Seed-Mediated Growth of Nearly Monodisperse Palladium Nanocubes with Controllable Sizes Wenxin Niu,†,‡ Zhi-Yuan Li,§ Lihong Shi,†,‡ Xiaoqing Liu,†,‡ Haijuan Li,†,‡ Shuang Han,†,‡ Jiuan Chen,†,‡ and Guobao Xu*,†
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 12 4440–4444
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China, Graduate UniVersity of the Chinese Academy of Sciences, and Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China ReceiVed March 4, 2008; ReVised Manuscript ReceiVed May 6, 2008
ABSTRACT: Nearly monodisperse Pd nanocubes with controllable sizes were synthesized through a seed-mediated growth approach. By using Pd nanocubes of 22 nm in size as seeds, the morphology of the as-grown nanostructures was fixed as single-crystalline, which enabled us to rationally tune the size of Pd nanocubes. The formation mechanism of initial 22 nm nanocubes was also discussed. The size-dependent surface plasmon resonance properties of the as-synthesized Pd nanocubes were investigated. Compared with previous methods, the yield, monodispersity, perfection of the shape formation, and the range of size control of these nanocubes are all improved. These Pd nanocubes may have potential interests in surface-enhanced Raman scattering, sensors, catalysis, study of size-dependent properties, and fabrication of high-order structures. Introduction Metallic nanoparticles show many properties that are significantly different from those of their bulk counterparts.1 To study their size- and shape-dependent properties, considerable attention has been paid to developing new synthetic methods,2,3 and significant progress has been made while many subjects remain unexplored. Palladium, well-known for its high hydrogen solubility4 and effective catalytic activity5 for Stille, Heck, and Suzuki reactions, has received much attention in recent research of shape-controlled synthesis of metallic nanostructures. Various Pd nanostructures have been synthesized;6 a polyol process7 and a water-based synthesis developed recently8 have been proven to be very versatile methods. Pd nanocubes have been synthesized through several methods;9-12 however, most of these methods still need improvement in terms of yield, monodispersity, perfection of cube formation, and size control. In this study, we report a facile process to prepare small Pd nanocubes of 22 nm in size with a high yield of 95.5%. Using the assynthesized small Pd nanocubes as seeds, nearly monodisperse Pd nanocubes with controllable sizes from 37 to 109 nm could be obtained through a seed-mediated growth approach. Compared with previous methods, the yield, monodispersity, perfection of the shape formation, and the range of size control of these nanocubes are all improved. The formation mechanisms of the Pd nanocubes were investigated and discussed. The surface plasmon resonance (SPR) properties of Pd nanocubes were investigated and compared with theoretical calculations using the discrete dipole approximation (DDA). Experimental Section Chemicals and Materials. Cetyltrimethylammonium bromide (CTAB, 99.0%) was obtained from Shanghai Sagon Company (China). LAscorbic acid (99.7%) was obtained from Beijing Chemical Reagent Company (China). Sodium borohydride (NaBH4, 96%) and palladium(II) chloride (PdCl2, anhydrous, 59% as Pd) were obtained from * Author to whom correspondence should be addressed. E-mail: guobaoxu@ ciac.jl.cn. Tel: +86-431-85262747. Fax: +86-431-85262747. † Changchun Institute of Applied Chemistry. ‡ Graduate University of the Chinese Academy of Sciences. § Institute of Physics, Chinese Academy of Sciences.
Sinopharm Chemical Reagent Co., Ltd. (China). Cetyltrimethylammonium chloride (CTAC, 98.0%) was obtained from Tianjin Guangfu Fine Chemical Research Institute (China). All the chemicals were analytical grade reagents and used without further purification. Double distilled water was used throughout the experiments. A ten millimolar H2PdCl4 solution was prepared by dissolving 0.1773 g of PdCl2 in 10 mL of 0.2 M HCl and further diluting to 100 mL with double distilled water. Synthesis of 22 nm Pd Nanocubes. 0.5 mL of 10 mM H2PdCl4 solution was added to 10 mL of 12.5 mM CTAB solution under stirring, and then the solution was heated at 95 °C for 5 min before 80 µL of freshly prepared 100 mM ascorbic acid solution was added. The reaction was allowed to proceed for 30 min. Seed-Mediated Growth of Bigger Pd Nanocubes. 125 µL of 10 mM H2PdCl4 solution was added to 5 mL of 50 mM CTAB solution kept at 40 °C, different volumes (400 µL for 37 nm, 200 µL for 44 nm, 80 µL for 56 nm, 40 µL for 76 nm, 10 µL for 109 nm nanocubes) of the as-synthesized 22 nm Pd nanocubes solution were added, and then 25 µL of freshly prepared 100 mM ascorbic acid solution was added and mixed thoroughly. The resulting solution was placed in a water bath at 40 °C. The reactions were stopped after 14 h by centrifuging (12000 rpm, 10 min). The precipitates were redispersed in deionized water for UV-visible extinction spectra characterization. Two more centrifugations (12 000 rpm, 10 min) were applied to the samples for scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterization. Control Experiments. (a) One-step synthesis of Pd nanoparticles (CTAC-capped Pd seeds) in the presence of CTAC. All procedures were the same as those described for synthesis of 22 nm Pd nanocubes except that CTAC was used instead of CTAB. (b) Seed-mediated growth of bigger Pd nanoparticles with CTAC-capped Pd seeds in CTAB solution. All procedures were the same as those described for seed-mediated growth of bigger Pd nanocubes except that 80 µL of CTAC-capped Pd seed solution was added as seeds. (c) Seed-mediated growth of bigger Pd nanoparticles with CTAC-capped Pd nanoparticle as seeds in CTAC solution All procedures were the same as those described for seedmediated growth of bigger Pd nanocubes except that 80 µL of CTACcapped Pd seed solution was added as seeds and CTAC solution was used instead of CTAB solution. (d) Seed-mediated growth of bigger Pd nanoparticles with smaller Pd nanoparticles of 3-4 nm in size as seeds. All procedures were the same as those described for seedmediated growth of bigger Pd nanocubes except that 2.5 µL of smaller Pd nanoparticle solution was added as seeds. Instrumentation. TEM studies were performed with a HITACHI H-8100 TEM operated at 200 kV. SEM images were taken using a FEI XL30 ESEM FEG scanning electron microscope operating at 25 kV. Selected area electron diffraction studies were performed on a Tecnai G2 20 S-TWIN TEM operated at 200 kV. At least 100 nanocubes
10.1021/cg8002433 CCC: $40.75 2008 American Chemical Society Published on Web 09/25/2008
Growth of Nearly Monodisperse Pd Nanocubes
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Figure 2. SEM image of the assembly of 56 nm Pd nanocubes (scale bar: 1 µm).
Figure 1. (a) SEM image of initial small Pd nanocubes of 22 nm in size. The inset shows corresponding TEM image (scale bar: 100 nm). (b-f) SEM images of 37 nm, 44 nm, 56 nm, 76 nm, and 109 nm Pd nanocubes (scale bar: 200 nm). were measured for size distribution. UV/vis extinction spectra were taken at room temperature on a CARY 500 Scan UV/vis/near IR spectrophotometer using a quartz cuvette with an optical path of 1 cm. X-ray diffraction measurements were obtained with a Philips PW1710 X-ray diffractometer (Cu KR radiation).
Results and Discussion Initial small Pd nanocubes were synthesized through a onestep reduction of H2PdCl4 with ascorbic acid in 12.5 mM CTAB aqueous solution at 95 °C. The reduction process was rather fast and the color of the solution turned brownish yellow within 30 s. The reaction was allowed to proceed for 30 min. Figure 1a shows SEM and TEM images of these nanocubes. The yield for these nanocubes was 95.5%, and byproducts included polyhedral (3.8%) and rodlike (0.6%) nanoparticles. The average size and size distribution of these nanocubes are 22 nm and 3.6%, respectively. A corresponding histogram of size distribution is shown in Figure S1a, Supporting Information. To grow bigger Pd nanocubes, a seed-mediated growth approach was adopted using the as-synthesized 22 nm nanocubes as seeds. A seed-mediated growth approach separates the nucleation and growth stages of nanoparticles, which provides better control over the size, size distribution, and shape evolution of nanoparticles.3 It has been reported that the growth of Pd nanoparticles could proceed in a controlled manner at relatively low temperatures.11,13,14 Therefore, our growth reactions were performed at 40 °C in 50 mM CTAB solution with ascorbic acid as reductant, and were stopped by centrifugation after 14 h. The final edge length of nanocubes could be tuned by adding different volumes of seed solution. Figure 1b-f shows SEM images of five batches of the enlarged nanocubes obtained by adding different volumes of seed solutions. By comparison, the
Figure 3. TEM images and corresponding SAED patterns of (a, b) a single seed Pd nanocube and (c, d) a single seed-mediated grown Pd nanocube (scale bar: 20 nm).
volume increment of a single Pd nanocube increased about 36 times as the volumes of the added seed solution decreased from 400 to 10 µL. Figure S1b-f, Supporting Information shows histograms of size distributions for the five batches of nanocubes. Table S1, Supporting Information shows corresponding average sizes, size distributions, yields, and byproducts of the nanocubes. The average sizes (size distributions) of the five batches of nanocubes are 37 nm (3.7%), 44 nm (4.4%), 56 nm (5.0%), 76 nm (7.1%), and 109 nm (7.0%), respectively. The yields for the five batches of nanocubes are 92.4%, 90.2%, 93.3%, 75.2%, and 70.6%, respectively. 37, 44, 56, and 76 nm Pd nanocubes shared the features of nearly perfect sharp corners and edges, while 109 nm Pd nanocubes were slightly truncated. Some of these nanocubes could assemble into large-scale ordered twodimensional arrays on the ITO substrates (Figure 2), which also demonstrated their monodispersity. In accordance with previous reports, residual CTAB was found to be essential for the formation of ordered assembly.2g,12 If the sample was washed three times (once more than the normal procedure), ordered assembly of the nanocubes could not form (Figure S2, Supporting Information). However, the residual CTAB could affect the quality of the SEM images; therefore, Figure S2, Supporting Information are slightly clearer than Figure 1. Figure 3 shows TEM images and corresponding selected area electron diffraction (SAED) patterns of a single seed Pd nanocube and a single seed-mediated grown Pd nanocube. The square spot arrays of SAED patterns show that they both are
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Figure 4. UV/vis extinction spectra of Pd nanocubes of different sizes: (a) 22 nm; (b) 37 nm; (c) 44 nm; (d) 56 nm; (e) 76 nm; (f) 109 nm (all spectra are normalized against the intensities of their main adsorption peaks).
single crystals bound by {100} facets; such SAED patterns have been observed in the case of silver and gold nanocubes.15 The X-ray diffraction (XRD) pattern of Pd nanocubes of ∼50 nm in size has an abnormally intense (200) peak (Figure S3, Supporting Information), which suggests that most of the Pd nanocubes were preferentially oriented with their {100} facets parallel to the substrate. Size control is an efficient method to tune the SPR properties of Pd nanocubes.9 Here we investigated these properties in a broader size range. Figure 4 shows that the SPR peaks of Pd nanocubes progressively shift from 290 to 580 nm as the sizes of Pd nanocubes increase from 37 to 109 nm. In contrast to smaller ones, the SPR properties of 109 nm nanocubes showed some new features. The extinction peak became extremely broad, and a shoulder peak appeared at about 400-450 nm. Theoretical extinction cross-section curves of a Pd nanocube of 109 nm calculated using DDA16,2c are displayed in Figure 5b. The location of the calculated peaks roughly matches with the experimentally measured spectrum shown in Figure 5a. We consider that the deviation is due to a relatively low yield (70.6%) for 109 nm Pd nanocubes. Moreover, truncation of 109 nm Pd nanocubes may also lead to a blue-shift of the SPR bands.17 According to the DDA calculations, we can also identify that the shoulder peak of experimentally measured spectrum is from adsorption while the main peak is an overlap of scattering and adsorption. Moreover, 109 nm Pd nanocubes show strong scattering properties compared with 50 nm ones reported previously.9 Two considerations were taken into account to understand the formation mechanisms of Pd nanocubes, that is, why the nanocrystals were formed as single crystals and why the nanocrystals were formed bound by {100} facets. In the case of growth of 22 nm Pd nanocubes, controlled experiments were carried out to understand why the nanocrystals were formed as single crystals. We found the reaction became slow when the concentration of CTAB was increased to 50 mM or 100 mM while keeping other parameters unchanged. The final products were polydisperse nanoparticles (Figure S4a,b, Supporting Information). Similar results were obtained when the reaction temperature was decreased to 40 °C (Figure S4c, Supporting Information). Therefore, relatively fast nucleation favors the formation of Pd nanocubes.7,18 Fast growth may have reduced the time available for the formation of twin defects,19 resulting in the formation of single crystal Pd nanoparticles. Oxidative etching process has been
Figure 5. (a) UV/vis extinction spectra of 109 nm Pd nanocubes. (b) Extinction (Qext), scattering (Qsca), and absorption (Qabs) cross sections calculated for a Pd nanocube with an edge length of 109 nm using the DDA method (Qext ) Qabs + Qsca).
used to grow single-crystalline nanoparticles by selectively dissolving twinned nanoparticles with chloride or bromide in the presence of oxygen.20 It may not be responsible for the formation of single-crystalline Pd nanocubes herein for the following reasons: (1) In oxidative etching process, Cl- is known to be more corrosive than Br-;20c,21 therefore, it was supposed to get single-crystalline nanoparticles in the presence of cetyltrimethylammonium chloride (CTAC) if the reaction followed the oxidative etching mechanism. However, if CTAB was replaced with CTAC, the final product was a mixture of polydisperse nanoparticles of irregular shapes (Figure S5a, Supporting Information). Seed-mediated growth of larger Pd nanoparticles by using these CTAC-capped nanoparticles as seeds in 50 mM CTAB solution also produced various nanostructures, which suggests the nonuniformity of crystal structures of the initial CTAC-capped seeds (Figure S5b, Supporting Information). (2) Oxygen is necessary in the oxidative etching process.20 However, ascorbic acid is an oxygen scavenger22 and has been used to prevent copper nanoparticles from being oxidized.23 It may play the same role for Pd nanoparticles. (3) Oxidative etching could be partially eliminated at low temperatures such as 70 °C.18 However, we have obtained nanocubes in high yields at temperatures as low as 65 °C (Figure S6, Supporting Information). In the seed-mediated growth procedure, the adoption of 22 nm Pd nanocubes as seeds was responsible for preservation of the single-crystalline nature of the as-synthesized larger Pd nanocubes. To demonstrate the importance of seeds, we prepared smaller Pd nanoparticles of 3-4 nm according to Berhault et al., 80% of these nanoparticles were supposed to be singlecrystalline.11 When these smaller Pd nanoparticles were used as seeds, the product was a mixture of multiple shapes (Figure S7, Supporting Information). This result suggests that the
Growth of Nearly Monodisperse Pd Nanocubes
utilization of large single-crystalline seeds was critical for the formation of nanocubes in high yields in the seed-mediated growth procedure. It is believed that crystal structure of seeds fluctuates at very small sizes, but the structure will be fixed as single-crystalline or multitwinned as the size of the crystal increases.24 The size of the seeds used in our synthesis was large enough to avoid twinning, which enabled us to rationally tune the size of Pd nanocubes through the seed-mediated growth procedure. The as-synthesized Pd nanocubes are bound by {100} facets. It has been reported that CTAB could preferentially bind to the {100} crystal facets of palladium and promote the formation of {100} facets of palladium.6g,13 In a control experiment CTAC was used instead of CTAB in the whole seed preparation and seed-mediated growth procedures; the as-synthesized nanoparticles were polydisperse, and most of these nanoparticles had rough surfaces (Figure S5c, Supporting Information). The appearance of these irregular shapes implies that the presence of bromide is essential for the formation of well-defined {100} facets. A previous report has studied the adsorption of bromide on the surface of Pd nanostructures in detail.21 Bromide was found to be capable of chemisorbing onto the surface of palladium seeds to promote the formation of {100} facets. Compared with bromide, chloride cannot efficiently stabilize and promoted the formation of {100} surfaces of Pd. Another report also suggested that bromide ions from trimethyl(tetradecyl)ammonium bromide effectively stabilize the {100} faces of rhodium, platinum, and palladium.25 Both reports and the control experiment with CTAC as surfactant suggest that Brfrom CTAB played an important role in the formation of nanocubes with well-defined {100} surfaces. Finally, we note that the ability of producing Pd nanocubes larger than 109 nm by this seed-mediated growth procedure is still limited. Unlike seed-mediated growth of gold and silver nanostructures,3 catalytical growth of Pd on preformed seeds is quite slow at 40 °C. If too few seeds were added to the growth solution, spontaneous nucleation would occur because there were too few deposition spots. In an experiment with only 1 µL of seeds added, the initial seeds grew into large cuboctahedra of 160 nm in size while spontaneous nucleation resulted in the formation of Pd nanorods, right bipyramids, and small nanocubes (Figure S8a,b, Supporting Information). It is noteworthy that Pd cubooctahedron are bound by {100} and {111} facets.20b,21 The formation of Pd cubooctahedron suggests that the growth rate of Pd along and directions could be modulated by adding different volumes of seed solution. Recent reports also showed that Pd octahedral and icosahedra bound by {111} facets could be synthesized in the presence of CTAB.11 These results suggest that finer shape control over the nanocrystals may be achieved through selective stabilization of crystal facets and growth kinetic modulation. However, spontaneous nucleation added much complexity to the reaction; further studies are still needed to fully understand the growth mechanism of Pd cubooctahedron. Conclusion In conclusion, the seed-mediated growth approach was demonstrated as an efficient method to synthesize nearly monodisperse palladium nanocubes in high yields with broad size control. By using 22 nm Pd nanocubes as seeds, twinning was successfully avoided during the growth process. The sizedependent surface plasmon resonance properties of the assynthesized Pd nanocubes were investigated. 109 nm Pd nanocubes show strong scattering properties compared with 50
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nm ones. These Pd nanocubes may have potential applications in surface-enhanced Raman scattering, SPR sensors, catalysis, and fabrication of high-order structures. Acknowledgment. We gratefully acknowledge support from the National Natural Science Foundation of China (No. 20505016), Department of Sciences & Technology of Jilin Province (20070108), and Hundred Talents Program of Chinese Academy of Sciences. Z.-Y.L. thanks the National Natural Science Foundation of China (No. 10525419 and 60736041) for financial support. Supporting Information Available: XRD pattern of Pd nanocubes of ∼50 nm, histograms of size distribution of Pd nanocubes of different sizes, SEM images of Pd nanoparticles synthesized at different conditions (Figures S1-S8), and size and shape distributions of Pd nanocubes of different sizes (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.
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CG8002433
