Stereoaligned epitaxial growth of single-crystalline platinum nanowires by chemical vapor transport.

Abstract

Epitaxial Pt nanowire (NW) arrays are synthesized for the first time by a chemical vapor transport method by using a metal halide as a precursor. Here we report that the epitaxial growth direction of NWs can be steered by seed crystal morphology. Octahedral seeds grow into inclined NWs possessing six growth directions, whereas half-octahedral seeds grow into vertical and horizontal NWs. Interfacial energies between the seed material and the substrate are critical in determining the morphology of seed crystals. We also demonstrate that non-SERS-active Pt NWs can show strong surface-enhanced Raman scattering (SERS) spectra by placing them on Ag films. The active SERS observation would help to elucidate platinum-catalyzed chemical reactions.

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DOI: 10.1002/asia.201100028
Stereoaligned Epitaxial Growth of Single-Crystalline Platinum Nanowires by
Chemical Vapor Transport
Youngdong Yoo,[a]Sol Han,[a]Minjung Kim,[b]Taejoon Kang,[a]Juneho In,[a]and
Bongsoo Kim*[a]
Introduction
Platinum is employed as one of the major catalysts in indus-
trial reactions, such as catalytic conversion of automobile
pollutant gases, oxygen reduction reaction in fuel cells, and
hydrogenation reactions.[1–3]As catalytic activity and selec-
tivity are strongly affected by the surface atomic structure of
the nanomaterial,[4]Pt nanowire (NW) arrays with well-de-
fined facets can be employed for detailed investigation of
surface-dependent catalytic properties. Freestanding Pt NWs
epitaxially grown on a substrate would be optimum for this
purpose.
Recently, we reported that epitaxial Au, Pd, and AuPd
NWs can grow through a physical vapor transport method
by using a metal slug or powder as the source.[5,6]This
method, however, could not be employed for the synthesis
of Pt NWs because of a low vapor pressure of Pt. Although
various methods for growth of Pt NWs have been devel-
oped,[7–13]freestanding and single-crystalline Pt NWs without
defects that have high aspect ratios and well-defined facets
have not been reported yet. We have synthesized epitaxially
grown and single-crystalline Pt NWs for the first time by
employing a metal halide as a precursor. This synthetic
method is based on simple modification of the van-Arkel
method used for the separation or purification of metals
through vapor transport.[14]Because metal halides have a
relatively high vapor pressure at a low temperature, this
method could be employed more generally to noncatalytic
growth of NWs composed of other metals possessing low
vapor pressures.
We have already reported that Au NWs grow vertically or
horizontally from half-octahedral seeds, depending on the
magnitude of atom flux.[5]In this study, we observed that
NW growth can also be initiated from octahedral seeds, re-
sulting in six inclined NW growth directions. So far epitaxial
growth directions of NWs have been controlled by changing
the crystal orientation of the substrate surface or by chang-
ing the atom flux. Here we suggest epitaxial growth direc-
tions of NWs can be steered by seed crystal morphology.
Vapor-phase NW growth initiated from seeds reported in
this paper is distinctly different from conventional solution-
phase seeded NW growth. While seed crystals in solution-
phase NW growth are added independently,[15,16]seed crys-
tals in our methods are formed spontaneously in the initial
stage of supplying atomic building blocks and then aniso-
Abstract: Epitaxial Pt nanowire (NW)
arrays are synthesized for the first time
by a chemical vapor transport method
by using a metal halide as a precursor.
Here we report that the epitaxial
growthdirection
steered by seed crystal morphology.
Octahedral seeds grow into inclined
NWs possessing six growth directions,
ofNWscanbe
whereas
into vertical and horizontal NWs. Inter-
facial energies between the seed mate-
rial and the substrate are critical in de-
half-octahedral seedsgrowtermining the morphology of seed crys-
tals. We also demonstrate that non-
SERS-active Pt NWs can show strong
surface-enhanced
(SERS) spectra by placing them on Ag
films. The active SERS observation
would help to elucidate platinum-cata-
lyzed chemical reactions.
Raman scattering
Keywords: epitaxial growth · gold ·
nanowires · platinum · Raman spec-
troscopy
[a] Y. Yoo, S. Han, T. Kang, J. In, Prof. B. Kim
Department of Chemistry
KAIST
Daejeon 305-701 (Korea)
Fax: (+ +82)42-350-2810
E-mail: bongsoo@kaist.ac.kr
[b] M. Kim
Graduate School of Nanoscience and Technology
KAIST
Daejeon 305-701 (Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100028.
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tropic nanostructures grow from these seed crystals.[5]Thus,
seed formation in our methods should be understood as an
initial step in the total growth process.
After the PtCl2vapor collides with the substrate, Pt atoms
condense on a sapphire substrate to form half-octahedral
seeds and octahedral seeds. The seed formation process is
strongly affected by the interfacial energy between the seed
material and the substrate. As these seeds are epitaxially
formed, depending on the crystallographic relationship with
the substrate, the NWs grown from these seeds also have
specific growth directions and thus epitaxial growth of NWs
can be accomplished.
We have also investigated surface enhanced Raman scat-
tering (SERS) enhancement on Pt NWs. By fabricating a
novel SERS platform composed of a well-faceted Pt NW on
an Ag film, we obtained high-quality SERS spectra of the
molecular species adsorbed on non-SERS-active Pt NW sur-
faces. Whereas Ag and Au are widely utilized as effective
materials for SERS, Pt has been commonly known as a non-
SERS-active material.[17,18]SERS enhancement on Pt surfa-
ces can be quite valuable to in-depth studies of platinum-
catalyzed chemical reactions.[19–21]
Results and Discussion
Growth of Pt NWs from Half-Octahedral Seeds: Vertical
and Horizontal Growth
When PtCl2precursor temperature is maintained at 4008 8C,
vertically grown Pt NWs are synthesized on a c-cut sapphire
substrate (Figure 1a). Lengths of NWs are 5–10 mm and di-
ameters of NWs are 70–150 nm. The morphology of the
NWs is similar to those of Au, Pd, and AuPd NWs previous-
ly reported.[5]These vertical NWs are aligned in three orien-
tations at 1208 8 to one another (Figure 1b), and the epitaxial
relationship between a vertical NW and c-cut sapphire is
(110) Pt//(0001) sapphire. The geometry and orientation of
half-octahedral nanocrystals are the same as those of verti-
cal NWs (compare the insets in Figure 1a and c); this sug-
gests that the vertical Pt NWs grow from the half-octahedral
nanocrystals (seeds). TEM data and the XRD pattern con-
firm that as-synthesized NWs are Pt with a face-centered
cubic structure and have a single crystalline nature and a
[110] growth direction (see Figures S1 and S2 in the Sup-
porting Information).
We reported that the growth direction (vertical or hori-
zontal) of Au, Pd, and AuPd NWs can be selected by chang-
ing the deposition flux.[5]These NWs grow vertically or hori-
zontally from the same half-octahedral seeds, depending on
experimental conditions. We can control the growth direc-
tion of Pt NWs through the same methods. When we in-
crease Pt deposition flux by raising the precursor tempera-
ture to 8008 8C, horizontal NWs instead of vertical NWs grow
on a c-cut sapphire substrate (Figure 1d). A magnified SEM
image shows that the NW is well-faceted and has a shape of
an elongated half-octahedron (the inset in Figure 1d).
Growth of Au and Pt NWs from Octahedral Seeds: Inclined
Growth
Although both half-octahedral seeds and octahedral seeds
are formed on the substrate, it has been observed that verti-
cal and horizontal growth are initiated from only half-octa-
hedral seeds. We have observed that epitaxial NW growth
can also be initiated from the octahedral seeds under appro-
priate experimental conditions. Here we report the inclined
growth of Au and Pt NWs initiated from octahedral seeds.
Figure 2a shows the inclined Au NWs synthesized at the
end edge part of the substrate at a source temperature of
11008 8C. The top-view image in Figure 2b indicates three in-
clined growth directions (actually six directions, as discussed
below) with in-plane components at 1208 8 to one another.
The inset in Figure 2b indicates that the inclined NW is
well-faceted and has an elongated octahedral shape. Inclined
NWs grow along <110> directions of an octahedral seed
and are enclosed by the most stable {111} top and side
facets. The angle between the NW grown along <110> di-
rections and the substrate is calculated to be 54.748 8 (Fig-
ure 2c).
Figure 2d shows octahedral Au nanocrystals found on the
substrate. The octahedral seed can have one of two orienta-
tions, labeled X and Y in Figure 2d, as the triangular base
can sit on the c-cut sapphire substrate in two equivalent ori-
Figure 1. Pt NWs grown vertically and horizontally on a c-cut sapphire
substrate. a) 458 8 tilted SEM image of the vertical Pt NWs. The inset is a
magnified SEM image showing the tip of the NW (scale bar: 100 nm).
b) Top-view SEM image of the vertical Pt NWs. The inset is a magnified
top-view SEM image of the vertical NW (scale bar: 100 nm). c) Top-view
SEM image of a half-octahedral Pt seed (gray circle) and an octahedral
Pt seed. The inset is a 458 8 tilted SEM image of a half-octahedral seed
(scale bar: 100 nm). d) Top-view SEM image of the horizontal Pt NWs.
The inset is a magnified SEM image showing the tip of the NW (scale
bar: 100 nm).
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entations (see Figure S3 in the Supporting Information).
Considering the correlations of the geometry and orienta-
tions of the octahedral nanocrystals with those of the in-
clined NWs, we can visualize that the inclined NWs grow
from octahedral seeds along three possible <110> direc-
tions normal to three {110} edges of the top face of the seed.
Thus we have a total of six inclined growth directions start-
ing from the two seed orientations (X and Y), as illustrated
by the six top-view diagrams in Figure 2e. These appear as
three growth directions in the top-view image of Figure 2b.
Similar inclined growth of Pt NWs is also observed at the
end edge part of the substrate (Figure 3a). The PtCl2precur-
sor temperature was 4008 8C. Octahedral Pt nanocrystals are
observed on the same substrate in two orientations at 1808 8
to one another (Figure 3b). These nanocrystals are consid-
ered to be seeds of the inclined NWs. Figure 3c shows a top-
view SEM image of an inclined NW and an octahedral seed.
Epitaxial Crystalization of Octahedral Seeds and Half-
Octahedral Seeds on Substrates
The Pt atoms arriving on the substrate will diffuse and nu-
cleate to form small nanocrystals. The shape of the nano-
crystals is determined by the surface energies of the nano-
crystal facets and the interface energy between the nano-
crystal and the substrate. When Pt nanocrystals have a (111)
interface with the substrate, octahedral seeds can be formed.
When Pt nanocrystals have a (110) interface with the sub-
strate, half-octahedral seeds can be obtained.
Figure 4a is a structural model (top view) of an octahe-
dral seed, all facets of which are made up of {111} facets.
Figure 4c illustrates atomic planes at the epitaxial interface
between the octahedral Pt seed and c-cut sapphire. The lat-
tice mismatch between a {111} bottom plane of Pt and c-cut
sapphire is 1.16% along the Pt <110> direction and 0.84%
along the Pt <112> direction. Figure 4b is a structural
model (458 8 tilted view) of a half-octahedral seed. The half-
octahedral seed can be obtained by bisecting the octahedral
seed, and possesses {110} bottom plane. Figure 4d illustrates
atomic planes at the epitaxial interface between the half-oc-
tahedral Pt seed and c-cut sapphire. The lattice mismatch
between {110} Pt and c-cut sapphire is 5.02% along the Pt
<100> direction and 52.52% along the Pt <110> direc-
tion (the lattice mismatch is 2.70% along the Pt <110> di-
rection when we consider a larger domain, in which five
layers of Pt are matched with three layers of sapphire). Seed
formation is strongly affected by the interfacial energy cor-
related with the lattice mismatch between the seed material
Figure 2. Au NWs grown in a tilted direction on a c-cut sapphire sub-
strate. a) 458 8 tilted SEM image of inclined Au NWs. b) Top-view SEM
image of inclined Au NWs. The inset is a magnified top-view SEM image
of an inclined Au NW (scale bar: 100 nm). c) Structural model of the in-
clined NW grown on the substrate. d) Top-view SEM image of octahedral
Au seeds. The upper-left inset shows a 458 8 tilted and magnified SEM
image and the lower-right inset shows a top-view magnified SEM image
of an octahedral seed (scale bars: 100 nm). The octahedral seed can have
two orientations labeled X and Y. e) Schematic illustration of the growth
pathways from octahedral seeds to inclined Au NWs. Numbered six top-
view images of inclined NWs represent the growth along the six possible
<110> directions that are normal to six (110) edges (correspondingly
numbered) of the top face of the seed for X and Y orientations.
Figure 3. Pt NWs grown in a tilted direction on a c-cut sapphire substrate.
a) 458 8 tilted SEM image of the Pt NWs. The inset is a magnified SEM
image showing the tip of the NW (scale bar: 100 nm). b) Top-view SEM
image of the octahedral Pt seeds. The octahedral seed can have two ori-
entations labeled X and Y. c) Top-view SEM image of an inclined Pt NW
and its octahedral seed.
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and the substrate.[22]Thus, the number of octahedral Pt
seeds formed on the c-sapphire substrate is 10–20 times
larger than that of half-octahedral Pt seeds because of the
better lattice match between the octahedral seed and c-cut
sapphire.
In the case of Au, the lattice mismatch between the octa-
hedral seed and c-cut sapphire is about 4.6% (Figure 4e).
The lattice mismatch between the half-octahedral Au seed
and c-cut sapphire is 0.98% along the Au <100> direction
and 49.21% along the Au <110> direction (the lattice mis-
match is 0.84% along the Au <110> direction when we
consider a larger domain, in which five layers of Au are
matched with three layers of sapphire; Figure 4 f). The
number of octahedral Au seeds formed on the c-sapphire
substrate is approximately 2–3 times larger than that of half-
octahedral Au seeds.
Formation of half-octahedral Pt seeds is more difficult
than that of half-octahedral Au seeds because of worse lat-
tice mismatch on the c-cut sapphire substrate (compare Fig-
ure 4d with f). On the other hand, formation of octahedral
Pt seeds is easier than that of octahedral Au seeds, because
of better lattice mismatch (compare Figure 4c with e).
Growth Mechanism of Noble Metal NWs from Octahedral
and Half Octahedral Seeds
As the density of half-octahedral Pt seeds is lower than that
of half-octahedral Au seeds, the densities of vertical and
horizontal Pt NWs grown from half-octahedral seeds are
lower than those of vertical and horizontal Au NWs, respec-
tively. Because the density of octahedral Pt seeds is higher
than that of octahedral Au seeds, the density of inclined Pt
NWs grown from octahedral seeds is higher than that of in-
clined Au NWs.
Why is vertical growth from half-octahedral seeds mostly
observed in spite of dominant formation of octahedral seeds
at the central part on the substrate? NW growth from the
seeds is strongly affected by local flow conditions. At the
central part on the substrate, the material flux direction
would be nearly perpendicular to the substrate, whereas at
the edge of the substrate turbulent flow might occur due to
the geometrical shape. When the material flux direction is
perpendicular to the substrate, vertical growth from the
half-octahedral seed would be more favored than inclined
growth from the octahedral seed because of more advanta-
geous solid angle of collision for NW growth. Inclined
growth from octahedral seeds, however, is often observed
near the end edge part of the substrate.
NW growth from seeds can occur along <110> directions
of half-octahedral and octahedral seeds, because the NWs
can be enclosed by the energetically most stable {111} facets
in this case. Thus, half-octahedral seeds with {110} bottom
facets can grow vertically or horizontally and octahedral
seeds with {111} bottom facets can grow along the three in-
clined directions (54.748 8 with respect to the substrate). Be-
cause the octahedral seeds have two orientations at angles
1808 8 to one another, a total of six inclined growth directions
are possible.
SERS Enhancement of Pt NWs
The SERS enhancement is strongly dependent on the de-
tailed morphology of metal nanostructures. Fabricating well-
defined and reproducible SERS platforms is greatly desira-
ble for SERS applications. Pt NWs possessing a single-crys-
talline nature and atomically smooth surfaces can be utilized
for fabricating a well-defined and reproducible SERS plat-
form.
To obtain SERS spectra of the molecular species ad-
sorbed on the non-SERS-active Pt NW, we fabricated a
novel SERS platform composed of a single Pt NW on an Ag
film. After incubation of Pt NWs in a benzenethiol (BT) or
brilliant cresyl blue (BCB) solution, the Pt NWs are cast on
a smooth Ag film (see the inset in Figure 5a). Figure 5a,b
shows polarization-dependant SERS spectra of BT and BCB
adsorbed on a single Pt NW on a Ag film. Strong enhance-
Figure 4. a) Structural model (top-view) of an octahedral seed. b) Struc-
tural model (458 8 tilted view) of a half-octahedral seed. c) Schematic of
atomic planes at the epitaxial interface between the octahedral Pt seed
and c-cut sapphire. d) Schematic of atomic planes at the epitaxial inter-
face between the half-octahedral Pt seed and c-cut sapphire. e) Schematic
of atomic planes at the epitaxial interface between the octahedral Au
seed and c-cut sapphire. f) Schematic of atomic planes at the epitaxial in-
terface between the half-octahedral Au seed and c-cut sapphire.
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ment of the SERS signal is observed when the polarization
direction is perpendicular to the NW axis (q=908 8), but the
SERS signal almost disappears when the polarization direc-
tion is parallel to the NW axis (q=08 8). A strongly enhanced
electric field is induced at the gap between the NW and the
film by light with perpendicular polarization, but this en-
hancement is not induced by light with parallel polariza-
tion.[23–25]
Conclusions
By employing a chemical vapor transport method by using a
metal halide as a precursor, we have epitaxially grown ster-
eoaligned Pt NW arrays on a c-cut sapphire substrate. Verti-
cal and horizontal NWs grow from half-octahedral seeds
and inclined NWs from octahedral seeds. The epitaxial
growth direction of NWs is determined by the morphology
of seed crystals. In addition, high-quality SERS spectra of
BT and BCB adsorbed on non-SERS-active Pt NW surfaces
are obtained by fabricating a novel SERS platform com-
posed of a Pt NW on an Ag film.
Experimental Section
Synthesis and Characterization of Pt NWs
Pt NWs are synthesized in a horizontal two-zone furnace with a 1 inch di-
ameter inner quartz tube via a chemical vapor transport method by using
PtCl2powder as a precursor without using any catalysts (see Figure S4 in
the Supporting Information). The PtCl2precursor is evaporated at 400–
8008 8C and transported to a sapphire substrate at the high-temperature
(10008 8C) region by carrier gas (Ar), at which Pt is deposited after the
dissociation of PtCl2on a hot substrate (PtCl2!Pt+ +Cl2). Ar gas flowed
at a rate of 300 sccm with the chamber pressure maintained at 760 Torr.
The reaction time was 30–60 min. Field emission SEM images were taken
on a Phillips XL30S. Samples were coated with gold to avoid charging
effect during the SEM observation. TEM images, HRTEM images, and
selected area electron diffraction (SAED) patterns were taken on a
TECNAI F30 TEM operated at 300 kV.
Raman Spectroscopy
Smooth Ag films were fabricated on Si substrates by electron beam-as-
sisted deposition of 10 nm of Cr followed by 300 nm of Ag. For BT ex-
periments, Pt NWs were incubated in a 1 mm BT solution in ethanol for
24 h. For BCB experiments, Pt NWs were incubated in a 1 mm BCB solu-
tion in ethanol for 30 min. A drop of an incubated Pt NW solution was
cast on an Ag film. The SERS platforms were rinsed with an excess of
ethanol and purged with nitrogen to remove the excess solvent. SERS
spectra were obtained by using a home-built micro-Raman system. The
633 nm light of a He-Ne laser (Melles Griot) was utilized as an excitation
source and the laser light (~500 nm diameter) was focused on a sample
through a ?100 objective (Mitutoyo). The SERS signal was collected
with the same objective. The polarization direction of laser light was con-
trolled by rotating a half-wave plate.
Acknowledgements
This research was supported by KOSEF through NRL (20100018868),
SRC (2011-0001335), and “Center for Nanostructured Material Technolo-
gy” under the “21st Century Frontier R&D Programs” (2011K000210) of
the MEST, Korea.
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Received: January 15, 2011
Published online: May 18, 2011
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