Facile synthesis of five-fold twinned, starfish-like rhodium nanocrystals by eliminating oxidative etching with a chloride-free precursor

Article abstract of DOI:10.1002/anie.201002546

Five-fold twinned, starfish-like Rh nanocrystals consisting of five arms (see TEM) have been synthesized in high yields with the use of [{(CF ₃COO)₂Rh}₂] as a precursor to completely eliminate oxidative etching from this system. The as-prepared Rh nanocrystals were shown with great performance as a substrate for SERS. (Figure Presented).

Full text of DOI:10.1002/anie.201002546

Communications
DOI: 10.1002/anie.201002546
Pentapod Nanostructures
Facile Synthesis of Five-fold Twinned, Starfish-like Rhodium
Nanocrystals by Eliminating Oxidative Etching with a Chloride-Free
Precursor**
Hui Zhang,* Xiaohu Xia, Weiyang Li, Jie Zeng, Yunqian Dai, Deren Yang, and Younan Xia*
Rhodium is widely used as a catalyst in a rich variety of
reactions such as hydrogenation, hydroformylation, hydro-
carbonylation, CO oxidation, and hydrogen generation.[1] It is
also of great interest for potential application in surface-
enhanced Raman scattering (SERS).[2] In recent years,
controlling the size and shape of Rh nanocrystals has
attracted extensive attention because these two parameters
allow one to tailor their intrinsic properties and thus enhanc-
ing their performance in various applications.[3] There has
been some great success in using micelles, dendrimers, and
other types of soft templates to reduce the size of Rh
nanocrystals and thus increase their catalytic activity.[4]
However, in comparison with other noble metals such as
Au, Ag, Pt, and Pd, there are only a few reports on the
synthesis of Rh nanocrystals with well-defined and control-
lable shapes or morphologies. So far, only single-crystal Rh
nanocrystals such as cubes, octahedrons, tetrahedron, or
multipods have been reported. For example, the Tilley and
Somorjai groups reported the syntheses of Rh nanocrystals in
the form of tripods, tetrapods, cubes, horns, and cubooctahe-
drons through seed-mediated growth by reducing RhCl3 with
a polyol under Ar protection.[5] When trimethyl(tetradecyl)-
ammonium bromide (TTAB) was employed as a capping
agent, relatively uniform, sub-10 nm Rh nanocubes have also
been obtained.[6] In a related study, we reported a polyol
process for synthesizing Rh tripods using Na3RhCl6 as a
precursor under Ar protection.[7] The Rh tripods exhibited
interesting SERS properties because the electromagnetic
field could be greatly enhanced at the tips of the tripod
nanostructures. Interestingly, no twinned Rh nanocrystal has
ever been reported with a reasonable yield. This result can be
attributed to the fact that either RhCl3 or Na3RhCl6 has been
employed as a precursor to the metal and it is impossible to
completely eliminate oxidative etching by simply bubbling an
inert gas through the reaction system.[5–7]
It is worth pointing out that RhCl3 and Na3RhCl6 are not
good precursors for synthesizing Rh nanocrystals because the
Clꢀ ions released from the precursors can combine with the
O2 from air to cause oxidative etching during both the
nucleation and growth processes.[8] As shown for a number of
noble-metal systems, the etching process can cause size
polydispersity for the final nanocrystals as etching (like the
corrosion of a metal piece) tends to occur in a non-uniform
pattern.[9] The etching can also make it very difficult to
generate nanocrystals with a twinned structure as the twin
defects are highly susceptible to oxidation and dissolution.[10]
Although the oxidative etching can be blocked to some extent
by protection with an inert gas or through the use of an
antioxidant capping agent (e.g., citrate ions or citric acid),[11] it
will be a great advantage if we can completely eliminate it by
choosing a proper precursor that does not contain the
necessary ligand required for oxidative etching.
Herein, we report a facile synthesis of starfish-like Rh
nanocrystals with five twined arms in a polyol system as well
as their excellent performance as substrates for SERS. The
nanocrystal grows from the corners of a five-fold twinned,
decahedral seed. The idea is based upon our previous study
which showed that Rh tripods could only prevail when oxygen
was excluded from a reaction system to block oxidative
etching.[7] In this study, we completely eliminate the oxidative
etching by using [{(CF3COO)2Rh}2] instead of Na3RhCl6 as a
precursor. Due to the exclusion of Clꢀ ions from the reaction
system, five-fold twinned Rh nanocrystals with five branched
arms could be obtained in high yields.
[*] Dr. H. Zhang, X. Xia, W. Li, J. Zeng, Y. Dai, Prof. Y. Xia
Department of Biomedical Engineering, Washington University
Saint Louis, MO 63130 (USA)
E-mail: xia@biomed.wustl.edu
Dr. H. Zhang, Prof. D. Yang
In a typical synthesis, [{(CF3COO)2Rh}2] and poly(vinyl
pyrrolidone) (PVP) were dissolved separately in ethylene
glycol (EG), and these two solutions were then injected
simultaneously using a syringe pump into EG held at a
specific temperature. The color of the solution immediately
turned from deep blue to black, indicating the formation of
Rh nanocrystals due to the reduction of [{(CF3COO)2Rh}2] by
EG (see Experimental Section for details). Figure 1a and b
shows transmission electron microscopy (TEM) images of a
typical sample prepared at 1808C for 6 h. These TEM images
clearly show that most of the Rh nanocrystals consisted of five
arms (like a starfish), with an angle of 728 between adjacent
ones. The Rh arms were 4–10 nm in width and 6–12 nm in
State Key Laboratory of Silicon Materials, and Department of
Materials Science and Engineering, Zhejiang University
Hangzhou, Zhejiang 310027 (People’s Republic of China)
E-mail: msezhanghui@zju.edu.cn
[**] This work was supported in part by a DOE subcontract from the
University of Delaware (DE-FG02-03 ER15468) and startup funds
from Washington University in St. Louis. As a visiting scholar from
Zhejiang University, H.Z. was also partially supported by the “New
Star Program” of Zhejiang University. Part of the work was
performed at the Nano Research Facility (NRF), a member of the
National Nanotechnology Infrastructure Network (NNIN), which is
supported by the NSF under award ECS-0335765.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002546.
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Figure 2. TEM images of Rh nanocrystals obtained at 1808C after
Figure 1. a,b) TEM images, c) high-resolution TEM image, and d) XRD
pattern of a typical sample of starfish-like Rh nanocrystals prepared
using polyol reduction at 1808C for 6 h.
injection of precursor for different periods of time: a) 0; b) 1 min;
c) 10 min; and d) 20 h. The insets show high-resolution TEM images
taken from the corresponding samples.
length, whereas the overall size of the nanocrystal was on the
order of 30 nm. A small number of icosahedral Rh nano-
crystals of 6–10 nm in size also coexisted with the starfish-like
nanocrystals in the final product (see Supporting Information
for structural characterization, Figure S1). Figure 1c shows
high-resolution TEM image of an individual starfish-like Rh
nanocrystal, where periodic lattice fringes could be clearly
resolved. The lattice spacing of each group of parallel fringes
was about 0.22 nm, corresponding to the {111} planes. Further
analysis indicates that the arms grew from the five corners of a
five-fold twinned, decahedral core along the < 110 > direc-
tion to generate a starfish-like structure, which is considerably
different from the growth behavior of other noble metals such
as Ag. In the case of Ag, a decahedral seed would grow into a
five-fold twinned nanorod and then a nanowire, rather than a
starfish-like nanocrystal probably due to preferential stabili-
zation of the newly formed {100} side faces by PVP through
selective chemisorption.[12] We believe that the different
growth behaviors for Rh and Ag decahedral seeds are most
likely induced by the different chemisorption capabilities of
PVP on the {100} faces of these two noble metals. Figure 1d
shows an X-ray diffraction (XRD) pattern of the starfish-like
Rh nanocrystals. All the diffraction peaks could be indexed to
the face-centered cubic Rh (JCPDS No.001-1214). The
broadening of the diffraction peaks can be attributed to the
relatively small sizes of the Rh arms in the nanocrystals.[13]
In order to clarify the formation mechanism of the
starfish-like Rh nanocrystals, a series of TEM images were
taken from the samples prepared at different reaction times,
as shown in Figure 2. In the initial stage of the reaction
(Figure 2a), nanocrystals with sizes of several nanometers
were formed. Analysis by high-resolution TEM imaging (see
inset of Figure 2a for a typical example) revealed that the
sample was dominated by Rh decahedrons, together with
some icosahedrons as marked by arrows. We believe that the
use of a high temperature (1808C) and the absence of Clꢀ ions
were responsible for the formation of decahedral and
icosahedral nanocrystals because they are supposed to be
the most stable species for a face-centered cubic crystal when
the reduction is conducted under thermodynamic control.[14]
The UV/Vis spectra (Figure S2) taken from the solution at
different stages of the reaction indicate that the precursor was
consumed in 1 min due to rapid reduction and the monomer
was quickly depleted in the solution, confirming that the
synthesis was under a thermodynamic control.[15] Tiny tips
also started to grow from the corners of some decahedrons
along the h110i direction. As the reaction was continued
(Figure 2b), more and longer tips were observed to protrude
from the corners of the decahedral seeds, and some small
starfish-like nanocrystals could be clearly resolved. Figure 2a
and b clearly show that the growth of different arms, even
those from the same seed, might take place at different time
points, resulting in different lengths for the arms in the
starfish-like nanocrystals. Since the precursor was completely
depleted in 1 min, the growth of the starfish-like nanocrystals
was mainly driven by Ostwald ripening at the expense of
icosahedral nanocrystals (as well as some small Rh clusters
that might also existed in the solution). This growth mech-
anism was consistent with the fact that icosahedrons are
usually stable at small sizes, and decahedrons at medium
sizes.[16] As a result, the growth rate of the starfish-like
nanocrystals was strongly reduced in comparison to the
formation of the decahedral or icosahedral nanocrystals in the
initial stage. Even after 10 min of reaction, only a few starfish-
like nanocrystals were formed, as shown in Figure 2c. With
extension of the reaction time to 6 h, most of the decahedral
Rh nanocrystals had been transformed into starfish-like
nanocrystals. Further extension to 20 h (Figure 2d) did not
result in any obvious morphological change, indicating that
the starfish-like Rh nanocrystals were highly stable in EG at
an elevated temperature due to the exclusion of oxidative
etching.
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The aforementioned results indicate that the formation of
the starfish-like Rh nanocrystals was thermodynamically
controlled in the initial stage. As a parameter critical for the
control of reduction rate, the reaction temperature should
play an important role. Figure 3 shows TEM images of the
Figure 4. TEM images of Rh nanocrystals prepared at 1808C for 6 h
using a) Na3RhCl6, b) [{Rh(CF3COO)2}2] + HCl, c) [{Rh(CH3COO)2}2],
d) [{Rh(CF3COO)2}2] + citric acid as precursor.
studies, some Rh nanoplates were formed in the presence of
citric acid, as shown in Figure 4d (indicated by arrows), due to
their strong binding to the {111} facets.[17] As a coordination
ligand, citric acid could also decrease the reduction rate of the
precursor by altering the equilibrium potential of divalent
Rh,[18] and thus strongly inhibiting the formation of the
starfish-like Rh nanocrystals.
The branched morphology intrinsic to the starfish-like Rh
nanocrystals make them attractive for use as SERS substrates
because in many cases branching can lead to larger surface
areas and thus stronger SERS signals.[19] Figure 5 compares
the SERS activities of Rh thin films consisting of three types
of Rh nanocrystals with different morphologies: a) the star-
fish-like nanocrystals (Figure 1a), b) the embryonic Rh
Figure 3. TEM images of Rh nanocrystals prepared at different temper-
atures for 6 h: a) 120, b) 160, c) 170, and d) 1908C.
products obtained at other temperatures. It is clear that only
nanoparticles with sizes less than 10 nm were formed when
the temperature was set to 1208C (Figure 3a). In contrast, an
increase of the temperature to 1608C (Figure 3b) led to the
formation of the starfish-like Rh nanocrystals. Significantly,
the amount of the starfish-like nanocrystals was increased as
the reaction temperature was further increased to 170 and
1908C, as shown in Figure 3c and d. These temperature-
dependent experiments further confirmed that the growth of
the starfish-like Rh nanocrystals was a result of thermody-
namically controlled reduction.
We also systematically investigated the effect of the
precursor type on the morphology of the Rh nanocrystals. In
the presence of Clꢀ ions, for example, with the use of
Na3RhCl6 (Figure 4a) or [{Rh(CF3COO)2}2] + HCl (1:1
molar ratio, Figure 4b) as a precursor, both irregularly shaped
Rh nanocrystals and tripods were obtained due to oxidative
etching, which was in agreement with our previous report.[7]
In the absence of Clꢀ, for example, with [{Rh(CH3COO)2}2]
(Figure 4c) or [{Rh(CF3COO)2}2] + citric acid (Figure 4d) as
the precursor, twinned nanocrystals containing at least one
twin defect were obtained due to the elimination of oxidative
etching. These twinned seeds could further evolve into
bipyramids, decahedrons, or icosahedrons under thermody-
namic control. The slight difference between the ligands in
[{Rh(CF3COO)2}2] and [{Rh(CH3COO)2}2] led to subsequent
evolution of the decahedral seeds into the starfish-like
nanocrystals or other forms. Although the exact mechanism
is yet to be elucidated, the different reduction rates associated
with these two precursors might be responsible for the
different pathways. Moreover, in agreement with previous
Figure 5. SERS spectra (cps=counts per second) of 4-mercaptopyr-
idine on films of Rh nanocrystals: a) starfish-like Rh nanocrystals (see
Figure 1a), b) embryonic Rh multipods (see Figure 4a), and c) spher-
ical Rh nanocrystals (see Figure 3a).
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carbon-coated copper grid and drying under ambient conditions.
TEM imaging was performed using a Phillips CM100 microscope
operated at 120 kV. High-resolution HRTEM images were obtained
tripods (Figure 4a), and c) the spherical nanocrystals (Fig-
ure 3a). We chose 4-mercaptopyridine (4-MP) as a probe
molecule because it has a large Raman scattering cross
section[20] and has been well established.[21] From the films of
Rh nanocrystals that had been modified with 4-MP, it was
found that the starfish-like nanocrystals gave the strongest
SERS signal with an enhancement of 5 and 20 times stronger
than that for the embryonic tripods and spherical nano-
crystals, respectively. These results suggest a trend of stronger
SERS signal with sharper morphological features on the Rh
nanocrystals. Recent studies have shown that metal nano-
crystals with sharp corners or edges were especially SERS
active since greater field enhancements are observed near the
sharpest surface features.[22] This phenomenon was also
observed in our previous work, in which the Rh multipods
with higher aspect ratios exhibited stronger SERS activities.[7]
The SERS results shown here are purely qualitative and exact
quantitative analysis for detecting small molecules needs
further exploration.
In summary, we have demonstrated a facile polyol
approach to the synthesis of starfish-like Rh nanocrystals by
replacing the conventional RhCl3 or Na3RhCl6 precursor with
[{Rh(CF3COO)2}2] to completely eliminate oxidative etching.
Under thermodynamically controlled conditions, decahedral
Rh nanocrystals were initially formed, and then arms
gradually protruded from the five corners of the decahedral
nanocrystals along the h110i direction to form the starfish-like
Rh nanocrystals, as driven by Ostwald ripening at the expense
of icosahedral nanocrystals. The growth parameters such as
temperature and precursor type both play important roles in
controlling the morphology of the product. The as-prepared
starfish-like Rh nanocrystals gave a 5 and 20 times signal
enhancement as SERS substrate in comparison to embryonic
Rh tripods and spherical Rh nanocrystals due to the sharper
morphological features. Considering their superior SERS
activities, it is anticipated that the starfish-like Rh nano-
crystals presented here may be a promising candidate for the
in situ monitoring of catalytic reactions.[23]
using
a JEOL 2100F operated at 200 kV. XRD analysis was
performed on Rigaku Geigerflex D-MAX/A Diffractometer using
CuKa radiation. The UV/Vis spectra were recorded with a Cary 50
spectrometer (Varian) using a quartz cuvette with an optical path
length of 1 cm. The substrate for SERS was prepared by drying a
5 mL aliquot of the final product on a 50 nm thick Au film supported
on a Si wafer. The sample was then incubated in an aqueous solution
of 4-mercaptopyridine (5 mm) for 1 h, rinsed with deionized water,
and dried in air. The SERS spectra were recorded using a Renishaw
inVia confocal Raman spectrometer coupled to a Leica microscope
with 50 ꢀ objective (N.A. = 0.90) in backscattering configuration. The
785 nm excitation (8 mW at the sample) was from a semiconductor
diode laser and used with a holographic notchfilter having a grating of
1200 lines per millimeter. The backscattered Raman signals were
collected on a thermoelectrically cooled (ꢀ608C) CCD detector. The
scattering spectra were recorded in the range of 550–1800 cmꢀ1, and
collected after 60 s of accumulation in one acquisition. Each SERS
spectrum was subtracted from the background line that was taken
from the clean Au film. OriginLab software (Northampton, MA,
USA) was used for SERS spectra baseline-correction. For the
baseline correction, a fourth order polynomial function was used to
fit and subtract the raw Raman spectrum. The spectra were then
smoothed by adjacent averaging at an average number of 4.
Received: April 28, 2010
Published online: June 23, 2010
Keywords: polyol approach · rhodium · SERS ·
.
starfish-like nanostructures
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Experimental Section
Synthesis of starfish-like Rh nanocrystals: In a typical synthesis, 7 mL
of ethylene glycol (EG; J. T. Baker, 9300-01) was placed in a three-
neck flask (equipped with a reflux condenser and a magnetic Teflon-
coated stirring bar), preheated in air at 1108C for 2 h, and then
ramped to the designated temperature (120, 160, 170, 180, or 1908C).
Meanwhile, 0.01 mmol of rhodium(II) trifluoroacetate dimer ([{Rh-
(CF3COO)2}2], Aldrich, 399191–250 mg) and 0.3 mmol of poly(vinyl
pyrrolidone) (PVP; MW ꢁ 55000, Aldrich, 856568–100 g) were sepa-
rately dissolved in 2 mL of EG at room temperature. These two
solutions were then injected simultaneously into the flask through a
syringe pump at a rate of 2 mLminꢀ1. Heating of the reaction at the
desired temperature was continued for 6 h. In order to clarify the
growth mechanism of the starfish-like Rh nanocrystals, a series of
samples were taken over the course of each synthesis with a glass
pipet. The product was collected by centrifugation and washed with
acetone and ethanol several times to remove EG and excess PVP.
Characterizations: The as-obtained samples were characterized
by transmission electron microscopy (TEM), powder X-ray diffrac-
tion (XRD), and UV/Vis spectroscopy. TEM samples were prepared
by placing a drop of the final product (suspended in water) on a
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