Shape-controlled synthesis of surface-clean ultrathin palladium nanosheets by simply mixing a dinuclear PdI carbonyl chloride complex with H2O

Article abstract of DOI:10.1002/anie.201303772

Simple shape control: Ultrathin Pd nanosheets are readily fabricated by simply mixing [Pd₂(μ-CO)₂Cl₄]^2- with H₂O at ambient temperature. The as-prepared Pd nanosheets are surface clean and serve as an excellent platform to evaluate the effect of organic capping agents on the catalytic and electrocatalytic properties of Pd nanocrystals. Copyright

Full text of DOI:10.1002/anie.201303772

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Communications
Pd Nanosheets
Simple shape control: Ultrathin Pd
nanosheets are readily fabricated by
simply mixing [Pd2(m-CO)2Cl4]2À with H2O
at ambient temperature. The as-prepared
Pd nanosheets are surface clean and
serve as an excellent platform to evaluate
the effect of organic capping agents on
the catalytic and electrocatalytics proper-
ties of Pd nanocrystals.
H. Li, G. X. Chen, H. Y. Yang, X. L. Wang,
J. H. Liang, P. X. Liu, M. Chen,
N. F. Zheng*
&&&& — &&&&
Shape-Controlled Synthesis of Surface-
Clean Ultrathin Palladium Nanosheets by
Simply Mixing a Dinuclear PdI Carbonyl
Chloride Complex with H2O
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DOI: 10.1002/anie.201303772
Pd Nanosheets
Shape-Controlled Synthesis of Surface-Clean Ultrathin Palladium
Nanosheets by Simply Mixing a Dinuclear PdI Carbonyl Chloride
Complex with H2O**
Huan Li, Guangxu Chen, Huayan Yang, Xingli Wang, Jinghong Liang, Pengxin Liu, Mei Chen,
and Nanfeng Zheng*
During the past decades, shape control of noble metal (NM)
nanocrystals has been extensively demonstrated as an effec-
tive means to tailor their properties for a wide range of
applications such as catalysis, optics, spectroscopy, biological
labeling, and photothermal therapy.[1–9] A number of synthetic
strategies including photochemical,[10] electrochemical,[11] and
templating methods[9,12] have been developed to achieve
shape control of NM nanocrystals. Wet-chemical synthesis of
NM nanocrystals with well-defined shape usually requires
strict control over the kinetics and thermodynamics of the
systems.[2,3,13] Therefore, typical shape-controlled syntheses of
NM nanocrystals involve the use of additives, such as
surfactants, polymer capping agents, small adsorbates, and
even biomolecules.[13–20] Although much effort has been
devoted to investigating the relationship between the addi-
tives and the shape of the obtained NM nanocrystals, it is still
a great challenge to address how the additives control the
shape of NM nanocrystals at the molecular level.
To gain a better understanding of how the shape control of
NM nanocrystals is achieved chemically, our group has
recently focused on the use of small strong adsorbates (e.g.,
CO, amines) for the shape control of Pt and Pd nano-
crystals.[13,21–28] For example, CO molecules behave differently
in the controlled synthesis of Pd and Pt nanostructures. CO
prefers to adsorb on the Pd{111} surface to facilitate the
growth of ultrathin Pd nanosheets and tetrapod/tetrahedral
nanocrystals having {111} as the main exposure surface.[21,26]
But for Pt, the preferential adsorption of CO on Pt{100}
induces the formation of Pt nanocubes.[23,25] However,
together with small adsorbates, these reactions typically
involved the use of weakly binding polymeric capping
agents (e.g., polyvinylpyrrolidone (PVP)). The presence of
polymeric capping agents in the systems would likely raise the
following two issues: 1) The surface of the as-prepared NM
nanocrystals is coated by polymeric capping agents. Since the
catalytically active sites on the NM nanocrystals would be
blocked more or less by the capping agents, their influence on
the catalytic performance of NM nanocrystals should not be
simply ignored. 2) The co-presence of small adsorbates and
polymeric capping agents makes it impossible to discuss how
the shape control of NM nanocrystals is achieved without
considering the role of polymeric capping agents.
We demonstrate here a facile shape-controlled synthesis
of ultrathin Pd nanosheets by simply mixing a Pd carbonyl
complex, [Pd2(m-CO)2Cl4]2À, with H2O in the absence of any
organic capping agents. This Pd carbonyl complex bearing
two bridging CO ligands belongs to a very large family of
carbonyl complexes of noble metals.[29,30] The first dipalla-
dium(I) carbonyl complex, [Pd2(m-CO)2Cl4]2À, was discovered
in 1942,[31] followed by the spectroscopic and X-ray crystal-
lography study to confirm its structure.[32,33] Until now, most
studies have focused on its structure and reactivity;[30] the
possibility of fabricating Pd nanostructures with well-defined
shape from this kind of complex has not been attempted. In
this work, we have prepared [Pd2(m-CO)2Cl4]2À by treating
[H2PdCl4] in CO atmosphere, and characterized it by X-ray
single-crystal analysis and X-ray absorption fine structure
(XAFS) spectroscopy. From the 13C isotopic studies, we found
that [Pd2(m-CO)2Cl4]2À reacted with H2O to yield ultrathin Pd
nanosheets while releasing CO2. In the absence of organic
capping agents, the as-prepared Pd nanosheets have clean
Pd{111} as their main exposure surfaces, which provides
a great opportunity to study the influence of organic capping
agent on the (electro)catalytic performances of NM nano-
crystals.
In a typical experiment, 30 mL of 1m H2PdCl4 aqueous
solution was added to 10 mL anhydrous DMF. The mixture
was treated under 1 atm CO for 15 min, during which the
orange solution turned light yellow. After the CO atmosphere
was removed, 1 mL H2O was added to the reaction mixture
and within the first minute a dark blue color was clearly
observed (Figure 1a). After 15 min, the product was sepa-
rated and characterized. All operations were conducted at
ambient temperature and no organic capping agent was used.
[*] Dr. H. Li, G. X. Chen, H. Y. Yang, X. L. Wang, J. H. Liang, P. X. Liu,
M. Chen, Prof. N. F. Zheng
State Key Laboratory for Physical Chemistry of Solid Surfaces
Collaborative Innovation Center of Chemistry for Energy Materials
and Department of Chemistry
College of Chemistry and Chemical Engineering
Xiamen University, Xiamen 361005 (China)
E-mail: nfzheng@xmu.edu.cn
Homepage: http://chem.xmu.edu.cn/person/nfzheng/index.html
[**] We thank the MOST of China (2011CB932403, 2009CB930703) and
the NSFC (21131005, 21021061, 20925103, 20923004) for financial
support. We also thank the XAFS station (BL14W1) of the Shanghai
Synchrotron Radiation Facility (SSRF).
2À
As shown in Figure 1b, the brown–red PdCl4 precursor
displayed UV/Vis absorption peaks at 336 nm and 436 nm,
corresponding to d–d transition and ligand-to-metal charge
transfer. In contrast, the yellow intermediate showed two
discernible absorptions at 329 and 369 nm, indicating the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303772.
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to lie flat on the TEM grids, making it impossible to directly
measure their thickness. To detect the thickness of the Pd
nanosheets, carbon nanotubes (CNTs) were introduced to
dispersions of the Pd nanosheets in DMF to allow the
attachment of nanosheets on the outer surface of the nano-
tubes. As observed in Figure 2c, the Pd nanosheet has
a thickness of 1.8 nm, which corresponds to less than ten
atomic layers. According to our previous calculations using
the discrete dipole approximation (DDA) method, the ultra-
thin nature is the main reason why Pd nanosheets have a well-
defined surface plasmon resonance absorption feature.[21]
Ultrathin Pd nanosheets were nicely fabricated by simply
mixing the solution of the yellow Pd intermediate with H2O.
This interesting phenomenon motivated us to gain a deeper
understanding of the formation mechanism of the Pd nano-
sheets. Thus, we directed our efforts to resolving the structure
of the yellow intermediate. As shown in Figure 3a, the FTIR
spectrum of the yellow solution displays two bands at
1905 cmÀ1 and 1965 cmÀ1. A signal at d = 193.8 ppm in
13C NMR spectrum (Figure 3b) indicated the presence of
a bridging 13CO ligand in the yellow intermediate obtained by
Figure 1. Photographs (a) and UV/Vis spectra (b) of PdCl42À in DMF,
the yellow intermediate prepared by treating PdCl42À under CO, and
the Pd nanosheets. c) TEM image of the Pd nanosheets. The inset
shows the size distribution of Pd nanosheets.
change of the coordination environment of Pd. The dark blue
solution formed after the addition of H2O exhibited a broad
and strong absorption particular in the near-infrared region,
2À
distinct from the absorptions of PdCl4 and the yellow Pd
intermediate but similar to the absorption of our previously
reported metallic Pd nanosheets.[21]
The transmission electron microscopy (TEM) image in
Figure 1c shows that the blue solution indeed contained
hexagonal Pd nanosheets which had an average diameter of
23 nm, the long diagonal of the hexagon. The high-resolution
TEM (HRTEM) image of an individual Pd nanosheet shows
the lattice fringes with interplanar spacing of 0.24 nm (Fig-
ure 2a), corresponding to 1/3(422) fringes of face-centered
cubic (fcc) Pd.[21] The fcc structure of the metallic Pd
nanosheets was also supported by their X-ray diffraction
Figure 3. a) FTIR and b) 13C NMR spectra of the solution of the yellow
intermediate [Pd2(m-13CO)2Cl4]2À in DMF before (bottom) and after
(top) addition of H2O. c) The crystal structure of [Pd2(m-CO)2Cl4]2À
(counterion PPh4+ not shown). d) Curve fitting for the EXAFS spectrum
of the yellow Pd intermediate.
Figure 2. a) HRTEM image of a Pd nanosheet lying flat on the TEM
grid. The inset is the corresponding Fourier transform pattern. b) XRD
pattern of Pd nanosheets. c) TEM image of a single Pd nanosheet
attached on a carbon nanotube.
(XRD) pattern (Figure 2b).The presence of 1/3(422) reflec-
tions suggests the presence of (111) stacking faults in the Pd
nanosheets. These results are consistent with our previous
observations on Pd nanosheets made from PdII acetylaceto-
nate.[21] The upper and lower surfaces of the Pd nanosheets
are Pd(111) facets while their side walls are Pd(100). Due to
their ultrathin nature, the as-prepared Pd nanosheets tended
using isotopically labeled 13CO.[33] To determine its structure,
we introduced tetraphenylphosphonium chloride (PPh4Cl) to
crystallize the yellow intermediate, and yellow crystals
suitable for X-ray single crystal analysis were successfully
obtained (see the Supporting Information (SI) for details).
Single-crystal analysis indicated that the obtained yellow
crystals contain [Pd2(m-CO)2Cl4]2À clusters (Figure 3c and
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Figure S1 (SI)) co-crystallized with PPh4+ in the triclinic space
2051 cmÀ1 associated with stretching modes of the bridging
and linear-bonded CO on the Pd surface.[36] Taken together,
after the addition of H2O, one half of the bridging CO ligands
on [Pd2(m-CO)2Cl4]2À reduce PdI to Pd0, and the other half
help to protect Pd{111} and promote the formation of the
ultrathin Pd nanosheets rather than irregular nanoparticles.
The most important feature of the Pd nanosheets
prepared by the reaction of H2O with [Pd2(m-CO)2Cl4]2À is
probably that their surface is free of any organic capping. To
prove this, atomic force microscopy (AFM) was applied to
measure the thickness of the Pd nanosheets obtained from
[Pd2(m-CO)2Cl4]2À (Figure 4a,b). An average thickness of
1.8 nm was observed from AFM. This thickness is the same as
the result from TEM but significantly different from the value
acquired from the PVP-capped Pd nanosheet (3.9 nm),[21]
suggesting that the Pd nanosheets made from [Pd2(m-
CO)2Cl4]2À possess rather clean surfaces at least on their
upper and lower sides.
To reveal the true properties of NM nanocrystals, many
strategies like plasma cleaning,[37] oxidation–reduction,[38] and
acid[39] and UV–ozone treatments[40] have been used to
remove the capping agents on their surfaces. Hence, the
high-surface-area Pd nanosheets free of organic capping we
report here are desirable systems for evaluating the influence
of organic capping agents on the catalytic performances of
NM nanocrystals. The catalytic activity of the Pd nanosheets
was assessed for the hydrogenation of styrene by supporting
the nanosheets on carbon nanotubes (Pd/CNT) (see the
group P1.[34] Each [Pd2(m-CO)2Cl4]2À cluster has a Pd–Pd unit
ꢀ
bridged by two CO ligands. The two Pd atoms have
a separation of 2.6673(7) ꢀ, indicating the presence of
metal–metal bond. Besides two bridging CO ligands, each
Pd is coordinated with two chloro ligands. Therefore, the Pd
atoms in the [Pd2(m-CO)2Cl4]2À cluster have an oxidation state
of + 1. The IR absorption bands at 1905 and 1965 cmÀ1
(Figure 3a and Figure S2 (SI)) are assigned to asymmetric
and symmetric stretching modes of bridging CO ligands in
[Pd2(m-CO)2Cl4]2À [32,33]
.
Although a [Pd2(m-CO)2Cl4]2À salt crystallizes, the yellow
solution does not necessarily contain only this anion. To
confirm that [Pd2(m-CO)2Cl4]2À was the only product from the
2À
reaction of PdCl4
with CO, we conducted an X-ray
absorption study on this intermediate solution under trans-
mission mode at the Pd k-edge (24.35 KeV). The Fourier
transform of the k2 weighted extended X-ray absorption fine
structure (EXAFS) spectrum showed three dominant peaks
at roughly 1.4, 2.0, and 2.4 ꢀ, indicating three different
coordination shells of the Pd atoms (Figure 3d). Bond
information obtained from our single-crystal data was used
to fit the EXAFS curve, and satisfying physical parameters
from fitting were obtained (see the Supporting Information
for details). The coordination numbers of Pd–Cl, Pd–C, and
Pd–Pd from the fitting results are 2.2, 1.9, 0.9, respectively,
close to the theoretical values (Table S1 (SI)). The bond
lengths and Debye–Waller factors are all in reasonable
ranges. The presence of only [Pd2(m-CO)2Cl4]2À in the
yellow intermediate indicates that CO can only reduce
PdCl42À to PdI in DMF at room temperature.
Upon addition of H2O at room temperature, [Pd2(m-
CO)2Cl4]2À was converted into uniform metallic Pd nano-
sheets. Our inductively coupled plasma mass spectrometry
(ICP-MS) analysis on the product solution after removal of
metallic Pd nanosheets suggested a nearly complete reduction
of PdI to Pd0. Moreover, the observations from the following
two experiments indicated that free CO dissolved in DMF
was not necessary for the formation of Pd nanosheets:
1) Although N2 was bubbled through the yellow solution for
30 min to remove dissolved CO before introduction of H2O,
Pd nanosheets were still produced right after the addition of
H2O. 2) The addition of acidified H2O to the DMF solution of
single crystals of (PPh4)2[Pd2(m-CO)2Cl4] also led to the
formation of a dark blue colloidal solution of Pd nanosheets.
After the addition of H2O to the DMF solution of [Pd2(m-
13CO)2Cl4]2À, as revealed in 13C NMR spectrum (Figure 3b),
a new 13C peak at 124.7 ppm ascribed to 13CO2 was detected,
while the 13C peak at 193.8 ppm corresponding to the bridging
13CO ligands disappeared.[35] This result suggests that CO
ligand in [Pd2(m-CO)2Cl4]2À serves as the reductant to reduce
PdI while it is oxidized to CO2. The oxidation of one CO
molecule to CO2 provides two electrons to reduce both PdI
sites in one [Pd2(m-CO)2Cl4]2À unit to Pd0. As a result, while all
[Pd2(m-CO)2Cl4]2À clusters in the DMF solution are converted
into metallic Pd for the formation of Pd nanosheets, there is
still enough CO to bind and protect the surface of the Pd
nanosheets. The FTIR spectrum of the as-prepared Pd
nanosheets (Figure 3a) shows two bands at 1961 and
Figure 4. a) A representative AFM topographic image of the Pd nano-
sheets. b) Height profile along the dashed line in (a). c) Time profile
of the conversion of styrene to ethylbenzene catalyzed by Pd nano-
sheets on CNT (squares), PVP-capped Pd nanosheets after purification
(circles), PVP-capped Pd nanosheets (triangles). Styrene (8.66 mmol),
5 wt% Pd/CNT (5.6 mg), H2 (1 atm), 258C, 15 mL ethanol. d) Cyclic
voltammetry curve recorded in an aqueous solution containing 0.5m
H2SO4 and 0.25m HCOOH at a scan rate of 50 mVsÀ1 by using freshly
made Pd nanosheets and in the presence of different amounts of PVP.
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Supporting Information for details). For comparison, PVP-
capped Pd nanosheets with an average diameter of 23 nm
(same as that of Pd nanosheets reported here) were also
synthesized by the previously reported procedure (TEM in
Figure S3 (SI)). As shown in Figure 4c, Pd/CNT gave a con-
version of 76.2% during the first hour under the given
conditions, while the PVP-capped nanosheets gave only
a 19.5% conversion at the same period. Since the two batches
of Pd nanosheets have similar morphology, we ascribed this
difference in activity to the partial blocking of active sites by
PVP. Similar observations were made by other groups.[40]
After careful cleaning using a mixed solvent of ethanol and
acetone (see the Supporting Information for details), the
purified PVP-capped Pd nanosheets exhibited an increase in
catalytic activity, further indicating the negative effect of PVP
blocking in catalysis. Recycling the Pd/CNT for seven times
did not lead to a decrease in the catalytic activity (Figure S4
(SI)). After 10 cycles, the activity was only slightly decreased
to 92%.
Compared with liquid-phase catalysis, electrocatalysis is
even more sensitive to the surface cleanness of catalysts. For
PVP-capped NM nanoparticles, tedious cleaning steps are
typically required before they can serve as efficient catalysts.
Being free of organic capping agent, the as-synthesized
organic-free Pd nanosheets readily display excellent electro-
catalytic performances in the oxidation of formic acid without
any required pretreatment (see the Supporting Information
for details). As shown in Figure 4d, the surface-clean Pd
nanosheets gave a maximum current density of 1420 mAmgÀ1
at the peak potential of 0.04 V, 0.1 V more negative than the
value (0.14 V) acquired by PVP-capped Pd nanosheets.[21]
This indicated that the surface-clean Pd nanosheets are
better catalysts in HCOOH oxidation than the PVP-capped
ones. And as expected, when the Pd nanosheets were mixed
with PVP in PVP/Pd mass ratios of 1:4 and 1:2, the maximum
current densities were obtained back at a higher potential
(0.14 V) and decreased to 310 and 60 mAmgÀ1, respectively.
These results clearly suggest a deleterious effect of PVP on
the (electro)catalysis of Pd nanocrystals and demonstrate the
importance of developing synthetic methods to produce
surface-clean Pd nanocatalysts.
Keywords: electrocatalysis · hydrogenation · nanosheets ·
palladium
.
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In summary, we have demonstrated a facile synthetic
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Received: May 2, 2013
Published online: && &&, &&&&
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