Carbon monoxide-assisted synthesis of single-crystalline Pd tetrapod nanocrystals through hydride formation

Article abstract of DOI:10.1021/ja3006429

Carbon monoxide can adsorb specifically on Pd(111) to induce the formation of unique Pd nanostructures. In the copresence of CO and H₂, single-crystalline Pd tetrapod nanocrystals have now been successfully prepared. The Pd tetrapods are enclosed by (111) surfaces and are yielded through hydride formation. Density functional theory calculations revealed that the formation of PdH_x in the presence of H₂ reduces the binding energy of CO on Pd and thus helps to decrease the CO coverage during the synthesis, which is essential to the formation of the PdH_x tetrapod nanocrystals. In addition to tetrapod nanocrystals, tetrahedral nanocrystals were also produced in the copresence of CO and H₂ when the reaction temperature was ramped to further lower the CO coverage. Upon aging in air, the as-prepared PdH_x nanocrystals exhibited a shape-dependent hydrogen releasing behavior. The conversion rate of PdH_x tetrapod nanocrystals into metallic Pd was faster than that of tetrahedral nanocrystals.

Full text of DOI:10.1021/ja3006429

Article
pubs.acs.org/JACS
Carbon Monoxide-Assisted Synthesis of Single-Crystalline Pd
Tetrapod Nanocrystals through Hydride Formation
Yan Dai, Xiaoliang Mu, Yueming Tan, Kaiqiang Lin, Zhilin Yang, Nanfeng Zheng,* and Gang Fu*
†
†
†
†
‡
,†
,†
†State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005, China
‡
Department of Physics, Xiamen University, Xiamen 361005, China
*
S Supporting Information
ABSTRACT: Carbon monoxide can adsorb specifically on
Pd(111) to induce the formation of unique Pd nanostructures.
In the copresence of CO and H , single-crystalline Pd tetrapod
2
nanocrystals have now been successfully prepared. The Pd
tetrapods are enclosed by (111) surfaces and are yielded
through hydride formation. Density functional theory
calculations revealed that the formation of PdHx in the
presence of H reduces the binding energy of CO on Pd and
2
thus helps to decrease the CO coverage during the synthesis,
which is essential to the formation of the PdH tetrapod
x
nanocrystals. In addition to tetrapod nanocrystals, tetrahedral nanocrystals were also produced in the copresence of CO and H2
when the reaction temperature was ramped to further lower the CO coverage. Upon aging in air, the as-prepared PdHx
nanocrystals exhibited a shape-dependent hydrogen releasing behavior. The conversion rate of PdH tetrapod nanocrystals into
x
metallic Pd was faster than that of tetrahedral nanocrystals.
INTRODUCTION
The properties of noble metal nanocrystals are highly
determined by their size, shape, and surface structures.
contrast, the surface plasmon resonance (SPR) properties of
noble metal nanocrystals are more sensitive to their
■
20−24
1
−3
morphology rather than exposure surfaces.
Since the
specific adsorption of small adsorbates controls only the surface
structure of Pd and Pt nanocrystals, other factors beyond the
specific adsorption have been utilized to enrich the morphology
of noble metal nanocrystals. Two recent examples are (1) the
preferential growth of micrometer-long five-fold twinned Pd
nanowires over Pd nanocubes under slow reduction in the
The controlled synthesis of noble metal nanocrystals has thus
attracted increasing research attention, with the aim of tailoring
their properties for applications in devices, photonics, sensing,
1
−3
and catalysis.
strategies have been developed to control the morphology of
In the past decades, a number of synthetic
2
−5
noble metal nanocrystals.
For Pd and Pt nanocrystals, more
−
25
presence of I , and (2) the production of corolla-like Pd
recently, the strong and specific surface adsorption of small
3+
mesocrystals in the copresence of Fe as the oxidative etchant
adsorbates has emerged as a power strategy to control the
16
6
−15
and CO as the surface controller. In these two examples, the
control over the reduction kinetics and introducing oxidative
etching to disturb the growth of seeds enrich the morphology
of noble metal nanocrystals. However, the question remains
whether there are further avenues to enrich the shape of
nanocrystals in the presence of specific strong surface
adsorbate.
surface structure of their nanocrystals.
For instance, the
preferential adsorption of halides on Pd{100} promotes the
9,10
formation of Pd nanocubes enclosed by Pd{100} facets. The
coadsorption of CO and amines makes Pt{100} more
energetically stable than Pt{111} and thus induces the
1
1,13
formation of Pt nanocubes.
CO prefers to adsorb on
Pd{111} surface and facilitates the growth of ultrathin Pd
nanosheets having {111} as the main exposure surface and
Herein, we now demonstrate that, while CO specifically
adsorbs on Pd(111), the adsorption strength and coverage of
1
2,16
displaying unique optical properties.
The adsorption of
CO on Pd can be essentially varied by introducing H to
methylamine stabilizes the stepped sites and therefore assists
the formation of Pt octapods enclosed by {411} facets
2
facilitate the formation of single-crystalline Pd tetrapod and
tetrahedral nanocrystals enclosed by (111) surfaces. In the
1
4
exhibiting extraordinary electrocatalytic properties.
copresence of CO and H , the freshly prepared tetrapod and
Although, for face-centered cubic (fcc) metals, the
morphology and surface of their nanocrystals are closely
related issues, their properties depend differently upon the
2
tetrahedral nanocrystals were β-PdHx but not pure Pd
nanocrystals. Upon aging in air, the as-prepared β-PdHx
2
−4,17
morphology and surface structure of the nanocrystals.
For
example, as a surface property, catalysis of noble metal
Received: January 19, 2012
Published: April 3, 2012
2
,18,19
nanocrystals concerns more of their surface structure.
In
©
2012 American Chemical Society
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Journal of the American Chemical Society
Article
nanocrystals released hydrogen and were converted gradually to
Pd nanocrystals. As revealed by density functional theory
electrochemically active surface areas (ECSA) of catalysts in the
electrode, which was estimated from the integrated reduction charge of
2
surface palladium oxide by assuming a charge of 424 μC/cm for the
(
DFT) calculations, the formation of palladium hydride in the
2
6
reduction of palladium oxide monolayer. The Pd contents in the
electrode were quantitatively analyzed by inductively coupled plasma.
To test the stability of the catalysts in the electro-oxidation of formic
acid, the cyclic voltammograms were recorded at a sweep rate of 200
mV/s in 0.5 M H SO + 0.25 M formic acid, between −0.2 and 0.8 V
presence of H weakens the chemisorption of CO and therefore
2
helps to lower the CO coverage to induce the formation of Pd
tetrapod nanocrystals.
2
4
EXPERIMENTAL SECTION
Chemicals and Materials. Palladium(II) acetylacetonate [Pd-
(vs SCE) for 100 cycles.
■
(
(
Catalysis. Catalytic hydrogenation of styrene was carried out in a
well-stirred glass pressure vessel (75 mL) at 40 °C. Dispersed in 10
mL ethanol, 0.5 mg of the catalysts (i.e., Pd tetrapod nanocrystals and
Pd black) were mixed with 20 mmol (2.3 mL) styrene in the pressure
acac) ] (99%) was purchased from Alfa Aesar. Poly(vinylpyrrolidone)
2
PVP, MW = 58 000, AR), tetrabutylammonium bromide (TBAB),
and N,N-dimethylformamide (DMF) were purchased from Sinopharm
Chemical Reagent Co. Ltd. (Shanghai, China). Pd black was
purchased from Aldrich Chemical Co. All reagents were used as
received without further purification.
vessel. H flow was applied into the vessel for several minutes to
2
remove oxygen. The vessel was then pressurized by 2.0 bar H . The
2
reaction was allowed to proceed, and samples were withdrawn at
regular intervals, filtered, and analyzed by gas chromatography (GC)
and gas chromatography−mass spectrometry (GC−MS).
Synthesis of Pd Tetrapod Nanocrystals. In a typical synthesis
of Pd tetrapod nanocrystals, 10 mg of Pd (acac) , 90 mg of PVP were
2
Computational Details. Periodic DFT calculations were carried
dissolved in 10 mL of DMF. The resulting homogeneous solution was
transferred to a glass pressure vessel. After being charged with CO:H2
2
7,28
out with the Vienna ab initio simulation package (VASP).
The
electron exchange and correlation were treated within the generalized
gradient approximation using the Perdew, Burke, and Ernzerhof (PBE)
=
1:4 to 2 bar, the vessel was heated from room temperature to 100 °C
in 20 min and kept at this temperature for another 160 min with
stirring. The resulting products were collected by centrifugation and
washed several times with ethanol and acetone.
2
9
functional. The projector augmented plane wave (PAW) method
was used for describing electron−ion interactions, with an energy
3
0,31
cutoff of 400 eV.
The convergence criteria for the energy
Synthesis of Pd Tetrahedral Nanocrystals. In 10 mL of DMF,
calculation and structure optimization were set to a self-consistent field
1
0 mg of Pd (acac) and 90 mg of PVP were dissolved. The resulting
2
−
4
(
SCF) tolerance of 1.0 × 10 eV and a maximum Hellmann−
homogeneous solution was transferred to a glass pressure vessel. After
Feynman force tolerance of 0.01 eV/Å.
being charged with CO:H = 1:4 to 2 bar, the vessel was heated from
2
The CO-Pd(111) system has been extensively studied over the past
room temperature to 140 °C in 20 min and kept at this temperature
for another 160 min with stirring. The resulting products were
collected by centrifugation and washed several times with ethanol and
acetone.
2
0 years. Experimentally, a set of ordered CO adsorption patterns have
32,33
been determined.
In addition, CO adsorption on Pd surfaces was
3
4−36
32−34
also a subject of much theoretical work.
Three key modes,
such as, √3 × √3 R30° (0.33 ML), C(4 × 2)-2CO (0.50 ML) and
C(2 × 2)-3CO (0.75 ML), were considered in our calculations. The
Pd(111) surface was modeled by five-layer slabs with different supper-
lattice sizes, i.e., √3 × √3, C(4 × 2), and C(2 × 2). During structural
optimization, the bottom two layers were fixed at bulk truncated
position, while the top three layers and CO were fully relaxed. The k-
points sampling was generated following the Monkhorst−Pack
procedure with a 7 × 7 × 1 mesh for √3 × √3 R30° while 5 × 5
Synthesis of Pd Nanosheet Seeds. With DMF (10 mL) and
water (2 mL), 50.0 mg of Pd(acac) , 160.0 mg of PVP, and 161 mg of
2
TBAB were mixed together. The resulting homogeneous solution was
transferred to a glass pressure vessel. The vessel was then charged with
CO to 1 bar and heated at 70 °C for 4.0 h before it was cooled to
room temperature.
Seeded Growth of Pd Nanosheets. In 2 mL of DMF, a 2 mL
solution of the above nanosheet seeds were washed once and
dispersed. The seed solution was then mixed with an 8 mL DMF
×
1 mesh for C(4 × 2)-2CO and C(2 × 2)-3CO.
CO−H interaction on Pd(111) surface has also attracted much
solution containing 32 mg of Pd(acac) . The resulting homogeneous
2
37−40
attention.
The main conclusion is that CO coadsorption will
solution was transferred to a glass pressure vessel. After being charged
cause adsorbed H atoms desorbing as H or dissolving into Pd lattice.
with CO or H to 1 bar or CO/H (1:4) mixture to 2 bar, the vessel
2
2
2
Morkel et al. further pointed that for Pd nanoparticles, a hydride phase
is more easily formed, and dissolved hydrogen starts to desorb only
was heated at 60 °C for 2.0 h with stirring for the seeded growth.
Characterization. TEM (including high-resolution transmission
electron microscope, HRTEM) studies were performed on a TECNAI
F-30 high-resolution transmission electron microscope operating at
37
after a considerable amount of CO has been removed. All these
indicated that the β-PdH phase should be relatively stable under our
x
synthesis condition. According to the neutron diffraction experi-
3
00 kV. The samples were prepared by dropping ethanol dispersion of
4
1,42
ment,
H atoms locate on the octahedral position in the bulky Pd
samples onto 300-mesh carbon-coated copper grids and immediately
evaporating the solvent. The powder X-ray diffraction (XRD)
measurements were performed with an X’pert PRO instrument
PANalytical) using Cu Kα radiation (λ = 0.15418 nm). The
thicknesses of nanosheets were measured by supporting the
lattice, with a saturation concentrate of x ≈ 0.64. For simplicity, we
assume that all octahedral sites are occupied by H atoms, giving an
ideal monohydride PdH. The computational parameters for CO/
PdH(111) system are similar to those for CO/Pd(111). At the level of
PBE/PAW, our calculation showed that the lattice constant is
expanded by 4%, from 3.936 to 4.091 Å (bulk Pd and PdH,
respectively), consistent with experimental observations. The
adsorption energies for CO on Pd(111) and PdH(111) at different
(
nanosheets on carbon nanotubes.
Electrochemical Measurements. Ethanol dispersion of purified
nanoparticles was deposited on a glassy carbon electrode to obtain the
working electrodes after the solvent is dried by an IR lamp. A saturated
calomel electrode (SCE) and a platinum foil were used as the
reference and counter electrode, respectively. For the CO stripping
voltammetry measurements, CO gas (99.99%) was bubbled at an open
coverages are listed in Table 1. The average adsorption energy ΔE is
ads
defined as follows:
ΔEads = ΔEads/n
(1)
circuit for 15 min through 0.1 M H SO4 solution in which the
2
electrode immersed. The electrode was quickly moved to a fresh
solution, and the CO stripping voltammetry was recorded at a sweep
rate of 2 mV/s.
with
ΔEads = E(slab/n·CO) − E(slab) − n·E(CO)
For the electro-oxidation of formic acid, the cyclic voltammograms
were recorded at a sweep rate of 50 mV/s in 0.5 M H SO + 0.25 M
(2)
2
4
formic acid. Before cyclic voltammetry (CV) measurements, two
in which E(slab/CO) is the energy of the slab with CO adlayer;
E(slab) stands for the energy of the slab, and E(CO) the energy of CO
in the gas phase. And n is the number of adsorbed CO in one unit cell.
cycles of potential sweeps between −0.25 and 1.2 V at a sweep rate of
2
50 mV/s were applied. The current is normalized by the
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To elucidate the influences of palladium hydride formation on the
growth mechanism of Pd tetrapod nanocrystal, we further consider
following equilibrium:
surface + nadsCO(gas) ↔ [surface − nadsCO]
(3)
The related Gibbs free energy in one unit cell at a given temperature
4
3,44
and CO pressure is
ΔGads = G(surface − nadsCO, solid) − G(surface, solid)
nadsμ(CO, gas)
−
(4)
The solid Gibbs free energies and gas phase chemical potential can
be written as
G(solid) = H − RTln(Q vib)
0
(5)
(6)
μ(gas) = H − RTln(Q vib·Q rot·Q trans) + RT
0
where
H = E + E
0
el
ZPE
(7)
Here H is the enthalpy at 0 K, which is equal to total electronic
0
energy (E ) plus the zero point correction energy (EZPE), and Qvib,
el
Q , and Q
are the partition functions for vibrations, rotations, and
rot
trans
translations. Additionally, RT replaces the volume work of ideal gas.
For gas phase molecule, these data can be obtained by using
45
Gaussian03 code. In this case, PBE functional combined with
Figure 1. (a) Large-area and (b) enlarged TEM images of as-
synthesized Pd nanocrystals. (c) HRTEM image of an individual Pd
tetrapod nanocrystal oriented along the ⟨100⟩ direction (inset) and
(d) its corresponding SAED pattern.
2
9,46
Dunning’s correlation consistent basis set, cc-pVQZ, is employed.
Here, we neglect the internal vibrational contribution of solid and
assume that both EZPE and RT ln Qvib are the same whenever CO is in
gas phase or adsorbed on the surface. Accordingly, eq 4 can be
rewritten as
like {111} faces and also 12 regular triangular {111} faces. All
exposure surfaces for the tetrapods are {111} facets. A set of
tilted TEM images (Figure S2, Supporting Information) of the
tetrapod nanocrystals were taken to verify the proposed
ΔGads = ΔEads − n·(−RTlnQ rot(CO, gas) − RTlnQ trans
(CO, gas) + RT)
(8)
(9)
Finally, surface free energy can be obtained by
ϕ = ΔG /N
ads
surf
where Nsurf stands for the number of surface Pd atoms in the slab.
According to eq 9 and the Boltzmann distribution, the
concentration and types of CO adsorption patterns on the Pd(111)
and PdH(111) can be estimated.
RESULTS AND DISCUSSION
■
Structure of Pd Tetrapod Nanocrystals. The represen-
tative electron microscopic images of the as-made Pd
nanocrystals are illustrated in Figure 1 and Supporting
Information. It was clearly revealed that tetrapod Pd nano-
crystals were the dominant products. Both the selected area
electron diffraction (SAED) and HRTEM measurements on a
single tetrapod clearly indicate their single-crystalline nature.
The HRTEM image (Figure 1c) of the tetrapod nanocrystals
shows lattice fringes with an interplanar spacing of 0.195 nm,
corresponding to {200} planes of fcc Pd. The SAED pattern
(Figure 1d) of an individual tetrapod Pd nanocrystal (inset of
Figure 1c) contains spots from an fcc structure viewed down
the ⟨100⟩ zone axis. Based on the observed TEM images, we
proposed a geometric model for the as-made Pd tetrapod
nanocrystals.
In the proposed model (Figure 2a), four rhombohedra are
jointed together by sharing one tetrahedron at the center to
form the tetrapod. As illustrated in Figure 2b, each
rhombohedron is enclosed by six rhombus-like {111} faces.
Sharing one tetrahedron at the center drives the four
rhombohedra together to the concave tetrahedron structure.
Overall, an ideal tetrapod nanocrystals consists of 12 rhombus-
Figure 2. (a) An ideal 3D geometrical model of a tetrapod nanocrystal.
(b) Model of one rhombohedral arm of the tetrapod. (c,f) TEM
images, (d,g) SAED patterns, and (e,h) geometric models of an
individual Pd tetrapod nanocrystals oriented along the (c−e) [111]
and (f−h) [211] directions.
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structure model. For individual tetrapod nanocrystals oriented
along [111] or [211] zone axis, their TEM images and SAED
patterns also match well with the corresponding geometric
models (Figure 2c−h). The presence of {111} exposure surface
in the as-prepared tetrapod nanocrystals was also confirmed by
the CO stripping experiments in H SO solution. The main
strong surface confinement effect of CO, ultrathin nanosheets
are selectively produced because they have more growth active
sites to fight against the CO confinement and therefore are
more favored by growth kinetics. However, due to their
ultrathin feature, the exposed surface area of the nanosheets
increases linearly with volume. The Pd nanosheets are not
thermodynamically stable products. Why were Pd tetrapods
2
4
peak observed in the CO stripping curve of the Pd tetrapods
(
Figure S3, Supporting Information) is indeed attributed to the
yielded only when H was introduced together with CO? We
2
47
CO stripping on Pd(111) facet.
believe that such a H effect is induced by the modified
2
Based on the above structural dissection, one should be
aware that the tetrapod structure reported here significantly
differs from those previously reported. So far, the reported
tetrapod structures were mainly limited to II−VI semi-
conductors (e.g., CdSe, CdS, MnS, CdTe, ZnTe, ZnS,
chemisorption property of CO on Pd(111) in the presence of
H .
2
It is well-known that metallic Pd is catalytically active to split
H2 molecules to hydrogen atoms which can exclusively
53−55
permeate into Pd to form metal hydride.
Although our
4
8−51
ZnO).
A number of studies have revealed that these
TEM studies revealed that all detected tetrapods were metallic
Pd, one should not exclude the possibility that the observed Pd
nanostructures were converted from palladium hydrides,
particularly under the high-vacuum conditions. In fact, the
powder XRD measurements (Figure 4) on the freshly prepared
compositions can be used to fabricate tetrapod structure
because they have both cubic (zinc blend) and hexagonal
(
wurtzite) phases. While the cubic structure is important to
create the core having four active sites for growth of four arms,
the hexagonal structure is essential to the anisotropic growth of
each arm. In comparison with II−VI semiconductors, the
tetrapods reported here are single-crystalline nanocrystals made
of fcc Pd. Although single-crystalline Au tetrapod nanocrystals
5
2
were observed, the Pd tetrapod reported here represents a
new type of morphology having {111} as exposure surfaces.
Formation Mechanism of Pd Tetrapod Nanocrystals.
To better understand how the tetrapod Pd nanocrystals were
produced in our system, the intermediate Pd nanocrystals
yielded at different reaction times were investigated by TEM
(Figure 3). The Pd tetrapod produced in a 30 min reaction had
Figure 4. XRD patterns of Pd tetrapod nanocrystals: (a) freshly
prepared samples and after being aged in air for (b) 9, (c) 18, and (d)
6
3 min.
Pd nanocrystals confirmed that the as-prepared tetrapods were
56
β-PdH but not pure metallic Pd. However, after being
x
Figure 3. TEM images of the Pd tetrapod nanocrystals produced at
a) 30 min, (b) 1.0 h, and (c) 6.0 h.
purified and aged in ambient conditions for 1 h, the as-prepared
(
β-PdH nanocrystals were completely converted into Pd. The
x
hydride formation is thus considered as a key factor to
determine the formation of the tetrapod nanocrystals.
a side length (tip-to-tip distance) of ∼35 nm. These
nanocrystals continued to grow within 3 h. Their average side
length was increased to 51 nm at 1 h and 60 nm at 3 h (Figure
Reduction of the CO Coverage on Pd(111) by Hydride
Formation. In order to investigate how the hydride formation
influences the growth of the nanocrystals, we further carried out
periodic DFT calculations to compare the chemisorption of
CO on Pd(111) and PdH(111) in three different CO
adsorption patterns [i.e., √3 × √3 R30° (0.33 ML), C(4 ×
2)-2CO (0.50 ML), and C(2 × 2)-3CO (0.75 ML)] (Figure
5a−c for models). For all these structures, the average CO
adsorption energies are significantly reduced by 0.3−0.5 eV
(Table 1) when PdH is formed. This is reasonable as the bare
Pd(111) surface is partially saturated by the interstitial H atom,
thus weakening the interaction between Pd surface and CO.
The temperature effect has also been considered in our
calculations. As shown in Figure 5d,e, surface free energies
(φsurf) of CO covered Pd(111) and PdH(111) are both
increased, and the high coverage adsorption patterns will
gradually convert into low coverage one with the increase of the
temperature. Interestingly, the adsorption behavior of
PdH(111) surface is rather different from that of Pd(111),
especially at relatively high temperature. Above 330 K, C(4 ×
1
b). The tetrapod feature readily appeared at 30 min and
became more and more obvious with the size enlargement as
the reaction proceeded. The tetrapod feature of the nanocryst-
als became evident after 1 h. No further change on both the size
and the morphology of the nanocrystals was observed beyond 3
h. These results imply that the concave feature of the Pd
tetrapod is not produced by etching, although etching has been
demonstrated as an effective method to create branched noble
16
metal nanostructures.
In the synthesis of Pd tetrapods, the copresence of CO and
H is essential. No Pd tetrapods were obtained in absence of
2
either CO or H (Figure S4, Supporting Information). While
2
the absence of CO produced Pd nanoaggregates with ill-defined
structure, the absence of H yielded ultrathin Pd nanosheets. In
2
the presence of pure CO, as demonstrated in our previous
studies, the production of ultrathin Pd nanosheets having (111)
as the dominating exposed surface is mainly due to the strong
12
and preferential chemisorption of CO on Pd(111). Under the
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Journal of the American Chemical Society
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CO-protected PdH (111) surfaces makes it possible to yield the
more thermodynamically stable tetrapod structure with lower
surface-to-volume ratio than nanosheets.
Our FT-IR measurements (Figure S5, Supporting Informa-
tion) on the freshly prepared tetrapod nanocrystals revealed
that CO adsorbed on the nanocrystals displayed only a
−1
characteristic C−O stretching band near 1920 cm that can be
assigned to CO bound on fcc or hcp 3-fold hollow sites. In
contrast, in addition to the C−O stretching band at ∼1920
−1
cm , the Pd nanosheets obtained in the presence of pure CO
−1
displayed C−O stretching bands at ∼2083 cm . In surface
science, many studies have demonstrated that high coverage of
CO (above 0.6 ML) on Pd(111) would give rise to the
presence of linear (atop) CO having a C−O stretching band
−1
57−59
above 2000 cm .
The absence of C−O stretching band
corresponding to atop CO further confirms the relatively low
coverage of CO on the surface of tetrapod nanocrystals as
predicted by our DFT calculations.
To experimentally demonstrate that Pd atoms can directly
deposit on CO-capped Pd(111) in the presence of H , uniform
Figure 5. The three CO adsorption patterns on PdH(111): (a) √3 ×
2
√3 R30° (0.33 ML), (b) C(4 × 2)-2CO (0.50 ML), and (c) C(2 ×
1.8 nm-thick Pd nanosheets with an average edge size of 35 nm
were first prepared in pure CO and used as seeds for the further
growth studies (Figure S6, Supporting Information). Larger Pd
nanosheets (edge size 51 nm), without increasing their
thickness (1.8 nm), were readily prepared from small
nanosheets (edge size 35 nm) in CO, further confirming that
freshly reduced Pd atoms did not deposit on the Pd(111)
2
)-3CO (0.75 ML). Dark-cyan balls, Pd; white balls, H; red balls, O;
and gray balls, C. Surface free energies (φsurf) of (d) Pd(111)-CO and
e) PdH(111)-CO as a function of temperature for different CO
(
coverages (i.e., 0.33, 0.50, and 0.75 ML).
Table 1. Calculated C−O Distances (dC−O) and Average CO
Adsorption Energies (ΔE ) on Pd(111) and PdH(111) at
ads
highly covered by CO. In the copresence H and CO, however,
2
Different Coverages
the seeded growth of the small nanosheets produced
nanosheets with a thickness increased to ∼3.2 nm. The
increased thickness of the nanosheets was a result of the
deposition of Pd atoms on Pd(111) surface allowed by the
(
√3 × √3 R30°)- C(4 × 2)-2CO, C(2 × 2)-3CO,
CO, θ = 0.33 ML,
hollow hcp
θ = 0.50 ML,
θ = 0.75 ML,
facets
coverage
fcc + hcp
top + fcc + hcp
1
.188
1.182(f),
1.151(t),
1.183(f),
1.182(h)
presence of H . However, it should be noted that the formation
2
1
.181(h)
of tetrahedral or tetrapod nanocrystals was also observed in the
seeded growth of the small nanosheets in the copresence of H2
and CO. Both tetrapod and tetrahedral nanocrystals have {111}
as their exposure surfaces. As discussed above, the allowed
dC−O
a
(Å)
1.189
1.182(f),
1.151(t),
Pd(111)
a
1
.182(h)
1.183(f),
a
1
.182(h)
ΔEads
−1.89
−1.75
−1.85
1.179(f),
−1.44
−1.46
1.151(t),
1.180(f),
1.182(h)
deposition of Pd atoms on CO-capped Pd(111) was mainly due
a
a
a
(
eV)
−2.01
.180
to the lowered CO coverage by PdH formation. Following this
x
1
dC−O
Å)
mechanism, the production of pure Pd tetrahedra would be
expected when the CO coverage on Pd(111) could be further
reduced during the synthesis. Indeed, when the reaction
temperature was increased to 140 °C, we did obtain Pd
1
.180(h)
(
PdH(111)
ΔE
−1.50
−1.39
−1.14
ads
eV)
(
a
Data from ref 34.
tetrahedra in the copresence of CO and H (Figure 6).
2
Optical Properties of Pd Tetrapod Nanocrystals. In the
copresence of CO and H , the freshly prepared nanocrystals
2
)-2CO becomes more stable than C(2 × 2)-3CO on
2
were CO-capped PdH but not pure metallic Pd nanocrystals.
PdH(111) surface, while C(2 × 2)-3CO is still dominant
among the three adsorption structures on Pd(111) until 420 K.
According to the Boltzmann distribution, we can estimate the
relative concentration of different coverages at different
temperatures (Table S1, Supporting Information). At 100 °C,
our calculations demonstrated that on PdH(111), there exists
x
To investigate the hydrogen releasing from PdH tetrapods,
x
6
0
.5% [√3 × √3 R30°, 0.33 ML] + 62.0% [C(4 × 2)-2CO,
.50 ML] + 31.5% [C(2 × 2)-3CO, 0.75 ML]; while on
Pd(111), the relative ratios are 0.7%, 33.7%, and 65.6%,
respectively. The high CO coverage on Pd(111) makes it very
difficult to have freshly reduced Pd atoms deposited on (111)
surfaces and explains why ultrathin nanosheets were yielded in
pure CO. On the other hand, for PdH(111) surface, low CO
coverage (i.e., 0.33 and 0.50 ML) can also be energetically
competitive with those high coverages (i.e., 0.75 ML),
providing a chance for Pd atoms to be directly deposited on
PdH (111) surfaces. The allowed deposition of Pd atoms on
Figure 6. TEM images of the Pd tetrahedral nanocrystals synthesized
at 140 °C in the presence of 1:4 CO/H2.
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Journal of the American Chemical Society
Article
UV−vis NIR spectra of freshly prepared PdHx tetrapod
nanocrystals were monitored. As shown in Figure 7a, the
Shape-Dependent Conversion of PdH Nanocrystals.
x
It should be noted that the red-shift of SPR peaks after
hydrogen adsorption were also previously reported on
62
supported Pd nanodisks. Before 30 min, the intensity of the
SPR peak of the Pd tetrapods at 620 nm increases linearly,
indicating a continuous and sustained release of hydrogen from
the as-prepared PdH tetrapod nanocrystals within 30 min.
x
After 30 min, the hydrogen release is much slower and becomes
negligible beyond 60 min. As revealed by XRD measurements
(
Figure S7, Supporting Information), the hydrogen release
kinetics from PdHx nanocrystals is shape-dependent. The
conversion of hydride was characterized by the fraction of the
Pd(111) diffraction peak over the sum of those PdH (111) and
x
Pd(111). Upon aging in air, as shown in Figure 8a, the
Figure 7. (a) UV−vis NIR spectra of the as-prepared PdH tetrapod
x
nanocrystals upon aging in ethanol for different times. (b) The
calculated extinction, absorption, and scattering coefficients of Pd
tetrapod nanocrystals using the DDA method.
freshly prepared PdH tetrapods display a broad absorption
x
peak centered at ∼810 nm. Aging the ethanol solution of the
PdH tetrapods results in the appearance of an absorption peak
x
at ∼620 nm.
According to our discrete dipole approximation (DDA)
6
0
Figure 8. (a) XRD of Pd tetrahedral nanocrystals. (b) Conversion of
calculations, the peak at 620 nm can be attributed to surface
plasmon resonance (SPR) of metallic Pd tetrapods. In our
DDA calculations, the dielectric constant of palladium is
directly taken from the experimental data by P. B. Johnson and
PdH tetrahedron and tetrapod nanocrystals upon aging in air as
x
characterized by time-domain XRD measurements.
6
1
conversion rate of the fresh PdH tetrahedral nanocrystals
x
R. W. Christy. As illustrated in Figure 7b, the calculated
orientation-averaged extinction spectrum for Pd tetrapods
dispersed in ethanol medium (n = 1.36) shows a broad
extinction peak at ∼610 nm. It should also be noted that the
significant contribution to the extinction spectrum of Pd
tetrapods is caused by absorption, and the scattering coefficient
is negligibly small. It is well-known that the LSPR modes of Pd
nanospheres are typically located in the UV region. For
example, the SPR peak of a Pd nanosphere with a diameter of
prepared at 140 °C into pure Pd is much slower than the
tetrapods (Figure 8b). The much faster hydrogen releasing
kinetics from tetrapod nanocrystals than from tetrahedral
nanocrystals can be explained by the presence of more edge
and tip sites in the Pd tetrapods.
Electrocatalysis and Catalysis by the Pd Tetrapod
Nanocrystals. The electrocatalysis performances of as-
obtained tetrapod Pd nanocrystals were studied by the
oxidation of formic acid. The commercial Pd black catalyst
was used as the reference for comparison. The ECSA of the as-
6
0 nm is calculated at ∼340 nm. However, in a tetrapod
nanocrystal with a side length of 60 nm (tip-to-tip distance),
the symmetry breaking results in a red-shifted and broader
LSPR peak at ∼610 nm. From the experimental results and
theoretical analysis, it is predicted that the LSPR modes can be
controlled through the whole visible region, even in the near-
infrared region, if the shape of Pd nanoparticles are well
designed.
2
prepared Pd tetrapods was measured to be 15.9 m /g, which is
2
4
5.6% of the commercial Pd black catalyst (34.9 m /g) (Figure
S8, Supporting Information). The ECSA of Pd tetrapods is
2
60.7% of the theoretical maximum surface area (26.2 m /g)
calculated from an ideal tetrapod geometry, suggesting that the
as-prepared Pd tetrapods have a rather clean surface. Figure 9a
7
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Journal of the American Chemical Society
Article
nanocrystals in air. Through DFT calculations, the formation of
palladium hydride during synthesis was found essential to
manipulate CO coverage on Pd to obtain the tetrapod
nanocrystals. Together with the use of specific strong
adsorbates to control the surface structure of Pd nanocrystals,
the shape-control strategy reported here could provide a new
synthetic avenue to further enrich the shape of Pd nanocrystals.
ASSOCIATED CONTENT
■
*
S
Supporting Information
Additional characterization data. This material is available free
of charge via the Internet at http://pubs.acs.org.
Figure 9. (a) Comparison of electrocatalytic properties of the Pd
tetrapods and Pd black in the oxidation of formic acid. The CV curves
were recorded in an aqueous solution containing 0.5 M H SO and
AUTHOR INFORMATION
■
Corresponding Author
nfzheng@xmu.edu.cn; gfu@xmu.edu.cn
2
4
0.25 M HCOOH at a scan rate of 50 mV/s. (b) Comparison of
catalytic properties of the Pd tetrapods and Pd black in the
hydrogenation of styrene. Catalysis conditions: 0.5 mg of Pd catalyst
Notes
The authors declare no competing financial interest.
and 20 mmol styrene in 10 mL of ethanol under 2 bar H at 40 °C.
2
ACKNOWLEDGMENTS
We thank the MOST of China (2011CB932403,
009CB930703), the NSFC (21131005, 21021061,
0925103, 20973139, 21133004), the Fok Ying Tung
■
2
2
compares the CV curves for the electro-oxidations of formic
acid on both catalysts. For straightforward comparison of the
surface activity, the current densities were normalized to
ECSAs. The maximum current density by the Pd tetrapods is
Education Foundation (121011), and the Fundamental
Research Funds for the Central Universities for financial
support.
2
3
.84 mA/cm , twice of that by the Pd black catalyst (1.92 mA/
2
cm ). Moreover, the Pd tetrapods have a lower peak potential
at 0.140 V (vs SCE) than the Pd black (0.276 V vs SCE), which
can be attributed to the presence of mainly Pd(111) facets in
the tetrapod nanocrystals. The Pd tetrapod nanocrystals also
displayed better stability during the electrochemical measure-
ments (Figures S9, Supporting Information). After repetitive
REFERENCES
63
■
(
1) Sau, T. K.; Rogach, A. L.; Jackel, F.; Klar, T. A.; Feldmann, J. Adv.
Mater. 2010, 22, 1805.
(2) Tao, A. R.; Habas, S.; Yang, P. Small 2008, 4, 310.
1
00 CV scans between −0.2 and 0.8 V (vs SCE), the peak
(3) Xia, Y. N.; Xiong, Y. J.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int.
current density of the Pd tetrapods was reduced by 9%. In
comparison, a 26% drop in the electrocatalysis activity was
observed on the commercial Pd black.
Ed. 2009, 48, 60.
(4) Sau, T. K.; Rogach, A. L. Adv. Mater. 2010, 22, 1781.
(5) Peng, Z. M.; Yang, H. Nano Today 2009, 4, 143.
(6) Chen, M.; Wu, B.; Yang, J.; Zheng, N. Adv. Mater. 2012, 24, 862.
In addition to their excellent electrocatalysis in the oxidation
of formic acid, the as-prepared Pd tetrapods exhibit high
catalytic performance in the hydrogenation of styrene without
the need to do any pretreatment. As shown in Figure 9b, the Pd
tetrapods show a comparable catalytic activity to the
commercial Pd black catalyst containing the same weight of
Pd. A full conversion of styrene was achieved by both catalysts
in 105 min at 40 °C. But it is worth noting that although the Pd
black showed a slightly better performance in the first 30 min,
its catalytic activity was decreasing with the proceeding of the
reaction. But instead of a decrease, a slight increase of the
catalytic activity was even observed from the Pd tetrapod
nanocrystals during the catalysis. Considering the Pd tetrapods
have a smaller surface area than the Pd black, the TOF for the
surface atoms on the Pd tetrapods is essentially much higher
than that on the Pd black catalyst. Based on the conversion at
(
7) Habas, S. E.; Lee, H.; Radmilovic, V.; Somorjai, G. A.; Yang, P.
Nat. Mater. 2007, 6, 692.
8) Song, H.; Kim, F.; Connor, S.; Somorjai, G. A.; Yang, P. D. J.
Phys. Chem. B 2005, 109, 188.
9) Xiong, Y. J.; Cai, H. G.; Wiley, B. J.; Wang, J. G.; Kim, M. J.; Xia,
(
(
Y. N. J. Am. Chem. Soc. 2007, 129, 3665.
(10) Huang, X.; Zhang, H.; Guo, C.; Zhou, Z.; Zheng, N. Angew.
Chem., Int. Ed. 2009, 48, 4808.
(11) Wu, B.; Zheng, N.; Fu, G. Chem. Commun. 2011, 47, 1039.
(
12) Huang, X.; Tang, S.; Mu, X.; Dai, Y.; Chen, G.; Zhou, Z.; Ruan,
F.; Yang, Z.; Zheng, N. Nat. Nanotechnol. 2011, 6, 28.
13) Chen, G.; Tan, Y.; Wu, B.; Fu, G.; Zheng, N. Chem. Commun.
012, 48, 2758.
14) Huang, X.; Zhao, Z.; Fan, J.; Tan, Y.; Zheng, N. J. Am. Chem.
Soc. 2011, 133, 4718.
15) Huang, X.; Tang, S.; Zhang, H.; Zhou, Z.; Zheng, N. J. Am.
(
2
(
(
Chem. Soc. 2009, 131, 13916.
(16) Huang, X.; Tang, S.; Yang, J.; Tan, Y.; Zheng, N. J. Am. Chem.
9
0 min, the turnover frequencies of the surface atoms
(
calculated from ECSAs) on the Pd tetrapods and the Pd
Soc. 2011, 133, 15946.
(17) Zhou, Z. Y.; Tian, N.; Li, J. T.; Broadwell, I.; Sun, S. G. Chem.
Soc. Rev. 2011, 40, 4167.
−1
black were estimated to be 92 700 and 41 600 h , respectively.
CONCLUSION
(18) Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem., Int. Ed. 2005,
■
4
(
1
4, 7852.
19) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109,
2663.
In conclusion, single-crystalline tetrapod Pd nanocrystals with
uniform diameter were successfully prepared in the presence of
hydrogen and carbon monoxide mixture. The as-made Pd
tetrapod nanocrystals are bound with {111} facets. Studies
revealed that the freshly obtained tetrapod nanocrystals were β-
(20) Eustis, S.; El-Sayed, M. A. Chem. Soc. Rev. 2006, 35, 209.
(21) Xia, Y.; Halas, N. J. MRS Bull. 2005, 30, 338.
(22) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.;
PdH nanocrystals and gradually transformed into pure Pd
Van, D. R. P. Nat. Mater. 2008, 7, 442.
x
7
079
dx.doi.org/10.1021/ja3006429 | J. Am. Chem. Soc. 2012, 134, 7073−7080

Journal of the American Chemical Society
Article
(
23) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Acc.
(57) Kaichev, V. V.; Prosvirin, I. P.; Bukhtiyarov, V. I.; Unterhalt, H.;
Rupprechter, G.; Freund, H.-J. J. Phys. Chem. B 2003, 107, 3522.
(58) Unterhalt, H.; Rupprechter, G.; Freund, H.-J. J. Phys. Chem. B
2002, 106, 356.
(59) Szanyi, J.; Kuhn, W. K.; Goodman, D. W. J. Vac. Sci. Technol., A
1993, 11, 1969.
(60) Draine, B. T.; Flatau, P. J. J. Opt. Soc. Am. A 1994, 11, 1491.
(61) Johnson, P.; Christy, R. Phys. Rev. B. 1974, 9, 5056.
(62) Langhammer, C.; Zoric, I.; Kasemo, B.; Clemens, B. M. Nano
Lett. 2007, 7, 3122.
Chem. Res. 2008, 41, 1578.
(
(
(
(
(
(
24) Liz-Marzan, L. M. Langmuir 2006, 22, 32.
25) Huang, X.; Zheng, N. J. Am. Chem. Soc. 2009, 131, 4602.
26) Rand, D. A. J.; Woods, R. J. Electroanal. Chem. 1972, 35, 209.
27) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 48, 13115.
28) Kresse, G.; Furthmu
29) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77,
̈
ller, J. Phys. Rev. B 1996, 54, 11169.
3
(
(
(
865.
30) Blo
31) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758.
32) Tushaus, M.; Berndt, W.; Conrad, H.; Bradshaw, A. M.; Persson,
̈
chl, P. E. Phys. Rev. B 1994, 50, 17953.
(63) Baldauf, M.; Kolb, D. M. J. Phys. Chem. 1996, 100, 11375.
̈
B. Appl. Phys. A: Mater. Sci. Process. 1990, 51, 91.
33) Rose, M. K.; Mitsui, T.; Dunphy, J.; Borg, A.; Ogletree, D. F.;
Salmeron, M.; Sautet, P. Surf. Sci. 2002, 512, 48.
(
(
34) Loffreda, D.; Simon, D.; Sautet, P. Surf. Sci. 1999, 425, 68.
(
35) Sautet, P.; Rose, M. K.; Dunphy, J. C.; Behler, S.; Salmeron, M.
Surf. Sci. 2000, 453, 25.
36) Yudanov, I. V.; Sahnoun, R.; Neyman, K. M.; Ro
Hoffmann, J.; Schauermann, S.; Johanek, V.; Unterhalt, H.;
Rupprechter, G.; Libuda, J.; Freund, H.-J. J. Phys. Chem. B 2003,
07, 255.
37) Morkel, M.; Rupprechter, G.; Freund, H.-J. Surf. Sci. 2005, 588,
L209.
38) Morkel, M.; Rupprechter, G.; Freund, H.-J. J. Chem. Phys. 2003,
19, 10853.
39) Rupprechter, G.; Morkel, M.; Freund, H.-J.; Hirschl, R. Surf. Sci.
004, 554, 43.
40) Cerda, J. I.; Santos, B.; Herranz, T.; Puerta, J. M.; de la Figuera,
J.; McCarty, K. F. J. Phys. Chem. Lett. 2011, 3, 87.
41) Worsham, J. E., Jr.; Wilkinson, M. K.; Shull, C. G. J. Phys. Chem.
Solids 1957, 3, 303.
42) Drexel, W.; Murani, A.; Tocchetti, D.; Kley, W.; Sosnowska, I.;
Ross, D. K. J. Phys. Chem. Solids 1976, 37, 1135.
43) Anez, R.; Sierraalta, A.; Martorell, G.; Sautet, P. Surf. Sci. 2009,
03, 2526.
(
̈
sch, N.;
́
1
(
(
1
(
2
(
́
(
(
(
̃
6
(
44) Hofmann, A.; Sauer, J. J. Phys. Chem. B 2004, 108, 14652.
(
45) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J.
B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J.
J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision
D.01; Gaussian, Inc.: Wallingford, CT, 2004.
(
(
46) Woon, D. E.; Dunning, J. T. H. J. Chem. Phys. 1995, 103, 4572.
47) Hara, M.; Linke, U.; Wandlowski, T. Electrochim. Acta 2007, 52,
5
(
1
733.
48) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000,
22, 12700.
(
49) Peng, X. Adv. Mater. 2003, 15, 459.
(50) Wang, D.; Lieber, C. M. Nat. Mater. 2003, 2, 355.
(51) Newton, M. C.; Warburton, P. A. Mater. Today 2007, 10, 50.
(52) Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J.
Am. Chem. Soc. 2003, 125, 16186.
(
(
(
(
53) Palczewska, W. Adv. Catal. 1975, 24, 245.
54) Goltsova, M. V. Int. J. Hydrogen Energy 2006, 31, 223.
55) Jewell, L. L.; Davis, B. H. Appl. Catal., A 2006, 310, 1.
56) Schirber, J. E.; Morosin, B. Phys. Rev. B 1975, 12, 117.
7
080
dx.doi.org/10.1021/ja3006429 | J. Am. Chem. Soc. 2012, 134, 7073−7080