Synthesis of palladium nanoparticles by sonochemical reduction of palladium(II) nitrate in aqueous solution

Article abstract of DOI:10.1021/jp0535801

The sonochemical synthesis of stable palladium nanoparticles has been achieved by ultrasonic irradiation of palladium(II) nitrate solution. The starting solutions were prepared by the addition of different concentrations of palladium(II) nitrate in ethyle

Full text of DOI:10.1021/jp0535801

J. Phys. Chem. B 2006, 110, 383-387
383
Synthesis of Palladium Nanoparticles by Sonochemical Reduction of Palladium(II) Nitrate
in Aqueous Solution
Abderrafik Nemamcha,† Jean-Luc Rehspringer,*,‡ and Djameledine Khatmi†
Laboratoire d’Analyse Industrielle et Ge´nie des Mate´riaux, De´partement de Chimie Industrielle, Faculte´ des
Sciences et de l’Inge´nierie, UniVersite´ 08 Mai 1945 de Guelma, B.P. 401, Guelma 24000, Alge´rie, and Groupe
des Mate´riaux Inorganiques, Institut de Physique et de Chimie des Mate´riaux de Strasbourg, UMR 7504
CNRS/ULP 23, rue de Loess, 67034 Strasbourg Cedex 2, France
ReceiVed: June 30, 2005; In Final Form: October 28, 2005
The sonochemical synthesis of stable palladium nanoparticles has been achieved by ultrasonic irradiation of
palladium(II) nitrate solution. The starting solutions were prepared by the addition of different concentrations
of palladium(II) nitrate in ethylene glycol and poly(vinylpyrrolidone) (PVP). The resulting mixtures were
irradiated with ultrasonic 50 kHz waves in a glass vessel for 180 min. The UV-visible absorption spectroscopy
and pH measurements revealed that the reduction of Pd(II) to metallic Pd has been successfully achieved and
that the obtained suspensions have a long shelf life. The protective effect of PVP was studied using Fourier
transform infrared (FT-IR) spectroscopy. It has been found that, in the presence of ethylene glycol, the
stabilization of the nanoparticles results from the adsorption of the PVP chain on the palladium particle surface
via the coordination of the PVP carbonyl group to the palladium atoms. The effect of the initial Pd(II)
concentration on the Pd nanoparticle morphology has been investigated by transmission electron microscopy.
It has been shown that the increase of the Pd(II)/PVP molar ratio from 0.13 × 10-3 to 0.53 × 10-3 decreases
the number of palladium nanoparticles with a slight increase in particle size. For the highest Pd(II)/PVP
value, 0.53 × 10-3, the reduction reaction leads to the unexpected smallest nanoparticles in the form of
aggregates.
1. Introduction
generate novel palladium nanoparticles with a much smaller size,
higher surface area, and narrower size distribution than those
Nanoscale materials have received considerable attention
because the particles in the nanometric size range are thought
of as a bridge between molecules and bulk materials. These
nanomaterials often exhibit very interesting chemical, optical,
electronic, and magnetic properties that are unachievable in bulk
materials.1,2 Moreover, ultrafine particles of noble metals have
attracted particular interest because their increased number of
edges, corners, and faces gives them a high surface/volume ratio
and therefore they are useful in various fields of chemistry.3,4
The development of the preparation of uniform palladium
nanoparticles becomes a very important issue in their application
to heterogeneous catalysis and to the water denitrification
processes.5 For this purpose, the catalytic particles have to be
as small as possible with a high accessible surface. Aggregates
have to be vanished. Several synthetic approaches and different
metal precursors have been applied to generate palladium
nanoparticles having different shapes and sizes: chemical
reduction of PdCl2 by NaBH46 and by arc-discharge,7 magnetic
stirring at 80 °C of Pd(OAc)2,8 reduction in supercritical carbon
dioxide of Pd(OAc)2,9 thermally induced reduction of Pd-
(Fod)2,10 and sonochemical reduction of PdCl2.11,12 To prevent
the formation of undesired agglomerates of palladium nano-
particles, the processes have been performed in the presence of
various surfactant molecules.2,3,13-16
prepared by other methods.11,12 However, few studies have been
performed on the preparation and characterization of palladium
nanoparticles produced by reduction of palladium(II) nitrate and
on the effect of protective agents.17,18
In this work, we present a study of the characteristics of the
palladium nanoparticles obtained by sonochemical reduction of
palladium(II) nitrate in aqueous solution with ethylene glycol
and poly(vinylpyrrolidone) as reducing and stabilizer agents,
respectively. The formation, size, and shape of palladium
nanoparticles will be discussed as a function of metal precursor
concentration and of the effect of the surfactant molecules.
2. Experimental Procedure
2.1. Materials. The metal precursor palladium(II) nitrate
(Pd(NO3)2) was purchased from Aldrich with a purity of 99%,
ethylene glycol (EG) from Prolabo was used as a reducing agent
and solvent,13 and poly(vinylpyrrolidone) (PVP) (K-30, average
molecular weight 40 000) from Aldrich was used as a protective
agent. All reagents were used as received without further
purification. Double deionized water was also employed during
this procedure.
2.2. Preparation of the Palladium Nanoparticle Suspen-
sions. The palladium nitrate solution was prepared by dissolving
1.25 mg of Pd(NO3)2 in 30 mL of deionized water. A mixture
of 40 mL of ethylene glycol and 5 × 10-6 mol (0.2 g) of PVP
was mixed under magnetic stirring in a glass vessel for 15 min.
After that, different amounts of Pd(NO3)2 solutions0.5, 1, 1.5,
and 2 mLswere added to the mixture, corresponding to 0.66
The ultrasonic reduction method of palladium salts (chlorides,
acetates) in various media (aqueous, polyols) has been used to
* Corresponding author. E-mail: rehsp@ipcms.u-strasbg.fr. Phone: (33)
388 107 190. Fax: (33) 388 107 247.
† Universite´ 08 Mai 1945 de Guelma.
‡ Institut de Physique et de Chimie des Mate´riaux de Strasbourg.
10.1021/jp0535801 CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/08/2005

384 J. Phys. Chem. B, Vol. 110, No. 1, 2006
Nemamcha et al.
Figure 1. Schematic of the ultrasonic irradiation experiment used in
this study.
Figure 2. pH changes of the sample solutions obtained by different
palladium(II) nitrate contents.
× 10-3 mol of Pd(NO3)2 in sample A, 1.33 × 10-3 mol of
Pd(NO3)2 in sample B, 2 × 10-3 mol of Pd(NO3)2 in sample
C, and 2.66 × 10-3 mol of Pd(NO3)2 in sample D, respectively.
The covered vessel was fixed and irradiated with ultrasonic
waves, necessary for the nitrate reduction,19 for 180 min using
a multiwave ultrasonic generator (50 kHz). The experimental
setup is schematically shown in Figure 1.
the reduction of palladium(II) ions could occur according to
the following reactions:
H2O ultrasonic irradiation8 •OH + •H
(1)
HOCH2CH2OH + •OH (•H) f HOCH2C• HOH + H2O
(H2) (2)
2.3. Characterization of Samples. The UV-visible absorp-
tion spectra of the prepared suspensions were obtained by an
UV-visible spectrophotometer from Hitachi with quartz cu-
vettes over wavelengths from 250 to 750 nm. The acidity of
the irradiated solution was measured by a pH meter (Crison
pH 25). Equivalent amounts of suspension were diluted in a
constant volume of distilled water and then analyzed at room
temperature. The protective action of stabilizer agent was
investigated using Fourier transform infrared (FT-IR) spectros-
copy. For the measurements, a drop of the colloidal suspensions
was deposited on a KBr pellet and analyzed using an ATI
Matson Genesis series FT-IR instrument.
nPd(II) + 2nHOCH2C• HOH f nPd(0) +
2nHOCH2CHO + 2nH+ (3)
The pH measurements on the obtained suspensions have been
carried out to get information about the reduction of Pd(II) ions
to palladium atoms Pd(0).21 The pH changes of the irradiated
solutions in the presence of EG and PVP are presented in Figure
2. The pH values of the samples of palladium solution decrease
remarkably with the metal in precursor concentration. This
variation is presumably attributed to the amount of produced
palladium atoms which increases the H+ ion concentration in
the irradiated solutions, according to eq 3. The pH curve shape
indicates clearly that the acidity of the solution decreases
progressively from low concentration in Pd(II) (sample A) to
high concentration (sample D). This result confirms the
dependence of Pd(II) reduction on the precursor concentration
in the starting solution in the presence of organic radicals.
The palladium nanoparticle formation has been investigated
using UV-visible spectroscopy in the 250-750 nm range.
Figure 3 shows the absorption spectra of palladium colloidal
suspensions after 180 min of ultrasonic irradiation and the
absorption of Pd(NO3)2 solution, used as a reference sample,
for comparison. The absorption bands presented in the reference
sample spectrum are attributed to the absorption of palladium(II)
species in the starting solution.2,8,11,22 The absence of the
absorption peaks above 300 nm in all of the samples shows the
full reduction of the initial Pd(II) ions.8,11 Yonezawa et al.23
have ascribed the absence of absorption bands to the total
reduction of Pd(II). The same assignment was made by Ho et
al.10 during thermally reduced-induced reduction of Pd(fod)2.
In addition, the spectra of the irradiated samples present broad
continuous absorptions in the UV-visible range. These absorp-
tions are typical of those of palladium nanoparticles.3,8,11,12,23,24
Thus, it can be assumed that we have obtained palladium colloid
having a particle size of less than 10 nm.1
The shape and size of the palladium particles were investi-
gated using transmission electron microscopy (TEM, Topcon
002B operating at 200 kV). Samples for TEM were prepared
by dissolving a palladium nanoparticle suspension in ethanol
and placing a drop of the obtained mixture onto a carbon-coated
copper grid, followed by the natural evaporation of the solvent
at room temperature.
3. Results and Discussion
After ultrasonic irradiation of the palladium(II) nitrate
mixtures, the color of the solutions turned from the initial pale
yellow to a dark brown. This observation indicates the reduction
of Pd(II) ions by organic radicals (eqs 1-3) according to the
mechanism proposed by Okitsu et al.11 and adopted by Chen et
al.12 It has been well established that the chemical effects of
ultrasound irradiation are, primarily, attributed to its acoustic
cavitation: formation, growth, and implosive collapse of bubbles
in the irradiated liquid. During the collapse, there is a high
concentration of energy from the conversion of the kinetic
energy of the liquid’s motion into heating of the contents of
the bubble.11,12,18,19 In addition, Rae et al.20 reported that the
presence of shear forces of polymers in aqueous solution may,
also, initiate the sonochemical reaction in the fluid. In this study,

Sonochemical Synthesis of Palladium Nanoparticles
J. Phys. Chem. B, Vol. 110, No. 1, 2006 385
Figure 3. UV-visible spectra of palladium nanoparticle suspensions
prepared with different concentrations of palladium(II) nitrate solution
after 180 min of ultrasonic irradiation: (A) 0.66 × 10-3 mol; (B) 1.33
× 10-3 mol; (C) 2.0 × 10-3 mol; (D) 2.66 × 10-3 mol and Pd(NO3)2
solution.
Figure 5. FT-IR spectra restricted to the PVP carbonyl absorption
region for the sample solutions obtained by different palladium(II)
nitrate concentrations after 180 min of ultrasonic irradiation: (A) 0.66
× 10-3 mol; (B) 1.33 × 10-3 mol; (C) 2.0 × 10-3 mol; (D) 2.66 ×
10-3 mol.
can protect the metal nanoparticles via the carbonyl group.
According to these results, one can suggest that the stabilization
of the formed palladium nanoparticles may result (i) mainly
from the adsorption of the PVP chain on the palladium particle
surface via the coordination of the PVP carbonyl group to the
palladium atoms and (ii) additionally from the steric effect of
the PVP polymer long chain.
In Figure 5, one can observe a slight shift of the PVP carbonyl
peak position toward higher frequencies from 1645 to 1651 and
1658 cm-1, with the increase in Pd(II) ion concentration from
0.66 × 10-3 mol (A) to 1.33 × 10-3 mol (B) and to 2 × 10-3
mol (C), respectively. This behavior can be explained by the
fact that the number of adsorbed PVP molecules on the particle
surface is restricted by the long carbon chain of the polymer.
Therefore, the absorption position of the PVP carbonyl depends
strongly on the particle number in the suspension. In fact, at
low Pd(II) concentration, the interaction between Pd nanopar-
ticles and the PVP molecules is strong. All of the particles are
protected with an optimal number of PVP molecules. However,
when the Pd(II) concentration increases, the total signal of the
CdO (PVP) increases and reaches a high value for sample C.
This means that the total interaction between CdO (PVP) and
Pd nanoparticle surfaces is low and suggests that the average
Pd particle covered surface decreases with an increase of Pd(II)
content. Unexpectedly, for the highest Pd content, sample D
(2.66 × 10-3 mol), the CdO (PVP) absorption shifts slightly
back to a lower frequency (1656 cm-1), as can be expected for
an increase of the interaction between PVP molecules and Pd
nanoparticles.
Figure 4. Photographs of palladium nanoparticle suspensions: (A)
0.66 × 10-3 mol; (B) 1.33 × 10-3 mol; (C) 2.0 × 10-3 mol; (D) 2.66
× 10-3 mol.
The stability of nanoparticle suspensions was studied through
the direct observation method reported by Ca´rdinas-Trivin˜o et
al.24 As shown in Figure 4, it was clearly found that the obtained
palladium nanoparticle suspensions absorb visible light strongly
and thus have a black color. Furthermore, no segregation
between the solvent and the black palladium particles was
observed for more than 15 months.
The stabilization of nanoparticles has been investigated using
FT-IR spectroscopy. The obtained spectra of palladium nano-
particle solutions are shown in Figure 5. In all spectra, the
absorption bands appearing in the 1665-1640 cm-1 range are
assigned to the stretching vibration mode of the poly(vinylpyr-
rolidone) carbonyl in the nanoparticle suspensions. Sze´raz et
al.25 have studied the solvent effect on the adsorption of PVP
molecules on γ-alumina. The shift of the carbonyl group position
toward low frequencies has been assigned first to the interaction
between the EG and the PVP and second to the adsorption of
PVP molecules on the γ-alumina surface. Moreover, during the
preparation of a PVP/sodium montmorillonite nanocomposite,
Koo et al.26 attributed the shift of the CdO (PVP) absorption
toward low frequencies to the interaction between CdO (PVP)
and the silicate surface. In our case, the shift of the PVP carbonyl
position in the EG-PVP-Pd suspension, compared to the Cd
O (PVP) adsorption position at 1679 cm-1 and to the Cd0
(PVP-EG) position at 1669 cm-1, is essentially due to the
interaction of the CdO (PVP) with the palladium nanoparticle
surface. This result is in good agreement with those reported in
earlier studies,22,24,25 where it has been concluded that the PVP
The palladium nanoparticle shape and size in the colloidal
solutions and their distribution have been investigated using
transmission electron microscopy. Figure 6 illustrates the
morphology of the prepared palladium nanoparticles. First, for
all samples, the sonochemical reduction of Pd(II) ions leads to
a sharp distribution of nanoparticles which are clearly separated
and have, in general, a rounded shape. Besides this, the
corresponding electron diffraction pattern is shown in Figure
6E. The values of interplanar distance dhkl calculated from the
diffraction rings are in good agreement with the standard ASTM
values for bulk palladium. This indicates the existence of a face-
centered cubic structure of palladium in the nanoparticles.

386 J. Phys. Chem. B, Vol. 110, No. 1, 2006
Nemamcha et al.
Figure 6. Transmission electron micrographs of the samples prepared from different concentrations of palladium(II) nitrate solution after 180 min
of ultrasonic irradiation: (A) 0.66 × 10-3 mol; (B) 1.33 × 10-3 mol; (C) 2.0 × 10-3 mol; (D) 2.66 × 10-3 mol. (E) Electron diffraction pattern
of the obtained palladium nanoparticles.
The TEM micrographs show that the size of our palladium
nanoparticles is affected by the changes in initial Pd(II) content.
For sample A (0.66 × 10-3 mol), the resulting Pd nanoparticles
are highly dispersed and have a diameter of about 3-4 nm
(Figure 6A). A further increase in initial Pd(II) concentration
leads to the formation of nanoparticles having sharp shapes and
particle diameters of about 4-5 and 5-6 nm for samples B
(1.33 × 10-3 mol) and C (2 × 10-3 mol), respectively (Figure
6B,C). For the highest Pd content, sample D (2.66 × 10-3 mol),
the nanoparticles are always packed in larger light aggregates
with an average diameter close to 2-3 nm but the grains seem
not to be stacked together. At this point, we have to stress that
mean particle size is not only related to the initial Pd(II)
concentration but also seems to be in relation with the relative
amount of PVP to the available metallic surface.
For a better interpretation of the obtained results, the mean
particle number and the global particle surface have been
roughly estimated using the following formulas:
6M[Pd(II)]
Nparticles
)
)
πFd3
6M[Pd(II)]
Fd
Sparticles
where [Pd(II)] is the initial Pd content, d is the mean particle
diameter observed by TEM, M is the Pd molecular weight, and
F is the Pd bulk density.
The number of PVP molecules available for each Pd
nanoparticle (PVPn/Nparticles) and the ratio between the number

Sonochemical Synthesis of Palladium Nanoparticles
J. Phys. Chem. B, Vol. 110, No. 1, 2006 387
TABLE 1: Characteristics of the Palladium Nanoparticle Suspensions
mean measured
diameter (nm)
calculated number
of particles
global particle
PVPn/Sparticles
surface (nm2)
PVPn/Nparticles
(molecules/nm2)
A (0.66 × 10-3 mol)
B (1.33 × 10-3 mol)
C (2.0 × 10-3 mol)
D (2.66 × 10-3 mol)
3.5
4.5
5.5
2.5
0.26 × 1018
0.24 × 1018
0.19 × 1018
2.84 × 1018
9.87 × 1018
15.46 × 1018
17.74 × 1018
55.67 × 1018
11.74
12.38
16.12
1.06
0.31
0.19
0.17
0.05
of PVP molecules and the global metallic surface (PVPn/Sparticles
)
(3) The initial palladium ion concentration affects the number,
the dispersion, and the size of palladium nanoparticles. It has
been shown that the increase of the Pd(II)/PVP molar ratio
decreases the number of palladium nanoparticles and increases
their size from 3 to 6 nm for low Pd(II) content. They are coated
by a shell of protective PVP and are unaggregated. In contrast,
for the highest value of Pd(II)/PVP, the formation of a high
number of particles with a PVPn/Nparticles value of ≈1 occurs
and PVP is unable to protect each particle. This leads to the
formation of aggregates of the smallest nanoparticles (mean size
<3 nm).
have been calculated for each sample. The obtained values for
the nanoparticle suspensions are given in Table 1. It appears
clearly that the increase of initial [Pd(II)] surprisingly decreases
the number of Pd particles for samples A-C. For sample D, a
strong increase is observed instead; the total nanoparticle surface
increases slightly for samples A-C, but for sample D, it
increases dramatically.
According to our results, it is found that, at the lowest Pd(II)/
PVP molar ratio (sample A), the colloid is formed by a high
number of stabilized nanoparticles with small diameters. For
higher Pd(II)/PVP molar ratios (samples B and C), the number
of stabilized nanoparticles decreases and the mean particle
diameter increases slightly but with an increase in global particle
surface. Unexpectedly, for high initial Pd(II)/PVP (sample D),
the obtained suspension is formed by aggregation of smaller
nanoparticles with a strong increase in total particle surface.
The FT-IR analysis shows that the interaction between CdO
(PVP) and the Pd nanoparticles is lower and close to that of
the PVP-EG solution and the calculated PVPn/Nparticles value
of about 1. All of these results suggest that an over PVP
concentration is enough to protect the palladium particle until
a definite number of PVP for each particle surface to be covered.
In other words, if the number of PVP molecules is in sufficient
quantity to overcoat all of the particle surfaces with a PVPn/
Nparticles value in the range 11-16 molecules by particle, each
particle can be stabilized and can grow when new Pd(0) is
produced. However, if the number of palladium nuclei generated
is so high that the PVPn/Nparticles value is less than 11, or when
PVPn/Sparticle is less than 0.16 molecules/nm2, aggregation occurs.
However, the adsorption of ethylene glycol on the particle
surface prevents them from sharing a contact surface and avoids
grain sintering and abnormal grain growth, as reported by Okitsu
et al.11 PVP is therefore expelled on the aggregate surface.
Acknowledgment. The authors thank S. Joulie´ for TEM
observations and Guelma University for its financial support.
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(2) The PVP protects the palladium nanoparticles by the
adsorption of its molecules on the particle surface via the
coordination of the CdO group with the palladium atoms.