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: azhyun@yonsei.ac.kr

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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