Template Synthesis of Noble Metal Nanocrystals with Unusual Crystal

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Template Synthesis of Noble Metal Nanocrystals with Unusual Crystal Structures and Their Catalytic Applications Zhanxi Fan and Hua Zhang* Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore CONSPECTUS: Noble metal nanocrystals own high chemical stability, unique plasmonic and distinctive catalytic properties, making them outstanding in many applications. However, their practical applications are limited by their high cost and scarcity on the earth. One promising strategy to solve these problems is to boost their catalytic performance in order to reduce their usage amount. To realize this target, great research efforts have been devoted to the size-, composition-, shape- and/or architecture-controlled syntheses of noble metal nanocrystals during the past two decades. Impressively, recent experimental studies have revealed that the crystal structure of noble metal nanocrystals can also significantly affect their physicochemical properties, such as optical, magnetic, catalytic, mechanical, electrical and electronic properties. Therefore, besides the wellestablished size, composition, shape, and architecture control, the rise of crystal structure-controlled synthesis of noble metal nanocrystals will open up new opportunities to further improve their functional properties, and thus promote their potential applications in energy conversion, catalysis, biosensing, information storage, surface enhanced Raman scattering, waveguide, near-infrared photothermal therapy, controlled release, bioimaging, biomedicine, and so on. In this Account, we review the recent research progress on the crystal structure control of noble metal nanocrystals with a template synthetic approach and their crystal structure-dependent catalytic properties. We first describe the template synthetic methods, such as epitaxial growth and galvanic replacement reaction methods, in which a presynthesized noble metal nanocrystal with either new or common crystal structure is used as the template to direct the growth of unusual crystal structures of other noble metals. Significantly, the template synthetic strategy described here provides an efficient, simple and straightforward way to synthesize unusual crystal structures of noble metal nanocrystals, which might not be easily synthesized by commonly used chemical synthesis. To be specific, by using the epitaxial growth method, a series of noble metal nanocrystals with unusual crystal structures has been obtained, such as hexagonal close-packed Ag, 4H Ag, Pd, Pt, Ir, Rh, Os, and Ru, and face-centered cubic Ru nanostructures. Meanwhile, the galvanic replacement reaction method offers an efficient way to synthesize noble metal alloy nanocrystals with unusual crystal structures, such as 4H PdAg, PtAg, and PtPdAg nanostructures. We then briefly introduce the stability of noble metal nanocrystals with unusual crystal structures. After that, we demonstrate the catalytic applications of the resultant noble metal nanocrystals with unusual crystal structures toward different chemical reactions like hydrogen evolution reaction, hydrogen oxidation reaction and organic reactions. The relationship between crystal structures of noble metal nanocrystals and their catalytic performances is discussed. Finally, we summarize the whole paper, and address the current challenges and future opportunities for the template synthesis of noble metal nanocrystals with unusual crystal structures. We expect that this Account will promote the crystal structure-controlled synthesis of noble metal nanocrystals, which can provide a new way to further improve their advanced functional properties toward their practical applications.

1. INTRODUCTION Noble metal nanocrystals have received extensive research interest recently owing to their remarkable performance in various promising applications, e.g., catalysis,1−6 surface enhanced Raman scattering (SERS),7 energy conversion,8 near-infrared photothermal therapy,9 and biomedicine.10 The last two decades have witnessed the impressive development in the precise control of size, shape, composition and architecture of noble metal nanocrystals, which are key parameters affecting their functional properties.11 However, the crystal structurecontrolled synthesis of noble metal nanocrystals still remains great underdeveloped and challenges. Notably, recent exper© XXXX American Chemical Society

imental investigations demonstrated that the chemical and physical properties of noble metal nanocrystals, such as optical,12 mechanical,12 catalytic,13 magnetic,13 electrical,14 and electronic properties,12 can also be remarkably changed by simply tuning their crystal structures. For instance, it was observed that 4H Ag nanowire film exhibits significantly enhanced localized surface plasmon resonance absorption in the visible region as compared to that of their common facecentered cubic (fcc) counterpart.12 Hence, the crystal structure Received: October 20, 2016

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DOI: 10.1021/acs.accounts.6b00527 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research control of noble metal nanocrystals is of high importance and will shed new light on their potential applications.15 Normally, the crystal structure controlled-synthesis of noble metal nanocrystals is realized under extreme conditions, such as high pressure16 and high temperature,13 which may induce the aggregation/degradation of ultrasmall noble metal nanostructures and thus cannot be widely used. Until recently, several synthetic methods, e.g,. anodized aluminum oxide-assisted electrochemical deposition,17 graphene oxide-mediated synthesis,18−20 ligand exchange,21 and colloidal method,22 have been developed for the crystal structure-controlled synthesis of noble metal nanocrystals at mild conditions. Compared to these synthetic methods, template synthesis, i.e., the direct growth of one noble metal on another presynthesized noble metal nanocrystal, provides a simple, versatile and relatively straightforward way to prepare noble metal nanocrystals with unusual crystal structures since the growth process is separated from the complicated nucleation process.23 Consequently, a wide range of noble metal nanocrystals with unusual crystal structures, such as hexagonal close-packed (hcp) Ag,21 4H Ag, Pd, Pt, Ir, Rh, Os, Ru, PdAg, PtAg, and PtPdAg22,24,25 and fcc Ru monometallic nanostructures or alloys,26−30 has been obtained by using this template synthetic method, making it possible to study their crystal structure-dependent properties. Note that the noble metal nanocrystals might not follow their bulk phases and bulk phase diagrams because of the size, morphology, and segregation effects.31 In this Account, we will give an overview of the recent advances in the template synthesis of noble metal nanocrystals with unusual crystal structures for catalytic applications. First, we will introduce representative template synthetic methods for the crystal structure control of noble metal nanocrystals. Then, we will show the catalytic applications of noble metal nanocrystals with unusual crystal structures in various chemical reactions, e.g., hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), and organic reactions. Finally, we will give a brief conclusion and provide a perspective on the future development of this research direction.

Table 1. Crystal Structure and Lattice Constant of Noble Metal Bulk Crystalsa noble metal

crystal structure

Au Ag Pt Pd Ir Rh Os Ru

fcc fcc fcc fcc fcc fcc hcp hcp

lattice constant a a a a a a a a

= = = = = = = =

4.0796 4.0855 3.9237 3.8902 3.8312 3.7956 2.7359 2.7042

Å Å Å Å Å Å Å, b = 4.3186 Å Å, b = 4.2816 Å

a

Data taken from the Inorganic Crystal Structure Database (ICSD). The Collection Codes (Coll. Codes) of Au, Ag, Pt, Pd, Ir, Rh, Os, and Ru in the ICSD are 44362, 44387, 52250, 52251, 659854, 650221, 52262, and 52261, respectively.

Recently, for the first time, we found that the hcp AuSSs can be used as a template to grow Ag nanostructure with an unusual hcp phase (Figure 1).21 Briefly, the Ag nanostructure is deposited on the surface of hcp AuSSs by reducing AgNO3 with oleylamine at 38 °C for 20 h, leading to the formation of hcp/ fcc Au@Ag core−shell square sheets because of a partial hcpto-fcc phase transformation (Figure 1A). Transmission electron microscope (TEM) image and the corresponding selected area electron diffraction (SAED) pattern reveal the presence of fcc structure in AuSSs after the Ag coating (Figure 1B, C). The coexistence of hcp and fcc structures is directly observed in high-resolution TEM (HRTEM) images (Figure 1D, E). Crosssectional high angle annular dark field-scanning TEM (HAADF-STEM) images show the lattice fringes coherently extend from inside to outside (Figure 1F, H), indicating the epitaxial relationship between the Au template and Ag shell, which is also confirmed by the corresponding fast Fourier transform (FFT) patterns (Figure 1G, I). Note that the addition of oleylamine is critically essential to the synthesis of hcp Ag nanostructure as a complete hcp-to-fcc phase transformation occurs in the absence of oleylamine after the reduced Ag was coated on hcp AuSSs.21 Moreover, the growth of Pd and Pt on hcp AuSSs can also induce a total phase change from hcp to fcc phases.34 Recently, for the first time, our group reported the colloidal synthesis of micrometer-long Au nanoribbons (NRBs) with an unprecedented 4H hexagonal structure, which has a characteristic stacking sequence of “ABCB” along the close-packed direction of [001]4H.22 Importantly, by using the 4H Au NRBs as template, we have been able to synthesize 4H Ag nanostructure via the epitaxial growth of Ag on their surface (Figure 2A−G).22 Note that the lattice mismatch between Au and Ag is quite small and neglectable (∼0.2%).32 Remarkably, 4H Pd (Figure 2H−N) and 4H Pt (Figure 2O−U) nanostructures can also be obtained through the epitaxial growth of Pd and Pt on 4H Au NRBs, respectively, although the lattice mismatches between Au and Pd, and Au and Pt are relatively large.32 Similar to Ag, the coating of Pd and Pt on 4H Au NRBs just induces a partial phase transformation from 4H to fcc structures, which is different from that of the aforementioned hcp AuSSs.22,34 This phenomenon reveals that the 4H structure of Au is more stable compared to the hcp structure. Importantly, for the first time, the 4H nanostructures of Ir, Rh, Os and Ru have also been obtained via the epitaxial growth of Ir, Rh, Os and Ru on the surface of 4H Au NRBs, respectively.24 In addition, this method can also be extended to

2. TEMPLATE SYNTHETIC METHODS FOR NOBLE METAL NANOCRYSTALS WITH UNUSUAL CRYSTAL STRUCTURES 2.1. Epitaxial Growth

Epitaxial growth refers to the oriented coating of one crystalline material (overlayer/shell) on another crystalline material (substrate/template).32 Usually, to achieve the epitaxial growth of the shell material on the template material, the lattice mismatch between these two materials should be small.32 Owing to the relatively small lattice mismatch between noble metals (Table 1), the epitaxial growth method has been successfully used for the solution-phase synthesis of various noble metal core−shell nanocrystals with well-defined size, shape, composition and architecture at mild conditions.33 Moreover, it is also possible to synthesize noble metal nanocrystals with unusual crystal structures via this epitaxial growth strategy when the common crystal structure of shell material is different to the crystal structure of template material. In order to grow one noble metal nanocrystal with unusual crystal structure, an effective way is to use another noble metal nanocrystal with novel crystal structure as the template.21,22,24 In 2011, our group first reported the wet-chemical synthesis of pure hcp Au square sheets (AuSSs) on graphene oxide sheets.20 B


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Figure 1. (A) Schematic illustration for the synthesis of hcp/fcc Au@Ag core−shell square sheets from hcp Au square sheets. (B) TEM image, (C) SAED pattern, and (D, E) HRTEM images of hcp/fcc Au@Ag square sheets along the [110]h/[101]f zone axes. (F, H) Cross-sectional HAADFSTEM images and (G, I) the corresponding FFT patterns along the [110̅ ]h/[1̅2̅1]f and [001]h/[1̅11]f zone axes, respectively. Reprinted with permission from ref 21. Copyright 2015 Nature Publishing Group.

tetrahedra can be tuned by adjusting the ratio of Pt and Ru precursors. Interestingly, Li et al. demonstrated that the epitaxial synthesis of fcc Ru nanostructure can also be realized by using fcc Pd−Cu alloy nanocrystals as the template.28 Compared to the single metal nanocrystal template, the lattice parameter of alloy metal nanocrystals is highly tunable via simply changing the atomic ratio between different metal elements. As the lattice mismatch is a key factor that greatly affects the epitaxial growth, the growth mode of Ru nanostructure on the Pd−Cu alloy template can be modulated from the epitaxial growth to nonepitaxial growth by adjusting the lattice mismatch between the Pd−Cu alloy template and Ru shell.28 It was observed that Pd−Cu alloy templates with a minor lattice mismatch regarding to fcc Ru, e.g., PdCu3 and PdCu2.5, favor the epitaxial growth of fcc Ru shell. The deviation from fcc Ru, e.g., Pd, PdCu2 and Cu, will induce the formation of hcp Ru shell. However, recent two studies have successfully synthesized fcc Ru nanostructures by using pure fcc Pd nanocrystals as templates, i.e., fcc Ru nanoframe on fcc Pd truncated octahedra (Figure 4A−C) and fcc Ru cubic

the synthesis of common transition metal nanostructure with unusual crystal structure, e.g. 4H hexagonal Cu nanostructure.24 Moreover, a noble metal nanocrystal with common crystal structure can also be used as template for the synthesis of another noble metal nanocrystal with unusual crystal structure via the epitaxial growth approach.26−30 Note that the crystal structures of bulk Au, Ag, Pt, Pd, Ir, and Rh are fcc, while the bulk Os and Ru adopt the hcp structure (Table 1). In 2015, by using fcc Pt nanocrystals as templates, Lee et al. reported the epitaxial growth of fcc Ru nanostructures on their surface.26 Interestingly, the shapes of Ru nanostructures are greatly affected by those of Pt nanocrystals. The fcc Ru octahedral boxes and octapods can be synthesized when the Pt nanocubes and Zn-doped Pt concave nanocubes are used as the templates, respectively. Meanwhile, another study found that fcc Pt@Ru core−shell tetrahedra can be synthesized via a one-step hydrothermal method (Figure 3).27 Detailed growth mechanism study revealed that after single-crystalline fcc Pt nanoparticles are formed, they are used as template to direct the growth of fcc Ru nanostructures in an epitaxial way. Importantly, the ratio between Pt and Ru in fcc Pt@Ru C


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Figure 2. (A, B) TEM images, (C) SAED pattern, (D) HRTEM image, (E) HAADF-STEM image, and (F, G) the corresponding STEM-EDS elemental mappings of typical 4H/fcc Au@Ag core−shell nanoribbons. (H, I) TEM images, (J) SAED pattern, (K) HRTEM image, (L) HAADFSTEM image, and (M, N) the corresponding STEM-EDS elemental mappings of typical 4H/fcc Au@Pd core−shell nanoribbons. (O, P) TEM images, (Q) SAED pattern, (R) HRTEM image, (S) HAADF-STEM image, and (T, U) the corresponding STEM-EDS elemental mappings of typical 4H/fcc Au@Pt core−shell nanoribbons. Reprinted with permission from ref 22. Copyright 2015 Nature Publishing Group.

Figure 3. (A) TEM image of fcc Pt10@Ru90 core−shell tetrahedra. Inset: EDS spectrum of Pt10@Ru90 tetrahedra. (B, C) HRTEM images, the corresponding FFT patterns and atomic structure models of Pt10@Ru90 tetrahedra along [110] and [001] zone axes, respectively. Reprinted with permission from ref 27. Copyright 2015 American Chemical Society.

nanocages on fcc Pd nanocubes (Figure 4D-G).29,30 These experimental results suggest that other factors involved in the reaction systems, such as solvent, pressure and surfactants, may also greatly affect the epitaxial growth of noble metal nanocrystals besides the crystal lattice mismatch. Notably, since Ru owns much higher chemical stability than do the Pd− Cu and Pd, the free-standing fcc Ru nanocages and nanoframes have been obtained by selectively etching the Pd−Cu and Pd templates.28−30

reaction was reported by Xia et al.,36 this method has been widely used for the preparation of various hollow noble metal nanomaterials with definite size, composition, shape and architecture.35,37,38 Because of the interdiffusion of metal atoms, the galvanic replacement reaction is also effective to generate alloy metal nanostructures.35 Very recently, for the first time, our group reported the crystal structure-controlled synthesis of noble metal alloy nanostructures using the galvanic replacement reaction method under mild conditions.25 Typically, by using polytypic 4H/fcc Au@Ag core−shell NRBs as templates, 4H PdAg alloy nanostructures have been synthesized via the galvanic replacement reaction between Ag and Pd(NO3)2 (Figure 5A−D). Note that the reduction potentials of Pd2+/Pd and Ag+/Ag are 0.95 and 0.80 V, respectively.35 Therefore, the

2.2. Galvanic Replacement Reaction

Galvanic replacement reaction is an electrochemical process in which one metal precursor is reduced by another metal having a lower reduction potential.35 After the synthesis of Au hollow nanostructures from Ag nanocrystals with galvanic replacement D


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Figure 4. (A) TEM image of fcc Pd@Ru core-frame octahedra. Inset: atomic structure model of a Pd@Ru octahedron. (B) STEM-EDS line scanning profiles of individual Pd@Ru octahedra. (C) HAADF-STEM image of a typical Pd@Ru octahedron along the [110] zone axis. Reprinted with permission from ref 29. Copyright 2016 American Chemical Society. (D) TEM and (E, F) HRTEM images of fcc Pd@Ru core−shell nanocubes. (G) HAADF-STEM image and the corresponding STEM-EDS elemental mappings of a typical Pd@Ru nanocube. Reprinted with permission from ref 30. Copyright 2016 American Chemical Society.

Figure 5. (A−C) TEM images, (D) SAED pattern, and (E, F) HRTEM images of 4H/fcc Au@PdAg core−shell nanoribbons. (G−I) TEM images, (J) SAED pattern, and (K) HRTEM image of 4H/fcc Au@PtAg core−shell nanoribbons. (L−N) TEM images, (O) SAED pattern, and (P) HRTEM image of 4H/fcc Au@PtPdAg core−shell nanoribbons. Reprinted with permission from ref 25. Copyright 2016 American Chemical Society.

Au@Ag NRBs as templates (Figure 5L−P). Moreover, fcc PdCu@Ru yolk−shell nanoparticles have also been obtained by combination of etching and the galvanic replacement reaction between the Cu in PdCu nanoparticle templates and RuCl3.28

galvanic replacement reaction between Ag and Pd(NO3)2 is thermodynamically favored, which can happen at ambient conditions. The HRTEM images clearly show that the 4H PdAg alloy nanostructure is epitaxially grown on the Au core (Figure 5E, F). Importantly, this method can be extended to the synthesis of 4H PtAg alloy nanostructures via the galvanic replacement reaction between Ag and Pt(NO3)2 (Figure 5G− K). In addition, through the simultaneous galvanic replacement reaction of Ag with Pd(NO3)2 and Pt(NO3)2, trimetallic 4H PtPdAg alloy nanostructure can be obtained by using 4H/fcc

3. STABILITY OF NOBLE METAL NANOCRYSTALS WITH UNUSUAL CRYSTAL STRUCTURES The stability of noble metal nanocrystals with unusual crystal structures is an important prerequisite for their practical applications. On the one hand, although the unusual crystal E


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Figure 6. (A) HER polarization curves of Pd black, 4H/fcc Au@PdAg nanoribbon (NRB) and Pt black. (B) Onset potentials and overpotentials (at J = 10 mA/cm2) of Pd black, 4H/fcc Au@PdAg NRB, and Pt black. (C) Tafel plots for various catalysts derived from (A). (D) Durability test of 4H/ fcc Au@PdAg NRB: polarization curves acquired before and after 10 000 potential cycles between +0.448 and −0.152 V (vs RHE). All measurements were conducted in 0.5 M H2SO4 aqueous solution. Reprinted with permission from ref 25. Copyright 2016 American Chemical Society.

metal nanocrystals can also be remarkably enhanced via tuning their crystal structures.13 In this section, we will briefly introduce the catalytic applications of noble metal nanocrystals with unusual crystal structures in the HER, HOR, and organic reactions.

structures of noble metal nanocrystals are metastable, they can be well preserved under certain conditions. For example, we have found that the polytypic 4H/fcc structure of Au@PdAg NRBs can be maintained after 10 000 potential cycles toward the electrocatalytic HER.25 Another study showed that the chemical stability of FePt nanoparticles is significantly enhanced by changing their crystal structure from fcc to the unusual face-centered tetragonal phases.13 On the other hand, we have also observed that the hcp AuSSs undergo an hcp-tofcc structure transition upon the high-energy electron beam irradiation.20 Meanwhile, the surface ligand exchange can also induce a phase transformation of hcp AuSSs from the hcp to fcc structures.21 Besides the crystal structure effect, the structure instability of hcp AuSSs may also arise from their ultrasmall thickness.20 Therefore, further experiments on the stability of noble metal nanocrystals with unusual crystal structures are urgently required to have a comprehensive understanding on it.

4.1. Hydrogen Evolution Reaction

Hydrogen gas, which can be generated from water via the HER, has been considered as an efficient, abundant and clean energy carrier for a sustainable planet.40 Noble metal nanocrystals are active catalysts for the HER.13,40 However, owing to the scarcity and high cost of noble metals, the further improvement of their catalytic activity toward the HER is essential to their wide applications. Recent studies suggest that the catalytic properties of noble metal nanocrystals can be modulated by adjusting their crystal structures.13,15 Therefore, highly active noble metal nanocatalysts for the HER may be obtained by the rational design and synthesis of novel crystal structures. As a proof-of-concept application, our group used the polytypic 4H/fcc Au@PdAg core−shell NRBs as electrocatalysts for the HER (Figure 6).25 Impressively, it was found that the onset potential and overpotential (at 10 mA/cm2) of 4H/fcc Au@PdAg NRBs are 2.0 and 26.2 mV, respectively, which are much lower than those of the commercial Pd black (onset potential, 85.0 mV; overpotential, 135.6 mV) and even close to those of the commercial Pt black (onset potential, 0.5 mV; overpotential, 16.5 mV) (Figure 6A, B). Significantly, the 4H/fcc Au@PdAg NRBs showed a very low Tafel slope (30 mV/decade), which is also very close to that of the commercial Pt black (27 mV/decade) (Figure 6C). Moreover, the 4H/fcc Au@PdAg NRBs exhibit an excellent catalytic durability toward

4. CATALYTIC APPLICATIONS OF NOBLE METAL NANOCRYSTALS WITH UNUSUAL CRYSTAL STRUCTURES As known, the compositional architecture and alloying of noble metal nanocrystals can benefit their catalytic performance by changing their electronic structures, resulting in the shift of their d-band centers and increase of their binding energies for adsorbate precursors.1,5,33,39,40 Interestingly, the recent research has proven that the crystal structure of noble metal nanocrystals can also greatly affect their intrinsic electronic structures,12 which would open up a new avenue for further improving their catalytic activities. Furthermore, the catalytic durability of noble F


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Figure 7. (A) HOR polarization curves of fcc Pt@Ru nanocrystals/C (solid lines), as-prepared Ru/C (black dashed line), 500 °C annealed Ru/C (brown dashed line), and the commercial Pt/C (gray dashed line) obtained in 0.1 M HClO4 aqueous solution saturated with H2. (B, C) Plots of the kinetic current densities of various catalysts normalized to the total metal mass and electrochemical active surface area (ECSA) depending on overpotential in the linear-current potential region near 0 V, respectively. (D) Specific surface area (SSA) calculated from Cu underpotential deposition stripping curves and inductively coupled plasma atomic emission spectroscopy. (E) Mass- and (F) ECSA-normalized exchange current densities of various catalysts. Reprinted with permission from ref 27. Copyright 2015 American Chemical Society.

the HER as no obvious degradation of catalytic activity is observed even after 10 000 potential cycles, indicating their potential capability in practical applications (Figure 6D). The enhanced catalytic activity of 4H/fcc Au@PdAg NRBs is attributed to their unusual crystal structure, as well as their unique nanodendritic surface morphology with rich atomic steps and kinks. Very recently, another study also showed that the catalytic activity of Ru nanocrystals toward the dehydrogenation of ammonia borane can be remarkably enhanced by changing their crystal structure from hcp to fcc phases.29

mA/cm2) (Figure 7E, F). Importantly, by increasing the crystallinity of hcp Ru nanoparticles, the specific activity of hcp Ru nanoparticles/C can be increased to 0.12 mA/cm2, which is still much lower than that of the fcc Pt10@Ru90 tetrahedra/C. Theoretical calculations indicate that high crystallinity and maximum exposure of {111}f facets, which are relatively inert to the adsorption of oxygen and have moderate adsorption energy of hydrogen, impart the outstanding HOR catalytic performance of fcc Pt@Ru tetrahedra/C.27

4.2. Hydrogen Oxidation Reaction

Organic reaction is a chemical reaction that involves organic compounds. Because of the unique electronic structure of noble metals, noble metal nanocrystals have been used as catalysts in various kinds of organic reactions for long time.42 It is wellknown that the catalytic performance of noble metal nanocrystals toward organic reactions can be adjusted by changing their size, shape, composition, and architecture.11 Interestingly, recent studies reveal that the crystal structure of noble metal nanocrystals could also significantly affect their catalytic activity in organic reactions.28,29 As a typical example, Li et al. demonstrated that the catalytic behavior of Ru nanocrystals toward different hydrogenation reactions is highly dependent on their crystal structures.28 For the hydrogenation of 4-nitrochlorobenzene, the fcc Ru nanocrystals show much higher catalytic activity with over 99% conversion of 4-nitrochlorobenzene in 1 h, while only 61% conversion was obtained for the hcp Ru nanocrystals. However, for the hydrogenation of styrene, the catalytic activity of hcp Ru nanocrystals is much higher than that of the fcc Ru nanocrystals. It was observed that the conversion of styrene catalyzed by hcp and fcc Ru nanocrystals is over 98% and 53% in 4 h, respectively. This phenomenon may arise from the difference in the adsorption behavior of substrate molecules on the surface of fcc and hcp Ru nanocrystals.28 Besides

4.3. Organic Reaction

HOR is an anodic half reaction of the proton exchange membrane fuel cell, which is being considered as low pollution and high efficiency power generators and thus holds great promise to replace the conventional internal combustion engine.39 Noble metal nanocrystals, especially the Pt- and Rubased nanostructures, show high catalytic activity toward the HOR.39,41 However, previous efforts to improve the catalytic performance of the Pt- and Ru-based nanocrystals for the HOR are limited to their size, shape and composition control.39,41 Recently, Zhang et al. found that the HOR catalytic activity of noble metal nanocrystals can be significantly enhanced by simply manipulating their crystal structure as well as the crystallinity (Figure 7).27 It was observed that the fcc Pt@Ru tetrahedra/C shows much higher catalytic activity than does the hcp Ru nanoparticles/C, which is quite close to that of the commercial Pt/C catalysts (Figure 7A−C). The electrochemical active surface areas of different catalysts are shown in Figure 7D. Specifically, the fcc Pt10@Ru90 tetrahedra/C shows the highest catalytic activity among various fcc Pt@Ru tetrahedra/C catalysts, i.e., 0.19 A/mg and 0.30 mA/cm2, which are significantly higher than that of the as-prepared hcp Ru nanoparticles/C (0.016 A/mg and 0.017 mA/cm2) and comparable with the commercial Pt/C (0.23 A/mg and 0.27 G


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noble metal alloy nanocrystals with uncommon crystal structures is quite promising but remains primitive. Note that the synergistic effect between different metal elements in alloy nanocrystals can simultaneously enhance their catalytic properties and chemical stability.1,33 Third, the critical thickness with which the deposited noble metal nanostructures can follow the unusual crystal structures of templates is still not clear. A better understanding on it is of fundamental importance and could help to promote the template synthesis of noble metal nanocrystals with novel crystal structures via the precise size control of the deposited noble metal nanostructures. Fourth, the stability of noble metal nanocrystals with unusual crystal structures is an important prerequisite for their potential applications, but it is yet to be fully explored. Fifth, besides the catalysis, using noble metal nanocrystals with unusual crystal structures in many other applications, such as near-infrared photothermal therapy, SERS, waveguide, energy conversion, controlled release, biosensing, and biomedicine, is still far from investigation. Last but not the least, the yield and purity as well as the large-scale synthesis of noble metal nanocrystals with unusual crystal structures via the template synthetic strategy need to be improved and achieved in order to fully realize their practical applications. We believe that new properties and novel applications of noble metal nanocrystals could be discovered through the rational design and template synthesis of unusual crystal structures. As for the noble metal nanocrystals, although their size-, composition-, shape-, and/or architecture-based properties and applications have been well studied, we believe that their crystal phase-based properties and applications in catalysis, SERS, waveguide, photothermal therapy, chemical and biosensing, and so forth are critically important not only fundamentally, but also practically. It opens up a new and important research direction for the (noble) metal nanomaterials.

hydrogenation reactions, Xia et al. found that the crystal structure of Ru nanocrystals can also greatly affect their catalytic performance in the reduction of p-nitrophenol (Figure 8).29

Figure 8. (A, B) Comparison of the catalytic activity of fcc Ru nanoframes (NFs) and hcp Ru nanowires (NWs) toward the reduction of p-nitrophenol. The decrease of peak intensity at 400 nm suggests the reduction of −NO2 to −NH2 groups (inset in A). (C) Normalized extinction (against the initial point) at 400 nm for pnitrophenol as a function of reaction time. (D) Plots of −ln Iext versus time derived from E. Iext denotes the normalized extinction at 400 nm. Reprinted with permission from ref 29. Copyright 2016 American Chemical Society.

Briefly, the reaction catalyzed by fcc Ru nanoframes is much faster than that of the hcp Ru nanowires (Figure 8A−C). The calculated reaction rate constants for fcc Ru nanoframes and hcp Ru nanowires are about 0.022 and 0.005 min−1, respectively, indicating the superior catalytic activity of fcc Ru nanoframes over that of hcp Ru nanowires (Figure 8D).

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AUTHOR INFORMATION

Corresponding Author

5. CONCLUSIONS AND PERSPECTIVES In this Account, we have summarized the template synthesis and catalytic applications of noble metal nanocrystals with unusual crystal structures. Two kinds of template synthetic methods, i.e., epitaxial growth and galvanic replacement reaction, have been described in detail. The successful crystal structure-controlled synthesis of noble metal nanocrystals has made it possible to study the relationship between crystal structure and their catalytic property in different chemical reactions, such as HER, HOR, and organic reaction. Although the research on the template synthesis of noble metal nanocrystals with unusual crystal structures is promising, there are still many challenges and opportunities in this new research field. First of all, more novel free-standing noble metal nanocrystals with uncommon crystal structures used as templates should be discovered. Currently, templates used for the synthesis of noble metal nanocrystals with uncommon crystal structures are still quite limited. Take Au for an example, only uncommon crystal structures of hcp (the 2H type) and 4H phases have been synthesized so far.20,22 It is highly expected that novel crystal structures of Au, especially the 6H hexagonal structure that has a typical stacking order of “ABCACB’’ in the [001]6H close-packed direction, could be obtained by systematically tuning the experimental conditions like temperature, solvent and surfactant.15 Second, the template synthesis of

*E-mail: [email protected]. Website: http://www.ntu.edu.sg/ home/hzhang/. ORCID

Hua Zhang: 0000-0001-9518-740X Notes

The authors declare no competing financial interest. Biographies Zhanxi Fan received his B.S. degree in Chemistry from Jilin University in 2010, and completed his Ph.D. under the supervision of Prof. Hua Zhang at Nanyang Technological University in 2015. Currently, he is working as a Research Fellow in the same group. His research interests include the wet-chemical synthesis, characterization, and applications of low-dimensional noble metal based nanomaterials. Hua Zhang obtained his B.S. and M.S. degrees from Nanjing University in 1992 and 1995, respectively, and completed his Ph.D. with Prof. Zhongfan Liu at Peking University in 1998. As a Postdoctoral Fellow, he joined Prof. Frans C. De Schryver’s group at Katholieke Universiteit Leuven (Belgium) in 1999, and then moved to Prof. Chad A. Mirkin’s group at Northwestern University in 2001. After working at NanoInk Inc. (USA) and the Institute of Bioengineering and Nanotechnology (Singapore), he joined Nanyang Technological University in July 2006. His current research interests H


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focus on the synthesis of ultrathin two-dimensional nanomaterials (e.g., metal nanosheets, graphene, metal dichalcogenides, metal− organic frameworks, etc.) and their hybrid composites for various applications in nano- and biosensors, clean energy, (opto-)electronic devices, catalysis, and water remediation; controlled synthesis, characterizationm and application of novel metallic and semiconducting nanomaterials, and complex heterostructures; etc.

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ACKNOWLEDGMENTS This work was supported by MOE under AcRF Tier 2 (ARC 26/13, No. MOE2013-T2-1-034; ARC 19/15, No. MOE2014T2-2-093; MOE2015-T2-2-057), AcRF Tier 1 (RG5/13), and NTU under Start-Up Grant (M4081296.070.500000) in Singapore.

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Template Synthesis of Noble Metal Nanocrystals with Unusual Crystal

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