Crystallographic control of noble metal nanocrystals

Nano Today (2011) 6, 265—285

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journal homepage: www.elsevier.com/locate/nanotoday

REVIEW

Crystallographic control of noble metal nanocrystals Wenxin Niu a,b, Guobao Xu a,b,∗ a

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, 5625 Renmin street, Changchun, Jilin 130022, China b Graduate University of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, China Received 28 December 2010; received in revised form 3 April 2011; accepted 21 April 2011 Available online 24 May 2011

KEYWORDS Crystal growth; Noble metal; Shape control; Nanocrystals; Crystallinity; Crystal facet

Summary This review provides an overview of recent developments in the controlled synthesis of well-defined noble metal nanocrystals from the viewpoint of crystallographic control. Discussions are focused on the relationship between the shapes of noble metal nanocrystals and their internal and external crystal structures. Representative strategies for the crystallographic control of noble metal nanocrystals are introduced and discussed from the aspects of internal and external crystallographic control. Typical examples of the enhanced properties of noble metal nanocrystals by crystallographic control are highlighted. © 2011 Elsevier Ltd. All rights reserved.

Introduction The study of noble metal nanocrystals has become a very exciting research topic in the past decade. Significant progress has been made in the application of noble metal nanocrystals in many highly interdisciplinary subjects, such as plasmonics, spectroscopy, biological labeling and imaging, medical diagnostics and therapy, catalysis, and solar energy conversion [1—6]. Most of these applications require a tight control of the size and crystal structure of the metal nanocrystals, because the properties of noble metal nanocrystals are strongly dependent on their sizes and crystal structure. Therefore, the rational controlled synthesis of metallic nanocrystals is of vital importance to understanding

∗ Corresponding author at: State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, 5625 Renmin Street, Changchun, Jilin 130022, China. Tel.: +86 431 85262747; fax: +86 431 85262747. E-mail address: [email protected] (G. Xu).

their growth mechanism and properties, which has a large impact on their potential industrial application. Most noble metals crystallize in a face-centered cubic (fcc) lattice, which is one of the most common and simplest systems found in crystals and minerals. It’s difficult to imagine that there are so many shapes of noble metal nanocrystals for such a simple system. Noble metal nanocrystals with shapes such as spheres, spheroids, various polyhedrons, rods, wires, and even stars can be chemically synthesized. Therefore, one can easily realize that the formation mechanisms behind these nanocrystals are very complicated. Although crystallization is a century-old process for material preparation, the existing theories cannot be systematically and fully applied to metal crystals in the nanoscale regime. Many existing shapes of metal nanocrystals are hard to predict. Theoretically, the equilibrium shape of an fcc metal nanocrystal is a truncated octahedron in an inert gas or vacuum [7]. In realistic chemical synthesis, especially in solution-phase, many critical parameters are responsible for the final shapes of noble metal nanocrystals. In gen-

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266 eral, these parameters could be divided into two categories: thermodynamic and kinetic parameters. Thermodynamic parameters include temperature and reduction potential, while kinetic parameters include reactant concentration, diffusion, solubility, and reaction rate. By controlling these parameters, it is possible to tune the nucleation and growth stages of metal nanocrystals and achieve crystallographic control. In this review, we first introduce the possible shapes of noble metal nanocrystals according to their crystal structures: single-crystalline, singly twinned, multiple planartwinned, cyclic penta-twinned, and multiple-twinned icosahedral structures. We focus on the relationship between the shapes of noble metal nanocrystals and their internal and external crystal structure. In the second part of the review, we introduce synthetic methods for the crystallographic control of noble metal nanocrystals, such as the seed-mediated growth method, polyol process, electrochemical method, and photochemical method. Each method is discussed from the aspects of internal and external crystallographic control. In the third part, our discussions are focused on how internal crystal structure (single-crystalline, twinned, and twin domain size) and external crystal structure (crystal facets) can alter the properties of noble metal nanocrystals. Finally, the perspectives of future research in the crystallographic control of noble metal nanocrystals are given.

Morphology and crystal structure of noble metal nanocrystals The shapes and properties of noble metal nanocrystals are closely related to their crystal structures, which are intrinsically determined by the overall arrangements of atoms in the nanocrystals. According to the internal and external arrangements of atoms in the nanocrystals, the crystallographic control of metal nanocrystals can be categorized into two aspects: the internal and external crystallographic control. Internal crystallographic control mainly refers to single-crystalline and twinned structures. Figure 1A shows a high-resolution transmission electron microscopy (HRTEM) image of a single-crystalline gold nanocrystal. For a single-crystalline metal nanocrystal, the crystal lattice of the entire nanocrystal is continuous and unbroken to the edges of the sample; there are no grain boundaries. A twinned nanocrystal includes two or more inter-grown crystals that share some of the same crystal lattice points in a symmetrical manner [7]. In a twinned nanocrystal, one of the inter-grown crystals is a repetition of the other through geometrical operations [8]. Typical twinned structures include singly twinned, multiple planar-twinned, cyclic penta-twinned, and multiple-twinned icosahedral structures. Their HRTEM images are presented in Figure 1B—F. External crystallographic control refers to the enclosing facets of noble metal nanocrystals [12]. In solution chemistry, fcc metal nanocrystals enclosed by low-index {1 0 0}, {1 1 1}, and {1 1 0} facets are more commonly observed because of their relatively low surface energy [13]. Highindex facets, which are denoted by a set of Miller indices (h k l) with at least one index being larger than the unit, have

W. Niu, G. Xu a much higher density of low-coordination number stepped atoms, ledges, and kinks and thus their surface energy is high [14]. Therefore, the rate of crystal growth in the direction perpendicular to a high-index facet is generally much faster than that along the normal direction of a low-index facet, which results in rapid elimination of the highindex facets with the addition of atoms during nanocrystal formation.

Single-crystalline noble metal nanocrystals The single-crystalline noble metal nanocrystal is one of the simplest nanostructures. Figure 2 shows the geometrical models of several typical single-crystalline metal nanocrystals enclosed by the low-index {1 0 0}, {1 1 1}, and {1 1 0} facets [15]. In each of these geometrical models, faces made of the same crystals facets are all equivalent. In Figure 2, the three vertexes of the triangle represent the shapes of single-crystalline metal nanocrystals enclosed completely by equivalent {1 0 0}, {1 1 1}, and {1 1 0} facets, which are cube, octahedron, and rhombic dodecahedron (RD), respectively. Examples of TEM images, corresponding SAED patterns, and HRTEM images for palladium nanocrystals with the three typical shapes are presented in Figure 3. It is important to note that tetrahedral nanocrystals are also a form of single-crystalline nanocrystals enclosed by four identical {1 1 1} facets [16,17]. Other shapes lying in the sidelines and located inside the triangle in Figure 2 are made of two and three kinds of facets, respectively. These shapes are actually derivatives of cubic, octahedral, and RD nanocrystals with varying degrees of edge- and cornertruncation. Although noble metal nanocrystals bound by high-index facets are challenging to synthesize because of their high surface energies, there have recently been several reports about their synthesis [18—24]. In principle, there are four typical single-crystal shapes with high-index surfaces: tetrahexahedron covered by {h k 0}, trapezohedron or deltoidal icositetrahedron by {h k k}, trisoctahedron by {h h l}, and hexoctahedron by {h k l} (h > k > l) [12,13]. The models of these shapes are shown in Figure 4. Geometrically, the shapes of noble metal nanocrystals can be categorized into convex polyhedron and concave polyhedron. For a convex polyhedron, there is a line connecting any two points on the surface of the nanocrystals, which always lies in the interior of the polyhedron [25]. For a concave polyhedron, there exist at least two vortexes inside the polyhedron. Most of the reported noble metal nanocrystals are convex polyhedral nanocrystals; concave polyhedral nanocrystals are also theoretically possible and some of them have already been synthesized. The trisoctahedron is a typical concave polyhedron. Trisoctahedral gold nanocrystals have been synthesized through seed-mediated growth methods [19,22]. Concave tetrahedral/trigonal bipyramidal palladium nanocrystals and concave cubic gold nanocrystals have also been reported [26,27]. The anisotropic growth of metal nanocrystals can greatly expand the family of metal nanocrystals. However, it is difficult to force single-crystalline noble metal colloidal nanocrystals to grow into anisotropic shapes because of the

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Figure 1 HRTEM images of spherical noble metal nanocrystals with different crystal structures: (A) a single-crystalline gold nanocrystal, (B) a singly twinned silver nanocrystal (reproduced with permission from Ref. [10], ©2006 American Chemical Society), (C) a multiple planar-twinned silver nanocrystal (reproduced with permission from Ref. [11], ©2009 WILEY—VCH Verlag GmbH & Co.), (D) a cyclic penta-twinned gold nanocrystal, (E and F) a multiple-twinned icosahedral gold nanocrystal. Other images are reproduced with permission from Ref. [9], ©2010 WILEY—VCH Verlag GmbH & Co.

high symmetry of the fcc noble metal system [28]. Therefore, there is no internal driving force for anisotropic growth for single-crystalline nanocrystals; most of the nanocrystal shapes are isotropic and their faces of the same kind of crystal facet are equivalent. Localized oxidative etching, careful manipulation of growth kinetics, and the selection of an adsorptive reagent can promote the formation of anisotropic metal nanocrystals [29—36]. Nanorods and nanobars are typical one-dimensional anisotropic nanocrystals. Gold nanorods have very complicated structures. Figure 5A shows one of the proposed geometrical models of gold

nanorods [33,36,37]. Gold nanorods could lie on a substrate in the [1 1 0] and [1 0 0] directions (Figure 5C and D). Figure 5E shows a HRTEM study of a cross-section of a gold nanorod. Based on this result, Liz-Marzán and coworkers proposed that the side facets of gold nanorods are dominated by high-index {2 5 0} facets [37]. Nanobars have relatively simple structures (Figure 5F). For nanobars, all six faces are made of {1 0 0} facets [29,38]. Figure 5G shows the SEM images of silver nanobars [39]. The TEM and HRTEM of palladium nanobars are shown in Figure 5H and I, respectively [38]. If the single-crystalline seeds

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Figure 2 Geometrical models of several typical singlecrystalline noble metal nanocrystals enclosed by the {1 0 0}, {1 1 1}, and {1 1 0} facets. The {1 0 0}, {1 1 1}, and {1 1 0} facets are shown in green, blue, and purple, respectively (reproduced with permission from Ref. [15], ©2010 American Chemical Society). Figure 4 Geometrical models of noble metal nanocrystals enclosed by high-index facets. The Miller indices {h k l} obey the order h > k > l (reproduced with permission from Ref. [13], ©2007 WILEY—VCH Verlag GmbH & Co.).

Figure 3 (A—C) TEM images of the RD, cubic, and octahedral palladium nanocrystals, respectively (scale bars: 200 nm). (D—F) TEM images (scale bars: 50 nm), (G—I) corresponding SAED patterns, and (J—L) HRTEM images (scale bars: 5 nm) of a single RD, cubic, and octahedral nanocrystal, respectively (reproduced with permission from Ref. [15], ©2010 American Chemical Society).

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Figure 5 One-dimensional single-crystalline noble metal nanocrystals: (A) geometrical model of gold nanorods (reproduced with permission from Ref. [37], ©2010 WILEY—VCH Verlag GmbH & Co.), (B) TEM images of gold nanorods (reproduced with permission from Ref. [36], ©1999 American Chemical Society), (C and D) HRTEM image of a gold nanorod oriented in the [1 1 0] direction and [1 0 0] direction, respectively (reproduced with permission from Ref. [33], ©2006 American Chemical Society), (E) HRTEM image of a standing gold nanorod, showing the cross-section of gold nanorods (reproduced with permission from Ref. [37], ©2010 WILEY—VCH Verlag GmbH & Co.), (F) geometrical model of nanobars (reproduced with permission from Ref. [38], ©2009 WILEY—VCH Verlag GmbH & Co.), (G) SEM image of a silver nanobar (reproduced with permission from Ref. [39], ©2007 American Chemical Society), (H) TEM image of palladium nanobars, (I) HRTEM image of a single palladium nanobar and the corresponding Fourier transform pattern (inset) (reproduced with permission from Ref. [38], ©2009 WILEY—VCH Verlag GmbH & Co.).

are directed to grow into two-dimensional nanostructures, then nanoprisms, nanoplates, or nanodisks can be obtained [40—42]. These planar nanocrystals usually have two {1 1 1} facets: one at the top and one at the bottom.

Twinned noble metal nanocrystals Twinned noble metal nanocrystals are also an important family of noble metal nanocrystals [43,44]. Common twinned structures of noble metal nanocrystals, such as singly twinned [10,45], multiple planar-twinned [11,46—48], cyclic penta-twinned [49—60], and multiple-twinned icosahedral structures [59—64], have well-defined shapes and have been chemically synthesized. Singly twinned noble metal nanocrystals are one of the simplest twinned structures. Right triangular bipyramids and nanoprisms (or nanoplates) can be grown from singly twinned seeds

(Figure 1B). Xia and coworkers reported the synthesis of silver right triangular bipyramids [10]. A right triangular bipyramid consists of two right tetrahedra symmetrically joined base-to-base, with six exposed {1 0 0} facets and a {1 1 1} twin plane that bisects its two tetrahedra halves. This structure has two transverse and three longitudinal vertices. Figure 6A and B shows the SEM and TEM images of silver right triangular bipyramids. Singly twinned noble metal nanocrystals primarily enclosed by {1 1 1} facets usually take a plate-like morphology [43], as presented in Figure 6C, and are usually called nanoplates or nanoprisms. Such structures have reentrant grooves on the sides of the crystal plates and can provide nucleation sites for adsorbing new crystal layers and driving plate-like crystal growth [65]. Singly twinned noble metal nanocrystals enclosed by {1 1 0} facets take the shape of squashed dodecahedra. The squashed dodecahedron is an irregular dodecahedron consisting of six rhombi and six trapezoids. Wang et al. reported the synthesis of

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Figure 6 Singly twinned and multiple planar-twinned noble metal nanocrystals: (A) SEM image of silver right triangular bipyramids, (B) HRTEM of a single silver right triangular bipyramid (reproduced with permission from Ref. [10], ©2006 American Chemical Society), (C) geometrical models of singly twinned nanoplates (reproduced with permission from Ref. [43], ©2006 The Royal Society of Chemistry), (D and E) SEM images of squashed dodecahedral gold nanocrystals (reproduced with permission from Ref. [66], ©2010 The Royal Society of Chemistry), (F) SEM images of silver nanobeams, (G) TEM image of a microtomed sample of silver nanobeams showing their cross-sectional profile (reproduced with permission from Ref. [45], ©2006 American Chemical Society), (H) TEM image of silver nanoplates, (I) HRTEM images taken from the side face of a silver nanoplate (reproduced with permission from Ref. [46], ©2007 The Royal Society of Chemistry).

squashed dodecahedral gold nanocrystals [66]. Their SEM images are shown in Figure 6D and E. By slowing the rate of atomic addition to singly twinned seeds, silver nanobeams, a type of one-dimensional singly twinned nanostructure, can also be synthesized (Figure 6F) [45]. The HRTEM image of a microtomed sample of silver nanobeams in Figure 6G suggests that the nanobeam is bisected by a twin plane parallel to the base. Right triangular bipyramids enclosed by {1 0 0} facets and plate-like nanoprisms can also be grown from multiple planar-twinned seeds. Mirkin and coworkers used a photochemical method to synthesize silver right bipyramid nanocrystals [11,27]. The HRTEM results show that these right triangular bipyramids were grown from multiple planar-twinned seeds, as shown in Figure 1C. Similarly, plate-like silver nanoplates can also be grown from multiple planar-twinned seeds (Figure 6H and I) [46]. McEachran and Kitaev observed that planar-twinned sliver nanoplates

could be transferred into either silver nanocubes or silver right triangular bipyramids [47]. Based on their experiments, they proposed that the deciding factor for the formation of bipyramids is seeds with a single or an odd number of planar twinning defects, while the deciding factor for the formation of cubes is seeds with an even number of planar twinning defects. Noble metal nanocrystals exhibiting five-fold symmetry are quite common during chemical synthesis [67]. Most of these nanocrystals have cyclic penta-twinned structures [49—60]. Typical penta-twinned nanocrystal shapes include decahedral nanocrystals, nanorods, nanowires, bipyramids, and starfish-like nanocrystals. As shown in Figure 7A, the decahedral nanocrystal consists of five juxtaposed tetrahedral crystallites bounded by {1 1 1} facets [43]. These five tetrahedra are joined together along a common [1 1 0] edge. A decahedral nanocrystal has 7 vertices, 10 faces, 15 edges, and one five-fold symmetry axis. Figure 7B and C show the

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Figure 7 Noble metal nanocrystals with cyclic penta-twinned structures: (A) geometrical model of a decahedral nanocrystal (reproduced with permission from Ref. [43], ©2006 The Royal Society of Chemistry), (B and C) HRTEM image and a representative selected area electron diffraction pattern of a single decahedral silver nanocrystal (reproduced with permission from Ref. [49], ©2002 Elsevier B.V.), (D) geometrical model of a penta-twinned nanorod (reproduced with permission from Ref. [43], ©2006 The Royal Society of Chemistry), (E and F) SEM images of silver nanorods (reproduced with permission from Ref. [56], ©2009 American Chemical Society), (G) TEM image of a gold bipyramid (reproduced with permission from Ref. [68], ©2005 American Chemical Society), (H) TEM image of a starfish-like gold nanocrystal (reproduced with permission from Ref. [57], ©2008 WILEY—VCH Verlag GmbH & Co.), (I) SEM image of an icosahedral gold nanocrystal (reproduced with permission from Ref. [61], ©2004 WILEY—VCH Verlag GmbH & Co.).

TEM image of an individual decahedral nanocrystal and its corresponding selected area electron diffraction (SAED) pattern with the electron beam directed along the five-fold axis of the decahedron [49]. The five twinned boundaries can be clearly identified. The complicated SAED pattern can be interpreted as an interpenetrated set of five individual diffraction patterns corresponding to the [1 1 0] zone axis, in which each pattern is obtained by rotating the former one by approximately 72◦ . If a decahedral nanocrystal is elongated in one dimension, its shape can be changed into nanorods and nanowires with pentagonal cross-sections [50,51,53,56]. Figure 7D shows a geometrical model of penta-twinned nanorods [43]. The penta-twinned nanorod is enclosed by five {1 0 0} side facets and 10 {1 1 1} end facets. The SEM images of silver nanorods grown from decahedral silver nanocrystals are shown in Figure 7E and F, featuring their

pentagonal cross-sections [56]. Other penta-twinned structures include bipyramid [33,68,69], starfish-like [57,58], and icosahedral nanocrystals [59—64]. The bipyramid nanostructure can be viewed as an elongated decahedron (Figure 7G). Gold bipyramids have a pentagonal base, two sharp apexes, and 10 high-index facets, such as {1 1 7} facets [68]. Starfish-like nanocrystals have five branches grown from the five twinned boundaries of a decahedral nanocrystal along the [1 1 0] direction [57]. These nanocrystals are also generally enclosed by high-index facets, such as {3 3 1} facets (Figure 7H). The icosahedral nanocrystal is one of the platonic nanocrystals [61] and is a multiple-twinned nanocrystal that consists of 20 tetrahedra with twinning on their {1 1 1} planes. This nanocrystal has 12 vertices, 20 faces, 30 edges, 6 fivefold axes passing through the vertices, and 10 axes of the third order crossing the centers of the faces. The

272 SEM image of an icosahedral gold nanocrystal is shown in Figure 7I.

Methods for crystallographic control of noble metal nanocrystals A number of synthetic methods have been developed that allow control of the internal crystallinity and external crystal facets of noble metal nanocrystals. This review focuses on the synthesis of noble metal nanocrystals by solutionbased chemical synthetic strategies. Solution-phase based methods have greater capability and flexibility to produce metal nanocrystals with well-defined morphologies with crystallographic control. The scale up process is easier using these methods, and there is no need for specialized equipment. Moreover, these methods facilitate direct solution-based processing and assembly. The seedmediated growth method, polyol process, electrochemical method, and photochemical method are representative solution-based methods. A higher degree of crystallographic control has been achieved with these methods. In addition to the above methods, several other methods, such as the sonochemical, microwave, and hydrothermal methods, have been developed. However, their versatility still needs improvement at current stages and they are not the focus of this review. To understand the mechanism of crystallographic control as a whole, the controlled growth of nanocrystals was illustrated from the aspects of internal and external crystallographic control within each method.

Seed-mediated growth method In the past 10 years, the seed-mediated growth method has been developed into one of the most versatile methods for crystallographic control of noble metal nanocrystals [70]. A typical seed-mediated growth process involves the preparation of small metal nanoparticles and their subsequent growth in reaction solutions [71]. During the growth of the seeds, the reduction of metal salts can only catalytically take place on the seed surface to produce larger nanocrystals. Compared with other methods, the seed-mediated growth method can separate the nucleation and growth stages of the nanocrystals, thus providing better control over the size, size distribution, and crystallographic evolution of the metal nanocrystals. Because of the high controllability, the seed-mediated method is promising in the providing of mechanistic insights into the growth mechanisms of noble metal nanocrystals. The seeding strategy was used to synthesize large colloidal gold nanoparticles with improved monodispersity in the 1990s, and this strategy was based on the colloidal gold surface-catalyzed reduction of gold salts by hydroxylamine [72—74]. Early attempts at using the seed-mediated growth method for shape control of metal nanocrystals were started by the Murphy group to grow silver and gold nanorods in 2001 [75,76]. They used a three-step seeding protocol that yielded long gold nanorods [76]. These gold nanorods have penta-twinned structures with five {1 0 0} side facets and 10 {1 1 1} end facets [50]. Twinning would break the cubic symmetry of the fcc lattice of gold and induce the elongation of the nanocrystals. The actual yield of gold nanorods was

W. Niu, G. Xu approximately 5—10% percents [77]. In 2002, the Nikoobakht and El-Sayed made significant progress in the seed-mediated growth of gold nanorods [31]. By using single-crystalline gold seeds prepared in cetyltrimethylammonium bromide (CTAB) solution and adding silver nitrate to the growth solution, they were able to produce exclusively single-crystalline gold nanorods with yields approaching 100%. Moreover, the aspect ratio of gold nanorods could be tuned from 1.5 to 10 by adding different amount of silver nitrate and accordingly the longitudinal plasmon band could be tuned between 600 and 1300 nm. By using cetyltripropylammonium bromide and cetyltributylammonium bromide as surfactants, Kou et al. extended this method to the synthesis of gold nanorods with aspect ratios as large as 70 [78]. In 2004, the Sau and Murphy further developed the seed-mediated growth method into an elegant and versatile method to synthesize multiple shapes of gold nanocrystals, such as rectangle-, hexagon-, cube-, triangle-, and star-like morphologies [79]. Since then, the seed-mediated growth method has become a powerful approach for the synthesis of metal nanocrystals with high quality and reproducibility. Our group has studied the effect of the size and crystal structure of seeds on the seed-mediated growth of single-crystalline gold nanocrystals [80]. We chose three types of gold seeds to investigate their effects on growth results: ∼41.3 nm cetylpyridinium chloride (CPC) capped single-crystalline gold seeds, ∼3 nm citrate-capped twinned gold seeds, and ∼1.5 nm CTAB-capped single-crystalline gold seeds. When these seeds are added into a growth solution containing CPC, potassium bromide, ascorbic acid, and gold salts, gold nanocrystals mainly enclosed by {1 0 0} facets are formed because the {1 0 0} facets are more stable in the presence of bromide. Different growth results are shown in Figure 8. When CPC-capped single-crystalline gold seeds are used, cubic gold nanocrystals with high yields (95.2%) are produced. In contrast, only 4.0% and 26.1% of the products are cubic nanocrystals when ∼3 nm twinned nanoparticles and ∼1.5 nm single-crystalline nanoparticles are used, respectively. These results prove that both the single-crystalline nature and the relatively large sizes of the CPC-capped seeds play important roles in the growth of single-crystalline gold nanocrystals with high yields. The crystal structure of seeds fluctuates at very small sizes, whereas their structure will be fixed as single-crystalline or multi-twinned as the size of the crystals increases [43]. Because of their relatively large sizes, the CPC-capped single-crystalline seeds can avoid twinning during the growth process, which consequently leads to the formation of exclusively single-crystalline nanocrystals. This ‘‘large seeds’’ strategy has enabled us to study the detailed correlations between growth conditions and the shapes of the singlecrystalline gold nanocrystals during the growth stage, and we can selectively synthesize single-crystalline rhombic dodecahedral, octahedral, and cubic gold nanocrystals by exploring different surfactants, growth kinetics, and adsorbates [80]. The ‘‘large seeds’’ strategy has also been applied to synthesize single-crystalline palladium nanostructures [15,81,82]. Iodide and pseudo-halide thiocyanate ions have been used to control the crystallographic structure of palladium nanocrystals. Rhombic dodecahedral (RD), cubic, and octahedral palladium nanocrystals, as well as their deriva-

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Figure 8 SEM images of gold nanocrystals synthesized with different types of seeds: (A) 41.3 nm CPC-capped single-crystalline gold seeds, (B) ∼3 nm citrate-capped twinned gold seeds, and (C) ∼1.5 nm CTAB-capped single-crystalline gold seeds (all scale bars: 500 nm) (reproduced with permission from Ref. [80], ©2009 American Chemical Society).

tives with varying degrees of edge- and corner-truncation, can be synthesized by a seed-mediated method with CTAB as a surfactant, KI as an additive, and ascorbic acid as a reductant. Single-crystalline Pd nanocubes (22 nm) were used as seeds, because they can avoid twinning during the growth procedure. An orthogonal experimental design was applied to determine the correlation between the shape and the reaction conditions, including reaction temperature and the concentration of KI. Figure 9 shows the SEM images of palladium nanocrystals obtained at different conditions. In the absence of KI, nanocubes mainly enclosed by {1 0 0} palladium facets are favored. In the presence of KI, the concentration of KI and the reaction temperature play an important role in the formation of palladium enclosed by different facets. RD nanocrystals mainly enclosed by {1 1 0} facets are favored at relatively high temperatures and medium KI concentrations. Octahedral nanocrystals mainly enclosed by {1 1 1} facets are favored at relatively low temperatures and medium KI concentrations. Nanocubes mainly enclosed by {1 0 0} palladium facets are favored at very low KI concentrations. For the formation of palladium nanocubes enclosed by the {1 0 0} facets in the absence of KI or at very low KI concentrations, CTAB plays a dominant role in their shape evolution. Bromide anions from CTAB are believed to adsorb onto the surface of palladium nanoparticles and promote the formation of {1 0 0} facets. When a medium concentration of KI is introduced in the growth solution, the iodide anions replace the bromide anions on the surface of the palladium nanocrystals and the {1 1 0} facets become more stable. RD nanocrystals enclosed by {1 1 0} palladium facets were synthesized using this method. Temperature has a profound effect on the growth of RD nanocrystals. RD palladium nanocrystals were produced at relatively high temperatures while octahedral palladium nanocrystals were produced at relatively low temperatures. This result suggests that a relatively high temperature can facilitate the deposition of palladium atoms on the {1 1 1} facets and consequently lead to the disappearance of the {1 1 1} facets. Gold nanorods can also be used as seeds in the seedmediated growth method. Many interesting shapes have been obtained because of the highly anisotropic nature of gold nanorods [83—87]. Carbó-Argibay et al. studied the crystallographic and morphological structure change of gold nanocrystals during the controlled growth of

single-crystalline and penta-twinned gold nanorods in DMF, respectively [83,84]. In dimethylformamide (DMF) solvent, PVP can alter the surface energies of bare gold facets in the order {1 1 1} < {1 1 0} < {1 0 0}. For single-crystalline gold nanorods, the morphology first turned into nanostructures with a square cross-section, with four rectangular lateral {1 1 0} facets and two sharp tips composed of {1 1 1} facets. As the growth proceeded, the nanocrystals eventually converted into octahedrons completely enclosed by the most-stable {1 1 1} facets. In the case of penta-twinned gold nanorods, the preferential addition of gold atoms on the {1 0 0} facets lead to the formation of truncated decahedral nanocrystals enclosed by {1 1 1} and {1 1 0} facets. In both cases, the final nanocrystals preserved the initial internal structure of the seeds. Moreover, single-crystalline gold nanorods can also be used as seeds to direct the growth of a single-crystalline palladium or silver shells on them [85,86]. The use of anisotropic gold nanorods as seeds is an effective way to fabricate different shapes for novel metal nanocrystals, especially bimetallic structures. If noble metal nanoprisms are used as seeds, more exotic nanocrystal shapes are anticipated. Recently, a few noble metal nanocrystals enclosed by high-index facets have also been synthesized by the seedmediated method. Tetrahexahedral gold nanocrystals with high-index {5 2 0} facets [20,24], concave cubes with highindex {7 2 0} facets [27], and concave trisoctahedral with high-index {h h l} facets [22,23] were synthesized by the seed-mediated growth method. Overall, the seed-mediated growth method is a very versatile process. The interplay between kinetic and thermodynamic factors can be manipulated to control the facets and crystallinity of noble metal nanocrystals.

Polyol process The polyol process was originally developed by Fievet et al. [88]. A polyol synthesis involves heating polyol solutions with a metal precursor and a polymeric capping agent. Polyvinylpyrrolidone (PVP) is commonly used as a stabilizer and shape directing reagent for the nanocrystals. In the past decade, the polyol process has become a versatile method to crystallographically control the synthesis of noble metal nanocrystals, such as silver, gold, platinum, palladium, and rhodium [89—93].

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Figure 9 SEM images of polyhedral palladium nanocrystal samples synthesized under different conditions (scale bar: 200 nm). In columns A—E, the reaction temperatures are 30, 40, 50, 60, and 80 ◦ C, respectively. In rows 1—5, 5 ␮L of 100 mM, 5 ␮L of 10 mM, 25 ␮L of 1 mM, 5 ␮L of 1 mM, and 5 ␮L of 0.1 mM KI solutions were added to the growth solutions, respectively. In row 6, no KI was added (reproduced with permission from Ref. [15], ©2010 American Chemical Society).

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Figure 10 Photographs and TEM images of the silver nanocrystals produced by polyol synthesis in the presence of air and NaCl: (A and B) the reaction time is 10 min, (C and D) the reaction time is 44 h (reproduced with permission from Ref. [93], ©2004 American Chemical Society).

Because the polyol process is generally performed at elevated temperatures and in air, oxygen has a significant effect on it. With the assistance of halide ions, the oxidation potential of noble metals can be reduced and the oxidation of noble metals by oxygen is greatly facilitated. Through detailed studies on the function of halide and oxygen, the Xia group proposed an important concept in controlling the crystallinity of noble metal nanocrystals—–oxidative etching [93]. During the synthesis of silver nanocrystals by polyol synthesis in the presence of air and NaCl, they found that the number of twined nanocrystals gradually decreased, while the number of single-crystalline nanocrystal increased (Figure 10). They proposed that the defect zones in twinned seeds have much higher energy relative to the single-crystal regions. Therefore, the twinned seeds are more susceptible to an oxidative environment and their atoms are easily attacked by the etchant and dissolved into the solution. In contrast, single-crystal seeds are more resistant to oxidative etching and left in the solution. By taking advantage of this selectivity, the population of different seed types in the reaction solution can be manipulated. By using a weaker etchant, bromide and oxygen, silver seeds with multiple twin defects could be removed; seeds with only one twin defect can resist the etching and are left in the solution [10]. Thus singly twinned right bipyramids can be produced. Palladium nanocubes can be produced by using FeCl3 as an oxidative etchant [94]. Xia and coworkers also discovered that localized oxidative etching by bromide and oxygen

on one specific face of palladium nanocrystals can initiate preferential growth on this face and thus break the cubic symmetry and induce the formation of anisotropic palladium nanorods and nanobars [29]. The introduction of citric acid or citrate ions can block oxidative etching and promote the formation of multiple-twinned palladium nanocrystals, because of the strong binding of these species to the palladium surface [62]. As a result, multiple-twinned palladium icosahedra were readily obtained in high yields in the presence of citric acid. The crystal facet control in the polyol process depends on a variety of parameters. A straightforward method involves the addition of molecules including surfactants, polymers, small molecules, and atomic species. These species can selectively adsorb to specific crystal facets of the noble metals and promote the formation of certain facets. A well-known example is based on the Au/Ag+ underpotential deposition (UPD) system [61,95,96]. The underpotential shift of silver is sensitive to the surface structure of the gold facets. Both theoretical calculations [97] and experiments [98] suggest that silver preferentially deposits on the gold {1 0 0} facets, rather than the {1 1 1} facets. In the polyol synthesis of gold nanocrystals, silver atoms reduced from Ag+ were preferentially deposited onto the {1 0 0} gold seed surface to form silver layers, which suppressed epitaxial overgrowth of the gold layers during the reaction and promoted the formation of gold {1 0 0} facets. Therefore, a series of gold nanocrystals with the shape of octahe-

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Figure 11 Noble metal nanocrystals synthesized by polyol process: (A—C) gold (reproduced with permission from Ref. [95], ©2006 American Chemical Society), (D—F) platinum (reproduced with permission from Ref. [99], ©2005 American Chemical Society), and (G—I) silver (reproduced with permission from Ref. [100], ©2006 WILEY—VCH Verlag GmbH & Co.) cubic, cuboctahedral, and octahedral nanocrystals.

dra, truncated octahedra, cuboctahedra, cubes, and higher polygons were synthesized by incremental changes of silver nitrate concentration (Figure 11A—C) [95]. The shape of platinum nanocrystals could also be controlled by adding silver nitrate (Figure 11D—F, [99]). A more versatile approach to control the crystal facet in the polyol process is to control the growth kinetics. Yang and coworkers have shown that silver nanocrystals can evolve from cubes to cuboctahedra and then to octahedra with an increasing ratio of {1 1 1} to {1 0 0} facets (Figure 11G—I) [100]. To produce these silver nanocrystals, silver nitrate and copper(II) chloride dissolved in 1,5-pentanediol and PVP dissolved in 1,5-pentanediol were injected periodically into a solution of hot pentanediol. Depending on the reaction time, specific polyhedral shapes including cubes, truncated cubes, cuboctahedra, truncated octahedra, and octahedral can be obtained in high yields. Both {1 0 0} and {1 1 1} silver facets are stable in the presence of PVP, indicating that shape control may not be explicitly dictated by the capping polymer. Preferential crystal growth promoted by kinetically limited equilibrium is believed to control the facet formation.

Ethylene glycol, 1,5-pentanediol, and di(ethylene glycol) are commonly used as the solvent and reductant for the polyol process [29,100]. DMF can also be used as a good solvent and reductant for the crystallographic control of noble metal nanocrystals [101]. Such a reaction system can also be used to synthesize gold and silver nanocrystals in a similar manner. Several morphologies including nanoprisms, decahedra, tetrahedra of silver, and nanoplates, nanocubes, rhombic dodecahedra, squashed dodecahedra, and octahedral of gold can be synthesized in DMF [54,66,102—104]. In DMF, the interaction between PVP and the metal facets is different from the reactions in ethylene glycol. In the presence of PVP, the silver {1 0 0} facet is more stable in ethylene glycol solvent [105], but the {1 1 1} facet is favored in DMF solvent [54,102,106].

Electrochemical methods The electrochemical method was first used to produce metal nanoparticles by Reetz and Helbig [107]. The process generally explores a two-electrode setup. The sacrificial anode

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Figure 12 SEM images of noble metal nanocrystals synthesized by electrochemical methods: (A and B) tetrahexahedral platinum nanocrystals (reproduced with permission from Ref. [18], ©2007 American Association for the Advancement of Science), (C and D) flower-like platinum nanocrystals, (E and F) concave hexoctahedral platinum nanocrystals, (G) tetrahexahedral palladium nanocrystals (reproduced with permission from Ref. [21], ©2010 American Chemical Society), (H) a trapezohedral palladium nanocrystal, (I) palladium nanorods (reproduced with permission from Ref. [109], ©2009 The Royal Society of Chemistry). Other images are reproduced with permission from Ref. [12], ©2008 American Chemical Society.

consists of the bulk metal to be transformed into metal nanoparticles. In 1997, the electrochemical method was successfully used to synthesize single-crystalline gold nanorods [36,108], and for the first time, the aspect ratio-dependent optical property of gold nanorods was observed. Recently, the electrochemical method has also been applied to the crystallographic control of noble metal nanocrystals. The Sun pioneered the synthesis of platinum and palladium nanocrystals with high-index facets using an electrochemical method [12]. For example, singlecrystalline tetrahexahedral Pt nanocrystals with {7 3 0} high-index facets were synthesized by treating electrodeposited ∼750 nm Pt nanospheres with square-wave potential in a solution of H2 SO4 and ascorbic acid (Figure 12A and B, [18]). Repetitive adsorption/desorption of oxygen generated by square-wave potential are believed to play a key role in the formation of high-index facets. Oxygen atoms preferentially adsorb at high-index facets because highindex facets contain many step atoms with low-coordination

numbers. Therefore, high-index facets are preserved during the electrochemical treatment. Changing the reaction parameters of the electrochemical method can tune the shapes and crystal facets of the nanocrystals [12]. For example, if the distribution density of the original Pt nanospheres on the glassy carbon substrate is decreased by one order of magnitude and the treatment time of the square-wave potential is prolonged to 1 h, flower-like Pt nanocrystals can be obtained (Figure 12C and D). These nanocrystals have similar surface facets to the tetrahexahedral Pt nanocrystals. If the ascorbic acid is replaced by 50 mM sodium citrate, concave hexoctahedral Pt nanocrystals bounded by 48 {h k l} high-index facets are obtained (Figure 12E and F). The electrochemical method can also be extended to synthesize tetrahexahedral palladium nanocrystals (Figure 12G) [21], trapezohedral palladium nanocrystals (Figure 12H) [12], and five-fold twinned palladium nanorods with {h k k} or {h k 0} high-index facets (Figure 12I) [109].

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Photochemical methods Light can be used to induce the reduction of metal salts to metal nanocrystals. For example, high yields of gold nanorods were obtained after a solution containing CTAB, HAuCl4 , AgNO3 , acetone, and cyclohexane was irradiated with a 254-nm UV light for 30 h [110]. The aspect ratio of the final rods is dictated by the amount of silver ions added to the system. Niidome et al. improved this photochemical method by adding ascorbic acid [111]. Gold nanorods can be produced within several minutes by this method. Light can also be used to induce the shape transformation of metal nanocrystals. The Mirkin group discovered a photoinduced method for converting large quantities of silver nanospheres into triangular nanoprisms [112]. This group could controllably synthesize monodisperse nanoprisms with desired edge lengths in the 30—120 nm range by using dualbeam illumination of the silver nanoparticles (Figure 13A—C) [113]. The process is believed to be driven by surface plasmon excitations. The plasmon excitations may fuse the nanoprisms in an edge-selective manner or keep the

W. Niu, G. Xu nanoprisms growing until they reach their light-controlled final size. The edge lengths of the nanoprisms can also be controlled by changing the pH of the growth solutions [114]. These silver nanoprisms have multiple planar-twinned structures [11,27]. The elaborate selection of stabilizer and structuraldirecting reagents could result in a better control over the crystal structure of silver nanocrystals during the lightdriven conversion process. For example, Zhou and coworkers used disodium tartrate as a structural-directing reagent and prepared tetrahedral silver nanocrystals in a large scale with relatively high yields [17]. The tetrahedral silver nanocrystals are single crystals enclosed by four {1 1 1} facets (Figure 13D and E). In the early stage, the addition of tartrate helps the silver seeds nucleate into tetrahedral rudiments under irradiation of a sodium lamp. Then, citrate acts as photoreducing reagent and promoter for the {1 1 1} facets and thus promotes the formation of tetrahedral silver nanocrystals. By using PVP and citrate as stabilizers and arginine as a photochemical promoter, Pietrobon and Kitaev were able to synthesize decahedral silver nanocrystals with

Figure 13 Silver nanocrystals synthesized by photochemical methods: (A—C) TEM images of silver nanoprisms with different sizes (reproduced with permission from Ref. [113], ©2003 Nature Publishing Group), (D) TEM image of tetrahedral silver nanocrystals, (E) TEM and corresponding SAED pattern of a tetrahedral silver nanocrystal (reproduced with permission from Ref. [17], ©2008 American Chemical Society), (F) TEM image of decahedral silver nanocrystals (reproduced with permission from Ref. [55], ©2008 American Chemical Society), (G—I) SEM images and geometrical models of right triangular bipyramids, truncated right triangular bipyramids, and triangular prisms, respectively (reproduced with permission from Ref. [27], ©2010 American Chemical Society).

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excellent shape selectivity [55]. The decahedra size can be varied in the range of 35—120 nm by adjusting the intensity, spectral properties of irradiating light, and photochemical regrowth (Figure 13F). In the photochemical methods, the formation of crystal facets of the nanocrystals is sensitive to the growth kinetics [27]. For the photomediated synthesis of silver nanocrystals in the presence of bis(p-sulfonatophenyl)phenylphosphine (BSPP), planar-twinned seeds could be converted into either right triangular bipyramids enclosed by {1 0 0} facets or triangular prisms mainly enclosed by {1 1 1} facets. The selective synthesis of these two shapes was realized through control of the reaction rate. A high pH (>10) and a [BSPP]/[Ag+ ] ratio close to 1 lead to faster reaction rates, and the deposition of silver occurs preferentially on {1 1 1} facets. Consequently, planar-twinned seeds will grow into {1 0 0}-faceted right triangular bipyramids (Figure 13G). A lower pH or a higher [BSPP]/[Ag+ ] ratio leads to a slow reaction rate, and the deposition of silver occurs preferentially on {1 0 0} facets. Thus, the formation of {1 1 1} facets is facilitated, and a mixture of truncated bipyramids with both {1 0 0} and {1 1 1} facets or triangular prisms with mostly {1 1 1} facets are obtained (Figure 13H and I).

trol of noble metal nanocrystals. For example, a variety of nanocrystals produced by different methods can be explored as seeds for the seed-mediated growth method. Pietrobon et al. used decahedral silver nanocrystals prepared by photochemical methods as seeds for the seed-mediated growth method [56]. The width of the silver nanorods was determined by the size of the starting decahedral nanocrystals, while the length could be tuned from 50 nm to 2 ␮m by the amount of silver salts added to the growth solution. On the other hand, the seed-mediated growth method can also adopt the growth conditions of other methods. For instance, Xia and coworkers used single-crystalline silver seeds to direct the growth of silver nanocubes in the polyol process [115]. When the precursor silver nitrate is reduced to elemental Ag, the byproduct HNO3 can inhibit both the homogeneous nucleation and evolution of single-crystal seeds into twinned nanoparticles. Thus they were able to control the edge length of the resultant Ag nanocubes by simply varying the amount of silver seeds added.

Combined methods

Driven by the scientific mantra of ‘‘structure dictates function’’, scientists in the field of noble metal nanocrystals are always searching for new properties after they obtain a new type of nanocrystal. It is essential to understand the prop-

A new concept that combines different methods has proven to be an alternative approach for the crystallographic con-

Enhanced properties of noble metal nanocrystals by crystallographic control

Figure 14 Chemical transformation of 10.5 nm silver single-crystalline and multiple-twinned silver nanocrystals to Ag2 Se nanostructures: (A) typical HRTEM image (left) and SAED (lower right inset) of single-crystalline nanocrystals and schematic diagram of atom diffusion paths (middle); typical HRTEM image (right) and SAED (lower left inset) of single-crystalline Ag2 Se nanocrystals with hollow core, (B) typical HRTEM image (left) and SAED (lower right inset) of multiple-twinned silver nanocrystals and schematic diagram of different atom diffusion paths (middle); typical HRTEM image (right) and SAED (lower left inset) of solid single-crystalline Ag2 Se nanocrystals (reproduced with permission from Ref. [116], ©2007 Nature Publishing Group).

280 erties on the crystallographic level rather than on the shape and size level because noble metal nanocrystals with identical shapes could have entirely different crystal structures. For example, silver nanocubes can have single-crystalline and twinned structures [47,105]. Fcc metal nanocubes are generally enclosed by six {1 0 0} facets, but fcc metal nanocubes with four side {1 1 0} facets and two end {1 0 0} facets can also be theoretically predicted. Herein, our discussions are focused on how internal crystal structure (single-crystalline, twinned, and twin domain size) and external crystal structure (crystal facets) can alter the properties of noble metal nanocrystals. Single-crystalline and twinned metal nanocrystals have different atomic energy sites on their surface and therefore have different chemical activities. Tang and Ouyang reported that a nanoscale chemical transformation could be manipulated by defect engineering within the noble metal nanocrystal templates [116]. They first found that the crystallinity of monodispersed silver nanocrystals could be well controlled by a judicious choice of functional groups of molecular precursors. (PPh3 )3 Ag-NO3 leads to the formation of multiple-twinned nanocrystals, while (PPh3 )3 Ag-Cl leads to the formation of single-crystalline nanocrystals.

W. Niu, G. Xu Then the chemical transformation reactions between these silver nanocrystals and selenium suspensions were investigated. Solid and perfect single-crystalline Ag2 Se were obtained for the multiple-twinned nanocrystal template (Figure 14B), whereas Ag2 Se nanoshells were produced for the reaction that started with single-crystalline nanocrystals (Figure 14A). The authors suggest that the formation of Ag2 Se nanoshells from single-crystalline silver nanocrystals follows a mechanism analogous to the Kirkendall effect [117]. Because the diffusion of silver atoms is faster than that of selenium atoms during the reaction, a vacancy in the center of the nanocrystals will form as the silver atoms move out. In the case of multiple-twinned nanocrystals, the twinning boundaries can act as the outward and inward transport channels for atoms, in a similar manner to macroscopic ‘pipe diffusion’. Owing to the large defect-volume ratio of the silver nanocrystals, atom transport along the twinning boundaries can balance the outward and inward atom flows and suppresses the formation of vacancies via transport along the surface and through the lattice. Therefore, solid and perfect singlecrystalline Ag2 Se were obtained from multiple-twinned nanocrystals.

Figure 15 (A and B) Luminescence, Rayleigh scattering and overlay images of the luminescent silver nanoparticles created by solidphase synthesis and those of commercially available silver nanoparticles (20 nm) made by solution-phase synthesis, respectively, (C and D) HRTEM images of a nonluminescent silver nanocrystal created by solution-phase synthesis and a luminescent silver nanocrystal created by solid-phase synthesis, respectively, (E) domain size distributions obtained from luminescent (red) and nonluminescent (green) silver nanocrystals (reproduced with permission from Ref. [118], ©2008 American Chemical Society).

Crystallographic control of noble metal nanocrystals Zheng and coworkers studied the different luminescent properties of twinned silver nanocrystals with different internal crystal domain sizes [118]. This group developed a unique method to synthesize silver nanocrystals with bright luminescence. The number of photons emitted from these nanocrystals exceeded that from quantum dots or dye molecules by two or five orders of magnitude, respectively (Figure 15A). The synthetic process involved the thermal reduction of silver ions in a glycine matrix. It is possible to control the nucleation and migration of reduced silver atoms by taking advantage of the solid-state matrix. The HRTEM results show that the silver nanocrystals display a polycrystalline structure with numerous small domains with domain sizes primarily in the 1—2 nm range (Figure 15C). Metal nanocrystals with sizes on the same order as those of the electron Fermi wavelength (∼0.5 nm for silver and gold) often display strong single-electron excitations and emit fluorescence [119,120]. The small domains present in the silver nanocrystals likely result in similar discrete energy states and their bright luminescence results from single-electron excitations. In contrast, for

281 silver nanocrystals of similar sizes (∼20 nm) produced by solution-phase synthesis, only ∼2% of the nanocrystals emit luminescence (Figure 15B). The HRTEM results show that these nanocrystals are typically single-crystalline or twined structures with domain sizes averaging approximately 8 nm (Figure 15D). For silver nanocrystals at this domain size range, collective excitations of electrons (plasmon resonance) become dominant as particle size approaches the electron mean free path length (∼50 nm for silver and gold) [121]. The catalytic activity of noble metal nanocrystals is highly dependent on the nature of their surface structures [5]. Nanocrystals enclosed by different crystal facets may have different densities of low-coordinated surface atoms. Noble metal nanocrystals with open surface structures, especially high-index facets, are generally superior in catalytic activity to low-index facets that are composed of closely packed surface atoms. Therefore, crystallographic control of noble metal nanocrystals is an important way to enhance the catalytic activity of nanocrystals.

Figure 16 Catalytic activity of silver nanocrystals for styrene oxidation: (A—C) TEM images and corresponding structural models of truncated triangular nanoplates, nanocubes, and near-spherical silver nanocrystals, respectively, (D) catalytic performance of the silver nanocrystals in the oxidation of styrene with tert-butyl hydroperoxide, (E) specific reaction rate of styrene conversion over truncated triangular, near-spherical, and cubic silver nanoparticles (reaction time: 3 h) (reproduced with permission from Ref. [122], ©2006 WILEY—VCH Verlag GmbH & Co.).

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Figure 17 Catalytic selectivity of palladium nanocrystals for benzene hydrogenation: (A) TEM image and geometrical model cubic platinum nanocrystals, (B) TEM image and geometrical model cuboctahedral platinum nanocrystals, (C) turnover rates of cyclohexane and cyclohexene formation on cubic and cuboctahedral platinum nanocrystals, (D) turnover rates of cyclohexane and cyclohexene formation on Pt {1 0 0} and Pt {1 1 1} single-crystals (reproduced with permission from Ref. [123], ©2007 American Chemical Society).

Xu et al. studied the activity of the catalytic oxidation of styrene over three types of silver nanocrystals: truncated triangular nanoplates, near-spherical nanocrystals, and nanocubes [122]. TEM characterization suggests that the silver nanocubes are bounded by six {1 0 0} crystal facets (Figure 16C), truncated triangular nanoplates are primarily bounded by the most-stable {1 1 1} crystal facets (Figure 16A), and near-spherical nanocrystals are mainly composed of numerous small {1 1 1} and {1 0 0} facets (Figure 16B). The rate of epoxidation of styrene with tert-butyl hydroperoxide on nanocubes is over 14 times higher than that on nanoplates and four times higher than that on near-spherical nanocrystals (Figure 16E and F). N2 -adsorption experiments show that the surface area is highest for the truncated triangular nanoplates and lowest for near-spherical nanocrystals. However, near-spherical nanoparticles have many {1 1 1} and {1 0 0} facets, and they are supposed to have many more edges and corners at the interface than the nanocubes and nanoplates. Therefore, the surface area and the atoms on the corners and edges of the silver nanocrystals can be excluded as having a major contribution to their catalytic activities. The catalytic

performance for styrene oxidation is mainly determined by the crystal facets of silver nanocrystals. This result indicates that the {1 0 0} facets have a higher surface energy and can facilitate the adsorption and activation of the reactant and thus have a higher activity. The catalytic selectivity could also be strongly affected by the crystal facets of the noble metal nanocrystals [5]. The hydrogenation of benzene yields only cyclohexane on bulk single-crystalline platinum with {1 0 0} surface and yields both cyclohexene and cyclohexane on bulk single-crystalline platinum with {1 1 1} surface [123]. Such dependence on the surface structure also works in the case of platinum nanocrystals (Figure 17). The hydrogenation of benzene forms both cyclohexene and cyclohexane on cuboctahedral platinum nanocrystals enclosed by both {1 1 1} and {1 0 0} facets and only forms cyclohexane on cubic platinum nanocrystals enclosed by {1 0 0} facets.

Summary and outlook We have discussed the controlled growth of the nanocrystals from the aspects of internal and external crystallographic

Crystallographic control of noble metal nanocrystals control, described the representative synthetic method, and illustrated typical examples of the enhanced properties of noble metal nanocrystals by crystallographic control. Obviously, a fundamental understanding of the growth mechanisms of noble metal nanocrystals is crucial to the rational design of metal nanocrystals with tailored shapes and desired properties. However, the growth mechanisms of noble metal nanocrystals are not currently well understood. There is a need to develop new strategies and adopt new tools to reveal the growth mechanisms. For example, in situ synchrotron-based X-ray diffraction techniques [124] and transmission electron microscopy [125] are promising methods to track the structure evolution and understand the nucleation and growth kinetics of metal nanocrystals. An integration of more advanced in situ monitoring tools, computational modeling, and experimental studies will allow for a comprehensive understanding of the growth mechanisms. In the last decade, we have witnessed the rapid development of various methods for the crystallographic control of various noble metal nanocrystals. From the viewpoint of synthesis, there are many substantial challenges to develop new synthetic methods, to synthesize new nanocrystals, and to construct nanocrystal superlattices. For example, the shape-controlled synthesis of nanocrystals of Ru, Ir, and Rh that are widely used for industrial applications remains a great challenge. It is expected that nanocrystals with high surface energies may have good catalytic ability; however, the synthesis of nanocrystals with high surface energies, particularly metal nanocrystals enclosed by high-index facets, has been met with limited success [126—128]. Multimetal nanocrystals have very unique properties that make them attractive for magnetic, catalytic, optical, and electronic applications, whereas there are still few reports about the crystallographic control of alloy metal nanocrystals [129—133]. The properties of nanocrystals are also dependent on the surface chemistry. Chemical modification of the noble metal nanocrystals, such as by the use of adatoms to decorate the surface of the nanocrystals on the nanocrystals, will provide an alternative versatile avenue for the development of new nanocrystal materials with desired properties [134]. Because the properties of nanocrystal superlattices can be as different from their individual components as the physical properties of nanoparticles are from bulk materials, nanocrystal assembly has received much attention and is a rapidly developing field of research [135,136]. While much progress has been made in the synthesis of nanocrystal, there remain several critical issues that need to be addressed before the practical application of noble metal nanocrystals is possible. First, syntheses need to be scaled up to kilograms or more with high yields. Second, there is a need for the development of cost-effective techniques for collecting nanocrystals and avoiding the aggregation of metal nanocrystals. Third, many shapes of noble nanocrystals can undergo shape transformation, especially at high temperatures. Therefore, the shape retention of noble metal nanocrystals is an important research subject in this field and more methods focused on the improvement of the stability of noble metal nanocrystals need to be developed [137]. Also, it is necessary to develop synthetic methods for the crystallographic control within a sub 10 nm scale for cost-effective catalysis applications [138—142].

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Acknowledgements We are grateful to the National Natural Science Foundation of China (No. 20875086 and 20505016), the Ministry of Science and technology of the People’s Republic of China (No. 2006BAE03B08), the Department of Sciences & Technology of Jilin Province (20070108 and 20082104), and Hundred Talents Programme of Chinese Academy of Sciences for financial support.

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Y.L. Liu, A.R.H. Walker, Angew. Chem. Int. Ed. 49 (2010) 6781. J. Yang, J.Y. Ying, Nat. Mater. 8 (2009) 683. J. Yang, J.Y. Lee, J.Y. Ying, Chem. Soc. Rev. 40 (2011) 1672. W. Chen, R. Yu, L.L. Li, A.N. Wang, Q. Peng, Y.D. Li, Angew. Chem. Int. Ed. 49 (2010) 2917. F.J. Vidal-Iglesias, J. Solla-Gullón, E. Herrero, A. Aldaz, J.M. Feliu, Angew. Chem. Int. Ed. 49 (2010) 6998. Z.W. Quan, J.Y. Fang, Nano Today 5 (2010) 390. F.R. Fan, D.Y. Liu, Y.F. Wu, S. Duan, Z.X. Xie, Z.Y. Jiang, Z.Q. Tian, J. Am. Chem. Soc. 130 (2008) 6949. V. Mazumder, M.F. Chi, K.L. More, S.H. Sun, J. Am. Chem. Soc. 132 (2010) 7848. S.S. Cheong, J.D. Watt, R.D. Tilley, Nanoscale 2 (2010) 2045. C.K. Tsung, J.N. Kuhn, W.Y. Huang, C. Aliaga, L.I. Hung, G.A. Somorjai, P.D. Yang, J. Am. Chem. Soc. 131 (2009) 5816. C. Wang, H. Daimon, T. Onodera, T. Koda, S.H. Sun, Angew. Chem. Int. Ed. 47 (2008) 3588. Z.M. Peng, H. Yang, Nano Today 4 (2009) 143. J.Y. Chen, B. Lim, E.P. Lee, Y.N. Xia, Nano Today 4 (2009) 81. Wenxin Niu was born in Liaoning, China, in 1981. He received a B.S. degree from the University of Science and Technology of China in 2004 and a Ph.D. from the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (with Professor Guobao Xu) in 2010. His research interests include shape-controlled synthesis and solar energy conversion applications of noble metal and semiconductor nanocrystals.

Guobao Xu received his BSc in chemistry from Jilin University, his MSc from Xiamen University, and his PhD from the Changchun Institute of Applied Chemistry. After postdoctoral research at the University of Hong Kong, the Hong Kong Polytechnic University, and NTT Microsystem Integration Laboratories, he joined Changchun Institute of Applied Chemistry as a Professor. His research interests include the synthesis and applications of metal nanocrystals, electrochemiluminescence, and nanomaterial-based electrochemistry. He won the Young Scholar Award of the Chinese Chemical society in 2005 and the Second Class Award of National Natural Science of China in 2009.