Palladium nanostructures synthesized by radiolysis or by photoreduction

Article abstract of DOI:10.1039/b719517f

Palladium nanostructures formed by nanowires, nanoplates or having a flower-like shape were synthesized by slow reduction via radiolysis or via photoreduction of palladium acetylacetonate Pd(acac)₂ in 2-propanol under CO atmosphere. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b719517f

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Palladium nanostructures synthesized by radiolysis or by photoreductionw
Tanafit Redjala,a Gabriela Apostolecu,a Patricia Beaunier,b Mehran Mostafavi,a
c
d
d
a
´
Arnaud Etcheberry, Denis Uzio, Cecile Thomazeau and Hynd Remita*
Received (in Montpellier, France) 19th December 2007, Accepted 25th February 2008
First published as an Advance Article on the web 15th April 2008
DOI: 10.1039/b719517f
Palladium nanostructures formed by nanowires, nanoplates or having a flower-like shape were
synthesized by slow reduction via radiolysis or via photoreduction of palladium acetylacetonate
Pd(acac)2 in 2-propanol under CO atmosphere.
nanocatalysts Pd@MCM-48 present remarkable selectivity
for the cleavage of benzyl ethers, demonstrating that nano-
structured mesoporous materials exhibit interesting catalytic
activity through cooperative properties.27
1
Introduction
Nanostructured metals are attracting much attention because
of their potential applications in catalysis,1 electronics,2,3 and
optics.4 The synthesis of well-controlled shapes and sizes of
nanoparticles and nanomaterials is often critical for their
performances5 as it is the case in catalysis or electro-catalysis
where the activity strongly depends on the size and shape of
the metal nanoparticles.6–9
Radiolytic reduction is a powerful method to synthesize
metallic nanoparticles and nanostructures.28–31 The specificity
of the radiation-induced reduction of metal ions into atoms
lies in generating radiolytic species of strongly reducing po-
tential. The final structure and size of the nanoparticles result
from the growth process which can be controlled by the nature
of the stabilizers (ligands, surfactants or polymers), the salt
and stabilizer concentrations, and the dose rate. In particular,
we have shown that the dose rate, which fixes the reduction
kinetics, has an effect on the mean size and the structure of the
nanoparticles formed by irradiation.32–34 The kinetic control
of the reduction and growth processes is critically important to
control the nanoparticle size and shape.
Palladium is one of the most efficient metals in catalysis,
especially for the formation of C–C bonds10 in organic reac-
tions such as Suzuki, Heck and Stille coupling11–15 and for the
hydrogenation of polyunsaturated hydrocarbons.16,17 Palla-
dium is also known as an efficient electro-catalyst for ethanol
oxidation for fuel cell applications18,19 and also displays a
remarkable performance in H2 storage and sensing.20
It has been shown that the catalytic activity of platinum and
palladium nanostructures highly depends on the morphology
of the nanoparticles.9,21 Pd nanoparticles of different sizes and
morphologies have been synthesized in the presence of ligands,
surfactants or polymers.22 Hexagonal plates and cubic parti-
cles of Pd have been obtained via RNA-mediated growth
control.23 Xia and co-workers have synthesized Pd triangles
and nanoplates by the polyol process and have shown that
these nanostructures are active substrates for surface-en-
hanced Raman scattering (SERS).24 Rectangular palladium
nanoparticles have been recently obtained via the reduction of
PdII by ascorbic acid in the presence of a surfactant, cetyl-
trimethylammonium bromide and trisodium citrate.25 Arrays
of palladium nanostructures have been synthesized using
mesoporous silica (MCM-48 and SBA-15) as templates by
chemical vapour infiltration,26 and the ball-shaped palladium
Photoreduction of metal salts in solutions or in soft or hard
templates is also an efficient way to synthesize metal nano-
particles or nanostructures.35–38
Herein, we describe the synthesis of 1D, 2D and 3D Pd
nanostructures by radiolysis or by photoreduction in 2-pro-
panol solution under CO atmosphere. New Pd flower-like
nanostructures comprised of thin foils as well as hexagonal
nanoplates and nanowires were obtained.
2
Experimental
Solutions of Pd(acac)2 (purchased from Johnson Matthey) in
pure 2-propanol (concentration range: 1 Â 10À4 to 2 Â 10À3
mol LÀ1) were transferred in small Pyrex flasks closed with a
rubber plastic septum and were carefully degassed and satu-
rated with carbon monoxide (purchased from Air Liquide) at
atmospheric pressure by CO bubbling during 15 min. As CO is
very toxic, CO bubbling was achieved with high caution in a
well-ventilated hood with external evacuation for CO. The
Pd(acac)2 solutions were irradiated quickly after CO bubbling
in order to avoid the direct very slow reduction of PdII by CO
which induces the formation of large black particles after
several days.
a
´
Laboratoire de Chimie Physique, Universite Paris XI, UMR 8000-
CNRS, Baˆt. 349, 91405 Orsay, France. E-mail: hynd.remita@
lcp.u-psud.fr; Fax: (+33) (0)1 69 15 30 55;
Tel: (+33) (0)1 69 15 72 58
´
Laboratoire de Reactivite de Surface, Universite Paris-VI, UMR
7609-CNRS, 4 Place Jussieu, 75252 Paris Cedex 05, France
b
c
´
´
´
Institut Lavoisier de Versailles, Universite Versailles Saint-Quentin-
en-Yvelines, UMR 8180-CNRS, 45 ave des Etats-Unis, 78035
Versailles Cedex, France
´
IFP, Direction Catalyse & Separation, IFP-Lyon, BP no 3, 69390
Vernaison, France
d
The g-irradiation source was a 60Co gamma-facility of 7000
Curies with a dose rate of 3000 Gy hÀ1 at Orsay. Electron
beam irradiations were performed with a 20 kW and 10 MeV
electron accelerator (CARIC-Ionisos Society) delivering trains
w Electronic supplementary information (ESI) available: Fig. S1:
Absorption spectrum evolution with UV irradiation of a solution
containing 10À3 mol LÀ1 Pd(acac)2 in 2-propanol under 1 atm of CO.
See DOI: 10.1039/b719517f
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of 14 ms pulses (10–350 Hz) through a scanning beam
(1–10 Hz) of mean dose rate 2200 Gy sÀ1 (7.9 MGy hÀ1).
For those experiments the solutions were irradiated in glass
vessels with a rubber plastic septum.
absorbance in the infrared. The absorption band obtained at
275 nm is due to free acetylacetone.
The solutions containing PdII(acac)2 are stable under CO
for many hours: there is no evolution of the absorption
spectrum and large black Pd particles (which precipitate)
appear only after a few days. This indicates that, at room
temperature and under our conditions of concentrations, CO
reduces PdII very slowly and the direct reduction by CO is
negligible at the scale of hours.
For photoreduction, Pd(acac)2 solutions were irradiated in
quartz cells under CO atmosphere with an Oriel 300 W Xenon
UV-visible lamp.
UV-visible absorption spectra were recorded with a double
beam Perkin Elmer Lambda 19 spectrophotometer.
Transmission electron microscopy (TEM) observations
were performed with a JEOL 100CXII transmission electron
microscope at an accelerating voltage of 100 kV and for high-
resolution measurements with a JEOL JEM 2010 at 200 kV.
Drops of the irradiated solutions were deposited on carbon-
coated copper grids and dried under N2 flow.
The radiolytic reduction of PdII(acac)2 at 10À3 mol LÀ1 in
2-propanol under CO atmosphere leads to Pd nanostructures.
Fig. 2 shows TEM images of the samples at different doses and
their corresponding selected area electron diffraction (SAED)
pattern. These Pd nanostructures are composed of thin
crumpled foils (50–100 nm large) with a few nanowires as
shown in Fig. 2(c) and (d). At the end of reduction (3 kGy
corresponding to 1 h irradiation), these nanostructures appear
as flower-like structures with a core from which different foils
spread out. A few individual nanoplates are also observed
(Fig. 2(c)). The formation of the plates is observed at low dose.
Different foils seem to originate from the same seed. It is known
that nanoplates exhibit surface plasmon features different from
those of cuboctahedra or twinned nanoparticles. Pd nanodisks
exhibit broad localized surface plasmons with higher sensitivity
of the plasmon to the disk ratio compared to Ag.43 Xia and co-
workers24 reported a broad peak at 530 nm for Pd nanoplates.
The broadness of the peak has been attributed to the dielectric
function of Pd and to the low thickness of the plates.
At lower concentration of PdII complex (10À4 mol LÀ1), as
shown in Fig. 3, more elongated nanostructures with more
thin nanowires are obtained. The diameter of the nanowires is
2 nm and their length lies between a few nm to 100 nm. The
selected area electron diffraction (SAED) from a region of the
TEM image shows a ring-like pattern indicating a polycrystal-
line structure (Fig. 3(a) inset). At higher magnification, high-
resolution (HRTEM) micrographs confirm that the Pd nano-
structures and nanowires are crystalline (Fig. 3(c)). The fast
Fourier transformation (FFT) of this image reveals one main
diffraction ring and two symmetric points (Fig. 3(c) inset). The
reciprocal distances found, d1 = 0.225 nm and d2 = 0.198 nm,
correspond to the (111) and (200) planes lattice of cubic Pd.
XPS analysis shows that the Pd nanostructures (obtained at
10À3 or 10À4 mol LÀ1 PdII) exhibit two reproducible peaks (at
The XPS analysis was performed on In foils. Sample drops
were deposited on the foils and dried under N2 flow. The XPS
analyzer was a Thermo Electron ESCALAB 220i-XL. Either a
non-monochromatic or a monochromatic X-ray Al-Ka line
was used for excitation. The photoelectrons were detected
perpendicularly to the support. A constant analyzer energy
mode was used with pass energy of 20 eV.
3
Results and discussion
Radiolytic synthesis
2-Propanol solutions containing PdII(acac)2 were irradiated
under CO atmosphere. PdII can be reduced by solvated
electrons and 2-propanol radicals (CH3)2–CꢁOH39–41 pro-
duced by the solvent radiolysis and by COÀ formed by the
fast reaction of CO with solvated electrons (k = 2.8 Â 109 L
molÀ1 sÀ1 in ethanol).42 The solutions after irradiation under
CO are dark blue.
With increasing irradiation dose, the peak at 326 nm
associated with the PdII(acac)2 complex decreases in intensity
and a new band appears at 275 nm (Fig. 1). Three quasi-
isosbestic points are found at around 255, 310 and 335 nm.
The surface plasmon spectra of spherical palladium nanopar-
ticles are known to exhibit a resonance band in the UV range
at around 205 nm. In addition to this plasmon band, a broad
absorption in the region 400–1200 nm is observed with highest
335.68 and 340.94 eV), features which are specific to the 3d5/2
/
3d3/2 spin–orbit splitting. The asymmetric shape of each
contribution is always present which agrees with the metallic
character of the Pd nanoparticles and completes the informa-
tion given by the binding energy positions (Fig. 4). However a
small additional Pd 3d3/2/3d5/2 contribution at 336.92 and
342.49 eV must be added to achieve a good simulation. This
high energy contribution can be interpreted as resulting from a
thin film coverage by acetylacetone (which is a versatile
ligand)44 or CO.45 Both electron diffraction and XPS analysis
of dried samples confirm that the nanostructures are com-
posed of completely reduced palladium.46 It should be noted
that these structures are stable in solution under CO over
weeks as proved by microscopy and XPS measurements.
The same solutions, when irradiated with gamma rays under
N2 atmosphere, are brown. The shape of the final absorption
Fig. 1 Absorption spectrum evolution with g-irradiation dose of a
solution containing 10À3 mol LÀ1 Pd(acac)2 in 2-propanol under 1 atm
of CO. Dose rate 3 kGy hÀ1. Optical path = 1 mm.
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Fig. 2 TEM micrographs of Pd nanostructures obtained by g-irradiation (dose rate: 3 kGy hÀ1) of a solution containing 10À3 mol LÀ1 Pd(acac)2
in 2-propanol under CO atmosphere: (a) dose = 0.5 kGy, (b) dose = 1.4 kGy, (c, d) dose = 3.6 kGy. The inset to (c) shows the indexed
corresponding SAED pattern.
spectrum below 350 nm is very similar in the UV region to that
obtained under CO but no absorption is observed above 400
nm. In the presence of N2, 2 nm spherical particles were
obtained (as shown in Fig. 5(a)) indicating the importance of
Fig. 3 TEM micrographs of Pd nanostructures obtained by g-irradiation (dose rate: 3 kGy hÀ1, dose: 360 Gy) (a–c) or by photoreduction (2 h) (d)
of a solution containing 10À4 mol LÀ1 Pd(acac)2 in 2-propanol under CO atmosphere. (a) Inset: the indexed SAED pattern. (c) High-resolution
micrograph of the Pd nanostructures obtained under CO atmosphere and inset the corresponding fast Fourier transformation (FFT).
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Fig. 4 XPS signals of the Pd nanostructures deposited on In foils and
obtained by g-irradiation of 2-propanol solutions of PdII(acac)2 (10À3
mol LÀ1) under CO atmosphere, dose = 3 kGy. The thick black line
corresponds to the Pd3d experimental signal and the thin black line is
the Shirley background. The experimental signal is fitted by two
double (3/2–5/2) components. The low binding energy (LE) compo-
nents (--, blue) are constructed using an asymmetric shape with the
parameters of a metallic Pd standard while the high energy (HE)
components (-Á-, red) are constructed with a symmetrical shape; the
area ratio PdHE/PdLE is 0.08.
CO in the formation of the new Pd nanostructures as
previously described.
Fast radiolytic reduction of PdII (acac)2 in 2-propanol
solution under CO atmosphere performed at much higher
dose rate (accelerated electron source; dose rate: 7.9 MGy
h
À1) leads to spherical particles with a narrow size distribution
(diameter 2 nm). These observations show that the dose rate,
which fixes the number of seeds and the reduction kinetics, has
an influence on the structure growth. As for other face-
centered cubic (fcc) noble metals, the thermodynamically
favourable shapes of Pd nanocrystals are cuboctahedra and
multiply twinned particles. The synthesis of Pd nanoplates or
nanofoils requires the control of the reduction kinetics, parti-
cularly at the seeding stage.47 Xia and co-workers have
reported the formation of Pd nanoplates in a slow reduction
process.24 Silver triangles were also synthesized by slow re-
duction of Ag+ by EDTA on radiolytically induced seeds.48
Other literature results report the opposite process i.e. the
formation of anisotropic nanostructures with a fast reduction
process.49 Here, it is clear that the presence of CO and the use
of a low irradiation dose rate favour the formation of these
anisotropic nanostructures.
Fig. 5 TEM micrographs of Pd nanoparticles obtained by g-radi-
olysis (dose = 3.6 kGy) (a) or by photoreduction (5 h irradiation) (b)
of a solution containing 10À3 mol LÀ1 Pd(acac)2 in 2-propanol under
N2 atmosphere. The inset of (a) shows a magnified image where the
2 nm Pd particles are clearly seen.
(111) planes.50 The borders appear sometimes as a rolling up
of the nanoplates. In this case, the selected area electron
diffraction presents an additional distance consistent with
the (200) plane lattice of Pd (Fig. 6(c) inset). Other shapes
such as triangles or truncated triangles and very few nanowires
are also obtained (Fig. 6(d)). The XPS spectra of these
nanoplates are similar to those of the nanostructures obtained
by g-radiolysis.
The spectrum evolution under UV-visible irradiation is
close to that obtained with g-radiolysis and the final spectrum
shows a broad absorption from the visible to the infrared
(400–1200 nm) (see Fig. S1 in ESIw).
Synthesis by photoreduction
Similar experiments were conducted using slow photoreduc-
tion. TEM observations have shown Pd nanostructures
formed by nanoplates or nanowires depending on the initial
PdII concentration. At 10À3 mol LÀ1 of palladium under CO
atmosphere, the complete photoreduction required 6 h and led
to palladium foils arising from the same core (Fig. 6(a))
or to individual nanoplates with regular mainly hexagonal
shapes (Fig. 6(b) and (c)). These hexagonal platelets
(70–100 nm) are mono-crystalline and the ED pattern
(Fig. 6(c) inset) shows six-fold symmetry of the diffraction
spots indicating that the hexagonal faces are composed by
The concentration of the Pd precursor has also an influence
on the final structure: in the case of very dilute solutions
(10À4 mol LÀ1) irradiated with UV light, more nanowires and
fewer nanoplates are obtained (Fig. 3(d)).
The photoreduction of palladium under N2 atmosphere
leads to large nanoparticles of 8–10 nm as shown in
Fig. 5(b) indicating that the presence of CO is necessary to
obtain these 1D or 2D nanostructures.
Experiments are in progress to understand the growth
process.
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Fig. 6 TEM micrographs (a–c) of Pd nanostructures obtained by photoreduction of PdII in 2-propanol under CO atmosphere: [PdII] = 10À3 mol
L
À1, irradiation time 6 h. The inset to (c) shows the indexed SAED pattern of individual nanoplates. (d) [PdII] = 10À4 mol LÀ1, irradiation time 2 h.
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4
Conclusion
Palladium nanostructures formed by nanowires, nanoplates or
having a flower-like shape were synthesized by slow reduction
either by photoreduction or by radiolysis of palladium acet-
ylacetonate in 2-propanol under CO atmosphere. Upon fast
radiolytic reduction with electron beams 2 nm nanoparticles
were obtained. It is clear that the presence of CO and acac
ligands and the reduction kinetics are key factors on the final
structure. These new types of Pd nanostructures could find
applications in different fields such as catalysis, electro-cata-
lysis and SERS.
Acknowledgements
The IFP is acknowledged for supporting this research. We also
thank the European Commission for the fellowship awarded
to Gabriela Apostolescu in the framework of the Marie Curie
Training Site program (contract No HPT-CT-20000-00023).
The authors thank Jackie Vigneron (Institut Lavoisier de
Versailles) for his assistance for XPS experiments.
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