 mmol) was immersed
The products of the various reactions were analyzed by X-ray
diffraction and by electron-microscopy techniques and are
described herein.
3
in 14 mL of 1 M AgNO solution (14 mmol) and after several hours
formation of some silver coating was observed. A SEM image of a
sample that was immersed for 72 h is shown in Fig. 1, in two mag-
nifications. Fine coral-like deposits of silver were formed on the
surface of the solid gallium, and the EDS analysis showed major
signals for silver and gallium.
2
. Experimental
2.1. Chemicals
Gallium (99.99%), silver nitrate (99.99%), copper acetate monohydrate (99.98%),
copper sulfate (99.93%), gold (III) chloride trihydrate (99.95%), nickel sulfate
99.98%), nickel nitrate (99.98%) and zinc acetate (99.8%) were purchased from
Sigma–Aldrich. Solutions of all the salts were prepared with double distilled water.
Experiments with molten gallium under ultrasonic irradiation
(
were conducted in 0.1–1.0 M AgNO
range 25–55 °C (Table 1). In each experiment, a granule of gallium
ca. 0.5 g) was inserted into a test tube containing 14 mL of the sil-
ver nitrate aqueous solution, which was heated in a water bath for
0 min (except in one case) for melting the gallium. Ultrasonic
3
solutions, in the temperatures
(
2.2. Experimental setup and procedure
The Ultrasonic transducer (model VCX 750, frequency 20 kHz, and amplitude
0%, 230 V AC, the horn was made of Titanium alloy Ti–6Al–4V horn). The sonicator
4
4
irradiation for 3 min caused dispersion of the molten gallium into
suspensions of small particles, which were separated by
centrifugation, rinsed with pure water and dried in a glove box
under Ar atmosphere. The resulting powders were examined by
SEM, and were found to have a porous microstructure that
seemed to be formed by aggregation of smaller particles (Fig. S1,
see the Supporting information). Essentially there were no
structural differences between the products formed at different
was manufactured by Sonics and Materials Inc., USA. The sonication experiments of
molten gallium in solutions of the various metals were performed in a glass test
tube with a spherical bottom, which was dipped in a water bath that was usually
kept at 55 °C for melting the gallium. The tip of the ultrasonic transducer was sus-
pended in the solution, ca. 2 cm above the gallium. Ultrasonic irradiation was
applied for 3 min, causing dispersion of the gallium and formation of a grey suspen-
sion of particles. These were separated by centrifugation at 6000 rpm for 10 min,
followed by washing with pure water and drying in the anti-chamber of the glove
box under Ar atmosphere.
3
Fig. 1. SEM images of spontaneous deposition of silver from 1 M AgNO on metallic gallium. Inset: EDS elemental analysis of the surface.

5
40
V.B. Kumar et al. / Journal of Alloys and Compounds 637 (2015) 538–544
Table 1
The quantities of the starting materials, the compositions of the products and the assessment factors for the various experiments. The % RBragg values indicate the mean error of
2
the diffraction intensities. The global Chi is a general statistical index (1-excellent, 5-fair).
Sample
Ga
g)
AgNO
(M)
3
T (°C)
Ag
Ag
2
Ga
AgRBragg (%)
Ag
2
GaRBragg (%)
Global Chi2
(
mmol
mmol
wt.%
at.%
wt.%
at.%
Ag1 (27)
Ag2 (30)
Ag3 (31)
Ag4 (32)
Ag5 (33)
0.56
0.56
0.48
0.48
0.51
0.50
0.49
8.0
6.9
6.9
6.9
7.3
7.3
7.0
1.0
1.0
1.0
1.0
0.5
0.25
0.1
14
14
14
14
7
95
98
99
98
88
80
90
98.0
99.2
99.6
99.2
95.0
91.4
96.0
5
2
1
2.0
0.8
0.4
0.8
5.0
8.6
4.0
1.2
1.0
0.5
1.3
3.1
8.4
2.7
22.1
43.7
33.5
43.6
20.2
35.8
40.5
2.72
2.54
2.7
3.2
2.2
55
25
35
45
55
55
2
12
20
10
a
Ag6 (34)
3.5
1.4
5.8
3.0
a
Ag7 (35)
a
The quantitative ratios are for the crystalline phases only.
3
Fig. 2. Elemental mapping of a sample of the silver–gallium product obtained in a 1.0 M solution of AgNO . (A) Normal image. (B) Gallium surface distribution. (C) Silver
surface distribution.
Ag concentrations. EDS analysis showed that the major component
was silver but gallium was also present as a minor component. The
prominent presence of silver means that in these cases the reduc-
tion of silver ions was very fast, unlike in the previous case where
no irradiation was applied. This can be explained by the converting
the bulk of molten gallium into particles, thus exposing a new sur-
face area free of the oxide passivation layer. Moreover, the particles
had an enormous surface area, which also enhanced the rate of the
heterogeneous reduction of the silver ions. Mapping the elemental
distribution on the surface of the product was done using high res-
olution SEM. An example is presented in Fig. 2, showing a comple-
mentary combination of gallium and silver. Some sites which are
silver depleted are gallium rich and vice versa. On other sites, nearly
even distribution of the elements was observed.
Rietveld’s refinement obtained for the samples formed in 0.5 M
AgNO (Ag5) (Fig. S2) shows excellent matching between the
observed data to the calculate spectrum and to the database for
the major (Ag) and the minor (Ag Ga) phases. It provides that
the cell parameter for the crystalline silver was 0.4089(2) nm, indi-
cating that the gallium has not been dissolved in the silver.
Shaking of the solid products, such as that shown in Fig. S1,
caused disintegration of the agglomerates into fine particles, which
were examined by TEM. Fig. 4 presents images of the products that
3
2
3
were formed in 1.0 and 0.1 M solutions of AgNO . In both cases a
variety of sizes could be observed, ranging between 3 and 50 nm.
EDS analysis (Fig. S3A) of these two samples showed the presence
of both gallium and silver. Selected-area diffraction (Fig. S3B)
yielded mixed patterns of spots and rings; the spots indicating
the crystalline structure of these particles and the rings arise from
the large number of particles, forming a polycrystalline diffraction
pattern.
X-ray diffraction was employed to study the crystallinity and
the composition of the various products. The diffractograms for
the products that were formed in the solution of 1 M AgNO
various temperatures (25–55 °C) showed pure crystalline silver
with traces of Ag Ga peaks (Fig. 3). No effect of the temperature
3
at
3
A unique phenomenon was observed in the 0.1 M AgNO solu-
2
tion: After separation of the solid product the supernatant
appeared to be milky-white rather than clear and colorless as in
the other cases. A droplet was applied on a carbon-coated TEM grid
and dried in a vacuum chamber at room temperature. The TEM
images (Fig. 5) showed uniform longitudinal particles, ca. 350 nm
long and 90 nm wide. Electron diffraction (ED) revealed that they
were crystalline, and the measured values of the d-spacing
matched with the database for GaO(OH). This compound was
formed by the reaction of the molten gallium with dissolved oxy-
gen and with OH radicals [9,18] that are formed by some
decomposition of the water during sonication [19]. In the super-
natants of the higher-concentrations solutions only few particles
were observed. Examination of these particles by HRTEM sug-
gested that they were hollow or very thin, as the full shapes of
the particles which lie underneath them can be seen through
(Fig. 5. At a high magnification the crystal lattice can be observed.
EDS analysis showed the presence of gallium and oxygen in the
sample.
was observed; even the experiment that was performed without
heating yielded a similar diffractogram, indicating that the heat
developed by the cavitation process was sufficient for melting
the gallium. The product that was formed in the 0.25 M and
0
.5 M solution included a mixture of pure silver with a minor
hexagonal phase of Ag Ga [16], (marked). The combination of such
2
two solid phases is considered as a heterogeneous alloy. The vari-
ous phases of the Ag–Ga systems were analyzed by Baron [17]. In
the solution of lower AgNO concentration (0.10 M) the products
3
included also amorphous gallium, as indicated by the broad signal
between 2h values of 32–40°. Due to the presence of liquid gallium,
the quantitative phase-analysis data for these samples are less
accurate than for all other samples. In these two cases the gallium
was in nominal molar excess with respect to the silver ions
(Table 1), so some of the unreacted gallium could be included in
the alloy, although according to the stoichiometric ratio (Eq. (1))
gallium was in excess in all cases. An Example of a calculated

V.B. Kumar et al. / Journal of Alloys and Compounds 637 (2015) 538–544
541
the formation of Cu–Ga
of CuSO
Elemental analysis of the product obtained in experiment Cu1
was done OES–ICP. A sample was dissolved in 1 M HNO on a
2
intermetallic compound in the presence
4
.
3
hot plate at 60 °C. After full dissolution, most of the acid was
evaporated, double-distilled water was added and most of the liq-
uid was evaporated again. This was repeated three times in order
to dilute the acid before the OES–ICP analysis. The molar ratio of
Ga:Cu was found to be 1:1.76, which corresponds to ca. 64 at.%
Cu and ca. 36 at.% Ga. In comparison, the elemental ratio that
was calculated from the XRD product distribution in this sample
(Table 2, row 1) is ca. 79 at.% Cu and 21 at.% Ga. Considering the
variance in the analytical methods, this can be considered as fair
agreement, showing that the major element is Cu. Examination
of the supernatant that was left after the centrifugation was done
by TEM. A droplet was applied on a C-coated Ni grid and dried at
ambient temperature. Bright field images showed aggregates of
particles, and EDS analysis revealed that they were composed of
Cu, Ga and O. ED showed a pattern of several single crystals.
Fig. 3. X-ray diffractograms of the products obtained by ultrasonic irradiation of
molten gallium in solutions of 1.0–0.1 M AgNO at 45–55 °C. At the 1.0 M solution
almost pure silver was obtained. The arrows mark the positions of the small peaks
blue color) for Ag Ga. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
3
(
2
3.3. Gold–gallium
The occurrence of a sonication-induced reaction in the gold–
3
.2. Copper–gallium
gallium system was examined using a similar procedure as in the
previous systems, with two initial proportions of gold to gallium.
In the first case, 0.34 g Ga (4.9 mmol) were immersed in 10 mL of
0.1 M HAuCl4 (1 mmol Au), so the Au:Ga ratio was ca. 1:5. In the
second case, 0.37 g Ga (5.3 mmol) were immersed in 14 mL of
40 mM HAuCl4 (0.56 mol Au), giving an Au:Ga ratio of ca. 1:10.
SEM images of the dried product (Fig. 8) show again porous mate-
rial, which seems to be formed by aggregation of small particles.
XRD analysis revealed that three crystalline components were pre-
Spontaneous reduction of copper ions by metallic gallium was
also found to be slow. A 0.5 g granule of gallium was immersed
in 14 mL of 1 M CuSO which were heated to 50 °C. After 24 h no
copper was observed, but after a week some red particles
appeared, separated and identified as copper. Experiments with
the copper–gallium system under sonication were performed in a
similar manner to the silver–gallium couple, in solutions of 0.1–
4
1
.0 M CuSO
4
3 2
or Cu(CH COO) . The temperature within the test tube
sent: gold, gallium and the intermetallic compound AuGa . Their
2
was measured during the sonication by a thermocouple, showing
only a moderate increase up to 63 °C. A red–brown suspension
was formed upon sonication, and the particles were separated by
centrifugation, rinsed twice with pure water and dried in a vacuum
chamber. No crystallites of GaO(OH) were found in the super-
natant. The experimental conditions of the five experiments
together with the product distributions, according to XRD analysis,
are summarized in Table 2.
Examples of SEM images of the products that were formed in
copper sulfate (Fig. 6A) and copper acetate (Fig. 6B) show porous
materials that seem to be composed of aggregates of smaller par-
ticles. The product in Fig. 7B seems to contain also some unreacted
gallium, which has not been dispersed into particles. EDS spectra of
both samples show two major signals for Ga and Cu. Fig. 7A shows
relative quantities depended on the initial ratio between gold
and gallium, as determined by the Rietveld analysis of the XRD
data (Fig. 9). AuGa2 is one of three known gold–gallium com-
pounds [15,20].
After precipitation of the particles, the supernatant appeared to
be ruby red, indicating the presence of gold nanoparticles. A dro-
plet was applied on a C-coated TEM grid and after evaporation of
the water at ambient temperature it was examined by HRTEM.
Two micrographs are presented in Fig. 10, showing spherical parti-
cles in the size-range of 5–20 nm. On Fig. 10A a halo can be
observed around each particle, which may be interpreted as a
core–shell structure, where the core is crystalline gold and the
shell is amorphous gallium. This interpretation is supported by
the selected area electron diffraction, showing a pattern of single
Fig. 4. TEM images of the Ag and Ag
B) 0.1 M AgNO
2 3
Ga nanoparticles that were obtained from disintegration of the initially-separated agglomerates that were formed in: (A) 1.0 M AgNO .
(
3
.

5
42
V.B. Kumar et al. / Journal of Alloys and Compounds 637 (2015) 538–544
3
Fig. 5. (A) TEM images of the particles that were found in the Ga/0.1 M AgNO solution after sonication and separation of the solid product. Inset: ED pattern of three single
crystals. (B) HRTEM images of the particles that were present in the Ga–0.1 M AgNO solution after sonication. Inset: The crystal lattice of a single particle of GaOOH.
3
Table 2
The conditions and the products distribution in the various experiments with the Cu–Ga system. (oC8) and (cI12) is a different phase of Ga.
Sample
Conditions
Reactants
mmol)
Products (according to XRD)
Zero shift O
2
(
Cu
CuGa
2
Ga (oC8)
wt.%
Ga (cI12)
wt.%
Ga
Cu2+
wt.%
at.%
wt.%
at.%
31
at.%
at.%
2.7
Cu1
Cu2
Cu3
Cu4
Cu5
15 ml 0.25 M CuSO
4
3 min sonication
No son., 50 °C, 30 days
8.5
7.2
8.0
7.1
6.7
3.75
3.75
3.75
7.5
41
97
35
92
41
69
97.3
37
97.3
48.8
59
–
–
8
18
–
–
–
3
–
–
–
ꢀ0.11
ꢀ0.3
15 mL 0.25 M CuSO
15 mL 0.25 M Cu(CH
15 ml 0.50 M CuSO
14 mL 0.10 M CuSO
4
3
CO
)
2 2
3 min sonication
65
Trace
41
63
ꢀ0.37
ꢀ0. 25
ꢀ0.3
4
No son., 50 °C, 30 days
3 min sonication
2.6
6.7
4
1.4
44.5
4 3 2
Fig. 6. SEM images of the products that were formed in (A) 0.25 M CuSO and (B) 0.25 M Cu(CH COO) . Inset: EDS spectrum of sample (A).
4 3 3
Fig. 7. X-ray diffractograms for the products obtained in (A) CuSO and (B) Cu(CH COO) solutions. The red peaks corresponding to unreacted Cu are marked. The rest of the
peaks correspond to CuGa . (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2

V.B. Kumar et al. / Journal of Alloys and Compounds 637 (2015) 538–544
543
4
Fig. 8. SEM images of the product obtained by sonication of molten gallium in a 40 mM HAuCl .
crystals (Fig. S4A) and by EDS analysis, showing the presence of
gold together with some gallium. The crystalline nature of the gold
nanoparticles can be seen in Fig. S4B (see the Supporting informa-
tion), showing twinning and the crystal lattice of a single particle.
In addition the ruby red of AuGa system was analyzed by UV–
Visible spectrophotometry. The UV–Visible spectrum (Fig. S5)
explained the ruby red solution contains the gold particles of
Plasmon peak and large shifting (ꢂ100 nm) is due to covering with
the metallic gallium.
3.4. Zinc–Gallium
The standard reduction potential of zinc is more negative by
02 mV than that of gallium and therefore zinc ions cannot be
2
reduced by metallic gallium, as in the cases above. The following
experiment was aimed at exploring whether it is also so under
ultrasonic irradiation, where the cavitation process creates
extreme conditions of pressure and temperature. In other words,
could the external energy applied in the form of acoustic waves
and heat act as external voltage for this couple.
The experiment was performed with molten gallium in a solu-
3 2
tion of Zn(CH COO) , by the same procedure as in the cases above.
After sonication the products were separated, rinsed and dried, and
analyzed by XRD. The diffractogram (Fig. S6) shows that the major
phase is crystalline gallium. Some minor signals were identified as
2 4
traces of the compound Ga O Zn, in which zinc exists in the +2 oxi-
dation state. No signal for metallic zinc was observed, so it can be
concluded that no reduction of zinc ions by gallium occurred. The
presence of just traces of zinc compound in the solid product
means that most of the zinc has remained as ions in the solution.
Fig. 9. Rietveld analysis of the XRD data of the solid products that were obtained by
sonication of molten gallium in aqueous HAuCl solutions. (A) Au:Ga initial ratio
:5. (B) Au:Ga initial ratio 1:10. The composition of each of the products is
indicated.
3
1
Fig. 10. HRTEM images of gold nanoparticles that were found in the supernatant of the Au–Ga system, after precipitation of the large particles. (A) Particles coated with a
halo. (B) A single particle in which twinning and the crystal lattice can be seen.

5
44
V.B. Kumar et al. / Journal of Alloys and Compounds 637 (2015) 538–544
On the contrary, doing a similar experiment with nickel ions, of
which the standard reduction potential is 303 mV more positive
than gallium, yielded metallic nickel, gallium oxide hydroxide
standard potential for this reaction is ꢀ0.202 V, which in terms
0
of the Gibbs free energy
(
D
G = ꢀnFE ) is equivalent to
117 kJ/mol. Eventually, this energy barrier is too high for such
reduction, in spite of the high temperature that develops at the
potential site for its occurrence.
nanoparticles and traces of a Ni
. Discussions
In this work we studied the combined effect of ultrasonic cav-
2
Ga intermetallic compound.
4
5
. Conclusions
itation and heterogeneous reduction in binary systems that
included molten gallium and ionic solutions of one of several
metals. Three of these metals, silver, copper and gold, have higher
reduction potential than gallium so their ions can be sponta-
neously reduced by metallic gallium. Zinc, however, has a more
negative reduction potential and thus zinc ions cannot be reduced
by metallic gallium. Immersion of a granule of gallium in solutions
of silver or copper ions caused indeed precipitation of these metals
on the surface of the gallium, but this process was very slow.
Melting the gallium, followed by applying ultrasonic irradiation
for 2–3 min induced dispersion of the molten gallium into particles
which reacted instantaneously with the metal ions. Examination of
the compositions of the dried solid products was done by X-ray
diffraction, and it revealed that in all the cases of the reducible
ions, the free metals (Ag, Cu, Au) were obtained together with
(1). Dispersion of gallium into microparticles enhances tremen-
dously the reduction rate of metals ions of higher reduction
potentials.
(2). The ultrasonic-aided reduction by Ga forms also intermetal-
2 2 2
lic compounds Like Ag Ga, CuGa and AuGa .
(3). Even the high temperature that develops during cavitation
cannot overcome the energetic barrier associated with
2
+
0
reduction of Zn by Ga (
D
G = ꢀnFE ).
Acknowledgements
We acknowledge the help of Dr. Y. Koltypin of the Department
of chemistry, Bar-Ilan University, Israel.
some intermetallic compounds with the gallium (Ag
2
Ga, CuGa
2
Appendix A. Supplementary material
2
and AuGa ). In the case of zinc, however, no reduction was
observed.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jallcom.2015.02.
The reduction with the dispersed molten gallium was instanta-
neous, due to the creation of enormous surfaces of unoxidized gal-
lium particles that could readily react with the metal ions. The
formation of some intermetallic compounds between the gallium
and the reduced metals can be attributed to the extreme local con-
ditions of pressure (ca. 500 atm.) and temperatures (ca. 5000 K)
which develop for extremely short times during the cavitation pro-
cess [21]. Thus, the ultrasonic cavitation induced not only fast
reduction of the metal ions but also fast reactions between the
reducing and the formed metals. The extent of these reactions
was found to be a function of the relative molar quantities of the
1
71.
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