Controlled Synthesis of Palladium Concave Nanocubes with Sub-10

Jan 22, 2014 - After another 5 min, 80 μL of 0.1 M ascorbic acid solution was quickly added. .... 10 mM, (b) 20 μL, 10 mM, (c) 100 μL, 1 mM, (d) 50...
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Controlled Synthesis of Palladium Concave Nanocubes with Sub-10Nanometer Edges and Corners for Tunable Plasmonic Property Wenxin Niu, Weiqing Zhang, Shaik Firdoz, and Xianmao Lu* Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585 S Supporting Information *

ABSTRACT: Developing new strategies for tuning the plasmonic properties of palladium nanostructures is of both fundamental and technological interest due to their potential applications in plasmonic hydrogen sensing, in situ surfaceenhanced Raman spectroscopy for catalysis, and solar energy harvesting. In this work, a new strategy of tuning the localized surface plasmon resonance (LSPR) property of Pd nanocrystals by selectively sharpening their edges and corners is reported. Through a Cu(II)-assisted seed-mediated growth approach, sub-10-nm sharp edges and corners were grown on regular Pd nanocubes. The LSPR peaks of the as-formed concave Pd nanocubes could be tuned across the visible spectrum by simply controlling their sizes. Cu(II) was found to selectively activate the fast growth of Pd atoms along the [110] and [111] directions of the cubic Pd seeds and promote the formation of this new type of Pd concave nanocubes. This strategy of building Pd sharp edges and corners may be applicable for the design of new plasmonic nanostructures by using seeds of different metals, sizes, shapes, and crystal structures.

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INTRODUCTION Controlling the shape of plasmonic noble metal nanocrystals (NCs) opens up opportunities to tailor their optical properties for various applications including biosensing, photothermal therapy, and solar energy harvesting.1−6 These applications are highly dependent on the localized surface plasmon resonances (LSPR) of noble metal nanostructures.7 For example, by tuning the size and shape of gold, silver, and their alloy nanostructures, their LSPR peak can be systematically tuned from the visible to the near-infrared (NIR) spectral range. In addition to Au and Ag, fine-tuning the shape of plasmonic Pd NCs has received increasing attention due to the unique properties of Pd.8−11 First, Pd has a significantly higher bulk melting point than that of Au or Ag, endowing Pd NCs with superior photothermal stability.10,12 Second, Pd NCs are well-known for their high catalytic activity for carbon−carbon coupling reactions, reduction of automobile pollutants, and electro-oxidation of formic acid.13,14 When these applications are coupled with enhanced plasmonic properties in the visible region, a few novel applications of Pd NCs can appear. For example, plasmonic Pd nanostructures could be directly used to monitor Pd-involved catalytic reactions by surface-enhanced Raman spectroscopy without the assistance of Au or Ag partners.15−17 With extinction spectra in the visible region, plasmonic Pd nanostructures can be used to harvest solar energy for the activation of Pd-catalyzed chemical reactions.18,19 Third, plasmonic Pd NCs can also be directly used as plasmonic hydrogen sensors due to the superior hydrogen absorption properties of metallic Pd.20,21 These potential applications have © 2014 American Chemical Society

made plasmonic Pd NCs an attractive family of nanomaterials. However, compared with Au and Ag, the plasmonic properties of Pd nanostructures have been much less explored. This is because a grand challenge still resides in developing a strategy to continuously tune the LSPR of Pd NCs in the visible range. Concave noble metal NCs have received considerable attention in recent years due to their high catalytic activities.17,22−34 One of the most well-known examples is the concave nanocubes. They can be viewed as regular nanocubes with the centers of the six square faces “pushed in”, giving a negative curvature of the NCs with sharp edges and corners. In the case of Pd nanocubes, several methods have been developed to fine-tune their concave structures. For example, Xia et al. reported the synthesis of Pd concave nanocubes enclosed by high-index {730} facets through preferential overgrowth on Pd cubic seeds through a kinetically controlled growth procedure.35 Zheng and co-workers reported the synthesis of concave cubic Pd NCs with high-index {730} facets and vicinal {hk0} facets based on the synergism between two capping reagents, cetyltrimethylammonium chloride and cetyltrimethylammonium bromide.36 While both studies employed ascorbic acid as the reducing agent, Wang et al. developed a nonseeded method for concave palladium nanocubes by introducing a relatively strong reducing agent of sodium ascorbate.37 However, these studies are focused on the enhanced catalytic activities of Pd concave nanocubes. The Received: January 19, 2014 Published: January 22, 2014 2180

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Figure 1. Electron microscopy images of Pd concave nanocubes: (a, b) SEM images of assembled and randomly arranged Pd concave nanocubes, respectively. Inset of (b) illustrates the concave structure of the Pd nanocubes. (c) TEM image of Pd concave nanocubes. (d, e) TEM image and corresponding SAED pattern of a Pd concave nanocube recorded along [001] zone axis. (f) HRTEM image of a corner of the Pd concave nanocube in Figure 1d. Scale bars: (a) 500, (b) 100, (c) 200, (d) 20, and (f) 5 nm. by dissolving 0.1773 g PdCl2 in 10 mL of 0.2 M HCl solution and further diluting to 100 mL with DI water. Synthesis of Pd Cubic Seeds. Cubic Pd seeds were synthesized according to published procedures.38,39 In a typical synthesis, 45.6 mg of CTAB was dissolved in 10 mL water in a 20 mL vial and heated at 95 °C under electromagnetic stirring. After 5 min, 0.5 mL of 10 mM H2PdCl4 solution was added. After another 5 min, 80 μL of 0.1 M ascorbic acid solution was quickly added. The reaction was stopped after 10 min and stored at 30 °C for future use. Seed-Mediated Growth of Concave Palladium Nanocubes. In a typical synthesis, 100 μL of 1 mM CuSO4 solution was added to 5 mL of 0.1 M CTAB solution kept in a 40 °C water bath. Then 125 μL of 10 mM H2PdCl4 solution and 40 μL of the Pd cubic seed solution were added. After gentle mixing, 100 μL of 0.1 M ascorbic acid solution was added and the solution was gently mixed. The resulting solution was placed in a water bath at 40 °C without disturbance. After 12 h, the products were collected by centrifugation (10000 rpm, 5 min). The sizes of Pd concave nanocubes were controlled by changing the mount of seeds during the growth. For the 67-, 91-, and 109-nm Pd concave nanocubes, 40, 20, and 15 μL of the Pd cubic seed solution were added, respectively. The precipitates were redispersed in water. The centrifugation/redispersion procedures were repeated for another two times and the final product was redispersed in 0.5 mL of water. Seed-Mediated Growth of Regular Palladium Nanocubes. All the procedures were the same as the synthesis of the Pd concave nanocubes except that no CuSO4 was introduced. For the 43-, 56-, and 65-nm Pd regular nanocubes, 115, 54, and 34.5 μL of the Pd cubic seed solution were added, respectively. Instrumentation. Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, and selected-area electron diffraction (SAED) were acquired using a JEOL JEM-2100F operating at 200 kV. Scanning electron microscopy (SEM) images

unique plasmonic properties of Pd concave nanocubes have not been explored, although the plasmonic properties may be very sensitive to the sharp edges and corners of Pd concave nanocubes. This is probably because sub-10-nm sharp edges and corners are highly desired to investigate the plasmonic properties of Pd concave nanocubes. But it is still challenging to develop a synthetic method for Pd concave nanocubes with tight size control and tunable sub-10-nm sharp features. Herein, we report a chemical sharpening method to tailor the LSPR properties of Pd NCs. Using regular Pd nanocubes as templates, sub-10-nm edges and corners are grown on the nanocubes through Cu(II)-assisted growth. It is found that Cu(II) ions can selectively activate the fast growth of Pd along the [110] and [111] directions and promote the formation of this new type of Pd concave nanocubes. More importantly, the LSPR peaks of sharpened Pd nanocubes could be tuned across the visible spectrum from 350 to 715 nm. These monodisperse plasmonic Pd nanostructures may find applications in plasmonenhanced catalysis, plasmonic hydrogen sensing, and SERS.

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EXPERIMENTAL SECTION

Materials. Palladium(II) chloride (PdCl2), copper(II) sulfate (CuSO4), copper(II) chloride (CuCl2), copper nitrate (Cu(NO3)2), and L-ascorbic acid were obtained from Sigma-Aldrich. Cetyltrimethylammonium bromide (CTAB) was obtained from Acros Organics. Hydrochloric acid (HCl, 37.5%) was obtained from Fisher Scientific. All chemicals were used as received without further purification. Ultrapure deionized (DI) water (Barnstead, 18.2 MΩ cm−1) was used throughout the experiments. A 10 mM H2PdCl4 solution was prepared 2181

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Figure 2. SEM images of Pd concave nanocubes with different edge lengths: (a, b) 67 nm, (c, d) 91 nm, (e, f) 109 nm. Scale bars: (a, c, e) 200 and (b, d, f) 100 nm. were taken using a JEOL JSM-6700F operating at 20 kV. UV−visible (UV−vis) spectra were recorded using a Shimadzu UV-1601 spectrometer with plastic cuvettes of 1 cm path length at room temperature. X-ray photoelectron spectroscopy (XPS) was performed on a Kratos AXIS Hsi spectrometer using monochromatic Al Kα X-ray source (1486.6 eV). C 1s at 285 eV was used as the charge reference to determine core level binding energies.

nanocube recorded along [001] zone axis, further confirming the single crystallinity of the concave nanocubes. Size Control of Pd Concave Nanocubes and Their Tunable Plasmonic Properties. It has been reported that plasmonic Pd nanostructures are more stable than Au and Ag nanostructures under illumination of light, due to the higher bulk melting point of Pd.10 However, the plasmonic properties of Pd nanostructures are much less investigated than that of Au and Ag. The plasmonic properties of Ag and Ag are highly dependent on their structural characteristics, especially the degree of sharpness of their edges and corners. For example, by simply truncating the corners of Ag nanocubes, their SPR extinction peaks could be significantly blue-shifted. However, such an effect has been seldomly investigated in the case of plasmonic Pd nanostructures. To systemically investigate this effect, Pd concave nanocubes with different sizes were prepared by changing the amount of cubic seeds added to the growth solution. By decreasing the amount the amount of seeds, concave nanocubes with edge lengths around 67, 91, and 109 nm were obtained (Figure 2a−e). All these concave nanocubes are produced with high purity and display sharp corners and edges in the sub-10-nm range. These concave nanocubes show red-shifted extinction spectra with increased size. For the 67, 91, and 109-nm concave nanocubes, the extinction peaks are located at 502, 614, and 715 nm, respectively. To demonstrate the effect of the sub-10-nm edges and corners on the plasmonic properties of the concave nanocubes, we also synthesized regular Pd nanocubes with edge lengths of 43, 56, and 65 nm (Figure S2, Supporting Information), which are close to the sizes of the cubic cores of the 67-, 91-, and 109-nm concave nanocubes, respectively. Compared with regular nanocubes

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RESULTS AND DISCUSSION Cu(II)-Assisted Growth of Pd Concave Nanocubes. The synthesis of Pd concave nanocubes was performed through a seed-mediated growth approach. Cubic Pd seeds with an average size of 15 nm were employed as the seeds (Figure S1, Supporting Information). Protruded edges and corners were grown on the Pd cubic seeds by reducing H2PdCl4 with ascorbic acid in the presence of cetyltrimethylammonium bromide (CTAB) and a small amount of CuSO4 (see Experimental Section for details). Scanning electron microscopy (SEM) images show that the resultant Pd NCs have an overall cubic shape with remarkable purity and uniform size (Figure 1a and b). However, unlike regular nanocubes, their edges and corners protrude up, while their faces curve in, forming a concave structure (Figure 1b). For each nanocube, 12 sharp edges with a thickness around 6 nm are formed on the cubic seed and every three adjacent edges join together to form a sharp corner. Figure 1d and e show a transmission electron microscopy (TEM) image and the corresponding selected area electron diffraction (SAED) pattern of a Pd concave nanocube. The characteristic square spot array of the SAED pattern indicates a single-crystalline nature and confirms that the inner faces of the concave nanocubes are enclosed by {100} facets.40 Figure 1f shows the HRTEM image of the Pd concave 2182

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Cu(II) salts, namely CuCl2 and Cu(NO3)2. It was found that CuCl2 and Cu(NO3)2 are also capable to produce similar Pd concave nanocubes (Figure S3, Supporting Information). These results prove that Cu(II) cations are responsible for the formation of the Pd concave nanocubes. We further investigated the effect of Cu(II) concentration on the growth of Pd concave nanocubes. As shown in Figure 4, the formation of concave structure is sensitive to the concentration of Cu(II) ions: the reaction with 100 μL of 1 mM Cu (II) gave the best result. Either increasing or decreasing the amount Cu(II) will lead to less sharp or complete disappearance of protruded edges and corners. Besides the concentration of Cu(II), the growth kinetics was also found to be critical for the growth of concave nanocubes. The growth kinetics could be simply tuned by changing the concentration of the reducing agent, ascorbic acid. Figure 5 shows that with 15 μL of 0.1 M ascorbic acid added, truncated Pd nanocubes with {111} and {100} facets were formed. When the amount of ascorbic acid was increased to 25 μL, regular Pd nanocubes with well-defined edges and corners were obtained. When the amount of ascorbic acid was increased to 50 μL, slightly sharpened edges and corners could be observed. When the amount of ascorbic acid was further increased to 100−400 μL, concave nanocubes with sharp edges and corners were produced. These results suggest that a fast growth rate favors the growth of the sharp corners and edges on the Pd concave nanocubes. The effect of Cu(II) on the growth of Pd concave nanocubes show several features that are different from Ag(I)-assisted growth of Au NCs. First, the UPD of Ag follows a facetblocking mechanism, therefore, the reaction rates of the growth is slowed down after the addition of Ag(I).53 However, in our case, the addition of Cu(II) accelerates the reaction and cause a faster growth of Pd NCs, evidenced by a faster color change of the growth solution. Second, the stabilization of high-energy facets by the UPD of Ag is favored under slow growth kinetics.41,54 Fast kinetics will disturb the UPD of Ag and discourage the formation of well-defined crystal facets. In contrast, Cu(II)-assisted formation of Pd concave nanocubes only works at fast growth kinetics. Third, UPD usually stabilizes high-energy facets.54 In our case, although different concentrations of Cu(II) were investigated, the concave nanocubes are still primarily enclosed by low-index facets. These clues suggest that the formation of Pd concave nanocubes by Cu(II)-assisted growth may not follow a UPD mechanism. To identify the possible role of Cu(II), X-ray photoelectron spectroscopy (XPS) was performed for the Pd concave nanocubes. As shown in Figure 6a, the Pd 3d5/2 peak is centered at 335.6 eV, corresponding to metallic Pd.55 The Cu 2p3/2 peak is centered at 932.1 eV, which can attributed to Cu(I) species,56 indicating that Cu(II) ions were reduced to Cu(I) during the reaction. Gou and Murphy have studied the reduction of CuSO4 with sodium ascorbate in the presence of CTAB.57 They found that CuSO4 can only be reduced to Cu(I), not Cu(0), even with the addition of NaOH and at a higher reaction temperature. It has been reported that the oxidation of ascorbic acid is highly sensitive to trace amounts of copper ions.58 The presence of Cu(II) can considerably expedite the oxidation of ascorbic acid in aqueous solution − Cu(II) and ascorbic acid first form a Cu(II)-ascorbate complex which subsequently breaks down to form ascorbic acid radicals and Cu(I) species.59 Ascorbic acid radical is a much stronger reducing agent than ascorbic acid and causes fast reduction of

with sizes similar to their cubic cores, the concave nanocubes show dramatic peak shifts. For the 67, 91, and 109-nm concave nanocubes, redshifts of 152, 227, and 302 nm were observed, respectively. While a spectral peak shift of more than 300 nm was observed for the concave nanocubes, for the regular nanocubes it was below 70 nm. These results demonstrate that the Pd concave nanocubes are another new type of plasmonic nanostructures that are capable to cover the entire visible spectrum. Compared with the 65-nm common nanocubes, the 67-nm concave nanocubes showed a dramatic red-shift of the extinction peak. Such a sharpness-dependent phenomenon has been investigated in the case Ag nanocubes.7 It is the first time that the dependence of the LSPR on the corner/edge sharpness is observed for Pd nanocubes. In general, this shift can be attributed to the accumulation of surface charges at the sharp features of plasmonic nanostructures. The increased charge separation reduces the restoring force for electron oscillation, causing the resonance peak to shift to lower energy. Concave Pd nanocubes with larger sizes have broader extinction peaks than smaller ones. As the size of Pd nanocubes increases, the scattering component of their extinction spectra becomes larger and the difference between the peak position of absorption and extinction increases.39 Therefore, the extinction spectra of Pd nanocubes with larger sizes exhibit broader peaks.

Figure 3. Normalized UV−vis extinction spectra of regular and concave Pd nanocubes with different sizes.

Growth Mechanism of Pd Concave Nanocubes. The introduction of foreign metal ions during the growth of noble metal NCs is an effective strategy to control the shapes of noble metal NCs.23,41−46 For example, the introduction of Ag ions during the seed-mediated growth of Au NCs has been extensively used to produce Au NCs with high-energy facets.47,48 The underpotential deposition (UPD) of Ag on Au NCs can selectively stabilize facets with more open structures, such as {310} and {730} high index facets. Recently, copper(II) ions have been also investigated during the growth of noble metal NCs. Copper salts have been used to assist the growth of hexoctahedral Au−Pd alloy NCs,49 Pd nanorods,50 Pd tripods,51 polyhedral Ag NCs from cubes to octahedra,52 and Ag NCs with concave surfaces.45 Herein, we first found that CuSO4 can assist the formation of Pd concave nanocubes with sub-10-nm edges and corners. To elucidate the role of CuSO4 on the growth of Pd concave nanocubes, experiments were conducted with another two 2183

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Figure 4. SEM images of Pd nanocubes synthesized with different amounts of CuSO4: (a) 40 μL, 10 mM, (b) 20 μL, 10 mM, (c) 100 μL, 1 mM, (d) 50 μL, 1 mM, (e) 25 μL, 1 mM, and (f) 0. Scale bar: 100 nm.

Figure 5. SEM images of Pd NCs synthesized with different volumes of 0.1 M ascorbic acid: (a) 15, (b) 25, (c) 50, (d) 100, (e) 200, and (f) 400 μL. Scale bar: 100 nm. 2184

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Figure 6. (a) Pd 3d and (b) Cu 2p XPS spectra of Pd concave nanocubes (red) synthesized with CuSO4 and regular Pd nanocubes (blue) synthesized without CuSO4.

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Pd precursors.60 The fast reduction of Pd precursors by ascorbic acid radicals tends to occur on the edges (in the [110] directions) and corners (in the [111] directions) of the cubic Pd seeds and leads to the formation of Pd concave nanocubes. In addition, CTAB also plays an essential role in stabilizing the {100} facets and favors the formation of the cubic cores of the concave nanocubes.

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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CONCLUSION In summary, the effect of edge and corner sharpening on the plasmonic properties of Pd nanocubes was first investigated. By forming sub-10-nm edge and corner on Pd nanocubes, dramatic redshifts were observed for their extinction spectra. Introduction of Cu(II) species could selectively activate the growth of sharp corners and edges of Pd nanocubes during the growth. Twelve sharp edges and 8 sharp corners with sub-10-nm thickness could be formed by tuning the reaction kinetics. The role of copper species in the growth of the Pd concave nanocubes was discussed. Our results not only resulted in a class of new plasmonic nanostructures with potential application for plasmonic hydrogen sensing, in situ surfaceenhanced Raman spectroscopy for catalysis, and solar energy harvesting, but also suggested a new approach of forming Pd sharp features on noble metal NCs.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of cubic Pd seeds, SEM images of regular Pd nanocubes with different sizes, SEM images of Pd concave nanocubes synthesized in the presence of CuCl2 and Cu(NO3)2, SEM images of Pd nanostructures synthesized with different concentrations of CTAB, and TEM images of products obtained at different intervals during the growth of Pd concave nanocubes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2185

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