Enriching Silver Nanocrystals with a Second Noble Metal - Accounts of

Jul 5, 2017 - Yiren Wu is pursuing her Ph.D. in Materials Science and Engineering (MSE) at Georgia Tech. She received her B.E. in Polymer Science and ...
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Enriching Silver Nanocrystals with a Second Noble Metal Yiren Wu,† Xiaojun Sun,† Yin Yang,‡ Jumei Li,§ Yun Zhang,† and Dong Qin*,† †

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 211816, P. R. China § School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan, Hubei 430205, P. R. China ‡

CONSPECTUS: Noble-metal nanocrystals have received considerable interests owing to their fascinating properties and promising applications in areas including plasmonics, catalysis, sensing, imaging, and medicine. As demonstrated by ample examples, the performance of nanocrystals in these and related applications can be augmented by switching from monometallic to bimetallic systems. The inclusion of a second metal can enhance the properties and greatly expand the application landscape by bringing in new capabilities. Seeded growth offers a powerful route to bimetallic nanocrystals. This approach is built upon the concept that preformed nanocrystals with uniform, well-controlled size, shape, and structure can serve as seeds to template and direct the deposition of metal atoms. Seeded growth is, however, limited by galvanic replacement when the deposited metal is less reactive than the seed. The involvement of galvanic replacement not only makes it difficult to control the outcome of seeded growth but also causes degradation to some properties. We have successfully addressed this issue by reducing the salt precursor(s) into atoms with essentially no galvanic replacement. In the absence of self-nucleation, the atoms are preferentially deposited onto the seeds to generate bimetallic nanocrystals with controlled structures. In this Account, we use Ag nanocubes as an example to demonstrate the fabrication of Ag@M and Ag@Ag−M (M = Au, Pd, or Pt) nanocubes with a core−frame or core−shell structure by controlling the deposition of M atoms. A typical synthesis involves the titration of Mn+ (a precursor to M) ions into an aqueous suspension containing Ag nanocubes, ascorbic acid, and poly(vinylpyrrolidone) under ambient conditions. In one approach, aqueous sodium hydroxide is introduced to increase the initial pH of the reaction system. At pH = 11.9, ascorbic acid is dominated by ascorbate monoanion, a much stronger reductant, to suppress the galvanic replacement between Mn+ and Ag. In this case, the M atoms derived from the reduction by ascorbate monoanion are sequentially deposited on the edges, corners, and side faces to generate Ag@M core−frame and then core−shell nanocubes. The other approach involves the use of ascorbic acid as a relatively weak reductant while Mn+ is cotitrated with Ag+ ions in the absence of sodium hydroxide. At pH = 3.2, when the molar ratio of Ag+ to Mn+ is sufficiently high, the added Ag+ ions can effectively push the galvanic reaction backward and thus inhibit it. As a result, coreduction of the two precursors by ascorbic acid produces Ag and M atoms for the generation of Ag@Ag−M core−frame nanocubes with increasingly thicker ridges. The Ag@Ag−Pd core−frame nanocubes can serve as a dual catalyst to promote the stepwise reduction of nitroaromatics to aminoaromatics and then oxidation to azo compounds. The consecutive reactions can be monitored using surface-enhanced Raman scattering (SERS). The Ag@Au core−shell nanocubes with Au shells of three or six atomic layers exhibit plasmonic peaks almost identical to those of the Ag nanocubes while the chemical stability and SERS activity are substantially augmented. For both types of bimetallic nanocubes, the Ag cores can be selectively removed to generate nanoframes and nanoboxes.

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INTRODUCTION Silver is perhaps the best choice of material for plasmonics and related applications owing to its relatively low cost and favorable dielectric functions.1 Over the past two decades, significant progress has been made in the synthesis of Ag nanocrystals with controlled shapes and sizes to tailor their properties and thus optimize their performance in a range of applications.2,3 In particular, Ag nanocrystals have been prepared with sharp features on the surface to drastically augment their surface-enhanced Raman scattering (SERS) activity.4 However, the sharp features tend to vanish due to the high susceptibility of Ag toward oxidative etching.5 As another © 2017 American Chemical Society

pitfall, Ag is limited in terms of catalytic application as it only shows activity toward oxidation reactions such as epoxidation,6 not reduction reactions. One can address the aforementioned limitations of Ag nanocrystals by introducing a second noble metal (M) such as Au, Pd, or Pt to generate Ag−M bimetallic nanocrystals.7 In principle, Ag−M bimetallic nanocrystals can take at least three different configurations, in the form of alloy, core−frame, and core−shell, respectively. An alloy is a solid solution, in which Received: April 30, 2017 Published: July 5, 2017 1774

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Figure 1. Schematic illustration of two proposed pathways for the deposition of a second metal M on a Ag nanocube seed for the generation of (top) a Ag@M core−frame and then core−shell nanocube and (bottom) a Ag@Ag−M core−frame nanocube, together with the resultant M-based nanoframe/nanobox and Ag−M nanoframes with ridges of different thicknesses after the removal of Ag core, respectively.

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the Ag and M atoms are miscible to each other at the atomic scale. In the case of Ag and Au, alloy nanocrystals with tunable compositions can be obtained via a coreduction route.8 However, it is challenging to control the shape taken by the alloy nanocrystals, primarily due to the lack of a capping agent that can selectively bind to the same facet of different metals. As for the Ag@M core−frame and core−shell nanocrystals, they can be readily prepared using seeded growth by simply depositing M atoms on the surface of preformed Ag nanocrystals. This approach immediately benefits from the large number of Ag nanocrystals that have been prepared with well-controlled shapes.3 When the M atoms are selectively deposited on the edges of a Ag nanocrystal, for example, a Ag@ M core−frame nanocrystal is formed.9 In this structure, the excellent plasmonic and SERS properties of the Ag core are still retained while the deposited M brings in new catalytic capabilities. Alternatively, when the M atoms are conformally deposited on the entire surface, a Ag@M core−shell nanocrystal is created.10 In this case, the M shell can greatly improve the chemical stability of the particle, in addition to the new catalytic properties associated with M. If the shell is kept below 1−2 nm thick, the excellent plasmonic and SERS properties of the Ag core can still be leveraged. Significantly, both SERS and catalytic properties can be integrated in the core−frame and core−shell nanocrystals to offer a unique probe for in situ detection and analysis of catalytic reactions by SERS.

SEEDED GROWTH AS A GENERAL APPROACH TO THE CORE−FRAME AND CORE−SHELL BIMETALLIC NANOCRYSTALS AND THEIR DERIVATIVES

Seeded growth offers a powerful route to the synthesis of bimetallic nanocrystals with a core−frame or core−shell structure.2 Despite its remarkable versatility to deposit another metal on Pd or Pt seeds,7 its capability is limited by the possible galvanic replacement between the precursor to M and Ag seeds.11−13 When the seeds are destroyed to a large extent, they can no longer serve as physical templates to control the growth pattern and thus obtain the desired products. Many groups had attempted to suppress the galvanic replacement between HAuCl4 and Ag nanocrystals,14−19 but it remained a challenge to apply seeded growth to the fabrication of Ag@M nanocrystals until a few years ago when we successfully developed an effective strategy.9,10 Our strategy relies on the use of a parallel reaction to compete with and thus suppress the galvanic reaction. Specifically, when Ag nanocrystals are mixed with a salt precursor to the second metal in the presence of a reducing agent, the added precursor can be reduced by both the Ag seeds (via galvanic replacement at a rate of Rgal) and reducing agent (via chemical reduction at a rate of Rred). Under Rred > Rgal, the precursor will be primarily reduced by the reducing agent instead of participating in the galvanic replacement. If selfnucleation is eliminated by titrating the precursor in a dropwise fashion, the metal atoms derived from the chemical reduction can be directed to nucleate from the surface of the Ag seeds, generating bimetallic nanocrystals with a core−frame or core− shell structure. 1775

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Figure 2. (A, B) TEM images of the samples prepared by titrating 0.2 and 0.8 mL, respectively, of 0.1 mM HAuCl4 into an aqueous suspension of Ag nanocubes in the presence of H2Asc, PVP, and NaOH at pH = 11.9. (C, D) HAADF-STEM images of a core−shell nanocube in (B), showing a Au shell of six atomic layers. Reprinted with permission from refs 20 and 10. Copyright 2017 and 2014 American Chemical Society.

galvanic reactions than the original precursor. In this case, galvanic replacement can still occur between the first few drops of AuCl4− precursor and the Ag atoms situated at the corners. We argue that the released Ag+ ions can react with OH− instantaneously to generate Ag2O patches at the corner sites, protecting the underlying Ag from further galvanic oxidation. After a few drops, the precursor is mostly reduced by HAsc− for the generation of Au atoms, followed by their deposition on the edges, side faces, and then corners for the generation of Ag@Au core−frame and then core−shell nanocubes. The core−frame nanocubes can be converted to Au-based nanoframes by directly etching away the Ag. For the core−shell nanocubes, they can be transformed into Au-based nanoboxes with welldefined openings at corners by dissolving the Ag2O patches with a weak acid, followed by the removal of Ag. The second approach involves the cotitration of aqueous Ag+ and Mn+ ions into an aqueous mixture of Ag nanocubes, H2Asc, and PVP at a pH around 3.2.9 When the added Ag+ ions are able to push the galvanic replacement backward, Rgal can be reduced to attain Rred > Rgal even at the slow reduction rate of H2Asc rather than HAsc−. Again, the Ag and M atoms derived from chemical coreduction by H2Asc are initially deposited on the edges, followed by sequential deposition on the corners and side faces, respectively. Upon the selective removal of Ag, Ag− M alloy nanoframes with different ridge thicknesses are obtained.

Using Ag nanocubes as a model system, Figure 1 illustrates two approaches to the fabrication of core−frame and core− shell nanocrystals in an aqueous solution under ambient conditions. The first approach involves the titration of aqueous Mn+ precursor into an aqueous mixture of Ag nanocubes, ascorbic acid (H2Asc, a reducing agent), poly(vinylpyrrolidone) (PVP, a colloidal stabilizer), and NaOH in the pH range of 10.3−11.9. The presence of OH− can affect the deposition of M atoms in a number of ways.20 For example, the H2Asc should be neutralized by OH− to give ascorbate monoanion (HAsc−) under alkaline condition, the actual reducing agent associated with H2Asc, achieving the condition of Rred > Rgal needed for suppressing the galvanic reaction. Because PVP binds more strongly to the {100} facets on Ag nanocubes, the specific surface free energies of the low-index facets on Ag nanocubes decrease in the order of γ(110) > γ(111) > γ(100).2 As such, the M atoms derived from the reduction by HAsc− are sequentially deposited on the edges, corners, and side faces for the formation of Ag@M core−frame and then core−shell nanocubes. It is worth mentioning that the site-selective deposition may also arise from a bipolar mechanism, in which reduction of Mn+ and oxidation of HAsc− occur at different sites on the surface of a seed.21 In the case of Au deposition, with the involvement of OH−, the titrated AuCl4− precursor can undergo ligand exchange to generate AuCl(OH)3− and Au(OH)4− with lower reduction potentials and thus slower rates for both the reduction and 1776

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Figure 3. (A) TEM image of Au-based nanoframes obtained by etching the sample shown in Figure 2A with 3% aqueous H2O2. (B) TEM image of Au-based nanoboxes after treating the sample shown in Figure 2B with aqueous H2Asc and then 3% aqueous H2O2. (C) Atomic-resolution HAABFSTEM image taken from the corner region of a nanobox shown in (B). (D) TEM image of Au-based nanoboxes with an outer edge length of 20 nm. Reprinted with permission from refs 20 and 22. Copyright 2017 and 2016 American Chemical Society.

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Ag@Au CORE−FRAME AND CORE−SHELL NANOCUBES AND DERIVATIVES

When the sample prepared using 0.2 mL of HAuCl4 was treated with 3% aqueous H2O2, we obtained Au-based nanoframes (Figure 3A). In the case of 0.8 mL of HAuCl4, we obtained Au-based nanoboxes with well-defined openings at corners (Figure 3B) when the sample was treated with a weak acid and then aqueous H2O2.22 The aberration-corrected high angle annular bright-field HAABF-STEM image taken from one of the corners of the nanobox indicates a wall thickness around 2 nm (Figure 3C). We have also successfully fabricated Aubased nanoboxes as small as 20 nm in edge length (Figure 3D).

As illustrated in Figure 1, we can follow Approach I to produce Ag@Au core−frame and core−shell nanocubes using 38 nm Ag nanocubes as seeds under ambient conditions.10,20 Specifically, at an initial pH of 11.9, the Au atoms derived from the reduction by HAsc− were sequentially deposited on the edges, side faces, and corners of the nanocubes. Figure 2A and B shows transmission electron microscopy (TEM) images of the samples obtained at the titration volumes of 0.2 and 0.8 mL, respectively, for 0.1 mM HAuCl4. The resultant particles are essentially identical to the parental Ag nanocubes, except for the increase in edge length from 38.6 ± 1.3 to 38.9 ± 1.7 and 40.3 ± 1.9 nm, respectively. Figure 2C shows the high-angle annular dark-field scanning TEM (HAADF-STEM) image of a nanocube obtained at 0.8 mL of HAuCl4. The clear contrast between the Ag core and the Au shell supports the conformal deposition of Au atoms on the entire surface of the nanocube. The dark contrast at the corner sites may indicate the presence of Ag2O patches. Figure 2D shows the atomic-resolution HAADF-STEM image along the [001] zone axis, revealing that six layers of Au atoms were deposited on the side faces of the Ag nanocube. The Au shell thickness was proportionally reduced to three atomic layers when the volume of HAuCl4 was reduced from 0.8 to 0.4 mL at a fixed number of Ag nanocubes.

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Ag@Ag-M (M = Au OR Pd) CORE−FRAME NANOCUBES AND DERIVATIVES As illustrated by the Approach II in Figure 1, we can obtain Ag@Ag−M core−frame nanocubes by cotitrating aqueous Ag+ and Mn+ into an aqueous mixture containing 38 nm Ag nanocubes, H2Asc, and PVP at pH = 3.2.9,23 For the fabrication of Ag@Ag−Au nanocubes,23 if the molar ratio of AgNO3 to HAuCl4 is greater than three, the galvanic reaction will be suppressed. The resultant Ag and Au atoms are concurrently deposited on the Ag nanocubes. Figure 4A−C shows SEM images of the products obtained after the cotitration of 0.2, 0.4, and 0.8 mL for each precursor. The absence of pits on the particles confirmed the successful suppression of galvanic replacement. If the titration volume was further increased, the nanocubes became significantly truncated at the corners, 1777

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Figure 4. (A−C) SEM images of Ag@Ag−Au nanocubes prepared by cotitrating aqueous AgNO3 (0.3 mM) and HAuCl4 (0.1 mM), at 0.2, 0.4, and 0.8 mL for each precursor, into an aqueous suspension of Ag nanocubes in the presence of H2Asc and PVP. The insets show models of the corresponding structures. (D−F) TEM images of the resultant structures after etching of the samples shown in (A−C) with 3% aqueous H2O2. Reprinted with permission from ref 23. Copyright 2015 Royal Society of Chemistry.

Figure 5. (A−C) TEM images of Ag@Ag−Pd nanocubes prepared by cotitrating aqueous Na2PdCl4 (0.2 mM) and AgNO3 (0.1 mM), at 0.1, 0.2, and 0.3 mL for each precursor, into an aqueous suspension of Ag nanocubes in the presence of H2Asc and PVP. (D−F) TEM images of the resultant structures after etching of samples shown in (A−C) with 3% H2O2. The scale bars in the insets are 20 nm. Reprinted with permission from ref 9. Copyright 2015 American Chemical Society.

primarily due to the preferential deposition of atoms along the edges.24 When the Ag cores were removed by etching with

aqueous H2O2, as shown by the TEM images in Figure 4D−F, the as-obtained nanocubes were transformed into Ag−Au 1778

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Figure 6. HAADF-STEM images of a sample prepared by reacting Ag nanocubes with 30 μL of 0.2 mM H2PtCl6 in the presence of H2Asc and PVP. (A−C) Images taken from one of the nanocubes when it was oriented along the ⟨100⟩ axis. (E−F) Images taken from one of the nanocubes shown in (D) when it was oriented along the ⟨110⟩ axis. Reprinted with permission from ref 26. Copyright 2017 American Chemical Society.

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Ag@Au CORE−SHELL CONCAVE CUBOCTAHEDRA By following the protocol used for the deposition of Au on Ag nanocubes at pH = 11.9, we examined the deposition of Au on Ag cuboctahedra whose surface is covered by both {100} and {111} facets at a surface area ratio of 1.7.27 Figure 7A outlines a distinct deposition pathway. Initially, the Au atoms derived were deposited on entire surface of a cuboctahedron for the formation of a Ag@Au core−shell cuboctahedron. Once a complete Au shell was formed, the Au atoms would be preferentially deposited on the Au{100} facets due to the selective capping of Au{111} facets by PVP in an aqueous solution,28 resulting in the formation of Ag@Au concave cuboctahedron. Figure 7B−D, shows SEM images of products obtained at the titration volumes of 0.2, 0.4, and 0.8 mL, respectively, for 0.1 mM HAuCl4. Figure 7E shows the HAADF-STEM image of an individual concave cuboctahedron that was orientated along the ⟨110⟩ zone axis. We noticed that the projected thickness of Au overlayers on {100} facets was greater than that of the overlayers on {111} facets. Figure 7F shows HAADF-STEM image of another concave cuboctahedron that was aligned along the ⟨100⟩ zone axis. Again, one could easily differentiate Au shell from Ag core by their contrasts. Figure 7G shows the STEM-electron energy-loss spectroscopy (EELS) mapping with the M4,5 edges of Au and Ag, respectively, confirming the uniform deposition of Au on {100} facets.

nanoframes with thin ridges and no coverage at the corners, Ag−Au nanoframes with thickened ridges and coverage at corner sites, and nanocages with pores on the side faces, respectively. We have also extended the standard cotitration protocol to the fabrication of Ag@Ag−Pd nanocubes.9,25 Because the reduction potential of PdCl42− is lower than that of AuCl4− and the Pd(II) precursor is added as a salt rather than an acid, the lower limit of molar ratio between AgNO3 and Na2PdCl4 can be reduced to 0.5.9 By varying the titration volume for each precursor, we could vary the elemental composition of the resultant structures. Figure 5A−C shows TEM images of the Ag@Ag−Pd nanocubes, where the Pd content was varied in the range of 2.2−7.3%. Figure 5D−F shows the corresponding Ag− Pd nanoframes and nanocages after the removal of Ag cores by etching with aqueous H2O2. Cotitration protocol can be extended to the fabrication of Ag@Ag−Pt core−frame nanocubes. Recently, we reported route to Ag@Pt core−frame nanocubes by reacting Ag nanocubes with H2PtCl6 in the presence of H2Asc and PVP.26 Figure 6A shows a HAADF-STEM image taken from a nanocube that was orientated along the [001] zone axis. Figure 6B shows the columns of the Ag atoms located on the {100} facets, together with some unclear features of atoms at the corner. By refocusing the electron beam from the top to the bottom of the cube, Figure 6C indicates the presence of Pt atoms located on the edges. Figure 6D shows the HAADFSTEM images from a nanocube that was orientated along the [110] zone axis. Figure 6E and F confirmed the deposition of Pt atoms on the edges of the Ag nanocube by the bright contrast.

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PLASMONIC PROPERTIES We used UV−vis spectroscopy to monitor changes to the plasmonic properties during the transformation from Ag nanocubes to Ag@Au core−frame and core−shell nanocubes. 1779

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Figure 7. (A) Schematic illustration showing the transformation from a Ag cuboctahedron into a Ag@Au cuboctahedron and Ag@Au concave cuboctahedron. (B−D) SEM images of products prepared by reacting Ag cuboctahedra with different volumes of 0.1 mM HAuCl4 at 0.2, 0.4, and 0.8 mL, respectively, in the presence of H2Asc, PVP, and NaOH at pH = 11.9. The scale bars in the insets are 20 nm. (E, F) HAADF−STEM images recorded from two of the Ag@Au concave cuboctahedra shown in (D) when they were oriented along the ⟨110⟩ and ⟨100⟩ zone axes, respectively. The insets show models of the concave cuboctahedra in the appropriate orientations. (G) STEM−EELS mapping of Au (red) and Ag (green) for the cuboctahedron shown in (F). Reprinted with permission from ref 27. Copyright 2016 American Chemical Society.

closest composition with dielectric constants available in literature. As shown in Figure 8C, the resonance peaked at 1100 nm, in agreement with the experimental value at 1080 nm. Owing to the ultrathin wall thickness, the optical resonance of the nanobox is dominated by absorption, with a peak cross section of 31.6 × 10−15 m2, five times greater than those reported for Au nanocages with a similar edge length that were prepared using the galvanic replacement route.29

As an example, Figure 8A shows UV−vis spectra of an aqueous suspension of 38 nm Ag nanocubes before and after they had reacted with different volumes of 0.1 mM aqueous HAuCl4 in the standard protocol at pH = 11.9.10,20 In responding to Au deposition, the resonance peak of the Ag nanocubes was only shifted from 430 to 433, 442, and 445 nm, indicating that the plasmonic properties of the original Ag nanocubes could be largely retained. When the Ag@Au core−shell nanocubes with Au shells of six atomic layers were treated with H2Asc, they could be further etched with aqueous H2O2 to generate Au nanoboxes with openings at the corners (Figure 3B).22 Elemental analyses by EDX and ICP-MS gave Au to Ag atomic ratios of 1.2:1 and 1:1, respectively. As shown in Figure 8B, the resonance peak of the Ag@Au core−shell nanocubes was shifted from 447 to 1080 nm after the selective removal of Ag cores. We also used the discrete dipole approximation (DDA) method to calculate the optical spectra for a nanobox with an outer edge length of 40 nm, a wall thickness of 2 nm, and triangular pores (10 nm in edge length) at all corners. In this calculation, we selected an atomic composition of 65%Au and 35%Ag because it was the

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SERS PROPERTIES We benchmarked the Ag@Au core−shell nanocubes against their parental Ag nanocubes. Figure 9A shows SERS spectra of 1,4-benzenedithiol (1,4-BDT) adsorbed on Ag nanocubes with an edge length of 38 nm and the resultant core−shell nanocubes with Au shells of three atomic layers thick, respectively. The SERS enhancement factor of the core−shell nanocubes was 5.4-fold greater than that of the Ag nanocubes, which could be attributed to the chemical (CHEM) enhancement due to a stronger bonding strength for Au−S than for Ag−S.30 The result indicates that the 1,4-BDT molecules adsorbed on the ultrathin Au shell can still feel the strong 1780

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Figure 9. SERS spectra collected from 1,4-BDT on (A) Ag nanocubes and Ag@Au core−shell nanocubes (with three atomic layers of Au) and (B) Ag cuboctahedra, Ag@Au cuboctahedra, and Ag@Au concave cuboctahedra. The laser excitation was 785 nm. Reprinted with permission from refs 10 and 27. Copyright 2014 and 2016 American Chemical Society.

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CATALYZING AND PROBING STEPWISE REACTIONS IN SITU Although others have demonstrated the use of bimetallic nanocrystals as a SERS probe for monitoring the catalytic reduction of 4-nitrothiophenol (4-NTP) by NaBH4 to produce 4-aminothiophenol (4-ATP),33 we are among the first to report the use of Ag@Ag−Pd core−shell nanocubes as a dual catalytic system capable of probing the Pd-catalyzed reduction of 4-NTP to 4-ATP by NaBH4 and the subsequent Ag-catalyzed oxidation of 4-ATP to 4,4′-dimercaptoazobezene (trans-DMAB) by the O2 from air.34 Figure 10A shows the time-resolved SERS spectra collected from the 4-NTP adsorbed on the Ag@Ag−Pd core−frame nanocubes containing 2.2 wt % of Pd. At t = 0 min, we observed three characteristic bands of 4-NTP at 1108 cm−1 (νCN), 1336 cm−1 (νNO2), and 1572 cm−1 (νCC).35 At t = 2 min after the introduction of NaBH4, the νNO2 band was shifted from 1336 to 1330 cm−1, while other bands remained essentially the same. At t = 6 min, a shoulder peak emerged at 1595 cm−1, which can be assigned to the νCC of 4-ATP.36 As the reaction further proceeded to 20 min, the νCC band of 4ATP increased in intensity as three bands of 4-NTP decreased in intensity. At t = 30 min, both the νCN and νNO2 bands of 4NTP could no longer be resolved. By 40 min, all the three bands of 4-NTP disappeared and the remaining peaks can be assigned to the νCS, βC−H, and νCC of 4-ATP, respectively, indicating the complete conversion from 4-NTP to 4-ATP. In the next 20 min, the three peaks of 4-ATP remained the same in peak position while their intensities were increased. At t = 70 min, we observed three new peaks at 1142, 1388, and 1429 cm−1, which can be assigned to the βCH + νCN, νNN + νCN, and νNN + βCH of trans-DMAB, respectively.37 These bands

Figure 8. (A) UV−vis spectra of 38 nm Ag nanocubes before and after reacting with different volumes of 0.1 mM HAuCl4 in the presence of H2Asc, PVP, and NaOH at pH = 11.9. (B) UV−vis-NIR spectra recorded from 40 nm Ag@Au nanocubes (with six atomic layers of Au) and the resulting Au-based nanoboxes. (C) Extinction spectra calculated using the DDA method for a 40 nm Au-based nanobox. The propagation direction (k-vector) and electric field (E-field) were perpendicular and parallel to the (100)-facet of the cubic box. Reprinted with permission from refs 10 and 22. Copyright 2014 and 2016 American Chemical Society.

electromagnetic (EM) field enhancement from the Ag core,31 resulting in a EM-based SERS activity comparable to that of Ag nanocubes. We also benchmarked the SERS activities of the Ag@Au cuboctahedra and concave cuboctahedra against their parental Ag cuboctahedra. As shown in Figure 9B, the enhancement factors of the Ag@Au cuboctahedra and concave cuboctahedra were 15 and 74 times greater than that of the Ag cuboctahedra. In addition to the stronger Au−S bonding for CHEM-based enhancement, we believe that the sharp features associated with a concave surface also make additional contributions to the EM-based enhancement.32 1781

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Figure 10. (A) Time-dependent SERS spectra for monitoring the reduction of 4-NTP by NaBH4 on Ag@Ag−Pd nanocubes with 2.2 wt % Pd. (B) Stepwise reactions involving the Pd-catalyzed reduction of 4-NTP by NaBH4 and Ag-catalyzed oxidation of 4-ATP by the O2 from air. Reprinted with permission from ref 34. Copyright 2016 John Wiley and Sons.

parallel reduction reaction to compete with and thus suppress the galvanic reaction. In one approach, ascorbic acid is used at pH around 11.9 for generating ascorbate monoanion with a strong reduction power and thus defeating the galvanic replacement. In this case, the precursor can be reduced to atoms by ascorbate monoanion, followed by their sequential deposition onto the edges, side faces, and corners of Ag nanocubes, for the formation of core−frame and then core− shell nanocubes. Alternatively, the precursor is cotitrated with Ag+ ions into a suspension of Ag nanocubes in the presence of ascorbic acid at a pH around 3.2. In this case, the presence of sufficient Ag+ ions can push back the galvanic reaction, facilitating the deposition of an alloy onto the edges, corners, and part of the side faces of Ag nanocubes for the generation of core−frame nanocubes with increasingly thicker ridges. In both cases, the plasmonic properties of the Ag nanocubes are essentially preserved in the core−frame and core−shell nanocubes while the second metal offers new capabilities. For example, the Ag@Au core−shell nanocubes exhibit enhanced SERS activity and chemical stability. As a unique application, we have demonstrated that the Ag@Ag−Pd core−frame nanocubes can serve as a dual catalyst for the Pd-catalyzed reduction of nitroaromatics to aromatic amines and then the Ag-catalyzed oxidation of amines to azo compounds while both reactions can be monitored and analyzed using SERS. The Ag

remained unaltered up to 90 min, except for slight increase in intensity. The SERS data suggests that the 4-ATP adsorbed on the surface could be oxidized by the O2 from air to generate trans-DMAB. Based on the SERS data, Figure 10B proposes a plausible mechanism. In the first step, the 4-NTP molecules adsorb on the Ag surface through the Ag−S linkage. Because of the formation of trans-DMAB rather than cis-DMAB, we assume that molecules are orientated with a configuration parallel to the surface. When NaBH4 is introduced to an aqueous solution under ambient conditions, it will decompose to produce H2, followed by their adsorption and dissociation on the Pd surface to generate H atoms.38 The atomic hydrogen can quickly reduce 4-NTP to 4-ATP. In the second step, the 4-ATP molecules remain on the surface of the catalyst until the NaBH4 is completely decomposed in the reaction solution. In the final step, the Ag surface activates the O2 from air,39 enabling Agcatalyzed oxidation of 4-ATP to trans-DMAB.

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CONCLUDING REMARKS We have discussed two general methods for depositing a second metal on the surfaces of Ag nanocrystals without involving galvanic replacement and thus facile fabrication of bimetallic nanocrystals with a well-defined core−frame or core−shell structure. The success relies on the introduction of a 1782

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ACKNOWLEDGMENTS This work was supported in part by the National Science Foundation (CHE-1412006), start-up funds from Georgia Tech, and 3M nontenured faculty award. We are grateful to our collaborators for their invaluable contributions to this work.

cores can also be selectively removed by etching to transform the core−frame and core−shell nanocubes into nanoframes and nanoboxes with a highly open structure and novel properties. Ultimately, this work will contribute to the rational design and synthesis of bi- and multimetallic nanocrystals with diversified compositions regardless of the possible complication from galvanic replacement.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dong Qin: 0000-0001-5206-5912 Notes

The authors declare no competing financial interest. Biographies Yiren Wu is pursuing her Ph.D. in Materials Science and Engineering (MSE) at Georgia Tech. She received her B.E. in Polymer Science and Engineering from Zhejiang University, China (2014). Her research interest includes the design and synthesis of bifunctional nanocrystals for in situ SERS analysis of chemical reactions. Xiaojun Sun received his B.S. in MSE from Shanghai Jiao Tong University, China, in 2013. He is currently working toward completion of his Ph.D. in MSE at Georgia Tech. His research interest includes the rational synthesis of bimetallic nanocrystals for applications in plasmonics and catalysis. Yin Yang is an Associate Professor in the Institute of Advanced Synthesis, Nanjing Tech University, China. He received his B.S. in Materials Science (2009) and Ph.D. in Materials Physics and Chemistry (2015) from Fudan University, China. He was a visiting Ph.D. student in the Qin lab (2012−2014). His research interest includes the rational synthesis of nanomaterials for applications in energy storage and catalysis. Jumei Li is an Associate Professor at Wuhan Institute of Technology, China. She received her B.S. (1999) and M.S. (2002) from Sichuan University, China, and her Ph.D. (2012) from Fudan University. She was a visiting scholar in the Qin lab (2013−2015). Her research interest includes the design and synthesis of nanomaterials for SERS applications. Yun Zhang received her B.S. in Environmental Science (2005) and Ph.D. in Environmental Chemistry (2011) from Lanzhou University. She is currently working in the Qin lab under the support of an International Postdoctoral Exchange Fellowship from China Scholarship Council (2016−2018). Her research interest includes the design and synthesis of nanomaterials for applications in SERS and environmental science. Dong Qin is an Associate Professor of MSE at Georgia Tech, with an adjunct appointment in the School of Chemistry & Biochemistry. Her academic records include a B.S. in Chemistry from Fudan University (1990), a Ph.D. in Physical Chemistry with Professor Hai-Lung Dai from University of Pennsylvania (1996), a postdoctoral stint with Professor George M. Whitesides at Harvard University (1996−1997), and an MBA from the University of Washington (2003). Before joining Georgia Tech in 2012, she held administrative positions as Associate Dean for Research in the School of Engineering and Applied Science at Washington University in St. Louis (2007−2011) and Associate Director of Center for Nanotechnology at the University of Washington (2002−2007). Her current research focuses on the unique properties and applications of nanoscale materials and systems. 1783

DOI: 10.1021/acs.accounts.7b00216 Acc. Chem. Res. 2017, 50, 1774−1784

Article

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