Synthesis and characterization of Pd(0), PdS, and Pd@PdO core-shell nanoparticles by solventless thermolysis of a Pd-thiolate cluster

Article abstract of DOI:10.1016/j.jssc.2010.07.013

Colloids of palladium nanoparticles have been prepared by the solvated metal atom dispersion (SMAD) method. The as-prepared Pd colloid consists of particles with an average diameter of 2.8±0.1 nm. Digestive ripening of the as-prepared Pd colloid, a process involving refluxing the as-prepared colloid at or near the boiling point of the solvent in the presence of a passivating agent, dodecanethiol resulted in a previously reported Pdthiolate cluster, [Pd(SC₁₂H₂₅)₂]₆ but did not render the expected narrowing down of the particle size distribution. Solventless thermolysis of the Pdthiolate complex resulted in various Pd systems such as Pd(0), PdS, and Pd@PdO coreshell nanoparticles thus demonstrating its versatility. These Pd nanostructures have been characterized using high-resolution electron microscopy and powder X-ray diffraction methods.

Full text of DOI:10.1016/j.jssc.2010.07.013

Journal of Solid State Chemistry 183 (2010) 2059–2067
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Journal of Solid State Chemistry
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Synthesis and characterization of Pd(0), PdS, and Pd@PdO core–shell
nanoparticles by solventless thermolysis of a Pd–thiolate cluster
Deepa Jose, Balaji R. Jagirdar n
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Colloids of palladium nanoparticles have been prepared by the solvated metal atom dispersion (SMAD)
method. The as-prepared Pd colloid consists of particles with an average diameter of 2.870.1 nm.
Digestive ripening of the as-prepared Pd colloid, a process involving refluxing the as-prepared colloid at
or near the boiling point of the solvent in the presence of a passivating agent, dodecanethiol resulted in
a previously reported Pd–thiolate cluster, [Pd(SC12H25)2]6 but did not render the expected narrowing
down of the particle size distribution. Solventless thermolysis of the Pd–thiolate complex resulted in
various Pd systems such as Pd(0), PdS, and Pd@PdO core–shell nanoparticles thus demonstrating its
versatility. These Pd nanostructures have been characterized using high-resolution electron microscopy
and powder X-ray diffraction methods.
Received 16 April 2010
Received in revised form
13 June 2010
Accepted 12 July 2010
Available online 15 July 2010
Keywords:
Palladium nanoparticles
Thermolysis
Solvated metal atom dispersion
Core–shell
& 2010 Elsevier Inc. All rights reserved.
Colloid
1. Introduction
a-alkylation of ketones and ring-opening alkylation of cyclic
1,3-diketones with primary alcohols [11], semi-hydrogenation of
phenylacetylene [12], hydrogenation of olefins [13] and unsatu-
rated alcohols [14], hydrodechlorination of chloroorganic
compounds [15], etc. They exhibit excellent hydrogen uptake
properties too [16]. PdO nanoparticles in porous silica micropores
were found to be very good catalysts for the aerobic oxidation of
Solventless thermolysis of precursor metal complexes has
been found to be a useful synthetic route for metal nanoparticles,
metal oxides, metal chalcogenides, etc., of various shapes such as
rods, disks, wires, belts, and platelets. The precursor serves as the
source of the metal, chalcogen, and the capping agent [1–9]. The
products obtained by solventless thermolysis are monodisperse
which is due primarily to minimal inter-particle collision [1].
Korgel and co-workers employed this methodology to prepare a
variety of nanorods and nanowires of Bi2S3 [2], nanobelts of
Pb3O2Cl2 [3], nanorods, nanoplatelets, and nanodisks of Cu2S [4],
and nanorods and triangular nanoprisms of NiS [5]. Cha et al. [6]
obtained Fe3O4 and Ag nanoparticles by the solventless thermo-
lysis of an oleate complex in the absence of a surfactant under low
pressure. Synthesis of uniform Cu2S nanowires via decomposition
benzyl alcohol in water [17]. PdO/CeO2 supported on
w Al2O3 was
found to be a very good catalyst for the oxidation of methane [18].
Supported PdO catalysts bring about the transformation of CH4 to
C2H6, CO, CO2, H2, and H2O [19]. Pd–PdO nanoparticles anchored
uniformly on the inner surface of TiO2 nanotubes were found to
show excellent photocatalytic activity [20]. PdO/TiO2 films show
enhanced visible light absorption during light illumination [21].
PdO/zirconia nanophase materials show unusual reversible
hydrogen storage properties [22]. PdS is
a
semiconductor
material, with
a
band gap energy ꢀ2 eV, having potential
of
a Cu thiolate precursor has been reported by Wu and
applications in lithographic films and plates, photographic films,
catalytic photoelectrodes and solar cells [23]. Thin films of PdS
were found to be very useful materials in photovoltaic cells due to
their high absorption coefficient, band gap of 1.6 eV, n-type
conductivity, and adequate donor density [24].
Motivated by the diversity of applications of various palladium
nanophases, we attempted to explore the thermolytic behavior of
a palladium thiolate cluster, [Pd(SC12H25 2 6
co-workers [7]. Trioctylphosphine oxide (TOPO) capped PdS
nanoparticles have been synthesized by thermolysis of
[Pd(S2CNMe(Hex))2] (Hex¼n-hexyl) [8]. Thermolysis of Pd(II)
complexes with xanthate ligands also gave PdS nanoparticles [9].
Palladium nanophases such as Pd(0), PdO, and PdS have
potential applications in catalysis, electronics, and hydrogen
storage [10–24]. Palladium nanoparticles are effective catalysts
for Suzuki reactions in both aqueous and non-aqueous solvents [10],
) ] . Herein, we report
the synthesis and characterization of a variety of Pd nanophase
materials such as Pd(0), Pd@PdO core–shell, and PdS by the
solventless thermolysis of [Pd(SC12H25)2]6 by a judicious choice
of conditions. The Pd–thiolate complex was obtained by the
n Corresponding author. Fax: +91 80 2360 1552.
E-mail address: jagirdar@ipc.iisc.ernet.in (B.R. Jagirdar).
0022-4596/$ - see front matter & 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2010.07.013

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disintegration of Pd nanoclusters by dodecanethiol from
Pd-4-tert-butyltoluene-dodecanethiol colloid under reflux conditions.
a
complex were obtained from a concentrated solution of the
complex in toluene by allowing the solvent to evaporate very
slowly.
2. Experimental section
2.5. Preparation of Pd(0), Pd@PdO core–shell, and PdS nanoparticles
2.1. Materials
The Pd–thiolate powder was heated in air at 298 1C in air for
3 h resulting in free-standing Pd(0) nanoparticles. Heating the
thiolate complex in air at higher temperatures like 325, 425 1C for
3 h gave free-standing Pd@PdO core–shell nanoparticles. The PdS
nanoparticles were prepared by the solventless thermolysis as
well as by solvothermolysis. In the solventless method, the
Pd–thiolate complex was sealed under Ar atmosphere in a glass
tube and it was heated at 430 1C for 3 h. We also prepared PdS
nanoparticles from the Pd–thiolate complex by the solution
thermolysis wherein the thiolate complex was dissolved in a
minimum amount of 4-tert-butyltoluene and the resulting
solution was refluxed vigorously at 198 1C for 6 h under H2
during which time, PdS nanoparticles precipitated out as a black
powder.
Palladium foil (0.3 mm thickness) (99.95%) was purchased
from Arora-Matthey, Kolkata, India. Tungsten crucibles were
obtained from R.D. Mathis Company, California. 2-Butanone
(HPLC grade), dodecanethiol, and 4-tert-butyltoluene were pur-
chased from Aldrich. 2-Butanone was dried over K2CO3 and
degassed by several freeze–pump–thaw cycles. Dodecanethiol
and 4-tert-butyltoluene were used after purging with Ar for
30 min before each experiment.
2.2. Instrumentation
UV–visible spectra were recorded using
a Perkin-Elmer
Lambda 35 UV/vis spectrometer. Solventless thermolysis experi-
ments were carried out using a programmable Thermolyne
furnace. The TEM bright field images, plasma filtered TEM images,
electron diffraction patterns, HRTEM, EF-TEM, and EDX analysis
were obtained using a TECNAI F30 transmission electron micro-
3. Results and discussion
3.1. Behavior of the as-prepared Pd–butanone-4-tert-butyltoluene–
doecanethiol colloid under reflux conditions
˚
scope with a spatial resolution of 2 A operating at 300 kV. The
TEM samples were prepared by placing a 1 L drop containing
m
the powder redispersed in EtOH on a Formvar-coated copper grid.
The powder X-ray diffraction measurements were carried out
using a Philips powder X-ray diffractometer. The TGA measure-
ments were carried out using a Netzsch TG209 F1 instrument. The
1H NMR and MALDI mass spectra were recorded using an Avance
Bruker 400 MHz spectrometer and ULTRALEX-TOF/TOF Bruker
Daltonics spectrometer, respectively.
Colloids of palladium nanoparticles were prepared by
the solvated metal atom dispersion (SMAD) method [26]. The
as-prepared Pd–butanone-4-tert-butyltoluene–thiol colloid was
blackish brown in color consisting of spherical Pd nanoparticles
[26]. Removal of butanone from the as-prepared colloid followed
by refluxing under an atmosphere of Ar resulted in a yellow
colored solution that showed absorption maxima at 415 and
313 nm with a shoulder at 369 nm (Fig. 1). The TEM bright field
image of this sample did not show the presence of any
nanoparticles (see Supplementary material). The unambiguous
characterization of the product was established using NMR,
MALDI mass, and FTIR spectroscopy, powder and single crystal
X-ray crystallography as a tiara-shaped Pd–thiolate complex
[Pd(SC12H25)2]6 (see Supplementary material). The same
2.3. Synthesis of Pd–butanone-4-tert-butyltoluene–dodecanethiol
colloid
The Pd–butanone-4-tert-butyltoluene–dodecanethiol colloids
were prepared by the solvated metal atom dispersion (SMAD)
method. The SMAD setup is described in detail in Ref. [25]. In a
typical experiment the crucible was loaded with about 200 mg of
Pd foil. Dodecanethiol (ꢀ9 mL) and 4-tert-butyltoluene (ꢀ20 mL)
were taken at the bottom of the reactor after purging with Ar for
30 min. The Pd to dodecanethiol ratio was about 1:20 mmol. The
crucible was heated resistively in vacuo and the metal vapor
was cocondensed with 2-butanone (ꢀ100 mL) on the walls of the
reactor which was kept at liquid N2 temperature. The Pd/
butanone matrix was allowed to melt down under Ar atmosphere
after the complete vaporization of the metal and stirred with
dodecanethiol and 4-tert-butyltoluene to get the Pd–butanone-
4-tert-butyltoluene–dodecanethiol colloid. This colloid was
siphoned under Ar atmosphere into a Schlenk tube.
2.4. Synthesis of [Pd(SC12H25)2]6
Butanone was removed from the as-prepared Pd colloid
(ꢀ100 mL) in vacuo and then the colloid was refluxed under Ar
atmosphere for 3 h. Ethanol (250 mL) was added to this Pd sample
and the mixture was allowed to stand at room temperature
overnight. Complete precipitation of the Pd–thiolate complex
[Pd(SC12H25)2]6 was effected during this time. The pure product
was obtained from the precipitate as an air stable yellow colored
powder by column chromatography using n-hexane as the eluent.
The yield was about 90% (0.850 g). Single crystals of the thiolate
Fig. 1. UV–visible spectrum of Pd–4-tert-butyltoluene–dodecanethiol colloid after
refluxing under Ar atmosphere for 3 h.

D. Jose, B.R. Jagirdar / Journal of Solid State Chemistry 183 (2010) 2059–2067
2061
complex was previously synthesized by Klabunde and co-workers
by a different route [27].
of sulfur moiety in both air and under Ar at ꢀ325 1C. Then a small
increase in weight owing to the oxidation of Pd(0) to PdO was
noted, when the TG analysis was carried out in air at ꢀ400 1C.
However, the percentage weight increase in air (ꢀ4.5%, inset
Fig. 2) does not account for the complete conversion (the increase
in wt% if complete oxidation takes place is ꢀ15%) of Pd(0) to PdO
suggesting that some Pd(0) still remains unoxidized even at high
temperature. No oxidation of Pd(0) takes place when the thermal
studies were carried out under Ar atmosphere. The grayish black
powder obtained after TG analysis of [Pd(SC12H25)2]6 in air was
confirmed as Pd(0) by powder XRD studies (see Supplementary
material).
Taking clues from the TGA studies, we attempted to arrest the
individual steps by thermolyzing the Pd–thiolate complex at
different temperatures in the absence of any solvent. Thus, we
were able to realize various Pd systems like Pd(0), PdS, and
Pd@PdO core–shell nanoparticles. The details of solventless
thermolysis and solvothermolysis of the Pd–thiolate complex
are summarized in Table 1.
Refluxing the as-prepared polydisperse dodecanethiol capped
SMAD-prepared Au, Ag, Cu, and Zn colloids gave highly mono-
disperse samples [28] via a process called as digestive ripening,
wherein large clusters break down and the smaller clusters grow
in size until a uniform size is reached for the entire colloid [29].
However, refluxing the dodecanethiol capped Pd colloids in
4-tert-butyltoluene did not result in a monodisperse sample,
instead,
a Pd–thiolate complex formed. The alkanethiolates
protected Pd nanoparticles were found to undergo a dynamic
growth process where initially larger particles are quickly formed
followed by a decomposition into smaller clusters and finally to
Pd ionic complexes [30]. Tsukuda et al. proposed a mechanism for
the decomposition of thiolate protected Pd clusters in which the
thiolate moieties detach from the Pd cores preferentially from the
edge and vertex sites [31]. The free thiol ligand can further act as
an etchant against the vertices and edges where the packing
density is less due to self-dissociation.
Removal of butanone in effect replaces butanone, the initial
capping agent with dodecanethiol as the final capping agent on Pd
nanoparticles. Presence of excess thiol results in ligand exchange
between the free thiol and the thiol bound to Pd nanoparticles.
Schmid observed that the free and the bound surfactants undergo
rapid exchange in the case of PPh3 capped Au nanoparticles [32].
During the ligand exchange process, the bound thiol takes with it
the surface atom or atoms from the Pd cluster due to the weak
Pd–Pd interactions [33]. A redox reaction involving oxidation
of Pd and reduction of the thiol resulted in the formation of the
Pd–thiolate complex. Essentially, the thiol causes the leaching of
Pd clusters to give the Pd–thiolate complex. An experiment in
which the Pd:dodecanethiol ratio was 1:1 gave only a small
amount of the Pd(II)–thiolate complex. This indicates that the
presence of excess thiol is required for the complete leaching
of Pd clusters. The transformation of the Pd nanoparticles to the
Pd–thiolate complex is schematically shown in Scheme 1.
3.3. Pd(0) nanoparticles
As is evident from the data in Table 1, the thiolate complex is
stable up to 270 1C. Thermolysis of the compound at 270 1C left
the powder XRD pattern unchanged from that of the thiolate
complex (Fig. 3). Raising the thermolysis temperature to 298 1C
resulted in the formation of bare Pd(0) nanoclusters. The color of
the powder changed from yellow to black. The powder XRD
3.2. Solventless thermolysis of [Pd(SC12H25)2]6
We examined the thermal behavior of the [Pd(SC12H25)2]6
complex in the temperature range 30–1000 1C in air (Fig. 2)
as well as under an Ar atmosphere (see Supplementary material).
Initially, the loss of the hydrocarbon part (ꢀ68%) of the
Pd–thiolate complex was noted at ꢀ285 1C, followed by the loss
Fig. 2. TGA of [Pd(SC12H25)2]6 in air. Inset: weight increase during heating in air.
Table 1
Thermolysis of [Pd(SC12H25)2]6.
Temperature (1C)
Atmosphere
Product
200
270
298
325
425
430
198b
Air
Pd–thiolatea
Pd–thiolate
Pd(0)
Air
Air
Air
Pd@PdO core–shell
Pd@PdO core–shell
PdS
Air
Argon
Hydrogen
PdS
a
Scheme 1. Transformation of Pd nanoparticles into the Pd–thiolate complex
Starting material unchanged at these temperatures.
Solvothermolysis.
b
[Pd(SC12H25)2]6.

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pattern shows that the sample is not fully crystalline having
some amorphous material (Fig. 4a). Redispersion of the Pd(0)
nanoparticles in ethanol followed by TEM studies (Fig. 4b)
revealed the presence of elongated, oval-shaped particles of size
ranging from 13 to 22 nm. The polydispersity of the particles is
due to the absence of a surfactant, use of which would result in
tuning the particle size. Ring-like diffraction patterns with spots
corresponding to the (111), (200), (220), (311), (222), (400), (331)
and (420) planes of Pd(0) were noted in the SAED pattern (Fig. 4c).
The EDX analysis (Fig. 4d) also confirmed the presence of Pd.
3.4. Pd@PdO core–shell nanoparticles
Solventless thermolysis of [Pd(SC12H25)2]6 complex at 325 1C
in air led to a Pd/PdO system as per the powder XRD studies
(red trace, Fig. 5). The powder XRD pattern showed both FCC Pd
Fig. 3. Powder XRD pattern of [Pd(SC12H25)2]6 before and after thermolysis at 200
and 270 1C in air.
Fig. 5. Powder XRD pattern of Pd@PdO formed by the solventless thermolysis of
[Pd(SC12H25)2]6 at 325 and 425 1C in air.
Fig. 4. (a) Powder XRD pattern, (b) TEM bright field image, (c) SAED pattern, and (d) EDX analysis of Pd(0) formed by the solventless thermolysis of [Pd(SC12H25)2]6 at
298 1C in air.

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2063
and tetragonal PdO phases: reflections from the (111), (200) and
(220) planes of Pd(0) and (101), (110), (112), (103) and (211)
planes of PdO were observed (JCPDS file numbers 461043 and
850713). Complete oxidation of Pd to PdO did not take place even
at higher temperatures (425 1C) suggesting a core–shell structure
consisting of a shell of PdO protecting a Pd(0) core (green trace,
Fig. 5). The average crystallite size calculated from the powder
XRD pattern using the Scherrer formula was found to be about
54 nm.
The Pd@PdO core–shell structure was confirmed by detailed
electron microscopy studies. The TEM bright field image (Fig. 6a)
showed a broad distribution of polygonal shaped particles with
size varying from 20 to 100 nm. The SAED pattern (Fig. 6b)
confirmed the presence of both Pd and PdO phases. The HRTEM
image (Fig. 6c) showed lattice fringes indicating the crystalline
nature of the sample. The interplanar distance calculated from the
the presence of both Pd and O in the same cluster. Oxygen was
found to be concentrated on the surface of the cluster. The bright
contrast in Fig. 6d corresponds to Pd and the bright contrast in
Fig. 6e corresponds to O. Thus the combined powder XRD and
electron microscopy results revealed the formation of a core–shell
structure of Pd@PdO.
Once Pd(0) formed, dissociative adsorption of oxygen takes
place on the surface of the particles. This is followed by the
diffusion of oxygen atoms into the Pd lattice by forcing apart the
Pd atoms to give PdO at 425 1C. The surface oxidation of Pd to PdO
has been studied by others [34]. Complete oxidation of Pd(0) to
PdO was not observed in our case and this could be due to the
diffusion of oxygen only onto the surface layers. The PdO layer
was found to be quite stable, heating the core–shell structure at
higher temperatures did not result in the oxidation of the core
which suggested that PdO formed a good protective shell over the
Pd core [35].
˚
HRTEM image gave two different d values: 2.25 A of the (111)
˚
plane of Pd in the core and 2.63 A of the (101) plane of PdO in the
shell region. Further, the existence of both Pd and PdO phases in
the same cluster was confirmed by EDX analysis. The EDX analysis
of an isolated cluster showed the presence of both Pd and O in 96
and 4 wt%, respectively (see Supplementary Material). In addition,
the energy-filtered TEM (EFTEM) images (Fig. 6d and e) showed
3.5. PdS nanoparticles
Solventless thermolysis of the Pd–thiolate complex at 430 1C
under Ar resulted in PdS nanopowder. The powder X-ray
Fig. 6. (a) TEM bright field image, (b) SAED pattern, (c) HRTEM image, (d) EFTEM Pd mapping, and (e) EFTEM O mapping of Pd@PdO formed by the solventless thermolysis
of [Pd(SC12H25)2]6 at 425 1C in air.

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Fig. 7. (a) Powder XRD pattern, (b) TEM bright field image, (c) histogram showing particle size distribution, (d) HRTEM image, and (e) SAED pattern of PdS formed by the
solventless thermolysis of [Pd(SC12H25)2]6 at 430 1C under Ar.
diffraction pattern (Fig. 7a) of the black powder showed
reflections corresponding to the tetragonal phase of PdS and
could be indexed to the (102), (210), (211), (212), (222) and (132)
planes (JCPDS#780206). The TEM bright field image (Fig. 7b)
revealed the presence of spherical PdS nanoclusters with a
size of 9.270.9 nm (Fig. 7c). The sample was found to be
highly crystalline as evidenced by the presence of clear lattice
fringes in the HRTEM image (Fig. 7d). In addition, the SAED
pattern (Fig. 7e) could be indexed to that of PdS. The EDX analysis
of several isolated clusters gave an average composition of 77% Pd
and 23% S (see Supplementary Material).
3.6. Solvothermal hydrogenolysis of Pd–thiolate complex
Solvothermolysis of Pd–dodecanethiolate complex at 259 1C in
diphenyl ether under Ar atmosphere gave PdS nanoparticles [27].
In our hands, solvothermolysis carried out in tert-butyltoluene

D. Jose, B.R. Jagirdar / Journal of Solid State Chemistry 183 (2010) 2059–2067
2065
ꢀ
Fig. 8. (a) UV–visible spectrum, (b) TEM bright field image, (c) SAED pattern, (d) HRTEM image showing crystal growth along the /I01S direction, (e) HRTEM image
showing crystal growth along the /010S direction, (f) FFT pattern corresponding to the HRTEM image in (d), and (g) FFT pattern corresponding to the HRTEM image in (e)
of PdS formed by the solvothermolysis of [Pd(SC12H25)2]6 at 198 1C under H2.

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D. Jose, B.R. Jagirdar / Journal of Solid State Chemistry 183 (2010) 2059–2067
via solventless thermolysis methods. Such versatility is unprece-
dented.
Acknowledgments
We are grateful to the Council of Scientific and Industrial
Research, India, for financial support. We thank the I.I.Sc. Institute
Nanoscience Initiative and the Solid State and Structural Chem-
istry Unit, I.I.Sc. for allowing us to access the microscopic facilities
and the powder X-ray diffractometer, respectively. D.J. thanks the
CSIR for a fellowship.
Appendix A. Supporting materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.jssc.2010.07.013.
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The Pd–thiolate complex is a very versatile precursor for the
synthesis of various Pd nanophase materials such as Pd(0), PdS,
and Pd@PdO core–shell systems devoid of any capping agents

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