An Effective Strategy for Room-Temperature Synthesis of Single

Mar 18, 2009 - All reactions were performed at room temperature. .... 1214, and 1588 cm−1 and assigned to the ring breathing (v1), C−H in-plane de...
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CRYSTAL GROWTH & DESIGN

An Effective Strategy for Room-Temperature Synthesis of Single-Crystalline Palladium Nanocubes and Nanodendrites in Aqueous Solution Feng-Ru Fan, Adel Attia, Ujjal Kumar Sur, Jian-Bin Chen, Zhao-Xiong Xie, Jian-Feng Li, Bin Ren, and Zhong-Qun Tian*

2009 VOL. 9, NO. 5 2335–2340

State Key Laboratory of Physical Chemistry of Solid Surfaces and College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, China ReceiVed NoVember 4, 2008; ReVised Manuscript ReceiVed January 28, 2009

ABSTRACT: Nearly monodisperse single-crystalline palladium (Pd) nanocubes and nanodendrites have been successfully prepared in aqueous solution at room temperature, for the first time, mainly by utilizing the equilibrium between the dissolution and precipitation of the Pd-cetyltrimethylammonium bromide (Pd-CTAB) complexes. The morphology of the obtained Pd nanocrystals can be tuned by the addition of foreign halide ions (Cl- and Br-). The corresponding selected area electron diffraction (SAED) and X-ray diffraction (XRD) patterns confirmed that the synthesis of nanocubes and nanodendrites are single-crystalline pure Pd structures with fcc crystal lattice. A preliminary formation mechanism based on the dissolution and precipitation of the Pd-CTAB complexes and competitive adsorption between different ions on the nanoparticle surface has been proposed. Introduction Metal nanoparticles have been extensively studied for many years because they usually exhibit unique properties and can be potentially used in many applications, including optics,1 catalysis,2 biodiagnostics,3 and surface-enhanced Raman scattering (SERS).4,5 The chemical and physical properties of metal nanoparticles depend not only on their size but also on their morphology.6 Many diverse shapes of metal nanoparticles have been synthesized and the nanocube is one of the most important shape-controlled nanoparticles.7 For instance, the silver nanocubes show much higher styrene oxidation activity than near-spherical nanoparticles and nanoplates because of their more-reactive {100} planes.8 In 2002, Sun and Xia reported the possibility of synthesizing Ag nanocubes in ethylene glycol (EG) in the presence of poly (vinyl pyrrolidone) (PVP).9 Since then, many successful attempts to prepare Au,10 Pd,11 Pt,12 and Rh13 nanocubes in the EG/PVP organic system have been reported. Metal nanocubes can also be obtained in aqueous solution by different ways,14-16 which builds a good foundation for the application of metal nanocubes. Pd nanoparticles act as a primary catalyst for many organic reactions such as Suzuki and Heck reactions17-21 and have played an important role in many catalytic and industrial applications.2,22,23 As one kind of potential important nanomaterial, Pd nanocubes have attracted much attention from researchers. For example, Xia’s group succeeded in the synthesis of monodisperse Pd nanocubes in EG/PVP organic system, through a seed etching process by using FeCl3 as etchant where the cube dimension was found to be dependent on the concentration of this etchant.11 Such kind of cubic nanostructures can be employed as a template for the formation of nanoboxes and nanocages used as SERS substrates.24,25 Rafailovich and co-workers synthesized the rectangular Pd nanoparticles, including nanocubes and nanorods, under the assistance of CTAB and trisodium citrate.26 Berhault et al. investigated the factors influencing kinetic- or thermodynamic-controlled Pd nanocrystals synthesis on the basis of seed-mediated growth with * To whom correspondence should be addressed. E-mail: [email protected]. Fax: 86-592-2085349.

CTAB.27 Recently, a great effort has been made to investigate the influence of halide ions on the seed-mediated growth of Au nanocrystals.28 In contrast, there is almost no report on shapecontrolled synthesis of Pd nanocrytals tuned by foreign halide ions. Most recently, high-yield monodisperse Au@Pd core-shell nanocubes and pure Pd nanocubes have been obtained by our group and Xu’s group, respectively, using a seed-mediated growth mechod.29 However, there is no report on the preparation of pure Pd nanocubes with one step in aqueous solution at room temperature. A typical report indicated that Pt nanocubes prepared in aqueous solution and caped by cetyltrimethylammonium bromide (CTAB) have the highest activities of catalysis compared with Pt nanocubes prepared in EG solution and caped by PVP.30 If the single-crystal Pd nanocubes can be obtained in aqueous solution easily and quickly by effectively controlling the shape of Pd nanocrystals, then such Pd nanocubes can be utilized in diverse applications. In this work, we took a simple approach to synthesize nearly monodisperse Pd nanocubes and nanodendrites in water at room temperature. The morphology can be tuned by the addition of some different concentration of foreign halides. Moreover, the synthesized nanocubes and nanodendrites are single crystal with face-centered cubic (fcc) structure and different sizes can be obtained. We have also utilized these Pd nanocubes and nanodendrites as possible SERS substrates. Experimental Section Chemical. Palladium(II) chloride (PdCl2), cetyltrimethylammonium bromide (CTAB), sodium chloride (NaCl), and ascorbic acid (AA) were purchased from Shanghai Chemicals Co. Ltd. All the chemicals used are of analytical-grade reagents and used without any further purification. Water purified with a Milli-Q system was used throughout the study. Synthesis. Pd nanocubes were synthesized as follow: First, a 10 mM Na2PdCl4 aqueous solution was prepared by completely dissolving 44.5 mg of PdCl2 in 5 mL of 100 mM HCl and 19 mL of deionized water, followed by the addition of 1 mL of 500 mM NaOH. Typically, the Na2PdCl4 and CTAB solutions were added to the AA solution in the following two different ways. In the first method (method A), 1 mL of 10 mM Na2PdCl4 was mixed with 1 mL of 10 mM CTAB, and the mixture was then dripped into a flask containing 4 mL of 25 mM

10.1021/cg801231p CCC: $40.75  2009 American Chemical Society Published on Web 03/18/2009

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Figure 1. (a) SEM image of Pd nanocubes; (b) XRD pattern of the as-prepared Pd nanocube; (c) TEM image (inset) and the corresponding SAED pattern taken with the incident beam perpendicular to one facet of the cube; (d) typical HRTEM image of a selected corner of an individual Pd nanocube. AA over a period of 5 min. The whole solution was kept under stirring for an hour. In the second method (method B), 1 mL of 10 mM Na2PdCl4 and 1 mL of 10 mM CTAB were injected simultaneously into a flask containing 4 mL of 25 mM AA with a homemade twochannel syringe pump at a rate of 12 mL/h. The whole solution was then kept under stirring for another 30 min. All reactions were performed at room temperature. In another set of experiments, Pd nanodendrites were obtained by the addition of 1 mL of 0.1 M NaCl to Na2PdCl4 prior to addition of the CTAB and AA, and other conditions were the same as described in method A. When it is necessary, NaI and NaBr were used occasionally instead of NaCl. The products from each method were centrifuged at 4000 rpm for 20 min; the precipitates were then collected and redispersed in water, followed by centrifugation at the same speed for 20 min. This step was then repeated twice to remove the unreacted AA and CTAB. Finally, the precipitates were dispersed in water for further analyses. Characterization. The composition and crystal phase of the obtained samples were checked by X-ray diffraction (XRD, PANalytical X-pert diffractometer with Cu KR radiation). The size and morphology of asprepared samples were characterized by scanning electron microscopy (SEM, LEO1530) with a field-emission electron gun. The highresolution transmission electron microscopic (HRTEM) observations and selected area electron diffraction (SAED) were performed on a FEI TECNAI F30 microscope operated at 300 kV. The optical absorption was recorded by using a Shimadzu-2100 UV-visible spectrophotometer. The SERS measurements were performed on a confocal microprobe Raman system (LabRam I) using a He-Ne laser (632.8 nm) as the excitation source.

Results and Discussion Figure 1a shows the typical SEM images of sample prepared by method A. The sample consists of cubical particles and has eight sharp corners with a very uniform size of about 60 nm. The yield for these nanocubes is nealy

60-70% with high monodispersity. The XRD pattern of the as-prepared Pd nanocubes is shown in Figure 1b. All of the peaks can be indexed to face-centered cubic (fcc) bulk Pd metal (JCPDS Card no. 05-0681). The absence of other diffraction peaks other than Pd metal indicates that this material was synthesized successfully as a pure phase. SAED pattern of the corresponding nanoparticle (inset in Figure 1c), which was obtained by directing the electron beam to one of the facets of the cube, infers that the particle is a single crystal enclosed by six {100} faces. Figure 1d shows the HRTEM image of a selected corner of an individual Pd nanocube and the lattice fringes of the image were measured to be 0.144 nm in agreement with the {220} lattice spacing of the fcc crystal, which also indicates that the individual cube is a single crystal. Besides the cubic morphology, there are also a small amount of nanodendrites that can be observed during the synthesis of Pd nanocubes. The ratio between nanocube and nanodendrites can be easily controlled by controlling the concentration of NaCl. Figure 2a shows the typical SEM images of Pd nanodendrites obtained by adding 1 mL of 0.1 M NaCl in method A. HRTEM images and electron diffraction patterns reveals that each regular Pd nanoderdrite is also a single crystal with a fcc structure. Figure 2b-d shows the SAED pattern and TEM image of an individual nanodendrite and the HRTEM image of a selected corner. The lattice fringes of the image were measured to be 0.227 nm which is in agreement with the {111} lattice spacing of Pd crystal.

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Therefore, these dendrite-shaped particles are also single crystals indicated by the spot pattern of SAED and HRTEM image. There are many factors that can directly influence the morphology of the Pd nanoparticles. Usually, the surfactant plays the most important role in shape-controlled nanoparticle synthesis. CTAB and PVP have been widely used as regulating agents for the selective growth of nanocrystals with well-defined shapes. As an ionic surfactant, CTAB is used in the chemical synthesis of many types of metal nanoparticles where it is mainly considered as a stabilizer or capping agent with a role of preventing nanoparticles to agglomerate.31 In method A, CTAB plays another important role; it is reacted with Na2PdCl4, forming complex-surfactant postmicellar aggregates (Figure 3a). Upon the addition of CTAB to Na2PdCl4, the color of the solution changed from yellow to orange and then transformed into orange precipitate. When an excess of AA was added dropwise into the colloid, the dissociative organic salt was reduced. The equilibrium between the dissolution and precipitation of complex-surfactant salt is anticipated to move slowly toward dissolution and the concentration of the dissociative organic salt during this process is in equilibrium and is believed to be stable until these reactions are completed. The equations representing these reactions can be described as follows

[PdCl4-nBrn]2-(aq) + 2CTA+(aq) a (CTA)2[PdCl4-nBrn](s)

(1)

[PdCl4-nBrn]2-(aq) + C6H6O4(OH)2(aq) f Pd(NPs) + C6H6O6(aq) + (4 - n)Cl-(aq) + 2H+(aq) + nBr-(aq) (2) Upon mixing CTAB with Na2PdCl4, the Br- dissociated from CTAB can replace Cl- and bind to Pd2+,32 then react with CTA+ forming an organic salt (eq 1). The exact composition of the organic salt was unclear until now, because a series of mixed ligand complexes can be formed, [PdCl4-nBrn]2-, where the n value lies between 0 and 4 and depends on the relative concentrations of the mixture of two halides (Cl- and Br-) in aqueous solutions. 33 To verify and explore the above process, we have analyzed the UV-visible spectra of the solution at different stages of the reaction between Na2PdCl4 and CTAB (panels c and d in Figure 3). At high [Na2PdCl4]/[CTAB] molar ratios, the peak at 290 nm indicating the formation of [PdCl4-nBrn]2- became broader and accordingly, the peak position red-shifted, which can be attributed to the coexistence of various kinds of Pd complexes. Upon storage, an organic salt (CTA)2[PdCl4-nBrn] precipitated and the peaks of the solution disappeared when the [Na2PdCl4]/[CTAB] molar ratio was about 0.5. The peaks at 209 and 238 nm assigned to [PdCl4]2- then emerged gradually and became noticeable. We suggest that the reaction described by eq 1 is reversible and Pd nuclei can be directly generated from the Pd-CTAB complexes. With the reduction of [PdCl4-nBrn]2- by AA, the equilibrium of reaction 1 turns to the left until the reactions are completed. Figure 3b shows representative energy-dispersive X-ray spectroscopy (EDX) spectra of the organic complexes, which clearly indicates the presence of complex elements (Br and Cl) in

Figure 2. (a) SEM image of Pd nanodendrites; (b, c) SAED pattern and TEM image of an individual Pd nanodendrites; (d) typical HRTEM image of a selected corner of an individual Pd nanodendrite.

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Figure 3. (a, b) SEM image and EDX spectrum of the Pd-CTAB complexes. The inset of B shows a typical EDX pattern of the prepared Pd nanocubes, the peak of Si results from the silicon substrate. (c, d) UV-vis spectra of the solution with different [Na2PdCl4]/[CTAB] molar ratios.

organic salt and C to be originated from the alkyl chains of CTAB. The inset shows the EDX spectra of Pd nanocubes; the Si appearing in the EDX is from the silicon wafer substrate. In this pattern, there are no signals of other elements except Pd and Si, which further confirms the high purity of synthesized Pd nanocubes. CTAB usually acts as a particle stabilizer, and the surface group may preferentially bind to certain crystal faces of the particles for controlling the shape of nanoparticles.34 However, the exact binding nature between these capping reagents and the specific crystal faces is still elusive, and there are no generalized mechanisms to interpret various metal nanocrystal shape-control experiment.12 At this point, a question on the role of the type of ions and their adsorption characteristics affecting the binding nature between capping reagent and the crystal facets comes up. We choose chloride ions as competitive ions to investigate the possibility of this role. In the experiment, many more dendriteshaped nanoparticles are obtained by the addition of 1 mL of 100 mM NaCl solution into the flask. Figure 2a shows the typical SEM image of dendrite-shaped Pd nanoparticles, which have larger size compared to Pd nanocubes. The ratio of dendrite to cube structure can be increased by increasing the amount of NaCl. Such a phenomenon can be described by a kinetic rather than thermodynamic control of the reaction, which can be attributed to the competitive adsorption of foreign ions and original adsorbates. Selective adsorption of CTAB on certain growing faces of nanoparticles may lead to an anisotropic structure.34 When CTAB and an excess amount of chloride ions coexist in solution, chloride ions may adsorb more strongly on the nanoparticle surfaces compared to CTAB alone. As an inorganic ion, chloride ion does not have remarkable stabilization and selective effect on adsorption, which leads to the formation of an isotropic structure. The competitive adsorption

between CTAB and chloride ions determines the shapes of the produced nanostructures. This finding is in agreement with the work of Xiong et al.11 However, they attributed the increase in the Pd nanocubes to the FeCl3 (which acts as an etchant) and not to the chloride ions. When bromide or iodide ions are used, Pd nanospheres with irregular shape are formed (images a and b in Figure 4). The coagglomeration of nanospheres indicates that CTAB was replaced by bromide or iodide ions to adsorb on the surfaces of nanoparticles and CTAB did not have the function as particle-stabilizer anymore. This is due to the strong adsorption of bromide or iodide ions on Pd surface compared to chloride ions. Unfortunately, an accurate quantitative value of the relative adsorbability of chloride ions to CTAB has been unavailable until now. A more detailed mechanism can be elucidated by further investigation. The strategy of the synthesis of Pd nanocubes can affect the size of the formed Pd nanocubes, which is shown in Figure 4. For example, Pd nanocubes of about 60 nm were produced by method A (Figure 4c), whereas the dimension of the nanocube was around 35 nm when synthesized by method B (Figure 4d). The smaller dimensions of Pd nanocubes synthesized by method B are due to more seeding compared to that in method A, which is followed by reduction by AA. This can be a new method to control the size of metal nanoparticles by changing the precursor. It is well-known that the SERS activity strongly depends on the size and shape of metal nanoparticles. Therefore, it would be of great interest to examine the SERS activity of the synthesized Pd nanocubes and nanodendrites. In the present study, SERS substrates were prepared by using a simple dropcoat method. A droplet of the Pd nanoparticles sol was dropped on a smooth Pd electrode surface and completely dried in air. Figure 5 shows the SERS spectra of pyridine adsorbed on nanocubes with average size of 60 nm and nanodendrites respectively. As can be seen, the three major bands located at

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Figure 4. SEM images of Pd nanoparticles obtained (a) by adding 1 mL of 0.1 M NaBr in procedure A; (b) by adding 1 mL of 0.1 M NaI in procedure A; (c) in procedure A; (d) in procedure B.

is much more limited in the case of dendrites compared to the cubes. The whole area of the cube in the gap region has enhanced uniformly. This fact results in a stronger SERS signal from nanocubes than that from nanodendrites. The above results indicate that metal nanoparticles with well-defined facet have high potential and advantages in the SERS-based sensors. Pd nanocube is also anticipated to be of importance to both theoretical investigations and practical applications, such as electrocatalysis and hydrogen storage. Further experiments and theoretical calculation based on this subject are under investigation in our laboratory. Conclusions

Figure 5. SERS spectra of pyridine on the nanoparticle-coated electrodes: (a) 60 nm Pd nanocubes obtained in procedure A and (b) nanodendrites. The inset shows a typical SEM image of the nanocubecoated surface.

about 1002, 1214, and 1588 cm-1 and assigned to the ring breathing (v1), C-H in-plane deformation (v9a) and ring stretching (v8a) vibration respectively. These Pd nanocubes exhibit high SERS activity similar to the electrochemically roughened Pd electrode,35 but the nanodendrites have the lower SERS activity. This result is consistent with our previous work on the study of SERS from Pt nanocubes and nanospheres systems.36 The electromagnetic field coupling between nanocubes plays an important role that can bring on the increase in electromagnetic enhancement. The effective area in the nanoparticle junction that can offer the largest SERS enhancement

Single-crystalline, nearly monodisperse Pd nanocubes and nanodendrites have been synthesized successfully in aqueous solution at room temperature. Pd nanocubes can be obtained from two distinct routes, and the shape of nanoparticles can be changed from cubic to dendrite by simply adding foreign chloride ions. A preliminary mechanism based on the dissolution and precipitation of the Pd-CTAB complexes and competitive adsorption between different ions on the nanoparticle surface are proposed. This simple one-step method can be further extended to other transition metals for preparing different shapecontrolled metal nanoparticles. Acknowledgment. This work was supported by the Natural Science Foundation of China (Grant 20620130427) and Ministry of Science and Technology (Grant 2007CB815303). F.R.F. also thanks the Department of Chemistry, Xiamen University, for the Undergraduate Research Award Program (Grant J0630429).

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