Functionalization of ZrO2 nanofibers with Pt nanostructures: The effect of surface roughness on nucleation mechanism and morphology control

Article abstract of DOI:10.1016/j.cplett.2009.05.075

ZrO₂ nanofibers with different surface roughness were employed as supports for the deposition of Pt nanostructures. When the nanofibers had a smooth surface, both homogeneous and heterogeneous nucleation took place, producing Pt nanoparticles d

Full text of DOI:10.1016/j.cplett.2009.05.075

Chemical Physics Letters 476 (2009) 56–61
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Functionalization of ZrO2 nanofibers with Pt nanostructures: The effect of
surface roughness on nucleation mechanism and morphology control
Eric Formo a, Pedro H.C. Camargo a, Byungkwon Lim a, Majiong Jiang b, Younan Xia a,
*
a Department of Biomedical Engineering, Washington University, St. Louis, MO 63130, United States
b Department of Chemistry, Washington University, St. Louis, MO 63130, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
ZrO2 nanofibers with different surface roughness were employed as supports for the deposition of Pt
nanostructures. When the nanofibers had a smooth surface, both homogeneous and heterogeneous
nucleation took place, producing Pt nanoparticles deposited on the nanofibers and free-standing Pt nano-
stars in the solution. By reducing the concentration of Pt precursor, Pt nanowires were formed on the
nanofibers. For nanofibers with a rough surface, Pt nanoparticles were formed on the nanofibers through
heterogeneous nucleation. These results demonstrate that the surface roughness had a significant impact
on the nucleation mechanism and the morphology of the resultant Pt nanostructures.
Ó 2009 Elsevier B.V. All rights reserved.
Received 13 May 2009
In final form 29 May 2009
Available online 6 June 2009
1. Introduction
ation. The successful use of electrospun nanofibers as supports
for catalytic nanoparticles has been demonstrated for a number
Platinum (Pt) is an invaluable catalyst in a wealth of industrial
applications, including the CO/NOx oxidation in catalytic convert-
ers, production of nitric acid, petroleum cracking, and hydrogena-
tion reactions [1]. Moreover, Pt is by far the most effective
electrocatalyst for both the oxygen reduction reaction (ORR) and
oxidation of the fuel (e.g., hydrogen, methanol, ethanol, and formic
acid) in fuel cell technologies [2]. These applications demand the
utilization of Pt in a finely divided state, in which the activity
and/or selectivity of Pt-based catalysts can be greatly improved
by controlling the size, shape, and morphology [3]. Therefore, tre-
mendous efforts have been directed towards the synthesis of Pt
nanostructures with well-controlled shapes (or facets), including
nanocubes, octahedrons, tetrahedrons, nanowires, multipods, and
nanostars [3].
For practical applications in heterogeneous catalysis, Pt nano-
structures are usually deposited on a solid support in order to
achieve their homogeneous dispersion and thus maximum possi-
ble surface site availability [4]. To this end, ceramic supports pro-
vide a robust platform for the incorporation of noble-metal
nanoparticles [5]. The utilization of oxide supports such as TiO2,
SnO2, CeO2 and ZrO2, for example, have shown great promise to
prevent aggregation and improve CO tolerance of Pt-based cata-
lysts, thus enhancing their long term stabilities [6–11].
of reactions [12–15]. Previously, we have developed a method for
depositing Pt nanostructures on the electrospun nanofibers made
of TiO2 and ZrO2 through a simple polyol reduction approach, in
which the nanofibers were added to a polyol reduction bath con-
taining PVP and [PtCl6]2À [16,17]. In this method, the reduction
of [PtCl6]2À generates Pt atoms that nucleate and grow onto the
surface of the nanofibers. In our previous studies, the nanofibers
were calcined at 550 °C for at least 6 h in order to obtain the sur-
face roughness suitable for heterogeneous nucleation of the Pt
atoms [16,17]. Here, we would like to investigate how shorter cal-
cinations times would affect the deposition of Pt nanostructures
over the ZrO2 nanofibers. The utilization of shorter calcination
times, for example, would be very attractive from an economic
point of view in the large-scale production of the supported cata-
lyst. On the other hand, a shorter calcination time is expected to
generate nanofibers with a smoother surface, which may have a
strong impact on the nucleation (homogeneous vs. heterogeneous)
of Pt atoms. The occurrence of homogeneous nucleation during the
deposition step, for example, could lead to lower density of nano-
particle coating on the nanofibers and aggregation of the nanopar-
ticles in the solution phase, causing detrimental effects over the
catalytic performance [18].
In this work, we report on the utilization of a nonoven mat com-
posed of ZrO2 nanofibers having a smooth surface as a support for
the deposition of Pt nanostructures via a polyol reduction ap-
proach. Here, the calcination temperature and time were reduced
to 500 °C and 3 h, respectively, and the conditions for Pt deposition
were similar to those reported in our previous studies (conducted
with nanofibers calcined at 550 °C for 6 h). Interestingly, we found
that the surface roughness of electrospun ZrO2 nanofibers had a
Electrospun nanofibers have proven to be effective catalytic
supports owing to their high porosity and large surface area. The
high porosity of a nonwoven mat of nanofibers usually enables di-
rect growth of catalytic nanostructures via heterogeneous nucle-
* Corresponding author.
E-mail address: xia@biomed.wustl.edu (Y. Xia).
0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2009.05.075

E. Formo et al. / Chemical Physics Letters 476 (2009) 56–61
57
significant impact not only on the nucleation mechanism, but also
on the morphology of the resultant Pt nanostructures.
ethanol and water to remove EG and excess PVP. In order to inves-
tigate the effect of the concentration of Pt precursor on the nucle-
ation and morphology of the deposited Pt nanostructures, the same
procedure was used, with the exception that the initial concentra-
tions of PVP and H2PtCl6 were decreased to 200 mM and 40 mM,
respectively.
2. Experimental
2.1. Fabrication of ZrO2 nanofibers by electrospinning
2.3. Characterization
The fabrication of ZrO2 nanofibers was achieved by electrospin-
ning a solution containing 1.5 g zirconium acetylacetonate (Al-
drich), 1 mL acetic acid (EMD), 0.15 g PVP (Mw ꢀ 1.3 Â 106,
Aldrich), and 2.5 mL ethanol (Pharmco). The solution was passed
through a syringe with a 21 gauge stainless steel needle at the
tip. The needle was electrified using a high-voltage DC power sup-
ply (ES30P-5 W, Gamma High Voltage Research Inc., Ormond
Beach, FL) and a voltage of 20 kV was applied. The solution was in-
jected continuously using a syringe pump (KDS-200, Stoelting,
Wood Dale, IL) at a rate of 0.4 mL/h. The nanofibers were collected
on a grounded aluminum foil and left overnight in air to fully
hydrolyze. The as-prepared composite nanofibers containing
amorphous ZrO2 and PVP were then calcined in air at 500 °C for
3 h to generate the polycrystalline ceramic fibers. For comparison,
another batch of ZrO2 nanofibers was prepared following the same
procedure, with the exception that the calcination step was con-
ducted at 550 °C for 6 h.
The SEM samples were prepared by placing a drop of the mate-
rial onto a silicon substrate and allowing it to dry under ambient
conditions. Images were taken using a field-emission SEM (FEI
NovaNano 230, Hillsboro, OR) operated at an accelerating voltage
of 5 kV for the ZrO2 nanofibers and 15 kV for the ZrO2 nanofibers
after Pt functionalization. X-ray diffraction was conducted with a
Geigerflex D-MAX/A Diffractometer (Rigaku, Berlin, Germany)
using Cu-K radiation at 35 kV and 35 mA. The TEM samples were
prepared by drop-casting a dispersion of the fibers onto carbon-
coated copper grids (Formvar/Carbon, 200 mesh, Ted Pella). TEM
images were acquired using a Tecnai G2 Spirit Twin (FEI, Hillsboro,
OR) operated at 120 kV. High-resolution HRTEM was performed
using a JEM-2100F (JEOL, Tokyo, Japan) operated at 200 kV.
3. Results and discussion
2.2. Functionalization of the ZrO2 nanofibers with Pt nanostructures
The ZrO2 nanofibers were prepared using a previously devel-
oped protocol, which involves electrospinning a polymer solution
containing a sol–gel precursor (zirconium acetylacetonate) onto a
flat grounded electrode in the form of a nonwoven mat [17,19].
Fig. 1A shows SEM image of the nonwoven mat of ZrO2 nanofibers
that were generated after calcination at 500 °C for 3 h. As depicted
in Fig. 1A, the nanofibers were uniform and their diameter was
ꢁ150 nm. The XRD pattern confirmed that the ZrO2 nanofibers
were crystallized in the tetragonal phase (Fig. 1B). The TEM image
(Fig. 1C) confirms that the nanofiber had a smooth and regular sur-
face that was composed of nanocrystalline grains <5 nm in size. It
In a typical procedure, 4 mL of ethylene glycol (EG, J.T. Baker)
containing 10 mg of ZrO2 nanofibers was injected into a 3-neck
flask fitted with a reflux condenser and a Teflon-coated stir bar, fol-
lowed by heating in air at 110 °C for 1 h. 400 mM PVP
(Mw ꢀ 5.5 Â 104, Aldrich) and 80 mM H2PtCl6 (Aldrich) were dis-
solved separately in 1 mL aliquots of EG at room temperature.
These two solutions were then added simultaneously into the flask
over a period of 1.5 min. The reaction mixture was further heated
at 110 °C for 15 h. The final samples were washed thoroughly with
Fig. 1. (A) SEM image and (B) XRD pattern for the nonwoven mat of ZrO2 nanofibers prepared by calcination in air at 500 °C for 3 h. (C) TEM image of an individual ZrO2
nanofiber from the product displayed in (A). In this case, the surface of the nanofibers was smooth and made of nanocrystalline grains <5 nm in size. (D) TEM image of an
individual ZrO2 nanofiber that was obtained after calcination at 550 °C for 6 h. In this case, the ZrO2 nanofiber displayed a rough and irregular surface comprised of bigger
crystalline grains.

58
E. Formo et al. / Chemical Physics Letters 476 (2009) 56–61
is worth pointing out that the calcination at 500 °C for 3 h resulted
in some substantial differences over the ZrO2 nanofiber surface
morphology as compared to calcination at 550 °C for 6 h. As shown
in Fig. 1D and previously reported by our group, the ZrO2 nanofi-
bers that were calcined at 550 °C for 6 h presented a significantly
rough and irregular surface together with increased crystallite
sizes [17].
After the ZrO2 nanofibers had been synthesized, they could un-
dergo a simple and versatile post-treatment approach to allow the
deposition of Pt nanostructures onto their surface. In this case, the
deposition of Pt onto the nanofibers was accomplished by immers-
ing the nanofibers into a polyol reduction bath containing [PtCl6]2À
and PVP for 15 h [3,20]. In our initial studies, we employed similar
conditions as those previously reported for the ZrO2 nanofibers
that had been calcined at 550 °C for 6 h [17]. Fig. 2 A and B, displays
TEM images of the resultant ZrO2 nanofibers after their post-treat-
ment in a polyol reduction bath. It can be observed that Pt nano-
particles 5 nm in diameter were deposited on the surface of the
nanofiber as a packed array with a sub-monolayer coverage. In
addition to their incomplete coverage, regions containing small
aggregates of Pt nanoparticles were observed on the surface of
the nanofiber (Fig. 2B). Interestingly, Pt nanostars (branched Pt
nanostructures) were co-produced in solution during the deposi-
tion step, as shown in Fig. 2C. Here, each Pt nanostar contained
2–6 branches, rounded edges and an overall size of 10 nm. HRTEM
images of the Pt nanostars (Fig. 2D) revealed that they were single-
crystalline, suggesting that they were formed via an overgrowth
mechanism rather than through random aggregation of pre-
formed small particles. It is important to note that the Pt nanostars
could be easily isolated from the other materials by simply passing
the reaction solution through a 200 nm membrane filter, which
was able to selectively collect the relatively large Pt-decorated
nanofibers.
These results suggest that both homogeneous and heteroge-
neous nucleation took place when ZrO2 nanofibers having a
smooth surface were employed as a support for the deposition of
Pt nanostructures via a polyol approach. Specifically, the Pt atoms
nucleated both in solution and on the nanofiber surface. After the
nucleation step, as more Pt atoms were produced by the reduction
of [PtCl6]2À, the seeds that had nucleated onto the surface of the
nanofiber (via heterogeneous nucleation) grew until they became
5 nm in diameter. Meanwhile, an overgrowth mechanism was ob-
served for the seeds that self-nucleated in solution, leading to the
formation of the Pt nanostars. In this case, the evolution to a
Fig. 2. (A and B) TEM images of an individual ZrO2 nanofiber (calcined at 500 °C for 3 h) that was coated with Pt nanoparticles by immersing the nanofibers in a polyol
reduction bath for 15 h. (C) TEM image of the Pt nanostars 10 nm in size that were co-produced in solution during the Pt deposition step. The scale bar in the inset
corresponds to 2 nm. (D) HRTEM image of a Pt nanostar, revealing that they were single-crystalline. (E and F) TEM images of an individual ZrO2 nanofiber that was calcined at
550 °C for 6 h and coated with Pt nanoparticles using a similar method as described in (A).

E. Formo et al. / Chemical Physics Letters 476 (2009) 56–61
59
branched morphology seems to result from the overgrowth of ini-
tially formed cuboctahedral Pt seeds at their corner sites. The over-
growth mechanism for Pt nanostructures has been observed at low
concentration of seeds and high concentration of Pt atoms [20–25].
In our system, it is plausible that the concentration of seeds formed
in solution via homogeneous nucleation was relatively low enough
as compared to the concentration of the generated Pt atoms during
the reduction of [PtCl6]2À. As some Pt seeds had been formed in
solution, further generation of Pt atoms could be accelerated via
an autocatalytic process, contributing to sustain their high concen-
tration in solution and enable the overgrowth mechanism [21–26].
It is important to note that only heterogeneous nucleation was
favored when nanofibers with a rough surface (calcined for at
550 °C for 6 h) were used under similar experimental conditions.
In this case, the TEM images of the Pt-decorated nanofibers
(Fig. 2E and F) indicate that their surface was completely covered
by a sheath of densely packed Pt nanoparticles and no material
was co-produced in the solution. The higher coverage of Pt nano-
particles on the ZrO2 nanofibers supports this observation
(Fig. 2F). Fig. 3 shows a schematic summarizing the effect of the
nanofiber’s surface roughness on the deposition of Pt nanostruc-
tures. It has been reported that highly irregular and rough surface
can serve as primary nucleation sites for the growth of noble-metal
nanostructures because of the higher surface energies associated
with irregularities such as indentions, step edges, or protrusions
which significantly lower the barrier for heterogeneous nucleation
[27,28]. Therefore, for ZrO2 nanofibers with a rough surface
(Fig. 3A), the increased number of nucleation sites for the deposi-
tion of Pt atoms associated with the increased surface area of the
nanofiber enables heterogeneous nucleation to become more
favorable while homogeneous nucleation was suppressed. Con-
versely, when the nanofiber surface was relatively smooth and reg-
ular (Fig. 3B), its corresponding lower surface area provided a
significantly lower number of nucleation sites for the growth of
Pt nanostructures. Under the experimental conditions we used, it
is possible that the energy barrier for heterogeneous nucleation
could not be lowered enough so that homogeneous nucleation
would be suppressed, enabling both homogeneous and heteroge-
neous nucleation to occur.
bath. Fig. 4 displays the product obtained after the ZrO2 nanofibers
were immersed in a polyol reduction bath for 15 h, in which the
concentration of [PtCl6]2À and PVP were decreased by two times
as compared to the product depicted in Fig. 2. Fig. 4A and B, shows
SEM images of the product, indicating that it was comprised of
agglomerate structures composed of Pt nanowires supported on
the ZrO2 nanofibers. From these images, it is clear that the as-pre-
pared Pt nanowires were uniform in size distribution. Moreover,
regions containing only the bare ZrO2 nanofiber could be observed
in the image. Fig. 4C shows a TEM image of the Pt nanowires lo-
cated at the surface of the Pt agglomerates, indicating that the
nanowires were 5 nm in diameter, with lengths up to 120 nm.
The HRTEM image (Fig. 4D) confirmed that the nanowire grew
along the {1 1 1} direction. We have previously observed similar
hierarchically agglomerate structures containing Pt nanowires that
were formed upon the addition of Fe2+ or Fe3+ species into polyol
synthesis of Pt nanostructures [20,29,30]. In our previous studies,
the Fe2+/Fe3+ pair acted as an oxidative etchant that was able to
oxidize the Pt atoms and nuclei back to PtII. This reduction in the
number of Pt atoms in the solution forced the reduction kinetics
to an extremely slow rate, inducing the Pt atoms to nucleate and
grow into uniform nanowires. Interestingly, here we observed
the formation of hierarchically structures containing Pt nanowires
in the presence of ZrO2 nanofibers without the introduction of any
Fe2+/Fe3+ species. It is possible that, by decreasing the concentra-
tion of [PtCl6]2À (thus decreasing the number of Pt seeds), the sub-
sequent reduction reaction to produce Pt atoms could be slowed
down below a critical level that was sufficient to induce aniso-
tropic growth. In this case, the decrease in the concentration of
Pt seeds (by decreasing the concentration of Pt precursor) would
have a similar effect over the reduction kinetics as the addition
of Fe2+/Fe3+ species into the polyol bath. Also, as [PtCl6]2À releases
ClÀ into the reaction solution during the formation of Pt atoms, it is
possible that the O2/ClÀ pair could have acted as an oxidative etch-
ant, contributing to the slow down of reduction kinetics that led to
anisotropic growth [26,31–33]. The mechanism for the production
of Pt nanowires may involve overgrowth on one of the eight {1 1 1}
facets of a cuboctahedral seed via an autocatalytic process. Another
mechanism would involve the formation of long chain PtII or PtIV
complexes such as [PtCl4]2À or its water-substituted form
[PtCl4(H2O)2]2À prior to their reduction to Pt atoms. Upon reduc-
tion, these long chain complexes could evolve into the nanowires.
In order to explore the effect of the concentration of Pt precur-
sor on the formation of Pt nanostructures, we decided to decrease
the concentration of [PtCl6]2À employed in the polyol reduction
Fig. 3. A schematic summarizing the effect of surface roughness on the deposition of Pt nanostructures. (A) When the nanofibers had a rough and irregular surface, Pt
nanoparticles were deposited via heterogeneous nucleation over the nanofiber’s surface as a densely packed array or a complete sheath. (B) Conversely, when the nanofibers
had a relatively smooth surface, both homogeneous and heterogeneous nucleation was observed. While the heterogeneous nucleation led to the decoration of the nanofibers
with Pt nanoparticles 5 nm in size as a sub-monolayer, homogeneous nucleation resulted in the formation of free-standing Pt nanostars.

60
E. Formo et al. / Chemical Physics Letters 476 (2009) 56–61
Fig. 4. (A and B) SEM images of the product obtained by decreasing the concentration of [PtCl6]2À by two times. Here, hierarchical agglomerates composed of Pt nanowires
supported on the ZrO2 nanofibers were produced. (C) TEM image taken from the surface of an agglomerate, depicting that the Pt nanowires were uniform and had a diameter
of 5 nm and lengths up to 120 nm. (D) HRTEM of an individual Pt nanowire, indicating that growth took place along the {1 1 1} direction.
It is important to note that this could also explain why the growth
direction for the nanowires was different from those observed for
other fcc noble-metals (that typically grow along the {1 0 0} direc-
tion) [3,34–37]. The formation of agglomerate structures contain-
ing Pt nanowires supported on the ZrO2 nanofibers may also
involve both homogeneous and heterogeneous nucleation. In this
case, the produced Pt atoms nucleated both onto the nanofiber sur-
face and in the solution. As the overall number of seeds produced
during the nucleation step was decreased (due to a lower concen-
tration of Pt precursor), anisotropic growth was induced. The Pt
nanowires then aggregated both in solution and onto the nanofi-
ber’s surface to produce micrometer-sized agglomerate structures
containing Pt nanowires together with the nanofibers. Interest-
ingly, Pt nanowires were not obtained when nanofibers with a
rough surface (calcined for at 550 °C for 6 h) were employed under
similar experimental conditions. In this system, only heteroge-
neous nucleation took place and the product was composed of
ZrO2 nanofibers decorated with Pt nanoparticles.
nanowires supported on the ZrO2 nanofibers were formed. In this
case, the reduction kinetics could be slowed down below a critical
level that was sufficient to induce anisotropic growth by simply
decreasing the number of Pt seeds produced during the nucleation
step. The results presented herein suggest that the surface rough-
ness of the support ZrO2 nanofibers had a profound impact not
only on the Pt nucleation mechanism, but also the morphology of
the resultant Pt nanostructures.
Acknowledgements
This work was supported by a research grant from the NSF
(DMR-0804088) and startup funds from Washington University
in St. Louis. P.H.C.C. was partially supported by the Fulbright Pro-
gram and the Brazilian Ministry of Education (CAPES).
References
[1] G. Ertl, Handbook of Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2008.
[2] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, Appl. Catal. B: Environ. 56
(2005) 9.
[3] J. Chen, B. Lim, E. Lee, Y. Xia, Nano Today 4 (2009) 81.
[4] X.W. Yu, S.Y. Ye, J. Power Sources 172 (2007) 145.
4. Summary
[5] X. He, D. Antonelli, Angew. Chem. Int. Ed. 41 (2002) 214.
[6] M. Gustavson, H. Ekstrom, P. Hanarp, L. Eurenium, G. Lindbergh, E. Olsson, B.
Kasemo, J. Power Sources 163 (2007) 671.
[7] M. Hepel, I. Dela, T. Hepel, J. Luo, C. Zhong, Electrochim. Acta 52 (2007) 5529.
[8] L. Yang, W. Yang, Q. Cai, J. Phys. Chem. C 111 (2007) 16613.
[9] L. Jiang et al., J. Phys. Chem. B 109 (2005) 8774.
We have examined the utilization of ZrO2 nanofibers as a sup-
port for the deposition of Pt nanostructures through a polyol
reduction approach. When the nanofibers had a rough and irregu-
lar surface, heterogeneous nucleation took place and the nanofi-
bers could be readily functionalized with a complete sheath
composed of densely packed array of Pt nanoparticles. Conversely,
when the nanofibers had a smooth surface, both homogeneous and
heterogeneous nucleation was observed under similar experimen-
tal conditions. While heterogeneous nucleation led to the decora-
tion of the nanofibers with Pt nanoparticles 5 nm in size with a
sub-monolayer coverage, homogeneous nucleation led to the for-
mation of Pt nanostars 10 nm in size via an overgrowth mecha-
nism. Moreover, when the concentration of Pt precursor was
reduced by two times, agglomerate structures composed of Pt
[10] J.R. Croy, S. Mostafa, J. Liu, Y. Sohn, H. Heinrich, B.R. Cuenya, Catal. Lett. 119
(2007) 209.
[11] M.V.M. Souza, N.F.P. Ribeiro, M. Schmal, Int. J. Hydrogen Energ. 32 (2007) 425.
[12] M. Graeser, E. Pippel, A. Greiner, J. Wendroff, Macromolecules 40 (2007) 6032.
[13] S. Zhan, D. Chen, X. Jiao, Y. Song, Chem. Commun. 20 (2007) 2043.
[14] A. Patel, S. Li, C. Wang, W. Zhang, Y. Wei, Chem. Mater. 19 (2007) 1231.
[15] M.M. Demir et al., Macromolecules 37 (2004) 1787.
[16] E. Formo, E. Lee, D. Campbell, Y. Xia, Nano Lett. 8 (2008) 668.
[17] E. Formo, M.S. Yavuz, E.P. Lee, L. Lane, Y. Xia, J. Mater. Chem. 19 (2009) 3878.
[18] B.C. Gates, Chem. Rev. 95 (1995) 511.
[19] D. Li, Y. Xia, Nano Lett. 3 (2003) 555.
[20] J. Chen, T. Herricks, Y. Xia, Angew. Chem. Int. Ed. 44 (2005) 2589.

E. Formo et al. / Chemical Physics Letters 476 (2009) 56–61
61
[21] J. Ren, R.D. Tilley, J. Am. Chem. Soc. 129 (2007) 3287. [30] J. Chen, T. Herricks, M. Geissier, Y. Xia, J. Am. Chem. Soc. 126 (2004) 10854.
[22] M.A. Mahmoud, C.E. Tabor, M.A. El-Sayed, Y. Ding, Z.L. Wang, J. Am. Chem. Soc.
130 (2008) 4590.
[23] B. Lim, X. Lu, M. Jiang, P.H.C. Camargo, E.C. Cho, E.P. Lee, Y. Xia, Nano Lett. 8
(2008) 4043.
[31] B. Wiley, T. Herricks, Y. Sun, Y. Xia, Nano Lett. 4 (2004) 1733.
[32] Y. Xiong, J. Chen, B. Wiley, Y. Xia, S. Aloni, Y. Yin, J. Am. Chem. Soc. 127 (2005)
7332.
[33] B. Wiley, Y. Sun, Y. Xia, Langmuir 21 (2005) 8077.
[34] L. Colombi Ciacchi, W. Pompe, A. De Vita, J. Am. Chem. Soc. 123 (2001) 7371.
[35] M. Mertig, L.C. Ciacchi, R. Seidel, W. Pompe, A. De Vita, Nano Lett. 2 (2002) 841.
[36] L.C. Ciacchi, M. Mertig, W. Pompe, S. Meriani, A. De Vita, Platinum Met. Rev. 47
(2003) 98.
[24] J. Ren, R.D. Tilley, Small 3 (2007) 1508.
[25] T. Herricks, J. Chen, Y. Xia, Nano Lett. 4 (2004) 2367.
[26] Y. Xia, Y. Xiong, B. Lim, S. Skrabalak, Angew. Chem. Int. Ed. 48 (2009) 60.
[27] A. Gouldstone, K.J. Van Vliet, S. Suresh, Nature 411 (2001) 656.
[28] H. Brune, Surf. Sci. Rep. 31 (1998) 121.
[37] L.C. Ciacchi, W. Pompe, A. De Vita, J. Phys. Chem. B 107 (2003) 1755.
[29] E.P. Lee, Z. Peng, D.M. Cate, H. Yang, C.T. Campbell, Y. Xia, J. Am. Chem. Soc. 129
(2007) 10634.