Direct Synthesis of Palladium Nanocrystals in Aqueous Solution with

Palladium octahedra, truncated octahedra, cuboctahedra, truncated cubes, and nanocubes with sizes of tens of nanometers have been synthesized in an aq...
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Direct Synthesis of Palladium Nanocrystals in Aqueous Solution with Systematic Shape Evolution Shu-Ya Liu, Yuan-Ting Shen, Chun-Ya Chiu, Sourav Rej, Po-Heng Lin, Yu-Chi Tsao, and Michael H. Huang* Department of Chemistry and Frontier Research Center on Fundamental and Applied Science of Matter, National Tsing Hua University, Hsinchu 30013, Taiwan S Supporting Information *

ABSTRACT: Palladium octahedra, truncated octahedra, cuboctahedra, truncated cubes, and nanocubes with sizes of tens of nanometers have been synthesized in an aqueous mixture of H2PdCl4 solution, cetyltrimethylammonium chloride (CTAC) surfactant, KBr solution, dilute KI solution, and ascorbic acid solution at 35 °C for 30 min. By tuning the amount of dilute KBr solution introduced, particle shape control can be achieved. Adjusting the volumes of the Pd precursor and KBr solutions added, smaller and larger Pd nanocrystals were obtained with excellent shape control. Extensive structural and optical characterization of these nanocrystals has been performed. Two absorption bands in the ultraviolet region can be discerned for these Pd nanocrystals. Concave Pd cubes can also be prepared. Pd cubes were found to grow at a faster rate than that for the formation of octahedra. The concentrations of KBr and KI in the solution are so low that spectral shifts were not detected upon their addition to the solution. The Pd nanocrystals can readily be used for various applications after simple removal of surfactant.

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factors affecting the particle shape.27,28 Thus, it is still important to develop a simple and direct method to make Pd nanocrystals in aqueous solution with tunable shape and size. In this study, a simple method has been developed for the direct formation of Pd nanocrystals in aqueous solution with systematic shape evolution from octahedral to cubic structures through fine control of the amount of KBr introduced. Particles with tunable size can be achieved. Concave Pd nanocubes were also prepared using this method. Formation of different particle shapes has been shown to be related to their different growth rates. The importance of addition of tiny amounts of KBr and KI during particle synthesis has been verified.

INTRODUCTION Palladium nanostructures are highly useful nanomaterials because they have been shown to efficiently catalyze organic coupling reactions such as Suzuki and Sonogashira reactions.1,2 They also catalyze hydrogenation and formic acid oxidation reactions.3−11 A wide variety of Pd nanocrystal morphologies has been prepared including rods, plates, and polyhedra.12 With regard to the examination of facet-dependent catalytic properties of Pd nanocrystals, it is still necessary to synthesize Pd nanoparticles exposing specific surface facets such as cubes and octahedra.13−16 With respect to the synthesis of polyhedral Pd nanocrystals, an important direction of research is to grow Pd nanocrystals with systematic shape evolution. This is because factors controlling the particle shape can be easily identified and the synthesized nanocrystals are most suitable for facetdependent property studies.17,18 Although Au−Pd core−shell nanocrystals with systematic shape evolution from cubic to octahedral structures have been achieved, it is still desirable to grow pure Pd nanocrystals with this series of shape evolution.1,19 Preparation of Pd cuboctahedra, truncated octahedra, and octahedra from the oxidative etching of preformed Pd cubes and redeposition of the dissolved Pd ions have been demonstrated, but this approach is more complicated.20 Use of poly(vinylpyrrolidone) (PVP) as a stabilizer in a polyol system may require an additional PVP removal process.20,21 Further growth of cubic Pd seeds is successful in producing Pd cubes, octahedra, and rhombic dodecahedra, but more experimental steps are needed.22−26 More recently, direct synthesis of Pd cubes, octahedra, and rhombic dodecahedra has been reported, yet the reagents used and their amounts are less systematic to clearly identify the © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Chemicals. Palladium(II) chloride powder (PdCl2, 99%, Aldrich), cetyltrimethylammonium chloride (CTAC, 95%, TCI), L(+)-ascorbic acid (AA, 99.7%, Aldrich), potassium bromide (KBr, Merck), potassium iodide (KI, Showa), and hydrochloric acid (HCl, 37%, Sigma-Aldrich) were used as received. A 10 mM H2PdCl4 solution was prepared by completely dissolving 0.0178 g of PdCl2 powder in 10 mL of 20 mM HCl solution. Synthesis of Palladium Nanocrystals. For the growth of Pd octahedra, truncated octahedra, cuboctahedra, truncated cubes, and cubes, 9.175, 9.150, 9.145, 9.135, and 9.125 mL of deionized water were respectively introduced into sample vials. Subsequently, 0.048 g of CTAC and 0.7 mL of 10 mM H2PdCl4 solution were added to each of the sample vials. The vials were kept in a water bath set at 35 °C throughout the particle synthesis. Then 0, 25, 30, 40, and 50 μL of 1.0 Received: April 14, 2015 Revised: May 31, 2015

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Figure 1. SEM images of (a1−a3) Pd octahedra with average particle sizes of 56, 64, and 69 nm, (b1−b3) truncated octahedra with average sizes of 49, 58, and 61 nm, (c1−c3) cuboctahedra with average sizes of 52, 55, and 60 nm, (d1−d3) truncated cubes with average sizes of 47, 54, and 57, and (e1−e3) nanocubes with average sizes of 45, 48, and 61 nm. All scale bars are equal to 100 nm. × 10−3 M KBr solution were introduced to make Pd octahedra, truncated octahedra, cuboctahedra, truncated cubes, and cubes, respectively. The vials were left undisturbed in the water bath for 15 min for halide ligand replacement reaction to complete.1 Next, 5 μL of 1.0 × 10−3 M KI solution was introduced, and the solution was left undisturbed for another 15 min. Finally, 120 μL of 0.05 M AA was added with thorough mixing, and the mixture was kept in the water bath for another 30 min to form Pd nanocrystals. The solution color turned light brown quickly within 30 s of introduction of AA and became dark brown in 30 min. Finally, the particles were collected by centrifugation at 4000 rpm for 10 min. The precipitate was centrifuged one more time with 10 mL deionized water at 4000 rpm for 10 min to remove the surfactant. Experimental conditions for the formation of Pd nanocrystals with smaller and larger sizes are given in the Supporting Information. Synthesis of Pd Concave Cubes. First, 0.048 g of CTAC, 8.795 mL of deionized water, and 0.7 mL of 10 mM H2PdCl4 solution were

mixed in a sample vial. The vial was kept in a water bath set at 60 °C. Then 50 μL of 1.0 × 10−3 M KBr solution was introduced, and the vial was left undisturbed for 15 min. Next, 5 μL of 1.0 × 10−3 M KI solution was introduced, and the solution was left undisturbed for another 15 min. After 450 μL of 0.05 M AA was added, the solution was thoroughly mixed. The mixture was kept in the water bath set at 60 °C for 30 min to form the concave cubes. The particles were collected by centrifugation at 4000 rpm for 10 min. The precipitate was centrifuged again with 10 mL of deionized water at 4000 rpm for 10 min. Instrumentation. Transmission electron microscopy (TEM) characterization was performed on a JEOL JEM-2100 electron microscope with an operating voltage of 200 kV. Scanning electron microscopy (SEM) images of samples were obtained using a JEOL JSM-7000F electron microscope. Powder X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-6000 diffractometer with Cu Kα radiation. UV−vis spectra were collected using a JASCO V-670 B

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Langmuir spectrophotometer. X-ray photoelectron spectroscopy (XPS) characterization was carried out on a ULVAC-PHI Quantera SXM highresolution XPS spectrometer. Data were recorded with a monochromatized Al anode as the excitation source. The C 1s peak was chosen as the reference peak.

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RESULTS AND DISCUSSION In this study, we have developed a method to directly grow Pd nanocrystals with shape control by mixing an aqueous solution of H2PdCl4, CTAC surfactant, KBr, KI, and ascorbic acid at 35 °C for 30 min. By simply tuning the volume of 10−3 M KBr solution from 0 to 50 μL, Pd nanocrystals with systematic shape evolution from octahedral to cubic structures were obtained. Figure 1 shows SEM images of the synthesized Pd octahedra, truncated octahedra, cuboctahedra, truncated cubes, and cubes (panels a2−e2) with sizes of 64 ± 5, 58 ± 6, 55 ± 5, 54 ± 4, and 48 ± 2 nm, respectively. Pd particles with high size and shape control have been produced, such that they readily formed ordered assembly structures. Figure S1 in the Supporting Information gives the size distribution histograms of these nanocrystals. It is also important to be able to tune the particle dimensions. To make smaller Pd nanocrystals, the amounts of H2PdCl4, KBr, and KI solutions used have been reduced. Similarly, the volumes of these same reagents were increased to enlarge the particle sizes while still maintaining the particle shapes. Figure 1 also presents SEM images of the smaller Pd octahedra (average size of 56 nm), truncated octahedra (49 nm), cuboctahedra (52 nm), truncated cubes (47 nm), and cubes (45 nm) (see panels a1−e1). SEM images for the larger Pd octahedra (69 nm), truncated octahedra (61 nm), cuboctahedra (60 nm), truncated cubes (57 nm), and cubes (61 nm) are also shown in Figure 1 (panels c3−e3). Again, particles in all these samples are highly uniform in size and shape. The tiny amount of KI introduced (5 μL of 10−3 M KI) was found to be critical to the formation of uniform octahedral Pd nanocrystals and possibly other particle shapes. In the absence of KI, large platelike structures were produced (see Figure S2). Iodide, acting as a strong binding ligand, is believed to bind to the Pd precursor by replacing some of its chloride ligands, thus affecting the reduction potential and kinetics of the starting Pd precursor. Interestingly, using the same condition to make medium-sized Pd cubes, but raising the volume of ascorbic acid added to 340 and 450 μL and the reaction temperature to 60 °C, Pd cubes with slight concave faces were obtained (see Figure S3). Use of a larger amount of ascorbic acid led to the formation of smaller concave cubes. Pd concave cubes possess high-index facets and can show an enhanced catalytic activity.1,29−31 Pd concave cubes with sharply raised edges and corners have also been reported.32 Further structural characterization of the Pd nanocrystals has been performed through XRD and TEM analysis. Figure 2 displays the XRD patterns of the medium-sized Pd nanocrystals with different shapes. An exceptionally strong (200) reflection peak has been recorded for Pd cubes and truncated cubes due to their preferential orientation of deposition on the substrate with their {100} faces parallel to the substrate surface. Cuboctahedra yield approximately equal intensity for the (111) and (200) peaks because of their significant {100} and {111} faces. As expected, octahedra and truncated octahedra show a strong (111) peak compared to that of the (200) peak because of their predominant {111} surfaces. TEM characterization of the Pd nanocrystals is presented in Figure 3. The labeled zone axes indicate the viewing directions of the

Figure 2. XRD patterns of the synthesized Pd nanocrystals with medium particle sizes. A standard XRD pattern of Pd is also provided.

particles. The selected-area electron diffraction (SAED) patterns reveal the single-crystalline nature of these nanocrystals. Corner truncations of the cuboctahedra and truncated cubes are clearly identifiable. The concave cubes show slightly depressed faces. Particles with some size distribution are observed in the TEM images because a dilute droplet of Pd nanocrystals was added to a TEM grid. When a drop of concentrated polyhedral particles is added to a substrate, they are more likely to assemble with some particle size sorting, as seen in the SEM images. High-resolution TEM images showing the lattice fringes of Pd nanocrystals and their d-spacing values are available in Figure S4, confirming that a Pd octahedron is bound by (111) lattice planes and a cube is bound by the (200) planes. The synthesis of Pd nanocrystals with systematic shape evolution and tunable size allows a more complete analysis of their optical properties. Figure 4 presents UV−vis extinction spectra of the Pd nanocrystal samples. Two absorption bands can be discerned for these Pd nanostructures. The first band has a fixed position at 250 nm. With increasing particle sizes, the second absorption band becomes more red-shifted from 290 nm to beyond 350 nm. For cubic particles with an average particle size similar to that of octahedral particles, they generally show a more red-shifted band position, possibly because cubic particles have a greater volume than octahedral particles. It is nice to know that simple adjustment of a small volume of KBr solution introduced is effective at tuning the Pd nanocrystal morphology. Figure 5 shows photographs of the solutions as a function of reaction time in the growth of Pd nanocubes and octahedra. The solution color turns dark more quickly in the formation of Pd nanocubes than for the growth of octahedra, showing cubes are produced at a faster rate. Again formation of nanocrystals of different shapes is related to their relative growth rates.1,17 Particle growth appears to be essentially complete within 10 min of reaction, as no further color change was observed. Figure 6 displays the corresponding UV−vis spectra of the solutions. The 226 and 286 nm peaks from the formation of [CTA+]2[PdCl42−] complex decrease progressively, while the band at around 350 nm from the formation of Pd particles emerges. The absorbance vs time plots clearly show that the descending rates of 226 and 286 nm peaks are appreciably faster in the formation of Pd cubes. The effectiveness of KBr to tune the Pd particle morphology should C

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Figure 3. TEM images of single Pd nanocrystals, their corresponding SAED patterns, and TEM images of the particles over larger areas: (a1−a3) Pd octahedra, (b1−b3) truncated octahedra, (c1−c3) cuboctahedra, (d1−d3) truncated cubes, (e1−e3) cubes, and (f1−f3) concave cubes.

be due to the partial replacement of chloride with bromide for the Pd precursor to form [PdBrxCl4−x2−].1,33 Iodide ions should also replace the chloride ligands, but the volume of KI solution used is fixed. The reduction potentials of PdBr42− (0.49 V vs SHE) and PdI42− (0.2279 V) to Pd are lower than that of PdCl42− (0.59 V).34 The Pd precursors used for the formation of Pd octahedra contain some iodide ligands, while the Pd precursors used for the growth of cubes have more bromide

ligands attached. This may explain the relatively faster reduction rate observed in the growth of Pd nanocubes. However, the tiny amounts of KI and KBr introduced, and their simultaneous presence in the solution makes the prediction of relative particle growth rates difficult. With the tiny difference in the volume of 10−3 M KBr solution added to control the particle shape, it is interesting to see if any spectral shift can be detected. Figure 4f presents UV− D

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Figure 4. UV−vis extinction spectra of the synthesized Pd (a) octahedra, (b) truncated octahedra, (c) cuboctahedra, (d) truncated cubes, and (e) cubes with different sizes. The arrows indicate the measured particle sizes. (f) UV−vis spectra taken at various stages of reagent addition in the preparation of the Pd nanocrystal solution. No spectral change can be detected upon the addition of KI and KBr solutions.

Figure 5. Photographs of the solutions taken during the synthesis of (a) Pd octahedra and (b) nanocubes.

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Figure 6. UV−vis extinction spectra of the final solutions taken as a function of the reaction time during the synthesis of (a) Pd octahedra and (b) nanocubes. The 226 nm band absorbance initially decreases with time and eventually becomes steady. (c, d) Plots showing the changes in the absorbance values of the (c) 226 nm band and the (d) 286 nm band as a function of the reaction time. The line at 6 min is drawn for comparison of the relative consumption rates of the [CTA+]2[PdCl42−] complex.

unwashed solution after Pd nanocube synthesis and the solution after one time washing were taken (see Figure S6). The unwashed sample shows only noise-level amount of bromide, while carbon, chloride, and nitrogen signals from the surfactant are significant. Interestingly, the appearance of Pd shoulders at ∼337 and 342 eV indicates the presence of a small amount of Pd(II) species in the solution, which should coordinate to chloride and some bromide ions.36 Iodide is undetectable. After particle washing, bromide, nitrogen, and chloride signals are undetectable. The carbon peak intensity has reduced drastically. The Pd(II) shoulders also disappear. The data show CTAC and residual Pd precursors have been cleanly removed after one washing cycle. Again bromide concentration on the particle surfaces is ultralow and undetectable in the synthesis of Pd nanocubes.

vis spectra of the species added in the synthesis of mediumsized Pd nanocubes. Upon mixing CTAC surfactant and H2PdCl4 solution to form the [CTA+]2[PdCl42−] complex, no further spectral shift was observed after the addition of KI and KBr solutions, revealing that the ultrasmall amounts of KI and KBr solutions used are insufficient in producing any spectral shift. In the absence of CTAC surfactant, still no spectral shift can be detected upon the introduction of the tiny amounts of KBr and KI solutions (see Figure S5). It is remarkable that addition of such a dilute KBr solution is effective at tuning the particle growth rate and shape. The results imply that trace halide impurity in the reagents can indeed lead to irreproducible metal nanostructures. In this study, the formation of Pd nanocrystals of different shapes should not be attributed to adsorption of bromide ions on selective faces because the tiny amounts of KBr solution used here are far from sufficient for capping crystal surfaces (for example, 600 mg of KBr added in the synthesis of 18 nm Pd cubes).35,36 Moreover, Pd shell growth on polyhedral gold cores has been found to proceed via incorporation of surrounding irregularly shaped Pd structures before yielding a well-defined shape.37 Molar ratios of H2PdCl4:KBr:KI are 1400:10:1 in the synthesis of Pd nanocubes. Pd nanocubes can still be synthesized in the absence of bromide ions in the solution.28 Au nanocrystals with various shapes have been produced by adding the same amounts of NaBr.38 Bromide ligand replacement tunes the reduction potential of the precursor, which in turn adjusts the reduction rate and particle growth rate. Previously no Br signals were detected in the XPS spectrum of gold nanocubes synthesized using twice the amount of NaBr than that used here to grow Pd nanocubes, so bromide adsorption on the washed nanocube and cuboctahedra surfaces is negligible.39 To prove this, XPS spectra of the

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CONCLUSIONS Pd nanocrystals with systematic shape evolution from octahedral to cuboctahedral and cubic morphologies have been synthesized in aqueous solution by gradually increasing the amount of KBr solution introduced. Only a trace amount of KBr beyond optical and XPS detection was used. CTAC surfactant and a tiny amount of KI solution were also added into the reaction mixture. By adjusting the volumes of H2PdCl4 and KBr solutions added, particle sizes can be tuned, while still maintaining their well-defined shapes. Extensive structural characterization of the Pd nanocrystals has been performed. Optical analysis reveals more red-shifted absorption bands for Pd cubes than for octahedra of similar sizes. Visual and spectral observation shows that Pd nanocubes are formed at a faster rate than that for the growth of octahedra. The simple synthetic procedure used here means that the Pd nanocrystals can be F

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(12) Lim, B.; Jiang, M.; Tao, J.; Camargo, P. H. C.; Zhu, Y.; Xia, Y. Shape-Controlled Synthesis of Pd Nanocrystals in Aqueous Solutions. Adv. Funct. Mater. 2009, 19, 189−200. (13) Kim, M.; Kim, Y.; Hong, J. W.; Ahn, S.; Kim, W. Y.; Han, S. W. The Facet-Dependent Enhanced Catalytic Activity of Pd Nanocrystals. Chem. Commun. 2014, 50, 9454−9457. (14) Wu, B.; Zheng, N. Surface and Interface Control of Noble Nanocrystals for Catalytic and Electrocatalytic Applications. Nano Today 2013, 8, 168−197. (15) Wang, R.; He, H.; Liu, L.-C.; Dai, H.-X.; Zhao, Z. ShapeDependent Catalytic Activity of Palladium Nanocrystals for the Oxidation of Carbon Monoxide. Catal. Sci. Technol. 2012, 2, 575−580. (16) Xu, Z.-N.; Sun, J.; Lin, C.-S.; Jiang, X.-M.; Chen, Q.-S.; Peng, S.Y.; Wang, M.-S.; Guo, G.-C. High-Performance and Long-Lived Pd Nanocatalyst Directed by Shape Effect for CO Oxidative Coupling to Dimethyl Oxalate. ACS Catal. 2013, 3, 118−122. (17) Chiu, C.-Y.; Huang, M. H. Achieving Polyhedral Nanocrystal Growth with Systematic Shape Control. J. Mater. Chem. A 2013, 1, 8081−8092. (18) Huang, M. H.; Rej, S.; Hsu, S.-C. Facet-Dependent Properties of Polyhedral Nanocrystals. Chem. Commun. 2014, 50, 1634−1644. (19) Chiu, C.-Y.; Yang, M.-Y.; Lin, F.-C.; Huang, J.-S.; Huang, M. H. Facile Synthesis of Au−Pd Core−Shell Nanocrystals with Systematic Shape Evolution and Tunable Size for Plasmonic Property Examination. Nanoscale 2014, 6, 7656−7665. (20) Liu, M.; Zheng, Y.; Zhang, L.; Guo, L.; Xia, Y. Transformation of Pd Nanocubes into Octahedra with Controlled Sizes by Maneuvering the Rate of Etching and Regrowth. J. Am. Chem. Soc. 2013, 135, 11752−11755. (21) Wang, Y.; Xie, S.; Liu, J.; Park, J.; Huang, C. Z.; Xia, Y. ShapeControlled Synthesis of Palladium Nanocrystals: A Mechanistic Understanding of the Evolution from Octahedrons to Tetrahedrons. Nano Lett. 2013, 13, 2276−2281. (22) Niu, W.; Zhang, L.; Xu, G. Shape-Controlled Synthesis of Single-Crystalline Palladium Nanocrystals. ACS Nano 2010, 4, 1987− 2010. (23) Zhang, L.; Niu, W.; Xu, G. Seed-Mediated Growth of Palladium Nanocrystals: The Effect of Pseudo-Halide Thiocyanate Ions. Nanoscale 2011, 3, 678−682. (24) Niu, W.; Li, Z.-Y.; Shi, L.; Liu, X.; Li, H.; Han, S.; Chen, J.; Xu, G. Seed-Mediated Growth of Nearly Monodisperse Palladium Nanocubes with Controllable Sizes. Cryst. Growth Des. 2008, 8, 4440−4444. (25) Jin, M.; Zhang, H.; Xie, Z.; Xia, Y. Palladium Nanocrystals Enclosed by {100} and {111} Facets in Controlled Proportions and Their Catalytic Activities for Formic Acid Oxidation. Energy Environ. Sci. 2012, 5, 6352−6357. (26) Xia, X.; Xie, S.; Liu, M.; Peng, H.-C.; Lu, N.; Wang, J.; Kim, M. J.; Xia, Y. On the Role of Surface Diffusion in Determining the Shape or Morphology of Noble-Metal Nanocrystals. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 6669−6673. (27) Zhang, H.-X.; Wang, H.; Re, Y.-S.; Cai, W.-B. Palladium Nanocrystals Bound by {110} and {100} Facets: From One Pot Synthesis to Electrochemistry. Chem. Commun. 2012, 48, 8362−8364. (28) Zhang, J.; Feng, C.; Deng, Y.; Liu, L.; Wu, Y.; Shen, B.; Zhong, C.; Hu, W. Shape-Controlled Synthesis of Palladium Single-Crystalline Nanoparticles: The Effect of HCl Oxidative Etching and FacetDependent Catalytic Properties. Chem. Mater. 2014, 26, 1213−1218. (29) Sreedhala, S.; Sudheeshkumar, V.; Vinod, C. P. Structure Sensitive Chemical Reactivity by Palladium Concave Nanocubes and Nanoflowers Synthesized by a Seed Mediated Procedure in Aqueous Medium. Nanoscale 2014, 6, 7496−7502. (30) Collins, G.; Schmidt, M.; O’Dwyer, C.; McGlacken, G.; Holmes, J. D. Enhanced Catalytic Activity of High-Index Faceted Palladium Nanoparticles in Suzuki−Miyaura Coupling Due to Efficient Leaching Mechanism. ACS Catal. 2014, 4, 3105−3111. (31) Zhang, J.; Zhang, L.; Xie, S.; Kuang, Q.; Han, X.; Xie, Z.; Zheng, L. Synthesis of Concave Palladium Nanocubes with High-Index

easily prepared for catalytic activity examination. They may also be used as building blocks for the fabrication of supercrystals.

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ASSOCIATED CONTENT

S Supporting Information *

Exact reagent amounts used for growing Pd nanocrystals with size control, particle size distribution histograms, additional SEM and TEM images of the synthesized Pd nanocrystals, UV−vis spectra, and XPS data. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01337.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.H.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology of Taiwan for support of this research (NSC 101-2113-M-007-018-MY3, NSC 102-2811-M-007-003, and NSC 102-2633-M-007-002).

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