Preparation of Au/Pd Bimetallic Nanoparticles by a Microwave-Induced
Plasma in Liquid Process
Hiroaki Shirai,1 Mai Thanh Nguyen,1 David Čempel,1 Hiroki Tsukamoto,1Tomoharu Tokunaga,2 Ying-Chih Liao,1,3 and Tetsu Yonezawa*1
1

Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University,
Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628
2
Department of Quantum Engineering, Graduate School of Engineering, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603
3
Department of Chemical Engineering, Faculty of Engineering, National Taiwan University,
No. 1, Roosevelt St. Sec. 4, Taipei 10617, Taiwan

E-mail: tetsu@eng.hokudai.ac.jp
Received: September 30, 2016; Accepted: November 19, 2016; Web Released: November 25, 2016

Tetsu Yonezawa
Professor Tetsu Yonezawa graduated from the Graduate School of Engineering, The University of Tokyo andreceived his Ph.D. in 1994. After spending 2 years as a post-doc in Tokyo and France, he was appointed as
an assistant professor of the Kunitake Lab., Kyushu University. After staying at Nagoya University and The
University of Tokyo as an associate professor, he has now been working at the Division of Materials Scienceand Engineering, Hokkaido University since 2008. He received the SPSJ Hitachi Chemical Award 2011,
Xingda Lectureship from Peking University in 2015 and some other awards. He was selected as a Fellow of
the Royal Chemical Society (UK) (FRSC) in 2016. He has published 200 original papers and more than 20
books.

Abstract
Au/Pd alloy nanoparticles (NPs) were successfully synthesized using HAuCl4, H2PdCl4, and α-thioglycerol as precursors
and a stabilizer, respectively, by microwave-induced plasma
in liquid process (MWPLP). After only several minutes of
microwave irradiation, metallic NPs were obtained in aqueous environment without any reducing agent. The reduction
mechanism of metal ions was conﬁrmed by UV­vis spectralchanges. The results from HAADF-STEM and EDS indicatedthat synthesized NPs were bimetallic Au/Pd. Moreover, XRD
patterns of synthesized NPs implied that the composition of
bimetallic NPs could be controlled by changing the initial ratioof metal precursors.

1. Introduction
Bimetallic nanoparticles (NPs) with alloy or core-shell structure are currently of intense interest due to their size dependent magnetic, optical, electronic, and catalytic properties.1­6
In particular, they have been an area of interest because they
exhibit catalytic performance superior to monometallic catalysts.6­20 Their high performance is attributed to ensemble andligand eﬀects. The ensemble eﬀect is the result of surface
atomic arrangements and surface strain, while ligand eﬀects are
Bull. Chem. Soc. Jpn. 2017, 90, 279–285 | doi:10.1246/bcsj.20160333

induced by electronic charge transfer due to electronic interactions between two metallic elements. It is widely acceptedthat the activity, stability and selectivity of the catalyst areenhanced by these eﬀects.6,7 However, control of chemicalcomposition of bimetallic NPs is still demanding.9,20­22
Since Pd has high catalytic activity for ethanol electrooxidation, Pd can be applied in catalysts for direct alcohol fuelcells (DAFCs).7,8 DAFCs with the electro-oxidation of liquidorganic molecules as the anode reaction have attracted muchattention because of the higher energy density of liquid fuels
compared to gaseous fuels such as hydrogen. Ethanol as arenewable biofuel is nontoxic and can be produced by biomass
fermentation.7 That is the reason why DAFCs are considered agreen energy source. Pd NPs are also intensively studied as anactive catalyst for hydrogenation. Polyvinylpyrrolidone (PVP)or surfactant-stabilized Pd NPs studied by us have been used
for selective hydrogenation of diene to monoene.6,10
Recently, Au/Pd alloy NPs have been studied in order toimprove catalytic activity and stability of Pd. Catalytic activityof Au/Pd alloy NPs is superior to pure palladium for reductionof 2-nitrophenol,9 CO oxidation,13 cyclohexane oxidation,14
solvent free oxidation of primary alcohols to aldehydes,15­18
and Suzuki­Miyaura coupling reaction.19 Combination of Au
and Pd is interesting since their bulk cohesive energies are very
similar which indicate that both metals are expected to have© 2017 The Chemical Society of Japan | 279

high miscibility at the nanoscale.23 Pd is widely known as ahighly active catalyst as described above, whereas Au is considered an inactive catalyst except under certain conditions.
However, Au in Au/Pd alloys can modify the strength of thesurface adsorption.11 In Au/Pd catalyst, the beneﬁcial eﬀect of
Au is noticeable in the decrease of poisoning in metal catalyst,
which is induced by intermediates diluting the surface ofPd.16­18
The synthesis of Au/Pd NPs has also been described inother reports. Lee et al.21 prepared Au-Pd NPs from aqueous
solutions of HAuCl4/K2PdCl4 mixtures with ascorbic acid asa reducing agent. Zhang et al.24 synthesized Au/Pd alloy NPs
by coreduction of AuCl3 and Pd(OAc)2 with mixed dodecylamine and tetraoctylphosphonium bromide as a stabilizer, andborane-tert-butylamine as a reducing agent. Moreover, Xu
et al.19 obtained formation of nanoﬂowers with Au core andPd petals by reduction of Pd ions in the presence of Au NPs
and PVP. We have also prepared PVP-stabilized Au/Pd NPs
for hydrogenation catalyst and Toshima et al. used EXAFS tounderstand their special core-shell structures.6,20
Many methods for synthesis of metal NPs and nanoclusters
such as chemical reduction,19,21,24,25 plasma in liquid,26­28 laser
aberration,29,30 and spattering4,31,32 have been reported. Amongthem, microwave-induced plasma in liquid process (MWPLP)
is a useful and simple method which does not need any
reducing agents and metal salts to control the conductivity ofthe solution.26,27 In this method, metal ions are reduced byhydrogen radicals and electrons which are generated by decomposition of water molecules in the plasma at ambient pressure.
PLP provides rapid reaction using activated chemicals species
and radicals. From the above, microwave induced plasma inliquid process can produce metal NPs without any reducing
agents in a short time at atmospheric pressure. These are advantages compared to chemical reduction etc.
Preparation of NPs such as Au,26,28 Ag,27 Pt,27,34 Ni,34
ZnO,34,35 CuO,36 and Cu2O36 by PLP have been reported sofar. However, there are only few reports about alloy NPs prepared by PLP. Hu et al.37 reported Au/Pt alloy clusters fabricated by PLP using a metallic rod as a metal source, but
composition of Au and Pt was dependent on the chosen particle.
In general, alloy composition may be controlled by changing theratio of metal sources in the initial solution. In this paper, we
synthesized Au/Pd alloy from metal ions using a home-mademicrowave induced liquid plasma apparatus, and tried to control
their composition.

Figure 1. Schematic illustration of the plasma reactor
used for microwave-induced plasma in liquid process(MWPLP).

2.1. Microwave-Induced Liquid Plasma Apparatus.
Figure 1 shows a schematic illustration of the reactor of the
microwave induced PLP apparatus.26,27,35 2.45 GHz microwave
was emitted from the magnetron (Micro denshi UW-1500)
and passed through a WRJ-2 rectangular waveguide (109.22 ©
54.61 mm), a power meter, a tuner, and a waveguide to a
coaxial adaptor. The coaxial electrode made of tungsten with asharp tip was ﬁxed in the middle of the waveguide and the tipof the electrode is projected inside of the reactor. The microwave energy, which propagates through the coaxial electrodeand a high electric ﬁeld, was concentrated on the tip of thecoaxial electrode. In this way, the tip of the coaxial electrodewas heated by joule heat and surrounding liquid was evaporated. Plasma is generated inside of the bubbles in the liquid.
Metal ions (gold and palladium hydroxide complexes) werereduced to metal by hydrogen radicals and electrons generated
by the decomposition of water molecules in the plasma. The
reactor was made of stainless steel and was coated inside with
PTFE. During reaction, the solution was cooled by refrigerant
which ﬂows in the cooling rod. We observed inside of thereactor via a quartz window on the side wall of the reactor. The
volume of the reactor was 500 cm3.
2.2. Materials.
Tetrachloroauric(III) acid hydrate(HAuCl4¢nH2O n = 3.6 Kojima, Japan), palladium (II) chloride (PdCl2, Kojima), and α-thioglycerol (TCI) were used as
precursors and the stabilizer. Hydrochloric acid (HCl, Junsei,
GR) was used to prepare tetra chloropalladate (II) acid
(H2PdCl4). Sodium hydroxide (NaOH, Junsei, GR) was used
to adjust the pH of the solution. All chemicals were used
as received without further puriﬁcation. Deionized water(Organo/ELGA purelabo system 18.2 M³ cm) was used during
this study.
2.3. Synthesis of Au/Pd Alloy NPs. HAuCl4 and H2PdCl4
were dissolved in 250 cm3 of deionized water respectively andtotal metal ion concentration was adjusted to 2.0 mM. H2PdCl4
was prepared by mixing PdCl2 and HCl. The molar initial of
Au to Pd was adjusted to 0:10, 2:8, 4:6, 6:4, 8:2, and 10:0.
We adjusted the pH of these two solutions to 11 by additionof 50 wt % aqueous NaOH solution. Then, they were stirred
overnight. After that, they were mixed, and 0.433 cm3 of αthioglycerol was added to the solution. We again adjusted thepH of this solution to 11 and stirred for 60 min in order to mix
uniformly. The mixed solution was introduced into the reaction vessel and then Au/Pd alloy, Au or Pd NPs were obtained
during 18 min of plasma irradiation. Plasma occurred at 1300 to1500 W and the output of the microwave was kept at 800 W
during the reaction in order to obtain a continuous plasma
discharge. Plasma reaction was stopped when solution temperature reached 90 °C as the temperature continued to increaseduring plasma reaction even though the solution was cooled.
That is the reason why the reaction time in this study wasestablished at 18 min.
2.4. Characterizations.
The emission spectrum of theplasma was measured by UV­vis spectrometry (Avantes

280 | Bull. Chem. Soc. Jpn. 2017, 90, 279–285 | doi:10.1246/bcsj.20160333© 2017 The Chemical Society of Japan

2. Experimental

AvaSpec-2048). UV­vis extinction spectra of the obtained
colloidal dispersion of metal nanoparticles were collected witha Shimadzu UV-1800. UV­vis extinction spectra were alsomeasured to observe the reduction of gold and palladium ionsby plasma irradiation.
The structures and sizes of Au-Pd bimetallic, Au, and Pd
NPs were observed using transmission electron microscopy(TEM Hitachi H-9500, acceleration voltage of 300 kV, LaB6
ﬁlament). STEM observations were carried out with a JEOLJEM-ARM-200F (acceleration voltage: 200 kV) equipped withan EDX detector. Samples for TEM and STEM were preparedby dropping aliquots of the NP dispersions onto carbon-coatedcopper grids after dialysis with cellulose tubes. Dialysis was
carried out to scavenge alkali ions and tungsten ions form thedispersions because the dispersion was treated with alkaline
solution in order to eliminate tungsten oxide nanoparticles
generated from the tungsten electrodes during the plasma
irradiation.
XRD measurements of the NPs were carried out with a
Rigaku MiniFlexII with Cu Kα radiation operated at a scanning
rate of 5° min¹1. X-ray photoelectron spectroscopy measurements were performed with XPS JEOL JPS-9200 in order toelucidate the electronic state of NPs. Mg Kα source (1253.6eV) was used as X-ray source and the Si 2p peak (99.2 eV) of
silicon wafer was used as correction of charge up. Powder
samples for XRD and XPS were obtained after condensation ofalkaline treated solution, centrifugation and vacuum drying.
3. Results and Discussion
Figure 2a shows the plasma emission spectrum which was
induced when pure Au NPs were synthesized. Hγ (435 nm), Hβ(486 nm), sodium (589 nm), Hα (656 nm), and oxygen radical(777, 845 nm) peaks were identiﬁed. No obvious emission peak
was observed from the tungsten electrode or other materials
constituted plasma source. Unfortunately, we cannot see thepeak of solvated electrons at ca. 720 nm. No organic additives
were added into this reaction solution.
Figure 2b shows UV­vis extinction spectral change withtime of the reaction solution after only 5 min of plasma irradiation. 0 min in Figure 2b is an UV­vis absorption spectrum of
the solution immediately after 5 min of plasma irradiation. All
reaction solutions were alkaline (pH 11) and HAuCl4 solution became colorless under such an alkaline condition. Even
with no plasma irradiation, the extinction increases with time.
Immediately after 5 min plasma reaction, a surface plasmon
resonance (SPR) shoulder peak of Au NPs could be observed ataround 520 nm, this indicates that Au NPs were successfully
synthesized by microwave-induced plasma in liquid process(PLP). The extinction intensity continued to increase with time
without plasma irradiation. After 15 min, the SPR peak became
clear with higher extinction intensity. After that, absorption intensity continued to increase, but the SPR peak becameslightly unclear.
When plasma is generated in water, many reducing andoxidizing species are generated by dissociation, solvation,
dimerization and recombination resulting from the decomposition of water molecules (eq 1).38 Hydrogen radicals, oxygen
radicals, as well as hydroxyl radicals are generated as radicals.
Furthermore, strong light, heat, as well as electrons are ejected
Bull. Chem. Soc. Jpn. 2017, 90, 279–285 | doi:10.1246/bcsj.20160333

Figure 2. (a) Plasma emission spectrum: (b) time change ofUV­vis extinction spectra of the HAuCl4 reaction solution
after 5 min of plasma irradiation. α-TG: α-thioglycerol
solution.

into the system. In these cases, hydrogen radicals and electrons
are direct reduction species of Au3+ to Au(0). The hydrated
electrons (E°(H2O/eaq¹) = ¹2.87 V)39 and hydrogen radicals(E°(H+/H•) = ¹2.31 V)39 are strong reducing species of Au3+.
However, the lifetime of hydrogen radicals should be in theorder of ms. Hydration electrons have a longer lifetime of μs.
These results show hydrogen radicals and electrons induced by
the plasma reduce metal ions during plasma reaction.
H2 O ! OH• þ H•

ð1ÞIn the second step, the radicals and solvated electrons canreact together by dimerization and recombination.40 In Table 1,
the possible reactions of the second step are listed.40 One caneasily ﬁnd that hydrogen molecules (H2) can be produced via
combination of hydrogen radicals. Secondary formed species
such as hydrogen gas molecules can also reduce metal ionsafter plasma.
We have successfully synthesized Au/Pd alloy, Au and Pd
NPs by microwave-induced plasma in liquid process (PLP).
Figure 3a shows TEM images of thus obtained Au, Pd and Au/
Pd alloy NPs. Each initial molar ratio of Au to Pd was adjusted
to (a) 0:10, (b) 2:8, (c) 4:6, (d) 6:4, (e) 8:2, and (f ) 10:0,
respectively. Their average diameters were (a) 3.02 « 0.53 nm,(b) 2.98 « 0.51 nm, (c) 2.30 « 0.61 nm, (d) 2.47 « 0.58 nm, (e)2.68 « 0.45 nm, and (f ) 6.26 « 1.81 nm, respectively. Changeof average diameters of NPs synthesized with various molar
ratios of Au to Pd are indicated in Figure 3b. The amount of αthioglycerol was kept constant and the average diameters of all© 2017 The Chemical Society of Japan | 281

Table 1. Second step reactions (dimerization and recombination) adapted from Ref. 40
Reaction
e¹aq + e¹aq ¼ H2 + 2OH¹
e¹aq + H• ¼ H2 + OH¹
H• + H• ¼ H2
e¹aq + H3O+ ¼ H2O + H•
e¹aq + OH• ¼ OH¹OH• + OH• ¼ H2O2
OH• + H• ¼ H2O
H2O+ + OH¹ ¼ 2H2O

Reaction rate constant
k/1010 mol dm¹3 s¹1
0.54
2.5
1.3
2.3
3.0
0.53
3.2
14.3

e¹aq: Solvated electron (e¹ + nH2O ¼ e¹aq).

NPs except for Au NPs were almost identical. The average
diameter of Au NPs was twice as large as that of other NPs. In
the case of Au/Pd alloy NPs, when both ions were reduced in
the same time under plasma, the sizes of the obtained NPs were
smaller. Shi et al.9 synthesized Au/Pd bimetallic nanodendrates using ascorbic acid as a reducing agent and pluronic as astabilizer. They mentioned that in the case of higher content of
Pd, the nanodendrites became smaller, which indicated that Pd
plays an important role in the formation of nanodendrites while
ﬁnely tuning the molar ratio of Au to Pd. Because of thediﬀerent standard redox potential of Au and Pd ions (AuCl4¹/
Au, 1.002 V, PdCl42¹/Pd, 0.591 V), the variation in the atomic
ratio of Au to Pd inﬂuences the nucleation process, and aﬀects
the size of primary particles.9 Their report showed that the Au
NPs were larger than the NPs synthesized with Pd, which
agrees well with our results.
Figure 4 shows photographs of solutions including NPs.
Pd and Au/Pd NPs dispersed in water and color of the dispersions was brown (Figures 4a­4e), which was due to extinctionof NPs by scattering. However Au NPs precipitated in water(Figure 4f ). These results indicate that NPs-including Pd could
disperse in water, but Au NPs did not. This is probably due tothe diﬀerence in the particle sizes, i.e., the size of Au NPs waslarger than that of Pd or Au/Pd NPs (Figure 3).
Figure 5 shows XRD patterns of the obtained NPs after
ﬁltration. Peak positions of (111) and (200) planes of Au and
Pd are also shown in the ﬁgure. In the case of alloy, in particular solid solution alloy, the XRD peak position of alloy
often appears between the peak positions of the corresponding
pure metals. In other words, the lattice spacing of alloy should
be between the lattice spacings of the corresponding metals.
Pure Pd (Figure 5a) and Au (Figure 5f ) NPs shows the XRD
peaks corresponding to that of the references. In the XRD
patterns (b)­(e), relatively strong peaks corresponding to Au
and Pd NPs were also observed. These peaks are considerably
broad but relatively symmetric and the peak maxima are identical with that of the pure Au (f ) and Pd (a) NPs. Noticeably, ineach pattern of Au/Pd NPs with diﬀerent initial molar ratios(Au:Pd of 2:8, 4:6, 6:4, and 8:2), a weak peak between the(111) peaks of Au and Pd is observed. In addition, the positionof this peak shifts from Au towards Pd with an increase in theinitial molar ratio of Pd used in the synthesis (red dashed line).
These results suggest that besides very small Au and Pd monometallic NPs, our products contained Au/Pd alloy NPs and thecomposition of alloy NPs could be controlled by adjusting themolar ratio of metal sources in the initial solution.
X-ray photoelectron (XP) spectra of the prepared NPs after
condensation, centrifugation and vacuum drying are grouped in
Figure 6. Vertical dotted lines show the peak positions of Au 4f
or Pd 3d of monometallic Au and Pd NPs. Although some XPS
spectra for Pd 3d were not clear, Au 4f peaks and Pd 3d peaks ofthe obtained NPs are shifted to lower binding energy comparedto pure Au NPs (A(f )) and pure Pd NPs (B(a)). These results
strongly indicate that Au/Pd alloy NPs were generated in the

282 | Bull. Chem. Soc. Jpn. 2017, 90, 279–285 | doi:10.1246/bcsj.20160333© 2017 The Chemical Society of Japan

Figure 3. TEM images of Au, Pd and Au/Pd alloy NPs
synthesized using diﬀerent initial molar ratio of Au to Pd(Au:Pd): (a) 0:10, (b) 2:8, (c) 4:6, (d) 6:4, (e) 8:2, and (f )10:0; (g) Average diameter of synthesized NPs. Insets of
Figures 3a­3f show the particle size distributions.

Figure 4. Photographs of synthesized solutions including
NPs. The using initial molar ratios (Au:Pd) are (a) 0:10,(b) 2:8, (c) 4:6, (d) 6:4, (e) 8:2, and (f ) 10:0.

Figure 6. XPS spectra of synthesized NPs: (A) Au 4f,(B) Pd 3d.
Figure 5. XRD patterns of NPs synthesized using diﬀerent
initial molar ratios (Au:Pd): (a) 0:10, (b) 2:8, (c) 4:6,(d) 6:4, (e) 8:2, and (f ) 10:0, respectively. The reference
patterns of Au (JCPDS no. 04-0784) and Pd (JCPDS
no. 05-0681) are shown. The dashed lines marked theposition of (111) peaks for Au and Pd are for visual guide.
The red dashed line is used to connect the (111) peak
position of the Au/Pd alloy.

products. Generally, Pd in Au/Pd alloy is more electronegative
than Au. That is the reason why Pd gains d electrons from Au,
which is supposed to be compensated by the depletion of sp
electron count in Pd.13,14 The negative shift of Au 4f is causedby the net charge ﬂowing into Au, thereby an increase in s andp electron density at Au sites.13,14 The charge compensation
phenomenon occurring in the Au/Pd alloy system resulted in
the negative shifts for both Au 4f and Pd 3d core levels andclearly show that the alloy formation occurred. The XPS signal
from the 3d core level of Pd was weaker than from the 4f corelevel of Au. It is possible that Au was rich at the surface ofalloy samples. Yi et al. have discussed the surface compositionof Au/Pd alloy for various Au/Pd compositions and at various
temperatures.3 They found that an Au rich surface occurred at
low annealing temperature, which was attributed to the lower
surface free energy of Au compared with that of Pd. The dominance of Au at the surface helps reduce the surface energy
Bull. Chem. Soc. Jpn. 2017, 90, 279–285 | doi:10.1246/bcsj.20160333

of alloy. In addition, Xu et al. reported the relative limited
solubility between Au and Pd which was evident from XRD
results.13 We also observed monometallic Au and Pd peaks
beside Au/Pd alloy peaks in XRD patterns (Figure 5) of Au/
Pd samples. Then, in order to analyze at the atomic level,
HAADF-STEM and the EDS were used.
The HAADF-STEM image and EDS elemental maps of NPs,
whose initial molar ratio of Au to Pd was 4 to 6, are shownin Figure 7. These colored elemental mapping images show that
Au atoms (red) and Pd atoms (green) were heterogeneously
distributed over the entire NP structure. Parts Au rich and Pd
rich were observed which indicates that Au and Pd formed
alloy partially changing composition. Although as shown in
Figure 2c, average particle diameter was 2.30 « 0.61 nm, theparticle in Figure 7a looks larger. We are not sure if thisnanostructure in Figure 7a was formed by some pure Au andpure Pd NPs whose size was from 2 to 3 nm or not. In order toensure that synthesized NPs were alloy, synthesized NPs were
carefully investigated by using HAADF-STEM.
Image intensity of the image collected by HAADF-STEM
is directly proportional to thickness, density, and the square of
atomic number. When the same amount of Au (atomic number,79) and Pd (atomic number, 46) are included in the sample,
intensity ratio of Au to Pd should be about 3 to 1. A magniﬁed
HAADF image of NPs, whose initial molar ratio of Au to Pd© 2017 The Chemical Society of Japan | 283

(a)Figure 7. (a) A HAADF-STEM image of synthesized NPs
whose initial molar ratio of Au and Pd was 4 to 6, (b) Au
EDS mapping image, (c) Pd EDS mapping image,(d) overlap image of Au and Pd mapping.

was 4 to 6, are shown in Figure 8a. In this ﬁgure, clear dot
shape contrasts can be observed because of atomic arrangement. These dot shape contrasts are atomic columns whichmean that atoms were lining up toward the depth direction ofthis image. It can be revealed if NPs are alloy or pure metal
by investigating the intensity of these dot shape contrasts.
Figure 8b shows image intensity of the area surrounded by ared square in Figure 8a. Increase of image intensity from left toright was observed. As described above, this image intensity
was inﬂuenced by thickness. The reason why image intensity
increased from the left to the right was that thickness of observed NP was not uniform because its shape is spherical. Even
though the thickness of NP has changed, these results are useful
because great change of image intensity was not observed in
the area of the x-axis from 1.2 to 2.4, which implied thatthickness is similar in this area. In this area, the intensity of thepart marked by a circle was higher than that of the part marked
by triangle. As described above, when intensity of the column
formed by pure Au and that formed by pure Pd were analyzed, the intensity ratio of Au column to Pd column should be
3 to 1, but the intensity ratio of higher to lower in Figure 8b
was 1.3 to 1. Results indicate that these columns were formedby both metals, the columns marked by circle were Au-rich,and the columns marked by triangle were Pd-rich. Thus, it was
concluded that this NP formed the alloy because diﬀerenceof image intensity of columns, which adjoin each other, wasobserved.
Au/Pd alloy NPs were synthesized by microwave induced
plasma in liquid process, but pure Au and pure Pd NPs werealso formed judging from XRD analyses. Shi et al.9 successfully synthesized Au/Pd alloy nanodendrites using ascorbic
acid as a reducing agent and pluronic as a stabilizer after 1 hourof reaction. Moreover, Long et al.14 successfully prepared Au/
Pd alloy NPs using NaBH4 and PVP as a reducing agent and astabilizer, respectively. Their reaction took 6 hours. On theother hand, in our case, some alloy NPs were formed, however
284 | Bull. Chem. Soc. Jpn. 2017, 90, 279–285 | doi:10.1246/bcsj.20160333

Figure 8. (a) Magniﬁed HAADF-STEM image of synthesized NP, whose initial molar ratio of Au to Pd was 4 to 6,(b) a line proﬁle image of the area surrounded by red
square in Figure 8a.

pure Au and pure Pd NPs were also formed. Their reaction
times are longer than that of our case (18 min). It is consideredthat 18 min of plasma irradiation was not enough to form all
alloy NPs.
4. Conclusion
In this paper, Au/Pd alloy, Au, and Pd NPs were successfully prepared by microwave-induced plasma in liquid process(PLP) with changing the initial molar ratios of Au to Pd. It
was revealed that hydrogen radicals induced by plasma reduce
metal ions, by measurement using a spectrometer and a UV­vis
spectrophotometer. Sizes of the synthesized NPs were several
nanometers. As we can see in XRD patterns, it was implied thatcomposition of alloy NPs synthesized by microwave-induced
plasma in liquid process can be controlled by changing initialmolar ratio of precursors. It was conﬁrmed by STEM that Au/
Pd alloy NPs were successfully synthesized although pure Au
and pure Pd NPs were also present.
This work is supported by Hokkaido University. Partial
support from Murata Foundation is highly appreciated. YCL
thanks Hokkaido University for a ﬁnancial support for his stayin Sapporo. A part of this work was conducted at HVEM center© 2017 The Chemical Society of Japan

of Hokkaido University and a HVEM laboratory, IMaSS of
Nagoya University supported by “Nanotechnology Platform”
Program of the Ministry of Education, Culture, Sports, Scienceand Technology (MEXT), Japan.
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