Etching growth under surface confinement: An effective strategy to prepare mesocrystalline Pd nanocorolla

Article abstract of DOI:10.1021/ja207788h

An etching growth strategy was developed to prepare corolla-like Pd mesocrystals consisting of unidirectionally aligned, well-spaced, and connected ultrathin (1.8-nm-thick) Pd nanosheets. The combined use of CO and Fe ³⁺ is critical to the successful synthesis of the branched corolla-like Pd mesocrystals. While CO functions as the surface-confining agent to allow anisotropic growth of the 1.8-nm-thick Pd nanosheets as branches, Fe³⁺ etches the Pd seeds at the early stage of the reaction to induce formation of the branched structure. Inheriting the unique properties of 1.8-nm-thick Pd nanosheets, the as-prepared Pd mesocrystals display well-defined surface plasmon resonance absorption in the near-infrared region, a high electrochemically active surface area, and a significant photothermal effect when irradiated with a near-infrared laser. Owing to the presence of internal voids and increased apparent thickness, the Pd mesocrystals also exhibit several features superior to those of single-domain Pd nanosheets, making them promising for electrocatalysis and cancer photothermal therapy applications.

Full text of DOI:10.1021/ja207788h

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pubs.acs.org/JACS
Etching Growth under Surface Confinement: An Effective Strategy To
Prepare Mesocrystalline Pd Nanocorolla
Xiaoqing Huang, Shaoheng Tang, Jing Yang, Yueming Tan, and Nanfeng Zheng*
State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005, China
S Supporting Information
b
ultrathin (1.8-nm-thick) Pd nanosheets. In the synthesis of CPdMs,
CO was applied as the surface-confining agent to ensure the
ABSTRACT: An etching growth strategy was developed to
prepare corolla-like Pd mesocrystals consisting of unidir-
ectionally aligned, well-spaced, and connected ultrathin
growth of ultrathin sheet-like branches having (111) as the main
3+
exposed surface, and a trace amount of Fe was used to etch the
Pd seeds and induce the formation of branched structure. The
as-prepared Pd mesocrystals nicely inherit the unique proper-
ties of 1.8-nm-thick Pd nanosheets: (1) a well-defined surface
plasmon resonance (SPR) absorption feature in the near-infrared
(NIR) region, (2) a high electrochemically active surface area
(
1.8-nm-thick) Pd nanosheets. The combined use of CO
3+
and Fe is critical to the successful synthesis of the
branched corolla-like Pd mesocrystals. While CO functions
as the surface-confining agent to allow anisotropic growth of
3+
the 1.8-nm-thick Pd nanosheets as branches, Fe etches the
Pd seeds at the early stage of the reaction to induce
formation of the branched structure. Inheriting the unique
properties of 1.8-nm-thick Pd nanosheets, the as-prepared
Pd mesocrystals display well-defined surface plasmon reso-
nance absorption in the near-infrared region, a high electro-
chemically active surface area, and a significant photo-
thermal effect when irradiated with a near-infrared laser.
Owing to the presence of internal voids and increased
apparent thickness, the Pd mesocrystals also exhibit several
features superior to those of single-domain Pd nanosheets,
making them promising for electrocatalysis and cancer
photothermal therapy applications.
2
(EASA) (67 m /g), and (3) a significant photothermal effect in
24
response to a NIR laser. More importantly, CPdMs offer the
following three advantages over single-domain 1.8-nm-thick Pd
nanosheets with a similar dynamic size in solution: (1) mass
transfer in electrodes made of CPdMs is significantly enhanced
by the branched structure, (2) CPdMs are more easily taken up by
cancer cells, resulting in much-enhanced NIR photothermal therapy
efficacy, and (3) CPdMs are more easily separated from solution by
centrifugation owing to their increased apparent thickness.
To prepare CPdMs, Pd(acac) , poly(vinylpyrrolidone), and a
2
small amount of FeCl were mixed together with benzyl alcohol
3
(see Supporting Information for details). The resulting homo-
geneous solution was transferred to a glass pressure vessel. After
being charged with CO to 1 bar, the vessel was heated from room
temperature to 100 °C in 30 min and kept at this temperature
for 1.5 h with stirring. As the reaction progressed, the mixture
experienced a significant color change from yellow to light
blue, and finally deep blue. The resulting blue colloidal products
were collected by centrifugation and washed several times with
ethanol and acetone. The production yield of CPdMs was
esocrystals, single-crystal-like inorganic superstructures
made of highly aligned crystalline subunits, represent an
M
emerging new class of colloidal crystals that have unique catalytic,
1
ꢀ5
magnetic, electronic, and optical properties.
These highly
ordered superstructures serve as structural bridges between
1,2
nanoscale matter and macroscale materials. With the efforts
∼
90%, which was measured by inductively coupled plasma
spectrometry (ICP).
of a large number of research groups, mesocrystals have been
6
ꢀ8
prepared in a wide range of compositions, such as CaCO ,
3
9
5,10
11ꢀ13
14 15
2 5 3 4 2
16
Representative electron microscopy images of the as-made
CPdMs are illustrated in Figure 1. The images show that all
particles surveyed were corolla-like with multibranched subunits,
demonstrating high-quality production of CPdMs. The size of
CPdMs ranged narrowly from 170 to 200 nm, with an average
diameter of 185 nm (Figure S1). The number of branches on
each CPdM ranged up to 30, with an average of 22. The corollar
feature of CPdMs was further confirmed by high angle annular
dark field-scanning transmission electron microscopy (HAADF-
STEM) analysis (Figures 1c, S2). Both selected area electron
diffraction (SAED) and high-resolution transmission electron
microscopy (HRTEM) measurements (Figure 1d,e) on indivi-
dual CPdMs indicated the single-crystalline nature of these
BaCO , ZnO, TiO2,
V O , NiO, Ca (PO ) , and
CaMoO4. To date, most of the reported mesocrystals are
3
1
7
oxides or carbonates. Reports on the synthesis of metallic
18,19
mesocrystals are rare.
However, owing to the presence of
internal voids and therefore high internal surface area, metallic
mesocrystals are expected to have some unique properties or
advantages that cannot be possessed by single-domain metallic
nanocrystals. In recent years, nanostructures of palladium, a key
catalytic element, have attracted considerable interest among
researchers. Various Pd nanostructures with well-defined mor-
23
2
0
21,22
phologies, such as cubes, polyhedra,
concave polydedra,
27
2
4,25
26,27
plates,
rods/wires,
and branched nanocrystals, have
been successfully prepared. However, it remains a great challenge
to find a method to fabricate mesocrystalline Pd nanostructures.
We now report here an etching growth strategy to prepare
corolla-like Pd mesocrystals (CPdMs) consisting of connected
Received: August 17, 2011
Published: September 15, 2011
r 2011 American Chemical Society
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|J. Am. Chem. Soc. 2011, 133, 15946–15949

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Figure 2. TEM images of Pd nanocrystals prepared under reaction
conditions similar to those in Figure 1, except that the reactions were
conducted in the absence of FeCl (a) and in the absence of CO (b).
3
the importance of CO in forming sheet-like structure in CPdMs.
It should be noted that, in the literature, branched Pd nanocrystals
27
2+
were also prepared when Cu was incoporated in the synthesis.
While the reaction with CO but without FeCl gave a blue
3
colloidal solution within 20 min, the reaction mixture containing
FeCl remained yellow, and no separable particles were obtained
3
from the reaction until 20 min. This phenomenon indicates that
FeCl greatly retarded the reduction of Pd(acac) . It is believed
Figure 1. (a) Representative low-magnification TEM, (b) high-magni-
fication TEM, and (c) HAADF-STEM images of as-prepared CPdMs.
3
2
that the retarded reduction is due to the oxidative etching effect
(d) SAED pattern of a single CPdM. Bottom-left inset shows the cor-
3
+
25,30
2+
induced by Fe , which has been previously demonstrated
responding TEM image. (e) HRTEM image on a branch/corolla of
the CPdM.
3
+
and can be explained by the higher redox potential of Fe /Fe
ꢀ
31
than Pd(acac) /Pd, acac . While CO facilitates the growth
2
3+
of uniform Pd nanosheets, Fe etches the Pd nanosheets that
highly branched Pd nanostructures. The SAED pattern showed a
series of diffraction spots with six-fold rotational symmetry. The
HRTEM image showed lattice fringes with interplanar spacing of
are formed. In our synthesis, CO was supplied continuously, but
only a limited amount of FeCl was supplied. As the reaction
3
3+
3
proceeded, Fe species were continuously consumed. Once Fe
+
0
.24 nm, corresponding to 1/3(422) fringes of face-centered
was depleted, Pd nanostructures were expected to grow to form
separable CPdMs. Indeed, after 20-min reaction time, the reac-
tion mixtures started to become blue, implying that a dramatic
amount of Pd(II) got reduced to metallic Pd.
cubic (fcc) Pd (Figure 1e). The fcc structure of the Pd nano-
sheets is supported by their X-ray diffraction pattern (Figure S3).
The observation of fcc Pd 1/3(422) fringes indicated that the
24,28,29
palladium nanosheet branches have (111) as basal planes.
To better understand the formation process of CPdMs, the
intermediate nanocrystals produced at different reaction times
were investigated by TEM. As illustrated in Figure 3, as a result of
the reductive-oxidative etching cycles caused by the co-presence
As verified by electrochemical CO stripping experiments in
H SO , the exposed surface of the as-prepared CPdMs was
2
4
indeed dominated by (111) (Figure S4).
3+
Because of their 2D structure, CPdMs tend to lie flat on the
TEM grids, making it difficult to directly analyze their side
structure. By mixing CPdMs with carbon nanotubes, fortunately,
some CPdMs can attach on the carbon nanotubes, enabling us to
obtain the side structure of CPdMs. As clearly illustrated in the
TEM image (Figure S5), the side of each CPdM consists of
parallel-aligned Pd nanosheets with uniform thickness, narrowly
distributed at 1.8 nm. As a whole, the thickness of each CPdM
was measured to be more than 10 nm. Based on our previous
work, CO binds strongly on the Pd (111) surface and therefore
confines the growth of the Pd nanostructures to maximize (111)
of CO and Fe , the products collected at 25 min were heavily
etched at their edges. The average size of the obtained particles
was 67 nm. These nanocrystals continued to grow for 60 min.
The average size of the particles increased to 142 nm at 30 min, to
168 nm at 45 min, and to 185 nm at 60 min. During the growth of
the particles, their branched feature also became more and more
prominent with increasing size of each branch. Beyond 1 h, no
further change in the size or the morphology of the obtained
nanocrystals was observed (Figure S9). These results imply that
3+
the etched sites resulting from Fe oxdiation at the early stage of
the reaction served as the growth sites for forming the sheet-like
branches.
2
4
surface exposure. CO is thus considered essential to the
anisotropic growth of sheet-like morphology of each branch of
Thus, the formation of CPdMs reported here undergoes a
CPdMs. In the presence of CO but the absence of FeCl , the
mechanism of etching growth under surface confinement. CO
and Fe are two key components in controlling the final
3+
3
3+
obtained sheet-like Pd nanostructures did not have the branched
feature (Figure 2a), nor was branched structure obtained when
FeCl was replaced by CTAC or ZnCl (Figure S6). Thus, it is
morphology of the CPdMs. Fe is involved in the oxidative
etching process that modifies the laterals of the Pd nanostruc-
tures at the early stage and provides active sites for further
growth. CO is a decisive factor that confines the growth of the
nanosheet branches of CPdMs throughout the growth process.
The strong binding of CO on Pd (111) ensures the continuous
growth of 2D Pd nanosheets at the etched sites along the original
3
2
3+
reasonable to propose that the presence of Fe is the main cause
of forming the branched structure in CPdMs. To support such a
proposal, a reaction was carried out with Fe but without CO. As
a result, uniform dendritite Pd nanostructures with non-sheet-
like branches were formed (Figures 2b, S7, S8), again confirming
3+
1
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Journal of the American Chemical Society
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Figure 4. (a) Absorption spectra of the as-prepared CPdMs and Pd
nanosheets. (b) Photothermal effect of the CPdMs. The temperature vs
time plots were recorded from the CPdMs with various concentrations
upon irradiation by a 1-W laser. (c) Viability of healthy liver cells
incubated for 48 h with different concentrations of the CPdMs. (d,e)
Micrographs of cancer cells after 12-h incubation with CPdMs and
Figure 3. TEM images of CPdMs produced in (a) 25, (b) 30, (c) 45,
(d) 60, and (e) 90 min reactions. (f) TEM image of CPdMs obtained
under 0.05 mL of FeCl (13.5 mg/mL).
3
Pd nanosheets, respectively, and also after 2-min-irradiation by a 2-W,
2
8
08-nm laser. The power density is 1.4 W/cm . (f) Pd uptake amount of
CPdMs and Pd nanosheets as measured by ICP.
growth direction, making it possible to form single-crystalline-
like Pd nanocorolla.
2
On the basis of the above growth mechanism, it is expected
that the number of branches of CPdMs should be controllable by
67.2 m /g, close to the optimized number obtained on individual
24
Pd nanosheets. Second, similar to individual Pd nanosheets, the
CPdMs dispersed in water display a SPR peak in the NIR region
(Figure 4a), although the peak is slightly blue-shifted. Resulting
from the photothermal effect of CPdMs induced by the NIR SPR
absorption, the temperature of a 1-mL solution containing
30 ppm Pd rose from 26.6 to 50.4 °C after 10-min irradiation
by an 808-nm laser at 1 W (Figure 4b). Such a photothermal
3
+
the amount of Fe . Such an expectation was indeed confirmed
experimentally. As shown in Figure 3f, while maintaining the
3+
other reaction conditions the same, reducing the Fe concen-
tration from 0.50 mM to 0.25 mM decreased the average number
of branches/corollas in the Pd mesocrystals from 22 to 10. Together
with the reduced number of branches, the size of the CPdMs was
also reduced from 185 to 105 nm (Figure S10). The decreased
CPdMs size can be explained by the fact that having less etchants
24
effect is comparable to that of Pd nanosheets. Third, the
CPdMs also exhibit excellent biocompatibility. The viable cell
count for healthy liver cells was reduced by only 10% after 48-h
exposure to a 450 μg/mL solution of the CPdMs (Figure 4c).
Besides inheriting the unique properties of ultrathin Pd
nanosheets, the CPdMs also possess properties that do not
appear in the single-domain ultrathin Pd nanosheets. For exam-
ple, the CPdMs are more easily separated by centrifugation than
unconnected Pd nanosheets with similar dynamic size (∼190 nm)
(Figures S11, S12). Furthermore, when deposited on a flat
electrode, the CPdMs created a highly porous architecture to give
3
+
(
Fe ) in the reaction produced more nuclei at the early stage.
Therefore, to some extent, the mesocrystalline Pd nanocorolla
can be manufactured with desired size and number of branches
3+
by controlling the reaction duration and the Fe concentration.
Two important structural features are associated with the
CPdMs: (1) they consist of ultrathin Pd nanosheets, and (2) the
Pd nanosheets in each particle are unidirectionally aligned and
well spaced but connected to give intraparticle void and increased
apparent thickness as compared to individual Pd nanosheets.
As a result of the ultrathin nature of the building components,
the CPdMs inherit several unique properties of ultrathin Pd
nanosheets. First, the EASA of the CPdMs was measured to be
2
a specific EASA constant of ∼67 m /g that does not heavily
depend ontheamountdeposited (FigureS12). Incomparsion, the
specific EASA of Pd nanosheets deposited on a flat electrode can
1
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Journal of the American Chemical Society
COMMUNICATION
only be optimized when a small amount of nanosheets is depos-
ited. Due to the possible unfavorable mass tranfer caused by close
stacking of the Pd nanosheets, increasing the deposited amout of
Pd nanosheets tends to decrease the specific EASA (Figure S13).
As discussed above, the CPdMs display strong NIR SPR
absorption and therefore a significant NIR photothermal effect,
making them promising for photothermal tumor therapy. After
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2-h incubation with CPdMs (20 μg/mL Pd), ∼100% of liver
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(
2
laser(1.4 W/cm ) (Figure 4d). In comparison, the same irradiation
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(
2
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CO and Fe was found critical to the synthesis of the branched
CPdMs. The as-prepared Pd mesocrystals not only nicely inherit
the unique properties of 1.8-nm-thick Pd nanosheets but also
display some outstanding properties that cannot be achieved
with single-domain Pd nanosheets, making them promising for
electrocatalysis and photothermal therapy applications.
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’
ASSOCIATED CONTENT
2
009, 131, 9114.
S
Supporting Information. Experimental details and addi-
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tional characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
(
(
’
AUTHOR INFORMATION
(
Corresponding Author
nfzheng@xmu.edu.cn
system:
Fe3 þ eꢀ f Fe
þ
2þ
;
E ¼ 0:771V
0
’
ACKNOWLEDGMENT
PdðacacÞ þ 2eꢀ f Pd þ 2acacꢀ; E ¼ 0:149V
0
2
We thank the NSFC (21131005, 21021061, 20925103, 20871100),
the Fok Ying Tung Education Foundation (121011), the MOST
of China (2011CB932403, 2009CB930703), the NSF of Fujian
Province (Distinguished Young Investigator Grant 2009J06005),
and the Key Scientific Project of Fujian Province (2009HZ0002-1)
for the financial support.
3
+
So, Fe can oxidize a Pd atom back to Pd(acac) .
(
4
2
32) Tang, S. H.; Huang, X. Q.; Zheng, N. F. Chem. Commun. 2011,
7, 3948.
’
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