Microwave-assisted synthesis of palladium nanocubes and nanobars

Article abstract of DOI:10.1016/j.materresbull.2009.09.028

Microwave was employed in the shape-controlled synthesis of palladium nanoparticles. Palladium nanocubes and nanobars with a mean size of about 23.8 nm were readily synthesized with H₂PdCl₄ as a precursor, tetraethylene glycol (TEG) as both a solvent and a reducing agent in the presence of PVP and CTAB in 80 s under microwave irradiation. The structures of the as-prepared palladium nanoparticles were characterized by transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and ultraviolet-visible (UV-vis) absorption spectroscopy. The formation of PdBr₄^2-due to the coordination replacement of the ligand Cl^- ions in PdCl₄^2- ions by Br^- ions in the presence of bromide was responsible for the synthesis of Pd nanocubes and nanobars. In addition, a milder reducing power, a higher viscosity and a stronger affinity of TEG were beneficial to the larger sizes of Pd nanocubes and nanobars.

Full text of DOI:10.1016/j.materresbull.2009.09.028

Materials Research Bulletin 45 (2010) 159–164
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Microwave-assisted synthesis of palladium nanocubes and nanobars
Yanchun Yu a, Yanxi Zhao a, Tao Huang a, , Hanfan Liu
*
a,b,**
a Key Laboratory of Catalysis and Material Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei Province, College of Chemistry and Material Science,
South-Central University for Nationalities, Wuhan 430074, China
b Institute of Chemistry, Chinese Academy of Science, Beijing 100080, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Microwave was employed in the shape-controlled synthesis of palladium nanoparticles. Palladium
nanocubes and nanobars with a mean size of about 23.8 nm were readily synthesized with H2PdCl4 as a
precursor, tetraethylene glycol (TEG) as both a solvent and a reducing agent in the presence of PVP and
CTAB in 80 s under microwave irradiation. The structures of the as-prepared palladium nanoparticles
were characterized by transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and
ultraviolet–visible (UV–vis) absorption spectroscopy. The formation of PdBr42Àdue to the coordination
replacement of the ligand ClÀ ions in PdCl42À ions by BrÀ ions in the presence of bromide was responsible
for the synthesis of Pd nanocubes and nanobars. In addition, a milder reducing power, a higher viscosity
and a stronger affinity of TEG were beneficial to the larger sizes of Pd nanocubes and nanobars.
ß 2009 Elsevier Ltd. All rights reserved.
Received 5 June 2009
Received in revised form 4 September 2009
Accepted 24 September 2009
Available online 2 October 2009
Keywords:
A. Metals
A. Nanostructures
B. Chemical synthesis
C. Electron diffraction
1. Introduction
alkylammonium ions are the most widely used mediating
materials to serve as surface-regulating agents or stabilizers in
Shape-controlled synthesis of metal nanostructures has been
paid much attention for decades owing to their size- and shape-
dependent properties [1–5]. So, special nanostructure with uniform
size and well-defined shape is required to regulate their properties
for various applications. As one of the most studied metallic
nanomaterials, Pd has attracted much interest for its applications in
many fields. For instance, it has been widely used as the catalysts for
some organic reactions [6–9], hydrogenation [10,11] and the low-
temperature reduction of pollutants exhausted from automobiles
[12,13]. Pdnanoparticles werealsousedaselectrocatalystsfordirect
alcohol oxidation in alkaline media [14–17]. It was also used as a
central component in hydrogen sensing due to its exceptional
sensitivity toward hydrogen [18,19]. Since the catalytic efficiency of
Pdnanoparticleshighlydependsonbothitssizeanditsshape,alotof
efforts have been devoted in the past decade to size and shape-
controlled synthesis of Pd nanostructures.
metallic shaped-nanoparticle synthesis [24,25]. Some hydroxyl
compounds such as polyols, citric acid, vitamin C, vitamin B as well
as some alkylamines or alkanolamine can be used as reducing
agents in reactions. Among them, ethylene glycol (EG) is one of the
most often used reducing agents. Generally, however, mix
structures with various morphologies of Pd nanoparticles were
produced. For instance, Pd colloidal nanoparticles coexisting with
various geometric shapes such as triangular, pentagonal, hexago-
nal, square, and rhombohedral Pd as well as truncated cubes or
octahedral, decahedra, and icosahedra were obtained by fast
reduction of Pd salt with methanol [26] and ethanol [27]. Pd
nanobars and nanorods could be prepared with high yields and
good uniformity by using a modified polyol process in which
[PdCl4]2À were reduced by EG [28]. Recently, Pd nanoparticles with
nanobelt, nanoplate, and nanotree morphologies were readily
prepared with PdCl2 as starting materials, vitamin B1 as a reducing
agent and water as a solvent [29]. The palladium spherical
nanoparticles, multitwinned particles, and spherical spongelike
particles were successfully prepared by a sonoelectrochemical
method in the presence of different surfactants or polymers [30].
Pd nanostructures formed by nanowires, nanoplates or flower-like
shapes were synthesized by radiolysis or photoreduction [31].
Fivefold twinned nanorods and right bipyramids of palladium
were synthesized in an aqueous medium with ascorbic acid as the
reducing agent and in the presence of bromide [21] and single-
crystal nanocubes and nanobars were also obtained [28]. Pd
icosahedra with a high yield of 80% were selectively obtained in an
aqueous solution with citric acid as a reducing agent [32]. So far,
Pd nanoparticles with various morphologies have been
prepared in the presence of some materials, such as surfactants,
polymers, coordinating ligand, DNA or RNA, and some templates
[20–23]. For instance, polyvinylpyrrolidone (PVP) or some
* Corresponding author. Tel.: +86 27 67842752; fax: +86 27 67842752.
** Corresponding author at: College of Chemistry and Material Science, South-
Central University for Nationalities, Wuhan 430074, China. Tel.: +86 27 67842752;
fax: +86 27 67842752.
E-mail addresses: huangt6628@yahoo.com.cn (T. Huang), hfliu@mail.iccas.ac.cn
(H. Liu).
0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2009.09.028

160
Y. Yu et al. / Materials Research Bulletin 45 (2010) 159–164
however, there have been few reports on the synthesis of
polygonal nanocrystal by microwave method.
3. Results and discussion
Microwave dielectric heating has many advantages compared
with conventional heating, such as prompt start up, uniform
heating, very short heating time, easy heat control (on and off), low
cost, etc. It has been extensively applied to the synthesis of metallic
nanostructures [33,34]. In this paper, microwave irradiation was
employed to synthesize Pd nanocubes and nanobars with
tetraethylene glycol (TEG) as both a reducing agent and a solvent,
H2PdCl4 as a precursor and PVP/cetyltrimethylammonium bro-
mide (CTAB) as a co-stabilizer. The effect of CTAB on the shape of
palladium nanoparticles was also investigated.
3.1. UV–vis absorption spectra of the reaction process
Fig. 1 shows the time-dependent UV–vis absorption spectra for
the formation of Pd nanoparticles in the above typical synthesis
process from 0 to 100 s. As shown in Fig. 1, at the beginning the
reaction solution showed two strong characteristic absorption
2À
peaks at around 300 and 420 nm, corresponding to that of PdCl4
ion. In addition, another peak at around 530 nm for PdCl42À ion was
also observed but too weak to identify. It can be seen that when the
reaction solution was heated by microwave irradiation for 40 s, the
2À
characteristic absorption peaks of PdCl4 ion reduced obviously.
2. Experimental
When the reaction proceeded for 60 s, accompanied by a slight
blue-shift, the peak at 300 nm reduced continuously, while the
peak at 420 nm disappeared almost completely with appearance of
a surface plasmon scattering, indicating that a Pd colloid began to
form. With increasing the irradiation time, the plasmon scattering
further enhanced due to the increase of Pd nanoparticles. When the
reaction was continuously irradiated by microwave for 80 s, the
plasmon absorption reached the maximum. Even though the
irradiation was prolonged to 100 s, the absorption was as the same
as that for 80 s. This confirmed that the reaction has finished
completely at 80 s.
2.1. Materials
PVP (average molecular weight, Mw = 360,000, Fluka Chemicals),
CTAB, CTAC, palladium chloride (PdCl2, 59.0%) (Shanghai Chemicals
Co., China), TEG (Acros Chemicals) and other chemicals were all of
the analytical grade and used without further purification.
H2PdCl4ÁnH2O was prepared by treating PdCl2 with concentrated
hydrochloric acid at 35 8C, in which the molar ratio of HCl/PdCl2 was
2/1. H2PdCl4ÁnH2O was dissolved in TEG before use and a brownish
red homogeneous H2PdCl4 solution was obtained, in which the
concentration of Pd(II) was kept at 0.03 mol LÀ1. PVP, CTAB, CTAC,
KBr and KCl were also dissolved in TEG to make up a certain
concentration of the corresponding solution in TEG, respectively.
3.2. TEM characterization
Fig. 2 shows TEM images of the as-prepared Pd nanoparticles in
the above typical synthesis, in which the precursor concentration
was 3 mmol LÀ1 and the molar ratio of H2PdCl4/CTAB/PVP was 1/4/
4. As can be seen, uniform and well-defined Pd nanocubes and
short nanobars were obtained (Fig. 2a). The average size was about
23.8 nm by statistic calculation. Fig. 2b displays a typical high-
resolution TEM image of a single Pd nanocube. The interplanar d
2.2. Preparation of Pd nanocubes and nanobars
In a typical synthesis, 1 mL of 0.03 mol LÀ1 H2PdCl4 solution,
1 mL of 0.12 mol LÀ1 PVP (in monomeric unit) solution and 1.0 mL
of 0.12 mol LÀ1 CTAB solution were added into a 50 mL round
bottom flask. Then, 7 mL of TEG was added to keep the final volume
of the mixture at 10 mL. In the reaction system, the molar ratio of
H2PdCl4/CTAB/PVP was 1/4/4. After thorough mixing, the solution
was then put into a modified domestic microwave oven (Galanz,
900 W) and heated for 80 s with 100% output of the power. The
color of the solution turned from pale yellow to brownish black,
and palladium colloid was obtained. The microwave oven was
modified without changing the power as described as in Ref. [34]: a
water-cooled condenser outside the oven’s cavity was connected
by a glass joint of a glass round-bottomed flask set inside. A Teflon
stirrer was set in the flask and was driven by a motor. The power
output was adjusted by the microwave machine with a routine on–
off manner and the temperature didn’t have to be controlled.
˚
spacing is about 2 A, corresponding to that of the {1 0 0} lattice
planes of fcc Pd. The inset of Fig. 2b shows a Fast Fourier Transform
(FFT) pattern of the selected Pd nanocube. The presence of the two
distinct spots, corresponding to the planes of fcc Pd, indicates that
the as-prepared Pd nanocubes are single crystalline structure.
3.3. XRD pattern of Pd nanocubes and nanobars
The XRD pattern of the Pd nanocubes and nanobars obtained in
the above typical experiment is shown in Fig. 3. Four characteristic
peaks of Pd at 2u = 40.58, 46.88, 68.48, 82.38, corresponding to the
{1 1 1}, {2 0 0}, {2 2 2}, {3 1 1} lattice planes, are observed. All the
diffraction peaks can be well-indexed to fcc Pd according to the
JCPDS card no. 00-001-1201, indicating that the as-prepared Pd
nanocubes and nanobars have a high purity and high crystallinity.
2.3. Characterization
Ultraviolet–visible (UV–vis) absorption spectra were measured
on a Lambda BIO35 spectrophotometer. Transmission electron
microscopy (TEM) and high-resolution transmission electron
microscopy (HRTEM) images were taken on a FEI Tecnai G2 20
transmission electron microscopy operated at 200 kV. Diffracto-
grams of HRTEM were obtained by Fourier transformation. The
sample for TEM observation was prepared by placing a drop of the
colloidal dispersion onto a copper grid coated with a perforated
carbon film, followed by evaporating the solvent at ambient
temperature. The average particle size and the distribution were
determined from the enlarged micrographs on the basis of the
measurement of about 300 particles. X-ray powder diffraction
(XRD) patterns were recorded on a Bruker D8 advance X-ray
diffractometer employing Cu Ka radiation with 40 kV and 50 mA.
The mean size of Pd nanoparticles was also estimated by using the
Scherrer’s equation.
Fig. 1. UV–vis absorption spectra of the reaction system at different stages.

Y. Yu et al. / Materials Research Bulletin 45 (2010) 159–164
161
Fig. 2. TEM and high-resolution TEM images of Pd nanoparticles. (a) TEM image, the inset arrows show some of the nanobars; (b) HRTEM image, the inset is a fast Fourier
transform (FFT) pattern of the selected Pd nanocube.
The average particle size is estimated by the Scherrer’s equation to
be about 23.6 nm, which is excellently consistent with the
observation by TEM.
and PVP. When PVP concentration was 12 mmol LÀ1, the most
appropriate range of CTAB concentration for the control synthesis
of Pd nanocubes and nanobars was 12–24 mmol LÀ1. In other word,
the most appropriate range of the molar ratio of H2PdCl4/CTAB/PVP
was 1/4/4 to 1/8/4.
3.4. Effect of CTAB and PVP
The synthesis was typically conducted under microwave
irradiation in TEG with H2PdCl4 as a precursor in the presence
of PVP and CTAB. TEG served as both a solvent and a reducing
agent. No doubt PVP plays an important role in controlling the
morphologies of final Pd nanoparticles. The PVP not only serves as
a colloidal stabilizer to eliminate random agglomeration, but also
directs the particles to grow into the well-defined shapes during
the synthesis [32]. It is well known that CTAB is widely used as a
shape controller in nanostructure synthesis. Nevertheless, Prof.
Xia’s group has confirmed that bromide played a critical role in the
formation of Pd nanobars and nanorods [28]. They demonstrated
that bromide can chemisorb onto the surface of Pd seeds and alter
the order of surface free energies for different facets so the
formation of {1 0 0} surface can be greatly promoted to generate
nanocubes and nanobars. To further understand our experimental
results, we used KBr, CTAC, and KCl instead of CTAB, respectively, in
the same reaction system under the same condition. Fig. 5a–d
shows the TEM image of the as-prepared Pd colloid corresponding
to CTAB, CTAC, KBr, and KCl system, respectively. Obviously, the
shapes of the as-obtained Pd nanoparticles for CTAC system
(Fig. 5b) are different from that of CTAB system (Fig. 5a). The
cooperation of CTAC and PVP resulted in the formation of an
icosahedral Pd nanostructure, which illustrates a hexagonal shape
under TEM, as shown in Fig. 5b. Similar results were also observed
for KBr and KCl system. As it can be seen, Pd nanocubes and
nanobars were also obtained for the reaction system with KBr
(Fig. 5c) instead of CTAB, while Pd icosahedra were obtained for
that with KCl (Fig. 5d) instead of CTAC. Synthesis in the presence of
either CTAB or KBr all generated nanocubes and nanobars, while
that in the absence of bromide or in the presence of chloride mainly
produced icosahedra. Since CTAB and CTAC, as well as KBr and KCl,
have a same cation, the difference in the morphology of Pd
nanoparticles can be attributed to their different counter ions, BrÀ
and ClÀ, respectively. Thus the formation of Pd nanocubes and
nanobars should be ascribed to the existence of BrÀ anions.
It is noteworthy that an appropriate amount of CTAB and PVP
plays an important role in controlling the morphologies of the final
Pd nanoparticles. Fig. 4 shows TEM images of Pd nanoparticles with
different amounts of CTAB and PVP. As shown in Fig. 4a, an obvious
agglomeration was observed without the addition of PVP when the
other conditions were kept the same as that in the above typical
experiment in which the precursor concentration was 3 mmol LÀ1
and H2PdCl4/CTAB molar ratio was 1/4. When the reaction system
contained 12 mmol LÀ1 of PVP, in which the molar ratio of H2PdCl4/
CTAB/PVP was 1/4/4, Pd nanocubes and nanobars with uniform
size and well dispersion were obtained, as shown in Fig. 4b.
However, when the concentration of the precursor and PVP was
kept unchanged, the morphologies of Pd nanoparticles varied with
the concentration of CTAB. As shown in Fig. 4c and d, mix
structures with various morphologies of Pd nanoparticles were
produced when the molar ratio of H2PdCl4/CTAB/PVP was 1/3/4
and 1/10/4, respectively. It was found that the absence of either
PVP or CTAB in the reaction system would hardly produce Pd
nanocubes and nanobars. These results show that the formation of
Pd nanocubes and nanobars depends on the coexistence of CTAB
2À
It has been reported that PdCl4 ion is not the real precursor
species in the reduction process [35]. It has been proved that the
major function of bromide in the synthesis is its chemisorption on
nanocrystal surface and it also contributed to the slow reduction
Fig. 3. XRD pattern of the typical Pd nanocubes and nanobars.

162
Y. Yu et al. / Materials Research Bulletin 45 (2010) 159–164
Fig. 4. TEM images of Pd nanoparticles with different amounts of CTAB and PVP. (a) H2PdCl4/CTAB = 1/4, without PVP; (b) H2PdCl4/CTAB/PVP = 1/4/4; (c) H2PdCl4/CTAB/
PVP = 1/3/4; (d) H2PdCl4/CTAB/PVP = 1/10/4. In all cases, the concentration of the precursor was 3 mmol LÀ1
.
due to the formation of a more stable coordinated anion PdBr42À in
the reaction system after the addition of bromide. The potential of
H2PdCl4 is lower than that of H2PdBr4. The addition of bromide
reduced the potential of palladium ion so that could reduce the
reduction rate. To realize the effect of the coordination replace-
ment on the reduction rate and nanocrystal growth under
microwave irradiation, UV–vis absorption spectroscopy was
employed. Fig. 6 shows the UV–vis absorption spectra of the
palladium precursor in TEG with different additives. Two
the addition of bromide, while the addition of CTAC or KCl did not
bring about any change of ligand. In addition, the same red shift of
the absorption band for CTAB and KBr systems indicates that the
2À
peak shift is independent on the formation of surfactant-PdBr4
2À
complex due to the interaction between surfactant and PdBr4
,
but the formation of coordinated ion PdBr42À. This result is
consistent with the TEM observation of similar nanocubes and
nanobars (Fig. 5a and c) for both the CTAB and KBr systems. That is
to say, the formation of the Pd nanocubes and nanobars depends on
the reduction kinetics and adsorption of kinetics of the BrÀ ions on
the surface of growing Pd nanoparticles. The formation of the
coordinated ion PdBr42À reduces the redox potential as well as the
reduction rate, while the preferential adsorption of BrÀ ions on the
surface of growing Pd nanoparticles promotes the formation of
{1 0 0} facets. This is in agreement with the reports by Xia’s group
[21,28].
2À
characteristic absorption peaks corresponding to those of PdCl4
ion were observed at 260 and 340 nm for H2PdCl4 solution in TEG
without any other additive due to the ligand metal charge transfer.
Moreover, though PVP and CTAC (or KCl) was introduced into the
H2PdCl4-TEG solution, not any change of the absorption bands was
observed. However, when the H2PdCl4-TEG solution contained PVP
and CTAB or KBr with the same concentration, the absorption
bands enhanced and a red shift occurred. The two ligand metal
charge transfer peaks are shifted to 300 and 420 nm, respectively,
and the red shift is independent on PVP for the same amount of
CTAB and KBr. The red shift of the absorption bands can be
attributed to the change of the palladium complex from ligand ClÀ
to ligand BrÀ. In other word, a change of the coordination bond or
ligand in coordinated palladium ion took place. These results
demonstrated that the coordinated ion PdBr42À was formed due to
the coordination replacement of the ligand ClÀ ion by BrÀ ion after
Furthermore, it was found that the red-shift distance was
dependent on the concentration of bromide. Withthe increase of the
bromide, the characteristic peaks at 260 and 340 nm shifted
gradually to 300 and 420 nm, as shown in Fig. 7. When the molar
ratio of H2PdCl4/KBr was 1/4, with continuously increasing the
amount of KBr, the red shift became unobvious. This meant the 4-
fold BrÀ ions can substitute most ClÀ ions in the PdCl42À ions to form
PdBr42À ions. Similar phenomenon was observed for CTAB instead of
KBr. Thus as can be seen, the effect of CTAB or KBr comes from the

Y. Yu et al. / Materials Research Bulletin 45 (2010) 159–164
163
Fig. 5. TEM images of Pd nanoparticles. (a) H2PdCl4/CTAB/PVP = 1/4/4; (b) H2PdCl4/CTAC/PVP = 1/4/4; (c) H2PdCl4/KBr/PVP = 1/4/4; (c) H2PdCl4/KCl/PVP = 1/4/4. In all cases,
the concentration of the precursor was 3 mmol LÀ1
.
formation of Br complex. The function of CTAB or KBr is just toform a
more stable Br complex to lower the redox potential. These results
confirm that the form of precursor is important in determining the
morphology of Pd nanoparticles, and also explain that why the
uniform Pd nanocubes and nanobars produced only with the molar
ratio of 1/4/4 to 1/8/4 of H2PdCl4/CTAB/PVP. Though the complete
substitution of all chloride in the PdCl42À ions needs 10-fold BrÀ ions
[35], an appropriately low concentration of bromide is more
favorable for the formation of single-crystal nanocubes and
nanobars [28]. In the present case, 4- to 8-fold BrÀ ions are suitable
for Pd nanocubes and nanobars. These UV–vis absorption char-
acteristics are consistent with the TEM observation.
In addition, TEG may be another essential factor for the shape-
and size-control of Pd nanocubes and nanobars. It was found that
Pd nanocubes and nanobars with mono-morphology and well
dispersion were not obtained if EG was used as a solvent and a
reducing agent instead of TEG under the same conditions. The
reason may be ascribed to the structure, viscosity and reducing
ability of TEG. Firstly, TEG has four ethylene-oxygen units which
demonstrate a certain affinity for metal ions and metal nanopar-
ticles so that it can be adsorbed on the surfaces of Pd particles more
strongly than EG [36,37]. Secondly, TEG has a higher viscosity
(20 8C, 59 cPs) than EG (20 8C, 22.1 cPs) [38]. Thirdly, TEG has
weaker reducing power than EG [39], though it also has two
terminal hydroxyl groups similar to EG. Both a milder reduction
and a higher viscosity of TEG would slow down the nucleation rate
and the growth rate of Pd nanoparticles, and may be beneficial to
the larger sizes of Pd nanocubes and nanobars, whereas the
stronger affinity is beneficial to stabilizing the dispersion of Pd
nanoparticles.
Fig. 6. UV–vis absorption spectra of the palladium precursor in TEG with different
additives. (A) H2PdCl4; (B) H2PdCl4 + PVP; (C) H2PdCl4 + PVP + CTAC; (D)
H2PdCl4 + PVP + KCl; (E) H2PdCl4 + PVP + KBr; (F) H2PdCl4 + PVP + CTAB; (G)
H2PdCl4 + CTAB; (H) H2PdCl4 + KBr. All contain 0.3 mmol LÀ1 of H2PdCl4.

164
Y. Yu et al. / Materials Research Bulletin 45 (2010) 159–164
Science Foundation of South-Central University for Nationalities
(Grant No. YZZ05002).
References
[1] L. Au, Y. Chen, F. Zhou, P.H.C. Camargo, B. Lim, Z.-Y. Li, D.S. Ginger, Y. Xia, Nano Res.
1 (2008) 441–449.
[2] J.B. Jackson, N.J. Halas, J. Phys. Chem. B 105 (2001) 2743–2746.
[3] R. Narayanan, M.A. El-Sayed, Nano Lett. 4 (2004) 1343–1348.
[4] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chem. Rev. 105 (2005) 1025–1102.
[5] Y. Wang, Y. Wei, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nano-
technology, 5, American Scientific Publishers, California, USA, 2004, pp. 337–368.
[6] M.T. Reetz, E. Westermann, Angew. Chem. Int. Ed. 39 (2000) 165–168.
[7] S.W. Kim, M. Kim, W.Y. Lee, T. Hyeon, J. Am. Chem. Soc. 124 (2002) 7642–7643.
[8] S.U. Son, Y. Jang, J. Park, H.B. Na, H.M. Park, H.J. Yun, J. Lee, T. Hyeon, J. Am. Chem.
Soc. 126 (2004) 5026–5027.
[9] Y. Li, X.M. Hong, D.M. Collard, M.A. El-Sayed, Org. Lett. 2 (2000) 2385–2388.
[10] T. Ikawa, H. Sajiki, K. Hirota, Tetrahedron 61 (2005) 2217–2231.
[11] N. Semagina, E. Joannet, S. Parrva, E. Sulman, A. Renken, L. Kiwi-Minsker, Appl.
Catal. A: Gen. 280 (2) (2005) 141–147.
[12] M. Fernandez-Garcia, A. Martinez-Arias, L.N. Salamanca, J.M. Coronado, J.A.
Anderson, J.C. Conesa, J. Soria, J. Catal. 187 (1999) 474–486.
Fig. 7. UV–vis absorption spectra of H2PdCl4 in TEG with different amounts of KBr.
All contain 0.3 mmol LÀ1 of H2PdCl4.
4. Summary
[13] Y. Nishihata, J. Mizuki, T. Akao, H. Tanaka, M. Uenishi, M. Kimura, T. Okamoto, N.
Hamada, Nature 418 (2002) 164–167.
[14] P.K. Shen, C.W. Xu, Electrochem. Commun. 8 (2006) 184–188.
[15] C.W. Xu, H. Wang, P.K. Shen, S.P. Jiang, Adv. Mater. 19 (2007) 4256–4259.
[16] H. Wang, C.W. Xu, F.L. Cheng, S.P. Jiang, Electrochem. Commun. 9 (2007) 1212–
1216.
[17] M. Wang, D.J. Guo, H.L. Li, J. Solid State Chem. 178 (2005) 1996–2000.
[18] A. Anso´n, E. Lafuente, E. Urriolabeitia, R. Navarro, A.M. Benito, W.K. Maser, M.T.
Martı´nez, J. Phys. Chem. B 110 (2006) 6643–6648.
[19] Y. Sun, Z. Tao, J. Chen, T. Herricks, Y. Xia, J. Am. Chem. Soc. 126 (2004) 5940–5941.
[20] Y. Xiong, J. Chen, B. Wiley, Y. Xia, Y. Yin, Z.-Y. Li, Nano Lett. 5 (2005) 1237–1242.
[21] Y. Xiong, H. Cai, Y. Yin, Y. Xia, Chem. Phys. Lett. 440 (2007) 273–278.
[22] Y. Xiong, Y. Xia, Adv. Mater. 19 (2007) 3385–3391.
[23] Y. Wang, H. Liu, Y. Jiang, J. Chem. Soc. Chem. Commun. 24 (1989) 1878–1879.
[24] S. Komarneni, D. Li, B. Newalkar, H. Katsuki, A.S. Bhalla, Langmuir 18 (2002) 5959–
5962.
[25] D. Li, S. Komarneni, J. Am. Ceram. Soc. 89 (2006) 1510–1517.
[26] H. Choo, B. He, K.Y. Liew, H. Liu, J. Li, J. Mol. Catal. A: Chem. 244 (2006) 217–228.
[27] D. Li, S. Komarneni, J. Nanosci. Nanotechnol. 8 (2008) 3930–3935.
[28] Y. Xiong, H. Cai, B.J. Wiley, J. Wang, M.J. Kim, Y. Xia, J. Am. Chem. Soc. 129 (2007)
3665–3675.
[29] M.N. Nadagouda, V. Polshettiwar, R.S. Varma, J. Mater. Chem. 19 (2009) 2026–
2031.
[30] Q. Shen, Q. Min, J. Shi, L. Jiang, J.-R. Zhang, W. Hou, J.-J. Zhu, J. Phys. Chem. C 113
(2009) 1267–1273.
Under microwave irradiation, Pd nanocubes and nanobars with
a mean size of 23.8 nm were readily synthesized with H2PdCl4 as a
precursor, TEG as both a solvent and a reducing agent by means of
the cooperation of PVP and CTAB in 80 s. The effect of CTAB on the
shape of palladium nanoparticles was investigated by using KBr,
KCl or CTAC instead of CTAB in the reaction system. The results
showed that the BrÀ ions play a crucial role in the formation of
palladium nanocubes and nanobars. The addition of CTAB resulted
in a change of the composition of the precursor. The coordinated
ion PdBr42À was formed due to the coordination replacement of the
ligand ClÀ ions in PdCl4 ions by BrÀ ion after the addition of
2À
bromide, while the addition of CTAC or KCl did not bring about any
2À
change of ligand. The formation of PdBr4
reduced the redox
potential so that the rate of reduction as well as the nucleation and
growth of nanoparticles was reduced. Meanwhile, the absorption
of BrÀ ions on the surface of Pd nanoparticles promoted the
formation of {1 0 0} facets. In addition, a milder reducing power, a
higher viscosity and a stronger affinity of TEG were also beneficial
to the formation of larger Pd nanocubes and nanobars.
[31] T. Redjala, G. Apostolecu, P. Beaunier, M. Mostafavi, A. Etcheberry, D. Uzio, C.
Thomazeau, H. Remita, New J. Chem. 32 (2008) 1403–1408.
[32] Y. Xiong, J.M. McLellan, Y. Yin, Y. Xia, Angew. Chem. Int. Ed. 46 (2007) 790–794.
[33] S. Komarneni, R. Pidugu, Q.H. Li, R. Roy, J. Mater. Res. 10 (1995) 1687–1692.
[34] W. Tu, H. Liu, J. Mater. Chem. 10 (2000) 2207–2211.
Acknowledgements
[35] B. Veisz, Z. Kirly, Langmuir 19 (2003) 4817–4824.
[36] F. Vogtle, E. Weber, Angew. Chem. Int. Ed. 18 (1979) 753–776.
[37] C. Wu, Chemistry of Crown Ether, (in Chinese), Scientific Press, Beijing, 1992.
[38] G.O. Curme, F. Johnston, Glycols, Reinhold Publishing Co., New York, 1952.
[39] R.P. Bandwar, C.P. Rao, Carbohydr. Res. 277 (1995) 197–207.
This research was supported by the National Nature Science
Foundation of China (Grant No. 20673146), the Nature Science
Foundation of Hubei Province (Grant No. 2006ABA366) and the