Self-organization of spherical, core-shell palladium aggregates by laser-induced and thermal decomposition of [Pd(PPh3)4]

Article abstract of DOI:10.1002/anie.200503408

(Figure Presented) Metal in the middle: Uniform, spherical, core-shell aggregates (100 ± 20 nm; see picture) are obtained by photodecomposition of [Pd(PPh₃)₄] to form the metal core, followed by aggregation of smaller Pd nanocrystals

Full text of DOI:10.1002/anie.200503408

Communications
Nanoparticles
the solution was recorded with a Shimadzu UV-2550 spec-
trometer to check its absorptivity around 532 nm, which is the
pulsed-laser wavelength available in our laboratory
(Figure 1). The broad absorption spectrum (from < 300 to
DOI: 10.1002/anie.200503408
Self-Organization of Spherical, Core–Shell
Palladium Aggregates by Laser-Induced and
Thermal Decomposition of [Pd(PPh3)4]**
Enyi Ye, Hua Tan, Shuping Li, and Wai Yip Fan*
Metallic nanoparticles serve many functions. For example,
palladium and platinum metal nanoparticles stabilized by
surfactants and solvents can act as catalysts for hydrogena-
tion[1,2] and Heck, Suzuki, and Stille cross-coupling reac-
tions.[3–5] Nanoparticles are also potential building blocks for
designing larger organized structures for chemical sensing
applications and magnetic and electronic devices.[6] This is
fundamentally important because these structures display
properties that are distinct from those of the individual
constituent particles.[7] Several methods have been applied to
obtain 3D aggregates of metal nanoparticles,[8–11] one of the
more efficient being the “brick-and-mortar” (“BM”) strategy,
which involves the use of multifunctional organic ligands,
polymers, or biomolecules as the cohesion “mortar”.[12,13]
Naka et al. have also used cubic silsesquioxanes as the
cross-linker for generating spherical palladium aggregates.[14]
Recently we have embarked on a project to assemble
metal nanoparticles by the laser decomposition of transition
metal carbonyls or phosphanes, with particular interest in
understanding the reaction pathways that lead to nanoparticle
formation. We chose the photolysis of organometallic species
in which the metal is already present in the zero oxidation
state, thereby eliminating the need for reducing agents and
possibly high temperatures. Herein a facile method for
assembling palladium nanoparticles into spherical, core–
shell aggregates by visible laser decomposition of commer-
cially available tetrakis(triphenylphosphane)palladium is pre-
sented. In this two-step procedure, palladium nanocores (15–
25 nm) are first generated by laser decomposition and then
smaller palladium nanocrystals (< 5 nm) are formed by the
thermal decomposition of the remaining [Pd(PPh3)4] aggre-
gated around the Pd core.
Figure 1. The absorption spectrum of the precursor [Pd(PPh3)4] (a) and
the as-preparedcore andcore–shell palladium nanoparticles (b).
650 nm) obtained showed that it should be possible to induce
changes in [Pd(PPh3)4] by irradiation with the laser. The
solution was then irradiated with a 532-nm pulsed YAG laser
(Continuum Surelite III-10, 80–100 mJ per 10-ns pulse), with
vigorous stirring, for 30 min. A change of color from orange to
black occurred in around 10 minutes, thereby indicating the
decomposition of the precursor and formation of palladium
nanoparticles. This could also be seen from the changes in the
UV/Vis spectrum (Figure 1). An aliquot of this solution
(labeled as product A) was removed immediately for charac-
terization, while the remainder (labeled as product B) was
kept in the dark and maintained under ambient conditions for
24 h without shaking or stirring. This allowed the precursor to
decompose thermally in the dark and generate more Pd
particles. Both A and B were centrifuged and rinsed with
DMF and ethanol several times to remove the free PPh3. The
remaining black solids were then collected and redispersed in
ethanol for further investigation. The UV/Vis absorption
spectrum of B dispersed in ethanol is essentially the same as
that of A.
Figure 2 shows a scheme for the formation of our core–
shell palladium spheres. The first step requires the laser
photodecomposition of [Pd(PPh3)4] into Pd atoms and free
PPh3. We monitored the progress of the photodecomposition
by 31P NMR spectroscopy and, indeed, the signals of the
precursor (d = 29.8 ppm) decrease in intensity with a con-
comitant increase of the signal for free PPh3 (d = À4.75 ppm).
ATEM image taken of the Pd nanocores (A) with a JEOL
TEM-2010F instrument (field emission, 200 keV) is shown in
Figure 3. A narrow distribution of core sizes, with diameters
ranging from 15 to 25 nm, is achieved upon laser photo-
decomposition of [Pd(PPh3)4]. The selective area electron
diffraction (SAED) pattern obtained during the acquisition of
the TEM image shows the cores to be polycrystalline
palladium corresponding to a face-centered cubic (fcc)
In a typical experiment, [Pd(PPh3)4] (11.5 mg, 0.01 mmol;
99% Sigma-Aldrich) was dissolved in 5 mL of DMF (HPLC
grade, Aldrich) and 5 mL of ethanol (AR grade, Aldrich) to
form an orange solution. The UV/Vis absorption spectrum of
[*] E. Ye, H. Tan, S. Li, Dr. W. Y. Fan
Department of Chemistry
National University of Singapore
3 Science Drive 3, Singapore 117543 (Singapore)
Fax: (+65)6779-1691
E-mail: chmfanwy@nus.edu.sg
[**] We thank Binghai Liu for the TEM imaging andthe electron
diffraction data analysis, and Shuhua Wang for the XRD measure-
ments. This work was carriedout under an ASTAR nanoscience
grant (143-000-198-305).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Figure 2. Formation of the core–shell palladium nanostructures.
NP=nanoparticle.
Figure 4. a) TEM image of the core–shell structure; b) andc) high-
magnification TEM images showing the core–shell structure clearly;
d) an HRTEM image of the shell showing that it is composed of
palladium nanoparticles.
formation of the Pd nanoparticles by revealing the character-
istic diffraction peaks of the palladium fcc phase (Figure 5).[15]
Figure 3. TEM image showing the cores formedduring the laser-
induced decomposition of [Pd(PPh3)4]; the insert shows the SAED
pattern of these core Pdparticles.
structure. Smaller palladium nanoparticles of the order of
5 nm can also be seen individually or as aggregates on the
surface of the larger core particles upon closer inspection.
Further thermal decomposition of the remaining [Pd-
(PPh3)4], in the absence of irradiation, produces many more
of these small Pd nanoparticles (< 5 nm) and, in turn, induces
even more aggregation on the surface of the core Pd
nanoparticles. This results in the formation of spherical
core–shell palladium structures (B; Figure 4a). These TEM
images of product B show that the overall core–shell spheres
are larger but maintain a fairly narrow size distribution of
100 Æ 20 nm. Incidentally, the core diameters remain
unchanged at about 20 nm. With the higher magnification
provided by a JEOL TEM-3010 (300 keV) instrument, the
shells were seen to be composed of dark spots (Figure 4b,c).
These dark spots were determined to be individual Pd
nanocrystals whose lattice spacing of 0.23 nm corresponds
to the (111) distance of fcc Pd (Figure 4d). Almost every
nanoparticle smaller than about 5 nm appears to be a discrete
entity in the nanoshell of the core–shell structure, although
some merging of the particles could also be observed upon
closer inspection.
Figure 5. The XRD pattern of the core–shell palladium spheres (prod-
uct B).
An energy-dispersive X-ray (EDX) analysis of the core–
shell structure revealed the presence of phosphorus within the
shell with a P/Pd ratio of around 0.06. Therefore, the shell
most likely consists of at least two types of PPh3, namely free
PPh3 in the space surrounding the small Pd nanoparticles and
PPh3 adsorbed on the nanoparticle surface. Both these types
of PPh3 play a key role in the aggregation process during
formation of the core–shell structure. Unfortunately we did
not obtain an NMR signal for the adsorbed PPh3 because of
the low concentration of the core–shell nanoparticles.
Although it is likely that [Pd(PPh3)4] would also be trapped
upon Pd nanoparticle formation, this species would decom-
pose within the nanoparticle in a manner similar to the
thermal decomposition in solution. Hence, [Pd(PPh3)4] rep-
resents, at most, a minor contribution to the composition of
the core–shell Pd structure, especially if the nanoparticles
have been left in the dark for a few days.
An X-ray diffraction (XRD) analysis with a Siemens
Powder XRD D5005 diffractometer also confirmed the
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When the EDX beam was directed towards the center of
It is worth pointing out that the outer-shell aggregates of
palladium nanoparticles are formed spontaneously without
the influence of any external forces. As seen in Figure 4b, the
core–shell structures are spherical. According to the diffu-
sion-controlled-growth theory, every nucleus gathers the
neighboring primary particles within its individual field of
attraction, which extends as a circular arc around a spherical
particle to a defined distance from the center of the particle. It
is therefore not surprising that the nanostructures obtained
here are spherical when this formation process is consid-
ered[17] as the particles tend to grow into a spherical shape in
order to minimize their surface energy.[18] In the present case,
the shape of the spontaneously formed core–shell aggregates
also becomes spherical in order to minimize their surface
energy.
In summary, stable core–shell palladium nanoparticles
(100 Æ 20 nm) are formed by laser decomposition followed by
thermal decomposition of a solution containing [Pd(PPh3)4].
The core (20 Æ 5 nm) consists of pure, metallic palladium
while the shell is an aggregate of smaller Pd particles (< 5 nm)
interspersed with phosphanes. If the palladium precursor is
allowed to decompose thermally only the shell structure is
formed, whereas if the laser irradiation is prolonged only the
core structure is observed. This provides a convenient method
for tuning the structure of palladium nanoparticles by making
use of both photochemical and thermal decomposition path-
ways.
the core–shell structure, the P/Pd ratio was found to decrease
to 0.02. We believe that the detected phosphorus, albeit in
lower amounts, originates from the shell, since the beam still
has to sample this region around the core. A simple
calculation shows that for a core thickness of about 20% of
the whole structure (as estimated from Figure 4b), the P/Pd
ratio should decrease slightly to about 0.05 assuming that the
core consists of only Pd atoms of the same density as the shell.
However, a larger decrease is seen, which strongly suggests
that the packing of Pd atoms within the core is denser, closely
resembling that of the pure metal, and that phosphorus
species are absent. Hence, EDX analysis supports the
formation of an almost pure Pd core, or at least one with a
much higher Pd density, surrounded by aggregates of smaller
Pd nanoparticles interspersed with phosphanes.
We also found that when the solution of [Pd(PPh3)4] was
irradiated with the pulsed laser for a longer time (> 2 h), no
core–shell structures could be obtained even after the
solution had been stored in the dark for a few days. During
such a prolonged irradiation period the precursor would have
decomposed completely to give only the core palladium
nanoparticles. If the duration of the photolysis is shortened
not all of the precursor decomposes, thus allowing the
remainder to undergo gradual thermal decomposition to
form the smaller palladium nanoparticles, which then aggre-
gate on the core surface. In another experiment, [Pd(PPh3)4]
was allowed to decompose slowly in the absence of any laser
or light irradiation under ambient conditions and without any
heating, shaking, or stirring. The solution gradually became
turbid and within 24 to 40 h turned from orange to black.
After performing the required workup to isolate the nano-
particles, we found that they were spherical aggregates with
no Pd core. These spherical aggregates are larger, with
diameters of up to 400 nm.
Received: September 27, 2005
Published online: December 30, 2005
Keywords: aggregation · nanostructures · palladium ·
.
photochemistry · self-assembly
The formation of larger aggregates of Pd nanoparticles in
the absence of irradiation is due to the smaller number of
“seed” nuclei in the solution. The precursor can only
decompose slowly in the dark to form smaller Pd nano-
particles (< 5 nm), which, in turn, are adsorbed by the free
PPh3 released during the decomposition process. These
individual nanoparticles aggregate, perhaps through intermo-
lecular p–p interactions, and their growth may be diffusion-
controlled.[16] Laser photolysis greatly influences the size of
the Pd aggregates formed. In the initial step, efficient
decomposition of [Pd(PPh3)4] accelerated by the intense
laser pulse releases more palladium atoms during each pulse
and causes the aggregation into larger palladium nanoparti-
cles (20 Æ 5 nm). Since no further triphenylphosphane is
added, there is insufficient adsorbate to prevent the formation
of these large nanocores. However, once the irradiation is
stopped, and providing some Pd precursors remain, slower
thermal decomposition can take place instead to form much
smaller nanoparticles. Interestingly, these smaller particles
tend to move towards the larger, laser-generated Pd cores to
form core–shell assemblies. We are not aware of any previous
work that reports the formation of such a core–shell structure
for palladium, although aggregates resembling the shell
structure have been observed previously.[14]
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