Hollow and cage-bell structured nanomaterials of noble metals

Article abstract of DOI:10.1021/ja302518n

Mastery of the structure of nanomaterials enables control of their properties to enhance their performance for a given application. Herein we demonstrate the synthesis of metal nanomaterials with hollow interiors or cage-bell structures based on the inside-out diffusion of Ag in core-shell structured nanoparticles. It begins with the synthesis of core-shell Ag-M or core-shell-shell M_A-Ag-M_B nanoparticles in an organic solvent. Ag is then extracted from the core or the inner shell by bis(p-sulfonatophenyl)phenylphosphane, which binds strongly with Ag(I)/Ag(0) to allow the complete removal of Ag in 24-48 h, leaving behind an organosol of hollow or cage-bell structured metal nanomaterials. Because of their relatively lower densities, which usually translate to a higher surface area than their solid counterparts, the hollow and cage-bell structured metal nanomaterials are especially relevant to catalysis. For example, cage-bell structured Pt-Ru nanoparticles were found to display outstanding methanol tolerance for the cathode reaction of direct methanol fuel cells (DMFCs) as a result of the differential diffusion of methanol and oxygen in the cage-bell structure.

Full text of DOI:10.1021/ja302518n

Article
pubs.acs.org/JACS
Hollow and Cage-Bell Structured Nanomaterials of Noble Metals
Hui Liu,† Jianglan Qu,† Yunfa Chen,† Jianqiang Li,† Feng Ye,† Jim Yang Lee,*,‡ and Jun Yang*,†,§
†State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing,
China 100190
‡Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore
119260
§Institure of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669
S
* Supporting Information
ABSTRACT: Mastery of the structure of nanomaterials enables control
of their properties to enhance their performance for a given application.
Herein we demonstrate the synthesis of metal nanomaterials with
hollow interiors or cage-bell structures based on the inside-out diffusion
of Ag in core−shell structured nanoparticles. It begins with the synthesis
of core−shell Ag−M or core−shell−shell MA−Ag−MB nanoparticles in
an organic solvent. Ag is then extracted from the core or the inner shell
by bis(p-sulfonatophenyl)phenylphosphane, which binds strongly with
Ag(I)/Ag(0) to allow the complete removal of Ag in 24−48 h, leaving
behind an organosol of hollow or cage-bell structured metal nano-
materials. Because of their relatively lower densities, which usually
translate to a higher surface area than their solid counterparts, the hollow and cage-bell structured metal nanomaterials are
especially relevant to catalysis. For example, cage-bell structured Pt−Ru nanoparticles were found to display outstanding
methanol tolerance for the cathode reaction of direct methanol fuel cells (DMFCs) as a result of the differential diffusion of
methanol and oxygen in the cage-bell structure.
template the synthesis of well-defined hollow Pt nanostructures
which were excellent catalyst for the methanol oxidation
reaction.19 The authors also demonstrated reusability of the
template by an acid−base treatment. A general strategy for the
fabrication of hollow or CBS structures is template-assisted
selective etching of core−shell particles. The core particle in this
case is overlaid with a single or double shell of a different material.
The core or the inner shell is then selectively removed by
calcination or with a solvent. Over the past 20 years, various
methods based on sacrificial templates of polymer and inorganic
spheres,20−26 liquid droplets,27 vesicles,28−30 and microemulsion
droplets,31−33 have been developed for the synthesis of hollow
and CBS nanomaterials, and were reviewed recently by the Lou,
Jiang, and Lu groups.34−36 These preparative methods are
however system specific, more successful for metal oxides than
for noble metals. Specific methods based on other principles such
as galvanic replacement,37,38 Kirkendall effect,8,39 Ostwald
ripening,40,41 and layer-by-layer assembly23,42 have also been
used for the synthesis of hollow or cage-bell nanomaterials.
These methods are only applicable to a small number of metals
(mostly Au and Pt) and there are significant limitations. For
example, the core and the shell of cage-bell nanostructures
prepared this way are usually made of the same material, and
there is no effective control of the shell thickness and shell
1. INTRODUCTION
Noble metal nanomaterials with customizable internal structures
and shell compositions have garnered sustained research interest
due to their immense potential for catalysis.1−3 For instance, Pd
nanoparticles with a hollow interior exhibit good catalytic activity
in Suzuki cross-coupling reactions and can be reused seven times
without the loss of catalytic activity.1 Liang and co-workers
showed that Pt hollow nanospheres are twice as active as solid Pt
nanoparticles of roughly the same size for methanol oxidation.4
The increase in activity could be attributed mainly to the larger
surface area of the hollow structure, where the porous shell allows
the internal surface of the catalyst to be accessible to the
reactants. The continuing research efforts in this area have also
given rise to the recent possibility of creating hybrids of core−
shell and hollow structures, or a new class of core−shell
structures with a distinctive core-void-shell configuration, which
are called cage-bell, yolk-shell, or rattle-type structures. With the
unique properties of a movable core, interior void spaces, and
controlled porosity and composition of the shell, cage-bell
structured (CBS) nanomaterials have great potential for a diverse
range of applications, such as nanoreactors,5−7 drug delivery
systems,8−10 lithium-ion batteries,11−15 photocatalysis,16 and
photonics.17 For example, polymeric hollow spheres with a
movable Au nanoparticle core have been synthesized, which
allow the optical sensing of chemicals diffused into the cavity.18
Most recently, Fan and co-workers developed a photocatalytic
approach using densely packed optically active porpyrins to
Received: March 15, 2012
© XXXX American Chemical Society
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structure.43,44 In summary, a general approach to rationally
fabricate hollow and cage-bell structures for a sufficiently wide
spectrum of noble metals is still lacking today and it poses
significant challenges.
stirring under flowing Ar or N2. After reaction, the Ag, Au, and Pt
nanoparticles were purified by precipitation with methanol, followed by
centrifugation and washing with methanol, and then redispersed in
20 mL of toluene.
2.3. Synthesis of Core−Shell Ag−M (M = Ru, Rh, Pd, Pt, Os, Ir,
PtRu, PtRh, PtOs, PtRuOS, or PtRhOs) Nanoparticles. For the
synthesis of core−shell nanoparticles with a Ag core and a monometallic
shell, 85 mg of AgNO3 was added to 20 mL of oleylamine in a three-
necked flask fitted with a condenser and a stir bar. The solution was
heated and kept at 150 °C under flowing Ar or N2 for 1 h for the
reduction of Ag+ by oleylamine. Then, 104 mg of RuCl3, or 105 mg of
RhCl3, 88 mg of PdCl2, 148 mg of OsCl3, 220 mg of IrCl4, 208 mg of
K2PtCl4 was added swiftly, followed by heating and keeping at the target
temperature (160 °C for Pd and Pt, 250 °C for Rh, 320 °C for Ru, Os,
and Ir) for 1 h under flowing Ar or N2 for the reduction of the shell metal
precursor. After reaction, the core−shell Ag−M nanoparticles were
purified by precipitation with methanol, centrifugation, washing with
methanol, and redispersed in 20 mL of toluene.
For the synthesis of core−shell nanoparticles with a Ag core and an
alloy shell, 85 mg of AgNO3 was added to 20 mL of oleylamine in a
three-necked flask fitted with a condenser and stir bar. The solution was
heated to and maintained at 150 °C under flowing Ar or N2 for 1 h for
the reduction of Ag+ by oleylamine. Then, 104 mg K2PtCl4/52 mg
RuCl3, or 104 mg K2PtCl4/52 mg RhCl3, 104 mg K2PtCl4/74 mg OsCl3,
70 mg K2PtCl4/35 mg RuCl3/50 mg OsCl3, 70 mg K2PtCl4/35 mg
RhCl3/50 mg OsCl3 was added swiftly, followed by heating and keeping
at the target temperature (250 °C for PtRh, 320 °C for PtRu, PtOs,
PtRuOs, and PtRhOs) for 1 h under flowing Ar or N2 for the reduction
of the shell metal precursors. After reaction, the core−shell nano-
particles were purified by precipitation with methanol, centrifugation,
washing with methanol, and redispersed in 20 mL of toluene.
2.4. Inside-out Diffusion of Ag in Core−Shell Ag−M Nano-
particles. Typically, the inside-out diffusion of Ag in core−shell Ag−Ru
nanoparticles, core−shell Ag−Rh nanoparticles, and core−shell Ag−Pt
nanoparticles was observable by transmission electron microscopy
(TEM) after aging the core−shell nanoparticles in toluene at room
temperature for 6, 8, and 7 months, respectively.
2.5. Synthesis of Hollow Metal Nanoparticles. For the syntheses
of hollow metal nanoparticle based on the inside-out diffusion of Ag in
core−shell nanoparticles, 4 mL of core−shell Ag−M nanoparticle
solution was diluted with toluene to 100 mL. An aqueous BSPP solution
was prepared by dissolving 500 mg of BSPP in 100 mL of water. The
diluted core−shell Ag−M nanoparticles in toluene and the aqueous
solution of BSPP were then mixed together. The molar ratio of BSPP/
Ag in the mixture was calculated to be ∼10/1 (this ratio was also used in
subsequent synthesis of cage-bell structured metal nanomaterials). The
mixture was aged for 48 h under vigorous stirring at room temperature
to remove Ag from the core−shell Ag−M nanoparticles. The mixture
was left to stand, and the upper toluene phase was collected (nominally
0.5 mM of Ru, Rh, Os, Ir, or Pt; 0.25 mM of alloy PtRu, PtRh, or PtOs;
0.17 mM of alloy PtRuOs, or PtRhOs).
2.6. Synthesis of Ag2X (X = S, Se, or Te) Nanoparticles. For the
synthesis of Ag2S nanocrystals, 1 mL of 50 mM aqueous solution of Na2S
was added swiftly to 50 mL of any of the aqueous phases obtained in
section 2.5, which contained the complex formed by BSPP and AgI ions.
The mixture was stirred for 1 h at room temperature. The color of the
mixture turned brown from colorless, indicating the formation of Ag2S
hydrosol.
Herein, we report a facile generic approach, which is based on
the inside-out diffusion of Ag in core−shell structures, for the
fabrication of hollow or cage-bell noble metal nanomaterials. In
this strategy, core−shell Ag−M nanoparticles or core−shell−
shell MA−Ag−MB nanoparticles are first prepared in an organic
solvent, followed by the removal of Ag from the core or the inner
shell with bis(p-sulfonatophenyl)phenylphosphane dihydrate
dipotassium salt (BSPP), which binds strongly with Ag(I)/
Ag(0) to promote the inside-out diffusion of Ag in the core−shell
structure, which is typically completed in 24−48 h. In this report,
the preparation of monometallic nanoparticles with a hollow
interior will be demonstrated first, followed by the preparation of
hollow nanoparticles with a bimetallic or trimetallic alloy shell,
and finally CBS metal nanomaterials with different core and shell
compositions. The hollow and CBS nanoparticles prepared as
such have relatively lower densities, which usually translate to a
higher surface area than their solid counterparts; hence, a higher
degree of metal utilization (for example, in catalysis) can be
expected. In particular, CBS Pt−Ru nanoparticles were found to
exhibit outstanding methanol tolerance for the cathode reaction
of the direct methanol fuel cell (DMFC). This may be attributed
to the difference in the diffusion of methanol and oxygen through
the shell of the CBS nanoparticles. Hence, a diffusion-limited
design, rather than the intrinsic properties of the catalytic metals,
may be used to deliver the desired catalyst selectivity for the
mitigation of the methanol crossover problem in DMFCs.
2. EXPERIMENTAL SECTION
2.1. Materials and Common Procedures. Ruthenium(III)
chloride (RuCl3, Ru content 45−55%), rhodium(III) chloride (RhCl3,
98%), palladium(II) chloride (PdCl2, 99%), silver nitrate (AgNO3,
99%), osmium(III) chloride (OsCl3, 99.9%), iridium(IV) chloride
(IrCl4, 98%) potassium tetrachloroplatinate(II) (K2PtCl4, 98%),
gold(III) chloride (AuCl3, 99%), oleylamine (70%, technical grade),
sodium sulfide nonahydrate (Na2S·9H2O, ≥98%), selenide powder (Se,
≥99.5%), tellurium powder (Te, 99.8%), sodium borohydride (NaBH4,
98%), sodium citrate dehydrate (HOC(COONa)(CH2COONa)2
·
2H2O, ≥99%), dodecylamine (DDA, 98%), aqueous HClO4 solution
(70%, ACS reagent), and Nafion 117 solution (5% in a mixture of lower
aliphatic alcohols and water) from Sigma-Aldrich; methanol (99%) and
ethanol (99.5%) from Merck; toluene (99.5%) from J. T. Baker; bis(p-
sulfonatophenyl)phenylphosphane dihydrate dipotassium salt (BSPP,
97%) from Strem Chemicals; hexadecyltrimethylammonium bromide
(CTAB, 98%) from Lancaster; and Vulcan XC-72 carbon powders (XC-
72C, BET surface area = 250 m2/g and average particle size = 40−
50 nm) from Cabot were used as received. All glassware and Teflon-
coated magnetic stir bars were cleaned with aqua regia, followed by
copious rinsing with deionized water before drying in an oven.
2.2. Synthesis of Ag, Au, and Pt Nanoparticle Seeds. In a typical
synthesis of Ag or Au nanoparticles, 85 mg of AgNO3 (or 80 mg of
AuCl3) was dissolved in 20 mL of oleylamine placed in a three-necked
flask equipped with a condenser and stir bar. The solution was heated to
150 °C under flowing Ar or N2 and kept at this condition for 1 h for the
reduction of Ag+ (or Au3+) ions by oleylamine, which also served as the
capping agent. No other reducing agent and stabilizer were used to form
the Ag or Au nanoparticles (Supporting Information Figure S1a−d). For
the synthesis of Pt nanoparticles, 70 mg of K2PtCl4 and 2 mg of AgNO3
were added to 20 mL of oleylamine. A small amount of AgNO3 was
needed to promote the formation of spherical Pt nanoparticles
(Supporting Information Figure S1e,f). In the absence of Ag, Pt
nanotetrapods would be formed instead (Supporting Information
Figure S2). The mixture was then heated and kept at 160 °C for 1 h with
For the synthesis of Ag2Se or Ag2Te nanocrystals, 1 mL of 50 mM
aqueous solution of Na2Se or Na2Te (prepared by reacting 79 mg of Se
or 127 mg of Te powder with 38 mg of NaBH4 in 20 mL of water) was
added swiftly to 50 mL of any of the aqueous phases obtained in section
2.5. The mixture was heated to 80 °C and kept at this temperature with
stirring for 1 h to form the Ag2Se or Ag2Te hydrosol.
The sulfonic groups of BSPP imparted negative charges to the Ag2X
(X = S, Se, or Te) nanocrystals, allowing us to manipulate the solubility
of the Ag2X nanocrystals in an organic phase using electrostatic
interactions. The Ag2X hydrosol was mixed with 50 mL of toluene
solution of CTAB (100 mM) and the mixture was stirred for 3 min.
CTAB/BSPP-stabilized Ag2X nanocrystals were extracted to the toluene
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layer rapidly, leaving behind a colorless aqueous solution. The ion pair
formation on the Ag2X nanocrystals surface was induced by the
electrostatic interaction between the negatively charged sulfonic anions
on the surface of Ag2X nanocrystals and the incoming ligand,
hexadecyltrimethylammonium cations.
analysis began with concentrating 5 mL of the toluene solution of the
metal nanoparticles to 0.5 mL using flowing Ar. Ten milliliters of
methanol was then added to precipitate the metal nanoparticles. The
nanoparticles were then recovered by centrifugation and washed with
methanol several times to remove nonspecifically bound oleylamine.
The nanoparticles were then dried at room temperature in vacuum.
2.10. Electrochemical Measurements. Electrochemical measure-
ments were carried out in a standard three-electrode cell, which was
connected to a PGSTAT 30 potentiostat. A leak-free Ag/AgCl
(saturated with KCl) electrode was used as the reference. The counter
electrode was a platinum mesh (1 × 1 cm2) attached to a platinum wire.
For the loading of the catalyst on Vulcan XC-72 carbon support, a
calculated amount of carbon powder was added to the toluene solution
of CBS Pt−Ru nanoparticles. After stirring the mixture for 24 h, the CBS
Pt−Ru nanoparticles/C (∼20 wt % Pt on carbon support) were
collected by centrifugation and washed 3 times with methanol. They
were then dried at room temperature in vacuum.
A thin layer of Nafion-impregnated catalyst cast on a vitreous carbon
disk was used as the working electrode. Ten milligrams of the CBS Pt−
Ru nanomaterials/C was ultrasonically dispersed in 10 mL of aqueous
solution containing 4 mL of ethanol and 0.1 mL of the Nafion solution.
A calculated volume of the ink was dispensed onto the 5 mm glassy
carbon disk electrode to produce a nominal catalyst loading of 20 μg cm−2
(Pt basis). The carbon electrode was then dried in a stream of warm air
at 70 °C for 1 h.
A solution of up to 1 M methanol in 0.1 M HClO4 was used for testing
the methanol tolerance of the CBS Pt−Ru nanoparticles. The rotation
rate of the glass carbon rotating disk electrode (RDE) was 1600 rpm.
Negative-going linear sweep voltammograms were recorded from 1 to
0 V at 20 mV·s−1 at room temperature in the presence of bubbling
ultrapure oxygen to maintain a saturated oxygen atmosphere near the
working electrode. Current densities were all based on the geometric
area (0.196 cm2) of the glassy carbon electrode.
For comparison, the catalytic activity of a commercial Pt/C catalyst
for ORR in 0.1 M HClO4 in the presence of 0.1 M methanol was also
evaluated. The commercial Pt/C catalyst was a E-TEK catalyst with
20 wt % Pt nanoparticles (3.2 nm) on Vulcan XC-72 carbon support.
2.7. Synthesis of Core−Shell−shell MA−Ag−MB (MA = Au or
Pt; MB = Ru, Os, Ir, Pt, or PtRh) Nanoparticles. For the synthesis of
core−shell−shell Au−Ag−MB (MB = Os, Ir, or Pt) nanoparticles, 80 mg
of AuCl3 was dissolved in 20 mL of oleylamine in a three-necked flask
fitted with a condenser and stir bar. The solution was heated to and
maintained at 150 °C under flowing Ar or N2 for 1 h for the reduction of
Au3+ ions by oleylamine. Then, 85 mg of AgNO3 was added swiftly and
the reaction mixture was kept at 150 °C under flowing Ar or N2 for
another hour for the growth of Ag on the existing Au seeds. Then,
148 mg of OsCl3, 220 mg of IrCl4, or 208 mg of K2PtCl4 was added
swiftly, followed by heating to and keeping at the target temperature
(320 °C for Os and Ir, 160 °C for Pt) for 1 h under flowing Ar or N2 for
the reduction of the metal precursors in the presence of previously
formed core−shell Au−Ag nanoparticles. After the reaction, the core−
shell−shell Au−Ag−MB nanoparticles were purified by precipitation
with methanol, centrifugation, washing with methanol, and redispersed
in 20 mL of toluene.
For the synthesis of core−shell−shell Pt−Ag−MB (MB = Ru, Os, Ir,
Pt, or PtRh) nanoparticles, 70 mg of K2PtCl4 and 2 mg of AgNO3 were
added to 20 mL of oleylamine. The mixture was heated to and
maintained at 160 °C for 1 h under flowing Ar or N2 and stirring to
prepare the Pt seeds. Then, 85 mg of AgNO3 was added swiftly and the
temperature of the reaction mixture was lowered to 150 °C and
maintained at this temperature under flowing Ar or N2 for another hour
for the growth of Ag on existing Pt seeds. Then, 104 mg of RuCl3, 148
mg of OsCl3, 220 mg of IrCl4, 208 mg of K2PtCl4, or 104 mg of K2PtCl4/
52 mg of RhCl3 was added swiftly, followed by heating and keeping the
reaction mixture at the target temperature (320 °C for Ru, Os, and Ir;
160 °C for Pt, 250 °C for PtRh) for 1 h under flowing Ar or N2 for the
reduction of the metal precursors in the presence of previously formed
core−shell Pt−Ag nanoparticles. After the reaction, these core−shell−
shell Pt−Ag−MB nanoparticles were purified by precipitation with
methanol, centrifugation, washing with methanol, and redispersed in
20 mL of toluene.
2.8. Synthesis of Cage-Bell Structured (CBS) Metal Nano-
particles. For the syntheses of CBS metal nanomaterials based on the
inside-out diffusion of Ag in core−shell nanoparticles, 4 mL of core−
shell−shell MA−Ag−MB nanoparticle solution was diluted with toluene
to 100 mL. An aqueous BSPP solution was prepared by dissolving
500 mg of BSPP in 100 mL of water. The diluted core−shell−shell MA−
Ag−MB nanoparticles in toluene and the BSPP aqueous solution were
then mixed together. The mixture was aged for 48 h under vigorous
stirring at room temperature. The mixture was left to stand, and the
upper toluene phase was collected.
2.9. Characterizations. TEM, high-resolution TEM (HRTEM),
and high-angle annular dark-field scanning TEM (HAADF-STEM)
were performed on a FEI Tecnai G2 F20 electron microscope operating
at 200 kV with a supplied software for automated electron tomography.
For the TEM measurements, a drop of the nanoparticle solution was
dispensed onto a 3-mm carbon-coated copper grid. Excess solution was
removed by an absorbent paper, and the sample was dried under vacuum
at room temperature. Average particle size and particle size distribution
were sampled from a few randomly chosen areas in the TEM image
containing ∼200 nanoparticles each. An energy dispersive X-ray
spectroscopy (EDX) analyzer attached to the TEM operating in the
scanning transmission electron microscopy (STEM) mode was used to
analyze the element distributions in the core−shell, hollow, and cage-
bell structured nanoparticles. The electron beam was only 0.7 nm in
diameter, capable of providing a high-resolution analysis. UV−visible
spectra of the core−shell nanoparticle solution before and after the
BSPP treatment were collected on a Shimadzu UV-2450 spectropho-
tometer. X-ray photoelectron spectroscopy (XPS) was conducted on a
VG ESCALAB MKII spectrometer. Powder X-ray diffraction (XRD)
patterns were recorded on a Rigaku D/Max-3B diffractometer, using Cu
Kα radiation (λ = 1.54056 Å). Sample preparation for XPS and XRD
3. RESULTS AND DISCUSSION
We discovered recently that Ag could diffuse out from the core or
the inner shell of Ag-containing single or double shell core−shell
metal nanoparticles. We will use core−shell Ag−Pt nanoparticles
to illustrate this interesting diffusion phenomenon. The TEM
images in Figure 1a,b show the initial uniform core−shell Ag−Pt
nanoparticles. After storage in toluene for 7 months at room
temperature, Ag diffused out from the interior of the core−shell
Ag−Pt nanoparticles and TEM showed a product very different
from the original nanoparticles (Figure 1c,a): there was
increasing presence of isolated Ag nanoparticles and hollow Pt
nanoparticles in a mixture of nanoparticles. The high-resolution
TEM (HRTEM) image of Figure 1d reveals the structures of the
isolated Ag and hollow Pt nanoparticles: two different sets of
lattice fringes with separations of 0.236 and 0.228 nm,
corresponding well with the {111} planes of face-centered
cubic (fcc) Ag and Pt, were found in the solid and hollow
nanoparticles, respectively. Two more examples of the inside-out
diffusion of Ag may be given. They occurred in core−shell Ag−Ru
and core−shell Ag−Rh nanoparticles and are illustrated in
Supporting Information Figures S7 and S8, as seen in Part III of
Supporting Information (SI).
The inside-out diffusion of Ag was promoted by the structure
of the Ag seeds. Figure S1a,b (SI Part I) are the TEM and
HRTEM images of Ag nanoparticles synthesized in oleylamine at
150 °C, which were used as seeds in the preparation of core−
shell Ag-M nanoparticles. These Ag seeds were ∼9 nm multiply
twinned decahedral nanoparticles.45 The twinned structure had a
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grow slowly even in the presence of etchant at the expense of the
twinned nanoparticles. Thus, the inside-out diffusion of Ag in
core−shell Ag−Pt nanoparticles can be rationalized by the
scheme shown in Figure 1e. Because of the concentration
gradient between the interior of the core−shell particles and the
surrounding solution, the AgI ions generated from the O2/Cl−
etching of twinned Ag seeds diffused out through the
discontinuous Pt shell, and were reduced by oleylamine to
form isolated single crystalline Ag nanoparticles in the colloidal
solution. With the progress of time, the core−shell Ag−Pt
nanoparticles would eventually disappear due to the etching and
outward diffusion of Ag to leave behind a physical mixture of
hollow Pt nanoparticles and single crystalline Ag nanoparticles.
Oleylamine was an indispensable multifunctional agent in this
case: It was simultaneously a particle stabilizing agent and the
reducing agent for the etched Ag ions. Without it, the nucleation
and growth of isolated single crystal Ag nanoparticles from
dissolved Ag twined particles would not be possible.
The scheme in Figure S9a (SI Part IV) may be used to generate
hollow metal nanoparticles by the inside-out diffusion
phenomenon of Ag in core−shell nanoparticles. In this protocol,
the preparation of core−shell Ag−M nanoparticles is an
important step preceding the fabrication of hollow noble metal
nanoparticles. Figure S4 (SI Part II) shows the TEM, HRTEM,
and scanning TEM (STEM) images of core−shell Ag−Ru,
Ag−Rh, Ag−Os, Ag−Ir, and Ag−Pt nanoparticles prepared by
the seed-mediated growth method. The formation of core−shell
nanoparticles was first suggested by brightness contrast (SI
Figure S4a−q) and then confirmed by energy-dispersive X-ray
(EDX, SI Figure S4d−t) analyses of arbitrarily chosen single
particles in the high-angle annular dark-field scanning TEM
mode. As indicated in SI Figure S4d−t, the Ru, Rh, Os, Ir, and Pt
signals were present throughout the particle, whereas the Ag
signal was detected only in the core region (∼9 nm).
However, Figure 1 and SI Figure S7 and S8 also show that the
inside-out diffusion of Ag could take months to complete under
normal circumstances. This process could however be
accelerated by introducing a chemical reagent BSPP, which
binds strongly with Ag as AgI−BSPP coordination complexes to
drive the outward diffusion of Ag in the core−shell nano-
particles,47,48 enabling the completion of the inside-out diffusion
of Ag in a much shorter time frame of 24−28 h (with agitation)
after mixing the core−shell Ag−M organosol with an aqueous
BSPP solution. The BSPP−AgI coordination compounds are
water-soluble and their continuous formation left behind an
organosol of hollow noble metal nanoparticles, shown by TEM
(Figure 2) as an increase in the image contrast between the core
and shell regions, and the development of visible discontinuity in
the metal shell. The removal of Ag was confirmed by EDX
analysis of the final products. As shown in Figure 2c,f,i,l,o, Ag
signal was not detected in the core−shell nanoparticles after
BSPP treatment. For the core−shell Ag−Pt nanoparticles, the
broad absorption in the visible light region due to the surface
plasmon resonance of Ag nanoparticles also disappeared after the
BSPP treatment (SI Figure S10), corroborating the elimination
of the Ag cores from the core−shell Ag−Pt nanoparticles. The
powder X-ray diffraction (XRD) patterns (SI Figure S11) of the
remaining hollow nanoparticles could be indexed to face-
centered cubic (fcc) Rh, Ir, or Pt and hexagonal Ru and Os.49
The only exception was Pd where nanoparticles with hollow
interiors could not be formed this way. The reduction of a Pd
precursor in the presence of Ag nanoparticle seeds could
not form core−shell nanoparticles; Ag−Pd alloy nanoparticles
Figure 1. Inside-out diffusion of Ag in core−shell Ag−Pt nanoparticles:
(a) TEM and (b) HRTEM images of starting core−shell Ag−Pt
nanoparticles; (c) TEM and (d) HRTEM images of core−shell Ag−Pt
nanoparticles after aging in toluene for 7 months at room temperature;
(e) schematic illustration of the mechanism for the inside-out diffusion
of Ag in core−shell Ag−Pt nanoparticles.
strong influence on the stability of the Ag nanoparticles and the
overlaid Pt shell. Generally, decahedral particles are mosaic
structures consisting of five single crystal tetrahedrons oriented
radially about a central axis so that all five tetrahedrons share a
common edge in the center, and each tetrahedron has two sides
in contact with its neighbors. However, this model is not a space
filling one. Since the angle between two {111} planes of a
tetrahedron is theoretically 70.53°, five tetrahedrons with
adjoining {111} planes will leave a gap of 7.35°. To make up
for this difference, significant lattice distortions and defects must
occur.46 These imperfections in the Ag seed nanoparticles were
disruptive to the epitaxial deposition of Pt atoms on the Ag seeds,
resulting in roughness and discontinuity in the Pt shell
subsequently formed. The twinned Ag nanoparticles were also
inherently unstable and could slowly be etched by dissolved O2
and Cl− dissociated from the Pt precursor. The crystallographic
defects in the twinned particles were the active sites for the
oxidative dissolution of the nanoparticles. The AgI ions released
by the O2/Cl− etching of twinned Ag nanoparticles could be re-
reduced by excess oleylamine in the solution to single crystalline
Ag nanoparticles. In principle, the transformation from twinned
to single crystalline particles is reversible since O2/Cl− etching
should also operate on single crystalline Ag nanoparticles.
However, as pointed out by Xia and co-workers,46 single
crystalline nanoparticles are less soluble and would continue to
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Figure 2. Hollow structured monometallic nanomaterials: (a, d, g, j, and m) TEM images, (b, e, h, k, and n) HRTEM images, and (c, f, i, l, and o) EDX
analyses of hollow (a−c) Ru, (d−f) Rh, (g−i) Os, (j−l) Ir, and (m−o) Pt nanoparticles.
(SI Figure S6) were the product instead. HRTEM imaging showed
excellent atomic ordering within each particle (SI Figure S6b)
indicating no lattice mismatches. The uniform contrast
throughout the particle is further proof that mixing of the
metallic components occurred at the atomic level. EDX analysis
of an arbitrary single particle in the STEM mode (SI Figure S6d)
also produced an uniform distribution of Ag and Pd signals across
the entire Ag−Pd nanoparticle. The formation of Ag−Pd alloy
nanoparticles in organic solution here is similar to the formation
of Ag−Au or Ag−Pd alloy nanoparticles by the replacement
reaction between Ag nanoparticles and Au or Pd precursors in
aqueous solution.50 Alloying rather than a core−shell con-
struction was caused by the rapid interdiffusion of Ag and Pd
atoms at an elevated synthesis temperature.
With a slight modification, the protocol can be used to
synthesize hollow alloy nanoparticles (SI Figure S9b). In this
case, co-reduction in the presence of preformed Ag seeds
involves two or more shell precursor metals in order to form an
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Figure 3. Hollow structured alloy nanoparticles: (a, e, i, m, and q) TEM images, (b, f, j, n, and r) HRTEM images, (c, g, k, o, and s) STEM images, and (d,
h, l, p, and t) element profiles of hollow (a−d) PtRu alloy, (e−h) PtRh alloy, (i−l) PtOs alloy, (m−p) PtRuOs alloy, and (q−t) PtRhOs alloy
nanoparticles.
alloy shell on the Ag core. The second step remains the same−
removal of the Ag core by BSPP treatment. SI Figure S5 shows
the TEM, HRTEM, and STEM images of core−shell Ag−PtRu,
Ag−PtRh, Ag−PtOs, Ag−PtRuOs, and Ag−PtRhOs nano-
suggesting that the removal of the Ag core from the core−shell
nanoparticles did not cause the collapse of the particle geometry.
It should be mentioned that the aqueous phase after the BSPP
treatment, which contained the coordinating compounds formed
by BSPP and AgI ions, is not a chemical waste. These BSPP−AgI
coordinating compounds were used to produce monodisperse
silver chalcogenide nanocrystals in aqueous solution by reactions
with Na2S, Na2Se, or Na2Te at room or elevated temperatures
(SI Part V). The TEM and HRTEM images of the Ag2S, Ag2Se,
and Ag2Te nanocrystals produced are shown in SI Figure S12.
XRD patterns (SI Figure S13) indicate that the Ag2S and Ag2Te
nanocrystals were monoclinic and the Ag2Se nanocrystals were
orthorhombic. These silver chalcogenide nanocrystals could be
further used to generate semiconductor-metal nanocomposites
for efficiently catalyzing methanol oxidation at room temper-
ature.51
particles prepared in this study. The formation of Agcore
−
alloyshell nanoparticles was again confirmed by EDX analysis of
single particles in the STEM mode. As shown in SI Figure S5d−t,
the Ag signal of Ag in all types of core−shell particles was
confined to the ∼9 nm core region, whereas the Pt and Ru in
Ag−PtRu; Pt and Rh in Ag−PtRh; Pt and Os in Ag−PtOs; Pt, Ru,
and Os for Ag−PtRuOs; and Pt, Rh, and Os for Ag−PtRhOs
were uniformly distributed throughout the nanoparticles. EDX
analysis in the STEM mode of core−shell Ag−PtRu, Ag−PtRh,
Ag−PtOs, Ag−PtRuOs, and Ag−PtRhOs after the BPSS
treatment (Figure 3d−t), on the other hand, confirmed the
complete removal of the Ag component, indicating the successful
generation of hollow PtRu, PtRh, PtOs, PtRuOs, and PtRhOs
alloy nanoparticles. TEM, HRTEM, and STEM images of the
hollow nanoparticles are given in Figure 3. The particle size and
morphology were virtually unchanged by the BSPP treatment,
The protocol can be further modified to synthesize noble
metal nanoparticles with a cage-bell structure. The cage-bell
structure in this work refers to a movable core enclosed by a shell
with nanochannels. In principle, the core and shell components
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Figure 4. Cage-bell structured (CBS) metal nanomaterials: (a, e, i, m, q, and u) TEM images, (b, f, j, n, r, and v) HRTEM images, (c, g, k, o, s, and w)
STEM images, and (d, h, l, p, t, and x) element profiles of (a−d) CBS Au−Os, (e−h) CBS Au−Ir, (i−l) CBS Au−Pt, (m−p) CBS Pt−Ru, (q−t) CBS
Pt−Os, and (u−x) CBS Pt−Pt nanoparticles.
may be made of metals or alloys. In the modification illustrated in
SI Figure S16, the formation of core−shell−shell MA−Ag−MB
nanoparticles, in which the inner Ag shell serves as the sacrificial
component, is most critical. In the current study, Au and Pt were
chosen as the core component (MA), with Ru, Os, Ir, Pt and alloy
PtRh as the outermost shell component (MB). The syntheses
and characterizations of 13 nm Au and 4 nm Pt seed nano-
particles are described in Experimental Section. The successful
preparation of the Au and Pt seeds was confirmed by X-ray
photoelectron spectroscopy (XPS) (SI Figure S3): a doublet in
the XPS spectrum of Au at 83.6 and 87.3 eV assignable to Au at
the zero valence state,52 and two pairs of doublets in the XPS
spectrum of Pt. The more intense doublet at 70.8 and 74.1 eV are
characteristic of Pt in the zero valence state. The second and
weaker doublet at 71.2 and 74.5 eV, with binding energies 1.4 eV
higher than those of the zerovalent Pt, may be assigned to PtO.52
It should be mentioned that a small amount of AgNO3 was used
in the synthesis of Pt nanospheres, without which Pt nano-
tetrapods were formed instead (SI Figure S2).
The procedures in SI Figure S16 were then used to form the
CBS nanoparticles. The seed nanoparticles (MA, Au and Pt) were
overlaid with Ag first, followed by the growth of another metal
(MB) shell to form MA−Ag−MB nanoparticles with the requisite
core−shell−shell structure. Electron microscopy images and the
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results of STEM-EDX analysis of core−shell−shell Au−Ag−Os,
Au−Ag−Ir, Au−Ag−Pt, Pt−Ag−Ru, Pt−Ag−Os, Pt−Ag−Ir,
Pt−Ag−Pt, and Pt−Ag−PtRh nanoparticles are given in SI
Figures S14 and S15, respectively. The core−shell−shell
structure was confirmed by the element profiles in SI Figure
S14d−l and Figure S15d−t, which show different distributions
for different metals across the particle. After BSPP treatment, the
inner Ag layer was removed from the MA−Ag−MB nanoparticles,
leaving behind Au−Os, Au−Ir, Au−Pt, Pt−Ru, Pt−Os, Pt−Ir,
Pt−Pt, and Pt−PtRh nanoparticles with the cage bell structure.
SI Figure S17 shows the UV−visible spectra in each stage of the
preparation of CBS Au−Pt and Pt−Ru nanoparticles, where
changes in optical properties after the BSPP treatment may be
used an indirect evidence for the elimination of the inner Ag shell
from the core−shell−shell MA−Ag−MB nanoparticles. More
direct evidence was provided by the disappearance of Ag signal in
the EDX analysis of core−shell−shell MA−Ag−MB after the
BSPP treatment (Figure 4d−x and SI Figure S18d−h). Electron
microscopy images (Figure 4 and SI Figure S18) show the
preservation of the size and morphology of the core−shell−shell
nanoparticles in the CBS nanoparticles. The void space between
the core and the outer shell regions, formed upon the elimination
of the Ag inner shell by BSPP, is discernible by the strong
brightness contrast in TEM, HRTEM, and STEM images.
Most of the materials in this work are valuable and important
catalytic metals (Pd, Pt, Rh, Ru, PtRh, PtRu, etc); thus, one
would expect the hollow and CBS metal nanoparticles to yield
significant applications for catalysis due to their advantage of low
density, which translated to a higher surface area and would be
beneficial to the catalytic reaction.
An unique application was raised by the CBS nanoparticles
prepared in this work. The anode and cathode catalysts of current
DMFCs are commonly based on Pt. These catalysts are not
selective to methanol oxidation reaction (MOR) or oxygen
reduction reaction (ORR) and hence any methanol crossover
from the anode to the cathode through the proton exchange
membrane (PEM) can be oxidized by the cathode catalyst. This
results in the creation of a mixed potential at the cathode which
degrades the fuel cell performance.53−55 We discovered that the
CBS Pt−Ru nanoparticles have none of the failing of common
Pt-based catalysts, and are effectively methanol tolerant in
oxygen reduction. As proof, Figure 5a shows the polarization
curves of ORR on CBS Pt−Ru nanoparticles in the presence of
different methanol concentrations. Even with the presence of
methanol in concentrations as high as 1.0 M in the electrolyte,
the catalytic reduction of oxygen on the CBS Pt−Ru nano-
particles was hardly affected, demonstrating the effective
inhibition of methanol oxidation on the CBS Pt−Ru nano-
particles. For comparison, oxygen reduction on a commercial E-
Tek Pt/C catalyst (3−5 nm nanoparticles on carbon powder, SI
Figure S19) with and without methanol was also measured
(Figure 5b). The ORR polarization curve in this case was clearly
affected even at a low methanol concentration of 0.1 M: a valley
was formed at the potential for methanol oxidation. In the CBS
Pt−Ru catalyst, the catalytically active metal, that is, Pt, is located
in the core region shielded by a porous Ru shell. Reactants must
diffuse through the porous shell of the CBS nanoparticles to
access the active core for catalysis to occur. In this case, the
selectivity for ORR may be caused by the porous shell permitting
only the passage of small molecule reactants. The situation is
depicted by the scheme in Figure 5c. Methanol and oxygen must
diffuse into the CBS nanoparticle interior through the porous Ru
shell for MOR and ORR to occur. However, a methanol
Figure 5. Inhibition of methanol oxidation by CBS Pt−Ru nano-
particles: (a) polarization curves of ORR on CBS Pt−Ru nanoparticles
in the presence of (black) 0.0 M methanol, (red) 0.1 M methanol,
(green) 0.5 M methanol, and (blue) 1.0 M methanol; (b) polarization
curves of ORR on commercial Pt/C catalyst in the presence of (black)
0.0 M methanol and (red) 0.1 M methanol; (c) schematic illustration of
the differential diffusion and reaction of reactants in CBS Pt−Ru
nanoparticles.
molecule is larger than an oxygen molecule (the diameters of
methanol and oxygen molecules are 0.44 and 0.34 nm,
respectively). Hence, the diffusion of O2 is faster than the
diffusion of methanol in CBS Pt−Ru nanoparticles, rendering the
oxidation of methanol on CBS Pt−Ru a noncompetitive event.
By tailoring the structures (e.g., the size of Pt core and the
porosity of Ru shell) of the CBS nanoparticles, one would expect
the ORR catalytic activity and methanol-tolerant property of
CBS Pt−Ru nanoparticles could be further enhanced.
4. CONCLUSION
In summary, we have demonstrated a facile general strategy for
the synthesis of noble metal nanoparticles with hollow or cage-
bell structures based on the inside-out diffusion of Ag in
precursor core−shell composite metal nanoparticles. A selective
BSPP treatment was used to accelerate the inside-out diffusion of
Ag. The synthesis was carried out at room temperature. The
synthesis byproduct of Ag(I)−BSPP complexes could be used for
the preparation of silver chalcogenide nanocrystals. The
recycling of silver minimizes the environmental footprint and
also improves the overall economics of the process. As an unique
property, CBS Pt−Ru nanoparticles displayed excellent methanol
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