Platinum Metal Catalysts of High-Index Surfaces ... - ACS Publications

J. Phys. Chem. C 2008, 112, 19801–19817

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FEATURE ARTICLE Platinum Metal Catalysts of High-Index Surfaces: From Single-Crystal Planes to Electrochemically Shape-Controlled Nanoparticles Na Tian, Zhi-You Zhou, and Shi-Gang Sun* State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, China ReceiVed: May 7, 2008; ReVised Manuscript ReceiVed: September 7, 2008

Nanoparticles of platinum group metals (PGM) supported on diverse substrate materials are widely used catalysts in many important fields such as modern chemical industry, petrochemical industry, automobile exhaust purification, and fuel cells. Due to the extremely high cost and rare reserve of the PGM on the earth, to further improve the catalytic activity, stability, and utility efficiency of PGM nanoparticles is the key issue in relevant industrial development as well as the challenge of basic research of science and technology. This feature article summarizes at first the relationship between surface structure and catalytic functionality gained by using metal single-crystal planes as model electrocatalysts, which reveals that high-index planes, i.e., the planes denoted by a set of Miller indices (hkl) with at least one index being larger than unit, with high density of atomic steps and kinks, exhibit generally high catalytic reactivity and stability. Next, guided by the knowledge acquired in model electrocatalysis, we put emphasis upon the electrochemically shape-controlled synthesis of Pt and Pd nanocrystals (NCs) bounded by highindex facets, including tetrahexahedral NCs with 24 {hk0} facets, trapezohedral NCs with 24 {hkk} facets, concave hexoctahedral NCs with 48 {hkl} facets, and multiple twinned nanorods with {hk0} facets. Finally, challenging issues and future prospects in this exciting field are outlined. 1. Introduction Catalysts of platinum group metals (PGM: Pt, Pd, Rh, Ir, Ru, and Os) are of technological importance and are used indispensably in modern chemical industry, petrochemical industry, automobile exhaust purification, and fuel cells, owing to their excellent activity and stability.1-4 However, the price of PGM is extremely high due to the rare reserve on the earth, and the performances of existing PGM catalysts are always behind the expectation of fast development of relevant industries. For example, the failure of large-scale commercialization of low-temperature fuel cells is due to unacceptably high cost mainly caused by high loading of Pt catalysts.5,6 To find and design new types of PGM catalysts with enhanced activity and stability is therefore the key issue in development of the above momentous fields. It is well-known that the catalytic properties of metal nanoparticles depend strongly on their size, shape, and chemical composition,7-17 among which the shape of a nanoparticle determines its surface atomic arrangement and coordination. As an example, a cubic Pt nanoparticle is bounded by six {100} facets, on which surface atoms are arranged in 4-fold symmetry with coordination numbers of eight. During the past decades, surface scientists have systematically studied the surface structure-catalytic functionality by using metal single-crystal planes as model catalysts and discovered that those surfaces with open * Corresponding author. Fax: +86 0592 2183047. E-mail: sgsun@ xmu.edu.cn.

structure usually exhibit much enhanced activity and stability.18-21 As for face-centered cubic (fcc) metals of PGM (i.e., Pt, Pd, Rh, Ir), the high-index planes, denoted by a set of Miller indices (hkl) with at least one index being larger than unit, are open-structure surfaces, which have high density of atomic steps and kinks.22,23 These low-coordinated atoms can easily interact with reactant molecules and serve as active sites for breaking chemical bonds.18,24-26 Consequently, the catalytic activity of high-index planes is generally superior to that of low-index planes, which consist of densely packed atoms, such as {111} and {100}.21,22 It has been reported that, for instance, the Pt(210) surface possesses an extremely high catalytic activity for electroreduction of CO2 and also for electrooxidation of formic acid,27,28 and the Pt(410) plane exhibits unusual activity for catalytic decomposition of NO that is a major pollutant of automobile exhaust.29 Moreover, some high-index planes with very high density of atomic steps, e.g., Pt(210), exhibit high thermal and chemical stability under both reducing and oxidizing conditions and even at elevated temperature.30 From the point of view of applications, high-index planes of bulk metals can not be used as practical catalysts due to their high cost and low surface area, as well as their effortless reconstruction under reaction conditions. Instead, metal nanoparticles with high-index facets are novel types of catalysts that are promising to satisfy the requirements of fast development of important fields employing PGM catalysts. Unfortunately, it is rather challenging to synthesize metal nanoparticles enclosed by high-index facets. It is well-

10.1021/jp804051e CCC: $40.75  2008 American Chemical Society Published on Web 11/12/2008

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Na Tian is an assistant professor at the Department of Chemistry, Xiamen University, China. She received her B.Sc. Degree in 1999 from Qufu Normal University, her M.Sc. Degree in 2002 from Hebei Normal University, and her Ph.D. in 2007 from Xiamen University. Her current research interests include shape-controlled synthesis of nanomaterials and electrocatalysis.

Tian et al.

Shi-Gang Sun is a professor at the State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, China. He received his B.Sc. Degree of Chemistry in 1982 from Xiamen University and Doctorat d’Etat in 1986 from Universite´ Pierre et Marie Curie (Paris VI) and carried out post-doctoral research at the Laboratoire d’Electrochimie Interfaciale du CNRS, France. He is currently a Fellow of the Royal Society of Chemistry, U.K., and a Fellow of the International Society of Electrochemistry. His research interests include electrocatalysis, electrochemical surface science, spectroelectrochemistry, and electrochemical energy conversion and storage.

In this feature article, we summarize at first the state-ofthe-art developments in studies of surface structure-catalytic functionality using single-crystal planes as model catalysts. Next, the emphasis is put upon our recent breakthrough in the synthesis of Pt nanocrystals with high-index facets and their high catalytic activity toward electrooxidation of small organic molecules.51-54 Finally, the challenging issues and future prospects along this exciting new avenue are outlined. Zhi-You Zhou is an associate professor at the Department of Chemistry, Xiamen University, China. He received his B.Sc. Degree in 1998 and Ph.D. in 2004 from Xiamen University. His current research interests include electrocatalysis, nanomaterials, fuel cells, and electrochemical in situ FTIR spectroscopy.

known that the surface energy of different crystalline planes of a fcc metal is increased in the order of γ{111} < γ{100} < γ{110} < γ{hkl}.31-34 As a consequence, the rate of crystal growth in the direction perpendicular to a high-index facet with high surface energy is much faster than that along the normal direction of a low-index facet, which results in rapid disappearance of high-index facets during nanoparticle formation.35 Over the past decade, although a variety of metal nanoparticles with polyhedral shapes have been synthesized through the development of shape-controlled synthesis methods, nearly all of them are bounded by low-index facets, such as tetrahedron, octahedron, decahedron, and icosahedron bounded by {111} facets,36-38 cube by {100},39,40 cuboctahedron by the mixture of {111} and {100},41 and rhombic dodecahedron by {110}.42 As for the most important catalytic metal of platinum, after a pioneering work of El-Sayed and co-workers in 1996 for the synthesis of cubic and tetrahedral Pt nanoparticles through chemical reduction of K2PtCl4 by H2 in the presence of polyacrylate,39 various Pt nanocrystals with different shapes have been obtained by changing Pt precursors, reducing reagents, stabilizing reagents, and solvents.41,43-50 Nevertheless, the surface structure of all the synthesized Pt nanoparticles is limited to low-index facets of {111} and {100}, and no Pt nanoparticles bounded by high-index facets are obtainable by chemically shapecontrolled synthesis methods.

2. State-of-the-Art Developments of Surface Structure-Catalytic Functionality Studies Using Metal Single-Crystal Planes as Model Catalysts 2.1. Metal Single-Crystal Planes and Their Surface Atomic Arrangement. Metal single-crystal planes can provide a variety of surface structures with well-defined atomic arrangements, depending on their orientation. As for fcc metals, such as Pt, a unit stereographic triangle (Figure 1) is conventionally used to illustrate the coordinates of different crystal planes.55 The atomic arrangement models of several typical planes are also shown in Figure 1. Three vertexes of the triangle represent the three low-index planes or basal planes, i.e., (111), (100), and (110). Among them, the (111) and (100) planes are atomic-scale flat with closely packed surface atoms, whereas the (110) plane is rough with step atoms. The coordination numbers (CN) of toplayer atoms on (111), (100), and (110) are 9, 8, and 7, respectively. Other planes lying in the sidelines and locating inside the triangle are high-index planes. The three sidelines of the triangle represent [011j], [11j0], and [001] crystallographic zones, in which the planes exhibit terrace-step structure and are thus also called stepped surfaces. To illustrate the terrace-step structure straightforwardly, a microfacet notation is often introduced to denote the stepped surfaces.56 For example, Pt(310) can be expressed as Pt(s)-[3(100) × (110)], indicating a stepped surface composed of a terrace of three atomic width of (100) symmetry, separated by a monatomic step of (110) symmetry. It is worthwhile to note that the step atoms on surfaces lying in the [001] zone are kink atoms with CN of 6, whereas the step atoms on surfaces belonging to [011j] and [11j0] zones hold a CN of 7 without the kink feature. The densities of step atoms (CN ) 6-7) on the stepped surfaces with monatomic

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Figure 1. Unit stereographic triangle of fcc metal single-crystal and models of surface atomic arrangement. (From ref 55.)

TABLE 1: Densities of Step Atoms and Configuration of Steric Sites on Pt Stepped Surfaces

†

a is lattice constant. For Pt, a ) 0.3924 nm. ‡ CN is the coordination number.

step structure in the three crystallographic zones are listed in Table 1. In the [001] zone, Pt(210) ()2(100) × (110)) possesses the highest density (5.81 × 1014 cm-2) of step atoms with CN of 6. Along with increasing the width of the (100) terrace, the density of step atoms decreases to 4.11 × 1014 cm-2 on Pt(310) ()3(100) × (110)) and to 2.55 × 1014 cm-2 on Pt(510) ()5(100) × (110)). The planes inside the triangle are distinguished by chirality and kink atoms with CN of 6. It has been well-established that the orientation, ordered domains, and population of step and terrace sites of a Pt singlecrystal surface can be readily characterized by electrochemistry using its voltammogram of hydrogen adsorption/desorption.57-62 For example, Furuya et al. have provided a series of voltammograms of Pt single-crystal planes including basal, stepped, and kinked surfaces in both acidic and basic solutions and shown that the features of hydrogen desorption varied regularly with Miller indices.62,63 As for Pt nanocrystals, Feliu and co-workers have demonstrated that irreversible adsorption of foreign adatoms is a sensitive probe to characterize the surface structure; e.g., the irreversible adsorption of bismuth can be used to characterize (111) ordered domains, and the irreversible adsorption of germanium may be employed to monitor (100) ordered domains.61 It is worthwhile pointing out that the surface

structural information of the Pt electrode extracted from voltammograms is phenomenological and thus indirect. Since surface relaxation, reconstruction, and even faceting may occur to a Pt electrode during experiments, the development of surface structural sensitive technology other than electrochemistry is always important. In an ultrahigh vacuum (UHV), low-energy electron diffraction (LEED) can provide, in reciprocal space, definitive structure information of Pt singe-crystal planes.30 In electrochemical environments, synchrotron surface X-ray scattering (SXS) and scanning tunneling microscopy (STM) are two powerful tools to determine the surface structures of Pt singlecrystal electrodes, and several excellent reviews have been presented by Adzic,64 Markovic,65 and Itaya.66 In the case of Pt low-index planes, electrochemical SXS and STM have confirmed that Pt(111) and Pt(100) hold a (1 × 1) structure,67-70 whereas Pt(110) exhibits a (1 × 1) or a (1 × 2) structure, depending on the cooling conditions after thermal annealing.71 The structural characterization of Pt high-index surfaces is very rare so far. SXS has the advantage that it can determine underlying structures as well as topmost structures and can monitor surface structure during electrochemical reactions in real time. Very recently, Hoshi et al. reported their successful characterization of two Pt high-index planes in 0.1 M HClO4

19804 J. Phys. Chem. C, Vol. 112, No. 50, 2008 solution by using SXS and found that the Pt(310) surface presented a pseudo (1 × 1) structure and that a kinked structure of this plane existed definitely on both clean and CO adsorbed surfaces,72 while the Pt(311) surface reconstructed to a (1 × 2) structure under the same conditions.73 2.2. Surface Structure Effects in Electrocatalysis. Numerous efforts have been made, in the past decades, to study the intrinsic relationship between surface structure and electrocatalytic properties by using metal single-crystal electrodes with well-defined surface structures.21,22,65,74 Several selected examples are reviewed below. 2.2.1. Effects of Surface Structure on Catalytic ActiWity. (1) Electrooxidation of Small Organic Molecules. Direct fuel cells (DFCs) based on polymer electrolyte membrane are very promising power sources for portable electronic devices and transportation vehicles, due to their high energy density, modest operating conditions, and safety.75,76 Anodic reactions of DFCs are electrooxidation of small organic molecules (SOMs), such as methanol, ethanol, ethylene glycol, and formic acid, on Ptbased electrocatalysts. These reactions are highly sensitive to surface structure since they involve cleavage of strong chemical bonds of C-C, C-H, and C-O.77-82 To provide a current density that is of practical and technical significance, these reactions have to take place at an overpotential as high as hundreds of millivolts, which reduces the output cell voltage of DFCs by the same value. Stimulated by this fact, the electrooxidation of SOMs on Pt single-crystal planes has been extensively studied to find out the intrinsic relationship between surface structure and catalytic functionality for designing and screening better catalysts. It is generally accepted that the electrooxidation of SOMs on Pt surfaces is via a dual-path mechanism, i.e., reactive intermediates (direct pathway) and poisoning intermediates (indirect pathway).83,84 The poisoning intermediates are determined mainly as adsorbed CO (COad) species, which are derived from the dissociative adsorption of SOMs and can be scarcely stripped out until electrode potential is above 0.6 V (RHE), where oxygen-containing species are generated on Pt surfaces. High-index planes can catalyze both the direct and the indirect pathways. Therefore, at relative high potential, high-index planes exhibit much higher activity than that of the basal planes, although sometimes lower activity may also occur at low potential due to the quick formation of the poisoning intermediates. In the case of electrooxidation of methanol on Pt basal planes, the catalytic activity is in an order of Pt(110) > Pt(100) > Pt(111).85 Introducing atomic steps on the surfaces generally promotes the overall oxidation rate, although the formation of poisoning CO species derived from dissociative adsorption of methanol on the step sites may be also increased.86,87 Koper et al.87-89 have studied methanol oxidation on Pt single-crystal planes lying in the [11j0] zone and found that the catalytic activity of the investigated Pt single-crystal planes is in the order of Pt(111) < Pt(110) < Pt(554) ()9(111) × (110)) < Pt(553) ()4(111) × (110)). Although the stepped surfaces are deactivated faster, they still remain a much higher activity than the Pt(111) does. Tripkovic et al.90 have studied methanol oxidation on Pt stepped surfaces belonging to the [011j] zone and demonstrated that Pt(755) held the highest activity, while the catalytic activity decreased by further increasing the density of step sites, i.e., Pt(755) ()6(111) × (100)) > Pt(211) ()3(111) × (100)) > Pt(311) ()2(111) × (100)), due to the poisoning effect. Moreover, density functional theory (DFT) calculations

Tian et al. also indicate that the initial steps of C-H and O-H cleavage of methanol decomposition are more favorable at the Pt step sites.91 For the electrooxidation of formic acid, the catalytic activity of Pt single-crystal basal planes was ranked in an order of Pt(110) > Pt(111) > Pt(100), and the corresponding apparent active energies were determined as 10.1, 25.8, and 32.2 kJ · mol-1, respectively.92 Figure 2a shows cyclic voltammograms (CVs) of formic acid oxidation on Pt stepped surfaces lying in the [001] zone, i.e., Pt(110), Pt(210), Pt(310), and Pt(610), in 0.1 M HCOOH + 0.5 M H2SO4.28 In the negativegoing potential sweep, two current peaks appearing at 0.80 and 0.40 V (RHE) correspond to formic acid oxidation on (110) steps and (100) terraces, by which the activity of these two types of surface sites can be assessed quantitatively. It was found that the activity of (110) step sites on the stepped surfaces is considerably different from that of the (110) sites on the Pt(110) electrode, depending on the width of the (100) terraces. The activity of the (110) step sites is increased by 2 times on Pt(210) and increased by 1.5 times on Pt(310), while it decreased by about 70% on Pt(610). This effect has been attributed to the particular surface sites consisting of (110) steps and (100) terraces. On Pt(210), the height of (110) steps and the width of (100) terraces are comparative, resulting in a high density of six-atom chair-sites (Figure 2b) that display a very high catalytic activity toward formic acid oxidation. As for Pt stepped surfaces belonging to the [011j] and [11j0] zones, Adzic et al.81 have found that the oxidation current of formic acid increased by introducing atomic steps on Pt(100) and Pt(110) surfaces but decreased by bringing steps on the Pt(111) surface. Besides, Hoshi et al.93,94 have studied the electrooxidation of formic acid on Pd basal planes and Pd(S)-[n(100) × (111)] (n ) 2-9) stepped surfaces. They illustrated that the catalytic activity of the basal planes was in the order of Pd(110) < Pd(111) < Pd(100), and the highindex plane of Pd(911) (n ) 5) exhibited the highest activity. (2) Electrooxidation of CO. Adsorbed CO (COad) usually acts as poisoning species involved in electrooxidation of SOMs; therefore, the study of electrooxidation of COad is of great interest in DFC development. The adsorption structures of CO on Pt low-index planes have been revealed by SXS and STM. 95,96 It has found that Pt high-index planes also exhibit high catalytic activity toward CO oxidation. Lebedeva et al.23,97 have studied the oxidation of COad on stepped surfaces of Pt(S)[n(111) × (111)] with n ) 30, 10, and 5 (i.e., Pt(15,15,14), Pt(554), and Pt(553)) and on basal planes of Pt(111) and Pt(110) in 0.5 M H2SO4. They demonstrated that the overpotential for the oxidation of a saturated CO adlayer, as well as CO submonolayer, is increased in the sequence of Pt(553) < Pt(554) < Pt(111), and the apparent rate constant is proportional to the step fraction on the Pt surfaces. These results have confirmed that the active sites for COad oxidation consist of step atoms. By using the same Pt single-crystal surfaces, Garcia et al.98 illustrated that, in alkaline solutions, the activity for COad oxidation is in the order of kinks (defects) > steps > terraces, based on the observation of multipeaks of CO oxidation in voltammograms. Mikita et al.99 studied the electrooxidation of COad on another series of stepped surfaces of Pt(S)-[n(100) × (110)] with n ) 2, 5, and 9 (i.e., Pt(210), Pt(510), and Pt(910)) and found that Pt(210) exhibits the highest activity, and the onset potential on Pt(210) is 0.20 V lower than that on Pt(510) and Pt(910). The high activity of Pt(210) was ascribed to the highest density of kink atoms. The Pt high-index surfaces also exhibit high activity for bulk (dissolved) CO oxidation, and the overpotential is linearly decreased with increasing step den-

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Figure 2. (a) Cyclic voltammograms of formic acid oxidation on Pt(110), Pt(210), Pt(310), and Pt(610) electrodes. The current peaks at about 0.80 and 0.40 V correspond to formic acid oxidation on (110) steps and (100) terraces, respectively. (b) Atomic model of Pt(210) surfaces and steric chair sites with high activity. (From ref 28.)

sity.100 The enhanced catalytic activity has been correlated with the preferential formation of oxygen-containing species at the step sites.97,101 (3) Electroreduction of O2. Oxygen reduction reaction (ORR) is one of the most important electrochemical reactions because of its wide application in electrochemical energy conversion and corrosion science. The catalytic activity of Pt basal planes for the ORR has been determined in the order of Pt(110) > Pt(100) > Pt(111).102 Recently, Feliu et al. have studied the structure sensitivity of the ORR on Pt high-index planes lying in the [011j] and [11j0] zones. They illustrated that the Pt(111) surface exhibits the lowest catalytic activity, while stepped surfaces, irrespective of their step site symmetry, display always higher catalytic activity (Figure 3).103,104 The catalytic activity increases linearly with increasing the density of step atoms. In the [011j] zone, the Pt(211) surface gives the highest activity that is over 1 order of magnitude higher than that of Pt(111) and 4-fold higher than that of Pt(100) in 0.5 M H2SO4. However, in 0.1 M HClO4 solution, the difference in activity becomes less significant; i.e., the activity of the Pt(211) is only three times that of Pt(111) and about two times that of Pt(100). This has been correlated to the inhibition effects of anion adlayers as studied by surface X-ray scattering.105,106 It has been illustrated that, for Pt single-crystal planes lying in the [11j0] zone, the Pt(331) displays the highest electrocatalytic activity.104 (4) Electroreduction of CO2. Electrochemical reduction of CO2 can transform inert CO2 into hydrocarbons, alcohols, carboxylic acid, and CO, depending on the nature of electrode materials and applied polarization conditions.107-110 It has been

Figure 3. Oxygen reduction activity (exchange current density) of Pt single-crystal electrodes lying in the [011j] zone in (9) 0.5 M H2SO4 and (O) 0.1 M HClO4. (From ref 103.)

reported that the catalytic activity of Pt basal planes for CO2 reduction is in an order of Pt(110) > Pt(100) . Pt(111).110-113 Yeager and co-workers demonstrated by using in situ FTIR spectroscopy that the CO2 reduction could produce linear- and bridge-bonded CO on Pt(100) and only linear-bonded CO on Pt(110).110 Hoshi et al.27,114-116 have systematically studied the kinetics of CO2 reduction on a series of Pt high-index planes

19806 J. Phys. Chem. C, Vol. 112, No. 50, 2008 and demonstrated that the catalytic activity is in the order of Pt(111) < Pt(100) < Pt(S)-[n(111) × (100)] < Pt(S)-[n(100) × (111)] < Pt(S)-[n(111) × (111)] < Pt(110) < Pt(S)-[n(110) × (100) ] < Pt(S)-[n(100) × (110)] < Pt(210). The high activity of the surfaces lying in the [001] zone was attributed to the presence of kink atoms in (110) steps.27 They further changed the kink density by using Pt single-crystal planes inside the stereographic triangle, e.g., Pt(532), Pt(431), and Pt(531), and found that the initial rate of CO2 reduction is almost increased linearly with the increase of density of kink atoms.116 Sun and co-workers have studied the surface processes of CO2 reduction on Pt and Rh single-crystal electrodes with different surface microscopic structures.117-120 They found that, for the Pt highindex planes with well-defined structure, the electrocatalytic activity toward CO2 reduction was in the order of Pt(510) < Pt(310) < Pt(210), i.e., with the increase of the density of (110) steps.119 When the surfaces were reconstructed by oxygen adsorption/desorption, the catalytic activity of all three electrodes had been further increased. Although the order of catalytic activity remains unchanged, the enhancement is more significant for the surface with higher density of (110) steps, e.g., the Pt(210). It was suggested therefore that the more open the surface structure is, the more active the Pt single-crystal electrode will be. 2.2.2. Effects of Surface Structure on Catalytic SelectiWity. The studies of hydrocarbon reactions (e.g., hydrogenolysis and isomerization) on Pt single-crystal planes have revealed that the product selectivity highly depends on surface atomic arrangement and the step atoms, especially kink atoms, which can promote the breaking of the C-C bond.18 This knowledge is very helpful in designing catalysts toward electrooxidation of SOMs containing C-C bonds. For example, ethanol is a promising alternative fuel to methanol due to its lower toxicity and easy availability from biomass.121-124 However, the electrooxidation of ethanol is quite incomplete, as it involves the cleavage of the C-C bond. The main products are acetaldehyde and acetic acid, and the yield of CO2 is even below 1% on the most effective electrocatalysts of Pt-Sn alloy.125 Obviously, the utilization efficiency of fuel is very low in the direct ethanol fuel cell. Tarnowski et al.126 have investigated the effects of step atoms on the selectivity of ethanol electrooxidation by using Pt(533), Pt(755), and Pt(111) electrodes. The results indicated that by introducing (100) step on (111) terrace the yield of CO2 has been boosted, while the yield of acetic acid on the Pt(533) was only about 1/3∼1/4 of that on the Pt(111). As for electrooxidation of isopropanol, Sun and co-workers127 found that Pt(610) was the most active surface for yielding CO2. The order of activity was Pt(610) > Pt(111) > Pt(100) > Pt(211) > Pt(110) for producing CO2 and Pt(100) > Pt(610) > Pt(211) > Pt(111) > Pt(110) for generating acetone. These results demonstrated that high-index surfaces with a high density of step and kink atoms will significantly promote the complete electrooxidation of SOMs containing C-C bonds. 2.2.3. Effects of Surface Structure on Stability. High-index planes usually exhibit high stability for electrooxidation reactions, which may be attributed to the existence of short-ranged steric sites as illustrated in Table 1 and to the reconstructionresistant property. Sun and co-workers have studied Pt highindex planes toward electrooxidation of ethylene glycol (EG).20,128,129 Figure 4a and 4b depict the first and tenth CVs of EG oxidation on Pt(111), Pt(332) ()5(111) × (110)), Pt(331) ()2(111) × (110)), and Pt(110) electrodes in 0.1 M EG + 0.5 M H2SO4 solutions, respectively.20 The catalytic activity was characterized by the peak current measured in negative-going

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Figure 4. Cyclic voltammograms of ethylene glycol oxidation on Pt(111), Pt(332), Pt(331), and Pt(110) electrodes. (a) The first cycle. (b) The tenth cycle, showing that Pt(331) has high activity and stability. (From ref 20.)

potential sweep, where the poisoning effect of COad was eliminated. In the first cycle, peak currents of 0.26, 1.27, 2.58, and 2.44 mA cm-2 in the negative-going potential sweep were measured on Pt(111), Pt(332), Pt(331), and Pt(110), respectively (Figure 4a). Obviously, the initial activities are in a decreasing order of Pt(331) > Pt(110) > Pt(332) > Pt(111). Along with the increase of potential cycles, the activity of all these electrodes decreased. In the tenth cycle, the corresponding peak currents were decreased to 0.22, 0.99, 2.33, and 0.95 mA cm-2; i.e., the activities were decreased by about 15%, 22%, 9%, and 61%, respectively (Figure 4b). This fact indicates that the stability of these electrodes is in the order of Pt(331) > Pt(111) > Pt(332) > Pt(110). The Pt(110) electrode exhibits very high initial activity due to its favorite stereographic matching in EG adsorption and oxidation, but its stability is the worst due to its surface reconstruction under testing conditions. In contrast, on the Pt(331) electrode, the (110) steps are in a short-ranged domain, which can resist the surface reconstruction. Therefore, the Pt(331) presents both the highest activity and stability. 2.2.4. Chiral Properties of High-Index Surfaces. High-index planes inside the stereographic triangle hold intrinsic chirality at kink sites and have received considerable attention due to their potential for heterogeneous asymmetric synthesis and separation.130-132 Attard et al.133,134 have studied the enantioselective electrooxidation of D- and L-glucose on the chiral single electrodes of Pt{643}R, Pt{531}R, Pt{431}R, and Pt{321}R as well as their corresponding enantiomorph surfaces. Slight

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Figure 5. (a) Structures of atomic arrangement and (b) histogram of catalytic activity toward ammonia synthesis on various Fe single-crystal surfaces. (From ref 138.)

enantiomeric response was observed for the electrooxidation of D- and L-glucose on the Pt{643}S, and diastereomeric product excess was about 60% (50% would be zero enantiomeric effect). As the kink sites increase, the electrochemical enantioselectivity increases; e.g., the diastereomeric product excess is as high as ∼80% on the Pt{531}R.134 2.3. Surface Structure Effects in Heterogeneous Catalysis. Surface structure effects in heterogeneous catalysis are usually more significant than that in electrocatalysis since there is no competition adsorption of electrolyte species on step atoms in heterogeneous catalysis. Taking ammonia synthesis as an example, it is an extremely high structure sensitivity reaction since it involves the dissociation of a very strong NtN bond. Ru and Fe are the two best elementary metal catalysts for this reaction.135 Ru is of hexagonal closest packing (hcp) lattice, and Ru(0001) is the closest-packed surface; while Fe is of bodycentered cubic (bcc) lattice, and Fe(110) is the closest-packed surface. It has been determined that the activity of step sites (defects) on the Ru(0001) surface is at least 9 orders of magnitude higher than that of (111) terrace sites at 500 K.136 Somorjai and co-workers137,138 have investigated the ammonia synthesis on Fe single-crystal surfaces and revealed that the reactive activity is in the order of Fe(111) > Fe(211) > Fe(100) > Fe(210) > Fe(110); i.e., the more open the surface structure is, the higher the activity will be (Figure 5). The catalytic activity of Fe(111) and Fe(211) is a few orders of magnitude higher than that of the other surfaces, which may be attributed to the facile adsorption of N2 on the top-layer atoms (CN ) 4) and the second- and third-layer atoms (CN ) 7) in sequence to weaken the NtN bond gradually on such open surfaces by DFT calculations.139,140 As discussed above, the open-structure surfaces generally exhibit high catalytic activity and stability, especially for electrooxidation of SOMs, which may be attributed to the existence of steric sites.20,28,117 Table 1 lists different possible steric sites on Pt stepped surfaces, which consist of the first, second, and even third outmost layer atoms near the atomic steps. There may exist “chair” sites and “folded-pentagon” sites on the Pt(S)-[n(100) × (110)] and Pt(S)-[n(110) × (100)] stepped surfaces, respectively. The combination of step and terrace atoms leads to the increase in catalytic activity and stability. The catalytic properties of a surface depend on the density of the steric sites.28 The densities of steric sites are normally equal to those of step atoms on the stepped surfaces except on Pt(S)-[n(110) × (100)] and Pt(S)-[n(110) × (111)] with (110) sites as terraces. The former amounts to 4/(a28n(n

Figure 6. Unit stereographic triangle of polyhedral nanocrystals bounded by different crystal planes. (From ref 53.)

- 1) + 3), and the latter equals 2/(a22n(n - 1) + 1), where a is the lattice constant of the Pt unit cell. 3. Nanoparticle Catalysts Practical catalysts are usually composed of metal nanoparticles supported on different substrate materials. As stated above, investigations of model catalysts using metal single-crystal planes revealed that high-index planes with open structure display convincing advantages in both electrocatalysis and heterogeneous catalysis. Therefore, a bridge linking the fundamental studies of model catalysis and practical applications consists of a challenge of design and preparation of nanoparticle catalysts with desirable open-structure surfaces or high-index facets. 3.1. Nanocrystal Shape and Surface Structure. By analogy with the unit stereographic triangle for fcc single-crystal planes, a triangle of polyhedral nanocrystals (NCs) bounded by corresponding surfaces is illustrated in Figure 6.141,142 The polyhedral NCs situated at the three vertexes are bounded by facets of single-crystal basal planes, i.e., cube by {100}, octahedron by {111}, and rhombic dodecahedron by {110}. The polyhedral NCs lying in the sidelines of the triangle are bounded by highindex facets: tetrahexahedra (THH) covered by {hk0}, trapezohedra by {hkk}, and trisoctahedra by {hhl}. These three kinds of polyhedra have 24 facets. The polyhedra inside the triangle are hexoctahedra bounded by {hkl} (h > k > l > 0) and have


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TABLE 2: Projections and Geometrical Parameters of Polyhedral Nanocrystals Bounded by Different High-Index Facets

48 facets. The polyhedra bounded by high-index facets belong to Catalan solids or Archimedean duals.141 Clearly, their shapes are unconventional and quite complex. For identifying them quickly, the THH NC bounded by 24 {hk0} facets can be considered as a cube with each face capped by a square pyramid; the trisoctahedron bounded by 24 {hhl} facets can be thought of as an octahedron with each face capped by a pyramid, etc. To identify the Miller indices of tetrahexahedral, trapezohedral, and trisoctahedral NCs, one approach is to measure the angles of projections along an appropriate crystallographic axis, as illustrated in Table 2. For both tetrahexahedral and trapezohedral NCs, the optimal projective direction is along [001], resulting in an octagon with 4-fold symmetry. Two angles R and β, along and direction, respectively, can be used to determine the Miller indices. The relationship between the R and Miller indices ({hk0} for tetrahexahedral NCs and {hkk} for trapezohedral NCs) can be expressed as follows.

R ) 2 arctan(h ⁄ k)

(1)

As for the trisoctahedral NCs, the optimal projective direction is along [110], resulting in an octagon with 2-fold symmetry. Three different angles, R, β, and γ, are dependent on the Miller indices. For example, the relationship between γ along the [1j10] direction and the Miller indices is below

( )

γ ) 180 ° -2 arctan

l

√2h

(2)

Similar correspondence of Miller indices with projective angles of hexoctahedral NCs can also be formulated, but it is more complex. According to the correlations between Miller indices and geometric parameters, polyhedral NCs can be bounded by all high-index facets depicted in the unit stereographic triangle (Figure 1), just by varying their geometric parameters while maintaining their shapes as tetrahexahedron, trapezohedron, trisoctahedron, or hexoctahedron. This important characteristic provides a convenient instruction of synthesizing metal nanoparticle catalysts with particular surface structure and of desirable catalytic properties for specified reactions. Taking the THH NCs as an example, the relationship between the Miller indices {hk0} and the geometric parameters, i.e., the height of the square pyramid (m) and the side length of the cube (n), can be expressed as follows

m k ) n 2h

(3)

If we could control the geometric parameters of the THH NCs,

i.e., m/n, their surface structure can be varied from {110} (m/n ) 1/2, i.e., the rhombic dodecahedron) to {210} (m/n ) 1/4), {310} (m/n ) 1/6), {520} (m/n ) 1/5), {730} (m/n ) 3/14), etc., and finally to {100} (m/n ) 0, i.e., the cube). It is worthwhile mentioning that the THH-shaped metal crystals in nature exist occasionally in Cu, Ag, and Au minerals,143,144 but they are generally unfamiliar to the nanomaterials community since it is quite difficult to synthesize the THH-shaped metal nanocrystals due to their high surface energy. 3.2. THH Pt Nanocrystal Catalysts Synthesized by the Electrochemical Square-Wave Potential Method. 3.2.1 Synthesis of THH Pt Nanocrystal Catalysts. As mentioned above, Pt NCs enclosed by high-index facets could not be synthesized through conventional chemical methods since the growth rate in the direction perpendicular to high-index facets is very fast, leading to the disappearance of high-index facets on the nanocrystals. We have therefore developed an electrochemical method for synthesis of THH Pt NCs with {hk0} high-index facets.51 As illustrated schematically in Figure 7A, Pt nanospheres of ∼750 nm in diameter were deposited electrochemically on the glassy carbon (GC) substrate and then subjected to a treatment of square-wave potential at 10 Hz, with upper potential of 1.20 V (SCE) and lower potential between -0.10 and -0.20 V, in a solution of 0.1 M H2SO4 + 30 mM ascorbic acid for 5-60 min. THH Pt NCs were grown exclusively on the GC surface at the expense of Pt nanospheres. Previously, Arvia et al. have employed electrochemical squarewave potential routes to facet bulk Pt electrode,145,146 which could yield several high-index planes of bulk crystals in some cases.147 Here, the use of Pt nanospheres deposited on the GC substrate instead of bulk Pt is vital to the growth of THH Pt NCs because GC is an inert substrate, on which isolated Pt NCs grow in island (Volmer-Weber) mode but not in columnar or dendritic mode on the bulk Pt.148,149 SEM images of the as-prepared THH Pt NCs are shown in Figure 7 B, C, and D.51 Three perfect square pyramids in Figure 7C and a nearly octagonal project in Figure 7D can be seen clearly, which confirm the THH shape of the Pt NCs. The yield of the THH Pt NCs in the final products is over 90%. The averaged sizes of the THH Pt NCs can be varied from 20 to 220 nm by controlling the growth time, and the size distribution is relatively narrow with relative standard deviation (RSD) ranging from 10% to 15%. The surface structure (Miller indices) of the THH Pt NCs has been identified as mainly {730} facets by high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED), as demonstrated in Figure 8. Besides, some THH Pt NCs bounded by {210}, {310}, or {520} were also observed. The atomic arrangement of the Pt(730) surface (Figure 8D) is periodically composed of two (210) microfacets followed by one (310) microfacet, i.e., a multipleheight stepped structure.22 The density of step atoms is as high as 5.1 × 1014 cm-2, i.e., 43% of the total surface atoms. These (210) and (310) microfacets have been directly observed in the HRTEM image; however, most high-index facets of THH Pt NCs trend to surface relaxation, as demonstrated in Figure 9.52 Two major types of relaxation can be identified. One is along the [1j1j0] direction as indicated by dark arrowheads, with an inward displacement as large as 10%a, where a is the lattice constant of the Pt unit cell (a ) 0.3924 nm); another is dominated in the [010] direction as indicated by green arrowheads, with an outward displacement normally below 7%a. The atoms taking [1j1j0] inward displacement are located at the step sites and show comparably light contrast, which indicates

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Figure 7. (A) Scheme of electrochemical preparation of THH Pt NCs from Pt nanospheres. (B, C, D, and F) SEM images of THH Pt NCs with a growth time of 60 min. (E) A model of a THH. (From ref 51.)

Figure 8. Surface structure of THH Pt NC. (A) TEM, (B) SAED, and (C) HRTEM showing the THH Pt NC is bounded by {730} facets. (E) Atomic model of Pt(730) surfaces. (From ref 51.)

their lower site occupancy. It is more precise to call these locations kinks instead of steps. High percentages of the atoms taking [010] outward displacement are located at the terraces and next to the step atoms. They have comparably higher atomic coordination than those of step, kink, and ledge atoms. What are the crucial factors for the formation of high-index facets? The ascorbic acid is excluded since THH Pt NCs can still be harvested in ascorbic acid-free solution, although the quality of THH shape degrades slightly. It was proposed that repetitive adsorption/desorption of oxygen generated by squarewave potential played a key role.51,63,150 During the electrochemical treatment, oxides or hydroxides (Oad, OHad), originated from the dissociation of H2O in solution, can readily form on

the surface of Pt NCs at upper potential (1.20 V vs SCE). The {111} and {100} facets are smooth with high coordinated surface atoms (their CNs are 9 and 8, respectively), so oxygen atoms preferentially diffuse/invade into the lattice and replace Pt atoms.151 After desorption of oxygen atoms from the lattice at lower potential (e.g., -0.20 V), those displaced Pt atoms cannot always return to their original positions, so the ordered surface structure will be destroyed (Figure 10). In contrast, as high-index facets contain many step atoms with low CN, the oxygen atoms preferentially adsorb at such sites without replacing them, so the ordered surfaces are preserved. Additionally, Han et al.152 have proved recently that chemisorption energy of Oad and OHad on Pt nanoparticles increases signifi-


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Figure 9. Surface relaxation of THH Pt NC. (a) HRTEM image of one surface facet of THH Pt NC; (b) a duplication of (a) with the center of each atom column marked by “+”; (c) the step configurations of (110), (210), (730), (310), and the real surfaces received from (b). (From ref 52.)

Figure 10. Schematic illustration of different effects of oxygen adsorption/desorption on the surfaces of Pt(111) and Pt(730) subjected to the treatment of square-wave potential. (From ref 51.)

cantly with decreasing CN through first-principles DFT calculations and found that the adsorption preferentially occurs on the edge- and vertex-atoms with low CN. Therefore, the oxygen adsorption/desorption should play the key role in the formation of high-index facets. This mechanism has been further confirmed by study of different effects of oxygen adsorption/desorption on different surface structures of Pt(111) and Pt(210). Figure 11a illustrates a cyclic voltammogram of the Pt(111) electrode in 0.5 M H2SO4, the spike at 0.18 V indicating the well-defined

(111) structure. After the oxygen adsorption/desorption by scanning potential to 1.20 V (SCE), the spike at 0.18 V disappears and a new current peak at -0.17 V appears, corresponding to hydrogen adsorption/desorption on (110) step sites. This fact indicates that the ordered (111) structure has been disturbed. In contrast, the hydrogen region characters on the Pt(210) electrode only changed slightly after oxygen adsorption/desorption cycles (Figure 11b). The results clearly demonstrate that the high-index planes (e.g., Pt(210)) are much more stable than the basal planes (e.g., Pt(111)) during oxygen adsorption/desorption. Recently, Seriani et al. have predicted that the oxygen adsorption can reduce surface energy and increase the fraction of open facets on the surface of Pt nanoparticles by using first-principles calculation.153 It may be accordingly concluded that the growth of THH Pt NCs during a treatment of square-wave potential results from three major effects: (1) the dynamic oxygen adsorption/desorption mediated by the square-wave potential; (2) the significant impediment effect in place-exchange between oxygen and Pt surface atoms on high-index surfaces; and (3) the decrease in surface energy

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Figure 12. Comparison of electrocatalytic activity toward (A, B) formic acid and (C, D) ethanol oxidation on THH Pt NCs (81 nm), Pt nanospheres (115 nm), and commercial Pt/C (3.2 nm) catalysts. The inset in (B) is a SEM image of a THH Pt NC after reaction, indicating the preservation of shape. (From ref 51.)

Figure 11. Effects of oxygen adsorption/desorption on the cyclic voltammograms of (a) Pt(111) and (b) Pt(210) electrodes in 0.5 M H2SO4.

of the Pt NCs by electrochemical adsorption of hydrogen at low potential and oxygen species at high potential. 3.2.2. Electrocatalytic Properties of the THH Pt NCs. The THH Pt NCs (81 nm) possess a catalytic activity superior to the spherical Pt nanoparticles with similar size (115 nm) and the commercial 3.2 nm Pt/C catalyst toward electrooxidation of SOMs.51 The current density (j), obtained from oxidation current normalized to electroactive Pt surface area, was used to characterize the catalytic activity. The steady-state current density of formic acid oxidation on the THH Pt NCs is 1.6∼4.0 times higher than that on the Pt nanospheres and 2.0∼3.1 times larger than that on the commercial Pt/C catalyst, depending on applied electrode potentials (Figure 12B). For ethanol electrooxidation, the enhancement factor of the catalytic activity obtained on the THH NCs varies from 2.0 to 4.3 referring to that of Pt nanospheres and from 2.5 to 4.6 to that of the commercial catalyst (Figure 12D). In addition, at a given oxidation current density of technical interest, the corresponding potential on the THH Pt NCs is much lower than that on the Pt nanospheres or the commercial catalyst. In the case of formic acid oxidation, the potential on THH Pt NCs is shifted negatively by ca. 60 mV as compared to that on Pt nanospheres at the same current density of 0.5 mA cm-2; while for ethanol oxidation, the negative shift is ca. 80 mV at a current density of 0.2 mA cm-2. The negative shift was even larger in comparison with the commercial catalyst. These results demonstrate that the THH Pt NCs exhibit much enhanced catalytic activity per unit surface area for the oxidation of SOMs. It

should be pointed out that if the catalytic activity is calculated by weight of Pt, the THH Pt NCs are considerably inferior to the commercial Pt/C catalyst (∼10% as active) since their sizes are different (81 nm vs 3.2 nm). Therefore, it is quite desirable to synthesize THH Pt NCs with a size comparable to commercial catalysts to improve the utility efficiency of Pt. No appreciable morphological change could be observed on the THH Pt NCs after electrooxidation of formic acid (the inset to Figure 12B), indicating that the THH Pt NCs display a good chemical stability. Moreover, the THH Pt NCs grow under a condition of intensive reduction and oxidation (e.g., square wave potential of -0.2∼1.2 V vs SCE), where polycrystalline Pt nanoparticles are unstable and tend to dissolve as indicated by the dissolution of Pt nanospheres of ∼750 nm, which suggests that the THH Pt NCs possess a high stability at oxidation conditions and may be very promising catalysts for O2 reduction. The THH Pt NCs present also a high thermal stability. In situ TEM experiment demonstrated that the THH Pt NCs were stable to temperatures above 800 °C with the preservation of their shape and facets.51 Very recently, Ma et al.154 investigated the thermal stability of THH Pt nanoparticles (∼7.8 nm) with high-index facets such as (730), (210), (310), and (520) by using molecular dynamic simulations. They indicated that in the lowtemperature range (0∼860 K) both surface atoms and interior atoms oscillate slightly around their equilibrium positions, and atomic steps on the high-index surface are maintained regularly. If the temperature is raised up to 860 K, surface diffusion will become dramatic, and the atomic steps may start to disappear, though the general shape of the nanoparticles is still retained to a high temperature of 1100 K or so. 154 3.3. Pt NCs of Other Shapes with High-Index Facets. Our recent progress confirmed that the electrochemical square-wave potential route can be further extended to synthesize Pt NCs of different shapes other than the THH with high-index facets. (1) Flowerlike Pt NCs. When the distribution density of original Pt nanospheres on GC substrate was decreased by 1 order of magnitude (about 1 × 106 cm-2) and the treatment time of square-wave potential was prolonged to 1 h, flowerlike Pt NCs can be obtained, as shown in Figure 13a. In a magnified SEM image (inset to Figure 13a), this shape can be seen as a cube with THH-like shapes located at vertices and small square pyramids sited at the face centers. The gradual evolution of the


Figure 13. (a) SEM image of flowerlike Pt NCs. (b) SEM image showing the evolution of the Pt NCs from THH to flowerlike shape.

THH Pt NCs into the flowerlike Pt NCs may be illustrated in Figure 13b, where the Pt NCs with different growth stages were captured. Clearly, the shape evolution was accompanied by increasing the particle size. The flowerlike Pt NCs consisting of square pyramids are likely to be bounded by {hk0} facets since their growth conditions are very similar to those of the THH Pt NCs. The formation of the flowerlike Pt NCs may be mainly related to the diffusion-controlled growth. Since the original Pt nanospheres on the GC substrate were sparse as in the present case, the solution concentration of Pt ions dissolved from the Pt nanospheres is very dilute; the growth of the Pt NCs is controlled by the diffusion. According to spherical diffusion law, the vertices of THH Pt NCs can gain more Pt ions from the solution, and the growth rate along the vertices is much faster than that of the other parts of the Pt NCs. Besides, under the conditions of square-wave potential, only high-index facets can survive. These two factors lead to the formation of the flowerlike Pt NCs with {hk0} facets. (2) ConcaWe Hexoctahedral Pt NCs. In the electrochemical square-wave potential route, when the ascorbic acid is replaced by sodium citrate of 30 mM concentration, THH Pt NCs can be still obtained (Figure 14a). However, when the concentration of sodium citrate was increased to 50 mM, the shape of Pt NCs was changed to a complex concave polyhedron (Figure 14b),53 on which six facets intersect at a point (A-point) on a 3-fold axis and eight facets intersect at another point (B-point) on a 4-fold axis. These symmetrical characters are identical to those of the convex hexoctahedron shown in Figure 6, which indicates that the Pt NCs are of concave hexoctahedral shape and are bounded by 48 {hkl} high-index facets. The inset to Figure 14b illustrates a model of concave hexoctahedron bounded by 48 facets of {321}. From a geometric point of view, tetrahexahe-

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Figure 14. SEM images of (a) THH Pt NCs grown in 30 mM sodium citrate + 0.1 H2SO4 and (b) concave hexoctahedral Pt NCs with {hkl} facets grown in 50 mM sodium citrate + 0.1 H2SO4. The inset is a model of concave hexoctahedron bounded by {321} facets. A-point and B-point are the intersections of six and eight facets, respectively, showing the characteristic symmetry of hexoctahedron. (From ref 53.)

dron can change into concave hexoctahedron by shrinking along the direction. As a result, every {h′k′0} facet is split into two {hkl} facets. It is worthwhile to note that the edges along two A-points are very faint. If the edges disappear completely, the facets become {hhl}. So in the Miller indices of the concave hexoctahedral Pt NCs, the values of h and k must be very close. (3) Multiple Twinned Pt Nanorods. Besides single crystalline Pt NCs, 5-fold twinned Pt nanorods with {hk0} high-index facets have also been synthesized by the electrochemical square-wave potential method (Figure 15).53 The preparation processes are very similar to those for the THH Pt NCs except that the GC electrode loading with Pt nanospheres was exposed in air for 3∼5 h prior to the treatment of square-wave potential. Such exposure could cause the GC surface to be hydrophobic or inert, on which a new Pt nucleus could hardly generate; instead, the nucleus preferentially forms on the surface of Pt nanospheres and grows into nanorods during the treatment of square-wave potential. The length of the Pt nanorods is about 1 µm, and their diameter is not uniform along the growth direction, broadest at the middle and gradually tapering to both ends. In a high-magnification SEM image (Figure 15b), zigzag-arranged facets (pyramidal islands) can be observed on the surface of the middle part, and five facets can be discerned at both ends, indicating a decagonal pyramidal shape of the ends. In addition, the two ends are asymmetrical with one more acuate than the other. The crystalline structure of the Pt nanorods was identified as 5-fold twinned crystals (five subcrystals twinned at {111} facets, growing along direction) by SAED and HRTEM.53 The shape of a typical 5-fold twinned nanorod is a pentagonal prism

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Figure 15. SEM images of Pt nanorods with high-index facets synthesized by the electrochemical square-wave potential method. (a) Low magnification and (b) high magnification, showing fine surface facets. (From ref 53.)

with (100) facets, capped by two pentagonal pyramids with (111) facets. Obviously, the present Pt nanorods exhibit significant differences: (1) the end is not a pentagonal pyramid but an elongated decagonal pyramid; (2) the side faces are not smooth {100} facets but zigzag-arranged fine facets (Figure 15b). Since the growth conditions of the Pt nanorods are similar to those of the THH Pt NCs, these facets are very likely to be {hk0} highindex facets. By measuring the geometric parameters (e.g., cone angle) of the Pt nanorods, we have determined the Miller indices of these facets as illustrated in Figure 16, i.e., {410} facets for the sharp end, {320}, {210}, or {730} facets for the obtuse end, and mainly zigzag-arranged {520} facets for the middle part. The densities of step atoms are considerablly different on these facets, among which the {320} facet holds the highest density, while the {410} facet has the lowest one. The gradual decrease in the density of step atoms from the bottom to the top of the Pt nanorod may be correlated with the interplay between growth and surface reconstruction induced by oxygen adsorption/desorption. The sharp end has the highest growth rate since it grows outward and can receive more Pt ions from the solution. In contrast, the obtuse end rooted on the Pt nanosphere has the lowest growth rate due to the competition of Pt ions between the nanorod and the Pt nanosphere, i.e., interparticle diffusion coupling.155 As dicussed above, the formation of {hk0} facets was induced by the oxygen adsorption/ desorption. As a consequence, the surface with slower growth rate underwent a more intensive surface reconstruction by oxygen adsorption/desorption, resulting in {hk0} facets with higher density of step atoms, such as {320} and {210} facets. This result indicates that the Miller indices of {hk0} facets of Pt NCs can be further tuned by controlling the growth rate in the electrochemical squarewave potential route. It is interesting to note that Liu et al. have synthezied 5-fold twinned Au bipyramids with {711} high-index facets by the assistance of Ag+ underpotential deposition (UPD) on the Au surface during chemical reduction of HAuCl4 by ascorbic acid.156 3.4. Pd NCs with High-Index Facets. Pd nanoparticles are also very important catalysts in different applications, such as electrooxidation of formic acid157,158 and the Suzuki and Heck coupling. 159,160 Therefore, shape-controlled synthesis of Pd nanocrystals has also attracted great interest. In recent years, Xia et al.

J. Phys. Chem. C, Vol. 112, No. 50, 2008 19813 have synthesized a series of Pd NCs with different shapes, such as cube, octahedron, decahedron, and icosahedron.37,38,161-166 However, these Pd NCs are all bounded by low-index facets. By using the electrochemical square-wave potential method, we have synthesized Pd NCs bounded by high-index facets.53 Figure 17a illustrates a SEM image of a trapezohedral Pd NC, on which four facets surrounded by eight facets can be seen clearly. This feature is well consistent with the model of trapezohedron bounded by 24 {hkk} facets (Figure 17b). By varying the Miller indices (i.e., h, k) of the trapezohedral models and comparing them with the trapezohedral Pd NC, we found that the trapezohedral Pd NC was most likely to be bounded by {311} facets. Figures 17c and 17d depict the SEM images of concave hexoctahedral Pd NCs, on which the edges along with two adjacent vertices can be seen more clearly in comparison with that of the concave hexoctahedral Pt NCs illustrated in Figure 14b. Figure 17e is a model of concave hexoctahedron with {321} facets, whose shape is fairly similar to that of the Pd NCs. Besides, we have also obtained 5-fold twinned Pd nanorods with {hk0} or {hkk} type high-index facets. Although the exact surface structure, i.e., Miller indices of the above Pd nanocrystals, have not been identified due to their complex shapes, these preliminary results demonstrate clearly that the electrochemical square-wave potential route is a powerful tool to synthesize metal nanoparticles bounded by high-index facets. 4. Summary and Future Trends This feature article has put emphasis upon the surface structurecatalytic functionality of metal catalysts. The fundamental studies using metal single-crystal planes as model catalysts revealed that the open-structure planes, which have a high density of atomic steps and kinks, generally exhibit superior catalytic activity and stability to that of the flat planes with closely packed surface atoms for important reactions, such as electrooxidation of small organic molecules, electroreduction of CO2 and O2, ammonia synthesis, etc. However, metal single-crystal planes could not be used as practical catalysts due to their high cost and ease of reconstruction under reaction conditions. The catalysts used in most important fields are in fact those of nanoparticles loaded on diverse supports. To control the surface structure of nanoparticle catalysts, especially to synthesize metal nanocrystals with open-structure surface (highindex facets in the case of Pt nanocrystals), is therefore the key issue to further improve the performance of practical catalysts. In the past decade, chemically shape-controlled synthesis methods were limited to synthesis of platinum group metal nanoparticles bounded by low-index facets. The synthesis of nanoparticles bounded by high-index facets is rather challenging due to their high surface energy. The electrochemically shape-controlled synthesis method, reviewed in the current paper, has successfully overcome the obstacle. The dynamic oxygen adsorption/desorption on Pt induced by the square-wave potential and the significant impediment effect in place-exchange between oxygen and surface atoms on open-structure planes play key roles in the formation of high-index facets. By this method, Pt and Pd nanocrystals bounded by different high-index facets, including tetrahexahedron by 24 {hk0} facets, trapezohedron by 24 {hkk} facets, and concave hexoctahedron by 48 {hkl} facets, have been synthesized successfully. Moreover, multiple twinned nanorods of Pt and Pd with highindex facets have also been obtained. These results demonstrated clearly that the electrochemically shape-controlled synthesis is a versatile method to synthesize metal nanocrystals bounded by highindex facets and opens a new prospect avenue in shape-controlled synthesis of metal nanoparticle catalysts of high performances.167,168 As expected, the Pt nanocrystals bounded by high-index facets exhibit high catalytic activity and stability. The tetrahexahedral Pt


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Figure 16. Miller indices and stepped atom densities of {hk0} facets at different parts of the Pt nanorod. (From ref 53.)

Figure 17. SEM images of trapezohedral Pd NCs with {hkk} facets (a) and concave hexoctahedral Pd NCs with {hkl} facets (c and d) synthesized by the electrochemical square-wave potential method and supported on ITO substrate. The models of trapezohedron with {311} facets (b) and concave hexoctahedron with {321} facets (e) are also shown. (From ref 53.)

nanocrystals showed a catalytic activity up to 400% higher than that of Pt nanospheres of similar sizes or commercial Pt/C catalyst for electrooxidation of small organic fuels, such as formic acid and ethanol. The tetrahexahedral nanocrystals are both thermally stable (up to 800 °C) and chemically stable during electrocatalytic reactions. Further challenges concerning the shape-controlled synthesis of platinum group metal nanocrystal catalysts with high-index facets may consist of both fundamental aspects and applications. In the fundamental aspects, the further development of the electrochemically shape-controlled synthesis method is certainly put in the first place. The electrochemical method has shown the ability to produce platinum group metal nanocrystals with diverse high-index facets. It has long been the catalysis community′s dream to obtain nanocrystals with controllable and uniform surface structures, to obtain the best catalytic properties or to study systemically the structure-activity relationship of nanocrystal catalysts. To achieve this target, the electrochemical method should combine with chemical or even physical techniques to improve the controllability

and uniformity of surface structures of nanocrystals. The characterization of the Miller indices of the complex nanocrystals also presents a key issue. Besides HRTEM analysis, the development of convenient electrochemical or spectroscopic methods, e.g., surface orientation specific reactions (such as UPD or irreversible adsorption of some metal ions on nanoparticles), is required. The exploration of different properties, including catalytic, chemical, and physical properties of the nanocrystals bounded by high-index facets will inspire new discoveries. As for the applications, two important steps should be addressed, consisting of decreasing the size of nanocrystals with high-index facets down to a size comparable to commercial nanocatalysts and of developing techniques for mass production, which certainly need theoretical advances and technical innovations. Prospective and exciting research directions of electrochemically shape-controlled synthesis can be predicted as follows: (1) to explore appropriate conditions, such as parameters of square-wave potential, electrode substrates, and electrolyte, etc., for preparation of nanocrystals with diverse shapes and different high indices as coordinated in the triangle

Feature Article shown in Figure 6; (2) to extend the electrochemically shapecontrolled synthesis methods to prepare nanocrystals with highindex facets of metals (such as Au, Ag, and Fe) other than the platinum group metals, which need certainly the assistance of theoretical calculation to screen other atoms or molecules as an oxygen atom that can preferentially adsorb on high-index surfaces to reduce their surface energy; (3) to apply the nanocrystal catalysts with high-index facets in direct fuel cells, electrosynthesis, and relevant important reaction systems. It is expected that the studies will contribute considerably to the design and synthesis of metal nanocrystal catalysts at microscopic scale and surface structure of atomic arrangement and to the advancement of relevant industries. Acknowledgment. This work was supported by the Natural Science Foundation of China (grant Nos. 20673091, 20433060, 20503023, 20873113, and 20833005) and Ministry of Science and Technology of China (Grant Nos. 2009CB220102 and 2007DFA40890). References and Notes (1) Bell, A. T. Science 2003, 299, 1688–1691. (2) Zaera, F. Appl. Catal. A-Gen. 2002, 229, 75–91. (3) Larminie, J.; Dicks, A., Fuel Cell Systems Explained, 2nd ed. John Wiley & Sons Chichester: West Sussex, 2003. (4) Heck, R. M.; Farrauto, R. J. Appl. Catal. A-Gen. 2001, 221, 443– 457. (5) Service, R. F. Science 1999, 285, 682–685. (6) Berger, D. J. Science 1999, 286, 49–49. (7) Arenz, M.; Mayrhofer, K. J. J.; Stamenkovic, V.; Blizanac, B. B.; Tomoyuki, T.; Ross, P. N.; Markovic, N. M. J. Am. Chem. Soc. 2005, 127, 6819–6829. (8) Takasu, Y.; Itaya, H.; Iwazaki, T.; Miyoshi, R.; Ohnuma, T.; Sugimoto, W.; Murakami, Y. Chem. Commun. 2001, 341–342. (9) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025–1102. (10) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663–12676. (11) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343–1348. (12) Vidal-Iglesias, F. J.; Solla-Gullon, J.; Rodriguez, P.; Herrero, E.; Montiel, V.; Feliu, J. M.; Aldaz, A. Electrochem. Commun. 2004, 6, 1080– 1084. (13) Bratlie, K. M.; Lee, H.; Komvopoulos, K.; Yang, P. D.; Somorjai, G. A. Nano Lett. 2007, 7, 3097–3101. (14) Gurau, B.; Viswanathan, R.; Liu, R. X.; Lafrenz, T. J.; Ley, K. L.; Smotkin, E. S.; Reddington, E.; Sapienza, A.; Chan, B. C.; Mallouk, T. E.; Sarangapani, S. J. Phys. Chem. B 1998, 102, 9997–10003. (15) Gotz, M.; Wendt, H. Electrochim. Acta 1998, 43, 3637–3644. (16) Reddington, E.; Sapienza, A.; Gurau, B.; Viswanathan, R.; Sarangapani, S.; Smotkin, E. S.; Mallouk, T. E. Science 1998, 280, 1735– 1737. (17) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M. Nat. Mater. 2007, 6, 241–247. (18) Somorjai, G. A.; Blakely, D. W. Nature 1975, 258, 580–583. (19) Somorjai, G. A. Science 1985, 227, 902–908. (20) Sun, S. G.; Chen, A. C.; Huang, T. S.; Li, J. B.; Tian, Z. W. J. Electroanal. Chem. 1992, 340, 213–226. (21) Baltruschat, H.; Bussar, R.; Ernst, S.; Hernandez, F. From Stepped Single Crystal Surfaces to Ordered Bimetallic Electrodes: Adsorption and Electrocatalysis as Studied by DEMS and STM. In In-situ Spectroscopic Studies of Adsorption at the Electrode and Electrocatalysis; Sun, S. G., Christensen, P. A., Wieckowski, A., Eds.; Elsevier Science B.V: Amsterdam, 2007; pp 471-537. (22) Somorjai, G. A. Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithaca, 1981. (23) Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; van Santen, R. A. J. Phys. Chem. B 2002, 106, 12938–12947. (24) Bernasek, S. L.; Somorjai, G. A. Surf. Sci. 1975, 48, 204–213. (25) Hammer, B.; Norskov, J. K. Theoretical surface science and catalysis - Calculations and concepts. In AdVances in Catalysis; Academic Press Inc: San Diego, 2000; Vol. 45, pp 71-129. (26) Janssens, T. V. W.; Clausen, B. S.; Hvolbaek, B.; Falsig, H.; Christensen, C. H.; Bligaard, T.; Norskov, J. K. Top. Catal. 2007, 44, 15– 26. (27) Hoshi, N.; Kawatani, S.; Kudo, M.; Hori, Y. J. Electroanal. Chem. 1999, 467, 67–73. (28) Sun, S. G.; Clavilier, J. Chem. J. Chin. UniV. 1990, 11, 998– 1002.

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