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Nano Today (2013) 8, 168—197
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REVIEW
Surface and interface control of noble metal
nanocrystals for catalytic and electrocatalytic
applications
Binghui Wu, Nanfeng Zheng ∗
State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen 361005, China
Received 11 December 2012; received in revised form 8 February 2013; accepted 22 February 2013
Available online 11 April 2013
KEYWORDS
Surface structure;
Interfacial structure;
Noble metal;
Nanocrystal;
Catalyst;
Electrocatalyst;
Shape control
Summary Catalysis and electrocatalysis by noble metal (NM) nanomaterials is typically surface and interface-sensitive. Effective surface and interface control over NM nanomaterials
provides important foundation for studies of structure-dependent catalysis which is critical to
the design of NM nanocatalysts with optimized catalytic performances for practical applications.
In this review, we focus on recent progress in developing wet-chemical strategies to control the
surface and interfacial structures of NM nanocrystals for catalytic and electrocatalytic applications. Approaches to control the surface structures of NM nanocrystals are first summarized
and demonstrated by representative examples. We then focus discussions on how to control
three different interfaces (i.e., metal—metal, metal—oxide and metal—organic interface) on
the surface of NM nanocrystals. Finally, conclusions and perspectives are given to propose the
challenges in catalysis-driven surface and interface control of NM nanocrystals.
© 2013 Elsevier Ltd. All rights reserved.
Introduction
Over the past decades, noble metal (NM) nanomaterials
have attracted increasing research attention for diverse
applications in fields of catalysis, biology, and nanotechnology [1—5]. At the length scale of nanometers, the
surface-area-to-volume ratio significantly increases, so that
surface/facet and interface effects become predominant
and even significantly modify the macroscopic properties
∗ Corresponding author. Tel.: +86 592 2186821;
fax: +86 592 2183047.
E-mail address: [email protected] (N. Zheng).
of the NM nanomaterials to enhance their catalytic performance. Catalysis has been considered as one of the
central fields of nanoscience and nanotechnology [6]. In
surface science, single-crystal surfaces of metal or metal
oxide have been widely adopted by surface scientists as
model catalysts or supports to systematically study the
effect of surface/interface structure on catalysis reactions, such as ammonia synthesis, isomerization of light
alkanes, hydrogenolysis of alkanes, electrocatalysis and so
on [7—16]. Well-defined single-crystal catalysts are usually
studied by applying physical techniques under ultralowpressure conditions to gain deep understanding of the
factors that influence their catalysis performances, which
leads to the pressure and material gaps between many
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Surface and interface control of noble metal nanocrystals
Figure 1
169
Typical surface and interfacial structures involved in NM nanocrystal-based catalysts.
model catalysis studies and real catalysis [15,17,18]. Owing
to the high surface-area-to-volume ratio, nanomaterials
with well-define surface/interface structure are ideal materials serving as novel model catalysts to bridge these gaps in
heterogeneous catalysis [19—23].
Heterogeneous catalysis occurs with the reactants
adsorbed to the surface of a catalyst. When NM nanocrystals are used in heterogeneous catalysis, reactant molecules
have to find paths to interact with the catalytic surface of
the NM nanocrystals. NM nanocrystals applied in catalysis
can be broadly classified into four categories: (1) supportless NM nanocrystals with clean surfaces; (2) supportless NM
nanocrystals that are surface-capped by organic species;
(3) supported NM nanocrystals with clean surfaces; and
(4) supported NM nanocrystals with organic-modified surfaces. Factors that influence their catalytic performances
are varied for NM nanocrystal catalysts lying in different categories. In all cases, it has been well-documented that the
size, composition, electronic effect (e.g., introducing a second metal to form alloy or intermetallics) and structure of
NM nanocrystals are important factors in determining their
catalysis.
In this review, we focus on the structural aspects of
NM nanocrystal catalysts as shown in Fig. 1. When only
structural aspects are taken into account, supportless NM
nanocrystals with clean surfaces represent the most ideal
catalysts in which surface structure is the only determining
factor in their catalysis. That is to say, when their surfaces
are clean, the performance of NM nanocrystals depends
strongly on their exposed facets. Different facets have
exposed atoms with different coordination numbers, and
thus exhibit different catalytic properties [24]. Numerous
studies that focus on facet-controlled synthesis and facetdependent catalytic properties on NM nanocrystals have
thus been carried out by a large number of research groups
[22,25—30]. For face-centered cubic (fcc) metals, the
surface energy of different single-crystal facets is increased
in the order of {1 1 1} < {1 0 0} < {1 1 0} < {h k l} [31].
As a result, many catalysis reactions are facet dependent.
For example, in catalytic hydrogenation of benzene, Bratlie
et al. [25] have found that Pt nanocubes exposed with
{1 0 0} facets catalyzed benzene to cyclohexane, while
{1 1 1} and {1 0 0}-bounded Pt nanocuboctahedra resulted
in both cyclohexane and cyclohexene, suggesting the same
product selectivities obtained on Pt{1 1 1} and Pt{1 0 0}
single crystals in earlier studies. Tian et al. have successfully applied electrochemical methods to prepare NM
nanocrystals for the study of enhanced electrocatalysis by
high-index facets [28]. NM nanocrystals prepared by electrochemical methods are probably the closest case to ideal
surface-clean NM nanocrystals [28]. It should be noted that
most shaped NM nanocrystals prepared by wet-chemical
reduction methods are essentially capped by protecting
agents. During the studies of facet effect, in some cases,
the surface-protecting agents are not taken into account
because they do not much influence the catalysis of NM
nanocrystals. In other cases, the surface-protecting agents
can be cleaned by chemical or physical treatments to
exclude their influence on the facet effect studies.
In most cases, the presence of surface-protecting agents
is considered deleterious to the catalytic performance of
NM nanocrystals and, therefore surface-protecting agents
are cleaned out before catalysis. However, many recent
studies have demonstrated the important roles of surfaceprotecting agents (e.g., organic ligands, polymers) in
creating metal—organic interfaces that can significantly
improve the catalytic performance, both activity and selectivity, of supportless organic-capped nanocrystals. The
reported metal—organic interfacial effects are mainly due
to the electronic and steric contributions of organic capping on metals [32—40]. Compared with supportless NM
nanocrystals, more structural factors are associated with
the catalytic performances of NM nanocrystals that are
deposited on non-inert supports such as semiconducting
oxides or other metals to have new metal—oxide and
metal—metal interfaces created. For example, metal—oxide
interfaces (e.g., Au—TiO2 , Pt—FeOx , Au—Fe3 O4 ) have been
found able to enhance catalysis in CO oxidation [21,41—43],
selective oxidation or hydrogenation [44—47], water—gas
shift [14,48,49], electrocatalytic H2 O2 reduction [50,51],
etc. The Pt—Au interface can facilitate CO oxidation [52]
and also stabilize the Pt electrocatalysts against dissolution
under potential cycling regimes [53]. Desired metal—organic
interfaces were also built up on many supported NM
nanocrystal catalysts by functionalizing the surface of NM
nanocrystals with organic modifiers. For example, supported
Pt nanocatalysts have been modified by chiral cinchona alkaloids for the enantioselective hydrogenation of ␣-ketoesters
[54—61].
170
Owing to the importance of surface and interfacial structure in the catalytic performances of NM nanocrystals, over
the past two decades considerable research efforts have
been directed towards controlling the surface and interfacial structures of NM nanocrystals to search for activity
determining factors of NM catalysts under real reaction conditions [3,62—69]. In this review, we focus on recent progress
in developing strategies to better control the surface and
interfacial structures of NM nanocrystals for optimizing their
catalysis performances. Several key strategies to control
the surface structures of NM nanocrystals are first summarized. These strategies include facet-selective capping,
electrochemical method, underpotential deposition (UPD)
and seeded growth. In each strategy, important representative examples are given and discussed on how surface
structures of NM nanocrystals are controlled and influence
catalytic performances. In the part of interfacial structure
control, we mainly focus our discussions on three different interfaces present on the surface of NM nanocrystals:
(1) metal—metal interface, (2) metal—oxide interface and
(3) metal—organic interface. Strategies to control the three
interfacial structures for improving catalytic and electrocatalytic performances of NM nanocrystals are highlighted.
Finally, conclusions and perspectives are given to propose
the challenges in catalysis-driven surface and interface control of NM nanocrystals.
Surface control of NM nanocrystals for
enhanced catalysis
Surface control by facet-selective capping
Utilizing the selective adsorption of a facet-specific ligand
to lower the surface energy of the targeted facet has
been developed as the most direct way to produce shaped
nanocrystals exposed with a particular facet. From the
thermodynamic aspect, several example strategies are summarized below to discuss how surfactants/polymers, small
adsorbates and biomolecules control exposure facets of NM
nanocrystals.
Surfactant/polymer-assisted facet control
A large variety of surfactants/polymers have been found to
play certain roles in the facet control of NM nanocrystals.
There have been several published reviews and feature articles on the important role of surfactants in assisting the
shape control of NM nanocrystals, mainly in the hydrophilic
system [62,63,70—72]. Only a few examples are featured
herein. For example, Zeng et al. [73] found that poly(vinyl
pyrrolidone) (PVP) played an unambiguous role in the shapecontrolled synthesis of Ag nanocubes/nanobars enclosed by
{1 0 0} facets. In the presence of PVP, in order to selectively obtain Ag octahedrons enclosed by {1 1 1} facets, the
use of sodium citrate was required. It was demonstrated
that cetyltrimethylammonium bromide (CTAB), a cationic
surfactant, could act as a ‘‘soft template’’ to direct the
formation of Au nanorods and stabilize their side {1 0 0}
facets [74—76]. Niu et al. reported that gold nano-octahedra
bounded by {1 1 1} facets were synthesized at relatively
high concentrations of cetylpyridinium chloride (CPC), suggesting that the capping of CPC promoted the formation of
B. Wu, N. Zheng
{1 1 1} facets of Au [77]. In the hydrophobic system, there
have also been a large number of reports on the effect of
surfactants (e.g., alkylamine, carboxylic acid) on shape control of NM nanocrystals [64,78—97]. For example, several
groups have successfully prepared ultrathin Au nanowires in
the presence of olyelamine under mild reaction conditions
[81—84]. The formation of one-dimensional Au+ —oleylamine
complex strands was proposed as the reason for the formation of Au nanowires [81,82]. However, it should be noted
that many of those reports on the effect of surfactants on
shape-controlled synthesis of NM nanocrystals involved the
use of other adscititious agents, such as metal carbonyls
[86—90], CO gas [91—93], H2 gas [94—96], and Fe(III) etching
agent [97]. Although the selective synthesis of NM nanocrystals with certain shapes can be achieved by the selective
use of surfactants/polymers, it still remains unclear how
these capping agents selectively bind on the specific facets
[62,63,98] and how the adscititious agents are involved in
the shape control. In the particular case of ionic surfactants,
the co-presence of both cationic and anionic components
makes it difficult to resolve their essential roles in the shape
control.
Small adsorbate-assisted facet control
Beside surfactants or polymers, many small molecules or
ions have been found to have a profound impact on the
facet evolution during the synthesis of NM nanocrystals. The
selective binding of those small adsorbates on specific facets
of NMs is widely considered as the main contributor to the
shape-controlled synthesis of NM nanocrystals. Compared
with surfactants or polymers, using small strong adsorbates
allows better understanding of how the facet evolution is
achieved chemically. More importantly, the selective use of
small adsorbates (e.g., halide anions, CO, amines) makes it
possible to synthesize NM nanocrystals with not only welldefined facets but also morphologies that have not been
achieved by other synthetic strategies [99,100]. Several
important examples are summarized below to highlight how
small but strong adsorbates induce the formation of wellshaped NM nanocrystals.
Carbon monoxide. CO is one of the most important probes
studied in surface science, and exhibits strong adsorption
on various single-crystal surfaces of NMs, particularly Pd
and Pt [101—103]. However, CO molecules behave differently in the controlled synthesis of Pd and Pt nanostructures
[99,100]. CO prefers to adsorb on Pd{1 1 1} surface to
facilitate the growth of ultrathin Pd nanosheets and tetrapod/tetrahedral nanocrystals having Pd{1 1 1} as the main
exposure surface. But for Pt, the preferential adsorption
of CO on Pt{1 0 0} induces the formation of Pt nanocubes
[91,104]. Very recently, when introduced into the synthesis of Pd nanocrystals, CO was demonstrated as an excellent
adsorbate to facilitate the formation of Pd nanocrystals with
Pd{1 1 1} as their exposure surfaces. Experimentally, Huang
et al. [105] introduced commercial pure CO gas into the
synthesis of Pd nanocrystals in the presence of halide salt
and PVP (Fig. 2A—G). After 3-h reaction at 100 ◦ C, monodisperse, ultrathin Pd nanosheets (∼1.8 nm, Fig. 2C) were
obtained. The Pd nanosheets were shaped in well-defined
hexagon with {1 1 1} facets as their basal planes and {1 0 0}
facets as their six side planes (Fig. 2F). Electrochemical CO
Surface and interface control of noble metal nanocrystals
171
Figure 2 CO-assisted synthesis of {1 1 1}-faceted Pd nanocrystals. (A) Transmission electron microscopy (TEM) image of the Pd
nanosheets synthesized in the presence of CO, PVP and halide at 100 ◦ C (Inset: photograph of an ethanol dispersion of the asprepared palladium nanosheets in a cuvette). (B) Selected area electron diffraction (SAED) pattern of a single Pd nanosheet (shown
in the inset). (C) TEM image of the assembly of Pd nanosheets perpendicular to the TEM grid (Inset: thickness distribution of the
Pd nanosheets). (D and E) CO stripping voltammetry of (D) the as-prepared uniform hexagonal Pd nanosheets (edge length 60 nm)
and (E) the irregular Pd nanosheets (prepared in the presence of PVP but in the absence of halide) in 0.1 M H2 SO4 solution at a
scan rate of 2 mV/s. (F) The adsorption model of halide and CO on a Pd nanosheet. (G) Comparison of electrocatalytic properties
of Pd nanosheets and Pd black in 0.5 M H2 SO4 + 0.25 M HCOOH aqueous solution at a scan rate of 50 mV/s. (H) TEM image of
faceted Pd tetrapod nanocrystals synthesized at 100 ◦ C in the presence of 1:4 CO/H2 . (Inset: an ideal 3D geometrical model of a
tetrapod nanocrystal). (I) Time-dependent XRD patterns of Pd tetrapod nanocrystals aged in air. (J) TEM image of the Pd tetrahedral
nanocrystals synthesized at 140 ◦ C in the presence of 1:4 CO/H2 .
Source of A—G: Reproduced with permission from Ref. [105], © 2011 Macmillan Publishers Limited; Source of H—J: Reproduced with
permission from Ref. [110], © 2012 American Chemical Society.
stripping measurements revealed that the basal {1 1 1} surfaces of the Pd nanosheets were adsorbed by CO, and the
side {1 0 0} surfaces were bonded by halide (Fig. 2D—E).
In comparison with the preferential and strong adsorption of CO on the upper and lower {1 1 1} facets, the
adsorption of halide on the side {1 0 0} facets was not so
strong. As a result, the side surfaces of the hexagonal Pd
nanosheets were the growth active sites where the freshly
reduced Pd species were deposited. Such a shape control mechanism was different from the emulsion/mesophase
mechanism proposed for the formation of Pd nanosheets
in the synthesis systems containing surfactants [106,107].
The surface-confined growth mechanism proposed by Huang
et al. made it possible to fine tune the diameters of Pd
172
nanosheets while keeping their 1.8-nm thickness [105]. However, why the thickness of the Pd nanosheets was fixed
at 1.8 nm remains unanswered by their studies. Owing to
their high surface area, the as-prepared nanosheets exhibited electrocatalytic activity for the oxidation of formic
acid that is 2.5 times greater than that of commercial
palladium black catalyst (Fig. 2G). Moreover, the ultrathin
Pd nanosheets displayed well-defined strong surface plasmon resonance (SPR) absorption peaks in near NIR region
that were tunable by the side-length of the Pd nanosheets.
Together with their high photothermal stability and biocompatibility, the significant NIR-induced photothermal effect
of the Pd nanosheets makes themselves promising candidates for photothermal therapy using low-power NIR laser.
Moreover, CO molecules adsorbed on Pd surface can be
easily removed by electrochemical oxidation or applying
a high temperature, making it ready to get surface-clean
Pd nanocrystals for various applications. The Pd nanosheets
were successfully used as templates for the epitaxial growth
of Ag to form Pd@Ag core—shell particles [108]. The Pd@Ag
core—shell particles displayed tunable SPR peaks that are
blue-shifted with increased coating thickness of Ag. However, with Pd nanosheets in the core, the prepared Pd@Ag
particles exhibited significantly enhanced photothermal stability as compared with pure Ag nanostructures.
The use of CO was essential to the synthesis of Pd
nanosheets having the thickness of 1.8 nm. Further studies
by Zheng and co-workers revealed that the combined used
of CO and Fe3+ led to the formation of branched mesocrystalline Pd nanocorolla [109]. The prepared Pd nanocorolla
consisted of unidirectionally aligned, well spaced and connected ultrathin (1.8-nm thick) Pd nanosheets. A mechanism
of etching growth under surface confinement was proposed
to explain the formation of Pd nanocorolla. In the proposed mechanism, CO functions as the surface confining
agent to allow the anisotropic growth of the 1.8-nm-thick Pd
nanosheets as the branches, and Fe3+ etches the Pd seeds at
the early stage of the reaction to induce the formation of the
branched structure. Consistent with their proposed etching
growth mechanism, the number of branches and apparent
thickness of each nanocorolla increased with the elevated
concentration of Fe3+ . The synthesized Pd mesocrystals
exhibited properties superior to those of single-domain Pd
nanosheets owing to the presence of internal voids and
increased apparent thickness. For example, the branched
Pd nanocorolla were more easily separated from solutions by
centrifugation than unconnected Pd nanosheets. The electrode made of Pd nanocorolla showed a better mass transfer
performance than that of Pd nanosheets. Pd nanocorolla
could be also uptaken by cancer cells more easily, resulting
in a much enhanced NIR photothermal therapy efficacy.
In addition of ultrathin nanosheets and mesocrystalline
nanocorolla made of ultrathin nanosheets, the preferential and strong adsorption of CO on Pd{1 1 1} can also
assist the formation of single-crystalline Pd tetrapods and
tetrahedra (Fig. 2H—J) [110]. In the selective synthesis of single-crystalline Pd tetrapods and tetrahedra, the
combined use of H2 and CO was found critical. Structurally, both Pd tetrapods and tetrahedra also have Pd{1 1 1}
as their exposure surfaces. However, Pd tetrapods and
tetrahedra are not favorable morphologies by growth kinetics because the growth of all their surfaces is confined by CO.
B. Wu, N. Zheng
Careful X-ray diffraction (XRD) studies revealed that the
freshly obtained tetrapod/tetrahedral nanocrystals were
actually ␤-PdHx nanocrystals but not metallic Pd (Fig. 2I).
The ␤-PdHx nanocrystals were gradually transformed into
metallic Pd nanocrystals in air without changing their shape
and exposed facets. Through density functional theory (DFT)
calculations, it was found that the formation of palladium
hydride in the presence of H2 reduces the binding energy
of CO on Pd as expected and thus helps to decrease the CO
coverage during the synthesis. The reduced CO coverage on
Pd{1 1 1} was thus proposed as the most essential aspect why
tetrapod and tetrahedral Pd nanocrystals were synthesized
in the co-presence of H2 and CO.
Besides the controlled synthesis of Pd nanocrystals
having Pd{1 1 1} exposure surfaces, CO could also assist
shape-controlled synthesis of Pt nanocrystals [100]. In the
literature, there have been quite many reports on the controlled synthesis of Pt nanocubes using metal carbonyls, such
as Fe(CO)5 [87], W(CO)6 [88], Co2 (CO)8 [89] and Cr(CO)6 [90].
In those studies, the zero-valent metals (i.e., Fe, Co, W)
in the metal carbonyls were considered to play an essential
role in the shape-controlled synthesis of Pt nanocubes, however, the possible role of CO decomposed from the metal
carbonyls was entirely ignored. The importance of CO in
the controlled synthesis of Pt nanocubes was later recognized independently by two research groups, Zheng’s group
[91,104] and Murray’s group [93]. In their synthesis of Pt
nanocubes, gaseous CO instead of possible decomposed CO
from metal carbonyl was used. While Murray and co-workers
emphasized the role of CO as a reducing agent in controlling
the reaction kinetics, Zheng et al. considered the selective
binding of CO on Pt{1 0 0} as the main reason for the preferential formation of Pt nanocubes in the presence of CO
gas.
In Zheng’s studies, Pt nanocubes bounded by {1 0 0}
facets were obtained by thermal decomposition of Pt(acac)2
using commercial pure CO gas in an oleylamine (OAm)/oleic
acid (OLA) mixed solvent (Fig. 3A—D) [91]. The as-prepared
Pt nanocubes displayed an obvious CO peak at 2082 cm−1
and a week CO peak around 1802 cm−1 in the Fourier transform infrared (FT-IR) spectra (Fig. 3D). On the contrary,
in the absence of any CO or CO-containing compounds,
the formation of Pt nanodendrites were observed and no
CO signal appeared in the range ∼2100—1800 cm−1 of FTIR spectra. The results implied that the important role of
CO in the formation of Pt nanocubes could originate from
the strong binding of CO on Pt. Although both OAm and
OLA were used, only OAm was present in the as-prepared
Pt nanocubes, evidenced by the IR studies. The energetics of various co-adsorption structures of CO and amine on
Pt(1 0 0) and Pt(1 1 1) facets were evaluated by DFT calculations. The DFT calculations revealed that the co-adsorption
of CO and amine made Pt(1 0 0) became more stable than
Pt(1 1 1) when the coverages of CO and amine were both
higher than 0.25 monolayer (ML), which nicely explained the
successful synthesis of Pt nanocubes in the presence of CO.
More interestingly, if CO coverage was high enough (i.e.,
≥0.5 ML), DFT calculations predicted that Pt(1 0 0) could
be more stable than Pt(1 1 1) without the need to introduce amine as co-adsorbent [104]. Based the prediction,
supported or un-supported Pt nanocubes were indeed experimentally prepared in the presence of CO but in the absence
Surface and interface control of noble metal nanocrystals
173
Figure 3 CO-assisted synthesis of Pt nanocrystals bounded with {1 0 0} facets and shaped Pt3 —TM (TM = Fe, Co, Ni) alloyed
nanocrystals. (A and B) TEM image of Pt nanodendrites synthesized without CO and Pt nanocubes prepared under CO flow, respectively. (C) HRTEM and fast Fourier transform (inset) of an individual Pt nanocube. (D) IR spectra of as-received OAm and the Pt
nanocubes prepared in OAm/OLA mixed solvent. (E) TEM image of OAm-free surface-clean Pt nanocubes supported on CNTs. (F) Cyclic
voltammograms for methanol oxidation reactions catalyzed by cubic Pt/CNTs and commercially available Pt/C in 0.5 M H2 SO4 + 1 M
CH3 OH aqueous solution. (G—I) HRTEM images of the Pt3 Fe nanocube, Pt3 Co truncated nano-octahedron and Pt3 Ni nano-octahedron,
respectively, synthesized under similar condition except different precursors.
Source of A—D: Reproduced with permission from Ref. [91], © 2011 The Royal Society of Chemistry; Source of E—F: Reproduced
with permission from Ref. [104], © 2012 The Royal Society of Chemistry; Source of H: Reproduced with permission from Ref. [33],
© 2012 WILEY-VCH Verlag GmbH & Co.
of any amine as surface capping agent (Fig. 3E) [104]. The
as-prepared Pt nanocubes were surfactant-free and could
be surface-clean after being exposed in air for hours. Consequently, the prepared supported Pt catalysts contained
clean Pt{1 0 0} facets and thus exhibited enhanced electrocatalytic activities (Fig. 3F).
Gaseous CO were also successfully used to synthesize Ptbased nanoalloys with exposing facets of {1 0 0} and even
{1 1 1} dependent on the transition metal (TM) component
or reaction kinetics (Fig. 3G—I) [33]. Yang and co-workers
reported that CO could assist the synthesis of cubic Pt—TM
(TM = Fe, Co, Ni, Pd) alloyed nanocubes terminated with
{1 0 0} facets and octahedral or icosahedral Pt—TM (TM = Ni,
Au, Pd) alloy nanocrystals enclosed with {1 1 1} facets,
based on a gas reducing agent in liquid solution (GRAILS)
method [92,111]. Thermal decomposing of both Pt(acac)2
and Co(acac)2 in the presence of CO gas led to the formation of truncated octahedral Pt3 Co nanocrystals which were
exposed with both {1 0 0} and {1 1 1} facets (Fig. 3H) [33].
CO controlled the formation of different facets-exposed
174
Pt—TM nanocrystals under various conditions, which could
be due to surface composition variation or reaction kinetics
[22,92,99,100]. Pt3 Ni nanocrystals in the shape of icosahedra or octahedra have Pt3 Ni {1 1 1} exposure surface
and thus showed enhanced electrocatalytic activity than
{1 0 0}-terminated Pt3 Ni nanocubes or the highly active Pt/C
catalyst [22,92,111]. The results were consistent with the
previous studies on Pt-skin Pt3 Ni {1 1 1} single-crystal surfaces. Pt-skin Pt3 Ni {1 1 1} single-crystal surfaces exhibited
an enhanced oxygen-reduction reaction (ORR) activity that
was 8-fold higher than Pt3 Ni{1 0 0} and 90-fold higher than
the Pt/C catalysts [13].
Halide anions. Many studies in surface science have
demonstrated that halides adsorb tightly on clean singlecrystalline Pd surfaces and the chemisorption ability of
halides increases in an order of Cl− < Br− < I− [112,113]. The
strong binding of halides on metallic Pd surfaces could alter
the surface energies of different Pd facets and thus achieve
possible shape control. In 2007, Xiong et al. first reported
the effect of halide anions on the shape-controlled synthesis of Pd nanocrystals (Fig. 4A—C) [114]. By introducing KBr
into polyol system and using Na2 PdCl4 as the Pd precursor
and PVP as the capping agent, Pd nanocubes and nanobars
enclosed by {1 0 0} facets were successfully synthesized. In
comparison, the use of KCl instead of KBr led to the formation of cuboctahedra bounded by a mix of {1 1 1} and
{1 0 0} facets. The addition of KI resulted in much smaller
nanoparticles with poorly defined shape due to the much
stronger chemisorption of I− on the nanocrystal surface.
Later, they further reported that Pd nanocubes could be synthesized in an aqueous solution by reducing Na2 PdCl4 with
L-ascorbic acid in the presence of extra Cl− and Br− ions as
the capping agents to promote the formation of Pd{1 0 0}
facets [115,116]. Without introducing extra Cl− and Br− ,
Pd octahedra exposed with {1 1 1} facets were prepared
by heating a water—ethanol mixture containing PVP, citric
acid, and Na2 PdCl4 [115]. The effect of halide on the facet
control was similar to their previous report [114]. Both the
as-prepared Pd nanocubes and octahedra were used to test
the ORR activities, which revealed that {1 0 0}-terminated
Pd nanocubes was one order of magnitude higher than that
of {1 1 1}-enclosed Pd octahedra [115].
However, the halide effect reported by Xia is not completely consistent with the order of chemisorption ability of
halides. It remained unclear why replacing Br− with I− did
not yield nanocrystals fully enclosed by {1 0 0} facets. From
the viewpoint of energetics, the stronger chemisorption of
I− on Pd{1 0 0} than Br− should make it easier to prepare Pd
nanocubes enclosed with {1 0 0} facets. To avoid the influence of Cl− , another Pd precursor, Pd(acac)2 , was chosen
by Zheng and co-workers to study the effect of halides on
the shape control of Pd nanocrystals (Fig. 4D—F). Interestingly, by heating N,N-dimethyl formamide (DMF) solution
containing PVP and Pd(acac)2 , Pd nanocubes were successfully synthesized only in the presence of I− [117]. Replacing
I− with Cl− or Br− , multiply twinned nanoparticles were
yielded. Due to the presence of only one kind of halide
in the synthesis, it was convincing to tell the halide effect
on the shape control. Although different from the observation in the polyol synthesis, the production of Pd nanocubes
bounded by {1 0 0} facets in the presence of I− is expected
from the viewpoint of thermodynamics. Similarly, 5-fold
B. Wu, N. Zheng
twinned Pd nanowires [118], which were also bounded by
{1 0 0} facets as the side surfaces, were synthesized in the
presence of I− by simply decreasing the reaction kinetics
(i.e., increasing the feed of I− and PVP) and using water
instead of DMF as the reaction solvent [117]. However, the
strong binding of I− on Pd{1 0 0} significantly suppressed the
electrocatalytic properties of Pd nanocrystals. Fortunately,
very recently, an effective cleaning method to make clean
Pd{1 0 0} surfaces from I− -capped Pd nanocubes has been
successfully developed by Zhang et al. [119].
The important effect of halide on the facet control of NM
nanocrystals was also recognized in many other studies. For
example, the combined use of Cl− and CTA+ was required
to yield trisoctahedral Au nanocrystals exposed with {2 2 1}
facets [120]. Rh, Pt or Pd nanocubes were successfully
prepared when TTAB was added in the polyol synthesis
(Fig. 4G) [121]. Selective choosing I− or Cl− as the shapedirecting agent, {1 0 0}-facet-enclosed PtPd nanocubes
and {1 1 1}-facet-bounded PtPd octahedra/tetrahedra were
respectively obtained, and the former exhibited higher
activity in the hydrogenation of nitrobenzene (Fig. 4H—I)
[29]. Yin et al. reported that a combination of Br− and I−
ions was critical to synthesize Pt—Pd nanocubes [122].
Amines. Amine serves as another important type of facetselective capping agent for NM nanocrystals. In 2011, Huang
et al. demonstrated a solvothermal synthesis of Pt concave
nanocrystals with high-index {4 1 1} facets and a unique
octapod morphology by introducing methylamine into a DMF
mixture containing PVP and hexachloroplatinic (IV) acid
(Fig. 5A—E) [123]. The octapod nanocrystals were revealed
to be bounded with 24 kite-like high-index {4 1 1} facets.
An ideal Pt(4 1 1) facet is made of (1 1 1) and (1 0 0) subfacets, and thus has a high density of seven-coordinated
step sites (Fig. 5E). Concave octapod nanocrystals were
also obtained when methylamine was substituted with other
amines (i.e., ethylamine, butylamine, 4-methylpiperidine,
trimethylamine) or NH3 . The concave feature was concomitant with the whole growth process, suggesting that
the creation of high-index facets was not due to etching processes. The evolution of high-index {4 1 1} facets
in the presence of amines was explained by the stabilizing effect of the strong selective coordination of amines on
the seven-coordinated Pt sites. Although amines bond to Pt
strongly under neutral and basic conditions, they are readily
protonated by acids and removed from the Pt surface.
After simple acid treatments, the synthesized Pt octapod
nanocrystals displayed excellent activities in electrocatalytic oxidation of formic acid. The specific activity of the
concave Pt nanocrystals normalized by electrochemically
active surface area (ECSA) was 2.3 and 5.6 times greater
than those of commercial Pt black and Pt/C, respectively,
owing to the presence of active sites with low coordination
numbers on the surface of Pt{4 1 1} facets.
Zhang et al. [124] also reported a facile synthesis of multipod Pt nanocrystals bounded by {2 1 1} (Fig. 5F—I) and
concave Pt nanocrystals by {4 1 1} facets via a solvothermal
method using Pt(acac)2 as the Pt precursor, 1-octylamine
as the solvent and capping agent, and formaldehyde as
an additional surface structure regulator. The authors suggested that pure amine tended to stabilize the monoatomic
step edges, resulting in the exposed {2 1 1} facets, while
the co-adsorption of amine and CO molecules formed by
Surface and interface control of noble metal nanocrystals
175
Figure 4 Halide anion-assisted synthesis of Pd, Rh, and PtPd nanocrystals. (A—C) TEM images of Pd nanocrystals synthesized in
a mixture of ethylene glycol and water, with Na2 PdCl4 as precursor and PVP as capping agent, in the presence of (A) Cl− , (B) Br− ,
or (C) I− . (D—F) TEM images of Pd nanocrystals synthesized in DMF, with Pd(acac)2 as precursor and PVP as capping agent, in the
presence of (D) Cl− , (E) Br− , or (F) I− . (G) TEM and HRTEM (inset) images of as-obtained Rh nanocubes synthesized in ethylene glycol
solution, with RhCl3 as precursor, PVP as capping agent and Br− as shape-directing agent. (H and I) TEM images of PtPd nanocubes
and octahedra/tetrahedra synthesized in a mixture of DMF and water, with K2 PtCl4 and Na2 PdCl4 as precursors, PVP as capping
agent, in the presence of (H) NaI or (I) NaCl.
Source of A—C: Reproduced with permission from Ref. [114], © 2007 American Chemical Society; Source of D—F: Reproduced with
permission from Ref. [117], © 2009 WILEY-VCH Verlag GmbH & Co.; Source of G: Reproduced with permission from Ref. [121],
© 2008 American Chemical Society; Source of H—I: Reproduced with permission from Ref. [29], © 2012 American Chemical Society.
decomposition of formaldehyde made the {1 0 0} terraces
more stable, resulting in the {4 1 1}-exposed facets. In
the electro-oxidation of ethanol, the catalytic activities
of Pt nanocrystal surfaces decreased in the sequence of
{4 1 1} > {2 1 1} > {1 0 0}.
Niu et al. [125] demonstrated that Pt—Ni bimetallic
nanobundles with branched morphology and stepped {2 1 1}
facets could be prepared by a seed-based diffusion route
in pure octadecylamine, using branched Pt as seeds and
nickel nitrate as Ni source (Fig. 5J—K). The multipod Pt
seeds were pre-synthesized in pure octadecylamine using
hexachloroplatinic (IV) acid as Pt precursor at 230 ◦ C. The
branched Pt seeds had {2 1 1} exposure surfaces. The Ni
species were diffused into the pre-synthesized multi-armed
Pt seeds at an elevated temperature (250 ◦ C). The side surface of the Pt—Ni branch contained a high density of steps
and defects, parts of which could be indexed as {2 1 1}.
In the electro-oxidation of methanol, the branched Pt—Ni
nanocrystals showed higher activity than conventional Pt
nanoparticles, due to the high density surface atomic steps
and the presence of surface Ni species.
Formaldehyde. Apart from acting as a strong reductant,
formaldehyde plays an obvious role in the facet control.
Huang et al. recently reported that the use of formaldehyde
could lead to the formation of concave tetrahedral/trigonal
bipyramidal Pd nanocrystals bounded with {1 1 1} and {1 1 0}
facets (Fig. 6A—C) [126]. The synthesis was carried out in
benzyl alcohol containing Pd(acac)2 , PVP and formaldehyde
176
B. Wu, N. Zheng
Figure 5 Amine-assisted synthesis of shaped Pt-based nanocrystals. (A) SEM image of concave Pt nanocrystals with exposing {4 1 1}
facets synthesized in the presence of methylamine. (B) High-magnification SEM image of a single concave Pt nanocrystal. (C) TEM
image, SAED pattern and geometric model of an individual concave Pt nanocrystal oriented along the [1 1 0] direction. (D) HRTEM
image of the region indicated by the box in (C). (E) Atomic model corresponding to the region indicated by the box in (D). (F) TEM
image and corresponding SAED pattern of a {2 1 1}-faceted multipod Pt nanocrystal synthesized in the presence of 1-octylamine. (G)
Cross-section TEM image and the corresponding SAED pattern of an individual nanorod projected along the [1 1 1] direction. (H and I)
HRTEM image of the edge region of the nanorod and the corresponding atomic model of the Pt(2 1 1) surface atomic arrangements,
both projected along the [0 1 1] direction. (J) TEM image of randomly chosen {2 1 1}-faceted Pt—Ni nanobundles synthesized in the
presence of octadecylamine. (K) HRTEM image of a single branch of the Pt—Ni nanobundles and atomic model of the {2 1 1} plane
with a high density of surface atomic steps.
Source of A—E: Reproduced with permission from Ref. [123], © 2011 American Chemical Society; Source of F—I: Reproduced with
permission from Ref. [124], © 2012 Springer; Source of J—K: Reproduced with permission from Ref. [125], © 2012 The Royal Society
of Chemistry.
via a solvothermal method. Replacing formaldehyde with
benzaldehyde, the reactions also led to the formation of
concave Pd nanocrystals, whereas the absence of formaldehyde led to the formation of Pd nanocrystals with mixed
morphologies, suggesting that the aldehyde group played
an important role in the formation of concave structures.
The concave tetrahedron could be structurally described
as a tetrahedron with each face excavated by a trigonal pyramid (Fig. 6B). The concave surfaces of the Pd
tetrahedra/bipyramids were assigned to {1 1 0} facets. The
formaldehyde played a key role for the formation of concave
Pd nanocrystals, since the degree of concavity was increased
with increasing the feed of formaldehyde. Correspondingly,
for formic acid oxidation, the electrocatalytic activity of Pd
nanocrystals is also increased with incremental proportion
of {1 1 0} facets.
Niu et al. [127] also reported that formaldehyde could
play a unique role in selective formation of various Pd
nanocrystals with {1 1 1} exposure surfaces (Fig. 6D—G).
Icosa-, deca-, octa-, tetrahedral, and triangular platelike Pd nanocrystals all enclosed with {1 1 1} facets were
successively synthesized by increasing the feed of OAm
in the presence of formaldehyde as a reducing agent.
Control experiments showed that while replacing formaldehyde with methanol resulted in the formation of elongated
Pd nanoparticles without well-defined {1 1 1} facets, benzaldehyde led to multi-armed Pd nanostructures. Jin et al.
[30] demonstrated that using Pd nanocubes as seeds, Pd
polyhedra including truncated cubes, cuboctahedra, truncated octahedra, and octahedra with increasing the ratio of
{1 1 1}/{1 0 0} were successfully prepared in the presence
of formaldehyde. While the important role of formaldehyde
was well-documented in the literature, the intrinsic role of
formaldehyde is still unclear. When formaldehyde adsorbs
on Pd surfaces, it can be catalytically decomposed into CO
and H2 , making it more complex and hard to distinguish the
real shape-directing agent and necessary to do more control
experiments to uncover this. The successful synthesis of concave tetrahedral/trigonal bipyramidal Pd nanocrystals in the
presence of CO was a nice attempt trying to reveal that the
role of formaldehyde in the shape control might originate
from CO [128].
Other small adsorbates. In recent years, some other small
adsorbates have also emerged as effective surface controllers in the shape-controlled synthesis of NM nanocrystals.
For instance, Habas et al. [26] demonstrated that NO2 could
Surface and interface control of noble metal nanocrystals
177
Figure 6 Formaldehyde-assisted synthesis of shaped Pd nanocrystals. (A) TEM image of concave tetrahedral/trigonal bipyramidal
Pd nanocrystals bounded with {1 1 1} and {1 1 0} facets. (B) High-magnification TEM image of a single concave tetrahedron (Top-right
and bottom-right insets show the corresponding SAED pattern and the ideal structure model of the concave tetrahedron). (C) HRTEM
image of the squared area indicated in (B). (D—G) TEM images of icosa-, deca-, octa- and tetrahedral Pd nanocrystals, respectively,
all enclosed with {1 1 1} facets (Insets are the corresponding ideal structure models).
Source of A—C: Reproduced with permission from Ref. [126], © 2009 American Chemical Society; Source of D—G: Reproduced with
permission from Ref. [127], © 2011 WILEY-VCH Verlag GmbH & Co.
stabilize Pd{1 1 1} facets. Increasing the ratio of NO2 to Pd
in the synthesis resulted in an increasing ratio of {1 1 1}
to {1 0 0} facets in the obtained Pd nanocrystals. HS− /S2−
ions were revealed able to facilitate the formation of Ag
nanocubes enclosed by {1 0 0} facets [129]. A relatively high
ratio of Ag+ to Pt in the synthesis led to the formation of
cuboctahedral and octahedral Pt nanocrystals bounded by
{1 1 1} facets [130]. In addition, C2 O4 2− were used to synthesize {1 1 1}-terminated Pt—Pd tetrahedra or icosahedra
[122,131].
Based on above discussions, one should agree that the
use of a small adsorbate represents an effective approach
to shape-controlled synthesis of NM nanocrystals bounded
with certain facets. By the use of small strong adsorbates
(e.g., CO, I− , amines), it is possible to fabricate diverse
NM nanocrystals with not only well-defined facet structure but also morphologies that have not been achieved by
other synthetic strategies. In spite of this, there are not
enough insights into the origin of small adsorbate-assisted
shape/facet control and the facet-dependence on catalytic
activity for NM nanocrystals. It is still very difficult to rationally identify or design a capping agent to prepare NM
nanocrystals exposed with specific facets. To address these
issues, more diverse types of small adsorbates should be considered. The binding ability of small adsorbate on different
facets of a NM nanocrystal should be investigated first at the
atomic level through a combination of DFT calculations and
various surface characterization techniques.
Biomolecule-assisted facet control
Several groups have demonstrated that biomolecules are
also capable of controlling the shape of NM nanocrystals.
Au nanoprisms/nanoplates and silver nanowires have been
successfully prepared by different research groups by using
plant extracts as the reducing agent [132—137]. For example, Xie et al. [135] demonstrated the synthesis of triangular
and hexagonal Au nanoplates with exposed {1 1 1} facets by
reacting aqueous chloroauric acid with the extract of the
unicellular green alga Chlorella vulgaris at room temperature. They suggested that an isolated and purified protein
from the algal extract with a molecular weight of ∼28 kDa
was the primary gold shape-directing protein (GSP) in the
synthesis of Au nanoplates. Au et al. [138] reported that
{1 1 1}-bounded Au microplates were synthesized by reducing aqueous chloroauric acid with the hydroxyl groups in
both serine and threonine of bovine serum albumin (BSA),
which is a globular protein in its native state. They found
that the surfaces of the as-synthesized Au microplates were
covered by a dense array of BSA bumps. However, the above
observations still show a lack of predictable control over
the resulting structures and exposed facets. It is required to
178
B. Wu, N. Zheng
Figure 7 Biomolecule-assisted synthesis of shaped Pt nanocrystals. (A) and (B) Schematic illustrations of facet-specific peptide
sequence selection and peptide directed shape-controlled nanocrystal synthesis. TEM images in (B): Pt nanocubes obtained from
the T7-regulated reaction and Pt nanotetrahedra obtained from the S7-regulated reaction, respectively; scale bars, 5 nm.
Source of A—B: Reproduced with permission from Ref. [139], © 2011 Macmillan Publishers Limited.
decode the RNA/protein sequence for further understanding
of the selective binding mechanism of the biomolecules.
Recently, Huang and co-workers have achieved the
controllable synthesis of Pt nanocrystals with various morphologies using peptide molecules (Fig. 7) [139—142].
Peptides with specific binding to a desirable Pt surface
were selected using the phage display library approach
(Fig. 7A). Pt nanocubes (bounded by six {1 0 0} facets),
octahedra (bounded by eight {1 1 1} facets) were pre-made
and deposited on silicon substrates for the selection of
peptides to recognize Pt{1 0 0} and {1 1 1}, respectively
(Fig. 7A). Through such a method, the Pt-{1 0 0} binding
peptide (T7: Ac-Thr-Leu-Thr-Thr-Leu-Thr-Asn-CONH2 ) and
Pt-{1 1 1} binding peptide (S7: Ac-Ser-Ser-Phe-Pro-Glu-ProAsp-CONH2 ) were selected [139]. Ac-Thr-Leu-His-Val-SerSer-Tyr-CONH2 (BP7A) was also selected as a specific Pt
binding peptide to stabilize single-twinned seeds formed
during nucleation [140]. Owing to specific binding of different selected peptide molecules, predictable shaped
nanocrystals enclosed by particular facets were made by
introducing more than one kind of peptides at different
growth stages (Fig. 7B) [139]. Unfortunately, the peptide
selection required the use of pre-made Pt nanocrystals with
well-defined surface as the substrates and has been carried out so far only for low-index Pt facets. However, the
great potential of biomolecules in the surface control of NM
nanocrystals should not be underestimated due to the great
diversity of biomolecules.
Facet control by electrochemical method
A unique electrochemical method to synthesize NM
nanocrystals with high-index facets has been recently
Surface and interface control of noble metal nanocrystals
179
Figure 8 High-index-faceted Pt and Pd nanocrystals synthesized with electrochemical square-wave potential methods. (A) Scheme
of preparation of THH—Pt nanocrystals. (B) Low-magnification and (C, E, F) high-magnification SEM images of THH—Pt nanocrystals.
(D) Geometrical model of an ideal THH. (G) TEM image and (H) SAED of a THH—Pt nanocrystal recorded along the [0 0 1] direction.
(I) HRTEM image recorded from the boxed area marked in (G). (J) Atomic model of Pt(7 3 0) plane with a high density of stepped
surface atoms. (K) SEM image of THH—Pd nanocrystals (The inset is the geometrical model of a THH). (L and M) TEM image of a
THH—Pd nanocrystal recorded along the [0 0 1] direction and HRTEM image recorded from the boxed area in (L), showing some
{2 1 0} and {3 1 0} steps that have been marked by red dots. (N) Aberration-corrected HRTEM images of HIF-Pt/C catalysts, showing
the high density of atomic steps. (O) Model of various atomic steps along a <1 0 0> crystal zone axis for comparison with (N).
Source of A—J: Reproduced with permission from Ref. [28], © 2007 Science; Source of K—M: Reproduced with permission from Ref.
[146], © 2010 American Chemical Society; Source of N—O: Reproduced with permission from Ref. [147], © 2010 WILEY-VCH Verlag
GmbH & Co.
developed by Sun and co-workers [4,28,143—148]. Benefiting from the exposed high-energy and clean surfaces,
the NM nanocrystals usually display high electrocatalytic
activity. Tetrahexahedral (THH) Pt nanocrystals with 24
high-index facets such as {7 3 0}, {2 1 0}, and/or {5 2 0}
were transformed from spherical Pt nanoparticles on glassy
carbon by a square-wave potential electrochemical treatment, that is, repeated sequences of electro-oxidation and
electro-reduction (Fig. 8A—J) [28]. It was suggested that the
adsorbed oxygen and hydroxyl species played an essential
role in the formation of these unusual platinum nanocrystals. At 1.2 V, the Pt(0) atoms on the surface of spherical Pt
nanoparticles were oxidized and covered by oxygen species
(Oad and OHad ) originated from the dissociation of H2 O in
solution. The Pt atoms on low-index facets of nanocrystals exhibited relatively high coordination numbers (CNs),
and in order to be stabilized by adsorbed oxygen species,
the surface Pt atoms had to undergo a place exchange
with oxygen atoms to form a Pt—O lattice. After reduction
in the voltage window of −0.20 V and −0.10 V, these displaced Pt atoms could not always return to their original
positions, leading to disorders in surface structures. However, the high-index planes with low CNs could preferentially
adsorb oxygen on stepped atoms and might not undergo the
place exchange reaction between oxygen and Pt, so that
ordered surfaces are preserved. Thus, after the rapid treatment of square-wave potential, only high-index planes with
a high proportion of stepped Pt atoms, such as the {7 3 0}
and {2 1 0} facets (Fig. 8I—J), would survive to produce
these unusual THH—Pt nanocrystals. The THH—Pt nanocrystals were thermally stable up to temperatures of ∼800 ◦ C.
It’s well known that high-index facets of Pt exhibit an
open structure [4]. In the case of the {7 3 0} surface, the
density of stepped atoms is as high as 5.1 × 1014 cm−2 , 43%
of the total number of atoms on the surface, which is considered to provide high catalytic activity. Experimentally,
THH—Pt nanoparticles indeed exhibited much more active
in formic acid/ethanol electro-oxidation (per unit Pt surface
180
area) than Pt nanospheres and commercial Pt/C catalyst
[28].
By a very similar electrochemical method, Sun’s group
also prepared THH—Pd nanocrystals exposed with {7 3 0}
high-index facets, using PdCl2 as precursor (Fig. 8K—M)
[146]. The as-prepared THH—Pd nanocrystals exhibited
4—6 times higher catalytic activity per unit surface area
than a commercial Pd black catalyst toward ethanol electrooxidation in alkaline media. Pt or Pd nanocrystals
with other high-index facets were also successfully prepared [143—145]. However, the high-index-faceted Pt/Pd
nanocrystals prepared by the electrochemical method were
relatively large in size (>20 nm) and deposited on glassy
carbon electrode, which precluded their practical applications due to low metal utilization efficiency. To address this
issue, Sun et al. have attempted the following two methods. In the first strategy, large Pt nanospheres were replaced
by insoluble Cs2 PtCl6 on carbon black as precursor [147].
The similar electrochemical square-wave potential method
produced high-index-faceted Pt nanocrystals supported on
carbon black (HIF-Pt/C), with a size (2—10 nm) comparable
to that of commercial Pt catalysts (Fig. 8N—O). Compared
to THH—Pt, the size of HIF-Pt was greatly decreased, making HIF-Pt more practical. Aberration-corrected HRTEM and
cyclic voltammetric characterizations confirmed that the
HIF-Pt/C nanocatalysts contained multiple {h k 0} facets and
a much higher density of stepped atoms than commercial
Pt/C catalysts (Fig. 8N). A high density of low-coordinate
atomic steps, such as {1 1 0}, {2 1 0}, {3 1 0}, and {5 1 0}
steps, could be identified. The as-prepared HIF-Pt/C catalysts also exhibited superior performance including specific
activity and selectivity for ethanol oxidation, compared
to commercial Pt/C catalysts. In their second strategy,
∼10 nm Pt nanocubes were used as the precursors for the
electrochemical production of sub-20 nm tetrahexahedral
nanocrystals exposed with {3 1 0} facets [148]. In spite of the
successful size reduction in the electrochemical approach,
more efforts should be made on the scale-up production of
these high-index-faceted nanocatalysts for practical applications in fuel cells, because they are high-energy and
surface-clean.
Facet control by UPD
In recent years, UPD has been developed to control shape
evolution and exposure facets of monometallic and alloy
nanostructures. UPD is a phenomenon that a foreign metal
submonolayer or monolayer can be deposited onto a given
metal substrate at a potential significantly less negative than
for bulk deposition [149]. On the basis of the theory of Kolb
et al., UPD occurs only when the work function of the bulk
of the adlayer metal is lower than that of the substrate, and
the underpotential shift is proportional to the work function difference [150]. For example, the work function of Ag
is more than 0.5 eV lower than that of Au. The UPD of Ag
over Au is therefore favored, which is actually one of the
earliest discovered UPD systems [151]. Moreover, different
surfaces have different work functions and thus different
UPD shifts. It is concluded that the UPD shift of silver on
gold facets increases in the order {1 1 1} < {1 0 0} < {1 1 0},
because a more open surface can easier stabilize the
B. Wu, N. Zheng
adsorbate layer due to more neighbors and therefore
stronger attractive potential [70,152].
UPD of impurity metals can provide an alternative strategy to control the shape evolution and facet exposing of
metallic nanostructures [153—155]. For example, the monolayer formed by Ag UPD could selectively protect a specific
facet of Au from further growth, resulting in that the modified facets grow more slowly and become dominant exposure
surfaces in the final structures. Using a combination of Ag+
and Br− ions (from CTAB), Ming et al. reported the synthesis of elongated THH—Au nanocrystals enclosed by 24
high-index {7 3 0} facets in a high yield via seed-mediated
growth (Fig. 9A—D) [156]. Halide ions (Cl− , Br− ) were found
to alter the UPD behavior of Ag+ ions on the surface of Au
nanocrystals owing to the interaction between halide and
Ag+ ions. When CTAC rather than CTAB was used, Mirkin and
co-workers found that Au concave nanocubes enclosed by 24
high-index {7 2 0} facets could be obtained even though the
rest of the conditions were identical (Fig. 9E—G) [157]. Similarly, Ag+ and CPC has been used to synthesize Au truncated
ditetragonal nanoprisms (TDPs) exposed with {3 1 0} facets
(Fig. 9H—J) [158]. The authors suggested that the formation of the Au nanocrystals bounded by different high-index
facets could be attributed to the synergistic effect of Ag
UPD, halide ions, and the surfactants.
More interestingly, Personick et al. demonstrated that
different facets could be selectively stabilized by strategically varying the Ag+ concentration in the presence
of CTAC [154]. Such a manipulation led to four different gold nanostructures with increasing concentrations of
Ag+ : {1 1 1}-terminated octahedra, {1 1 0}-enclosed rhombic dodecahedra, {3 1 0}-faceted TDPs, and {7 2 0}-bounded
concave cubes [154,155]. It was explained that increasing concentrations of Ag+ in the growth solution could
stabilize more open surface facets. Such a preferential
deposition of Ag adatoms may remarkably affect the growth
rates of Au nanocrystals along different crystallographic
directions and thus their facets exposed on the surface
[154,155,157,159,160]. In addition to a single component
of Ag+ ions, a synergistic function of two foreign metal ionic
species, Ag+ and Pd2+ , was also used on shape-controlled synthesis of single-crystalline Au nanocrystals either partially or
entirely enclosed by high-index {3 1 0} facets, depending on
the ratio of Ag+ /Pd2+ [161]. However, when introduced separately, Ag+ and Pd2+ ions led to {1 1 0} and {1 0 0} truncations
of Au octahedra, respectively.
With the assistance of Cu UPD followed by galvanic
replacement reaction, Zhang et al. reported the synthesis
of hexoctahedral (HOH) AuPd alloy nanocrystals bounded
with high-index {4 3 1} facets (Fig. 9K—L) [162]. It was found
that the Cu UPD on Au could be employed as a bridge
to improve the alloying of Pd into Au lattice, since redox
potential of Au/Cu UPD (0.67 V) was just located between
the standard reduction potentials of Au3+ /Au (1.00 V) and
Pd2+ /Pd (0.59 V). The control experiment confirmed that
phase separation would occur in the absence of Cu2+ ions.
Combined with Cu UPD, octadecyl trimethyl ammonium
chloride (OTAC) was necessary to the special shape of
HOH Au—Pd alloy nanocrystals, whereas porous Au—Pd alloy
nanospheres were produced without OTAC. By utilizing Cu
UPD [162—164], most recently Xia et al. reported that Ag
concave octahedrons could be synthesized based on Ag
Surface and interface control of noble metal nanocrystals
181
Figure 9 High-index-faceted Au and AuPd nanocrystals synthesized by the UPD method. (A and B) SEM images of the side and
end surfaces of elongated THH—Au nanocrystals synthesized by Ag UPD, respectively. (C) Schematic showing the formation of a
THH from a cube. (D) HRTEM image of the edge of the high-index {7 3 0} facet and the corresponding atomic model from [1 0 0]
direction. (E) SEM image of concave cubic Au nanocrystals synthesized by Ag UPD. (F) Model images of concave cubes with different
tilted angles to illustrate the concave surfaces. (G) HRTEM image of the edge of the high-index {7 2 0} facet and the corresponding
atomic model projected from [0 0 1] direction. (H) SEM image of TDP—Au nanocrystals synthesized by Ag UPD. (I) High-magnification
SEM image stressing typical TDP profiles in random orientations. (J) Model of an ideal TDP enclosed by {3 1 0} facets. (K) SEM image
of HOH—AuPd alloy nanocrystals synthesized by Cu UPD. (L) A series of high-magnification SEM images and corresponding models of
the HOH nanocrystals with exposed {4 3 1} facets in different orientations.
Source of A—D: Reproduced with permission from Ref. [156], © 2009 American Chemical Society; Source of E—G: Reproduced with
permission from Ref. [157], © 2010 American Chemical Society; Source of H—J: Reproduced with permission from Ref. [158], © 2011
American Chemical Society; Source of K—L: Reproduced with permission from Ref. [162], © 2011 American Chemical Society.
nanocubes seeds in the presence of Cu2+ ions [163]. They
suggested that Cu2+ ions were responsible for promoting the
growth of {1 1 1} facets or retarding the growth of {1 0 0}
facets of the cubic seeds, related to the concept of UPD.
Besides facet control, UPD can also be directly used
to design a highly catalytic monolayer on different-shaped
metal nanocrystals [165—168], particularly those with highindex facets, for enhanced catalytic activity. Lu et al. [158]
reported the use of Au TDPs enclosed by 12 high-index {3 1 0}
facets as substrate and nano-facet activator to deposit a
Pt monolayer by Cu UPD and subsequent galvanic replacement reaction. The as-resulted Au(TDP)-Pt(ML) exhibited an
enhanced catalytic activity compared to pure TDP—Au.
Seeded growth
Seeded growth has been widely used to prepare monometallic or core—shell bimetallic nanocrystals with exposing
well-defined facets of the shell metal. Many studies
have demonstrated that monometallic metal nanocrystals
with well-defined shapes and thus exposed facets can
be prepared from metal nanocrystals with uniform size.
Depending on the seeded growth conditions, the exposure
facets developed can be different from that of the seeds
[163,169—173]. For example, Ag cubic seeds were successfully used as seeds for the preparation of Ag nanocrystals
with concave surfaces [163]. Single-crystal Au nanorods
were also applied as the seeds for the growth of perfect
Au octahedra [169]. Tetrahedral Pt nanocrystals were also
used as seeds for the preparation of branched Pt nanostars
[171]. For the seeded growth of the second metal on the
metal seeds, there are three typical growth models: the
layered growth (Frank—van der Merwe mode, F—M mode),
the island growth (Volmer—Weber mode, V—W mode), and
the intermediate type of growth (Stanski—Krastanow mode,
S—K mode) [174]. Lattice mismatch (e.g., 0.77% for Pt/Pd,
4.08% for Pt/Au) plays a critically important role in the
overgrowth of the secondary metal. High lattice mismatch
prevents the conformal overgrowth [26,175]. In order to get
182
B. Wu, N. Zheng
Figure 10 Core—shell nanocrystals with faceted Pd shells synthesized via seeded growth. (A) Schematic showing the growth of
THH Au—Pd core—shell nanocrystals from THH—Au nanocrystals and SEM image of the resulted nanocrystals. (B) Schematic showing
the growth of TOH Au—Pd core—shell nanocrystals from TOH—Au nanocrystals and SEM image of the resulted nanocrystals. (C—E)
TEM images of Pt—Pd core—shell nanocrystals from Pt nanocubes with increasing ratio of NO2 to Pd (Insets are the corresponding
models). (F and G) SEM image of THH Au—Pd core—shell nanocrystals from Au nanocubes (the inset is an ideal 3D geometrical model)
and scheme illustration of their formation mechanism, respectively.
Source of A—B: Reproduced with permission from Ref. [177], © 2010 American Chemical Society; Source of C—E: Reproduced with
permission from Ref. [26], © 2007 Macmillan Publishers Limited; Source of F—G: Reproduced with permission from Ref. [188],
© 2010 American Chemical Society.
an intact epitaxial overgrowth shell on a given seed, rather
than a rough, polycrystalline shell or binary structure, small
lattice mismatch and a strong bond between seed and shell
atoms should be satisfied in general. Moreover, a slow reduction rate of the precursor of the shell metal should be kept
[176,177]. A rapid reduction rate will break the near equilibrium condition and result in the island growth [178—181].
We focus here on the seeded growth of core—shell bimetallic nanostructures with well-defined exposed facets in the
F—M mode, of which both the core and shell metals are
single-crystalline. Epitaxial overgrowth on seed nanocrystals with well-defined shape provides a straightforward
and effective route to control of exposure surfaces of the
obtained core—shell metal heterostructures. While the
epitaxial overgrowth helps to inherit the exposure surfaces
of the seeds, the outermost surfaces of the obtained
core—shell nanocrystals can be easily switched from one
metal to another [177,178,182—187]. For instance, starting
from Au triangular nanoprisms, Au@Ag triangular core—shell
nanoprisms with {1 1 1}-exposed facets and highly tunable
Surface and interface control of noble metal nanocrystals
SPR could be synthesized under irradiation with 1064-nm
light [182] or by a chemical reduction with ascorbic acid
[183]. {1 1 1}-bounded Pd@Pt nanoplates with hexagonal
and triangular shapes were obtained in the epitaxial growth
of Pt shells on Pd nanoplates via the slow reduction associated with the mild reducing power of citric acid [184]. In
these two cases, the lattice mismatch of Au—Ag and Pd—Pt
is less than 1%, which is suitable for seeded growth in the
F—M mode. More importantly, metal shells bounded with
high-index facets were also successfully fabricated based
on given metal templates with high-index facets [177,186].
Wang and co-workers reported the template-directed epitaxial overgrowth of Pd on THH and TOH Au nanocrystals with
high-index facets (Fig. 10A—B) [177]. The {7 3 0} and {2 2 1}
facets of the THH and TOH Au seed nanocrystals were nicely
inherited by the grown Pd nanoshells, due to the slow reduction of the Pd precursor. The obtained Au@Pd nanocrystals
possessed a large number of coordinatively unsaturated Pd
atoms at steps on their high-index facets. As a result, the
prepared obtained Au@Pd nanocrystals displayed much better catalytic performances in the Suzuki coupling reaction
than those with the low-index {1 0 0} facets.
In the seeded growth method, the final core—shell
nanocrystals do not necessarily adopt the morphologies
of the seed nanocrystals. The morphology of the final
nanocrystals highly depends on the synthetic conditions
and is strongly guided by facet-specific capping agents
used in the overgrowth process [26,175,188—190]. Habas
et al. [26] demonstrated that the epitaxial overgrowth of
lattice-mismatched Au on cubic Pt seeds gave anisotropic
growth of Au rods, whereas the epitaxial overgrowth of
lattice-matched Pd on cubic Pt seeds led to conformal
shape-controlled core—shell particles (Fig. 10C—E). It was
further found that introducing more NO2 in the synthesis resulted in an increasing ratio of {1 1 1} to {1 0 0} in
the obtained Pd nanocrystals, due to preferential binding
of NO2 on Pd{1 1 1}. The as-prepared Pt@Pd nanocubes,
cuboctahedra and octahedra were ready to investigate the
facet-dependent properties of Pd. Their electrocatalytic
studies revealed that Pd{1 0 0} exhibited better catalytic
activity in electro-oxidation of formic acid than Pd{1 1 1}.
When Pt nanocubes were used as seeds, Pt@Rh concave nanocubes were successfully prepared through a slow
seeded growth route in the presence Br− [176] because of
preferential binding of Br− on Rh{1 0 0} [121,191].
Seeded growth was also reported as an effective strategy to create catalytically highly active surfaces from metal
seeds with low-index facets. For example, Lu et al. [188]
successfully prepared Au@Pd core—shell THHs with exposed
{7 3 0} facets from Au nanocubes (Fig. 10F—G). While the
Au nanocubes were enclosed by {1 0 0} facets, the obtained
Au@Pd core—shell THHs possessed high-index {7 3 0} exposure surfaces. The large lattice mismatch between the Au
core and the Pd shell, and the oxidative etching process
involved in the seeded growth process were considered
as the two important factors for the formation of THH
nanocrystals. Since the lattice mismatch between Au and Pd
is as high as 4.6%, the Pd shell with an increasing thickness
has to bear more and more strain. The authors proposed that
the developed THH morphology with all high-index facets
was to release the build-up of this lattice strain. Moreover,
in the presence of dissolved oxygen and Cl− , continuous Pd
183
reduction and partial oxidative etching might also promote
the Ostwald ripening of Pd and thus facilitate the formation of {7 3 0} high-index facets. Similarly, Au@Pd concave
nanocubes were prepared from Au octahedron seeds in the
presence of dissolved oxygen and Br− [192].
Up to now, there are a large number of reports on
the successful formation of faceted Au@Ag [175,182,183],
Au@Pd [175,177,185—188,190,192], Pd@Au [189,193,194],
Pd@Ag [194], Pd@Pt [178,184], Ag@Pt [195] heterogeneous
core—shell NM nanostructures by seeded growth. In the
seeded epitaxial growth of core—shell metal nanostructure,
the roles of several factors, such as atomic radii, bond
dissociation energies and electronegativity of core/shell
metals, have been discussed [175]. In order to further enrich
core—shell metal nanostructures and prepare more welldefined facets for catalytic applications, the use of alloy
cores [195] or alloy shells, instead of single component,
should be considered in the future studies.
Besides common facet-control methods summarized
above, in the literature there are some other methods
of facet control of NM nanocrystals, such as chemical etching (the etchants could be H2 O2 , Fe3+ , O2 /Cl− ,
etc.) [97,114,117,196—199] and kinetically-controlled overgrowth [163,176,188,200—202]. For example, the galvanic
replacement of Ag nanocubes with HAuCl4 led to the formation of Au nanocages in a cube shape [197]. The I− -assisted
galvanic replacement of Pd nanocubes with Pt(acac)2
yielded hollow PtPd nanocubes [117]. Different-shaped Ag
nanocrystals were fabricated from Ag nanooctahedra by
using a highly anisotropic etching process [198]. Concave
Pt—Ni alloy particles were successfully prepared from octahedron shaped Pt—Ni alloy particles in the presence of
dimethylglyoxime and air [199]. Sometimes two or more
methods accompany the nanocrystal-growth process. With
the developed methods, numerous NM nanocrystals and also
their nanoalloys with well-defined facets have prepared for
the facet-dependent catalysis studies. Such research efforts
would eventually help us to design NM nanocatalysts having
optimized catalytic performances.
Interface control of NM nanocrystals for
enhanced catalysis
Apart from surface sites, interfacial sites between NM
nanoparticles and concomitant components are usually
where catalytic reactions take place. The interface induced
by a foreign component may endow the NM nanocatalysts
with enhanced catalytic activity, selectivity and stability. In
general, to develop nanostructured NM catalysts with high
catalytic performance, three types of interface interactions
should be considered, including metal—metal, metal—oxide
and metal—organic interfaces. Specific concomitant components (metal, oxide, or organic adsorbates) and also the
primary NM need to be appropriately chosen to steer the
electronic effect or steric effect for optimizing the catalytic
performances of NM nanocatalysts.
Metal—metal interface
When two different metals are brought together to form
bimetallic nanostructures, the following architectures are
184
possible [203]: (1) alloyed structure; (2) core—shell structure with only one metal exposed; (3) heterojunction
structure with both two metals and their interface exposed.
The two metal components in a bimetallic nanostructure
can interplay with each other to induce synergetic effect
in catalysis. Synergetic effects in bimetallic nanocatalysts
are often introduced by the change of electronic structure due to charge transfer or structural strain between
metals, interfacial collaboration by two metals, and interfacial stabilization. There have been several nice reviews
summarizing the alloy and structural effects on catalysis by bimetallic nanomaterials [204—207]. Here we focus
our discussions of metal—metal interface on heterojunction structures which have both their metal components and
their interface exposed. Many studies have demonstrated
that the metal—metal interfaces in metal—metal heterojunctions are responsible for their enhanced performance
in various catalytic applications [52,208—210]. There are
three routes to fabricate metal—metal heterostructures: (1)
seed-mediated synthesis via V—W or S—K mode, depending on the interplay among surface, interfacial and strain
energies [118,208,209,211—224]; (2) simultaneous or successive reduction of metal precursors in one-pot synthesis
[210,225—228]; and (3) selective growth followed by etching
or galvanic replacement [53,229,230]. Metal—metal heterostructures are ideal structures to study metal—metal
interfacial effect in catalysis.
Metallic heterojunction nanomaterials exhibit enhanced
catalytic activities in many liquid organic catalysis reactions, such as glucose oxidation and catalytic reduction of
p-nitrophenol by NaBH4 . It was found that charge transfer
between the primary catalytic metal component and concomitant metal plays an important role in the high catalytic
activity [210,230,231]. For example, Pd-Au nanomaterials
with crown-jewel structure exhibited significantly enhanced
activity in catalytic oxidation of glucose. The crown-jewel
structured Pd-Au catalysts were carefully prepared by deposition of top Au atoms on Pd mother nanoparticles via
the replacement reaction method [231]. While XPS confirmed a negative charge on Au due to the negative shift
of the Au 4f binding energy, DFT calculations revealed that
the Pd atoms donated electrons to the Au atoms. The
dramatically enhanced activity of the Pd-supported Au nanomaterials was attributed to the high negative charge density
of Au atoms. Guo et al. [230] synthesized dumbbell-like
Ag-tipped Au nanorods via lateral etching of the Au@Ag
core—shell nanorods by FeCl3 at room temperature for
p-nitrophenol hydrogenation (Fig. 11A—B). The Ag-tipped
Au nanorods showed higher reaction rate in the reduction of p-nitrophenol than the Au nanorods and the Au@Ag
core—shell nanorods. Due to the lower ionization potential
of Ag (7.58 eV) than that of Au (9.22 eV), they suggested
that the electronic charge would transfer from Ag to Au
in the Ag-tipped Au nanorods. The increased electron density of Au caused by the charge transfer was considered as
the main factor for the observed enhanced catalysis by Agtipped Au nanorods. Huang et al. [210] demonstrated that
dumbbell-like Cu—Ag nanostructures displayed enhanced
catalytic activity towards hydrogenation of p-nitrophenol
when compared with individual Cu or Ag nanostructures
or the mixture of Cu and Ag nanostructures (Fig. 11C—D).
The Cu—Ag nanodumbbells were obtained by sequential
B. Wu, N. Zheng
formation of Ag and Cu in a novel one-pot, seed-free strategy
with both silver and copper precursors fed at the same time.
It was suggested that the enhanced activity by the Cu—Ag
dumbbell nanostructures was due to the electron transfer
from Cu to Ag and thus increased localized electron density
of Ag segments.
In electrocatalysis, appropriate metal—metal heterojunctions can help to improve both activity and stability
of NM nanocatalysts. During the past decade, a significant amount of research efforts has been devoted towards
designing cost-effective Pt electrocatalysts for fuel cells.
Pt-alloyed and Pt-monolayer nanostructures have thus
attracted much research attention [22,44,46,111,232—237].
Pt—Pd and Pt—Au are among the most demonstrated
metal—metal heterojunctions for enhanced electrocatalysis. In single-crystal model catalysis, a significantly
enhanced electrocatalysis of Pt in ORR was already reported
by depositing Pt monolayer on Pd(1 1 1) surface [11]. Both
Pd and Pt have a face-centered cubic (fcc) phase with a similar unit length of 3.92 Å for Pt and 3.89 Å for Pd. The small
lattice mismatch means that the full epitaxial growth should
be favored [208]. In the literature, together with core—shell
structured or alloyed nanoparticles, Pt—Pd heterojunction
nanomaterials (e.g., nanodendrites [208,217,218,220,228],
dumbbell-like heterojunctions [214] and concave nanocubes
[229]) with both metals partially exposed for electrocatalysis have been well-documented. In many of the
developed Pt—Pd heterojunctions, the Pd—Pt interface were
built up by using nanoparticles [208,214,217,218,228,229],
or nanowires [220] as support for the deposition of
nanoparticles of the second metal to respectively form
particle-on-particle and particle-on-wire structures for the
studies on the interfacial effect. In 2008, Yang and coworkers synthesized binary Pt/Pd nanoparticles by localized
Pd overgrowth on cubic Pt seeds and proved that particleon-particle structure exhibited much less self-poisoning and
a lower activation energy relative to single Pt component
in the electro-oxidation of formic acid [214]. In 2009, Lim
et al. reported the synthesis of Pd—Pt bimetallic nanodendrites consisting of a dense array of Pt branches on a Pd
core (Fig. 12A—C) [217]. The Pd—Pt nanodendrites exhibited enhanced electrocatalysis in the ORR. Pt—Pd bimetallic
heteronanostructures were also prepared by several other
groups and found to exhibit enhanced electrocatalytic activity and stability in ORR [208,229] and methanol oxidation
[218,220,228].
In addition to Pt—Pd heterostructures, Pt—Au nanostructures have also attracted much attention in the
electrocatalysis. Xu et al. found that the catalytic activity of Pt nanoparticles could be greatly enhanced when
Pt nanoparticles were deposited onto the surface of Au
nanoparticles [211,224,238]. Zhang et al. have successfully demonstrated the stabilization of Pt oxygen-reduction
fuel-cell electrocatalysts against Pt dissolution by modifying Pt nanoparticles with Au clusters (Fig. 12D—F) [53].
The Au clusters were deposited onto the carbon-supported
Pt nanoparticles through a galvanic replacement reaction.
The as-prepared Au-Pt/C catalyst showed a negligible loss
of 4% in ECSA after 30,000 cycles of the durability test
[53]. Similarly, Tan et al. found that particle-on-wire nanomaterials with Pt and Pt3 Ni nanoparticles grown on Au
nanowires displayed much better activity and durability
Surface and interface control of noble metal nanocrystals
185
Figure 11 Particle-on-particle nanostructures and their catalytic properties in liquid hydrogenation. (A) TEM image of Ag-tipped
Au nanorods. (B) Plots of ln(Ct /C0 ) versus reaction time during the course of reduction of p-nitrophenol catalyzed by different
nanorods. (C) TEM image of dumbbell-like Cu—Ag nanostructures. (D) The linear relationship between ln(Ct /C0 ) and reaction time
during the course of reduction of p-nitrophenol catalyzed by Cu nanostructures, Ag nanostructures, the mixture of Cu and Ag
nanostructures, and dumbbell-like Cu—Ag nanostructures.
Source of A—B: Reproduced with permission from Ref. [230], © 2012 American Chemical Society; Source of C—D: Reproduced with
permission from Ref. [210], © 2012 WILEY-VCH Verlag GmbH & Co.
in ORR than commercial Pt/C catalyst (Fig. 12G—I) [222].
The Au component endowed stability by raising the
Pt oxidation potential [53], or suppressed the formation of poisoning intermediate CO [215]. Thus, Pt-on-Au
nanostructures usually have higher electrocatalytic activities [53,211,215,221,222,224,225,227,238] and durabilities
[53,211,221,222,224,238] in ORR and also alcohol oxidation
than the state-of-the-art Pt/C catalysts.
The metal—metal interface also plays an outstanding
role in gas-phase catalysis. It was reported that Pt-on-Au
dendritic nanostructures with a substantial interface exhibited enhanced CO tolerance for hydrogen oxidation in CO/H2
mixtures, compared to pure Pt [52]. The nanodendrites
were active H2 oxidation catalysts and showed high activity
at 90 ◦ C in the presence of 1000 ppm CO. The improved CO
tolerance is resulted from the interfacial collaboration by
two metals. Considering that CO did not bind strongly to
large Au nanoparticles (11 nm), the authors speculated that
the Au core most likely activated O2 and facilitated CO
oxidation at the Au—Pt interface. By oxidizing the CO at
the Au—Pt interface, the Pt components were cleansed of
CO contaminates and resumed their primary function as H2
oxidation catalysts.
Metal—oxide interface
NM nanoparticles supported on oxides form the basis of
many important catalysts for energy technology, pollution
prevention, and environmental cleanup [239—242]. In
186
B. Wu, N. Zheng
Figure 12 Particle-on-particle/wire nanostructures and their electrocatalytic properties. (A and B) TEM images of (A) truncated
octahedral Pd seeds and (B) the resulted Pd—Pt nanodendrites synthesized in the presence of truncated octahedral Pd seeds shown
in (A). (C) Comparison of electrocatalytic properties (mass activity at 0.9 V versus RHE) of the Pd—Pt nanodendrites, Pt/C catalyst
(E-TEK) (20% by weight of 3.2-nm Pt nanoparticles on carbon support), and Pt black (Aldrich) (fuel-cell grade). (D) High-resolution
TEM image of an Au/Pt/C catalyst made by displacement of a Cu monolayer by Au. (E) Polarization curves for the O2 reduction
reaction on Au/Pt/C catalysts on a rotating disk electrode, before and after 30,000 potential cycles (Sweep rate, 10 mV/s; rotation
rate, 1600 rpm). (F) Voltammetry curves for Au/Pt/C catalysts before and after 30,000 cycles (Sweep rate, 50 mV/s). (G and H) TEM
images of (G) Pt/Au nanowires and (H) Pt3 Ni/Au nanowires synthesized in the presence of Au nanowire seeds. (I) ORR polarization
curves for Pt/Au, Pt3 Ni/Au and Pt/C catalysts recorded at room temperature in an O2 -saturated 0.1 M HClO4 aqueous solution
(Sweep rate, 10 mV/s; rotation rate, 1600 rpm).
Source of A—C: Reproduced with permission from Ref. [217], © 2009 Science; Source of D—F: Reproduced with permission from Ref.
[53], © 2007 Science; Source of G—I: Reproduced with permission from Ref. [222], © 2011 The Royal Society of Chemistry.
oxide-supported NM nanocatalysts, the metal—oxide interfaces play an important role in their catalytic performances
and also stabilizing NM nanoparticles from sintering [243].
Many studies have assigned the origin of the catalytic activities of supported NM catalysts to the perimeter interfaces
between NM nanoparticles/nanoclusters and the oxide
support (Fig. 13) [42,239]. In the literature, the importance
of metal—oxide interfaces, such as Au—TiO2 [42,244—248],
Au—ZnO [43,249], Au—Fe3 O4 [50,250,251], Ag—CeO2 [252],
Pt—TiO2 [47,253], Pt—CeO2 [48,49,254,255], Pt—Fe3 O4
[256], Pd—CeO2 [257], Pd—Fe3 O4 [258], PtPd—Fe3 O4 [51], in
both catalysis and electrocatalysis has been widely recognized. Similar to the case of metal—metal interface, charge
transfer might also occur across the nanoscale interface,
either from metal to oxide (e.g., Au—Fe3 O4 [50,259]) or
from oxide to metal (e.g., Pt—Fe3 O4 [256]) to promote the
catalysis. In the case of Au—TiO2 interface, for example, a
charge transfer from TiO2 to Au occurs when TiO2 surfaces
are defective with oxygen vacancies, and vice versa [260].
Although multiple mechanisms have been proposed to
explain the high catalytic activity of oxide-supported Au for
CO oxidation at low temperatures, it is generally accepted
that the reaction occurs on metal sites at the Au—oxide
interface (Fig. 13) [41,42,239,240,261]. Both the size of
metal nanoparticles [262—264] and the type of oxide supports [48,265—268] influence the metal—oxide interactions.
Surface and interface control of noble metal nanocrystals
187
Figure 13 The mechanisms of CO oxidation on TiO2 -supported Au nanocatalysts. (A) Probable pathways reported by Haruta for CO
oxidation over supported Au catalysts; CO oxidation only occurs around the perimeter of Au—TiO2 . (B) Schematic of the mechanism
of low-temperature CO oxidation over an Au/TiO2 catalyst at a perimeter zone of reactivity, reported by Yates; experiments directly
observing CO/TiO2 and CO/Au surface species show that processes 2 and 3 are fast compared with process 4.
Source of A: Reproduced with permission from Ref. [240], © 2004 Springer; Source of B: Reproduced with permission from Ref. [42],
© 2011 Science.
However, most conventional oxide-supported NM catalysts are prepared by impregnating an oxide support with
a soluble metal precursor, followed by drying and reduction [239,240,261]. Due to different interactions between
supports and metal precursors, it is challenging to prepare
supported metal nanoparticle catalysts having the same
size of metal nanoparticles while supported on different
substrates. It is thus difficult to individually extract the
influencing factors of a supported metal nanocatalyst. To
simplify the study over the effect of metal-support interfaces on catalysis, attempts have been made in the design
of NM nanocatalysts to at least keep the metal nanoparticles in the same size. Several developed strategies are
as follows: (1) colloidal deposition of pre-made the samesized NM nanoparticles on different supports [269,270];
(2) post-modification of NM nanocatalysts to introduce
new metal—oxide interfaces [271—279]; (3) building up an
NM—oxide interface (usually dumbbell-like structures) to
stabilize NM nanoparticles from sintering and then bringing
the created NM—oxide particles onto supports to generate
the second NM-support interface [50,245,250,251,256,280];
(4) encapsulating pre-synthesized NM nanoparticles in hollow porous spheres to form ‘‘yolk-shell’’ or ‘‘rattle-type’’
metal@oxide nanostructures [48,257,281—287].
Pre-making NM nanoparticles with well-defined size and
composition and then depositing them on various oxidesupports represents one of the most effective methods
to extract the metal—metal oxide effect in catalysis. For
example, by depositing ∼3 nm Au nanoparticles on various
supports (i.e., TiO2 , Al2 O3 , ZrO2 , and ZnO) via a colloidal
deposition method, Comotti et al. found that TiO2 was
the most effective support to make the Au nanoparticles the most active in CO oxidation [269]. Zheng et al.
have developed an organic method to deposit sub-10 nm
organic-capped Au nanoparticles on different types of
oxide supports. The deposition method was not supportdependent [270]. However, it should be noted that pre-made
uniform NM nanoparticles are typically surface bound by
organic capping agents. Thermal treatments are usually
required to activate deposited NM nanoparticles to gain
decent catalysis. During the thermal treatments, however,
NM nanoparticles might sinter and change their size from
the pre-made size, making it again difficult to attribute the
observed catalysis difference to the metal—oxide interface.
During past several years, post-modification of pre-made
NM nanocatalysts with an inorganic coating has been demonstrated as an effective alternative to study the effect
of metal—oxide interface on catalysis [271—279]. In this
post-modification method, the pre-made NM nanocatalysts
can be prepared by already developed preparation methods. For example, Horváth et al. [271] first deposited
gold sols onto SiO2 surface and then used water-soluble
Ti(IV) bis(ammoniumlactato)-dihydroxide as a precursor
to prepare TiO2 /Au/SiO2 nanocatalysts. The introduced
Au—TiO2 interfaces resulted in the greatly increased CO oxidation activity compared to that of the reference Au/SiO2 .
Dai and co-workers [276] prepared MnOx /Au/SiO2 nanocatalysts by soaking Au(en)2 Cl3 -derived Au/SiO2 nanocatalysts
with KMnO4 solutions, followed by treatment in O2 —He
at 300—600 ◦ C. The yielded amorphous MnOx near gold
nanoparticles created additional Au—MnOx interfaces which
were found to boost the catalytic performance of the Au
nanocatalysts in low-temperature CO oxidation. When the
secondary metal—oxide interface is built up during the post
treatment, the size of original NM nanoparticles and their
interface with the primary oxide support should not be
altered, which is the key to make conclusive claim on the
effect of secondary metal—oxide interface.
188
B. Wu, N. Zheng
Figure 14 Metal-on-oxide structures and their interfaces. TEM images and proposed interfacial structures of (A, D) 6.7—15.2-nm
Au—Fe3 O4 /P25-TiO2 prepared by the colloidal deposition method, (B, E) 6.7—15.2-nm Au—Fe3 O4 /TiO2 prepared by the thermal
decomposition method, and (C, F) 6.7—4.9-nm Au—Fe3 O4 /TiO2 prepared by the thermal decomposition method.
Source of A—F: Reproduced with permission from Ref. [245], © 2009 Springer.
In addition to enhance catalytic activity, many
metal—oxide interfaces also help to stabilize NM
nanoparticles from sintering, which is essential to individually study the size and support effects of supported NM
nanocatalysts. NM—metal oxide dumbbell nanoparticles
that contain NM components are an ideal system to separate
the size and interface effects [50,245,250,251,256,280].
Dumbbell nanoparticles with NM nanoparticles in specific
size can be prepared through colloidal methods. When
dumbbell nanoparticles are attached onto oxide supports,
two metal—oxide interfaces are simultaneously built up.
One is the interface inside the dumbbell nanoparticles.
The other is interface between NM nanoparticles and the
oxide support. While the interface inside the dumbbell
nanoparticles can be used to stabilize NM nanoparticles from sintering, the NM-support can be manipulated
to optimize the catalytic activity. Au—Fe3 O4 dumbbell
nanoparticles have been thus applied to study the interface
effect of Au nanocatalysts in the past several years. For
example, Yin et al. prepared highly active Au catalysts from
Au—Fe3 O4 dumbbell nanoparticles for CO oxidation through
colloidal deposition method [250]. The Au nanoparticles
had an average size of ∼3 nm. Without any support, their
Au—Fe3 O4 dumbbell nanoparticles were highly active for
CO oxidation at low temperature. Similar high activities
were also observed when supported on inert supports,
such as SiO2 and carbon. However, TiO2 -supported particles
were less active and showed complete CO conversion above
∼120 ◦ C. These results of Au—Fe3 O4 dumbbell nanoparticles
of ∼3-nm Au nanoparticles were in sharp contrast to
Au—Fe3 O4 of 6.7-nm Au nanoparticles. Wu et al. prepared
Au—Fe3 O4 dumbbell nanoparticles with Au particle size of
6.7 nm and did careful studies over the effects of supports
on their catalytic activity in CO oxidation (Fig. 14) [245].
Their studies showed that the Au—Fe3 O4 interface could
effectively prevent the Au nanoparticles from sintering up
to 550 ◦ C, which is thus possible to exclude the size effect
while studying the influence effect. When supported on
activated carbon, the supported 6.7—15.2 nm Au—Fe3 O4
dumbbell nanoparticles displayed a negligible catalytic
activity, indicating that, without creating appropriate
Au—oxide interface, 6.7-nm Au nanoparticles alone are catalytically inert. However, the catalytic activity of supported
Au—Fe3 O4 dumbbell nanoparticles increased with increased
contact between Au and TiO2 support (Fig. 14), suggesting
the essential role of Au—TiO2 interface in the supported
6.7-nm Au nanoparticles. The dramatic difference between
∼3-nm and 6.7-nm Au nanoparticles alerts us that the size
and metal—oxide effect in a supported NM nanocatalyst
could be tightly related and therefore should be carefully
evaluated.
Encapsulating pre-synthesized NM nanoparticles in hollow porous spheres to form so-called ‘‘yolk-shell’’ or
‘‘rattle-type’’ nanostructures has been demonstrated as an
Surface and interface control of noble metal nanocrystals
effective method to study the effect of metal—oxide interface on catalysis too [48,257,281—289]. The ‘‘yolk-shell’’ or
‘‘rattle-type’’ nanostructures contain void space between
the NM cores and the outer porous shells and can effectively suppress the aggregation and sintering of fine metal
nanocatalysts. For the encapsulated NM nanocatalysts, the
metal—oxide interfacial effect can be systematically studied by keeping the NM core the same while changing the
compositions of the shell.
Oxide-supported NM nanocatalysts are generally considered to have a ‘‘metal-on-oxide’’ structure. ‘‘Oxide-onmetal’’ is another structure type to study the metal—oxide
interfacial effect in the nanoscale. By using single-crystal
metal surfaces, the oxide-on-metal model catalysts (socalled ‘‘inverse catalyst’’) have been made and studied
for more than 20 years [290—298]. Recently, Bao’s group
successfully extended the oxide-on-metal interface studies from single-crystal model systems to nanoscale systems
(Fig. 15) [21]. Based on surface science measurements and
density functional calculations, they first found that CO
bonding energy at coordinatively unsaturated ferrous (CUF)
sites on the FeO1 − x /Pt(1 1 1) interface was smaller than
that on FeO-free Pt surface (Fig. 15A). And the interfaceconfined CUF sites between Pt and nano-FeO1 − x were found
as the active center for the activation of O2 (Fig. 15B).
Such a finding was then successfully applied to elaborately
prepare Pt-FeO1 − x catalysts with highly dispersed small
Fe oxide species on the surface of Pt nanoparticles. The
prepared supported Pt-FeO1 − x nanocatalysts afforded the
largest amount of active sites at the perimeter interfaces
of Fe oxide nano-islands on Pt nanoparticles and therefore exhibited remarkable activity in the catalytic selective
oxidation of CO (Fig. 15D). By using the same concept,
they have prepared various ‘‘TMOx -on-Pt’’ (TM = Fe, Co, and
Ni) nanocatalysts with Pt nanoparticles decorated with different highly dispersed oxides, and systematically studied
the Pt—TMOx interfacial effects in both preferential oxidation of CO in excess H2 (PROX) and CO oxidation in the
absence of H2 (COOX) [299—303]. Similar studies on ‘‘oxideon-metal’’ nanocatalysts were also reported by other groups
[304—307].
For supported NM catalysts, an ideal structure should
have a maximum utilization of NM, which means that the
NM species should be dispersed as small clusters or even single atoms on oxide supports. Towards this goal, increasing
research efforts have been recently directed made in developing oxide-supported single-atom NM catalysts [308—312].
In oxide-supported single-atom NM catalysts, it should be
expected that the metal—oxide interface will make an even
more important influence on their overall catalytic performance.
Metal—organic interface
It is well known that metal—ligand (or metal—organic)
interactions can influence the optical properties of
organometallic compounds due to the ligand-to-metal or
metal-to-ligand charge transfer [313,314]. Together with
charge transfer, steric properties of ligands also play
critical roles in modifying the reactivity and selectivity of metal complexes in various homogeneous catalysis,
189
such as cross-coupling reactions, cyclization, rearrangement, cycloisomerization [39,315—322]. Although steric
and electronic factors are considered as the main two
effects of ligands on the catalytic performance of metal
complexes, these two effects are sometimes not easily separated.
Similarly, in the field of heterogeneous catalysis,
metal—ligand interfacial interactions have been largely
studied. In many cases, the presence of capping ligands on
the surface of NM nanocrystals was found to completely
hinder their catalytic activity. However, many studies have
also demonstrated that capping ligands on the surface of
nanoparticles can effectively steer the chemoselectivity
and enantioselectivity in various nanoparticle catalyzed
liquid-phase reactions [59,60,323,324]. A well-documented
example is the Orito’s catalytic system in which the
platinum catalysts were surface-modified with cinchona
alkaloid derivatives for the asymmetric hydrogenation of
␣-ketoesters [54—61]. The cinchona modifiers not only render the catalysts enantioselective but strongly accelerate
their hydrogenation activities. Many other chiral ligands
were also found to have interfacial effect on the heterogeneous catalysis, such as chiral diphosphite [325,326],
tartaric acid [56,327]. Changes in conformational, steric,
and electronic properties of chiral ligands could lead to
dramatic variations of reactivities and enantioselectivities
of the modified nanoparticulate catalysts.
Moreover, it has been recently found that capping achiral
ligands, such as alkylamine and alkanethiol, on NM nanocatalysts can also modify their activities and selectivities for
heterogeneous catalysis, such as chemoselective hydrogenation [33—35,37,40] or silane alcoholysis [36]. When amines
was used as the surface modifiers, for example, Wu et al.
have demonstrated that the chemoselectivities of Pt3 Coalloy nanoparticles in the selective hydrogenation of C O
bonds of ␣,␤-unsaturated aldehydes (e.g., cinnamaldehyde,
citral) [33] varied with the carbon chain length of the capping amines (Fig. 16A). Detailed studies and periodic DFT
calculations revealed that primary amines with longer carbon chains at the surface of the Pt3 Co-alloy nanoparticles
gave higher selectivity for the hydrogenation of C O bonds
and less hydrogenation of C C bonds. The high hydrogenation selectivity was attributed to the steric effect of the
long-chain amines that favors adsorption of the substrate
molecules on the catalytic surface of the nanoparticles via
the end C O groups rather than adsorption of the C C
bonds located in the middle of the molecule. Similarly, Kwon
et al. found that primary amines could act as selectivity
switchers in alkyne hydrogenation reactions on Pt and Co/Pt
nanoparticles (Fig. 16B) [37]. While ligand-free nanoparticles catalyzed complete hydrogenation of alkynes into
alkanes, addition of primary alkylamines onto the surface
of Pt and Pt3 Co nanoparticles could drastically increase the
product selectivity towards alkenes from 0 to more than 90%
with ∼99.9% conversion. The balance between the adsorption energetics of substrates and interfacial capping ligands
was claimed to determine the selectivity and activity of the
catalysts.
Several recent studies also revealed that both selectivity
modification and activity enhancement can be realized due
to strong metal—thiol interactions. For instance, Marshall
et al. [35] demonstrated that self-assembled monolayer
190
B. Wu, N. Zheng
Figure 15 Oxide-on-metal structure and the catalytic properties in PROX. (A) Calculated adsorption energy for CO and O2
molecules on Pt(1 1 1) and FeO1 − x /Pt(1 1 1) surfaces. (B) Schematic structure of the CUF sites and calculated transition states
of O2 dissociation (the inset shows the top view) at the boundary between FeO and Pt(1 1 1). (C and D) PROX reaction of the (C)
Pt/SiO2 and (D) Pt-FeO1 − x /SiO2 nanocatalysts under the conditions 1% CO, 0.5% O2 , and 98.5% H2 . Space velocity is 36,000 mL g−1 h−1 ;
pressure = 0.1 MPa.
Source of A—D: Reproduced with permission from Ref. [21], © 2010 Science.
coatings of n-alkane thiols on Pd nanocatalysts improved
the selectivity of 1-epoxybutane formation from 1-epoxy-3butene from 11% to 94% under equivalent reaction conditions
and with equivalent conversions compared to the uncoated
catalyst (Fig. 16C). Although sulfur species were generally considered as poisons of NM catalysts, the thiol-coated
Pd nanocatalysts still exhibited epoxybutane formation
rates that were approximately 40% of the rate for the
uncoated catalyst. The same group also revealed that
the selectivity modification with thiol could be extendable to supported Pt nanocatalysts [328]. More recently,
Taguchi et al. [36] reported an unprecedented catalyticrate-enhancement effect of alkanethiol on Au nanoparticle
catalyzed silane alcoholysis reactions at room temperature
(Fig. 16D). A metalloenzyme-like catalytic-enhancement
mechanism based on molecular encapsulation in the alkanethiol interface around the Au nanoparticles was put
forward by the authors. Alkanethiol capping on Au nanoparticles helped to create space to encapsulate substrate
molecules, based on molecular recognition through intermolecular hydrophobic interactions.
Besides Pt—amine, Pd—thiol and Au—thiol interfaces,
other metal—organic interactions were also reported to
play an important role on the catalytic performance of NM
nanocatalysts, such as Pt-thiol [34,328], Au-bipyridine [32],
Au-quinoline [40]. The presence of metal—organic interface
may lead to enhanced selectivity and/or activity, due to
steric effect [32—34,36], electronic effect [35,37] or their
combination [38—40].
Recently, a new kind of metal—organic interface, based
on ionic liquids (ILs), has received much attention for
the construction of high-performance ORR electrocatalysts.
ILs typically possess high thermal and chemical stabilities with considerable electrical conductivity and wide
electrochemical windows [329]. Three types of metal—IL
systems, Pt—IL [330—335], Au—IL [336,337] and Pd—IL
[338], have been demonstrated to own improved ORR
activity [330—334,336—338] and stability [330,334—336].
The enhanced electrocatalysis was attributed to high
O2 solubility in certain ILs [330,334], enhanced anti-CO
poisoning [332], and methanol-tolerant [334] properties of the IL-modified NM catalysts. Snyder et al.
[330] developed a composite electrocatalyst consisting of
nanoporous-NiPt alloy impregnated with a hydrophobic,
high-oxygen-solubility and protic IL ([MTBD][beti]) that had
extremely high mass activity for the ORR compared with any
other catalyst for this reaction. Nanoporous-NiPt alloy itself
had a relatively high kinetic current density jk , 7 mA cm−2
at 0.9 V versus RHE. Interestingly, when impregnating the
NiPt alloy with IL, jk rised to 18.2 mA cm−2 at 0.9 V, equal to
the best single-crystal model catalysts of Pt3 Ni(1 1 1) [13].
The excellent role of IL was further proved by Zheng’s group
[334]. They found the IL [MTBD][bmsi] is more oxygenphilic
and less methanol-philic than the exterior aqueous solution
(Fig. 16E). When impregnated with [MTBD][bmsi], graphenesupported Pt nanoparticles can exhibit both enhanced
electrocatalytic activity and excellent methanol tolerance
for ORR. These examples suggest that the metal—IL interfaces represent a class of interfaces that can be easily
built up to enhance the electrocatalytic performance of
NM nanocatalysts. The effectiveness of NM—IL interfaces
in enhancing catalytic performance of NM nanocatalysts in
catalysis other than electrocatalysis should be investigated
in future studies.
Surface and interface control of noble metal nanocrystals
191
Figure 16 Metal—organic interfaces and their (electro)catalytic properties. (A) Optimized structure of cinnamaldehyde adsorption
(ball-and-stick) on the Pt3 Co surface-capped by OAm (line) and the resulted selective hydrogenation of cinnamaldehyde. Orange, the
terminal oxygen atom; blue, Pt atom; purple, Co atom. (B) Schematic diagram of selective hydrogenation of alkyne on alkylaminecapped Pt and Co/Pt nanoparticles or on ligand-free nanoparticles. (C) The structure of a propanethiol on Pd(1 1 1) and the resulted
selective hydrogenation of 1-epoxy-3-butene to 1-epoxybutane on thiol-capped Pd nanocatalysts. (D) Schematic illustration of the
metalloenzyme-like catalytic-enhancement mechanism based on molecular encapsulation in the alkanethiol—SAM interface around
the Au nanoparticles.
Source of A: Reproduced with permission from Ref. [33], © 2012 WILEY-VCH Verlag GmbH & Co.; Source of B: Reproduced with
permission from Ref. [37], © 2012 American Chemical Society; Source of C: Reproduced with permission from Ref. [35], © 2010
Macmillan Publishers Limited; Source of D: Reproduced with permission from Ref. [36], © 2012 WILEY-VCH Verlag GmbH & Co.;
Source of E: Reproduced with permission from Ref. [334], © 2012 The Royal Society of Chemistry.
Conclusions and perspectives
Controlling the exposed facet and interface of NM nanomaterials provides a powerful tool for realizing their enhanced
catalytic properties (i.e., activity, selectivity and stability). By choosing the synthetic methods, such as using
the selective facet capping agents (e.g., surfactant, small
adsorbate, biomolecule), electrochemical method, UPD, or
seeded growth, NM nanocrystals exposed with controllable
facets can be obtained by wet chemistry. NM nanocrystals with the exposed facets with abundant active sites,
especially low-coordinated stepped sites, are significant
in catalysis and electrocatalysis. In spite of the reported
facet controlling strategies, further attention should be paid
to the following research themes in the near future: (1)
large-scale continuous production of small NM nanocrystals
exposed with desirable crystal facets, (2) stabilizing the
desired exposure facets of small NM nanocrystals under real
catalytic conditions; (3) combination of DFT calculations
and various surface characterization techniques to get a
molecular level understanding on the selective facet binding
and facet-dependent catalysis. Moreover, the surface reconstruction [339—341] and surface segregation [342—344] of
NM nanocrystals can be induced by the substrates and thus
should be considered in the catalytic process. The effect
of surface/lattice strain [111,131,345] on catalytic performance is still to be further exploited.
Apart from facet control, interface control via building up and tuning the metal—metal, metal—oxide, and
metal—organic interactions of NM-based nanomaterials can
also optimize the catalytic performance of NM nanocrystals. The interactions between NM and foreign components
usually arise from steric or electronic effects, and characterization means, such as IR-CO probe [42,346] should be
developed and applied to accurately recognize the interfacial interaction. The presence of interfaces between NM
and foreign component (i.e., metal, oxide, ligand) may bring
new possibilities for tuning the structural properties and
catalytic behaviors of NM nanocrystals. In addition, studies on other interfaces, such as metal—hydroxide [347,348],
metal—CNTs/graphene [349—354], should be also envisaged. For example, NM—graphene nanocomposites exhibited
unexpected catalytic activity for the Suzuki—Miyaura coupling reaction [349,351] and electrochemical applications
[350,352—354].
Although the last decade has witnessed significant
progress in the facet-controlled or interface-controlled synthesis of NM nanomaterials, NM nanomaterials with good
simultaneous control of both facet and interfacial structures
are less reported. It is considered that future generations of
192
NM nanocatalysts will have highly selective structures with
specific components and well-defined surface and interface structures, and thus exhibit high catalytic activity and
selectivity, meeting the standards of green chemistry [6].
To pursue such NM nanocatalysts, the traditional ‘‘trialand-error’’ approaches to making nanocatalysts have to
be replaced by rational ‘‘reflection in action’’ or ‘‘design
and synthesis’’. Collaborative efforts across different fields,
such as catalysis, synthetic chemistry, surface science, electrochemistry and computational chemistry are required to
achieve the goal of designing high-performance NM nanocatalysts.
Acknowledgments
We thank the MOST of China (2011CB932403,
2009CB930703), the NSFC (21131005, 21021061, 20925103,
20923004), and the Fok Ying Tung Education Foundation
(121011) for the financial support.
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Binghui Wu received his B.S. degree from Beijing Institute of Technology in 2006. He then
obtained his Ph.D. degree under the supervision of Professor Nanfeng Zheng from Xiamen
University in 2012. His research interests
include the synthetic methodologies and catalytic applications of noble metal and metal
oxide nanomaterials.
197
Nanfeng Zheng received his Ph.D. degree
from University of California — Riverside in
2005. After 2-year postdoctoral research at
University of California — Santa Barbara, he
moved to Xiamen University as a full professor in 2007. He is currently a Changjiang Chair
professor at Xiamen University. In 2009, he
received the Distinguished Young Investigator
Award from NSF-China. His research interests focus on advanced functional materials
for both fundamental research and practical
applications, particularly in the fields of catalysis and biology.