Controlled formation of concave tetrahedral/trigonal bipyramidal palladium nanocrystals

Article abstract of DOI:10.1021/ja9059409

(Figure Presented) Novel concave Pd nanocrystals with uniform diameter were successfully prepared in the presence of formaldehyde. While the outer surfaces of the as-prepared concave Pd nanocrystals are {111}, the faces concave toward the polyhedral cente

Full text of DOI:10.1021/ja9059409

Published on Web 09/10/2009
Controlled Formation of Concave Tetrahedral/Trigonal Bipyramidal Palladium
Nanocrystals
Xiaoqing Huang, Shaoheng Tang, Huihui Zhang, Zhiyou Zhou, 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
Received July 16, 2009; E-mail: nfzheng@xmu.edu.cn
In the past decade, the controlled synthesis of noble metallic
nanocrystals with uniform geometries has attracted enormous
interest, because it could provide an effective route for tuning the
excavated tetrahedral nanocrystal consists of four {111} faces
shaped in a hollow equilateral triangle and 12 isosceles triangle-
like {110} faces concave toward the center of the tetrahedron. The
outer and inner side lengths of the hollow equilateral triangle face
determine the degree of concavity of the nanocrystals and therefore
the fraction of {110} facets in the as-prepared concave tetrahedra
(Figure S2, Table S1). While the concave tetrahedra are single-
1
-3
catalytic, optical, electronic, or magnetic properties of metals.
Noble metal nanocrystals with a large variety of shapes have been
synthesized by different techniques, such as template-directed
4,5
6,7
8-12
fabrication, electrochemical, or chemical synthesis.
How-
ever, the majority of metal nanocrystals that have been prepared
crystalline, the concave trigonal bipyramids are single twinned with
1
3,14
15
16
22,23
to date are spheres, cubes,
rods/wires, limited to the flat or convex shapes. Reports on
the synthesis of concave metal nanocrystals are rare.
tetrahedra, octahedra, and
a stacking fault along the {111} plane (Figure S3).
The presence
1
7-19
of both {111} and {110} facets in the as-prepared concave
nanocrystals was confirmed by the CO stripping experiments in
2
0
We report here a facile synthetic strategy to prepare concave
polyhedral Pd nanocrystals. The as-prepared Pd nanocrystals are
enclosed by {111} and reactive {110} facets. The electrocatalytic
activity of the concave nanocrystals in formic acid oxidation exhibits
dependency on the percentage of {110} facets which are control-
lable by varying reaction conditions.
both H
2 4 4
SO and HClO solutions. The peaks observed in both CO
stripping curves (Figure S4) are well attributed to the CO stripping
on Pd (110) and (111) facets.
2
4
2
In a typical synthesis of concave Pd nanocrystals, Pd(acac) , PVP,
and formaldehyde solution were mixed together with benzyl alcohol.
The resulting homogeneous solution was transferred to a Teflon-
lined stainless-steel autoclave. The sealed vessel was then heated
at 100 °C for 8 h. The resulting products were collected by
centrifugation and washed several times with ethanol and acetone.
The representative electron microscopic images of the as-made
Pd nanocrystals are illustrated in Figure 1. It was clearly revealed
that concave polyhedral nanocrystals were the dominant products
with a typical yield of >80%. The concave nanocrystals had a
uniform side length and were present in two forms with tetrahedral
and trigonal bipyramidal shapes which were in a ratio of ∼3:1
(
Figure S1). Their side lengths were 45 ( 2 nm and 35 ( 2 nm,
respectively. The concave feature of the as-prepared nanocrystals
was also confirmed by the HAADF-STEM and SEM analysis
(
Figure 1c, 1d). The most striking feature of the as-made concave
nanocrystals is that the concave surfaces are essentially {110} whose
surface energies are however the highest among all low-index facets
2
1
(
i.e., {111}, {100}, and {110}) of fcc metals.
Both the selected area electron diffraction (SAED) and high
Figure 1. (a) Large-area and (b) enlarged TEM, (c) HAADF-STEM, and
(d) SEM images of as-synthesized concave Pd nanocrystals. (e) High-
magnification TEM image of a single concave tetrahedron. Top-right and
bottom-right insets show the corresponding SAED pattern and the ideal
structure model of the concave tetrahedron. (f) High-resolution TEM image
of the squared area indicated in (e).
resolution TEM (HRTEM) (Figure 1e, f) measurements on single
concave tetrahedra clearly indicated their single-crystalline nature.
While the SAED pattern (Figure 1e inset) was indexed to the [111]
zone axis of an fcc Pd single crystal, the HRTEM images showed
lattice fringes with an interplanar spacing of 0.139 nm, correspond-
ing to {220} planes of fcc Pd. These fringes were running along
the edge of the tetrahedron. Together with the SAED and HRTEM,
the HAADF-STEM and SEM analysis suggested good geometric
agreement between the concave tetrahedral nanocrystals and the
model shown in Figure 1e. The model can be better visualized as
an excavated tetrahedron with a trigonal pyramid excavated at the
center of each face of the tetrahedron. Each cut-out pyramid has a
To better understand the formation process of concave polyhedra,
the intermediate nanocrystals produced at different reaction times
were investigated by TEM. Figure 2 details the morphological
evolution of the Pd nanocrystals with the reaction time. The Pd
polyhedra produced in a 1-h reaction had a side length of ∼16 nm
(Figure S5). These nanocrystals continued to grow within 8 h. Their
average side length was increased to 24 nm at 2 h, 38 nm at 4 h,
and 45 nm at 8 h. Associated with the size enlargement, the concave
feature was shaping up. While hardly observed at 1 h, the feature
{
111} face and three {110} faces exposed. As a result, each
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3916 9 J. AM. CHEM. SOC. 2009, 131, 13916–13917
10.1021/ja9059409 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
became discernible at 2 h and evident at 4 h and thereafter. Beyond
h, no further change on both the size and morphology of the
8
nanocrystals was observed. These results imply that the concave
faces were developed at the very beginning and concomitant with
the growth process. When formaldehyde was substituted with
benzaldehyde, the reactions also led to the formation of concave
Pd nanocrystals, indicating the correlation of the concave structure
formation to the aldehyde group (Figure S6).
Figure 3. (a-c) TEM images and (d) electrocatalytic properties of Pd
nanocrystals with different percentages of {110} facets. The CV curves
were recorded in 0.5 M H
of 50 mV/s.
2 4
SO + 0.25 M HCOOH solution at a scan rate
In conclusion, concave Pd nanocrystals with uniform diameter
were successfully prepared in the presence of formaldehyde. The
as-made concave Pd nanocrystals are bound with {111} and {110}.
Their electrocatalytic activity increases with the percentage of {110}
facets which correlates with their degree of concavity.
Figure 2. TEM images of the concave polyhedral Pd nanocrystals produced
in (a) 1.0-, (b) 2.0-, (c) 4.0-, (d) 8.0-, and (e) 15-h reactions. (f) Summary
curve of the time-dependent side length.
Acknowledgment. We thank NSFC (20871100, 20721001),
NSFC for Distinguished Young Investigator Grant, MSTC
92009CB930703), RFDP(200803841010), NSF of Fujian for
(
While the absence of formaldehyde led to the formation of Pd
nanocrystals with mixed morphologies (Figure S7), the concentra-
tion of formaldehyde controlled the degree of concavity of the
nanocrystals. When the formaldehyde solution (40%) supplied was
reduced from 0.1 to 0.07, 0.04, 0.02, and 0.01 mL, the concave
feature gradually faded out and finally disappeared (Figures S8,
S9). Based on the ideal concave tetrahedral model, the percentage
of {110} facets was approximately calculated decreasing from
Distinguished Young Investigator Grant (2009J06005), and the Key
Scientific Project of Fujian Province (2009HZ0002-1).
Supporting Information Available: Experimental details and data.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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JA9059409
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