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Shape-Controlled Nanocrystals for Catalytic Applications
Hyunjoo Lee • Cheonghee Kim • Sungeun Yang •
Joung Woo Han • Jiyeon Kim
� Springer Science+Business Media, LLC 2011
Abstract The activity, selectivity, and long-term stability
of catalyst nanoparticles can be enhanced by shape mod-
ulation. Such shaped catalytic nanocrystals have well-
defined surface crystalline structures on which the cleavage
and recombination of chemical bonds can be rationally
controlled. Metal and metal oxide nanocrystals have been
synthesized in various shapes using wet chemistry tech-
niques such as reducing metal precursors in the presence of
the surface-capping agents. The surface-capping agents
should be removed prior to the catalytic chemical reaction,
which necessitates clean catalytically active surface. The
removal process should be performed very carefully
because this removal often causes shape deformation.
A few examples in which the surface-capping agents
contribute positively to the chemical reactions have been
reported. The examples described in this review include
shaped metal, metal composite, and metal oxide nano-
crystals that show enhanced catalytic activity, selectivity,
and long-term stability for various gas-phase, liquid-phase,
or electrocatalytic reactions. Although most of the studies
using these shaped nanocrystals for catalytic applications
have focused on low-index surfaces, nanocrystals with
high-index facets and their catalytic applications have
recently been reported. By bridging surface studies with
nanoparticle catalysts using shape modulation, catalysts
with improved properties can be rationally designed.
Keywords Shape-control � Nanocrystal � Catalysts �Platinum � Surface-capping agents
1 Introduction
Heterogeneous catalytic reactions occur when reactants are
adsorbed on a surface and undergo a cleavage or recom-
bination of their chemical bonds. Therefore, the surface
structure of catalysts strongly affects their catalytic activ-
ity, selectivity, and long-term stability. The atomic
arrangement on such a surface differs depending on its
crystalline structure. For example, a {111} surface has a
hexagonal atomic arrangement, whereas a {100} surface
has a square atomic arrangement for face-centered cubic
(FCC) metals such as Pt, Au, Ag, Pd, and Rh. For high-
index surfaces, where one of the indices h, k, or l is greater
than one for an {hkl} surface, there are more steps or kinks
compared with low-index surfaces. For decades, numerous
surface studies have reported changes in catalytic activity
or selectivity when these reactions occur on various single-
crystalline surfaces. Somorjai and co-workers [1] reported
that aromatization from hexane to benzene or from heptane
to toluene occurred on a Pt(111) surface much more than
on a Pt(100) surface, as shown in Fig. 1. In the hydrog-
enolysis of methylcyclopentane, the cyclic bond was bro-
ken much more frequently on a Pt(100) surface, producing
many more fragments, than on a Pt (111) surface, on which
aromatization occurred more often to yield benzene [2].
Different catalytic activities on various single-crystalline
surfaces have also been reported for electrocatalytic reac-
tions. When electrochemical formic acid oxidation was
performed on Pt(100), Pt(111), and Pt(110) surfaces, the
Pt(100) surface showed much higher activity in a reverse
scan, whereas the Pt(100) surface was more vulnerable to
surface poisoning in a forward scan, as illustrated in Fig. 2
[3]. However, these single-crystalline surface studies were
usually performed in exceptionally clean conditions far
removed from practical applications. Practical catalysts are
H. Lee (&) � C. Kim � S. Yang � J. W. Han � J. Kim
Department of Chemical and Biomolecular Engineering,
Yonsei University, Seoul 120-749, Republic of Korea
e-mail: [email protected]
123
Catal Surv Asia
DOI 10.1007/s10563-011-9130-z
commonly produced in nanometer-sized particles, and
often must endure more demanding conditions, e.g. higher
pressure and more reaction components.
Recently, as synthetic techniques for nanomaterials have
been actively developed [4–8], various types of shaped
nanocrystals with well-defined surfaces have been realized.
When cubic nanocrystals are synthesized from FCC metals,
the facets consist solely of {100} surfaces, whereas octa-
hedral or tetrahedral nanocrystals have only {111} sur-
faces. The surface structure of such nanocrystals can thus
be controlled by changing their shape. Therefore, catalysts
with higher activity and selectivity can be realized by
bridging surface studies with the synthesis of nanoparticle
catalysts by shape modulation. In this review, we introduce
the various methods developed for the synthesis of shaped
nanocrystals, and provide examples showing the enhance-
ment of catalytic activity, selectivity, and long-term sta-
bility with the use of shaped nanocrystals. We hope that
this short review will provide valuable insight into the high
potential of shaped nanocrystals for catalytic applications.
2 Synthesis of Shaped Nanocrystals Using Wet
Chemistry
The formation of nanoparticles involves a nucleation stage,
wherein a seed is formed, and a subsequent growth stage.
Nanoparticle morphology is often determined by the shape
of the seeds, the direction of growth, and the growth rate
[9, 10]. For example, single-crystal seeds can grow into
cubes, cuboctahedra, and octahedra with single crystalline
natures, whereas multi-twinned seeds can grow into deca-
hedrons and icosahedrons with several crystalline domains
[10, 11]. The rates and direction of crystal growth can be
controlled by the addition of various reducing agents,
surface-capping agents, absorptive small molecules, or
inorganic ions. The reaction temperature and precursor
concentration often play an important role in controlling
the final shape of nanoparticles. It is thus important to
determine an optimal balance among various shape-con-
trolling factors.
2.1 Control of Reducing Rate: Reducing Agents,
Temperature, and Methods for Supplying Metal
Precursors
Reducing power can be varied by using different reducing
agents such as NaBH4, ascorbic acid, diols, or citric acid.
Altering the pH value can also modulate the reducing
power of a given reducing agent. Xia and co-workers
[12, 13] obtained differently shaped Pd nanocrystals by
using different reducing agents in the presence of polyvi-
nylpyrrolidone (PVP). When ethylene glycol was used as a
reducing agent, Pd nanorods were obtained, but when PVP
dissolved in water was used as a weak reducing agent, Pd
nanobars with lower aspect ratios were synthesized. Cubic,
cuboctahedral, and dendritic Pt nanocrystals shown in
Fig. 3 were also synthesized by varying the reducing
agents in the presence of tetradecyltrimethylammonium
bromide (TTAB) [14]. Higher pH values resulted in a
Fig. 1 Differences in catalytic activity for the aromatization reac-
tions of n-hexane or n-heptane depending on the platinum surface
structure (adapted from Ref. [1])
Fig. 2 Formic acid electrooxidation on Pt(100), Pt(111), Pt(110), and
polyoriented Pt single crystals electrodes (adapted from Ref. [3])
H. Lee et al.
123
slower reduction rate with the selective growth of {100}
facets of cubes when NaBH4 was used as a reducing
agents. When NaBH4 was used together with H2 at lower
pH, cuboctahedra with {100} and {111} facets were pro-
duced. When a different reducing agent (ascorbic acid) was
used, dendritic Pt nanocrystals were obtained. Nogami and
co-workers [15] have obtained porous single-crystalline Pt
nanocubes by adjusting pH values in the presence of eth-
ylene glycol, HCl, and PVP.
In addition, the reaction temperature and the method of
providing a metal precursor can be important factors in
controlling the reducing rate. Ren and Tilley demonstrated
that the morphology of platinum nanocrystals is highly
dependent on the reduction temperature. When the tem-
perature is increased, the number of branches in the Pt
nanocrystals increases, producing tripods, octapods, or
multipods [16]. The reducing rate can also be slowed by
adding the metal precursor over a long period of time rather
than dissolving all the precursor at the initial stage. Yang
and co-workers obtained shaped Ag nanocrystals by adding
a mixture of Ag precursor and PVP over long time, as
illustrated in Fig. 4. The selective overgrowth in the h100idirection on these Ag cubes produced much larger Ag
octahedra in the final stage [17].
2.2 Control of Overgrowth Direction by Selective
Adsorption: Surface-Capping Agents
Surface-capping agents play two major roles in synthesiz-
ing shaped nanocrystals; (1) stabilizing the surface of
nanoparticles by preventing the further growth and Ostwald
ripening and (2) hindering the growth in a specific direction
by the selective adsorption of the surface-capping agents
on a certain facet. Metallic nanocrystals have been syn-
thesized in various shapes by changing the type and con-
centration of surface-capping agents [13, 18–22]. Figure 5
shows that a shape evolution of Cu2O crystals, going from
cubes to truncated octahedra, octahedra, and finally to
nanospheres, was obtained by varying the concentration of
PVP when a copper citrate complex solution was reduced
with glucose. The high concentration of PVP hindered the
growth in the h111i direction, generating {111} facets
exclusively [21]. Highly monodisperse cubic platinum
nanocrystals have been synthesized by tuning the carbon
length of alkylamine additives [18]. Recently, Li and co-
workers [19] obtained Pd nanocrystals of various shapes
that are enclosed with well-defined {111} facets, including
icosahedra, decahedra, octahedra, tetrahedra, and triangular
plates, by changing the concentration of oleylamine, which
played a crucial role in shape control by balancing crystal
strain and surface energy.
2.3 Control of Overgrowth Direction by Selective
Adsorption: Small Molecules or Inorganic Ions
Small molecules or inorganic ions selectively adsorbed on
a specific facet have been studied for shape control [13,
23–29]. Yang and co-workers demonstrated that Pd shells
overgrown on cubic Pt nuclei showed various shapes, e.g.,
cubes, cuboctahedra, and octahedra, depending on the
amount of NO2 added. As the NO2 concentration was
Fig. 3 High resolution TEM
images of platinum nanocrystals
obtained by using NaBH4/H2
(cuboctahedron), NaBH4
(cubes), or ascorbic acid
(dendrites) as a reducing agent.
Scale bar 3 nm (adapted from
Ref. [14])
Fig. 4 SEM images of shaped Ag nanocrystals obtained by adding
Ag precursor over an extended period. Scale bar 100 nm (adapted
from Ref. [17])
Shape-Controlled Nanocrystals for Catalytic Applications
123
increased, the shape of the Pd nanocrystal evolved from
cubes to octahedra due to the stabilization of the {111}
facets [28]. Adsorptive inorganic ions were also introduced
as a shape-controlling factor. When metal precursors
(typically Pt or Au) were reduced in an ethylene glycol
solution in the presence of polymeric capping agents, a
trace amount of Ag species can be selectively adsorbed on
{100} facets, promoting overgrowth in the h100i direction
[24, 25, 30]. The shapes of Pt and Au nanocrystals evolve
from cubes to octahedra with an increasing amount of Ag
ions. The Pt case is depicted in Fig. 6 [25]. Sun and co-
workers [26] obtained Pt cubes by adding Fe(CO)5 when Pt
precursors were reduced in the presence of oleylamine and
oleic acid. Additionally, the Br- ions typically present in
the surfactants such as alkyltrimethylammonium bromide
stabilize {100} facets of Rh nanocrystals, preferentially
yielding the cubic shape [31].
3 Effect of Surface-Capping Agents on Catalytic
Properties
Heterogeneous catalytic reactions occur on the surfaces of
nanoparticle catalysts. A clean surface on the catalyst is
essential for active reactions. Because the shape control of
nanoparticle catalysts is usually achieved by utilizing
organic surface-capping agents, removing these capping
agents is important in procuring the expected catalytic
properties, although the complete removal of the capping
agent is rarely achieved. The residual surface-capping
agents on a surface often interfere in the surface reaction,
with either negative or less often positive effects.
3.1 Activity Modulation by Various Surface-Capping
Agents
Although different catalyst nanoparticles may have the same
shape, their synthetic route can significantly affect their cat-
alytic properties. The effect of surface-capping agents was
tested for electrocatalytic H adsorption/desorption, ethylene
hydrogenation, benzene hydrogenation, and liquid-phase
p-nitrophenol reduction. Pt cubes with the same shape were
synthesized using PVP or TTAB as surface-capping agents.
PVP has very long alkyl chains, with a molecular weight of
55,000, and the carbonyl groups in the backbone structure are
known to have strong interactions with Pt surfaces. Con-
versely, TTAB has relatively short alkyl chains consisting of
14 carbons and a molecular weight of 336, and it has no groups
having particularly strong interactions with Pt surfaces. As
presented in Table 1, the activity of these two nanocrystals
showed little difference for liquid-phase p-nitrophenol
reduction because the surface-capping agents spread out
throughout the liquid media, minimizing the effect of
molecular length. However, for the other electrocatalytic or
gas-phase reactions, TTAB-capped Pt cubes showed a supe-
rior activity to PVP-capped Pt cubes, especially with larger
reactants, in the following order of activity ratio (TTAB/
PVP): H \ C2H4 \ C6H6. Because the PVP molecules lying
on the Pt surface covered the active sites more than the TTAB
molecules, TTAB-capped Pt cubes have a cleaner surface
Fig. 5 SEM images of variously shaped Cu2O crystals depending on PVP concentration (adapted from Ref. [21])
Fig. 6 High-resolution TEM
images of platinum nanocrystals
obtained by adding Ag species.
A shape evolution was observed
from cubes to cuboctahedra to
octahedra (adapted from Ref.
[25])
H. Lee et al.
123
with uncovered Pt atom ensembles, which is essential for
surface-catalyzed reactions [14, 32, 33]. Therefore, the effect
of surface-capping agents should be carefully considered
when shaped nanocrystals are used as catalysts.
3.2 Removal of Surface-Capping Agents
Because surface-capping agents often block the active sites
on a catalytic surface and thus have a detrimental effect on
catalytic activity, most previous studies used shaped
nanocrystals as catalysts after the removal of the surface-
capping agents. To remove the residual capping agents, the
treatments with UV-ozone or plasma, repetitive oxidation/
reduction, calcination, and electrochemical imposition of
high voltage have been utilized [34–41]. These processes,
however, often induce changes in nanocrystal shape. Inaba
et al. [37] demonstrated the deformation of Pt cubes after
repetitive potential cycling in the range of 0.05–1.4 V. The
peak at 0.12 V in Fig. 7a represents Pt(110) facets,
whereas the peak at 0.23 V represents Pt(100) facets.
Initially, these Pt cubes showed a large peak only at
0.23 V, but the peak at 0.12 V became larger after repet-
itive cycling. The changing shape of the Pt nanoparticles
was also clearly observed by TEM. The imposition of high
voltage has often been applied to remove residual impuri-
ties from electrocatalysts, but the possibility of shape
deformation should be carefully considered in these cases.
The extent of shape deformation is also affected by
surface-capping agents. Previously, we prepared Pt cubes
using three different kinds of surface-capping agents: PVP,
TTAB, and oleylamine. The Pt cubes were supported in a
TEM grid, and underwent thermal treatment at various
temperatures (100–300 �C) and chemical environments
(air, H2, or N2). The point at which the shape began to
show deformation differed depending on the surface-cap-
ping agent used. Oleylamine-capped Pt cubes stayed
unchanged even at 300 �C under air, whereas the other
cubes showed severe deformation and aggregation, as
illustrated in Fig. 8 [39]. It should be also noted that the
removal of surface-capping agents does not necessarily
guarantee a clean metallic surface. When the removal of
TTAB from Pt nanoparticles was tracked by increasing
temperature using in situ IR, the ammonium head group
remained on the surface at temperatures above 300 �C,
whereas the hydrocarbon tail group was easily removed
below 200 �C [42].
3.3 Unique Roles of Surface-Capping Agents
In certain cases, a surface-capping agent can play a unique
role in the catalytic reaction. For example, Pt nanoparticles
encapsulated with bacterial aminopeptidase (PepA) were
synthesized in a previous study [43]. Here, PepA not only
acted as a surface-capping agent to stabilize the Pt nano-
particles in aqueous solution, but it also enabled enzymatic
Table 1 Turnover frequency (TOF) for various catalytic reactions
using PVP- and TTAB-capped Pt nanocubes as catalysts (adapted
from Ref. [33])
PVP-capped
Pt nanocubes
TTAB-capped
Pt nanocubes
TTAB/
PVP
H desorption
(% active site)
13.3 50.0 3.8
C2H4 hydrogenation
(TOF, S-1) [14]
2.8 20.0 7.1
Benzene hydrogenation
(TOF, S-1) [32]
0.07 0.94 13.4
p-Nitrophenol
hydrogenation
(TOF, S-1)
0.59 0.58 0.98
Fig. 7 a Cyclic
voltammograms of Pt
nanocubes, and TEM images
b before and c after repeated
potential cycling in 0.5 M
H2SO4 at a scan rate of 50 mV/s
(adapted from Ref. [37])
Shape-Controlled Nanocrystals for Catalytic Applications
123
function, demonstrating its own catalytic reaction. Glu-
tamic acid p-nitroanilide was hydrolyzed by PepA, and the
p-nitroanilide product was subsequently converted into
p-phenylenediamine by the Pt nanoparticle; see Fig. 9.
Both the enzymatic surface layer and the Pt nanoparticle
participated in the chemical conversion by catalyzing
sequential reactions.
Additionally, the surface-capping agent can help to pre-
serve the original metallic state of the nanocrystal when
ligand-capped nanoparticles are loaded onto oxide supports.
When dodecylamine-capped Pt nanoparticles were supported
on Fe3O4 for use as catalysts in the preferential oxidation
(PROX) of CO, these catalysts showed higher catalytic
activity than the corresponding ligand-free Pt/Fe3O4 catalyst.
Because dodecylamine protected the nanoparticle surfaces,
the Pt surfaces remained in a metallic state, whereas unpro-
tected Pt nanoparticles underwent more surface oxidation.
The authors suggested that such metal-support interactions
can be tuned and optimized by using protective layers on
nanoparticle surfaces [44].
4 Catalytic Activity Enhancement by Shape
Modulation
Many studies have reported an enhancement of catalytic
activity by altering the shape of catalyst nanoparticles.
In comparison with conventional catalysts with poorly-
defined surfaces, such shaped nanoparticles have regularly
arranged atomic surface configurations. The activity
improvement predicted by single-crystalline surface studies
or molecular simulation results can be realized by shape
modulation. Several examples are shown below for metals,
metal composites, and metal oxide catalysts.
4.1 Metal Catalysts
Numerous examples displaying activity enhancement by
shape modulation have been reported for metal catalysts.
For instance, Pt nanocubes showed a higher activity than
polyhedron nanoparticles for an oxygen reduction reaction
in a sulfuric acid medium [26]. Sulfate ions were adsorbed
Fig. 8 TEM images of a PVP-, b TTAB-, and c oleylamine-capped Pt nanocubes without treatment, d PVP-, e TTAB-, and f oleylamine-capped
Pt nanocubes after heat treatments at 300 �C in air (adapted from Ref. [39])
Fig. 9 a TEM image of PepA–Pt nanoparticles complexes and b reaction schemes for the PepA–Pt nanoparticle-catalyzed productions of p-
phenylenediamine from Glu-p-nitroanilide by proteolysis and hydrogenation (adapted from Ref. [43])
H. Lee et al.
123
on {111} facets more strongly and blocked the active sites
for an oxygen reduction reaction, whereas the activity for
various crystalline surfaces showed less difference in a
perchloric acid solution [45]. The dendritic shape of Pt
nanocrystals also displayed a good activity for an oxygen
reduction reaction. Although most shaped Pt nanoparticles
displayed only enhanced specific activity (i.e., activity per
unit surface Pt atom), Pt dendrites presented a mass activity
(i.e., activity per unit Pt mass) three times higher than
conventional E-Tek Pt/C catalysts, in addition to five times
higher specific activity [46]. The stronger Pt–Pt bond strain
on a surface with a high curvature weakens Pt bonding with
oxygen spectators during the oxygen reduction reaction,
and resulting in improved activity.
Shaped Pd nanocrystals provide another example.
A single-crystalline surface study showed that the Pd(100)
surface is highly active for electrocatalytic formic acid
oxidation compared with the Pd(111) and Pd(110) surfaces
[47]. When cubic, cuboctahedral, and octahedral Pd
nanocrystals were prepared (these crystal morphologies are
illustrated in Fig. 10), the Pd nanocubes showed much
higher activity than the other shapes for formic acid
oxidation [28]. In contrast, the sharp peak at *0.4 V in a
reverse scan of the cyclic voltammograms represents the
reduction of oxidized Pd on the surface (Fig. 10). The
shaper peak of the Pd nanocubes implies that the surfaces
of cubic nanocrystals are more susceptible to surface oxi-
dation than those on octahedral nanocrystals.
Pd spheres, tetrahedra, and multipods were also tested
for cyclohexene hydrogenation [48]. Pd multipods showed
the highest activity, and Pd spheres presented the lowest
activity. The multipods have a kinetically favored struc-
ture, with a high density of surface defects, but the con-
ventional Pd spheres have a thermodynamically favored
structure, with a low density of surface defects. The high
density of surface defects on the multipods has been
hypothesized to cause the enhanced catalytic activity.
4.2 Composite Metal Catalysts
Catalytic activity can be improved by using composite
materials. For example, when Pd was locally overgrown on
Pt nanocubes, the activity for formic acid oxidation was
enhanced, and surface poisoning was reduced [49]. Formic
Fig. 10 TEM and high-angle annular dark-field (HAADF) scanning
TEM (STEM) images of Pd nanocrystals with Pt seeds. Cyclic
voltammograms for electrochemical formic acid oxidation was
performed in 0.1 M H2SO4 and 0.2 M formic acid with a scan rate
of 50 mV/s (adapted from Ref. [28])
Shape-Controlled Nanocrystals for Catalytic Applications
123
acid oxidation can follow two different routes. One is a
dehydration pathway (HCOOH ? H2O ? COads), where
the CO poisons the Pt surface, and the other is a dehy-
drogenation pathway (HCOOH ? CO2 ? 2H? ? 2e-),
with no surface poisoning species. It was predicted that a
Pt(100) surface decorated with Pd should have significantly
less surface poisoning caused by the change in the path-
way, whereas a Pt(111) surface would display no differ-
ence before and after Pd decoration [50]. Pt nanocubes with
{100} surfaces showed severe poisoning upon formic acid
oxidation following the dehydration pathway, and required
higher potentials to oxidize COads. Conversely, when Pd
was locally overgrown on Pt nanocubes, the dehydroge-
nation route became favored, and less poisoning and lower
oxidation potentials were observed, as expected from the
single-crystalline surface study.
Shaped Au–Pt composite metal catalysts have been used
for electrocatalytic methanol oxidation, formic acid oxi-
dation, and oxygen reduction reaction [51, 52]. As shown
in Fig. 11, Pt was overgrown on shaped Au nanocrystals.
When the concentration of Pt precursors and overgrowth
time were controlled, full shells or shells partially over-
grown on Au(100) facets were obtained. When these Au–Pt
composite nanocrystals were tested for electrocatalytic
reactions, they generally showed improved activity com-
pared with Pt black. It should be noted that Au alone has no
activity for electrocatalytic reactions. Spherical hollow Pt
shells were also synthesized by etching Au cores selec-
tively using a NaCN solution. The hollow Pt shells showed
a higher activity for formic acid oxidation than Pt black.
Branched nanocubes were prepared by overgrowing Pt
nanocubes in the presence of a Pt/Co precursor solution
[53]. Selective overgrowth occurred at the corners. Elec-
trochemical CO stripping showed that there are two
different kinds of active sites: Pt-abundant regions and
Co-abundant regions. The branches probably have Co more
than the cubic cores. The branched nanocubes showed a
better activity for methanol oxidation than the Pt nano-
cubes. Co in the branch region appears to enhance the
catalytic activity by modifying the electronic structure of Pt
and lowering the oxidation potential of CO.
4.3 Metal Oxide Catalysts
In addition to metal-based catalysts, metal oxide catalysts
have also shown enhanced activity due to shape modula-
tion. TiO2 nanorods were used for the photocatalytic
decomposition of formic acid [54]. Nanorods have higher
aspect ratios than nanoparticles and display better catalytic
activity. The higher aspect ratio results in a decreased
charge transfer resistance and capacitive reactance, leading
to more photocatalytic reactions.
CeO2 nanoparticles with modulated shapes showed
different catalytic activities in CO oxidation. The {100}
facets of CeO2 nanoplates were the most active for CO
oxidation [55]. Additionally, when metals such as Cu, Au,
or Pt were deposited on shape-controlled ceria, the metal/
ceria catalysts had varying catalytic activities for prefer-
ential oxidation of CO in excess H2 depending on the shape
of the ceria support used [56].
Fig. 11 a–c TEM images of Pt overgrown on Au nanocrystals with
octahedral, cubic and spherical shapes. d TEM image of hollow Pt
sphere after etching Au from (c). Electrocatalytic formic acid
oxidation of e Au–Pt composites and f hollow Pt spheres. Cyclic
voltammograms were taken in 0.1 M HClO4 and 0.1 M formic acid
with a scan rate of 50 mV/s (adapted from Ref. [51, 52])
H. Lee et al.
123
Co3O4 nanorods have also been shown to be good cat-
alysts for low-temperature CO oxidation; see Fig. 12
[57, 58]. Co3O4 nanorods mainly consist of {110} facets,
whereas Co3O4 nanoparticles have both {111} and {100}
facets. The {110} surfaces contain Co3? cations, which are
well-known active sites for CO oxidation, whereas the
{111} and {100} surfaces contain only Co2? cations,
which are nearly inactive. Co3O4 nanoparticles also shows
shape-dependent catalytic activity in methane combustion
reactions, with Co3O4 nanosheets with {112} facets having
the highest activity [59].
5 Catalytic Selectivity Enhancement by Shape
Modulation
Catalytic selectivity can also be controlled by shape
modulation. Surface crystalline structure strongly affects
the breaking and recombination of chemical bonds, leading
to different selectivities. Aromatization readily occurs on a
Pt(111) surface [1]; however, a C–C bond can be broken
more easily on a Pt(100) surface [2]. As an example, a prior
study on a Pt single-crystalline surface reported that when
benzene is hydrogenated, fully hydrogenated cyclohexane
is produced on a Pt(100) surface, whereas partially
hydrogenated cyclohexene is also produced on a Pt(111)
surface [60]. This difference in selectivity was confirmed in
a nanoparticle system [32]. When Pt nanocubes with only
Pt(100) facets were used as catalysts, only cyclohexane
was produced, but when Pt cuboctahedra with both {100}
and {111} facets were used, a significant amount of
cyclohexene was also detected.
The electrocatalytic hydrogenation of 2-cyclohexenone
was tested with Pt nanocrystals of different shapes: Pt cubes,
cuboctahedra, and dendrites [61]. C=C bonds were hydroge-
nated more easily than C=O bonds in all cases, but the ratio of
cyclohexanone to cyclohexanol differed significantly
depending on nanocrystal shape. The Pt dendrites produced
fully hydrogenated cyclohexanol preferentially, whereas the
Pt cubes yielded more of the intermediate product, cyclo-
hexanone. The hydrogenation of C=O bonds occurs more
readily on a surface with many steps, whereas it occurs less
often on a Pt(100) surface. The selectivity can thus be con-
trolled by changing the shape of the Pt nanocrystals.
Zaera and co-workers [62] reported tuning the selec-
tivity for cis- vs. trans-olefins by using Pt tetrahedra with
Pt(111) facets. A catalytic process for the selective
formation of cis-olefins would ideally minimize the pro-
duction of unhealthy trans-fats during the partial hydro-
genation of edible oils. Whereas the isomerization of the
trans- form to the cis- form is promoted on a Pt(111)
surface, the trans- form product is favored on open sur-
faces. Figure 13 demonstrates that tetrahedral Pt nanopar-
ticles promoted isomerization from the trans- form to the
cis- form. When the nanoparticle shape was deformed by
heat treatment, the transition from the cis- form to the
trans- form occurred more.
Relatively fewer examples showing changes in selec-
tivity due to shape modulation have been reported for metal
oxide nanocatalysts. Shape-controlled Cu2O nanocrystals
were used for the conversion of iodobenzene to 1-phenyl-
imidazole [63]. When iodobenzene and imidazole were
reacted with Cu2O catalyst, a product yield of 80.6% was
obtained for Cu2O cubes, whereas a 96.7% yield was
obtained for Cu2O octahedra. Thus, {111} facets seemed to
be more advantageous for this reaction.
6 Long-Term Stability Enhancement by Shape
Modulation
In addition to activity and selectivity, long-term stability of
nanoparticle catalysts can be improved by using shaped
nanocrystals. The stability of the crystalline surface upon
the adsorption of chemicals differs among various crys-
talline structures. For example, when CO is adsorbed on a
ceria surface, the surface stability is estimated to follow
the order of {111} [ {110} [ {100} based on molecular
simulations [64]. Inspired by this study, shaped ceria
nanocrystals were synthesized as rods with {110} facets,
cubes with {100} facets, and octahedra with {111} facets.
After copper was deposited onto the shaped ceria, the Cu/
Fig. 12 High-resolution TEM images and model of Co3O4 nanorods
with {110} facets. The Co3O4 nanorods displayed better activity than
Co3O4 nanoparticles for CO oxidation (adapted from Ref. [57])
Shape-Controlled Nanocrystals for Catalytic Applications
123
ceria catalysts were tested for preferential CO oxidation in
the presence of excess H2 [56]. As shown in Fig. 14,
octahedral Cu/ceria catalyst displayed the highest activity
for CO conversion and also the best long-term stability
over 100 h of reaction. As predicted by molecular simu-
lations, cubic Cu/ceria with {100} facets showed the
largest drop in conversion over 100 h.
When Pt dendrites were used as electrocatalysts for oxygen
reduction reactions, they showed higher activity than com-
mercial catalysts, as described in Sect. 4.1. Additionally, Pt
dendrites showed enhanced long-term stability [46]. The
commercial Pt/C catalysts usually experience severe sintering
problems at extended reaction times, leading to a large
decrease in the electrochemically active surface area. Pt
nanoparticles in the commercial Pt/C catalyst have very small
sizes of 1–3 nm, and these small nanoparticles can easily
migrate on a carbon support and aggregate. In contrast, Pt
dendrites have much larger sizes, at 13–53 nm, although they
have numerous branches on the scale of 1–3 nm. This large
size of Pt dendrites minimizes the sintering problem,
enhancing long-term stability. On comparing the electro-
chemically active surface area before and after 5,000 cyclic
voltammograms in an oxygen reduction reaction, the decrease
in surface area was 40% for the commercial Pt/C, whereas the
reduction was much smaller, at 19%, for Pt dendrites with an
average size of 53 nm.
7 Nanocrystals with High-Index Surfaces
The synthesis of nanocrystals with high-index surfaces has
been a challenging problem due to their high surface
energies. Recently, several groups have presented methods
for the synthesis of metal and metal oxide nanocrystals
with high-index surfaces. Nanocrystals with high-index
Fig. 13 Isomerization of cis- and trans-2-butene promoted by tetrahedral Pt-xerogel SiO2 catalysts (adapted from Ref. [62])
Fig. 14 Long-term stability results for preferential CO oxidation in
the presence of excess H2 over 4 wt% Cu/CeO2 at 140 �C (adapted
from Ref. [56])
H. Lee et al.
123
surfaces display optical properties and catalytic activities
different from nanocrystals with low-index surfaces such as
{100} or {111}. In the following, the properties of nano-
crystals with high-index surfaces are introduced, concen-
trating on their synthetic methods.
7.1 Wet-Chemistry Methods
Shaped nanocrystals are generally synthesized by wet
chemistry using metal precursors, surface-capping agents,
reducing agents, and solvents. The synthetic routes yield-
ing shaped metal nanocrystals with high-index surfaces are
summarized in Table 2. Among these nanocrystals with
high-index surfaces, Au nanocrystals are the most often
reported [65–69]. Xie and co-workers [65] presented the
synthesis of trisoctahedral (TOH) gold nanocrystals
enclosed by twenty-four {221} facets by reducing a
HAuCl4 solution with ascorbic acid in the presence of
cetyltrimethylammonium chloride (CTAC). Wang and co-
workers [66] reported the production of tetrahexahedral
(THH) gold nanocrystals enclosed by twenty-four {037}
facets using a seed-mediated growth method. Au seed was
first prepared by reducing a HAuCl4 solution with NaBH4
in the presence of cetyltrimethylammonium bromide
(CTAB), then overgrowth was initiated by injecting Ag?
ions, CTAB, and ascorbic acid, resulting in the formation
of THH Au nanocrystals. Similarly, Guo and co-workers
[67] also synthesized THH gold nanocrystals enclosed by
{520} facets. CTAB and DDAB (didodecyldimethy-
lammonium bromide) were used as surface-capping agents.
Lee and co-workers [68] synthesized concave TOH Au
nanocrystals bounded by {221}, {331} and/or {441} fac-
ets. These nanocrystals were also synthesized by a seed-
mediated method using ascorbic acid and CTAC. Mirkin
and co-workers [69] synthesized concave nanocubes
enclosed by twenty-four {720} facets. Au seed was
prepared using CTAC and then overgrowth occurred in the
presence of Ag? ions, ascorbic acid, and CTAC. The
production of shaped Pd nanocrystals with high-index
surfaces using Au seeds has also been reported. Huang and
co-workers [70] synthesized THH Au@Pd (Au core–Pd
shell) nanocrystals bounded by {730} facets from Au cubic
seeds. Similarly, Lee and co-workers [71] synthesized
THH Au@Pd nanocrystals enclosed by various facets
({210}, {520} & {310}, {720} & {410}) using Au TOH
seeds and NaBr solutions. Xia and co-workers [72] repor-
ted synthesizing Pd concave nanocubes bounded by {730}
facets from Pd cubic seeds in the presence of PVP. Pt
nanocrystals with high-index surfaces also have been
reported [73, 74]. Zheng and co-workers [73] synthesized
concave polyhedral Pt nanocrystals having {411} facets, as
shown in Fig. 15. The nanocrystals were prepared by
reducing a H2PtCl6 solution in the presence of PVP and
methylamine. The concave Pt nanocrystals showed
enhanced electrocatalytic activity in the oxidation of for-
mic acid and ethanol compared with commercial Pt cata-
lysts. Xia and co-workers [74] reported the production of
concave Pt nanocubes enclosed by {510}, {720} and {830}
facets by reducing a K2PtCl4 solution with NaBH4 in the
presence of KBr and Na2H2P2O7.
7.2 Electrochemical Methods
The first platinum nanocrystals with high-index surfaces
were synthesized in 2007 by an electrochemical treatment
of Pt nanospheres supported on glassy carbon using a
square-wave potential [75]. THH Pt nanocrystals enclosed
by twenty-four high-index surfaces ({730}, {210} and/or
{520}) were prepared. These THH Pt nanocrystals exhib-
ited much higher electrocatalytic activity for the oxidation
of formic acid and ethanol than commercial Pt catalysts.
Surprisingly, the THH Pt nanocrystals, at *200 nm in
Table 2 Metal precursors, reducing agents, and surface-capping agents used for the synthesis of the nanocrystals with high-index surfaces
Metal Precursor Reducing agent Surface-capping agent Shape/facets Refs.
Au HAuCl4 Ascorbic acid (AA) CTAC Trisoctahedra/{221} [65]
HAuCl4 NaBH4 (seed), AA CTAB, Ag? ions Tetrahexahedra/{037} [66]
HAuCl4 NaBH4 (seed), AA CTAB, DDAB Tetrahexahedra/{520} [67]
HAuCl4 NaBH4 (seed), AA CTAB(seed), CTAC Trisoctahedral/{221}, {331} [68]
HAuCl4 NaBH4 (seed), AA CTAC, Ag? ions Concave nanocube/{720} [69]
Pd Au nanocubes (core), H2PdCl4 AA CTAC Tetrahexahedra/{730} [70]
Au trisoctahedra (core),
H2PdCl4
AA NaBr Tetrahexahedra/{210}, {520}, {310},
{720}, {410}
[71]
Na2PdCl4 AA KBr, PVP Concave nanocube/{730} [72]
Pt H2PtCl6 – PVP, methylamine Concave polyhedral/{411} [73]
K2PtCl4 NaBH4 KBr, Na2H2P2O7 Concave nanocube/{510}, {720}, {830} [74]
CTAC cetyltrimethylammonium chloride, CTAB cetyltrimethylammonium bromide, DDAB didodecyldimethylammonium bromide
Shape-Controlled Nanocrystals for Catalytic Applications
123
size, showed shape stability upon thermal treatment up to
815 �C. THH Pd nanocrystals enclosed by {730} facets
were synthesized by the programmed electrodeposition
method, which reduced the Pd precursor to form Pd nuclei
and then caused the nuclei to be overgrown with high-
index surfaces [76]. These nanocrystals showed 4–6 times
higher electrocatalytic activity than a commercial Pd black
catalyst. Fivefold-twinned Pd nanorods bound by high-
index {hk0} or {hkk} surfaces were also prepared by an
electrochemical method [77]. The reason that these nano-
crystals with high-index surfaces have exceptionally high
thermal stability should be investigated further.
7.3 Metal Oxide Nanocrystals with High-Index
Surfaces
Recently, several groups have reported the synthesis of
metal oxide nanocrystals with high-index surfaces [78–80].
Yang and co-workers [79] presented the synthesis of ana-
tase TiO2 crystals with {105} high-index surfaces, as
shown in Fig. 16. According to both calculated and
experimental results, these TiO2 {105} facets have the
ability to cleave water into hydrogen gas photocatalyti-
cally. Wang and co-workers [80] synthesized polyhedral
50-facetted Cu2O microcrystals partially enclosed by
{311} facets. These polyhedral Cu2O nanocrystals exhib-
ited enhanced catalytic activity for CO oxidation. Xie and
co-workers [78] synthesized octahedral SnO2 nanocrystals
with exposed {221} facets. These nanocrystals showed
great gas-sensing performance due to the high chemical
activity of the {221} facets.
8 Conclusions
The synthesis of shape-controlled nanocrystals can be a
good design strategy for the preparation of catalysts with
enhanced activity, selectivity, and long-term stability.
These shaped nanocrystals are usually synthesized using
wet chemistry methods by reducing metal precursors in the
presence of surface-capping agents and additives. As the
nanocrystals are formed via nucleation and overgrowth
stages, the direction of overgrowth can be controlled by
changing the reducing rate or by blocking a specific facet
with organic or inorganic molecules, leading to shaped
nanocrystals. The surface-capping agents used for these
syntheses should be removed to yield catalytically active
surfaces prior to catalytic applications, and extra care
should be taken in this process. However, there are a few
reported examples demonstrating that the surface-capping
agents themselves can participate in the chemical reaction
or promote the catalytic reaction. Catalytic activity,
selectivity, and long-term stability can be enhanced by
using shaped metal or metal oxide catalysts for various gas-
phase, liquid-phase, or electrocatalytic reactions, as often
predicted by single-crystalline surface studies. Recently,
nanocrystals with high-index surfaces have been realized,
showing improved catalytic activity.
In order to apply these shaped nanocrystals for practical
catalysts in industry, the economic aspect should also be
Fig. 15 TEM images, selected area electron diffraction patterns,
geometric models, an high-resolution TEM image, and an atomic
model of concave Pt nanocrystals (adapted from Ref. [75])
Fig. 16 SEM images and schematic shape of anatase TiO2 crystals
bound by high-index {105} facets (adapted from Ref. [79])
H. Lee et al.
123
considered. Mass production of the shaped nanocrystals
and long-term stability of the shape should especially be
achieved prior to practical application. The shape control
of the nanocrystals can be obtained in the lab-scale, but
scale-up is often not possible. Simpler synthetic procedure
would be favored and its modification for process devel-
opment should also be investigated. The shape should be
preserved during the reaction over a long operation time for
practical application. Only a few studies have reported the
shape stability over a long time. Strategies to obtain the
shape stability at more harsh reaction conditions, e.g. high
temperature, strong oxidation or reduction conditions,
should be developed as well. Although there are still many
obstacles to overcome for practical applications, the shaped
catalytic nanocrystals would remain fascinating with a high
potential for rational design of better practical catalysts and
for fundamental understanding of catalytic reactions.
Acknowledgments This work was supported by the DAPA/ADD,
the National Research Foundation of Korea (NRF-2009-C1AAA001-
0092926), the New & Renewable Energy (No. 20093021030021) and
the Human Resources Development (No. 20104010100500) programs
of the Korea Institute of Energy Technology Evaluation and Planning
(KETEP) grant funded by the Korea government Ministry of
Knowledge Economy.
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