Shaping Gold Nanocrystals in Dimethyl Sulfoxide: Toward

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Shaping Gold Nanocrystals in Dimethyl Sulfoxide: Toward Trapezohedral and Bipyramidal Nanocrystals Enclosed by {311} Facets Wenxin Niu,*,† Yukun Duan,‡ Zikun Qing,‡ Hejin Huang,‡ and Xianmao Lu*,§,⊥ †

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore § Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China ⊥ National Center for Nanoscience and Technology (NCNST), Beijing 100190, China ‡

S Supporting Information *

ABSTRACT: The remarkable synthetically tunable structural, electronic, and optical properties of gold nanocrystals have attracted increasing interest and enabled multidisciplinary applications. Over the past decades, nearly all the possible fundamental shapes of faceted Au nanocrystals have been synthesized, except for only one missingthe trapezohedron enclosed by {hkk} facets. In this report, the unprecedented synthesis of trapezohedral Au nanocrystals with {311} crystal facets was realized. Dimethyl sulfoxide (DMSO) was discovered as a solvent for shaping Au nanocrystals with {311} crystal facets for the first time. Mechanistic studies, together with previous DFT and STM studies, attribute the unique role of DMSO to its ambidentate nature, where both sulfur and oxygen of DMSO can coordinate to gold surface, endowing its unique role in stabilizing high-index {311} facets through a “two center bonding” mode. The DMSO-based synthesis provides a new synthetic tool toward the synthesis of a series of unreported Au nanocrystals with new structures. In particular, a new type of gold bipyramids, the octagonal bipyramids, was first synthesized with additional plasmonic tunability while simultaneously retaining their {311} facets. The application of these new Au nanocrystals in surface-enhanced Raman scattering spectroscopy was investigated, and their shape-dependent performances were demonstrated. These results highlight the tremendous potential of using ambidentate molecules as shape- and surface-directing agents for metal nanocrystals and offer the promise of enabling new synthetic tools toward atomically precise control of surface structures of metal nanocrystals.

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{hkl} facets24,25 have been reported (h > k > l > 0). Trapezohedral Au nanocrystals enclosed by {hkk} facets, which is the only missing basic shape of gold nanocrystals, remains one of the key unsolved problems of gold nanocrystal synthesis. This challenge stresses the need to develop new strategies for controlling the surface facets of Au nanocrystals. During the past decades, increasing attention is being devoted to develop new strategies to control the surface structures of noble metal nanocrystals,26,27 especially for those enclosed by high index facets.28−30 Sun et al. developed an electrochemical strategy and opens up the possibility of rationally synthesizing high-index noble metal nanocrystals.31,32 In this approach, repetitive oxygen adsorption/desorption stimulated by electrochemical square wave-potential treatment was used to promote the formation of step or kink sites on the high-index facets.33 While the electrochemical method achieved tremendous success for the synthesis of platinum-group metal nanocrystals,34 it is still

INTRODUCTION The remarkable synthetically tunable structural, electronic, and optical properties of gold nanocrystals have attracted increasing interest over the past decades. As a result, exquisite control of their properties over the size, shape, and surface of gold nanocrystals has been achieved in recent years.1−5 Owing to these accomplishments, gold nanocrystals have found widespread applications in catalysis, plasmonics, spectroscopy, biomedicine, and energy conversion.6−12 Precise control of exposed facets in Au nanocrystals is pivotal in formulating structure−activity relationships in many of these applications.13 Toward this goal, intense research efforts have been devoted to the synthesis of Au nanocrystals of controllable facets. Figure 1 summarizes the theoretically predicted geometric models of isotropic single-crystalline Au nanocrystals enclosed by different low-index and high-index crystal facets. Among these theoretically predicted Au nanocrystals, Au octahedron enclosed by {111} facets,14,15 cube by {100} facets,16,17 rhombic dodecahedron by {110} facets,18,19 trisoctahedron by {hhk} facets,20,21 tetrahexahedron by {hk0} facets,22,23 and hexoctahedron by © 2017 American Chemical Society

Received: January 2, 2017 Published: April 6, 2017 5817

DOI: 10.1021/jacs.7b00036 J. Am. Chem. Soc. 2017, 139, 5817−5826

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single-crystalline nanorods as seeds, providing a method of imparting additional plasmonic tunability to Au nanocrystals while simultaneously retaining their {311} high-index crystal facets.

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EXPERIMENTAL SECTION

Chemicals and Materials. Dimethyl sulfoxide (DMSO), dimethyl sulfone (DMSO2), gold(III) chloride trihydrate (HAuCl4·3H2O), formic acid, silver nitrate (AgNO3), polyvinylpyrrolidone (PVP, average Mw ∼55 000), polydiallyldimethylammonium chloride (PDDA, average Mw 200 000−350 000, 20 wt % in H2O), cetylpyridinium chloride (CPC), ascorbic acid, potassium bromide (KBr), hexadecyltrimethylammonium bromide (CTAB), hydroquinone, and 4-nitrothiophenol were purchased from Sigma-Aldrich. N,N-Dimethylformamide (DMF) was purchased from Merck. Ethylene glycol (EG) was obtained from J. T. Baker. All chemicals were used as received without further purification. Synthesis of Cubic Au Seeds. CPC-capped Au seeds were synthesized according to procedures published elsewhere.19,64 For the growth of cubic Au seeds, 5 mL of 0.1 M KBr solution, 10 mL of 10 mM HAuCl4 solution, 2.5 mL of 0.1 M ascorbic acid solution, and 10 mL of the CPC-capped Au seed were subsequently added to 500 mL of 0.1 M CPC solution under gentle stirring. The reaction products were centrifuged after 30 min and redispersed in 250 mL solution containing 2 mM CTAB and 60 mM PVP. After overnight incubation in a 40 °C water bath, the nanocubes were then washed with ethanol twice and finally dispersed in 10 mL of 0.1 M PVP solution in DMSO. Synthesis of Octahedral Au Seeds. Au octahedra seeds were prepared by adopting published methods with slight modification.65,66 Typically, 25 μL of 0.5 M HAuCl4 solution and 200 μL of PDDA solution were added to 10 mL EG under stirring. The resulting mixture was heated in an oil bath of 200 °C for 30 min. The Au octahedra seeds were diluted to 25 mL with 0.1 M PVP aqueous solution. Then the mixture was centrifuged and washed with 10 mL PVP aqueous solution. Finally, the Au octahedra seeds were redispersed in 3 mL of 0.1 M PVP solution in DMSO. Synthesis of Au Trapezohedral Nanocrystals. In a typical synthesis, 1.2 g of PVP was dissolved in 14 mL of DMSO in a 20 mL glass vial. Next, 250 μL of formic acid and 20 μL of 0.5 M HAuCl4 aqueous solution were added and the mixture was heated at 120 °C for 10 min. Subsequently, 2 mL of the Au cubic seeds or 1 mL of the octahedral Au seeds was added. Then the reaction was heated at 120 °C for another 20 min. The products were centrifuged and washed with ethanol twice for further characterization. Synthesis of Au Nanorods as Seeds for Au Bipyramids. Gold seeds for Au nanorods were prepared as described elsewhere.67 To synthesize Au nanorods, AgNO3 solution (1.75 mL, 10 mM) and HAuCl4 solution (2.5 mL, 10 mM) were added to 45 mL of 0.1 M CTAB solution, followed by the addition of hydroquinone solution (2.5 mL, 0.1 M). The resulting mixture was gently mixed by inversion until it became colorless. Subsequently, 0.8 mL of the as-prepared Au seed solution was added, and the reaction was allowed to react at 30 °C overnight. The nanorods were centrifuged, treated with the same procedures of the cubic Au seeds, and finally dispersed in 10 mL of 0.1 M PVP solution in DMSO. Synthesis of Single-Crystalline Au Bipyramids. In a typical synthesis, 1.2 g of PVP was dissolved in 14 mL of DMSO. Next, 250 μL of formic acid and 20 μL of 0.5 M HAuCl4 was added to the mixture. The mixture was then heated at 120 °C for 10 min before 1 mL of the Au nanorod seeds was added. The mixture was subsequently heated at 120 °C for 20 min. The products were collected by centrifugation and washed with ethanol twice for further characterization. Preparation of SERS Substrates. Five μL of Au trapezohedra or bipyramid solution was dropped on a silicon wafer. After dried in room temperature, the substrates were incubated in 0.4 mL of 10 mM 4nitrothiophenol ethanolic solution for 1 h. The substrate was then rinsed with copious amounts of ethanol and dried under nitrogen flow. Instrumentation. Scanning electron microscopy (SEM) studies were performed on a JEOL JSM-6700F SEM operating at 20 kV.

Figure 1. Geometric models of Au nanocrystals enclosed by different crystal facets: octahedron, red; cube, blue; rhombic dodecahedron, green; trapezohedron, purple; trisoctahedron, yellow; tetrahexahedron, cyan; hexoctahedron, gray.

limited in the case of gold. In recent years, tuning reaction kinetics has become another versatile approach to synthesize metal nanocrystals with thermodynamically unstable facets.35−38 Fast reduction rates, which will lead to higher supersaturation, can result in the formation of nanocrystals with high surface energy, as deduced by Xie et al.39 For example, trisoctahedral Au nanocrystals enclosed by {221} facets and hexoctahedral Au nanocrystals enclosed by {321} facets have been synthesized under fast reduction kinetics.20,24,35,40 Another compelling strategy of controlling the surface structures of noble metal nanocrystals is based on the strong and specific surface adsorption of small adsorbates on metal nanocrystals.41−45 A typical example is the underpotential deposition (UPD) of silver on Au nanocrystals.46−48 The UPD of silver atoms prefers to occur on the high-energy facets of Au with more open surface structures, and thus stabilizes the asformed high-indexed facets.49 Au nanocrystals with different types of {hk0} facets have been synthesized through this approach.22,50−53 Small molecules such as carbon monoxide,54−56 alkylamine,57−59 oxalate,60,61 and nitrogen dioxide,62 represent another important class of small adsorbates in synthesizing noble metal nanocrystals of different facets. However, Most of these molecules can only stabilize low-index facets, except for alkylamines that have been recently demonstrated as ligands to stabilize the {hkk} facets of platinum58,59 and the {hkl} facets of gold.63 While these strategies have advanced the facet control of nanocrystals and envisioned many applications, the synthesis of {hkk} facets of gold still constitutes a formidable challenge, which may demand an ability to discover new interfacial reactions beyond our present capabilities. In this paper, the exploitation of dimethyl sulfoxide (DMSO) as a solvent for Au nanocrystal growth was investigated and the unique role of DMSO molecules in stabilizing the Au {311} facets was discovered, which results in the unprecedented synthesis of trapezohedral Au nanocrystals enclosed by {311} facets. As an ambidentate molecule, DMSO has both sulfur and oxygen coordination to transition metals, endowing its unique role in stabilizing high-index {311} facets. On the basis of our discovery, a new form of single-crystalline Au bipyramids enclosed by {311} facets were further discovered by exploiting 5818

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Figure 2. SEM images of trapezohedral Au nanocrystals. (a) A large-scale SEM image of ordered assembly of trapezohedral Au nanocrystals and corresponding FFT pattern inset. (b−d) SEM images of trapezohedral Au nanocrystals at different magnification. (e−l) SEM images of trapezohedral Au nanocrystals of different orientations and their corresponding geometric models: (e, i) a trapezohedral nanocrystal lying with one of its faces parallel to the substrate, (f, j) a trapezohedral nanocrystal viewed from its 4-fold axis, (g, k) a trapezohedral nanocrystal viewed from its 3-fold axis, (h, l) a trapezohedral nanocrystal viewed from its 2-fold axis. An animation showing a rotating trapezohedral nanocrystal is also available in Supporting Information. Scale bars: (a) 500 nm, (b) 200 nm, (c, d) 100 nm, (i−l) 50 nm.

induce deep restructuring of gold.72,73 In addition, thiol ligands are notoriously difficult to remove once they are adsorbed to the surface of gold, often blocking chemical reactions at the surface and limit the application of gold nanocrystal in many fields.74 Another interesting but scarcely demonstrated example of organosulfur compound in the field of gold nanocrystal synthesis is DMSO. In contrast to thiol ligands, DMSO only has a moderate bonding affinity to gold through electron lone-pairs on both the oxygen and sulfur atoms.75 As a popular polar and aprotic solvent, DMSO is advantageous for nanoparticle syntheses in several respects. DMSO can dissolve both polar and nonpolar compounds, and is miscible in a wide range of organic solvents as well as water.76 In addition, it has low toxicity and a relatively high boiling point.77 Owing to these merits, DMSO has been tentatively used as a solvent for metal nanoparticle synthesis.78−81 In spite of these reports, DMSO has not been used to control the facets of metal nanocrystals; and their interaction with different crystals facets has yet to be explored. Herein, a seed-mediated growth approach was exploited to examine DMSO as a solvent for the growth of gold nanocrystals.

Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, and selected-area electron diffraction (SAED) were acquired using a JEOL JEM-2100F TEM operating at 200 kV. Xray diffraction (XRD) patterns were obtained on a Bruker D8 Discover with GADDS and Cu Kα radiation. UV−visible (UV−vis) spectra were recorded using a Shimadzu UV-1800 spectrometer with plastic cuvettes of 1 cm path length at room temperature. Surface-enhanced Raman studies (SERS) were performed on an XploRA PLUS Raman microscope (Horiba/JY, France) with an excitation source of a 785 nm laser. The power on the sample was set at about 0.1 mW. A 100× magnification long working distance (8 mm) objective was used to collect SERS signals.

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RESULTS AND DISCUSSION Synthesis and Structure of Trapezohedral Au Nanocrystals. Organosulfur compounds represent an important family of reagents during the development of gold nanocrystals.68−70 In particular, thiol ligands are commonly used as stabilizing agents which bind to the surface of the Au nanocrystals by forming gold−thiolate bonds. Thiol ligands, however, usually provide little control over the atomic structures of nanoparticle surfaces,71 due to their strong affinity to gold and capacity to 5819

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Figure 3. (a) A TEM image of trapezohedral Au nanocrystals. (b, c) A TEM image of a single trapezohedral Au nanocrystal lying flatly on the copper grid and its corresponding geometric model, (d) A SAED pattern obtained from the trapezohedral nanocrystal in (b), showing a typical diffraction pattern along the [113] direction, (e, f) A TEM image of a trapezohedral nanocrystal with four of its {311} faces aligned perpendicular to the copper grip and its corresponding geometric model. (g, h) A HRTEM image of the {311} facet of the trapezohedral nanocrystal in (e) and its corresponding atomic model. Scale bars: (a) 200 nm, (b, e) 50 nm, (g) 1 nm.

inspection of the SEM images reveals important geometric characteristics of the Au nanocrystals. Each of the Au nanocrystals are enclosed by characteristic kite-shaped faces, which is a typical feature of trapezohedral nanocrystals enclosed by {hkk} facets (h > k > 0) for face-centered cubic metals.29,34 Geometrically, trapezohedron is an unconventional polyhedron with 24 equal kite-shaped faces.85,86 A trapezohedron has 48 edges and 26 vertices.87 Moreover, it has 3 different types of axes: 6 of 4-fold axes (Figure 2f), 8 of 3-fold axes (Figure 2g), and 12 of 2-fold axes (Figure 2h). According to these structural features of trapezohedra, models of trapezohedra enclosed by different {hkk} (h > k > 0) facets were theoretically constructed (Figure S5). By comparison with these built models, it is possible to determine the exact Miller indices of the trapezohedral Au nanocrystals. Single-particle SEM images of trapezohedral nanocrystals of different orientations were obtained and shown in Figure 2i−l. For a trapezohedral nanocrystal lying with one of its faces parallel to the substrate, it shows a roughly elongated octagonal shape, which matches well the trapezohedral models enclosed by {311} facets in Figure 2e. Furthermore, trapezohedral nanocrystals viewed from their 4-fold, 3-fold, and 2-fold axes were also examined by SEM and compared with trapezohedral models enclosed by {311} facets, respectively. The close similarity further suggests the trapezohedral Au nanocrystals are enclosed by {311} facets. To confirm the proposed structure, the trapezohedral Au nanocrystals were also thoroughly characterized by TEM. Selected area electron diffraction (SAED) patterns from a flatly lying trapezohedral Au nanocrystal (i.e., with one of its faces parallel to the substrate) can be indexed to face-centered cubic gold along the [113] zone axis (Figure 3b−d), providing an unambiguous evidence that the trapezohedral nanocrystals are enclosed by {311} facets. HRTEM was used to directly image the

Specially, PVP (as a stabilizing reagent), formic acid (as a mild reducing agent), and gold(III) chloride were first added to DMSO and heated at 120 °C for 10 min. During this induction period, Au(III) was reduced to Au(I) by formic acid. Subsequently, cubic Au seeds (Figure S1) were introduced to initiate the growth of trapezohedral nanocrystals. A mild reducing agent is necessary because DMSO did not show reducing capability in the present synthetic conditions (Figure S2a). Note that formic acid is not essential for the growth of trapezohedral nanocrystals, replacing formic acid with another mild reducing agent, PVP with lower molecular weight (10k),82,83 could yield similar results of trapezohedral nanocrystals (Figure S3), albeit with slightly truncation and lower uniformity. High-quality trapezohedral Au nanocrystals were favored at a high concentration of PVP and a relatively low reaction temperature. A high concentration of PVP can prevent the aggregation of nanocrystals during the growth process. Deceasing the concentration of PVP led to the formation of aggregated nanocrystals (Figure S2b and S2c). At a relatively low reaction temperature (100 °C), well-defined trapezohedral nanocrystals were still obtained with longer reaction time (Figure S2d and S2e). While at a higher reaction temperature (140 °C), trapezohedral nanocrystals were obtained with many small nanoparticles attached to their surfaces due to spontaneous nucleation (Figure S2f). Representative SEM images of the trapezohedral Au nanocrystals were shown in Figure 2. It was clearly revealed that the asobtained Au nanocrystals have remarkable monodispersity (Figure 2a and S4). In addition, all the nanocrystals have welldefined shapes and crystal facets. Although the trapezohedral nanocrystals are not energetically favorable to form a hexagonal close-packed array,84 they can still form glassy close-packed array, evidenced by the FFT pattern in the inset of Figure 2a. Closer 5820

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octahedral Au nanocrystals show exclusively strong (200) and (111) reflections, indicating they are enclosed by {100} and {111} facets, respectively. These results collectively uncover the formation of {311} facets on the trapezohedral Au nanocrystals. Growth Process of Trapezohedral Au Nanocrystals. Both cubic and octahedral Au nanocrystals (Figure S7) can been used as seeds for the growth of trapezohedral Au nanocrystals (Figure 5). To better understand the growth process of the

atomic arrangement of the {311} facets of the trapezohedral nanocrystals. For this particular purpose, a trapezohedral nanocrystal projected along the [110] direction was selected (Figure 3e, f), which allows 4 of its {311} facets aligned parallel to the electron beam of TEM (i.e., perpendicular to the copper grid). In this respect, we can use HRTEM to direct study the atomic arrangement of {311} facets, as shown in Figure 3g. On the basis of the microfacet notation,88 Au {311} facets can be expressed as Au(s)-[2(100) × (111)], indicating a stepped surface composed of a terrace of 2 atomic width of (100) symmetry, separated by a monatomic step of (111) symmetry (Figure 3h). As illustrated in Figure 3g, the {311} crystal facet made of subfacets of (111) and (100) planes can be clearly observed and match well with the corresponding atomic model in Figure 3h. The Miller indices of the trapezohedral Au nanocrystals were also determined by measuring the dihedral angles of a trapezohedral Au nanocrystals viewed from its 4-fold axis (Figure S6). The measured α and β are 143.17° and 126.83°, respectively, are in excellent agreement with the theoretical values of 143.2° and 126.8° calculated from {311} crystal facets.85 Taken together, these observations confirmed that the trapezohedral nanocrystals are enclosed by well-defined {311} facets. X-ray powder diffraction was also used to investigate the structural information on the trapezohedral Au nanocrystals. On the basis of Bragg’s Law, the (311) plane of face-centered cubic gold with all odd h, k, and l Miller indices (h > k > l > 0) is one of the allowed reflections in the XRD patterns of gold. Essentially, a pronounced (311) diffraction peak was observed in the XRD patterns of trapezohedral NCs, indicating the abundance of {311} crystal facets of the trapezohedral Au nanocrystals (Figure 4). Compare with trapezohedral nanocrystals, the cubic and

Figure 5. Shape evolution of trapezohedral gold nanocrystals. (a−d) Evolution from cubic Au seeds, reaction time: 0 min, 8 min, 14 min, 20 min, respectively. (e−h) Evolution from octahedral Au seeds, reaction time: 0 min, 10 min, 16 min, 20 min. Scale bars: 50 nm.

trapezohedral Au nanocrystals, the temporal shape evolution during the growth of trapezohedral Au nanocrystals was monitored by SEM. In the case of cubic Au seeds, the nanocubes first grew into nearly spherical shapes with multiple small facets to minimize their surface energy (Figure 5b). Due to the stabilizing effect of PVP,15,89,90 {111} facets were also observed. As the reaction proceeded, the {311} facets began to dominate the nanocrystals and leaving only a small portion of {110} and {111} facets (Figure 5c). Finally, the {110} and {111} faces gradually disappeared and trapezohedral nanocrystals with smooth faces and well-defined edges and corners were obtained (Figure 5d). In the case of octahedral seeds, the nanocrystal started with growing square pyramids with 4 {311} facets on each vertices of the octahedral seeds, resulting in the transformation of each triangle {111} faces of the octahedral seeds into a hexagon (Figure 5f). As the reaction time was extended, the {311} facets became larger and the hexagonal {111} facets first shrunk into triangles (Figure 5g) and then full disappeared (Figure 5h). In both cases, DMSO has played a decisive role in the final formation of {311} facets. Formation Mechanism of Trapezohedral Au Nanocrystals. In order to elucidate the role of DMSO for the formation of trapezohedral nanocrystals, two different solvents with similar structures or properties to DMSO, were also used for the growth of Au nanocrystals. The first one is dimethyl sulfone (DMSO2), which is also known as methylsulfonylmethane.91 Compared with DMSO, the central sulfur atom of the DMSO2 molecule is twice double bonded to oxygen atoms, thus blocking the direct interaction of sulfur atom with gold surface. Figure 6d shows that cuboctahedral Au nanocrystals enclosed by lowindexed {111} and {100} facets were obtained by using DMSO2 as the solvent (Figure 6d−f). In the case of N,N-dimethylformamide (DMF),92 a similar polar aprotic solvent to DMSO, polyhedral nanocrystals with shapes close to the small rhombicuboctahedron were obtained, which is enclosed by low-indexed {111}, {100}, and {110} facets (Figure 6g−i). These results highlight the unique capability of sulfoxide group in

Figure 4. Normalized XRD patterns obtained from trapezohedral, cubic, and octahedral Au nanocrystals. The major peaks of the samples have been labeled by different colors indicating the abundance of the respective facets. 5821

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conditions do not involve specific reagents to stabilize the highenergy (110) subfacets, the Au (331) and (310) facets may not be favored in the current system. Synthesis and Structure of Au Bipyramids Enclosed by {311} Facets. During the past two decades, significant attention has been given to the study of the plasmonic properties of Au nanocrystals as a result of their countless new applications based on surface plasmon resonance.7,10,97 Among different plasmonic nanostructures, Au nanobipyramids exhibit several key advantageous properties over others, such as strong local electric field enhancements, suppressed plasmonic spectral broadening, and high refractive index sensitivities.46,98,99 Benefiting from these merits, Au nanobipyramids exhibited better surface-enhanced Raman scattering (SERS) enhancements than Au nanorods.98 Despite these desirable features, current synthetic methods toward Au nanobipyramids either lead to bipyramids with relatively low purity with large spectral broadening or need complicated or multistep separation schemes,53,98,100−102 which stresses the need to develop new synthetic approaches. Herein, single-crystalline Au nanorods67 (Figure S8) were exploited as seeds for the growth of Au nanocrystals under identical conditions as the synthesis of trapezohedral nanocrystals. Figure 7 shows a schematic illustration of a possible pathway

Figure 6. Molecular models of different solvents and their use in the growth of Au nanocrystals, respectively: (a−c) DMSO, (d−f) DMSO2, (g−i) DMF. Atoms in the models are represented as spheres with different color coding: hydrogen (white), carbon (gray), oxygen (purple), sulfur (yellow), nitrogen (green). Scale bars: (b, e, h) 100 nm, (c, f, i) 50 nm.

DMSO molecule in the formation of {311} high-index facets, indicating that both the sulfur and oxygen atoms should be accessed during the growth of trapezohedral nanocrystals. DMSO is an ambidentate ligand of a pyramidal form with an oxygen atom, a sulfur atom, and two methyl groups at the apexes.93 The lone-pair electrons on both the sulfur and oxygen atoms are capable of coordinating to transition metals.94,95 Previous scanning tunneling microscopy (STM) studies show DMSO molecules preferentially decorated the step edges of the Au (111) surfaces, suggesting DMSO molecules have stronger affinity to step atoms with low coordination number.75 A more recent study gives a clearer image of the interactions between gold and DMSO.96 For close-packed Au (111) planes, density functional theory (DFT) calculations indicate that the sulfur atom of DMSO binds much stronger to the metal surface than the oxygen atom, and the most favorable adsorption configuration for a single adsorbed DMSO is the on-top site, with the sulfur atom bound to the metal surface. In contrast, for gold planes with low-coordinated Au atoms, DMSO molecules tend to interact with low-coordinated Au atoms in a “two center bonding” mode, where lone-pair electrons on the O and S atoms interact with the low-coordinated Au atom and the terrace atom on (111) plane, respectively. Such a stable configuration has been verified by both DFT calculations and STM studies.96 Note that electron lone-pair of sulfur atom contributes primarily to the capping effect; therefore, DMSO2 cannot not facilitate the formation of high-index facets. These studies suggest that DMSO molecules can preferentially stabilize the low-coordinated Au atoms of the {311} facets, decrease the surface energy of {311} facets, and thus make trapezohedral Au nanocrystals a favorable shape. Compared with the {311} facets, other similar stepped facets such as the {331} and {310} facets have (110) planes as subfacets (Table S1), which have much higher surface energy than the (111) and (100) planes.18,19 Since the current synthetic

Figure 7. Schematic illustration of growing nanorods into bipyramids and the elongation of trapezohedra into bipyramids, as well as a typical model of a bipyramid and its cross-section view. An animation showing a bipyramid built from a trapezohedron is available in Supporting Information.

of growing nanorods into Au nanocrystals enclosed by {311} facets. Single-crystalline Au nanorods may induce the formation of elongated {311} facets on their side facets and lead to the formation of Au bipyramids. As depicted in Figure 7, such an octagonal bipyramid can be built from a trapezohedron by elongation along one of its 4-fold axis. For clarity, two vertices lie on the 4-fold axis are indicated as vertices A and B in Figure 7. During the elongation, 4 faces sharing vertex A disappear, while the adjacent 8 faces of these 4 faces extend along the 4-fold axis and meet at one point, which leads to the formation of one sharp end of the bipyramid. Similar transformation on vertex B leads to the formation of the other end of the bipyramid. As a result, this new type of bipyramid has an octagonal cross-section. SEM and TEM images of the Au nanobipyramids are shown in Figure 8 and S9. Each nanobipyramid is symmetrical and wellfaceted. SEM images of single Au bipyramid match well with our proposed model with slight truncation, proves that each bipyramid has an octagonal cross section and 16 identical side faces (Figure 8c−f). Single-particle TEM, SAED, and HRTEM analysis of an Au bipyramid viewed along the [100] direction is shown in in Figure 8i, j. Both the continuous crystal lattice and the characteristic square spot array of the SAED pattern prove that the Au bipyramid is of single-crystalline nature. SAED of a single Au bipyramid with one of its facets parallel to copper grid shows a typical diffraction pattern along the [113] direction, confirming that Au bipyramids are essentially enclosed by {311} 5822

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Figure 8. Growth of single-crystalline octagonal Au bipyramids in DMSO: (a, b) SEM images of Au bipyramids, (c−f) SEM images of Au bipyramids of different orientations and their corresponding geometric models, (g, h) TEM images of Au nanobipyramids, (i, j) TEM and HRTEM images of a single Au bipyramid view in the [001] direction. The inset shows the corresponding SAED pattern along the [001] direction. (k, l) TEM and HRTEM images of a single Au bipyramid view in the [113] direction. The inset shows the corresponding SAED pattern along the [113] direction. Scale bars: (a, g) 100 nm, (b) 50 nm, (e, f, h, i, k) 20 nm, (j, l) 5 nm.

facets (Figure 8k, l). In contrast to previous reports of Au bipyramids,46,53,98,102 which all are multiple-twinned; the present bipyramids represent a new form of single-crystalline bipyramids free of twin defects. Compared with previous synthetic methods,44,51,94,98 the current method allows the creation of bipyramids without introducing any foreign metals, which could affect the intrinsic properties of high index gold facets. Plasmonic Properties of Trapezohedral and Bipyramidal Au Nanocrystals. The present method toward Au bipyramids successfully imparts additional plasmonic tunability to Au nanocrystals while simultaneously retaining their {311} high-index crystal facets. Compared with trapezohedral Au nanocrystals, the single-crystalline Au bipyramids have two plasmonic peaks (Figure 9a), which are arisen from the longitudinal and transverse modes of elongated Au nanocrystals, respectively.98 In addition, the single-crystalline Au nanobipyramids have much narrower full width at half-maximum (fwhm) values (96.8 nm) than that of trapezohedral Au nanocrystals (212.9 nm). More importantly, the fwhm of the Au bipyramids is also smaller than that of well-studied Au nanorods.98 For plasmonic applications, Au nanobipyramids with sharp tips are supposed to have strong local electromagnetic

Figure 9. (a) Normalized UV−vis spectra of trapezohedral and bipyramidal Au nanocrystals, respectively. (b) SERS spectra of 4nitrothiophenol on monolayer assembly of trapezohedral and bipyramidal Au nanocrystals, respectively.

enhancement and may have better performance in SERS. To test this hypothesis, SERS substrates were prepared by forming nanocrystal monolayer of Au trapezohedra (Figure S4a, S4b) and bipyramids (Figure S9a, S9b) on silicon, respectively. The extinction spectra of the monolayers of trapezohedral and bipyramidal nanocrystals on the substrates show strong extinction in the near-IR range due to plasmon coupling 5823

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Journal of the American Chemical Society effects,103 therefore, a 785 nm laser was used for the SERS studies (Figure S10). In addition, the use of the 785 nm laser can minimize the interband transition of gold and avoid plasmon damping of gold nanostructures, and thus is a better choice for SERS studies of gold nanostructures.104−106 4-Nitrothiophenol was used as probe molecules and adsorbed to the SERS substrates. As shown in Figure 9b, the Au nanobipyramid monolayer have stronger SERS signal than the trapezohedra monolayer. The average EF for the trapezohedra monolayer is calculated to be 2.45 × 106, while the average EF for the bipyramid monolayer is calculated to be 7.85 × 107. The density of hot spots from nanocrystal coupling was counted for each monolayer as well. The density of coupling hotspots for the bipyramid monolayer is 2.8 times as large as that of the trapezohedra monolayer. In comparison, the SERS enhancement factor of the bipyramid monolayer is 32 times as large as that of the trapezohedra monolayer. This comparison suggests that in addition to a higher density of coupling hot spots, the stronger local electromagnetic enhancement from plasmonic coupling of bipyramids with sharp tips107 plays a major role in the high SERS performance of the bipyramid monolayer.

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Wenxin Niu: 0000-0002-0835-3295 Yukun Duan: 0000-0003-3299-0822 Xianmao Lu: 0000-0002-7422-2867 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ministry of Education Singapore (Grant# R279-000391-112), Singapore National Research Foundation (Grant# R279-000-337-281), and French-Singaporean joint MERLION program (Grant# R279-000-334-646) for the financial support of this work.

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CONCLUSIONS In conclusion, dimethyl sulfoxide was investigated as a solvent for the shape control of Au nanocrystals for the first time. Through a simple seed-mediated growth process, the unprecedented synthesis of trapezohedral Au nanocrystals was demonstrated. Extensive structural characterization unambiguously demonstrates the formation of {311} high-index facets. Though control experiments with other solvents, we revealed that the sulfinyl group in DMSO is essential for the formation of {311} facets. Combined with previous DFT calculations and STM studies, we proposed that DMSO molecules can stabilize low-coordinated Au atoms on the {311} facets in a “two center bonding” mode, where lone-pair electrons on the O and S atoms are simultaneously interacting with the low-coordinated Au atom and the terrace atom, respectively. Moreover, by exploiting single-crystalline nanorods as seeds, the unexpected formation of a new form of Au bipyramids was first demonstrated, featuring their narrow full width at half-maximum and high SERS performance. These results highlight the tremendous potential of using ambidentate molecules as shape- and surface-directing agents for metal nanocrystals. We envisioned that by introducing multiple potential donor atoms in shape-directing ligands,41,108 a new family shape-directing reagents for synthesizing metal nanocrystals with otherwise inaccessible features can be discovered and these reagents may forward the atomically precise control of surface structures of metal nanocrystals.

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Video S1: Animation showing a rotating trapezohedron (AVI) Video S2: Animation showing an octagonal bipyramid built from a trapezohedron (AVI)

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b00036. TEM and SEM images of cubic and octahedral seeds, geometric models of trapezohedral Au nanocrystals enclosed by different {hkk} facets, large scale SEM images of trapezohedral and bipyramidal Au nanocrystals, SEM images of Au trapezohedral nanocrystal synthesized using octahedral seeds, SEM images of Au trapezohedral nanocrystal synthesized using PVP 10K as the reducing agent, and TEM images of single-crystalline Au nanorods (PDF) 5824

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