A simple route to water-soluble size-tunable monodispersed Pd nanoparticles from light decomposition of Pd(PPh3)4

Article abstract of DOI:10.1016/j.cplett.2006.07.018

Water-soluble monodispersed palladium nanoparticles of diameters 2.5, 5 and 10 nm controlled by varying the concentration of CTAB (hexadecyltrimethylammonium bromide) micelles have been prepared from light decomposition of Pd(PPh₃)₄. The size and structure of these single crystalline particles have been confirmed using UV-vis spectroscopy, high resolution transmission electron microscope, energy dispersive X-ray analysis and powder X-ray diffraction methods.

Full text of DOI:10.1016/j.cplett.2006.07.018

Chemical Physics Letters 428 (2006) 352–355
www.elsevier.com/locate/cplett
A simple route to water-soluble size-tunable monodispersed
Pd nanoparticles from light decomposition of Pd(PPh3)4
Hua Tan, Tong Zhan, Wai Yip Fan *
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
Received 15 March 2006; in final form 30 May 2006
Available online 14 July 2006
Abstract
Water-soluble monodispersed palladium nanoparticles of diameters 2.5, 5 and 10 nm controlled by varying the concentration of
CTAB (hexadecyltrimethylammonium bromide) micelles have been prepared from light decomposition of Pd(PPh . The size and struc-
3 4
)
ture of these single crystalline particles have been confirmed using UV–visible spectroscopy, high resolution transmission electron micro-
scope, energy dispersive X-ray analysis and powder X-ray diffraction methods.
ꢀ
2006 Elsevier B.V. All rights reserved.
1
. Introduction
prevent uses in biological applications and catalytic uses
in the organic reactions conducted in the pure water phase
[9–11]. Although there are some reports on the preparation
of Pd nanoparticles in water using certain water-soluble
surfactants or polymers, the size distributions of these Pd
nanoparticles have been found to be broad which then lim-
its the understanding of the size effect of nanoparticles on
the various chemical processes under investigation [12,13].
Recently, we have prepared core-shell palladium nano-
particles in which the shell is made up of small palladium
particles interspersed among phosphine ligands [14]. On
the other hand, the core itself is made up of pure palla-
dium atoms. However these particles are water-insoluble
and only limited size-controlled was achievable. In this
work, we have found that by simply adding CTAB surfac-
tants, monodispersed Pd nanoparticles in water could be
conveniently prepared from light decomposition of tetra-
kis(triphenylphosphine) palladium, Pd(PPh3)4 at room
temperature and open atmospheric conditions. Since pal-
ladium is already present in the zero oxidation state, there
is also no need to add any reducing agent. In addition,
the free phosphine produced from the light dissociation
often precursor is insoluble in water, thus minimising
the number of unwanted components in the aqueous
solution.
In recent years, much effort has been directed towards
the preparation of metal nanoparticles with fine size
control which exhibit unique electronic, optical, and cata-
lytic properties due to the quantum size effect [1,2]. These
nanostructured materials are currently under intensive
investigation for applications in optoelectronic devices,
ultrasensitive chemical and biological sensors, and as cata-
lysts in chemical and photochemical reactions [3,4]. Palla-
dium is a very important metal catalyst in the field of
organic and inorganic synthesis for the past three decades
[
5]. Palladium metal with nanometer-scale dimension is of
particular interest due to its catalytic ability, thus the
synthesis of uniform-sized Pd nanoparticles has became a
very important and attractive research area. However, even
though there are many papers on the synthesis of Pd
nanoparticles [6–8], only little work has resulted in the pro-
duction of monodispersed Pd nanoparticles. Phosphine-
stabilized monodispersed Pd nanoparticles have been
synthesized previously but they could only be dispersed
in organic solvents or a mixture of alcohol/water, which
*
Corresponding author. Fax.: +65 67791691.
E-mail address: chmfanwy@nus.edu.sg (W.Y. Fan).
0
009-2614/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.07.018

H. Tan et al. / Chemical Physics Letters 428 (2006) 352–355
353
2
. Experiment
3. Results and discussion
Pd(PPh ) , DMF (HPLC grade), ethanol (AR grade)
Upon light irradiation of Pd(PPh ) in an aqueous solu-
3
4
3 4
and hexadecyltrimethylammonium bromide (CTAB) were
obtained from Sigma–Aldrich and used as received.
Pd(PPh3)4 was added into 10 cm of 0.16, 0.08 and
tion containing CTAB, the initial orange solution gradu-
ally turned dark brown. The UV–vis spectrum (Fig. 1)
3
shows that Pd(PPh ) in toluene has a broad absorption
3 4
0
8
.04 M CTAB aqueous solutions in glass cells. After
h of vigorous stirring, a portion of the palladium pre-
peak centered at 433 nm, while in CTAB micelles, this peak
has been red-shifted slightly to 438 nm. After irradiation,
the peak disappeared and the whole spectrum exhibits a
broad continuous absorption which can be matched to
the spectrum of Pd nanoparticles in previous literature
[13,15–17].
Fig. 2 shows the TEM images of samples taken from
solutions containing 0.16 M, 0.08 M and 0.04 M of CTAB.
These images showed that having different concentrations
of CTAB lead to different sizes of nanoparticles (2.5, 5
and 10 nm). Larger diameter particles tend to be produced
for the more dilute CTAB solution. For all three images,
highly monodispersed nanoparticles were seen. XRD dif-
fraction patterns taken of the samples showed only strong
signals due to face-centred-cubic (fcc) palladium phase
while EDX analysis of the particles revealed only palla-
dium peaks with no phosphine contaminants (see Fig. 3).
A high resolution TEM image (Fig. 2b) acquired for the
2.5 nm nanoparticles shows a lattice spacing of 0.22 nm
corresponding to the (111) plane.
cursor was dissolved in the solution. The mass of this
portion corresponded to the difference between the mass
of the undissolved solid Pd(PPh3)4 separated by filter
paper subtracted away from the initial mass of solid
Pd(PPh ) placed into the CTAB solution. The concen-
trations of three prepared saturated Pd(PPh ) –CTAB
solution are thus 1.2 · 10 mol/L, 9.3 · 10 mol/L and
5
aqueous solutions according to the actual amount of dis-
solved Pd precursor. The three saturated Pd(PPh ) –
CTAB solution in the glass cell were placed directly
under visible light radiation from a 200 W xenon lamp
3
4
3
4
À3
À4
À4
.4 · 10 mol/L in 0.16, 0.08 and 0.04 M CTAB
3
4
(
300–800 nm) for 24 h (see Fig. 1). After irradiation,
3
1
0 cm chloroform was mixed with the three sample solu-
tions in order to remove the dissociated PPh . The
3
resulting solutions were then centrifuged for 30 mins,
the precipitate collected and redispersed in deionized
water.
The transmission electron microscope (TEM) images
and electron diffraction (ED) patterns were acquired using
JEOL-2010(200 keV) and JEOL-3010(300 keV) imaging
microscopes. The atomic elements present in the sample
were recorded by an Energy Dispersive X-ray (EDX)
instrument attached as part of the TEM microscope. The
powder X-ray diffraction (XRD) spectrum was recorded
by SIEMENS D5005 diffractometer. The progress of the
reaction was monitored using a SHIMADZU UV-2550
spectrometer.
We have also conducted a series of experiments to deter-
mine the role of CTAB in controlling the size of the Pd
nanoparticles. When Pd(PPh ) precursor was added in
the water or toluene without CTAB, Pd nanoparticle could
still be obtained but they are of varying size distribution.
Similarly, when surfactants such as PVP were used instead,
the resulting Pd nanoparticles were also not monodi-
spersed. These experiments showed that CTAB is indeed
a key factor in the formation of monodispersed nanoparti-
cles. CTAB is a well-known surfactant which forms spher-
3
4
ical micelles in water with a critical micelle concentration
À4
(
cmc) of 8.0 · 10 M [18,19]. Since the CTAB concentra-
tions used in this work were far above cmc, most or all
the CTAB are in micellar form of certain sizes. Generally,
a lower concentration of CTAB tends to produce larger
spherical micelles.
In this work, we found that the micelle was able to trap
water-insoluble compounds such as the palladium precur-
sor in its cavity with the ammonium end of the surfactant
facing the water solvent molecules while the hydrophobic
alkyl ends are oriented towards the palladium in the micelle
cavity. Pd(PPh ) are then decomposed into Pd and free
3
4
PPh molecules inside the micelles under visible light irradi-
3
ation. Since phosphines were not detected in the EDX spec-
trum, we believe that as the Pd nanoparticle is gradually
forming inside each of the micelle, the free phosphines
are expelled and eventually precipitated out of the aqueous
solution. Alternatively, chloroform can be added to the
CTAB solution to remove the phosphine. The size of the
Pd nanoparticles is governed by the micelle cavity which
in turn is simply dictated by the CTAB concentration in
Fig. 1. UV–visible spectra of (a) Pd(PPh
.08 M CTAB solution before irradiation and (c) Pd nanoparticles formed
after irradiation.
3 4 3 4
) in toluene, (b) Pd(PPh ) in
0

354
H. Tan et al. / Chemical Physics Letters 428 (2006) 352–355
Fig. 2. TEM images of monodispersed palladium nanoparticle of different diameters. (a) 2.5 nm (in 0.16 M CTAB solution), (b) high resolution image of a
single particle of 2.5 nm size with lattice spacing of 0.22 nm (c) 5 nm (in 0.08 M CTAB solution) and (d) 10 nm (in 0.04 M CTAB solution).
Although only three CTAB concentrations have been
used in this work, we believe that in principle, a whole
range of Pd nanoparticle sizes could be conveniently pre-
pared. This would certainly render the study of the effect
of nanoparticle sizes on the various types of reactions catal-
ysed by palladium to be much more systematic.
Acknowledgements
H. Tan and T. Zhan thank the National University of
Singapore for research studentships. The work is funded
under an Agency for Science and Technology Research
(ASTAR) Grant No. 143-000-198-305.
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