Effect of surface modification on de-alloying of Pd70Ag30 nanoparticles

Article abstract of DOI:10.1016/j.jallcom.2006.02.023

Heating of surface modified Pd₇₀Ag₃₀ alloy nanoparticle pellets in air was conducted and the effect of surface modification on selective oxidation of Pd and surface segregation of Ag on the surface was examined. For unmodified nanopa

Full text of DOI:10.1016/j.jallcom.2006.02.023

Journal of Alloys and Compounds 426 (2006) 341–344
Effect of surface modification on de-alloying of Pd Ag nanoparticles
7
0
30
a
a
b
b
a,∗
Kuan-Wen Wang , Shu-Ru Chung , Ling-Yun Jang , Jyh-Fu Lee , Tsong-Pyng Perng
a
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan
b
National Synchrotron Radiation Research Center, Hsinchu, Taiwan
Received 19 November 2005; accepted 8 February 2006
Available online 3 April 2006
Abstract
Heating of surface modified Pd70Ag30 alloy nanoparticle pellets in air was conducted and the effect of surface modification on selective
◦
oxidation of Pd and surface segregation of Ag on the surface was examined. For unmodified nanoparticles at 250 C, free Ag was observed because
of preferential oxidation of Pd. Modification with stearic acid reduced the degree of oxidation and the amount of free Ag. With polyethylene
glycol, the oxidation of Pd was further suppressed. No free Ag was formed on the pellet surface, instead, more segregation of Ag within the alloy
◦
nanoparticles to the surface occurred. The effectiveness of these two surfactants gradually diminished at 350 C.
©
2006 Elsevier B.V. All rights reserved.
Keywords: Pd70Ag30 nanoparticles; Surface modification; Stearic acid; Polyethylene glycol
1
. Introduction
and smaller free Ag particles. The compositional reconstruction
on surface was determined by the competition between selective
oxidation of Pd and segregation of Ag [12]. In this paper, the
change of driving forces for surface segregation and oxidation
in the same Pd70Ag30 nanoparticles by surface modification was
studied. The behaviors of segregation of Ag and oxidation of Pd
in the surface modified alloy were examined and compared with
those of unmodified sample.
Surface modification is a common material processing tech-
nique and is often used in many fields of materials technology
1,2]. For nanomaterials, modification by surfactants may sub-
[
stantially change the surface properties because of the large
surface area [3–5]. From thermodynamic point of view, there
is a tendency for material systems to reach the minimum surface
energy state. If a small amount of surfactant (low surface energy
component) is added to a sample material, it will easily spread
over the surface to lower the surface energy of that material [5,6].
In alloy nanoparticles, de-alloying may take place during
annealing by out-diffusion of one component from the interior
2. Experimental
The alloy nanoparticles Pd70Ag30 were prepared by a chemical precipita-
tion method [13]. Pd(NO3)2 and AgNO3 were mixed at the atomic ratio of
70:30, and then reduced by formaldehyde in a basic environment. Since the
[
7,8]. Such a process may be induced by the change of mobil-
reduction potentials of Pd and Ag are different in the solution, the composi-
tion within the as-prepared Pd70Ag30 alloy particles is inhomogeneous [14,15].
The average size of the particles was 8 nm [13–15]. The alloy particles were
then surface modified with 1 wt% of stearic acid (SA) with a chemical for-
mula CH3(CH2)16COOH (Merck Co.) and polyethylene glycol (PEG) with
a formula H(OCH2CH2)nOH (Showa Chemical Co.) and a molecular weight
near 200 g/mol. The modified samples are named Pd70Ag30-s and Pd70Ag30-p,
respectively.
ity of one of the components in the alloy when the other one is
preferentially oxidized in air [9,10], or one of the components is
attracted to the surface by the atmospheric gas so that significant
diffusion takes place [10,11].
Previously, heating of Pd70Ag30 nanoparticle pellets in air at
◦
2
50–450 C has been studied [12]. It was observed that Pd was
◦
preferentially oxidized at 250–300 C, resulting in formation of
free Ag, in addition to the alloy phase. Heating at higher temper-
atures caused more intensive oxidation of Pd and yielded fewer
The nanoparticles were then pressed as pellets and heated in air at
◦ ◦
50–350 C for 1 h. The heating rate was 10 C/min. The phase structure of the
2
nanoparticles after heating was examined by X-ray diffraction (XRD, Rigaku)
with a Cu K␣ source. The surface morphology of the pellets was examined
by field emission scanning electron microscopy (FESEM, JEOL JSM-6500F).
Electron spectroscopy for chemical analysis (ESCA, Physical Electronics PHI
1600) equipped with a dual (Al/Mg) anode X-ray source and a spherical capaci-
tor analyzer was used to study the surface composition of the alloy nanoparticles.
∗
Corresponding author. Tel.: +886 3 5742634; fax: +886 3 5723857.
E-mail address: tpperng@mx.nthu.edu.tw (T.-P. Perng).
0
925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.02.023

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K.-W. Wang et al. / Journal of Alloys and Compounds 426 (2006) 341–344
X-ray absorption near edge spectrum (XANES) was measured on the BL15B
Tender X-ray beamline at the National Synchrotron Radiation Research Center
discussed previously [12], and is similar to that in the Au–Cu
alloy system where preferential oxidation of Cu results in a de-
alloying phenomenon [10]. Compared to the strong and sharp
(
NSRRC), Taiwan. The L3-edges of Pd and Ag were obtained in a fluorescence
mode to determine the surface-enriched component in the alloy pellets.
◦
Ag peaks in the pattern for unmodified Pd70Ag30 at 250 C,
the peaks for free Ag in Pd70Ag30-s are weaker and the size of
Ag is smaller, implying relatively less oxidation of Pd, less de-
alloying, and less sintering of Ag to form free Ag crystallites in
this sample. On the other hand, much lower PdO is formed in the
Pd70Ag30-p pellet, probably because active surface segregation
of Ag inhibits the oxidation of Pd and prevents de-alloying. No
split of the peaks in the pattern of Pd70Ag30-p suggests that the
alloy nanoparticles remain as solid solution but with a gradi-
ent structure of Pd core/Ag shell. Therefore, the two surfactants
have different effects on surface segregation of Ag and oxidation
of Pd in the alloy nanoparticles.
3
. Results and discussion
Fig. 1 shows the FESEM micrographs of the three pellet sam-
◦
ples after heating at 250 C. As seen in Fig. 1(a), many white
particles on the surface are observed for the unmodified sam-
ple. As reported previously, these white particles are all pure
Ag [12]. For Pd70Ag30-s, fewer and smaller particles of Ag are
present in Fig. 1(b). On the contrary, the surface is smoother and
is basically free of such individual particles for Pd70Ag30-p in
Fig. 1(c).
Fig. 2 displays the XRD patterns of the three nanoparticle
pellets after heating in air. For the unmodified nanoparticles
heated at 250 C, formation of palladium oxide and free Ag is
observed, which results in a peak shift of the alloy phase. From
the peak positions, the composition calculated by Vigard’s law
is approximately Pd:Ag = 44:56 [12]. The sharp peaks of Ag
imply the extraction of Ag from the alloy due to the oxidation
of Pd. Large crystallites of Ag are formed, which is consistent
with the SEM observation, Fig. 1(a). This phenomenon has been
When the unmodified Pd70Ag30 was heated from room tem-
perature to 350 C, the composition has different reconstruction
◦
◦
due to more intensive oxidation of Pd. The oxidation of Pd
becomes more dominant than the surface segregation of Ag. No
or very little free Ag is observed, and because of depletion of Pd
in the alloy particles, the peak positions of the alloy phase are
shifted closer to those of Ag. For the modified samples heated
at this temperature, most of the surfactants have desorbed or
decomposed [15]. Therefore, the difference in XRD patterns
◦
Fig. 1. SEM micrographs of three Pd70Ag30 nanoparticle pellets after heating in air at 250 C: (a) Pd70Ag30, (b) Pd70Ag30-s, and (c) Pd70Ag30-p.

K.-W. Wang et al. / Journal of Alloys and Compounds 426 (2006) 341–344
343
Fig. 3. The Ag (3d3/2 and 3d5/2) and Pd (3d3/2 and 3d5/2) peaks from ESCA
◦
analysis for various Pd70Ag30 samples before and after heating at 250 C. The
surface composition of Ag (excluding the oxygen content) in each sample is
shown on the curve.
◦
adsorbed by Pd. After heating to 250 C, the surface Ag concen-
tration is increased in all samples. For unmodified Pd70Ag30,
the content of Ag (excluding the oxygen content) increases from
2
0.6 to 28.8 at%, and only oxide state of Pd is observed on the
◦
surface, due to oxidation of Pd at 250 C. For Pd70Ag30-s, the
increase of Ag is smaller (26.5 at%), and only Pd in oxide state
is observed as well. For Pd70Ag30-p, however, the surface Ag
has increased to 52.3 at%. In the meantime, both metallic and
oxide states of Pd are present, with the metallic state more domi-
nant. Therefore, surface modification by PEG has enabled more
surface segregation of Ag and indirectly suppressed oxidation
of Pd. Since no free Ag crystallites are observed from the SEM
micrograph, the enrichment of Ag could occur near the surface
within the alloy nanoparticles or yield a continuous layer on the
very top surface of the pellet.
Fig. 2. XRD patterns of three Pd70Ag30 nanoparticle pellets after heating in air
◦
◦
at different temperatures: (a) 250 C and (b) 350 C.
In Fig. 4, the L3 edges of Ag at 3351eV in XANES spectra for
◦
the three samples heated to 250 C are compared. The dramatic
between modified and unmodified nanoparticles has become
limited and smaller, and the peak positions of the alloy phase
are all about the same except that there is relatively less PdO
and more alloy phase in the Pd70Ag30-p sample. It is noted that
the asymmetric peaks for the alloy phase in Pd70Ag30-s may
be composed of two sets of peak, one from sharp free Ag and
the other from broader alloy phase. This suggests that the PEG
treatment is more persistent than the SA one.
jump of the Ag edge for Pd70Ag30-p implies the remarkable
enrichment of Ag on the pellet surface [16,17]. Note that Ag
is easy to partially oxidize on the surface at room tempera-
ture. When it was analyzed by XANES at room temperature
and ambient pressure, some AgxO could be detected in the
spectrum, as seen from the Ag edges in Fig. 4. With moder-
ate pre-sputtering under high vacuum in the ESCA analysis, no
oxide was detected. In conjunction with the surface composi-
tions analyzed by ESCA, the result of XANES tells that the
significant surface enrichment of Ag in Pd70Ag30-p is on both
the particle and pellet surface. According to these chemical com-
position analyses and the XRD patterns and SEM micrographs,
both surface and bulk oxidations of Pd are inhibited by the mod-
ification, especially by PEG.
Fig. 3 shows the surface compositions by ESCA analysis
for various Pd70Ag30 samples, as prepared and after heating
◦
at 250 C. For the as-prepared sample, the surface is Pd-rich
because of the lower reduction potential of Pd in chemical pre-
cipitation [14,15]. The split of Pd peaks indicates that the surface
Pd is partially oxidized at room temperature since oxygen is

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K.-W. Wang et al. / Journal of Alloys and Compounds 426 (2006) 341–344
influence of surface modification is reduced when the surfactants
◦
are gradually desorbed or decomposed at 350 C.
Acknowledgements
We acknowledge the support of this work by the Ministry
of Education of Taiwan under Contract No. A-91-E-FA04-1-4
and National Science Council of Taiwan under Contract No.
NSC-91-2120-E-007-003.
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◦
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